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Conformational Properties of Nickel(II) Octaethylporphyrin in Solution. 2. A Low-Temperature Optical Absorption Spectroscopy Study

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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 acco...