Unmasking Underlying Molecular Photophysical Mechanisms Using Quantum Chemistry

Abstract

Quantum chemistry is performed on biological chromophores in light harvesting and synthetic chromophores in targeted photochemical applications to elucidate how molecular electronic structure impacts their underlying function. This function is revealed to manifest from the interplay between light-absorbing bright states and light- transmitting dark electronic excited states. The role played by these states, representing either different spatial or different spin symmetries, is discerned using multireference perturbation theory methodologies. Through these methods, molecular mechanisms of spin manipulation underlying the photophysical complexity observed in experimental spectroscopy are unveiled. These mechanisms hinge on correlations entangling pairs of spin-triplet eigenfunctions or spin-orbit coupling connecting formally spin singlet and spin triplet states. The former is unveiled to be the key intermediate initiator for multiplying molecular charge production from light, while the latter is unveiled to be the key driving force for multiplying light production from charges initiated by electrical stimulation. These processes are simulated at the molecular level using nonadiabatic dynamics that tracks electronic population transfer between states driven by nuclear motion in time. The dynamics are directed by couplings that may be modulated through definable nuclear distortion traceable to the activation of specific vibrational degrees of freedom. These computed vibrationally-assisted electronic dynamics across model molecular systems enable the discrimination between intrinsic intramolecular photophysics and the effects of the surrounding molecular environment in naturally- occurring supramolecular pigment-protein complexes or device active layers.