Chem. Mater. 1994,6, 2309-2316
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Molecular Metals Based on 1,2,7,8-Tetrahydrodicyclopenta[cd:Zm]peryleneand Iodine, (CPP)2(13)l-a Jorge Morgado,t>$Isabel C. Santos,? Rui T. Henriques,? Marc FourmiguB,§ Pedro Matias,l Luis F. Veiros,$ Maria J. Calhorda,$J M. Teresa Duarte,$ Luis Alcacer,$ and Manuel Almeida*>t Departamento de Quimica, ICEN, Instituto Nacional de Engenharia e Tecnologia Industrial, P-2686 Sacavkm Codex, Portugal; Departamento de Engenharia Quimica, Instituto Superior Tkcnico, P-1096 Lisboa Codex, Portugal; Laboratorire de Physique des Solides, Universitt? de Paris-Sud, F-91405 Orsay Ckdex, France; and Instituto de Tecnologia Quimica e Biolbgica, Rua da Quinta Grande 6, Apartado 127, P-2780 Oeiras, Portugal Received April 7, 1994. Revised Manuscript Received September 29, 1994@ The synthesis and characterization of molecular metals derived from 1,2,7,8-tetrahyd-
rodicyclopenta[cd:Zmlperylene (CPP) by partial oxidation with iodine and with general formula (CPP)2(13)1-~,6 = 0-0.13, are reported. Single crystals, obtained either by electrocrystallization or by diffusion-controlled iodine oxidation of CPP, present two types of morphologies, elongated diamond or thinner needle-shaped crystals, both with a monoclinic cell, space group P21/a, a = 4.3757(9), b = 19.3681(11),c = 10.0860(11) A, /3 = 98.050(8)", V = 846.4(2) Hi3, 2 = 2. The structure of the diamond-shaped crystals was solved by X-ray diffraction to a final R(F) = 0.096, R,(F) = 0.069. It consists of regular stacks of CPP molecules along a with a 3.41 A spacing and uncorrected one-dimensional chains Of Is- located in channels between four CPP stacks corresponding to (cpP)2(13)0.892. The thin needle crystals have the same unit cell but a n unspecified and slightly different iodine content. Band structure calculations in this structure by the extended Huckel method indicate a onedimensional conduction band 0.55 eV wide. These thin needle crystals present, at room temperature, a n electrical conductivity along the a axis o,(RT) = 200 S/cm and thermopower S,(RT) = 30 ,uV/K, while for the diamond-shaped crystals o,(RT) = 2 S/cm and S,(RT) = -8 pV/K. These transport coefficients for both types of crystals indicate a metallic behavior from room temperature down to -63 K, where a metal-to-insulator (M-I) transition takes place. EPR studies in single crystals show a n almost isotropic line at g = 2.0044 and with a width of -6 G in the range 80-300 K and without significant differences between the two types of crystals. Static magnetic susceptibility measurements i n polycrystalline samples emu/ of the thin needles show a paramagnetic contribution of xp(RT) = (1.2 T 0.4) x mol slightly decreasing upon cooling until the M-I transition where it vanishes, ascribed to the coLdu&ion electrons i n the CPP stacks
Introduction Since the early report in 1954 by Akamatu et a1.l of relatively high electrical conductivity in a perylenebromine complex, the perylene (Per) molecule has been widely used in the preparation of several molecular compounds exhibiting high electrical conductivity and even metallic properties. As in many other molecular conductors, it is now known that the metallic properties are provided by regular stacks of partially oxidized planary perylene molecules whose n-electrons interact with the neighbor molecules extending, at least in one direction, along the whole solid and forming partially filled bands. The partial oxidation required to form partially filled bands is made through charge transfer from the stacked molecules to acceptor counterions. Counterions as C104- or AsFs- 2,3 or even metal comInstituto Nacional de Engenharia e Tecnologia Industrial. Instituto Superior Tecnico. Universite de Paris-Sud. Instituto de Tecnologia Quimica e Biol6gica. Abstract published in Advance ACS Abstracts, November 1,1994. (1) Akamatu, H.; Inokuchi, H.; Matsunaga, Y. Nature 1954,173, 168. +
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plexes of the type M(mntI2 (mnt = maleonitrile dithihave been olate or cis-2,3-dimercapto-2-butenedinitrile) used to render metallic properties to the perylene chain^.^-^ Partial oxidation with halogens, particularly with bromine and iodine, is a general method of obtaining molecular metals.8 However, these halogens can present different oxidation states and quite often they tend to form disordered structures that in some cases do not have a very stable and/or a well-defined halogen c0ntent.~9~ In the case of the perylene-iodine com(2)Endres, H.; Keller, H. J.; Muller, B.; Schweitzer, D. Acta Crystallogr. 1985,C41,607. (3) Keller, H. J.; Nothe, D.; Pritzkow, H.; Wehe, D.; Werner, M.; Koch, P.; Schweitzer, D. Mol. Cryst. Liq. C y s t . 1980,62,181. (4)Gama, V.; Almeida, M.; Henriques, R. T.; Santos, I. C.; Domingos, A,; Ravy, S.; Pouget, J. P. J . Phys. Chem. 1991,95, 4263. ( 5 ) Gama, V.; Henriques, R. T.; Bonfait, G.; Pereira, L. C.; Waerenborgh, J. C.; Santos, I. C.; Duarte, M. T.; Cabral, J. M. P.; Almeida, M. Inorg. Chem. 1992,31,2598. (6) Gama, V.; Henriques, R. T.; Bonfait, G.; Almeida, M.; Meetsma, A.; van Smaallen, S.; de Boer, J. L. J . Am. Chem. SOC.1992,114,1986. (7) Gama, V.;Henriques, R. T.; Bonfait, G.; Almeida, M.; Ravy, S.; Pouget, J. P.; AlcPcer, L. Mol. Cryst. Liq. Cryst. 1993, 234,171. (8) Marks, T. J.; Kalina, D. W. In Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum Press: New York, 1982;Vol. 1, Chapter 6.
