Photophysical and electrochemical properties and temperature dependent geometrical isomerism in alkyl quinacridonediimines

Article abstract of DOI:10.1039/c3nj00477e

Quinacridone diimines 1-10 were synthesized by the condensation of anilines with alkyl substituted quinacridones (QA). Photophysical and electrochemical properties of the compounds were investigated. Unconventional behavior of absorption spectra suggested a decrease in π-conjugation within the QA skeleton as well as lack of extended π-conjugation between the QA skeleton and the N-phenyl rings. A computational study of compounds 1-10, a variable temperature ¹H NMR study of compounds 2, 7 and 10 (for instance), and single crystal X-ray analysis of 2, 3, 6, 7, 8 and 10 indicated that the anomalous behavior is due to the buckled, non-planar structure of the quinacridones. Moreover the molecules existed in two interconvertible geometric isomeric forms at different temperatures. Molecular orbital calculations were performed at B3LYP/6-31+G(d), B3PW91/6-31G(d) and PBEPBE/6-31G(d) levels of theory at B3PW91/6-31G(d) optimized structures for both isomers of all compounds (1-10); the results obtained are in close agreement with the experimentally determined values.

Full text of DOI:10.1039/c3nj00477e

NJC
PAPER
Photophysical and electrochemical properties and
temperature dependent geometrical isomerism in
alkyl quinacridonediimines†
Cite this: New J. Chem., 2014,
38, 752
Iqbal Javed,*abc Ayub Khurshid,*de Muhammad Nadeem Arshadf and Yue Wang*a
Quinacridone diimines 1–10 were synthesized by the condensation of anilines with alkyl substituted quina-
cridones (QA). Photophysical and electrochemical properties of the compounds were investigated. Uncon-
ventional behavior of absorption spectra suggested a decrease in p_conjugation within the QA skeleton as
well as lack of extended p_conjugation between the QA skeleton and the N-phenyl rings. A computational
1
study of compounds 1–10, a variable temperature H NMR study of compounds 2, 7 and 10 (for instance),
and single crystal X-ray analysis of 2, 3, 6, 7, 8 and 10 indicated that the anomalous behavior is due to the
buckled, non-planar structure of the quinacridones. Moreover the molecules existed in two interconvertible
geometric isomeric forms at different temperatures. Molecular orbital calculations were performed at
B3LYP/6-31+G(d), B3PW91/6-31G(d) and PBEPBE/6-31G(d) levels of theory at B3PW91/6-31G(d) optimized
structures for both isomers of all compounds (1–10); the results obtained are in close agreement with the
experimentally determined values.
Received (in Montpellier, France)
6th May 2013,
Accepted 20th November 2013
DOI: 10.1039/c3nj00477e
www.rsc.org/njc
Jones and co-workers have reported the crystal structure of
QA.7 Schmidt and co-workers have reported different poly-
Introduction
Quinacridone (QA) and its derivatives are widely used organic morphic forms of QA that differ significantly in terms of their
pigments that display excellent fastness properties as well as coloristic properties.8 Our group has explored the polymorphs
pronounced photovoltaic and photoconductive activities.1–3 and pseudopolymorphs of N,N-di (n-butyl) quinacridone.9
Many investigations on QA derivatives have been performed
The literature reveals that a great deal of effort has been devoted
to explore the effects of different structural parameters on their to the synthesis and characterization of various QA derivatives;
physical properties. For example, Mu¨llen and co-workers have however little attention has been paid to functionalization of the
carefully studied the phase-formation behavior and aggregation carbonyl group.10 To fully exploit the potential applications of such
properties of some soluble QAs.4,5 Nakahara and co-workers functional compounds in organic material based devices, it is
have applied alkyl QA derivatives in Langmuir–Blodgett films to necessary to design and synthesize some new types of QA derivatives
control the orientation and packing of the chromophores.6 with different molecular structures. A careful understanding of the
relationship between molecular structures, conformations and opti-
cal and electronic properties in solution and condensed states will
lead to the development of new strategies for the preparation of
high-performance organic optical and electronic materials.
a State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun 130012, P. R. China.
E-mail: javedkhattak79@gmail.com, yuewang@jlu.edu.cn;
A series of QA diimines were synthesized by the condensa-
tion of alkyl substituted quinacridones with aromatic amines.
The introduction of the aniline base backbone generally not
only enhances the conducting and optoelectronic properties of
the derivatives but also broadens the absorption band due to
extended p-conjugation but this requires coplanarity, at least to
within 151.11 Physical properties are closely related to structure
but after measuring the photophysical properties of the syn-
thesized compounds the coplanarity was questionable as they
show anomalous behavior in their physical properties.
Tel: +46-704763801
b Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden
c Department of Chemistry & Biochemistry, University of Agriculture, Faisalabad,
Pakistan
d Department of Chemistry, College of Science, King Faisal University,
Al-Ahsa 31982, Kingdom of Saudi Arabia. E-mail: kayub@kfu.edu.sa
e Department of Chemistry, COMSATS Institute of Information Technology,
Abbottabad, KPK, Pakistan 22060
f Centre of Excellence for Advanced Materials Research (CEAMR),
King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
† Electronic supplementary information (ESI) available: 1H NMR spectra, cyclic
voltammetric and crystallographic data. CCDC 923157 (2), 923158 (3), 923159 (6),
923160 (7), 923161 (8) and 923162 (10). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3nj00477e
For in-depth structural exploration of these compounds,
single crystals have been grown at different temperatures for
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some compounds and single crystal X-ray analysis is carried on F2. The positions of hydrogen atoms were calculated and
out. Density Functional Theory (DFT) calculations are also refined isotropically.
performed in an attempt to attain a certain level of predictability
Synthesis
in these systems. To get an unclouded picture we embarked on
the study of the variable temperature 1H NMR spectrum of Quinacridone was purchased from Tokyo Kasei Kogyo Com-
compounds 2, 7 and 10 for instance. We also evaluated the pany. 3,5-Dimethylaniline, 1-bromobutane and 1-bromooctane
relationship between the structure, conformation and opto- were obtained from Acros Organics. Diethyl-2, 5-dihydroxy-1,
electronic properties of the QA diimines.
