Synthesis and structural properties of 2-((10-alkyl-10H-phenothiazin-3-yl)methylene)malononitrile derivatives; a combined experimental and theoretical insight
© Al-Zahrani et al. 2016
Received: 31 October 2015
Accepted: 29 February 2016
Published: 15 March 2016
Donor acceptor moieties connected through π-conjugated bridges i.e. D-π-A, in order to facilitate the electron/charge transfer phenomenon, have wide range of applications. Many classes of organic compounds, such as cyanine, coumarin carbazole, indoline, perylene, phenothiazine, triphenylamine, tetrahydroquinoline and pyrrole can act as charge transfer materials. Phenothiazines have been extensively studied as electron donor candidates due to their potential applications as electrochemical, photovoltaic, photo-physical and DSSC materials.
Two phenothiazine derivatives, 2-((10-hexyl-10H-phenothiazin-3-yl)methylene)malononitrile (3a) and 2-((10-octyl-10H-phenothiazin-3-yl)methylene)malononitrile (3b) have been synthesized in good yields and characterized by various spectroscopic techniques like FT-IR, UV–vis, 1H-NMR, 13C-NMR, and finally confirmed by single crystal X-ray diffraction studies. Density functional theory (DFT) calculations have been performed to compare the theoretical results with the experimental and to probe structural properties. In order to investigate the excited state stabilities the absorption studies have been carried out experimentally as well as theoretically.
KeywordsPhenothiazine X-ray DFT MEP NBO NLO
In few years, a great interest has developed in molecules having electron donor–acceptor (D–A) properties and their modern applications as dye sensitized solar cells (DSSC) , photosensitizers  and redox sensitizers . The metal based donor–acceptor (D–A) complexes are well known where a metal atom behaves as an electron acceptor and ligands as electron donor species [4–6]. Ruthenium metal is a key contributor in the synthesis of such complexes. To avoid the cost of metal and its environmental hazards there is a space for the synthesis of new organic donor–acceptor molecules. A salient feature of such organic based (D–A) molecules is that donor acceptor moieties are connected through π-conjugated bridges i.e. D-π-A, in order to facilitate the electron/charge transfer phenomenon . The classes of organic compounds that have been evaluated as (D–A) candidates include cyanine , coumarin , carbazole , indoline , perylene , phenothiazine , triphenylamine , tetrahydroquinoline  and pyrrole .
Molecules containing phenothiazine as electron donor part have been extensively studied due to their electrochemical , photovoltaic , photo-physical  and DSSC applications . The synthesis of phenothiazine derivatives and their DSSC applications were claimed by many investigators, and the best results were produced in the solar cells where phenothiazine was used as electron donor and boradiazaindacene as electron acceptor candidates . In addition to their physical applications, phenothiazine derivatives have been recognized as potent anti-psychotic , anti-infective , antioxidant, anti-cancer  and anti-Parkinson agents . These were also qualified as valuable MALT1 protease , cholinesterase , and butyryl-cholinesterase enzyme inhibitors .
Results and discussion
The synthesis of two phenothiazine derivatives 3a and 3b has been accomplished in three steps beginning from 10-phenothiazine resulting in good yields (details are given in the experimental section). These compounds have been characterized by 1H-NMR, 13C-NMR, FT-IR and UV–vis. spectroscopic techniques, and finally their structures have been confirmed by X-ray diffraction analysis. Computational studies have been carried out to compare the theoretically calculated spectroscopic properties with the experimental results, and to investigate some structural properties as well.
