Ultrafast
Hole/Electron Transfer Dynamics in a CdSe
Quantum Dot Sensitized by Pyrogallol Red: A Super-Sensitization System
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Abstract
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