During the last two decades research in photoinitiators concentrated on new cleavable structures, but only few systems were established on the market [1]. Modification of well known PIs gave improved performance properties in some cases, but the overall efficiency was not drastically changed. This could be explained by the limited reactivity of the formed radical towards polymerizable double bonds. New chromophores should not only increase this reactivity, also light absorption and performance properties should be adjusted.

By investigating pyridin analogous hydroxyalkylphenones, it has been found that the benzoyl moiety is replaceable by suitable chromophores [2-4].

Other work focuses on cross-conjugated photoinitiators. Beside high reactivity and low initiator concentrations (0.1 wt%), amine based coinitiators can be avoided [5-7]. Unusually high activity has been found in Two Photon induced Photopolymerization. The concept of triple bond containing initiators has also been realized in the field of cationic initiators based on sulfonium and iodonium salts.

Nowadays, especially long wavelength initiators are of high interest, not only for dental composites but also in the field of ceramic additive manufacturing.

[1] R. Liska: 'New chromophores for photoinitiators' (bookchapter) in "Photochemistry and UV Curing", Research Signpost (2006)

[2] R. Liska: "New photocleavable structures I: Synthesis of hydroxyalkylphenone analogues electron-rich heterocycles"; Heterocycles, 55 (2001), S. 1475 - 1486.

[3] R. Liska, D. Herzog: "New Photocleavable Structures: II. alpha-Cleavable Photoinitiators Based on Pyridines"; Journal of Polymer Science Part A: Polymer Chemistry, 42 (2004), S. 752 - 764.

[4] B. Seidl, R. Liska, G. Grabner: "New photocleavable structures III. Photochemistry and photophysics of pyridinol and benzoyl-based photoinitiators"; Journal of Photochemistry and Photobiology A - Chemistry, 180 (2006), S. 109 - 117.

[5] R. Liska, B. Seidl: "1,5-Diphenyl-1,4-diyn-3-one: A Highly Efficient Photoinitiator"; Journal of Polymer Science Part A: Polymer Chemistry, 43 (2005), S. 101 - 111.

[6] B. Seidl, R. Liska: "Mechanistic investigations on a diynone type photoinitiator"; Macromolecular Chemistry and Physics, 208 (2007), 1; S. 44 - 54.

[7] B. Seidl, K. Kalinyaprak-Icten, N. Fuß, M. Höfer, R. Liska: "Photoinitiators with Double and Triple Bonds"; Journal of Polymer Science Part A: Polymer Chemistry, 46 (2008), S. 289 - 301.