J . Phys. Chem. 1990, 94, 4521-4528
4521
Gaussian Approximation to the Unique Heterogeneous Langmuir-Hinshelwood Type Fluorescence Quenching at the Silica Gel Gas/Solid Interface: Pyrene and 9,lO-Diphenylanthracene Singlet Quenching by Oxygen R. Krasnansky, K. Koike, and J. K. Thomas* Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: September 6 , 1989; In Final Form: January 16, 1990)
The temperature dependency of the oxygen quenching of the singlet excited states of both pyrene and 9,lO-diphenylanthracene on nonporous silica gel has been studied by both laser-induced time-resolved and steady-state fluorescence spectroscopy. A Gaussian distribution of rate constants is presented to account for the heterogeneous kinetics. Oxygen adsorption isotherms on the silica gel surface above the gas/liquid critical temperature are determined; oxygen assimilation times of 23.2, 44.1, 99.5, and 238 ps have been calculated at -47, -64, -80, and -98 "C, respectively. A dynamic Langmuir-Hinshelwood model for the bimolecular quenching reaction is presented.
Introduction The restricted geometries provided by and the chemical reactivity occurring at solid/gas or solid/liquid interfaces have recently been of much interest to the industrial' as well as the academic2 community. Key parameters of surface reactivity revolve around the nature and homogeneity of the surface adsorption, the mode of molecular approach, and the bulk transport rate to and from the surface interface. Studies incorporating geometrically unique solids such as silica gels,3 zeolites: Vycor glass,s and clays6 have led to a number of novel theories involving fractals,' restricted adsorptions,* and restricted particle g r ~ w t h . ~ The nonporous silica gel CAB-0-SIL allows us to examine the silica gel surface by fluorescence probing techniques without complicating the system with a partitioning of the surface-bound fluorophore between external surface and internal porous surface. The addition of a second molecular entity, known as the quencher, opens an additional path of relaxation for the electronically excited state of the probe. When quenching molecules are present in both the gaseous and adsorbed states, the possibility of either Langmuir-Rideal,I0 reaction between a gas-phase molecule and a surface-bound molecule, or Langmuir-Hinshelwood,'l reaction between two surface-bound molecules, types of reactivity exists. Interpretation of both steady-state spectra and time-resolved fluorescence decay traces in the absence and presence of various amounts of quenchers at various temperatures enables us to comment on the nature of the probe adsorption and mode of bimolecular approach of quencher to probe. The fluorophores incorporated in this study are pyrene, monitoring the IBs, 'A, transition,12 and 9,lO-diphenylanthracene (DPA), monitoring tke IB2" IAl, transition,I2-l3while the
-
-
( I ) Baum, T. H.;Jones, C. R. Appl. Phys. Left. 1985, 475, 538. (2) Thomas, J. K.J . Phys. Chem. 1987, 91, 268. Beck, G.; Thomas, J. K.Chem. Phys. Left. 1983, 94 (6), 553. Kessler, R. W.; Willkinson, F. J . Chem. Soc.. Faraday Trans. I 1981, 77, 309. ( 3 ) Hite, P.; Krasnansky, R.; Thomas, J. K. J. Phys. Chem. 1986, 90, 309. Milosavljevic, B. H.; Thomas, J. K.J . Phys. Chem. 1988.92, 2997. Yoshihira, D.; Miyake, H.; Yokota, A.; Soga, K. Inorg. Chim. Acta 1985, 105, 69. Kaufman, V . R.; Avnir, D. Langmuir 1986, 2, 717. (4) Liu, X.; lu, K.-K.;Thomas, J . K. J . Phys. Chem. 1989, 93, 4120. Turro, N . Pure Appl. Chem. 1984, 59, 7705. (5) Wolfgang, S.;Gafney, H.D. J . Phys. Chem. 1983,87,5395. Piciulo, P. L.; Sutherland, J. W. J . Am. Chem. SOC.1979, IO/,3124. (6) Thomas, J. K.Acc. Chem. Res. 1988,2/, 275. Rajanski, D.; Huppert, D.; Bale, H.D.; Dacai, X.; Schmidt, P. W.; Farin, D.; Seri-Levy, A.; Avnir, D. Phys. Reu. Lett. 1986, 56, 2505. ( 7 ) Avnir, D.; Pfeifer, P. J . Chem. Phys. 1983, 79 (7), 3558. Avnir, D.; Farin, D.; Pfeifer, P. J . Chem. Phys. 1983, 79 (7). 3566. (8) Levy, A.; Avnir, D.; Ottolenghi, M. Chem. Phys. Lett. 1985, 122, 233. Levitz, P.; Drake, J. M. Phys. Reo. Lett. 1987, 58. 686. Avnir, D.; Wellner, E.; Ottolenghi, M. J . Am. Chem. SOC.1989, I l l , 2001. (9) Liu. X.; Thomas, J. K.Chem. Phys. Lett. 1988, 144, 286. (IO) Rideal, E. K. Proc. Cambridge Philos. SOC.1939, 35, 130. ( I I ) Hinshelwd, C. N . Kinetics of Chemical Changes, Clarendon: Oxford, U.K., 1940; p 187. (12) Pariser, R. J . Chem. Phys. 1955, 24 (2), 250.
