UV light, belt 5a displayed a fluorescence emission band at 500 nm. With the increase of the belt size, the emission bands were found to blue-shift to 475 nm (5b) and 456 nm (5c). In the emission spectra of 5c, fluorescence peaks with vibrational structures were observed (Fig. ). The quantum yields of 5a-c (F), which were determined using fluorescein (0.1 N NaOH solution) as the reference, were 8.5%, 4.9% and 13.0%, respectively. The time-resolved fluorescence measurements showed the fluorescence lifetimes (s) of 4.9 ns, 1.0 ns and 1.7 ns for 5a-c, respectively (Supporting Information). Following the equations F = krad × s and s = (krad + knonrad) -1 , the radiative (krad) and nonradiative (knonrad) decay rate constants were calculated (Fig. ), which indicated a decreased order of radiative decay rate from a large to a small molecular belt. eV is the energy level of ferrocene/ferrocenium below the vacuum level) and LUMO [ELUMO CV = EHOMO CV + Eg opt (eV)] were estimated and listed in Fig. .