Saswati Lahiri

Vibronic interaction and photophysics of chalcone derivatives

Abstract A comprehensive photophysical study of 1-(4-methoxy-1-phenyl)-3-(1-naphthyl) prop-2-en-1-one (MPNPO) and naptho (1, 2b) furan (NF) in different solvents at room temperature and at 77 K in ethanolic and methylcyclohexane glass shows a strong phosphorescence emission in NF. The phosphorescence emission and excitation spectra of NF along with their polarizations and phosphorescence lifetimes suggest that the perturbation of the zero point level of the emitting state ( 3 π, π∗) by a nearby ( 3 n,π∗) state leads to a number of spectral features, which are not expected from ( 3 π, π∗) phosphorescence. It is concluded that the strong solvent dependent triplet quantum yield and lifetime of NF at 77 K is due to the dependence of radiationless transitions on vibronic spin-orbit coupling between ( 1 n, π∗ ) and ( 3 π, π∗) states.

Tetracyclo[6.4.0.03,5.04,8]dodecan-6-ones: A novel construction of tricyclo[7.2.1.01,6]dodecan-10-one skeleton by acid-catalyzed cyclopropyl bond cleavage

Abstract The tetracyclo[6.4.0.0 3,5 .0 4,8 ]dodecan-6-one derivatives were prepared from tricyclic precursors by direct photoinduced oxa-di-pi-methane (ODPM) rearrangements. Diastereoselective syntheses of these tricyclic precursors have been described. Introduction of keto-substitution at CI initiated the first example of C1–C8 bond cleavage in such tricyclo[3.3.0.0 2,8 ]octan-3-one systems.

Photoreactivity in δ-keto-β, γ-enones: oxa-di-π-methane (ODPM) rearrangements from states other than T1(π, π∗)

Abstract The phototransformations of tricyclic β, γ-enones with substitutions on the π bond ( 10a–10c ) have been studied. All of them were found to yield only the corresponding oxa-di-π-methane-rearranged products 11a–11c on direct as well as on sensitized irradiation. Quenching studies suggest that states other than the lowest triplet are involved in the cases of 10a and 10b .

A Convenient Route To 1,2-Dibenzoyl-1-Arylethylenes

Abstract Short and convenient syntheses of 1,2-cis-dibenzoyl-1-ary 1-ethylenes 5a-e, 6 and 8a-d from the corresponding aryl ethers and dibenzoyl acetylene have been developed using boron trifluoride-etherate or aluminium chloride as catalyst.

Regioselectivity in ketyl radical promoted ring cleavage of configurationally restricted α,α-diketocyclopropanes under SET and PET conditions

Abstract Regioselective bond scission of cyclopropyl ketones 1a-d in single electron transfer (SET) reductive process with tributytin hydride has been found to be controlled by radical stabilizing effect. In the case of 1e and 11 stereoelectronic effect was also operative. Under photoinduced electron transfer (PET) condition similar regioselectivity was also observed. A ring expansion product 16 via intramolecular ketone-cyclopropane interaction was obtained from 1b in this case.

Non-radiative depletion of the excited electronic states of 9-cyanoanthracene in presence of tetrahydronaphthols.

Both steady state and time resolved spectroscopic measurements reveal that the prime process involved in quenching mechanism of the lowest excited singlet (S1) and triplet (T1) states of the well known electron acceptor 9-Cyanoanthracene (9CNA) in presence of 5,6,7,8-tetrahydro-1-naphthol (TH1N) or 5,6,7,8-tetrahydro-2-naphthol (TH2N) is H-bonding interaction. It has been confirmed that the fluorescence of 9CNA is not at all affected in presence of 5,6,7,8-tetrahydro-2-methoxy naphthalene (TH2MN) both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) media. This indicates that the H-bonding interaction is crucial for the occurrence of the quenching phenomenon observed in the present investigations with TH1N (or TH2N) donors and 9CNA acceptor. In ACN solvent both contact ion-pair (CIP) and solvent-separated (or dissociated) ions are formed due to intermolecular H-bonding interactions in the excited electronic states (both singlet and triplet). In NH environment due to stronger H-bonding interactions, the large proton shift within excited charge transfer (CT) or ion-pair complex, 1 or 3(D+-H...A-), causes the formation of the neutral radical, 3(D+H-A)*, due to the complete detachment of the H-atom. It is hinted that both TH1N and TH2N due to their excellent H-bonding ability could be used as antioxidants.

Intramolecular Charge Transfer in Naphthalene Analogues of Chalcone

Intramolecular charge transfer (ICT) bands were observed in the emission spectra of naphthalene analogues of chalcones of type A, B and C. In C, a naphthalene excimer band was also observed because of increased coplanarity in such molecules. Neither any phosphorescence band nor any triplet transient could be detected from either of these molecules.

Unusual cyclization of 1,2-dibenzoylcyclohexa-1,4-diene during catalytic hydrogenation

Abstract Catalytic hydrogenation of 1,2-dibenzoylcyclohexa-1,4-dienes gave the corresponding dihydro- and tetrahydroisobenzofuran derivatives possibly via a 1,4-addition followed by water-elimination.

Solid-state photolysis of sterically congested cis-1,2-dibenzoylalkenes: isolation and characterization of vinylketenes

Hitherto non-isolable vinylketenes from the solid state photolysis of cis-1,2-dibenzoylalkene compounds have been isolated and characterized and their stabilities were found to be enhanced with the extent of steric congestion in the molecules.