Induction of smectic polymorphism in bent-core derivatives of the 6-oxoverdazyl by partial fluorination of alkyl chains

With the increasing number and variety of all-organic paramagnetic liquid crystals the focus is shiing towards optimization of their properties for specic applications. Materials based on p-delocalized radicals are of particular interest in the context of molecular electronics and spintronics due to their photophysical and electrochemical properties. In this context we have been working with 6-oxoverdazyl and demonstrated that it is a suitable paramagnetic structural element for disc-like and bent-core, such as 1[n] (Fig. 1), liquid crystalline materials. The latter compounds exhibit ambipolar photoconductivity, electro-optical effects, a novel Tet3D phase, and their electronic absorption covers the visible region of the spectrum depending on the substituent at the C(3) position. The bent-core mesogens 1[n] typically exhibit a nematic phase. For electronic applications, however, more organized lamellar phases with close intermolecular contacts are necessary. As the rst step towards this goal we have focused on uorofobic interactions to induce nanosegregation. In this context we have investigated a series of mesogens 2[m,n] bearing partially uorinated alkyl chains (Fig. 1). Here we discuss the synthesis, thermal and optical investigation of series 2[m,n], and analysis of mesophases by powder XRD and EPR methods.


Introduction
With the increasing number and variety of all-organic paramagnetic liquid crystals 1 the focus is shiing towards optimization of their properties for specic applications.Materials based on p-delocalized radicals are of particular interest in the context of molecular electronics and spintronics 2 due to their photophysical and electrochemical properties. 3In this context we have been working with 6-oxoverdazyl and demonstrated that it is a suitable paramagnetic structural element for disc-like [4][5][6][7][8][9] and bent-core, such as 1[n] (Fig. 1), 10,11 liquid crystalline materials.The latter compounds exhibit ambipolar photoconductivity, electro-optical effects, a novel Tet 3D phase, 10 and their electronic absorption covers the visible region of the spectrum depending on the substituent at the C(3) position. 11he bent-core mesogens 1[n] typically exhibit a nematic phase.For electronic applications, however, more organized lamellar phases with close intermolecular contacts are necessary.As the rst step towards this goal we have focused on uorofobic 12 interactions to induce nanosegregation. 13In this context we have investigated a series of mesogens 2[m,n] bearing partially uorinated alkyl chains (Fig. 1).
Here we discuss the synthesis, thermal and optical investigation of series 2[m,n], and analysis of mesophases by powder XRD and EPR methods.The diphenol precursors 3 were obtained using the general Milcent protocol 14 following functional group transformations, 15 as described recently. 11The required benzoyl chlorides 4[m,n] were prepared from the corresponding 4alkoxybenzoic acids 16 and SOCl 2 .

Thermal analysis
Optical microscopy and DSC analysis revealed that all derivatives in series 2[m,n] exhibit liquid crystalline behavior, with the exception of 2 [6,4]a (Table 1).Thus, all mesogens form a SmA phase, while in derivatives 2[m,n]a (X ¼ CF 3 ) also a monotropic SmC phase was detected by optical microscopy (Fig. 2 and 3) and conrmed by powder XRD (vide infra).This is in sharp contrast with behavior found in the non-uorinated series 1[n]: only the shortest member of the 1[n]a series, derivative 1 [8]a, Fig. 1 The structures of bent-core 6- oxoverdazyls 1[n] and 2[m,n].exhibits a monotropic SmA phase, 10 while other derivatives show nematic and tetragonal phases or no mesogenic behavior at all (1[n]b and 1[n]c). 10,11Thus, it appears that removal of the -C 6 H 4 COO-group and partial uorination of the alkoxy chain in 1[n] is an effective tool for induction of a broad-range mesogenic behavior in 6-oxoverdazyls 2[m,n] irrespectively on the nature of substituent X at the C(3) position.For example, while the thien-2-yl derivative 1 [12]b melts at 153 C and has a virtual N-I transition ([T NI ]) at 138 C, 11 the uorocarbon analogue 2 [6,6]b exhibits a SmA phase between 107 C and 197 C (Table 1).
Analysis of data in Table 1 demonstrates that the mesophase stability strongly depends on the size of the semirigid peruoroalkane segment.For instance, increasing the number of the CF 2 units from 6 to 8 induces smectic behavior in 2 [8,4]  a, and additional two CF 2 units increase SmA and SmC phase stability by about 30 K in 2 [10,4]a with a modest increase of melting temperature.Similar behavior is observed in the pair 2 [6,6]a and 2 [8,6]a in which elongation of the uorocarbon by two units results in stabilization of the SmA phase by nearly 60 K.
Data in Table 1 also permits analysis of the effect of the degree of alkyl chain uorination on transition temperatures.Thus, increasing the number of uorine atoms or changing the F/H ratio in the C 12 chain from 1 : 1 in 2 [6,6]a to 2 : 1 in 2 [8,4]a increases the SmA-I transition temperature by 59 K.In another pair with the C 14 tail (2 [8,6]a and 2 [10,4]a), changing the F/H ratio from 4 : 3 to 5 : 2 increases SmA phase stability by 34 K.
Finally a comparison of derivatives in series 2 [6,6] demonstrates that the effectiveness of the C(3)-X substituent in stabilization of the SmA follows the order CF 3 < C 6 H 5 < C 6 H 4 F-m < 2-thienyl, which is different from that established for the virtual N-I transition temperature in series 1 [12]: 11 2-thienyl < CF 3 < C 6 H 5 < C 6 H 4 F-m.This stark difference in the position of the 2-thienyl substituent in the two series suggests that other molecular factors, such as the p-p interactions, are important in the stabilization of the nematic and smectic phases.

