Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores

Keto-enol equilibrium: stable tautomers of ortho-, meta-, and para-hydroquinones in large aromatics

The synthesis of [6]helicene para-quinone starting from the 1,4-dimethoxy-[6]helicene derivative is presented. The demethylation reaction with boron tribromide led to unexpected results. Instead of the expected para-hydroquinone, the diketone tautomeric form was isolated. In contrast to the 1,4-hydroquinone and 1,4-dihydroxynaphthalene, the stable tautomers for the [4] and [6]helicenes were the aromatic diketones. These experimental results were corroborated by calculations. Additional calculations showed that these tautomeric species were indeed the stable forms of 1,4 and 1,3-hydroquinones when present in larger aromatics, in drastic contrast with 1,2-dihydroxy-aromatics (known as catechol).

Ferrocene appended 1,8-naphthalimide chromophores: Synthesis, theoretical, the effect of phenyl substitution on aggregation-induced enhanced emission and second-order nonlinear optical studies

A new ferrocene appended linear donor-π-acceptor (D-π-A) type 1,8-naphthalimide chromophores [FcPhNap-n-butyl (1) and Fc(Ph)₂Nap-n-butyl (2)] have been synthesized and characterized using various analytical and spectroscopic techniques. The chromophores 1 and 2 show the one-electron transfer process, which was examined through cyclic voltammetric (CV) method. The solvatochromic studies show red shift by increasing the solvent polarity from non-polar to polar for both the chromophores 1 and 2, due to the better stabilization of the more polarized excited state than the ground state in the polar solvent. The fluorescence studies show low fluorescence emission, which was enhanced by aggregation in THF/water mixture when the water ratio is increased upto 60%, due to the restricted intramolecular rotation (RIR) process in the aggregated state. Also, the quantum yield increases 3.4 (1) and 6.8 (2) times in the aggregation induced enhanced emission (AIEE) state than THF solution for both the chromophores [THF/AIEE = 0.05/0.17 for 1 and 0.03/0.22 for 2]. The second-order non-linear optical properties of the chromophores 1 and 2 were studied using Kurtz and Perry powder technique, chromophore 2 shows 1.2 times higher SHG efficiency than 1, owing to the presence of additional phenyl group in 2, which further supports the effective charge transfer process from donor to acceptor. Furthermore, optical and non-linear optical properties of chromophores 1 and 2 were analyzed by DFT/TD-DFT calculations using different functionals (B3LYP, long-range corrected CAM-B3LYP and LC-BLYP) with 6-31+G** level of theory. The B3LYP functional shows higher hyperpolarizability values [β₀ = 109.2 × 10^-30 esu (1), 299.7 × 10^-30 esu (2)], due to the overestimation of the dipole moment, polarizability and hyperpolarizabilities between the donor-π-acceptor systems in B3LYP-hybrid functional, which leads to incorrect long-range charge transfer.

Ferrocene-appended boronated ester: effect of cyanovinylene group on the nonlinear optical properties and colorimetric detection of fluoride ion

A centrosymmetric ferrocenyl boronated ester shows second-order NLO effect, selective and sensitive detection of fluoride ion.

Electronic structure and second-order nonlinear optical properties of lemniscular [16]cycloparaphenylene compounds

Chiral organic compounds are excellent second-order nonlinear optical (NLO) materials due to their inherent non-symmetric electronic structures combined with the advantages of organic compounds. At present, density functional theory (DFT) has become a powerful tool for predicting the properties of novel materials. In this paper, based on chiral lemniscular [16]cycloparaphenylene, three novel compounds are designed by introduction of donor/acceptor units and their combinations. The geometrical/electronic structure, electronic absorption, and the second-order NLO properties of these compounds have been systematically investigated by DFT/TDDFT theory. The simulated UV-Vis/CD spectra of compound 1 are in good agreement with the experimental ones, enabling us to assign their electronic transition characteristics and absolute configuration with high confidence. The investigations show that energy gaps, absorption wavelength and second-order NLO response may be effectively tuned by the introduction of the donor or acceptor units or their combinations. For instance, the second-order NLO value of compound 4 is about 207 times as large as the average second-order polarizability of the organic molecule urea. Thus, the studied compounds are expected to be potential large second-order NLO materials.

