J. Phys. Chem. 1985,89, 3521-3526 of the vibrational predissociation is comparable with that of the fast decay component, that is, in the order of lo9 s-I. On the other hand, Knight et al. have obtained the rate constants of the collisional quenching of the pyrimidine fluorescence with rare gas partner, which is in the range of lo7 torr-' s - ' . ~ Therefore, one can make a condition in which the collisional quenching competes with the fast fluorescence decay if rare gas is added up to 100 torr. In such conditions, Knight et al. observed the collision-induced vibrational relaxation from the 12' level of pyrimidine. However, at the lower pressures, they could not detect the vibrational relaxation because the collision-induced electronic relaxation is dominant, resulting in the fluorescence quenching. Therefore, only when the rate of the collision is comparable with that of the fast decay can the vibrational relaxation be induced. From these considerations, it becomes clear that only competitive
-
3521
processes with the fast decay can result in vibrationa1 relaxation within the singlet state. The agreement between the rate,constant of the vibrational predissociation of the pyrimidine-Ar complex and that of the collision at the onset of the collision-induced vibrational relaxation is not accidental. This implies that the intrinsic intersystem crossing of pyrimidine which causes the fast fluorescence decay is not perturbed by collisionsg and even by complexation. Acknowledgment. We thank Professor R. Shimada of Kyushu University for providing us with pyrimidine-d4. We also thank Drs.T. Ebata and N. Gonohe for their stimulating discussions. This work was partially supported by Toray Science and Technology Grants. Registry No. Ar, 7440-37-1; N2, 7727-37-9; pyrimidine, 289-95-2.
Fluorescent Probes for Silica and Reversed-Phase Silica Surfaces: 1,3-Di-I-pyrenylpropane and Pyrene David Avnir,*t Reinhard Busse,* Michael Ottolenghi,*x Edna Wellner,tt and Klaas A. Zachariasse*s Departments of Organic and Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel, and Max- Planck- Institut ftir biophysikalische Chemie, Abt. Spektroskopie, 0-3400 Gdttingen, Federal Republic of Germany (Received: January 2, 1985)
The fluorescence of the bifunctional probe molecule 1,3-di- 1-pyrenylpropane(Py(3)Py), adsorbed on silica (in the presence and absence of coadsorbed l-octanol) and on octadecylsilica, is investigated by utilizing photostationaryand single-photonaunting methods. It is concluded that dynamic (intramolecular) excimer formation, taking place after excitation, o&urs at low probe concentrations in the reversed octadecyl phase. A study of temperature effects yields a value of -40 kJ/mol for the effective activation energy of this process, indicating a relatively high viscosity for the reversed phase. Dynamic intramolecular excimers are also observed on silica surfaces in the presence of 1-octanol as coadsorbate. No excited-state rearrangements are observed for Py(3)Py on untreated (or partially C18silylated) silica, where the only path to excimers originates in ground-state intermolecular aggregates. Comparative experiments are also carried out with pyrene (Py) in the same systems. Py(3)Py appears to be a most convenient indicator for the degree of mobility freedom of adsorbed molecules.
Introduction The photochemistry and photophysics of adsorbed molecules have recently gained considerable attention.'