Discrete π-Stacks of Perylene Bisimide Dyes within Folda-Dimers: Insight into Long- and Short-Range Exciton Coupling

Unsymmetrical S-annulated perylene diester imide, stabilizing room temperature columnar phase as a dopant for greenish-yellow OLEDs with an outstanding EQE of 6.9

The quest for highly luminescent, cost-effective, and metal-free materials is driving innovation in organic light-emitting diode (OLED) technology. Herein, we unveil a novel unsymmetrical bay S-incorporated perylene diester imide (PEI-SST), stabilizing a room temperature columnar hexagonal phase over a wide thermal range, that was prepared by microwave-assisted synthesis. PEI-SST demonstrates a remarkable photoluminescence quantum yield of 83%. As a dopant in a CBP-based host-guest OLED, at 1.0 wt%, PEI-SST delivered a promising greenish-yellow electroluminescence with an exceptional EQEmax of 6.9% and a brightness of 2529 cd m-2.

Chiral ring-in-ring complexes with torsion-induced circularly polarized luminescence

We introduce a class of supramolecular precursors, termed 'folda-bonders', which utilize macrocycles to fold and precisely align reactive groups, effectively acting as bonding facilitators. This design enables highly efficient, selective, and mild 'click-like' reactions, making them particularly well-suited for the modular synthesis of complex structures. In this study, we highlight the versatility of folda-bonders in the one-pot aqueous synthesis of chiral ring-in-ring complexes exhibiting torsion-induced circularly polarized luminescence (CPL). Cucurbit[8]uril macrocycles facilitate the pseudostatic pre-folding of bis(4-phenyl pyridinium) derivatives, which act as folda-bonders, enabling efficient, purification-free covalent cyclization mediated by an axially chiral fragment. The single-crystal structure, obtained directly from the product solution, confirms the formation of a chiral ring-in-ring configuration. The macrocycle-imparted rigidity, combined with the tunable flexibility of alkyl linkers, drives the emergence of distinct chiroptical properties in the ring-in-ring complexes. Remarkably, torsion within the strained shorter alkyl linker is responsible for generating CPL, whereas longer linkers retain chirality, as evidenced by CD signals, but do not exhibit CPL. These findings demonstrate the potential of integrating noncovalent and covalent strategies to design sophisticated molecular architectures with tailored functional properties.

Photoinduced stepwise charge hopping in π-stacked perylene bisimide donor-bridge-acceptor arrays

The mechanistic understanding of light-driven charge separation and charge-carrier transport within the frameworks of π-conjugated molecules is imperative to mimic natural photosynthesis and derive synthetic materials for solar energy conversion. In this regard, since the late 1980s, the distance and solvent dependence of stepwise (incoherent) charge-carrier hopping versus single-step (coherent) superexchange transport (tunnelling) have been studied in detail. Here we introduce structurally highly defined cofacially stacked donor-acceptor perylene bisimide arrays, which offer a high resemblance to natural systems. Similarity is achieved through controlling energy and electron transfer processes via intermolecular interactions between the π-stacked perylene bisimide subunits. Selective excitation of the donor induces electron transfer to the acceptor unit in polar solvents, facilitated by a 'through-stack' wire-like charge hopping mechanism with a low attenuation factor β = 0.21 Å-1, which suggests through-stack as being equally supportive for long-distance sequential electron transfer compared to the investigated 'through-bond' transfer along π-conjugated bridges.

Precise Control of Noncovalent Dimerization via Cucurbit[n]uril-Based Host-Guest Complexation

Upon complexation between differently sized cucurbit[n]uril (n = 7, 8, or 10) hosts and a fluorenone derivative with various binding modes, the aggregation state of the fluorenone guest is precisely switched from J-aggregates to the monomer, H-dimer, and separated "H-dimer", thus enabling the construction of multicolor luminescent materials in an aqueous solution.

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.

Multiple effects of aromatic substituents on excited-state properties and singlet fission process in azaquinodimethane systems

Singlet fission (SF) could offset the thermalization loss of high-energy photons via multiexciton generations, thus holding great potential in improving the power conversion efficiency of solar cells. However, the development of SF-based devices has basically remained stagnant so far owing to the limited scope of practical SF materials. Therefore, designing and developing practical SF material systems have been imperative, yet challenging so far. In this work, we comprehensively investigated the effects of aromatic substituents on excited-state properties and SF process of azaquinodimethane systems. Results indicated that the aromatic substituents have a significant influence on molecular diradical characters, thereby determining the excited-state energetics of the SF material system, including optical band gaps and triplet energy. Moreover, the aromatic substituents influenced charge transfer coupling interactions by adjusting molecular packing in the aggregate state to shunt the excited-state population to exert SF process or trap in excimer species. These results not only offer a deep insight into the multiple regulatory effects of the aromatic substituents on excited-state properties and SF process but also provide a practical SF material system, which could lay the foundation for the discovery of new SF-active chromophores and practical applications of new-generation light-harvesting materials.

Morphology-Controlled Long-Range Photogenerated Charge Carrier Transfer Pathway for Enhanced Photocatalytic Hydrogen Production

Achieving precise control over the construction of efficient charge transport channels through self-assembly engineering represents a highly effective strategy for the synthesis of organic supramolecular photocatalysts. Herein, tetragonal zinc meso-5,10,15,20-tetra(4-pyridyl) porphyrin (ZnTPyP) nanorods (T-ZnTPyPs) and hexagonal ZnTPyP nanowires (H-ZnTPyPs) were synthesized by varying the assembly temperature. H-ZnTPyPs demonstrated a photocatalytic hydrogen production rate (183 mmol/g/h) that was 14.62 times greater than that of T-ZnTPyP (13 mmol/g/h). This significantly enhanced activity is primarily attributed to the distinct and well-defined molecular arrangements of H-ZnTPyPs, which support continuous linear long-range electron transfer pathways through effective π-π stacking. Conversely, the heat manipulation used in the synthesis of T-ZnTPyPs limits the participation of water molecules in the crystalline stacking arrangements, leading to lattice distortions that disrupt the π-π stacking interactions and significantly impede long-range electron transfer pathways. This research presents a novel strategy for modulating π-π stacking to optimize charge transport in supramolecular photocatalysts.

