Liquid crystalline radicals: discotic behavior of unsymmetrical derivatives of 1,3,5-triphenyl-6-oxoverdazyl

A series of six 6-oxoverdazyl (1[10]) substituted with a total of three 3,4,5-tri(decyloxy)phenyl and/or 3,4,5-tri(decylsulfanyl)phenyl groups was investigated by thermal, XRD, spectroscopic, magnetic and photovoltaic methods. The compounds exhibit columnar hexagonal (1[10]c, Colh), columnar hexagonal ordered (1[10]b, Colh(o)), columnar hexagonal 3D (1[10]a, 1[10]e and 1[10]f, Colh(3D)) or a sequence of two phases (1[10]b, Colh–Colh(3D)). The mesophase structure and stability and also thermochromism were investigated as a function of the number and distribution of decyloxy and decylsulfanyl substituents in the molecule. Thermal analysis demonstrated that the presence of the 3,4,5-tri(decyloxy)phenyl substituent in the C(3) position increases the phase stability. Spectroscopic analysis showed that only all-decyloxy derivative 1[10]b exhibits a hypsochromic shift upon Colh(o) formation, while all other compounds in the series show a modest bathochromic shift in the columnar phase relative to the isotropic phase. Magnetization investigation of 1[10]d demonstrated the paramagnetic behavior of isolated spins in isotropic and columnar phases. Negligible photocurrent was detected for 1[10]d in the columnar phase.


Introduction
Liquid crystalline radicals with p-delocalized electron spins 1 constitute an emerging class of materials for fundamental studies of spin-spin interactions in supramolecular assemblies and potential applications as organic semiconductors. [2][3][4][5] In this context, we have focused on the verdazyl as a centerpiece of mesogenic compounds, [2][3][4]6 and recently reported two series of discotic derivatives 1[n]a and 1[n]b (Chart 1). The former series, 2,6 1,3,5-tris(3,4,5-trialkylsulfanylphenyl)-6-oxoverdazyls (1[n]a, n ¼ 6, 8, 10), exhibits a 3-dimensional columnar hexagonal phase (Col h(3D) ) 6 below 60 C and a broad absorption band in the visible range resulting in a dark blue-violet color. In contrast, the alkoxy analogues 1[n]b form a broad-range ordered columnar hexagonal phase (Col h(o) ) with clearing temperatures below 130 C, and exhibit a pronounced thermochromism: in the isotropic phase they are dark green, while in the discotic phase they become red. 3 In both series of compounds photocurrent was detected, and the hole mobility m h was foundc3tc31984a-f to be in the order of 10 À3 cm 2 V À1 s À1 in the mesophase. 2,3 Tuning of properties, such as the type, range and stability of the mesophase and thermochromism, in this class of discogens could be achieved by "combining" aryl fragments from each of the series. For this purpose we chose the symmetric derivatives 1 [10]a and 1 [10]b and prepared analogous "mixed" derivatives containing both the 3,4,5-tri(decylsulfanyl)phenyl and 3,4,5tri(decyloxy)phenyl substituents. Here we report four "mixed" derivatives 1[10]c-1 [10]f (Chart 1) and analyze structural effects on the mesophase and electronic absorption. We also investigate the derivative 1 [10]d for its magnetic and photovoltaic properties.

Electronic absorption
Hexane solutions of all radicals 1 [10] exhibit low intensity broad absorption bands in the visible range with maxima at about 610 nm and 500 nm (Fig. 1). The UV portion of the spectra exhibits absorption bands characteristic of the 3,4,5-trialkylsulfanylphenyl ($260 nm) and 3,4,5-trialkoxyphenyl ($210 nm) groups. According to TD-DFT results for models 1 [1], the low energy absorption bands originate from several p / p* electronic transitions that involve 5 highest occupied b MOs, localized mostly on the benzene rings, to the b-LUMO, localized solely on the verdazyl unit, as shown for 1 [1]c in Fig. 2. The lowest energy excitations at about 600 nm have a small contribution from the SOMO (a-HOMO) to the a-LUMO transition, in which both orbitals are associated with the verdazyl unit. 10 Further analysis revealed that the verdazyl unit contains the bulk spin density and little is delocalized into benzene rings ( Fig. 3) in agreement with other results for verdazyls. 11

