Tetraacylgermanes as highly efficient photoinitiators for visible light cured dimethacrylate resins and dental composites Norbert Moszner, 1 Urs Karl Fischer, 1 Iris Lamparth, 1 Pascal F € assler, 1 Judith Radebner, 2 Anna Eibel, 3 Michael Haas, 2 Georg Gescheidt, 3 Harald Stueger 2 1 Ivoclar Vivadent AG, Schaan FL-9494, Liechtenstein 2 Institute of Inorganic Chemistry, Technische Universit€ at Graz, Graz A-8010, Syria 3 Institute of Physical and Theoretical Chemistry, Technische Universit€ at Graz, Graz A-8010, Syria Correspondence to: N. Moszner (E - mail: norbert.moszner@ivoclarvivadent.com) ABSTRACT: Tetrabenzoylgermane 1 and various substituted tetrabenzoylgermanes 2–7 were investigated as visible light (VL) photoini- tiators (PIs) for dental dimethacrylate resins and dimethacrylate-based composites. The tetrabenzoylgermanes 1–7 show a very strong VL absorption between 400 and 450 nm. Substituents on the benzoyl chromophore strongly influence their properties such as melting point, solubility, absorption behavior, or PI reactivity. A good photobleaching behavior and a very high reactivity as VL PI was found in photo-differential scanning calorimeter experiments for selected tetrabenzoylgermanes. Composite pastes containing only 0.1 wt % of Ge-PI exhibited a sufficient photocuring due to the high PI-reactivity of the tetraacylgermanes. Among the investigated germane PIs, tetrakis(2-methylbenzoyl)germane 2 shows the best performance as VL PI for restorative composites and enables the composites to be photocured using an LED with an emission maximum of 500 nm. V C 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46115. KEYWORDS: composites; photopolymerization; radical polymerization; resins Received 23 November 2017; accepted 28 November 2017 DOI: 10.1002/app.46115 INTRODUCTION Today, durable esthetic tooth-colored resin-based filling compo- sites in combination with efficient enamel-dentin adhesives are very popular and play an important role in modern dentistry. Resin-based dental composites are used as luting cements (LCs) and as restorative filling materials. LCs with a filler content between 55 and 70 wt % are used for the cementation of indi- rect restorations such as inlays, onlays, crowns, or veneers and show high tensile and compressive strengths as well as the low- est solubility compared to conventional cements, such as glass ionomer cements. Different resin-based direct filling composites are distinguished mainly by their viscosity in condensable/pack- able composites and flowable composites. Packable composites exhibit a higher viscosity and filler load (75–85 wt %), are con- densable like amalgam, and enable the formation of tight inter- proximal contacts. Flowable composites are characterized by a lower viscosity and filler load (65–75 wt %) and have the ability to be syringed into a cavity preparation with a needle tip. 1,2 Recently, bulk–cure composites have been developed. They are distinguished by a shorter curing time and improved curing depth. 3 All these direct restorative composites are mainly based on mixtures of different fillers, crosslinking and functionalized methacrylates, and are mostly cured by visible light (VL) photoinitiators (PIs). 4 Most of the VL cured dental materials are still based on bimolecular PI systems, which are combina- tions of camphorquinone (CQ) with suitable tertiary amines (A) as coinitiator. 5 CQ-based PI systems allow fast curing of rel- atively thick layers, compared to ultraviolet (UV) curing. How- ever, disadvantages of the CQ/A PIs concern the yellowing of the cured materials caused by the oxidation of amine impurities and the relatively low quantum yield of radical formation. Fur- thermore, in CQ/A PIs as bimolecular photoinitiating systems, the interaction of the partners is strongly influenced by the vis- cosity of the medium and therefore such binary PIs tend to generate significant inhibition layers. Finally, in water-based acidic dental formulations, such as enamel–dentin adhesives, the acid–base reaction of the frequently used strongly acidic adhe- sive monomers with the amine coinitiator affects the initiating efficiency of the CQ/A PIs. 6 In this context, it could be shown that benzoyltrimethylgermane (BTMGe) and dibenzoyldiethyl- germane can be used as VL PIs for the photopolymerization of dimethacrylate mixtures and dimethacrylate-based compo- sites. 7–10 Further structure optimizations resulted in the devel- opment of the PI bis-(4-methoxybenzoyl)diethylgermane (BMBDGe), which was commercialized under the brand name Ivocerin. 11,12 The bisacyldialkylgermanes are amine-free mono- molecular PIs, which show very good solubility in V C 2017 Wiley Periodicals, Inc. J. APPL. POLYM. SCI. 2018, DOI: 10.1002/APP.46115 46115 (1 of 7)