4882
Langmuir 1996, 12, 4882-4888
Langmuir-Schaefer Films of Distyrylphenanthrolines and Rhenium Tricarbonyl Chloride Complexes: Headgroup Influence on Anisotropy K. K. Balasubramanian and V. Cammarata* Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312 Received April 4, 1996X Seven new compounds, four 4,7-bis(p-substituted-styryl)-1,10-phenanthrolines and three rhenium tricarbonyl chloride (diamine) complexes, were synthesized and characterized. These compounds which lack alkyl chains, formed stable Langmuir films. These films were transferred from the air-water interface by horizontal lifting to a variety of solid surfaces such as Au/glass, Ge, and quartz with transfer ratios of 1.02 ( 0.10. UV/vis and three FTIR methods (transmission IR, grazing angle reflectance IR, and attenuated total reflectance IR) were used to analyze the transferred films. [4,4′-Bis[p-(methacryloyloxy)styryl]2,2′-bipyridine]rhenium tricarbonyl chloride and 4,7-bis(p-nitrostyryl)-1,10-phenanthroline showed the largest anisotropy in the Langmuir-Schaefer (LS) films. The orientation of the other compounds varied from moderately anisotropic to totally isotropic. Hence, the net orientation of these materials in LS films is greatly influenced by the terminating functional groups.
Introduction
* Address correspondence to this author. E-mail: Cammavi@ mail.auburn.edu. Telephone: (334) 844-6962. Fax: (334) 844-6959. X Abstract published in Advance ACS Abstracts, September 15, 1996.
Katz et al. formed multilayers from rigid diphosphonic acids such as quaterthiophenediphosphonic acid.9 To achieve a better understanding of new LB and LS films having designed structures and properties, it is necessary to do comparative studies of various systems with different functional groups. The systematic control of aggregate formation, molecular orientation, and electronic structure of the resultant films should be useful in designing new materials. To our knowledge, there have been only a few reports dealing with substituent-dependent structural changes in LBS films in the literature.6,10 Our previous work involved the comparative study of different functional groups in the distyrylbipyridine system where we showed that changing the end group drastically affected the net orientation of resultant LS films.11 From the isotherm data, we speculated that the molecules existed in the transoid conformation. In this paper, we extend the study by using phenanthroline instead of bipyridine and by complexing the bipyridine and phenanthroline nitrogens with a metal carbonyl complex (Chart 1). In this system, the molecules can exist only in the cisoid configuration and, in principle, should show different solution and thin film properties, since the molecular structure is different from that of the transoid molecules. There are a number of papers reported in the literature where transition metal complexes were used to make LB films. Ruthenium with various ligands such as aliphatic ester derivatives of 2,2′-bipyridine12 and 4,7-diphenyl-1,10-phenanthroline13 was reported to form LB films when mixed with fatty acids. De Armond and co-workers incorporated similar Ru complexes from the subphase into fatty acid LB films.14 There are other organometallic
(1) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (2) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum: New York, 1990. (3) Kuhn, H. Thin Solid Films 1989, 178, 1. Ringsdorf, H.; Schmidt, G.; Schneider, J. Thin Solid Films 1987, 152, 207. Wegner, G. Ber. Bunsen.-Ges. Phys. Chem. 1991, 95, 1326. (4) Song, Y. P.; Petty, M. C.; Yarwood, J.; Feast, W. J.; Tsibouklis, J.; Mukherjee, S. Langmuir 1992, 8, 257. (5) Terashita, S.; Ozaki, Y.; Iriyama, K. J. Phys. Chem. 1993, 97, 10445. Yamamoto, M.; Wajima, T.; Kameyama, A.; Itoh, K. J. Phys. Chem. 1992, 96, 10365. Ulman, A.; Scarringe, R. P. Langmuir 1992, 8, 894. (6) Cammarata, V.; Atanansoska, L.; Miller, L. L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992, 8, 876. (7) Muller, J.; Base, K.; Magnera, T. F.; Michl, J. J. Am. Chem. Soc. 1992, 114, 9721.
(8) Kobayashi, N.; Lam, H.; Nevin, W. A.; Janda, P.; Leznoff, C. C.; Koyama, T.; Monden, A.; Shirai, H. J. Am. Chem. Soc. 1994, 116, 879. Ogawa, K.; Kinoshita, S. I.; Yonehara, H.; Nakahara, H.; Fukuda, K. J. Chem. Soc., Chem. Commun. 1989, 477. (9) Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Mater. 1991, 3, 699. (10) Lehmann, U. Thin Solid Films 1988, 160, 257. Saito, K.; Ikegami, K.; Kurodan, S.; Tabe, Y.; Sugi, M. J. Appl. Phys. 1992, 71, 1401. Enomoto, S.; Ozaki, Y.; Kuramoto, N. Langmuir 1993, 9, 3219. (11) Balasubramanian, K. K.; Cammarata, V.; Wu, Q. Langmuir 1995, 11, 1658. (12) Gaines, G. L., Jr.; Behnken, P. E.; Valenty, S. J. J. Am. Chem. Soc. 1978, 100, 6549. Yamada, S.; Nakano, T.; Matsuo, T. Thin Solid Films 1994, 245, 196. (13) Murakata, T.; Miyashita, T.; Matsuda, M. J. Phys. Chem. 1988, 92, 6040.
