Crystal structure of 9,9′-{(1E,1′E)-[1,4-phenyl­enebis(aza­nylyl­idene)]bis­(methanylyl­idene)}bis­(2,3,6,7-tetra­hydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol)

Faizi, M.S.H.; Ali, A.; Potaskalov, V.A.

doi:10.1107/S205698901601344X

research communications

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 72| Part 10| October 2016| Pages 1366-1369

https://doi.org/10.1107/S205698901601344X

Open

access

Crystal structure of 9,9′-{(1E,1′E)-[1,4-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol)

Md. Serajul Haque Faizi,^a Akram Ali ^b and Vadim A. Potaskalov ^c ^*

^aDepartment of Chemistry, College of Science, Sultan Qaboos University, PO Box 36 Al-Khod 123, Muscat, Sultanate of , Oman, ^bDepartment of Chemistry, Indian Institute of Technology Kanpur, Kanpur, UP 208 016, India, and ^cDepartment of General and Inorganic Chemistry, National Technical University of Ukraine, Kyiv Polytechnic Institute, 37 Prospect Peremogy, 03056 Kiev, Ukraine
^*Correspondence e-mail: potaskalov@xtf.kpi.ua

Edited by H. Stoeckli-Evans, University of Neuchâtel, Switzerland (Received 9 August 2016; accepted 22 August 2016; online 5 September 2016)

The whole molecule of the title compound, C₃₂H₃₄N₂O₂, is generated by inversion symmetry; the central benzene ring being situated about the crystallographic inversion center. The aromatic ring of the julolidine moiety is inclined to the central benzene ring by 33.70 (12)°. There are two intramolecular O—H⋯N hydrogen bonds in the molecule, generating S(6) ring motifs. The conformation about the C=N bonds is E. The fused non-aromatic rings of the julolidine moiety adopt half-chair conformations. In the crystal, adjacent molecules are linked by pairs of C—H⋯π interactions, forming a ladder-like structure propagating along the a-axis direction.

Keywords: crystal structure; julolidine; Schiff base; 8-hydroxyjulolidine-9-carboxaldehyde; p-phenylenediamine; hydrogen bonding; C—H⋯π interactions.

CCDC reference: 1500381

1. Chemical context

8-Hydroxyjulolidine-9-carboxaldehyde is a well-known chromophore used in fluorescence chemosensors; chemosensors with the julolidine moiety are usually soluble in aqueous solutions (Narayanaswamy & Govindaraju, 2012 ; Maity et al., 2011 ; Na et al., 2013 ; Noh et al., 2013 ). Compounds containing the julolidine group display chromogenic naked-eye detection of copper, zinc, iron, and aluminium ions as well as fluoride ions (Choi et al., 2015 ; Wang et al., 2013a ,b ; Kim et al., 2015 ; Jo et al., 2015 ). There are many reports in the literature on 8-hydroxyjulolidine-9-carboxaldehyde-based Schiff bases and their applications as sensors for metal ions (Park et al., 2014 ; Lee et al., 2014 ; Kim et al., 2016 ). Intramolecular C—H⋯N hydrogen bonds have been observed in a julolidine-derived structure (Barbero et al., 2012 ). Julolidine dyes exhibiting excited-state intramolecular proton transfer (Nano et al., 2015 ) and julolidine ring-containing compounds are also fluorescent probes for the measurement of cell-membrane viscosity. The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of new organic and polynuclear coordination compounds (Faizi & Sen 2014 ; Faizi et al., 2016 ). Recently Choi et al. (2016 ) have reported on a new chemosensor, similar to the title compound, which is a fluorescent chemosensor for the selective detection of Zn²⁺ in aqueous solution. This was synthesized by a condensation reaction of 8-hydroxyjulolidine-9-carboxaldehyde with 2-(aminomethyl)benzeneamine in ethanol at room temperature. We report herein on the synthesis and crystal structure of the title julolidine derivative.

