3-Substituted 6-oxoverdazyl bent-core nematic radicals: synthesis and characterization

Abstract

A series of bent-core derivatives of 6-oxoverdazyl 1[12] was synthesized and mesogenic properties were investigated in the pure form and in binary mixtures. Results demonstrate that the effectiveness of the C(3) substituent in nematic phase stabilization follows the order: COOMe > m-FC₆H₄ > Ph > thienyl > o-FC₆H₄, which is consistent with steric parameters established with DFT computational methods and opposite to the order of the appearance of a re-entrant isotropic phase. The effect of alkyl chain elongation on mesogenic properties and the effect of C(3) substituent on electronic absorption spectra are also investigated.

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Introduction

There is growing interest in liquid crystalline stable radicals as potential organic semiconductors and also materials for spintronics and fundamental studies of spin–spin interactions in organized media.^1–4 In this context, we have demonstrated 6-oxoverdazyl to be a suitable paramagnetic core element of mesogenic compounds and reported discotics bearing alkoxy^5,6 and alkylsulfanyl substituents.^7–9 These mesogenic radicals exhibit 3-dimensional columnar hexagonal phases up to 130 °C and substantial hole mobility of about 10⁻³ cm² V⁻¹ s⁻¹.^5,7 In search for new types of supramolecular architectures and hence types of intermolecular interactions, we turned to bent-core derivatives in which the 6-oxoverdazyl is the central angular paramagnetic element. We prepared series 1[n]a containing the CF₃ group at the C(3) head position and demonstrated rich polymorphism, photo-induced ambipolar charge transport, and linear reorientation of optical axis in electric field in the series.¹⁰

It has been demonstrated that chemical modifications of the head position in bent-core compounds affects their electronic properties and mesogenic behavior.^11,12 For this reason, we set out to investigate verdazyl derivatives containing a functional group at the C(3) position that can be modified chemically. We also envisioned a substituent with an extended π system for modification of the electronic structure of the heterocycle and intermolecular π–π interactions. Here we report a series of derivatives 1[12] containing carboxyl group (1[12]b) and also thienyl (1[12]c) and phenyl (1[12]d) rings (Fig. 1). We also investigate steric and polar effects of substitution of the phenyl ring with fluorine (1[12]e and 1[12]f), and alkyl chain elongation (1[16]c and 1[16]d) on phase properties. The new compounds are investigated in the pure form and as binary mixtures with 1[12]a, and results are rationalized with the help of DFT calculations.

Fig. 1 The structure of bent-core 6-oxoverdazyls 1[12].

Results and discussion

Synthesis

The preparation of bent-core derivatives 1[n] follows the Milcent protocol¹³ and general functional group interconversion methods¹⁴ for 6-oxoverdazyls (Scheme 1), as was reported earlier for 1[n]a.¹⁰ Thus, reactions of 4-substituted phenylhydrazine hydrochlorides 2 and 3 with appropriate aldehydes gave crude hydrazones 4b and 5c–5f, which were converted to carbamoyl chlorides 6b and 7c–7f, respectively, upon treatment with triphosgene in the presence of pyridine. After purification, the chlorides were reacted with appropriate arylhydrazine 2 or 3 in hot ethanol to give tetrazines 8b and 9c–9f, respectively. The crude tetrazines were subsequently oxidized to radicals 10b and 11c–11f, under PTC reaction conditions using K₃Fe(CN)₆ (method A) or NaIO₄ (method B). The resulting 6-oxoverdazyl radicals were converted into the corresponding phenols 12b–12f by catalytic debenzylation (benzyl ether 10b) or basic hydrolysis (benzoates 11c–11f). The diphenols were acylated with appropriate acid chlorides 13[n]^15,16 in the presence of DMAP to provide compounds 1[n] in yields of about 70%.