The Langmuir-Blodgett/Langmuir-Schaefer (LBS) techniques are two effective methods of forming artificial molecular assemblies.1 It is possible to form highly anisotropic films by these methods that are useful in applications such as nonlinear optics, photovoltaics, and sensors.2 Since one can expect to develop new aspects of chemical, physical, and biological processes in this uniquely ordered environment, extensive research has been done on these films in the past several years.3 It is typical in Langmuir-Blodgett (LB) research to use a molecule which has an active group, e.g. a chromophore or electrophore, with a long alkyl chain.1,2 The long alkyl chain which provides organization within each layer either is mixed with the active group as an independent entity4 or is incorporated onto the active group itself.5 Our interest is to expand the scope of LB and Langmuir-Schaefer (LS) molecules that lack these long alkyl chains.6 Removal of alkyl chains would be expected to increase the density of the active groups in each layer. The question is whether or not such compounds will lead to organized and transferable films and, if so, how they are organized. There are several reports where LB films are composed of molecules lacking long alkyl chains. Michl and coworkers synthesized rigid-rod oligo-p-carboranes and fabricated them into LB films.7 There are several reports of LB films formed from phthalocyanines in the literature.8
S0743-7463(96)00323-X CCC: $12.00
© 1996 American Chemical Society
Headgroup Influence on Anisotropy in LS Films Chart 1
complexes such as cobalt phthalocyanine,15 liquid-crystalline-behaving crown ether-substituted phthalocyanines,16 and Cu, Zn, and Co porphyrin complexes17 which have been reported to form LB films. The electrochemical properties of LB films made from some of these complexes are reviewed by Goldenberg.18 The bipyridine analogues of rhenium carbonyl complexes are known to catalytically reduce carbon dioxide.19 Rhenium(I) halotricarbonyl complexes of polypyridines were synthesized, and their photophysical and electrochemical properties were studied by Juris and co-workers.20 There are also various other applications of these complexes reported in the literature.21,22 In this paper, we describe the synthesis and structural characterization of novel LS films composed of all aromatic materials. We investigate the average anisotropy of planar aromatic materials and their organometallic analogues as a function of para substituents. The goal is to provide information to establish general rules for manipulating non-alkylbased oriented films. Experimental Section Synthesis. All aldehydes (Aldrich) were used as received. 4,7-dimethyl-1,10-phenanthroline monohydrate was purchased from GFS chemicals and used as received. Rhenium pentacarbonyl chloride (98%) (Aldrich), 1,2-dichlorobenzene (Aldrich), acetic anhydride (Aldrich), anhydrous potassium acetate (Fischer), and trifluoroacetic acid (Aldrich) were used as received. To remove acidic impurities, chloroform (Fischer) was passed through a column of adsorption grade alumina. 13C and 1H NMR spectra were obtained on a Bruker 250 multiprobe spectrometer. Elemental analyses were performed by Atlantic Microlabs. Mass spectrometry data were provided by the Auburn Mass Spectrometry Laboratory. The 1,10-phenanthroline derivatives were (14) Samha, H.; De Armond, M. K. Langmuir 1994, 10, 4157. De Armond, M. K. Coord. Chem. Rev. 1991, 111, 73. (15) Palacin, S.; Ruaudel-Teixier, A.; Barraud, A. J. Phys. Chem. 1989, 93, 7195. (16) Nostrum, C. F.; Picken, S. J.; Schouten, A.; Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 9957. (17) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsay, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (18) Goldenberg, L. M. J. Electroanal. Chem. 1994, 379, 3. (19) Leidner, C. R.; Sullivan, B. P.; Reed, R. A.; White, B. A.; Crimmins, M. T.; Murray, R. W.; Meyer, T. Inorg. Chem. 1987, 26, 882. Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A.; Higgins, S. J. Chem. Soc., Faraday Trans. 1994, 90, 459. Cook, R. L.; MacDuff, R. C.; Sammells, A. F. J. Electrochem. Soc. 1990, 137, 187. (20) Juris, A.; Campagna, S.; Bidd, I.; Lehn, J.; Ziessel, R. Inorg. Chem. 1988, 27, 4007. (21) Wrighton, M. S.; Wolf, M. O. Chem. Mater. 1994, 6, 1526. Christ, C. S.; Yu, J.; Zhao, X.; Palmore, T. R.; Wrighton, M. S. Inorg. Chem. 1992, 31, 4439. (22) Bard, A. J.; Zhang, X. J. Phys. Chem. 1988, 92, 5566.
