Luminescence properties of the structure built from 3-cyano-4-di­cyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olate and caesium(I)

The structure of caesium(I) 3-cyano-4-di­cyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olate (CsA), Cs⁺·C₈HN₄O₂⁻, is related to its luminescence properties. The structure of CsA (triclinic, P) is not isomorphous with previously reported structures (monoclinic, P2₁/c) of the KA and RbA salts. Nevertheless, the coordination numbers of the metals are equal for all salts (nine). Each anion in the CsA salt is connected by pairs of inversion-related N—H...O hydrogen bonds to another anion, forming a centrosymmetric dimer. The dimers are linked into infinite ribbons, stacked by means of π–π inter­actions, thus building up an anionic wall. Time-dependent density functional theory calculations show that the formation of the dimer shifts the wavelength of the luminescence maximum to the blue region. Shortening the distance between stacked anions in the row [from 3.431 (5) Å for RbA to 3.388 (2) Å for KA to 3.244 (10) Å for CsA] correlates with a redshift of the luminescence maximum from 574 and 580 nm to 596 nm, respectively.

S1. Comment

Establishing the correlation between physical properties of individual molecules and their crystals is essential in the search for new materials. In the present paper, we study the correlation between the structure (Fig. 1) and luminescence properties of the caesium salt, (I), of 3-cyano-4-dicyanomethylene-2,5-dioxopyrrolidin-3-ine (A). It was shown in a previous report (Tafeenko et al., 2009) that the luminescence spectra of three salts of A with alkali metals (Na, K and Rb) in solution do not depend on the nature of the cation but correlate with the dielectric constant of the solvent. In the solid state, the luminescence maximum (λ max) varies with structural parameters: the value of redshift of the maximum of luminescence increases with a decrease of the distance between the stacked anions. It was also shown that in the isostructural potassium and rubidium salts all exocyclic heteroatoms of A are involved in the formation of a nearly ideal tricapped trigonal prism that encloses the cation, and the anions are arranged in stacks as a result of π–π interactions. The caesium cation has a larger ioAnic radius than the potassium and rubidium cations, so replacing potassium or rubidium by caesium in the tricapped trigonal prism allows us to enlarge its volume and the distance between adjacent anions in the stack. According to our previous results, a blueshift in the luminescence spectrum of the salt was expected. Contrary to expectations, the luminescence spectrum of the caesium salt in the solid state showed a maximum at 596 nm; it is redshifted as compared with the potassium (λ max = 580 nm) and the rubidium (λ max = 574 nm) salts.

In the CsA salt, as in the RbA and KA salts, the coordination number of the metal is 9 and all external atoms of anion A are involved in the formation of the coordination polyhedron. However, the coordination polyhedron for caesium could be hardly classified as a tricapped trigonal prism because it does not contain any parallel faces (see Fig. 2). Each polyhedron is connected to a neighboring polyhedron via a common, nearly rectangular, face, thus forming double polyhedra. Coupled polyhedra are connected by four edges with neighboring ones to form a layer lying in the ab plane (Fig. 3). The shortest distance between cations in the coupled polyhedron is 4.5860 (11) Å [Cs1···Cs1^iv; symmetry code: (iv) -x, 1 - y, -z], while the shortest distances between cations located in different double polyhedra are 5.0909 (11) Å [Cs1 ··· Cs1^vii; symmetry code: (vii) -x, -y, -z] and 5.8737 (12) Å [Cs1 ··· Cs1ⁱⁱⁱ; symmetry code: (iii) -x + 1, -y + 1, -z]. The cation–apex distances in the polyhedron vary in the range 3.157 (5)–3.527 (9) Å. These values are much larger than the values of 2.833 (1)–3.173 (2) Å reported for the the potassium salt and those for the rubidium salt [2.964 (2)–3.271 (3) Å]. Each anion is linked by two N1—H1···O1^viii [N1···O1 = 2.850 (9) Å, H1···O1ⁱ = 2.00 Å and N1—H1···O1ⁱ = 171°; symmetry code: (i) -x, -y + 2, -z + 1] hydrogen bonds to another anion, thus forming a centrosymmetric dimer. Adjacent dimers are connected by –CN···NC– dipole–dipole and π–π interactions, thus forming infinite, essentially planar ribbons (Fig. 4). Since each ribbon interacts with two adjacent ribbons by means of π–π interactions, we may consider the dimers to be molecular building blocks of anionic walls (Fig. 4). The ribbons of adjacent walls are parallel, in contrast to the crystal structure of the ammonium salt (Tafeenko et al., 2005), where ribbons of adjacent walls form a dihedral angle of 53.70 (4)°. The distance between the anions in the stacks of the caesium salt is 3.244 (10) Å. This value is shorter than the corresponding distances in the potassium and the rubidium salts – 3.388 (2) and 3.431 (5) Å, respectively. The correlation between these values and the luminescence maximum wavelength is clear – the shorter the distance, the larger the redshift of the luminescence maximum (574, 580 and 596 nm for the rubidium, potassium and caesium salts, respectively). However, in contrast to the structures of the potassium and rubidium salts, anions A in the structure of the caesium salt form centrosymmetric dimers by means of N—H···O hydrogen bonding. Hydrogen bonding may alter the luminescence properties of the salts in the solid state.

