Department of Chemistry
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Item Substituted diphenyl butadiynes: a computational study of geometries and electronic transitions using DFT/TD-DFT(RSC, 2014) Pati, Avik K.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.Item On the photophysics of butadiyne bridged pyrene–phenyl molecular conjugates: multiple emissive pathways through locally excited, intramolecular charge transfer and excimer states(RSC, 2015-01) Pati, Avik K.The present work describes the photophysical properties of a group of butadiyne bridged pyrene–phenyl molecular hybrids having different substitutions with varying donor and acceptor abilities. In addition to emission from the locally excited (LE) state originating from the pyrene moiety, intramolecular charge transfer (ICT) emissions were observed in molecules with donor–acceptor character. The positions of the ICT emission maxima varied over a wide range of wavelengths (475–600 nm). Pyrene behaved as a donor when a strong acceptor group (–CN) was attached to the phenyl ring and it behaved as an acceptor when the phenyl group contained a strong donor group (–NMe2). In mixed aqueous solvents at higher percentages of water (80–99%), the derivatives showed emissions from the aggregate state in addition to the LE and ICT states. Emissions from the aggregate states of the derivatives were centred in the range 510–560 nm. The aggregate state emissions were found to originate from static excimers involving pyrene moieties. A detailed structure–property relationship of the butadiynyl derivatives was revealed in this study.Item Photophysics and Photoreactivity of Cross-conjugated Enediynyl Aggregates: Applications to Multi-parametric Sensing of Microheterogeneity and Reversible Fluorescence Switching(Elsevier, 2020-01) Pati, Avik K.Fluorescent aggregates find wide-spread applications in modern optoelectronics. Here, we study the photophysics of organic fluorophores composed of benzene, pyrene, naphthalene, and anthracene moieties connected via a Y-shaped enediyne π-spacer in water-acetonitrile solvent mixtures and solid forms. The fluorophores show a predominant aggregate emission band, which is ca. 4000 cm−1 red-shifted compared to their locally excited (LE) emission energies in water-acetonitrile. The aggregate emission is however blue-shifted compared to an intramolecular charge transfer (ICT) emission band. The interplay of the LE, ICT, and aggregate emissive states depends on peripheral aromatic units, electronic substituents and microenvironments. The LE to aggregate emission spectral shift is exploited to sensitively probe micro-heterogeneity in bile salts, which are complemented by fluorescence intensity and fluorescence anisotropy changes. The solution phase aggregate emissions closely resemble the solid state aggregate emission bands, which are attributed to excimer states. The aggregate fluorescence states in the solids are altered by external stimuli and reversibly switched multiple of cycles. The aggregates undergo photoreactions in mixed-aqueous solvents, but they remain unreactive in solid powder forms.