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Specific Indolo[3,2,1-jk]carbazole Conducting Thin-Film Materials Production by Selective Substitution John B. Henry, Stuart I. Wharton, Elanor R. Wood, Hamish McNab, and Andrew R. Mount* School of Chemistry, The University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, U.K.
bS Supporting Information ABSTRACT: Selectively substituted indolo[3,2,1-jk] carbazole (IC) molecules have been synthesized through flash vacuum pyrolysis (FVP) and then electro-oxidized, resulting in the formation of redox-active and electronically conducting thin films consisting exclusively of three highly luminescent dimer species, the 2,20 -, 2,100 -, and 10,100 -coupled dimers. DFT calculation has enabled both the accurate calculation of monomer oxidation potentials and the prediction of the nature of the resulting dimers through consideration of the coupling of the oxidized monomer radical cations. This demonstrates that substituted ICs represent a class of molecules able to form redox-active and conducting dimer films of controlled composition upon oxidation and that DFT calculations can be used to inform the synthesis of specific IC monomers most likely to both produce electronically conducting thin-film materials and yield specific luminescent dimers with desirable materials properties.
’ INTRODUCTION Redox-active and photoactive N-heteroaromatic oligomers and polymers such as pyrroles,1 indoles,2 and carbazoles3 have received a great deal of attention in recent years due to their attractive properties. They generally are extended π-conjugated thin-film materials, giving efficient molecular luminescence and semiconducting to conducting electronic and redox-active film properties at relatively low cost. This makes them attractive for a variety of applications, including the active film in organic electronic devices, sensors, supercapacitors, and electroluminescent materials.4 There has been much recent interest in the chemical synthesis and characterization of carbazole, isoindole, and indolocarbazole derivative films for their efficient hole transport properties5,6 in organic thin film transistors, their fluorescence in light-emitting devices,7 and their enhanced quantum yield for light-emitting diode applications.8 Indolo[3,2-b]carbazole has recently been the subject of interest both as a building block for highly efficient electroluminescent materials9 and within copolymers for use in photovoltaic applications.10 Indolo[2,3a]carbazole has also been investigated as a potential anion sensing material,11 and when fused with thiadiazole, it has been shown to undergo electropolymerization.12 We have recently reported that indolo[3,2,1-jk]carbazole (IC) (1) (see Figure 1) can be readily synthesized using flash vacuum pyrolysis (FVP), that it can be coupled electrochemically to form a film that consists exclusively of three specific redox-active dimers,13 and that its electrochemical properties along with other heteroaromatics can also be calculated accurately using a density functional theory (DFT) methodology.14 This is interesting as, typically, selective small-molecule thin-film production is far from facile. Electrochemical oligomer formation r 2011 American Chemical Society
is often unspecific and/or readily leads to polymer formation; in this case, IC is thought to exclusively form dimer films due to the unusually high oxidation potential of the dimer and perhaps also the unusually low dimerdimer coupling rates.13 This avoids the necessity for specific dimer chemical synthesis (through selective substituted monomer formation and site-specific coupling), which is often complex, costly, and a low-yield approach. It also avoids the chemical synthesis difficulty of establishing optimum molecular ordering when depositing thin films from solution.15 There is therefore real interest in electrogenerating molecular film systems of controlled composition as small molecules (both monomers and oligomers) can form crystalline or homogeneous multilayers due to their efficient packing, which should result in enhanced materials properties.16 In this paper, we therefore probe the effect of modification of the IC core on the synthesis and coupling of IC monomers. Specific substitution in the 5- position (2) and aza-group insertion (3) are shown computationally and confirmed experimentally to give IC films that all display similar selective oligomer thin-film formations to parent IC, with the properties and proportions of these oligomers being controlled by the substituent. Such substitution therefore provides a route to oligomeric thin-film production with control of composition as well as making possible the ready incorporation of a variety of chemical functionalities through substitution to tune the resultant film and oligomer properties.
Received: December 2, 2010 Revised: March 27, 2011 Published: May 10, 2011 5435
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’ RESULTS AND DISCUSSION Synthesis of Substituted Indolo[3,2,1-jk]carbazoles. The substituted indolo[3,2,1-jk]carbazoles 2ad were prepared by the two-step method previously described for the parent compound 113 (see Scheme 1). Thus, the nitroarylcarbazoles 6ad were made by treatment of carbazole 5 with an appropriately substituted 3-halogenonitrobenzene 4ad in the presence of base (see Experimental Section). FVP of these nitroaryl compounds 6ad at 875 C provided the indolo[3,2,1-jk]carbazoles 2ad in 5767% yield via cleavage of the nitro group and cyclization of the resulting aryl radical.17 5-bromoIC 2a13 and 7-azaIC 318 were synthesized by the FVP methods described elsewhere. Computational Studies of IC Systems. The coupling of the IC monomer to form dimers had previously been shown to be consistent with coupling via monomer radical cations.13 It has been argued that the most likely coupling sites are those with the
Figure 1. Structures of indolo[3,2,1-jk]carbazole (1), 5-substituted indolo[3,2,1,-jk]carbazole, where X = Br (2a), CH3 (2b), NH2 (2c), and CN (2d), and 7-azaindolo[3,2,1-jk]carbazole (3).