X-ray diffraction analysis
Crystal data and structure refinement parameters of 3a and 3b
Wave length Å
Diffraction radiation type
0.340 × 0.140 × 0.060
0.41 × 0.13 × 0.11
2θ range for data collection
5.756 to 59.036°
5.7 to 59.02°
−8 ≤ h ≤ 10, −17 ≤ k ≤ 17, −21 ≤ l ≤ 22
−21 ≤ h ≤ 22, −21 ≤ k ≤ 23, −23 ≤ l ≤ 24
4728 [R (int) = 0.0988]
20,881 [R (int) = 0.0574]
Goodness-of-fit on F2
Final R indexes [I >=2σ (I)]
R1 = 0.0659, wR2 = 0.1162
R1 = 0.0752, wR2 = 0.1475
Final R indexes [all data]
R1 = 0.2559, wR2 = 0.1809
R1 = 0.2263, wR2 = 0.2183
Largest diff. peak/hole/e Å−3
The thiazine rings are not planar having the root mean square (rms) deviation values of 0.1721 (1) Å, 0.1841 (2) Å, 0.2184 (3) Å, 0.1392 (2) Å and 0.1593 (2) Å for compounds 3a and 3b (molecule A, molecule B, molecule C, molecule D) respectively. In compound 3a, the two aromatic rings are oriented at a dihedral angle of 24.80(1)°, while the thiazine ring is oriented at dihedral angles of 13.33 (1)° and 12.56 (1)° with reference to ring 1 (C1–C6) and ring 2 (C7–C12), respectively.
In 3b, having four molecules A, B, C and D in the asymmetric unit, the dihedral angles between the two aromatic rings are 24.85 (1)°, 32.41 (2)°, 18.83 (2)° and 23.80 (2)°. The observed orientation angles of thiazine rings with adjacent aromatic rings are 14.51 (2)°, 11.88 (2)° in molecule A, 16.28 (2)°, 16.49 (2)° in molecule B, 10.03(2)°, 10.16(2)° in molecule C and 13.63 (2)°, 11.74 (2)° in molecule D. These values are comparable with the already reported related structures [33–36], the difference is merely due to a variety of substituted groups on aromatic ring and nitrogen atom. The crystal structures revealed that the malononitrile group (NC–CH–CN) was not co-planar with the aromatic rings but was twisted at dihedral angles of 21.21 (2)°, 3.02 (5)°, 7.54 (5)°, 14.96 (4)° and 13.05 (5)° in 3a and 3b (A, B, C, D) respectively. The puckering parameters for molecule 3a are QT = 0.424 Å, θ = 77.8 (5)° and φ = 4.1 (6)°, and in 3b puckering parameters (QT, θ and φ) are 0.4533 Å, 76.37°, 5.12 ° for molecule A, 0.5377 Å, 98.01°, 185.47° for molecule B, 0.3427 Å, 104.29°, 188.85° for molecule C and 0.3922 Å, 75.42°, 9.84° for molecule D. These values differentiate the four independent molecules in the asymmetric unit of crystal structure of compound 3b, Additional file 1: Table S1. From the X-ray crystallographic studies, a weak C–H···N intermolecular interaction has been observed in 3a. As a result of this interaction, a dimer is formed generating sixteen membered ring motifs R 1 1 (16) (see Additional file 1: Fig. S1). Molecules A and B in 3b form dimers to generate sixteen membered ring motifs R 1 1 (16) Additional file 1: Fig. S2. The π-π interaction has not been observed either in 3a or in 3b.
In the past decade, methods based on DFT have got the attention of researchers because of their accuracy and wide applications. The DFT investigations of both compounds 3a and 3b have been performed not only to validate X-ray results, but also to compare and investigate other spectroscopic and structural properties. The structures of both 3a and 3b have been optimized by using B3LYP/6-31G (d, p) level of theory, and the the optimized geometries are shown in Fig. 3. A comparison of bond angles and bond lengths for both compounds are listed in Additional file 1: Tables S2, S3. Although the packing diagram of 3b shows four molecules in asymmetric unit, yet only molecule A has been considered for comparison. The experimental and simulated bond lengths/bond angles of all atoms for compounds 3a and 3b (A) are correlated nicely. A deviation of 0.001–0.036 Å in bond lengths has been appeared for both compounds. Maximum deviations of 5.4° and 4.2° in dihedral angles from C14–C13–C5 bonds in 3a and from C23–C22–C21 bonds in 3b have been observed.