Langmuir, Vol. 12, No. 20, 1996 4883 synthesized by modified Perkin reactions,11 and the rhenium complexes were synthesized by the procedure described in the literature.19 4,7-Bis(p-(acetyloxy)styryl)-1,10-phenanthroline (1). 4,7Dimethyl-1,10-phenanthroline (5 mmol) was refluxed with 4-hydroxybenzaldehyde (15 mmol) in the presence of potassium acetate (5 mmol) and acetic anhydride (10 mmol) with a trace amount of iodine for 48 h. Absolute ethanol was used to extract the crude product. Repeated precipitation in hot ethanol yielded the desired product. Yield ) 45%. NMR (CDCl3): s 9.04 (2H), d 9.33-9.36 (2H), d 8.73-8.76 (2H), d 8.15-8.28 (4H), d 7.93 (4H), d 7.32 (4H), s 2.5 (6H). IR (cm-1): 1752, 1583, 1563, 1509, 1427, 1371, 1226, 1197, 1166, 1016, 968, 912, 842, 788. Mass spec (FAB): (M + 1)+ ) 501. UV/vis (nm) CHCl3: (>2 µM) 322, CHCl3/TFA 318, sh 363, 431. Elemental analysis. Found: C, 75.91; H, 4.74; N, 5.61. Calcd: C, 76.77; H, 4.83; N, 5.59. 4,7-Bis(p-nitrostyryl)-1,10-phenanthroline (2). A procedure similar to that for 1 was employed using 4-nitrobenzaldehyde. Yield ) 41%. NMR (CDCl3/TFA-d): d 9.20 (2H), s 8.56 (2H), d 8.37 (4H), 8.33 (2H), d 8.05 (2H), d 7.89 (4H), d 7.73 (2H). IR (cm-1): 1592, 1511, 1340, 1106, 960, 952, 844. Mass spec (EI): M+ ) 474. UV/vis (nm) CHCl3: (>2 µM) 341, CHCl3/TFA 322, 354, 391. Elemental analysis. Found: C, 71.04; H, 3.91; N, 5.80. Calcd: C, 70.86; H, 3.83; N, 5.90. 4,7-Bis(p-cyanostyryl)-1,10-phenanthroline (3). A procedure similar to that for 1 was used with 4-cyanobenzaldehyde. Yield ) >80%. NMR (CDCl3): d 9.22 (2H), s 8.23 (2H), d 7.95 (2H), 7.83 (2H), s 7.75 (8H), d 7.35 (2H), s 7.25 (2H). IR (cm-1): 2219, 1600, 1565, 1554, 1509, 1411, 1328, 1172, 964, 950, 833, 804. Mass spec (EI): M+ ) 434. UV/vis (nm) CHCl3: (>2 µM) 322, CHCl3/TFA 318, 354, 404. Elemental analysis. Found: C, 82.67; H, 4.29; N, 12.74. Calcd: C, 82.94; H, 4.14; N, 12.9. 4,7-Bis(p-(methylcarboxy)styryl)-1,10-phenanthroline (4). A procedure similar to that for 1 was followed with 4-(methylcarboxy)benzaldehyde. Yield ) >80%. NMR (CDCl3): d 9.20 (2H), s 8.25 (2H), d 8.11 (4H), d 7.95 (2H), d 7.83 (2H), d 7.70 (4H), d 7.38 (2H), s 3.96 (6H). IR (cm-1): 1714, 1604, 1565, 1509, 1434, 1415, 1282, 1187, 1110, 1018, 962, 844, 769. Mass spec (EI): M+ ) 500. UV/vis (nm) CHCl3: (>10 µM) 322, CHCl3/TFA 318, 363, 418. Elemental analysis. Found: C, 76.79; H, 4.83; N, 5.60. Calcd: C, 76.77; H, 4.83; N, 5.59. [4,4′-Bis(4-(acetyloxy)styryl)-2,2′-bipyridine]rhenium Tricarbonyl Chloride (5). Rhenium pentacarbonyl chloride (0.415 mmol) and 0.458 mmol of 4,4′-bis(4-(acetyloxy)styryl)-2,2′bipyridine11 were refluxed in 50 mL of 1,2-dichlorobenzene for 11 h and stirred for 1 h during cooling. The initially formed orange solution turned into a yellow precipitate. The solid was allowed to settle, filtered and washed with 3 × 10 mL portions of ethyl ether. The reaction vessel was always covered with aluminum foil to avoid any possible photochemical decomposition of the complex. Yield: 55%. NMR (CDCl3): d 8.56 (2H), s 8.36 (2H), d 7.77 (4H), d 7.53 (2H), d 7.27 (4H), d 6.95 (2H), d 6.75 (2H), s 2.37 (6H). Mass spec (FAB): M+ ) 782. IR (cm-1): 2017, 1916, 1891, 1756, 1610, 1506, 1427, 1371, 1218, 1195, 1166, 1014, 989, 916, 846. UV/vis (nm) CHCl3: (>10 µM) sh 290, 335, sh 408. Elemental analysis. Found:; C, 50.74; H, 3.16; N, 3.53; Cl, 6.0. Calcd (including 0.18 units of solvent): C, 50.61; H, 3.06; N, 3.46; Cl, 5.96. [4,4′-Bis[4-(methacryloyloxy)styryl]-2,2′-bipyridine]rhenium Tricarbonyl Chloride (6). A procedure similar to that for 5 was used with the ligand 4,4′-bis[4-(methacryloyloxy)styryl]2,2′-bipyridine.23 After the cooling period, a yellow solution was obtained. The complex was precipitated out by adding 100 mL of hexane. The yellow precipitate obtained was filtered, washed with 3 × 10 mL portions of ether, and dried. Yield ) 64%. Mass spec (FAB): (M+) ) 834. IR (cm-1): 2017, 1916, 1893, 1731, 1612, 1596, 1506, 1455, 1423, 1317, 1294, 1205, 1165, 1124, 1014, 973, 946. UV/vis (nm) CHCl3: (>10 µM) sh 305, 327, sh 408. Elemental analysis. Found: C, 52.80; H, 3.45; N, 3.35; Cl, 7.31. Calcd (including 0.4 units of solvent): C, 52.90; H, 3.13; N, 3.31; Cl, 7.15. [4,7-Bis(4-(acetyloxy)styryl)-1,10-phenanthroline]rhenium Tricarbonyl Chloride (7). The procedure was similar to that used for 5 with the ligand 4,7-bis(4-(acetyloxy)styryl)(23) Balasubramanian, K. K.; Cammarata, V. Langmuir 1996, 12, 2035.
