Debasish Saha

Jadavpur University, Kolkata, West Bengal, India

Are you Debasish Saha?

Claim your profile

Publications (12)44.95 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A heterobimetallic complex of the composition [(bpy)(2)Ru(Imdc)Os(bpy)(2)](ClO4), where Imdc(3-) = imidazole-4,5-dicarboxylate(3-) ion and bpy = 2,2'-bipyridine, was synthesized and characterized using standard analytical and spectroscopic techniques, including X-ray crystallography. Steady state and time resolved luminescence data at room temperature showed that an efficient intramolecular electronic energy transfer takes place from the Ru center to the Os based component in the dyad. The cation binding properties of the complex were thoroughly investigated in solution using UV-Vis absorption, steady state and time-resolved emission spectroscopic techniques. It is interesting to note that the luminescence intensity, quantum yield as well as the lifetime of the complex were enhanced substantially in the presence of selective cations, showing cation-induced molecular switching behavior. The effect of different cations on the rate of photoinduced intramolecular energy transfer in the dyad was also explored. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies were utilized to investigate the structural and electronic properties of the dyad. The optimized geometrical parameters for the dyad, computed both in the gas phase and in solution, are reported and compared with the single crystal X-ray data. Computational (TD-DFT) studies also provide insight into the nature of the ground and excited states, with detailed assignments of the orbitals involved in the absorption and emission transitions. In particular, the blue-shifts of the absorption and emission bands in the presence of selective cations are reproduced by our calculations.
    Full-text · Article · Jul 2014 · Polyhedron
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A mixed-ligand bimetallic ruthenium(II) complex of composition [(bpy)2Ru(H2PyImPhen)Ru(bpy)2](ClO4)4 (1), where H2PyImPhen = 2,6-di(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)pyridine and bpy = 2,29-bipyridine has been synthesized and characterized using standard analytical and spectroscopic techniques. The X-ray crystal structure of its N–H deprotonated form (2) has been determined which showed that the compound crystallized in monoclinic form with the space group P21/c. The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the available X-ray data. The absorption spectra, redox behavior and luminescence properties of the complexes were thoroughly investigated. Complex 1 displays strong luminescence at room temperature with lifetimes in the range of 160–500 ns (aerated condition), depending upon the nature of the solvent. The complex is found to undergo one reversible oxidation in the positive potential window and three successive quasi-reversible reductions in the negative potential window. Both the anion and cation binding properties of 1 were thoroughly investigated in acetonitrile solution using absorption, steady state and time-resolved emission spectral studies. The anion sensing studies revealed that the receptor acts as a sensor for F2, AcO2 and H2PO4 2. It is evident that in the presence of excess F2 and AcO2 ion, deprotonation of the imidazole N–H fragments of 1 occurs. Anion-induced lifetime quenching by F2 and AcO2 and enhancement by H2PO4 2, makes the receptor 1 a suitable lifetime-based sensor for selective anions. Coordination of metal ions such as Mn2+, Co2+, Cu2+ to the secondary coordination sphere of 1 trigger emission quenching, while metal ions having a completely filled d orbital (such as Zn2+, Cd2+ or Hg2+) do not quench; rather some finite enhancement of the fluorescence intensity of the receptor occurs. Density functional theory (DFT) and time-dependent DFT (TD-DFT) study provides insight into the nature of the ground and the excited states involved in absorption and emission transitions. In particular, the changes in the absorption and emission spectral characteristics of 1 on interaction with anions and cations are also reproduced by our calculations.
    Full-text · Article · Jul 2013 · RSC Advances
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Experimental results coupled with computational studies were utilized to investigate the structural and electronic properties of mixed-ligand bimetallic ruthenium(II) and osmium(II) complexes of composition [(bpy)2M(Imdc)M(bpy)2](+) [M = Ru(II) (1) and M = Os(II) (2)], where H3Imdc = imidazole-4,5-dicarboxylic acid and bpy = 2,2'-bipyridine. The X-ray crystal structures of both the bimetallic complexes were determined which showed that compound 1 crystallizes in monoclinic form with space group P2(1)/c, while 2 is obtained in orthorhombic form with the space group Pca2(1). The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the single-crystal X-ray data. The absorption spectra, redox behaviors, and luminescence properties of the complexes were thoroughly investigated. The complexes display very intense, ligand-centered absorption bands in the UV and moderately intense MLCT bands in the visible regions. While the Ru(II) complex displays moderately strong luminescence, the corresponding Os(II) complex does not luminesce at room temperature. Both the bimetallic complexes show two successive one-electron reversible metal-centered oxidations. The effect of alkali, alkaline earth, and transition metal cations on the absorption and emission spectral behavior of the complexes has also been studied in detail. As compared to the luminescence intensities and the quantum yields of the free complexes, those of the complexes were enhanced substantially in the presence of selective cations showing cation-induced molecular switching behaviors. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies provide insight into the nature of the ground and excited states with resulting detailed assignments of the orbitals involved in absorption and emission transitions. In particular, the blue-shifts of the absorption and emission bands in the presence of cations are also reproduced by our calculations.
