298
Langmuir 1988,4, 298-304
Langmuir-Blodgett Films of Donor-Urethane-TCNQ and Related Moleculest,* R. M. Metzger," R. R. Schumaker, and M. P. Cava Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487-9671
R. K. Laidlaw,$ C. A. Panetta, and E. Torres Department of Chemistry, University of Mississippi, University, Mississippi 38677 Received July 28, 1987 New molecules of the type D-a-A have been shown to form Pockels-Langmuir monolayers at the air-water interface and Langmuir-Blodgett f i i s over glass or Al substrates (D, oneelectron donor; a, covalent bridge (urethane);A, strong one-electron acceptor BHTCNQ or weak two-electron acceptor HMTCAQ). Assemblies M11D-a-AIM2(Ml, Mz, conventional metallic thin films; D, strong one-electron donor; A, strong one-electron acceptor) had been predicted in 1973 to be one-molecule-thick rectifiers of electrical current. When A is BHTCNQ, monolayers are formed if D is either pyrene (strong donor) (area per molecule at film collapse, A, = 53 A2/molecule;differential surface tension at collapse, n, = 28.2 mN/m), (dodecy1oxy)phen 1 (weak n, = donor) ( A , = 50 A2, I'I, = 20.2 mN/m), or (bis(dodecy1)amino)phenyl (medium donor) (A, = 54 45.9 mN/m). When A is HMTCAQ, monolayers are formed if D is (bis(dodecy1)amino)phenyl ( A , = 58 A2, n, = 22.3 mN/m). Thus, greasy tails are found to be generally helpful in orienting D-a-A molecules in monolayers (even though they may slow down electron conduction through the molecules).
H2,
I. Introduction The formation of monolayers and Langmuir-Blodgett films of molecules of the type donor-urethane-acceptor is reported below. From the experience gathered in this study, it is hoped that rectification of electrical current may be observed across monolayers of such molecules. In 1973 TTFTCNQ, the 1:l salt of tetrathiafulvalene (TTF, 1)lwith 7,7,8,8-tetracyanoquinodimethan(TCNQ, 2)2was found to have high electrical condu~tivity.~Since then, the field of lower dimensional organic conductors has grown rapidly, and several significant milestones have been established: single-crystal maximum conductivities of up to lo00 S/cm, superconductivity with critical temperatures as high as 8 K, and polymers with promising potential applications (lightweight batteries, photovoltaics, et^.).^ In 1973, Ari Aviram and collaborators at IBM Research (Yorktown) suggested5 that electrical rectification might be observed in organized assemblies of compounds of the type TTF-bicyclooctane-TCNQ (3) or, more generally, of the type D-PA, where D represents an organic one-electron donor, A an organic one-electron acceptor, and a a covalent, nonconjugated a bridge. If these D-a-A compounds could be organized as a monolayer between two conventional metal films M1 and M2 as ordered structures M11D-a-AIM2(4), then the asymmetry of the molecular orbitals of the D-a-A molecules would ensure that electron transfer is preferred in one direction over the other. The Aviram proposal was studied theoretically by Aviram and Ratner6 and reviewed since that time.' Our experimental progress toward the realization of Aviram's idea has been reported elsewhere.8-18 'Dedicated to Harden Marsden McConnell on the occasion of his 60th birthday. Supported in part by the National Science Foundation, Grants DMR-84-17563and CHE-86-07458. Current address: Department of Chemistry, University of Alabama, Tuscaloosa, AL 35487-9761. Permanent address: Science Department, Laramie County Community College, Cheyenne, WY
*
82007--3299.
0743-7463/88/2404-0298$01.50/0
,N C ,
CN
4
If all conditions for synthesis and assembly of device 4 can be met, then the mechanism for rectification is6p9 forward bias: go MlID-a-AIM2+ M1-ID+-a-A-IM2+ la -ITF+ M~-ID-u-AIM~+ (1) lb reverse bias: no go M1ID-a-AIM2 -XXX+ M~+ID--u-A+IM~- (2) reverse bias: go? M~ID-u-AIM~ -1TR ---* M1ID+-a-A-IMz 3a
(1) Wudl, F.; Kaplan, M. L.; Hufnagel, E. J.; Southwick, E. W., Jr. Chem. Commun. 1970, 1453-1454. (b) Cooper, W. F.; Kenny, N. C.; Edmonds, J. W.; Nagel, A.; Wudl, F.; Coppens, P. Chem. Commun. 1971, 889-890. (c) Melby, L.R.; Hartzler, H. D.; Sheppard, W. A. J.Org. Chem. 1974,39, 2456-2458. (d) Wudl, F.; Kaplan, M. L.; Hufnagel, E. J.; Southwick, E. W., Jr. J. Org. Chem. 1974,39,3608-3609. (2)(a) Acker, D. S.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Melby, L. R.; Benson, R. E.; Mochel, W. E. J. Am. Chem. SOC.1960, 82, 1962,84, 6408-6409. (b)Acker, D.S.; Hertler, W. R. J . Am.Chem. SOC. 337c-3374. (3)(a) Ferraris, J.; Cowan, D. 0.; Walatka, V., Jr.; Perlstein, J. H. J . Am.Chem. Sac.1973,95,948-949.(b) Miles, M. G.; Wilson, J. D.; Cohen, M. H. US.Patent 3799814,Dec 18, 1973. (c) Coleman, L. B.; Cohen, M. J.; Sandman, D. J.; Yamagishi, F. G.; Garito, A. F., Heeger, A. J. Solid State Commun.1973,12,1125-1132. (4)See, e.g.: Synth. M e t . 1987,17-19. (5)Aviram, A.; Freiser, M. J.; Seiden, P. E.; Young, W. R. U.S. Patent 3 953 874,April 27, 1976. (6)Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974,29,277-283. (7)Aviram, A.; Seiden, P. E.; Ratner, M. A. In Molecular Electronic Deuices;Carter, F. L., Ed.; Marcel Dekker: New York, 1982,p 5. (8) Metzger, R. M.; Panetta, C. A. J . Phys. (Les Ulis, Fr.) 1983,44, C3-1605-C3-1611.
