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 degrees +/- 3 degrees 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 films, and annealing of these films has little effect on their absorption spectra. However, spin-coating from chloroform or pyridine or exposure of the films to these solvents in their vapor phases changes their ordering presumably due to incorporation of residual solvent molecules. Annealing yields absorption spectra identical to those of films spin-coated from toluene. The absorption spectra are insensitive to atmospheric moisture, in contrast to those of zinc tetraphenylporphyrin films lacking octyl substituents.

Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 2. Transport properties of singlet excitation.

Exciton diffusion has been studied in 5-25-nm-thick films of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides by intentional doping with quenchers using steady-state as well as time-resolved fluorescence spectroscopy. The fluorescence spectra of the films are very similar to those of solutions, indicating emission from localized exciton states. From the dependence of the fluorescence quenching on the quencher concentration and fluorescence lifetime measurements, the exciton diffusion can be concluded to be quasi-one-dimensional with an exciton diffusion length of 9 +/- 3 nm and an intrastack energy-transfer rate constant of 10(11)-10(12) s(-1). From fluorescence anisotropy decay measurements, we conclude that neighboring stacks aggregate in a herringbone structure, forming ordered domains that are randomly oriented in the substrate plane. These measurements indicate an interstack energy-transfer rate constant of (7 +/- 2) x 10(10) s(-1).