Subrata Sinha

Bio: Subrata Sinha is an academic researcher from Visva-Bharati University. The author has contributed to research in topics: Excited state & Electron transfer. The author has an hindex of 14, co-authored 40 publications receiving 759 citations. Previous affiliations of Subrata Sinha include National Institute of Advanced Industrial Science and Technology & Indian Association for the Cultivation of Science.

Origin of the stabilization energy of perylene excimer as studied by fluorescence and near-IR transient absorption spectroscopy

Abstract: The origin of the stabilization energy of perylene excimer in toluene has been studied. In order to measure the fluorescence spectra and its decay profiles under the condition free from the reabsorption effect, we designed a thin optical cell with a path length of about 10 μm. From the results, the photophysical parameters of perylene excimer such as the formation rate constant, the peak position and the lifetime and the quantum yield of fluorescence can be obtained. Using these values, the binding energy of perylene excimer can be evaluated as 0.44 eV from the temperature dependence of the transient absorption. By analyzing the band shape of the charge transfer absorption in near-IR wavelength range, the transfer integral V between a neutral excited state and an ion-pair state can be estimated as 0.37 eV. From these results, we conclude that the stabilization energy of perylene excimer consists of two components: exciton interaction (70%) and charge transfer interaction (30%). We also discuss the origin of the stabilization energy of other aromatic excimers on the basis of the results on perylene excimer.

Excited-state processes in the carotenoid zeaxanthin after excess energy excitation.

Abstract: Aiming for better understanding of the large complexity of excited-state processes in carotenoids, we have studied the excitation wavelength dependence of the relaxation dynamics in the carotenoid zeaxanthin. Excitation into the lowest vibrational band of the S2 state at 485 nm, into the 0−3 vibrational band of the S2 state at 400 nm, and into the 2Bu+ state at 266 nm resulted in different relaxation patterns. While excitation at 485 nm produces the known four-state scheme (S2 → hot S1 → S1 → S0), excess energy excitation led to additional dynamics occurring with a time constant of 2.8 ps (400 nm excitation) and 4.9 ps (266 nm excitation), respectively. This process is ascribed to a conformational relaxation of conformers generated by the excess energy excitation. The zeaxanthin S* state was observed regardless of the excitation wavelength, but its population increased after 400 and 266 nm excitation, suggesting that conformers generated by the excess energy excitation are important for directing the popu...

Synthesis and electron transfer studies of ruthenium-terpyridine-based dyads attached to nanostructured TiO2.

Abstract: A series of bis(terpyridine)Ru-II complexes have been prepared, where one of the terpyridines is functionalized in the 4'-position by a phosphonic or carboxylic acid group for attachment to TiO2. The other is functionalized, also in the 4'-position, by a potential electron donor. In complexes 1a, 3a, and 4a,b, this donor is tyrosine or hydrogen-bonded tyrosine, while in 2a it is carotenoic amide. The synthesis and photophysical properties of the complexes are discussed. On irradiation with visible light, the formation of a long-lived charge-separated state was anticipated, via primary electron ejection into the TiO2, followed by secondary electron transfer from the donor to the photogenerated Ru-III. However, such a charge-separated state could be observed with certainty only with complex 2a. To explain the result, quantum chemical calculations were performed on the different types of complexes.

Electrophosphorescence and delayed electroluminescence from pristine polyfluorene thin-film devices at low temperature.

Abstract: Intrinsic long-lived electrophosphorescence and delayed electroluminescence from a conjugated polymer (polyfluorene) thin film is observed for the first time at low temperature. From bias offset voltage dependent measurements, it is concluded that the delayed fluorescence is generated via triplet-triplet annihilation. A fast and efficient triplet exciton quenching by charge carriers is found to occur in the active polymer layer of the working devices.

Population and decay of keto states in conjugated polymers

Abstract: Using time-resolved and steady-state photoluminescence techniques, fluorene/fluorenone copolymers have been studied to investigate the role of keto defects in degraded polyfluorene. Keto sites can be populated via migration from polyfluorene singlets, thereby quenching the polyfluorene fluorescence, and via direct photon absorption. In the former case, the migration process dominates all thermal and interchain variability in the efficiency of quenching. No annihilation process of fluorenone triplets and no interchain processes such as excimer formation participate in the defect emission itself.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Abstract: A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

Abstract: A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).

Recent advances in white organic light-emitting materials and devices (WOLEDs).

Abstract: WOLEDs offer new design opportunities in practical solid-state lighting and could play a significant role in reducing global energy consumption. Obtaining white light from organic LEDs is a considerable challenge. Alongside the development of new materials with improved color stability and balanced charge transport properties, major issues involve the fabrication of large-area devices and the development of low-cost manufacturing technology. This Review will describe the types of materials (small molecules and polymers) that have been used to fabricate WOLEDs. A range of device architectures are presented and appraised.

Photoluminescence and Conductivity of Self-Assembled π–π Stacks of Perylene Bisimide Dyes

Abstract: The self-assembly of a new, highly fluorescent perylene bisimide dye 2 into pi stacks, both in solution and condensed phase, has been studied in detail by NMR spectroscopy, vapor pressure osmometry (VPO), UV/Vis and fluorescence spectroscopy, differential scanning calorimetry (DSC), optical polarizing microscopy (OPM) and X-ray diffraction. The NMR and VPO measurements revealed the formation of extended pi-pi stacks of the dye molecules in solution. The aggregate size determined from VPO and DOSY NMR measurements agree well with that obtained from the concentration and temperature-dependent UV/Vis spectral data by employing the isodesmic model (equal K model). In the condensed state, dye 2 possesses a hexagonal columnar liquid crystalline (LC) phase as confirmed by X-ray diffraction analysis. The columnar stacking of this dye has been further explored by atomic force microscopy (AFM). Well-resolved columnar nanostructures of the compound are observed on graphite surface. A color-tunable luminescence from green to red has been observed upon aggregation which is accompanied by an increase of the fluorescence lifetime and depolarization. The observed absorption properties can be explained in terms of molecular exciton theory. The charge transport properties of dye 2 have been investigated by pulse radiolysis-time resolved microwave conductivity measurements and a 1D charge carrier mobility up to 0.42 cm(2) V(-1) s(-1) is obtained. Considering the promising self-assembly, semiconducting, and luminescence properties of this dye, it might serve as a useful functional material for nano(opto)electronics.