Complicated Fermi-type vibronic resonance: Untangling of the single-site quasi-line fluorescence excitation spectra of a methylated dibenzoporphin

Abstract

The quasi-line low-temperature (4.2 K) fluorescence excitation spectra of 2,3,12,13-tetramethyldibenzo[ g,q]porphin introduced into an n-octane matrix have been measured in the range of the S 2 ← S 0 electronic transition at selective fluorescence monitoring for the two main types of impurity centers (sites). A characteristic feature of these spectra is that a conglomerate of quasi-lines – a structured complex band – is observed instead of one 0–0 quasi-line of the S 2 ← S 0 transition. In this band, the intensity distributions for the two main sites considerably differ from each other. The occurrence of such conglomerates is interpreted as a result of nonadiabatic vibrational–electronic interaction between the vibronic S 2 and S 1 states (the complex vibronic analogue of the Fermi resonance). The frequencies and intensities of individual transitions determined from the deconvolution of complex conglomerates are used as the initial data for solving the inverse spectroscopic problem: the determination of the unperturbed electronic and vibrational levels of states involved in the resonance and the vibronic-interaction matrix elements between them. This problem is solved with a method developed previously. The experimental results and their analysis are compared to the analogous data obtained earlier for meso-tetraazaporphin and meso-tetrapropylporphin. The energy intervals between the S 2 and S 1 electronic levels ( Δ E S 2 S 1 ) of the two main types of impurity centers formed by molecules of a given porphyrin in the crystal matrix are found to significantly differ from each other, the values of this difference ( δ Δ E S 2 S 1 ) being considerably greater for tetramethyldibenzoporphin, δ Δ E S 2 S 1 = 228 cm - 1 , than for the two other porphyrins. At the same time, the energies of the unperturbed vibrational states of the S 1 electronic level participating in the resonance are very close to each other for these two sites.