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Photoionization of naphthalene, trans-stilbene and anthracene adsorbed on the external and internal surfaces of zeolites
I. Photochenz. Pholobiol. A: Chem,
79 (1994) lOf107
103
Photoionization of naphthalene, tram-stilbene and anthracene adsorbed on the external and internal surfaces of zeolites Kai-Kon
Iu, Xinsheng
Liu and J. Kerry Thomas+
Department of Chemistry and Biochemimy, University of Notre Dame, Notre Dame, IN 46556 (USA) (Received
July 19, 1993; accepted November
9, 1993)
Abstract Laser photolysis of anthracene, naphthalene and tins-stilbene on zeolite NaA and in zeolite NaX produces electrons trapped in N%“+ clusters, singlet and triplet excited states and radical cations of the arenes. Only Na, ‘+ is observed in NaX where the arena is located inside the supercage; however, other trapped species, such as Na,‘+, are also observed in NaA where the arene is located solely on the external surface. Thermal decomposition of franr-stilbene in NaX produces an unknown brown product, which shortens the lifetime of the electrons trapped in the sodium ionic clusters. The photoionized electrons from arenes on NaA are trapped by sodium cation clusters inside the sodalite cage next to the surface and close to the arene radical cation. No oxygen quenching of the trapped species, Na,‘+ and Na,3+, in the arene-NaA system is observed due to the exclusion of 0, by the small entry pore of the sodaiite cage (approximately 2.6 A).
2. Experimental section
1. Introduction important materials in petroleum cracking [l], ion exchange [2] and as molecular sieves [3]. They also provide constrained environments for chemical reactions at the molecular level. Reviews on chemical reactions utilizing the constrained character of zeolites are well documented [4]. Recently, studies in our laboratory [5] have shown that several arenes in zeolites can be photoionized to produce the arene cation and trapped electron species which are remarkably similar to those reported in thermal studies. The electron trapping sites are clusters of sodium ions, Nad4+, Nas3+ and Na,‘+, which lead to trapped electrons of the form Naq3+, Na3’+ and Na2+ respectively. Other reports [6] have shown that 3” ray irradiation of zeolites also gives rise to trapped electrons of the type Na43’, Na,” and Na,+. In this paper, we report the photoionization of arene in the internal and on the external surfaces of zeoIites; the data reveal that the photoionized electrons are trapped by the surrounding sodium cations within a short distance (less than 10 A) of the cation. Zeolites
are
fAutbor to whom correspondence
should be addressed.
lOlO-6030/94/$07.00 @ 1994 Elsevier Sequoia. All rights reserved SSDK lOlO-6030(93)03739-4
Zcolites X (NaX, Si/AI= 1.4) and A (NaA, Si/ Al = 1.0) were obtained from Aldrich. Naphthalene and anthracene (gold label), trans-stilbene (96%), n-pentane and cyclohexane (both spectral grade) were also obtained from Aldrich. buns-Stilbene was further recrystallized from MeOH-H,O mixed solvent. The maximum loading of the arene into the zeolite was 1% by weight and this was verified by checking the absorbance of the supernatant. The time-resolved diEuse reflectance apparatus and data treatment have been described in detail previously [5a]. The excitation source was an XeCl excimer laser (308 nm). 3. Results and discussion
Figure 1 shows the transient diffuse reflectance spectra under vacuum (symbol 0) and in air (symbol *) of naphthalene in NaX (Fig. l(a)) and on NaA (Fig. l(b)). Basically, two species are observed in zeolite X under vacuum: the triplet excited state of naphthalene (3Np*, at 410 nm) and the trapped electron (Naa3+, at 550 nm). Under 02, 3Np* and Nad3+ are removed and weaker absorption of the radical cation of naphthalene (NP’+ , at 370,570 and 680 nm) is observed (symbol * in Fig. l(a)). Similar species are produced in
104
K.-K IU
et al. / Photoionization of naphthalene, tram-st&ne
and anthmcene on and in zeolites
*
00
00 * 350
440
530 Wavelength
620
800
710
(id
o.oon 350 (01
I
x
**
** I
I
440
I
I
*
*
*
*NY** I
530 Wavelength
I
620 (nm)
I
I
710
I
8
io -
!O-
IQ-
Q-
OOaOoOO-
***we*** o.oob 1 1 350
* *
*
440
(bl
’
1 5x Wavelength
’ 620 [nml
1 710
0
8
Fig. 1. Time-resolved laser phototysis spectra of zeolite loaded with 1% w/w of naphthalene: (a) NaX; (b) NaA. Spectra were taken 200 ns after the laser flash. Symbol 0 represents a sample under 10m4 Torr; symbol * represents a sample under 250 Torr of oxygen.
