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.

Modulation of aluminum sensing properties of a sulphone group containing chemosensor and its biological applications.

A chemosensor with two binding pockets facilitates binding of one metal ion in either of the pockets providing a better chance for the interaction and hence recognition of the cation. We report here a chemosensor, namely 2,2'-(1E)-(5,5'-sulfonylbis(2-hydroxy-5,1-phenylene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)dinaphthalen-1-ol (H4L-naph), for selective sensing of Al3+ in DMF- HEPES buffer (1:4, v/v, pH 7.4). It shows almost 100-fold fluorescence enhancement at 532 nm (λex = 482 nm) in the presence of Al3+. Its quantum yield and excited state lifetime enhances significantly with the cations. H4L-naph forms a 1:2 complex with Al3+ with an association constant value of 2.18 × 104 M-2. Fluorescence enhancement may be attributed to CHEFF mechanism and restriction of >CN isomerization. Effect of the presence of naphthyl rings instead phenyl ring of a previously reported probe has resulted shifting of excitation/emission peak towards longer wavelength. The probe has been applied to image Al3+ in L6 cells with no significant cytotoxicity.

Synthesis, Structural Characterization, and Luminescence Switching of Diarylethene-Conjugated Ru(II)-Terpyridine Complexes by trans-cis Photoisomerization: Experimental and DFT/TD-DFT Investigation.

We synthesized and thoroughly characterized a new family of diarylethene-conjugated mononuclear Ru(II)-terpyridine complexes and investigated in detail their photophysical, electrochemical, and spectroelectrochemical behaviors. Interestingly, the compounds show moderately strong room-temperature luminescence predominantly from their 3MLCT state with luminescence lifetime varying between 8.43 and 22.82 ns. Because of the presence of diarylethene unit, all the monometallic complexes underwent trans-to-cis photoisomerization upon interaction with UV light with substantial changes in their absorption and luminescence spectra. Reverting back from the cis to the trans form is also made possible upon treatment with visible light or by heat. Trans-to-cis isomerization leads to almost complete quenching of luminescence, while backward cis-to-trans isomerization gives rise to restoration of the original luminescence for all the complexes. Thus, "on-off" and "off-on" emission switching was made possible upon successive interaction of the complexes with UV and visible light. Computational investigation involving density functional theory (DFT) and time-dependent DFT methods was done for proper assignment of the experimental absorption and emission spectral bands in the complexes. Finally, experimentally observed trend on the absorption and emission spectral behaviors of the complexes upon photoisomerization was also compared with the calculated results.

Synthesis, Photophysics, and Switchable Luminescence Properties of a New Class of Ruthenium(II)-Terpyridine Complexes Containing Photoisomerizable Styrylbenzene Units.

We report here the synthesis and structural characterization of a new class of homoleptic terpyridine complexes of Ru(II) containing styrylbenzene moieties to improve room-temperature luminescence properties. Solid-state structure determination of 2 was done through single-crystal X-ray diffraction. Tuning of photophysical properties was done by incorporating both electron-donating and electron-withdrawing substituents in the ligand. The complexes exhibit strong emission having lifetimes in the range of 10.0-158.5 ns, dependent on the substituent and the solvent. Good correlations were also observed between Hammett σp parameters with the lifetimes of the complexes. Styrylbenzene moieties in the complexes induce trans-trans to trans-cis isomerization accompanied by huge alteration of their spectral profiles upon treating with UV light. Reversal of trans-cis to trans-trans forms was also achieved on interacting with visible light. Change from trans-trans to the corresponding trans-cis form leads to emission quenching, whereas trans-cis to the corresponding trans-trans form leads to restoration of emission. In essence, "on-off" and "off-on" photoswitching of luminescence was observed. Calculations involving density functional theory (DFT) and time-dependent-DFT methods were performed to understand the electronic structures as well as for appropriate assignment of the absorption and emission bands.

Ru-Os dyads based on a mixed bipyridine-terpyridine bridging ligand: modulation of the rate of energy transfer and pH-induced luminescence switching in the infrared domain.

