48
Langmuir 1989,5, 48-54
Surface Geometry and Pore Size Effects on Photoinduced Charge-Transfer Interactions between Pyrene and Diethylaniline on Silica Surfaces Hanna Birenbaum, David Avnir," and Michael Ottolenghi* The Institute of Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received June 15, 1988. I n Final Form: August 4, 1988 The charge-transfer fluorescence quenching of pyrene (Py) by N,"-diethylaniline (DEA) adsorbed on silica surfaces was investigated for silicas with average pore sizes (aps) ranging from 60 (Si-60) to 1000 8, (Si-1000). For all silicas a complete lack of quenching activity was observed below the respective DEA monolayer value. Such inertness of DEA as a charge-trader quencher is interpreted in terms of its strong adsorption interactions with surface silanol groups. Above monolayer coverage the slopes of the quenching (Stern-Volmer) plots markedly increase with decreasing aps, reaching a solutionlikeefficiency in the case of Si-60. The effect is attributed to a gradual nonhomogeneous pore volume fii up by DEA, which is sensitive to the aps distribution. The nature of the surface-quenchingmechanism is discussed in light of the Py-DEA exciplex fluorescence wavelengths and relative yields. It appears that the irregularityof the surface affects the mutual lPy*-DEA geometry as well as the effective local polarity. Consequently, the relative weight of the exciplex, versus electron-transfer pathways, is markedly aps dependent.
Introduction A central issue in the analysis of chemical reactions in heterogeneous interface environments concerns the role of the geometry of the solid support. There is growing evidence that by careful selection of specific features of the surface geometry one can dramatically affect reaction pathways.' The corresponding changes in reaction routes are the outcome of a complex interplay between a number of parameters such as geometrical restrictions on diffusional pathways, reactant aggregation phenomena, ground-state and excited-state distortion of molecular conformations, and preferential orientations of molecular moieties. All these are markedly affected by the very existence of a distribution in the magnitude of environmental physical properties (e.g., polarity, cage size, energetic profile for molecule/surface interactions, etc.). To gain further insights into the general problem of geometry/chemistry relationships, we have chosen, in this study, a combination of a basic geometric feature and a well-characterized photochemical process, namely, the average pore size (aps) of the support, silica (Si) in our case, and a charge-transfer (CT) process from excited singlet pyrene ('Py*) to diethylaniline (DEA). Earlier studies on the effects of pore size on several surface photoprocesses have indeed pointed out the importance of this parameter in relation to cage effe~ts,~B surface irregularity e f f e ~ t s ,and ~ , ~molecular reorientation and molecular association^.^^' However, most of the progress in the understanding of CT photoprocesses is based on studies carried out in homogeneous solutions (for reviews, see ref 8 and 9) or in microenvironments such as (1)For recent reviews of these effects in photochemistry, see: (a) Turro, N. J. Pure Appl. Chem. 1986,58 1219. (b) Oelkrug, D.; Fleming, W.; Fullman, R.; Gunther, R.; Honen, W.; Krabichler, G.; Schafer, M.; Uhl, S. Pure Appl. Chem. 1986,58,1207. (c) de Mayo, P. Pure Appl. Chem. 1982,54,1623. (d) Thomas, J. K. J. Phys. Chem. 1987,91,267. (2) Turro, N. J.; Chang, C.-C.; Abrams, L.; Corbin, D. R. J. Am. Chem. SOC. 1987,109,2449 and earlier reports from that laboratory. (3)Wellner, E.; Rojanski, D.; Ottolenghi, M.; Huppert, D.; Avnir, D. J . Am. Chem. SOC. 1987,109,575. (4)Pines, D.; Huppert, D. J.Phys. Chem. 1987,91,569. (5)Avnir, D. J.Am. Chem. SOC. 1987,109,2931. (6)Roxolo, C.B.; Deckman, H. W.; Abeles, B. Phys. Reu. Lett. 1986, 57, 2462. (7)Avnir, D.; Busse, R.; Ottolenghi, M.; Wellner, E.; Zachariasse, K. J. Phys. Chem. 1985, 89,3521. Wellner, E.;Ottolenghi, M.; Avnir, D.; Huppert, D. Langmuir 1986,2,616.
0743-7463/89/2405-0048$01.50/0
Table I. DEA Monolayer Values (DEA,) on Porous Silicas and the Corresponding "Critical"DEA-Py* Quenching Values (DEA,) Derived from Figure 2 DEA,, (mol/g) X 10' B-point Langmuir Si-60 8.8 f 1.0 11.5 + 1.5 3.8 + 0.5 Si-200 3.0 f 0.5 0.22 + 0.05 Si-500 0.22 f 0.05 Si-1000 0.20 f 0.05 0.21 + 0.03
DE&, (mol/g) 106 10.5 + 1.5 2.5 + 0.3 0.25 + 0.03
Sa
(sib 110) 14 1.5 0.3 0.1
Stern-Volmer slopes.
