Looking at Photoinduced Charge Transfer Processes in the IR: Answers to Several Long-Standing Questions

Accelerating Symmetry-Breaking Charge Separation in an Angular versus Linear Perylenediimide Dimer through the Modulation of Coulombic Coupling

Achieving faster charge separation in organic systems capable of mimicking the electron transfer events in natural photosynthesis has been an exciting research topic for several decades. Herein, we demonstrate the orientation-dependent acceleration of symmetry-breaking charge separation (SB-CS) in an angular (A-PDI2) versus linear (L-PDI2) perylenediimide dimer. Femtosecond transient absorption measurements reveal ultrafast SB-CS in A-PDI2 (τCS = 6.3 ps) with charge separation ∼20 times faster than in L-PDI2 (τCS = 127.9 ps). Nanosecond transient absorption measurements establish the negligible population of triplet excited-states in L-PDI2 (ϕT < 1%), whereas a significant triplet excited-state population (ϕT = 35.9%) is quantified in A-PDI2. The theoretically computed Coulombic coupling strength in A-PDI2 (|JCoul| = 14.9 cm-1) and L-PDI2 (|JCoul| = 438.4 cm-1) is rationalized as the crucial factor modulating the SB-CS rates. The current investigation could be beneficial for designing light harvesting materials capable of faster charge separation for efficient optoelectronic devices.

Recent Advances in Probing Electron Delocalization in Conjugated Molecules by Attached Infrared Reporter Groups for Energy Conversion and Storage

This review article reports an overview of the recent developments in the field of electron delocalization study in organic conjugated molecules by utilizing the vibration frequencies exhibited by the attached functional groups such as nitrile (-C≡N), alkyne (-C≡C-), or carbonyl (-C=O). A brief introduction to electron delocalization, methods for study, and their importance is given first, followed by the application of infrared spectroscopy in organic molecules. Details of molecules with various infrared reporter groups have been explained in respective subsections based on the functional groups. All the reported organic molecules have been structured and presented with the electron delocalization properties studied using an infrared reporter group. Finally, an outlook on this recently promising, exciting, and interesting field of probing electron delocalization using infrared reporter groups is provided.

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

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

Aggregation promotes charge separation in fullerene-indacenodithiophene dyad

Fast photoinduced charge separation (CS) and long-lived charge-separated state (CSS) in small-molecules facilitate light-energy conversion, while simultaneous attainment of both remains challenging. Here we accomplish this through aggregation based on fullerene-indacenodithiophene dyads. Transient absorption spectroscopy reveals that, compared to solution, the CS time in aggregates is accelerated from 41.5 ps to 0.4 ps, and the CSS lifetime is prolonged from 311.4 ps to 40 μs, indicating that aggregation concomitantly promotes fast CS and long-lived CSS. Fast CS arises from the hot charge-transfer states dissociation, opening up additional resonant channels to free carriers (FCs); subsequently, charge recombination into intramolecular triplet CSS becomes favorable mediated by spin-uncorrelated FCs. Different from fullerene/indacenodithiophene blends, the unique CS mechanism in dyad aggregates reduces the long-lived CSS dependence on molecular order, resulting in a CSS lifetime 200 times longer than blends. This endows the dyad aggregates to exhibit both photoelectronic switch properties and superior photocatalytic capabilities.

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

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

Intramolecular charge transfer enhanced optical limiting in novel hydrazone derivatives with a D1-D-Ai-π-A structure

The present paper investigates one of the hydrazone derivatives (BTH with a D-π-A structure) based on density functional theory. With the computation results of ground state absorption (GSA), excited-state absorption (ESA) and multi-photon absorption (MPA), the optical limiting effect observed in the experiment for the BTH molecule can be well predicted and elucidated by the MPA-ESA mechanism. The analysis of the hole-electron and the electron density differences between two transition states reveal that the main transitions involved in the GSA and ESA of BTH could be recognized as local excitation. Based on these observations, four novel hydrazone derivatives based on the BTH unit with a D1-D-Ai-π-A structure were designed to promote intramolecular charge transfer (ICT). It shows that the ICT effect is well improved by adding the D1 and Ai units. Compared with the original BTH molecule, the main bands of GSA and ESA of D1-D-Ai-π-A molecules are both red-shifted. In addition, GSA, ESA and MPA probabilities are all improved because the obvious charge transfer character results in the transition dipole moment change from localized to delocalized. Accordingly, the optical limiting effect in these hydrazone derivatives is well enhanced. These observations provide guidance for designing novel optical limiting materials based on the hydrazone derivatives.

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.

