BITS Faculty Publications

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Subject: search.filters.subject.Charge transfer

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    The study of the electronic structures and properties of pure and transition metal-doped silicon nanoclusters: a density functional theory approach
    (Taylor & Francis, 2009-03) Bandyopadhyay, Debashis
    This report presents the study of ab initio electronic structure and properties of pure and transition metal (TM = Ti, Zr and Hf)-doped silicon clusters, TM@Si(n), by using density functional theory with a polarised basis set (LanL2DZ) within the spin-polarised generalised gradient approximation for different values of n varying from 8 to 20. As the first step of the study, different optimised geometries of pure and doped clusters are calculated. These optimised clusters are then used to calculate different structural and physical parameters of the clusters, like binding energy, Highest Occupied Molecular Orbital – Lowest Unoccupied Molecular Orbital (HOMO–LUMO) gap, charge transfer, etc. In order to check the stability of the clusters, the second-order difference in the energy of the optimised structures is calculated. To study the optical behaviour of the clusters, infrared and Raman spectra are also calculated. Further calculations are also done on cation and anion clusters of both pure and doped nanoclusters to obtain their ionisation potential, electron affinity and chemical potential. An effort has been made to correlate the variation of different calculated parameters with the size of the clusters to explain the real existence and stabilities of different TM-doped clusters.
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    Superior Grafting and State-of-the-Art Interfacial Electron Transfer Rates for Newly Designed Geminal Dicarboxylate Bound Ruthenium(II)– and Osmium(II)–Polypyridyl Dyes on TiO2 Nanosurface
    (ACS, 2014-02-04) Banerjee, Tanmay
    Two new Ru(II)–/Os(II)–polypyridyl based sensitizer dyes with geminal dicarboxylic acid group as the binding unit for superior grafting of the dye to TiO2 have been designed and synthesized. Steady-state photochemical studies of the two sensitizer dyes in presence of TiO2 in water confirm strong binding of the dyes to TiO2. Femtosecond transient absorption studies of these newly synthesized dyes on TiO2 nanosurface have been carried out in water and the results have been compared with those for the corresponding 4,4′-dicarboxy-2,2′-bipyridine analogues of the dyes. While the charge recombination rates are considerably slower, interestingly, the electron injection rates are very fast for multiple saturated C–C linkages present between the chromophoric core and the anchoring moiety. The origin and the consequences of such profound effects on the ultrafast interfacial dynamics are discussed. This is the first report on the ultrafast transient absorption studies of dyes with geminal dicarboxylic acid binding groups, which we believe will add significantly to the present research efforts toward the development of robust and efficient dyes for use in dye solar applications.
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    Ultrafast Electron Injection, Hole Transfer, and Charge Recombination Dynamics in CdSe QD Super-Sensitized Re(I)–Polypyridyl Complexes with Catechol and Resorcinol Moiety: Effect of Coupling
    (ACS, 2015-01-26) Banerjee, Tanmay
    Ultrafast charge-transfer dynamics have been demonstrated in CdSe quantum dots (QD) using two Re(I)–polypyridyl complexes having pendent catechol (Re1,2) and resorcinol (Re1,3) as the sensitizer molecules. The energy level diagram of CdSe QD and Re1,2 and Re1,3 sensitizer reveals that photoexcited hole of CdSe QD can be transferred to both Re1,2 and Re1,3 molecule, and photoexcited Re1,2 and Re1,3 can inject electron in the conduction band, which has been confirmed by steady-state and time-resolved photoluminescence studies with selective photoexcitation. Femtosecond transient absorption studies have been carried out to monitor charge-transfer dynamics in early time scale. Transient absorption spectra show formation of cation radicals for both Re1,2 and Re1,3 in the 550–650 nm region with a peak at 590 nm region and broad absorption in the 650–1000 nm region, which can be attributed to photoexcited electron in the conduction band of CdSe QD. Charge recombination was determined by monitoring the decay of cation radicals as well as decay of an electron and found to be slower in the Re1,3/CdSe system as compared to that of the Re1,2/CdSe system, which is due to weaker electronic coupling in the former system.
