3824
J . Phys. Chem. 1985, 89, 3824-3828
Structural Features of 2,2‘-Bipyridyl through Optical and ODMR Studies K. Vinodgopal and Willem R. Leenstra* Department of Chemistry, University of Vermont, Burlington, Vermont 05405 (Received: February 25, 1985; In Final Form: April 19, 1985)
The first excited triplet state of 2,2’-bipyridyl has been investigated by low-temperaturephosphorescence and optically detected magnetic resonance (ODMR) spectroscopy in the pseudocrystalline Shpolskii matrices, n-heptane, n-hexane, and n-octane. The emission at 1.8 K is narrow and well-resolved,with the vibronic progression arising mostly from the totally symmetric Raman vibrations of the molecule. Some nontotally symmetric vibronic progressions are also observed in the emission spectrum, indicating some distortion away from planarity. The ODMR studies reveal more structural information about the molecule. Two sets of internally consistent ODMR resonances are observed when the emission from each and every vibronic peak in the phosphorescence spectra is monitored. Site selective ODMR studies also give the same results of a doubling. Previous theoretical studies using different computational procedures indicate that the molecule in its ground state can take on both a trans and a quasi-cis conformation. Our experimental evidence strongly corroborates the existence of two conformations in both the ground and excited states, and we assign the two observed triplet states as arising from the two geometrical isomers.
Introduction The double molecule 2,2’-bipyridyl has presented a continuing challenge to spectroscopists; transition-metal complexes of the molecule have been the most widely studied, since they have important applications in energy storage.’ Our interest in the molecule arises from earlier observations on other double molecules such as 2,2’-biquinoline* and 2,2’-biq~inoxaline,~ which exhibit localized and delocalized triplet states. This led us to search for a similar phenomenon in 2,2’-bipyridyl, since structural inferences from such observations would be of consequence to the behavior of bipyridyl as a ligand. In addition, the corresponding localized triplet state should be a “pyridine-like” state, which has heretofore not been directly accessible to spectroscopic inve~tigation.~ Spectroscopic studies on this molecule to date have often been confusing and contradictory. High-field triplet state EPR studies on bipyridyl in poly(viny1 alcohol) (PVA) films seem to indicate the presence of two distinct conformations of the m o l e c ~ l e , ~ - ~ whereas similar studies in glassy matrices show only one conformation.* X-ray crystallographic studies on the other hand suggest that the molecule maintains a trans planar geometry in the neat c r y ~ t a l . The ~ presence of dimerlike splittings in the first excited singlet state have also been reported.I0 We have examined in detail the lowest triplet state of the 2,2’-bipyridyl molecule on the basis of its low-temperature phosphorescence and zero-field optically detected magnetic resonance (ODMR) in pseudocrystalline Shpolskii matrices. The presence of the quadrupolar I4N nuclei in this molecule should in principle allow us to characterize the observed triplet states on the basis of the hyperfine structure in the zero-field ODMR transitions. Experimental Section 2,2’-Bipyridyl obtained from Aldrich Chemicals was extensively (1) Wrighton, M. S. J. Chem. Educ. 1983, 60, 877. (2) Clarke, R. H.; Mitra, P.; Vinodgopal, K. J . Chem. Phys. 1982, 7 7 , 5288. (3) Vinodgopal, K.; Fleischman, S. H.; Leenstra, W. R. J. Phys. Chem. 1984, 88, 3982. (4) Motten, A. G.; Kwiram, A. L. J. Chem. Phys. 1981. 75, 2608. ( 5 ) Ito, T.; Higuchi, J. Chem. Lett. 1974, 12, 1519. (6) Ito, T.; Higuchi, J. Chem. Phys. Lett. 1977, 46, 477. (7) Higuchi, J.; Yagi, M.; Iwaki, T.; Bunden, M.; Tanigaki, K.; Ito, T. Bull. Chem. SOC.Jpn. 1980, 890, 53. (8) Gondo, Y.; Maki, A. J. Phys. Chem. 1968, 72, 32 IS. (9) Merrit, L. L.; Schroeder, E. D. Acta Crystallogr. 1956, 9, 1981. (10) McAlpine, R. D. J . Mol. Spectrosc. 1971, 38, 441. (1 1) Harris, C. B.; Buckley, M. J. In ”Advances in Nuclear Quadrupole Resonance”; Smith, J. A. S., Ed.; Heyden: London, 1975; Vol. 2. (12) Dinse, K. P.; Wins.com, C. J. In ‘Triplet State ODMR Spectroscopy”; Clarke, R. H., Ed.; Wiley-Interscience: New York, 1982. (13) Dinse, K. P.; Winscom, C. J. J. Chem. Phys. 1978, 68, 1337. (14) Leenstra, W. R.; Vinodgopal, K. Chem. Phys. Lett. 1985,115, 3 1 1 .