Scheme 1. Reagents and Conditions: (i) base; (ii) FVP 875 C
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highest electron spin density, where there will be the highest probability of radicalradical coupling and bond formation.14,19 In support of this, (1) (Figure 2b) has previously been shown to couple in the 2-, 5-, and 10-positions,13 which correspond to the calculated regions of highest electron spin density. Calculated peak oxidation potentials calculated using the B3LYP method and 6-311þG(d,p) basis set with temperature correction given by frequency calculations have previously been found to be in excellent agreement with the experimentally measured values for a variety of heteroaromatics, including 1, with an overall root-mean-square error (RMSE) of 37 mV.14 Although it can often be difficult to establish a precise experimental value of Eθ for those species that display chemically irreversible oxidation peaks due to chemical coupling, it is, however, possible to determine Eθ from the measured peak potential, Ep, and vice versa, for electrochemically reversible systems. The test of electrochemical reversibility, which can still be applied when there is chemical irreversibility on oxidation, is the separation of the peak potential, Ep, and the half-peak potential, Ep/2, as|Ep Ep/2| = 57 mV for a one-electron reversible redox reaction at 298 K.20 As with this previous work,14 the measured value of Ep Ep/2 for oxidation at low substituted IC concentration (1 mM) in acetonitrile was found to be within a few millivolts of this value at sweep rates below 100 mV s1, even when the data was not compensated for iR drop. In this case, Eθ is readily interrelated to Ep/2 and hence Ep, to within single millivolt accuracy, through 1=2 ! RT DO θ 1:09 þ ln Ep=2 ¼ E þ ð1Þ F DR where DO and DR are the diffusion coefficients of the oxidized and reduced species, respectively.20 Often for aromatics with extensive electronic conjugation, DO ≈ DR, which removes the second term in parentheses in eq 2. Having synthesized 2ad and 3, it is satisfying that their measured peak oxidation potentials also agree to within twice the RMSE with those calculated (Figure 3 and Supporting Information; for CV data, see the Supporting Information). This ability to accurately calculate peak oxidation potentials gives confidence
Figure 2. (a) The orientation of the skeleton structure substituted ICs and the calculated electron spin density mapped onto this skeleton structure for (b) 1, (c) 2a, (d) 2b, (e) 2c, (f) 2d, and (g) 3. The coloring is red, orange, yellow, green, and blue, with blue indicating the highest electron spin density (0.003) and red the lowest electron spin density (0.001). The same range of spin density was used for all six maps. 5436
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The Journal of Physical Chemistry A in the applicability of this computational method for modeling the electronic properties of this set of substituted ICs. It is also clear (see Figure 3 and Supporting Information) that both the monomer and thin-film redox potentials correlate with the Hammett substituent constant, which indicates that this substitution can be used to tune monomer and film redox properties while retaining IC redox behavior. This is consistent with the calculated electron spin density maps for 2a, 2b, and 2d (Figure 2), which, as that for 1, show the 2-, 10-, and 5-positions to be the positions of highest electron spin density in these systems (as with 1, one excludes the high electron spin density 7- and 8-positions as sites of coupling for ICs due to steric constraints).21 This therefore predicts that coupling will occur solely at the 2- and 10-positions in 2 as substitution will preclude 5-coupling through steric hindrance of the initial coupling and/or as 5-coupling cannot then lead to stabilization through the production of an aromatized dimer of
Figure 3. Calculated (b) and measured (9) peak oxidation potentials, Ep, versus the Fc/Fcþ couple for 5-substituted ICs versus the Hammett substituent constant (σþ). The nature of the 5-substituent is shown in each case.
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reduced or zero charge through Hþ loss; these will then favor dissociation to regenerate monomer radical cations. In addition, the electron spin density map for 3 shows an asymmetric spin density arrangement due to the introduction of the aza-functionality, with maximum spin density coupling positions again at the 2- and 10-positions. We therefore predict that, as that for 1 under electro-oxidative coupling, 2a, 2b, 2d, and 3 will each form a dimeric film on the electrode surface, consisting exclusively of the three isomeric dimers obtained from coupling in the 2- and 10-positions, the 2,20 -, 2,100 -, and 10,100 dimers. Production and Characterization of Dimer Films. As with 1, reproducible coupling through monomer oxidation at a rotating disk electrode (RDE) was indeed found for 2a, 2b, 2d, and 3, giving high-quality redox films.13,21 The resulting currenttime transients (Figure 4) show an initial current that rises and then falls, consistent with nucleation and growth of the redox film, first on selective nucleation centers on the platinum and then as the film grows progressively thicker on the previously deposited redox film. There is some observed variation in these early time currenttime transients both between experiments and between IC monomers, consistent with the sensitivity of film nucleation and growth to the nature of the platinum surface and the monomer. However, following this initial film formation, as seen previously for indoles2b and 1,13 reproducible essentially steadystate currents were observed in all cases (where the current was observed to vary by less than 1% across several repeat experiments at a fixed rotation speed), consistent with constant-rate film growth across the entire electrode surface. These essentially steady-state currents, i, were also observed for several hundred seconds. Given that these insoluble films result from efficient monomer coupling to form insoluble redox-active dimers (vide infra), an estimate of the film thickness can be readily obtained by integrating these currenttime transients, calculating the number of dimers formed and estimating their molecular size. As for 1,13 this indicates the highly reproducible growth of films on the
Figure 4. Film formation and onset of steady-state currents produced upon electro-oxidation at 1 Hz of 5 mM solutions of (a) 2d at þ1.3 V and (b) 2b at þ1.1 V and (c) a 10 mM solution of 3 at þ1.28 V in 0.1 M LiClO4/MeCN. Electro-oxidation was initiated by carrying out a potential step from 0.10 V near t = 2 s, to give the observed steeply rising current. 5437
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Figure 5. KouteckyLevich plots for the reciprocal of the steady-state currents (i1) versus W1/2 for the electro-oxidation of (a) 2d and (b) 2b. The data presented are for bulk monomer concentrations of 9, 1.25; b, 2.5; 2, 5; and 1, 10 mM. For these and subsequent data, a common diffusion coefficient is fitted to these data from eq 2 and then used to draw the fit lines for all data in each plot at all bulk concentrations. As predicted by eq 1, these data therefore all show an inverse relationship of the gradient with the bulk monomer concentration within experimental error.
