Ferrocene conjugated donor-π-acceptor malononitrile dimer: Synthesis, theoretical calculations, electrochemical, optical and nonlinear optical studies

Journal Pre-proof Ferrocene conjugated donor-π-acceptor malononitrile dimer: Synthesis, theoretical calculations, electrochemical, optical and nonlinear optical studies Selvam Prabu, Ezhumalai David, Thamodharan Viswanathan, J.S. Angelin Jinisha, Malik Richa, K. Rudharachari Maiyelvaganan, Muthuramalingam Prakash, Nallasamy Palanisami PII:

S0022-2860(19)31411-5

DOI:

https://doi.org/10.1016/j.molstruc.2019.127302

Reference:

MOLSTR 127302

To appear in:

Journal of Molecular Structure

Received Date: 5 July 2019 Revised Date:

25 October 2019

Accepted Date: 26 October 2019

Please cite this article as: S. Prabu, E. David, T. Viswanathan, J.S. Angelin Jinisha, M. Richa, K. Rudharachari Maiyelvaganan, M. Prakash, N. Palanisami, Ferrocene conjugated donor-π-acceptor malononitrile dimer: Synthesis, theoretical calculations, electrochemical, optical and nonlinear optical studies, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127302. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical Abstract

Ferrocene conjugated donor-π-acceptor malononitrile dimer has been synthesized and spectroscopically characterized. The band gap values were obtained from experimental as well as theoretical calculations (M06/6-31+ G**). The SHG efficiency was determined by Kurtz-Perry powder technique which shows 0.35 times compared with reference para-nitroaniline.

Ferrocene conjugated donor-π-acceptor malononitrile dimer: Synthesis, theoretical calculations, electrochemical, optical and nonlinear optical studies Selvam Prabu,a Ezhumalai David,a Thamodharan Viswanathan,a J.S. Angelin Jinisha,a Malik Richa,a K. Rudharachari Maiyelvaganan,b Muthuramalingam Prakashb and Nallasamy Palanisami*a a

Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology,

Vellore 632014, Tamilnadu, India. b

Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur - 603203,

Tamilnadu, India. Corresponding author: E-mail: [email protected]; Tel: +91 98426 39776 Abstract The donor-π-acceptor ferrocene conjugated malononitrile dimer [C5H5-Fe-C5H4C(H)=C(CN)-C(NH2)=C(CN)2] (1) has been synthesized and characterized with the aid of analytical and spectroscopic techniques [FT-IR, GC-Mass, 1H and

13

C NMR]. The thermal

stability of the compound 1 was investigated using thermogravimetric analysis (TGA) and differential thermal analysis (DTA), exhibit the compound is stable up to 180

o

C. The

photophysical properties were performed by UV-visible and fluorescence spectroscopic methods and the compound 1 exhibited weak fluorescence emission. In addition, the optical band gap (Eg) was calculated using diffused reflectance spectroscopic technique which shows the band gap value of Eg = 3.6 eV. The redox wave of the compound 1 was determined by cyclic voltammetry, revealed that one electron transfer ability of the ferrocene to ferrocenium ion. The experimentally observed HOMO and LUMO values are good agreement with the theoretical calculation by DFT method using M06/6-31+ G** as basis set. The solvatochromic studies of compound 1 shows negative solvatochromism due to the high ground state polarizability in presence of cyano acceptors. Further, the nonlinear optical properties were investigated by Q-switched Nd-YAG laser and second harmonic generation efficiency (SHG) using para-nitroaniline (p-NA) as a standard and the results of SHG efficiency is 0.35 times comparable with p-NA. Keywords: Ferrocene, malononitrile dimer, nonlinear-optics, DFT studies, solvatochromism. 1

1. Introduction Chromophores with donor-π-acceptor show non-linear optical (NLO) properties which are the great interest for future in the fields of optical data storage, optical communication, optical transferring and NLO bioimaging [1]. The D-π-A arrangement has effective intramolecular charge transfer (ICT) between the donor to acceptor moieties and it creates a dipolar push-pull structure have strong CT absorption and featuring low-energy and the molecules become polarized [2]. The polarizability and the respective optical linear and nonlinear properties of these systems depend on their chemical structure and electronic behavior of the appended donor and acceptor and length of the π-conjugated linker [3]. When compared with organic chromophores, the transition metal based organometallic chromophores have a most extensive interest because of changing the oxidation state of the metal atom which tune the polarizability [4]. Among them the ferrocene chromophores are having great interest owing to their peculiar properties like a one electron charge transfer from the iron atom. It undergoes a facile interconversion between (Fe2+

