Chiral discotic derivatives of 1,3,5-triphenyl-6-oxoverdazyl radical

Two derivatives of 1,3,5-aryl-6-oxoverdazyl containing two 3,4,5-trioctyloxyphenyl and one 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl group in position C(3) (1b) or N(1) (1c) were investigated by thermal, X-ray diffraction and magnetic methods. The results were compared to those obtained for achiral derivative 1a containing three 3,4,5-trioctyloxyphenyl groups. All three compounds exhibit an ordered columnar hexagonal phase, Colh(o), and for chiral derivatives, 1b and 1c, a superstructure with doubled periodicity was found. The introduction of the three chiral alkoxy substituents in 1a lowered the mesophase stability by about 50 K and induced a Colh phase in 1c. Thermochromic analysis showed a hypsochromic shift upon formation of the Colh(o) phase similar for all three derivatives 1 (~0.3 eV), which coincides with a 5% drop in effective magnetic moment, μeff, for 1c. Analysis of magnetisation data in a range of 2–370 K at 200 Oe revealed weak antiferromagnetic interactions (θ = – 4 K) in the Colh(o) phase.


Introduction
Among the many characteristics of discotic liquid crystals sought for practical applications, [1][2][3] semiconduction and ability to generate photocurrents [4][5][6] are the most desired. The efficiency of the charge transport in such soft materials depends, among others, on the organisation of molecules, defects and proximity (overlap) of the π-systems. [7] Conformational and steric requirements of substituents in molecules forming discotic phases typically prevent close contact of the πsystems. In some achiral systems, e.g. triphenylenes [8] and hexabenzocoronenes, [7,9] the close molecular packing in columnar ordered and plastic phases results from a spontaneous helical twist in the columns (spontaneous symmetry breaking). [10] Overall, for nonchiral molecules, helical columns with opposite handiness are formed in equal amounts giving rise to a racemic bulk material. For materials that already have twisted columns formed by achiral molecules, a uniform sense of handiness of the helical twist is achieved by using peripheral alkyl chains containing stereogenic centres. [7] The same substitution in some other discogens transforms the columnar hexagonal (Col h ) to a columnar rectangular (Col r ) phase. [11] Recently, we demonstrated that substituted triphenyl derivatives of verdazyl, a π-delocalised radical, form discotic hexagonal phases. [12][13][14] We have investigated three series of compounds containing 3,4,5-trialkoxyphenyl [12] (such as 1a), 3,4,5-trialkylsulfanylphenyl [13] and a mixture of both substituents. [14] The results demonstrated that the alkylsulfanyl derivatives form a disordered columnar hexagonal 3D phase (Col h(3D) ) below 60°C, presumably with a helical structure. In contrast, the alkoxy derivatives, including the octyloxy derivative 1a, form a broad-range ordered columnar hexagonal phase (Col h(o) ), with a correlation length along the columns of at least 370 Å at 30°C and pronounced thermochromism upon the formation of the mesophase. In both series of compounds, photocurrent was detected, and the hole mobility μ h was found to be in the order 10 −3 cm 2 V −1 s −1 in the mesophase. The tighter molecular packing in the ordered phase is also evident from magnetisation studies, which demonstrated a 4% decrease of the effective magnetic moment, μ eff , at the I→Col h(o) transition. Further improvement of the π-π overlap and hence increase of the photocurrent and spin-spin interactions in this type of compounds could be achieved by using stereogenic centres in the side chains.
Here, we report two derivatives 1b and 1c carrying three alkyl chains with stereogenic centres, the (S)-3,7dimethyloctyl (C 10 H 21 *), and analyse their effect on the supramolecular structure and phase stability. We probe the phase structure by the X-ray diffraction (XRD) method and investigate its impact on molecular packing through thermochromic and magnetic methods.

Thermal analysis
Analysis of compounds in series 1 by thermal differential scanning calorimetry (DSC) and polarized optical microscopy (POM) methods demonstrated that both red, waxy chiral derivatives, 1b and 1c, exhibit liquid crystalline behaviour at ambient temperature and above (Table 1), with clearing temperatures lower than that for the achiral derivative 1a by about 50 K. In contrast to 1a and 1b, chiral compound 1c exhibits two mesophases as shown in Figure 1. The higher temperature, narrow-range phase has a small enthalpy of Col-Iso transition (6.5 kJ mol −1 ) and relatively small thermal hysteresis of about 3 K. In contrast, the Col-Col phase transition has an enthalpy nearly four times greater than   that of the Col-Iso transition and a significantly larger hysteresis (11 K, Figure 1). Optical textures obtained on cooling of an uncovered thick sample of 1c or 1b between glass plates are typical for a columnar mesophase ( Figure 2). Textures observed for two phases of 1c are essentially identical (Figure 2a and 2b). Similarly, little change in the texture is observed at the Col-Col phase transition for 1c in a 5 μm planar cell (Figure 2c).

