Selenylated-oxadiazoles as promising DNA intercalators: Synthesis, electronic structure, DNA interaction and cleavage

Abstract

A series of selenylated-oxadiazoles were prepared and their interaction with DNA was investigated. The photophysical studies showed that all the selenylated compounds presented absorption between 270 and 329 nm, assigned to combined n→π* and π→π* transitions, and an intense blue emission (325–380 nm) with quantum yield in the range of Φ_F = 0.1–0.4. DFT and TD-DFT calculations were also performed to study the likely geometry and the excited state of these compounds. Electrochemical studies revealed the ionization potential energies (−5.13 to −6.01 eV) and electron affinity energies (−2.25 to −2.83 eV), depending directly on the electronic effect (electron-donating or electron-withdrawing) of the substituent attached to the product. Finally, the UV–Vis DNA interaction experiments indicated that the compounds can interact with the DNA molecule due to intercalation, except for 3g (which interacted via electrostatic interaction). Plasmid cleavage assay presented positive results only for 3f that presented the strongest interaction results. These results made the tested selenylated-oxadiazoles as suitable structures for the development of drugs and the design of structurally-related therapeutics.

Graphical abstract

1. Introduction

In the last few decades, there has been rising attention around the medicinal properties of organoselenium compounds, as these compounds exhibit fascinating biological characteristics [1,2]. In particular, the fact that certain enzymes that contain selenium can feature antioxidant properties. Several reports highlight their antioxidant, anti-inflammatory, antitumor, and antiviral activities [[3], [4], [5], [6]]. Notably, some organoselenium compounds display the mimetic activity of these enzymes, making them attractive synthetic targets [7,8]. Considering the current pandemic of corona virus (COVID-19), a very interesting study just appeared in the literature where an organoselenium compound (Ebselen) presented the strongest antiviral effect at a concentration of 10 μM treatment in COVID-19 virus infected Vero cells [9]. Therefore, the construction of the C–S/Se bond seems to be a very important target in organic synthesis and has attracted considerable attention in the last years [[10], [11], [12], [13], [14], [15]].

Similarly, the 1,3,4-oxadiazole (ODZ) scaffold is a ubiquitous heterocycle [16]. The oxadiazole core is well established as a “privileged scaffolds” and is widely used for pharmaceutical purposes [17]. ODZs are bioisosteres which increase pharmaceutical activity. A wide range of biological activity is associated with ODZs, including, analgesic, anti-viral, anti-inflammatory, anticancer, angiogenesis inhibition, anticonvulsant and immune-stimulatory [18,19]. This moiety is present in several commercially available drugs, I-IV (Fig. 1 ) [20,21], highlighting the importance of this nucleus. Additionally, ODZ derivatives are useful in material sciences (e.g. V, Fig. 1) as well as in the organic synthesis [[22], [23], [24]].

Fig. 1.

Structure of compounds containing the heterocycle 1,3,4-oxadiazole (ODZ) with biological activity I-IV and applications in material sciences V.

Small organic molecules that can interact with DNA have been of interest in medicinal chemistry and have helped in the development of new drugs for the treatment of different diseases [25]. Considering the biological relevance of organoselenium compounds as well as the therapeutic properties of ODZ, there are studies available that highlight the individual DNA intercalation properties of both [26,27]. However, there are only a few reports in the literature involving the molecular hybridization of these structures [[28], [29], [30]]. To the best of our knowledge, there has been no report, up till now, which demonstrates the interaction with DNA as well as biological studies of these hybrid structures.

Furthermore, the application of small organic compounds in optoelectronics is an emerging area that has been extensively investigated as this could lead to the development of more flexible and lower cost smaller devices e.g. solar cells, OLEDs, OFETs, OTFTs [[31], [32], [33], [34]]. In this regard, heteroaromatic small molecules and hybrid compounds also have been largely investigated [35].

As part of our continuing research interest in the synthesis of biologically relevant organoselenium compounds [[36], [37], [38]] and material chemistry [[39], [40], [41], [42]], herein we report the synthesis of selenylated-ODZ, their characterization, DNA interaction, and plasmid DNA cleavage activity. Furthermore, the photophysical, thermal and electrochemical properties of the selenylated products were explored and were also supported by theoretical studies. All of these studies will help in the design of structurally-related therapeutics.

