【作者】 王金婷；

【导师】 张立敏； 俞书勤；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2009， 博士

【摘要】 时间分辨激光诱导瞬态吸收光谱是探测光化学反应中产生的短寿命中间体的有效方法。它被广泛应用于各个领域,是研究超快反应的强有力的工具之一。通过吸收光谱随光激发的延迟时间变化和测得的反应动力学常数可以推测反应的机理。本文的工作包括对我室自行搭建的仪器进行改进,并利用这套仪器研究了一些体系的瞬态光化学过程,主要包括以下四部分:(1)对我室自行搭建的仪器进行了改进。以前这套仪器只能采取描点法来获得光谱。通过在不同的延迟时间设门,采用单色仪和A/D数据采集卡同步扫描的办法,可以获得瞬态连续光谱。(2)在乙腈溶剂中研究了呫吨酮的激发三重态的性质，并得到了呫吨酮激发三重态和胺类、醇类以及酚类反应的瞬态吸收光谱和猝灭速率常数(kq)。除了苯胺和3-硝基苯胺被认为是能量转移外,呫吨酮和其余胺类的反应随着自由能变的减小,log kq逐渐增大,由此认为发生了电子转移反应。对于DMT、TMA、DMA、TEA来说,通过瞬态吸收光谱的变化可以知道既有电子转移反应又有氢转移反应发生。呫吨酮和醇类只发生氢转移反应，其猝灭速率矹痛嫉摩?C-H的键能有关。由于有不同的机理,呫吨酮和醇类反应的猝灭速率矹群头永喾?应的小得多。(3)在1:1乙腈/水溶液中得到了核苷酸和芳香酮的瞬态吸收光谱,通过瞬态吸收光谱的变化研究了鸟苷酸、腺苷酸、胞苷酸、胸腺苷酸猝灭二苯甲酮、呫吨酮激发三重态的反应捎谑笛橹猩闪顺榍庾杂苫鸵趵胱幼杂苫贫?出芳香酮和鸟苷酸、腺苷酸的反应机理是先发生电子转移后发生质子转移。在芳香酮和胞苷酸、胸腺苷酸的反应中没有观察到相应自由基的瞬态吸收峰,由此推断出它们和胞苷酸、胸腺苷酸没有发生电子转移和质子转移反应。对瞬态吸收峰处的时间衰减曲线进行拟合得到了核苷酸猝灭芳香酮的速率常数,可以看到随着反应自由能变?G的增大,反应速率常数逐渐减小。(4)用时间分辨激光闪光光解的方法在乙腈、乙酸乙酯、甲苯、环己烷四种溶剂中研究了芴酮(FL)激发三重态的性质,得到芴酮激发三重态和胺类以及酚类反应的瞬态吸收光谱和猝灭速率常数(kq)。在芴酮和胺类的反应中,瞬态吸收光谱中相应反应物的阳离子自由基和阴离子自由基吸收峰的出现表明芴酮激发三重态和胺发生了电子转移反应。芴酮和三乙烯二胺(DABCO)、三苯胺(TPA)以及三种N,N-二甲基苯胺衍生物的反应速率常数随着溶剂极性减小逐渐变小,但芴酮和三种N,N-二甲基苯胺衍生物的反应速率常数下降的程度比DABCO和TPA慢。这是由于芴酮和N,N-二甲基苯胺衍生物能形成弱氢键,而和DABCO、TPA不能形成氢键。在同一种溶剂中,三种苯胺的猝灭速率常数的变化可以用Hammett常数取代基效应来解释。芴酮和苯胺类衍生物以及酚类的反应猝灭速率常数随着溶剂极性减小而逐渐增大。这是因为芴酮和苯胺形成的NH…O氢键以及与酚类形成的OH…O氢键都比和N,N-二甲基苯胺衍生物形成的CH…O氢键强,而溶剂极性的减小有利于氢键的形成,所以猝灭速率常数逐渐增加。芴酮与苯胺类衍生物和酚类衍生物的猝灭速率常数在四种溶剂中都随着自由能变增大而逐渐减小。通过相应的自由基吸收峰的生成判断芴酮和苯胺类化合物发生了电子转移和质子转移反应,而和酚类化合物发生了耦合电子-质子转移反应。更多还原