The data for all monomers are therefore consistent with the general radical coupling mechanism typically observed for N-heteroaromatics, which results in the coupling of oxidized monomer M to form an insoluble (Mx/Mxþ redox-active) oxidized oligomer or polymer with n = (1 þ x1).13 xe
xHþ
e
xM f xMþ• f ðMÞx a ðMÞþ x þe
Figure 6. KouteckyLevich plots for the reciprocal of the steady-state currents (i1) versus W1/2 for the electro-oxidation of 3. The data presented are for bulk monomer concentrations of 9, 5; b, 10; and 2, 20 mM.
order of several micrometers in thickness under hydrodynamic control. The maintenance of constant currents over such time scales indicates that these films are electronically conducting. These steady-state current data for 2 (Figure 5) and 3 (Figure 6) at various concentrations and rotation speeds, W, in Hz, were found to obey the KouteckyLevich equation22 1 ¼ i
0:6435v1=6 2=3 nFAcm Dm W 1=2
! þ
1 i¥
ð2Þ
where n is the number of electrons for monomer oxidation, F is the Faraday constant, A is the electrode area, Dm is the diffusion coefficient of the monomer, cm is the bulk concentration of the monomer, v is the kinematic viscosity of the electrolyte, and i¥ is the mass-transport current observed as W tends to infinity. Such behavior has again been found previously for indoles2b and 1.13 As eq 1 applies when first-order mass-transport-dependent and independent reactions occur in series, such film formation behavior has been interpreted as the mass-transport-dependent transport of the monomer to the electrode surface to give masstransport-independent first-order adsorption, oxidation, and then radicalcation coupling, resulting in the redox film product.
Least-squares fitting of all data to eq 1 at all cm and W are tabulated in the Supporting Information. As with 1, FAB mass spectrometry on the dissolved films gave parent ion peaks at 531, 509, and 483 Da for 2d, 2a, and 3, respectively, corresponding to the mass of a protonated dimer for each monomer species. This again suggests exclusive dimer product formation with x = 2 and, given the demonstrated redox activity of each film, a value of n = 1.5 in eq 1. In each case, the charge obtained from the area under the CV redox peaks (see Supporting Information) of the resulting films was found to be one-third of that passed during electrooxidation and film formation within experimental error, consistent with this mechanism. The mean values for Dm obtained for these substituted ICs are all in the range of (2 ( 1) 105 cm2 s1, consistent with that obtained for 113 and with values expected for hetoraromatics of this size in acetonitrile background electrolyte. The i¥ corresponds to the coupling current obtained when W f ¥, and the surface monomer concentration is equal to cm. These intercepts decrease with increasing bulk concentration and apparently asymptotically approach a limiting value, as has been seen for indole2b and IC.13 This has been modeled previously for indoles as being consistent with Langmuir-type monomer surface adsorption, electro-oxidation, coupling, and redox film formation,23 for which the limiting value corresponds to the current for the coupling of an adsorbed monolayer of radical cations. The values of i¥ are all in the range of 26 mA, which, as that for 1, is consistent with relatively efficient surface coupling and film formation.13 By contrast, electro-oxidation of 2c did not give film formation. This is consistent with the markedly different electron spin density distribution calculated for 2c (Figure 6c), emphasizing that this difference has a marked effect on the resulting coupling products. The difference in electrochemical properties can also be found in the breakdown of the relationship between both the calculated and observed peak oxidation potentials and the Hammett substituent constant, which further emphasizes the markedly different electronic distribution in 2c. 5438
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Figure 8. Electron spin density maps for radical cations of modified ICs. The darkest blue areas show the regions of highest spin density. (a) Skeleton structure and orientation of aza-ICs. Electron spin density mapped onto the skeleton structure of (b) 5-aza-IC, 7; X = N; Y,Z = CH; (c) 7-aza-IC, 3, X,Z = CH; Y = N; and (d) 7,8-diaza-IC, 8, X = CH; Y, Z = N. (The coloring is schematic as follows: blue indicates a positive spin density (0.003), while red shows a negative spin density (0.001). The same range of spin density was used for all four maps).