2. Experimental

2.1. Materials and spectroscopic characterization

All reagents were used as received. The solvents were purchased from commercial sources and purified according to the standard procedures. During synthesis, the reactions were monitored by thin-layer chromatography (TLC) using Silica Gel 60 F254 and were detected by exposure to ultraviolet light or to iodine vapours/acidic vanillin solution. The selenylated products were purified by column chromatography using flash silica gel. Fourier transform infrared (FTIR) spectra (4000-600 cm⁻¹) were recorded on a Bruker Alpha using KBr pellets. Proton and Carbon nuclear magnetic resonance spectra (¹H and ¹³C NMR) were recorded in CDCl₃ at 400 and 100 MHz, respectively, on a Varian AS-400. The chemical shifts (δ) are reported in parts per million (ppm) relative to TMS (0.00 ppm), and the coupling constants J are reported in hertz (Hz). Selenium-77 nuclear magnetic resonance spectra (⁷⁷Se NMR) at 38.14 MHz on a Bruker AC-200 NMR spectrometer. Spectra were recorded in CDCl₃ solutions. Chemical shifts are reported in ppm, referenced to diphenyl diselenide as the external reference (463.61 ppm). High-resolution mass spectra were recorded on a Bruker micrOTOF-Q-II (APPI⁺ mode) and, mass spectrometer equipped with an automatic syringe pump for sample injection. Melting points were determined on a Microquimica MQRPF-301 digital model apparatus.

2.2. Cyclic voltammetry

Cyclic voltammetry (CV) experiments were performed in 0.01 mol L⁻¹ solutions of tetra-n-butylammonium hexafluorophosphate (TBAPF₆) in CH₂Cl₂ as a supporting electrolyte. Before each measurement, the cell was deoxygenated by purging with argon and the ferrocene/ferricenium (Fc/Fc⁺) redox couple was used as an internal standard. The measurements were carried out in a cell with three electrodes, composed of a glassy carbon electrode (GCE) as the working electrode, a platinum wire as a counter electrode and, an Ag/Ag⁺ electrode as reference. The electrochemical cell was connected to a PalmSens3 potentiostat/galvanostat (Palm Instruments BV).

2.3. Thermal analysis

The thermogravimetric analysis (TGA) of the samples was carried out using a Shimadzu TGA–50 thermogravimetric analyzer. The experiments were performed in an N₂ atmosphere in the temperature ranging from 30 to 800 °C at a heating rate of 10 °C min⁻¹. The nitrogen flow rate was 100 mL min⁻¹.

2.4. Photophysics

For the photophysical studies, spectroscopic-grade solvents and a quartz cell with 10 mm path length were used. The UV–Vis absorption spectra were recorded with a Shimatzu spectrophotometer model UV–1800 with a diode array detector in a quartz cell with a 10 mm path length. The following procedure was applied in all experiments performed.

The emission spectra for samples were recorded on a Hitachi F4500 spectrofluorimeter equipped with a thermostatic cell holder set at 25.0 ± 0.1 °C using a quartz cell of 10 mm path length in CH₂Cl₂. The slit width settings of both the excitation and emission monochromators were adjusted to 5 nm. The samples were excited at a different wavelength and the emission spectra recorded as required for each sample.

2.5. DFT modeling of the structures

Geometry optimizations for the selenylated-ODZs 3a-h were carried out in vacuum with the Orca 4.2.1 software package [43] at the DFT level using the B3LYP functional [44,45] and, the basis set chosen was Def2-TZVP(-F) for all atoms [[46], [47], [48]]. The RIJCOSX algorithm was employed to accelerate the evaluation of the functionals, using the resolution of identity approximation for the Coulomb part (RIJ), and the chain of spheres approach for the Fock exchange (COSX) [49,50]. RIJCOSX requires the specification of an auxiliary basis set for the Coulomb part and that of a numerical integration grid for the exchange part discussed elsewhere. The DFT grid was set to GRID5, the COSX grid was GRIDX5. In order to simulate the absorption spectra, time-dependent density functional theory under the Tamm-Dancoff approximation (TD-DFT/TDA) [51] was employed to obtain the first 30 excitations, using the same calculations protocol. To include solvent effects in the excited state energies, the conductor-like polarizable continuum model (CPCM) [52] was used, adopting CH₂Cl₂ as a solvent. Images of the complex geometries were obtained using the Chemcraft program [53].

2.6. Spectrophotometrically DNA-complex interaction measurements

Absorption titration measurements were done by varying the concentration of the compounds from 0 to 50 μmol L⁻¹ but the CT DNA concentration was kept constant at 50 μmol L⁻¹ dissolved in 100 μmol L⁻¹ HEPES buffer. The baseline was performed with the mixed solvent and parallel measurements with the solvent and the compounds to eliminate the absorbance of compounds itself. The solutions were mixed and the absorption spectra were recorded immediately. The experiments were repeated three times and the results obtained were plotted in tables and can be seen on graphics [54].