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plexes, a t least three phases are known, with relatively poor stability due to iodine vaporization; in addition to the phase (Per)2(12)3,one of the earliest reported organic conductors,1°-12 (Per)(I2)3and (Per)(Iz)shave also been r e p ~ r t e d . ~In J ~these compounds iodine has been formulated as I2 with a degree of charge transfer that has remained questionable until latter resonance Raman and 1291 Mossbauer spectroscopy studies14in (Per)z(I2)3 showed the presence of reduced polyiodides and the mixed valence nature of iodine as I2 and Is-. The perylene derivative 1,2,7,84etrahydrodicyclopenta[cd:Zm]perylene (CPP) only more recently, and in a much smaller extent, has been used to prepare conduct-
ing compounds.15 In this molecule, four outer hydrogen atoms of perylene are replaced by the two bulkier -(CHz-CHz)- groups, which do not change the n-system. It is therefore interesting to compare with perylene, the ability of CPP to make conducting compounds. In this paper we report the synthesis and characterization of metallic compounds obtained by partial oxidation of CPP with iodine. Preliminary data on these compounds were recently reported16 showing the existence of two phases. As in many other iodine-containing molecular conductors, the iodine is disordered in the structure. However and interestingly, it is found that the bulkier -(CH2-CH2)- groups of CPP not only do not significantly reduce the strength of the electronic interactions in the solid when compared with those of perylene but also do render the iodine compositions more stable.
Experimental Section Sample Preparation. CPP was synthesized as previously described17 and purified by multiple recrystallization in toluene. Small single crystals of (CPP)2(13)1-~ were obtained either by the reaction of CPP with iodine or by electrochemical oxidation of CPP in the presence of tetrabutylammonium triiodide. The first method of chemical oxidation was best carried out using a slow diffusion technique under anaerobic conditions in H-shaped diffusion cells and using as solvent dichloromethane (Merck Uvasol) t h a t was previously dried with molecular sieves and, just before use, passed through a n alumina column. After being left undisturbed at room temperature for a period of several days, the diffusion cells were opened and black thin needles (-5 x 0.05 x 0.05 mm3) were obtained by filtration, washed with dichloromethane, and dried. (9) Coppens, P. In Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum Press: New York, 1982; Vol. 1, Chapter 7. (10) Kommandeur, J.; Hall, F. R. J . Chem. Phys. 1961,34,129. (11) Akamatu, H.; Uchida, T. Bull. Chem. SOC.Jpn. 1961,34,1015. (12) Akamatu, H.; Uchida, T. Bull. Chem. SOC.Jpn. 1962,35,981. (13) Kao, H.-C. I.; Jones, M.; Labes, M. M. J . Chem. SOC.,Chem. Commun. 1979,329. (14)Teitelbaum, R. C.; Ruby, S. L.; Marks, T. J. J . A m . Chem. SOC. 1979,101, 7568.
(15) Lapuyade, R.; Morand, J. P.; Chasseau, D.; Hauw, C.; Delhaes, P. J . Phys. (Paris) Coll. 1983,44,C3-1235. (16)Morgado, J.; Alcacer, L.; Henriques, R. T.; Lopes, E. B.; Almeida, M.; FourmiguB, M. Synth. Met. 1993,55-57,1735. (17) Tanaka, N.; Kasai, T. Bull. Chem. SOC.Jpn. 1981,54,3026.