0022-3654/85/2089-3824$01.50/0
purified, first by recrystallization and then by zone refining. Gold label n-alkanes obtained from Aldrich Chemicals were purified by distillation, followed by passing the hydrocarbon several times through columns of activated silica gel. Samples of bipyridyl in M and quickly alkane were made up to concentrations of frozen in a quartz sample tube. The phosphorescence spectra were obtained at 1.8 K with a 0.5-mJarrel-Ash monochromator by excitation with the filtered output of a 200-W high-pressure Hg lamp. Band-pass was set at 10 cm-l, a value less than the molecular line width of -20 cm-l. Instrumental details for ODMR experiments were essentially the same as described earlierS3All of the ODMR experiments were carried out at 1.8 K. Microwave sweep rates were dictated by molecular dynamics at 1-2 MHz.
-
Results Phosphorescence. While previous workers have reported the phosphorescence of bipyridyl in glassy solvents such as cyclohexane,15the present work constitutes the first attempt to obtain a resolved emission spectrum in an appropriate Shpolskii matrix. The 1.8 K phosphorescence spectrum of bipyridyl in n-heptane is shown in Figure 1. The emission is highly structured and well-resolved, with the 0-0 band at 20490 cm-l being the most intense line in the spectrum. The line width of the phosphorescence origin was calculated to be about 20 cm-l. Castellucci et al. have previously reported the detailed vibrational spectrum of the 2,2’-bipyridyl molecule in a neat Their analysis shows that, as expected for a molecule with a center of symmetry, the infrared and Raman vibrations are mutually exclusive. We have used their results to make a detailed vibronic analysis of our phosphorescence spectrum in n-heptane. The analysis shows that the n-heptane spectrum can be resolved into an emission arising from just one major site. The spectrum also reveals major vibronic bands at uW +1214, +1459, +1565, and +1585 cm-I, which correlate well with the Raman active ag vibrations at 1236, 1446, 1569, and 1587 cm-I, respectively. Some fairly intense nontotally symmetric vibrations were also observed in the emission spectrum: the vibronic bands at vW +110, +805, and +882 cm-I correlate well with the infrared-active a, vibrations at 11 1 and 808 cm-’ and the b, vibration at 890 cm-I, respectively. A complete assignment of the entire phosphorescence vibronic spectrum is given in Table I. (15) Gondo, Y.; Kanda, Y. Bull. Chem. SOC.Jpn. 1965, 38, 1187. (16) Castellucci, E.; Angeloni, L.; Neto, N.; Sbrana, G.Chem. Phys. 1979, 43, 365. (17) Neto, N.; Muniz-Miranda, M.; Angeloni, L.; Castellucci. E. Spectrochim. Acta., Part A 1983, 39A, 97. (18) Muniz-Miranda, M.; Castelucci, E.; Keto, N.; Sbrana, G.Spectrochim. Acta., Parr A 1983, 39A, 107.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 18, 1985 3825
Structural Features of 2,2’-Bipyridyl TABLE I: Phosphorescence Analysis of Bipyridyl in n-Heptane direction E, vibr of ODMR wavelength, A cm-’ intens assign. mode signal ~~
4257.0 4267.0 4297.0 4315.0 4326.0 4338.0 4370.0 4376.0 4391 .O 4398.0 4408.0 4423.0 4441.8 4462.0 4470.0 4489.0 4504.0 4509.0 4532.5 4539.0 4554.0 4561.0 4565.0 4571.5 4577.8 4599.6 4604.3 46 10.6 4616.9 4627.8 4632.5 4648.1 4660.6 4679.4 4695.0 4700.0 4705.0 4715.0 4725.0 4734.5 4737.0 4746.0 4755.0 4760.0 4770.0 4775.0 4780.0 4787.0 4804.0 4811.0 4825.0 4830.0 4845.0 4854.0 4869.0 4880.0 4894.0 4905.0 4915.0 4919.0
