BITS Faculty Publications
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Item Contrasting Solid-State Fluorescence of Diynes with Small and Large Aryl Substituents: Crystal Packing Dependence and Stimuli-Responsive Fluorescence Switching(ACS, 2015-09) Pati, Avik K.There has been a significant current interest in solid state luminescence of organic molecules and their stimuli responsive fluorescence switching behavior. Although small organic derivatives with olefinic, acetylenic, phenylenevinylenic, phenyleneethynylenic spacers are widely documented as solid state emitters in the literature, the solid state photophysics of organic derivatives with “butadiyne” spacer still remains unexplored. We provide detailed investigation on the solid state fluorescence properties of a series of butadiynyl fluorophores. Replacement of a phenyl ring, which is at periphery of the butadiyne bridge, with a large moiety such as pyrenyl group furnishes contrasting emissions in the solid state. While the butadiyne bridged phenyl derivatives show a blue shift of emission maxima in the solid powder with respect to monomer spectra in solution state, the butadiyne bridged pyrenyl derivatives exhibit a red shift in the solid state. The blue shift of the emission maxima of the butadiyne bridged phenyl derivatives in the solid powder is attributed to allowed excitonic transition in aggregates with nearly parallel transition dipoles. On the other hand, formation of pyrenyl excimer accounts for the red shift of the butadiyne bridged pyrenyl derivatives in the solid powder. In addition to that, the solid state fluorescence of the pyrenyl analogues is reversibly switched between two aggregate forms through external heating and rubbing stimuli.Item Photophysics of Diphenylbutadiynes in Water, Acetonitrile–Water, and Acetonitrile Solvent Systems: Application to Single Component White Light Emission(ACS, 2016-07) Pati, Avik K.Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer, and intramolecular charge transfer (ICT) states in acetonitrile–water solvent systems. The LE, excimer, and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (−NMe2)–pull (−CN) substituents and those are closely overlapped for non-push–pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In the case of the push (−NMe2)–pull (−CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push–pull diphenylbutadiyne together cover the visible region (400–700 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push–pull character.Item White Light Emission in Butadiyne Bridged Pyrene–Phenyl Hybrid Fluorophore: Understanding the Photophysical Importance of Diyne Spacer and Utilizing the Excited-State Photophysics for Vapor Detection(ACS, 2016-07) Pati, Avik K.Generation of white light emission (WLE) from a single organic fluorophore is challenging because of the need to get fluorescence covering the visible region (400–700 nm) upon excitation of the dye at near-ultraviolet wavelength. Herein, we report WLE from a butadiyne bridged pyrene–phenyl hybrid fluorophore in mixed-aqueous solvents as well as in polymer film matrices. The ability of the butadiynyl dye to emit from multiple excited states such as locally excited (LE; 400–500 nm), aggregate (excimer type; 475–600 nm), and charge transfer (CT; 500–750 nm) states spanning the emission almost throughout the visible range has made the generation of the white light to be possible. In highly polar solvent such as acetonitrile, the butadiynyl dye emits from the LE and CT states, and the WLE is achieved through a control of the dye concentration such that intermolecular CT (exciplex type) contributes along with the intramolecular CT and LE emissions. In mixed-aqueous systems such as water–acetonitrile and water–N,N-dimethylformamide, the CT emission is red-shifted (because of the high dielctric constant of water), and the contribution of the aggregate emission (originated because of the poor solvent water) is important in maintaining the relative distribution of the fluorescence intensities (LE, excimer, and CT) in the entire visible region. The significance of the diyne spacer in achieving the WLE is delineated through a control study with a single acetylenic analogue. The LE, aggregate, and CT emissions are involved in generating bluish-white light in a poly(vinyl alcohol) film matrix of the butadiynyl dye. Blue emission is noted in a poly(methyl methacrylate) (PMMA) film matrix of the dye with a major contribution from the LE and a minor contribution from the aggregate state. Exposure of the PMMA film of the dye to polar aprotic vapors assists in gaining the CT state emission such that the LE, aggregate, CT emissions cover the entire visible region to produce the WLE. This opens a new strategy for selective vapor sensing.