Experimental and simulated vibrational (cm−1) values of 3a and 3b
3a Calc. (intensity)
3b Calc. (intensity)
The simulated vibrations above 1700 cm−1 have been scaled by using a scaling factor of 0.958 and for less than 1700 cm−1 scaling factor is 0.9627 . In the table only those simulated vibrations are given whose intensities are more than ten. For both compounds, the vibrations arise mainly from aromatic C–H, double bond C=C, C–N, C–S, nitrile, CH2, and CH3 functional groups. From Table 2, it is clear that there exists an excellent agreement between the experimental and theoretical vibrations.
Aromatic (CH), (C=C) and aliphatic (C=C) vibrations
The aromatic (CH) vibrations generally appear in the region 2800–3100 cm−1 . The bands appeared in this region are normally of very low intensity, and not much affected by substituents. In the simulated spectra, the aromatic CH stretching vibrations of both compounds 3a and 3b have been predicted at 3086, 3077 cm−1 and 3085, 3077 cm−1 respectively. The calculated aromatic CH stretching vibrations coincide well with the experimental value appearing at 2916 cm−1 for both compounds. The symmetric and asymmetric stretching vibrational regions of aromatic ring (C=C) usually lie in between 1600–1200 cm−1 . The experimental scans of 3a and 3b show aromatic C=C stretching vibrations at 1574, 1402 cm−1 and 1570, 1405 cm−1 respectively. The simulated aromatic stretching C=C peaks are found in strong correlation and appear at 1603, 1568, 1526, 1395 cm−1 for compound 3a, and 1594, 1526, 1395 cm−1 for compound 3b. An aliphatic C=C group in conjugation with aromatic ring is also present in both compounds and appears at 1559 cm−1 experimentally whereas this stretching vibration appears at at 1553 cm−1 for both 3a and 3b.
Aromatic in-plane and out of plane CH bending vibrational regions are usually weak and are observed in the range 1000–1300 cm−1 and 650–900 cm−1 respectively . In the simulated spectra, in plane CH (aromatic) bending vibrations are observed in the range of 1428–1286 cm−1 for compound 3a, and in the region of 1352–1139 cm−1 for compound 3b. The corresponding experimental values are depicted at 1218 cm−1 for compound 3a and 1220 cm−1 for compound 3b. The prominent out of plane CH (aromatic) bending vibrations of compound 3a are observed at 1163, 927, 810 and 735 cm−1 in the simulated spectrum, and for compound 3b these are observed in the range 927–740 cm−1. These out of plane bending vibrations are well supported by the experimental values of both compounds having their values noticed at 805 and 814 cm−1 respectively. The calculated out of plane bending vibrations of phenyl ring in compound 3a are in the range 741–429 cm−1, and for 3b in the range 709–429 cm−1. These simulated values are very nicely correlated with the experimental values of the both compounds.
CH2 and CH3 group vibrations
The simulated stretching (symmetric/asymmetric) CH2 vibrations appear in the range of 3001–2895 cm−1, and 3005–2893 cm−1 for compounds 3a and 3b respectively. These simulated values appear in nice agreement with the experimental values having appeared at 2848 cm−1 for compound 3a, and 2847 cm−1 for compound 3b. Along with the stretching vibrations, several scissoring, in-plane and out of plane bending, methylene (CH2) and methyl vibrations are observed in the simulated and experimental spectra and a nice agreement is found between them.
Both compounds 3a and 3b show the CH2 scissoring vibrations in the range 1456–1448 cm−1 and 1453–1448 cm−1 respectively and these are correlated well with the experimental 1458 and 1462 cm−1 values respectively. The in-plane bending CH2 vibrations are observed in the range 1337–1275 cm−1 and 1337–1287 cm−1 for 3a and 3b respectively. These bending vibrations are in agreement with the experimental counterparts having appeared at 1317 cm−1, 1218 and 1323, 1228 cm−1 for 3a and 3b respectively.
