Synthesis and properties of a series of new asymmetrical tetrathiafulvalene derivatives

ELSEVIER

Synthetic

Metals 90 ( 1997) 49-55

Synthesis and properties of a series of new asymmetrical tetrathiafulvalene derivatives Hong-Qi Li *, Zhong-Qi Yao, Dan Liu, Gan-Zu Tan, Xian-Da Yu Lnrt:hoi~

Instime

of Chemical PhsJics, Chitxse Academy of Scieuces, Larxhou Received

730000,

China

12 June 1997; accepted 26 June 1997

Abstract

A seriesof new asymmetricallysubstitutedtetrathiafulvalene(TTF) derivatives.a chargetransfer(CT) complex,andseveralrelated symmetricalTTF derivarivesweresynthesized. Thecompounds werecharacterized by elementalanalysis, ‘H NMR spectra, IR spec:ra, and electron ionization (EI) or negative-ion fast atom bombardment (FAB)- mass spectral analysis. UV-Vis spectra of the new asymmetrical TTF derivativesandthe CT complexin solutionwere investigated. In addition, mixed conducting Langmuir-Blodgett (LB) films were constructed from the asymmetric21 TTF derivatives by admixing with an equimolar arachidic acid. 0 1997 Elsevier Science S.A. Keyvords:

TeKathiafulvalene

derivatives;

Synthesis:

Langmuir-Blodgett

films

1. Introduction

physical properties of thesenew asymmetricalTTF derivatives and a CT complex.

Since the discovery that the crystalline 1:1 chargetransfer (CT) complex formed by thedonor tetrathiafulvalkle (TTF) and the acceptortetracyanoquinodimethane(TCNQ) exhibited metallic behaviour [ 11, the past two decadeshave witnessedunabatedinterest in the synthesisof TTF derivatives that display unusualsolid-stateproperties[ 2,3]. For the synthesis of novel electroactive materialsand the achievement of highly conducting Langmuir-Blodgett (LB) films, attention is now being focusedincreasingly on asymmetricalTTF derivatives [4-S] and on the incorporation of TTF molecule( s) into more complex molecular architectures[ 9, lo]. So far many conducting LB films constructedfrom asymmetrical TTF derivatives [ 1l-171 or bis-TTF derivatives [ 181 have been reported. In connection with our current interest in the synthesisof functionalized TTF derivatives [ 191andthepreparationof mixed conductingLB filmsbased on symmetrical TTF derivatives [ 20,211, we have synthesized a seriesof new asymmetricalTTF derivatives. In the present work we report the synthesis,characterization and * Corresponding 841 7088.

author. Tel.:

+ 86 931 882 7971 3003: fax:

0379.6779/97/$17.00 0 1997 Elsevier PIZSO379-6779(97)03917-9

t86

Science S.A. All rights reserved

931

2. Experimental 2.1. Reagents rind instwnents

4,5-Bis(benzylthio)-1,3-dithiole-2-thione (1) was synthesized by a procedure reported previously [22]. Other reagentswere commercially available. ~~ Melting point was determinedon a PHMK micro melting point apparatuswith the thermometeruncorrected.Elemental analysis was performed on a Carlo-Erba model 1106 elemental analyser. ‘H NMR spectrawere recorded on a FT80A-NMR instrument and the chemical shift was given in ppm relative to TMS with CDCl, as the solvent. Electron ionization massspectra(EI-MS) (70 eV) and negative-ion fast atom bombardment mass spectra (FAB-MS) were obtained on a VG 7070 spectrometer.FT-IR spectrawere recordedon a Nicolet 10DX FT-IR spectrometerin theregion of 400-4000 cm-’ as KBr pelIets. UV-Vis spectrawere obtainedon anUV-365 spectrophotometer.X-ray diffraction was determinedusing a D/MAX-RB automatic instrument.

50

H.-Q.

