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Examination of covalently bound polymeric stationary phases by luminescence spectroscopy
Analytica Chimica Acta, 200 (1987) 143-150 Elsevrer Science Publishers B.V., Amsterdam - Printed in The Netherlands
EXAMINATION OF COVALENTLY BOUND POLYMERIC STATIONARY PHASES BY LUMINESCENCE SPECTROSCOPY
C. H. LOCHMULLER*
and M. T. KERSEY
P. M. Gross Chemzcal Laboratory,
Duke University, Durham, NC 27706
(U.S.A.)
(Received 3rd March 1987)
SUMMARY Lummescence spectroscopy is used to probe silica-gel surfaces derivatized with [3-(3pyrenyl)propyl] methyldichlorosilane. Evidence of solution pre-polymerization and subsequent derivatization to accessible surface silanols is presented. Conformational changes of the bound ligands in polar and non-polar solvents indrcate a dynamic surface which minimizes its surface area in hostile solvents.
Reversed-phase bonded stationary phases for liquid chromatography are generally classified into two categories, brush and polymeric. They are produced by reacting monofunctional or polyfunctional silanes, respectively, to accessible surface silanols of silica gel [l-5]. Early chromatographic experiments predominantly used brush-phase columns because of the greater reproducibility in the bonded-phase derivatization and higher column efficiencies. Recently, polymeric stationary phases were shown to provide a more selective separation of polycyclic aromatic hydrocarbons with similar efficiencies when compared to brush phases [ 2,6,7] . Verzele and Mussche [ 31 argued that, chromatographically, there are no differences between brush and polymeric phases; however, Sander and Wise [2] have demonstrated selectivity changes of polycyclic aromatic hydrocarbons for the two bonded-phase types. More information is required to characterize these chemically modified polymeric surfaces. Several spectroscopic methods have been developed to probe the physicochemical micro-environment of these complex surfaces. Nuclear magnetic resonance and infrared spectroscopy have been used to characterize the shape and solvent-dependent conformations of chemically bound silanes [8-111. Lochmiiller et al. [12-l 51 introduced steady-state and time-dependent luminescence spectroscopy to examine the micro-environment of the bound *Charles H. Lochmtiller is Professor of Chemistry and Biochemical Engineering at Duke University. His roughly 100 publications include work on robotics, proton-induced x-ray emission and n.m.r. but his main thrust has been in fundamental studies of separation mechanisms. He received the Pioneer Award for his work in robotics in 1985 and is the recipient of the 1987 Chromatography Award of the American Chemical Society. 0003-2670/87/$03.50
0 1987 Elsevier Science Publishers B.V.
144
silanes. These techniques were used to examine the distribution of surface silanols, the conformational changes of chemically modified brush phases in common liquid-chromatographic solvents and the surface polarity of endcapped bonded phases [12-l 51. In the present work, [ 3-(3-pyrenyl)propyl] methyldichlorosilane-bonded phases were examined by using luminescence spectroscopy. The data reported indicate solution pre-polymerization of the silane oligomer. Data from mass spectral and supercritical fluid chromatographic analyses suggest formation of low-molecular-weight polymers. Luminescence data indicate a structural rearrangement of these low-molecular-weight polymeric phases in polar and non-polar solvents. EXPERIMENTAL
Materials Whatman Partisil-10 silica gel (BET, Nz surface area 323 m2 g-l, mean pore diameter 93 A, and mean particle size 10 pm) was used as the support. Synthesis of the pyrene silane and subsequent hydrosilylation reactions were done in this laboratory under conditions previously described [12]. Methyldichlorosilane (Petrarch Systems) was used without further purification in the synthesis of the difunctional pyrene silane. Spectral-grade hexane and methanol (Mallinckrodt) were used as contact solvents. Fluorescence studies Steady-state studies. All steady-state fluorescence spectra were obtained by using a Perkin-Elmer Model MPF-66 spectrophotometer. Emission spectra were collected from 360 to 530 nm at an excitation wavelength of 315 nm. Excitation spectra were collected from 270 to 370 nm at emission wavelengths of 390 nm (monomer) and 480 nm (excimer). AI1 spectra were measured at 22°C. Carbon and hydrogen determinations (M. H. W. Lab., Phoenix, Arizona) were run on all bonded phases and these were diluted with underivatized silica to avoid inner-filter effects. Derivatized silica samples were prepared for data acquisition by adding approximately 20 mg of the derivatized silica to 2 ml of solvent. The solvent/silica slurry was freeze/pump/thawed a total of three times to avoid oxygen quenching of the fluorescence signal. After the final thaw, the cell was inverted and the silica settled into the quartz tube. The cell was then placed into a specially designed holder which fits into both spectrometers. The cell and holder were designed in this laboratory and are illustrated in Fig. 1. Time-dependent studies. Instrumentation, data collection, and numerical analysis have been discussed [ 131. Data were acquired at an excitation wavelength of 315 nm and an emission wavelength of 390 nm. An average instrument response of 4.3 ns (FWHM) was recorded when 0.5 atm of nitrogen (National Specialty Gases) was used in the flashlamp.
