ExploringNon-Condon Effects in a Covalent TetraceneDimer: How Important Are Vibrations in Determining the ElectronicCoupling for Singlet Fission?

Singletfission (SF) offers opportunities for wavelength-selectiveprocessing of solar photons with an end goal of achieving higher efficiencyinexpensive photovoltaic or solar-fuels-producing devices. In orderto evaluate new molecular design strategies and for theoretical explorationof dynamics, it is important to put in place tools for efficient calculationof the electronic coupling between single-exciton reactant and multiexcitonproduct states. For maximum utility, the couplings should be calculatedat multiple nuclear geometries (rather than assumed constant everywhere,i.e., the Condon approximation) and we must be able to evaluate couplingsfor covalently linked multichromophore systems. With these requirementsin mind, here we discuss the simplest methodology possible for rapidcalculation of diabatic one-electron coupling matrix elementsbasedon Boys localization and rediagonalization of molecular orbitals.We focus on a covalent species called BT1 that juxtaposes two tetraceneunits in a partially cofacial geometry via a norbornyl bridge. InBT1, at the equilibrium C2vstructure,the “nonhorizontal” couplings between HOMOs and LUMOs(tHLand tLH) vanish by symmetry. We then explore the impact of molecular vibrationsthrough the calculation of tABcouplinggradients along 183 normal modes of motion. Rules are establishedfor the types of motions (irreducible representations in the C2vpoint group) that turn on tHLand tLHvalues as wellas for the patterns that emerge in constructive versus destructiveinterference of pathways to the SF product. For the best modes, calculatedelectronic coupling magnitudes for SF (at root-mean-squared deviationin position at 298 K), are within a factor of 2 of that seen for noncovalenttetracene dimers relevant to the molecular crystal. An overall “effective”electronic coupling is also given, based on the Stuchebrukhov formalismfor non-Condon electron transfer rates. [ABSTRACT FROM AUTHOR]