Investigations of the 9, 10-Diphenylacridyl Radical as an Isostructural

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Investigations of the 9,10-Diphenylacridyl Radical as an Isostructural Dopant for the Molecular Semiconductor 9,10-Diphenylanthracene Thomas P. Vaid,* Abigail K. Lytton-Jean,† and Brian C. Barnes Department of Chemistry, Washington University, St. Louis, Missouri 63130 Received July 17, 2003

The synthesis and characterization of the 9,10-diphenylacridyl radical (2), the isostructural n-dopant for 9,10-diphenylantracene (1), are described. Crystals of 1 doped with up to 10% 2 are grown from tetrahydrofuran-heptane solutions and are single phase and isomorphous with crystals of pure 1 grown from the same solvent system. The doped crystals do not have significant room-temperature electrical conductivity, and solution-phase electrochemical measurements demonstrate why this is so. Investigations of 2 reveal that it is monomeric with one unpaired electron in solution, displays unusual magnetic behavior in the solid state, and is a modest electrical conductor as a pressed pellet. The solution-phase ESR and NMR spectra of 2 are presented and interpreted.

Introduction We present a novel class of dopants for organic molecular semiconductors that will potentially enable their use as the active component of any type of traditional semiconductor device. The present paper describes the synthesis and characterization of the 9,10-diphenylacridyl radical, an isostructural n-dopant for the molecular semiconductor 9,10-diphenylanthracene. Many organic molecules with large, conjugated π-electron systems form molecular crystals that are modest conductors of electricity.1 The electronic structure of these crystals is often described with an energy band model, traditionally associated with inorganic solid-state materials. When there are relatively strong intermolecular interactions in a crystal, the molecular orbitals on individual molecules spread into delocalized bands. The highest occupied molecular orbital (HOMO) becomes the valence band and the lowest unoccupied molecular orbital (LUMO) becomes the conduction band. The band gap of the crystal is approximated by the HOMO-LUMO gap of an individual molecule. The small HOMO-LUMO gap of molecules with large conjugated π-systems leads to a small band gap in the solid statesone reason that they might be described as semiconductors, while most molecular crystals are insulators. The representation of these molecular materials as semiconductors leads naturally to the question of whether the large variety of semiconductor devices made from inorganic semiconductors can also be created with organic molecular semiconductors. Nearly all inorganic semiconductor devices are created from various arrangements of junctions between

metals, insulators, n-type semiconductors, and p-type semiconductors.2 Thus, the ability to dope molecular semiconductors in a controlled manner to create n-type or p-type materials will potentially allow the creation of any type of traditional semiconductor device in a manner analogous to inorganic semiconductor device construction. Many devices based on molecular or polymeric materials are already known, the most prominent among these being light-emitting diodes (LEDs)3 and organic field-effect transistors.4 Both of these devices utilize nontraditional device architectures that do not require doped semiconductors. For example, organic LEDs generally use two electrical contacts with very different work functions, one for the injection of electrons and one for the injection of holes, and often employ two or more organic materials. In contrast, a typical inorganic LED is simply a gallium arsenide p-n junction with two ohmic contacts. A method for p- and n-doping of molecular semiconductors will obviate the need to create new architectures for devices based on molecular materials and may expand the variety of semiconductor devices that can be constructed from molecular materials. A note on the use of semiconductor terminology to describe molecular materials should be made at this point. Molecular and polymeric materials are sometimes called p-type or n-type, even though they have not been purposefully doped with an electron donor or acceptor. Molecules that are easily oxidized, such as triarylaminecontaining species, are occasionally termed “p-type” and ones that are easily reduced are sometimes called “ntype”. However, the true inorganic semiconductor analogues to these two materials are an intrinsic (i.e.,

* To whom correspondence should be addressed. E-mail: vaid@ wustl.edu. † Current address: Department of Chemistry, Northwestern University, Evanston, IL 60208. (1) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers; Oxford University Press: Oxford, 1999.

(2) Sze, S. M. Modern Semiconductor Device Physics; John Wiley & Sons Inc.: New York, 1998. (3) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-128. (4) Katz, H. E.; Bao, Z. J. Phys. Chem. B 2000, 104, 671-678.

10.1021/cm034646c CCC: $25.00 © 2003 American Chemical Society Published on Web 10/07/2003

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Figure 2. Molecules 9,10-diphenylanthracene (1) and 9,10diphenylacridyl (2). Atom numbering is shown for 2.

