Bas Wegewijs - Academia.edu (original) (raw)

Papers by Bas Wegewijs

Physical Review B, 2000

We have determined the excess polarizability of the relaxed first excited-singlet-state S 1 on is... more We have determined the excess polarizability of the relaxed first excited-singlet-state S 1 on isolated chains of phenylene-vinylene oligomers and polymers by measuring the transient change in the microwave dielectric constant that occurs on flash photolysis of dilute solutions. The isotropic value of the excess polarizability volume, ⌬␣Ј(S 1), increases from 250 Å 3 for a dimer to 960 Å 3 for an octamer, and 1600 Å 3 for high molecular weight polymers. The value for the isolated polymer is close to that found for the unrelaxed S 1 state in electroabsorption ͑Stark effect͒ measurements on solid samples of poly͑phenylene-vinylene͒, indicating that exciton delocalization is not strongly influenced by either post-excitation relaxation of the polymer backbone geometry, or by interchain interactions.

Recueil des Travaux Chimiques des Pays-Bas, 2010

ABSTRACT The solvent dependence of the radiative-rate constant of both inter- and intramolecular ... more ABSTRACT The solvent dependence of the radiative-rate constant of both inter- and intramolecular donor-acceptor systems reveals the occurrence of significant intensity borrowing from one or more local transitions. This phenomenon appears to be especially pronounced in systems of the type D-3σ-A, where donor and acceptor interact via through-bond interaction over a relay of three σ bonds. Within the context of a three-state model, involving the no-bond state (DA), the charge transfer state (D+A−) and a single locally excited state [(DA)*], a simplified method is given to analyse the electronic coupling elements between these states.

The Journal of Physical Chemistry A, 1998

Time-resolved laser-induced optoacoustic spectroscopy was used to study the charge separation pro... more Time-resolved laser-induced optoacoustic spectroscopy was used to study the charge separation process taking place in the excited state of a donor−bridge−acceptor (D-br-A) compound in alkane solvents. This molecule contains a D−A pair separated by a rigid saturated hydrocarbon bridge 6 sigma bonds long. Excitation at 308 nm results in very efficient long-range charge separation, leading to formation of a charge-transfer (CT) state with a dipole moment of 38 D. By monitoring the pressure waves generated during the decay of the excited species, we could discern three consecutive relaxation processes. To separate the contributions of enthalpy and structural volume changes (ΔVstr), experiments were carried out with a series of n-alkanes having different photothermal properties. However, the usual separation method clearly failed for this system, and a new approach was developed to ensure that ΔVstr remained constant across the alkane series. This consisted of selecting conditions where the solvent compressibi...

The Journal of Physical Chemistry A, 2001

Journal of the American Chemical Society, 2001

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to in... more The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S(1) state of 9,9'-bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bianthryl (CAAC). Changes in both the real, Deltaepsilon' (dielectric constant), and imaginary, Deltaepsilon' ' (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S(1) for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, flip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca. 20 D determined from the solvatochromic shifts in the fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred picoseconds and is controlled mainly by diffusional rotation of the molecules. The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results.

Journal of Physical Organic Chemistry, 1996

The process of intramolecular charge separation was studied in three nearly identical donor-bridg... more The process of intramolecular charge separation was studied in three nearly identical donor-bridge-acceptor compounds (1-3) under jet-cooled conditions. Each consists of a vinylcyanonaphthalene group as a powerful electron acceptor and an anilino derivative as an electron donor, separated by a rigid hydrocarbon bridge. The only difference between the three compounds is the substituent at the para position of the anilino group, which directly influences the ionization potential of the donor. For compound 1, which has the weakest donor of the three, it is found that excitation at (or very near) the electronic origin of the acceptor chromophore results exclusively in local emission, while excitations with an excess energy >60 cm" lead to a distinct red shift. Comparison with 2 and 3 shows that the red-shifted band can be attributed to a charge-separated state with an extended conformation, in which an electron has been transferred across three sigma bonds. Interestingly, this charge separation process appears to be more efficient in 2 and 3 than in 1, resulting in complete quenching of the local emission even at the lowest energy excitations. These observations are interpreted in terms of intramolecular radiationless transition theory and a comparison is made with the behaviour of (semi)flexibly bridged systems in which charge separation was shown earlier to be linked to a major, electrostatically driven reorganization.

