Sankar Chakravorti

Quest for Mode of Binding of 2-(4-(Dimethylamino)styryl)-1-methylpyridinium Iodide with Calf Thymus DNA

The mode of binding of 2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide (DASPMI) with calf thymus DNA as revealed from different steady state and time-resolved emission spectroscopic measurements has been reported in this paper. Fluorescence enhancement of DASPMI and its quenching by potassium iodide (KI) points to groove binding of dye with ct-DNA, rather than intercalation in the ct-DNA helix. An increase in steady state anisotropy and fluorescence lifetime hints at binding with ct-DNA. The value of binding constant from emission and association constant from circular dichroic spectrum also indicates weak binding. The strong dependence on ionic strength or salt in controlling the binding of DASPMI with ct-DNA by electrostatic interaction confirms groove binding. The high semicone angle of DASPMI in ct-DNA certainly rules out the possibility of intercalated bonding. A theoretical modeling shows that the probe is bound to ct-DNA as a crescent with a curvature of 11.35 A, which is the previously known curvature of probe in the minor groove.

Excited state photodynamics of 4-N,N-dimethylamino cinnamaldehyde: A solvent dependent competition of TICT and intermolecular hydrogen bonding

Abstract This paper describes the dual fluorescence of 4-N,N-dimethylamino cinnamaldehyde (DMACA) in various solvents. Based on major findings the first fluorescence band has been assigned to be arising out of delocalized excited state (DE) and the anomalous fluorescence band in polar aprotic solvents has been assigned to twisted intramolecular charge transfer (TICT) state whereas, in protic solvent it is arising out due to hydrogen bonding interaction between hydrogen donor part of the solvent and carbonyl group of the probe molecule. This hydrogen bonding being an efficient fluorescence quenching channel trammels the TICT formation and thus an apparent competition exists between TICT and hydrogen bonding formation in protic solvent. The calculated absorption spectrum generated from CNDO calculations follows nicely the experimental absorption band. The AM1 calculations corroborate the large dipole moment change and the charge transfer in orthogonal position as found from experimental observations.

Photosensitizers: comprehensive photophysics/photochemistry and theory of coumarins, chromones, their homologues and thione analogues

The photophysical–photochemical behaviour, including absorption spectra, fluorescence spectra and lifetimes, phosphorescence spectra, triplet–triplet transient spectra, triplet lifetimes and quantum yields and sensitized singlet oxygen formation and yields, has been determined for many of a group of 22 carbonyl and thione compounds. These include coumarins, psoralens, chromones and furochromone and their thiones. Many of the thiones were synthesized for the first time. Two types of theoretical calculations were carried out on the majority of the compounds. The principal goals of the research for the thiones, were (1) to produce or improve the photosensitizer capabilty of the compounds in vivo, (2) to shift the spectra of the compounds to significantly longer wavelength and (3) to increase the triplet and singlet oxygen quantum yields. Most of these have been largely achieved. The photosensitizing abilities of two of the thiones have been evaluated using a unique testing technique employing a series of genetically engineered bacteria.

Modification of a styryl dye binding mode with calf thymus DNA in vesicular medium: from minor groove to intercalative.

This paper reports an interesting transformation of binding mode of 2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide (DASPMI) with calf thymus DNA from minor groove binding in buffer solution to intercalative binding when the dye is encapsulated inside a vesicle formed by the interaction of 1,8-naphthalimide (a charge transfer dye) with the supramolecular association of sodium dodecyl sulfate and block-copolymer polyethylene-b-polyethylene glycol. The pre-encapsulated dye in the vesicular interior binds intercalatively to ct-DNA, as evinced by the high value of equilibrium binding constant of DASPMI-DNA complex, changes in CD-spectra of DNA and isosbestic point, along with downshift and hypochromicity of absorption band. Increase in anisotropy decay by 1.5 times with a single component strongly confirms restricted motion of the probe inside ct-DNA confirming intercalative binding. The compaction of ct-DNA caused by the interaction of the vesicle allows DASPMI to bind ct-DNA in the intercalative mode. However, the groove binding mode in ct-DNA-DASPMI remains unaffected by the retro-addition of the vesicles to the already bound dye to ct-DNA.

Modulation of photophysics due to orientational selectivity of 4-N,N-dimethylamino cinnamaldehyde β-cyclodextrin inclusion complex in different solvents

Abstract This paper delineates some results on the twisted intramolecular charge transfer (TICT) dynamics of 4- N , N -dimethylamino cinnamaldehyde (DMACA) encapsulated in β-cyclodextrin in aqueous and non-aqueous solvents at room temperature. 4- N , N -Dimethylamino cinnamaldehyde is found to form a 1:1 inclusion complex with β-CD in both aqueous and non-aqueous solvents with a binding constant higher in aqueous solvent. The most important feature of the inclusion complexes of 4- N , N -dimethylamino cinnamaldehyde, as revealed from photophysics of normal and twisted intramolecular charge transfer bands, is that there is a preferential orientation of 4- N , N -dimethylamino cinnamaldehyde inside β-CD cavity in aqueous solvent with dimethyl group sticking outside and in non-aqueous solvents it is just the opposite.