4884 Langmuir, Vol. 12, No. 20, 1996 1,10-phenanthroline. Yield ) 63%. Mass spec (FAB): (M+) ) 806. NMR (CDCl3): d 9.2 (2H), s 8.1 (2H), d 7.8 (2H), d 7.6 (4H), d 7.5 (2H), d 7.37 (2H), d 7.22 (4H), 2.25 (6H). IR (cm-1): 2017, 1893, 1764, 1621, 1592, 1562, 1508, 1427, 1371, 1209, 1166, 1014, 966, 914, 846. UV/vis (nm) (>10 µM) CHCl3: 308, 356. Elemental analysis. FOund: C, 51.05; H, 2.94; N, 3.53; Cl, 4.40. Calcd: C, 52.17; H, 3.00; N, 3.48; Cl, 4.40. Langmuir Trough and FTIR Spectroscopy Experiments. A symmetric compression KSV3000 Langmuir-Blodgett trough was used to perform LS experiments. Chloroform or a 10% v/v TFA/CHCl3 mixture was used as the spreading solvent. The typical concentration used in all the experiments was 1 mg/mL. The evaporation time was about 15-30 min before the compression. The horizontal lifting technique was used to transfer the monolayers from the air-water interface to various substrates.24 All the other parameters used were the same as in our previous paper.11 Spectroscopy. Au-coated borosilicate glass was used as the substrate for grazing angle incidence experiments. Quartz microscopic slides were the substrates for UV/vis experiments and Ge ATR crystals were used for ATR experiments. Polarized transmission spectra were recorded by mounting the Ge crystal onto a KBr pellet holder. Different polarizations were obtained by using a Cambridge Sciences KRS-5 substrate/0.12 µm aluminum wire grid polarizer. The specific conditions and the instrumentation are described elsewhere.11 For the tilt angle calculations the peak heights over the baseline were used. Baseline corrections were done to the spectra reported, but no smoothing or filtering was done.
Results and Discussion Syntheses. Four new compounds, 1-4, based on distyrylphenanthrolines were synthesized with different functional groups at the para position of the styryl functionality. All the compounds were synthesized via a modified Perkin reaction11 with yields about 50% for all the reactions. All the compounds were characterized by IR, NMR, UV/vis, elemental analysis, and mass spectrometric methods. Two rhenium tricarbonyl chloride complexes 5 and 6 based on styryl bipyridines and another complex based on distyryl-1,10-phenanthroline, 7, were also synthesized and characterized as above. Dichlorobenzene was used as the solvent in this reaction, and the yields were similar for all the complexes made (∼60%). Satisfactory NMR results were obtained for all the compounds with CDCl3 or TFA-d as the solvent. Mass spectra of all the compounds showed the molecular ion peaks, except for that of 1, which showed the (M + 1)+ peak at 501. Satisfactory elemental analysis results were obtained for all the compounds except that complexes 5 and 6 showed a higher percentage of Cl than the theoretical value. The experimental value agreed better with the theoretical value when a percentage of the solvent molecules was included in the calculation. The bulk infrared spectra in KBr pellets showed the expected vibrational frequencies for all compounds based on our previous work.11 The solubility of these compounds varied with the substitution. The (acetyloxy)-, cyano-, and (methylcarboxy)phenanthroline derivatives were soluble in chloroform. The nitro derivative needed 15% TFA in chloroform to increase the solubility to the millimolar level. This concentration is necessary for our Langmuir film fabrication work, since very low concentrations require large volumes of spreading solution and increase the possibility of trace contamination.25 The increase in solubility with addition of TFA is due to the protonation of pyridine nitrogens.26 All the metal complexes were soluble in CHCl3 up to the millimolar level. The increase in solubility may (24) Langmuir, I.; Schaefer, V. J. J. Am. Chem. Soc. 1938, 60, 1351. (25) Gaines, G. L., Jr., Insoluble Monolayers at Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966. (26) Summers, L. A. Adv. Heterocycl. Chem. 1984, 35, 281.
