Ferrocene/non-ferrocene conjugated linear eight-membered borasiloxanes: Structural, theoretical, optical and non-linear optical studies

Abstract

A linear donor-π-acceptor-π-donor (D-π-A-π-D) type 4-methoxyphenyl 1 and 4-ferrocenylphenyl 2 conjugated borasiloxanes were synthesized and characterized by various spectroscopic techniques (FT-IR, NMR and HRMS). Further, the molecular structure of 4-methoxyphenyl borasiloxane 1 was confirmed by the single-crystal X-ray diffraction method, shows a monoclinic crystal system in the centrosymmetric space group (P21/c) with various non-covalent interactions in the crystal packing. The intramolecular charge transfer (ICT) process of both the borasiloxanes were studied using the solvatochromic technique, which shows negative solvatochromism for 1 and 2, due to the high ground state dipole moment than the excited states. The second-order nonlinear optical (NLO) properties of the borasiloxanes were studied using Kurtz and Perry powder method. Compound 1 was crystallized in a centrosymmetric crystal system, and it exhibits second-order NLO response, because of the distorted silicon environment in an eight-membered borasiloxane ring with various C–H...π interactions in the crystal packing. The 4-ferrocenylphenyl borasiloxane 2 shows 1.5 times higher SHG efficiency than 1, due to the ferrocene moiety is conjugated in the para position of the borasiloxane ring, which involves an effective one-electron transfer process (FeII ↔ FeIII). Furthermore, the experimental results of optical and nonlinear optical properties of the borasiloxanes 1 and 2 were supported by the DFT and TD-DFT calculations using the B3LYP/6-31+G** level of theory.