Theoretical calculations of the electronic absorption spectra of oxotitanium(IV) phthalocyanine in the solid state

Oxotitanium(IV) phthalocyanine (OTiPc) has been reported to be a near-IR-active photoconductor, where crystal modifications termed phases I, II, and Y exist. In this paper, solid-state electronic absorption spectra of OTiPc have been theoretically calculated by considering three origins (i.e., exciton, charge transfer (CT), and molecular distortion) to clarify the crystal form dependence on the large red shift of the Q absorption band and high photoconductivity of the photoactive phase II in particular. In these calculations, intermolecular interactions are included in the form of configuration interactions between exciton and CT configurations, and matrix elements were calculated using the ZINDO/S Hamiltonian in order to calculate 3D metallocomplexes. By considering 243 neighboring OTiPc molecules, our theoretical calculations have succeeded in reproducing the experimental trends of the solid-state optical properties, such as the large red shift of the Q absorption band, dependence on the incident light direction, phase dependence, and CT character, indicating the reliability of our calculations. In addition, it is demonstrated that the photoconductive properties correlate well with intermolecular resonance integrals. Calculations shown here are useful not only for understanding the solid-state electronic structures of OTiPc but also for designing new functional molecular crystals.