Heavy-atom free BODIPY photosensitizer small molecules capable of forming triplet excited states via the intersystem crossing (ISC) process are critically important for various technologies: from biomedicine to photocatalysis. In this paper a comparative analysis of tetra- and pentamethyl-substituted BODIPY dyes (1–6) photosensitizing efficiency in various organic solvents solutions are presented. α-Unsubstituted BODIPY luminophores ability to forming triplet excited states via the intersystem crossing (ISC) process in the absence of heavy element atoms and donor substituents in the molecular structure has been demonstrated. Analysis of DFT calculations of tetramethyl- and pentamethyl-substituted BODIPY excited states showed, that the position of methyl substitutions plays major roles on the ΔES-T value and the efficiency of energy transfer to the triplet state and, accordingly, the photosensitization efficiency of 1–6 dyes. For the first time it has been shown that α,α-unsubstituted BODIPY 1 has a favorable combination of effective absorption (logε from 4.55 to 4.64), fluorescence (Փfl varies from 47 to 66 %) and singlet oxygen generation (ΦΔ varies from 14 to 40 %), long fluorescence lifetimes (τfl from 5.75 to 6.13 ns) and small Stokes shift (212 to 537 cm−1) in various nature organic solvent solutions along with the small molecule size and synthesis simplicity.
