Effect of TiO2 particles on normal and resonance Raman spectra of coumarin 343: a theoretical investigation

It is well known that interfacial structures and charge transfer in dye-sensitized solar cells are extremely important for the enhancement of cell efficiency. Here, the normal Raman spectra (NRS) and resonance Raman spectra (RRS) of a C343-sensitized TiO2 cluster (Ti9O18) are theoretically predicted from combined electronic structure calculations and a vibrationally-resolved spectral method to reveal the relationship between interfacial geometries and excited-state dynamics. The results show that although the NRS of free C343 and the C343–TiO2 cluster correspond to the vibrational motions of C343 in a high frequency domain, their mode frequencies show obvious differences due to the interaction of the TiO2 cluster on C343, and several new Raman active fingerprint modes, such as bidentate chelating bonding modes, can be used to determine interfacial geometries. However, the resonance Raman activities of low-frequency modes are significantly enhanced and several modes from the TiO2 cluster can be observed, consistent with experimental measurements. Furthermore, the RRS from a locally excited state and a charge transfer state of C343–TiO2 are dramatically different, for instance, new Raman active modes with 1212 cm−1, 1560 cm−1 and 1602 cm−1, corresponding to the motions of CH2 rocking, CC/C–N/CO stretching and CO/CC stretching, appear from the charge transfer state. The obtained information on mode-specific reorganization energies from these excited states is greatly helpful to understand and control interfacial electron transfer.