Diethoxycarbonyl-BODIPYs as heavy-atom-free photosensitizers for holographic recording in cellulose acetate photopolymer

A series of new heavy-atom-free photosensitizers using 2,6-diethoxycarbonyl-BODIPY was designed and synthesized. Photoinduced electron transfer between the BODIPY and meso-aryl subunits followed by the formation of BODIPY triplet excited states via spin-orbit charge transfer intersystem crossing (SOCT-ISC) was studied by steady-state and ultrafast pump-probe transient optical spectroscopy. Highly efficient photosensitization was observed for dyads bearing pyrene and anthracene substituents, which exhibited singlet oxygen generation with quantum yield of up to 94%. Notably, charge transfer and ISC were observed in non-polar solvent (toluene) due to the increased electron accepting ability of the diethoxycarbonyl-BODIPY as compared to alkyl-substituted BODIPY scaffold. Studied dyads were used as photosensitizers for creation of volume phase transmission gratings in a photopolymerizable material based on cellulose acetate - polyethylene glycol (CA-PEG) matrix. In combination with N-phenylglycin (NPG) as a co-initiator, new BODIPYs showed the ability to initiate a free-radical polymerization of acrylamide monomers under 532 nm irradiation. This allowed for holographic recording of diffractive structures with diffraction efficiency of up to 56% obtained for CA-PEG layers with BODIPY-pyrene dyad as compared to a reference heavy-atom-containing diiodo-BODIPY dye (27%). The materials sensitized with diethoxycarbonyl-BODIPYs showed refractive index modulation of up to 2.3 × 10-3 during holographic recording at the spatial frequency of 800 lines/mm which demonstrate the potential of SOCT-ISC photosensitizers in photopolymerization and holographic recording applications.