华东理工大学田禾团队报道了N，N′-二苯基二氢二苯并[a，c]吩嗪荧光团中的顺序多步激发态结构转变。相关研究成果发表在2023年5月15日出版的国际学术期刊《德国应用化学》。

研究发现，单个多环π-支架可以沿着弯曲、平面和扭曲的构象发生连续的多步激发态结构演化，这些构象共存，在室温溶液中产生本征多重荧光发射。通过在N，N′-二苯基二氢二苯并[a，c]吩嗪（DPAC）的邻位上安装甲基或三氟甲基，增强的空间效应将DPAC的荧光发射从主要的红光带改变为分辨率良好的三带。邻位取代的DPAC的超宽带三重发射范围从~350到~850 nm，这对于分子量<500的小荧光团来说是前所未有的。

超快光谱和理论计算清楚地表明，上述剧烈变化源于空间位阻对激发态势能面（S1 PES）形状的影响。与亲代DPACs的陡峭的S1 PES相比，邻位取代基的引入使S1中的结构演化路径更宽、更平坦，因此邻位取代的衍生物表现出从弯曲到平面再到扭曲形式的较慢的结构转变，产生本征三重发射。

研究结果提供了弯曲、平面和扭曲发射态可以在相同的S1 PES中共存的概念证明，这大大扩展了对激发态结构弛豫的理解。

附：英文原文

Title: Sequential Multistep Excited-State Structural Transformations in N,N′-Diphenyl-dihydrodibenzo[a,c]phenazine Fluorophores

Author: Xin Jin, Shiyan Guo, Xueli Wang, Muyu Cong, Jinquan Chen, Zhiyun Zhang, Jianhua Su, Da-Hui Qu, He Tian

Issue&Volume: 2023-05-15

Abstract: We demonstrate that a single polycyclic π-scaffold can undergo sequential multistep excited-state structural evolution along the bent, planar, and twisted conformers, which coexist to produce intrinsic multiple fluorescence emissions in room-temperature solution. By installing a methyl or trifluoromethyl group on the ortho-site of N,N′-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC), the enhanced steric effects change the fluorescence emission of DPAC from a dominant red band to well-resolved triple bands. The ultra-broadband triple emissions of ortho-substituted DPACs range from ~350 to ~850 nm, which is unprecedented for small fluorophores with molecular weight of < 500. Ultrafast spectroscopy and theoretical calculations clearly reveal that the above dramatic changes originate from the influence of steric hindrance on the shape of excited state potential energy surface (S1 PES). Compared to the steep S1 PES of parental DPAC, introduction of ortho-substituent is shown to make the path of structural evolution in S1 wider and flatter, so the ortho-substituted derivatives exhibit slower structural transformations from bent to planar and then to twisted forms, yielding intrinsic triple emission. The results provide the proof of concept that the bent, planar, and twisted emissive states can coexist in the same S1 PES, which greatly expand the fundamental understanding of the excited-state structural relaxation.

DOI: 10.1002/anie.202305572

Source: https://onlinelibrary.wiley.com/doi/10.1002/anie.202305572