Rationalizing substituent effects in 1-azathioxanthone photophysics

Abstract

The influence of an electron donating substituent on the photophysical properties of 1-azathioxanthone dyes has been investigated using optical spectroscopy and theoretical models. The motivation behind the study is based on the fact that thioxanthones are efficient triplet sensitizers, and thus promising sensitizers for lanthanide centered emission. By adding an aza group to one of the phenyl ring systems, direct coordination to a lanthanide center becomes possible, which makes azathoixanthones great candidates as antenna chromophores in lanthanide(III) based dyes. Here, three 1-azathioxanthone derivatives have been synthesized targeting efficient triplet formation following absorption in the visible range of the spectrum. This is achieved by adding methoxy groups to the 1-azathioxanthone core. The derivatives were characterized using absorption, emission, and time-gated emission spectroscopy, where fluorescent quantum yields, singlet and triplet excited states lifetimes were determined. The experimentally determined photophysical properties of the three 1-azathioxanthone compounds are contrasted to those of the parent thioxanthone and is rationalized using the Strickler-Berg equation, Hückel MO theory, and Dewar's rules in combination with computational chemistry. We find that the transition energies follow predictions, but that the overall photophysical properties are determined by the relative energies as well as the nature of the involved states in both the singlet and the triplet excited state manifolds.

π‐Expanded Thioxanthones – Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide‐Based Luminescent Probes with Visible Excitation

“…Methyl substitution and addition of aza‐bridges do not change the photophysics of the parent thioxanthone chromophore . We have previously explored the effect on the triplet state by adding electron donating groups to the azathioxanthone scaffold, see Scheme compounds 8 a – c . The results confirmed that the absorption and fluorescent properties of the thioxanthone chromophore (S 1 ) behave as predicted by extended Hackle theory (see the Supporting Information for details).…”

“…Even though the thioxanthone chromophore has been explored as a sensitizer for lanthanide(III) centre emission, and though thioxanthones with various substitution patterns have been made, thioxanthone derivatives with strong absorption beyond 400 nm and a small singlet‐triplet gap have yet to be identified.…”

“…Table includes two donor substituted azathioxanthones, 8 b with a red‐shifted main absorption band and 8 c with a blue‐shifted main absorption band. While the addition of donor groups in the correct positions achieve the desired redshift, the result is a strongly fluorescent dye. As cursory inspection of Table reveals, this is not an issue with the π‐extended thioxanthones that all remain poor fluorophores with similar photophysics to the parent thioxanthone chromophore.…”

“…One of the most explored chromophores is thioxanthone ( 1 , Scheme ),– a chromophore that has also been shown to be one of the best as a sensitizer for lanthanide centred emission, in particular the coordinating azathioxanthones . Methyl substitution and addition of aza‐bridges do not change the photophysics of the parent thioxanthone chromophore . We have previously explored the effect on the triplet state by adding electron donating groups to the azathioxanthone scaffold, see Scheme compounds 8 a – c .…”

“…The results confirmed that the absorption and fluorescent properties of the thioxanthone chromophore (S 1 ) behave as predicted by extended Hackle theory (see the Supporting Information for details). Unfortunately, the properties of the phosphorescent T 1 state are less easily predicted, and the addition of electron donating group in 8 b results in the desired redshift, but also unfavorable strong fluorescence . Here, we chose to modulate the photophysical properties of thioxanthone by expanding the conjugated system of thioxanthone as illustrated in Scheme compounds 2 , 3 , and 9 .…”