Photoremovable Protecting Groups Based on Electrocyclization with Leaving Group Expulsion Via a Proposed Zwitterion

The triplet excited state of thioxanthone produced by photolysis undergoes reversible triplet energy transfer with a trimethylene - linked benzothiophene-2-carboxanilide ring system. The ensuing electrocyclic ring closure of the anilide moiety produces a putative zwitterionic intermediate that is capable of expelling leaving groups (LG-) from the C-3 position of the benzothiophene ring. Stern-Volmer quenching studies with cyclohexadiene as quencher furnish the rate constants for the triplet excitation transfer in the forward and reverse directions, which can be expressed as an equilibrium constant K = 0.058. Overall, the rate of triplet excited state reaction becomes K x kr = 5.7 x 104 s-1 for LG- = Cl-, where kr is the triplet decay rate of the C-3 chloro – substituted benzothiophene-2-carboxanilide, found through Stern-Volmer quenching. The high quantum efficiencies found for the trimethylene - linked systems are due to K x kr being competitive with triplet excited state decay of the thioxanthone of kd = 7.7 x 104 s-1. From isc = 0.68, the overall expected quantum yield for direct photolysis should be 0.50 for LG- = Cl- as compared to 0.41 at 25 °C experimentally. decreases with increasing basicity of the leaving group (LG- = (EtO)2PO2-, PhCH2CO2-, PhS-, PhCH2S-, Cl¬-). Photoremovable protecting groups are of great importance in biological, biochemical, and physiological applications. The current work has discovered a highly versatile photoremovable protecting group that can release a wide variety of potentially biologically active leaving groups with high efficiencies at long wavelengths of 395 nm, which will minimize damage to cells and tissue. The released leaving groups have a wide range of basicities. Thus, both weak bases such as carboxylates and phosphates and relatively strong bases such as thiolates are photochemically released