Fluorescence Enhancement of Aromatic Macrocycles by Lowering Excited Singlet State Energies

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 (S_1MIN) was required to facilitate efficient fluorescence.