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

Abstract

SummaryPhenothiazinyl 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.

Review on the Ugi Multicomponent Reaction Mechanism and the Use of Fluorescent Derivatives as Functional Chromophores

“…This strategy has been used, for instance, in the synthesis of 14 derivatives of donor−acceptor systems where the phenothiazine acts as the donor, whereas the 9,10anthraquinone is the acceptor moiety (Scheme 12). 19 The absorption characteristics of the synthesized compounds have been evaluated and showed similar behavior to a system lacking electronic communication with the acceptor moiety in the ground state. In the excited state a quenching could account for the fast electron transfer from the donor to the acceptor as depicted by femtosecond spectroscopy.…”

Phase Behavior of a New Class of Anthraquinone-Based Discotic Liquid Crystals

“…In the case of compound 4.1 , a strong absorption band at 313 nm was observed (Figure ) which corresponds to π–π* transition and a less intense peak at 403 nm indicates the n−π* (forbidden) transition of the central anthraquinone core . The other shoulder peak at 295 nm may be because of the π–π* transition of the benzene ring, as shown in Figure .…”

“…In the case of compound 4.1, a strong absorption band at 313 nm was observed (Figure 8) which corresponds to π−π* transition and a less intense peak at 403 nm indicates the n−π* (forbidden) transition of the central anthraquinone core. 39 The other shoulder peak at 295 nm may be because of the π−π* transition of the benzene ring, as shown in Figure 8. Interestingly, for compound 4.4 with dodecyloxy chain length, the absorption maximum (λ max ) was observed at 292 nm related to the π−π* transition of the benzene ring connected through a triple bond with the central anthraquinone core.…”

Unimolecular Exciplexes by Ugi Four-Component Reaction

“…The Ugi four-component reaction (4CR) (Dömling and Ugi, 2000; Dömling, 2006; Biggs-Houck et al, 2010) generates the chemically robust α-aminoacyl amide scaffold with a high level of diversity and has widely and successfully been applied in biological and medicinal chemistry (Dömling et al, 2012). Encouraged by Ugi 4CR syntheses of a phenothiazine-anthraquinone-based PIET system and related dyads (Bay et al, 2013, 2014; Bay and Müller, 2014), characterized by cyclic voltammetry, steady-state UV/vis, and fluorescence spectroscopy, as well as femtosecond transient absorption spectroscopy for identification of the desired charge separated state after light excitation, we decided to transpose the Ugi 4CR to a novel synthesis of N,N -dimethylaniline-anthracene dyads, which are known to form exciplex emitters (Hui and Ware, 1976).…”

“…As part of our program to methodologically develop concise syntheses of functional chromophores by multicomponent reactions (MCR) (Levi and Müller, 2016a), we are particularly interested in MCR syntheses of fluorophores (Levi and Müller, 2016b; Riva et al, 2016; Merkt and Müller, 2018; Müller, 2018) and donor-acceptor systems that interact in the excited state via photo-induced electron transfer (PIET) (Kavarnos, 1993; Lemmetyinen et al, 2011; Wenger, 2011; Ricks et al, 2012; Vauthey, 2012), as shown by non-radiative fluorescence quenching (Bucci and Müller, 2006; Bay et al, 2013, 2014; Bay and Müller, 2014). This can be considered as chromophore-chromophore interactions at short distances, that is, between Dexter and Förster radii.…”