Ultrafast time-resolved fluorescence studies of Excited-State Proton Transfer Dynamics in 3-cyano-4-methyl-7-hydroxycoumarin complexes

• Main Authors: Chen, Tzu Chun, 陳姿均
• Other Authors: Cheng, Po Yuan
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/67587882491110745079

碩士 === 國立清華大學 === 化學系 === 104 === We employed a broadband ultrafast time-resolved fluorescence (TRFL) spectrometer implemented by optical Kerr gating (OKG) to study the excited-state proton transfer (ESPT) dynamics in 3-cyano-4-methyl-7-hydroxycoumarin (3CN4MU) complexes. We chose 3CN4MU as the proton donor and two bases, triethylamine (TEA) and 1-methylimidazole (1MI), of different proton affinities (PA) as the proton acceptors. We used two solvents, ethyl acetate (EA) and toluene (TL), of different polarities to study the solvent effect in ESPT. It was conclude from by steady-state spectra that the ground-state 3CN4MU transfers a proton from its phenolic group to TEA, which possesses stronger PA. Therefore, it is difficult to study ESPT in the 3CN4MU-TEA complex. On the other hand, 3CN4MU forms ground-state hydrogen-bonded complexes with 1MI (weaker PA) which suggesting that no proton-transfer reaction occurs in the ground state. Excited states are reached by 383 nm femtosecond laser pulse excitation. The observed TRFL spectra reveal that ESPT does not occur in 3CN4MU-1MI complex in TL, due to the weak solvation effect in nonpolar solvent. On the contrary, solvation-controlled proton transfer in excited state is observed in 3CN4MI-1MI complex in polar solvent EA. We used a total fluorescence intensity function P(t) to measure the excited-state population and transition moment evolution with time during the ESPT process. The P(t) function of 3CN4MU-1MI in EA can be described by three time constants. The fast initial decay component (0.8 ps) can be assigned to first step of proton transfer, which is controlled by solvation effect. The second decay component (30 ps) is assigned to the ion-pair structural relaxation to reduce the overall free energy. In the end, the 1700 ps component is assigned to the lifetime of the excited-sate proton-transferred ion pair.