Li et ai. /Sytlwtic

Electrical conductivity of the LB films was measured in air with a ZC-36 lOI Q resistance instrument by the standard two-contact method using silver paint contacts. 2.2. Syilthesis 2.2.1. Preparatiorl of4,5-bis(alkylthio)-1,3-dithiole-2-

thiorle (3-d) A mixture of 1 (20.3 g, 0.05 mol), sodiummethoxide (5.4 g, 0.10 mol) and absolute methanol (250 ml) was stirred until the solid disappearedcompletely to give a dark red solution of compound 2 in methanol. To the solution was added dropwise 23.0 g (0.12 mol) iodomethanein 30 min. The mixture wasstirred for another 30 min andleft for 20 h. A yellow precipitate was obtained by filtration. Recrystallization from CHClJC,H,OH ( 1:1, vol./vol.) gave 8.9 g 4,5bis( methylthio) - 1,3-dithiole-2-thione (3) asyellow needles; yield 78.6%; m.p. 102.0-103.0 “C (lit. [23] 101 “C). Anal. Calc. for C,H,&: C, 26.53; H, 2.67. Found: C, 26.53; H, 2.60%. IR spectra( KBr pellet, v,,,~~,cm- ’ ) : 2979,291O (CH), 1059, 1050 (C=S), 965, 896 (C-S). ‘H NMR spectra ( CDC13,5, ppm) : 2.50 (s. 6 H, CH,) . EI-MS (70 eV) : nt/z 226 (M’), 150, 135, 103,91. Compounds4-6 were synthesized similarly with n-decyl bromide, I-bromohexadecaneand benzyl bromide replacing iodomethane,respectively. 4,5-Bis( decylthio) - 1,3-dithiole-2-thione (4) : yellow pellets; yield 53.3%; m.p, 40.0-41.0 “C (lit. [24] 40 “C). Atzal. Calc. for C23HIizS5:C, 57.68; H, 8.84. Found: C, 58.10; H, 8.56%. IR spectra (KBr pellet, v,,,,. cm-‘): 2954, 2917, 2847 (C-H), 1468 (C-H), 1059, 1027 (C=S), 887, 717 (C-S). ‘H NMR spectra (CDCl,, 6, ppm): 0.87 (t, 6 H, CH,), 1.25 (m, 36 H, CH,) . 4,5-Bis( hexadecylthio) -1,3-dithiole-2-thione (5) : yellow pellets; yield 78.5%; m.p. 69.0-70.0 “C. Anal. Calc. for CJ5 H&: C, 64.95; H, 10.28. Found: C, 65.31; H, 9.79%. IR spectra(KBr pellet, vnlax,cm- ‘) : 2957,2922,2852 (C-H), 1468 (C-H), 1059, 1032 (C=S), 889 (C-S). ‘H NMR spectra (CDC13, 6, ppmj: 0.86 (t, 6 H, CH,), 1.25 (m, 60 H, CM. 4,5-Bis( benzylthio) -1,3-dithiole-2-thione (6) : golden needles; yield 64.9%; m.p, 84.0-85.0 “C. Anal. Calc. for C,,H,,S,: C, 53.93; H, 3.73. Found: C, 53.54; H, 3.53%. IR spectra(KBr pellet, vmilx,cm-‘) : 3091 (Ph-H) ,2957,2922, 2852 (C-H), 1468 (C-H), 1059,1032 (C=S), 839 (C-S). ‘H NMR spectra ( CDCl,, 6, ppm) : 4.12 (s, 4H, CH,) ,7.3 1 (s, 10 H, Ph-H). EI-MS (70 eV): in/z 378 (M’), 287 (M-CH,Ph), 9 1 (CH,Ph) . 2.2.2. Jjxthesis of TTF deuivati\~es(7-13) Freshly distilled tributyl phosphite (8 ml) was addedto a flask containing compound3 (0.50 g, 2.2 mmol) and6 (0.83 g, 2.2 mmol) . The flask was evacuatedand filled with nitrogen repeatedly. Then the mixture was stirred at 90-100 “C for 7 h. The reaction mixture was allowed to cool. Tributyl phosphite was removed by evaporation under reducedpres-

