Excited-state Structural Relaxation Research Articles

Ultrafast Charge Separation Driven by Solvation-Coupled Intramolecular Torsion.

Photoinduced intramolecular charge separation in a pyrene- and triarylamine-based donor-acceptor dyad was studied by polarization-dependent femtosecond time-resolved transient absorption (TA) spectroscopy in polar solvents. Photoexcitation forms an excited state with charge transfer (CT) character due to the intrinsic electronic coupling between the triarylamine and pyrene groups, resulting in ultrafast charge separation (CS) in polar solvents. TA measurements reveal a correlation between the rate of CS and solvation dynamics, which implies that solvation is involved in the CS reaction. In addition, polarization-dependent TA spectroscopy was devoted to tracking the ultrafast anisotropy evolution of the cationic absorption band, which is attributed to intramolecular torsional motion and is proposed to be coupled to diffusive orientational solvent modes. The results therefore reveal that the evolution of the CT state in the condensed phase is driven by solvation-coupled excited-state structural relaxation. In other words, intramolecular torsional motion is directly confirmed to be involved in the reaction coordinate of the CS reaction in a strongly coupled donor-acceptor dyad.

Efficient Electron Transfer Driven by Excited-State Structural Relaxation in Corrole-Perylenedimiide Dyad.

A sterically encumbered trans-A2B-corrole possessing a perylenediimide (PDI) scaffold in close proximity to the macrocycle has been synthesized via a straightforward route. Electronic communication as probed via steady-state absorption or cyclic voltammetry is weak in the ground state, in spite of the corrole ring and PDI being bridged by an o-phenylene unit. The TDDFT excited-state geometry optimization suggests after excitation the interchromophoric distance is markedly reduced, thus enhancing the through-space electronic coupling between the corrole and the PDI. This is corroborated by the strong deviation of the emission spectrum originating from both PDI and corrole in the dyad. Selective excitation of both donor and acceptor units triggers efficient sub-picosecond electron transfer and hole transfer, respectively, followed by fast charge recombination. In comparison to previously studied corrole-PDI dyads, both charge separation and charge recombination occur faster, because of the structural relaxation in the excited state.

Ultrafast photophysics of an orange-red thermally activated delayed fluorescence emitter: the role of external structural restraint.

The application of thermally activated delay fluorescence (TADF) emitters in the orange-red regime usually suffers from the fast non-radiative decay of emissive singlet states (kSNR), leading to low emitting efficiency in corresponding organic light-emitting diode (OLED) devices. Although kSNR has been quantitatively described by energy gap law, how ultrafast molecular motions are associated with the kSNR of TADF emitters remains largely unknown, which limits the development of new strategies for improving the emitting efficiency of corresponding OLED devices. In this work, we employed two commercial TADF emitters (TDBA-Ac and PzTDBA) as a model system and attempted to clarify the relationship between ultrafast excited-state structural relaxation (ES-SR) and kSNR. Spectroscopic and theoretical investigations indicated that S1/S0 ES-SR is directly associated with promoting vibrational modes, which are considerably involved in electronic-vibrational coupling through the Huang-Rhys factor, while kSNR is largely affected by the reorganization energy of the promoting modes. By restraining S1/S0 ES-SR in doping films, the kSNR of TADF emitters can be greatly reduced, resulting in high emitting efficiency. Therefore, by establishing the connection among S1/S0 ES-SR, promoting modes and kSNR of TADF emitters, our work clarified the key role of external structural restraint for achieving high emitting efficiency in TADF-based OLED devices.

Manipulating Lone-Pair-Driven Luminescence in 0D Tin Halides by Pressure-Tuned Stereochemical Activity from Static to Dynamic.

The excellent luminescence properties and structural dynamics driven by the stereoactivity of the lone pair in a variety of low-dimensional ns2 metal halides have attracted growing investigations for optoelectronic applications. However, the structural and photophysical aspects of the excited state associated with the lone pair expression are currently open questions. Herein, zero-dimensional Sn-based halides with static stereoactive 5 s2 lone pairs are selected as a model system to understand the correlations between the distinctive lone pair expression and the excited-state structural relaxation and charge carrier dynamics by continuous lattice manipulation. Lattice compression drives 5 s2 lone pair active switching and self-trapped exciton (STE) redistribution by suppressing excited-state structural deformation of the isolated SnBr4 2- units. Our results demonstrate that the static expression of the 5 s2 lone pair results in a red broadband triplet STE emission with a large Stokes shift, while its dynamic expression creates a sky-blue narrowband emission dominated by the radiative recombination of singlet STEs. Our findings and the photophysical mechanism proposed highlight the stereochemical effects of lone pair expression in controlling light emission properties and offer constructive guidelines for tuning the optoelectronic properties in diverse ns2 metal halides.

