4917
J. Phys. Chem. 1986, 9 0 , 4 9 1 1 4 9 2 0
Widely Tunable Band Filling in a Molecular Metal. Chemical and Physical Consequences of Electrochemically Doping a Cofacially Joined Metallomacrocyclic Assembly John G. Gaudiello; Manuel Almeida; Tobin J. Marks,*+ William J. McCarthy,* John C. Butler,t and Carl R. Kannewurft Departments of Chemistry and Electrical Engineering and Computer Science, and the Materials Research Center, Northwestern University, Evanston, Illinois 60201 (Received: July 17 , 1986)
The cofacially joined metallomacrocyclic polymer [Si(Pc)O], (Pc = phthalocyaninato)can be reversibly oxidized electrochemically as a slurry to yield a “molecular metal”. As monitored by controlled potential coulometry, X-ray powder diffraction, infrared spectroscopy,specular reflectance spectroscopy, and thermoelectric power, the stoichiometry of the resulting ([Si(Pc)O](BF,),), polymers can be continuously varied from y = 0.00 to 0.50. This broad, incremental tuning of the band filling (the first time for a molecular metal) is accompanied by a continuous evolution of the optical and charge transport properties from those of a semiconductor to those of a low-dimensional metal-like material.
Controllable variation of band filling remains a major, largely unsolved problem in “molecular metal” A significant obstacle is undoubtedly the capricious nature of the molecular packing forces which strongly influence crystallization st~ichiometry.~We communicate here that the rigorously enforced subunit connectivity of a cofacially j ~ i n e dphthalocyanine ~,~ assembly (A) allows, via electrochemical doping, the first major variation in molecular metal band filling ( P ) . ~ We describe some of the physical properties of these materials, an electrochemical slurry doping methodology that appears to have considerable scope,6>’and observations of general relevance to understanding conductive polymer electrochemistry.
(1) (a) Pecile, C.; Zerbi, G.; Bozio, R.; Girlando, A., Eds. Proceedings of the International Conference on the Physics and Chemistry of Low-Dimensional Synthetic Metals (ICSM 84), Abano Terme, Italy, June 17-22, 1984; Mol. Cryst. Liq. Cryst. 1985, 117-121. (b) Williams, J. M. Prog. Inorg. Chem. 1985, 33, 183-220. (c) Wudl, F. Acc. Chem. Res. 1984, 17, 227-232. (d) Greene, R. L.; Street, G. B. Science 1984, 226, 651-656. (e) Miller, J. S., Ed. Extended Linear Chain Compounds;Plenum: New York, 1982; Vols. 1-3. (0 Epstein, A. J.; Conwell, E. M., Eds. Proceedings of the International Conference on Low-Dimensional Conductors, Boulder, CO, Aug. 9-1 4,1981; Mol. Cryst. Liq. Crysr. 1981-1982, 77, 79, 81, 83, 85, 86, Parts A-F. (g) Jerome, D.; Schulz, H. J. Adu. Phys. 1982, 31, 299-490. (2) (a) Miller, J. S.; Epstein, A. J. Science, in press. (b) Epstein, A. J.; Miller, J. S.; Pouget, J. P.; Comes, R. Phys. Reu. Lett. 1981, 47, 741-744. (c) In the most successful work to date, p = -0.50 -0.63 for (NMP),(Phen) ,_,(TCNQ) in the TCNQ-centered band.2a-b (3) (a) Wiygul, F. M.; Metzger, R. M.; Kistenmacher, T. J. Mol. Cryst. Liq. Cryst. 1984, 107, 115-131 and references therein. (b) Metzger, R. M. J . Chem. Phys. 1981, 75, 3087-3096. (c) Torrance, J. B.; Silverman, B. D. Phys. Reu. B: Solid State 1977, I S , 788-801. (d) Epstein, A. J.; Lipari, N. 0.; Sandman, D. J.; Nielsen, P. Phys. Reo. B Solid State 1976, 13, 1569-1579. (4) (a) Marks, T. J. Science 1985, 227, 881-889 and references therein. (b) Dirk, C. W.; Marks, T. J. Inorg. Chem. 1984, 23, 4325-4332. (c) Diel, B. N.; Inabe, T.; Lyding, J. W.; Schoch, K. F., Jr.; Kannewurf, C. R.; Marks, T. J. J . A m . Chem. SOC.1983, 105, 1551-1567. (d) Dirk, C. W.; Inabe, T.; Schoch, K. F., Jr.; Marks, T. J. J . A m . Chem. SOC.1983, 105, 1539-1550. ( 5 ) (a) Hanack, M. Mol. Cryst. Liq. Cryst. 