4470
Langmuir 2000, 16, 4470-4477
Facile Reduction of Zeolite-Encapsulated Viologens with
Solvated Electrons and Selective Dispersion of Inter- and
Intramolecular Dimers of Propylene-Bridged Bisviologen
Radical Cation
Yong Soo Park,† Kyungmi Lee,‡ Chongmok Lee,*,‡ and Kyung Byung Yoon*,†
Department of Chemistry, Sogang University, Seoul 121-742, Korea, and
Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea
Received November 1, 1999. In Final Form: February 11, 2000
Zeolite-Y (Y), zeolite-L (L), mordenite (M), and ZSM-5 exchanged with methyl viologen C1VC12+ and
N′,N′′-trimethylenebis(1-methyl-4,4′-bipyridinium) (henceforth bisviologen) C1V2+-C3-VC12+ were prepared.
Treatment of the dried, viologen-doped zeolites with solvated electrons in diethyl ether readily yields the
corresponding viologen radical cation, i.e., C1VC1•+ and C1V•+-C3-VC1•+ within the zeolite pores. Although
C1V•+-C3-VC1•+ readily folds to its intramolecular dimer C3[C1V•+]2 in solution, it remains in the open form
in the four zeolites. C1VC1•+ readily dimerizes to [C1VC1•+]2 at high concentrations in L but not in other
zeolites. However, C1V•+-C3-VC1•+ does not dimerize by itself in any of the four zeolites even at high
concentrations. C1VC1•+ readily dimerizes in Y, L, and M when the pores are filled with water. The hydrationinduced dimerization also works well for C1V•+-C3-VC1•+ in L and M. This leads to selective formation of
the unprecedented intermolecular dimer of bisviologen radical cation, i.e., [C1V•+-C3-VC1•+]2 in L and M.
The intramolecular analogue C3[C1V•+]2 can also be dispersed into Y, L, and M by direct occlusion of its
bisperchlorate salt from the acetonitrile solution. The broad ∼530 nm band of the viologen radical dimer
splits into two when the viologen radical moieties are forced to juxtapose in the collinear conformation
within the narrow and straight channels of L and M. The near-infrared band of the viologen radical dimer
progressively blue shifts with decreasing the interannular distance. This paper thus demonstrates a novel
use of zeolites to selectively disperse inter- and intramolecular dimers of diradical dication and to lock the
viologen radical dimers into the collinear conformation which allows us to delineate the spectral variation
of viologen radical dimers with changing the interannular conformation and distance.
Introduction
Zeolites are widely used as catalysts, ion exchangers,
sorbents, etc., in industry.1-3 For academic purposes, they
have also been extensively employed as organizing media
to induce long-lived charge separation in photochemistry,4-8 hosts for various semiconductor quantum dots,9
and nanoreactors for various organic transformations.10,11
The presence of repeating units of nano- to subnanometer
†
‡
Sogang University.
Ewha Womans University.
(1) (a) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974.
(b) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular
Sieves; Academic: London, 1978.
(2) (a) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape-Selective
Catalysis in Industrial Applications; Marcel Dekker: New York, 1989.
(b) Csicsery, S. M. In Zeolites Chemistry and Catalysis; Rabo, J. A., Ed.;
ACS Monograph 171; American Chemical Society: Washington, DC,
1976; p 680 ff. (c) Weisz, P. B.; Frilette, V. Z. J. Phys. Chem. 1960, 64,
382.
(3) Ruthven, D. M. Principles of Adsorption and Adsorption Process;
Wiley: New York, 1984.
(4) (a) Ramamurthy, V., Ed. Photochemistry in Organized and
Constrained Media; VCH: New York, 1991. (b) Holt, S. L., Ed. Inorganic
Reactions in Organized Media; ACS Symposium Series 177; American
Chemical Society: Washington, DC, 1981.
(5) (a) Borja, M.; Dutta, P. K. Nature 1993, 362, 43. (b) Dutta, P. K.;
Incavo, J. A. J. Phys. Chem. 1987, 91, 4443. (c) Dutta, P. K.; Turbeville,
W. J. Phys. Chem. 1992, 96, 9410. (d) Dutta, P. K.; Borja, M. J. Chem.
Soc., Chem. Commun. 1993, 1568. (e) Ledney, M.; Dutta, P. K. J. Am.
Chem. Soc. 1995, 117, 7687. (f) Dutta, P. K.; Ledney, M. Prog. Inorg.
Chem. 1997, 44, 2209. (g) Sykora, M.; Kincaid, J. R. Nature 1997, 387,
162.
(6) (a) Yonemoto, E. H.; Kim, Y. I.; Schmehl, R. H.; Wallin, J. O.;
Shoulders, B. A.; Richardson, B. R.; Haw, J. F.; Mallouk, T. E. J. Am.
Chem. Soc. 1994, 116, 10557. (b) Kim, Y. I.; Mallouk, T. E. J. Phys.
Chem. 1992, 96, 2879. (c) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E.
J. Am. Chem. Soc. 1988, 110, 8232. (d) Persaud, L.; Bard, A. J.; Campion,
A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Am. Chem.
Soc. 1987, 109, 7309.
sized crystalline voids is one of the key features that confer
zeolites such a broad applicability.
Viologens V2+ have been widely applied as herbicides,12
redox indicators,13 electron mediators for electrochromic
displays,14 and relays in the photochemical15 and biological
systems.16 The viologen radical cations V•+ have been
(7) (a) Sankararaman, S.; Yoon, K. B.; Yabe, T.; Kochi, J. K. J. Am.
Chem. Soc. 1991, 113, 1419. (b) Yoon, K. B.; Hubig, S. M.; Kochi, J. K.
J. Phys. Chem. 1994, 98, 3865.
(8) Fukuzumi, S.; Urano, T.; Suenobu, T. Chem. Commun. 1996,
213.
(9) See: Ozin, G. A.; Kuperman, A.; Stein, A. Angew. Chem., Int. Ed.
Engl. 1989, 28, 359 and references therein.
(10) (a) Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Acc. Chem. Res.
1992, 25, 299. (b) Ramamurthy, V.; Garcia-Garibay, M. A. In Comprehensive Supramolecular Chemistry; Bein, T., Ed.; Pergamon Press:
Oxford, 1996; Vol. 7, p 693.
(11) (a) Herron, N.; Corbin, D. R. Inclusion Chemistry within
Zeolites: Nanoscale Materials by Design; Kluwer Academic Press:
Holland, 1995. (b) Turro, N. J. In Crystallography of Supramolecular
Compounds; Tsoucaris, G., Atwood, J. L., Lipkowski, J., Eds.; Kluwer
Academic Press: Holland, 1996; p 429.