Sequential Multistep Excited-State Structural Transformations in N,N'-Diphenyl-dihydrodibenzo[a,c]phenazine Fluorophores.

We demonstrate that a single polycyclic π-scaffold can undergo sequential multistep excited-state structural evolution along the bent, planar, and twisted conformers, which coexist to produce intrinsic multiple fluorescence emissions in room-temperature solution. By installing a methyl or trifluoromethyl group on the ortho-site of N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC), the enhanced steric effects change the fluorescence emission of DPAC from a dominant red band to well-resolved triple bands. The ultra-broadband triple emissions of ortho-substituted DPACs range from ≈350 to ≈850 nm, which is unprecedented for small fluorophores with molecular weight of <500. Ultrafast spectroscopy and theoretical calculations clearly reveal that the above dramatic changes originate from the influence of steric hindrance on the shape of excited state potential energy surface (S1 PES). Compared to the steep S1 PES of parental DPAC, the introduction of ortho-substituent is shown to make the path of structural evolution in S1 wider and flatter, so the ortho-substituted derivatives exhibit slower structural transformations from bent to planar and then to twisted forms, yielding intrinsic triple emission. The results provide the proof of concept that the bent, planar, and twisted emissive states can coexist in the same S1 PES, which greatly expand the fundamental understanding of the excited-state structural relaxation.

Ultrafast Charge Separation Driven by Torsional Motion in Orthogonal Boron Dipyrromethene Dimer

In this work, the photoinduced charge separation (CS) via symmetry breaking in an orthogonal meso-β-linked boron dipyrromethene (BODIPY) dimer was investigated by polarized transient absorption spectroscopy. The time constant about 0.76 ps of the CS reaction determined in dimethyl sulfoxide is much faster than the solvation dynamics. The observed transient anisotropy of the BODIPY anion band implies that both hole and electron transfers occur with similar probabilities. The bidirectional charge transfer processes suggest that the locally excited state is weakly coupled to the polar solvent, and the solvation coupled excited-state structural relaxation within the BODIPY monomeric unit is rather limited. In combination with the electronic excitation analysis based on time-dependent density-functional theory calculations, we deduced that the CS in the orthogonal BODIPY dimer is enabled via the torsional motion associated with covalently connected BODIPY units, promoting the electronic coupling, and irrelevant to the dynamic solvent relaxation.

Energy Counterbalance to Harness Photoinduced Structural Planarization of Dibenzo[b,f]azepines toward Thermal Reversibility.

In sharp contrast to most photoinduced structural planarization (PISP) phenomena, which are highly exergonic and irreversible processes, we report here a series of a new class of PISP molecules, 9-phenyl-9H-tribenzo[b,d,f]azepine (PTBA) and its derivatives, where PISP is within the thermally reversible regime. The underlying foundation is the energy counterbalance along PISP, where upon electronic excitation the azepine core chromophore undergoes planarization to gain stabilization from a cyclic 4n π conjugation (n is an integer; Baird's rule). Concurrently, the C7═C8 fused benzene ring is prone to gain aromaticity, which conversely decreases the 4n π-electron resonance stabilization of the 9H-tribenzo[b,d,f]azepine, hindering a full planarization. The offset results in a minimum energy state (P*) along PISP that is in thermal equilibrium with the initially prepared state (R*). The relaxed structure of R* deviates greatly from the planar configuration commonly seen in PISP. PISP of PTBAs is thus sensitive to the solvent polarity, temperature, and substituents, causing prominent stimuli-dependent ratiometric fluorescence for R* versus P*. Exploitation of the energy counterbalance effect proves to be a practical strategy for harnessing excited-state structural relaxation.

The Observation of Interchain Motion in Self-Assembled Crystalline Platinum(II) Complexes: An Exquisite Case but By No Means the Only One in Molecular Solids.

In organic and organometallic solids, upon electronic excitation, most intermolecular structural relaxations follow a pathway along the π-π stacking direction or metal-metal bond with significant coupling strength. Differently, we discovered that the self-assembled platinum(II) complexes, Pt(fppz)2, exhibit an unusual interchain contraction. The ground-state and excited-state multiple local minima were distinguished by temperature-dependent excitation/emission spectra, indicating the existence of multiple local minima. The time-resolved emission decay revealed the excited-state structural relaxation lifetime with τobs = 41 ns at 298 K. Temperature-dependent X-ray diffraction analysis showed that the packing geometries contract 0.6 Å along the interchain direction (a-axis) at 50 K compared to the geometries at 298 K. Such structural displacements render the slow internal conversion rate in the excited states. We thus demonstrate the correlation between the packing geometries and the excited-state dynamics of the self-assembled Pt(II) complexes, shedding light on the unique direction of interchain structural deformation of the molecular aggregates.

