Synthesis and charge-transfer complex formations of 1,n-bis(3,6- diethylcarbazol-9-yl)alkanes with three π-acceptors

Article abstract of DOI:10.1016/j.molstruc.2013.02.026

Dimeric 1,n-bis(3,6-diethylcarbazol-9-yl)alkanes (where n = 1-5) were synthesized and their structures were characterized via spectroscopic techniques. Structures of two of the dimers, 1,2-bis(3,6-diethylcarbazol-9-yl) ethane (2b) and 1,4-bis(3,6-diethylcarbazol-9-yl)butane (2d), were investigated by single crystal X-ray crystallographic techniques. The crystal structures of 2b and 2d were solved in the monoclinic space groups C2/c and P2₁/n, respectively. The methylene chain adopted an anti conformation in 2b and a gauche-anti-gauche conformation in 2d, enabling coplanar orientations of carbazole rings in both structures. The molecular packing in both structures was stabilized by intermolecular π-π stacking. Charge transfer complexations of 2a-2e with the π-acceptors p-chloranil, tetracyanoethylene, and tetracyanoquinodimethane in solution were investigated by determining their stoichiometries, molar absorptivities, equilibrium constants, enthalpies, and entropies. All the dimers formed weakly associated complexes with each of the acceptors having equilibrium constants between 1.32-8.94 M^-1 in 1,2-dichloroethane. Complexations were driven by the slightly negative formation enthalpies between -2.00 and -4.24 kcal mol^-1.

Full text of DOI:10.1016/j.molstruc.2013.02.026

Journal of Molecular Structure 1040 (2013) 65–74
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and charge-transfer complex formations of
1,n-bis(3,6-diethylcarbazol-9-yl)alkanes with three p-acceptors
⇑
Erol Asker , Fahrettin Filiz
Balikesir University, Department of Chemistry Education, 10100 Balikesir, Turkey
h i g h l i g h t s
"
"
"
"
1,n-Bis(3,6-diethylcarbazol-9-yl)alkanes (n = 1–5) as the
They formed charge-transfer (CT) complexes with three
Thermodynamic properties of the complexes were determined.
Single crystal X-ray structures of the two of the donor molecules were determined.
p
-donors were prepared.
p-acceptors were observed.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 January 2013
Received in revised form 24 February 2013
Accepted 25 February 2013
Available online 5 March 2013
Dimeric 1,n-bis(3,6-diethylcarbazol-9-yl)alkanes (where n = 1–5) were synthesized and their structures
were characterized via spectroscopic techniques. Structures of two of the dimers, 1,2-bis(3,6-diet-
hylcarbazol-9-yl)ethane (2b) and 1,4-bis(3,6-diethylcarbazol-9-yl)butane (2d), were investigated by sin-
gle crystal X-ray crystallographic techniques. The crystal structures of 2b and 2d were solved in the
monoclinic space groups C2/c and P21/n, respectively. The methylene chain adopted an anti conformation
in 2b and a gauche-anti-gauche conformation in 2d, enabling coplanar orientations of carbazole rings in
Keywords:
both structures. The molecular packing in both structures was stabilized by intermolecular
p–p stacking.
1,n-Bis(3,6-diethylcarbazol-9-yl)alkanes
Charge-transfer complexations
Charge transfer complexations of 2a–2e with the -acceptors p-chloranil, tetracyanoethylene, and tetra-
p
cyanoquinodimethane in solution were investigated by determining their stoichiometries, molar absorp-
tivities, equilibrium constants, enthalpies, and entropies. All the dimers formed weakly associated
complexes with each of the acceptors having equilibrium constants between 1.32–8.94 Mꢀ1 in 1,2-
dichloroethane. Complexations were driven by the slightly negative formation enthalpies between
p
-Acceptors
Crystal structures
ꢀ2.00 and ꢀ4.24 kcal molꢀ1
.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
information about their complexations. For this purpose, studies
on CT complexations of dianthrylalkanes and dicarbazolylalkanes
had been conducted and their results had been published [16–
19]. In our previous works we have reported the results of the
studies on CT complexations of 1,n-di(9-ethylcarbazol-3-yl)alk-
anes [20] and 1,n-di(3-methylcarbazol-9-yl)alkanes [21]. We pro-
pose that electron donor alkyl groups would increase the
electron affinity (Ea) of carbazole chromophore and result in higher
Derivatives of naturally occurring carbazole are of interest for
two main reasons: one is due to their pharmacological activities
such as antibacterial, antifungal [1,2], anti-HIV, and anticancer
activities [3,4] and the other is their uses in industry and material
sciences due to their photoconducting properties [5–8]. Charge
transfer (CT) complexations of carbazole donors with certain elec-
tron acceptors gratefully enhance their photoconduction proper-
ties. In fact, the first industrially used organic photoconductor is
a CT complex between poly-9-vinylcarbazole (PVK) and trinitroflu-
orenone (TNF) [9]. There are numerous studies conducted on the
CT complexations of PVK with various electron acceptor molecules
[10–15]. Studying CT complexations of polymeric systems has its
drawbacks due to their low solubility. To overcome this limitation
researchers have used their dimeric model compounds to attain
equilibrium constants, K, and lower enthalpies of formation,
DHf.
In this contribution we extend our work to the syntheses and CT
complexations of 1,n-bis(3,6-diethylcarbazol-9-yl)alkanes, in
which each carbazole groups are separated with 1–5 –CH2– groups
and carry two ethyl groups on the C3 and C6 positions. We have
included two monomeric analogues, 9-ethylcarbazole (1a) and
3,6,9-triethylcarbazole (1b) for comparisons. As the electron
acceptors, tetracyanoethylene, (TCNE), para-chloranil, (p-CHL),
and tetracyanoquinodimethane (TCNQ) were selected. Structures
of the donor and acceptor molecules discussed in the present study
are given in Scheme 1.
