J O U R N A L O F C H E M I S T R Y Materials New semiconductors obtained by reaction of 4-imidazoline-2-selone† derivatives with TCNQ. Characterization and X-ray structure of (C9H12N4Se)2+(TCNQ)2- 3 Francesco Bigoli,a,b Paola Deplano,c Francesco A. Devillanova,c Alberto Girlando,*a Vito Lippolis,c M.-Laura Mercuri,c M.-Angela Pellinghellia,b and Emanuele F.Troguc aDip. Chimica Generale ed Inorganica, Chimica Analitica e Chimica Fisica, Parma University, 43100 Parma, Italy bCentro di Studio Strutturistica DiVrattometrica del CNR viale delle Scienze 78, 43100 Parma, Italy cDip.Chimica e T ecnologie Inorganiche e Metallo-organiche, Cagliari University, 09124 Cagliari, Italy The charge transfer complexes I, II and III of three new electron donors, 1,1¾-ethylenebis(3-methyl-4-imidazoline-2-selone) 1, 1,1¾-methylenebis(3-methyl-4-imidazoline-2-selone) 2 and 1,3-dimethyl-4-imidazoline-2-selone 3, with TCNQ have been synthesized.We report the X-ray crystal structure of II, (C9H12N4Se)(TCNQ)3, which crystallizes in the triclinic system, space group P1, with one molecule per unit cell and a=7.563(7), b=10.371(6), c=13.575(5) A ° , a=95.00(2), b=95.54(2), c=109.74(2)°. The comparison with the already described crystal structure of I and a detailed spectroscopic characterization of all three complexes, allows us to show that the change of the donor yields compounds characterized by quite diVerent charge distributions and stack types.The semiconducting properties, ranging from~10-2 to 6×10-4 S cm-1, are most likely due to the TCNQ stacks, where two negative charges are distributed among three TCNQ units.At room temperature, the charge is fully delocalized along the TCNQ stack of I, the semiconductor with the highest conductivity, whereas in II and III the charge is unevenly distributed among the TCNQ units. The donors show a strong tendency to form stable dications: in I and III they give rise to mixed-valence cations and a new dication with elimination of an Se atom is formed in II.Among the various classes of electron donor molecules cur- molecules are faced plane to plane in groups of three, closely resembling the trimerized stack structure of other TCNQ salts, rently synthesized in the field of molecular metals, the derivalike Cs2(TCNQ)3.4 The similarity between the bond distances of the TCNQs in the triads suggested that the dinegative charge is not localized on the TCNQ units, but because of the lack of single crystal vibrational spectra no definite conclusion on the charge distribution could be reached.1 N NMe Se R = 1 R-(CH2)2-R 2 R-CH2-R 3 R-CH3 tives of imidazoline-2-selone occupy a rather peculiar place. These molecules are good donors, having a first oxidation potential intermediate between those of TTF‡ and of BEDT–TTF,1 but also exhibit a quite varied and interesting chemistry.Reaction with diiodine or interhalogens, for instance, yields several diVerent types of compounds, such as insertion compounds, neutral adducts and dications.2,3 We recently started investigating the charge transfer (CT) complexes of imidazoline-2-selone derivatives with p-electron acceptor molecules.The reaction of 1 with TCNQ produces a mixed-valence N MeN N NMe Se Se N Me N N Me N Se MeN N Se NMe N Se Se 4+ 4 salt I formed by a tetracation (4) consisting of one centrosymmetrical molecule of the neutral ligand and two dications, In the present paper we report the structural and spectromutually related by a symmetry center and bearing an SeMSe scopic characterization of the complex II obtained from bridge.1 The neutral ligand and the two dications are held RM(CH2)MR (2) and TCNQ.Furthermore, we complete the together by an SeUSe donor–acceptor interaction between characterization of I through its single crystal polarized infraeach Se atom of 1 and the adjacent Se of each dication. Two red (IR) spectra from room temperature to 77 K.The comparidinegative triads of TCNQ act as counterions. The TCNQ son of the spectral data of I and II with those of the complex III