Morgado et al. The electrochemical oxidation was carried out, also under anaerobic conditions, in two-compartment cells using platinum electrodes and galvanostatic conditions (-2 pA/cm2).l8The cell was filled with a n almost saturated solution (-1.6 mmoyL) of CPP and -0.8 mmol/L of (n-CdH&NI3, freshly prepared as described in ref 19, in dichloromethane purified as described above. After approximately 7 days of electrocrystallization, black crystals were collected from the anode compartment by filtration, washed with dichloromethane, and dried. Two distinct types of crystal morphologies could be noticed, needleshaped crystals as those obtained by diffusion, but with slightly smaller dimensions, and also some elongated diamond shaped crystals (-1.5 x 0.2 x 0.02 mm3) which tend t o have irregular and less perfect faces. CHN analysis of these preparations using a Perkin-Elmer 240 elemental analyzer in the analytical service of our Laboratory in Sacav6m revealed results variable from sample to sample but consistent with a stoichiometry (CPP)Z(Id-6 with 6 in the range 0.0-0.13. Upon storage a t room temperature, even for periods of several months, it was never detected any sign of iodine loss. X-ray Crystallography. A black, with metallic shine, elongated diamond-shaped crystal, having approximate dimensions 0.6 x 0.2 x 0.1 mm3 was glued to the tip of a glass capillary and transferred t o a goniometer head mounted on an Enraf-Nonius TURBO CAD-4 difractometer, equipped with a rotating anode (50 kV,80 mA) using graphite monochromaI0.710 69 A). Cell dimensions were tized Mo Ka radiation (,= determined from the measured 8 values for 25 intense reflections with 12" < 8 < 16". From the cell dimensions and pattern of systematic extinctions, the crystal was seen to be monoclinic space group P21/a. This choice was confirmed by the solution and the successful refinement of the structure. Reduced cell calculations did not indicate any higher metrical lattice symmetry,20 and examination of the final atomic coordinates of the structure did not yield any extra symmetry element.21 The intensities of 2653 observations of 1920 unique reflections in the range 1.5" < 8 < 30" were measured by the w-28 scan mode a t room temperature. The data were corrected for Lorentz, polarization, and empirically for absorption effects (maximum and minimum transmission factors of 0.999 and 0.893). The atomic positions of one of the iodine sites plus those of all carbon atoms in the CPP cation were determined by a direct method with program MITHRILZ2using the 300 reflections with the highest normalized structure factor amplitudes. Since the CPP cation lies on a crystallographic inversion center, only one-half is included in the asymmetric unit, and on the basis of the expected stoichiometry, (CPP)2(13)1-6,each of the iodine sites in the asymmetric unit should have ca. 0.75 occupancy (i.e., 3/41. Least-squares refinement of this incomplete model, with isotropic thermal motion parameters converged t o R = 0.491, with large residual peaks near the known iodine position, suggesting positional disorder. Three equalweight disordered sites (including that previously known) were then included in the model, each having 0.25 occupany (i.e., 0.75/3), and the isotropic refinement converged t o R = 0.130, with residual peaks of only ca. 1 e/A3. Since from elemental analysis results it was expected that slightly less than one IS anion would be present in the structure per two CPP cations, the occupancy of the iodine disordered sites was refined along with the positional and structural parameters. The leastsquares refinement with anisotropic thermal motion parameters for all non-hydrogen atoms converged to R = 0.1106. The positions of all hydrogen atoms were then determined from a difference Fourier map and included in the refinement with (18) Engler, E. M.; Green, R.; Haen, P.; Tomkewicz, Y.; Mortensen, K.; Berezden, J. Mol. Cryst. Liq. Cryst. 1982,79,15. (19) Williams, J. M.; Emge, T. J.;Wange, H. H.; Beno, M. A.; Copps, P. T.; Hall, L. N.; Carlson, K. D.; Crabtree, G. W. Inorg. Chem. 1984, 23,2558. (20) Spek, A. L. J . Appl. Crystallogr. 1988,21,578. (21) (a) Le Page, Y. J . Appl. Crystallogr. 1987,20,264.(b)Le Page, Y . J . Appl. Crystallogr. 1988,21,983. (22) Gilmore, C. J. MITHRIL, A Computer Program for the Automatic Solution of Crystal Structures from X-ray Data; University of Glasgow, 1983.
Chem. Mater., Vol. 6, No. 12, 1994 2311
Molecular Metals (CPP)2(Idl-a Table 1. Crystal Data and Details of Structure Determination for CPP2(1& chemical formula formula wt, g/mol crystal system space group, No.Z4
CZ4H16(13)0.446
474.19 monoclinic P21/a, 14 a, A 4.3757(8) 19.3681(11) b, A 10.0860(10) c, A P, deg 98.050(8) v, A3 846.35(23) z 2 Dcale,dcm3 1.86 F(OOO),electrons 532 p(Mo Ka), cm-l 34.60 295 temp, K monochromator graphite total no. of data 2653 unique no. of data 1920 obsd data (F,2 2u(F0)) 1459 no. of refined parameters 169 final agreement factors, R, Rw 0.096, 0.058 weighting scheme w = l/(dF,) 0.000193(F0)2) residual electron density in final -0.62,0.69 difference Fourier map, e/A3
+
Table 2. Selected Bond Lengths (bi) and Angles (deg) for CPPZ(1S)1 4 1(2)-1(1) I(l)-I(la) 1(2)-1(2a) C(2)-C(1) C(12)-C(l) C(9)-C(2) C(5)-C(4) C(7)-C(6) C(9)-C(8) C(ll)-C(lO) C(12)-C(3b) H(4)-I(ld)* 1(3)-1(1)-1(2) 1(2)-1(3)-1(1) C(12)-C(l)-C(2) C(6)-C(2)-C(1) C(9)-C(2)-C(6) C(5)-C(4)-C(3) C(5)-C(6)-C(2) C(7)-C(6)-C(5) C(9)-C(8)-C(7) C(lO)-C(g)-C(2) C(ll)-C(lO)-C(9) C(ll)-C(l2)-C(l)
Bond Lengths (A) 1.470(6) 1(3)-1(1) 2.190 1(3)-1(2) 2.208(3) 1(3)-1(3a) 1.402(7) C(3)-C(1) 1.425(7) C(6)-C(2) 1.399(7) C(4)-C(3) 1.405(8) C(6)-C(5) 1.509(8) C(8)-C(7) 1.509(8) C(lO)-C(9) 1.399(8) C(12)-C(ll) 1.451(7) H(ll)-I(lC)* 3.051 H(11)-1(2e)* Bond Angles (deg) 7.1(5) 1(3)-1(2)-1(1) 165.6(9) C(3)-C(l)-C(2) 118.0(5) C(12)-C(l)-C(3) 124.2(5) C(9)-C(2)-C(l) 112.4(5) C(4)-C(3)-C(l) 122.9(5) C(6)-C(5)-C(4) 118.2(5) C(7)-C(6)-C(2) 133.0(4) C(8)-C(7)-C(6) 105.1(5) C(S)-C(S)-C(Z) 118.5(5) C(lO)-C(9)-C(8) 119.3(5) C(l2)-C(ll)-C(lO) 117.3(5)
0.750(5) 0.732(6) 2.220(4) 1.427(7) 1.403(7) 1.396(7) 1.377(8) 1.548(9) 1.375(7) 1.402(7) 3.095 3.125 7.3(5) 117.0(5) 125.0(5) 123.4(5) 118.5(5) 119.3(5) 108.8(5) 104.9(5) 108.8(5) 132.7(4) 123.4(5)
a (a) -x, 1 - y , -2; (b) 2 - x , -y, 2 - z ; (c) 1 - x , -y, 1 - z; (d) 1 - x , 1 - y, 1 - z ; (e) x , -1 y, 1 z ; (*) nonbonding distances.
