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M. Tromayer, M. Markovic, P. Gruber, K. Hölzl, W. Steiger, S. Naumov, T. Scherzer, J. Stampfl, A. Ovsianikov, R. Liska:
"Novel Two-Photon Initiators Based on Cleavable Diazosulfones";
Poster: 4th European Symposium of Photopolymer Science, Leipzig; 09-11-2016 - 09-14-2016; in: "ESPS 2016", (2016), P-21.

The process of two-photon polymerization (2PP) enables 3D free-form stereolithography with a resolution in the sub-micrometre range, used in rapid prototyping, the production of scaffolds for tissue engineering, photonic crystals, optical waveguides and microelectronic components.[1] Low achievable writing speeds and lack of highly efficient photoinitiators have been a critical obstruction for further advances in the development of 2PP. The proposed initiation mechanism of 2PP suggests radical formation in a bimolecular interaction between two-photon initiator (2PI) and the monomer. While highly reactive commercial one-photon initiators rely on efficient radical formation via a direct monomolecular cleavage process, their applicability in 2PP is limited by low two-photon absorption (2PA) cross sections (σ2PA).

Aryl diazosulfones exhibit strong absorption in the visible range, which could also be excited by 2PA at 800nm. While they have been used as radical polymerization initiators before[2] where they are cleaved either photolytically or thermolytically under release of nitrogen gas and generation of phenyl- and sulfonyl based radicals, their use in 2PP is novel and promising because of their ability to generate radicals independently of the monomer. Furthermore, due to fast cleavage mechanism under decomposition of the photoactive chromophore, a reduction of quenching by oxygen and bimolecular generation of reactive oxygen species is expected, which would be advantageous regarding biological applications, such as 2PP in the presence of living cells.
In this work, several diazosulfone-based compounds were synthesized and their potential as 2PI assessed. Investigations of structure-activity relationships were conducted via quantum chemical calculations and experimental tests including z-scan, UV-VIS spectroscopy, 2PP structuring tests and in some cases cytotoxicity assays.