Fe3+) which implies notable redox electrochemical

potential and ionization energy as compared to other best organic donors [5]. The donor property of ferrocene found to the electronic coupling between metal d-orbital and π* orbital of the acceptor unit that could be extensive NLO response by metal to ligand charge transfer (MLCT) [6]. Recently, our research group explored various ferrocene based linear [7a,19b,23b] and Yshaped derivatives and their NLO properties, these are evidence in their high second-order NLO response in solution determined by EFISH. The dispersion of this kind of chromophores as guests in a polymethylmethacrylate matrix (PMMA), can lead to a composite film with a good SHG response and the d33 value of Y-shaped ferrocenyl quinoxaline in PMMA is 5.27 pm V-1, it is highest ever reported for a host/guest system [7b]. The malononitrile dimer is an effective electron acceptor in push-pull chromophores due to their electron withdrawing nature of nitrile groups and powerful active methylene group [8]. The amino substituent present in the malononitrile dimer show specific reactivity which can be used for the development of new dyes [9]. The presence of an active methylene group induces to condense with carbonyl compounds and aromatic nitroso compounds [10]. In addition, it has polycyano groups which act as good acceptor and it makes effective D-π-A system with strong donor atoms like ferrocene which enables the second order non-linear optical response [11]. 2

Even though malononitrile dimer containing aromatic compounds are reported in literature [910], but their organometallic system yet to be reported. Recently, Shabbir et al., have reported the combined experimental and theoretical studies on structure relationship and nonlinear optical properties of organic, inorganic and organometallic chromophores. For theoretical calculations, various functional basis sets have been used, among them M06 function has given good accuracy for organometallic compounds like ferrocene derivatives [12, 13]. Inspired by the aforementioned considerations, we have reported ferrocene appended donor-π-acceptor malononitrile 1 and characterized with the aid of analytical and spectroscopic techniques [FT-IR, GC-Mass and (1H, 13C) NMR]. The optical properties have investigated with the aid of UV-visible, fluorescence spectroscopic techniques and non-linear optical properties were examined in the Kurtz-Perry powder method. The redox behavior was investigated with cyclic voltammetry. In addition, optimization of molecular structure, molecular orbital energy distributions, polarizability (α) and hyperpolarizability (β) of compound 1 was investigated using density functional theory (DFT) with M06/6-31+ G** basis set. 2. Experimental section 2.1 Materials and procedure All the chemicals were purchased from Sigma Aldrich Chemical Co. The solvents were used after purified by distillation. The chromatographic separations were carried out using silica gel 60 (AVRA, 100-120 mesh). The malononitrile dimer was synthesized according to reported procedures [14]. 2.2 General physical measurements The NMR spectra recorded on a BRUKER (400 MHz) spectrometer with DMSO-d6 as a solvent and tetramethylsilane as an internal standard the chemical shifts we reported in δ (ppm). A mass spectrum was recorded on the GC-MS instrument (PerkinElmer). FT-IR spectra of samples were obtained using a SHIMADZU IR Affinity-1 instrument equipped with highsensitivity DLATGS detector spectra were recorded KBr discs. The Electronic absorption spectra were recorded using a JASCO UV-visible spectrometer in a 1 cm2 quartz cuvette at 3