obtained by reacting 3 with TCNQ also gives useful hints †4-Imidazoline=2,3-dihydroimidazole. about the structure of the latter semiconductor. A preliminary ‡List of abbreviations: BEDT–TTF=bis(ethylenedithio)tetrathiafulvacomparison between the structure and spectroscopic properties lene; CT=charge transfer; IR=infrared; MEM=methylethylmorpholof the three CT complexes obtained by reacting 1–3 with inium; TCNQ=7,7,8,8-tetracyano-p-quinodimethane; TTF=tetrathiafulvalene.TCNQ is given in ref. 5. J. Mater. Chem., 1998, 8(5), 1145–1150 1145Table 1 Crystallographic data for compound II, (C9H12N4Se) (TCNQ)3 Experimental formula C45H24N16Se Materials Mw 867.75 crystal system triclinic The electron donors 1, 2 and 3 were prepared as previously space group P1 described.2 TCNQ, the solvents and the reagents were commera/ A ° 7.563(7) cial products (Aldrich), used without further purification. b/A ° 10.371(6) Compounds I, II and III are the reaction products of TCNQ c/A° 13.575(5) with 1, 2 and 3, respectively.The synthesis of I has already a(°) 95.00(2) been described.1 II and III were obtained by mixing CH2Cl2 b(°) 95.54(2) c(°) 109.74(2) solutions of 2 and 3 with a CH3CN solution of TCNQ in 152 U/A° 3 989(1) and 151 ratio, respectively. The mixed solutions were left to Z 1 stand at room temperature, and after several days well-shaped Dc/Mg m-3 1.457 crystals of II and lustrous dark-blue microcrystals of III were radiation Ni-filtered obtained in about 70% yield.Elemental analysis: II, Calc. for wavelength Cu-Ka(l=1.541838 A° ) C45H24N16Se: C 62.29, H 2.79, N 25.83; Found: C 62.38, H 2.62, T/K 295 m/cm-1 17.52 N 25.42%. III, Calc. for C17H12N6Se: C 53.79, H 3.16, N 22.15; h-range for intensity collection(°) 3–70 Found: C 54.09, H 3.31, N 22.64%.data collected ±h,±k,l no. of measured reflections 3760 Spectroscopic measurements no. of reflections with I>2s(I) 2226 no. of refined parameters 317 Single crystal polarized infrared spectra were obtained with a min/max height in final Dr map/eA° -3 -0.35/0.71 Bruker IFS66 FT-IR spectrometer, equipped with a Bruker largest shift/e.s.d. 0.20 A590 microscope, and a KRS5 grid polarizer (Specac).Low R=S|DF|/S|F| 0.0596 temperature measurements under the IR microscope were wR=ÓS w(DF )2/S wF2 0.0607 performed by using a Linkam FTIR-600 liquid nitrogen cold stage. The FT-Raman spectra were recorded on a Bruker RFS100 FTR spectrometer, operating with an excitation fre- temperature conductivities ranging from ~10-2 to quency of 1064 nm (Nd:YAG laser). The power level of the 6×10-4 S cm-1.An almost complete characterization of I, laser source was 20–40 mW, and the powdered samples were including the crystal structure, has already been described,1 as dispersed in KBr and packed into a glass capillary for 180° summarized in the introduction. scattering geometry. No sample decomposition was observed during the experiments.Crystal structure of II Compound II crystallizes in the triclinic system, space group Conductivity measurements P1, Z=1, and can be formulated as (C9H12N4Se)(TCNQ)3. A Conductivity measurements were made at room temperature summary of the crystallographic data is reported in Table 1. on pellets (thickness 0.5 mm, diameter 12 mm) using the two- The structure (Fig. 1) consists of homologous layers of alternatpoint probe method. s(I)=1.0×10-2; s(II)=5.9×10-4; ing (C9H12N4Se)2+ dications and (TCNQ)2- 3 anions, roughly s(III)=6.5×10-3 S cm-1. parallel to the ab plane, the cations and anions lying roughly parallel to the bc and ac planes, respectively. X-Ray diVraction measurements Compound II diVers from I in both the cation and the anion structure.In I, a mixed-valence cation is present, with dications The diVraction measurements on the complex II were made characterized by an SeMSe bridge and the formation of an on a Siemens AED diVractometer. The crystals to choose from were not of good quality; the one selected had dimensions of 0.05×0.21×0.36 mm. Accurate unit cell parameters and the orientation