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

A series of new heavy-atom-free photosensitizers based on the 2,6-diethoxycarbonyl-BODIPY scaffold was designed and applied for holographic recording in a photopolymerizable material. 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 quantum yields (Φ_Δ) of up to 94%. Charge transfer and SOCT-ISC were observed in non-polar solvent (toluene) due to the increased electron accepting ability of the diethoxycarbonyl-BODIPY. In combination with N-phenylglycine (NPG) as a co-initiator, new BODIPYs initiate a free-radical polymerization of acrylamide monomers under 532 nm irradiation that was used for creation of volume phase transmission gratings in a photopolymerizable material based on cellulose acetate and polyethylene glycol (CA-PEG). As a result of holographic recording, diffractive structures with diffraction efficiency of up to 56% were obtained for CA-PEG layers sensitized with BODIPY-pyrene dyad as compared to a reference heavy-atom-containing diiodo-BODIPY dye (27%). The developed materials showed refractive index modulation of up to 2.3 × 10⁻³, which demonstrates the potential of diethoxycarbonyl-BODIPYs photosensitizers for holographic recording applications.