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Anal. Chem. 1988. 60,564-568
for such systems would reflect this loss in current.
Observations on the Background Current at Carbon Paste Electrodes. Kakutani and Senda ( I I ) have shown theoretically that currents produced in the ac experiment due to strongly or irreversibly adsorbed species and species chemically bonded to the surface by other means are 90° out of phase with the applied ac potential for systems that are reversible in the ac sense, but kinetically slower systems have phase angles which are not in quadrature. Currents for the latter will pass through the lock-in amplifier. Figure 7A shows the first ac cyclic voltammogram at a freshly prepared carbon paste electrode cycled between 200 and 700 mV. On the anodic scan, a shoulder appears a t about 560 mV. On the return cathodic scan, a definite alternating current peak appears a t 480 mV. Figure 7B shows the second ac cyclic scan on the same carbon paste surface. A more defined set of peaks appears in this voltammogram with the peak potential for the anodic scan at 511 mV and that for the cathodic scan at 480 mV again. Holding the dc potential constant at 700 mV with a superimposed ac peak potential of 25 mV at 10 Hz for 2 h produced the cyclic voltammogram shown in Figure 7C. The change in the magnitude of the current at 540 mV after 2 h is approximately 5 nA. Other peaks have become evident in the background shown in Figure 7C. Graphite has been found to contain many types of carbon-oxygen surface groups such as quinones, quinhydrones, phenols, and carbonyls (12-14). It is reasonable that these currents are due to surface functional groups and the evidence in Figure 7 suggests that while anodization at a fixed dc potential may reduce the background current at that potential, surface currents may be increasing at another potential due to the change in the nature of the
surface functional group. The detail shown in the ac background scans in Figure 7 is much greater than can be observed in dc scanning experiments because the latter are obscured by the charging current, but the digital lock-in amplifier discriminates against a major portion of the charging current without current nulling. Registry No. C, 7440-44-0; o-dianisidine, 119-90-4.
LITERATURE CITED (1) Adams, R. N. ,Electrochemistry at Solid ,Electrodes; Marcel Dekker: New York. 1969. (2) McAiiister. D. L.: Drvhurst. G. I n Laboratow Techniuues in Electroanalytical Chemistry; Kjssinger, P. T., Heineman, W. R.: Eds.; Marcel Dekker: New York, 1984; pp 289-319. Stulik, K.; Pacakova, V.; Starkova, B. J . Chromatogr. 1981, 213, 47. Walker, D. E.; Adams, R. N.; Juliard, A. L. Anal. Chern. 1960, 32, 1528. Hanekamp, H. B.; Bos, P.; Vittori, 0. Anal. Chlm. Acta 1981, 131, 149. Kingsley, E. D. Ph.D. Dissertation, University of Massachusetts, Amherst, MA, 1982. Bard, A. J.. Faulkner. L. R. ,Electrochemical Methods in Analytical Chemistry; Marcel Dekker: New York, 1980; pp 288-389. Adams, R. N. Rev. Polarogr. 1963, 11, 71. Kissinger, P. T. I n Laboratory Technlques in ,Electroanalytical Chemis try; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984; Chapter 22, pp. 611-612. Wang, J. Talanta 1981, 28, 369. Kakutani, T.; Senda, M. Bull. Chem. Soc. Japn. 1979, 52, 3236. van der Linden, W. E.; Deiker, W. E. Anal. Chim. Acta 1980, 179, 25. Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136. Panzer. R. E.; Elving, P. J. J . Electrochem. Soc. 1972, 40, 99.
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RECEIVED for review June 26,1987. Accepted November 19, 1987. M.B.G. wishes to express his thanks for support in the form of a Grant-in-Aid of Research from Sigma Xi.
Solid-Surface Luminescence Interactions of Nitrogen Heterocycles Adsorbed on Silica Gel Chromatoplates Submerged in Chloroformln -Hexane Solvents G . J. Burrell and R. J. Hurtubise* Chemistry Department, University of Wyoming, Laramie, Wyoming 82071 The room-temperature fluorescence (RTF) and room-temperature phosphorescence (RTP) of benzo[f)quinoiine and benro[h]quinoiine, obtalned from the samples adsorbed on silica gel chromatoplates submerged in chioroform/n-hexane solvents, revealed several of the Interactions of the nitrogen heterocycles with the sdid matrix. The RTP resuits showed the emitting phosphor was protected from coillsionai deactivation by the matrlx and that the adsorbed chloroform mlnimally disrupted the phosphor adsorption Interactions. I n addition, at least two popuiatlons of phosphors were indicated. The RTF data and RTP data showed that different interactions were occurring in the singlet state compared to the triplet state. A comparison of RTF intensity and RTP intensity as a function of chromatographic solvent strength indicated that the protonated forms of the nitrogen heterocycles in their triplet states were interacting with the matrix more strongly than the protonated forms of the nitrogen heterocycles in their singlet states.
