Enhancement of intersystem crossing in asymmetrically substituted BODIPY photosensitizers†

We present a novel method to promote intersystem crossing (ISC) and triplet state formation in boron dipyrromethenes (BODIPYs) through the asymmetrical introduction of functional groups within the chromophore. This approach enables the development of new BODIPY photosensitizers without relying on the incorporation of heavy atoms or large electron-donating aromatic groups. Demonstrated on a series of 14 synthesized asymmetrical BODIPY (aBDP) compounds, it significantly enhances photosensitization efficiency compared to the reference symmetrical BODIPYs. In particular, the asymmetrical introduction of ethoxycarbonyl groups into pyrrolic rings of the BODIPY core lead to efficient ISC and singlet oxygen generation, with quantum yields reaching 0.76 in non-polar solvents. Quantum chemical calculations elucidated the ISC mechanism, revealing an S₁ → T₂ pathway facilitated by a reduced singlet–triplet energy gap (ΔE_S–T). The new photosensitizers were successfully applied in holographic recording through photopolymerization of acrylamide monomers in a cellulose acetate–based photopolymer, using irradiation with a 532 nm laser. The material containing an asymmetrical BODIPY–anthracene dyad exhibited the highest exposure sensitivity, achieving diffraction efficiencies up to 71% for volume transmission gratings at an exposure energy of 2.3 J cm⁻². The practical potential of these dyes was demonstrated by fabricating a holographic optical element – an off-axis lens – with a uniform diffraction efficiency of 38 ± 3% across the lens.