145
w
Teflon
o-
Vacuum SeaI
mg
7
Quartzto glass seal
Quartz tube
Fig. 1. Freeze/pump/thaw area of the holder.
cell and holder. The quartz tube is inserted through the shaded
Solution polymerization [ 3-( 3Pyrenyl)propyll methyldichlorosilane was polymerized by using the same bonded-phase reaction conditions (without silica) previously described [ 151. Polymerization was terminated by adding an excess of trimethylchlorosilane (Petrarch Systems). Samples of.the mixture were removed, solvent was evaporated by bubbling dry nitrogen through the mixture and the pale-green, viscous liquid was examined by mass spectrometry and supercritical fluid chromatography. The latter data were acquired on a Hewlett-Packard model 1082B liquid chromatograph modified for the purpose. Aliquots of the polymerized solution were diluted in chloroform to concentrations between 10e3 and 5 X lo-’ M in monomer. Allylpyrene solutions, within the same concentrations, were used to compare the polymeric solutions with a nonaggregating model compound. RESULTS
AND DISCUSSION
Difunctional silanes were used in this work because of the inherently less complex polymeric structures, compared to trifunctional silanes, formed during polymerization. Polymers synthesized with trifunctional silanes exhibit cross-linking; however, difunctional silane polymers only form linear or cyclic structures. Further, only linear structures bind to accessible surface silanols and the unreactive cyclic polymers are washed from the silica surface. Possible cyclic and linear structures formed by the condensation of three difunctional silane monomers are shown below (R = 3-propylpyrene). The photophysics of pyrene is well documented [16-191. Pyrene forms excited-state dimers (excimers) in a collisional quenching process involving an excited-state pyrene molecule and one in the ground state. Excimer formation decreases the quantum yield of the monomer emission (378 nm) and
146
gives rise to a broad, structureless emission (A_ = 470 nm). Figure 2 illustrates the loss in intensity of the monomer emission with a corresponding rise in the excimer emission for two covalently bound polymeric phases in contact with methanol. Similar spectra have been observed for pyrene in solution
lJ61. Steady-state fluorescence measurements of polymer solutions The aggregation of difunctional pyrenesilanes in solution was observed by using steady-state fluorescence spectroscopy. The association of difunctional silane monomers was monitored by measuring the excimer/monomer ratio of the polymer solutions and comparing that to measurements made with allylpyrene solutions. Figure 3 compares the excimer/monomer ratio for polymerized [3-(3-pyrenyl)propyl] methyldichlorosilane and allylpyrene solutions. A plot of the excimer/monomer ratio vs. allylpyrene concentration in solution approaches zero as the concentration of allylpyrene decreases. However, compared with the allylpyrene solutions, the polymerized difunctional silane solutions exhibit increased excimer formation. The greater excimer intensity observed for the difunctional silane solutions is indicative of silane polymerization because, upon dilution, the difunctional silane oligomers maintain the
Fig. 2 Steady-state fluorescence spectra. (A) 1.36% carbon diluted l/20 (B) 7.24% carbon diluted l/1000 m methanol.
m methanol,
1
Concentrctlon
2
3
4
Carbon
CM)
5
6
7
(%)
Fig. 3. Steady-state fluorescence excimer/monomer intensity ratios vs. concentration polymerized difunctional pyrenesilanes (m) and allylpyrene (0) in chloroform.
of
Fig. 4. Steady-state fluorescence excimer/monomer intensity ratios vs. percent carbon for difunctional propylpyrene bonded phases in hexane ( n ) and methanol (0 ).
