arachidic acid mixture

SYflTHETIIC Ilfl|TRLS ELSEVIER

Synthetic Metals 92 (1998) 265-268

Study on a novel mixed Langmuir-Blodgett film based on a new asymmetrical tetrathiafulvalene/arachidic acid mixture Hongqi Li *, Zhongqi Yao, Dan Liu, Ganzu Tan, Xianda Yu Lanzhou Institute of Chemical Physics, ChineseAcademy of Sciences, Lanzhou 730000, China

Received 28 April 1997; accepted 11 July 1997

Abstract A mixed semiconducting Langmuir-Blodgett (LB) film based on a new asymmetrical tetrathiafulvalene (TTF) derivative and arachidic acid is described. The LB film of the 1:1 (molar ratio) mixture of the TTF derivative dibenzylthio-dimethylthio-tetrathiafulvalene with arachidic acid was constructed and characterized by FT-IR and UV spectra, and X-ray diffraction. Measurement on conductivity of the mixed LB film relating to the number of layers showed that the conductivity of the film decreased with increasing layer number. After iodine oxidation the conductivity of the LB film increased by 2-3 orders of magnitude compared with that before iodine doping and reached a maximum of 2.5 × 10 3 S cm- 1 due to the formation of a new conducting phase, as demonstrated by PT-IR and UV spectral analyses. Results from X-ray diffraction indicate that the as-deposited LB film has an ordered lamellar structure with a thickness per layer of 2.41 nm. © 1998 Elsevier Science S.A. Keywords: Tetrathiafulvalene;Langmuir-Blodgettfilm; Conductivity

1. Introduction

In recent years, the interest in electroactive organic thin films prepared by the Langmuir-Blodgett (LB) technique has considerably increased [ 1 ]. Electrically conducting LB films are mainly based on charge transfer (CT) complexes containing the acceptor compound 7,7',8,8'-tetracyanoquinodimethane ( T C N Q ) [ 2 - 4 ] . However, it has been demonstrated that donor molecules [5,6], especially tetrathiafulvalene (TTF) derivatives [ 7 - 1 8 ] , are also suited. Though most of the LB films were constructed from amphiphilic molecules with long alkyl chains, recent studies on LB films based on non-amphiphilic TTF derivatives by Petty and co-workers [ 19,20] and ourselves [21,22] demonstrate that the attachment of long alkyl chains is not a prerequisite for the formation of conducting LB films of TTF-based CT complexes. In the present work we report the preparation, structural characterization, and electrical properties of a novel mixed LB film based on the mixture of a new asymmetrical TTF derivative, dibenzylthio-dimethylthio-tetrathiafulvalene and arachidic acid. * Corresponding author. Tel.: + 86 931 882 7971-3003; fax: + 86 931 841 7088. 0379-6779/98/$19.00 © 1998Elsevier Science S.A. All rights reserved P11S0379-6779( 97 )04106-4

2. Experimental

2.1. Synthesis

The title compound dibenzylthio-dimethylthio-tetrathiafulvalene (compound 1, the chemical structure of which is shown in Fig. 1 ) was synthesized by tributyl phosphitemediated cross-coupling reaction of 4,5-bis(benzylthio)1,3-dithiole-2-thione with 4,5-bis (methylthio)- 1,3-dithiole2-thione at 90-100 °C in 41.6% yield [23]. Compound 1: dark red solid. ~H NMR t5 (CDCI3): 2.50 ( s, 6H, CH3), 3.90 (s, 4H, CH2), 7.25 (s, 10H, P h - H ) ppm. IR (KBr pellet, v, c m - ~ ) : 3060, 3027 ( P h - H ) , 2956, 2918 ( C - H ) , 1600, 1493, 1453 ( C - H ) , 1424 ( C = C ) , 1069, 1028 ( C = S ) , 888, 769 ( C - S ) . M/Z: 540 (M ÷ ), 496 (M-CH2S), 449 (M-CH2Ph), 416 (M-PhCH2SH), 91 (CH2Ph). Anal. Calc. for C22H2oS8: C, 48.85; H, 3.73. Found: C, 48.79; H, 3.97%.

C6tBCI-12S""

"S/---~

/

"S CI-I3

1 Fig. 1. Molecular structure of compound 1.

266

H. Li et al. ~Synthetic Metals 92 (1998) 265-268

2.2. Film deposition The measurement of surface pressure-area (Tr-A isotherms was carried out in a KSV 5000 twin-compartment Langmuir trough. The subphase was dilute NaOH solution in deionized water. The pH value of the subphase was controlled by the concentration of the NaOH solution. The temperature was 18-20 °C and the compression speed was 30 mm rain when the 7r-A isotherms were measured. The surfaces of the substrates were successively cleansed with CHC13, C2HsOH, CH3COCH3 and ultrasonic waves. Monolayers from the l: 1 (molar ratio) mixture of compound 1 and arachidic acid ( A A ) were transferred onto CaF2, quartz and glass substrate at a surface pressure of 25 mN m ~for measurement of FTIR and UV spectra, and X-ray diffraction, respectively. The speed for up and down strokes was 3 mm rain ~. The transfer ratio was about 0.9 for both up and down strokes. Y-type deposition was achieved. FT-IR spectra were recorded on a Nicolet 10DX FT-IR spectrometer. UV spectra were recorded on a UV-365 spectrometer. X-ray diffraction was determined using a D / M A X RB automatic instrument. Electrical conductivity was measured using a ZC-36 1017 ~ resistance instrument. For LB film oxidation, the neutral LB film was put into a vessel containing iodine and left in it for 5-30 min at about 50 °C.

