Design of Novel Functional Materials Using Reactions of Quinones with Aromatic Amines

Abstract

This chapter presents examples for the successful use of Schiff base chemistry in the molecular design and synthesis of novel organic semiconductors composed of fused heterocyclic ladder oligomers as an alternative for oligoacenes. The materials resulting from reacting quinones with aryl amines (Schiff Bases) have proven to possess favorable properties for applications when compared with their oligoacene counterparts, including pentacene. Molecules obtained from reactions of 2,5- dichloro-1,4-benzoquinone and 2,3-dichloro-1,4-naphtoquinone with 2-aminophenols and 2-aminothiophenols discussed in this chapter. We also include one example of half a ladder molecule, which we usually use as a first step in the preparation of asymmetrical structures. Four molecules are discussed in this work. They are: benzo[5,6][1,4]thiazino[2,3-b]phenothiazine (1); 6,9-dichloro-5H-benzo[a]phenoxazin-5-one (2); benzo[a]benzo[5,6][1,4]thiazino [3,2-c]phenothiazine (3); and 13-nitrobenzo[a]benzo[5,6][1,4]oxazino[3,2-c]phenothiazine (4). Compounds 2 and 3 crystallize in the monoclinic space groups P21/c with four molecules in the unit cell each, whereas compound 4 crystallizes in the orthorhombic Fdd2 space group with 16 molecules in the unit cell. Some of the most significant features in the single-crystal structures of three of these are also discussed. π–π Stack distances of 3.085 and 3.290 Å were also observed in compounds 3 and 4.

1. Introduction

Conjugated organic molecules have attracted great interest due to their unique optical and electrical properties. They have potential for application devices such as field-effect transistors (FETs), light-emitting diodes (LEDs), and solar cells [1, 2, 3, 4]. Among the most studied materials in this context were oligoacenes, such as pentacene, a molecule that has been heavily investigated for use as organic semiconductor [5]. Pentacene is rather expensive and is prone to oxidation with poor solubility in common organic solvents. Such undesired properties rendered pentacene and other acenes less attractive for practical applications. Several structural modifications of oligoacenes have been examined, including introduction of alkyl groups and electron-withdrawing groups to remedy some of the abovementioned undesired properties [6]. We would like to emphasize that both molecular structure and the way molecules arrange in the solid state (in single crystals and thin films) determine the properties of molecular materials and that device performance in both thin film and single-crystal configuration heavily depends on sloid state packing [4].

We and others have explored the modification of the carbon skeleton by replacing carbon atoms in pentacene for instance with heteroatoms, such as nitrogen, oxygen, and sulfur, by employing Schiff base chemistry to construct these heterocyclic aromatic compounds as alternatives to oligoacenes [3, 4]. Upon examining the structure of pentacene, one easily recognizes that heterocyclic compounds shown in Figures 1 and 2 are reasonably accessible by employing Schiff base chemistry.

Figure 1.

Example of the synthesis of symmetrical linear ladder oligomers.

Figure 2.

Structural similarities between pentacene and symmetrical linear ladder oligomers readily accessible by reactions of aryl amines with 2,5-dichloro-1,4-benzoquinone.

Phenothiazine, phenazine and phenoxazine have been used in a wide range of applications. These materials have also found a wide range of applications in medicinal chemistry and materials science [7, 8, 9]. Due to their planar rigid highly conjugated structures, these materials usually possess interesting optical properties, including fluorescence and nonlinear absorption. Many molecular structures can be prepared using Schiff base chemistry using appropriate quinones and aryl amines. The starting materials of choice to achieve this task in preparing linear pentacene-like heterocyclic molecules, such as the ones shown in Figure 1, for example, is to use the reaction of 2,5- dichloro-1,4-benzoquinone with 2-aminophenol; 1,2-phenylene diamine or 2-aminothiophenol. Starting with substituted aryl amines at various positions, one can modify the unsubstituted structures shown in (Figures 1 and 2) [7, 8, 9]. However, if one uses 2,3-dichloro-1,4-naphthoquinone instead of 2,5-dichloro-1,4-benzoquinone, bent or Y-shaped structures are obtained (Figures 3 and 4) [4, 10].