⇑
Corresponding author. Tel.: +90 266 241 27 62x245; fax: +90 266 249 50 05.
E-mail address: asker@balikesir.edu.tr (E. Asker).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.02.026

66
E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
Scheme 1. Synthesis and structures of the title donor and acceptor compounds.
A mixture of HgCl2 (500 mg), Zn powder (10.0 g), concentrated HCl
2. Experimental
(2.5 ml, %36) and water (50 ml) was stirred at ambient tempera-
ture for 15 min to amalgamate pulverized zinc. Then, the liquid
phase was decanted and the zinc amalgam was washed three times
with 25 ml of water. To this, conc. HCl (50 ml) and 9-Ethyl-3,6-
diacetylcarbazole were added and the mixture was stirred for
2 h. Toluene (50 ml) was added and the mixture was refluxed for
12 h, during which time three times 10 ml of conc. HCl was added
to the flask. Then, the content of the flask was cooled to room tem-
perature and the resultant phases were separated, the aqueous
phase was washed with benzene and the organic phases were
combined, washed with water, dried with anhydrous Na2SO4 and
evaporated under reduced pressure. The crude product was chro-
matographed over a silicagel column using hexane/CH2Cl2 (10:1)
as eluent and recrystallized from methanol to give 2b as colorless
crystals (47% yield, m.p. 84 °C; lit. [24] 84–86 °C).
2.1. Instrumentation
1H and 13C NMR spectra were recorded on a Varian Mercury
300 MHz NMR spectrometer using tetramethylsilane (TMS) as
the internal reference and CDCl3 as the solvent. IR spectra were ta-
ken on a Perkin Elmer Spectrum 100 FT-IR spectrometer using
attenuated total reflection (ATR) sampling. Electronic absorption
measurements were recorded on a PG Instruments T80 + double
beam UV–Visible spectrophotometer in 3.5 ml, 1.0 cm path length
optical quartz cells with polytetrafluoroethylene (PTFE) stopper.
The instrument was equipped with a PTC-2 peltier temperature
controller to perform temperature studies.
2.2. Synthesis
2.2.1. Synthesis of 3,6,9-triethylcarbazole (1b)
2.2. Syntheses of 1,n-bis(3,6-diethylcarbazol-9-yl)alkanes (2a–2e)
Commercially available 9-ethylcarbazole (1a) was diacetylated
according to the general procedure described in the literature
Carbazole, recrystallized from aceton, (16.7 g) was diacetylated
according to the same procedure described above to yield 3,6-
diacetylcarbazole (15.6 g; 62.2%; m.p. 234, lit. [25] 235–237 °C).
Clemmensen reduction of 3,6-diacetylcarbazole (12.5 g) yielded
5.8 g (63.0%; m.p. 119 °C) of 3,6-diethylcarbazole as colorless crys-
tals after column chromatography (silica gel, 10:1 v/v hexane/ace-
tone as eluent) and recrystallization from aceton.
Syntheses of 2a, 2c–2e were achieved via nucleophilic substitu-
tion reactions between 3,6-diethylcarbazol-9-ide nucleophile and
corresponding 1,n-dibromoalkane substrates. In the case of 2b,
[22]. To
a mixture of 9.75 g (50 mmol) of 1a, 16.0 g of
(ꢁ0.12 mol) AlCl3, and 150 ml of dry CS2 was added 15.0 g of acetyl
bromide (ꢁ0.12 mol) drop-wise under an anhydrous atmosphere
in an ice bath. After refluxing for 3 h, the solvent was evaporated
off and the residue was treated with ice-dilute HCl, filtered and
washed with distilled water. 9-Ethyl-3,6-diacetylcarbazole was ob-
tained as pale yellow crystals (7.2 g) after recrystallization from
acetone (m.p. 184–186 °C; lit. [22] 184–190 °C). 9-Ethyl-3,6-diace-
tylcarbazole was reduced via Clemmensen reduction reaction [23].

E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
67
bromides had to be replaced with p-toluenesulfonate (-OTs) due to
very slow reaction rate. Procedure for the synthesis of 2c is given
here as an example. KOH (0.5 g) was added to a stirred solution
of 1.0 g (4.48 mmol) of 3,6-diethylcarbazole in 25 ml of acetone
at RT. To this 0.39 g (2.24 mmol) of CH2Br2 was added and the
resulting mixture was refluxed for 2 h; then the content of the flask
was poured onto crushed ice, filtered, washed with water and air
dried. The crude product was purified by column chromatography
(silica gel) using 20:1 v/v hexane/CH2Cl2 as eluent and recrystal-
lized from CH2Cl2 to give 2a as colorless, fine needles.
Ka radiation [26]. The program used for data collection and cell
refinement: CrystalStructure [27]; data reduction: SORTAV [28];
structure solution: SIR92 [29]; structure refinement with direct
methods: SHELXL97 [30]; molecular graphics: ORTEP-3 for Win-
dows [31] and Mercury CSD 2.0 [32]; preparation of materials for
publication: WinGX [33]. All non-hydrogen atoms were found
from E-map and refined anisotropically; the hydrogen atoms were
positioned geometrically from the difference Fourier map and al-
lowed to ride on their corresponding parent atoms with C–H dis-
tances of 0.93 Å (aromatic), 0.96 Å (methyl), and 0.97 Å
(methylene) with Uiso(H) = 1.5Ueq(C) of the parent atom for the
methyl group and 1.2Ueq(C) for the rest. The crystal data and de-
tails of the data collection and refinement for the compounds are
given in Table 1. CCDC numbers 915656 (2b) and 915657 (2d) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033).