(12) Summers, L. A. The Bipyridinium Herbicides; Academic: New
York, 1980.
(13) Michaelis, L. Chem. Rev. 1935, 16, 243.
(14) (a) Schoot, C. J.; Ponjee, J. J.; van Dam, H. T.; van Doorn, R. A.;
Bolwijn, R. T. Appl. Phys. Lett. 1973, 23, 64. (b) van Dam, H. T.; Ponjee,
J. J. J. Electrochem. Soc. 1974, 121, 1555. (c) Bruinink, J.; Kregting,
C. G. A.; Ponjee, J. J. J. Electrochem. Soc. 1977, 124, 1854. (d) Barna,
G. G.; Fish, J. G. J. Electrochem. Soc. 1981, 128, 1290. (e) Lee, C.; Sung,
Y. W.; Park, J. W. J. Electroanal. Chem. 1997, 431, 133.
(15) (a) Ebbesen, T. W.; Levey, G.; Patterson, L. K. Nature 1982, 298,
545. (b) Ebbesen, T. W.; Ferraudi, G. J. Phys. Chem. 1983, 87, 3717.
(c) Nakahara, A.; Wang, J. H. J. Phys. Chem. 1963, 67, 496. (d) Ebbesen,
T. W.; Manring, L. E.; Peters, K. S. J. Am. Chem. Soc. 1984, 106, 7400.
(e) Hoffman, M. Z.; Prasad, D. R.; Jones, II, G. Malba, V. J. Am. Chem.
Soc. 1983, 105, 6360. (f) Jones, II, G.; Malba, V. Chem. Phys. Lett. 1985,
119, 105.
(16) Fultz, M. L.; Durst, R. A. Anal. Chim. Acta 1982, 140, 1.
10.1021/la991430+ CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/21/2000
Zeolite-Encapsulated Viologens
known to readily dimerize in solution with increasing
the polarity of the solvent17 (or by decreasing the temperature) and with enforcing the hydrophobic interaction
of the radical cations by increasing the length of alkyl
chains.18 The characteristic purple color of the dimeric
forms [V•+]2 readily distinguish themselves from the blue
monomeric forms. Unlike monomers, the dimeric forms
are inefficient in mediating electrons during catalytic
generation of hydrogen from water.19 Accordingly, elucidation of the behavior of dimerization and the nature of
dimeric states of viologen radical cations has been a subject
of extensive research since it can provide insight into the
effective use of viologens as electron mediators for various
electrochromic and solar energy conversion devices. For
this matter, cyclodextrins and micelles have been tested
as the media to selectively induce or suppress dimerization
of V•+ and to organize the resulting radical dimers.18,20-22
None of the above-mentioned media, however, showed
the ability to selectively entrap the inter- and intramolecular dimers of bisviologen radical cation C1V•+-C3-VC1•+
and to orient the viologen radical cations to a specific
conformation in the dimeric states.
Various methods have been developed to reduce the
substrates encapsulated within zeolites. However, the
fundamental difficulties associated with the heterogeneity
of the reaction conditions limit their diversity. Nevertheless, various reduction methods have been developed by
now, and they can be classified into three types depending
on the source of electrons: physical, chemical, and
electrochemical. The physical methods involve exposure
of substrates-bearing zeolites to high-energy radiations
such as vacuum-UV,23 X- or γ-rays,24 electron beams,25-27
and electron sparks,28a often at low temperatures (77 K).
However, the reduced species obtained by physical
methods usually disappear upon warming to room temperature. The chemical reagents often used to reduce
intrazeolite substrates are hydrogen, carbon monoxide,
and hot alkali metal vapors.1,27,29,30 Since the required
reaction temperatures are usually high, thermally stable
(17) (a) Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86,
5524. (b) Wolszczak, M.; Stradowski, Cz. Radiat. Phys. Chem. 1989, 33,
355. (c) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Am. Chem. Soc.
1977, 99, 5882. (d) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Chem.
Soc., Perkin Trans. 2 1977, 1787.
(18) (a) Park, J. W.; Choi, N. H.; Kim, J. H. J. Phys. Chem. 1996, 100,
769. (b) Diaz, A.; Quintela, P. A.; Schuette, J. M.; Kaifer, A. E. J. Phys.
Chem. 1988, 92, 3537.
(19) (a) Grätzel, M. Acc. Chem. Res. 1981, 14, 376. (b) Sullivan, B.
P.; Dressick, W. J.; Meyer, T. J. J. Phys. Chem. 1982, 86, 1473. (c) Adar,
E.; Degani, Y.; Goren, Z.; Willner, I. J. Am. Chem. Soc. 1986, 108, 4696.
(d) Okuno, Y.; Chiba, Y.; Yonemitsu, O. J. Chem. Soc., Chem. Commun.
1984, 1638. (e) Meisel, D.; Mulac, W. A.; Matheson, M. S. J. Phys. Chem.
1981, 85, 179. (f) Kirch, M.; Lehn, J.-M,; Sauvage, J.-P. Helv. Chim.
Acta 1979, 62, 1345. (g) Fan, F.-R. F.; Reichman, B.; Bard, A. J. J. Am.
Chem. Soc. 1980, 102, 1488.
(20) Yasuda, A.; Kondo, H.; Itabashi, M.; Seto, J. J. Electroanal. Chem.
1986, 210, 265.
(21) (a) Lee, C.; Kim, C.; Park, J. W. J. Electroanal. Chem. 1994, 374,
115. (b) Lee, C.; M. S. Moon, Park, J. W. J. Electroanal. Chem. 1996,
407, 161. (c) Lee, C.; Lee, Y. M.; Moon, M. S.; Park, S. H.; Park, J. W.;
Kim, K. G.; Jeon, S.-J. J. Electroanal. Chem. 1996, 416, 139.
(22) (a) Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1985, 89, 4876. (b)
Park, J. W.; Paik, Y. H. J. Phys. Chem. 1987, 91, 2005. (c) Quintela, P.
A.; Diaz, A.; Kaifer, A. E. Langmuir 1988, 4, 663.
(23) Liu, X. S.; Thomas, J. K. J. Chem. Soc., Faraday Trans. 1995,
91, 759.
(24) Kasai, P. H. J. Chem. Phys. 1965, 43, 3322.
(25) (a) Iu, K.-K.; Liu, X. S.; Thomas, J. K. J. Phys. Chem. 1993, 97,
8165. (b) Liu, X. S.; Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1994, 98,
13720.
