r,ω-Bis(thioacetyl)oligophenylenevinylene Chromophores from Thioanisol Precursors Dwight S. Seferos,† David A. Banach,‡ Norma A. Alcantar,§,⊥ Jacob N. Israelachvili,§ and Guillermo C. Bazan*,†,‡ Departments of Chemistry and Biochemistry, Materials, and Chemical Engineering, Institute for Polymers and Organic Solids, University of California, Santa Barbara, California 93106
[email protected] Received November 11, 2003
The selective cleavage of arylmethyl thioethers provides a convenient protocol for the synthesis of all-E isomers of R,ω-bis(thioacetyl)oligophenyenevinylene molecules (OPVs). The S-methyl group is tolerant of Wittig-type and Heck-type reactions for forming OPV structures and can be converted to the S-acetyl group by treatment with sodium thiomethoxide and acetyl chloride. The thermal conditions of the deprotection/reprotection step concurrently isomerize the conjugated chromophore to the all-E isomer, regardless of the stereochemistry of the starting olefins. This approach is demonstrated for a variety of linear and [2.2]paracyclophane containing OPVs, which have been characterized by electrochemical and spectroscopic techniques. Additionally, these S-acetylterminated OPVs self-assemble on gold surfaces. Monolayers containing these molecules were characterized by water contact angle measurements, ellipsometry, and X-ray photoelectron spectroscopy. Introduction Self-assembled monolayers (SAMs) formed by organic thiols on gold surfaces are an area of intense scientific investigation.1,2 SAMs of alkane-thiolates on gold form spontaneously from solution and are of particular interest because they are ordered, densely packed, and have dimensions which are consistent with a single molecular layer. SAMs of aromatic-thiolates have also been investigated.3 Experimental evidence suggests that they are more stable toward oxidation4 but form less quickly5 and with less order6 when compared to their aliphatic counterparts. Also, electronic conjugation between the adsorbed thiolate and aromatic substituents has been shown to influence the binding to the surface, and the molecular dipole of the resultant adsorbed species.7 For example, adsorption to the surface is slowed when a †
Department of Chemistry and Biochemistry. Department of Materials. Department of Chemical Engineering. ⊥ Current address: Department of Chemical Engineering, University of South Florida, Tampa, FL 33620. (1) Selected reviews on self-assembled monolayers: (a) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press Inc: New York, 1991. (b) Dubois, L. H.; Nuzzo, R. G. Annu Rev. Phys. Chem. 1992, 43, 437. (c) Ulman, A. Chem. Rev. 1996, 96, 1533. (d) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (3) (a) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (b) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792. (4) Kolega, R. R.; Shlenoff, J. B. Langmuir 1998, 14, 5469. (5) Garg, N.; Friedman, J. M.; Lee, T. R. Langmuir 2000, 16, 4266. (6) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018. (7) Ulman, A. Acc. Chem. Res. 2001, 34, 855. ‡ §
conjugated thiol is functionalized by an electron-withdrawing group.8 Thiol (and dithiol)-functionalized molecules that contain a molecular fragment with a conjugated electronic structure are attractive because they allow for the attachment of well-defined organic semiconducting materials onto a metallic surface and are often referred to as molecular wires. Additionally, these rigid, functional molecules may be used as a template to assemble higher order structures. Previously, these concepts have been demonstrated by the fabrication and electronic characterization of metal-molecule junctions9 and through the formation of surface-nanoparticle10 or nanoparticlenanoparticle11 assemblies. Molecular structures that have been used in this respect are typically thiol- or thioacetyl(8) Liao, S.; Shnidman, Y.; Ulman, A. J. Am. Chem. Soc. 2000, 122, 3688. (9) (a) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (b) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (c) Cui, X. D.; Primak, A.; Zarate, X.; Torfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsey, S. M. Science 2001, 294, 571. (d) Ramachandran, G. K.; Hopson, T. J.; Rawlett, A. M.; Nagahara, L. A.; Primak, A.; Lindsey, S. A. Science 2003, 300, 1413. (e) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549. (f) Kushmerick, J. G.; Holt, D. B.; Yang, J. C.; Naciri, J.; Moore, M. H.; Shashidhar, R. Phys. Rev. Lett. 2002, 89, 086802-1. (g) Holmlin, R. E.; Hagg, R.; Chabinyc, M. L.; Imagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (10) (a) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (b) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (11) (a) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (b) Novak, J. P.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 3979. (c) Javier, A.; Yun, C. S.; Sorena, J.; Strouse, G. F. J. Phys. Chem. B 2003, 107, 435. (d) Ouyang, M.; Awschalom, D. D. Science 2003, 301, 1074. 10.1021/jo035664g CCC: $27.50 © 2004 American Chemical Society
1110
J. Org. Chem. 2004, 69, 1110-1119
Published on Web 01/20/2004
R,ω-Bis(thioacetyl)oligophenylenevinylene Chromophores SCHEME 1.
Dithioacetyl OPVs 1-7
terminated oligomers of polyphenylene12 (OP) and polyphenylenethynylene13 (OPE) because their synthesis and self-assembly have been well established. Thioacetyl groups are often the preferred terminal functionality for these structures because they behave much like thiols but are not as susceptible to oxidation. When exposed to gold surfaces thioacetyl groups are cleaved in situ to form gold thiolate monolayers identical with those formed from organic thiols.14 In this contribution we report a general and straightforward synthetic route to a variety of thioacetylterminated oligo(phenylenevinylene)s (OPVs). The OPV backbone is a superior semiconducting organic structure, relative to OP and OPE, because of its high degree of planarity and highly tunable optical properties.15 Few synthetic protocols that lead to thiol- or thioacetylterminated OPVs have been reported. Chidsey et al.16 reported a synthetic route to benzylthiol-terminated OPVs. However, the methylene spacer in the benzyl system introduces a point of saturation in an otherwise fully conjugated organic thiol system; this disrupts electronic delocalization from the OPV π-system to the thiol, and ultimately to the surface. Recently, the synthesis of thioacetyl-terminated stilbenes and distrylbenzenes was reported by Bjørnholm et al.17 In this effort, the synthetic intermediates were S-tert-butyl derivatives, which are not commercially available, and in many cases conversion to the corresponding S-acetyl-protected target compounds is difficult to achieve. The molecular structures reported in this contribution are shown in Scheme 1. In addition to the detailed synthetic protocol, we report on the electronic characterization of these molecules, and demonstrate the formation of self-assembled monolayers on gold surfaces. Our molecular building blocks include OPVs up to six and a half repeat units in length, which are soluble in common organic solvents. Furthermore, we include OPV chro(12) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; Woll, C. Langmuir 2002, 18, 7766. (13) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (14) Wasserman, S. R.; Biebuyck, H.; Whitesides, G. M. J. Mater. Res. 1989, 4, 886. (15) Sherf, U. Top. Curr. Chem. 1999, 201, 163. (16) Dudek, S. P.; Sikes, H. D.; Chidsey, C. E. D. J. Am. Chem. Soc. 2001, 123, 8033.
