Department of Chemistry
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Item Deciphering the Photophysical Role of Conjugated Diyne in Butadiynyl Fluorophores: Synthesis, Photophysical and Theoretical Study(ACS, 2013-07) Pati, Avik K.The present work focuses on the current interest in diyne bridged chromophores necessitating a clearer understanding of the photophysics of such molecules. The significance of the diyne moiety in the photophysics has been investigated by synthesizing simple substituted diphenyl butadiynyl derivatives following a quick and efficient microwave assisted Eglinton coupling of terminal alkynes. Emission of the fluorophores is observed from the usual locally excited (LE) state and intramolecular charge transfer (ICT) state. Separation of pure ICT emission from pure LE emission has been carried out by Gaussian/Lorentzian curve fitting. The vibronic coupling in the local transitions appears to be confined to the normal mode involving the C–C triple bond stretching of the diyne moiety. This implies that the LE transition involves the diyne moiety, a conclusion supported by quantum chemical calculations. The resolved ICT emission follows double linear dependence on ET(30) solvent polarity scale. The important role of the diyne moiety in the photophysics of this class of molecules is clearly discernible in this study.Item Small push-pull diacetylenes as emergent fluorophores(AIP, 2018-08) Pati, Avik K.Organic fluorophores containing acetylene spacers have gained significant current interest because of their wide-spread applications in optoelectronics. In this present review, we summarize our recent photophysical understanding on small organic dyes which contain a diacetylene conduit. Diphenylbutadiynes with push-pull substituents exhibited emissions from both locally excited (LE) and intramolecular (ICT) states in non-aqueous media. The LE emission was confined to the normal mode involving C≡C stretching of the diacetylene moiety. Quantum chemical calculations showed that the fluorophores are twisted and the diyne moiety deviates from the usual linearity (sp) in the ICT excited state. The diphenylbutadiynes with an acceptor group at meta position of the phenyl ring produced the lowest absorption energy compared to their ortho and para isomers, which was related to the ‘meta effect’ well-known in organic photochemical reaction. The ‘meta effect’ was not perceived when the chain length (n) was increased (n≥4). Butadiyne bridged pyrene-phenyl hybrid derivatives showed the LE emission originating from the pyrene core, which was in contrast to the observation of the LE emission in the diphenylbutadiynes. The dyes showed aggregate emission in mixed-aqueous solvents. In addition to the solution state emission, the fluorophores exhibited emission in solid powder form and showed reversible fluorescence switching in the solids. The solid state emission of the derivatives was either blue (excitonic coupling) or red (excimeric coupling) shifted with regard to the solution phase emission spectrum, depending on the size of the peripheral aromatic moiety. The diynes containing donor and acceptor peripheries displayed single component white light emission, which was exploited to polar aprotic vapor detection in a polymer film matrix. The photophysical outcomes of the diacetylenic dyes make them a promising class of important π-conjugated organic fluorophores.Item Tuning the Baird aromatic triplet-state energy of cyclooctatetraene to maximize the self-healing mechanism in organic fluorophores(PNAS, 2020-09) Pati, Avik K.Bright, photostable, and nontoxic fluorescent contrast agents are critical for biological imaging. “Self-healing” dyes, in which triplet states are intramolecularly quenched, enable fluorescence imaging by increasing fluorophore brightness and longevity, while simultaneously reducing the generation of reactive oxygen species that promote phototoxicity. Here, we systematically examine the self-healing mechanism in cyanine-class organic fluorophores spanning the visible spectrum. We show that the Baird aromatic triplet-state energy of cyclooctatetraene can be physically altered to achieve order of magnitude enhancements in fluorophore brightness and signal-to-noise ratio in both the presence and absence of oxygen. We leverage these advances to achieve direct measurements of large-scale conformational dynamics within single molecules at submillisecond resolution using wide-field illumination and camera-based detection methods. These findings demonstrate the capacity to image functionally relevant conformational processes in biological systems in the kilohertz regime at physiological oxygen concentrations and shed important light on the multivariate parameters critical to self-healing organic fluorophore design.