polyelectrolytes and micelles.1° Only a few observations, however, have been made in relation to intra- and intermolecular CT phenomena of molecules adsorbed on inert solid surfaces.ldJ1 Much of the extensive attention devoted to CT interactions between aromatic hydrocarbons and amines has been due to the role played by sandwichtype CT exciplexes and thus to the high sensitivity of the processes to solvent polarity and to steric restrictions imposed on the relative orientation of the reactants.8J2 As indicated by several investigations in micelles and lipos o m e ~such , ~ ~properties qualify a system such as Py-DEA to serve as a model probe for CT interactions in heterogeneous media. In the present study the same system is applied to solid porous silica surfaces with varying average pore size distribution (aps). Our study indicates that both the chemistry and geometry of the surface markedly affect the course of the charge-transfer process between the adsorbed reactants. (8)Mataga, N.; Ottolenghi, M. In Molecular Association; Foster, R., Ed.; Academic: New York, 1979;Vol. 11, pp 1-78. (9)Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986,86, 401. (10)For a collection of representative papers, see: J. Photochem. 1985, 29, 1-257. (11)Chandrasekaran, K.; Thomas, J. K. J. Am. Chem. SOC. 1983,105, 6383. (b) Wolfgang, S.;Gafney, H. J.Phys. Chem. 1983,87,5395. (c) Nakashima, N.;Phillips, D. Chem. Phys. Lett. 1983,97,337.(d) Levy, A.; Avnir, D.; Ottolenghi, M. Chem. Phys. Lett. 1985,121,233. (12)Okada, T.;Kubota, M.; Masaki, S.; Mataga, N.; Ide, R.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1972,14,536. Masaki, S.;Okada, T.; Mataga, N.; Sakata, Y.; Misumi, S. Bull. Chem. SOC. Jpn. 1977,49,859. Cheung, S.T.; Ware, W. R. J. Phys. Chem. 1983,87,466. Yorozu, T.; 1981,103,5480. Hayashi, K.; Irie, M. J. Am. Chem. SOC. (13)(a) Waka, Y.;Hamamoto, K.; Mataga, N. Photochem. PhotobioE. 1980,32,27. (b) Waka, Y.; Nataga, N.; Tanaka, F. Ibid. 1980,32,335. (c) Neumann, S.; Korenstein, R.; Barenholz, Y.; Ottolenghi, M. Isr. J. Chem. 1982,22,125.Thomas, J . K.; Hashimoto, S. New J. Chem. 1987, 11, 145.
0 1989 American Chemical Society
Geometry and Pore Size Effects on Charge Transfer
E
a
o p
I
I
I
0
1
2
3
I
4 [ D E A I , Mx103
I
I
5
6
Figure 1. Characteristic adsorption profile of DEA on Si-60. (a) The adsorption isotherm. The “B-point”(Le., the intersection of the initial and final slopes of the curve, see ref 14) is indicated. (b) The fit of the adsorption data in a to the Langmuir isotherm14 DEA/DEA,d = (b[DEA,])-’ + DEA/DEA, where [DEA] is the equilibrium concentrationin solution, is the amount adsorbed (mol/g), DEA, is the monolayer value, and b is a constant.
Experimental Section Materials. The following silicas (Si)were used (genericname, actual aps (A), NTBETsurface area (m2/g)):Si-60,60,550,Si-200, 180, 150; Si-500, 420, 50; Si-1000, 1290, 20. All Si were Merck Fractosils (particle size range 60-125 pm) except for Si-60, obtained from Woelm (particle size 32-60 pm). Pyrene (Py, Aldrich) was recrystallized from ethanol and purified over silica. N,”-diethylaniline (DEA,BDH) and 1-octanol (Riedel) were vacuum distilled. Cyclohexanewas a Fluka spectrograde product. Adsorption Isotherms and Monolayer Values. DEA adsorption isotherms were determined at room temperature by using 3 g of Si and 5 mL of DEA-cyclohexane solutions, allowing 24 h for equilibration. DEA equilibrium concentrations in solution were determined from the absorption at 258 nm. Figure 1shows a characteristic adsorption isotherm for Si-60 and a Langmuir-type plot of the data. The DEA monolayer values obtained either from a B-point analysis of the former or from the slope of the latter14 are collected in Table I. Sample Preparation. A solution of Py and DEA in cyclohexane (20-30 mL) was added to 0.3 g of silica followed by careful vacuum evaporation of the solvent, which did not result in any loss of the above adsorbates. Silica samples were dried before use by heating to 180 O C under vacuum for 10-15 h. The amount of pyrene on the surface was kept relatively low, 0.001 < Bpy < 0.005, where Bpy denotes pyrene surface coverage (monolayer fraction) so as to avoid complications due to self-quenching of the excited singlet lPy* via intermolecular interactions leading to excimers.7s16 This precaution is especially relevant since excimer generation is favored in the presence of coadsorbates which increase the mobility of pyrene on the surface.'^'^ No excimer emission is observed at these 0 values. Fluorescence Measurements. Steady-state fluorescence measurementswere carried out on a Perkin-Elmer L85 fluorimeter after the samples were flushed with dry nitrogen. Results Quenching of Pyrene Fluorescence on Porous Silica Surfaces with Varying Pore Size Distribution. The efficiencies of lPy* quenching by DEA on various silica surfaces are presented in Figure 2 in terms of Stern-Volmer plots,16 in which the relative fluorescence (14) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1982. (15) Bauer, R. K.; de Mayo, P.;Ware, W. R.; Wu, K. C. J. Phys. Chem. 1982,86,378. Bauer, R. K.; de Mayo, P.; Okada, K.; Ware, W. R.; Wu, K. C. J. Phys. Chern. 1983,87,460. Beck, G.;Thomas, J. K.Chern. Phys. Lett. 1983,94,553. Lochmuller, C. H.; Colbom, A. S.;Hunnicatt, M. L.; Harris, J. M. J . Am. Chem. SOC.1984,106,4077.
