Deep Red emitting dicyanovinylene isophorone based chromophores: Combined synthesis, optical properties, viscosity sensitivity, and DFT studies

Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112389

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Deep Red emitting dicyanovinylene isophorone based chromophores: Combined synthesis, optical properties, viscosity sensitivity, and DFT studies

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Sulochana Bhalekar, Kiran Avhad, Nagaiyan Sekar* Department of Dyestuff Technology, Institute of Chemical Technology, Matunga, Mumbai, 400 019, India

ARTICLE INFO

ABSTRACT

Keywords: N-ethy1 carbazole Triphenylamine MLR analysis NLO NBO

Deep red emitting dicyanovinylene isophorone based dyes are synthesized and studied for photophysical properties, viscosity sensitivity, and DFT. The dyes consist of Carbazole and Triphenylamine donor and thiophene spacer together with one auxiliary accepter, –CN leads to deep red emission towards the NIR region (∼600−700 nm) in polar solvent media. All dyes show good emission solvatochromism with an increase in solvent polarity. The dyes show intramolecular (ICT) as well as “Twisted Intramolecular Charge Transfer (TICT)” which is reinforced by Lippert Mataga, MacRae, Weller, and Rettig plots. Viscosity induced emission enhancement study in ethanol/polyethylene glycol (PEG 400) system shows 4.34–4.85 folds increases emission intensity. ICT is supported by Frontier Molecular Orbital (FMO) diagram and bandgap energy. The observed experimental findings are also correlated with the computed values using “Density Functional Theory (DFT)”. The magnitudes of “first order hyperpolarizability”(β0) are 500−1000 × 10−30.e.s.u. while those of the “second order hyperpolarizability”(γ) are 3000−8300 × 10-36 e.s.u. Hence they can act as very good NLOphores. Mulliken Hush analysis and Natural Bond Orbital (NBO) analysis were used to support charge transfer characteristics.

1. Introduction Due to enormous electro-optic (EO) activity, low dielectric constants, ultrafast response time, and exceptional processibility for large scale integrating photonics organic-polymeric variety of materials with good macroscopic nonlinear optical (NLO) properties are favorable candidates for broadband optical modulation and terahertz photonics [1–8]. In this area, for large μβ values (where μ is the dipole moment, and β is the first order polarizability) require strong intermolecular dipole-dipole interactions among the chromophoric groups in the polymeric system would interfere the induced noncentrosymmetric alignment [8]. Qualitative “two-level model” employed in the molecular engineering of Donor-π-Acceptor (D-π-A) has proved experimentally to understand the molecular first order hyperpolarizability β as well the second order hyperpolarizability γ as a function of geometry and substitution in D-π-A [9–11]. Both β and γ can be related to the three factors, the excess dipole moment (Δμge = μee − μgg), the transition dipole moment, (μge), and the energy gap (ΔEge) between the two levels under consideration, and the expressions connecting these values are,

µge (µge )2 ( Ege )2

⁎

< >

1 2 µ ( µ2 3 eg Eeg

2 µeg )

The progressive shift in ΔEge promotes enhancement in the optical nonlinearity of the chromophore.. Therefore, nowadays there is a great deal of interest in the designing chromophores exhibiting strong nonlinear optical properties in the red/near-infrared (NIR) region. [12]. Heteroaromatic π-spacer plays a vital role in tuning the photophysical properties of sensitizers [13–15]. Thiophene is, among the heteroaromatic π-spacer having excessive electrons [16], well explored in Dye Sensitized Solar Cells (DSSCs), nonlinear optics, and telecommunications [17–19]. The optical, electrochemical, and photovoltaic performance could be efficiently adjusted by changing the π-conjugation length, especially using the thiophene bridge [20]. Introducing thiophene and methine unit (HC]CH) having cyano (–CN) group in πsystem lengthens the π-conjugation leading to wide absorption in the visible region [21–23]. Moreover, thiophene has lower resonance energy leading to increasing the ICT, in turn increasing the red shifted emission [12]. Also, the presence of heterocyclic ring to increase the conformational stability enhances polarizability, increased chemical and thermal robustness, also it leads to noncentrosymetry [24,25]. Dicynoviynyleneisophoron acts as an efficient polling acceptor group and also provides long chain polymeric type of asymmetric molecule.

Corresponding author. E-mail addresses: [email protected], [email protected] (N. Sekar).

https://doi.org/10.1016/j.jphotochem.2020.112389 Received 25 October 2019; Received in revised form 12 January 2020; Accepted 14 January 2020 Available online 16 January 2020 1010-6030/ © 2020 Elsevier B.V. All rights reserved.

Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112389

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The chromophores having a dicyanovinylene acceptor show red shifted emission. NIR shifted absorption-emission, good chemical, and thermal stability [26]. Dicyanovinyleneisophorone acceptor groups that show deep red emission in the solid state were synthesized having an interest in the development of biochips and bioimaging [27]. Zeng and coworkers synthesized isophorone based heterocyclic rings and studied their influence on their electronic structure, AIE properties and molecular packing [28]. Isophorone based D-π-A red emitting compounds show strong ICT. Triphenylamine based multibranched isophorone derivatives with and without methoxy group were synthesized and characterized for viscosity sensitivity [29], bioimaging and strong two photon absorption [30,31]. Shrinivas et.al reported organic dyes with Dπ-A with carbazole as donor and thiophene ring as a spacer for DSCC [32]. Thiophene ring remains planar with its neighboring rings which increase the effective conjugation length and increases bathochromic shifts by decreasing the LUMO energy level [33–35]. Thiophene acts as a donor and hole transporting material used as building blocks for conjugated polyenes due to their thermal stability they have applications in optoelectronic devices, thin films crystallography and solar cells [36]. In this article, we are reporting deep red emitting dyes containing dicyanovinylene isophorone acceptor which consists of thiophene spacer along with auxiliary acceptor, -CN. These dyes show deep red emission towards the NIR region (∼600−700 nm) in polar solvents. These derivatives show very good emission solvatochromism and also they show viscosity sensitivity in the red region above 600 nm hence they can be used in biological applications and acts as fluorescent molecular rotors (FMRs). Solvent polarity plots suggest that compounds show ICT, as well as TICT, which is further supported by Multilinear regression analysis by Kamlet Taft and Catalan method [37,38], suggest that the red shift in emission is mainly due to the solvent acidity and dipolarity. Also, ICT in these derivatives is supported by DFT using FMO diagrams which clearly indicates that HOMO is mainly located on the electron donating group and LUMO is located on acceptor group and they have a low energy band gap which indicates effective intramolecular charge transfer. We have also done mulliken hush analysis study by solvatochromism which shows total delocalization of electrons from donor to acceptors. NBO calculations were done to investigate the charge transfer or interactions in the conjugative system using a filled bonding donor and an empty antibonding acceptor. All these derivatives show very high values of dipole moment, polarizability, first and second order hyperpolarizability which indicates that they can act as very good NLOphores.

