The orientation of porphins in n-alkane Shpolskii hosts

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)?, in excellent agreement with previous determinations.