Structural and Spectroscopic Study of 6, 7-Dicyano-Substituted

Article pubs.acs.org/JPCA

Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity Ken-ichi Sakai,*,† Kenta Nagahara,† Yuuya Yoshii,‡ Norihisa Hoshino,‡ and Tomoyuki Akutagawa‡ †

Department of Bio- and Material Photonics, Chitose Institute of Science and Technology (CIST), Chitose 066-8655, Japan Polymer Hybrid Materials Research Center, Institute of Multidisciplinary Research for Advanced Materials (IMRAS), Tohoku University, Sendai 980-8577, Japan

‡

S Supporting Information *

ABSTRACT: The introduction of cyano groups into lumazine (pteridine-2,4-(1H,3H)dione) at the C6 and C7 positions enhances its electron affinity, proton acidity, and solubility in solvents. As a result, 6,7-dicyanolumazine (DCNLH2) forms charge transfer (CT) complexes with donors such as tetrathiafulvalene, 2,3,5,6-tetramethyl1,4-phenylenediamine, and 3,3′,5,5′-tetramethylbenzidine and readily dissociates a proton from the N1 nitrogen to form a monoanionic salt with tetrabutylammonium (TBA+). Crystal structures of the CT complexes consist of mixed stacks in which DCNLH2 interacts with donors in face-to-face configurations, but they form intermolecular hydrogen bonds differently depending on the donor type. In the TBA+ salt, two deprotonated DCNLH− monoanions form a unique dianionic dimer connected by two centrosymmetric hydrogen bonds, N3−H···O−C2, which is electronically isolated by the presence of bulky TBA+ countercations and the absence of a proton at the N1 hydrogen-bonding site. This dimer fluoresces yellowish green (fluorescence quantum yield Φ = 0.04). Because the DCNLH− anion only shows weak blue fluorescence in aqueous solution (Φ < 0.01), we suggest that the dimer formation is responsible for the fluorescence enhancement with a large emission band shift to the low-energy side.

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INTRODUCTION Pteridine, a heterobicyclic compound composed of fused pyrimidine and pyrazine rings, is found in a range of biological systems and biochemical reactions. Among its derivatives (Scheme 1), isoalloxazine (10-substituted 2,3,4,10-

moiety, its redox and acid−base properties have drawn attention to understand the biochemical mechanisms involved.1,2 Similar attention has also been directed at other derivatives, such as pterin (2-aminopteridine-4(3H)-one),3−5 lumazine (pteridine-2,4-(1H,3H)dione; hereafter LH2),6−8 and alloxazine (benzo[g]pteridine-2,4-(1H,3H)dione), a tautomer of isoalloxazine.9−11 These derivatives have been studied particularly from the viewpoint of photophysical and photochemical properties that are sensitive to pH or to hydrogen bonds (H-bonds) with the surrounding molecules. For example, it has recently been shown that fluorescent LH2 with methyl groups at C6 and C7 positions is an effective probe for detecting DNA abasic sites, because selective binding to specific DNA bases through H-bonds causes its fluorescence to be quenched.12 LH2 exhibits a redox reaction associated with the transfer of two electrons and two protons.13 It has two ionizable protons at the N1 and N3 positions; thus, it can exist in neutral (LH2), monoanionic (LH−), and dianionic (L2−) forms depending on the pH of the aqueous solution. LH2 has pH-dependent absorption and fluorescence spectra because its three forms have different electronic structures.6 The ability of LH2 to convert into various redox and acid−base states is an important

Scheme 1. Representative Pteridine Derivatives

tetrahydrobenzo[g]pteridine-2,4-dione) is probably the most common; it is a component of riboflavin (vitamin B2), flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). FMN and FAD play a hydrogen-accepting (i.e., electron- and proton-accepting) role as a cofactor of flavoproteins in many dehydrogenation and hydroxylation reactions. Because these reactions occur at the isoalloxazine © 2013 American Chemical Society

Received: February 12, 2013 Revised: March 27, 2013 Published: April 9, 2013 3614

dx.doi.org/10.1021/jp401528c | J. Phys. Chem. A 2013, 117, 3614−3624

The Journal of Physical Chemistry A

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each of these complexes except TMBZ. The dimers in the CT complexes are negatively charged by CT from the donors, while the dimers in the TBA+ salt are composed of the monoanions with one proton dissociated. The different H-bonds formed in these dimers are discussed using X-ray and IR data and theoretical analysis using a density functional theory (DFT) method.

factor in its functions and roles in biological systems. Therefore, chemical modulation of its electron affinity and proton acidity levels is sure to yield better understanding of LH2. H-bonding interactions between the molecule and its surroundings are also important and have been the subject of many pteridine derivative (including LH2) investigations.10,11,14−16 In particular, the pyrimidine part of LH2, which has the same structure as the nucleotide base uracil, is likely to form H-bonds with itself, giving a cyclic H-bonded dimer structure in crystals.17,18 Pteridines, therefore, provide a suitable way of investigating Hbonds in solids.19 In this work, we focus on 6,7-dicyano-substituted LH2 (DCNLH2) and compare its acid−base and optical properties to those of LH2. Introducing cyano groups to LH2 should enhance its electron affinity and proton acidity because of the strong electron-withdrawing effect of the cyano groups. In fact, its higher electron affinity makes it possible for DCNLH2 to form charge transfer (CT) complexes with electron-donating molecules (Scheme 2), such as tetrathiafulvalene (TTF),