Optical Axial Chirality Enhancement and Transfer within Aromatic Micelles upon (Co-)encapsulation

Axial chirality is the key physiochemical element, yet its chiroptical utilities have been largely limited to covalent synthesis and infinitely assembled systems so far. Here we report a new application of axially chiral binaphthyls for efficient, optical chirality enhancement and transfer upon noncovalent encapsulation by achiral aromatic micelles in water. The CD activities of dialkoxy binaphthyls are significantly enhanced (up to 7-fold) upon encapsulation by an anthracene-based aromatic micelle. Large emission enhancement (∼4-fold) and efficient guest-to-guest, optical chirality transfer are achieved through coencapsulation of the binaphthyls with achiral cycloparaphenylenes, in a guest-within-guest fashion, by the micelle. The observed unusual properties are derived from the tight inclusion of the chiral guests into the macrocyclic guests, efficiently generated only in the aromatic cavity. Moderate CPL can be observed from the coencapsulated macrocycles within the ternary composites. Furthermore, more than ∼4-fold enhanced guest-to-guest chiroptical transfer is demonstrated with a functionalized cycloparaphenylene through the present coencapsulation strategy.

The first step of cyanine dye self-assembly: Dimerization

Self-assembling amphiphilic cyanine dyes, such as C8S3, are promising candidates for energy storage and optoelectronic applications due to their efficient energy transport properties. C8S3 is known to self-assemble in water into double-walled J-aggregates. Thus far, the molecular self-assembly steps remain shrouded in mystery. Here, we employ a multiscale approach to unravel the first self-assembly step: dimerization. Our multiscale approach combines molecular dynamics simulations with quantum chemistry calculations to obtain a Frenkel exciton Hamiltonian, which we then use in spectral calculations to determine the absorption and two-dimensional electronic spectra of C8S3 monomer and dimer systems. We model these systems solvated in both water and methanol, validating our model with experiments in methanol solution. Our theoretical results predict a measurable anisotropy decay upon dimerization, which is experimentally confirmed. Our approach provides a tool for the experimental probing of dimerization. Moreover, molecular dynamics simulations reveal that the dimer conformation is characterized by the interaction between the hydrophobic aliphatic tails rather than the π-π stacking previously reported for other cyanine dyes. Our results pave the way for future research into the mechanism of molecular self-assembly in similar light-harvesting complexes, offering valuable insights for understanding and optimizing self-assembly processes for various (nano)technological 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.

'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.

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.

Structure-Spectroscopy Correlation in the Self-Assembled Perylene Diimide-Based Dimers via Inter-Chromophore Coupling

The impact of conformational change on the ground and excited states of seven perylene diimide (PDI)-based dimeric systems is examined by introducing longitudinal shift, transverse shift, and rotation of one monomer with respect to another. The minimum energy conformations are compared via an energy decomposition analysis. The heteroatom-substituted dimeric systems, such as B2 N2-embedded PDI, trans-thio-PDI (trans-S2-PDI), and N-PDI, show BN···π, C═S···π, and N···H interactions that survive over a longer range of longitudinal and transverse shifts. The excitonic coupling analysis reveals that both Coulomb- and CT-mediated couplings are crucial for understanding aggregate absorption spectra. While the Coulomb coupling exhibits a monotonic behavior with conformation changes, the CT component changes significantly with minor geometrical deviations. The interplay between the two couplings leads to J-type, H-type, and null aggregates, depending on the conformations of the dimers. The overall trend of both couplings is consistent across all systems, although they differ in magnitude. The trans-S2-PDI shows the strongest Coulomb and CT couplings, while it is weak in perylene and B2N2-PDI dimers. The resonant model for strongly coupled Frenkel excitonic (FE) and CT states successfully characterizes the single- and double-band nature of absorption spectra in dimers. In strong coupling regions, the dimers show blue-shifted single-band excitation to the upper FE state. In contrast, excitation to the lower FE and upper CT states produces a red-shifted two-band spectrum in the weakly coupled regions. The intensity of the CT band diminishes with the monomer separation. In most cases, the perpendicularly stacked structures show null-aggregate behavior with no spectral shift due to the absence of Coulomb and CT couplings. The exciton relaxation pathway of the heteroatom-substituted PDIs is found to be influenced by the presence of nπ* states between the FE and CT states.

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.

Two-dimensional-lattice-confined single-molecule-like aggregates

Intermolecular distance largely determines the optoelectronic properties of organic matter. Conventional organic luminescent molecules are commonly used either as aggregates or as single molecules that are diluted in a foreigner matrix. They have garnered great research interest in recent decades for a variety of applications, including light-emitting diodes1,2, lasers3-5 and quantum technologies6,7, among others8-10. However, there is still a knowledge gap on how these molecules behave between the aggregation and dilution states. Here we report an unprecedented phase of molecular aggregate that forms in a two-dimensional hybrid perovskite superlattice with a near-equilibrium distance, which we refer to as a single-molecule-like aggregate (SMA). By implementing two-dimensional superlattices, the organic emitters are held in proximity, but, surprisingly, remain electronically isolated, thereby resulting in a near-unity photoluminescence quantum yield, akin to that of single molecules. Moreover, the emitters within the perovskite superlattices demonstrate strong alignment and dense packing resembling aggregates, allowing for the observation of robust directional emission, substantially enhanced radiative recombination and efficient lasing. Molecular dynamics simulations together with single-crystal structure analysis emphasize the critical role of the internal rotational and vibrational degrees of freedom of the molecules in the two-dimensional lattice for creating the exclusive SMA phase. This two-dimensional superlattice unifies the paradoxical properties of single molecules and aggregates, thus offering exciting possibilities for advanced spectroscopic and photonic applications.