23491 23 436 23 381 23 175 23 116 23 052 22 883 22852 22 774 22738 22 686 22 609 22513 22412 22371 22 277 22 203 22 178 22 063 22031 21 958 21 925 21 906 21 875 21 845 21 741 21 719 21 689 21 660 21 609 21 587 21 514 21 457 21 370 21 299 21 277 21 254 21 209 21 164 21 121 21 104 21 070 21 031 21 008 20 964 20 942 20921 20 890 20816 20 786 20 725 20 704 20 640 20 602 20538 20 49 1 20433 20 387 20 346 20 329
~
vs sh
m
s w vw w w w
m m s
s
m m s
m m m s
w
vs s w w w w m w
m sh w w m w w w w w
m w w m w w
m w w w
w w
m vw vw vw vw vw vw vw vw
0-0
0-0- 55 0-0- 110 0-0-316 0-0-375 0-0-439 0-0-608 0-0-639 0-0-717 0-0-753 0-0- 805 0-0- 882 0-0-977 0-0- 1083 0-0- 1119 0-0- 1214 0-0- 1288 0-0- 1313 0-0- 1428 0-0- 1459 0-0- 1532 0-0- 1565 0-0- 1585 0-0- 1617 0-0- 1646 0-0- 1750 0-0- 1772 0-0- 1801 0-0- 1831 0-0- 1882 0-0- 1904 0-0- 1977 0-0-2034 0-0-2120 0-0-2191 0-0-2214 0-0-2237 0-0-2282 0-0-2327 0-0-2369 0-0 - 2380 0-0-2420 0-0- 2460 0-0-2482 0-0-2526 0-0 - 2548 0-0- 2570 0-0-2601 0-0-2675 0-0- 2705 0-0-2765 0-0-2787 0-0-2851 0-0 - 2889 0-0-2953 0-0 - 2999 0-0- 3058 0-0-3103 0-0-3145 0-0- 3161
73, a, lll,a, 330,a, 355, b, 438, b, 612, a, 2x316 736, b, 763, aB 808, a, 890, b, 974, b, 1088, a, 1143,a, 1235,a, 1297,a, 1311,a, 1440,a, 1479,a, 3 1 6 + 1214 1569,a, 1587, a, 55 + 1565 55 + 1585 1313 + 4 3 8 1459 316 1428 375 1459 375 1565 316 1565 332
+ + + + + 1428 + 608 1313 + 805
+ + +
1313 882 1459 753 1428 805 1565 + 7 1 7
+
1565 805 805 + 1585 2 X 1214 882 1585
+
977
+ 1565
+ 1565 + 1565 + 1565 1440 + 1565 1479 + 1565
1214 1288 1313
2 X 1565 1565 1585
+
+ + + + + + + + -
TABLE 11: ODMR Frequencies and ZFS for 2,2’-Bipyridyl .. . in n-Heptane (Adct = 4257 A) 2E. MHz
triplet state I triplet state I1
763.5 777.5
0
+
0 0
+ + + + + + + + + + + + + + + +
We have also studied the emission in the Shpolskii matrices hexane and octane. In both of these hosts, the phosphorescence can be analyzed as arising from two major sites with the 0 4 bands occurring at 23 501 and 23 468 cm-’ in hexane and at 23 612 and 23 496 cm-’ in octane. The vibrational progressions observed in each of these matrices are essentially the same as those observed in heptane. Zero-Field ODMR. Microwave modulation of the emission in the heptane matrix at the 0-0 band resulted in three sets of resonances corresponding to the 2E, the D - E and the D E transitions. The ODMR data for the heptane matrix are summarized in Table 11. For all three transitions, the signals correspond to increases in phosphorescence intensity. It was also
+
D + E.. MHz 3753.2 3756.1
IDI. , . , cm-’
cm-’
0.1124 0.1253
0.0127 0.0130
IEl. I
I,
TABLE III: Frequencies of the 2E ODMR Transitions of the Two Conformers in Various Shpolskii Hosts detection wavelength, 2E ODMR freq, host nm MHz hexane 425.5 731.1 426.1
-
+ + + + + + + + + + + + + + + + + + + + + + + + + -
D - E. MHz 2991.3 2980.5
heptane
425.7
octane
423.5 425.6
743.0 813.0 817.6 763.5 777.5 837.3 842.8 783.3 809.4