(26) Edgell, M. J.; Mugford, S. C.; Castle, J. E. J. Catal. 1988, 433.
(27) (a) Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Schomaker, V. Discuss.
Faraday Soc. 1966, 41, 328. (b) Kasai, P. H.; Bishop, Jr., R. J. J. Phys.
Chem. 1973, 77, 2308.
(28) (a) Park, Y. S.; Um, S. Y.; Yoon, K. B. Chem. Phys. Lett. 1996,
252, 379. (b) Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1988, 110, 6586.
Langmuir, Vol. 16, No. 10, 2000 4471
metal ions or metal oxides have often been the subjects
of chemical reduction. The electrochemical methods, on
the other hand, have been demonstrated to be more
suitable for reduction of thermally sensitive species in
zeolites.31-34 The electrochemical methods also have
drawbacks arising from the basic requirement that zeolite
powders should be immobilized onto the electrode surface
and from the need of electrolytes which often leach the
positively charged electroactive species out of the zeolite
pores into solution. This method is also unsuitable for
large-scale preparations.
In this paper, we introduce a highly efficient method to
reduce zeolite-encapsulated viologens and a novel use of
zeolites to selectively disperse the inter- or the intramolecular dimer of C1V•+-C3-VC1•+ within zeolites and to lock
the viologen radical dimers in the collinear conformation
within channel-type zeolites.
Experimental Section
Materials. Zeolite-Y (LZY-52, Lot No. 968087061020-S),
zeolite-L (ELZ-L, Lot No. 961687041001-S), and mordenite (LZM5, Lot No. 962487061003-S) were purchased from Union Carbide.
ZSM-5 with the Si/Al ratio of 13.5 was a gift from ALSI-PENTA
Zeolite GmbH. For convenience, the former three are designated
as Y, L, and M, respectively. The zeolites were treated with
aqueous 1 M NaCl solution and then subsequently washed with
distilled deionized water until the silver ion test for chloride was
negative. For L, treatment with 1 M NaCl solution was repeated
for five times to exchange K+ with Na+ before the final washing
with distilled deionized water.
Methyl viologen dichloride (C1VC12+ 2Cl-) from Hannong
Chemical Co. was recrystallized repeatedly until colorless. N′,N′′Trimethylenebis(1-methyl-4,4′-bipyridinium) tetraiodide (C1V2+C3-VC12+ 4I-) was synthesized in a stepwise manner according
to the following procedure.35 Into the acetonitrile solution of 4,4′bipyridine, 0.5 equiv of 1,3-diiodopropane was added in a dropwise
manner over a period of 24 h while the temperature of the reaction
mixture was maintained at 70 °C. This yielded N,N′-(trimethylene)bis(4,4′-bipyridinium) diiodide (V+-C3-V+ 2I-). The yellow
precipitate was recrystallized over a hot water. 1H NMR (D2O):
δ ) 9.05 (4H, d), 8.77 (4H, d), 8.46 (4H, d), 7.90 (4H, d), 4.50 (4H,
t), 2.94 (2H, quin). The propylene-bridged bis(bipyridinium)
diiodide (V+-C3-V+ 2I-) was subsequently treated with methyl
iodide in N,N-dimethylformamide (DMF) at 90 °C over a period
of 6 h to yield C1V2+-C3-VC12+ 4I-. The red precipitate was
collected and recrystallized in cold water. 1H NMR (D2O): δ )
9.22 (4H, d), 9.08 (4H, d), 8.61 (4H, d), 8.52 (4H, d), 5.00 (4H, t),
4.50 (6H, s), 2.95 (2H, quin). The red iodide salt of C1V2+-C3VC12+ was converted to colorless perchlorate salt by metathesis
(29) (a) Harrison, M. R.; Edwards, P. P.; Klinowski, J.; Thomas, J.
M.; Johnson, D. C.; Page, C. J. J. Solid State Chem. 1984, 54, 330. (b)
Anderson, P. A.; Edwards, P. P. J. Am. Chem. Soc. 1992, 114, 10608.
(c) Haug, K.; Srdanov, V.; Stucky, G.; Metiu, H. J. Chem. Phys. 1992,
96, 3495. (d) Srdanov, V. I.; Haug, K.; Meiu, H.; Stucky, G. D. J. Phys.
Chem. 1992, 96, 9039. (e) Sun, T.; Seff, K.; Heo, N. H.; Petranovskii,
V. P. Science 1993, 259, 495.
(30) (a) Martens, L. R. M.; Grobet, P. J.; Jacobs, P. A. Nature 1985,
315, 568. (b) Martens, L. R. M.; Vermeiren, W. J. M.; Grobet, P. J.;
Jacobs, P. A. Stud. Surf. Sci. Catal. 1987, 31, 531. (c) Xu, B.; Chen, X.;
Kevan, L. J. Chem. Soc., Faraday Trans. 1991, 87, 3157.
(31) (a) Rolison, D. R. Chem. Rev. 1990, 90, 867. (b) Rolison, D. R.
Stud. Surf. Sci. Catal. 1994, 85, 543.
(32) (a) Calzaferri, G.; Lanz, M.; Li, J. J. Chem. Soc., Chem. Commun.
1995, 1313. (b) Li, J.; Pfanner, K.; Calzaferri, G. J. Phys. Chem. 1995,
99, 2119. (c) Li, J.; Calzaferri, G. J. Electroanal. Chem. 1994, 377, 163.
(d) Li, J.; Calzaferri, G. J. Chem. Soc., Chem. Commun. 1993, 1430.
(33) (a) Walcarius, A.; Lamberts, L.; Derouane, E. G. Electrochim.
Acta 1993, 38, 2257. (b) Walcarius, A.; Lamberts, L.; Derouane, E. G.
Electrochim. Acta 1993, 38, 2267.
(34) (a) Gemborys, H. A.; Shaw, B. R. J. Electroanal. Chem. 1986,
208, 95. (b) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49.
(35) (a) Furue, M.; Nozakura, S. Chem. Lett. 1980, 821. (b) Furue,
M.; Nozakura, S. Bull. Chem. Soc. Jpn. 1982, 55, 513. (c) Imabayashi,
S.-I.; Kitamura, N.; Tazuke, S.; Tokuda, K. J. Electroanal. Chem. 1988,
243, 143.
4472
Langmuir, Vol. 16, No. 10, 2000
with NaClO4 in water. Metallic potassium and 18-crown-6 were
purchased from Aldrich. DMF was used as received (Aldrich).