Figure 1. Formation of a p-n junction by contacting an n-type and a p-type semiconductor. Electrons are transferred from the n side to the p side, resulting in a net charge density in the depletion region (top), which leads to band bending (bottom). Ec is the conduction band edge, Ev is the valence band edge, and Ef is the Fermi level.

undoped) semiconductor with a low work function and one with a high electron affinity, respectively. They are simply different semiconductors, and a junction between the two is properly termed a heterojunction, not a p-n junction. It may be that for molecular semiconductors with low ionization energies, adventitious impurities will often oxidize some fraction of the molecules so that more free holes than free electrons are present, and in this way the material is indeed p-doped. The opposite may occur for molecular semiconductors with a high electron affinityssome fraction of the molecules are reduced by impurities and the material thereby becomes n-doped. However, the nature of the dopants and their concentration are not well-controlled in these situations and it is not clear how the materials will behave as components of junctions. Conjugated polymers and molecular semiconductors are often purposefully doped by reaction with a strong oxidizing agent or reducing agent. For example, pdoping is accomplished by the addition of I2 and ndoping by reaction with potassium. Iodine accepts electrons to leave behind holes in the semiconducting molecule and forms I3- ions that fit interstitially in the organic host’s crystal lattice. Similarly, n-doping with potassium creates interstitial K+ ions. Doping in this way has its closest analogue in inorganic semiconductors in the n-doping of silicon with lithium, which results in interstitial Li+ ions in the silicon lattice. For molecular semiconductors, however, this type of doping is not suitable for the formation of the semiconductor junctions necessary to create devices. An important aspect of the nature of doped inorganic semiconductors is that the dopant atoms are fixed in place in the crystal lattice. In a thought experiment in which a p-type semiconductor is brought into contact with an n-type semiconductor, the Fermi levels equalize through charge transfer from the n-type side to the p-type side (Figure 1). The charges that result are fixed on the dopant atom sites, and the electric field that results from the fixed charges in the depletion region causes band bending and the rectification of current in a diode. The K+ and I3- counterions in a doped organic material are mobile at ambient

temperaturessin fact, they are generally introduced by diffusion of the dopant into a pristine crystal of the organic molecule. The mobility of these counterions is a fundamental problem in the construction of devices. For example, if an iodine-doped crystal of pentacene were brought into contact with a potassium-doped crystal of pentacene, charge transfer would occur to create a p-n junction, but the potassium and triiodide would then migrate under the influence of the newly created electric field and form potassium triiodide at the interface, destroying the diode. The technique by which inorganic semiconductors are doped serves as the model for isostructural doping of organic molecular semiconductors. Silicon is n-doped by the substitution of a very small fraction of the silicon atoms by an atom of an element such as phosphorus, which has five valence electrons rather than the four of silicon. The crystal structure is maintained (phosphorus forms four bonds with its neighboring silicon atoms), and the crystal has no net charge. To create an n-dopant for a molecular hydrocarbon, one would substitute one carbon with a nitrogen while maintaining the structure of the molecule and charge neutrality of the molecule as a whole. A p-dopant results when a carbon is substituted by boron, once again while maintaining charge neutrality of the molecule. An n-type semiconducting material is produced by the cocrystallization of the parent hydrocarbon with a small amount of the n-dopant molecule. Because the isostructural dopant and the parent molecular semiconductor have the same skeletal structure, it is likely that they will easily cocrystallize. A related strategy for the doping of organic materials involves the covalent attachment of an ion to the dopant molecule or polymer. In the case of the small-molecule dopant, the attached ion has the opposite charge of that on the main part of the molecule, resulting in a Zwitterionic dopant.5 Conjugated polymers with covalently attached ions, sometimes called “self-doped” polymers, have been synthesized with the intent to incorporate them in stable rectifying junctions.6 The molecule 9,10-diphenylanthracene (1, Figure 2) has a HOMO-LUMO gap slightly smaller than that of anthracene and, therefore, presumably forms crystals with an electronic band gap smaller than anthracene. The neutral radical 9,10-diphenylacridyl (2, Figure 2) (5) Gregg, B. A.; Cormier, R. A. J. Am. Chem. Soc. 2001, 123, 79597960. (6) Lonergan, M. C.; Cheng, C. H.; Langsdorf, B. L.; Zhou, X. J. Am. Chem. Soc. 2002, 124, 690-701.

