Femtosecond Time-Resolved Raman Spectroscopy Reveals Structural Evidence for meta Effect in Stilbenols.

The meta effect in substituted aromatics plays a crucial role in their excited-state photophysical properties. Meta-substituted hydroxyarenes such as naphthols, stilbenols, and chromophoric constituents of green fluorescent proteins show unusual photoacidity and enhanced fluorescence lifetime and quantum yield when compared to their para-derivatives. Variation in the excited state features of the meta-derivatives when compared to the para-derivatives in stilbenols has been attributed to the enhanced torsional barrier for interconversion between the planar and the twisted perpendicular forms. Herein, we employed femtosecond time-resolved Raman spectroscopy to provide the direct structural evidence for the enhanced torsional barrier in meta-stilbenol. The Raman band profiles of the olefinic C═C stretch related to the torsional motion are found to decay with time constants of ∼750 and ∼13 ps in meta-stilbenol and para-stilbenol respectively, unraveling the structural evidence for the observed enhanced photoacidity originating from enhanced rates of excited-state proton transfer. Further, time-resolved fluorescence measurements are performed to elucidate the relaxation pathways of the excited states of the stilbenols.

Simultaneous binding of a cyclophane and classical intercalators to DNA: observation of FRET-mediated white light emission.

DNA-assisted Förster resonance energy transfer (FRET) between an anthracene-based cyclophane (CP) and mono- and bis-intercalators such as propidium iodide (PI) and ethidium homodimer-1 (EHD), respectively, has been studied using various photophysical and biophysical techniques. The cyclophane and PI exhibited simultaneous binding to DNA at all concentrations studied and showed DNA-assisted FRET from the excimer of cyclophane with a FRET efficiency of ca. 71%. On the other hand, the bis-intercalator EHD, only at lower concentrations (<3 µM), can act as an acceptor for the energy transfer process with a lower efficiency of ca. 44%. At higher concentrations (>15 µM), EHD, on account of its higher binding affinity, displaces cyclophane from the DNA scaffold. Employing the ternary system comprising of the cyclophane, DNA and PI and fine-tuning the concentrations of the components in a molar ratio of 1 : 0.75 : 0.05 (CP : DNA : PI) we have demonstrated white light emission with CIE coordinates (0.35, 0.37).

White photoluminescence and electroluminescence from a ternary system in solution and a polymer matrix.

Efficient white photoluminescence was obtained from a ternary system in solution and a polymer matrix when excited at 380 nm whereas at 560 nm, it showed intense red emission. The LED device fabricated based on this system exhibited white electroluminescence with a low turn on voltage of 3 V.

DNA-assisted white light emission through FRET.

We observed FRET between an excimer of a partially intercalated cyclophane and ethidium bromide, a classical intercalator in presence of DNA and by fine-tuning the molar concentrations of these three components, we could generate white light emission in the aqueous and non-aqueous media.

Effect of bridging units on photophysical and DNA binding properties of a few cyclophanes.

A few novel anthracene-based cyclophanes CP-1, CP-2 and CP-3 were synthesized and their interactions with DNA were investigated employing photophysical and biophysical techniques. In methanol and acetonitrile, these systems exhibited optical properties characteristic of the anthracene chromophore. However, in the aqueous medium, the symmetric cyclophane CP-1 showed a dual emission having lambda(max) at 430 and 550 nm, due to the monomer and excimer, respectively. In contrast, the cyclophanes CP-2 and CP-3 in the aqueous medium showed structured anthracene absorption and emission spectra similar to those obtained in methanol and acetonitrile. DNA binding studies indicate that CP-1 undergoes efficient nonclassical partial intercalative interactions with DNA resulting in the exclusive formation of a sandwich-type excimer having enhanced emission intensity and lifetimes. The cyclophane CP-2 having one anthracene moiety exhibited nonclassical intercalative binding with DNA, albeit with less efficiency compared with CP-1. In contrast, CP-3, having sterically bulky viologen bridging group showed DNA electrostatic as well as groove binding interactions. These results demonstrate that the nature of the bridging unit plays an important role in the binding mode of the cyclophanes with DNA and in the formation of the novel sandwich-type excimer.