Polarized Absorption in Crystalline Pentacene: Theory vs Experiment

The polarized absorption spectra of crystalline pentacene are obtained for excitation normal to the ab herringbone plane by measuring transmitted light in ultrathin crystals. The spectral line shapes for excitation polarized along b and orthogonal to b are analyzed theoretically using a Holstein-like Hamiltonian which includes both Frenkel and charge transfer (CT) excitons represented in a multiparticle basis set. The model agrees with prior estimates regarding the strong CT contribution (≈45%) of the exciton responsible for the b-polarized lower Davydov component. The polarization resolution allows one to also establish the nature of the upper Davydov component, which is found to contain far less CT content (≈15%), as well as the natures of the higher-energy vibronic excitons, which are found to consist of a complex mixture of Frenkel one- and two-particle states and CT excitons. Generally, the spectrum polarized along b displays J-aggregate-like vibronic signatures while the spectrum polarized orthogonal to b displays H-aggregate-like vibronic signatures. The assignment is entirely consistent with the calculated exciton band dispersions which agree well with the measured ones.