observed that as the microwave power was gradually attenuated from the 20 mW generated by the sweep oscillator to less than 1 l W , what was a single ODMR peak began to separate itself into two distinct transitions, Le., a doublet for each zero-field ODMR transition. (The 2E transitions were observable at applied microwave powers as low as 0.2 bW.) However, applying even lower microwave power exposed no distinct hyperfine satellites on each of the doublets. All six transitions are shown in Figure 2, while the corresponding frequencies are listed in Table 11. Addition of the 2E and D E doublet component frequencies gave the corresponding frequencies of the D E transition, implying thereby that each component of the observed doublet arises from a distinct triplet state. ODMR experiments done on the various vibronic bands gave rise to the same doublet, identical both in the observed frequencies and in intensity ratio with that observed at the 0-0 band. One major difference was observed for the ODMR transitions detected at those vibronic bands, corresponding to progressions of the infrared-active vibrations, mentioned in the preceding section. While the transitions were again observed at exactly the same frequencies, they corresponded to decreases in phosphorescence intensity, unlike those observed on the other vibronic lines. For a few vibronic lines, no ODMR signals (or an extremely weak one) were observed. The direction of the 2E ODMR signal at each vibronic band is indicated in the last column of Table I by the symbols +/-/0, denoting respectively an increase/decrease/no change in emission intensity. We have used this type of information to assist in the assignment of the vibronic contributions, the details of which are presented in the subsequent discussion. We have also carried out the ODMR studies in other Shpolskii matrices, such as hexane and octane. Here again, low applied microwave power studies indicated the presence of a doublet, as was the case in heptane. As indicated in the previous section, two distinct sites are observed in the emission in each of these matrices, and the corresponding ODMR frequencies observed while monitoring the phosphorescence from these two sites differ by 30 MHz on the average. The frequencies of the 2E transition in all three hosts are summarized in Table 111. We shall henceforth call the triplet states giving rise to the observed doublets in the ODMR transitions as triplet state I and triplet state 11, the former having lower values for the zero-field parameters D and E than the latter.
+
Discussion An interesting feature of the phosphorescenceis the anomalously large full width at half-maxima (fwhm) of the individual lines, as compared to that observed for biquinoline and biquinoxaline. It is reasonable to believe however, that the bipyridyl molecule will have more torsional flexibility than biquinoline or biquinoxaline, so that the small bipyridyl molecule should have
3826 The Journal of Physical Chemistry, Vol. 89, No. 18, 1985
I
l
l
1
I
I
446
436
426
I
I
466
I
486
1
1
Vinodgopal and Leenstra
I
476
I
486
WAIIELB)((TH I
1
1
406
I
I
606
1
1
S16
1
1
626
1
I
636
nn
Figure 1. Phosphorescence spectrum of 2,2’-bipyridyl in n-heptane at 1.8 K.
FEQLBCYIWZ Figure 2. ODMR spectra of 2,2’-bipyridyl in n-heptane at 1.8 K. From top to bottom, spectra represent the D + E , D - E, and 2E transitions. All signals represent increases in phosphorescence intensity.
