Mixed Electronic States in Molecular Dimers: Connecting Singlet Fission, Excimer Formation, and Symmetry-Breaking Charge Transfer

Fluorescence-Detected Pump-Probe Spectroscopy for Artifact-Free Detection of Stokes Shift Dynamics

Fluorescence-detected pump-probe (F-PP) spectroscopy is a recently developed method to study excited-state dynamics. F-PP combines the temporal resolution of conventional transient absorption (TA) spectroscopy with the sensitivity of fluorescence detection. In this work, we demonstrate inherently phase-stable F-PP spectroscopy using 20 fs pulses to monitor the ultrafast Stokes shift dynamics of a solvated fluorophore (Y12). We observed a shift in the stimulated emission maximum with a time constant of 84 fs. In contrast to TA, F-PP provides a coherent artifact-free view of this process. Using quantitative signal background subtraction, as discussed in this work, F-PP uncovers the pure stimulated emission spectrum and its ultrafast dynamics. This signal isolation is a clear advantage over TA, where different contributions often overlap heavily. We compare results from F-PP and TA on an equal footing using the same excitation pulses, emphasizing the features and advantages of the F-PP technique.

A Light-Responsive Metal-Organic Framework with Perchlorinated Nanographene Ligands

Harnessing water as a sustainable electron source for artificial photosynthesis remains a significant challenge. This work presents Alice-MOF-1, a novel zirconium metal-organic framework (MOF) incorporating hexatopic ligands with a perchlorinated hexa-peri-hexabenzocoronene (HBC) core, as a photocatalyst for CO2 reduction using water as the terminal electron donor. Contortion of the ligand, induced by edge chlorination, minimizes π-stacking and enhances solubility, enabling direct MOF synthesis. The controlled arrangement of chromophores within Alice-MOF-1 is crucial for enabling the complex multielectron redox reactions. The unique ligand architecture within the MOF promotes symmetry-breaking charge transfer (SBCT), a mechanism observed in natural photosynthesis, leading to efficient charge separation with minimal energy loss. Femtosecond transient absorption spectroscopy and time-resolved electron paramagnetic resonance spectroscopy (EPR) confirm the formation of long-lived radical ions, providing direct evidence for efficient SBCT and negligible charge recombination. These findings demonstrate the power of MOF-based chromophore assemblies to mimic nature's light-harvesting strategies for sustainable energy conversion.

Ultrafast excited-state dynamics and "three-in-one" phototheranostic properties of a phenanthroline-carbolong photosensitizer

The favorable excited-state dynamics, nonlinear optics, and extraordinary phototheranostic capabilities of conjugated metallaaromatics are attractive topics of research. A promising and multifunctional photosensitizer, double-phenanthroline-carbolong DPC, was investigated comparatively. It featured strong two-photon absorption (2PA) properties within the near-infrared (NIR) range, with a maximum 2PA cross-section of ∼7417 GM at 770 nm in MeOH. Time-dependent density functional theory and ultrafast excited-state dynamics illustrated that fast charge transfer coupled with intersystem crossing to the stable triplet state outcompeted radiative decays. The "three-in-one" phototherapeutic effect included NIR-wavelength 2PA excitation, photodynamic therapy, and photothermal therapy in DPC, as illustrated subsequently. The significant contribution from the intrinsic intramolecular charge communication along the DPC skeleton provided the possibility for moderate photothermal conversion (η = 36.8%) and photodynamic synergistic therapy (Φ Δ = 8.4%). Interestingly, singlet oxygen generation from DPC was also observed when irradiated at two-photon excitation wavelengths. In vitro experiments demonstrated the synergistic phototoxicity of DPC in 4T1 cancer cells. This work offers insights into extraordinary carbolong metallaaromatics and highlights their potential applications in the fields of nonlinear optics and multifunctional phototheranostics.

Measurement of Anisotropic Exciton Transport Lengths in Organic Crystals Using Photoetching

Measuring anisotropic exciton transport in organic crystals goes beyond just assessing one-dimensional (1D) transport. It offers a deeper understanding of how molecular packing and interactions affect exciton transport in different dimensions. However, achieving nanoscale precision in measuring anisotropic exciton transport lengths and linking them to specific crystalline directions remains a formidable challenge. Here the development of a photoetching method is reported to visualize the exciton transport distances as gaps within two-dimensional (2D) crystals, which in turn allows for the use of a scanning electron microscope (SEM) to precisely measure the sizes. The photoetching method combined with hetero-seeded self-assembly enables the use of conventional fluorescence spectrometry for precise determination of anisotropic exciton transport lengths in 2D structures at the nanoscale. Relying on this novel method, It is unexpectedly found that increasing intermolecular interactions in one crystal direction not only improves exciton transport in that dimension but also enhances exciton transport in the other dimension. These findings provide valuable insights for engineering organic materials that require efficient exciton transport across extended distances.

Tuning Excited-State Charge Transfer Character in Cofacial Core-Substituted Perylenediimide Dimers

Understanding the interplay between excimer formation and symmetry-breaking charge separation is important for optimizing charge separation in organic photovoltaic materials. To explore this connection, we synthesized four 1,6,7,12-tetrakis(p-X-phenoxy)perylene-(3,4:9,10)-bisdicarboximide cofacially stacked dimers, where X = MeO, tert-butyl, Br, and CF3. Steady-state spectroscopy reveals H-type aggregation and excimer formation in all four dimers, while transient absorption spectroscopy shows relatively small changes in their excited-state absorptions. However, time-resolved fluorescence (TRF) spectroscopy shows that relaxation occurs from an initial Frenkel exciton-dominated excimer state to one in which charge transfer (CT) character contributes. Relaxation to the lower-lying state with CT character is attributed to a combination of structural and charge distribution changes elicited by varying the substituents. This study illustrates how subtle changes in charge distribution and structure can combine to influence the excited state dynamics that influence charge separation in molecular dimers.

Modulating the Charge Transfer Coupling in Boron-Dipyrromethene Homodimers by π-Bridge Units

To mimic the excitation energy conversion mechanisms observed in natural light-harvesting systems, we have extensively investigated photoinduced symmetry-breaking charge separations (SBCSs) in various multichromophoric model systems have been extensively investigated. However, designing multichromophoric model systems capable of simultaneously achieving ultrafast and complete SBCS remains a significant challenge. In this study, we employed benzene, thiophene, and furan as π-bridges to develop a series of boron dipyrromethene (BODIPY) homodimers. Spectral analysis, together with an estimation of the π-bridge-dependent charge transfer (CT) coupling using the fragment charge difference method, reveals that π-bridge units with different electron-donating abilities can effectively modulate the CT coupling between chromophores. Notably, the furan-based π-bridge, exhibiting the most pronounced electron-donating character, facilitates symmetry-breaking charge transfer (SBCT), i.e., excimer formation with a time constant of about 12 ps in weak polar toluene. Furthermore, a dramatic increase in the SBCS rate constant was observed in highly polar acetonitrile, improving from 60.4 ps for the benzene-bridged homodimer to 2.9 ps for the furan-bridged counterpart. These findings underscore the potential of π-bridge units in tuning the photophysical properties of covalent molecular aggregates by optimizing such systems for specific applications such as organic photovoltaics and photocatalysis.

