ABSTRACT

Photochromism is the reversible structural change of the molecules upon light irradiation having different spectral pattern before and after irradiation. Liquid crystal (LC) is a long-range orientational order of the molecules in between solid and isotropic liquid phases which share the properties of the crystal and liquid phases. 1-Alkyl-2-(arylazo)imidazoles, (Raai-C_nH_2n+1, R = H, Me and n = 10, 12, 14, 16, 18, 20, 22) exhibits photochromism and two of them (Haai-C₁₈H₃₇ and Haai-C₂₂H₄₅) show Liquid crystal (LC) properties. One of these compounds, Meaai-C₁₆H₃₃ is structurally characterized by single crystal X-ray diffraction study. The compounds upon irradiation with UV light show E-to-Z isomerisation. Quantum yields (f_E®Z) of E-to-Z isomerisation vary with the molar mass of the compounds. The reverse transformation, Z-to-E, is recorded in thermal condition. The activation energy (E_a) of isomerisation is determined by the controlled temperature reaction. Liquid crystal property of the molecules is examined by polarizing optical microscopy (POM) and DSC. The carbon chain length n = 18 shows nematic (N)-isotropic (I) phase transition and an unknown Smectic (Sm) phase is observed with n = 22 on heating cycle.

1. Introduction

Smart molecules can respond to a specific external stimulus in a specific manner and are of increasing interest in areas as diverse as microbiology [1], microfluidics [2], organic electronics [3,4], high density optical data storage devices [5,6] etc. Using of light as external stimulating agent has manifold advantages. Fritzsche (1867) observed the bleaching of an orange colored tetracene solution in daylight and then re-colored at night [7]. However, during the 1950s, the research groups of Hirshberg and Fischer carried out comprehensive studies on light irradiated color changes and the subsequent physical and chemical changes of different organic molecules [8]. The research field has received a renewable thrust in the 1980s when the photo-induced reversible transformation was observed for organic molecules like spirooxazines, diarylethenes etc. without photodegradation. This process is known as photochromism. It is defined as “reversible transformation of a chemical species induced in one or both directions by the absorption of electromagnetic radiation between two forms having different absorption spectra” [9]. The photochromic compounds have become appealing for their potential advantages in the construction of photo-switching devices and supramolecular architectures [9-15]. Various organic compounds show cis(Z)/trans(E) isomerization, pericyclic reactions, H atom transfer reactions, and dissociation processes. The molecules have been used as photoresponsive functional devices utilized as smart polymers [16], liquid crystals [17], molecular switches [18] and machines [19].

Azobenzene is used for a wide range of applications, from dye indicators and nonlinear optical chromophores to quantum interferences [20]. The molecule shows photoinduced E/Z isomerization by UV or visible light. More recently the synthesis and the characterization of heterocyclic azo dyes such as arylazopyridines have been extensively reported [21-24]. The prospect of azobenzene has encouraged us for synthesizing arylazoimidazoles [25] which acts as a potential switching group in biology and in coordination chemistry, since imidazole is a ubiquitous and essential group in biology, especially as a metal coordinating. The photochromism of 1-alkyl-2- (arylazo)imidazole (RaaiR) [26,27] and some of their complexes [28-31] are also reported.

The liquid crystal (LC) is a long-range orientational order and lies in between solid and isotropic liquid phases. The LC phase (mesophase) shares properties of the crystal and liquid phases. Some examples of anisotropic properties are birefringence, alignment in electric and magnetic fields (electrical permittivity and magnetic susceptibility), elasticity, viscosity, and conductivity [32,33]. Liquid crystals are widely used in information display technology and the anisotropic fluid states of rigid polymers are used for processing of high strength fibers. Liquid crystals (LCs) should also be useful in bio-related fields because the self-organized structures of lyotropic liquid crystals are compatible with those in living systems [34].