A series of heterobimetallic complexes of compositions [(bpy/phen)2Ru(dipy-Hbzim-tpy)Os (tpy-PhCH3/H2pbbzim)]4+ (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine and H2pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine)), derived from a heteroditopic bpy-tpy bridging ligand, were synthesized and thoroughly characterized in this work. The heterometallic complexes exhibit two successive one-electron reversible metal-centered oxidations corresponding to OsII/OsIII at lower potential and RuII/RuIII at higher potential. All the four dyads exhibit very intense, ligand centered absorption bands in the UV region and moderately intense MLCT bands in the visible region. The dyads also show intense infrared emission with the emission maximum spanning between 734 nm and 775 nm with reasonably long room temperature lifetimes varying between 30 ns and 104 ns. Both steady state and time resolved luminescence spectroscopic investigations indicate that efficient and fast intramolecular energy transfer from the 3MLCT state of the Ru(ii) center to the Os-center takes place in all the four dyads. In addition, the rate of energy transfer was found to depend on the terminal ligand on the Os-site. Due to the presence of a number of imidazole NH protons in the dyads, significant modulation of both the ground and excited state properties of the complexes was made possible by varying the pH of the solution. By varying the terminal ligand, pH-induced "on-off", "off-off-on" and "on-off-on" emission switching of the complexes was nicely demonstrated in the infrared region.

Demonstration of intramolecular energy transfer in asymmetric bimetallic ruthenium(ii) complexes.

A new family of bimetallic Ru(ii) complexes derived from an asymmetric bridging ligand (tpy-Hbzim-dipy) consisting of both bipyridine and terpyridine chelating sites covalently connected via phenyl-imidazole spacer were designed in this work to demonstrate intramolecular energy transfer from one component to the other in asymmetric dyads. To fine tune the photo-redox properties, both bidentate and tridentate terminal ligands in the complexes were varied systematically. Both steady state and time-resolved luminescence spectral results indicated photo-induced intramolecular energy transfer from the excited MLCT state of the [(bpy/phen)2RuII(dipy-Hbzim-tpy)] component to the MLCT state of the tpy-containing unit [(dipy-Hbzim-tpy)RuII(tpy-PhCH3/H2pbbzim)] in dyads with rate constant values on the order of 106-107 s-1. Temperature-dependent luminescence studies indicated an enhancement in the luminescence intensity and excited state lifetimes upon decreasing the temperature.

Pyrene and imidazole functionalized luminescent bimetallic Ru(II) terpyridine complexes as efficient optical chemosensors for cyanide in aqueous, organic and solid media.

We report in this work the anion recognition and sensing aspect of a new family of bimetallic Ru(ii) complexes derived from a symmetrical bridging 5,11-bis(4-([2,2':6',2''-terpyridine]-4'-yl)phenyl)-4,12-dihydropyreno[4,5-d:9,10-d']diimidazole (tpy-H2PhImzPy-tpy) terpyridine ligand in solution as well as in the solid sate through different channels such as absorption, steady state and time-resolved emission, and (1)H NMR spectroscopic techniques. Interestingly, the complexes exhibit luminescence in the red region with moderately long lifetimes compared with the related terpyridine complexes of Ru(ii). In DMSO, complexes 1 and 2 act as sensors for F(-) and to a lesser extent for AcO(-), CN(-) and H2PO4(-), whereas 3 acts as a sensor for F(-), AcO(-), CN(-) and to some extent for H2PO4(-). In contrast to DMSO, all the complexes exhibit very high selectivity towards cyanide ions in the presence of an excess of other anions in aqueous medium. The complexes display visual detection of cyanide with the detection limit lying in the range of 1.01 × 10(-7) to 9.79 × 10(-8) M. Equilibrium constants for the interaction of the complexes with the anions were evaluated from absorption and emission titration profiles and were found to lie in six orders of magnitude. It is observed that the excited-state lifetimes of the complexes were modulated to a significant extent by the selected anions in all the three media proving the utility of such complexes to act as lifetime-based sensors for anions. The fact that all the complexes can selectively sense cyanide in the presence of other anions with their detection limits lying in the range of 10(-7) M-10(-8) M in aqueous solution is particularly important for their practical applicability. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) studies were performed to understand the nature of the ground and excited states of the complexes with detailed assignments of the orbitals involved in absorption transitions. In particular, the red-shifts of the absorption and emission bands in the presence of selective anions have been well reproduced by computations.