Intramolecular Singlet Fission in Individual Graphene Nanoribbons─Competition with a Charge Transfer

Graphene nanoribbons (NRs) constitute a versatile platform for developing novel materials, where their structure governs their optical, electronic, and magnetic properties while also shaping their excited-state dynamics. Here, we investigate a set of three twisted N-doped molecular NRs of increasing length, obtained by linearly fusing perylene diimide to pyrene and pyrazino- or thiadiazolo-quinoxaline residues. By employing various temperature-dependent time-resolved spectroscopy techniques, we reveal how the flexible twisted NR geometry promotes the formation of a mixed electronic state with varying contributions from locally excited and charge-transfer (CT) states. The fate of this mixed state is highly sensitive to the molecular geometry, length, and solvent polarity. For the shortest NR, intersystem crossing dominates the deactivation pathway, efficiently generating triplets in low-polarity solvents. In contrast, for the extended NRs, intramolecular singlet fission (SF) takes place within a single nanoribbon. This is enabled by enhanced superexchange coupling due to a pronounced push-pull nature and the existence of multiple localized π-electron states caused by heteroatom doping, thereby circumventing the need for dimeric interactions typically associated with conventional SF systems. In higher-polarity environments, evidence of a (diabatic) CT state emerges. These findings underscore the intricate relationship between geometry, energy levels, and excited-state dynamics in twisted N-doped NRs.

Proximal Oblique-Packing of Heptamethine Cyanines through Spiro-Connection Boosts Triplet State Generation in Near-Infrared

Near-infrared (NIR) triplet dyes are the cornerstones of cutting-edge biomedical and material applications. The difficulty in rational development of triplet dyes increases exponentially as the absorption wavelength shifts deeper into the NIR range. Although classical H-/J-typed packing of NIR dyes has the potential to enhance intersystem crossing (ISC) compared with that in single-chromophore dyes, the triplet state quantum yields remain limited in such strategy. Herein, proximal oblique-packed (V-shaped) heptamethine cyanines (SZ780) through spiro-connection were achieved. Multi-channel ultrafast ISC were direct observed in SZ780 and a record high ISC rate constant (up to ~1011 s-1) is registered among all the reported NIR triplet dyes. SZ780 exhibits a triplet state quantum yield of 18.9 % upon excitation at 750 nm, which is almost an order of magnitude higher than that of the monomer (IR780, 2.1 %) and nearly threefold increase compared to that of the H-packed dimer (SC780) (6.7 %). Moreover, SZ780 efficiently generates singlet oxygen under 808 nm light irradiation, inducing cancer cell apoptosis in vivo. These findings demonstrate that constructing V-aggregated dyes system by spiro-connection offers a powerful approach for the design of high-performance NIR triplet sensitizers.

Reversible gating of singlet fission by tuning the role of a charge-transfer state

Stimulus-responsive triplet excited states and multiexcitonic logic gates have garnered increasing interest. Singlet fission is an efficient multiple exciton generation process, in which one singlet converts into two triplets. Singlet fission is, however, rarely reported to be switchable by external stimuli. Here we design a meta-diethynylphenylene-linked tetracene dimer featuring pyridyl endgroups that function as an acid/base-responsive switch, enabling the reversible modulation of singlet fission. In its neutral form, the interchromophore charge-transfer state facilitates singlet fission and promotes the formation of a correlated triplet-pair state. Upon treatment with acid, protonation of the pyridyl nitrogens generates a more strongly electron-accepting pyridinium, leading to an intra-chromophore charge-transfer state, which inhibits singlet fission. Finally, an IMPLICATION logic gate is constructed by using acid and base as inputs and monitoring the formation of triplet excited states based on singlet fission.

Structural Dynamic Impact of Chromophores on Singlet Fission

Singlet fission (SF) represents a unique mechanism that allows a single high-energy photon to split into two triplet excitons, significantly enhancing the quantum efficiency of photovoltaic and optoelectronic materials. Therefore, SF shows great potential for applications in solar cells and optoelectronic devices. Despite significant progress in recent years, synergistic effects of various factors that govern the structural dynamics of solvated chromophores individually or jointly and lead to the complicated dynamics of SF still require further exploration. This Perspective systematically analyzes various factors that affect and even modulate the efficiency and mechanism of SF, especially the structural dynamics of solvated chromophores, including molecular vibrations, chromophore dynamic interactions, solvent and environmental fluctuations, temperature and thermodynamic variations, and pressure and physical changes. The inherent dynamic characteristics of these factors dominate the structural and electronic properties and interchromophore interactions, thereby manipulating the energetics and kinetic pathways of SF and ultimately determining the SF efficiency and feasibility. The discussions presented in this Perspective provide dynamics insights, important foundations, and strategies for future design of materials and performance optimization of optoelectronic devices through considering dynamic factors.

Supramolecular rosette intermediated homochiral double helix

Precise organization of organic molecules into homochiral double-helix remains a challenge due to the difficulty in controlling both self-assembly process and chirality transfer across length scales. Here, we report that a type of bisnaphthalene bisurea molecule could assemble into chirality-controlled nanoscale double-helices by a supramolecular rosette-intermediated hierarchical self-assembly mechanism. A solvent-mixing self-assembly protocol is adopted to direct bisnaphthalene bisurea cyclization into chiral discrete rosettes through cooperative intramolecular and intermolecular hydrogen bonds. Controlled hexagonal packing of rosettes at higher concentrations gives one-dimensional single-stranded nanofibers, which intertwine into well-defined double-helix nanostructures with preferred chirality that depends on the absolute configurations of bisnaphthalene bisurea. The hierarchical organization of bisnaphthalene bisurea molecules enables effective excitation energy delocalization within the double-helix, which contributes to near-unity energy transfer from double-helix to adsorbed acceptor dyes even in donor/acceptor ratios over 1000, leading to bright circularly polarized luminescence from the originally achiral acceptor. The experimental and theoretical simulation results not only provide a hierarchical strategy to fabricate homochiral double-helix but also bring insights in understanding the high-efficiency light-harvesting process in photosystem II.

A comparative investigation on excimer fluorescence toward its bright future

Must excimers quench fluorescence? This study aims to clarify the misconception that excimers are defective species with weak fluorescence. For this purpose, we utilized a rigid xanthene template to connect anthracene units for constructing an inter-excimer and an intra-excimer. Their photophysical properties were systematically investigated in solution and crystal forms, representing dynamic and static environments, respectively. In solutions, the inter- and intra-excimers exhibited low fluorescence efficiencies due to the limited formation and ease of dissociation of the excimers. In crystals, the inter- and intra-excimers both demonstrated a significant increase in fluorescence efficiency, which was ascribed to the greatly suppressed non-radiation for the static excimer in a rigid environment. Furthermore, the efficiency of the inter-excimer was higher than that of the intra-excimer, which arose from the more stable excited state for more effective non-radiative suppression. Therefore, it was concluded that the probability and stability of excimer formation are the key factors for improving excimer fluorescence efficiency. Overall, their fluorescence efficiencies can be ranked as follows: dynamic inter-excimer < dynamic intra-excimer < static intra-excimer < static inter-excimer, which is subjected to environmental rigidity and excimer stability. This work will provide a comprehensive understanding of excimers and propose a novel design strategy to achieve high-efficiency fluorescent materials for innovative organic photo-functional applications.

