English

• 1. INTRODUCTION

  Room temperature phosphorescence (RTP) has attracted considerable attention in the fields of organic light-emitting diodes, biological imaging and sensing devices due to their long-lived emission, large stokes shifts and high quantum yields^[1]. Heretofore, most of the room temperature phosphorescence compounds contain noble metals, such as Ir(Ⅲ), Ru(Ⅲ) and Pt(Ⅱ)^[2]. Considering the intrinsic problems of noble metals including high cost and human toxicity, it is vital to develop highly efficient metal-free pure organic compounds with RTP. However, because of the inefficient intersystem crossing caused by intramolecular motions and collisions with quenching species of oxygen and moisture, achieving organic room temperature phosphorescence is still of great challenge^[3].

  Supramolecular organic frameworks^[4], a kind of crystalline solids constructed via non-covalent interactions (e.g. hydrogen bonds, halogen bonds, van der Waals contacts and π-π stacking) to form one-, two-, or three-dimensional infinite networks, have attracted immense attention not only because of their intriguing architectures but also of their exploitable properties for potential applications in the fields of absorption^[5], luminescence^[6], sensing^[7], catalysis^[8], molecular recognition^[9], and so on. According to literatures, in a self-assembly system the intermolecular interactions such as hydrogen bonds can provide a rigid molecular environment for suppressing non-radiative transition to enhance the phosphorescence^[10]. With this in mind, a native nuclo-bases cytosine with rigid plane organic molecular posing both acceptor and donor functionalities was selected to construct hydrogen bonding net with complementary carboxylic acid^[11], and it is a supramolecular framework with isophthalic acid (C-H₂IA) synthesized in water media. The crystal structure redetermination and room temperature phosphorescence are reported here.

  2. EXPERIMENTAL

  2.1 Materials and methods

  Cytosine (C) and isophthalic acid (H₂IA) were purchased from Energy Chemical without further purification. The IR spectrum was recorded in the range of 4000~400 cm⁻¹ on a Nicolet IS50 FTIR spectrometer with ATR accessory. A thermogravimetric analysis (TGA) experiment was performed using a Mettler Toledo TG/DSC3+ instrument (heating rate of 10 K·min^-1, argon stream) from room temperature to 1000 ℃. Powder X-ray diffraction measurements were performed with a Rigaku Ultima Ⅳ at room temperature. The luminescence and phosphorescence spectra for the solid samples were recorded at room temperature on an Edinburgh Instrument FLS980 luminescence spectrophotometer.

  2.2 Synthesis and crystallization

  The supramolcular framework of C-H₂IA was synthesized from the mixture of cytosine (C) and isophthalic acid (H₂IA) in an aqueous solution at 120 ℃. A 25 mL oven-dried two-necked round bottom flask was charged with cytosine (0.0555 g, 0.5 mmol), isophthalic acid (0.0831 g, 0.5 mmol) and deionized water (15 mL). After being heated in an oil bath at 120 ℃ for 30 min, the turbid solution got clear. Then it was cooled to room temperature naturally, obtaining colorless acicular crystals (yield: 72.6%). IR (cm^-1): 3000 (m, br), 1695 (vs), 1658(vs), 1446 (w), 1370(w), 1253 (m), 1202 (m), 1150 (m), 1073 (w), 973 (w), 930 (w), 792 (w), 735 (m), 873 (w), 645 (w), 568 (w).

  2.3 X-ray crystallography

  Data of compound C-H₂IA were collected on a Rigaku Saturn 724 CCD diffractometer by a graphite-monochromatized MoKα radiation (λ = 0.71073 Å) by using an ω (2.62≤θ≤29.43º) scan mode at 293(2) K. A total of 10529 reflections with 2716 independent ones (R_int = 0.0391) were collected, in which 2208 were observed (I > 2σ(I)). The structure was solved by direct methods and refined with full-matrix least-squares methods on F² by using the Olex2 program package. The final R = 0.0408 and wR = 0.1061 with 185 parameters; S = 1.122, (Δ/σ)_max = 0.000, (Δρ)_max = 0.261 and (Δρ)_min = –0.264 e/ų. All non-hydrogen atoms were refined anisotropic. C-bound H atoms were positioned geometrically, with C–H = 0.93 Å and U_iso(H) = 1.2U_eq(C). Other hydrogen atoms were located in difference Fourier maps, with U_iso(H) = 1.5U_eq(N) and U_iso(H) = 1.5U_eq(O), respectively. Distances and angles of hydrogen bonds for C-H₂IA are given in Table 1.