The observation of room-temperature phosphorescence (RTP) from molecules adsorbed on solid surfaces has been
shown to depend on several factors such as how rigidly the phosphor is held, the extent of hydrogen bonding with the surface, and other factors ( I , 2). Schulman and Walling ( 3 ) observed strong R T P from ionic organic molecules adsorbed on filter paper. I t was concluded that hydrogen bonding interactions between the molecules and hydroxyl groups on the surface were important in the restriction of collisional and other nonradiative deactivation processes ( 4 ) . An enhancement in the RTP from benzolflquinoline (BMQ) was observed when the carboxylate functionality of a polyacrylate binder in a commercial thin-layer chromatoplate was converted to the acid form (5). Strong hydrogen bonding between B m Q and the carboxyl groups in the binder resulted in the strong R T P signal. Recent work has demonstrated the importance of a rigidly held mechanism to PABA adsorbed on sodium acetate (6). R T P has also been observed from a system in which no hydrogen bonding is possible. Smith and Hurtubise ( 7 ) observed strong R T P from the dianion of terephthalic acid adsorbed on the sodium salt of polyacrylic acid. With this system it would not be possible to form hydrogen bonds. Other mechanisms for enhancing RTP have been suggested. Matrix packing (8),where added sugars or salts effectively
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block quenching agents such as oxygen or moisture from colliding with the phosphors, has been postulated. A matrix isolation mechanism has been suggested as responsible for the RTP observed from adsorbates on cellulose (9). In this mechanism the cellulose fibers swell because of the solvent used to adsorb the phosphors. The phosphor molecules are entrapped by the cellulose upon subsequent drying and this effectively shields the phosphors from external quenchers. In this work, the RTF and RTP from BMQ and benzo[hlquinoline (B[h]Q), which were adsorbed on EM silica gel thin-layer chromatoplate sections, were investigated ( 5 , l O ) . The chromatoplate sections were submerged in a series of binary solvents with different chromatographic solvent strengths. Changes in the luminescence properties of these adsorbates were investigated as a function of several solvent parameters. EXPERIMENTAL SECTION Reagents. BMQ, phenanthrene, and naphthalene (Gold Label, Aldrich Chemical Co., Milwaukee, WI) and reagent grade hydrochloric acid (J.T. Baker, Phillipsburg, NJ) were used as received. B[h]Q and fluoranthene (Aldrich 97%) were recrystallized from ethanol/water; triphenylene (Aldrich 97%) was recrystallized from chloroform/ethanol. Ethanol was purified by distillation. Hexane (Baker, 97% n-hexane, HPLC grade) and chloroform (Baker, 0.015% amylene preserved, HPLC grade) were used as received. Aluminum-backed silica gel thin-layer chromatoplates (Merck, Darmstadt, FRG) were developed twice with distilled ethanol to elute impurities to one end, then dried 30 min at 110 "C prior to adsorbate application. Spectrometer. RTF, RTP, and solution fluorescence intensities and spectra were obtained with a Perkin-Elmer LS-5 luminescence spectrometer equipped with a Model 3600 data station (Perkin-Elmer, Norwalk, CT). Excitation radiation was provided by a Xe flash lamp pulsed at the line frequency. Scan rate (240 nm min-'), response factor 2, ordinate scaling, mini-floppy disk spectral storage and data manipulation were accomplished through the use of a PECFS applications program. Excitation and emission slit widths were manually controlled. The following slit settings were used: solution fluorescence measurements, 5-nm excitation slit, 5-nm emission slit; RTF measurements, 15-nm excitation slit, 3-nm emission slit; RTP measurements, 10-nm excitation slit, 5-nm emission slit. Phosphorescence delay time (3.0 ms) and gate time (7.0 ms) were also operated manually. A Perkin-Elmer model 660 printer was used to obtain hard copies of spectra. Apparatus. A sample compartment device constructed for use in a Perkin-Elmer LS-5 luminescence spectrometer was used to secure a 1-cm-square quartz cell containing a 2.0 X 1.4 cm section of the thin-layer chromatoplate spotted with adsorbate (11). Thin-layer chromatography (TLC) experiments were performed in conventional 30 cm X 30 cm X 10 cm glass TLC tanks fitted with glass lids. Procedures. Solid-Surface Luminescence Measurements. Separate 100-ngsamples of BMQ, B[h]Q, BMQH+, and B[h]QH+ were adsorbed on the chromatoplate sections from 1-pL aliquots of ethanolic or 0.1 M HC1 ethanolic solutions of the individual compounds. The chromatoplates were dried 30 min at 110 "C and stored in a desiccator. For luminescence measurements, a chromatoplate section was placed diagonally in a 1-cm-square quartz cell, which was in the cell compartment apparatus (11). Luminescence signals were maximized through positional optimization of the cell. Background luminescence from blank areas of the chromatoplate was also monitored and subtracted out when necessary. Binary solvents containing the following volume percentages of chloroform in n-hexane were prepared: l % , 5%, 30%, 50%, 75%, and 100%. Three milliliter volumes of these solvents were pipetted into the cells containing the spotted chromatoplate sections. The luminescence intensities and spectra of the adsorbates were obtained while the chromatoplate sections were in the solvents. RTP lifetime data for B[flQH+ and B[h]QH+in the presence of the solvents were obtained by using a computer program de-
