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

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 (Phi(+)(H)) as determined by the electro-chemical and absorption properties of the PIs, and the introduction of an electron-pushing group could increase Phi(+)(H) due to the enlarged energy gap between the singlet excited states S-1 and S-2. Interestingly, the wavelength dependence of Phi(+)(H) was due to the nature of the transition. The pi-pi* transition showed high Phi(+)(H) values (0.56-0.66) under excitation by UV LEDs, while the pi-sigma* transition showed low efficiency under visible LED irradiation. Intramolecular charge transfer from the pyrazoline ring to sulfonium salts induced efficient photolysis and high Phi(+)(H). The sulfonium salts could efficiently initiate cationic polymerization and free radical polymerization. In addition, their high two-photon absorption cross-sections (delta(780 nm) = 75-105 GM) reduced the threshold of two-photon microfabrication. These PIs should have potential in LED photocuring and two-photon 3D microfabrication.