Excitation‐Wavelength‐Dependent Emission and Delayed Fluorescence in a Proton‐Transfer System.

Abstract: Manipulating the relaxation pathways of excited states and understanding mechanisms of photochemical reactions present important challenges in chemistry. Here we report a unique zinc(II) complex exhibiting unprecedented interplay between the excitation‐wavelength‐dependent emission, thermally activated delayed fluorescence (TADF) and excited state intramolecular proton transfer (ESIPT). The ESIPT process in the complex is favoured by a short intramolecular OH⋅⋅⋅N hydrogen bond. Synergy between the excitation‐wavelength‐dependent emission and ESIPT arises due to heavy zinc atom favouring intersystem crossing (isc). Reverse intersystem crossing (risc) and TADF are favoured by a narrow singlet–triplet gap, ΔEST≈10 kJ mol−1. These results provide the first insight into how a proton‐transfer system can be modified to show a synergy between the excitation‐wavelength‐dependent emission, ESIPT and TADF. This strategy offers new perspectives for designing ESIPT and TADF emitters exhibiting tunable excitation‐wavelength‐dependent luminescence. [ABSTRACT FROM AUTHOR]