1984, 105, 133-149. (b) Diel, B. N.; Inabe, T.; Jaggi, N.; Lyding, J. W.; Schneider, 0.;Hanack, M.; Kannewurf, C. R.; Marks, T. J.; Schwartz, L. J. J. Am. Chem. SOC.1984,106, 3207-3214. (c) Moyer, T. J.; Schechtman, L. A,; Kenney, M. E. Polym. Prepr. (Am. Chem. Soc., Diu. Polym. Chem.) 1984,25,234-235. (d) Hanack, M. Chimia 1983, 37, 238-247. (e) Nohr, R. S.; Kuznesof, P. M.; Wynne, K. J.; Kenney, M. E.; Siebenman, P. G. J . Am. Chem. SOC.1981, 103, 4371-4377. (6) Reported in part at the Workshop on Conductive Polymers, Brookhaven National Laboratory, Upton, NY, Oct. 7, 1985. (7) In addition to the BF4- results reported herein, preliminary experiments also indicate that a wide range of anions/dianions (e.g., PF6-, p-toluenesulfonate, CF3SOI-, CF3(CF2)7S03-,S042-,Cr2072-)is amenable to such an approach. Furthermore, reversible reductive doping produces conductive n-doped materials with counterions such as Li+ and Bu4N+ (Gaudiello, J. G.; Kellogg, G . E.; Marks, T. J., unpublished results). (8) (a) A brief note has reported galvanostatic doping of [Si(Pc)O], pellets: Orthmann, E. A.; Enkelmann, V.; Wegner, G. Markromol. Chem., Rapid Commun. 1983, 4, 687-692. (b) For electrochemical studies of Si(Pc)(OR), and RO[Si(Pc)O],R complexes in solution, see: Wheeler, B. L.; Nagasubramanian, G.; Bard, A. J.; Schechtman, L. A,; Dininny, D. R.; Kenney, M. E. J. A m . Chem. SOC.1984,106,7404-7410; Mezza, T. M.; Armstrong, N . R.; Ritter, G. W., 11; Iafalice, J. P.; Kenney, M. E. J . Electroanal. Chem. 1982, 137, 227-237. (c) Results were independent of particle size. Times following voltage steps for current decay to background levels were ca. 10-20 h. (d) The basic three-compartment cell design is given in: Smith, W. H.; Bard, A. J. J . A m . Ckem. SOC.1975, 97, 5203-5210. (9) A larger scale, preparative analogue of “electrochemical voltage spectroscopy”: (a) Shacklette, L. W.; Toth, J. E.; Murthy, N. S.; Baughman, R. H. J. Electrochem. SOC.1985,132, 1529-1535. (b) Kaufman, J. H.; Mele, J. E.; Heeger, A. J.; Kaner, R.; MacDiarmid, A. G. J. Electrochem. SOC.1983, 130, 571-574. Thompson, A. H. Reu. Sci. Instrum. 1983, 54, 299-237. Thompson, A. H. J . Electrochem. SOC.1979, 126, 608-616.
-
x-
&+
common in the electrochemistry of conductive polymers and are thought to be structural in origin,I3 the present results provide
c y
A
Insoluble [Si(Pc)0ln4 can be doped as a finely ground slurry (5-30-pm particle size by SEM) by using controlled potential coulometry (CPC).8-g A standard three-compartment cellsdwith a large-area (8 cm2) Pt gauze working e!ectrode, rapid stirring with a Telfon-coated magnetic stir bar, and rigorously purified acetonitrile/Bu4N+BF4- was employed. Measurable levels of oxidation were not observed a t potentials less than 1.60 V vs. SSCE. At potentials greater than 1.80 V, the material is oxidized to {[Si(Pc)O][BF4loso), as determined by coulometry and elemental analysis. X-ray powder diffraction shows that, upon doping, this highly crystalline polymer changes from an orthorhombicM to a tetragonal crystal structurek3l0 similar to {[Si(Pc)O] (BF4), 36)n prepared by chemical doping.’ Diffraction data for these samples differ principally in the intensity of reflections arising from electron density in off-axis counterion channels (true for all ([S~(PC)O]XJ,“.~~). {[Si(Pc)O][BF4]0,50)n can be completely undoped electrochemically at -0.200 V to regenerate the neutral polymer. Interestingly, this [Si(Pc)0],lz retains a tetragonal structure. Subsequent reoxidation of this tetragonal [Si(Pc)O], begins a t potentials ca. 1.25 V less positive than the original orthorhombic [Si(Pc)O],. As illustrated in Figure la, once in the tetragonal structure, wide variation of p can be effected without major structural reorganization. While such overpotential (“break-in”) phenomena are ‘Department of Chemistry and the Materials Research Center. 8 Department of Electrical Engineering and Computer Science and the Materids Research Center.
