Effect of substitution on light-curable properties in diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide: DFT and TD-DFT calculations

Abstract

Acylphosphine oxide-based photoinitiators have been used in digital light processing (DLP) 3D printing slurry to prepare ceramic materials. However, the limitation is how to improve the absorption efficiency of photoinitiator in UV–Vis light and its initiation efficiency. In this study, through the DMol3 module of Materials Studio, the hydrogen atom is substituted by different electron-donating substituents (-NH2, -OCH3, -OH), substitution positions (ortho, meta, para) and tri-substitutions positions (2,4,6 and 3,4,5) were systematically studied respectively, including the change from the ground state to the first excited state, energy gap, ultraviolet-visible spectrum, the excitation energy, absorption wavelength, and oscillator strength. These results show that the order of electron-donating ability of these four groups is as follows: -NH2 > -OCH3 > -OH> -H. When the initiation wavelength λ≤ 425 nm, enhancing the electron-donating ability of the substituent and substitute para-substitution position can improve the oscillator strength and cause a blue-shift of the absorption peak. 4-TPO-NH2 which has the strongest electron-donating ability group substituted in para position shows the highest oscillator strength (21.5%) and blue shift (22 nm) compared with unsubstituted TPO. When the initiation wavelength λ≥ 425 nm, enhancing the electron-donating ability of the substituent, substitute para-substitution or ortho-substitution position, and tri-substituents of 2,4,6 positions, is the most beneficial to improve the oscillator strength and the red-shift of the absorption peak. Compared to unsubstituted TPO, 2,4,6-TPO-NH2 has 9.00 times increase in oscillator strength with 122 nm red-shift, and 2-TPO-NH2 has 134 nm red-shift with 2.53 times increase in oscillator strength. Furthermore, these studies can improve the development of acylphosphine oxide-based photoinitiators, and will also help to expand the application of DLP 3D printing in ceramic fields.