Regulating the NLO response of anthraquinone-supported thiourea-linked crown ether macrocycle by introducing metal cations: A DFT study

Recently, an anthraquinone-supported thiourea group linking a 1-aza-18-crown-6 macrocycle L has been the subject of extensive attention due to the perfect affinity towards metal cations. This work systematically researched the effects of different metal cations ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]) on the second-order nonlinear optical (NLO) properties of macrocycle L by density functional theory (DFT). DFT calculations revealed that the values of first hyperpolarizabilities ([Formula: see text] decrease significantly when alkaline earth metal cations ([Formula: see text] and [Formula: see text]) were injected into macrocycle L due to the smaller charge transfer (CT) transition and larger transition energy. Conversely, the variations of [Formula: see text] values in alkali metal cations ([Formula: see text] and [Formula: see text] and transition metal cations ([Formula: see text] and [Formula: see text]) derivatives are not obvious compared to the [Formula: see text] value of macrocycle L. Therefore, the NLO properties of macrocycle can be effectively regulated by alkaline earth metal cations. Furthermore, we found that the [Formula: see text] value of anion-controlled complex Na(L)(ClO4) is larger than that of L*Na+ complex because the anion [Formula: see text] improves the planarity of anthraquinone-supported thiourea group leading to the enhancement of the CT ability. In addition, the influence of frequency-dependent on the first hyperpolarizabilities is weak for the current systems. Hence, we look forward to the conception of this work will offer a fundamental guideline and reference for further research for novel NLO materials.

Interaction and selectivity of 14-crown-4 derivatives with Li+, Na+, and Mg2+ metal ions

The interactions between crown ether ligands (14-crown-4, 14C4; 4,4,5,5-tetramethylbenzo-14-crown-4, BC₄H₁₂-14C4; 4,4,5,5,9,9,10,10-octamethyl-14-crown-4, C₈H₂₄-14C4; dibenzo-14-crown ether-4, DB14C4) and alkaline and alkaline earth metal ions (Li⁺, Na⁺, Mg²⁺) were investigated using density functional theory modeling at the M062X/def2SVP and def2TZVP level. The condensed softness analysis of crown ethers, a condensed Fukui function, a condensed dual descriptor, and frontier molecular orbital theory were used to analyze the reactivities of the complexes. The complex stability was analyzed in terms of the binding energies, standard Gibbs free energy of formation, and energy decomposition of the interaction in aqueous solution. The results show that the active sites were mainly located at the carbon atoms of the benzene ring and oxygen atoms. The reactivities of DB14C4 and BC₄H₁₂-14C4 are higher than those of 14C4 and C₈H₂₄-14C4. The electrostatic interaction is the principal factor determining the stability of the complexes. The complexes containing Li⁺ has the greatest stability in aqueous solution among the complexes containing Li⁺, Na⁺, and Mg²⁺. BC₄H₁₂-14C4 shows selective adsorption toward Li⁺ in a mixed solution of Li⁺, Na⁺, and Mg²⁺. To evaluate the stability of complexes containing Mg²⁺, the solvent effect must be accurately described. An energy decomposition analysis was used to evaluate the stability of complexes containing Li⁺, Na⁺, and Mg²⁺, and the solvent effects were considered.

The role of π-linkers and electron acceptors in tuning the nonlinear optical properties of BODIPY-based zwitterionic molecules

Intramolecular charge transfer process can play a key role in developing strong nonlinear optical (NLO) response in a molecule for technological application. Herein, two series of boron dipyrromethene (BODIPY)-based push–pull systems have been designed with zwitterionic donor–acceptor groups, and their NLO properties have been evaluated using a density functional theory-based approach. Different π-conjugated linkers and electron acceptor groups were used to understand their roles in tuning the NLO properties. The molecules were analyzed through HOMO–LUMO gaps, frontier molecular orbitals, polarizabilities, hyperpolarizabilities, Δr indices, transition dipole moment densities, ionization potentials, electron affinities and reorganization energies for holes and electrons. These observations correlated well with the computed absorption spectra of the molecules. It is found that with the introduction of different π-linkers in the molecule, planarity is maintained and the HOMO–LUMO gap is systematically decreased, which leads to a large NLO response. It was noted that the electronic absorption wavelength maxima were found in the near-infrared region (934–1650 nm). The results show that compared to the pyridinium acceptor group, the imidazolium acceptor group in the BODIPY systems amplifies the NLO response to a larger extent. It is also observed that the BODIPY-based dye with an imidazolium acceptor and thienothiophene π-linker shows the highest first hyperpolarizability value of 3194 × 10⁻³⁰ esu. Furthermore, the charge transfer occurs in the z-direction, as the z-component of the first hyperpolarizability is the dominant factor in this system. Here, the designed molecules show a characteristic reorganisation energy value, which is a deciding factor in the rate of hole/electron transport for favourable intermolecular coupling. As a whole, this theoretical work highlights that π-conjugated linkers and electron acceptor groups can be used judiciously to design new molecular systems for optoelectronic applications.

BODIPY-based zwitterionic molecules with pyridinium and imidazolium electron acceptors and thienothiophene π-linkers reveal significant first hyperpolarizability.

A cation-selective and anion-controlled benzothiazolyl-attached macrocycle for NLO-based cation detection: variational first hyperpolarizabilities

The addition of Hg²⁺ leads to a significant decrement in the NLO response with respect to the corresponding macrocycle L.

First hyperpolarizabilities of Pt(4-ethynylbenzo-15-crown-5)2(bpy) derivatives with the complexation of mono-cations (Li+, Na+, K+) and di-cations (Mg2+, Ca2+): development of a cation detector

The coordination of mono-cations and di-cations onto the crown merits the design of the NLO-based cation detector.