Ultrafast Charge Transfer Dynamics in a Slip-Stacked Donor-Acceptor-Acceptor System

Photoexcitation of molecular electron donor and/or acceptor chromophore aggregates can greatly affect their charge-transfer dynamics. Excitonic coupling not only alters the energy landscape in the excited state but may also open new photophysical pathways, such as symmetry-breaking charge separation (SB-CS). Here, we investigate the impact of excitonic coupling on a covalent donor-acceptor-acceptor system comprising a perylene donor (Per) and two perylenediimide (PDI) acceptor chromophores in which the three components are π-stacked in a geometry that is slipped along their long axes (Per-PDI2). Following selective photoexcitation of PDI, femtosecond transient absorption data for Per-PDI2 is compared to that for the single-donor, single-acceptor Per-PDI system, and the PDI2 dimer, which both have the same interchromophore geometry as Per-PDI2. The data show that electron transfer from Per to the lower exciton state of the PDI dimer is slower than that of the single PDI acceptor system. This is due to the lower free energy of the reaction for charge separation because of the electronic stabilization afforded by the excitonic coupling between the PDIs. While PDI2 was shown previously to undergo ultrafast SB-CS, the strong π-π electronic interaction of Per with the adjacent PDI in Per-PDI2 breaks the electronic symmetry of the PDI dimer, resulting in the oxidation of Per rather than SB-CS. These results show that the electronic coupling between molecules designed to accept charges produced by SB-CS in molecular dimers and the chromophores comprising the dimer must be balanced to favor SB-CS.

Triple-Bond Vibrations: Emerging Applications in Energy and Biological Sciences

Triple bonds, such as that formed between two carbon atoms (i.e., C≡C) or that formed between one carbon atom and one nitrogen atom (i.e., C≡N), afford unique chemical bonding and hence vibrational characteristics. As such, they are not only frequently used to construct molecules with tailored chemical and/or physical properties but also employed as vibrational probes to provide site-specific chemical and/or physical information at the molecular level. Herein, we offer our perspective on the emerging applications of various triple-bond vibrations in energy and biological sciences with a focus on C≡C and C≡N triple bonds.

Photoinduced asymmetric charge trapping in a symmetric tetraazapyrene-fused bis(tetrathiafulvalene) conjugate

In fused donor-acceptor (D-A) ensembles, rapid charge recombination often occurs because the D and A units are spatially close and strongly coupled. To the best of our knowledge, a long-lived charge separated (CS) state is still elusive in such systems. The results presented here show that symmetric annulation of two tetrathiafulvalene (TTF) donors to a central tetraazapyrene (TAP) acceptor via two quinoxaline units leads to a CS state lifetime of a few ns. A detailed study of the electronic interactions between TTF and TAP units in the ground and excited states was performed and compared with the asymmetric counterpart by cyclic voltammetry, optical absorption and ultrafast transient absorption spectroscopy. The results demonstrate that the photoinduced asymmetric charge trapping between two TTFs significantly stabilizes the CS state, which is also verified theoretically.

Coherent Vibronic Wavepackets Show Structure-Directed Charge Flow in Host-Guest Donor-Acceptor Complexes

Designing and controlling charge transfer (CT) pathways in organic semiconductors are important for solar energy applications. To be useful, a photogenerated, Coulombically bound CT exciton must further separate into free charge carriers; direct observations of the detailed CT relaxation pathways, however, are lacking. Here, photoinduced CT and relaxation dynamics in three host-guest complexes, where a perylene (Per) electron donor guest is incorporated into two symmetric and one asymmetric extended viologen cyclophane acceptor hosts, are presented. The central ring in the extended viologen is either p-phenylene (ExV²⁺) or electron-rich 2,5-dimethoxy-p-phenylene (ExMeOV²⁺), resulting in two symmetric cyclophanes with unsubstituted or methoxy-substituted central rings, ExBox⁴⁺ and ExMeOBox⁴⁺, respectively, and an asymmetric cyclophane with one of the central viologen rings being methoxylated ExMeOVBox⁴⁺. Upon photoexcitation, the asymmetric host-guest ExMeOVBox⁴⁺ ⊃ Per complex exhibits directional CT toward the energetically unfavorable methoxylated side due to structural restrictions that facilitate strong interactions between the Per donor and the ExMeOV²⁺ side. The CT state relaxation pathways are probed using ultrafast optical spectroscopy by focusing on coherent vibronic wavepackets, which are used to identify CT relaxations along charge localization and vibronic decoherence coordinates. Specific low- and high-frequency nuclear motions are direct indicators of a delocalized CT state and the degree of CT character. Our results show that the CT pathway can be controlled by subtle chemical modifications of the acceptor host in addition to illustrating how coherent vibronic wavepackets can be used to probe the nature and time evolution of the CT states.

A Symmetry-Broken Charge-Separated State in the Marcus Inverted Region

We report a long-lived charge-separated state in a chromophoric pair (DC-PDI₂ ) that uniquely integrates the advantages of fundamental processes of photosynthetic reaction centers: i) Symmetry-breaking charge-separation (SB-CS) and ii) Marcus-inverted-region dependence. The near-orthogonal bichromophoric DC-PDI₂ manifests an ultrafast evolution of the SB-CS state with a time constant of τ S B - C S ${{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ =0.35±0.02 ps and a slow charge recombination (CR) kinetics with τ C R ${{\tau }_{{\rm C}{\rm R}}}$ =4.09±0.01 ns in ACN. The rate constant of CR of DC-PDI₂ is 11 686 times slower than SB-CS in ACN, as the CR of the PDI radical ion-pair occurs in the deep inverted region of the Marcus parabola ( - Δ G C R ${{-{\rm \Delta }G}_{{\rm C}{\rm R}}}$ >λ). In contrast, an analogous benzyloxy (BnO)-substituted DC-BPDI₂ showcases a ≈10-fold accelerated CR kinetics with τ C R / τ S B - C S ${{\tau }_{{\rm C}{\rm R}}/{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ lowering to ≈1536 in ACN, by virtue of a decreased CR driving force. The present investigation demonstrates a control of molecular engineering to tune the energetics and kinetics of the SB-CS material, which is essential for next-generation optoelectronic devices.