0022-3654/86/2090-4917$01.50/0 0 1986 American Chemical Society
4918
Letters
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
initial doping
undoping
0.60
b
h
C
2 c
e
0.50
-
m
E 0
3 .- 0.40 c L
m
a
0.30 0)
t
0,
9)
0.20
oxidation
2.
7
-
reduction
0.10
I
2.20
I
2.00
I
1.80
I
1.60
I
I
1.40
1.20
I
1.00
I
0.80
I
I
0.60
0.40
0.20
I
0.00
POTENTIAL (volts v s SSCE)
Figure 1. (a) Structural/potential relationships in the electrochemistry of [Si(Pc)O], in acetonitrile/tetrabutylammonium tetrafluoroborate. Vertical lines indicate the threshold for doping and undoping processes. Slanted lines indicate equilibrium between phases. (b) Controlled potential coulometry of tetragonal [Si(Pc)O], in acetonitrile/tetrabutylammonium tetrafluoroborate.
the most persuasive evidence to date that this is indeed the case. Furthermore, unlike the orthorhombic polymer which is converted (10) (a) Alrneida, M.; Kanatzidis, M. G.; Liang, W.-B.; Marcy, H. 0.; McCarthy, W. J.; Kannewurf, C. R.; Marks, T. J., submitted for publication. (b) Inabe, T.; Liang, W:B.; Lornax, J. F.; Nakamura, S.; Lyding, J. W.; McCarthy, W. J.; Carr, S. H.; Kannewurf, C. R.; Marks, T. J. Synth. Met. 1986, 13, 219-229. (c) Inabe, T.; Nakamura, S.; Liang, W.-B.; Marks, T . J.; Burton, R. L.; Kannewurf, C. R.; Imaeda, K. I. J. Am. Chem. SOC.1985, 107, 7224-7226. (d) Inabe, T.; Marks, T. J.; Burton, R. L.; Lyding, J. W.; McCarthy, W. J.; Kannewurf, C. R.; Reisner, G. M.; Herbstein, F. H. Solid State Commun.1985,54, 501-503. (e) Martinsen, J.; Palmer, S. M.; Tanaka, J.; Greene, R.C.; Hoffman, B. M. Phys. Rev. E : Condem. Matrer 1984,30, 6269-6276. (1 1) (a) Inabe, T.; Gaudiello, J. G.; Moguel, M. K.; Lyding, J. W.; Burton, R. L.; McCarthy, W. J.; Kannewurf, C. R.; Marks, T.J. J . Am. Chem. SOC., in press. (b) Inabe, T.;Moguel, M. K.; Marks, T.J.; Burton, R. L.; Lyding, J. W.; Kannewurf, C. R. Mol. Crysr. Liq. Cryst. 1985, 118, 349-352. (c) Inabe, T.; Lyding, J. W.; Moguel, M. K.; Kannerwurf, C. R.; Marks, T. J. Mol. Cryst. Liq. Cryst. 1983, 93, 355-367. (d) Analysis techniques were described previously."*d Data for ([Si(Pc)O]Xjn/Ni(Pc)X pairs (the latter characterized by single-crystal techniqueslOa*c,d) differ principally in the interplanar spacings. (1 2) Thermal undoping of ((Si(Pc)O]I, ,in4' yields a diffractometrically, spectroscopically, and electrochemically indistinguishable material.
from [Si(Pc)O], to ([Si(Pc)O] [BF4]o,so),over a narrow 200-mV range, tetragonal [Si(Pc)O], can be doped and undoped over a ca. 900-mV potential range. This behavior was investigated more quantitatively by CPC (Figure lb). At each set potential, the doping level determined by coulometry is, within experimental error, identical with that determined by elemental analysis. At any point on the CPC curves, the doping level can be reproducibly adjusted by changing the potential according to Figure 1b. These results argue convincingly that the tetragonal ([Si(Pc)O][BF,],J,/[Si(Pc)O], system is at the maximum possible degree of oxidation for the set potential at every point on the CPC plot. The smooth shape of the dopinglundoping curves argues that these processes are relatively homogeneous from y = 0.0 to 0.50. Abrupt steps in y as a function of potential (as observed for doping of orthorhombic [Si(Pc)O],) would indicate doped phases of sig(13) (a) Pickup, P. G.; Osteryoung, R. A. J . Am. Chem. SOC.1984, 106, 2294-2299 and references therein. (b) Kaufrnan, F. B.; Schroeder, A. H.; Engler, E. M. Kramer, S. R.; Chambers, J. Q. J . Am. Chem. SOC.1980, 102, 483-488.