Nitrile and C–N Group vibrations
The nitrile symmetric stretching vibrations of very high intensity appear at 2245 cm−1 in the simulated spectra for 3a and 3b. The nitrile asymmetric stretching vibrations of low intensity also appear at 2230 and 2231 cm−1 for both compounds. In the experimental scans, the nitrile vibrations appear at 2214 and 2215 cm−1 for 3a and 3b respectively, and are found in excellent correlation with the simulated values. The simulated C–N–C stretching frequency appear at 1483 cm−1 for both 3a and 3b and is in full agreement with its experimental counterpart observed at 1472 and 1474 cm−1 respectively.
The assignments of N-Ph stretching modes are difficult, as there are problems to discriminate them from other aromatic ring vibrations. For substituted aromatic rings, Silverstein et al.  defined the N-Ph stretching bands in the range 1200–1400 cm−1. In the present study of compound 3a, the observed N-Ph symmetric stretching bands appear at 1338 and 1279 cm−1 in the simulated spectrum and are in very good agreement with the experimental 1363 cm−1 value. Similarly, the calculated N-Ph stretching frequencies of 3b appearing at 1337 and 1279 cm−1 also show good agreement with the experimental band at 1363 cm−1.
Nuclear magnetic resonance (NMR) studies
Comparison of experimental and simulated 1H-NMR of 3a and 3b (ppm) in CDCl3
Both phenothiazine derivatives (3a and 3b) mainly have aromatic and aliphatic protons. In the experimental 1H-NMR spectra, aromatic and double bonded protons appear in the range 7.74–6.83 ppm (compound 3a) and 7.75–6.83 ppm (compound 3b). The computed aromatic C–H signals (with respect to TMS) appear in the range 8.88–7.18 ppm (3a)/8.93–7.16 ppm (3b), and are found in nice agreement with the experimental values. The calculated chemical shift values for methylene and methyl hydrogen atoms of both 3a and 3b are found in the range 4.24–0.55/4.22–0.81 respectively, and are proved in good agreement with the experimental counterparts which appear in the range of 3.87–0.88 (3a)/3.87–0.87 (3b).
Frontier molecular orbital analysis and UV–vis absorption studies
Frontier molecular orbital analysis has proved very helpful in understanding the electronic transitions within molecules and analyzing the electronic properties, UV–vis absorptions and chemical reactivity as well . The FMO analysis also plays an important role in determining electronic properties such as ionization potential (I. P.) and electron affinity (E. A.). The HOMO (highest occupied molecular orbital) represents the ability to donate electrons and its energy corresponds to ionization potential (I. P.), whereas the LUMO (lowest unoccupied molecular orbital) acts as electron acceptor and its energy corresponds to electron affinity (E. A.) . Frontier molecular orbital (FMO) analysis is carried out at the same level of theory as used for the geometry optimization, applying pop = full as an additional keyword. The HOMO and LUMO surfaces along with the corresponding energies and energy gaps are shown in Additional file 1: Fig. S6. Compound 3a contains 93 filled orbitals, whereas 3b contains 103 filled orbitals. The HOMO–LUMO energy difference in both 3a and 3b has been found to be 2.96 eV. The kinetic stabilities of compounds can be assigned on the basis of HOMO–LUMO energy gap . A low HOMO–LUMO energy gap means less kinetic stability and high chemical reactivity. It is clear that the HOMO–LUMO energy gaps in compounds 3a and 3b are very less, indicating that electrons can easily be shifted from HOMO to LUMO after absorbing energy.
Experimental and simulated UV–vis. λmax (nm) values of 3a and 3b measured in DCM, chloroform, methanol and DMSO
Theoretical [TD-SCF/B3LYP/6-31G (d, p)]
λmax1 (osc. strength)
λmax2 (osc. strength)
Significant donor–acceptor interactions of 3a/3b and their second order perturbation energies calculated at B3LYP level using 6-31G (d, p) basis set
Donor (i) (occupancy)
EDA, % EDB, %
Acceptor (j) (occupancy)
EDA, % EDB, %
Ej–E i b
F (i, j) (a.u.)
