Diels−Alder Adduct of Pentacene and Maleimide: Crystal Growth and

Diels-Alder Adduct of Pentacene and Maleimide: Crystal Growth and the Influence of Solvent Molecules on Structure and Hydrogen Bonding David B. Mitzi* and Ali Afzali

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 691-697

IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 ReceiVed August 21, 2006; ReVised Manuscript ReceiVed January 9, 2007

ABSTRACT: Crystals of the soluble Diels-Alder adduct of pentacene and maleimide, C26H17NO2, as well as of the corresponding 1:1 inclusion compounds with 1,2-dichlorobenzene and toluene and the 2:3 compound with pyridine, have been grown by either solution growth techniques (1, C26H17NO2‚C6H4Cl2; 2, C26H17NO2‚C6H5CH3; and 3, 2C26H17NO2‚3C5H5N) or by vacuum sublimation (4, C26H17NO2). Each of the crystal structures are comprised of discrete C26H17NO2 molecules, with the maleimide moiety adding to the third (central) ring of the original pentacene molecule. The structures present several distinct hydrogen-bonding patterns, depending upon the character of the solvent used. Whereas the C26H17NO2 molecules in 4 (no solvent) are arranged in extended hydrogen-bonded (N-H‚‚‚O) chains, in 1 and 2, the C26H17NO2 molecules form hydrogen-bonded (N-H‚‚‚O) dimers, alternating with the included solvent molecules. In 3, the N-H‚‚‚O bonds are replaced with N-H‚‚‚N and weak C-H‚‚‚O interactions between C26H17NO2 and pyridine molecules, leading to hydrogen-bonded C26H17NO2‚‚‚C5H5N pairs. The inclusion compounds thermally decompose in the temperature range 125-160 °C, losing the solvent molecules and leaving behind C26H17NO2, which principally sublimes rather than undergoing the retro-Diels-Alder decomposition in the temperature range 275-340 °C. Introduction The Diels-Alder (D-A) reaction of acenes with dienophiles has generated substantial recent interest, as a result of potential application in the solution-deposition of semiconducting organic films,1-4 examination of RNA-diene interactions,5 creation of new dendritic macromolecules,6 thermal cross-linking, chain extension, and property modification of copolyesters,7 as well as for more fundamental mechanistic and structural studies.8-11 Reaction of relatively insoluble pentacene with N-sulfinylacetamide, for example, leads to a soluble precursor that can be spin coated or drop cast onto a substrate.1 Upon heating of the precursor film, a low-temperature retro D-A reaction begins at ∼120 °C, leading to reconversion to pentacene. High-quality semiconducting pentacene films have been deposited from the D-A precursor solution, providing field-effect mobilities of as high as 0.9 cm2 V-1 s-1, among the highest reported values for spin-coated organic semiconductors.1 The choice of dienophile can also influence the functionality of the resulting adduct, as in the case of using N-sulfinyl-tert-butylcarbamate rather than sulfinylacetamide, which yields a soluble and photopatternable pentacene precursor.2 Appropriately tailored acene and dienophile species have been employed to design crystals that undergo topochemical reaction. For example, a heat-induced solid-state [4+2] cycloaddition between anthracene and bis(N-ethylimino)-1,4-dithiin within a charge-transfer crystal provides an unusual example of a heteromolecular single-crystal-to-single-crystal transformation.11 The success of this transformation depends on the careful selection of reactant species, so as to yield a molecular crystal with the component molecules held in a precise geometry to facilitate reaction with minimal movement of the molecules. Choice of dienophile may also influence the inclusion chemistry of resulting D-A adducts.9,10 In the case of the maleimideanthracene adduct 9,10-dihydro-9,10-ethanoanthracene-11,12dicarboxamide, addition of a functional “clathratogenic group” (an inclusion-promoting group, such as an amine) to the * Corresponding author. E-mail: [email protected].

