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

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• 作者：Rajan Vatassery ; Jonathan A. Hinke ; Antonio Sanchez-Diaz ; Ryan Hue ; Kent R. Mann ; David A. Blank ; Wayne L. Gladfelter
• 刊名：The Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：May 23, 2013
• 年：2013
• 卷：117
• 期：20
• 页码：10708-10715
• 全文大小：411K
• 年卷期：v.117,no.20(May 23, 2013)
• ISSN：1932-7455

文摘

Oleate-capped CdS nanocrystals (NCs) dispersed in dichloromethane were found to quench the excited-state fluorescence of the terthiophene derivative 3鈥?4鈥?dibutyl-5鈥?phenyl-[2,2鈥?5鈥?2鈥?terthiophene]-5-carboxylic acid (1-CO₂H). Infrared and ¹H NMR spectroscopies provided evidence that 1-CO₂H substitutes for oleate on the surface of the CdS NCs. Upon binding, the fluorescence of 1-CO₂H is quenched, and the ¹H NMR lines from 1-CO₂H 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-CO₂H, Stern鈥揤olmer plots indicated a nearly linear increase in I₀/I as the CdS NCs鈥?concentration was increased, but as the dye/NC ratio reached 20/1, I₀/I reached a maximum of 8 and began to decrease. By a dye/NC ratio of 2/1, the I₀/I reached a steady value of 2.5. The peak in the Stern鈥揤olmer 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鈥檚 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.