  Table 1

  

  Table 1.  Hydrogen Bond Lengths (Å) and Bond Angles (°) for C-H₂IA

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  D–H⋅⋅⋅A	d(D–H)	d(H⋅⋅⋅A)	d(D⋅⋅⋅A)	< (DHA)
  N(4)–H(4)⋅⋅⋅N(4)#1	0.88	1.96(3)	2.829(2)	174
  N(6)–H(6A)⋅⋅⋅O(2)#1	0.85	2.03	2.8670(15)	171
  O(9)–H(9)⋅⋅⋅O(9)#2	1.23	1.23	2.4612(18)	180
  N(1)–H(1)⋅⋅⋅O(2)#3	0.93	1.91	2.8304(15)	173
  O(11)–H(11)⋅⋅⋅O(10)#4	0.92	1.80	2.6179(14)	147
  Symmetry transformations used to generate the equivalent atoms: #1: –x+1, –y+2, –z+1; #2: –x, –y+1, –z+1; #3: –x+2, –y+2, –z+1; #4: x+1, y, z

  3. RESULTS AND DISCUSSION

  3.1 Structural description

  According to literatures, the nucleobase cytosine molecule can be protonated under acidic conditions to form three types of motifs, cytosinium ion (CH⁺)^[12], cytosine-cytosinium ion dimer (C·CH⁺) which was first observed by Eichhorn et al. with acetyl cytosine^[13] and cytosinium ion-cytosinium ion dimer (CH⁺·CH⁺), the only one example reported by Pedireddi et al. previously^[14]. However, it is difficult to form a cytosinium ion-cytosinium ion dimer due to the electrostatic repulsion between two face-to-face protonated cytosine molecules. The crystal structure of C-H₂IA was reported by Pedireddi et al. previously (Cambridge Structural Database with a refcode TAZXAU)^[14] containing a cytosinium ion-cytosinium ion dimer. In this previous structure determination, one hydroxyl proton of carboxylic acid transferred to the nitrogen atom of pyrimidine ring, and the short inter H···H contact 1.2227 Å for the adjacent pyrimidine rings and the short inter O···O contact 2.4585 Å for the adjacent carboxyl groups imply the unambiguous protonation state of the compound considering electrostatic repulsion. As a result, it is important to redetermine the crystal structure of cytosinium isophthalate, not only to reveal the actual position of proton but also help to better understand the biological processes for the fact that many cytosine derivatives are active pharmaceutical ingredients.

  The crystal structure of C-H₂IA was determined by single-crystal X-ray diffraction (SC-XRD). As depicted in Fig. 1, single-crystal X-ray diffraction analysis revealed that the supramolecular framework of C-H₂IA crystallizes in triclinic space group P$ \overline 1 $ with a = 8.5825(4), b = 8.9568(5), c = 9.0642(5) Å, α = 100.843(5), β = 105.240(5), γ = 113.056(5)°, containing one protoned cytosine molecule and one deprotoned isophthalic acid in the asymmetric unit, revealing proton transfer from the carboxylate group to the pyrimidine ring occurs. In contrast to the previous structure determination, the hydrogen atom labeled H(9) which located to one of the carboxylate group oxygen atoms O(9) lies on an inversion center shared by two crystallo-graphically equivalent oxygen atoms O(9). Simultaneously, the hydrogen atom labeled H(4) bound to the pyrimidine N(4) atom disorder across a crystallographic inversion center with the occupancy of 0.5 rather than 1.0. As shown in Fig. 2, the cytosine dimeric unit is formed via complementary triple hydrogen bonds utilizing its dual nature of acceptor and donor properties, involving heterocyclic N–H, carbonyl, and the amino groups, thus resembling a Watson-Crick like pattern, by forming N–H···O and N–H···N hydrogen bonds with the N···O and N···N distances of 2.8670(15) and 2.829(2) Å, respectively. In the dimers, the half occupancy hydrogen atom residing on N(2) atom is disordered and shared by two cytosine molecules. The adjacent triplet hydrogen-bonded dimers are connected via N–H···O hydrogen bonds performed by pyridyl NH and oxygen atom with the N···O distances of 2.8304(15) Å, forming a one-dimensional infinite tape structure of [HC₂]_nⁿ⁺ along the [1 0 0] direction. As shown in Fig. 3, the neighboring isophthalic acids are connected via O–H–O symmetric hydrogen bonds to form a one-dimensional lattice like infinite tape of [H(HIA)₂]_n^n- with O···O distances of 2.4612(18) Å. In the symmetric hydrogen bond, the H(9) atom is equally shared between two oxygen atoms, and the donor and acceptor can not be distinguished. The short distances between two crystallographically equivalent oxygen atom O(9) indicate a strong symmetric hydrogen bond, which is not found in the previous report[14] due to the incorrect proton determination. The organic base tapes and organic acid tapes are stacked one with another through π-π stacking interactions involving benzene and pyrimidine rings. π-π interaction can be observed between two neighboring parallel benzene rings with the centroid-centroid distance of 3.9378(9) Å stacking between the pyrimidine and benzene rings involved with the centroid-to-centroid (Cg−Cg) distance of 3.7061(9) Å and dihedral angle of 9.38(7)° (Fig. 4).

