Unraveling Molecular Assembly and Tracking Lipid Droplets Dynamics using Fluorescent Phenanthroimidazole Derivatives

A judicious structural modification of molecular building units imparts significant variation in the evolution dynamics of supramolecular self-assembled architectures and their functional properties. The incorporation of alkyl chains into the rigid and π-conjugated molecular backbone not only directs the self-assembly but also enhances the hydrophobicity of the probe, facilitating specific interactions with hydrophobic organelle like lipid droplets (LDs). Such a fluorescent molecular probe is ideally suited for elucidation of complex self-assembly processes and to decipher the intracellular dynamics of LDs. Invoking the concept, the main skeleton of 1,4-bis(1H-phenanthro[9,10-d]imidazol-2-yl)benzene abbreviated as BPIB, was functionalized with a varying number of octyl chains to obtain BPIB1 and BPIB2. The alkyl chains were crucial to circumvent the π-π stacking leading to the strong fluorescence in aggregates and solid-state. Contrary to BPIB and BPIB2, intermolecular interactions-driven spontaneous self-association of BPIB1 having an amine nitrogen center and a long alkyl chain resulted in a stimuli-responsive fluorescent organogel. We deciphered the reversible morphological transformation between supramolecular fibers and spherical nanoaggregates using fluorescence lifetime imaging microscopy. The gradual progression in the fluorescence lifetime provides a unique strategy for exploring the dynamics of the self-assembly process. Furthermore, BPIB1 was found to be a specific marker for LDs in multiple cell lines. The fluorescence correlation spectroscopy revealed the microenvironments near LDs. Highly photostable BPIB1 was employed for the real-time tracking of the LDs dynamics in live cells. Thus, a combined microscopic and spectroscopic approach demonstrated in the present study opens up new avenues for further exploration of intriguing molecular aggregation and intracellular dynamics.