774
P. E(. WONGAND A. 0. ALLEN
Charge Transfer to Molecules on the Surface of Irradiated Porous Glass'
by P. K. Wong and A. 0. Allen Chemistry Department, Brookhaven National Laboratory, Upton, hTew York 11975
(Received August 1.6, 1969)
Tetracyanoethylene (TCNE) adsorbed on porous Vycor glass forms under y rays at room temperature its radical anion TCNE- with a yield G = 6.5 ions per 100 eV absorbed in the glass, independent of the surface concentration of TCNE. When the glass is first irradiated alone and the TCNE then added, the anion still forms with a yield G = 4.2. Presence of TCNE during irradiation prevents formation of the sharp trappedelectron spin resonance (esr) signal characteristic of irradiated glass, but when TCNE is added to irradiated glass, the trapped-electron signal is not affected,although the signal characteristic of TCYE- appears with an intensity around 5 times that of the trapped electron signal. Thus the electron-donating centers formed on the glass surface are not the trapped electrons seen by esr. Tetra-N-methyl-phenylenediamine (TMPD), triphenylamine, and perylene adsorbed on the glass form under y rays radical cations stable at room temperature. Methylaniline, diethylaniline, and triphenylphosphine yield cations, and acetone, butanone, and acetophenone yield anions when irradiated on the glass at 77'K, but these ions disappear when warmed to room temperature.
The radiation chemistry of surfaces has now been the subject of many publications. The studies involve irradiation of a molecular substance, usually organic, adsorbed on a mineral oxide or halide. The results appear to fall into two classes, depending on whether (a) energy transfer or (b) charge transfer from the mineral to the adsorbed molecules is involved. In the first case, the molecules acquire activation energy from the solid during irradiation which causes their decomposition; the yield of the reaction increases gradually with concentration of molecules on the surface, leveling off only after a complete monolayer is formed, and very little reaction occurs if the solid is irradiated first and the substrate added subsequently. Examples include the radiolysis in the adsorbed state of aliphatic hydrocarbons,2 a ~ o e t h a n e ,and ~ ethan01.~ I n the second case, molecules are chosen which are good electron acceptors or donors, and the reaction products result from a one-electron reduction or oxidation. Here it is generally found that the radiolytic yield remains constant down to very low surface concentrations of the substrate, and considerable yield is in some cases obtained if the solid is irradiated first and the substrate then added. Among the examples are the production of radical anions or cations from benzene, naphthalene, biphenyl, galvinoxyl,6 and oxygen,6 and dealkylation of isopropylbenzene.' Sometimes the reaction follows different courses if the substrate is present during irradiation or is added later; thus GO2gives the radicaI COZ- if present during irradiation but yields the radical 0 2 - if added after the irradiation.8 It thus appears that the surface of silica or similar solids is activated by radiation to a state which can transfer electric charge to a passing molecule if the molecule happens to be readily liable to one-electron oxidation or reduction, but cannot generally activate other molecules. During irradiaThe Journal of Physical Chemistry
tion, however, these active charge-donating centers must frequently be neutralized by further migrating charges continually forming in the material, and the energy of neutralization thus released can activate an adsorbed hydrocarbon or other molecule in the immediate neighborhood and bring about its decomposition. The present work is a study of transfer of both positive and negative charge from irradiated porous Vycor glass. This material was chosen in part because it is known to yield, under irradiation, centers which react with added hydrogen and other s u b s t a n ~ e s , ~ but mainly because its transparency allows accurate quantitative determination of reaction products by optical absorption spectrophotometry.
Experimental Section Materials. N,N,N',N'-Tetramethyl-p-phenylenediamine (TMPD) was purified as described previously. lo Triphenylamine (TPA) from Baker Chemical was purified by crystallization in methanol and sublimation. (1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) (a) J. M. Caffrey and A. 0. Allen, J. Phys. Chem., 62, 33 (1968); (b) J. W. Sutherland and A. 0. Allen, J . Amer. Chem. Soc., 83, 1040 (1961); (c) N. H. Sagert and D. J. Dyne, Can. J . Chem., 45, 616 (1967). (3) J. G. Rabe, B. Rabe, and A. 0. Allen, J . Phys. Chem., 70, 1098 (1966). (4) L. Abrams and A. 0. Allen, ihid., 73, 2741 (1969). ( 5 ) (a) P. K. Wong and J. E. Willard, ibid., 72, 2623 (1968); (b) 0. Edlund, P.-0. Kinell, A. Lund, and A. Shimiau, J . Chem. Phys., 46, 3679 (1967). (6) P. H. Kasai, ihid., 43,3322 (1965). (7) (a) E. A. Rojo and R. R. Hentz, J . Phys. Chem., 70, 2919 (1966); (b) R. R. Hentz and D. K. Wickenden, ihid., 73, 817 (1969). (8) P. K. Wong and J. E. Willard, ihid., 73, 2226 (1969). (9) G. M. Muha and D. J. C. Yates, ibid., 70,1399 (1966). (10) C. Capellos and A. 0. Allen, ibid., 72,4265 (1968).