+
+
isotropic thermal motion parameters and C-H bond lengths restrained t o l.OO(2) A. The final refinement was carried out with a weighting scheme of the form W = l.O/(uz(Fo) + 0.000193(F,)2), and converged to R(F) = 0.096, RdF) = 0.058 for 1459 reflections with F, > 2dF0) and 169 refined parameters. The structure refinement calculations were carried out using the program SHELX-76.23 The iodine occupation factor converged t o 0.2231(6), which corresponds t o a stoichiometric factor of 0.892(2) for the 13 anion. Crystal data and experimental details of the structure determination are given in Table 1, final fractional atomic coordinates and equivalent isotropic displacement parameters for the non-hydrogen atoms are given in Table 2, and selected bond distances and angles (23) (a) Sheldrick, G. M. SHELX, A Crystallographic Calculation Program; University of Cambridge, 1976. (b) Sheldrick, G. M. SHELXS86, Program for Crystal Structure Solution; University of Gijttingen: Gottingen, Germany, 1986. (24) International Tables for Crystallography; Hahn, T., Ed.; Reidel: Dordrecth (present distributor Kluwer Academic Publishers, Dordrecht), 1983; Vol. A, Space Group Symmetry.
Table 3. Fractional Atomic Coordinates (A x lo4) and Equivalent Isotropic Thermal Parameters (k)for CPP2(13)1-a X Y Z B,"" I(1) I(2) I(3) C(1) C(3) c(2)
c(4) C(5) C(6) c(7) c(8) C(9) C(10) C(11) C(12)
3878(19) 534(19) 2207(19) 7734(11) 5457(11) 9072(11) 8025(12) 5733(12) 4448(11) 2063(12) 1711(12) 3957(11) 4691(11) 6972(12) 8558(12)
2522(2) 2576(2) 2597(3) 345(2) 701(2) 693(2) 1356(2) 1696(2) 1370(2) 1587(2) 956(3) 427(2) -232(2) -598(2) -331(2)
3925(4) 3946(4) 3954(5) 9157(4) 8307(5) 10344(5) 10588(5) 9710(5) 8552(5) 7399(5) 6446(5) 7106(5) 6753(5) 7579(5) 8773(4)
4.73(08) 4.85(08) 5.00(08) 2.04(09) 2.10(09) 2.24(09) 1.95(09) 2.50(10) 2.64(11) 2.46(11) 2.83(10) 2.40(10) 2.59(10) 2.37(10) 2.75(10)
a Be, = 8~~'(~/3(Uii + Uzz + U33 + 2 COS BUi3)). are collected in Table 3. Tables of thermal displacement parameters and comprehensive lists of bond angles and distances are given as supplementary material. Atomic scattering factors and anomalous dispersion factors for iodine were taken from International Tables.25 All calculations were carried out on a VAX 9000 at Instituto Superior TBcnico, using MITHRIL,z2 PARSTZ6(calculation of geometrical data), and O R T E P I P (preparation of illustrations) programs. Further X-ray diffraction studies, in order to achieve a better characterization of the iodine disorder in this compound, were performed, a t room temperature in the same large elongated diamond-shaped single crystal used for the CAD-4 data collection, by oscillation photographic techniques using Cu K a filtered radiation. Band Structure Calculations. The interaction energies between pairs of CPP molecules, ,&,were calculated by means of the extended Huckel methodz8with modified H i s z gand the band structure of the CPP stacks was obtained with the tight binding approach of the same m e t h ~ d . ~ O Standard ,~l parameters were used. All calculations were performed with the CPP coordinates taken from the crystal structure. Electrical Transport Measurements. The thermoelectric power was measured along the needle a axis in the range 20-300 K by a slow ax. technique using a n apparatus similar t o that described by Chaikin and K ~ a kattached , ~ ~ to the cold stage of a closed-cycle helium refrigerator and operated under computer control.33 The thermal gradients used were kept below 1 K and were monitored by Au (0.07 at. % Fe)-chrome1 thermocouple measured with a Keithley 181 nanovoltmeter. A similar thermocouple was used to measure the sample temperature with a precision of 0.1 K. The extremities of the single-crystal sample were glued directly with platinum paint (Demetron 308A) to 99.99% pure q5 = 25 pm gold wires. The thermoelectric voltage was measured using a Keithley 181 nanovoltmeter and the absolute thermopower was calculated after correction for the absolute thermopower of gold using the data of H ~ e b n e r . ~ ~ Electrical conductivity measurements were performed in the same cell used for thermopower measurements and using as samples the same crystals in which thermopower was previously measured. After thermopower measurements, without
(25) International Tables for X-ray Cristallography; Kynoch Press: Birmingam, England, 1974; Vol. IV. (26) Nardelli, M. Comput. Chem. 1983, 7, 95. (27) Johnson, C. K. ORTEPII, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976. (28) (a) Hoffmann, R.; Lipscomb, W. N. J . Chem. Phys. 1962, 36, 2179; 1962,37,2872. (b) Hoffmann, R. J . Chem. Phys. 1963,39,1397. (29) Ammeter, J. H.:Burpi, H.-B.: Thibeault, J. C.; Hoffmann, R. J . A m . Chem. SOC.1978, 106 3686. (30) (a) Whangbo, M.-H.; Hoffmann, R. J. A m . Chem. SOC.1978, 100,6093. (b) Whango, M.-H.;Walsh, Jr., W. M.; Haddon, R. C.; Wudl, F. Solid State Commun. 1982, 43, 637. (31) Whangbo, M.-H.; Williams, J. M.; Beno, M. A.; Dorfman, J. R. J . A m . Chem. Soc. 1983,105,645. (32) Chaikin, P. M.; Kwak, J. F. Rev. Sei. Znstrum. 1975, 46, 218. (33) Lopes, E. B., internal report, LNETI-SacavBm,Portugal, 1991. (34) Huebner, R. P. Phys. Rev. 1964, 135,A1281.