Cross-Transition-Dipole Stacking of Conjugated Organic Molecules: Structure, Exciton Behavior and Optoelectronic Property

Organic conjugated molecules have gained widespread application as organic semiconductors due to their unique optoelectronic properties. The rigidity of these large conjugated structures facilitates strong intermolecular interactions, which significantly influence their properties in the solid state through various molecular arrangements. The study of the relationship among molecular arrangement, exciton behavior, and optoelectronic properties is an eternal research topic. Cross-dipole stacking is a specific molecular arrangement that demonstrates unique characteristics and has received continuous attention over the past decades. This mini-review will first discuss the unique exciton behaviors in cross-dipole stacking based on exciton models, including weak exciton coupling and suppression of Förster resonance energy transfer. These exciton behaviors, determined by molecular stacking arrangements, are fundamental to the optoelectronic properties of cross-dipole stacking systems. Next, we will introduce well-defined cross-dipole systems and summarize their design principles from a molecular structure perspective. Finally, we will present the specific optoelectronic properties of cross-dipole stacking systems and their outstanding performance, such as high solid-state luminescence, good charge carrier mobility, and significant CD/CPL. Through this mini-review, we hope to enhance the understanding of cross-dipole stacking, contributing to the construction of such systems, the exploration of excited-state behaviors, and the discovery of high-performance materials.

Self-Assembled Bent Perylenediimides

The properties of π-functional materials are predominantly influenced by both their molecular structures and interactions between π-systems. Recent advancements have focused on modifying the geometry or topology of π-molecules from planar to nonplanar conformations to tailor molecular properties. However, the interactions among nonplanar π-molecules remain largely unexplored, likely due to the significant reduction in contact surfaces arising from their curved structures. Herein, we investigated the electro-optical properties and π-stacking behaviors of a series of bent perylenediimides (B-PDIs) with gradual changes in bending angles, achieved by altering the lengths of linear alkyl chains connecting the two nitrogen positions of each PDI. Curvature-dependent self-assembly of these bent PDIs is observed, which is primarily driven by dipole-dipole interactions rather than dispersion forces. More importantly, fine-tuning intermolecular coupling through bending enables excited-state symmetry-breaking charge separation in [n]B-PDIs (n = 16, 12) in the crystalline solid state.

Insight into the Excited States in Monomers and π-Stacked Dimers of Azulene-Fused Acenes: ADC(2) and TD-DFT Studies

Charge transfer (CT) states in polycyclic aromatic hydrocarbons play crucial roles in determining their electronic properties and their potential applications in organic electronics. In this work, we investigate the nature of the excited states in monomers and π-stacked dimers of azulene-fused naphthalene and anthracene systems, focusing on the interplay between structure and excited-state properties. Four different isomers for azulene-fused naphthalene (NapAz-A, NapAz-B, NapAz-C, and NapAz-D) and anthracene (AntAz-A, AntAz-B, AntAz-C, and AntAz-D) are considered. The excited-state studies are performed at the SCS-ADC(2) level and at the TD-DFT level using CAM-B3LYP, SCS-ωB2GP-PLYP, and SCS-RSX-QIDH functionals. For the monomers, the SCS-ADC(2) results reveal that states with CT characters are different in naphthalene- and anthracene-based systems. In π-stacked dimers, a few of the excited states are of the charge resonance (CR) type in NapAz-A, NapAz-B, and NapAz-C and the intermolecular CT type in NapAz-D. Similarly, AntAz-A, AntAz-B, and AntAz-D have some CR type excited states, whereas the AntAz-C isomer has intramolecular CT type excited states. Overall, among the three DFT functionals considered, CAM-B3LYP has been found to reproduce well the SCS-ADC(2) excited results in both monomers and π-stacked dimers.

Symmetry-Breaking Charge-Separation in a Subphthalocyanine Dimer Resolved by Two-Dimensional Electronic Spectroscopy

Understanding the role of structural and environmental dynamics in the excited state properties of strongly coupled chromophores is of paramount importance in molecular photonics. Ultrafast, coherent, and multidimensional spectroscopies have been utilized to investigate such dynamics in the simplest model system, the molecular dimer. Here, we present a half-broadband two-dimensional electronic spectroscopy (HB2DES) study of the previously reported ultrafast symmetry-breaking charge separation (SB-CS) in the subphthalocyanine oxo-bridged homodimer μ-OSubPc2. Electronic structure calculations and 2D cross-peaks reveal the dimer's excitonic structure, while ultrafast evolution of the multidimensional spectra unveils subtle features of structural relaxation, solvation dynamics, and inhomogeneous broadening in the SB-CS. Analysis of coherently excited vibrational motions reveals dimer-specific low-frequency Raman active modes coupled to higher-frequency vibrations localized on the SubPc cores. Finally, beatmap amplitude distributions characteristic of excitonic dimers with multiple bright states are reported and analyzed.

'Invisible' Molecular Dynamics Revealed for a Conformationally Chiral π-Stacked Perylene Bisimide Foldamer

Whilst energetic and kinetic aspects of folding processes are meanwhile well understood for natural biomacromolecules, the folding dynamics in so far studied artificial foldamer counterparts remain largely unexplored. This is due to the low energy barriers between their conformational isomers that make the dynamic processes undetectable with conventional methods such as UV/Vis absorption, fluorescence, and NMR spectroscopy, making such processes 'invisible'. Here we present an asymmetric perylene bisimide dimer (bis-PBI 1) that possesses conformational chirality in its folded state. Owing to the large interconversion barrier (≥116 kJ mol-1), four stereoisomers could be separated and isolated. Since the interconversion between these stereoisomers requires the foldamer to first open and then to re-fold, the transformation of one stereoisomer into others allowed us to 'visualize' the dynamics of folding with time and determine its lifetimes and the energetic barriers associated with the folding process. Supported by quantum chemical calculations, we identified the open structure to be only a fleetingly metastable state of higher energy. Our experimental observation of the kinetics associated with the molecular dynamics in the PBI foldamer advances the fundamental understanding of folding in synthetic foldamers and paves the way for the design of smart functional materials.

Distance-Dependent Symmetry-Breaking Charge Transfer in 9,10- Bis(phenylethynyl)anthracene Dimers

To investigate the effect of the through-bond coupling strength on the symmetry-breaking charge separation (SB-CS) dynamics and mechanism, three 9,10-bis((4-hexylphenyl)ethynyl)anthracene dimers with varying distances, viz., a single-bond linked dimer (0-dimer), a phenylene linked dimer (1-dimer) and a para-biphenylene linked dimer (2-dimer), were synthesized and studied systematically using steady-state and transient spectroscopy. Steady-state absorption spectra revealed that the electronic coupling strength decreased gradually with the increase of the inter-chromophore distance, and the transition of S0→S1 includes the dark charge transfer (CT) excitation. fs-TA spectra demonstrated that SB-CS could be conducted in both weakly and highly polar solvents for 0-dimer, but the SB-CS dynamics has a significant difference. In weakly polar solvents, SB-CS only produces the partial CT (PCT) state, but it could generate the CT state via the PCT state in highly polar solvent. In comparison, SB-CS is only proceeded in highly polar solvents in 1-dimer and 2-dimer to produce the CT state directly. These results demonstrate that the SB-CS dynamics is strongly dependent on the inter-chromophore electronic coupling, and the relatively strong electronic coupling is crucial for the occurrence of SB-CS in weakly polar environment that is commonly presented in photoelectric devices.

Enabling Ultrafast Intramolecular Singlet Fission in Perylene Diimide Tetramer with Saddle-Shaped Linker

Intramolecular singlet fission (SF) in multichromophore systems is of high interest for photovoltaic application. As an attractive candidate for SF-based devices, enabling efficient SF in covalent oligomers of perylene diimide (PDI) still remains challenging. In this work, inter-PDI SF with τSF = ∼150 ps and ∼150% triplet yield in a covalent tetramer COTh-FPDI was facilitated by employing a saddle-shaped cyclooctatetrathiophene (COTh) core and fused linking with PDIs. In comparison, ultrafast symmetry-breaking charge separation (τCS = ∼100 fs) was observed for the tetramer COTh-αPDI with flexible linking. Taking advantage of rigid linking and minimized excited-state structural relaxation, unique inter-PDI geometry in COTh-FPDI can be fully defined by the topological characteristic of COTh, which plays a key role for inter-PDI electronic coupling required by SF. Our work provides a new strategy for enabling intramolecular SF by predefining interchromophore geometry by a rigid structure, which might be inspired for future designing of multichromophore SF systems.