dicarboximido moiety significantly impacts the ability of the host structure to incorporate molecular species. Factors that influence inclusion with respect to these functional groups include geometric constraints of the resulting molecule, as well as the possibility of noncovalent interactions (e.g., hydrogen bonding and van der Waals interactions) between the host structure and the included guest species.9,10 In this study, we consider the D-A reaction of maleimide and pentacene in the formation of a new soluble pentacene adduct. In contrast to the sulfinyl-related pentacene adducts,1,2 the resulting maleimide adduct is more thermally stable and therefore less prone to the retro D-A reaction upon heating. The pentacene-maleimide adduct is examined with respect to inclusion chemistry using various solvents, some of which have chemical groups that can effectively hydrogen bond to the maleimide component of the D-A adduct. The structural flexibility of the D-A adduct is demonstrated by comparing the molecular structure in different chemical environments. Experimental Section General. Pentacene and maleimide were purchased from Aldrich Chemical Co. and were used as received. NMR spectra were obtained on a 400 MHz Bruker spectrometer using d6-DMSO as solvent. Chemical analyses were performed for each product (in duplicate) by Galbraith Laboratories. Synthesis. (a) C26H17NO2‚C6H4Cl2 (1). Pentacene (835 mg, 3 mmole) was added to a solution of maleimide (291 mg, 3 mmole) in 20 mL of 1,2-dichlorobenzene (Aldrich, 99%), and the mixture was heated at reflux under a stream of nitrogen for 3 h. After cooling the mixture to room temperature, the precipitate was filtered, washed several times with toluene and then hexane, and finally dried. Recrystallization from 1,2-dichlorobenzene afforded the 1:1 adduct (1.05 g, 67% yield). A final slower recrystallization was performed by slow cooling (2 °C/h) of a solution of 194 mg of 1 in 10 mL of 1,2-dichlorobenzene (Aldrich, 99%, anhydrous), from 115 to 20 °C, providing an approximately quantitative yield of colorless rodlike crystals. The powder X-ray diffraction pattern of the product could be indexed to that calculated for the single-crystal structure determined for 1 (see below), with no unindexed reflections. Chemical anal. Observed: C, 73.7; N, 2.7; H, 3.8. Calcd for 1: C, 73.6; N, 2.7; H, 4.1. 1H NMR (d6-DMSO, 400

10.1021/cg060559w CCC: $37.00 © 2007 American Chemical Society Published on Web 03/15/2007

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Table 1. Crystallographic Data for C26H17NO2‚C6H4Cl2 (1), C26H17NO2‚C6H5CH3 (2), 2C26H17NO2‚3C5H5N (3), and C26H17NO2 (4) fw space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g/cm3) λ (Å) absorp. coeff. µ (cm-1) Rfa Rwb a

1

2

3

4

522.420 P21/c (No. 14) 11.8175(6) 11.9876(6) 17.9070(9)

467.557 P21/c (No. 14) 11.710(1) 11.453(1) 18.356(2)

375.419 P21/n (No. 14) 14.209(1) 7.6337(6) 18.656(2)

92.109(1)

90.262(2)

2535.0(4) 4 1.369 0.71073 2.9 6.8 8.7

2461.8(5) 4 1.262 0.71073 0.8 4.3 5.1

988.137 P1h (No. 2) 10.920(2) 15.549(2) 15.558(2) 102.799(3) 95.801(3) 91.164(3) 2560.4(6) 2 1.282 0.71073 0.8 5.7 5.7

108.395(1) 1920.1(5) 4 1.299 0.71073 0.8 4.4 4.9

Rf ) ∑(|Fo| - |Fc|)/∑(|Fo|). b Rw ) {∑w(|Fo| - |Fc|)2/∑(w|Fo|2)}1/2.