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veloped in this work for the collection and display of RTP intensity as a function of time after manual closure of the incident beam shutter. RTP lifetime ( T J values were calculated as the inverse of the slope of a In (RTP intensity) vs time plot. Solution Fluorescence Measurements. The solutions in which the chromatoplates were submerged were analyzed for the amount of desorbed compound. This was conveniently performed by simply removing the chromatoplate section and analyzing the remaining solvent. Solution fluorescence calibration curves, and excitation and emission spectra were obtained for 1pg/mL BMQ and B[h]Q in the binary solvents. Thin-Layer Chromatography of Polycyclic Aromatic Hydrocarbons to Determine Chromatoplate Actiuity (a)for the Calculation of Binary Solvent Eluotropic Strengths (cab). Phenanthrene, naphthalene, fluoranthene, and triphenylene were eluted with n-hexane on EM chromatoplates treated as for the luminescence studies to determine the activity of the chromatoplate as described by Snyder (12). Visualization of the luminescence of the migrated compound spots was accomplished with an ultraviolet handlamp by illumination of the chromatoplate, which was submerged in liquid nitrogen. Chromatoplate Scatter Measurements. The relative amount of scattered emission radiation reaching the detector was measured for blank chromatoplate sections in the binary solvents. For the measurements in the phosphorescence mode the following instrumental parameters were used: A,, = 516 nm, A,, = 516 nm; 10-run excitation slit, 5-nm emission slit; 3.0-ms delay time, 7.0-ms gate time. The instrumental parameters for the measurements in the fluorescence mode were: A,, = 425 nm,, A, = 440 nm; 10-nm excitation slit, 3-nm emission slit; a Corning 8364 neutral density filter (Esco Products, Oak Ridge, NJ) was placed in the cell compartment window leading to the emission detector, and the incident and observed scattered wavelengths were offset slightly because the intense scatter from the chromatoplates saturated the detector in the fluorescence mode. RESULTS AND DISCUSSION
RTP from Adsorbed B[f]QH+ and B[h]QH+ Exposed to Solvents. RTP excitation and emission spectra, RTP intensities, and RTP lifetimes ( T ~ were ) obtained for 100-ng samples of B[flQH+ (I) and B[h]QH+ (11) adsorbed on EM chromatoplate sections submerged in the binary solvents. The
H+
(I)
(I I)
phosphorescence lifetimes for B[flQH+ (1.1 f 0.02 s) and B[h]QH+ (0.73 f 0.02 s) remained essentially constant on the dry chromatoplate and the chromatoplates submerged in the solvents listed in Table I. Also, the corresponding R T P excitation and emission wavelengths for the two compounds on the dry chromatoplate and submerged in all the solvents listed in Table I showed no change or very small changes. These results indicated that the solvents were not greatly perturbing the interactions between the phosphors and the chromatoplate. The RTP intensities of both compounds decreased significantly when exposed to n-hexane, yet an analytical useful signal was still observed (Table I). Collisional deactivation by adsorbed or solution-phase solvent molecules did not totally eliminate the RTP. As shown in Table I, with increasing volume percentages of chloroform, further decreases in the R T P intensities of the adsorbates were observed. The background intensity from blank areas of the chromatoplate was unaffected by the prescence of any of the solvents. By measuring the solution fluorescence from the n-hexane to 50% (v/v) CHCl, solvents, after removal of the chromatoplate sections, it was determined that the RTP intensity changes were not caused by desorption of the adsorbates from
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Table I. CHC18/n-Hexane Solvent Effects on the RTP and RTF Relative Intensities from B[f]QH+ and B[b]QH+ Adsorbed on EM Chromatoplates conditions dry chromatoplate n-hexane 1% CHC13 5% CHC13 30% CHCl3 50% CHC13 75% CHClSC
CHC13d
RTP re1 intens B[hlQH+ BUJQH+ 3.4 2.2 2.1 2.0 2.0 1.9 1.8 1.5
3.1 2.1 2.0 2.0 1.9 1.9 1.6 1.0
RTF re1 intens B[flQH*" B[h]QH+" 173 237 215 199 142 140 136 107
117 163 155 148 136 126 121 100
solvent strength
(cab)*
0.000 0.011 0.047 0.16 0.20 0.24 0.26
"RTF relative intensities are not directly comparable to RTP intensities. However, the RTF intensities are about 100 times the RTP intensities. *Solventstrength as defined in ref 16. cAmount of BWQH+and B[h]QH+desorbed was