critical interaction distance for excimer formation. Consequently, a percentage of the polyfunctional silane must be covalently bound to one another. The results of supercritical fluid chromatography and mass spectral analyses indicate, in addition to individual difunctional silanes, the presence of dimers and trimers. Evidence of larger-molecular-weight polymers was not observed. Steady-state
fluorescence
measurements
of polymeric
bonded
phases
Steady-state spectra of polymeric bonded phase samples ranging from 1.36% to 7.24% carbon were collected in contact with hexane and methanol. Surface coverages (r_lmol mV2) of these polymeric bonded phases cannot be determined because an average molecular weight of the bound polymer is unknown. Figure 4 illustrates the changes observed in the excimer/monomer ratio for polymeric phases in the two solvents. The ratio increases with increasing percent carbon. This trend has also been observed for both pyrene in solution [16] and for monofunctional pyrenesilanes bound to silica gel 1131. Differences in the excimer/monomer ratios for methanol and hexane are observed above 2% carbon and increase with increasing pyrenesilane surface coverage. The larger excimer/monomer values in methanol are attributed to the collapse of the bound ligands which, at higher percent carbon, have a greater probability of coming within the critical interaction distance to form excimers. The greater degree of excimer emission observed in methanol for similar carbon loadings compared with hexane has also been demonstrated for brush phases [12,13]. Changes observed for the excimer/monomer ratio in methanol and hexane may be explained by solvent-induced conformational changes in the bonded phase caused by “solvophobic” interactions between the bound ligands and solvent [20]. This experiment demonstrates that
148
polymeric bonded phases, like their monofunctional silane counterparts, appear to reduce the total surface area of the bound ligand moieties in hostile solvents. These results support a dynamic model for polymeric phases which suggests that the ligands swell in good solvents and shrink in hostile solvents. Time-dependent measurements of polymeric bonded phases Values of the pre-exponential factors (A,), lifetimes (Ti) and percent carbon in hexane and methanol are listed in Table 1. The validity of the numerical analysis and model based upon the pre-exponential factors has been defended [ 131. When the “component increment&ion” of Isenberg et al. [21] was used, it was concluded that difunctional pyrene bonded phases fit to a sum of three exponentials. Figure 5(a) illustrates the pre-exponential factors for the series of difunctional bonded phases, 1.36% to 7.24% carbon, in methanol. One observes that the population of bound pyrene molecules which form excimers is approximately 80% and increases from 80 to about 95% between 5 and 7.24% carbon. The data appear to indicate that up to 5% carbon, the degree of eximer formation for difunctional propylpyrene bonded phases is independent of carbon loading. Increased pre-exponential factors for carbon loadings TABLE 1 Normalized pre-exponential percent carbon Carbon (%) coverageb (pm01 m-‘) Hexane A, A* A, 71 71 73
factors (Ai)
1.36 0.19
1.99 0.28
0.736 0.062 0.176 4.7 100.3 157.5
0.637 0.083 0,280 2.1 60.5 138.2
0.846
0.837
and lifetimesa (7i) as a function of solvent and
2.51 0.35
0.451 0.368 0.173 20.2 53.5 97.0
3.48 0.50
4.43 0.64
0.468 0.390 0.143 14.9 45.1 101.2
0.835 0.154 0.011 12.8 40.3 135.8
4.91 0.71
5.11 0.74
1.24 1.10
0.086 0.009 9.3 32.3 119.9
0.801 0.097 0.102 3.4 42.8 123.3
0.91 0.07 0.01 1.6 33.7 123.9
-
0.843 0.086 0.071 1.3 33.5 106 3
0.05 0.00 6.0 30.0 117.8
0.904
Methanol A, A, A, 71 71 73
0.94
*Lifetimes are in ns. bThese values are based on calculations which assume that the predominant surface oligomer is monomeric (and as if monomeric chemistry was used); it is not the intent of this calculation to suggest that this is the case here and the values are presented solely for comparison to other published work where “monomeric” chemistry indeed was used.
149
1
2
3
4
5
Carbon
PA)
6
7
-I 8
8
Corbon
(%I
Fig. 5. Plots of the pre-exponential factors vs. percent carbon for difunctional propylpyrene bonded phases in methanol (a) and in hexane (b): (A) A,, (B) A,, (C)A,.
