CRYSTAL GROWTH & DESIGN
Hydrogen-Bond-Assisted, Crossed Dipole π-Stacking in 1,4-Bis(phenylethynyl)benzene
2006 VOL. 6, NO. 6 1253-1255
Nibedita Sanyal and Paul M. Lahti* Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003 ReceiVed January 12, 2006; ReVised Manuscript ReceiVed March 21, 2006
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: 2,5-Bis(2-hydroxyethoxy)-1,4-bis(phenylethynyl)benzene (BHE-PE2.5) was synthesized and characterized by X-ray singlecrystal diffraction analysis: formula ) C26H22O4, M ) 398.44, orthorhombic unit cell, Pbcn, a ) 22.5520(9) Å, b ) 12.9259(4) Å, c ) 7.2463(2) Å, R ) β ) γ ) 90.00(0)°, V ) 2112.33(12) Å3, Z ) 4, R1 ) 0.1194, wR2 ) 0.1455. The molecules are somewhat deplanarized, with a 19.8° interplane torsion between phenyl rings. They form π-stacks with a centroid-centroid distance of 3.62 Å along the stack axis, a cant angle of the central phenyl ring of 13.2° relative to the π-stack c-axis, and a 99.2° rotation of each molecule along the π-stack, yielding crossed-dipole stacks in which the molecular long axes alternate nearly perpendicularly to one another. This unusual crossed dipole pattern is induced by a network of hydrogen bonds that assembles the BHE-PE2.5 molecules into the stacks and further associates the stacks into two-dimensional sheets. Phenyleneethynylene-based systems have been intensely studied, especially in the area of organic electronic materials, whose properties can be strongly affected by π-stacking.1 Much work has been devoted to the identification of structural motifs that affect π-stacking in these and related systems. This communication describes the synthesis and crystallography of 2,5-bis(2-hydroxyethoxy)-1,4-bis(phenylethynyl)benzene (BHE-PE2.5), a 2.5-mer of poly(1,4-phenyleneethynylene) that incorporates pendant hydrogenbonding groups at the central phenylene ring; the hydrogen-bonding network assembles the molecules into unusual cross-dipole π-stacks that are of potential interest for future materials electronic applications. Scheme 1 shows the synthesis of BHE-PE2.5. Acylation of commercially available 1,4-bis(2-hydroxyethoxy)benzene, bisiodination, and Sonogashira coupling with phenylacetylene yielded diacetate BAc-PE2.5. Hydrolysis of the diacetate yielded the bishydroxyethoxy functionalized BHE-PE2.5, which was readily characterized.2 Samples for X-ray single-crystal diffraction analysis3 were prepared by recrystallization from hot acetone to yield pale yellow, shiny, and fragile flakes. Table 1 summarizes various structural parameters obtained4 from the analysis, and Figure 1 shows an ORTEP5 representation of the structure. BHE-PE2.5 crystallizes in an orthorhombic Pbcn unit cell.3 It exhibits similar bond lengths for its phenyl and ethynyl carboncarbon bonds (Table 1) to its unsubstituted analogue 1,4-bis(phenylethynyl)benzene (PE2.5),6 which has aryl r(CdC) ) 1.3761.405 Å, r(CtC) ) 1.201-1.202 Å, and r(C-C) ) 1.426-1.433 Å. The conjugated framework in BHE-PE2.5 is somewhat twisted, with a relative interplane angle between the terminal and central phenyl rings of 19.8°; by comparison, the rings in PE2.5 are planar6 to within 1.5°. Save for this modest difference in the plane-plane torsion between terminal in central phenyl rings the two molecules have very similar structures. The pendant side chains in BHE-PE2.5 are symmetrically disposed due to the inversion center in the central ring. They exhibit a gauche conformation about the C(12)-C(13) bond, not s-trans extended. This is unlike the extended conformers seen in bishexyloxy,7 bis-dodecyloxy,7 and related8,9 PE2.5 analogues with side chain subsitution. The terminal hydroxyl groups of the pendant chains form two sets of O(H)‚‚‚O(H) hydrogen bonds, where each OH group is both a hydrogen-bond donor and an acceptor. The O(H)‚‚‚O(H) contacts along the c-axis, r[O2‚‚‚O2′] ) 2.539 Å, form * To whom correspondence should be addressed. E-mail: lahti@ chem.umass.edu.
Scheme 1.