mophore dimers that incorporate a [2.2]paracyclophane unit as a point of attachment. The molecular structure of these paracyclophane chromophores can serve to study how a well-defined through-space delocalized unit influences electronic communication between a pair of electroactive organic units.18 This property should prove significant when examining the fabrication of molecularscale junctions19 and devices.20 Results and Discussion Synthesis and Characterization. The synthetic challenge in building thiol-terminated OPVs lies in the incorporation of the highly reactive thiol terminus, or termini, into the growing molecular backbone. Although previous synthetic strategies employ protecting the thiol as an S-acetyl21 or S-tert-butyl17 derivative, we chose a more general procedure. Several considerations led us to incorporate S-methyl derivatives. First, many derivatives are commercially available, allowing this route to be widely applicable. Second, S-acetyl derivatives are less tolerant of synthetic manipulations that require nucleophilic bases, such as trialkylamines. As a result, S-acetyl derivatives are difficult to use in palladium-mediated cross-coupling protocols, such as Heck-type reactions. Third, a general method is available for the selective dealkylation of methylaryl thioethers.22 The reaction of methylaryl thioethers with sodium thiomethoxide in polar solvent leads to the selective cleavage of the S-CH3 bond.22 Under these conditions, nucleophilic attack by sodium thiomethoxide on the terminal methyl group of the thioether produces dimethyl sulfide and generates an aryl sulfur anion. Although typical protocols involve acidic workup, we demonstrate that workup with acetyl chloride installs the acetyl group at the sulfur terminus, and that these S-acetyl deriva(17) Stuhr-Hansen, N.; Chrisensen, J. B.; Harrit, N.; Bjørnholm, T. J. Org. Chem. 2003, 68, 1275. (18) Bartholomew, G. P.; Bazan, G. C. Acc. Chem. Res. 2001, 34, 30. (19) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (20) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (21) Hsung, R. P.; Babcock, J. R.; Chidsey, C. E. D.; Sita, L. R. Tetrahedron Lett. 1995, 36, 4525. (22) (a) Tiecco, M.; Tingoli, M.; Testiferri, L.; Chianelli, D.; Maiolo, F. Synthesis 1982, 478. (b) Testaferri, L.; Tiecco, M.; Tingoli, M.; Chianelli, D.; Montanucci, M. Synthesis 1983, 751.
J. Org. Chem, Vol. 69, No. 4, 2004 1111
Seferos et al.
tives are much more soluble than the corresponding S-methyl or S-H counterparts. This additional stability is significant because it allows the final, surface-reactive compounds to be purified by standard chromatographic techniques. Additionally, the thermal conditions of the deprotection/reprotection step yield exclusively the all-E isomer, eliminating the need for a subsequent isomerization step, regardless of the stereochemistry of the starting material. Because they are more linear, and thus more fully conjugated, the all-E isomers of OPVs are the most desirable structures. It should be noted that the isomerization of the final conjugated targets often complicates the synthesis of OPV-type molecules. In our efforts, many of the methylated precursors can only be obtained as a mixture of E/Z isomers. However, the concurrent isomerization process during the final conversion to the thioacetyl product allows one to use all isomers obtained in the previous manipulations to maximize overall yields and avoid tedious chromatographic separations. The synthetic route to alkoxy-substituted OPV-SAc molecules is shown in Scheme 2. In the first example, commercially available 4-(methylthio)benzaldehyde and 1,4-bis[(methyl)triphenylphosphonium chloride]-2,5-dihexyloxybenzene were reacted under Wittig-type conditions23 to form the distyrylbenzene derivative 1,4dihexyloxy-2,5-bis[4′-(methylthio)styryl]benzene (1a) in 93% yield. The S-methyl groups were cleaved by treatment with sodium thiomethoxide to form a dianionic intermediate, which was quenched in situ with acetyl chloride. After standard workup and routine chromatography, 1,4-dihexyloxy-2,5-bis[4′-(acetylthio)styryl]benzene (1) was isolated in 39% yield. We note that the temperature (140 °C) of this deprotection/reprotection step provides the all-E isomer, as determined by using absorption and NMR (1H and 13C) spectroscopies. For the synthesis of 1,4-dihexyloxy-2,5-bis[4′-(4′′(acetylthio)styryl)styryl]benzene (2), which contains an OPV structure with five aromatic rings and four olefinic linkages, 4-(methylthio)benzaldehyde was extended by one repeat unit to 4-(4′-(methylthio)styryl)benzaldehyde (8). This conversion is readily accomplished by the Horner-Emmons-Wadsworth reaction24 of 4-(methylthio)benzaldehyde with diethyl 4-(4,4,5,5-tetramethyl1,3-dioxolan-2-yl)benzylphosphonate, followed by hydrolysis of the terminal acetal with dilute hydrochloric acid to provide 8 in 82% yield. Compound 8 reacts under similar conditions as in the synthesis of 1 to yield 1,4dihexyloxy-2,5-bis[4′-(4′′-(methylthio)styryl)styryl]benzene (2a) in 61% yield. Subsequent deprotection/acylation provided 2 in 87% yield. The synthesis of 1,4-bis[2′,5′-dihexyloxy-4′-(4′′-(4′′′(acetylthio)styryl)styryl)styryl]benzene (3) was accomplished in four steps. First, 4-(methylthio)benzaldehyde was converted to 4-(4′-(methylthio)styryl)styrene (9) in 32% yield by a Horner-Emmons-Wadsworth reaction with diethyl-4-(vinyl)benzylphosphonate. Next, Pd catalyzed coupling with 4-iodo-2,5-dihexyloxybenzaldehyde under phosphine-free conditions25 afforded 2,5-dihexyloxy-4-(4′-(4′′-(methylthio)styryl)styryl)benzaldehyde (10) (23) Maercker, A. Org. React. 1965, 14, 270. (24) Wadsworth, W. S. Org. React. 1977, 25, 73. (25) Heck, R. F. Org. React. 1982, 27, 345.
1112 J. Org. Chem., Vol. 69, No. 4, 2004
in 61% yield. 1,4-Bis[2′,5′-dihexyloxy-4′-(4′′-(4′′′-(methylthio)styryl)styryl)styryl]benzene (3a) was obtained after coupling of 1,4-bis[methyl(triphenyl)phosphonium bromide]benzene and 2 equiv of 10. Finally, 3a was converted to the S-acetyl-terminated derivative 3 in the same manner described above for 1. The synthesis of the paracyclophane derivatives and the unsubstituted distyrylbenzenes, shown in Scheme 3, adopts a simple two-step protocol. In the first step, the methyl derivatives were formed and isolated after either Heck or Wittig-type reactions. Then, in the one-pot deprotection/reprotection procedure described for 1-3, the methyl group was cleaved, and the anion was acylated. For example, 4,12-bis[4′-(acetylthio)styryl][2.2]paracyclophane (6) was made available after Heck coupling of 4-(methylthio)styrene with pseudo-p-dibromo[2.2]paracyclophane, which provides 4,12-bis[4′-(methylthio)styryl][2.2]paracyclophane (6a) in 37% yield,followed by deprotection/reprotection to form 6 in 78% yield. 2,5-Bis[4′-(acetylthio)styryl]benzene (4) was prepared by (1) a Horner-Wittig reaction of 4-(methylthio)benzaldehyde and 1,4-bis[methyldiethylphosphonate]benzene to form 2,5-bis[4′-(methylthio)styryl]benzene (4a), followed by (2) deprotection/reprotecton to form the S-acylterminated product 4, in 65% and 49% yield, receptively. In this example a 49% yield of the deprotection/reprotection step was achieved on a multiple gram scale. 4,7,12,15-Tetra[4′-(acetylthio)styryl][2.2]paracyclophane (5) and 4,4′-bis[4-(acetylthio)styryl]-1,1′-dibenzene (7) were afforded in an analogous sequence to 4 in 77% and 76% overall yield, starting from 4,7,12,15-tetra[(methyl)diethylphosphonate][2.2]paracyclophane and 4,4′-bis[(methyl)triphenylphosphonium bromide]-1,1′-dibenzene, respectively. In the synthesis of 1-7, the solubility of the methyl derivatives 1a-7a is substantially lower, relative to that of the S-acetyl compounds. Solubility is further diminished after isomerization to the all-E isomers, which was necessary for spectral assignment of the 1H and 13C NMR resonances. As a result of this low solubility, 13C NMR spectroscopy assignments of S-methyl derivatives are reported only for 1a, 5a, and 6a, the most soluble intermediates. Compounds 4a and 7a were the least soluble S-methyl-terminated OPVs, and could only be isolated by trituration. In the case of 7a, no 1H NMR data are available, and only the 1H NMR data are given for the final target compound 7. The low solubility of the intermediates highlights the efficiency of the deprotection/reprotection step. Spectroscopy and Electrochemical Measurements. Spectroscopic data for 1-7 and electrochemical measurements of 1a-6a are summarized in Table 1. For the electrochemical measurements, we chose to work with all S-methyl derivatives, instead of the S-acetyl counterparts, to rule out adsorption onto the electrode surfaces and avoid reduction of the carbonyl functionality. It is assumed that the carbonyl does not contribute to the OPV π-system and that it is removed once the molecule is self-assembled onto a surface.26 In the case of 7a, no electrochemical data could be obtained due to low solubility. (26) Price, D. W., Jr.; Dirk, S. M.; Rawlett, A. M.; Chen, J.; Wang, W.; Reed, M. A.; Zacarias, A. G.; Seminario, J. M.; Tour, J. M. Mater. Res. Soc. Proc. 2001, 660, JJ9.4.1/D7.4.1.
