J . Phys. Chem. 1989, 93, 2337-2341
2337
Intramolecular Hydrogen Bonding. 8. Comparison of Free Jet and Shpol'skii Matrix Spectra of 1-Aminoanthraquinone N. Balakrishnan and G. D. Gillispie* Department of Chemistry, North Dakota State University, Fargo, North Dakota 581 05 (Received: August 1, 1988)
Dispersed fluorescence spectra have been measured for excitation of various SI vibronic levels of 1-aminoanthraquinone in a free jet expansion. The emission spectrum from the vibrationless level of SI agrees completely with the Shpol'skii matrix spectrum, whereas several features of the jet excitation spectrum are doubled compared to the matrix spectrum and there is a new active excited-state fundamental vibration at 118 cm-I. The differences in the matrix and gas-phase spectra are likely associated with large-amplitude motion of the intramolecular hydrogen bond chelate ring. The single vibronic level emission spectra reveal a large Duschinsky effect despite the seemingly good agreement between the frequencies of active So and SI fundamental vibrations. Deuterium substitution for the intramolecularly hydrogen bonded proton shifts the Sl-So 0-0 band 81 cm-I to the blue. This value is surprisinglylarger than the 49-cm-' shift found in the Shpol'skii matrix experiments.
Introduction Previous papers in this series have focussed OR the vibronic spectra of molecules with intramolecular hydrogen bonds and the information these spectra yield on excited-state proton-transfer potential functions. The goal is to identify the electronic and structural factors that control whether or not proton transfer m u r s and on what time scale. The Shpol'skii matrix fluorescence and fluorescence excitation spectra of 1-aminoanthraquinone (1-NH2-AQ) were reported previously.' The absence of excited-state proton transfer in 1-NH,-AQ provides an interesting contrast to the a-hydroxyanthraquinones, which clearly undergo facile proton transfer.2 In fact, the intramolecular hydrogen bond seemed to have almost no influence on the 1-NH,-AQ spectra. A reasonable mirror image symmetry between the fluorescence and excitation spectra exists, and deuterium isotope effects on the vibronic structure were found to be small. The Shpol'skii matrix paper also included a preliminary free jet excitation spectrum of the normal isotopic species; some small differences with the matrix excitation spectrum were noted. Now we have explored the jet spectra of 1-aminoanthraquinone in greater detail. Dispersed fluorescence spectra from about a dozen SI vibrational levels within 600 cm-l of the S1origin are presented. This added information reveals that Duschinksy effects (vibrational mode mixing) associated with electronic excitation are more severe than previously thought. We have also examined the effect of deuterium substitution in the hydrogen bond on the jet spectra. Experimental Section A C W argon expansion seeded with 1-NH2-AQvapor (nozzle diameter 250 pm) was crossed 10-1 5 nozzle diameters downstream with the output of an excimer pumped tunable dye laser (Lumonics EX-520 excimer laser and Hyperdye tunable dye laser) operating with Coumarin 460 dye. The detection system for the excitation spectra included an f / l aspheric lens to collect the emission, a 500-nm glass cut-on filter to reject scattered exciting light, and an uncooled Hamamatsu R-928 photomultiplier tube. Signals were processed with a Stanford Research Systems boxcar averager. The emission spectra were dispersed in an Instruments SA HR-640 monochromator equipped with 1800 grooves/mm holographic grating. Slit widths of 250-500 ym were used, giving an effective resolution of 0.25-0.50 nm. Results The free jet excitation spectrum of 1-aminoanthraquinone is shown in Figure 1. Bands A through F obviously correspond to the similarly designated fundamentals in the Shpol'skii matrix *Author to whom correspondence should be addressed.
TABLE I: Vibrational Frequencies of Active Fundamentals in the Free Jet Electronic Spectra of l-Aminoanthraquinoneosb
1-NHZ-AQ band
SO
A
218 (227) 320 (322) 346 (350) 385 (389) 475 (478) 554 (547)
B C D E F
SI 211/216 (222) 307/312 (312) 343 (342) 380 (380) 499/495 (493) 547 (546)
1-ND,-AQ SI
213 (220) 300/304 (308) 330/335 (338) 373 (377) 486 (490)
- (541)
'In cm-'. bValues in parentheses are for condensed phase; see ref 1.