room temperature using acetonitrile as a solvent. The emission spectra were recorded using a Hitachi F‐7000 FL spectrophotometer in acetonitrile solvent. The fluorescence lifetime measurements were performed using a single photon counting method. The fluorescence decay curves were carried by the molecule to 280 nm using 150 ps light of a nano-LED. The cyclic voltammogram of the compound 1 was carried out on a CH-Instruments Model CHI620E in acetonitrile solvent (1x 10-3 M) that containing 0.1 M tertbutyl ammonium perchlorate as supporting electrolyte at the scan rate 0.1 Vs-1. A platinum wire as a counter electrode, glassy carbon is the working electrode and reference electrode is a saturated Ag/AgCl. Thermogravimetric analysis was done by TGA SDT Q 600 V20.9 Build 20 instrument under N2 atmosphere at heat rate 20 oC min-1 (0-800 oC). 2.3 Synthesis of ferrocene conjugated malononitrile dimer compound 1 The compound 1 was synthesized by Knoevenagel condensation reaction. The ferrocene carboxaldehyde (1 mmol, 0.214 g), malononitrile dimer (1.19 mmol, 0.158 g) and piperidine (1.19 mmol, 0.1 ml) in methanol (10 ml) were kept under reflux for 3 hours. After complete the reaction solvent was evaporated and make a crude. Further, the crude was purified by column chromatography using hexane and ethyl acetate (8:2). Yield:70%. MP = > 300 oC. C17H12FeN4: Calculated. C, 62.22; H, 3.69; N, 17.07; Observed. C, 61.82; H, 3.44; N, 16.60. GC-MS (m/z): Calculated mass: 328.15, Observed mass: 328.18. 1H- NMR (400 MHz, DMSO-d6, δ, ppm): 8.84 (s, 2H), 7.844 (s, 1 H), 4.935(s, 2H), 4.773(s, 2H), 4.308 (s, 5H). 13C NMR (100 MHz, DMSOd6, δ, ppm): 166.40, 157.64, 116.86, 116.76, 115.73, 96.66, 79.73, 75.05, 74.34, 71.59, 70.22, 69.24, 48.93. FT-IR (ʋ cm-1): 3230(m), 3321(m), 2193(s), 1714(m), 1653(m), 1553(m), 1056829(s). 674-472(s), Absorption (λmax): 276, 325, 567 nm and Emission (λmax): 302 nm. 2.4 Theoretical Calculations The electronic structure and molecular properties of compound 1 was investigated using density functional theory (DFT) to understand bonding patterns, the electronic charge and molecular orbital energy distributions. The geometry of the compound 1 was optimized using M06 functional theory. Compare to B3LYP functional M06 has given good accuracy for ferrocene derivatives [13]. The DFT/M06 level of computational calculations was carried out using the 6-31+G** basis set for finding the global minimum energy structure of chromophore 4

and its molecular properties calculation. Further, analyze the polarizability (α0) and hyperpolarizability (β0) which was help to study the NLO properties of compound 1. All computation calculations were carried out using the GAUSSIAN 16 package. The frontier molecular orbital structures and electronic geometries were taken by Gauss View 6.0 [15]. 2.5 Determination of the quantum yield To determine the fluorescence quantum yields (Φ) using the equation (1) reference is the tryptophan (Φst = 0. 14 in water) [16] Φx = Φst X

[஺஺

ೞ೟ ೣ

ி

ఎమ

X ிೣ X ఎమೣ ೞ೟

ೞ೟

]

(1)

Here, Φx and Φst stands for unknown and standard (tryptophan) sample respectively. Φ for quantum yield, A is absorbance, F for the total area of the fluorescence spectra, η is the refractive index of the solvents. The fluorescence lifetime measurement was carried out using a single photon counting method. The fluorescence decay curve was measured by exciting the molecule to 280 nm using 150 ps light of a nano-LED. 2.6 Non-liner optical measurement The Kurtz-Perry powder method is a commonly used technique to analyze the second harmonic generation (SHG) efficiency of NLO compounds. The compound 1 is made into a fine powder with the same particle size and it is packed in a micro-capillary tube. The packed powder sample of compound 1 was analyzed by the fundamental beam of 1064 nm from a Q-switched Nd: YAG laser. 10 mJ per pulse input beam energy was incident on the sample [17]. 3. Results and discussion 3.1 Synthesis and characterization of compound 1 The compound 1 was synthesized by Knoevenagel condensation reaction with ferrocene carboxaldehyde and malononitrile dimer in the presence of piperidine and methanol as a solvent shown in scheme1. Further, compound 1 was characterized by FT-IR, GC-Mass, 1H, 13C NMR and the thermal stability was examined by thermogravimetric analysis (TGA).

