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 (S1MIN) was
required to facilitate efficient fluorescence.