Comparing with azobenzene photochrome, arylazoimidazoles have several advantages of possessing both the photoactivity related to the azo unit, the capability of self-assembly through the imidazolyl group and coordinating ability to metal ions. Azoimidazoles display surprisingly a strong tendency toward self-assembly, which offers a new and efficient way to make photoactive liquid crystal (LC) materials. Attachment of long chain alkyl groups to aromatic molecules may develop liquid crystal (LC) property. The E form of azo derivatives is a rod and is nematic LCs (NLCs) while the Z is bent and tends to destabilize the phase structure. Therefore, E/Z photoisomerization of the azobenzene in the N phase can disorganize the phase structure of NLCs, resulting in a nematic-isotropic (N-I) isothermal phase transition (photochemical phase transition). In this article, we describe a preliminary account on photochromic LC (PLC) activity of some long chain alkyl group substituted 1-alkyl-2- (arylazo)imidazoles, Raai-C_nH_2n+1 (n = 10, 12, 14, 16, 18, 20, 22).

2. Experimental

2.1. Materials

1-Alkyl-2-(arylazo)imidazoles were synthesized by reported procedure [27]. 1-Bromo-n-alkanes, n-C₁₀H₂₁-1- Br, n-C₁₂H₂₅-1-Br, C₁₄H₂₉-1-Br and 1-chloro-n-alkanes, C₁₆H₃₃-1-Cl, C₁₈H₃₇-1-Cl, C₂₀H₄₁-1-Cl and C₂₂H₄₅-1-Cl were purchased from Sigma-Aldrich and used as such. All other chemicals and solvents were analytical reagent grade as received.

2.2. Physical Measurements

Microanalytical data (C, H, N) were collected on PerkinElmer 2400 CHNS/O elemental analyzer. Spectroscopic data were obtained using the following instruments: UV-Vis spectra from a Perkin Elmer Lambda 25 spectrophotometer; IR spectra (KBr disk, 4000 - 400 cm⁻¹) from a Perkin Elmer RX-1 FTIR spectrophotometer; photo excitation has been carried out using a Perkin Elmer LS-55 spectrofluorimeter and ¹H NMR spectra were recorded from a Bruker (AC) 300 MHz FTNMR spectrometer. The liquid crystalline properties were established by thermal microscopy (Nikon polarizing microscope LV100POL attached with Linkam Temperature controlled stage model Examina-THMS600, and the phase transitions were confirmed by differential scanning calorimetry (Perkin-Elmer Diamond DSC Pyris1 system).

2.3. Synthesis of Compounds

2-(Arylazo)imidazoles were synthesized by the diazotization of aniline or p-toluidine in NaNO₂/HCl at low temperature (0˚C - 5˚C) and followed by coupling with imidazole in aqueous sodium carbonate solution (pH-7). The alkylation was carried out by adding alkyl halide in dry THF solution to the corresponding 1-alkyl-2-(arylazo) imidazole in presence of NaH (Scheme 1). Long chain alkyl groups are appended at imidazole motif to synthesise present series of molecules. The compounds are 1-alkyl-2-(arylazo)imidazole [Raai-C_nH_2n+1, where R = H (a), Me (b); C_nH_2n+1—where n = 10 (C₁₀H₂₁, 1), 12 (C₁₂H₂₅, 2), 14 (C₁₄H₂₉, 3), 16 (C₁₆H₃₃, 4), 18 (C₁₈H₃₇, 5), 20 (C₂₀H₄₁, 6), 22 (C₂₂H₄₅, 7)]. Detail synthesis for Meaai-C₁₀H₂₁ (1b) is given below.

To dry THF solution (75 ml) of 2-(p-tolylazo)imidazole (2 g, 10.8 mmol), NaH (50% paraffin) (0.56 g) was added in small portion and stirred at cold condition on ice bath for 0.5 h. 1-Bromodecane (n-C₁₀H₂₁-1-Br; 2.50 g, 11.31 mmol) in dry THF (25 ml) was added slowly through pressure equalizing system under stirring condition for a period of 1 h and then warm for another 1 h on steam bath. The solution was evaporated to dryness, extracted with CH₂Cl₂, washed with NaOH solution (10%, 10 ml × 3) and finally by distilled water (20 ml × 3). The CH₂Cl₂ extract was chromatographed over silica gelcolumn (45 × 1 ml) prepared in petroleum ether (40˚C - 60˚C). The elution was performed by ethylacetate-pe-

troleum ether (1:3 v/v) mixture and evaporated under reduced pressure. An orange crystalline product was obtained and then dried over P₄O₁₀ under vacuum. Yield of the product was 2.28 g (65%).

