Ferrocene/non-ferrocene conjugated linear eight-membered borasiloxanes: Structural, theoretical, optical and non-linear optical studies
Introduction
Inorganic ring system with donor-π-acceptor (D-π-A) type molecules have been a great interest in research due to its wide range of applications in the field of nonlinear optics (NLO) [1], [2], [3]. Thus, it can be exploited in different optical applications such as optical data processing, electro-optical devices, optical communication, rewritable optical data storage and NLO bio-imaging [4], [5], [6]. Among the various inorganic ring systems, borasiloxane (B
OSi) ring frameworks have been pulled into impressive consideration because of the molecular building block structure of the material [7,8]. The synthesis and structural characterization of cyclic-borasiloxanes were well addressed as six, eight, ten, and twelve membered ring systems [8], [9], [10]. However, the photophysical and material applications of borasiloxanes were lacking and yet to be more explored [7,11]. In the borasiloxane family, the three coordinated boron atom is isoelectric and isostructural with positively charged carbocation, because the vacant Pz orbital of the boron atom acts as a π-electron acceptor [12]. The boron atom with tri- and tetra-coordinated compounds have a wide range of uses in functional materials such as nonlinear optics, fluorescent sensors and organic light-emitting diodes (OLEDs) [13]. In addition, the silicon-containing materials show a low HOMO-LUMO energy gap with good electron mobility and high fluorescence efficiency, which are effectively utilized for the preparation of high performed optoelectronic devices [14]. Cyclic and cage borasiloxanes were used for the preparation of various type of borosilicate materials, because of their unique thermal and chemical robustness and these materials can be utilized for various applications such as flame retardants [15], colorimetric sensors [7] and solid-state luminescent materials [16], etc.
In the field of nonlinear optics, the ferrocene molecule acts as not only a redox mediator, but also an effective donor molecule, because of the possible interconversion between two different oxidation states (FeII↔FeIII) and its inertness towards air and moisture [17]. Moreover, ferrocene conjugated heteroaromatic scaffolds show higher optical and nonlinear optical efficiencies than the organic molecules, due to the coupling between π*-orbital of the heteroaromatic unit with d-orbitals of the Fe2+ in ferrocene. In addition, the heteroatom acts as an ancillary donor as well as an acceptor, and the overall dipole moment and polarizability of the molecule will be enhanced [18], [19], [20]. Also, the 4-methoxy- and 4-(N,N-dimethyl)phenyl were the most widely used organic donor species in push−pull chromophores, which are comparable with ferrocene.
The hyperpolarizability (β) and dipole moment (µ) value of the molecule depends on the strength of the donor and acceptor moieties. The careful choice to design the molecules with large β values will promote the bulk and active second-order nonlinear optical (NLO) signal of the materials. Notably, the molecule with the absence of symmetry is one of the vital requirements for making a higher NLO response of the inorganic-organic hybrid materials. Our group has been reported ferrocene appended push-pull chromophores with different shapes like linear [19,21,22], Y-shaped quinoxalines [18], and imidazoles [20] for nonlinear optics. Further, we have shown the Y-shaped ferrocenyl chromophores as a guest in a polymethylmethacrylate (PMMA) grid can incite a composite film with a large SHG efficiency (d33 = 5.27 pm V−1), which is the highest host/guest framework in an organometallic system. In addition, optical and nonlinear optical properties of the centrosymmetric eight-membered borasiloxanes, and its enhancement of SHG efficiencies by broken symmetry approach were investigated compressively by both experimental and theoretical methods were first reported by us [24]. The second-order NLO signals were observed in centrosymmetric borasiloxanes due to the distorted environment of the silicon atom in the borasiloxane ring and non-covalent interactions in the centrosymmetric crystals. It can be further improved by extension of π-conjugation in the borasiloxane ring, in order to enhance the charge transfer process. In this present work, we have reported the synthesis and structural characterization of ferrocene and non-ferrocene (methoxyphenyl) appended linear borasiloxanes with extended π-conjugation, and the role of donors (ferrocene and methoxyphenyl at para-position) in the second-order nonlinear optical effect. Also, the experimentally observed optical and nonlinear optical properties of the borasiloxanes 1 and 2 were correlated by DFT and TD-DFT calculations using the B3LYP/6-31+G** level of theory.
Section snippets
Materials and procedures
1-(4-bromophenyl)ferrocene and 4-ferrocenylphenylboronic acid was synthesized from the reported procedure [25] and 4-methoxyphenyl borasiloxane was crystallized from CHCl3 solvent. All solvents were of reagent grade and distilled prior to use. All other chemicals were purchased from Sigma Aldrich and TCI chemicals.
General physical measurements
The NMR spectra were recorded on a BRUKER (400 MHz and 500 MHz) spectrometer. Chemical shifts were reported in δ (ppm), and ESI-mass spectra were recorded under LC/MS, 6230B Time of
Synthesis and spectral characterization
The 4-methoxyphenyl borasiloxane 1 and 4-ferrocenylphenyl borasiloxane 2 were synthesized by the condensation reaction [34,35] with an equal molar ratio of boronic acids (4-methoxyphenyl or 4-ferrocenylphenyl) and diphenylsilanediol in toluene under reflux using a Dean-Stark trap to remove water molecule and the borasiloxanes were obtained in good yield, as shown in scheme 1.
The 1H, 13C, 11B and 29Si NMR spectra of borasiloxanes 1 and 2 were recorded using CDCl3 at room temperature (Fig.
Conclusion
In summary, we have designed and synthesized 4-methoxy (1) and 4-ferrocene (2) substituted linear borasiloxanes and characterized using analytical and spectroscopic techniques. The single-crystal X-ray diffraction studies confirmed the molecular structure of 4-methoxyphenyl borasiloxane (1), which is a monoclinic crystal system with a centrosymmetric space group (P21/c). The intramolecular charge transferability of both the borasiloxanes were studied using the solvatochromic technique, shows
Declaration of Competing Interest
The authors declare that they have no conflicts of interest with the contents of this article.
Acknowledgments
We gratefully acknowledge the financial support from the Council of Scientific and Industrial Research (CSIR), Government of India [Ref. No. 01(2914)/17/EMR-11]. T. V thanks CSIR for the Senior Research Fellowship [File No. 09/844/(0097)/2020-EMR-I]. The authors thank Prof P. K. Das from Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore for SHG measurements. We are very grateful to the STIC-India, Cochin, for providing the single-crystal XRD data. The
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2022, Spectrochimica Acta - Part A: Molecular and Biomolecular SpectroscopyCitation Excerpt :Our group have also reported ferrocene appended malononitrile dimer [29], Y-shaped quinoxalines [30] and imidazoles [31] for NLO properties. Also, the NLO efficiencies of centrosymmetric borasiloxanes were studied [32,33] and it was enhanced by the broken symmetry approach [34]. Further, we have extended both NLO and AIEE studies on ferrocenyl conjugated Schiff base [35], pyrazoles [36,37], linear multidonor-π-acceptor type chromophores [38], β-ketoimines and its BF2 complexes [39].