0 1988 American Chemical Society
LB Films of Donor-Urethane-TCNQ
Langmuir, Vol. 4, No. 2, 1988 299
Under zero or moderate forward electrical bias, electron transfer (ET) can occur by elastic, through-space (TS)21 tunneling from D to MI and from Mz to A (eq la). Next, a through-bond (TB)19tunneling (inelastic tunneling) of the electron (call it ITF) can occur from A- to D', thus effecting charge separation (eq lb). The reverse process (eq 2) is unlikely, since D is a poor one-electron acceptor and A is a poor electron donor. Indeed, in the gas phase the energy required to form the ions TTF+(g) and TCNQ-(g) a t infinite mutual separation is known to require I D - AA = 6.83 - 2.8 = 4.0 eV, whereas the cost of forming the ions T T F ( g ) and TCNQ+(g) is estimated a t over 9 eVa9 Under reverse bias, D-a-A may form the zwitterion by an inelastic tunneling, ITR (eq 3a). If eq l b is more likely than eq 3a, and also more likely than TS tunneling from M1 to Mz, or from Mz to M1, then rectification will occur.6 The appeal of the organic rectifier is its proposed size: the proposed molecule 4 is only 2 nm long, so one could conceive of a M11D-a-AJM2sandwich of thickness (working length) 5 nm, which is much shorter than the 100-500-nm design rules used in state-of-the-art inorganic Si or GaAs electronic devi~es.~J' In our experimental approach to the Aviram-Ratner rectifier we assumed that the best technique for its assembly is the Langmuir-Blodgett film technique.m For this, the D-a-A molecule must be an ampiphilic molecule, i.e., either the D end or the A end is made hydrophilic, the opposite end must be hydrophobic. On a Langmuir trough, or film balance, the amphiphilic molecules self-assemble as a monolayer (hereinafter Pockels-Langmuir (PL) mon~layer);'~ at maximum reversible compression (at a differential surface tension n,) the monolayer will occupy minimum surface area Ac.19 The monolayer must transfer well (Le., with a transfer ratio close to 1.00) to a solid substrate, thus forming a Langmuir-Blodgett (LB) film.20 Another method for assembly of the rectifier is binding the D-a-A molecule through silane bonds to a solid sub~ t r a t e . ~ ~ ~ ~ ~ The synthetic and device assembly criteria for the organic rectifier can be s ~ m m a r i z e d ~as~ folrows: ~J~ (9) Metzger, R. M.; Panetta, C. A. In Molecular Electronic Devices; Carter. F. L.. Ed.: Marcel Dekker: New York. 1987: Vol. 11. D 5. (10) Panetta, C. A.; Baghdadchi, J.; Metzger, R. M.Mol. 'dryst. Liq. Cryst. 1984,107, 103-113. (11) Metzger, R. M.; Panetta, C. A,; Heimer, N. E.; Bhatti, A. M.; Torres. E.: Blackburn. G. F.:. Trbathv. - S. K.: Samuelson. L. A. J.Mol. Electron. 1986. 2. 116124. (12) Metzger, R. M.; Panetta, C. A.; Miura, Y.; Torres, E. Synth. Met. 1987. is. iw-802. - (13) Torres, E.; Panetta, C. A.; Metzger, R. M. J. Org. Chem. 1987,52, 2944-2945. (14) Laidlaw, R. K.; Miura, Y.; Panetta, C. A.; Metzger, R. M. Acta Crystallogr., Sect. C: Cryst. Struct. Comqun., in press. (15) Metzger, R. M.; Panetta, C. A. Proceedings of the Eighth Winter Meeting on Low-Temperature Physics; Cuernavaca, Mexico, January 1987, in press. (16) Laidlaw, R. K.; Baghdadchi, J.; Panetta, C. A.; Miura, Y.; Torres, E.; Metzger, R. M. Acta Crystallogr.,Sect. E Struct. Crystallogr. Cryst. Chem., in press. (17) Miura, Y.; Laidlaw, R. K.; Panetta, C. A.; Metzger, R. M. Acta Crystallogr., Sect. C: Cryst. Struct. Commun., in press. (18) Metzger, R. M.; Laidlaw, R. K.; Torres, E.; Panetta, C. A., submitted for publication in J. Crystallogr. Spectrosc. Res. (19) See, e.g.: Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (20) (a) Blodgett, K. B. J . Am. Chem. SOC.1935,57, 1007-1022. (b) Blodgett, K. B.; Langmuir, I. Phys. Reu. 1937,51,964-982. (c) Kuhn, H.; Moebius, D.; Buecher, H. In Techniques of Chemistry. Vol. I. Physical Methods of Chemistry. Part V. Determination of Thermodynamic and Surface Properties; Weissberger, A., Rossiter, B. W., Eds.; Wiley-Interscience: New York, 1972, pp 577-702. (d) Kuhn, H. Pure Appl. Chem. 1979,51,341-352. (e) Kuhn, H. Pure Appl. Chem. 1981,53,2105-2122. (0 See, e.g.: Thin Solid Films 1980, 68, 1983, 99, 1985, 132-134. (21) Hoffman, R. Acc. Chem. Res. 1971,4, 1-9. (22) Murray, R. W. Acc. Chem. Res. 1980,13, 135-141. (23) Maoz, R.; Sagiv, J. Thin Solid Films 1985, 132, 135-151. I
