Conjugated Bifunctional Carbazole-Based Oxime Esters: Efficient and Versatile Photoinitiators for 3D Printing under One- and Two-Photon Excitation

The rapid development of 3D printing using visible light as an irradiation source requires efficient visible light photoinitiators to realize a fast and reliable 3D printing process. Based on the photocleavage concept, a series of symmetric bifunctional photoinitiators comprising oxime-ester moieties as latent initiation functionalities and conjugated carbazole as chromophores was synthesized. The impact of substitution position and the types of radicals on photophysical and photochemical behavior was investigated via experimental tests combined with quantum-chemical calculations. The conjugated carbazole-based oxime esters exhibit broad absorption bands from 250 to 400 nm and an interesting photobleaching property was observed during photolysis. Under 405 nm LED irradiation, the photoinitiation efficiency of the novel photoinitiators is comparable to that of the commercial visible light photoinitiator TPO. A vertical resolution of 50 mu m was obtained when using oxime esters as photoinitiators in DLP 3D printing to fabricate delicate objects. The A-pi-D-pi-A core structures impart sufficient two-photon cross sections (102-136 GM) to the oxime esters, enabling the construction of complex 3D microstructures of nanometer spatial resolution with two-photon 3D printing technology. Notably, the threshold energy of the formulation containing 4d is lower than that of the commercial two-photon resists IP-L. Also, the thermal stability of the PIs (>160 degrees C) is sufficient for daily storage.