Mono‐Heteroatom Substitution for Harnessing Excited‐State Structural Planarization of Dihydrodibenzo[a,c]phenazines

• Published in: Chemistry : a European journal Vol. 25; no. 72; pp. 16755 - 16764
• Main Authors: Chen, Yi, Chen, Deng‐Gao, Chen, Yi‐An, Wu, Cheng‐Ham, Chang, Kai‐Hsin, Meng, Fan‐Yi, Chen, Meng‐Chi, Lin, Jia‐An, Huang, Chun‐Ying, Su, Jianhua, Tian, He, Chou, Pi‐Tai
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 20.12.2019
• Subjects: Chemistry; Chemistry, Multidisciplinary; fluorescence; heterocycles; photoinduced planarization; Physical Sciences; Science & Technology; sensors; substituent effects; SYSTEM; ISOMERIZATION; VIBRONIC RELAXATION; MOLECULES; sensors; fluorescence; heterocycles; ATOM-TRANSFER; photoinduced planarization; ABSORPTION; substituent effects; CHARGE-TRANSFER; PROTON-TRANSFER; EXCHANGE
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201904900

With the aim of generalizing the structure–properties relationship of bending heterocyclic molecules that undergo prominent photoinduced structural planarization (PISP), a series of new dihydrodibenzo[ac]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 dihydrodibenzo[ac]phenazines, for which the driving force is elongation of the π delocalization to gain stabilization in the excited state. Plane to see: A series of dihydrodibenzo[ac]phenazine derivatives in which one N atom is replaced by O (PNO), S (PNS), Se (PNSe), or CMe2 (PNC) was synthesized. The nonplanar ground‐state geometries of PNO, PNS, and PNSe undergo photoinduced structural planarization (PISP) to give more or less planar conformers with large, but different, emission Stokes shifts and hence colors. Moreover, PNSe, which has the largest PISP barrier, was shown to be a highly sensitive viscosity probe.