The effect of hydrogen bonding on the luminescence spectrum maximum was clarified by means of quantum chemistry methods. The monomer A, C₈HN₄O₂^-, and centrosymmetric dimer [C₈HN₄O₂^-]₂ were taken as models to study. (Because of the time-consuming computing procedure and for simplification, the sodium cation instead of caesium was chosen as a counter-ion.) We used density functional theory (DFT) for the ground-state (S₀) and time-dependent density functional theory (TDDFT; Bauernschmitt & Ahlrichs, 1996) for the excited-state (S_n) equilibrium structure optimization and vertical transition energy calculations between excited and ground states of the monomer and dimer (computational details are described in the Experimental section). Vertical excitation energies and oscillator strengths for singlet–singlet S₀ → S_n transitions are listed in the Table 1. It was found that S₀ → S₁ and S₀ → S₃ transitions with high oscillator strength may be attributed to the optically allowed excitation electronic transitions for the monomer and the dimer, respectively. We calculated the equilibrium configurations of the nuclear skeleton of the S₁ and S₃ states for monomer and dimer. Vertical transition energies and oscillator strengths of the S₁ → S₀ and S₃ → S₀ transitions for the monomer S₁ state and dimer S₃ state of an equilibrium structure are listed in Table 2. In addition, we optimized the S₁ state and S₂ state equilibrium structures of the dimer and evaluated the S₁ → S₀ and S₂ → S₀ transition energies. These values correspond to the infrared range of the spectrum. However, it was shown experimentally that the luminescence maxima of both the caesium and the sodium salts are in the visible range of the spectrum. Consequently, luminescence of both caesium and sodium salts corresponds to the S₃ → S₀ transition in dimers composed of anions A. Data presented in Table 2 show that, for the dimer, the S₃ → S₀ transition is blueshifted by 56.8 nm compared with the monomer S₁ → S₀ transition.

Therefore, formation of the centrosymmetric dimer of anions A as a result of hydrogen bonding results in a significant blueshift effect on the luminescence maximum. The formation of dimers in the crystal structure diminishes the effect of redshift caused by π–π interaction between stacking anions. However, for the CsA salt, the π–π `stacking effect' prevails over the `dimers effect', resulting in a redshift of the luminescence maximum value of 16 nm, as compared with the KA salt.

Related literature

For related literature, see:

Tafeenko, V. A., Panin, G. N., Baranov, A. N., Bardasov, I. N. & Aslanov, L. A. (2007). Acta Cryst. C63, m541–m547.