Figure 3.

Structures of symmetrical and asymmetrical bent or Y-shaped ladder oligomers readily accessible by reactions of aryl amines with 2,3-dichloro-1,4-naphthoquinone.

Figure 4.

Example of the synthesis of symmetrical bent or Y-shaped ladder oligomers.

The double substitution when benzoquinones or naphthoquinones are used results in the extended linear and bent or Y-shaped structures shown above (Figures 1 and 4). These reactions have been well studied and many factors affecting yields and product distribution under various conditions of temperature, catalysts, and solvents were reported, and as such, that aspect will not be emphasized herein [7, 8, 9]. The focus of this chapter, however, is to showcase how Schiff base chemistry can be employed in preparing and characterizing a few examples of ladder oligomers both linear and bent. The impact of shape and heteroatoms on solid state structures as revealed by single-crystal structural analysis and the observed inter- and intramolecular interactions will be discussed.

2. Results and discussion

All materials were prepared according to the published procedures [7, 8, 9, 10]. In the choice of molecules to target, we attempted to address the impact of molecular geometry (linear vs. bent) and the presence of different heteroatoms on solid state structures of these materials and the types of inter- and intramolecular interactions in these structures. Surprisingly, despite this intensely researched area in terms of synthesis, biological activity, and optical properties, structural studies of these materials are scarce. In this chapter, we highlight our structural studies by means of single-crystal structural analysis on a group of known ladder-type oligomers. Four molecules, which were synthesized using Schiff base chemistry, are considered in this study. They are: benzo[5,6][1,4]thiazino[2,3-b] phenothiazine (1); 6,9-dichloro-5H-benzo[a]phenoxazin-5-one (2); benzo[a]-benzo[5,6][1,4] thiazino[3,2-c]phenothiazine (3); and 2-nitrobenzo[a]benzo [5,6][1,4]oxazino[3,2-c]phenothiazine (4) (Figure 5).

Figure 5.

Structures of compounds 1-4.

We started with targeting compound 1, which has a backbone similar to the prototype organic semiconducting material, pentacene. The crystal structure of this molecule is known and was reported [11]. It was reported to crystallize in the monoclinic space group P21/c, a = 10.472(2), b = 5.430(3), and c = 12.356(10)A, Z = 2 [11]. We did not find a deposited CIF file associated with this structure or publication in the Cambridge Structural Database. The molecule is reported to be quasi planar with the central benzene ring of the molecule assuming a quinonoid structure with unequal C-C bond lengths. The authors also reported that the molecules of compound 1 are arranged in the unit cell in alternate parallel layers, with the planes of neighboring layers almost perpendicular. Despite numerous efforts, we were unable to grow single crystals suitable for analysis. We then attempted to target the Y-shaped or bent (3-arm) ladder oligomer, compound 2 (the doubly substituted naphthoquinone); however, we always ended up with only half a ladder probably due to the reduced reactivity of the initial product as reported by Kafor [9]. We intend to use it for building other symmetric full ladders in future work. We, therefore, turned toward using the more reactive aminothiophenol and targeted the molecule benzo[5,6][1,4]thiazino[2,3-b]phenothiazine (3). This reaction worked smoothly and gave deep blue powder. This turned out to be an approach worthy of our efforts, as the molecule readily formed crystals both from solution (dichloromethane) and by sublimation (Figure 6). The calculated dipole moment for compound 3 is 0.65 Debye (DFT-B3LYP) compared with an essentially zero dipole moment for 1 [12]. We reported earlier on a single-crystal field-effect transistor using compound 3 [4].

Figure 6.

Crystals of compound 3 grown by physical vapor transport.