-" This relatively novel field appears to be rapidly developing as a new tool, leading to a deeper insight into the nature of selected photoprocesses, as well as to better understanding of solid interfaces. The extensive use of pyrene (Py) in such studies is due to its unusually long fluorescence lifetime, to its ability to form excimers, and to the sensitivity of its structured fluorescence spectrum to the polarity of the environment? The wide variety of solid surfaces which have been studied with Py as a probe includes ~ i l i c areversed-phase ,~ silica! a l ~ m i n a ,titania ~ and other semiconductors,* calcium f l ~ o r i d eclays,l0 ,~ and zeoIites." In the present study we have applied the bifunctional pyrene derivative, 1,3-di-1-pyrenylpropane (Py(3)Py), as a probe for silica surfaces. The major advantage of this molecule is associated with its ability to form excimers intramolecularly. Consequently, exceedingly low concentrations may be used, avoiding experimental difficulties due to ground-state aggregation' which highly complicate the excimer formation mechanism in the case of Py. Especially relevant, in this respect, were previous studies in which the intramolecular excimer formation process in Py(3)Py was successfully applied to determine the local viscosity of heterogeneous systems such as micelles, biomembranes, and microemulsions.'* In his pioneering surface photochemistry studies, Leermakers detected the intramolecular excimer emission of 1,3-diphenylDepartment of Organic
propane in silica/cyclohexane However, in the absence of a clear distinction between solvated and adsorbed probe (1) R. K. Bauer, R. Bornstein, P.De Mayo, K. Okada, M. Rafalska, W. R. Ware, and K. C. Wu, J . Am. Chem. SOC.104,4635 (1982). (2) Z. Grauer, H. Daniel, and D. Avnir, J. Colloid. Interface Sci., 96,411 (1983); Z. Grauer, D. Avnir, and S.Yariv, Can. J . Chem., 62, 1889 (1984); R. Reisfeld, N. Manor, and D. Avnir, Sol. Energy Mater., 8, 399 (1983); D. Avnir, D. Levy, and R. Reisfeld, J . Phys. Chem., 88, 5956 (1984). (3) K. Hara, P. De Mayo, W. R. Ware, A. C. Weedon, G. S. K. Wong, and K. C. Wu, Chem. Phys. Lett., 64, 105 (1980). (4) K. Kalyanasundaram and J. K. Thomas, J. Am. Chem. SOC.,99,2039 (1977). (5) (a) P.De Mayo, L. V. Natarajan, and W. R. Ware, Chem. P h p . Lett., 107, 187 (1984); (b) P. Levitz, H. Van Damme, and P. Keracis, J . Phys. Chem., 88,2228 (1984); (c) L. W. Weis, T. R. Evans, and P.A. Leermakers, J . Am. Chem. SOC.,90,6109 (1968). (6) (a) C. Francis, J. Lin, and L. A. Singer, Chem. Phys. Lett., 98, 553 (1983); (b) R. G. Bogar, J. C. Thomas, and J. B. Callis, Anal. Chem., 56, 1080 (1984); (c) C. H. Lochmiiller, A. S.Colborn, M. L. Hunnicutt, and J. M. Harris, Anal. Chem., 55, 1344 (1983); (d) C. H. Lochmiiller, A. S.Colbom, M. L. Hunnicutt, and J. M. Harris, J . Am. Chem. SOC.,106,4077 (1984). (7) D. Oelkrug and M. Radjaipour, Z . Phys. Chem. (Wiesbuden), 123, 163 (1980). (8) K. Chandrasekaran and J. K. Thomas, J. Colloid Interface Sci., 100, 116 (1984). (9) R. K. Bauer, P. De Mayo, W. R. Ware, and K. C. Wu, J. Phys. Chrm., 86, 3781 (1982). (10) R. A. Delaguardia and J. K. Thomas, J. Phys. Chem., 88,964 (1984). (11) B. H. Baretz and N. J. Turro, J . Pfiotochem.,24, 201 (1984). (12). (a) K. A. Zachariasse, Chem. Phys. Letr., 57,429 (1978); (b) K. A. Zachariasse, W. Kiihnle, and W. Welkr, Chem. Phys. Lett., 73,6 (1980); (c) K. A. Zachariasse, W. L. C. Vaz, C. Sotomayor, and W. Kiihnle, Biochim. Biophys. Acta, 688, 323 (1982); (d) L. M. Almeida, W. L. C. Vaz, K. A. Zachariasse, and V. M. C. Madeira, Biochemistry, 21, 5972 (1982); (e) K. Kano, T.Yamaguchi, and T. Ogawa, J. Phys. Chem., 88,793 (1984); (f)K. A. Zachariasse, B. Kozankiewicz, and W. Kiihnle in "Photochemistry and Photobiology", Vol. 11, A. H. Zewail, Ed., Harwood, London, 1983, p 941.
0022-3654/85/2089-3521$01.50/00 1985 American Chemical Society
Avnir et al.
3522 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
-
A
B
C
- Xem 3 9 6 n m
u1 c .-
-____ Xem 475 nm
C 3
X nm
360 400
440
480
nm Figure 1. Effect of concentration on the fluorescence spectrum of Py(3)Py adsorbed on silica gel: (A) 3.6 X lod, (B) 2.7 X lod, (C) 1.8 X lod, (D) 0.9 X lo", and (E) 0 . 4 X lod mol of Py(3)Py/g of silica. X
Figure 2. Excitation spectra of (A) 1.8 X lod mol/g of Py(3)Py adsorbed on silica gel, (B) 5.4 X lov7mol/g of Py(3)Py adsorbed on silica with 1.2 X lo-) mol/g of 1-octanol, and (C) 1.6 X mol/g of Py adsorbed on Si-Cls: (-) &, = 399 nm; ( - - - ) A,, = 470 nm.
