Exploring viscosity sensitivity of π-extended coumarin fluorogen-based PRPGs for precise photorelease of valproic acid: detection and defibrillation of TDP-43 aggregation.

Alteration of cellular microenvironment viscosity by protein aggregation plays a crucial role as a biophysical parameter that reflects abnormal cellular behaviour, leading to neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), etc. Herein, we report the design and development of a series of coumarin fluorogen-based photoremovable protecting groups (PRPGs, 5a-d) with variations in substitutions tethered with a π-extended linker that integrate viscosity sensing with visible-light-triggered release of bioactive molecules. By introducing π-extended conjugation and systematic substitution, the coumarin fluorogen-based PRPGs exhibit twisted intramolecular charge transfer (TICT)-based fluorescence modulation in response to microenvironmental viscosity. Comprehensive photophysical and photochemical investigations, supported by theoretical calculations, identified PRPG 5d as the most sensitive viscosity-responsive system with green-light absorption. Under viscous conditions, restricted bond rotation suppresses nonradiative decay and photoisomerization, enabling efficient photorelease of the neuroprotective agent valproic acid. The versatility of PRPG 5d was demonstrated in biologically relevant in vitro models, including TDP-43 protein aggregation and Parkinson's disease induced SH-SY5Y neuroblastoma cells. In both extracellular and intracellular neurodegenerative environments, increased viscosity was effectively sensed, triggering light-mediated valproic acid release and subsequent defibrillation. Overall, this work establishes coumarin fluorogen-based PRPGs as a promising platform for viscosity-guided, spatiotemporally controlled drug release, offering potential applications in the diagnosis and targeted therapy of neurodegenerative diseases.