4318
J. Phys. Chem. 1986, 90, 4318-4322
SURFACE SCIENCE, CLUSTERS, MICELLES, AND INTERFACES Solvent-Induced Conforrnatlonal Changes on Chemically Modlfled Silica Surfaces C. H. Lochmuller* and M. L. Hunnicuttt P . M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706 (Received: July 18, 1985; In Final Form: April 16, 1986)
The time-dependent luminescence of (l0-(3-pyrenyl)decyl)dimethylmonochlorosilane (3PDS) chemically bonded to microparticulate silica was investigated to elucidate further the solvent-induced conformation changes observed in bonded (3-(3-pyrenyl)propyl)dimethylmonochlorosilane (3PPS). The excimer decay profiles for low-surface-coverage 3PDS silica derivatized exhaustivelywith n-octadecyldimethylmonochlorosilane indicate that a large fraction of the molecules form excimers instantaneously when in contact with poorly solvating or ‘hostile” solvents. A smaller fraction of the molecules appear to form excimers by a quasi-diffusion-controlledmechanism similar to that for intramolecular excimer formation in free solution.
Introduction The photophysics of organic molecules adsorbed on or chemically bound to silica gel has been the subject of recent attention. Research in this area has focused on the restriction to motion imposed by the surface via specific, directed molecular interactions between the adsorbate and adsorbent. The distribution of molecules chemically bound to silica and their organization in contact with different solvents have also been recently investigated. Ware et al. presented data supporting intra- and intergranular motion of aromatic hydrocarbons adsorbed on dry silica gel,’ as well as the occurrence of bimolecular ground-state complex formation for adsorbed p ~ r e n e . ~ .Excitation ~ of these stable ground-state dimers resulted in “excimer-like” emission, but adsorbed pyrene exhibited true excimer emission only in the presence of long-chain alcohols. Ware et al. concluded that “excimer-like” emission resulted from surface-enhanced intramolecular ground-state complex formation and that coadsorbed molecules deactivate polar surface silanols on silica.4 Lochmiiller et al. observed similar results for (3-(3-pyrenyl)propy1)dimethylmonochlorosilane (3PPS) chemically bound to silica and in contact with solventS except that true excimer emission-excited-state dimer formation vs. ground-state dimer excitation-was observed. Those results also suggested that clustering of bound molecules was significant at less than 10% of the total attainable surface coverage. Furthermore, the time-dependent luminescence of surface-bound 3PPS molecules in contact with several different solvents indicated that the organization of surface-bound molecules is primarily determined by the proximity and distribution of chemically reactive silanols. These experiments also indicated a solvent-induced conformation change resulting in up to a 20% change in the fraction of associated 3PPS molecules. We report here on time-dependent luminescent studies of silicas reacted with (10-( 3-pyrenyl)decyl)dimethylmonochlorosilane (3PDS), which were undertaken to investigate further the solvent-induced conformation changes observed for the propylpyrene analogue. Differences in the excimer rise times are shown to have their origin in the degree of solvent-hydrocarbon chain interaction. The results contribute to an improved physicochemical model of the surface structure of n-alkyl chemically modified to studies of polymers grafted on surface^,'^-^^ and to the pho‘Current address: Department of Chemistry, University of Utah, Salt Lake City, UT 841 12.
0022-3654/86/2090-4318$01 .50/0
tophysics of intramolecular e x c i m e r ~ . ” ~ ’ ~ Experimental Section The silica gel used was Whatman Partisil-10 (Clifton, NJ) (N2 surface area, 323 m2/g; pore diameter, 93 A; mean particle diameter, 10 km). The pyrene silane was synthesized and its structure confirmed in our laboratory. Reagent-grade chloroform was dried by distillation from and stored over calcium hydride prior to its use. Spectral-grade methanol, acetonitrile, water, hexane, and tetrahydrofuran were used without further purification, although special precautions were taken to ensure that the tetrahydrofuran did not form peroxides. The silica derivatization procedure and subsequent treatment are detailed in a previous paper.5 The surface concentration of bound silane (pmol/m2) was derived from elemental analysis and the values of the expected average distance between chemically bound pyrene molecules (Dexp)were calculated by the method of Unger et al.I9 Pyrenylalkaneln-alkane silicas were prepared by treating the 0.14 (1) Bauer, R. K.; de Mayo, P.; Ware, W. R.; Okade, K.; Rafalska, M.; Wu, K. C. J . Am. Chem. SOC.1982, 104,4635. (2) Hara, K.; de Mayo, P.; Ware, W. R.; Weedon, A. C.; Wong, G. S. K.; Wu, K. C. Chem. Phys. krf.1980, 69, 105. (3) Bauer, R. K; De Mayo, P.; Ware, W. R.; Wu, K. C. J . Phys. Chem. 1982, 86, 3781.
