Impact of Redox Stimuli on Ferrocene–Buckybowl Complexes: Switchable Optoelectronic and Nonlinear Optical Properties

The Renaissance of Ferrocene-Based Electrocatalysts: Properties, Synthesis Strategies, and Applications

The fascinating electrochemical properties of the redox-active compound ferrocene have inspired researchers across the globe to develop ferrocene-based electrocatalysts for a wide variety of applications. Advantages including excellent chemical and thermal stability, solubility in organic solvents, a pair of stable redox states, rapid electron transfer, and nontoxic nature improve its utility in various electrochemical applications. The use of ferrocene-based electrocatalysts enables control over the intrinsic properties and electroactive sites at the surface of the electrode to achieve specific electrochemical activities. Ferrocene and its derivatives can function as a potential redox medium that promotes electron transfer rates, thereby enhancing the reaction kinetics and electrochemical responses of the device. The outstanding electrocatalytic activity of ferrocene-based compounds at lower operating potentials enhances the specificity and sensitivity of reactions and also amplifies the response signals. Owing to their versatile redox chemistry and catalytic activities, ferrocene-based electrocatalysts are widely employed in various energy-related systems, molecular machines, and agricultural, biological, medicinal, and sensing applications. This review highlights the importance of ferrocene-based electrocatalysts, with emphasis on their properties, synthesis strategies for obtaining different ferrocene-based compounds, and their electrochemical applications.

Inorganic electrides of alkali metal doped Zn12O12 nanocage with excellent nonlinear optical response

Finding new materials with exceptionally large nonlinear optical response is an interesting and challenging avenue for scientific research. Here, we report the alkali metal doped Zn₁₂O₁₂ nanocages as inorganic electrides with excellent nonlinear optical response. Density functional theory calculations have been performed for geometric, electronic and nonlinear optical response of exo- and endohedrally alkali metal doped Zn₁₂O₁₂ nanoclusters. For exohedral doping, all different possible doping sites are considered for decoration of alkali metal on the nanocage. The electride nature of the complexes is highly dependent on the position of alkali metal doping. All exohedral complexes except for alkali metal doping on six membered ring (r₆) are electride in nature, as revealed from frontier molecular orbital analysis. Interaction energies reveal that all doped nanoclusters except endo-K@Zn₁₂O₁₂ are thermodynamically stable. The exothermic encapsulation of alkali metals in Zn₁₂O₁₂ nanocages is in marked contradiction with other inorganic fullerenes where encapsulation is an endothermic process. The barriers for boundary crossing are also evaluated in order study the interconversion of exo- and endohedral complexes. Doping of alkali metal significantly influences the properties of nanocages. HOMO-LUMO (H-L) gap is reduced significantly whereas hyperpolarizability is increased several orders of magnitude. The NLO response of exohedrally doped complexes is higher than the corresponding endohedral complexes, which is in mark contradiction with the behavior of phosphide or nitride nanocages. The highest first hyperpolarizability of 1.0 × 10⁵ au is calculated for K@r₆-Zn₁₂O₁₂ complex. Third order NLO response of these complexes is calculated and compared with the best systems reported in the literature at the same level of theory.

A computational study on second-order nonlinear optical properties based on bis-cyclometalated Ir(iii) complexes: redox and substituent effects

A series of neutral Ir(iii) complexes that possess cyclometalated ligands (C^N) and different ancillary ligands, N-heterocyclic carbene (NHC) and their ionic complexes 1+/−–5+/− have been investigated using density functional theory.

Designing a novel material with considerable nonlinear optical responses based on the bicorannulenyl molecule

In this work, a series of molecules decorated with Li atom and donor/acceptor have been theoretically designed based on bicorannulenyl molecule, where incorporating Li and different substitution are used as an effective strategy for enhancing nonlinear optical response. The mixed method is constructed through incorporating the Li and NO₂/NH₂ substitution. To ensure accuracy, results were compared with another two functionals. As expected, data from three different functional approximations indicate that these molecules have large first hyperpolarizability. The calculation proves that these molecules exhibit large first hyperpolarizability in the range of 1956-37,758 au. For Li doped systems, by analyzing NBO, charge transfer occurs in studied molecules, which helps to get large nonlinear optical response. It is revealed that when Li atom is introduced into the molecule with only NO₂/NH₂ substitution, the first hyperpolarizability increases significantly. Compared with Li doped and NO₂ substitution, incorporating Li and NH₂ substitution can be more powerful in increasing the first hyperpolarizabilities of bicorannulenyl molecule. In addition, the number of NH₂ substitutions can more effectively enhance the first hyperpolarizability. We hope that this study could provide a new idea for designing nonlinear optical materials using bicorannulenyl molecule.