MHz) 3.41, s 2H; 4.96, s 2H; 7.36-7.39, m 2H; 7.44-7.48, m 4H; 7.60-7.64, m 2H; 7.82, s 2H; 7.85-7.92, m 4H; 7.97, s 2H and 10.80, bs 1H. (b) C26H17NO2‚C6H5CH3 (2). A suspension of pentacene (835 mg, 3 mmol) and maleimide (291 mg, 3 mmol) in 10 mL of toluene (Aldrich, 99.8%, anhydrous) was heated in a sealed tube at 170 °C for 3 h. The reaction mixture was cooled to room temperature and the white solid was filtered off, washed with toluene, and dried in a vacuum oven at 60 °C. Recrystallization from toluene afforded a white crystalline adduct (1.0 g, 71%). A final slower recrystallization was performed by slow cooling (2 °C/h) a solution of 194 mg of 2 in 25 mL of toluene (Aldrich, 99.8%, anhydrous) from 115 to 20 °C, providing ∼165 mg of colorless crystals. The powder X-ray diffraction pattern of the product could be indexed to that calculated for the singlecrystal structure determined for 2 (see below), with no unindexed reflections. Chemical anal. Observed: C, 84.6; N, 3.1; H, 5.5. Calcd for 2: C, 84.8; N, 3.0; H, 5.4. 1H NMR (d6-DMSO, 400 MHz) 3.42, s 2H; 4.95, s 2H; 7.12-7.17, m 3H; 7.22-7.26, m 2H; 7.44-7.48, m 4H; 7.82, s 2H; 7.85-7.87, m 4H; 7.97, s 2H and 10.78, bs 1H. (c) 2C26H17NO2‚3C5H5N (3). Crystals of 3 were prepared by dissolving 200 mg of 2 in 0.7 mL of pyridine (Aldrich, 99.8%, anhydrous) upon heating to 115 °C. Slow cooling the solution from 115 to 20 °C at 2 °C/h yielded approximately 120 mg of colorless crystals (57% yield). The powder X-ray diffraction pattern of the product could be indexed to that calculated for the single-crystal structure determined for 3 (see below). Chemical anal. Observed: C, 80.9; N, 7.2; H, 5.1. Calcd for 3: C, 81.4; N, 7.1; H, 5.0. 1H NMR (d6-DMSO, 400 MHz) 3.44, s, 2H; 4.96, s, 2H; 7.36-7.39, m 3H; 7.447.48, m 4H; 7.78, t, j ) 6 Hz 1.5 H; 7.82, s 2H; 7.85-7.87, m 4H; 7.98, s 2H; 8.56, d, j ) 6 Hz, 3H; and 10.82 bs, 1H. (d) C26H17NO2 (4). Crystals of 4 were prepared from C26H17NO2‚C6H4Cl2 (1) by first dissociating the dichlorobenzene, heating it to 225 °C in a flowing nitrogen atmosphere. Approximately 40 mg of the resulting white product was then loaded into an evacuated quartz tube (prebaked to remove water). The sealed tube was placed into a tube furnace such that one end (the end containing the white product) was in the hot zone of the furnace at 300 °C, whereas the other end was outside the furnace. After approximately 24 h, the white product at the hot end of the tube had sublimed down the tube toward the cool end, yielding colorless, well-formed crystals. The powder X-ray diffraction pattern of the product could be indexed to that calculated for the single-crystal structure determined for 4 (see below), with no unindexed reflections. Chemical anal. Observed: C, 83.3; N, 3.9; H, 4.3. Calcd for 4: C, 83.2; N, 3.7; H, 4.6. 1H NMR (d6-DMSO, 400 MHz) 3.40, s 2H; 4.96, s 2H; 7.44-7.48, m 4H; 7.82, s 2H; 7.857.87, m 4H; 7.98, s 2H and 10.79 s, 1H. X-ray Crystallography. A colorless C26H17NO2‚C6H4Cl2 (1) (C26H17NO2‚C6H5CH3 (2)/2C26H17NO2‚3C5H5N (3)/C26H17NO2 (4)) crystal, with the approximate dimensions 0.16 mm × 0.22 mm × 0.53 mm (1) (0.16 mm × 0.19 mm × 1.4 mm (2)/0.04 mm × 0.11 mm × 0.79 mm (3), 0.16 mm × 0.24 mm × 0.44 mm (4)), was selected under a microscope and attached to the end of a quartz fiber with 5 min epoxy. Intensity data were collected with a Bruker SMART CCD diffractometer equipped with a fine focus 2.4 kW sealed tube X-ray source (Mo KR

