The Many Facets of RuII(dppe)2 Acetylide Compounds

In this contribution, we describe the various research domains in which RuII alkynyl derivatives are involved. Their peculiar molecular properties stem from a strong and intimate overlap between the metal centered d orbitals and the π system of the acetylide ligands, resulting in plethora of fascinating properties such as strong and tunable visible light absorption with a strong MLCT character essential for sensing, photovoltaics, light-harvesting applications or non-linear optical properties. Likewise, the d/π mixing results in tunable redox properties at low potential due to the raising of the HOMO level, and making those compounds particularly suited to achieve redox switching of various properties associated to the acetylide conjugated ligand, such as photochromism, luminescence or magnetism, for charge transport at the molecular level and in field effect transistor devices, or charge storage for memory devices. Altogether, we show in this review the potential of RuII acetylide compounds, insisting on the molecular design and suggesting further research developments for this class of organometallic dyes, including supramolecular chemistry.

Phosphorescent metallaknots of Au(I)-bis(acetylide) strands directed by Cu(I) π-coordination

Knots containing metal atoms as part of their continuous strand backbone are termed as metallaknots. While several metallaknots have been synthesized through one-pot self-assembly, the designed synthesis of metallaknots by controlling the arrangement of entanglements and strands connectivity remains unexplored. Here, we report the synthesis of metallaknots composed with Au(I)-bis(acetylide) linkages and templated by Cu(I) ions. Varying the ratio of the building blocks results in the switchable formation of two trefoil knots with different stoichiometries and symmetries (C2 or D3) and an entangled metalla-complex. While the entangled complex formed serendipitously, the strand ends can be subsequently linked through coordinative closure to generate a 41 metallaknot in a highly designable fashion. The comparable structural characteristics of resulting metalla-complexes allow us to probe the correlations between their topologies and photophysical properties, showing the backbone rigidity of knots endows complexes with excellent phosphorescent properties. This strategy, in conjunction with the coordinative closure approach, provides a straightforward route for the formation of highly phosphorescent metallaknots that were previously challenging to access.

Pyridine appended pyrimidine bis hydrazone: Zn2+/ATP detection, bioimaging and functional properties of its dinuclear Zn(II) complex

A pyridine functionalized pyrimidine-based system, H2P was successfully synthesized, characterized, and evaluated for its remarkable selective characteristics towards Zn2+ and ATP ions. The chemical sensing capabilities of H2P were demonstrated through absorption, fluorescence, and NMR spectroscopic techniques. The probe exhibited outstanding sensitivity when interacting with the ions, demonstrating relatively strong association constants and impressively low detection limits. The comprehensive binding mechanism of H2P with respect to Zn2+ and ATP ions was investigated using a combination of analytical methods, including Job's plot, NMR spectroscopy, mass spectrometry, and density functional theory (DFT) experiments. The interesting sensing ability of H2P for Zn2+/ATP ions was harnessed for live cell bioimaging and other diverse on-site detection purposes, including paper strips, cotton swabs, and applications involving mung bean sprouts. Further, the fluorescent probe demonstrated its effectiveness in detecting Zn2+ and ATP within live cells, indicating its significant potential in the realm of biological imaging applications. Moreover, the molecular configuration of the zinc complex (H2P-Zn2Cl4), derived from H2P, was elucidated using X-ray crystallography. This complex exhibited intriguing multifunctional attributes, encompassing its capability for detecting picric acid and for reversible acid/base sensing responses. The enhanced conducting behavior of the complex as well as its resistance properties were investigated by performing I-V characteristics and electrochemical impedance spectroscopic (EIS) experiments respectively.