0022-3654/90/2094-452 1$02.50/0
quencher is oxygen. This work demonstrates that the adsorption of fluorophore onto the CAB-0-SIL surface is not homogeneous, but rather, can be characterized by a distribution of adsorption sites; each adsorption site presents an environment which is reflected by the unimolecular decay rate of fluorophore residing at that site. The distribution of fluorophore unimolecular decays is modeled to a Gaussian in natural logarithmic space about a mean unimolecular decay rate. The observable excited-state decay rate in the presence of quencher also demonstrates a Gaussian distribution. Through quenching temperature and coadsorbed species dependencies, the oxygen quenching of surface-bound excited-state fluorophore is assigned as predominantly Langmuir-Hinshelwood in nature.
Experimental Section Steady-state fluorescence spectra were obtained on a SLM/ Aminco SPF-500 spectrofluorometer in conjunction with a Zenith 2-368 computer. A neutral-density filter, OD = 1.0, was placed in the excitation line to prevent photodecomposition of surfacebound fluorophore. Time-resolved fluorescence decays were obtained for fluorophores excited with a PRA Nitromite nitrogen flow laser, Model LN-100, with a 0.12-11s fwhm 70-pJ 337.1-nm pulse. Emitted light was collected at 90' to the excitation, cutoff filters removed collected scattered light, and the monitoring wavelength, Xob, was selected with a Bausch and Lomb 33-86-02 monochromator equipped with a 1350 grooves/mm grating and detected by a Hamamatsu R-1644 microchannel plate with a response time of 0.2 ns. The signal was digitized via a Tektronix 7912HB programmable digitizer equipped with a 7B10 timebase and either a 7A16A amplifier, response time 1.6 ns, or a 7A23 amplifier, response time 0.7 ns. Decay profile simulations were performed on a Zenith 2-368 computer by a nonlinear least-squares fitting method. Samples were prepared by exposing the silica gel, previously dried at 150 "C for 24 h, to either cyclohexane or pentane solutions containing the proper amounts of the fluorophore. The solvent was carefully removed under vacuum; the silica gel/probe sample was placed in a quartz cell equipped with a stopcock. Less than 0.07% of the silica surface was covered by the probe. Immediately preceding data collection the sample was evacuated under vacuum at 125-1 30 OC. The total dehydration procedure would be sufficient to remove the physisorbed water while leaving the surface silanol functionality intact.I4 Selected amounts of oxygen were introduced into a constant-position sample cell by a series of stopcock manipulations and a vacuum line. ( 1 3) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970; p 71. (14) Iler, R.K.The Chemistry of Silica Gel: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; Wiley: New York, 1979 p 630. Sindorf, D. W.; Maciel, G.E. J . Am. Chem. Soc. 1983, 105, 1487.
0 1990 American Chemical Society
4522
The Journal of Physical Chemistry. Vol. 94, No. 11, I990
Various temperatures were achieved by incorporation of a quartz Dewar and a stream of chilled nitrogen gas. The chilling of the nitrogen was achieved by passing the gas through a coiled copper tube submerged in a liquid nitrogen bath. The sample temperature was monitored via a thermocouple attached to the sample cell wall and varied by regulating the nitrogen gas flow. Exterior Dewar fogging was prevented with a second room temperature stream of nitrogen gas. O2 adsorption isotherms were obtained with a differential pressure analysis apparatus. The apparatus consists of equal volume spheres connected by a 1 m high U-tube filled halfway with distilled and degassed dimethylpoly(si1oxane) and a series of three-way stopcocks. Each sphere possessed a cell port equipped with a two-way stopcock. The total volumes were calibrated such that the volume of the left equaled the volume of the right. A sample of known weight was placed in one of the cells and the whole system was evacuated. With the two-way stopcocks closed the system was equilibrated with a given amount of oxygen; the left and right spheres were then isolated and the two-way stopcocks were opened. Both the sample and the empty reference cells were equivalently submerged into various chilling baths. The amount of gas adsorbed was determined from the difference in the heights of the dimethylpoly(siloxane) columns and a previously obtained calibration curve. The bulk pressure was measured with a Hastings vacuum gauge equipped with a DV-300 Raydist gauge tube. HS-5 CAB-0-SIL possessing a surface area of 325 m2/g and a particle diameter of 0.008 mm was graciously donated by the Cabot Corp. Kieselgel-60 (surface area of 500 m2/g, pore volume of 0.75 cm3/g, pore diameter of 60 A, and mesh of 70-230) was purchased from Aldrich. Pyrene was purchased from Aldrich and passed three times down an activated silica gel/cyclohexane column. 9,lO-Diphenylanthracene (99%), HPLC grade cyclohexane, and gold label pentane were used as received from Aldrich. Oxygen was used as received from Mittler. Dimethylpoly(siloxane) was purchased from Sigma. A vacuum of lo4 Torr was achieved with a Duo-seal Model 1400 vacuum pump.
Results and Discussion Gaussian Distribution Model. As reported earlier,15 neither the pyrene nor the DPA fluorophore yields monoexponential unimolecular decays once adsorbed onto the silica gel surface. From the low surface area coverage and the absence of pyrene excimers, the surface probes can be seen as existing as isolated species. Previously, deviations from first-order decay kinetics were either treated by ignoring the early portions of the signal or by modeling a computer-generated decay profile to the unimolecular decay profile by use of a multiexponential fit,16 (eq I ) , where I(0) I ( r ) = I(O)(A exp(-k,t) + [ I - A ] exp(-k2r)) (I) is the initial fluorescence intensity and I(t) is the fluorescence intensity at time equals r. The physical interpretation of the given biexponential model mandates that the probe molecule is partitioned by a percent factor A between two uniquely identifiable environments characterized by the two rate constants kl and k2. The biexponential model is weakened by its intrinsic assumption of two possible environments. Each additional exponential term added to the model also brings hand-in-hand an additional partitioning factor. Where, in the absence of two chemically unique adsorption sites, true representation of multiple adsorption sites on a surface would require the summation of each adsorption site, the number of variables in the decay simulation quickly increases to the point where an interpretation of those values becomes questionable. Albery et al.” approached the problem of mathematically simulating heterogeneous systems by expanding the work of Scott
Krasnansky et al.