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EXPERIMENTAL SECTION Preparation of (DCNLH2)3(H2O)2 (1). DCNLH2 was prepared according to a published method,20 and single crystals were obtained by slow cooling a 9:1 ethanol:H2O solution. Anal. Calcd for C24H10N18O8 (1): C 42.49%, H 1.49%, N 37.16%; found: C 42.66%, H 1.73%, N 37.18%. Preparation of (TMBZ)(DCNLH 2)(CH3CN) (2) and (TMPD)1.5(DCNLH2) (3). 1 (50 mg) and TMBZ (50 mg) or TMPD (50 mg) were placed at the bottom of each side of an H-shaped tube. Their diffusion in acetonitrile (40 mL) for a week yielded black needle crystals of the CT complexes. Polymorphic crystals are present in the TMPD complex. Although the 1:1 complex is the great majority, X-ray singlecrystal analysis was successfully achieved for the 1.5:1 complex (3). Anal. Calcd for C26H25N9O2 (2): C 63.02%, H 5.09%, N 25.44%; found: C 62.89%, H 5.28%, N 24.97%. Anal. Calcd for C18H18N8O2 (the 1:1 complex of TMPD and DCNLH2): C 57.14%, H 4.79%, N 29.61%; found: C 57.38%, H 4.62%, N 29.72%. Preparation of (TTF)(DCNLH2)2 (4). An acetonitrile solution (2 mL) of TTF (50 mg) was added to an acetonitrile solution (2 mL) of 1 (50 mg). The solution changed from pale yellow to black. After filtration, the filtrate was left standing overnight in a refrigerator to yield black block-shaped crystals. Anal. Calcd for C11H4N6O2S2 (4): C 41.77%, H 1.27%, N 26.57%; found: C 41.85%, H 1.46%, N 26.55%. Preparation of (TBA+)(DCNLH−) (5). An acetonitrile solution (5 mL) of TBA+ acetate (150 mg) was added to a hot acetonitrile solution (5 mL) of 1 (100 mg). The solution

Scheme 2. Electron Donors Used in This Study

2,3,5,6-tetramethyl-1,4-phenylenediamine (TMPD), and 3,3′,5,5′-tetramethylbenzidine (TMBZ). On the other hand, its acidity is also raised, leading to the crystallization of the DCNLH− salt with tetrabutylammonium (TBA+). From X-ray single-crystal analyses, H-bonded dimers were confirmed in Table 1. Crystallographic Data for 1−5 1 formula F.W. crystal system space group crystal size/mm a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z Dc/g cm−3 T/K μ/cm−1 total reflections unique data (Rint) R1 wR2 GOF

C24H10N18O8 678.46 monoclinic P21/c (#14) 0.20 × 0.10 × 0.05 5.5993(1) 24.2233(5) 19.9883(4)

2 C26H25N9O2 495.54 orthorhombic Pna21 (#33) 0.15 × 0.03 × 0.03 12.4315(5) 25.145(2) 7.7790(3)

91.9071(8) 2709.57(9) 4 1.663 100 11.402 30504 4950(0.0294) 0.0397 (I > 2.00σ(I)) 0.1121 (all reflections) 1.003

2431.6(2) 4 1.354 100 7.441 26463 4345(0.0586) 0.0476 (I > 2.00σ(I)) 0.0810 (I > 1.20σ(I)) 0.869

3 C23H26N9O2 460.52 triclinic Pl (#2) 0.15 × 0.10 × 0.02 7.0980(2) 8.8860(2) 17.7107(4) 91.290(1) 94.207(2) 93.309(1) 1111.79(4) 2 1.376 100 7.653 12755 3939(0.0515) 0.0630 (I > 2.00σ(I)) 0.2047 (all reflections) 1.029 3615

4

5

C11H4N6O2S2 316.32 monoclinic P21/c (#14) 0.30 × 0.20 × 0.05 12.4709(3) 6.7324(2) 15.9897(3)

C24H37N7O2 455.60 monoclinic C2/m (#12) 0.20 × 0.10 × 0.10 19.5748(4) 12.6306(3) 10.3215(3)

113.5340(7)

95.112(2)

1230.81(4) 4 1.707 113 4.441 13169 2239(0.0580) 0.0361 (I > 2.00σ(I)) 0.0954 (all reflections) 1.071

2541.75(9) 4 1.190 100 6.303 14692 2438(0.0555) 0.0762 (I > 2.00σ(I)) 0.2391 (all reflections) 0.975



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changed from pale to dark yellow. Slow cooling yielded yellow block-shaped crystals. Anal. Calcd for C24H37N7O2 (5): C 63.27%, H 8.19%, N 21.52%; found C 63.42%, H 8.30%, N 21.56%. Optical Measurements. For the absorption titration experiments, aqueous solutions were adjusted to different pHs using a citric acid/phosphate buffer (pH 3−6), a tris(hydroxymethyl)aminomethane/HCl buffer (pH 7−9), and NaOH (pH > 10). The pH was measured using a pH meter (Thermo Orion model 230). Absorption and fluorescence spectra were recorded on an absorption spectrometer (Shimadzu UV2400PC, Japan) and a fluorescence spectrometer (Jasco FP-6500, Japan), respectively. The fluorescence quantum yield (Φ) in aqueous solutions was determined by a relative method using quinine sulfate in 0.5 M sulfuric acid (Φ = 0.52) as a standard,21 whereas for solids, a spectrometer equipped with an integrating sphere (Labsphere QEMS-2000PL) was used. IR spectra were measured on KBr pellets using a spectrometer (Shimadzu FT-IR 8700, Japan). X-ray Crystallography. Temperature-dependent crystallographic data were collected using a diffractometer (Rigaku RAPID-II, Japan) equipped with a rotating anode fitted with a multilayer confocal optic, using Cu Kα (λ = 1.541 87 Å) radiation from a graphite monochromator. Structure refinements were carried out using the full-matrix least-squares method on F2. Calculations were performed using Crystal Structure software packages. Parameters were refined using anisotropic temperature factors, except for the hydrogen atom. The crystallographic data are shown in Table 1. Computational Method. All calculations were performed on the GAUSSIAN 09W package22 using a DFT method with the B3LYP hybrid functional and the 6-311+G(d,p) basis sets.

