The orientation of porphins inn-alkane Shpolskii hosts

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Abstract
When a solution of a porphin in n-octane is slowly frozen the guests predominantly occupy a single site (A) in which the porphin has been shown to replace a number of host molecules displaced along the b-direction; if the solution is quickly frozen the optical spectra reveal the presence of a second site (B). In the present paper it is shown that the occurrence of these two types of sites, A and B, is a common phenomenon throughout the series from n-heptane (C7) to n-decane (C 10) as hosts and phenomenological criteria are formulated to distinguish between the two kinds of sites. The orientation of palladiumporphin (PdP) in the A and B sites of a C7 host is determined from Zeeman experiments on the S 1← S 0 and T 0 →S 0 transitions in a single crystal of C7 doped with PdP. The porphin nuclei in the B sites replace a number of alkanes displaced along the a-axis. In the course of the experiments signals related to a third type of site, B', grew in intensity in the spectra. This site is coplanar with the B site and it differs from the latter by an in-plane rotation of the trapped molecule over 20°. The origins of the S 1 ←S 0 transition for the two types of site are assigned for H2P, ZnP and PdP as solutes in C7, C8, C9 and C10 as hosts. Finally, the Zeeman shifts observed in the S 1 ←S 0 transition of PdP in the three sites of C7 yield an effective orbital angular momentum for PdP in the state S 1 of (4·4 ± 0·1)ħ, in excellent agreement with previous determinations.