4-dicarboxylate was purchased from Aldrich. The chemicals
On the basis of single crystal X-ray analysis, molecular were used directly without further purification.
modeling calculations and the variable temperature 1H NMR
The CnQA (n = 4,8), N,N-di(n-butyl)-1,3,8,10-tetramethyl-
spectrum study, herein we report that these QA diimines exist quinacridone (TMQA) and N,N-di(n-butyl)-2,9-difluoroquinacri-
in different geometrical isomeric forms at different temper- done (DFDBQA) were synthesized according to the similar
atures with the buckled QA skeleton in a non-planar fashion.
method reported in the literature.5
Synthesis of N, N-diphenyl-N, N0-di(n-butyl)-
quinacridonediimine (1)
Computational methods
Aniline (4 mmol) and triethylamine (TEA) 1.2 mL (8.5 mmol)
were dissolved in 20 mL of toluene. 0.6 mL (5.4 mmol) of
titanium chloride (TiCl4) in 5 mL of toluene was added drop-
wise over 15 min using an addition funnel. The addition funnel
was rinsed with 3 mL of toluene. DBQA (1 mmol) was added
using a powder addition funnel at once. The funnel was rinsed
with 3 mL toluene. The reaction mixture was refluxed for about
8 hours. The reaction mixture was cooled and filtered, the
filtrate obtained was dried and loaded on the silica gel column
and eluted with CH2Cl2: diethyl ether (50 : 1). The desired
product was obtained as a red solid in 50% yield.
DFT calculations were performed using the Gaussian 03 suite of
programs.12 Geometries of both isomers of QA diimines, 1–10,
were optimized using the hybrid functional B3PW91 method,
which consists of Becke’s three-parameter (B3) hybrid exchange
functional in conjunction with the correlation functional of the
Perdew and Wang (PW91) method. Frequency calculations were
performed in order to confirm these structures as true minima
(absence of an imaginary frequency). Molecular orbital calcula-
tions were performed at B3LYP/6-31+G(d), B3PW91/6-31G(d)
and PBEPBE/6-31G(d) levels of theory at B3PW91/6-31G(d)
optimized structures.
Compound 1
1H NMR (CDCl3, ppm) (500 MHz): d 8.50 (s, 2H), 7.43 (d, 2H),
7.34 (t, 6H), 7.11 (m, 2H), 7.01 (t, 4H), 6.90 (d, 4H), 4.32 (t, 2H),
3.40 (m, 2H), 1.99 (m, 2H), 1.55 (m, 4H), 1.01 (m, 6H), 0.89
(t, 2H), MS: m/z 574.31 [M]+. Anal. Calcd for C40H38N4; C, 83.49;
H, 6.66; N, 9.75. Found: C, 83.78; H, 6.46; N, 9.70.
Experimental section
Instrumentation
1H NMR spectra were recorded on a Bruker AVANCE 500 MHz
spectrometer with tetramethylsilane as the internal standard.
Elemental analysis and mass spectroscopy were carried out on
Flash EA 1112 and GC/MS mass spectrometers, respectively.
UV-vis absorption spectra were recorded using a PE UV-Vis
lambdazo spectrometer.
The same procedure was adopted for compounds 2–10 and
reaction conditions are tabulated in Table S1 (ESI†).
N, N-Bis(4-fluorophenyl)-N, N0-di(n-butyl)quinacridonediimine (2)
1H NMR (CDCl3, ppm) (500 MHz): d 8.43 (s, 2H), 7.54 (d, 4H),
7.34 (m, 2H), 7.03 (t, 6H), 6.80 (m, 4H), 4.32 (t, 2H), 3.43 (t, 2H),
1.90 (m, 2H), 1.35–1.65 (m, 6H), 1.01–1.01 (m, 6H), MS: m/z
612.29 [M]+. Anal. Calcd for C40H36 F2N4; C, 78.66; H, 5.94; N,
9.17. Found: C, 78.66; H, 5.95; N, 9.17.
Electrochemical measurements
Cyclic voltammetry was performed on a BAS 100 W instrument
with a scan rate of 100 mV sꢀ1. A three-electrode configuration
was used for the measurement: a platinum electrode as the
working electrode, a platinum wire as the counter electrode,
and an Ag/Ag+ electrode as the reference electrode. A 0.1 M
solution of tetrabutylammonium perchlorate (TBAP) in CH2Cl2
was used as the supporting electrolyte.
N,N-Diphenyl-N,N0-di(n-butyl)-2,9-difluoroquinacridonediimine (3)
1H NMR (CDCl3, ppm) (500 MHz): d 8.19 (s, 2H), 7.59 (s, 2H),
7.37 (t, 4H), 7.21 (d, 2H), 7.04 (t, 4H), 6.90 (d, 4H), 4.30 (t, 2H),
3.37 (d, 2H), 1.97 (m, 2H), 1.59 (m, 2H), 1.31 (m, 4H), 1.00
(m, 6H), MS: m/z 612.30 [M]+. Anal. Calcd for C40H36 F2N4 C,
X-ray crystallography
Single crystals suited for X-ray structural analysis were obtained 78.66; H, 5.94; N, 9.17. Found: C, 78.74; H, 5.95; N, 8.73.
by slow diffusion of petroleum ether into the chloroform
N, N-Bis(4-fluorophenyl)-N, N0-di(n-butyl)-2,9-
difluoroquinacridonediimine (4)
phite monochromator) in the o rotation scan mode. The struc- 1H NMR (CDCl3, ppm) (500 MHz): d 8.16 (s, 2H), 7.56 (s, 2H),
ture determination was done using direct methods by using 7.22 (m, 2H), 7.07 (t, 6H), 6.85 (m, 4H), 4.31 (t, 2H), 3.47 (d, 2H),
SHELXTL 5.01v and refinements with full-matrix least-squares 1.94 (m, 2H), 1.94 (m, 2H), 1.54 (m, 2H), 1.32 (m, 4H), 1.00
solution of compounds. Diffraction data were collected on
a Rigaku R-AXIS RAPID diffractometer (Mo Ka radiation, gra-
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(m, 6H), MS: m/z 646.30 [M]+. Anal. Calcd for C40H34F4N4; C,
74.29; H, 5.30; N, 8.66.