C----*
molecules, the interpretation of his data remains a m b i g u ~ u s . ' ~ A similar study was carried out by Nakashima and Phillips in the case of 9,9'-bianthryl adsorbed on porous Vycor glass.14 Preliminary experiments with adsorbed Py(3)Py were reported by Bauer et al.Is
PY(3IPY/Sl
0 -4
Experimental Section
log C (mole /gr )
Materials. 1,3-Di-1-pyrenylpropane (Py(3)Py) was synthesized as described previously.I6 Pyrene (Py) (Riedel De Haen) was purified chromatographically. All solvents were spectroscopic grade. 1-Octanol was distilled before use. Octadecyldimethylchlorosilane was purchased from Petrarch Systems Inc. Silica gel was a Woelm product (Lot No. 1192): particle size, 32-63 pm; surface area, 500-600 m2 g by the nitrogen BET method; pore volume, 0.87-0.89 cm3/g;l surface fractal dimension?l 2.97 f 0.04.'s,19 Reversed-phase silica was octadecyldimethylsilica (Si-Cls, LiChroprep RP-18) (Merck, Lot No. 0090509): particle size, 40-63 pm; surface area, 150 m2/g; average pore diameter, 60 A. Adsorption. The adsorbent was added to a mixture of cyclohexane and the desired amount of adsorbate. The solvent was removed at 40 OC by slow evaporation, initially under 20 mmHg and subsequently under high vamum. Surface equilibration was achieved by prolonged tumbling of the sample. Sample deaeration was obtained either by evacuation or by flushing with dry nitrogen. Monolayer Coverage. The monolayer,value of Py on unmodified silica (Si) was determined from the adsorption isotherm in a Si/cyclohexane slurry (see Hoffmann et a1.22) as nl = 0.2 mmol/g. The monolayer value of Py(3)Py on silica (n2) was calculated from the ratio of the cross-sectional areas of Py and
c -
(13) D. Avnir, P. De Mayo, and I. Ono, J . Chem. SOC.,Chem. Commun., 1109 (1978). (14) N. Nakashima and D. Phillips, Chem. Phys. Lerr., 97, 337 (1983). (15) R. C. Bauer, P. De Mayo, K. Okada, W. R. Ware, and K. C. Wu, J . Phys. Chem., 87,460 (1983). (16) K. A. Zachariasse and W. Ktihnle, Z . Phys. Chem. (Wiesbaden),101, 267 (1976). (17) Information provided by Woelm Pharma. (18) P. Pfeifer, D. Avnir, and D. Farin, J . Srar. Phys., 36, 671 (1984).
This value, which was determined from adsorption data, was confirmed by small-angle X-ray scattering~. experiments (P. W. Schmidt and H. D. Bale, private c&"nication). (19) D. Farin, A. Volpert, and D. Avnir, J . Am. Chem. SOC.,107, 3368 (19851 ,---,. (20) (a) J. E. Amoore, Ann. N.Y. Acad. Sci., 116,457 (1964); (b) A. L. McClellan and H. F. Harnsberger, J. Colloid Interface Sci., 23, 577 (1967). (21) D. Avnir, D. Farin, and P. Pfeifer, Nature (London), jo8,261 (1984). (22) R. L. Hoffmann, D. G. McConnell, G. R. List, and C. D. Evans, Science, 157, 550 (1967).
Py/Si
Figure 3. Dependence of the relative excimer fluorescence intensity {Z'/(I Z')] on the probe concentration (C) for a variety of adsorbed systems: (01, Py/Si; (A), P y ( W y / S i ; (e), Py/Si-C,,; (A), P Y ( ~ ) P Y / S ~ - C I B .
+
Py(3)Py (uI and u2, respectively, determined from silhouette areasz0):
where D is the fractal dimension of the surface.2' Substituting u 1 / u 2= 0.52 and D = 3.0, we obtain n2 = 0.07 mmol/g. The 1-octanol monolayer value on silica was found to be 1.8 mmol/g by adsorption on Si/toluene s l ~ r r i e s . ' ~ , ~ ~ Surface Silylation. For comparative purposes, Woelm's silica (Si-60) was partially (-65%) silylated by using a previously described procedure.23 Spectroscopy. Fluorescence emission spectra were measured with Perkin-Elmer LS-5 and SLM 4800 fluorometers, in 2-mm quartz cells (Hellma) in a front-face excitation geometry. Fluorescence kinetic parameters were determined at 25 O C by utilizing the method of time-correlated single-photon counting. The experimental setup, employing an Edinburgh Instruments 199F nanosecond flashlamp (filled with nitrogen), and the data analysis procedure have been described before.24 The excitation wavelength was 337 nm.