Novel inorganic aromatic mixed-valent superalkali electride CaN3Ca: an alkaline-earth-based high-sensitivity multi-state nonlinear optical molecular switch

Focusing on innovative high-performance single-pole double-throw nonlinear optical (NLO) molecular switches, two C3v configurations (1 and 3) and one D3h configuration (2) of bipyramidal CaN3Ca have been obtained by using quantum mechanical methods. Not only are 1, 2, and 3 alkaline-earth-based aromatic superalkalis, but they are also interesting electrides. The salt-like electronic structures of e-Ca2+N33-Ca2+ (1) and Ca2+N33-Ca2+e- (3) with localized redox centres are rare inorganic Robin-Day class II-type structures, and e0.5-Ca2+N33-Ca2+e0.5- (2) with a delocalized structure is a class III-type mixed-valent superalkali electride. Under a small external electric field of ±0.0110 a.u. (0.565 V Å-1), the short-distance hopping of Ca atoms in CaN3Ca from the D3h configuration with in-plane aromaticity to each C3v configuration with out-of-plane aromaticity brings about the long-range transfer of half an electron from one Ca atom to another. And, subsequently, a large dipole moment (μ0) and remarkable static first hyperpolarizability (β0) occur. μz and βzzz range from 0 (D3h, off form) to -12.1 or 12.1 D (C3v, on forms) and from 0 (D3h, off form) to -19 428 or 19 428 a.u. (C3v, on forms), respectively. These extremely large differences in μz and βzzz values between the D3h and each of the C3v configurations confirm the potential of these inorganic aromatic Robin-Day-type superalkali electrides for applications in high-sensitivity multi-state nonlinear optical switches.

Metal ions doped into merocyanine form of coumarin derivatives: nonlinear optical molecular switches

In the present study, DFT calculations are carried out on domestically designed 7-methyl-2-phenyl-5'H-spiro[chromene-4,2'-chromeno[3,4-e][1,3]oxazin]-5'-one spiropyran and merocyanine derivatives to recognize alkali and alkaline earth metal ions. Detection of these metal ions can be attained by exploiting the variation of the second-order nonlinear optical properties. Merocyanine forms of these derivatives exhibit the ability to complex with different metal ions (Li⁺, Na⁺, K⁺, and Ca²⁺), which is associated with large contrasts in the hyper-Rayleigh scattering (HRS) response as a function of metal size and charge. Interestingly, in this study, Mero-Li⁺ shows significant nonlinear optical response with dynamic HRS first hyperpolarizability amounting to 7607 a.u., which is about nine times higher than its corresponding spiro form (846 a.u.) at the CAM-B3LYP/6-311G* level of theory. The present investigation clarifies the effect of metal nature on the enhancement of the first hyperpolarizability between the closed and open forms of the studied coumarin derivatives. Graphical abstract The coumarin-based compound 3 demonstrate the higher second-order NLO responses as a function of metal cation size and charges. Complexation of smaller alkali metal ions leads to the formation of stronger metal-ligand bonds, larger geometrical relaxations and significant enhancement of the HRS first hyperpolarizabilities. This present investigation elucidates the effect of metal nature on the enhancement of the first hyperpolarizability between the closed and open forms of studied coumarin derivatives.

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.

