Exciplexes photonics on the silica surface

Processes of photoinduced electron transfer and formation of bimolecular complexes on the surface of nonporous dispersed silica and aerosil have been explored. Complexes of the two types, with a dissociative (acene + electron donor) and a stable (acene + acceptor) ground state have been obtained. Formation of the anthracene-N,N-diethylaniline exciplex on the surface proceeds in a diffusion mode. Increasing the quencher concentration to σ = 1% is accompanied by exciplex formation. Formation and stabilization of Mullican’s (dark) complexes has been noted with σ = 10%. The reaction of complexation on the surface is stimulated by heating. A suggestion of CTC localization in places of the highest energetic uniformity of the surface has been made.     

Interparticle diffusion of aromatic hydrocarbons on aerosil surface

It has been demonstrated by the photobleaching method that the photostability of pyrene molecules on the surface of pyrogenic nonporous SiO₂ (aerosil) is independent of the surface concentration of the fluophor. Through a kinetic analysis of the recovery of fluorescence of the luminophore and solution of the two-dimensional diffusion equation, coefficients of interparticle diffusion were estimated for the molecules of pyrene and N,N-dimethylaniline. Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 31 Nauki Prospect, Kiev, Ukraine 252039. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 1, pp. 23–26, January–February, 1998.     

Thermal activation of complexation reaction on aerosil surface

The influence of temperature in the 20–100°C range on processes of bimolecular interaction in the aerosil-phenanthrene-trinitrobenzene system has been studied. Temperature-stimulated CTC accumulation on the surface is found. Activation energies of complex formation are estimated. Quenching of phenanthrene fluorescence by acceptor has a mixed static and dynamic character.     

Exciplex luminescence studies of the surface polarity of end-capped bonded phases

The fluorescence intensities and emission maxima of surface-bound exciplexes were studied to determine the relative polarity of microparticulate silica before and after derivatization with trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDS). The results indicate that HMDS yields a bonded phase with greater average polarity relative to TMCS at equivalent surface coverages. This difference is attributed to the formation of basic sites on the surface by chemisorption of ammonia generated during derivatization with HMDS. The observation of exciplex emission on silica gel is novel and strongly supports clustering of surface-bound molecules into high-density regions.     

Photophysics of pyrene adsorbed on aerosil

The spectral luminescence properties of pyrene adsorbed on the surface of aerosil have been investigated at different thermal activation temperatures (453, 773, and 1073°K) and different degrees of surface coverage (0.01–10% of monolayer). The differences from the properties of pyrene adsorbed on silica gel are dictated by the high polarity of the aerosil surface, the less significant rearrangement of its hydroxyl coverage in thermal treatment, and its pronounced energetic discrete homogeneity at low degrees of coverage.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 121–125, January–February, 1987.     

Effect of gold nanoparticles on an aerosil surface on the fluorescence and Raman spectra of adsorbed tryptophan

We discuss the fluorescence and Raman spectra of the amino acid tryptophan (Trp) in the presence of gold nanoparticles in solution and on the surface of highly dispersed silica (aerosil) containing gold nanoparticles (Au-SiO₂). The fluorescence of Trp is efficiently quenched in the presence of gold nanoparticles both in solution and on the SiO₂ surface. The fluorescence and excitation spectra contain bands for molecular Trp and a charge transfer complex between Trp and the nanoparticles. In the Raman spectra of Trp with gold nanoparticles, considerably enhanced intense vibrations appear for the carboxyl and amino groups and also for the benzene and pyrrole rings. The effect of gold nanoparticles on the Raman spectra of Trp in a heterogeneous system is considerably weakened due to strong light scattering by the dispersed silica.     

It spectroscopic study of the interaction of chlorosilanes with the surface of aerosil

The surface-chemical reactions of the chlorosilanes

$$\left( {\begin{array}{*{20}c} {} & {} & {} & {{\text{CH}}_{\text{3}} } & {} & {} & {{\text{CH}}_{\text{3}} } & {} \\ {} & {} & {} & {} & {} & {} & {} & {} \\ {{\text{CL}}_{\text{3}} {\text{SiH,}}} & {{\text{CL}}_{\text{3}} {\text{Si}}} & {} & {} & {,{\text{ ClSi}}} & {} & {} & {\text{H}} \\ {} & {} & {} & {} & {} & {} & {} & {} \\ {} & {} & {} & {\text{H}} & {} & {} & {{\text{CH}}_{\text{3}} } & {} \\ {} & {} & {} & {} & {} & {} & {} & {} \\ \end{array} } \right)$$     

Electronic donor-acceptor interactions of anthracene with a dehydroxylated surface of silica gel and aerosil

The ability of a dehydroxylated surface of silica gel and aerosil to enter into donor-acceptor interactions with poly-acenes was demonstrated by methods of steadystate and kinetic spectrofluorometry. The anthracene molecule forms EDA complexes with electron-acceptor sites of silica gel and aerosil in the ground state (charge-transfer complex, CTC) and the excited state (exciplex). At a degree of coverage of the silica gel and aerosil surfaces by polyacene molecules of 10^–1–10^–3% of a monolayer, energetic and structural inhomogeneity of the OH groups of silica gel appears. Silica gel possesses stronger electron acceptor sites; the electron-acceptor sites of aerosil are more accessible to EDA interactions and more monotypic. Vacancy defects of the SiO₂ matrix are suggested as accepting sites.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 693–698, November–December, 1986.     