Polarity of Ordered Solvent Molecules in 9,9'-Bianthracene Single Crystals Selects between Singlet Fission or Symmetry-Breaking Charge Separation

Singlet exciton fission (SF) and symmetry-breaking charge separation (SB-CS) are both photophysical processes that can occur between two organic chromophores and are both of interest to improve solar energy conversion. Here, we tuned the photophysics of a 9,9'-bianthracene (BA) single crystal between SF and SB-CS using solvent intercalation to change the electric field within the crystal. Crystals of BA were grown in o-xylene, chlorobenzene, o-dichlorobenzene, and benzonitrile, as well as solvent-free from a melt. The crystals were studied by X-ray diffraction, steady-state optical spectroscopy, and transient absorption microscopy to elucidate the role of the intercalated solvent molecules. The crystals with no solvent in the structure undergo fast SF (<2 ps), while the crystals with intercalated moderately polar solvents o-xylene, chlorobenzene, and o-dichlorobenzene show evidence of charge-transfer-mediated SF. Finally, the crystals containing highly polar benzonitrile undergo SB-CS instead of SF. These results demonstrate that controlling solvation of BA in the crystal structure can tune its photophysics between SF and SB-CS.

Unusual Hydrophobic Property of Blue Fluorescent Amino Acid 4-Cyanotryptophan

It is a common belief that the negative heat capacity change (ΔCp) associated with protein folding, which is a manifestation of the hydrophobic effect, results from a decrease in the solvent accessible hydrophobic surface area. Herein, we investigate the conformational energy landscape and dynamics of a tetrapeptide composed of two glycine and two 4-cyanotryptophan residues using time-resolved fluorescence spectroscopy, molecular dynamics simulations, and density functional theory calculations and find that, contrary to this expectation, the hydrophobic association of two 4-cyanotryptophan side chains leads to a positive ΔCp (approximately 543 J K-1 mol-1). Furthermore, we find that promoting one of the 4-cyanotryptophans to its excited electronic state strengthens this self-association. Taken together, our results provide not only insight into how modification of an aromatic amino acid can affect its hydrophobicity but also a potential strategy for designing protein sequences that can fold (unfold) at high (low) temperatures.

Conformational dynamics of the pyrene excimer

The conformational dynamics of the pyrene excimer play a critical role in its unique fluorescence properties. Yet, the influence of multiple local minima on its excited-state behavior remains underexplored. Using a combination of time-dependent density functional theory (TD-DFT) and unsupervised machine learning analysis, we have identified and characterized a diverse set of stable excimer geometries in the first excited state. Our analysis reveals that rapid structural reorganization towards the most stable stacked-twisted conformer dominates the excimer's photophysics, outcompeting radiative relaxation. This conformer, which is primarily responsible for the characteristic red-shifted, structureless fluorescence emission, reconciles experimental observations of long fluorescence lifetimes and emission profiles. These findings provide new insights into the excited-state dynamics of excimers. They may inform the design of excimer-based materials in fields ranging from organic electronics to molecular sensing.

Coherence in Chemistry: Foundations and Frontiers

Coherence refers to correlations in waves. Because matter has a wave-particle nature, it is unsurprising that coherence has deep connections with the most contemporary issues in chemistry research (e.g., energy harvesting, femtosecond spectroscopy, molecular qubits and more). But what does the word "coherence" really mean in the context of molecules and other quantum systems? We provide a review of key concepts, definitions, and methodologies, surrounding coherence phenomena in chemistry, and we describe how the terms "coherence" and "quantum coherence" refer to many different phenomena in chemistry. Moreover, we show how these notions are related to the concept of an interference pattern. Coherence phenomena are indeed complex, and ambiguous definitions may spawn confusion. By describing the many definitions and contexts for coherence in the molecular sciences, we aim to enhance understanding and communication in this broad and active area of chemistry.

Light/X-ray/ultrasound activated delayed photon emission of organic molecular probes for optical imaging: mechanisms, design strategies, and biomedical applications

Conventional optical imaging, particularly fluorescence imaging, often encounters significant background noise due to tissue autofluorescence under real-time light excitation. To address this issue, a novel optical imaging strategy that captures optical signals after light excitation has been developed. This approach relies on molecular probes designed to store photoenergy and release it gradually as photons, resulting in delayed photon emission that minimizes background noise during signal acquisition. These molecular probes undergo various photophysical processes to facilitate delayed photon emission, including (1) charge separation and recombination, (2) generation, stabilization, and conversion of the triplet excitons, and (3) generation and decomposition of chemical traps. Another challenge in optical imaging is the limited tissue penetration depth of light, which severely restricts the efficiency of energy delivery, leading to a reduced penetration depth for delayed photon emission. In contrast, X-ray and ultrasound serve as deep-tissue energy sources that facilitate the conversion of high-energy photons or mechanical waves into the potential energy of excitons or the chemical energy of intermediates. This review highlights recent advancements in organic molecular probes designed for delayed photon emission using various energy sources. We discuss distinct mechanisms, and molecular design strategies, and offer insights into the future development of organic molecular probes for enhanced delayed photon emission.

Intramolecular benzene excimer formation in 13,14-diphenyldibenzo[b,j][4,7]phenanthroline

13,14-diphenyldibenzo[b,j][4,7]phenanthroline (DBP3) in various solvents was studied by time-resolved fluorescence and fs transient absorption (fs-TA) spectroscopy. An intramolecular benzene excimer is demonstrated to form within DBP3; it exhibits strong redshifted emission with maximum at 540-640 nm. "Intrinsic" fluorescence from DBP3 is dramatically quenched down to τ = 50-400 fs in all the solvents studied. Fs-TA and time-resolved fluorescence spectra have proved that relaxed intramolecular benzene excimer is formed from S1 state via hot excimer state with three lifetime components: 50 fs, ∼3.5 ps, and ∼25 ps, which are of the inertial (electronic) and diffusive parts of the relaxation due to solute-solvent interaction. Formation of triplet states via intersystem crossing was observed directly from the upper excited electronic states of DBP3.

Ultrafast and Coherent Dynamics in a Solvent Switchable "Pink Box" Perylene Diimide Dimer

Perylene diimide (PDI) dimers and higher aggregates are key components in organic molecular photonics and photovoltaic devices, supporting singlet fission and symmetry breaking charge separation. Detailed understanding of their excited states is thus important. This has proven challenging because interchromophoric coupling is a strong function of dimer architecture. Recently, a macrocyclic PDI dimer was reported in which excitonic coupling could be turned on and off simply by changing the solvent. This presents a useful case where coupling is modified without synthetic changes to tune supramolecular structure. Here we present a detailed study of solvent dependent excited state dynamics in this dimer by means of coherent multidimensional spectroscopy. Spectral analysis resolves the different coupling strengths, which are consistent with solvent dependent changes in dimer conformation. The strongly coupled conformer forms an excimer within 300 fs. The low-frequency Raman active modes recovered from two-dimensional electronic spectra reveal frequencies characteristic of exciton coupling. These are assigned to modes modulating the coupling from the corresponding DFT calculations. Further analysis reveals a time dependent frequency during excimer formation. Analysis of two-dimensional "beatmaps" reveals features in the coupled dimer which are not predicted by the displaced harmonic oscillator model and are assigned to vibronic coupling.

