华东理工大学张志云团队报道了空间效应和芳香性逆转的相互作用扩大二氢吩嗪的结构/电子响应。相关研究成果于2022年3月8日发表于国际顶尖学术期刊《美国化学会杂志》。

为了深入了解构象和电子变量的耦合，研究人员利用空间位阻，在光激发芳香性反转的指导下，通过S0态的弯曲构象和S1态的扭曲构象来调节多环骨架。多环5,10-二氢吩嗪（DHP）在S0中采用弯曲结构，但由于8π电子中心环的激发态芳香性，在S1中涉及弯曲到平面的转变。DHP骨架的N，N′-位和1，4，6，9-位为微调分子内空间位阻提供了一个多功能的化学手柄。具体而言，N，N′-二苯基-5,10-二氢吩嗪（DPP-00）及其衍生物DPP-10–DPP-22在1,4,6,9-位上合成了不同数量的甲基。X射线晶体分析表明，随着甲基数目的增加，DPP-00–DPP-22的DHP骨架沿N··N轴具有更多弯曲构型。随着弯曲促进的π-共轭中断，DPP-00–DPP-22的吸收光谱从416 nm显著蓝移到324 nm。

相比之下，随着甲基取代基数量的增加，发射谱带呈现出从波长459 nm到584 nm的反向偏移。理论计算表明，甲基的引入使S1中的平面DHP骨架沿N··N轴进一步扭曲，从而形成扭曲的高应变构象。空间位阻越大的结构的斯托克斯位移越大，这可归因于释放了更大的应变和芳香稳定能。

该研究强调了在单一荧光团中空间效应和芳香逆转相互作用的潜在前景。

附：英文原文

Title: Interplay of Steric Effects and Aromaticity Reversals to Expand the Structural/Electronic Responses of Dihydrophenazines

Author: Xin Jin, Sifan Li, Lifang Guo, Jianli Hua, Da-Hui Qu, Jianhua Su, Zhiyun Zhang, He Tian

Issue&Volume: March 8, 2022

Abstract: To gain insights into the coupling of conformational and electronic variables, we exploited steric hindrance to modulate a polycyclic skeleton with a bent conformation in the S0 state and a twisted conformation in the S1 state under the guidance of photoexcited aromaticity reversals. Polycyclic 5,10-dihydrophenazine (DHP) adopted a bent structure in S0 but involved a bent-to-planar transformation in S1 due to the excited-state aromaticity of the 8π-electron central ring. The N,N′-locations and 1,4,6,9-sites of the DHP skeleton provided a versatile chemical handle for fine-tuning intramolecular steric hindrance. Specifically, N,N′-diphenyl-5,10-dihydrophenazine (DPP-00) and its derivatives DPP-10–DPP-22 were synthesized with different numbers of methyl groups on the 1,4,6,9-sites. X-ray crystal analyses suggested that the DHP skeletons of DPP-00–DPP-22 had more bending configurations along the N···N axis with an increase in the number of methyl groups. Following the bending-promoted interruption of π-conjugation, the absorption spectra of DPP-00–DPP-22 significantly blue-shifted from 416 to 324 nm. By contrast, the emission bands exhibited a reverse shift to longer wavelengths from 459 to 584 nm as the number of methyl substituents increased. Theoretical calculations revealed that introducing methyl groups caused the planar DHP skeleton in S1 to further twist along the N···N axis, resulting in a twisted high-strain conformation. The greater Stokes shift of the more steric-hindered structure can be attributed to the release of larger strain and aromatic stabilization energy. This research highlighted the potential promise associated with the interplay of steric effects and aromaticity reversals in a single fluorophore.

DOI: 10.1021/jacs.1c12610

Source: https://pubs.acs.org/doi/10.1021/jacs.1c12610