Crystal structures and conducting properties of TTM-TTP salts

Crystal structures and conducting properties of TTM-TTP salts

SL,?ATIITII[ IE=TBLLS I Synthetic Metals 70 (1995) 1153-1154 ELSEVIER Crystal structures and conducting properties of TTM-TTP salts Y. Misaki, a H. ...

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SL,?ATIITII[ IE=TBLLS I Synthetic Metals 70 (1995) 1153-1154

ELSEVIER

Crystal structures and conducting properties of TTM-TTP salts Y. Misaki, a H. Nishikawa, a T. Yamabe, a T. Mori, b H. Mori c and S. Tanaka c aDivision of Molecular Engineering, Graduate school of Engineering, Kyoto University, Yoshida, Kyoto 606-01, J a p a n bDepartment of Organic and Polymeric Materials, Faculty of Engineering, Tokyo Institute of Technology, O-okayama, Tokyo 152, J a p a n CInternational Superconductivity Technology Center, Shinonome, Tokyo 135, J a p a n Abstract Conducting properties of various cation radical salts of the title donor have been investigated. Among them ( T T M - T T P ) I 3 and ~I-(TTM-TTP)2I 3 show metallic conductivity down to TMI = 160 and 20 K, respectively. The results of X-ray crystal structure analysis of (TTM-TTP)I 3 and (TTM-TTP)(PF6)0.267(THF)0. 6 are also presented. 1. I N T R O D U C T I O N A bis-fused TTF, 2,5-bis(1,3-dithiol-2-ylidene)1,3,4,6-tetrathiapentalene (BDT-TTP) and its derivatives are of considerable interest as a donor component for organic metals because several of them have afforded charge-transfer salts showing metallic conducting property down to low t e m p e r a t u r e (<4.2 K) [1,2]. In particular, several bis(methylthio) substituted BDT-TTPs have two-dimensional molecular arr a n g e m e n t [2], although the methylthio group has b e e n r e g a r d e d as an u n d e s i r a b l e s u b s t i t u e n t for realizing two-dimensional metals owing to the steric hindrance. In this Proceeding we report conducting properties of cation radical salts based on tetrakis(methylthio) derivative of BDT-TTP (TTM-TTP, depicted as below) and crystal s t r u c t u r e s o f ( T T M TTP)I 3 [3] and (TTM-TTP)(PF6)0.267(THF)0. 6.

MeST S~::~ST S~:=:~ST SMe MeSaS

S.~ ~ ' . S

S..r-~SMe

TTM-TTP 2. C O N D U C T I N G P R O P E R T I E S Single crystals of TTM-TTP salts were prepared by electrochemical oxidation in the presence of the corresponding t e t r a b u t y l a m m o n i u m salts at a cons t a n t current of 0.3-0.5 9A in T H F or 1,1,2-trichloroe t h a n e (TCE). On the other hand, two 13 salts, (TTM-TTP)I 3, a - ( T T M - T T P ) 2 I 3 were obtained by diffusion technique with Bu4NI 3 in THF-TCE (1:1, v/v). The composition and conducting property of t h e m are s u m m a r i z e d in Table 1. Most of salts show semiconductive behavior, an.d3 their conductivity is relatively low values of 10" -10 S cm- at room temperature. On the other hand, (TTM-TTP)I 3 and ~0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved 0379-6779(94)02797-3

SSD1

(TTM-TTP)2I 3 display_ exceptionally high conductivity of 700 and 200 S cm -1, respectively. Both salts are metallic down to 160 and 20 K, respectively, below of which become semiconductors. Whereas u - ( T T M TTP)2I 3 is a low conductive ( a r t = 0.03 S cm "1) semiconductor with a large activation energy of 0.3 eV. 3. CRYTAL S T R U C T U R E S 3.1. (TTM-TTP)I 3 As shown in Figure 1, this salt has uniform columns of donor molecules [4], therefore, the calculated overlap of i n t r a s t a c k is a very large value of -26.1x10 -3. On the other hand, the interstack interaction is less t h a n 1/20 of the i n t r a s t a c k interaction because the effective overlap along the t r a n s v e r s e direction is prohibited owing to the steric hindrance of the methylthio groups t h a t are b e n t toward the outside from the molecular plane. It seems to be surprising t h a t the present salt shows metallic conductivity in spite of 1:1 salt. However, it has one donor molecule in a unit cell, therefore there exists only one (half-filled) energy band, which m a y result in metallic conduction. F u r t h e r m o r e , the reduced onsite Coulomb repulsion and the c o m p a r a t i v e l y large bandwidth (ca. 1 eV) probably avoid the formation of a Mott-insulator [5].