R,ω-Bis(thioacetyl)oligophenylenevinylene Chromophores SCHEME 2.
Synthetic Route to OPVs 1-3a
a Reagents and conditions: (i) NaH, THF. (ii) (1) NaH, toluene; (2) HCl (aq), THF. (iii) NaH, THF. (iv) Pd(OAc) , Et N, DMF. (v) 2 3 1,4-Bis[(methyl)triphenylphosphonium chloride]benzene, NaH, THF. (vi) (1) NaSCH3, DMF; (2) AcCl.
Examination of the absorption and fluorescence spectra of 1-3 reveals a red shift as the conjugation length is systematically increased. The λmax ) 447 nm for 3 is close to the saturation maximum (λmax ≈ 460 nm) observed for OPVs with eight aryl rings and seven double bonds.28 Compound 1 is red-shifted compared to the unsubstituted distyrylbenzene derivative 4, which is attributed to the
donating properties of alkoxy substituents.29 The linear spectra of compounds 4 and 7 are nearly identical, (27) Cervini, R.; Li, X.-C.; Spencer, G. W. C.; Holmes, A. B.; Moratti, S. C.; Friend, R. H. Synth. Met. 1997, 84, 359. (28) Maddux, T.; Li, W.; Yu, L. J. Am. Chem. Soc. 1997, 119, 844. (29) Dottinger, S. E.; Hohloch, M.; Segura, J. L.; Steinhuber, E.; Hanack, M.; Tompert, A.; Oelkrug, D. Adv. Mater. 1997, 9, 223.
J. Org. Chem, Vol. 69, No. 4, 2004 1113
Seferos et al. SCHEME 3.
Synthetic Route to OPVs 4-7a
a Reagents and conditions: (i) 1,4-Bis[(methyl)triphenylphosphonium chloride]benzene, NaH, THF. (ii) 4,7,12,15-Tetra[(methyl)diethylphosphonate][2.2]paracyclophane, KOt-Bu, DMF. (iii) Methyltriphenylphosphonium bromide, NaH, THF. (iv) Psuedo-pdibromo[2.2]paracyclophane, Pd(OAc)2, tri-o-tolylphosphine, Et3N, DMF. (v) 1,1′-Bis[(methyl)triphenylphosphonium brominde]-4,4′dibenzene, KH, THF. (vi) (1) NaSCH3,DMF; (2) AcCl.
TABLE 1. Electrochemical and Spectroscopic Data for OPVs 1-7 compda
λmaxb (nm)
PLb (nm)
Egc (eV)
IPd (eV)
EAe (eV)
1 2 3 4 5 6 7
402 428 447 365 408 336 359
460 489 505 423 467 425 427
2.75 2.58 2.48 3.06 2.56 3.22 3.08
5.19 5.25 5.02 5.32 5.13 5.28
2.44 2.67 2.54 2.23 2.57 2.06
a The structures are given in Scheme 1. b Measured in DCM. Refers to the optical band-gap measured from the absorption edge at 10% maximum intensity. d Refers to the ionization potential determined from the first anodic wave, referenced internally to ferrocene, then the absolute value was calculated.27 e Refers to the electron affinity calculated from the ionization potential and optical band-gap. c
indicating little contribution from the added aryl ring in the biphenyl, instead of the phenyl, core. By examining the absorption and fluorescence spectra of 4-6, insight can be gained into the contributions from through-space and through-bond delocalization within 1114 J. Org. Chem., Vol. 69, No. 4, 2004
the [2.2]paracyclophane-containing chromophores. The large stokes shift of 90 nm in 6 is attributed to internal conversion from the excited “stilbene like” chromophore to the inter-ring excited state centered on the [2.2]paracyclophane core, which has a very low oscillator strength.30 Compound 5 can be viewed as a dimer of 4. When comparing 5 to 4, a red shift in both the absorption and fluorescence is observed, and thus the S1 state differs in the two molecules. The lower energy of the S0 f S1 transition in 5 is attributed to a mixed excited state that has contributions from through-space as well as throughbond delocalization.31 Self-Assembly and Characterization of Monolayers. Examples of S-acetyl-terminated organic molecules and their reactivity with gold surfaces are numerous. Whitesides et al. observed that S-acetyl-terminated molecules have the same binding energy as thiols when allowed to react with gold surfaces and that the structure (30) Oldham, W. J., Jr.; Miao, Y.-J.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1998, 120, 419. (31) Wang, S.; Bazan, G. C.; Tretiak, S.; Mukamel, S. J. Am. Chem. Soc. 2000, 122, 1289.
R,ω-Bis(thioacetyl)oligophenylenevinylene Chromophores TABLE 2. Advancing and Receding Water Contact Angle and Layer Thickness Data for Treated Gold Substrates thickness (Å) samplea
ΦAc (deg)
ΦRd (deg)
calcde
obsdf
ODT 1 2 3 4 4 + NH4OHb 5 6 7
110 ( 1 84 ( 1 82 ( 2 81 ( 2 71 ( 1 68 ( 7 71 ( 1 73 ( 2 72 ( 4
97 ( 2 59 ( 2 62 ( 1 57 ( 2 48 ( 3 24 ( 3 51 ( 2 52 ( 4 56 ( 3
22 19.8-22.5 32.5-37.2 43.2-49.5 19.8-22.5
19 ( 1 3(0 8(3 12 ( 4 12 ( 1
14.3-20.8 21.2-24.2 24.8-28.4
11 ( 2 7(2 22 ( 1
a Refers to a gold substrate treated with a solution of the corresponding compounds (structures are given in Scheme 1). ODT was adsorbed from absolute ethanol, all other samples from DCM. b Measured after the 4-treated surface was immersed in concentrated NH4OH for 2 min. c Advancing water contact angle. d Receding water contact angle. e Calculated from the Au surface to the farthest hydrogen atom of the molecule, optimized by MMFF calculations (see the Experimental Section). f Measured by ellipsometry assuming a refractive index of 1.55 for the adsorbed layer.