pH-Induced processes in wire-like multichromophoric homo- and heterotrimetallic complexes of Fe(II), Ru(II), and Os(II).

In this work we studied the influence of pH on the absorption, steady state and time-resolved emission spectroscopic behaviors of recently reported multichromophoric trimetallic complexes of the forms [(bpy)2M(phen-Hbzim-tpy)M'(tpy-Hbzim-phen)M(bpy)2](6+) (M = Ru(II) or Os(II), and M' = Fe(II), Ru(II), and Os(II)) derived from a heteroditopic phenanthroline-terpyridine bridge, 2-(4-(2,6-di(pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5-f][1,10]phenanthroline (tpy-Hbzim-phen) and 2,2'-bipyridine (bpy) as the auxiliary ligand. For purposes of comparison, the UV-vis absorption and emission titrations of three monometallic model compounds [(bpy)2Ru(phen-Hbzim-tpy)](ClO4)2 (1), [(bpy)2Os(phen-Hbzim-tpy)] (ClO4)2 (2) and [(tpy-PhCH3)Ru(tpy-Hbzim-phen)](ClO4)2 (3), where tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine) were studied under the same experimental conditions. The absorption titration data were used to determine the ground state pKa values, whereas the luminescence and lifetime data were utilized for the determination of excited state pKa* values of the complexes. The evolving factor analyses of the set of absorption spectra of the complexes obtained by varying the pH of the solution confirm that only three absorbing species exist in the pH window of 2-12. Moreover, the modulation of the rate of the intramolecular energy transfer among the components in the homo- and heterotrimetallic complexes as a function of pH of the solution was also demonstrated.

Ruthenium(II) and osmium(II) mixed chelates based on pyrenyl-pyridylimidazole and 2,2'-bipyridine ligands as efficient DNA intercalators and anion sensors.

We report herein the synthesis and characterization of two monometallic ruthenium(II) and osmium(II) complexes of composition [(bpy)2M(HImzPPy)] (ClO4)2 derived from pyrenylimidazole-10-pyridin-2-yl-9H-9,11-diazacyclopenta[e]pyrene (HImzPPy) and 2,2'-bipyridine (bpy) ligands. X-ray crystallographic study shows that both crystals belong to the triclinic system having space group P1̅. The photophysical properties of 1 and 2 in acetonitrile indicate that the metal-to-ligand charge-transfer excited state is mainly centered in the [M(bpy)2](2+) moiety of the complexes and slightly affected by the extended conjugation of the pyrenylimidazole moiety. Both complexes display one-electron reversible metal-centered oxidative processes and a number of quasi-reversible reductive processes. The binding affinities of the complexes toward calf-thymus DNA (CT-DNA) were thoroughly studied through different methods such as absorption, emission, excited-state lifetime, circular dichroism, and thermal denaturation of DNA and a relative DNA binding study using ethidium bromide. All of these experiments account for the intercalative nature of both 1 and 2 toward CT-DNA as well as their light-switch behavior. The anion recognition study through different spectroscopic techniques reveals that both complexes act as "turn-on" luminescence sensors for H2PO4(-) and "turn-off" sensors toward F(-) and AcO(-). The imidazole N-H proton of the receptors gets deprotonated with the excessive addition of F(-) and AcO(-), while it interacts with H2PO4(-) through hydrogen-bonding interaction. Theoretical calculations (DFT and TD-DFT) were also performed to understand the photophysical properties of the metalloreceptors.