    Full-text · Article · Jun 2013 · Inorganic Chemistry
  • Shyamal Das · Debasish Saha · Sourav Mardanya · Sujoy Baitalik
    [Show abstract] [Hide abstract]
    ABSTRACT: Experimental results coupled with computational studies were utilized to investigate the structural and electronic properties of mixed-ligand monometallic osmium(ii) complexes of composition [(bipy)(2)Os(H(2)Imdc)](+) (), the N-H deprotonated form [(bipy)(2)Os(HImdc)] (), and the COOH deprotonated form [(bipy)(2)Os(Imdc)](-) (), where H(3)Imdc = imidazole-4,5-dicarboxylic acid and bipy = 2,2'-bipyridine. The X-ray crystal structures of [(bipy)(2)Os(H(2)Imdc)](+) () and [(bipy)(2)Os(HImdc)] () have been determined, which showed that compound crystallizes in a monoclinic form with the space group P2(1)/c, while is obtained in a triclinic form with the space group P1[combining macron]. The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the available X-ray data. The influence of pH on the photophysical and redox properties of the complexes has been thoroughly investigated. As compared to protonated complex (), which undergoes reversible oxidation at 0.50 V (vs. Ag/AgCl) in acetonitrile, the redox potential of the fully deprotonated complex () is shifted to a much lower value, 0.16 V. The proton-coupled redox activity of (+) has been studied over the pH range 2-12 in an acetonitrile-water (3 : 2) medium. From the pH versus E(1/2) profile, the equilibrium constants of the complex species in the protonated/deprotonated forms and the metal ion in +2/+3 oxidation states have been determined. Using these values the bond dissociation free energies for the imidazole N-H and COOH bonds have also been estimated. The pK(a) values for in the +2 state have also been determined spectrophotometrically. Substantial red shifts in the MLCT bands and the large shift in the E(1/2) value to a less positive potential that occur on deprotonation are energetically correlated. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies provide insight into the nature of the ground and excited states, with resulting detailed assignments of the orbitals involved in the absorption and emission transitions.
    No preview · Article · Aug 2012 · Dalton Transactions
  • Debasish Saha · Shyamal Das · Sourav Mardanya · Sujoy Baitalik
    [Show abstract] [Hide abstract]
    ABSTRACT: The X-ray crystal structure of a mixed-ligand bimetallic ruthenium(II) complex of composition [(bipy)(2)Ru(H(2)Impib)Ru(bipy)(2)](ClO(4))(4) (1), where H(2)Impib = 1,3-di(1H-imidazo[4,5-f][1,10]phenanthroline-2-yl)benzene and bipy = 2,2'-bipyridine, has been determined and showed that the compound crystallized in monoclinic form with the space group P2(1)/c. The absorption, steady state and time-resolved luminescence spectral properties of the complex were thoroughly investigated in different solvents. The compound displays strong luminescence at room temperature with lifetimes in the range of 140-470 ns, depending upon the nature of the solvent. Solvent-induced lifetime tuning makes the complex a suitable solvatochromic probe. The complex is found to undergo one simultaneous two-electron reversible oxidation in the positive potential window (0 to +1.6 V) and four quasi-reversible reductions in the negative potential window (0 to -2.2 V). Spectroelectrochemical studies have also been carried out for the bimetallic compound in the range of 300-1600 nm. With stepwise oxidation of the Ru(ii) centers replacement of MLCT bands by LMCT bands occur with the development of a broad band at λ(max) = 1260 nm, which is ascribed to inter-valence charge-transfer (IVCT) transition for the mixed-valence Ru(II)Ru(III) species. The anion sensing properties of the receptor were thoroughly investigated in acetonitrile solution using absorption, steady state and time-resolved emission spectroscopic studies. The anion sensing studies revealed that the receptor acts as sensor for F(-), AcO(-) and H(2)PO(4)(-). It is evident that in the presence of excess F(-) and AcO(-) ions, deprotonation of the imidazole N-H fragments of the receptor occurs, an event which is signaled by the change of color from yellow to orange visible to the naked eye. From the absorption and emission titration studies the binding/equilibrium constants of the receptor with the anions have also been determined. Anion-induced lifetime quenching by F(-) and AcO(-) and enhancement by H(2)PO(4)(-) makes the receptor a suitable lifetime-based sensor for selective anions. Cyclic voltammetry (CV) measurements of the compound carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses with F(-) and AcO(-) ions.