of the adsorbed species was indistinguishable from that obtained with the corresponding free thiols.14 It was also noted that the acetate group was no longer present in the adsorbed layer. Later, Frisbie et al. observed this reaction in similar experiments, and again noted the similarity of the monolayers formed from both thiol- and thioacetate-containing molecules.32 The reactivity of other organic protected thiols toward gold surfaces has also been examined.33 The examination of gold surfaces after treatment with organic thiols typically relies on contact angle measurements, ellipsometry, reflectance Fourier transform infrared spectroscopy (FTIR), electrochemistry, X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM).34 For our analysis we used water drop contact angle measurements, ellipsometry, and XPS to determine monolayer quality. Also, we used AFM to examine surface roughness before and after treatment with a solution of surfaceactive compound. For comparison with the new molecules reported here, and to test the accuracy of the measurements, octadecane thiol (ODT) was used because it has been demonstrated to form dense, ordered monolayers.35 Table 2 contains the water contact angle measurements and ellipsometry data for gold substrates after treatment in 0.1 mM solutions of 1-7 in dichloromethane (DCM) for 24 h, followed by rinsing. Substrates treated with 4-7 gave advancing water contact angles ranging from 71° to 73°, in agreement with the literature value of 70° for monolayers formed from ω-S-acetyl-functionalized thiols.2 The receding water contact angle for (32) Skulason, H.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 9750. (33) Gryko, D. T.; Clausen, C.; Lindsey, J. S. J. Org. Chem. 1999, 64, 8635. (34) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (c) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (d) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirken, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (35) For a recent paper containing wettablility, ellipsometry, electrochemical measurements, FTIR, and SPM data of this SAM see: Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116.
substrates treated with 4-7 ranged from 48° to 56°. When the substrate treated with 4 was treated with concentrated ammonium hydroxide and rinsed with water, the advancing water contact angle decreased by 3°, and the receding water contact angle decreased by 24°, indicating further derivatization of the surface. Because a decrease in the water contact angle indicates more hydrophilic surface groups,36 the hydrolysis of the protruding thioacetate esters to thiophenol termini upon exposure to aqueous base is likely to cause this change. In previous studies involving thioacetyl-terminated conjugated molecules, ammonium hydroxide has been shown to completely hydrolyze the thioacetate group.13 Substrates treated with 1-3 gave water contact angles ranging from 81° to 83° and 62° to 57° for advancing and receding drops, respectively. These higher values, relative to 4-7, may be attributed to the more hydrophobic character of the hexyloxy side chains contained within these molecules. Although the side chains are not surface groups, disorder on the surface and/or poor packing within these monolayers could make contributions from these side chains more significant. Ellipsometry measurements suggest that compounds 4, 5, and 7 form the most complete layers, where the observed thickness is closest to the calculated range for the three molecules. The calculated range refers to an orientation where the molecule coincides with the surface normal, or is tilted up to 30° from the surface normal, although some investigators have reported a more tilted geometry for aromatic thiols.6 Further, we note that a systematic error in observed monolayer thickness due to advantageous contamination on untreated substrates could be responsible for some discrepancies, as previously observed,2 and confirmed experimentally (vide infra). We emphasize that none of the layers appear to be as ordered or dense as the alkane thiol example ODT. Substrates treated with 1-3, whose structures contain hexyloxy side chains, generally gave low observed thicknesses, although there is a correlation between thickness and the molecular dimension. A thickness of 11 Å for the substrate treated with 5 suggests that this molecule is oriented in an upright fashion, relative to the plane of the gold surface, despite the fact that coordination of three thiolates, resulting in a less upright orientation, is also possible when considering the structure of this tetra(thioacetyl) molecule. The substrate treated with 6, whose thickness is 7 Å, appears to be less dense than the layers that contain the more linear structures 4 and 7. The atomic concentrations derived from the XPS survey scans for the different compounds are given in Table 3. Evidence of a thin contamination layer (∼5 Å) is provided by the existence of ∼22% carbon on the gold surface prior to treatment. For a dense monolayer, one would expect an increase in the carbon concentration compared to the untreated gold since the XPS spectra survey the top 1-5 nm of the surface, which is on the order of the monolayer thickness. A significant (26%) increase in the carbon content was observed after treating the substrate with a solution of ODT, indicating the presence of a dense monolayer. Smaller, but significant (5-16%) increases in carbon concentration compared to (36) Johnson, R. E., Jr.; Dettre, R. H.; Brandreth, D. A. J. Colloid Interface Sci. 1977, 62, 205.
J. Org. Chem, Vol. 69, No. 4, 2004 1115
Seferos et al. TABLE 3. Elemental Composition of Treated Gold
TABLE 4. High-Resolution XPS Deconvolution of the
Substrates Determined by XPS
Aromatic and Aliphatic Contributions to the C1s Peaks Observed in Gold Substrates
samplea
C (%)
Au (%)
O (%)
untreated ODT 1 2 3 4 5 6 7
21.8 47.8 27.7 30.7 25.6 36.8 32.4 27.8 38.9
26.3 35.2 26.5 47.2 38.1 45.9 25.7 27.4 27.0
51.9 14.6 45.8 19.6 36.3 14.1 40.3 43.8 32.4
S (%) 2.4 2.5 3.1 1.5 1.0 1.7
a Refers to a gold substrate treated with a solution of the corresponding compounds (structures are given in Scheme 1). Untreated samples were cleaned and handled exactly as treated samples (see the Experimental Section). ODT was adsorbed from absolute ethanol, all other samples from DCM.
samplea
aromaticb (%)
aliphaticb (%)
untreated ODT 4
18.8
61 93.6 43.5
39.2
a
Refers to a gold substrate treated with a solution of the corresponding compounds (structures are given in Scheme 1). Untreated samples were cleaned and handled exactly as treated samples (see the Experimental Section). ODT was adsorbed from absolute ethanol, 4 from DCM. b Abundance determined by deconvolution of the C1s peaks (see the Experimental Section).
TABLE 5. Surface Roughness of Gold Substrates after Treatment in Various Concentrations of 4 samplea
rmsb
Rac
untreated 0.1 mM 1.0 mM 10 mM
0.385 0.930 0.841 0.900
0.076 0.276 0.595 0.193
a Untreated samples measured immediately after evaporation. 0.1, 1.0, and 10 mM refer to the concentration of 4 in DCM that the substrate was treated with. b Refers to the root-mean-square surface roughness obtained from AFM measurements. c Refers to the average surface roughness obtained from AFM measurements.
FIGURE 1. High-resolution XPS spectra showing the C1s peaks for a substrate treated with a solution containing 4 (a), ODT (b), and the substrate prior to treatment (c). The vertical line is at 284.7 eV.
the original gold surface were observed for samples treated with compounds 1-7. Additionally, the increase in the carbon signal correlated with the presence of sulfur, which was detected using the S2p peak located at 162.3 eV. This peak was assigned as the metal-thiolate species.37 Substrates treated with 4-7 generally gave higher abundances of carbon and sulfur than those treated with 1-3. High-resolution spectra of the C1s peaks from untreated gold substrates and substrates treated with ODT and 4 were taken and curve-fitted after referencing to Au(4f7/2) at 84.0 eV and are presented in Figure 1. At 20 eV pass energy, the main component of the C1s peak was too broad (fwhm ) 1.85 eV) to be considered a single component and was fitted by using the following assignments: aromatic; aliphatic; C R to carbonyl; C-O (alcohols, ethers, carboxylic acids); and carbonyl acids at 284.4, 284.7, 285.8, 287.2, and 289 eV, respectively.2,38 Although all components were used to deconvolute the C1s peaks, contributions from the aliphatic and aromatic components were the most significant, and are tabulated in Table 4. A small aromatic component (18.8%) was (37) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697 (38) (a) Briggs, D. Surface analysis of polymers by XPS and static SIMS; Cambridge University Press: Cambridge, U.K., 1998. (b) Cumpston, B. H.; Jensen, K. F. J. Appl. Polym. Sci. 1998, 69, 2451.
