Albert W. Marsman

The determination of first hyperpolarizabilities β using hyper-Rayleigh scattering : a caveat

Abstract A comparison of first hyperpolarizabilities β determined by hyper-Rayleigh scattering (HRS; β HRS ) and electric field-induced second-harmonic generation (EFISH; β EFISH ) for a series of donor-acceptor substitutetd π-conjugated organic compounds shows a marked discrepancy between β HRS and β EFISH values for NLO-chromophores which fluoresce in the frequency-doubled wavelength region (532 nm). For MONS and DANS it is unequivocally established that two-photon absorption (TPA) at the fundamental wavelength (1064 nm) occurs followed by Stokes and anti-Stokes fluorescence around the second-harmonic wavelength. Hence, hyper-Rayleigh scattering at 1064 nm is not suited to determining the β of these fluorescent NLO-chromophores.

Distance-dependent electron transfer in Au/spacer/Q-CdSe assemblies

CdSe quantum dots, in the form of colloidal nanocrystals, are bound to the surface of a gold electrode by bisulfide spacer molecules. The distance dependance of the electron transfer rate between the CdSe and the electrode was investigated by a time-resolved optoelectronic method and related to the length r of the spacer molecule 1–3 [Eq. (1)]. For s, a value of 0.50±0.05 A-1 was found.

Solid state structures in spin-coated assemblies of oligo(cyclohexylidene) oxime derivatives and (partly) saturated analogues: an atomic force microscopy study

Abstract The deposition mechanism of a series of oligo(cyclohexylidene) oximes and their (partly) saturated analogues is studied, using atomic force microscopy (AFM). Hydrophilic silicon wafers have been spin-coated with chloroform solutions of a number of compounds which are denoted as 1( n ) , with n =1 or 2 , and as 2 – 6 . The compounds 1(1) and 2 formed either micro-crystals or multilayer structures, depending on the amount dissolved in the spin-coating solution. Compounds 1(2) and 3 – 6 had a different deposition behavior. In the beginning of the deposition process, the films were not rigid enough to reveal structural characteristics with AFM. Under ambient conditions initially small crystals were formed which slowly grew to micro-meter-sized crystals. High-resolution AFM images of these micro-crystals revealed the molecular packing. Single-crystal X-ray data of 1(1) , 2 and 3 showed that the surface structure of the micro-crystals was consistent with that of a terminated bulk crystal. The relations between the molecular structure of the various compounds and the characteristic times for the nucleation, the crystallization and the ripening stages of the deposition process are qualitatively discussed. We compare the present results with structural and stability data of Langmuir monolayers, in cases where the latter are accessible.

Second-order nonlinear optical properties of stilbene,benzylideneaniline and azobenzene derivatives. The effect of π-bridgenitrogen insertion on the first hyperpolarizability

The second-order nonlinear optical (NLO) and electronic properties of stilbenes (CC bridge), benzylideneanilines (CN and NC bridges) and azobenzenes (NN bridge) containing either an N,N-dimethylamino donor and/or a nitro acceptor were investigated using EFISH, UV spectroscopy, cyclic voltammetry and PPP/SCF calculations. It appeared that although first hyperpolarizabilities of the ethylene and azo bridged donor–acceptor compounds are of comparable magnitude, substitution of one carbon by a nitrogen atom reduces the NLO activity. Differences in hyperpolarizabilities were rationalized with the aid of a two-level model, which revealed that they find their origin in the redox activity of the nitrogen-containing bridges.

Through-Bond Orbital Coupling in End-Functionalized Bicyclohexylidenes − Photoelectron Spectroscopy and Ab initio SCF−MO Calculations

To establish whether through-bond (TB) orbital interactions occur between the functional groups and the hydrocarbon skeleton in a series of end-functionalized oligo(cyclohexylidenes) 1−12, their HeI photoelectron (PE) spectra were measured and analyzed. Vertical ionization energies, Ivj, of the highest occupied molecular orbitals (MOs) of 1−12 were assigned using ab initio RHF/6−31G* MO energies (−εj) in combination with Koopmans’ theorem. Excellent to good agreement was found between the PES and RHF/6−31G* results. In addition, the Ivj, assignments were further corrobor-

Electronic and second-order nonlinear optical properties ofconformationally locked benzylideneanilines and biphenyls. The effect ofconformation on the first hyperpolarizability

The first hyperpolarizability β and electronic properties of N,N-dimethylamine and/or nitro-substituted benzylideneanilines and biphenyls are compared with those of conformationally locked model compounds, viz. identically substituted 2-phenyl-3,3-dimethyl-3H-indoles and fluorenes, respectively. Although PM3 semi-empirical calculations indicate that a small to moderate increase in β can be achieved by locking the π-systems in a planar conformation, experimental results show that the actual gain is either small or negligible. This mainly finds its origin in relatively narrow electronic absorption bands of the conformationally locked compounds, which are related to their rigidity. Moreover, single crystal X-ray structures of the donor–acceptor substituted 3H-indoles demonstrate that the presence of a saturated bridge does not necessarily lead to a more planar structure. Furthermore, it is shown that the difference in β of the two N,N-dimethylamino-nitro-substituted benzylideneaniline isomers is of electronic rather than of conformational origin.

Confirmation of syn—anti isomerism of 2,5-dimethoxydicyanoquinonediimine radical anions in solution

Abstract The conformational behaviour of 2,5-dimethoxydicyanoquinonediimine radical anions has been studied in polar solvents with ESR and proton ENDOR spectroscopy. Both syn and anti isomers of the radical anions have been identified. The two conformers differ in molecular symmetry and hyperfine coupling between the unpaired electron with nearby protons. Separate spectra of the conformers have been obtained with ENDOR-induced ESR. Finally, from electronic spin relaxation, the activation energy for the interconversion process between the syn and anti conformers was estimated at 28.5 kJ/mol (6.8 kcal/mol).