2.7. Plasmid DNA cleavage assay

Plasmid DNA pBSK II (Stratagene) was obtained and purified according to standard techniques [54]. The DNA cleavage ability of the compounds diluted in acetonitrile was examined to establish their ability to cleavage pBSK II supercoiled DNA (FI) to the open circular (F II) and, linear DNA (F III) through single and double-strand breaks respectively [54].

In general, 300 ng of pBSK II DNA (30 μmol L⁻¹bp) in 10 mmol L⁻¹ HEPES buffer pH 7.0 was treated with compounds at concentrations of 0 (negative control) 250 and 500, μmol L⁻¹ in a final concentration of 25% acetonitrile at 37 °C in the absence of light (AL) for 24 h. All the assays were conducted using freshly prepared solutions. Thereafter, each reaction was quenched by adding 4 μL of a loading buffer solution (50 mmol L⁻¹ Tris-HCl pH 7.5, 0.01% bromophenol blue, 50% glycerol, and 250 mmol L⁻¹ EDTA) and then subjected to electrophoresis on a 1.0% agarose gel containing 0.3 μg mL⁻¹ of ethidium bromide in 0.5 × TBE buffer (44.5 mmol L⁻¹ Tris pH 8.0, 44.5 mmol L⁻¹ boric acid, and 1 mmol L⁻¹ EDTA) at 90 V for 1.5 h. The resulting gels were visualized and digitized using a DigiDoc-It gel documentation system (UVP) (KODAK). The proportion of plasmid DNA in each band was quantified using the GelAnalyzer version 2010a program (freeware) [55]. The quantification of supercoiled DNA (F I) was corrected by a factor of 1.47 since the ability of ethidium bromide to intercalate into this DNA topoisomeric form decreases in relation to open circular and linear DNA. The results are expressed as graphic representations of the best correlation of the concentration in order to maximize Form II and III (circular and linear respectively).

2.8. Statistical analysis: DNA interaction assays

Statistical analysis of plasmid experiments was conducted by ANOVA followed by Dunnett's multiple comparison test. The GraphPad Prism 6.0 software was employed and a p-value under 0.05 was considered significant [54].

2.9. Synthesis

The starting material, oxadiazoles 1, was synthesized according to the previous report [56]. To prepare the series of selenylated oxadiazoles 3a-h, the previous method (reported by us) was used [30].

The desired compounds were obtained by mixing in a Schlenck tube with a magnetic stir bar, appropriate 1,3,4-oxadiazole 1a-f (0.5 mmol), iodoarene 2a-c (1.0 mmol), selenium 200 mesh (1.0 mmol), KHCO₃ (1.0 mmol) and CuI (0.0125 mmol) were added, followed by the addition of DMSO (2.0 mL). The reaction was heated up to 120 °C in a vegetal oil bath for 12 h, with vigorous stirring. After this, the reaction mixture was brought to room temperature and diluted with ethyl acetate (25 mL), then it was washed with distilled water (2 × 10 mL) and a saturated solution of NaCl (1 × 10 mL). The organic phase was separated, dried over MgSO₄ and concentrated under reduced pressure. The crude residue was purified by column chromatography and eluted with an appropriate mixture of hexane/ethyl acetate.

2-(phenylselanyl)-5-(p-tolyl)-1,3,4-oxadiazole (3a). Off White solid; Yield: 84%; mp: 59–61 °C (59–60 °C) [30]; Purified using: (95:5) hexane/ethyl acetate.¹H NMR (400 MHz, CDCl₃) δ 7.84 (d, J = 8.2 Hz, 2H), 7.76 (dd, J = 7.9, 1.5 Hz, 2H), 7.44–7.35 (m, 3H), 7.26 (d, J = 8.0 Hz, 2H), 2.39 (s, 3H)·¹³C NMR (100 MHz, CDCl₃) δ 167.44, 155.79, 142.48, 135.01, 129.89, 129.79, 129.62, 126.85, 124.49, 120.83, 21.72. ⁷⁷Se NMR (38.14 MHz, CDCl₃) δ = 365.59.