Ion Exchange. C1VC12+ ion was introduced into zeolites by
aqueous ion exchange of Na+. The exchanged amounts of C1VC12+ into zeolites were controlled to be 0.5 per unit cell (puc)
for lowly loaded zeolites.36 For highly loaded zeolites, the
incorporated amounts were increased to 2 per supercage for
zeolite-Y and 1 puc for channel-type zeolites. The unexchanged
amounts of C1VC12+ ion in the wash were quantified by UV-vis
spectrophotometry monitored at λ ) 257 nm ( ) 2.0 × 104 cm-1
M-1).12 The amounts of C1V2+-C3-VC12+ exchanged into zeolites
were controlled to be ∼0.25 puc. The unexchanged amounts were
quantified similarly with ) 4.4 × 104 cm-1 M-1 at 257 nm.35b
All the viologen-doped zeolites were dried in the air at ambient
temperatures. The air-dried zeolites were then briefly washed
with methanol prior to subsequent evacuation (<10-5 Torr) for
2 h at room temperature.37 The dehydration temperatures were
then slowly increased to 200 °C.38 The samples were evacuated
at the final temperatures for an additional period of 15 h and
kept in the glovebox.
Reduction of Zeolite-Encapsulated Viologens with Solvated Electrons. 18-Crown-6 was purified by recrystallization
in hot water. Diethyl ether was distilled over sodium and
benzophenone. All the air- or moisture-sensitive compounds were
handled in the golvebox charged with high-purity argon.
Typically, 5 mL of diethyl ether was introduced into a vial
containing 0.5 g of a dried, viologen-doped zeolite in the glovebox.
To this slurry, metallic potassium and 18-crown-6 were added.
The added amount of potassium was controlled to be slightly
larger than 1 equiv with respect to the amount of C1V2+ moiety
in the zeolite since excessive amounts result in reduction of
viologen moiety even to the neutral state. The added amount of
crown ether was controlled to be 1 equiv to potassium. The slurry
was then stirred until the supernatant solution stopped developing blue color originating from solvated electrons. The obtained
blue zeolites were filtered over glass frits and washed for three
times with 5 mL of fresh diethyl ether. The blue zeolites were
dried by evacuating for 10 min at room temperature.
Occlusion of C1V•+-C3-VC1•+ 2ClO4- into Zeolites. Acetonitrile was treated with KMnO4 and distilled over P2O5. The
distilled solvents were stored in Schlenk flasks under argon and
transferred into a glovebox. The perchlorate salt of C1V2+-C3VC12+ was reduced by 2e- to generate the corresponding salt of
diradical dication C1V•+-C3-VC1•+ 2ClO4- by treating the parent
bisviologen salt with zinc amalgam in acetonitrile solution until
the absorption at 260 nm due to C1V2+ moiety disappeared
completely. Since this bisviologen radical cation exists in the
intramolecular dimeric form, it should better be designated as
C3[C1V•+]2. The concentration of the filtered solution of C3[C1V•+]2
2ClO4- was determined by UV-vis spectrophotometric measurements at 540 nm using the reported molar extinction
coefficient of 2.7 × 104 M-1 cm-1.39
Zeolites with Na+ as the countercation were dehydrated at
300 °C for 15 h and kept in the glovebox prior to salt occlusion.
Into an aliquot (0.3 g) of each dried zeolite, 2 mL of fresh CH3CN
was added to soak the dried zeolite before addition of the purple
diradical solution. This was done for a precautionary measure
to prevent possible decomposition of the diradical by excessive
heat that generates during the course of soaking zeolites with
polar solvents. Into the soaked zeolite, 5 mL of acetonitrile
solution of C3[C1V•+]2 2ClO4- (60 mM) was added. The purple
slurry was stirred at room temperature for 15 h, and development
(36) The length of channel in a unit cell is 7.5 Å whereas the estimated
length of C1VC12+ is 13 Å.
(37) The brief washing of the air-dried zeolites with the more volatile
methanol prior to evacuation was intended to facilitate dehydration
since the final temperatures were not high enough to complete
dehydration.
(38) The viologens encapsulated within the four zeolites are stable
below 200 °C in a vacuum. Thus, the 260-270 nm band characteristic
of the dried viologen in the zeolite media remains intact as long as the
temperature is kept under 200 °C. However, at higher temperatures
the zeolites turn blue due to formation of viologen radical cation. The
zeolites even turn brown or gray at much higher temperatures. See:
Park, Y. S.; Um, S. Y.; Yoon, K. B. J. Am. Chem. Soc. 1999, 121, 3193.
(39) Atherton, S. J.; Tsukahara, K.; Wilkins, R. G. J. Am. Chem. Soc.
1986, 108, 3380.
Park et al.
Figure 1. Diffuse reflectance UV-vis-NIR spectra of the C1VC12+-doped (0.5 puc) zeolites (as indicated) after treatment
with solvated electrons in the dry state (left) and after hydration
(right).
of either purple or blue color on zeolite powders was visually
monitored. The colored zeolites were filtered over glass frits and
washed for three times with 5 mL of fresh acetonitrile. The colored
zeolites were dried by evacuating for 30 min at room temperature.
Spectral Measurements. In the glovebox, an aliquot (0.25
g) of each colored zeolite was transferred into a flat cylindrical
quartz cell capped with a grease stopper for the diffuse reflectance
UV-vis spectrum. After spectral measurements, moisturization
of the dry samples was carried out by adding a few drops of water
into the cell under a counterflow of argon. The moistened zeolite
samples were equilibrated at room temperature for several hours
before spectral measurements. An alumina disk was used as the
reference for obtaining the backgrounds of zeolite samples. The
diffuse reflectance UV-vis spectra of the colored zeolites were
recorded on a Shimadzu UV-3101PC (0.1 nm resolution) equipped
with an integrating sphere (ISR 3100). A photomultiplier tube
(Hamamatsu R-928) and PbS cell were used as the detectors for
UV-vis and near-infrared (NIR) regions, respectively. The
wavelength accuracies were less than (0.3 nm and (1.6 nm in
the UV-vis and NIR regions, respectively. NMR spectra were
recorded from a Varian Gemini 300 spectrometer.