BD C3–C4 1.97721
BD* C2–C3 0.02660
BD C4–C5 1.97419
BD* C3–C4 0.01233
BD C4–C5 1.59136
BD* C13–C14 0.24073
BD C2–C3 1.97034
BD* C3–C4 0.01233
BD C2–C3 1.60070
BD* C4–C5 0.42336
BD C1–C2 1.97300
BD* C2–C3 0.02660
BD C1–C6 1.97721
BD* C5–C6 0.02189
BD C1–C6 1.71641
BD* C2–C3 0.40194
BD C5–C6 1.97016
BD* C4–C5 0.02494
BD C7–C12 1.97320
BD* C11–C12 0.02533
BD C7–C8 1.97672
BD* C7–C12 0.03387
BD C7–C8 1.69501
BD* C11–C12 0.38891
BD C11–C12 1.66680
BD* C9–C10 0.33937
BD C9–C10 1.66550
BD* C7–C8 0.38725
BD C13–C14 1.81237
BD* C15–N2 0.08582
BD C13–C14 1.81237
BD* C16–N3 0.08857
LP N1 1.69519
BD* C2–C3 0.40194
LP N1 1.69519
BD* C11–C12 0.38891
LP S1 1.84528
BD* C1–C6 0.34392
LP S1 1.84528
BD* C7–C8 0.38725
Different solvents covering a wide range of polarity and dielectric constant have been selected in order to explore the solvent effect on the absorption maxima, but no significant difference has been observed. The experimental UV–vis. spectra of both compounds show mainly two absorption bands. In dichloromethane, λmax1 and λmax2 values for compound 3a appear at 320 and 474 nm corresponding to the π–π* and n–π* transitions respectively , and for 3b the values appear at 321 nm and 474 nm. In chloroform the absorption maxima of 3a are found at 321 nm (λmax1), 478 nm (λmax2) and for 3b they have been appeared at 321 nm (λmax1), 478 (λmax2). Similarly, the absorption maxima values appear at 317 nm (λmax1), 478 nm for compound 3a, and 317 nm (λmax1), 463 nm (λmax2), for compound 3b in methanol (polar protic) and DMSO (polar aprotic) respectively. The gas phase simulated spectrum of compound 3a show absorption maxima λmax1 and λmax2 at 300.4 nm (oscillating strength, f = 0.37) and 476.4 nm (f = 0.21) respectively. On the other hand, compound 3b shows λmax1 at 300.4 nm (f = 0.36) and λmax2 at 475.7 nm (f = 0.21). The details of the simulated absorption values along with the oscillating strengths of both compounds in gas, dichloromethane (DCM), chloroform, methanol and DMSO are given in Table 4.
Molecular electrostatic potential (MEP)
Natural bond orbital (NBO) analysis
First hyperpolarizability parameters of 3a and 3b
β × 10−30 (esu)
The pi-bond of ethylenic moiety (C13–C14) also shows an average of ~20 kcal/mol stabilization energy when it is delocalized to either acetonitrile group. The strongest stabilization energy to the system by 31.28 kcal/mol is due to the lone pair donation of nitrogen atom N (1) to the antibonding π* (C2–C3) orbital. On the other hand, the same lone pair gives a stabilization energy of 24.09 kcal/mol when it is conjugated with the antibonding π* (C11–C12) orbital of the aromatic ring. This clearly shows that the delocalization of lone pair of nitrogen N (1) is more towards that aromatic ring which has extended conjugation due to presence of electron withdrawing acetonitrile groups. The lone pair donation from sulfur atom (S1) to the antibonding π* (C1–C6) and (C7–C8) orbitals of both phenyl rings results in the stabilization energies of 12.09 and 11.23 kcal/mol respectively. The occupancy of lone pair electrons in sulfur atom (S1) is 1.84 as compared to 1.69 of lone pair on nitrogen atom (N1). As a consequence, the stabilization energies arising from the lone pair donation of sulfur atom to the antibonding π* (C–C) bonds of phenyl rings are comparatively smaller than those arising from lone pair donation of N1 atom. A plausible reason could be due to the deviation of sulfur atom from planarity because of its larger size. All σ to σ* transitions involving C–C bonds correspond to the weak stabilization energies in the range of ~2.53–4.58 kcal/mol.
