pH value-dependent electronic absorption and Ru(III/II) potential of bis-tridentate pincer ruthenium complexes

Luminescent Ruthenium-Terpyridine Complexes Coupled with Stilbene-Appended Naphthalene, Anthracene, and Pyrene Motifs Demonstrate Fluoride Ion Sensing and Reversible Trans-Cis Photoisomerization

A new family of luminescent heteroleptic Ru(II)-terpyridine complexes coupled with stilbene-appended naphthalene, anthracene, and pyrene motifs is reported. Each of the complexes features moderately intense emission at room temperature having a lifetime of 16.7 ns for naphthalene and 11.4 ns for anthracene, while a substantially elevated lifetime of 8.3 μs was observed for the pyrene derivative. All the three complexes display a reversible couple in the positive potential window due to Ru2+/Ru3+ oxidation but multiple reversible and/or quasi-reversible peaks in the negative potential domain because of the reduction of the terpyridine moieties. All the complexes selectively sense F- among the studied anions via the intermediary of different noncovalent interactions. The interaction event is monitored through absorption, emission, and 1H and 19F NMR spectroscopy. Additionally, upon utilizing the stilbene motif, reversible trans-cis isomerization of the complexes has been undertaken upon alternate treatment of visible and UV light so that the complexes can act as potential photomolecular switches. We also carried out the anion sensing characterization of the cis form of the complexes. Theoretical calculation employing density functional theory is also executed for a selective complex (naphthalene derivative) to elucidate different noncovalent interactions that are operative during the complex-fluoride interplay.

Design of molecular sensors and switches based on luminescent ruthenium-terpyridine complexes bearing active methylene and triphenylphosphonium motifs as anion recognition sites: experimental and DFT/TD-DFT investigation

Synthesis, characterization and thorough investigation of the photophysical and electrochemical properties of a new category of emissive homo- and heteroleptic Ru(II)-complexes derived from the [4'-(p-triphenylphosphonium methyl phenyl)-2,2':6',2''-terpyridine]bromide (tpy-PhCH2PPh3Br) ligand have been executed in this work. Incorporation of the PhCH2PPh3+Br- group at the terpyridine motif appropriately adjusts the triplet metal-to-ligand charge transfer (3MLCT) and metal-centered (3MC) excited states so that the complexes luminesce at room temperature (RT) having lifetimes within the range of 6.82-9.63 ns. The RT emission characteristics of the complexes get further enhanced via aggregation phenomena through the use of different solvent/non-solvent mixtures (DMSO/H2O and DMSO/PhCH3 mixtures). Temperature dependent emission spectral measurements indicate that the emission intensity, quantum yield and lifetime increase upon dropping down the temperature, thereby designated as the on-state, while the increase of temperature causes a reduction of the said properties, indicating the off-state and the process is fully reversible. Taking advantage of the active methylene group coupled with a phosphonium motif, anion sensing characteristics of the complexes are investigated systematically in DMSO through the use of various optical channels and spectroscopic tools. The complexes are very much sensitive to fluoride and to a lesser extent acetate and dihydrogen phosphate among the studied anions. In essence, the complexes function as sensors for temperature and fluoride ion. Computational investigations were also executed via density functional theory (DFT) and time-dependent (TD)-DFT to obtain a clear understanding of the electronic structures of the metalloreceptors, appropriate assignment of the spectral bands and their mode of interaction with selected anions.

Tuning of photo-redox behaviours and thermodynamic and kinetic aspects of intercomponent energy transfer in trimetallic complexes of Ru(II) and Os(II) by exploiting their second coordination sphere

This paper deals with a thorough investigation of pH-induced tuning of the ground and excited state photophysical as well as electrochemical behaviours of two series of our recently reported homo- and heterotrimetallic complexes of the type [(bpy)₂Ru(d-HIm-t)M(t-HIm-d)Ru(bpy)₂]⁶⁺ and [(bpy)₂Os(d-HIm-t)M(t-HIm-d)Os(bpy)₂]⁶⁺ (M = Ru^II and Os^II) derived from a heteroditopic bpy-tpy (d-HIm-t) type bridging ligand through the exploitation of their second coordination sphere. A small bathochromic shift of the absorption and emission spectral band along with substantial alteration of emission intensity and lifetime of the triads is noted upon deprotonation of the NH motifs at elevated pH values. The lowering of the half wave potential of a M³⁺/M²⁺ couple is also observed upon removal of the NH protons. Both ground and excited state pK_a values of the triads are estimated from their absorption/emission versus pH spectral profiles. In addition, the variation of the free energy change (ΔG) and the rate of intercomponent energy transfer (k_en) in the triads upon stepwise deprotonation of the NH motifs are also addressed in the present study.