greater than 5% indicates the interaction of oligomeric units on the more densely packed surface. Polymeric phases may exhibit large excimer-forming populations at low percent carbon because either linear polymers form in solution and then bind to accessible surface silanols and/or, as Sander and Wise [2] suggest, the polymeric network is initiated on the surface and “grows out” into solution. The large excimer-forming population at low percent carbon is attributed to solution pre-polymerization and subsequent reaction of the polymerized silane to surface silanols. The extent of silane-surface initiated polymerization at low carbon coverages would be less than the competitive process of silane reaction to the more accessible surface silanols for two reasons: first, the probability of a pyrenesilane ligand in solution encountering an accessible surface silanol is greater than one reacting with a bound silane; and second, the interaction of a bound silane and one in solution is more hindered and less favorable than the competitive process of binding to a more accessible surface silanol. The model of polymer formation with polyfunctional silanes suggested by Sander and Wise may be more applicable to higher carbon loadings because there are fewer surface silanols to compete with the residual silane silanols. Figure 5(b) illustrates the pre-exponential factors for the polymeric bonded phases in hexane. At low percent carbon, a greater degree of excimer formation than expected from brush-phase results [13] is observed. However, the excimer population decreases up to 3% carbon and then increases. This anomalous behavior may indicate the difficulty in reproducibly synthesizing polymeric phases. Examination of the lifetimes, r2 and r3, listed in Table 1 indicate a concentration-quenching kinetic model of the pyrene monomer emission up to 3% carbon [16]. The constant lifetimes, within experimental error, for greater than 3% carbon is a departure from true, diffusion-controlled behavior. This might be expected in constrained systems where the ligands are bound to the surface.
150
Comparison of the lifetimes in hexane and methanol supports a dynamic stationary-phase model for polymeric bonded phases. The lifetimes, 72 and TV, are longer in methanol than those in hexane below 2.5% carbon. The shorter lifetimes observed in hexane correspond with increased solvation of the alkyl chains. This solvation gives the silane a greater degree of motion which, on average, decreases the relaxation time of the pyrene molecule. Conversely, methanol, a O-solvent, restricts the ligand motion increasing the average relaxation time of the hydrophobic pyrene ligands. Pyrene crystal quantum yields and lifetimes have been observed to increase with decreasing temperature [17]. Above approximately 3% carbon, there are no significant differences in the lifetimes, r2 and 73, between the two contact solvents, which indicates that the ligands are motionally constrained to the same degree. The differences between TV in hexane and methanol can be attributed to the degree of solvation of the pyrene ligands. Silanes solvated by hexane take longer, on average, to form excimers than in methanol because of the increased mobility of the propyl chain and the larger separation between pyrene molecules. The shorter lifetimes observed in methanol may be indicative of the collapsed state of the ligands which put the pyrene molecules within the critical interaction distance for excimer formation. We gratefully thank L. Mink for the supercritical fluid chromatographic data. This project was supported by the National Science Foundation, Grant CHE85-00658 (to C.H.L.) REFERENCES 1 R. E. Majors, J. Chromatogr., 180 (1980) 488. 2 L. C. Sander and S. A. Wise, Anal. Chem., 56 (1984) 504. 3 M. Verzele and P. Mussche, J. Chromatogr., 254 (1983) 117. 4H. Hemetsberger, M. Kellermann and H. Ricken, Chromatographia, 10 (1977) 726. 5 H. Colin and G. Guiochon, J. Chromatogr., 141 (1977) 289. 6 S. A. Wise and W. E. May, Anal. Chem., 55 (1983) 1479. 7 S. A. Wise and L. C. Sander, J. High Resolut. Chromatogr. Chromatogr. Commun., 8 (1985) 248. 8 R. K. Gilpin, J. Chromatogr. Sci., 22 (1984) 371. 9 C. H. Lochmiiller, D. B. Marshall and D. R. Wilder, Anal. Chem., 52 (1980) 19. 10 L. C. Sander, J. B. CaIlis and L. R. Field, Anal. Chem., 55 (1983) 1068. 11 J. D. Miller and H. Ishida, Anal. Chem., 57 (1985) 283. 12C. H. LochmiiIler, A. S. Colborn and M. L. Hunnicutt, Anal. Chem., 55 (1983) 1344. 13 C. H. Lochmiiller, A. S. Colborn, M. L. Hunmcutt and J. M. Harris, J. Am. Chem. Sot., 106 (1984) 4077. 14 C. H. Lochmiiller, D. B. Marshall and D. R. Wilder, Anal. Chim. Acta, 130 (1981) 31. 15C. H. LochmiiIIer, M. T. Kersey and M. L. Hunnicutt, Anal. Chim. Acta, 175 (1985) 267. 16 J. B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, New York, 1970, p. 302. 17 J. B. Birks, Rep. Prog. Phys., 38 (1975) 903. 18 G. Marconi and P. R. Salvi, Chem. Phys. Lett., 123 (1986) 254. 19 T. Azumi, A. T. Armstrong and S. P. McGlynn, J. Chem. Phys., 41 (1964) 3839. 20 C. Horvath and W. J. Melander, Chromatogr. Sci., 15 (1977) 393. 21 I. Isenberg, R. D. Dyson and R. Hanson, Biophys. J., 13 (1973) 1090.