Langmuir, Vol. 5, No. I , 1989 49 intensities, Io/I, are plotted against the amounts of added DEA in units of moles of DEA/gram of silica. There are two major features of the above plots that deserve special consideration. First, it is clearly evident in the cases of Si-60, Si-200, and Si-500 that, in variance with homogeneous solutions, quenching does not start from the very beginning of surface coverage by DEA but only after an “induction” or “critical” amount of adsorbed DEA is reached. (Due to the accuracy level of the experiments, the phenomenon could not be clearly demonstrated in the case of Si-lOOO.) Moreover, as shown in Table I, the critical DEA quenching values coincide, within the limits of our experimental accuracy, with the corresponding DEA monolayer value as determined by the B-point or Langmuir plots analysis. It may, therefore, be concluded that, independently of the specific silica sample, no lPy* quenching by DEA is observed as long as monolayer coverage is not attained. The second characteristic of the quenching plots relates to the quenching efficiencies ( I o / I )observed after monolayer coverage by DEA is exceeded. These are shown in Figure 2 and are replotted in Figure 3 in terms of DEA monolayer equivalents, 6DW. Several features are relevant in this respect. First, it is evident that Si-60 (Figure 2a) differs markedly from Si-200, Si-500, and Si-1000 in exhibiting two distinct slopes (s). A sharp initial rise with s = A(Io/I)/A6 > 110, accounting for -90% of the change in I,,/I, is followed by a considerably less efficient process (with s = 14) which accounts for the residual -10%. In variance with this behavior, only a single slope is observed in the cases of Si-200, Si-500, and Si-1000 for which no sharp initial rise may be observed within the limits of our experimental accuracy. This “normal” Stern-Volmer behavior is associated, however, with a marked effect of the aps on the I o / I vs DEA coverage (6DEA) plots. As shown in Figure 3 and summarized in Table I, the slopes increase progressively with the increase in average pore size values: s = 1.5 for Si-200, s = 0.3 for Si-500, and s = 0.1 for Si-1OOO. It should finally be noted that these slopes are maintained constant over a range of DEA equivalents which by far exceeds monolayer coverage. For example, in the case of Si-500, only -35% quenching occurs upon adding 1 (second) monolayer equivalent, and the slope of the curve is constant over an average of 10 monolayer equivalents. Exciplex Generation. In homogeneous solutions, quenching of ‘Py* by DEA is accompanied by the appearance of the characteristic red-shifted emission (in n-hexane X = 450 nm) of the charge-transfer exciplex, l(Py-DEAqT? The exciplex is generated with -100% efficiency in a nonpolar solvent such as hexane but contributes less than 1% in, e.g., methanol or acetonitrile, where ion-pair formation efficiently competes with exciplex generation8 Figure 4 shows a characteristic emission spectrum of a Py/DEA Si-200 system, indicating a substantial contribution of the red-shifted exciplex band. Table I1 gives the contributions of the pyrene and exciplex emission intensities for the various silica samples relative to the unquenched pyrene intensity in the respective DEA-free systems. Exciplex emission maxima are also given in Table 11. Effects of Coadsorption with 1-Octanol. Being effective competitors for adsorption sites on silica surfaces, alcohols have been shown to affect photoprocesses of adJ ~ effects sorbed pyrene, such as excimer g e n e r a t i ~ n . ~The of coadsorbed 1-octanol on the reaction between excited pyrene and DEA on Si-60 are shown in Figure 5. Addition (16) Stem, 0.;Volmer, M. Phys. Z . 1919,20,183. Turro, N. J. Modern Molecular Photochemistry; Benjamin: London, 1987.
50 Langmuir, Vol. 5, No. 1, 1989
Birenbaum et al. no
w.0,
I
-
a ) Si-60
b ) Si
-
7.5
I
-
5.0
3
-
P.5
0.0
e.5
5.0
7.5
10.0
D L A ( m o ~ o i p r )RIO'
3.0
4
00 00
J
d ) Si-lOOO 0
i
50 IO 0 D E I (mol@/prlilO'
m.0
I5 0
0.0
16.0
7.6
28.6
SOB
D E I (mWrr/gr 1 I 10'
Figure 2. Stern-Volmer plots of the quenching of the ppene fluorescence ae a function of added (adsorbed) DEA on silicas with varying average pore size. Ppene surface coverages: Si-60, 8, = 2.6 X lo-! Si-200, Opy = 5 X Si-600, Opy = 1.7 X Si-1000, e, = i o x io-*.
