Ultrafast Time-Domain Spectroscopy Reveals Coherent\nVibronic Couplings upon Electronic Excitation in Crystalline Organic\nThin Films

The coherent coupling\nbetween electronic excitations and vibrational\nmodes of molecules largely affects the optical and charge transport\nproperties of organic semiconductors and molecular solids. To analyze\nthese couplings by means of ultrafast spectroscopy, highly ordered\ncrystalline films with large domains are particularly suitable because\nthe domains can be addressed individually, hence allowing azimuthal\npolarization-resolved measurements. Impressive examples of this are\nhighly ordered crystalline thin films of perfluoropentacene (PFP)\nmolecules, which adopt different molecular orientations on different\nalkali halide substrates. Here, we report polarization-resolved time-domain\nvibrational spectroscopy with 10 fs time resolution and Raman spectroscopy\nof crystalline PFP thin films grown on NaF(100) and KCl(100) substrates.\nCoherent oscillations in the time-resolved spectra reveal vibronic\ncoupling to a high-frequency, 25 fs, in-plane deformation mode that\nis insensitive to the optical polarization, while the coupling to\na lower-frequency, 85 fs, out-of-plane ring bending mode depends significantly\non the crystalline and molecular orientation. Comparison with calculated\nRaman spectra of isolated PFP molecules in vacuo supports this interpretation\nand indicates a dominant role of solid-state effects in the vibronic\nproperties of these materials. Our results represent a first step\ntoward uncovering the role of anisotropic vibronic couplings for singlet\nfission processes in crystalline molecular thin films.