Induction of columnar discotic behavior in verdazyl radicals with alkylsulfanyl substituents

Article abstract of DOI:10.1080/10426507.2012.736896

Substitution of the 1,3,5-triphenyl-6-oxoverdazyl radical with n-alkylsulfanyl groups lead to derivatives 1[n], which exhibit a columnar rectangular phase (Colr) below 60 °C. Compounds 1[n] have a broad absorption band in the visible region with maxima at 540 and 610 nm and redox potentials E^0/+1 1/2 = +0.99 V and E^0/-1 1/2 =-0.45 V vs. SCE. Time-of-flight (TOF) investigation of 1[8] revealed hole mobility of μh = 1.52 × 10^-3 cm² V^-1 s^-1 in the columnar phase with an activation energy Ea = 0.06 ± 0.01 eV. Magnetization studies of 1[8] demonstrated nearly ideal paramagnetic behavior in both solid and fluid phases above 200 K and weak antiferromagnetic interactions at low temperatures. Verdazyl derivatives 1[n] were prepared in a sequence of reactions starting from 1-bromo-3,4,5-trifluorobenzene by alkylsulfanylation, followed by hydrazinylation, and finally 6-oxoverdazyl ring assembly using the Milcent method.

Full text of DOI:10.1080/10426507.2012.736896

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Phosphorus, Sulfur, and Silicon and the
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Induction of Columnar Discotic Behavior
in Verdazyl Radicals with Alkylsulfanyl
Substituents
Aleksandra Jankowiak a , Damian Pociecha b , Hirosato Monobe c ,
Jacek Szczytko d , Żaneta Dębska e , Jarosław Romański e & Piotr
Kaszyński a e
a Department of Chemistry , Vanderbilt University , Nashville , TN ,
37235 , USA
b Department of Chemistry , University of Warsaw , 02-089 Warsaw ,
Poland
c Faculty of Physics , University of Warsaw , Hoża 69, 00-681
Warsaw , Poland
d National Institute of Advanced Industrial Science and Technology,
AIST Kansai Centre , Ikeda , Osaka 563-8577 , Japan
e Faculty of Chemistry , University of Łódź , Tamka 12, 91403 , Łódź
, Poland
Accepted author version posted online: 25 Oct 2012.Published
online: 29 May 2013.
To cite this article: Aleksandra Jankowiak , Damian Pociecha , Hirosato Monobe , Jacek Szczytko ,
Żaneta Dębska , Jarosław Romański & Piotr Kaszyński (2013) Induction of Columnar Discotic Behavior
in Verdazyl Radicals with Alkylsulfanyl Substituents, Phosphorus, Sulfur, and Silicon and the Related
Elements, 188:4, 418-426, DOI: 10.1080/10426507.2012.736896
To link to this article: http://dx.doi.org/10.1080/10426507.2012.736896
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Phosphorus, Sulfur, and Silicon, 188:418–426, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.736896
INDUCTION OF COLUMNAR DISCOTIC BEHAVIOR
IN VERDAZYL RADICALS WITH ALKYLSULFANYL
SUBSTITUENTS
Aleksandra Jankowiak,1 Damian Pociecha,2 Hirosato Monobe,3
4
5
5
˙
Jacek Szczytko, Zaneta Debska, Jarosław Roman´ski,
ꢀ
and Piotr Kaszyn´ski1,5
1Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA
2Department of Chemistry, University of Warsaw, 02-089 Warsaw, Poland
3Faculty of Physics, University of Warsaw, Hoz˙a 69, 00-681 Warsaw, Poland
4National Institute of Advanced Industrial Science and Technology, AIST Kansai
Centre, Ikeda, Osaka 563-8577, Japan
5Faculty of Chemistry, University of Ło´dz´, Tamka 12, 91403 Ło´dz´, Poland
GRAPHICAL ABSTRACT
Abstract Substitution of the 1,3,5-triphenyl-6-oxoverdazyl radical with n-alkylsulfanyl groups
lead to derivatives 1[n], which exhibit a columnar rectangular phase (Colr) below 60 ◦C.
Compounds 1[n] have a broad absorption band in the visible region with maxima at 540 and
610 nm and redox potentials E0/+1 = +0.99 V and E0/−1 = −0.45 V vs. SCE. Time-of-
1/2
1/2
flight (TOF) investigation of 1[8] revealed hole mobility of μh = 1.52 × 10−3 cm2 V−1 s−1 in
the columnar phase with an activation energy Ea = 0.06 0.01 eV. Magnetization studies of
1[8] demonstrated nearly ideal paramagnetic behavior in both solid and fluid phases above
200 K and weak antiferromagnetic interactions at low temperatures. Verdazyl derivatives 1[n]
were prepared in a sequence of reactions starting from 1-bromo-3,4,5-trifluorobenzene by
alkylsulfanylation, followed by hydrazinylation, and finally 6-oxoverdazyl ring assembly using
the Milcent method.
Received 30 September 2012; accepted 1 October 2012.
This work was supported by Vanderbilt University Discovery Grant and NSF grant (CHE-1214104). We
thank Prof. Andrzej Twardowski for funding SQUID measurements and Ms. Ola Kruczkowska for her technical
assistance.
Address correspondence to Dr Piotr Kaszynski, Department of Chemistry, Vanderbilt University, Nashville,
TN 37235, USA. E-mail: piotr.kaszynski@vanderbilt.edu
418

