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Langmuir 1993,9, 22-24
Crystal Engineering in Two Dimensions: Close-Packing Applied to a Langmuir-Blodgett Film D.R. Swanson and C . J. Eckhardt' Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304 Received March 25, 1992. In Final Form: October 30, 1992 The cloae-packing principle, well-established for molecular crystals, is applied to a model for twodimensional lattices previously reported by Skabardonis and Sukenik" for [22.3.3]propellaues. Atomatom potential calculations for two propellane films predict a phase of planar group p2gg, in contrast to previous predictions, but support previous conclusionsregarding the effect of amphiphile conformational change on lattice parameters. Formation and possible applications of different planar lattices by various amphiphiles are discussed. it is certainly arguable that the ring conformation is affected by the puckering of the fused cyclopentane base, it is instructive to isolate the effect of the base conformational change. These calculatione limit the conformational difference between an anti,anti propellane diol and a propellane dione to the hydrophilic end.
Introduction The potential for "crystal engineering" planar lattices was recently demonstrated using a computational approach which successfullyreproducesthe packing of wellcharacterized amphiphiles.2 The method is analogous to the rigid body approximation utilized in the study of molecular crystals. The translational symmetry of rigidified molecules is sufficiently simplified by considering a monolayer as a two-dimensional lattice that the design of a specific lattice using particular molecules becomes plausible. Work is presently underway to investigate the engineering of yet unobserved planar lattices. This research has two objectives: (1)to engineer new thin film materiale utilizing collective properties enhanced by specific translational symmetries and (2) to investigate solid-statephenomena in dimensionallyreduced systems. To test the computational methodology and the practicality of the former objective, calculations have been performed and compared to isotherm data from films composed of a series of [22.3.3lpropellanes synthesized and characterizedby Skabardonisand Sukenik' (see Figure 1). In particular, the [22.3.3lpropellane-16,19-dione(1) had been hypothesized' to pack in one of two configurations: (1) a square lattice with neighboring molecules oriented at right angles relative to one another about the film normal (planar group p4) or (2) a slipped packing of c o n f i a t i o n 1(pl). The proposed structures are valuable starting points for calculationssince both yield coareas in reasonable agreement with those measured for the dione film. Nonetheless, if these were the energetically favored lattices, they would be unusual because the principle of closest packing is apparently not obeyed. This system was therefore studied using atom-atom potential calculations for quantitative comparison of the calculated to the proposed packing arrangements. The series of propellanes of Skabardonis and Sukenik is enlightening because the large hydrophobic region of the [22.3.3lpropellanes, which normally contributes primarily to the intermolecular packing, is compositionally constant, while the coarea found for the various films is not. Figures 2 and 3 show the reported pressurearea isothermsof the seriesof compoundswith a reported coarea of 64 A* for the anti,anti diol. The smoothness of the isothermsindicates no complex phase transition behavior. The molecular conformationsof anti,anti diols have been shown to differ from that of diones by a puckering of the fused cyclopentane rings3*'which has been proposed to account for the smaller coareas of the diol films.' While
Calculational Method Assuming a rigid amphiphile simplifies computations, since internal degrees of freedom may be neglected. Crystal structures exist for the [10.3.3lpropellanediol,an anti,anti diol, and the [12.3.3lpropellanedione with conformations differing primarily in the structure of the amphiphilebaee.3 In order to obtain computationalresulta with conformationaldifferenceslimited to the hydrophilic ends of the amphiphiles,the [12.3.3lpropellanedioneused in the model of Skabardonisand Sukenikwas investigated along with a hypothetical hybrid diol. This hybrid is composed of the alkyl ring of the [12.3.3lpropellanedione joined to the puckered ring system of the anti,anti diol mentioned above. Thus, any difference in predicted packing between the dione and the hybrid diol will result solely from the change in conformation of the base. The rigid molecule approximation has been shown to be useful for quite different amphiphile~~*~ and is a rational starting point for the investigation of these systems with respect to the various conformations known for the fused cyclopentane base.' The detailed methodology of the calculations has been described elsewhere.2 Here the oxygens are fixed in the plane of the film,and the molecule is initially oriented along the normal to the film plane in a physically reasonable manner. This latter constraint is subsequently released and the energy of the film is minimized with respect to the lattice parameters of the 17 poesible twodimensional lattices which include no more than four molecules per unit cell. The preponderance of X-ray evidence supports this assumption."1° Furthermore, the (3) Natrnjan, A.; Ferrara, J. D. Hays, J. D.; Khot, M.; Colonell, J.; Youngs, W. J.; Sukenik, C. N. J. Org. Chem. 1990,55,2164-2169. (4) Khot, M. S.;Smith, D. A.; McMillan, G. R.; Sukenik, C. N. J. Org. Chem. 1992,57,3799. (6) Eckhardt,C.J.;Peachey,N.M.;S~n,D.R.;Kim,J.-H.;Uphaue, R. A.; Lutzand, G.P.; Beak, P. Langmuir 1992,8,2591. (6) Kjner, K.; Ale-Nielsen, J.; Helm, C. A.; Laxhuber, L. A.; Mbhwald, H. Phys. Reu. Lett. 1987,68, 2224. (7)Dutta, P.; Peng, J. B.; Lin, B.; Kettemn, J. B.; Prakaah, M.; Georgopoul~,P.;Ehrlich, s. phys. Reo. Lett. 1987,58, 2228. (8) Feigen, L. A.; Lvov, Y. M.; Troitaky, V. I. Sou. Sci. Rev., Sect. A. 1 9 l , l I , 286. (9) Kenn, R. M.; B o b , C.; B i b , A. M.; Peterson, I. R.; Mbhwald, H.; Ale-Nielsen, J.; Kjaer, K. J. Phycr. Chem. 1991,96, 2092. (10) Jaquemain, D.; Wolf,S. G.;Leveiller, F. Angew. Chem.,Int. Ed. Engl. 1992,31, 130.