Multichromophoric bimetallic Ru(II) terpyridine complexes based on pyrenyl-bis-phenylimidazole spacer: synthesis, photophysics, spectroelectrochemistry, and TD-DFT calculations.

A symmetrical bridging ligand, 5,11-bis(4-([2,2':6',2″-terpyridine]-4'-yl)phenyl)-4,12-dihydropyreno[4,5-d:9,10-d']diimidazole (tpy-H2PhImzPy-tpy), containing terpyridyl coordinating units connected via a pyrenyl-bis-phenylimidazole spacer have been designed to synthesize a new class of light harvesting bimetallic Ru(II) complexes. The electronic properties of this complexes can be fine-tuned by varying tridentate terminal ligands. Full characterization of the compounds has been done with the help of (1)H NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Geometry optimization of the complexes was also carried out with density functional theory (DFT). Electronic absorption spectra exhibit a number of very intense π-π* and n-π* transitions in the UV and moderately intense MLCT and ILCT transitions in the visible region. Interestingly, the present bimetallic complexes exhibit moderately strong luminescence in the range between 657 and 703 nm and lifetimes (long component) between 5.8 and 67.0 ns at room temperature showing the dependence of the emission characteristics upon the type of terminal ligand and solvent. Detailed temperature-dependent emission measurements showed that an overall enhancement of photoluminescence intensity and lifetime occur in all three cases upon lowering of temperature. The redox behavior of the compounds is characterized by a single reversible anodic wave corresponding to two closely spaced one-electron processes. The appearance of intervalence charge transfer transition (IVCT) bands in the NIR region on electrochemical generation of Ru(II)Ru(II)/Ru(II)Ru(III) species indicates the presence of substantial electronic communication among the two ruthenium centers in the bimetallic complexes. DFT and TDDFT calculations were also done for better understanding of the absorption and emission spectral characteristics of the complexes.

Demonstration of multiple logic operations in a heteroditopic pyrene-phenylimidazole-terpyridine conjugate based on optical responses by selective anions and cations: an experimental and theoretical investigation.

Combined experimental and density functional theory (DFT) and time-dependent density functional theory (TD-DFT) studies were carried out to investigate the structural and electronic properties of a terpyridyl-phenylimidazole system covalently linked to pyrene, 10-(4-[2,2':6',2"-terpyridine]terpyridin-4'-yl-phenyl)-9H-9,11-diazacyclopenta[e]pyrene (tpy-HImzPy). X-ray crystal structure determination shows that the compound crystallized in monoclinic form with the space group P21/c. The anion and cation sensing properties of tpy-HImzPy were thoroughly studied in dimethyl sulfoxide and in mixed dimethyl sulfoxide-water medium through different channels such as absorption, steady-state and time-resolved emission, and (1)H NMR spectroscopic methods. In this work, by grafting the pyrene moiety into the terpyridyl chelating unit, an intraligand-charge-transfer sensitive chromophore has been developed whose absorption and emission behaviors are highly sensitive to selective anions and cations, and the response profiles in terms of absorption or emission intensity and wavelength toward the tested ions varied quite a lot. On the basis of these observations, we developed a unique molecular system capable of performing multiple logic functions such as INHIBIT, OR, XOR, NOR, and XNOR by simply varying the combination and level of various ionic inputs in a systematic manner. In this work, we will also be particularly interested to see the effect of selective anion and cation on the optical properties of receptor by means of computational studies and correlate them with the experimentally observed data.

Light harvesting and directional energy transfer in long-lived homo- and heterotrimetallic complexes of Fe(II), Ru(II), and Os(II).

