Through-Space Interaction: What, Where and How?
Electronic conjugation through covalent bonds is generally considered as the basis for the electronic transition of organic luminescent materials. Tetraphenylethylene (TPE), an efficient fluorophore with aggregation-induced emission (AIE) character, its blue photoluminescence in aggregate state is always ascribed to the through-bond conjugation (TBC) among the four phenyl rings and the central C=C bond. Herein, systematic spectrometry studies and ab initio theoretical simulation were conducted for TPE and its derivatives, and intramolecular through-space interaction (TSI) between two vicinal phenyl rings is proved as the origin of the blue emission. Furthermore, aided by the evaluation of excited-state decay dynamics, the non-luminescent nature of TPE in solution is revealed as the result of excited-state evolution towards conical intersections via isomerization and cyclization. In aggregate state, the excited-state TSI (ESTSI) is stabilized by the restriction of intramolecular motions, and strong blue emission from through-space conjugation is induced. The mechanistic model of ESTSI delineated in this work provides a new strategy to design luminescent materials beyond the traditional theory of TBC, and expands the quantum understanding of molecular behavior into the aggregate level.