Direct Observation of a Photochemical Alkyne–Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in a Donor–Acceptor Dyad

Mechanoluminescence from Amorphous Organic Luminogens

The ability of mechanoluminescent (ML) materials to convert mechanical energy into visualizable patterns through light emission offers a wide range of applications in advanced stress sensing, human-machine interfaces, biomedical science, etc. However, the development remains in its infancy, and more importantly, the reliance on specific crystalline structures in most existing ML materials limits their processability and practical utility. Here, we introduce a series of purely organic amorphous ML materials incorporating flexible skeletons and twisted donor-acceptor-acceptor' structures designed to enhance dipole moment and flexibility. These materials exhibit multicolor ML in amorphous states and possess low glass transition temperatures, allowing facile and in situ regeneration and processing. The stress-induced short-range molecular ordering within the amorphous phase generates local piezoelectricity, enabling ML without crystallinity. This approach overcomes the limitations of traditional crystalline ML materials, facilitating the development of flexible ML films and expanding the practical utility of organic ML systems.

Investigation of Vibrational Cooling in a Photoexcited Dichloro-Ruthenium Charge Transfer Complex Using Transient Electronic Absorption Spectroscopy

Vibrational cooling of molecules in excited electronic states is ubiquitous in photochemical reactions in solution but challenging to infer in time-resolved electronic absorption experiments. We report the ultrafast photophysics of cis-dichlorobis(2,2'-bipyridine)ruthenium(II), Ru(bpy)2Cl2, a precursor molecule commonly utilized in synthetic modifications of a vast array of ruthenium complexes. Femtosecond time-resolved electronic absorption spectroscopy is used to track an ultrafast spectral narrowing of the excited-state absorptions at 475 nm (21,050 cm-1) and 505 nm (19,800 cm-1) due to the reduced ligand in the photoexcited molecular complex. These sharp features, which overlap with a broader ground-state bleach spanning 450 nm (22,220 cm-1) to 600 nm (16,670 cm-1), evolve rapidly with time constants of 16 ± 5, 15 ± 3, and 18 ± 2 ps, respectively, for ligand-centered (π → π*, 266 nm) and charge-transfer (t2 → π*, 400 and 550 nm) excitations and constitute a direct signature of picosecond vibrational cooling.

Structural and solvent modulation of symmetry-breaking charge-transfer pathways in molecular triads

Whereas the photoinduced charge-transfer properties of electron donor-acceptor dyads are now well understood, those of symmetric conjugated architectures containing several identical donor-acceptor branches have started to be scrutinised much more recently. Here, we report on our investigation of the charge-transfer dynamics of a series of formally centrosymmetric triads consisting of a quadrupolar dihydropyrrolopyrrole core substituted with two identical diphenylethynyl lateral branches. Using a combination of time-resolved electronic and vibrational spectroscopies, we show that these molecules exhibit rich excited-state dynamics, which includes three different types of symmetry-breaking charge-transfer processes depending on the nature of the end substituents on the core and branches as well as on the solvent: (i) excited-state symmetry breaking within the core; (ii) charge transfer from the core to one of the two branches; (iii) charge transfer between the two branches. This investigation illustrates how the excited-state properties of symmetric conjugated molecules, including the nature and location of the exciton, can be controlled by fine tuning structural as well as environmental parameters.

Real-Time Tracking of Hot Carrier Injection at the Interface of FAPbBr3 Perovskite Using Femtosecond Mid-IR Spectroscopy

