Laser Flash Photolysis Studies On Some Organic Molecules

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.