Electronic absorption spectra and thermochromism
Further information about the supramolecular structures of the mesophase was obtained for 1b and 1c by UV-vis spectra recorded for thin-film samples at ambient temperature.
Solution spectra of all three derivatives 1 are identical, resulting from the same chromophore. They exhibit low-intensity absorption bands in the visible range, with maxima at about 610 nm and 500 nm and a moderate-intensity absorption band at about 210 nm, characteristic for the 3,4,5-trialkoxyphenyl group ( Figure 3).
Visible spectra recorded for thin films of 1 in the isotropic phase and mesophase demonstrated a substantial thermochromic effect upon the formation of the ordered hexagonal phase in all three samples. Results shown in Table 2 demonstrate that for 1a the formation of a Col mesophase is associated with a  hypsochromic shift of 310 meV, which is largest in the series and also larger than that previously observed for the decyloxy analogue (282 meV). [12] A replacement of three octyloxy groups in 1a with three *C 10 H 21 O substituents at the C(3) position has relatively little effect on the shift, while placing chiral chains at the N(1) position results in slightly smaller hypsochromic shift by several meV. This trend is consistent with decreasing π-π overlap caused by increasing steric demands of the liquid-like larger alkyl chains. A plot of the low-energy absorption band maximum as a function of temperature ( Figure 4) provides further details of the thermochromic effect in 1c. At low temperature in the hexagonal ordered phase (Col h(o) ), the λ max value is about 543 nm. Upon heating, the position of the maximum slightly shifts to 542 nm and at the phase transition to the columnar disordered phase, in a range of 2 K shifts to 634 nm. In the middle of the Col h →Col h(o) phase transition range, at 67°C the spectrum represents an approximate superimposition of spectra obtained for both ordered and disordered phases ( Figure 3). Further heating leads to a decrease of the λ max value to 627 nm in the isotropic phase. This small bathochromic shift between the Iso and Col h phases is typical for other discotic verdazyls. [14] Upon cooling of the sample from Iso to Col h(o) phase, λ max changes in the opposite direction with about 5 K hysteresis ( Figure 4).

X-ray diffraction
XRD analysis confirmed the existence of two columnar hexagonal phases in derivative 1c by revealing the sixfold symmetry of the structure clearly visible in the pattern obtained for an aligned sample (for details see Supplemental data). The upper temperature phase can be identified as a disordered type, Col h , since its diffractogram contains only one diffused signal in the highangle range, reflecting liquid-like ordering of molecules along columns and a sharp Bragg reflection in the lowangle range ( Figure 5). The lattice parameter (column diameter) is comparable to the molecular size ( Table 3). The XRD pattern of the lower temperature phase is much richer and exhibits a number of sharp signals that can be indexed to a 2D hexagonal lattice with the lattice parameter doubled with respect to the Col h phase ( Table 3). Fitting of the XRD pattern demonstrates that all except one sharp reflections are due to the 2D hexagonal structure of columns (for details see Supplemental data); the only remaining signal at 3.9 Å can be attributed to molecular core correlation along the column stack. [12] Fitting procedure revealed also the presence of a broad signal in the high-angle region reflecting molten state of aliphatic tails. It should be    Note: a Ref. [12].
stressed, however, that the main electron density modulations are still related to the presence of columns having single molecule in the cross-section; diffraction signals related to superstructure with doubled periodicity are much weaker than signals originating from the fundamental structure. The reflection related to intermolecular distance along the column axis is sharp in the lower temperature phase, which points to an ordered type phase, Col h(o).

1
In derivative 1b, XRD methods revealed only one mesophase (for details see Supplemental data), which was identified as an ordered columnar hexagonal phase, Col h(o) , similar to the lower temperature phase of 1c. Also, the doubling of the columnar structure with respect to molecular diameter is observed. The observed XRD patterns do not permit assignment nor exclude the presence of a helical structure in the columns in both derivatives.
Hexagonal phases with a superlattice are rare. One example of such a phase was observed for the decyloxy analogue of 1a, [12] in which doubling of the unit diameter was manifested by the appearance of sub-harmonic of the main (100) signal of the hexagonal lattice. In the present case, the modulation of the electron density in 1b and 1c is different, and the subharmonic of the main signal of the hexagonal lattice is absent; instead sub-harmonic of the (110) signal is clearly visible (for details see Supplemental data).