N, N-Diphenyl-N, N0-di(n-butyl)-1,3,8,10-tetramethylquin-
acridonediimine (5)
1H NMR (CDCl3, ppm) (500 MHz): d 7.28 (t, 4H), 6.96 (t, 2H),
6.75 (d, 8H), 3.41–3.44 (m, 4H), 2.75–2.84 (m, 4H), 2.36 (s, 6H),
1.34–1.55 (m, 6H), 1.02 (t, 6H), MS: m/z 630.30 [M]+. Anal. Calcd
for C44H46N4; C, 83.77; H, 7.35; N, 8.88. Found: C, 83.44; H,
7.31; N, 8.76.
N, N-Bis(4-fluorophenyl)-N, N0-di(n-butyl)-1,3,8,10-
tetramethylquinacridonediimine (6)
1H NMR (CDCl3, ppm) (500 MHz): d 7.13 (s, 2H), 6.98 (m, 4H),
6.72 (m, 8H), 3.46 (m, 4H), 2.83 (m, 4H), 2.30 (s, 6H), 1.33–1.57
(m, 10H), 1.03 (s, 6H), MS: m/z 666.40 [M]+. Anal. Calcd for
C
44H44 F2N4; C, 79.25; H, 6.65; N, 8.40. Found: C, 79.44; H, 6.41;
N, 8.52.
N, N-Bis(2,3,4,5,6-pentafluorophenyl)-N, N0-di(n-butyl)-
quinacridonediimine (7)
1H NMR (CDCl3, ppm) (500 MHz): d 8.55 (s, 2H), 7.61 (t, 4H),
7.41 (d, 2H), 7.04 (m, 2H), 4.25 (mt, 4H), 1.88 (m, 4H), 1.56
(m, 4H), 1.11 (t, 6H), MS: m/z 754.8 [M]+. Anal. Calcd for C40H28
F10N4; C, 63.66; H, 3.74; N, 7.42. Found: C, 63.99; H, 3.73;
N, 7.43.
N, N-Bis(3,5-dimethylphenyl)-N, N0-di(n-butyl)-2,9-
difluoroquinacridonediimine (8)
1H NMR (CDCl3, ppm) (500 MHz): d 8.17 (s, 2H), 7.73 (s, 2H),
7.13 (d, 4H), 6.68 (s, 2H), 6.52 (s, 4H), 6.90 (d, 4H), 4.16–4.36
(m, 2H), 3.46 (s, 2H), 2.29 (s, 12H), 1.23–1.41 (m, 8H), 0.98
(t, 6H), MS: m/z 666.31 [M]+. Anal. Calcd for C44H44F2N4; C,
79.25; H, 6.65; N, 8.40. Found: C, 79.76; H, 6.31; N, 8.46.
N, N-Dinaphthyl-N, N0-di(n-butyl)quinacridonediimine (9)
1H NMR (CDCl3, ppm) (500 MHz): d 8.55–8.75 (m, 2H),
8.15–8.18 (m, 2H), 7.89 (d, 2H), 7.54 (t, 6H), 7.38–7.41
(m, 6H), 7.04–7.10 (m, 4H), 6.72 (s, 2H), 3.39–3.59 (m, 2H),
1.54 (d, 2H), 1.25–1.31 (m, 4H), 0.74–0.90 (m, 10H), MS: m/z
674.30 [M]+. Anal. Calcd for C48H42N4; C, 85.43; H, 6.27; N, 8.30.
Found: C, 85.10; H, 6.14; N, 8.10.
N, N-Bis(4-cyanophenyl)-N, N0-di(n-octyl)quinacridonediimine (10)
1H NMR (CDCl3, ppm) (500 MHz): d 7.83–8.04 (m, 4H), 7.62
(d, 8H), 7.53 (t, 2H), 7.28 (s, 2H), 6.95 (t, 6H), 3.89–4.38 (m, 4H),
1.73–2.00 (m, 4H), 1.31–1.42 (m, 22H), 0.90 (t, 4H), MS: m/z
736.20 [M]+. Anal. Calcd for C50H52N6; C, 81.49; H, 7.11; N,
11.40. Found: C, 81.15; H, 6.99; N, 11.41.
Results and discussion
Synthesis
Scheme 1 Synthesis procedure for quinacridone derivatives (1–10) from
alkyl substituted QAs.
The synthetic procedure is outlined in Scheme 1. Alkyl substituted Introduction of the alkyl chain at the N–H position prevents the
QAs were synthesized according to the standard procedure.5 hydrogen bonding between CQO and N–H, therefore the
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solubility of QA is increased. The soluble alkyl substituted QAs
were treated with anilines to form a new series of QA deriva-
tives. These compounds are soluble in common organic sol-
vents, such as toluene, chloroform and tetrahydrofuran (THF)
etc. The chemical structures were verified by 1H NMR, mass
spectrometry and elemental analysis. Structures of some of
the compounds were further confirmed by single crystal X-ray
analysis.
Fig. 2 Absorption in acetone (a) and toluene (b).
Photophysical properties
the maximum absorption wavelength shifts to a longer wave-
length with decreasing polarity, was not observed in regularity.
This means that different substituents obviously affect the
absorption of substituted QA.
Absorption spectra of these derivatives as thin films on
quartz from CHCl3 solution were also recorded (Fig. 1). The
lmax is slightly red shifted and this spectral displacement on
going from solution to the solid state is in accordance with the
molecular exciton theory of Kasha.13 It is also important to note
that the spectral shape in the solid state is quite similar to that
in solution and the numbers of absorption bands are the same
indicating that the molecular nature is not changed.