dihedral geometries. A similar argument has been made to explain the spectroscopic features of f ~ r i 1 . l ~ The detailed vibrational analysis of Castelucci et a1.I6l8 allows us to carry out a rigorous analysis (within the limitations of our line widths) of the phosphorescence spectrum in the heptane matrix. All of the vibronic bands have intensities that are weaker than the 0-0 band, indicating thereby that there is no large geometry change in the molecule on being excited from the ground singlet state to the first excited triplet state. While most of the vibr6nic bands observed in the emission spectrum correspond to the Raman-active totally symmetric vibrations, the presence of the infrared-active vibrations cited earlier suggests that the molecule has undergone some distortion from planarity, consistent with the torsional flexibility mentioned above. This is borne out particularly by the presence of the a, vibration at 110 cm-l, which Castellucci et a1.16’* have identified as an out-of-plane inter-ring a, vibration. It is significant that such out-of-plane nontotally symmetric interring vibrations are not observed in the emission spectrum of other 2,2’ double molecules such as biquinoline and biquin~xaline.~,~ The empirical observation of negative ODMR signals being correlated with the nontotally symmetric vibrations at 110, 805, and 882 cm-’ hgve helped us assign combination vibronic bands at 2191, 2369,#2380,and 2460 cm-I, where an asymmetric contributor leads to a net decrease of zero change in signal intensity. One exception is the low-frequency torsional mode at 55 cm-I, which can easily be explaihpl as being the result of its position as a shoulder on the strong 0-0 band. Our kinetic measurements indicate an-overall triplet lifetime of 1.0 s. This suggests that the observed triplet states are of the a-a* type. Such long decay times are characteristic of many analogous aromatic heterocyclic systems such as quinoline and quinoxaline,20where the lowest excited triplet state is known to be AT*. Further, as will become clear in the subsequent discussion, the observed ZFS for the two triplet states are in line with those expected for a a-a* state. The ODMR studies lead to further structural conclusions. The most interesting feature of the ODMR spectra is the doubling of each zero-field transition on each site. There are various possible rationalizations for this phenomenon and we shall consider each one separately. (a) It is possible that the observed doubling arises from two electrostatically distinct sites, in the heptane matrix, with different ZFS parameters. These could be hidden under the inhomogeneous line widths of the optical transitions but are resolved in the ODMR spectra. However, our phosphorescence and ODMR studies of the bipyridyl molecule in the Shpolskii matrices hexane and octane rule out such a possibility. In both hexane and octane, we do observe multiple sites. As Table I11 shows, the ODMR frequencies observed for the different sites are separated by as much as 80
considerable freedom for molecular motion in a pseudocrystalline lattice. Thus, the large observed line width is suggestive of an ensemble of bipyridyl molecules with some minor distribution of
(19) Sandroff, C. J.; Chan, I. Y.Chem. Phys. Lett. 1983, 97,60. (20) Ziegler, S. M.; El-Sayed, M. A. J . Chem. Phys. 1970, 52, 3257.
I
3700
3750
I
I
750
1
I
I
3ooo
2980
I
I
770
I
I
790
Structural Features of 2,2’-Bipyridyl MHz. Such variations are consistent with numbers obtained for such diverse systems as porphyrins2’ and biquinoline,2 where numerous sites are observed in the optical spectrum. It is extremely unlikely that two different sites would give rise to identical ODMR transitions (both in frequency and intensity ratio between the two components of the doublet) on each and every vibronic band of the emission spectrum, as we have observed. (b) It is also possible to invoke the triplet dimer mode122*23 and consider the bipyridyl molecule to be a dimer of two strongly interacting pyridine units in a plane-parallel configuration. An intramolecular excitonic interaction between the two pyridine halves could thus give rise to dimer states. There are two possible interpretations, however, using the dimer model. In the first case, the two transitions observed could be considered to arise separately from the monomer and dimer manifolds, Le., the high-frequency transition originates from a triplet state with spin density localized on one half of the molecule only (the monomer state), while the low-frequency transition originates from a triplet state with spin density delocalized over the entire double molecule (the dimer state). Such states have been observed in the double molecules 2,2’-biquinoline2 and 2,2’-biq~inoxaline.