Effects of C3-aromatic heterocycles on 1,3,5-triaryl-2-pyrazoline sulfonium salt photoacid generators as light-emitting diode-sensitive cationic photoinitiators

Four 1,5-diphenyl-3-aromatic heterocyclyl-2-pyrazoline-based sulfonium salt photoacid generators (PAGs) with different aromatic heterocycles substituted on C3 atom and dimethyl sulfonium group on C5 atom were synthesized. These PAGs were highly photosensitive in the 365-425 nm light-emitting diode region, and the intramolecular charge transfer from the pyrazoline ring to sulfonium salts induced efficient photolysis and high phi(+)(H). The heterocycles as well as their substituted positions significantly influenced the energy of the S-2 orbital, which was determined by the electrochemical and absorption properties of the PAGs. The raising of the S-2 orbital energy enlarged the energy gap of S-0-S-2 and S-1-S-2, resulting in blue shift of the absorption spectra and increase in the quantum yield of photoacid generation (phi(+)(H)), respectively. When the energy of excited electrons was higher than that of the S-2 orbital, the transition from S-0 to S-2 (pi-pi*) occurred before the C-S cleavage on S-1 and the PAGs showed high phi(+)(H) values (0.52-0.72). The transition from S-0 to S-1 (pi-sigma*) occurred when the energy of electrons is lower than that of the S-2 orbital, and the PAGs showed low phi(+)(H) value. The photopolymerization kinetics demonstrated that these PAGs were highly efficient cationic photoinitiators.

Automated summary

The pyrazoline-based sulfonium salt photoacid generators showed high photosensitivity in the 365-425 nm light-emitting diode region, with efficient photolysis and high phi(+)(H) induced by intramolecular charge transfer. The energy of the S-2 orbital was significantly influenced by the heterocycles and their substituted positions, impacting the absorption spectra and quantum yield of photoacid generation.