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    Single-Site Photocatalytic H2 Evolution from Covalent Organic Frameworks with Molecular Cobaloxime Co-Catalysts
    (ACS, 2017-10-12) Banerjee, Tanmay
    We demonstrate photocatalytic hydrogen evolution using COF photosensitizers with molecular proton reduction catalysts for the first time. With azine-linked N2-COF photosensitizer, chloro(pyridine)cobaloxime co-catalyst, and TEOA donor, H2 evolution rate of 782 μmol h–1 g–1 and TON of 54.4 has been obtained in a water/acetonitrile mixture. PXRD, solid-state spectroscopy, EM analysis, and quantum-chemical calculations suggest an outer sphere electron transfer from the COF to the co-catalyst which subsequently follows a monometallic pathway of H2 generation from the CoIII-hydride and/or CoII-hydride species.
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    H2 Evolution with Covalent Organic Framework Photocatalysts
    (ACS, 2018-01-05) Banerjee, Tanmay
    Covalent organic frameworks (COFs) are a new class of crystalline organic polymers that have garnered significant recent attention as highly promising H2 evolution photocatalysts. This Perspective discusses the advances in this field of energy research while highlighting the underlying peremptory factors for the rational design of readily tunable COF photoabsorber–cocatalyst systems for optimal photocatalytic performance.
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    Material Profiling for Photocrystallography: Relating Single-Crystal Photophysical and Structural Properties of Luminescent Bis-Cyclometalated Iridium-Based Complexes
    (ACS, 2013-03) Laskar, Inamur Rahaman
    The photophysical properties of seven luminescent iridium complexes are characterized in their single-crystal form, and the photoactivity is related to their molecular structures. Specifically, solid-state optical emission spectra and associated lifetimes are determined from single crystals of iridium complexes containing three bidentate ligands: two variously substituted 2-phenylbenzothiazoles and either a 2,4-pentadione (acetylacetone) or 2-pyridinecarboxylic (picolinic) acid. All complexes studied exhibit emissive behavior in the solid-state which originates from 3π–π* and metal-to-ligand-charge-transfer (MLCT) electronic transitions; this is supported by density functional theory. Phosphorescence is observed in all cases with microsecond lifetimes, ranging from 0.30 to 2.4 μs at 298 K and 1.4–4.0 μs at 100 K. Structure–property relationships are established which are relevant to the potential solid-state application of this series of luminescent complexes as organic light emitting diodes (OLED) material components. In addition, these materials are assessed for their suitability to time-resolved pump–probe photocrystallography experiments, which will reveal their photoexcited state structure. Accordingly, the design process by which materials are selected and technical parameters are defined for a photocrystallography experiment is illustrated. This family of complexes presents a case study for this photocrystallography material profiling. Results show that the time-resolved photoexcited state structure, featuring the MLCT transition is, in principle at least, viable for two of these complexes.
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    Effect of Geometrical Distortion on the Electronic Structure: Synthesis and Characterization of Monoradical-Coordinated Mononuclear Cu(II) Complexes
    (ACS, 2016) Roy, Ram Kinkar
    Ligand H3SamiMixed(tBu) was composed of two different compartments, a redox-active 2-aminophenol and a salen salicylidene. Both compartments were linked via a benzyl linker. The ligand reacted with CuCl2·2H2O under air in the presence of Et3N and provided the corresponding monoradical-coordinated mononuclear Cu(II) complex (1). Complex 1, in solution, reacted with air and provided complex 2 via ligand-centered oxygenation at the benzyl-CH2 position. Both complexes were characterized via IR, mass spectrometry, X-ray single-crystal diffraction, variable-temperature magnetic susceptibility, cyclic voltammograms (CVs), and UV–vis/NIR spectroscopic techniques. X-ray crystallographic analyses clearly showed almost equally distorted square planar geometry around the Cu(II) atom in both complexes. However, the bending of the radical-containing C6 ring compared to the N1–Cu1–O1 plane was different in both complexes. While complex 1 was paramagnetic and showed a ferromagnetic coupling between the dx2–y2 magnetic orbital of Cu(II) ion and the pz orbital of coordinated π-radical, complex 2 was diamagnetic by experiencing a strong antiferromagnetic coupling between the two magnetic orbitals. UV–vis/NIR spectra of the complexes were dominated by charge-transfer transitions. CVs of the complexes showed two reversible one-electron oxidations and one reversible one-electron reduction. E1/2ox2 and E1/2red1 potentials were different in both complexes, while E1/2ox1 values were almost the same and the process corresponded to the formation of phenoxyl radical. Theoretical studies were also performed to understand the magnetic coupling phenomena, and TD-DFT calculations were employed for the assignment of charge-transfer absorption bands.