Molecular Configuration Fixation with C–H···F Hydrogen Bonding for Thermally Activated Delayed Fluorescence Acceleration

Summary The conflict between improving reverse intersystem crossing (RISC) and singlet radiation is one of the key fundamental issues for thermally activated delayed fluorescence (TADF) materials. Here, we demonstrate that the excited-state structural relaxation can be effectively suppressed through fixing the molecular configuration with intramolecular C–H···F hydrogen bonds. The resulted TADF emitter 7CzFDCF3DPh reveals the nearly identical ground state, the first singlet (S1) and triplet (T1) excited-state configurations. Therefore, the singlet-triplet energy gap is reduced to

Driving high quantum yield NIR emission through proquinoidal linkage motifs in conjugated supermolecular arrays.

High quantum yield NIR fluorophores are rare. Factors that drive low emission quantum yields at long wavelength include the facts that radiative rate constants increase proportional to the cube of the emission energy, while nonradiative rate constants increase in an approximately exponentially with decreasing S0–S1 energy gaps (in accordance with the energy gap law). This work demonstrates how the proquinoidal BTD building blocks can be utilized to minimize the extent of excited-state structural relaxation relative to the ground-state conformation in highly conjugated porphyrin oligomers, and shows that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-bridged (porphinato)zinc (PZnn) arrays, and 4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E) spacers that are integrated into the backbone of these compositions, elucidate new classes of impressive NIR fluorophores. We report the syntheses, electronic structural properties, and emissive characteristics of neoteric PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD fluorophores. Absolute fluorescence quantum yield (ϕf) measurements, acquired using a calibrated integrating-sphere-based measurement system, demonstrate that these supermolecules display extraordinary ϕf values that range from 10–25% in THF solvent, and between 28–36% in toluene solvent over the 700–900 nm window of the NIR. These studies underscore how the regulation of proquinoidal conjugation motifs can be exploited to drive excited-state dynamical properties important for high quantum yield long-wavelength fluorescence emission.

Nonradiative Excited-State Decay via Conical Intersection in Graphene Nanostructures.

Chemical groups are known to tune the luminescent efficiencies of graphene-related nanomaterials, but some species, including the epoxide group (-COC-), are suspected to act as emission-quenching sites. Herein, by performing nonadiabatic excited-state dynamics simulations, we reveal a fast (within 300 fs) nonradiative excited-state decay of a graphene epoxide nanostructure from the lowest excited singlet (S1 ) state to the ground (S0 ) state via a conical intersection (CI), at which the energy difference between the S1 and S0 states is approximately zero. This CI is induced after breaking one C-O bond at the -COC- moiety during excited-state structural relaxation. This study ascertains the role of epoxide groups in inducing the nonradiative recombination of the excited electron-hole, providing important insights into the CI-promoted nonradiative de-excitations and the luminescence tuning of relevant materials. In addition, it shows the feasibility of utilizing nonadiabatic excited-state dynamics simulations to investigate the photophysical processes of the excited states of graphene nanomaterials.

Intermolecular Hydrogen Bonding Modulates O-H Photodissociation in Molecular Aggregates of a Catechol Derivative.

The catechol functional group plays a major role in the chemistry of a wide variety of molecules important in biology and technology. In eumelanin, intermolecular hydrogen bonding between these functional groups is thought to contribute to UV photoprotective and radical buffering properties, but the mechanisms are poorly understood. Here, aggregates of 4-t-butylcatechol are used as model systems to study how intermolecular hydrogen bonding influences photochemical pathways that may occur in eumelanin. Ultrafast UV-visible and mid-IR transient absorption measurements are used to identify the photochemical processes of 4-t-butylcatechol monomers and their hydrogen-bonded aggregates in cyclohexane solution. Monomer photoexcitation results in hydrogen atom ejection to the solvent via homolytic O-H bond dissociation with a time constant of 12ps, producing a neutral semiquinone radical with a lifetime greater than 1ns. In contrast, intermolecular hydrogen bonding interactions within aggregates retard O-H bond photodissociation by over an order of magnitude in time. Excited state structural relaxation is proposed to slow O-H dissociation, allowing internal conversion to the ground state to occur in hundreds of picoseconds in competition with this channel. The semiquinone radicals formed in the aggregates exhibit spectral broadening of both their electronic and vibrational transitions.