5

SCHEME 1. Synthesis of compound 1

The 1H and

13

C NMR spectrum of compound 1 was recorded in DMSO-d6 at room

temperature and the spectra were shown in Figures S1 and S2 respectively. The 1H-NMR spectrum of compound 1, the unsubstituted cyclopentadienyl ring (η5-C5H5) protons are as appeared at singlet in the region of 4.35 ppm and substituted cyclopentadienyl ring (η4-C5H4) assigned at 4.81 and 4.98 ppm. The olefinic proton (C-H) resonating at 7.84 ppm as a singlet, the -NH2 protons are appeared at 8.84 ppm as a singlet. The 13C NMR spectrum of compound 1 the Fc-CH carbon resonate at 69.24-79.73 ppm, −C=C− carbon resonate at 48.93, 96.66, 157.64, 166.40 and the three carbons of nitrile (−C≡N) are assigned at 115.73, 116.76, 116.86 ppm respectively. In the FT-IR spectrum of compound 1 shown in Fig. S3 and it shows the N-H stretching assigned in the region of 3230 and 3321 cm-1, the −C≡N stretching appeared at 2193 cm-1. 3.2 Thermal analysis The thermal stability of compound 1 was analyzed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermogram was obtained at a heating rate of 20 oC/min up to 800 oC under N2 atmosphere. In the TGA and DSC curves of compound 1, is shown in Fig. S5 it was stable up to 180 oC. Further, it gradually decreased up to 700 oC. TGA result shows 75% weight loss at 700 oC and the DSC curve shows an endothermic peak at 96.35 oC owing to the removal of moisture and another endothermic peak occurs at 697.43 oC which is corresponding to the decomposition of the organic moieties. 3.3 DFT Calculation To understand the structural and electronic properties of ferrocenyl malononitrile dimer 1 using density functional theory (DFT) and time dependent- density functional theory (TD-DFT) with basic set of M06/6-31+G** and calculations were done Gaussian 16 software [14,15]. The 6

Fig. 1 shows electronic density distributions of the HOMO and LUMO energy levels and ground state optimized geometry of the compound 1. In order to ascertain the electron density, HOMO is mainly located at ferrocene moiety and π- spacer double bond whereas LUMO is located at the substituted cyano acceptor of compound 1 is shown in Fig. 1a. Single crystal X-ray data of the compound 1 was not obtained due to the poor diffraction quality of the crystals. Thus, the geometrical parameters of compound 1 was determined by DFT with M06/6-31+ G** basis set as shown in Fig. 1b. The calculated energy gap and dipole moment of compound 1 is 3.94 eV and 9.40 respectively. When the dipole moment of compound 1 is compared with paranitroaniline it shows 1.29 times higher (7.24 at M06/6-31+ G** level) values [18], because the compound 1 have strong D-π-A system due to the presence of ferrocene as a donor and three strong -cyano acceptor groups along with extended π-conjugation which makes the more polar in nature compared with para-nitroaniline. The TD-DFT results of selected transition and frontier orbitals as shown in Tables 1.

(a)

(b)

Fig 1. (a) HOMO-LUMO, energy gap and (b) geometrical structure of compound 1 calculated at M06/6-31+G** basis set

7

Table 1 Density surfaces of the frontier orbitals involved in the electronic transitions of the compound 1 from TDDFT at iso surface value of 0.02 a. u Orbitals

HOMO-3

HOMO-2

HOMO-1

HOMO

LUMO

LUMO+2

LUMO+3

LUMO+4

LUMO+5

LUMO+6

Compound 1 Solvent phase (acetonitrile) LUMO+1

3.4 Absorption studies The absorption spectrum of compound 1 was carried in acetonitrile solution and it shows three absorption bands (Fig. 2b) and the first absorption band in the range of 276 nm at a higher energy level which is due to π-π* transition and originated to a ligand-centred transition. The medium energy band at 325 nm is due to the n-π* transition that can be described by intramolecular charge transfer (ICT) excitations. The absorption at 567 nm occurs for d-d transition (assigned to 1E1g
1A1g), and it shows the low energy [19]. The theoretical UV-visible spectrum of compound 1, in solvent phase (acetonitrile) using the TD-DFT with basis set M06/6–31+G** in gaussian G16 Package. The experimental and theoretical spectrum are shown in Fig. 2a. We observed two major transitions which are intramolecular charge transfer and d-d transition. The experimentally observed wavelengths at 325 nm and 567 nm have the same trend with theoretical values at 329 nm and 625 nm. Since the solvent has less effect in theoretical studies, it shows red shift when compared with experimental results [20]. The corresponding transitions are HOMO → LUMO (0.42) and HOMO-3 → LUMO (0.68) [21], and the values are summarized in Table 2. The transitions have selected only those oscillator strength (f) is > 0.001.