Metals 90 (1997) 49-55

sure.Column chromatography (silica gel 200-300 mesh) of the residue using CHCl,--petroleum ether (b.p. 60-90 “C) (1:l m 10, vol./vol.) as eluent gave 0.50 g 2,3-bis(methylthio) -6,7-bis( benzylthio) tetrathiafulvalene(7) (and the symmetrical TTF derivatives 10 and 13 asby-products) as dark red solid; yield 42.0%; m.p. 63.5-64.5 “C. Anal. Calc. for &H2”S8: C, 48.85; H, 3.73. Found: C, 48.79; H, 3.97%. UV-Vis spectra (CHCI,, Amax,nm) : 237, 257,3 13,335. IR spectra(KBr pellet, Y,,,~~,cm- ‘) : 3060,3027 ( Ph-H)) 2956, 2918 (C-H), 1600, 1493, 1453 {C-H), 1424 (C=C), 888, 769 (C-S). ‘H NMR spectra (CDCI,, 6, ppm): 2.50 (s, 6 H, CHj), 3.90 (s, 4 H, CH2), 7.25 (s, 10 H, Ph-H) EI-MS (70eV):m/z540(M+),496(M-CH,S),449(M-CH,Ph), 416 (M-PhCH,SH), 91 (CH,Ph). Compounds8 and 9 were synthesizedby a similar procedure, with the symmetrical TTF derivatives 11 and 13, 12 and 13 asby-products, respectively. 2,3-Bis (decylthio j -6,7-bis(benzylthio) tetrathiafulvalene (8) : dark red crystal; yield 3 1.1%; m.p. 57.0-59.0 “C. AtzaE. Calc. for CG0H5&: C, 60.55; H, 7.11. Found: C, 60.38; H, 7.38%. UV-Vis spectra (CHCl,, A,,,,,, nm) : 202, 244, 336, 368. IR spectra ( KBr pellet, u,,,, cm-‘) : 3057, 3028 (PhH), 2918 (C-H), 1493, 1467, 1452 (C-H), 1415 (C=C), 888,768 (C-S). ‘H NMR spectra(CDCl,, 6, ppm): 0.87 (t, 6 H, CH,), 1.22 (m, 36 H, CH,), 3.88 (s, 4 H, PhCH,) ,7.25 (s, 10 H, Ph-H). FAB-MS: m/z 792 (M+), 668 (MPhCH,SH) ,9 1 (CH?Ph) . 2,3-Bis (hexadecylthio) -6,7-bis( benzylthio) tetrathiafulvalene (9) : dark red hair-shapedcrystal; yield 32.8%; m.p. 80.0-82.0 “C. Anal. Calc. for C52H80S8: C, 64.94; H, 8.38. Found: C, 64.73; H, 8.73%. UV-Vis spectra (CHCl,, A,,,,,, nm): 201, 241, 310. IR spectra (KBr pellet, v,,,, cm-‘): 3056,3027 (Ph-H), 2917 (C-H), 1494,147O(C-H), 1415 (C=C), 887,768 (C-S). ‘H NMR spectra( CDCl,, 6, ppm) : 0.88 (t, 6 H, CH,), 1.25 (m, 60 H, CH,), 3.88 (s, 4 H, PhCH2), 7.25 (s, 10 H, Ph-H) . FAB-MS: m/z 960 (M+ ), 883 (M-Ph),778 (M-2PhCH,),614 (M-C&CH,Ph),510 (M-2&H&, 154 (Ph-Ph). Tetrakis( methylthio) tetrathiafulvalene (10): yellow needles;m.p. 95.0-97.0 “C (lit. 125194.5-96.0 “C) .Anal. Calc. for C,,H,,S,: C, 30.90; H, 3.11. Found: C, 31.11; H, 3.19%. IR spectra ( KBr pellet. I’,,,,, cm-‘): 2987, 2918 (C-H), 1530 (C-H), 966,883 (C-S). ‘H NMR spectra(CDQ, 6, ppm):2.59(s, 12H,CH,j.E1-MS (70eV):m/z398 (M’), 373 (M-CH3), 341 (M-SCH,), 238, 118,91. Tetrakis( decylthio) tetrathiafulvalene (11) : orange needles; m.p. 57.0-58.0 “C (lit. [26] 57-59 or 58-60 “C). Anal. Calc. for C,,H,&: C, 61.82; H, 9.47. Found: C, 61.79; H, 9.83%. IR spectra (KBr pellet, ZI,,,, cm-‘): 2955, 2919, 2850 (C-H), 1468(C-H), 995,890 (C-S). ‘H NMR spectra (CDC13,6, ppm) : 0.87 (t, 12H, CH,), 1,25 (m, 72 H, CH,). FAB-MS: m/z 892 (M’), 752 (M-C3S4C1,,H2,). Tetrakis( hexadecylthio) tetrathiafulvalene ( 12) : yellow crystal; m.p. 85.0-86.0 “C (lit. 1221 about 84 “C). Anal. Calc. for C,0H132S8: C, 68.34; H, 10.81.Found: C, 68.44; H, 10.89%. IR spectra (KBr pellet, v,,,,, cm-‘): 2955, 2918