Facilitating intrinsic delayed fluorescence of conjugated emitters by inter-chromophore interaction

Delayed fluorescence (DF) is a unique emitting phenomenon of great interest for important applications in organic optoelectronics. In general, DF requires well-separated frontier orbitals, inherently corresponding to charge transfer (CT)-type emitters. However, facilitating intrinsic DF for local excited (LE)-type conjugated emitters remains very challenging. Aiming to overcome this obstacle, we demonstrate a new molecular design strategy with a DF-inactive B,N-multiple resonance (MR) emitter as a model system. Without the necessity of doping with heavy atoms, we synthesized a co-facial dimer in which an excimer-like state (Sexc) was expected to facilitate efficient reverse intersystem crossing (RISC, T1 → Sexc) and intrinsic DF. Benefiting from greatly enhanced SOC and reduced ΔE ST, the proof-of-concept emitter Np-2CzB exhibited k RISC up to 6.5 × 105 s-1 and intrinsic DF with >35% contribution (Φ DF/Φ F) in dilute solution. Further investigation indicated that Sexc state formation relies on an optimized co-facial distance (d = ∼4.7 Å), strong inter-chromophore interaction (J coul > 450 cm-1) and a rigid structure (Γ S1→S0 < 350 cm-1). Although our strategy was demonstrated with a B,N-MR emitter, it can be applicable to many LE-type conjugated emitters without intrinsic DF. By triggering potential DF emission, many classic emitters might play a more important role in optoelectronics.

Surface Loading Dictates Triplet Production via Singlet Fission in Anthradithiophene Sensitized TiO2 Films

Singlet fission, the process of transforming a singlet excited state into two lower energy triplet excited states, is a promising strategy for improving the efficiency of dye-sensitized solar cells. The difficulty in utilizing singlet fission molecules in this architecture is understanding and controlling the orientation of dyes on mesoporous metal oxide surfaces to maximize triplet production and minimize detrimental deactivation pathways, such as electron injection from the singlet or excimer formation. Here, we varied the concentration of loading solutions of two anthradithiophene dyes derivatized with either one or two carboxylic acid groups for binding to a metal oxide surface and studied their photophysics using ultrafast transient absorption spectroscopy. For the single carboxylic acid case, an increase in dye surface coverage led to an increase in apparent triplet excited-state growth via singlet fission, while the same increase in coverage with two carboxylic acids did not. This study represents a step toward controlling the interactions between molecules at mesoporous interfaces.

Distinct vibrational motions promote disparate excited-state decay pathways in cofacial perylenediimide dimers

A complex interplay of structural, electronic, and vibrational degrees of freedom underpins the fate of molecular excited states. Organic assemblies exhibit a myriad of excited-state decay processes, such as symmetry-breaking charge separation (SB-CS), excimer (EX) formation, singlet fission, and energy transfer. Recent studies of cofacial and slip-stacked perylene-3,4:9,10-bis(dicarboximide) (PDI) multimers demonstrate that slight variations in core substituents and H- or J-type aggregation can determine whether the system follows an SB-CS pathway or an EX one. However, questions regarding the relative importance of structural properties and molecular vibrations in driving the excited-state dynamics remain. Here, we use a combination of two-dimensional electronic spectroscopy, femtosecond stimulated Raman spectroscopy, and quantum chemistry computations to compare the photophysics of two PDI dimers. The dimer with 1,7-bis(pyrrolidin-1'-yl) substituents (5PDI2) undergoes ultrafast SB-CS from a photoexcited mixed state, while the dimer with bis-1,7-(3',5'-di-t-butylphenoxy) substituents (PPDI2) rapidly forms an EX state. Examination of their quantum beating features reveals that SB-CS in 5PDI2 is driven by the collective vibronic coupling of two or more excited-state vibrations. In contrast, we observe signatures of low-frequency vibrational coherence transfer during EX formation by PPDI2, which aligns with several previous studies. We conclude that key electronic and structural differences between 5PDI2 and PPDI2 determine their markedly different photophysics.

Constructing Heavy-Atom-Free Photosensitizers for Hypoxic Tumor Phototherapy Based on Donor-Excited Photoinduced Electron-Transfer-Driven Type-I and Type-II Mechanisms

The spin-orbit charge transfer intersystem crossing (SOCT-ISC) photophysical process has shown great potential for constructing heavy-atom-free photosensitizers (PSs) for photodynamic therapy (PDT) of tumors. However, for almost all such PSs reported to date, the SOCT-ISC is driven by the acceptor-excited photoinduced electron transfer (a-PeT). In this work, for the first time the donor-excited photoinduced electron transfer (d-PeT)-driven SOCT-ISC mechanism is utilized to construct the heavy-atom-free PSs for PDT of tumors by directly installing the electron-deficient N-alkylquinolinium unit (as an electron acceptor) into the meso-position of the near-infrared (NIR) distyryl Bodipy chromophore (as an electron donor). In the less polar environment, the PSs exist as the monomer and promote the production of singlet oxygen (1O2) (Type-II) relying on the d-PeT-driven population of the triplet excited state via SOCT-ISC, whereas in the aqueous environment, they exist as nanoaggregates and induce the generation of superoxides (O2-•) and hydroxyl radicals (HO•) (Type-I) via the d-PeT-driven formation of the delocalized charge-separated state. The PSs could rapidly be internalized into cancer cells and induce the simultaneous production of intracellular 1O2, O2-•, and HO• upon NIR light irradiation, endowing the PSs with superb photocytotoxicity with IC50 values up to submicromolar levels whether under normoxia or under hypoxia. Based on the PSs platform, a tumor-targetable PS is developed, and its abilities in killing cancer cells and in ablating tumors without damage to normal cells/tissues under NIR light irradiation are verified in vitro and in vivo. The study expands the design scope of PSs by introducing the d-PeT conception, thus being highly valuable for achieving novel PSs in the realm of tumor PDT.

Overcrowded 14,14'-Bidibenzo[a,j]anthracenes: Challenges in Syntheses and Atypical Property of Lacking Symmetry-Breaking Charge Transfer (SBCT)

14,14'-Bidibenzo[a,j]anthracenes (BDBAs) were prepared by iridium-catalyzed annulation of 5,5'-biterphenylene with alkynes. The molecular geometries of overcrowded BDBAs were verified by X-ray crystallography. The two dibenzo[a,j]anthryl moieties are connected through the sterically hindered 14 positions, resulting in highly distorted molecular halves. The conformation with a small twist angle between two molecular halves can minimize steric conflicts between the substituents at 1 and 13 positions and the carbon atoms of the central axis, as well as steric clashes between those substituents. One such example is octafluoro-substituted BDBA, where the interplanar angle between two anthryl moieties is approximately 31° (currently the lowest reported value, cf. 81° in 9,9'-bianthracene). The intramolecular interactions and electronic couplings between two molecular halves resulted in upfield 1H NMR signals, redshifted absorption and emission bands, and a reduced HOMO-LUMO gap. Photodynamic investigations on BDBAs indicated that the formation of the conventional symmetry-breaking charge transfer (SBCT) state was suspended by restricted rocking around the central C-C bond. Such a mechanism associated with this highly constrained conformation was examined for the first time.