Morgado et al.
2312 Chem. Mater., Vol. 6, No. 12, 1994
Figure 2. Partial ORTEP projection of the CPP2(1&-6 structure along c showing a n almost continuous distribution of the iodine positions in channels between the CPP molecules.
a
a
Figure 1. ORTEP projection of the CPP2(Id-d structure along
a. removing the sample, two additional contacts were made on the sample attaching similar gold wires with platinum paint to the inner part of the crystals in order to obtain a four-inline configuration. A low-frequency (77 Hz) 1pA current was used, and the voltage measured with a lock-in amplifier (EG&G PAR Model 5301). Samples were checked for unnestednested voltage ratio as defined by Schaffer et al.,35and those with ratios larger than 5% were rejected. EPR EPR spectra were obtained in a conventional X-band spectrometer (Bruker ESP 300) equipped with a helium flow cryostat (Oxford Instruments ESR-900) which enabled measurements in the range 4-300 K with a stability of 0.2 K. A selected single crystal was attached with a minimum amount of Apiezon grease t o a signal-free Teflon tubing and placed inside a quartz tube which in turn was inserted in the cavity so that the needle axis, a,of the samples was perpendicular to the static magnetic field. The sample temperature was measured with a n accuracy of 1 K by a Au (0.07 at. % Fe)chrome1 thermocouple placed close to the sample. g values were determined by the simultaneous measurement of the field using a n NMR gaussmeter (ER-035M) and the microwave frequency (HP-5350B frequency counter). Magnetic Susceptibility. A longitudinal Faraday system (Oxford Instruments) with a 7 T superconducting magnet was used to measure the magnetic susceptibility of polycrystalline samples in the range 4-300 K. Several runs were performed using typically 5-10 mg of the thin needle crystals placed in a thin-wall Teflon bucket previously measured in the same temperature range. The force was measured with a microbalance (Sartorius S3D-V) applying downward and reverse gradients of 5 T/m. At several different temperatures, in the range 10-300 K, the dependence of the magnetization with the field was checked up to 7 T. A linear behavior was observed only up to -4 T, and therefore systematic temperaturedependent magnetic susceptibility measurements were made at 3.5 T.
Results The crystal structure of the (CPP)2(I&-s elongated diamond-shaped crystal is based on a unit cell with two equivalent and centrosymmetricCPP molecules that are regularly stacked along the a axis (Figure 1). The iodine (35) Schaffer, P. E.; Wudl, F.; Thomas, G. A.; Ferraris, J. P.; Cowan,
D.0 .Solid State Commun. 1974, 14, 347.
atoms are located in channels of this structure, surrounded by four stacks of CPP molecules, as shown in Figures 1and 2. The X-ray oscillation photographs in this type of elongated diamond-shaped crystal revealed, in addition to the Bragg layer lines corresponding to the above-referred unit cell, the presence of diffusive scattering layers perpendicular to the a axis at distances corresponding to d = 10.2 A, as well as their equally sharp second-, third-, and fourth-order planes, the third one being particularly strong. These diffuse scattering planes have only minor intensity modulations in the b*,c* plane, clearly denoting the existence of onedimensional order in this solid that is attributed to the iodine atoms. This 10.2 A spacing is only slightly above typical values found in many other triiodide containing solids, usually in the range 9.4-9.9 A,9r36and it can be taken as a clear indication of the presence of triiodide in this compound. This is further confirmed by the strong intensity of the third-order layer. Other iodine species, or a mixture of them, can be excluded since they would imply a different repeat unit, for instance, on the order of 15 A for The modulation of the average iodine positions along b, shown in Figure 2, denotes the tight fitting small deviations to linearity. In fact distances slightly shorter than the sum of the van der Waals radii (3.35 A for H-I- or 3.15 A for H-II3' are observed between iodine ositions and CPP hydrogen atoms: H(ll)-I(la) 3.09 H(4)-I(lb) 3.05 A, H(11)-1(2c) 3.12 A (Table 2). The CPP molecules are almost planar with a very slight chairlike distortion of the -CHz-CHz- groups, the C7 and C8 atoms presenting a deviation of 0.051 and 0.037 A, respectively, from the average central perylene plane. The overlap mode of CPP molecules is shown in Figure 3 and the slight chairlike distortion previously referred, allows a maximum contact between the central perylene rings in a stack that are separated by 3.41 A, with a minimum repulsion of the -(CH2CH2)- groups. The normal to the average CPP plane makes an angle of 38.7" with the a axis. The CPP overlap mode is the same as that observed in perylene (Per) based molecular metals like the members of the a-(Per)zM(mnt)zseries with M = Pt, Au, Pd, Ni, Cu, Fe, and Co or (Per)Co(mnt)2(CHzCl2)0,5, where the interpla-
1,
(36) Herbstein, F. H.; Kaftory, M.; Saenger, W. 2.Kristallogr. 1980, 154. 11. (37) Huheey, J. E. Inorganic Chemistry, 3rd ed.; Harper & Row: New York, 1983.
