Molecular reaction dynamics with molecular identification by vibrational spectrum of excited state from time-resolved fluorescence (VSETRF)

Abstract

Absorption and emission spectra for an electronic transition may provide certain dynamical information under ideal circumstances. For detailed dynamical information including various relaxation and chemical reaction rates, one resorts to the time domain spectroscopies, the most typical of which are pump/probe transient absorption (TA) and time-resolved fluorescence (TRF) based on electronic transitions. The absorption and emission spectra arising from an electronic transition of polyatomic molecules in liquid are in general characterized by a broad featureless band that represents a superposition of vibronic transitions, and therefore, structural information from these spectroscopies are hard to come by. Femtosecond pulses can launch nuclear vibrational wave packets in the electronic ground and excited states by the impulsive electronic transitions. An ultrafast chemical reaction can also generate nuclear wave packets in certain cases. Motion of the nuclear wave packets can be probed directly in time domain, which may provide the information on related relaxation processes and chemical reactions. The wave packet motion manifests itself in the time domain signals as intensity or frequency modulations. Therefore, Fourier analysis of the time domain signal provides a vibrational spectrum of the ground and/or excited states, which can be utilized for the identification of the chemical species responsible for the electronic transition. That is, both structural as well as dynamical information can be obtained from highly time-resolved spectroscopies. In this thesis, highly time-resolved fluorescence is used as a major tool to investigate ultrafast chemical reaction dynamics in liquids. The chemical reactions studied are excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) reactions. Because these chemical reactions occur in the excited states, TA is used as auxiliary method as the TA signal may be generated from both the ground and excited states, whereas the TRF signal arises from the excited state exclusively. High time resolution is required to observe the nuclear wave packet motions arising from high frequency molecular vibrations. Two photon absorption (TPA) can be exploited as an excitation method in TRF to obtain the high time resolution, which was shown to provide high time resolution as high as 30 fs. Unfortunately, it is not possible to excite all molecules by TPA. The ability of TPA is characterized by TPA cross-section (TPACS) value. In chapter 6, TPACS is a measure for the probability of an absorption process. TPACS can be measured by two photon absorption induced fluorescence (TPAIF) method. The TPACS values can be calculated by comparing the well-known TPACS values of TPA molecule such as coumarin and rhodamine series. Z-scan method can also be used for the TPACS. In chapter 3, ESIPT dynamics of two oxadiazole-based model compounds, 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenol (SOX) and 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-benzene-1,4-diol (DOX) have been investigated by TRF. SOX and DOX have one and two intramolecular hydrogen bond moieties, respectively, where ESIPT may occur. TRF fully resolves the ESIPT dynamics and establishes the two state conversion between the enol and keto isomers in the S1 potential energy surface. The apparent inconsistency between the ESIPT rate and the steady state spectrum is settled by invoking the conformational inhomogeneity including the intermolecular hydrogen bonding with protic solvents. The ESIPT rates of SOX and DOX are not as fast as those of typical ESIPT molecules, which indicates a finite barrier for the reaction. For DOX, ESIPT is enabled for only one hydroxyl moiety, and the ESIIPT rate is twice as fast as that of SOX. We propose a symmetric potential energy surface along the two proton transfer coordinates, and bifurcation of the initial probability density of the enol isomer into the two potential minima representing the keto isomer In chapter 4, I have investigated excited state dynamics of 1-hydroxy-2-acetonaphthone (HAN) by time-resolved fluorescence with a resolution high enough to record the nuclear wave packet motions in the excited state. ESIPT of has been in controversy, mainly because its Stokes shift is small compared to those of typical ESIPT molecules. Population dynamics of both the normal and tautomer forms were recorded together with the wave packet motions of the tautomer in the excited state, which confirm the ESIPT of HAN. The population dynamics of the normal and tautomer forms imply that the ESIPT dynamics is biphasic with two time constants <25 and 80 fs. Theoretical analysis of the vibrational modes of the tautomer excited impulsively reveals that major part of the change for the ESIPT reaction is on the naphthalene ring. In chapter 5, I investigate the ICT dynamics of (6-dimethylaminonaphthalen-2-yl)ethan-1-one (acedan). The structural effect of electron donor with comparison between acedan and acedan analogue, where the methyl group of electron donor is substituted for the cyclohexanol, is studied in acetonitrile and ethanol by using TRF and the femtosecond time-resolved fluorescence spectra (TRFS) method and quantum mechanical calculations. The ICT reaction rates are interpreted by two path reaction dynamics. Also, the result shows no dependence of the structural effect of electron donor implying the planar ICT (PICT) in ICT dynamics. To investigate the role of naphthalene ring in the course of the ICT dynamics of acedan, substituted acedan-D and acedan analogue-D where the two hydrogens of the naphthalene ring is replaced by deuterium, were compared with acedan and acedan analogue experimentally and theoretically. The dominant wave packet oscillations of 508 and 510 cm-1 are observed in the TRF signals of acedan and acedan analogue, respectively. Also the dominant wave packet oscillations of 504 and 508 cm-1 are generated in the TRF signals of the substituted acedan-D and acedan analogue-D, respectively. The isotope effect of the naphthalene ring is clearly observed. Coherent nuclear wave packet motion of ~500 cm-1, which can be assigned to the vibrational motion of the carbon atoms in the naphthalene ring, is observed in the red region of fluorescence of acedan and acedan analogue in ethanol. From the theoretical and experimental studies, it is concluded that the naphthalene ring is strongly coupled to the ICT reaction of acedan.