Figure 1. DCNLH2 absorption spectra in buffered aqueous solution at different pH values: (a) pH 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0, (b) pH 8.0, 9.0, 10.0, 10.5, and 11.5.

Scheme 3. Acid−Base Equilibrium of DCNLH2 and Its Assumed Molecular Forms in Aqueous Solution

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RESULTS AND DISCUSSION Spectroscopic Determination of Acid Dissociation Constants of DCNLH 2 . The absorption spectra for DCNLH2 are measured at different pHs. Under acidic condition, absorption bands are observed at 278 and 350 nm (Figure 1a). However, the intensity of these bands decreases with pH increasing up to 8.0 while new bands begin to appear at 316 and 398 nm, with isosbestic points at 292, 333, and 372 nm. Further increase in the pH to 11.5 causes the disappearance of the 316 and 398 nm bands and the appearance of new bands at 274 and 366 nm, with isosbestic points at 291, 333, and 388 nm (Figure 1b). These spectral changes certainly indicate the occurrence of two-step proton dissociation, as shown in Scheme 3. The three spectral types observed at pH 3.0, 8.0, and 11.5 are from DCNLH2, DCNLH−, and DCNL2−, respectively. The plot of peak absorbance against pH provides sigmoid-shaped titration curves, as shown in Figure 2. The pKa1 and pKa2 values for DCNLH2 are found to be 5.6 and 10.4, respectively, from the curve midpoints. Comparing the pKa values for DCNLH2 and LH2 (pKa1 = 7.8, pKa2 = 12.6),6 it is clear that the acidity of LH2 is greatly increased by the presence of the electron-withdrawing cyano substituents. Consequently, the three DCNLH2 acid− base states are attainable under weakly acidic to weakly basic conditions. pH-Dependent Fluorescence Spectral Changes for DCNLH2. The fluorescent properties of DCNLH2 also change depending on its acid−base state. Figure 3a shows the fluorescence spectral changes between pH 3.0 and 11.5, along with a plot of the 415 nm peak intensity against pH (inset).

Figure 2. DCNLH2 absorption titration curves against pH, obtained by monitoring absorbance at 316 and 366 nm.

Although the fluorescence is very weak in acidic and neutral conditions, it rapidly increases above pH 9.0. The increase in the peak intensity plot is in good accordance with the increase in the second titration curve in Figure 2. Therefore, fluorescence enhancement results from the formation of DCNL2−. At pH 11.5, when the dianion predominates, Φ is determined to be 0.32. In contrast, Φ is less than 0.01 at pH 3.0 and 8.0, implying that both DCNLH2 and DCNLH− are poorly 3616



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Figure 3. (a) Fluorescence spectral changes in a DCNLH2 aqueous solution with increasing pH and a plot of peak intensities against pH (inset). (b)−(d) Absorption, fluorescence, and excitation spectra at pH 3.0 (b), at pH 8.0 (c), and at pH 11.5 (d). In (b)−(d), solid lines at the left and right are absorption and fluorescence spectrum, respectively. Each fluorescence spectrum was obtained by excitation at the absorption peak wavelength. Dotted lines are excitation spectra monitored at fluorescence maximum wavelength. In (b), a dashed line is the excitation spectrum monitored at 518 nm.

fluorescent. Comparing these values with those for LH2,6 Φ of DCNL2− is 10 times higher than Φ of L2− (0.03), and Φ of DCNLH− is much lower than Φ of LH− (0.24). As has been reported, it is complicated and difficult to identify the LH2 fluorescent species present, especially when the species is in a neutral (LH2) or a monoanionic (LH−) form, because these forms are likely to undergo reactions in the excited state, such as proton association/dissociation and watermediated proton tautomerism.6,23,24 This also seems to be true for DCNLH2. The fluorescence spectrum at pH 3.0 exhibits two bands with peaks at 413 and 518 nm (Figure 3b). Both excitation spectra for these two excited species are in complete agreement with the absorption spectrum with a peak at 351 nm, suggesting that their origins are the same. These spectral characteristics are quite similar to the characteristics previously reported for LH2 under acidic conditions, where the lowerenergy band is ascribed to emission from a monocationic LH3+ form with the N8 nitrogen protonated.6 Therefore, the excited species that give the 518 nm band might be derived from the DCNLH3+ monocation. On the other hand, a large disagreement between the excitation and absorption spectra is found at pH 8.0 (Figure 3c). The excited species showing fluorescence at 440 nm has a peak at 362 nm in its excitation spectrum, which is far from the absorption maximum at 397 nm. A straightforward explanation for this disagreement cannot be found, but it is possible that the solvent (water) effect is a contributing factor. In contrast, the spectral characteristics of DCNL2− at pH 11.5 are more understandable, probably because reactions involving protons are difficult under basic conditions. The excited species showing fluorescence at 414 nm has a peak at 372 nm in the excitation spectrum, which is almost in agreement with the absorption maximum at 366 nm (Figure 3d). Crystal Structure of (DCNLH2)3(H2O)2 (1). LH2 is poorly soluble in any solvent, which makes the fabrication of a single LH2 crystal difficult.17 However, its solubility is greatly

