11 Radical-Cation Salts of Arenes
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A New Family of Organic Metals V. Enkelmann Max-Planck-Institut für Polymerforschung, Welder-Weg 11, D-6500 Mainz, Federal Republic of Germany
Stable radical-cation salts of a variety of simple arenes can be prepared by anodic oxidation in the presence of suitable counterions, for example, ClO - , BF - , PF - , AsF - , and SbF -. These highly conducting crystals of the compositionA r - . + X -(where Ar - is a radical cation of 2-Y aromatic hydrocarbons and X- is an anion), which are deposited on the anode during the electrocrystallization as shiny black needles, can be considered as a new family of organic metals built up from simple, easily obtainable building blocks. The packing found between the aromatic rings in the radical-cation salts can be regarded as a model for interchain interactions in conducting polymers, for example, doped polyacetylene and poly(p-phenylene). 4
4
6
6
6
2
y
2
y.+
Electrocrystallization Stable radical-cation salts of many aromatic hydrocarbons (Ar) can be pre pared by anodic oxidation of a solution in C H C 1 , tetrahydrofuran (THF), chlorobenzene, or C H C 1 C H C 1 i n the presence of a suitable supporting electrolyte, for example, ( N B u ) X - , where N B u is tetra-n-butylammonium and X- is ClO - , BF - , P F - , A s F - , or SbF - (1-5). As shown in Figure 1, crystals form directly on the anode, which can be various shapes, sizes, and materials (e.g., Pt, A u , N i , C u , Ge, or graphite). The electro chemical processes that lead to crystal growth can be split into three inde pendent main reactions (3, 6): 2
2
4
4
2
2
4
+
4
6
Ar
6
->
6
Ar
.+
+e
-
0065-2393/88/0217-0177$06.75/0 © 1988 American Chemical Society
In Polynuclear Aromatic Compounds; Ebert, Lawrence B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
(1)
178
POLYNUCLEAR AROMATIC COMPOUNDS Ar" Ar ' 2
+
+
+ Ar ±5 Ar "
(2)
+ X " - » Ar X
(3)
2
2
The key reaction of these three is the dimerization (reaction 2) in which the short-lived intermediate monomer radical cation is stabilized. The equilib rium constant (K ) of this reaction determines not only the stability of the reactive intermediates but also the kinetics of crystal growth and the com position of the crystals. Fluoranthene (FA) is one example in which K is very high. In this case, under all experimental conditions tested so far, crystals of the ideal com position A r X are always obtained. This situation is not the case with pyrene (Py), perylene, and many other arenes. Depending on the solvent, tem perature, counterion, applied voltage, and current density, a variety of dif ferent complexes are formed. A number of well-characterized radical-cation salts are listed i n Table I as well as some details of the experimental con ditions. Inspection of this table shows that if the composition deviates from A r X , the arene-to-counterion ratio is always co 0
CM
^ ^
^ CO
^CO
^-'^-'
CD 05
^
CD 05 CO CD CM i - H
CO ΙΟ ï> ^ 05 © d l > cô i o TP H H H «M
κ
u s
2
CD
NOTE: The values in parentheses are the estimated standard deviation. The experimental density given by Chiang et al. (I) (1.37 g/cm ) does not correspond to the density calculated for Py Cl a density of 1.38 g/cm was calculated.
(4) 91.08 109.31 (2)
(3) 103.0
CO.
Ο fa
90.9 (3) 75.97 (2)
B: Pyi (SbF ) , and C: Pe(PF ) (CH Cl )o, (where Pe is perylene). 2
2
6
7
6
L1
2
2
6
8
In Polynuclear Aromatic Compounds; Ebert, Lawrence B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
6
2
POLYNUCLEAR AROMATIC COMPOUNDS
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196
R a d i c a l - C a t i o n Salts as M o d e l s for C o n d u c t i n g Poly mers. Polymers that have an extended ir-electron system in their back bones, for example, polyacetylene (PA) and poly(p-phenylene) (PPP), can be transformed by oxidation or reduction in the solid state (doping) to derivatives that exhibit metallike conductivity (24, 25). These materials are usually i n soluble and infusible and exhibit a very complicated morphology that cannot be changed by subsequent treatment. The lack of knowledge about the structure and state of order is the cause of the current controversy about the conduction mechanism in doped polymers. The interactions found between the aromatic rings in the radical-cation salts can be regarded as models for interchain interactions in conducting polymers. The concept that uses the structural principles found in the model compounds to construct models for the conducting polymer derivatives is illustrated in Figure 14. The radical cations created on the polymer backbone in the oxidation are thought to stabilize themselves by forming complexes with neutral chain segments in their vicinity according to reaction 2. The stack-forming elements are part of the main polymer chain. Consequently, the derived structural models can be characterized as intercalation structures in which layers of polymer chains and counterions alternate. Many of the
Of
)•
50•Φ S(>0
• ο
. Polymer Segment
Figure 14. Analogy of the packing in radical-cation salts (left) and in conducting polymer salts (right).
In Polynuclear Aromatic Compounds; Ebert, Lawrence B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
11.
ENKELMANN
197
Radical-Cation Salts of Arenes
structural properties of conducting polymers have been successfully ex plained by using this concept. Radical-cation salts of oligomers of PPP, for example, terphenyl and quaterphenyl (QP) and their substituted analogues, have been prepared in order to use the packing found in the oligomers as a model for doped P P P (8, 10). The crystal structure of the radical-cation salt of QP, Q P Q P ( S b F ) , is shown in two projections in Figure 15. The structure consists of stacks of Q P molecules that are separated by layers of SbF ~ ions. A l l Q P units are nearly aligned in one direction; thus, the packing found here could seemingly be a reasonable model for the polymer salt. The projection perpendicular to the stack reveals that an additional (neutral) Q P molecule is packed in the anion layer to fill empty space. In contrast with all other salts, each Q P molecule carries one positive charge. In terms of the structural principles of organic conductors given earlier, this salt should be an insulator. The relatively high conductivity observed could be explained by a hybrid of the valence bond forms shown in Chart I. This situation could be interpreted like the polymer chain; that is, more than one radical-cation site is created on one extended molecule. 3
6
3
Downloaded by UNIV OF PITTSBURGH on February 2, 2016 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch011
6
Chart I
The motif in the smaller dashed cell shown in Figure 15 was used to construct a model for the P P P salt. Both the unit cell dimensions and the composition ( C H ) ( S b F ) were used without further adjustment and re produced the experimental doping level (40% per phenyl ring) and the X ray and neutron scattering (10). In a similar way, the packing found in the radical-cation salts could be used to solve the crystal stucture of the S b F " salt of PA, [(CH)(SbF ) o ] (26). A projection of this structure along the chain direction is shown in Figure 16. A l l details of the quite complicated structure will not be discussed here. The structure is incommensurate; that is, it can be described by the two sublattices also shown in Figure 16. Two types of polymer chains are found: Type 2 is located in rows of counterions filling the empty space, and type 1 (shown in black) is closely packed in a plane between these rows. The interplanar spacing (3.44 Â) allows charge transport not only along the polymer chain but also perpendicular to it by interchain charge transfer. This condition is important because the conjugation length in PA is limited 6
4
8
6
3
6
In Polynuclear Aromatic Compounds; Ebert, Lawrence B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
6
0
