Designing Excited States: Theory-Guided Access to Efficient Photosensitizers for Photodynamic Action

Energy-funneling-based broadband visible-light-absorbing bodipy-C60 triads and tetrads as dual functional heavy-atom-free organic triplet photosensitizers for photocatalytic organic reactions

C60-bodipy triads and tetrads based on the energy-funneling effect that show broadband absorption in the visible region have been prepared as novel triplet photosensitizers. The new photosensitizers contain two or three different light-harvesting antennae associated with different absorption wavelengths, resulting in a broad absorption band (450-650 nm). The panchromatic excitation energy harvested by the bodipy moieties is funneled into a spin converter (C60), thus ensuring intersystem crossing and population of the triplet state. Nanosecond time-resolved transient absorption and spin density analysis indicated that the T1 state is localized on either C60 or the antennae, depending on the T1 energy levels of the two entities. The antenna-localized T1 state shows a longer lifetime (τ(T)=132.9 μs) than the C60-localized T1 state (ca. 27.4 μs). We found that the C60 triads and tetrads can be used as dual functional photocatalysts, that is, singlet oxygen ((1)O2) and superoxide radical anion (O2(.-)) photosensitizers. In the photooxidation of naphthol to juglone, the (1)O2 photosensitizing ability of the C60 triad is a factor of 8.9 greater than the conventional triplet photosensitizers tetraphenylporphyrin and methylene blue. The C60 dyads and triads were also used as photocatalysts for O2(.-)-mediated aerobic oxidation of aromatic boronic acids to produce phenols. The reaction times were greatly reduced compared with when [Ru(bpy)3Cl2] was used as photocatalyst. Our study of triplet photosensitizers has shown that broadband absorption in the visible spectral region and long-lived triplet excited states can be useful for the design of new heavy-atom-free organic triplet photosensitizers and for the application of these triplet photosensitizers in photo-organocatalysis.

Photosensitizer that selectively generates singlet oxygen in nonpolar environments: photophysical mechanism and efficiency for a covalent BODIPY dimer

Photosensitizers that selectively generate singlet oxygen in non polar/polar microenvironments are highly desirable for photodynamic therapy of tumor but not yet reported. BODIPY (boron-dipyrromethene complexes) covalent dimer 1 is such a photosensitizer that forms singlet oxygen only in hexane, cyclohexane, and toluene significantly but not in polar solvents. Its corresponding monomer is not photoactive in any solvents for forming singlet oxygen. To reveal the mechanism, we measured the excited triplet-, singlet-, and ground-state properties as well as singlet oxygen generation capability with laser flash photolysis, fluorescence spectroscopy, time-correlated single photon counting, and absorption spectroscopy in various solvents. The striking difference is due to the fact that the excited dimer (excimer) undergoes very fast intramolecular charge transfer (ICT) that makes intersystem crossing noncompetitive in polar solvents, while ICT is negligible in nonpolar solvents.

Thiosquaraine rotaxanes: synthesis, dynamic structure, and oxygen photosensitization

Thiosquaraine dyes have sulfur atoms instead of oxygens on the central squaraine core, and they are powerful singlet oxygen photosensitizers. Stability studies show that they are susceptible to attack by nucleophiles. This problem was circumvented by preparing a mechanically interlocked thiosquaraine rotaxane. NMR studies of the rotaxane indicate an unusual dynamic molecular structure due to a nonsymmetrical coconformation. Upon irradiation with red light, the thiosquaraine rotaxane generates the same amount of singlet oxygen as the known photosensitizer methylene blue.