Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 1. Optical spectroscopy of excitonic interactions involving the soret band.

The photophysical properties of excited singlet states of zinc tetra-(p-octylphenyl)-porphyrin in 5−25-nm-thick films spin-coated onto quartz slides have been investigated by optical spectroscopy. Analysis of the polarized absorption spectra using a dipole−dipole exciton model with two mutually perpendicular transition dipole moments per molecule shows that the films are built from linear aggregates, i.e., stacks with a slipped-deck-of-cards configuration. The molecular planes of the porphyrins in the stacks are found to be perpendicularly oriented with respect to the substrate plane. Assuming a value of 2−3 for the dielectric constant of the film, from the excitonic shift, an angle of 44° ± 3° and an interplanar distance of 0.35−0.36 nm between adjacent porphyrins are calculated, close to the ground-state geometry in solution. The ordering in these films was further investigated by the effects of various solvents and temperature annealing. Spin-coating from toluene as a solvent results in highly ordered ...