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Purc, Anna,Koszarna, Beata,Iachina, Irina,Friese, Daniel H.,Tasior, Mariusz,Sobczyk, Krzysztof,Pę,dziń,ski, Tomasz,Brewer, Jonathan,Gryko, Daniel T. Royal Society of Chemistry 2017 ORGANIC CHEMISTRY FRONTIERS Vol.4 No.5
<▼1><P>Benzofuran has been proven to be the versatile substituent for tuning the optics of diketopyrrolopyrroles.</P></▼1><▼2><P>An in-depth investigation of the reaction of substituted salicylaldehydes with chloroacetonitrile led to the development of new conditions for the synthesis of 2-cyanobenzofurans. The crucial improvement lies in the use of phase-transfer catalysis in the second step, <I>i.e.</I>, intramolecular aldol type condensation. In a two-step process, the reactants were transformed into a library of 3,6-bis(benzofuran-2-yl)diketopyrrolopyrroles. We show that the presence of a methyl group in a position adjacent to the cyano functionality only slightly decreased the yield of diketopyrrolopyrroles (to 30–57%). An analysis of the relationship between the degree of polarization/planarization of aryl-diketopyrrolopyrroles and their one- and two-photon spectroscopic properties is reported. Careful design of the desired dyes and enhanced control of their ability to assume a planar molecular structure resulted in interesting photophysical properties, such as absorption and emission in the so-called biological window. Despite having less promising linear spectroscopic properties, the deplanarized molecules possess pretty strong two-photon absorbing properties. Placing methyl groups at adjacent positions to the linkage between benzofuran and the DPP core caused the formation of yellow-emitting dyes with almost quantitative fluorescence quantum yield, moderate Stokes shift and reasonable two-photon absorption cross-sections.</P></▼2>