improved by introducing the cyano groups, and a crystal of 1 is easily obtained. In the crystal, DCNLH2 forms a complicated H-bonding network with water molecules (Figure 4a). Although DCNLH2 has many sites capable of H-bonding, the following four sites in the pyrimidine part are particularly effective: N1−H and N3−H as proton donors and O−C2 and O−C4 as proton acceptors. Using these sites, two crystallographically independent DCNLH2 molecules (A and B in Figure 4b) are connected to form H-bonded dimers, where the N3−H and O−C2 sites of A are linked to the O−C2 and N1− H sites of B, respectively. The dimers, therefore, have two asymmetric H-bonds. The bond lengths of each independent DCNLH2 and the H-bond parameters are summarized in Table 2 and 3, respectively. There is a large difference between the two N···O distances (2.76 Å for N3···O and 2.99 Å for N1···O), reflecting the fact that the molecular plane of A is inclined with respect to that of B by about 13°. Crystal Structures of DCNLH2-Based CT Complexes. To verify whether the electron affinity of LH2 is enhanced by dicyano modification, we first perform cyclic voltammetry (CV) measurements on DCNLH2. However, the shape of an irreversible reduction wave observed (data not shown) is similar to that previously reported for LH2, which is ascribed to the dihydrogenation reaction involving two electrons and two protons13 rather than one-electron reduction. This indicates that the same reaction must occur for DCNLH2. Therefore, instead of using CV, we estimate the electron affinity of DCNLH2 from the LUMO energy level. For the sake of comparison, DFT calculations are also carried out for LH2 and representative acceptors and donors used in this study; these results are listed in Table 4. We predict that dicyano modification to LH2 can make its LUMO energy lower by about 1.3 eV. The DCNLH2 LUMO value would, therefore, be close to that of chloranil (CA) but still higher by 0.3 eV. 7,7,8,8Tetracyanoquinodimethane (TCNQ) and CA have high electron affinities for reacting with a TTF donor, giving 3617



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comparison of the donor HOMO levels, the electron-donating ability is considered to be in the order TMPD > TTF > TMBZ. DCNLH2 eventually forms CT complexes with all of these donors. The crystal packing diagrams of 2−4 are shown in Figure 5. In the TMBZ (2) and TMPD (3) complexes, donors (D) are alternately stacked with DCNLH2 acceptors (A) in a DADA arrangement, while the TTF complex (4) rather favors an ADAADA arrangement, in which two A molecules located above and below D are crystallographically equivalent. These mixed stacks bring about orbital mixing between the HOMO of D and the LUMO of A, i.e., CT interaction. Figure 6 shows an overlapping mode for each complex. The interplanar distances in 2, 3, and 4 are 3.16, 3.22, and 3.47 Å, respectively. D and A will be located such that the HOMO−LUMO orbital overlap between D and A is maximized. There is no way of knowing how much charge is transferred from D to A in TMBZ and TMPD complexes. However, in TTF complexes, CT can be estimated from the bond length ratio of the central CC bond and the adjacent C−S bond of TTF.30 The CC and C−S bond lengths in 4 are 1.357 and 1.759 Å, respectively. Using this method, we estimate a charge of +0.1 per TTF and a charge of −0.05 per DCNLH2, showing that 4 is in an almost neutral state because of weak CT interaction. As represented by TTF−CA, the mixed-stack CT complexes provide the possibility of developing unique physical properties originating from the neutral-to-ionic phase transition.27−29,31−34 Exploring this possibility for the DCNLH2-based CT complexes 2−4 is our next subject of study. Intermolecular H-bonding is also different for each of the CT complexes (Figure 7). In the TMBZ complex (2), the N3 nitrogen and the keto oxygen at the C4 position form H-bonds with TMBZ, while the N1 nitrogen binds to CH3CN. As a result, no contact between DCNLH2 molecules is observed (Figure 7a). However, one can see the DCNLH2 dimer structures within TMPD (3) and TTF (4) complexes. In the former, the dimer is constructed by two centrosymmetric N− H···O type H-bonds between the N3 nitrogen of each side and

Figure 4. (a) Crystal-packing view and (b) intermolecular H-bonding in 1.

functional CT complexes, such as the first organic metal TTF− TCNQ25,26 and TTF−CA, which is now becoming important in ferroelectricity research.27−29 In fact, DCNLH2 also formed a CT complex with TTF, but LH2 did not. Therefore, our predictions based on DFT calculations seem to be valid. From a

Table 2. Bond Lengths of DCNLH2 in 1−4 and DCNLH− in 5 Determined by X-ray Crystallography 1 N1−C2 C2−N3 N3−C4 C4 C4a C4a−N5 N5−C6 C6−C7 C7−N8 N8−C8a C8a−Nl C8a−C4a C2−O C4−O C6−C N−C[−C6] C7−C N−C[−C7] Nl−H N3−H

A

B

C

1.383(3) 1.375(3) 1.367(3) 1.486(3) 1.319(3) 1.337(3) 1.400(3) 1.334(3) 1.341(3) 1.365(3) 1.411(3) 1.222(3) 1.216(3) 1.443(3) 1.148(3) 1.454(3) 1.143(3) 0.88 0.88

1.368(3) 1.372(3) 1.370(2) 1.485(3) 1.325(3) 1.332(3) 1.403(3) 1.329(3) 1.331(3) 1.370(2) 1.411(3) 1.227(2) 1.214(3) 1.446(3) 1.142(3) 1.453(3) 1.145(3) 0.88 0.88

1.366(3) 1.381(3) 1.381(3) 1.487(3) 1.322(3) 1.341(3) 1.396(3) 1.337(3) 1.336(3) 1.360(3) 1.409(3) 1.222(2) 1.211(2) 1.452(3) 1.147(3) 1.449(3) 1.149(3) 0.88 0.88 3618

2

3

4

5

1.372(4) 1.386(4) 1.379(4) 1.488(4) 1.333(3) 1.338(4) 1.401(4) 1.333(4) 1.343(4) 1.355(3) 1.406(4) 1.220(3) 1.208(3) 1.451(4) 1.142(4) 1.450(4) 1.146(4) 0.84(3) 0.93(3)