Bis(3,6-diethylcarbazol-9-yl)methane (2a): 72%, m.p. 254 °C; FTIR
(ATR) u (cmꢀ1): 3052, 3025, 2970. 2953, 2864, 1610, 1492, 1493,
ꢀ
1463, 1367, 1309, 1229, 1216, 1156, 1053, 872, 806, 752, 677; 1H
NMR (300 MHz, CDCl3), d: 7.89 (s, 4H), 7.36 (d, J = 8.50 Hz, 4H),
7.22 (d, J = 8.50 Hz, 4H), 6.53 (s, 2H), 2.80 (q, J = 7.61 Hz, 8H),
1.32 (t, J = 7.61 Hz, 12H); 13C NMR: (75 MHz, CDCl3) d: 139.1,
135.9, 126.5, 123.9, 119.4, 109.3, 53.0, 29.1, 16.7; UV–Vis, (1,2-
dichloroethane), kmax/nm (
(36.3), 332 (9.1), 346 (9.7).
e
ꢂ 10ꢀ3 l molꢀ1 cmꢀ1): 264 (31.6), 298
1,2-Bis(3,6-diethylcarbazol-9-yl)ethane (2b): 54%, m.p. 178 °C;
FTIR (ATR) u (cmꢀ1): 3054, 3025, 2960, 2924, 2859, 1609, 1489,
ꢀ
2.4. Absorption measurements
1475, 1453, 1361, 1307, 1258, 1197, 1142, 1055, 883. 796, 740,
657; 1H NMR (300 MHz, CDCl3), d: 7.89 (s, 4H), 7.35 (d,
J = 8.44 Hz, 4H), 7.21 (d, J = 8.48 Hz, 4H), 4.58 (s, 4H), 2.85 (q,
J = 7.61 Hz, 8H), 1.35 (t, J = 7.61 Hz, 12H); 13C NMR: (75 MHz,
CDCl3) d: 139.1, 135.4, 126.3, 123.5, 119.5, 108.0, 41.3, 29.2, 16.9;
2.4.1. Determination of complexation stoichiometries
The stoichiometries of the complexations of the donor mole-
cules with the three p-acceptors were determined using Job’s plots
(method of continuous variation) [34]. In 10 ml volumetric flasks,
10 mM of carbazole donor unit (i.e. 5 mM dicarbazoles) and
10 mM of acceptor molecules in 1,2-dichloroethane were prepared
separately. These solutions were mixed in 2.0 ml volumetric flasks
where the total concentration of the components held constant
while their mole fractions differed from 0.1 to 0.9. Acceptor solu-
UV–Vis, (1,2-dichloroethane), kmax/nm
(e
ꢂ 10ꢀ3 l molꢀ1 cmꢀ1):
266 (43.8), 300 (40.1), 340 (9.6), 354 (10.7).
1,3-Bis(3,6-diethylcarbazol-9-yl)propane (2c): 67%, m.p. 61 °C;
FTIR (ATR) u (cmꢀ1): 3047, 3014, 2959, 2927, 2869, 1609, 1492,
ꢀ
1472, 1453, 1376, 1345, 1323, 1262, 1247, 1219, 1192, 1148,
1056, 993, 875, 803, 748, 722; 1H NMR (300 MHz, CDCl3), d: 7.89
(s, 4H), 7.25 (d, J = 8.42 Hz, 4H), 7.16 (d, J = 8.51 Hz, 4H), 4.24 (t,
J = 7.64 Hz, 4H), 2.82 (q, J = 7.64 Hz, 8H), 2.39 (quintet,
J = 7.63 Hz, 2H), 1.32 (t, J = 7.63 Hz, 12H); 13C NMR: (75 MHz,
CDCl3) d: 139.2, 135.1, 126.1, 123.3, 119.4, 108.4, 41.0, 29.2, 28.3,
Table 1
Summary of general and crystal data with data collection and structure refinement of
2b and 2d.
Compound
2b
2d
16.9; UV–Vis, (1,2-dichloroethane), kmax/nm (
e
ꢂ 10ꢀ3 l molꢀ1
-
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C
34H36N2
C36H40N2
500.70 g/mol
293(2) K
0.71073 Å
Monoclinic
P21/n
cmꢀ1): 266 (43.2), 300 (38.6), 340 (9.1), 354 (8.4).
472.65 g/mol
293(2) K
0.71073 Å
Monoclinic
C2/c
1,4-Bis(3,6-diethylcarbazol-9-yl)butane (2d): 67%, m.p. 141 °C;
FTIR (ATR) u (cmꢀ1): 3048, 3014, 2957, 2925, 2861, 1609, 1576,
ꢀ
1491, 1476, 1450, 1350, 1325, 1307, 1256, 1215, 1179, 1147,
1056. 884, 798, 755, 735; 1H NMR (300 MHz, CDCl3), d: 7.89 (s,
4H), 7.24 (d, J = 8.49 Hz, 4H), 7.16 (d, J = 7.49 Hz, 4H), 4.12 (s,
4H), 2.82 (q, J = 7.62 Hz, 8H), 1.90 (s, 4H), 1.35 (t, J = 7.62 Hz,
12H); 13C NMR: (75 MHz, CDCl3) d: 139.3, 134.9, 126.0, 123.1,
119.3, 108.6, 43.0, 29.2, 27.2, 16.8; UV–Vis, (1,2-dichloroethane),
Unit cell dimensions
A
B
C
B
22.9058(9) Å
7.0733(2) Å
17.0765(8) Å
97.7671(23)°
2741.34(18) Å3
4
12.4306(7) Å
9.0721(4) Å
13.6954(3) Å
109.598(27)°
1454.98(14) Å3
2
Volume
Z
kmax/nm
(7.1), 356 (7.9).