3. Experimental section Synthesis of 9-ethyl-9H-carbazole-3-carbaldehyde and 4-(diphenylamino)-2-methoxybenzaldehyde as given in references [30,42]. Synthesis of 2A 2B 2C is synthesized using the reported procedure given in the literature [43]. 3.1. General procedure for the synthesis of 2A, 2B, and 2C 3.1.1. (E)-3-(9-Ethyl-9H-carbazol-3-yl)-2-(thiophen-2-yl) acrylonitrile (2A) To the solution of 1A, 0.2 g (89.6 mmol) and 2-(thiophen-2-yl) acetonitrile (0.11 g 89.6 mmol) add the catalytic amount of potassium tertiary butoxide in 20 ml ethanol. The mixture was refluxed for 15 min and allow to cool the reaction mixture. The yellow colored solid was obtained which was filtered and washed with a small amount of ethanol, recrystallized with ethanol The crude product was purified with 2% Ethyl acetate Hexane mixture to get a pure yellow colored product. Yield: 0.23 g 85%. Melting point-120−122 °C. NMR-1H NMR (500 MHz, CDCl3) δ 8.58 (d, J = 8.5 Hz, 1 H), 8.16 (d, J = 7.5 Hz, 1 H), 8.10 (d, J = 8.5 Hz, 1 H), 7.57 (d, J = 8.5 Hz, 1 H), 7.51 (d, J = 8.5 Hz, 1 H), 7.45 (dd, J = 7.5, 1.5 Hz,2 H), 7.37 (dd, J = 8.2 7.5 Hz, 1 H), 7.30 (m, J = 7.5,1.5 Hz,2 H), 7.08 (dd, J = 7.5, 7.5 Hz, 1 H), 4.39 (q, J = 8 Hz,2 H), 1.47 (t, J = 8 Hz, 3 H). 13 C NMR (125 MHz, CDCl3) δ 141.2, 128.0, 126.8, 126.5, 125.9, 125.1, 122.5, 120.8, 119.8, 108.9, 37.7, 13.8. Elemental Analysis Expected- C-76.80 H-4.91 N8.53 Chemical Formula: C21H16N2S Found C 76.84 H 4,90 N 8.54. 3.1.2. (E)-3-(2-Ethoxy-9-ethyl-9H-carbazol-3-yl)-2-(thiophen-2-yl) acrylonitrile (2B) To a solution of 1B (0.2 g 74.9 mmol) and one equivalent of 2(thiophen-2-yl) acetonitrile (0.092 g 74.9 mmol), a catalytic amount of potassium tertiary butoxide in 20 ml ethanol was added. The mixture was refluxed for 15 min and allow to cool the reaction mixture. Yellow solid was obtained which was filtered and washed with a small amount of ethanol, recrystallized with ethanol The crude product was purified with 2% Ethyl acetate Hexane mixture to get a pure yellow colored product.Yield: 0.22 g 85%. Melting point-125−127 °C. NMR-1H NMR (500 MHz, CDCl3) δ 8.10 (d, J =7.7 Hz, 1 H), 8.03 (s, 1 H), 7.42 (m, J = 7.5 Hz,1 H), 7.36 (d, J = 7.2 Hz, 2 H), 7.28 – 7.22 (dd,J = 7.7,7.2 Hz, 3 H), 7.08 (dd, J = 7.2,7.5 Hz 1 H), 6.79 (s, 1 H), 4.31 (q, J = 7.2 Hz, 2 H), 4.22 (q, J =7 Hz, 2 H), 1.55 (t, J =7 Hz, 3 H), 1.44 (t, J = 7.2 Hz, 3 H). 13 C NMR (125 MHz, CDCl3) δ 157.2, 142.6, 140.8, 140.3, 136.0, 127.9, 125.6, 125.1, 124.9, 123.3, 120.3, 120.3, 119.9, 118.1, 117.2, 116.78, 115.3, 91.4, 64.6, 37.6, 14.8, 13.7. Elemental Analysis Expected- C, 74.16; H, 5.41; N, 7.52 Chemical Formula: C23H20N2OS Found C 74.18 H 5.40 N 7.53.

2. Materials and methods 2 Thiophene acetonitrile potassium tertiary butoxide was purchased from Sigma Aldrich India. The synthetic solvents were purchased from Palav Mumbai. Reactions are monitored using TLC and detected in UV (long and short). The compounds were purified using column chromatography by using 2% ethyl acetate in 98% hexane. 1H NMR and 13 CNMR spectra were recorded on “Agilent 500 MHz NMR spectrophotometer” using TMS as standard reference and in CDCl3 solvent. Mass spectra are measured in IIT SAIF Bombay using instrument 410 Prostar Binary LC with 500 MS IT PDA Detectors, Varian Inc USA. Quantum yield was obtained by using Rhodamine 6 G (ΦF = 0.94 in ethanol) as a standard reference. Density Functional Theory”(DFT)and Time Dependent Density Functional Theory (TDDFT) [39] with B3LYP functional and 6-31++G(d,p) basis set as implemented in Gaussian 09 software [39]. For nonlinear optical properties like dipole moment linear polarizability, first and second order hyperpolarizability, CAMB3LYP, and BHandHLYP functionals were used with basis set 6-31+ +G(d,p) [40,41] Fig. 1 Scheme 1.

3.1.3. (E)-3-(4-(Diphenylamino)-2-methoxyphenyl)-2-(thiophen-2-yl) acrylonitrile (2C) To a solution of 1C (0.2 g 66 mmol) and one equivalent of 2-(thiophen-2-yl) acetonitrile (0.081 g 66 mmol), catalytic amount of potassium tertiary butoxide in 20 ml ethanol was added. The mixture was refluxed for 15 min and allow to cool the reaction mixture. Yellow solid was obtained which was filtered and washed with small amount of ethanol, recrystallized with ethanol The crude prduct was purified with 2% Ethyl acetate Hexane mixture to get pure yellow colored product. Yield:0.21 g 80%. Melting Point-135−137 °C 1 H NMR (400 MHz, CDCl3) δ 8.09 (d, J = 8.7 Hz, 1 H), 7.77 (s, 1 H), 7.35 (m, J = 7.5 Hz,7.3 Hz 5 H), 7.28 (s, 1 H), 7.25 (d, J = 8.7 Hz, 1 H), 7.19 (m, J = 7.3 Hz,4 H), 7.13 (d, J = 7.3 Hz, 1 H), 7.06(d, J = 8.7 Hz 2

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Fig. 1. Molecular structures of dicyanovinyleneisophoron based chromophores (5A, 5B and 5C).