2. Structural commentary

The molecular structure of the title compound is illustrated in Fig. 1. The whole molecule of the title compound is generated by crystallographic inversion symmetry. The conformation about the azomethine C4=N1 bond [1.285 (3) Å] is E. The C3—N1—C4—C5 torsion angle is 172.9 (2)°. The molecule is non-planar, with the dihedral angle between the central benzene ring and the aromatic ring of the julolidine moiety being 33.70 (12)°. Depending on the tautomers, two types of intramolecular hydrogen bonds are observed in Schiff bases: O—H⋯N in phenol–imine and N—H⋯O in keto–amine tautomers. The present analysis shows that the title compound exists in the phenol–imine form (Fig. 1). It exhibits two intramolecular O1—H1A⋯N1 [d(N⋯O) 2.579 (3) Å] hydrogen bonds, which generate S(6) ring motifs (Fig. 1 and Table 1).

Table 1
Hydrogen-bond geometry (Å, °)

Cg is the centroid of the C5–C7/C11/C15/C16 ring.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1A⋯N1	0.82	1.85	2.579 (3)	148
C10—H10B⋯Cgⁱ	0.97	2.68	3.603 (3)	160
Symmetry code: (i) x+1, y, z.

Figure 1
The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 40% probability level. Unlabelled atoms are generated by the symmetry operation −x, −y + 1, -z. The intramolecular O—H⋯N hydrogen bonds (see Table 1

) are shown as dashed lines.

3. Supramolecular features

In the crystal, adjacent molecules are linked by a pair of C—H⋯π interactions (Table 1 and Fig. 2), forming a ladder-like structure propagating along the a-axis direction (Fig. 3).

Figure 2
A view of the C—H⋯π interactions, shown as dashed lines (see Table 1

), in the crystal of the title compound.

Figure 3
A view along the a axis of the crystal packing of the title compound.

4. Database survey

There are very few examples of similar compounds in the literature and, to the best of our knowledge, the new fluorescent chemosensor for the selective detection of Zn²⁺ in aqueous solution, mentioned in the Chemical context section (Choi et al., 2016) has not been characterized crystallographically. A search of the Cambridge Structural Database (CSD, Version 5.37, update May 2016; Groom et al., 2016 ) gave 120 hits for the julolidine moiety. Of these, six have an OH group in position 8, and four also have a C=N group in position 1. Of the latter, one compound, viz. 9-{[(4-chlorophenyl)imino]methyl}-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol (CSD refcode: IGALUZ; Kantar et al., 2013 ), resembles the title compound and also exists in the phenol–imine form with an intramolecular O—H⋯N hydrogen bond.

5. Synthesis and crystallization

An ethanolic solution of 8-hydroxyjulolidine-9-carboxaldehyde (100 mg, 0.46 mmol) was added to p-phenylenediamine (25 mg, 0.23 mmol) in absolute ethanol (3 ml). Two drops of HCl were added to the reaction solution and it was stirred for 30 min at room temperature. The resulting yellow precipitate was recovered by filtration, washed several times with small portions of ice-cold EtOH and then with diethyl ether to give 199 mg (85%) of the title compound. Crystals suitable for X-ray diffraction analysis were obtained within three days by slow evaporation of a solution in methanol.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The OH and C-bound H atoms were included in calculated positions and treated as riding atoms: O—H = 0.82 and C—H = 0.93-0.97 Å, with U_iso(H) = 1.5U_eq(O) and 1.2U_eq(C) for other H atoms.

Table 2
Experimental details

Crystal data
Chemical formula	C₃₂H₃₄N₄O₂
M_r	506.63
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	5.1776 (3), 27.9346 (17), 8.7893 (6)
β (°)	96.203 (2)
V (Å³)	1263.79 (14)
Z	2
Radiation type	Mo Kα
μ (mm⁻¹)	0.08
Crystal size (mm)	0.20 × 0.15 × 0.12

Data collection
Diffractometer	Bruker SMART APEX CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2003 )
T_min, T_max	0.783, 0.990
No. of measured, independent and observed [I > 2σ(I)] reflections	15125, 2243, 1469
R_int	0.073
(sin θ/λ)_max (Å⁻¹)	0.596

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.050, 0.127, 1.02
No. of reflections	2243
No. of parameters	173
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.33, −0.22
Computer programs: SMART and SAINT (Bruker, 2003), SIR97 (Altomare et al., 1999 ), DIAMOND (Brandenberg & Putz, 2006 ), SHELXL97 (Sheldrick, 2008 ) and PLATON (Spek, 2009 ).