Pyrazinacene conjugated polymers: a breakthrough in synthesis and unraveling the conjugation continuum

Pyrazinacenes are next generation N-heteroacenes and represent a novel class of stable n-type materials capable of accepting more than one electron and displaying intriguing features, including prototropism, halochromism, and redox chromism. Astonishingly, despite a century since their discovery, there have been no reports on the conjugated polymers of pyrazinacenes due to unknown substrate scope and lack of pyrazinacene monomers that are conducive to condensation polymerization. Breaking through these challenges, in this work, we report the synthesis of previously undiscovered and highly coveted conjugated polymers of pyrazinacenes. In order to understand the intricacies of conjugation extension within the acene and along the polymer backbone, a series of electronically diverse four pyrazinacene conjugated polymers were synthesized. Polymers synthesis required optimizing a few synthetic steps along the 12-step synthetic pathway. The generated pyrazinacene monomers are not amenable to the popular condensation polymerizations involving Pd or Cu catalysts. Gratifyingly, Pd and Cu free dehydrohalogenation polymerization of the monomer with HgCl2 resulted in high molecular weight organometallic conjugated pyrazinacene polymers within a few minutes at room temperature. The dual role played by the Hg(ii) during the polymerization, combined with the self-coupling of the RHgCl (intermediate), is at the core of successful polymerization. Notably, the self-coupling of intermediates challenges the strict stoichiometric balance typically required for step-growth polymerization and offers a novel synthetic strategy to generate high molecular weight conjugated polymers even with imbalanced monomer stoichiometries. A combination of electrochemical studies and DFT-B3LYP simulations indicated that the presence of the reduced pyrazine ring promotes interacene π-conjugation through the metal center, in contrast to completely oxidized tetrazaazaanthracene. The extension of conjugation results in ca. 2 eV lower reduction potential for polymers compared to the monomer, placing the LUMO energy levels of these polymers on par with some of the best-known n-type polymers. Also, the presence of NH protons in the pyrazinacene polymers show ionochromism and red-shift UV-vis absorption maximum by ca. 100 nm. This work not only shows a way to realize highly desirable and elusive pyrazinacene conjugated polymers but also paves the way for a library of n-type conjugated polymers that can undergo multi-electron reduction.

Poly(ferrocenylene iminoborane): an inorganic-organic hybrid polymer comprising a backbone of moderately interacting ferrocenes

The first poly(ferrocenylene iminoborane), that is, a polyferrocene-based metallopolymer featuring CC-isoelectronic/-isosteric BN linking units, and a series of monodisperse ferrocenylene iminoborane oligomers are presented. Our studies provide important insight into the structural and electronic nature of this novel class of hybrid materials.

Spin Crossover and Fluorine-Specific Interactions in Metal Complexes of Terpyridines with Polyfluorocarbon Tails

In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding FeII and CoII complexes. The CoII complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59 Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F ⋯ ${\cdots }$ F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing.

Synthesis of platinum(II)-complex end-tethered polymers: spectroscopic properties and nanostructured particles

Although metal-containing polymers have been widely studied as a novel class of functional soft materials, the microphase separation between polymeric segments and metal-ligand complexes has been less addressed, which is critical to control their structures and functions. To do this, short-chain polystyrenes (PSs) have been end-functionalized with nanosized square-planar platinum(II) complexes. The platinum(II)-comprising polymers were found to show significant luminescence enhancement in chloroform/methanol solvent mixtures upon increasing the methanol composition. By modulating both the PS length and solvent quality, various self-assembled morphologies formed controllably in the mixed solvents and typical examples include nanofibers, nanoellipsoids, and nanospheres. More interestingly, the inside structures of these polymer particles are shown to be lamellar with sub-10 nm spacings, wherein the PS blocks are alternatively aligned with the platinum(II) units. Such a luminescence enhancement and hierarchical nanostructured particles originate from a subtle combination of directional Pt(II)⋯Pt(II) and/or π-π stacking interactions between the platinum(II) units and the solvophobic effect between the PS blocks. This work suggests that by microphase separating polymer chains with nanosized metal-ligand complexes, metal-containing polymers can self-assemble to form sub-10 nm scale nanostructures showcasing desired properties and functions.

Bottom-Up Synthesis of Metalated Carbyne Ribbons via Elimination Reactions

Elimination reactions are one of the most important reactions in organic synthesis, especially in the formation of alkenes and alkynes. Herein, based on scanning tunneling microscopy, we report the bottom-up synthesis of one-dimensional carbyne-like nanostructures, metalated carbyne ribbons with the incorporation of Cu or Ag atoms, through α- and β-elimination reactions of tetrabromomethane and hexabromoethane on surfaces. Density functional theory calculations demonstrate a width-dependent band gap modulation within these ribbon structures, which is affected by interchain interactions. Moreover, mechanistic insights into the on-surface elimination reactions have also been provided in this study.