  Figure 1

  

  Figure 1.  A perspective view with atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level

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  Figure 2

  

  Figure 2.  A view of infinite tape of [HC₂]_nⁿ⁺ connected via hydrogen bonds (dashed line). Hydrogen atoms on carbons and one moiety of disorder hydrogen atoms are omitted for clarity

  DownLoad: Full-Size Img PowerPoint

  Figure 3

  

  Figure 3.  A view of infinite tape of [H(HIA)₂]_n^n- connected via hydrogen bonds (dashed line). The hydrogen atoms on benzene ring are omitted for clarity

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  Figure 4

  

  Figure 4.  (a) Three-dimensional supramolecular net formed by π-π stacking of tapes; (b) Illustration of the molecular packing in C-H₂IA

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  3.2 Powder X-ray diffraction (PXRD)

  To verify the sample purity after crystallization process, X-ray powder diffractogram of C-H₂IA was performed at room temperature. As shown in Fig. 5, experimental X-ray powder diffractograms are almost in agreement with the simulated pattern calculated from signal-crystal diffraction data using the Mercury software^[15], stating the purity of the compound. The intensity difference between both diffractograms could be attributed to the preferred orientation effects.

  Figure 5

  

  Figure 5.  Powder X-ray diffraction (PXRD) patterns of C-H₂IA

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  3.3 Thermogravimetric analysis (TGA)

  In order to estimate the thermal stability of C-H₂IA, thermogravimetric analysis (TGA) was performed under argon gas atmosphere. In the TGA curve of C-H₂IA (Fig. 6), there was no obvious weight loss above temperature 200 ℃, showing a considerable thermal stability of C-H₂IA. The rapid massive weight loss from 250 to 350 ℃ can be attributed to the decomposition of organic compound. The remainder after treating under 1000 ℃ can be attributed to the carbon formed at high temperature.

  Figure 6

  

  Figure 6.  Thermogravimetric analysis (TGA) curves of C-H₂IA

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  3.4 Photoluminescence analysis

  The photoluminescence properties of C-H₂IA in crystalline state were first explored under atmospheric conditions. Exposed to the radiation of a UV-lamp, the crystals showed a blue light emission. Interestingly, after UV-light cessation, the solid sample displayed nearly 10 seconds lasting green glow which can be easily observed by naked eyes. As shown in Fig. 7b, the steady solid-state photoluminescence spectrum showed a blue emission band with peaks at 350 and 380 nm and a green emission band with peaks at 480 and 500 nm observed from room temperature phosphorescence spectrum (Fig. 7b green line). Further researches on time-resolved photoluminescence decay of C-H₂IA were carried out. As shown in Fig. 7c, the blue emission at 375 nm decay plot was fitted by a tri-exponential curve with lifetime of 10 ns, which can be assigned to the fluorescence properties. As shown in Fig. 7d, the ultra long-lived green emission at 500 nm decay plot was fitted by a bi-exponential curve with lifetime of 0.7 s, which can be assigned to the room temperature phosphorescence properties.

