Time-Dependent Approach to Resonance Raman Spectra Including Duschinsky Rotation and Herzberg–Teller Effects: Formalism and Its Realistic Applications

Efficient quantum dynamical and electronic structure approaches are presented to calculate resonance Raman spectroscopy (RRS) with inclusion of Herzberg–Teller (HT) contribution and mode-mixing (Duschinsky) effect. In the dynamical method, an analytical expression for RRS in the time domain is proposed to avoid summation over the large number of intermediate vibrational states. In the electronic structure calculations, the analytic energy-derivative approaches for the excited states within the time-dependent density functional theory (TDDFT), developed by us, are adopted to overcome the computational bottleneck of excited-state gradient and Hessian calculations. In addition, an analytic calculation to the geometrical derivatives of the transition dipole moment, entering the HT term, is also adopted. The proposed approaches are implemented to calculate RR spectra of a few of conjugated systems, phenoxyl radical, 2-thiopyridone in water solution, and free-base porphyrin. The calculated RR spectra show the evident HT effect in those π-conjugated systems, and their relative intensities are consistent with experimental measurements

Spectral Characteristics of Chemical Enhancement on SERS of Benzene-like Derivatives: Franck–Condon and Herzberg–Teller Contributions

A systematically theoretical investigation has been performed to study the dynamic chemical enhancement of surface-enhanced Raman spectroscopy (SERS) of pyridine, pyrimidine, 2-mercapto­pyridine, and 4-mercapto­pyridine absorbed on a silver cluster of Ag₂₀. The influences of different structural configurations (V and S), different intermolecular charge-transfer (CT) excited states, and the different approximations [Franck–Condon (FC) or FC<i> + </i>Herzberg–Teller (FCHT) approximations] to spectral cross sections have been examined. It is found that the photoexcitation can easily produce the intermolecular CT excited states, leading to the absorption maxima red-shift, and their intensities decrease compared with that of Ag₂₀. Furthermore, we observe that the absolute Raman intensities are sensitive to the systems’ structural configurations, exchange-correlation functionals, CT excited states, and the FC/FCHT approximations as well. However, the relative Raman intensities and dominant vibrational structures of CT resonance RS (RRS) are mainly determined by the adsorbates. The modes which can have a larger enhancement in all CT RRS are those related to ring stretch and ring breathing. The ring-stretching mode at around 1600 cm^–1 in four molecule–Ag₂₀ systems is evidently enhanced compared to that of bare molecules which can be considered as a hint of the presence of the CT resonance enhancement. Additionally, we observe that HT effects dominate the resonance enhancement and it could explain the coupling between the plasmonic and chemical enhancement mechanisms, but the FCHT approximation does not significantly change the relative RRS intensities obtained through the FC approximation

Vibronic Coupling Effect on the Vibrationally Resolved Electronic Spectra and Intersystem Crossing Rates of a TADF Emitter: 7‑PhQAD

Assessing and improving the performance of organic light-emitting diode (OLED) materials require quantitative prediction of rate coefficients for the intersystem crossing (ISC) and reverse ISC (RISC) processes, which are determined not only by the energy gap and the direct spin–orbit coupling (SOC) between the first singlet and triplet excited-states at a thermal equilibrium position of the initial electronic state but also by the non-Condon effects such as the Herzberg–Teller-like vibronic coupling (HTVC) and the spin-vibronic coupling (SVC). Here we apply the time-dependent correlation function approaches to quantitatively calculate the vibrationally resolved absorption and fluorescence spectra and ISC/RISC rates of a newly synthesized multiple-resonance-type (MR-type) thermally activated delayed fluorescence (TADF) emitter, 7-phenylquinolino­[3,2,1-de]­acridine-5,9-dione (7-PhQAD), with the inclusion of the Franck–Condon (FC), HTVC, and Duschinsky rotation (DR) effects. The SVC effect on the rates has also been approximately evaluated. We find that the experimentally measured ISC rates of 7-PhQAD originate predominantly from the vibronic coupling, consistent with the previous reports on other MR-type TADF emitters. The SVC effect on ISC rates is about 10 times larger than the HTVC effect, and the latter increases the ISC rates by more than 1 order of magnitude while it slightly affects the vibrationally resolved absorption and fluorescence spectra. The discrepancy between the theoretical and experimental results is attributed to inaccurately describing excited-states calculated by the time-dependent density functional theory as well as to not fully accounting for the complex experimental conditions. This work provides a demonstration of what proportion of ISC and RISC rate coefficients of a MR-type TADF emitter can be covered by the HTVC effect, and it opens design routes that go beyond the FC approximation for the future development of high-performance OLED devices