Hyperpolarizability and non-linear optical properties
Recently, compounds having non-linear optical (NLO) properties have got appreciable attention of researchers because of their wide applications in optoelectronic devices of telecommunications, information storage, optical switching and signal processing . Molecules containing donor acceptor groups along with pi-electron conjugated system are considered as strong candidates for possessing NLO properties .
In each 3a and 3b, the phenothiazine moiety is connected to a nitrile group through a conjugated double bond, and these molecules are anticipated to show non-linear optical (NLO) properties. For the estimation of NLO properties, the first hyperpolarizability (βo) analysis for compounds 3a and 3b has been performed by employing same level of theory as for geometry optimization i.e. 6-31G (d, p) along with POLAR as an additional keyword. The first hyperpolarizability, a third rank tensor, is always described by a 3 × 3 × 3 matrix. The total 27 components of the 3D matrix can be reduced to 10 components as a result of Kleinman symmetry . From the Gaussian output file ten components of 3D matrix have been identified as β xxx, β xxy, β xyy, β yyy, β xxz, β xyz, β yyz, β xzz, β yzz and β zzz respectively, and the values are given in Table 6.
All analytical grade chemicals and solvents were purchased from BDH, and used without further purification. Stuart Scientific (SMP3, version 5.0, UK) melting point apparatus was used to record the melting point, and the reported m. p. were uncorrected. 1H-NMR spectra were recorded on a Bruker-AVANCE-III 600 MHz at 300 K, and chemical shifts were reported in ppm with reference to the residual solvent signal. FT-IR spectra were recorded under neat conditions on Thermo Scientific NICOLET iS 50 FT-IR spectrometer (Thermo Scientific). UV–visible studies were performed by using Evolution 300UV/VIS spectrophotometer (Thermo Scientific).
Sample crystals were mounted on Agilent Super Nova (Dual source) Agilent Technologies Diffractometer, equipped with graphite-monochromatic Cu/Mo Kα radiation source. The data collection was accomplished by using CrysAlisPro software  at 296 K. Structure solution was performed using SHELXS–97 and refined by full–matrix least–squares methods on F2 using SHELXL–97 , in-built with X-Seed . All non–hydrogen atoms were refined anisotropically by full–matrix least squares methods . All the C–H hydrogen atoms were positioned geometrically and treated as riding atoms with C–H = 0.93 Å and Uiso (H) = 1.2 Ueq (C) for aromatic carbon atoms. The methyl and methylene hydrogen atoms were also positioned geometrical with C methyl –H = 0.96 Å and C methylene –H = 0.97 Å and Uiso (H) = 1.5 Ueq (C) and Uiso (H) = 1.2 Ueq (C) for methyl and methylene carbon atoms respectively. The figures were drawn using ORTEP III , PLATON  and OLEX2  programs. The cifs of both molecules have been assigned CCDC numbers 1028273 & 1028274 and these data files can be obtained free of charge on application to CCDC 12 Union Road, Cambridge CB21 EZ, UK. (Fax: (+44) 1223 336-033; e-mail: email@example.com. ac.uk).
