J . Phys. Chem. 1991, 95, 506-509
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S l n g b M o n Ionization of Pyrene and Anthracene Wing Trapped Electrons In AlksMiWaltaCCatExchanged Zedltes X and V. A Direct RmeReedved Diffuse Reflectance Study Kai-Kong Iu and J. K. Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: September 13, 1990)
Time-resolved diffuse reflectance spectroscopy is used to investigate the photolysis of anthracene- or pyrenoimpregnated alkali-metalation-exchanged zeolite X and Y with a nitrogen laser (337 nm). The studies suggest that anthracene or pyrene molecules are distributed in at least two different sites (active and nonactive) in the zeolites X and k'. Excitation of anthracene or pyrene adsorbed in nonactive sites produces singlet excited states of anthracene or pyrene, which can either be photoionized by absorbing a second photon or decay to the triplet excited states through intersystem crossing; however, anthracene or pyrene adsorbed in active sites is photoionized through a single-photon process. The products of the photoionization are cation radicals of arene and trapped electrons (e.g., N a l + for Nay). The broad absorption band for the trapped electron is around 550 nm for both NaX and Nay: around 620 nm for both LiX and LiY and around 710 nm for KX.
Introduction Zeolites are used extensively in catalysis, petroleum refining, water treatment, and ionic and molecular sieving. In contrast to the amorphous nature of most adsorbents and solid catalysts, zeolites exhibit unique uniforr. i *iicframeworks with pores and tunnels (3-8 A).l-5 Several ;e, arts on the remarkable effects of constrained zeolites on various photochemical and photophysical processes have bcm do:um:nted.6 The possible formation of ar. ionic cluster inside the zeuite, which could be used in catalysis process, is also of importance. Kasai first identified by ESR the trapped electron as Na43+in NaY and as Nab5+in NaX, after the y- or X-ray irradiation.' Recently, Koch? et al. have reported a high yields of Na43+in NaX following treatment of the dehydrated NaX with a solution of n-butyllithium-hexane at room temperature; the trapped species (e+, Na43+or K43+)9are also found in exposure of the zeolite to metal vapor at high temperature. We have previously reported some photophysical properties of pyrene in zeolites, such as surface polarities, dimer formation, Cu2+,TI+, and oxygen quenching of singlet excited pyrene and coadsorbed water effects on the reactions of 'pyrene*.lo In this report, we employ time-resolved diffuse reflectance spectroscopy to identify the short-lived intermediates in the photolysis of anthracene and pyrene in zeolites. Experimental Section Zeolites and chemicals: Sodium-exchanged zeolite X (Si/Al = 1.4) and Y (Si/AI = 2.5) were obtained from Aldrich and UOP, respectively. Lithium and potassium chloride (ACS certified grade) were obtained from Fisher. Pyrene (99%), anthracene (99.9%), ar?d n-pentane (anhydrous, 99+%) were ordered from Aldrich. The purification of pyrene had been described.lh All other chemicals were used as received. Zec !ite sump'?! preparafion: Lithium- and potassium-exchanged zeolites wet ,prc+red from the sodium-exchanged form via ion exchange with a stock solution of the chlorides of the metal. No attempt was made to verify the degree of ion exchange." The loading method of anthracene or pyrene into the zeolite had been described.1° The arene concentration was less than 1 X 10-6 mol/g, and all samples were contained in 2-mm beam path standard quartz cells. Time-resolved diffused reflectance measurement: A detailed description of the diffuse reflectance experimental setup has been reported.I2 Except for the laser intensity study, all data were recorded with a laser (337 nm) intensity of 4 mJ/cm*, at which bleaching of the ground-state pyrene does not occur (see Figure 4B). Unlike the solution-phase work, where the concentration * T o whom correspondence should be addressed.