8

Fig 2. The absorption spectrum of compound 1 in acetonitrile both theoretical (a) and experimental (b) Table 2 Absorption data from experimental and theoretical (TD-DFT method at M06/6–31+G**) studies. Excitation energy (eV) 2.6

Theoretical

1.9 (2.1)a

625

3.4

363

3.7 (3.7)a

329

a

Wavelength (nm) Experimental

471 567

325

Oscillator strength (f)

Orbital transitions

0.0033

HOMO-2→ LUMO (13%) HOMO-1 → LUMO+4 (50%)

0.0135

HOMO → LUMO (35%) HOMO → LUMO+6 (24%)

0.1408

HOMO-2→ LUMO (12%) HOMO → LUMO (37%)

0.3016

HOMO-3 → LUMO (93%)

experimental excitation energy value

In order to examine the optical band gap, we used diffuse reflectance spectroscopic (DRS) technique for compound 1 and the wavelength range is 200-2500 nm. It is useful to understand the optical absorption or transmittance window and cut-off wavelength of the compound. The charge transfers involving the compound was 300-1200 nm which evidences that the absorption in most of the visible region in Fig. 3. The Kubelka- Munk equation calculated the absorption co-efficient (α) [22]. (α/S) = (1-R)2 /2R …………... (1) 9

Where α is absorption coefficient, R is the diffused reflectance at certain energy and S is the scattering coefficient which situated between the localized states near the mobility edges according to Mott and Davis model of the density of states. The optical energy band gap (Eg) of highly degenerate semiconducting material was calculated the following equation [23] ‐υ = A (‐υ - Eg) ……………. (2) Where ‐ is Plank’s constant, υ is the frequency of incident photons and A is a constant. The Eg of investigated compound 1 can plot (α‐υ)2 versus the photon energy (‐υ) using the Tauc’s relationship as shown in Fig. 3. The energy gap of compound 1 was found to be 3.6 eV which coinciding with theoretical calculations. It suggests that compound 1 may be used for optical applications. bb a λ=774nm

Fig 3. (a) UV-Vis transmission spectrum and (b) Tauc’s plots of (αhυ)2 versus photon energy (hυ) of compound 1

3.5 Emission studies The fluorescence emission spectrum was taken in acetonitrile solution and the emission occurs from charge transfer nature of the compound 1. When it was excited at 276 nm it shows weak fluorescence because of electron withdrawing nature of three-cyano groups which stabilizes the excited state of compound 1. The emission was observed at 322 nm (n-π*) it may be metal-to- ligand or ligand-to-metal charge transfer as shown in Fig. 4 (a). The quantum yields were calculated in acetonitrile solvent using tryptophan as a standard [16]. The observed quantum yield for the compound 1 was Φx = 0.06 which is less value when compared 10

with the standard tryptophan (Φst = 0. 14) [16]. It may be explained, the ferrocene shows extensive fluorescence quenching because of the photo induced electron-transfer process and cyano groups in malononitrile have electron withdrawing nature which stabilizes the excited state [24-26]. Fluorescence lifetime (τ) has been investigated using time-correlated single photon counting method with exciting the sample at 280 nm, 150 ps pulse using nano-LEDs. By this technique one can deduce the exciting lifetime of the molecule, that is how long the molecule stays excited state. Moreover, the measurement of fluorescence lifetime is more robust than the extent of fluorescence intensity, because it depends neither on the strength of excitation nor the concentration of the fluorophore. The fluorescence lifetime of compound 1 is 2.63ns shown in Fig. 4 (b). a

b

Fig 4. (a) Emission spectrum in acetonitrile and (b) life time fluorescence emission spectra for compound 1

