3.1 Structural commentary
The tautomeric structures of CA has been investigated by many workers[2][20]. It is interesting and important to understand the tautomeric form in which the CA is involved in compound formation with bicyclo [2.2.2] octane. The C1-O2 and C2-O3 have bond lengths of 1.2250 and 1.2278Å, but C3-O1 has a bond length of 1.2155Å. This indicates that C3-O1 has a C = O character whereas C1-O2 and C2-O3 are involved in delocalization of π electrons (Fig. 1). The N1 hydrogen of CA is involved in salt bridge with the N4 nitrogen of the bicyclo ring system of DABCO. The negative charge developed on N1 is delocalized over (Fig. 2) N1-C1-O2. The lone pair and the C = O of N2, C2 = O3 and N3 are involved in the delocalization. So the bond lengths follow C2-O3 > C1-O2 > C3-O1. This assignment has been further corroborated by the C-N bond lengths. The C3-N3 and C1-N2 bond lengths are 1.3866 and 1.3857Å which are longer than C2-N2 and C2-N3 with almost equal bond lengths of 1.3612 and 1.3637Å showing partial single and double character of the C-N bond. The C1-N1 bond length is 1.3540Å which is the least indicating the delocalization of negative charge on N1 nitrogen over N1-C1-O2 (diagram). C3-N3⁓C1-N2 > C2N3⁓C2-N2⁓C3-N1 > C1-N1. The N2-H2 and N3-H3 are at a distance of 0.86Å whereas N1-H1 has a bond length of 1.27Å. N1 hydrogen of CA is involved in the salt bridge of the DABCO ring.
The N1-H1 and N4-H1 have bond lengths of 1.27 and 1.42Å with d(D…A) being 2.69 Å (Table.2) indicate that intermolecular H-bonding and salt bridge between N1-H1…N4. The bond angle is 178° showing a linear disposition. The N2-H2 and N3-H3 hydrogen atoms are involved in the intermolecular hydrogen bonding with O2 and O3 of the adjacent molecules along the C-axis (Fig. 3). Therefore the compound exhibits a supramolecular assembly through intermolecular hydrogen bonding.
The O2-C1-N1 (122°), O2-C1-N2 (120.8°) and N1-C1-N2 (116°) indicated the interaction of C1\(\stackrel{⃛}{-}\)O2 with C1\(\stackrel{⃛}{-}\)N1 is greater than C1\(\stackrel{⃛}{-}\)O1 with C1\(\stackrel{⃛}{-}\)N2 σ-bond. Both interactions decrease N1-C1-N2 angle. The O3-C2-N3 and O3-C2-N2 have 122° bond angle due to C1\(\stackrel{⃛}{-}\)O3 interaction with partially delocalized π-bonds on N2-C2-N3. Due to lone pair and π interactions, N3-C2-N2 becomes a smaller bond angle of 115°. The O1-C3-N1 is 123°, due to π-delocalized-σ interactions whereas O1-C3-N3 is lesser 120.8°. The N1-C3-N3 is 116° due to σ and π bond (C3 = O1) interactions. The C2-N3-H3 and C3-H3-N3 have a bond angle of 117°. The H2-N2-C2 and H2-N2-C1 are 180° indicating that the lone pair has more effect on the N-H bond rather than ring bond angles. The hydrogen is involved in the hydrogen bonding with N4 nitrogen of the bicyclo ring.
O2-C1-N1-C3 and O2-C1-N2-C2 are almost in the same plane as shown by the torsion angle nearer to 180°. But there is a small difference first one is -178.81°and the second one is 177.76°. Indicating that their atoms are not exactly at the same plane and the ring is slightly tilted. Similarly, O1-C3-N1-C1; O1-C3-N3-C2 and O3-C2-N2-C1; O3-C2-N3-C3 are also almost planar but show a slight deviation from 180°. N2-C1-N1-C3 and N2-C2-N3-C3 are at an angle of 0.03 and − 0.17° indicating that they lie in a different plane. Similarly, N3-C3-N1-C1 (0.91°); N3-C2-N2-C1 (1.23°) and N1-C1-N2-C2 (-1.18°); N1-C3-N3-C2 (-0.86°) indicated that they lie in different planes. In the DABCO the bicyclic ring is in the form of two cyclohexane rings in chair conformation as indicated by torsion angles.