H.-Q.

Li er al. /Synthetic

Merds

2.3. LBjlrn

(C-H), 1469 (C-H), 887, 718 (C-S). ‘H NMR spectra (CDCl,, 6, ppm): 0.86 (t, 12 H, CH,), 1.25 (m, 120 H, CM. Tetrakis( benzylthio) tetrathiafulvalene (13) : ruby rodshaped crystal; m.p. 171.0-172.0 “C. Anal. Calc. for C34H28S8: C, 58.92; H, 4.07. Found: C, 58.51; H, 4.18%. IR spectra ( KBr pellet, v,,,,, cm - ‘) : 3059,3024 (Ph-H), 2962, 2926 (C-H), 1599, 1452 (C-H), 893,767 (C-S). ‘H Nn/rR spectra (CDCl,, 6, ppm): 3.94 (s, 8 H, CH,), 7.28 (s, 20 H, Ph-H). FAB-MS: nz/z 694 (M + ), 91 ( PhCHJ .

51

49-5.5

deposition

Monolayer formation was achieved by spreading a solution of 7-9 (1.0 mmol/l) and arachidic acid (1.0 mmol/l) in CHCl, on the surface of double-distilled water (pH = 4.8 i 0.1). Surface pressure-area isotherms were measured at 19+1 “C and a compression speed of 30 mm/mm. The surfaces of the substrates were successively cleansed with CHCl,, C2HjOH, CH,COCH,, and ultrasonic wave. Monolayers were transferred onto glass substrates with or without two aluminium electrodes at a dipping pressure of 25 mN/m for measurement of electrical conductivity and X-ray diffraction, respectively. The speed for up and down strokes was 3 mm/min. The transfer ratio was about 0.9 for both up and down strokes. Y-type deposition was achieved. The doping of the LB films was carried out by exposure to iodine vapour for several minutes in a sealed vessel.