A new family of trimetallic complexes of the form [(bpy)2 M(phen-Hbzim-tpy)M'(tpy-Hbzim-phen)M(bpy)2](6+) (M=Ru(II), Os; M'=Fe(II), Ru(II), Os; bpy=2,2'-bipyridine) derived from heteroditopic phenanthroline-terpyridine bridge 2-{4-[2,6-di(pyridin-2-yl) pyridine-4-yl]phenyl}-1H-imidazole[4,5-f][1,10]phenanthroline (phen-Hbzim-tpy) were prepared and fully characterized. Zn(2+) was used to prepare mixed-metal trimetallic complexes in situ by coordinating with the free tpy site of the monometallic precursors. The complexes show intense absorptions throughout the UV/Vis region and also exhibit luminescence at room temperature. The redox behavior of the compounds is characterized by several metal-centered reversible oxidation and ligand-centered reduction processes. Steady-state and time-resolved luminescence data show that the potentially luminescent Ru(II)- and Os(II)-based triplet metal-to-ligand charge-transfer ((3)MLCT) excited states in the triads are quantitatively quenched, most likely by intercomponent energy transfer to the lower lying (3)MLCT (for Ru and Os) or triplet metal-centered ((3)MC) excited states of the Fe(II) subunit (nonluminescent). Interestingly, iron did not adversely affect the photophysics of the respective systems. This suggests that the multicomponent molecular-wire-like complexes investigated here can behave as efficient light-harvesting antennas, because all the light absorbed by the various subunits is efficiently channeled to the subunit(s) in which the lowest-energy excited states are located.

Photoinduced intramolecular energy transfer and anion sensing studies of isomeric RuIIOsII complexes derived from an asymmetric phenanthroline-terpyridine bridge.

Two heterobimetallic Ru(II)-Os(II) complexes of compositions [(bpy)2M(II)(phen-Hbzim-tpy)M'(II)(tpy-PhCH3)](4+), where M(II) = Ru and M'(II) = Os (4) and M(II) = Os and M'(II) = Ru (5), phen-Hbzim-tpy = 2-(4-(2,6-di(pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5][1,10]phenanthroline, bpy = 2,2'-bipyridine, and tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine have been synthesized and characterized by elemental analyses, ESI mass spectrometry, and (1)H NMR and UV-vis absorption spectroscopy. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated and compared with that of monometallic model compounds [(bpy)2M(II)(phen-Hbzim-tpy)](2+) [M(II) = Ru (1) and M(II) = Os (2)] and [(phen-Hbzim-tpy)Ru(II)(tpy-PhCH3)](2+) (3). The complexes display very intense, ligand-centered absorption bands in the UV and moderately intense MLCT bands in the visible regions. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations, whereas the monometallic complexes display one-electron oxidation in the positive potential window. Steady state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer takes place from the Ru-center to the Os-based component in both the heterometallic dyads in all the solvents. The complexes under investigation contain an imidazole NH proton which became appreciably acidic due to metal coordination and can be utilized for recognition of selective anions in solution either via hydrogen bonding interaction or by proton transfer. Accordingly, the anion binding properties of the two heterobimetallic complexes as well as parent bridging ligand, phen-Hbzim-tpy, have been studied in solutions using absorption, steady state and time-resolved luminescence spectral measurements. The metalloreceptors act as sensors for F(-), CN(-) and AcO(-) ions. It is evident from sensing studies that in the presence of excess of selective anions, deprotonation of the imidazole N-H proton occurs in all cases.

Ru(II) and Os(II) complexes based on terpyridyl-imidazole ligand rigidly linked to pyrene: synthesis, structure, photophysics, electrochemistry, and anion-sensing studies.