Results and Discussion Unmodified Porous Silica Surfaces. Emission spectra of Py(3)Py adsorbed on unmodified silica (Py(3)Py/Si) are shown in Figure 1. A pronounced concentration effect on the relative intensities of the monomer ( I , at 396 nm) and excimer (1',at 470 nm) bands is evident: The excimer emission is clearly favored at relatively high probe concentrations. The general patterns are ~ the analogous to those observed for Py in the same ~ y s t e m .In (23) (a) H. Hemetsberger, P.Behrensmeyer,J. Henning, and H. Ricken, Chromarographia, 12, 71 (1974); (b) P. Larsen and 0. Schou, Chromatogruphia, 16, 204 (1982). (24) K. A. Zachariasse, G. Duveneck, and R. Busse, J . Am. Chem. Soc., 106, 1045 (1984). (25) K. A. Zachariasse, R.Busse, G. Duveneck, and W. Kiihnle, J. Phofochem., 28, 235 (1985).
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3523
Fluorescent Probes for Silica Surfaces
TABLE I: Decay Time Parameters (1/A in ns) and Their Amplitudes A , at 25 ‘C, for the Monomer (373 nm) and Excimer (520 nm) Fluorescence of Py(3)Py and Py on Silica Surfaces
system Py(3)Py/ 1-octanol/silica
mol/g of Si 5.4 x 10-7
A, nm
PY(WY/S~-CIS Py/Si-C I
3 x 10“ 6.5 X 10“
520
373 520 343 520
case of Py, the broad, red-shifted band (maximum near 480 nm) was attributed to ground-state aggregation, leading to excimer formation in a static process not involving diffusion. That a similar conclusion is also valid in the case of Py(3)Py is indicated by the excitation spectra shown in Figure 2A. The monomer excitation peak at 348 nm is clearly blue-shifted relative to that of the excimer at 355 nm. In spite of such basic analogies, a quantitative analysis of fluorescence intensity ratios is indicative of differences between the Py and Py(3)Py systems. Thus, as shown in Figure 3, the Py concentration required to obtain a given Z’/(Z + Z’) ratio is about 10 times higher than in the case of Py(3)Py. In other words, Py(3)Py more easily forms intermolecular aggregates. The effect substantially exceeds the simple stoichiometric ratio between the two probes, which, all other factors being equal, would predict a factor of 2 between the respective concentrations. A plausible explanation for this large concentration effect can be sought in the bifunctional nature of Py(3)Py. The presence of two pyrene moieties in a single molecule may allow the formation of “domino-like” chains in which pyrene moieties of each molecule are associated with those of their close neighbors. An alternative explanation may be based on the nonhomogeneous nature of the adsorbing sites; Le., at high probe concentrations adsorption will also take place in regions where the crevices of the surface induce Py(3)Py to deviate from an essentially stretched ground-state conformation into a bent structure, favoring excimer formation. Complementary to the steady-state fluorescence measurements, single-photon-counting experiments were carried out in the same Py(3)Py/Si systems. No growing-in of the excimer emission at 520 nm was observed within the time resolution (-0.5 ns) of the experiments. This observation is in keeping with our previous conclusion, attributing the excimer emission to ground-state aggregates. The Effect of Coadsorbed I-Octanol. Of primary importance in surface science is the profile of gradual changes in a multilayered adsorbate as function of the distance from the solid surface.26 Of special interest are parameters such as microviscosity, degree of order, polarity, and hydrophilicity. Such properties may, in principle, be approached by using appropriate probe molecules. Following the preliminary work of Bauer et al.,I5 we have utilized Py(3)Py to obtain an insight into the effective microviscosity of 1-octanol layers adsorbed on a porous silica surface. Figure 4B shows the effect of the amount of coadsorbed 1octanol on the excimer to monomer fluorescence intensity ratio of Py(3)Py. It is evident that a sharp increase in the relative intensity of the excimer band takes place around 2 X mol of octanol/g of silica. The effect was found to be independent of the probe concentration in the 10”-10-7 M range. The corresponding monomer and excimer excitation spectra shown in Figure 2B are not indicative of ground-state association, thus suggesting a predominantly dynamic excimer formation process. A dynamic excimer was previously postulated by Bauer et al.I5 for Py adsorbed on decanol/silica surfaces. The effect of I-octanol is best rationalized by comparing the dependence of the relative intensity parameter Z’/(Z + Z’) on the alcohol concentration (Figure 4B) with the 1-octanol adsorption curve on silica (Figure 4A). The adsorption curve indicates that a monolayer coverage of the surface is obtained at 1.8 X 10-3 mol of octanol/g of silica. The slight increase in the Z’/(Z + Z’) ratio observed at this concentration indicates that only a limited
-
(26) P. G . de Gennes and P. Pincus, J . Phys., Leu.,44,L241 (1983).