One of the most effective approaches to optimizing the performance of perovskite solar cells is to fully understand the ultrafast carrier dynamics at the interfaces between absorber and transporting layers at both the molecular and atomic levels. Here, the injection dynamics of hot and relaxed charge carriers at the interface between the hybrid perovskite, formamidinium lead bromide (FAPbBr3), and the organic electron acceptor, IEICO-4F, are investigated and deciphered by using femtosecond (fs) mid-infrared (IR), transient absorption (TA), and fluorescence spectroscopies. The visible femtosecond-TA measurements reveal the generation of hot carriers and their transition to free carriers in the pure FAPbBr3 film. Meanwhile, the efficient extraction of hot carriers in the mixed FAPbBr3/IEICO-4F film is clearly evidenced by the complete disappearance of their spectral signature. More specifically, the time-resolved results reveal that hot carriers are injected from FAPbBr3 to IEICO-4F within 150 fs, while the transfer time for the relaxed carriers is about 205 fs. The time-resolved mid-IR experiments also demonstrate the ultrafast formation of two peaks at 2115 and 2233 cm-1, which can be attributed to the C≡N symmetrical and asymmetrical vibrational modes of anionic IEICO-4F, thus providing crystal clear evidence for the electron transfer process between the donor and acceptor units. Moreover, photoluminescence (PL) lifetime measurements reveal an approximately 10-fold decrease in the donor lifetime in the presence of IEICO-4F, thereby confirming the efficient electron injection from the perovskite to the acceptor unit. In addition, the efficient electron injection at the FAPbBr3/IEICO-4F interface and its impact on the C≡N bond character are experimentally evidenced and align with density functional theory (DFT) calculations. This work offers new insights into the electron injection process at the FAPbBr3/IEICO-4F interface, which is crucial for developing efficient optoelectronic devices.

Comparative Observation of Distinct Dynamic Stokes Shifts in Diaryl BODIPY Triads with Broadband Two-Photon Absorption

Time-resolved evolution of excited states in the twist-conjugated chromophores is of great fundamental interest for photoluminescent applications. The four diaryl BODIPY triads modified with diverse end-cappers at 2,6-positions were investigated properly, and considerable two-photon absorption capabilities in the first biological spectral window were obtained. Fast relaxations from the initially twisted conformation to the planarized conformation in the excited state were resolved spectrally and kinetically, accompanied by the discernible phenomenon of the fluorescence dynamic Stokes shift (DSS). Along with increasing electron donating capabilities and solvent polarities, the characteristics of structural rearrangement and intramolecular charge transfer have been estimated by enhanced DSS behaviors. Especially, the blue-shifted DSS was rationalized as the sequence conversion between the planarized state and the twisted charge transfer state. A molecular-level picture for relaxation pathways in different polarities was depicted and supported by the theoretical simulations. Significant and fast structural motions in this work contribute to the excited-state dynamics and rational development of versatile BODIPY chromophores.

Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs

ConspectusSymmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environment in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimer's structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer, the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups.By understanding trends for how rates of SBCT and charge recombination depend on a dimer's internal structure and its environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications and can serve as a model for light-induced charge separation in biological systems.

Serial crystallography captures dynamic control of sequential electron and proton transfer events in a flavoenzyme

Flavin coenzymes are universally found in biological redox reactions. DNA photolyases, with their flavin chromophore (FAD), utilize blue light for DNA repair and photoreduction. The latter process involves two single-electron transfers to FAD with an intermittent protonation step to prime the enzyme active for DNA repair. Here we use time-resolved serial femtosecond X-ray crystallography to describe how light-driven electron transfers trigger subsequent nanosecond-to-microsecond entanglement between FAD and its Asn/Arg-Asp redox sensor triad. We found that this key feature within the photolyase-cryptochrome family regulates FAD re-hybridization and protonation. After first electron transfer, the FAD^•- isoalloxazine ring twists strongly when the arginine closes in to stabilize the negative charge. Subsequent breakage of the arginine-aspartate salt bridge allows proton transfer from arginine to FAD^•-. Our molecular videos demonstrate how the protein environment of redox cofactors organizes multiple electron/proton transfer events in an ordered fashion, which could be applicable to other redox systems such as photosynthesis.

Designed polymeric conjugation motivates tunable activation of molecular oxygen in heterogeneous organic photosynthesis

Photocatalytic oxidative organic reactions are important synthetic transformations, and research on reaction selectivity by reactive oxygen species (ROS) is significant. To date, however, there has rarely been any focus on the directed generation of ROSs. Herein, we report the first identification of tunable molecular oxygen activation induced by polymeric conjugation in nonmetallic conjugated microporous polymers (CMP). The conjugation between these can be achieved by the introduction of alkynyl groups. CMP-A with an alkynyl bridge facilitates the intramolecular charge mobility while CMP-D, lacking an alkynyl group enhances the photoexcited carrier build-up on the surface from diffusion. These different processes dominate the directed ROS generation of the superoxide radical (O₂^-) and singlet oxygen (¹O₂), respectively. This theory is substantiated by the different performances of these CMPs in the aerobic oxidation of sulfides and the dehydrogenative coupling of amines, and could provide insight into the rational design of CMPs for various heterogeneous organic photosynthesis.

