Surveying the Silica Gel Surface with Excited States - Advances in

11 Surveying the Silica Gel Surface with Excited States R. Krasnansky and J. K. Thomas

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch011

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Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556

Time-resolved and steady-state fluorescence probing was used to study gas-solid and liquid-solid silica gel interfaces. The molecular surveying probes pyrene,1-aminopyrene,pyrenecarboxylic acid, and 9,10-diphenylanthracene adsorbed at the silica gel surface give information about probe environment, mobility, and accessibility. Reactions of surface-bound arene singlet excited states with molecular oxygen were monitored over a range of temperatures; the reactions were assigned to a unique Langmuir-Hinshelwood mechanism. The kinetics reflect the details of oxygen adsorption and movement on the silica gel surface.

SURFACES ARE O F INTEREST BECAUSE O F their media-thickening tendency

(I), chromatographic separation ability (2), and catalytic activity (3). Although surfaces have been long exploited, understanding the nature of surfaces has not always kept pace. Key questions about surface phenomena revolve around the understanding of the chemical environment present at, the mobility found on, and the molecular accessibility allowed at a surface. Answers to the key questions about surface phenomena have been obtained by use of luminescence probing techniques. In general, a lumophore is isolated at an interface. O n the basis of steady-state and timeresolved spectral properties of the lumophore i n neat solution or i n simple organized media (4), the energy, spectral shape, and time-resolved decay Current address: Rohm and Haas Company, Research Laboratories, 727 Norristown Road, Spring House, PA 19454 Corresponding author

0065-2393/94/0234-0223$08.36/0 © 1994 American Chemical Society

Bergna; The Colloid Chemistry of Silica Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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profile of a probe's emission are interpreted to address the key questions of local environment, mobility, and accessibility. Specific modifications of the lumophore probe impinge unique discriminating abilities on the probe. By virtue of their photophysical properties, pyrene and anthracene derivatives have gained a significant place among luminescence probes. Because of its surface functionality, thermal stability, and at times porous intraparticle structure, silica gel presents an attractive surface for photophysical and photochemical study. This chapter deals with both surface functionality identification and with molecular mobility and reactivity occurring at the silica gel surface. Pyrene has a nonallowed 0 —» 0 transition (ground state to excited state) that is markedly medium-dependent. Media effects are readily monitored through pyrene*s fluorescence vibrational structure (5). The compounds 1-aminopyrene (1-AP) and 1-pyrenecarboxylic acid (PCA) each possess two chromophores that interact in the excited state; the extent of the interaction depends on the media. The lone pair of the amine chromophore of 1-AP acts as an internal switch for photophysics. Previous work (6) showed that the photophysical behavior of 1-AP is dominated by protonation, or " b l o c k i n g " , of the lone pair (Figure 1). Protonation of the amino group leads to a complex, 1-APH+, that exhibits photophysical properties similar to those of pyrene; the free 1-AP system exhibits different photophysics. In similar work (7), the photophysical properties of P C A have been correlated to its acid-base properties and media effects. The compounds 1-AP and P C A have been used to distinguish the relative population of geminal and nongeminal silanol functionality at the silica gel surface. The nonporous silica gel Cab-O-Sil is a convenient system to study molecular geometry during the fluorescence quenching of ^ e n e s * by oxygen at the gas-solid interface without the complication of partitioning

Figure 1. Acid-base photophysical behavior of 1-AP.


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Surveying the Silica Gel Surface

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the probe between an external and an internal porous surface. W h e n quenching molecules are present in both the gaseous and the adsorbed states, either a Langmuir-Rideal (8) reaction between a gas-phase mole cule and a surface-bound molecule or a Langmuir-Hinshelwood (9) reaction between two surface-bound molecules may occur. The fluorophores incorporated for the oxygen-quenching study were pyrene, monitoring the —> * A i transition (10), and 9,10-diphenylanthracene (DPA), monitoring the * B 2 « —* A\ transition (10, II). The adsorption of fluorophores onto the Cab-O-Sil surface is not homogeneous, but rather can be characterized by a distribution of adsorption sites; each adsorption site presents a microenvironment that is reflected by the unimolecular decay rate of a fluorophore residing at the site. The distribution of fluorophore unimolecular decays is modeled by a Gaussian distribution in natural logarithmic space about a mean unimolecular decay rate. The observable excited-state decay rate in the presence of quencher also has a Gaussian distribution. The oxygen quenching of surface-bound excited-state fluorophore is considered predominantly Langmuir-Hinshel wood. g