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.

Switchable Dual Circularly Polarized Luminescence in Through-Space Conjugated Chiral Foldamers

Organic materials with switchable dual circularly polarized luminescence (CPL) are highly desired because they can not only directly radiate tunable circularly polarized light themselves but also induce CPL for guests by providing a chiral environment in self-assembled structures or serving as the hosts for energy transfer systems. However, most organic molecules only exhibit single CPL and it remains challenging to develop organic molecules with dual CPL. Herein, novel through-space conjugated chiral foldamers are constructed by attaching two biphenyl arms to the 9,10-positions of phenanthrene, and switchable dual CPL with opposite signs at different emission wavelengths are successfully realized in the foldamers containing high-polarizability substitutes (cyano, methylthiol and methylsulfonyl). The combined experimental and computational results demonstrate that the intramolecular through-space conjugation has significant contributions to stabilizing the folded conformations. Upon photoexcitation in high-polar solvents, strong interactions between the biphenyl arms substituted with cyano, methylthio or methylsulfonyl and the polar environment induce conformation transformation for the foldamers, resulting in two transformable secondary structures of opposite chirality, accounting for the dual CPL with opposite signs. These findings highlight the important influence of the secondary structures on the chiroptical property of the foldamers and pave a new avenue towards efficient and tunable dual CPL materials.

Dynamic two-dimensional covalent organic frameworks

Porous covalent organic frameworks (COFs) enable the realization of functional materials with molecular precision. Past research has typically focused on generating rigid frameworks where structural and optoelectronic properties are static. Here we report dynamic two-dimensional (2D) COFs that can open and close their pores upon uptake or removal of guests while retaining their crystalline long-range order. Constructing dynamic, yet crystalline and robust frameworks requires a well-controlled degree of flexibility. We have achieved this through a 'wine rack' design where rigid π-stacked columns of perylene diimides are interconnected by non-stacked, flexible bridges. The resulting COFs show stepwise phase transformations between their respective contracted-pore and open-pore conformations with up to 40% increase in unit-cell volume. This variable geometry provides a handle for introducing stimuli-responsive optoelectronic properties. We illustrate this by demonstrating switchable optical absorption and emission characteristics, which approximate 'null-aggregates' with monomer-like behaviour in the contracted COFs. This work provides a design strategy for dynamic 2D COFs that are potentially useful for realizing stimuli-responsive materials.

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.

Flexible molecular crystals for optoelectronic applications

The cornerstones of the advancement of flexible optoelectronics are the design, preparation, and utilization of novel materials with favorable mechanical and advanced optoelectronic properties. Molecular crystalline materials have emerged as a class of underexplored yet promising materials due to the reduced grain boundaries and defects anticipated to provide enhanced photoelectric characteristics. An inherent drawback that has precluded wider implementation of molecular crystals thus far, however, has been their brittleness, which renders them incapable of ensuring mechanical compliance required for even simple elastic or plastic deformation of the device. It is perplexing that despite a plethora of reports that have in the meantime become available underpinning the flexibility of molecular crystals, the "discovery" of elastically or plastically deformable crystals remains limited to cases of serendipitous and laborious trial-and-error approaches, a situation that calls for a systematic and thorough assessment of these properties and their correlation with the structure. This review provides a comprehensive and concise overview of the current understanding of the origins of crystal flexibility, the working mechanisms of deformations such as plastic and elastic bending behaviors, and insights into the examples of flexible molecular crystals, specifically concerning photoelectronic changes that occur in deformed crystals. We hope this summary will provide a reference for future experimental and computational efforts with flexible molecular crystals aimed towards improving their mechanical behavior and optoelectronic properties, ultimately intending to advance the flexible optoelectronic technology.

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.

Axially Chiral Organic Semiconductors for Visible-Blind UV-Selective Circularly Polarized Light Detection

Technologies that detect circularly polarized light (CPL), particularly in the UV region, have significant potential for various applications, including bioimaging and optical communication. However, a major challenge in directly sensing CPL arises from the conflicting requirements of planar structures for efficient charge transport and distorted structures for effective interaction with CPL. Here, a novel design of an axially chiral n-type organic semiconductor is presented to surmount the challenge, in which a binaphthyl group results in a high dissymmetry factor at the molecular level, while maintaining excellent electron-transporting characteristics through the naphthalene diimide group. Experimental and computational methods reveal different stacking behaviors in homochiral and heterochiral assemblies, yielding different structures: Nanowires and nanoparticles, respectively. Especially, the homochiral assemblies exhibit effective π-π stacking between naphthalene diimides despite axial chirality. Thus, phototransistors fabricated using enantiomers exhibit a high maximum electron mobility of 0.22 cm2 V-1 s-1 and a detectivity of 3.9 × 1012 Jones, alongside the CPL distinguishing ability with a dissymmetry factor of responsivity of 0.05. Furthermore, the material possesses a wide bandgap, contributing to its excellent visible-blind UV-selective detection. These findings highlight the new strategy for compact CPL detectors, coupled with the demonstration of less-explored n-type and UV region phototransistors.

