2242
J. Phys. Chem. B 2000, 104, 2242-2250
Donor-Acceptor Complexes of Alkylcarbazole and Dicarbazolylalkane Donors with the Acceptors Tetracyanoethylene and Tetranitromethane Gary J. Haderski, Zhenhua Chen, Randolph B. Krafcik, John Masnovi,* Ronald J. Baker, and Robert L. R. Towns Department of Chemistry, CleVeland State UniVersity, CleVeland, Ohio 44115 ReceiVed: September 24, 1999; In Final Form: NoVember 29, 1999
Electron donor-acceptor complexes of conformationally flexible 1,n-dicarbazolylalkanes (C12H8N-(CH2)nNC12H8), where n ) 1-5, were examined. Carbazole, methylcarbazole, ethylcarbazole, and cyanoethylcarbazole also were studied as monochromophoric analogues for comparison. In dichloromethane solution, the dicarbazolylalkanes form 1:1 complexes with the terminal carbazolyl chromophores acting as independent donors when n g 2. With the acceptor tetranitromethane (TNM), the carbazoles form contact complexes displaying small positive enthalpies of formation. In contrast, stable complexes form with the acceptor tetracyanoethylene (TCNE). Crystalline TCNE complexes were isolated for the bichromophoric donors with n ) 2-4. The solid complexes and their uncomplexed donor components were analyzed by single-crystal X-ray diffraction. The solid-state stoichiometries of (carbazolyl donor):(TCNE acceptor) were found to depend on the donor conformation. Dicarbazolylalkane donors separated by two or four methylene units exhibit a 1:1 donor:acceptor ratio and form stacked arrays of alternating donor and acceptor groups. A three-carbon bridging alkyl chain leads to local sandwich-type complexes in the solid state with a resulting donor:acceptor ratio of 4:1.
Introduction
SCHEME 1
Electron donor-acceptor (EDA) complexes of carbazole derivatives have received much attention. Polymeric donors, such as poly(vinylcarbazole) doped with various electron acceptors, have been used as photosemiconductors in xerography1 and have generated interest in the mechanisms of charge formation and propagation.2 Other studies have concerned the nonlinear optical properties of such complexes.3 We have investigated the complexation thermodynamics of a series of carbazole donors with two electron acceptors, tetracyanoethylene (TCNE) and tetranitromethane (TNM) in order to elucidate the properties of the complexes. Carbazole (MH) and three N-alkylcarbazoles (M1-M3) were selected for comparison with five di(N-carbazolyl)alkanes (D1-D5), which have one to five methylene units separating the two carbazolyl groups, respectively (Scheme 1). These compounds were chosen as low molecular weight models of poly(N-vinylcarbazole) or (PVC).4 The acceptors, TCNE and TNM, were selected because they form well-resolved charge-transfer absorptions with the carbazole donors, yet differ in their complexing characteristics in solution. Tetracyanoethylene, which has been used as a dopant for PVC,3,5 forms relatively stable EDA complexes with carbazoles, allowing investigation of the solid-state structures of several complexes.6,7 In contrast, TNM forms weak complexes that are photochemically reactive.8,9 We have described previously the behavior of ion pairs formed by direct excitation of carbazole-TNM complexes and donor-TCNE complexes.9,10 The reactivities of carbazole radical cations formed in this manner were observed to depend markedly on the number of methylene units separating the carbazolyl groups in the dicarbazolylalkanes. Such an effect was ascribed to the extent and nature of the intramolecular interactions that occurred between the two carbazolyl groups.9 The
present study was undertaken in order to determine whether similar behavior might be observed for the ground-state electron donor-acceptor complexes of the dicarbazolylalkanes. This represents the first systematic study of the thermodynamic properties of complexes involving bichromophoric carbazole donors in solution and including the solid state.
