On the role of vibrational-electronic couplings in dictating the ultrafast photophysics of molecular dyads

Abstract

Ultrafast energy and charge transfer driven by coupled vibrational–electronic (“vibronic”) motions is a central topic in much of photochemistry and photophysics. Accurately describing the quantum dynamics requires an exact treatment of vibrational degrees of freedom in the system Hamiltonian. However, including multiple vibrational modes in extended aggregates is computationally challenging due to a rapid increase in the number of vibronic basis states. Recent advances, including effective-mode approaches, enable the exact simulation of quantum dynamics with a large set of intramolecular modes. Yet, calculating nonlinear spectroscopic observables, such as four-wave mixing signals, often requires n-particle approximations. Often, vibronic dynamics is driven by specific ‘promoter’ vibrational modes, for example, the tuning coordinate in the classic example of conical intersections. It is therefore crucial to develop theoretical approaches that intuitively connect spectroscopic observables to the dynamics of promoter versus spectator modes. This thesis introduces an approach to understand vibronic coupling in the multidimensional vibrational phase space in terms of a few effective modes that drive the electronic delocalization of the vibronic wavepacket. This effective mode approach reduces vibrational dimensionality while preserving exact vibronic dynamics and provides a physically intuitive toy model understanding of complex photophysics. Using this understanding, the thesis later applies a suite of impulsive optical probes to analyse two distinct ultrafast photophysical processes – singlet exciton fission (SEF) and symmetry-breaking charge separation (SBCS), highlighting the promoter modes in both cases. New experimental observations, not available with non-impulsive excitation, such as cross-peaks in 2D spectra and strongly wavelength-dependent SEF, near-instantaneous generation of electronically mixed singlet-charge transfer or singlet-triplet intermediates, negligible electronic reorientation during SEF, and selective hole transfer during SBCS are reported. Together, these experimental observations highlight the power of multidimensional electronic spectroscopy, impulsively generated vibrational wavepackets, and polarization control in deciphering the vibronic dynamics that drive ultrafast photophysics in complex molecular systems.