Pradipta Purkayastha

Role of rotamerisation and excited state intramolecular proton transfer in the photophysics of 2-(2′-hydroxyphenyl)benzoxazole, 2-(2′-hydroxyphenyl)benzimidazole and 2-(2′-hydroxyphenyl)benzothiazole: a theoretical study

Semiempirical (AM1-SCI) calculations have been performed to rationalise the experimental findings in relation to the photophysics of 2-(2′-hydroxyphenyl)benzoxazole (HBO), 2-(2′-hydroxyphenyl)benzimidazole (HBI) and 2-(2′-hydroxyphenyl)benzothiazole (HBT). The calculations reveal that, while for HBO and HBI, two rotameric isomers are present in the ground state, there is only one stable species in the S0 state of HBT. Excited state intramolecular proton transfer (ESIPT) reaction is, however, operative in the lowest excited singlet (S1) and triplet (T1) states for all the three molecular systems; resulting altogether three fluorescence bands for HBO and HBI and two for HBT. The excitation, fluorescence and phosphorescence bands have been assigned theoretically. The calculated results agree well with the existing experimental reports. The potential energy surfaces (PES) have been generated for the intramolecular proton transfer (IPT) reactions. The PES reflect that although the IPT process is not favourable in the ground state, the ESIPT process is feasible, both thermodynamically as well as kinetically, for all the three molecular systems in the S1 as well as T1 states.

Photophysics of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine: emission from two states

Steady state photophysics of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ) has been studied in different pure and mixed solvents. The fluorometric behaviour of AODIQ is modified dramatically with a change in the solvent polarity. The fluorescence quantum yield, when plotted against the solvent polarity parameter (E T (30)), passes through a maximum. Plot of the fluorescence parameters in a graded series of dioxane-water mixture against E T (30) shows segmented linearity with two independent slopes; reflecting a lower sensitivity in the less polar region and a much higher sensitivity in higher polarity range. The observations lead to the proposition that the fluorescence of AODIQ originates from two different states, viz., a locally excited (LE) and a charge transfer (CT) one. Studies of the phosphorescence and heavy atom quenching of the fluorescence reveal that the CT state lies above the lowest triplet state even in a very polar environment.

Fragmentation of molecule-induced γ-cyclodextrin nanotubular suprastructures due to drug dosage

The guest-host concentration has already been proved to be a very important factor in the drug delivery process. In the present work we demonstrate the formation of compound induced gamma-cyclodextrin nanotubular suprastructure. The nanotubes formed are found to be highly sensitive to the concentration of the guest molecule. The increasing concentration of the compound in solution initiates a competition toward their existence inside the core of the nanotubes affecting the extent of nanotubular cluster formation. The hydrogen bonding responsible for the building of the cyclodextrin nanotubes is found to be partially disrupted because of this increasing competition. The continuous replacement of the guest molecules inside the nanochannels is supposed to be responsible for the instability in some of the hydrogen bondings that develop during the primary and the secondary interactions between the formed nanotubes resulting into fragmentation of the suprastructures. The steady state and time-resolved fluorescence experiments coupled with fluorescence anisotropy and atomic force microscopy illustrate the guest concentration dependence of the formation of the gamma-cyclodextrin nanotubes.

Kinetics of the photoconversion of diphenylamine in β-cyclodextrin environments

Abstract Photochemical conversion of diphenylamine (DPA) to carbazole (CAZL) has been studied fluorometrically in aerated aqueous and aqueous β-cyclodextrin (β-CD) environments. The fluorophore forms a 1:1 inclusion complex with β-CD. The quantum yield of the photoreaction is found to remain unaltered in both the environments. However, estimation of the rate constants for the photochemical conversion of DPA to CAZL in the two environments reveals that within the CD environment, rate of the photoreaction is affected remarkably. This has been rationalized from the restriction in the intramolecular rotation for accessing the reaction centres responsible for the photocyclization process. This has also been corroborated from a comparison of the cavity diameter of the β-CD and the optimized geometry of DPA obtained from the semiempirical (AM1-SCI) calculation.

Defining the different phases of premicellar aggregation using the photophysical changes of a surface-probing compound.

Photophysical changes of a cylindrical compound undergoing twisted intramolecular charge transfer may be used as a surface probe to study the different phases of premicellar aggregate formation. The probe molecule, trans-2-[4-(dimethylamino)styryl] benzothiazole (DMASBT), attaches itself to the premicellar and the micellar aggregates of cationic, anionic, and neutral surfactants in different orientations because of its dipolar nature in the excited state. The micelle formation is preceded by a few typical rearrangements of the surfactant molecules. These events need proper inspection that can only be done by compounds that sense environmental changes by residing in the vicinity of the surface of those aggregates. Steady-state and time-resolved fluorescence spectroscopy coupled with steady-state fluorescence anisotropy measurements serve as a very useful tool to monitor premicellar aggregate formation. The dipolar interaction of DMASBT with the surface of the aggregate and its extraordinary capability to sense the polarity of the environment make it a very efficient molecule to use for the purpose.

