Thioxanthone-benzothiophenes as photoinitiator for free radical polymerization $ Nurcan Karaca a , Nuket Ocal a , Nergis Arsu a, *, Steffen Jockusch b, * a Department of Chemistry, Yildiz Technical University, Davutpasa Campus, Istanbul 34210, Turkey b Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027, United States A R T I C L E I N F O Article history: Received 3 November 2015 Received in revised form 12 January 2016 Accepted 14 January 2016 Available online xxx Keywords: Thioxanthone Photoinitiator Photopolymerization Laser flash Photolysis A B S T R A C T Photoinitiators for free radical polymerization based on the thioxanthone chromophore that contains benzothiophene were synthesized and characterized. Compared to thioxanthone, these compounds show a bathochromic shifted absorption up to 460 nm. High quantum yields for intersystem crossing generate sufficient amounts of triplet states. Initiator radicals are generated by reaction of the triplet states with tertiary amines, such as diethanolamine with high rate constants (2–6  10 9 M 1 s 1 ) as determined by laser flash photolysis. Photoinitiated polymerization experiments of MMA showed efficient polymerization with initiator concentrations as low as 0.1 mM. ã 2016 Elsevier B.V. All rights reserved. 1. Introduction Light-mediated polymerizations have gained importance in recent years, especially for surface coatings and surface patterning. The key ingredient in these photopolymerization formulations is the photoinitiator [1–3]. An important criterion for selection of the photoinitiator is a good match of the absorption properties of the photoinitiator with the spectral characteristics of the light source. The most popular photo-curing tools contain light sources with emission in the near-UV spectral region (350–400 nm) [2]. Bisacylphosphine oxides and benzoin based photoinitiators absorb in this spectral region, but with low molar absorptivities. However, thioxanthone based photoinitiators often show molar absorptiv- ities of 4000 M 1 cm 1 and higher at 380 nm [4]. In thioxanthone based photoinitiators, free radicals for the initiation of radical polymerization are in most cases generated in the reaction of triplet excited states of thioxanthones with a co-initiator, such as amines (Eq. (1)). The initial electron transfer reaction generates a radical ion pair, which rapidly undergoes a proton transfer resulting in carbon centered radicals [5]. In addition to bimolecular reactions to generate initiator radicals from thioxanthone derivatives [4,6], there are a few examples were initiator radicals are generated in an $ Dedicated to Professor Yoshihisa Inoue for his retirement from Osaka University and his contribution to photochemistry. * Corresponding authors. E-mail addresses: narsu@yieldiz.edu.tr (N. Arsu), sj67@columbia.edu (S. Jockusch). http://dx.doi.org/10.1016/j.jphotochem.2016.01.017 1010-6030/ ã 2016 Elsevier B.V. All rights reserved. Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx G Model JPC 10116 No. of Pages 7 Please cite this article in press as: N. Karaca, et al., Thioxanthone-benzothiophenes as photoinitiator for free radical polymerization, J. Photochem. Photobiol. A: Chem. (2016), http://dx.doi.org/10.1016/j.jphotochem.2016.01.017 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal homepa ge: www.elsev ier.com/locate/jphotochem