Potential Energy Surfaces of Environment-Sensitive Flapping Fluorophores: FLAP

A series of FLAP molecules are one of the functional fluorescent probes that can sense subtle changes in local environments such as viscosity, polymer free volume, and external mechanical force. In this work, potential energy surfaces (PESs) of a representative Flexible Aromatic Photofunctional (FLAP) molecule, FLAP0, which bears anthraceneimide wings fused with a cyclooctatetraene (COT) core, have been investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The calculation results indicate that, upon excitation with 350-400 nm light, the accessible states are the second (S2) and third (S3) singlet excited states. FLAP0 in these states undergoes nonadiabatic transitions to the first excited (S1) state via S3/S2 and S2/S1 conical intersections (CIX). Notably, a transition state (TS) on the S1 PES near the S2/S1 CIX exhibits a symmetry-breaking motion rather than flapping of the anthraceneimide wings, depriving the driving force for planarization. After reaching the S1 state with a shallow V-shaped geometry, therefore, FLAP0 planarization is thermally induced by COT hydrogen wagging. Due to the low barrier for this motion, the V-shaped and planar forms exist in thermal equilibrium on the S1 surface rather than undergoing unidirectional transformation. This equilibrium facilitates access to diverse energy minima around the planar S1 geometry, leading to the structured green emission bands. Consequently, the putative "vibronic structure" in solution is revealed to be the sum of emissions from these distinct planar forms.