Bending-Induced Vibrational Landscape Reorganization Governs Energy Dissipation in Perylene Bisimides.

Structural distortion is widely recognized to suppress emissions in organic chromophores; however, the mechanistic origin of the associated enhancement in nonradiative decay remains unresolved. Here, we employ a series of perylene bisimide derivatives with systematically controlled bending to directly elucidate the structural origin of distortion-enhanced nonradiative relaxation. A pronounced transition is observed from near-unity emission to strongly enhanced nonradiative decay within the singlet manifold, while intersystem crossing remains negligible. Time-resolved electronic spectroscopy excludes triplet-mediated pathways and establishes internal conversion as the dominant decay channel. Crucially, time-resolved Raman measurements provide direct insight into the underlying structural dynamics, revealing that bending suppresses the electronically coupled aromatic skeletal mode, reorganizes the low-frequency vibrational manifold, and accelerates vibrational dephasing. The results herein demonstrate that bending enhances internal conversion not primarily through energy-gap reduction, but by transforming the excited-state vibrational landscape into a more dissipative environment that facilitates efficient energy relaxation. More broadly, this work identifies vibrational dissipation and coherence loss as key determinants of nonradiative decay and establishes a general structure-vibrational dynamics framework for controlling excited-state energy flow in π-conjugated systems.