Thermochromic discotic 6-oxoverdazyls

Article abstract of DOI:10.1039/c2cc33051b

6-Oxoverdazyls 1b[n], substituted with three 3,4,5-trialkoxyphenyl groups (n = 8 and 10), exhibit a columnar hexagonal phase (Col_h) below 130 °C. They display reversible color change from red to green at the transition to the isotropic phase. XRD and magnetization data do not support dimerization of the radicals in the mesophase. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc33051b

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 7064–7066
www.rsc.org/chemcomm
COMMUNICATION
Thermochromic discotic 6-oxoverdazylsw
Aleksandra Jankowiak,a Damian Pociecha,b Hirosato Monobe,c Jacek Szczytkod and
ae
´
Piotr Kaszynski*
Received 27th April 2012, Accepted 29th May 2012
DOI: 10.1039/c2cc33051b
6-Oxoverdazyls 1b[n], substituted with three 3,4,5-trialkoxyphenyl
groups (n = 8 and 10), exhibit a columnar hexagonal phase (Colh)
below 130 8C. They display reversible color change from red to green
at the transition to the isotropic phase. XRD and magnetization data
do not support dimerization of the radicals in the mesophase.
We have now prepared the oxy analogue of 1a[n], the achiral
3,4,5-trialkoxyphenyl derivatives 1b[n] (Chart 1), which exhibit a
broad range enantiotropic columnar hexagonal phase (Colh) and
unusual thermochromism that is unrelated to that observed in
some chiral phases (such as N*). Here we describe the detailed
investigation of the first two members of this series, 1b[8] and 1b[10].
Radicals 1b[n] were synthesized similarly to 1a[n]1 from
hydrazines 2b[n]2 and benzaldehydes3,4 3b[n] according to the
Milcent method5 (Scheme 1). Thus, crude hydrazines2 were
converted to hydrazones 4b[n], which were reacted with
triphosgene to give carbonyl chlorides 5b[n]. After isolation and
purification, the chlorides were reacted with hydrazines 2b[n].
The resulting tetrazanes 6b[n] were partially purified and oxidized
either with PbO2 or K3Fe(CN)6 to give radicals 1b[n] in about
10% overall yield.
Liquid crystalline radicals in which spin is delocalized in the p
system are still very rare and sought after for fundamental studies
and applications in photovoltaics. Recently we reported that
1,3,5-tris(3,4,5-trialkylsulfanylphenyl)-6-oxoverdazyls (1a[n], n =
6, 8, and 10, Chart 1) form a monotropic columnar rectangular
phase (Colr) below 60 1C.1 The compounds are dark blue due to
a broad absorption band in the visible region (lmax = 540 nm
and 610 nm in hexane). Their redox potentials are E0/+1
=
1/2
+0.99 V and E0/ꢀ1 = ꢀ0.45 V vs. SCE, and photovoltaic
1/2
Hexane solutions of radical 1b[8] exhibit low intensity
absorption bands in the visible range (maxima at 486 nm
and 614 nm, Fig. 1). According to TD-DFT results for the
1b[1] model, the observed bands originate from several p - p*
studies of 1a[8] revealed a hole mobility of mh E 1.5 ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1 in the mesophase with an activation energy
Ea = 0.06 ꢂ 0.01 eV. While, electrochemical and photophysical
properties of 1a[n] are favorable, the metastable nature of the
discotic phase is problematic for potential applications.
Chart 1
a Department of Chemistry, Vanderbilt University, Nashville,
TN 37235, USA. E-mail: piotr.kaszynski@vanderbilt.edu;
Tel: +1-615-322-3458
b Department of Chemistry, University of Warsaw,
Zwirki i Wigury 101, 02-089 Warsaw, Poland
c National Institute of Advanced Industrial Science and Technology,
AIST Kansai Centre, Ikeda, Osaka 563-8577, Japan
d Faculty of Physics, University of Warsaw, Hoz_a 69, 00-681 Warsaw,
Poland
Scheme 1 Synthesis of 1b[n]. Reagents and conditions: (a) EtOH, cat.
AcOH, reflux; (b) CO(OCCl3)2, pyridine, CH2Cl2, rt; (c) 2b[n], Et3N,
benzene, 50 1C; (d) PbO2, Na2CO3, toluene/MeCN or K3Fe(CN)6,
0.5 M Na2CO3, [Bu4N]+Brꢀ (cat.), CH2Cl2.
e
´
´
´
´
Faculty of Chemistry, University of Łodz, Tamka 12, 91403 Łodz,
Poland
w Electronic supplementary information (ESI) available: Synthetic,
measurements, and theoretical details. See DOI: 10.1039/c2cc33051b
c
7064 Chem. Commun., 2012, 48, 7064–7066
This journal is The Royal Society of Chemistry 2012