(
e
ꢂ 10ꢀ3 l molꢀ1 cmꢀ1): 268 (37.6), 300 (31.4), 342
Calculated density
1.145 g/cm3
1.143 g/cm3
0.066 mmꢀ1
540
Absorption coefficient 0.066 mmꢀ1
1,5-Bis(3,6-diethylcarbazol-9-yl)pentane (2e): 78%, m.p. 99 °C;
F(000
)
1016
FTIR (ATR) u (cmꢀ1): 3048, 3014, 2960, 2933, 2965, 1608, 1575,
ꢀ
Crystal size
h Range for data
collection
0.7 ꢂ 0.5 ꢂ 0.2 mm
0.6 ꢂ 0.5 ꢂ 0.3 mm
3.02–30.03°
2.71–25.14°
1491, 1475, 1456, 1351, 1326, 1268, 1251, 1234, 1203, 1172,
1150, 1057, 882, 803, 752, 724; 1H NMR (300 MHz, CDCl3), d:
7.92 (s, 4H), 7.29 (d, J = 7.76 Hz, 4H), 7.25 (d, J = 7.76 Hz, 4H),
4.20 (t, J = 7.04 Hz, 4H), 2.86 (q, J = 7.32 Hz, 8H), 1.88 (quintet,
J = 7.62 Hz, 4H), 1.46 (quintet, J = 6.61 Hz, 2H), 1.37 (t, J = 7.32 Hz,
12H); 13C NMR: (75 MHz, CDCl3) d: 139.3, 134.8, 126.0, 123.0,
119.3, 108.5, 43.1, 29.2, 25.3, 16.8; UV–Vis, (1,2-dichloroethane),
Limiting indices
ꢀ32 6 h 6 31
0 6 k 6 10
0 6 l 6 24
ꢀ14 6 h 6 14
0 6 k 6 10
0 6 l 6 16
Reflections collected/
unique
39953/3997
[R(int) < 0.0341]
28160/2578
[R(int) < 0.001]
Refinement method
Full-matrix least squares Full-matrix least squares
kmax/nm
(8.7), 356 (8.7).
(
e
ꢂ 10ꢀ3 l molꢀ1 cmꢀ1): 268 (45.3), 300 (35.7), 342
on F2
on F2
Data/restraints/
parameters
3997/0/73
2578/0/77
Goodness-of-fit on F2
Final R indices
2.637
R1 = 0.2044,
2.405
R1 = 0.1494,
2.3. Single-crystal X-ray diffraction measurements
[I > 2
R indices (all data)
r
(I)]
wR2 = 0.6061
R1 = 0.2075,
wR2 = 0.5325
R1 = 0.1529,
The single crystals were grown from 1:1 dichloromethane/hex-
ane solution. X-ray diffraction measurements of the compounds 2b
and 2d were carried out at ambient temperature on a Rigaku RAX-
IS-RAPID S diffractometer equipped with CCD detector, using Mo
wR2 = 0.6087
wR2 = 0.5367
Largest diff. peak and
hole
0.113 and ꢀ0.421 e Åꢀ3
0.075 and ꢀ0.487 e Åꢀ3

68
E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
Fig. 1. ORTEP drawings of 2b and 2d with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level.
Table 2
Selected bond lengths (Å), bond and torsion angles (°) for the compounds 2b and 2d.
2b
2d
C1–C9a
C3–C11
C4A–C4a
C6–C13
1.386 (5)
1.502 (7)
1.462 (4)
1.507 (7)
1.435 (5)
1.535 (7)
116.5 (4)
123.9 (4)
118.0 (4)
105.6 (3)
122.8 (4)
116.6 (4)
109.6 (3)
108.1 (3)
93.1 (5)
C1–C9a
C3–C12
C4a–C4b
C6–C14
1.390 (7)
1.526 (9)
1.426 (6)
1.503 (8)
1.466 (6)
1.501 (9)
115.9 (5)
123.8 (6)
119.2 (5)
106.4 (4)
123.0 (5)
117.8 (5)
108.4 (4)
109.2 (4)
ꢀ83.1 (6)
101.8 (5)
67.8 (9)
N9–C10
N9–C10
C10–C10i
C11–C11i
C9A–C1–C2
C1–C2–C3
C2–C3–C4
C8a–C4b–C4a
C6–C7–C8
C7–C8–C8a
N9–C8a–C4b
C9a–N9–C8a
C9a–N9–C10–C10i
C8a–N9–C10–C10i
C2–C3–C11–C12
C4–C3–C11–C12
C5–C6–C13–C14
C7–C6–C13–C14
C2–C1–C9a
C1–C2–C3
C2–C3–C4
C8a–C4b–C4a
C8a–C6–C7
C8a–C8–C7
N9–C8a–C4b
C9a–N9–C8a
C9a–N9–C10–C11
C8a–N9–C10–C11
C2–C3–C12–C13
C4–C3–C12–C13
C5–C6–C14–C15
C7–C6–C14–C15
ꢀ89.2 (5)
90.4 (6)
ꢀ86.8 (6)
89.8 (7)
ꢀ82.9 (7)
ꢀ108.0 (8)
ꢀ103.6 (7)
74.7 (7)
Symmetry code: (i) ꢀx + 3/2, ꢀy ꢀ 1/2, ꢀz. Symmetry code: (i) ꢀx + 1, ꢀy, ꢀz + 2.
tions of the same concentration, as they were in the complex solu-
tion, were used as the blank to eliminate the absorption due to the
acceptor. The average absorptions of three scans at kmax of the CT
complexes were used in the plots.
Fig. 2. Diagrams showing p–p overlaps in the structures of 2b and 2d.
2.4.2. Determination of molar absorptivities and complexation
constants
trations of the donor molecules were kept high and diluted with
0.6 mM acceptor solutions.