Real scenario of metal ion sensor: is conjugated polymer helpful to detect hazardous metal ion

Metal ions from natural and anthropogenic sources cause pollution to society and the environment is major concern in the present scenario. The deposition and contamination of metal ions in soil and water affect the biogeochemical cycles. Thus, it threatens the everyday life of living and non-living organisms. Reviews on the detection of metal ions through several techniques (Analytical methods, electrochemical techniques, and sensors) and materials (Nanoparticles, carbon dots (quantum dots), polymers, chiral molecules, metal-organic framework, carbon nanotubes, etc.) are addressed separately in the present literature. This review reveals the advantages and disadvantages of the techniques and materials for metal ion sensing with crucial factors. Furthermore, it focus on the capability of conjugated polymers (CPs) as metal ion sensors able to detect/sense hazardous metal ions from environmental samples. Six different routes can synthesize this type of CPs to get specific properties and better metal ion detecting capability in vast research areas. The metal ion detection by CP is time-independent, simple, and low cost compared to other materials/techniques. This review outlines recent literature on the conjugated polymer for cation, anion, and dual ion sensors. Over the last half decades published articles on the conjugated polymer are discussed and compared.

Bis-Alkynyl Complexes of Fe(III) Tetraaza Macrocycles─A Tale of Two Rings

Reported herein are new Fe bis-alkynyl complexes [Fe^III(L)(C₂R)₂]BPh₄ based on tetraimine macrocycle (L = HMTI = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; 1a-1c; R = C₆H₅ (a), C₁₀H₉ (b), SiMe₃ (c)) and tetraamine macrocycle (L = HMC = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; 2a-2c). These complexes have been characterized using single-crystal X-ray diffraction, electronic absorption spectroscopy, and cyclic and differential pulse voltammetry. Spectroelectrochemical studies of 1a and 2a allowed for investigation of the Fe^II oxidation state, which revealed a strong dependence on the nature of the macrocycle for both the energies of the Fe^II to C₂Ph metal-to-ligand charge transfer (MLCT) and the ν(C≡C). The ν(C≡C) was further influenced by the oxidation state, though sensitivity to the formal metal oxidation state was much higher in the case of 2a than in 1a. These findings are rationalized on the basis of the relative energies of the formally metal-centered orbitals via density functional theory calculations.

Conjugated polymers based on metalla-aromatic building blocks

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet-visible (UV-Vis) absorption bands. Bulky ligands on the metal suppress π-π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV-Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

Synthesis of Mixed Arylalkyl Tertiary Phosphines via the Grignard Approach

Trialkyl and triaryl phosphines are important classes of ligands in the field of catalysis and materials research. The wide usability of these low-valent phosphines has led to the design and development of new synthesis routes for a variety of phosphines. In the present work, we report the synthesis and characterization of some mixed arylalkyl tertiary phosphines via the Grignard approach. A new asymmetric phosphine is characterized extensively by multi-spectroscopic techniques. IR and UV-Vis spectra of some selected compounds are also compared and discussed. Density functional theory (DFT)-calculated results support the formation of the new compounds.

Highly emissive supramolecular gold(I)-BTD materials

Herein we report the synthesis of three light emitting rod-shaped gold(I) complexes by combining ethynyl-functionalized 2,1,3-benzothiadiazole (BTD), with different N-heterocyclic (imidazole, benzimidazole and phenantroimidazole) carbene gold(I) complexes. As could be determined by single crystal structure determination, the size of the N-heterocyclic carbene affects the relative disposition of the two ligands in the linear gold complexes (nearly perpendicular in the benzimidazole vs. planar in the phenantroimidazole derivative) which translates to different types of interactions between neighbouring units. In fact, the planar conformation in the more π-extended phenantroimidazole carbene allows Au(I) atoms to get sufficiently closer favouring aurophilic interactions. This compound is obtained as two different colour emitting solids which is ascribed to changes in packing modes probably leading to differences in the strength of aurophilic interactions. Interestingly, while the emission in the solid state is almost quenched, the incorporation of the compounds in polymer matrixes enhances the emissive properties of the compounds reaching near unity quantum yields.