Figure 7. (a) 360 MHz 1H NMR spectrum of the 5-methyl-IC film dissolved in [D6]acetone/[D6]DMSO. (b) 600 MHz 1H NMR spectrum of a 7-aza-IC film dissolved in [D6]acetone/[D6]DMSO. (c) The dimer species present, as deduced from these spectra and from those in the Supporting Information, where X = CH and R = CH3 for 2b and X = N and R = H for 3. Also shown are chemical shift values in ppm for 113 and the corresponding values obtained for those protons adjacent to the coupling site in each dimer as 2b, 3 for comparison.
Dimer Film Characterization. HPLC analyses of these films (Supporting Information) indicate that there are three distinct products for each monomer oxidative coupling. The films formed from 2b and 3 were characterized by a range of NMR techniques (Figure 7a,b and Supporting Information); the NMR spectra can clearly be seen to be rich in peaks, consistent with a mixture of three different dimeric isomers. Given the similarity of the electrochemical and computational characteristics to 1, it is satisfying that these NMR spectra can be analyzed and their peaks attributed by analogy with 1 product spectra.13 As the characteristics of the dimer products from 1 were confirmed by specific dimer synthesis and characterization, this is strong evidence for equivalent specific dimer formation. In each case, three dimers are observed, as for 1, from radical cation coupling at the 2- and 10-positions; however, unlike 1, there is no coupling at the 5-position, as predicted by steric blocking in 2b and by computation in 3. All of the monomers and dimers studied were found to be highly fluorescent, with quantum yields of 0.50 and 0.31 for 3 and
2b, respectively. All monomers emitted in the ultraviolet region (see Supporting Information) with peak emission wavelengths of 385, 369, and 365 nm, respectively, for 2b, 2d, and 3, while the peak emission wavelengths of 410, 408, and 406 nm, respectively, for the dissolved dimer films correspond to the emission of blue light. These attractive luminescence properties and the redox activity make this class of molecular films and their constituents of interest for fluorescence and UV light emission applications. Prediction of Substituted IC Properties. Having experimentally validated the calculation of substituted IC properties by synthesizing selected systems, calculations were extended to predict the peak oxidation potentials and electron spin density maps of other substituted ICs that have thus far not been synthesized. These extend the linear relationship found between the peak oxidation potential and Hammett substituent constant for these additional 5-substituents (Figure 3), as well as giving similar electron spin density distributions as those for 2a, 2b, and 2d. This indicates that a wide range of 5-substituents are effectively conjugated into the aromatic system and that 5-substitution can be used to tune both monomer oxidation and film redox properties. There is also the question of whether selective production of a single dimer species is possible. To address this, Figure 8 also shows the electron spin density maps of three more ICs containing aza- functionality. 3 has been discussed in detail earlier in this work, and it is interesting that the calculated peak oxidation potentials of 5-aza-IC, 7, and 7-aza-IC, 3, are predicted to be similar. However, the electron spin density map (Figure 7) for 7 shows the greatest electron spin density primarily in the 2-position, while insertion of the second aza-group in 8 leads to a spin density predominantly in the 10-position. These calculations suggest that selective aza-functionalization could offer a route to selective film formation with potentially single-dimer production, giving exclusive 10,100 -coupling for 3 and 2,20 -coupling for 7. It is noticeable that the electron spin density map of the 7,8-diaza-IC, 8, 5439
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The Journal of Physical Chemistry A shows spin density isolated in the 2-position and would possibly make a good candidate for forming an isolated 2,20 -dimer; the presence of the second aza-group greatly increases the peak oxidation potential and may cause a barrier to practical film formation in standard solutions. We are currently synthesizing these systems to confirm these predictions.
