Interfacial Electron Transfer Dynamics in a Single CdTe Quantum Dot-Pyromellitimide Conjugate

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 center dot- 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.