Excited-State Quenching Mechanism of a Terthiophene Acid Dye Bound to Monodisperse CdS Nanocrystals: Electron Transfer versus Concentration Quenching

Abstract

Oleate-capped CdS nanocrystals (NCs) dispersed in dichloromethane were found to quench the excited-state fluorescence of the terthiophene derivative 3′,4′-dibutyl-5″-phenyl-[2,2′:5′,2″-terthiophene]-5-carboxylic acid (1-CO 2 H). Infrared and 1H NMR spectroscopies provided evidence that 1-CO 2 H substitutes for oleate on the surface of the CdS NCs. Upon binding, the fluorescence of 1-CO 2 H is quenched, and the 1H NMR lines from 1-CO 2 H are broadened. The importance of the carboxylate group in binding to the CdS NC was further established by examining the behavior of a similar fluorophore where the carboxylic acid group was replaced with a bromo substituent (1-Br). The CdS NCs had no influence on the fluorescence intensity or NMR line shapes of 1-Br. For 1-CO 2 H, Stern–Volmer plots indicated a nearly linear increase in I 0/I as the CdS NCs’ concentration was increased, but as the dye/NC ratio reached ∼20/1, I 0/I reached a maximum of ∼8 and began to decrease. By a dye/NC ratio of 2/1, the I 0/I reached a steady value of ∼2.5. The peak in the Stern–Volmer plot at a 20/1 ratio was consistent with a maximum in the contribution from concentration quenching at this coverage. On the basis of the appearance of the dye’s radical cation spectrum at low dye/NC ratios, ultrafast transient absorption spectroscopy confirmed electron transfer from the singlet excited state of the dye to the CdS NC with a lifetime of 16 ps. At higher dye/NC ratios, the signal from the radical cation was much less dominant, and the decay of the singlet excited state was dominated by the concentration quenching process having a 1 ps lifetime.

Effect of extending conjugation via thiophene-based oligomers on the excited state electron transfer rates to ZnO nanocrystals

“…Further quencher addition caused a decrease in emission until a plateau was reached at a dye:ZnO NC of approximately two. The general shape of the Stern Volmer graphs observed for these dyes paralleled those reported in earlier studies, where the suggested causes were concentration quenching 21 or competition between dye binding sites that were or were not quenched. 3,4 The current work focused on the ultrafast quenching kinetics at a dye to ZnO NC of 2:1 in order to minimize any complications attributed to dye-dye interactions.…”

Dynamics of Energy Transfer from CdSe Nanocrystals to Triplet States of Anthracene Ligand Molecules

“…Using eq , with k q = 1.5 × 10 7 s –1 , we calculate that m max = 16 could achieve P NC→9‑ACA = 0.95 if the fast 134 ps decay component is neglected. Possible experimental approaches to increase m max include changing the NC preparation conditions so the 9-ACA displaces a carboxylic acid ligand rather than a phosphonic acid ,, or changing the ligand exchange conditions to boost [9-ACA] LEX and/or make displacement of the ODPA more favorable. We anticipate that optimization of the ligand exchange conditions will increase m max and the UC yield as well.…”

Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore–Catalyst Assembly Anchored to TiO2

“…35 This is well below the anodic peak potential determined for T3 absorbed on planar indium-tin oxide (ITO) in the same media, E p = 1.03 V vs SCE (Figure S8), and the resulting couple formed upon continued anodic scanning, E 1/2 = 0.91 V vs SCE (Figure S9). 36,37 At the TiO 2 -T3 photoanode, photocurrent arises from the thermodynamically favorable oxidation of H 2 Q to semiquinone, HQ •+ , then to quinone, Q, by two sequential T3 •+ radical cation reductions. T3 •+ is formed after electron injection by the chromophore's excited state, T3*, into the conduction band of TiO 2 .…”

“…This is well below the anodic peak potential determined for T3 absorbed on planar indium-tin oxide (ITO) in the same media, E p = 1.03 V vs SCE (Figure S8), and the resulting couple formed upon continued anodic scanning, E 1/2 = 0.91 V vs SCE (Figure S9). , …”