View Online
temperature peak is more pronounced. These results indicate
the formation of another, high-temperature metastable phase.
The radicals have limited thermal stability. Repeated cycling
between 100 1C and 135 1C at a rate of 5 K minꢀ1 demonstrated
progressively lower transition temperature and peak broadening.
For instance, in the third cycle the Col - I transition was 1.4 K
lower with a 40% smaller enthalpy.
The observed clearing temperature of 1b[10] is higher by
70 K than that observed1 for the sulfur analogue 1a[10] and 24 K
lower than that reported for the analogous triazine derivative.7
The oxygen and sulfur analogues 1b[8] and 1a[8] are immiscible
and their 4 : 1 mixture phase separates below 125 1C.
Optical analysis of a thin film of 1b[n] revealed that the texture
of the phase obtained on cooling is typical of a columnar discotic
phase Colh (Fig. 3b). Surprisingly, at the transition to the
isotropic phase, the sample undergoes a reversible color change
from red to green (Fig. 1 and Fig. 3a), as revealed by temperature-
dependent absorption spectroscopy studies. Detailed analysis
demonstrated that the change in color corresponds to a 78 nm
shift of the low energy absorption band from 552 nm in Colh
to 630 nm in the isotropic phase. The latter absorption
wavelength is similar to that observed for 1b[n] in hexane solutions
(lmax = 614 nm). Thus, self-assembly of the molecules in columns
results in a significant alteration of the electronic structure of the
radicals and suggests close p–p interactions in the columnar phase.
Verdazyls typically do not form p-dimers or close p associates,8
however a rare example of an intramolecular dimer was described
recently.9 1,10-Bis(6-oxoverdazyl)ferrocene (7) crystallizes with the
two radicals forming a co-facial dimer in the solid-state, which is
accompanied by a 70 nm blue-shift of the low energy absorption
band. This suggests that the formation of columns by 1b[n] has a
similar effect as enforcing co-facial orientation of the verdazyls by
geometry of ferrocene in 7.
Fig. 1 Electronic absorption spectrum of 1b[8] in hexane. The inset
shows enlarged low-energy portion of the spectrum and solid-state
spectra of 1b[10] in isotropic (green) and columnar (red) phases.
Table 1 Transition temperatures (1C) and enthalpies (kJ molꢀ1, in
italics) for 1b[n]a
n
Phase transition data
8
10
Cr o 25 Colh(o) 127 (55.5) I
Cr o Colh(o)’ 20 (23.3) Colh(o) 121 (57.3) I
a
Determined by DSC (5 K minꢀ1) in the heating mode: Cr = crystal;
Colh(o) = ordered columnar hexagonal; I = isotropic.
electronic transitions involving mainly the 5 highest occupied
MOs, localized mostly on the benzene rings, to the LUMO,
localized solely on the verdazyl unit.6 The two lowest energy
excitations at 614 nm and 514 nm have a small contribution
from the SOMO to LUMO transition, associated with the
verdazyl unit. However, the excitation due mostly to this
transition is calculated at 427 nm (f = 0.004).
Powder XRD analysis for 1b[8] and 1b[10] confirmed the
existence of a columnar hexagonal phase (Colh) either in
samples obtained from solutions or from the melt. Diffractograms
of the mesophase obtained above 30 1C showed a series of sharp
reflections that can be indexed assuming a 2D hexagonal lattice
(Fig. 4 and Table 2). The wide angle region of the diffractograms
shows two broad halos at around 4.5 A and 3.9 A and a
distinct reflection at about 4.3 A. The diffused signals correlate
with the mean distance between the molten alkyl chains
(former) and the mean separation between benzene rings
(latter). The sharper signal at about 4.3 A indicates a long-range
positional correlation along the column length (correlation
length 210 A at 110 1C and 370 A at 30 1C for 1b[10]), and is
possibly related to verdazyl–verdazyl distance. No evidence for
dimer formation was found.
Compounds 1b[8] and 1b[10] come out from AcOEt as a dark
red waxy substance. DSC analysis revealed only one transition
on heating at 127 1C for the former and two transitions at 20 1C
and 118 1C with moderate endotherms for 1b[10] (Table 1 and
Fig. 2). However, on cooling DSC analysis for both compounds
revealed two exothermic transitions above 100 1C, which for
1b[10] appeared as a broad peak with a maximum at 115 1C and
a sharp peak at 111 1C with a total exotherm of 54.3 kJ molꢀ1
.
The ratio of the two peaks is cooling rate dependent. At low
cooling rate (1 K minꢀ1) only the high temperature peak is
observed, while at higher rates (8 K minꢀ1) the sharp low
Fig. 3 Optical textures of 1b[10]: (a) growth of a red Colh phase
out of the green isotropic phase (no polarizers) and (b) fully grown
Colh phase.
Fig. 2 DSC trace of 1b[10]. The heating and cooling rates are 5 K minꢀ1
.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7064–7066 7065