Molecular Metals (CPP)z(Id1-6
Chem. Mater., Vol. 6, No. 12, 1994 2313
Figure 3. Overlap mode of the CPP molecules in CPP2(13)1-6.
nar distances are slightly shorter, in the range 3.323.36 A for the first series4,5r38,39 and 3.28 A in the last compound.6 The structural information available for the thin needles was limited by their small size and reduced diffracting power. Therefore, it was possible to check in the same four-circle X-ray diffractometer with rotating anode only that their unit-cell parameters were equal to those observed in the elongated diamondshaped crystal measured as described above. The transport properties, specially thermopower results (see below) support the existence of two distinct phases corresponding to the two morphologies observed. The fact that the unit cells are exactly the same indicates that the differences between them are primarily associated with the iodine content and/or their different degree of ordering. The electronic dimensionality of this solid was tested through the closest contacts between CPP neighbor molecules. Given the crystal structure formed by parallel stacks of CPP molecules and the space-group symmetry, three different contacts can be found, involving pairs of adjacent molecules. These interactions exist along the directions of the crystallographic axes, namely, the contacts along the CPP stacks, LA,parallel to a , and other two, IB and IC, corresponding to the interactions along b and c, respectively (Figure 4). The C-C intermolecular distances were calculated for each of the three contacts, IA, IB, and IC, and extended Huckel calculation^^^,^^ were performed. Their relative strength was evaluated through the magnitude of the interaction energies, p~ = (q,iIHefflqj),between the HOMOs pi, pj, calculated for the three-nearest neighbors CPP pair^.^^,^^ The interaction energies are listed in Table 4, and as (38) Alclcer, L.; Novais, H.; Pedroso, F.; Flandrois, S.;Coulon, C.; Chasseau, D.; Gaultier, J. Solid State Commun. 1980,35,945. (39) Domingos, A.; Henriques, R. T.; Gama, V.; Almeida, M.; LopesVieira, A.; Alcacer, L. Synth. Met. 1989,27,B411. (40) Whangbo, M.-H.; Williams, J. M.; Leung, P. C. W.; Beno, M. A.; Emge, T. J.; Wang, H. H. Znorg. Chem. 1986,24, 3500. (41) Whangbo, M.-H.; Williams, J. M.; Leung, P. C. W.; Beno, M. A.; Emge, T. J.;Wange, H. H.; Carlson, K. D.; Crabtree, G. W. J . A m . Chem. SOC.1985,107, 5815.
Figure 4. Crystallographic independent interactions between pairs of nearest-neighbors CPP molecules. Table 4. Shorter C-C Intermolecular Contacts, d, and Interaction Energies, /?,between Nearest-NeighborsCPP Pairs interaction
IA IB IC
dcc(&' 3.40, 3.59, 3.42, 3.56, 3.59, 3.42, 3.39 3.80b 3.70b
(meV) 312.5 1.3 8.6
Pii
a C-C distances below 3.6 A (sum of the van der Waals radii37 plus 5%). Shortest intermolecular C-C distances.
,-LUMO 1.6 eV 0.7 eV
HOMO
Figure 5. Valence orbitals of a CPP molecule.
expected they show that the only effective contacts are along the stacking direction a, IA, with b y about 2 orders of magnitude greater than the other two, IB and IC. Thus, the solid being electronically l D , the corresponding band structure was calculated for a stack of CPP molecules using the tight-binding a p p r o a ~ h ~ l , ~ ~ of the extended Huckel method. The resulting valence band is a simple sinusoidal band 0.55 eV wide. This band is essentially built from the HOMO of the CPP molecule, owing to the great energy separation between this orbital and other valence orbitals of the isolated CPP molecule (Figure 51, and the small band dispersion values characteristic of molecular conductors (0.55 eV in this case). As a consequence of this energy separation, the width of this band is 4t as expected from the tight binding theory, t = 0.137 eV being the calculated transfer integral for the HOMOs of the next-neighbor CPP molecules along the stack. This calculated width
Morgado et al.
2314 Chem. Mater., Vol. 6, No. 12, 1994 400,
-2
I
L-........
Temperature (K)
-
1-
I
.ma.---------.
