Interaction of aza-aromatic compounds with porous silica beads and controlled porous glasses as studied by absorption and fluorescence spectroscopy

Abstract

In this work, a comparative study of the surface acidities of different micro-porous solid media based on silicon dioxide (non-modified silica beads, silica beads modified with C30 alkyl chains, silica modified with polymers, and controlled porous glasses) is presented. For this purpose, conjugated molecules with additional electron-donor and proton-acceptor centres are used as adsorbates. Acridine (AC) and some of its derivatives 1,2,7,8-dibenzacridine (1-DBA), 3,4,5,6-dibenzacridine (3-DBA), and acridine orange (AO) are chosen for this study. The absorption and fluorescence spectra of the probe molecules in solution are used to compare with the spectra of the adsorbed species, and to distinguish between various surface binding sites of different acidity. The mobilities of the adsorbed probe molecules on the surfaces are determined by the fluorescence anisotropy. The kinetics of the adsorption of the probes at the surfaces are measured by time-dependent absorbance and fluorescence spectroscopy. The accessibility of the adsorbed species at the surfaces to oxygen and the reaction with active groups on chemically modified silica surfaces (mainly amino groups) are also studied by fluorescence spectroscopy. In the case porous Vycor glass surface as adsorbent and acridine as adsorbate, the excitation and emission spectra must be clearly assigned to the acridinium cation, i.e., the Vycor glass surface protonates acridine in the ground-state. The fluorescence lifetime of adsorbed acridine at Vycor glass is more strongly quenched by oxygen than the respective species in acidic water. This means that the acridine at the solid/air interface are more exposed to oxygen than in solution. The reaction equilibrium is not observed for the adsorption of acridine at porous Vycor glass. 1,2,7,8-dibenzacridine is adsorbed at Vycor porous glass surfaces also exclusively in its potonated form as a result from the interaction between acidic surface centers and the free lone pair on the nitrogen. The depolarizition measurements reveal that the adsorbed protonated 1,2,7,8-dibenzacridine is immobilized at the Vycor glass surfaces. The fluorescence lifetime of adsorbed 1,2,7,8-dibenzacridine at Vycor glass is more quenched by oxygen than the respective species in solution (in acidic ethanol). The reaction equilibrium can not be observed in case of adsorption of 1,2,7,8-dibenzacridine at porous Vycor glass surfaces. The steady state fluorescence and fluorescence decays reveal that 3,4,5,6-dibenzacridine is adsorbed at Vycor porous glass and forms the protonated species at the surface. The fluorescence anisotropy spectrum demonstrates that the protonated species of 3,4,5,6-dibenzacridine are immobilized at the Vycor glass surface. A finite equilibrium is observed when 3,4,5,6-dibenzacridine is adsorbed at the porous Vycor glass surface. The kinetics of adsorption of acridien, 1,2,7,8-dibenzacridine and 3,4,5,6-dibenzacridine at the porous Vycor glass surfaces are studied by measuring the absorbance of the adsorbed protonated species at porous Vycor glass surfaces. By comparing the values of the total rate constant of adsorption, which is calculated in a first-order approximation, it is noticed that the rates of the adsorption of the three compounds at PVG decrease in the following order; acridine > 1,2,7,8-dibenzacridine >> 3,4,5,6-dibenzacridine. This is due to the difference in the effective diffusion coefficient of the three compounds into the glass sheet. Since the effective diffusion coefficient which obtained from the algorithm in section 2.6. (theory part) shows that effective diffusion coefficient of the three compounds is decreases according to the following order: acridine > 1,2,7,8-dibenzacridine >> 3,4,5,6-dibenzacridine. This means that the adsorption of the three probes at porous Vycor glass surfaces is diffusion controlled. 1,2,7,8-dibenzacridine is adsorbed at the Geltech porous glass as protonated molecule and as H-bonded species which both are immobilized at the surface. Upon excitation the H- bonded species is transformed to the protonated form. Non-modified silica beads are applied as adsorbent for acridine from solution in cyclohexane and the main surface product is H-bonded acridine which is protonated in the excited singlet state. The fluorescence anisotropy spectrum demonstrates that the adsorbed acridine is immobilized at silica surface. The fluorescence lifetime of adsorbed acridine, at non-modified silica is more strongly quenched by oxygen than the respective species in solution (in water at pH = 1 for AC). 1,2,7,8-dibenzacridine is mainly assigned to protonated cations and hydrogen-bonded complexes at non-modified silica surface. The ratio between the two species at the silica surface depends on the water coverage of the silica surface. The hydrogen-bonded species undergo protonation upon excitation. The fluorescence excitation anisotropy spectra and the fluorescence life times value it possible to distinguish between the two adsorbed species (protonated and hydrogen-bonded) of 1,2,7,8-dibenzacridine at the silica surface. In case of non-modified silica as adsorbent and acridine and /or 1,2,7,8-dibenzacridine as adsobates, a finite equilibrium is observed and calculated. This reaction equilibrium depends on the water covrage of the non-modified silica surface (and thus on the humidity of air). The kinetic studies for the adsorption of acridine and 1,2,7,8-dibenzacridine at the non-modified silica beads surface from cyclohexane solution show that, the rate of adsorption of acridine is faster than the rate of adsorption of 1,2,7,8-dibenzacridine. This is may be due to the difference in the diffusion coefficient of acridine and 1,2,7,8-dibenzacridine into the silica particles. The effective diffusion coefficient of acridine which obtained from the algorithm in section 2.6. (theory part) is higher than that of 1,2,7,8-dibenzacridine. 3,4,5,6-dibenzacridine is not adsorbed from cyclohexane and not protonated at the silica beads surface, since the free electron pair on the nitrogen is sterically shielded by the benzo groups. AO is adsorbed at non-modified silica and is immobilized at the surfaces. 1,2,7,8-dibenzacridine is adsorbed at silica C30 from cyclohexane solution. There are two species formed at the silica C30 surface; the main one is the protonated form of 1,2,7,8-dibenzacridine and the other is the hydrogen-bonded species. The fluorescence excitation anisotropy distinguishs the two adsorbed species on silica C30 surface. The oxygen quenching rate constant for protonated 1,2,7,8-dibenzacridine at silica C30 surface is slightly lower with respect to the non-modified surface and also with respect to the acidic solution. This is due to the lower mobility of oxygen in the interphase since, oxygen is not capable of penetrating between the alkyl chains. Acridine organe is still adsorbed at silica modified with 200 mg of polymerized divinylbenzene (DVB-coated silica) and formes the protonated molecule due to the interaction with residual silanol groups. On the other hand, a very small amount of acridine orange is adsorbed at TBB-coated silica less than 5%. This means that the TBB coating shields most of the acidic surface silanol groups from the fluorescence analysis, whereas the DVB- coating shields only some of the surface silanol groups. The decrease of accessible surface silanol groups is in the series: non-modified silica > or = silica C30 > DVB-coated silica > TBB-coated silica. 5-Dimethyl amine naphthalene-1-sulfon (DANSyl) chloride is used to quantify the reaction of silica modified with amino-groups, since DANSyl chloride is a non-fluorescent probe in dry dichloromethane. DANSyl chloride reacts with the amino-groups at the silica surface and forms DANSyl amide, which is a fluorescent probe. DANSyl chloride dissolved in dry dichloromethane reacts readily and compeletly with the amino group in 3-aminopropyl, 3-aminopropyl + octyl, 3-aminopropyl + phenyl and 3-aminopropyl + butyric silica surfaces to form DANSyl amide. The DANSyl amide which is formed at the surface shows an anisotropy value much higher than that of DANSyl amide in dic hloromethane (r in DCM = 0) which proofs that the formed DANSyl amide is actually immobilized at the silica surface. The kinetics of reaction of DANSyl chloride with the amino groups at the modified silica surface is measured by the time-dependent increase in the fluorescence intensity of the product, DANSyl amide at the surface. The reaction kinetics measurements show that the 2nd–order rate constants are in order of 2 M-1s-1 which is not significantly different from that in homogeneous solution.