Theoretical studies were performed by using Gaussian 09 software at density functional theory (DFT) level, as instituted in program . The visualization of the results/optimized geometries was achieved by using Gauss view 05 . The energy minima optimization of both compounds was carried out at B3LYP/6-31G (d, p) and B3LYP/6-311 + G (2d, p) levels of theory (the later was used further for nuclear magnetic studies). Frequency simulations were performed at the same level, to confirm the optimized geometries as a true minimum (no imaginary frequency). In addition, frequency simulations at B3LYP/6-311G (d, p) level were used for vibrational analysis. Nuclear magnetic resonance studies were performed at B3LYP/6-311 + G (2d, p) level, by adopting GIAO method in chloroform solvent and applying polarizable continuum model (PCM) for the solvent consideration. Chemical shift values were referred by using the internal reference standard i.e., tetramethylsilane. UV–vis absorption studies were simulated by using TD-DFT method and at B3LYP/6-31G (d, p) level of theory. MEP, NBO, FMO and first hyperpolarizability analyses were simulated at B3LYP/6-31G (d, p) level of DFT.
The synthesis of both phenothiazine derivatives was carried out in three steps starting from simple phenothiazine. First step was alkylation of nitrogen, followed by subsequent aldehyde formation and then conversion to final product (Fig. 1).
General procedure for the synthesis of N-alkylated phenothiazine (1a, 1b)
In a round bottom flask a mixture of potassium hydroxide (2.003 g, 0.0357 mol), 10-phenothiazine (2.91 g, 0.0119 mol), 1-bromohexane (for 1a) or 1-bromooctane (0.0179 mol) (for 1b) and potassium iodide (in catalytic amount) in 50 ml dimethyl sulfoxide (DMSO) were taken. The reaction mixture was stirred for 5 h at room temperature and water (200 ml) was added. The crude product was extracted with CHCl3 (3 × 50 ml) and the organic layer was washed with saturated ammonium chloride solution and then with water. The organic layer was dried over anhydrous sodium sulfate and filtered, after removing the solvent under reduced pressure, crude product was purified by flash column chromatography (eluent: n-hexane) to obtain colorless oil 1a in 88.68 % yield, and 1b in 86.15 % yield.
General procedure for synthesis of 10-alkyl-10H phenothiazine-3-carbaldehyde (2a, 2b)
To an ice cooled flask containing N, N-dimethylformamide (86 ml), POCl3 (53.5 ml) was added drop wise under stirring. After complete addition, the solution was stirred at room temperature for 90 min. Then the reaction mixture was cooled in an ice bath and already synthesized compound (1a or 1b) (65 mmol) was added. The reaction mixture was warmed gradually up to 75 °C for 2 h. Then the mixture was cooled to room temperature and poured into ice water, basified (sat. aqueous K2CO3 solution) and extracted with CHCl3 (4 × 30 ml). Organic layer was washed, dried over MgSO4, filtered, evaporated and purified by flash silica gel column chromatography using petroleum ether/ethyl acetate (80/20) as eluent system to obtain yellow solids, 2a in 92 % yield and 2b in 91 % yield.
Synthesis of 2-((10-hexyl-10H-phenothiazin-3-yl)methylene)malononitrile (3a) and 2-((10-octyl-10H-phenothiazin-3-yl)methylene)malononitrile (3b)
A mixture of (2a or 2b) (3 mmol) and malononitrile (3 mmol) in basic ethanolic solution (10 ml) was stirred at room temperature overnight. The precipitates formed were filtered off and purified by recrystallization from methanol affording final products, 3a in 78 % yield, and 3b in 73 % yield.