excitation spectrum.' Except for the lowest frequency band (A), the agreement is to within 5 cm-' (Table I). However, A, B, and E are all doublets in the jet spectrum, and the fundamental at 118 cm-' is unambiguously missing in the matrix spectrum; numerous other weaker features are observed, e.g., the cluster around the base of band D. Conventional wisdom might suggest that these are merely a consequence of the greater resolution and freedom from phonon sidebands afforded by the gaseous environment, compared to the Shpol'skii matrix. However, the matrix excitation features are quite narrow (3-4 cm-l fwhm) and phonon sidebands are weak. There is no possibility whatsoever that comparable splittings are actually present in the matrix spectra. The excitation spectrum of a partially deuterated 1-NH,-AQ sample is shown in Figure 2. The origin band of the undeuterated l-NH,-AQ is observed at the extreme left of the figure. Almost all of the bands in Figure 2 are therefore attributable to one or more deuterium-carrying species. A satisfactory vibrational analysis develops with the assignment of the strongest band a t 470.28 nm as an electronic origin. The vibrational intervals of fundamentals A-F are only slightly different than in the normal isotopic species (Table I). Several points should be noted about the deuterium substitution effects: (1) the origin shift of 8 1 cm-I to the blue is substantially greater than the 49-cm-' shift found in the Shpol'skii matrix spectrum; (2) the deuterium substitution has more of an effect on the relative intensities than it does on the frequencies of the active fundamentals; (3) a low-frequency fundamental near 120 cm-I is present again. At the laser step size (0.01 nm) and scale of Figures 1 and 2, the rotational structure of the individual bands is not evident. Rotational contours for the 0-0 bands of the deuterated and undeuterated species are shown in Figure 3. The width of the sharp Q-branch in Figure 3A is limited by the laser line width; (1) Carter, T. P.; Van Benthem, M. H.; Gillispie, G. D. J . Phys. Chem. 1983,87, 1891. (2) Balakrishnan, N.; Gillispie, G. D., to be submitted for publication.
0022-3654/89/2093-2337%01.5010 0 1989 American Chemical Society
Intramolecular Hydrogen Bonding
2338 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 FREE JET EXCITATION 1-AMINOANTHRAQUINONE
E
F
I
C
400
200
0
600
800
AF (cm-1) Figure 1. Uncorrected fluorescence excitation spectrum of l-NH2-AQ in an argon free jet expansion. The spectrum was taken at wavelength increments of 0.01 nm but is presented here in wavenumber displacements from the origin band at 472.07 nm. Band labels A-F correspond
to notation introduced for matrix spectra (ref 1). The fundamental at 118 cm-' is not observed in condensed phase. FREE JET EXCITATION SPECTRUM DEUTERATED 1-AMINOANTHRAQUINONE
I
4
2
0
-2
-4
A0 (cm-? Figure 3. Rotational contours of origin bands taken at a wavelength step size of 0.0004 nq7 (A) Normal isotopic species. The width of the central Q-branch is laser limited, ca. 0.002 nm. (B) Partially deuterated sample over the same region as in (A). The feature with Q-branch at approximately + I cm-I is assigned to 1-NHD-AQ, Le., a monodeuterated species with the deuteron nor in the intramolecular hydrogen bond. (C) Origin region near 470.28 nm of species in which hydrogen-bonded
proton has been isotopically replaced. 3200 7
FREE JET FLUORESCENCE OF 1-NHTAQ EXCITED AT SI ORIGIN 472.09 nm 2400 x
0
200
400
2
E
I
600
A I (cm-1) Figure 2. Fluorescence excitation spectrum of a deuterated sample. Zero wavenumber displacement represents the origin band at 470.28 nm of a deuterium-carrying species. The band at extreme left (-81 c d ) is the origin band of the normal isotopic species. The other doublet marked with an asterisk is fundamental E of 1-NH2-AQ.