(4) Ware, W. R.; Bauer, R. K.; de Mayo, P.; Okada, K.; Wu, K. C. J . Phvs. Chem. 1983. 87. 460. ,~ (5) Lochmiiller; C. H.; Colbourn, A. S.; Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1983, 55, 1344. (6) Lochmiiller. C. H.; Colborn. A. S.; Hunnicutt, M. L.; Harris. J. M. J . Am. Chem. SOC.1984, 106, 4077. (7) Gilpin, R. K.; Gangoda, M. E.; Krishen, A. E. J . Chromatogr. Sci. 1982, 20, 345. (8) Gilpin, R. K.; Gangoda, M. E. J . Chromatog. Sci. 1983, 21, 352. (9) Lochmiiller, C. H.; Wilder, D. R. J . Chromarogr. Sci. 1979, 17, 574. (10) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068. ( 1 1) Hansen, S. J.; Callis, J. B. J . Chromatogr. Sci. 1983, 21, 560. (12) Sindorf, D. W.; Maciel, G. E. J . Am. Chem. SOC.1983, 105, 1848. (13) Tazuke, S.; Ooki, H.; Sato, K. Macromolecules 1982, 15, 400. (14) Anderson, V. C.; Craig, B.B.; Weiss, R. G. J . Am. Chem. SOC.1982, 104, 2972. (15) Johnson, G. E. Macromolecules 1980, 13, 839. (16) de Gennes, P.-G. Macromolecules 1980, 13, 1069. (17) Goldenberg, M.; Emert, J.; Morzwetz, H. J . A m . Chem. SOC.1978, 100, 7171. (1 8) Snare, M. J.; Thistlethwaite, P. J.; Ghiggino, K. P. J . Am. Chem. SOC. 1983, 105, 3328. ( 1 9) Unger, K. K.; Roumeliotis, P. J . Chromatogr. 1978, 149, 21 1. (20) Scott, R. P. W.; Kucera, P. J . Chromatogr. 1978, 149, 93. ( 2 1 ) Scott, R. P. W.; Kucera, P. J . Chromatogr. 1979, 171, 37.
0 1986 American Chemical Society
Conformational Changes on Silica Surfaces pmol/m2 (low-surface-coverage) 3PDS silica with a 5 M excess of either hexamethyldisilazane (HMDS) or n-octadecyldimethylmonochlorosilane (ODS), respectively. The surface concentrations of these pyrenylalkane/n-alkane-modifiedsilicas were calculated from the elemental carbon content before and after secondary derivatization and are (0.14 pmol/(m2 3PDS) 2.84 Kmol/(m2 HMDS)] and [0.14 pmol/(m2 3PDS) + 2.12 pmol/(m2 ODs)]. Time-dependent measurements were made with a Photochemical Research Associates System 2000 nanosecond fluorimeter. All silica samples were subjected to 3 cyces of freeze-pumpthaw to remove oxygen and were packed into a quartz column flow cell described previo~sly.~The packed-flow cell was placed into a nitrogen-purged sample compartment, and no change in count rate was observed during the analysis time. The excitation monochromator was set at 305 nm and the emission monochromator was set at 390 or 480 nm to detect monomer and excimer emission, respectively. The bandwidth of the excitation and emission monochromators was 16 nm. Ultrahigh-purity hydrogen was used in the flash lamp and gave an average instrument response function of 3.2 ns (full width at half-maximum). Numerical analysis of the fluorescence decay data performed and the statistical and experimental criteria used to judge the goodness-of-fit of the model decay law (eq 1) were extensively described
+E
+
T f ( t )= A,et/T,
+
A z e t / ~ 2 A3et/~3
(1)
in a previous paper.6 A three-component model provided the best statistical fit to the monomer emission data over the surface concentrations studied.