radiation). A detector distance of approximately 5.0 cm was employed in the collection of 2272 frames with increasing ω and an exposure time of 20 s (1) (40 s (2)/60 s (2)/30 s (4)) per frame. The increment in ω between each frame was 0.3°. Final unit-cell parameters (Table 1) and the crystal orientation matrix were obtained by a least-squares fit of 8058 (1) (4363 (2)/2763 (3)/ 6454 (4)) reflections. An empirical absorption correction based on equivalent reflections was applied to the intensity data.12 The structure was solved and refined using the NRCVAX 386 PC version program.13 First, the C, N, O, and Cl atoms (for 1) were located using direct methods and Fourier difference maps. All heavy atoms (C, N, O, and Cl) were refined anisotropically. Hydrogen atoms were located from difference maps and fully refined using isotropic thermal parameters, with the exception of the dichlorobenzene hydrogens for 1 and the hydrogens for the non-hydrogen-bonded pyridine in 3, for which only the thermal parameters were refined. For 1, the dichlorobenzene solvent molecule exhibited some disorder, which was modeled using two sets of chlorine atoms Cl1, Cl2 and Cl1a, Cl2a (displaced by 0.4-0.5 Å) with occupancies of 0.65 and 0.35, respectively. Similarly, the toluene molecule in 2 exhibited relatively large thermal parameters and irregular C-H distances as a result of disorder and thermal motion of the solvent molecules in the structure. The minimum and maximum peaks in the final difference Fourier maps corresponded to -0.53 and 0.85 e/Å3 (1) (-0.18 and 0.24 e/Å3 (2), -0.26 and 0.28 e/Å3 (3), -0.15 and 0.18 e/Å3 (4)). No additional symmetry was detected for the refined structures using the MISSYM program.14 Crystallographic results for the three compounds are summarized in Table 1. A complete listing of crystallographic data (in CIF format) is given in the Supporting Information. Thermal Analysis. Thermogravimetric analysis (TGA) scans were performed, using a TA Instruments TGA-2950 system, in a flowing nitrogen atmosphere and with a 1 °C/min ramp to 375 °C.

Results and Discussion Crystal Structures. Each of the crystal structures for 1-4 consist of discrete C26H17NO2 molecules (Figure 1), with the maleimide moiety adding to the third (central) ring of the original pentacene molecule. In 4 (no solvent), the dihedral angle formed by the best-fit planes through the two nominally planar arms of the C26H17NO2 molecule, defined by the carbon atoms (C7-C18) and (C1-C7, C18-C22), respectively, is 126.5°. The three inclusion compounds 1-3 present similar dihedral angles, 121.7, 128.1, and 117.9/124.4° (two independent molecules in 3), respectively. Analogous dihedral angles for some anthracenebased Diels-Alder adducts range from 120.8 to 128.9°.9,15 The ∼10° range of dihedral angles observed in the C26H17NO2 compounds suggests that, despite the relatively rigid nature of the polycyclic structure, there is still significant mechanical flexibility to this molecule. The C26H17NO2 molecules in 4 (no solvent molecules incorporated) form into extended hydrogen-bonded zigzag

Diels-Alder Adduct of Pentacene and Maleimide

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Figure 1. Pentacene-maleimide adduct, showing atom labels and thermal parameters for C26H17NO2 (4). The thermal ellipsoids for the non-hydrogen atoms are drawn at the 50% probability level, whereas the hydrogen atoms are drawn as spheres with arbitrary size.

chains directed down [010] (Figure 2). Each C26H17NO2 molecule exhibits N-H‚‚‚O hydrogen bonds to two different C26H17NO2 molecules across the chain (N‚‚‚O, N-H‚‚‚O distances of 2.930(2) and 2.09(2) Å, respectively, and N-H‚‚ ‚O angle of 160(2)°). As a comparison, in the analogous maleimide (C4H3NO2) structure (i.e., not reacted with pentacene),16 each maleimide is similarly involved in two N-H‚‚‚O hydrogen bonds, although in this case, both are directed to the same opposing maleimide molecule rather than to two separate molecules. The resulting hydrogen-bonded maleimide dimers have N‚‚‚O and N-H‚‚‚O distances ranging from 2.851(7) to 2.917(7) Å and 1.98 to 2.05 Å, respectively, slightly shorter than those observed in 4. In 4, only one imido carbonyl oxygen (O1) participates in N-H‚‚‚O hydrogen bonding, whereas the other oxygen (O2) has a weak C-H‚‚‚O interaction with an adjacent hydrogen-bonded chain (3.437(2) Å C26‚‚‚O2 distance; 2.61(2) Å C26-H‚‚‚O2 distance; 146(1)° C26-H‚‚‚O2 bond angle). In addition to hydrogen bonding, there are several intermolecular C‚‚‚C interactions, which are shorter than or comparable to the sum of the van der Waals radii for two aromatic carbon atoms (2 × 1.77 Å ) 3.54 Å)17 and link the hydrogen-bonded chains together into a more three-dimensional network. These interactions include C2‚‚‚C10* (3.462(3) Å) and C10‚‚‚C23* (3.57(2) Å), where the asterisks denotes a carbon atom on a different C26H17NO2 molecule and chain. The dichlorobenzene- and toluene-containing compounds 1 and 2 provide a different hydrogen-bonding motif than that observed in 4 (Figure 3). Each C26H17NO2 molecule has two N-H‚‚‚O hydrogen bonds to a single opposing molecule (rather than to two different molecules), leading to an isolated dimer configuration similar to that found in maleimide.16 The hydrogen bonds in 1 are 2.877(4) Å (for the N‚‚‚O distance) and 1.95(4) (for the N-H‚‚‚O distance), with a N-H‚‚‚O bond angle of 167(3)°. As for 4, only O1 forms a N-H‚‚‚O hydrogen bond, with the other oxygen, O2, participating in a weak C-H‚‚‚O interaction (3.290(4) Å C3‚‚‚O2 distance; 2.60(4) Å C3-H‚‚‚ O2 distance; 126(3)° C3-H‚‚‚O2 angle) to an adjacent hydrogenbonded dimer. The reduced connectivity of the N-H‚‚‚O hydrogen-bonded network allows for more facile integration of the additional solvent (guest) molecules (one dichlorobenzene molecule per C26H17NO2). With respect to the solvent, the only intermolecular C‚‚‚C interactions shorter than 3.60 Å in 1 are between the dichlorobenzene molecule and arm 2 of C26H17NO2. The offset face-to-face interaction between these moieties yields four relatively short intermolecular C‚‚‚C distances ranging from 3.56(6) to 3.58(7) Å. The dihedral angle