Phenylene as an efficient mediator for intermetallic electronic coupling

The new compound [(NC)Ru₂(ap)₄]₂(μ-1,4-C₆H₄) (ap = 2-anilinopyridinate) was prepared to address the open question of whether a 1,4-phenylene bridge can mediate intermetallic electronic coupling. As a manifestation of strong coupling, hole delocalization between the Ru₂ centers on the IR time scale (10^-14 s) was established using spectroelectrochemistry. An orbital mechanism for coupling was elaborated with DFT analysis.

Structural Transformation of Copper Coordination Complexes with Inducing Supramolecular Chirality

Four copper coordination complexes [CuL(CH3OH)]n (1), [CuL(C2H5OH)]n (2), [(CuL)·0.5H2O]n (3), and [CuL(H2O)·0.5H2O]n (4) with a tridentated Schiff base ligand (H2L = (E)-2-((4-fluoro-2-hydroxybenzylidene)amino)acetate) were obtained in different solvents and reaction times....

A Ferroelectric Metallomesogen Exhibiting Field-Induced Slow Magnetic Relaxation

Magnetoelectric (ME) materials exhibiting coupled electric and magnetic properties are of significant interest because of their potential use in memory storage devices, new sensors, or low-consumption devices. Herein, we report a new category of ME material that shows liquid crystal (LC), ferroelectric (FE), and field-induced single molecule magnet (SMM) behaviors. Co(II) complex incorporating alkyl chains of type [Co(3C₁₆ -bzimpy)₂ ](BF₄ )₂ (1; 3C₁₆ -bzimpy=2,2'-(4-hexadecyloxy-2,6-diyl)bis(1-hexadecyl-1H-benzo[d]imidazole)) displayed a chiral smectic C mesophase in the temperature range 321 K-458 K, in which distinct FE behavior was observed, with a remnant polarization (88.3 nC cm^-2 ). Complex 1 also exhibited field-induced slow magnetic relaxation behavior that reflects the large magnetic anisotropy of the Co(II) center. Furthermore, the dielectric property of 1 was able to be tuned by an external magnetic field occurring from both spin-lattice coupling and molecular orientational variation. Clearly, this multifunctional compound, combining LC, FE, and SMM properties, represents an entry to the development of a range of next-generation ME materials.

Heterometal Grafted Metalla-ynes and Poly(metalla-ynes): A Review on Structure-Property Relationships and Applications

Metalla-ynes and poly(metalla-ynes) have emerged as unique molecular scaffolds with fascinating structural features and intriguing photo-luminescence (PL) properties. Their rigid-rod conducting backbone with tunable photo-physical properties has generated immense research interests for the design and development of application-oriented functional materials. Introducing a second d- or f-block metal fragment in the main-chain or side-chain of a metalla-yne and poly(metalla-yne) was found to further modulate the underlying features/properties. This review focuses on the photo-physical properties and opto-electronic (O-E) applications of heterometal grafted metalla-ynes and poly(metalla-ynes).

Boraiminolithium: An Iminoborane-Transfer Reagent

BN/CC isosterism can give rise to attractive molecules with unique physical or chemical properties. We report here the synthesis, characterization, and reactivities of the boraiminolithium species 2, a room-temperature-stable crystalline solid accessible through a facile dehydrohalogenation/deprotonation reaction. This species, bearing a polarized B≡N triple bond and an anionic N center, is the first example of a BN analogue to the well-known alkynyllithium molecules (lithium acetylides). It has demonstrated a remarkable ability for iminoborane-transfer reactions, which allows for the isolation of a series of unprecedented N-functionalized iminoboranes as well as novel main-group heterocycles. Stable boraiminolithium reagents may become powerful tools in the exploration of new BN-containing building blocks for synthetic chemistry and materials science.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar-Ligand-Substituent Motion

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh-terpy)₂ ](BPh₄ )₂ ⋅3ac (1⋅3 ac; FPh-terpy=4'-((3-fluorophenyl)ethynyl)-2,2':6',2''-terpyridine) and is dependent on the degree of 180° flip-flop motion of the ligand's polar fluorophenyl ring. Single crystal X-ray structures at several temperatures confirmed the flip-flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid-state ¹⁹ F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip-flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin-state conversion being dependent on the molecular motion of a ligand-appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule-based magnetoelectric materials.