Results and Discussion
Reduction of Intrazeolite C1VC12+ and C1V2+-C3VC12+ with Solvated Electrons. The colorless C1VC12+doped zeolites (0.5 puc) immediately turned blue upon
treating with solvated electrons in diethyl ether. Diffuse
reflectance spectra of the blue zeolites (dried) revealed
the presence of methyl viologen radical cation C1VC1•+
within the zeolites as demonstrated by its characteristic
absorption bands shown in Figure 1 (left) for the four
prototypical zeolites, Y, L, M, and ZSM-5. It is worth
noting that the spectrum of C1VC1•+ is entirely different
from that of solvated electrons in diethyl ether shown in
Figure 2. From the fact that the absorption spectrum of
C1VC1•+ gives characteristic bands at 396 and 602 nm
while that of the dimer [C1VC1•+]2 gives the corresponding
maxima at 360, 530, and 850 nm,17,18,21 the absence of
absorption bands at around 850 nm in the spectra allowed
us to conclude that C1VC1•+ exists only in the monomeric
form in all the four dry zeolites. This procedure also worked
equally well for a 10 g scale of C1VC12+-doped zeolites. It
is important to note that none of the methods listed in
Introduction are suitable for this large, preparative scale.
Small amounts of unreduced C1VC12+ were usually noticed
in the diffuse reflectance spectra of the blue samples
judging from the residual absorption band at 260 nm due
to parent dication (not shown). We attribute the incomplete
Zeolite-Encapsulated Viologens
Langmuir, Vol. 16, No. 10, 2000 4473
Figure 2. UV-vis-NIR spectrum of solvated electron generated in diethyl ether from metallic potassium and 18-crown-6.
Figure 4. Diffuse reflectance UV-vis-NIR spectra of the
C1V2+-C3-VC12+-doped (0.25 puc) zeolites (as indicated) after
treatment with solvated electrons in the dry state (left) and
after hydration (right).
Figure 3. Diffuse reflectance UV-vis-NIR spectra of the C1VC12+-doped L (1 puc) after treatment with solvated electrons
in the dry state.
reduction of the parent dication to the intrinsic instability
of solvated electrons in ethereal solvents and to the
presence of unidentified electron consumers such as
residual water and protons in the zeolites.
The tendency of C1VC1•+ to exist in the monomeric form
within the zeolite pores persisted even in the highly loaded
zeolites (2 per supercage for Y or 1 puc for channel-type
zeolites)39 with the single exception of L. For instance,
even the Y incorporating two C1VC12+ ions within a
supercage turned intense blue (due to monomers) but not
purple (due to dimers) upon reduction. The diffuse
reflectance spectrum of the deep blue zeolite showed
essentially the same spectrum with the one shown in
Figure 1A (left) except higher intensity. The same result
was obtained from the highly loaded M and ZSM-5. Thus,
C1VC1•+ always exists in the monomeric form within the
above three zeolites regardless of the degree of loading,
as long as they are kept dry. In contrast, the highly loaded
L (1 puc) readily turned purple when treated with solvated
electrons. Consistent with this, the diffuse reflectance
spectrum of the reduced samples confirmed the presence
of substantial amounts of [C1VC1•+]2 as well as the
monomeric form as judged by the characteristic absorption
bands at 360, 530, and 850 nm as shown in Figure 3. The
estimated dimer-to-monomer ratio was 0.82, where the
concentration of monomer in the mixture was calculated
by extracting the monomer component from the observed
spectrum as described in the previous papers.18a,21b Correction of the dimer-to-monomer ratio was made using
the corresponding molar extinction coefficients assuming
they are the same with those obtained from aqueous
solutions. The above procedure was carried out using 360
(dimer) and 390 nm (monomer) bands.
The colorless, C1V2+-C3-VC12+-doped (0.25 puc) zeolites
also readily turned blue when treated with solvated
electrons. In fact, the two C1V•+ moieties linked by the
propylene (C3) unit are known to rapidly dimerize in
solution to generate an intramolecular dimer, i.e., C3[C1V•+]2 almost quantitatively.35,39,40 By contrast, the
diffuse reflectance spectra of the blue zeolites, in particular
those of Y and ZSM-5 (Figure 4, A and D), did not show
even a trace of the absorption band at ∼850 nm for C3[C1V•+]2. Similar results were obtained from L and M.
However, scrutiny of the spectra of L and M at longer
wavelengths region revealed the presence of weak absorption bands at ∼900 and ∼1000 nm, respectively, for
L and M due to dimeric forms of the bisviologen radical
cation. Since the spatial restrictions imposed by the zeolite
pores (<13 Å) do not allow the large bisviologen radical
cation (∼ 29 Å) to fold into the intramolecular dimeric
conformation, the spectra of the blue zeolites shown in
Figure 4, A through D (left), are concluded to arise from
the open (unfolded) form of the bisviologen radical cation,
i.e., C1V•+-C3-VC1•+. This is a unique situation to freeze
the bisviologen radical cation only in the open form.
The above results also demonstrate that solvated
electrons generated from metallic potassium and 18crown-6 in diethyl ether are very convenient and efficient
for reduction of zeolite-encapsulated viologens. We believe
this method can be extended for reduction of various other
substrates encapsulated within zeolites. In fact, the use
of solvated electrons for reduction of intrazeolite species
was first introduced by Suib and Stucky and their coworkers.41 They generated solvated electrons by dissolving
europium metal in liquid ammonia. Edwards and coworkers later used sodium and primary alkylamines as
for the sources of solvated electrons.42 We introduced a
more convenient way to generate solvated electrons by
using alkali metals and crown ethers in ethereal solvents.43
The solvated electrons have also been shown to be highly
effective for generation of alkali metal ionic clusters within
zeolites. The use of liquid ammonia and primary alkylamines does not bear any problem, for such purposes.
However, the highly basic media are not suitable for
(40) (a) Geuder, W.; Hünig, S.; Suchy, A. Tetrahedron 1986, 42, 1665.
(b) Neta, P.; Richoux, M.-C.; Harriman, A. J. Chem. Soc., Faraday Trans.
2 1985, 81, 1427.
(41) (a) Suib, S. L.; Zerger, R. P.; Stucky, G. D.; Emberson, R. M.;
Debrunner, P. G.; Iton, L. E. Inorg. Chem. 1980, 19, 1858. (b) Stucky,
G. D.; Iton, L.; Morrison, T.; Shenoy, G. J. Mol. Catal. 1984, 27, 71.
(42) Anderson, P. A.; Barr, D.; Edwards, P. P. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1501.
(43) Park, Y. S.; Lee, Y. S.; Yoon, K. B. J. Am. Chem. Soc. 1993, 115,
12220.
4474
Langmuir, Vol. 16, No. 10, 2000
reduction of zeolite-encapsulated substrates such as
viologens that are very vulnerable to nucleophiles. In this
regard, the use of highly inert diethyl and crown ethers
to generate solvated electrons is more applicable for a
wider range of substrates encapsulated within zeolites.
