Heribert Reis

Ground and excited state dipole moments of coumarin laser dyes: Investigation by electro-optical absorption and emission methods

Abstract Modified electro-optical absorption and emission methods were used to measure the dipole moments of four efficient coumarin laser dyes (CU1, CU4, CU120, CU334) in the equilibrated ground, excited Franck-Condon and equilibrated excited states. The measurements were performed in cyclohexane (CU1, CU120, CU334) and dioxane (all CUs) at room temperature. Our results show that the charge distribution in CU4 differs substantially from that in the other CUs. The equilibrated ground and excited state dipole moments measured by electro-optical methods are compared with those derived from other measurement techniques and from semiempirical calculations. The possible role of twisted intermolecular charge transfer (TICT) state formation is discussed.

Dipole moments of aminophthalimides determined by modified electro-optical absorption and emission measurements

Modified electro-optical absorption and emission methods were used to measure the electric dipole moments of five aminophthalimides in their ground and excited states. The equilibrated ground and excited state dipole moments measured by these methods are in reasonable agreement with those derived from other measurement techniques, confirming the general validity of these methods. A difference is observed between the properties of 3-aminophthalimide (3AP) and 4-amino-N-methylphthalimide (4ANMP) in different solvents. The equilibrated dipole moment of 3AP in its excited state is practically independent of the solvent polarity in contrast with 4ANMP. The possible mechanism of this effect is discussed.

Spectral and electrooptical absorption and emission studies on internally hydrogen bonded benzoxazole `double' derivatives: 2,5-bis(benzoxazolyl)hydroquinone (BBHQ) and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Single versus double proton transfer in the excited BBPC revisited

Abstract Ground and excited state dipole moments and polarizabilities of 2,5-bis(benzoxazolyl)hydroquinone (BBHQ) and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC) are determined by means of electrooptical absorption and emission measurements. BBHQ is found to exhibit a small, while BBPC a large increase of the static polarizability in the Franck–Condon (FC) excited singlet state. The change of the dipole moments upon excitation to the FC state is zero within experimental error. However, both molecules show dipole moments in the fluorescent states of their phototautomers, of about 5 D, the major component being parallel to the long molecular axis. The experimental and theoretical results strongly suggest that the fluorescent species of BBHQ and BBPC are monoketo-tautomers created by single proton transfer in the course of the excited state relaxation. For BBPC this was confirmed by a comparative photophysical study with its monomethoxy-derivative (MeBBPC), in which one active proton is replaced by the CH3 group. These results lead to the revision of previous conclusions [Grabowska et al., Chem. Phys. Lett. 177 (1991) 17] stating that BBPC undergoes a double proton transfer in the excited state. For BBHQ the previously found single proton transfer mechanism of tautomerization is fully confirmed.

Electric field-induced anisotropy spectra

Abstract A novel method for the representation of results from the measurement of the effect of an external electric field on the absorption or fluorescence of solute molecules is derived from Liptays formalism so that the pure field-induced anisotropy of the absorbance or fluorescence intensity of solute compounds can be represented over the respective wavenumber interval. The method is demonstrated with 4-N,N-dimethylamino-4′-nitrostilbene and applied to three derivatives of dimethylaminobenzonitrile, namely 1-methyl-5-cyanoindoline, 2,6,N,N-tetra-methyl-4-cyanoaniline, and 6-cyanobenzo-quinuclidine.

Electro-optical and laser spectrofluorimetry study of coumarins 7 and 30: evidence for the existence of the close-lying electronic states and conformers

Our experiments and calculations reveal a set of anomalous and interesting properties of coumarin 7 (CU7) and coumarin 30 (CU30) in solution. The spectral dependence of electro-optical coefficients and, consequently, dipole moments indicates that the absorption band is a superposition of several (at least two) electronic transitions. Probably, there are two close-lying π,π* singlet electronic excited states which contribute to the first absorption band. The calculations using the MM2 program show that both dyes in the ground state have two stable plane anti conformers. From time-resolved spectrofluorimetric measurements we obtained evidence for the existence of conformers in paraffin oil solutions of CU30.