11x1 20
11x2
43 51
11x3 146 106
37 36 45
129 230 186
254 392 389
27
A,I
An
A,,
0.60 -2.04
0.50
0.04 1.22
-1.7
3.1 1.01
0.05 1.44 1.14
0.15 -3.61
0.99 2.92
A
B z
4.50
-
225-
I
1
mmles adsorbed
-
4.05
-
2.83
-
1.61
-
I
-P -A X
c (
\
c (
0.39 0.00
I IO
2 20 mmoles added
3 30
4 40
Figure 4. (A) Adsorption curve of 1-octanol on silica gel. (B) Dependence of the relative excimer fluorescence intensity {I’/(I + I?] on the octanol concentration. The arrow denotes the value in octanol solution.
intramolecular probe mobility leading to excimer formation takes place in the monolayer. It is not until the octanol concentration has increased to twice the monolayer coverage that Z’/(Z’ + r) reaches its maximum value, which is essentially identical with that characterizing Py(3)Py in homogeneous 1-octanol solutions. The dynamic nature of the excimer generation process for Py(3)Py on octanol/silica surfaces was confirmed by fluorescence decay time measurements. Figure 5 shows the results in the case of a bilayer coverage. The excimer fluorescence recorded at 520 nm shows a clear growing-in phase, initiating practically from zero, reaching its maximum at about 50 ns. The figure indicates that three exponentials are required to fit the fluorescence response functions i M ( t )(monomer) and iD(t)(excimer):
+ + i D ( t ) = A21e-Alr + AZ2e+?+ A 23
i M ( t )= Alle-XLf A12e-X2r AI3e+3I
(2) (3)
As in homogeneous solutions,24 fits with two exponentials lead to considerably larger errors. The decay parameters, 1/A, and the corresponding amplitudes, A, are listed in Table I. The sum of the excimer amplitudes CIA2,is close to zero, a condition required when excimer formation is completely dynamic. This is the case in the absence of an unresolved growing-in component due to the contribution of excimers promptly formed from ground-state aggregates. In homogeneous Py(3)Py solutions the three-exponential fit was interpreted in terms of a mechanism involving two structurally distinct e x c i m e r ~ We . ~ ~note, ~ ~ ~however, that the fit to two decay phases with positive amplitudes (A,*and Ai3)is also observed for
3524
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
$1
Avnir et al.
$1
373nm
NSEC 20 43 AMPL(I0-’1 0.60 0.50
520nm
NSEC 27 51 106 AMPL(IO-’) -2.04 0.99 1.22
I46 0.04
-%l i
L -
Y x
w= =
d
= 2 mnR
-
0
m
WTD. D E V I R T I O N
5
‘0’
’
152
’
2h
’
3ks ’ rbs ’ 6’10 ’ HTD.DEVIRTION
7i2
’ sku ’ s’x
Y
c:
0-
are also indicated.