Supporting information

CCDC reference: 1500381

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S205698901601344X/su5322sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S205698901601344X/su5322Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S205698901601344X/su5322Isup3.cml

checkCIF report

Computing details top

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenberg & Putz, 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

9,9'-{(1E,1'E)-[1,4-Phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol) top

Crystal data top

C₃₂H₃₄N₄O₂	F(000) = 540
M_r = 506.63	D_x = 1.331 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3371 reflections
a = 5.1776 (3) Å	θ = 2.4–26.5°
b = 27.9346 (17) Å	µ = 0.08 mm⁻¹
c = 8.7893 (6) Å	T = 100 K
β = 96.203 (2)°	Block, yellow
V = 1263.79 (14) Å³	0.20 × 0.15 × 0.12 mm
Z = 2

Data collection top

Bruker SMART APEX CCD diffractometer	2243 independent reflections
Radiation source: fine-focus sealed tube	1469 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.073
/w–scans	θ_max = 25.0°, θ_min = 2.8°
Absorption correction: multi-scan (SADABS; Bruker, 2003)	h = −6→6
T_min = 0.783, T_max = 0.990	k = −33→33
15125 measured reflections	l = −10→10

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.050	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.127	H-atom parameters constrained
S = 1.02	w = 1/[σ²(F_o²) + (0.0483P)² + 0.7868P] where P = (F_o² + 2F_c²)/3
2243 reflections	(Δ/σ)_max < 0.001
173 parameters	Δρ_max = 0.33 e Å⁻³
0 restraints	Δρ_min = −0.22 e Å⁻³