INDUCTION OF COLUMNAR DISCOTIC BEHAVIOR
419
INTRODUCTION
One-dimensional charge transport along the self-assembled columns in discotic liq-
uid crystals is a desirable property for applications in molecular electronics1 and light
harvesting devices.2,3 Therefore, a number of aromatic and heteroaromatic discotic meso-
gens have been investigated for their photophysical properties.4 More facile generation of
an electron-hole pair is expected in neutral π-radicals than in closed-shell aromatics due to
their relatively high-lying SOMO and low Ip.5 Recently, first examples of liquid crystalline
π delocalized stable radicals, derived from the triphenylmethyl, were reported.6 This dis-
covery provided new impetus for the development of other materials of this type, in which
magnetic properties can be coupled with anisotropic charge transport, dielectric switching,
chirality, and photo-alignment.
Verdazyls,7 including 6-oxoverdazyls, are among several π-delocalized stable rad-
icals. They exhibit broad absorption bands in the visible part of the spectrum and an
electrochemical window of about 1.5 V.8 Some verdazyl derivatives have been investigated
as components of paramagnetic semiconductors,9 and in photo-excited molecular systems10
intended for photo-magnetic devices.11 With this in mind have prepared a series of dis-
cotic derivatives of 6-oxoverdazyl 1[n] (Chart 1) and investigated their liquid crystalline,
electrochemical, magnetic, and photovoltaic properties.12
SR
SR
SR
1[n]
N
N
N
N
n = 6, R = C6H13
n = 8, R = C8H17
n = 10, R = C10H21
n = 12, R = C12H25
SR
SR
SR
O
SR
SR
SR
Chart 1
RESULTS AND DISCUSSION
Synthesis. Radicals 1[n] were obtained from hydrazines 2[n]13 and benzaldehydes
3[n]14 according to the Milcent method15 (Scheme 1). Thus, crude hydrazines were con-
verted to hydrazones 4[n], which were reacted with triphosgene to give carbonyl chlorides
5[n]. After isolation and purification, the chlorides were reacted with hydrazine 2[n] in ben-
zene. The resulting tetrazanes 6[n] were partially purified and oxidized with K3Fe(CN)6
under PTC conditions or with PbO2 to give radicals 1[n] in 10–20% overall yield.12