’ CONCLUSIONS It is clear from the work presented in this paper that either selected peripheral or framework aza-substitution of IC results in the production of electronically conducting thin films which, as with IC, consist of three specific and highly luminescent dimer species. This extends the variety of N-heteraromatic systems that can couple to form conducting and redox-active thin films while opening up the prospect of the ready formation of films of controlled structure, composition, and morphology. Given the interest in such N-heteroaromatics, it also indicates that substituted IC thin films are worthy of further study for a variety of thin-film materials applications. Computation and experimental characterization has shown that the redox and positional coupling of these substituted ICs can also be both predicted and explained through the coupling of two oxidized monomer radical cations at their positions of highest radical electron density. This establishes that substituted ICs represent a class of molecules that display attractive selective coupling and luminescence characteristics, whose properties (e.g., solubility and redox potential) can, in principle, be readily tuned by changing the substituent nature. The effect of variation of this substitution has also therefore been predicted in silico, which has motivated the specific synthesis of monomers to form molecular films of controlled composition and with properties tailored to materials application. This combination of experiment and computation is generic as it offers the potential for efficient in silico prediction of thin-film materials and redox properties in order to identify the most promising candidates for synthesis. This should reduce the complexity and cost of monomer variant synthesis for these and similar systems, as well as lead to the more efficient exploration of structure property relationships. ’ EXPERIMENTAL SECTION Routine 1H and 13C NMR spectra were recorded at 250 or 63 MHz respectively for solutions in [2H]chloroform unless otherwise stated. Coupling constants are quoted in Hz. Mass spectra were recorded under electron impact conditions. 9-(4-Methyl-2-nitrophenyl)carbazole 6b. A solution of carbazole 5 (2.76 g, 16.4 mmol) and 5-methyl-2-fluoronitrobenzene 4b (2.61 g, 16.8 mmol) in DMSO (50 cm3) was stirred with cesium carbonate (5.38 g, 27.9 mmol) for 15 h at 100 C. The mixture was diluted with water (50 cm3), and the resulting suspension was extracted with dichloromethane (3 100 cm3). The combined organic extracts were washed with water (3 100 cm3) and dried (MgSO4), and the solvent was removed to give crude 9-(4-methyl-2-nitrophenyl)carbazole 6b, which was recrystallized from ethanol/ethyl acetate (2.74 g, 55%), mp = 101103 C (lit.,24 104106 C) (Found: C, 75.3; H, 4.5; N, 9.2. C19H14N2O2 requires C, 75.5; H, 4.6; N, 9.3%. Found: Mþ 302.1062. C19H14N2O2 requires M = 302.1050); δH 8.03 (2H, dd, 3J = 7.0 and 0.8), 7.85 (1H, s), 7.50 (1H, d, 3J = 8.1), 7.38 (1H, d, 3J = 8.1), 7.337.14 (4H, m), 7.00 (2H, dd, 3J = 6.8 and 4 J = 0.5) and 2.45 (3H, s); δC (63 MHz, CDCl3) 147.54 (quat), 141.39 (2 quat), 140.53 (quat), 135.35 (CH), 131.58 (CH),
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128.90 (quat), 126.61 (4 CH), 124.15 (2 quat), 120.91 (4 CH), 109.55 (CH) and 21.55 (CH3); m/z 302 (Mþ, 100%), 254 (42), and 167 (50). 9-(4-Amino-2-nitrophenyl)carbazole 6c. Carbazole 5 (1.00 g, 6 mmol) and 4-fluoro-3-nitroaniline 4c (0.93 g, 6 mmol) were dissolved in DMSO (20 cm3). Cesium carbonate (2.15 g, 6.6 mmol) was added to the solution with stirring. The suspension was stirred for 18 h at 140 C before being cooled and diluted with brine (50 cm3). The mixture was extracted with dichloromethane (4 100 cm3). The combined organic layers were washed with water (2 100 cm3), dried (MgSO4), and concentrated under reduced pressure. The residue was purified by dry flash chromatography (hexane/ethyl acetate) to yield 9-(4-amino-2-nitrophenyl)carbazole 6c (1.51 g, 83%), mp = 162164 C (lit.,25 164165 C); δH (d6-DMSO) 8.22 (2H, d, 3J = 7.9), 7.417.37 (4H, m), 7.27 (2H, t, 3J = 7.6), 7.127.07 (3H, m) and 6.24 (2H, br, s); δC (63 MHz, DMSO) 150.55 (quat), 147.99 (quat), 141.31 (2 quat), 131.20 (CH), 126.36 (2 CH), 122.73 (2 quat), 120.60 (2 CH) 119.99 (2 CH), 118.95 (CH), 116.17 (quat), 109.33 (2 CH) and 108.96 (CH); m/z 303 (Mþ, 22%), 256 (15), 167 (85), 156 (88), 110 (85), 98 (32), 83 (100), 58 (53), and 43 (78). 