Ultrafast hole/electron transfer dynamics in a CdSe quantum dot sensitized by pyrogallol red: a super-sensitization system

To find a suitable hole-transporting adsorbate for CdSe quantum dots (QDs), a pyrogallol red (PGR) molecule was chosen where PGR also can sensitize CdSe QDs. Energy level diagrams suggest that the photoexcited hole can be transferred to PGR and photoexcited PGR can inject an electron into CdSe QDs. Steady-state and time-resolved emission studies suggest that the photoexcited hole is transferred to PGR; however, the process is too fast to monitor with the subnanosecond time-resolution spectroscopic technique. Femtosecond transient absorption spectroscopy has been employed to monitor the charge-transfer behavior of the above system in an early time scale. Photoexcitation of pure PGR and CdSe QDs at 400 nm laser light gives the transient absorption due to the photoexcited singlet state of PGR and charge carriers (electron/hole) in CdSe QDs, respectively, in the visible/near-IR region of the absorption spectra. However, on photoexcitation of the CdSe/PGR composite at 400 nm, the PGR cation radical and electron in the CdSe QD were detected in the transient absorption spectra. Hole transfer time from the photoexcited CdSe QD to PGR is found to be 500 fs. The transient signal due to the PGR cation and electron in the CdSe QD also contributed to photoexcitation of PGR on the CdSe QD, where electron injection is found to be <150 fs. Charge recombination dynamics were found to be very slow with time constants of 4 ps (15%) and >200 ps (85%) confirming a grand charge-separated state in the CdSe/PGR composite system