Powder XRD analysis
XRD analysis conrmed the presence of a SmA phase in all mesogenic compounds in series 2[m,n] and a SmC phase in derivatives 2[m,n]a (see the ESI, † and X-ray diffractogram for 2 [8,4]a in Fig. 4).All compounds exhibit a typical layer thickness dependence on temperature: it increases in SmA phase and then decreases in the SmC phase with decreasing temperature.Analysis of the thermal expansion coefficient k, obtained by tting the datapoints to a quadratic function d 01 ¼ kT 2 + b, demonstrates that the layer thickness change depends on the substituent at the C(3) position: it is similar for the aryl groups and twice larger for   the CF 3 group and follows the order: 2-thienyl < C 6 H 5 $ C 6 H 4 F-m < CF 3 (Table 2).This indicates that derivatives 2[m,n]a undergo the most substantial molecular reorganization in the SmA phase presumably due to the bulk of the CF 3 substituent.Conversely, least reorganization in the SmA phase is observed in the thienyl derivative 2 [6,6]b, which coincides with its highest SmA-I transition temperature in the series (Table 1).Data in Table 2 also suggests that the higher degree of uorination in the alkyl tail (longer R f component or higher F/H ratio) the stronger temperature effect on the layer thickness.
A comparison of the layer thickness with DFT-calculated molecular size indicates a partial (up to 20%) interdigitation of the molecules in the SmA phase as shown for 2 [6,6]b in Fig. 5.
The typical lack of intralayer molecular correlation in the observed SmA and SmC phases is consistent with no photoconductivity observed in a sample of 2 [8,4]a.

EPR analysis
In contrast to standard SQUID magnetometry, EPR spectroscopy provides information on local spin-spin interactions in a greater temperature range and is suitable for studying of high temperature paramagnetic liquid crystals, 17 such as series 2[m,n].Thus, analysis of 2 [8,6]a demonstrated a monotonic broadening of the single line EPR spectrum with decreasing temperature resulting from increasing spin-spin exchange interactions in the neat isotropic phase (Fig. 6).At the I-SmA transition increased molecular organization in the lamellar structure results in a subtle increase of the line width (DH pp ).Maximum line width is observed for the SmA phase just before the SmA-SmC transition at 116 C at which the phase has the highest order.In the SmC phase the line width, DH pp , decreases due to progressing molecular tilt and changing spin-spin exchange interactions.At 96 C the monotropic SmC phase crystallizes abruptly changing the strength of spin-spin interactions between the paramagnetic centres in the rigid crystalline phase.Thus, the variable temperature EPR spectroscopy offers a convenient tool for analysis of molecular organization in uid phases. 17

Conclusions
Partial uorination of the alkyl chains in bent-core derivatives of 6-oxoverdazyl 1[n] induced SmA and SmC behavior in series 2[m,n]; unfortunately, more organized lamellar phases desired for electronic applications were not observed.The mesophase stability in the series depends on the degree of alkyl chain    uorination, and increases with increasing size of the per-uoroalkyl segment.
Variable temperature EPR was demonstrated to be a useful tool in studying of local spin-spin interactions as a function of the phase structure.The search for banana phases in the 6oxoverdazyl derivatives with intralayer molecular correlation continues.

Computational details
Quantum-mechanical calculations were carried out using Gaussian 09 suite of programs. 18Geometry optimizations for unconstrained molecules at the pseudo-C 2 symmetry were undertaken at the B3LYP/6-31G* level of theory using default convergence limits.

General
Solvents and reagents were purchased and used as received without further purication.Products were puried by ash chromatography on silica gel (230-400 mesh, Merck or Fluka).IR spectra were measured in KBr pellets with a FT-IR NEXUS spectrometer.Mass spectrometry was performed with a Finnigan MAT-95 or a Varian 500-MS LC Ion Trap instrument.Thermal analysis was performed with differential scanning calorimeter DSC-1 Mettler Toledo and onset temperatures are reported in Table 1.
X-ray diffraction experiments in the wide-angle range were performed with Bruker D8 GADDS system, and Bruker Nanostar system was used for small angle diffraction measurements.
Variable temperature EPR spectra for neat radical 2 [8,6]a were recorded using an X-band Bruker spectrometer every 2-3 K on cooling allowing for 2 min stabilization.The line width was measured as a difference in position of the maximum and minimum of the EPR signal.
If not stated otherwise, reactions were carried out under argon in a ame-dried ask with addition of the reactants via syringe; subsequent manipulations were conducted in air.
The synthesis of diphenols 3 was reported recently.(5 mL) containing DMAP (84 mg, 0.69 mmol) was stirred at room temperature until the diphenol and the intermediate mono-acylated derivative were fully consumed (about 20-30 min).The mixture was quenched with H 2 O, and the products extracted with CH 2 Cl 2 (3 Â 10 mL).The organic layers were dried (MgSO 4 ), and the solvents were removed under reduced pressure.The resulting crude product was puried by chromatography followed by repeated recrystallization.Reported yields refer to analytically pure samples.

Fig. 4 X
Fig. 4 X-ray diffractogram for SmA and SmC phases of 2[8,4]a at 140 C and 100 C, respectively, obtained by integration of the 2D patterns.The insert shows temperature dependence of the d 01 signal.

Fig. 6
Fig.6EPR line width as a function of temperature recorded for neat 2[8,6]a on cooling.

Table 1
Thermal properties of 2[m,n] a

Table 2
Layer spacing d 01 , molecular length L, degree of interdigitation I, and thermal expansion coefficient k a Layer spacing measured at T ¼ T AI À 10 K. b The length of the molecule measured as the F/F distance.c Degree of interdigitation I ¼ (L À d 01 )/ L. d Thermal expansion coefficient for the SmA phase from the tting the kT 2 + b function.See the ESI.e Layer spacing measured at T ¼ T AC À 6 K.