Excited-State Descriptors for High-Throughput Screening of Efficient Electro-Fluorescent Materials

Electro-fluorescent materials with 100% internal quantum efficiency (IQE) and short fluorescence lifetimes (τf, ∼ ns) are desirable and achievable by harvesting high-lying triplet excitons. However, the IQE governed by exciton utilization efficiency (EUE) and photoluminescence quantum yield (PLQY) remains low. Herein, from the electro-fluorescence process involving triplet excitons and energy gap law, we established two excited-state descriptors (triplet–triplet energy gap ΔETT and oscillator strength f) to characterize EUE, PLQY and τf, by taking a series of hot exciton compounds as prototypes. Subsequently, these descriptors were employed to perform high-throughput screening of over 5000 fluorophores, predicting 19 candidate molecules with a high IQE (>90%). We stressed that the large values of these descriptors are conducive to high EUE and PLQY and short τf. This work presents a guideline to design and screen efficient electro-fluorescent materials beyond the spin-statistical limit

Excited-State Descriptors for High-Throughput Screening of Efficient Electro-Fluorescent Materials

Electro-fluorescent materials with 100% internal quantum efficiency (IQE) and short fluorescence lifetimes (τf, ∼ ns) are desirable and achievable by harvesting high-lying triplet excitons. However, the IQE governed by exciton utilization efficiency (EUE) and photoluminescence quantum yield (PLQY) remains low. Herein, from the electro-fluorescence process involving triplet excitons and energy gap law, we established two excited-state descriptors (triplet–triplet energy gap ΔETT and oscillator strength f) to characterize EUE, PLQY and τf, by taking a series of hot exciton compounds as prototypes. Subsequently, these descriptors were employed to perform high-throughput screening of over 5000 fluorophores, predicting 19 candidate molecules with a high IQE (>90%). We stressed that the large values of these descriptors are conducive to high EUE and PLQY and short τf. This work presents a guideline to design and screen efficient electro-fluorescent materials beyond the spin-statistical limit

Theoretical Investigations on the Roles of Intramolecular Structure Distortion versus Irregular Intermolecular Packing in Optical Spectra of 6T Nanoparticles

It is of vital importance to theoretically understand unique nanoparticle size-tunable and excitation wavelength-dependent multiple optical properties in organic nanoparticles. In this work, we proposed a theoretical protocol to calculate the optical spectrum of the organic nanoparticles, which combines molecular dynamics (MD) simulation, the quantum mechanics/molecular mechanics (QM/MM) approach, and vibronic-coupled Frenkel exciton spectrum theory. By using the protocol, we explored the relationship between intramolecular structure distortion, irregular intermolecular packing, and optical spectra in α-sexithiophene nanoparticles. Two representative clusters cutting from the simulated amorphous nanoparticle were investigated and found to exhibit a blue shift for absorption and emission spectra compared to the solution, which is totally different from the blue-shifted absorption and red-shifted emission in crystal. For the cluster with distorted monomer and disordered packing, the blue shift results from the higher excitation energy and larger vibronic coupling of low-frequency vibration modes, while for the cluster with planar monomer and ordered packing, the blue shift is induced by the synergism of vibronic coupling and excitonic coupling. Strikingly, the superposition of the spectra of two clusters reproduces the experimental spectra and well explains the unusual blue-shifted emission observed for α-sexithiophene nanoparticles. Our theoretical protocol is general and applicable to other organic nanoparticles, thus aiding the rational design of high-quality organic nanoparticles

Electrostatic Interaction-Induced Room-Temperature Phosphorescence in Pure Organic Molecules from QM/MM Calculations

Room temperature phosphorescence (RTP) from pure organic material is rare due to the low phosphorescence quantum efficiency. That is why the recent discovery of crystallization induced RTP for several organic molecules aroused strong interests. Through a combined quantum and molecular mechanics CASPT2/AMBER scheme taking terephthalic acid (TPA) as example, we found that electrostatic interaction not only can induce an enhanced radiative decay T₁ → S₀ through the dipole-allowed S₁ intermediate state, but also can hinder the nonradiative decay process upon crystallization. From gas phase to crystal, the nature of S₁ state is converted to ¹(π,π*) from ¹(n,π*) character, enhancing transition dipole moment and serving as an efficient intermediate radiative pathway for T₁ → S₀ transition, and eventually leading to a boosted RTP. The intermolecular packing also blocks the nonradiative decay channel of the high-frequency CO stretching vibration with large vibronic coupling, rather than the conventional low-frequency aromatic rotation in crystal. This mechanism also holds for other organic compounds that contain both ketones and aromatic rings