Thermal analysis
Analysis of compounds in series 1 [10] by thermal (DSC) and optical methods (POM) demonstrated that all exhibit liquid crystalline behavior ( Table 1). The most stable mesophase was found for the tris(3,4,5-tridecyloxyphenyl) derivative 1 [10]b, for which the Col h(o) phase becomes isotropic at 121 C. 3 Replacement of the aryl substituent at the N(1) position of 1 [10]b with the 3,4,5-tri(decylsulfanyl)phenyl group in 1 [10]c destabilized the mesophase by 30 K and eliminated the ordered character of the columnar phase, as evident from XRD analysis (vide infra). The texture of the mesophase, containing pseudoisotropic areas with only a few birefringent domains (e.g. Fig. 4a) is typical for columnar hexagonal phases. Interestingly, the  same substitution at the C(3) position of 1 [10]b has a more pronounced effect on the mesophase: the columnar mesophase in compound 1 [10]d, an isomer of 1 [10]c, has lower thermal stability at 11 K, and the Col h(3D) phase, characteristic of series 1[n]a, is formed below the narrow range Col h phase (Table 1 and Fig. 5). This is in agreement with the expectations that the Col h(3D) phase is more organized than the Col h phase. Optical analysis of both phases in 1 [10]d shows that bright domains visible in the texture of the high temperature Col h phase loose birefringence upon phase transition to the Col h(3D) phase (Fig. 4).
Substitution of two 3,4,5-tri(decylsulfanyl)phenyl groups and one 3,4,5-tri(decyloxy)phenyl group into the verdazyl core induces only the Col h(3D) phase in derivatives 1 [10]e and 1 [10]f (Table 1) for which two characteristic textures, obtained on slow and fast cooling from the isotropic phase, are shown in Fig. 6. Again, the mesophase of the isomer with the 3,4,5-tri(decyloxyl)phenyl group at the C(3) position of the verdazyl ring exhibits higher thermal stability.
Thus, the results demonstrate that derivatives 1 [10] with one 3,4,5tri(decylsulfanyl)phenyl group exhibit the Col h phase and with two such groups only the Col h(3D) phase is present. Also isomers with the 3,4,5-tri(decyloxy)phenyl group at the C(3) position have higher thermal stability; such derivatives are presumably more anisometric due to near coplanarity of two alkoxy substituents with the benzene ring and the whole aryl substituent with the central heterocycle (Fig. 2).

Thermochromism
Visible spectra were recorded for thin lms of 1 [10] placed between glass slides in the isotropic phase and mesophase approximately 10 K away from the Col-Iso transition. Results shown in Table 2 demonstrate that the formation of a mesophase is associated with a large (282 meV) hypsochromic shi only for 1 [10]b. In contrast, all other compounds that contain sulfur atoms exhibit a modest bathochromic shi of the lowest energy absorption maximum. For instance, in the all-sulfur analogue 1 [10]a, the shi is 26 nm (82 meV) and is the largest among the 5 compounds. The smallest, nearly negligible bathochromic shi of 3 nm was measured for 1 [10]d. Considering that only 1 [10]b forms a Col h(o) ordered phase in the series, the hypsochromic shi is associated with tight packing of molecules in the column, which is absent in other members of the series.