Scheme 1 Preparation of 6-oxoverdazyls 1[n]. Reagents and conditions: (i) RCHO or RCH(OH)₂, (ii) CO(OCCl₃)₂, pyridine, CH₂Cl₂, rt; (iii) hydrazine hydrochloride 2 or 3, Et₃N, EtOH, 60 °C; (iv) method A: K₃Fe(CN)₆, Na₂CO₃, [Et₄N]⁺Br⁻ (cat.), CH₂Cl₂–H₂O, rt; or method B: NaIO₄, [Et₄N]⁺Br⁻ (cat.), CH₂Cl₂–H₂O, rt; (v) for X = OCOPh : KOH (0.1 M in MeOH), CH₂Cl₂; for X = OBn : H₂ (3 atm), Pd/C, THF-EtOH; (vi) C_nH_2n+1OC₆H₄COOC₆H₄COCl (13[n]), DMAP, CH₂Cl₂, rt.

Thermal analysis

Analysis of series 1[12] by optical (POM) and thermal (DSC) methods revealed that all compounds melt above 130 °C and only three out of six derivatives form a mesophase (Table 1). Thus, methoxycarbonyl derivative 1[12]b and the previously reported CF₃ derivative 1[12]a exhibit an enantiotropic nematic phase, while the m-fluorophenyl 1[12]e forms a monotropic nematic phase (Fig. 2a). The overall stability of the nematic phase follows the order m-FC₆H₄ > COOMe > CF₃ and the re-entrant isotropic phase was observed only in the CF₃ derivative 1[12]a. Extension of the alkyl chain in the 3-thienyl and 3-phenyl derivatives did not induce mesogenic behavior in 1[16]c and 1[16]d, and had little effect on the melting point.

Table 1 Thermal properties of pure compounds and binary mixtures^a

Compound	Transition temperatures/°C	Virtual transition temperatures/°C^b
		[TNI]	[TNIre]
a Cr = crystal, N = nematic, I and Ire = isotropic.b Extrapolated from ∼10 mol% solutions in 1[12]a; [TNI] linear extrapolation, [TNIre] nonlinear (x^1/2) extrapolation. See the ESI for details.c Ref. 10.d Monotropic transition.e Peak transition temperatures.f Decomposition.g Not observed.
1[12]a^c	Cr 136.9 (Ire 121.6)^d N 152.3 I^e	—	—
1[12]b	Cr 139 N 161 I^f	231	^g
1[12]c	Cr 153 I	138	136
1[12]d	Cr 157 I	163	139
1[12]e	Cr 177 (N 169)^d I	194	110
1[12]f	Cr 155 I	82	144
1[16]c	Cr 149 I	91	150
1[16]d	Cr 160 I	102	140

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Fig. 2 Schlieren texture of nematic phase (a) for 1[12]e on slow cooling from the isotropic phase, and a sequence of photomicrographs taken upon cooling of a 8.5 mol% solution of 1[12]d in 1[12]a: nematic phase (b), re-entrant isotropic phase growing from the nematic phase (c), and crystalline phase growing from I_re (d).

In order to understand the broader impact of the C(3) substituent on mesogenic properties, all derivatives were investigated as low concentration additives (∼10 mol%) to 1[12]a. Results demonstrate that all mixtures exhibit an enantiotropic nematic phase (N) and all, except for 1[12]b–1[12]a, show a re-entrant isotropic phase (I_re), as shown for a 1[12]d–1[12]a mixture in Fig. 2 and 3. Linear extrapolation of the N → I and parabolic extrapolation^10,17 of the N → I_re transition temperatures gave virtual temperatures [T_NI] and [T_NIre], respectively, shown in Table 1. Analysis of the results indicates that the effectiveness of the C(3) substituent in nematic phase stabilization follows the order: COOMe > m-FC₆H₄ > Ph > thienyl > o-FC₆H₄. Not surprisingly, the virtual N–I_re transition temperatures follow the exactly opposite order, and substituents that most stabilize the nematic phase also most suppress the re-entrant isotropic phase.

Fig. 3 DSC trace for a 8.5 mol% solution of 1[12]d in 1[12]a.

Elongation of the alkyl chain significantly suppresses the virtual N–I transition, and the effect is stronger for the Ph derivative 1[16]d than for the thienyl analogue 1[16]c.

