One trinucleus dimethine cyanine dye: Experimental and theoretical studies on molecular structure as well as absorption and fluorescence properties

Journal of Molecular Structure 1039 (2013) 84–93

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One trinucleus dimethine cyanine dye: Experimental and theoretical studies on molecular structure as well as absorption and fluorescence properties D.D. Zhang a, L.Y. Wang a,⇑, J.J. Su a, X.F. Zhang a, Y.B. Lei a, G.H. Zhai a, Z.Y. Wen b a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, PR China b Institute of Modern Physics, Northwest University, Xi’an 710069, PR China

h i g h l i g h t s " A kind of trinucleus dimethine cyanine dye 1 was synthesized and characterized. " The structure of dye 1 was experimentally and theoretically studied. " The electronic emission spectra were predicted in four different solvents with CIS/PCM. " The simulated fluorescence spectra duplicated the experimental one for each solvent.

a r t i c l e

i n f o

Article history: Received 25 September 2012 Received in revised form 25 January 2013 Accepted 26 January 2013 Available online 8 February 2013 Keywords: 1-Methyl-2,6-bis[2-(furan-2yl)vinyl]pyridinium iodide DFT X-ray diffraction Molecular structure Absorption and fluorescence properties

a b s t r a c t A kind of trinucleus dimethine cyanine dye: 1-methyl-2,6-bis[2-(furan-2-yl)vinyl]pyridinium iodide (1) was synthesized and characterized by 1H NMR, 13C NMR, IR, MS, UV–Vis spectroscopy and elemental analysis. The crystals of dye 1, obtained from slow evaporation of solvent acetone, crystallized in the triclinic space group P  1 with a = 9.6501(16) Å, b = 10.2308(17) Å, c = 10.7341(17) Å, V = 887.2(3) Å3, and Z = 2 (at 298(2) K), and it was stabilized by the hydrogen bonds and intermolecular face-to-face p  p aromatic stacking interactions. Crystallographic, IR, 1H NMR and UV–Vis data of dye 1 were compared with the results of density functional theory (DFT) method, and the calculated molecular geometries, vibrational bands, 1H NMR chemical shifts and UV–Vis maximum absorption were consistent with the experimental results. The fluorescence spectra were predicted in four different solvents with CIS/PCM methods. Compared with experimental values, the absolute deviations of emission maxima were 17.4 nm in chloroform, 6.3 nm in DMSO, 4.9 nm in methanol, and 6.8 nm in water, respectively. And the experimental fluorescence spectra were nicely reproduced by the simulated fluorescence spectra for each solvent. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Methine cyanine dyes have been widely researched and explored as sensitizers in photography [1] and optical recording materials in laser disk [2]. Recently, methine cyanine dyes have attracted attention owing to their excellent fluorescence properties [3–5] as well as their potential applications as probes for DNA in living cells [6–10]. Among these, trinucleus dimethine cyanine dyes, containing three heteroaromatic rings joined by two vinyl chains, possess large conjugated systems and have two donor-pacceptor (D–p–A) structures, which have attracted extensive attention in recent years. In a preceding paper [11], we studied the fluorescence properties of six synthetic trinucleus dimethine cyanine dyes with pyridine nucleus in solution and in presence ⇑ Corresponding author. Tel.: +86 29 88302604; fax: +86 29 88303798. E-mail address: [email protected] (L.Y. Wang). 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.01.064

of DNA, and significant enhancement of the fluorescent quantum yield was observed in the presence of DNA, which could be proposed as fluorescent probes for DNA detection. Giuseppe Musumarra et al. [12] reported the synthesis of water-soluble trinucleus dimethine cyanine dyes and spectroscopic (UV/Vis, CD, NMR) studies on their interactions with the decamer d(CGTACGTACG)2, which provided clear evidence for the DNA binding ability of trinucleus dimethine cyanine dyes. Their further studies [13] found that these dyes demonstrated in vitro antitumor antiproliferative effects, particularly, distinct for MCF7 mammary adenocarcinoma cells. Abbotto et al. [14] synthesized two novel heteroaromatic-based chromophores classified trinucleus dimethine cyanine dyes, and investigated their nonlinear optical characterization by the Z-scan technique. The results showed that the dyes exhibited large two-photon absorption (TPA) values in the femtoseconds regime, which proved TPA-based applications potentially. Shindy et al. [15] prepared some new dimethine