For the determination of molar absorptivity, e, and equilibrium
constant, K, values of CT complexations, Benesi–Hildebrand tech-
nique was used [35]. Stock solutions of 0.6 mM of carbazole unit
(i.e. 0.6 mM monomers and 0.3 mM dimmers) were prepared in
25 ml flasks. Another solution containing 0.6 mM carbazole unit
and 60 mM TCNE was prepared in a 2 ml flask and 1 ml of this
2.4.3. Determination of enthalpy and entropy changes of
complexations
The enthalpy and entropy changes (DH and DS) of complex for-
mations were determined using the van’t Hoff equation. In 2.0 ml
volumetric flasks, solutions containing 5 mM acceptor and 5 mM
carbazole unit at 25.0 °C in 1,2-dichloroethane were prepared.
Absorbances of the complexes at 10–45 °C ( 0.1 °C) in 5 °C inter-
vals, at three data points around kCT were recorded at three inde-
pendent scans. When the solvent was dichloromethane
was transferred into a quartz UV cell using a 250
ll syringe. This
solution was diluted with 250 l, 0.6 mM donor solution 10 times.
l
After each dilution absorbances at three data points around kCT
were recorded in three scans. In the complexations where p-CHL
and TCNQ were acceptors, due to their low solubilities, the concen-

E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
69
Fig. 3. Molecular packing diagrams of 2b and 2d. Color changes represent the halves generated by symmetry operations. H atoms have been omitted for clarity. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 2. Representation of CT process between HOMO’s of an electron donor and
LUMO of an electron acceptor molecule.
absorbances were recorded at temperatures ranging between 7
and 32 °C ( 0.1 °C) in 5 °C intervals. Concentration changes due
to the expansion/contraction of the solvents [36] at changing tem-
peratures were taken into account in calculating the thermody-
namic constants.
3. Results and discussion
3.1. Synthesis of the donor compounds
For the diacetylation of carbazole, which exhibits the chemical
behaviors of secondary aromatic amines, was achieved via Fri-
edel–Crafts method. Nagai and Huang [24] proved that using acetyl
bromide instead of acetyl chloride gives much higher yield of 3,6-
diacetylcarbazole compared to monoacetylated derivative in CS2.
This is considered due to the weaker C–Br bond in acetyl bromide
than C–Cl bond in acetyl chloride and stabilization of acylium ion
by the polar solvent. 3,6-Diacetylcarbazole was easily reduced to
3,6-diethylcarbazole via Clemmensen reduction reaction. Synthe-
sis of bis(3,6-diethylcarbazol-9-yl)alkanes were achieved via
nucleophilic substitution reaction of 3,6-diethylcarbazole with
Fig. 4. Electronic spectra of (A) 2c (3 ꢂ 10ꢀ5 M), p-CHL, TCNE, and TCNQ
(6 ꢂ 10ꢀ5 M) and (B) complexes of 2c with the acceptors p-CHL, TCNE, and TCNQ
in 1,2-dichloroethane at 25 °C.

70
E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
Table 3
Thermodynamic properties of CT complexes of the donor compounds with the acceptors p-CHL, TCNE, and TCNQ.
d
e
kc
kCT
K
eCT (Mꢀ2 cmꢀ1
)
Kg(Mꢀ1
)
D
H (kcal molꢀ1
)
D )
S (cal molꢀ1 Kꢀ1
TCNEa
M1
M2
D1
D2
D3
347
356
346
354
354
356
356
596
635
612
625
626
627
628
3705 42
11260 225
2800 28
6990 141
7250 138
8265 152
8285 129e
2.94
8.94
2.22
5.55
5.75
6.56
6.58g
ꢀ2.00 0.06
ꢀ2.69 0.19
ꢀ2.03 0.09
ꢀ2.29 0.17
ꢀ2.31 0.09
ꢀ2.20 0.08
ꢀ2.56 0.13
ꢀ2.63 0.20
ꢀ3.62 0.65
ꢀ1.80 0.30
ꢀ1.55 0.57
ꢀ1.51 0.29
ꢀ1.02 0.29
ꢀ2.04 0.42
D4
D5
TCNEb
M1
M2
D1
D2
D3
347
356
346
354
354
356
356
596
635
612
625
626
627
628
7861 100
19886 363
6441 99
12640 315
12911 361
13534 271
15370 245f
6.24
15.78
5.11
10.03
10.25
10.74
12.20h
ꢀ2.29 0.09
ꢀ3.13 0.14
ꢀ1.82 0.10
ꢀ2.89 0.05
ꢀ2.91 0.10
ꢀ3.37 0.03
ꢀ3.49 0.02
ꢀ7.60 0.30
ꢀ8.79 0.49
ꢀ5.08 0.33
ꢀ7.35 0.17
ꢀ7.31 0.34
ꢀ8.69 0.09
ꢀ8.81 0.07
D4
D5
p-CHLa
M1
M2
D1
D2
D3
347
356
346
354
354
356
356
527
559
550
553
553
554
554
2645 23
3480 44
2850 32
3180 59
3280 48
3560 84
3915 62f
2.91
3.83
3.13
3.50
3.60
3.91
4.30i
ꢀ2.26 0.15
ꢀ2.78 0.19
ꢀ2.09 0.15
ꢀ2.55 0.22
ꢀ2.58 0.11
ꢀ2.71 0.07
ꢀ2.87 0.09
ꢀ5.05 0.49
ꢀ6.09 0.60
ꢀ2.74 0.52
ꢀ3.95 0.72
ꢀ4.02 0.37
ꢀ4.50 0.23
ꢀ4.72 0.29
D4
D5
TCNQa
M1
M2
D1
D2
D3
347
356
346
354
354
356
356
590
625
608
621
621
623
624
7965 194
10915 221
3260 26
6235 78
7475 68
9545 89
9075 179
3.21
4.40
1.32
2.51
3.01
3.85
3.66
ꢀ3.56 0.12
ꢀ4.03 0.16
ꢀ2.32 0.07
ꢀ3.93 0.16
ꢀ3.82 0.08
ꢀ4.14 0.09
ꢀ4.24 0.10
ꢀ7.16 0.38
ꢀ7.85 0.53
ꢀ2.47 0.22
ꢀ7.22 0.53
ꢀ6.16 0.26
ꢀ7.00 0.31
ꢀ7.07 0.33
D4
D5
a
b
c
d
e
f
Experiments performed in 1,2-dichloroethane.