Results and Discussion 3PDS Proximity and Distribution. The values obtained for lifetime and preexponential factor are in fact average or firstmoment estimates. No information is derived as to the shape of the distribution of lifetimes or preexponentials in the analysis. As Ware2* has shown, the analysis of such data in the presence of underlying heterogeneity is a complex issue. Simple decay models may mask significant underlying heterogeneity and lead to oversimplified models of the microscopic nature of the surface environment. While the microscopic state of bound pyrene is undoubtedly complex, it is likely to be less so than in the case of adsorbed pyrene. All translational motion is severely restricted by the covalent anchoring, eliminating ordinary surface diffusion and ”site-hopping” as complicating factors. Adsorption itself is probably very limited in the presence of solvent. In the case of trimethylsilylated (“end-capped”) pyrene silicas, all chemically available, strong adsorption sites have been eliminated. The absence of any indication of surface-enhanced, ground-state complexes4 is also indicative of a lack of adsorption of the pyrene terminus. The degree of heterogeniety is likely to be more like that of the solvent-moderated behavior of adsorbed pyrene than like an adsorbed molecule on activated silica. There is also no evidence of strong quenching of monomer emission that is not directly correlated to ”excimer emission” in any of the cases studied. The following discussion assumes that the average properties derived with a simple three-component model (eq 1) could be oversimplifying a more complex microheterogeneity. Certainly, a brute-force application of this approach to other, less well-defined systems is to be avoided. Figure 1 is representative of the typical monomer decay profiles obtained with 3PDS at various surface concentrations. These are actual tetrahydrofuran data, but the methanol and acetonitrile experiments are also characterized by the concentration quenching of the long-lived monomer component with increasing surface coverage (more silanols reacted). The results are similar to those expected in highly constrained system, where excimer formation is controlled by the local, rather than bulk fluorophore concen(22) James, D. R.; Liu, Y.-W.; DeMayo, P.; Ware, W. R. Chem. Phys. Lett. 1985, 120(4,5), 460.
.. x X
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00
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X
X
X
X . X
X .
-__
. -&i.=3
NANOSECONDS
X
.
. . .
X X X X X
a X X
d56.44
713.84
Figure 1. Monomer decay profile for 3PDS bonded on Partisil 10: solvent, tetrahydrofuran;surface concentration (kmol/m2) (A) 0.14, (B) 0.17, (C) 0.32, (d) 0.60.
TABLE I: Monomer Normalized Preexponential Factors ( A i ) and Lifetimes (T,, ns) as a Function of Solvent and Surface Concentration surface concentration, kmol/m2 parameter 0.12 0.14 0.17 0.32 0.60 Methanol AI A2 A3 71
72
73
0.25 0.20 0.55 6.3 43.5 135.6
0.29 0.20 0.51 6.6 49.1 155.9
0.47 0.27 0.26 4.0 24.4 104.3
0.68 0.26 0.06 2.3 10.6 54.6
0.85 0.10 0.05 1.2 5.5 63.9
Acetonitrile AI A2
A3 71
72 73
0.26 0.15 0.59 7.1 35.5 102.7
0.29 0.19 0.52 4.3 32.8 131.2
0.51 0.27 0.22 4.1 24.2 81.1
0.82 0.15 0.03 2.3 13.1 55.0
Tetrahydrofuran AI A2 A3 71
72 73
0.25 0.16 0.59 9.6 43.1 156.7
0.22 0.24 0.54 9.3 49.8 143.7
0.45 0.32 0.23 8.4 40.5 125.1
0.63 0.30 0.07 4.6 15.1 64.0
0.85 0.13 0.02 2.6 9.0 70.9
tration.6 The lifetimes and normalized amplitude factors obtained by fitting the monomer decay data are contained in Table I. Analysis of these data suggests that as in the case of 3PPS silicas there are three “populations” (but two emitters) at lower coverages: those forming excimers, those decaying as monomers but that will form excimers at higher surface coverage, and those that will never form excimers a t achievable surface coverage. The average lifetime for the longer lived monomer emission obtained from 0.12 pmol/m2 in contact with tetrahydrofuran ( 1 6 5 ns) agrees with those obtained for (3-(3-pyrenyl)propyl)trimethylsilane (PPT) in free solution. This implies that this fraction of the 3PDS molecules is fully solvated and experiences no surface interference at low surface coverages. Interestingly, all of the lifetimes decrease with increasing surface concentration due to enhanced collisional quenching by adjacent 3PDS molecules. A comparison of the A I and A3 amplitude factors for 3PDS and 3PPS in contact with methanol is presented in Figure 2. These amplitude factors exhibit the same monotonic increase and
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The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
Lochmiiller and Hunnicutt TABLE II: Excimer Rise Times as a Function of Solvent and Surface Concentration rise times, ns surface concn, aceto- tetrahydrofimol/m2 methanol nitrile furan hexane water 3-Pyrenyldecylsilane 0.12 7.7 5.2 13.1 0.14 5.8 4.6 12.1 11.0 0.5 0.17 5.2 4.0 10.0 9.0 0.32 2.2 2.0 5.9 4.9 0.60
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