3.6 Electrochemical studies The Electrochemical studies were carried out in cyclic voltammetry technique. The compound 1 was performed in acetonitrile solution with containing 0.1 M tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte at a scan rate of 100 mV s−1. The counter electrode as a platinum wire, glassy carbon as a working electrode and Ag/AgCl electrode act as a reference electrode. The cyclic voltammogram of compound 1 was shown in Fig. 5. It exhibits the quasi-reversible and current ratio was (ipa/ipc) equal to unity for the electrochemical assessment. The oxidation potential of the compound 1 shows 840 mV and ferrocene 11

carboxaldehyde is 910 mV in the same experimental conditions. The redox potential of compound 1 was observed E1/2 = 828 mV [(Epa+Epc)/2] and ferrocene carboxaldehyde redox potential is E1/2 = 913 mV. This implies that compound 1 shifted towards left hand side due to presence of electron withdrawing nature of the substituted cyano groups on compared with the ferrocene carboxaldehyde as shown in Fig. 5. The HOMO and LUMO energy levels were derived by the formula EHOMO = -e (Eox + 4.4) and ELUMO = (Ered + 4.4) [27]. The experimentally calculated HOMO, LUMO and band gap values are -5.28, -1.92 and 3.36 respectively. These values are good agreements with the theoretical calculation of compound 1 as shown in Table 3 which plays an important role in NLO properties.

Fig 5. Cyclic voltammogram of ferrocene carboxaldehyde and compound 1 Table 3. Experimental and theoretical HOMO, LUMO, band gap and dipole moment Experimental data Calculated TD-DFT data λonset EHOMO ELUMO EHOMO ELUMO Band Egoptical Dipole gapc (nm)a (eV)a (eV)a (eV)b (eV)c (eV)c Momentc compound 1 368 -5.28 -1.92 3.36 -6.54 -2.64 3.94 9.40 a ௢௡௦௘௧ Calculated from cyclic voltammetry using EHOMO= -e (‫ܧ‬௢௫ +4.4). ௢௣௧௜௖௔௟ Eoptical onset value obtained from cyclic voltagram (i.e.) oxidation peak ELUMO=‫ܧ‬௚ +EHOMO. S. No

௢௣௧௜௖௔௟

b c

Calculated as optical band gap from absorption onset/edge using equation (‫ܧ‬௚ Theoretical value calculated by DFT.

) =1240/ λonset.

3.7 Solvatochromism The solvatochromic technique is a preliminary study for intramolecular charge transfer process and NLO response of the chromophores. We have carried out solvatochromism 12

for compound 1 in various solvents from non-polar to polar with the concentration of 1×10−5 M as shown in Fig. 6. It shows negative solvatochromism when the solvent polarity increases from non-polar to polar, the absorbance shifted the longer to shorter wavelength 325-318 nm. The polar solvent like DMSO shows the shortest wavelength at λmax = 318 nm, whereas nonpolar solvent like toluene shows the longest wavelength at λmax = 325 nm. It is from the intramolecular solute-solvent interactions at high dipolar ground state [28]. The Kamlet-Taft correlation analysis also shows the same effect in solvent from non-polar to polar based on ∆ῠmax [29]. It can be calculated from the absorption bands (λmax) of two different solvent polarities, [∆ῦmax = ῦmax longest hypsochromic shift − ῦmax shortest bathochromic shift; here, ῦmax = 1/ λmax x C, where C is the concentration] [7a]. In this study the longest λmax in toluene at 325 nm and the shortest λmax in DMSO at 318 nm from this ∆ῦmax was calculated and the data related to compound 1 is listed in table S1.

Fig 6. UV–visible absorption spectra of compound 1 in solvents of various polarities

3.8 Non-linear optical studies The Kurtz-Perry powder method is commonly used to analyze the second harmonic generation (SHG) efficiency of compounds. The compound 1 was made into a fine powder with the same particle size and it is packed in a micro-capillary tube within the uniform pore. The packed powder sample of compound 1 was analyzed by the fundamental beam of 1064 nm from a Q-switched Nd: YAG laser. The 1.0 mJ per pulse input beam energy was incident on the sample [17]. The bright green light emission was observed that implies the SHG efficiency of 13

compound 1. The SHG efficiency was compared with para-nitroaniline because it has similar to donor-π-acceptor system of compound 1, shows 0.35 times less than the standard (p-NA) as shown in Fig. 7. The computational parameters of polarizability (α) and hyperpolarizability (β ) are calculated for compound 1 using the M06/6-31+G** level of theory and the calculation details of

α and β were provided (see Supplementary). The polarizability (α) and

hyperpolarizability (β ) of compound 1 is 69.832 x 10-24 esu and 179.237 x 10-33 esu respectively and it is comparable with para-nitroaniline in gas phase (132.744 x 10-33 esu at M06/6-31+G** level) [18] due to absence of intermolecular interactions [30].