Ultrafast Aggregation-Induced Tunable Emission Enhancement in a Benzothiadiazole-Based Fluorescent Metal-Organic Framework Linker

Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor-acceptor-donor-type benzodiazole-based molecules (the organic building block in metal-organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10^-11 to 2.48 ns at 2.0 × 10^-3 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the C≡C vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.

Photo-Induced Partially Aromatized Intramolecular Charge Transfer

Diverse models of intramolecular charge transfer (ICT) have been proposed for interpreting the origin of the charge-transfer (CT) state in donor-acceptor (D-A) dyes. However, a large variety of fused-heterocyclic dyes containing a pseudo-aromatic ring in the rigid structure have shown to be incompatible with them. To approximate a solution within the ICT concept, we reported a novel ICT model called partially aromatized intramolecular charge transfer (PAICT). PAICT involves the generation of a CT state from an ICT that occurred within a pre-excited D-A fused-heterocyclic structure possessing a pseudo-aromatic or unstable aromatic ring as the acceptor moiety. The model was proposed from the multiple-emissive mesomeric D-A N¹-aryl-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one, whose excited mesomeric states, which are defined by the aromatic and pseudo-aromatic forms of the pyrindin-4(1H)-one ring, led to a common partial aromatized CT state upon excitation via PAICT. The latter was supported through theoretical calculations on the excited mesomeric states, one-dimensional (1D) and two-dimensional (2D) excitation-emission measurements in different solvents, and the detection of three excited states by lifetime measurements upon 370 nm excitation. The existence of mesomerism was supposed from: (i) two overlapping bands at 370-390 (or 400-420 nm) in UV-vis spectra, (ii) the direct interaction between the pyridinic nitrogen of one molecule and the carbonylic oxygen of the other found in the solid state and, (iii) the detection of three excited states by lifetime measurements. The PAICT opens new perspectives for interpreting the charge-transfer phenomenon in fused-heterocyclic dyes, in particular, those containing a pseudo-aromatic or unstable aromatic ring as an acceptor moiety.

Mode-Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers

Exciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode-specific kinetics in the S₁ Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

NIR Photosensitizer for Two-Photon Fluorescent Imaging and Photodynamic Therapy of Tumor

Preparation of near-infrared (NIR) emissive fluorophore for imaging-guided PDT (photodynamic therapy) has attracted enormous attention. Hence, NIR photosensitizers of two-photon (TP) fluorescent imaging and photodynamic therapy are highly desirable. In this contribution, a novel D-π-A structured NIR photosensitizer (TTRE) is synthesized. TTRE demonstrates near-infrared (NIR) emission, good biocompatibility, and superior photostability, which can act as TP fluorescent agent for clear visualization of cells and vascular in tissue with deep-tissue penetration. The PDT efficacy of TTRE as photosensitizer is exploited in vitro and in vivo. All these results confirm that TTRE would serve as potential platform for TP fluorescence imaging and imaging-guided photodynamic therapy.

Electron-Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer

Although photoinduced proton-coupled electron transfer (PCET) plays an essential role in photosynthesis, a full understanding of the mechanism is still lacking due to the complex nonequilibrium dynamics arising from the strongly coupled electronic and nuclear degrees of freedom. Here we report the photoinduced PCET dynamics of a biomimetic model system investigated by means of transient IR and two-dimensional electronic-vibrational (2DEV) spectroscopies, IR spectroelectrochemistry (IRSEC), and calculations utilizing long-range-corrected hybrid density functionals. This collective experimental and theoretical effort provides a nuanced picture of the complicated dynamics and synergistic motions involved in photoinduced PCET. In particular, the evolution of the 2DEV line shape, which is highly sensitive to the mixing of vibronic states, is interpreted by accurate computational modeling of the charge separated state and is shown to represent a gradual change in electron density distribution associated with a dihedral twist that occurs on a 120 fs time scale.

