Synergetic Nano-bending, Photo-switching, and Macrocycle Shuttling Effects of Vibration-Induced Emission Lassos on Photoluminescence Properties of Hydrazone-Based [1]Rotaxanes

Abstract 9,14-Diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) fluorophores exhibited large Stokes shifts and vibration-induced emission (VIE) with bistable cyan (at 475 nm) and orange (at 600 nm) emissions, whereas hydrazone fluorophores (cyan emission at 475 nm) underwent reversible Z/E photo-isomerization, enabling efficient photo-switching. Herein, we introduced synergetic nano-bending and photo-switchable effects on optical switchable hydrazone linkers and macrocycle shuttling effects of VIE-active DPAC stoppers in [1]rotaxanes, wherein mechanically interlocked architecture-imposed spatial constraints could modulate photoluminescence (PL) behaviors in response to multiple external stimuli. Surprisingly, the highest lifetime τavg1 = 11.8 ns (cyan emission lifetime monitored at 475 nm) was obtained in the loosened lasso owing to stronger intramolecular hydrogen bonds of hydrazone units with minor constraints of nano-bending. While macrocycle shuttling was acid/base-triggered between the loosened lasso with a near-white-light emission [CIE (0.26, 0.32)] and the tightened lasso with an orange emission [CIE (0.37, 0.36)], [1]rotaxanes could tune synergetic PL emissions by modulating bent/planar conformations of DPAC units via acid/base controls and Z/E photo-isomerization of hydrazone units upon 365/450 nm exposures in isopropanol (IPA), respectively. The non-interlocked analogue exhibited the largest lifetime decrease (Δτavg1 = –1.9 ns), whereas the loosened and tightened lassos showed smaller decreases of Δτavg1 = –1.6 and –0.6 ns, respectively, demonstrating that nano-bending constraints suppressed hydrogen-bond disruption during Z/E photo-isomerization. Furthermore, the loosened lasso exhibited temperature- and viscosity-responsive emissions due to hydrogen-bonding disruptions of hydrazone units and progressive planarization of DPAC units upon heating to induce lower viscosity. These results highlight the potential of [1]rotaxanes as stimuli-responsive luminescent molecular systems, enabling proof-of-concept demonstrations in temperature and viscosity sensing as well as optical information encoding.