We report in this work a new family of bis-tridentate ruthenium(II) and osmium(II) complexes bearing a terpyridyl ligand rigidly link to pyrenyl-benzimidazole moiety (tpy-HImzPy = 10-(4-[2,2':6',2''-terpyridine]terpyridin-4'-yl-phenyl)-9H-9,11-diaza-cyclopenta[e]pyrene) along with other tridentate ligands such as 4'-(2-naphthyl)-2,2':6',2″-terpyridine (tpy-NaPh) and 2,6-bis(benzimidazole-2-yl)pyridine (H2pbbzim). All the complexes are thoroughly characterized by their elemental analysis, ESI mass spectrometry, and (1)H NMR spectroscopy. The molecular structures of two complexes [Ru(tpy-HImzPy)2](ClO4)2 (3) and [(pbbzim)Ru(tpy-HImzPy)] (2a) in the solid state were determined by X-ray crystallography. The absorption, steady-state, and time-resolved luminescence and electrochemical properties of all the four compounds have been studied. On excitation at their MLCT bands, all four compounds exhibit moderately strong room-temperature luminescence with lifetimes ranging between 3.8 and 161.1 ns in aerated condition, whereas in the deaerated (N2 purged) condition, the lifetimes vary between 8.2 and 199.1 ns, depending upon the nature of the solvents. The presence of imidazole N-H protons in all the complexes motivates us to study anion sensing properties of the complexes in solution through different channels. Spectrophotometeric, fluorometric, (1)H NMR spectroscopic, and cyclic voltammetric studies of the complexes in presence of anions reveal that the complexes sense principally F(-), CN(-), and to a lesser extent for AcO(-). Multichannel anion sensing studies also indicate that anion-induced deprotonation of the imidazole N-H protons occur in all four compounds. The equilibrium constant of this deprotonation steps have been estimated from UV-vis absorption and emission titration data. Anion-induced modulation of lifetimes makes all the four complexes suitable for lifetime-based sensors for selective anions.

Synthesis, structural characterization, and photophysical, spectroelectrochemical, and anion-sensing studies of heteroleptic ruthenium(II) complexes derived from 4'-polyaromatic-substituted terpyridine derivatives and 2,6-bis(benzimidazol-2-yl)pyridine.

Heteroleptic bis-tridentate ruthenium(II) complexes of composition [(H2pbbzim)Ru(tpy-Ar)](ClO4)2, where H2pbbzim = 2,6-bis(benzimidazol-2-yl)pyridine and tpy-Ar = 4'-substituted terpyridine ligands with Ar = phenyl (2), 2-naphthyl (3), 9-anthryl (4), and 1-pyrenyl (5) groups, have been synthesized and characterized by using standard analytical and spectroscopic techniques. The X-ray crystal structures of the complexes [(H2pbbzim)Ru(tpy-Naph)](ClO4)2 (3), [(pbbzim)Ru(tpy-Naph)]·(CH3)2CO·H2O (3a), and [(H2pbbzim)Ru(tpy-Py)](ClO4)2 (5) have been determined. The absorption, steady-state, and time-resolved luminescence spectral properties of the complexes were thoroughly investigated in dichloromethane. The compounds display strong luminescence at room temperature with lifetimes (τ2) in the range of 5.5-62 ns, depending upon the nature of the polycyclic aromatic moiety as well as the solvents. The complexes are found to undergo one reversible oxidation in the positive potential window (0 to +1.5 V) and four successive quasi-reversible reductions in the negative potential window (0 to -2.4 V). The anion-sensing properties of the receptors were thoroughly investigated in acetonitrile/dichloromethane (1/9 v/v) solutions (2 × 10(-5) M) using absorption, steady-state, and time-resolved emission spectroscopic studies. (1)H NMR titration experiments, on the other hand, were carried out in either CD3CN or DMSO-d6. The anion-sensing studies revealed that the receptors act as sensors for F(-), CN(-), AcO(-), and SO4(2-) and to some extent for HSO4(-) and H2PO4(-). It is evident that, in the presence of excess anions, deprotonation of the imidazole N-H fragments of the receptors occurs, which is signaled by the change of color from yellow-orange to violet visible with the naked eye. From the absorption and emission titration studies the binding/equilibrium constants of the receptors with the anions have also been determined. Anion-induced lifetime quenching and/or enhancement make the receptors suitable lifetime-based sensors for selective anions. Cyclic voltammetric (CV) measurements of the compounds carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses with F(-) and AcO(-) ions. Spectroelectrochemical studies have also been carried out for both the protonated and deprotonated forms of the complexes in the range of 300-1200 nm. With successive oxidation of the Ru(II) center, replacement of MLCT bands by LMCT bands occurs gradually with observation of sharp isosbestic points in all cases.

