Mechanism and deactivation kinetics of S2-xanthione in acetonitrile, a quenching solvent, and of S2-exciplex measured by pico- and femtosecond laser spectroscopy (original) (raw)
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Femtosecond and nanosecond transient absorption and picosecond time-correlated single photon counting techniques have been used to study the mechanism and dynamics of the efficient quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT) in the S 2 state, by acetonitrile. The results suggest the involvement of two aborted processes in the quenching mechanism: exciplex formation and hydrogen abstraction. The occurrence of the latter process is supported by the observation of a clear isotope effect on going from acetonitrile to deuterated acetonitrile.
Chemical physics letters, 1992
by 2−Solaser−inducedfluorescenceexcitationspectroscopyinasupersonicjet.Theadatomislocatedoverthepyranthioneringinthe1:1complex,andthe2:1complexhasasymmetricalsandwichstructure.Ananalysisofthemicroscopicsolventshiftsindicatesthatthethioneshavesubstantiallysmallerelectricdipolemomentsinthe2-So laser-induced fluorescence excitation spectroscopy in a supersonic jet. The adatom is located over the pyranthione ring in the 1 : 1 complex, and the 2: 1 complex has a symmetrical sandwich structure. An analysis of the microscopic solvent shifts indicates that the thiones have substantially smaller electric dipole moments in the 2−Solaser−inducedfluorescenceexcitationspectroscopyinasupersonicjet.Theadatomislocatedoverthepyranthioneringinthe1:1complex,andthe2:1complexhasasymmetricalsandwichstructure.Ananalysisofthemicroscopicsolventshiftsindicatesthatthethioneshavesubstantiallysmallerelectricdipolemomentsinthe2 state than in the ground state, in qualitative agreement with previous semi-empirical calculations. The dipole moment of xanthione $2 is estimated to be only 2.0 D, so that thione-adatom interactions are almost exclusively dispersive in the upper state.
Chemical physics letters, 1993
The S&It, fluorescence excitation spectra of xanthione (XT) and benzopyranthione (BPT) and their 1: 1 and 1: 2 complexes with N2 and CO have been measured in a supersonic expansion. Remarkably long, harmonic progressions in van der Waals modes are observed and are assigned on the basis of the structural and dipole moment changes in the chromophores. The microscopic salvation shifts are rationalized in terms of dip&r and dispersive contributions to the binding energies.
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The primary physical and chemical processes in the photochemistry of 1-(trideuteromethyl)-2,3,4-trideutero (1) and 1-acetoxy-2-methoxy-(2) 9,10-anthraquinones were studied using femtosecond transient absorption spectroscopy and computational chemistry. Excitation of 1 and 2 at 270 nm results in the population of a set of highly excited singlet states which decay within the laser pulse by internal conversion and vibrational energy redistribution. The transient absorption spectra of the lowest singlet and triplet excited states of substituted anthraquinones 1 and 2 as well as the triplet excited and ground states of the products were detected. The assignments of the transient absorption spectra were performed on the basis of quantum chemical calculations of the electronic absorption spectra of the intermediates. Time-dependent density functional theory or CASSCF/CASPT2 procedure were used to calculate the spectroscopic properties of the intermediates.
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Relaxation dynamics of the excited singlet states of 2,5-bis-(N-methyl-N-1,3-propdienylaniline)-cyclopentanone (MPAC), a ketocyanine dye, have been investigated using steady-state absorption and emission as well as femtosecond time-resolved absorption spectroscopic techniques. Following photoexcitation using 400 nm light, the molecule is excited to the S 2 state, which is fluorescent in rigid matrices at 77 K. S 2 state is nearly non-fluorescent in solution and has a very short lifetime (0⋅5 ± 0⋅2 ps). In polar aprotic solvents, the S 1 state follows a complex multi-exponential relaxation dynamics consisting of torsional motion of the donor groups, solvent re-organization as well as photoisomerization processes. However, in alcoholic solvents, solvent re-organization via intermolecular hydrogen-bonding interaction is the only relaxation process observed in the S 1 state. In trifluoroethanol, a strong hydrogen bonding solvent, conversion of the non-hydrogen-bonded form, which is formed following photoexcitation, to the hydrogen-bonded complex has been clearly evident in the relaxation process of the S 1 state.
The Journal of Chemical Physics, 2009
The ultrafast relaxation kinetics of all-trans--carotene homologs with varying numbers of conjugated double bonds n͑n =7-15͒ and lycopene ͑n =11͒ has been investigated using femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies, both carried out under identical excitation conditions. The nonradiative relaxation rates of the optically allowed S 2 ͑1 1 B u + ͒ state were precisely determined by the time-resolved fluorescence. The kinetics of the optically forbidden S 1 ͑2 1 A g − ͒ state were observed by the time-resolved absorption measurements. The dependence of the S 1 relaxation rates upon the conjugation length is adequately described by application of the energy gap law. In contrast to this, the nonradiative relaxation rates of S 2 have a minimum at n = 9 and show a reverse energy gap law dependence for values of n above 11. This anomalous behavior of the S 2 relaxation rates can be explained by the presence of an intermediate state ͑here called the S x state͒ located between the S 2 and S 1 states at large values of n ͑such as n =11͒. The presence of such an intermediate state would then result in the following sequential relaxation pathway S 2 → S x → S 1 → S 0 . A model based on conical intersections between the potential energy curves of these excited singlet states can readily explain the measured relationships between the decay rates and the energy gaps.
We have experimentally demonstrated conclusive evidence of solvent-to-chromophore excited state proton transfer (ESPT) as a deactivation mechanism in a binary complex isolated in the gas phase. The above was achieved by determining the energy barrier of the ESPT processes, qualitatively analysing the quantum tunnelling rates and evaluating the kinetic isotope effect. The 1:1 complexes of 2,2’-pyridylbenzimidazole (PBI) with H2O, D2O and NH3, produced in a supersonic jet-cooled molecular beam, were characterised spectroscopically. The vibrational frequencies of the complexes in the S1 electronic state were recorded using a resonant two-colour two-photon ionization method coupled to a Time-ofFlight mass spectrometer set-up. In the PBI-H2O, the ESPT energy barrier of 43110 cm-1 was measured using UV-UV hole-burning spectroscopy. The exact reaction pathway was experimentally determined by isotopic substitution of the tunnelling proton (in PBI-D2O) and increasing the width of the proton...
The Journal of Physical Chemistry A, 2005
The properties and reactivities of the xanthone (Xn) ketyl radical (XnH•) in the doublet excited state (XnH•-(D 1)) were examined by using two-color two-laser flash photolysis. The absorption and fluorescence of XnH•-(D 1) were observed for the first time. Several factors governing the deactivation processes of XnH•(D 1) such as interaction and reaction with solvent molecules were discussed. The remarkable change of reactivity of XnH•(D 1) compared with that in the ground state (XnH•(D 0)) was indicated from the experimental results. The rapid halogen abstraction of XnH•(D 1) from some halogen donors such as carbon tetrachloride (CCl 4) was found to occur. The halogen abstraction occurred more efficiently in the polar solvents than in the nonpolar solvents. It is suggested that the polar solvents promote the spin distribution of XnH•(D 1) of the phenyl ring favorable to the halogen abstraction.