Haridas Pal

Cooperative Metal Ion Binding to a Cucurbit[7]uril−Thioflavin T Complex: Demonstration of a Stimulus-Responsive Fluorescent Supramolecular Capsule

We report an intriguing noncovalent interaction of thioflavin T (ThT), a fibril diagnostic dye, with the versatile macrocyclic host molecule cucurbit[7]uril (CB7) in the presence of metal cations. ThT forms both 1:1 (CB7.ThT) and 2:1 [(CB7)(2).ThT] complexes with CB7 host, leading to specific structural arrangements. Addition of competitive guests like metal cations to the 1:1 stoichiometric complex displays expected competitive binding interactions with CB7, leading to decreased fluorescence intensity from ThT. However, addition of metal ions to the 2:1 complex leads to unusual enhancement in the fluorescence emission ( approximately 270-fold in the presence of Ca(2+) and approximately 160-fold in the presence of Na(+)). These contrasting observations on the fluorescence enhancement with change in the stoichiometric equilibrium have been investigated explicitly for a feasible binding model. Detailed photophysical characterization with supporting data from NMR and anisotropy measurements has led to the revelation of a novel stimulus-responsive cooperative metal ion binding to the stoichiometrically selected (CB7)(2).ThT complex, demonstrating a highly fluorescent supramolecular nanocapsule. The first example of a noncovalently packed fluorescent complex became feasible due to the structural arrangement of the host-guest complex in the 2:1 stoichiometry with two CB7 portals providing strong negative charge density for the metal ions to group and seal the complex, thus protecting the incorporated dye. To further strengthen the usefulness of the supramolecular capsule established here, rupture of the capsular complex has been demonstrated with a strong competitive guest, 1-amantadine hydrochloride, which helped in disrupting the capsule to release the dye. It is proposed here that by judicious design of the chromophore (guest) structure, such capsular assemblies can be explored for the binding and release of drug molecules, for fluorescence on-off systems, and as building blocks for molecular architectures displaying unique properties.

Photophysical properties of coumarin-120: Unusual behavior in nonpolar solvents

Photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and picosecond laser flash photolysis (LFP) and nanosecond pulse radiolysis (PR) techniques. C120 shows unusual photophysical properties in nonpolar solvents compared to those in other solvents of moderate to higher polarities. Where the Stokes shifts (Δν=νabs−νfl), fluorescence quantum yields (Φf), and fluorescence lifetimes (τf) show more or less linear correlation with the solvent polarity function Δf={(e−1)/(2e+1)−(n2−1)/(2n2+1)}, all these parameters are unusually lower in nonpolar solvents. Unlike in other solvents, both Φf and τf in nonpolar solvents are also strongly temperature dependent. It is indicated that the excited singlet (S1) state of C120 undergoes a fast activation-controlled nonradiative deexcitation in nonpolar solvents, which is absent in all other solvents. LFP and PR studies indicate that the intersys...

Photodynamics of the S1 state of some hydroxy- and amino-substituted naphthoquinones and anthraquinones

Photodynamics of the S1 state of the 1,4- and 1,8-disubstituted hydroxyquinones and aminoquinones have been studied in different organic solvents by absorption and fluorescence spectroscopy. The internal conversion process is the major deactivation path for the S1 state. The solvent and temperature dependence and deuterium isotope effect on the fluorescence dynamics suggest that hydrogen stretching vibrations in different intra- and inter-molecular hydrogen bonds are responsible for very fast non-radiative decay processes. Among the intra-molecular hydrogen-bonded molecules studied only the S1 state of 1,8-dihydroxy-9,10-anthraquinone showed the evidence of excited-state intramolecular proton-transfer process. Preliminary observations on the interaction of the S1 state of these quinones with benzene and other aromatic hydrocarbon solvents via the formation of an exciplex have also been reported.

Triplet excited states and semiquinone radicals of 1,4-disubstituted anthraquinones

Abstract The triplet state properties of 1,4-dihydroxy-9,10-anthraquinone (quinizarin, QNZ), 1-amino-4-hydroxy-9,10-anthraquinone (AHAQ) and 1,4-diamino-9,10-anthraquinone (DAAQ) were investigated in cyclohexane and isopropanol solutions using nanosecond laser flash photolysis. TT absorption maxima, extinction coefficients, triplet quantum yields and kinetic parameters were measured. The corresponding properties of the neutral semiquinone radicals were also determined in isopropanol. Pulse radiolysis data were used to supplement the measurements. The possibility of dimerization in solution and its effect on the photophysics of the triplet state are discussed for these quinones.

Noncovalent interaction of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin with Cucurbit[7]uril: a supramolecular architecture.

Noncovalent interaction of two water-soluble synthetic macromolecules, Cucurbit[7]uril (CB7) and 5,10,15,20-tetrakis(4- N-methylpyridyl)porphyrin (TMPyP), has been studied from the viewpoint of organizing them through supramolecular interactions and thereby modulating their functional activities. Steady-state and time-resolved fluorescence measurements along with NMR results establish that CB7 crowns the N-methylpyridyl group of the TMPyP in a 1:4 stoichiometry. The overall binding constant was evaluated to be approximately 4.5 x 10 (19) M (-4). The high binding affinity, promoting a stable and extendable molecular assembly in aqueous solution, could open new frontiers in the design and synthesis of higher-order supramolecular structures with photofunctional moieties and project their utility in therapeutic applications.

