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Vibronic coupling in metalloporphyrins
486
Abstracts
163
VIBRONIC COUPLING IN METALLOPORPHYRINS L.L.Gladkov* and A.NLShulga ~' ° Byelorussian State University, ScarynaAv., 4,Minsk,220050, Belarus b Institute ofMolecular andAtomic Physics of Belarus Academy of Sciencies, Scaryna Av. 70, 220072 Minsk, Belarus
Resonance Raman spectra, absorption and fluorescence spectra of porhyrins are forming partially due to vibronic borrowing from active elctronic transitions. This factor depends on activity of electronic transitions and energy gap between corresponding electronic states. It was believed early in spectroscopy of porphyrins that Sorer band transitions took part in vibronic couplin~ But introduction of metal atom in the center of porphyrin molecule leads to degeneration of $1 and $2 states and role of $2 state was not negligible. In this report we discuss role of vibronic coupling between $I and $2 electronic states. Because the moments of the vibronic transitions are oriented in the sameway as the moments of the electronic transitions donating intensity than polarization of zero-phonon lines in fluorescence spectra is a marker of the donating electronic transition and vibronic borrowing can be studied by polarized fluorescence methods. We recorded fine-structure fluorescence spectra and made up polarization measurements under siteselective excitation and 4,2 K for structurally similar metalloporphyrins with isocyclic substitution. Absorption spectra of this compounds are practically identical and energy gap between low intensive So->S2 and more intensive So -> $2 transitions is about 300 cm "I . However, the polarization of So -> $2 transitions in absorption spectra is completely different for this cases. We have found, that the frequencies and relative intensities of lines have been practically identical in fine-structure spectra of fluorescence of this metalloporphyrins. In contrast the polarization of a number of lines have opposite signs and that correlates with the polarization of So-> $2 transitions. It points out that the vibronic interaction of quasi-degenerate S~ and $2 levels plays a important role in forming the vibrational structure of the fluorescence spectra in the region of stretching vibrations of the macrocycle. Because the energies of vibrational quanta are substantially higher than the energy gap between the S1 and $2 levels, the vibronic interaction is nonadiabatic. The analysis of number of finestructure fluorescence and absorption spectra of metalloporphyrins is shown that the character of this type of vibronic interaction involving the participation of a second electronic level, is universal for metalloporphyrins. If a porphyrin molecule has effectively a tetragonal symmetry, D4h , the vibrations of -~-2g sym~letry species is active in spectra due to this vibronic interaction. The recearch described herein was supported by International Science Foundation (Grant Nr.RWZ000).
Abstracts
163
VIBRONIC COUPLING IN METALLOPORPHYRINS L.L.Gladkov* and A.NLShulga ~' ° Byelorussian State University, ScarynaAv., 4,Minsk,220050, Belarus b Institute ofMolecular andAtomic Physics of Belarus Academy of Sciencies, Scaryna Av. 70, 220072 Minsk, Belarus
Resonance Raman spectra, absorption and fluorescence spectra of porhyrins are forming partially due to vibronic borrowing from active elctronic transitions. This factor depends on activity of electronic transitions and energy gap between corresponding electronic states. It was believed early in spectroscopy of porphyrins that Sorer band transitions took part in vibronic couplin~ But introduction of metal atom in the center of porphyrin molecule leads to degeneration of $1 and $2 states and role of $2 state was not negligible. In this report we discuss role of vibronic coupling between $I and $2 electronic states. Because the moments of the vibronic transitions are oriented in the sameway as the moments of the electronic transitions donating intensity than polarization of zero-phonon lines in fluorescence spectra is a marker of the donating electronic transition and vibronic borrowing can be studied by polarized fluorescence methods. We recorded fine-structure fluorescence spectra and made up polarization measurements under siteselective excitation and 4,2 K for structurally similar metalloporphyrins with isocyclic substitution. Absorption spectra of this compounds are practically identical and energy gap between low intensive So->S2 and more intensive So -> $2 transitions is about 300 cm "I . However, the polarization of So -> $2 transitions in absorption spectra is completely different for this cases. We have found, that the frequencies and relative intensities of lines have been practically identical in fine-structure spectra of fluorescence of this metalloporphyrins. In contrast the polarization of a number of lines have opposite signs and that correlates with the polarization of So-> $2 transitions. It points out that the vibronic interaction of quasi-degenerate S~ and $2 levels plays a important role in forming the vibrational structure of the fluorescence spectra in the region of stretching vibrations of the macrocycle. Because the energies of vibrational quanta are substantially higher than the energy gap between the S1 and $2 levels, the vibronic interaction is nonadiabatic. The analysis of number of finestructure fluorescence and absorption spectra of metalloporphyrins is shown that the character of this type of vibronic interaction involving the participation of a second electronic level, is universal for metalloporphyrins. If a porphyrin molecule has effectively a tetragonal symmetry, D4h , the vibrations of -~-2g sym~letry species is active in spectra due to this vibronic interaction. The recearch described herein was supported by International Science Foundation (Grant Nr.RWZ000).