Balasubramanian and Cammarata
Figure 1. Pressure-area isotherms at 6 °C of compounds 1 and 6. The spreading solutions were 1 mg/mL in chloroform onto a 18 MΩ H2O subphase. Table 1. Mean Molecular Areas of Substituted 1,10-Phenanthrolines and Rhenium Carbonyl Complexes R
MMA (Å2)
temp (°C)
OCOCH3 (1) NO2 (2) CN (3) COOCH3 (4) (Re bpy)-OCOCH3 (5) (Re bpy)-OCOC(CH3)CH2 (6) (Re phen)-OCOCH3 (7)
42 ( 2 42 ( 1 42 ( 2 45 ( 1 50 ( 2 59 ( 1 65 ( 1
6 6 6 6 6 6 6
be due to the weaker π-stacking intermolecular forces in these complexes. The diameter of the metal carbonyl group is larger than the thickness of the aromatic rings. Langmuir Isotherms and Depositions. All four of the phenanthroline derivatives 1-4 formed stable Langmuir films at the air-water interface. Figure 1 shows an example of a pressure-area isotherm of 1. Upon compression, the film pressure increases until such a point that the film appears to break. We use the extrapolation of the linear portion to the baseline as a measure of the mean molecular area. Table 1 gives the mean molecular areas of all the compounds studied in this work. In our previous work,11 all the bipyridines had mean molecular areas of 30-35 Å2. Molecular models predict that the smallest projected mean molecular area for the transoid conformation of the bipyridine ring is 32 Å2 and that for the cisoid conformation is 45 Å2. We proposed earlier that the bipyridines assume the transoid conformation at the air-water interface. These results are consistent with our previous proposal, since all the phenanthroline derivatives show extrapolated mean molecular areas about 40-45 Å2, in agreement with the mean molecular area obtained for the cisoid bipyridine. These molecules behave in a manner similar to the bipyridine systems in that the compression curve does not trace the expansion curve.11 When these films are compressed and held at a constant pressure (Π ) 15 dyn/cm), the film area decreases less than 5% per hour. This result shows that the films are stable at the air-water interface and do not solubilize or collapse. These films can be transferred intact to various substrates, except the cyano derivative of phenanthroline, which hydrolyzed during the Langmuir work. In these films we do not observe a cyano stretch but observe a carbonyl stretch in the LS film (see below). Transfer to various substrates such as Au/glass, quartz, and Ge ATR crystals was done using the horizontal lifting process.24 Table 2 shows the transfer ratio for all the phenanthroline compounds onto various substrates. We observe that the transfer ratios of these molecules for all the substrates are approximately 1.0 and that the single-layer transfers were consistent up to 45 layers for compound 1.
Headgroup Influence on Anisotropy in LS Films
Langmuir, Vol. 12, No. 20, 1996 4885
Table 2. Transfer Ratios to Various Substrates R
Au/glass
Ge (GeOx)
quartz
OCOCH3 (1) NO2 (2) COOCH3 (4) OCOCH3 (Re bpy) (5) OCOC(CH3)CH2 (Re bpy) (6) OCOCH3 (Re phen) (7)
0.96 ( 0.09 1.00 ( 0.07 1.00 ( 0.08 1.08 ( 0.10 1.00 ( 0.10
0.96 ( 0.06 1.25 ( 0.15 0.98 ( 0.08 1.06 ( 0.08 1.03 ( 0.11
1.18 ( 0.09 1.02 ( 0.09 0.93 ( 0.02 0.92 ( 0.05 0.86 ( 0.08
1.03 ( 0.06
0.92 ( 0.03
1.16 ( 0.08
Figure 3. UV/vis spectra of thin films of 4 on quartz slides: (top) LS film of a six-layer thick deposited at Π ) 15 mN/m; (bottom) film evaporated from concentrated chloroform solution.
Figure 2. UV/vis spectra of solution of 4: (top) 1.2 × 10-5 M concentration in a 1 cm path length cell; (bottom) 5.7 × 10-8 M concentration in a 10 cm path length cell.
All the rhenium complexes, 5-7 were soluble in CHCl3, could be applied to the air-water interface, and formed steep pressure-area curves. Figure 1 shows the pressure-area isotherm of 6. Extrapolation of the pressurearea isotherms gave mean molecular areas ranging from 50 to 65 Å2 (Table 1). The higher value of the mean molecular area compared to those of the bipyridine and phenanthroline ligands is due to the larger size of the metal carbonyls attached to them. Thus the “thickness” of the phenanthroline or bipyridine ring limits the area of films composed solely of those heterocycles. These complexes also formed stable Langmuir films upon constant pressure, and transfer was accomplished with transfer ratios of approximately 1.0 (Table 2). UV/vis. The solution UV/vis spectra of 4 are shown in Figure 2. The top portion is the solution spectrum of 4 in chloroform at 12 µM. The band maximum was centered at 322 nm at this concentration, and upon dilution (Figure 2 bottom), a red shift occurs with the lowest energy band absorbing at 372 nm. This is an indication of the presence of H aggregates. Compounds 1, 3, and 4 show similar aggregation behavior where 2 does not show a shift with dilution. This phenomenon where the nitro derivative does not aggregate was also seen in the bipyridine system.11 The detailed account of the UV/vis spectra of these compounds in solution will be reported elsewhere.27 The UV/vis spectra of thin films of 4 are shown in Figure 3. The top portion of the figure is a six-layer LS film of 4 transferred to a quartz slide, and the bottom portion is an isotropic film formed by the evaporation of a chloroform solution of 4. Comparing Figures 2 and 3, it can be clearly seen that the thin film spectra match the H aggregated solution spectrum. The process of spreading the solution of 4 onto the Langmuir trough and reorganization through compression leads to the same aggregation that exists in the chloroform solution. Since H aggregation occurs in the solution before application to the trough and after LS deposition onto the quartz, we conclude that the aggrega(27) Balasubramanian, K. K.; Cammarata, V. Macromolecules, in preparation.
