Dimethylamino)-4‘-nitrostilbene Research Articles

Peculiarities of electric field alignment of nonlinear optical chromophores incorporated into thin film polymer matrix

The electric field poling process of free-standing films of poly(methyl methacrylate) (PMMA) matrix doped with the nonlinear optical compound 4-(dimethylamino)-4'-nitrostilbene (DMANS) was investigated by molecular simulation methods. The influence of the vacuum/bulk interfacial regions on static and dynamic properties, including the glass transition temperature Tg and the field-induced chromophore reorientation, was studied by employing films of three different thicknesses and by comparison with previous work on the bulk system. The interfacial region, defined as the region, where the local density increases from zero to the bulk density, is about 2 nm wide, independent of the film thickness. Tg decreases with decreasing film thickness, in accord with previous experimental work and theoretical predictions. The resistance against field-induced chromophore reorientation in the liquid state is found to increase strongly with the film thickness.

Study of secondary relaxations of poly(ethylene terephthalate) by photoluminescence technique

The α and β relaxation processes in two types of poly(ethylene terephthalate) with different degrees of crystallinity were studied by means of three methods, differential scanning calorimetry, dynamic-mechanical analysis and fluorescence spectroscopy. Information provided is complementary in the mean that every method sense phenomena that may occur at different times and length scales. Several probes, Coumarin 152 (C152), Coumarin 153 (C153), Coumarin 337 (C337) and 4′-dimethylamino-4-nitrostilbene (DMANS), were adsorbed in polymer films, and their fluorescence analysed over the temperature range from −150 to 150 °C. In general, a decrease in fluorescence intensity of probes as temperature increase was observed. This behaviour has been explained as a consequence of the enhancement of the free volume fraction that favoured the radiationless process of the lowest excited singlet state. Plots of fluorescence intensity versus temperature showed changes around the secondary relaxation temperatures. Therefore, good correlations between fluorescence and dynamic mechanical and calorimetric analysis were established. The obtained results indicated that the fluorescence from the probes incorporated to the material was dependent on the crystallinity of polymer. It would indicate that the fluorescence emission from those probes can be used to analyse annealing processes in semicrystalline polymers.

Molecular Dynamics Simulations of Electric Field Poled Nonlinear Optical Chromophores Incorporated in a Polymer Matrix

The electric field poling process of nonlinear optical chromophores embedded in an amorphous polymer matrix was studied using molecular dynamics (MD) simulations. Three systems were considered, consisting of a poly(methyl methacrylate) matrix doped with the following chromophores: N,N-dimethyl-p-nitroaniline (DPNA), 4-(dimethylamino)-4‘-nitrostilbene (DMANS), and N,N‘-di-n-propyl-2,4-dinitro-1,5-diaminobenzene (DPDNDAB). The cooling process in the presence of a poling electric field was simulated at constant NPT conditions using simulated annealing. The rotational dynamics of the dopants was investigated in the unpoled and poled states above Tg and in the poled state below Tg. The short-time behavior with respect to the back-relaxation to the unpoled state following removal of the poling field was examined for the systems below Tg and was found to deviate from the single-exponential model. The electric field effects, during and following poling, were examined by computing the angle between the dipole mom...

Effects of a Paracyclophane Linker on the Charge-Transfer Transition of 4-(Dimethylamino)-4‘-nitrostilbene

Resonance Raman intensities of the push-pull chromophore 4-(dimethylamino)-4′-nitrostilbene (DANS) have been measured in dichloromethane, acetonitrile, and methanol at excitation wavelengths spanning its strong visible charge-transfer absorption band. The effects of inserting a paracyclophane moiety between the donor and the acceptor are investigated by performing similar measurements on a second molecule, PCP-DANS, which consists of p-dihexylaminostilbene and p-nitrostilbene moieties attached through their unsubstituted rings by a paracyclophane linker. The Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine reorganization energies along the Ramanactive normal coordinates. Excitation to the nominal charge-transfer state generates a much greater change in geometry in the vicinity of the nitro acceptor group in DANS than in PCP-DANS, while the geometry changes around the dialkylamino donor end are similar. Comparison with other push-pull chromophores coupled with the results of ZINDO electronic structure calculations both suggest that the transition in DANS is well-described as an intramolecular charge-transfer transition, whereas the transition in PCP -DANS has less charge-transfer character.

Isolated Building Blocks of Photonic Materials: High-Resolution Spectroscopy of Excited States of Jet-Cooled Push-Pull Stilbenes

The lowest excited singlet states of 4-(dimethylamino)-4‘-cyanostilbene (DCS), 4-(dimethylamino)-4‘-nitrostilbene (DANS), and 4-di(hydroxy-ethyl)amino-4‘-nitrostilbene (DANS-diol) have been investigated by high-resolution fluorescence excitation and dispersed emission spectroscopy on samples seeded in supersonic expansions. Using ab initio calculations of the harmonic force fields of the electronic ground state as a starting point for the analysis of vibrational modes and frequencies, detailed vibrational assignments of the rich line structure of the S1 ← S0 transition in these three push−pull stilbenes are reported. From the experimental and theoretical analysis, it becomes apparent that the excitation spectra are dominated by low-frequency vibrational modes of the stilbene-like backbone, albeit that a characterization in terms of trans-stilbene vibrational terminology is not very appropriate. The fluorescence excitation and emission spectra show significant Franck−Condon factors for transitions involvin...

Determination of first hyperpolarizability of nonlinear optical chromophores by second harmonic scattering using an external reference

First hyperpolarizabilities (β) of two tricyanovinylthiophene nonlinear optical chromophores were determined using second harmonic, hyper-Rayleigh, scattering. The use of an external standard in the determinations is compared with the previous use of an internal standard. The first hyperpolarizability of the well-known chromophore 4 dimethylamino 4′ nitrostilbene (DANS) was also determined using the external standard method and its value is compared to those in the literature. A new method of analyzing the hyper-Rayleigh scattering signal by recording a histogram of the scattered energy is reported. This method is shown to give more reliable results in a shorter period of time than the usual static gate method. The histogram function provides additional information in the form of the histogram width which is shown to be an indication of the mean-square concentration fluctuations of the chromophores in solution. The effects of molecular interactions on the concentration dependence of the hyper-Rayleigh scattering signal is discussed. It is shown that depending on the concentration range, the β value of para-nitroaniline, used as the external standard, can agree with two different values reported in the literature. Results on the depolarization ratio of the hyper-Rayleigh scattered light from one of the tricyanovinylthiophene chromophores are presented. At low concentration the depolarization ratio agrees with the theoretically predicted value based on C2v symmetry. However, at higher concentration the measured depolarization ratio increases indicating molecular interactions.

Molecular second-order optical nonlinearities of metallocenes

The nonresonant quadratic molecular hyperpolarizabilities of several ferrocene and ruthenocene derivatives were studied by dc electric-field-induced second-harmonic generation (EFISH) experiments using fundamental radiation at 1.91 pm. Hyperpolarizabilities approaching that of 4-(dimethylamino)-4'-nitrostilbene (DANS) were observed, indicating that the ferrocene moiety can act as an effective donor. It was found that the dipole projections of the B tensors of ferrocene complexes