Absorption properties of all the derivatives were measured both
as thin films on quartz and in CHCl3 as a solution (Fig. 1). In
the solution state, these compounds cover the absorption range
from 450 nm to 600 nm and their maximum absorption wave-
length lmax values are summarized in Table S2 (ESI†).
It is worth noting that lmax of the majority of these com-
pounds is blue shifted with respect to their parent non-
iminated compounds (alkyl substituted QA), which is quite
opposite to the expected red shifting as a result of extended
p-conjugation. A generally accepted concept, for the relation-
ship between structure and absorption, requires maximum
planarity for producing the red shift because of increase of
p-conjugation. From the observed absorption spectra it can be
deduced that the N-phenyl ring might not be in the plane of
the quinacridone nucleus, so it is not involved in extended
p-conjugation. This conjecture was confirmed by single crystal
X-ray analysis. Single crystal structure analysis of six com-
pounds reveals that the QA nucleus is not planar but slightly
buckles to a ‘‘butterfly’’ conformation. In addition, the N-phenyl
rings are twisted out of the plane of the central ring system and
prefer to be in conjugation with the nitrogen lone pair and not in
extended p-conjugation. The QA skeleton is buckled to a large
extent in compounds 5 and 6, which results in large blue shifts
in the UV-Vis spectrum and this is confirmed by the crystal
structure of 6 where the QA skeleton is buckled by 31.681 (Fig. 8).
Compound 7 is the only molecule which exhibits a slight red
shift in lmax by 17 nm as each N-phenyl ring is substituted by five
fluorine atoms, which enhances intermolecular interactions.
Absorption spectra were also recorded in acetone and
toluene in order to observe the effect of the polarity of solvents
on absorption wavelengths (Fig. 2). Generally, QAs form more
stable aggregates in less polar solvents and, as a result, the
absorption maxima are red shifted. On the other hand, the
absorption maxima are blue shifted in more polar solvents,
probably due to the loss of aggregation. The solvent effect, i.e.,
Twisting of the N-phenyl ring and buckling of the QA skeleton
From the analysis of absorption spectra it is deduced that the
N-phenyl ring is not in the plane of the central ring system and
therefore not in conjugation with the p-system of the QA
skeleton. The twisting of the N-phenyl ring out of the plane
might be due to the steric hindrance of the peri-hydrogen of the
central ring system with ortho hydrogen of the N-phenyl ring14
(Fig. 3a).
QA carbonyl groups are less reactive towards nucleophilic
attack than aliphatic carbonyl groups as these are the part of a
conjugation system. The second carbonyl group left after mono-
substitution was found to be more reactive than the first one.
This is inferred from the fact that the mono-substituted pro-
duct (Fig. 3d) was very difficult to obtain and the reaction
generally yielded a mixture of starting material and the disub-
stituted product with very small or negligible amounts of
Fig. 1 Absorption spectra in the CHCl3 solution state (left) and in the thin
Fig. 3 Quinacridone diimines in (a) sterically hindered form, (b) the ‘‘(A)’’
film state (right).
isomer, (c) the ‘‘(B)’’ isomer and (d) the monosubstituted product.
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mono-substituted product. A proposed explanation is that when
the first imine is formed after mono-substitution, the QA system
buckles and moves the second carbonyl group out of conjuga-
tion, making it more reactive than the first one. After disubstitu-
tion the QA nucleus is buckled and the N-phenyl ring is out of
the plane of the QA nucleus. This buckling and non-planarity of
the N-phenyl ring are evident in X-ray structures of molecules.
Variable temperature NMR studies
The 1H NMR spectrum of the synthesized compounds was
complicated and difficult to interpret in full detail. The aro-
matic region contains several overlapping peaks due to the QA
skeleton and the N-phenyl protons. This is not what would be
expected if only one isomeric form were present as in the crystal
structure. We postulate that complex spectra of the compounds
may be due to different conformational and geometrical iso-
mers being present in solution which can be differentiated at
different temperatures (Fig. 4).15
A variable temperature 1H NMR study of 2, 7 and 10 gives an
evidence of different geometrical isomers (ESI†).
Fig. 5 Variable temperature 1H NMR study of 2 in CDCl3 (upper) and C6D6
A variable temperature 1H NMR study of compound 2
revealed interesting molecular dynamics, one at lower temperature
(200–230 K) and the other at higher temperatures (298–335 K). The
1H NMR spectrum of 2 (in chloroform-d3) at ambient temperature
shows two broad singlets at d 3.50 and 4.30, with a relative ratio of
1.0 : 0.58 and are attributed to ZZ(2B) (major) and EE(2A) (minor)
isomers, respectively (vide infra). The broad signals become sharp
when the temperature is reduced however at further lower tem-
peratures, broadening of the signals is also observed (Fig. 5 and
ESI†). The signal at d 3.43 is broadened at 233 K and it splits at
223 K into two signals at d 3.08 and 3.72 in a relative ratio of 1 : 1.
The coalescence temperature is expected to be somewhere between
233 and 223 K. These new signals cannot be attributed to the EZ
isomer because at a further lower temperature (213 K) the signal at
d 4.33 also splits in a similar fashion into two signals at d 4.24 and
4.48 in a relative ratio of 1 : 1. The splitting of both peaks at
lower temperatures indicates some process other than E–Z
interconversion, mainly because of the fact that both isomers
show a similar splitting pattern but different peaks appear in
the NMR spectrum. The energy of the dynamic process is
(lower).
