Kaustuv Das

Excited-state intramolecular proton transfer and rotamerism of 2-(2'-hydroxyphenyl) benzimidazole

Abstract Excited-state intramolecular proton transfer (ESIPT) and rotamerism of 2-(2′-hydroxyphenyl benzimidazole (HPBI) have been studied using steady-state and time-resolved emission spectroscopy and by semi-empirical quantum-chemical methods. Two ground-state rotamers ( I and II ) with distinct excitation and emission spectra and lifetimes have been identified. Excitation of one of them ( I ) produces the normal emission while the other after excitation undergoes ultrafast ESIPT to form the tautomer ( III ) with Stokes-shifted emission. At 77 K the tautomer emission is markedly supressed as the rotamer, II , responsible for it, is less stable than I . CNDO/S-CI calculations were performed optimising the ground-state geometry by the AM1 method. These calculations give good estimates of the ground- and excited-state energies of the rotamers I and II and the tautomer, III .

Photodynamic action of Rose Bengal silica nanoparticle complex on breast and oral cancer cell lines.

Rose Bengal, an anionic photosensitizer was conjugated to organically modified silica nanoparticles having 3‐amino propyl groups by electrostatic or covalent interaction. The drug‐nanoparticle complexes were characterized by FTIR, light scattering and zeta potential measurements. Significant changes were observed in the spectroscopic properties of the drug when it is conjugated with nanoparticles. The toxicity of the free drug and drug‐nanoparticle complex was studied against oral (4451) and breast (MCF‐7) cancer cell lines. Both complexes with nanoparticles were more phototoxic than free Rose Bengal, with the covalent complex being the more effective. Studies carried out on cellular uptake, photostability and singlet oxygen generation suggest that enhanced phototoxicity is primarily due to the enhanced uptake of the drug‐nanoparticle complex.

Picosecond spectral relaxation of curcumin excited state in a binary solvent mixture of toluene and methanol.

Picosecond spectral relaxation of the excited state of curcumin in a binary solvent mixture of toluene and MeOH (or MeOH-d4) is reported with an instrument time resolution of ∼40 ps. With increasing mole fraction of MeOH (MeOH-d4) the fluorescence intensity and lifetime of curcumin increase to a maximum at a MeOH (MeOH-d4) mole fraction of 0.14 (0.40) and then decrease. In addition, fluorescence decays taken at the red edge of the emission spectrum started to show measurable rise times (170 to 30 ps), the magnitude of which decreased gradually with increasing alcohol mole fraction. This is attributed to the modulation of the nonradiative rates associated with the excited-state intermolecular H(D) bonding between the pigment and the polar protic solvent. As a consequence, the solvation times in the binary mixture were observed to slow down considerably (20-40 times) at certain solvent compositions compared to neat MeOH. The fact that three Gaussian components are needed to adequately represent the steady-state emission spectra and two isosbestic points are observed in the time-resolved area normalized emission (TRANE) spectra of the pigment suggests the existence of at least three species in the excited state. The observed results are rationalized with a scheme where ground state of the pigment exists in free and H-bonded (intermolecular) state. Optical excitation results in a mixture of these species in the excited state and the observed spectral relaxation correspond to the conversion of these two species in to a third species where dipolar solvation and intermolecular H-bonding have been optimized.

Salt effect on the hydrophobic binding of p-toluidino naphthalene sulphonate with cyclodextrins

Lithium chloride, lithium perchlorate, cesium bromide, tetra-n-butyl ammonium bromide (TBBr) and guanidine hydrochloride (GnCl) affect the fluorescence properties of 2, 6-p-toluidino naphthalene sulphonate (TNS) bound to α- and β-cyclodextrins (CD) in strikingly different ways. This indicates that the salts have considerable influence on the hydrophobic binding of TNS molecules with CDs. The salting-in agents (LiClO4, CsBr and GnCl) markedly reduce the fluorescence quantum yield (Φf) and lifetime (τf) of CD-bound TNS anion. It is suggested that they directly interact with the bound TNS ions. The salting-in agent TBBr increases Φf and τf. This fluorescence enhancement indicates that the local polarity is reduced by wrapping the four n-butyl groups around the CD-bound TNS ions. The salting-out agent LiCl causes a marginal increase (< 15%) and that at a high concentration (7 M). In the absence of CD in aqueous solution on addition of 0.1 M TBBr; the fluorescence yield of TNS increases sixfold. This is attributed to clustering of TB cations around the TNS anion.