Mechanism of Charge Separation and Frontier Orbital Structure in Graphitic Carbon Nitride and Graphene Quantum Dots.

Graphene quantum dots (GQDs) and carbon nitride quantum dots (CNQDs), the latest addition to the carbon material family, are promising materials for numerous novel applications in optical sensing, photocatalysis, biosensing, and photovoltaics. However, understanding the photocatalytic capability of CNQDs compared to GQDs requires investigations of the charge behavior on the excited state energy surface. In this work, through time-dependent density functional tight binding (TD-DFTB) calculations, we show that CNQDs exhibit superior ground state frontier orbitals (FOs) localization. Strong localization of the FOs and excited state charge separation observed in the first excited state are caused by the relaxation of the structure. Excited energy surface investigations reveal spatial confinement of FOs to the stretched C-N bonds due to excited state structural relaxation. On the other hand, no such localized FOs structure was found for GQDs, presumably caused by its strong π-conjugated configuration not allowing large structural changes upon excitation. The optical absorption and emission of CNQDs is sensitive to size and does not show large variations with the shape of the QD. Our approach provides an explanation for the origin of the enhanced photocatalytic performance of CNQDs over GQDs and their characteristic FOs localization.

Put Your Backbone into It: Excited-State Structural Relaxation of PffBT4T-2DT Conducting Polymer in Solution

Conformational and energetic disorder in organic semiconductors reduces charge and exciton transport because of the structural defects, thus reducing the efficiency in devices such as organic photovoltaics and organic light-emitting diodes. The main structural heterogeneity is because of the twisting of the polymer backbone that occurs even in polymers that are mostly crystalline. Here, we explore the relationship between polymer backbone twisting and exciton delocalization by means of transient absorption spectroscopy and density functional theory calculations. We study the PffBT4T-2DT polymer which has exhibited even higher device efficiency with nonfullerene acceptors than the current record breaking PCE11 polymer. We determine the driving force for planarization of a polymer chain caused by excitation. The methodology is generally applicable and demonstrates a higher penalty for nonplanar structures in the excited state than in the ground state. This study highlights the morphological and electronic changes in conjugated polymers that are brought about by excitation.

Excited-state structural relaxation and exciton delocalization dynamics in linear and cyclic π-conjugated oligothiophenes.

π-Conjugated oligothiophene is considered a chain segment of its polymeric counterpart, whose size and shape can be precisely controlled. Because of its simplified structure, it is possible to understand complex excited-state dynamics of the π-conjugated polymers by employing a bottom-up approach. We review theoretical and experimental aspects of π-conjugated oligothiophenes by summarizing recent works employing time-resolved spectroscopy. The extent of exciton delocalization, which is a prerequisite to efficient charge generation at organic heterojunctions, is described sequentially in model linear and cyclic oligothiophenes, and their analogues. The heterogeneous nature of these systems is highlighted by illustrating the results at both ensemble and single-molecule levels. Exciton dynamics that arise in the polymers are also covered and the signifcance of exciton and charge delocalization in photovoltaic materials is highlighted.

2,5-bis[2-(2-phenyl-1,3-oxazol-5-yl)phenyl]-1,3,4-oxadiazole – new sterically hindered high Stokes shift fluorophore sensitive to media viscosity

Abstract New sterically hindered high Stokes shift polyheteroaromatic fluorophore (2,5-bis[2-(2-phenyl-1,3-oxazol-5-yl) phenyl]-1,3,4-oxadiazole, ( 1 ) belonging to the ortho -POPOP family was synthesized and its molecular structure was confirmed by X-ray and NMR data. Like any other ortho -POPOPs, the title molecule is substantially non-planar in the ground state owing to repulsion of its oxazole/oxadiazole cycles introduced in ortho -positions of the inner-chain benzene rings. Thus the intramolecular π-conjugation in 1 molecule is weakened in respect to its planar para -substituted analogs. Intermolecular π – π contacts in the crystalline lattice result in asymmetrization of the oxadiazole–phenyl–oxazole subunits of 1 , which reflects itself in significantly different angles between the planes of azole and their neighboring benzene rings. Quantum-chemical modeling of 1 revealed for this molecule the ground state symmetrization in fluid media with alignment of the inter-ring angles. The significant excited state planarization of the title molecule, which was predicted by our TD-DFT calculations as well, results in partial restoration of the disturbed ground state conjugation, in decrease of the structurally relaxed excited state energy and finally – in theoretically predicted and experimentally observed abnormally high fluorescence Stokes shifts. The excited state conformational changing of 1 requires high amplitude intramolecular motions, which should be affected by the local media viscosity. This was confirmed in our time-resolved fluorescence experiments, which demonstrate decrease of the excited state structural relaxation rate in alcohols of different viscosity: 2-propanol, ethylene glycol and glycerol. Special experiments in toluene–polystyrene system allow us to recommend the investigated compound as potential fluorescent viscosity sensor.