1.380(4) 1.389(4) 1.369(5) 1.482(5) 1.321(5) 1.347(4) 1.397(5) 1.334(5) 1.340(4) 1.353(5) 1.421(4) 1.211(4) 1.230(4) 1.431(5) 1.151(5) 1.439(5) 1.156(5) 0.94(5) 0.94(5)

1.375(2) 1.380(2) 1.369(5) 1.485(2) 1.332(2) 1.338(2) 1.406(3) 1.334(2) 1.339(2) 1.365(2) 1.410(3) 1.220(2) 1.212(2) 1.441(3) 1.150(2) 1.450(2) 1.143(2) 0.82(3) 0.79(2)

1.372(6) 1.360(7) 1.406(5) 1.465(6) 1.334(5) 1.351(6) 1.401(6) 1.319(5) 1.353(6) 1.369(5) 1.429(6) 1.259(5) 1.175(5) 1.441(6) 1.139(6) 1.447(7) 1.140(7) 1.58(4)



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Table 3. Hydrogen Bond Parameters for a DCNLH2 Dimer in 1, 3, and 4 and for a DCNLH− Dimer in 5 and the Reference Data for an LH2 Dimer compd

N−H···O−C

N···O (Å)

N−H (Å)

H···O (Å)

O−C (Å)

N−H···O (deg)

1

[B]N1−H···O−C2[A] [A]N3−H···O−C2[B] N3−H···O−C4 N3−H···O−C2 N1−H···O−C4 N3−H···O−C2 N3−H···O−C2 N3−H···O−C2

2.9948(19) 2.7633(19) 2.873(4) 2.8474(18) 2.9138(19) 2.816(5) 2.878(3) 2.847(3)

0.880 0.880 0.940 0.790 0.820 1.58(4) 0.96(3) 0.97(3)

2.13 1.92 1.95(5) 2.06(3) 2.18(3) 1.41 1.94(3) 1.88(3)

1.222(3) 1.227(2) 1.230(4) 1.220(2) 1.212(2) 1.259(5) 1.229(3) 1.232(3)

166.76 161.14 166(4) 176(3) 148(3) 140(3) 167(2) 177(2)

3 4 5 (LH2)2(3H2O)a a

Reference 18.

Table 4. DFT Estimation of the HOMO and LUMO Levels (eV) of the Molecules Used in This Study LH2 DCNLH2 CA TCNQ TTF TMPD TMBZ

HOMO

LUMO

−7.58 −8.42 −7.97 −7.69 −4.77 −4.39 −4.97

−2.88 −4.20 −4.52 −5.15 −1.18 −0.40 −0.41

Figure 6. Overlapping modes between DCNLH2 and a donor in (a) 2, (b) 3, and (c) 4.

the keto oxygens at the C4 position of the opposite sides (Figure 7b). These dimers are isolated from each other because the N1 nitrogen and the keto oxygen at the C2 position are used for H-bonds with TMPD molecules. In contrast, in the latter, the N3 nitrogens and the keto oxygens at the C2 position participate in dimer construction, giving two centrosymmetric N−H···O type H-bonds and aligning the two DCNLH2 molecules (Figure 7c). These dimers are connected to neighboring dimers through H-bonds between the N1 nitrogen of one side and the keto oxygen at the C4 position of the other side and form a ladderlike structure. In both the dimers in 3 and 4, two DCNLH2 monomers are in an almost planar arrangement. The bond lengths and H-bond parameters are summarized in Tables 2 and 3, respectively. We find DCNLH2 to be a promising building block for crystal engineering because it provides various H-bonded crystals. Particularly, in the case of fabricating CT complexes with donors, DCNLH2 crystal structures seem to be dependent on the donor characteristics such as molecular size, electron-donating ability, and the presence of H-bonding sites. Crystal Structure of (TBA+)(DCNLH−) Salt (5) and its Solid-State Fluorescence. One of the points of interest in LH2 under basic conditions is identifying which protons on the N1 and N3 positions dissociate more easily to form an LH− monoanion. Although LH2 is more likely to release a proton from N1 rather than N3, the situation appears to vary depending on the molecule environment.35 So far, there have been no reports on the structural determination of LH− by Xray crystallography, except for where LH− is built in a zinc complex as the ligand.36 Because of the enhancement of acidity by dicyano modification, monoanionic species predominantly exist around neutral pH. In addition, such a modification

Figure 5. Crystal-packing views of DCNLH2-based CT complexes: (a) 2, (b) 3, and (c) 4. Hydrogen atoms are omitted for clarity. 3619



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Figure 7. Intermolecular H-bonding in (a) 2, (b) 3, and (c) 4.

improves solubility. Our success in crystallizing DCNLH− is largely attributed to these factors. Figure 8a shows packing view of 5. No face-to-face stacking of DCNLH− is observed because of bulky TBA+ countercation