Magnetisation measurements
Magnetic studies at 200 Oe revealed paramagnetic behaviour of 1c in the liquid crystalline and isotropic phases, with increasing antiferromagnetic interactions upon phase transition and lowering temperature ( Figure 6).
Assuming ideal paramagnetic behaviour in the isotropic phase, diamagnetic and impurity correction was established using the Curie-Weiss law with θ = -4 K (for details see Supplemental data). The effective magnetic moment (μ eff ) in the isotropic phase (>353 K) is close to the value of 1.732 for an ideal paramagnet and corresponds to 99 ± 0.5% of spins ( Figure 6). Upon phase transition to the Col h phase at 359 K, the μ eff value continuously decreases to about 1.63 at 330 K at the phase transition to Col h(o) phase. These changes in μ eff exhibit hysteresis (for details see Supplemental data) consistent with that observed in thermal analysis in Figure 1.
Cooling of the sample below 300 K in the Col h(o) phase results in a small, but reproducible maximum at about 225 K, followed by a rapid decrease below 50 K due to increasing antiferromagnetic interactions. The average value of μ eff in the Col h(o) in a temperature range of 150-320 K was found to be 1.63 ± 0.01, which corresponds to 94 ± 0.5% of paramagnetic spins. These results are similar to those found [12] for 1a and indicate that spins remain largely isolated at >150 K.

Summary and conclusions
Experimental data demonstrate that substitution of three chiral groups C 10 H 21 * for C 8 H 17 groups in 1a results in significant destabilisation of the columnar Col h(o) phase and induction of a disordered Col h phase. The Col h(o) phase in the two chiral derivatives exhibits superstructure with a doubled cell constant, which is absent in 1a. The Col h -Col h(o) and Iso-Col h(o) phase transitions are accompanied by strong thermochromism and a small change of effective magnetic moment μ eff . The two regioisomers, C-chiral (1b) and N-chiral (1c), differ in their thermal properties: the Col h(o) phase is more destabilised for N-chiral (1c) than for C-chiral (1b) isomer. Overall thermochromism and magnetisation data indicate comparable intracolumn intermolecular interactions for all the three derivatives 1 in the Col h(o) phase, which appear stronger than those in the decyloxy analogue of 1a. Thus, impact of chiral alkyl chains on molecular packing in the Col h(o) phase of 1a is minimal.

Experimental part 4.1. General
Reagents and solvents were obtained commercially. Reactions were carried out under Ar, and subsequent manipulations were conducted in air. Nuclear Magnetic Resonance (NMR) spectra were obtained at 128 MHz ( 11 B) and 400 MHz ( 1 H) in CDCl 3 . 1 H NMR spectra were referenced to the solvent. Details and additional results of XRD and magnetisation measurements are provided in the Electron Spray Ionisation (ESI). Optical microscopy and phase identification were performed using a polarised microscope equipped with a hot stage. Thermal analysis was obtained using a TA Instruments DSC using small samples of about 0.5-1.0 mg.

Electronic absorption spectra
UV-vis spectra for 1b were recorded in spectroscopic grade hexane at a concentration of 5-35 × 10 −6 M. Extinction coefficients were obtained by fitting the maximum absorbance at 211 nm against concentration in agreement with Beer's law. Visible spectra for neat 1 that was placed between two glass slides were obtained on cooling at temperatures about 10 K above and then 10 K below the Col-Iso phase transition for each compound using a hot stage mounted in a UV spectrometer. For 1c, the spectrum of Col h phase was recorded in the middle of the phase range.

Powder XRD measurements
X-ray diffraction experiments in broad-angle range were performed with Bruker D8 GADDS (Cu Kα radiation, Göbel mirror, point collimator, Vantec 2000 area detector) equipped with a modified Linkam heating stage. For small-angle diffraction experiments, Bruker Nanostar system was used (Cu Kα radiation, cross-coupled Göbel mirrors, three pinhole collimation, Vantec 2000 area detector). Samples were prepared in the form of a thin film or a droplet on heated surface. The X-ray beam was incident nearly parallel to sample surface.

Note
1. This phase could also be considered as soft crystalline; however, a broad signal observed in the high-angle region that could be attributed to the molten alkyl chains points to the liquid crystalline, although ordered, nature of this columnar phase.

Supplemental data
Supplemental data for this article can be accessed here.