The quest for a G-quadruplex specific fluorescent sensor among other DNA forms under physiological salt conditions has been addressed in this article. We demonstrate for the first time the application of a water-soluble fluorogenic dye, Thioflavin T (ThT), in a dual role of exclusively inducing quadruplex folding in the 22AG human telomeric DNA, both in the presence and absence of Tris buffer/salt, and sensing the same through its fluorescence light-up having emission enhancement of the order of 2100-fold in the visible region. Appropriate conditions allow an apparent switch over of the parallel quadruplex structure in 22AG-ThT (50 mM Tris, pH 7.2) solution to the antiparallel form just by the addition of K(+) ions in the range 10-50 mM. Moreover, addition of ThT cooperatively stabilizes the K(+) induced antiparallel quadruplexes by a ΔT(m) ∼11 °C. The distinction of ThT as a quadruplex inducer has been contrasted with the erstwhile used structurally related dye, Thiazole Orange (TO), which did not induce any quadruplex folding in the 22AG strand in the absence of salt. The striking fluorescence light-up in ThT on binding to the human telomeric G-quadruplex is shown to be highly specific compared to the less than 250-fold enhancement observed with other single/double strand DNA forms. This work has implication in designing new generation dyes based on the ThT scaffold, which are highly selective for telomeric DNA, for potential diagnostic, therapeutic, and ion-sensing applications.

The excited-state dynamics of a transition metal complex, tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck-Condon singlet excited state in this molecule was missing. In this study, emission from the Franck-Condon excited singlet state of [Ru(bpy)(3)](2+) has been observed for the first time, and its lifetime has been estimated to be 40 +/- 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited (1)(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 +/- 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin-orbit coupling is proposed. This enhances the radiative rate constant, k(r), of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56-1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.

The photophysical properties of the phenazine-based dye neutral red were investigated in aqueous solution in the presence of the macrocyclic host molecule cucurbit[7]uril (CB7) using ground-state absorption as well as steady-state and time-resolved fluorescence measurements. The results are contrasted to those previously obtained for beta-cyclodextrin (beta-CD; Singh et al. J. Phys. Chem. A 2004, 108, 1465). Both the neutral (NR) and cationic (NRH+) forms of the dye formed inclusion complexes with CB7, with the larger binding constant for the latter (K = 6.5 x 10(3) M(-1) versus 6.0 x 10(5) M(-1)). This result differed from that for beta-CD, where only the neutral form of the dye was reported to undergo sizable inclusion complex formation. From the difference in binding constants and the pK(a) value of protonated neutral red in the absence of CB7 (6.8), an increased pK(a) value of the dye when complexed by CB7 was projected (approximately 8.8). This shift differed again from the behavior of the dye with beta-CD, where a decreased pK(a) value (ca. 6.1) was reported. The photophysical properties of both NR and NRH+ forms showed significant changes in the presence of CB7. Fluorescence anisotropy studies indicated that the inclusion complexes of both forms of the dye rotate as a whole, giving rotational relaxation times much larger than that expected for the free dye in aqueous solution. The thermodynamic parameters for the NRH+.CB7 complex were investigated in temperature-dependent binding studies, suggesting an entropic driving force for complexation related to desolvation of the cation and the removal of high-energy water molecules from the CB7 cavity.

This Feature Article provides an account of the recent work by our research group (with some reference to relevant work of other groups) on the spectacular physico-chemical properties of cucurbituril-based supramolecular assemblies of chromophoric dyes having technological and biological importance. Simultaneous association of multiple hosts or guests either by cooperative binding or competitive displacement is an applied strategy to construct novel functional assemblies with predesignated characteristics. Here, effort has been made to briefly discuss the diverse photophysical characteristics of several host-guest complexes and the dynamic responses of such molecular assemblies towards external stimulants like metal ions. The detailed experimentation on these assemblies project their applications in the controlled uptake and release of drugs, photofunctional devices, aqueous-based dye lasers, and molecular architectures.