Aggregation-Driven Photoinduced α-C(sp3)-H Bond Hydroxylation/C(sp3)-C(sp3) Coupling of Boron Dipyrromethene Dye in Water Reported by Near-Infrared Emission

Molecular aggregation is a powerful tool for tuning advanced materials' photophysical and electronic properties. Here we present a novel potential for the aqueous-solvated aggregated state of boron dipyrromethene (BODIPY) to facilitate phototransformations otherwise achievable only under harsh chemical conditions. We show that the photoinduced symmetry-breaking charge separation state can itself initiate catalyst-free redox chemistry, leading to selective α-C(sp3)-H bond activation/Csp3-Csp3 coupling on the BODIPY backbone. The photoproduction progress was tracked by monitoring the evolution of the strong Stokes-shifted near-infrared emission, resulting from selective self-assembly of the terminal heterodimeric photoproduct into well-ordered J-aggregates, as revealed by X-ray structural analysis. These findings provide a facile and green route to further explore the promising frontier of packing-triggered selective photoconversions via supramolecular engineering.

Impact of Local and Nonlocal Vibronic Coupling on the Absorption and Emission Spectra of J- and H-Dimers

The impact of exciton-vibrational (EV) coupling involving low-energy ("slow") intermolecular vibrations and higher-energy ("fast") intramolecular vibrations on the absorption and emission spectra of H- and J-dimers is studied theoretically for a pair of chromophores with excitonic coupling dominated by transition dipole-dipole coupling, JC. Exact quantum-mechanical solutions based on a Frenkel-Holstein-Peierls Hamiltonian reveal a fascinating interplay between the two coupling sources in determining the spectral line widths, Stoke shifts and radiative decay rates. It is shown that the ratio rules derived from the vibronic progression of the fast mode in molecular dimers remain valid under the influence of slow-mode EV coupling under most conditions. However, a highly unusual aggregate behavior occurs when the product of local and nonlocal couplings, |gLgNL|, exceeds 2ℏωs|JC|, where ℏωs is the energy of the slow mode. In this regime and when gL and gNL are in-phase, an H-dimer (JC > 0) becomes strongly emissive and can even be super-radiant, while a J-dimer (JC < 0) with out-of-phase gL and gNL values becomes subradiant. Such behaviors are in marked contrast to the predictions of Kasha theory and demonstrate the richness of the photophysical behavior resulting from EV coupling involving inter- and intramolecular vibrations.

Symmetry-Breaking Charge Separation Mediated Triplet Population in a Perylenediimide Trimer at the Single-Molecule Level

Herein, we demonstrate triplet excited-state population in a conformationally rigid perylenediimide trimer (PDI-T) via intramolecular symmetry-breaking charge separation (SB-CS) at the single-molecule level. The single-molecule fluorescence intensity trajectories of PDI-T in nonpolar polystyrene matrix (ε = 2.60) exhibit prolonged fluorescence with infrequent dark states, representing the triplet and/or the charge transfer states. In contrast, in a poly(vinyl alcohol) matrix (ε = 7.80), erratic blinking dynamics resulting in low photon counts were observed, corroborating the feasibility of charge separation in a polar environment. In agreement with the single-molecule measurements, transient absorption spectroscopy of PDI-T reveals ultrafast SB-CS (τCS < 5 ps) in polar tetrahydrofuran (ε = 7.58) and acetone (ε = 20.70), with the population of the triplet excited-state through charge recombination. The current investigation shows the utility of rigid and weakly coupled molecular constructs in controlling triplet generation and SB-CS for potential applications in optoelectronic devices.

Sculpting photoproducts with DNA origami

Natural light-harvesting systems spatially organize densely packed dyes in different configurations to either transport excitons or convert them into charge photoproducts, with high efficiency. In contrast, artificial photosystems like organic solar cells and light-emitting diodes lack this fine structural control, limiting their efficiency. Thus, biomimetic multi-dye systems are needed to organize dyes with the sub-nanometer spatial control required to sculpt resulting photoproducts. Here, we synthesize 11 distinct perylene diimide (PDI) dimers integrated into DNA origami nanostructures and identify dimer architectures that offer discrete control over exciton transport versus charge separation. The large structural-space and site-tunability of origami uniquely provides controlled PDI dimer packing to form distinct excimer photoproducts, which are sensitive to interdye configurations. In the future, this platform enables large-scale programmed assembly of dyes mimicking natural systems to sculpt distinct photophysical products needed for a broad range of optoelectronic devices, including solar energy converters and quantum information processors.

Ultrafast symmetry-breaking charge separation in Perylenemonoimide-embedded multichromophores: impact of regioisomerism

Symmetry-breaking charge separation (SB-CS) has recently evolved as an emerging concept offering its potential to the latest generation of organic photovoltaics. However there are several concerns that need to be addressed to reach the state-of-the-art in SB-CS chemistry, for instance, the desirable molecular geometry, interchromophoric distance and extent of electronic coupling. To shed light on those features, it is reported herein, that ortho-functionalized perylene monoimide (PMI) constituted regioisomeric dimer and trimer derivatives with varied molecular twisting and electronic conjugation have been synthesized. In steady-state photophysical studies, all the dimers and trimer derivatives exhibit a larger bathochromic shift in the emission spectra and a significant reduction of fluorescence quantum yield in polar DMF. Among the series of multichromophores, ortho- and self-coupled dimers display the strikingly different optical feature of SB-CS with a very fast charge separation rate (τCS = 80.2 ps) upon photoexcitation in DMF, which is unveiled by femtosecond transient absorption (fs-TA) studies. The SB-CS for two dimers is well-supported by the formation of PMI˙+ and PMI˙- bands in the fs-TA spectra. Further analysis of fs-TA data revealed that, among the other multichromophores the trimer also exhibits a clear charge separation, whereas SB-CS signatures are less prominent, but can not be completely disregarded, for the meta- and para-dimers. Additionally, the charge separation dynamics of those above-mentioned PMI derivatives are devoid of a kinetically favorable excimer or triplet formation. The evidence of a profound charge transfer phenomenon in the ortho-dimer is characterized by density functional theory (DFT) calculations on excited state electronic structures. The excitonic communications in the excited state electronic arrangements unravel the key role of dihedral twisting in SB-CS. The thermodynamic feasibility of CS (ΔGCS) and activation barrier (ΔG≠) of the derivatives in DMF are established from the Rehm-Weller equation and Marcus's theory, respectively. This work is an in-depth study of the effect of mutual orientation of PMIs and regioisomerism in determining sustainable guidelines for using SB-CS.

A Terrylene Bisimide based Universal Host for Aromatic Guests to Derive Contact Surface-Dependent Dispersion Energies

π-π interactions are among the most important intermolecular interactions in supramolecular systems. Here we determine experimentally a universal parameter for their strength that is simply based on the size of the interacting contact surfaces. Toward this goal we designed a new cyclophane based on terrylene bisimide (TBI) π-walls connected by para-xylylene spacer units. With its extended π-surface this cyclophane proved to be an excellent and universal host for the complexation of π-conjugated guests, including small and large polycyclic aromatic hydrocarbons (PAHs) as well as dye molecules. The observed binding constants range up to 108 M-1 and show a linear dependence on the 2D area size of the guest molecules. This correlation can be used for the prediction of binding constants and for the design of new host-guest systems based on the herewith derived universal Gibbs interaction energy parameter of 0.31 kJ/molÅ2 in chloroform.