0+---7x-
20150
'
20950 I
"
'
1
2?350
Wave h u m b e r
similar contours are observed in 1 3 - and 1,s-dihydroxyanthraquinone.2 In Figure 3B,C one observes two vibronic bands in the vicinity of the origin region for the normal and deuterated species, thereby confirming our previous assignment] of the deuterium shifts in the Shpol'skii matrix. Isotopic substitution of the nonhydrogen-bonded amino proton shifts the spectrum less than 1 cm-I compared to 81 cm-' for the hydrogen-bonded proton. We now present a series of emission spectra excited at the prominent features of Figure 1. The dispersed fluorescence from the vibrationless level in SIis shown in Figure 4. It totally agrees with the matrix fluorescence spectrum in every way. The active fundamentals at 218, 320, 346, 385, 475, and 554 cm-I (labeled as A-F) match those in the matrix spectrum both in relative intensity and in wavenumber interval. The largest discrepancy is the frequency reduction from 227 cm-' in the matrix environment to 218 cm-' in the gas phase for the lowest frequency active fundamental. None of the labeled fundamentals are doublets. This spectrum was rerun at higher resolution (5 cm-I), and we are quite confident that there are no splittings comparable to those in the excitation spectrum. The emission from the vibrationless level of SI in l-ND,-AQ (not shown) is in similar good agreement
Figure 4. Dispersed fluorescencespectrum from the SI vibrationless level. Spectral resolution is approximately 10 cm-I. Bands are labeled according to the scheme used in ref 1.
with the matrix fluorescence. In neither 1-NH2-AQ nor 1ND,-AQ is there fluorescence activity corresponding to the 1 18/ 117-cm-I vibration active in excitation. The fluorescence spectra from the excited vibrational levels of SI provide a more definitive test of the So-Sl normal-mode correlations. If a particular excited-state normal mode is nearly parallel to one in the ground state, then the emission spectrum from its u = 1 SI level should show enhanced intensity of the ground-state vibrational levels involving one or more quanta of that mode. The band representing one quantum of that vibration in the ground state then acts like a false origin; Le., there will be displacements from it corresponding to the active vibrations in the fluorescence from the vibrationless level in SI. The 216-, 312-, and 547-cm-' SI vibrations, matched with 218-, 317-, and 55 1-cm-' ground-state vibrations, respectively, exemplify this behavior. Consider the emission from 0 + 216 cm-I in Figure 5. The band of greatest intensity (other than the feature at the
Balakrishnan and Gillispie
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2339
I
E
I
E
0+216 (A)
0+312 (B)
0+495 (E)
0+547 (F)
i 0+499 (E')
Figure 5. Fluorescence spectra from the SI levels with vibrational energies 0, 216, 312, and 547 cm-I. In the three lower spectra the 1-1 transitions of vibrations A, B, and F have been aligned with the 0 band. Top spectrum taken at 0.25-nm resolution and others at 0.50 nm.
I
E
iP
1
CB
I
Figure 7. Fluorescence spectra from the SI levels with vibrational energies 495 and 499 cm-I. The similarity of these spectra indicates a Fermi resonance mechanism is responsible for the 4951499-cm-l doublet in the excitation spectrum. anything else. Notice how intense is the one-quantum displacement of vibration E. The one-quantum displacement of fluorescence fundamental D is stronger in the emission from 0 380 cm-' than it is from 0 343 cm-I, but only modestly so. The two-quantum displacements from the exciting frequency provide stronger evidence that 343 cm-I in SI has its parentage in the ground-state mode of nearly the same frequency, and likewise for 0 380 cm-I. There must be a very significant Duschinsky effect; Le., the normal coordinate for the 343-cm-l mode in SI is a linear combination of the normal coordinates of the 345-cm-' vibration and 475-cm-' vibration normal coordinates in So. The 380-cm-' vibration in SI has substantial contributions from both of these ground-state modes and 385 cm-' in So. Even though there are So and, SI fundamentals at nearly the same frequency and with comparable activity for transitions from the corresponding vibrationless level in the other electronic state, there is substantial mode mixing. The emission spectra for excitation into the SI doublet at 495/499 cm-' makes this point even more evident (Figure 7). The emission spectra are very similar but show almost no activity of the 475-cm-I ground-state vibration even though it is the most intense fundamental in the fluorescence from the S, vibrationless level. The apparent match between 475 cm-' in So and ca. 495 cm-' in SI is an illusion; the 475-cm-' ground-state normal mode is largely redistributed into the 343- and 380-cm-' upper state modes. Of course, this is always possible, but it is surprising in a case where the number of active vibrations, their intensities, and the frequency displacements match so well in the spectra from the vibrationless levels, Le., the kind measured for Shpol'skii matrices. As for the doubling in the excitation spectrum, this is probably a Fermi resonance effect, since the emission spectra from the pair are so similar. A near degeneracy between two zerothorder levels with one carrying very little oscillator strength must be invoked. The 307/312-cm-' pair in S, (Figure 8) is probably another case of Fermi resonance. We have already shown in Figure 3 that 312 cm-l in SI matches well with the 317-cm-' ground-state vibration. Excitation into 0 + 307 cm-' gives about the same emission spectrum as from 0 312 cm-' but with a second set of bands displaced about 35 cm-' (to lower frequency displacement). We propose the existence of a ground-state mode near