Thomas and co-workers generated hydrated (solvated)
electrons within X and Y by pulse radiolysis.44 They
reported that the electrons trapped in the clusters of water
migrate to the clusters of alkali metal ions. Although this
method appears to be useful to study dynamics of electron
transfer within zeolites, it looks to be unsuitable to achieve
permanent reduction of various organic substrates incorporated within zeolites, since pulse radiolysis simultaneously generates hole centers, protons, and hydroxy
radicals which are powerful oxidants.
Hydration-Induced Dimerization of C1VC1•+ and
Intermolecualr Dimerization of C1V•+-C3-VC1•+ within
Zeolites: The Effect of Pore Size and Shape. Upon
hydration of the blue, C1VC1•+-containing Y, the color
immediately turned purple. The diffuse reflectance UVvis spectrum of the purple zeolite showed 360, ∼550, and
∼900 nm bands characteristic of [C1VC1•+]2 with weak
shoulder bands at 396 and 600 nm due to residual (small)
amounts of monomeric C1VC1•+ as shown in Figure 1A
(right). This result confirmed that the spherical supercage
(13 Å) of Y is large enough to accommodate [C1VC1•+]2.
Similar results were observed from the channel-type
zeolites with intermediate pore sizes, i.e., L (7.1 Å) and
M (6.5 × 7.0 Å) as shown in Figure 1 (B and C, right). The
relative fraction of monomer is higher in M than in L. In
contrast, even hydration was incapable of inducing
dimerization of the radical cation within the narrowest
channels of ZSM-5 (∼5.5 Å) as demonstrated by the
retained perfect monomer spectrum in Figure 1D (right)
after hydration.
The dimer-to-monomer ratio gradually decreased with
decreasing the pore size of the zeolite. Thus, the estimated
ratios were 2.8, 2.2, and 1.3 respectively on going from Y
to M.45 The relative intensity of the near-infrared (NIR)
band at ∼850 nm increases significantly (with respect to
the 360 nm band) in the channel type zeolites (0.94 for L
and 0.59 for M) than in the spherical cavity of Y (0.28) or
in water (0.11). It is also interesting to note that the
bandwidth and position of the NIR band change with
varying the pore size. Thus, whereas the NIR band appears
at 920 nm in the spherical cage of Y with the bandwidth
(fwhm) of 0.69 eV, it appears at 830 and 760 nm
respectively in the channels of L and M with a much
narrower bandwidth (fwhm ) 0.34 eV). The absorption
band of [C1VC1•+]2 at ∼530 nm in L has a shoulder at 500
nm, which has never been observed in solution. This
phenomenon will be discussed later in terms of the change
of dimer conformations.
Hydration also readily induced dimerization of the open
(unfolded) form of the bisviologen radical cation, C1V•+C3-VC1•+, but only within the medium-sized channels of
L and M. Thus, as strongly contrasted in Figure 4 (B and
C vs A and D), the characteristic dimer bands at 360, 550,
and ∼850 nm appeared only in L and M but not in Y and
ZSM-5. The dimers generated from the unfolded bisviologen radical cations in L and M are proposed to be
intermolecular, i.e., [C1V•+-C3-VC1•+]2 based on the reasoning that the long unfolded bisviologen radical cation
is unable to fold to form intramolecular dimer, C3[C1V•+]2,
(44) (a) Thomas, J. K. Chem. Rev. 1993, 93, 301. (b) Liu, X. S.; Zang,
G. H.; Thomas, J. K. J. Phys. Chem. B 1997, 101, 2182.
(45) The ratio was estimated on the basis of the intensity ratio at 360
(dimer) and 390 nm (monomer).
Park et al.
within the restricted channels (<7.1 Å) of L and M, unlike
in solution. Zeolites thus provide a valuable opportunity
to investigate the characteristic features regarding the
intermolecular dimer of bisviologen radical cation which
otherwise undergoes rapid folding in solution.
The unabled formation of intermolecular dimer of C1V•+C3-VC1•+ in ZSM-5 is understandable from the fact that
the ZSM-5 channels are too narrow to overlap two viologen
units in one channel. The comparison of the kinetic
diameter of each viologen unit (5.85 Å) with the size of the
channels (5.6 × 5.3 or 5.6 × 5.1 Å) clearly supports the
reasoning. However, the failure to form [C1V•+-C3-VC1•+]2
in Y is rather surprising. This may arise if the bisviologen
radical cations exist in the bent conformation with the
propylene bridge residing in the window while the two
C1V•+ units are confined separately in the two adjacent
supercages.
The hydration-induced dimerization of C1VC1•+ and
C1V•+-C3-VC1•+ was found to be reversible within zeolite
pores. Thus, the purple hydrated zeolites readily recoverd
the original blue color upon dehydration under vacuum.
Accordingly, the diffuse reflectance spectra also restored
the initial monomeric forms.
It has been known that C1VC1•+ readily dimerizes in
aqueous solutions but not in any organic solvents, including methanol.17a This phenomenon was attributed to the
hydrophobic interaction of the radical cations in such a
high-dielectric medium since the monocationic form of
the large organic cation is more hydrophobic than its
dicationic analogue. We ascribe the hydration-induced
intermolecular dimerization of C1VC1•+ and C1V•+-C3-VC1•+
to the similar reasons discussed above. For this reasoning
to be viable there should be experimental proof regarding
that the actual micropolarity of zeolite pores exerting on
the radical cations is less than that of water. Our previous
results have indeed suggested that the actual micropolarity of the supercage of Y is comparable to those of 50%
aqueous acetonitrile or 50% aqueous DMF.28b,46 Ramamurthy and co-workers also found that thionin and the
related dyes readily dimerize in the supercage of Y upon
hydration.47 The high dielectric constant of water was
proposed to be responsible, at least partially, for the above
phenomenon.
On the contrary, neutral aromatic compounds, such as
anthracene and pyrene, intercalated within zeolite pores
are known to exist in the dimeric states in the anhydrous
condition at high loading but dissociate into separate
molecules upon hydration.48-50 These results are not
directly relevant to ours since neutral guest molecules
may well undergo aggregation in the anhydrous zeolite
pores.
Subtraction of the monomer component from the
composite (monomer and dimer) spectra in hydrated M
(Figures 1C and 3C, right) revealed that the resulting
dimer spectra show an extra band at 625 nm as shown in
Figure 5, A and D. The strong hyperbaric effect of the
pore lining water may be responsible for the appearance
of the unusual band.51
Incorporation of C3[C1V•+]2 within Zeolite Pores
by Salt Occlusion. The closed form, C3[C1V•+]2, was also
effectively dispersed into the zeolites by salt occlusion of
(46) Yoon, K. B.; Kochi, J. K. J. Phys. Chem. 1991, 95, 1348.