In an attempt to generate a wide temperature range nematic material with broad absorption in the visible range (vide infra), three-component mixtures of 3-thienyl, 3-phenyl, and 3-CF₃ derivatives were briefly investigated. Thus, addition of a nearly equimolar mixture of 1[12]c and 1[12]d to 1[12]a, as the nematic host, in a 1 : 4 ratio gave a material with nematic phase range of 131–150 °C and transition to I_re at 123 °C. Increasing of the content of the 3-aryl components to 1 : 1 ratio with 1[12]a modestly suppressed the melting point to 125 °C and significantly destabilized the nematic phase by 12 K. Also the re-entrant I_re phase was suppressed by 19 K in favour of the supercooled nematic phase. Further increase of the ratio to 2 : 1 gave a non-homogenous material with a high melting point of 142 °C and without an enantiotropic nematic phase.

Molecular modelling

For a better understanding of properties of derivatives 1[n], molecular and electronic structures of diphenols 12 set at the pseudo C₂ symmetry were investigated at the CAM-B3LYP/6-31G(2d,p) level of theory.

The resulting equilibrium structures, shown in Fig. 4 for 12b and 12c, are close to that obtained experimentally for 1,3,5-triphenyl-6-oxovedazyl.¹⁸ The 4-hydroxyphenyl rings are twisted off the co-planarity with the verdazyl ring and dihedral angles N(2)–N(1)–C_Ph–C_Ph and C(6)–N(1)–C_Ph–C_Ph are 33.9 ± 1.0° and 40.1 ± 0.9°, respectively. The substituent at the C(3) position also lacks coplanarity with the heterocycle, and the dihedral angle θ is smallest for the thienyl derivative 12c (3.3°, Table 2, Fig. 4) and the largest, as expected, for the o-FC₆H₄-derivative 12f (30.2°).

Fig. 4 Two views of equilibrium pseudo C₂ symmetry geometries for 12b (left) and 12c (right) obtained at the CAM-B3LYP/6-31G(2d,p) level theory.

Table 2 Calculated substituent-verdazyl dihedral angle θ and the wavelength λ_max of the lowest energy electronic excitation for series 12

12	a	b	c	d	e	f
a CAM-B3LYP/6-31G(2d,p) level of theory.b TD-B3LYP/6-31G(2d,p)//CAM-B3LYP/6-31G(2d,p) level of theory in CH2Cl2 dielectric medium.
θ/^a	—	11.0	3.3	8.2	6.3	30.2
λmax/nm^b	547	534	584	551	550	542

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A comparison of the results in Tables 1 and 2 demonstrates that the large twist angle θ of 30.2° calculated for 12f correlates with the exceptionally low [T_NI] (82 °C) and particularly high [T_NIre] values (144 °C) found for 1[12]f derivative (Table 1). On the other hand, a relatively low [T_NI] value observed for the thienyl derivative 1[12]c (138 °C) despite a high degree of co-planarity (θ = 3.3°), is presumably related to the large size of the sulphur atom. Conversely, very high [T_NI] value for derivative 1[12]b (231 °C, Table 1) is related to the presence of a small planar semi-flexible polar COOMe group at the C(3) position with a moderate θ angle (11°).

Electronic absorption spectra

All radicals 1[n] are intense red in solutions of typical organic solvents with the exception of thienyl derivatives 1[n]c, which are dark green. Analysis of 1[12]a in CH₂Cl₂ solutions demonstrated a broad, low intensity absorption band in the visible range with a maximum at 508 nm (Fig. 5). Replacement of the CF₃ group at the C(3) position with the thienyl in 1[12]c resulted in broadening of the band and a strong batochromic shift to λ_max = 598 nm. The m-FC₆H₄ derivative 1[12]e absorbs in the intermediate range of wavelengths and shows two distinguishable maxima at λ_max = 540 and 566 nm. Thus, absorption of all three compounds effectively covers the range of 450–650 nm.

Fig. 5 Low energy portion of electronic absorption spectra for 1[12]a (red), 1[12]c (green) and 1[12]e (black) in CH₂Cl₂.

