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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 11-1012 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) × 1010 s-1.

Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 3. Fluorescence detected magnetic resonance of triplet states

Fluorescence detected magnetic resonance (FDMR) has been applied to ～25-nm-thick porphyrin films, containing ordered domains of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides. Illuminating the films at 1.4 K with 457.9-nm light from a continuous wave Ar+ laser produces at least two different, Jahn-Teller-distorted, ZnTOPP triplet species, labeled i and ii. Microwave-induced magnetic resonance of i and ii in the absence or presence of an externally applied magnetic field affects the fluorescence intensity of ZnTOPP, thus allowing FDMR. For triplet species i, formed in films spin-coated from toluene solution, the zero-field splitting (ZFS) parameters were determined as |D| = (316.9 ± 0.1) × 10-4 cm-1 and |E| = (32.0 ± 0.5) × 10 -4 cm-1. By exposure of the spin-coated films to chloroform vapor at room temperature, triplet i is converted into species ii, with |D| = (295 ± 3) × 10-4 cm-1 and |E| = (121 ± 3) × 10-4 cm-1. For the excited triplet state of ZnTOPP in a toluene glass, ZFS parameters with values of |D| = (295 ±1) × 10-4 cm-1 and |E| = (91 ±1) × 10-4 cm-1 are found. From a combined study of the FDMR- and microwave-induced fluorescence spectra, i and ii are identified as unligated and ligated ZnTOPP triplet species, respectively. From the asymmetrically shaped zero-field FDMR signals of i, we conclude that the local crystal field perturbations of the stacked molecules are anisotropic. The FDMR results of the ZnTOPP films are compared with those for a film of zinc tetraphenylporphyrin (ZnTPP), which lacks the octyl substituents, and therefore is nonordered. Upon illumination, the ZnTPP films contain only a single, ligated, triplet species with ZFS parameters very similar to those of ligated ZnTOPP. At ～5 K, the lifetime of triplet i is considerably shortened compared to that of ZnTOPP in a glass at the same temperature.

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 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.

Nonisotropic excitation energy transport in organized molecular systems: Monte Carlo simulation-based analysis of fluorescence and fluorescence anisotropy decay

An improved application is presented of the Monte Carlo method including simultaneous parameter fitting to analyze the experimental time-resolved fluorescence and fluorescence anisotropy decay of two organized molecular systems exhibiting a number of different, nonisotropic energy transfer processes. Using physical models and parameter fitting for these systems, the Monte Carlo simulations yield a final set of parameters, which characterize the energy transfer processes in the investigated systems. The advantages of such a simulation-based analysis for global parametric fitting are discussed. Using this approach energy transfer processes have been analyzed for two porphyrin model systems, i.e., spin-coated films of zinc tetra-(octylphenyl)-porphyrins (ZnTOPP) and the tetramer of zinc mono(4-pyridyl)triphenylporphyrin (ZnM(4-Py)TrPP). For the ZnTOPP film energy transfer rate constants of ～1 × 1012 s-1 and ～80 × 109 s-1 have been found, and are assigned to intra- and interstack transfer, respectively. For the tetramers, the transfer rate constants of 38 × 109 and 5 × 109 s-1 correspond to energy transfer to nearest and next nearest neighbor molecules, respectively. The results are in agreement with a Foerster type energy transfer mechanism.