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is an isostructural n-dopant for 1. The radical was first reported in a pair of papers in 1912 and 1914,7,8 but has not been reinvestigated since those initial reports. It is produced by the reduction of the 9,10-diphenylacridinium cation, and 2 is therefore a reasonably good reducing agent. Thus, 2 has, in a general sense, the correct electronic properties to act as an n-dopant for 1. Experimental Section General Procedures and Materials. Triphenylamine was purchased from City Chemical. Benzoic acid, polyphosphoric acid, zinc powder, and sodium borohydride were purchased from Aldrich and used as received. Tetrabutylammonium hexafluorophosphate was purchased from Aldrich and recrystallized twice from ethanol. Acetonitrile was distilled from P2O5 and stored over activated 3-Å sieves. Tetrahydrofuran was distilled from purple Na/benzophenone, and benzene and heptane were distilled from Na/benzophenone with added tetraglyme. 1H and 13C NMR spectra were recorded on a Varian Mercury 300 NMR spectrometer. Infrared spectra were recorded as Nujol mulls on a Perkin-Elmer Spectrum BX FTIR. UV-vis spectra were recorded on a Varian Cary 1E spectrophotometer; for air-sensitive compounds the solutions were prepared in a drybox in an airtight cuvette. ESR spectra were obtained on a Bruker ER200 X-band spectrometer with 0.10 G modulation. X-ray powder diffraction patterns were recorded on a Rigaku Geigerflex D/max diffractometer interfaced to a PC. Elemental analysis was carried out by the University of Illinois microanalysis laboratory, Urbana, IL. 9,10-Diphenylacridinium Chloride (3+Cl-). Polyphosphoric acid (210 g), triphenylamine (24.0 g, 97.8 mmol), benzoic acid (6.00 g, 49.1 mmol), and a magnetic stir bar were placed in a 250-mL round-bottomed flask with a condenser attached. The suspension was heated to 200 °C with stirring while open to the ambient atmosphere; no cooling water was passed through the condenser. The flask was swirled occasionally to wash down a white sublimate. After 30 min at 200 °C, the resulting hot brown liquid was poured into a 1-L Erlenmeyer flask and 800 mL of water was added. The suspension was filtered and washed with 200 mL of water. To the yellow filtrate was added 5.0 M NaOH(aq) until the yellow color was completely quenched and a precipitate of the acridanol had formed (∼1200 mL of NaOH(aq)). The mixture was filtered; the collected acridanol was washed into a new flask with 250 mL of 1.0 M HCl(aq) to yield a dark yellow solution of the acridinium chloride. Water and HCl were removed from the solution by rotary evaporation to yield a yellow-brown solid. The solid was dissolved in 150 mL of CHCl3 and a few grams of powdered CaCO3 was added to neutralize excess HCl. The suspension was filtered and the filtrate diluted to 400 mL with CHCl3. Addition of 300 mL of hexane gave a yellow precipitate. The precipitate was dissolved in 400 mL of CHCl3 and slow addition of 150 mL of hexane yielded 9.125 g of 9,10diphenylacridinium chloride as yellow microcrystals. A second crop of crystals from CHCl3/hexane yielded 2.827 g, for a total yield of 66% based on benzoic acid. mp ∼320 °C (dec). 1H NMR (CDCl3, ref. CHCl3 at 7.27 ppm): δ 8.21 (td, 2H, J ) 8.0 Hz, J ) 1.4 Hz), 8.11 (dd, 2H, J ) 8.8 Hz, J ) 1.4 Hz), 8.00-7.75 (m, 10H), 7.70-7.64 (m, 2H), 7.60 (d, 2H, J ) 9.1 Hz). 13C NMR (CDCl3, ref. CDCl3 at 77.23 ppm): δ 163.3, 142.0, 139.3, 136.7, 132.6, 132.2, 132.0, 130.9, 130.2, 130.0, 129.2, 128.6, 127.9, 126.0, 119.7. UV-vis in CH3CN, 6.4 × 10-6 M; nm, (L‚mol-1‚ cm-1): 194, 7.0 × 104; 262, 6.8 × 104; 345, 7.5 × 103; 361, 1.6 × 104; 428, 5.1 × 103. 9,10-Diphenylacridyl Radical (2). All manipulations excluded air and water. A suspension of zinc powder (6.00 g, 92 mmol) in a solution of 9,10-diphenylacridinium chloride in 40 mL of dry acetonitrile was stirred at 22 °C for 2 h; the (7) Cone, L. H. J. Am. Chem. Soc. 1912, 34, 1695-1706. (8) Cone, L. H. J. Am. Chem. Soc. 1914, 36, 2101-2110.