N,N-dimethyl-4-(5-(phenylselanyl)-1,3,4-oxadiazol-2-yl)aniline (3b). Orange solid; Yield: 71%; mp: 85–86 °C (85–86 °C) [30]; Purified using (90:10) hexane/ethyl acetate. ¹H NMR (400 MHz, CDCl₃) δ 7.83–7.78 (m, 2H), 7.74 (dd, J = 7.8, 1.7 Hz, 2H), 7.42–7.33 (m, 3H), 6.69 (d, J = 9.1 Hz, 2H), 3.04 (s, 6H)·¹³C NMR (100 MHz, CDCl₃) δ 168.12, 154.21, 152.55, 134.73, 129.86, 129.39, 128.40, 125.15, 111.63, 110.65, 40.20.

2-(3,5-dimethoxyphenyl)-5-(phenylselanyl)-1,3,4-oxadiazole (3c). Yellow solid; mp: 78–80 °C (79–81 °C) [30]; Yield: 43%; Purified using (90:10). hexane/ethyl acetate. ¹H NMR (400 MHz, CDCl₃) δ 7.76 (dt, J = 6.5, 1.5 Hz, 2H), 7.45–7.35 (m, 3H), 7.09 (d, J = 2.3 Hz, 2H), 6.58 (t, J = 2.3 Hz, 1H), 3.82 (s, 6H)·¹³C NMR (100 MHz, CDCl₃) δ 167.27, 161.21, 156.37, 135.15, 129.95, 129.75, 125.06, 124.33, 104.69, 104.49, 55.73.

2-(benzo[d][1,3]dioxol-5-yl)-5-(phenylselanyl)-1,3,4-oxadiazole (3d). Yellow solid; mp: 82–84 °C (82–84 °C) [30]; Yield: 76%; Purified using (95:5) hexane/ethyl acetate.¹H NMR (400 MHz, CDCl₃) δ 7.75 (dd, J = 8.0, 1.5 Hz, 2H), 7.49 (dd, J = 8.2, 1.7 Hz, 1H), 7.44–7.36 (m, 4H), 6.87 (d, J = 8.2 Hz, 1H), 6.04 (s, 2H)·¹³C NMR (100 MHz, CDCl₃) δ 167.07, 155.63, 150.80, 148.33, 135.07, 129.93, 129.68, 124.46, 122.10, 117.43, 108.91, 106.99, 101.95.

2-(4-nitrophenyl)-5-(phenylselanyl)-1,3,4-oxadiazole (3e). Yellow solid; mp: 129–134 °C; Yield: 15%; Purified using (90:10) hexane/ethyl acetate. ¹H NMR (400 MHz, CDCl₃) δ 8.34 (d, J = 9.0 Hz, 2H), 8.15 (d, J = 9.0 Hz, 2H), 7.80 (dd, J = 8.2, 1.4 Hz, 2H), 7.53–7.40 (m, 3H)·¹³C NMR (100 MHz, CDCl₃) δ 165.50, 158.35, 149.65, 135.58, 130.18, 130.13, 129.07, 127.80, 124.49, 123.61. IR ν _max: 3103, 1550, 1522, 1453, 1338, 1066, 865, 853, 711. APPI-HMRS:m/z [(M + H)]⁺ calculated for C₁₄H₁₀N₃O₃Se: 347.9882, found 347.9880.

2-(naphthalen-1-yl)-5-(phenylselanyl)-1,3,4-oxadiazole (3f). Orange solid; mp: 115–118 °C (112–114 °C) [30]; Yield: 65% Purified using (95:5) hexane/ethyl acetate.¹H NMR (400 MHz, CDCl₃) δ 9.06 (d, J = 8.5 Hz, 1H), 8.01 (dd, J = 7.3, 1.2 Hz, 1H), 7.96 (d, J = 8.3 Hz, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.80 (dt, J = 6.5, 1.5 Hz, 1H), 7.62–7.38 (m, 7H)·¹³C NMR (100 MHz, CDCl₃) δ 167.27, 156.28, 135.36, 133.80, 132.74, 129.94, 129.93, 129.79, 128.71, 128.41, 128.16, 126.75, 126.09, 124.87, 124.19, 120.08.

4-((5-(p-tolyl)-1,3,4-oxadiazol-2-yl)selanyl)aniline (3g). Brown solid; Yield: 65%; mp: 160–163 °C (166–168 °C) [30]; Purified using: (75:25) hexane/ethyl acetate.¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 8.0 Hz, 2H), 7.55 (d, J = 8.1 Hz, 2H), 7.25 (d, J = 7.4 Hz, 2H), 6.66 (d, J = 8.1 Hz, 2H), 3.92 (s, 2H), 2.40 (s, 3H)·¹³C NMR (100 MHz, CDCl₃) δ 167.19, 157.17, 148.40, 142.30, 137.69, 129.78, 126.85, 121.08, 116.12, 110.63, 21.75.