    No preview · Article · Jun 2012 · Dalton Transactions
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Combined experimental and DFT-TD-DFT computational studies were utilized to investigate the structural and electronic properties of mixed-ligand monometallic ruthenium(II) complexes of compositions [(bpy)(2)Ru(H(2)Imdc)](+) (1(+)), its N-H deprotonated form [(bpy)(2)Ru(HImdc)] (1), and COOH deprotonated form [(bpy)(2)Ru(Imdc)](-) (1(-)), where H(3)Imdc = imidazole-4,5-dicarboxylic acid and bpy = 2,2'-bipyridine. The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the previously reported X-ray data. The influence of pH on the absorption, emission, and redox properties of [(bpy)(2)Ru(H(2)Imdc)](+) (1(+)) has been thoroughly investigated. The absorption titration data were used to determine the ground state pK values, whereas the luminescence data were utilized for the determination of excited state acid dissociation constants. The proton-coupled redox activity of 1(+) has been studied over the pH range 2-12 in acetonitrile-water (3:2). From the E(1/2) versus pH profile, the equilibrium constants of the variously deprotonated complex species in Ru(II) and Ru(III) oxidation states have been determined. As compared to the protonated complex (1(+)), which undergoes reversible oxidation at 0.96 V (vs Ag/AgCl) in acetonitrile, the redox potential of the fully deprotonated complex (1(-)) is shifted to a much lower value, viz., 0.52 V. Density functional theory (DFT) and time-dependent DFT (TD-DFT) study provides insight into the nature of the ground and excited states with resulting detailed assignments of the orbitals involved in absorption and emission transitions. In particular, the red-shifts of the absorption and emission bands and the cathodic shift in the oxidation potential of 1(+) compared to 1 and 1(-) are also reproduced by our calculations.
    Full-text · Article · May 2012 · The Journal of Physical Chemistry A
  • [Show abstract] [Hide abstract]
    ABSTRACT: A series of hetero- and homoleptic tridentate ruthenium(II) and osmium(II) complexes of compositions [(tpy-PhCH(3))Ru(tpy-HImzphen)](ClO(4))(2) (1), [(H(2)pbbzim)Ru(tpy-HImzphen)](ClO(4))(2) (2), and [M(tpy-HImzphen)(2)](ClO(4))(2) [M = Ru(II) (3) and Os(II) (4)], where tpy-PhCH(3) = 4'-(4-methylphenyl)-2,2':6',2"-terpyridine, H(2)pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-HImzphen = 2-(4-[2,2':6',2"]terpyridine-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole, have been synthesized and characterized by using standard analytical and spectroscopic techniques. X-ray crystal structures of three complexes 2, 3, and 4 have been determined. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated. All of the complexes display moderately strong luminescence at room temperature with lifetimes in the range of 10-55 ns. The effect of solvents on the absorption and emission spectral behavior of the complexes has also been studied in detail. The anion sensing properties of all the complexes have been studied in solution using absorption, emission, and (1)H NMR spectral studies and by cyclic voltammetric (CV) measurements. It has been observed that the complexes 1, 3, and 4 act as sensors for F(-)only, whereas 2 acts as sensor for F(-), AcO(-), and to some extent for H(2)PO(4)(-). It is evident that in the presence of excess of anions deprotonation of the imidazole N-H fragment(s) occurs in all cases, an event which is signaled by the development of vivid colors visible with the naked eye. The receptor-anion binding/equilibrium constants have been evaluated.
    No preview · Article · Nov 2011 · Inorganic Chemistry
  • Debasish Saha · Shyamal Das · Dinesh Maity · Sujoy Baitalik
    [Show abstract] [Hide abstract]
    ABSTRACT: Mixed-ligand complex of composition [(bpy)2Os(H 3Imbzim)](ClO4)2·2H2O (1), where H3Imbzim = 4,5-bis(benzimidazole-2-yl)imidazole and bpy = 2,2′;-bipyridine, has been synthesized and characterized using standard analytical and spectroscopic techniques. The X-ray crystal structure of the complex has been determined which showed that it crystallized in triclinic form with the space group P-1. The anion binding properties of the complex has been thoroughly investigated in solution using absorption, emission, and 1H NMR spectral measurements. The metalloreceptor acts as sensor for F-, AcO-, and to some extent for H2PO 4- anions. Although at relatively lower concentration of anions the 1:1 H-bonded adduct is formed, in the presence of excess of anions deprotonation of the imidazole N-H fragments occurs which is accompanied by a distinct change of color. Double deprotonation is also observed in the presence of hydroxide. The binding affinities of different anions towards the receptors have been evaluated. Cyclic voltammetry measurements carried out in acetonitrile have provided evidence in favor of an anion-dependent electrochemical response of the receptor with F- and AcO- ions.
