An anthracene-based molecular rotor as a theranostic agent for viscosity sensing and imaging-guided photodynamic therapy.

Viscosity variations within cellular microenvironments are closely associated with pathological states such as cancer, motivating the development of molecular probes that integrate environmental sensing with therapeutic function. Here, we report two anthracene-fused heptamethine cyanine molecular rotors, ASCy7 (asymmetric) and SCy7 (symmetric), designed as heavy-atom-free near-infrared (NIR) materials for combined viscosity sensing and imaging-guided photodynamic therapy (PDT). Anthracene π-extension enhances intramolecular charge transfer and spin-orbit coupling, thereby promoting intersystem crossing and efficient singlet oxygen generation under 850 nm irradiation. Both probes exhibit strong viscosity-dependent fluorescence via the twisted intramolecular charge transfer (TICT) mechanism and show selective mitochondrial localization in HepG2 cells. Upon NIR activation, ASCy7 and SCy7 induce reactive oxygen species-mediated apoptosis with IC50 values as low as 0.39 µM, while maintaining excellent dark biocompatibility. This work establishes π-extension engineering as an effective materials design strategy to integrate environmental responsiveness and photodynamic activity within a single NIR cyanine platform for cancer theranostics.