2-((4-bromophenyl)selanyl)-5-(p-tolyl)-1,3,4-oxadiazole (3h). Off White solid; mp: 105–106 °C (105–106 °C) [30]; Yield: 65%; Purified Using (95:5) hexane/ethyl acetate. ¹H NMR (400 MHz, CDCl₃) δ 7.83 (d, J = 8.2 Hz, 2H), 7.61 (d, J = 8.5 Hz, 2H), 7.50 (d, J = 8.4 Hz, 2H), 7.26 (d, J = 8.0 Hz, 2H), 2.39 (s, 3H)·¹³C NMR (100 MHz, CDCl₃) δ 167.47, 155.18, 142.57, 136.46, 133.00, 129.78, 126.80, 124.39, 123.10, 120.58, 21.70.

3. Results and discussion

3.1. Synthesis

For the synthesis of selenylated-ODZs, an efficient and sustainable approach was used, reported previously by us [30]. The method involves the one-pot approach using oxadiazole 1, elemental selenium and, aryl-iodides 2. Under the optimized conditions CuI (2.5 mol%) was used as an effective catalyst and 2 M equiv. of KHCO₃ was applied as a base in DMSO at 120 °C for 12 h under atmospheric air (Scheme 1 ). After the completion of the reaction, the reaction mixture was quenched with 5 mL of brine and extracted with ethyl acetate. The organic phase was evaporated under reduced pressure and was purified by column chromatography, resulting in the desired selenylated-ODZs. All of the synthesized products (3a–h) were characterized by ¹H NMR,¹³C NMR and for new compounds, HRMS and IR were also performed.

Scheme 1.

Synthesis of selenylated-ODZs 3a-h.^{[a],[b]. [a]} Reaction conditions: 1 (0.5 mmol), Se, 100 mesh (1 mmol), 2 (1 mmol), CuI (2.5 mol %), KHCO₃ (2 M equiv.), DMSO (2 mL). ^[b] Isolated yields.

By applying this strategy, a series of selenylated-ODZs were synthesized with electron-donating groups on the oxadiazole skeleton, such as the dimethylamino portion 3b, 2,5-dimethoxy 3c, 1,3-benzodioxole 3d portion. The product with ODZ core with Electron withdrawing group 3e and bulky group 3f were also obtained. The aryl moiety was attached with the selenide by electron-donating as well as electron-withdrawing groups that were also prepared, 3g and 3h respectively (Scheme 1).

3.2. Electrochemical behavior

Electrochemical measurements were conducted to determine the influence of the electronic effect (electron-donationand and electron-withdrawing groups) on the redox properties of the selenylated-ODZs. All molecules showed electroactivity in the investigated redox window, both in the region of anodic and cathodic potential (see Fig. 2 ). As expected, compounds 3a-h presented a reduction process at the most negative potentials, probably associated with the oxadiazole group [57]. Compound 3e showed different reduction behavior due to the electroactivity of the nitro group. Scanning at positive potential values the compounds 3a-h demonstrated irreversible oxidation processes (Fig. 2) [58,59]. Compounds 3b and 3g exhibited different oxidation behavior than that of the other compounds due to the presence of the amino group.

Fig. 2.

Cyclic voltammograms of 3 a-h (concentration 0.2 mg mL⁻¹ in CH₂Cl₂). The scanning rate is 100 mV s⁻¹. Three electrodes electrochemical cell: GCE (working), Pt wire (auxiliary) and Ag/Ag⁺ (reference). Supporting electrolyte: 0.01 molL^-1 TBAPF₆ in CH₂Cl₂ (purged with argon).

The electrochemical HOMO and LUMO energy levels were determined respectively, by the ionization potential (IP) and electron affinity (EA), which could be correlated with redox processes accessed by CV. To calculate the absolute energies, the redox data were corrected with an Fc/Fc⁺ couple and, the standard reduction potential of the couple was found at 0.25 Vvs. Ag/Ag⁺. The optical band gap of 3a-h was determined from the absorption onset in the solution. The IP and EA value was calculated using the empirical equations [60]: IP = −($E_{oxi}^{onset}$ + 4.44) eV, EA = −($E_{red}^{onset}$ + 4.44) eV, where $E_{oxi}^{onset}$and $E_{red}^{onset}$ represents the oxidation onset potential and reduction onset potential, respectively. The optical and electrochemical data obtained from these studies are summarized and compared in Table 1 .