1116 J. Org. Chem., Vol. 69, No. 4, 2004
observed in the untreated gold sample, which was most likely due to adsorbed aromatic contaminants. A significantly larger aromatic component (39.2%) was seen in the sample treated with 4, signifying the presence of aromatic species in the monolayer. No evidence of aromatic carbon was observed in sample treated with ODT. The enhancement of the aromatic component of the C1s peak in the sample treated with 4 was also accompanied by a decrease in aliphatic components by the same magnitude. Surface analyses of gold substrates before and after treatment with 4 were measured by AFM. For this experiment the surfaces were immersed for 24 h in DCM solutions of 4 with concentrations varying from 0.1 to 10 mM. From the section analysis of the AFM images the surface roughness values were obtained and are given in Table 5. In all cases, surface roughness increased after deposition of 4. The morphology of the untreated sample was essentially featureless, substrates treated in 0.1 and 1.0 mM solutions showed slightly more surface features, while the sample obtained from the 10 mM solution had large, smooth features within the 1 µm × 1 µm image (see the Supporting Information). Examination of the sectional analysis of the plane images reveals the height of the features in each sample. Untreated substrates had features with heights that were less than the calculated height of the SAM of 4, substrates treated with both 0.1 and 1.0 mM solutions of 4 had features that were commensurate with the molecular dimensions of the SAM, and substrates treated with 10 mM solutions of 4 had features that were significantly larger than the molecular dimensions of the SAM (see the Supporting Information). These data suggest over-deposition of organic material at higher concentrations. Summary and Conclusion We have demonstrated a facile synthetic approach for the synthesis of R,ω-bis(thioacetyl)phenylenevinylene
R,ω-Bis(thioacetyl)oligophenylenevinylene Chromophores
oligomers. In this method, the S-acetyl group is masked as an S-methyl derivative. Most significantly, from a molecular structure elaboration point of view, the Smethyl-terminated aryl group can be used in Heck and Wittig or Horner-Wittig-type reactions, which are often used for building OPV structures. We have also shown how the S-methyl terminus or termini can be converted to the corresponding S-acetyl derivative, while concurrently thermally isomerizing the olefinic linkages to produce solely all-E products. The methodology was demonstrated by creating a series of new thioacetylterminated OPVs. In addition to the more classic linear OPV systems, examples of chromophore dimers with [2.2]paracyclophane points of attachment were also synthesized and examined. Spectroscopic and electrochemical measurements were performed. We note the unique optical properties of molecules that contain the [2.2]paracyclophane core that exhibit through-space, as well as through-bond, delocalization. Finally, we demonstrate the utility of these molecules to chemically adsorb on gold surfaces. Molecules with linear hexyloxy-substituted OPV structures 1-3 form disordered SAMs. The [2.2]paracyclophanecontaining OPVs 5 and 6 form significantly better SAMs than examples with side groups, and to our knowledge these are the first examples of surface-bound [2.2]paracyclophane-containing molecules. The best structures for obtaining a monolayer are the unsubstituted OPVs 4 and 7, but appear to be less complete than SAMs prepared from ODT, an alkane thiol of approximately the same molecular dimension. Given the ease of preparation, interesting electronic properties, and reactivity toward gold surfaces, these compounds are promising surface-modifying reagents and molecular-wire candidates.
Experimental Section Substrate Preparation. For contact angle measurements, ellipsometry, and XPS, 1500 Å of gold (99.999%) was E-beam evaporated on heavily doped (n+) silicon wafers with 2000 Å of thermal SiO2 and a 20 Å titanium adhesion layer. After evaporation, the samples were annealed for 1 h in a vacuum oven at 260 °C and stored under absolute ethanol until use. For AFM measurements, 50 Å of gold (99.999%) was E-beam evaporated on freshly cleaved mica with a 20 Å chromium adhesion layer. Preparation of Monolayers on Gold. Working in a laminar flow cabinet, a gold substrate was irradiated in a UVO cleaner (20 m), then immersed in absolute ethanol (20 m) to remove any transient oxide. The substrate was then rinsed with ethanol, blown dry with a stream of nitrogen, and immersed in a 0.1 mM solution of the acetylthiol in DCM. After the formation of a monolayer (24 h), the sample was removed, rinsed with DCM and absolute ethanol, and stored under absolute ethanol. Prior to analysis, the sample was removed and blown dry with nitrogen. 1,4-Dihexyloxy-2,5-bis[4′-(methylthio)styryl]benzene (1a). To a two-neck round-bottom flask equipped with a needle valve and a magnetic stir-bar was added 4-(methylthio)benaldehyde (0.25 mL, 1.87 mmol), 1,4-bis[(methyl)triphenylphosphonium chloride]-2,5-dihexyloxybenzene (750 mg, 0.83 mmol), and THF (15 mL) under an atmosphere of argon. The mixture was cooled to 0 °C, sodium hydride (45 mg, 1.87 mmol) was added under an argon purge, and the reaction was allowed to warm to room temperature and stirred overnight. The reaction was quenched with water, rinsed into ether (100 mL), washed with three portions of brine, dried, and concentrated.
