N-to-S substitution induced fluorescence-to-phosphorescence dominant emission with excitation-dependent intersystem crossing

To date, most of the reported pure organic room-temperature phosphorescent (ORTP) molecule crystals still show fluorescence-dominant emission. However, their RTP mechanism is still in debate. Herein, we demonstrated that a small variation in the molecular structure induces a drastic change from fluorescence-dominant to phosphorescence-dominant emission with excitation-dependent intersystem crossing (ISC). 6-Bromo-N-ethylcarbazole-3-carboxaldehyde (BECA) exhibits fluorescence-dominant emission in the solid state. In contrast, RTP becomes predominant in its dibenzothiophene counterpart (BDTA). Furthermore, both BDTA polymorphs exhibit excitation-dependent fluorescence and phosphorescence (ExDFPh) emissions with multiple ISC channels. Significantly, two kinds of phosphorescence-emitting species were observed and verified, which result in an unexpected generation of pure RTP even when the excitation wavelength is much longer than the fluorescence peak wavelength. The latter is possibly due to changes in excitation and ISC pathways. Alternatively, nearly pure RTP could also be achieved through high-energy excitation light, which is attributed to the contribution of ISC from a higher-lying singlet excited state. Therefore, RTP could be generated at a wide excitation range. Crystal structure analyses revealed that molecule packing switches to dense H-aggregation from BECA and BDTA, accounting for drastic photophysical differences between them. This study is expected to shed light on the RTP mechanism and also promote the development of ExDFPh materials. A small variation in a molecule structure induces a drastic change from fluorescence (F)-dominant to phosphorescence (P)-dominant dual emission with excitation-dependent properties.