Vaid et al. yellow solution quickly turned dark red-brown. The acetonitrile was removed under vacuum. Benzene was added and the mixture was filtered and washed with benzene. The volume of the red-brown filtrate was reduced to 45 mL and 30 mL of acetonitrile was added to precipitate some solid. Heating to reflux and then cooling to 0 °C afforded 1.040 g of red-brown microcrystals that were collected by filtration. A second crop of 524 mg was collected from 20 mL of benzene/20 mL of acetonitrile for a total yield of 1.564 g (58%). mp ∼245 °C (dec). 1 H NMR (C6D6, ref. C6D5H at 7.16 ppm): δ 47 (vvbr, 4H), 25 (vbr, 1H), 17.5 (br, 2H), 4.52 (1H), -2.6 (br, 2H). UV-vis in THF, 9.6 × 10-6 M; nm, (L‚mol-1‚cm-1): 215, 6.6 × 104; 258, 4.6 × 104; 275, 2.7 × 104; 283, 4.9 × 104; 352, 1.2 × 104; 490, 5.1 × 103; 524, 6.3 × 103. IR (Nujol, cm-1): 1579 (m), 1311 (s), 1225 (w), 734 (s), 698 (m). Anal. Calcd (found) for C25H18N: C, 90.33 (89.82); H, 5.46 (5.11); N, 4.21 (4.38). Cocrystallization of 1 and 2. A mixture of 270 mg of 1 and 30 mg of 2 was dissolved in 20 mL of THF to form a homogeneous dark red-brown solution. Heptane (15 mL) was added; no precipitate formed. Solvent was slowly removed from the stirred solution at 22 °C until about 7 mL remained. The red microcrystals that had formed were collected by filtration. Isolated yield: 224 mg. 9,10-Diphenylacridan (5). The product, 5, is mildly airand light-sensitive, but the following operations can be performed on the benchtop with nitrogen-sparged solvents. To a yellow solution of 3+Cl- (500 mg, 1.36 mmol) in 40 mL of water was added a solution of NaBH4 (60 mg, 1.59 mmol) in 5 mL of water. A white precipitate formed and the yellow color of the solution was quenched. The fine white precipitate of 5 was most easily isolated by extraction of the aqueous suspension with 40 mL of toluene. Evaporation of the toluene yielded 5 (236 mg, 52%). 1H NMR (CDCl3, ref. CHCl3 at 7.27 ppm): δ 7.65 (tt, 2H, J ) 7.1 Hz, J ) 1.6 Hz), 7.53 (tt, 1H, J ) 7.4 Hz, J ) 1.4 Hz), 7.37 (dd, 2H, J ) 8.2 Hz, J ) 1.4 Hz), 7.28 (d, 4H, J ) 4.4 Hz), 7.18 (m, 1H), 7.15 (dd, 2H, J ) 7.4 Hz, J ) 1.1 Hz), 6.98 (td, 2H, J ) 7.4 Hz, J ) 1.4 Hz), 6.84 (td, 2H, J ) 7.4 Hz, J ) 1.4 Hz), 6.32 (dd, 2H, J ) 8.2 Hz, J ) 1.1 Hz), 5.43 (s, 1H). 13C NMR (CDCl3, ref. CDCl3 at 77.23 ppm): δ 147.7, 141.6, 141.1, 131.4, 131.0, 129.6, 128.8, 128.5, 127.9, 127.1, 126.5, 124.5, 120.9, 114.5, 48.0. Solution-Phase Magnetic Susceptibility.9 A Wilmad #517 coaxial tube (i.d. 2.3 mm) and two #517 spacers were placed in a standard 5-mm NMR tube. A weighed sample of 2 was diluted to 1.00 mL with C6D6 to create a solution of known concentration, a portion of which was added to the coaxial tube, and neat C6D6 was placed in the outer annulus. Measurements were made at ambient temperature and the actual temperature of the probe was recorded. The difference in the chemical shift of C6D5H in the solution of 2 and the reference was used for calculations. Solid-State Magnetic Susceptibility. Measurements were made on a Quantum Design MPMS. Samples were about 18 mg and were loaded in an airtight quartz sample holder. An applied field of 1000 or 10000 G was used for variabletemperature susceptibility measurements. Electrochemistry and Conductivity. Cyclic voltammetry and solid-state conductivity measurements were performed with a EG&G/PAR 263A potentiostat with electrical connections to the inside of a nitrogen-filled drybox, where all materials and solutions were maintained during the measurements. Cyclic voltammetry was performed in dry THF with 0.10 M [Bu4N][PF6] supporting electrolyte, 0.010 M analyte, and 0.005 M ferrocene internal standard. Scans were run at 50 mV/s. Platinum disks of 0.50-mm diameter were utilized as working electrode and pseudoreference electrodes, with a 5.0-mm platinum disk counter electrode. Solid-state conductivity of pressed powders was measured in a two-electrode configuration on a home-built apparatus that consists of a Delrin block with a 6.35-mm cylindrical hole and two 6.35mm copper or stainless steel cylindrical contacts. The material to be studied was inserted between the two contacts within (9) Evans, D. F. J. Chem. Soc. 1959, 1959, 2003-2005.


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Scheme 1. Synthesis and Reactivity of the 9,10-Diphenylacridinium Cation, 3+

the Delrin block and a mass of 3.85 kg was placed on the top contact, thus applying 11.9 bar of pressure. X-ray Crystallography. Measurements were performed at the University of Houston on a Siemens SMART platform diffractometer equipped with a 1 K CCD area detector. Crystals were mounted in mineral oil and data were collected at -50 °C. An empirical absorption correction was applied. Structures were solved with SHELXS-97 and refined with SHELXL-97 within the WinGX10 interface.