Molecule 4 was prepared in two steps, according to our published procedure, where half ladder was first prepared by reacting 2,3-dichloro-1,4-naphthoquinone with 4-nitro-2-aminophenol followed by reacting the resulting product with 2-aminothiophenol to access the asymmetrical molecule 4 (Figure 7) [10].

Figure 7.

Synthesis of asymmetric molecules (4, R = NO₂).

3. Solid state structures

Upon purification and crystallization from suitable solvent except for 3, which was grown by sublimation, suitable crystals for X-ray diffraction were used. Compounds 2 and 3 crystallize in the monoclinic space groups P2₁/c with 4 molecules in the unit cell each, whereas compound 4 crystallizes in the orthorhombic Fdd2 space group with 16 molecules in the unit cell. To the best of our knowledge, none of these structures were reported. The main common features of the three structures are that these molecules are nearly planar and tend to form π-stacks in the solid state. Both characteristics are favorable features for better device performance as they facilitate charge transfer.

Molecular structures and packing within the unit cells for compounds 2, 3, and 4 are shown in Figure 8. Compound 2 crystallizes in the monoclinic space group P2₁/c with unit cell dimensions of: a = 16.4331(13) Å; b = 3.7613(3) Å; c = 20.6941(17) Å, and β = 98.5260(10)°; α = γ = 90° and 4 molecules in the unit cell. Compound 3 crystallizes in the monoclinic space group P2₁/c with unit cell dimensions of: a = 11.7444(3) Å; b = 3.83000(10) Å; c = 35.8138(9) Å, and β = 97.5590(10)°; α = γ = 90° and 4 molecules in the unit cell. Compound 4 crystallizes in the orthorhombic space group Fdd2 with unit cell dimensions of: a = 6.7497(9) Å; b = 52.478(7) Å; c = 18.832(3) Å and α = β = γ = 90° and 16 molecules in the unit cell.

Figure 8.

Molecular structure and packing in the unit cell of 2, 3, and 4.

The dominant intermolecular interactions observed included: Cl…Cl close contacts of 3.729 Å and Cl…H 3.265 Å in compound 2; C-H…S short distances of 2.942 and 3.221 Å in 3; and N–O…H–C distance of 2.43 Å and C-H…N of 3.726 Å in 4. π–π Stack distances of 3.085 and 3.290 Å were also observed in compounds 3 and 4, respectively, while the optical band gap for 4 was estimated to be 1.85 eV (Figures 8 and 9) [10].

Figure 9.

π-π stacking in compounds 3 and 4.

Finally, we have also carried out density functional theory (DFT) at the B3LYP/6-31G* level calculations for compounds 1-5 as gas phase isolated molecules, using Spartan 20 software [12]. The main features of these calculations are as follows: (i) Calculated molecular geometries were compatible with the experimentally determined ones. (ii) Linear ladder oligomer appears to have lower calculated band gaps compared to bent ones (Figure 10). It is worth noting that these calculations are carried out on isolated gas phase molecules.

Figure 10.

DFT calculated HOMO-LUMO levels (B3LYP) for compounds 1 (TPDT) and 3 (DDBA) compared with pentacene.

4. Conclusions

In conclusion, we have demonstrated that Schiff base chemistry can be successfully applied to prepare interesting new materials suitable for various applications. The shape, (linear versus bent) of fused heterocycles containing N and S, plays a crucial role impacting the tendency of these materials to form crystals suitable for device fabrication. The presence of chlorine atoms results in Cl…Cl interactions, dominating the close intermolecular interactions within the unit cell. DFT calculations indicate that linear heteroacenes would have lower calculated band gaps. For example, compound 1 (linear) has a lower calculated band gap of 0.12 eV when compared with the bent molecule (3).

Acknowledgments

Funding from the Pennsylvania State University Research Development Grants and Alfaisal University IRG #18421 is acknowledged. The authors also acknowledge V. G. Young, Jr., and the X-Ray Crystallographic Laboratory, Department of Chemistry at the University of Minnesota, and C. Fiester, A, Bradely, and A. Nazzal of Wilkes University for help with synthesis of compound 4.