Electronic and steric effects of platinum(ii) di-yne and poly-yne substituents on the photo-switching behaviour of stilbene: experimental and theoretical insights

A series of mono-, di-, and poly(platina-ynes) incorporating stilbene spacer units with the formulae trans-[R-C[triple bond, length as m-dash]C-Pt(PBu₃)₂-C[triple bond, length as m-dash]C-R] (R = (E)-1,2-diphenylethene), trans-[(Ph)-(Et₃P)₂PtC[triple bond, length as m-dash]C-R-C[triple bond, length as m-dash]CPt(PEt₃)₂(Ph)] (R = (E)-1,2-diphenylethene), and trans-[-(PⁿBu₃)₂PtC[triple bond, length as m-dash]C-R-C[triple bond, length as m-dash]C-]_n (R = (E)-1,2-diphenylethene), respectively, have been synthesized and characterized to explore the effects of ligand topology on the photoisomerization and photophysical properties of these materials. The structural and photophysical properties of the complexes have been investigated and compared with those of the previously reported mono-, di- and poly(platina-ynes) incorporating azobenzene spacers. We found that the organometallic species 1M, 2M and 1P undergo topology-dependent reversible trans-to-cis photoisomerization in CH₂Cl₂ solution. Computational modelling supported the experimental findings.

Utilization of a Pt(ii) di-yne chromophore incorporating a 2,2'-bipyridine-5,5'-diyl spacer as a chelate to synthesize a green and red emitting d-f-d heterotrinuclear complex

A new heterotrinuclear (d-f-d) complex [Eu(btfa)₃1c] (btfa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione and 1c = [(Ph)(Et₃P)₂Pt-C[triple bond, length as m-dash]C-R-C[triple bond, length as m-dash]C-Pt(Et₃P)₂(Ph)] (R = 2,2'-bipyridine-5,5'-diyl) has been synthesized by utilizing the N,N-donor sites of the organometallic chromophore. The complex was characterized by analytical and spectroscopic methods. Photophysical properties of the complex were analysed in detail using both steady-state and time-resolved emission and excitation spectroscopy. The optical absorption spectrum of the complex is dominated by the spin allowed π-π* transitions of the btfa and 1c units in the UV-visible region (200-418 nm) and thus is excitable over a wide range of wavelengths across the UV into the visible region of the electromagnetic spectrum. The complex displays typical red Eu(iii) emission when excited at 345 nm. However, it also shows green emission when excited at 464 nm and, thus could be an interesting candidate for full colour display applications. The change in the colour could be a result of the high value of the energy back-transfer rate (6.73 × 10⁵ s^-1) from the triplet state of the organometallic chromophore to the ⁵D₁ state of Eu(iii). Judd-Ofelt (J-O) intensity parameters (Ω₂ and Ω₄), radiative (A_R), non-radiative (A_R) decay rates and intrinsic quantum yield (Q) have been calculated.

Two Is Better than One? Investigating the Effect of Incorporating Re(CO)3Cl Side Chains into Pt(II) Diynes and Polyynes

Pt(II) diynes and polyynes incorporating 5,5'- and 6,6'-disubstituted 2,2'-bipyridines were prepared following conventional Sonogashira and Hagihara dehydrohalogenation reaction protocols. Using Pt(II) dimers and polymers as a rigid-rod backbone, four new heterobimetallic compounds incorporating Re(CO)₃Cl as a pendant functionality in the 2,2'-bipyridine core were obtained. The new heterobimetallic Pt-Re compounds were characterized by analytical and spectroscopic techniques. The solid-state structures of a Re(I)-coordinated diterminal alkynyl ligand and a representative model compound were determined by single-crystal X-ray diffraction. Detailed photophysical characterization of the heterobimetallic Pt(II) diynes and polyynes was carried out. We find that the incorporation of the Re(CO)₃Cl pendant functionality in the 2,2'-bipyridine-containing main-chain Pt(II) diynes and polyynes has a synergistic effect on the optical properties, red shifting the absorption profile and introducing strong long-wavelength absorptions. The Re(I) moiety also introduces strong emission into the monomeric Pt(II) diyne compounds, whereas this is suppressed in the polyynes. The extent of the synergy depends on the topology of the ligands. Computational modeling was performed to compare the energetic stabilities of the positional isomers and to understand the microscopic nature of the major optical transitions. We find that 5,5'-disubstituted 2,2'-bipyridine systems are better candidates in terms of yield, photophysical properties, and stability than their 6,6'-substituted counterparts. Overall, this work provides an additional synthetic route to control the photophysical properties of metallaynes for a variety of optoelectronic applications.