-3
_"
0
50
I50
100
200
250
300
Temperature (K)
Figure 7. Temperature-dependent thermoelectric power of CPP2(13)1-6 single crystals; ( 0 )thin needles, (0) elongated diamond-shaped crystals. 4
.
- I
9
0
I
*.
50
100
150
200
250
300
Temperature (K)
Figure 8. Paramagnetic susceptibility,xp, of CPP2(13)1-6 as a function of temperature.
(42) Calhorda, M. J.;Veiros, L. F.; Canadell, E. Inorg. Chem. 1994,
33, 4290.
(43) Gama, V.; Henriques, R. T.; Almeida, M.; Veiros, L. F.; Calhorda, M. J.; Meetsma, A.; de Boer, J. L. Znorg. Chem. 1993,32, 3705.
values of the thermopower in the metallic regime are however significantly different for the two types of crystals, clearly denoting the existence of two phases; the thin needles present larger positive values (-30 pVl K) while the elongated diamond-shaped crystals have smaller and negative values (-4pVlK). In both cases at lower temperatures, below the M-I transition, thermopower first decreases fast to a minimum --40 pV/K and then increases toward larger positive values upon cooling, indicative of semiconducting behavior in agreement with resistivity data and as expected from the opening of a gap at the Fermi level. The paramagnetic susceptibility of randomly aligned thin needles prepared by diffusion is shown in Figure 8. These results were obtained from the experimental measurements after a subtraction of a diamagnetic contribution which, assuming the stoichiometry (CPP12(I3), was estimated from tabulated Pascal constants as (4.5 x emdmol. Different preparations gave essentially the same temperature-dependent results, with an average value for the room-temperature paramagnetic susceptibility of xp(RT) = (1.2 F 0.4) x emdmol. Upon cooling there is a small decrease of x p until approaching the M-I transition where a sudden decrease is observed. At lower temperatures the susceptibility becomes dominated by a Curie tail corresponding approximately to 1% of S = impurities o r defects in case of Figure 8. It should be noted that the present results are of significantly better quality than the preliminary data previously published,16 not only due to a smaller Curie tail but also because previous results were obtained with a field of 5 T, under which some nonlinear behavior is already noticeable, especially a t low temperatures.
Chem. Mater., Vol. 6, No. 12, 1994 2315
Molecular Metals (CPP)kI3)1-8 "I
I o a.
0
h
o
O
'
o0
0
5
3
0.
4-
0
0
% . 2 0
0
2i o a?
8,
,
EPR showed no differences in the temperature dependence of the signal obtained either with elongated diamond or thin needle shaped crystals. In all samples, the EPR signal consisted of one single Lorentzian line. The g values were found almost isotropic when the magnetic field, H , was perpendicular to a and the crystal rotated along this axis, with g 2.0044 and no significant temperature dependence down to liquid helium temperatures. These g values are identical to those found in the (CPP)ZPF&H&~Z radical salt.15 The width of the signal (Figure 9) is -6 G at room temperature, remaining temperature independent upon cooling until ca. 80 K. A fast decrease is observed around 60 K and a minimum of 0.6 G is reached at 25 K. The intensity of the EPR signal, as simply estimated by the product of the second derivative maxima by the square of the linewidth, approximately follows the temperature dependence of the static paramagnetic susceptibility.
-
Discussion In view of the same lattice parameters of the two types of crystals, indicating the same type of CPP packing, their different properties are certainly related to a different degree of ordering of the iodine. Given the size of the channels in the structure and the type linear 13- size (9.59 Ag,36),the maximum amount of iodine allowed in this structure considering linear Ischains without tilting is slightly less than one triiodide for each two CPP molecules (-0.92). Higher iodine contents with a stoichiometry closer to (CPP1213 as suggested by some of the elemental analysis results are only possible either considering the tilting or bending of the 13-ions or slightly stronger interactions between the iodine species or a combination of these effects. The 13-ions in elongated diamond-shpaed crystals are well ordered at relatively long range with a repeat d = 10.19 A. Most probably the smaller thin needles are formed in conditions further from equilibrium and therefore without the long-range order of Is- ions in the elongated diamond-shaped crystals. A better description of the iodine disorder in these compounds especially in the thin needles, requires additional X-ray diffraction work. The main difference in the transport properties of the two types of crystals lies in the thermopower that, as previously mentioned, was systematically measured in several samples prior to electrical resistivity measurements. Different electrical properties as a consequence of different degrees of disorder of triiodide ions are not
