Tuning Hole and Electron Transfer from Photoexcited CdSe Quantum Dots to Phenol Derivatives: Effect of Electron‐Donating and ‐Withdrawing Moieties

Abstract

Charge-transfer processes from photoexcited CdSe quantum dots (QDs) to phenol derivatives with electron- donating (4-methoxy) and -withdrawing (4-nitro) moieties have been demonstrated by using steady-state and time- resolved emission and femtosecond transient absorption spectroscopy. Steady-state and time-resolved emission studies suggest that in the presence of both 4-nitrophenol (4NP) and 4-methoxyphenol (4MP) CdSe QDs luminescence is quenched. Stern-Volmer analysis suggests both static and dynamic mechanisms are active for both the QD/phenol composites. Cyclic voltammetric analysis recommends that photoexcited CdSe QDs can donate electrons to 4NP and holes to 4MP. To reconfirm both electron- and hole-transfer mechanisms, CdSe/CdS quasi-type II and CdSe/CdTe type II core-shell nanocrystals were synthesized and photoluminescence quenching was monitored in the absence and presence of both 4NP and 4MP, for which hole and electron transfer were systematically restricted. Results suggest that indeed electron and hole transfer take place from photoexcited CdSe to 4NP and 4MP, respectively. To monitor the charge-transfer dynamics in both systems on an early timescale, femtosecond transient absorption spectroscopic techniques have been employed. Electron and hole transfer and charge-recombination dynamics are discussed and the effect of electron-donating and -withdrawing groups has been demonstrated.

Electrochemical Evaluation of Dopant Energetics and the Modulation of Ultrafast Carrier Dynamics in Cu-Doped CdSe Nanocrystals

“…To understand the paradoxical behavior of nitro- and aminophenols to quench the dopant emission and confirm charge transfer processes, TAS measurements were carried out in the presence of phenols. In case of hole quenching ligands, the ground state bleach recovery tends to become slower due to decoupling of hole from the electron. ,− For o-AP we do envision slower bleach recovery indicating energetically favorable hole transfer from Cu–CdSe to HOMO of o-AP as shown in Figure B (the multiexponential fitting parameters are tabulated in Table S3, Supporting Information). On the other hand, in the case of electron quenchers the bleach recovery becomes faster due to fast depopulation of the CB electron to the LUMO of quencher. ,, Interestingly, from TAS we do not find any evidence of electron transfer from doped CdSe to o-NP as the ground state bleach recovery remains similar in the presence of o-NP as shown in Figure B.…”

“…On the other hand, in the case of electron quenchers the bleach recovery becomes faster due to fast depopulation of the CB electron to the LUMO of quencher. 57,59,61 Interestingly, from TAS we do not find any evidence of electron transfer from doped CdSe to o-NP as the ground state bleach recovery remains similar in the presence of o-NP as shown in Figure 4B. This surprising behavior can be explained considering the mechanism of Curelated emission as depicted in Scheme 1.…”

“…Two substituted phenols with electron-rich amino functional group (o-AP) and with electron withdrawing nitro group at the ortho position (o-NP) were chosen to elucidate their role in Cu-related emission. It is known in literature that depending on the energetics amino-substituted molecules act as hole quenchers whereas nitro-substituted molecules are typical electron quenchers for CdSe NCs. − From CV measurements (Figure S7, Supporting Information) we also found that the lowest unoccupied molecular orbital (LUMO) of o-NP can energetically accept electrons, while the o-AP has suitable highest occupied molecular orbital (HOMO) level for capturing the hole from Cu–CdSe (Scheme ).…”

Alkaline Phosphatase Immobilized CdTe/ZnS Quantum Dots for Dual-Purpose Fluorescent and Electrochemical Detection of Methyl Paraoxon

“…45 The transfer of photogenerated electrons from CdTe/ZnS QDs to p-nitrophenol is thermodynamically feasible because the lowest unoccupied molecular orbital (−3.89 eV) of p-nitrophenol is lower as compared to the conduction band (−3.9 eV) of QDs. 46,47 3.4. Paper-Based Fluorescence Sensor for Methyl Paraoxon Detection.…”

Excitons with Reverse Rectifying Characteristics for Superior Solar Photocatalysis

“…To realize the situation of the electron and hole in the ZnS@CdS inverted type‐I core/shell upon photoexcitation, we have performed electron and hole quenching experiments in the presence of 4‐NP and 4‐MP, respectively. It is extensively reported in the literature that 4‐NP and 4‐MP are molecules that quench electrons and holes, respectively …”