Solvation dynamics in a solid host. Coumarin 480 in zeolite 13X

The dynamics of solvation of coumarin 480 guest molecule in supercages of a faujasite zeolite 13X is studied using picosecond time-resolved emission spectroscopy. The guest molecule exhibits a time-resolved Stokes shift in zeolite 13X. The decay of the response function, C(t), is nonexponential with the major components on a nanosecond timescale. The average solvation time is observed to be 8.13 ns. The nanosecond solvation is attributed to the self-motion of the probe solute and the motion of the sodium ions present in the zeolite host.

Non-radiative pathways of anilino-naphthalene sulphonates: Twisted intramolecular charge transfer versus intersystem crossing

Abstract The effect of solvent polarity and external heavy atom (ethyl iodide) on the fluorescence properties of 2-tolouidino-6-naphthalene-sulphonate (TNS) and 1-anilino-8-naphthalene-sulphonate (ANS) have been studied. For ANS with rise in polarity, a monotonic decrease of fluorescence yield (α f ) and lifetime (τ f ) is observed while ethyl iodide was found to have negligible effect. For TNS at low polarity α f and τ f are found to increase with rise in polarity, while ethyl iodide causes a decrease in τ f . At high polarity [ E T (30) > 51] behaviour of TNS is more or less similar to that of ANS. It is proposed that polarity-dependent, twisted intramolecular charge transfer is the main non-radiative process for ANS at all polarities [ E T (30) = 36–63] and for TNS only at high polarity. At low polarity intersystem crossing is the main non-radiative pathway for TNS.

Effect of Curcumin on the Diffusion Kinetics of a Hemicyanine Dye, LDS-698, across a Lipid Bilayer Probed by Second Harmonic Spectroscopy

The diffusion kinetics of a hemicyanine dye, LDS-698, across model membrane bilayers was studied in real time by the surface specific second harmonic technique. Using liposomes made from different headgroups, it has been established that the diffusion is initiated by electrostatic adsorption of the positively charged dye to the outer surface of negatively charged liposomes and its time constant is affected by the rigidity of the bilayer. In the presence of the liphophilic drug curcumin (curcumin/lipid mole ratio ~ 0.2), the diffusion of LDS-698 was observed to be faster by ~56 times (from 780 to 14 s) at 25 °C. Under similar curcumin concentration, when cholesterol containing liposomes are used at 2 °C, the observed diffusion time constant increases from 14 to 65 s, showing that the effect of curcumin is superior to the effect of increasing bilayer rigidity on the diffusion process. Control experiments with other lipophilic molecules such as DPH and Nile Red showed that the effect of liposomal curcumin is superior. Consistent with previous reports of curcumin affecting the bilayer organization, this study additionally demonstrates increased permeability of liposomal curcumin, in particular against organic cations. It is speculated that origin of this enhanced membrane permeability by lipophilic molecules may depend upon the interaction of the molecule with the polar headgroup region of the lipid which, in turn, is expected to depend on the chemical structure of the molecule.

Diffusion of chlorin-p6 across phosphatidyl choline liposome bilayer probed by second harmonic generation.