EXAMINATION OF COVALENTLY BOUND POLYMERIC STATIONARY PHASES BY LUMINESCENCE SPECTROSCOPY
C. H. LOCHMULLER*
and M. T. KERSEY
P. M. Gross Chemzcal Laboratory,
Duke University, Durham, NC 27706
(U.S.A.)
(Received 3rd March 1987)
SUMMARY Lummescence spectroscopy is used to probe silica-gel surfaces derivatized with [3-(3pyrenyl)propyl] methyldichlorosilane. Evidence of solution pre-polymerization and subsequent derivatization to accessible surface silanols is presented. Conformational changes of the bound ligands in polar and non-polar solvents indrcate a dynamic surface which minimizes its surface area in hostile solvents.
Reversed-phase bonded stationary phases for liquid chromatography are generally classified into two categories, brush and polymeric. They are produced by reacting monofunctional or polyfunctional silanes, respectively, to accessible surface silanols of silica gel [l-5]. Early chromatographic experiments predominantly used brush-phase columns because of the greater reproducibility in the bonded-phase derivatization and higher column efficiencies. Recently, polymeric stationary phases were shown to provide a more selective separation of polycyclic aromatic hydrocarbons with similar efficiencies when compared to brush phases [ 2,6,7] . Verzele and Mussche [ 31 argued that, chromatographically, there are no differences between brush and polymeric phases; however, Sander and Wise [2] have demonstrated selectivity changes of polycyclic aromatic hydrocarbons for the two bonded-phase types. More information is required to characterize these chemically modified polymeric surfaces. Several spectroscopic methods have been developed to probe the physicochemical micro-environment of these complex surfaces. Nuclear magnetic resonance and infrared spectroscopy have been used to characterize the shape and solvent-dependent conformations of chemically bound silanes [8-111. Lochmiiller et al. [12-l 51 introduced steady-state and time-dependent luminescence spectroscopy to examine the micro-environment of the bound *Charles H. Lochmtiller is Professor of Chemistry and Biochemical Engineering at Duke University. His roughly 100 publications include work on robotics, proton-induced x-ray emission and n.m.r. but his main thrust has been in fundamental studies of separation mechanisms. He received the Pioneer Award for his work in robotics in 1985 and is the recipient of the 1987 Chromatography Award of the American Chemical Society. 0003-2670/87/$03.50
0 1987 Elsevier Science Publishers B.V.
144
silanes. These techniques were used to examine the distribution of surface silanols, the conformational changes of chemically modified brush phases in common liquid-chromatographic solvents and the surface polarity of endcapped bonded phases [12-l 51. In the present work, [ 3-(3-pyrenyl)propyl] methyldichlorosilane-bonded phases were examined by using luminescence spectroscopy. The data reported indicate solution pre-polymerization of the silane oligomer. Data from mass spectral and supercritical fluid chromatographic analyses suggest formation of low-molecular-weight polymers. Luminescence data indicate a structural rearrangement of these low-molecular-weight polymeric phases in polar and non-polar solvents. EXPERIMENTAL
Materials Whatman Partisil-10 silica gel (BET, Nz surface area 323 m2 g-l, mean pore diameter 93 A, and mean particle size 10 pm) was used as the support. Synthesis of the pyrene silane and subsequent hydrosilylation reactions were done in this laboratory under conditions previously described [12]. Methyldichlorosilane (Petrarch Systems) was used without further purification in the synthesis of the difunctional pyrene silane. Spectral-grade hexane and methanol (Mallinckrodt) were used as contact solvents. Fluorescence studies Steady-state studies. All steady-state fluorescence spectra were obtained by using a Perkin-Elmer Model MPF-66 spectrophotometer. Emission spectra were collected from 360 to 530 nm at an excitation wavelength of 315 nm. Excitation spectra were collected from 270 to 370 nm at emission wavelengths of 390 nm (monomer) and 480 nm (excimer). AI1 spectra were measured at 22°C. Carbon and hydrogen determinations (M. H. W. Lab., Phoenix, Arizona) were run on all bonded phases and these were diluted with underivatized silica to avoid inner-filter effects. Derivatized silica samples were prepared for data acquisition by adding approximately 20 mg of the derivatized silica to 2 ml of solvent. The solvent/silica slurry was freeze/pump/thawed a total of three times to avoid oxygen quenching of the fluorescence signal. After the final thaw, the cell was inverted and the silica settled into the quartz tube. The cell was then placed into a specially designed holder which fits into both spectrometers. The cell and holder were designed in this laboratory and are illustrated in Fig. 1. Time-dependent studies. Instrumentation, data collection, and numerical analysis have been discussed [ 131. Data were acquired at an excitation wavelength of 315 nm and an emission wavelength of 390 nm. An average instrument response of 4.3 ns (FWHM) was recorded when 0.5 atm of nitrogen (National Specialty Gases) was used in the flashlamp.