  Figure 7

  

  Figure 7.  (a) Photographs of C-H₂IA crystals with a hand-held 300 nm UV lamp irradiation on and off; (b) Normalized photoluminescence (PL, blue line) and room temperature phosphorescence (green line) spectra of C-H₂IA under 300 nm excitation; (c) Time-resolved fluorescence decay (blue dot) and fitting curves (red line) of the emission bands at 375 nm; (d) Time-resolved phosphorescence decay (green dot) and fitting curves (red line) of the emission bands at 500 nm

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  Suppressing the motion of organic molecules to increase the possibility of radiative transition and decrease the non-radiative transitions plays a key role to the room temperature phosphorescence in this report. Thus, Hirshfeld surface calculation was performed with CrystalExplorer program^[16] to analyze the non-covalent interactions in the crystal lattice. Fig. 8 shows the Hirshfeld surface of C-H₂IA mapped over d_norm used a red-white-blue color scheme. In the Hirshfeld surface, the dark red color highlights shorter contacts with the negative d_norm values which are shorter than the sum of van der Waals radii, whereas the dark blue color represents the contacts longer than the van der Waals radii with the positive d_norm values and white represents the contact around the van der Waals radii. The hydrogen bonding interactions, such as N–H···O and N–H···N interactions in the cytosine-cytosinium ion dimer, N–H···O interactions between cytosine-cytosinium ion dimers and O–H···O interactions between isophthalic acid molecules, can be observed in the Hirshfeld surface plot as the bright red shaded areas. The Hirshfeld surface plot is in coincidence with the result of single-crystal analysis.

  Figure 8

  

  Figure 8.  Hirschfeld surfaces for C-H₂IA mapped with d_norm

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  4. CONCLUSION

  In summary, a supramolecular framework cytosinium isophthalate was synthesized by self-assembly of cytosine and isophthalic acid in aqueous media and the crystal structure was redetermined revealing the partial proton transfer from carboxylate group to the pyrimidine ring. In the supramolecular framework C-H₂IA, the hydrogen bonds take over to form the one-dimensional infinite cytosine and isophthalic acid tape structures. On this basis, the adjacent arranged organic base and acid tapes are further extended into a three-dimensional network through π-π interactions. Interesting, the framework C-H₂IA shows a green phosphorescence emission visible to the naked eyes under ambient conditions. The room temperature solid-state photoluminescence analysis reveals C-H₂IA possesses the ultra-long emission lifetime up to 0.7 s and is thus a potential organic luminescent material with ultra-long phosphorescence feature.

  
  
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• 

  Figure 1  A perspective view with atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level

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  Figure 2  A view of infinite tape of [HC₂]_nⁿ⁺ connected via hydrogen bonds (dashed line). Hydrogen atoms on carbons and one moiety of disorder hydrogen atoms are omitted for clarity

  下载: 全尺寸图片 幻灯片
  

  Figure 3  A view of infinite tape of [H(HIA)₂]_n^n- connected via hydrogen bonds (dashed line). The hydrogen atoms on benzene ring are omitted for clarity

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  Figure 4  (a) Three-dimensional supramolecular net formed by π-π stacking of tapes; (b) Illustration of the molecular packing in C-H₂IA

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  Figure 5  Powder X-ray diffraction (PXRD) patterns of C-H₂IA

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  Figure 6  Thermogravimetric analysis (TGA) curves of C-H₂IA

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  Figure 7  (a) Photographs of C-H₂IA crystals with a hand-held 300 nm UV lamp irradiation on and off; (b) Normalized photoluminescence (PL, blue line) and room temperature phosphorescence (green line) spectra of C-H₂IA under 300 nm excitation; (c) Time-resolved fluorescence decay (blue dot) and fitting curves (red line) of the emission bands at 375 nm; (d) Time-resolved phosphorescence decay (green dot) and fitting curves (red line) of the emission bands at 500 nm

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  Figure 8  Hirschfeld surfaces for C-H₂IA mapped with d_norm

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  Table 1.  Hydrogen Bond Lengths (Å) and Bond Angles (°) for C-H₂IA

  D–H⋅⋅⋅A	d(D–H)	d(H⋅⋅⋅A)	d(D⋅⋅⋅A)	< (DHA)
  N(4)–H(4)⋅⋅⋅N(4)#1	0.88	1.96(3)	2.829(2)	174
  N(6)–H(6A)⋅⋅⋅O(2)#1	0.85	2.03	2.8670(15)	171
  O(9)–H(9)⋅⋅⋅O(9)#2	1.23	1.23	2.4612(18)	180
  N(1)–H(1)⋅⋅⋅O(2)#3	0.93	1.91	2.8304(15)	173
  O(11)–H(11)⋅⋅⋅O(10)#4	0.92	1.80	2.6179(14)	147
  Symmetry transformations used to generate the equivalent atoms: #1: –x+1, –y+2, –z+1; #2: –x, –y+1, –z+1; #3: –x+2, –y+2, –z+1; #4: x+1, y, z

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