Rotamerisation and intramolecular proton transfer of 2-(2′-hydroxyphenyl)oxazole, 2-(2′-hydroxyphenyl)imidazole and 2-(2′-hydroxyphenyl)thiazole: a theoretical study

Abstract Semi-empirical (AM1–SCI) calculations have been performed on 2-(2′-hydroxyphenyl)oxazole (HPO), 2-(2′-hydroxyphenyl)imidazole (HPI) and 2-(2′-hydroxyphenyl)thiazole (HPT) to rationalise the photophysical behaviour of the compounds exhibiting intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The calculations reveal that there is a gradual variation in the properties from HPO to HPT through HPI so far as the existence of the rotational isomers in the ground state is concerned. While HPO gives rise to two stable rotamers ( I and II ) in all the common solvents, there is only one stable species for HPT in the S 0 state. For HPI, rotamer II is possible only in the isolated state and/or in solvents of low polarity, but in high polar solvents it gives rise to the normal form ( I ) only. For all the molecules in the series, however, intramolecular proton transfer (IPT) takes place in the lowest excited singlet ( S 1 ) and the triplet ( T 1 ) states. Combination of the rotamerism and ESIPT gives rise to multiple fluorescence bands for the fluorophores. Theoretical assignments have been made for the excitation, fluorescence and phosphorescence bands. Simulated potential energy curves (PEC) in different electronic states reveal that the IPT process is feasible in either of the S 1 and T 1 states but not in the ground state. The ESIPT reaction has been found to be favoured both thermodynamically and kinetically in these electronic states compared to the ground state. However, quantum mechanical tunnelling has been proposed for the prototropic reaction to proceed in the excited states.

Photoluminescence of 2-(2′-hydroxy-5′-methylbenzoyl)-1,5-diphenylpyrrole in aqueous and β-cyclodextrin environments: manifestation of open and closed conformers

Abstract Steady-state fluorometric studies have been performed on 2-(2′-hydroxy-5′-methylbenzoyl)-1,5-diphenylpyrrole (HMBDPP) in aqueous and aqueous β-cyclodextrin (β-CD) environments at ambient temperature. The fluorophore mostly shows a single emission in aqueous solution. Addition of β-CD to the aqueous solution of the fluorophore results in the development of another emission band at higher energy. The difference in the fluorometric behaviour is assigned to a remarkable change in the polarity of the microenvironment within the supramolecular structural environment compared to that of the bulk aqueous phase. Semi-empirical calculation (AM1-SCI) rules out the possibility of intramolecular proton transfer reaction in any of the S 0 , S 1 and T 1 states of the fluorophore. It is proposed that HMBDPP exists mostly in the intermolecularly hydrogen-bonded form (open conformer) in aqueous solution while within β-CD environment, it is the intramolecularly hydrogen-bonded form (closed conformer) that predominates.

Orientation of a TICT probe trapped in the peripheral confined water created by ionic surfactant envelope around silver nanoparticles.

Ionic surfactants are known to aggregate around the surface of a nanoparticle as a single layer in premicellar and a double layer in micellar concentrations. This motif of arrangement indicates the development of a layer of confined water of lower polarity than bulk water around the surface of the nanoparticle. We have demonstrated the behavior of a twisted intramolecular charge transfer (TICT) probe, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT), in the confined aqueous layer developed at the surface of spherical silver nanoparticles (Ag NPs) at and above the critical micellar concentrations (CMC) of a cationic and an anionic surfactant, namely, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). It is observed that the presence of charged surfactant head groups affects DMASBT differentially in the ground and the excited states. In presence of CTAB, DMASBT turns over in the excited state and interacts with the Ag NP surface, whereas in SDS the probe remains in its original orientation during the interaction. Steady-state and time-resolved fluorescence spectral studies provide enough evidence for orientation of the TICT probe in the peripheral water of Ag NP created by the surfactants. The results were confirmed by steady-state anisotropy measurements. The data show the difference between the properties of the confined peripheral water and the bulk aqueous environment. The TICT probe, DMASBT, is proved to be an excellent marker for the phenomenon.

A tetraphenylmethane based dendritic tolan–anthracene dyad: synthesis and energy transfer properties

In order to synthesize a peripherally rigid dendritic donor-acceptor dyad for energy transfer studies, a tritolan dendron based on a tetraphenylmethane scaffold was prepared from New Fuchsin. The dendron showed a small degree of homoconjugation but a large hypochromic effect. Coupling of two such dendrons with an anthracene core led to a dendritic tolan-anthracene dyad whose steady state photophysical studies (UV, PL, PLE) showed vectorial transfer of excitation energy from the surface tolan units to the anthracene core.

Photocyclization of triphenylamine: an investigation through time-resolved photoacoustic calorimetry

Energetics of formation of the intermediate produced during the photocyclization of triphenylamine (TPA) in alkane and alcoholic solvents have been investigated by employing the time-resolved photoacoustic calorimetric technique. The study, in conjugation with flash photolytic observations, confirms that the long lived intermediate N-phenyldihydrocarbazole (NPDHC) is formed through a cascade of processes. The endothermicity for the process of NPDHC formation from TPA was determined to be ∽53 kcal mol−1 in all the environments studied. The nature and structure of NPDHC have been assigned for the first time. The experimental value of the enthalpy of reaction resembles well that calculated from the semi-empirical (AM1-SCI) method.