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1. Time-resolved vibronic spectra with nuclear–electronic orbital time-dependent configuration interaction

   https://doi.org/10.1063/5.0243394  

   Garner, Scott M; Upadhyay, Shiv; Li, Xiaosong; Hammes-Schiffer, Sharon (January 2025, The Journal of Chemical Physics)

   Time-resolved spectroscopy is an important tool for probing photochemically induced nonequilibrium dynamics and energy transfer. Herein, a method is developed for the ab initio simulation of vibronic spectra and dynamical processes. This framework utilizes the recently developed nuclear–electronic orbital time-dependent configuration interaction (NEO-TDCI) approach, which treats all electrons and specified nuclei quantum mechanically on the same footing. A strategy is presented for calculating time-resolved vibrational and electronic absorption spectra from any initial condition. Although this strategy is general for any TDCI implementation, utilizing the NEO framework allows for the explicit inclusion of quantized nuclei, as illustrated through the calculation of vibrationally hot spectra. Time-resolved spectra produced by either vibrational or electronic Rabi oscillations capture ground-state absorption, stimulated emission, and excited-state absorption between vibronic states. This methodology provides the foundation for fully ab initio simulations of multidimensional spectroscopic experiments. 

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2. Development of Interface-Specific Two-Dimensional Vibrational–Electronic (i2D-VE) Spectroscopy for Vibronic Couplings at Interfaces

   https://doi.org/10.3390/spectroscj3010001  

   Qian, Yuqin; Huang-Fu, Zhi-Chao; Brown, Jesse B; Rao, Yi (March 2025, Spectroscopy Journal)

   Bulk 2D electronic–vibrational (2D-EV) and 2D vibrational–electronic spectroscopies (2D-VE) were previously developed to correlate the electronic and vibrational degrees of freedom simultaneously, which allow for the study of couplings between electronic and vibrational transitions in photo-chemical systems. Such bulk-dominated methods have been used to extensively study molecular systems, providing unique information such as coherence sensitivity, molecular configurations, enhanced resolution, and correlated states and their dynamics. However, the analogy of interfacial 2D spectroscopy has fallen behind. Our recent work presented interface-specific 2D-EV spectroscopy (i2D-EV). In this work, we develop interface-specific two-dimensional vibrational–electronic spectroscopy (i2D-VE). The fourth-order spectroscopy is based on a Mach–Zehnder IR interferometer that accurately controls the time delay of an IR pump pulse pair for vibrational transitions, followed by broadband interface second-harmonic generation to probe electronic transitions. We demonstrate step-by-step how a fourth-order i2D-VE spectrum of AP3 molecules at the air/water interface was collected and analyzed. The line shape and signatures of i2D-VE peaks reveal solvent correlations and the spectral nature of vibronic couplings. Together, i2D-VE and i2D-EV spectroscopy provide coupling of different behaviors of the vibrational ground state or excited states with electronic states of molecules at interfaces and surfaces. The methodology presented here could also probe dynamic couplings of electronic and vibrational motions at interfaces and surfaces, extending the usefulness of the rich data that are obtained. 

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3. Theory for proton-coupled energy transfer

   https://doi.org/10.1063/5.0217546  

   Cui, Kai; Hammes-Schiffer, Sharon (July 2024, The Journal of Chemical Physics)

   In the recently discovered proton-coupled energy transfer (PCEnT) mechanism, the transfer of electronic excitation energy between donor and acceptor chromophores is coupled to a proton transfer reaction. Herein, we develop a general theory for PCEnT and derive an analytical expression for the nonadiabatic PCEnT rate constant. This theory treats the transferring hydrogen nucleus quantum mechanically and describes the PCEnT process in terms of nonadiabatic transitions between reactant and product electron–proton vibronic states. The rate constant is expressed as a summation over these vibronic states, and the contribution of each pair of vibronic states depends on the square of the vibronic coupling as well as the spectral convolution integral, which can be viewed as a generalization of the Förster-type spectral overlap integral for vibronic rather than electronic states. The convolution integral also accounts for the common vibrational modes shared by the donor and acceptor chromophores for intramolecular PCEnT. We apply this theory to model systems to investigate the key features of PCEnT processes. The excited vibronic states can contribute significantly to the total PCEnT rate constant, and the common modes can either slow down or speed up the process. Because the pairs of vibronic states that contribute the most to the PCEnT rate constant may correspond to spectroscopically dark states, PCEnT could occur even when there is no apparent overlap between the donor emission and acceptor absorption spectra. This theory will assist in the interpretation of experimental data and will guide the design of additional PCEnT systems. 

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4. The impact of stacking and phonon environment on energy transfer in organic chromophores: computational insights

   https://doi.org/10.1039/D3TC00479A  

   Mukazhanova, Aliya; Negrin-Yuvero, Hassiel; Freixas, Victor M.; Tretiak, Sergei; Fernandez-Alberti, Sebastian; Sharifzadeh, Sahar (April 2023, Journal of Materials Chemistry C)

   Energy transfer in organic materials is extensively studied due to many applications in optoelectronics. The electronic and vibrational relaxations within molecular assemblies can be influenced by stacking arrangements or additions of a backbone that unites them. Here, we present the computational study of the photoexcitation dynamics of a perylene diimide monomer, and face-to-face stacked dimer and trimer. By using non-adiabatic excited-state molecular dynamics simulations, we show that the non-radiative relaxation is accelerated with the number of stacked molecules. This effect is explained by differences in the energy splitting between states that impacts their corresponding nonadiabatic couplings. Additionally, our analysis of the vibronic dynamics reveals that the passage through the different conical intersections that participate in the relaxation of the stacked systems, activate a positive feedback mechanism. This effect involves a narrow set of vibrational normal modes that accelerate the process by increasing the efficiency of its vibronic dynamics. In contrast, an addition of a biologically inspired backbone slows down the relaxation rate due to its participation in the vibronic dynamics of the molecular stacking arrangements. Our results suggest the stacking arrangements and common backbones as strategies to modulate the efficiency of electronic and vibrational relaxation of diimide-based systems and other molecular aggregates. 

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5. Ab Initio Methodology To Describe the Static Mechanism of Electrodipolar Luminescence in Lanthanides

   https://doi.org/10.1021/acs.jctc.5c01752  

   Dergachev, Vsevolod D; Chibotaru, Liviu F; Varganov, Sergey A (January 2026, Journal of Chemical Theory and Computation)

   Quantification of the electron-vibrational couplings in lanthanide complexes and identification of the vibrations that are strongly coupled to electronic transitions are important for applications of the optical lanthanide spectroscopy. While the information about the electron-vibrational couplings can be extracted from the emission spectra, this is not common because of the challenges associated with interpreting the complex vibronic structures of the spectra. To overcome this challenge, we develop a fully ab initio methodology for predicting the vibronic peaks in the emission spectra of lanthanide complexes by calculating the electron-vibrational couplings associated with individual vibrational modes. We show that the energy gradients of the emitting and ground spin-orbit states, which are the key quantity required for calculating the electron-vibrational couplings, can be obtained analytically from the energy gradients of the spin-diabatic states and the corresponding nonadiabatic coupling matrix elements. To illustrate this methodology, we calculate the 4F9/2→4I15/2 emission spectrum of the erbium trensal complex, provide the full decomposition of the vibronic structure of the spectrum, and investigate the effects of spin-orbit interaction and metal-ligand hybridization on the electron-vibrational couplings. In addition, to validate our methodology, we calculate and compare with experiment the vibronic structure of the 4S3/2→4I15/2 green emission band. 

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