HETEROCYCLES

Communication | Special issue | Vol. 84, No. 1, 2012, pp. 315-321
Received, 10th June, 2011, Accepted, 19th July, 2011, Published online, 22nd July, 2011.
DOI: 10.3987/COM-11-S(P)33

■ Solid-State Fluorescence Properties and Crystal Structures of 7-(Diethylamino)coumarin Derivatives

Kanji Kubo,* Taisuke Matsumoto, Keiko Ideta, Haruko Takechi, and Hajime Takahashi

School of Dentistry, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hokkaido 061-0293, Japan

Abstract

7-(Diethylamino)-3-phenylcoumarin (1) gave strong emission band at 477 nm in the solid state when excited at 400 nm, but ethyl 2-[7-(diethylamino)-2-oxo-2H-1-benzopyran-3-yl]-4-thiazolecarboxylate (2) had no fluorescence in solid state. The crystal structures of 1 and 2 were analyzed by X-ray crystallography. The crystal of 1 had intermolecular C-H···O, C-H···N and C-H···π interactions and that of 2 had intermolecular C-H···O, π···π, and S···π interactions.

Materials containing a coumarin (2H-1-benzopyran-2-one) component have been useful in many fields such as fluorescence materials and laser dyes, nonlinear optical materials, photorefractive materials, photo resists, intermediates for drug synthesis, luminescence materials, and analytical reagents etc.1 Although the fluorescence of the coumarin itself is weak, the introduction of substituent group into coumarin increase the fluorescence intensity. Recently, we have reported the synthesis and fluorescence properties of 7-diethylaminocoumarin derivatives as fluoroionohores accessible for analytical purposes in the fields of analytical and biological chemistry.2 The crystal structure analyses of 7-diethylaminocoumarin,3 7-diethylamino-3-dimethylaminocoumarin,4 methyl 4-(7-diethylamino-2oxo-2H-1-benzopyran-3-yl)- benzoate,5 and cholesteryl 4-(7-diethylamino-2oxo-2H-1-benzopyran-3-yl)benzoate6 have been reported.
We now report the solid-state fluorescence properties and crystal structures of 7-(diethylamino)-3-phenyl-2H-1-benzopyran-2-one (1) and ethyl 2-[7-(diethylamino)-2-oxo-2H-1-benzopyran-3-yl]-4-thiazolecarboxylate (2) in order to elucidate the relationship between the solid-state fluorescence and molecular packing.

The structures of 17 and 28 were confirmed by X-ray crystallographic analysis as shown in Figure 1. Phenyl derivative (1) adopts a twisted conformation. The dihedral angle between the coumarin ring defined by C1–C9/O1/O2 and the phenyl ring defined by C14–C19 is 42.41(1)º. Thiazole derivative (2) adopts an almost flattened conformation. The thiazole ring defined by C14–C16/N2/S1 of 2 made angles of 2.31(4)º and 12.15(5)º with coumarin ring and ethoxycarbonyl plane defined by C17–C19/O3/O4, respectively.

The C-C and C-O bond lengths of the coumarin ring of 19 and 210 are similar to those of 7-diethylaminocoumarin3 and distinct from those of coumarin itself.11 The C7-N1 bond lengths of 1 and 2 are close to that (1.376(5) Ǻ)4 of 7-diethylamino-3-dimethylaminocoumarin and Csp2-Nsp2 planar bond length [1.355 Ǻ].12 The respective deviations of each atom from the least squares plane defined by C7/C10/C12/N1 atoms are within the range of -0.002–0.005(1) Ǻ for 1 and -0.001–0.003 Ǻ for 2, respectively. This means that the diethylamino group substituted at C-7 position effects on the conjugation system of coumarin. The two ethyl groups of diethylamine group are syn for 1 and anti for 2 with respect to one another.
There are some intermolecular C-H···O hydrogen bonds of 113 and 214 as shown in Figure 2 and 3. The H···O distances of 1 and 2 are similar to those of methyl 4-(7-diethylamino-2-oxo-2H-1-benzopyran-3-yl)benzoate (2.49–2.88 Ǻ)5 and cholesteryl 4-(7-diethylamino-2oxo-2H-1-benzopyran-3-yl)benzoate (2.40–2.66 Ǻ).6 Intermolecular C-H···N hydrogen bonds is observed in 1 as shown in Figure 2. The H···N distance13 of 1 is shorter to that [2.91 Ǻ]15 of 2,2'-bi-2-imidazoline.

Intermolecular C–H···π interactions are observed in 1, as shown in Figure 4. The distances between H10 and the centroid (Cg1iii) (Symmetry code: (iii) x, 1/2-y, 1/2+z) of the pyrane ring (C1–C4/C9/O1) and between H7 and the centroid (Cg2iv) (Symmetry code: (iv) 1-x, 1/2+y, 1/2-z) of the benzene ring (C16–C19) are 2.691 and 3.133 Å, respectively, which are similar to the intermolecular C–H···π interaction [2.835, 3.079 Ǻ] observed in cholesteryl 4-(7-diethylamino-2-oxo-2H-1-benzopyran-3-yl)benzoate.6

Intermolecular π···π interactions between the coumarin planes of 2 are observed in Figure 5. The distances between intermolecular coumarin planes are 3.288(2) Ǻ for C1···C3i and 3.064(2) Ǻ for C1···O2i (symmetry codes: (iii) -x, 2-y, 1-z, (iv) 1-x, 2-y, 1-z), which is within the range associated with π···π interaction [3.3-3.8 Ǻ].16,17
Interestingly intermolecular S···π contact (3.372 Ǻ) between S1 atom and the centroid of benzene ring (C4–C9) of 2 is observed.
Figure 6 illustrates the fluorescence spectral behavior of 1 and 2 in CHCl3 (1.0 x 10–5 M) and in the solid state. Coumarin derivatives (1 and 2) in CHCl3 solution gave a strong emission band around 464 nm (when excited at 400 nm) for 1 and 480 nm (when excited at 452 nm) for 2, respectively. The emission intensity of 2 was larger than that of 1. Interestingly, phenyl derivative (1) gave a strong emission band at longer wavelength (477 nm) in the solid state when excited at 400 nm but thiazole derivative (2) had no fluorescence in solid state.

The strong emission at longer wavelength of 1 would be explained on the intermolecular interaction between the coumarin rings by the formation of C-H···π interaction in the crystal state. While no fluorescence of 2 in solid state might be explained on the intermolecular interaction between the coumarin rings by the formation of π···π and S···π interactions in the crystal state. In conclusion, the solid-state fluorescence properties of 7-diethylaminocoumarin derivatives were dependent on the molecular arrangement in the crystals.

ACKNOWLEDGEMENT
This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University)”.

References

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