The 2-benzoyl xanthone/triethylamine system as a type II photoinitiator: A laser flash photolysis and computational study [1]

2-Benzoylxanthone (BzX) was synthesized, characterized and used as type II photoinitiator (PI) in combination with triethylamine for the polymerization of methylmethacrylate (MMA). The photophysical/photochemical behaviour of the photoinitiator, the involved excited state and the reaction with the amine co-initiator was studied by means of absorption and nanosecond time-resolved absorption spectroscopy. Upon irradiation with 266 or 355 nm laser light, the triplet state 3BzX* (λmax = 355 nm and 530 nm) was generated as the only transient (lifetime of 22.7 μs) in nitrogen saturated MeCN solution. 3BzX* was confirmed through quenching experiments with oxygen, 2-methylbutadiene, perylene and MMA and spectral similarity to benzophenone triplet (3BP*). The quantum yield of its formation (ΦT = 0.8) and the molar absorption coefficient (ɛ = 7500 L mol−1 cm−1) was measured in MeCN. The triplet was photoreduced by triethylamine (TEA) via photoinduced electron/proton transfer giving the corresponding ketyl and α-amino ethyl radical (CHMe–NEt2). From the reduction potential of BzX measured via cyclic voltametry (two cathodic peaks at −1.63 V and −1.91 V vs. Ag/AgCl), an exergonic electron transfer reaction results. The ketyl radical resembles the well-known benzophenone ketyl. In conclusion, the triplet 3BzX* corresponds to an n → π* transition localized on the benzoyl substituent and resembles the benzophenone triplet 3BP* but deviates from the triplet state of xanthone (3X*). This is supported through DFT/B3LYP calculations, viz., (i) fully ground and triplet state geometry optimizations show that charge and spin densities are localized on the benzoyl group, and (ii) calculation of the electronic transitions via TD-DFT at B3LYP/631G+(d) and PB1BPE/631G+(d) levels of theory shows the local character. Agreement with experiment is better by applying the conductor like polarized continuum model (CPCM) to consider the solvent effect (MeCN).The effectiveness of BzX as photoinitiator for the polymerization of MMA is found to be double that of the unsubstituted xanthone (X). The photopolymerization rates (Rp) were found to be 7.06 × 10−4 mol L−1 s−1 in the case of BzX and 3.04 × 10−4 mol L−1 s−1 in the case of BX. This is attributed to the fact that triplet 3BzX* behaves like the benzophenone triplet 3BP* and deviates from that of 3X*, i.e., it is the localization of the triplet excitation on the benzoyl subunit which renders BzX a good type-II photoinitiator.

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► The bichromophoric aromatic ketone, 2-benzoyl-xanthone, was found to be much more effective type-II photoinitiator than the unsubstituted xanthone.
► The triplet state is localized to the benzoyl and not to xanthoyl subunit.
► Irrespective of which chromophore is originally excited, the lower energy triplet state is finally populated.
► The quantum mechanical calculations are in complete accordance to the experimental results.