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2sigma(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
O1	0.5852 (3)	0.64668 (6)	0.15270 (19)	0.0327 (5)
H1A	0.4785	0.6259	0.1246	0.049*
N2	1.2746 (4)	0.67853 (7)	0.5427 (2)	0.0250 (5)
N1	0.3470 (4)	0.56564 (7)	0.1548 (2)	0.0259 (5)
C3	0.1732 (4)	0.53146 (8)	0.0816 (3)	0.0216 (6)
C11	1.0954 (4)	0.64728 (8)	0.4704 (3)	0.0208 (6)
C7	1.0677 (4)	0.60049 (8)	0.5312 (3)	0.0212 (6)
C1	−0.2269 (4)	0.51736 (9)	−0.0764 (3)	0.0249 (6)
H1	−0.3807	0.5292	−0.1271	0.030*
C15	0.9341 (4)	0.66163 (8)	0.3388 (3)	0.0229 (6)
C16	0.7394 (5)	0.63101 (9)	0.2775 (3)	0.0250 (6)
C6	0.8730 (4)	0.57163 (9)	0.4650 (3)	0.0245 (6)
H6	0.8546	0.5412	0.5055	0.029*
C2	−0.0546 (4)	0.54825 (9)	0.0030 (3)	0.0240 (6)
H2	−0.0918	0.5808	0.0039	0.029*
C5	0.7015 (5)	0.58567 (8)	0.3400 (3)	0.0241 (6)
C4	0.5029 (5)	0.55395 (9)	0.2728 (3)	0.0277 (6)
H4	0.4865	0.5239	0.3161	0.033*
C8	1.2531 (5)	0.58325 (9)	0.6635 (3)	0.0277 (6)
H8A	1.3963	0.5663	0.6250	0.033*
H8B	1.1643	0.5611	0.7251	0.033*
C12	1.3361 (5)	0.72291 (9)	0.4669 (3)	0.0305 (6)
H12A	1.4648	0.7165	0.3971	0.037*
H12B	1.4101	0.7456	0.5429	0.037*
C10	1.4682 (5)	0.66195 (9)	0.6634 (3)	0.0305 (6)
H10A	1.5310	0.6889	0.7264	0.037*
H10B	1.6144	0.6484	0.6181	0.037*
C14	0.9740 (5)	0.70887 (9)	0.2652 (3)	0.0307 (6)
H14A	0.8078	0.7212	0.2203	0.037*
H14B	1.0841	0.7046	0.1837	0.037*
C13	1.0976 (5)	0.74444 (9)	0.3793 (3)	0.0313 (6)
H13A	1.1453	0.7730	0.3264	0.038*
H13B	0.9744	0.7535	0.4499	0.038*
C9	1.3577 (5)	0.62498 (9)	0.7617 (3)	0.0309 (6)
H9A	1.2190	0.6390	0.8130	0.037*
H9B	1.4917	0.6138	0.8393	0.037*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0348 (11)	0.0320 (11)	0.0288 (10)	−0.0051 (8)	−0.0072 (9)	0.0013 (8)
N2	0.0219 (11)	0.0267 (12)	0.0257 (12)	−0.0030 (10)	−0.0006 (9)	−0.0002 (9)
N1	0.0213 (11)	0.0324 (13)	0.0233 (11)	−0.0016 (10)	−0.0003 (10)	−0.0044 (10)
C3	0.0198 (12)	0.0261 (13)	0.0198 (13)	−0.0057 (11)	0.0060 (11)	−0.0059 (11)
C11	0.0174 (12)	0.0239 (14)	0.0219 (13)	−0.0019 (10)	0.0057 (11)	−0.0050 (10)
C7	0.0213 (13)	0.0239 (14)	0.0192 (13)	0.0009 (11)	0.0061 (11)	−0.0041 (11)
C1	0.0187 (13)	0.0316 (15)	0.0241 (14)	0.0004 (11)	0.0016 (11)	−0.0006 (11)
C15	0.0248 (13)	0.0244 (13)	0.0202 (13)	−0.0008 (11)	0.0049 (11)	0.0019 (11)
C16	0.0229 (13)	0.0350 (15)	0.0165 (12)	0.0061 (12)	−0.0010 (11)	−0.0004 (11)
C6	0.0273 (14)	0.0252 (14)	0.0217 (13)	0.0020 (11)	0.0065 (11)	−0.0004 (11)
C2	0.0239 (13)	0.0233 (14)	0.0251 (14)	−0.0017 (11)	0.0042 (11)	−0.0038 (11)
C5	0.0263 (14)	0.0236 (14)	0.0234 (14)	−0.0039 (11)	0.0070 (12)	−0.0052 (11)
C4	0.0312 (14)	0.0263 (14)	0.0270 (14)	0.0006 (12)	0.0098 (12)	−0.0024 (12)
C8	0.0297 (15)	0.0305 (15)	0.0229 (14)	0.0056 (12)	0.0030 (12)	0.0031 (11)
C12	0.0275 (14)	0.0250 (14)	0.0394 (16)	−0.0064 (12)	0.0048 (12)	−0.0054 (12)
C10	0.0253 (13)	0.0350 (15)	0.0296 (15)	0.0023 (12)	−0.0043 (12)	−0.0082 (12)
C14	0.0289 (14)	0.0323 (15)	0.0305 (15)	−0.0034 (12)	0.0015 (12)	0.0033 (12)
C13	0.0356 (15)	0.0255 (14)	0.0329 (15)	−0.0034 (12)	0.0047 (13)	0.0045 (12)
C9	0.0292 (14)	0.0384 (16)	0.0233 (14)	0.0096 (13)	−0.0056 (11)	−0.0031 (12)