+
+
+
- 8
1000
500
0
A V (cm-1) Figure 6. Fluorescence spectra from the SI levels with vibrational energies 0, 343, and 380 cm-'. The spectra are presented in terms of wavenumber displacement from the exciting light which appears to the right of each trace. excitation wavelength which contains a large contribution from scattered exciting light) occurs at 218 cm-' away from the excitation, a displacement also prominent in the fluorescence spectrum from the SI vibrationless level. The 218-cm-' band acts like a false origin for further displacements corresponding to the other fundamentals active in Figure 4. The emission from fundamentals B and F in SI shows similar behavior. Excitation of the 343- or 380-cm-' SI fundamentals gives a more complicated pattern, as is shown in Figure 6 along with the emission spectrum from the SI vibrationless level for comparison. We have chosen to align the exciting frequencies of each spectrum. This form of presentation emphasizes the great similarity in the emission from 0 343 and 0 380 cm-I. Although the 343- and 380-cm-l frequency intervals are very close to ground-state vibrations at 346 and 385 cm-', respectively, labeling 343 cm-' as C and 380 cm-' as D in SI is clearly more a convenience than
+
+
+
2340
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
h
8:
Intramolecular Hydrogen Bonding
Discussion This and similar studies aim to deduce the energetics of intramolecular hydrogen bonds. Owing to a particular interest in proton-transfer processes, much of the focus has been on the first excited singlet state since the proton transfer occurs more commonly in excited than in ground states. Traditional spectroscopic (infrared and Raman) and structural (X-ray, neutron, and electron diffraction) techniques are limited to ground electronic states. Vibrationally resolved electronic spectra are the only source of information about the excited states. The SI So absorbance spectrum (whether recorded directly or in the form of luminescence excitation or resonance-enhanced multiphoton ionization spectroscopy) identifies vibrational frequencies for a select set of SI modes, namely, ones which are not almost identical with a corresponding ground-state mode. The vibronic intensities are the measure of the differences between the So and S, vibrational modes. These differences take the form of normal-mode rotations (the so-called Duschinsky effect) and normal-mode displacements, the latter limited to totally symmetric vibrations. The most tractable case for large molecules is that of a methyl or phenyl group internal rotation. Often the geometry change (i.e., change in dihedral angle) of this coordinate dwarfs all others, and the mode is insignificantly coupled with other vibrations. Several examples in the literature3s4 demonstrate how well excited-state potential functions can be extracted in such “quasidiatomic” situations. These are situations in which Duschinsky rotation can safely be ignored. Warren, Hayes, and Small have presented spectra for /3-methylnaphthalene5 and phenanthrene,6 which require Duschinksy rotation (as well as Herzberg-Teller type coupling) be included to satisfactorily explain the spectra. Undoubtedly, the hope has been that the A-H stretching and bending vibrational modes for A-H-B hydrogen bond situations would fall in the “quasi-diatomic” category so the excited-state hydrogen bond details could be extracted. This hope has not been realized. Isotopic substitution of the hydrogen-bonded proton generally leads to moderate to large modification of many different features in the vibronic spectra, not major changes in just a few. If anything, the spectra presented in this paper demonstrate that Duschinsky effects are even more severe than in previous examples. Subbi has reached the same conclusion for q ~ i n i z a r i n . He ~ has recently published8 a simplified approach to calculate the Franck-Condon factors when normal-mode rotation is large, but no applications have yet appeared. Since quinizarin is the only other compound with intramolecular hydrogen bonds studied at a comparable level of detail to the work here, most conclusions are speculative. We can, however, anticipate some points to guide future work. First, let us consider the greater number of bands in the free jet excitation spectrum compared to the condensed-phase work. A similar situation obtains for quinizarin, where many nontotally symmetric modes appear only in the gas-phase spectrum. One might have expected just the opposite; Le., the site symmetry in the matrix would be lower than the molecular point group, thereby making more vibrations allowed. There appears to be a trend that the additional features of the gas-phase spectra are mostly associated with low-frequency vibrations; it must be conceded that this is possibly an illusion since the region closest to the origin band is generally the easiest to assign. In the case of quinizarin many nontotally symmetric modes, both in-plane and out-of-plane, have been assigned in the jet spectra. Their nonappearance in the matrix spectrum was explained by invoking vibronic coupling of Sl to Rydberg ~ t a t e s . ~ . ~ In the matrix environment the large molecular volume Rydberg
-
1000
500
0
v
A (cm-1) Figure 8. Fluorescence spectra from the S1 levels with vibrational energies of 312 and 307 cm-l. Each of the prominent bands in the upper spectrum also appear in the lower one, but accomplished by additional bands ca. 35 cm-l to lower wavenumber displacement.