(1)The donor D must have a low I D (C7 eV) and must be fairly flat. (2) The acceptor A must have a high AA (>2 eV) and must be fairly flat. (3) The a bridge must be long enough to prevent the scrambling of the molecular orbitals of D with those of A (greater than three carbon-like atoms) yet short enough to prevent the curling of the A end over the D end (less than nine carbon-like atoms): thus, five to six carbon-like atoms seems optimal. The a bridge must be largely saturated (so as to prevent scrambling through conjugation) and fairly flat (for good lateral LB packing). (4) The D-a-A molecule must pack well in monolayers. The lateral T-T attractive interactions, and maybe a possible mixed-valence ground state, may reduce the cost of ionization from over 4 eV in the gas phase to 1-2 eV. (5) The I D and AA values must match as closely as possible the work functions of M1 and M2 (Al, 3.74; Au, 4.58; Pt, 5.29 eV).24 Thus T T F ( I D = 6.83 eV),25TMPD ( I D = 6.25 eV)26or pyrene ( I D = 7.41 eV)27are good candidates for D, and TCNQ (AA = 2.8 eV)2s is the best candidate for A. (6) The bridge-building organic reaction 4 leading to D-a-A must be more efficient, or more likely, than competing salt-forming reactions such as eq 5: D-X Y-A D-a-A (4)
+
-
X-D+ Y-A(5) D-X + Y-A (7) The Franck-Condon reorganization of the molecular geometry of D to D+and of A to A-29 must be small, or fast, so that the overall E T within D-a-A does not become s ~ o w . ~Under ~ ~ ' the right conditions, E T could be faster than 1 ns.30-33 (8) The self-assembly of the rectifier films a t the airwater interface requires that, either by themselves or with the aid of pendant hydrophilic or hydrophobic groups, the D-a-A molecules arrange themselves as compact, polar monolayers that do not allow the head-to-tail stacking of A-a-D next to D-PA. (9) The monolayers must be close-packed and defect-free for a t least a few micrometers in the lateral direction (to prevent electrical shorting of metal layer Mz to layer Ml). Over hydrophobic, oxide-free metals (such as Pt, Au, Ag) cadmium arachidate films have larger defects (probably a t domain b o u n d a r i e ~ and ) ~ ~maybe a disordered, fan-like structure;35 over hydrophilic, oxide-covered metals (Al, SnO, Si02, etc.) the defects seem smaller, allowing for a percolative current j (whose voltage dependence is log j = a V / 4 , 3 6not log j = b W 3 ' ) through d i s ~ l i n a t i o n s . ~ ~
--.
(24) American Institute of Physics Handbook, 2nd ed.; Gray, D. R., Ed.; McGraw-Hill: New York, 1963; pp 9-147-9-149. Ferraris, J. P. J.Electron (25) Gleiter, R.; Schmidt, E.; Cowan, D. 0.; Spectrosc., Relat. Phenom. 1973, 2, 207-210. (26) Batley, M.; Lyons, L. E. Mol. Cryst. 1968, 3, 357-374. (27) Clar, E.; Robertson, J. M.; Schloegl, R.; Schmidt, W. J. Am. Chem. SOC.1981,103, 1320-1328. (28) Compton, R. N.; Cooper, C. D. J. Chem. Phys. 1977, 66, 4325-4329. (29) Marcus. R. A. Discuss.Faradav SOC.1960.29. 21-31. (30) Calcaterra, L. T.; Closs, G. L.;Miller, J. R. J.'Am. Chem. SOC. 1983. 105., 670-671. - - (31) Miller, J. R.; Calcatarra, L. T.; Closs, G. L. J. Am. Chem. SOC. 1984.106. 3047-3049. (32) H k h , N.-& Paddon-Row, M. N.: Cotsaris, E.; Oevering, H.; Verhoeven, J. W.; Heppener, M. Chem. Phys. Lett. 1985, 117,fi-ll. (33) Dutton, P. L.; Prince, R. C.; Tiede, D. M.; Petty, K. M.: Kaufmann, K. J.; Netzel, K. J.; Rentzepis, P. M. Brookhauen Symp. Biol. 1977, 28, 213-237. (34) Peterson, I. R.; Russell, G. J. Thin Solid Films 1985,134,143-152. (35) Garoff, S.; Deck", H. W.; Dunsmuir, J. H.; Alvarez, M. S.; Bloch, J. M. J.Phys. (Les Ulis, Fr.) 1986, 47, 701-709. (36) Roberta, G. G.; Vincett, P. S.; Barlow, W. A. J. Phys. C 1978,11, 2077-2085. ~
~
~
300 Langmuir, Vol. 4, No. 2, 1988
Metzger et al.
