Fluorescence and phosphorescence spectroscopy of C 70 in toluene at 5 K: site dependent low lying excited states

Abstract

We have recorded the dispersed fluorescence and phosphorescence spectra of C 70 in toluene matrices at 5 K upon excitation with the green Ar + laser line ( λ=514 nm), along with phosphorescence excitation spectra and also site selected phosphorescence spectra. The dispersed phosphorescence spectrum shows spectral features that originate from at least 14 distinct different sites for C 70 within the toluene matrix. Selective excitation of the lowest singlet–singlet transition at 665.8 nm generated a spectrum from a single phosphorescent site which shows prominent Herzberg–Teller induced e 1 ′ and e 2 ′ modes in a 3 A 2 ′– 1 A 1 ′ transition. The dispersed fluorescence spectrum showed Herzberg–Teller induced e 1 ′ and a 1 ″ modes that were inferred to originate from a 1 A 2 ′– 1 A 1 ′ transition as well as bands in the origin region that were interpreted to come from 1 E 1 ′– 1 A 1 ′ and 1 A 2 ″– 1 A 1 ′ transitions. The selective phosphorescence excitation spectra revealed that the character, the absolute energy and the order of the two lowest excited singlet states 1 A 2 ′ and 1 E 1 ′ are site dependent. An orbitally allowed and thermally activated T 2 state lying 240 cm −1 higher in energy from the T 1 state is revealed by temperature dependent experiments. Site selective phosphorescence excitation spectra showed evidence of phonon sidebands 12–14 cm −1 from the corresponding zero-phonon line. The energy difference between the S 1 and T 1 states was determined to be about 2936 cm −1 , however the exact splitting energy is matrix and site dependent.