Theoretical Insights into the Excited State Decays of a Donor-Acceptor Dyad: Is the Twisted and Rehybridized Intramolecular Charge-Transfer State Involved?

The twisted intramolecular charge transfer has been proposed for a number of years and widely accepted to explain the excited-state dynamics of organic molecules. Recently, a new state termed as "twisted and rehybridized intramolecular charge transfer" has been proposed to explain the excited-state dynamics of an aniline-triazine electron donor-acceptor dyad with an alkyne spacer based on ultrafast time-resolved spectroscopy. However, the change of the geometries along the excited-state decay pathway remains unknown. In this study, by optimization of the excited-state geometry of the donor-acceptor dyad and potential energy surface scan along the twisting angle, we successfully reproduce the experimentally observed band in time-resolved infrared absorption spectroscopy. Our calculation results demonstrated that the rehybridization process is not involved and only the twisted intramolecular charge transfer state is formed. Moreover, we located a minimum energy conical intersection between the ground and first excited-state of the donor-acceptor dyad, which is easily reached and corresponding to the primary nonradiative decay pathway of the donor-acceptor dyad. The energy of minimum energy conical intersection is solvent-dependent and consistent with the experimentally observed solvent-dependent lifetime of excited state.

Solvent tuning of photochemistry upon excited-state symmetry breaking

The nature of the electronic excited state of many symmetric multibranched donor–acceptor molecules varies from delocalized/multipolar to localized/dipolar depending on the environment. Solvent-driven localization breaks the symmetry and traps the exciton in one branch. Using a combination of ultrafast spectroscopies, we investigate how such excited-state symmetry breaking affects the photochemical reactivity of quadrupolar and octupolar A–(π-D)_2,3 molecules with photoisomerizable A–π–D branches. Excited-state symmetry breaking is identified by monitoring several spectroscopic signatures of the multipolar delocalized exciton, including the S₂ ← S₁ electronic transition, whose energy reflects interbranch coupling. It occurs in all but nonpolar solvents. In polar media, it is rapidly followed by an alkyne–allene isomerization of the excited branch. In nonpolar solvents, slow and reversible isomerization corresponding to chemically-driven symmetry breaking, is observed. These findings reveal that the photoreactivity of large conjugated molecules can be tuned by controlling the localization of the excitation.

Symmetric multibranched donor-acceptor molecules are promising photoactive materials for diverse applications. Here the authors show that, in octupolar and quadrupolar dyes, excited-state symmetry breaking occurs efficiently in polar solvents only and results in a concentration of the excitation that may trigger fast photochemical reactions.

Dramatic changes in the excited-state behaviour of the green fluorescent protein chromophore by a strong π-donating group through significantly lowering the excited-state potential energy surface with photoinduced intramolecular charge transfer

A strong π-donating group like p-NMe₂ dramatically changes the excited-state behavior of the green fluorescent protein (GFP) chromophore, such as realizing charge-transfer absorption and executing significant photoinduced intramolecular charge transfer (ICT), which makes a planar first singlet (S₁) excited-state minimum disappear and significantly lowers the S₁ excited-state potential energy surface (PES), leading to barrierless τ-torsion and φ-torsion excited-state relaxation and eliciting the φ-torsion S₁ excited-state minimum. This finding is critical since a strong π-donating group like p-NMe₂ may do the same things to other fluorophores.

Hydrogen bond configuration and protonation of ground and lowest excited triplet states of 4‑amino‑4'‑nitrobiphenyl based on nanosecond transient absorption spectroscopy