A Chirally Locked Bis-perylene Diimide Macrocycle: Consequences for Chiral Self-Assembly and Circularly Polarized Luminescence

Macrocycles containing chiral organic dyes are highly valuable for the development of supramolecular circularly polarized luminescent (CPL) materials, where a preorganized chiral framework is conducive to directing π-π self-assembly and delivering a strong and persistent CPL signal. Here, perylene diimides (PDIs) are an excellent choice for the organic dye component because, alongside their tunable photophysical and self-assembly properties, functionalization of the PDI's core yields a twisted, chiral π-system, capable of CPL. However, configurationally stable PDI-based macrocycles are rare, and those that are also capable of π-π self-assembly beyond dimers are unprecedented, both of which are advantageous for robust self-assembled chiroptical materials. In this work, we report the first bay-connected bis-PDI macrocycle that is configurationally stable (ΔG⧧ > 155 kJ mol-1). We use this chirally locked macrocycle to uncover new knowledge of chiral PDI self-assembly and to perform new quantitative CPL imaging of the resulting single-crystal materials. As such, we discover that the chirality of a 1,7-disubstituted PDI provides a rational route to designing H-, J- and concomitant H- and J-type self-assembled materials, important arrangements for optimizing (chir)optical and charge/energy transport properties. Indeed, we reveal that CPL is amplified in the single crystals of our chiral macrocycle by quantifying the degree of emitted light circular polarization from such materials for the first time using CPL-Laser Scanning Confocal Microscopy.

Stimulating Phonon Bottleneck Effect in Organic Semiconductors by Charge-Transfer-Mediated J-Aggregation

Hot carriers rapidly lose kinetic energies on a subpicosecond time scale, posing significant limitations on semiconductors' photon-conversion efficiencies. To slow the hot carrier cooling, the phonon bottleneck effect is constructed prevalently in quantum-confined structures with discrete energy levels. However, the maximum energy separation (ΔEES) between the energy levels is in a range of several hundred meV, leading to unsatisfactory cooling time. To address this, we design a novel organic semiconductor capable of forming intermolecular charge transfer (CT) in J-aggregates, where the lowest singlet excited state (S1) splits into two states due to the significant interplay between the Coulomb interaction and intermolecular CT coupling. The ΔEES between the two states can be adjusted up to 1.02 eV, and an extremely slow carrier cooling process of ∼72.3 ps was observed by femtosecond transient absorption spectroscopy. Moreover, the phonon bottleneck effect was identified in organic materials for the first time, and CT-mediated J-aggregation with short-range interactions was found to be the key to achieving large ΔEES. The significantly prolonged carrier cooling time, compared to <100 fs in the isolated molecule (10-6 M), highlights the potential of organic molecules with diversified aggregation structures in achieving long-lived hot carriers. These findings provide valuable insights into the intrinsic photophysics of electron-phonon scattering in organic semiconductors.

Unveiling the topology of partially disordered micro-crystalline nitro-perylenediimide with X-aggregate stacking: an integrated approach

Profound knowledge of the molecular structure and supramolecular organization of organic molecules is essential to understand their structure-property relationships. Herein we demonstrate the packing arrangement of partially disordered nitro-perylenediimide (NO2-PDI), revealing that the perylenediimide units exhibit an X-shaped packing pattern. The packing of NO2-PDI is derived using a complementary approach that utilises solid-state NMR (ssNMR) and 3D electron diffraction (3D ED) techniques. Perylenediimide (PDI) molecules are captivating due to their high luminescence efficiency and optoelectronic properties, which are related to supramolecular self-assembly. Increasing the alkyl chain length on the imide substituent poses a more significant challenge in crystallizing the resulting molecule. In addition to the alkyl tails, other functional groups, like the nitro group attached as a bay substituent, can also cause disorder. Such heterogeneity could lead to diffuse scattering, which then complicates the interpretation of diffraction experiment data, where perfect periodicity is expected. As a result, there is an unmet need to develop a methodology for solving the structures of difficult-to-crystallize materials. A synergistic approach is utilised in this manuscript to understand the packing arrangement of the disordered material NO2-PDI by making use of 3D ED, ssNMR and density functional theory calculations (DFT). The combination of these experimental and theoretical approaches provides great promise in enabling the structural investigation of novel materials with customized properties across various applications, which are, due to the internal disorder, very difficult to study by diffraction techniques. By effectively addressing these challenges, our methodology opens up new avenues for material characterization, thereby driving exciting advancements in the field.

Squaraine Appended with Benzodipyrrole for Fluorescent Sensing of Methanol: Exciton Coupling Controls Photophysical Properties

The photophysical properties of dyes composed of two squaraine chromophores fused with a benzodipyrrole central moiety (BS1 and BS2), were investigated using steady-state absorption, fluorescence, and transient absorption spectroscopy. The dyes exhibit solvent-independent split electronic absorption due to exciton-coupling. Interestingly significant solvent-dependent fluorescence properties were observed. In toluene, they emit from the lowest excited state, while in methanol, they show weak emission in the higher energy region. In the low-temperature glass matrix, emission from the lowest excited state dominates similarly to that in toluene. The transient absorption spectra exhibit similar ground-state bleaching in toluene and methanol, revealing the formation of delocalized excited states by exciton coupling independent of solvent. However, the excited state deactivates rapidly in ultrafast time scale in methanol, likely due to solvent interaction, leading to rapid non-radiative deactivation. The PEG film doped with the exciton-coupled bis-squaraine shows a distinct fluorescence response to methanol vapor.

Engineering Soluble Diketopyrrolopyrrole Chromophore Stacks from a Series of Pd(II)-Based Ravels

A strategy to engineer the stacking of diketopyrrolopyrrole (DPP) dyes based on non-statistical metallosupramolecular self-assembly is introduced. For this, the DPP backbone is equipped with nitrogen-based donors that allow for different discrete assemblies to be formed upon the addition of Pd(II), distinguished by the number of π-stacked chromophores. A Pd3 L6 three-ring, a heteroleptic Pd2 L2 L'2 ravel composed of two crossing DPPs (flanked by two carbazoles), and two unprecedented self-penetrated motifs (a Pd2 L3 triple and a Pd2 L4 quadruple stack), were obtained and systematically investigated. With increasing counts of stacked chromophores, UV/Vis absorptions red-shift and emission intensities decrease, except for compound Pd2 L2 L'2 , which stands out with an exceptional photoluminescence quantum yield of 51 %. This is extraordinary for open-shell metal containing assemblies and explainable by an intra-assembly FRET process. The modular design and synthesis of soluble multi-chromophore building blocks offers the potential for the preparation of nanodevices and materials with applications in sensing, photo-redox catalysis and optics.