【Abstract】 Time-resolved laser induced transient absorption spectra are an effective technique to probe short lived transients of photochemistry. It has been used in different research fields and also is one of the powerful techniques to study the fast reactions. By analyzing the changes of absorption spectra at different delay time and the quenching rate constants, the mechanism of reactions can be obtained. Improving our homemade laser flash photolysis instrument is one of my works. Transient photochemistry processes of some systems are studied using this instrument. This work includes four parts:(1) Improving our homemade laser flash photolysis instrument. Previously the spectra were obtained by point-by-point manner. After improvement, several gates were marked at different delay time in the time-resolution signal. The transient absorption spectra were obtained from the integrated intensity of each gate signal recorded through scan of monochromator and A/D data acquisition system synchronously.(2) The property of the lowest excited triplet states of xanthone (XT) in acetonitrile has been investigated by using time-resolved laser flash photolysis at 355nm. The transient absorption spectra and the quenching rate constants (kq) of the excited xanthone with several amines have been obtained. Good correlation between log kq and the driving force (?G) of the reactions suggests the electron transfer mechanism, except aniline (AN) and 3-nitroaniline (3-NO2-A) which are presented by energy transfer mechanism. With the appearance of ketyl radical, hydrogen atom transfer also happened between xanthone and dimethyl-p-toluidine (DMT), 3,5,N,N-tetramethylaniline (TMA), N,N-dimethylaniline (DMA), triethylamine (TEA). So, both electron transfer and H-atom transfer existed in these systems. Great discrepancies of kq values are discovered in H-atom abstraction reactions for alcohols and phenols, which can be explained by different abstraction mechanisms. The quenching rate constants (kq) between xanthone and alcohols correlate well with the α-C-H bonding energy of alcohols.(3) The rate constants for electron transfer from guanosine 5’-monophosphate (GMP), adenosine 5’-monophosphate(AMP), cytidine 5’-monophosphate(CMP), thymidine 5’- monophosphate(TMP) to the excited benzophenone(BP), xanthone(XT) have been determined in 1:1 CH3CN/H2O solution. From the changes of the transient absorption spectra, we can conclude that photoinduced proton transfer occur following the electron transfer from GMP, AMP to excited BP, XT, which is in accordance with free energy change of the reactions. For CMP and TMP, electron transfer and proton transfer do not appear because no ketyl and anion radical are generated. In addition, there is a good correlation between kq and the free energy changes(?G).(4) The property of the lowest excited triplet state of fluorenone (FL) in acetonitrile, ethyl acetate, toluene, cyclohexane has been investigated using time-resolved laser flash photolysis. The appearances of corresponding cation radical and anion radical suggest that the electron transfer reaction occurs between 3FL* and amines. In the reactions of FL with 1,4-diazobicyclo[2.2.2]-octane (DABCO), triphenylamine (TPA) and three N,N-dimethylaniline derivatives, there is a decrease in the quenching rate constants along with reducing solvent polarity. The quenching rate constants of 3FL* with the three N,N-dimethylaniline derivatives decrease more slowly than that with 1,4-diazobicyclo[2.2.2]-octane (DABCO) and triphenylamine (TPA) in virtue of formation of weak hydrogen bonding between 3FL* and DMT, TMA, N,N-DMA. Furthermore, the change of quenching rate constants of FL excited triplet state with the three N,N-dimethylaniline derivatives can be explained using Hammett constant. In the reactions of FL with aniline derivatives and Phenol derivatives, the quenching rate increase with reducing solvent polarity. Because the NH…O and OH…O hydrogen bonding formed between FL with anilines and phenols respectively are stronger than the CH…O hydrogen bonding formed between FL with N,N-dimethylaniline derivatives. And reducing solvent polarity will increase the strength and extent of hydrogen-bonding. The quenching rate constants between FL with aniline and phenol derivatives decrease with increase of the free energy change (ΔG) in the four solvents. According to the appearance of corresponding radical absorption peak, we can conclude that electron transfer and proton transfer reactions occur between FL and aniline derivatives and coupled electron-proton transfer reactions occur between FL and phenol derivatives.更多还原