View Online
Time-of-Flight (ToF) measurements determined that the
hole mobility (mh) in an unaligned sample of 1b[8] was 3.3 ꢂ
0.2 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 in the temperature range of 40–110 1C
and showed weak temperature dependence. However, the
charge photogeneration was inefficient and the resulting
photocurrent was only 6.0 ꢁ 10ꢀ13 S cmꢀ1 at 90 1C, comparable
with dark current (4.0 ꢁ 10ꢀ13 S cmꢀ1 at 90 1C). No photo-
conductivity was detected in the isotropic phase of 1b[8].
Electrochemical analysis of 1b[8] in CH2Cl2 (10ꢀ3 M)
revealed two quasi-reversible redox pairs. The measured potentials,
E0/+11/2 = +0.83 V and E0/ꢀ11/2 = ꢀ0.60 V vs. SCE, are shifted
cathodically by about 0.16 V relative to those reported for 1a[6].1
The observed cell potential Ecell = 1.43 V is consistent with a
relatively low disproportionation energy calculated for 1b[1]
(rigid, 107.5 kcal molꢀ1; relaxed, 92.3 kcal molꢀ1).
Fig. 4 2D XRD pattern for 1b[8] at 115 1C.
Table 2 Selected X-ray diffraction data for Colh phase of 1b[n]
In conclusion, achiral liquid crystalline 6-oxoverdazyl radicals
1b[n] exhibit unprecedented reversible thermochromism at the
transition from the hexagonal columnar (Colh) to the isotropic
phase. XRD and magnetization results do not support dimeriza-
tion of the radicals in the Colh phase, although the discs exhibit
significant correlation length along the column. In contrast to
the sulfur analog 1a[8], charge photogeneration in 1b[8] was
inefficient; however, charge mobility in both compounds is
comparable. Further studies to understand the origin of the
observed thermochromism and low efficiency of charge photo-
generation are currently in progress.
n
Temp/1C Miller indices hk d spacing/A Lattice parameter/A
8
115
10
10
10
22.81
4.35
24.56
4.26
25.07
4.33
a = 26.35
a = 28.35
a = 28.95
10 30
110
Analysis of the low temperature phase observed by DSC for
1b[10] revealed another signal in the small angle region
indicating the formation of a superlattice with doubling the
basic lattice periodicity, occasionally observed in some Colh
phases.10 This is consistent with a low hysteresis of the
transition, suggesting minimal molecular reorganization.
The cell constant a = 26.35 A for 1b[8] (Table 2) is smaller
than the calculated molecular size of 1b[8] (32 A). For 1b[10],
the cell constant a increases with temperature at a rate of
This work was supported by Vanderbilt University Discovery
Grant and NSF grant (DMR-0907542). We thank Prof. Andrzej
Twardowski for funding SQUID measurements and Ms Ola
Kruczkowska for her technical assistance.
Notes and references
1 A. Jankowiak, D. Pociecha, J. Szczytko, H. Monobe and
5.49 ꢁ 10ꢀ3 A Kꢀ1
.
P. Kaszyn
2 A. Jankowiak and P. Kaszyn
3 A. Nowak-Krol, D. Gryko and D. T. Gryko, Chem.–Asian J.,
2010, 5, 904.
´
ski, J. Am. Chem. Soc., 2012, 134, 2465.
´
ski, Beilstein J. Org. Chem., 2012, 8, 275.
Magnetic studies at 500 Oe revealed paramagnetic behavior
of 1b[8] in the liquid crystalline phase and do not support
formation of dimers in the columnar phase. The effective
magnetic moment (meff) in the mesophase was found to be
1.591(4) in a temperature range of 200–360 K, which is close to
the value of 1.732 for an ideal paramagnet and corresponds to
92 ꢂ 1% of spins. The number of spins increases to 96% near
the isotropic transition. Cooling of the sample from the nearly
isotropic phase to 2 K at 500 Oe showed that molar susceptibility
wm is well described by the Curie law down to 200 K where
antiferromagnetic interactions gradually increase. The results are
similar to those found for 1a[8] and indicate that spins remain
largely isolated at >200 K.
´
4 H. Kretzschmann, K. Muller, H. Kolshorn, D. Schollmeyer and
¨
H. Meier, Chem. Ber., 1994, 127, 1735.
5 R. Milcent, G. Barbier, S. Capelle and J.-P. Catteau, J. Heterocycl.
Chem., 1994, 31, 319.
6 B3LYP/6-31G(d,p) level of theory. See ESIw for details.
7 H. Lee, D. Kim, H.-K. Lee, W. Qiu, N.-K. Oh, W.-C. Zin and
K. Kim, Tetrahedron Lett., 2004, 45, 1019.
8 R. G. Hicks, in Stable Radicals Fundamentals and Applied Aspects
of Odd-Electron Compounds, ed. R. G. Hicks, Wiley&Sons, 2010,
pp. 245–280, and references therein.
9 B. D. Koivisto, A. S. Ichimura, R. McDonald, M. T. Lemaire,
L. K. Thompson and R. G. Hicks, J. Am. Chem. Soc., 2006, 128, 690.
10 T. N. Y. Hoang, D. Pociecha, M. Salamonczyk, E. Gorecka and
R. Deschenaux, Soft Matter, 2011, 7, 4948.
c
7066 Chem. Commun., 2012, 48, 7064–7066
This journal is The Royal Society of Chemistry 2012