Effect of positional isomerism on the excited state charge transfer dynamics of anthracene-based D-π-A systems

Understanding the dynamics of the back electron transfer (BET) rate of ion pairs from the electronically excited state of donor-acceptor systems is crucial for developing materials for organic electronics. The structure-property relationships in the organic molecular architectures play a key role in controlling the BET rate and have been utilized as a criterion to design systems with a reduced BET rate. Here, we examine the influence of isomerism on the BET rate in anthracene based systems, viz., (E)-2-(2-(anthracen-9-yl)vinyl)benzonitrile (ortho-CN) and (E)-3-(2-(anthracen-9-yl)vinyl)benzonitrile (meta-CN) with N,N-diethylaniline (DEA) in methylcyclohexane using time-resolved spectroscopy. The radical cation (DEA˙⁺) and the radical anion (ortho-CN˙^- or meta-CN˙^-) generated after photoexcitation show synchronous decay kinetics, and the rate constant of back electron transfer (k_BET) for the DEA/ortho-CN pair was 6.6 × 10⁴ s^-1, which is ca. 2 orders of magnitude lower compared with the DEA/meta-CN pair. The role of isomerism in providing resonance stabilization for the organic radicals is expected to have implications for strategies that retard charge recombination in photovoltaics. The role of the molecular structural features that dictate the kinetics for charge recombination has been further identified using quantum calculations.

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.

Alkoxy-Substituted Quadrupolar Fluorescent Dyes

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 μM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

Synthesis and Characterizations of 5,5'-Bibenzo[rst]pentaphene with Axial Chirality and Symmetry-Breaking Charge Transfer

Exploration of novel biaryls consisting of two polycyclic aromatic hydrocarbon (PAH) units can be an important strategy toward further developments of organic materials with unique properties. In this study, 5,5'-bibenzo[rst]pentaphene (BBPP) with two benzo[rst]pentaphene (BPP) units is synthesized in an efficient and versatile approach, and its structure is unambiguously elucidated by X-ray crystallography. BBPP exhibits axial chirality, and the (M)- and (P)-enantiomers are resolved by chiral high-performance liquid chromatography and studied by circular dichroism spectroscopy. These enantiomers have a relatively high isomerization barrier of 43.6 kcal mol^-1 calculated by density functional theory. The monomer BPP and dimer BBPP are characterized by UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry, and femtosecond transient absorption spectroscopy. The results indicate that both BPP and BBPP fluoresce from a formally dark S₁ electronic state that is enabled by Herzberg-Teller intensity borrowing from a neighboring bright S₂ state. While BPP exhibits a relatively low photoluminescence quantum yield (PLQY), BBPP exhibits a significantly enhanced PLQY due to a greater S₂ intensity borrowing. Moreover, symmetry-breaking charge transfer in BBPP is demonstrated by spectroscopic investigations in solvents of different polarity. This suggests high potential for singlet fission in such π-extended biaryls through appropriate molecular design.

Symmetry-breaking charge separation in a nitrogen-bridged naphthalene monoimide dimer

The photophysical properties of 4-aminonaphthalene-1,8-imide-based derivatives, bis-ANI, consisting of two naphthalimide (NI) units linked by a butylamine bridge and its monomer ANI have been intensively investigated by steady-state and transient spectroscopy combined with quantum chemical calculations. The excited state relaxation dynamics of the two molecules are studied in three solvents of varying polarity - from hexane to tetrahydrofuran to acetone. A strong reduction in the fluorescence quantum yields and larger red shifts of the emission spectra are observed when going from the monomer ANI to dimer bis-ANI with increasing solvent polarity. It is found that the presence of the central amino linker in bis-ANI facilitates the formation of an asymmetric CS state between the ANI and NI moieties in bis-ANI, where NI˙^- is the dominant radical anion unit after CS, evidenced by the femtosecond transient absorption measurements and spectroelectrochemistry in polar solvents. Femtosecond transient absorption spectra and quantum chemical calculations reveal the conformational change after the formation of the symmetry-breaking charge separation (SBCS) state upon photoexcitation, while a near-orthogonal structure in the excited state of bis-ANI retards charge recombination. In addition, it is evidenced that the rate of SBCS can be tuned by changing the different polar solvents.

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.