2.2.3. Preparation of the CT complex tetrakis(hexadecylthio)tetrathiaful~~aler~e-TCNQ (14) A solution of compound 9 (48.0 mg, 0.05 mmol) and TCNQ (10.2 mg, 0.05 mmol) in 10 ml of CHCl,/CH,CN (l:I, vol./vol.) was heated to reflux for 5 h. The reaction mixture was allowed to cool and left at room temperature for 24 h. A yellow crystal was obtained by filtration of the solution. Subsequent washing of the precipitate with CH,CN and CH& successively gave 54.2 mg product 14 as yellow needles; yield 93.1%; m.p. 83.0-84.0 “C. Anal. Calc. for C64Ha,N,SS: C, 65.93; H, 7.26; N, 4.81. Found: C, 65.41; H, 7.73; N, 5.02%. UV-Vis spectra (CHCI,, h maxInm) : 255, 337, 378, 404. IR spectra ( KBr pellet, vmax, cm-‘): 3056, 3027 (Ph-H), 2918 (C-H), 1495, 1470 (C-H), 1414 (C=C), 887, 769 (C-S). ‘H NMR spectra (CDCl,, 6, ppm): 0.88 (t, 6 H, CH,), 1.25 (m, 60 H, CH,), 3.88 (s, 4 H, PhCH,), 7.25 (s, 14 H, Ph-H).

3. Results and discussion 3.1. Synthesis of the asym~netrical7TF The synthetic routes of the Scheme 1. The asymmetrical pared by the cross-coupling ‘half-units’ in the presence of

2

1

90 (1997)

3 R=CH, 4 R = G+$I,~, 5 R q G+$),,~, 6 R= G-i,Ph

7 R=CH,

lOR=C+

13

8 R = (G-i@-i3

11 R = (C+$),cH,

9 R = (CH&CH,

1 2 R = (C+$),,~3

14

Scheme 1.

derivatives

title compounds are shown in TTF derivatives 7-9 are preof two different 1,3-dithiole tributyl phosphite in 30-40%

52

H.-Q.

Li et nl. /Sythrtic

Met&

yields. Though the cross-coupling reaction produces a mixture of an asymmetrical TTF derivative and two symmetrical TTF compounds and, thus, the products need to be separated by column chromatography which is complicated and timeconsuming, it is straightforward and gives high yields compared with other routes to such compounds [27-291, including the methods developed recently such as Me,AIpromoted reactions of organotin compounds with esters [ 81 and Horner-Wadsworth-Emmons methodology [ 61. So the cross-coupling of two different 1,3-dithiole molecules is still the most general method for synthesis of asymmetrical TTF derivatives. If the separation procedures of the products are improved, for example, to purify the products by recrystallization instead of column chromatography, the cross-coupling method would be more facile and useful for the synthesis of a vast variety of compounds including asymmetrical TTF derivatives, functionalized and oligomeric TTF systems. We have found on the basis of preliminary studies that the mixed products obtained from the cross-coupling of two specific 1,3-dithiole ‘half-units’ can be separated by multi-step crystallization instead of column chromatography. Further studies on this aspect are in progress.

90 (1997)

49-55

400 nm. They are assigned to the IT + T* transitions of the TTF ring. Upon UV (365 nm) irradiation, a unique phenomenon was observed in the spectra. Ten minutes after irradiation, the intensities of the absorption bands decreased sharply and some of the bands even disappeared completely. However, no new absorption band was visible until the detection wavelength of 700 nm. With increasing irradiation time, the intensities of the absorption bands decreased further. These changes have never been observed for TTF derivatives. This phenomenon is so complicated and even incredible that we can hardly put forward a reasonable explanation. Obviously, the phenomenon results from neither a cis-rmns isomerization nor a molecular aggregation or polymerization, for in both cases at least a new absorption band should appear. Perhaps the only plausible explanation of the uniquephenomenon is to attribute it to the gradual photodegradation of the TTF derivatives, in which rhe resulting small molecules do not show UV-Vis spectral absorption in the range of 190700 nm. We hope to be able to use these TTF derivatives as new optical materials after further investigation. 3.3. LB films Pure compounds 7-9 are non-amphiphilic and so can only form a stable monolayer at the air/water interface by admixing an equimolar arachidic acid (AA). The surface pressurearea isotherms for the mixtures of compounds 7-9 with AA ( 1: 1, molar ratio) are shown in Fig. 2. Collapse of the monolayers was not observed up to a surface pressure of 40 mN/ m. The compressed monolayers were all readily transferred onto solid substrates at a surface pressure of 25 mN/m to give Y-type layers. For all of the pristine LB films based on 7-9/AA (l:l, molar ratio), X-ray diffraction patterns exhibit several evi-

