Flower-like morphology of naphthalene diimides containing tetra-L- and D-alanine

Abstract

Two structural isomers of naphthalene diimide (NDI) chromophores NDI-3 and NDI-4, which are NDI imide position functionalised with four L-/D-alanine subunits. Density functional theory calculations revel that HOMO molecular orbital is localized on N-phenyl substituent and LUMO on NDI core. Further, the UV-vis, fluorescence measurements show that both the derivatives i.e. NDI-3 and NDI-4 exhibit changes in optical and photophysical properties with the addition of non-polar i.e. methylcyclohexane in chloroform as well as water in tetrahydrofuran. From these results, it is clear that the structural isomers NDI-3 and NDI-4 shows a significant solvatochromic effect, which ascribed to the aggregation effect via Pi-Pi stacking of NDI core along with hydrogen-bonding between alanine side chains. Scanning electron microscopy (SEM) and polarized optical microscopy images clearly demonstrate formation of flower like morphology in THF:H sub(2)O and fiber nanostructure in CHCl sub(3):MCH for both NDI-3 and NDI-4. Interestingly, only NDI-4 in CHCl sub(3):MCH (4:6, v/v) solvent mixture produces rose-like flower microstructures.