3. Results and discussion

3.1. Surface pressure-area isotherms of compound 1/AA mixture Pure compound 1 is non-amphiphilic and cannot form a stable monolayer at the air/water interface so a solution of compound 1 ( 1.0 × l 0 - 3 tool l - ~) and AA ( 1.0 × l 0 - 3 tool 1- ~) in CHC13 was used as the spreading solution. The 7r-A isotherms of compound 1 with equimolar ratio of AA at different pH values (4,7, 6.7, or 8.5) of the subphase were recorded as shown in Fig. 2 ( a ) - ( c ) , respectively. The col-

lapse pressures are 41-42 mN m ~, whereas the mean areas per molecule are about 28, 24 and 32 ~2 for curves ( a ) - ( c ) , respectively. This shows that a stable monolayer is formed at the air/water interface by admixing AA. Though the collapse pressure is almost constant at different pH values of the subphase, the mean molecular occupied area changes significantly with the change of pH value of the subphase. The smallest occupied area per molecule of 24 ,~2 at pH = 6.7 indicates that the neutral condition of the subphase is more favorable to the film formation of the mixture than the acidic ( p H = 4 . 7 ) or basic ( p H = 8 . 5 ) condition. This is because that AA tends to form CH3(CHz)~sCOOH + or C H 3(CH2) jsCOO in an acidic or basic condition and leads to the increase of intermolecular Coulomb repulsion and a loose arrangement of the molecules.

3.2. UV spectra The electronic absorption spectra of compound 1 / A A ( 1:1, molar ratio) LB film before and after iodine oxidation are shown in Fig. 3. The as-deposited LB film exhibits three absorption bands at 310, 340 and 460 nm, respectively. These bands are attributed to the w --* w* transitions of the TTF ring. The UV spectrum of the film changes markedly upon iodine doping. Immediately after exposure to iodine vapor in a sealed container for 10 min, strong absorption bands at 225, 300 and 420 nm were observed. The last band is ascribed to the red shift of the peak at 340 nm due to the interactions between compound 1 and iodine. The other two are assigned to intramolecular transitions of the conjugated T-system of 1 "+ radical cation [9]. After oxidation (10-15 rain), no marked change was observed, but for a blue shift of the band at 420 nm to 390 or 380 nm. However, 120 h after doping the spectrum of the film is significantly different: the band at 225 nm has disappeared and the bands at 300, 370, 460 and 580 nm are visible. The new absorption at 580 nm is assigned to the CT band.

50 ~, 4C

L

b

a ....

30 o~ @ o

20

m 10

0

10

20

30

40

50

Area per molecule ( A 2 ) Fig. 2. rr-A isotherms of compound 1/AA ( 1:1 ) at 18-20 °C: (a) pH = 4.7; (b) pH=6.7; (c) pH=8.5.

260

360

460

560

600

y00

soo

,00

Wavelength (rim) Fig. 3. UV spectra of compound l/AA ( 1:1 ) LB film: (a) before I 2 oxidation; (b) immediatelyafter I2 oxidation; (c) 10 rain after lz oxidation; (d) 15 rain after 12oxidation; (e) 120 h after I2 oxidation.

H. Li et al. / Synthetic Metals 92 (1998) 265-268

267

3.3. FT-IR spectra Complementary changes were observed in FT-IR spectra of compound 1/AA ( 1:1, molar ratio) LB film upon iodine doping (Fig. 4). Before iodine doping, the LB film shows not only the usual absorption peaks belonging to the methylene groups (2916, 2849 and 1491, 1452 cm-* symmetrical and asymmetrical stretching and scissoring lines, together with the wagging series: 1243 and 1172 c m - *) and the carbonyl groups in AA molecules ( 1738 and 1666 c m - *stretching lines), but also the band at 1540 cm-1 which is assigned to the C = C stretching vibration of the TTF ring. When recorded 30 min after iodine oxidation, the band at 1540 cm * disappears and two new absorption bands at 1333 and 1311 cm-~, respectively, are visible. According to the evidence from the UV spectra the disappearance of the band at 1540 c m - ~ indicates that the TTF skeleton is oxidized to the corresponding radical cation 1 + upon iodine doping [21 ]. Following Meneghetti et al. [24] and Vandevyver et al. [12] we assigned the two new bands at 1333 and 1311 cm 1 to

o

[...