I
1 I
0+211
I
240 I
z
I
196
,
1000
0
500 ’
AF
(cm-1)
Figure 9. Fluorescence spectra from the SI levels with vibrational energies of 118 and 21 1 cm-’.
280 cm-’ which correlates with an excited-state vibration whose frequency is almost the same as that of B. This vibration carries little oscillator strength and is therefore silent in the emission from the vibrationless level of S,. In the excited state the zeroth-order frequency of this mode increases, bringing it into near resonance with vibration B. The emission spectrum from 0 118 cm-’, the feature with no counterpart in the matrix spectrum, is shown in Figure 9 along with the emission from 0 + 21 1 cm-’. For the former, low-frequency intervals of 80, 123, and 164 cm-’ from the exciting light are observed with displacements corresponding to vibrations A-E built on these origins. Possibly, 80 cm-’ is the ground-state fundamental frequency corresponding to 118 cm-l in S1 with 164 cm-’ in the emission thus representing a two-quantum transition. The signal-to-noise ratio is not high enough for a examination for the three-quantum transition near 240 cm-’ to test this hypothesis. In the case of the emission from 0 + 21 1 cm-I, there are lowfrequency intervals at 196 and 240 cm-’, neither of which corresponds to bands observed in the emission from the S1 vibrationless level. It does not seem possible to assign 21 1 cm-l to a Fermi resonance with 216 cm-I, as was used to explain the other doublets in the excitation spectrum.
+
(3) Ito, M. J . Phys. Chem. 1987, 91, 517, and references therein. (4) Werst, D.W.; Brearley, A. M.; Gentry, W. R.; Barbara, P. F. J . A m . Chem. SOC.1987, 109, 32. (5) Warren, J. A.; Hayes, J. M.; Small, G. J. Chem. Phys. 1986, 102, 313. (6) Warren, J. A.; Hayes, J. M.; Small, G. J. Chem. Phys. 1986, 102, 325. (7) Subbi, J. Chem. Phys. Lett. 1984, 109, 1. (8) Subbi, J. Chem. Phys. 1988, 122, 157. (9) Smulevich, G.; Amirav, A.; Even, U.; Jortner, J. Chem. Phys. 1982, 73, I
J. Phys. Chem. 1989, 93, 2341-2341 states are effectively "squeezed" out of existence. However, these extra bands should also then be present in the emission spectrum from the SI vibrationless level if their source is vibronic coupling. The spectrum of Figure 4 shows no splittings or additional bands over those found in the matrix fluorescence spectrum. The hypothesis of coupling to Rydberg states requires that the mechanism be specific to the anthraquinones, also, since there is generally good agreement between the gas-phase and condensed-phase spectra of benzene, naphthalene, anthracene, etc. An alternative interpretation is that the molecules undergo large-amplitude motions in the gas phase and that these modes are dampened out in the matrix environment. A puckering motion of the six-membered pseudoring containing the intramolecular hydrogen bond is a good candidate for such a mode. Furthermore, it seems reasonable that such a mode would couple fairly strongly to other vibrations more or less localized in the pseudoring. On the other hand, there is no evident activity of modes highly localized in the hydrogen bond, i.e., the N-H stretch or in- and out-of-plane bends. The change in the origin isotope shift from 49 cm-I in condensed phase to 8 1 cm-I in the gas phase is curious. Many authors have ascribed the shift entirely to the A-H stretching mode of the A-H-B hydrogen bond arrangement, although a good demonstration the procedure is justified is conspicuously lacking. It is easy to show that the origin shift is 0.146Aw in this approximation, where A w is the reduction of the A-H stretching frequency in the excited-state over the ground-state value. The additional shift of 32 cm-' represents an environmental contribution of almost
2341