Keeping the chromophores crossed: evidence for null exciton splitting

Fundamental understanding of the supramolecular assemblies of organic chromophores and the development of design strategies have seen endless ripples of interest owing to their exciting photophysical properties and optoelectronic applications. The independent discovery of dye aggregates by Jelley and Scheibe was the commencement of the remarkable advancement in the field of aggregate photophysics. Subsequent research warranted an exceptional model for defining the exciton interactions in aggregates, proposed by Davydov, Kasha and co-workers, independently, based on the long-range Coulombic coupling. Fascinatingly, the orthogonally cross-stacked molecular transition dipole arrangement was foretold by Kasha to possess null exciton interaction leading to spectroscopically uncoupled molecular assembly, which lacked an experimental signature for decades. There have been several attempts to identify and probe atypical molecular aggregates for decoding their optical behaviour. Herein, we discuss the recent efforts in experimentally verifying the unusual exciton interactions supported with quantum chemical computations, primarily focusing on the less explored null exciton splitting. Exciton engineering can be realized through synthetic modifications that can additionally offer control over the assorted non-covalent interactions for orchestrating precise supramolecular assembly, along with molecular editing. The task of attaining a minimal excitonic coupling through an orthogonally cross-stacked crystalline architecture envisaged to offer a monomer-like optical behaviour was first reported in 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br2). The attempt to stitch molecules covalently in an orthogonal fashion to possess null excitonic character culminated in a spiro-conjugated perylenediimide dimer exhibiting a monomer-like spectroscopic signature. The computational and experimental efforts to map the emergent properties of the cross-stacked architecture are also discussed here. Using the null aggregates formed by the interference effects between CT-mediated and Coulombic couplings in the molecular array is another strategy for achieving monomer-like spectroscopic properties in molecular assemblies. Moreover, identifying supramolecular assemblies with precise angle-dependent properties can have implications in functional material design, and this review can provide insights into the uncharted realm of null exciton splitting.

Symmetry-Breaking Charge Separation in a Chiral Bis(perylenediimide) Probed at Ensemble and Single-Molecule Levels

Chiral molecular assemblies exhibiting symmetry-breaking charge separation (SB-CS) are potential candidates for the development of chiral organic semiconductors. Herein, we explore the excited-state dynamics of a helically chiral perylenediimide bichromophore (Cy-PDI2) exhibiting SB-CS at the ensemble and single-molecule levels. Solvent polarity-tunable interchromophoric excitonic coupling in chiral Cy-PDI2 facilitates the interplay of SB-CS and excimer formation in the ensemble domain. Analogous to the excited-state dynamics of Cy-PDI2 at the ensemble level, single-molecule fluorescence lifetime traces of Cy-PDI2 depicted long-lived off-states characteristic of the radical ion pair-mediated dark states. The discrete electron transfer and charge separation dynamics in Cy-PDI2 at the single-molecule level are governed by the distinct influence of the local environment. The present study aims at understanding the fundamental excited-state dynamics in chiral organic bichromophores for designing efficient chiral organic semiconductors and applications toward charge transport materials.

Enantiopure J-Aggregate of Quaterrylene Bisimides for Strong Chiroptical NIR-Response

Chiral polycyclic aromatic hydrocarbons can be tailored for next-generation photonic materials by carefully designing their molecular as well as supramolecular architectures. Hence, excitonic coupling can boost the chiroptical response in extended aggregates but is still challenging to achieve by pure self-assembly. Whereas most reports on these potential materials cover the UV and visible spectral range, systems in the near infrared (NIR) are underdeveloped. We report a new quaterrylene bisimide derivative with a conformationally stable twisted π-backbone enabled by the sterical congestion of a fourfold bay-arylation. Rendering the π-subplanes accessible by small imide substituents allows for a slip-stacked chiral arrangement by kinetic self-assembly in low polarity solvents. The well dispersed solid-state aggregate reveals a sharp optical signature of strong J-type excitonic coupling in both absorption (897 nm) and emission (912 nm) far in the NIR region and reaches absorption dissymmetry factors up to 1.1 × 10^-2. The structural elucidation was achieved by atomic force microscopy and single-crystal X-ray analysis which we combined to derive a structural model of a fourfold stranded enantiopure superhelix. We could deduce that the role of phenyl substituents is not only granting stable axial chirality but also guiding the chromophore into a chiral supramolecular arrangement needed for strong excitonic chirality.

Intramolecular and Intermolecular Interaction Switching in the Aggregates of Perylene Diimide Trimer: Effect of Hydrophobicity

The research on perylene diimide (PDI) aggregates effectively promotes their applications in organic photovoltaic solar cells and fluorescent sensors. In this paper, a PDI fabricated with three peripheral PDI units (N, N’-bis(6-undecyl) perylene-3,4,9,10-bis(dicarboximide)) is investigated. The trimer shows different absorption and fluorescence properties due to hydrophobicity when dissolved in the mixed solvent of tetrahydrofuran (THF) and water. Through comprehensive analysis of the fluorescence lifetime and transient absorption spectroscopic results, we concluded that the trimer underwent different excited state kinetic pathways with different concentrations of water in THF. When dissolved in pure THF solvent, both the intramolecular charge-transfer and excimer states are formed. When the water concentration increases from 0 to 50% (v/v), the formation time of the excimer state and its structural relaxation time are prolonged, illustrating the arising of the intermolecular excimer state. It is interesting to determine that the probability of the intramolecular charge-transfer pathway will first decrease and then increase as the speed of intermolecular excimer formation slows down. The two inflection points appear when the water concentration is above 10% and 40%. The results not only highlight the importance of hydrophobicity on the aggregate properties of PDI multimers but also guide the further design of PDI-based organic photovoltaic solar cells.