Stimuli-Responsive Molecular Switches and Logic Devices Based on Ru(II)-Terpyridyl-Imidazole Coordination Motif

We report herein the synthesis, photophysics, and electrochemistry of three Ru(II)-terpyridine complexes derived from a new terpyridyl-imidazole ligand (tpy-HImzPh₃F₂) and study their pH- and temperature-responsive behaviors toward the fabrication of molecular switches. The complexes emitted at room temperature (RT) have a lifetime within the 4.5-49.0 ns domain, depending on the auxiliary ligand and the solvent used. In the acidic region, the complexes exhibit emission, indicating the "on-state", while in the basic condition, the emission is totally quenched, indicating the "off-state". Similarly, when the temperature is lowered, the emission intensity and lifetime are enhanced, demonstrating the on-state, while increase of temperature leads to quenching of the emission intensity and lifetime, designated as the off-state. In both cases, the process is reversible. The bathochromic shift of the spectral band together with the emission quenching and lowering of the Ru³⁺/Ru²⁺ potential is also observed upon deprotonation at elevated pH. In addition, systematic variation of the absorption spectral behaviors upon variation of pH helps in evaluation of the pK_a's of the complexes. In essence, the complexes can act as switches emanated from a huge change in their absorption, emission, and redox behaviors as a function of their acidity/basicity (pH) and temperature. Moreover, their emission spectral responses as a function of pH and temperature were utilized for the fabrication of two-input binary logic gates. Density-functional theory (DFT) and time-dependent density-functional theory (TD-DFT) computations are performed for appropriate interpretation of the spectral bands.

Ru(II)-Based Metallo-Supramolecular Polymer with Tetrakis(N-methylbenzimidazolyl)bipyridine for a Durable, Nonvolatile, and Electrochromic Device Driven at 0.6 V

Low-voltage operation, high durability, and long memory time are demanded for electrochromic (EC) display device applications. Metallo-supramolecular polymers (MSPs), composed of a metal ion and ditopic ligand, are one of the recently developed EC materials, and the ligand modification is expected to tune the redox potential of MSP. In order to lower the redox potential of MSP, tetrakis(N-methylbenzimidazolyl)bipyridine (L_Bip) was designed as an electronically rich ligand. Ru-based MSP (polyRu-L_Bip) was successfully synthesized by 1:1 complexation of RuCl₂(DMSO)₄ with L_Bip. The molecular weight (M_w) was high (8.8 × 10⁶ Da) enough to provide a simple ¹H NMR spectrum, of which the proton peaks could be assigned by the comparison with the spectrum of the corresponding mono-Ru complex. The redox potential (E_1/2) between Ru(II/III) was 0.51 V versus Ag/Ag⁺, which was much lower than the redox potential of previously reported Ru-based MSP with bis(terpyridyl)benzene (0.95 V vs Ag/Ag⁺). The polymer film exhibited reversible, distinct color changes between violet and light green-yellow upon applying very low potentials of 0 and 0.6 V vs Ag/Ag⁺, respectively. The appearance and disappearance of the metal-to-ligand charge transfer absorption by the electrochemical redox between Ru(II/III) were confirmed using in situ spectro-electrochemical measurement. A solid-state EC device with polyRu-L_Bip was revealed to have large optical contrast (ΔT 54%), fast response time (1.37 s for bleaching and 0.67 s for coloration), remarkable coloration efficiency (571 cm²/C), and high durability for the repeated color changes more than 20,000 cycles. The device also showed a long optical memory time of up to 19 h to maintain 40% to the initial contrast under the open circuit conditions. It is considered that the stabilization of the Ru(III) state by L_Bip suppressed the self-coloring to Ru(II) inside the device.

pH-Responsive colorimetric, emission and redox switches based on Ru(ii)-terpyridine complexes

We have undertaken a thorough investigation on pH-responsive optical and redox switching behaviors of our recently reported trans form of bis-tridentate Ru(ii) luminophores, [(H₂pbbzim)Ru(tpy-pvp-X)]²⁺ where X = H, Me, Cl, NO₂, and Ph. The complexes possess two benzimidazole protons in their second coordination sphere, which became acidic upon coordinating influence of Ru²⁺ and could be successively deprotonated with the increase of pH. The effect of pH on photophysical and electrochemical behaviours of the complexes was thoroughly studied. Substantial quenching of emission together with the red-shift of both absorption (color change) and emission bands is noticed for all complexes upon dissociation of NH protons. Absorption vs. pH data were employed for determination of ground-state pK_a values, while excited-state pK_a (pK_a*) values were estimated by employing the Förster cycle based equation. The electronic nature of X induces a small but finite effect on the pK_a values and a linear correlation is found by plotting pK_avs. Hammett σ_p parameters of X. Proton-coupled electrochemical behaviours were investigated within the pH range of 1-10. From the E_1/2vs. pH plot, acid dissociation constants in different protonation states of the complexes were estimated in both Ru²⁺ and Ru³⁺ states. Compared with their protonated forms which exhibit reversible oxidation within 0.91-0.95 V, the oxidation potential of the doubly deprotonated forms shifted remarkably to the cathodic region (0.61-0.66 V). In essence, the present complexes act as potential pH-responsive colorimetric, emission and redox switches.