Spectra of Polycrystalline Phthalocyanines in the Visible Region

Transmission spectra of sublimed thin films of the α‐crystalline forms of H₂PC, CuPC, NiPC, CoPC, and ZnPC in the 0.5 to 0.9‐μ region changed sharply as a result of $\mathrm{\alpha \to \beta }$ transformation induced by heat treatment above 300°C. The VOPC was unaffected. In the β‐crystalline forms the absorption maxima (cm⁻¹) were: H₂PC 13 970, 15 500; CuPC 13 800, 15 540; NiPC 14 430, 15 900; CoPC 14 120, 15 730; and ZnPC 13 340, 15 430. The positions and contours of these bonds were to be expected on the basis of polarized single‐crystal results reported by Lyons, Walsh, and White and Fielding and MacKay. Digital computer calculations of Davydov splittings for finite‐sized “block”‐ and “bar”‐shaped crystallites of CuPC based upon pairwise dipole–dipole interactions compared well with earlier results for the infinite sphere model in H₂PC. Assignments for the four observed exciton components in β‐H₂PC must not only correctly predict the band positions and relative intensities, but should also be in accord with a red shift of each of the $B_{\text{2u}}$ and $B_{\text{3u}}$ vapor components. In the $\beta$ crystal of the metal complex an intramolecular distortion or site symmetry should split the $E_g$ molecular states. These in turn should also be red shifted and split by the crystal environment. The results, including studies of mixtures of several phthalocyanines in CuPC, are consistent with the interpretation.