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cyanine dyes and tested the antimicrobial activity of selected dyes against some bacterial strains. It was shown that one of the trinucleus dimethine cyanine dyes bearing quinoline ring acted out good antibacterial property. Yang et al. [16] found that the synthetic trinucleus dimethine cyanine dyes had reverse saturable absorption (RSA) and optical limiting effect under nanosecond pulse irradiation. However, there were few reports concerning crystal structures of the trinucleus dimethine cyanine dyes, and they were faced with the limitation of sufficient structural data, which were needed in order to get more reliable structure–property relationships. In this paper, the crystal structure of a trinucleus dimethine cyanine dye: 1-methyl-2,6-bis[2-(furan-2-yl)vinyl]pyridinium iodide (Scheme 1) was reported. And the comparative studies on the theoretical and experimental molecular geometry as well as spectroscopy (IR, 1H NMR, UV–Vis and fluorescence) for dye 1 were described. The geometric parameters of dye 1 in the ground state were calculated by the DFT/B3LYP and DFT/PBE1PBE methods, using 6-31G, and 6-31G⁄ basis sets, respectively. The theoretical chemical shifts of 1H NMR were gained by DFT/GIAO model. The UV–Vis spectrum was calculated at PBE1PBE/6-31G and B3LYP/631G level. The fluorescence spectra in four different solvents were obtained by CIS/PCM method. The significance of this work was to get a better understanding on the usefulness of quantum-chemical methods to predict and evaluate the spectroscopic properties of these important dyes. 2. Experimental 2.1. Reagents and physical measurements All reagents were obtained from commercial sources and used without further purification. All chemicals were of analytical grade. Melting points were taken on a XT-4 micromelting apparatus and uncorrected. IR spectra in cm1 were recorded on Bruker Equiox55 spectrometer. 1H NMR spectra were recorded at 400 MHz on a Varian Inova-400 spectrometer and chemical shifts were reported relative to internal Me4Si. 13C NMR spectra were recorded at 100 MHz on a Varian Inova-400 spectrometer and chemical shifts were reported relative to internal Me4Si. Elemental analysis was performed with Vario EL-III instrument. The electron impact (EI) mass spectra were recorded at 70 eV with a GCMS-QP2010 system equipped with the solid sample direct insertion probe. The absorption spectra were recorded on a Shimadzu UV-1700 UV–Vis spectrometer. Fluorescence measurements were carried out on a Hitachi F-4500 spectrofluorimeter. 2.2. Synthesis The 1-methyl-2,6-bis[2-(furan-2-yl)vinyl]pyridinium iodide (1) was synthesized according to a reported literature procedure [11]. The general route for the synthesis of dye 1 was shown in Scheme 1. 1-methyl-2,6-dimethylpyridinium iodide (0.20 g, 0.8 mmol) and furaldehyde (0.62 g, 6.4 mmol) were dissolved in H2O (18 mL). NaOH (20%, 4.6 mL) was added as a catalyst. The reaction mixture was stirred over 30 min at room temperature. The resulting precipitate was filtered off, washed with cold H2O and purified by

Scheme 1. Synthesis of dye 1.

recrystallization from H2O. Pure khaki powder 0.27 g was obtained (yield: 62%), m.p.: 214–215 °C; UV–Vis (methanol) kmax: 397 nm; 1 H NMR (CDCl3, 400 MHz): d 4.37 (s, 3H, N+CH3), 6.54–6.55 (m, 2H, CH@CH), 6.93–6.94 (m, 2H, furan-H), 7.21–7.27 (m, 2H, CH@CH), 7.50–7.56 (m, 4H, furan-H), 8.08 (d, 2H, J = 8.8 Hz, pyridine-H), 8.28 (t, 1H, J = 8.8 Hz, pyridine-H). 13C NMR (DMSO-d6, 100 MHz): d 41.1, 112.7, 115.4, 115.5, 122.8, 128.4, 142.3, 145.7, 150.6, 152.2. IR (KBr) t: 3061 (t@CAH), 1602, 1561(tC@C), 1463, s 1384 (s, tCAC, tCAN), 1305 (dCAH) 1247 (tas CAOAC ) 1068 (tCAOAC ), 951, 982, 878, 757 (m, d@CAH). MS: EI (70ev) m/z(%):264 (8MACH3I), 263 (39MACH3IAH), 142, 127. Found: C, 53.31; H, 3.60; N, 3.76. Anal. Calcd. for C18H16NIO2 = 405.02: C, 53.35; H, 3.46; N, 3.98. Single crystals of dye 1 for X-ray diffraction experiments were grown from acetone. 2.3. X-ray crystallography A khaki crystal with approximate dimensions of 0.31  0.25  0.15 mm3 was selected for data collection. The Xray diffraction data were collected on a Bruker SMART APEX II CCD diffractometer equipped 0 with a graphite monochromated Mo Ka radiation (k = 0.71073 Å A) by using x  2h scan technique at room temperature. The structure was solved by direct methods with SHELXS-97 [17], and refined using the full-matrix leastsquares method on F2 with anisotropic thermal parameters for all non-hydrogen atoms using SHELXL-97 [18]. Hydrogen atoms were generated geometrically. The crystal data, details concerning data collection and structure refinement were given in Table 1. Atomic coordinates and equivalent displacement parameters were listed in Table 2. Molecular illustrations were prepared using the XP package [19]. CIF file containing complete information on the studied structure were available as Electronic Supplementary Publication from Cambridge Crystallographic Data Centre (CCDC782114). 2.4. Computational In this work, the molecular structures of 1-methyl-2,6-bis[2(furan-2-yl)vinyl]pyridinium iodide at the ground state were optimized by the B3LYP [20,21] and PBE1PBE [22] functionals, using

Table 1 Crystal data and structure refinement for dye 1. Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions (Å)

Volume (Å3) Z Calculated density (mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm3) h Range for data collection Limiting indices h, k, l Refinement method Reflections collected/unique/Rint Completeness to h = 25.10° Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) Largest diff. peak and hole (eÅ3)

C18H18INO3 423.23 298(2) 0.71073 Triclinic P1 a = 9.6501(16) b = 10.2308(17) c = 10.7341(17) 887.2(3) 2 1.577 1.818 416 0.31  0.25  0.15 2.08–25.10 11/11, 12/11, 12/10 Full-matrix least-square on F2 4486/3101/0.0180 98.30% 3101/0/208 1.044 R1 = 0.0349, wR2 = 0.0880 R1 = 0.0426, wR2 = 0.0942 0.293 and 0.528