Table 1.

Optical and electrochemical properties of compounds 3a-h where $E_{oxi}^{onset}$ is the onset potential of oxidation; $E_{red}^{onset}$ is the onset potential of reduction; IP (HOMO) is the ionization potential; EA (LUMO) is the electron affinity; $E_{gap}^{ele}$ and $E_{gap}^{opt}$ are the electrochemical and optical band gap, respectively; and λ_onset is the absorption onsets wavelength.

Parameters	Compounds
	3a	3b	3c	3d	3e	3f	3 g	3 h
$E_{oxi}^{onset}$ (V)a	1.35	0.79	1.36	1.32	1.57	1.48	0.69	1.42
$E_{red}^{onset}$ (V)a	−1.90	−2.19	−2.07	−2.14	−1.61	−1.94	−2.18	−2.15
IP (HOMO) (eV)b	−5.79	−5.23	−5.80	−5.76	−6.01	−5.92	−5.13	−5.86
EA (LUMO) (eV)c	−2.54	−2.25	−2.37	−2.30	−2.83	−2.50	−2.26	−2.29
$E_{gap}^{ele}$(eV)	3.25	2.98	3.43	3.46	3.18	3.42	2.87	3.57
λonset (nm)	311.2	359.2	394.4	330.1	369.2	347.8	395.9	385.9
$E_{gap}^{opt}$ (eV)d	3.98	3.45	3.14	3.76	3.36	3.56	3.13	3.21

^aVersus NHE.

^bIP = −($E_{oxi}^{onset}$E+ 4.44) eV.

^cEA = − ($E_{red}^{onset}$E+ 4.44) eV.

^dOptical band gap calculated on the onset of the absorption spectrum ($E_{gap}^{opt}$= 1240/λ_onset).

Analysis of the data listed in Table 1 leads to the conclusion that the IP and EA energies depend directly on the electron-donating or electron-withdrawing characteristics of the substituent group. The introduction of electron donor groups increases the electron density and therefore decreases the oxidation potential and the IP [57,61], 3g with the dimethylamino substituent in the phenyl portion directly linked to the selenium atom, showed this effect more pronouncedly. This effect was also observed on molecules 3b, 3c and 3d, in which oxidation starts at less anodic values, however the electron donor substituent is linked to the phenyl portion closest to oxadiazole. For compounds 3a and 3f, with even less donating substituents, this same behavior was also observed. The insertion of a nitro group, due to its electron-withdrawing effect, caused an increase in the oxidation potential of compound 6e. A similar effect was observed for compound 6h [61]. The increase in electron density caused by the introduction of electron donor groups decreases the reduction potential and the EA. The data suggest that this substitution alters the LUMO energy level, i.e., the energetic state of the electrons inserted during the reduction process, leading to a change in the reduction potential. Observing the reduction potentials of the studied molecules indicates that the introduction of electron-withdrawing groups into selenylated-oxadiazoles derivatives can increase their redox potential due to the decrease in the LUMO level [61]. These values are in agreement with those obtained in the theoretical modeling below, where the influence of the substituent groups in the frontier orbitals was confirmed.

3.3. Thermal behavior

The thermal properties of the 3a-h were investigated by thermogravimetric analysis (TGA) under N₂ atmosphere. The results indicate that all of the compounds are stable at room temperature, with degradation starting above 260 °C (Fig. 3 ). A second degradation event can be observed for 3g and 3b around 330 °C and, a last one occurs for all compounds around 550 °C, except for 3a.

Fig. 3.

TGA thermograms curves of compounds 3a-h.

3.4. Photophysical properties

The UV–Vis absorption and emission spectra of compounds 3a-h obtained in CH₂Cl₂ are presented in Fig. 4 and the results summarized in Table 2 . All compounds exhibited an intense absorption band around 270–352 nm which are attributed to combined n→π* and π→π* transitions. Furthermore, for compounds 3c, 3e and 3g are possible to observe a weak band around to 330 nm, which should be associated with an n→π* due to the presence of the nonbonding electron pairs over these molecules. All the compounds 3a-h exhibited an intense luminescence emission after excitation at their intense absorption band cited above, with the emission maxima centered at 325–380 nm. This emission is safely attributed to be dominantly fluorescent due to the small Stokes shift and because they were measured at room temperature without purging.

Fig. 4.