The structure involves the diazabicyclo[2.2.2]octane (DABCO) sandwiched between two cyanuric acid moieties. The N1-H1 is involved in salt bridge with each N4 of the DABCO ring arranged linearly. The pKa2 and pKa3 values of the cyanuric acid are 6.88, 11.40 and 13.5 respectively. When N1-H1 is involved in salt bridge, where as other N-H bands involve in intermolecular hydrogen bonding. On the other side, the intermolecular hydrogen bonding extends between the CA molecules giving rise to a supramolecular assembly. The salt bridge exists between the two cyanuric acid molecules and are DABCO(2:1), which comprises one molecular formula unit (370 g mol− 1). The cyanuric acid moiety decomposes initially and follows the decomposition of DABCO in TG-DTA analysis.
Table 2
Hydrogen bonds for TTDO [Å and °].
| D-H...A | d(D-H) | d(H...A) | d(D...A) | <(DHA) |
| N2-H2...O2#4 | 0.86 | 1.93 | 2.7874(13) | 175.4 |
| N3-H3...O3#5 | 0.86 | 1.96 | 2.8075(12) | 170.1 |
| N1-H1...N4 | 1.27(3) | 1.42(3) | 2.6899(12) | 178(2) |
| Symmetry transformations used to generate equivalent atoms: |
| #1 -x + 1,y,-z + 3/2 #2 -x + 1/2,-y + 3/2,-z + 1 #3 -x + 1,-y + 2,-z + 1 |
| #4 -x + 1/2,-y + 1/2,-z + 1 #5 -x + 1/2,y + 1/2,-z + 1/2 |
3.2 Hirshfeld surface analysis
The intermolecular interactions in the crystal packing have been studied[21] by Crystal Explorer 21.5 program[22]. It shows that H...H (33.1%) and O…H/H…O (42.1%) contact makes a more important contribution to the Hirshfeld surface (Fig. 4). The other considerable intermolecular interactions on the molecules are C…H/H…C(7.2%) and N…H/H…N(9.1%) and O…O(3.1%). The O…H interaction is the greatest as expected for the H-bonded solids.
3.3 FT-IR Analysis
The FTIR spectrum (Fig. 5) of TTDO is recorded in the range of 400–4000 cm− 1. The bands at 3430 and 3406 cm− 1 as a doublet are attributed to ν(N2-H) and ν(N3-H)[23]. The ν(C-H) stretching vibration of DABCO is observed at 2899 cm− 1[24]. The peaks at 1769 and 1742 cm− 1 are due to ν(C = O) stretching vibrations[25]. The medium absorptions at 1600 cm− 1 are due to characteristic ν(C\(\stackrel{⃛}{-}\)N) in the FTIR spectrum as shown in the resonating structures. Other peaks are due to CH2 and C-N group vibrations.
3.4 Absorption spectrum
The UV-Visible absorption spectra of cyanuric acid in different pH solutions concluded that cyanuric acid exists in the keto (tri-oxo) form in acidic and neutral solutions[26]. They found that when cyanuric acid is treated in a strongly alkaline solution enol form can be achieved[27]. The UV–Visible absorption spectrum[15] was measured using a UV-1600 Series spectrophotometer (Fig. 6). The peaks observed at 224 and 211 nm are due to n→π* transitions of nonbonding electrons residing on the triazine nitrogens (keto form). The absorption (A) was used to calculate the absorption coefficient (α) using the formula: α = 2.303 A/t and (αhν)2 = A(Eg - hν), where ν is the incident radiation frequency, A is a constant, Eg is the energy gap and h is the Planck’s constant[28]. The energy gap (Eg) is calculated as 5.33 eV from the plot of (αhν)2 vs hν as in the inset in Fig. 6.