calculated to be 10.2 kcal molꢀ1 which is much lower than
expected for the E–Z isomerization around a double bond.15
Since the signals due to N–CH2 are affected at lower temper-
ature, we believe that the variable NMR behavior may be
associated with some conformational process involved around
N–CH2. At room temperature, the N-alkyl chains are free rotat-
ing however at lower temperature, free rotation of N-alkyl
chains may be restricted due to interaction with the peri-
hydrogens. Due to the restricted rotation, both hydrogens on
N–CH2 (Ha and Hb) become diastereotopic due to the absence
of the symmetry plane in the molecule, and behave differently
in NMR measurements. Due to the restricted rotation, one alkyl
chain is restricted above the plane of quinacridone whereas the
other is restricted below the plane of the quinacridone. Some
signals in the aromatic region (d 7.5 and 8.4) are also broa-
dened which may be assigned to the peri-hydrogens. We
realized that the NMR signals at d 3.46 and 4.34 are quite sharp
at a temperature range of 263–283; however broadening is also
observed at 303 (vide supra), which clearly points to some dynamic
process just above room temperature. Since chloroform-d3 boils at
333 K, we have carried out high temperature NMR measurements
1
in benzene. The H NMR spectrum of 2 (in benzene-C6D6) at
ambient temperature shows two sharp singlets at d 3.26 and
3.89, with a relative ratio of 1.0 : 0.58, which are attributed to
ZZ(2B) (major) and EE(2A) (minor) isomers, respectively. How-
ever the dynamic processes in benzene proceed at different
rates compared to chloroform. For example, the signals at
d 3.26 and 3.89 start broadening at 310 K and coalesced at
330 K. This indicates that the dynamic process is quite fast in
benzene compared to the rate in chloroform. Since the EZ isomer
is quite stable over the temperature range of ca. 320–335 K
(the temperature range under study), it is a quite stable species
Fig. 4 Geometrical isomers ‘‘(A)’’ and ‘‘(B)’’ of quinacridone diimines.
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on the potential energy surface. These findings are consistent 3.04 and the other at 3.50, which is quite opposite to the behavior
with the results from the computational analysis where pre- of 2 and 7 in variable temperature NMR. The protons due to the
dicted relative energy of the EZ isomer is not very different from Z-component of the EZ isomer appear downfield compared to the E
the EE isomer. The energy difference between ZZ(2B) and ZE is fragment, whereas in 2 and 7 a reverse phenomenon is observed. At
relatively large whereas the difference in relative energies lower temperature, each of the peaks split into two, which is
between EZ and EE(2A) isomers is small (vida infra).
attributed to restricted conformation (vide supra).
The variable temperature NMR behavior of compounds 7 and 10
The rate constant, k, for the interconversion of the isomers
is also analyzed (ESI†). The room temperature NMR spectrum (in at the coalescence temperature (Tc) 330 K for the broad NCH2
benzene) of 7 shows a single peak at d 4.38 which is a composite signals in benzene-d6 was calculated from the Gutowsky–Holm
signal from the coalesced NCH2 groups of the isomer mixture due equation, and the imine interconversion barrier (DG‡) was
to rapid interconversion on the NMR time-scale at room tempera- calculated to be 15.42 kcal molꢀ1 by using the Eyring equation.
ture. These observations point towards the fact is that the ZZ–EE
To further confirm the ‘‘(A)’’ and ‘‘(B)’’ isomers of QA
isomerization is much faster in 7 compared to 2. At ambient diimines we grew crystals of these compounds at ambient
temperature the interconversion rate is fast on the NMR time scale. and low temperatures. Importantly, we got single crystals of
Occurrence of the ZZ(7B) isomer of compound 7 cannot be observed 3, 6, 8 and 10 at low temperature with ‘‘(B)’’ conformation and
in the variable temperature NMR spectrum at any temperature of 2 and 7 with ‘‘(A)’’ conformation at ambient temperature,
under study. Not only the signal at d 4.38 is broad at 298 K, but also which shows that QA diimines exist in two isomeric forms and
the signals at d 7.8–8.8. At 260 K, two broad singlets at d 3.70 and more likely interconvert through intermediates.
4.40 are also observed which coalesce at 275 K. The signals at d 3.70
Computational study
and 4.40 may be attributed to the EZ isomer because two signals are
expected for the unsymmetrical isomer and they both coalesce at A lack of the expected enhanced properties of QA derivatives
the same temperature (275 K) and are converted into a broad singlet 1–10 stimulated an in-depth structural exploration through
at d 4.38. The coalescence temperature is believed to be around single crystals X-ray analysis and simulation models. Geome-
275 K. At further lower temperature (245 K), the peaks at d 3.70 and tries of the QA derivatives were optimized using the hybrid
4.40 start splitting into two peaks with equal intensity. This process functional B3PW91 and the structural analysis reveals close
is very similar to the restricted conformation process described resemblance to the X-ray structure. The phenyl ring of the
above for 2. However the splitting of the peak at d 4.38 is not imine moiety makes an average angle of ca. 50–511 with the
observed at temperatures as low as 230 K, which indicates that the QA skeleton (B) for the internal isomers compared with 53–541
restriction in conformational flexibility is achieved earlier for the Z for the external analogue. Another striking difference is the
component of the EZ isomer or for the ZZ(7B) isomer. This is very orientation of the butyl group: the butyl group is bent towards
much consistent with the structural analysis. For the ZZ(7B) isomer inside in the case (A) (external), whereas in (B) (internal) it is
(or the Z component of the EZ isomer), the phenyl ring of the imine bent towards outside due to steric interaction with the phenyl
part is bent towards the alkyl chain which pushes the latter towards group of the imine group, which is bent inside (Fig. 8–13).
the peri-hydrogens and therefore causes more steric hindrance for
In order to evaluate the stability of the EZ isomer relative to
rotation. Therefore based on the structural analysis, the order of EE(A) and ZZ(B) isomers, we have calculated relative energies of
restricted conformational flexibility should be ZZ(B) > EZ > EE(A) all these isomers for compounds 7 and 10. For both these
and this is also observed experimentally. The high temperature molecules, the EE isomer is the most stable isomer however
NMR spectrum of compound 7 (in benzene) from 298–338 K is also the EZ isomer is unstable relative to the EE isomer by 2.55 and
shown in the ESI.† No significant change in the NMR of 7 could be 1.54 kcal molꢀ1 for 7 and 10, respectively. The ZZ isomers for 7
observed in this temperature range, which indicates that the and 10 lie relative to the EE isomer at 5.9 and 5.33 kcal molꢀ1
dynamic (E–Z) isomerization processes complete at 298 K (vide respectively (see Tables S6 and S7, ESI†).