Optical absorption (left) and emission (right) spectra of compounds 3a-h in CH₂Cl₂ solution. Emission spectra were obtained by excitation at the low energy region of the first intense absorption band.

Table 2.

Photophysical data for the compounds 3a-h.

	λabs (nm)	Solution[a]
		λem (nm)	ΦF[b]
3a	270	327	0.35
3b	329	372	0.20
3c	282, 352	334	0.22
3d	307	342	0.16
3e	291, 352	333	0.10
3f	315, 335	370	0.42
3 g	274, 352	325	0.10
3 h	268, 317	380	0.19

^aIn CH₂Cl₂.

^bUsing anthracene as standard.

The relative photoluminescence quantum yields (Φ _F) were measured using anthracene as standard (0.36 in cyclohexane) [62]. High Φ _F was obtained in CH₂Cl₂ for compound 3a (Φ _F = 0.35) and 3f (Φ _F = 0.42), both containing the fewer donor groups methyl and naphthyl, respectively. The lowest Φ _F = 0.10 were observed for 3e and 3g. The other compounds showed intermediate values.

3.5. Theoretical modeling of the electronic structure

To have a better understanding of the electronic and emissive properties of compounds 3a-h, we first carried out calculations using DFT to obtain their optimized ground-state structures (see Fig. S19) and the calculated frontier orbitals HOMO and LUMO along with their energies are shown in Fig. 5 . The HOMO for compounds 3b, 3d and 3g containing electron donor groups is centered over these groups and the closest phenyl ring, while for compounds 3a, 3c, 3f, and 3e where the donating power decreases, the HOMO is mostly a π orbital spread throughout the phenyl and oxadiazole moieties with the contribution of a non-bonding pair in the selenium atom. In contrast, for 3h with an electron-withdrawing group, the HOMO is a π orbital centered on both oxadiazole moiety and a non-bonding pair at the selenium atom. Except for compound 3h, the LUMO orbital for the other compounds is π* spread throughout the oxadiazole moiety and the benzene ring attached to it. On the other hand, on compound 3h the LUMO is centered in the nitro group.

Fig. 5.

Frontier molecular orbitals and their energies of 3a-h calculated using B3LYP/def2-TZVP(-f).

TD-DFT was used to simulate the absorption spectra to understand the emission. Based on these calculations, for compound 3g with dimethylamine group, a high donating power substituent in the phenyl moiety directly bonded to the selenium atom, the first intense band is assigned as HOMO-1→LUMO transition. For compounds 3b and, 3d also with the higher donating power substituents, but now in the phenyl moiety closest to the oxadiazole, the intense transition that generates the emission of these compounds is assigned as HOMO→LUMO transition. For compounds 3a, 3c and 3f with less donating substituents, the intense band results from two transitions involving the frontier orbitals, HOMO-1, HOMO, LUMO and, LUMO+1, with an associated transition dipole moment smaller than that found for the above-mentioned compounds. On the other hand, for 3e and 3h with the most electron-withdrawing substituents, the first intense transition also has its origin in a HOMO→LUMO configuration, with a higher transition dipole for 3e and a lower one to 3h, demonstrating that the substituents can have a significant impact on the configuration of the transition with larger amplitude, which ultimately influences the emission. Table S18 displays some of the calculated low energy transitions.

Since fluorescence generally occurs from the lowest excited state to the ground state and its rate depends on the transition matrix element of the dipole moment operator, it is possible to rationalize the measured quantum yield of compounds 3a-h. For compounds 3b, 3c, 3d, and 3g the first excited state has an associated transition dipole moment much smaller than that found for the first excitation for compounds 3a, 3f, and 3e which exhibited the higher quantum yield. For compound 3h with a nitro group even with a high associated transition dipole moment associated with the first excited state, this state has lower energy when compared with the other compounds which should facilitate the non-radiative path, decreasing the quantum yield.

3.6. Spectrophotometrically DNA-complex interaction measurements

The spectrophotometrically DNA-compound interaction measurements are presented in Fig. 6 as graphics showing the results of titration of the compounds on the DNA UV absorbance peak (260 nm). Compounds 3a, 3b, 3c, 3e and 3h presented only a small hypochromic effect while 3g presented hyperchromic one. The compound 3d showed hipochromism associated with bathochromism (blue shift) and compound 3f showed the stronger hypochromic effect associated with ipsochromism (redshift).

Fig. 6.

Graphic representation from compound-DNA interactions observed by UV-Spectrophotometric analysis. All compounds showed some ability to interact with DNA through intercalation except for 3g that presented characteristics of groove binding interaction.

