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 temperatur