,
supra). Quite contrary to variable temperature of 2, no significant
difference in the rates of dynamic processes is observed with the and MO energies and isodensity plots of HOMO ꢀ 2 to LUMO +
change of solvent from chloroform to benzene. 2 are shown in Fig. 6. Molecular orbitals of both isomers have
Molecular orbitals of both isomers ((A) and (B)) are analyzed
The variable temperature NMR spectrum of compound 10 is strong resemblance among themselves regarding isodensity
more complicated probably because in this case a number of however their energies are quite different (see Table S3, ESI†).
conformational and geometrical isomerization processes are taking For 1a, the HOMO (ꢀ5.0 eV at B3PW91) is located primarily on
place simultaneously (ESI†). Compound 10 is structurally different the QA skeleton. The next highest occupied MOs (HOMO ꢀ 1
than compounds 2 and 7 in two ways; it contains a long more and HOMO ꢀ 2) are 0.43 and 0.53 eV below the HOMO and are
flexible alkyl chain and a cyano group on the phenyl ring. The centered mainly on the phenyl ring of the imine group. The
electron withdrawing effect of cyano probably speeds up the imino lowest unoccupied orbital (LUMO) lies 2.99 eV above the
nitrogen inversion process and lowers the barrier to EZ isomerisa- HOMO. The LUMO and the next higher orbitals (LUMO + 1)
tion. The proton NMR spectral analysis of compound 10 reveals are centered mainly on the QA core, however LUMO + 2 is
some flips of the chemical shifts. At room temperature, the signal exclusively based on the QA core. The internal orientation of
at d 3.54 is attributed to the EE(10A) isomer. EE–EZ isomerization is the phenyl ring of imines has little effect on the isodensity of all
observed at 260 K, however the EZ isomer has two peaks, one at of these orbitals.
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geometrical isomer were obtained at low temperature (ꢀ10 1C
for 3, 6, 8 and 10). It is worth noting that except 10, which has
CN groups as electron withdrawing groups, all other crystals
obtained have fluorine atoms in the skeleton, which betokens
that halogens may induce crystallization.
N, N-Bis(4-fluorophenyl)-N, N0-di(n-butyl)quinacridonediimine (2)
The crystal structure for 2 is shown in Fig. 8 and the molecules
in the unit cell lie at the inversion centre. In the crystal form
both phenyl groups at N are out of the plane of the QA body by
53.68(5)1 indicating that phenyl rings prefer to be in conjuga-
tion with a lone pair of nitrogen and both N-phenyl rings are
oriented away from the alkyl chain, which confirmed the
‘EE(2A)’ form of conformation at ambient temperature. A small
contortion is observed in the N-phenyl ring and is mainly due to
the electron withdrawing character of fluorine. The angle
between carbons a, b and c (122.73(15)1) is slightly larger than
expected for pure sp2 hybridization at C b (Fig. 7a). A C2–H2ꢁ ꢁ ꢁF1
type weak hydrogen bonding interaction has been observed with
dHꢁ ꢁ ꢁF = 2.53 Å and symmetry operation is 2 ꢀ x, ꢀy, ꢀz.
The bond distances in this aromatic ring are shorter than
the rg value for benzene (1.399 ꢂ 0.001 Å) and this is also
attributed to the fluorine substituent at the para position of the
aromatic ring. It is also worth noting that the QA skeleton is not
planar, but is buckled by 16.51(2)1 and the angle between the
QA body and the N-phenyl ring is 126.32(7)1.
As far as molecular packing is concerned molecules are con-
nected to each other by N-phenyl rings through hydrogen bonding
in such a way that the QA skeleton seems to be embedded perpen-
dicularly between N-phenyl rings of different molecules.
Fig. 6 MO energies and isodensity plots of HOMO ꢀ 2 to LUMO + 2.
A comparison of eigenvalues of HOMOs and LUMOs of QAs
1–10 (A) and (B) is shown in Table S3 (ESI†) at three different
levels of theory. B3PW91/6-31G(d) and B3LYP/6-31+G(d) meth-
ods are quite good at predicting the energy of HOMOs (see
Table S4, ESI†) in comparison with the experimental methods,
which is consistent with the fact that hybrid functionals
are better at predicting the HOMO eigenvalues than those
without hybrid functionals (PBEPBE);16,17 although the former
(B3PW91) slightly underestimates the energies, the latter
(B3LYP) overestimates them. The simulated LUMO eigenvalues
are much higher (for both hybrid and non-hybrid functionals)
than the experimental values and this discrepancy is because of
the poor description of virtual orbitals by time independent
DFT methods. Hybrid functionals although significantly
increase the accuracy of the HOMO eigenvalues, however they
are less accurate in computing the HOMO–LUMO gap when
compared with non-hybrid functionals PBEPBE (see Table S3,
ESI†). The difference between theoretical and experimental
HOMO–LUMO gaps decreases with increasing size of hydro-
carbons.18 QAs are big enough to show the accurate HOMO–
LUMO gap through calculations, however we think that the
shape of the molecule and planarity are also key features. The
QA skeleton is not even planar and it shows buckling effect.
N,N-Diphenyl-N,N0-di(n-butyl)-2,9-difluoroquinacridonediimine (3)
The crystal structure of 3 is shown in Fig. 9 and crystallographic
parameters are given in the ESI.† This crystal was grown at low
temperature and therefore it exists in ‘ZZ(3B)’ form of the
isomer as N-phenyl rings are perpendicular to the QA skeleton
pointing towards alkyl chains. The end-on-view of the crystal
structure in Fig. 9 reveals that the QA skeleton is buckled to a
different extent (7.721 and 11.181) at the points of substitution.