Journal of Photochemistry and Photobiology A: Chemistry, 1994

In this paper the fluorescent behaviour is studied under jet-cooled conditions as well as in solu... more In this paper the fluorescent behaviour is studied under jet-cooled conditions as well as in solution of two donor-bridge-acceptor molecules (1 and 2) in which the donor (D) and acceptor (A) are held rigidly apart by an extended saturated hydrocarbon bridge with an effective length of three sigma-bonds. It was found that even in the isolated molecules no excess excitation energy at all is needed to induce charge separation and that charge transfer (CT) fluorescence can be observed from both these molecules. Furthermore, broad red-shifted excitation bands are observed indicating that excitation can take place directly from the ground state into the CT state, even at energies considerably lower than that required to reach any of the locally (in D or A) excited states. Comparison of the radiative rate constants for CT fluorescence in media of different polarity indicates important intensity borrowing from local transitions in media of low polarity. Nevertheless, the admixture of locally excited configurations in the CT state is too small to diminish significantly the CT nature of the emissive state even in the gas phase.

been supported by the European Commission through the Life Long Learning (LLL) Programme – Erasmu... more been supported by the European Commission through the Life Long Learning (LLL) Programme – Erasmus programme – Multilateral Projects of its Directorate-General for Education and Culture. This publication reflects the views of the authors only, and the European Commission cannot be held responsible for any use which may be made of the information contained therein. © Nuffic / TUNING Association Higher Education Institutions are free to test and use the material in this publication, provided that the source is clearly acknowledged. No part of this publication, including the cover design, may be reproduced, stored or transmitted in any form or any means, whether electronical, chemical, mechanical, optical, by recording or photocopying, without prior permission of the publisher. © Publicaciones de la Universidad de Deusto

Chemical Physics Letters, 1993

Charge separation in isolated supennolecules, consisting of an aniline or Cmethoxy aniline donor ... more Charge separation in isolated supennolecules, consisting of an aniline or Cmethoxy aniline donor and a cyanonaphthalene acceptor connected by a semirigid bridge, is described in terms of irreversible sequential intramolecular relaxation involving electron transfer from an electronically excited molecular state and electrostatically driven folding, Sequential folding of the charge transfer state extends the energy range of this state, which is accessible by radiationless electron transfer. The analysis of the intramolecular electron transfer dynamics led to agreement between the distance scale for charge separation inferred from energetic and structural data and accounted for the excess energy dependence of the nuclear Franck-Condon vibrational overlap factors,

Recueil des Travaux Chimiques des Pays-Bas, 1995

Physical Review B Condensed Matter and Materials Physics 65, Jun 13, 2002

The pulse-radiolysis time-resolved microwave conductivity technique was used to study charge tran... more The pulse-radiolysis time-resolved microwave conductivity technique was used to study charge transport in three binary mixtures of triphenylene derivatives and in three of these components separately. In the liquid-crystalline mesophase the hole mobilities in the mixtures are found to be an order of magnitude higher than those in the separate components. This is partly due to the more stable columnar structure in the mixtures and partly due to efficient ``alloy band formation.'' In the crystalline phase the mobility in the mixtures is only a few times lower than that of the separate components. The ionization energies of the components as measured by cyclic voltammetry are up to ~0.5 eV apart. It turns out that this energy difference is easily compensated by the relatively large charge-transfer integrals for hole transport in the binary mixtures, which was obtained from ab initio Hartree Fock calculations. The mobilities estimated theoretically for ordered systems largely exceed the experimental values. This is most likely due to structural disorder along the columns in the material. Theoretical estimations of the hole mobility suggest that mobilities in excess of 1 cm2/V s could be attainable in well-ordered crystalline triphenylene samples.

Synthet Metal, 2001

ABSTRACT

Chem Phys Lett, 2000

The emission of electrons from conjugated polymer films occurs on laser photoexcitation at relati... more The emission of electrons from conjugated polymer films occurs on laser photoexcitation at relatively low photon energies, < 4 eV, and light intensities, < 10 15 photons/ cm2 in a ca. 5 ns pulse. The transient conductivity resulting from the highly mobile electrons in the gas phase depends on the nature and pressure of the gaseous environment and can dominate the intrinsic photoconductivity in the bulk of the polymer. The photoelectron contribution to the overall photoconductivity can be quenched by a gaseous environment of CO2+10% SF6 at atmospheric pressure.

Journal of Physical Chemistry B, 1997

Advances in Chemical Physics, 1999

The Journal of Physical Chemistry, 1996