The crude product was purified by column chromatography eluting with 50% dichloromethane in hexanes to yield 0.448 g (93%) of green, florescent powder, which contained all three isomers of the title compound. For spectroscopic analysis the product was converted to the all-E isomer by degassing and refluxing in toluene with a few crystals of iodine. 1H NMR (CDCl3) δ 7.463 (d, 3J ) 8.37 Hz, 4H), 7.447 (d, 3Jtrans ) 16.75 Hz, 2H), 7.251 (d, 4H), 7.116 (s, 2H), 7.093 (d, 2H), 4.057 (t, 3 J ) 6.42 Hz, 4H), 2.52 (s, 6H), 1.879 (m, 4H), 1.559 (m, 4H), 1.399 (m, 8H), 0.937 (t, 3J ) 6.98 Hz, 6 H). 13C NMR (CDCl3) δ 151.3, 137.7, 135.2, 128.3, 127.1, 127.0, 126.9, 123.061, 110.7, 69.8, 31.8 29.7, 26.2, 22.9, 16.1, 14.3. HRMS-EI 574.2920, ∆ ) 3.4 ppm. 1,4-Dihexyloxy-2,5-bis[4′-(acetylthio)styryl]benzene (1). In an inert atmosphere glovebox, a 10-mL round-bottom flask fitted with a magnetic stir-bar and needle valve was charged with 1a (200 mg, 0.35 mmol) and sodium thiomethoxide (60 mg, 0.86 mmol). The apparatus was attached onto a Schlenck line, 3 mL of DMF was added, and the reaction was heated to 140 °C, at which time the solution became dark red. The reaction was stirred at 140 °C for 20 h. Upon cooling to 0 °C, excess acetyl chloride (0.25 mL, 3.5 mmol) was added all at once, the ice bath was removed, and the mixture was stirred for an additional hour. The solution was diluted with chloroform (100 mL), washed with three portions of brine, dried, and concentrated. Column chromatography eluting with chloroform afforded 87 mg (39%) of the title compound as a pale green solid, with only the all-E isomer present. 1H NMR (CDCl3) δ 7.570 (d, 3J ) 8.37, 4H), 7.532 (d, 3Jtrans ) 16.47 Hz, 2H), 7.410 (d, 4H), 7.146 (d, 2H), 7.127 (s, 2H), 4.068 (t, 3J ) 6.42 Hz, 4H), 2.448 (s, 6H), 1.887 (m, 4H), 1.570 (m, 4H), 1.406 (m, 8H), 0.942 (t, 3J ) 7.26 Hz, 6H). 13C NMR (CDCl3) δ 194.5, 151.4, 139.4, 134.9, 128.1, 127.4, 127.0, 126.6, 125.215, 110.8, 69.7, 31.8, 30.4, 29.639, 26.2, 22.9, 14.3. HRMS-EI 630.2848, ∆ ) 1.6 ppm. 4-(4′-(Methylthio)styryl)benzldehyde (8). To a 50-mL round-bottom flask fitted with a magnetic stir-bar was added 4-(methylthio)benzaldehyde (380 mg, 2.4 mmol), diethyl 4-(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)benzylphosphonate (1.00 g, 2.8 mmol), and toluene (30 mL) under an argon atmosphere. Sodium hydride (240 mg, 10.0 mmol) was added at 0 °C, and the reaction was stirred at 65 °C for 16 h. After cooling, the contents of the flask were rinsed into toluene (80 mL), washed with water, dried, and concentrated to give a white powder. The crude product was taken up in a solution of THF (50 mL) and aqueous hydrochloric acid (10%, 100 mL) and stirred at 60 °C for 1 h. The THF was evaporated, and the remaining aqueous solution was extracted with three portions of DCM, dried, and concentrated. The crude solid was passed through a short silica plug (DCM elutent) to yield 520 mg (82%) of the title compound as a white solid. 1H NMR (CDCl3) δ 9.987 (s, 1H), 7.879 (d, 2H), 7.633 (d, 2H), 7.465 (d, 2H), 7.253 (d, 2H), 7.152 (d, 2H), 2.512 (s, 3H). 13C NMR (CDCl3) δ 191.8, 143.6, 139.5, 135.4, 133.5, 131.7, 130.4, 127.5, 127.0, 126.7, 126.6, 15.7. HRMS-EI 254.0767, ∆ ) 0.6 ppm. 1,4-Dihexyloxy-2,5-bis[4′-(4′′-(methylthio)styryl)styryl]benzene (2a). 8 (400 mg, 1.57 mmol) and 1,4-bis[(methyl)triphenylphosphonium chloride]-2,5-dihexyloxybenzene (635 mg, 0.71 mmol) were reacted, worked up, and purified in the same manner described above. Chromatography afforded 340 mg (61%) of the title compound as a yellow, florescent powder. 1 H NMR (CDCl3) δ 7.516 (m, 12H), 7.461 (d, 3J ) 8.37 Hz, 4H), 7.256 (d, 4H), 7.146 (s, 2H), 7.089 (s, 4H), 4.080 (t, 3J ) 6.42 Hz, 4H), 2.523 (s, 6H), 1.900 (m, 4H), 1.580 (m, 4H), 1.415 (m, 8H), 0.951 (t, 3J ) 7.25 Hz, 6H). HRMS-EI 778.33908, ∆ ) 3.8 ppm. 1,4-Dihexyloxy-2,5-bis[4′-(4′′-(acetylthio)styryl)styryl]benzene (2). 2a (150 mg, 0.19 mmol) and sodium thiomethoxide (32 mg, 0.46 mmol) were reacted, quenched with excess acetyl chloride, worked up, and purified in the same manner as described above. Chromatography afforded 138 mg (87%) of the title compound as a yellow, florescent powder. 1H NMR
J. Org. Chem, Vol. 69, No. 4, 2004 1117
Seferos et al. (CDCl3) δ 7.541 (m, 14H), 7.413 (d, 3J ) 8.37 Hz, 4H), 7.150 (m, 8H), 4.083 (t, 3J ) 6.42 Hz, 4H), 2.448 (s, 6H), 1.804 (m, 4H), 1.583 (m, 4H), 1.420 (m, 8H), 0.954 (t, 3J ) 7.26). 13C NMR (CDCl3) δ 194.4, 151.4, 138.8, 138.0, 136.2, 134.9, 130.1, 128.5, 127.5, 127.4, 127.2, 127.1, 126.9, 123.8, 110.7, 69.7, 31.9, 30.4, 29.7, 26.2, 22.9, 14.3. LRMS-FAB calcd 834, found 834. 4-(4′-(Methylthio)styryl)styrene (9). A round-bottom flask fitted with a gas inlet tube and a magnetic stir-bar was charged with 4-(methylthio)benzaldehyde (1.00 g, 7.15 mmol), diethyl-4-(vinyl)benzylphosphonate (2.00 g, 7.87 mmol), sodium hydride (240 mg, 10.00 mmol), and toluene (50 mL) under an argon atmosphere. The reaction was heated at 65 °C for 2.5 h. Upon cooling, the mixture was diluted with toluene (150 mL), washed with water, dried, and concentrated to yield an orange powder. The crude material was passed through a silica plug and recrystallized in a solution of 50% benzene in petroleum ether to afford 580 mg (32%) of the title compound. 1H NMR (CDCl3) δ 7.477 (d, 3J ) 8.37 Hz, 2H), 7.448 (d, 3J ) 8.37 Hz, 2H), 7.411 (d, 2H), 7.250 (d, 2H), 6.726 (dd, 3Jcis ) 10.89 Hz, 3Jtrans ) 17.56 Hz, 1H), 5.777 (dd, 2J ) 0.84 Hz, 1H), 5.261 (dd, 1H), 2.518 (s, 3H). 13C NMR (CDCl3) δ 138.1, 137.081, 137.0, 136.7, 134.5, 128.2, 127.8, 127.1, 126.9, 126.8, 126.8, 113.9, 16.0. HRMS-EI 252.0961, ∆ ) 4.6 ppm. 2,5-Dihexyloxy-4-(4′-(4′′-(methylthio)styryl)styryl)benzaldehyde (10). In an inert atmosphere glovebox, a two-neck round-bottom flask fitted with a reflux condenser, needle valve, and magnetic stir-bar was charged with 2,5-dihexyloxy-4iodobenzaldehyde (0.99 g, 2.3 mmol), 9 (0.69 g, 2.7 mmol), and palladium(II) acetate (0.01 g, 0.046 mmol). The apparatus was attached to a Schlenk line and triethylamine (0.39 mL, 2.8 mmol) and DMF (5 mL) were added. The reaction was heated to 100 °C for 24 h with vigorous stirring. Upon cooling, the mixture was diluted with ether (500 mL), washed with water, dried, and concentrated. Column chromatography on silica eluting with DCM afforded 0.77 g (61%) of the title compound. 1H NMR (CDCl3) δ 10.462 (s, 1H), 7.533 (m, 4H), 7.468 (m, 3H), 7.339 (s, 1H), 7.254 (m, 3H), 7.188 (s, 1H), 7.094 (m, 2H), 4.124 (t, 2H), 4.085 (t, 2H), 2.519 (s, 3H), 1.870 (m, 4H), 1.576 (br s, 4H), 1.385 (br s, 8H), 0.937 (t, 6H). 