This might be due to the fact that at the temperature of
crystallization the molecule has not fully flipped to ‘ZZ(3B)’
form of conformation. It is also worth noting that the buckling
in this case is smaller than 2, this is due to the substituent
effect at the N-phenyl ring in 2. In addition N-phenyl rings are
X-ray single crystal structure determination
Suitable single crystals of one geometrical isomer were
Fig. 7 Different kinds of angles in QA diimine structure.
obtained for 2 and 7 at ambient temperature and for the other
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Fig. 8 Crystal structure of compound 2 where the additional ‘‘a’’ letters
in the atom labels indicate that these atoms are at equivalent positions
(ꢀx, ꢀy, 1 ꢀ z), showing C–Hꢁ ꢁ ꢁF hydrogen bonding interaction and unit
cell packing.
Fig. 10 Crystal structure of compound 6 where the additional ‘‘a’’ letters
in the atom labels indicate that these atoms are at equivalent positions
(ꢀx, 2 ꢀ y, ꢀz), showing C–Hꢁ ꢁ ꢁF and C–Hꢁ ꢁ ꢁN hydrogen bonding
interactions and unit cell packing.
The molecule has line of symmetry and lies at the inversion
centre in its crystal structure. In this molecule the substituent
factor is of more concern as QA skeleton side rings are substituted
with four methyl groups while N-phenyl rings have fluorine at the
para position so the central QA skeleton is buckled by 31.68(3)1,
which is 15.17(2)1 larger than 2. The N-phenyl rings are out of the
plane of the central QA skeleton by 53.41(8)1 and are rotated out of
the central plane at an angle of 126.59(7)1. The crystal structure in
Fig. 10 shows that each molecule is connected to four molecules
with two types of hydrogen bonding between C22–H22AꢁꢁꢁF1 and
C19–H19AꢁꢁꢁN1 having dHꢁꢁꢁX (F/N) bond distances of 2.51 Å
and 2.72 Å and symmetry operations are 1 + x, y, z ꢀ 1 and
1 ꢀ x, 2 ꢀ y, ꢀz respectively.
N, N-Bis(2,3,4,5,6-pentafluorophenyl)-N, N0-di(n-butyl)-
quinacridonediimine (7)
In the crystal structure of 7, shown in Fig. 11, molecules lie at
the inversion centre. In the crystal form both N-phenyl groups
at N are out of the plane of the QA body by 54.29(9)1 indicating
that phenyl rings prefer to be in conjugation with the lone pair
of nitrogen and both N-phenyl rings are oriented away from the
alkyl chain which confirmed the ‘EE(7A)’ form of conformation
at ambient temperature.
A contortion is observed in the N-phenyl ring, which is quite
larger than the previous compounds due to the electron with-
drawing character of the five fluorine atoms. The angles are
slightly larger than expected for pure sp2 hybridization. The bond
distances in this aromatic ring are shorter than the rg value of
benzene (1.399 ꢂ 0.001 Å) and this may be attributed to fluorine
atoms substituted at the aromatic ring. In this molecule, the QA
skeleton is also not planar, and is buckled by 20.52(2)1 and the
angle between the QA body and the N-phenyl ring is 125.71(8)1.
As far as molecular interactions are concerned each mole-
Fig. 9 Crystal structure of compound 3 showing C–Hꢁ ꢁ ꢁF hydrogen
bonding interaction and unit cell packing.
rotated out of the central plane at different angles (128.90(9)1
and 130.35(7)1). Atoms C(38) and C(2) in the molecule at (x, y, z)
act as hydrogen-bond donors, via atoms H(38) and H(2) to
atoms F(1) and F(2) at (x ꢀ 1, y ꢀ 1, z) and (1 + x, 1 + y, z)
respectively, though the side rings of the QA skeleton are the
only interactions which join the molecules together in such a
way that the QA skeleton is framed linearly with N-phenyl rings
perpendicular to it on both sides, where dHꢁ ꢁ ꢁF = 2.48 Å and
2.50 Å for both interactions, respectively.
N, N-Bis(4-fluorophenyl)-N, N0-di(n-butyl)-1,3,8,10-
tetramethylquinacridonediimine (6)
This crystal was grown at low temperature and it also exists
in ‘ZZ(6B)’ form at low temperature as shown in Fig. 10. cule is connected to four other molecules through two types
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grown at low temperature. The molecule lies at the inversion
centre in the crystal structure of 8. The central QA skeleton is
not planar like other crystals and is buckled by 27.64(2)1 in a
butterfly fashion. Also the N-phenyl rings do not lie in the
plane of the central ring system but are twisted out of the
plane by approximately 51.43(9)1. As far as molecular packing
is concerned this crystal has different behavior than rest of
the crystals as the central QA skeleton of one molecule stacks
with other molecule’s QA skeleton by CH2ꢁ ꢁ ꢁp interactions.
Molecules are commingled linearly by hydrogen bonding
between the side rings of the QA skeleton. The interesting
feature of this crystal is that the molecules are stacked
together in a staircase in such a way that large cavities are
generated among the molecules. The weak hydrogen bonding
interaction through C2–H2ꢁ ꢁ ꢁF1, where the flourine atom is
situated at (ꢀx, 1 ꢀ y, ꢀz) with respect to the donor carbon
atom at (x, y, z) via H2, helps to stabilize the crystal structure of
the molecule, where dHꢁ ꢁ ꢁF = 2.51.
Fig. 11 Crystal structure of compound 7 where the additional ‘‘a’’ letters
in the atom labels indicate that these atoms are at equivalent positions
(ꢀx, 1 ꢀ y, 1 ꢀ z), showing C–Hꢁ ꢁ ꢁF hydrogen bonding interaction and unit
cell packing.
N, N-Bis(4-cyanophenyl)-N, N0-di(n-octyl)quinacridonediimine (10)
This molecule is also non-planar, lies at the inversion centre as
it has a plane of symmetry and is triclinic, belonging to space
group p1.
