Multicomponent synthesis of 7-(diethylamino)coumarin–pyrrolo[3,4-b]pyridin-5-one conjugates and modulation of their twisted intramolecular charge transfer (TICT) processes

By coupling an Ugi-Zhu three-component reaction to a cascade sequence (aza Diels-Alder cycloaddition/N-acylation/decarboxylation/dehydration) into a one-pot process, seven new 7-(diethylamino)coumarin-pyrrolo[3,4-b]pyridin-5-one bis-heterocyclic conjugates were synthesized in moderate overall yields (15–38 %). Next, the photophysical properties of all products were determined using UV–Vis and fluorescence spectroscopy. These products exhibited fluorescence quantum yields (FQY) ranging from 17 % to 25 %. In contrast, one of their precursors, the free 7-(diethylamino)-3-carbaldehyde displayed a low FQY of 2 %, and, also exhibited dual emission across all solvents used for the measurements. The increase in quantum yield observed upon incorporation of the coumarin moiety into MCR products was attributed to the inhibition of the twisted intramolecular charge transfer (TICT) process. This one naturally occurs in the free coumarin but was suppressed in the products due to a decrease in the acceptor strength of the substituent at position 3 of the coumarin moiety. Furthermore, the electronic structure of the compounds was computed by DFT/TD-DFT calculations allowing to determine which molecular orbitals (MO) were involved in the electronic transitions.