DNA-Induced Novel Optical Features of Ethyl Viologen-Tethered Perylenediimide Triad

Perylenediimide (PDI) chromophores with redox-active groups have facilitated the construction of several novel functional materials. For the first time, unusual spectroscopic changes and differential binding behavior of a tetracationic ethyl viologen tethered PDI derivative, PDEV, with calf-thymus DNA (ct-DNA) is reported here and is ascertained as due to the DNA compaction. From an initial aggregation on the DNA backbone, resulting in quenching in the emission intensity (turn off), with increase in ct-DNA concentration, a novel and strong fluorescence band (turn on) emerged in the blue region. Distinct absorption spectral changes having sharp features in vibronic patterns, increased fluorescence lifetime (from 0.33 to 4.5 ns), anisotropy values are also displayed on titration with ct-DNA corroborating the changes in microenvironment of PDEV in commensurate with structural transition from coil state to compact state. The structural changes in the DNA are also established from the circular dichroism spectra, viscosity measurements, and DNA melting data. The compaction is also directly visualized in the atomic force microscopy, scanning electron microscopy, and fluorescence microscopy images. Such compact DNA morphologies with redox-responsive donor-acceptor moieties are potential to create desired nanostructures with stimuli responsive functionalities such as on-off switch, biosensor, drug delivery, and other optoelectronic devices.