Wolfgang Liptay

Tautomerization in fluorescent states of bipyridyl-diols: A direct confirmation of the intramolecular double proton transfer by electro-optical emission measurements

Abstract Integral electro-optical emission measurements (IEOEM) and INDO/S calculations were carried out for 2,2′-bipyridyl- 3,3′-diol (BP(OH) 2 ), and its 5-methyl- (MeBP(OH) 2 ) and 5,5′-dimethyl- (Me 2 BP(OH) 2 ) derivatives. All three molecules were reported earlier as effective lasing dyes [5,6] undergoing photoautomerization in the S 1 state. From the IEOEM experiment an upper limit of about 1.0 D (debye) is derived for the fluorescent state dipole moment, μ e , in each case, while the INDO/S calculations predict μ e of the excited singlet (π, π ∗ ) state of the diketo and monoketo forms in the range: 0.0–0.5 D and 4.0–4.9 D, respectively. Comparison of both results leads to an unequivocal discrimination between the two possible photoreactions, single or double proton transfer (SPT or DPT), and shows that the fluorescent species are diketo tautomers produced by DPT.

Excited state dipole moments and polarizabilities of centrosymmetric and dimeric molecules. I. Model study of a bichromophoric molecule

Abstract Refractometric, dielectric and electro-optical absorption measurements are reported for 1-dimethylamino-2,6-dicyano-4-methyl-benzene (I) and 1,4-bis(4′-dimethylamino-3′,5′-dicyanophenyl)bicyclo[2.2.2]octane (II). The evaluation leads to dipole moments and polarizabilities of the ground state as well as the first dipole allowed singlet state. The experimental res excellently substantiate the method of electro-optical absorption measurements in solution. It is shown that the excited dimer wavefunctions of the bichromophoric molecule II localize by solvent induced local site perturbations.

Excited state dipole moments and polarizabilities of centrosymmetric and dimeric molecules. II. Polyenes, polyynes and cumulenes

Abstract Electro-optical absorption spectra are measured for a series of polyenes, polyynes and cumulenes with centrosymmetric π-chromophores in cyclohexane solution at 298 K. For all molecules the long-axis component of the polarizability tensor is considerably larger in the first dipole-allowed singlet state compared to the ground state. The transition moments are found to be parallel to the long molecular axis. All polyenes and one cumulene show a linear Stark component indicating a long-axis excited state dipole moment. Both the dipole moments and the polarizabilities are corrected within the extended Onsager model for solvent cavity and reaction field effects. It is suggested that symmetry lowering solvent perturbations are the reason for the apparent excited state dipole moments.

Excited state dipole moments and polarizabilities of centrosymmetric and dimeric molecules. III. Model calculations for 1,8-diphenyl-1,3,5,7-octatetraene

Abstract A vibronic coupling model is used for the quantitative interpretation of both the 1 1 B u ← 1 1 A g absorption and the 2 1 A g → 1 1 A g fluorescence spectra of 1,8-diphenyl-1,3,5,7-octatetraene (DPO) in cyclohexane solution at 298 K in the absence and in the presence of an external electric field (electro-optical spectra). It is assumed that the coupling of the close lying 2 1 A g and 1 1 B u states is mainly due to static perturbations by the solvent environment and occurs via two totally symmetric CC and CC bond stretching vibrations. The solvent perturbations give rise to non-zero dipole moments of the vibronic excited states and influence the excited state polarizabilities. Both quantities strongly depend on the considered vibronic level. This implies that the electro-optical absorption band is not homogeneous with respect to the electro-optical properties of the underlying transitions. Some general aspects of the evaluation of heterogeneous electro-optical spectra are discussed for a two-band system. The long axis polarizability of the fluorescent state of DPO is determined by integral electro-optical emission measurements.

Experimental determination of the S1 torsional potential of 9,9’‐bianthryl in 2‐methyl‐butane by simultaneous Franck–Condon and band shape analysis of temperature dependent optical fluorescence spectra

Optical fluorescence spectra of anthracene (AN) and of 9,9’‐bianthryl (BA) have been measured in the apolar solvent 2‐methyl‐butane (2MB) at seven different temperatures in the range 123 K to 298 K. The AN spectra are strongly structured and consist of simple Franck–Condon (FC) progressions. The BA spectra on the other hand are broad and show unusual temperature dependencies. These effects are interpreted within a two state model with five vibrational and one torsional degree of freedom. A FC analysis is carried out using vibronic band shape functions which are convolutions of two functions. The first one is a Gaussian representing the spectral broadening due to librations and intermolecular interactions. The second one results from a semiclassical treatment of the intramolecular torsional mode (large amplitude motion, LAM). Optical transitions are assumed to occur from ensembles which are thermally equilibrated with respect to their nuclear degrees of freedom. The Boltzmann distribution with respect to t...

Solvent dependence of effective S1 torsional potentials in 9,9′-bianthryl and 9-phenylanthracene

Optical fluorescence spectra of 9,9′-bianthryl (BA) and 9-phenyl-anthracene ( 9PA ) were studied in 2-methylbutane ( 2 MB ) at various temperatures in the range 110–298 K and in 2MB/benzene (BE) solvent mixtures at 298 K. Effective S1 torsional potentials of BA and 9PA in the different solvents were determined by means of a simultaneous Franck-Condon and bandshape analysis of the experimental spectra. The solvent influence on the excited state torsional potential is observed to be stronger in the case of BA as compared to 9PA. The model calculations suggest that the barrier of the S~ double minimum potential of BA is increased from280 cm−1 in 2MB to465 cm−1 in BE solution whilst the minimum angle min is shifted from 62° to 58°.

Anisotropic fluorescence of Michler's ketone in liquid solutions

Abstract In solutions of Michlers ketone in ethanol at 295 K, the fluorescence is almost completely anisotropic. This phenomenon is caused by the unusually short lifetime of the excited singlet state due to a short intrinsic lifetime and processes leading to a very low quantum yield. The absorption, fluorescence and phosphorescence spectra, their degree of anisotropy and the values of the quantum yield at 295 K, 203 K and 100 K are reported.

Model Substances for Electro-optical and Dielectric Studies, Part I Synthesis and Structure of 1,4-Bis(4'-dimethylamino-3',5'-dicyanophenyl)bicyclo[2.2.2]octane

A synthesis for a new 1,2,6-donor-acceptor-substituted chromophoric benzene system is given. This chromophore is incorporated in a bicyclo[2.2.2]octane system to build up model compounds for electro-optical and dielectrical studies. Furthermore a preparative convenient route to derivatives of 1,4-bis(diphenyl)bicyclo[2.2.2]octane is worked out. The structures of the new compounds are proven by spectroscopical means.

The Determination of Molecular Quantities from Measurements on Macroscopic Systems.II. The Determination of Electric Dipole Moments

Abstract Some bulk quantities appropriate for the description of dielectric and refractometric measurements on macroscopic systems are defined and their properties are discussed. Based on three molecular models (Lorentz model, Onsager model in spherical approximation, and in ellipsoidal approximation) model molar quantities are introduced, which depend on intrinsic properties of the molecule, i.e. the electric dipole moment and the polarizability. The relations will be applied for the evaluation of the results of dielectric and refractometric measurements on eight aromatic molecules.