Resonance Raman Spectra of a Perylene Bis(dicarboximide) Chromophore in Ground and Lowest Triplet States

Resonance Raman spectroscopy is employed to probe the ground (S-0) and lowest triplet (T-1) excited states of a perylene bis(dicarboximide) (PDI) dimer. Four bands at similar to 1324, 1507, similar to 1535, and 1597 cm(-1) are signatures of the T-1 excited state; a fifth band at similar to 1160 cm(-1) is tentatively assigned. Density functional calculations of an asymmetrically substituted PDI monomer match the experimental bands of the PDI dimer in both S-0 and T-1 states. The match supports a T-1 excited state that is localized on a single PDI moiety of the dirtier. The normal modes of the asymmetrically substituted PDI are correlated with ones calculated for the unsubstituted PDI in the D-2h point group. Patterns in the Raman intensities are consistent with an A-term mechanism of enhancement The positions of six bands are predicted for the resonance Raman spectrum of unsubstituted PDI in its T-1 excited state. The spectra and normal-mode analysis reported here are expected to facilitate future studies of singlet fission in PDI crystals or other assemblies.