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Table 2 Atomic coordinates (104) and equivalent isotropic displacement parameters (Å2  103) for dye 1. Atom

x

y

z

U(eq)

I(1) N(1) O(1) O(2) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) C(16) C(17) C(18) O(3)

2522(1) 340(3) 2037(4) 3744(4) 2643(7) 3587(7) 3613(6) 2650(5) 2180(5) 1085(5) 553(4) 888(5) 309(5) 617(5) 958(4) 1923(5) 2550(5) 3466(5) 4169(5) 4931(6) 4654(7) 613(6) 3603(5)

1730(1) 3140(3) 2492(4) 5768(4) 2233(8) 1490(6) 1210(6) 1836(5) 1855(5) 2374(5) 2304(4) 1370(5) 1272(5) 2088(5) 3043(4) 3932(5) 3965(4) 4851(5) 5005(5) 6052(6) 6472(7) 4203(5) 899(5)

6814(1) 2687(3) 7738(3) 1242(3) 9110(5) 9470(5) 8293(5) 7270(4) 5862(4) 4988(4) 3594(4) 3147(4) 1836(4) 965(4) 1375(4) 481(4) 821(4) 1712(4) 3030(4) 3393(5) 2300(6) 3134(5) 3635(4)

73(1) 52(1) 86(1) 88(1) 103(2) 90(2) 82(1) 65(1) 64(1) 63(1) 56(1) 66(1) 69(1) 68(1) 56(1) 64(1) 59(1) 61(1) 72(1) 82(1) 104(2) 81(1) 108(1)

U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.

6-31G, and 6-31G⁄ basis sets, respectively. Analytical frequency calculations were done to confirm the optimized structures to be an energy minimum. In our calculation, the anion, which would not affect the spectra of the cationic dyes, was neglected. The IR spectrum was calculated at B3LYP/6-31G⁄ level. The 1H NMR data were obtained from two different methods: PBE1PBE and B3LYP, using 6-311++G⁄⁄ basis set. The UV–Vis spectrum was calculated at PBE1PBE/6-31G and B3LYP/6-31G level in methanol solution, respectively. The fluorescence spectrum was obtained by CIS/ PCM [23] method. All calculations were performed by Gaussian 03 and GaussView molecular visualization program packages running under Windows XP [24,25].

3. Results and discussion 3.1. Crystal structure The molecular structure and atom numbering of 1-methyl-2,6bis[2-(furan-2-yl)vinyl]pyridinium iodide (1) were shown in Fig. 1, and Table 3 listed some selected bond lengths, bond angles and torsion angles obtained from X-ray diffraction and DFT calculation for the dye molecule. It could be found from the X-ray results that the bond lengths of the carbon–carbon, carbon–nitrogen and carbon–oxygen for dye 1

were basically intermediate between typical CAC single (1.54 Å) and C@C double (1.34 Å) bonds, typical CAN single (1.47 Å) and C@N double (1.27 Å) bonds as well as typical CAO single (1.42 Å) and C@O double (1.21 Å) bonds, respectively, which demonstrated that the bonds of the carbon–carbon, carbon–nitrogen and carbon– oxygen on the dye molecular skeleton had definite double bond character. This meant that the bond lengths of the carbon–carbon, carbon–nitrogen and carbon–oxygen in the framework molecule had a tendency of averages, and the p orbits had overlapped and crossed-cover, and the p electrons in the whole dye molecule were delocalized. It could also be seen that most bonds had a partial double-bond character, however, some bonds had a much more pronounced double bond character than others, and these corresponded precisely to the bonds marked as double in the empirical Lewis structure presented in Scheme 1. In short, furan rings, pyridine ring, together with two vinyl chains of the dye molecules formed a big conjugated system. All bond angles on pyridine ring were close to 120°, and 108° on furan rings, which meant that the heterocyclic rings of the dye molecules had no deformation. The torsion angles of C(5)AC(6)AC(7)AN(1), C(5)AC(6)AC(7)AC(8) and C(1)AO(1)AC(4)AC(5) were 166.1°, 15.8° and 177.2°, which showed that the dye molecular framework was slightly distorted. It could be seen from Table 3 that the computed bond lengths, bond angles and dihedral angles fitted the experimental data in general, and the mean absolute deviations of bond lengths, bond angles and torsion angles were 0.03 Å, 0.58° and 5.89° for B3LYP/ 6-31G, 0.02 Å, 0.49° and 6.41° for B3LYP/6-31G⁄, 0.02 Å, 0.62°, and 6.37° for PBE1PBE/6-31G, and 0.02 Å, 0.53°, and 6.82° for PBE1PBE/6-31G⁄, respectively. We had also drawn graphic correlations between the experimental and the theoretical bond angles of dye 1 (Fig. 2). As shown in Fig. 2, the computed bond angles obtained by four different methods were in good agreement with experimental data. It appeared that the B3LYP/6-31G⁄ method correctly reproduced the signs of the bond angles, which was useful for investigating the characteristic of some structurally analogous dyes. 3.2. Crystal packing The crystal packing of dye 1 was presented in Fig. 3. The investigated compound crystallized in the P  1 space group with two dye molecules and two molecules of crystal water per unit cell. In crystal packing along a axes, there were four kinds of hydrogen bonds between molecules (Table 4). They were C(1)AH(1)  O(2), formed by C(1)AH(1) in furan ring and O(2) in furan ring of another dye molecule, C(5)AH(5)  O(3), formed by C(5)AH(5) in vinyl group and O(3) of crystal water, C(13)AH(13)  I(1), formed by C(13)AH(13) in vinyl group and I(1), and C(17)AH(17)  O(3), formed by O(3) of crystal water and C(17)AH(17) in furan ring of another dye molecule, respectively. In the crystal packing along c axis, adjacent molecules were stacked through hydrogen bond between O(3)AH(3B) of crystal water and I(1), coulombic attraction between I and dye molecules, and p  p interaction force with face-to-face distances of 3.370 Å. That is, the pyridine ring from one molecular interacted with furan ring from another molecular and they formed paralleled planes. In summary, supramolecular interactions, including hydrogen bonds, coulombic attractions and p  p stacking, made the dye form a steric configuration of three-dimensional extension. 3.3. IR spectrum

