Broadening the Horizon of the Bell–Evans–Polanyi Principle towards Optically Triggered Structure Planarization

Abstract Finding a relationship between kinetics and thermodynamics may be difficult. However, semi‐empirical rules exist to compensate for this shortcoming, among which the Bell–Evans–Polanyi ( B‐E‐P ) principle is an example for reactions involving bond breakage and reformation. We expand the B‐E‐P principle to a new territory by probing photoinduced structure planarization (PISP) of a series of dibenz[ b , f ]azepine derivatives incorporating bent‐to‐planar and rotation motion. The latter involves twisting of the partial double bond character, thereby inducing a barrier that is substituent dependent at the para N ‐phenyl position. The transition‐state structure and frequency data satisfy and broaden the B‐E‐P principle to PISP reactions without bond rearrangement. Together with dual emissions during PISP, this makes possible harnessing of the kinetics/thermodynamics relationship and hence ratiometric luminescence properties for excited‐state structural transformations.