tSi-""
25.0
00
50 IO 0 D E A C O V E R A G E (Oorn)
15'0
20 0
Figure 3. Average pore size effects on the Stern-Volmer slopes (same data ae in Figure 2) expressed in terms of DEA monolayer equivalents (ODm). (Note that the plot for Si-60 emphasizes the low-sloperegion (s = 14)rather than the initial high-slope region (Si > 110),which accounts for most of the quenching fraction.) of an equivalent of one octanol monolayer (curve d) substitutes the flat region, observed in the octanol-free system = 1 (curve c), with a relatively weak dependence below of I o / I on the amount of DEA. An increase in the slope is observed above ODEA = 1,which is the turning point in the octanol-free system. A continuous linear curve is obtained in the presence of the eqpivalent of two octanol monolayers (curve b). Curve a compares this behavior with the Stern-Volmer plot in a homogeneous octanol solution. Figure 6 shows the effect of varying the octanol amount at a constant eDEA value. Representative data concerning exciplex parameters in the above systems are given in Table I1 along with those of the octanol-free Si-60 system.
I
350
I
I
410
I
I
I
470
I
X (nm)
I
I
530
I
590
650
Figure 4. Emission spectrum of a pyrene (epy= 0.0025)/DEA (2.6 monolayer equivalents) system on Si-200.
Discussion Absence of 'Py* Quenching below = 1. It is difficult to account for the complete lack of quenching activity of DEA below ODEA = 1 in terms of accessibility on the surface. Thus, other 'Py* quenching processes such as excimer generation are efficient even a t very low OPr ~ a l u e s . ~ +(In ' J ~analogy to the effects of coadsorbed alcoh01s,'J2J5 DEA coadsorption is expected to increase the 'Py* mobility.) Moreover, when eDEA = 1is approached static quenching pathways requiring little msbility are
Langmuir, Vol. 5, No. 1, 1989 51
Geometry and Pore Size Effects on Charge Transfer
Table 11. Relative Intensity of Exciples Fluorescence (Zex, Characteristic Values) for Py/DEA on Porous Silicas“ BDM, equiv F L X bfeX , , A nm cm-’ x -0.01 495 f 5 (20.2) 90-96 -0.01 Si-60 >1 0.24 465 f 5 (21.5) 0.5 0.12.7 Si-200 1.1 0.13 465 f 5 (21.5) 0.08 1.5 0.62 0.06-0.05 0.07-0.05 485 f 5 (20.6) 2-3.5 0.80-0.90 465 f 5 (21.5) 0.18 0.72 Si-500 1.5 0.25 465 f 5 (21.5) 0.08 0.72 2.5 0.25 0.05-0.04 0.08-0.05 485 f 5 (20.6) 4.0 0.6-0.75 0.93 480 f 10 (20.8) Si-1000 6 30 28 480 f 10 (20.8) 50-70 20 0.04-0.03 10-15 0.83 0.021 0.03 490 f 10 (20.4) Si-60 1.29 0.94 0.03 0.03 490 f 10 (20.4) (1 octanol monolayer) 2.0 0.08 490 f 10 (20.4) Si-60 1.02 0.95 0.08 0.06 490 f 10 (20.4) (2 octanol monolayers) 1.6 0.98 0.06
(e,
“ I,, is the exciplex fluorescence intensity relative to the unquenched fluorescence of Py in the respective DEA-free system. Both values are recorded at the wavelength of maximum emission intensity and are scaled one to each other by using the IPy* (unquenched) and ‘(Py-DEA+)* (totally quenched) emission maximum in n-hexane. b!x is the relative exciplex fluorescence intensity, per each quenched ‘Py*, calculated as biX= Io,/F, where F is the fraction of lPy* quenched. , X is the wavelength of maximum exciplex fluorescence intensity. [ D E A I M x IO3 ( a )
40
30
50
100
150
200
I
1
I
1
-/
11
//i 250
300
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yo 2 0 I
IO
01 0
0
--.
0
I
50
J 75
OCTANOL MONOLAYER EQUIVALENTS 1
0
1
25
0.5 1.0 DEA C O V E R A G E (&E&)
1.5 (b,c,d)
2.0
Figure 5. Quenching of pyrene fluorescence by DEA in systems containing 1-octanol: (a) pyrene in an homogeneous 1-octanol solution; (b) pyrene (0, = 0.0025) on Si-60 in the presence of 2 monolayer equivalents of 1-octanol; (c) same as b but in the absence of 1-octanol (curve identical with Figure 2a); (d) same as b but with 1 1-octanol monolayer equivalent. The lower scale (6DM) represents the D M monolayer equivalents (for bd) while the upper scale gives the (same) DEA amount in total (molar) concentrations units (for a comparison with a).
