期刊
JOURNAL OF PHYSICAL CHEMISTRY B
卷 121, 期 10, 页码 2308-2318出版社
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcb.6b11020
关键词
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资金
- Fundo Europeu de Desenvolvimento Regional (FEDER) through Programa Operacional Factores de Competitividade (COMPETE)
- University degli Studi di Perugia
- Fundacao para a Ciencia e a Tecnologia (FCT), Portuguese Agency [PEst-OE/QUI/UI0313/2014]
- FCT [UID/QUI/00100/2013]
- SunStorage Harvesting and storage of solar energy, [POCI-01-0145-FEDER-016387,]
- European Regional Development Fund (ERDF), through COMPETE 2020 - Operational Programme for Competitiveness and Internationalisation (OPCI)
- national funds, through FCT
- LaserlabEurope EC's Seventh Framework Programme [284464]
- [SFRH/BPD/108469/2015]
Excited-state proton transfer (ESPT) in Indigo and its monohexyl-substituted derivative (Ind and NHxInd, respectively) in solution was investigated experimentally as a function of solvent viscosity, polarity, and temperature, and theoretically by time-dependent density functional theory (TDDFT) calculations. Although a single emission band is observed, the fluorescence decays (collected at different wavelengths along the emission band using time-correlated single photon counting (TCSPC)) are biexponential, with two identical decay times but different pre-exponential factors, which is consistent with the existence of excited-state keto and enol species. The femtosecond (fs)-transient absorption data show that two similar decay components are present, in addition to a shorter (<3 ps) component associated with vibrational relaxation. From TDDFT calculations it was shown that with both Ind and NHxInd, the reaction proceeds through a single ESPT mechanism driven by an Arrhenius-type activation through a saddle point, which is enhanced by tunneling through the barrier. From the temperature dependence of the steady-state and time-resolved fluorescence data, the activation energy for the process was found to be, similar to 11 kJ mol(-1) for Ind and kJ mol(-1) for NHxInd, in close agreement with the values calculated by TDDFT: 12.3 kJ mol(-1) (Ind) and 3.1 kJ mol(-1) (NHxInd). From time resolved data, the rate constants for the ESPT process in dimethyl sulfoxide were found to be 9.24 X 10(10) s(-1) (Ind) and 7.12 X 10(10) (NHxInd). The proximity between the two values suggests that the proton transfer mechanism in indigo is very similar to that found in NHxInd, where a single proton is involved. In addition, with NHxInd, the TDDFT calculations, together with the viscosity dependence of the fast component, and differences in the activation energy values between the steady-state and time-resolved data indicate that an additional nonradiative process is involved, which competes with ESPT. This is attributed to rotation about the central carbon-carbon bond, which brings the system close to a conical intersection (CI). The CI is of the sloped type, where the seam is reached through an OH stretching vibration.
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