Fig. 2. Time-resolved laser photolysis spectra of zeolite loaded with 1% w/w of rnnz.r-stilbene: (a) NaX; (b) NaA. Spectra were taken ZOOns after the laser flash.Symbol0 representsa sample under 10e4 Torr; symbol * represents a sample under 250 Torr of oxygen.
NaA, ia. the excited triplet 3Np* and the trapped electrons. However, the trapped electrons exist in two forms, Na,‘+ and Na3*+, as reported recently [7], where Na,‘+ shows a broad absorption band with a maximum at 640 nm. Under 250 Torr of O2 the triplet and some of the trapped electrons are removed, while the weak absorption at 370 nm is indicative of the arene cation. The broad trapped electron band tends to obscure much of the naphthalene species. Figure 2 shows similar studies with the arene Puns-stilbene. Again, the spectrum in NaX under 250 Torr of O2 shows only the rramr-stilbene radical cation (t-SW+, at 480 and 710 nm), while the spectrum under vacuum also shows the trapped electron (Nad3 , at 550 nm). A weak absorption at 360 nm is attributed to the triplet excited state
of trans-stilbene (3t-Sb*). Again fast photolysis of t-Sb on NaA is similar to Fig. l(b), namely, t-+ ‘Mb* and two types of trapped electron $,;+ and Na3’+) are inferred from the spectrum. It is important to note that both two-photon and one-photon ionization of the arene occur in the above systems. Evidence of the one-photon ionization of the arene-zeolite system has been reported [5,8]. One-photon ionization ls illustrated by the linear dependence of the cation yield on the laser power, and confirmed by an additional method involving the decrease in the ‘arene* lifetime relative to the laser flash. This was achieved using a high pressure of 02, whcrc the ‘arene* lifetime was reduced to less than 0.2 ns (oxygen quenching rate of pyrene fluorescence is (1.32f 0.05) x lo7 Torr-’ s-’ in zeolite NaX [9]),
l
K-K
III et al. I Photoionization of naphthnlene.
thus markedly reducing the possibility that, during the pulse, intermediates will absorb a second photon leading to ionization [5]. AI1 spectra (with and without 0,) in both Figs. 1 and 2 indicate a significant amount of one-photon ionization of the arene. The spectral absorption of Np’+ and t-Sb’+ is significant at 500-600 nm, which clouds a clear interpretation of the trapped electron spectrum, particularly in the case of NaA where several trapped species coexist. Therefore another arene, anthracene, in which the triplet and cation exhibit little absorption at around 500-600 nm, was used to overcome these difficulties. Figure 3 shows the spectra of anthracene on NaA with and without 250 Torr of 02. Basically, the spectrum again shows
o.oo+ 400
480
560 Wavelength
(0 1
640 hm)
720
800
l.OO-
0.75
-
0.50.. .. t” ,,+ .A:,..,
tram-stilhene and anthmcene on and in zeolites
three transient species, 3An* (410 nm), Ati+ (710 run) and a broad band between 480 and 650 run due to the trapped electrons. In recent studies of time-resolved pulse radiolysis [7c], two trapped electron species, Nad3+ and Nas2+, were produced in NaA. The species Na3’+ is quenched by oxygen with a quenching rate constant of 26.08 k6.42 Torr-’ s-r. However, in the present photolysis study, little oxygen quenching of this species is observed (see Fig. 3(b)). The trapped electron signal at 650 run exhibits very similar decay rates in 710 Torr of oxygen (k = 536 s-l) and in vacuum (k=352 s-l) (the decay rates were obtained by curve fitting using the gaussian distribution model [lo]). The decrease in the yield of the trapped electron in oxygen (see Fig. 3(b)) is due to the elimination of the two-photon ionization process (see Scheme 1). The time-resolved pulse radiolysis data [7c] indicate two different locations for the two trapped electron species, Na,3+ and Na32C, in NaA, Nag3+ resides inside the sodalite cage and is not accessible to o gen due to the small entry pore of the cage (2.6 4 ), while Na,‘+ resides inside the tiage and is accessible to oxy