Phosphorus(V) tetrapyrazinocorrolazines bearing axial aryloxy groups as pH-sensitive fluorophores and photosensitizers

Phosphorus(V) complexes of octaphenyltetrapyrazinocorrolazine bearing two aryloxy groups in the axial position, [TPyzCAP(OAr)₂] (2a-c, Ar = phenyl (2a), 4-dimethylaminophenyl (2b), and 4-hydroxyphenyl (2c)), were prepared using a one-pot procedure by consecutive treatment of the dihydroxidophosphorus(V) derivative, [TPyzCAP(OH)₂] (1), with SOCl₂ and then with the corresponding phenol ArOH. Complex 2a containing axial PhO groups is fluorescent in all studied solvents (toluene, CH₂Cl₂, THF, and DMSO, Φ_F ∼ 0.16-0.31) and is efficient to generate singlet oxygen (Φ_Δ = 0.55 (THF), 0.68 (toluene)). The introduction of NMe₂ and OH groups in the para-position of the axial ArO ligands strongly affects the fluorescence parameters and photosensitizing properties due to the appearance of the solvent-sensitive and pH-switchable effects of photoinduced electron transfer (PET). The PET effect of NMe₂ groups completely quenches the excited state of 2b in all solvents, but it is switched-OFF upon their protonation, and in the presence of acid traces, the fluorescence of 2b becomes bright and singlet oxygen generation is strongly enhanced. The PET effect of the OH group is increased upon its deprotonation and in the presence of base 2c as well as 1 becomes non-fluorescent. Specific solvation in THF and DMSO increases the ionic character of the OH bonds, and the fluorescence and photosensitizing properties of 1 and 2c are strongly decreased in these solvents. According to the results of DFT calculations performed using the B3LYP functional with the cc-pVDZ basis set and cyclic voltammetric studies, the molecular orbitals localized on aryloxy ligands are destabilized upon the introduction of OH and especially NMe₂ groups and their close position to the HOMO of corrolazine macrocycle (above HOMO in 2b and between HOMO and HOMO-1 in 2c) leads to the appearance of the PET effect.