Table I. Names and Acronyms of Compounds Used in This
--./
Studv
PY P-c-Me DMAP-C-Me cholestanyl TTF
pyrene methyl phenylcarbamate 7 methyl (4-(dimethylamino)(pheny1)carbamate 8 4,5-dimethyltetrathiafulvaleno[4',5'-b] cholestane 5 6
BHTCNQ
9
HMTCAQ
10
TTF-C-BHTCNQ
11
P-C-BHTCNQ DDOP-C-BHTCNQ
12 13
DMAP-C-BHTCNQ
14
BDDAP-C-BHTCNQ
15
Py-C-BHTCNQ DMAP-C-HMTCAQ
16
17
BDDAP-C-HMTCAQ 18 Py-C-HMTCAQ
19
2-bromo-5-(2-hydroxyethoxy)-7,7,8,8tetracyanoquinodimethan 2-(hydroxymethyl)-ll,ll,l2,12tetracyanoanthraquinodimethan (tetrathiafulvaleny1)carbamateBHTCNQ phenylcarbamate-BHTCNQ (4-(dodecyloxy)phenyl)carbamateBHTCNQ (4-(dimethylamino)phenyl)carbamate-BHTCNQ (4-(bis(dcdecyl)amino)phen y 1)carbamate-BHTCNQ pyrenylcarbamate-BHTCNQ (4-(dimethylamino)phenyl)carbamate-HMTCAQ (4-(bis(dodecyl)amino)phenyl)carbamate-HMTCAQ pyrenylcarbamate-HMTC AQ
5
fL
L
NCvCN
.
9
O
NCVCN
R
NCYcN
.
NC,,CN
c H 3 1 " 0 N > 0 CH',
-0
(10) The T B tunneling, must, at 2-nm distance, be much more likely than the TS tunneling through the monolayer. In inelastic electron tunneling s p e ~ t r o s c o p yfor ~ ~random films of insulating materials between Al and Pb electrodes at 4 K, the fraction TB/TS tends to be only 0.01. For the NC. .CN organic rectifier to succeed, this fraction, for "crystalline" H 3 c ' N e N H monolayer domains, must be much closer to 1.0. H3C' bo (11)The process of laying down the second layer (Al) 1z NCACN as a sputtered film over the organic layer, and the exoNC- CN thermicity of the T B tunneling from D+-a-A-to D-a-A, or other heating processes, should not heat the monolayer much above 100 "C, or else chemical degradation may N C ~ C N ensue. Similarly, high voltages may lead to dielectric breakdown. It should be noted parenthetically that molecules of the general type D-a-A (D, porphyrin; A, quinone) are also of la great interest for the study of electron transfer in chloroplasts and in artificial photosynthetic systems.& Figure 1. Structures of molecules 5-19. For the past few years we have worked toward the reinfrared regions. Two potential new bands are of interest: alization of an Aviram rectifier.&l8 We report below on (i) the intramolecular charge-transfer or intervalence band our experiences in forming PL monolayers, and LB films, (IVT), whose probability is lowered by the presumed coof some of the D-a-A molecules synthesized. Some of the P L and LB results have been given e l s e ~ h e r e ~ but ~ ~ J ~ J ~planarity of the D orbitals with the A orbitals, and consequent negligible overlap, and (ii) the intermolecular are collected here to provide a comprehensive view of our charge-transfer band (ICT), which would be observable progress. only in very concentrated solutions. The presence of the A competing process to the self-assembly of PL monoICT would imply the head-to-tail orientation of D-a-A layers (head-to-head alignment of the D-a-A molecules) dimers or clusters in solution. at the air-water interface is the formation of head-to-tail (intermolecular Mulliken charge-transfer) dimers. Experimental Section Therefore we also studied the optical spectra of D-a-A molecules in nonaqueous solvents in the visible and near Synthesis. The relevant molecular structures are given in (37) Mann, B.; Kuhn, H. J. Appl. Phys. 1971, 42, 4398-4405. (38) Petetson, I. R. J.Mol. Electron, submitted. (39) Tunneling Spectroscopy: Capabilities, Applications, and New Techniques; Hansma, P. K., Ed., Plenum: New York, 1982. (40) (a) Lindsay, J.; Mauzerall, D.; Linschitz, H. J. Am. Chem. SOC. 1983,105,65284529. (b) McIntosh, A. R.; Bolton, J. R.; Connolly, J. S.; Marsh, K. L.; Cook, D. R.; Ho, T.-F.;Weedon, A. C. J.Phys. Chem. 1986, 90,5640-5646. (c) Joran, A. D.; Leland, B. A.; Geller, G. G. Hopfield, J. J.; Dervan, P. B. J.Am. Chem. SOC.1984,106,6090-6092. (d) Nishitani,
S.; Kurata, N.; Sakata, Y.; Misumi, S.; Karen, A.; Okada, T.; Mataga, N. (e) Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A.; Lehman, W. R.; Nemweth, G. A.; Moore, A. L. Nature (London) 1984,307,630-632. (0 Krieger, C.; Weiser, J.; Staab, H. A. Tetrahedron Lett. 1985, 26, 6055-6058.
Figure 1, and the chemical names are given in Table I. The synthesis of the cholestanyl TTF (13) was prepared by crosscoupling the appropriate dithiocarbonates in triethyl phosphite solvent a t the refluxing temperature. Ita synthesis and properties will be the subject of a future communication. The synthesis of BHTCNQ (9) was first reported by Hertler;41 its crystal structure has been determined.13 The synthesis of HMTCAQ (10) is described elsewhere.16 The carbamates were all synthesized by using the isocyanates of the corresponding donors and coupling them to the alcohols BHTCNQ and HMTCAQ in acetonitrile, under
J. Am. Chem. SOC.1983, 105, 7771-7772.
(41) Hertlrr, W. R. J. Org. Chem. 1976,41, 1412-1416. (42) Engler, E. M.; Scott, B. A.; Etemad, S.; Penny, T.; Patel, V. V. J. Am. Chem. SOC.1977,99, 5909-5916.
