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 functio n 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 a ttribute the low-frequency subband to a conformer with a nonplanar mac rocycle structure, whereas the high-frequency subband is interpreted a s resulting from a planar conformer. The subbands' intensity ratios ex hibit a solvent-dependent van't Hoff behavior between 300 and 160 K. C rystallization of CH2Cl2 prevents measurements at lower temperatures. For Ni(II) octaethylporphyrin in the glass-forming isopentane/ethyl et her mixture, the intensity ratio bends over in a region between 150 an d 100 K and remains constant below. These data can be fitted by a modi fied 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 t herefore dominantly occupied at low temperatures. Furthermore we found that the Sorer band's profile is Voigtian with a temperature-dependen t Gaussian contribution. The latter results from a bath of low-frequen cy modes to which the electronic transition into the B state is vibron ically coupled. This most likely comprises out-of-plane modes of the p orphyrin, in particular those involving the central metal atom, and mo lecular motions within the liquid environment. At temperatures above t he glass transition of the solvent, the amplitudes of these motions in crease above the values predicted by a purely harmonic model. This is indicative of strong nonharmonic contributions to their potential ener gy.