220 cm-' to Aw, which is an enormous amount for what must be largely still a localized intramolecular mode. A more detailed investigation into this point is hampered by low signal to noise in the spectra. We attempted to measure the fluorescence lifetime of l-NH,-AQ in the gas phase but could not discern any deviation of the emission from the temporal profile of the laser. This places the lifetime at under 1-2 ns. In solution the lifetime (and fluorescence quantum yield) is solvent dependent,I0 but the longest lifetime of 1.75 ns in benzene only corresponds to a fluorescence quantum yield of 0.058. Efforts are under way to improve the collection efficiency of the optics and to convert to a pulsed nozzle for greater emission signal. Then a better study of the emission from the low-frequency excitation features and their dependence on isotopic substitution can be carried out. We also note that we have not been able to find any genuine fluorescence of 2-aminoanthraquinone, which might have been expected to shed some light on the question of the differences in matrix vs gas-phase spectra and the role of the intramolecular hydrogen bond. Acknowledgment. This work was supported by a grant from the National Science Foundation, and we gratefully acknowledge their support. Registry No. 1-Aminoanthraquinone, 82-45-1; deuterium, 7782-39-0. (10) Inoue, H.; Hida, M.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1982, 86, 3184.
Ab Initio Force Constants and the Reassignment of the Vibrational Spectra of all-fransand a//-cis - 1,3,5,7- Oct at et raene Tracy P. Hamilton**+ and Peter Pulay* Department of Chemistry, University of Arkansas, Fayetteville. Arkansas 72701 (Received: August 9, 1988)
Complete scaled ab initio force fields and frequencies have been calculated for the all-trans and all-cis isomers of 1,3,5,7octatetraene. Several peaks in the infrared and Raman spectra have been reassigned, and a medium intensity IR peak at 1580 cm-' is predicted. The agreement between the experimental and theoretical C-C stretching frequencies is very good. We believe that the force constants are superior to previous theoretical ones. A detailed comparison with the results from an extended Pariser-Parr-Pople configuration interaction theory is made. The effect of cis-trans isomerization is discussed. The perdeuterated frequencies are also presented in anticipation of future experimental measurements.
Introduction The vibrational spectra of small linear polyenes has been much studied because many larger molecules, such as carotenoids, visual pigments, and polyacetylene polymers contain linear conjugated chains. Doped polyacetylenel may become an important material in the future; it can be twice as conductive as copper on a weight basis.* The study of the structure and force field of smaller polyenes is essential for understanding the more complex systems. An example of the importance of the vibrational spectroscopy of these molecules is the estimation of the defect density in polya ~ e t y l e n e . ~This is based on the fact that the frequency of the strongest Raman band in linear polyenes decreases smoothly with increasing chain length.4 Both all-cis- and all-trans-polyacetylene are known; their electrical properties are remarkably different. Most carotenoids either are all-trans or contain a single cis-C=C bond. In this Present address: Center for Computational Quantum Chemistry, School of Chemical Sciences, The University of Georgia, Athens, GA 30602.
0022-365418912093-2341$01.50/0
paper, we consider only the all-trans and all-cis forms of octatetraene. Theoretical calculations are essential to the understanding of the vibrational force fields of polyenes because of the long-range coupling caused by the conjugated x system. The latter precludes models with force constants transferred from simple compounds. Obtaining full harmonic force fields from experimental data becomes progressively more difficult as the molecular size increases. For a molecule of the size of octatetraene, it is practically impossible to obtain an experimental force field without the help of a theoretical model. The latter is also important to correct the experimental misassignments that are almost inevitable for a molecule this large. We would like to emphasize the importance (1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. B.; Chiang, C. K.; Heeger, A. J. J. Chem. SOC.,Chem. Commun. 1977, 578. (2) Naarman, H.; Theophilou, N. Synth. Met. 1987, 22, 1. (3) Kuzmany, H. Pure Appl. Chem. 1985, 2, 235. (4) Rimai, L.; Heyde, M. E.; Gill, D. J. Am. Chem. SOC.1973, 95, 4493.
0 1989 American Chemical Society
