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Dive into the research topics where Vikas S. Padalkar is active.

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Featured researches published by Vikas S. Padalkar.


RSC Advances | 2015

Highly emissive excited-state intramolecular proton transfer (ESIPT) inspired 2-(2′-hydroxy)benzothiazole–fluorene motifs: spectroscopic and photophysical properties investigation

Vikas S. Padalkar; Daisuke Sakamaki; Norimitsu Tohnai; Tomoyuki Akutagawa; Ken-ichi Sakai; Shu Seki

Tuning or switching of the solid state luminescence of organic materials is an attractive target for both basic research and practical applications. In the present study, solid state emissive compounds with very high quantum efficiencies (ΦF up to 68%) were achieved by chemical alteration of the excited state intramolecular proton transfer (ESIPT) 2-2′-hydroxy benzothiazole (HBT) unit. Five ESIPT inspired compounds based on fluorene were synthesized via Suzuki coupling reaction. Their photophysical properties were studied by means of steady state absorption, emission spectra and a time resolved emission method in solid as well as in solution of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region with large Stokes shift (∼232 nm). Efficient yellow emissive compounds showed very high quantum yields (ΦF = 55–68%) in the solid state, which are the highest quantum yields in the solid state to the best of our knowledge, for fluorene based ESIPT molecules. The fluorescence lifetime in the solid state is between 3.48–5.21 ns, while it is 5–10 fold less in chloroform (0.52–0.75 ns) solution. The optical properties of these compounds are sensitive towards the polarity of the medium. The structural properties, such as X-ray single crystal analyses, DSC and TGA were studied, and the lack of stacking and/or hydrogen bonding interactions around HBT motifs reveals enough room for ESIPT in the series of molecules even in their solid state. The DFT computations were performed to support experimental results and the calculations are well in line with the experimental results. These suggest high quantum efficiency ascribed to the large orbital energy difference between HOMOs and LUMOs of enol and keto forms transformed via ESIPT, and hence, singlet energy localization onto the keto form. The intra-molecular charge transfer nature between fluorene and HBT units plays a key role for the localization of energy on HBT motifs in their excited states.


RSC Advances | 2016

AIE active triphenylamine–benzothiazole based motifs: ESIPT or ICT emission

Vikas S. Padalkar; Daisuke Sakamaki; Kenji Kuwada; Norimitsu Tohnai; Tomoyuki Akutagawa; Ken-ichi Sakai; Shu Seki

Two novel donor–π bridge–acceptor compounds containing excited state intramolecular proton transfer (ESIPT) and non-ESIPT units based on triphenylamine–benzothiazole were synthesized via Suzuki coupling reaction. Their photophysical properties were studied in the solid state as well as in solutions with solvents of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region in both solutions and solid state. The optical properties of these compounds are highly dependent on solvent polarity. Significant positive solvatochromism (∼30 nm absorption and ∼80 nm emission red shift in polar solvents) were observed for both the compounds. Large Stokes shift (∼15 000 cm−1), polarity sensitive optical properties and very high quantum efficiencies (∼90%) in solvents and the solid state are the striking features of the synthesized compounds. Intramolecular charge transfer (ICT) characteristics of the compounds were supported experimentally and computationally. Halochromism and the intermolecular charge transfer phenomenon were used for investigation of ESIPT emission for compound 9 over ICT emission.


Langmuir | 2015

Fabrication of Thermoresponsive Nanoactinia Tentacles by a Single Particle Nanofabrication Technique.

Masaaki Omichi; Hiromi Marui; Vikas S. Padalkar; Akifumi Horio; Satoshi Tsukuda; Masaki Sugimoto; Shu Seki

Nanowires that are retractable by external stimulus are the key to fabrication of nanomachines that mimick actinia tentacles in nature. A single particle nanofabrication technique (SPNT) was applied over a large area to the fabrication of retractable nanowires (nanoactinia tentacles) composed of poly(N-isopropylacrylamide) (PNIPAM) and poly(vinylpyrrolidone) (PVP), which are thermoresponsive and hydrophilic polymers. The nanowires were transformed with increasing temperature from rod-like- to globule-forms with gyration radii of ∼1.5 and ∼0.7 μm, respectively. The transformation of the nanowires was reversible and reproducible under repeated cycles of heating and cooling. The reversible transformation was driven by hydration and dehydration of PNIPAM, the thermoresponsive segments, resulting in coil-to-globule transformation of the segments. The nanoactinia tentacle systems trapped the nanoparticles as a model of living cells under thermal stimulation, and the trapping was controlled by temperature. We present herein a unique nanomachine system which can be applicable to nanoparticle filtering/sensing systems and expandable to large-area functionalization and demonstrate polymer-based nanoactuators via scaling of molecular level coil-to-globule transformation into micron-sizes.


