Preparation of amino-substituted anthraquinone: study of the intersystem crossing and application as efficient photoinitiators for photopolymerization

Abstract

We prepared a series of amino-substituted 9,10-anthraquinone (AQ) derivatives to study the photophysical properties, especially the intersystem crossing (ISC) efficiency and the triplet excited states. Monoamino-substituted AQ derivatives show red-shifted absorption bands centered at 509 nm, as compared to the chloro-AQ derivatives (λ_abs = 340 nm). Interestingly, attaching an extra amino substituents on the AQ chromophore, i.e. the diamino substituted AQ derivatives, does not induce further red-shifting of the absorption band. We found that only diamino AQ derivatives show decent ISC efficiency (singlet oxygen quantum yield,Φ_Δ, is up to 33.3%), whereas the monoamino-AQ derivatives show negligible Φ_Δ values. Nanosecond transient absorption spectra indicate the formation of a triplet excited state, and the triplet excited state lifetime was determined to be ca. 2.5 μs. This relatively short triplet excited state lifetime is attributed to the n–π* character of the T₁ states of the diamino substituted AQ derivatives, which is supported by DFT computations. We show that the bisamino-substituted AQs can be used as efficient photoinitiators for photopolymerization of alkene monomers, whereas the AQ derivatives with poor ISC capability only show a negligible photopolymerization effect. The mechanism is supposed to be hydrogen abstraction by the photoinitiators at triplet excited states. These results indicate that compounds showing efficient ISC and T₁ state with n–π* character can be developed as efficient photoinitiators for radical polymerization of alkene monomers.