420
A. JANKOWIAK ET AL.
SR
SR
SR
RS
RS
RS
RS
SR
RS
CHO
3[n]
c
d
a
1[n]
+
HN
N
NH
N
N
RS
RS
RS
SR
SR
RS
RS
N
Y
RS
NHNH2
2[n]
O
4[n], Y = H
6[n]
SR
SR
SR
RS
b
5[n], Y = COCl
Scheme 1a
aReagents and conditions: (a) EtOH, cat. AcOH, reflux; (b) CO(OCCl3)2, pyri-
◦
dine, CH2Cl2, rt; (c) 2[n], Et3N, benzene, 50 C; (d) PbO2, Na2CO3, Toluene/MeCN
or K3Fe(CN)6, Na2CO3 0.5 M, [Bu4N]+Br− (cat), CH2Cl2.
Scheme 2a
The requisite hydrazines 2[n] and aldehydes 3[n] were obtained from trialkylsul-
fanylbromobenzenes 7[n] as shown in Scheme 2. Thus, bromobenzenes 7[n] were treated
with t-BuLi and the resulting aryllithiums were reacted with di-t-butyl azodicarboxylate
(DTBAD) to give hydrazides 8[n]. Hydrazines 2[n] were prepared by removal of the Boc
groups from hydrazides 8[n] with five equivalents of TfOH in a CF3CH2OH/CH2Cl2 mix-
◦
ture at −40 C and isolated as darkening viscous oils in purities >90% and yields of
60–86%.13
aReagents and conditions: (a) n-BuLi, THF; (b) DMF; (c) aq HCl; (d) t-BuLi, THF;
(e) t-BuOOCN NCOOBu-t; (f) CF3SO3H, CF3CH2OH/CH2Cl2, −40 ◦C, <2 min.
Benzaldehydes 3[n] were prepared in a routine way by lithiation of 7[n] with n-BuLi
and subsequent reaction of the aryllithium with dry DMF at −78 ◦C (Scheme 2).14
3,4,5-Trialkylsulfanylbromobenzes 7[n] were obtained by reacting 1-bromo-3,4,5-
trifluorobenzene with sodium alkanethiolate in dry DMSO or a DMSO/THF mixture for
higher homologues (Scheme 3).14 Tetraalkylsulfanylbenzenenes 9[n] by products were
formed in this reaction in the amounts of 20–40%, and they were separated by chromatog-
raphy. However, complete purification of bromobenzenes 7[n] was not necessary, since
Scheme 3a

INDUCTION OF COLUMNAR DISCOTIC BEHAVIOR
421
Figure 1 Electronic absorption spectrum of 1[8] in hexane.
the by products 9[n] are inert in the lithiation process and were readily separated from
aldehydes 3[n] or hydrazides 8[n].
aReagents and conditions: (a) RSNa, DMSO, or DMSO/THF, 60–80 ◦C.
Electronic Absorption Spectroscopy
Hexane solutions of radicals are blue due to low-intensity absorption bands in the
visible range with maxima at 540 nm and 610 nm (Figure 1). According to TD-DFT results
for 1[1] models, the absorption bands originate from several electronic transitions involving
mainly the highest formally doubly occupied MOs, localized on the benzene rings, to the
LUMO localized on the verdazyl unit (Figure 2). The excitation involving the SOMO to
LUMO transition is calculated at 529 nm (f = 0.03) for 1[1].
Electrochemistry
Electrochemical analysis of 1[8] in CH2Cl2 (10−3 M) showed two quasi-reversible
redox pairs with potentials of E0/+11/2 = +0.99 V and E0/−11/2 = −0.45 V vs. SCE (Figure 3).
The cell potential Ecell = 1.43 V is typical for other 6-oxoverdazyl derivatives.
Figure 2 Contours and energies β-FMOs involved in the low energy excitations. (Color figure available online).

422
A. JANKOWIAK ET AL.
Figure 3 The cyclic voltammograms of 1[8]: 1 mM, [Bu4N]+[PF6]−, 0.1 M, 100 mV s–1
.
Liquid Crystalline Behavior
Thermal analysis of a freshly crystallized sample of 1[8] revealed a melting transition
with a peak at 65 ◦C (Table 1, “heating 1” in Figure 4). Upon cooling the sample underwent
◦
a transition at 56 C to a birefringent phase with a texture characteristic for a discotic
◦
columnar phase (Figure 5) that supercools to −40 C. The monotropic phase undergoes
◦
transition to an isotropic phase at 60 C on heating with an enthalpy comparable to that
recorded on cooling. Crystalline polymorph of 1[8] obtained from melt is different from
that obtained from solutions. It melts with a peak at 52 ◦C forming an enantiotropic phase
with a transition to the isotropic phase at 60 ◦C (“heating 2” in Figure 4). Similar mesogenic
behavior is observed for 1[6] and 1[10] homologues (Table 1), while compound 1[12] does
not exhibit liquid crystalline properties.
Powder X-ray diffraction (XRD) analysis for the three lower homologues of 1[n]
revealed formation of a columnar rectangular phase (Colr) consistent with the polarized
microscopy texture (Figure 5). The fast-cooled samples gave XRD patterns with two sharp
diffraction signals in the small angle region that can be indexed as (11) and (02), and a typical
broad halo at 4.2 Å in the wide-angle region (Figure 6, Table 2). Upon further cooling, the
sample crystallized, that manifested itself in significant changes in the XRD patterns; the
sharp signals disappeared and pronounced small-angle scattering of the X-ray beam was
observed instead. Larger crystallites (or another crystalline polymorph) could however be
observed for slowly cooled samples-in this case the diffraction patterns exhibited similar
features to that of the fast-cooled sample in columnar phase with additional low intensity
sharp reflections in the entire range of the diffractogram.
Table 1 Transition temperatures (◦C) for 1[n]a
n
1a[n]
6
8
10
12
Cr 39 Colr 50 I
Cr 62 (Colr 60) I
Cr 62 (Colr 55) I
Cr 63 I
aDetermined by DSC (5 K min−1) in the heating
mode: Cr = crystalline; Colr = columnar rectan-
gular; I = isotropic.