3000

2200

1800

1400

1000

800

600

Wavenuraber ( cm q ) Fig. 4. FT-IR spectra of compound 1 / A A (1:1) LB film: (a) before Iz doping; (b) 30 rain after I2 doping.

the central C = C stretching vibrational mode of the conjugated nuclei activated in the dimeric form ( 1 + ) 2 . More precisely this corresponds to the out-of-phase combination of the individual stretching mode of each molecular constituent of the dimer. 3.4. X-ray diffraction For the pristine LB film of compound I / A A ( 1:1, molar ratio), X-ray diffraction patterns exhibit six (00l) reflection lines (Fig. 5), which demonstrates that the LB film has an ordered lamellar structure. According to Bragg's equation: 2 door sin 0o~l = A D = 1 d0ol = A 1/2 sin 0ool where A is the wavelength of the incident wave (Cu K~ ray, 0.154 18 nm), 1 denotes the diffraction number, 0ool the incident angle and D the identity period of the film, the thickness per layer (d) of the LB film can be calculated, which is

|

3

5

|

7

a

9

t

a

11 13 15

2 0 (degree)

Fig. 5. X-raydiffractionpattern of compound 1/AA ( 1:t ) LB film. Table 1 Data and results from X-ray diffraction of compound 1 / AA ( 1: 1 ) LB film hlk

20 (°)

D (nm)

d (nm)

002 003 004 005 006 007 Mean value

3.63 5.88 7.37 9.15 10.70 12.55

4.88 4.5 l 4.80 4.83 4.96 4.94 4.82

2.44 2.26 2.40 2.42 2.48 2.47 2.41

one-half of D because of the Y-type of the film. The data as well as the results are listed in Table 1. The thickness per layer (d) of the LB film is 2.41 nm calculated from Bragg's equation. This value is larger than the molecular length of compound 1 ( 1.22 nm) but smaller than that of AA (2.67 nm), which were estimated from the Corey-Pauling-Koltun (CPK) model. This means that compound 1 is embedded into AA molecules which leads to a tilted arrangement of AA molecules. 3.5. Electrical conductivity of the LB film Two AI electrodes were deposited on the hydrophilic substrate by the vacuum deposition technique and then each monolayer was deposited on the substrate. Voltage was applied to measure the resistance of the LB film and, from the two electrodes, the electrical conductivity was calculated according to the following equation [ 25 ] : o" = d/RA where o- represents conductivity, R the resistance, d the distance between two electrodes and A the area through which the electric current passes. The conductivity of the LB film as a function of the number of layers before and after iodine doping is summarized in Table 2. From Table 2, it is seen that the conductivity decreases with increasing layer number, the highest conductivity being 2.5 × 10 3 S c m - ~after iodine doping. The results indicate that the conductivity of the LB film depends on the layer number of the LB film, similar to

268

H. Li et al. ~Synthetic Metals 92 (1998) 265-268

Table 2 Electrical conductivity of the LB film vs. layer number without and with iodine doping Layer no.

cr without I_, doping (S c m - ' )

cr with I2 doping (Scm ')

1 5 11 15 21 25

2.5×10 5 8.5)<10 ~ 9.0×10 7

2.5×10 1.3×10 5.0)<10 4.1 × 10 2.3)<10 1.4)<10

2 . 2 × 10 - 7

4.5)<10 .7 1.7X 10 -7

3 3 4 4 4 4

that of EDT-TTF(SC~8) 2/behenic acid LB film [ 11 ] and the result reported by Bourgoin et al. [26]. It suggests that carriers can only traverse a few layers and conductive paths can only be formed in a few layers of the LB film [27], thus the conductivity decreases with increasing layer number. After iodine doping the conductivity increases by 2-3 orders of magnitude compared with that before iodine doping, perhaps due to the interactions between compound 1 and iodine, which leads to the formation of a new conducting phase [ 22], consistent with the FT-IR and UV spectral analyses.

4. Conclusions A new asymmetrically substituted TTF derivative dibenzylthio-dimethylthio-tetrathiafulvalene (compound 1) was synthesized. The mixture of compound 1 with AA ( I: 1, molar ratio) could form a stable monolayer at the air/water interface. Absorption in the UV spectra of compound 1 / A A ( 1 : 1, molar ratio) LB film showed red shift immediately after iodine oxidation and then blue shift together the appearance of a new absorption at 580 nm which was assigned to the CT band. The above UV spectral analysis was supported by the complementary changes observed in the FT-IR spectra of compound 1 / A A ( l:l ) LB film, in which the band at 1540 cm Jdisappeared and two new absorption bands at 1333 and 13 l 1 c m - 1 were observed after iodine doping, indicating that the neutral TTF derivative was oxidized to the corresponding radical cation 1 "÷ (to give (1 "÷)2) upon iodine doping. Results from X-ray diffraction patterns of compound 1 / A A ( 1:t ) LB film demonstrated that the as-deposited LB film had an ordered lamellar structure with a thickness per layer of 2.41 nm. The conductivity of the mixed LB film depended on the number of layers and reached a maximum of 2.5 × 10 -3 S c m - ~ after iodine doping.

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