9-(4-Cyano-2-nitrophenyl)carbazole 6d. 4-Chloro-3-nitrobenzonitrile 4d (2.00 g, 11 mmol), carbazole 5 (1.83 g, 11 mmol), and potassium carbonate (1.80 g, 11 mmol) were heated, with stirring, in nitrobenzene (5 cm3) for 15 h at 180 C. The nitrobenzene was removed by bulb-to-bulb distillation under vacuum. The residue was dissolved in ether and extracted with water (3 100 cm3). The combined organic layers were dried (MgSO4), and the solvent was removed under reduced pressure. The residue was purified by dry flash chromatography (hexane/toluene) to give 9-(4-cyano-2-nitrophenyl)carbazole 6d (2.20 g, 64%), mp = 171173 C (lit.,25 172174 C); δH 8.33 (1H, s), 8.00 (2H, d, 3 J = 7.1), 7.92 (1H, dd, 3J = 8.3, 4J = 1.9), 7.73 (1H, d, 3J = 8.3), 7.327.32 (4H, m), and 7.04 (2H, d, 3J = 7.3); δC 147.97 (quat), 140.18 (2 quat), 137.60 (CH), 135.90 (quat) 132.41 (CH), 130.54 (CH), 127.20 (2 CH) 124.88 (2 quat), 124.22 (2 CH), 121.35 (2 CH), 116.59 (quat), 112.93 (quat), and 109.25 (2 CH); m/z 313 (Mþ, 7%), 266 (6), 167 (100), 91 (81), and 77 (9). Indolo[3,2,1-jk]carbazole Derivatives 2. The precursors 6 were subjected to FVP by distillation under vacuum through a silica pyrolysis tube (35 2.5 cm), which was heated by a laboratory tube furnace. The crude indolocarbazoles 6 were collected in a coldfinger trap cooled by dry ice/acetone situated as close as possible to the exit point of the furnace. Upon completion of the pyrolysis, the trap was allowed to warm up to room temperature under nitrogen, and the product was removed from the trap by dissolution in dichloromethane. The solution of product was concentrated under reduced pressure, and the product was purified by dry flash chromatography using a gradient of hexane/ethyl acetate as the eluent unless otherwise stated. Pyrolysis parameters are quoted as follows: mass of precursor, furnace temperature (Tf), inlet temperature (Ti), pressure (range if appropriate) (P), and reaction time (t). Indolo[3,2,1jk]carbazole 1 and 5-bromoindolo[3,2,1-jk]carbazole 2a were prepared as previously described,13 as was the azaindolocarbazole 3.17 The following indolocarbazoles were made by this method. 5-Methylindolo[3,2,1-jk]carbazole 2b. FVP of 6b (0.500 g, Tf = 875 C, Ti = 140160 C, P = 3.5 102 Torr, t = 120 min) gave 5-methylindolo[3,2,1-jk]carbazole 2b (0.300 g, 60%), mp = 101103 C (lit.,26 110112 C) (Found: C, 88.6; H, 4.9; N, 5.2. C19H13N requires C, 89.1; H, 5.1; N, 5.5%. Found: Mþ 5440
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The Journal of Physical Chemistry A 255.1035. C19H13N requires M = 255.1043); δH 7.95 (1H, d, 3 J = 7.8), 7.83 (2H, m), 7.73 (1H, s), 7.64 (1H, dd, 3J = 8.0 and 4 J = 0.6) 7.52 (1H, d, 3J = 8.2), 7.37 (2H, m), 7.16 (2H, m), and 2.37 (3H, s); δC (63 MHz, CDCl3) 144.35 (quat), 139.06 (quat), 137.22 (quat), 131.53 (2 quat), 130.54 (quat), 130.27 (quat), 127.97 (CH), 126.95 (CH), 123.80 (CH), 123.45 (CH), 122.97 (CH), 121.76 (CH), 120.57 (2 CH), 119.62 (quat), 112.40 (CH), 112.02 (CH), and 21.95 (CH3); m/z 255 (Mþ, 100%) and 178 (54). 5-Aminoindolo[3,2,1-jk]carbazole 2c. FVP of 6c [0.550 g, Tf = 875 C, Ti = 220260 C, P = 2.0 102 Torr, t = 50 min] gave 5-aminoindolo[3,2,1-jk]carbazole 2c (0.266 g, 57%) after chromatography, mp = 134137 C; (Found: Mþ 256.0996, C18H12N2 requires M = 256.1001); δH (360 MHz) 8.15 (1H, dt, 3 J = 7.8, 4J = 1.1, 5J = 0.7), 8.05 (1H, d, 3J = 7.4), 7.98 (1H, d, 3 J = 7.3), 7.83 (1H, dt, 3J = 7.9, 4J = 1.1, 5J = 0.7), 7.67 (1H, d, 3 J = 7.4), 7.57 (1H, t, 3J = 7.4), 7.56 (1H, td, 3J = 7.4, 4J = 1.2), 7.44 (1H, d, 4J = 2.0), 7.35 (1H, dd, 3J = 7.7, 4J = 1.0), 6.91 (1H, dd, 3J = 8.4, 4J = 2.3), and 3.45 (2H, br s); δC (91 MHz) 143.97 (quat), 141.14, (quat), 138.46 (quat), 132.57 (quat), 130.93 (quat), 129.31 (quat), 123.38 (CH), 122.88 (CH), 122.13 (CH), 120.82 (CH) 119.20 (CH), 119.07 (CH), 118.25 (quat), 118.18 (quat), 114.70 (CH), 112.32 (CH), 111.51 (CH), and 109.52 (CH); m/z 256 (Mþ, 35%), 187 (35), 167 (6), 143 (24), 99 31), 87 (100), 71 (13), and 55 (45). 