Design of high efficiency for two-photon polymerization initiator: Combination of radical stabilization and large two-photon cross-section achieved by N-benzyl 3,6-bis(phenylethynyl)carbazole derivatives

Novel A-π-D-π-A V-shaped 3,6-bis(phenylethynyl)carbazole based chromophores were designed and synthesized as two-photon polymerization (TPP) initiators combining a large two-photon absorption cross-section with facilitated radical formation. 9-Benzyl-3,6-bis(4-nitrophenylethynyl)carbazole (4d) shows a strong two-photon absorption around 800 nm and exhibits very high two-photon polymerization initiating sensitivity with a threshold power of 0.8 mW at the concentration of 0.18 mol%, which is much lower than the threshold power of 6.37 mW found for benzil. The corresponding threshold of laser exposure intensity for TPP is 3.0 × 10⁷ mJ cm^-2. The lowest loading of 4d is up to 0.012 mol% with a threshold power of 3.2 mW.