LAUSR

In sharp contrast to most photoinduced structural planarization (PISP) phenomena, which are highly exergonic and irreversible processes, we report here a series of a new class of PISP molecules, 9-phenyl-9H-tribenzo[b,d,f]azepine (PTBA) and its derivatives, where PISP is within the thermally reversible regime. The underlying foundation is the energy counterbalance along PISP, where upon electronic excitation the azepine core chromophore undergoes planarization to gain stabilization from a cyclic 4n π conjugation (n is an integer; Baird's rule). Concurrently, the C7═C8 fused benzene ring is prone to gain aromaticity, which conversely decreases the 4n π-electron resonance stabilization of the 9H-tribenzo[b,d,f]azepine, hindering a full planarization. The offset results in a minimum energy state (P*) along PISP that is in thermal equilibrium with the initially prepared state (R*). The relaxed structure of R* deviates greatly from the planar configuration commonly seen in PISP. PISP of PTBAs is thus sensitive to the solvent polarity, temperature, and substituents, causing prominent stimuli-dependent ratiometric fluorescence for R* versus P*. Exploitation of the energy counterbalance effect proves to be a practical strategy for harnessing excited-state structural relaxation.