Conjugated Ketocarbazoles as Efficient Photoinitiators: From Facile Synthesis to Efficient Two-photon Polymerization

Exploration of straightforward synthetic strategies to construct efficient two-photon initiators (2PIs) containing a long conjugate length is essential to promote the development of two-photon photopolymerization (2PP). This paper described a series of ketocarbazoles with C=C bonds as π bridges, cyclic ketones as electron acceptors, and 9-alkyl substituted carbazoles as electron donors. The conjugated 2PIs were prepared straightforwardly via classical aldol condensation reaction and the impact of the central ring size on the photoactivity was systematically studied. The maximum absorption of the 2PIs was located at ~440 nm and good photobleaching behaviors were observed. The initiator with cyclopentanone held the largest two-photon absorption cross section of 377 GM. All the conjugated ketocarbazoles can be used as efficient 2PIs to induced 2PP and exhibited lower threshold energy compared to the widely used commercial photoinitiator TPO and the two- photon resists IP-L.