One/two-photon sensitive sulfonium salt photoinitiators based on 1,3,5-triphenyl-2-pyrazoline

Abstract

Three 1,3,5-triphenyl-2-pyrazoline-based sulfonium salt photoinitiators (PIs) were synthesized, and the relationship between their molecular structures and photophysical and photochemical properties under one/two-photon irradiation were presented. The sulfonium salts had absorption peaks from 343 nm to 372 nm that overlapped the emission of 365–425 nm light-emitting diode (LED) light sources. The energy levels of molecular orbitals were closely related to the quantum yield of photoacid generation (ΦH+) as determined by the electrochemical and absorption properties of the PIs, and the introduction of an electron-pushing group could increase ΦH+ due to the enlarged energy gap between the singlet excited states S1 and S2. Interestingly, the wavelength dependence of ΦH+ was due to the nature of the transition. The π–π* transition showed high ΦH+ values (0.56–0.66) under excitation by UV LEDs, while the π–σ* transition showed low efficiency under visible LED irradiation. Intramolecular charge transfer from the pyrazoline ring to sulfonium salts induced efficient photolysis and high ΦH+. The sulfonium salts could efficiently initiate cationic polymerization and free radical polymerization. In addition, their high two-photon absorption cross-sections (δ780 nm = 75–105 GM) reduced the threshold of two-photon microfabrication. These PIs should have potential in LED photocuring and two-photon 3D microfabrication.