Resonance Raman Characterization of Tetracene Monomer and Nanocrystals: Excited State Lattice Distortions With Implications For Efficient Singlet Fission
The characterization of specific phonon modes and exciton states that lead to efficient singlet fission (SF) may be instrumental in the design of the next generation of high-efficiency photovoltaic devices. To this end, we analyze the absolute resonance Raman (RR) cross sections for tetracene (Tc) both as a monomer in solution and as a crystalline solid in an aqueous suspension of nanocrystals. For both systems, a time-dependent wavepacket model is developed that is consistent with the absolute RR cross sections, the magnitude of the absorption cross sections, and the vibronic line shapes of the fluorescence. In the monomer, the intramolecular reorganization energy is between 1500 and 1800 cm –1 and the solvent reorganization energy is 70 cm –1 . In nanocrystals, the total reorganization is diminished to less than 600 cm –1 . The lowest energy exciton has an estimated intramolecular reorganization energy between 300 and 500 cm –1 while intermolecular librational phonons have a reorganization energy of about 130 cm –1 . The diminished reorganization energy of the nanocrystal is interpreted in the context of the delocalization of the band-edge exciton onto about ∼7 molecules. When electron and electron–hole correlations are included within many-body perturbation theory, the polarized absorption spectra of crystalline Tc are calculated and found to be in agreement with experiment. The low-lying exciton states and optically active phonons that contribute to the polarized crystal absorption are identified. The likely role of coherent exciton phonon evolution in the SF process is discussed.