(47) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Am. Chem.
Soc. 1993, 115, 10438.
(48) Liu, X.; Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1989, 93, 4120.
(49) Hashimoto, S.; Ikuta, S.; Asahi, T.; Masuhara, H. Langmuir
1998, 14, 4284.
(50) (a) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Phys.
Chem. 1993, 97, 13380. (b) Ramamurthy, V.; Sanderson, D. R.; Eaton,
D. F. Photochem. Photobiol. 1992, 56, 297.
Zeolite-Encapsulated Viologens
Figure 5. Components of [C1VC1•+]2 (A) and [C1V•+-C3-VC1•+]2
(B) in M obtained by subtracting the corresponding monomer
component from the diffuse reflectance spectra shown in Figures
1C (right) and 4C (right), respectively. For comparison, the
corresponding spectra in L (B and D) are also shown.
Langmuir, Vol. 16, No. 10, 2000 4475
Neither hydration nor subsequent dehydration of the
salt-occluded Y altered the spectrum of the purple zeolite.
Thus, the spectrum shown in Figure 6A essentially does
not change even after hydration as shown in the inset.
Knowing the fact that the unfolded C1V•+-C3-VC1•+ generated in situ within Y cannot form intermolecular dimer
even with the aid of hydration, it is concluded that the
only possible species that can give rise to the absorption
of dimeric viologen radical cations is the occluded C3[C1V•+]2.
Interestingly, the spectrum of purple Y also includes
weak absorption bands of monomeric C1V•+ moiety at 390
and 600 nm in the diffuse reflectance spectrum (Figure
6A). This strongly contrasts with the spectrum of C3[C1V•+]2 in acetonitrile which shows only that of the dimeric
form. Since it will be difficult to generate two monomeric
C1V•+ moieties (if once enters) by unfolding from C3[C1V•+]2
within the supercages of Y (due to steric reasons), the
absorption bands of the monomeric V•+ moiety should
rather be concluded to arise from the open form of the
bisviologen radical cation, i.e., C1V•+-C3-VC1•+ which
entered supercages as such. This strongly indicates that
equilibrium must exist between the open and the closed
forms in acetonitrile (eq 1) although this is not so apparent
in solution due to the absence of the monomeric form in
the spectrum.
C3[C1V•+]2 h C1V•+-C3-VC1•+
Figure 6. Diffuse reflectance UV-vis-NIR spectra of the
colored zeolites (as indicated) after equilibrating with C3[C1V•+]2
2ClO4- in acetonitrile. The dashed line in (A) shows the
corresponding spectrum of zeolite-A treated with C3[C1V•+]2
2ClO4- in acetonitrile. The inset in (A) shows the spectrum of
Y occluding C3[C1V•+]2 2ClO4- after hydration.
the dication together with its anions. For instance, the
colorless powder of dry Y readily picked up purple color
upon stirring in the purple solution of C3[C1V•+]2 2ClO4in acetonitrile (see Experimental Section for details). The
purple color of Y gradually intensified with time. The
filtered, washed, and subsequently vacuum-dried Y
showed brilliant purple color, and the diffuse reflectance
spectrum of the purple zeolite showed strong absorption
bands at around 360, ∼550, and ∼850 nm characteristic
of the dimer as shown in Figure 6A.52 In contrast, zeolite-A
(Na+ form) with only 4 Å pore opening did not pick up the
purple color under the same experimental conditions
despite that the zeolite has a higher number of exchange
sites than Y (higher Si/Al ratio) on the external surface.
Accordingly, the washed and dried zeolite-A remained
white and showed only flat background in the visible region
as represented by the dashed line in Figure 6A. This result
thus strongly indicates that the intense coloration of Y
with purple arises from the salt occlusion of the dimer
cation into the supercages of Y but not from the externally
exchanged or adsorbed ones.
(51) Yoon, K. B.; Huh, T. J.; Kochi, J. K. J. Phys. Chem. 1995, 99,
7042.
(52) Scrutiny of the obtained spectra also revealed the presence of
V2+ moiety. This indicates that oxidation of some of bisviologen radical
cations also occurs during salt occlusion. Reduction of the resulted C1V2+
moiety with solvated electrons did not alter the spectra shown in Figure
6.
(1)
The estimated ratio for closed to open forms within Y was
1.4. Similar results were observed from L and M (Figure
6B,C), and the corresponding ratios were estimated to be
∼1.5 for both channel-type zeolites. From the very high
propensity of bisviologen radical cation to exist in the
closed (folded) form in acetonitrile, the presence of the
open form as much as 40% in the zeolite pores reflects
much more facile incorporation of the open form from the
solution into the zeolites. In the case of L, it is also
interesting to note that the intensity of the NIR band
dramatically increases and concomitantly blue shifts to
813 nm from the value of 850 nm in solution.
In the case of ZSM-5 with the channels being too small
to accommodate dimeric forms of viologen radical cations,
the developed color was blue than purple. The diffuse
reflectance spectrum of the blue zeolite showed that the
absorption bands arises mainly from the open form. This
result further demonstrated the preference of C3[C1V•+]2
to enter the zeolite pores in the open form as the pore size
decreases. In the case of Y with large spherical cages, the
equilibrium between the tightly closed and the slightly
open forms may also exist, and this may contribute to the
absorption of the monomeric V•+ moieties in the observed
spectrum (Figure 6A) if the corresponding equilibrium
constant is high in the supercages of Y.
Relationship between the Conformation of the
Dimeric Viologen Radical Cations and Observed
Spectra. The ∼530 nm band of [C1VC1•+]2 usually shows
up as a broad, single absorption band in solution as shown
in Figure 7A.53 So was the absorption band of the dimer
in the large spherical cages of Y (Figure 7B). In contrast,
however, the ∼530 nm band accompanies a shoulder band
at ∼500 nm when the dimer was incorporated within the
straight channels of L and M as shown in Figure 7C,D.
The spectra of C3[C1V•+]2 and [C1V•+-C3-VC1•+]2 assembled
in the straight channels of L and M also showed the
(53) The ∼530 nm band of [C1VC1•+]2 in water was obtained by
subtracting the corresponding spectrum of the monomer which exists
in equilibrium with the dimer in water.
4476
Langmuir, Vol. 16, No. 10, 2000
Park et al.