    No preview · Article · Sep 2011 · Indian journal of chemistry
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    No preview · Article · Jan 2011 · Inorganic Chemistry
  • [Show abstract] [Hide abstract]
    ABSTRACT: A series of heteroleptic tridentate ruthenium(II) complexes of composition [(H(2)pbbzim)Ru(tpy-X)](PF(6))(2) (1-7), where H(2)pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-X = 4'-substituted terpyridine ligands with X = H, p-methyl phenyl (PhCH(3)), p-bromomethylphenyl (PhCH(2)Br), p-dibromomethylphenyl (PhCHBr(2)), p-cyanomethylphenyl (PhCH(2)CN), p-triphenylphosphonium methylphenyl bromide (PhCH(2)PPh(3)Br), and 4'-phenylformyl (PhCHO) groups, has been synthesized and characterized by using standard analytical and spectroscopic techniques. These compounds were designed to increase the excited-state lifetime of ruthenium(II) bisterpyridine-type complexes. The X-ray crystal structure of a representative compound 2, which crystallized with monoclinic space group P2(1)/c, has been determined. The absorption spectra, redox behavior, and luminescence properties of the ruthenium(II) complexes have been thoroughly investigated. All of the complexes display moderately strong luminescence at room temperature with lifetimes in the range of 10-58 ns. Correlations have been obtained for the Hammett sigma(p) parameter with their MLCT emission energies, lifetimes, redox potentials, proton NMR chemical shifts, etc. The anion binding properties of all the complexes as well as the parent ligand H(2)pbbzim have been studied in acetonitrile using absorption, emission, and (1)H NMR spectral studies, and it has been observed that the metalloreceptors act as sensors for F(-), AcO(-), and to some extent H(2)PO(4)(-). At a relatively lower concentration of anions, a 1:1 H-bonded adduct is formed; however, in the presence of an excess of anions, stepwise deprotonation of the two benzimidazole N-H fragments occurs, an event which is signaled by the development of vivid colors visible with the naked eye. The receptor-anion binding constants have been evaluated. Cyclic voltammetric (CV) measurements carried out in acetonitrile-dimethylformamide (9:1) provided evidence in favor of anion (F(-), AcO(-)) concentration dependent electrochemical responses, enabling 1 - 7 to act as suitable electrochemical sensors for F(-) and AcO(-) ions.
    No preview · Article · Jun 2010 · Inorganic Chemistry
  • [Show abstract] [Hide abstract]
    ABSTRACT: Mixed-ligand monometallic ruthenium(II) and osmium(II) complexes of composition [(bipy)(2)M(H(2)Imdc)](ClO(4)), where H(3)Imdc = imidazole-4,5-dicarboxylic acid and bipy = 2,2'-bipyridine, have been synthesized and characterized using standard analytical and spectroscopic techniques. The X-ray crystal structures of the ruthenium(II) complexes, [(bipy)(2)Ru(H(2)Imdc)](ClO(4)) (1) and its corresponding N-H deprotonated form [(bipy)(2)Ru(HImdc)] (3) have been determined. Compound 1 crystallizes in monoclinic form with space group P2(1)/c while 3 is obtained in triclinic form with the space group P1. The anion binding properties of the complexes 1 and 2 have been thoroughly investigated in acetonitrile solution using absorption, emission, and (1)H NMR spectral measurements. Both of the metalloreceptors act as sensors for F(-), AcO(-) and H(2)PO(4)(-) anions. Although at relatively lower concentration of anions, the 1 : 1 H-bonded adduct is formed, in the presence of excess of anions, deprotonation of the imidazole N-H fragment occurs and is accompanied by distinct change of colour. Cyclic voltammetry (CV) measurements carried out in acetonitrile have provided evidence in favour of anion-dependent electrochemical responses of 1 and 2 with F(-) and AcO(-) ions.
    No preview · Article · May 2010 · Dalton Transactions
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ruthenium(II) complexes of compositions [(bpy)2Ru(H3Imbzim)]2+ and [(bpy)2Ru(H2Imbzim)Ru(bpy)2]3+, where H3Imbzim = 4,5-bis(benzimidazol-2-yl)imidazole and bpy = 2,2′-bipyridine, can act as colorimetric sensors for F−, for AcO−, and, to some extent, for H2PO4− ions in solution. The anion binding affinities of the metalloreceptors were also evaluated by monitoring their respective UV−vis and steady-state emission properties, 1H NMR chemical shifts, and redox potentials, as a function of the anion concentration.
    No preview · Article · Mar 2010 · Inorganic Chemistry