en due to the larger entry pore of the cage (4 x ). The present photolysis data, for which arene molecules are located on the external surface of NaA, indicate that oxygen does not quench the Nas2+ species. Oxygen quenching requires close contact of oxygen with the excited arene; any blocking of the small entry pore of the rr-cage (4 A) will inhibit the oxygen quenching. For these reasons, special care has been taken to avoid blocking the entry apertures by high anthracene loading or by any residual solvent, i.e. n-pentane. The loading of anthracene was therefore limited to 2x lo-’ mol g -’ on NaA, which corresponds to a separation of 80 8, between two anthracene molecules, and cyclohexane was used as the solvent. The larger cyclohexane molecule does not penetrate into the internal cages due to size exclusion. Nevertheless, little kinetic quenching of the trapped species (see
i’‘~.,,:-: ,”......,,., ‘..” 0.25-
o.oo+0 Ibl
‘f-: ..,._i...7”,.,.;. i,..c
:
_:
I 0.4
I
1 I 1 0.8 1.2 Timelmiliiseconds)
Arenet+ :..,‘r..~‘i .‘i” .... :i
1 I.6
105
e- (Trowed
.,..._.:. . .P.
2
Fig. 3. (a) Time-resolved laser photolysis spectra of zcolite A loaded with 2x 10m7mol-‘g-’ of anthracene. Spectrawere taken 200 ns after the laser flash. Symbol 0 represents a sample under lo-’ Torr; symbol + represents a sample under 2.50 Torr of oxygen. (b) Decay signals at 650 nm with (dotted line) and without (full line) 710 Torr of oxygen.
AWle NONACTIVE
AWIM SITE
ACTIVE
SITE
Scheme 1. The hvo-photon ionization process.
by No+
K-K Iu d aL I Phoroionizatian of naphthaiene, tram-stilbene and anthracene on and in zeolites
106
Fig. 3(b)) in the An-NaA system by oxygen was observed. The photon energy at 308 nm is 4.02 eV and the ionization energy for anthracene in the gas phase is 7.37 eV [Sal; therefore the kinetic energy of the two-photon ionization will be low (the potential energy of the lowest singlet excited state of anthracene is 3.35 eV (last absorption peak of anthracene, 370 nm); therefore, the total energy of two absorbed photons is 7.37 eV (3.35 + 4.02 eV) for anthracene), and this low-energy electron ejected from the arene is trapped by the sodium cluster close to the arene radical cation as shown in Scheme 1. The sodalite cage of NaA is cubic packed [ll] and one a-cage is surrounded by eight smallersized sodalite cages as shown in Fig. 4. Arene molecules are adsorbed on the external surface of NaA due to exclusion by the small entry pore of the a-cage (4 A). The photoionized electrons of the arene will first travel through the surface’s sodalite cage in order to penetrate into the internal surface of NaA. Due to the low kinetic energies, these photoionized electrons are all trapped within the sodalite cage to form Na43+ or NaJZ+. The formation of both Na32+ and Na,3+ clusters inside the sodalite cage of zeolite by high-energy radiation has been reported previously [7]. The lifetime of the trapped electron in arene-NaX systems is affected by several factors, such as the nature of the arene cation radical, the incubation time of the samples, impurities and the residual solvent present in the supercage. Figure 5 gives, as an example, the decay signals of the trapped electrons in the t-Sb-NaX and Np-NaX systems, showing the effect of arene cation radicals on the lifetime. A significantly shorter lifetime is observed in the r-Sb-NaX system than in the Np-NaX system. Because the zeolite surface . “reactive” for the arenes of low ionization Fotential, prolonged incubation of samples causes decomposition, and creates species that efficiently quench trapped electrons. For this reason, samples
1.00
0.75 .z : 2 ‘Z Q n
0.25
.; 0.00 0.50
a .0
I
20
I
I
I
J
60 40 Time (microseconds)
I
I
80
0
Fig. 5. Time-resoled transient signals of electrons trapped in sodium ionic clusters of NaX. The broken line represents a sample of NaX containing 1% w/w of naphthalene; the full line represents a sample of NaX containing 1% w/w of tranr-stilbene.
