Optimization of a photocyclic initiating system for visible light using of holographic recording and RT-FTIR experiments

Light induced polymerization reaction “photopolymerization” is employed in quite different technical applications that have become beneficial to humans. These applications include microelectronics, information technologies, optical fibers, dental materials, printing inks, paints, varnishes.

Among these applications, photopolymer materials have revealed to be the best choice for holographic recording, promoting holography as a compelling choice for next-generation data storage needs.3 In holography and display devices, photopolymers were developed primarily for display of single images due to the refractive index variations or relief profiles generated by an optical interference pattern.4 Compared to inorganic photorefractive materials, photopolymers have much higher sensitivity due to chemical amplification effects, large dynamical range, high versatility of the formulations. Additionally, this is a relatively low cost one step process..

Tailoring of photopolymer materials is one specific application of visible light photoinitiating systems (PIS).2,4, Basically, visible light PIS comprises a dye chosen to absorb at a specific irradiation wavelength and a coinitiator that produces free radicals able to initiate the polymerization reaction.2, However, these two-component PIS have moderate photopolymerization efficiency, and the addition of a third component was found to be beneficial for improving the performance of these systems. In that case, secondary redox reactions take place after the primary photochemical reaction, and a higher yield of initiating species is obtained. Therefore, three-component photoinitiating systems have been found to be by far more efficient than corresponding two-component systems.2.

Pyrromethene dyes present encouraging properties as efficient photosensitizers. Their photophysical properties are well known and they show attractive laser performance.11-13 Strong absorption and fluorescence bands in the visible region were observed for this family of dyes.14-17 Many applications were developed with these compounds in various fields such as optoelectronics,18-19 visible laser imaging 14,20 and photoinitiators for free radical photopolymerization.

In this paper, a photopolymerizable resin optimized as an efficient holographic recording medium was used to probe the reactivity of photoinitiating systems based on a pyrromethene dye (EMP). The efficiency of the combination of three different coinitiators considering an amine (EDB) as electron donor and an electron acceptor which could be either a triazine derivative (TA) or an iodonium salt (I250) was investigated. Real-time FTIR spectroscopy (RT-FTIR) was used to characterize the photopolymerization kinetics of these systems by following the evolution of monomer conversion during the photopolymerization reaction. The influence of the experimental conditions (irradiation intensity and irradiation time) on the performance of these PIS using holographic recording was also examined by recording the diffraction efficiency as a function of irradiation time. Moreover, time resolved spectroscopy experiments were used to describe the behaviour of the different photoinitiating systems and to propose the mechanism that takes places.