S2. Experimental

The title salt was obtained by mixing the alkali caesium iodide, CsI, in aqueous solution with a suspension of 2,2,3,3–tetracyanocyclopropanecarboxylic acid in propan–2–ol, in a molar ratio of 1:1. The reaction was carried out at room temperature, and the water and propan–2–ol v/v ratio was taken as 1:1. Orange powder was obtained from the reaction mixture after solvent evaporation. The orange powder was washed with diethyl ether and dissolved in a water–ethanol mixture (1:1 v/v). The resulting solution was left aside at 318 K. Upon slow evaporation over a period of 7 d, dark-red crystals of the caesium salt were grown.

S3. Refinement

Atom H1 was treated as riding on the parent (N1) atom, with an N1—H1 distance of 0.86 Å and a U_iso(H1) value equal to 1.2U_eq(N1). For DFT and TDDFT calculations we used the B3LYP (Becke, 1993) exchange–correlation functional with the 6–311++G** (Krishnan et al., 1980; McLean & Chandler, 1980) basis set. Calculations were performed with PC GAMESS/Firefly (Granovsky, 2008) and the GAMESS (US) QC packages (Schmidt et al., 1993) for the DFT and TDDFT methods, respectively.

Computing details

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software (Enraf–Nonius, 1989); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2000); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The atom-numbering scheme in the title caesium salt, with displacement ellipsoids drawn at the 50% probability level.
	Fig. 2. The polyhedron surrounding a Cs^+ cation can hardly be considered as a tricapped trigonal prism. The coordination number of the caesium cation is nine. [Symmetry codes: (i) -x + 1, -y + 1, -z + 1; (ii) x - 1, y, z; (iii) -x + 1, -y + 1, -z; (iv) -x, -y + 1, -z; (v) x, y - 1, z - 1; (vi) x - 1, y - 1, z - 1.]
	Fig. 3. Part of the crystal structure showing how polyhedra coupled by common faces are connected by the edges N3/N3^iii and N4^i/N4^vi, thus forming an essentially planar layer lying in the ab plane. The cations lie within 0.082 (1) Å of the least-squares plane. The shortest distances between cations are listed in the text. [Symmetry codes: (i) -x + 1, -y + 1, -z + 1; (iii) -x + 1, -y + 1, -z; (iv) -x, -y + 1, -z; (vi) x - 1, y - 1, z - 1; (vii) -x, -y, -z.]
	Fig. 4. The arrangement of the centrosymmetric anionic dimers (blocks) in ribbons. Adjacent blocks in each ribbon are connected by –CN···NC– dipole–dipole and π–π interactions. Adjacent ribbons are interconnected via π–π interactions, forming an anionic wall. The interplanar distance is 3.18 (1) A°. Hydrogen-bonding parameters are listed in the text.

caesium(I) 3-cyano-4-dicyanomethylene-2,5-dioxopyrrolidin-3-ide

Crystal data

Cs^+·C8HN4O2^−	Z = 2
Mr = 318.04	F(000) = 296
Triclinic, P1	Dx = 2.219 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 5.8737 (9) Å	Cell parameters from 25 reflections
b = 9.2927 (11) Å	θ = 12–16°
c = 9.6759 (11) Å	µ = 3.87 mm^−1
α = 113.040 (11)°	T = 295 K
β = 90.834 (13)°	Prism, dark-red
γ = 100.401 (12)°	0.08 × 0.06 × 0.05 mm
V = 475.90 (11) Å^3

Data collection

Enraf–Nonius CAD-4 diffractometr	1665 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.047
Graphite monochromator	θmax = 28.0°, θmin = 2.3°
non–profiled ω scan	h = −7→7
Absorption correction: part of the refinement model (ΔF) (Walker & Stuart, 1983)	k = −12→11
Tmin = 0.084, Tmax = 0.538	l = 0→12
2424 measured reflections	2 standard reflections every 60 min
2290 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.053	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.134	H-atom parameters constrained
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0693P)^2 + 0.1745P] where P = (Fo^2 + 2Fc^2)/3
2290 reflections	(Δ/σ)max < 0.001
137 parameters	Δρmax = 0.93 e Å^−3
0 restraints	Δρmin = −1.75 e Å^−3

Special details

Experimental. Luminescence spectra of all the salts were recorded by microspectrophotometer QDI 2010 (CRAIC Technologies). Excitation wavelength was 365?nm.