Intramolecular charge transfer (ICT) is an important photochemical process. In contrast to those in singlet manifold, triplet ICT states were less studied. In this paper, the lowest excited triplet state (T₁) of 4‑amino‑4'‑nitrobiphenyl (NH₂-Bp-NO₂) was recorded with nanosecond transient absorption spectroscopy in acidic acetonitrile and alcoholic solutions. By employing the Kamlet-Taft model to analyze the correlation between absorption maxima and alcohol solvent properties including polarity/polarizability, abilities of hydrogen bond donating and hydrogen bond accepting, hydrogen bond configuration in the ground state (S₀) and T₁ was resolved. The results suggest that the hydrogen bond between amino H and alcohol is dominant in S₀, while in T₁, hydrogen bonds between amino H and alcohol, between nitro O and alcohol have comparable contribution. By examination of the 1‑naphthol quench effect on T₁, the hydrogen bond between nitro O and alcohol was confirmed present. Theoretical calculation results on the model of NH₂-Bp-NO₂-(MeOH)₃ also indicate that hydrogen bonds between amino H and alcohol, between nitro O and alcohol are both much stronger in T₁ than in S₀. In acidic acetonitrile solution, in S₀ of NH₂-Bp-NO₂ the amino group is protonated with pK_a of 4.5, meanwhile in T₁ the nitro group is much easier to be protonated than in S₀. Its conjugated acid was measured to have a pK_a of 3.1.

Cascade cyclization and acyl migration of propargylic esters with isocyanides: rapid access to substituted furans

A novel rearrangement of ester-activated propargylic acetate with isocyanide has been disclosed. This protocol enables a quick synthesis of polysubstituted furan derivatives.

Structural dynamics of an excited donor–acceptor complex from ultrafast polarized infrared spectroscopy, molecular dynamics simulations, and quantum chemical calculations

Infrared anisotropy experiments and mixed quantum/classical computations demonstrate large scale reorientation following excitation of a donor/acceptor complex.

Enhancement of intramolecular charge transfer strength in diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives

The intramolecular charge transfer characteristic of two diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives (DPAOXD and DPAOXDBEN) was studied through a combination of experimental techniques and theoretical calculations. Significant enhancement of intramolecular charge transfer strength has been found in both these compounds through molecular structure modification. The experimental result found only a small red shift in the absorption spectra (∼15 nm) but a very large red shift in the emission spectra (∼114 nm for DPAOXD and ∼140 nm for DPAOXDBEN) with increasing solvent polarity, indicating a large extent charge transfer occurred in their excited state. The increase of molecular dipole moment from the ground state to the charge transfer excited state is calculated to be 22.10 D in DPAOXD and 26.67 D in DPAOXDBEN, respectively. Theoretical calculations present clear evidence that electrons transfer from the terminal diphenylamine to the bi-1,3,4-oxadiazole rings in DPAOXD, and the two 1,3,4-oxadiazole rings and central benzene ring in DPAOXDBEN. As compared to the methoxy group, the substitution by a diphenylamine group could increase both the transferred charge and distance, which could substantially strengthen the charge transfer character. Further introduction of a central benzene ring in DPAOXDBEN could further increase the transferred distance, and then the charge transfer strength. These findings could provide good guidance for the design of molecules with high intramolecular charge transfer characteristics.

Weak interactions but potent effect: tunable mechanoluminescence by adjusting intermolecular C-H···π interactions

A new mechanoluminescent material (4-(diphenylamino)phenyl)(4-(diphenylphosphanyl)phenyl)methanone (CDpP), which displays tunable mechanoluminescent emission colors, has been designed and successfully synthesized. CDpP shows two distinct mechanoluminescent colors (blue and green) in different crystalline states. Single-crystal analyses and femtosecond transient emission studies reveal that the striking tunable mechanoluminescence properties of CDpP mainly originate from the different C-H···π interactions in the crystal structures. CDpP crystals with more C-H···π interactions show blue mechanoluminescence (ML), and the emission is attributed to the locally excited (LE)-state because the twisting process for the excited state is restricted by C-H···π interactions. Conversely, CDpP crystals with fewer C-H···π interactions display green ML, in which the red-shifted emission band originates from the twisted intramolecular charge transfer (TICT) excited state because the diphenylamine moiety is relatively free to rotate. The manipulation of weak intermolecular interactions in the crystalline state is a useful and reliable strategy for the tuning of the ML emission wavelengths.

Influence of the hydrogen-bond interactions on the excited-state dynamics of a push–pull azobenzene dye: the case of Methyl Orange

H-bonding with the solvent affects significantly the photoisomerisation of Methyl Orange.

Why does 4-biphenyl carbonyl azide have ultra-short lived excited states? An ultrafast UV-vis spectroscopic and computational study

A new model is presented for ultrashort-lived excited states of carbonyl azides, their solvent dependence and other experimental observations.