Direct Observation of Ultrafast Relaxation Dynamics of a Mixed Excimer State in Perylene Monoimide Dimer by Femtosecond Transient Absorption

A J-type dimer PMI-2, two perylene monoimides linked by butadiynylene bridger was prepared, and its excited-state dynamics was studied using ultrafast femtosecond transient absorption spectroscopy, along with steady-state spectroscopy and quantum chemical calculations. It is evidently demonstrated that the symmetry-breaking charge separation (SB-CS) process in PMI-2 is positively mediated by an excimer, which is mixed by localized Frenkel excitation (LE) and an interunit charge transfer (CT) state. Kinetic studies show that, with the polarity increasing of the solvent, the transformation of excimer from a mixture to the CT state (SB-CS) is accelerated, and the recombination time of the CT state is reduced obviously. Theoretical calculations indicate that these are due to PMI-2 obtaining more negative free energy (ΔG_cs) and lower CT state energy levels in highly polar solvents. Our work suggests that the mixed excimer can be formed in a J-type dimer with suitable structure, in which the charge separation the process is sensitive to the solvent environment.

Engineering giant excitonic coupling in bioinspired, covalently bridged BODIPY dyads

Strong excitonic coupling in photosynthetic systems is believed to enable efficient light absorption and quantitative charge separation, motivating the development of artificial multi-chromophore arrays with equally strong or even stronger excitonic coupling. However, large excitonic coupling strengths have typically been accompanied by fast non-radiative recombination, limiting the potential of the arrays for solar energy conversion as well as other applications such as fluorescent labeling. Here, we report giant excitonic coupling leading to broad optical absorption in bioinspired BODIPY dyads that have high photostability, excited-state lifetimes at the nanosecond scale, and fluorescence quantum yields of nearly 50%. Through the synthesis, spectroscopic characterization, and computational modeling of a series of dyads with different linking moieties, we show that the strongest coupling is obtained with diethynylmaleimide linkers, for which the coupling occurs through space between BODIPY units with small separations and slipped co-facial orientations. Other linkers allow for broad tuning of both the relative through-bond and through-space coupling contributions and the overall strength of interpigment coupling, with a tradeoff observed in general between the strength of the two coupling mechanisms. These findings open the door to the synthesis of molecular systems that function effectively as light-harvesting antennas and as electron donors or acceptors for solar energy conversion.

Synthesis of a Wheel-Shaped Nanographdiyne

Graphdiyne, a sp- and sp²-hybridized 2D π-conjugated carbon material with well-dispersed pores and unique electronic properties, was well investigated and applied in catalysis, electronics, optics, and energy storage and conversion. Graphdiyne fragments with conjugation in 2D can provide in-depth insights for understanding the intrinsic structure-property relationships of graphdiyne. Herein, an atomic precise wheel-shaped nanographdiyne composed of six dehydrobenzo [18] annulenes ([18]DBAs, the smallest macrocyclic unit of graphdiyne), was realized through the sixfold intramolecular Eglinton coupling in the hexabutadiyne precursors obtained by the sixfold Cadiot-Chodkiewicz cross-coupling of hexaethynylbenzene. Its planar structure was revealed by X-ray crystallographic analysis. The full cross-conjugation of the six 18π electron circuits yields the π-electron conjugation along the giant π core. This work provides a realizable method for the synthesis of future graphdiyne fragments with different functional groups and/or heteroatom doping, as well as the study of the unique electronic/photophysical properties and aggregation behavior of graphdiyne.

Covalent post-assembly modification of π-conjugated supramolecular polymers delivers structurally robust light-harvesting nanoscale objects

A two-component stapling strategy is used to covalently tether a new class of water-soluble supramolecular polymers built from bay-functionalized perylene bisimide (PBI) units. By leveraging a novel combined strategy where excitonic coupling and fluorescence data are exploited as spectroscopic reporters, structural design principles are established to form light-harvesting superstructures whose ground-state electronic properties are not sensitive to solvation environments. Moreover, we interrogate the structural properties of stapled superstructures by capitalizing on the drastic changes in fluorescence quantum yields against parent supramolecular assemblies. In essence, our work shows that the combination of excitonic coupling measurements and photoluminescence experiments delineates a more accurate understanding of the design principles required to limit the degree of structural defects and magnify short- and long-range electronic couplings between redox-active units in this new class of solvated nanoscale objects. These results highlight that the fragile conformation of non-covalent assemblies, which are regulated by weak secondary interactions, can be preserved by post-assembly modification of preformed supramolecular polymers. These synthetic and spectroscopic principles can in turn be codified as experimental handles to parameterize the optoelectronic properties of light-harvesting nanoscale objects.

Ultra-Narrowband Near-Infrared Responsive J-Aggregates of Fused Quinoidal Tetracyanoindacenodithiophene

Narrowband photoresponsive molecules are highly coveted in high-resolution imaging, sensing, and monochromatic photodetection, especially those extending into the near-infrared (NIR) spectral range. Here, a new class of J-aggregating materials based on quinoidal indacenodithiophenes (IDTs) that exhibit an ultra-narrowband (full width half maxima of 22 nm) NIR absorption peak centered at 770 nm is reported. The spectral width is readily tuned by the length of the solubilizing alkyl group, with longer chains resulting in significant spectral narrowing. The J-aggregate behavior is confirmed by a combination of excited state lifetime measurements and single-crystal X-ray diffraction measurements. Their utility as electron-transporting materials is demonstrated in both transistor and phototransistor devices, with the latter demonstrating good response at NIR wavelengths (780 nm) over a range of intensities.