Fig. 1. The molecular structure diagram of dye 1.

Vibrational spectroscopy was extensively used in organic chemistry for the identification of functional groups of organic compounds, the study of molecular conformations, reaction kinetics,

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D.D. Zhang et al. / Journal of Molecular Structure 1039 (2013) 84–93 Table 3 The selected bond lengths (Å), bond angles (°) and torsion angles (°) for dye 1 determined by X-ray diffraction and DFT calculations. Parameters

X-ray

B3LYP 6-31G

Bond lengths N(1)AC(7) N(1)AC(11) N(1)AC(18) O(1)AC(4) O(1)AC(1) O(2)AC(14) O(2)AC(17) C(1)AC(2) C(2)AC(3) C(3)AC(4) C(4)AC(5) C(5)AC(6) C(6)AC(7) C(7)AC(8) C(8)AC(9) C(9)AC(10) C(10)AC(11) C(11)AC(12) C(12)AC(13) C(13)AC(14) C(16)AC(17) Mean absolute deviation Bond angles C(7)AN(1)AC(11) C(7)AN(1)AC(18) C(4)AO(1)AC(1) C(14)AO(2)AC(17) C(2)AC(1)AO(1) C(1)AC(2)AC(3) O(1)AC(4)AC(3) O(1)AC(4)AC(5) N(1)AC(7)AC(8) N(1)AC(7)AC(6) C(9)AC(8)AC(7) C(10)AC(9)AC(8) C(9)AC(10)AC(11) N(1)AC(11)AC(10) N(1)AC(11)AC(12) C(13)AC(12)AC(11) C(12)AC(13)AC(14) C(15)AC(14)AO(2) O(2)AC(14)AC(13) Mean absolute deviation Dihedral angles C(2)AC(3)AC(4)AC(5) O(1)AC(4)AC(5)AC(6) C(5)AC(6)AC(7)AN(1) C(5)AC(6)AC(7)AC(8) C(1)AO(1)AC(4)AC(5) C(18)AN(1)AC(11)AC(12) Mean absolute deviation a

1.371 1.376 1.475 1.341 1.375 1.358 1.367 1.294 1.405 1.345 1.435 1.324 1.445 1.390 1.372 1.370 1.386 1.448 1.337 1.414 1.313

122.5 118.3 105.4 106.4 111.3 106.8 110.0 118.8 118.3 120.2 120.4 120.0 121.0 117.8 119.5 124.9 125.5 108.7 118.9

177.3 5.6 166.1 15.8 177.0 3.0

1.394 1.394 1.489 1.406 1.387 1.406 1.387 1.372 1.428 1.384 1.422 1.364 1.449 1.402 1.392 1.392 1.402 1.449 1.364 1.422 1.372

122.29 118.80 106.98 106.97 110.03 106.99 108.49 118.54 118.07 119.21 120.74 119.80 120.74 118.07 119.22 123.91 124.67 108.49 118.54

179.8 1.3 157.1 24.6 179.8 10.9

PBE1PBE a

AD

6-31G

0.02 0.02 0.01 0.06 0.01 0.05 0.02 0.08 0.02 0.04 0.01 0.04 0.00 0.01 0.02 0.02 0.02 0.00 0.03 0.01 0.06 0.03

1.386 1.386 1.479 1.375 1.355 1.375 1.355 1.369 1.419 1.382 1.422 1.363 1.448 1.399 1.389 1.389 1.399 1.448 1.363 1.422 1.369

122.39 118.75 107.01 107.01 111.04 105.94 109.18 118.96 118.15 119.27 120.61 119.77 120.61 118.15 119.27 123.74 125.23 109.18 118.96

0.21 0.50 1.58 0.57 1.27 0.19 1.51 0.26 0.23 0.99 0.34 0.20 0.26 0.27 0.28 0.99 0.83 0.21 0.36 0.58 2.50 4.31 9.00 8.82 2.82 7.90 5.89

⁄

179.9 1.6 155.8 26.0 179.9 11.4

AD 0.01 0.01 0.00 0.03 0.02 0.02 0.01 0.08 0.01 0.04 0.01 0.04 0.00 0.01 0.02 0.02 0.01 0.00 0.03 0.01 0.06 0.02 0.11 0.45 1.61 0.61 0.26 0.86 0.82 0.16 0.15 0.93 0.21 0.23 0.39 0.35 0.23 1.16 0.27 0.48 0.06 0.49 2.60 4.04 10.30 10.15 2.92 8.44 6.41

6-31G 1.385 1.385 1.476 1.396 1.378 1.396 1.378 1.369 1.424 1.380 1.419 1.360 1.445 1.398 1.389 1.389 1.398 1.445 1.360 1.419 1.369

