Fluorescence and phosphorescence spectroscopy of C70 in toluene at 5 K: site dependent low lying excited states Original Research Article

We have recorded the dispersed fluorescence and phosphorescence spectra of C70 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 C70 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 e1′ and e2′ modes in a 3A2′–1A1′ transition. The dispersed fluorescence spectrum showed Herzberg–Teller induced e1′ and a1″ modes that were inferred to originate from a 1A2′–1A1′ transition as well as bands in the origin region that were interpreted to come from 1E1′–1A1′ and 1A2″–1A1′ transitions. The selective phosphorescence excitation spectra revealed that the character, the absolute energy and the order of the two lowest excited singlet states 1A2′ and 1E1′ are site dependent. An orbitally allowed and thermally activated T2 state lying 240 cm−1 higher in energy from the T1 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 S1 and T1 states was determined to be about 2936 cm−1, however the exact splitting energy is matrix and site dependent.