’
s-
0
c
13
ie
15
14
17
18
Ln ( k I s - ’ ) Figure 1. Pictorial representation of the Gaussian distribution model in In ( k ) space.
et a].’* Albery points out that an observed heterogeneity of a system can be approximated by a Gaussian distribution. Since an observed rate constant is a consequence of the free energy of a system, the distribution in the natural logarithm of observed rate constants is described by a Gaussian distribution; the distribution can be characterized through a mean rate constant, k , and a distribution parameter, y. The dispersion in the first-order rate constants for --m -< x -< m then becomes In ( k ) = In
( k ) + yx
(2)
The observed decay profile is composed of the summation of the contributions from each microscopic species. Integration over the distribution, exp(-x2), yields the following equation when the fluorescence intensity is proportional to the probe’s excited-state concentration
J - m
-
where
11
exp(-x2) dx = d2
and x is the integration range. After transformation of the variable, x = In (A) for x < 0 and x = -In (A) for x > 0, the integration of the numerator of eq 3 can be carried out using the extended Simpson’s rule;19 division of this integration by the denominator yields eq 4.
g(A) = A-’ expl-[In (A)]2)(exp(-ktAv)
+ exp(-ktA-v))
0.2 -I ( f-) - --(2[g(O.1) + g(0.3) + g(0.5) + g(0.7) + I(0) 37r’/* g(O.9)] + g(0.2) + g(0.4) + g(0.6) + g(0.8) + exp(-kt)]
(4)
Whereas the observed decay profile no longer is characterized by a single decay rate, the steady-state fluorescence intensity becomes dependent on both yob and kobs. The typical SternVolmer plot is no longer represented by eq 5 , but rather, by eq
6; see derivation 1, where kOb is defined by eq 7B, k b is the (15) Bauer, R. K.; de Mayo, P.; Okada. K.; Ware, W. R.; Wu, K . C. J . Phys. Chem. 1983, 87, 460.
(16) Lochmuller, C. H.; Colborn, A. S . ; Hunnicatt, M . L.; Harris, J. M . J . Am. Chem. SOC.1984, 106. 4077. (17) Albery, W. J.; Bartlett. P. N.; Wilde, C. P.; Darwent, J. R. J . Am. Chem. SOC.1985. 107, 1854.
bimolecular quenching rate constant, KO is the mean probe’s ex(18) Scott, K. F. J . Chem. SOC.,Faraday Trans. I 1980, 76, 2065. (19) Riddle, D. F. Calculus and Analytic Geometry, 3rd ed.; Wadsworth: Belmont, CA, 1979; p 219.
The Journal of Physical Chemistry, Vol. 94, No. 1 I, I990 4523
Quenching at the Silica Gel Gas/Solid Interface TABLE I: Gaussian Model Fit Parameters for the Unimolecular Decays of CAB-0-SIL Bound Fluorophores
L". s-I
svstem 4.74 x 10-7 mol/g of pyrene on CAB-0-SIL 4.76 x 10-7 mol/g of DPA
on CAB-0-SIL
Y
(4.38 f 0.02) (3.97 f 0.04) (3.67 f 0.06) (3.33 f 0.03) (6.72 f 0.14) (6.60 f 0.19) (6.35 f 0.1 1) (6.33 f 0.07) (6.47 f 0.04)
X X X X X X X X X
IO6 IO6 lo6 IO6 IO' IO' IO7 IO7 IO7
temp. "C
0.496 k 0.017 0.696 k 0.026 0.852 i 0.034 0.884 f 0.030 0.493 f 0.042 0.445 f 0.025 0.420 f 0.060 0.469 f 0.026 0.430 f 0.024
7.50
19
-26 -66 -92
--.
18 -1 5
0
-28 -50 -8 5
cited-state unimolecular decay rate constant, Lob is the mean observed decay rate constant, yo is the distribution parameter of the Gaussian for the unimolecular decay, and yobsis the distribution parameter for the observed unimolecular decay rate. A pictorial representation of the Gaussian model is shown in Figure I. Deriuation I. Steady-state fluorescence intensity is defined as the integration of the fluorescence decay function I ( t ) . I =
1:
I(-t') dt'
I(-t') is defined in the Gaussian distribution model through eq 3; note that the time transformation o f t = -t'has been made for convenience.
I =
Am15 6,
I
exp(-x2) exp[-kt exp(yx)] dx dt
Since these mathematical functions are continuous and can be differentiated, we can change the order of integration to
The first absolute integration over t can be performed exactly to yield I =
1
exp(-x2 - yx) dx
The second absolute integration over x can be converted to a Gaussian error function through the transformation of z = x (y / 2). Integration yields
+
i=
t
exp(
G)
Therefore, the final steady-state fluorescence expression becomes
Io = exp( I
- (7Obd2
)-
Lobs
LO
Application of the Gaussian model to the evacuated pyrene/ CAB-0-SIL and DPA/CAB-0-SIL systems yields the photophysical parameters displayed in Table I; the presented results represent several sets of experiments. As in solution,20 mean observable unimolecular decay rate constants of the excited state decrease with decreasing temperature for the pyrene system while remaining relatively constant for the DPA system. These observations reflect the fact that the quantum yield of fluorescence approaches unity for DPA while the quantum yield of fluorescence is only 0.32 for pyrene in cyclohexane.21 The low degree of intersystem crossing, typically the predominate temperature-dependent component of the unimolecular decay rate, accounts for the temperatue insensitivity of the DPA system. The longer lifetime of excited state of pyrene, as compared to DPA, increases (20) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970; p I80 and references therein. (21) Berlman, 1. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic Press: New York, 1971, p 383.