Experiments performed in dichloromethane.
Lowest energy absorption maximum (nm) of the donor molecule.
Lowest energy CT maximum (nm).
Acceptor in excess.
Donor in excess.
g
h
i
e
e
e
= 910 Mꢀ1 cmꢀ1 at 25 0.1 °C.
= 1260 Mꢀ1 cmꢀ1 at 25 0.1 °C.
= 2480 Mꢀ1 cmꢀ1 at 25 0.1 °C.
corresponding 1,n-dibromoalkanes (for n = 1, 3–5) and ethylene
ditosylate (for n = 2). As the nucleophile 3,6-diethylcarbazolide an-
ion was formed by abstracting the proton attached to carbazole
nitrogen by hydroxide ion in acetone. The much lower reaction
rate when 1,2-dibromoethane was used is due to lower partial po-
gauche conformation. These conformations allowed the carbazole
moieties position themselves in parallel planar geometries. The
ethyl groups on C3 and C6 positions point the same direction in
2b, whereas they point opposite directions in 2d. The molecular
packings in both structures were mainly determined by p–p inter-
sitive charge on the
a
-carbon to the nucleophile because of the
actions between carbazole moieties. The dihedral angle of
0.44(14)° between the mean planes of central pyrollic rings of
the neighboring 2b molecules at (x, y, z) and (1 ꢀ x, 1 ꢀ y, ꢀz) with
a ring-centroid separation of 3.525(2) Å and interplanar spacings of
ca. 3.3.331 Å corresponding to a ring-centroid ofset of ca. 1.153 Å
indicates this partial overlap. A similar overlap was observed in
the structure of 2d having a dihedral angle between the mean
planes of pyrrolic rings of the molecules at (x, y, z) and (1 ꢀ x,
existence of the second electronegative atom on the neighboring
carbon atom. The use of -OTs as a better leaving group increased
the reaction rate considerably.
3.2. Crystal structures
The crystal structures of the compounds 2b and 2d were solved
in the monoclinic space groups C2/c and P21/n, respectively. In the
structures of the both compounds, the asymmetric unit contains
half a molecule. The other halves of the molecules are generated
from the first ones by the crystallographic inversion centers, which
were lying between the central C–C bonds of the alkylene separa-
tors. The atom numbering scheme of the compounds 2b and 2d are
shown in Fig. 1. The selected bond lengths, bond and torsion angles
are listed in Table 2. Bond lengths and angles are comparable to
those of related compounds reported in the literature [37]. In both
structures C1–C9a bond is somewhat shorter and C4a–C4b bond is
longer than the other C–C bonds in the carbazole rings.
1 ꢀ y, ꢀz) is 0.18(18)° with
a ring centroid separation of
3.580(4) Å and interplanar spacings of ca. 3.492 Å corresponding
to a ring-centroid ofset of ca. 0.789 Å. These interactions account
for semiconducting properties and charge transfer complexations
of carbazole derivatives with
p-accpetors. Intermolecular p–p
interactions and molecular packing of 2b and 2d are shown in
Figs. 2 and 3, respectively.
3.3. Charge-transfer complexations
Charge transfer bands in the electronic spectra of EDA com-
plexes are observed in solutions due to absorbed photon energies
to transfer electrons from higher energy, occupied molecular
In the structure of 2b the conformation of the C–C connector is
anti while in 2d the C–C–C–C chain is arranged in a gauche-anti-

E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
71
Scheme 3. Representation of three of the ground state HOMO’s of carbazole donors
and LUMO’s of the acceptor molecules.
orbitals of the donor molecules to the lowest lying molecular orbi-
tal of the acceptors (Scheme 2). Carbazole donors studied in this
work gave maximum absorbances of the light with k = 527–
635 nm depending on the acceptor. Color changes upon mixing do-
nor and acceptor solutions are clear indication of CT complexation.
As a representative, the CT spectra of the complexes formed be-
tween the donor 2c and the acceptors p-CHL, TCNE, and TCNQ
are shown in Fig. 4.
Absorbances due to CT formation are outside the region where
the individual donor or acceptor molecules absorb light. Hence, the
absorbances around 527–635 nm could be related to the concen-
tration of the CT complex. Lowest energy kmax and kCT are listed
in Table 3.
The energy associated with charge-transfer is related to elec-
tron affinity (Ea) of the acceptor and ionization potential (Ip) of
the donor molecules and electrostatic attraction between the do-
Fig. 5. Job plots of CT complexes of the donor compounds with (A) p-CHL, (B) TCNE,
and (C) TCNQ in 1,2-dichloroethane. X is the mole fraction of the carbazole unit in
the solution.
the absorptions of uncomplexed acceptors at these kCT values.
The transition bands due to HOMO-2 and HOMO-1 of carbazoles
and LUMO of acceptors appear as two overlapping peaks resulting
in a broad shoulder having kCT values around 550 (p-CHL), 620
(TCNQ), and 625 nm (TCNE).
nor and acceptor molecules according to the equation,
DE = Ea -
ꢀ Ip + J. Measured Ea values for TCNE, TCNQ and p-CHL found in
the literature are 3.17 0.2 [38], 2.8 0.2 [39,40] and
1.37 0.1 eV [41], respectively. Measured Ip values are around
7.6–8.0 [42,43] for carbazole and 7.41 eV [44] for ethylcarbazole.
