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

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 (lambda(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,phi(Delta), is up to 33.3%), whereas the monoamino-AQ derivatives show negligible phi(Delta) 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 mu s. This relatively short triplet excited state lifetime is attributed to the n-pi* character of the T-1 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 T-1 state with n-pi* character can be developed as efficient photoinitiators for radical polymerization of alkene monomers.

Automated summary

We synthesized a series of amino-substituted 9,10-anthraquinone derivatives to investigate their photophysical properties, particularly the intersystem crossing (ISC) efficiency and the triplet excited states. Monoamino-substituted derivatives exhibited a red-shifted absorption band at 509 nm compared to chloro-anthraquinone derivatives (lambda(abs) = 340 nm). Interestingly, additional amino substituents on the anthraquinone chromophore, i.e. diamino-substituted derivatives, did not induce further red-shifting of the absorption band. Only diamino derivatives showed decent ISC efficiency (singlet oxygen quantum yield, phi(Delta), up to 33.3%), while monoamino derivatives exhibited negligible phi(Delta) values. Nanosecond transient absorption spectra confirmed the formation of a triplet excited state with a lifetime of approximately 2.5 mu s. This short triplet excited state lifetime was attributed to the n-pi* character of the T-1 states in the diamino-substituted derivatives, as supported by DFT computations. We demonstrated that bisamino-substituted anthraquinones can serve as efficient photoinitiators for photopolymerization of alkene monomers, whereas derivatives with poor ISC capability showed minimal photopolymerization effects. The mechanism likely involves hydrogen abstraction by the photoinitiators in their triplet excited states. These findings suggest that compounds exhibiting efficient ISC and T-1 states with n-pi* character could be developed as efficient photoinitiators for radical polymerization of alkene monomers.