Results and Discussion Synthesis. Our synthesis of 2 follows Staskun’s11 preparation of 9,10-diphenyl-9-acridanol (4) from triphenylamine and benzoic acid in hot polyphosphoric acid (Scheme 1). The reaction proceeds by the electrophilic benzoylation of triphenylamine at a carbon ortho to the nitrogen, followed by ring closure with dehydration.11 A water-soluble 9,10-diphenylacridinium cation (3+) is formed by the reaction. The cation 3+ can be (10) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838. (11) Staskun, B. J. Org. Chem. 1968, 33, 3031-3036.

precipitated from aqueous solution as 4 by the addition of base, and 4 can be converted back to 3+ by aqueous acid. Staskun synthesized 4 and did not isolate the cation, while we did not isolate and purify 4 but isolated 3+Cl- as yellow crystals from chloroform/hexane. Reduction of 3+Cl- by zinc in anhydrous CH3CN yields the deep red-brown radical 2, which can be crystallized from hot CH3CN/benzene. Crystals of 2 maintain their dark red-brown color for long periods in air, while solutions of 2 quickly turn yellow upon exposure to air. Attempted Crystal Structure of 2. Crystals of 2 grown by slow sublimation under vacuum or by slow evaporation of a THF/heptane solution diffract X-rays well. However, in both cases structure solution revealed a disordered structure in which a fraction of the molecules had one phenyl group bent significantly out of the acridine plane. This was judged to be due to contamination by 9,10-diphenylacridan (5, Scheme 1). An authentic sample of 5 was synthesized by the reaction of 3+Cl- with NaBH4 in water.12 A sealed ampule of 2 was heated to the sublimation temperature

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Table 1. Calculated Relative Energies and Boltzmann Populations of 2 with Various Twist Angles between the N-Phenyl Plane and the Acridine Plane N-phenyl dihedral angle

relative energy (kcal/mol)

fractional population

90 85 80 75 70 65 60 55 50 45

0 0.05 0.197 0.436 0.778 1.252 1.839 2.573 3.465 4.504

0.282 0.259 0.201 0.134 0.075 0.033 0.012 0.003 0.001 0.000

of 160 °C, and 1H NMR of the contents confirmed that some 2 had been converted to 5. The origin of 5 in the solution-grown crystals of 2 is unclear.13 Calculations. To aid in interpretation of the ESR and 1H NMR spectra of 2, electronic structure calculations were performed. The optimized geometry (NWChem,14 B3LYP/6-311G**) is planar throughout the acridine ring system, not including the phenyl groups. Both phenyl rings are twisted at 90° to the acridine ring system in the optimized geometry, but the potential for rotation around the acridine-phenyl bonds is quite shallow. The unpaired electron spin density on the phenyl rings changes significantly with their twist angle, and the NMR and ESR parameters discussed below depend directly on the unpaired spin density at each nucleus. Therefore, a range of twist angles for both rings were investigated. Because protons from the N-phenyl ring appear in the 1H NMR spectrum (see below), and the potential for rotation around both acridine-phenyl bonds is similar, the N-phenyl rotation will be discussed here. The dihedral angle between the N-phenyl ring and the acridine ring system was fixed at angles between 90° and 45° at 5° intervals. At each fixed angle the remainder of the molecular geometry was optimized, and the relative energies of these structures are given in Table 1. The population of each of these geometries can be obtained from their Boltzmann factor, and if they are the only allowed geometries, the total population can be set to 1, yielding the fractional populations given in Table 1. The population-weighted average of the torsion angles is 82°, so a configuration with both phenyl rings at a twist angle of 80° is representative of the molecule in solution. A picture of the calculated (Gaussian 98,15 B3LYP/EPR-II with a polarized continuum (12) Roberts, R. M. G.; Ostovic, D.; Kreevoy, M. M. Faraday Discuss. Chem. Soc. 1982, 257-265. (13) Crystal data for 2: Monoclinic, space group P121/n1, a ) 9.0154(6) Å, b ) 20.9910(14) Å, c ) 9.8587(7) Å, β ) 108.696(1)°. V ) 1767.24(7) Å3, Z ) 4, dcalc ) 1.25 g/cm3. (14) Straatsma, T. P.; Apra, E.; Windus, T. L.; Dupuis, M.; Bylaska, E. J.; de Jong, W.; Hirata, S.; Smith, D. M. A.; Hackler, M. T.; Pollack, L.; Harrison, R. J.; Nieplocha, J.; Tipparaju, V.; Krishnan, M.; Brown, E.; Cisneros, G.; Fann, G. I.; Fruchtl, H.; Garza, J.; Hirao, K.; Kendall, R.; Nichols, J. A.; Tsemekhman, K.; Valiev, M.; Wolinski, K.; Anchell, J.; Bernholdt, D.; Borowski, P.; Clark, T.; Clerc, D.; Dachsel, H.; Deegan, M.; Dyall, K.; Elwood, D.; Glendening, E.; Gutowski, M.; Hess, A.; Jaffe, J.; Johnson, B.; Ju, J.; Kobayashi, R.; Kutteh, R.; Lin, Z.; Littlefield, R.; Long, X.; Meng, B.; Nakajima, T.; Niu, S.; Rosing, M.; Sandrone, G.; Stave, M.; Taylor, H.; Thomas, G.; van Lenthe, J.; Wong, A.; Zhang, Z. NWChem, A Computational Chemistry Package for Parallel Computers, Version 4.5; Pacific Northwest National Laboratory: Richland, WA, 2003.