matrix for data collection were obtained from leastsquares refinements, using the setting angles of 29 reflections (18°<h<31°).The check on the standard reflection showed no significant crystal decay under irradiation. Intensities were corrected for Lorentz and polarization eVects. No correction for absorption was applied. The structure was solved and refined using the SHELX-766 and SHELX-867 computer programs.Only the non-hydrogen atoms of the dication were refined anisotropically. All hydrogen atoms were placed at their geometrical position (CMH: 0.96 A ° ) and refined by ‘riding‘ on the corresponding atoms with unique isotropic thermal parameters [U=0.0969(86) A° 2]. In the last stages of refinement the best result was obtained using unit weights. Scattering factors and correction for anomalous dispersion eVects were taken from ref. 8. Full crystallographic details, excluding structure factors, have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Information for Authors, J. Mater. Chem., 1998, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/84.Results Compounds I, II and III, obtained by reacting TCNQ with 1, Fig. 1 Projection of the structure of II along [001] 2 and 3, respectively, are molecular semiconductors with room 1146 J. Mater. Chem., 1998, 8(5), 1145–1150Table 3 Raman spectra of the compounds I, II and IIIa I II III n� /cm-1 n� /cm-1 n� /cm-1 tentative assignmentb 227 w 229 w 226 m 308 w 332 m 331 m 328 m T, agn9 433 w 490 w 567 w 609 w 605 m 600 m T, agn8 705 w 708 w 706 w 967 m 970 w,br 962 m T, agn6 1156 m D 1196 s 1195 s 1195 s T, agn5 1374 m,br 1388 m 1384 w T, agn4 Fig. 2 Perspective view of (C9H12N4Se)2+ cation in II, showing the 1413 s 1417 s 1417 s T, agn4 atomic numbering scheme. The thermal ellipsoids are drawn at the 1451 m 1443 m D? 30% probability level. 1603 s 1601 s 1601 s T, agn3 2202 m 2205 m,br 2205 m,br T, agn2 eight-membered central ring (see structural formula 4 in the aQualitative relative intensities indicated by: s, strong; m, medium; introduction).In II, the donor 2 gives rise to a dication with w, weak; br, broad; sh, shoulder. bD: donor; T: TCNQ. For the elimination of one Se atom, and the formation of a six- numbering of the normal modes see ref. 13. membered ring. The molecular structure and atomic labelling scheme of the dication are shown in Fig. 2; the bond lengths and angles are given in Table 2. The central ring of when it is used in the analysis of an homologous series of (C9H12N4Se)2+ has a boat conformation, with h and w9 equal compounds, as in the present case. We have recorded the to 86.6(9)° and -6.1(9)°, respectively, whereas the dihedral powder Raman spectra and the single crystal polarized IR angle between the two planar imidazoline rings is of 50.2(5)°. spectra of I, II and III, and the interpretation of the data will The dication structural data are in agreement with those be reported in that order.Tables 3 and 4 report the frequencies observed in the diiodine neutral adducts of 2,2 and of its sulfur observed in the Raman and IR spectra (polarization perpenanologue, 10 and in the insertion derivatives of 2 with chlorine.11 dicular to the stack) for the three compounds.The TCNQ stacks of the complex I are formed by triads in a Partial vibrational data for I have already been published, zig-zag arrangement. In II, on the other hand, the TCNQ but no definite conclusion could be reached about the charge stack is better described as being made up of TCNQ dimers distribution on the TCNQ sites.1 The single crystal IR spectra (A and C¾ in Fig. 1), separated by a third TCNQ (B), shifted up to 2500 cm-1 are reported in Fig. 3. The spectrum polarized in the a direction. The dihedral angles between the meanparallel to the stack is similar to that of the powders,1 being weighted planes of the non-exactly planar TCNQ molecules dominated by the electronic CT transition (maximum around are in the range 1.5(2)–3.9(2)°. 