Avik Kumar Pati

Substituted diphenyl butadiynes: a computational study of geometries and electronic transitions using DFT/TD-DFT.

This work is aimed at theoretical understanding of electronic absorption and emission energies of a series of substituted diphenyl butadiynes through an assessment of several TDDFT functionals and a detailed study of solvent effects on their ground and excited state structures and properties. Out of a series of functionals examined, the coulomb attenuated DFT functional CAM-B3LYP is found to be most successful in predicting charge transfer absorption and emission energies of such derivatives. However, TDDFT potential energy surfaces obtained from hybrid functionals such as B3LYP and PBE0 are found to give a good description of the stability of locally excited (LE) and intramolecular charge transfer (ICT) states as a function of torsional angle, for the butadiynyl fluorophores. Interesting structural variations are observed in the ground and excited state optimized geometries of the fluorophores. The ICT emission of the butadiynyl fluorophores is observed to originate from the twisted state where the two phenyl rings in the diphenyl butadiyne get twisted around the butadiyne moiety. A bending of the butadiyne moiety is noted for some of the butadiynyl derivatives in the ICT emissive state. In addition, the direction of absorption and emission transition dipole moment vectors of the butadiynyl fluorophores is found to depend on the nature of substituents present at the periphery of the diphenyl butadiyne moiety.

Deciphering the Photophysical Role of Conjugated Diyne in Butadiynyl Fluorophores: Synthesis, Photophysical and Theoretical Study

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.

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.

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.

Photophysics of Diphenylbutadiynes in Water, Acetonitrile-Water, and Acetonitrile Solvent Systems: Application to Single Component White Light Emission.

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.

Photoinduced solid state keto–enol tautomerization of 2-(2-(3-nitrophenyl)-4,5-diphenyl-1H-imidazol-1-yloxy)-1-phenylethanone

Excited state intramolecular proton transfer (ESIPT) plays an important role in biological systems and has also recently found applications in electronic devices such as transducers, switches etc. In this paper we report the synthesis and solid state photochromic behavior of 2-(2-(3-nitrophenyl)-4,5-diphenyl-1H-imidazol-1-yloxy)-1-phenylethanone (II) due to ESIPT. Compound II exhibits yellow color in dark and red color in light, with the yellow form attributed to the keto derivative and the red form assigned to its enol derivative The color change in the presence of light is thus attributed to the keto–enol tautomerism through ESIPT. The color change from yellow to red is a photochemical process which thermally decays to the yellow form in the dark. The solid state stability of the enol form upon phototautomerization of the keto form is a noteworthy phenomenon, and its stability has been substantiated by our experimental findings. In the solution state, the yellow form (keto) is stable in chloroform while the red form (enol) is stable in DMSO. Theoretical calculations have been performed to understand the geometries and electronic transitions of the keto and enol forms. In addition, ground and excited state equilibrium constants for the keto–enol tautomerism were calculated.