Structural modification on tetraphenylpyrazine: from polarity enhanced emission to polarity quenching emission and its intramolecular charge transfer mechanism†
Abstract
Strong polarity usually causes emission quenching; however, the polarity induced emission enhancement with great importance for applications in some polar environments is rarely reported. A structural and theoretical proof-of-concept to realize polarity enhanced emission materials is necessary. In this research, we have achieved polarity enhanced emission by studying the intramolecular charge transfer mechanism: (1) the weak emission in non-polar solvents is attributed to the obvious hole–electron overlap on the flexible tetraphenylpyrazine unit; (2) the planarized intramolecular charge transfer could increase the hole–electron population gradually and reduce the overlap degree on tetraphenylpyrazine to achieve emission enhancement in highly polar solvents; (3) twisted conformation with excessive charge separation (twisted intramolecular charge transfer) could cause fluorescence quenching in highly polar solvents. In addition, these findings can offer guidelines for the design of new molecular systems with polarity enhanced emission behavior.