I
0 0
1.44e-7
I
2.BBe-7
I
4.32e-7
1 5 . 7 6 ~ - 7 7.20e-7
[02]bulk/ko 1 (mol m-3 5) Figure 2. Comparison of steady-state oxygen quenching of CAB-0-SIL bound probe, oxygen concentrations scaled by probe's unimolecular decay mol/g of DPA, X, = 365 nm, A,, = 390-520 nm, rate. ( 0 )4.76 X 23 "C, and (0) 4.74 X IO-' mol/g of pyrene, A,, = 337 nm, Aob = 340-520 nm, 25 O C .
the sensitivity of the pyrene's unimolecular decay to surface perturbations. Chilling the system leads to a stronger association of probe and surface. It is suggested that this stronger association magnifies the heterogeneity of the adsorption sites as reflected in the increasing y value with a decrease in temperature. The DPA system, possessing a short-lived excited state and a more flexible carbon structure as compared to pyrene, does not demonstrate the increase in y with a decrease in temperature. The physical interpretation of this model in these systems would be that each probe experiences slightly different geometric perturbations or surface interactions due to surface irregularities on the scale of the probe's cross-sectional area. The Gaussian model allows us to remove the constraint of a very limited number of types of adsorption sites found in the biexponential model while leaving only two variable parameters, Lobs and y, in our mathematical simulation of the decay profile. Oxygen Quenching of Singlet Excited States of Pyrene and DPA. Bimolecular quenching reactions of excited states are typically represented by pseudo-first-order reaction kinetics, eq 7A kob = ko + k,[quencher] (7A) When there exists a distribution of probe environments, such as demonstrated in our systems, one must recognize the difference between the macroscopic fluorescence decay profile observed and microscopic fluorescence decay rates present. The composite distribution of both unimolecular decay rates and respective bimolecular reaction rates are combined in the distribution of observable decay rates. Hence, the parameters in eq 7A have to be redefined in the Gaussian distribution terminology to yield eq 7B Lobs = Lo+ kb[quencher] (7B) Due to the dramatic difference in unimolecular decay rates of pyrene and DPA, a large concentration range of oxygen could be examined and compared. Steady-state and time-resolved oxygen quenching studies were performed at various temperatures and correlated as a function of bulk gaseous oxygen concentration. Oxygen pressures were converted to concentration units through the ideal gas equation to account for density changes at the various temperatures. Since the concentration of quencher required to observe a given extent of quenching is intimately related to the probe's unimolecular decay rate, one can comment on the similarities of the approach of the quencher to two different probes by correlating Io/[ for each quenching system to [02]/ko. Figure 2 shows that the room temperature Stern-Volmer plots scaled as discussed above for the Ipyrene/CAB-0-SIL and for the 'DPA/CAB-0-SIL systems superimpose; this superimposing indicates that the dif-
Krasnansky et al.
The Journal of Physical Chemistry, Vol. 94, No. 11. 1990
4524
-.
9
0 00
12 0
I
I
24 0
360
0 0 0 L--000 480
1 480
9 60
14 4
192
24
0
60 0
[02lbulk / (mol m - 3 ) Faure 3. Steady-stateoxygen quenching of 7.76 X lo-' mol/g of DPA on CAB-0-SIL, kx= 365 nm and = 390-520 nm at (A) 23 "C, (B) -27 OC, (C) -50 "C, and (D) -90 OC.
Figure 5. Time-resolved oxygen quenching 4.76 X lo-' mol/g of DPA, A,, = 337 nm, Aob = 430 nm response of kobr to bulk gaseous oxygen concentration at (A) 18 OC, (B) -28 OC, (C) -50 OC, and (D) -83 "C.
40
u) H
m
0
U 3 n
in
=u
-
401
400
'
r ,
5l
6 I
I
I
1
1
,
1
1 \
0
Q
4
0
a
0 00
05
15
IO I021
0
20
40
60
80
100
Time I ns Figure 4. Transient fluorescence decay profile of 4.76 X mol/g of DPA on CAB-0-SIL, with Gaussian model simulated decay profiles, at 18 OC and various pressures of oxygen, A,, = 337 nm and &b, = 430 nm.
