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

We measured the polarized resonance Raman spectra of Cu(II)-2,2,7,8,12,13,1 7,18-octamethylchlorin in CS2 at various excitation wavenumbers in a spectr al region covering the Q(y), Q(x) and B-x optical absorption bands. Additio nally, we measured the FTIR-Raman spectrum of the highly overcrowded spectr al region between 1300 and 1450 cm(-1). The spectral decomposition was carr ied out by a self-consistent global fit to all spectra obtained. The thus i dentified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)-octaethylporphyrin, by utilizing their depolarizat ion ratio dispersions and by a normal mode analysis. The latter was based o n a modified transferable molecular mechanics force field of Ni(Il)-octaeth ylporphyrin [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 an d Cu(II)-octaethylporphyrin revealed that some modes are significantly dist orted by the reduction of the pyrrole ring, in accordance with results whic h 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 compa rable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyr ins is also applicable to metallochlorins. Finally, we performed a prelimin ary analysis of the absorption spectrum and the resonance excitation profil es and depolarization ratio dispersions of some Raman lines. The results sh ow that the electronic properties of Cu(Il)-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 Al modes is determined by interferences between Franck-Condon coupling and int erstate Herzberg-Teller coupling between Q,(Qy) and B,(By) states. The B-2 modes are resonance enhanced by Herzberg-Teller coupling between Q and Qy a nd between Q (Qy) and By(B,). Franck-Condon coupling of Al modes with large contributions from CalphaCm stretching vibrations is comparatively strong for Q(x). This is interpreted as reflecting the expansion of the chlorin ma crocycle by an electronic transition into this excited state. Copyright (C) 2001 John Wiley & Sons, Ltd.