Stilbene molecules

Firstly, we focus on cofacial dimers formed by stilbene molecules in such conformations, the amplitude of interchain interactions is expected to be maximized [57], Table 4-1 collects the INDO/SCl-calculated transition energies and intensities of the lowest two excited states of stilbene dimers for an interchain distance ranging from 30 to 3.5 A. 

Regarding the emission properties, AM I/Cl calculations, performed on a cluster containing three stilbene molecules separated by 4 A, show that the main lattice deformations take place on the central unit in the lowest excited state. It is therefore reasonable to assume that the wavefunction of the relaxed electron-hole pair extends at most over three interacting chains. The results further demonstrate that the weak coupling calculated between the ground state and the lowest excited state evolves in a way veiy similar to that reported for cofacial dimers. 

Table 4-1. INDO/SCI-calculalcd iransilion energies, intensities, and Cl dcscriplions of llie lowest two excited stales (S and S2, respectively) of a cofacial dimer formed by Iwo stilbene molecules for various interchain distances.

Riedel D, Cranney M, Martin M, Guillory R, Dujardin G, Dubois M, Sautet P (2009) Surface-isomerization dynamics of trans-stilbene molecule adsorbed on Si(100)-2xl. J Am ChemSoc 131 5414... 

This obvious supposition occured to be false—the addition of stilbene causes no changes in the results of the reaction (Chang and Johnson 1965, 1966). It follows that an effective contact between PhCH=CHPh and (PhC=CPh) , 2Na is less than probable. As known, (PhC=CPh) , 2Na is a polymeric aggregate [(PhC=CPh) , 2Na+] . Bulk stilbene molecules are incapable of diffusing inside the aggregate. At the same time, the penetration of proton (or methanol) seems to be possible. 

The ii-stilbene molecule is planar in crystalline form, in gas phase, and presumably in solutions, although the phenyl group may be rotated as much as 32° to reduce nonbonded repulsions between hydrogen atoms (Waldek 1991, Meier 1992). This modest twisting still allows sufficient... 

H. Hamaguchi What information do you have concerning the structure and dynamics of the hexane-dressed stilbene molecule How flexible or how rigid is the structure ... 

E. Poliak The RRKM theory and Kramers theory and its later generalizations by Grote, Hynes, and other are two sides of the same coin. In the spatial diffusion limit, one can show that Kramers s rate expression is identical in form to the RRKM expression, that is, a ratio of equilibrium unidirectional flux and density of reactants. The difficult problem in the application of RRKM theory to the stilbene molecule with a few attached benzenes is whether the equilibration of energy occurs fully on the time scale of the isomerization. One should also... 

The fluorescence of coumarin compounds has been well known since 1911, when it was observed that the absorption band of coumarin at 311 nm was bathochromically shifted by one of several auxochromic groups, such as hydroxyl or amino, in the 3- or 7-position. A comparison of the stilbene molecule (88) with 3-phenylcoumarin (93) indicates the similarity of structure, and basically similar substitution patterns in the latter molecule have produced similarly useful FBAs (69FRP1568007). 

M s in DMSO-water in the presence and absence of amylose. Thus quenching is reduced about 30-fold for iodide by amylose incorporation of the stilbene chromophore. While it is somewhat uncertain as to what precisely the nature of the quenching of stilbene by iodide is, it is reasonable to assume that the reduced quenching constants imply a more difficult approach of the iodide ion to the complexed stilbene than to the free. We are currently exploring many aspects of reactivity of amylose-incorporated chromophores. We find for example that amylose is able to extract totally insoluble hydrophobic stilbene molecules into water and we are presently trying to obtain crystal structural data on the complex molecule. The dynamics of complex formation and dissociation are currently under investigation. 

Let us mention the rotational beats which have recently been observed (with a trans-stilbene molecule in a gasodynamic jet) and interpreted by Felker and Zewail [54, 145]. The essence of this interesting phenomenon may be described in a simplified way as follows. Let a picosecond laser pulse be capable of coherently exciting from some rotational state J f of a... 

The light-induced reversible trans-cis photoisomerisation of franv-stilbene molecule in condensed media includes at least four macroscopic stages excitation of the stilbene chromophore, radiative deactivation of the excited state with the rate constant kr, medium relaxation around the excited stilbene molecule with the rate constant km (to provide space for torsional distortion during the photoisomerisation process) and eventually, twisting transition with the rate constant kt c. 

If the rate-limiting stage in an overall photoisomerisation process of the excited stilbene molecule in a viscous medium is the medium relaxation (kr km) and the isomerization after the relaxation proceeds faster than the excited-state fluorescence decay (k,.c kr), the apparent steady-state rate constant of the overall trans-cis photoisomerisation process may be expressed as follows (Likhtenshtein et al., 1992, 1996 Likhtenshtein, 1993 Papper and Likhtenshtein, 2001) ... 

Han et al. (1995) measured hole mobilities of a series of enamines doped into a PC. In comparing the results with similar hydrazone, aiylamine, and stilbene molecules, it was noted that the enamines showed higher mobilities. Describing the results by Gill s expression gave iQ between 1 and 7 x 1(H cm2/Vs, A0 between 0.41 and 0.47 eV, and T0 between 375 and 388 K. 

Theoretical calculations have been made on stilbene which are relevant to photoisomerization dynamics. MNDO calculations of stilbene potential energy properties shows no evidence of a doubly excited "phantom" state but a singly excited state with adiabatic rotation around the central ethylene bond has only a small barrier on this path23T Calculations of dipole moments, optical spectra, and second order hyperpolarizability coefficients of some mono- and disubstituted stilbene molecules allows the design of useful nonlinear optical molecules 38. 

F re 4-4. Schematic representation of the one-electron structure of a single stilbene molecule and that of a cofacial dimer formed by two chains separated by 4 A. The INDO-calculated energy spUt of the HOMO and LUMO levels when going from the isolated molecule to the dimer are also given. 

Figure 4-5. INDO/SCI-calculated transition energies of the lowest two optical transitions of a cofacial dimer formed by two stilbene molecules as a function of interchain distance. The horizontal line refers to the transition energy of the isolated molecule. Note that the upper value reported at 3.5 A corresponds to the transition to the fifth excited state, which provides the lowest intense absorption feature.

Table 4-2. Ground -state and lowest-excited state AMl/CI-optimized C-C bond lengths (in A) in a cofacial dimer formed by two stilbene molecules separated by 3.5, 4.0, and 4.5 A, respectively. The geometrical parameters for single molecules are reported in the right-hand-side column.

Figure 4-9. INDO/SCI-simulated absorption and emission spectra of two stilbene molecules with a huge interchain distance (solid lines) and those of a cofacial dimer formed by two stilbene chains separated by 4 A (dotted lines).

---