5000 cm-1, not shown in Fig. 3) along the TCNQ stack. We have not been able to obtain single crystals of III Superimposed on it the vibronic bands typical of the TCNQ suitable for X-ray analysis, so that all the information on the molecule13 are clearly identified.These bands correspond to complex III formed by reacting 3 with TCNQ relies on the the totally symmetric (ag) modes of TCNQ, the intramolecular spectroscopic measurements reported in the following section. modes which, modulating the on-site energies, are coupled to the CT electron and borrow IR intensity from the CT trans- Spectroscopic studies ition.12 As for the powder spectra, these bands are normal absorptions below ca. 1000 cm-1, but above this frequency Vibrational spectroscopy is an invaluable tool for a detailed characterization of organic semiconductors,12 particularly they appear as indentations in the continuum single-electron transition. This well known phenomenon14 allowed us to when it is associated with X-ray structure determination or Table 2 Selected bond distances (A ° ) and angles (°) for the dication in complex II (e.s.d.in parentheses) bond length/A° Angle(°) SeMC(11) 1.908(13) C(21)MN(21)MC(51) 127.1(12) SeMC(12) 1.865(13) C(11)MN(21)MC(51) 124.6(11) N(11)MC(11) 1.319(17) C(11)MN(21)MC(21) 108.2(11) N(11)MC(31) 1.402(16) C(12)MN(12)MC(32) 109.7(11) N(11)MC(41) 1.440(14) C(41)MN(12)MC(32) 127.3(12) N(21)MC(11) 1.348(14) C(41)MN(12)MC(12) 122.6(12) N(21)MC(21) 1.353(21) C(22)MN(22)MC(52) 126.3(15) N(21)MC(51) 1.468(18) C(12)MN(22)MC(52) 125.3(13) N(12)MC(41) 1.468(21) C(12)MN(22)MC(22) 108.2(12) N(12)MC(12) 1.345(20) N(11)MC(11)MN(21) 107.5(11) N(12)MC(32) 1.366(20) SeMC(11)MN(21) 128.0(9) N(22)MC(12) 1.351(20) SeMC(11)MN(11) 124.1(9) N(22)MC(22) 1.387(23) N(21)MC(21)MC(31) 110.1(13) N(22)MC(52) 1.446(27) N(11)MC(31)MC(21) 103.5(12) C(21)MC(31) 1.358(21) N(11)MC(41)MN(12) 108.7(11) C(22)MC(32) 1.353(31) N(12)MC(12)MN(22) 107.4(13) C(11)MSeMC(12) 88.4(5) SeMC(12)MN(22) 128.7(10) C(31)MN(11)MC(41) 126.6(11) SeMC(12)MN(12) 123.8(9) C(11)MN(11)MC(41) 122.5(11) N(22)MC(22)MC(32) 107.5(16) C(11)MN(11)MC(31) 110.7(10) N(12)MC(32)MC(22) 107.1(15) J.Mater. Chem., 1998, 8(5), 1145–1150 1147Table 4 Infrared spectra, polarized perpendicularly to the stack, of the TCNQ units. However, in trimerized stacks the ag frequencies compounds I, II and IIIa may be perturbed by electron–phonon coupling.15 A safe and accurate estimate of r must then rely on the frequencies of the I II III normally IR active ungerade modes.The IR spectra of I n� /cm-1 n� /cm-1 n� /cm-1 tentative assignmentb polarized perpendicularly to the stack (Fig. 3 and Table 4) 626 w yield a straightforward assignment of the main in-plane (b1u 646 s and b2u) vibrations of TCNQ. The CON stretching frequencies 675 m 2D2+ around 2200 cm-1 have often been used to determine the 707 w D average charge on the TCNQ units.16 The spectra of I show 734 m 2D2+ two CON stretching bands, one of b1u and the other of b2u 751 m 747 m 744 m D symmetry, again indicating uniform charge distribution. 759 w 800 w 805 m However, since several authors have reported deviations from 830 m the expected linear dependence of the CON frequencies from 1033 w 1023 w 2D2+ r,17–19 we prefer to use the CC stretching mode b1un20.17 Its 1079 w frequency occurs at 1517 cm-1 (Table 4), indicating an average 1097 w 1092 m D charge of 0.68.We can now safely conclude that in I the 2e- 1122 m 1115 w charge is uniformly distributed among the three TCNQ units 1147 w 1134 w 1132 m D 1176 m 1170 m in the trimerized stack. This finding explains well the remark- 1209 s 1203 s T, b2un36 able conductivity of I: about two orders of magnitude higher 1230 m 1222 s D than that of Cs2(TCNQ)3.20 1251 w We have also obtained single crystal polarized IR spectra at 1333 m 1334 s D, 2D2+ low temperature, to detect the presence of phase transitions in 1375 s 1349 m 1372 w D? the complex I.Fig. 4 compares the CON stretching region 1405 w 1442 w 1453(perpendicular polarization) at 300 and 77 K. It is seen immedi- 1480 w 1492 s 2D2+ ately