of &Sb-Nax incubated for extended times exhibit faster decays of the trapped electrons. The formation of a new species with a brown color and a broad absorption at 450 nm is associated with enhanced electron decay. An earlier report [8] has indicated that the photoexcitation of Np and t-Sb in zeolite NaX leads to arene cations, but no trapped electrons were observed. There are many reports, using different techniques, which all clearly show that electrons are trapped in Na+ clusters in zeolites. The present work also shows the presence of trapped electrons on excitation of Np and t-Sb in zeolites. The conflict with ref. 8 can be partly explained by the observation of reactive products produced thermally in r-Sb-zeolites, and by the presence of a trace amount of oxygen in the arene-NaX system where no trapped electrons in the form of sodium ionic clusters are observed particularly when the spectra are recorded at a long time scale (millisecond).
4. Conclusions
Sodolite
cage
Fig. 4. Schematic drawing of the structure of zeolite A.
The photoionization of arenes, such as naphthalene, anthracene and tmns-stilbene, in zeolite NaX and on zeolite NaA is observed. The ionization products are the radical cations of the arenes and trapped electrons (e.g. Na_,‘+ in NaX, Na_,3+ and Na3’+ in NaA). The thermal decomposition of arenes with unsaturated side chains such as frans-
K-K
lu et al I Photoionization of naphthakne,
stilbene, shortens the lifetime of the trapped electron. Photoionization of arenes on the external surface of NaA results in electron trapping, which takes place exclusively inside the sodalite cage, and due to the low kinetic energy of the photoionized electron the electron locates near to the ionized arene. In these systems, oxygen accessibility of the sodium ionic cluster is eliminated by the small entry pore of the sodalite cage (approximately 2.6 A).
Acknowledgment We wish to thank the National Science Foundation for financial support of this work.
References 1 (a) J.A. Rabo (ed.), Zeokte Chem&y and Cot&s& ACS Monograph 171. Washington DC, 1976. (b) R.M. Barrer, Hjdrothennal Chetitty of Zeolites, Academic Press, New York, 1982.