Photoinduced electron and energy transfer and pH-induced modulation of the photophysical properties in homo- and heterobimetallic complexes of ruthenium(II) and rhodium(III) based on a heteroditopic phenanthroline-terpyridine bridge.

Homo- and heterobimetallic complexes of compositions [(bpy)2Ru(II)(phen-Hbzim-tpy)Ru(II)(tpy/tpy-PhCH3/H2pbbzim)](4+) and [(bpy)2Ru(II)(phen-Hbzim-tpy)Rh(III)(tpy-PhCH3/H2pbbzim)](5+), where phen-Hbzim-tpy = 2-[4-(2,6-dipyridin-2-ylpyridin-4-yl)phenyl]-1H-imidazole[4,5-f][1,10]phenanthroline, bpy = 2,2'-bipyridine, tpy = 2,2':6',2"-terpyridine, tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2"-terpyridine, and H2pbbzim = 2,6-bis(benzimidazol-2-yl)pyridine, have been synthesized and characterized by elemental analyses, electrospray ionization mass spectrometry, and (1)H NMR spectroscopy. The absorption spectra, redox behavior, and luminescence properties of these bimetallic complexes have been thoroughly investigated and compared with those of monometallic [(bpy)2Ru(II)(phen-Hbzim-tpy)](2+) and [(tpy-PhCH3)Rh(III)(tpy-Hbzim-phen)](3+) model compounds. The electrochemistry of the complexes shows a reversible Ru(II/III) oxidation in the anodic region and an irreversible Rh(III/I) reduction and several ligand-based reductions in the cathodic region. Steady-state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer from the metal-to-ligand charge-transfer (MLCT) excited state of the [(bpy)2Ru(II)(phen-Hbzim-tpy)] chromophore to the MLCT state of the tpy-containing chromophore [(phen-Hbzim-tpy)Ru(II)(tpy/tpy-PhCH3/H2pbbzim)] occurs in all three unsymmetrical homobimetallic complexes. On the other hand, for both heterometallic dyads, an efficient intramolecular photoinduced electron transfer from the excited ruthenium moiety to the rhodium-based unit takes place. The rate constants for the energy- and electron-transfer processes have been determined by time-resolved emission spectroscopy. The influence of the pH on the absorption, steady-state, and time-resolved emission properties of complexes has been thoroughly investigated. The absorption titration data were used to determine the ground-state pK values, whereas the luminescence data were utilized for determination of the excited-state acid dissociation constants. In effect, deprotonation of the azole NH moieties of the complexes leads to a substantial lowering of the MLCT absorption and emission band energies.

Luminescent bis-tridentate ruthenium(II) and osmium(II) complexes based on terpyridyl-imidazole ligand: synthesis, structural characterization, photophysical, electrochemical, and solvent dependence studies.

Homo- and heteroleptic bis-tridentate ruthenium(II) and osmium(II) complexes of compositions [(tpy-PhCH(3))Ru(tpy-HImzPh(3))](ClO(4))(2) (1), [(H(2)pbbzim)Ru(tpy-HImzPh(3))] (ClO(4))(2) (2) and [M(tpy-HImzPh(3))(2)](ClO(4))(2) [M = Ru(II) (3) and Os(II) (4)], where tpy-PhCH(3) = p-methylphenyl terpyridine, H(2)pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-HImzPh(3) = 4'-[4-(4,5-diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2':6',2'']terpyridine, have been synthesized and characterized by using standard analytical and spectroscopic techniques. These compounds were designed to increase the room temperature excited-state lifetimes of bisterpyridine-type ruthenium(II) and osmium(II) complexes. The X-ray crystal structures of two homoleptic complexes 3 and 4 have been determined and show that both the compounds crystallized in orthorhombic form with space group Fddd. The photophysical and redox properties of the complexes have been thoroughly investigated. All the complexes display moderately strong luminescence at room temperature with lifetimes in the range of 6-35 ns. The complexes are found to undergo one reversible oxidation in the positive potential window (0 to +1.6 V) and one irreversible and two successive quasi-reversible reductions in the negative potential window (0 to -2.0 V). The influence of solvents on the photophysical properties of the complexes has also been investigated in detail.