Al-Balushi R, Rather JA, Munam A, Ilmi R, Raithby PR, Zhang Y, Fu Y, Xie Z, Chen S, Islam SM, Wong WY, Skelton JM, Khan MS. Synthesis, characterization, and optoelectronic properties of phenothiazine-based organic co-poly-ynes

We present the synthesis of seven new organic co-poly-ynes P1–P7 incorporating phenothiazine (PTZ) motif and evaluate their optoelectronic properties and performance in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs).

Oxidized divinyl oligoacene-bridged diruthenium complexes: bridged localized radical characters and reduced aromaticity in bridge cores

A series of bimetallic ruthenium vinyl complexes 1-5 bridged by oligoacenes were synthesized and characterized in this study. Comparative cyclic voltammetry results from 1-5 indicated that the first oxidation potential decreased gradually with the extension of conjugate ligands. Upon oxidation to singly oxidized species 1+-5+, rather small ν(CO) changes in the infrared (IR) spectra and the characteristic bands of metal-to-ligand charge transfer absorptions in the near IR (NIR) region predicted via time-dependent DFT calculations suggested that strong bridged ligands participate in redox processes. NIR absorptions were not observed in complexes 4+ and 5+ possibly because of instability in their twisted and noncoplanar geometry. Electron paramagnetic resonance results and spin density distribution demonstrated that the bridged localized degrees of 1+-5+ successively increased with the extension of oligoacene from benzene to tetracene. Further comparative analysis of neutral molecules and monocations to the aromaticity and π-electron density of bridge cores indicated a step-by-step transformation process from an aromatic to quinoidal radical upon oxidation.

Role of Substituents at 3-position of Thienylethynyl Spacer on Electronic Properties in Diruthenium(II) Organometallic Wire-like Complexes

A series of organometallic complexes [Cl(dppe)₂ Ru-C≡C-(3-R-C₄ H₂ S)-C≡C-Ru(dppe)₂ Cl] (3-R-C₄ H₂ S=3-substituted thienyl moiety; R=-H, -C₂ H₅ , -C₃ H₇ , -C₄ H₉ , -C₆ H₁₃ , -OMe, -CN in 5 a-5 g respectively) have been synthesized by systematic variation of 3-substituents at the thienylethynyl bridging unit. The diruthenum(II) wire-like complexes (5 a-5 g) have been achieved by the reaction of thienylethynyl bridging units, HC≡C-(3-R-C₄ H₂ S)-C≡CH (4 a-4 g) with cis-[Ru(dppe)₂ Cl₂ ]. The wire-like diruthenium(II) complexes undergo two consecutive electrochemical oxidation processes in the potential range of 0.0 - 0.8 V. Interestingly, the wave separation between the two redox waves is greatly influenced by the substituents at the 3-position of the thienylethynyl. Thus, the substitution on 3-position of the thienylethynyl bridging unit plays a pivotal role for tuning the electronic properties. To understand the electronic behavior, density functional theory (DFT) calculations of the selected diruthenium wire-like complexes (5 a-5 e) with different alkyl appendages are performed. The theoretical data demonstrate that incorporation of alkyl groups to the thienylethynyl entity leaves unsymmetrical spin densities, thus affecting the electronic properties. The voltammetric features of the other two Ru(II) alkynyl complexes 5 f and 5 g (with -OMe and -CN group respectively) show an apparent dependence on the electronic properties. The electronic properties in the redox conjugate, (5 a⁺ ) with K_c of 3.9×10⁶ are further examined by UV-Vis-NIR and FTIR studies, showing optical responses in NIR region along with changes in "-Ru-C≡C-" vibrational stretching frequency. The origin of the observed electronic transition has been assigned based on time-dependent DFT (TDDFT) calculations.