Suppression of optical absorption and emission by aggregation-induced structural bending

The optical response of organic molecular aggregates is a cornerstone of next-generation optoelectronics, yet the microscopic mechanisms governing the intrinsic renormalization of the transition-dipole moment (TDM) upon aggregation remain elusive. Here, we combine a one-dimensional tight-binding model with first-principles calculations to elucidate the physical origins of the suppression of optical absorption and emission by aggregation-induced structural bending. Derived from the tight-binding analysis, we identify two distinct mechanisms driving the collapse of the TDM: (i) TDM vector reorientation, where the TDM vectorial summation of non-collinear local TDMs results in a reduced net magnitude; and (ii) electronic spatial confinement, arising from the weakened hopping integral (t cos θ) at the structural kink, which forces frontier orbitals to segregate toward opposite termini and minimizes their spatial overlap. Validating these predictions with a D-π-A type hydrazone derivative molecule as a prototype, we show that aggregation-induced spontaneous bending leads to a precipitous drop in optical strength. These findings establish a rigorous quantum framework for molecular aggregate photophysics and offer critical design principles for optimizing light-harvesting and emissive materials.