uncommon as observed for instance in the well-studied case of TMA(TCNQ)I.44-48A positive thermopower, almost proportional to temperature, as observed in CPP2(13)1-~thin needles at high temperatures, is the behavior predicted by the tight binding approach, neglecting Coulomb correlation effects, for a onedimensional metal with bandfilling larger than onehalf.49 In this case the stoichiometry imposes a band filling slightly larger than 3/4. A similar behavior has been observed also in the 3/4 filled band systems a-(Per)zM(mnt)z, with M = Pt, Au, Pd, Ni, Cu, Fe, and Co, where at room temperature the thermopower is in The negative thermopower the range 32-42 ,uV/K.4,5,7,50 with smaller values, as observed in the elongated diamond-shaped crystals, reflects the perturbation of the simple sinusoidal tight-binding band by the external potential of the more ordered triiodide ions. Such a perturbation is smoothed by the lack of long-range order in the thin needles. The M-I transition observed for both types of crystals at ca. 63 K is most probably a Peierls transition due to a distortion of the CPP chains. This is suggested by the maximum of d In e/d(l/T) at -63 K, denoting the opening of a gap at the Fermi level at this temperature, a t variance with a gradual increase of the electrical resistivity due to localization at low temperatures that has been observed especially in other disordered molecular conductors. Further evidence for a Peierls mechanism at the origin of this transition, is provided by the vanishing of the small Pauli-like susceptibility observed in the metallic regime of the thin needles, at the M-I transition temperature, indicating that both charge and spin degrees of freedom are frozen at this transition. The low-temperature resistivity data do not clearly approach a constant value of d In e/d(l/T), and therefore it is difficult to accurately obtain the lowtemperature energy gap. This behavior is most probably due to defect or impurity states in the gap, estimated as -25 meV for both types of crystals. The clear metallic properties of both phases in an extended high-temperature range, as confirmed by thermopower, also support the indication that the measured electrical conductivity in elongated diamondshaped crystals, 1.2 S/cm, is a lower limit of the intrinsic value. In fact a comparison with other molecular conductors show that clear metallic properties, as observed in this compound, are almost always associated with electrical conductivities at least 1 order of magnitude larger ('50 S / ~ m ) . ~As~ previously 2~~ noted, this difference can be explained by unfavorable geometrical conditions and extreme anisotropy. (44)Coulon, C.; Flandrois, S.; Delhaes, P.; Hauw, C.; Dupuis, P. Phys. Reu. B 1981,23,2850. (45)Filhol, A,; Gallois, B.; Langier, J.; Dupuis, P.; Coulon, C. Mol. Cryst. Liq. Cryst. 1982,84, 17. (46)Ikari.. T.: , Jandl., S.:, Aubin., M.:, Truone. K. D. Phvs. Rev. B 1983. 28,3859. (47)Coppens, P.; Leung, P.; van Tilborg, P.; Murphy, IC;Epstein, A.; Miller, J . S. Mol. Cryst. Liq. Cryst. 1980,61, 1. (48)Filhol, A.; Rovira, M.; Hauw, C.; Gaultier, J.; Chasseau, D.; Dupuis, P. Acta Crystallogr., Sec. B 1979,35, 1652. (49)Kwak, J. F.; Beni, G.; Chaikin, P. M. Phys. Reu. B 1976,13, 641. (50) Henriques, R. T.; Almeida, M.; Matos, M. J.; Alcacer, L.; Bourbonnais, C. Synth. Met. 1987,19, 379. (51)Epstein, A. J.; Conwell, E. M.; Miller, J. S. Ann. N.Y. Acad. Sci. 1978,313,183. (52)Delhaes, P. In Lower Dimensional Systems and Molecular Electronics; Metzger, R. M., Day, P., Papavassiliou, G. C., Eds.; NATOAS1 Series B248; Plenum Press: New York, 1991;p 43. I
I
2316 Chem. Mater., Vol. 6, No. 12, 1994
The EPR results show that the small Pauli-like paramagnetic susceptibility is due to the delocalized electrons in the CPP stacks. The similarity between the EPR results in elongated diamond-shaped and thinneedle crystals suggests in both forms similar static susceptibility. The paramagnetic susceptibility observed in this case is smaller than those in the a-(Per)N~ ~ C O ,where ~ (mnt)z compounds with M = A u , ~C U , and room-temperature values are in the range (1.5-2.8) x emu/mol. In these compounds, the M(mnt)zunits are diamagnetic and the susceptibility is due only to the Pauli-like contribution of the perylene conduction electrons forming a 3/4 filled band. Since both the smaller bandwidth and larger band filling of the CPP compound would favor a larger density of states at the Fermi level, and consequently a larger Pauli paramagnetism, the smaller xp value in the CPP compound is taken as an indication of reduced Coulomb repulsion effects that tend to enhance the paramagnetic susceptibility from the Pauli limit for uncorrelated electrons. Such reduction is attributed to the large polarizability of triiodide ions that screen the repulsion effects. In conclusion we have shown that iodine partially oxidized regular stacks of CPP afford two types of crystals, CPP2(13)1-s7both having the same unit cell and (53) Gama, V.; Henriques, R. T.; Almeida, M.; Alcacer, L. J . Phys. Chem. 1994,98, 997.
Morgado et al. presenting metallic properties comparable t o other perylene compounds. The details of their different metallic properties reflect different degrees of iodine disorder. Despite the larger volume of the -(CHzCHd- groups, the CPP molecules present in this solid the same type of overlap and only slightly larger intermolecular spacings than perylene molecules in many other comparable molecular metals, giving rise to conduction bands of similar width and also with high electrical conductivity values. Furthermore, the CPP hydrogen atoms interact with the triiodide anions locking them in this solid with a stable composition at variance with (Per)z(Iz)3,where iodine content is not stable and it is mixed valence between 12 and
Acknowledgment. This work was partially supported by Junta Nacional de InvestigaqBo Cientifica e Tecnol6gica (Portugal) under Contracts PMCT/C/ PMF798/907PBIC/C/CEN1157/92,and PMCT/C/CEN367/ 90 and by EC Espirit Basic Research Action MOLCOM No. 3121. SupplementaryMaterial Available: Tables 1-8, giving crystal data and details of structure determination, atomic coordinates, atomic displacement parameters, bond distances and angles, selected nonbonded distances, and deviations to molecular planes (7 pages); tables of F, and F, (7 pages). Ordering information is given on any current masthead page.