Linear and nonlinear optical properties of some organoxenon derivatives

We employ a series of state-of-the-art computational techniques to study the effect of inserting one or more Xe atoms in HC2H and HC4H, on the linear and nonlinear optical (L&NLO) properties of the resulting compounds. It has been found that the inserted Xe has a great effect on the L&NLO properties of the organoxenon derivatives. We analyze the bonding in HXeC2H, and the change of the electronic structure, which is induced by inserting Xe, in order to rationalize the observed extraordinary L&NLO properties. The derivatives, which are of interest in this work, have been synthesized in a Xe matrix. Thus the effect of the local field (LF), due to the Xe environment, on the properties of HXeC2H, has also been computed. It has been found that the LF effect on some properties is significant. The calculations have been performed by employing a hierarchy of basis sets and the techniques MP2 and CCSD(T) for taking into account correlation. For the interpretation of the results we have employed the complete active space valence bond and CASSCF/CASPT2 methods.

Polarizabilities and second hyperpolarizabilities of ZnmCdn clusters

Static average polarizabilities, α, and second hyperpolarizabilities, γ, of Zn m Cd n nanoclusters for m+n=2–6 have been calculated, at the ab initio level, using effective core potentials (ECP). We have developed in a systematic way a series of basis sets. Employing a hierarchy of methods (HF, MP2, CCSD, CCSD(T)), we have computed the polarizability of Cd. The α value we computed is in excellent agreement with the best theoretical and experimental values. From the set of the designed basis sets we selected one, for the calculation of the polarizabilities of the considered clusters, which gives a satisfactory α value for Cd, at a low computational cost. The basis set for Zn has been reported in our recent study of the properties of Zn m . We have also employed the basis sets developed by Kellö and Sadlej in order to compute the relativistic correction to α, which has been compared with the corresponding correlation contribution to α of Zn m Cd n . We have employed two ECPs, having a small and a large core. The polarizability results for Cd m , which have been computed by employing them, agree at the HF level, but at the MP2 level have a remarkable difference. The variation of α/m as a function of m for Zn m and Cd m , has been discussed. The average second hyperpolarizabilities of all clusters, at the MP2 level, are well described by a linear function of the composition of the clusters using constant atomic increments.

Excited-state acidity of bifunctional compounds

Fluorescence emission and excitation spectra and experimental dipole moments are presented for 5-(2-hydroxyphenyl)-3-phenyl-1,2,4-oxadiazole and 3-(2-hydroxyphenyl)-5-phenyl-1,2,4-oxadiazole. These data indicate that the long-wavelength emission of the former, in non-hydrogen bonding solvents, is due to a tautomeric PT structure. The latter exhibits only the ‘normal’ band. The striking difference in behavior between these two compounds of very similar structure is explained on the basis of coplanarity between the aryl ring in position 5 and the heterocycle ring, the aryl ring in the 3 position not being in the same plane.

Electro optical emission measurements on a non-conjugated bichromophoric donor-acceptor molecule

Electro optical emission measurements have often been used for the determination of dipole moments of fluorescent excited states. In this publication results from such measurements on a non-conjugated bichromophoric donor-acceptor molecule in cyclohexane, benzene and dioxane solutions are reported and compared to literature values determined by quite different time resolved microwave conductivity (TRMC) measurements. The dipole moment of the first excited state is found to be around 80 × 10-30 cm, in benzene and in dioxane which is in very good agreement with the results from TRMC measurements, if a comparable description of the in solution effective electric field is used with both methods. The results in cyclohexane do not agree, most probably because of the short excited state life time in this solvent. In addition, the direction of the transition moment was determined from the electro optical measurements to be parallel to the excited state dipole moment. This information cannot be derived from TRMC m...

Electro-optical and fluorescence lifetime studies of aminophthalimide probes in liquids

The modified electro-optical absorption and emission methods are described as well as their use for measurements of electrical dipole moments of five aminophthalimides in ground and excited states. It is found that there exists a principal difference between properties of 3- aminophthalimide (3AP) and 4-amino-N-methylphthalimide (4ANMP) in different solvents. The equilibrated dipole moment of 3AP in its excited state is practically independent on the solvent polarity, in comparison with 4ANMP. The possible mechanism of this effect is discussed.