hunts-stilbene and anthmcene on and in zeolites
107
2 W.H. Flank and T.E. Wbyte Jr. (eds.), Perspectives in Mokular Sieve Science, ACS Syuuxxium Series 368, 1988. 3 (a) D.W. Breck, Zeoiite‘Molecular Sieves, %Iey, New York, 1974. (b) J.R. Katzer led.), MoZecularSiewsll ACSSvmposium - Series.40, Washington CC, 1977. 4 (a)N.J.Turro,P Chem.,59(1986)1219,andreferences cited therein. (b) L. Kevan, Rev. Chem. Zntertned., 8 (1987) 53. (c) V. Ramamurthy (ed.), PhorochemLrtry in Otgonized and Constmined Media, VCH, New York, 1991. (d) J.K. Tbomag Chem. Rev., 93 (1993) 301. 5 (A) K.-K. Iu and J.K. Thomas, J. phys. Chem., 95 (1991) 506. (b) K.-K Iu and J.K. Thomas, Co/./& Sur$, 63 (1992) 39. 6 (a) P.H. Kasai, I. Chem Phys., 43 (1965) 3322. (b) J.A. Rabo, C.L. AngeU, P.H. Kasai and V. Schomaker, Discu.w. Fumday Sot., 41 (1966) 328. (c) G.A. Kuranora, High Energy Chem., 2.5 (1991) 91. 7 (a) X. Liu and J.K. Thomas, Lungmuir, 8 (1992) 1750. (b) X. Liu and J.K. Thomas, Chem. Phys. Lett., 192 (1992) 555. (c) K-K Iu, X. Liu and J.K Thomas, 1. Phys. Chem., 97 (1993) 8165. 8 F. Gassner and J.C. Sciano,J. Photochem. PholobioL A: Chem., 67 (1992) 91. 9 X. Liu, K-K. Iu and J.K Thomas, J. Phys. Chem., 93 (1989) 4120. 10 (a) W.J. Albery, P.N. Bartlett, C.P. Wilde and J.R. Darwent, J. Am Chem Sot., 107 (1985) 1854. (b) R.K Krasnansky, K. Koike and J.K. Thomas, J. whys. Chenz., 94 (1990) 4521. (c) R.B. Draper and M.A. Fox, Lmngmuir, 6 (1990) 1396. 11 D.W. Breck, Zeoiite Molecular Sieves, Wiley, New York, 1974, p. 83.
79 (1994) lOf107
103
Photoionization of naphthalene, tram-stilbene and anthracene adsorbed on the external and internal surfaces of zeolites Kai-Kon
Iu, Xinsheng
Liu and J. Kerry Thomas+
Department of Chemistry and Biochemimy, University of Notre Dame, Notre Dame, IN 46556 (USA) (Received
July 19, 1993; accepted November
9, 1993)
Abstract Laser photolysis of anthracene, naphthalene and tins-stilbene on zeolite NaA and in zeolite NaX produces electrons trapped in N%“+ clusters, singlet and triplet excited states and radical cations of the arenes. Only Na, ‘+ is observed in NaX where the arena is located inside the supercage; however, other trapped species, such as Na,‘+, are also observed in NaA where the arene is located solely on the external surface. Thermal decomposition of franr-stilbene in NaX produces an unknown brown product, which shortens the lifetime of the electrons trapped in the sodium ionic clusters. The photoionized electrons from arenes on NaA are trapped by sodium cation clusters inside the sodalite cage next to the surface and close to the arene radical cation. No oxygen quenching of the trapped species, Na,‘+ and Na,3+, in the arene-NaA system is observed due to the exclusion of 0, by the small entry pore of the sodaiite cage (approximately 2.6 A).
2. Experimental section
1. Introduction important materials in petroleum cracking [l], ion exchange [2] and as molecular sieves [3]. They also provide constrained environments for chemical reactions at the molecular level. Reviews on chemical reactions utilizing the constrained character of zeolites are well documented [4]. Recently, studies in our laboratory [5] have shown that several arenes in zeolites can be photoionized to produce the arene cation and trapped electron species which are remarkably similar to those reported in thermal studies. The electron trapping sites are clusters of sodium ions, Nad4+, Nas3+ and Na,‘+, which lead to trapped electrons of the form Naq3+, Na3’+ and Na2+ respectively. Other reports [6] have shown that 3” ray irradiation of zeolites also gives rise to trapped electrons of the type Na43’, Na,” and Na,+. In this paper, we report the photoionization of arene in the internal and on the external surfaces of zeoIites; the data reveal that the photoionized electrons are trapped by the surrounding sodium cations within a short distance (less than 10 A) of the cation. Zeolites
are
fAutbor to whom correspondence
should be addressed.