Results of TD-DFT calculations are consistent with experimental data and show a single, major (f ∼ 0.09–0.19) low energy excitation in the visible range related mainly to the delocalized β-HOMO to the β-LUMO, concentrated on the verdazyl ring, transition (Fig. 6). Analysis of the theoretical values in Table 2 demonstrates that the wavelength of the lowest energy excitation follows the order: COOMe < o-FC₆H₄ < CF₃ ∼ m-FC₆H₄ ∼ Ph < thienyl. Interestingly, the coplanarity of the Ph ring with the verdazyl appears to be important for modifying the HOMO–LUMO gap, as evident from comparison of the C(3)-o-FC₆H₄ (12f) and C(3)-m-FC₆H₄ (12e) derivatives (Table 2).

Fig. 6 B3LYP/6-31G(2d,p)-derived contours of β-HOMO (left) and β-LUMO (right) orbitals for 12c and relevant to low energy excitations in 1[12]c.

EPR spectroscopy

EPR spectrum measured for dibenzyloxy derivative 10b (Fig. 7) exhibits 9 broad lines with an average hyperfine coupling constant a_N = 5.62 G, which is typical for this class of verdazyls⁷ and consistent with spin density distribution in the molecule, as shown for 12c in Fig. 7. The fundamental nine-line pattern is due to coupling to four quadrupolar ¹⁴N nuclei of the verdazyl system and the lines are broadened by minor coupling to hydrogen atoms of the attached aromatic rings.

Fig. 7 Left: EPR spectrum of dibenzyloxy 10b recorded in benzene. Right: CAM-B3LYP/6-31G(2d,p) calculated spin density in 12c (blue: alpha spin density, green: beta spin density).

Conclusions

Synthesis of several diphenols 12 and their acylation to form bent-core compounds 1[n] has been demonstrated, which opens up possibilities of incorporation of these angular structural elements to other, more complex molecular and supramolecular systems. Analysis of series of bent-core derivatives 1[12] revealed that some compounds form a rare nematic phase, but no ordered phases were observed even in higher homologues 1[16]. Results show that the carboxy group in 1b is a promising functionality for further manipulation with the supramolecular structure of the mesogens by esterification with more complex, including partially fluorinated alcohols. Investigation of series 1[n] as ∼10 mol% binary mixtures with nematogen 1[12]a demonstrated that the trend in virtual [T_NI] is consistent with steric properties of the C(3) substituent obtained with DFT methods. Spectroscopic analysis revealed that electronic absorption of a ternary mixture of compounds 1[12]a, 1[12]c, and 1[12]d covers most of the visible range, which makes it attractive for designing of materials for photovoltaics. Further investigation of derivatives of diphenols 12 and induction of banana phases is underway.

Computational details

Quantum-mechanical calculations were carried out using Gaussian 09 suite of programs.¹⁹ Geometry optimizations for unconstrained model compounds at the pseudo-C₂ symmetry were undertaken at the CAM-B3LYP/6-31G(2d,p) level of theory using tight convergence limits. Electronic excitation energies were obtained at the B3LYP/6-31G(2d,p)//CAM-B3LYP/6-31G(2d,p) level using time-dependent DFT calculations²⁰ supplied in the Gaussian package. Solvent effect on electronic excitations was included using the PCM model²¹ [keywords: SCRF(PCM, solvent = CH₂Cl₂)]. Equilibrium geometries and partial output data for TD-DFT calculations are provided in the ESI.†

Experimental section

General

Solvents and reagents were purchased and used as received without further purification. Products were purified by flash chromatography on silica gel (230–400 mesh, Merck or Fluka). NMR spectra were recorded with Bruker AVIII 600 instrument. Chemical shifts are reported relative to solvent residual peaks (¹H NMR, δ = 7.26 ppm [CDCl₃], δ = 2.50 ppm [DMSO-d₆]; ¹³C NMR, δ = 77.00 ppm [CDCl₃], δ = 39.52 ppm [DMSO-d₆]). Substitution patterns of the carbon atoms were determined with 2D NMR spectroscopy (COSY, HMQC) and are indicated as ¹³C NMR peak multiplicity. IR spectra were measured in KBr pellets or thin films. Mass spectrometry was performed with a Finnigan MAT-95 or a Varian 500-MS LC Ion Trap instrument. Melting points were determined in capillaries and are uncorrected. UV-vis spectra were recorded in spectroscopic grade CH₂Cl₂ at concentrations of 1–20 × 10⁻⁶ and fitted to the Beer–Lambert law. If not stated otherwise, reactions were carried out under argon in a flame-dried flask with addition of the reactants by using syringe; subsequent manipulations were conducted in air.