Quantum dynamics of the photoinduced charge separation in a symmetric donor–acceptor–donor triad: The role of vibronic couplings, symmetry and temperature

The photoinduced charge separation in a symmetric donor–acceptor–donor (D–A–D) triad is studied quantum mechanically using a realistic diabatic vibronic coupling model. The model includes a locally excited DA*D state and two charge-transfer states D+A−D and DA−D+ and is constructed according to a procedure generally applicable to semirigid D–A–D structures and based on energies, forces, and force constants obtained by quantum chemical calculations. In this case, the electronic structure is described by time-dependent density functional theory, and the corrected linear response is used in conjunction with the polarizable continuum model to account for state-specific solvent effects. The multimode dynamics following the photoexcitation to the locally excited state are simulated by the hybrid Gaussian-multiconfigurational time-dependent Hartree method, and temperature effects are included using thermo field theory. The dynamics are connected to the transient absorption spectrum obtained in recent experiments, which is simulated and fully assigned from first principles. It is found that the charge separation is mediated by symmetry-breaking vibrations of relatively low frequency, which implies that temperature should be accounted for to obtain reliable estimates of the charge transfer rate.

Accelerating symmetry-breaking charge separation in a perylenediimide trimer through a vibronically coherent dimer intermediate

Understanding the photophysics and photochemistry of molecular π-stacked chromophores is important for utilizing them as functional photonic materials. However, these investigations have been mostly limited to covalent molecular dimers, which can only approximate the electronic and vibronic interactions present in the higher oligomers typical of functional organic materials. Here we show that a comparison of the excited-state dynamics of a covalent slip-stacked perylenediimide dimer (2) and trimer (3) provides fundamental insights into electronic state mixing and symmetry-breaking charge separation (SB-CS) beyond the dimer limit. We find that coherent vibronic coupling to high-frequency modes facilitates ultrafast state mixing between the Frenkel exciton (FE) and charge-transfer (CT) states. Subsequently, solvent fluctuations and interchromophore low-frequency vibrations promote CT character in the coherent FE/CT mixed state. The coherent FE/CT mixed state persists in 2, but, in 3, low-frequency vibronic coupling collapses the coherence, resulting in ultrafast SB-CS between the distal perylenediimide units.

Super-Exchange Charge Transfer in One-Photon and Two-Photon Absorption of Multibranched Compounds

In this work, density functional theory is used to study organic molecules in a donor-acceptor (D-A) system centered on phenothiazine with strip and trigonal structures. The transition modes of the one-photon absorption (OPA) and two-photon absorption (TPA) processes of the two molecules are studied. The calculations show that the molar absorption coefficient of OPA for trigonal molecule TPPO and the cross section of TPA are both larger than those for strip molecule M1 due to the increase in the number of branches of the system. A special local excitation-enhanced charge-transfer excitation appears in strip-type molecule M1. In the charge-transfer process of trigonal D-A structure molecule TPPO, there are not only local excitation-enhanced charge-transfer excitation but also super-exchange charge transfer between the three branches that occurs due to the increase in the planarity of the system.

Probing effect of solvation on photoexcited quadrupolar donor-acceptor-donor molecule via ultrafast Raman spectroscopy

The symmetric and quadrupolar donor-acceptor-donor (D-A-D) molecules usually exhibit excited-state charge redistribution process from delocalized intramolecular charge transfer (ICT) state to localized ICT state. Direct observation of such charge redistribution process in real-time has been intensively studied via various ultrafast time-resolved spectroscopies. Femtosecond stimulated Raman spectroscopy (FSRS) is one of the powerful methods which can be used to determine the excited state dynamics by tracking vibrational mode evolution of the specific chemical bonds within molecules. Herein, a molecule, 4,4′-(buta-1,3-diyne-1,4-diyl)bis( N, N-bis(4-methoxyphenyl)aniline), that consists of two central adjacent alkyne (-C≡C-) groups as electron-acceptors and two separated, symmetric N, N-bis(4-methoxyphenyl)aniline at both branches as electron-donors, is chosen to investigate the excited-state photophysical properties. It is shown that the solvation induced excited-state charge redistribution in polar solvents can be probed by using femtosecond stimulated Raman spectroscopy. The results provide a fundamental understanding of photoexcitation induced charge delocalization/localization properties of the symmetric quadrupolar molecules with adjacent vibrational markers located at central position.

Bridge-Length- and Solvent-Dependent Charge Separation and Recombination Processes in Donor-Bridge-Acceptor Molecules

The photoinduced intramolecular charge separation (CS) and charge recombination (CR) phenomena in a series of donor-bridge-acceptor (D-B-A) molecules are intensively investigated as a means of understanding electron transport through the π-B. Pyrene (Pyr) and triarylamine (TAA) moieties connected via phenylene Bs of various lengths are studied because their CS and CR behaviors can be readily monitored in real time by femtosecond transient absorption (fs-TA) spectroscopy. By combining the steady-state and fs-TA spectroscopic measurements in a variety of solvents together with chemical calculations, the parameters that govern the CS behaviors of these dyads were obtained, such as the solvent effects on free energy and the B-length-dependent electronic coupling (V_DA) between D and A. We observed the sharp switch of the CS behavior with the increase of the solvent polarity and B-linker lengths. Furthermore, in the case of the shortest distance between D and A when the electron coupling is sufficiently large, we observed that the CS phenomenon occurs even in low-polar solvents. Upon increasing the length of B, CS occurs only in strong polar solvents. The distance-dependent decay constant of the CS rate is determined as ∼0.53 Å^-1, indicating that CS is governed by superexchange tunneling interactions. The CS rate constants are also approximately estimated using Marcus electron transfer theory, and the results imply that the V_DA value is the key factor dominating the CS rate, while the facile rotation of the phenylene B is important for modulating the lifetime of the charge-separated state in these D-B-A dyads. These results shed light on the practical strategy for obtaining a high CS efficiency with a long-lived CS state in TAA-B-Pyr derivatives.

IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions

Conjugated molecular chains have the potential to act as "molecular wires" that can be employed in a variety of technologies, including catalysis, molecular electronics, and quantum information technologies. Their successful application relies on a detailed understanding of the factors governing the electronic energy landscape and the dynamics of electrons in such molecules. We can gain insights into the energetics and dynamics of charges in conjugated molecules using time-resolved infrared (TRIR) detection combined with pulse radiolysis. Nitrile ν(C[triple bond, length as m-dash]N) bands can act as IR probes for charges, based on IR frequency shifts, because of their exquisite sensitivity to the degree of electron delocalization and induced electric field. Here, we show that the IR intensity and linewidth can also provide unique and complementary information on the nature of charges. Quantifications of IR intensity and linewidth in a series of nitrile-functionalized oligophenylenes reveal that the C[triple bond, length as m-dash]N vibration is coupled to the nuclear and electronic structural changes, which become more prominent when an excess charge is present. We synthesized a new series of ladder-type oligophenylenes that possess planar aromatic structures, as revealed by X-ray crystallography. Using these, we demonstrate that C[triple bond, length as m-dash]N vibrations can report charge fluctuations associated with nuclear movements, namely those driven by motions of flexible dihedral angles. This happens only when a charge has room to fluctuate in space.

Vibrational Dephasing along the Reaction Coordinate of an Electron Transfer Reaction

The role of molecular vibration in photoinduced electron transfer (ET) reactions has been extensively debated in recent years. In this study, we investigated vibrational wavepacket dynamics in a model ET system consisting of an organic dye molecule as an electron acceptor dissolved in various electron donating solvents. By using broad band pump-probe (BBPP) spectroscopy with visible laser pulses of sub-10 fs duration, coherent vibrational wavepackets of naphthacene dye with frequencies spanning 170-1600 cm-1 were observed in the time domain. The coherence properties of 11 vibrational modes were analyzed by an inverse Fourier filtering procedure, and we discovered that the dephasing times of some vibrational coherences are reduced with increasing ET rates. Density functional theory calculations indicated that the corresponding vibrational modes have a large Huang-Rhys factor between the reactant and the product states, supporting the hypothesis that the loss of phase coherence along certain vibrational modes elucidates that those vibrations are coupled to the reaction coordinate of an ET reaction.

Symmetry-Breaking Charge Separation in Phenylene-Bridged Perylenediimide Dimers

Perylenediimides (PDIs) are important molecular building blocks that are being investigated for their applicability in optoelectronic technologies. Covalently linking multiple PDI acceptors at the 2,5,8,11 (headland) positions adjacent to the PDI carbonyl groups is reported to yield higher power conversion efficiencies in photovoltaic cells relative to PDI acceptors linked at the 1,6,7,12 (bay) positions. While the photophysical properties of PDIs linked via the bay positions have been investigated extensively, those linked at the headland positions have received far less attention. We showed previously that symmetry-breaking charge separation (SB-CS) in PDIs hold promise as a strategy for increasing photovoltaic efficiency. Here we use transient absorption and emission spectroscopies to investigate the competition between SB-CS, fluorescence, and internal conversion in three related PDI dimers linked at the headland positions with o-, m-, and p-phenylene moieties: o-PDI₂, m-PDI₂, and p-PDI₂, respectively. It is found that o-PDI₂ supports SB-CS yielding PDI^•+-PDI^•-, which is in equilibrium with the o-PDI₂ first excited state in a polar solvent (CH₂Cl₂) while m-PDI₂ and p-PDI₂ exhibit accelerated internal conversion due to the motion of the linker along with subnanosecond intersystem crossing (ISC). Electronic coupling and structural dynamics are shown to play a significant role, with o-PDI₂ being the only member of the series that exhibits significant through-bond interchromophore coupling. The pronounced o-PDI₂ steric congestion prevents the free internal rotation that leads to rapid deactivation of the excited state in the other dimers.

