The ESIPT-suppressed 2-(2′-hydroxyphenyl)benzoxazole derivative as a new photoinitiator for multiphoton polymerization

Abstract

The quest for new photoinitiators (PIs) capable of efficiently absorbing multiple photons, particularly three photons, has intensified due to the multiphoton polymerization (MPP) technique. MPP enables the fabrication of next-generation photonic micro- and nanodevices with resolution beyond the diffraction limit and without shape limitation, offering considerable advantages over other methods. However, designing organic compounds acting as effective multiphoton PIs while enabling smart functionalization of manufactured structures remains challenging. In this contribution, we report using a 2-(2′-hydroxyphenyl)benzoxazole (HBO) derivative, named HBO-NBu₂, as a PI for two- and three-photon polymerization. HBO fluorophores are well-known to undergo an excited-state intramolecular proton transfer (ESIPT) process, but in this structure, the combination of a planar and linear push–pull dipolar structure, with an extensive π-conjugation, led to a complete suppression of ESIPT, subsequently exhibiting an excellent nonlinear response. Herein, we investigate the linear photophysical properties, excited-state dynamics, and multiphoton absorption of HBO-NBu₂, supported by quantum chemical calculations. Our results demonstrate that HBO-NBu₂ exhibits structural rigidity, resulting in a fluorescence quantum yield of 62% in dichloromethane solution, owing to the absence of ESIPT and substantial intramolecular charge transfer (ICT) due to its push–pull nature. Moreover, this molecule shows significant two- and three-photon absorption cross-sections of 80 GM (at 800 nm) and 4 ± 1 × 10⁻⁸¹ cm⁶ (s per photon)² (at 1030 nm), respectively. Finally, we demonstrate a proof of concept of MPP using HBO-NBu2 as a PI, showing that even at low concentrations, it outperforms widely used PIs.