Figure 2. Detailed structure of C26H17NO2 (4). (a) Single hydrogenbonded chain of C26H17NO2 molecules, progressing along [010], with the hydrogen bonding indicated using dashed lines. Atoms are represented as spheres with uniform sizes selected for each atom type (white spheres, carbon atoms; red spheres, oxygen atoms; blue spheres, nitrogen atoms; and small gray spheres, hydrogens). (b) Overall stacking of the C26H17NO2 molecules, using a skeletal drawing for clarity (hydrogen atoms are not shown). The hydrogen-bonded chain shown in (a) is highlighted in (b) using a bolder line font. The chains are extending into the plane of the figure.

between the best planes formed by the dichlorobenzene carbons and the naphthalene moiety associated with arm 2 of C26H17NO2 is 4.6(2)°, indicating a nearly parallel relationship between these species (parts a and b of Figure 4). The hydrogen-bonded C26H17NO2 dimer in 2 is essentially identical to that in 1, with similar hydrogen-bonding distances and angles: 2.859(2) Å (N‚‚‚O distance), 1.97(2) Å (N-H‚‚‚O distance), and 175(2)° (N-H‚‚‚O bond angle). As for 1, only the O1 oxygen is involved with N-H‚‚‚O hydrogen bonding, whereas O2 has C-H‚‚‚O interactions with two adjacent C26H17NO2 dimers (C3‚‚‚O2 distance, 3.373(2) Å; C3-H‚‚‚O2 distance: 2.65(2) Å, C3-H‚‚‚O2 angle: 131(1)°; C1‚‚‚O2 distance: 3.392(2) Å, C1-H‚‚‚O2 distance, 2.45(2) Å; C1-H‚‚‚ O2 angle, 163(1)°). Note that in 1, the C1‚‚‚O2 interaction found in 2 is less important, as the C1‚‚‚O2 distance is shifted to 3.672-

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Figure 4. Relationship between solvent and C26H17NO2 molecules in 1 (a and b) and 2 (c and d). Two views, rotated 90° from each other, are shown for each compound. Atoms are represented as spheres with uniform sizes selected for each atom type (hydrogen atoms are not shown). The carbon atoms associated with the solvent molecules (dichlorobenzene and toluene) and arm 2 of the C26H17NO2 molecule are shaded gray to facilitate viewing of the relative configuration of these moieties. Figure 3. Detailed structure of C26H17NO2‚C6H4Cl2 (1). (a) Single pair of hydrogen-bonded C26H17NO2 molecules, with the hydrogen bonding indicated using dashed lines. Atoms are represented as spheres with uniform sizes selected for each atom type (white spheres, carbon atoms; red spheres, oxygen atoms; blue spheres, nitrogen atoms; and small gray spheres, hydrogens). (b) Overall stacking of the C26H17NO2 (black line) and C6H4Cl2 (gray line) molecules, using a skeletal drawing for clarity (hydrogen atoms are not shown). The hydrogen-bonded pair shown in (a) is highlighted in (b) using a bolder line font.