Geometric parameters (Å, º) top

O1—C16	1.358 (3)	C6—H6	0.9300
O1—H1A	0.8200	C2—H2	0.9300
N2—C11	1.378 (3)	C5—C4	1.435 (3)
N2—C10	1.454 (3)	C4—H4	0.9300
N2—C12	1.459 (3)	C8—C9	1.515 (3)
N1—C4	1.285 (3)	C8—H8A	0.9700
N1—C3	1.418 (3)	C8—H8B	0.9700
C3—C2	1.383 (3)	C12—C13	1.508 (3)
C3—C1ⁱ	1.394 (3)	C12—H12A	0.9700
C11—C15	1.410 (3)	C12—H12B	0.9700
C11—C7	1.425 (3)	C10—C9	1.499 (4)
C7—C6	1.370 (3)	C10—H10A	0.9700
C7—C8	1.505 (3)	C10—H10B	0.9700
C1—C2	1.376 (3)	C14—C13	1.505 (3)
C1—C3ⁱ	1.394 (3)	C14—H14A	0.9700
C1—H1	0.9300	C14—H14B	0.9700
C15—C16	1.386 (3)	C13—H13A	0.9700
C15—C14	1.494 (3)	C13—H13B	0.9700
C16—C5	1.403 (3)	C9—H9A	0.9700
C6—C5	1.392 (3)	C9—H9B	0.9700

C16—O1—H1A	109.5	C7—C8—H8A	109.5
C11—N2—C10	120.75 (19)	C9—C8—H8A	109.5
C11—N2—C12	119.8 (2)	C7—C8—H8B	109.5
C10—N2—C12	115.88 (19)	C9—C8—H8B	109.5
C4—N1—C3	120.5 (2)	H8A—C8—H8B	108.1
C2—C3—C1ⁱ	118.6 (2)	N2—C12—C13	111.4 (2)
C2—C3—N1	117.6 (2)	N2—C12—H12A	109.3
C1ⁱ—C3—N1	123.7 (2)	C13—C12—H12A	109.3
N2—C11—C15	120.5 (2)	N2—C12—H12B	109.3
N2—C11—C7	119.9 (2)	C13—C12—H12B	109.3
C15—C11—C7	119.6 (2)	H12A—C12—H12B	108.0
C6—C7—C11	118.7 (2)	N2—C10—C9	111.4 (2)
C6—C7—C8	121.2 (2)	N2—C10—H10A	109.4
C11—C7—C8	120.1 (2)	C9—C10—H10A	109.4
C2—C1—C3ⁱ	120.6 (2)	N2—C10—H10B	109.4
C2—C1—H1	119.7	C9—C10—H10B	109.4
C3ⁱ—C1—H1	119.7	H10A—C10—H10B	108.0
C16—C15—C11	119.0 (2)	C15—C14—C13	111.3 (2)
C16—C15—C14	120.4 (2)	C15—C14—H14A	109.4
C11—C15—C14	120.6 (2)	C13—C14—H14A	109.4
O1—C16—C15	117.0 (2)	C15—C14—H14B	109.4
O1—C16—C5	120.8 (2)	C13—C14—H14B	109.4
C15—C16—C5	122.1 (2)	H14A—C14—H14B	108.0
C7—C6—C5	123.1 (2)	C12—C13—C14	110.0 (2)
C7—C6—H6	118.4	C12—C13—H13A	109.7
C5—C6—H6	118.4	C14—C13—H13A	109.7
C1—C2—C3	120.8 (2)	C12—C13—H13B	109.7
C1—C2—H2	119.6	C14—C13—H13B	109.7
C3—C2—H2	119.6	H13A—C13—H13B	108.2
C6—C5—C16	117.3 (2)	C10—C9—C8	109.7 (2)
C6—C5—C4	121.2 (2)	C10—C9—H9A	109.7
C16—C5—C4	121.4 (2)	C8—C9—H9A	109.7
N1—C4—C5	122.3 (2)	C10—C9—H9B	109.7
N1—C4—H4	118.8	C8—C9—H9B	109.7
C5—C4—H4	118.8	H9A—C9—H9B	108.2
C7—C8—C9	110.7 (2)

Symmetry code: (i) −x, −y+1, −z.

Hydrogen-bond geometry (Å, º) top

Cg is the centroid of the C5–C7/C11/C15/C16 ring.

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···N1	0.82	1.85	2.579 (3)	148
C10—H10B···Cgⁱⁱ	0.97	2.68	3.603 (3)	160

Symmetry code: (ii) x+1, y, z.

Acknowledgements

The authors are grateful to the National Taras Shevchenko University, Department of Chemistry, Volodymyrska str. 64, 01601 Kyiv, Ukraine, for financial support.