The 0.37 eV difference between the Ea values of TCNE and p-CHL
is also supported by the 72 nm mean difference between their
respective kCT values. However, according to kCT values TCNQ
should have an Ea value close to 3.05 eV, assuming these three
acceptors exhibit similar or very close J values related to electro-
static attractions with the donors. Introduction of alkyl subtituents
on carbazole rings reduces Ip values up to 0.3 eV [45]. There are
two kCT bands observed from the spectra of the CT complexes of
the donor compounds with TCNE around 416 nm and 630 nm.
The peak around 416 nm is assigned to the transition from donors’
HOMO-3 to TCNE’s LUMO (Scheme 3) [46]. In the CT spectra of the
carbazole donors with p-CHL and TCNQ are not resolvable due to
Carbazoles form donor–acceptor complexes with the electron
acceptors according to following equation:
D þ A¡DA
ð1Þ
Equilibrium constant, K of the CT complex formation is a func-
tion of concentration of D, A and DA species at equilibrium (Eq.
(2)).
K ¼ ½DAꢃ=ð½Dꢃ½AꢃÞ
ð2Þ
In photometric studies the concentration of DA could be related
to its absorbance by Beer’s law and used to deduce the concentra-
tion of D and A using Benesi Hildebrand equation (Eqs. (3a) and

72
E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
(3b)). This method requires initial concentration of one of the com-
ponents to be very high and changing compared to the low and
fixed concentration of the other component.
that are not interacting with either of the components need to be
used. The solvents used in this study obey this criteria but com-
plexation in CH2Cl2 is more favored compared to CH2ClCH2Cl.
Although they have similar dipole moments [36] the size of CH2-
ClCH2Cl seems prevented close approach of the donor acceptor
molecules. The K values obtained in CH2Cl2 were found roughly
twice those determined in CH2ClCH2Cl. The effect of number of al-
kyl substituents present on the carbazole rings on complexation is
attained by comparing the results of this study with our previous
works [20,21]. 3,6,9-Trialkylcarbazoles form complexes with TCNE
in CH2Cl2 with 5.8 and 2.8 Mꢀ1 higher K values compared to 9-al-
kyl- and 3,9-dialkylcarbazoles. Apparently, lowered Ip’s of carba-
zole rings by the electron donating alkyl groups played a notable
effect on the complexation.
ð½Dꢃ0lÞ=A ¼ ðK
and
e
Þꢀ1ð1=½Aꢃ0Þ þ eꢀ1 when ½Aꢃ0 ꢄ ½Dꢃ0
ð3aÞ
ð½Aꢃ0lÞ=A ¼ ðK
e
Þꢀ1ð1=½Dꢃ0Þ þ eꢀ1 when ½Dꢃ0 ꢄ ½Aꢃ0
ð3bÞ
where A is the absorbance at kCT, e is the molar absorption coeffi-
cient of the complex, l is the optical path length (1.0 cm), [A]0 is
the initial concentration of the acceptor and [D]0 is the initial con-
centration of the donor. From ꢀth1e linear Eqs. (3a) and (3b), (K
e)
ꢀ1 is
obtained as the slope and ( as the intercept. When the donor
e
)
compounds are dimers acting as two individually behaving mono-
mers Eqs. (3a) and (3b) becomes
ð½Dꢃ0lÞ=A ¼ ð2K
and
e
Þꢀ1ð1=½Aꢃ0Þ þ ð2
e
Þꢀ1 when ½Aꢃ0 ꢄ ½Dꢃ0
ð4aÞ
ð4bÞ
ð½Aꢃ0lÞ=A ¼ ð2K
e
Þꢀ1ð1=½Dꢃ0Þ þ eꢀ1 when ½Aꢃ0 ꢄ ½Dꢃ0
ꢀ1
A plot of [D]0l/A vs. (1/[A]0) in Eq. (4a) would yield (2K
the slope and (2
(1/[D]0) in Eq. (4a) would yield (2K
e
)
as
ꢀ1
e
)
as the intercept, ꢀw1hereas a plot of [A]0l/A vs.
ꢀ1
e
)
as the slope and (
e)
as
the intercept. Note that, since these equations are in linear forms,
they are usable only when donors and acceptors form complexes
in 1:1 M ratio. Therefore, the stoichiometries of the complexations
should be determined beforehand and should be proved that Bene-
si–Hildebrand (B–H) technique is applicable. Job plots (Fig. 5) show
that all monomers and dimers gave the highest absorbances when
they are mixed in 1:1 donor unit–acceptor molar ratio. These
experiments also show that the dimers behave like two individual
monomers which form complexes independent from each other.
The B–H plots are given in Fig. 6 and results of the calculations
of
e
and K values are given in Table 3.
