Enhanced synergistic catalysis by a light-harvesting binary organic dyes system based on FRET for cross-dehydrogenative-coupling reaction

Abstract

Rose bengal (RB) showed photocatalytic performance for cross dehydrogenative-coupling (CDC) reaction. However, single organic dye only absorbs light of specific wavelength which results a lower light utilization efficiency. In this paper, we proposed a simple and effective approach based on Förster resonance energy transfer (FRET), to improve the performance of RB through the incorporation of rhodamine 6G (Rh6G) into RB solution and constructing a binary dyes system rhodamine 6G-rose bengal (Rh6G-RB). The energy transfer efficiency and antenna effect of Rh6G-RB were up to 58% and 10.8, respectively. The synergistic effect of RB and Rh6G exhibited excellent catalytic performance under low concentration (10−5 M). The catalytic rate constant (kd) for Rh6G-RB reached 0.026 mol/(L·min) at 35 °C for aza-Henry reaction of tertiary amines with nitromethane, which was 2.03 and 5.52 times higher than that of single RB and Rh6G respectively. Moreover, the synergy of Rh6G and RB was also observed in Mannich reaction of N-tertiary amine derivatives with acetone, which all afforded the coupling products with good to excellent conversions. Finally, the Rh6G-RB system was also demonstrated an efficient visible light photoredox catalyst in continuous flow fashion. The research opened a new avenue and convenient for developing of binary or ternary organic dyes systems based on FRET and advance the application of these dye systems in photocatalytic organic synthesis.