lOlO-6030/94/$07.00 @ 1994 Elsevier Sequoia. All rights reserved SSDK lOlO-6030(93)03739-4
Zcolites X (NaX, Si/AI= 1.4) and A (NaA, Si/ Al = 1.0) were obtained from Aldrich. Naphthalene and anthracene (gold label), trans-stilbene (96%), n-pentane and cyclohexane (both spectral grade) were also obtained from Aldrich. buns-Stilbene was further recrystallized from MeOH-H,O mixed solvent. The maximum loading of the arene into the zeolite was 1% by weight and this was verified by checking the absorbance of the supernatant. The time-resolved diEuse reflectance apparatus and data treatment have been described in detail previously [5a]. The excitation source was an XeCl excimer laser (308 nm). 3. Results and discussion
Figure 1 shows the transient diffuse reflectance spectra under vacuum (symbol 0) and in air (symbol *) of naphthalene in NaX (Fig. l(a)) and on NaA (Fig. l(b)). Basically, two species are observed in zeolite X under vacuum: the triplet excited state of naphthalene (3Np*, at 410 nm) and the trapped electron (Naa3+, at 550 nm). Under 02, 3Np* and Nad3+ are removed and weaker absorption of the radical cation of naphthalene (NP’+ , at 370,570 and 680 nm) is observed (symbol * in Fig. l(a)). Similar species are produced in
104
K.-K IU
et al. / Photoionization of naphthalene, tram-st&ne
and anthmcene on and in zeolites
*
00
00 * 350
440
530 Wavelength
620
800
710
(id
o.oon 350 (01
I
x
**
** I
I
440
I
I
*
*
*
*NY** I
530 Wavelength
I
620 (nm)
I
I
710
I
8
io -
!O-
IQ-
Q-
OOaOoOO-
***we*** o.oob 1 1 350
* *
*
440
(bl
’
1 5x Wavelength
’ 620 [nml
1 710
0
8
Fig. 1. Time-resolved laser phototysis spectra of zeolite loaded with 1% w/w of naphthalene: (a) NaX; (b) NaA. Spectra were taken 200 ns after the laser flash. Symbol 0 represents a sample under 10m4 Torr; symbol * represents a sample under 250 Torr of oxygen.
Fig. 2. Time-resolved laser photolysis spectra of zeolite loaded with 1% w/w of rnnz.r-stilbene: (a) NaX; (b) NaA. Spectra were taken ZOOns after the laser flash.Symbol0 representsa sample under 10e4 Torr; symbol * represents a sample under 250 Torr of oxygen.
NaA, ia. the excited triplet 3Np* and the trapped electrons. However, the trapped electrons exist in two forms, Na,‘+ and Na3*+, as reported recently [7], where Na,‘+ shows a broad absorption band with a maximum at 640 nm. Under 250 Torr of O2 the triplet and some of the trapped electrons are removed, while the weak absorption at 370 nm is indicative of the arene cation. The broad trapped electron band tends to obscure much of the naphthalene species. Figure 2 shows similar studies with the arene Puns-stilbene. Again, the spectrum in NaX under 250 Torr of O2 shows only the rramr-stilbene radical cation (t-SW+, at 480 and 710 nm), while the spectrum under vacuum also shows the trapped electron (Nad3 , at 550 nm). A weak absorption at 360 nm is attributed to the triplet excited state
of trans-stilbene (3t-Sb*). Again fast photolysis of t-Sb on NaA is similar to Fig. l(b), namely, t-+ ‘Mb* and two types of trapped electron $,;+ and Na3’+) are inferred from the spectrum. It is important to note that both two-photon and one-photon ionization of the arene occur in the above systems. Evidence of the one-photon ionization of the arene-zeolite system has been reported [5,8]. One-photon ionization ls illustrated by the linear dependence of the cation yield on the laser power, and confirmed by an additional method involving the decrease in the ‘arene* lifetime relative to the laser flash. This was achieved using a high pressure of 02, whcrc the ‘arene* lifetime was reduced to less than 0.2 ns (oxygen quenching rate of pyrene fluorescence is (1.32f 0.05) x lo7 Torr-’ s-’ in zeolite NaX [9]),
l
K-K
III et al. I Photoionization of naphthnlene.
