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  • Stark effect and Spectral Hole-Burning: Solvation of Organic Dyes in Polymers
    E. Vauthey, K. Holliday, C. Wei, A. Renn and U.P. Wild
    Chemical Physics, 171 , 1993, p253-263
    DOI:10.1016/0301-0104(93)85148-2 | unige:3016 | Abstract | Article PDF
Spectral hole-burning studies of nile red and cresyl violet in polyvinylbutyral and polyvinylformal films have been performed. From the shape of spectral holes under the influence of an electric field, the dipole moment difference between the ground and excited state of both dyes has been determined. The Stark effect was investigated at different positions in the inhomogeneously broadened absorption band of the guest molecules. The observed dipole moment difference decreases with increasing wavelength. This variation is caused by the matrix induced dipole moment. For nile red, which is a neutral and polar molecule, the distribution of induced dipole moments is strongly correlated with the orientation of its ground state dipole moment. In the case of cresyl violet perchlorate, which is a salt, this distribution is anisotropic for guests absorbing in the blue part of the inhomogeneous band but becomes more isotropic as the absorption wavelength increases. The wavelength dependence of the observed dipole moment is much stronger and is ascribed to the existence of the cresyl violet perchlorate salt in different states of solvation.
Holographic detection of spectral holes is demonstrated in a crystalline host material with signal-to-noise ratios of up to 104. Hole burning occurs in two Pr3+ sites in the Y2SiO5 lattice, in both cases due to population redistribution between the ground-state quadrupole levels. The signal contains contributions due to a resonant hole and several side holes and antiholes, a phenomenon not previously observed using the holographic technique. The diffracted spectrum is modeled in two ways. In the first case the transmission spectrum is used to determine the population gratings and thus the diffraction efficiency. In the second case the transition probabilities between ground- and excited-state Kramer's doublets are used to model the population gratings. The technique is applied to pseudo-Stark-effect measurements from which the crystallographic sites as determined by x-ray analysis are matched to the spectroscopic data presented here. The time decay of the diffracted signal is used to study nuclear spin-lattice relaxation. It is shown that at 1.6 K temperature-dependent phonon-induced processes make no contribution to this decay. The nonexponential time decay of the population upon radio-frequency irradiation resonant with a ground-state quadrupole splitting is attributed to Pr-Pr cross relaxation
  • The Reliability of Free Ion Yield in Photoinduced Electron Transfer Reactions. The Model System 9,10-Dicyanoanthracene/Biphenyl in acetonitrile
    E. Vauthey, D. Pilloud, E. Haselbach, P. Suppan and P. Jacques
    Chemical Physics Letters, 215 (1-3) , 1993, p264-268
    DOI:10.1016/0009-2614(93)89298-V | unige:3019 | Abstract | Article PDF
A detailed study of the separation efficiency in the photoinduced electron transfer reaction between 9,10-dicyanoanthracene and biphenyl in acetonitrile is presented. Both transient absorption and photoconductivity indicate a separation efficiency of about 0.4. This value is in discrepancy with two of three previously reported efficiencies. The problems arising with too large donor concentrations and with the use of a secondary donor to determine the separation efficiency are discussed.
A study of the hole-burning mechanisms of bis[4-(diethylamino)-2-hydroxyphenyl]squaraine (DEAH) and bis[4-(disethylamino)-phenyl]squaraine (DEA) in hydrogen-bonding and non-hydrogen-bonding polymers is presented. Intramolecular H-bonding is only possible for DEAH. In all systems, the spectral holes are not persistent and decay with a distribution of rates ranging from 10-5s-1 to about 1 s-1, the time resolution of the experiment. In H-donating matrices, this distribution varies with the burning wavelength. From the hole-burning efficiencies and the kineticsof the hole refilling, four different types of nonphotochemical hole-burning mechanisms are postulated. The efficiency of these mechanisms depends mainly on the occurrence of processes slowing down the relaxation to the initial product state.
The rotational dynamics of nile red has been studied in polar protic, polar aprotic and non-polar solvents. In the non-polar and the aprotic solvents, with the exception of long alkanenitriles, the rotation dynamics is consistent with the prediction of the Stokes—Einstein—Debye hydrodynamics theory for slip and close to the stick boundary condition, respectively. However in protic solvents, the rotation dynamics can be explained in terms of the Stokes—Einstein—Debye hydrodynamics theory under stick boundary condition only if solvent attachment via hydrogen bonding is assumed. The anomalous behaviour observed in longer alkanenitriles has been assigned to the formation of a reverse micelle-like solvation layer around nile red.
  • Molecular Conformation and Excited-State Dipole Moments of Di- and Tetramethylaminobenzonitrile (DMABN and TMABN)
    W. Rettig, D. Braun, P. Suppan, E. Vauthey, K. Rotkiewicz, R. Luboradzki and K. Suwinska
    Journal of Physical Chemistry, 97 (51) , 1993, p13500-13507
    DOI:10.1021/j100153a014 | unige:3026 | Abstract | Article PDF
The conformational analysis of TMABN by three different methods X-ray analysis, photoelectron spectroscopy, and UV molar absorption coefficient yields a twist angle of the dimethylamino group of 60-70° in the ground state, whereas DMABN is not far from planar in qualitative agreement with the predictions from force field calculations (QCFF/PI and MM3). Dipole moment determinations by the thermochromic method agree with those from other methods (solvatochromism, electrochromism and time resolved microwave absorption) in that the excited state dipole moment of TMABN is very large, as well as that of the TICT state of DMABN. Its value increases somewhat with solvent polarity. This is explained by a nuclear polarizability model. The force field calculations are used to predict twist angle values for various sterically hindered DMABN derivatives



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Eric Vauthey

Physical Chemistry Department - Sciences II - University of Geneva
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