"Awakening" of the S2 Emission of Tricarbocyanine Dyes Stimulated by Interaction with Carbon Quantum Dots.

Abstract

Anti-Kasha emission (i.e., the emission from Sn (n > 1) excited levels) of infrared chromophores which possess intensive absorption and S1 emission in the near-infrared region, but which are spectrally silent in the visible, is a challenging task for relevant applications such as energy conversion, bioimaging, sensitization of solar cells, optical sensors, and so on. Here we demonstrate a dual emission of near-infrared tricarbocyanine dyes with a bright green S2 fluorescence, whose quantum yield increases by 2-4 times together with a strong enhancement of the spontaneous rate of S2 fluorescence, whereas the quantum yield of S1 emission decreases by 2-7 times, respectively, as a result of immobilization of the dye molecule via interaction with carbon quantum dots. The enhanced immobilization-induced S2 emission is shown to occur because of planarization of the molecule and freezing its rotational degrees of freedom as indicated by suppression of the dye hot-band absorption-assisted anti-Stokes S1 emission.