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of the transience can be simply described by Lambert-Beer law, the concentration of the absorption species in the diffuse reflectance experiment follows the K&elke-Munk equation." However, the Kubelka-Munk equation requires R,, which is the sample reflectance measured against a standard materia! (e& MgO). For the sake of experimental simplicity, we adoptw Bn approach by Wilkinson et al.I4 in which the percentage of absorption (1 R,) is employed to descrik the absorbed species concentration. R, is equal to the reflectivity with the laser excitation divided by the reflectivity without the laser excitation ( R , = J,/Jo).
Results and Dlsclwsions Photolysis of arene in dehydrated alkali-metd-cation-exchanged zeolites: We have detected directly the transient optical absorption signal of the Nal)+ in NaY following a laser flash excitation of anthracene-impregnated zeolites. Figure 1 B shows a broad absorption band (symbol X with solid line) centered a t 550 nm in a NaY sample that is similar to Kasai's report of a Na43+ absorption band in y-irradiated The transient diffuse reflectance spectra of Li+-, Na+-, and K+-exchanged form of zeolite X and Y samples are also included in parts A and B of Figure 1, respectively. All spectra were taken at 100-ps full (1) Breck, D. W. Zedire Mdecular Sieves; Wiley: New York, 1974. (2) Katzcr, J. R., Ed. Molecular Sieves 11. ACT Symp. Ser. 1977. No. 40. (3) Rabo, J. A., Ed. Zeolite Chemistry and Catalysis; ACS Monograph Ser. 1976, 171. (4) Barrer, FRS, R. M. Hydrorhermal Chemistry of Zeolires; Academic Press: New York, 1982. (5) Flank, W. H., Whyte, Jr., T. E., Eds. Perspectives in Molecular Sieve Science. ACS Symp. Ser. 1988, No. 368. (6) (a) Turro, N . J. fure Appl. Chem. 1986,59, 1219-1228. (b) Kevan, L. Rev. Chem. Inrermed. 1987, 8. 53-58 and references therein. (7) (a) Kasai, P. H. J . Chem. fhys. 1965,43, 3322-3327. (b) Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Schomaker, V. Discuss. Faraday Soc. 1966, 41. 328-349. (8) Yoon, K. 8.; Kochi, J. K. J . Chem. Soc., Chem. Commun. 1988, 510-511. (9) (a) Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Schomaker, V. Dlscuss. Farado- SOC.1966. 41. 328-349. Ib) Harrison. M. R.: Edwards. P. P.: Klinow&i, J.; Thomas, J.'M.; Johnson,-D.C.; Page, C.J. J . kolid Srare Chem:
.---
1984.51. 330-341. . ..,. -
(IO) (a) Liu, X.; lu, K.-K.; Thomas, J. K. J . fhys. Chem. 1989. 93, 4120-4128. (b) lu, K.-K.; Thomas, J. K. Langmuir 1990,6,471-478. ( 1 1) Zccording to the technical note on ion exchange in molecular sieves supplied by the UOP, the procedure we used should provide at least 90% exchange. (12) Manuscript submitted to Iler's memorial symposium at 200th ACS National Meeting in Washington, D.C., 1990. (13) Kubelka, P.; Munk, F. Z . Tech. Phys. 1931, 12, 593-604. (14) Oelkrug. D.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J . Chem. Soc., Faraday Trans. 2 1987.83, 2081-2095.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 507
Letters SCHEME I An" or Py"
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Figure 1. Diffuse reflectance spectra of dehydrated anthracene-impregnated alkali-metalation-exchanged zeolite immediately after the laser excitation: (A) zeolite X samples under vacuum ( IV3Torr); (B) zeolite Y samples under vacuum Torr). ( X ) Na-exchanged zeolite; (0) Li-exchanged zeolite; (*) K-exchanged zeolite.
