Simultaneous Determination of the Adsorption Constant and the Photoinduced Electron Transfer Rate for a Cds Quantum Dot–Viologen Complex

Abstract

Transient absorption (TA) spectroscopy of solution-phase mixtures of colloidal CdS quantum dots (QDs) with acid-derivatized viologen molecules, N-[1-heptyl],N'-[3-carboxypropyl]-4,4'-bipyridinium dihexafluorophosphate (V(2+)), indicates electron transfer occurs from the conduction band of the QD to the LUMO of V(2+) after photoexcitation of a band-edge exciton in the QD. Analysis of the magnitude of the ground state bleach of the QD as a function of the molar ratio QD:V(2+) yields the QD-ligand adsorption constant, K(a) (4.4 × 10(4) M(-1)) for V(2+) ligands adsorbed in geometries conducive to electron transfer. The value of K(a), together with the measured rates of (i) formation of the V(+•) electron transfer product and (ii) recovery of the ground state bleach of the QD, enables determination of the intrinsic rate constant for charge separation, k(CS,int) ~ 1.7 × 10(10) s(-1), the rate for a single QD-V(2+) donor-acceptor pair. This analysis confirms previous reports that the number of ligands adsorbed to each QD is well-described by a Poisson distribution. This is the first report where the QD-ligand charge transfer and binding equilibria are quantitatively investigated simultaneously with a single technique.

Efficiency of Hole Transfer from Photoexcited Quantum Dots to Covalently Linked Molecular Species

“…(7) A growing body of spectroscopic work has examined charge transfer rates from QDs to molecular charge acceptors typically physisorbed onto the QD surface, exploring the parameter space in the Marcus equation. (8) Electron transfer studies (8)(9)(10)(11)(12)(13) outnumber hole studies, (14)(15)(16)(17)(18)(19) despite hole transfer being the limiting factor in the efficiencies of QD sensitized solar cells and in QD-based colloidal photocatalytic hydrogen evolving systems. (20,21) To establish a sound model for charge transfer from nanocrystals to molecular acceptors, we must address the features of this system that make the process more difficult to characterize than that of the pure molecular case.…”

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

“…As shown in Figure 1, the TA spectra of free CdS QDs consist of a long-lived bleach of 1S exciton band due to the state filling of the 1S electron level and derivative like features caused by the presence of the exciton. 29 The spectra of CdS-MB + complexes show a much faster 1S exciton bleach recovery (compared with free QDs without molecular acceptors)…”

“…25,30 For the CdS-MV 2+ and CdS-AQ complexes, the recovery of 1S bleach is accompanied by the formation of reduced adsorbates (MV +á and AQ -radicals) signals at ~600 nm and ~630 nm, respectively ( Figure 1D and 1E). 29,[31][32][33] Therefore, the photoinduced ET processes from the excited QDs to adsorbates can be monitored by the kinetics of either the QD (1S exciton bleach recovery) or adsorbate (ground state bleach or radical formation) TA features, as shown in Figure S4.…”

“…29,34,35 To compare ET rates among different QD-acceptor complexes with different ratios, we determine an ÒintrinsicÓ ET rate (k 1 ) in 1:1 complexes from the measured ET kinetics (see supporting information SI5). The intrinsic ET rates for all QDacceptor complexes are listed in Table S2 and plotted as a function of radius in Figure S4.…”

Communication: Uncovering molecule-TiO2 interactions with nonlinear spectroscopy

“…[1][2][3] One vital area of research aims to understand how molecule-semiconductor interactions influence the primary events initiated by light absorption (e.g., electron transfer, nuclear relaxation). [4][5][6][7][8][9][10][11][12][13] Pioneering experimental work established <100 fs time scales for photoinjection in several dye-sensitized TiO 2 systems. 14,15 The observed dynamics conform (approximately) to the rate theory developed by Marcus and Gerischer decades ago in which photoexcitation of the molecular sensitizer and electron injection are regarded as sequential processes.…”