5-Cyanoindolo[3,2,1-jk]carbazole 2d. The pyrolysate from FVP of 6d (0.703 g, Tf = 875 C, Ti = 210240 C, P = 3.2 102 Torr, t = 45 min) was purified by dry flash chromatography using toluene/ethyl acetate to give 5-cyanoindolo[3,2,1-jk]carbazole 2d (0.400 g, 67%), mp = 191192 C (from toluene); (Found: C, 83.6; H, 3.8; N, 10.0. C19H10N2 3 0.3H2O requires C, 83.6; H, 3.9; N, 10.3%); δH (360 MHz) 8.27 (1H, d, 4J = 1.1), 8.08 (1H, dq, 3J = 7.8, 4J = 1.2, 5J = 0.7), 7.98 (1H, d, 3J = 7.4), 7.88 (1H, d, 3J = 7.5), 7.69 (1H, d, 3J = 8.1, 4J = 1.1, 5J = 0.7), 7.65 (1H, d, 3J = 7.4), 7.63 (1H, d, 3J = 7.4), 7.58 (1H, t, 3J = 7.5), 7.53 (1H, td, 3J = 7.5, 4J = 1.2), and 7.43 (1H, td, 3J = 7.5, 4J = 1.1); δC (91 MHz) 143.82 (quat), 139.57 (quat), 137.89 (quat), 130.12 (quat), 129.92 (CH), 129.77 (quat), 127.02 (CH), 126.87 (CH), 123.70 (CH), 123.23 (CH), 122.86 (CH), 120.42 (CH), 119.64 (CH), 119.63 (quat), 118.69 (quat), 116.56 (quat), 112.35 (CH), 112.09 (CH), and 104.30 (quat); m/z 266 (Mþ, 100%), 238 (4), 133 (16), 97 (8), and 73 (18). HPLC was carried out on the electrochemically formed films dissolved in DMF/acetone and diluted with water. The HPLC conditions used were Spherisorb S5-ODS1 column, eluent A = H2O/MeCN 9:1, þ1% NH4OAc; eluent B = MeCN. Solvent gradient: time 0 min = 5% B; gradient increase in B over 30 min to 95% B; eluent mixture maintained at this level until 55 min. Detection wavelength 254 nm. Fluorescence measurements were recorded using a Spex Fluoromax spectrometer with a 1 cm path length fused silica cuvette. Quantum yields were estimated by making a comparative measurement using indole as the standard, which has a known quantum yield of 40% in ethanol solution.27 Low (∼μM) comparable concentrations of sample and standard were used to ensure that inner filter effects were negligible. For both the sample and standard, the quantum efficiency is proportional to the ratio of the integrated emission intensity (the area under the measured emission spectrum) to the absorption at the excitation wavelength. Electrochemical measurements were performed in background electrolyte solutions of 0.1 M anhydrous LiClO4 in acetonitrile (dried; distilled). The reference electrode was made
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in-house and consists of a Ag wire dipped into a solution of AgClO4 (0.01 M) in background electrolyte solution, with a measured potential of 0.074 V versus the Fc/Fcþ couple. The counter electrode was a 2 cm2 Pt gauze, and the working electrode was a 0.387 cm2 Pt rotating disk electrode. These were controlled by an AUTOLAB PGSTAT30 potentiostat (Eco Chemie BV) equipped with GPES 4.9 software. All calculations were carried out using the software package Gaussian 0328 running on a SUSe 9.x Linux HPC cluster consisting of 68 AMD Opteron processing cores contained within the EaStCHEM Research Computing Facility Hare cluster. Default convergence criteria were used for all calculations (maximum force = 0.00045, rms force = 0.0003, maximum displacement = 0.0018, and rms displacement = 0.0012). The computational method employed was B3LYP, the Becke29 threeparameter hybrid functional, which utilizes the correlation functional of Lee, Yang, and Parr30 and includes both local and nonlocal terms. For calculations of the electron spin density, the unrestricted method uB3LYP was employed. The basis set used in all calculations was 6-311þG(d,p) with acetonitrile solvation modeled with the polarizable continuum model (PCM);31 the parameters used for the salvation model were the Gaussian defaults.28 For all molecules, a frequency calculation was performed with optimized geometries to ensure that a minimum in the potential energy hypersurface had been reached and to obtain temperature-corrected free energies for the molecules at 298 K. The spin density distribution was evaluated by generating cube files for the electron density and electron spin from the solution calculations. The spin density distribution plots were then made by mapping onto the 99% electron density surface. The output was viewed using Jmol, an open-source Java viewer for chemical structures in 3D (http://www.jmol.org/) and rendered using the Persistence of Vision Raytracer freeware (http://www.povray.org).
