Samiran Kar

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.

Selective fluorescence sensing of Cu(II) and Zn(II) using a simple Schiff base ligand: naked eye detection and elucidation of photoinduced electron transfer (PET) mechanism.

A simple Schiff base compound 2-((cyclohexylmethylimino)-methyl)-naphthalen-1-ol (2CMIMN1O) has been synthesized and characterized by (1)H NMR, (13)C NMR and FT-IR spectroscopic techniques. A significantly low emission yield of the compound has been rationalized in anticipation with photo-induced electron transfer (PET) from the imine receptor moiety to the naphthalene fluorophore unit. Consequently, an evaluation of the transition metal ion-induced modification of the fluorophore-receptor communication reveals the promising prospect of the title compound to function as a chemosensor for Cu(2+) and Zn(2+) ions selectively, through remarkable fluorescence enhancement as well as visual changes. While perturbation of the PET process has been argued to be the plausible mechanism behind the fluorescence enhancement, the selectivity for these two metal ions has been interpreted on the grounds of an appreciably strong binding interaction. Particularly notable aspects regarding the chemosensory activity of the compound is its ability to detect the aforesaid transition metal ions down to the level of micromolar concentration (detection limit being 2.74 and 2.27ppm respectively), along with a simple and efficient synthetic procedure.

Excited State Lactim to Lactam Type Tautomerization Reaction in 5-(4-Fluorophenyl)-2-Hydroxypyridine: Spectroscopic Study and Quantum Chemical Calculation

The photophysical properties of 5-(4-fluorophenyl)-2-hydroxypyridine (FP2HP) have been studied by steady state and time resolved spectroscopy in combination with quantum chemical calculations. The molecule FP2HP exists as lactim and lactam form in the ground state due to small stability difference but does not undergo lactim to lactam isomerisation due to high barrier energy. Whereas in the excited state the lactim form undergoes tautomerization producing red shifted emission of the lactam tautomer along with the local emission of the lactim form. In polar protic solvents, the barrier for lactim-lactam tautomerization rapidly decreases forming the lactam tautomer only. Temperature has pronounced effect on the excited state tautomerization equilibrium and is clearly reflected in the measured equilibrium constant (Ktau0) and free energy change (ΔG0). Structural calculations at Hartree Fock and Density Functional Theory levels, calculated stability of the isomers in different solvents using Polarized Continuum Model and the theoretical potential energy surfaces for the ground and excited states along the proton transfer coordinate are reported for the tautomeric equilibrium of FP2HP.

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.

Photoinduced properties of methyl ester of 2,6-dimethyl-1-amino-4-benzoic acid: Evidence of charge transfer emission from a weak primary amino donor

Photophysical properties of methyl ester of 2,6-dimethyl-1-amino-4-benzoic acid (M26DMB) have been investigated using steady state absorption and emission spectroscopy and time-resolved emission spectroscopy. Interestingly, not only in polar solvents, the molecule M26DMB having a weak primary amino donor group shows broad red-shifted emission band even in non-polar environment which in all probability arises from closely spaced local and charge transfer (CT) states. Clear dual fluorescence in polar protic solvents comprises of less intense local emission band and strong red-shifted CT band. The position of the red-shifted emission band is dependent on both the polarity and the hydrogen-bonding ability of the solvent.