tion is strong enough to remain throughout the LS film formation process. This aggregation phenomenon within the Langmuir film explains the irreversibility of the compression-expansion curve. Compression of smaller domains into larger ones results in a contiguous film while the reverse process (on a fast time scale) fractures the film instead of expanding it. This results in the immediate decrease of the pressure. Recompression traces the original expansion/fracture curve. The evaporated film also shows a shoulder at ∼404 nm that is not observed in either the solution spectra or the LS film spectra. We interpret this as formation of J aggregates upon evaporation of solvent. The formation of both J and H aggregates on the surface, we believe, precedes three-dimensional crystallite formation. Interestingly enough little or no J aggregation is observed in the LS film, implying that interlayer interactions are insignificant relative to intralayer interactions. We have observed mixed aggregation in thin films previously in our bipyridine systems and speculate that the interlayer interactions may be stronger.11 FTIR Spectroscopy. Three different infrared techniques were used to study the transferred LS films: transmission, grazing angle reflectance (GIR), and attenuated total reflectance (ATR) IR. For the phenanthrolines 1, 2, and 4 we focused our analysis on the characteristic bands of the substituents and the common bands of the distyrylphenanthroline backbone. Carbonyl stretches at 1720 and 1760 cm-1 were the main focus for 1 and 4, and for 2, the asymmetric and symmetric stretches of the nitro group were observed at 1517 and 1344 cm-1, respectively. The peaks at ∼1590 cm-1 were assigned to the ring breathing mode along the pyridine axis. For the complexes, the three very strong metal carbonyl peaks were observed at 2017, 1910, and 1890 cm-1. Defining the molecular plane as the plane containing the aromatic rings, the single band appearing at 2017 cm-1 is assigned to the stretching vibration of the out of plane carbonyl group and the doublet at 1916 and 1893 cm-1 is assigned to the symmetric and asymmetric stretches of the two in-plane carbonyls, respectively. The assignments of primary bands are given in Table 3. The laboratory coordinates are defined so that the y-axis refers to the plane of the film/substrate in the direction that the float moved on the trough, the x-axis is the other coordinate perpendicular and in the plane, and the z-axis is perpendicular to the plane of the film/substrate.11 Transmission IR. In the polarized transmission IR, 0° and 90° polarization represent the electric field vectors in the x and y film directions, respectively. By comparing the intensities from both polarizations, one can obtain
4886 Langmuir, Vol. 12, No. 20, 1996
Balasubramanian and Cammarata
Table 3. ATR-FTIR Results of Distyrylphenanthrolines and Rhenium Carbonyl Complexes cm-1
reda (5)
remma (6)
rephda (7)
band assignment
2017
58 ( 2
68 ( 6
64 ( 1
1916
59 ( 1
72 ( 1
65 ( 4
1893
62 ( 1
86
67 ( 4
out of plane Re carbonyl sym in-plane Re carbonyl asym in-plane Re carbonyl CdO str CdO str CdO str CdC str along pyr asy NO2 phO ph oblique str CH3 sym sym NO2 C-O-ph bnd C-O-ph bnd C-O-ph bnd ? ? HCdCH op bnd in-plane anisotropy
1760 1733 1720 1606 1594 1517 1508 1432 1369 1342 1280 1189 1166 1110 1016 964 Ax/Ay a
phda (1)
phdn (2)
phes (4)
65 ( 3
66 ( 1
62 ( 3 82 ( 6
64 ( 1 64 ( 2 70 ( 3 77 ( 2 71 ( 2 66 ( 2 67 ( 3
50 ( 5 67 ( 4
61 ( 1 59
69 ( 5a 65 ( 3a
66 ( 4 59
82 ( 4
68 ( 4
62 ( 5 63 ( 5
68
77 ( 1 58 ( 7 59 ( 5
74 ( 2 75 ( 1
58 ( 4 55 ( 4
67 ( 4 69 ( 5 69 ( 5
68 ( 4 69 ( 4
0.98
1.11
1.03
61 ( 6 70 ( 4 70 ( 6 1.07
1.10
54 ( 5 1.00
Peaks overlap significantly.
