Photochromism of Spirooxazines in Homogeneous Solution and Phospholipid Liposomes

The photochromic behavior of several spirooxazines (SO) containing phenanthrene or phenanthroline moieties in the oxazine part of molecules has been investigated in several solvents and phosphatidylcholine (PC) liposomes. The solvatochromic properties of the merocyanine (MC) forms of these dyes were used to probe their location within the PC membrane. Transient spectroscopic measurements revealed that, when first formed by photoexcitation, the MC forms of phenanthroline-containing spirooxazines were located at relatively nonpolar sites within the membrane, but they subsequently moved to a more polar environment typical of the aqueous−organic interface. The characteristic time for this intersite movement was τ ≈ 10^-3 s, corresponding to a diffusion coefficient of D ≈ 10^-11 cm² s^-1. In contrast, these spectral shifts were not observed when PC liposome-bound SO containing the phenanthrene moiety were photoexcited, suggesting that either intersite diffusion was more rapid for these compounds or the initially formed MC (and its spiro precursor) were located in a more polar microenvironment. The rate of thermal ring-closing following UV photoexcitation decreased modestly when either an electron-withdrawing group was present on the MC oxazine ring or an electron-donating group was present on the MC indoline ring. A dramatic increase in the ring-closing rate was observed for an o-phenanthroline-containing SO coordinated to a Ru(bpy)₂²⁺ metal center, an effect attributable to strong donation of electron density from the Ru(II) d-orbitals into the ligand π*-orbitals.