Synthesis and characterization of diphenol radicals 12 and other precursors and intermediates are provided in the ESI.†

Preparation of binary mixtures

A mixture of known amounts of additive 1[n] (∼1 μmol) and the trifluoromethyl derivative 1[12]a (∼10 μmol) was placed in a small vial and 1,2-dichloroethane (0.1 mL) was added. The mixture was heated and stirred with a spatula at 50 °C (hot stage) to give a homogenous solution. The solvent was removed under stirring at about 80 °C and the resulting mixture was analyzed by POM to confirm homogeneity. Typically, each mixture was analyzed by DSC in 3 heating–cooling cycles and reproducible results (within 0.5 K) were averaged. The resulting transition peak temperatures were used to obtain virtual transition temperatures by linear ([T_NI] = 152.3 + ax_i) or non-linear ([T_NIre] = 121.6 + ax_i^1/2) extrapolation to the pure compound (x_i = 1), as demonstrated elsewhere for similar mixtures.¹⁷

Synthesis of bent-core verdazyls 1[n]

To a mixture of diphenol radical 12 (0.20 mmol) and appropriate acid chloride 13[n] (0.60 mmol) in 8 mL of dry CH₂Cl₂, solid DMAP (98 mg, 0.80 mmol) was added in one portion. The mixture was stirred at room temperature until the starting material and the by-product mono-acylated derivative were fully consumed (typically 2–5 min). The mixture was quenched with H₂O, diluted with CH₂Cl₂ and the organic layer was washed with 2% HCl (10 mL), 5% aq. NaHCO₃ (10 mL) and with H₂O (3 × 25 mL). The organics were dried over MgSO₄, filtered, and the solvents were removed under reduced pressure. If not stated otherwise, the resulting crude product was flash chromatographed (SiO₂, CH₂Cl₂/EtOAc 95 : 5), and recrystallized 4–6 times from the CH₂Cl₂/EtOAc or CH₂Cl₂/MeCN mixtures.

1,5-bis{4-[4-(4-Dodecyloxybenzoyloxy)benzoyloxy]phenyl}-3-trifluoromethyl-6-oxoverdazyl (1[12]a). The synthesis and properties were described previously.¹⁰ UV-vis (CH₂Cl₂) λ_max (log ε) 269 (4.86), 311 (4.20), 341 (3.87), 508 (3.68).

1,5-bis{4-[4-(4-Dodecyloxybenzoyloxy)benzoyloxy]phenyl}-3-methoxycarbonyl-6-oxoverdazyl (1[12]b). Crude product was washed with several portions of cold methanol, then two times chromatographed (SiO₂, CH₂Cl₂/EtOAc 60 : 1 gradient to 30 : 1) and three times recrystallized from EtOAc/MeCN mixture to give red crystals (155 mg, 67%); IR (KBr) ν 1735 (C=O), 1605, 1510, 1255, 1200, 1160, 1060 cm⁻¹. Anal. calcd for C₆₈H₇₇N₄O₁₃ (1157.5): C 70.51, H 6.70; found: C 70.58, H 6.94.

1,5-bis{4-[4-(4-Dodecyloxybenzoyloxy)benzoyloxy]phenyl}-6-oxo-3-(thien-2-yl)verdazyl (1[12]c). Green solid (165 mg, 70%), mp 153 °C; IR (KBr) ν 1735 (C=O), 1690, 1605, 1505, 1260, 1200, 1160, 1065 cm⁻¹; UV-vis (CH₂Cl₂) λ_max (log ε) 272.5 (4.92), 335 (4.18), 598 (3.42); MALDI-MS (m/z): 1205.5 (100, [M + Na]⁺), 1182.5 (15, [M + H]⁺), 1153.5 (47, [M − CO]⁺). Anal. calcd for C₇₀H₇₇N₄O₁₁S (1181.5): C 71.10, H 6.56, N 4.74, S 2.71. Found: C 70.89, H 6.81, N 4.65, S 2.67.