Usually compounds that bind with DNA through intercalation show hypochromism and when it is strong, bathochromism can also be present [63]. The extent of the hypochromism is usually consistent with the strength of intercalative interaction because this interaction mode involves a stacking interaction between an aromatic chromophore and the base pair of DNA [64].

The hyperchromic effect observed when the electrostatic attraction between the compound and DNA is happening reflects the corresponding changes of DNA in its conformation and structure after the compound–DNA interaction has occurred [64].

As can be observed, compounds 3a, 3b, 3c, 3d, 3e, and 3h presented by UV–Vis spectrophotometric analysis showed only a very poor ability to interact with DNA due to intercalation. Compound 3g interacts with DNA probably due to electrostatic interaction of the protonated amine group in solution. Compound 3f demonstrated a stronger hipochromism associated with a redshift effect characterizing it as an intercalating compound with strong activity.

3.7. Plasmid DNA cleavage assay

The plasmid cleavage activity assays were conducted as described and the results appear in Fig. 7 . Only compound 3f exhibited plasmid cleavage activity. These results corroborate those obtained with the spectrophotometry DNA-compound interaction analysis were the compound 3f shown the highest hipochromism and was the only one that presented a redshift characteristic of intercalators, as described before. Such activity can have a relation with the planar structure present on the compound, as can be observed in other complexes carrying 2,2-bipyridine or 1,10-phenanthroline in which the second one gives stronger results when similar structures are compared [65]. Compounds 3a, 3b, 3c are the ones that presented the weakest UV–Vis DNA interaction characteristics and no plasmid DNA cleavage activity, those are the more suitable for developing drugs [55]. Compound 3f showed strong DNA interaction and its ability to cleave may be used for developing DNA markers or anti-cancer drugs [54,65].

Fig. 7.

Graphic representation from compound-DNA interactions plasmid DNA cleavage assay.*p < 0,05.

4. Conclusions

A series of selenylated-oxadiazoles (ODZs) were synthesized in good yields and characterized by ¹HNMR, ¹³C NMR and IR. All the selenylated products were thermally stable, presenting initial decomposition only in temperatures above 260 °C. The photophysical investigation revealed that all compounds displayed absorption in the UV region (270–329 nm), which are attributed to combined n→π* and π→π* transitions. These compounds showed emission in the blue region with a high Φ _F for compound 3a (Φ _F = 0.35) and 3f (Φ _F = 0.42) displayed emission with the naphthyl and methyl groups, respectively. Theoretical methods were used to obtain their ground state geometry and TD-DFT results demonstrated an influence of the substituent groups in transition dipole moment associated with the first excited state, which seems to impact directly in the quantum yield of these compounds. Electrochemical studies revealed that the magnitude of oxidation potentials (0.69–1.57 V vs. NHE) and reduction potentials (−1.61 to −2.18 V vs. NHE) and also the IP energies (−5.13 to −6.01 eV) and EA energies (−2.25 to −2.83 eV) depend directly on electron-donating or electron-withdrawing effects of the substituents in agreement with the calculated frontier orbitals. UV–Vis DNA interaction experiments indicate that the compounds are able to interact with the DNA molecule in different ways, mostly due to intercalation with the exception of compound 3g that presented groove binding characteristics. Selenylated-ODZ 3a, 3b, and 3c are the ones that presented the weakest UV–Vis DNA interaction characteristics and no plasmid DNA cleavage activity.

These studies will help in the development of structurally-related series of compounds as well as in the design of new efficient molecules in future drug development programmed by identifying hit and lead compounds.

CRediT authorship contribution statement

Jamal Rafique: Writing - original draft, Formal analysis. Giliandro Farias: Formal analysis. Sumbal Saba: Writing - original draft, Formal analysis. Eduardo Zapp: Formal analysis. Ismael Casagrande Bellettini: Formal analysis. Cristian Andrey Momoli Salla: Formal analysis. Ivan Helmuth Bechtold: Formal analysis. Marcos Roberto Scheide: Investigation, Methodology. José Sebastião Santos Neto: Investigation, Methodology. David Monteiro de Souza Junior: Investigation, Methodology. Hugo de Campos Braga: Investigation, Methodology. Luiz Fernando Belchior Ribeiro: Investigation, Methodology. Francine Gastaldon: Investigation, Methodology. Claus Tröger Pich: Investigation, Methodology. Tiago Elias Allievi Frizon: Writing - original draft, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the Supplementary data to this article: