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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. 

Monitoring conical intersection and aromaticity changes in photo-relaxation of cyclooctatetraene by TRUECARS and TRXD. 

We present NEXMD version 2.0, the second release of the NEXMD (Nonadiabatic EXcited-state Molecular Dynamics) software package. Across a variety of new features, NEXMD v2.0 incorporates new implementations of two hybrid quantum-classical dynamics methods, namely, Ehrenfest dynamics (EHR) and the Ab-Initio Multiple Cloning sampling technique for Multiconfigurational Ehrenfest quantum dynamics (MCE-AIMC or simply AIMC), which are alternative options to the previously implemented trajectory surface hopping (TSH) method. To illustrate these methodologies, we outline a direct comparison of these three hybrid quantum-classical dynamics methods as implemented in the same NEXMD framework, discussing their weaknesses and strengths, using the modeled photodynamics of a polyphenylene ethylene dendrimer building block as a representative example. We also describe the expanded normal-mode analysis and constraints for both the ground and excited states, newly implemented in the NEXMD v2.0 framework, which allow for a deeper analysis of the main vibrational motions involved in vibronic dynamics. Overall, NEXMD v2.0 expands the range of applications of NEXMD to a larger variety of multichromophore organic molecules and photophysical processes involving quantum coherences and persistent couplings between electronic excited states and nuclear velocity. 

Light-harvesting and intramolecular energy funneling are fundamental processes in natural photosynthesis. A comprehensive knowledge of the main structural, dynamic, and optical properties that regulate the efficiency of such processes can be deciphered through the study of artificial light-harvesting antennas, capable of mimicking natural systems. Dendrimers are some of the most explored artificial light-harvesting molecules. However, they have to be well-defined and highly branched conjugated structures, creating intramolecular energy gradients that guarantee efficient and unidirectional energy transfer. Herein, we explore the contributions of the different mechanisms responsible for the highly efficient energy funneling in a large, complex poly(phenylene–ethynylene) dendrimer, whose architecture was particularly designed to conduct the initially absorbed photons toward a spatially localized energy sink away from its surface, avoiding its quenching by the environment. For this purpose, the nonradiative photoinduced energy relaxation and redistribution are simulated by using nonadiabatic excited state molecular dynamics. In this way, the two possible direct and indirect pathways for exciton migrations, previously reported by time-resolved spectroscopy, are defined. Our results stimulate future developments of new synthetic dendrimers for applications in molecular-based photonic devices in which an enhancement in the photoemission efficiency can be predicted by changes in the detailed balance between the different intramolecular energy transfer pathways.