The Journal of Physical Chemistry, Vol, 90, No. 21, 1986 4919
a - 1.0 -
TABLE I: Optical Reflectivity Data for jISi(Pc)Ol(BF,),I.
2.0
1.6 1.4 W
Y 20 Y
:a:
1.2
Materials
Aw,, cm-I
I
Y
up,cm-I
exptl"
theortlb
0.27 0.36
4140 4700 4980 5400
-560
-735
280
285 568
0.41 0.50
700
'Observed displacement in upfrom y = 0.36 sample. *Theoretical displacement in upfrom y = 0.36 sample calculated from eq 2.
-
y
= 0.41 1751
1.0 -
y = 0.36
0.8 0.6 -
I
v
L
0.4 -
y
0.2 -
= 0.13
r 2.0
2.5
3.0
3.5
4.0
4.5
LOG FREQUENCY (anI)
,
y = 0.50
-25
5.0
I
I
1
1
I
0
100
200
300
400
,
TEMPERATURE (K)
Figure 3. Variable-temperature thermoelectric power ( S ) data for ([Si(Pc)O](BF,),J, samples a t various doping levels. l o o 1
changes in lattice parameters (+0.27 (6) 8, in a; -0.03 (2) A in c), consistent with a homogeneous, reversible doping process which does not destroy the polymer structure. Over this range, transmission IR spectra exhibit the characteristic4"growth of conduction electron absorption. This is completely discharged upon electrochemical undoping, and there is no detectable displacement or change in Si(Pc) skeletal or Si-0 chain modes, arguing further that the [Si(Pc)O], structure remains intact. ([Si(Pc)O](BF4)J, optical reflectance spectra show the development of a typical molecular metal plasma edge upon incremental doping (Figure 2a). At y 2 0.27, the edgelike feature is sufficiently well-resolved to allow a n a l y s i ~ using ~ ~ ~a~Drude ~ ~ ' ~model for the dielectric function.'6 Assuming a simple, one-dimensional tight-binding band, the plasma frequency, up,can be related to the band filling (eq 1) where t is the transfer integral (bandwidth/4), c is the interplanar spacing, and N, is the carrier d e n ~ i t y . " J ~ ~Assuming '~
..;q
tNc sin (np/2)
h
0.00
0.10
0.20
0.30
0.40
0.50
Y (degree of partial oxidation)
Figure 2. (a) Optical specular reflectance of polycrystalline ([Si(Pc)O](BF4)y]1 samples a t various doping levels. For ease of viewing each spectrum is displaced by 0.3 reflectance unit from the one below. (b) Four-probe electrical conductivity vs. dopant level for compressed polycrystalline ([Si(Pc)O](BF,),), samples a t 25 O C .
nificantly different free energies and/or structures.*J4 Irreversible decomposition of the polymer begins at potentials >+ 2.50 V vs. SSCE. The electrochemical behavior of ([Si(Pc)O](BF,),!,, allows the preparation of bulk samples for investigating physical characteristics as a function of wide excursions in band filling. As y = 0.00 0.50, diffraction data reveal small monotonic, reversible
-
(14) Whittingham, M. S. Ann. Chim. (Paris) 1982, 7, 204-214 and references therein.
]
112
(1)
t and c remain approximately constant, this relationship reduces to eq 2.'' Data for ([Si(Pc)O](BF,),], samples are set out in Table up
-
[sin (xp/2)11/*
I. It can be seen that wp shifts with increasing y in excellent agreement with theory. This provides additional evidence that the electrochemical doping process is homogeneous and that simple tight-binding band theory is useful in describing the electronic structure'* of these metallomacrocyclic assemblies. In contrast, (15) McCarthy, W. J.; Kannewurf, C. R.; Inabe, T.; Marks, T. J.; Burton, R. L. In Basic Properties of Optical Materials; Feldman, A., Ed.; National Bureau of Standards: Washington, DC, 1985; NBS Spec. Publ. 697, pp 54-57.
