The Ugi four-component reaction as a concise modular synthetic tool for photo-induced electron transfer donor-anthraquinone dyads

Phenothiazinyl and carbazolyl-donor moieties can be covalently coupled to an anthraquinone acceptor unit through an Ugi four-component reaction in a rapid, highly convergent fashion and with moderate to good yields. These novel donor-acceptor dyads are electronically decoupled in the electronic ground state according to UV-vis spectroscopy and cyclic voltammetry. However, in the excited state the inherent donor luminescence is efficiently quenched. Previously performed femtosecond spectroscopic measurements account for a rapid exergonic depopulation of the excited singlet states into a charge-separated state. Calculations of the Gibbs energy of photo-induced electron transfer from readily available UV-vis spectroscopic and cyclovoltammetric data applying the Weller approximation enables a quick evaluation of these novel donor-acceptor dyads. In addition, the X-ray structure of a phenothiazinyl-anthraquinone dyad supports short donor-acceptor distances by an intramolecular π-stacking conformation, an important assumption also implied in the calculations of the Gibbs energies according to the Weller approximation.