3.5 Thermal Analysis
The TG curve shows two stages of weight loss in the TTDO crystal. DTA curve shows two exothermic peaks corresponding to two oxidative decomposition stages[23]. The first stage of decomposition starts at 144°C and ends at 260°C with a derivative peak at 250°C with the elimination of 34.37% due to loss of cyanuric acid moiety with a broad exothermic peak (Fig. 7). The second stage of weight loss is observed between 280 to 361°C with a mass loss is about 65.63%. This is due to the decomposition of bicyclo [2.2.2] octane moiety. The ring system is stable and requires a higher temperature for decomposition. The formation of (NH4)2CO3 and its decomposing to NH3, CO2 and water has been observed as an endothermic peak at 358°C. The compound decomposes leaving nil residue.
3.6 Photoluminescence
Organic crystals are essential materials for light-emitting device (LEDs) applications. The luminescent properties of the TTDO crystals were analyzed by the PL spectrum. Analyzing the optoelectronic characteristics and electronic structure of the compounds can be done using the photoluminescence spectroscopic technique. The crystalline material is excited with UV radiation of wavelength 261 nm. The PL emission peak at 532 nm in the green region is exhibited by the TTDO crystals (Fig. 8). The emission intensity depends on the structural perfection as well as defects in grown crystal specimens [23]. The cyanuric acid moiety has two different π-delocalization in the present structure. They are the π1 delocalization over O3-C2-N2-N3 and π2 over N1-C1-O2 and π3 over C3 = O1. In the electronic transition n-π* and π-π* are possible. Since there are different π energy levels several n-π* and π-π* transitions can occur. In the PL spectrum where the molecules are excited by a particular wavelength, the electrons may get excited to π1*, π2* and π3*, therefore, giving several transitions. When the light is emitted from these excited states the molecule may undergo vibration relaxation and comes back to the ground state. This is the reason for different emitted wavelengths depending upon the excitation wavelength. The molecules can be tuned to emit the light in the required region by varying the excitation wavelength.
Chromaticity
Chromaticity is an objective specification of the quality of a colour regardless of its luminescence. Photoluminescence colour-tuning (PLCT) and control of emission chromaticity are critical for the continued development of functional organic and hybrid materials[29]. The chromaticity of TTDO crystal coordinates (x, y) were found to be x = 0.20239, y = 0.66914 have been determined from the tristimulus values using the following expression,
\(x=\frac{X}{X+Y+Z}\) --------(1)
\(y=\frac{Y}{X+Y+Z}\) --------(2)
The chromaticity of TTDO confirms that it is suitable for green LED applications (Fig. 9). Therefore the emission spectrum shows the band at 532 nm which confirms that it is suitable for green light emitting optical applications.
3.7 Antibacterial Activity
The antibacterial activity of 1,4-Diazabicyclo[2.2.2]octane cyanuric acid was evaluated using Gram-positive (Staphylococcus aureus) and Gram-negative (Salmonella typhi) bacteria(Fig.S1). Antibacterial activity based on 1,4-diazabicyclo[2.2.2]octane bis-quaternary salts with dodecyl substituents bound by linker groups containing four carbon atoms was analysed and all the compounds displayed activity against the test strains[30]. Antibacterial activity was determined by the disc diffusion method using Muller-Hinton Agar (MHA) medium. The disc was placed in MHA plates and 20 µl of the sample (Concentration: 1000µg, 750µg and 500 µg) was added in the disc (Table.3). The plates were incubated at 37ºC for 24 h. The bacteria staphylococcus aureus showed the inhibition zone with all three concentrations of TTDO.
Table 3
Antibacterial activity of TTDO expressed as inhibition zone in mm.
| Organisms | Concentration of TTDO (µg/ml) Zone of Inhibition (mm) | Ampicillin (1mg/ml) |
| 1000 | 750 | 500 |
| Staphylococcus aureus | 9 | 7 | 7 | 22 |
| Salmonella typhi | 8 | 7 | 7 | 27 |
