ABSTRACT The utilization of high‐energy excited states in organic luminescence offers a method for bypassing the efficiency limits imposed by Kasha's rules. Herein, we report a novel A‐π‐A molecule, PIFPI ((2‐(4'‐(4,5‐diphenyl‐1‐(4‐(trifluoromethyl)phenyl)‐1 H ‐imidazol‐2‐yl)‐1,1'‐biphenyl‐4‐yl)‐1‐phenyl‐1 H ‐benzo d imidazole)), which achieves efficient single‐molecule white‐light emission via an excited‐state intermolecular proton transfer (ESInterPT) process in an ethyl acetate/acetic acid medium. Steady‐state studies confirmed that the dual emission is independent of the excitation wavelength, complying with Kasha's rule for emissive states. However, ultrafast spectroscopy provides direct evidence that the ESInterPT event itself occurs on an ultrafast timescale (0.78 ps) directly from a high‐energy singlet state (S n , n ≥ 2), effectively bypassing competing nonradiative relaxation. This mechanism is independently verified by light‐induced acidification, as UV irradiation drastically lowered the pH. We constructed a leakage‐free, single‐molecule‐based white‐light‐emitting gel. The external quantum efficiency of the resulting gel device ( GWLED ) is 3.53%, a significant improvement over that of its solution counterpart (0.49%). It also delivers high‐quality white light with Commission Internationale de l’ Eclairage coordinate of (0.3135, 0.3355) and a color‐rendering index of 91.2. This study provides direct experimental evidence for ultrafast proton transfer from high‐energy states and demonstrates its potential for advanced white‐light materials and practical devices.
Wang et al. (Thu,) studied this question.