expected. We thus conclude that due to specific interactions of DEA with the surface the intrinsic rate constant between lPy* and DEA is reduced to a value orders of magnitude below that of the (diffusion-controlled) rate constant in solution. The observed lack of DEA charge-transfer reactivity on silica surface a t eDm < 1 is consistent with the strong adsorption interactions of its n-electrons with the acidic hydrogens of the surface silanol gr0ups.l’ Spectroscopic studies with aromatic amines have shown that interactions with the silanols strongly reduce the resonant coupling of the n-electrons with the aromatic ring.I8 The strength of the amine-silanol hydrogen-bond interactions may be sufficient to cause complete protonation of the amine.”-19 (17) Bauer, G.; Strober, W. Kolloid 2. 1959, 167, 27. Bartell, F. E.; Dabay, J. J.Am. Chem. SOC. 1950,72,4388. Tanabe, K. In Solids, Acids and Bases; Academic: New York, 1970. Rochester, C. H.; Yang, G. H. J. Chem. Faraday Trans 1 1980,76,1158. Griffith, D. M.; Marshall, K.; Rochester, C. H. Zbid. 1974, 70, 400. (18) Kotov, E. I. Opt. Spektrosk. 1956,1,500; 1957, 79,5139. Robin, M.; Trueblood, K. N. J. Am. Chem. SOC.1957, 79, 5139. (19) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
Figure 6. Effect of added 1-octanol (monolayer equivalents) on the quenching of lPy* ( 0 , = 0.0025) by DEA (eDm = 0.25) on Si-60.
The ratio between fully protonated and hydrogen-bonded residues depends on the pK, value of both the amine and the silan01s.~~ The latter may vary over a relatively wide range (e.g., 6.5 < pK, < 9.2) depending on the nature of the silanol group (geminal, vicinal, etc.).20 However, in spite of this heterogeneity, the coincidence between the DEA monolayer values and the respective “critical” DEA quenching amount (Table I) clearly indicates that the DEA-silanol interactions are sufficiently strong so as to reduce the reactivity of essentially all DEAs in the first monolayer. We attribute this behavior to stabilization of the nitrogen lone pair of electrons, which in turn causes an increase in the DEA ionization potential to a level that completely inhibits its charge-transfer interaction with ‘Py*. It is only after the monolayer value is exceeded, when “free” DEA molecules become available, that the lPy*-DEA quenching process is initiated. We note that, due to its high sensitivity to the DEA concentration, this process might be used as a method for “counting” free silanols on silica surfaces. Pore Size E f f e c t s on the Quenching E f f i c i e n c y above = 1. There are several principal features of the quenching patterns above eDEA= 1which need special (20) For example, see: ref 18, p 660. Somorjai, G. D. In Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithica, 1981; pp 58-63. Morrison, S. R. In The Chemical Physics of Surfaces; Plenum: New York, 1977. Urger, K. In Porous Silica; Elsevier: Amsterdam, 1974; pp 130-133. Vystahii, 2. Z.; Strazhesko, D. N. In Adsorption and Adsorbents; Struzhensko, D. N., Ed.; Wiley-Interscience: New York, 1973; VOl. 1, p 55.
Birenbaurn et al.
52 Langmuir, Vol. 5, No. 1, 1989
consideration: First, the question arises in the cases of Si-200, Si-500, and Si-1000 as to why constant-slope lines are obtained in the Stern-Volmer plots even when the amount of DEA increases over several monolayer equivalents. Moreover, the Stern-Volmer slopes (s) of such plots decrease monotonicallywith the increase in the aps value. Our present explanation of the quenching maximum is that on porous silica only the first, strongly interacting monolayer is formed homogeneously over the whole available surface. Additional amounts of DEA are not equally and randomly distributed on the surface, but there is a gradual pore volume fillup, propagating from left (the micropore region) to right in the distribution curve. It is further assumed that Py is distributed all over the surface. As more DEA is added above the average OD, = 1 value, increasing DEA amounts reach (by equilibration) the larger pores in which the quenching process is taking place. We attribute the increase in the value of s with the decrease in aps to the fact that small-aps silicas are relatively richer in micropores that the large-aps silica^.'^ Thus, if in the former Py adsorbs in relatively smaller pores (in which a second DEA monolayer is more easily built up), relatively higher s values will be observed. The above arguments should be considered in view of the Py quenching behavior on Si-60. The latter shows basic features consistent with those of the higher aps silicas. Namely, the lack of quenching below O D m = 1and the higher Stern-Volmer slope above ODEA = 1, which is in keeping with the general trend observed with the higher aps silicas. However, Si-60 differs from the latter in that its Stern-Volmer plot is both very steep and biphasic: an initial sharp rise (s > 110) is followed by a secondary slope (s = 14). The basic behavior on Si-60 is in keeping with the fact that Si-60 is very rich in micropores of the kind which according to our postulated mechanism are readily filled up by DEA. Thus, if Py resides in such pores, a high concentration of DEA quencher will build up immediately after ODEA = 1 is exceeded, leading to a highly efficient quenching process. It is difficult, however, to account, at present, for the more quantitative aspects of the quenching behavior on Si-60, especially for the sharp transition between the two s values (rather than a gradual change in the Stern-Volmer slope, corresponding to a continuous distribution of Py over a series of adsorption sites). Also of interest is the sharp jump in s, over more than 2 orders of magnitude, while passing from Si-200/Si-500/Si-l000 to Si-60. This should be considered along with the observation (see Figure 5a,c) that the initial Stern-Volmer slope (si) on Si-60 is close to that observed in a homogeneous (1-octanol) solution. These observations may all be associated with the very high fractal dimension, D,, of the surface available for adsorption in Si-60, which in several independent measurements21i22was found to be about 3 for a molecular size range. The very high surface convolution which leads to the high D, value may thus be responsible for the high (solutionlike) quenching efficiency, in that 'Py* is effectively surrounded by DEA molecules in three dimensions. (See ref l l e for a similar argument rationalizing the efficient environmental relaxation of an excited aromatic amine adsorbed on Si-60.) Accordingly, the secondary (21) Farin, D.; Volpert, A.; Avnir, D. J. Am. Chem. SOC.1985, 107, 3368, 5319. Avnir, D.; Pfeifer, P. Nouv. J. Chim. 1983, 7, 71. (22) (a) Pines-Rojanski, D.; Huppert, D.; Avnir, D. Chem. Phys. Lett. 1982,139,109. Pines, D.; Huppert, D.; Avnir, D. J. Chem. Phys. 1988, 89, 1177. Rojanski, D. et al. Phys. Rev. Lett. 1983, 56, 2505. (b) Christensen, H.; Topsoe, H., private communication, Haldor Topsoe Co., Denmark. (c) Drake, J. M.; Levitz, P.; Sinha, S. Mater. Res. SOC.,Syrnp. Proc. 1986, 73, 305.