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Experimental Details Instrumentation. Steady-state fluorescence spectra were obtained on an S L M - A m i n c o SPF-500 spectrofluorometer equipped with an L X 3 0 0 U V illumina tor, a 1200-grooves per millimeter grating, and a Hamamatsu R 928P photomultiplier tube in conjunction with a Zenith Z-368 computer. A neutral density filter, optical density 1.0, was placed in the excitation line to prevent photodecomposition of surface-bound fluorophore. Time-resolved fluorescence decay profiles were obtained with a P R A Nitromite nitrogen flow laser, model L N - 1 0 0 , with a 0.12-ns full width at half-maximum, 70-μΙ, 337.1-nm pulse. Emitted light was collected at 90° to the excitation line; a Kopp 4-96 band-pass filter removed collected scattered light. The monitoring wavelength, Aob, was selected with a Bausch and Lomb 33-86-02 monochromator equipped with a 1350-grooves per millimeter grating and detected by a Hamamat su R-1644 microchannel plate with a response time of 0.2 ns. The signal was digitized via a Tektronix 7 9 1 2 H B programmable digitizer equipped with a 7B10 time base and either a 7 A 1 6 A amplifier (response time 1.6 ns) or a 7A23 amplifier (response time 0.7 ns). Decay profile simulations were performed on a Zenith Z368 computer by a nonlinear least-squares fitting method. Sample Preparation. Samples were prepared by exposing the silica gel, previously dried at 150 °C for 24 h, to either cyclohexane or pentane solutions containing selected amounts of the fluorophore. The solvent was carefully removed under vacuum when required. Complete probe adsorption of liquid-solid samples was verified with absorption spectroscopy. Less than 0.07% of the silica surface was typically covered by the probe. Immediately before data collection, dry samples were evacuated under vacuum at 125-130 °C for 30 min. The total dehydration procedure was sufficient to remove the physisorbed water while leaving the surface silanol functionality intact (12). Selected amounts of oxygen were introduced into a


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constant-position sample call by a series of stopcock manipulations and a vacuum line. L i q u i d - s o l i d samples were deoxygenated ([Chlfinai < 10~ M) by bubbling the samples with solvent-saturated nitrogen for 30 min. Various temperatures were achieved by incorporation of a quartz Dewar flask and a stream of chilled nitrogen gas. Nitrogen was cooled by passing the gas through a coiled copper tube submerged in liquid nitrogen. The sample temperature was monitored via a thermocouple attached to the sample cell wall and varied by regulating the nitrogen gas flow. Exterior Dewar fogging was prevented with a second, room-temperature stream of nitrogen. Oxygen adsorption isotherms were obtained with a differential pressure analysis apparatus. The apparatus consisted of equal-volume spheres connected by a meter-high U-tube, filled halfway with distilled and degassed dimethylpolysiloxane, and a series of three-way stopcocks. Each sphere possessed a cell port equipped with a two-way stopcock. The total volumes were calibrated such that the volume of the left sphere equalled that of the right. A sample of known weight was placed in one of the cells, and the whole system was evacuated. W i t h the two-way stopcocks closed, the system was equilibrated with a given amount of oxygen; the left and the right spheres were then isolated, and the two-way stopcocks were opened. The sample and the empty reference cells were equivalently submerged into various chilling baths. The amount of gas adsorbed was determined from the difference in the heights of the dimethylpolysiloxane columns and a calibration curve. The bulk pressure was measured with a Hastings vacuum gauge equipped with a D V - 3 0 0 Raydist gauge tube. HS-5 Cab-O-Sil having a Brunauer-Emmett-Teller surface area (SBET) of 325 ± 25 m /g, an accessible pyrene surface area (S rene) of 348 ± 40 m /g, and a particle diameter of 0.008 mm was donated by the Cabot Corporation. Mathenson Coleman and Bell silica gel (MCB) possesses an SBET of 672 ± 70 m /g,


