Mono-Heteroatom Substitution for Harnessing Excited-State Structural Planarization of Dihydrodibenzo[a,c]phenazines |
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Authors: | Yi Chen Deng-Gao Chen Yi-An Chen Cheng-Ham Wu Kai-Hsin Chang Fan-Yi Meng Meng-Chi Chen Jia-An Lin Chun-Ying Huang Jianhua Su He Tian Prof Pi-Tai Chou |
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Institution: | 1. Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology Shanghai, 200237 Shanghai, P. R. China;2. Department of Chemistry, National Taiwan University, Taipei, 10617, R.O.C. Taiwan |
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Abstract: | With the aim of generalizing the structure–properties relationship of bending heterocyclic molecules that undergo prominent photoinduced structural planarization (PISP), a series of new dihydrodibenzoac]phenazine derivatives in which one nitrogen atom is replaced by oxygen ( PNO ), sulfur ( PNS ), selenium ( PNSe ), or dimethylmethanediyl ( PNC ) was strategically designed and synthesized. Compounds PNO , PNS , and PNSe have significantly nonplanar geometries in the ground state, which undergo PISP to give a planarlike conformer and hence a large emission Stokes shift. A combination of femtosecond early relaxation dynamics and computational approaches established an R*→I* (intermediate)→P* sequential kinetic pattern for PNS and PNSe , whereas PNO undergoes R*→P* one-step kinetics. The polarization ability of the substituted heteroatoms, which is in the order O<S<Se, correlates with their increase in π conjugation, and hence the Stokes shift of the emission is in the order PNO < PNS < PNSe . Compound PNSe with the largest PISP barrier was shown to be a highly sensitive viscosity probe. Further evidence for heteroatom-harnessing PISP is given by PNC , in which the dimethylmethanediyl substituent lacks lone pair electrons for π extension, showing the normal emission of the bent structure. The results led to the conclusion that PISP is ubiquitous in dihydrodibenzoac]phenazines, for which the driving force is elongation of the π delocalization to gain stabilization in the excited state. |
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Keywords: | fluorescence heterocycles photoinduced planarization sensors substituent effects |
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