M. p. 84–85 °C IR (neat, cm−1): υmax = 2916, 2848, 2214, 1559, 1472, 1458, 1402, 1360, 1218, 805, 740, 607; 1 H-NMR (CDCl3, ppm): 7.74, (1H, dd, Ar–H, J = 1.8 Hz, 1.2 Hz), 7.53 (1H, d, Ar–H, J = 2.4 Hz), 7.47 (1H, s, Ar–H), 7.17 (1H, m, Ar–H), 7.08 (1H, dd, Ar–H, J = 1.8 Hz, 1.2 Hz), 6.98 (1H, m, Ar–H), 6.88 (1H, d, Ar–H, J = 8.4 Hz), 6.84 (2H, d, Ar–H, J = 9 Hz), 3.87 (2H, t, CH2, J = 7.2 Hz, 9.8 Hz), 1.44 (2H, pent, CH2), 1.32 (2H, pent, CH2), 1.81 (4H, pent, CH2), 0.88 (2H, t, CH3, J = 0.6 Hz, 1.2 Hz), 13 C-NMR (CDCl3, ppm): 157.3, 150.8, 142.4, 131.4, 129.5, 127.8, 127.6, 125.1, 124.9, 122.9, 116.0, 114.8, 114.7, 113.6, 48.2, 31.3, 26.6, 26.4, 22.5, 14.0, UV–vis (DMSO): λmax = 319.5 nm, 470.5 nm.
M. p. 90–92 °C IR (neat, cm−1): υmax = 2916, 2848, 2215, 1570, 1559, 1461, 1405, 1364, 1220, 930, 814, 740, 608; 1 H-NMR (CDCl3, ppm): 7.74 (1H, dd, Ar–H, J = 2.4 Hz, 1.8 Hz), 7.54 (1H, d, Ar–H, J = 2.4 Hz), 7.47 (1H, s, Ar–H), 7.17 (1H, m, Ar–H), 7.08 (1H, dd, Ar–H, J = 1.2 Hz, 1.2 Hz), 6.98 (1H, m, Ar–H), 6.88 (1H, d, Ar–H, J = 9 Hz), 6.84 (2H, d, Ar–H, J = 9 Hz), 3.88 (2H, t, CH2, J = 2.4 Hz, 1.8 Hz), 1.81 (2H, pent, CH2), 1.44 (2H, pent, CH2), 1.30 (8H, m, CH2), 0.87 (2H, t, CH3, J = 6.6 Hz, 7.2 Hz), 13 C-NMR (CDCl3, ppm): 157.3, 150.8, 142.4, 131.4, 129.5, 127.8, 127.6, 125.1, 124. 9, 124.1, 122.9, 116.0, 114.9, 114.71, 113.5, 48.2, 31.7, 29.1, 29.1, 26.7, 26.6, 22.6, 14.1, UV–vis. (DMSO); λmax = 320 nm, 471 nm.
In this study, two novel phenothiazine derivatives 2-((10-hexyl-10H-phenothiazin-3-yl)methylene)malononitrile (3a) and 2-((10-octyl-10H-phenothiazin-3-yl)methylene)malononitrile (3b) have been synthesized and characterized by using FT-IR, UV–vis, 1H, 13C-NMR spectroscopic techniques and finally their structures are confirmed by single crystal X-ray diffraction studies. The DFT studies have shown a strong agreement between the simulated and experimental results. The optimized geometries of the both compounds at 6-31G (d, p) level have been used further for investigating structural properties. Frontier molecular orbital analysis shows that both the molecules have very low HOMO–LUMO energy gap, and therefore are kinetically less stable. The molecular electrostatic potential investigations reveal that electronegative region in both the compounds is spread over the nitrile groups. The high first hyperpolarizability values signify that these compounds can have very good nonlinear optical responses. The phenothiazine derivatives have very wide applications not only in dye sensitized solar cells but also in clinical field, and hopefully the results of this study will increase the interest of researchers working in this field.
FAA, AMA and RME synthesized the compounds. AMA and MNA did the crystallographic studies. TM and MAG performed the theoretical calculations. All authors have contribution in write-up. All authors read and approved the final manuscript.
This Project was funded by the King Abdulaziz City for Science and Technology (KACST) through National Science, Technology and Innovation Plan (NSTIP) under grant number 8-ENE198-3. The authors, therefore, acknowledge with thanks KACST for support for Scientific Research. Also, the authors are thankful to the Deanship of Scientific Research (DSR), King Abdulaziz University for their technical support.
The authors declare that they have no competing interests.
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