Figure 4. Polarized transmission spectra of a 12-layer LS film of 6 on a Ge crystal. The deposition conditions were Π ) 15 mN/m and 6 °C: (top) 0° polarized spectrum corresponding to the electric field vector in the x direction; (bottom) 90° polarized spectrum corresponding to the electric field vector in the y direction.
information about the in-plane anisotropy. In our previous work on the bipyridine system, we observed no in-plane anisotropy.11 Figure 4 shows the polarized spectra of a 12-layer LS sample of 6 deposited at Π ) 15 mN/m. Using eq 1, we can calculate
Dx/y ) Ax/Ay
(1)
Dx/y is the in-plane dichroic ratio, where Ax and Ay are the absorbances of a given band at 0° and 90°, respectively. This ratio, Dx/y, is a measure of the in-plane anisotropy and is presented in Table 3. The dichroic ratios of the most intense bands varied slightly and were numerically averaged to obtain the average in-plane anisotropy. This average was incorporated into the calculations of the ATRIR experiments below. Although this anisotropy is small, it is significant for analyzing the data from the ATR experiments. The conclusion is, in both phenanthroline and rhenium tricarbonyl chloride complex systems, that the films were not isotropic within the film plane.
It is interesting that the bipyridine systems showed no in-plane anisotropy while it is observable in the phenanthroline systems. Michl and Thulstrup proposed that the critical parameter in aligning molecules upon stretching was the aspect ratio.28 Our proposed transoid structure perpendicular to the surface for bipyridines and the cisoid structure for phenanthrolines would have in-plane molecular aspect ratios of 2 and 2.8, respectively. We propose that the increased aspect ratio of the phenanthroline derivatives accounts for the observable in-plane anisotropy. The key point is compression as well as expansion (stretching) can lead to anisotropic behavior. Only in a few systems has in-plane anisotropy been measured.6,29 ATR-FTIR Spectroscopy. In ATR-IR spectroscopy, the absorption spectrum of a sample is recorded by contacting the sample with a transparent material of high refractive index. If the light is introduced into the higher refractive index medium, at the proper angle, it can be totally internally reflected. The interaction of the evanescent surface wave with the sample material results in a decrease of energy of certain frequencies characteristic of the sample material. Harrick originally developed fundamental theories of ATR spectroscopy.30 Using the same treatment described in our previous work,6,11 average tilt angles for various vibrational dipoles within the molecular films were derived. Similar analyses for inplane anisotropic films have been proposed by other investigators.31 Figures 5 and 6 show the polarized spectra of a randomly oriented sample of 6 produced by evaporating a chloroform solution and a six-layer sample deposited with the LS technique, respectively. Consistent with the theory for a random sample, all the bands at x + z polarization were of higher intensity than those at y polarization. The dichroic ratios varied from ∼1.2 to 1.4, which is close to the theoretical value (1.3) for a film with a refractive index of ∼1.5. Table 3 gives the average tilt (28) Michl, J.; Thulstrup, E. W. Acc. Chem. Res. 1987, 20, 192. (29) Sauer, T.; Wegner, G. Mol. Cryst. Liq. Cryst. 1988, 162B, 97. Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357. Kalachev, A. A.; Sauer, T.; Vogel, V.; Plate, N. A.; Wegner, G. Thin Solid Films 1990, 188, 341. (30) Harrick, N. J. Internal Reflection Spectroscopy; John Wiley & Sons, Inc.: New York, 1967. (31) Ahn, D. J.; Franses, E. I. J. Phys. Chem. 1992, 96, 9952. Ahn, D. J.; Franses, E. I. Thin Solid Films 1994, 244, 971.
Headgroup Influence on Anisotropy in LS Films
Figure 5. Polarized ATR-IR spectra of a six-layer LS film of 6 on a Ge ATR crystal. The deposition conditions were Π ) 15 mN/m and 6 °C: (top) 0° polarized spectrum corresponding to the electric field vector in the x and z directions; (bottom) 90° polarized spectrum corresponding to the electric field vector in the y direction.
Langmuir, Vol. 12, No. 20, 1996 4887
Figure 7. Grazing angle IR spectra of the compound 6: (top) film evaporated from chloroform solution onto Au-coated glass slides; (bottom) six-layer LS film deposited at Π ) 15 mN/m onto Au-coated glass slides.
measure of the tilt angles for all the bands, we use eq 2,11 random 2 Arandom /A2017 ) µ2x cos2 54.7°/µ2017 cos2 54.7° x LS 2 2 2 2 assumed ALS x /A2017 ) µx cos θ/µ2017 cos θ2017
Figure 6. Polarized ATR-IR spectra of a thin film of 6 evaporated from chloroform solution onto a Ge ATR crystal: (top) 0° polarized spectrum corresponding to the electric field vector in the x and z directions; (bottom) 90° polarized spectrum corresponding to the electric field vector in the y direction.
angles calculated for different vibrational dipoles in the LS films obtained. No calculations were done for transferred films of 3, since we believe it hydrolyzed during the Langmuir work. Two new peaks were observed at 1725 and 1280 cm-1 that are not present in the cast film or KBr spectra but are consistent with the carboxylic derivative of the distyrylphenanthroline compound. Grazing Angle Reflectance IR (GIR). According to the fundamental theory, GIR spectroscopy only couples reflected radiation to vibrational dipoles perpendicular to the substrate,4,32 that is, in our coordinate system, the z direction. Comparison of GIR spectra of randomly oriented, cast films of 6 with a four-layer LS film of the same compound is shown in Figure 7. Qualitatively, we determined that the metal carbonyls are relatively perpendicular to the substrate compared to the other vibrational dipoles, since their relative intensity was much larger compared to the other peaks. To get a quantitative (32) Chollet, P. A.; Messier, J. J. Chem. Phys. 1982, 73, 235.