As this single crystal was grown at low temperature, it exists
in the ‘ZZ(10B)’ form of conformation (Fig. 13). There is no
pꢁ ꢁ ꢁp stacking or intermolecular hydrogen bonding involved
between molecules. The only interactions which might be
involved are van der Waals forces. The molecule is buckled by
22.74(1)1 and the N-phenyl ring is twisted out of the plane of the
central core by 51.5(9)1. This large buckling and twisting effect
is due to the presence of the electron withdrawing ‘CN’ group at
the N-phenyl ring. As far as molecular packing is concerned the
central core of the molecules is located at the edges of the unit
cell while the N-phenyl ring is embedded at the center of the
unit cell.
Fig. 12 Crystal structure of compound 8 where the additional ‘‘a’’ letters
in the atom labels indicate that these atoms are at equivalent positions
(1 ꢀ x, 1 ꢀ y, ꢀz), showing C–Hꢁ ꢁ ꢁF hydrogen bonding interaction and unit
cell packing.
of hydrogen bonds. Atoms C(14) and C(20) in the molecule at
(x, y, z) act as hydrogen-bond donors, via atoms H(14) and
H(20b) to atoms F(1) and F(3) at (1 + x, y, z) and (x, 1 + y, 1 + z)
respectively to form the dimers (Fig. 12). The dHꢁ ꢁ ꢁA distance in
C
14–H14ꢁ ꢁ ꢁF1 and C2–H20bꢁ ꢁ ꢁF3 is 2.59 Å and 2.53 Å respectively.
As far as molecular packing is concerned molecules are
connected to each other by N-phenyl rings through hydrogen
bonding in such a way that the QA skeleton seems to be
embedded perpendicularly between N-phenyl rings of different
molecules.
N, N-Bis(3,5-dimethylphenyl)-N,N0-di(n-butyl)-2,9-
difluoroquinacridonediimine (8)
Fig. 13 Crystal structure of compound 10, where the additional ‘‘a’’ letters
in the atom labels indicate that these atoms are at equivalent positions
(2 ꢀ x, 1 ꢀ y, ꢀz), showing C–Hꢁ ꢁ ꢁF hydrogen bonding interaction and unit
cell packing.
X-ray crystal structure determination of 8 shown in Fig. 12
reveals the ZZ(8B) form of conformation as this crystal is also
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3 H. Nakahara, K. Kitahara, H. Nishi and K. Fukuda, Chem.
Lett., 1992, 711.
Redox properties
To study the effect of different groups on the electrochemical
properties of these QA derivatives, cyclic voltammetry (CV) was
performed in anhydrous CH2Cl2 solution with 0.1 M TBAP as a
supporting electrolyte. The potentials relative to standard ferro-
cene (Fc/Fc+) are summarized in Table S5 (ESI†). The cyclic
voltammograms of all the derivatives are shown in S-2 (ESI†).
Most of the derivatives exhibit a reversible reduction and oxidation
wave potential except (5 and 8) which display irreversible oxidation
and (1) which displays irreversible reduction waves. The first half
`
4 S. De Feyter, A. Gesquiere, F. C. De Schryver, U. Keller and
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9 Y. Fan, Y. Zhao, L. Ye, B. Li, G. Yang and Y. Wang, Cryst.
Growth Des., 2009, 9(3), 1421.
wave reduction and oxidation potentials of the compounds are
1/2
(E = ꢀ1.98 V–1.20 V) and (E1o/x2 = 0.22 V–0.68 V), respectively. The
red
electrochemical data give an unclouded picture of the effect of
electron donating and electron withdrawing groups on electro-
10 (a) I. Javed, T. Zhou, F. Muhammad, J. Guo, H. Zhang and
Y. Wang, Langmuir, 2012, 28(2), 1439; (b) I. Javed, Z. Zhang,
T. Peng, T. Zhou, H. Zhang, M. I. Khan, Y. Liu and Y. Wang,
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chemistry of the compounds. Compounds 7 and 10 with strong
1/2
electron withdrawing groups on N-phenyl rings have higher E
red
(ꢀ1.45 V, ꢀ1.20 V) and E1o/x2 (0.67 V, 0.62 V) while compound 5 with
four methyl groups as electron donating groups on the QA
1/2
red
1/2
ox
skeleton has a relatively low E (1.98 V) and E (0.40 V). This
effect is also translated in the LUMO and HOMO levels, com-
pounds having electron withdrawing groups have relatively low
LUMO and HOMO levels.
12 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
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A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
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Conclusions
A series of QA diimines 1–10 were synthesized by the condensation
of anilines with alkyl substituted QAs (QA) and their photophysical
and electrochemical properties were investigated. Unconventional
behavior of absorption spectra suggested the lack of p-conjugation
within the QA skeleton as well as the lack of extended p-conjugation
between the QA skeleton and N-phenyl rings. A computational
1
study, a variable temperature H NMR study of compounds 2, 7
and 10 (for instance) and single crystal X-ray analysis of 2, 3, 6, 7, 8
and 10 gave an unclouded picture of anomalous behavior, indicat-
ing a buckled, non-planar structure and the existence of molecules
in two different isomeric forms at different temperatures. A theore-
tical study was carried out for both isomers of all compounds using
a computational approach; results obtained are in close agreement
with the experimentally determined values.
13 M. Kasha, in NATO Advanced Study Institute Series, Series B,
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14 H. W. Boone, M. A. Bruck, R. B. Bates, A. B. Padias and
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (50773027 and 50733002), the Major State
Basic Research Development Program (2009CB939700), and
111 Project (B06009). We acknowledge Tobias Sparrman,
Department of chemistry,Umeå University, SE-901 87 Umeå,
Sweden for VT NMR facilities. K.A acknowledges Queen’s Uni-
versity, Canada, for the computational facility.
17 G. Zhang and C. B. Musgrave, J. Phys. Chem. A, 2007,
111, 1554.
18 S. S. Naghavi, T. Gruhn, V. Alijani, G. H. Fecher, C. Felser,
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Notes and references
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2 T. Shichiri, M. Suezaki and T. Inoue, Chem. Lett., 1992, 1717.
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