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

Abstract

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.

Keywords: absorption spectroscopy; chromophores; cyclic voltammetry; fluorescence; multicomponent reactions; photo-induced electron transfer.

Figure 1

Phenothiazine–anthraquinone dyad 1, donor-only (2) and acceptor-only (3) models assembled by Ugi 4CR.

Scheme 1

Ugi 4CR synthesis of donor–anthraquinone dyads 8.

Scheme 2

Ugi 4CR synthesis of donor-only reference systems 10.

Figure 2

Molecular structure of S(O)-1 (left) (30% ellipsoids, except for the CH₃CH₂ end of the hexyl group, the disordered water molecules were omitted for clarity) and the intra- and intermolecular π–π-stacking interactions between an inversion symmetry related pair (right, hydrogen atoms were omitted for clarity).

Figure 3

Cyclic voltammogram of dyad 8c (recorded in CH₂Cl₂, T = 298 K, c (8c) = 0.1 mol·L⁻¹, Pt working electrode, Pt counter electrode, Ag/AgCl reference electrode, electrolyte N(n-Bu)₄PF₆, scan rate of 250 mV·s⁻¹).

Figure 4

DFT-computed (B3LYP, 6-311G*) frontier molecular orbitals HOMO (bottom) and LUMO (top) of the phenothiazine–anthraquinone dyad 1.

Figure 5

Normalized absorption spectra of the phenothiazine–anthraquinone dyad 8c (recorded in CH₂Cl₂, c (8c) = 2.5∙10⁻⁵ mol·L⁻¹, T = 298 K).

Figure 6

Absorption spectra of Do–anthraquinone dyads 8c (top) and 8e (bottom) with the corresponding references 3 and 10, and their addition spectra (Do + Acc) (recorded in CH₂Cl₂, T = 298 K).

Figure 7

Normalized absorption and emission spectra of Ugi-donor compounds 2 and 10 (recorded in CH₂Cl₂, T = 298 K, λ_max,exc (2) = 311 nm, λ_max,exc (10a) = 320 nm, λ_max,exc (10b) = 298 nm).

Figure 8

Emission spectra of donor-only system 2 phenothiazine–anthraquinone dyads 8a,b (top), and the donor-only system 10a and the phenothiazine–anthraquinone dyads 8c,d (bottom) (recorded in CH₂Cl₂, T = 298 K, c = 0.7–2.9∙10⁻⁶ mol·L⁻¹).