slope of the Stern-Volmer plot reflects the pore volume which is outside the D, N 3 fractal domain, for which pore radii considerations (see above) become relevant. Taking into account that most (65%) of the area of Si-60 is hidden in pores of diameter < 10 A,23 we note that two DEA molecules on opposing walls and a Py molecule in between can account for such a diameter size. It is in this domain, where pore volume and surface area (by tiling) become equivalent, that indeed most of the quenching occurs for the D, 3 materials. Indirect estimates of the D values of the other Si, obtained from Forster-type energy transfer between adsorbates,4,228 indicate significantly lower surface irregularity (D 2.4). Direct D, measurements of the high-aps materials are in progress. Quenching Mechanism: Polarity vs Geometry Considerations. After discussing the general features of the diffusion-controlled 'Py*-DEA process on the heterogeneous Si surfaces, we now turn to considering the mechanistic details of the above CT process on the solid interface. Generally, the quenching of the fluorescence of organic molecules, via donor (D)-acceptor (A) interactions, is based on the following scheme:s
-
-
'A*
+
D -.-
(~A*...D)
(A-D+)
5 l(A-D+)*
-
A
+
D
in which '(A*-D), '(A-D+)*, and (A-D') represent the locally excited encounter complex, the fluorescent exciplex, and the (geometrically correlated) ion pair, respectively. (The scheme omits intersystem crossing pathwa s to triplet states.) In nonpolar solutions k,, >> ki and k,,ty + k i r >> k:? (where ktx and k i r are the rate constants for fluorescent and nonfluorescent decay pathways and k:? measures the exciplex dissociation route). In polar solutions (e.g., alcohols) the situation is reversed, leading to a marked decrease in the relative exciplex fluorescence yield, The solvent polarity also affects the wavelength of maximum exciplex emission, Due to its dipolar nature (relative to the nonpolar ground state), the exciplex is stabilized in polar solvents, resulting in a red-shifted emission. In Figure 7 we have collected some relevant literature data on homogeneous solvent polarity effects on both $,: and A,,. On this we have superimposed the corresponding surface parameters as measured on Si surfaces. The latter are quantitatively presented in Table 11. In the following sections we compare the two sets of parameters with the purpose of gaining insights into the quenching mechanism on the porous silica surfaces. A comparison of the x:$ and A,, values on Si-60 (above DEA monolayer coverage) with those characteristic of homogeneous solutions clearly shows that no simple analogies between the two systems can be drawn. Thus, the low exciplex fluorescence yield, +is, on Si-60 is comparable to that in a highly polar solvent such as ethanol ( 6 23.9). However, the exciplex emission maximum on Si-60 (A,, = 495 f 5 nm) is considerably blue-shifted relative to ethanol (A,, = 537 nm), being indicative of a relatively nonpolar (e 5) environment. This implies that the low exciplex yield of Si-60 cannot be attributed to a high local surface polarity. This conclusion is in keeping with work applying an intramolecular charge-transfer probe as a surface polarity indicator for silica.lld It was
$tx.
(23) Shields, J. E.; Lowell, S. Powder Technol. 1983, 36, 1.
(24) Knibbe, H. Ph.D. Thesis, Free University of Amsterdam, The Netherlands, 1960.
