Size-dependent fluorescence properties of [n]cycloparaphenylenes (n = 8–13), hoop-shaped π-conjugated molecules

Hoop-shaped π-conjugated molecules such as cycloparaphenylene (CPP) have attracted the attention of many chemists because they exhibit interesting properties due to the distorted π-electron system. To gain a systematic understanding of the properties that result from distorted π-electron systems, it is important to know precisely how these properties depend on the hoop size. In the present study, we have investigated the size dependence of the fluorescence properties of CPPs. The fluorescence spectra of smaller CPPs showed red-shifted fluorescence peaks, smaller fluorescence quantum yields, and longer lifetimes, when compared to those of larger ones. One of the important factors that gave rise to these fluorescence properties of smaller CPPs was greater structural relaxation from the Franck–Condon state, which is a postulation supported by theoretical calculations. The structural relaxation in the picosecond domain was experimentally detected by the fluorescence upconversion method. The present results are an important example that confirms steric factors strongly governing the fluorescence properties of a molecule.