Photophysical properties of tin ethyl etiopurpurin I (SnET2) and tin octaethylbenzochlorin (SnOEBC) in solution and bound to albumin

The photophysical characteristics of two second-generation PDT photosensiti zers, tin ethyl etiopurpurin I (SnET2) and tin octaethylbenzochlorin (SnOEB C), have been measured in homogeneous solution and when bound to bovine ser um albumin (BSA), The ground state and triplet state absorption spectra hav e been characterized, as have triplet lifetimes and quantum yields for inte rsystem crossing, singlet oxygen formation and photobleaching. In total, th ese parameters provide a complete set of data that can be used to quantitat ively compare the photosensitizing efficiencies of these molecules. The pho tobleaching quantum yield of SnET2 is increased dramatically when it is bou nd to BSA, thus limiting the production of singlet oxygen at incident fluen ces above 1 J/cm(2). In contrast, the quantum yield of photobleaching of Sn OEBC is at least an order of magnitude lower than that of SnET2 under these conditions and does not significantly limit the photosensitization process for typical in vivo or in vitro fluences. This difference is expected to p lay a significant role in determining the relative photosensitizing ability of these compounds in vivo.