LB Films of Donor-Urethane-TCNQ a nitrogen atmosphere, in the presence of dibutyltin dilaurate catalyst.g13 The carbamate linking reaction yields a single easily purified adduct if the D-u-A molecule involves a weak electron donor (phenyl, (dodecyloxy)phenyl),l0J1a moderate electron donor ((dimethylamino)phenyl,etc.), or the strong donor pyrene.” However, with the strong donor TTF, two carbamate products 11A and 11B (one “ionic”, one “neutral”) are observed, whose purification is very difficu1t.&l0 Also, when bis(dodecy1)amino “hydrophobic tails” are present, there is always some urea side product. The crystal structures of P-C-BHTCNQ (12) and DMAP-C-HMTCAQ (17) have been determined:l0Js their molecular geometry is extended, not folded. The visible and ultraviolet spectra were determined by using Varian Cary 2200 (200-900 nm) and Varian Cary 17 (200-3000 nm) spectrophotometers. Unfortunately, the limited amount of sample available for each compound (typically 10 mg) limited the range of solution concentrations which could be studied. Film Balance Work. A Lauda film balance (Langmuir trough), Model FW-1, was used to obtain pressure-area (TI-A) isotherms; LB films were obtained by using the vertical film dipping mechanism of a JoyceLoebl f i i balance, suspended over the Lauda film balance with a home-built swinging mount. The film balance was housed in a special room equipped with a high efficiency particulate analysis filter to extract dust and oil particulates from the room heating/air conditioning unit; the system was run under positive pressure, so that no dust would enter the f i i balance room. For the aqueous subphase, a Millipore Milli-Q Model ZO40 system, supplied with house deionized water, provided pyrogen-free water with a resistivity of 18 MQ cm. For certain studies, the subphase was acidified with a reagent-grade dilute HC1 solution or made basic with reagent-grade dilute NaOH solution. In certain cases the subphase was made 1.0 X M in Cd2+by using reagent-grade CdIz (Aldrich). The temperature of the subphase (10-50 “C) was controlled to 0.1 “C by a Lauda Model RM6 variable-temperature bath. A Brinkmann BR 1101 XY-recorder provided a graph of the II-A curves. The compounds to be studied were weighed on a Mettler M5SA microbalance and dissolved in reagent-grade chloroform. The solutions were spread on the subphase (“cast”)by a Hamilton 50-pL syringe. With the syringe held above, and perpendicular, to the subphase small drops were dispersed at the air-water interface, as the syringe was passed over the subphase. Typical quantities of solution were 20 pL, containing approximately ioi7 molecules. To assure quantitative transfer, the syringe was rinsed twice with solvent, and the wash solvent was spread over the subphase. The microscope slides used for LB studies were soaked in soap and Milli-Q water for one day and rinsed with Milli-Q water before use. There are several ways of defining molecular areas from a pressure-area isotherm.lS One can define A. as the area per molecule at extrapolated zero differential surface tension. One can also define A, as the area per molecule at the collapse point, Le., at the point in the II-A isotherm where the pressure is maximum (“collapsepressure”, II,) and the area is minimum (at areas below A,, the E A curve is not reversible, and an overlap of the “ice floes” of the individual monolayer domains occurs). One can also define A , as the area at the midpoint pressure II, = O.5IIc. For LB film studies on aluminum or glass substrates, the substrate was dipped at the maximum speed setting (1.0 mm/s) while a constant film pressure of approximately 15-20 mN/m was maintained.
Results The optical spectra of the compounds are summarized in Table 11. The data for pyrene agree well with literature ~ a l u e s . 4The ~ spectra for D-PA molecules were assigned to the D or to the A moieties, whenever feasible. Some peaks remain unassigned (see Table 11). However, no IVT could be clearly or definitely identified in this study (as a new band of unknown polarization). The solutions were (43)Jones, R. N.;Gogek, C. J.; Sharpe, R. W. Can. J. Res. Sect. B 1948,26B,719-727.
Langmuir, Vol. 4, No. 2, 1988 301
0
1 10
1
20
I
I
I
30
40
50
I
60
70
BO
Area I ( X 2 1 molecule)
Figure 2. Pressure-area isotherm for DDOP-C-BHTCNQ (13) at 292 K, redrawn from ref 11.
40
30
mNlm
10
’
10
20
30 40 50 Area I ( X 2 1 molecule)
60
70
80
Figure 3. Pressure-area isotherm for Py-C-BHTCNQ (16) at 283 K, redrawn from ref 11.
6o
t
10-
Area I(Xzlmolecule)
Figure 4. Pressure-area isotherm for BDDAP-C-BHTCNQ (20) and 293 K (curve 1) and at 303 K (curve 2). probably too dilute to allow for the detection of an ICT band. However, for both P y (which is known to dimerize in solution) and for Py-C-HMTCAQ the concentration dependence of the visible spectra in the range 10-5-10-4 M shows moderate deviations from Beer’s law, which may indicate some Py-Py dimer formation. The PL and LB film results obtained are given in Table 111. Figures 2-5 display the II-A isotherms for DDOPC-BHTCNQ,” BDDAP-C-BHTCNQ, Py-C-BHTCNQ,” and BDDAP-C-HMTCAQ, respectively. The formation of monolayers was determined on the basis of the measured area per molecule. For the BHTCNQ molecule16one estimates a van der Waals width of 3.9 A (because of the size of the Br atom) and a van der
302 Langmuir,Vol. 4, No. 2, 1988
Metzger et al.