1 H), 6.64 (d, J = 8.7 Hz, 1 H), 6.55 (s, 1 H), 3.70 (s, 3 H) 13 C NMR (101 MHz, CDCl3) δ 158.9, 155.3, 151.5, 146.6, 139.3, 134.0, 129.5, 128.8, 127.9, 127.1, 125.8, 125.7, 125.1, 124.4, 124.3, 115.7, 113.7, 103.4, 102.1, 55.5. Elemental Analysis Expected- C, 76.44; H, 4.93; N, 6.86 Chemical Formula: C26H20N2OS C 76.47 H 4.94 N 6.87.

1.49 (t, J = 7.5 Hz, 3 H). 13 C NMR= (126 MHz, CDCl3) δ 182.4, 149.6, 144.6, 142.1, 141.8, 140.6, 137.1, 127.6, 126.8, 126.3, 123.7, 123.6, 123.5, 122.8, 120.9, 120.3, 117.2, 109.1, 109.1, 100.8, 37.9, 13.9. Elemental Analysis C, 74.13; H, 4.52; N, 7.86 Chemical Formula: C22H16N2OS Found C 74.16 H 4.53 N 7.87

3.2. General procedure for synthesis of aldehydes 3A, 3B and 3C

3.2.2. (E)-3-(2-Ethoxy-9-ethyl-9H-carbazol-3-yl)-2-(5-formylthiophen-2yl) acrylonitrile (3B) A mixture of POCl3 (0.10 ml 161.2 mmol) and DMF (0.13 ml 107.5 mmol) was cooled for 15 min at 0 °C.A solution of 2B (0.2 g, 53.7 mmol) in DMF slowly at 0OC.then stir the reaction mixture for 15 min at room temperature and then reflux at 80OC for 2 h. On completion of the reaction the mixture was cooled to room temperature and then poured in to ice cooled water to get red solid it was filtered and the crude product was purified by column chromatography in 5% ethyl acetate hexane mixture Red colored solid obtained. Yield: 0.35 g (62 %); Melting point = 140−142 °C. 1 H NMR (500 MHz, CDCl3) δ 9.85 (s, 1 H), 8.04 (d, J = 1.5 Hz,2 H), 7.73 (d,J = 8.2,8 Hz 2 H), 7.40 (dd, J = 8.2,8 Hz, 2 H), 7.26 (d, J = 7.8 HZ 1 H), 7.18 (d J = 7.8 Hz, 1 H), 6.77 (s,1 H), 4.23 (t,J = 7.8 Hz, 2 H), 4.17 (q, J = 7.5 Hz, 2 H), 1.57 (t,J = 7.8 Hz,3 H), 1.45 (t, J = 7.5 Hz,3 H). 13 C NMR (126 MHz, CDCl3) δ 182.58, 157.7, 150.6, 144.3, 139.2,

3.2.1. (E)-3-(9-Ethyl-9H-carbazol-3-yl)-2-(5-formylthiophen-2-yl) acrylonitrile 3A A mixture of POCl3 (0.11 ml 121.9 mmol) and DMF (0.14 ml 182.29 mmol) was cooled for 15 min at 0OC. A solution of 2A (0.2 g 61 mmol) in DMF was slowly added at 0 °C, and then the reaction mixture was stirred for 15 min at room temperature and then refluxed at 80 °C for 2 h. On completion of the reaction the mixture was cooled to room temperature and then poured in to ice cooled water to get red solid it was filtered and the crude product was purified by column chromatography in 5% ethyl acetate hexane mixture Red colored solid obtained. Yield : 0.35 g (62 %); Melting point = 132−134 °C. 1 H NMR = (500 MHz, CDCl3) δ 9.90 (s, 1 H), 8.67 (s, 1 H), 8.17 (d, J = 8.5 Hz, 8 Hz 2 H), 7.76 (s, 1 H), 7.74 (d, J = 8.5 Hz, 1 H), 7.55 (d,J = 7.5 Hz, 1 H), 7.48 (dd, J = 8.5 Hz, 1 H), 7.45 (dd, J = 8.5,8 Hz, 1 H), 7.33(d, J = 8 Hz, 1 H), 7.27 (s, 1 H), 4.42 (q, J = 7.5 Hz, 2 H),

Scheme 1. Synthesis of 5A, 5B, and 5C. 3

Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112389

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137.3, 135.6, 129.8, 125.9, 125.3, 125.3, 122.1, 120.7, 120.3, 119.8, 119.7, 108.7, 91.4, 64.7, 37.7, 14.8, 13.8. Elemental Analysis Expected C, 71.98; H, 5.03; N, 6.99 Chemical Formula: C24H20N2O2S Found C 71.99 H 5.04 N 6.98

crude product was purified by column chromatography in 10% ethyl acetate hexane mixture. Red colored solid obtained as pure product.Yield=0.2 g 70%. Melting Point-148−149 °C 1 H NMR (500 MHz, CDCl3) δ 8.96 (s, 1 H), 8.12 (d, J = 7.5 Hz, 1 H), 8.06(s,1 H),7.92 (d, J = 7.8HZ, 1 H), 7.51 (d, J = 7.8 Hz 1 H), 7.45 (d, J = 8 Hz 1 H), 7.33 (d, J = 8 Hz, 1 H), 7.26 (d, J = 8.5 Hz 1 H), 7.22 (d, J = 8.5,7.5 Hz 2 H), 7.15 (s, 1 H), 6.90 (d, J =14.2 Hz, 1 H), 6.79 (d, J =14.2 Hz, 1 H), 4.79 (q, 2 H), 4.12 (q, 2 H), 3.66 (s, 2 H), 2.60 (d, 2 H), 1.65 (s6H), 1.25(d,6 H). 13 C NMR (201 MHz, CDCl3) δ 168.8, 157.5, 153.3, 143.7, 143.2, 140.8, 140.5, 136.9, 131.1, 129.5, 128.9, 126.7, 125.4, 123.5, 123.2, 120.4, 120.3, 120.2, 117.7, 117.1, 115.1, 113.6, 112.9, 108.6, 101.1, 91.4, 78.3, 64.8, 42.9, 39.1, 37.6, 32.0, 29.7, 28.0, 14.9, 13.7. HRMS- Expected m/z-568.2297 observed-m/z+1-569.2170 IR spectra – (KBr pellet) Cm−1-2929.67, 2218.25, 1662.52, 1596.96, 1448.44, 1371.29, 1257.50, 1081.99, 808.12, 746.40. Elemental Analysis Expected C, 76.03; H, 5.67; N, 9.85 Chemical Formula: C36H32N4OS Found C 76.06 H 5.67 N 9.86

