Sanjib Ghosh

S1 (n,π*), T1 (n,π*) and S2(n,π*) emissions in 3,6-diphenyl-s-tetrazine

Abstract Simultaneous emissions from S1(n,π*) and S2(n,π*) states in 3,6-diphenyl-s-tetrazinc (DPT) have been observed along with weak luminescence from T1 (n,π*). The occurrence of the S2(n,π*) fluorescence has been justified on the basis of the slow S2 XXX S1 internal conversion resulting from the large energy gap between the two states. This is the first case of dual fluorescence where both the emitting states are of (n,π*) nature.

Tuning of “Antenna Effect” of Eu(III) in Ternary Systems in Aqueous Medium through Binding with Protein

A simple ternary system containing a protein [human serum albumin (HSA)/bovine serum albumin (BSA)], tetracycline hydrochloride (TC), and Eu(III) in suitable aqueous buffer medium at physiological pH (= 7.2) has been shown to exhibit highly efficient antenna effect compared to the binary complex of TC with Eu(III) (Eu(3)TC). The ternary system containing E. coli alkaline phosphatase (AP), TC, and Eu(III), however, shows a slight enhancement of Eu(III) emission, although the binding constant of AP with TC is 2 orders of magnitude greater than with BSA/HSA. The enhanced emission of bound TC in the binary systems containing proteins and TC gets quenched in the ternary systems containing HSA/BSA, showing the efficient energy transfer (ET) from TC to Eu(III). Steady state and time-resolved emission studies of each component in all the ternary systems in H(2)O and in D(2)O medium reveal that Eu(III) is very well protected from the O-H oscillator in the ternary system containing HSA/BSA compared to that containing AP. The docking studies locating the binding site of TC in the proteins suggest that TC binds near the surface of AP. In the case of HSA/BSA, TC resides in the interior of the protein resulting in a large shielding effect of Eu(III). The rotational correlation time (θ(c)) determined from the anisotropy decay of bound TC in the complexes and the accessible surface area (ASA) of the ligand in the complexes obtained from the docking studies also support the contention that Eu(3)TC is more exposed to solvent in the case of the ternary system consisting of AP, TC, and Eu(III). The calculated radiative lifetime and the sensitization efficiency ratio of Eu(III) in all the systems clearly demonstrate the protein mediated tuning of antenna effect in Eu(III).

Thiocyanate mediated structural diversity in phenol based “end-off” compartmental ligand complexes of group 12 metal ions: Studies on their photophysical properties and phosphatase like activity

The reaction of a pentadentate compartmental ligand LH, namely 4-tert-Butyl-2,6-bis-[(2-pyridin-2-yl-ethylimino)-methyl]-phenol, with group 12 metal ions (ZnII, CdII, HgII) followed by addition of NaSCN afforded one discrete dinuclear complex [Zn2(L)(SCN)3](1), and two polymeric 1D species [Cd2.5(L)(SCN)3(AcO)]n (2) and [Hg2(L)(SCN)3]n (3). All the complexes have been structurally characterized by single crystal X-ray diffraction. The crystal structure of the complexes reveals different coordination modes of thiocyanate anion that affect the different topology detected in the compounds: the anions are μ1-NCS and μ1,1-NCS connected in complex 1, while μ1,3-NCS bridging mode is observed in 2, and μ1-SCN and μ1,3-NCS in 3. The polymeric Hg complex of the bicompartmental ligand system here reported is unprecedented. Detail study of their photophysical properties including the phosphorescence spectra at 77K has been done. Phosphatase like activity of all the three complexes has been performed in DMSO-H2O medium and their activity follows the order of 1>2>>3.

The low-lying electronic states of alloxan

Abstract A series of low-lying ( n ,π*) states of alloxan have been investigated through semiempirical calculations and absorption and emission studies. The energy of the lowest ( n ,π*) triplet state as calculated by UHF methodology agrees very well with that of the observed phosphorescent state.

Antenna effect and phosphorescence spectra to find the location of drug tetracycline in bovine β-lactoglobulin A

A ternary system comprising of a Eu(III) complex of the drug Tetracycline hydrochloride (Eu3TC) bound to bovine β-lactoglobulin variant A (BLGA) in aqueous buffer at physiological pH (pHu2009=u20097.4) has been investigated to exploit the enhanced “antenna effect” to locate the bound drug and find the microenvironment of the binding site. Steady-state and time-resolved emission studies at room temperature as well as at 77xa0K have been carried out to evaluate the binding parameters in the binary system consisting of BLGA and tetracycline hydrochloride (TC). Low-temperature phosphorescence studies at 77xa0K of pure BLGA confirm Trp 19 to be the emitting residue, while Trp 61 is silent. Enhancement of BLGA phosphorescence emission in the ternary system at 77xa0K indicates that Trp 19 is very close to Eu(III) in the Eu3TC complex. The molecular docking results further confirm that TC binds close to Trp 19 in a hydrophobic domain. The results thus obtained can provide guidelines to design and synthesize target-oriented drugs as well as suitable bio-probes.Graphical abstract

Features of partial encapsulation of an ESIPT probe 3-hydroxy-2-naphthoic acid (3HNA) in the nano cavities of β- and γ-cyclodextrin: comparative study with sequestered 3HNA in micelles and reverse micelle

The role of intramolecular H-bonding within an excited state intramolecular proton transfer (ESIPT) probe, the intermolecular H-bonding ability of the microenvironment with the probe in different organized assemblies, the polarity of the medium after encapsulation and the motional restriction of the probe have been investigated by monitoring the ESIPT emission as well as the localized π–π* emission of 3-hydroxy-2-naphthoic acid (3HNA) in various confined media like cyclodextrins, micelles and reverse micelle. The steady state and time-resolved fluorescence and also the time resolved anisotropy measurements of the encapsulated probe indicate the formation of a 1u2006:u20061 complex between the β- and γ-cyclodextrin hosts and the guest. The values of the binding constants and different thermodynamic parameters for complexation have also been reported. The results in various confined media have been correlated using control experiments on the ESIPT emission of 3HNA in mixed solvents consisting of dimethyl sulfoxide (DMSO) and water (mixture of an aprotic polar and a protic polar solvent) and also of acetonitrile (ACN) and dichloromethane (DCM) (mixture of an aprotic polar and a non-polar solvent). Theoretical calculation on the orientation of 3HNA within β- and γ-cyclodextrins has been carried out in order to explain the observed rotational correlation time of the probe in confined environments.