We systematically examined the mechanism of the solvent polarity dependence of the fluorescence ON/OFF threshold of the BODIPY (boron dipyrromethene) fluorophore and the role of photoinduced electron transfer (PeT). In a series of BODIPY derivatives with variously substituted benzene moieties at the 8-position, the oxidation potential of the benzene moiety became more positive and the reduction potential of the BODIPY fluorophore became more negative as the solvent polarity was decreased; consequently, the free energy change of PeT from the benzene moiety becomes larger in a more nonpolar environment. Utilizing this finding, we designed and synthesized a library of probes in which the threshold of fluorescence ON/OFF switching corresponds to different levels of solvent polarity. These environment-sensitive probes were used to examine bovine serum albumin (BSA) and living cells. The polarity at the surface of albumin was concluded to be similar to that of acetone, while the polarity of the internal membranes of HeLa cells was similar to that of dichloromethane.

Boron dipyrromethene (BODIPY), which is commonly used as an energy absorbing and transferring antenna molecule, has been modified to contain an electron donor moiety, 8-(2,4,5-trimethoxyphenyl)-4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (MEOPHBDP). The photoinduced electron transfer from a 2,4,5-trimethoxyphenyl moiety to a BODIPY moiety of MEOPHBDP in acetonitrile was observed by femtosecond laser flash photolysis measurements. The lifetime of the charge-separated state of MEOPHBDP was 59 ps at 298 K. The dye-sensitized solar cells (DSSC) were prepared using MEOPHBDP with carboxylic acid (MEOPHBDP-COOH) and a reference BODIPY dye having no electron donor moiety. The photovoltaic measurements were performed using a standard two-electrode system consisting of a working electrode and a Pt sputtered electrode in methoxyacetonitrile containing 0.5 M iodide and 0.05 M I(2). The photoelectrochemical properties of DSSC with MEOPHBDP are compared with those with a reference BODIPY dye.

Chromophore-assisted light inactivation is a promising technique to inactivate selected proteins with high spatial and temporal resolution in living cells, but its use has been limited because of the lack of a methodology to prevent nonspecific photodamage in the cell owing to reactive oxygen species generated by the photosensitizer. Here we present a design strategy for photosensitizers with an environmentsensitive off/on switch for singlet oxygen ( 1 O2) generation, which is switched on by binding to the target, to improve the specificity of protein photoinactivation. 1 O2 generation in the unbound state is quenched by photoinduced electron transfer, whereas 1 O2 generation can occur in the hydrophobic environment provided by the target protein, after specific binding. Inositol 1,4,5-trisphosphate receptor, which has been suggested to have a hydrophobic pocket around the ligand binding site, was specifically inactivated by an environmentsensitive photosensitizer-conjugated inositol 1,4,5-trisphosphate receptor ligand without 1 O2 generation in the cytosol of the target cells, despite light illumination, demonstrating the potential of environment-sensitive photosensitizers to allow high-resolution control of generation of reactive oxygen species in the cell.activatable photosensitizer ͉ boron dipyrromethene derivative ͉ electron transfer ͉ inositol 1,4,5-trisphosphate receptor C hromophore-assisted light inactivation (CALI)(1) is a technique with great potential to inactivate proteins with high spatial and temporal resolution by using an antibody to direct a suitable fluorophore specifically to the protein of interest. Illumination induces local generation of reactive oxygen species (ROS), which react chemically with the adjacent antigen and inactivate it. Although CALI is a powerful technique, its use has been limited by the complexity of the procedures (i.e., the need to deliver a labeled antibody into cells or to use a laser as the light source). Several groups have reported alternative approaches. Genetically targeted CALI is one such method, in which the target protein is tagged with a tetracysteine tag that is recognized by a membrane-permeant biarsenical chromophore (FlAsH) (2, 3), or tagged with GFP (4-6). However, these methods also cause nonspecific damage, owing to the nonspecific binding of the biarsenical chromophore to cysteine-rich proteins (3, 7) in FlAsH-mediated photoinactivation, or to the use of a relatively high-power laser in EGFP-mediated CALI (4, 5). Current implementations of the CALI technique leave much to be desired, and highly specific inactivation of a protein of interest would require a methodology to control ROS generation by the photosensitizer in the cells with high spatial resolution.We present here an approach for designing photosensitizers with an environment-controlled off/on switch for singlet oxygen ( 1 O 2 ) generation to improve the specificity of CALI. We have developed environment-sensitive photosensitizers (ESPers), which are activated by recognition of the hydrophob...