(5) Å (perhaps, in part, reflecting the longer b-axis dimension for 1). The included toluene molecule in 2 is significantly rotated relative to the dichlorobenzene orientation found in 1 (parts c and d in Figure 4). The resulting dihedral angle between the best planes formed by the toluene carbons (neglecting the methyl group) and the naphthalene moiety associated with arm 2 of C26H17NO2 is 40.6(2)°, leading to an edge-to-face (rather than face-to-face) interaction, with a closest C26H17NO2-toluene C‚ ‚‚C distance of 3.537(3) Å. In 3, pyridine effectively competes with adjacent C26H17NO2 molecules for hydrogen bond formation. As a result of this competition, there is no significant hydrogen-bonding between neighboring C26H17NO2 molecules (Figure 5) other than relatively weak C-H‚‚‚O interactions (e.g., C3-H‚‚‚O4*, C‚‚‚O and C-H‚‚‚O distances of 3.359(6) and 2.43(4) Å, respectively, and C-H‚‚‚O bond angle of 149(3)°; C40-H‚‚‚O4*, C‚‚‚O and C-H‚‚‚O distances of 3.312(7) and 2.37(4) Å, respectively, and C-H‚‚‚O bond angle of 158(3)°; C52-H‚‚‚O1*, C‚‚‚O and C-H‚‚‚O distances of 3.473(5) and 2.54(4) Å, respectively, and C-H‚‚‚O bond angle of 161(3)°). There are, however, numerous other noncovalent-type interactions, including a face-to-face contact between nearly parallel five-membered dicarboximide rings on adjacent C26H17NO2 molecules, with two C‚‚‚C contacts

as short as 3.371(6) Å for C24‚‚‚C25* and C25-C24*. A short C‚‚‚O distance of 2.973(5) Å is also found (not hydrogenbonding because there is no hydrogen atom involved) between carbonyls on neighboring dicarboximide rings, which are at a 75° dihedral angle with respect to each other. A very similar short C‚‚‚O contact (2.97 Å) is observed in the anthracenebased D-A adduct, N-carboxyethyl-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxamide methanol clathrate,9 between a carboxy CdO group and a carboximido CdO group, where the strongly dipolar nature of the CdO bond is enhanced by electron-withdrawing neighbors such as the imide N atom. The interaction therefore appears to be dipole-dipole in nature, as has been previously observed in organic crystals containing several carbonyl groups (e.g., as in the anhydrous alloxan structure).9,18 Instead of hydrogen-bonding to adjacent C26H17NO2 molecules through N-H‚‚‚O links, as in 1, 2, and 4, the structure for 3 exhibits a single N-H‚‚‚N linkage between each C26H17NO2 molecule and an opposing guest or pyridine, with N‚‚‚N distances of 2.842(6) Å (N1‚‚‚N4) and 2.952(6) Å (N2‚‚‚N3) and N-H‚‚‚N bond angles of 173(5) and 163(4)°, respectively (Figure 5). Only two of the three pyridines are hydrogen bonded to a C26H17NO2 molecule. Of the hydrogen-bonded pyridines, one (i.e., the one associated with the N4) has an offset faceto-face interaction with arm 1 of an adjacent C26H17NO2 molecule (6.5° dihedral angle between pyridine and the naphthalene component of C26H17NO2), with a closest intermolecular C‚‚‚C contact of 3.469(6) Å. The other hydrogen-bonded pyridine (i.e., the one associated with the N3) lacks this faceto-face interaction, but rather has a significant C-H‚‚‚O interaction to the same C26H17NO2, as shown in Figure 5a (C57‚

Diels-Alder Adduct of Pentacene and Maleimide

Figure 5. Detailed structure of 2C26H17NO2‚3C5H5N (3). (a) Two independent hydrogen-bonded (dashed lines) C26H17NO2‚C5H5N pairs are shown, using a skeletal representation for clarity. Selected secondary interactions (see text) are also indicated using dotted lines. The third non-hydrogen-bonded pyridine molecule is not shown for clarity. (b) Overall stacking of the C26H17NO2 and C5H5N molecules is shown (hydrogen atoms are not shown). The non-hydrogen-bonded pyridine molecule is shown using a lighter gray coloration to distinguish it from the hydrogen-bonded molecules.