The Carbene Cannibal: Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. Here, we present the first case of direct observation of bimolecular SB-CS in a transition metal complex, [Fe^IIIL₂](PF₆) (L = [phenyl(tris(3-methylimidazol-1-ylidene))borate]⁻). Photoexcitation of the complex in the visible region results in the formation of a doublet ligand-to-metal charge transfer (²LMCT) excited state (E_0–0 = 2.13 eV), which readily reacts with the doublet ground state to generate charge separated products, [Fe^IIL₂] and [Fe^IVL₂]²⁺, with a measurable cage escape yield. Known spectral signatures allow for unambiguous identification of the products, whose formation and recombination are monitored with transient absorption spectroscopy. The unusual energetic landscape of [Fe^IIIL₂]⁺, as reflected in its ground and excited state reduction potentials, results in SB-CS being intrinsically exergonic (ΔG_CS° ∼ −0.7 eV). This is in contrast to most systems investigated in the literature, where ΔG_CS° is close to zero, and the charge transfer driven primarily by solvation effects. The study is therefore illustrative for the utilization of the rich redox chemistry accessible in transition metal complexes for the realization of SB-CS.

Physical Mechanism of Photoinduced Charge Transfer in One- and Two-Photon Absorption in D-D-π-A Systems

The photoinduced charge transfer process of a D-π-A molecule (W1) and three D-D-π-A molecules (WS5–WS7) with triphenylamine as a donor was studied theoretically. D-D-π-A molecules are formed by inserting donors between the triphenylamine and π-linker (π-bridge) on the base of the W1 molecule. The results showed that donor insertion resulted in a red shift in the absorption spectrum, and the absorption intensity increased to a certain extent. A visualization method was used to observe the charge transfer of the four molecules in the process of one- and two-photon absorption (TPA). The local excitation enhanced charge transfer excitation in the TPA process was analyzed and discussed, and the insertion of the thiazolo[5,4-d]thiazole donor showed the largest TPA cross-section. This work contributed to the profound understanding of D-D-π-A molecules and the design of large cross-section TPA molecules.

Direct Tracking Excited-State Intramolecular Charge Redistribution of Acceptor-Donor-Acceptor Molecule by Means of Femtosecond Stimulated Raman Spectroscopy

Symmetric quadrupolar molecules generally exhibit apolar ground states and dipolar excited states in a polar environment, which is explained by the excited state evolution from initial charge delocalization over all molecules to localization on one branch of the molecules after a femtosecond pulse excitation. However, direct observation of excited-state charge redistribution (delocalization/localization) is hardly accessible. Here, the intramolecular charge delocalization/localization character of a newly synthesized acceptor-donor-acceptor molecule (ADA) has been intensively investigated by femtosecond stimulated Raman scattering (FSRS) together with femtosecond transient absorption (fs-TA) spectroscopy. By tracking the excited state Raman spectra of the specific alkynyl (-C≡C-) bonds at each branch of ADA, we found that the nature of the relaxed S₁ state is strongly governed by solvent polarity: symmetric delocalized intramolecular charge transfer (ICT) characters occurred in apolar solvent, whereas the asymmetric localized ICT characters appeared in polar solvent because of solvation. The solvation dynamics of ADA extracted from fs-TA is consistent with the time constants obtained by FSRS, but the FSRS clearly tracks the excited state intramolecular charge transfer delocalization/localization.

Ultrafast Excimer Formation and Solvent Controlled Symmetry Breaking Charge Separation in the Excitonically Coupled Subphthalocyanine Dimer

Knowledge of the factors controlling excited state dynamics in excitonically coupled dimers and higher aggregates is critical for understanding natural and artificial solar energy conversion. In this work, we report ultrafast solvent polarity dependent excited state dynamics of the structurally well‐defined subphthalocyanine dimer, μ‐OSubPc₂. Stationary electronic spectra demonstrate strong exciton coupling in μ‐OSubPc₂. Femtosecond transient absorption measurements reveal ultrafast excimer formation from the initially excited exciton, mediated by intramolecular structural evolution. In polar solvents the excimer state decays directly through symmetry breaking charge transfer to form a charge separated state. Charge separation occurs under control of solvent orientational relaxation.

Achieving high circularly polarized luminescence with push-pull helicenic systems: from rationalized design to top-emission CP-OLED applications

While the development of chiral molecules displaying circularly polarized luminescence (CPL) has received considerable attention, the corresponding CPL intensity, g lum, hardly exceeds 10-2 at the molecular level owing to the difficulty in optimizing the key parameters governing such a luminescence process. To address this challenge, we report here the synthesis and chiroptical properties of a new family of π-helical push-pull systems based on carbo[6]helicene, where the latter acts as either a chiral electron acceptor or a donor unit. This comprehensive experimental and theoretical investigation shows that the magnitude and relative orientation of the electric (μe ) and magnetic (μ m ) dipole transition moments can be tuned efficiently with regard to the molecular chiroptical properties, which results in high g lum values, i.e. up to 3-4 × 10-2. Our investigations revealed that the optimized mutual orientation of the electric and magnetic dipoles in the excited state is a crucial parameter to achieve intense helicene-mediated exciton coupling, which is a major contributor to the obtained strong CPL. Finally, top-emission CP-OLEDs were fabricated through vapor deposition, which afforded a promising g El of around 8 × 10-3. These results bring about further molecular design guidelines to reach high CPL intensity and offer new insights into the development of innovative CP-OLED architectures.