(16) Tanner, D. B. In Extended Linear Chain Compounds; Miller, J. S . , Ed.; Plenum: New York, 1982; Vol. 2, pp 205-258. (17) N , = pN, where N is the number density of Pc molecules.
4920
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
chemical doping of orthorhombic [Si(Pc)O],, is largely inhomogeneous$.ll and as expected, wp exhibits no detectable p dependence; the maximum p achievable is only ca. +0.35. The dependence of {[Si(Pc)O](BF,,),,),, electrical conductivity on y is presented in Figure 2b. The behavior is similar to that observed for inhomogeneous [Si(Pc)O], doping with halogens and nitrosonium Clearly, transport properties are rather insensitive to modest changes in carrier density, at least when sampled in polycrystalline specimens. In such cases, electrical behavior should be strongly influenced by interparticle contacts." Since thermoelectric power is a zero current transport coefficient, it should be less sensitive to interparticle effects and should more directly probe intrinsic transport chara~teristics.~J~*J~ The dependence of ([Si(Pc)O](BF,),), thermopower on y is shown in Figure 3. As y increases from 0 to ca. 0.27, there is a continuous (18) (a) Pietro, W. J.; Marks, T. J.; Ratner, M. A. J. Am. Chem. SOC. 1985, 107, 5387-5391. (b) Pietro, W. J.; Ellis, D. E.; Marks, T. J.; Ratner, M. A. Mol. Cryst. Liq. Cryst. 1984, 105, 273-287. (c) Ciliberto, E.; Doris, K. A,; Pietro, W. J.; Reisner, G. M.; Ellis, D. E.; Fragala, I.; Herbstein, F. H.; Ratner, M. A,; Marks, T. J. J . Am. Chem. SOC.1984, 106, 7748-7761. (d) Doris, K. A.; Fragala, I.; Ratner, M. A,; Marks, T. J. Isr. J . Chem., in press. (19) (a) Schweitzer, D.; Hennig, I.; Bender, K.; Endres, H.; Keller, H. J. Mol. Cryst. Liq. Cryst. 1985, 120, 213-220. (b) Mortensen, K.;Jacobsen, C. S.;Bechgaard, K.; Carneiro, K.; Williams, J. M. Mol. Cryst. Liq. Cryst. 1985, 119, 401-404. (c) Maaroufi, A.; Codon, C.; Flandrois, S.;Delhaes, P.; Mortensen, K.;Bechgaard, K.Solid Stare Commun. 1983, 48, 555-559 and references therein. (d) Khanna, S. K.; Fuller, W. W.; Chaikin, P. M. Phys. Rev. B: Condens. Matter 1981, 24, 2958-2963. (e) Chaikin, P. M.; Griiner, G.; Schegolev, I. F.; Yagubskii, E. B. Solid State Commun. 1979, 32, 1211-1214.
Letters evolution in thermopower behavior from p-type semiconducting (large, positive, increasing with decreasing temperature) to metal-like (small, tending to zero as T-+ 0). This is is qualitatively the response expected in a one-dimensional molecular metal tight-binding band description (e.g., eq 3, neglecting the energy I
27r2ke2TCOS (7rp/2) S=
3e(4r) sin2 ( 7 r p / 2 )
(3)
dependence of the scattering time) as the band filling is progressively depleted (in accord with the homogeneous doping p i c t ~ r e . l , l ~ * , l Interestingly, ~,~~ the onset of metal-like behavior in the thermoelectric power coincides approximately with the onset of the strong plasma edge in the optical spectra (Figure 2a). These results demonstrate the feasibility of widely tuning the band filling in a structure-enforc+ molecular metal. Further studies of collective property-electronic structure relationships in such systems are in progress. Acknowledgment. This research was supported by the N S F through the Northwestern Materials Research Center (Grant DMR82-16972, T.J.M. and C.R.K.) and by the Office of Naval Research (T.J.M.). M.A. thanks NATO for a postdoctoral fellowship. We thank Dr. W. J. Pietro for helpful comments. (20) Deviations from this simple model are expected in systems with strong electron correlations and relatively narrow bands: (a) Ihle, D.; Eifrig, Th. Phys. Status Solidi B 1919,137, 135-140. (b) Chaikin, P. M.; Beni, G. Phys. Rev. B Solid State 1976, 13, 647-651.