INDUCTION OF COLUMNAR DISCOTIC BEHAVIOR
423
Figure 4 DSC traces of 1[8]. The heating and cooling rates are 5 K min−1
.
Figure 5 Optical textures of a Colr phase of 1[8] upon fast cooling. (Color figure available online).
Figure 6 2D XRD pattern for 1[8] at 25 ◦C.

424
A. JANKOWIAK ET AL.
Table 2 X-ray diffraction data for 1[n]
1[n]
n
Temp /◦C
30
Miller indices hk
d spacing (Å)
Latticeparameters (Å)
6
11
02
11
02
11
02
19.7
17.1
21.9
18.8
23.5
19.6
a = 24.1
b = 34.2
a = 26.9
b = 37.6
a = 29.4
b = 39.2
8
30
30
10
Magnetic Properties
Magnetization studies of 1[8] (at 200 Oe) revealed paramagnetic behavior in all
phases. The effective magnetic moment (μeff) for 1[8] in the crystalline phase was found
to be 1.620 0.03 in a temperature range of 310–345 K, which is close to the value
of 1.732 for an ideal paramagnet and corresponds to 93 3% of spins (Figure 7). The
◦
number of spins increased to 95 2% at about 65 C, which coincides with melting to
an isotropic phase, Cr→I. No abrupt changes of magnetization were observed at the phase
transitions upon cooling (I→Colr) or heating (Colr→I), and the number of spins remained
approximately constant at 95 2%.
Cooling of the samples from the isotropic phase to 2 K in magnetic field showed that
molar susceptibility χm is well described by Curie law down to 200 K (Figure 8). At lower
temperatures antiferromagnetic interactions gradually reduce the observed magnetization.
These results indicate that the alkylsulfanyl groups prevent effective intermolecular close
π–π contacts, and the unpaired electrons remain largely isolated either in the rigid or fluid
phases. This is consistent with results of molecular modeling for 1[8], which show alkyl
chains effectively preventing blocking access to the verdazyl core (Figure 9).
Photovoltaic Properties
Time-of-flight (TOF) measurements found positive charge carrier (hole) mobility
(μh) in a fast-cooled, unaligned multidomain sample of 1[8] to be 1.52 × 10−3 cm2 V−1
s
−1 at 40 ◦C (Figure 10), which is in the range of typical values of 10−4–10−1 cm2 V−1 s−1
.
Figure 7 μeff vs. T plot for 1[8]. H = 200 Oe.

INDUCTION OF COLUMNAR DISCOTIC BEHAVIOR
425
Figure 8 Paramagnetic molar susceptibility χm vs. 1/T plot after diamagnetic correction. C = 0.357(1) cm3 K
mol−1. H = 200 Oe.
Figure 9 Two views of a molecule of 1[8] optimized with the UFF algorithm. Alkyl chains are in the all-trans
conformation. (Color figure available online).
Figure 10 The temperature dependence of positive carrier mobility obtained by the TOF method for 1[8] at
337 nm. Electric field strength 40 kVcm−1
.

426
A. JANKOWIAK ET AL.
The Arrhenius analysis of μh at several temperatures gave the activation energy Ea = 0.06
0.01 eV and is consistent with a hopping transport mechanism.
SUMMARY
The first series of liquid crystalline derivatives of the verdazyl radical has been
synthesized and characterized.12 Compounds in series 1[n] containing alkylsulfanyl chains
exhibit the Colr phase, however magnetic analysis demonstrated that the verdazyl units are
laregly isolated even in the solid phase. We have demonstrated charge photogeneration and
its transport in a liquid crystalline radical, which is comparable to that in other discotic
mesogens. Further studies of liquid crystalline derivatives of the verdazyl, such as oxygen
analogues of 1[n],16 are in progress in our laboratory.
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