References

Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119. Web of Science CrossRef CAS IUCr Journals Google Scholar
Barbero, N., Barolo, C., Marabello, D., Buscaino, R., Gervasio, G. & Viscardi, G. (2012). Dyes Pigments, 92, 1177–1183. Web of Science CSD CrossRef CAS Google Scholar
Brandenberg, K. & Putz, H. (2006). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Bruker (2003). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Choi, Y. W., Lee, J. J., You, G. R., Lee, S. Y. & Kim, C. (2015). RSC Adv. 5, 86463–86472. Web of Science CrossRef CAS Google Scholar
Choi, Y. W., You, G. R., Lee, J. J. & Kim, C. (2016). Inorg. Chem. Commun. 63, 35–38. Web of Science CrossRef CAS Google Scholar
Faizi, M. S. H., Gupta, S., Mohan, V. K., Jain, K. V. & Sen, P. (2016). Sens. Actuators B Chem. 222, 15–20. Web of Science CrossRef CAS Google Scholar
Faizi, M. S. H. & Sen, P. (2014). Acta Cryst. E70, m206–m207. CSD CrossRef IUCr Journals Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CSD CrossRef IUCr Journals Google Scholar
Jo, T. G., Na, Y. J., Lee, J. J., Lee, M. M., Lee, S. Y. & Kim, C. (2015). New J. Chem. 39, 2580–2587. Web of Science CrossRef CAS Google Scholar
Kantar, E. N., Köysal, Y., Akdemir, N., Ağar, A. A. & Soylu, M. S. (2013). Acta Cryst. E69, o883. CSD CrossRef IUCr Journals Google Scholar
Kim, Y. S., Lee, J. J., Choi, Y. W., You, G. R., Nguyen, L., Noh, I. & Kim, C. (2016). Dyes Pigm. 129, 43–53. Web of Science CrossRef CAS Google Scholar
Kim, Y. S., Park, G. J., Lee, J. J., Lee, S. Y., Lee, S. Y. & Kim, C. (2015). RSC Adv. 5, 11229–11239. Web of Science CrossRef CAS Google Scholar
Lee, S. A., You, G. R., Choi, Y. W., Jo, H. Y., Kim, A. R., Noh, I., Kim, S.-J., Kim, Y. & Kim, C. (2014). Dalton Trans. 43, 6650–6659. Web of Science CSD CrossRef CAS PubMed Google Scholar
Maity, D., Manna, A. K., Karthigeyan, D., Kundu, T. K., Pati, S. K. & Govindaraju, T. (2011). Chem. Eur. J. 17, 11152–11161. Web of Science CrossRef CAS PubMed Google Scholar
Na, Y. J., Hwang, I. H., Jo, H. Y., Lee, S. A., Park, G. J. & Kim, C. (2013). Inorg. Chem. Commun. 35, 342–345. Web of Science CSD CrossRef CAS Google Scholar
Nano, A., Gullo, M. P., Ventura, B., Armaroli, N., Barbieri, A. & Ziessel, R. (2015). Chem. Commun. 51, 3351–3354. Web of Science CrossRef CAS Google Scholar
Narayanaswamy, N. & Govindaraju, T. (2012). Sens. Actuators B Chem. 161, 304–310. Web of Science CrossRef CAS Google Scholar
Noh, J. Y., Kim, S., Hwang, I. H., Lee, G. Y., Kang, J., Kim, S. H., Min, J., Park, S., Kim, C. & Kim, J. (2013). Dyes Pigments, 99, 1016–1021. Web of Science CrossRef CAS Google Scholar
Park, G. J., Park, D. Y., Park, K.-M., Kim, Y., Kim, S.-J., Chang, P.-S. & Kim, C. (2014). Tetrahedron, 70, 7429–7438. Web of Science CSD CrossRef CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar
Wang, L., Li, H. & Cao, D. (2013a). Sens. Actuators B Chem. 181, 749–755. Web of Science CrossRef CAS Google Scholar
Wang, M., Wang, J., Xue, W. & Wu, A. (2013b). Dyes Pigments, 97, 475–480. Web of Science CSD CrossRef CAS Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.