X-ray diffraction
XRD analysis conrmed the existence of columnar hexagonal phases in all unsymmetric derivatives 1 [10]. The liquid crystalline character of these phases is evidenced by diffused signals in     a wide-angle range, originating from short-range positional correlations between neighboring molecules along the column axis (Fig. 7). The diffractogram of 1 [10]c with a single 3,4,5tri(decylsulfanyl)phenyl group in the N(1) position consists of two sharp signals in the small angle range and diffused halo at high angles. Such a pattern can be attributed to a Col h phase having 2D hexagonal lattice of disordered columns, i.e. with liquid like order of molecules along columns ( Table 3). The high temperature phase of 1[10]d (Fig. 7) was also identied as a Col h phase. In contrast, the low temperature phase of 1 [10]d gave a much richer XRD pattern, which could be indexed to a three-dimensional columnar hexagonal phase (Col h(3D) ), i.e. having a well dened periodicity along the columns. It should be stressed that this additional periodicity is an order of magnitude larger that the distance between neighboring molecules along the column axis, and that the positional correlations between neighboring molecules remains shortrange (the phase is of disordered type). The diffused wide angle XRD signal in the Col h(3D) phase is split into two, reecting a slightly different mean spacing between mesogenic cores and alkyl tails. The 3D columnar hexagonal phase was also identi-ed from XRD patterns for the two remaining derivatives 1[10]e and 1 [10]f.
The hexagonal lattice parameter a, related to the intercolumnar distance, is similar in both columnar phases of 1[10]d (Table 3) indicating very little structural reorganization during the Col h / Col h(3D) phase transition. The Col h(3D) phase has a small thermal expansion coefficient, and both lattice parameters a and c increase only by 0.12Å and 0.05Å, respectively, upon cooling by 33 K.
Analysis of the data in Table 3 indicates that the lattice parameter a is similar for all six derivatives and in a range of 27-29Å with the largest intercolumnar separation found for 1 [10]b (28.95Å). In contrast, periodicity along the column (parameter c) varies signicantly in the series and correlates with the Col h(3D) phase stability (Fig. 8). Thus, the largest value c was found for compound 1 [10]f, which shows the highest Col h(3D) -I transition temperature.
A possible representation of the proposed Col h(3D) phase is a helical structure within the column resulting from rotation of

Magnetic and photovoltaic characterization
Magnetic studies of 1 [10]d at 500 Oe revealed nearly ideal paramagnetic behavior in both the liquid crystalline and isotropic phases. Similar to previous observations for 1 [8]a, no abrupt changes were detected upon phase transitions, which indicates that the spins are isolated in both phases. 2 In contrast, a sample of 1 [8]b showed a small decrease of magnetization upon isotropic to mesophase transition. 3 Time-of-Flight (ToF) studies of an unaligned sample of 1 [10]d in cells of 12.5 mm or 4.75 mm gap (applied voltage of 30-50 kV cm À1 ) showed only a negligible transient photocurrent in the range of 80-30 C that was insufficient to calculate charge mobility. In the case of 1[8]a and 1 [8]b also weak photocurrent was detected and hole mobility m h was calculated to be about 3 Â 10 À3 cm 2 V À1 s À1 in the mesophase. 2,3 No photocurrent was detected in the isotropic phase of the latter compound.

Conclusions
A gradual replacement of the oxygen atoms in the decyloxy derivative 1 [10]b with sulfur atoms leads to change of columnar hexagonal phase organization from ordered (Col h(o) ) to disordered (Col h ) to 3 dimensional (Col h(3D) ). The Col h(3D) phase is more organized than the Col h phase, as evident from the phase sequence in 1 [10]d. Spectroscopic investigation of series 1 [10] revealed that only all-decyloxy derivative 1 [10]b exhibit hypsothermochromism, which is associated with the tight molecular packing in the ordered Col h(o) phase. Detailed studies of 1 [10]d demonstrated essentially isolated spins in all phases, and negligible photocurrent due to efficient charge traps.

Computational details
Quantum-mechanical calculations were carried out at the UB3LYP/6-31G(d,p) level of theory using the Gaussian 09 suite of programs. 15 Geometry optimizations were undertaken using default convergence limits and without symmetry constraints. No conformational search for the global minimum was attempted.
Electronic excitation energies for 1 [1] in a vacuum were obtained at the UB3LYP/6-31G(d,p) level using the timedependent DFT method 16 supplied in the Gaussian package.

Experimental part
General 1 H NMR spectra were obtained at 400 MHz ( 1 H) in CDCl 3 and referenced to the solvent unless stated otherwise. Thermal analysis was performed on a TA 2920 DSC using a typical heating rate of 5 K min À1 . Reactions were carried out under Ar, and subsequent manipulations were conducted in air. Details of magnetization and photoconductivity measurements were described previously. 3 Electronic absorption spectra UV-vis spectra for 1 [8]b were recorded in spectroscopic grade hexane at a concentration of 1-10 Â 10 À6 M. Extinction coefficients were obtained by tting the maximum absorbance at 260 or 261 nm against the concentration in agreement with Beer's law.
Visible spectra for neat 1 [10] placed between two glass slides were obtained at temperatures about 10 K above and then 10 K below the Col-I phase transition for each compound using a hotstage mounted in a UV spectrometer. For 1 [10]d the spectrum of the Col h phase was recorded in the middle of the phase range.