(2.816 Å) are shorter than those in the same type of H-bonded dimers (2.847 Å in 4, 2.878 and 2.847 Å in (LH2)2(H2O)3)18 (Table 3). Considering this, the assignment of proton positions must be reasonable, as two DCNLH− anions could not be placed so close together if no H-bonds exist between them. By comparing the bond length with DCNLH2 of 1 (Table 2), we found that the changes caused by N1-deprotonation are especially remarkable for the N3−C4, C2−O, and C4−O bonds. N3−C4 and C2−O bonds are lengthened from 1.37− 1.38 to 1.41 Å and from 1.21−1.23 to 1.26 Å, respectively, while the C4−O bond is shortened from 1.21−1.23 to 1.18 Å. These changes reflect the distribution of the negative charge caused by N1-deprotonation. In the structural formula of DCNLH2 (Scheme 3), the removal of a proton from the N1 position forces one either to draw [N1C2] double bond and to change [C2O] to [C2−O−] or to draw [N1C8a] and [C4C4a] double bonds and to change [C4O] to [C4− O−]. To explain the elongation of the C2−O bond, the former is more applicable; the elongation must result from negative charge delocalization on the keto oxygen at the C2 position. This explanation is also reasonable regarding the anomalous position of the proton, because the more negatively charged the keto oxygen, the more strongly it attracts the N3 proton from the other side. Although there are some examples of H-bonded dimers based on heterocycles such as 2-pyridone,37 7azaindole,38 and uracil,39 this is the first example, to our knowledge, of a dianionic H-bonded dimer composed of two monoanionic heterocycles. It should also be noted that the crystal of 5 exhibits yellowish-green fluorescence. Its spectrum is shown in Figure 9. Although the fluorescence maximum of DCNLH− in aqueous solution (pH 8.0, Figure 3c) is 440 nm, the fluorescence maximum in the crystal appears to be around 525 nm. Using an integrated sphere method, Φ is determined to be 0.04. Considering that DCNLH− is poorly fluorescent in aqueous

Figure 8. (a) Crystal packing view of 5. Hydrogen atoms are omitted for clarity. (b) Dianionic DCNLH− dimer structure within 5.

obstacles. In the side-by-side direction, DCNLH− forms dimers connected by two centrosymmetric H-bonds of N3−H···O−C2 (Figure 8b). Using a difference Fourier map, we can insert a proton between the N3 nitrogen and its counter keto oxygen, whereas the same cannot be effected at the N1 nitrogen. Therefore, an N1-deprotonated form is more favorable in the crystal of 5. However, the N3−H and H···O distances are 1.58 and 1.41 Å, respectively, which are quite anomalous compared with those in conventional H-bonds. The N3···O distances 3620



Article

neighbors. The bands around 3200 cm−1 are probably due to the N−H stretching in the proton donor side. On the other hand, the characteristics of the spectrum for 5 with DCNLH− monoanions are very different from those seen for 1−4. First, an intense band typical of CO stretching is not observed, but instead it seems to be split into three peaks. The lowest-energy peak is at 1645 cm−1, which is more than 50 cm−1 lower than that of 4. This is consistent with the large elongation of the C2−O bond in 5 confirmed by X-ray analysis. Next, the band corresponding to N−H stretching is not found around 3200 cm−1. This band can be hidden by the 2950 cm−1 centered band, which is ascribed to C−H stretching of TBA+. However, considering the centrosymmetric dimer structure formed by strong H-bonds in 5, it is plausible that the N−H modes are IRinactive. Such disappearance of hydrogen vibrations is often observed in symmetric systems with strong H-bonds.40,41 Finally, compared to the spectra of 1−4, the CN stretching band at 2224 cm−1 in 5 is shifted to the lower energy side by 20 cm−1. This suggests that the negative charge caused by N1 deprotonation is relatively spread over the cyano groups. Assessing H-Bonding State in a DCNLH− Dimer Using DFT Calculations. H-bonds seen for a DCNLH− dimer in 5 are characteristic of a very strong H-bond regime, in which the proton position is located near the center between the proton donor (D) and acceptor (A) atoms. Practically, such situations are quite rare and limited to a few known cases. One of these is when a very short H-bond is formed between homonuclear atoms, as in the case of intramolecular O···H···O H-bonds.42,43 Another is when the difference in pKa values between D and A approaches zero, even if the H-bond is constructed with heteronuclear atoms.44−47 In the former case, the two resonance structures are depicted as [X−H···X] and [X···H− X] because one cannot tell which side is D or A, whereas in the latter, the structures are depicted as [D−H···A] and [D−···H− A+]. Both of the H-bonds are strengthened by resonance between these two limiting structures, resulting in the proton shared by both sides being central between them.19, On the other hand, a very strong H-bond is also formed if either D or A is ionic.48,49 For A−, the H-bond is strengthened by the presence of a negative charge that attracts the proton more strongly and by the resonance between the [D−H···A−] and [D−···H−A] structures. The N3−H···O−C2 H-bonds in a DCNLH− dimer might be classified as this type of H-bond, because as mentioned above, negative charge seems to be delocalized on the keto oxygen at the C2 position. Therefore, we calculate the optimized geometries of DCNLH2 and DCNLH− using the DFT method and estimate how much negative charge is put on the keto oxygen by deprotonation of the N1 nitrogen. The optimized bond lengths and atomic charges (Mulliken and NBO charges) are summarized in Tables 5 and 6, respectively. From the X-ray results (Table 2), it is notable that the C2−O and C4−O bonds are lengthened and shortened, respectively, by N1 deprotonation; they are originally almost the same length. As a result, the C2−O bond becomes longer than the C4−O bond by 0.084 Å in DCNLH−. However, such a bond-length difference between these two bonds is not confirmed in the DFT results, even for DCNLH−. Instead, remarkable changes are seen in the C8a− N1 and N8−C8a bonds, which are related to the large inflow of negative charge (maximum −0.4) to the C8a atom that is positively charged before N1 deprotonation. The increase in negative charge on the O(−C2) atom is relatively small (maximum −0.1). The DFT calculations, therefore, did not

Figure 9. Fluorescence (right side) and excitation (left side) spectra for 5 in crystalline power (solid lines) and in water (dotted lines). Excitation spectra were monitored at fluorescence maximum wavelength.