Electronic structure and second-order nonlinear optical property of chiral peropyrenes

Discovery of novel materials with excellent second-order nonlinear optical (NLO) properties is a very attractive topic in chemistry and materials science. Recently, much more attention has been paid to chiral compounds due to their inherent asymmetric structure and intramolecular charge transfer. Currently, the density functional theory (DFT) has become a powerful methodology to rationalize experimental observations and to design new materials with desirable properties. In this work, on the basis of the reported chiral peropyrene, we designed another five compounds consisting of donor or acceptor moieties and the donor/acceptor combinations. We systematically studied their geometrical/electronic structures and electronic transition/second-order NLO properties. The measured UV-Vis/CD spectra of compound 1 are almost reproduced by our calculations, enabling us to assign its electronic transition property and absolute configuration. For these compounds, the different substituents have great effect on their photophysical properties (i.e., band gap, absorption wavelength, and NLO response). The charge transfer synergy provides some useful information for further performance improvement. Interestingly, compound 6 shows a remarkably large first hyperpolarizability value of 18.14 × 10^-30 esu. Our research enables an opportunity for understanding the structure-property relationship of chiral peropyrenes. Graphical abstract The nonlinear optical properties of the studied compounds were studied with the aid of the DFT calculations.

Redox-switching of ternary Ni(ii) and Cu(ii) complexes: synthesis, experimental and theoretical studies along with second-order nonlinear optical properties

The redox-switching behavior of the title compounds has been probed for the Ni(ii) derivatives using time-resolved spectroelectrochemistry under thin-layer conditions.

Spontaneous Symmetry Breaking Facilitates Metal-to-Ligand Charge Transfer: A Quantitative Two-Photon Absorption Study of Ferrocene-phenyleneethynylene Oligomers

Change of the permanent molecular electric dipole moment, Δμ, in a series of nominally centrosymmetric and noncentrosymmteric ferrocene-phenyleneethynylene oligomers was estimated by measuring the two-photon absorption cross-section spectra of the lower energy metal-to-ligand charge-transfer transitions using femtosecond nonlinear transmission method and was found to vary in the range up to 12 D, with the highest value corresponding to the most nonsymmetric system. Calculations of the Δμ performed by the TD-DFT method show quantitative agreement with the experimental values and reveal that facile rotation of the ferrocene moieties relative to the organic ligand breaks the ground-state inversion symmetry in the nominally symmetric structures.

Ferrocene amphiphilic D–π–A dyes: synthesis, redox behavior and determination of band gaps

We reported the synthesis, optical and redox behavior of six ferrocenyl D–π–A dyes. Optical and electrochemical band gaps were determined and corroborated by TD-DFT calculations. Compounds4exhibit the smallest band gap of this series.

Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications

Fourteen new dipolar cations have been synthesized, containing methoxy or tertiary amino electron donor groups attached to helquat (Hq) acceptors. These Hq derivatives have been characterized as their TfO^- salts by using various techniques including NMR and electronic absorption spectroscopies. UV-vis spectra show intense, relatively low energy absorptions with λ_max ≈ 400-600 nm, attributable to intramolecular charge-transfer (ICT) excitations. Single-crystal X-ray structures have been solved for two of the chromophores, one as its PF₆^- salt, revealing centrosymmetric packing arrangements (space groups Pbca and P1̅). Molecular quadratic nonlinear optical (NLO) responses have been determined directly by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and indirectly via Stark (electroabsorption) spectroscopy for the low energy absorption bands. The obtained static first hyperpolarizabilities β₀ range from moderate to large: (9-140) × 10^-30 esu from HRS in MeCN and (44-580) × 10^-30 esu from the Stark data in PrCN. The magnitude of β₀ increases upon either extending the π-conjugation length or replacing a methoxy with a tertiary amino electron donor substituent. Density functional theory (DFT) and time-dependent DFT calculations on selected tertiary amino chromophores confirm that the low energy absorptions have ICT character. Relatively good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the CAM-B3LYP functional with the 6-311G(d) basis set. The β_tot values predicted by using DFT at the same level of theory are large ((472-1443) × 10^-30 esu in MeCN). Both the theoretical and experimental results show that para-conjugation between Hq and electron donor fragments is optimal, and enlarging the Hq unit is inconsequential with respect to the molecular quadratic NLO response.

Synthesis and characterisation of push–pull flavin dyes with efficient second harmonic generation (SHG) properties

We report push–pull flavins bearing different donor groups that exhibit high hyperpolarisabilities (β = 355–9550 × 10⁻³⁰ esu) and decomposition temperatures thereby indicating their potential application as non-linear optical device materials.