Charge transfer interactions of pyrazine with Ag12 clusters towards precise SERS chemical mechanism

We have synthesized Ag12 nanoclusters (NCs) with mercaptosuccinic acid (H2SMA) as the ligand. This cluster is found to be water-soluble and has satisfactory stability with [Ag12(HSMA)6Na6]2+, as determined by high-resolution mass spectrometry. Interestingly, it is noted that both the H2SMA ligand and Ag12 clusters do not display interference Raman signals, suggesting that this material is a good candidate as a substrate for surface-enhanced Raman spectroscopy (SERS). As a result, we observe enhanced Raman activity of pyrazine molecules adsorbed on Ag12 NCs along with a large red-shift up to ∼27 cm-1. To fully demonstrate the charge transfer interactions between pyrazine and Ag12 clusters, by utilizing first-principles calculations, we estimate polarizability tensor and conduct electronic natural population analysis (NPA), natural bond orbital (NBO) analysis, deformation density analysis (DDA) and charge decomposition analysis (CDA). In view of the minimized contribution from local surface plasmon resonance (LSPR), such a comprehensive study of metal NCs, which are free of Raman interference, provides a modelling method towards the long-debated chemical mechanism in SERS theory.

Second-order nonlinear optical response of phenyl-substituted cationic BIS-cyclometalated iridium(III) complexes: Effect of different position

In this paper, a series of cationic iridium complexes [(2-phenylpyridine)2(2,2[Formula: see text]-bipyridine)Ir][Formula: see text] which substituted phenyl on different ligands position have been systematically investigated by density functional theory (DFT) method. Significantly, the first hyperpolarizability [Formula: see text] values can be enhanced by introducing phenyl on 2-phenylpyridine ligands R1 or R2, whereas substituting phenyl on 2,2[Formula: see text]-bipyridine ligands R3 result in a decreasing [Formula: see text] values. The [Formula: see text] values exhibit obvious connection with the corresponding HOMO and LUMO energy gap. Furthermore, the time-dependent (TD) DFT calculations suggest that the enhanced [Formula: see text] values are related to obvious charge transfer from 2-phenylpyridine ligands to 2,2[Formula: see text]-bipyridine ligands. The investigation of frequency-dependent first hyperpolarizability [Formula: see text] ([Formula: see text]; [Formula: see text], 0) and [Formula: see text] ([Formula: see text]; [Formula: see text], [Formula: see text]) shown less dispersion effect at the low-frequency region for all of the studied complexes. Overall, tuning phenyl on the different ligands position can be seen as an effective strategy to modulate the second-order nonlinear optical response for these iridium complexes, which is benefit to theoretical and experimental further investigation.

A structure-property interplay between the width and height of cages and the static third order nonlinear optical responses for fullerenes: applying gamma density analysis

To reveal a new structure-property relationship regarding the nonlinear optical (NLO) properties of fullerenes that are associated with gamma (γ) density, fullerenes I (C40, C50, C60 and C70), whose heights range from 4.83 to 7.96 Å, and II (C24, C36, C48 and C72), whose widths range from 4.45 to 8.22 Å, have been the research objects. Calculation of their geometric and electronic structures, absorption spectra, and the second hyperpolarizability (γ) and the γ density analysis have been performed. It is found that the electronic spatial extent and the polarizability (α) value increase linearly as the fullerenes increase by every 12 carbon atoms. Similarly, the γ values are also proportional to the fullerene size. It is worth noting that the relative magnitude of γ_xxxx and γ_zzzz was exactly consistent with that of the width and height of fullerenes. The analysis of γ density provides the essential reason for this result, that is, the magnitude of the contribution to γ values associated with γ densities is proportional to the density amplitudes multiplied by the distance between them. Larger fullerenes possess larger density amplitudes and longer distances, resulting in larger γ values with respect to smaller fullerenes. This work presents a new structure-property interplay between the width and height of the fullerenes and their second hyperpolarizability γ. Moreover, the γ density analysis provides a new insight to explore the nature of the relationship between the structure and the NLO properties.

Electric-field-induced nonlinear optical switches of all-metal spherical aromatic molecules with infrared transparency: a theoretical study

Two new all-metal spherical aromatic molecules Be₆Li₈ and Be₆Li₁₄ have been designed theoretically.

Redox-mediated reactions of vinylferrocene: toward redox auxiliaries

Chemical redox reactions have been exploited to transform unreactive vinylferrocene into a powerful dienophile for the Diels-Alder reaction and reactive substrate for thiol addition reactions upon conversion to its ferrocenium state. We have further investigated the ability of these reactions to facilitate redox-auxiliary-like reactivity by further hydrogenolyisis of the Diels-Alder adduct to the corresponding cyclopentane derivative.