Effect of Excited-State Structural Relaxation on Midgap Excitations in Co2+-Doped ZnO Quantum Dots

The unique midgap excited states of Co2+-doped ZnO quantum dots have given rise to new applications in photocatalysis, sensing, magneto-optics, and magnetoelectronics. However, the electronic characteristics of these midgap transitions are not fully understood, and the uncertain interplay between these transitions has led to disagreement in the literature. In this work, midgap excited states of Co2+-doped ZnO quantum dots are analyzed using linear response time-dependent density functional theory and the effective mass theory with a focus on the geometry relaxation in the donor-type photoionization excited state. Relaxation of the excited-state geometry lowers the charge-transfer transition energy to be in the vicinity of the prominent spin-allowed 4A2 → 4T1 Co2+ d–d transition that gives this material its characteristic color. For large quantum dots, the excited-state population distribution between this Co2+ d–d excited state and the charge-transfer excited state can be tuned by thermal energy, resultin...

The dynamics of excited state structural relaxation of 4‐dimethylaminobenzonitrile (DMABN) and related compounds

The excited state structural relaxation of 4‐dimethylaminobenzenes with various para‐acceptor substituents having double‐band emission, local excited (LE) and charge transfer (CT), has been investigated. Fluorescence measurements at different temperatures and in different solvents have confirmed the existence of viscosity‐dependent, temperature, and polarity‐activated relaxation. The kinetics analysis has shown that the radiative deactivation rate constants of the individual LE and CT states differ by 7–112‐fold. The dipole moment changes at the excitation for CT states are significantly larger than those for LE states. The spectral‐kinetics behavior of compounds studied agrees with the models A → A∗ → B∗ or A → A∗↔B∗, where A∗ is the local excited planar and B∗ is the relaxed twisted state of the molecule. The rate constants of the twisted state formation have been calculated in the temperature range 293–77 K. The activation energies of forward process for compounds studied have been estimated.

Excited state structural relaxation relaxation of N‐(1‐anthryl)‐2,4,6‐trimethyl‐pyridinium cation

The fluorescence spectrum of N‐(1‐anthryl)‐2,4,6‐trimethylpyridinium cation (1) has an anomalously high Stokes′ shift. The fluorescence spectra of 1 in ethanol and butyronitrile are shifted to shortwavelength region and fluorescence quantum yield increases as the temperature decreases. The fluorescence rate constant of this compound changes considerably (6 times in ethanol and 15 times in butyronitrile) as the temperature decreases from 293 K (relaxed state) to 77 K (mainly nonrelaxed state). It points out that at these temperatures the fluorescence takes place from two species with different structures. It is concluded that anomalously high fluorescence Stokes′ shift of 1 is caused by both solvent orientation relaxation and excited state structural relaxation consisting in the mutual rotation of anthracene and pyridinium fragments of the cation and resulting in the formation of a specie with different structure. The rates of these processes are determined by the temperature‐dependent viscosity of the medium.

Absolute Resonance Raman Intensity Analysis of Isopropyl Nitrate in the Condensed Phase

The first resonance Raman spectra of isopropyl nitrate (IPN) are reported. Absolute resonance Raman scattering cross-sections for IPN dissolved in cyclohexane and acetonitrile are measured at excitation wavelengths spanning the strong π−π* absorption located at ∼200 nm. Resonance enhancement is observed for seven vibrational coordinates, all involving the −ONO2 chromophore, demonstrating that the photoinduced excited-state structural evolution is dominated by structural changes localized to the −ONO2 group. The absorption and absolute resonance Raman cross-sections are modeled using the time-dependent formalism for absorption and Raman scattering. This analysis demonstrates that the photoinitiated excited-state structural-relaxation dynamics are multidimensional, and consistent with N−O bond cleavage. This observation is consistent with N−O bond dissociation being the dominant photodissociation pathway for larger (e.g., three or more carbon atoms) alkyl nitrates. Comparison of the results in cyclohexane a...