that the room temperature doublet becomes a complex 1503 w 1505 sh T, b1un20 structure at liquid nitrogen temperature, with at least five 1517 s 1524 s 1519 sh T, b1un20 maxima clearly identified.Indeed, at temperatures below 250 K 1566 s 1569 ms 1560 m D, 2D2+ the charge is no longer uniformly distributed on the TCNQ 1633 m 1629 w sites, as revealed by the structure of the CON stretching 2162 s 2158 s T, b2un33 2175 s 2176 s T, b1un19 region.The change appears to occur rather smoothly with 2191 s 2189 s T, b2un33 temperature variation, so it is hardly possible to speak of a 2194 s T, b2un33 phase transition, nor can we determine precisely the diVerence 2198 s 2198 s 2198 s T, b1un19 in charge among the TCNQ sites, which remains rather small. 2203 s 2207 sh T, b1un19 The intensity of the vibronic bands in the parallelel IR spectrum does not change significantly, indicating that the stack structure aQualitative relative intensities indicated by: s, strong; m, medium; w, weak; br, broad; sh, shoulder. bD: donor; T: TCNQ. For the is aVected very little by the localization of the charge. numbering of the normal modes see ref. 13. The crystal structure and conductivity show that II is a Fig. 3 Polarized IR spectra of I: (a) electric vector parallel to the stack axis; (b) electric vector perpendicular to the stack axis estimate the room temperature optical gap of I at ca. 1000 cm-1 (~0.12 eV).1 In the previous work,1 the frequency of the Raman active Fig. 4 Polarized IR spectra of I in the CON stretching region, at (i) TCNQ agn4 mode, observed for I at 1413 cm-1 (Table 3), room (300 K) and (ii) liquid nitrogen (77 K) temperature.Only the polarization perpendicular to the stack axis is shown. suggested a uniform charge distribution (r) among the trimeric 1148 J. Mater. Chem., 1998, 8(5), 1145–1150In the light of the above data, and of the X-ray analysis, we suggest that the TCNQ oVset of the stack (B in Fig. 2) bears a unit negative charge, whereas the other electron is slightly unevenly distributed among the A and C TCNQ units, as in the dimerized stack structure of MEM(TCNQ)2.22 As far as we know, the present stack structure and charge distribution have not been encountered so far in TCNQ stacks. The charge localization of II as opposed to the charge delocalization of I may well explain the large diVerence in room temperature conductivities (5.9×10-4 vs. 1.0×10-2 S cm-1) between the two compounds. We have not been able to obtain single crystals of III suitable for X-ray analysis, so that all the information on the complex formed by reacting 3 with TCNQ relies on the spectroscopic measurements. Fig. 6 shows the IR spectra polarized parallel and perpendicular to the stack axis, whereas Tables 3 and 4 report the Raman and IR frequencies.The IR spectrum polarized parallel to the stack shows an electronic transition centered around 4000 cm-1, lower than for I or II, but without trace of vibronic absorptions. Therefore the stack along which the CT transition occurs, most probably the TCNQ stack, is a regular one, i.e. a stack with constant distance between the molecular units.12 In such a case one would expect a uniform charge distribution, but the Raman spectrum in the TCNQ agn4 mode spectral region is rather Fig. 5 Polarized IR spectra of II: (a) electric vector parallel to the similar to that of II (Table 3), where the charge is localized. stack axis; (b) electric vector perpendicular to the stack axis Analogous information comes from the IR spectrum polarized perpendicular to the stack.The spectrum (Fig. 6 and Table 4) shows four bands in the CON stretching region, indicating quite diVerent complex from I. The spectroscopic measurements oVer further clues about the origin of the diVerence. two diVerently charged TCNQ units, one with charge ~1 and the other between 0.4 and 0.5. Careful analysis of the b1un20 Fig. 5 reports the IR absorption spectra polarized parallel and perpendicular to the stack axis. The spectra polarized parallel spectral