Vibrational analysis of Ni(II)‐ and Cu(II)‐octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopy

Abstract

AbstractWe measured the polarized resonance Raman spectra of Cu(II)‐2,2,7,8,12,13,17,18‐octamethylchlorin in CS2 at various excitation wavenumbers in a spectral region covering the Qy, Qx and Bx optical absorption bands. Additionally, we measured the FTIR‐Raman spectrum of the highly overcrowded spectral region between 1300 and 1450 cm−1. The spectral decomposition was carried out by a self‐consistent global fit to all spectra obtained. The thus identified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)‐octaethylporphyrin, by utilizing their depolarization ratio dispersions and by a normal mode analysis. The latter was based on a modified transferable molecular mechanics force field of Ni(II)‐octaethylporphyrin [E. Unger, M. Beck, R.J. Lipski, W. Dreybrodt, C.J. Medforth, K.M. Smith and R. Schweitzer‐Stenner, J. Phys. Chem. B 103, 10229 (1999)]. A comparison of normal mode patterns obtained for Cu(II)‐octamethylchlorin and Cu(II)‐octaethylporphyrin revealed that some modes are significantly distorted by the reduction of the pyrrole ring, in accordance with results which Boldt et al. reported earlier for Ni(II)‐octaethylchlorin [N.J. Boldt, F.J. Donohoe, R.R. Birge and D.F. Bocian, J. Am. Chem. Soc. 109, 2284 (1987)]. In contrast to conclusions drawn from this study, however, the results of our vibrational analysis and several further lines of evidence suggest that the normal modes of corresponding chlorines and porphyrins are still comparable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyrins is also applicable to metallochlorins. Finally, we performed a preliminary analysis of the absorption spectrum and the resonance excitation profiles and depolarization ratio dispersions of some Raman lines. The results show that the electronic properties of Cu(II)‐octamethylchlorin can still be described in terms of Gouterman's four orbital model [M. Gouterman, J. Chem. Phys. 30, 1139 (1959)]. In regions of the Q bands, Raman scattering of A1 modes is determined by interferences between Franck– Condon coupling and interstate Herzberg– Teller coupling between Qx(Qy) and Bx(By) states. The B2 modes are resonance enhanced by Herzberg– Teller coupling between Qx and Qy and between Qx(Qy) and By(Bx). Franck– Condon coupling of A1 modes with large contributions from CαCm stretching vibrations is comparatively strong for Qx. This is interpreted as reflecting the expansion of the chlorin macrocycle by an electronic transition into this excited state. Copyright © 2001 John Wiley & Sons, Ltd.