Table 11. Visible and Ultraviolet Spectral Peaks of D-a-A Molecules and of Their D and A Components molecule concn (io6 mol/L), [ref for synthesis] solventa UV-vis data,b X,/nm (log c) Donors, D heptane [42] 303 (4.09), 317 (4.04), 370 (3.23), 455 (2.90) 'M'F, 1 (strong D) 287 s (4.07), 298 (4.10), 315 s (4.09), 327 s (4.02) 1.32, hexane cholestanyl TTF, 8, (strong D) 243 (4.93), 254 (4.36), 264 (4.57), 275 (4.78), 296 s (4.11), 308 (4.32), Py, 5, (strong D) 1.97, CHC13 321 (4.61), 337 (4.77) 2.55, CH3CN 198 (4.56), 233 (4.24), 260-280 (2.80) P-C-Me, 6, (weak D) 3.53, CH3CN 195 (4.44), 265 (4.38), 311 (3.55) DMAP-C-Me, 7, (moderate D) [14] BHTCNQ, 9, red crystals [16] (strong A) HMTCAQ, 10, sepia powder (weak A) [13]
Acceptors, A 207 (4.26), 256 s (3.70), 281 (3.61), 377 s (4.36), 402 (4.57), 411 (4.59), 484 b (3.75) 2.53, CH3CN 197 (4.56), 213 s (4.35), 226 s (4.14), 279 s (4.41), 283 (4.46), 305 (4.25), 346 (4.37) 3.42, CH3CN
D-PA Molecules 291 (4.08), 401 (4.52),412 (4.54), 482 (3.67), 752 (3.27), 853 (3.40) CH3CN [lo]
TTF-C-BHTCNQ, 11A, neutral phase mp 105-108 OC [8-101 TTF-C-BHTCNQ, l l B , ionic phase mp 145-150 "C [&lo] P-C-BHTCNQ, 12, red-brown crystalline [lo1 DDOP-C-BHTCNQ, 13, black soft solid [ 111 DMAP-C-BHTCNQ, 14, black solids BDDAP-C-BHTCNQ, 15, red-brown solid Py-C-BHTCNQ, 16, red-brown solid [ l l ] DMAP-C-HMTCAQ, 17, black crystal [18] BDDAP-C-HMTCAQ, 18, black wax
3.20, CHC13 CHpClp[ l l ] 2.76, CH&N 1.57, CH3CN
Py-C-HMTCAQ, 19, black microcrystals
1.75, CHC13
CH&N [lo]
297 (2.47), 416 (4.631, 443 s (4.451, 763 (4.24), 853 (4.57)
3.89, CHCl,
257 (4.15),381 s (4.181, 402 (4.39),421 (4.47), 474 (3.74)c
CH3CN [ l l ] 1.75, CHC13
239 (3.39), 282 (2.91), 410 (3.64),475 (2.89) 251 s (4.16),269 (4.20), 313 (4.05), 342 s (3.86): 398 (3.691, 424 (3.691, 472 (3.70)c 271 (4.31),317 s (3.87), 405 (3.16), 485 (3.00)c 272 (4.22), 278 (4.22), 343 (4.16), 385 s (3.70),400 s (3.62),475 (3.60) 271 s (4.47), 281 (4.52),304 (4.21), 343 (4.31) 197 (4.79),213 s (4.51),226 s (4.25),276 s (4.63),282 (4.67), 305 (4.38), 331 (4.39): 340 (4.40)c 246 (4.78), 273 s (4.57),283 (3.761, 313 (4.44), 332 s (4.61), 344 (4.731, 354 s (4.68), 362 s (4.60); 382 s (4.27),vd
When a concentration is not listed, the spectrum was studied previously and reported elsewhere; the reference for previous optical study is in brackets. b s = shoulder; b = broad. Boldface, D-a-A peak assigned to neutral donor; italic, D-a-A peak assigned to neutral acceptor; plain type, D-a-A peak unassigned. CNonear IR absorption bands (800-1500 nm) were found. dunassigned peak; see Results section.
Table 111. Monolayer Data from Pressure-Area (II-A ) Isotherms for TTF-Cholestane and for D-a-A Molecules, at Water Subphase Temperature T " molecule PL? LB? T/K II,/(mN/m) A,/A2 A,/A2 AO/A2 cholestanyl TTF, 8 N N TTF-C-BHTCNQ, 11A neutral-phase, microcrystalline [8-101 Y Y ? 12.7 134 f 50 P-C-BHTCNQ, 12 deep-red needles crystalline, with solved X-ray structure [ 101 N N DDOP-C-BHTCNQ, 13 Y Y 292 20.26 50' 55b 60b pure brown solid, with well-defined cyclic 292 38.0c 26c 2gC 33c voltammogram [111 DMAP-C-BHTCNQ, 14 Nd Nd BDDAP-C-BHTCNQ, 15 47.3 57 69 82 wax, impure (urea contaminant?) Y Y 293 Y Y 303 45.9 54 70 82 Py-C-BHTCNQ, 16 28.2 53 60 66 pure brown solid I l l ] Y Y 283 DMAP-C-HMTCAQ, 17 black crystalline, with solved X-ray structure [18] N N BDDAP-C-HMTCAQ, 18 black wax Y Y 293 22.3 58 71 83 Py-C-HMTCAQ, 19 black microcrystals N N "The estimated precision in II is f O . l mN/m; the estimated precision in the area per molecule A,, A,, A, is *5%, except where indicated. An entry Y under PL? means that Pockele-Langmuir monolayers are formed; an entry Y under LB? means that Langmuir-Blodgett films are formed. bReinterpretation of Figure 2 of ref 11. CAsreported in ref 11. dForms thermodynamic mixed films (collapse pressure is a function of the mixture).lg
Waals length of 10.5 A (long end of TCNQ) or of 5.5 A (short end of TCNQ molecule). Therefore, the minimum area for the BHTCNQ molecule, viewed shorter edge on, is 21.5 A2 (but this estimate leaves no room for the alkyl chains that must extend outward toward the d mor end) or 41 A* (viewed longer edge on). Typically, the II-A isotherm yields ridiculously low apparent values for A, when nonmonolayer clusters formed spontaneously (pyrene or Py-C-HMTCAQ: these form bluish clusters as soon as
the chloroform solution is cast onto the water surface) or dissolved partially in the aqueous subphase (DMAP-CBHTCNQ and DMAP-C-HMTCAQ) or for other nefarious reasons (P-C-BHTCNQ). Monolayers form for TTF-C-BHTCNQ (but the molecules seem to lay "pancake down" on the ~ u b p h a s e ) ?for ~ Py-C-BHTCNQ," and for molecules bearing long greasy chains (DDOP-C-BHTCNQ,'l BDDAP-C-BHTCNQ, and BDDAP-C-HMTCAQ). The differential surface tension
LB Films of Donor-Urethane-TCNQ
2o
t
Langmuir, Vol. 4, No. 2, 1988 303 cholestanyl TTF could. However, as is seen in Table 111, this is not the case. Furthermore, when the TTF end of the molecule was oxidized to TIT+in CDC13solution with I,, the resulting dark-green solution of this amphiphilic anion failed to give a P L monolayer. Previously it was found that LB films could be formed with DDOP-C-BHTCNQ'l and with Py-C-BHTCNQ" it was found here that LB films can be formed routinely with BDDAP-C-BHTCNQ and with BDDAP-C-HMTCAQ on glass and also on A1/A1203.
