Toward probing of the local electron-phonon interaction in small-molecule organic semiconductors with Raman spectroscopy.

Electron-phonon interaction strongly affects and often limits charge transport in organic semiconductors (OSs). However, approaches to its experimental probing are still in their infancy. In this study, we probe the local electron-phonon interaction (quantified by the charge-transfer reorganization energy) in small-molecule OSs by means of Raman spectroscopy. Applying density functional theory calculations to four series of oligomeric OSs-polyenes, oligofurans, oligoacenes, and heteroacenes-we extend the previous evidence that the intense Raman vibrational modes considerably contribute to the reorganization energy in several molecules and molecular charge-transfer complexes, to a broader scope of OSs. The correlation between the contribution of the vibrational mode to the reorganization energy and its Raman intensity is especially prominent for the resonance conditions. The experimental Raman spectra obtained with various excitation wavelengths are in good agreement with the theoretical ones, indicating the reliability of our calculations. We also establish for the first time relations between the spectrally integrated Raman intensity, the reorganization energy, and the molecular polarizability for the resonance and off-resonance conditions. The results obtained are expected to facilitate the experimental studies of the electron-phonon interaction in OSs for an improved understanding of charge transport in these materials.