time scale (see Figure 3A), where the maximum signal is reached. From the experimental finding, we also observe 3An* (420 nm), An'+ (720 nm), and a broad absorption band of sodium ionic cluster (-550 nm) for both sodium-exchanged zeolites X and Y. The assignment of these absorption bands agrees with those in the l i t e r a t ~ r e . ~ *. A , ~new ~ * ~broad ~ absorption band around 620 nm was observed for both LiX and LiY. The existence of An" is confirmed by the observation of an An'+ spectrumi6under 300 Torr of oxygen (Figure 2A, open circles) when "n* and ionic cluster absorption bands are removed. Because of the less efficiency at 720 nm of the monochromator grating, we chose pyrene-a molecule that not only has a gas-phase ionization potential slightly higher than anthracene (7.55 vs 7.40 eV)17 but also exhibits a cation radical absorption at 450 nm, where the (IS) Kikuchi, K. JOEM Handbook I, Triplet-triplet Absorption Spectra; Bunrhin: Tokyo, 1989; pp SI-57. ( 16) Shida, T. Physicul Sciences Data 34, ElecrronfcAbsorption Spectra
of Radlcal Ions; Elsevier: New York, 1988;p 69. (17) Birks, J. B. Photophystcs ofrlromatic Molecules; Wiley: New York, 1970 p 6 9 .
Wavelength
(
nm 1
Figure 2. Diffuse reflectance spectra of dehydrated anthracene- (A) and pyrene- (B) impregnated NaX immediately after the laser excitation. Torr); (0)under 300 Torr of oxygen. (X) under vacuum
efficiency of the grating is higher. Figure 2B shows a clean pY.+ spectrumi8 under 300 Torr of oxygen (open circles) that further supports the photoionization of both anthracene and pyrene in zeolite X and Y. For the pyrene sample under vacuum, we also observed '4r* (415 nm),I9 Py" (450 nm),I* and N a t + (550 nm)," which is similar to the result of the anthracene sample. Since the gas-phase ionization potential of pyrene is slightly higher than anthracene (vide supra), photolysis of anthracene samples should follow the same fashion as the pyrene samples. Figure 3 shows the time-resolved diffuse reflectance signals of pyrene-impregnated NaX at 415,450, and 550 nm at fast (Figure 3A) and slow (Figure 3B) time scales. Figure 3C is the formation of the 550-nm reflectivity signal (Nad3+)of a pyrenaimpregnated NaX sample where the formation rate of this signal follows the excitation laser flash (10 ns). Figure 4 shows an oxygen pressure study (panel A) and laser intensity study at 300 Torr of oxygen (panel B) of the yield of the pyrene cation radical. From the result, we observe the pyrene cation yield at oxygen pressures higher than 300 Torr remains constant, which indicates that a single-photon (18) Reference 16, pp 85-86. (19) Reference 15, pp 153-156.
Letters
508 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 1.N
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pyrene cation radical yield of pyrene-impregnated NaX. The reflectivity of the sample at 450 nm under 300 Torr of oxygen is plotted against the laser intensity.
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Figure 4. (A) Oxygen pressure studies of the pyrene cation radical yield of pyrene-impregnated NaX. the reflectivity of the sample at 450 nm is plotted against the oxygen pressure. (B) Laser intensity studies of the
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Time-resolveddiffuse reflectance signal of pyrene-impregnated NaX at different wavelengths under vacuum. Solid line, 41 5 nm; long dashed line, 450 nm; short dashed line, 550 nm. (A) Fast time scale. (B) Slowest time scale. (C) 550-nm signal at fastest time scale without correction of background luminescence. No data point was digitized between 40 and 90 ns (shown as dotted line) because of the off-screen spike on the digitizer. Figure 3.
ionization of the pyrene occurs.2o At oxygen pressures lower than 300 Torr, a decrease of cation yield was observed as the oxygen pressure was increased. This indicates a biphotonic ionization of the pyrene may also occur, particularly at the oxygen pressures where the lifetime of the singlet excited pyrene is longer than the excitation laser pulse (10 ns). The striking result of the singlephoton ionization of pyrene in zeolites is further supported by the laser intensity study at 300 Torr of oxygen, at which a linear (20) From our previous reported oxygen quenchin rate of singlet excited pyrcne in zeolite X, kq = ( I .32 i 0.05) X IO' Torr' s-Q,1Oa the pyrene singlet excited lifetime is less than 0.3 ns at 300 Torr of oxygen. Because of the short-lived of the pynne singlet excited state compared to the IO-nslaser pulse, the probability of absorbing a second photon in order to ionize the pyrene is minimized under this condition.
