Structures, photoresponse properties and DNA binding abilities of 4-(4-pyridinyl)-2-pyridone salts

Abstract

Three salts [perchlorate (2), chloride (3) and tetrafluoroborate (4)] were synthesized from a 1-(2-aminoethyl)-6-hydroxy-2-oxo-1,2-dihydro-[4,4'-bipyridine]-3,5-dicarbonitrile compound (1) and characterized by spectroscopic and single crystal X-ray diffraction methods. Various noncovalent interactions (e.g., anion⋯π⁺, π⋯π, lp⋯π) are explored in the solid state crystal structure of the salts. Optical band gaps of all the four compounds were determined from their solid-state UV-vis spectrum. Electrical properties like electrical conductivity, photosensitivity, etc. were calculated and the results revealed that they have potential to act as optoelectronic devices. The values of the electrical parameters increase several times when they are exposed to visible light rather than in dark conditions. The light sensing properties of the salts (2-4) are enhanced compared to that of the mother organic compound 1 but the magnitude of this enhancement is not same for the three salts. This observation has been rationalized by theoretical considerations. Moreover, the DNA binding ability of one of the representative salts (compound 2) was examined to check the biological importance of the synthesized salts.

Fig. 1. Structures of compounds 1–4.

Fig. 2. X-ray structures of compounds 2 (a), 3 (b) and 4 (c) with indication of the bis-arene torsion angle.

Fig. 3. UV-vis absorption spectra (inset) and Tauc's plots for compounds 1–4.

Fig. 4. I–V characteristics curve for ITO/compound (1–4)/Al structured thin film devices under (a) dark, and (b) photo illumination condition.

Fig. 5. dV/d ln I vs. I curve for the compound 1–4 based thin film devices under (a) dark, and (b) photoillumination condition.

Fig. 6. H vs. I curves for the compound 1–4 based thin film devices under (a) dark, and (b) photo illumination condition.

Fig. 7. log I vs. log V curves for the compound 1–4 based thin film devices under (a) dark, and (b) irradiation condition.

Fig. 8. I vs. V ² curves for the compound 1–4 based thin film devices under (a) dark, and (b) irradiation condition.

Fig. 9. Capacitance vs. frequency graph for determination of dielectric constant.

Fig. 10. Electronic band structures of the ground state of 2, 3, and 4 crystals. Points of high symmetry in the first Brillouin zone are labeled as follows: Z = (0, 0, 0.5), G = (0, 0, 0), Y = (0, 0.5, 0), A = (−0.5, 0.5, 0), B = (−0.5, 0, 0), D = (−0.5, 0, 0.5), E = (−0.5, 0.5, 0.5), C = (0, 0.5, 0.5).

Fig. 11. Calculated partial density of states of 2, 3 and 4 crystal cell. The left panel shows PDOS of aromatic atoms of AHPD (Ar), ketone group (CO), nitrile group (CN) and perchlorate anions (ClO₄⁻) of 2 crystal cell. Middle panel shows of PDOS of aromatic atoms of AHPD (Ar), ketone group (CO), nitrile group (CN) and chloride anions (Cl⁻) of 3 crystal cell. The right panel shows the PDOS of aromatic atoms of AHPD (Ar), ketone group (CO), nitrile group (CN) and tetrafluoroborate anions (BF₄⁻) of 4 crystal cell.

Fig. 12. (A) A plot of real (small square) and imaginary (big square) parts of the dielectric function versus the photon energy of 2, 3 and 4 crystals. Blue, green and red lines specify the incident direction of polarized radiation in the ‘x’, ‘y’ and ‘z’ directions of the crystal, respectively. (B) A plot of real (big square) and imaginary (small square) parts of optical conductivity versus the photon energy of 2, 3 and 4 crystals. Blue, green and red lines specify the incident direction of polarized radiation in the ‘x’, ‘y’ and ‘z’ directions of the crystal, respectively.

Fig. 13. (a) Representative picture for the partial intercalation of cationic moiety into DNA base pairs. (b) 2-dimensional picture for the binding of cationic moiety with DNA.

Fig. 14. (a) Absorbance spectra of compound 2 35 μM in presence of 0, 196, 294, 417, 540 and 620 μM of DNA (plots 1 to 6) (inset: Benesi–Hildebrand analysis plot). (b) Relative absorbance change with increasing concentration of DNA.

Fig. 15. Intrinsic CD spectra of CT DNA (60 μM) in the presence of 0, 6, 12, 18, and 30 μM of compound 2.