solution (Φ < 0.01), the dimer formation must be responsible for the fluorescence enhancement associated with the large redshift. The existence of strong H-bonds within the DCNLH− dimer is supported by the fact that similar fluorescence is observed in an aqueous solution of 5 (Figure 9, dotted line). This shows that once the dimer is formed, it can exist in a stable form even in an aqueous solution. The excited species showing fluorescence at 525 nm has a peak at 450 nm in the excitation spectrum, both as a crystal and in solution, resulting in a Stokes shift of 3174 cm−1. This suggests that drastic structural change, such as proton transfer, does not occur in the excited state. A more detailed investigation of the DCNLH− dimer in solution is currently underway. IR Absorption Measurements. To obtain more information on the H-bonding states, IR spectra are measured using the KBr method. In each of the spectra for 1−4, shown in Figure 10, an intense absorption band around 1700 cm−1 is ascribed to CO stretching in DCNLH2. This band is split into two peaks, and the lower-energy side peaks in 1−4 are 1721, 1719, 1712, and 1697 cm−1, respectively. Because the CO bond is weakened more when a stronger H-bond is formed, we conclude that DCNLH2 in 4 is most strongly connected to its

Figure 10. IR absorption spectra of 1−5. 3621



Article

Table 5. DFT-Optimized Bond Lengths (Å) for DCNLH2 and DCNLH− N1−C2 C2−N3 N3−C4 C4−C4a C4a−N5 N5−C6 C6−C7 C7−N8 N8−C8a C8a−Nl C8a−C4a C2−O C4−O C6−C N−C[−C6] C7−C N−C[−C7] Nl-H N3−H

DCNLH2

DCNLH−

1.395 1.392 1.401 1.496 1.321 1.335 1.416 1.332 1.331 1.369 1.415 1.206 1.204 1.433 1.154 1.435 1.154 1.013 1.014

1.362 1.428 1.379 1.487 1.317 1.340 1.420 1.317 1.374 1.323 1.451 1.224 1.220 1.429 1.157 1.446 1.154

Figure 11. Potential curve for concerted displacements of two protons from the N3 to the O(−C2) side in the DCNLH− dimer, obtained using DFT single-point calculations using the X-ray geometrical parameters without optimization. Relative energies (kcal/mol) for each proton coordinate are plotted against the distance between N3 and H.

all, be used to give an interpretation for the anomalous proton positioning.

■

CONCLUSION Although the LH2 skeleton has the ability to convert into various redox and acid−base states, it is difficult to capture these states crystallographically. We found that modifying LH2 with cyano groups enhanced its electron affinity, proton acidity, and solubility in solvents. As a result, DCNLH2 can react with electron donors such as TTF, TMBZ, and TMPD, yielding mixed-stack CT complexes with DCNLH2 in a slightly reduced state. On the other hand, DCNLH2 readily loses a proton, giving the monoanionic salt (TBA+)(DCNLH−). In these crystals, a dimer structure is preferably formed by linking two monomers via two centrosymmetric H-bonds. Particularly, in the (TBA+)(DCNLH−) crystal, we confirmed a unique dianionic dimer with H-bonds showing an anomaly in the proton positions. We also revealed that such a dimer formation brings about yellowish green fluorescence. Therefore, DCNLH2 provides not only useful information for understanding the role

1.012

provide a reasonable explanation for the X-ray results concerning the bond-length changes from N1-deprotonation. This can lead to a misestimation of charge distribution, especially in the monoanionic state. To reproduce the experimental results, one probably needs to take into account the precise surrounding effects, such as H-bonding interactions and the Madelung energy in the crystal. Figure 11 shows the potential curve for the concerted displacements of two protons from the N3 to the O(−C2) side in the DCNLH− dimer, which is obtained when all of the atoms except for the two protons are fixed at the X-ray structure. The curve does not have a minimum near the midpoint of the H-bond, but two minima at the N3 and O(−C2) sides, where the former is higher in energy than the latter by 17 kcal/mol. This treatment could not, after

Table 6. Mulliken and NBO Charges for DCNLH2 and DCNLH− Mulliken charge Nl C2 N3 C4 C4a N5 C6 C7 N8 C8a O[−C2] O[−C4] C[−C6] N[−C−C6] C[−C7] N[−C−C7] H1 H3 a

NBO charge Δ

DCNLH2

DCNLH−

Δa

−0.32477 0.29379 −0.40815 0.33123 −0.02045 0.04892 1.21801 1.16309 0.02999 −0.16642 −0.36942 −0.35423 −1.07984 −0.23936 −1.27773 −0.21232

0.12597 −0.09109 0.04129 0.03006 0.03716 −0.04696 −0.00518 0.07777 0.01595 −0.29463 −0.09213 −0.10947 0.02697 −0.09486 −0.12716 −0.08062

−0.58748 0.77685 −0.64657 0.63668 0.05232 −0.33974 −0.01222 0.05232 −0.33974 −0.01222 −0.64822 −0.61417 0.28395 −0.33898 0.29141 −0.31179

0.01118 −0.03624 −0.00249 −0.01178 −0.02113 −0.03301 −0.08100 −0.07829 0.06890 −0.41454 −0.07278 −0.09306 0.03137 −0.10596 0.04038 −0.09367

0.36766

−0.04711

−0.59866 0.81309 −0.64408 0.64846 0.07345 −0.30673 0.06878 0.13061 −0.40864 0.40232 −0.57544 −0.52111 0.25258 −0.23302 0.25103 −0.21812 0.43470 0.43078

0.40177

−0.02901

DCNLH2

DCNLH

−0.45074 0.38488 −0.44943 0.30117 −0.05761 0.09587 1.22320 1.08532 0.01404 0.12821 −0.27730 −0.24476 −1.10681 −0.14450 −1.15057 −0.13170 0.36597 0.41477