A study on the spectroscopy and photophysics of N-phenyl pyrrole and N-phenyl pyrazole

Abstract Comprehensive spectroscopic and photophysical studies of N-phenyl pyrrole (PPr) and N-phenyl pyrazole (PPz) in different solvents show that a new emission band appears due to aggregation of PPr in water. The presence of β-cyclodextrin (β-CD) in aqueous solution of PPr causes deaggregation but the twisted intramolecular charge transfer emission ceases due to the cavity effect. In PPz, aggregation and TICT emission are absent. Protonation in PPz is restricted to a single site only at very high acid concentration. Both the molecules are planar in the ground state.

Dynamics of twisted intramolecular charge transfer process of 4-N,N-dimethylaminocinnamic acid in α-cyclodextrin environment

This Letter reports some interesting results on twisted intramolecular charge transfer emission of 4-N,N-dimethylaminocinnamic acid complexed with α-cyclodextrin. Salient features of this investigation, as revealed from twisted intramolecular charge transfer emission intensity and position dependence on α-CD concentration, are the formation of 1:2 barrel-shaped complex in addition to 1:1complex, while twisted intramolecular charge transfer (TICT) band intensity shows an increase and then a sharp decrease (bell-shaped variation) along with blue shift and initial addition of base to the solution obstructs the formation of 1:2 complex.

Photophysics of 4-dimethylamino cinnamic acid in different environments

Abstract 4-dimethylamino cinnamic acid (DMACA) has been studied by steady state and time-resolved fluorescence spectroscopy in different environments in order to get the information about its photophysics and photochemistry. Dual fluorescence in polar medium has been assigned to be arising out of delocalized excited state (DE) and twisted intramolecular charge transfer (TICT) state from different experimental and theoretical considerations. The computed excited state dipole moments in different twisted geometries evince that a twist of 90° of N(CH3)2 produces minimum energy state and maximum dipole moment change. The experimentally measured value (9.23 D) for dipole moment change from ground to excited state tallies with the corresponding value computed theoretically (7.5 D) for twisted geometry. Polarization studies in polar, nonpolar solvents and in β-CD cavity suggest that the emission transition moment is perpendicular to the molecular axis confirming the presence of twisted conformer. Weak acid protonates the carbonyl group while strong acid favors protonation of amino nitrogen. The solvent dependent absorption and emission spectra were found to be due to intramolecular structure change rather than ‘solvent coordinate’ effect. TICT band intensity is enhanced in β-CD inclusion complex due to modulation of photophysics by it while the normal (DE) band failed to appear. The fluorescence dependence of DMACA inclusion complex on polarity and viscosity has been discussed in detail. Quantum chemical calculations with AM1 Hamiltonian have been performed to find the actual geometry producing twisted intramolecular charge transfer state by twisting N(CH3)2 group to different angles and to confirm experimental findings qualitatively.

Photophysics of phenyl pyridines and bipyridines in different media

Abstract Analysis of absorption and emission spectra of 2,4′-bipyridine (2BPy), 4,4′-bipyridine (4BPy) and three isomeric phenyl pyridines (PhPy) in different solvents reveal that the general nature of lowest emitting state is π, π∗. A significant enhancement of fluorescence of 2PhPy at 77 K in relation to 3PhPy has been attributed to a possible through-space interaction of n - and π-electronic clouds. A discernible feature for phenyl pyridines is the increased basicity and polarity in the excited singlet state. The internal conversion seemed to be active in both phenyl pyridines and bipyridines in MCH solvent and for bipyridines in ethanolic solution, the second-order spin-orbit coupling comes into picture.

Exploring the location and orientation of 4-(N, N-dimethylamino) cinnamaldehyde in anionic, cationic and non-ionic micelles

Abstract This Letter reports probing of non-ionic, anionic and cationic micelles utilizing different dual emission properties of 4-(N,N-dimethylamino) cinnamaldehyde. Twisted intramolecular charge transfer (TICT) band is more enhanced and blue shifted in non-ionic micelle than those are in ionic micelles. In non-ionic micelle, the molecule enters in the core region whereas, in ionic micelles, it is anchored in the interfacial region with different orientations. Micellar–water interface electric field in ionic micelles was found to have profound effect on TICT decay time. Interestingly, inorganic salt-counterion binding helps the acceptor moiety to enter into the core region in anionic micelle.