Figure 3. Representation of the calculated unpaired electron spin density on 2 when the phenyl rings are twisted at 80° to the acridine ring system. Gray is positive spin density and black is negative spin density. Nitrogen is at the top, as in Figure 2.

model of solvent benzene) unpaired electron spin density on 2 with both phenyl rings set at a twist angle of 80° is given in Figure 3. 1H NMR. The 1H NMR spectrum of paramagnetic 2 is shown in Figure 4. Because some of the proton resonances were too broad to be observed, ferrocene was added to a separate sample as an internal standard for accurate absolute integrations. The chemical shift dispersion of 2 is determined almost completely by the unpaired electron spin density at the various hydrogen nuclei. For organic radicals, there is a linear relationship between an electron-nucleus hyperfine coupling constant (a, the hyperfine splitting observed in ESR) and the paramagnetic shift of that nucleus’s NMR resonance, that is, the chemical shift of the nucleus relative to the chemical shift of that nucleus in an analogous diamagnetic molecule.16 For a hydrogen nucleus at 295 K, the expression is

(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998. (16) Kreilick, R. W. In NMR of Paramagnetic Molecules; La Mar, G. N., Horrocks, W. D., Holm, R. H., Eds.; Academic Press: New York, 1973; pp 595-626.


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Figure 4. 1H NMR spectrum of 2 in C6D6. Resonances are labeled with the protons from which they result (see Figure 2 for numbering). Small peaks near 1 ppm are from hydrocarbon grease. Table 2. Electron-Nuclear Hyperfine Coupling Constants for 2 nucleusa

calc. ab

N 1, 8 (2 H) 2, 7 (2 H) 3, 6 (2 H) 4, 5 (2 H) 11, 15 (2 H) 12, 14 (2 H) 13 (1 H) 16, 20 (2 H) 17, 19 (2 H) 18 (1 H)

1.94 -4.11 1.21 -4.2 0.75 -0.46 0.45 -0.162 -0.136 0.158 -0.038

a

a from 1H NMRc

|a| (ESR simulation)d

-0.136 0.135 -0.041

3.178 2.86 0.551 3.496 0.508 0.161 0.153 0.114 0.118 0.145 0.040

Figure 5. Experimental (top) and simulated (bottom) ESR spectrum of 2 as a dilute solution in C6H6.

b

See Figure 2 for atom numbering. Calculated (B3LYP/EPRII) with both phenyl rings twisted at 80°. c Derived from 1H NMR spectrum with eq 1. d From best-fit simulation of ESR spectrum.

∆f ) 75.0aH

(1)

where ∆f is the paramagnetic shift of the 1H NMR frequency (in ppm) and aH is the hyperfine coupling constant (in gauss (G)). The aH for all hydrogens of 2 for a range of phenyl twist angles were calculated (Gaussian 98, B3LYP/EPR-II). The N-phenyl group always had the lowest unpaired electron spin density, and its hydrogens therefore had the lowest aH, and therefore the lowest ∆f, so they should lead to the least shifted and least broadened resonances in the 1H NMR. If one assumes a chemical shift of 7.5 ppm for all protons in the hypothetical diamagnetic analogue of 2, at a twist of 80° for both phenyl rings the calculated ∆f values for the N-phenyl hydrogens are approximately equal to those of the three 1H resonances in Figure 4, and they are labeled appropriately. Table 2 lists all the calculated aH for 2 with both phenyl rings twisted at 80°. (We also calculated aH for all hydrogens for all N-phenyl twist angles of Table 1 and then took their population weighted averages. The values were very similar to those calculated for the static structure with enforced 80° twists.) The aH’s derived from the NMR spectrum with eq 1 are given in Table 2, and there is reasonable agreement with the calculated values. From the calculated spin density on 2, it seems most likely that any other experimentally observable resonances would originate from the C-phenyl protons. However, the origin of the two other observed reso-

nances, (24.5 ppm, 1H) and (47 ppm, 4H), cannot be unambiguously determined. ESR. An ESR of a very dilute solution of 2 in benzene is shown in Figure 5. The extensive hyperfine coupling in the spectrum makes it difficult to model exactly. The calculated hyperfine couplings of Table 2 yield a simulated spectrum that bears little resemblance to the experimental spectrum. A Fourier transform of the spectrum has well-resolved peaks at 8.78, 8.50, and 6.90 G-1, indicating that hyperfine couplings of 0.114, 0.118, and 0.145 G are present. Trial-and-error variation of hyperfine couplings to obtain the best match between the experimental and simulated spectrum led to the couplings in the last column of Table 2, which yield the simulated spectrum shown in Figure 5.17 The hyperfine couplings from the Fourier transform were purposefully included. There should be an exact match between the hyperfine couplings of the ESR and those calculated from the NMR spectrum; a possible reason for the mismatch is the assumption of a chemical shift of 7.5 ppm for all hydrogens in the hypothetical diamagnetic version of the molecule, whereas the actual chemical shifts of such a hypothetical molecule cannot be determined experimentally. There is varying agreement between the ab initio-calculated couplings and those used in the simulation. The ai’s derived from the bestfit simulation must be regarded as somewhat tentative. Magnetic Susceptibility. i. Solution Phase. The Evans method9 was used to determine the magnetic susceptibility of 2 as a solution in C6D6. It has been noted that Evans’ original equations must be modified (17) WINEPR SimFonia 1.25, Shareware version; Bruker BioSpin Corporation: Billerica, MA, 1996.