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

	x	y	z	Uiso*/Ueq
Cs1	0.11717 (10)	0.28032 (6)	0.00688 (5)	0.0501 (2)
O1	0.1838 (10)	1.0426 (7)	0.6789 (6)	0.0471 (13)
O2	0.2633 (10)	0.6562 (7)	0.2258 (6)	0.0516 (14)
N1	0.1745 (11)	0.8535 (7)	0.4366 (6)	0.0396 (14)
H1	0.0595	0.8745	0.3970	0.048*
N2	0.9114 (13)	0.6411 (10)	0.6408 (8)	0.0543 (18)
N3	0.6723 (14)	0.4537 (11)	0.1623 (8)	0.064 (2)
N4	0.6895 (15)	0.9644 (10)	0.8648 (8)	0.063 (2)
C2	0.2614 (13)	0.9319 (8)	0.5884 (8)	0.0374 (15)
C3	0.4462 (13)	0.8582 (8)	0.6103 (7)	0.0359 (15)
C4	0.4774 (13)	0.7418 (8)	0.4743 (7)	0.0351 (14)
C5	0.2965 (13)	0.7389 (8)	0.3592 (8)	0.0381 (15)
C6	0.6344 (13)	0.6394 (9)	0.4330 (8)	0.0378 (15)
C7	0.6508 (13)	0.5373 (10)	0.2828 (8)	0.0437 (17)
C8	0.7912 (13)	0.6397 (9)	0.5473 (8)	0.0379 (15)
C9	0.5796 (15)	0.9157 (10)	0.7505 (8)	0.0461 (18)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs1	0.0533 (4)	0.0496 (3)	0.0461 (3)	0.0109 (2)	0.0029 (2)	0.01767 (19)
O1	0.043 (3)	0.052 (3)	0.050 (3)	0.020 (3)	0.006 (2)	0.020 (2)
O2	0.046 (4)	0.061 (3)	0.046 (3)	0.011 (3)	0.000 (2)	0.021 (2)
N1	0.031 (3)	0.047 (3)	0.045 (3)	0.011 (3)	0.004 (2)	0.021 (3)
N2	0.040 (4)	0.073 (5)	0.057 (4)	0.017 (4)	0.008 (3)	0.032 (3)
N3	0.050 (5)	0.089 (6)	0.051 (4)	0.026 (4)	0.010 (3)	0.021 (4)
N4	0.056 (5)	0.066 (5)	0.064 (4)	0.013 (4)	−0.005 (4)	0.023 (4)
C2	0.029 (4)	0.040 (4)	0.050 (4)	0.007 (3)	0.006 (3)	0.024 (3)
C3	0.032 (4)	0.042 (4)	0.040 (3)	0.006 (3)	0.009 (3)	0.023 (3)
C4	0.023 (4)	0.043 (4)	0.043 (3)	0.004 (3)	0.006 (3)	0.022 (3)
C5	0.032 (4)	0.038 (4)	0.049 (4)	0.000 (3)	0.004 (3)	0.025 (3)
C6	0.025 (4)	0.047 (4)	0.049 (4)	0.009 (3)	0.010 (3)	0.026 (3)
C7	0.024 (4)	0.061 (5)	0.052 (4)	0.008 (3)	0.010 (3)	0.029 (4)
C8	0.027 (4)	0.045 (4)	0.046 (3)	0.008 (3)	0.012 (3)	0.022 (3)
C9	0.044 (5)	0.053 (4)	0.051 (4)	0.016 (4)	0.008 (3)	0.027 (3)

Geometric parameters (Å, º)