relationship between the Py*+ yield and a laser intensity21 up to 8 mJ/cm2 and a plateauing off at the laser energy above 8 mJ/cm2, which may be due to the saturation of the ground-state pyrene a b ~ o r p t i o n were , ~ ~ observed. To summarize our experimental results of the dehydrated alkali-metal-ions-exchanged zeolite X and Y,we postulate Scheme I for the photolysis of the anthracene or pyrene in these zeolites. The existence of more than one site inside these zeolites is also supported by our early studies,1° in which the decay of pyrene fluorescence does not follow first-order decay kinetics under vacuum. We have shown in our early studies'O of the samples used in the previous study that oxygen can diffuse freely in the dehydrated zeolites and that the accessibility of oxygen to all the pyrene-occupied sites is equivalent. The biphotonic ionization of pyrene located a t nonactive sites involves an intermediate, 'Py', which is susceptible to oxygen quenching, while the monophotonic ionization of the pyrene located at active sites does not involve (21) Cautions has been taken to analyze the linear dependence of the l a w intensity study as stressed by Stein et The data shown in Figure 49, in which an intercept of zeta yield of pynne cation radical at zero laser intensity, strongly supports the singlephoton ionization of pyrene in the zeolitea studied. (22) Lachish, U.;Shafferman, A.; Stein, G. J . Chem. Phys. 1976,64, 4205-4211. (23)Our laser intensity of 337 nm varies from 1-15 mJ cm2. According to earlier calculation in ref 14, we can estimate that 20% o the ground-state pyrene was excited at a laser intensity 0.29 X IO-* mol of photon cm' (1 mJ/cm2), and all of the ground-state pyrene was excited at 2 X mol of photon/cm2 (7 mJ/cm2). This corresponds to the linear part of the data in Figure 4B.
/
Id
509
J. Phys. Chem. 1991, 95, 509-5 11 an intermediate that is susceptible to oxygen quenching, as shown in Scheme I. The ionization potential in zeolite can be expressed as
1, = Ig + P+ +
v,
where I , is the ionization potential in zeolite, I g is the gas-phase ionization potential (7.55 eV for pyrene), P+ is the polarization energy of the cation in the medium (e.g., zeolite in this case), and the V, is the energy state of the ionized electron.24 P+ can be calculated from the Born charging expression as follows:
P+ = (e2/2r)(1 - 1/e)
(2)
where the e is the charge of the electron and r is the interaction distance of the ion and the surrounding media. Normally, P+ is around -1.5 to -2.0 eV for a typical c (=2.0); therefore, the difference between gas-phase ionization potential and ionization potential in medium Is - Iz as in eq 1) is around 3.0-3.5 eV or larger if c is larger than 2. In fact, single-photon ionization of 3-aminoperylene with a green light (530 nm, 2.34 eV) had been reported in NaLS micelles system.26 Kasai reported the formation of NO'+ and NO2'- ion pair in NO-treated and our early studies on the pyrene fluorescence III/I ratio less than unity" all support a rather polar environment inside the zeolite X and Y. This polar environment may be the cause of the single-photon ionization of pyrene we observed. In other words, it may be the result of a large value of the dielectric constant inside the zeolite X and Y, the active site in particular. Although Richardson observed the ESR signal of arene radical cation in which may be caused by pro(24) This value is typically -1.5 eV in water.2s Presumably, it does not change much in zeolite particularly in fully hydrated samples. (25) Barker, G. C.; Bottura, G.; Cloke, G.; Gardner, A. W.; Williams, M. J. Ekcrroanal. Chem. Inrerfac. Elecrrochem. 1974, 50, 323. (26) Thomas, J. K.;Picuilo, P. J. Am. Chem. Soc. 1978,100,3239-3240. (27) Kasai, P. H.; Bishop, Jr., R. J. J . Am. Chem. SOC. 1972, 94, 5560-5566.