145
w
Teflon
o-
Vacuum SeaI
mg
7
Quartzto glass seal
Quartz tube
Fig. 1. Freeze/pump/thaw area of the holder.
cell and holder. The quartz tube is inserted through the shaded
Solution polymerization [ 3-( 3Pyrenyl)propyll methyldichlorosilane was polymerized by using the same bonded-phase reaction conditions (without silica) previously described [ 151. Polymerization was terminated by adding an excess of trimethylchlorosilane (Petrarch Systems). Samples of.the mixture were removed, solvent was evaporated by bubbling dry nitrogen through the mixture and the pale-green, viscous liquid was examined by mass spectrometry and supercritical fluid chromatography. The latter data were acquired on a Hewlett-Packard model 1082B liquid chromatograph modified for the purpose. Aliquots of the polymerized solution were diluted in chloroform to concentrations between 10e3 and 5 X lo-’ M in monomer. Allylpyrene solutions, within the same concentrations, were used to compare the polymeric solutions with a nonaggregating model compound. RESULTS
AND DISCUSSION
Difunctional silanes were used in this work because of the inherently less complex polymeric structures, compared to trifunctional silanes, formed during polymerization. Polymers synthesized with trifunctional silanes exhibit cross-linking; however, difunctional silane polymers only form linear or cyclic structures. Further, only linear structures bind to accessible surface silanols and the unreactive cyclic polymers are washed from the silica surface. Possible cyclic and linear structures formed by the condensation of three difunctional silane monomers are shown below (R = 3-propylpyrene). The photophysics of pyrene is well documented [16-191. Pyrene forms excited-state dimers (excimers) in a collisional quenching process involving an excited-state pyrene molecule and one in the ground state. Excimer formation decreases the quantum yield of the monomer emission (378 nm) and
146
gives rise to a broad, structureless emission (A_ = 470 nm). Figure 2 illustrates the loss in intensity of the monomer emission with a corresponding rise in the excimer emission for two covalently bound polymeric phases in contact with methanol. Similar spectra have been observed for pyrene in solution
lJ61. Steady-state fluorescence measurements of polymer solutions The aggregation of difunctional pyrenesilanes in solution was observed by using steady-state fluorescence spectroscopy. The association of difunctional silane monomers was monitored by measuring the excimer/monomer ratio of the polymer solutions and comparing that to measurements made with allylpyrene solutions. Figure 3 compares the excimer/monomer ratio for polymerized [3-(3-pyrenyl)propyl] methyldichlorosilane and allylpyrene solutions. A plot of the excimer/monomer ratio vs. allylpyrene concentration in solution approaches zero as the concentration of allylpyrene decreases. However, compared with the allylpyrene solutions, the polymerized difunctional silane solutions exhibit increased excimer formation. The greater excimer intensity observed for the difunctional silane solutions is indicative of silane polymerization because, upon dilution, the difunctional silane oligomers maintain the
Fig. 2 Steady-state fluorescence spectra. (A) 1.36% carbon diluted l/20 (B) 7.24% carbon diluted l/1000 m methanol.
m methanol,
1
Concentrctlon
2
3
4
Carbon
CM)
5
6
7
(%)
Fig. 3. Steady-state fluorescence excimer/monomer intensity ratios vs. concentration polymerized difunctional pyrenesilanes (m) and allylpyrene (0) in chloroform.
of
Fig. 4. Steady-state fluorescence excimer/monomer intensity ratios vs. percent carbon for difunctional propylpyrene bonded phases in hexane ( n ) and methanol (0 ).
