Triplet Exciton Energy Transfer Extends Lifetime

Transferring triplet exciton energy from semiconductor nanocrystals to surface-bound molecular acceptors has extended the lifetime of the originally prepared excited state by six orders of magnitude.

The finding has implications for fields ranging from solar energy conversion to photochemical synthesis to optoelectronics to light therapy for cancer treatment.

Excitons are the electron/hole pairs formed in semiconductor nanocrystals upon absorption of light, temporarily storing it as chemical energy. In solar cells, excitons transport energy through the material for collection and conversion into electricity.

Illustration depicting semiconductor nanocrystal to molecule triplet energy transfer and established subsequent reactions. Courtesy of Cedric Mongin.

A photochemical drawback to using most semiconductor nanocrystals as photosensitizers is their short excited-state lifetimes — typically tens of nanoseconds — which renders them inadequate to drive photochemical reactions.

Now, researchers from North Carolina State University have extended the semiconductor nanocrystal excited-state lifetime to time scales long enough to perform chemistry.

"The fundamental question was, 'Can we take a nanoparticle excited state with a lifetime of tens of nanoseconds and extend it through sensitization,'" said professor Felix Castellano. "If we take the original nanocrystal excited state and transfer its energy to a triplet acceptor on the surface of the nanomaterial, then the molecular triplet excited state you create should have a long enough lifetime to promote chemical reactions. This would also suggest that semiconductor nanocrystals exhibit molecular-like behavior."

Castellano's team used cadmium selenide (CdSe) nanocrystals capped with oleic acid, prepared by researchers at Bowling Green State University in Ohio. Some of the oleic acid was then replaced by the molecular triplet acceptor 9-anthacenecarboxylic acid (ACA). When the CdSe nanocrystal bearing ACA was struck with a green laser pulse, the exciton produced in the CdSe was transferred to the ACA, forming a molecular triplet exciton with a millisecond lifetime. This represents a lifetime extension of six orders of magnitude, enabling subsequent chemical reactivity.

The researchers also reported that net triplet energy transfer occurred from surface acceptors to freely diffusing molecular solutes, further extending the lifetime while sensitizing singlet oxygen in aerated solution.

"The other benefit is that by translating the exciton away from the nanoparticle surface, instead of involving the nanoparticle itself in the desired chemical reactions, you won't degrade the nanoparticle," Castellano said. "It can keep absorbing light and transferring the energy into the bulk solution."

The research was published in Science (doi: 10.1126/science.aad6378).