‚‚O4* and C57-H‚‚‚O4* distances of 3.43(1) and 2.55(5) Å, respectively, and a C57-H‚‚‚O4* angle of 148(3)°). The third pyridine (associated with N5) has no significant hydrogen bonding interaction with neighboring C26H17NO2 molecules (accounting for the larger degree of structural disorder for this molecule). However, there is a weak edge-to-face type contact to arm 1 of a C26H17NO2 molecule, with a closest C‚‚‚C interaction of 3.575(9) Å (C29‚‚‚C65*) and a dihedral angle between the pyridine and the naphthalene moiety (arm 1) of 61°. The structural results for 1-4, taken collectively, support the notion that, in addition to the impact of the primary N-H‚‚‚O and N-H‚‚‚N hydrogen-bonding, a wide variety of secondary interactions play an important role in determining the overall

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structure and molecular conformation observed for the C26H17NO2 compounds. Of particular interest, given the dicarboximide group on each C26H17NO2, are relatively weak C-H‚‚‚O hydrogen-bonding interactions, which have been shown to play an important role in supramolecular assembly, crystal packing, and molecular conformation.19-23 Significant C-H‚‚‚O interactions in the structures 1-4 have C‚‚‚O distances ranging from 3.29 to 3.47 Å, C-H‚‚‚O distances between 2.37 and 2.65 Å, and C-H‚‚‚O bond angles between 131 and 163°. These values fall roughly in the middle of the ranges reported for other known structures with structurally significant C-H‚‚‚O hydrogen bonding.19-23 Note that the tendency to form short C-H‚‚‚O contacts increases as the “donor strength” of C-H increases (neglecting steric hindrance).19 The donor strength depends in part on carbon hybridization and increases in the order C(sp1)-H > C(sp2)-H > C(sp3)-H. For a given hybridization, the donor strength increases when C is bonded to an electronegative atom (e.g., N), as is the case for a pyridine molecule. The C-H‚‚‚O bond is also strengthened when the O atom is associated with a strong hydrogen acceptor (e.g., CdO), as for the dicarboximide ring.19 Thermal Analysis. Beyond structural analysis, two additional considerations for the inclusion compounds 1, 2, and 3 are the temperature at which the included molecules dissociate from the sample and the decomposition or sublimation temperature of the remaining C26H17NO2. For comparison, the previously reported sulfinylacetamide-pentacene adduct undergoes a retroDiels-Alder reaction at temperatures of as low as 120 °C to yield pentacene and N-sulfinylacetamide.1 Figure 6a shows the thermogravimetric analysis (TGA) data for C26H17NO2‚C6H4Cl2 (1) upon being heated in a nitrogen atmosphere. At temperatures below 100 °C, 1 is relatively stable and exhibits little weight loss. Above 130 °C, the first weight loss process begins, corresponding to the loss of the dichlorobenzene molecule (note that the boiling point of 1,2-dichlorobenzene is ∼180 °C). This process is completed by approximately 150 °C, yielding a weight loss of 28.3%, in good agreement with the expected loss of 28.1% for the decomposition of C26H17NO2‚C6H4Cl2 to C26H17NO2. The second weight-loss transition begins at approximately 275 °C and is completed by 340 °C, resulting in essentially the complete loss of the sample. A quantity of C26H17NO2‚C6H4Cl2 (1), decomposed in the TGA up to 225 °C and then placed in an evacuated quartz tube at 300 °C, leads to transport of the colorless crystalline product to a cooler region of the tube (see the Experimental Section). The resulting crystals are C26H17NO2 (4), indicating that the weight loss observed above 275 °C is primarily sublimation rather than decomposition of the C26H17NO2 adduct. The pentacene-maleimide adduct is therefore substantially more thermally stable than the previously reported pentacenesulfinylacetamide system.1 The analogous toluene derivative, (2), exhibits virtually identical decomposition characteristics (Figure 6b) with, however, a smaller initial weight loss (19.9% observed versus 19.7% expected) than for the dichlorobenzene analog, as a result of the less massive solvent molecule evolving from the sample. Figure 6c shows the thermogravimetric analysis (TGA) data for 3C26H17NO2‚2C5H5N (3). Above 125 °C, the first weight-loss process begins (mostly completed by ∼140 °C), corresponding to the loss of the pyridine (note that the boiling point of pyridine is ∼115 °C). Despite a higher vapor pressure at a given temperature when compared with 1,2-dichlorobenzene, the solvent molecule evolves from the sample at approximately the same temperature, perhaps because of the stronger hydrogen