Py on a variety of Si surfaces (see Table I and ref 9). Moreover, the agreement between the corresponding monomer and excimer 1/X parameters (Table I) is inferior to that observed for Py(3)Py in homogeneous s o l u t i o r ~ s .It~is~ ~thus ~ ~ most likely that the triple exponential decay of adsorbed Py(3)Py is associated not only with the two s ~ g g e s t e excimers d ~ ~ ~ ~ but ~ also to other factors such as multiple adsorption sites, scattered light effect, etc. However, it is clearly evident that the lifetime data, in the case of Si/octanol are fully consistent with the photostationary experiments implying a dynamic (intramolecular) excimer generation mechanism. Reversed-Phase Octadecylsilica Surfaces. Emission and excitation spectra of Py(3)Py adsorbed on octadecylsilylated silica are shown in Figure 6. It is evident that the excitation spectra are markedly dependent on the probe concentration. At relatively high probe coverages (e.g., 2.6 X 10” mol of Py(3)Pylg of S i x l 8 ) , the excitation spectrum recorded for the excimer band is redshifted relative to that corresponding to the monomer (Figure 6A), whereas at lower concentrations (e.g., 2.6 X mol/g) the two spectra coincide (Figure 6B). These observations suggest that at relatively high concentrations the excimer is primarily due to a “static” process, originating from aggregates analogous to those invoked for untreated silica surfaces. A dynamic mechanism appears to prevail in the low concentration range. That such a dynamic path is intramolecular in nature is indicated by the presence of a residual concentration-independent excimer emission observed below lod mol/g of silica (see Figure 3). As shown in the figure, this behavior, characteristic of Py(3)Py/Si-C18, is not observed for either Py(3)Py or Py on unmodified silica, in keeping with the suggestion that the only path to excimer formation is that associated with ground-state aggregates. We note a t this point that a somewhat complex behavior characterizes Py on S i + + In this dase the shape of the concentration dependence of P / ( Z + Z’) shown in Figure 3 is similar to that characterizing the previously discussed Py(3)Py/Si-CI8 system. However, the corresponding excitation spectra (Figure 2C) are indicative of a superposition of both “static” and dynamic excimer formation mechanisms, essentially over the whole concentration range. This conclusion is based on the observation that even at relatively high Py concentrations the shift between monomer and excimer excitation maxima is considerably smaller than that observed in the other systems. The corresponding magnitudes of these shifts are 3 nm for Py/Si-C18, 9 nm for p ~ / S i 9, ~nm for Py(3)Py/Si (Figure 2), and up to 15 nm for Py(3)Py/Si-CIS.
/
I
,
I
I
,
I
360 400 440 400 520 300 340 300
- Xem - 396
----
Xem-475
-
360 400 440
480 520
300 340 300
X nm
Figure 6. Emission and excitation spectra of Py(3)Py adsorbed on Si-
c,*.Probe concentration:
(A)2.6 X lo4 mol/g; (B) 2.6 X lo-’ mol/g.
It is worth noting (see Figure 3) that the curves showing the concentration dependence of Z’/(Z + Z’) for both Py and Py(3)Py on Si-CI8 are shifted to higher concentrations relative to those of the same molecules on untreated silica. In agreement with the observations of Francis et a1.,6athis implies that the tendency of both probes to aggregate is weaker than on untreated silica.
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3525
Fluorescent Probes for Silica Surfaces
a
A
520nm
3
.-
.I
530nm
B
H
ir ,^?.
120 NS
‘3 I i n
I\
0 ’
’
I52
NSEC
’ Zh ’
3b
\ ’ ’ UbS
’
SI0
CHRNNEL
37 AMPL(IO-*)- 17
129 31
’
752
’ ah#
S h
254 0.45
NSEC
45 AMPL(IO-’l -3.6
186
389
2.9
1.1
3
‘yl-
O
NTD.DEVIRTION
i
6
x2 : 3.49
Figure 7. Rise and decay curve of the excimer fluorescence of (A) Py(3)Py adsorbed on Si-C18(3 X lod mol/g) and (B) Py adsorbed on Si-Cls (6.5 X
mol/g), fitted to three exponentials. See caption to Figure 5 .
