Resonance Raman Characterization of Tetracene Monomer and Nanocrystals: Excited State Lattice Distortions With Implications For Efficient Singlet Fission

Abstract

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 highefficiency 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.

Revealing ultrafast vibronic dynamics of tetracene molecules with sub-8 fs UV impulsive Raman spectroscopy

“…3(b) shows resonance Raman spectra at an incident photon energy of 3.13 eV, which is a similar condition to the resonance Raman spectroscopy of tetracene, except for the solvent. 19 From the figure, the resonance Raman spectra in the FCHT approximation appear at 504, 1192 and 1672 cm À1 , whereas these are not observed in the FC approximation. These modes belong to B 3g symmetry and are experimentally observed in resonance Raman spectroscopy, 19 indicating that the FCHT approximation is appropriate in the transition from the S 0 to S 1 states in tetracene.…”

“…These values are almost consistent with the previous resonance Raman study. 19 Fig. 4(b) shows the contour map of the Duschinsky matrix.…”

“…40 From these, it is a common insight that the contribution of molecular vibrations is important; for example, the multiple pulse excitation at 275 fs intervals improved the singlet fission yield by approximately 20%, 35 and resonance Raman experiments have also suggested that high and low frequency vibrational modes couple during internal conversion. 19 However, to the best of our knowledge, there are no reports that directly observe the vibronically hot states in the S 1 state in tetracene with the ultrashort time scale after photoexcitation.…”

“…21,22 Several mechanisms have been proposed to explain the singlet fission in tetracene, including the contribution of high energy levels in the S 0 state, 22 hot bands in the S 1 state, 22 entropy effects 23 and vibronic coupling. 21 Experimentally, tetracene and its derivatives, including monomers in solutions, thin films and crystals, have been investigated using resonance Raman spectroscopy, 19 time-resolved fluorescence spectroscopy, [23][24][25][26]29,31,34,[42][43][44] transient absorption spectroscopy, 26,[30][31][32]36,[44][45][46][47][48] two-dimensional (2D) electronic spectroscopy 28 and impulsive Raman spectroscopy. 40 From these, it is a common insight that the contribution of molecular vibrations is important; for example, the multiple pulse excitation at 275 fs intervals improved the singlet fission yield by approximately 20%, 35 and resonance Raman experiments have also suggested that high and low frequency vibrational modes couple during internal conversion.…”

Singlet Fission in Dideuterated Tetracene and Pentacene

“…For example, intramolecular vibrational modes between 300 and 1550 cm −1 have been found to be important for efficient SF in acenes [9,17,20,22–25] with a high sensitivity of SF rates to the frequency of the considered modes [17,22,25] and were found to induce vibronic coherences on timescales longer than the singlet fission process itself [19–21,26–28] . Intermolecular vibrations have been discussed as an additional contribution to charge transfer mediated SF via non‐adiabatic population transfer [11,20,28] and have been demonstrated to significantly affect SF rates and the subsequent triplet‐pair separation process [27,29–36] …”