Journal of Physical Chemistry B | 2017

Optical and Structural Properties of ESIPT Inspired HBT–Fluorene Molecular Aggregates and Liquid Crystals

Vikas S. Padalkar; Yusuke Tsutsui; Tsuneaki Sakurai; Daisuke Sakamaki; Norimitsu Tohnai; Kenichi Kato; Masaki Takata; Tomoyuki Akutagawa; Ken-ichi Sakai; Shu Seki

In bulk materials, positional isomers not only help in understanding how slight difference in molecular structure alters the crystal packing and optical properties, but also play a key role in developing new type of materials for functional applications. A detailed study on the photophysical properties of fluorene-HBT positional isomers in solution and in the solid state providing a molecular level understanding of the factors which influence fluorescence behavior is reported. Two molecules Ia and IIa were synthesized by Suzuki coupling reaction and their photophysical properties were compared to positional isomers Ib and IIb. Crystal structure analyses and density functional theory (DFT) computation studies were performed to understand structure-properties relation and the results reveal that changing substitution pattern has a marked influence on their packing modes and luminescence properties. Strong noncovalent interactions (π-π) in the solid state hamper the excited state intramolecular proton transfer (ESIPT) process which causes fluorescence quenching in the solid state (Ia and IIa = Φf, 28-40%; Ib and IIb = Φf, 55-67%). Compounds show solvent-responsive and aggregation induced emission (AIE) properties. Bent structures of Ia with double and symmetric substitution of ESIPT motifs exhibit particularly unique condensed phase upon heating, confirmed as a nematic liquid crystalline phase, and this is the first report on the ESIPT and AIE active liquid crystalline materials with a banana-shaped molecule.


Chemical Society Reviews | 2016

Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters

Vikas S. Padalkar; Shu Seki


ChemistrySelect | 2017

AIE Active Carbazole-Benzothiazole Based ESIPT Motifs: Positional Isomers Directing the Optical and Electronic Properties

Vikas S. Padalkar; Kenji Kuwada; Daisuke Sakamaki; Norimitsu Tohnai; Tomoyuki Akutagawa; Ken-ichi Sakai; Tsuneaki Sakurai; Shu Seki


Asian Journal of Organic Chemistry | 2016

π–π Interactions: Influence on Molecular Packing and Solid-State Emission of ESIPT and non-ESIPT Motifs

Vikas S. Padalkar; Daisuke Sakamaki; Kenji Kuwada; Akifumi Horio; Haruka Okamoto; Norimitsu Tohnai; Tomoyuki Akutagawa; Ken-ichi Sakai; Shu Seki


ACS Sensors | 2016

Fabrication of “Clickable” Polyfluorene Nanowires with High Aspect Ratio as Biological Sensing Platforms

Tuchinda Wasin; Kazuyuki Enomoto; Tsuneaki Sakurai; Vikas S. Padalkar; Hoi Lok Cheng; Michael T. Tang; Akifumi Horio; Daisuke Sakamaki; Masaaki Omichi; Akinori Saeki; Kazuya Kikuchi; Yuichiro Hori; Atsuya Chiba; Yuichi Saito; Tomihiro Kamiya; Masaki Sugimoto; Shu Seki


Journal of Polymer Science Part B | 2016

Anisotropic swelling of hydrogel nanowires based on poly(vinylpyrrolidone) fabricated by single-particle nanofabrication technique

Satoshi Tsukuda; Masaaki Omichi; Masaki Sugimoto; Akira Idesaki; Vikas S. Padalkar; Shu Seki


Journal of Photopolymer Science and Technology | 2016

Fabrication of Fluorescent Nanowires via High-Energy Particles-Triggered Polymerization Reactions

Akifumi Horio; Tsuneaki Sakurai; Vikas S. Padalkar; Daisuke Sakamaki; Tetsuya Yamaki; Masaki Sugimoto; Shu Seki

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Ken-ichi Sakai

Chitose Institute of Science and Technology

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Masaki Sugimoto

Japan Atomic Energy Agency

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