TTF derivatives such as metal-TTF hybrids have recently been targeted as potential new materials [ 30-33 ] for magnetic materials, molecular electronics, molecular optical switches, and other telecommunication devices. To investigate the optical properties of the asymmetrical TTF derivatives, we have determined the UV-Vis spectra of compounds 7-9 and the CT complex 14 in CHCI, solution before and after UV (365 nm) irradiation, as shown in Fig. 1. All of the four compounds give three to four absorption bands at 2007

Wavelength

Wavelength (nm)

(nm)

9

300 Wavelength Fig. 1. UV-Vis irradiation.

spectra of 7-9 and 14 in CHC13:

500 (MI)

700

(a) before UV irradiation,

(b)

Wavelength (nm) 10 min after UV (365 nm) irradiation,

and (c) 20 tin

after UV (365 nm)

H.-Q.

Li ei 01. / S~nrkric

Molecular area / nm’ Fig. 2. Surface pressure-area isotherms for the I:1 molar mixture (a),S/AA(b)andg/AA(c) inwatersubpha,e(pH=4.8iO.l)at 1 “C and a compression speed of 30 mmimin.

Mern1.c

90 (1997)

5

of 7/AA 19k

2 e(degr=) Fig. 5. X-ray

LB film 7/AA

(1:l)

Mean value S/AA (1:l)

5 Fig. 3. X-ray

diffraction

,

,

I

*

10 2 0 (degree) pafEriEif

7IAA

I

A15 ( 1:i)

LB film. Mean value g/AA (1:l)

Mean value

2 f3(degree) Fig. 4. X-ray

diffraction

patterns of S/AA

20 diffraction

Table 1 Data and results from X-ray

-I

I

53

49-55

( 1: 1) LB lilm.

dent (001) reflection lines (Figs. 3-5), demonstrating that the LB films have an ordered lamellar structure. According to the Bragg equation the thicknessper layer of the LB film (d) can be calculated, which is one-half of the identity period of the film (D) becauseof the Y-type of the LB film. The data aswell asthe resultsare listed in Table 1. The thickness per layer of 7/AA LB film (2.41 nm) is significantly larger than the molecular length of 7 ( 1.22 nm), but smallerthan that of AA (2.67 nm), which is estimated from the CPK model. This meansthat compound 7 is imbedded into AA moleculesand leadsto a tilted arrangementof AA molecules. For S/AA LB film, the thickness per layer (2.25 nm) is exactly the mean value of the molecular lengths of 8 (1.78

patterns of g/AA

diffraction

patterns

( 1: 1) LB film.

of 7-g/AA

hki

28(0)

D (nm)

d (nm)

002 003 004 005 006 007

3.62 5.88 7.31 9.15 10.70 12.55

001 002 003 004 005 006 007

1.96 3.92 5.86 8.58 10.00 10.60 14.01

003 005 007 009

5.16 8.48 11.84 15.08

4.88 4.5 I 4.80 1.83 4.96 4.91 4.82 4.50 4.50 4.52 4.12 1.42 5.00 4.11 1.50 5.13 5.2 1 5.40 5.29 5.26

2.44 2.26 2.40 2.12 2.48 2.47 2.41 2.25 2.25 2.26 2.06 2.2 1 2.50 2.2 1 2.25 2.57