tonation from the zeolite framework (Bronsted acid site, for example); we, however, did not detect any trace of pyrene cation radical in the zeolite samples studied here in the absence of the laser excitation by a UV-vis spectrometer. The active site for the single-photon ionization of pyrene could be a radical promoter (or electron acceptor) site proposed by Rabo et al. in KY29and recently by Shih in ZSM-5.30 This electron acceptor site could react with arene to form a ground-state electron-transfer complex as arene + EA site arene'+.-EA'Although we did not resolve the spectrum of this ground-state electron-transfer complex by UV-vis spectroscopy, it might be very similar to that of the ground state of either anthracene or pyrene. Excitation with the 337-nm laser could photolyze this complex and create the ionized electron and arene cation radical in the alkali-metal-cation-exchanged zeolite X or Y.
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Conclusion We have observed a series of alkali-metal ionic clusters inside the zeolites X and Y through photoionization of anthracene- or pyrene-impregnated samples. Both mono- and biphotonic ionization of pyrene were observed in these zeolites. The active site that provides the single-photon ionization process may be the electron-acceptor site a t which a ground-state charge-transfer complex between pyrene and the zeolite framework could be formed. Acknowledgment. We thank the National Science Foundation (Grant CHE-89-11906) for support of this work. We also thank Dr. D. E. Vaughan at Exxon Research Center for a helpful discussion on the properties of cation-exchange zeolites, Dr. X. Liu for commenting on the origin of the active sites, and R. T. Gajek at UOP for providing the Na-exchanged zeolite Y. (28) Richardson, J. T. J . Caral. 196'1.9, 172-177. (29) Rabo, J. A.; Poutsma, M.L. Ado. Chem. 1971, 102, 284. (30) Shih, S.J. Carol. 1983, 79, 390-395.
ntrinsic" Response of Polymer Liquid Crystals in Photochemical Phase Transition Tomiki Ikeda,* Takeo Sasaki, and Haeng-Boo Kim Photochemical Process Division, Research Laboratory of Resources Utilization. Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan (Received: August 17, 1990)
Time-resolved measurements were performed on the photochemically induced isothermal phase transition of polymer liquid crystals (PLC) with mesogenic side chains of phenyl benzoate (PAPB3) and cyanobiphenyl(PACB3) under conditions wherein the photochemical reaction of the doped photoresponsive molecule (4-butyl-4'-methoxyazobenzene,BMAB) was completed within -10 ns, and the subsequent phase transition of the matrix PLC from nematic (N) to isotropic (I) state was followed by time-resolved measurements of the birefringence of the system. Formation of a sufficient amount of the cis isomer of BMAB with a single pulse of a laser lowered the N-I phase transition temperature of the mixture, inducing the N-I phase transition of PLCs isothermally in a time range of -200 ms. This time range is comparable to that of low molecular weight liquid crystals, indicating that suppression in mobility of mesogens in PLCs does not affect significantly the thermodynamically controlled process.
Introduction Photochemical phase transition is an isothermal phase transition triggered by a photochemical reaction of photoresponsive molecules doped in the matrix liquid crystals (LC). There have been many reports on the photochemical phase transition of LCs in which 'To whom correspondence should be addressed.
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such photoresponsive guest molecules as azobenzene and spiropyran derivatives were used.'+ In the azobenzene derivative/LC ( I ) Haas, W. E.; Nelson, K.F.; Adams, J. E.; Dir, G. A. J. Electrochem. Sor. 121. ... 1914. . - - - , 1667. --(2) Ogura, K.;Hirabayashi, H.; Uejima, A,; Nakamura, K.Jpn. J. Appl. Phys. 1982, 21, 969. (3) Attard, G.; Williams, G. Narure 1987, 326, 544.
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0 1991 American Chemical Society
