Pyrene-cyanostyrene-pyridine triad: Multi-stimuli responsive fluorescent emitter and mitochondrial imaging

Abstract

Tuning the fluorescence of π-conjugated small organic molecules through the structural modifications and in the presence of external stimuli have found immense applications ranging from sensing, switching, optoelectronics to bioimaging. Herein, we demonstrated a simple design strategy for the synthesis of a pyrene-cyanostyrene-pyridine molecular triad (PYBNP) using a cost-effective, one-step Knoevenagel condensation between 1-pyrenecarboxaldehyde and 2-(4-(pyridin-4-yl)phenyl)acetonitrile. PYBNP exhibited strong solid-state emission having a quantum yield of ∼ 36 ± 3% due to the deactivation of nonradiative decay channels through the restriction of intramolecular rotations and reduced π-π stacking interactions as revealed from the crystal structure analysis. Similarly, PYBNP was found to be highly emissive in the constrained environments like viscous medium and aggregated state. The optical properties of PYBNP could be tuned by the UV light-mediated dynamic Z/E-isomerization as elucidated through fluorescence spectroscopy and NMR analysis. Additionally, the pyridinic nitrogen centre allowed the fabrication of methylated analog (PYBNPM; a new cationic photoresponsive species). The cationic PYBNPM showed a better solvatochromic behavior contrary to PYBNP due to the efficient intramolecular charge transfer (ICT). The different crystal packing of PYBNP and that of hexafluorophosphate salts of PYBNPM led to distinct fluorescent properties in the solid-state. As a preliminary exploration, the cationic PYBNPM(PF6) was employed for the specific imaging of the mitochondria through multiple fluorescence channels in an aqueous medium. The cost-effective and viable design strategy demonstrated for the fabrication of a simple molecular triad holds the promise for the emergence of new functional fluorescent molecules to molecular materials for task-specific applications.