fusional processes of each system are identical. It was also noticed that a linear Stern-Volmer plot is not obtained in Figure 2 as predicted by the simple Stern-Volmer model. The temperature dependency to the steady-state quenching is typified by those of DPA as seen in Figure 3. There is a dramatic increase in the degree of quenching at a given concentration of oxygen with a decrease in temperature. The Gaussian distribution model was applied to the decay profiles at various concentrations of oxygen. Time-resolved fluorescence decay profiles for the oxygen quenching of the DPA/CAB-0-SIL system are shown in Figure 4. Initial intensities of the fluorescence decay profiles are fixed at the value obtained from the decay profile in absence of quencher indicating that the quenching is dynamic in nature; short-time portions of the decay profiles are seen to be lost in the response time of the system. As the oxygen concentration increases there is an increase in the kOk, Figure 5, and an increase in the y. The mean observed rate constant is correlated through equation 7B to oxygen concentration, yielding a bimolecular rate constant k$. As seen before, when the quenching is plotted in terms of bulk gaseous oxygen concentrations, as would be appropriate for a Langmuir-Rideal scheme, there is an increase in quenching efficiency with a decrease
/ (mol
20
25
30
m-3)
0 Simulated from Time-Resolved Data 19' C 0 Actual Steady-State 25' C Figure 6. Comparison of (A) actual A,, = 337 nm, A d = 340-520 nm and (B) Gaussian simulated steady-state oxygen quenching from timeresolved fluorescence quenching 4.74 X IO-' mol/g of pyrene on CAB0-SIL, A,, = 337 nm, Aob = 340 nm,at 19 "C.
in temperature. The good agreement between the actual steady-state quenching and the simulation of the steady-state quenching derived from eq 7B and the calculated fluorescence decay fitting parameters is shown in Figure 6. Simulation of the fluorescence decay becomes difficult at high degrees of fluorophore quenching; the difficulties are seen to rise from the uncertainty in initial fluorescence intensity as observed with the instrument as instantaneous components enter into quenching scheme. Recently, Turro et a1.22derived the expressions for the bimolecular rate constant for a Langmuir-Rideal type quenching scheme both for a smooth surface, eq 8, and for a porous solid in the Knudsen regime, where the pore diameter is much less than the gas-phase mean free path, eq 9. -
1 d[probe*] =dt 4
- d[probe*] = -
[probe*]
a(u)ap,obc
kT
P,[probe*]
TO
4 *~ra&Rp(u)
dt
6
kT
+ - (8)
P,[probe*]
[probe*]
+(9) TO
where (22) Drake, J. M.; Levitz, P.; Turro, N. J.; Nitsche, K. S.; Cassidy, K . F. J . Phys. Chem. 1988. 92, 4680.
Quenching at the Silica Gel Gas/Solid Interface (u) =
(
The Journal of Physical Chemistry, Vol. 94, No. I I, 1990 4525
Y
%)‘I2
a is the efficiency of the bombardment reaction, uVok is the cross section area of the probe, rab is the interaction radius for the reactants, g is a geometric factor ( g > O), R , is the pore radius, T~ is the lifetime of the probe in the absence of quencher, P, is the bulk gaseous quenching pressure, m , is the molecular mass in kg/molecule of the gas, Tis the absolute temperature, and k is the Boltzmann constant. Converting pressures to concentration units through the ideal gas law and defining the mean velocity ( u ) we obtain eqs 10 and 1 1 , where R is the ideal gas constant,
k, = R u a ( 2 ~ m , k ) - ~ / ~ T ’ / ~
(10) o i O.OOe*O
I
5.00e-6
1.008-5
[ 1 -Hexadecanoll /
from eq 8 and 9, respectively. Equations 10 and 1 1 show that a gas-phase bombardment quenching mechanism would predict that the quenching efficiency should increase with the square root of temperature; both systems show decreasing oxygen quenching efficiencies with increasing temperatures. In addition to the observed temperature trends, the efficiency of a bombardment mechanism should be comparable to the efficiency of the completely gas-phase quenching reaction. Ware et al.23report a bimolecular quenching rate constant of 1.75 X IO8 mol-’ m3 s-l for oxygen quenching of singlet anthracene in the gas phase at 573 K. Collision theory would predict a bimolecular rate quenching rate constant of 6.38 X lo8 mol-l m3 s-I at 573 K using eq 12 and an interaction diameter of 7.1 A for
--)
k, = (2r*d2N( 8irkT an activationless collision-controlled process,24 where is the reduced mass of the probe and the quencher molecules. Ware also reports an increase in the k, to 1.88 X IO8 mol-l m3 S-I with a 40 K increase in temperature. This observation excludes any kinetic dominating role of arene/oxygen complex formation in the quenching scheme such as found in the self-quenching of pyrene in the gas phase observed by Pillings et aL2$ Parmenter26 et al. also observed a dramatically inefficient oxygen quenching of gaseous glyoxal. Though not stated directly, Parmenter’s data does yield a linear k, vs T112plot as again predicted from eq 12. The low efficiency of a gas-phase bimolecular quenching reaction is not unexpected. Not all collisions would be expected to yield favorable probe and quencher configurations or yield sufficiently long encounter times to facilitate readjustment of the encounter pair. In the absence of a mediating force, such as a solvent cage, increasing the encounter time and allowing the interconversion of translational, vibrational, and rotational energies, the probability of the quenching reaction decreases. The efficiency of the oxygen quenching in the system has been seen to be sensitive to the presence of coadsorbed inert molecules. Samples in which physisorbed water was not completely removed demonstrated lower quenching efficiencies than samples properly dried. The 111/1 ratios of the pyrene’s fluorescence spectra of such samples were, however, quite comparable. Since the III/I has been seen to reflect the physical environment the pyrene finds itself in,27the water can be seen as simply blocking lateral motion on the surface instead of blocking the facial attack of the exposed pyrene surface. Similarly, coadsorption of various amounts of alcohols onto the Kieselgel-60 silica gel surface has been seen to dramatically affect the efficiency of the oxygen quenching of (23) Ware, W. R.; Cunningham, P. T. J . Chem. Phys. 1%5,43 ( 1 l), 3826. (24) Wilkinson, F. Chemical Kinetics and Reaction Mechanisms; Van Nostrand Reinhold: New York, 1980; p 106. (25) Davis, A.; Pillings, M. J.; Westby, M.J. Chem. Phys. 1981, 63, 209. Davis, A.; Pillings, M. J. Chem. Phys. 1983, 79, 235. (26) Parmenter, C. S.; Seaver, M. Chem. Phys. 1980, 53. 333. (27) Kakyanasundaram, K.; Thomas, J. K. J . Am. Chem. Soc. 1977.99, 2039.