thus markedly reducing the possibility that, during the pulse, intermediates will absorb a second photon leading to ionization [5]. AI1 spectra (with and without 0,) in both Figs. 1 and 2 indicate a significant amount of one-photon ionization of the arene. The spectral absorption of Np’+ and t-Sb’+ is significant at 500-600 nm, which clouds a clear interpretation of the trapped electron spectrum, particularly in the case of NaA where several trapped species coexist. Therefore another arene, anthracene, in which the triplet and cation exhibit little absorption at around 500-600 nm, was used to overcome these difficulties. Figure 3 shows the spectra of anthracene on NaA with and without 250 Torr of 02. Basically, the spectrum again shows
o.oo+ 400
480
560 Wavelength
(0 1
640 hm)
720
800
l.OO-
0.75
-
0.50.. .. t” ,,+ .A:,..,
tram-stilhene and anthmcene on and in zeolites
three transient species, 3An* (410 nm), Ati+ (710 run) and a broad band between 480 and 650 run due to the trapped electrons. In recent studies of time-resolved pulse radiolysis [7c], two trapped electron species, Nad3+ and Nas2+, were produced in NaA. The species Na3’+ is quenched by oxygen with a quenching rate constant of 26.08 k6.42 Torr-’ s-r. However, in the present photolysis study, little oxygen quenching of this species is observed (see Fig. 3(b)). The trapped electron signal at 650 run exhibits very similar decay rates in 710 Torr of oxygen (k = 536 s-l) and in vacuum (k=352 s-l) (the decay rates were obtained by curve fitting using the gaussian distribution model [lo]). The decrease in the yield of the trapped electron in oxygen (see Fig. 3(b)) is due to the elimination of the two-photon ionization process (see Scheme 1). The time-resolved pulse radiolysis data [7c] indicate two different locations for the two trapped electron species, Na,3+ and Na32C, in NaA, Nag3+ resides inside the sodalite cage and is not accessible to o gen due to the small entry pore of the cage (2.6 4 ), while Na,‘+ resides inside the tiage and is accessible to oxy en due to the larger entry pore of the cage (4 x ). The present photolysis data, for which arene molecules are located on the external surface of NaA, indicate that oxygen does not quench the Nas2+ species. Oxygen quenching requires close contact of oxygen with the excited arene; any blocking of the small entry pore of the rr-cage (4 A) will inhibit the oxygen quenching. For these reasons, special care has been taken to avoid blocking the entry apertures by high anthracene loading or by any residual solvent, i.e. n-pentane. The loading of anthracene was therefore limited to 2x lo-’ mol g -’ on NaA, which corresponds to a separation of 80 8, between two anthracene molecules, and cyclohexane was used as the solvent. The larger cyclohexane molecule does not penetrate into the internal cages due to size exclusion. Nevertheless, little kinetic quenching of the trapped species (see
i’‘~.,,:-: ,”......,,., ‘..” 0.25-
o.oo+0 Ibl
‘f-: ..,._i...7”,.,.;. i,..c
:
_:
I 0.4
I
1 I 1 0.8 1.2 Timelmiliiseconds)
Arenet+ :..,‘r..~‘i .‘i” .... :i
1 I.6
105
e- (Trowed
.,..._.:. . .P.
2
Fig. 3. (a) Time-resolved laser photolysis spectra of zcolite A loaded with 2x 10m7mol-‘g-’ of anthracene. Spectrawere taken 200 ns after the laser flash. Symbol 0 represents a sample under lo-’ Torr; symbol + represents a sample under 2.50 Torr of oxygen. (b) Decay signals at 650 nm with (dotted line) and without (full line) 710 Torr of oxygen.
