Citrate ion, a commonly used reductant in metal colloid synthesis, undergoes strong surface interaction with
silver nanocrystallites. The slow crystal growth observed as a result of the interaction between the silver
surface and the citrate ion makes this reduction process unique compared to other chemical and radiolytic
synthetic methods. Addition of citrate ions to preformed silver colloids (Ag-capped SiO2) results in the
complexation of citrate with silver colloids. The difference absorption spectra of SiO2−Ag colloids in the
presence of citrate ions show an increase in the absorption at 410 nm with increase in concentration of citrate.
The apparent association constant as determined from these absorption changes is 220 M-1. Pulse-radiolysis
studies show that citrate ions complex with the silver seeds and influence the particle growth. For example,
one of the primary intermediates, Ag2
+ produced in the radiolytic reduction of silver ions, readily interacts
with citrate to form a complex absorbing in the 410-nm region. The complex, [Ag2
+−citrate], undergoes
slower transformations compared to uncomplexed Ag2
+. This slow transformation of the citrate complex
eventually leads to the formation of larger clusters of silver. Steady-state and pulse-radiolysis experiments
provide evidence for the multiple roles of citrate ions as a reductant, complexant, and stabilizer that collectively
dictate the size and shape of silver nanocrystallites.
The excited state interaction between CdSe nanocrystals and a hole acceptor, p-phenylenediamine (PPD), is probed using emission and transient absorption spectroscopies. The changes in the photophysical properties of CdSe nanocrystals arising from the interaction with PPD are compared with an aliphatic amine, n-butylamine (n-BA). At low concentrations (<0.5 mM) n-butylamine enhances the emission of CdSe quantum dots whereas PPD effectively quenches the emission at similar concentrations. The low oxidation potential of PPD (E°) 0.26 V vs NHE) enables it to act as an effective scavenger for photogenerated holes. A surface bound complexation equilibrium model has been proposed to explain the quenching phenomenon. The transient absorption measurements confirm the formation of PPD cation radical and subsequent formation of coupling product. Formation of such charged species at the surface extends the bleaching recovery over several microseconds.
Relative energies of the ground state isomers of 1,4-diphenyl-1,3-butadiene (DPB) are determined from the temperature dependence of equilibrium isomer compositions obtained with the use of diphenyl diselenide as catalyst. Temperature and concentration effects on photostationary states and isomerization quantum yields with biacetyl or fluorenone as triplet sensitizers with or without the presence of O(2), lead to significant modification of the proposed DPB triplet potential energy surface. Quantum yields for ct-DPB formation from tt-DPB increase with [tt-DPB] revealing a quantum chain process in the tt --> ct direction, as had been observed for the ct --> tt direction, and suggesting an energy minimum at the (3)ct* geometry. They confirm the presence of planar and twisted isomeric triplets in equilibrium (K), with energy transfer from planar or quasi-planar geometries (quantum chain events from tt and ct triplets) and unimolecular decay (k(d)) from twisted geometries. Starting from cc-DPB, varphi(cc-->tt) increases with increasing [cc-DPB] whereas varphi(cc-->ct) is relatively insensitive to concentration changes. The concentration and temperature dependencies of the decay rate constants of DPB triplets in cyclohexane are consistent with the mechanism deduced from the photoisomerization quantum yields. The experimental DeltaH between (3)tt-DPB* and (3)tp-DPB*, 2.7 kcal mol(-1), is compared with the calculated energy difference [DFT with B3LYP/6-31+G(d,p) basis set]. Use of the calculated DeltaS = 4.04 eu between the two triplets gives k(d) = (2.4-6.4) x 10(7) s(-1), close to 1.70 x 10(7) s(-1), the value for twisted stilbene triplet decay. Experimental and calculated relative energies of DPB isomers on the ground and triplet state surfaces agree and theory is relied upon to deduce structural characteristics of the equilibrated conformers in the DPB triplet state.
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