\
Discussion -
0
20
40
60
80
100
Area I ( X z l molecule)
Figure 5. Pressurearea isotherm for BDDAP-C-HMTCAQ (18) at 293 K.
II, is greatest for BDDAP-C-BHTCNQ. The II-A isotherms for DDOP-C-BHTCNQl' and BDDAP-C-HMTCAQ (Figures 2 and 5) contain two changes in slope, of which the first is the point of maximum compression (&, A,) and is reversible; the second, a t lower areas and higher pressures, was thought previouslyll to be the point of maximum compression and minimum area for DDOP-C-BHTCNQ. However, it is not reversible for BDDAP-C-HMTCAQ, and it may be the point of minimum area for only the "tails" of these molecules, not for the whole molecule; that is, a t the higher pressure part of the molecule this tail is all that remains in the monolayer, in an irreversible fashion. Thus in Table I1 the molecular area for DDOP-C-BHTCNQ is reinterpreted. Efforts were made to form PL monolayers of DMA-CHMTCAQ by varying the temperature, the pH, and the ionic strength of the aqueous subphase. It is likely that the dimethylamino end of the molecule is more hydrophilic and that a t low pH the nitrogen atom may quatemize, and thus the resulting cation could be more amphiphilic. However, a t pH 2.2 and 1.0 X lo4 M CdI,, most of the compound probably dissolved in the subphase, and no collapse point was reached. At pH 7, the apparent areas were A, = 23 (284K), 17 (298 K), 12 (308 K), and 9 A2 (318 K), and the apparent collapse pressure was around 59-65 mN/m. A t pH 10.4, the apparent molecular areas were 22, (291 K), 14 (295 K), 13 (298 K), and 10 A2 (310 K). In analogy to the effect of pH 5 and of 1.00 X lo-, M Cd2+ ions in stabilizing arachidate ions to form the most incompressible fatty acid PL monolayers,ls an experiment was performed in which the subphase was saturated with CaSO, at 293 K, in the hope of affecting the coordination of suitable subphase counterions to DMA-C-HMTCAQ; however, the apparent A, was a disappointing 17 A2. When it was found that P-C-BHTCNQ did not form monolayers, an experiment was performed in which the BHTCNQ moiety in P-C-BHTCNQ wm reduced with LiI in CH,CN; a green paramagnetic solution is formed. It was hoped that the A- ion would act as a hydrophilic end of a D-a-A- anion; however, a CH3CN solution of this Li+ salt, when cast onto a water subphase, did not yield a PL monolayer. Similarly, it was known in the 1930s that cholestanol and cholesterol form PL monolayers.44 Although cholestane is not known to form m ~ n o l a y e r s it , ~was ~ hoped that a ~~~
~~
(44) (a) Adam, N. K.; Askew, F. A.; Danielli, J. F. Biochem. J . 1935, 29,1786-1801. (b) Cadenhead, D. A.; Demchak, R. J. Biochem. Biophvs. Acta 1964, 176, 879.