122.38 118.77 107.14 107.14 110.05 106.86 108.65 118.43 118.21 119.12 120.60 119.71 120.60 118.21 119.11 123.56 124.46 108.65 118.43

179.8 1.4 155.4 26.3 179.8 10.5

AD 0.01 0.01 0.00 0.05 0.00 0.04 0.01 0.08 0.02 0.03 0.02 0.04 0.00 0.01 0.02 0.02 0.01 0.00 0.02 0.01 0.06 0.02 0.12 0.47 1.74 0.74 1.25 0.06 1.35 0.37 0.09 1.08 0.20 0.29 0.40 0.41 0.39 1.34 1.04 0.05 0.47 0.62 2.50 4.24 10.70 10.52 2.81 7.46 6.37

6-31G⁄ 1.376 1.376 1.467 1.365 1.346 1.365 1.346 1.366 1.414 1.378 1.420 1.358 1.444 1.395 1.386 1.386 1.395 1.444 1.358 1.420 1.366

122.48 118.72 107.08 107.08 111.14 105.77 109.38 118.89 118.29 119.17 120.46 119.71 120.46 118.29 119.16 123.35 125.04 109.38 118.89

179.9 1.6 154.3 27.4 179.9 11.1

AD 0.00 0.00 A0.01 0.02 0.03 0.01 0.02 0.07 0.01 0.03 0.02 0.03 0.00 0.01 0.01 0.02 0.01 0.00 0.02 0.01 0.05 0.02 0.02 0.42 1.68 0.68 0.16 1.03 0.62 0.09 0.01 1.03 0.06 0.29 0.54 0.49 0.34 1.55 0.46 0.68 0.01 0.53 2.60 3.97 11.80 11.59 2.90 8.05 6.82

AD: Absolute deviation.

etc. The observed and calculated data of the vibrational spectrum of dye 1 were given in Table 5. The results showed that the calculated IR spectrum data were slightly higher than the experimental value. The suggested reasons were as following: first, the harmonic oscillator approximation was considered in the calculation of vibrational spectroscopy; second, the computational data were gained when the dye molecules were in isolated state of vacuum condition, however, the experimental data were obtained when they were solid complex. After the calculated data were modified using 0.9613 [26] as the frequency scaling factor, the maximum error and the mean absolute deviation between calculating and experiment vibrational spectrum were 16 cm1 and 8 cm1,

respectively. And the plot of the calculated wavenumbers vs experimental wavenumbers, as shown in Fig. 4, showed a straight line. The values of the correlation coefficient (0.9999) indicated good linearity between the calculated and experimental wavenumbers. The observed experimental IR spectrum and the theoretically predicted IR spectrum were shown in Fig. 5. It could be found that the experimental vibrational frequency at 3479 cm1 referred to the tOH of water, which did not appear in the computed spectrum obviously. The reason was that the small residual moisture presented in the sample led to the sharp water band in this region. The typical vibration frequency of @CAH stretching mode at 3073 cm1 was slightly higher than the experimental band at

D.D. Zhang et al. / Journal of Molecular Structure 1039 (2013) 84–93

a

125

y = 0.95935x + 4.52033

B3LYP/6-31G

2

120

R = 0.98709

115 110

b

125

B3LYP/6-31G*

88

120

105

y = 0.95890x + 4.71230 2

R = 0.99010

115 110 105

105

110

115

120

105

125

110

125

d y = 0.94396x + 6.28851 2

120

PBE1PBE/6-31G*

PBE1PBE/6-31G

c

115

120

125

X-ray

X-ray

R = 0.98578

115 110

125 120

y = 0.94549x + 6.25123 2 R = 0.98799

115 110 105

105 105

110

115

120

125

X-ray

105

110

115

120

125

X-ray

Fig. 2. Graphic correlations between the experimental and the theoretical bond angles of dye 1 obtained by four different methods: (a) B3LYP/6-31G, (b) B3LYP/6-31G⁄, (c) PBE1PBE/6-31G and (d) PBE1PBE/6-31G⁄.

3061 cm1. The dye 1 with the conjugated chain predominantly involved carbon carbon bonds and the vibrational frequency was associated with carbon carbon bond stretching modes of the whole carbon skeleton. The vinyl C@C stretching vibration mode predicted at 1612 cm1 was in excellent agreement with the experimental observation at 1602 cm1. The computed vibrational frequencies in the range of 1565–1471 cm1 were ascribed to C@C and C@N resonance conjugated unsaturated stretching modes in the chromophore, which were in excellent agreement with the experimental observation at 1561–1463 cm1. The CAN stretching vibration was found at 1395 cm1, which was in keeping with the computed band at 1384 cm1. The CAOAC stretching vibrations of furan rings were calculated in the range of 1262–1084 cm1, which also fitted the experimental band at 1247–1068 cm1. The various carbon–carbon and carbon–hydrogen deformative vibrations were expected in the range of 758–959 cm1, which were satisfactorily consistent with the experimental band at 757–951 cm1. Moreover, the IR frequency band at 951 cm1 coincided satisfactorily with the computed data at 959 cm1, which was caused by rocking vibrations outside of the RCH@CHR plane. It also showed transconfiguration of methine chain in the molecular structure, which could be proved by the crystal structure of dye 1.