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables of experimentally measured and calculated peak oxidation potentials for Figures 3 and 8. Monomer and film CVs. Least-squares fitting data for KouteckyLevich analysis. HPLC data. NMR characterization data for 2b and 3. Fluorescence spectra for 2b. The complete reference details for ref 28. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT S.I.W. thanks EPSRC for a research studentship, J.B.H. thanks the School of Chemistry, The University of Edinburgh, for funding. This work has made use of the resources provided by the EaStCHEM Research Computing Facility (http://www. eastchem.ac.uk/rcf). This facility has been partially supported by the eDIKT initiative (http://www.edikt.org). The School of Chemistry is part of the EaStCHEM joint Chemistry Research School; we acknowledge the financial support of the Scottish Funding Council. 5441
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’ DEDICATION Dedicated to the memory of Professor Hamish McNab, following his untimely death on November 15, 2010. ’ REFERENCES (1) (a) Waltman, R. J.; Bargon, J. Can. J. Chem. 1986, 64, 76–95. (b) Sadki, S.; Schottland, P.; Brodie, N.; Sabouraud, G. Chem. Soc. Rev. 2000, 29, 283–293. (c) Higgins, S. J. Chem. Soc. Rev. 1997, 26, 247–257. (2) (a) Bartlett, P. N.; Dawson, D. H.; Farrington, J. J. Chem. Soc., Faraday Trans. 1992, 88, 2685–2695. (b) Jennings, P.; Jones, A. C.; Mount, A. R.; Thomson, A. D. J. Chem. Soc., Faraday Trans. 1997, 93, 3791–3797. (c) Xu, J.; Hou, J.; Zhou, W.; Nie, G.; Pu, S.; Zhang, S. Spectrochim. Acta, Part A 2006, 63, 723–728. (3) (a) Yapi Abe, S.; Bernede, J. C.; Delvalle, M. A.; Tregouet, Y.; Ragot, F.; Diaz, F. R.; Lefrant, S. Synth. Met. 2002, 1261–6. (b) Syed Abthagir, P.; Saraswathi, R. Org. Electron. 2004, 5, 299–308. (4) Boudreault, P. L. T.; Wakim, S.; Blouin, N.; Simard, M.; Tessier, C.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2007, 129, 9125–9136. (5) (a) Morin, J. F.; Drolet, N.; Tao, Y; Leclerc, M. Chem. Mater. 2004, 16, 4619–4626. (b) Li, J.; Dierschke, F.; Wu, J.; Grimsdale, A. C.; Mullen, K. J. Mater. Chem. 2006, 16, 96–100. (c) Drolet, N.; Morin, J. F.; Leclerc, N.; Wakim, S.; Tao, Y.; Leclerc, M. Adv. Funct. Mater. 2005, 15, 1671–1682. (6) (a) Wakim, S.; Bouchard, J.; Simard, M.; Drolet, N.; Tao, Y.; Leclerc, M. Chem. Mater. 2004, 16, 4386–4388. (b) Li, Y.; Wu, Y.; Gardner, S.; Ong, B. S. Adv. Mater. 2005, 17, 849–853. (c) Wu, Y.; Li, Y.; Gardener, S.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 614–618. (7) Mitsumori, T.; Bendikov, M.; Dautel, O.; Wudl, F.; Shioya, T.; Sato, H.; Sato, Y. J. Am. Chem. Soc. 2004, 126, 16793–16803. (8) Kobayashi, N.; Koguchi, R.; Kijima, M. Macromolecules 2006, 9102–9111. (9) Zhao, H.-P.; Wang, F.-Z.; Yuan, C.-X.; Tao, W.-T.; Sun, J.-L.; Zou, D.-C.; Jiang, M.-H. Org. Electron. 2009, 10, 925–931. (10) (a) Zhou, E.; Yamakawa, S.; Zhang, Y.; Tajima, K.; Yang, C.; Hashimoto, K. J. Mater. Chem. 2009, 19, 7730–7737. (b) Tsai, J.-H.; Chueh, C.-C.; Lai, M.-H.; Wang, C.-F.; Chen, W.-C.; Ko, B.-T.; Ting, C. Macromolecules 2009, 42, 1897–1905. (11) Curiel, D.; Cowley, A.; Beer, P. D. Chem. Commun. 2005, 236–238. (12) Balaji, G.; Shim, W. L.; Parameswaran, M.; Valiyaveettil, S. Org. Lett. 2009, 11, 4450–4453. (13) Wharton, S. I.; Henry, J. B.; McNab, H.; Mount, A. R. Chem. Eur. J. 2009, 15, 5482–5490. (14) Henry, J. B.; Mount, A. R. J. Phys. Chem. A 2009, 113, 13023–13028. (15) Li, Y.; Wu, Y.; Ong, B. S. Macromolecules 2006, 39, 6521–6527. (16) (a) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. Nature 2006, 444, 913–917. (b) Briseno, A. L.; Tseng, R. J.; Ling, M. M.; Falcao, E. H. L.; Yang, Y.; Wudl, F.; Bao, Z. Adv. Mater. 2006, 18, 2320–2324. (17) Crawford, L. A.; McNab, H.; Mount, A. R.; Wharton, S. I. J. Org. Chem. 2008, 73, 6642–6646. (18) Crawford, L. A.; McNab, H.; Mount, A. R.; Verhille, J.; Wharton, S. I. Synthesis 2010, 923–928. (19) Waltman, R. J.; Diaz, A. F.; Bargon, J. J. Phys. Chem. 1984, 88, 4343–4346. (20) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001; p 231. (21) Electrooxidation was performed with the application of a potential at least 100 mV higher than the measured peak oxidation potential to ensure that the reaction rate was not limited by the rate of electrochemical reaction at the electrode surface. (22) Koutecky, J.; Levich, V. G. Zh. Fiz. Khim. 1958, 32, 1565–1575. (23) Mackintosh, J. G.; Wright, S. J.; Langirdge-Smith, P. R. R.; Mount, A. R. J. Chem. Soc., Faraday Trans. 1996, 92, 4109–4114. (24) Tucker, S. H.; Preston, R. W. G. J. Chem. Soc. 1943, 659–661.
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(25) Tucker, S. H.; Dunlop, S. J. Chem. Soc. 1939, 1945–1956. (26) Buchanan, C.; Tucker, S. H. J. Chem. Soc. 1958, 2750–2755. (27) Kirby, E. P.; Steiner, R. F. J. Phys. Chem. 1970, 74, 4480–4490. (28) Frisch, M. J. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford CT, 2004. (29) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (30) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (31) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151–5158.
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