Langmuir, Vol. 5, No. 1, 1989 53
Geometry a n d Pore Size Effects o n Charge Transfer
22
-
6 "
-? \
20I
s,z,
]
21-q
19-
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- 450 Si - 200/500/1000
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-500
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- 470
- 530
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- 560
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I
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I
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Figure 7. P DEA exciplex emission energies (a) and relative intensities, Pf (b), in homogeneous solutions as a function of the solvent dielectric constant (e). Notations Si-60, Si-200, etc. denote (b) observed on silica surfaces (this the ranges of vEu (a) and work) a t relatively low DEA monolayer equivalent values (see Table I for details). Values reported for a are from ref 24 with the following solvents: n-hexane (e 1.9), toluene (e 2.4), DEA (e 5.2), ethyl acetate (e 6.0), 1,2-dimethoxyethane (e 6.8), 1,2-dichloroethane (e 10.4), amyl alcohol (t 14.8), 2-propanol (e 18.56), ethanol (e 23.9), methanol (e 32.02). Values for b are from ref 12b,c and references therein with solvents n-hexane, toluene, diethyl ether (e 4.36), dimethoxyethane, dichloromethane (e 8.96), dichloroethane, amyl alcohol, 2-propanol, ethanol, methanol, acetonitrile (e 37.00).
shown that the relatively high surface polarity, due to the free silanols, is markedly reduced in the presence of coadsorption with residues such as alcohols, which strongly interact with the silanols exposing a nonpolar aliphatic interface. In the case of DEA an analogous blanket of apolar phenyl groups will be formed due to the strong interaction of the diethylamino group with the surface hydroxyls. Seeking an explanation for the low &, and ,X, combination on the Si-60 surface (other than the dependency of k,/kj on polarity), we point to the accumulated evidence obtained in homogeneous solutions, showing that k,, is much more sensitive than ki to the relative orientation of 'A* and D in the ('A*-D) encounter pair? Thus, due to the geometrical requirements associated with the sandwichlike exciplex conformation, the value of k,, may be affected by a variety of steric factors which inhibit the formation of the exciplex.12 We suggest (see discussion below) that on the relatively nonpolar Si-GO/DEA (ODE* > 1) surface, the mutual orientation of lA* and D is unfavorable for exciplex generation, leading to deactivation via the ground-state ion pair (A-D+). The quenching consequences on Si-200, Si-500, and S i - l o o 0 markedly differ from those on Si-60, primarily in that relatively high 4:x values are observed for these high-aps silicas (Table I1 and Figure 7). Moreover, in the extreme case of Si-lo00 both the exciplex fluorescence yield and its emission frequency are comparable to the corresponding parameters observed in low-polarity homogeneous solutions, such as toluene (e 2.4). We conclude that on Si-lo00 the reaction between 'Py* and DEA takes place in a low-polarity environment and is free of geometrical restrictions for generating the exciplex. Specifically, for
the two extreme cases Si-lo00 and Si-60, the surface of the former is much smoother',4,22agenerating a first DEA monolayer in the form of an ordered apolar phenyl blanket. This allows sufficient freedom for optimal DEA-'Py* orientation, hence the agreement between a and b of Figure 7; ,@: and ,X, are both typical of an apolar, homogeneous environment. On the other hand, the above-mentioned extreme surface irregularity of Si-60 and the fact that most of its surface area lies in micropores not only distort the ordered phenyl blanket, increasing the effective polarity as indicated by the higher X,value, but also interfere with achieving the proper D-A orientation for exciplex formation. The transition between Si-60 and Si-lo00 appears to be gradual: as shown in Figure 7 and Table 11,Si-500 behaves similarly to Si-1000 but with a lower $tx value. Further decreases in 4iXare observed on Si-200, indicating a nonnegligible role of geometrical restrictions as discussed in the case of Si-60. It should be noted that the above arguments are strictly applicable to DEA coverage values, which are not far from a total (average) monolayer value. Thus, as shown in Table 11, A- increases and q$, decreases at higher DEA coverage. This effect is not fully understood but appears to be associated with an increased local polarity upon increasing the amount of polar DEA molecules on the first phenyl blanket monolayer. Our basic argument is therefore that the surface geometry of the silicas play an important role in dictating the ease at which the optimal exciplex alignment becomes possible (the smaller the aps, the more difficult it becomes to acquire the desired alignment) and that a correlation exists between the surface irregularity and the course of the CT process, namely, the efficiency of exciplex generation and the A,, values. Effects of Coadsorbed 1-Octanol on Si-60. As shown in Figure 5d, the presence of the equivalent of one l-octanol monolayer markedly affects the &/I vs 6 D m plot on Si-60. While the secondary (lower value) slope characteristic of the octanol-free system (Figure 5c) appears to show little (if any) change, the sharp rise in Zo/Z at 6DEA = 1is now absent, being replaced by a gradual rise, starting from 8Dm = 0. Such observations may be rationalized by assuming that 1-octanol competes with DEA on adsorption sites, inducing a DEA desorption, so that a small amount of "free" (effectively quenching) DEA molecules is present, even before the equivalent of 6DEA = 1 is reached. This explains the (low) initial slope of the curve starting from 6 D m = 0. The complete absence of the sharp rise at 6 D m > 1 (which accounts for -90% of the quenching in the octanol-free system) should be considered in terms of the explanation given above for this effect. Accordingly, it must be assumed that 1-octanol displaces free DEA molecules from the vicinity of 'Py* in those narrow pores in which the quenching process is carried out. In the presence of higher octanol monolayer equivalents, more DEA molecules should become unbound, leading to a higher initial slope. This is in fact demonstrated by Figure 5b, which reflects the situation in the presence of two octanol monolayers. Apparently, the DEA desorption effect is more pronounced than the DEA displacement effect. Indeed, Figure 6 shows that the octanol effect is even more complex. Thus, one would expect the interval between curve b (2 equiv of monolayer) and curve a (octanol solution) to be filled with linear curves of progressively higher slopes, as octanol is gradually added. However, as seen in Figure 6, the quenching efficiency as a function of added octanol passes through a maximum at about 3 equiv of an octanol monolayer and then drops back
54
Langmuir 1989,5, 54-58
to quenching inertness at higher octanol amounts. This behavior is most likely associated with the above (opposing) phenomena, i.e., the DEA displacement effect becoming progressively predominant as the amount of octanol increases. An analogous formulation of the same mechanism may invoke a preferential extraction of DEA (or Py) from the surface vicinity upon pore fill up with octanol. As to the exciplex parameters A, and & (see Table 11), it is indicated that the presence of l-octanol partially releases the restrictions imposed on the generation of the exciplex on Si-60. Again, the plausible explanation is that Py is partially displaced by the alcohol into larger pores, leading to values closer to those observed in, e.g., Si-200.