3.2.3. (E)-3-(4-(Diphenylamino)-2-methoxyphenyl)-2-(5-formylthiophen2-yl) acrylonitrile (3C) A mixture of POCl3 (0.09 ml 147 mmol) and DMF (0.11 ml 98 mmol) was cooled for 15 min at 0OC. A solution of 2C (0.2 g 49 mmol) in DMF slowly at 0OC.then stir the reaction mixture for 15 min at room temperature and then reflux at 80OC for 2 h. On completion of the reaction the mixture was cooled to room temperature and then poured in to ice cooled water to get red solid it was filtered and the crude product was purified by column chromatography in 5% ethyl acetate hexane mixture Red colored solid obtained. Yield : 0.35 g (62 %) Melting point = 165−167 °C. 1 H NMR (400 MHz, CDCl3) δ-9.89 (s, 1 H), 8.16 (s, 1 H), 7.80 (d J = 6.8 Hz, 1 H), 7.78 (d, J = 6.8 Hz, 1 H), 7.47 (m, 11 H), 7.02 (d, J = 7.2 Hz,1 H), 6.80 (s,1 H), 6.68 (d, J = 7.2,1 H), 3.75 (s, 3 H). 13 C NMR (126 MHz, CDCl3) δ 182.5, 159.7, 152.7, 146.1, 137.3, 137.1, 129.6, 129.3, 129.1, 126.2, 125.8, 125.0, 123.4, 123.2, 114.5, 112.9, 102.1, 99.7, 55.6. Elemental Analysis Expected C, 74.29; H, 4.62; N, 6.42; Chemical Formula: C27H20N2O2S. Found C 74.32 H 4.63 N 6.43

3.3.3. -(3-((E)-2-(5-((E)-1-Cyano-2-(4(diphenylamino) 2methoxyphenyl) vinyl) thiophen-2-yl) vinyl)-5, 5-dimethylcyclohex-2-en-1-ylidene) malononitrile (5C) Compound 3C (0.2 g 50 mmol) and 4 (0.128 g 68.8 mmol) were dissolved in 10 ml absolute ethanol, add 0.1mLof piperidine, reflux the reaction mixture for 2 h. On completion of the reaction, the mixture was cooled to room temperature. The reaction mixture was filtered and the crude product was purified by column chromatography in 10% ethyl acetate-hexane mixture. The red colored solid obtained as pure product Yield: 0.19 g 68%. Melting Point-150−152 °C 1 H NMR (500 MHz, CDCl3) δ 8.15 (s 1 H), 7.80 (d, J = 8.2 Hz, 1 H), 7.40 (dd, J = 8.2,7.5 Hz, 4 H), 7.29 (s,2 H), 7.22 – 7.08 (m, 7 H), 7.08(d,7.5 Hz,1 H),6.93 (d, J = 8.2 Hz, 1 H), 6.86(d,J = 14.5 Hz,1 H), 6.72 (d, J = 14 Hz, 1 H), 6.63(d, J = 7.2 Hz) 1 H), 6.52 (s, 1 H), 3.73 (s, 3 H), 3.51 (s, 1 H), 2.62 (s, 1 H), 2.47 (d, 1 H), 2.07 (s, 1 H), 1.28 (s, 3 H), 1.10 (s, 3 H). 13 C NMR (201 MHz, CDCl3) δ 168.8, 159.3, 158.9, 153.2, 152.1,146.3, 143.6, 140.8, 136.4, 135.6, 135.1, 133.9, 129.6, 128.8, 125.9, 124.8, 117.5, 115.3, 115.2, 113.3, 112.9, 105.0, 102.9, 102.6, 100.7, 96.1,78.2, 68.4, 55.7, 42.9, 39.0, 31.9, 29.7, 28.0. HRMS data-m/z-expected-604.2297 observed-m/z+23-627.2170 IR spectra – (KBr pellet) Cm−1-2927.74, 2214.13, 1668.31, 1591.16, 1487.01, 1332.72, 1282.57, 1120.57, 1031.85, 754.12, 696.25. Elemental Analysis Expected C, 77.45; H, 5.33; N, 9.26 Chemical Formula: C39H32N4OS Found C 77.48 H 5.34 N 9.26.

3.3. General procedure for 5A, 5B and 5C 3.3.1. -(3-((E)-2-(5-((E)-1-Cyano-2-(9-ethyl-9H-carbazol-3-yl) vinyl) thiophen-2-yl) vinyl)-5, 5-dimethylcyclohex-2-en-1-ylidene) malononitrile (5A) Compound 3A (0.2 g 56.2 mmol) and 4 (0.15 g 84.3 mmol) was dissolved in 10 ml absolute ethanol, add 0.1 ml of piperidine, and the reaction mixture was refluxed for 2 h On completion of the reaction the mixture was cooled to room temperature and then poured in to ice cooled water to get solid product The reaction mixture was filtered and the crude product was purified by column chromatography in 10% ethyl acetate hexane mixture. Red colored solid obtained as ure product.Yield: 0.16 g 56%.Melting Point-145−147 °C 1 H NMR (400 MHz, CDCl3) δ 8.66 (s, 1 H), 8.19 (d, J = 7.3 Hz, 1 H), 8.14 (d, J = 7.8 Hz, 1 H), 7.61 (s, 1 H), 7.54 (s, 2 H), 7.50 (d, J = 7.8 Hz, 3 H), 7.43 (s, 1 H), 7.32 (d, J = 7.5,7.8 Hz, 3 H), 7.16 (d, J = 7.5 Hz, 1 H), 7.05 (d, J =11.5 Hz, 1 H), 6.84 (d, J =11.5 Hz, 1 H), 6.79 (s, 1 H), 4.44 (d, J = 7.2 Hz, 2 H), 2.77 (s, 1 H), 2.63 (s, 1 H), 2.46 (s, 1 H), 2.19 (s, 1 H), 1.28 (s, 6 H), 1.11 (s, J = 7.2 Hz,3 H). 13 C NMR (201 MHz,CDCl3) δ 168.7, 152.9, 142.7, 142.3, 141.8, 141.5, 141.0, 140.6, 138.9, 134.9, 132.9, 130.9, 129.2, 127.3, 127.0, 126.7, 123.7, 122.9, 120.9, 120.1, 112.5, 110.1, 109.1, 101.4, 96.1, 70.6, 42.9, 39.1, 37.9, 30.9, 29.7, 28.0, 13.9. HRMS - m/z-expected-524.2035 Observed-m/z-1-523.2217 and m/z +1-525.2295 IR spectra – (KBr pellet) Cm−1- 2968.24, 2216.06, 1666.36, 1566.45, 1492.80, 1334.66, 1232.43, 1157.21, 1128.28, 802.33, 748.33 Elemental Analysis Expected C, 77.83; H, 5.38; N, 10.68 Chemical Formula: C34H28N4S Found C 77.86 H 5.39 N 10.69