Two-dimensional radial-π-stacks in solution

Highly organized π-aggregate architectures can strongly affect electronic couplings, leading to important photophysical behaviors. With the escalating interest in two-dimensional (2D) materials attributed to their exceptional electronic and optical characteristics, there is growing anticipation that 2D radial-π-stacks built upon radial π-conjugation nanorings, incorporating intra- and inter-ring electronic couplings within the confines of a 2D plane, will exhibit superior topological attributes and distinct properties. Despite their immense potential, the design and synthesis of 2D π-stacks have proven to be a formidable challenge due to the insufficient π-π interactions necessary for stable stacking. In this study, we present the successful preparation of single-layer 2D radial-π-stacks in a solution. Pillar-shaped radially π-conjugated [4]cyclo-naphthodithiophene diimide ([4]C-NDTIs) molecules were tetragonally arranged via in-plane intermolecular π-π interactions. These 2D π-stacks have a unique topology that differs from that of conventional 1D π-stacks and exhibit notable properties, such as acting as a 2D template capable of absorbing C60 guest molecules and facilitating the formation of 2D radial-π-stacks comprising [4]C-NDTI-C60 complexes, rapid exciton delocalization across the 2D plane, and efficient excitation energy funneling towards a trap.

Controlling Interchromophore Coupling in Diamantane-Linked Pentacene Dimers To Create a "Binary" Pair

Two isomeric pentacene dimers, each linked by a diamantane spacer, have been synthesized. These dimers are designed to provide experimental evidence to support quantum mechanical calculations, which predict the substitution pattern on the carbon-rich diethynyldiamantane spacer to be decisive in controlling the interpentacene coupling. Intramolecular singlet fission (i-SF) serves as a probe for the existence and strength of the electronic coupling between the two pentacenes, with transient absorption spectroscopy as the method of choice to characterize i-SF. 4,9-Substitution of diamantane provides a pentacene dimer (4,9-dimer) in which the two chromophores are completely decoupled and that, following photoexcitation, deactivates to the ground state analogous to a monomeric pentacene chromophore. Conversely, 1,6-substitution provides a pentacene dimer (1,6-dimer) that exhibits sufficiently strong coupling to drive i-SF, resulting in correlated triplet M(T1T1) yields close to unity and free triplet (T1 + T1) yields of ca. 50%. Thus, the diamantane spacer effectively switches "on" or "off" the coupling between the chromophores, based on the substitution pattern. The binary control of diamantane contrasts other known molecular spacers designed only to modulate the coupling strength between two pentacenes.

Elucidating Singlet-Fission-Born Multiexciton Dynamics via Molecular Engineering: A Dilution Principle Extended to Quintet Triplet Pair

Multiexciton in singlet exciton fission represents a critical quantum state with significant implications for both solar cell applications and quantum information science. Two distinct fields of interest explore contrasting phenomena associated with the geminate triplet pair: one focusing on the persistence of long-lived correlation and the other emphasizing efficient decorrelation. Despite the pivotal nature of multiexciton processes, a comprehensive understanding of their dependence on the structural and spin properties of materials is currently lacking in experimental realizations. To address this gap in knowledge, molecular engineering was employed to modify the TIPS-tetracene structures, enabling an investigation of the structure-property relationships in spin-related multiexciton processes. In lieu of the time-resolved electron paramagnetic resonance technique, two time-resolved magneto-optical spectroscopies were implemented for quantitative analysis of spin-dependent multiexciton dynamics. The utilization of absorption and fluorescence signals as complementary optical readouts, in the presence of a magnetic field, provided crucial insights into geminate triplet pair dynamics. These insights encompassed the duration of multiexciton correlation and the involvement of the spin state in multiexciton decorrelation. Furthermore, simulations based on our kinetic models suggested a role for quintet dilution in multiexciton dynamics, surpassing the singlet dilution principle established by the Merrifield model. The integration of intricate model structures and time-resolved magneto-optical spectroscopies served to explicitly elucidate the interplay between structural and spin properties in multiexciton processes. This comprehensive approach not only contributes to the fundamental understanding of these processes but also aligns with and reinforces previous experimental studies of solid states and theoretical assessments.

Tetracene Diacid Aggregates for Directing Energy Flow toward Triplet Pairs

A comprehensive investigation of the solution-phase photophysics of tetracene bis-carboxylic acid [5,12-tetracenepropiolic acid (Tc-DA)] and its related methyl ester [5,12-tetracenepropynoate (Tc-DE)], a non-hydrogen-bonding counterpart, reveals the role of the carboxylic acid moiety in driving molecular aggregation and concomitant excited-state behavior. Low-concentration solutions of Tc-DA exhibit similar properties to the popular 5,12-bis((triisopropylsilyl)ethynl)tetracene, but as the concentration increases, evidence for aggregates that form excimers and a new mixed-state species with charge-transfer (CT) and correlated triplet pair (TT) character is revealed by transient absorption and fluorescence experiments. Aggregates of Tc-DA evolve further with concentration toward an additional phase that is dominated by the mixed CT/TT state which is the only state present in Tc-DE aggregates and can be modulated with the solvent polarity. Computational modeling finds that cofacial arrangement of Tc-DA and Tc-DE subunits is the most stable aggregate structure and this agrees with results from 1H NMR spectroscopy. The calculated spectra of these cofacial dimers replicate the observed broadening in ground-state absorption as well as accurately predict the formation of a near-UV transition associated with a CT between molecular subunits that is unique to the specific aggregate structure. Taken together, the results suggest that the hydrogen bonding between Tc-DA molecules and the associated disruption of hydrogen bonding with solvent produce a regime of dimer-like behavior, absent in Tc-DE, that favors excimers rather than CT/TT mixed states. The control of aggregate size and structure using distinct functional groups, solute concentration, and solvent in tetracene promises new avenues for its use in light-harvesting schemes.

Impact of Packing Geometry on Excimer Characteristics and Mobility in Perylene Bisimide Polycrystalline Films

Efficient exciton transport is essential for high-performance optoelectronics. Considerable efforts have been focused on improving the exciton mobility in organic materials. While it is feasible to improve mobility in organic systems by forming well-ordered stacks, the formation of trap states, particularly the lower-lying states referred to as excimers, remains a significant challenge to enhancing mobility. The mobility of excimer excitons intricately depends on the strength of excitonic coupling in terms of Förster-type diffusive exciton transfer processes. Given that the formation and mobility of excimer excitons are highly sensitive to molecular arrangements (packing geometries), conducting comprehensive investigations into the structure-property relationship in organic systems is crucial. In this study, we prepared three types of polycrystalline films of perylene bisimide (PBI) by varying substituents at the imide and bay positions, which allowed us to tailor the properties of excimer excitons and their mobility based on packing geometries and excitonic coupling strengths. By utilizing femtosecond transient absorption spectroscopy, we observed ultrafast excimer formation in the higher coupling regime, while in the lower coupling regime, the transition from Frenkel to excimer excitons occurs with a time constant of 500 fs. Under high pump-fluence, exciton-exciton annihilation processes occur, indicating the diffusion of excimer excitons. Intriguingly, employing a three-dimensional diffusion model, we derived a diffusion constant that is 3000 times greater in the high coupling regime than in the low coupling regime. To investigate the optoelectronic properties in the form of a bulk system, we fabricated n-type organic field effect transistors and obtained 8000 times higher mobility in the high coupling regime. Furthermore, photocurrent measurements enable us to investigate the charge carrier transport by mobile excimer excitons, suggesting a 230-fold improvement in external quantum efficiency with tightly packing PBI molecules compared to the low coupling regime. These findings not only offer valuable insights into optimizing organic materials for optoelectronic devices but also unveil the intriguing potential of exciton migration within excimers.

Tetracene cyclophanes showing controlled intramolecular singlet fission by through-space orientations

Tetracene cyclophanes: a series of cyclic tetracene dimers bridged by two flexible ethylene glycol units demonstrated enhanced intramolecular singlet fission through through-space orientations by suppressing the H-type excited complex.