All other compounds were prepared and purified by identical procedure and the yield was varied 55% - 70%. The microanalytical data of the compounds are as follows: Haai-C₁₀H₂₁ (1a), 54˚C ± 1˚C, Anal. Found: C, 73.00; H, 9.00; N, 18.00 Calc. for C₁₉H₂₈N₄: C, 73.08; H, 8.97; N, 17.95. Meaai-C₁₀H₂₁ (1b), 53˚C ± 1˚C, Anal. Found: C, 73.70; H, 9.13; N, 17.23 Calc. for C₂₀H₃₀N₄: C, 73.62; H, 9.20; N, 17.18. Haai-C₁₂H₂₅ (2a), 54˚C ± 1˚C, Anal. Found: C, 74.20; H, 9.36; N, 16.40. Calc. for C₂₁H₃₂N₄: C, 74.12; H, 9.41; N, 16.47. Meaai-C₁₂H₂₅ (2b), 52˚C ± 1˚C, Anal. Found: C, 74.64; H, 9.55; N, 15.90 Calc. for C₂₂H₃₄N₄: C, 74.58; H, 9.60; N, 15.82. Haai-C₁₄H₂₉ (3a), 54˚C ± 1˚C, Anal. Found: C, 74.94; H, 9.84; N, 15.30 Calc. for C₂₃H₃₆N₄: C, 75.00; H, 9.78; N, 15.22. Meaai-C₁₄H₂₉ (3b), 53˚C ± 1˚C Anal. Found: C, 75.47; H, 9.92; N, 14.72 Calc. for C₂₄H₃₈N₄: C, 75.39; H, 9.95; N, 14.66. Haai-C₁₆H₃₃ (4a), 53 ± 1˚C, Anal. Found: C, 75.70; H, 10.05; N, 14.20 Calc. for C₂₅H₄₀N₄: C, 75.76; H, 10.10; N, 14.14. Meaai-C₁₆H₃₃ (4b)_, 54˚C ± 1˚C, Anal. Found: C, 76.12; H, 10.20; N, 13.60 Calc. for C₂₆H₄₂N₄: C, 76.09; H, 10.24; N, 13.66. Haai-C₁₈H₃₇ (5a), 59˚C ± 1˚C, Anal. Found: C, 76.50; H, 10.35; N, 13.30. Calc. for C₂₇H₄₄N₄: C, 76.42; H, 10.38; N, 13.21. MeaaiC₁₈H₃₇ (5b), 56˚C ± 1˚C, Anal. Found: C, 76.80; H, 10.55; N, 12.72 Calc. for C₂₈H₄₆N₄: C, 76.71; H, 10.50; N, 12.79. Haai-C₂₀H₄₁ (6a), 65˚C ± 1˚C, Anal. Found: C, 76.94; H, 10.66; N, 12.30 Calc. for C₂₉H₄₈N₄: C, 76.99; H, 10.62; N, 12.39. Meaai-C₂₀H₄₁ (6b), 63˚C ± 1˚C, Anal. Found: C, 77.31; H, 10.76; N, 12.06 Calc. for C₃₀H₅₀N₄: C, 77.25; H, 10.73; N, 12.02. Haai-C₂₂H₄₅ (7a), 65˚C ± 1˚C, Anal. Found: C, 77.60; H, 10.86; N, 11.70 Calc. for C₃₁H₅₂N₄: C, 77.51; H, 10.83; N, 11.67. Meaai-C₂₂H₄₅ (7b), 63˚C ± 1˚C, Anal. Found: C, 77.80 H, 10.88; N, 11.30 Calc. for C₃₂H₅₄N₄: C, 77.73; H, 10.93; N, 11.34%.

2.4. X-Ray Diffraction Study

The crystallographic data are shown in Table 1. Suitable single crystal of Meaai-C₁₆H₃₃ (4b) was mounted on a Siemens CCD diffractometer equipped with graphite monochromated Mo-K_a (l = 0.71073 Å) radiation. The unit cell parameters and crystal-orientation matrices were determined for two complexes by least squares refinements of all reflections. The intensity data were corrected for Lorentz and polarisation effects and an empirical absorption correction were also employed using the SAINT program. Data were collected applying the condition I > 2s(I). All these structures were solved by direct methods and followed by successive Fourier and difference Fourier syntheses. Full matrix least squares refinements on F² were carried out using SHELXL-97 with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were constrained to ride on the respective carbon or nitrogen atoms with an isotropic displacement parameters equal to 1.2 times the equivalent isotropic displacement of their parent atom in all cases. Complex neutral atom scattering factors were used throughout for all cases. All calculations were carried out using SHELXS 97 [35], SHELXL97 [36] and PLATON 99 [37].