Conclusions The capability of an aromatic amine such as DEA to serve as an effective electron donor in charge-transfer processes on silica surface drops virtually to zero by the strong adsorption interactions with the acidic silanol groups. An effective CT process is observed only when an excess of free (nonadsorbed) DEA molecules becomes available above monolayer coverage. The process is then markedly affected by the average pore size of the silica, most likely due to the nonhomogeneous distribution of the added DEA in the porous adsorbent. However, the details of the quenching mechanism are still unclear. Thus,
preliminary fluorescence lifetime measurements indicate that the population of quenched ‘Py* is heterogeneous. In the case of Si-100 most of the excited pyrene molecules react with DEA on a 50-200-ns time scale. However, with the other silicas considerable fractions are quenched over much shorter time scales which are below the resolution of our measurements (-10 ns). Further work, with subnanosecond time resolution, should be carried out for a quantitative classification of the pyrene populations participating in the DEA quenching process. Finally, it became evident that the course of the reaction of the surface is not governed simply by polarity effects as in the case of the same process in homogeneous solutions. Considerable steric restrictions on exciplex generation are clearly evident in the case of Si-60 which are completely absent in the case of Si-1O00. The effects are most likely associated with the details of the surface geometry, namely its porosity and irregularity. Further studies are in progress for elucidating the exact role of the surface fractal dimension and of the pore size distribution in controlling the course of the CT reactions.
Acknowledgment. Supported by the US-Israel Binational Fund and by the E. Berman Solar Energy Research Foundation. Registry No. Py, 129-00-0;DEA, 91-66-7; SiOz, 111-87-5; 1-octanol,7631-86-9.
Application of the Kirkwood-Buff Theory of Solutions to the Surface Phase between the Water-Ethanol Binary Liquid Mixture and Its Vapor E. Tronel-Peyroz,* J. M. Douillard, R. Bennes, and M. Privat Laboratoire de Physico-Chimie des Systcmes Polyphas6s, CNRS U A 330, Route de Mende, BP 5051, 34033 Montpellier, France Received January 29, 1988. In Final Form: August 2, 1988 From experimentaldata concerning adsorption at the liquid-vapor interface and a rough model for the transition layer, the Kirkwood-Buff integrals, Giju,have been calculated for the surface-phase mixture by using Ben-Naim’s procedure. The superficial local mole fractions were then calculated. Results obtained for the water-ethanol surface mixture are compared with those for the water-ethanol bulk mixture. It is shown that usual theoretical models offer no possibility of predicting the surface properties.
I. Introduction The classical method used for interpreting adsorption mechanisms combines both thermodynamic and nonthermodynamic pr0cedures.l Thus, starting from Gibbs’ equation and a determination of the adsorption isotherm, information concerning the orientation of the adsorbed molecules can be obtained by employing phenomenological models. One can go further with regards to the microscopic interpretation of these results by using the various molecular adsorption models, of which the adjustable parameters can be calculated as a function of the molecular properties of the adsorbed layers. Unfortunately, none of these models furnishes at the present time a satisfactory description of the structure of the superficial layer, and (1) Rangarajan,S . K.In Specialist Periodical Reports, Electrochemistry; Thorsk, H. R., Ed.;The Chemical Society: London, 1980; Vol. 7.
this is partly (as Nikitas has pointed out in his critical review) because “the various models have the same basic features. They idealize the adsorbed region to a monolayer for which an a priori lattice structure is assumed”.2 This relative failure is also due to the general difficulties encountered in explaining molecular interactions, the starting point of any microscopic model. In the exact theory of solutions developed by Kirkwood and BufP the essential parameter is the radial distribution function (rdf) gij(r),which is a measure of the importance of the correlation between the position and the orientation of i j pairs. This can be determined by classical scattering methods in a pure liquid but is difficult to determine in mixtures. (2) Nikitas, P. Electrochem. Acta 1985, 30, 1513. (3) Kirkwood, J. G.; Buff, F. P.J. Chem. Phys. 1951, 19, 774.
0743-7463/89/2405-0054$01.50/0 0 1989 American Chemical Society
