Single‐molecule fluorescent probe: The formation of the highly fluorescent 4‐NHOH form of a 3,4‐dinitrophenyl‐substituted boron dipyrromethane dye can be used to study interfacial electron transfer on individual TiO2 particles (see picture; B yellow, C gray, F green, H white, O red). Kinetic and image analyses of the resulting fluorescence bursts reveal temporal dynamics of molecular interactions and reactive‐site distributions.

Airborne singlet oxygen (1O2) molecules, which are generated during the TiO2 photocatalytic reactions and diffused from the surface into air, were detected at the opposite surface using terrylenediimide (TDI) molecules at the single-molecule level. The novel 1O2 nanosensor, which has a detectable number of about 1000 1O2 molecules in 70 x 70 square micrometers, can easily detect the single 1O2 molecule at a distance of over 1000 micrometers from the place of its creation in ambient air.

Charge transfer to and from semiconductor quantum dots (QDs) is of intense interest because of its important roles in QD-based devices, such as light emitting diodes and solar cells. We now report a mechanistic study of the electron transfer (ET) processes between CdTe QDs and a pyromellitimide derivative with a carboxylic acid (PI-CA) using ensemble and single-particle spectroscopies. First, to confirm whether the photochemical reactions occur by excitation of the QDs, the quenching of the QD emission by PI-CA as well as control substrates was examined using steady-state and time-resolved emission spectroscopies. It was found that the PI-CA molecules are strongly bound to the surface of the QDs and significantly quench the photoluminescence (PL) band near 675 nm. Nanosecond transient absorption measurements also revealed that PI•- was formed during the laser flash photolysis of the conjugates between CdTe QD and PI-CA in chloroform solution. The single-particle PL spectroscopy was utilized to clarify the interfacial ET processes within individual CdTe/PI-CA conjugates modified on the glass surface via thiol linkers. The influences of the modified PI-CA molecules on the luminescence intermittency, the so-called blinking phenomenon, of single QDs were discussed in terms of a diffusion-controlled electron transfer (DCET) theory for the probability distributions of the “on” or “off” events. Finally, on the basis of the experimental and analytical results, it was concluded that the blinking characteristics observed for individual CdTe/PI-CA conjugates are due to the energy diffusions away from and back to a resonance condition fulfilled by the energy of the acceptor states resulting in the intermittent changes in the interfacial ET redox turnover rates.

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Photoinduced electron transfer (ET) to and from semiconductor quantum dots (QDs) is of intense interest because of its important roles in QD-based devices, such as detectors, light-emitting diodes, and solar cells. In this study, we have investigated the solvent dependence of the interfacial ET between ZnS-capped CdSe (CdSe/ZnS) QDs and pyromellitimide (PI) at the ensemble and single-particle levels. The steady-state and time-resolved photoluminescence (PL) and absorption measurements revealed that the rates of ET from the excited CdSe/ZnS to the adsorbed PI-CA molecules in nonpolar solvents, such as octane and toluene, are higher than aprotic polar solvents, such as benzonitrile. Furthermore, it was found that two pathways are included in the ET processes between CdSe/ZnS and PI, that is, ET from the excited CdSe core to PI-CA molecules adsorbed on the surface of the CdSe or ZnS. The former process proceeds on the picosecond time scale, while the latter proceeds on the nanosecond time scale. The interfacial ET dynamics in individual CdSe/ZnS-PI conjugates in different solvents were next examined. The PL intermittency or blinking kinetics of single QDs were analyzed by adapting a diffusion-controlled electron transfer (DCET) theory for the probability distribution of the "on" events. The results, which are qualitatively consistent with that obtained from the bulk experiments, clearly showed that the probability of the ET events increases with the decreasing solvent polarity. Consequently, our findings provide new insight into the mechanism of the interfacial ET reactions on the semiconductor nanoparticles and potentially contribute to the development of the design of QD-based devices.

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