region, partially obscured by the presence of a strong absorption due to the donor at 1492 cm-1 (see below) confirms to the stack show the CT electronic absorption with a maximum beyond 5000 cm-1, over which the vibronic transitions the presence of both TCNQ- and TCNQ-0.5 units.The comparison of the IR spectrum of III (Table 4) with due to the TCNQ ag modes are superimposed. The optical gap can again be estimated from the frequency above which those of the neutral donor 3 and of the dication formed by the latter by reaction with Br211 indicates the presence of a mixed- the vibronic bands appear as indentations (~1000 cm-1 or ~0.12 eV).However, it should be noted that, at variance with valence counterion. Since analytical results indicate that III contains TCNQ and 3 in a 151 molar ratio, we suggest that what is observed in I, the structure of the indentations is rather complex. Apart from other diYcult to analyze features, a the complex is composed of a mixed-valence dication formed by the dimerized donor with an Se–Se bridge and a neutral doublet structure is clearly resolved, with a narrow component immediately below the main transition. This fact suggests that molecule of 3, and by a triad of TCNQs bearing two negative in II the 2e- charge is not uniformly distributed on the TCNQ sites,21 as the following analysis will confirm.As shown by the X-ray data, the donors give rise to a new type of dication, whose vibrational frequencies are of course unknown. Therefore, it is diYcult in some cases to disentangle the frequencies due to TCNQ from those of the dication. In the Raman spectrum (Table 3) we observe three bands (1451, 1417 and 1388 cm-1) in the spectral region of the TCNQ agn4 mode (see above).13 The first one would indicate the presence of neutral TCNQ, but since other Raman or IR bands attributable to this species are not seen, we prefer to attribute this band to the donor.If the other two bands are assigned to TCNQ, they indicate the presence of at least two diVerently charged TCNQs, one with intermediate charge and one fully charged. The IR spectra polarized perpendicularly to the stack (Fig. 5 and Table 4) confirm that in II the charge is unevenly distributed on the TCNQ sites. The CON spectral region shows six bands, instead of the two expected for uniform charge distribution, suggesting the presence of three diVerently charged TCNQs, one with r~1 and the other two both with a charge close to 0.5. The assessment of the precise estimate of r cannot rely on the CON stretching frequencies, however, 17–19 and we can reasonably assign only two bands in the C–C stretching region, at 1503 and 1524 cm-1.If they are both assigned to the TCNQ b1un20 mode, they would indicate a charge of r~1 and of r~0.5, respectively, consistent with what can be qualitatively deduced from the analysis of the Fig. 6 Polarized IR spectra of III: (a) electric vector parallel to the stack axis; (b) electric vector perpendicular to the stack axis CON stretching region.J. Mater. Chem., 1998, 8(5), 1145–1150 1149charges. At this point we can advance two hypotheses about by the National Research Council (CNR) under the Coordinate Project No. 96/215. the structure of the TCNQ stack. The first is that the stack somehow resembles that of II, with two TCNQ-0.5 and one TCNQ-1 units forming a repetitive trimeric pattern.In the References second hypothesis the TCNQ bearing unit negative charge 1 F. Bigoli, P. Deplano, F. A. Devillanova, A. Girlando, V. Lippolis, instead does not belong to the stack, which is made up solely M.-L. Mercuri, M.-A. Pellinghelli and E. F. Trogu, Inorg. Chem., from TCNQ0.5- units.We have performed single crystal low 1996, 35, 5403. temperature IR measurements to verify whether complex III 2 F. Bigoli, M.-A. Pellinghelli, P. Deplano, F. A. Devillanova, undergoes a Peierls transition. In such a case, the frequency V. Lippolis, M.