A terpyridyl-imidazole (tpy-HImzPh3) based bifunctional receptor for multichannel detection of Fe2+ and F- ions.

The X-ray crystal structures of the tridentate ligand, 4'-[4-(4,5-diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2':6',2'']terpyridine (tpy-HImzPh(3)) and its bis-homoleptic iron(ii) complex of composition [Fe(tpy-HImzPh(3))(2)](2+) have been determined, showing that the ligand crystallized in a monoclinic form with the space group P2(1)/c while its Fe(II) complex crystallizes in an orthorhombic form with space group Fddd. Both the anion and cation binding properties of the receptor were thoroughly investigated in dimethylformamide-acetonitrile (1 : 9) solution using absorption, emission, and (1)H NMR spectral studies which revealed that the receptor acts as a sensor for both F(-) and Fe(2+). In the presence of excess F(-) ion, deprotonation of the imidazole N-H fragment of the receptor occurs, an event which is signaled by the development of a yellow color visible with the naked eye. The estimated value of the equilibrium constant of the receptor with F(-) is 1.9 × 10(4) M(-1). Deprotonation is also observed in the presence of hydroxide. The receptor also shows colorimetric and fluorimetric sensing ability towards Fe(2+) ions. The binding site for the metal ion in the system has been unambiguously established by single-crystal X-ray diffraction studies of the Fe(II) complex of the receptor. The influence of solvents on the absorption and fluorescence spectra of the receptor has been investigated in detail. Cyclic voltammetric (CV) and square wave voltammetric (SWV) measurements carried out in dimethylformamide-acetonitrile (2 : 3) provided evidence in favor of cation (Fe(2+)) and anion (F(-)) concentration dependent electrochemical responses, enabling the ligand to act as a suitable electrochemical sensor for F(-) and Fe(2+) ions.

Synthesis, structural characterization, and photophysical, electrochemical, intercomponent energy-transfer, and anion-sensing studies of imidazole 4,5-bis(benzimidazole)-bridged Os(II)Os(II) and Ru(II)Os(II) bipyridine complexes.

Homo- and heterobimetallic complexes of composition [(bpy)(2)M(II)(H(2)Imbzim)M'(II)(bpy)(2)](ClO(4))(3)·nH(2)O, where M(II) = M'(II) = Os (1), M(II) = Ru and M'(II) = Os (2), H(3)Imbzim = 4,5-bis(benzimidazole-2-yl)imidazole, and bpy = 2,2'-bipyridine, have been synthesized and characterized using standard analytical and spectroscopic techniques. Both of the complexes crystallized in monoclinic form with the space group P2(1)/m for 1 and P2(1)/n for 2. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated. The complexes display very intense, ligand-centered absorption bands in the UV region and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations. The strong fluorescence of free H(3)Imbzim is completely quenched in the metal complexes by energy transfer to the metal-based units, which exhibit their characteristic MLCT phosphorescence. The luminescence data of the heterometallic complex 2 show that electronic energy transfer takes place from the ruthenium center to the osmium-based component. The anion binding properties of the complexes have been studied in solutions using absorption, emission, and (1)H NMR spectral measurements. The metalloreceptors act as sensors for F(-) and AcO(-) ions. Sensing studies indicate the presence of two successive anion-induced deprotonation steps, leading to the formation of [(bpy)(2)M(HImbzim)M'(bpy)(2)](2+) and [(bpy)(2)M(Imbzim)M'(bpy)(2)](+) species. Double deprotonation is also observed in the presence of hydroxide. The binding affinities of different anions toward the receptors have been evaluated. Cyclic voltammetry measurements carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses of the bimetallic metalloreceptors with F(-) and AcO(-) ions.