3.4. 1H NMR spectra The experimental spectrum data of dye 1 in CDCl3 with TMS as internal standard were obtained at 400 MHz and were displayed in Table 6. The absolute isotropic chemical shielding of dye 1: ddye(1H) was calculated by DFT/GIAO model at PBE1PBE and B3LYP level with the 6-311++G⁄⁄ basis set, respectively. Relative chemical shifts were then estimated using the corresponding TMS shielding dTMS(1H) calculated at the same theoretical level. Numerical values

of chemical shift dapred ¼ dTMS ð1 HÞ  ddye ð1 HÞ, together with calculated values of dTMS(1H) were tabulated in Table 6. The theoretical and experimental 1H chemical shift values (with respect to TMS) were compared, as shown in Table 6. It could be seen that the 1H chemical shift values were calculated to be 3.84–8.21 ppm with PBE1PBE level and 3.82–8.15 ppm with B3LYP level, which were observed to be 4.37–8.28 ppm. It stated that the calculated chemical shifts were mostly in agreement with the experimental 1H NMR data, and the comparison of absolute deviation was 0.98 to 1.67 ppm with PBE1PBE level and 0.90 to 1.03 ppm with B3LYP level. It appeared that B3LYP method was more suitable than PBE1PBE method for studying the numerical values of chemical shift. 3.5. The UV–Vis spectra and the dominant electronic excitations The dye stock solution (5.0  104 mol L1 in DMSO) was diluted with different solvents and resulted in working solution of dye (2.0  105 mol L1). The absorption spectra were examined at room temperature in different solvents and recorded using 1 cm quartz cells on a Shimadzu UV-1700 UV–Vis spectrometer. The physical constants and the UV–Vis data of dye 1 in different solvents were listed in Table 7. It could be found that dye 1 absorbed in the range of 399– 415 nm, and had molar extinction coefficients of 2.4  1042.9  104 L mol1 cm1 in different solvents, and the kmax of dye 1 decreased with the increasing of solvent polarity. The effect of the solvent polarity on the absorption maximum could be explained as follow: interactions between the dye molecules and the solvents made the ground state of dye molecules more stable by forming hydrogen bonds [27,28]. As a preliminary study of the solute–solvent interactions, the Bayliss function [29] [f(n, e) = (n2  1)/(2n2 + 1) + (e  1)/(e + 2)]

D.D. Zhang et al. / Journal of Molecular Structure 1039 (2013) 84–93

89

Fig. 3. Crystal packing of dye 1: (a) along the a axis; and (b) along the c axis.

Table 4 The geometry of DAH  A hydrogen bonds of dye 1. DAH  A

d(DAH) (Å)

D(H  A) (Å)

d(D  A) (Å)

\(DHA) (°)

C(1)AH(1)  O(2) C(13)AH(13)  I(1) C(5)AH(5)  O(3) C(17)AH(17)  O(3) O(3)AH(3B)  I(1)

0.929 0.929 0.930 0.930 0.850

2.530 3.121 2.665 2.571 2.896

3.288 4.009 3.409 3.375 3.659

139 160 138 145 150

was used, where n was the refractive index and e was the static dielectric constant of the solvents. Calculated f(n, e) values ranged from 0.169 for methanol, to 0.210 for chloroform. Plotting the results of 1/k with f(n, e) was shown in Fig. 6, and the correlation between 1/k and f(n, e) provided R2 value of 0.8740. This deviation of the linear correlation indicated that the maximum absorption wavelengths of dye 1 were not only related to the refractive index and the static dielectric constant of the solvents, but also to other properties of the solvents.

The observed absorption maxima and the main orbital compositions of the computed lower-lying singlet excited states of the dye molecules were given in Table 8. The TD-DFT calculations presented three transition states, and only one dominant transition with a maximum configuration interaction coefficient for the dye. From Table 8, it could be seen that the lowest energy absorption found at 398.0 nm belonged to the first dipole-allowed p ? p⁄ transition from HOMO to LUMO, which might be attributed to intramolecular charge transfer (ICT) [30], corresponding to 448.4 nm with a maximum configuration interaction coefficient of 0.86 calculated at B3LYP/6-31G level and 427.3 nm with a maximum configuration interaction coefficient of 0.87 calculated at PBE1PBE/6-31G level, respectively. It revealed that PBE1PBE method was more suitable than B3LYP method for studying the absorption spectrum of this dye. Fig. 7 presented the sketch of the MOs for dye 1 at PBE1PBE/6-31G level. It showed that HOMO was mainly localized on the whole molecular skeleton, and LUMO on the pyridine ring and double bond moieties. However, HOMO1 was

90

D.D. Zhang et al. / Journal of Molecular Structure 1039 (2013) 84–93

Table 5 Comparison of the observed and calculated vibrational spectrum of dye 1. Exp.

Calc.

Freq.

a

b

Int. (IR)

No scaled

Scaled

3061 m 3197 1602 s 1677 1561 vs 1628 1463 s 1530 1384 s 1451 1305 m 1355 1247 vs 1313 1068 m 1128 951 m 998 928 m 965 878 m 916 757 m 789 Mean absolute deviation a b c d

ADd

Vibratory feature c

3073 1612 1565 1471 1395 1303 1262 1084 959 928 881 758

Table 6 Theoretical and experimental 1H isotropic chemical shifts (all values in ppm) for the dye 1. Proton

Int. (IR) 0.97 139.8 8.77 303.48 7.96 55.18 627.85 5.93 77.86 6.14 1.32 113.47

t@CAH tC@C tC@C, tC@N tC@C, tC@N tCAN dCAH

d@CAH d@CAH d@CAH d@CAH

Calcd. PBE1PBE/6-311++G⁄⁄

12 10 4 8 11 2 15 16 8 0 3 1 8

tas CAOAC tsCAOAC

Exp.

dexp.