4298


Vaid et al.

Figure 6. Magnetic moment of microcrystalline 2 from 6 to 400 K.

for the magnet/sample geometry of superconducting NMR spectrometers.18,19 If one assumes that the volume diamagnetic susceptibility of the compound under study and that of the solvent are similar,20 corrections for the diamagnetism of the solvent and the analyte become unnecessary, and the equation for calculating the paramagnetic susceptibility of the analyte reduces to

x∆fT M

µeff ) 0.0437

(2)

where µeff is in Bohr magnetons, ∆f is the shift in frequency of the analyte solution resonance versus that of the neat reference (in ppm), T is the temperature in Kelvin, and M is the molarity of the solution of analyte. The assumption of the equality of volume diamagnetism of 2 and C6D6 seems reasonable, and we indeed found that separate addition and subtraction of the diamagnetism of the solvent and analyte canceled and gave results identical to those of eq 2. A 0.0647 M solution yielded a magnetic moment of 1.70 µB and a 0.100 M solution gave a magnetic moment of 1.69 µB. Both are close to the ideal value of 1.73 µB for one unpaired electron, so it appears that 2 is monomeric in benzene solutions with concentrations up to at least 0.10 M, which is near the saturation concentration at room temperature. ii. Solid State. The magnetic susceptibility of microcrystalline 2 was measured with a SQUID magnetometer. The molar susceptibility was calculated and a correction for diamagnetism of 2.33 × 10-4 emu/mol (from the sum of the diamagnetic susceptibilities of anthracene and biphenyl)21 was applied at all temperatures. The magnetic moment in Bohr magnetons was calculated with the formula22

µeff ) 2.828xχMT

Figure 7. Cyclic voltammetry of 1 (top) and 2 (bottom) in THF.

(3)

where χM is the molar susceptibility (corrected for diamagnetism) and T is the temperature in Kelvin. Figure 6 is a plot of the magnetic moment of 2 as a function of temperature. If there were no intermolecular (18) Live, D. H.; Chan, S. I. Anal. Chem. 1970, 42, 791-792. (19) Schubert, E. M. J. Chem. Educ. 1992, 69, 62. (20) Evans, D. F.; James, T. A. J, Chem. Soc., Dalton Trans. 1979, 723-726. (21) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986. (22) Cheetham, A. K., Day, P., Eds. Solid State Chemistry: Techniques; Oxford University Press: Oxford, 1987.

interactions, the magnetic moment would be the same as was found in solution, 1.70 µB, at all temperatures. Instead, it is about 1.3 µB from below 100 K to about 200 K and then steadily increases to about 1.6 µB at about 350 K, where it levels off. This behavior was observed in three different samples, but its origin is not yet understood. Electrochemistry. The solution-phase redox potentials of molecular species provide an estimate of their electronic energy levels in a solid-state molecular crystal.23 The solution-phase oxidation (0/+) potential of 1 approximates its valence band edge in the solid state, while the reduction (0/-) potential of 1 indicates its conduction band edge in the solid state. Redox potentials in volts are equivalent to solid-state energies in electronvolts. Thus, electrochemical measurements on 1 and 2 under the same conditions will allow the construction of an approximate energy level diagram for crystalline 1 doped with 2. Cyclic voltammograms of 1 and 2 in THF are given in Figure 7. In both cases ferrocene was added as a reference and the ferrocene+/0 potential was set to zero. The reversible reduction of 1 occurs at -2.45 V. There is a reversible oxidation of 2 at -0.88 V and an irreversible reduction of 2 at approximately -2.1 V. In a separate measurement, the potential for the oxidation of 1 in THF was measured to be approximately 0.85 V, although oxidation of the solvent began near this potential so the value is not exact. The approximate band energy level diagram of 1 doped with 2 derived from these redox potentials is given in Figure 8a. The desired arrangement of energy levels for a crystal of 1 doped with an ideal n-dopant D is given in Figure 8b. The energy difference between the +/0 couple of D and the 0/- couple of 1 should be as small as possible since it is the energy required for an electron transfer (23) Bouvet, M.; Silinsh, E. A.; Simon, J. Mol. Cryst. Liq. Cryst. C: Mol. Mater. 1995, 5, 255-277.