! 0.4 I .50e-5
(mol m-2)
0 l o / I ( 2 0 8 torr 0 2 ) 0 111/1 Figure 7. Dependency of both the degree of oxygen quenching incurred at 208 Torr and the III/I ratio in the absence of oxygen on the amount of coadsorbed I-hexadecanol, 2.3 X lo-’ mol/g of pyrene on Kieselgel-60 22
oc.
surface-bound pyrene. Figure 7 presents both the degree of quenching incurred at 208 Torr of oxygen and the III/I ratio of adsorbed pyrene in the absence of oxygen as a function of coadsorbed I-hexadecanol surface concentration. The vast majority of the decrease in oxygen quenching efficiency clearly occurs over a coadsorbed 1-hexadecanol loading level which leaves the pyrene’s local environment unaffected; this demonstrates to us that the coadsorbed alcohol does not act like a blanket for the pyrene but rather, as an obstacle to the surface bound oxygen at this loading level. A bombardment mechanism for oxygen quenching should have been insensitive to simple surface lateral obstacles. At higher alcohol loading levels, the local environment of the pyrene is systematically altered with surface I-hexadecanol surface concentration. Similar effects have been observed with coadsorbed cyclohexane and coadsorbed dimethylpoly(si1oxane) with the pyrene/silica gel system.28 The low efficiency of gas-phase quenching reactions, the observed temperature trends of the quenching reactions, and the sensitivity of the quenching efficiency to coadsorbed inert molecules mandates the closer examination of the surface-bound oxygen in the quenching of surface-bound probe. Measurements of the surface oxygen concentrations at the temperatures studies were therefore required. Oxygen Adsorption Isotherms. Oxygen adsorption onto the CAB-0-SIL surface at various temperatures has been measured by using the differential pressure analysis apparatus previously described. The isotherms obtained were found to follow a Langmuir-type adsorption at the temperatures examined. Computer simulations of the adsorption isotherm were accomplished by use of the Langmuir equation,29eq 13, where 0 is the fraction of adsorption sites occupied, K is the adsorption equilibrium constant, [021adis the surface oxygen concentration, [O,], is the amount of oxygen adsorbed at infinite bulk concentration, and [O,]bu]k is the bulk gaseous oxygen concentration.
where
Oxygen adsorption isotherms with computer simulations are shown in Figure 8 . Figure 9A shows that the amount of O2adsorbed ~~
(28) Krasnansky, R.; Thomas, J. K. Manuscript in preparation. (29) Adamson, A. W. Physical Chemistry ofSurface, Interscience: New York, 1960; p 465.
The Journal of Physical Chemistry, Vol. 94, No. 1 1 , I990
0
A
- 5 60e-8 I 5 We-7 I
is a frequency factor and Q is the heat of adsorption. A leastsquares fit of experimental results using eq 14 yields a heat of adsorption of 3.54 kcal/mol, Figure 9B; the value of Q falls within the regime of physical adsorption. Figure 9C presents a plot of K as a function of temperature. The average amount of time an adsorbed oxygen molecule resides on the CAB-0-SIL surface can be calculated through eq 15 (see Appendix I1 for derivation), where
"
I
J I
I
2.80e-7
140e-7
0 00
15 0
0 00
300
450
750
600
[OPlbulk / (mol m-3) Figure 8. Oxygen adsorption isotherms on CAB-0-SIL along with Langmuir model computer simulations at (A) -47 OC, (B) -64 OC, (C) -80 OC, and -98 OC.
on the silica gel surface dramatically increases with a decrease in temperature. The temperature dependency of K can be easily derived by defining the rate of oxygen adsorption and the rate of oxygen desorption; proper substitution of the equations and algebraic manipulations, see Appendix I, yields eq 14, where ko
-:4
Krasnansky et al.