2.5. Computational Study

The direction of the dipole of the compound 5a was calculated by Density Functional Theory (DFT), which may determine the presence of dipolar interaction within the molecules. All calculations have been performed by the Chem3D (version 10) [38] with GAUSSIAN 03 Interface [39]. The first optimization was done by the MM2 method, and finally DFT was performed as stated earlier.

3. Results and Discussion

3.1. Synthesis and Formulation

1-Alkyl-2-(arylazo)imidazole (Raai-C_nH_2n+1) are synthesized by controlled alkylation of 2-(arylazo)imidazole (RaaiH) by adding alkyl halide (C_nH_2n+1X) in dry THF using NaH. RaaiH are synthesized by diazo reaction of aryldiazonium salt with imidazole at pH 7 (Scheme 2). Alkyl halides of varying carbon chain (C₁₀ to C₂₂) are used to synthesize Raai-C_nH_2n+1 (n = 10 (C₁₀H₂₁, 1), 12 (C₁₂H₂₅, 2), 14 (C₁₄H₂₉, 3), 16 (C₁₆H₃₃, 4), 18 (C₁₈H₃₇, 5), 20 (C₂₀H₄₁, 6), 22 (C₂₂H₄₅, 7)). The compounds are purified by chromatographic separation in silica gel column using acetonitrile-benzene (8:2, v/v). The elemental composition is supported by microanalytical data and they are low melting solid (vide Experimental section).

3.2. Spectral Studies

A moderately intense stretching for n(C=N) is observed at 1585 - 1615 and n(N=N) at 1370 - 1374 cm⁻¹ (Table 2).

C_nH_2n+1 n = 10 (C₁₀H₂₁, 1), 12 (C₁₂H₂₅, 2), 14 (C₁₄H₂₉, 3), 16 (C₁₆H₃₃, 4), 18 (C₁₈H₃₇, 5), 20 (C₂₀H₄₁, 6), 22 (C₂₂H₄₅, 7)

The stretching frequency of –N=N- (exocylic) are insignificantly affected on comparing with –C=N- (imidazolyl) group. This may support effect of alkylation at N(1)-imidazolyl position of the compounds [27].

The absorption spectra of the compounds were recorded in MeOH solution in the wavelength range 200 - 900 nm (Figure 1, Table 2). The absorption spectra of Raai-C_nH_2n+1 show main absorption band at 355 - 375 nm with a molar absorption coefficient in the order of 10⁴

M⁻¹∙cm² and a weak band at 260 - 280 nm and 450 - 455 nm. The intense absorption band corresponds to p-p^* transitions, while the tail portion is due to n-p^* transition. With increasing chain length the absorption maxima have been shifted to longer wavelength which supports the enhancement of inductive effect of substituent with increase in number of bonded carbon centres at N(1)-imidazolyl group. The –N=Nis a part of a conjugated system of double bonds, both the n→π^* and the π→π^* bands are shifted to longer wavelength [40].

The ¹H NMR spectra of Raai-C_nH_2n+1 (1-7) are recorded in CDCl₃. The atom numbering is shown in Scheme 2. The spectral data are set out in Table 3. The proton signals are assigned on the basis of spin-spin interaction and effect of substitution. The alkylation of imidazole is supported by the disappearance of d(N–H) at ~10.30 ppm and the appearance of 1-alkyl signal at 0.85 - 4.40 ppm for -N–CH₂–(CH₂)n–CH₃. A triplet signal for -N–CH₂– appears at 4.40 ppm; and multiplet is observed for –(CH₂)n– at 0.80 - 1.50 ppm. Imidazolyl 4- and 5-H appear as broad singlet at 7.24 - 7.28 and 7.14 - 7.17 ppm, respectively. Broadening may be due to rapid proton exchange between these imidazolic protons. The aryl protons (7-H to 11-H) are upfield shifted on going from phenylazo (a) to p-tolylazo (b) which may be due to +I effect of –Me group.