775
MOLECULES ON IRRADIATED POROUS GLASB Perylene from K and K Laboratories was sublimed twice. N-Methylaniline and N,N'-diethylaniline from Eastman Chemicals were vacuum distilled. Triphenylphosphine (TPP) from Mallinckrodt, acetone, and 2,3-butanedione from Baker Chemicals were used without purification. Tetracyanoethylene (TCNE) from Aldrich Chemical was crystallized from chlorobenzene and twice sublimed according to Merrifield and Phillips.ll The surface area of the porous Vycor glass (No. 7939) from Corning was determined as 140 mS/g by the B E T method using nitrogen.12 The glass plates, 1 X 6 X 35 mm, were thermostated at 600" in a muffle oven before being evacuated from Torr at the same temperature. 12-16 hr to 3-5 X Adsorption. Liquids were adsorbed on the glass plates by bulb-to-bulb distillation. Solids were sublimed a t 80" and the vapor was brought in contact with the glass. The sample preparations were carried out in a grease-free system using Hoke valves with Teflon seats. The Vycor plates were irradiated in Pyrex ampoules to which either Suprasil windows or 4-mm od tubes were attached for optical or esr observation, respectively. The ampoules were positioned so that the Suprasil section was 8-9 in. away from the y source and received very little irradiation. Irradiation. Samples were irradiated at about 23°C or 77°K with a cobalt-60 y source operating at a dose rate of 0.858 Mrad/hr. Spectra. Optical spectra were obtained on a Cary 14 automatic recording spectrophotometer. For lowtemperature measurements, the samples were cooled in a copper cell-holder attached to a brass Dewar filled with liquid nitrogen. For electron spin resonance (esr) observations, we used a Varian Associates 4500 spectrometer equipped with 100-kHz field modulation and a V-4531 multiple purpose cavity. A standard Varian Associates quartz Dewar was used for measurements at 77"IL
Results Addition of Charge Scavengers to Preirradiated Vycor. The irradiated Vycor glass formed a brownish purple color, which has a rising absorption starting a t 360 nm into the uv region. Addition of charge scavengers produced optical bands in the region 400-700 nm; T M P D and TPA produced spectra similar to Figure 1. TCNE on preirradiated Vycor yielded a band closely resembling that of TCNE- generated by reducing the compound with 1,2-diazabicyclo(2,2,2)octane(Figure 2). T M P D and TPA on preirradiated Vycor gave broad and unresolved esr spectra, superimposed on which was a sharp asymmetric line characteristic of trapped electrons in irradiated silica.la When an irradiated sample of Vycor (Figure 3) was exposed to the vapor of TCNE, an extra esr signal appeared on the low-field side of the sharp asymmetric signal (Figure 4). The appearance of the new signal did
I I
II
0.81-
1
I
I
1
IO
70 0
800
Figure 1. Optical spectra of cation radicals on Vycor surface at room temperature: A, TMPD, 0.75 Mrad; B, TPA, 2.38 Mrad. 1.0
0.8
c
0.6
v)
z w
0 -I
5
k0
0.4
0.2
I
\
\ A
'\
\
'.
\
0
300
~
I 400 5 00 WAVELENGTH ( n m )
'\
600
Figure 2. Optical spectra of (TCNE)-at room temperature: A, before irradiation on Vycor; B, after irradiation on Vycor at 0.43 Mrad; C, by reduction with 1,2 diazabicyclo [2,2,2]octane in dioxane.
not affect the original esr line due to trapped electrons. The new signal is attributed to TCNE- and the yield of this species is about five times larger than that of trapped electrons. (11) R. E. Merrifield and W. D. Phillips, J . Amer. Cheh. Soc., 80, 2778 (1958). (12) D. M. Young and A. D. Crowell, "Physical Adsorption of Gases", Butterworth and Co., Ltd., London, 1962,p 150. (13) C. M. Nelson and R. A. Weeks, J . Amer. Ceram. Soc., 43, 399 (1960). VOlt4me 74, Number 1 February 10, 1970
P. K. WONGAND A. 0. ALLEN
776 1
2.0
-
_1
L-
.
I
I
I
I
.
a
1.5
u
n (L
5
+
H
5 G.
-5
1.0
0
.
.
r
Y-
0.5
0
I
20
[AanNE]
Figure 3. Esr spectrum of irradiated Vycor; dose, 0.57 Mrad.
i
I
I 60
I 40
' I
80
, MICZ0!1OLBS
100
120
PC? GRA!,
Figure 5. G values for cation radicals on Vycor as a function of substrate concentration. Dose, 0.75 Mrad: 0, TMPD irradiated on Vycor; 0,TMPD added to preirradiated Vycor; A TPA, irradiated on Vycor; A, TPA added to preirradiated Vycor.