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Figure 6. TGA scans for (a) C26H17NO2‚C6H4Cl2 (1), (b) C26H17NO2‚C6H5CH3 (2), and (c) 2C26H17NO2‚3C5H5N (3), each run using a 1 °C/min heating rate in a flowing nitrogen atmosphere. For each compound, the weight change associated with the solvent molecule dissociation is indicated by an arrow.

bonding between the solvent molecule and the C26H17NO2 molecule for the pyridine system (although note that one of the three pyridine molecules does not exhibit substantial hydrogen bonding). The observed weight loss (24.9%) confirms the 3:2 stoichiometry between C26H17NO2 and pyridine components, as previously established by the crystal structure determination and NMR analysis. Conclusion D-A reactions between acenes and dienophiles have attracted significant scientific and technological interest for the preparation of both novel and/or application-oriented organic compounds and films. In this study, we have reacted pentacene with maleimide to achieve a D-A adduct in which the maleimide moiety adds to the third (central) ring of the original pentacene molecule. The resulting D-A adduct is soluble in a variety of organic solvents including pyridine, N, N-dimethylformamide, 1,4-dioxane, 1,2-dichlorobenzene (with heating), and toluene (with heating). In addition, the resulting five-membered dicarboximide functional group enables a range of structure-directing interactions, from stronger more directional N-H‚‚‚O and N-H‚‚‚N hydrogen bonds to weaker C-H‚‚‚O hydrogen bonds and more isotropic van der Waals type interactions. C26H17NO2 is shown to act as a host for various guest (solvent) molecules. Crystal packing in these guest-containing structures follows from the relatively complex interplay of the various contacts among the C26H17NO2 and guest molecules.

Mitzi and Afzali

Three distinct hydrogen-bonding motifs have been isolated using the host C26H17NO2 molecules and 1,2-dichlorobenzene, toluene, or pyridine as guest molecules. The crystal structure for 4 (no solvent molecule) is found to consist of zigzag chains of C26H17NO2, linked by two N-H‚‚‚O hydrogen bonds per C26H17NO2. Incorporation of the solvents 1,2-dichlorobenzene and toluene in a 1:1 ratio with respect to C26H17NO2 (1 and 2, respectively) yields structures that consist of C26H17NO2 dimers, bound together by pairs of N-H‚‚‚O bonds across the dicarboximide rings. In the analogous 3:2 inclusion compound with pyridine, 3, competition for hydrogen bonding among pyridine and C26H17NO2 molecules leads to the replacement of C26H17NO2 with a pyridine in the hydrogen-bonded pair and correspondingly the replacement of N-H‚‚‚O with a N-H‚‚‚N interaction. The higher degree of solubility of C26H17NO2 in pyridine versus dichlorobenzene or toluene (at room temperature) likely arises because of the stronger interaction of the solvent with the C26H17NO2 molecules. In each of the C26H17NO2 structures, relatively weak C-H‚‚‚O hydrogen bonds are also shown to play a significant role in determining the structure, along with an array of other types of secondary interactions (e.g., arene-arene, van der Waals). Finally, the thermal properties of the D-A adducts have been examined, revealing much higher thermal stability compared with previous D-A adducts based on, for example, the reaction of sulfinylacetamide with pentacene. Upon heating the guestincorporated compounds, loss of the guest occurs in the temperature range 125-160 °C, leaving behind the C26H17NO2 molecule. In turn, C26H17NO2 is found to sublime, rather than decompose, in the temperature range 275-340 °C. The improved thermal stability and solubility of C26H17NO2 should enable thin films of this compound to be prepared via either solution-based or vacuum-based thermal evaporation processes. Acknowledgment. The authors gratefully acknowledge R. Tromp for useful discussions regarding this project. Supporting Information Available: X-ray crystallographic files, in CIF format, containing structural information for C26H17NO2‚C6H4Cl2 (1), C26H17NO2‚C6H5CH3 (2), 2C26H17NO2‚3C5H5N (3), and C26H17NO2 (4). This material is available free of charge via the Internet at http://pubs.acs.org.

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