3.3. Molecular Structure

The molecular structure of Meaai-C₁₆H₃₃ (4b) is shown in Figure 2. The bond parameters are listed in Table 4" target="_self"> Table 4. The bond length and angles of the fragment of the structure, (p-tolylazo)imidazole, are comparable to the reported structure of Haai-Me [27]. The azo N-N distance is 1.266(4) Å which is slightly longer than the reported value (1.257(6) Å) [27]. Two groups around N=N are not perfectly planar and the dihedral angle between the two least squares planes is avg. 32˚. The crowding of long chain, 1-C₁₆H₃₃ may be the reason for deviation. The

bonding strength between N and p-tolyl-C [C(8)–N(2), 1.421(4) Å] is weaker than that of N and imidazolyl-C [C(1)–N(1), 1.385(4) Å]. This suggests that the intramolecular interaction between azo-N and imidazolyl-C wave functions is stronger than that of p-tolyl orbitals. The C(3)-C(4) bond length (1.364(5) Å) of imidazolyl group is somewhat greater than reported data [41] which may be due to the steric demand of 1-C₁₆H₃₃ appended to imidazolyl group. The bond angle values at chain of C(15)-C(16)-C(17) (116.3(4)˚) C(20)-C(21)-C(22) (120.6 (6)˚), C(24)-C(25)-C(26) (123.6(8)˚) etc. show gradual increment which may be due to the removal of steric strain from the neighboring groups arranged in a zigzag manner. The imidazolyl-N is hydrogen bonded, N(3)-- -H(3)-C(3), (N(3)---H(3), 2.51 Å; N(3)---C(3), 3.390(5) Å and Ð N(3)---H(3)-C(3), 158˚; symmetry, -x, -1/2+y, 1/2-z) to imidazolyl-C-H giving 1D chain (Figure 3). Packing view in the unit cell shows π---π interaction (3.3477(16) Å; symmetry, 1-x, 2-y, -z) between adjacent imidazolyl groups to form 2D supramolecular structure (Figure 4).

3.4. Photochromism

The irradiation of UV light at l_max to a MeOH solution of the ligands has shown the absorption spectral change (Figure 5). The intense peak at l_max decreases, which is accompanied by a slight increase at the tail portion of the spectrum until a photostationary state (PSS-I) is reached. Subsequent irradiation at the newly appeared longer wavelength peak reverses the course of the reaction and the original spectrum is recovered up to a point, which is another photostationary state (PSS-II) under irradiation at the longer wavelength peak. The quantum yields of the E-to-Z isomerisation were determined following previously reported procedure [26,27] and the results are set out in Table 5.

The absorption spectra of the E-isomer of ligands have changed with isosbestic points upon excitation (Figure 5) to the Z-isomer. The ligands show little sign of degra-

dation upon repeated irradiation (Figure 5(b)). The quantum yields were measured for the E-to-Z (f_E®Z) photoisomerisation of these ligands in MeOH. The f_E®Z values are significantly dependent on nature of substituents, and chain length of -C_nH_2n+1. The Me substituent, HaaiC_nH_2n+1 to Meaai-C_nH_2n+1, and the alkyl substituent C_nH_2n+1 (n = 10, 12, 14, 16, 18, 20, 22) at N(1)-position reduce f_E®Z values. In general, increase in mass of the molecule reduces the rate of isomerisation, E-to-Z. The rotor volume and mass have significant influence on the

isomerisation rate and quantum yields.

Thermal Z-to-E isomerisation of the ligands was followed by UV-Vis spectroscopy in MeOH at 298 - 313 K (Table 6). The Eyring plots in the range 298 - 313 K gave a linear graph from which the activation energy was obtained (Table 6, Figure 6). With the increase of the alkyl chain length on N atom of the imidazole moiety, the E_as are reduced which means faster Z-to-E thermal isomerisation. The entropy of activation (DS^*) are high negative in the organic compounds having large alkyl groups than that of the compounds having short alkyl chain. This is also in support of increase in rotor volume in the organic fragments.