Chart 1
Chart 2
Figure 7. Splitting of the ∼530 nm band of the dimeric viologen
radical cations [V•+]2 upon changing the pore shape of the
surrounding medium from the nonrigid solvent cages of water
(A) and the large spherical supercages of Y (B) to the rigid
straight channels of L and M (C to H).
characteristic two-humped shape as demonstrated in
Figure 7E-H. In fact, it should be noted that even the
broad ∼530 nm band of [C1VC1•+]2 in solution is not single
Gaussian and can be decomposed into two absorption
bands. In this regard the splitting of the ∼530 nm bands
of [C1VC1•+]2, C3[C1V•+]2, and [C1V•+-C3-VC1•+]2 into ∼500
and ∼530 nm bands within the straight channels of L and
M is proposed to occur due to more pronounced displacement between the two closely overlapped bands in the
straight channels than in solution or in the large spherical
cage. The forced juxtaposition of the two viologen rings in
the parallel or collinear conformation from the energetically more favorable one is proposed to give rise to the
larger displacement between the two bands.
In a situation in which two V•+ moieties are linked by
two o-xylyl groups, the only possible conformation for the
interannular interaction is the collinear one.40 The
spectrum of this well-defined collinear structure also
showed the splitting of the ∼530 nm band, consistent with
our proposal. A similar tendency was reported for [C4VC4•+]2 in γ-cyclodextrin, where the bipyridine rings were
included only partially into the host.21c On the basis of
this reasoning, the intramolecular dimer C3[C1V•+]2 is
proposed to retain the collinear conformation both in
solution and in the large spherical cages of Y.
As a corollary, the crossing conformation is proposed to
be preferred in aqueous solutions and in the large spherical
cages of Y. Consistent with this, the reported crystal
structure of C1VC1•+PF6- shows that the radical cation
forms infinite stacks at an angle of 37 ° relative to the
adjacent ones. The diffuse reflectance spectrum of the
crystals also shows a broad ∼530 nm band without a
shoulder.54 Accordingly, we propose that the degree of
splitting of the ∼530 nm band can serve as a criterion for
the degree of collinear conformation of the dimeric viologen
radical cations [V•+]2.
The zeolites employed in this study can be categorized
into three types: Y with large spherical cages (type I), L
and M with intermediate channels (type II), and ZSM-5
with narrow channels (type III). On the basis of the results
described so far, the conformation of [C1VC1•+]2 can be
depicted according to Chart 1 (A and B), depending on the
pore structure. Thus, in type I zeolites, the dimer is allowed
to maintain the crossing conformation (A) as in the solid
state or in solution. This crossing conformation can be
rationalized to be more stable than the collinear one
considering the steric repulsion between the two methyl
groups and electrostatic repulsion between the nitrogen
centers that carry high positive charge densities.
(54) Bockman, T. M.; Kochi, J. K. J. Org. Chem. 1990, 55, 4127.
Chart 3
In type II zeolites with the pore diameters smaller than
7.1 Å, the collinear structure should be the only possible
conformation due to spatial restrictions (Chart 1B). In
type III zeolites (ZSM-5), in which dimerization of C1VC1•+ is impossible, the radical cation should rest alone
within the narrow channels as depicted in Chart 1C.
Thus, by employing adequate zeolites, C1VC1•+ can be
dispersed within the solid supports in two different
forms: monomeric and dimeric. In the case of the latter,
the conformation can further be controlled by varying the
shape of the pores. Zeolites thus provide a unique
opportunity to deliberately control dimerization behavior
of C1VC1•+ and conformation of its dimeric states.
A more versatile variation of dimer conformation is
expected from C1V•+-C3-VC1•+ when encapsulated within
zeolites. Thus, as depicted in Chart 2, either linear (A) or
bent (B) conformation of bisviologen radical cation can be
imagined in type I zeolites from their inability to yield
dimeric forms. In addition, intramolecular dimer (C) can
be readily incorporated into the zeolites. Thus, by choosing
appropriate pathways, two distinctively different conformations of bisviologen radical cation (closed vs open) can
be selectively confined within type I zeolites.
In the pores of type II zeolites in which even intermolecular dimerization is possible, three possible intermolecular association can be considered as illustrated in
Chart 3 (A, B, and C). However, there is no experimental
evidence as yet that can serve as the basis to differentiate
one from the other. Although the half-overlap conformation
Zeolite-Encapsulated Viologens
(B or C) was suggested to be unfavorable in solution,40a
all the three conformations are possible within type II
channels with some fractions of monomers. Thus, type II
zeolites can provide a unique opportunity to disperse
various intermolecular (A, B, and C) and intramolecular
(D) dimers of bisviolgen radical cation. In type III zeolites,
only the monomeric bisviologn radical cation should be
dispersed within the zeolites, as depicted in Chart 3E.
The sharp decrease of the bandwidth of the NIR band
from 0.69 (fwhm) in Y to 0.34 eV in L and M might directly
reflect the conformational difference of [C1VC1•+]2 within
the two different types of zeolite pores (I and II). In general,
it is believed that the NIR band arises from the face-toface charge-transfer (CT) interaction between the two V•+
moieties.17,35,40 Although there are numerous factors that
govern the spectral broadness for a given structure of a
CT complex,55 the geometrical variation such as the
variation of the interacting angle and the interannular
distance between the two planar systems should in
principle be taken into account for the observed spectral
width. Since the intermolecular distance cannot vary
significantly within the restricted confinements of the
(55) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley: New
York, 1969; Chapter 8.
Langmuir, Vol. 16, No. 10, 2000 4477
zeolite pores, the angle variation between the two long
plane axes is proposed to be partially responsible for the
observed unusually large bandwidth of the NIR band in
Y.
The progressive blue shift of the NIR band of [C1VC1•+]2
upon decreasing the pore size of the zeolite host (i.e., 920,
830, and 760 nm for Y, L, and M, respectively) is primarily
ascribed to the decrease of the interannular distance
between the two C1VC1•+. The same trend was observed
in a series of bisviologen radical cations linked by two
bridges.21c,40a However, the change in the geometry of the
dimeric forms that could occur while the pore size
decreases should also be considered to account for the
observed blue shift. Nevertheless, from the same hypsochromic shift of the NIR band it can be inferred that the
preferred site for [C1VC1•+]2 is the large spherical cage of
Y but not the connecting windows.
Acknowledgment. We thank the Korea Science and
Engineering Foundation (KOSEF) for supporting this
work (951-0303-024-2 and 97-05-01-04-01-3). K.B.Y. also
thanks the Creative Research Initiatives Program of the
Ministry of Science and Technology (MOST) of the Korean
Government for financial support.
LA991430+