(1) Shbardonie, J. G.;Sukenik, C. N. Longmuir 1988,41307-1311. (2) Eclrhardt, C. J.; Swaneon, D. R. Chem. Phys. Lett. 1992,187,233238.
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1993 American Chemical Society
Langmuir, Vol. 9, No.1, 1W 23
Letters
(1) Dione
(2) Syn Ketol
(5) Syn,Anfi Diol (4) Syn,Syn Diol Figure 1. [22.3.3lPropellanes synthesized by Skabardonis and Sukenik.’ 40
1
(3) Am’ Ketol
(6)Anli,Anh’ Diol
Table I. IsoL..srm Results (Sltabardonir ani SukenlL*) area/molde at comDound f i i collawle (A*) coarea (AZ) dione (1) 64 55 52 syn ketol (2) 61 49 anti ketol (3) 62 46 syn,syn diol (4) 64 anti,syndiol (5) 67 48 anti,anti diol (6) 64 44 ~~
30
50
70
so
MOLECULAR AREA (sq. Angstrems/md*oul*)
Figure 2. Pressurearea isotherms of dione (1) and ketols (2, 3h1 40
so
50
70
BO
MOLECULAR AREA (sq. Angstroms/mol*cul*)
Figure 3. Pressurearea isotherms of diols (4, 5, and 6).l
structure previously proposed1 for this film contains four moleculesper unit cell which necessitatesthe investigation of these packings. Since the propellanes exist as racemates, one requires the same composition of a bidimensional crystal. The assumption that the asymmetric unit consists of one molecule, reasonable for such asymmetric species, leads to the elimination of several of the planar groups. The oblique group p l and the square group p4 cannot accommodate racemic mixtures under these constraints, but the centered redangular groups pg and p2gg can. Chirality is fundamental to the engineering of desired translational symmetries. An oblique lattice may be formed by a pure enantiomer or a nonchiral amphiphile, but will not be formed by a racemic mixture barring an aggregated asymmetric unit. Most molecular crystals display monoclinic packing with a centered rectangular baee. Analogously,it appears most amphiphiles will pack
Table 11. Calculational Resultr (Mom) lattice molecular area (A21 energy (kcal/mol) n-A datal 64 square @4) 69.1 -8.47 slippacked @1) 58.7 -13.06 rectangular @s) 61.6 -10.40 -15.65 oblique @1) 53.1
in a centered rectangular monolayer, since this lattice accommodates both achiral and racemic molecules in closest packed configurations. By application of these ideas to the chiral propellanes,it is expectedthat a centered rectangular group will exhibit closest packing, i.e. the lattice of lowest energy. Resulte Experimental results are tabulated in Table I for propellane systems studied by Skabardonis and Sukenik. Computationalresults for the systemsconsidered are given in Table 11. The calculations confirm the conjecture of Skabardonis and Sukenik that a lattice of a form intermediate to the square @4) and slippacked @1)latticea will give the correct coarea (or nearly so)since calculations of the two forms bracket the experimental value (Table 11). Space group symmetry arguments given above, however, do not allow either of theeetwo lattices. Neither accommodates racematee, and the slip-packing requires an asymmetric unit of more than one molecule. Further investigationof allpoeeiblelatticegfulfillingthecomtrainta outlined above disclosed two lattices that are cloee in energy. The centered rectangular lattice pg is of lower energy than the above models and of lower molecular area than the reported coarea, but the centered redangular lattice p2gg provides the best minimum. Interestingly, this lattice is closer packed (lower potential energy) but results in a larger area per molecule in the plane. Thie apparent paradox is explained by a greater molecular tilt in the p2gg lattice, as shown in Figure 4 (middle). These results demonstrate that the previously proposed models are not closest packed configurations. Calculations involving amphiphiles consisting of the dione chain and the diol base gave the same qualitative results, but the area per molecule is considerably lower. The molecular area predicted in the lowest energy lattice
24 Langmuir, Vol. 9, No. 1, 1993
Figure 4. (top) Stereoscopic view of the p2gg unit cell viewed from below the film plane. (middle and bottom) Side views. In each case, only carbons are shown for clarity. In side views hindmost molecules are denoted by solid circles.
