Photoinduced Dynamics with Constrained Vibrational Motion: FrozeNM Algorithm.

Ab initio molecular dynamics (AIMD) simulation, analyzed in terms of vibrational normal modes, is a widely used technique that facilitates understanding of complex structural motions and coupling between electronic and nuclear degrees of freedom. Usually, only a subset of vibrations is directly involved in the process of interest. The impact of these vibrations can be evaluated by performing AIMD simulations by selectively freezing certain motions. Herein, we present frozen normal mode (FrozeNM), a new algorithm to apply normal-mode constraints in AIMD simulations, as implemented in the nonadiabatic excited state molecular dynamics code. We further illustrate its capacity by analyzing the impact of normal-mode constraints on the photoinduced energy transfer between polyphenylene ethynylene dendrimer building blocks. Our results show that the electronic relaxation can be significantly slowed down by freezing a well-selected small subset of active normal modes characterized by their contributions in the direction of energy transfer. The application of these constraints reduces the nonadiabatic coupling between electronic excited states during the entire dynamical simulations. Furthermore, we validate reduced dimensionality models by freezing all the vibrations, except a few active modes. Altogether, we consider FrozeNM as a useful tool that can be broadly used to underpin the role of vibrational motion in a studied process and to formulate reduced models that describe essential physical phenomena.

Electronic Energy Relaxation in a Photoexcited Fully Fused Edge-Sharing Carbon Nanobelt.

Carbon nanobelts are cylindrical molecules composed of fully fused edge-sharing arene rings. Because of their aesthetically appealing structures, they acquire unusual optoelectronic properties that are potentially suitable for a range of applications in nanoelectronics and photonics. Nevertheless, the very limited success of their synthesis has led to their photophysical properties remaining largely unknown. Compared to that of carbon nanorings (arenes linked by single bonds), the strong structural rigidity of nanobelts prevents significant deformations away from the original high-symmetry conformation and, therefore, impacts their photophysical properties. Herein, we study the photoinduced dynamics of a successfully synthesized belt segment of (6,6)CNT (carbon nanotube). Modeling this process with nonadiabatic excited state molecular dynamics simulations uncovers the critical role played by the changes in excited state wave function localization on the different types of carbon atoms. This allows a detailed description of the excited state dynamics and spatial exciton evolution throughout the nanobelt scaffold. Our results provide detailed information about the excited state electronic properties and internal conversion rates that is potentially useful for designing nanobelts for nanoelectronic and photonic applications.

Vibronic Quantum Beating between Electronic Excited States in a Heterodimer.

Energy transfer in multichromophoric molecules can be affected by coherences that are induced by the electronic and vibrational couplings between chromophore units. Coherent electron-vibrational dynamics can persist at the subpicosecond time scale even at room temperature. Furthermore, wave-like localized-delocalized motions of the electronic wave function can be modulated by vibrations that actively participate in the intermolecular energy transfer process. Herein, nonadiabatic excited state molecular dynamics simulations have been performed on a rigid synthetic heterodimer that has been proposed as a simplified model for investigating the role and mechanism of coherent energy transfer in multichromophoric systems. Both surface hopping (SH) and Ehrenfest approaches (EHR) have been considered. After photoexcitation of the system at room temperature, EHR simulations reveal an ultrafast beating of electronic populations between the two lowest electronic states. These oscillations are not observed at low temperature and have vibrational origins. Furthermore, they cannot be reproduced using SH approach. This periodic behavior of electronic populations induces oscillations in the spatial localization of the electronic transition density between monomers. Vibrations whose frequencies are near-resonant with energy difference between the two lowest electronic excited states are in the range of the electronic population beating, and they are the ones that contribute the most to the coherent dynamics of these electronic transitions.

Photoinduced non-adiabatic energy transfer pathways in dendrimer building blocks.

The efficiency of the intramolecular energy transfer in light harvesting dendrimers is determined by their well-defined architecture with high degree of order. After photoexcitation, through-space and through-bond energy transfer mechanisms can take place, involving vectorial exciton migration among different chromophores within dendrimer highly branched structures. Their inherent intramolecular energy gradient depends on how the multiple chromophoric units have been assembled, subject to their inter-connects, spatial distances, and orientations. Herein, we compare the photoinduced nonadiabatic molecular dynamics simulations performed on a set of different combinations of a chain of linked dendrimer building blocks composed of two-, three-, and four-ring linear polyphenylene chromophoric units. The calculations are performed with the recently developed ab initio multiple cloning-time dependent diabatic basis implementation of the Multiconfigurational Ehrenfest (MCE) approach. Despite differences in short time relaxation pathways and different initial exciton localization, at longer time scales, electronic relaxation rates and exciton final redistributions are very similar for all combinations. Unlike the systems composed of two building blocks, considered previously, for the larger 3 block systems here we observe that bifurcation of the wave function accounted by cloning is important. In all the systems considered in this work, at the time scale of few hundreds of femtoseconds, cloning enhances the electronic energy relaxation by ∼13% compared to that of the MCE method without cloning. Thus, accurate description of quantum effects is essential for understanding of the energy exchange in dendrimers both at short and long time scales.