Transition Dipole Moments of One-Dimensional Excitons in Soluble Phthalocyanine Thin Films

We report on polarization-resolved absorption and photoluminescence spectroscopy experiments that investigate the spatial orientation of excitonic transition dipoles at temperatures ranging from 4 K to room temperature in crystalline phthalocyanine thin films prepared with solution-based deposition methods. Octabutoxy phthalocyanines are quasi-1D systems with highly directional intermolecular interactions along a preferred crystalline axis. Experiments reveal the existence of redshifted delocalized bulk band gap exciton states at temperatures below 175 K. These states are the result of the strong pi-pi short-range coupling and long-range Coulomb coupling between nearest-neighbor molecules along the stacking axis. They are characterized by linearly polarized, nondegenerate dipoles that largely obey the Davydov selection rules. Photoluminescence studies reveal that these excitons couple to lattice and molecular vibrations, forming delocalized exciton-polarons at temperature below 175 K. Finally, by changing the incident light wave vector orientation, we find additional circularly and elliptically polarized states that most likely result from the mixing of HOMO-n (n > 1) orbitals with aza-nitrogen orbitals.