Several conclusions can be drawn from the results presented above: (1)The areas per molecule, A,, given in Table I11 (50-58 Az)are consistent with the minimum size of the BHTCNQ molecule estimated above; since the film thickness has not been measured, it is not meaningful a t present to discuss the potential slant of the D-a-A molecules in the monolayers. Since the HMTCAQ molecule is very bent in DMA-C-HMTCAQ,l8 it might be expected that the molecular area would be larger for BDDAP-C-HMTCAQ than for BDDAP-C-BHTCNQ, but this is not observed: the molecules, although bent, may still fit in some high-density packing geometry. However, the low II, indicates that the BDDAP-C-HMTCAQ monolayer is held by weaker cohesive forces than the BBDAP-C-BHTCNQ monolayer. (2) The presence of long hydrophobic alkyl chains assures the formation of PL monolayers and LB films. This is a shame, since electron conduction through a long alkyl chain must slow down the electron transfer through a D-a-A molecule. (3) Oxidizing the D end of a cholestane-substituted D, or reducing the A end of D-a-A, does make amphipilic ions D+-a-A or D-PA-; but if the parent neutral molecule does not form PL monolayers, neither does the daughter ion. Of course, the charge in D+-PA or in D-a-A- is distributed over several atoms of a ?r ring system. One should remember that arachidic acid already makes good monolayers a t neutral pH, but cadmium arachidate just makes stronger monolayers a t pH 5, thanks to the very concentrated negative charge close to the water subphase and to the favorable counterion atmosphere provided by the cadmium ions in the subphase.lg (4) Protonating the amine nitrogen in DMAP-CHMTCAQ, and then hoping that the pH of the subphase or a suitable counterion atmosphere of anions in the subphase will stabilize the monolayer, was unsuccessful. (5) The relatively flat, s-electron-rich molecules discussed in this study do not self-assemble in monolayers unless the saturated long alkyl chains are present. Not many s-electron systems are known to self-assemble as monolayers, but both quinquethienyl, 2045and the anthracene propionic acid, 21,& have been found to make PL monolayers and LB films. (6) While Py-C-BHTCNQ was found previously to form PL monolayers and LB films,ll Py-C-HMTCAQ would not form a monolayer. It is not known whether the orientation of the Py-C-BHTCNQ monolayer was D-a-A atop A-a-D (undesired geometry) or D-a-A atop D-PA." It is known that HMTCAQ is a poor two-electron acceptor and is a very bent molecule,4' while BHTCNQ is a very good (45) Schoeler, U.; Tews, K. H.; Kuhn, H. J. Chem. Phys. 1974, 61, 5009-5016. (46) Vincett, P. S.;Barlow, W. A.; Boyle, F. T.;Finney, J. A.; Roberts, G. G . Thin Solid Films 1979. 60. 265-277. (47) Kini, A. M.; Cowan, D'. 0.: Gerson, F.; Moeckel, R. J.Am. Chem. SOC.1985,107, 556-562.
304 Langmuir, Vol. 4, No. 2, 1988
one-electron acceptor and is almost flat.16 ( 7 ) Probably DDOP-C-BHTCNQ, BDDAP-CBHTCNQ, and BDDAP-C-HMTCAQ are ordered in monolayers a t relatively low llc but then can be compressed further, so that the alkyl chain thickness defines the packed region, while the aromatic rings and the carbamate bridge are extruded out of the packed region. Such an extrusion out of monolayers has been observed before for whole molecule^.^^ (8) No ICT or IVT band49has been identified for the solutions of the D-a-A molecules. Given the D to A distance and the extended geometry of the molecule, the difficulty in finding the IVT is not surprising. The lack of an ICT at moderate concentrations (except for the Py adduct) suggests that the head-over-tails dimer does not form very readily in dilute solutions. Of course, an optical absorption band can be unambiguously assigned to an IVT or ICT only if the specular reflectance optical spectra of single crystals of known structure are available, so that the polarization of the bands can be measured, as was done recently for picolyltricyanoquinodimethane (PSCNQ, 22).50151 (9) Of the molecules that form good monolayers, Py-CBHTCNQ, DDOP-C-BHTCNQ, and BDDAP-CBHTCNQ all contain the strong one-electron acceptor BHTCNQ, while BDDAP-C-HMTCAQ contains the ,weak two-electron acceptor HMTCAQ. Py-C-BHTCNQ contains a fairly strong donor, while BDDAP-C-BHTCNQ and BDAP-C-HMTCAQ contain a moderate donor, and (48) Kawabata, Y.;Sekiguchi, T.; Tanaka, M.; Nakamura, T.; Komizu,
H.; Matsumoto, M.; Manda, E.; Saito, M.; Sugi, M.; Iizima, S. Thin Solid
Films 1985, 133,175-180. (49) Hush, N. S. Progr. Inorg. Chem. 1967,8, 391-444. (50) Metzger, R. M.; Heimer, N. E.; Ashwell, G. J. Mol. Cryst. Liq. Cryst. 1984,107, 133-149. (51) Akhtar, S.;Tanaka, J.; Metzger, R. M.; Ashwell, G. J. Mol. Cryst. Lip. Cryst. 1986, 139, 353-364.
Metzger et al.
DDOP-C-BHTCNQ contains a weak donor.
Y O H
7
ZQ /
/
/
A general conclusion from (2)-(4) is that, to form monolayers, the "hydrophobic" interactions must first stabilize the lateral stacking; only after this is achieved can the ionicity of the polar end of the molecule (preferably centered on a very few atoms near the aqueous subphase) help in stabilizing the monolayer. Clearly, more synthetic and monolayer studies are needed to explore the self-assembling and electronic properties of the systems described here. In particular, we must seek a chemically convenient, fii-forming, strong one-electron donor, so that (strong donor)-a-(strong acceptor) compounds can be tested for electrical rectification.
Acknowledgment. We are grateful to our co-workers past and present for their assistance: Dr. Jamil Baghdadchi (ARCO), Dr. Sukant Tripathy (Lowell), Prof. Norman Heimer (Mississippi). R.M.M. is grateful to T. Penner (Kodak) for suggesting the reinterpretation of the film data for DDOP-C-BHTCNQ. We wish to thank the National Science Foundation for their support. Registry No. 10, 129-00-0;11, 2603-10-3;12, 78823-56-0;13, 111469-51-3;14,58268-31-8; 15,108562-20-5; 16, 91259-23-3;17, 111469-52-4;18, 108432-12-8;19, 111469-53-5;20, 111469-54-6; 21,108432-11-7;22,108562-22-7;23,111469-55-7;24,108562-23-8.