−

a

Δ = DCNLH− − DCNLH2 3622



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(14) Bradlley, L. H.; Swenson, R. P. Role of Hydrogen Bonding Interaction to N(3)H of the Flavin Mononucleotide Cofactor in the Modulation of the Redox Potentials of Clostridium beijerinckii Flavodoxin. Biochemistry 2001, 40, 8686−8695. (15) Schreier, W. J.; Pugliesi, I.; Koller, F. O.; Schrader, T. E.; Zinth, W.; Braun, M.; Kacprzak, S.; Weber, S.; Römisch-Margl, W.; Bacher, A.; Illarionov, B.; Fischer, M. Vibrational Spectra of the Ground and the Singlet Excited ππ* State of 6,7-Dimethyl-8-ribityllumazine. J. Phys. Chem. B 2011, 115, 3689−3697. (16) Cui, D.; Koder, R. L., Jr.; Dutton, P. L.; Miller, A.-F. 15N SolidState NMR as a Probe of Flavin H-Bonding. J. Phys. Chem. B 2011, 115, 7788−7798. (17) Goswami, S.; Maity, A. C.; Fun, H.-K. One-Step Synthesis of Lumazine and Xanthine: First Co-Crystal of Lumazine and Perchloric Acid with a Unique Monohydrated Hydronium Ion (H5O2+) Mediated Supramolecular Assembly of the Lumazine Dimer. Eur. J. Org. Chem. 2007, 4056−4064. (18) Norrestam, R.; Stensland, B.; Söderberg, E. The Crystal and Molecular Structure of Lumazine Hydrate. Acta Crystallogr. 1972, B28, 659−666. (19) Steiner, T. The Hydrogen Bond in the Solid State. Angew. Chem., Int. Ed. 2002, 41, 48−76. (20) Rothkopf, H. W.; Wöhrle, D.; Müller, R.; Koßmehl, G. Di- und Tetracyanpyrazine. Chem. Ber. 1975, 108, 875−886. (21) Crosby, G. A.; Demas, J. N. Measurement of Photoluminescence Quantum Yields. Review. J. Phys. Chem. 1971, 75, 991−1024. (22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. GAUSSIAN 09W; Gaussian, Inc.; Wallingford CT, 2009. (23) Presiado, I.; Erez, Y.; Gepshtein, R.; Huppert, D. Excited-State Intermolecular Proton Transfer of Lumazine. J. Phys. Chem. C 2010, 114, 3634−3640. (24) Denofrio, M. P.; Thomas, A. H.; Braun, A. M.; Oliveros, E.; Lorente, C. Comment on “Excited-State Intermolecular Proton Transfer of Lumazine”. J. Phys. Chem. C 2010, 114, 14307−14308. (25) Ferraris, J.; Cowan, D.; Walatka, W.; Perlstein, J. Electron Transfer in a New Highly Conducting Donor−Acceptor Complex. J. Am. Chem. Soc. 1973, 95, 948−949. (26) Jérome, D. Organic Conductors: from Charge Density Wave TTF-TCNQ to Superconducting (TMTSF)2PF6. Chem. Rev. 2004, 104, 5565−5592. (27) Okamoto, H.; Mitani, T.; Tokura, Y.; Koshihara, S.; Komatsu, T.; Iwasa, Y.; Koda, T.; Saito, G. Anomalous Dielectric Response in Tetrathiafulvalene-p-Chloranil As Observed in Temperature- and Pressure-Induced Neutral-to-Ionic Phase Transition. Phys. Rev. B 1991, 43, 8224−8232. (28) Giovannetti, G.; Kumar, S.; Stroppa, A.; Brink, J. v. d.; Picozzi, S. Multiferroicity in TTF- CA Organic Molecular Crystals Predicted through Ab Initio Calculations. Phys. Rev. Lett. 2009, 103, 266401−1− 266401−4. (29) Kagawa, F.; Horiuchi, S.; Tokunaga, M.; Fujioka, J.; Tokura, Y. Ferroelectricity in a One- Dimensional Organic Quantum Magnet. Nat. Phys. 2010, 6, 169−172. (30) Umland, T. C.; Allie, S.; Kuhlmann, T.; Coppens, P. Relation between Geometry and Charge Transfer in Low-Dimensional Organic Salts. J. Phys. Chem. 1988, 92, 6456−6460. (31) Torrance, J. B.; Vazquez, J. E.; Mayerle, J. J.; Lee, V. Y. Discovery of a Neutral-to-Ionic Phase Transition in Organic Materials. Phys. Rev. Lett. 1981, 46, 253−257. (32) Iwasa, Y.; Koda, T.; Tokura, Y.; Kobayashi, A.; Iwasawa, N.; Saito, G. Temperature-Induced Neutral−Ionic Transition in Tetramethylbenzidine−Tetracyanoquinodimethane (TMB−TCNQ). Phys. Rev. B 1990, 42, 2374−2377. (33) Matsuzaki, S.; Hiejima, T.; Sano, M. Pressure-Induced Neutral− Ionic Transition in a 2:1 Charge Transfer Crystal of Tetrathiafulvalene and Iodanil, (TTF)2IA. Solid State Commun. 1992, 82, 301−304.

of pteridines in vivo but also versatile platforms for the development of H-bonded molecular systems, functional CT complexes, and fluorescent materials.

■

ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic data in CIF format of 1−5. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-123-27-6054. Fax: +81-123-27-6054. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by a Grant-in-Aid and performed under the Cooperative Research Program of Network Joint Research Center for Materials and Devices, Institute of Multidisciplinary Research for Advanced Materials (IMRAS), Tohoku University.

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REFERENCES

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Structural and Spectroscopic Study of 6, 7-Dicyano-Substituted

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