Theoretical and Experimental Investigations on the Aggregation-Enhanced Emission from Dark State: Vibronic Coupling Effect

Aggregation-induced/-enhanced emission (AIE/AEE) has aroused broad interest. The mechanism behind is understood as the aggregation restriction of the nonradiative decay from electronically excited state to the ground state, either through interconversion or through conical intersection, leaving the dipole-allowed radiative decay channel relatively intact. Here, a report on an AEE phenomenon for 5,10-diphenylphenazine (DPhPZ) compound is presented, for which the experiment shows to be AEE active but the lowest excited state has been long known to be dark state, namely with zero transition dipole with the ground state according to Kasha's rule. The computational studies demonstrate that the optical emission stems from the dark state with emission intensity borrowed from higher-lying bright state through Herzberg-Teller vibronic coupling, and the resultant spectra are in good agreement with the experiment in terms of both line shape and peak position. The vibronic-coupling-induced radiative decay and the restricted nonradiative decay synergistically bring about highly efficient luminescence of DPhPZ in the solid phase. The findings open a new avenue for the development of solid-emissive luminophores.