Quantum Yield of Conversion of The Dental Photoinitiator Camphorquinone Yin-Chu Chen a , Jack L. Ferracane b , Scott A. Prahl a a Department of Biomedical Engineering, OHSU, Portland, OR b Division of Biomaterials & Biomechanics, OHSU, Portland, OR ABSTRACT The primary absorber in dental resins is the photoinitiators, which start the photo polymerization process. We studied the quantum yield of conversion of camphorquinone (CQ), a blue light photoinitiator, using 3M FreeLight LED lamp as the light curing unit. The molar extinction coefficient, ε 469 , of CQ was measured to be 46±2 cm −1 /(mol/L) at 469 nm. The absorption coefficient change to the radiant exposure was measured at three different irradiances. The relationship between the CQ absorption coefficient and curing lamp radiant exposure was the same for different irradiances and fit an exponential function: μ a469 (H)= μ ao exp(-H/H threshold ), where μ ao is 4.46±0.05 cm −1 , and H threshold =43±4 J/cm 2 . Combining this exponential relationship with CQ molar extinction coefficient and the absorbed photon energy (i.e., the product of the radiant exposure with the absorption coefficient), we plotted CQ concentration [number of molecules/cm 3 ] as a function of the accumulated absorbed photons per volume. The slope of the relationship is the quantum yield of the CQ conversion. Therefore, in our formulation (0.7 w% CQ with reducing agents 0.35 w% DMAEMA and 0.05 w% BHT) the quantum yield was solved to be 0.07±0.01 CQ conversion per absorbed photon. Keywords: Photo-cured dental composite, curing efficiency, molar extinction coefficient 1. INTRODUCTION Photo-cured composites have been widely used in dental restorations. 1 Generally, a composite consists of a mixture of resins with photoinitiators and silane-coated, inorganic filler particles. The photoinitiator absorbs light, and is promoted to an excited state that interacts with a photoreducer (a electron or proton donor molecule) to initiate a free radical addition polymerization of the resin monomers. Camphorquinone (CQ), a blue light photoinitiator, is commonly used in dental resin formulations. 2 CQ is di-2,3-diketo-1,7,7-trimethylnorcamphane with molecular weight of 166.2 and has an absorption peak around 469 nm. The photo-curing efficiency, defined as extent of cure per delivered photon, has been widely discussed in photo- cured composite systems by evaluating the extent of cure or curing depth for different composite formulations 3–5 or for different light curing units. 6–8 However, these studies were specific to a particular combination of curing units and materials and lacked the information of the actual number of photons absorbed for extent of cure. Some studies 7, 8 suggested a “integrated relative curing potential” (ICPrel) parameter defined as ICPrel =  λ2 λ1 E(λ)A(λ)dλ , where E(λ) is the spectral irradiance of the curing unit, A(λ) is the relative absorbance of photoinitiator, and λ 1 –λ 2 is the wavelength range of the curing unit. In fact, if we substitute above A(λ) with the absorption coefficient μ a (λ) of the photoinitiator, above equation represents the total absorbed energy per unit volume in the material (according to the CIE/ISO definition 9 ). This parameter gives the effective photon absorption in the material. However, not all the light absorbed by the materials is equally effective at inducing polymerization. The primary absorption in resins is by the photoinitiator and the absorption drops during the curing process, 10 which, in turn, may decrease the polymerization rate. Further author information: (Send correspondence to S. A. P.) Y.-C. C.: E-mail: yinchu@bme.ogi.edu, Tel: (503) 216-6830 S. A. P.: E-mail: prahl@bme.ogi.edu, Tel: (503) 216-2197 Proceedings of SPIE Vol. 5771, p. 256-266, Saratov Fall Meeting 2004: Optical Technologies in Biophysics and Medicine VI; Valery V. Tuchin; Ed.