RESUMEN
Resonance Raman spectroscopy is employed to probe the ground (S0) and lowest triplet (T1) excited states of a perylene bis(dicarboximide) (PDI) dimer. Four bands at ~1324, 1507, ~1535, and 1597 cm(-1) are signatures of the T1 excited state; a fifth band at ~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 S0 and T1 states. The match supports a T1 excited state that is localized on a single PDI moiety of the dimer. The normal modes of the asymmetrically substituted PDI are correlated with ones calculated for the unsubstituted PDI in the D2h 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 T1 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.
RESUMEN
The characterization of triplet excited states is essential for research on organic photovoltaics and singlet fission. We report resonance Raman spectra of two triplet oligothiophenes with n-alkyl substituents, a tetramer and hexamer. The spectra of the triplets are more complex than the ground state, and we find that density functional theory calculations are a useful starting point for characterizing the bands. The spectra of triplet tetrathiophene and hexathiophene differ significantly from one another. This observation is consistent with a T1 excitation that is delocalized over at least five rings in long oligomers. Bands in the 500-800 cm(-1) region are greatly diminished for an aggregated sample of hexathiophene, likely caused by fast electronic dephasing. These experiments highlight the potential of resonance Raman spectroscopy to unequivocally detect and characterize triplets in thiophene materials. The vibrational spectra can also serve as rigorous standards for evaluating computational methods for excited-state molecules.