Reduction-Activatable Fluorogenic Nanobody for Targeted and Low-Background Bioimaging

Environment-sensitive fluorogenic antibodies enable target-specific bioimaging with reduced unspecific background signal and improved spatiotemporal resolution. However, current strategies for the construction of fluorogenic antibodies are hard to handle due to challenges that lie in the prior design of fluorogenic probes and subsequent antibody labeling. Here, we report a simple strategy to generate a fluorogenic nanobody, which we term D-body, by in situ incorporation of a reduction-responsive Nile blue foldamer which is self-quenched via a dimerization-caused quenching mechanism. The D-body can be efficiently internalized by cells with high epidermal growth factor receptor expression levels and is highly fluorogenic upon lysosomal activation, allowing wash-free cell imaging with exquisite specificity and fast in vivo imaging with a high tumor-to-background ratio. The modular D-body is readily available and easy to handle, offering a platform that is highly tunable for bioimaging applications.

Advances in Molecular Design and Photophysical Engineering of Perylene Bisimide-Containing Polyads and Multichromophores for Film-Based Fluorescent Sensors

Film-based fluorescent sensors (FFSs) represent an important chemistry technology for meeting the urgent needs of on-site and real-time analysis, thereby enabling significant applications in environmental and health monitoring. As the core of FFSs, innovative design of sensing fluorophores and their intrinsic excited-state-related response nature endow FFSs with superior sensing performances in an endless expansion. In this Perspective, we specifically focus on perylene bisimide (PBI)-containing polyads and multichromophores with rigid configuration and notable photochemical stability for developing high-performance FFSs. These nonplanar structures mitigate aggregation and create abundant gaps for the sake of mass transfer and availability of the sensing units in the adlayer of the sensing films. We also comprehensively discuss how to adjust electronic coupling governing the excited-state events by appropriate functionalization strategies, thus providing a plethora of valuable insights for the exploration of the structure-property relationships in these orchestrated molecular systems. Throughout this Perspective, we also identify opportunities for FFSs in the future developments.

Connecting the dots for fundamental understanding of structure-photophysics-property relationships of COFs, MOFs, and perovskites using a Multiparticle Holstein Formalism

Photoactive organic and hybrid organic-inorganic materials such as conjugated polymers, covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and layered perovskites, display intriguing photophysical signatures upon interaction with light. Elucidating structure-photophysics-property relationships across a broad range of functional materials is nontrivial and requires our fundamental understanding of the intricate interplay among excitons (electron-hole pair), polarons (charges), bipolarons, phonons (vibrations), inter-layer stacking interactions, and different forms of structural and conformational defects. In parallel with electronic structure modeling and data-driven science that are actively pursued to successfully accelerate materials discovery, an accurate, computationally inexpensive, and physically-motivated theoretical model, which consistently makes quantitative connections with conceptually complicated experimental observations, is equally important. Within this context, the first part of this perspective highlights a unified theoretical framework in which the electronic coupling as well as the local coupling between the electronic and nuclear degrees of freedom can be efficiently described for a broad range of quasiparticles with similarly structured Holstein-style vibronic Hamiltonians. The second part of this perspective discusses excitonic and polaronic photophysical signatures in polymers, COFs, MOFs, and perovskites, and attempts to bridge the gap between different research fields using a common theoretical construct - the Multiparticle Holstein Formalism. We envision that the synergistic integration of state-of-the-art computational approaches with the Multiparticle Holstein Formalism will help identify and establish new, transformative design strategies that will guide the synthesis and characterization of next-generation energy materials optimized for a broad range of optoelectronic, spintronic, and photonic applications.

Molecular Tetris by sequence-specific stacking of hydrogen bonding molecular clips

A face-to-face stacking of aromatic rings is an effective non-covalent strategy to build functional architectures, as elegantly exemplified with protein folding and polynucleotide assembly. However, weak, non-directional, and context-sensitive van der Waals forces pose a significant challenge if one wishes to construct well-organized π-stacks outside the confines of the biological matrix. To meet this design challenge, we have devised a rigid polycyclic template to create a non-collapsible void between two parallel oriented π-faces. In solution, these shape-persistent aromatic clips self-dimerize to form quadruple π-stacks, the thermodynamic stability of which is enhanced by self-complementary N-H···N hydrogen bonds, and finely regulated by the regioisomerism of the π-canopy unit. With assistance from sufficient electrostatic polarization of the π-surface and bifurcated hydrogen bonds, a small polyheterocyclic guest can effectively compete against the self-dimerization of the host to afford a triple π-stack inclusion complex. A combination of solution spectroscopic, X-ray crystallographic, and computational studies aided a detailed understanding of this cooperative vs competitive process to afford layered aromatics with extraordinary structural regularity and fidelity.

Intermolecular Interactions and Charge Resonance Contributions to Triplet and Singlet Exciton States of Oligoacene Aggregates

Intermolecular interactions modulate the electro-optical properties of molecular materials and the nature of low-lying exciton states. Molecular materials composed by oligoacenes are extensively investigated for their semiconducting and optoelectronic properties. Here, we analyze the exciton states derived from time-dependent density functional theory (TDDFT) calculations for two oligoacene model aggregates: naphthalene and anthracene dimers. To unravel the role of inter-molecular interactions, a set of diabatic states is selected, chosen to coincide with local (LE) and charge-transfer (CT) excitations within a restricted orbital space including two occupied and two unoccupied orbitals for each molecular monomer. We study energy profiles and disentangle inter-state couplings to disclose the (CT) character of singlet and triplet exciton states and assess the influence of inter-molecular orientation by displacing one molecule with respect to the other along the longitudinal translation coordinate. The analysis shows that (CT) contributions are relevant, although comparably less effective for triplet excitons, and induce a non-negligible mixed character to the low-lying exciton states for eclipsed monomers and for small translational displacements. Such (CT) contributions govern the L_a/L_b state inversion occurring for the low-lying singlet exciton states of naphthalene dimer and contribute to the switch from H- to J-aggregate type of the strongly allowed B_b transition of both oligoacene aggregates.