The observation of dynamic excimer generation for both Py(3)Py and Py on Si-Ct8 should be considered in light of the recent data of Bogar et a1.6b Working with Py as a probe, these authors observed a dynamic excimer generation process on Si-C18surfaces, only upon wetting the bonded alkyl phase with a water (25%)/ ethanol (75%) mixture. We suggest that this apparent discrepancy is due to a relatively lower degree of surface modification in the system of Bogar et al. Thus, we have observed that no excimer formation is detectable with Py or Py(3)Py adsorbed on the partially silylated ( ~ 6 5 %silica ~ ~ surface. ) However, a substantial contribution of a dynamic excimer is observed by adding the water/methanol phase; e.g., in the case of 1.6 X mol/g of Py, the Z’/(+Z’) ratio (where Z’refers to the dynamic excimer) increases from -0 in dry partially silylated Si-C18to 0.64 in the presence of the liquid phase. It is generally agreed that only in densely packed systemsz7 or in the presence of a mobile liquid phase28 do the n-alkane chains assume parallel stacking.27 Our observations may be rationalized by invoking a relatively low microviscosity in the stacked systems, as compared to the disordered alkyl chain structure characterizing the surface in the case of dry, partially silylated surfaces. The contribution of dynamic excimer generation mechanism in the cases of Py and Py(3)Py on Si-C18 was confirmed by fluorescence lifetime measurements. This is indicated by the (partial) growing-in of the excimer fluorescence shown in Figure 7, as well as by the substantial contribution of the negative-amplitude component A i l . A detailed analysis of the concentration dependence of the relative contributions of the “static” and dynamic mechanisms in both systems is in progress. Temperature Effects. The conclusion that inter- and intramolecular motions, leading to excimer formation, may take place during the fluorescence lifetime of Py and Py(3)Py on derivatized silica surfaces calls for the consideration of the effects of temperature on the Z’/Z ratio. As shown in homogeneous liquids, as well as in micelles and biomembranes,I2** a linear dependence between In (Z’/Z) and 1 / T is observed in the low-temperature, high-viscosity range. The slope yields the activation energy, E,, of the respective excimer generation process. In Figure 6B we present the effect of temperature on the fluorescence of Py(3)Py/Si-C18 in the low concentration range, (27) A. Dawidowicz, J. Rays, and Z. Suprynowicz, Chromatographia, 17, 157 (1983); (b) K.K. Unger, J. Chromatogr. Libr., 16, (1979). (28) L. C. Sander, J. B. Callis, and L.R. Field, Anal. Chem., 55, 1068 (1983).
- 0.94
-2.58
-
-3.41
A
1
CI
\
E -1.42r
-C -1.81
-
-2.20
I
in which the dynamic excimer mechanism was shown to predominate. It is evident that the relative excimer contribution markedly increases with temperature. A similar behavior characterizes Py on Si-CL8, but no increase in the Z‘/I ratio could be detected for either molecule on unmodified porous silica. An analysis of such temperature effects in terms of Arrhenius plots of Z’/Z is shown in Figure 8. In spite of the precautions which should be taken in analyzing such curves (due to complications associated with two excimer species” and possible heterogeneities of adsorption sites), the linear fits provide an additional independent support to the dynamic excimer mechanism. The plots in Figure 8 yield apparent activation energy values, E , = 40 and 19 kJ/mol for the excimer generation processes of
3526
J . Phys. Chem. 1985,89, 3526-3530
Py(3)Py and Py, respectively. It is interesting to compare the E, value observed for Py(3)Py in the cl8 alkyl phase of (LiChroprep RP-18) Si-C18 with that determined in homogeneous liquids. It has been suggestedl2”Vcthat a numerical value for the microscopic fluidity of a medium may be obtained by using a calibration curve in which the value of Z’/Z, obtained for Py(3)Py in a series of hexadecane/liquid paraffin mixtures, is plotted against the corresponding (macroscopic) viscosity of the mixture. By applying such a curve, we obtain a value of 180 CPfor the effective viscosity of the n-alkyl phase of (LiChroprep RP-18) Si-C18 at 30 O C . It is thus indicated that the intramolecular mobility of Py(3)Py on the above Si-CI8 is phenomenologically similar to that in highly viscous environments such as liquid paraffin and Triton X-100 micelles.I2 The substantial difference between the E, values of Py and Py(3)Py on the same Si-CI8surface may originate from the presence of the trimethylene chain in Py(3)Py. Similar observations have been made in homogeneous solution^.^^^*^
Conclusions Application of the bichromophoric molecule Py( 3)Py as a fluorescent probe for silica surfaces shows complete immobilization of the Py moieties on unmodified silica with respect to the in-
tramolecular rearrangements required for excimer formation during the excited-state lifetime (- lo-’ s). Some intramolecular mobility of Py(3)Py on silica is induced by the presence of a monolayer of coadsorbed 1-octanol. Of special relevance, however, is the sharp rise in mobility associated with the addition of the equivalent of a second monolayer. Under such conditions the intramolecular mobility becomes identical with that in pure 1-octanol, suggesting the collapse of the monolayer structure. On a densely derivatized (c18) reversed-phase silica, dynamic excimer generation takes place without requiring the presence of coadsorbed solvents. Work is in progress aiming at utilizing Py(3)Py for further characterization of adsorption layers and inversed phases on solid surfaces, paying special attention to the effects of surface porosity and fractal dimension. Acknowledgment. This work was sponsored by a grant from the Israel-US. Binational Science Foundation. Support by the Niedersachsen (FRG)-Hebrew University cooperative research agreement and partial assistance by the Fritz Haber Center for Molecular Dynamics of The Hebrew University are acknowledged. Registry No. Py(3)Py, 61549-24-4; 1-octanol, 111-87-5.