13C NMR (CDCl3) δ 189.4, 156.4, 150.9, 138.3, 137.5, 136.7, 134.5, 134.3, 132.0, 127.7, 127.5, 127.1, 127.0, 126.8, 124.4, 122.9, 110.6, 110.3, 69.4, 69.3, 31.8, 29.43, 29.41, 26.1, 26.0, 22.84, 22.81, 16.0, 14.3. HRMS-EI 556.30096, ∆ ) 0.3 ppm. 1,4-Bis[2′,5′-dihexyloxy-4′-(4′′-(4′′′-(methylthio)styryl)styryl)styryl]benzene (3a). 10 (40 mg, 0.065 mmol), 1,4-bis[(methyl)triphenylphosphonium chloride]benzene (22.6 mg, 0.032 mmol), and sodium hydride (1.8 mg, 0.078 mmol) were reacted and worked up in the manner previously described. Column chromatography on silica gel eluting with 50% DCM in hexanes afforded 26 mg (70%) of the title compound. 1H NMR (CDCl3) δ 7.518 (br m, 16H), 7.462 (d, 3J ) 8.37 Hz, 4H), 7.258 (d, 4H), 7.153 (m, 8H), 7.091 (s, 4H), 4.85 (t, 8H), 2.524 (s, 6H), 1.905 (m, 8H), 1.570 (m, 8H), 1.421 (m, 16H), 0.955 (t, 12H). MALDI-TOFMS calcd 1183, found 1183. 1,4-Bis[2′,5′-dihexyloxy-4′-(4′′-(4′′′-(acetylthio)styryl)styryl)styryl]benzene (3). 3a (9 mg, 7.6 µmol) and sodium thiomethoxide (1.3 mg, 18.24 µmol) were reacted, quenched with excess acetyl chloride, worked up, and purified in the same manner as previously described. Chromatography afforded 5.5 mg (58%) of the title compound. 1H NMR (CDCl3) δ 7.545 (m, 20H), 7.415 (d, 3J ) 8.37 Hz, 4H), 7.157 (m, 12H), 4.088 (t, 8H), 2.449 (s, 6H), 1.907 (m, 8H), 1.586 (m, 8H), 1.421 (m, 16H), 0.955 (t, 12H). MALDI-TOFMS calcd 1239, found 1239. 2,5-Bis[4′-(methylthio)styryl]benzene (4a). To a flamedried three-neck flask fitted with a needle valve and magnetic stir-bar was added 1,4-bis[(methyl)dethylphosphonate]benzene (378 mg, 1.0 mmol), 4-(methylthio)benzaldehyde, and toluene (10 mL) under an argon atmosphere. The suspension was cooled to 0 °C, sodium hydride was added, and the reaction was heated to 65 °C for 24 h. Upon cooling, the solids were suspended in a solution of hexanes and chloroform (1:1),
1118 J. Org. Chem., Vol. 69, No. 4, 2004
washed with water, and collected by filtration. This procedure was repeated a total of three times to afford 244 mg (65%) of the title compound as a pale-yellow powder. Repeating on a 20-mmol scale afforded 3.183 g (37%). 1H NMR (CDCl3) δ 7.519 (d, 3J ) 8.93 Hz, 4H), 7.502 (s, 4H), 7.232 (d, 4H), 7.082 (s, 4H), 2.522 (s, 6H). Anal. Calcd for C24H22S2: C, 76.96; H, 5.92. Found: C, 76.12; H, 6.06. HRMS-EI 374.1155, ∆ ) 2.0 ppm. 2,5-Bis[4′-(acetylthio)styryl]benzene (4).17 4a (2.50 g, 6.68 mmol) and sodium thiomethoxide (1.12 g, 16.63 mmol) were reacted in DMF (100 mL) and quenched with excess acetyl chloride in the manner previously described. The contents of the flask were precipitated into 150 mL of methanol, filtered, and spun onto silica gel. Column chromatography eluting with chloroform afforded 1.42 g (49%) of the title compound. 1H NMR (CDCl3) δ 7.564 (d, 3J ) 8.37 Hz, 4H), 7.535 (s, 4H), 7.415 (d, 4H), 7.149 (m, 4H), 2.448 (s, 6H). 13C NMR (CDCl3) δ 194.4, 138.7, 136.8, 134.9, 129.9, 127.924, 127.4, 127.3, 127.0, 30.3. Anal. Calcd for C26H22O2S2: C, 72.52; H, 5.15. Found: C, 72.52; 5.22. HRMS-EI 430.1063, ∆ ) 0.4 ppm. 4,7,12,15-Tetra[4′-(methylthio)styryl][2.2]paracyclophane (5a). A 50-mL round-bottom flask fitted with a rubber septa and a magnetic stir-bar was back-filled with argon and charged with 4,7,12,15-tetra[(methyl)diethylphosphonate][2.2]paracyclophane (500 mg, 0.62 mmol), 4-(methylthio)benzaldehyde (0.5 mL, 3.72 mmol), and DMF (40 mL). A separate suspension of potassium tert-butoxide (347 mg, 3.1 mmol) in DMF was prepared and added to the flask at 0 °C. Upon addition of the base, there was an immediate color change to dark blue, and the reaction was allowed to warm to room temperature and stirred overnight. The contents of the flask were then dissolved in chloroform (700 mL), washed with brine, dried, and concentrated. Column chromatography on silica gel eluting with chloroform afforded 450 mg (91%) of the title compound. 1H NMR (CDCl3) δ 7.409 (d, 3J ) 8.30 Hz, 8H), 7.288 (d, 8H), 7.174 (d, 3Jtrans ) 16.04, 4H), 6.983 (s, 4H), 6.865 (d, 4H), 3.576 (m, 4H), 2.877 (m, 4H), 2.568 (s, 12H). 13C NMR (CDCl3) δ 138.2, 138.0, 136.8, 134.8, 128.23, 128.15, 127.2, 127.0, 124.9, 33.3, 16.1. LRMS-FAB calcd 800, found 800. 4,7,12,15-Tetra[4′-(acetylthio)styryl][2.2]paracyclophane (5). 5a (275 mg, 0.34 mmol) and sodium thiomethoxide (119 mg, 1.7 mmol) were reacted, quenched with excess acetyl chloride, worked up, and purified in the same manner as previously described. Chromatography afforded 263 mg (85%) of the title compound. 1H NMR (CDCl3) δ 7.492 (d, 3J ) 8.37 Hz, 8H), 7.443 (d, 8H), 7.263 (d, 3Jtrans ) 15.91 Hz, 4H), 6.975 (s, 4H), 6.887 (d, 4H), 3.58 (m, 4H), 2.88 (m, 4H), 2.480 (s, 12H). 13 C NMR (CDCl3) δ 194.4, 138.879, 138.3, 136.9, 135.0, 128.7, 128.1, 127.5, 127.1, 126.7, 33.3, 30.5. LRMS-FAB calcd 912, found 912. 4-(Methylthio)styrene.39 To a flame-dried three-neck roundbottom flask fitted with a magnetic stir-bar and back-filled with argon was added 4-(methylthio)benzaldehyde (5.72 g, 37.58 mmol), methyltriphenylphosphonium bromide (16.11 g, 45.10 mmol), and THF (300 mL). Sodium hydride (1.08 g, 45.10 mmol) was added under argon purge at 0 °C. The reaction mixture warmed to room temperature overnight with stirring. Volatile substances were removed under reduced pressure, and the remaining crude material was diluted with chloroform (300 mL), washed with brine (3 × 100 mL), and concentrated. Distillation under reduced pressure (75-80 °C) afforded 5.58 g (99%) of pure product. 1H NMR (CDCl3) δ 7.346 (d, 3J ) 8.652, 2H), 7.222 (d, 2H), 6.679 (dd, 3Jcis ) 10.89 Hz, 3Jtrans ) 17.59 Hz, 1H), 5.720 (d, 1H), 5.221 (d, 1H), 2.498 (s, 3H). 13C NMR (CDCl3) δ 138.2, 136.4, 134.7, 126.8, 126.8, 113.4, 16.0. LRMS-EI calcd 150, found 150. 4,12-Bis[4′-(methylthio)styryl][2.2]paracyclophane (6a). In an inert atmosphere glovebox, a 1-neck round-bottom flask (39) For an alternative preparation with additional spectroscopic characterization see: Hirao, A.; Shione, H.; Ishizone, T.; Nakahama, S. Macromolecules 1997, 30, 3728.