Generation of ion-radical chlorophyll states in the light-harvesting antenna and the reaction center of cyanobacterial photosystem I

The energy and charge-transfer processes in photosystem I (PS I) complexes isolated from cyanobacteria Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 were investigated by pump-to-probe femtosecond spectroscopy. The formation of charge-transfer (CT) states in excitonically coupled chlorophyll a complexes (exciplexes) was monitored by measuring the electrochromic shift of β-carotene in the spectral range 500-510 nm. The excitation of high-energy chlorophyll in light-harvesting antenna of both species was not accompanied by immediate appearance of an electrochromic shift. In PS I from T. elongatus, the excitation of long-wavelength chlorophyll (LWC) caused a pronounced electrochromic effect at 502 nm assigned to the appearance of CT states of chlorophyll exciplexes. The formation of ion-radical pair P₇₀₀⁺A₁^- at 40 ps was limited by energy transfer from LWC to the primary donor P₇₀₀ and accompanied by carotenoid bleach at 498 nm. In PS I from Synechocystis 6803, the excitation at 720 nm produced an immediate bidentate bleach at 690/704 nm and synchronous carotenoid response at 508 nm. The bidentate bleach was assigned to the formation of primary ion-radical state P_B⁺Chl_2B^-, where negative charge is localized predominantly at the accessory chlorophyll molecule in the branch B, Chl_2B. The following decrease of carotenoid signal at ~ 5 ps was ascribed to electron transfer to the more distant molecule Chl_3B. The reduction of phylloquinone in the sites A_1A and A_1B was accompanied by a synchronous blue-shift of the carotenoid response to 498 nm, pointing to fast redistribution of unpaired electron between two branches in favor of the state P_B⁺A_1A^-.

Time-Resolved Fluorescence Spectra of Untreated and Sodium Borohydride-Reduced Chromophoric Dissolved Organic Matter

Time-resolved fluorescence spectra of chromophoric dissolved organic matter (CDOM) from different sources were acquired using UV (280 and 375 nm) and visible light (440 and 640 nm) excitation to probe the structural basis of the emission properties of CDOM. Emission decays were faster at the blue and red edges, particularly at the red edge, relative to those acquired from 480 to 550 nm. Based on the lifetime distribution and multiexponential analysis of the emission decays recorded at different time resolution, current findings demonstrate that the components recovered based on a superposition model have no defined physical meaning. A substantial increase in steady-state fluorescence intensity and only small changes (<30%) of amplitude-weighted average lifetime caused by sodium borohydride reduction suggest that intramolecular fluorescence quenching occurs mainly through formation of ground state charge-transfer interactions. Short-lived species (lifetime < 100 ps) dominate the emission decays over wavelengths from 400 to 800 nm, particularly under excitation at long wavelengths (440 and 640 nm). Compared to locally excited (LE) states, the contribution of charge-transfer excited (ECT) states and other short-lived species to the steady-state emission is small because of their very rapid nonradiative relaxation. This study suggests that a careful choice of observation wavelength is needed to distinguish LE states from ECT states.

Change of Quadrupole Moment upon Excitation and Symmetry Breaking in Multibranched Donor-Acceptor Dyes

Upon photoexcitation, a majority of quadrupolar dyes, developed for large two-photon absorption, undergo excited-state symmetry breaking (ES-SB) and behave as dipolar molecules. We investigate how the change of quadrupole moment upon S₁ ←S₀ excitation, ΔQ, influences the propensity of a dye to undergo ES-SB using a series of molecules with a A-π-D-π-A motif where D is the exceptionally electron-rich pyrrolo[3,2-b]pyrrole and A are accepting groups. Tuning of ΔQ is achieved by appending a secondary acceptor group, A', on both sides of the D core and ES-SB is monitored using a combination of time-resolved IR and broadband fluorescence spectroscopy. The results reveal a clear correlation between ΔQ and the tendency to undergo ES-SB. When A is a stronger acceptor than A', ES-SB occurs already in non-dipolar but quadrupolar solvents. When A and A' are identical, ES-SB is only partial even in highly dipolar solvents. When A is a weaker acceptor than A', the orientation of ΔQ changes, ES-SB is observed in dipolar solvents only and involves major redistribution of the excitation over the D-π-A and D-A' branches of the dye.

Bimolecular photoinduced electron transfer in non-polar solvents beyond the diffusion limit

Electron transfer (ET) quenching dynamics in non-polar solvents are investigated using ultrafast spectroscopy with a series of six fluorophore/quencher pairs, covering a driving force range of more than 1.3 eV. The intrinsic ET rate constants, k₀, deduced from the quenching dynamics in the static regime, are of the order of 10¹²-10¹³ M^-1 s^-1, i.e., at least as large as in acetonitrile, and do not exhibit any marked dependence on the driving force. A combination of transient electronic and vibrational absorption spectroscopy measurements reveals that the primary product of static quenching is a strongly coupled exciplex that decays within a few picoseconds. More weakly coupled exciplexes with a longer lifetime are generated subsequently, during the dynamic, diffusion-controlled, stage of the quenching. The results suggest that static ET quenching in non-polar solvents should be viewed as an internal conversion from a locally excited state to a charge-transfer state of a supermolecule rather than as a non-adiabatic ET process.