~In fact, those observations led us initially to study bipyridyl, where we hoped to see a localized state arising from the pyridine half of the molecule. However, a localized state in bipyridyl would have pyridine-like ZFS: wh’ich would be considerably higher than those we obtained for either of the two transitions in the doublet. Further, in both biquinoline and biquinoxaline, as a result of exchange narrowing,24 the localized triplet states have 2 E line widths which are nearly twice that of the delocalized states, whereas in bipyridyl, both 2E transitions in the doublet have the same line width of approximately 3 MHz. Thus, we can fairly readily rule out the possibility that the two triplet states observed in bipyridyl arise from localized and delocalized states. The second possible interpretation, arising from the exciton model, is to consider the two sets of ODMR transitions as arising from the symmetric and antisymmetric dimer states, with a concomitant nonobservation of the monomer state. (McAlpine’O has reported the presence of such states in the low-temperature So-S1 absorption spectra of a mixed crystal of bipyridyl in biphenyl. However, the energies of the transitions reported lead us to believe that the observed system was not in fact bipyridyl.) Zewail and Harris25 have shown in their studies on 1,2,4,5tetrachlorobenzene (TCB), that spin-orbital anisotropy can lead to small but resolvable differences in observed ODMR frequencies for the two dimer states. The O D M R frequencies for the two triplet states in our case differ on the average by only 6 M H z (Table 11), which is comparable yith numbers reported for TCB dimers,2s where the observed ODMR transitions have been conclusively assigned to the + and - dimer states. However, dynamic arguments are of critical importance, since Cooper et aLZ6have shown that the and - dimer components should have appreciably different decay rates. Our pfeliminary dynamic measurements indicate no such difference. We thus eliminate this second excitonic interpretation since (i) the monomer state was not observed and (ii) dynamic arguments are contradictory. (c) The last possible explanation for the observed doubling of the ODMR transitions is the likely existence of two distinct conformations of the bipyridyl molecule in our Shpolskii matrices. As will be clear from the subsequent discussion, this represents the most likely explanation for the observed doubling. The presence of two possible conformations of bipyridyl has been suggested by various a ~ t h o r s . ~Using ~ - ~ ~different calpulational
+
(21) Langhoff, S.R.; Davidson, E.’R.; Gouterman, M.; Leenstra, W.R.; Kwiram, A. L. J . Chem. Phys. 1975, 62, 169. (22) Clarke, R. H.; Hobart, D.; Leenstra, W. R. J . Am. Chem. SOC.1979, 101, 2416. (23) Sternlicht, H.; McConnell, H. M. J . Chem. Phys. 1961, 35, 1793. (24) Hutchison, C. A.; King, J. S. J . Chem. Phys. 1973, 58, 892. (25) Zewail, A. H.; Harris, C. B. Phys. Rev. B 1975, 1 1 , 935. (26) Cooper, D.E.; Fayer, M. D. J. Chem. Phys. 1978,68, 229. (27) Agresti, A.; Bacci, M.; Castellucci, E.; Salvi, P. R. Chem. Phys. Lett. 1982, 89, 324.
The Journal of Physical Chemistry, Vol. 89, No. 18, 1985 3827 procedures, these authors have obtained the energy of the molecule as a function of the dihedral angle between the two halves of the double molecule. The potential energy diagrams clearly indicate that the molecule has two conformations (the trans planar and a quasi-cis) at energy minima, with the trans-planar arrangement being the more stable one of the two. (At the freezing temperature of the matrices concerned, a ratio of 1:l is predicted.) It must be emphasized that these calculations were carried out for the free molecule and the results therefore could be different from those for the molecule in the solid state, where its conformation is determined by the competition between short-range intramolecular forces and the long-range forces of the crystalline lattice. In fact, as mentioned earlier, the particular stacking forces in the neat crystal are such that only the trans form was observed via X-ray crystallography. We consequently do not attempt to make an exact description of the geometrical features, but certainly infer the validity of cis- and translike conformations. Only one experimental attempt at discerning the cis and trans conformations of bipyridyl exists in the literature. Higuchi and c o - w o r k e r ~ ~have - ~ identified two triplet states in a sample of bipyridyl in a stretched PVA film via high-field EPR, one of whose ZFS parameters matches one of our values (viz., our triplet state 11). However, their second triplet state, which they deem to arise from the cis conformation, is inferred from an EPR spectrum’ where