4. Result and discussion 4.1. Photophysical study Due to the presence of a long chain of π conjugation, we have studied the effect of electron donating, π-spacer and acceptor groups on the absorption and emission properties in different solvents. We have done the solvatochromism study in eleven different solvents. Emission spectra were recorded at their respective excitation spectra. Fig. 2 represents the normalized absorption and fluorescence spectra of 5A which shows the absorption spectra of are insensitive towards the solvent polarity while emission spectra are significantly influenced by solvent polarity and therefore bathochromically shifted emission was observed from nonpolar to polar solvents. Fig. S1 indicates the normalized absorption and emission spectra of 5B and 5C. They show two distinct peaks in which one peak around 350–450 is due to π-π*

3.3.2. -(3-((E)-2-(5-((E)-1-Cyano-2-(2-ethoxy-9-ethyl-9H-carbazol-3-yl) vinyl) thiophen-2-yl) vinyl)-5, 5-dimethylcyclohex-2-en-1-ylidene) malononitrile (5B) Compound 3B (0.2 g 50 mmol) and 4 (0.139 g 75 mmol) was dissolved in in 10 ml absolute ethanol, add 0.1 ml of piperidine, reflux the reaction mixture for 2 h, On completion of the reaction the mixture was cooled to room temperature. The reaction mixture was filtered and the 4

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diagrams and bandgap energy which suggests that there is effective ICT is observed and it is showing a very low bandgap value around 1.44. All these compounds show higher values of molar extinction coefficients (ε). Table 1 represents comparative values for all parameters. The compound 5A shows values of ε around 1.8 × 10^4, 5B shows around 27211-7.9 × 10^4 and 5C shows 3.5 × 10^4. Full-width half maxima (FWHM) is also showing higher values (160–260) for all the three compounds. Compound 5A and 5B show low values of quantum yield around 0.05 while compound 5C shows comparatively higher values of quantum yield (0.38) in nonpolar solvents while very low values of quantum yield (0.012) in polar solvents. The higher values are due to the auxiliary acceptor cyano group. Also, all these compounds show large values of oscillator strength as well as transition dipole moment which is further supported computationally which gives higher values of oscillator strength as well as dipole moment. Oscillator strength is calculated by using the equation

Fig. 2. Normalized absorption and emission spectra of 5A.

=

transition while another peak around 500 nm arising out of ICT [44]. Compound 5A shows two absorption peaks, one around 387 nm and another peak around 497 nm and emission ranging from 604−698 nm from hexane to DMF. (Fig. 3). Compound 5B shows two absorption peaks near 399 nm and 500 nm and emission ranging from 598−640 nm (Fig. 4). Compound 5C shows two absorption peaks around 427 nm and 507 nm while emission ranging from 555−730 nm which shows deep red emission (Fig. 5). It is noticeable that there is a slight red shift of absorption maxima due to the introduction of ethoxy and methoxy group in 5B and 5C, also there is a slight increase in molar extinction coefficient oscillator strength and transition dipole moment in both compounds [45]. The compound 5A shows emission solvatochromism around 94 nm shift and the compound 5B and 5C show 42 nm and 188 nm shift respectively in emission from nonpolar to polar solvents. The compounds 5A and 5B show Stokes shift in the range 92−206 nm, 5C shows Stokes shifts varying from 52 nm to 254 nm from nonpolar to polar solvents. In the case of 5C while going from nonpolar to polar solvent quenching of fluorescence was observed which may be due to the TICT state [46] and which is due to freely rotating phenyl groups of triphenylamine moiety. To explain this property we did viscosity study in ethanol and PEG 400 mixture in which again emission enhancement takes place while increasing the percentage of the viscous solution, as in viscous solution the free rotation of phenyl rings restricted and effective charge transfer takes place. In TICT state compounds get twisted and charge transfer is restricted where quenching of fluorescence along with bathochromic shift takes place in polar solvents. In polar solvents, TICT is stabilized which leads to the nonradioactive decay whose energy gap is less than the locally excited state and because of the low energy gap it leads to the bathochromic shift. It is further supported by using DFT using FMO

4.32 × 10 n

9

d

(1)

where ƒ is oscillator strength n is refractive index, ε is molar absorptivity and ʃενdν is integrated absorption coefficient (IAC). Also, the transition dipole moment is calculated by equation

µeg 2 =

f 4.72 × 10

7

× v¯

(2)

Where µeg2 is transition dipole moment, ῡ is absorption frequency in cm−1 and ƒ is oscillator strength. The compounds 5B and 5C show comparatively higher values of oscillator strength and transition dipole moment which may be due to the additional auxiliary ethoxy and methoxy donor group [45]. Photophysical parameters of 5A, 5B, and 5C are represented in Table S1, S2, and S3 respectively. Also, we have compared all three compounds with TMS [30]. and Dye 1 [47,48] which is without thiophene spacer. In our synthesized derivatives thiophene ring is added in between donor and acceptor. Thiophene ring remains planar with its neighboring rings which increase the effective conjugation length and increases bathochromic shifts by decreasing the LUMO energy level.All new derivatives shows bathochromic shift in absorption and emission due to presence of thiophene ring and they are comparede with TMS and Dye 1. All these compounds show a comparatively red shift in absorption and emission. (Table 2). 4.2. Solvatochromism and Lippert-Mataga analysis We observed positive solvatochromism in emission in all the compounds from nonpolar to polar solvents. The compound 5A in hexane, the emission is at 604 nm while in DMF 698 nm. For the compound 5C in hexane, the emission is at 555 nm while in acetonitrile the emission is at 743 nm. We have plotted a graph of Lippert-Mataga and MacRae

Fig. 3. Absorption and emission graphs of 5A. 5

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Fig. 4. Absorption and emission graphs of 5B-.