-L. Mercuri and E. F. Trogu, Gazz. Chim. Ital., and shape of the vibronic bands would help us to decide 1994, 124, 445. between the two hypotheses.However, we have not detected 3 F. Bigoli, F. Demartin, P. Deplano, F. A. Devillanova, F. Isaia, V. Lippolis, M.-L. Mercuri, M.-A. Pellinghelli and E. F. Trogu, phase transitions down to liquid nitrogen temperature. With Inorg. Chem., 1996, 35, 3194; F. Bigoli, P. Deplano, the lack of further data, and considering that the conductivity F. A. Devillanova, V.Lippolis, M.-L. Mercuri, M.-A. Pellinghelli and the electronic spectrum of III diVer from those of II, we and E. F. Trogu, Chem. Ber., in press. prefer the second of the two above-mentioned hypotheses, i.e. 4 C. J. Fritchie and P. Arthur, Acta Crystallogr., 1966, 21, 139. a TCNQ stack formed solely by TCNQ0.5- units. The fact 5 F. Bigoli, P. Deplano, F. A. Devillanova, A. Girlando, V.Lippolis, that we have not observed the Peierls transition, together with M.-L. Mercuri, A. Pelagatti, M.-A. Pellinghelli and E. F. Trogu, Synth.Met., 1997, 86, 1853. the problems encountered in obtaining the X-ray structure of 6 G. M. Sheldrick, SHEL X-76: Programs for Crystal Structure III, suggest the presence of structural disorder in the Determination, University of Cambridge, UK, 1976.compound. 7 G. M. Sheldrick, SHEL X-86: Program for the Solution of Crystal Structures, Universita�t Go� ttingen, Germany, 1986. 8 International Tables for X-ray Crystallography, The Kynoch Press, Conclusions Birmingham, 1974, vol. IV. 9 D. Cremer and J. A. Pople, J. Am. Chem. Soc., 1975, 97, 1354. The donors 1, 2 and 3, based on the same imidazoline-2-selone 10 F.Bigoli, P. Deplano, F. A. Devillanova, M.-L. Mercuri, group, react with TCNQ to produce new molecular semi- M.-A. Pellinghelli, A. Sabatini, E. F. Trogu and A. Vacca, J. Chem. conductors (I, II and III) showing conductivities ranging from Soc., Dalton T rans., 1996, 3583. ~10-2 to 6×10-4 S cm-1 at room temperature. Structural 11 F. Bigoli and P. Deplano, et al., in preparation.and spectroscopic results show that I, II and III complexes 12 C. Pecile, A. Painelli and A. Girlando,Mol. Cryst. L iq. Cryst., 1989, 171, 69. are quite diVerent. In I and III the donor gives rise to mixed- 13 R. Bozio, I. Zanon, A. Girlando and C. Pecile, J. Chem. Soc., valence cations, whereas a new dication with elimination of an Faraday T rans. 2, 1978, 74, 235. Se atom is formed in II.In any case the donors confirm the 14 M. J. Rice, L. Pietronero and P. Bruesch, Solid State Commun., tendency to form closed-shell dications. The imidazoline ring 1977, 21, 757. loses one electron in agreement with the 4n+2 rule, but then 15 A. Painelli, C. Pecile and A. Girlando,Mol. Cryst. L iq. Cryst., 1986, two rings associate through Se bridges. The charge transfer 134, 1. 16 J. S. Chappell, A. N. Bloch, W. A. Bryden, M. Maxfield, and conductivity are essentially associated with the TCNQ T. O. Poehler and D. O. Cowan, J. Am. Chem. Soc., 1981, 103, 2442. stacks. In all three complexes, two negative charges are distrib- 17 A. Girlando, A. Painelli and C. Pecile,Mol. Cryst. L iq. Cryst., 1984, uted among three TCNQ units. Yet, the TCNQ stack structure 112, 325. and charge distribution is diVerent, the most interesting case 18 M. Meneghetti and C. Pecile, J. Chem. Phys., 1986, 84, 4149. being that of I, the complex with highest conductivity. At 19 K. A. Hutchison, G. Srdanov, R. Menon, J.-C. P. Gabriel, 300 K the charge is fully delocalized along the trimerized B. Knight and F.Wudl, J. Am. Chem. Soc., 1996, 118, 13081. 20 K. D. Cummings, D. Tanner and J. Miller, Phys. Rev., 1981, B24, TCNQ stack, and becomes localized below 250 K. 4142. 21 R. Swietlik, Synth.Met., 1995, 74, 115. We wish to thank Dr C. Bellitto (Istituto di Chimica dei 22 M. J. Rice, V. M. Yartsev and C. S. Jacobsen, Phys. Rev., 1980, Materiali, CNR, Rome) for the conductivity measurements. B21, 3437. This work has been supported by the Ministry of University and of Scientific and Technological Research (MURST) and Paper 7/07223F; Received 6th October, 1997 1150 J. Mater. Chem., 1998, 8(5), 1145–