Colloid stationary phases containing aerosil

Summary Binary colloid systems, including aerosil and an organic modifier (e.g. polyethylene glycol 400 or squalane) are suggested as stationary phases for gas chromatography. The dependence of solute retention on the composition of the phases is studied and the influence of the sol — gel transition on the retention indicated.     

Studies on catalytically active surface compounds, X. Alkali metal induced slow tumbling of vanadyl complexes on the surface of aerosil

The effect of alkali metal admixtures (Li, Na, K, Rb, and Cs) on V/SiO₂ catalysts was studied by ESR spectroscopy. Alkali metals strongly change the VO²⁺ spectra. Reduction leads to spectra showing a slow rotational motion of VO²⁺ species on the support surface. The proposed explanation includes former structural models and the formation of salts of polyvanadium acid.

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(Li, Na, K, Rb Cs) V/SiO₂ . VO²⁺. , VO²⁺ . .

     

Electrooptical effect by aerosil suspension

Summary The electrooptic investigations carried out on suspensions of spherical isotropic particles (aerosil 0 × 50) of diameter 500 Å demonstrate the existence of reproducible effects in a wide interval of conditions of preparation and measurement. The observed effects are interpreted as an indication of the presence of anisodiametric particles the latter being aggregates of 12 spheres. The comparison of the data, obtained by the value of the electrooptic effect on one hand and by the velocity of the field-free decay of the effect on the other, shows that not more than 10% of the initial particle number is presented by single spheres. The importance is discussed of the model chain of spherical particles for the electrooptic investigations and for the study of the interaction between disperse particles.

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Zusammenfassung Die durchgeführten elektrooptischen Untersuchungen der Suspensionen von kugelförmigen isotropen Teilchen (Aerosil 0 X 50) mit einem Durchmesser von 500 Å zeigen das Vorhandensein reproduzierbarer Effekte in einem weiten Bereich von Herstellungsund Meßbedingungen. Der beobachtete Effekt wird als ein Hinweis auf die Anwesenheit anisometrischer Teilchen gedeutet, die Aggregate von 12 Kugeln darstellen. Der Vergleich der Daten, die aus der Größe des elektrooptischen Effekts erhalten wurden, mit diesen aus der Geschwindigkeit des Verschwindens des Effektes nach Ausschalten des Feldes zeigt, daß nicht mehr als 10% von der Anfangsanzahl der Teilchen als einzelne Kugel vorhanden sind. Es wird die Bedeutung des Modells Kette aus kugelförmigen Teilchen für die elektrooptischen Untersuchungen und für die Erforschung der Wechselwirkung zwischen dispersen Teilchen diskutiert.

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With 1 figure     

Guided-wave liquid-crystal photonics

In this paper we review the state of the art in the field of liquid-crystal tunable guided-wave photonic devices, a unique type of fill-once, molecular-level actuated, optofluidic systems. These have recently attracted significant research interest as potential candidates for low-cost, highly functional photonic elements. We cover a full range of structures, which span from micromachined liquid-crystal on silicon devices to periodic structures and liquid-crystal infiltrated photonic crystal fibers, with focus on key-applications for photonics. Various approaches on the control of the LC molecular orientation are assessed, including electro-, thermo- and all-optical switching. Special attention is paid to practical issues regarding liquid-crystal infiltration, molecular alignment and actuation, low-power operation, as well as their integrability in chip-scale or fiber-based devices.     

Theoretical molecular photonics

Theoretical concepts of molecular photonics are presented. Features of the photophysical processes in molecular systems and the relationship between their probability and the electronic structures of the molecules are discussed. Possible mechanisms for the photochemical decomposition of electron-excited molecular systems with chemical bond cleavage are considered. The effect adiabatic photochemical reactions have on the probability of nonradiative conversion processes is shown. Dependences of spectral luminescence properties, the photochemical and radiation chemical stability of the molecular systems, and the efficiency of molecules of photostabilizers of polymers on their structure are interpreted. Areas of application are indicated for molecular photonics.     

DNA molecular photonics

Synthetic DNA conjugates in which one or both ends of a short duplex is capped by a stilbene chromophore have been prepared and characterized crystallographically. Selective excitation of the chromophore can be used to initiate electron transfer processes in which a nucleobase serves as either an electron donor or an electron acceptor. These processes include hole- and electron injection and hole migration. The dynamics of these processes and its dependence on distance, driving force, and base sequence have been investigated by means of femtosecond time-resolved spectroscopy. Duplexes with identical chromophores at both ends have been used to study both the dynamics of electron transfer processes and exciton coupling between the two chromophores by means of circular dichroism spectroscopy. Duplexes with different chromophores can also be used to study distance dependence of both electron transfer and exciton coupling.     

Studies on catalytically active surface compounds: XVIII. Photoreduction of oxidic vanadium clusters on aerosil and their chemisorptive properties

Photoreduction by CO of silica supported vanadia catalysts of different cluster size has been studied. It was found that the rates of photoreduction and the highest attainable degrees of reduction depend on the size of vanadia clusters. The chemisorptive properties of photoreduced catalysts toward but-1-ene and O₂ are studied and discussed in terms of the distribution of V³⁺ ions in partially reduced clusters.

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. , -1 O₂. V³+ .