No abstract

In this paper, we have investigated the use of poly(aryl ether) dendron-based gel as a medium for resonance energy transfer (RET) from organic donors (phenanthrene, naphthalene, and pyrene) to lanthanide [Eu(III) and Tb(III)] ions. The gel has been prepared through self-assembly of glucose-cored poly(aryl ether) dendrons in a dimethyl sulfoxide/water mixture (1:9 v/v). The efficiency of RET was calculated by metal-centered emission quantum yield measurements in the gel medium. While there was no resonance energy transfer observed between the donor-acceptor pairs in solution, efficient RET has been observed in the gel medium. The metal-centered quantum yield values were 11.9% for phenanthrene-Eu(III), 3.9% for naphthalene-Eu(III), and 3.6% for pyrene-Eu(III) systems. Partial RET in the system has been utilized to generate white-light emission from the gel by incorporating an additional lanthanide ion, Tb(III), along with the organic donors and Eu(III). The CIE (Commission Internationale d'Eclairage) coordinates obtained for gels formed by phenanthrene-Tb(III)-Eu(III) (PTE), naphthalene-Tb(III)-Eu(III) (NTE), and pyrene-Tb(III)-Eu(III) (PyTE) were (0.33, 0.32) for PTE, (0.35, 0.37) for NTE, and (0.35, 0.33) for PyTE. The correlated color temperatures (CCT) for white-light-emitting gels were calculated, and the values (5520 K for PTE, 4886 K for NTE, and 4722 K for PyTE) suggest that the system generates cool white light.

Fluorescence quenching of CdS quantum dots (QDs) by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Förster resonance energy transfer (FRET), is studied by steady-state and time-resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge-transfer interactions. The bimolecular quenching rate constant estimated from the Stern-Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions.

The light-harvesting properties of both CdSe and CdTe nanocrystals are ideally suited for their use in quantum dot (QD)-sensitized solar cells. However, corrosion of the CdTe QD in an aqueous environment in the presence of sulfide/polysulfide electrolyte renders it unsuitable despite its better electron injection ability (compared to CdSe QD) to a large band-gap semiconductor like TiO2. In this work, we explore the stability of a CdTe QD, which we have developed exclusively for its use in ionic liquids, in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid in the presence of S2– and investigate the hole transfer process from this photoexcited QD to S2–. We not only demonstrate that an appropriate capping of the CdTe QD and use of an ionic liquid in place of the aqueous medium enhances the stability of the QD significantly in the presence of S2– but also provide evidence of hole transfer from a photoexcited QD to the sulfide salt using steady-state and time-resolved emission and ultrafast transient absorption measurements.