Figure 8. (a) Approximate energy diagram of 1 doped with 2, derived from solution-phase electrochemistry. (b) Ideal energy levels of 1 doped with a donor D.

from D to 1, that is, for D to act as an n-dopant for 1. (For phosphorus in silicon, the analogous energy differencesthe binding energy of an electron to the phosphorus dopantsis 0.045 eV.) Also, the reduction of D should occur at a potential more negative than that for the reduction of 1; otherwise, D will act as an electron trap in 1. So there are obvious problems with the n-doping of 1 with 2 if the energy levels correspond to those in Figure 8a. First, the donor level lies about 1.57 eV from the conduction band edge, an energy much larger than kT at room temperature, so an extremely small fraction of 2 in 1 will be ionized. Second, 2 is actually easier to reduce than 1, so 2 will act as an electron trap in 1. Nevertheless, we have grown crystals of 1 doped with 2 and examined their electrical properties, as described below. Solid-State Conductivity of 2. As a pure material, the radical 2 is a modest electrical conductor. A 6.0-mg sample was pressed between either two copper or two stainless steel contacts, and in each case a linear plot of current vs applied voltage was obtained. The resistance of the sample was calculated from the slope of the plot and did not depend on the contact electrode, although the measured values ranged from 230 to 450 MΩ. The resistivity of the material was determined assuming a density of 1.25 g/cm3 (the density of 2 calculated from X-ray diffraction data) and assuming that a uniform disk of material was formed between the circular contacts. The cross-sectional area of 0.3167 cm2 leads to a calculated sample thickness of 0.0152 cm (152 µm). The resistance values quoted above indicate a resistivity of 4.8 × 109 to 9.4 × 109 Ω‚cm for a pressed microcrystalline sample of 2. Cocrystallization of 1 and 2. An isostructural dopant and its parent molecular semiconductor have the same skeletal structure, which should lead to relatively easy cocrystallization of the two. The molecular structure of 1, determined by X-ray crystallography,24,25 and the calculated structure of 2 are in fact very similar. But while the averaged structure of 1 has no dipole moment, the calculated dipole of 2 is 2.0 D, which may (24) Adams, J. M.; Ramdas, S. Acta Crystallogr. B 1979, B35, 679683. (25) Langer, V.; Becker, H. D. Z. Kristallogr. 1992, 199, 313-315.

Chem. Mater., Vol. 15, No. 22, 2003 4299

manifest itself in somewhat different solubility properties of 1 and 2. Crystallization of 1 by cooling a benzene/heptane solution leads to crystals whose powder diffraction pattern is consistent with the unit cell of the previously reported crystal structure of solution-grown crystals.24,25 However, cooling a benzene/heptane solution of a 9:1 mixture of 1 and 2 under the same conditions yields a mixture of crystals isomorphous with 1 and at least one other crystalline phase. Evaporation of a heptane/THF solution of 1 leads to crystals isomorphous with the crystals of 1 described above. Evaporation of a heptane/THF solution of a 9:1 mixture of 1 and 2 (as described in the Experimental Section) yields only one crystalline phase, isomorphous with pure 1. Observation of these doped crystals under a microscope reveals that they are uniformly the dark red color characteristic of 2 (1 is pale yellow). A sample of the doped crystals of known mass was dissolved in THF and the UV-vis spectrum was recorded. The absorption at 524 nm, with the known absorptivity of 1 at 524 nm, implies a doping level of 9.4% (on a mole or mass basis). Neither pure 1 nor 1 doped with 2 displays any measurable electrical conductivity on our apparatus. Conclusions We have introduced isostructural doping, a novel concept for doping molecular semiconductors. For isostructural doping to be successful, two conditions must be fulfilled. First, the dopant must substitutionally cocrystallize with the host molecule while maintaining the host’s crystal structure. We have found simple solution-phase conditions that allow the cocrystallization of 1 with large percentages of 2 without altering the crystal structure of 1. The second condition for isostructural doping is that the dopant must have the correct electronic properties within a crystal of the host. Namely, for an n-dopant, its filled level must lie very close in energy to the empty conduction band of the host. In molecular terms, the dopant must be almost a strong enough reducing agent to spontaneously reduce the host molecule. For 1 and 2 this is not the case. However, we are currently synthesizing both n- and p-type isostructural dopants for other molecular semiconductors that will likely fulfill both of the conditions laid forth, and thereby make possible the construction of true p-n junctions with molecular semiconductors. Acknowledgment. We thank James Schilling for the use of his SQUID magnetometer and assistance in performing susceptibility measurements, Tom Lin for assistance in acquiring ESR spectra, Lev Gelb for the use of his computing facilities, and James Korp of the University of Houston for acquiring the single-crystal X-ray diffraction data. This work was supported by NSF grant CHE-0133068 and Research Corporation grant RI0697. CM034646C

Investigations of the 9, 10-Diphenylacridyl Radical as an Isostructural

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