Electrogenerated chemiluminescence of solutions, films, and nanoparticles of dithienylbenzothiadiazole-based donor-acceptor-donor red fluorophore. Fluorescence quenching study of organic nanoparticles.

ABSTRACT

We report here the electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) from a solution, film, and nanoparticles (NPs) of a red-emitting dithienylbenzothiadiazole molecular fluorophore [4,7-bis(4-(n-hexyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole, 1a], which has a donor-acceptor-donor configuration. In addition, the quenching of the fluorescence of the organic NPs by KI was investigated. The 1a film and NPs exhibit two absorbance peaks at 350 and ~504 nm that are red-shifted compared to those of 1a dissolved in solution (340 and 486 nm). Fluorescence quenching of 1a NPs does not follow a linear Stern-Volmer relationship; i.e., the fluorescence emission with excitation wavelength at either 350 or 504 nm decreased with increasing concentration of KI. Static quenching and heterogeneity related to the size distribution of the 1a NPs are proposed to explain the nonlinearity. A lifetime of 4.49 ± 0.04 ns was found for 1a organic NPs in water saturated with N2. After addition of KI, the fluorescence lifetime decreased to 3.1 ns. The fluorescence emission of 1a film/NPs is red-shifted (~17 nm) compared with that of 1a solution in dichloromethane (DCM). Solution ECL was generated in DCM through an annihilation reaction, while film and NP ECL could be generated in water through oxidation with a coreactant, tri-n-propylamine (TPrA). A film of 1a with thickness of 100-900 nm was prepared by drop-casting 1a in DCM on fluorine-doped tin oxide, and the ECL of the 1a film was found in phosphate-buffered saline solution with TPrA. Both 1a in solution and the 1a film produce strong ECL (I(film) = 0.14I(solution)). The ECL spectrum of 1a in solution, produced by electron-transfer annihilation of the reduced and oxidized forms, consists of a single peak with maximum emission at about 637 ± 4 nm, ~20 nm red-shifted from its fluorescence, while the ECL spectrum of 1a film produced by reaction with TPrA consists of a single peak with maximum emission at 642 ± 3 nm, a 10 nm red shift compared with the fluorescence of 1a film. Organic fluorescent 1a NPs were prepared by a reprecipitation method in water saturated with N2, and they were characterized by transmission electron microscopy, absorbance, fluorescence, and ECL. Strong ECL was also generated from the organic NPs in water by reduction with K2S2O8 coreactant.