Fig. 5. Absorption and emission graphs of 5C. Table 1 All photophysical parameters of 5A, 5B and 5C in DMF solvent. Dyes

λ

5A 5B 5C

nm 492 506 507

λ

abs

ems

nm 698 640 728

Stokes Shift nm 206 134 221

cm−1 5998.56 4137.85 5987.6

εmax

FWHM

φF

f

μeg Debye

L mol−1 cm−1 22455 51249 25682

nm 201 165 263

0.0732 0.0166 0.07

0.673 2.683 1.343

8.399 16.767 12.041

λabs-Absorption maxima. λems- Emission maxima. εmax- Molar extinction coefficient. FWHM-Full width half maxima. ??F-Quantum yield. f- Oscillator strength. μeg -transition dipole moment.

function versus Stokes shift and Weller and Rettig function versus emission values in cm−1. Lippert Mataga function is given by Eq. 3 [49]

a

LM

f= m1 LM ( , )

( , n) =

1 2 +1

n2 1 2n2 + 1

MR

Where m =

(3)

f= m1 LM ( , )

1 +2

n2 1 n2 + 1

(6)

2(µe µg )2 hca03

μe is dipole moment in excited state μg is dipole moment in the ground state, h is Planck's constant, c is the speed of light and a0 is Onsager radii. Also, Weller function is given by equation

(4)

Where ε is dielectric constant and n is the refractive index of the solvent. Also, Mac Rae equation is given by [50]

a

( , n) =

w

( , n) =

1 2 +1

n2 1 4n2 + 2

Weller equation considers the frequency of emission [51].

(5) 6

(7)

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Table 2 Comparison of all three compounds with TMS (without thiophene spacer). Compound

λabs

λems

Stokes shift

FWHM

εmax

фf

f

μeg

Kr×10^8

TMS Dye A 5A 5B 5C

480 508 386,491 396,508 408,509

630 637 644 621 649

150 129 153 113 140

103 105 197 169 258

L mol−1 cm−1 49300 46345 21226 48782 34518

0.025 0.027 0.0072 0.007 0.2

1.04 0.73 1.48 1.36 1.39

DEBYE 10.33 8.88 8.452 11.94 12.73

(S−1) 3.05 1.80 1.48 4.6 1.39

λabs- Absorption maxima. λems- Emission maxima. εmax- Molar extinction coefficient. FWHM-Full width half maxima. ??F-Quantum yield. f- Oscillator strength. μeg -transition dipole moment. Kr-Radiative rate constant.

f=

2µe2

hca03

w

4.3. Multilinear regression analysis using Kamlet-Taft and Catalan parameters

+ constant

Rettig equation is given by equation [52,53] R

1 +2

( , n) =

f=

2µe2

hca03

R

n2 1 2n2 + 4

Multilinear regression analysis (MLR) has been studied by using Kamlet Taft and Catalan method [54–58]. There is no much change is observed for the absorption of all compounds. For emission compound 5A, 5B and 5C show correlation coefficient for both Kamlet Taft 0.82,0.82,0.87 and Catalan 0.82 0.89,0.94 method respectively. We observed negative values for emission spectra for 5A, 5B, and 5C. The compounds 5A 5B and 5C show values of correlation coefficient around -5.02,-2.86 and -11.17 by the Catalan method and solvent dipolarity play an important role in the red shift. As we go from nonpolar to polar solvent negative values for correlation coefficient are observed for solvent acidity by Kamlet Taft method and solvent dipolarity by the Catalan method. Hence solvent acidity and dipolarity mainly affect the

(8)

+ constant

ῡƒ is emission in cm−1. The regression coefficient for all these compounds found around 0.9 which indicates very good charge transfer as well as ICT and TICT for all the compounds. Fig. 6 and Fig S2 and S3 represent the polarity plots for 5A, 5B, and 5C respectively.

Fig. 6. Solvent polarity plots of 5A. 7

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Table 3 comparative MLR VALUES of Emission of 5A, 5B and 5C by Kamlet and Catalan method. Kamlet-Taft

y0 × 103

aα

bβ

cπ*

r

5A 5B 5C Catalan 5A 5B 5C

16.71 ± 0.37 16.79 ± 0.22 18.39 ± 0.68 y0 × 103 18.18 ± 1.59 17.54 ± 0.95 (21.21) ± 2.4

(-0.25) ± 0.46 (-0.38) ± 0.27 (0.25) ± 0.86 aSA (-0.04) ± 0.73 (-0.33) ± 0.44 (1.71) ± 1.10

(-0.50) ± 0.72 (-0.22) ± 0.43 (-1.73) ± 1.35 bSB (0.55) ± 0.76 (0.37) ± 0.46 (0.16) ± 1.15

(-1.79) ± 0.64 (-0.99) ± 0.38 (-4.06) ± 1.19 cSdP (-5.02) ± 1.25 (-2.86) ± 0.75 (-11.17) ± 1.9

0.82 0.68 0.87 dSP (1.48) ± 2.22 (0.97) ± 1.32 (4.16) ± 3.34

red shift in all three compounds. Dipolarity indicates the solvents ability to stabilize the dipole or charge through nonspecific interactions (π*) and also hydrogen bond donor (α) and acceptor (β) strength which is given by Kamlet Taft equation, y = y0 + a α + bβ + cπ*

mixture (HAD) using MH analysis [12,59]. All these values are given in Table 4 and Table S4. The values of Cb2are found close to zero (around 0.17-0.34) for all three compounds which indicates that total delocalization of charge takes place in all the compound

(9)

5. Optimization of all compounds by DFT method

Where y is spectroscopic property a is hydrogen bond donor strength b is hydrogen bond acceptor strength and c is the strength of dipolarity/ polarizability. Catalan method is described by equation, y = y0 + aSASA + bSBSB + cSPSP + dSdPSdP

r 0.89 0.95 0.8

5.1. Frontier molecular orbital (FMO) We optimized all compounds in the gas as well as eleven solvents using B3LYP functional and 6-311++G(d,p) basis set [60–62] Fig.10. Represents HOMO and LUMO and their energy bandgap It is observed that HOMO is mainly located on electron donating group carbazole and triphenylamine ring along with thiophene ring and auxiliary acceptor CN group while LUMO is mainly located on thiophene ring and acceptor group isophorone ring which indicate effective charge transfer takes place from donor to acceptor. All the compounds show very low bandgap values (1.59 eV for 5A, 1.60 eV for 5B and 1.44 eV for 5C) which supports effective ICT takes place and maximum conjugation is observed in all compounds. The compound 5C shows comparatively less band gap energy(1,44 eV) which suggests more charge transfer compare to the other two compounds. ELeEH is the difference between HOMO and LUMO energies.