Synthesis of BHE-PE2.5
Table 1. Selected Structural Parameters for BHE-2.5PE distances
Å
angles
deg
r(C(7)≡C(8)) r(C(6)-C(7)) r(C(8)-C(9)) r(O(2)‚‚‚O(2′)) r(O(2)‚‚‚O(2′′)) r(stack)
1.195(5) 1.442(6) 1.441(6) 2.539(6) 2.686(6) 3.623(2)d
∠c/r(stack) (cant ∠) ∠O(1)C(12)-C(13)O(2) phenyl-phenyl torsion ∠C(9)C(8)-C(9′)C(8′)
13.2a 63.2b 19.8 99.2c
a Vector angle. b Intramolecular torsion. c Relative molecular rotation angle around one centroid to the next in a stack along the c-axis. d Onehalf of repeat distance along the c-axis. See Figures 1 and 2 for labeling.
Figure 1. ORTEP style diagram for BHE-2.5PE. Ellipsoids shown at 50% probability limits.
pleated chains, such that hydrogen bonds assemble molecules of BHE-PE2.5 into alternating, criss-crossed π-stacks. Within the π-stacks, the BHE-PE2.5 molecules are slightly canted (13.2°)
10.1021/cg060019z CCC: $33.50 © 2006 American Chemical Society Published on Web 04/18/2006
1254 Crystal Growth & Design, Vol. 6, No. 6, 2006
Communications polymeric conjugated systems that can π-stack well. Breaking the symmetry of cofacial π-stacks can decrease the tendency to quench, but Cornil et al. also noted11 that crossed-dipolar π-stacking “where the long axes of adjacent chains are perpendicular turns out to be the most efficient configuration to prevent any decrease in luminescence efficiency in condensed media.” Recent work by Xie et al. shows such crossed-dipolar stacking and crystalline fluorescence in some substituted 1,4-bis(styryl)benzenes.13 Crossed-dipolar π-stacking of this type is just what is formed in BHE-PE2.5, with assistance by its hydrogen-bond network. Neat microcrystalline films of BHE-PE2.5, deposited on quartz fluoresce visibly, with an emission maximum at 475 nm (excitation at 335 nm). The emission characteristics in the solid state of BHE-PE2.5 and related molecules are ongoing subjects of investigation in our labs.
Figure 2. Crystal packing of BHE-PE2.5.10 Oxygen atoms are shown as spheres, and ethynyl carbons are shown as smaller spheres. Hydrogen bonds within a π-stack are shown as double-headed arrows, and interstack hydrogen bonds are shown as dotted lines. W An interactive 3D image is available in CSD Mercury format (http:// www.ccdc.cam.ac.uk/products/csd_system/mercury/.
relative to the c-axis stack vector. The centroid-centroid distances between the molecules are about 3.6 Å along the c-axis, with a parallel plane-plane distance of rstack ) 3.509 Å between the central phenylene rings. The rotation angle from one molecule to the next along the π-stack is (99°. As a result, the long axes of the molecules alternate in a nearly perpendicular, crossed fashion going up the stack (along the c-axis). Figure 2 and Table 1 summarize the geometric features of the π-stacks. Each pendant OH group also participates in hydrogen bonds that link the π-stacks along the b-axis, r[O2‚‚‚O2′′] ) 2.686 Å. The linked stacks form two-dimensional sheets in the crystallographic bc-plane. Figure 2 shows how the sheets pack along the a-axis in a corrugated manner, with the terminal phenylethynyl groups interdigitated along the zone of packing between the sheets. The packing zone (where the sheets come together) incorporates some aryl CH to π-cloud interactions that are enabled by the canting of the BHE-PE2.5 molecules relative to the stack axis, and the slight torsion of the terminal phenyl rings relative to the central phenylene ring. A search of the Cambridge Crystallographic Database (version 5.26 from 2004) found 21 structures that incorporate a 2.5-mer of phenyleneethynylene. None shows criss-crossed π-stacks such as those exhibited by BHE-PE2.5, although some show varying degrees of aligned dipolar π-stacking. PE2.5 itself forms6 nearly perfectly cofacial π-stacks as dyads, in part due to a molecular geometry with all phenyl rings coplanar to within 1.5°. Although the PE2.5 molecules align along their long axes, the dyads stack in a strongly offset manner, with the central phenylene rings of one dyad overlapping with the terminal phenyl ring of the next dyads along the stack. They do not form extended stacks such as BHE-PE2.5 does. The criss-crossed π-stacking motif exhibited by BHE-PE2.5 is not only unusual but also of interest for applied electronic purposes, by comparison to cofacially aligned π-stacking (shown schematically below).11-13 Cornil et al. have noted11 in a computational study that highly cofacial, aligned-dipole π-stacking of oligo(paraphenylenevinylenes) (analogous to the dyad stacking in 1,4-bis(phenylethynyl)benzene) is expected to quench luminescence and is believed to contribute to poor solid-state luminescence in
The unusual, crossed-dipolar π-stacking that is induced in BHEPE2.5 by its hydrogen-bonding network would be useful to replicate for related conjugated molecules, given the potential for improving solid-state luminescence behavior by comparison to aligned, cofacially π-stacked analogues. Work is ongoing to find related systems with similar crossed-dipolar stacking that could give useful solid-state photonics behavior. Acknowledgment. The authors thank Dr. A. Chandrasekaran of the UMass-Amherst X-ray Structural Characterization Facility (NSF CHE-9974648) for assistance with the crystallographic analysis. Supporting Information Available: Synthesis and characterization, crystallographic summary in CIF format. This information is available free of charge via the Internet at http://pubs.acs.org.