6

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an Spyrene of 250

±50

m /g, and a mesh of 2 8 - 2 0 0 . Each silica has a silanol concentration of 4.9 ± 0.9 silanols per square nanometer (13). Pyrene was purchased from Aldrich and passed three times down an activated silica gel-cyclohexane column. A sample of 1aminopyrene (97%) was purchased from Aldrich, recrystallized from ethanol, and passed down an activated silica gel-benzene column. The 9,10-diphenylanthracene (99%), high-performance liquid chromatography grade cyclohexane, and gold label pentane were used as received from Aldrich. Oxygen was used as received from Mittler. Dimethylpolysiloxane was purchased from Sigma. A vacuum of 10~ torr (10~" Pa) was achieved with a Duo-seal model 1400 vacuum pump. 2

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Results and

Discussion

Photophysics of Pyrene on Various Silica Gel Surfaces. Pyrene gives information about its environment via changes in its fluorescence fine structure (14). Typically, five vibronic bands are identified; the ratio of the I H band at 392 nm to the I band at 372 nm, the IH/I ratio, increases in noninteracting (nonpolar) media with a concomitant increase in fluorescence lifetime (15). The versatility of the pyrene probe arises from the "forbiddenness" of the So —* Si transition; any intensity for the transition comes from vibronic coupling with higher excited states (10). Interactive or polar solvents, via their interaction with the arene, increase the intensity of the 0 —> 0 transition in both the absorption and the emission.


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This increase in intensity is reflected in the increase in peak I with respect to the change in peak III. Pyrene adsorbed on a silica gel surface exhibits a typical ΠΙ/Ι ratio of 0.58 (15). This value is between the values observed for pyrene in water and in methanol and reflects the interaction of the arene probe with the surface silanol functionality of the silica gel surface. Similar data are obtained on solid AI2O3. Photophysics of 1-Aminopyrene on Various Silica Gel Sur faces. Figure 2 presents the fluorescence spectra of 1-AP adsorbed on M C B and FS-662 silica gel in cyclohexane. The fluorescence spectrum of MCB-bound 1-AP is quite typical of 1-APH+, whereas the fluorescence spectrum of FS-662-bound 1-AP is quite typical of 1-AP. The surfacebound 1-APH+* decays with an inherent unimolecular lifetime of 135 ± 2 ns, and the surface-bound 1-AP* decays with an inherent unimolecular lifetime of 4.9 ± 0 . 1 ns. The different photophysical behavior of the 1-AP indicates that the adsorption sites for 1-AP are different on these silica gel samples at the given probe loadings. Moderate heat treatment of the M C B silica gel at 450 °C for 24 h, a temperature below the sintering temperature, alters the surface such that the fluorescence spectra of 1-AP adsorbed on this surface have characteris tics of both 1-AP and 1-APH+; a heating temperature of 650 °C yielded an adsorbed 1-AP spectrum identical to that of 1-AP adsorbed on FS-662. A several-hour concentrated nitric acid treatment of the FS-662 silica, followed by washing with water until the wash maintained a neutral p H and drying, alters the surface such that the fluorescence spectrum of 1-AP adsorbed to this surface resembles that obtained from the M C B surface. The thermal (16) and the chemical (17) treatments of the silica gels alter the degree of clustering of the silanol functionality on the silica surface. The available surface silanol functionality is shown in Figure 3. Hair et al. (17,18) showed that the geminal silanol configuration gives rise to an adsorption site that, when occupied by aniline, yields a protonated adsorbed form of aniline; a similar correlation would account for the observed photophysical behavior of 1-AP on the silica gel surfaces. Milosavljevic and Thomas (7) used P C A to probe the microacidity of the silanol functionality. Scheme I shows the photophysics of P C A . In particular, the observed fluorescence decay rate constant was diagnostic for the determination of the apparent p H of the microenvironment. M C B bound P C A demonstrated a neutral form, singly protonated carboxylic acid group fluorescence, whereas FS-662-bound P C A demonstrated a mixed anionic-neutral fluorescence. The observed fluorescence decay rate constants of 1.9 X 10 s and 3.8 X 1 0 s correspond to an apparent p H of 1.6 and 4.1 for the microenvironments of the geminal and nongeminal silanols, respectively. 8

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T H E C O L L O I D CHEMISTRY O F SILICA

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Surveying the Silica Gel Surface with Excited States - Advances in

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