Frequencies in cm1. m: Middle, s: strong, vs: very strong. Scaling factor using 0.9613. AD: Absolute deviation.

a b

TMS 3H(N + CH3)

4.37

2H(5C 6C CH@CH)

6.54

2H(furan-H)

6.93

2H(C12 C13 CH@CH)

7.24

4H(furan-H)

7.53

2H(pyridine-H)

8.08

1H(pyridine-H)

8.28

1

d( H) 31.1478 27.3094 25.9951 27.0489 24.1666 22.9422 23.5740 23.5019 23.9238 23.8803 23.0975 24.5943 24.4633 22.9928 23.8740 23.5475 23.7571

dpred 3.84 5.15 4.10 6.98 8.21 7.57 7.65 7.22 7.27 8.05 6.55 6.68 8.16 7.27 7.60 7.39

a

B3LYP/6-311++G⁄⁄ b

AD

0.53 0.78 0.27 0.44 1.67 0.64 0.72 0.02 0.03 0.52 0.98 0.85 0.63 0.81 0.48 0.89

d(1H) 31.3264 27.5046 26.1909 27.2530 24.1678 24.1658 23.8289 23.7359 24.4374 23.1814 23.3216 24.8306 24.6827 23.2053 24.1683 24.0650 23.8380

dpreda

ADb

3.82 5.14 4.07 7.16 7.16 7.50 7.59 6.89 8.15 8.00 6.50 6.64 8.12 7.16 7.26 7.49

0.55 0.77 0.30 0.62 0.62 0.57 0.66 0.35 0.90 0.47 1.03 0.89 0.59 0.92 0.82 0.79

1

dpred = dTMS( H)  ddye(1H). AD refers to the absolute deviation of the chemical shifts in this table.

3500

mainly localized on the furan and double bond moieties, while LUMO+1 was partially localized on the molecular skeleton. The HOMO and LUMO energy values were 6.20 eV and 2.72 eV for dye 1, respectively, and the calculated energy gaps were 3.48 eV, corresponding to experimental value 3.12 eV. The calculated results were slightly higher than the experimental results in methanol, because the PCM method did not consider any specific interactions between the dye and the solvent [31].

3000 2500

Cal.

2

R =0.9999 2000 1500

3.6. The fluorescence property

1000 500 500

1000

1500

2000

2500

3000

Exp.

Transmittance (%)

Fig. 4. Graphic correlation between the experimental and the scaled theoretical frequencies obtained by B3LYP/6-31G⁄ method.

0.7 0.6 0.5 600

Intensity

a

b

400 200 0 500

1000

1500

2000

2500

3000

3500

4000

Wavenumber (cm-1) Fig. 5. The experimental IR spectrum (a) and predicted IR spectrum and (b) at B3LYP/6-31G⁄ level of dye 1.

The dye stock solution (5.0  104 mol L1 in DMSO) was diluted with different solvents and resulted in working solution of dye (1.0  104 mol L1). Fluorescence measurements were carried out at room temperature on a Hitachi F-4500 spectrofluorimeter in 1 cm quartz cells. Fluorescence emission was excited at the maximum of the absorption spectrum. Excitation maxima, emission maxima and Stokes shift of dye 1 were summarized in Table 9. From Table 9, it could be found that dye 1 exhibited fluorescence properties at room temperature. The fluorescence maxima were located at 506.8–524.4 nm in different solvents. Compared with the absorption maxima of the dye 1, the emission spectra were shifted to the long wavelength region by 91.3–102.4 nm (Stokes shift). The Stokes shifts were large, which might be attributed to an excited-state intramolecular charge transfer between the donor and acceptor in the dye molecules. Large Stokes shift could be conducive to decrease self-quenching and measurement error by excitation light and scattered light [32]. In the present work, to obtain fluorescence properties of dye 1, the electronic emission spectra were predicted with CIS/PCM method at optimized geometric structure. We used the scaling factor 0.72 for the emission wavelengths obtained by CIS method, which had already been proved to remove these systematic errors [33]. The experimental and theoretical kem in four different solvents were summarized in Table 9 and the experimental and computational fluorescence spectra of dye 1 in four different solvents were visible in Fig. 8. In Table 9, the emission maxima of dye 1 were predicted at 506.2–524.2 nm. Compared with experimental values, the absolute deviations were 17.4 nm in chloroform, 6.3 nm in DMSO, 4.9 nm

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D.D. Zhang et al. / Journal of Molecular Structure 1039 (2013) 84–93 Table 7 The physical constants and the UV–Vis data of dye 1 in different solvents. Solvents

Refractive index (n)

Dielectric constant (e)

kmax (nm)

Molar extinction coefficient (e  104) (L mol1 cm1)

k1  103 (nm1)

Chloroform Ethanol Methanol Water

1.4459 1.3610 1.3288 1.3330

4.9 24.6 32.6 78.4

409.5 398.0 398.0 399.5

2.5 2.4 2.5 2.9

2.44 2.51 2.51 2.50

2.52

y = -1.6367x + 2.7892 2 R = 0.8740

λ-1 Χ 10-3

2.50

2.48

2.46

2.44 0.17

0.18

0.19

0.20

0.21

f (n) Fig. 6. Correlation of wave numbers 1/k in various solvents vs f(n, e) of dye 1.