6
i
t---i
5 G.
U Figure 4. Esr spectrum of irradiated Vycor on which TCNE has been adsorbed; dose, 0.57 Mrad.
The yields of the ion radicals (TMPDf, TPA+, and TCNE-) were estimated by their optical absorption. Beer's law gives OD = kc, where c is the concentration of the absorbing material in mol/g and OD is the optical density. The constant k is found for each species by adsorbing a known concentration of the material on Vycor and measuring its optical density. This was accomplished by oxidizing the TMPD and TPA by oxygen or iodine on the surface. A weighed quantity of TMPD or TPA was distilled onto the glass under vacuum; exposure at room temperature to either oxygen or iodine vapor immediately oxidized the material completely. TCNE- was prepared in CC14 according to T ~ r k e v i c h . ' ~When a Vycor plate was immersed in the solution (in air a t room temperature) the TCNEwas spontaneously absorbed into the Vycor. The solvent was then evaporated away and the optical density of the plate was measured. The radiolytic yields of The Journal of Physical Chemistry
ETCNE]
, !IICROEIOLCS
PER GRAM
Figure 6. G values for (TCNE) - on Vycor as a function of TCNE concentration. Dose, 0.75 Mrad: 0, TCNE irradiated on Vycor; 0,TCNE added to preirradiated Vycor.
TMPD f, TPA f, and TCNE- are given in Figures 5 and 6. G(TMPD+) (molecules formed per 100 eV adsorbed by the whole system) varies from 1 to 1.6, depending on the concentration of TMPD. G(TPA+) has a constant value of 0.31, independent of TPA concentration from 20 to 100 pmol/g. G(TCNE-) = 4.5 from 6 to 21 pmol/g of adsorbate. Figure 7 is a plot of TPA concentration against y dose. At a total dose of 7 Mrads, the concentration of TPAf attains 4.4 X 1017/g. The accumulation of ion radicals is independent of initial TPA concentration from 13 to 70 pmol/g. Irradiation of Vycor Containing Adsorbed Material. f
(14) D. N. Stamires and J. Turkevich, J . Amer. Chem. SOC.,85,2657 (1963).
777
MOLECULES ON IRRADIATED POROUS GLASS Table I : Absorption Bands of Cation Radicals c
4
5 t
(nm)Other Vyoor matrices
P X m s x
Species
DOSE (MRADS)
Figure 7. Concentration of (TPA)+ as a function of dose at 0.89 Mrad/hr (room temperature).
522 561 608
5380 Photolysis in 575 3-methylpen632 tane glass at 77'K
550 640
560b y Radiolysis in 640 methyltetrahydrofuran a t 77'K
425 -4450 453 455
Flash photolysis inHzSOc
450 -450b 468 -475
y Radiolysis in
(e&a)
320d Flash photolysis 330 inaqueous alcohol
535
5456 Chemisorption on silica alumina
H
When a substrate was present on the Vycor surface and the combination y-irradiated, colored species were generated which could be detected by the optical or the esr method. T M P D and TPA when irradiated in this manner produced spectra shown in Figure 1. Other oxidizable substrated produced optical bands and esr spectra similar to those of their cation radicals. Table I summarizes the optical results. The esr signals of cation radicals appear as poorly resolved broad spectra plus a sharp asymmetric line due to trapped electrons in the irradiated Vycor. The species attributable to TMPD+, TPA, + and perylene+ were stable at room temperature on Vycor; the rest in Table I were stable at 77"K, but disappeared when brought to room temperature. When electron scavengers were present on Vycor, the irradiated samples produced species that have optical bands and esr signals. Acetone, butanone, biacetyl, and acetophenone irradiated and measured at liquid nitrogen temperature, showed absorption peaks in the visible region. TCNE formed a t room temperature a band shown in Figure 2 and an esr
CClr a t 77°K
330 345 438
_ I
Figure 8. Esr spectra of (TCNE)- on Vycor: dose, 0.57 Mrad.
Preparation
a A. C. Albrecht and W. T. Simpson, J. Arner. Chern. Soc., 77, 4454 (1955). 'JM. Kondo, M. R. Ronayne, J. P. Guarino, and W. H. Hamill, ibid.,86,1297 (1964). c E. J. Land and G. Porter, Trans. Faraday Soc., 59, 2027 (1963). d H. I. Joschek and L. I. Grossweiner, J. Amer. Chem. SOC.,88, 3261. (1966). 8 W. K. Hall, J. Catal., 1, 53 (1962).
Table I1 : Absorption Bands of Anion Radicals r--Xmax, nm-
On Vycor
Inother matrices
[(CH&OI-
730
740"
[CgH$OCHJ-
775
808'
[CBHeCOCHJ
420
440'
[>c.-c
