Department of Chemistry
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Item Meta Effect of Absorption Energy in Donor–Acceptor Substituted Benzenoids: A Computational Study of Its Dependence on Acceptor Strength, Solvent Polarity, and Conjugation Length(ACS, 2014-08) Pati, Avik K.The present work focuses on theoretical understanding of electronic absorption energies of N,N-dimethylaniline with different ortho-, meta-, and para-substituted acceptor groups. The meta isomers exhibit the lowest absorption energy compared to the ortho and para derivatives. This unusual behavior of absorption energies of the meta isomers is related to the “meta effect” well-known in organic photochemical reactions. The meta effect of absorption energy of the derivatives is found to depend on the strength of acceptors, solvent polarity, and conjugation length. The meta derivatives with strong acceptor groups generally exhibit the lowest absorption energy over the other isomers irrespective of solvent polarity. However, the meta isomers with weak acceptor groups exhibit the meta effect only in highly polar solvents. The trend of the lowest absorption energies of the meta isomers is observed to change if the acceptor group is bridged through π conjugation unit (n) with the core moiety. The normal pattern of absorption energy that is the para isomer is of lowest energy is observed to occur for the derivatives where the repeated conjugation units (n) are between 2 and 4. The normal pattern of absorption energy is continued to observe from n > 4 for all the derivatives.Item Photophysical Impact of Diacetylenic Conjugation on Classical Donor–Acceptor Electronic Energy Pair(ACS, 2018-12) Pati, Avik K.Organic fluorophores with extended π-conjugation are important for their widespread applications. The present work provides photophysical insights into a diacetylene bridged classical donor–acceptor electronic energy pair, naphthalene–pyrene, in comparison with its constituents’ molecular structures, naphthyl and pyrenyl acetylenes, as well as parent naphthalene and pyrene chromophores. The diacetylenic dye loses the individual spectral identities of the donor and acceptor fluorophores exhibiting a locally excited (LE) emission (∼411 nm) from the overall molecular entity with high fluorescence quantum yields (0.55–0.84) in nonaqueous media. In contrast to the parent pyrene, the hybrid derivative shows a strongly allowed S0 → S1 transition. In mixed-aqueous media, the dye forms aggregates displaying a new red-shifted absorption (∼425 nm) as well as emission (∼510 nm) band. Unlike the hybrid dye, the naphthyl and pyrenyl acetylenes do not form aggregates. In the aggregate state of the hybrid fluorophore, electronic energy transfer takes place from the naphthyl moiety to pyrenyl ring. The excited-state photophysical properties of the dye are exploited in vapor sensing in the solid state.