R,ω-Bis(thioacetyl)oligophenylenevinylene Chromophores fitted with a needle valve and a magnetic stir-bar was charged with 4,12-dibromo[2.2]paracyclophane (250 mg, 0.68 mmol), 4-(methylthio)styrene (225 mg, 1.5 mmol), palladium(II) acetate (20 mmg, 0.09 mmol), tri-o-tolylphosphine (50 mg, 0.16 mmol), and DMF (5 mL). The apparatus was attached to a Schlenck line, and triethylamine (0.21 mL, 1.5 mmol) was added and the resulting mixture was degassed by 3 freezepump-thaw cycles and heated to 100 °C for 40 h under rapid stirring. Upon cooling, the contents of the flask were rinsed into chloroform (25 mL) and washed with brine (3 × 25 mL). Column chromatography, eluting with 50% chloroform in hexanes, afforded 126 mg (37%) of the title compound. 1H NMR (CDCl3) δ 7.523 (d, 3J ) 8.37 Hz, 4H), 7.261 (d, 4H), 7.198 (d, 3 Jtrans ) 16.19 Hz, 2H), 6.856 (d, 2H), 6.689 (d, 4J ) 1.68 Hz, 2H), 6.665 (dd, 3J ) 7.82 Hz, 2H), 6.423 (d, 2H), 3.606 (m, 2H), 3.168-2.895 (complex, 6H), 2.550 (s, 6H). 13C NMR (CDCl3) δ 139.7, 138.3, 137.9, 137.6, 135.1, 133.8, 130.3, 129.6, 128.8, 127.08, 127.06, 126.6, 34.7, 33.6, 13.2. HRMS-EI 504.19337, ∆ ) 2.3 ppm. 4,12-Bis[4′-(acetylthio)styryl][2.2]paracyclophane (6). 6a (80 mg, 0.158 mmol) and sodium thiomethoxide (27 mg, 0.38 mmol) were mixed in DMF (2 mL), quenched with excess acetyl chloride, worked up, and purified in the same manner as previously described. After chromatography, 69 mg (78%) of the title compound was isolated. 1H NMR (CDCl3) δ 7.629 (d, 3J ) 8.37 Hz, 4H), 7.462 (d, 4H), 7.280 (d, 3Jtrans ) 16.19 Hz, 2H), 6.901 (d, 2H), 6.712 (d, 4J ) 1.68 Hz, 2H), 6.667 (dd, 3 J ) 7.82 Hz, 2H), 6.436 (d, 2H), 3.600 (m, 2H), 3.2-2.9 (complex, 6H), 2.474 (s, 6H). 13C NMR (CDCl3) δ 194.5, 139.7, 139.3, 138.7, 137.2, 135.0, 133.8, 130.5, 130.1, 128.7, 128.5, 127.4, 126.8, 34.7, 33.6, 30.5. HREI-MS 560.183444, ∆ ) 1.7 ppm. 4,4′-Bis[4-(methylthio)styryl]-1,1′-dibenzene (7a). To a three-neck round-bottom flask equipped with a needle valve and a magnetic stir-bar was added 4-(methylthio)benzaldehyde (0.2 mL, 1.5 mmol), 4,4′-bis[(methyl)triphenylphosphonium bromide]-1,1′-dibenzene (432 mg, 0.5 mmol), and THF (25 mL) under an argon atmosphere. Potassium hydride (60 mg, 1.5 mmol) was added at 0 °C, and the reaction was allowed to warm to room temperature overnight with constant stirring. The suspension was poured into hexanes (100 mL), washed with water, then collected by filtration to afford 209 mg (93%) of pale yellow solid, which was used in the next step without further purification. 4,4′-Bis[4-(acetylthio)styryl]-1,1′-dibenzene (7). 7a (200 mg, 0.49 mmol) and sodium thiomethoxide (123 mg, 1.76 mmol) were reacted in DMF (7 mL), quenched with excess acetyl chloride, worked up, and purified in the same manner as previously described. After chromatography, 182 mg (82%) of the title compound was isolated. 1H NMR (CDCl3) δ 7.638 (m, 8H), 7.585 (d, 3J ) 8.37 Hz, 4H), 7.424 (d, 4H), 7.185 (m, 4H), 2.452 (s, 6H). Anal. Calcd for C32H26O2S2: C, 75.86; H, 5.17. Found: C, 75.01; H, 5.46. HREI-MS 506.13905, ∆ ) 3.2 ppm. Cyclic Voltammetry Measurements. Electrochemical measurements were obtained by using a standard threeelectrode cell cyclic voltammetry setup with a Pt disk, Ag wire, and Pt wire for the working, auxiliary, and reference electrodes, respectively. Measurements were made in solutions of 0.1 M tetrabutlyamonium phosphorushexafloride, in degassed
DCM, and at an analyte concentration of 0.5 mg/mL. Scans were measured nominally from 0 to 1.1 to 0 V vs Pt wire, at 200 mV/s, and referenced internally to Fc/Fc+. Water Contact Angle Measurements. Advancing and receding contact angles were obtained at room temperature and humidity on a Gaertaer Scientific Corporation Goniometer, using MilliQ purified water. Each sample substrate was measured three times and averaged. Ellipsometry. Layer thickness measurements were preformed on a Rudolf Auto EL instrument with a 632.8 nm light source at 70° angle of incidence. Measurements of the analyzer and polarizer angles of the substrate were made immediately before and after deposition in the thiol-containing solutions. The change in sample thickness was calculated assuming values of 1.55 and 0 for the n and k optical constants of the organic layer, respectively. Each sample substrate was measured 2-3 times before and after treatment, and averaged. The calculated thickness range was determined assuming a mean S-Au distance of 2 Å,1 and also that the molecules lie normal to the surface or have an average tilt of 30° from the surface normal. Molecular dimensions were minimized (MMFF) and measured with the Spartan software package. XPS. Surface chemical analysis was performed with a Kratos Axis Ultra photoelectron spectrometer with a monochromatic Al KR source at 1486.6 eV. A charge neutralizer was used for all samples. Survey scans were measured with a 160 eV pass energy at 0.5 eV intervals, 225 W electron beam power, and 700 µm by 300 µm spherical spot size. The data acquisition time for the survey scans was 5 min. All spectra were calibrated by using the Au(4f7/2) peak referenced at 84.0 eV. Peak areas from the survey scans were converted into atomic concentration percentages by using elemental sensitivity factors. High-resolution scans of the C1s peak were taken with use of 20 eV pass energy at 0.05 eV intervals, 300 W electron beam power, and 700 µm by 300 µm spherical spot size. The data acquisition time for the high-resolution scans was 4 min. The C1s peaks were curve-fitted with 70% Gaussian/30% Lorentzian peak shapes of variable widths with use of XPS data analysis software. The takeoff angle for all scans was 90°. AFM. Images of the gold substrates (1 × 1 µm scan size) were obtain by using a Digital Instruments Dimension 3000 microscope operating in tapping mode. Images were plane-fit and flattened.
Acknowledgment. Financial support form the National Science Foundation (DMR 0097611), the Office of Naval Research, and the Mitsubishi Chemical Center for Advanced Materials is gratefully acknowledged. AFM and XPS results were obtained at the MRL (Materials Research Laboratory at UCSB) Central Facilities supported by the National Science Foundation (DMR 96-32716). Supporting Information Available: General experimental methods, including references to OPV precursors, and AFM images: plane view and section analysis. This material is available free of charge via the Internet at http://pubs.acs.org. JO035664G
J. Org. Chem, Vol. 69, No. 4, 2004 1119