Structural distortion and electron redistribution in dual-emitting gold nanoclusters

Deciphering the complicated excited-state process is critical for the development of luminescent materials with controllable emissions in different applications. Here we report the emergence of a photo-induced structural distortion accompanied by an electron redistribution in a series of gold nanoclusters. Such unexpected slow process of excited-state transformation results in near-infrared dual emission with extended photoluminescent lifetime. We demonstrate that this dual emission exhibits highly sensitive and ratiometric response to solvent polarity, viscosity, temperature and pressure. Thus, a versatile luminescent nano-sensor for multiple environmental parameters is developed based on this strategy. Furthermore, we fully unravel the atomic-scale structural origin of this unexpected excited-state transformation, and demonstrate control over the transition dynamics by tailoring the bi-tetrahedral core structures of gold nanoclusters. Overall, this work provides a substantial advance in the excited-state physical chemistry of luminescent nanoclusters and a general strategy for the rational design of next-generation nano-probes, sensors and switches.

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.

Time-Resolved Resonant Raman Spectroscopy of the Photoinduced Electron Transfer from Ruthenium(II) Trisbipyridine to Methyl Viologen

We report the first time-resolved resonant Raman (TR3) spectra of photoinduced charge transfer from [Ru(bpy)₃]²⁺ to methyl viologen, with observations of vibrational structure. The presence of singly charged methyl viologen in solution is noted by the appearance of several spectroscopic lines, which are visible in the spectra following subtraction of reagent molecules. Assignments are confirmed using both density functional theory (DFT) calculations and literature values and are shown to be consistent with transient absorption spectroscopy data. This presents proof-of-concept for the application of TR3 in mechanistic studies of photocatalytic systems.

One- and two-photon absorption properties of quadrupolar A-π-D-π-A dyes with donors of varying strengths

In this work, the one- and two-photon absorption properties of two quadrupolar A-π-D-π-A dyes, featuring carbazole and triphenylamine as donors of varying strengths, are calculated using density functional theory. The characteristics of two-photon excitation are analysed by using the three-state model closest to the real physical process. By taking the intermediate state as a bridge, the charge transfer characteristics in the two-photon excitation process are studied in detail by using various visualisation methods. Comparing the intramolecular charge transfer (ICT) characteristics shows that increasing the ability of the push-pull electrons of the central substituents, thus increasing the intramolecular charge transfer, is a more efficient method to enhance the molecular σ_tp. Our study promotes a deeper understanding of the design of A-π-D-π-A molecules with large cross-sections in two-photon absorption.

Vibrational Spectroscopy Reveals Effects of Electron Push-Pull and Solvent Polarity on Electron Delocalization in Radical Anions of Donor-Acceptor Compounds

The nature of excess electrons has been studied in donor-acceptor (D-A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push-pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile ν(C≡N) vibrations of the fluorene acceptor unit in radical anions of these D-A compounds. Quantum calculations show that the push-pull capability translates the position of an excess electron while keeping its width relatively constant. On the other hand, solvation changes both, making an electron more compact in polar media. The current study points to the idea that solvation plays a more significant role in controlling the nature of excess electrons, while synthetic modification that influences electron push-pull capability enables further tuning.

Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer

Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO)₃(2,2'-bipyridine) were measured in a broad spectral range ∼40-2000 (4000) cm^-1 at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground- and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at density functional theory-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation (ν(ReCl) and ν(CC inter-ring) vibrations upward; ν(CC, CN) and ν(Re-C) downward) are consistent with the bonding/antibonding characters of the highest occupied molecular orbital and the lowest unoccupied molecular orbital involved in excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO)₃ → bpy charge transfer. FSRS spectra show a mode-specific temporal evolution, providing insights into the intersystem crossing (ISC) mechanism and subsequent relaxation. Most of the Raman features are present at ∼40 fs and exhibit small shifts and intensity changes with time. The 1450-1600 cm^-1 group of bands due to CC, CN, and CC(inter-ring) stretching vibrations undergoes extensive restructuring between 40 and ∼150 fs, followed by frequency upshifts and a biexponential (0.38, 21 ps) area growth, indicating progressing charge separation in the course of the formation and relaxation of the lowest triplet state. Early (40-150 fs) restructuring was also observed in the low-frequency range for ν(Re-Cl) and δ(Re-C-O) vibrations that are presumably activated by ISC. FSRS experimental innovations employed to measure low- and high-energy Raman features simultaneously are described and discussed in detail.

Modulation of circularly polarized luminescence through excited-state symmetry breaking and interbranched exciton coupling in helical push-pull organic systems

π-Helical push-pull dyes were prepared and their (chir)optical properties were investigated both experimentally and computationally. Specific fluorescent behaviour of bis-substituted system was observed with unprecedented solvent effect on the intensity of circularly polarized luminescence (CPL, dissymmetry factor decreasing from 10-2 to 10-3 with an increase in solvent polarity) that was linked to a change in symmetry of chiral excited state and suppression of interbranched exciton coupling. The results highlight the potential of CPL spectroscopy to study and provide a deeper understanding of electronic photophysical processes in chiral π-conjugated molecules.