References (1) See for example the following review articles and numerous citations therein: (a) Tour, J. M. Chem. ReV. 1996, 96, 537. (b) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (c) Edrington, A. C.; Urbas, A. M.; DeRege, P.; Chen, C. X.; Swager, T. M.; Hadjichristidis, N.; Xenidou, M.; Fetters, L. J.; Joannopoulos, J. D.; Fink, Y.; Thomas, E. L. AdV. Mater. 2001, 13, 421. (d) Zheng, J.; Swager, T. M. AdV. Polym. Sci. 2005, 177, 151. (e) Stone, M. T.; Heemstra, J. M.; Moore, J. S. Acc. Chem. Res. 2006, 39, 11. (2) BHE-PE2.5: mp 235-236 °C; HRMS calcd for C26H22O4 398.1177, found 398.1541. MS (electron impact, m/z): 398 (M, 100%), 354, 310, 281, 252, 250, 226, 155, 126, 87, 57. 1H NMR (acetone-d6): 4.01 (t, 4H, J ) 8 Hz), 4.26 (t, 4H, J ) 8 Hz), 7.29 (s, 2H), 7.497.51 (m, 6H), 7.63-7.65 (m, 4H). UV-vis (THF, nm): 306, 360 ( ) 25200 M-1 cm-1). Luminescence (nm): 400 nm (THF, λexc ) 365); 475 nm (neat solid; λexc ) 335). (3) Crystallography: orthorhombic, Pbcn, C26H22O4, formula wt ) 398.44, T ) 293(2) K, Z ) 4, a ) 22.5520(9) Å, b ) 12.9259(4) Å, c ) 7.2463(2) Å, V ) 2112.33(12) Å3, µ ) 0.084 mm-1, F(000) ) 840 (without hydroxyl hydrogens), 1853 total reflections, for 1842 reflections with F2 > 2σ(F?), using 136 parameters, R1 ) 0.1192, wR2 ) 0.1455. All hydrogens were allowed to ride on the parent atoms, except that hydroxyl hydrogens were not included in any calculations. CCDC deposition #295499. (4) Crystallographic analysis with SHELXL97 (Sheldrick, G.; SHELXTL97, Program for the Refinement of Crystal Structures; University of Go¨ttingen, Germany, 1997). (5) ORTEP-III for Windows, see Farrugia, L. J. J. Appl. Cryst. 1997, 30, 565. (6) Li, H.; Powell, D. R.; Douglas, R.; Firman T. K.; West, R. Macromolecules 1970, 3, 1093. (7) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules 1998, 31, 52.
Communications (8) Hoger, S.; Enkelmann, V.; Bonrad K.; Tschierske, C. Angew. Chem., Int. Ed. 2000, 39, 2268. (9) Bunz, U. H. F.; Enkelmann, V.; Koppenburg, L.; Jones, D.; Shimizu, K. D.; Claridge, J. B.; zur Loye. H-C.; Lieser, G. Chem. Mater. 1999, 11, 1416. (10) Figure prepared using the Mercury 1.4.1 freeware program from the Cambridge Crystallographic Data Centre. (11) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bre´das, J. L. J. Am. Chem. Soc. 1998, 120, 1289.
Crystal Growth & Design, Vol. 6, No. 6, 2006 1255 (12) He, F.; Xu, H.; Yang, B.; Duan, Y.; Tian, L. L.; Huang, K. K.; Ma, Y. G.; Liu, S. Y.; Feng, S. H.; Shen, J. C. AdV. Mater. 2005, 17, 2710. (13) Xie, Z.; Yang, B.; Li, F.; Cheng, G.; Liu, L.; Yang, G.; Xu, H.; Ye, L.; Hanif, M.; Liu, S.; Ma, D.; Ma, Y. J. Am. Chem. Soc. 2005, 127, 14152.
CG060019Z