(10)

Where SA is solvent acidity, SB is solvent basicity SP is solvent polarizability and SDP is solvent dipolarity. Table 3 represents the MLR values for absorption emission and stokes shift by using both methods. 4.4. Viscosity study of all compounds in EtOH: PEG400 mixture The compound 5C shows quenching of fluorescence while going from nonpolar to polar solvents due to TICT, hence to correlate experimental findings we have done viscosity study in EtOH: PEG400 mixture. In the 5 u M solution of 5C, we have increased the percentage of PEG in ethanol from 10 to 99%. We have observed a 4.85 fold increase in emission intensity for 5C and 4.34 fold increase in emission intensity for 5A. There is a slight increase is observed in absorption graphs. Viscosity sensitivity above 600 nm is essential because of its biological applications and viscosity sensitivity in the region of 600 nm and above is not reported much till now. The dyes described here show emission above 600 nm (deep red region) hence they can act as very good FMRs. (Figs. Fig. 77, Fig. 88, Fig. 99 ).

5.2. BLA and BOA of compounds All three compounds show in ground state BLA values are positive and around 0.08-0.09 which is close to zero indicates that structure approaching cyanine limit (0). Also, BOA values are negative and close to polyene limit (0.6) (Fig.11 and Table 5) 5.3. Natural bond orbital analysis

4.5. Mulliken Hush (MH) analysis

NBO is an efficient method to investigate the charge transfer or interactions in the conjugative system using filled bonding donor and empty antibonding acceptor and by calculating their energies by second order perturbation theory [63]. In 5A π electron delocalization is

MH analysis is used for the study of charge transfer. We have calculated oscillator strength (f), the degree of delocalization Cb2, the distance between donor and acceptor (RDA) and electronic coupling

Fig. 7. Viscosity absorption and emission graph of 5A. 8

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Fig. 8. Viscosity absorption and emission graph of 5B.

maximum around C8-C10,C11-C12.C33-C34,C35-C36,C47-C55,C47C55 distributed to π* antibonding of C9-C13,C9-C13,C16-C17,C40-C41 C44-C45 and C58-N59 with a stabilization energy of 22.73,230.34,128.01,82.4,105.81 and 22.03 KJ/mol respectively. Also, it shows the hyperconjugative interaction of LP N7 with antibonding C8-C10 with energy 47.22 KJ/mol with electron density (1.5994). All bonding to antibonding has a higher electron density around 1.96–1.97 which leads to strong charge delocalization takes place (Table S5). In the case of 5B π electron delocalization is distributed from C9-C13, C39-C40, C43-C44, and C46-C54 to N7-C8, C43-C44, C46-C54, and C55-N56 with stabilization energy of 53.64, 20.07, 28.51, 24.04 KJ/ mol with electron density 1.96–1.98 which indicates more charge delocalization takes place. From N7 LP to C8 LP energy is 110.74 with electron density 1.60KJ/mol.π*C32-C33, C34-C35 and C46-C54 NBO conjugated with π* C16-C17, C39-C40, and C43-C44 with stabilization energy 92.84,86.98,109.36 respectively which indicated more delocalization takes place (Table S6). For 5C charge delocalization from bonding C2-C4,C5-C6,C15-C16,C24-C25,C27-C35 to antibonding C5C6, C8-C9,C20-C21,C27-C35,C36-N37 with stabilization energy 27.75,20.32,20.17,29.01,24.46 respectively and they have high electron density around 1.96–1.98 KJ/mol. Also From antibonding C2C4,C5-C6,C13-C14,C15-C16,C27-C35 charge delocalization takes place to antibonding C3-C7,C3-C7,C8-C9,C20-C21,C24,C25 with enormous stabilization energy of 166.97,164.23,102.2,87.25,107.84 indicates higher charge delocalization takes place. (Table S7) (Fig S4) represents the graph of NBO analysis for all three compounds.

Table 4 MH analysis of 5A, 5B, and 5C. Compound

fabs

μge×10−18

Δμab×10−18

Cb2

HDA.

RDA

5A 5B 5C

0.732 2.42 1.26

e.s.u 12.12 10.52 20.72

e.s.u 8.67 15.87 11.59

0.21 0.34 0.17

(cm−1) 8501.24 9706.17 7412.61

(A°) 12.18 16.48 12.61

HDA-Electronic coupling mixture. ƒabs- Oscillator strength. Cb2-Degree of delocalization. RDA-Distance between donor and acceptor. μge –Transition dipole moment from the ground to excited state.

5.4. TD vertical excitation oscillator strength, the orbital contribution of compounds We have compared the experimental absorption, oscillator strength and dipole moment by using TDDFT. Table 6. We have observed that the transition takes place from HOMO-LUMO with a major contribution of 98–99 %. (Table S8) while from HOMO-1to LUMO with major orbital contribution 79–85 % for compounds 5A and 5C and From HOMO1 to LUMO the experimentally observed and computationally found values for absorption and vertical excitation matches and are in the same trend (Table S9). Also for compound 5A and 5C HOMO-1 to LUMO transition observed which shows comparatively less oscillator strength and orbital contribution.

Fig. 9. Viscosity absorption and emission graph of 5C.

9

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S. Bhalekar, et al.

Fig. 10. FMO diagrams with band gap energy.

