RAMAN DISPERSION SPECTROSCOPY ON THE HIGHLY SADDLED NICKEL(II)-OCTAETHYLTETRAPHENYLPORPHYRIN REVEALS THE SYMMETRY OF NONPLANAR DISTORTIONS AND THE VIBRONIC COUPLING STRENGTH OF NORMAL-MODES

We have measured the polarized Raman cross sections and depolarization ratios of 16 fundamental modes of nickel octaethyltetraphenylporphyri n in a CS2 solution for 16 fundamental modes, i.e., the A(1g)-type vib rations nu(1), nu(2), nu(3), nu(4), nu(5), and phi(8), the B-1g vibrat ions nu(11) and nu(14), the B-2g vibrations nu(28), nu(29), and nu(30) and the antisymmetric A(2g) modes nu(19), nu(20), nu(22), and nu(23) function of the excitation wavelength. The data cover the entire reson ant regions of the Q- and B-bands. They were analyzed by use of a theo ry which describes intra- and intermolecular coupling in terms of a ti me-independent nonadiabatic perturbation theory [E. Unger, U. Bobinger , W. Dreybrodt, and R. Schweitzer-Stenner, J. Phys. Chem. 97, 9956 (19 93)]. This approach explicitly accounts in a self-consistent way for m ultimode mixing with all Raman modes investigated. The vibronic coupli ng parameters obtained from this procedure were then used to successfu lly fit the vibronic side bands of the absorption spectrum and to calc ulate the resonance excitation profiles in absolute units. Our results show that the porphyrin macrocycle is subject to B-2u-(saddling) and B-1u-(ruffling) distortions which lower its symmetry to S-4 Thus, evid ence is provided that the porphyrin molecule maintains the nonplanar s tructure of its crystal phase in an organic solvent. The vibronic coup ling parameters indicate a breakdown of the four-orbital model. This n otion is corroborated by (ZINDO/S) calculations which reveal that sign ificant configurational interaction occurs between the electronic tran sitions into \Q]- and \1B]-states and various porphyrin-->porphyrin, m etal-->porphyrin, and porphyrin-->metal transitions. The intrastate co upling parameters are used to estimate the excited electronic states' displacements along the normal coordinates with respect to the ground state and their contributions to the reorganization energy, It turns o ut that the \B]-state is predominantly affected by symmetric A(1g)-dis placements, whereas the \Q]-state is subject to A(2g), B-1g, and B-2g displacements of its equilibrium configuration. While the former is in duced by the combined effect of ruffling and saddling, the latter aris es from Jahn-Teller coupling within the degenerate states. (C) 1997 Am erican Institute of Physics.