In dieser Arbeit wird eine vergleichende Untersuchung zur Oberflächenazidität verschiedener mikroporösen, fester Substanzen, die auf Siliciumdioxid basieren, präsentiert. Bei den Materialien handelt es sich um nicht modifizerte Siliciumkörner, Siliciumkörner mit C30-Alkylketten, Silicium modifiziert mit Polymeren sowie um Gläser mit kontrollierter Porengrösse. Konjugierte Moleküle mit zusätzlichen Elektrondonor- und Protonakzeptorzentren wurden als Adsorbate verwendet: Acridin (AC), 1,2,7,8-Dibenzacridin (1-DBA), 3,4,5,6-Dibenzacridin (3-DBA) sowie Acridinorange (AO). Die Apsorptions- und Fluoreszenzspektren der Probenmoleküle in Lösung dienten als Vergleich mit den Spektren der adsorbierten Spezies, um zwischen den verschiedenen Bindungsstellen mit unterschiedlicher Azidität auf der Oberfläche unterscheiden zu können. Die Beweglichkeiten der adsorbierten Probenmoleküle auf der Oberfläche wurde mit Hilfe von Fluoreszenzanisotropiemessungen ermittelt. Die Adsorptionskinetiken der Proben an den Oberflächen wurden durch zeitabhängige Absorptions- und Fluoreszenzspektroskopie bestimmt. Die Zugänglichkeit der adsorbierten Spezies in Bezug auf Sauerstoff sowie ihre Reaktivität mit aktiven Gruppen (in erster Linie Amino-Gruppen) auf den modifizierten Oberflächen wurde ebenfalls mittels Fluoreszenzspektroskopie untersucht.