Ion-π interaction in impacting the nonlinear optical properties of ion-buckybowl complexes

Ion-buckybowl complexes have received considerable attention in modern chemical research due to its fundamental and practical importance. Herein, we performed density functional theory (DFT) to calculate the geometical structure, binding interactions, dipole moments and the first hyperpolarizabilities (βtot) of ion-buckybowl complexes (ions are Cl(-) and Na(+), buckybowls are quadrannulene, corannulene and sumanene). It is found that the stabilities of ion-buckybowl compounds primarily originate from the interaction energy, which was proved by a new isomerization energy decomposition analysis approach. Plots of reduced density gradient mirror the ion-π weak interaction has been formed between the ions and buckybowls. Significantly, the buckybowl subunits cannot effectively impact the nonlinear optical (NLO), but the kind of ion has marked influence on the second-order NLO responses. The βtot values of Cl(-)-buckybowl complexes are all larger as compared to that of Na(+)-buckybowl complexes, which is attributed to the large charge-transfer (CT) from Cl(-) to buckybowl. Our present work will be beneficial for further theoretical and experimental studies on the NLO properties of ion-buckybowl compounds.

The effect of heterocyclic π bridges on second order nonlinear optical properties of compounds formed between ferrocenyl and corannulenyl

Among D–π–A type compounds, when imidazole acts as a π bridge (endo/exo-3), the β_tot values are almost 2 times as large as when the phenyl acts as a π bridge (endo/exo-2).

Variational first hyperpolarizabilities of 2,3-naphtho-15-crown-5 ether derivatives with cation-complexing: a potential and selective cation detector

When the crown ether and its cation derivatives display obvious contrasts in NLO properties, the NLO-based detection of cations is achieved.

Macrocyclic Se4N2[7,7]ferrocenophane and Se2N[10]ferrocenophane containing benzyl unit: synthesis, complexation, crystal structures, electrochemical and optical properties

Se₄N₂[7,7]ferrocenophane containing benzyl unit shows switch-on third-order NLO responses to Cu²⁺ and Hg²⁺ due to the formation of delocalized π-conjugated systems.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.

Understanding the photophysical properties of chiral dinuclear Re(i) complexes and the role of Re(i) in their complexes

We have investigated the electronic structure, UV-Vis/CD spectra and the second-order NLO properties of chiral dinuclear Re(i) complexes and elucidated structure–property relationships with the aid of DFT calculations.

Photoisomerization-switchable second-order nonlinear optical responses of dithienylethene-containing boron derivatives: a theoretical study

The switchable second-order nonlinear optical properties of a series of dithienylethene-containing boron compounds possessing reversible photochromic behavior have been systemically investigated based on density functional theory. The significant differences on the static first hyperpolarizabilities caused by the photoisomerization reaction confirm the switchable nonlinear optical behavior. Results show that the closed-ring 1c–4c possess large static first hyperpolarizabilitieswith respect to their corresponding open-ring 1o–4o, which is due to the better π-conjugation and a more obvious degree of charge transfer. Specifically, the static first hyperpolarizability values increase in the range of 2.4–5.2 times with the strengthening electron-withdrawing ability of the substituent R (R = H, CF3, NO2) on the pyridine and elongating the conjugated chains. It indicates that introducing boron-based functional groups to the dithienylethene monomer and modifying the pyridine ring can effectively improve the switching nonlinear optical responses.

Ferrocene/fullerene hybrids showing large second-order nonlinear optical activities: impact of the cage unit size

Ferrocene/fullerene complexes through face-to-face fusion enjoy the merits of both ferrocene and fullerene due to their strong donor–acceptor interactions.

Third order NLO properties of corannulene and its Li-doped dimers: effect of concave–convex and convex–convex structures

The convex–convex stacking motif of corannulene dimers enhances the third-order NLO response due to obvious interlayer charge transfer.

New donor–acceptor conjugates based on a trifluorenylporphyrin linked to a redox–switchable ruthenium unit

Spectroelectrochemical studies show four accessible redox states on dyad 3 with the first oxidation process taking place at the Ru(ii) centre and the second oxidation and first reduction processes at the porphyrin centre.