Cs1—O1^i	3.157 (5)	N1—C2	1.399 (8)
Cs1—N2^ii	3.209 (7)	N1—H1	0.8600
Cs1—O2	3.240 (6)	N2—C8	1.134 (9)
Cs1—N4^iii	3.301 (9)	N3—C7	1.145 (10)
Cs1—N4^ii	3.332 (8)	N4—C9	1.152 (10)
Cs1—N3^iv	3.379 (7)	C2—C3	1.438 (10)
Cs1—N3	3.399 (8)	C3—C4	1.378 (9)
Cs1—O2^v	3.432 (5)	C3—C9	1.411 (10)
Cs1—N3^vi	3.527 (9)	C4—C6	1.395 (9)
O1—C2	1.228 (8)	C4—C5	1.517 (9)
O2—C5	1.207 (8)	C6—C7	1.407 (10)
N1—C5	1.370 (9)	C6—C8	1.428 (10)
O1^i—Cs1—N2^ii	150.20 (17)	O2—Cs1—N3^vi	62.73 (16)
O1^i—Cs1—O2	139.78 (16)	N4^iii—Cs1—N3^vi	126.48 (19)
N2^ii—Cs1—O2	65.46 (18)	N4^ii—Cs1—N3^vi	139.2 (2)
O1^i—Cs1—N4^iii	66.67 (18)	N3^iv—Cs1—N3^vi	96.82 (19)
N2^ii—Cs1—N4^iii	99.4 (2)	N3—Cs1—N3^vi	73.7 (2)
O2—Cs1—N4^iii	146.09 (18)	O2^v—Cs1—N3^vi	61.43 (17)
O1^i—Cs1—N4^ii	87.63 (16)	C5—N1—C2	111.1 (6)
N2^ii—Cs1—N4^ii	63.39 (19)	C5—N1—H1	124.4
O2—Cs1—N4^ii	115.01 (16)	C2—N1—H1	124.4
N4^iii—Cs1—N4^ii	79.7 (2)	O1—C2—N1	122.8 (6)
O1^i—Cs1—N3^iv	133.46 (16)	O1—C2—C3	130.0 (6)
N2^ii—Cs1—N3^iv	64.38 (18)	N1—C2—C3	107.2 (6)
O2—Cs1—N3^iv	64.94 (19)	C4—C3—C9	129.0 (7)
N4^iii—Cs1—N3^iv	81.2 (2)	C4—C3—C2	109.2 (6)
N4^ii—Cs1—N3^iv	119.8 (2)	C9—C3—C2	121.5 (6)
O1^i—Cs1—N3	103.05 (16)	C3—C4—C6	132.3 (6)
N2^ii—Cs1—N3	76.01 (19)	C3—C4—C5	106.4 (6)
O2—Cs1—N3	58.25 (17)	C6—C4—C5	121.3 (6)
N4^iii—Cs1—N3	151.0 (2)	O2—C5—N1	125.3 (7)
N4^ii—Cs1—N3	72.6 (2)	O2—C5—C4	128.7 (7)
N3^iv—Cs1—N3	120.1 (2)	N1—C5—C4	106.0 (6)
O1^i—Cs1—O2^v	75.40 (13)	C4—C6—C7	123.3 (6)
N2^ii—Cs1—O2^v	126.79 (16)	C4—C6—C8	119.0 (6)
O2—Cs1—O2^v	93.22 (12)	C7—C6—C8	117.7 (6)
N4^iii—Cs1—O2^v	71.13 (17)	N3—C7—C6	177.4 (8)
N4^ii—Cs1—O2^v	150.21 (17)	N3—C7—Cs1	59.4 (5)
N3^iv—Cs1—O2^v	62.44 (15)	C6—C7—Cs1	122.9 (5)
N3—Cs1—O2^v	134.60 (19)	N2—C8—C6	178.1 (8)
O1^i—Cs1—N3^vi	78.29 (16)	N4—C9—C3	179.2 (10)
N2^ii—Cs1—N3^vi	127.99 (19)

Symmetry codes: (i) x, y−1, z−1; (ii) −x+1, −y+1, −z+1; (iii) x−1, y−1, z−1; (iv) x−1, y, z; (v) −x, −y+1, −z; (vi) −x+1, −y+1, −z.