Figure 1. Donor a r r a n g e m e n t of (TTM-TTP)I 3.

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Y. Misaki et aL / Synthetic Metals 70 (1995) 1153-1154

Table 1. Composition and Electrical Conductivity of (TTM-TTP)Ax Anion

Solvent

Form

C104 C104 ReO4 IO4 PF 6 PF 6 AsF 6 AsF 6 I3 I3 (a) 13 (~) AuC12 AuBr2

TCE b') THF TCE TCE TCE THF TCE TCE TCE-THF TCE-THF TCE THF THF

plate needle plate plate plate plate needle plate needle plate needle needle needle

x a)

0.8(Re) 0.8(I) 0.267(X) 1.2(As) 0.9(As) 1.0(X) 0.5(X) 0.54(I) 0.15(Au) 0.9(Au), 0.7(Br)

Ort / Scm "1

E a/eV

2 0.8 19 0.34 0.01 0.003 2.4 0.2 600 0.03 200 1.2 0.6

0.1 0.05 0.07 0.1 0.22 0.14 0.05 0.2 TMI = 160 K 0.3 TM! = 20 K 0.03 0.086

a) Determined by the energy dispersion spectroscopy from the ratio of sulfur and the elements designated in the parentheses. X designates the value determined from the single crystal X-ray structure analysis, b) TCE = 1,1,2-trichloroethane. 3.2. (TTM-TTP) (PF6)0.267 (THF)0. 6 There are three of crystallographically independent donor molecules (molecules A, B and C) [6]. The molecule C lies on a center of inversion, and the others are located on general positions. The crystal contains two i n d e p e n d e n t T H F molecules. TTM-TTP molecules form conducting sheets along ac plane, which are sandwiched by anion sheets composed of PF6 anions and T H F molecules. The donor molecules are stacking along [111] direction with fivefold period as A-B-C-B-A (Figure 2). Contrary to (TTM-TTP)I 3, the present salt has two-dimensional a r r a n g e m e n t of donors. All of the overlap modes of donors are the ring-over-bond type-(Figure 3). The angles between the molecular planes and the direction of the int r a s t a c k interaction are 90 ° for molecules A-B and 60 ° for A-A' and B-C. The overlap between molecules A and B (b2 ,= -25.1x10"~) i s a b o u t six times as l a r g e s s those of A-A (bl = -4.2x10 -3) and B-C (b3 = 4.1x10"3), resulting in strongly dimerized structure in the stack. On the other hand, the overlaps along the transverse direction are comparatively large (5.8-6.9x10") and the donors a r r a y uniformly along the t r a n s v e r s e direction, therefore, the electronic s t r u c t u r e of this salt is expected to be one-dimensional along c-axis. C

CO

.

b

t~

F i g u r e 2. D o n o r a r r a n g e m e n t TTP)(PF6)0.267(THF)0.6

of (TTM-

A-A'

B-C

Figure 3. Overlap modes of intrastack donors in (TTM-TTP)(PF6)0.267(THF)0.6

REFERENCES

1. Y. Misaki et al., Chem. Lett. (1993) 729; 2073; in press; J. Chem. Soc., Chem. Commun. (1994) 459; T. Mori et al., Bull. Chem. Soc., Jpn. in press. 2. T. Mori et al., Chem. Lett. (1993) 733; (1993) 2085. 3. T. Mori et al., Bull. Chem. Soc., Jpn. 67 (1994) 661. 4. C r y s t a l data: Triclinic, space group P - 1 , a = 11.225(3) b = 11.867(3), c = 5.788(1) A, a = 99~69(2), fl = 102.36(2), 7= 112.22(2) °, V = 670.2(3)/k 3, R = 0.047 using 1851 independent reflections ([Fol > 3o(F)). 5. Cyclic v o l t a m m e t r y reveals t h a t E 2 - E 1 value of TTM-TTP is a b o u t two-thirds of TTM-TTF: Y. Misaki et al., Chem. Lett. (1992) 2321. 6. Crystal data: Triclinic, space group P 1, a = 17.900(8) b = 20.336(8), c =o9.79(1) A, a = 82J6(6), fl = 104.44(6), 7 = 111.10(3) , V = 3219(64) ]k3, R = 0.104 using 3351 independent reflections ( I F o l > 3o(F)).