Fluorescence behavior of individual charge-transfer complexes revealed by single-molecule fluorescence spectroscopy: Influence of the host polymer matrix

Extremely pure polymer matrices were used for elucidating the fluorescence properties of singly isolated charge-transfer (CT) complexes formed between the donor N-ethylcarbazole and the acceptor 1,2,4,5-tetracyanobenzene. Simultaneous measurements (time traces of CT fluorescence intensities and lifetimes) using single-molecule fluorescence spectroscopy showed three patterns: (1) fluctuations in the fluorescence intensities and lifetimes seldom occurred, (2) the fluorescence intensities frequently fluctuated together with the lifetimes, or (3) in addition to the above fluctuations with time, blinking and/or off-states longer than 1 s were observed. For methacrylate polymers, both the degree of fluctuations in the CT fluorescence lifetimes and the percentage of the CT complexes showing off-states increased with the free volume of the host polymers. These results suggest that the degree of fluctuations in the relative geometrical arrangements of the donor and acceptor molecules is related to the availability of space in the host polymer, and that the free volume provides the necessary space for formation of non-fluorescent donor–acceptor geometries of the CT complexes and/or temporal dissociation of the CT complexes. Survival times of the CT fluorescence were also closely related with the free volume of the host polymers.