critical interaction distance for excimer formation. Consequently, a percentage of the polyfunctional silane must be covalently bound to one another. The results of supercritical fluid chromatography and mass spectral analyses indicate, in addition to individual difunctional silanes, the presence of dimers and trimers. Evidence of larger-molecular-weight polymers was not observed. Steady-state
fluorescence
measurements
of polymeric
bonded
phases
Steady-state spectra of polymeric bonded phase samples ranging from 1.36% to 7.24% carbon were collected in contact with hexane and methanol. Surface coverages (r_lmol mV2) of these polymeric bonded phases cannot be determined because an average molecular weight of the bound polymer is unknown. Figure 4 illustrates the changes observed in the excimer/monomer ratio for polymeric phases in the two solvents. The ratio increases with increasing percent carbon. This trend has also been observed for both pyrene in solution [16] and for monofunctional pyrenesilanes bound to silica gel 1131. Differences in the excimer/monomer ratios for methanol and hexane are observed above 2% carbon and increase with increasing pyrenesilane surface coverage. The larger excimer/monomer values in methanol are attributed to the collapse of the bound ligands which, at higher percent carbon, have a greater probability of coming within the critical interaction distance to form excimers. The greater degree of excimer emission observed in methanol for similar carbon loadings compared with hexane has also been demonstrated for brush phases [12,13]. Changes observed for the excimer/monomer ratio in methanol and hexane may be explained by solvent-induced conformational changes in the bonded phase caused by “solvophobic” interactions between the bound ligands and solvent [20]. This experiment demonstrates that
148
polymeric bonded phases, like their monofunctional silane counterparts, appear to reduce the total surface area of the bound ligand moieties in hostile solvents. These results support a dynamic model for polymeric phases which suggests that the ligands swell in good solvents and shrink in hostile solvents. Time-dependent measurements of polymeric bonded phases Values of the pre-exponential factors (A,), lifetimes (Ti) and percent carbon in hexane and methanol are listed in Table 1. The validity of the numerical analysis and model based upon the pre-exponential factors has been defended [ 131. When the “component increment&ion” of Isenberg et al. [21] was used, it was concluded that difunctional pyrene bonded phases fit to a sum of three exponentials. Figure 5(a) illustrates the pre-exponential factors for the series of difunctional bonded phases, 1.36% to 7.24% carbon, in methanol. One observes that the population of bound pyrene molecules which form excimers is approximately 80% and increases from 80 to about 95% between 5 and 7.24% carbon. The data appear to indicate that up to 5% carbon, the degree of eximer formation for difunctional propylpyrene bonded phases is independent of carbon loading. Increased pre-exponential factors for carbon loadings TABLE 1 Normalized pre-exponential percent carbon Carbon (%) coverageb (pm01 m-‘) Hexane A, A* A, 71 71 73
factors (Ai)
1.36 0.19
1.99 0.28
0.736 0.062 0.176 4.7 100.3 157.5
0.637 0.083 0,280 2.1 60.5 138.2
0.846
0.837
and lifetimesa (7i) as a function of solvent and
2.51 0.35
0.451 0.368 0.173 20.2 53.5 97.0
3.48 0.50
4.43 0.64
0.468 0.390 0.143 14.9 45.1 101.2
0.835 0.154 0.011 12.8 40.3 135.8
4.91 0.71
5.11 0.74
1.24 1.10
0.086 0.009 9.3 32.3 119.9
0.801 0.097 0.102 3.4 42.8 123.3
0.91 0.07 0.01 1.6 33.7 123.9
-
0.843 0.086 0.071 1.3 33.5 106 3
0.05 0.00 6.0 30.0 117.8
0.904
Methanol A, A, A, 71 71 73
0.94
*Lifetimes are in ns. bThese values are based on calculations which assume that the predominant surface oligomer is monomeric (and as if monomeric chemistry was used); it is not the intent of this calculation to suggest that this is the case here and the values are presented solely for comparison to other published work where “monomeric” chemistry indeed was used.
149
1
2
3
4
5
Carbon
PA)
6
7
-I 8
8
Corbon
(%I
Fig. 5. Plots of the pre-exponential factors vs. percent carbon for difunctional propylpyrene bonded phases in methanol (a) and in hexane (b): (A) A,, (B) A,, (C)A,.