1,5-bis{4-[4-(4-Hexadecyloxybenzoyloxy)benzoyloxy]phenyl}-6-oxo-3-(thien-2-yl)verdazyl (1[16]c). Green solid (163 mg, 63%), mp 149 °C; MALDI-MS (m/z) 1316.6 (32, [M + Na]⁺), 1294.7 (19, [M + H]⁺), 1265.6 (47, [M − CO]⁺). Anal. calcd for C₇₈H₉₃N₄O₁₁S (1293.7): C 72.36, H 7.24, N 4.33, S 2.48. Found: C 72.38, H 7.26, N 4.35, S 2.45.

1,5-bis{4-[4-(4-Dodecyloxybenzoyloxy)benzoyloxy]phenyl}-6-oxo-3-phenylverdazyl (1[12]d). Crude product was flash chromatographed through a silica gel pad (CH₂Cl₂ gradient to CH₂Cl₂/EtOAc 20 : 1). Analytically pure sample was obtained after column chromatography (CH₂Cl₂/EtOAc 50 : 1) as a light red solid (181 mg, 77%); mp 157 °C; IR (KBr) ν 1670 (C=O), 1600, 1515, 1225 cm⁻¹. Anal. calcd for for C₇₂H₇₉N₄O₁₁ (1175.6): C 73.51, H 6.77, N 4.76; found: C 73.24, H 6.70, N 4.73.

1,5-bis{4-[4-(4-Dodecyloxybenzoyloxy)benzoyloxy]phenyl}-3-(3-fluorophenyl)-6-oxoverdazyl (1[12]e). Crude product was flash chromatographed (SiO₂, CH₂Cl₂/EtOAc 50 : 1) and then purified by column chromatography (SiO₂, CH₂Cl₂). Pure sample was obtained after recrystallization from a CH₂Cl₂/MeOH mixture, followed by two recrystallizations from a CH₂Cl₂/MeCN mixture to give violet solid (189 mg, 79%); UV-vis (CH₂Cl₂) λ_max (log ε) 266 (4.90), 327 (4.21), 540 (3.52), 564 (3.53); IR (KBr) ν 1740 (C=O), 1690, 1605, 1505, 1260, 1200, 1160, 1070 cm⁻¹; MALDI-MS (m/z) 1194.3 (22, [M + H]⁺), 1165.3 (100, [M − CO]⁺). Anal. calcd for C₇₂H₇₈FN₄O₁₁ (1193.6): C 72.40, H 6.58. Found: C 72.23, H 6.50.

1,5-bis{4-[4-(4-Dodecyloxybenzoyloxy)benzoyloxy]phenyl}-3-(2-fluorophenyl)-6-oxoverdazyl (1[12]f). Crude product was chromatographed twice (SiO₂; first CH₂Cl₂/petroleum ether 10 : 1, then CH₂Cl₂/EtOAc 100 : 1) and recrystallized four times from a CH₂Cl₂/MeCN mixture to give a light violet solid (200 mg, 84%), mp 155 °C; IR (KBr) ν 1740 (C=O), 1690 (C=O), 1605, 1505, 1260, 1200, 1160, 1065 cm⁻¹. Anal. calcd for C₇₂H₇₈FN₄O₁₁ (1193.6): C 72.40, H 6.58, N 4.69. Found: C 72.39, H 6.33, N 4.64.

Acknowledgements

This work was supported by National Science Center (2013/09/B/ST5/01230) and National Science Foundation (CHE-1214104) grants. M.J. also thanks The University of Łódź Foundation.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Complete synthetic details for intermediates 4–12, mixture analysis, partial output data for TD-DFT calculations, and archive of optimized equilibrium geometries for 12. See DOI: 10.1039/c5ra04119h

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