--
-21 5
--
4.26~10.~ -2 ! 5 x l O - '
T is the assimilation time and Z is the collision frequency. The assimilation time of the oxygen as a function of temperature is shown in Figure 9D. Adsorption parameters above -47 OC could not be measured directly due to the sensitivity of the apparatus but could be extrapolated from Figure 9, A, C, and D. Langmuir-Hinshelwood Approach to Quenching. Surface oxygen concentrations for the various gaseous bulk oxygen concentrations were determined by using the adsorption isotherm parameters. From a steady-state point of view, the amount of oxygen adsorbed on the silica surface in the range where quenching occurs is quite low; at ambient temperatures, the surface quencher concentration even approaches the surface probe concentration. The amount of time a given oxygen resides on the surface is also quite short, anywhere from a few picoseconds to several hundred picoseconds depending on the temperature. Such short assimilation times would not be expected to yield the observed dynamic type of quenching if the oxygens initially present on the surface at the moment of probe excitation were completely responsible for the quenching of the probe's excited state. The probe's excited-state lifetime is immensely greater than the assimilation time of the oxygen. The number of times an adsorbed oxygen is replaced, at some other arbitrary surface location, by an oxygen from the gas phase is great during the lifetime of the probe. The random placement of oxygens on the surface would be crucial in describing the apparent surface migration distance incurred during the
1310
--
I -0 400
-*
Oar-
-'
--
I
' dSLOPE dY-:NTEF
376 -5 63
1 16~10-~ -5 35~:0-~
3-
---
-15 5 r
-1E 0 ' 0 00300
0 00347 -1.66~10-~~
---
= 0 00534 -1 3 0 x 1 ~ 1 6
2.82X10-5 11 0 L O O
I 0 00360
0 00420
0 004EO
0 00540
0 00300 0 00360 0 00420
1K / (K 1 ) 0 6 0 0 , - - ~ -
O F - -0 6 0 0 -1E 0
O/R L" 141
O
'
-
C EO0
* 1790 -30
'
dWR ! dLn l A l
I
0 00540
0 00600
-
S:OUe
5
-w
0-
I
T -
0 00480
1/T / ( K l )
,
-0 = 1 68~10 = -3 50x!0-9
-~
1 00
1
-6 00'
0 00600
2 62x10+ 11 0
= 1620 = - 4 30
i-lnter
u
500
E 00 r---
-
~ X ~ -3.07x10-'
i
0 00552
= 6 C
8
0
--
A I
0 0191 -3 68x!0-!6
= -3 4 3 x 1 0 - 1 5 = 2 82~10-6 = !I 0
= 9 41x10W7
Y
~
A
i-
-22.0 0.00400
A 0 00440
0 00480
0.00520
liT / ( K . 1 )
0 00560
0 00600
12 0 1 00
~
00 00300
1 00
' 0 00360
0 OCA20
1/T /
0 00480
0 00540
0 00600
(K-1)
Figure 9. (A, upper left) Plot of In [O& vs TI, oxygen adsorption on CAB-0-SIL. (B, lower left) Temperature dependency of the adsorption equilibrium I for constant for oxygen adsorption on CAB-0-SIL along with computer simulation was defined through eq 14. (C, upper right) Plot of In ( K ) vs T I for oxygen adsorption on CAB-0-SIL. oxygen adsorption on CAB-0-SIL. (D.lower right) Plot of In {assimilation time) vs T
O - ~ ~
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4527
Quenching at the Silica Gel Gas/Solid Interface 8 00e7
I
1
Q
-8.00e7
a -
1"oE!z3zz3
I
0
0
0
-1.60
I
600e8
-
000
L
-
Nin E
5 20e-9
1 04e-8
i 56e-8
208e-8
3.00e-3
2 60e-8
l02lsurface 1 (mol m-2) Figure 10. Time-resolved oxygen quenching 4.76 X IO-' mol/g of DPA, A,, = 337 nm, Aob = 430 nm, Gaussian fit, response of kobato surface oxygen concentration at (A) 18 OC, (B) -28 OC, (C) -50 OC, and (D) 5 00e6
30 0
I
0 00
-83 OC.
0?4642.0 '
7 L
3 60e-3
4.20e-3
4,809-3
5.40e-3
6 00e-3
1K I (K-1)
Arrhenius plot of oxygen quenching (A) 4.74 X lo-' mol/g of pyrene and (B) 4.76 X lo-' mol/g of DPA on CAB-0-SIL. Figure 12.
SCHEME I: Pictorial View of Oxygen Quenching of CAB-0-SIL Bound Fluorophore
: 1 0
Bulk Gaseous Oxygen
0"
o
d
2
o
-
0
I
0
0-0
0
d 0-0
I
I
000 0 00
J 2 20e-9
440e-9
660e-9
8 80e-9
1 10e-8
[O~Isurface/ (mol m-2) Figure 11. Time-resolvedoxygen quenching t.74 X IO-' mol/g of pyrene, A,, = 337 nm, Ad = 392 nm, response of kob to surface oxygen concentration at ( A ) 19 OC, (B) -26 OC, (C) -66 OC, and (D) -92 OC. lifetime of the probe's excited state. A large surface oxygen exchange rate also upholds pseudo-first-order conditions. Figures 10 and 1 1 show that the observed decay rate constants of both DPA and pyrene vary linearly with surface oxygen concentration; the bimolecular quenching rate constants obtained from the slopes of these plots decrease with a decrease in temperature as would be expected from a surface migration mechanism. Under higher probe surface coverage conditions, it has been shown through the formation of static, as opposed to dynamic, excimers of pyrene on the silica gel surface that the fluorophore does not migrate on the time scale of the excited state;30 all motion, therefore, arises from the motion of the oxygen. An Arrhenius treatment of the observed quenching rate constants, Figure 12, yields activation energies for the bimolecular Langmuir-Hinshelwood-type quenching reaction of 3.31 and 2.31 kcal/mol for (30) Hara, K.;De Mayo, P.; Ware, W. R.;Weedon, A. C.; Wong, G. S. K.;Wu, K. C. Chem. Phys. Left. 1980,69 ( I ) , 105. Krasnansky, R.;Thomas, J . K . Manuscript in preparation.