one of only two AM = 1 transitions were uniquely observed, one of which has a signal-to-noise ratio of approximately 1.5. These workers have also calculated 1El values for both the cis and trans conformations using semiempirical M O methods: trans IEI values are calculated to be larger than cis by a factor of 1.19. The calculated values furthermore contradict their experimental numbers for which trans is larger than cis by a factor of 2.67. These two concerns lead us to believe that their presumed cis form represents an unidentified impurity. On the other hand, our experimental results seem to be in good agreement with the calculations since our ODMR measurements reveal two triplets whose IEI values are different by only a factor of 1.02. Taking into account all of the above evidence, we believe that our observed doubling of ODMR transitions arises from the presence of the two distinct conformations and assuming that the
-
I
II
calculations predict a qualitative ordering, we assign our observed triplet states I and I1 to the cis and trans forms, respectively. Conclusions
From the foregoing discussion, it is clear that it is the presence of two isomers (cis and trans) at each optically resolvable site which is responsible for the observed doubling of the ODMR transitions. Site explanations for the doubling effect have been discounted since we do observe multiple sites that are far apart in ODMR frequencies. Furthermore, even optically unresolvable minor sites would not exhibit the doubling effect on each and every vibronic band for each zero-field transition. The lack of any identifiable hyperfine and quadrupolar satellites to the ODMR transition presents itself as a significant gap in our experimental data. A value for the hyperfine splitting constant A,,11-14would have helped us unambiguously identify the source of the doublet. Significantly lower values for A,, would imply a dimeric state,2s whereas comparable A,, values eliminate excitonic considerations. Unfortunately, the torsional freedom mentioned earlier has eliminated this avenue of corroboration. At any rate, dynamic data argue against excitonic considerations. (28) Benedix, R.; Birner, P.; Birnstock, F.; Hennig, H.; Hofmann, H. J. Mol. Strurr. 1979, 51, 99. (29) Barone, V.; Ley,F.; Cauletti, C.;Piancastelli, M. N. Mol. Phys. 1983, 49, 599.
3828
J. Phys. Chem. 1985,89, 3828-3833
In an attempt to provide further confirmation of the conformational model, we are presently carrying out similar experiments with symmetrically substituted bipyridyl derivatives. It is our hope that with the appropriate choice of substituent and its position on the bipyridyl molecule, we would be able to constrain the rotational freedom of the molecule such that it would be forced to exist in only a single conformation. Observation of only one set of ODMR transitions in such a case would clearly favor the
interpretation of two conformations.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for Padial SUPpOfi ofthis work, to the University of Vermont (UVM PS-Il), and to the Research COrPoration (9326). Registry No. Bipyridyl, 366-18-7.
Silica-Supported ZnSCdS Mixed Semiconductor Catalysts for Photogeneration of Hydrogen A. Ueno,+ N. Kakuta, K. H. Park, M. F. Finlayson, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber, and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: February 26, 1985)
A silica-supported mixed semiconductor catalyst, ZnS-CdS/Si02, immersed in aqueous sulfide solutions shows significant activity for hydrogen generation from water under illumination with either UV or visible light. The individually supported ZnS/SiO, and CdS/Si02 were much less active. A physical mixture of ZnS/Si02 and CdS/Si02 also did not improve the activity, even when the amounts of ZnS and CdS were the same as those in the ZnS.CdS/Si02 catalyst. This indicates that intimate contact between ZnS and CdS particles is necessary for photogeneration of hydrogen. Surface analysis (X-ray photoelectron spectroscopy and sputtering) indicates that the active catalyst had a layered structure around the silica particles with a CdS-rich layer coating the silica and a ZnS-rich layer overcoating the CdS. Samples prepared by sequential deposition of ZnS followed by CdS were much less active for hydrogen production than samples prepared either by sequential deposition in the opposite order or by coprecipitation. ZnSCdS on a number of other supports (e.g. Nafion, nylon, and alumina) is also quite active. Electron microscopy and X-ray diffraction show that neither bulk particle morphology nor support are critical. Overall, the critical factors are (1) CdS overcoating ZnS, (2) close proximity of Cds and ZnS, and (3) minimization of particle shadowing (light scattering).