Time-Resolved Spectroscopy for Dynamic Investigation of Photoresponsive Metal-Organic Frameworks

Photoresponsive MOFs with precise and adjustable reticular structures are attractive for light conversion applications. Uncovering the photoinduced carrier dynamics lays the essential foundation for the further development and optimization of the MOF material. With the application of time-resolved spectroscopy, photophysical processes including excimer formation, energy transfer/migration, and charge transfer/separation have been widely investigated. However, the identification of distinct photophysical processes in real experimental MOF spectra still remains difficult due to the spectral and dynamic complexity of MOFs. In this Perspective, we summarize the typical spectral features of these photophysical processes and the related analysis methods for dynamic studies performed by time-resolved photoluminescence (TR-PL) and transient absorption (TA) spectroscopy. Based on the recent understanding of excited-state properties of photoresponsive MOFs and the discussion of challenges and future outlooks, this Perspective aims to provide convenience for MOF kinetic analysis and contribute to the further development of photoresponsive MOF material.

Exciton interactions of chlorophyll tetramer in water-soluble chlorophyll-binding protein BoWSCP

The exciton interaction of four chlorophyll a (Chl a) molecules in a symmetrical tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP was analyzed in the pH range of 3-11. Exciton splitting ΔE = 232 ± 2 cm-1 of the Qy band of Chl a into two subcomponents with relative intensities of 78.1 ± 0.7 % and 21.9 ± 0.7 % was determined by a joint decomposition of the absorption and circular dichroism spectra into Gaussian functions. The exciton coupling parameters were calculated based on the BoWSCP atomic structure in three approximations: the point dipole model, the distributed atomic monopoles, and direct ab initio calculations in the TDDFT/PCM approximation. The Coulomb interactions of monomers were calculated within the continuum model using three values of optical permittivity. The models based on the properties of free Chl a in solution suffer from significant errors both in estimating the absolute value of the exciton interaction and in the relative intensity of exciton transitions. Calculations within the TDDFT/PCM approximation reproduce the experimentally determined parameters of the exciton splitting and the relative intensities of the exciton bands. The following factors of pigment-protein and pigment-pigment interactions were examined: deviation of the macrocycle geometry from the planar conformation of free Chl; the formation of hydrogen bonds between the macrocycle and water molecules; the overlap of wave functions of monomers at close distances. The most significant factor is the geometrical deformation of the porphyrin macrocycle, which leads to an increase in the dipole moment of Chl monomer from 5.5 to 6.9 D and to a rotation of the dipole moment by 15° towards the cyclopentane ring. The contributions of resonant charge-transfer states to the wave functions of the Chl dimer were determined and the transition dipole moments of the symmetric and antisymmetric charge-transfer states were estimated.

Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores

So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. , We show here that thanks to of fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Competitive Charge Separation Pathways in a Flexible Molecular Folda-Dimer

We report the photophysical properties of a molecular folda-dimer system PDI-AnEt2-PDI, where the electron-donating N,N-diethylaniline (AnEt2) moiety bridges two electron-accepting perylene diimide (PDI) chromophores. The conformationally flexible PDI-AnEt2-PDI adopts either an open (two PDIs far apart) or folded (two PDIs within π-stacking distance) conformation, depending on the solvent environment. We characterized the photoinduced charge separation dynamics of both open and folded forms in solvents of varying polarity. The open form undergoes charge separation to give PDI•--AnEt2•+-PDI (Bridge electron transfer) independent of solvent polarity. The folded form exhibits two charge separation photoproducts, yielding both PDI•--AnEt2•+-PDI and PDI•--AnEt2-PDI•+, the latter of which is formed via symmetry-breaking charge separation (SBCS) between the two π-stacked PDI chromophores. Our results further indicate that the conformational flexibility of the folda-dimer leads to unexpected excimer formation in some open form conditions. In contrast, no excimer formation is observed in the folded form, indicating that this geometry preferentially yields the SBCS instead. Our results provide insight into how conformationally flexible folda-dimer systems can be designed and built to tune competitive photophysical pathways.

The Effect of Torsional Motion on Multiexciton Formation through Intramolecular Singlet Fission in Ferrocene-Bridged Pentacene Dimers

A series of ferrocene(Fc)-bridged pentacene(Pc)-dimers [Fc-Ph(2,n)-(Pc)2 : n=number of phenylene spacers] were synthesized to examine the tortional motion effect of Fc-terminated phenylene linkers on strongly coupled quintet multiexciton (5 TT) formation through intramolecular singlet fission (ISF). Fc-Ph(2,4)-(Pc)2 has a relatively small electronic coupling and large conformational flexibility according to spectroscopic and theoretical analyses. Fc-Ph(2,4)-(Pc)2 exhibits a high-yield 5 TT together with quantitative singlet TT (1 TT) generation through ISF. This demonstrates a much more efficient ISF than those of other less flexible Pc dimers. The activation entropy in 1 TT spin conversion of Fc-Ph(2,4)-(Pc)2 is larger than those of the other systems due to the larger conformational flexibility associated with the torsional motion of the linkers. The torsional motion of linkers in 1 TT is attributable to weakened metal-ligand bonding in the Fc due to hybridization of the hole level of Pc to Fc in 1 TT unpaired orbitals.

Intramolecular Triplet Diffusion Facilitates Triplet Dissociation in a Pentacene Hexamer

Triplet dynamics in singlet fission depend strongly on the strength of the electronic coupling. Covalent systems in solution offer precise control over such couplings. Nonetheless, efficient free triplet generation remains elusive in most systems, as the intermediate triplet pair 1 (T1 T1 ) is prone to triplet-triplet annihilation due to its spatial confinement. In the solid state, entropically driven triplet diffusion assists in the spatial separation of triplets, resulting in higher yields of free triplets. Control over electronic coupling in the solid state is, however, challenging given its sensitivity to molecular packing. We have thus developed a hexameric system (HexPnc) to enable solid-state-like triplet diffusion at the molecular scale. This system is realized by covalently tethering three pentacene dimers to a central subphthalocyanine scaffold. Transient absorption spectroscopy, complemented by theoretical structural optimizations and steady-state spectroscopy, reveals that triplet diffusion is indeed facilitated due to intramolecular cluster formation. The yield of free triplets in HexPnc is increased by a factor of up to 14 compared to the corresponding dimeric reference (DiPnc). Thus, HexPnc establishes crucial design aspects for achieving efficient triplet dissociation in strongly coupled systems by providing avenues for diffusive separation of 1 (T1 T1 ), while, concomitantly, retaining strong interchromophore coupling which preserves rapid formation of 1 (T1 T1 ).

Excimer Formation Driven by Excited-State Structural Relaxation in a Covalent Aminonaphthalimide Dimer

Strongly coupled excimer formation from interchromophoric charge transfer driven by the ultrafast excited-state structural dynamics of a 5,5'-linked 4-amino-1,8-naphthalimide covalent homodimer was investigated by ultrafast transient spectroscopy and chemical calculations. Theoretical calculations indicate that the structural relaxation associated with the dihedral motion leads to significantly enhanced interchromophoric charge transfer (CT) coupling, which favors the formation of an excimer-like symmetry-broken CT state. The formation and relaxation dynamics of the excimer state in the dimer are identified via ultrafast transient absorption and fluorescence spectroscopy. The structural relaxation following the photoexcitation occurs in tens of picoseconds and stabilizes the dimer to the strongly coupled excimer state. The highly polar solvents further stabilize the excimer state and enhance the CT character, which enable efficient electron and excitation energy transport in covalent molecular aggregates.