Dual Fluorescence of 2-(4’-N,N-Dimethylaminophenyl) imidazo[4,5-b]pyridine: Effect of homogeneous and microheterogeneous environments

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Intermolecular charge transfer (ICT) is a common excited state phenomenon that occurs in several photochemical and photobiological processes. Organic molecules those consist of an electron donor and acceptor pair linked by a single bond may undergo ICT in the excited state. Such bichromophore compounds have received considerable attention as possible models for the investigation of the primary function of photoelectric devices and chemical energy storage. Though several models are proposed to explain the formation of ICT state, the twisted intramolecular charge transfer (TICT) model proposed by Grabowski et al. is widely accepted. In TICT model, upon excitation, within lifetime of the locally excited state of the molecule, transfer of an electron from the donor to the acceptor is accompanied by rotational relaxation to a twisted conformation of the donor relative to the acceptor to form the TICT state. At perpendicular geometry, the donor and the acceptor are completely decoupled and the electrostatic interaction between them stabilizes the TICT state. The formation of TICT state mostly results in dual fluorescence. The shorter wavelength emission band is called normal or locally excited fluorescence and the longer wavelength emission band is assigned as TICT band. The TICT emission is highly sensitive to environment. This environmental sensitivity of the TICT emission has rendered it useful as a microenvironment probe. Solvent polarity and viscosity usually affects the relative positions and intensities of the TICT fluorescence. Of special relevance to the present work are reports that H-bonding of the solvent plays an important role in formation and stabilization of the TICT state. Such hydrogen bonding may be important in facilitating proton-coupled charge transfer phenomena often observed in biological assemblies and other systems. The present thesis described the effect solvent, pH, metal ions N-alkylation and organized assemblies such cyclodextrin, normal and reverse micelle and proteins on the TICT of 2-(4’-N,Ndimethylaminophenyl) imidazo[4,5-b]pyridine (DMAPIP-b, Chart 1). Besides, investigating the effect of organized assemblies’ effort is made to understand the nature and characteristics of these heterogeneous systems...
Supervisor: G. Krishnamoorthy