Nanooptics and photonics

Immobilization-induced enhancement of S2 fluorescence of near-infrared dyes due to interaction with carbon quantum dots
Dimitriev O.P.1, Piryatinski Yu.P.2, Sharanov I.P.3, Slominskii Yu.L.3
1 V.Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, pr. Nauki 41, Kyiv, Ukraine. 
E-mail: dimitr@isp.kiev.ua
2 Institute of Physics NAS of Ukraine, pr. Nauki 46, Kyiv, Ukraine.
3 Institute of Organic Chemistry NAS of Ukraine, 5 Murmanska St., Kyiv, Ukraine.

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 (NIR) region, but which are spectrally silent in the visible, is a challenging task for relevant applications in actual fields, such as energy conversion, bio-imaging, sensitization of solar cells, optical sensors, etc. Here we demonstrate a dual emission of NIR tricarbocyanine dyes with a bright green S2 fluorescence, which is relatively weak for the free dye molecule but 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 above spectral changes are shown to depend on the terminal and meso-groups of the dye and the solvent used. The dye immobilization due to interaction with carbon quantum dots is shown to cause freezing of rotational degrees of freedom of the molecule as indicated by suppression of the dye hot band absorption-assisted anti-Stokes S1 emission. The observed enhancement of the S2 fluorescence and its rate is discussed in terms of immobilization-induced emission and a Purcell effect, respectively.