Effect of solvent polarity on the aggregation of C60

C60 forms aggregates in solution when the dielectric constant of the solvents exceeds some critical value viz. ⩾13. Further, for aggregation the C60 concentration has to exceed some critical value. The aggregation process is reversible and the aggregates exist in equilibrium with monomers. Picosecond laser flash photolysis experiments indicate that the excited (S1) aggregates undergo very fast relaxation without contributing towards the triplet formation.

Contrasting guest binding interaction of cucurbit[7-8]urils with neutral red dye: controlled exchange of multiple guests

Interactions among macrocyclic hosts and dyes/drugs have been explored extensively for their direct usage in controlled uptake and release of large number of potential drug molecules. In this paper we report the non-covalent interaction of cucurbit[8]uril macrocycle (CB8) with a biologically important dye, neutral red, by absorption and fluorescence spectroscopy. A comparative analysis with the complexation behaviour of the dye with CB7, the lower homologue of CB8, indicates contrasting guest binding behaviour with significant changes in the photophysical characteristics of the dye. While CB7 interaction leads to a 1 ratio 1 stoichiometry resulting in approximately 6 fold enhancement in the fluorescence emission of the dye, CB8 displays signatures for a 1 ratio 2 host-guest stoichiometry with drastic reduction in the fluorescence emission. Apart from the evaluation of approximately 2 unit shift in the protolytic equilibrium on complexation (pK(a) shift), the measurements with tryptophan established a selective guest exchange to favour a co-localized dimer inside the CB8 cavity. In a protein medium (BSA), the 1 ratio 2 complex was converted to a 1 ratio 1 ratio 1 CB8-NRH(+)-BSA complex. The finding that NRH(+) can be transferred from CB8 to BSA, even though the binding constant for NRH(+)-CB8 is much higher than NRH(+)-BSA, is projected for a controlled slow release of NRH(+) towards BSA. Since the release and activity of drugs can be controlled by regulating the protolytic equilibrium, the macromolecular encapsulation and release of NRH(+) demonstrated here provide information relevant to host-guest based drug delivery systems and its applications.

Interaction of C60 and C70 with aromatic amines in the ground and excited states. Evindence for fullerene—benzene interaction in the ground state

While C60 interacts with aromatic amines such as dimethylaniline in the ground state, C70 does not. Fluorescence spectroscopic studies, including lifetime measurements, show the formation of exciplexes of both C60 and C70 with aromatic amines in nonaromatic solvents such as methylcyclohexane. Exciplexes are however not formed in benzene solvent, due to π—π interaction between benzene and the fullerene. Based on spectroscopic absorption measurements, it is shown that both C60 and C70 do indeed interact with benzene in the ground state.

Intermolecular electron transfer between coumarin dyes and aromatic amines in Triton-X-100 micellar solutions: Evidence for Marcus inverted region

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amines has been investigated in Triton-X-100 micellar solutions and the results have been compared with those observed earlier in homogeneous medium. Significant static quenching of the coumarin fluorescence due to the presence of high concentration of amines around the coumarin fluorophore in the micelles has been observed in steady-state fluorescence studies. Time-resolved studies with nanosecond resolutions mostly show the dynamic part of the quenching for the excited coumarin dyes by the amine quenchers. A correlation of the quenching rate constants, estimated from the time-resolved measurements, with the free energy changes (DeltaG0) of the ET reactions shows the typical bell shaped curve as predicted by Marcus outer-sphere ET theory. The inversion in the ET rates for the present systems occurs at an exergonicity (-DeltaG0) of approximately 0.7-0.8 eV, which is unusually low considering the polarity of the Palisade layer of the micelles where the reactants reside. Present results have been rationalized on the basis of the two dimensional ET model assuming that the solvent relaxation in micellar media is much slower than the rate of the ET process. Detailed analysis of the experimental data shows that the diffusional model of the bimolecular quenching kinetics is not applicable for the ET reactions in the micellar solutions. In the present systems, the reactions can be better visualized as equivalent to intramolecular electron transfer processes, with statistical distribution of the donors and acceptors in the micelles. A low electron coupling (Vel) parameter is estimated from the correlation of the experimentally observed and the theoretically calculated ET rates, which indicates that the average donor--acceptor separation in the micellar ET reactions is substantially larger than for the donor--acceptor contact distance. Comparison of the Vel values in the micellar solution and in the donor--acceptor close contact suggests that there is an intervention of a surfactant chain between the interacting donor and acceptor in the micellar ET reaction.

Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70

Abstract Effect of solvent polarity on the aggregation behaviour of C70 has been investigated in several mixed solvents using optical absorption, fluorescence, dynamic light scattering and scanning electron microscopic measurements and compared with those observed for the other fullerene analogue, C60. It is seen that similar to C60, aggregation of C70 also requires the solvent polarity to exceed some critical value. In terms of solvent dielectric constant the critical solvent polarity, required for C70 aggregation is found to be in the range of ∼27–31, which is much higher than that required for C60 aggregation (∼12–14). The large difference in the critical solvent polarity required for C60 and C70 aggregation has been rationalized on the basis of the molecular shapes and the polarizabilities of two fullerene molecules.