Direct observation of triplet energy transfer from semiconductor nanocrystals

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Science  22 Jan 2016:
Vol. 351, Issue 6271, pp. 369-372
DOI: 10.1126/science.aad6378

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A different way to put triplets in play

Most molecules adopt a singlet spin configuration: All their electrons are arranged in pairs. Unpaired triplet states engage in a variety of useful reactions but are hard to produce. Quantum mechanics dictates that photo-excitation from singlet to triplet states is inefficient. Instead, chemists rely on sensitizers, which populate the triplet states of their neighbors through energy transfer after absorbing light themselves. Mongin et al. now show that certain nanoparticles can act as triplet sensitizers.

Science, this issue p. 369


Triplet excitons are pervasive in both organic and inorganic semiconductors but generally remain confined to the material in which they originate. We demonstrated by transient absorption spectroscopy that cadmium selenide semiconductor nanoparticles, selectively excited by green light, engage in interfacial Dexter-like triplet-triplet energy transfer with surface-anchored polyaromatic carboxylic acid acceptors, extending the excited-state lifetime by six orders of magnitude. Net triplet energy transfer also occurs from surface acceptors to freely diffusing molecular solutes, further extending the lifetime while sensitizing singlet oxygen in an aerated solution. The successful translation of triplet excitons from semiconductor nanoparticles to the bulk solution implies that such materials are generally effective surrogates for molecular triplets. The nanoparticles could thereby potentially sensitize a range of chemical transformations that are relevant for fields as diverse as optoelectronics, solar energy conversion, and photobiology.

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