in methanol, and 6.8 nm in water. It could be found that the kem obtained by CIS level in four different solvents was in good agreement with the experimental values. Moreover, simulated fluorescence spectra satisfactorily duplicated the experimental fluorescence spectra for each solvent, as shown in Fig. 8, which was obtained by SWizard program [34]. Therefore, the CIS/631++g fitted this kind of dye well and the scaling factor 0.72 used for scaling CIS method was reasonable. According to the CIS/6-31++g energy level and molecular orbital analysis (here methanol as an illustration, Fig. 8c), S1 ? S0 transition was mainly associated with the transition from the corresponding LUMO ? HOMO, and the maximum configuration coefficient was 0.80. From (Fig. 8c), it could be seen that the LUMO consisted primarily of pyridine ring and double bond moieties dominated by p⁄ antibonding orbital, while HOMO was partially localized on the molecular skeleton dominated by p bonding orbital, the emission peaks located at 512.2 nm should be described as p⁄ ? p, with ICT character.

Fig. 7. Molecular orbitals of dye 1.

Table 9 Excitation maxima, emission maxima, Stokes shift and emission spectra obtained by CIS/6-31++g of dye 1. Solvents

Chloroform DMSO Methanol Water

kex (nm)

415.5 422.0 410.5 415.5

kem (nm) Experimental

Stokes shift

Theoretical

Oscillator strengtha

506.8 524.4 512.2 513

91.3 102.4 101.7 97.5

524.2 518.1 507.3 506.2

2.0202(80%) 2.0263(80%) 2.0280(80%) 2.0317(80%)

a The proportion of the main transition (configuration interaction coefficient) was given in parentheses.

Table 8 Main calculated orbital transitions of dye 1 in methanol. Band

Calculated/kCal (nm)

Exp. (nm)

B3LYP/6-31G

1 2 3 a b

PBE1PBE/6-31G

Wavelength

fa

Transition characterb

Wavelength

f

Transition character

448.4 384.1 339.6

1.2054 0.0012 0.2278

HOMO ? LUMO(86%) HOMO1 ? LUMO(56%) HOMO ? LUMO+1(43%)

427.3 359.2 330.1

1.3004 0.0006 0.2319

HOMO ? LUMO(87%) HOMO1 ? LUMO(59%) HOMO ? LUMO+1(47%)

Oscillator strength. The proportion of the main transition (configuration interaction coefficient) was given in parentheses.

398.0

D.D. Zhang et al. / Journal of Molecular Structure 1039 (2013) 84–93

a

600

600 Experimental spectra Simulated spectra

400

400

300

300

100

200

200

100

100

450

500

550

100

0

0

0

0

600

450

500

Wavelength (nm)

550

600

Wavelength (nm)

c

d

Experimental spectra Simulated spectra

Experimental spectra Simulated spectra

200

100

100

0

0 450

Wavelength (nm)

Intensity, a.u.

F/a.u.

200

Intensity, a.u.

F/a.u.

200

Experimental spectra Simulated spectra

Intensity, a.u.

200

500

Intensity, a.u.

F/a.u.

500

b

F/a.u.

92

500

550

600

Wavelength (nm)

Fig. 8. Experimental and simulated fluorescence spectra (right y-axis) of dye 1 in four different solvents: (a) in chloroform, (b) in DMSO, (c) in methanol, and (d) in water.

4. Conclusions 1-Methyl-2,6-bis[2-(furan-2-yl)vinyl]pyridinium iodide dye was synthesized and characterized by X-ray diffraction, 1H NMR, 13 C NMR, IR, MS, UV–Vis spectroscopy and elemental analysis. Single-crystal X-ray diffraction study indicated that the investigated dye 1 crystallized in the P  1 space group with two dye molecules and two molecules of crystal water per unit cell. The face-to-face p  p aromatic stacking interactions, hydrogen bonds and coulombic attraction were displayed in the crystal packing. In addition, the calculated IR and 1H NMR were in good agreement with the experimental data. Both the excitation and emission spectra gave clear and definite results in experimentally and theoretically, respectively. The molecular orbital coefficient analysis suggested that the electronic spectrum was assigned to the p ? p⁄ electronic transitions. The fluorescence peaked at 512.2 nm belonged to the emission from S1 ? S0, and should be assigned to p⁄ ? p, ICT. Acknowledgement We appreciate the financial support for this research by a grant from the Scientific and Technological Research and Development Projects in Shaanxi Province (No. 2012K07-06), the Special Science Research Foundation of Education Committee (No. 11JK0558). References [1] T. Karatsu, M. Yanai, S. Yagai, J. Mizukami, T. Urano, A. Kitamura, J. Photochem. Photobiol. A 170 (2005) 123. [2] C.F. Zhao, R. Gvishi, U. Narang, G. Ruland, P.N. Prasad, J. Phys. Chem. 100 (1996) 4526. [3] V.B. Kovalska, K.D. Volkova, M.Yu. Losytskyy, O.I. Tolmachev, A.O. Balanda, S.M. Yarmoluk, Spectrochim. Acta Part A 65 (2006) 271. [4] T.G. Deligeorgiev, N.I. Gadjev, A.A. Vasilev, V.A. Maximova, I.I. Timcheva, H.E. Katerinopoulos, G.K. Tsikalas, Dyes Pigm. 75 (2007) 466.

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