PhotophysicalInvestigationsonPhotoinitiatorswithCovalentlyLinked Thioxanthone Sensitizer Moieties by KurtDietliker and StephanieBroillet 1 ) Ciba Specialty Chemicals Inc. , Schwarzwaldallee 215, CH-4002 Basel and Bruno Hellrung, Piotr Rzadek, GuentherRist, and JakobWirz Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel and DmytroNeshchadin and Georg Gescheidt* Institute of Physical and Theoretical Chemistry, Graz University of Technology, Technikerstrasse 4/I, A-8010 Graz In memoriam Hanns Fischer The photophysical properties of three photoinitiators with a covalently linked thioxanthone sensi- tizer unit absorbing up to 410 nm were investigated by laser-flash photolysis and CIDNP spectroscopy. These complementary techniques revealed two competing reaction pathways of the molecular dyads 1 – 3 : i) triplet-energy transfer from the sensitizer to the morpholine moiety followed by a-cleavage to yield a radical pair, which initiates radical polymerization, and ii) bimolecular electron transfer from the morpholine to the thioxanthone subunit followed by proton transfer. The relative efficiency of these routes is determined by the triplet energy of the photoinitiator moiety relative to that of the sensi- tizer. 1.Introduction. – Photoinitiators have been successfully utilized in the production of polymeric materials by radiation curing. Industrial applications of this technology providing considerable economic and ecological advantages are coatings, printing inks, electronic materials, as well as three-dimensional models and tools [1 – 3]. Although such initiators are rather efficient [4 – 12], it is possible to further enhance their activity by the addition of sensitizers [11] [13]. This is especially useful when the sensitizer can be excited with light of different wavelengths than the photoinitiator, thereby adapting the action spectrum to specific requirements of the application. A typ- ical case is the curing of pigmented formulations, where the excitation of the photoini- tiator has to be performed in a transmission window of the pigment to allow sufficient penetration of light into the coating layer. While sensitization is usually achieved through diffusive encounters, we introduce here photoinitiators for radical polymeriza- 1 ) Current address: UCB Farchim SA, Z. I. de Planchy, 10, Chemin de Croix Blanche 1, CH-1630 Bulle. # 2006 Verlag Helvetica Chimica Acta AG, Zürich Helvetica Chimica Acta – Vol. 89 (2006) 2211