greater than 5% indicates the interaction of oligomeric units on the more densely packed surface. Polymeric phases may exhibit large excimer-forming populations at low percent carbon because either linear polymers form in solution and then bind to accessible surface silanols and/or, as Sander and Wise [2] suggest, the polymeric network is initiated on the surface and “grows out” into solution. The large excimer-forming population at low percent carbon is attributed to solution pre-polymerization and subsequent reaction of the polymerized silane to surface silanols. The extent of silane-surface initiated polymerization at low carbon coverages would be less than the competitive process of silane reaction to the more accessible surface silanols for two reasons: first, the probability of a pyrenesilane ligand in solution encountering an accessible surface silanol is greater than one reacting with a bound silane; and second, the interaction of a bound silane and one in solution is more hindered and less favorable than the competitive process of binding to a more accessible surface silanol. The model of polymer formation with polyfunctional silanes suggested by Sander and Wise may be more applicable to higher carbon loadings because there are fewer surface silanols to compete with the residual silane silanols. Figure 5(b) illustrates the pre-exponential factors for the polymeric bonded phases in hexane. At low percent carbon, a greater degree of excimer formation than expected from brush-phase results [13] is observed. However, the excimer population decreases up to 3% carbon and then increases. This anomalous behavior may indicate the difficulty in reproducibly synthesizing polymeric phases. Examination of the lifetimes, r2 and r3, listed in Table 1 indicate a concentration-quenching kinetic model of the pyrene monomer emission up to 3% carbon [16]. The constant lifetimes, within experimental error, for greater than 3% carbon is a departure from true, diffusion-controlled behavior. This might be expected in constrained systems where the ligands are bound to the surface.
150
Comparison of the lifetimes in hexane and methanol supports a dynamic stationary-phase model for polymeric bonded phases. The lifetimes, 72 and TV, are longer in methanol than those in hexane below 2.5% carbon. The shorter lifetimes observed in hexane correspond with increased solvation of the alkyl chains. This solvation gives the silane a greater degree of motion which, on average, decreases the relaxation time of the pyrene molecule. Conversely, methanol, a O-solvent, restricts the ligand motion increasing the average relaxation time of the hydrophobic pyrene ligands. Pyrene crystal quantum yields and lifetimes have been observed to increase with decreasing temperature [17]. Above approximately 3% carbon, there are no significant differences in the lifetimes, r2 and 73, between the two contact solvents, which indicates that the ligands are motionally constrained to the same degree. The differences between TV in hexane and methanol can be attributed to the degree of solvation of the pyrene ligands. Silanes solvated by hexane take longer, on average, to form excimers than in methanol because of the increased mobility of the propyl chain and the larger separation between pyrene molecules. The shorter lifetimes observed in methanol may be indicative of the collapsed state of the ligands which put the pyrene molecules within the critical interaction distance for excimer formation. We gratefully thank L. Mink for the supercritical fluid chromatographic data. This project was supported by the National Science Foundation, Grant CHE85-00658 (to C.H.L.) REFERENCES 1 R. E. Majors, J. Chromatogr., 180 (1980) 488. 2 L. C. Sander and S. A. Wise, Anal. Chem., 56 (1984) 504. 3 M. Verzele and P. Mussche, J. Chromatogr., 254 (1983) 117. 4H. Hemetsberger, M. Kellermann and H. Ricken, Chromatographia, 10 (1977) 726. 5 H. Colin and G. Guiochon, J. Chromatogr., 141 (1977) 289. 6 S. A. Wise and W. E. May, Anal. Chem., 55 (1983) 1479. 7 S. A. Wise and L. C. Sander, J. High Resolut. Chromatogr. Chromatogr. Commun., 8 (1985) 248. 8 R. K. Gilpin, J. Chromatogr. Sci., 22 (1984) 371. 9 C. H. Lochmiiller, D. B. Marshall and D. R. Wilder, Anal. Chem., 52 (1980) 19. 10 L. C. Sander, J. B. CaIlis and L. R. Field, Anal. Chem., 55 (1983) 1068. 11 J. D. Miller and H. Ishida, Anal. Chem., 57 (1985) 283. 12C. H. LochmiiIler, A. S. Colborn and M. L. Hunnicutt, Anal. Chem., 55 (1983) 1344. 13 C. H. Lochmiiller, A. S. Colborn, M. L. Hunmcutt and J. M. Harris, J. Am. Chem. Sot., 106 (1984) 4077. 14 C. H. Lochmiiller, D. B. Marshall and D. R. Wilder, Anal. Chim. Acta, 130 (1981) 31. 15C. H. LochmiiIIer, M. T. Kersey and M. L. Hunnicutt, Anal. Chim. Acta, 175 (1985) 267. 16 J. B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, New York, 1970, p. 302. 17 J. B. Birks, Rep. Prog. Phys., 38 (1975) 903. 18 G. Marconi and P. R. Salvi, Chem. Phys. Lett., 123 (1986) 254. 19 T. Azumi, A. T. Armstrong and S. P. McGlynn, J. Chem. Phys., 41 (1964) 3839. 20 C. Horvath and W. J. Melander, Chromatogr. Sci., 15 (1977) 393. 21 I. Isenberg, R. D. Dyson and R. Hanson, Biophys. J., 13 (1973) 1090.