Fluorescence Enhancement of Aromatic Macrocycles by Lowering Excited Singlet State Energies

• Published in: Journal of organic chemistry 2020-01, Vol.85 (1), p.150-157
• Main Authors: Ikemoto, Koki, Tokuhira, Toshiki, Uetani, Akari, Harabuchi, Yu, Sato, Sota, Maeda, Satoshi, Isobe, Hiroyuki
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/acs.joc.9b02379

A series of cyclo-meta-phenylene congeners with a variation of interphenylene bridges was synthesized by adopting concise synthetic routes to investigate the structure–fluorescence relationships of macrocycles. With fundamental UV–vis absorption and fluorescence spectra, no unique effect of the macrocyclic structures was noted. However, the fluorescence quantum yields were dramatically affected by the macrocyclic structures and varied at a range of 5–92%. The quantum yields qualitatively depended on the number of the vinylene-bridged phenanthrenylene panels, and the theoretical investigations revealed the energetic and structural effects of the phenanthrenylene panels during nanosecond photodynamic processes. A high energy barrier along the S0/S1 internal conversion path to reach the minimum energy conical intersection was necessary to hamper a nonradiative process, and with the transition state energy level of the excited singlet state being insensitive to macrocyclic structures, a low energy level of excited singlet states (S1MIN) was required to facilitate efficient fluorescence.