Excited State Dynamics of Photoswitches: Molecular Motors, Photoacids and Vitamin B12

Abstract

The photochemistry and photophysics of three types of photoswitchable compounds have been investigated. Chiral overcrowded alkenes, which undergo unidirectional photoisomerization are examples of light driven molecular-scale motors. Photochromic photoacids based on the reversible light induced ring-closure and ring-opening of spiropyran and merocyanine release a proton causing a large drop in pH upon switching from the ring-open merocyanine to the ring-closed spiropyran form. Hydroxocobalamin and aquocobalamin, derivatives of vitamin B12, act as light activated hydroxyl radical catalysts under aerobic conditions in solution. Ultrafast UV-vis transient absorption spectroscopy has been used to investigate the details of excited state dynamics and photoproducts involved in the photochemistry of these molecules. Light driven molecular motors undergo 360° of internal rotation about their central carbon-carbon double bond in four steps: P-cisàM-transàP-transàM-cis, etc. The cis to trans and trans to cis steps are photoisomerizations, while the “M” to “P” steps are thermal conformational changes. Transient absorption spectroscopy and pump-repump probe (PrPP) spectroscopy were used to probe the excited state behavior of all four conformations of the molecular motor. We have found evidence for unintended backward isomerization in the M-cis conformation, previously thought to be insensitive to photoexcitation. Spiropyran and merocyanine can reversibly interchange upon absorption of light. Merocyanine is a planar π conjugated molecule with visible absorption bands. Spiropyran possesses a central “spiro” carbon breaking the conjugation between both sides of the molecule,and therefore has absorption bands in the UV. The indazole and phenol based versions of merocyanine/spiropyran investigated here display a large pKa difference between the merocyanine and spiropyran forms. As a result, light activated ring closure of merocyanine to form spiropyran is accompanied by a drop in pH of ~ 2 units. Transient absorption in aqueous solution and a polar aprotic solvent (DMSO) uncovered solvent effects in the photochemical formation of spiropyran. In the phenol-based merocyanine, water appears to extend the lifetime of the cis-merocyanine precursors to spiropyran. Hydroxocobalamin (HOCbl) and aquocobalamin (H2OCbl) are derivatives of vitamin B12 that have long been thought to be photostable. However, recent transient absorption studies and anaerobic photolysis experiments have demonstrated otherwise. Transient absorption experiments exciting with 269 nm light show homolytic bond cleavage to form hydroxyl radicals and cob(II)alamin with ~ 1.5% quantum yield. In addition, anaerobic photolysis with wavelengths ≤ 300 nm demonstrates homolysis as well. As a result, HOCbl and H2OCbl are potential hydroxyl radical photocatalysts.