Self-Assembling Behaviour of Perylene, Perylene Diimide, and Thionated Perylene Diimide Deciphered through Non-Covalent Interactions

The π-conjugated supramolecular polymers (SMP) have gained vast popularity in materials chemistry and biomedicine due to their spectacular self-assembling behaviour. A detailed account of the electronic structure and bonding through quantum theory of atoms-in-molecules, non-covalent interactions, and energy decomposition analysis (EDA) in the oligomers of perylene, perylene diimide (PDI), and thionated-PDI (t-PDI) is presented. The oligomers of all three molecules show a slip angle of θ≈62° thus forming H-aggregates. The stacking pattern in perylene oligomers prefers a slip-stacked brick-layer order, while the bulkier PDI and t-PDI prefer a parallel step-wise pattern in their oligomers. Successive addition of monomers leads to a consequent rise in the association energy, although to a much greater extent in PDI and t-PDI than in perylene. While the major contribution to this association energy comes from the dispersion interactions in all three systems, the steric interactions in t-PDI quench the cooperativity in its SMP formation. A detailed analysis of the non-covalent interactions reveals the presence of π-π, π-hole⋅⋅⋅O=C, and π-hole⋅⋅⋅S=C electrostatic interactions playing a crucial role in the self-assembly process, which can be further implemented on developing force field-based methods for understanding the self-assembling mechanism in higher degree of oligomers.

Activating charge-transfer state formation in strongly-coupled dimers using DNA scaffolds

Strongly-coupled multichromophoric assemblies orchestrate the absorption, transport, and conversion of photonic energy in natural and synthetic systems. Programming these functionalities involves the production of materials in which chromophore placement is precisely controlled. DNA nanomaterials have emerged as a programmable scaffold that introduces the control necessary to select desired excitonic properties. While the ability to control photophysical processes, such as energy transport, has been established, similar control over photochemical processes, such as interchromophore charge transfer, has not been demonstrated in DNA. In particular, charge transfer requires the presence of close-range interchromophoric interactions, which have a particularly steep distance dependence, but are required for eventual energy conversion. Here, we report a DNA-chromophore platform in which long-range excitonic couplings and short-range charge-transfer couplings can be tailored. Using combinatorial screening, we discovered chromophore geometries that enhance or suppress photochemistry. We combined spectroscopic and computational results to establish the presence of symmetry-breaking charge transfer in DNA-scaffolded squaraines, which had not been previously achieved in these chromophores. Our results demonstrate that the geometric control introduced through the DNA can access otherwise inaccessible processes and program the evolution of excitonic states of molecular chromophores, opening up opportunities for designer photoactive materials for light harvesting and computation.

Mitochondria-Driven Dye Rearrangement That Enables Spatiotemporally Controlled Photomedicine

Reversibly controlling the dye arrangements in living systems has great potential to realize spatiotemporally controlled photomedicine. However, tuning or even maintaining a certain arrangement of dyes in a complex living environments is extremely challenging due to the interference of the various biological species. Herein, a conceptual supramolecular strategy to engineer a switchable photosensitizer (PS) via mitochondria-mediated dynamic interconversion between monomer and J-aggregation, enabling specific activation of the mitochondria-targeting photodynamic therapy (PDT) and hibernation after mitochondria damage is presented. The presented mitochondria-mediated "activate-then-hibernate" PS design enables a fascinating spatiotemporally controlled PDT in which spatially controlled mitochondrial-targeting enhances therapeutic efficacy and temporally controlled activation-then-hibernation averts off-target damage during PDT and tissue damage after clinical treatment, thus offering significant potential for biological research and clinical needs.

Molecular Wires for Efficient Long-Distance Triplet Energy Transfer

We propose design rules for building organic molecular bridges that enable coherent long-distance triplet-exciton transfer. Using these rules, we describe example polychromophoric structures with low inner-sphere exciton reorganization energies, low static and dynamic disorder, and enhanced π-stacking interactions between nearest-neighbor chromophores. These features lead to triplet-exciton eigenstates that are delocalized over several units at room temperature. The use of such bridges in donor-bridge-acceptor assemblies enables fast triplet-exciton transport over very long distances that is rate-limited by the donor-bridge injection and bridge-acceptor trapping rates.

Excimer evolution hampers symmetry-broken charge-separated states

Achieving long-lived symmetry-broken charge-separated states in chromophoric assemblies is quintessential for enhanced performance of artificial photosynthetic mimics. However, the occurrence of energy trap states hinders exciton and charge transport across photovoltaic devices, diminishing power conversion efficiency. Herein, we demonstrate unprecedented excimer formation in the relaxed excited-state geometry of bichromophoric systems impeding the lifetime of symmetry-broken charge-separated states. Core-annulated perylenediimide dimers (SC-SPDI2 and SC-NPDI2) prefer a near-orthogonal arrangement in the ground state and a π-stacked foldamer structure in the excited state. The prospect of an excimer-like state in the foldameric arrangement of SC-SPDI2 and SC-NPDI2 has been rationalized by fragment-based excited state analysis and temperature-dependent photoluminescence measurements. Effective electronic coupling matrix elements in the Franck-Condon geometry of SC-SPDI2 and SC-NPDI2 facilitate solvation-assisted ultrafast symmetry-breaking charge-separation (SB-CS) in a high dielectric environment, in contrast to unrelaxed excimer formation (Ex*) in a low dielectric environment. Subsequently, the SB-CS state dissociates into an undesired relaxed excimer state (Ex) due to configuration mixing of a Frenkel exciton (FE) and charge-separated state in the foldamer structure, downgrading the efficacy of the charge-separated state. The decay rate constant of the FE to SB-CS (k FE→SB-CS) in polar solvents is 8-17 fold faster than that of direct Ex* formation (k FE→Ex*) in non-polar solvent (k FE→SB-CS≫k FE→Ex*), characterized by femtosecond transient absorption (fsTA) spectroscopy. The present investigation establishes the impact of detrimental excimer formation on the persistence of the SB-CS state in chromophoric dimers and offers the requisite of conformational rigidity as one of the potential design principles for developing advanced molecular photovoltaics.