AWle NONACTIVE
AWIM SITE
ACTIVE
SITE
Scheme 1. The hvo-photon ionization process.
by No+
K-K Iu d aL I Phoroionizatian of naphthaiene, tram-stilbene and anthracene on and in zeolites
106
Fig. 3(b)) in the An-NaA system by oxygen was observed. The photon energy at 308 nm is 4.02 eV and the ionization energy for anthracene in the gas phase is 7.37 eV [Sal; therefore the kinetic energy of the two-photon ionization will be low (the potential energy of the lowest singlet excited state of anthracene is 3.35 eV (last absorption peak of anthracene, 370 nm); therefore, the total energy of two absorbed photons is 7.37 eV (3.35 + 4.02 eV) for anthracene), and this low-energy electron ejected from the arene is trapped by the sodium cluster close to the arene radical cation as shown in Scheme 1. The sodalite cage of NaA is cubic packed [ll] and one a-cage is surrounded by eight smallersized sodalite cages as shown in Fig. 4. Arene molecules are adsorbed on the external surface of NaA due to exclusion by the small entry pore of the a-cage (4 A). The photoionized electrons of the arene will first travel through the surface’s sodalite cage in order to penetrate into the internal surface of NaA. Due to the low kinetic energies, these photoionized electrons are all trapped within the sodalite cage to form Na43+ or NaJZ+. The formation of both Na32+ and Na,3+ clusters inside the sodalite cage of zeolite by high-energy radiation has been reported previously [7]. The lifetime of the trapped electron in arene-NaX systems is affected by several factors, such as the nature of the arene cation radical, the incubation time of the samples, impurities and the residual solvent present in the supercage. Figure 5 gives, as an example, the decay signals of the trapped electrons in the t-Sb-NaX and Np-NaX systems, showing the effect of arene cation radicals on the lifetime. A significantly shorter lifetime is observed in the r-Sb-NaX system than in the Np-NaX system. Because the zeolite surface . “reactive” for the arenes of low ionization Fotential, prolonged incubation of samples causes decomposition, and creates species that efficiently quench trapped electrons. For this reason, samples
1.00
0.75 .z : 2 ‘Z Q n
0.25
.; 0.00 0.50
a .0
I
20
I
I
I
J
60 40 Time (microseconds)
I
I
80
0
Fig. 5. Time-resoled transient signals of electrons trapped in sodium ionic clusters of NaX. The broken line represents a sample of NaX containing 1% w/w of naphthalene; the full line represents a sample of NaX containing 1% w/w of tranr-stilbene.
of &Sb-Nax incubated for extended times exhibit faster decays of the trapped electrons. The formation of a new species with a brown color and a broad absorption at 450 nm is associated with enhanced electron decay. An earlier report [8] has indicated that the photoexcitation of Np and t-Sb in zeolite NaX leads to arene cations, but no trapped electrons were observed. There are many reports, using different techniques, which all clearly show that electrons are trapped in Na+ clusters in zeolites. The present work also shows the presence of trapped electrons on excitation of Np and t-Sb in zeolites. The conflict with ref. 8 can be partly explained by the observation of reactive products produced thermally in r-Sb-zeolites, and by the presence of a trace amount of oxygen in the arene-NaX system where no trapped electrons in the form of sodium ionic clusters are observed particularly when the spectra are recorded at a long time scale (millisecond).
4. Conclusions
Sodolite
cage
Fig. 4. Schematic drawing of the structure of zeolite A.
The photoionization of arenes, such as naphthalene, anthracene and tmns-stilbene, in zeolite NaX and on zeolite NaA is observed. The ionization products are the radical cations of the arenes and trapped electrons (e.g. Na_,‘+ in NaX, Na_,3+ and Na3’+ in NaA). The thermal decomposition of arenes with unsaturated side chains such as frans-
K-K
lu et al I Photoionization of naphthakne,
stilbene, shortens the lifetime of the trapped electron. Photoionization of arenes on the external surface of NaA results in electron trapping, which takes place exclusively inside the sodalite cage, and due to the low kinetic energy of the photoionized electron the electron locates near to the ionized arene. In these systems, oxygen accessibility of the sodium ionic cluster is eliminated by the small entry pore of the sodalite cage (approximately 2.6 A).
Acknowledgment We wish to thank the National Science Foundation for financial support of this work.
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hunts-stilbene and anthmcene on and in zeolites
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