(2)
random where, ALS are the absorbances of LS and x and Ax random samples of a band at frequency x whose tilt angle is to be calculated. One needs to know or assume one angle to use this equation to calculate the other tilt angles. In this analysis, we assumed an angle from the previous ATR experiments. From Table 3, it can be seen that the metal carbonyl peak at 2017 cm-1 is 68°. Table 4 shows the calculated tilt angles for various vibrations in the LS film of 6 and for the other compounds. Comparing Tables 3 and 4, it can be seen that the tilt angles obtained by both GIR and ATR experiments are in general agreement. There seems to be a disagreement in the tilt angles if the peaks overlap significantly, as in 6 with the 1606 and 1594 cm-1 bands. In the case of 2, the number of prominent peaks is very few; however, anisotropy is observed in both types of spectroscopy. Comparing the results from both GIR and ATR experiments, the anisotropy of these compounds differed with respect to the substitution. It varied from highly anisotropic to totally isotropic. Compounds 6 and 2 showed the largest anisotropy while the other compounds varied from moderately anisotropic for compound 1 and 7 to close to totally isotropic for compounds 4 and 5. Similar observations of variation in anisotropy with respect to substitution were made for our bipyridine system.11 There the order of anisotropy was NO2 ≈ O2CCH3 > CO2CH3.11 The similarity in order can be attributed to either organization by immersion of the substituent into the aqueous phase or the intermolecular forces, most likely due to dipole-dipole coupling, being stronger for some substituents than others, changing the degree of surface aggregation. In comparing the rhenium complex of 4,4′-bis[p-(methacryloyloxy)styryl]-2,2′-bipyridine with the ligand itself,23 films made from both have a large degree of anisotropy. Since we do not observe aggregation for the rhenium complexes,27 we argue that the immersion of the substituent into the subphase has the most influence on the anisotropy. Interestingly the complex does not have a phase transition in the isotherm similar to that of the pure ligand, which may support the delicate interplay
4888 Langmuir, Vol. 12, No. 20, 1996
Balasubramanian and Cammarata
Table 4. GIR-FTIR Results of Distyrylphenanthrolines and Rhenium Carbonyl Complexes cm-1
reda (5)
remma (6)
rephda (7)
band assignment
2017
52 ( 4
68 ( 1
62
1916
59
72
65
1893
62 ( 3
out of plane Re carbonyl sym in-plane Re carbonyl asym in-plane Re carbonyl CdO str CdO str CdO str -CdC str along pyr asy NO2 phO ph oblique str CH3 sym sym NO2 C-O-ph bnd C-O-ph bnd C-O-ph bnd ? ? HCdCH op bnd
1760 1733 1720 1606 1594 1517 1508 1432 1369 1342 1280 1189 1166 1110 1016 964 a
phda (1)
phdn (2)
phes (4)
65
70
63 ( 2
69
62 ( 5 64 78 ( 1 78 ( 1 61 ( 3 67 ( 4 65 ( 2
65 ( 4
77 ( 2 67 ( 5a 71 ( 4a
77 ( 3a 81 ( 1a
67a
56 ( 4 68 ( 2 59 ( 2
78 ( 2
70
62 ( 3 66 ( 3
78 ( 1 79 ( 1 79 ( 1
67
77 57 ( 5 66 ( 3 69 ( 4 62 ( 4 59 ( 4 63 ( 6
69 ( 2 76 ( 1
70 72
Peaks overlap significantly.
between area limitations of the substituent and the aromatic ring.23 Future work includes surface potential measurements to better understand the film structure at the air-water interface. Conclusions Four compounds based on 4,7-bis(p-substituted-styryl)1,10-phenanthroline and three metal carbonyl complexes based on 4,7-bis(p-substituted-styryl)-1,10-phenanthroline and 4, 4′-bis(p-substituted-styryl)-2,2′-bipyridine were synthesized and characterized. All formed stable Langmuir films which could be transferred onto various substrates with transfer ratios close to 1.02 ( 0.10. From UV/vis spectroscopy of solutions and thin films, it was found that the predominant intermolecular interactions present in the phenanthroline derivatives arise from H aggregates. Mixture of H and J aggregation was observed in evaporated thin films but not in LS films. From various
FTIR techniques, it was found that [4,4′-bis[p-(methacryloyloxy)styryl]-2,2′-bipyridine]rhenium tricarbonyl chloride and 4,7-bis(p-nitrostyryl)-1,10-phenanthroline showed maximum anisotropy in the LS films. The orientation of the other compounds varied from moderately anisotropic to totally isotropic. In-plane anisotropy was observed for all these compounds, and it was attributed to the higher aspect ratios compared to those of the bipyridine compounds. It is concluded that the net anisotropy of these materials in LS films is greatly influenced by the functional groups. Acknowledgment. We would like to acknowledge Dr. George Goodloe for obtaining the mass spectral data. We would like to acknowledge partial financial support from The Petroleum Research Foundation and the Department of Energy. LA9603232