We have investigated the diffusion of the photosensitizer Chlorin-p(6) (Cp(6)) across a egg lecithin lipid bilayer at different pH by the Second Harmonic Generation (SHG) method. Cp(6) has three ionizable carboxylic acid groups, and consequently, neutral and several ionic forms of Cp(6) are expected to be present in the pH range 3-8. The absorption spectra of Cp(6) get considerably modified in the presence of liposomes as the pH is decreased indicating that the drug liposome binding is pH dependent. The first pK(a) of interconversion (D-C) has been identified at pH ~7.0 by fluorescence measurement in an earlier work. In this work, the second pK(a) of interconversion (C-B) has been identified at pH ~4.8 by the hyper-Rayleigh scattering method. At acidic pH (3, 4, and 5), where species A, B, and C are dominant, the addition of liposomes to a Cp(6) solution generates an instantaneous rise (less than 1 s) in the second harmonic (SH) signal followed by decays whose time constants ranged from ten to hundreds of seconds. The instantaneous rise is attributed to the adsorption of Cp(6) to the outer lipid bilayer, and the decay is attributed to the diffusion of the neutral and charged (A and B) species of the drug. The observed fast and slow time constants for diffusion in the pH range 3-5 are attributed to the neutral (A) and ionic form (B) of Cp(6), respectively. At pH 6, the intensity of the generated SH signals on the addition of liposome reduced, and at physiological pH, it was too weak to be detected. These results are consistent with previous studies that show that the interaction between Cp(6) and egg-PC liposomes is pH dependent. At lower pH due to the presence of the hydrophobic species (A and B) of Cp(6), its interaction with liposomes is strong, and at higher pH, the abundance of the negatively charged hydrophilic species (C and D) decreases the interaction with the like charged liposomes. We have also studied the effect of increasing the bilayer rigidity by decreasing the temperature of the medium or by incorporating 50 mol % cholesterol in the lipid bilayer and observed that lowering of temperature has more profound effect on the diffusion rates. The characteristics of the SH signal changed significantly when liposomes incorporating 50 mol % cholesterol were used at a low (3 °C) temperature. Under these conditions, the SH signal consisted of an instantaneous (<1s) followed by a slower rise (10-90s), and then, it decayed on a much longer time scale. This slow rise of the SH signal at pH 3 and 4 may be attributed to the temperature dependent adsorption of the anionic species (B) of Cp(6) with the liposomes. Further investigations are required in order to understand clearly the pH dependent diffusion of this drug across lipid bilayers.

Spectroscopic investigations on the binding of the photosensitizer Chlorin p6 with amine-modified silica nanoparticles in aqueous media.

Absorption and emission properties of the amphiphilic photosensitizer Chlorin p6 were investigated in aqueous medium in the presence of silica nanoparticles (SiNPs) having positively charged amino groups. The results of these studies reveal that the acid–base ionization equilibrium of Chlorin p6 in aqueous medium is significantly affected as a result of strong electrostatic binding between the negatively charged drug and SiNP. The spectroscopic signature of the drug bound to SiNPs suggests that the tri‐anionic form of the drug remains bound to the positively charged SiNPs at pH 8.0. As the pH is progressively decreased the formation of hydrophobic aggregates is disrupted significantly due to the presence of electrostatic binding force, which competes with intermolecular hydrophobic forces. The interplay of hydrophobic and electrostatic forces in the drug–nanoparticle binding process might affect the relative uptake and photodynamic efficacy of the free drug and the drug–nanoparticle complex in cancer cells.

Dual emission of 2-(2'-hydroxyphenyl) -benzimidazole in reverse micelle

Abstract Picosecond time resolved and steady state emission spectroscopy were employed to study the dual emission phenomenon of 2-(2′-hydroxyphenyl)-benzimidazole (HPBI) in aerosol OT (AOT) reverse micelle in n -heptane. It is observed that the microenvironment inside the water pool markedly affects the dual emission (normal and tautomer) of HPBI. While the normal emission is negligible in n -heptane, on addition of AOT and water its intensity increases gradually and reaches a moderate value ( φ f =0.04) at a water-to-surfactant ratio, w 0 ≈40. The decay of the normal emission becomes faster with addition of water. The intensity and lifetime of the tautomer emission increase on addition of AOT. The increase in the intensity of the normal emission with w 0 is ascribed to the formation of the polyhydrates in which excited state intramolecular proton transfer is blocked. The increase in the intensity of the tautomer emission in reverse micelles, compared with n -heptane, is attributed to the reduction in the non-radiative rates. The polarity of the water pool inferred from both the emission parameters of the normal, and the tautomer emission is less than that of ordinary bulk water.