10.1021/jp9934174 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/16/2000
Donor-Acceptor Complexes of 1,n-Dicarbazolylalkanes
J. Phys. Chem. B, Vol. 104, No. 10, 2000 2243
Experimental Section Reagents. Carbazole (Lancaster) and N-ethylcarbazole (Aldrich) were purified by column chromatography using 100200 mesh silica gel (Fisher), eluting fractionally with hexane/ dichloromethane and recrystallizing several times from ethanol. The di(N-carbazolyl)alkanes, excluding 1,2-di(N-carbazolyl)ethane (D2), were prepared from the potassium salt of carbazole and the corresponding 1,n-dibromoalkanes according to the literature.11 A modification of this synthesis involving the ditosyl ester of ethylene glycol was used to prepare D2 in 40.5% yield.12 N-Methylcarbazole (Aldrich) was used as received. N-(2Cyanoethyl)carbazole was generously donated by the late Professor Fred A. Sheibley. This compound contained a small amount (∼7%) of anthracene, which was removed by DielsAlder reaction with TCNE. TCNE was purified by successive sublimation until colorless. TNM was synthesized according to the literature13 and purified by steam distillation followed by repeated extraction with distilled water and freeze-thaw degassing to remove traces of NO2. Dichloromethane (Aldrich, Gold Label) was used as received. Instrumentation. All absorbance measurements were made in 1 cm optical Pyrex sample cells using a Hewlett-Packard HP8452 diode array spectrophotometer equipped with a thermostated cell holder. The temperature of the cell holder was monitored by a calibrated digital thermometer. Samples containing TNM were measured in dim red light in order to prevent photochemical nitrations.8-10 Absorption Measurements. Typical absorption spectra of TCNE and TNM complexes are shown in Figure 1. Job plots14 (method of continuous variation) were obtained to determine the molecularity of the associations (Figure 2). Equilibrium constants and charge-transfer (CT) molar extinction coefficients () of the EDA complexes were determined by the BenesiHildebrand method.15 The TCNE:carbazole concentration ratios were varied from about 80:1 to 20:1 through a series of 20 dilutions with a stock solution of carbazole, with an initial TCNE concentration of 0.060 M. Additionally, two of the more soluble donors, M2 and D3, were examined using a TCNE:carbazole ratio that varied from 1:87 to 1:22. The TNM:carbazole concentration ratios were varied from approximately 125:1 to 30:1, with an initial TNM concentration of 0.60 M. Absorbance measurements were made at 21 ( 1 °C. No peak maxima were apparent for the donor-TNM complexes; only diffuse tailings to low energy were observed. Therefore, the absorbance measurements were made in the charge-transfer tails at the same relative energies observed for the well-resolved donor-TCNE maxima.16,17 For each complex, measurements were made at wavelengths where the uncomplexed donor and acceptor had no absorbance. Selected BenesiHildebrand plots are shown in Figures 3a and 4. To determine the dependence of EDA complexation on temperature, solutions containing 5 mM TCNE and 2.5 mM carbazole donor or 0.4 M TNM and 5 mM carbazole donor at (21.0 ( 0.1) °C were thermostated at six temperatures. Concentration changes due to solution volume change with temperature were explicitly taken into account. The shape of the absorption spectra remained essentially unchanged at these temperatures and concentrations.19 The equilibrium constants (K) for EDA complex formation are related to the optical density (A), the molar extinction coefficient (), and the initial concentrations of donor ([D]0) and acceptor ([A]0) by
K ) (A/)[([A]o - A/)([D]o - A/)]-1
(1)
Figure 1. Absorption spectra of (a) carbazole-TCNE complexes normalized at 588 nm and (b) carbazole-TNM complexes in dichloromethane for M3 (small dash), D1 (dash-dot), D3 (solid line), and D5 (long dash).
Figure 2. Job plots for the M2 (filled circle) and D3 (open circle) complexes with TCNE.
The integrated form of the van’t Hoff equation may then be written
-(∆H/R)T-1 + (∆S/R) ) ln(A/) - ln([A]o - A/) ln([D]o - A/) (2) The data are displayed graphically in Figure 5. The low solubility of 1,1-dicarbazolylmethane (D1) prevented accurate measurement of the temperature dependence of its complexation
2244 J. Phys. Chem. B, Vol. 104, No. 10, 2000
Figure 3. (a) Benesi-Hildebrand determination for TCNE complexes with M2 (diamond), M3 (inverted triangle), D3 (triangle), and D5 (circle). (b) Expansion of intercept for M2 at high [TCNE] (solid line) and high [donor] (dotted line) and for D3 at high [TCNE] (dashed line) and high [donor] (dash-dot line).
Haderski et al.
Figure 5. Temperature dependence of CT absorptions for (a) carbazole-TCNE complexes (2.5 mM donor and 5 mM TCNE) and (b) carbazole-TNM complexes (5 mM donor and 0.4 M TNM) for D4 (diamond), D2 (triangle), M3 (inverted triangle), M2 (circle), and D1 (square). 1H
Figure 4. Benesi-Hildebrand determination for TNM complexes with MH (triangle), M3 (square), D1 (inverted triangle), D3 (circle), and D5 (diamond).