3.5. Liquid Crystal Behaviour and Tansition Enthalpies of Haai-C_nH_2n+1 (n = 18 (5a) and 22 (7a))

The thermal properties of the compounds were studied using polarized light optical microscopy (POM) and differential scanning calorimetry (DSC). Out of fourteen compounds only two compounds, Haai-C₁₈H₃₇ (5a) and Haai-C₂₂H₄₅ (7a) show mesomorphic response. The sample is sandwiched between untreated slide and a coverslip and is under hot stage of POM. Other twelve compounds show common crystalline-isotropic transition and the DSC plots do not show significant changes. The compound, 5a exhibits nematic (N) texture at 55˚C (Figure 7) while 7a shows unknown Smectic (Sm) phase at 62˚C (Figure 8) and, finally shift to isotropic (I) phase at 58˚C (5a) and 65˚C (7a) respectively. The crystal and nematic phases are under thermal equilibrium. During cooling from isotropic phase, 5a shows strongly fluctuating textures of the N phase and has been transformed to textures in which “Schlieren” regions can be seen (Figure 7). Homeotropic and homogenous oriented domains cause this coexistence, which permits the assignment of the smectic phase in compound 7a. The transition temperature and associated enthalpies are Cr → N 37.4˚C (15.9 KJ∙mol⁻¹) → Iso 58˚C (0.5 KJ∙mol⁻¹/heating cycle) → N 44.1˚C (12.6 KJ∙mol⁻¹/cooling cycle) for 5a (Figure 9).

3.6. Computational Study

The Density Functional Theory (DFT) computation has been used to calculate dipole moment of 5a and 7a. It

may determine the presence of dipolar interaction within the molecules (Figure 10). The dipole moment of 5a is 2.997 Debye (X = −1.3726, Y = −2.4777, Z = 0.9814). After geometry optimization of the compound 5a by addition of atoms step by step, the optimized structure suggests that the molecule possess rod shape. The first optimization was done by the MM2 method, and finally DFT was performed. The maxima of dipole moment are in the negative Y-direction (−2.4777 D), so the direction of net dipole moment should be along the molecular axis (axial). Therefore, the direction of net dipole moment is axial (along molecular axis). Near-neighbor antiparallel correlation of molecules within the layer of N phase will occur in reality if the direction of dipole is axial or along the molecular axis. The direction of rod shaped compound, 5a in adjacent layers should be antiparallel, so that the layer polarization alternates from layer to layer; leading to a macroscopic apolar antiferroelectric structure which is applicable to N phases and sometime in case of reentrant Smectic phase with switching behavior. Within the layers the molecules are in most cases additionally non-tilted parallel to the polar direction in the case of the Nematic phase. DFT study suggests that the nearneighbour antiparallel correlation exists and in broad-on side position with respect to the direction of dipole within the layers. Based on the direction of the dipole moment, calculated from DFT we have constructed a model of supramolecular arrangement in Nematic phase which is depicted in Figure 11.

4. Conclusion

Seven pairs of 1-alkyl-2-(arylazo)imidazoles, RaaiC_nH_2n+1 (n = 10, 12, 14, 16, 18, 20, 22) with long alkyl chain at imidazolyl-N(1) centre are spectroscopically

characterised. As one of the compounds, 1-hexadcayl-2- (p-tolylazo)imidazole, Meaai-C₁₆H₃₃ is characterised by

single crystal X-ray diffraction study. Upon UV light irradiation, the compounds show photochromism through geometrical isomerisation, E (trans)-to-Z (cis) forms. The reverse process is followed by thermal route and potential energy barrier (E_a) is calculated. The presence of long chain alkyl group shows long-range orientational order in between solid (crystalline, Cr) and isotropic (I) liquid phases. In some cases Cr and Nematic (N) texture shows thermal equilibrium in heating/cooling cycle. The Schlieren texture of N phase observed in compound 5a and 7a exhibits unknown smectic pahse with symmetrical lines. This is a preliminary report of our work; 1, 3-dialkyl-2-(arylazo)imida-zolium salts of different anions and the metal complexes (transition and nontransition) are waiting in this series.

5. Acknowledgements

The authors express their sincere thanks to Department of Science and Technology, New Delhi for financial support. One of the authors (AN) thanks UGC for the award of Fellowship.

REFERENCES

Supplementary Material

Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Center; CCDC No. 854093. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK

(e-mail, deposit@ccdc.cam.ac.uk, or www, htpp://www.ccdc.cam.ac.uk).

NOTES

^*Corresponding author.