6. Nonlinear optical properties of comp by Computational method-

polarizability is a measure of electric dipole moment changes which is caused due to the external magnetic field. Polarizability is related to the structure and bonding properties of the material. Dipole moment calculated by using the equation

6.1. The linear polarizability

μ = (μx2+μy2+μz2)1/2

The interactions of materials with electromagnetic field to cause the alterations with phase frequency polarization and amplitude are called nonlinear optics.NLO materials have wide applications in the field of optical communication, dynamic image processing, optoelectronic devices. Extensive research is going on the synthesis of various readily available materials having large NLO properties. The linear

(11)

Linear polarizability can be calculated by using the equation α0 = 1/3(αxx+ αyy+ αzz) Δα = 2

−1/2

(12) 2

2

[(αxx− αyy) + ( αyy − αzz) + ( αzz − αxx) +

Fig. 11. Optimized geometry of 5A in G.S and E.S-.

10

2

6α2xx]1/2

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Table 5 BLA BOA analysis of 5A, 5B, and 5C.

Table 7 NLO values of 5A, 5B, and 5C in Toluene and Methanol solvents.

BLA(GS) Solvent Hexane Toluene 1,4Dioxane DCM THF EA Acetone DMF Acetonitrile Methanol Ethanol

5A 0.09436 0.09388 0.094 0.0921 0.0923 0.09247 0.0916 0.0914 0.09138 0.09141 0.0915

5B 0.095 0.0939 0.094 0.0921 0.0917 0.0925 0.0917 0.0915 0.09149 0.0915 0.0916

BOA(GS) 5C 0.0878 0.0899 0.0883 0.0883 0.0884 0.0886 0.0878 0.0877 0.0877 0.0877 0.0878

5A −0.4766 −0.4765 −0.4766 −0.476 −0.476 −0.476 −0.4762 −0.4762 −0.4762 −0.4762 −0.476

5B −0.4754 −0.4736 −0.4737 −0.473 −0.4731 −0.473 −0.473 −0.47 −0.473 −0.473 −0.473

5A 5C −0.476 −0.4772 −0.4774 −0.476 −0.476 −0.4765 −0.476 −0.476 −0.4759 −0.476 −0.473

5B (μ) (α0) Δα βo 5C (μ) (α0) Δα βo

Most of the push pull chromophores having large second order NLO properties are noncentrosymmetric and one-dimensional charge transfer. The first order hyperpolarizability enhances with increasing oscillator strength, transition dipole moment and decreasing energy bandgap. The values of β are dependent on the donor acceptor strength and conjugated π bridge. Values for β0 were calculated by using Eq. 14.

=

2 x

+

2 y

+

Bhand HLYP Methanol 18.95 132.45 162.03 960.12 6454.12

Toluene 16.32 123.49 150.21 780.23 5120 BHandHLYP Toluene 17.9518 110.93547 132.4602 552.08711 2843.6916 Bhand HLYP Toluene 17.27 123.49 150.21 747.98 4541.64

Methanol 22.1306 137.126067 162.778041 918.181712 5836.8406 Methanol 19.19 147.36 168.85 1241.53 8293.99

Methanol 18.34 132.23 163.85 999.48 6496.73 Methanol 21.2061 137.216967 159.825244 936.526851 5875.3708 Methanol 20.02 147.26 172.82 1218.73 8290.32

7. Conclusion In conclusion, we have successfully synthesized donor auxiliary acceptor-π spacer-acceptor derivatives and characterized by 1HNMR, 13 CNMR, HRMS analysis. All three compounds show a bathochromic shift in emission with a large stokes shift of around 92−206 nm. These dyes show deep red emission towards the NIR region (∼600−700 nm) in a polar medium. All these chromophores show very good solvatochromism which is confirmed solvent polarity plots, multilinear regression analysis which indicates that solvent acidity and dipolarity affect the red shift in the emission of all three chromophores. All these compounds show emission enhancement in a viscous solvent like PEG 400 above 600 nm (deep red region) hence they can act as very good FMRs that have biological applications. Delocalization of charge transfer characteristics was elucidated using MH analysis and NBO analysis. They show higher values of dipole moment (16–22Debye), linear polarizability (110−147 × 10^-24e.s.u.) First order hyperpolarizability (β0) around (500-1000×^-30 e.s.u) and second order hyperpolarizability (γ) around (3000−8300 × 10^-36e.s.u), hence they can be used as NLOphores.

2 z

βtotal = [(βxxx + βxyy + βxzz)2 + (βyyy + βxxy + βyzz)2 +(βzxx + βzyy + βzzz)2]1/2 (14) Second order hyperpolarizability is given by the equation, γ = (1/5) [(γxxxx + γyyyy + γzzzz + 2γxxyy + 2γyyzz + 2γzzxx)]

Toluene 16.02 122.35 148.23 785.21 5145 CAM B3LYP Toluene 17.2 110.902 130.109 513.72 2835.12 CAM B3LYP Toluene 17.012 123.01 146.25 749.231 4547.21

(μ) (α0) Δα βo

(13)

0

CAM B3LYP

(15)

We calculated the values of dipole moment, polarizability, first and second order hyperpolarizability was calculated in different nonpolar and polar solvents using hybrid and range separated CAM B3LYP and BHandHLYP functional and 6-311++G (d, p) basis set by using equations given in the literature [64]. The results of μ, Δα, α0, β are summarized in Table 7. All three compounds show higher values of dipole moment, linear polarizability first and second order hyperpolarizability. The dipole moment observed in the range of 16–22 Debye. The values of linear polarizability α0 are observed around 110−147 × 10−24e.s.u. First order hyperpolarizability (β0) around 500-1000×-30e.s.u and second order hyperpolarizability (γ) around 3000−8300 × 10-36e.s.u which are several folds greater than that of urea. Also, it is observed that as we going from nonpolar to polar solvent all the values go on increasing. for second order hyperpolarizability while going from nonpolar to polar solvent values observed are almost double. There is not much change is observed for CAMB3LYP and BHandHLYP. All values of second order hyperpolarizability of three compounds in a nonpolar and polar solvent are given in Fig.12.

Author statement All the co-authors are aware of this submission. Declaration of Competing Interest The authors declare no conflict of interest.

Table 6 Vertical excitation oscillator strength orbital contribution of 5A, 5B and 5C HOMO-LUMO level. Orbital Comp 5A 5B 5C

HOMO-LUMO 138-139 150-151 159-160

contribution 0.70455 0.70101 0.70502

ev 2.16 2.16 1.98

vertical

Oscillator strength

excitation 573.46 573.7 615

f DFT 2.04 1.78 1.80

11

f exp 0.696 1.1597 1.159

Abs (exp)

Major orbital

482 503 485

contribution 99.28 98.28 99.41

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Fig. 12. Graph of values of second order hyperpolarizability of 5A, 5B, and 5C.

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