Conformational properties of nickel(II) octaethylporphyrin in solution .2. A low-temperature optical absorption spectroscopy study

We have measured the absorption spectrum of Ni(II) octaethylporphyrin in CH2Cl2 and in a 50% v/v isopentane/ethyl ether mixture as a function of temperature between 150 and 300 K and 40 and 300 K, respectively. The Soret band can be decomposed into two subbands whose frequencies differ by 220 cm(-1). By analogy with resonance Raman results (Jentzen et al. J. Phys. Chem. 1996, 100, 14184-14191 (preceding paper)), we attribute the low-frequency subband to a conformer with a nonplanar macrocycle structure, whereas the high-frequency subband is interpreted as resulting from a planar conformer. The subbands' intensity ratios exhibit a solvent-dependent van't Hoff behavior between 300 and 160 K. Crystallization of CH2Cl2 prevents measurements at lower temperatures. For Ni(II) octaethylporphyrin in the glass-forming isopentane/ethyl ether mixture, the intensity ratio bends over in a region between 150 and 100 K and remains constant below. These data can be fitted by a modified van't Hoff expression which also accounts for the freezing of the above conformers into a nonequilibrium distribution below a distinct temperature T-f. The fit yields a freezing temperature of T-f = 121 K and a transition region of 52 K. In accordance with the Raman data we found that the nonplanar conformer has the lowest free energy and is therefore dominantly occupied at low temperatures. Furthermore we found that the Sorer band's profile is Voigtian with a temperature-dependent Gaussian contribution. The latter results from a bath of low-frequency modes to which the electronic transition into the B state is vibronically coupled. This most likely comprises out-of-plane modes of the porphyrin, in particular those involving the central metal atom, and molecular motions within the liquid environment. At temperatures above the glass transition of the solvent, the amplitudes of these motions increase above the values predicted by a purely harmonic model. This is indicative of strong nonharmonic contributions to their potential energy.