Introduction Photogeneration of hydrogen from water is of current interest and has been extensively studied by using semiconductors as optically active materials. Oxide semiconductors such as TiO2I and SrTi032have been attractive because of their stability with respect to photodriven corrosion. Chalcogenide semiconductors have also been used but require sacrificial reagents such as Na2S and Na2S03to limit ph~tocorrosion.~Among the chalcogenides, zinc4 and cadmium5 sulfides have been widely studied and their electronic and electrochemical properties are reasonably well described. Light conversion efficiencies of these semiconductor materials increase with increasing surface-to-volume ratios. Henglein et al.6*7studied the electronic properties and activities for photogeneration of hydrogen using colloidal ZnS and CdS. They reported that the electronic structure of these colloidal chalcogenides differed from that of larger particles. This was reflected in a shorter wavelength onset of the absorption spectra of the colloids. Reber et a].* have also reported an enhancement in the rate of hydrogen generation as the particle size decreased to colloidal dimensions. Gratzel et aL9 proposed an interparticle electron transfer to explain an enhancement in the rate of hydrogen production with a CdS/TiO, mixed semiconductor system. In earlier work we have shown that a ZnS-CdS mixed semiconductor catalyst incorporated into a Nafion film is a factor of 50 more active for photogeneration of hydrogen than either ZnS/Nafion or CdS/Nafion.'O The purpose of the present work was to study the morphology/structure of a ZnSCdS mixed semiconductor catalyst on SiO, and to compare these results with other supports, including Nafion.lO Resent address: Department of Materials Science, Toyohashi University of Technology, Toyohashi 440, Japan.
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Experimental Section ( 1 ) Catalyst Preparation. Several kinds of catalysts composed of ZnS and/or CdS were employed and their preparation procedures are given in the following paragraphs. (a) ZnS/SiO, and CdSlSiO,. The ZnS and CdS catalysts were prepared by conventional impregnation techniques. Silica powder (1 .OO g of Cab-0-Si1 300, Degussa) was immersed in 50 mL of mol of Zn(N03)2.6H20 aqueous solution containing 2.3 X (Fisher Scientific) or Cd(N03)2.4H20 (Fisher Scientific). The mixture was stirred for 2 h and then 50 mL of water saturated ( 1 ) S. N. Frank and A. J. Bard, J . Phys. Chem., 81, 1484 (1977); A. J. Bard, J . Photochem., 10, 59 (1979); T. Sakata and T. Kawai, Chem. Phys. Letr., 80, 341 (1981). (2) J.-M. Lehn, J.-P. Sauvage, and R. Ziessel, N o w . J . Chim.,4, 623 (1980); K. Domen, S. Naito, T. Onishi, and K. Tamaru, Chem. Phys. Letf., 92, 433 (1982). (3) T. Inoue, T. Watanabe, A. Fujishima, K. Honda, and K. Kobayakawa, J . Electrochem. Soc., 124,719 (1977); A. B. Ellis, S. W. Kaiser, and M. S. Wrighton, J . Am. Chem. Soc., 98, 6855 (1976). (4) R. E. Stephens, B. Ke, and D. Trivich, J. Phys. Chem., 59,966 (1976), S . Yanagida, T. Azuma, and H. Sakurai, Chem. Left., 1868 (1982). ( 5 ) M. Gutierrez and A. Henglein, Ber. Bunsenges. Phys. Chem., 87,474 (1983); J. R. Darwent, J. Chem. Soc., Faraday Trans. 2,77, 1703 (1981); M. Matsumura, Y. Saho, and H. Tsubomura, J. Phys. Chem., 87, 3807 (1983). (6) A. Henglein and M. Gutierrez, Ber. Bunsenges. Phys. Chem., 87, 852 (1983). (7) H. Weller, U. Koch, M. Gutierrez, and A. Henglein, Ber. Bunsenges. Phys. Chem., 88, 649 (1984). (8) J.-F. Reber and K. Meier, J. Phys. Chem., 88, 5903 (1984). (9) N. Serpone, E. Borgarello, and M. Gratzel, J. Chem. Sot., Chem. Commun., 342 (1984); E. Borgartllo, K. Kalyanasundarama, and M. Gratzel, Helv. Chim. Acta, 65, 243 (1982). (10) N. Kakuta, K.-H. Park, M. F. Finlayson, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber, and J. M. White J . Phys. Chem., 89, 732 (1985); A. W.-H. Mau, C. B. Huang, N. Kakuta, A. J. Bard, A. Campion, M. A. Fox, J. M. White, and S. E. Webber, J. Am. Chem. Soc., 106, 6537 (1984).
0 1985 American Chemical Society
