Photoinitiators for two-photon polymerisation: effect of branching and viscosity on polymerisation thresholds

Abstract

A series of multi-branched two-photon photoinitiators (PIs) based around the well-known triphenylamine donor core were synthesised for use in two-photon polymerisation (TPP) and are designated as compounds 6, 7 and 8. The use of a phenylene-vinylene π-system was used with an ethyl ester acceptor moiety which gives dipolar (6), quadrupolar (7) and octupolar (8) branching. Two-photon absorption cross-sections (δ_2PA) of 126 GM, 358 GM and 590 GM were measured at 780 nm for 6, 7 and 8, respectively. The fluorescence quantum yields (ϕ_F) were determined in both MeOH and the acrylate system employed for TPP, and demonstrate the impact of viscosity upon photophysical properties of multi-branched molecules. Excellent polymerisation thresholds were demonstrated in the μW region: namely 45 μW (6), 61 μW (7) and 27 μW (8) at a writing speed of 50 μm s⁻¹. Finally, an explanation for the disparity of polymerisation thresholds is proposed for these PIs and provides insight into the future development of low threshold PIs for TPP.