Fig 7. Bar diagram of NLO efficiency for compound 1

Conclusion We have successfully synthesized ferrocene conjugated donor-π-acceptor malononitrile dimer and characterized with the aid of different spectroscopic and analytical techniques. The compound 1 shows negative solvatochromism from non-polar to polar solvents due to high ground state polarizability and in presence of three -cyano groups. The emission spectra of compound 1 exhibit weak in acetonitrile because of the quenching nature of ferrocene moiety. The structure was optimized by DFT method using M06/6-31+G** level of theory and HOMOLUMO energy gab was calculated (3.9 eV), which is comparable with experimentally obtained results. The second order NLO studies of compound 1 was carried out Kurtz-Perry powder technique and the efficiency shows 0.35 times with para-nitroaniline. In addition, polarizability (α) and hyperpolarizability (β ) values were computed using M06/6-31+G** level of theory and the results are comparable with reference para-nitroaniline in gas phase. Further, the efficiency 14

of NLO will be enhanced by increasing the π conjugation of the compound and in this aspect currently we are working in our laboratory. Acknowledgement We gratefully acknowledge the financial support from the DST Indo-Italian Joint Project [No.INT/Italy/P-15/2016 (SP)]. The authors gratefully acknowledge the VIT-SIF for providing the instrument facilities. The authors would like to thank Dr. Kamini Mishra (VIT) for NLO studies. References 1. (a) M.G. Papadopoulos, A.J. Sadlej, J. Leszczynski, Non-linear optical properties of matter. Dordrecht: Springer, (2006); (b) A. Newell, Nonlinear optics, CRC Press (2018); (c) T. Schneider, Nonlinear Optics in Telecommunications, (2004); (d) S. Muhammad, H.L. Xu, R.L. Zhong, Z.M. Su, A.G. Al-Sehemi, and A. Irfan, Quantum chemical design of nonlinear optical materials by sp 2-hybridized carbon nanomaterials: issues and opportunities, J. Mater. Chem. C, 1 (2013) 5439-5449; (e) B.E. Urban, P. Neogi, K. Senthilkumar, S.K. Rajpurhit, P. Jagadeeshwaran, S. Kim, Y. Fujita and A. Neogi, Bioimaging using the optimized nonlinear optical properties of ZnO nanoparticles, IEEE J. Sel. Top. Quantum Electron, 18 (2012) 1451-1456. 2. (a) W. Chen, T. Salim, H. Fan, L. James, Y.M. Lam and Q. Zhang, Quinoxalinefunctionalized C60 derivatives as electron acceptors in organic solar cells, RSC Adv., 4 (2014) 2529; (b) C.Y. Jung, C.J. Song, W. Yao, J.M. Park, I.H. Hyun, D.H. Seong and J.Y. Jaung, Synthesis and performance of new quinoxaline-based dyes for dye sensitized solar cell, Dyes Pigm., 121 (2015) 204-210; (c) K. Pei, Y. Wu, H. Li, Z. Geng, H. Tian and W.H. Zhu, Cosensitization of D-A-π-A quinoxaline organic dye: efficiently filling the absorption valley with high photovoltaic efficiency, ACS Appl. Mater. Interfaces, 7 (2015) 5296-5304. 3. (a) F. Bures, H. Cermakova, J. Kulhanek, M. Ludwig, W. Kuznik, I.V. Kityk, T. Mikysek and A. Ruzicka, Structure-property relationships and nonlinear optical effects in donor-substituted dicyanopyrazine-derived push-pull chromophores with enlarged and varied π-Linkers, Eur. J. Org. Chem., 3 (2012) 529-538; (b) K. Pei, Y. Wu, A. 15

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Highlights
The donor-π-acceptor ferrocene conjugated malononitrile dimer has been synthesized.
The solvatochromic studies revealed that negative solvatochromism due to high ground state polarizability from three -cyano groups.
The DFT and TD-DFT calculations were done using M06/6-31+ G** basis set and correlated with experimental results.
The NLO efficiency was examined by Kurtz-Perry powder method.

Declaration of interests ✔ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

There is no conflict to declare.