2.61 2.70 2.65 2.63

nm) andAA (2.67 nm), indicating amore tilted arrangement of_AA moleculesand, thus, lessclosecontactsof the LB film. However, the thicknessper layer of 9/AA LB film (2.63 nm) is much larger than the molecular length of 9 (2.16 nm) but similar to that of AA (2.67 nm). We suggestthat the C16Hjj long alkyl chains stand upright on the water’s surface and form suchclosecontacts with AA that the thicknessper layer of the film approachesthe molecular length of AA. It is in agreement with the results obtained from the surface pressure-areaisothermof 9/AA mixture. The room-temperature conductivity values of 7-9/AA ( 1:1) LB films asa function of the number of layers before and after iodine doping arelisted in Table 2. The conductivity of 7/AA LB film decreaseswith increasing layer numbers. The highest conductivity values are 2.5X 10-j and 2.5 X lo-” S/cm before andafter iodine doping,respectively. The dependenceof the conductivity on the number of layers might suggestthat carriers could traverse only a few layers and conductive pathscould only be formed in a few layers of

54

H.-Q. Li et al. /Spthetic

Table 2 Electrical conductivity of 7-9/AA ( 1: 1) LB films vs. the number of layers at room temperature before and after iodine doping LB film

Number layers

7/AA(l:l)

1 5 11 15 21 25 5 23 1 5 11 21

S/AA(l:lj 9/AA(l:l)

of

Conductivity before iodine doping (S/cm)

Conductivity after iodine doping (S/cm)

2.5~ 10-j 8.5x10-” 9.0x 1w7 2.2x1o-7 %5x10-’ 1.7x lo-’ 2.6X 1o-s 2.9x10-* 1.3x1o-5 1.3 x 10-S 2.0x 1o-3 1.6X io-’

2.5 x lo-” 1,3x IO-” 5.0x 1o-4 4.1 x lo-” 2.3 x 10-j 1.4x lo+ 2.5X IO-’ 2.7X 10-h l.-IX lo-” 1.8X lo-’ 2.3X10-l 1.1x 10-l

the LB film, so the conductivity decreases with increasing layer numbers. The measurement on the conductivity of 5- and 23-layered S/AA LB film shows that the conductivity is hardly affected by the number of layers. The conductivity values are very low: 2.6-2.9X low8 S/cm before iodine doping and 2.52.7 X 10e6 S/cm even after iodine doping. We think that the loose arrangement of compound 8 molecules in the LB film might account for the low conductivity of the LB film. For 9/AA ( 1: 1) LB film the conductivity increases with increasing layer numbers and reaches the maxima 2.0 X lo-’ and 2.3 X 10-l S/cm before and after iodine doping, respectively, when the number of layers is 1I, and then decreases slightly. The results indicate that the conductivity of 9/AA ( 1: 1) LB film depends on the number of layers, similar to that of EDT-TTF( SC,,),/behenic acid LB film [ 341 and the results reported by Bourgoin et al. [35]. This might suggest that carriers could traverse several layers and conductive paths could be formed in a number of layers. Thus, the conductivity increases with increasing layer numbers until the number of layers reaches 11. For more layers, however, the quality of the layers deposited decreases slightly and leads to the slight decrease of the conductivity of 2 1-layered LB film compared with that of the 1l-layered one.

4. Conclusions Three new asymmetrical TTF derivatives, a CT complex and several related symmetrical lTF derivatives were synthesized by cross-coupling of two different 1,3-dithiole ‘halfunits’ in relatively high yields. These compounds were characterized by elemental analysis, ‘H NMR spectra, IR spectra, and EI-MS or FAB-MS analysis. UV-Vis spectra of the new asymmetrical TTF derivatives and the CT complex in CHC13 were studied, in which a unique phenomenon was observed that the intensities of the absorption bands decreased after UV (365 nm) irradiation. Surface pressure-

Metals

90 (1997)

49-55

area isotherm measurement showed that the new TTF derivatives could form a stable monolayer at the air/ water interface by admixing with an equimolar arachidic acid. The transfer ratio of the monolayers was unity, and Y-type deposition was achieved. Results from X-ray diffraction patterns of the pristine LB films demonstrated that the LB films had an ordered lamellar structure. The conductivity of the mixed LB films was studied as a function of the number of layers.

Acknowledgements The authors are grateful to the Laboratory of Organic Solids, Chinese Academy of Sciences, for partial financial support.

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