with TNM; therefore, the enthalpy of formation in this case was specified to be the mean (0.41 ( 0.15) found for the other seven donors. Structure Determinations. Stoichiometries of the solid-state TCNE complexes were determined by dissolution of the complex in CDCl3, followed by the addition of excess anthracene (which reacts quantitatively with the liberated TCNE to form a Diels-Alder adduct), and relative integration of the
NMR signals due to the carbazole derivative and the TCNEanthracene cycloadduct. For each compound, a single-crystal suitable for X-ray diffraction was selected and attached to the end of a glass capillary using quick-drying epoxy cement at room temperature. Crystals of the TCNE complexes were coated with the epoxy cement in order to retard decay of the crystalline integrity caused by TCNE sublimation. Intensity data were collected at ambient temperature for 1° e Θ e 25° in the (ω2Θ) scan mode with a variable scan rate of 2.3-5.5° min-1 using Mo KR radiation (wavelength 0.710 73 Å). For each crystal, three intensity controls were measured hourly. Fobs values were corrected for Lorentz and polarization effects. The criterion I > 2.5σ(I) was used for reflections to be considered statistically significant in all determinations. The structures were solved by direct methods and refined by full-matrix least-squares methods.20 The function minimized was Σw(|Fo| - |Fc|)2 with w ) 1.0/σ(Fo)2. Hydrogen atoms were located by difference Fourier synthesis or generated in chemically reasonable positions during later treatment. Crystallographic details can be found in the supplemental information and are summarized in Tables 2 and 3. Results and Discussion Solution Phase. The ability of TCNE and TNM to form EDA complexes with donors in solution is well documented.8-10,21
Donor-Acceptor Complexes of 1,n-Dicarbazolylalkanes TABLE 1: Thermodynamic Properties of EDA Complexes of TCNE and TNM with Carbazoles in CH2Cl2 TCNE λa λCTb MH M1 M2
335 600 346 592 347 596
M3 D1 D2 D3
340 336 344 345
D4 D5
346 590 347 592
TNM MH M1 M2 M3 D1 D2 D3 D4 D5
578 578 586 588
KCTc 5920 ( 30 7740 ( 20 6050 ( 30 (5950 ( 30)e 2950 ( 20 3950 ( 20 6930 ( 40 8060 ( 60 (8550 ( 40)e 9570 ( 50 10500 ( 50
Kd (M-1) ∆H (kcal/mol) 4.70 6.14 4.80 (4.72)e 2.34 3.13 5.50 6.40 (6.79)e 7.60 8.34
∆S (eu)
-2.91 ( 0.13 -6.50 ( 0.05 -3.01 ( 0.11 -6.47 ( 0.04 -3.01 ( 0.06 -6.85 ( 0.02 -2.59 ( 0.11 -2.71 ( 0.12 -2.85 ( 0.11 -2.95 ( 0.21
-7.38 ( 0.04 -6.52 ( 0.04 -5.78 ( 0.04 -6.10 ( 0.08
-3.07 ( 0.07 -6.14 ( 0.02 -3.13 ( 0.19 -6.18 ( 0.07
λ
λCT
KCTc
Kf
∆H (kcal/mol)
∆S (eu)
335 346 347 340 336 344 345 346 347
507 530 534 518 518 526 526 528 530
80.7 ( 0.3 99.6 ( 0.8 75.0 ( 0.4 64.8 ( 0.4 76.3 ( 0.8 137.0 ( 1.1 144.6 ( 0.7 163.5 ( 0.5 168.7 ( 0.5
0.212 0.262 0.197 0.171 0.201 0.361 0.381 0.430 0.444
+0.29 ( 0.07 +0.54 ( 0.06 +0.48 ( 0.07 +0.08 ( 0.02 0.41g +0.40 ( 0.08 +0.50 ( 0.08 +0.40 ( 0.06 +0.58 ( 0.07
-1.69 ( 0.02 -1.40 ( 0.02 -1.63 ( 0.02 -2.06 ( 0.01 -1.79 -0.53 ( 0.02 -0.20 ( 0.02 -0.14 ( 0.02 0.00 ( 0.02
a Lowest energy absorption maximum of uncomplexed carbazole (nm). b Lowest energy CT maximum (nm). c Acceptor in excess except where noted. d ) 1260 M-1 cm-1 at 21 ( 1 °C. e Carbazole in excess. f ) 380 M-1 cm-1 at 21 ( 1 °C. g Assumed.
TABLE 2: Crystallographic Data for 1,n-Di(N-carbazolyl)alkanes D2, D3 (Form II), and D4 compd formula fw space group dimensions (mm) index ranges
unit cell dimens a (Å) b (Å) c (Å) β (deg) vol (Å3) Z Dm (g cm-3) Dx (g cm-3) µ (cm-1) F(000) temp (K) R wR no. of unique reflns no. of significant reflns no. of variables extinction coeff k GoF highest peak (e Å-3) deepest hole (e Å-3) (∆/σ)max
D2 1/ (C H N ) 2 26 20 2 180.22 P21/n 0.3 × 0.3 × 0.2 -22 e h e 14 0eke6 0 e l e 17
D3 2(C27H22N2) 748.98 P21/n 0.46 × 0.39 × 0.14 -11 e h e 11 0 e k e 26 0 e l e 22
D4 C28H24N2 388.52 P21/n 0.3 × 0.3 × 0.3 -11 e h e 11 0 e k e 10 0 e l e 30
12.130(2) 5.175(2) 15.098(5) 92.54(1) 946.8(8) 4 1.26 1.264 0.690 380 302 0.052 0.058 1679 1137 128 0.42(4) 0.00015 2.53 0.27 -0.24