the pyrene-CAB-0-SIL/oxygen and the DPA-CAB-0-SIL/ oxygen systems, respectively. Agreement of these activation energies with the oxygen heat of adsorption, independently determined as 3.54 kcal/mol, reflects that the activation barrier to the quenching reaction is the removal of the oxygen from silica surface itself. A pictorial representation of the dynamics of the oxygen quenching is presented in Scheme I. The silica surface is seen to play an analogous role in the quenching scheme as the solvent cage in solution. The interaction time of the probe and quencher is elongated and excess translational, vibrational, and rotational energy of the oxygen is dissipated by the adsorption process. The adsorption process, thus, increases the quenching probability. ConcIusion
The heterogeneous adsorption and the bimolecular quenching with oxygen of the CAB-0-SIL bound fluorophores pyrene and DPA have been computer simulated by using a Gaussian distribution model. The bimolecular quenching rate constant was found to follow the opposite trend with teinperatue when the gas-phase concentration of oxygen was used as the independent variable as mandated by a Langmuir-Rideal mechanism. Oxygen adsorption isotherms for the CAB-0-SIL surface have been measured and found to be Langmuir in nature above oxygen's critical temperature. The bimolecular quenching rate constant was found to follow the expected trend with temperature when surface concentration of oxygen was used as the independent variable as mandated by a Langmuir-Hinshelwood mechanism.
J . Phys. Chem. 1990, 94, 4528-4535
4528
The quenching observed could not be accounted for by the oxygen present on the surface at the time of excitation; the exchange of surface-bound oxygens with oxygens from the bulk gas reservoir during the lifetime of the excited state had to be recognized. Coadsorption of inert obstacle molecules dramatically decreases the efficiency of the oxygen quenching. We assign the predominate quenching of the singlet excited states of pyrene and DPA located on the CAB-0-SIL surface by oxygen to a modified LangmuirHinshelwood scenario.
(when gas concentration units are used) In ( K ) + In ([021m) = -.
In
“1”’)
(’[
kO
- 0.5 In
~ X M ,
(i)+ 2
R T (14)
Appendix 11 Assimilation Time.
Acknowledgment. We are grateful to NSF for financial support of this work via grant No. CHE-8911906 and the Cabot Corp. for their donation of the CAB-0-SIL. We thank Dr. Victor G. Kuykendall, Mr. Kenneth C. Dowling, and Dr. Kai Kong Iu, with special thanks to Dr. B. H. Milosavljevic from the Boris Kidrich Institute for Nuclear Science in Belgrade, Yugoslavia, for their stimulating discussions. Appendix I Temperature Dependency of K .
let
rate constant of adsorption = k,, =
[
Z =
-
2::,]‘12
P (2nRTM,)
and 7
rate constant of desorption = kDff= Nooko exp( 9 ) RT
=1 exp($)
ko ZT[02l-
where
[O2Iad= r= Registry No.
[02]-
+ 27
[ 0 2 1 ad [ 0 2 1 -
z([021= - [021ad)
(15)
0, 7782-44-7; pyrene, 129-00-0; 9, IO-diphenyl-
anthracene, 1499-10-1.
Kinetics of the Reactions of IO Radicals with NO and NOp E. P.Daykin and P.H. Wine* Molecular Sciences Branch, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received: October 6, 1989: In Final Form: January 17, 1990)
A laser flash photolysis-long path absorption technique has been employed to study the kinetics of the reactions of IO radicals with NO and NO2 as a function of temperature and pressure. The IO + NO rate coefficient is independent of pressure over the range 40-200 Torr of N 2 , and its temperature dependence over the range 242-359 K is adequately described by exp[(328 f 71)/fl cm3 molecule-’ s-’ (errors are 20, precision only). the Arrhenius expression k , = (6.9 1.7) X These Arrhenius parameters are similar to those determined previously for the CIO + NO and BrO + NO reactions. The IO + NO2 association reaction is found to be in the falloff regime over the temperature and pressure ranges investigated (254-354 K and 40-750 Torr of N2). Assuming F, = 0.4 independent of temperature, a physically reasonable set of falloff parameters which adequately describe the data are ko = 7.7 X IO-”( T/300)-s,0cm6 molecule-2 s-I and k , = 1.55 X lo-” cm3 molecule-’ s-’ independent of temperature. The IO + NO2 rate coefficients determined in this study are about a factor of 2 faster than those reported in the only previous study of this reaction.
*
Introduction The potential role of iodine in tropospheric photochemistry has received considerable attention in recent years. It has been suggested that IO, chemistry can result in catalytic destruction of tropospheric ozone as well as perturbation of the tropospheric cycles of H,O,,,NO,, and sulfur.’-3 Iodine can potentially play a more important role in tropospheric photochemistry than other halogens for two reasons. First, unlike a majority of fluorine, chlorine, and bromine atom precursors, most iodine atom precursors of atmospheric importance are photosensitive at wave-
* Author to whom correspondence should
be addressed
0022-3654/90/2094-4528$02.50/0
lengths (>300 nm) which penetrate to the earth’s surface. Second, reactions of hydrogen-containing species with iodine atoms to form the reservoir species HI are, in general, endothermic and do not occur at atmospheric temperatures. (The I + HOz reaction is ( 1 ) Chameides, W. L.; Davis, D. D. J . Geophys. Res. 1980, 85, 7383. (2) Jenkin, M. E.; Cox, R. A,; Candeland, D. E. J . Amos. Chem. 1985, 2, 359. (3) Barnes, I.; Becker, K. H.; Martin, D.; Carlier, P.; Mouvier, G.; Jour-
dain, J. L.; Laverdet, G.; LeBras, G. In Biogenic Sul/ur in rhe Enuironment; Saltzman, E. S., Cooper, W. J., Eds.; ACS Symposium Series 393; American Chemical Society: Washington, DC, 1989; pp 464-475, and references therein.
0 I990 American Chemical Society