Surface Photochemistry: The Effects of Temperature on the Singlet Quenching of Pyrene Adsorbed on Silica Gel by 2-Bromonaphthalene Paul de Mayo,* Lalgudi V. Natarajan, and William R. Ware* Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 (Received: January 24, 1985)
The effect of temperature on the fluorescence quenching of pyrene adsorbed on silica gel by 2-bromonaphthalene has been studied. These studies have yielded activation energies for diffusion on a silica gel surface. Activation energies of 4 and 2 kcal mol-’ were obtained respectively for the quenching of pyrene on ‘dry” silica gel and for decanol-covered silica gel. The magnitudes of the activation energies indicate that the diffusion of pyrene and 2-bromonaphthalene is comparatively rapid when the silica gel surface is more uniform, whereas it is slower on “dry” silica gel. It is significant that the values of the activation energies are of the order of magnitude of hydrogen-bond energies and agree well with those reported in literature for pyrene diffusion in fluid solutions. For a decanol-covered surface, cooling results in the disappearanceof dynamic excimers and a single exponential decay for pyrene with a lifetime of 620 ns. Phosphorescence of pyrene was also observed on a decanol-covered silica gel surface of temperatures below 200 K.
Introduction The use of fluorescence spectroscopy as a probe of surface adsorbate interactions has been amply demonstrated in the study of photophysics and photochemistry of organic molecules adsorbed on oxide surfaces such as silica alumina,”’ and Vycor (1) Hara, K.;de Mayo, P.; Ware, W. R.; Weedon, A. C.; Wong, G. S . K.; Wu, K. C. Chem. Phys. Lett. 1980, 69, 105. (2) Bauer, R. K.; de Mayo, P.; Ware, W. R.; Wu, K. C. J. Phys. Chem. 1982, 86, 3781. (3) Bauer, R. K.; Borenstein, R.; de Mayo, P.; Okada, K.;Rafalska, M.; Ware, W. R.; Wu, K. C. J. Am. Chem. SOC.1982, 104, 4635. (4) Bauer, R. K.; de Mayo, P.; Okada, K.; Ware, W. R.; Wu, K. C. J. Phys. Chem. 1983,87, 460. (5) Bauer, R. K.; Natarajan, L. V.; de Mayo, P.; Ware, W. R. Can. J. Chem. 1984, 62, 1279. ( 6 ) de Mayo, P.; Natarajan, L. V.; Ware, W. R. Chem. Phys. Left. 1984,
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(7)de-Mayo, P.; Natarajan, L. V.; Ware, W. R. Adu. Chem. Ser., in press. (8) Francis, C.; Lin, J.; Singer, A. Chem. Phys. Lett. 1983, 94, 162. (9) Beck, G.; Thomas, J. K. Chem. Phys. Lett 1983, 94, 5 5 3 .
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glass.12 As compared to their gas-phase and solution counterparts the photochemistry and photophysics of organic molecules adsorbed on oxide surfaces are not well understood. We have recently demonstrated how variations in the heat treatment of silica gel prior to addition of an adsorbate affects the resolution of the fluorescence emission spectra and decay times of pyrene and na~hthalene.~ Heat treatment results in changes in the number and disposition of the silanol functions on the s ~ r f a c e . ’ ~ ,We ’~ have also shown that heat treatment followed by the addition of 1-decanol produces a surface resembling that of a two-dimensional liquid as judged by pyrene fl~orescence.~ The Occurrence of intra(10) Ishida, H.; Takahashi, H.; Sato, H.; Tsubomuro, H. J . Am. Chem. SOC.1970, 92, 275. (1 1) Oelkrug, D.; Radjaipour, M.; Erbse, H. 2. Phys. Chem. (Frankfurt am Main) 1974, 88, 23. (12) Nakashima, N.; Phillips, D. Chem. Phys. Lett. 1983, 97, 337. (13) Fripiat, J. J.; Uytterhoeven, J. J. Phys. Chem. 1962, 66, 800. (14) Iier, R. K. “The Chemistry of Silica, Solubility, Polymerization and Surface Properties and Biochemistry”; Wiley: New York, 1979.
0 1985 American Chemical Society