Instead of calculating values for each of the donor–acceptor
e
complexes, we have used their average and assigned the same va-
lue to all complexes to minimize experimental errors. This allowed
us calculate more reliable K values from their slopes. The average
e
values were determined as 1380 (195) Mꢀ1 cmꢀ1 for carbazole–
TCNE, 830 (99) Mꢀ1 cmꢀ1 for carbazole-p-CHL, and 2520
(324) Mꢀ1 cmꢀ1 for carbazole–TCNQ complexes in 1,2-dichloroeth-
ane. The numbers in the parentheses are the mean error of stan-
dard deviations (e.s.d.’s) of
analyses. Same experiments were done in dichloromethane for car-
bazole–TCNE complexations and the average value was deter-
mined as 1350 (102) Mꢀ1 cmꢀ1. The average
values determined
e values determined by the regression
e
e
in this study are close and in the range of e.s.d.’s to the values
determined in the previous studies [20,46]. Because the molar
absorptivity is an intrinsic property determined by the size of the
molecule and the probability that electronic transition will take
place [47], all the donors should have the same molar absorptivity
values for their complexes with the same acceptors. This assump-
tion is reasonable because the absorbances around 600 nm are
only due to
to the acceptor molecules. Hence, to be consistent and to compare
our results with the earlier studies same values reported are also
p–
pꢅ electronic transitions from the carbazole rings
e
used here. Among the acceptors, TCNE formed complexes with the
highest equilibrium constants. The formation constants of com-
plexes with p-CHL and TCNQ have similar values. Comparing the
donor molecules, as the length of the methylene chain increases
the dimers exhibit similar complexation behaviors to their mono-
meric analogue, 1b. Interference of one carbazole ring on the com-
plexation of the other was observed the most in 2a as its K values
suggest. This effect was decreased as the carbazole rings are sepa-
rated farther apart in 2c–2e. In CT complexations inert solvents
Fig. 6. Benesi–Hildebrand plots of the complexes of carbazole donors with (A) p-
CHL, (B) TCNE, and (C) TCNQ in 1,2-dichloroethane at 25 °C.

E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
73
The enthalpies of complex formation found between ꢀ1.82 and
ꢀ4.24 kcal/mol indicate that the process is slightly exothermic.
They also suggest that the interactions between donor and acceptor
molecules are weak, which is a common feature of complex forma-
tion. Comparing their
ated complexes than 1a. Among the dimeric donor compounds 2a
has the least negative H values because of the steric interference
of the second carbazole group on the same sp3-hybridized carbon.
As the length of the –CH2– chain increased H values became more
DH values, 1b formed more strongly associ-
D
D
negative, indicating that carbazole groups of the dimers behave like
two individual monomers. Among the acceptor molecules TCNQ
formed more strongly associated complexes than TCNE and p-CHL.
This is attributed to the size and the nodal properties of LUMO of
TCNQ and HOMO-2 of carbazole ring. More negative
DH values of
complex formation between carbazoles and TCNE were determined
in dichloromethane compared to 1,2-dichloroethane.
The negative
plexations are entropically disfavored and the process is driven by
the formation enthalpies. The S values increase and get closer to
those of monomeric analogues as the –CH2– chain length increases.
More negative S values for 1b compared to 1a indicate that ethyl
DS values of complex formations show that com-
D
D
groups on the carbazole ring increased the strength of the intermo-
lecular association and caused decrease in the randomness of the
complexes. The
dichloromethane were found to be more negative than in 1,2-
dichloroethane. The K, S, and S values suggest that the more
DS for the carbazole–TCNE complexations in
D
D
strongly the donor and acceptor compounds associate the more
temperature dependency they exhibit. Hence the complexations
at high temperatures are less favored and will cause less negative
D
G and lower K values.
4. Conclusion
1,n-Bis(3,6-diethylcarbazol-9-yl)alkanes as the dimeric model
compounds of poly-9-vinylcarbazoles were prepared and crystal
structures of two of them, 2b and 2d, were solved. The crystal
packing and intermolecular associations show that these two di-
mers adopted conformations along the methylene chains so that
p–p interactions between carbazole ring systems would be possi-
ble. From the results of the complexation studies it is concluded
that 1,n-bis(3,6-diethylcarbazol-9-yl)alkanes formed stable inter-
molecular CT complexes with the
p acceptors p-CHL, TCNE, and
TCNQ in 1,2-dichloroethane. The stoichiometries of complexation
determined as one donor unit to one acceptor molecule. The equi-
librium constants, K, of the complexations were determined by the
linear Benesi–Hildebrand method. It was observed that value of K
increases as the chain length separating the two carbazole groups
increases. Higher K values of complexations were determined
when dichloromethane was used as the solvent. The enthalpies
and entropies of complex formations were calculated using the
Fig. 7. The van’t Hoff plots of the complexes of carbazole donors with (A) p-CHL, (B)
TCNE, and (C) TCNQ in 1,2-dichloroethane.
3.4. Determination of the thermodynamic constants
van’t Hoff equation. Slightly more negative
DH and DS values of
complex formations between the donor compounds and TCNQ
were obtained compared to TCNE and p-CHL. These results imply
that complexes with TCNQ show more temperature dependency.
When comparing the solvents, more strongly associated complexes
with higher temperature dependency are formed in dichlorometh-
ane than 1,2-dichloroethane. Comparing the monomers, 3,6,9-tri-
The van’t Hoff equation combined with Beer’s law is used for
the determination of enthalpy and entropy changes of CT complex-
ations (Eqs. (5) and (6)).
ꢀRT ln K ¼
DH ꢀ T
D
S
ð5Þ
ethylcarbazole formed complexes with higher
K and more
negative H and S values than 9-ethylcarbazole. Electron donor
ethyl groups caused increase in K values and temperature depen-
dencies of the complexations.
D
D
lnðA=
¼ ꢀð
e
Þ ꢀ lnð½Dꢃ0 ꢀ ðA=
e
ÞÞ ꢀ lnð½Aꢃ0 ꢀ ðA=
S=RÞ
eÞÞ
D
H=RÞð1=TÞ þ ð
D
ð6Þ
A plot of lnK (the whole term on the left in Eq. (6)) vs. 1/T would
H/R as the slope and S/R as the intercept. The van’t Hoff
plots of carbazoles with p-CHL, TCNE and TCNQ are given in Fig. 7,
respectively and the thermodynamic constants are summarized in
Table 3.
yield ꢀ
D
D
Acknowledgements
Orhan Zeybek of Balikesir University, Mustafa Arslan of Sakarya
University and Gulsah Deniz of Istanbul University are thanked for

74
E. Asker, F. Filiz / Journal of Molecular Structure 1040 (2013) 65–74
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