of each amphiphilecompareswell with the isotherm coarea data. This supports the conjecturethat the conformation of the base of these propellanes has a significant effect upon the exhibited coarea (Figure 4, bottom). It is also consistent with the general premise that packing should be determined primarily by the larger hydrophobicregion of the molecule, since the lattice predicted is identical in each m e . The dione and hybrid diol incorporate the conformationalextremes of the fused cyclopentanebase? and it is therefore consistent that the coareas of the other propellaneslisted in Table I are intermediate to the dione and the anti,anti diol. The molecular conformations used in the calculations were found by X-ray diffraction? and the assumption of a rigid molecule gives good results even for conformationally labile molecules such as stearic acide2The model and forcefield employed s u d u l l y reproduce monolayer structurea,2fiand the force field has been used to reproduce various molecular crystal atructures.l1-13 This lends credibility to the prediction of the p2gg planar group for (11)Williamr, D.E. J. Chem. Phye. 1967,47,4680. (12) Pertsin, A. J.; Ki orodeky, A. I. The Atom-Atom Potential Method; Springer: B e r L z ermany, 1987.
Letter8 solid phases of these amphiphiles. Highly convoluted molecules show a liquid (disordered)phase when van der Waals attractions dominate the intermolecularpacking,14 but it is not possible to determine the predicted coarea for a disordered phase from this model. However, it is highly unlikely such a phase would exhibit a similar coarea unless the films were at least two-dimensional powders such as has been observed in more conventional f i l m ~ . ~ l ~ J ~ Conclusions Both the dione and the hybrid diol are predicted to occupy the same planargroup,p2gg,in contrast to packings predicted previously. This is consistent with a packing determined predominantly by a constant large methylene region of the molecule. The quantitative change in molecular area is dictated by the conformation of the base in agreement with previous arguments by Skabardonis and Sukenik. Thus, while knowledge of molecular conformation is essential to explain LB film structure, close packing principles and space group theory must be considered as well. This is illustrated by the example of the propellane racemates that necessarily cannot pack in several of the planar groups unless an unusual aggregate asymmetricunit is formed. The present restricted model satisfactorilyreproduces presently availabledata for these films. The lattice formed by a specific amphiphile may be effectively predicted when an amphiphile may be considered to be rigid. This in turn facilitates the deliberate construction of specific nets. These ideas have been applied to a series of [22.3.3lpropellanes; however, translational orderingis more readily exhibited by amphiphiles of more uniform cross sections.6J4 Nevertheless, the Skabardonis-Sukenik propellanes do demonstrate the efficacy of 2D crystal engineering through control of amphiphilegeometry. By designingamphiphileswith less convoluted configurations, it should be poesible to more systematicallysynthesizespecificlatticesby appropriately designed molecules through proper considerationof closepacking principle^.^ Acknowledgment. Major support of this research by ARO Grant DAAL03-89-G-0094 is gratefully acknowledged. This research was also conducted using the Cornell National SupercomputerFacility,a resource of the Center for Theory and Simulations in Science and Engineering (Cornell Theory Center), which receives major funding from the National Science Foundation and IBM Corporation, with additional support from New York State and members of the Corporate Research Institute. D.R.S. gratefully acknowledges fellowship support by the University of Nebraska. (13) Kulver, R.; Eckhardt, C. J. Phys. Reu. B 1988,37,5951. B m e , K.-H.; Luty, T.; Eckhardt, C. J. J. Chem. Phys. 1990,93,2016. (14) Roberta, G., Langmuir-BlodgettFilms;Plenum: New York, 1990. (15) Schloeeman, M.L.; Schwartz, D. K.; Penhan, P. S.; Kawamoto, E. H.; Kellog, G. J.; Lee,S.Phys. Rev. Lett. 1991, 66, 1599.
