A fluorescent molecular rotor showing vapochromism, aggregation-induced emission, and environmental sensing in living cells

Abstract

Among the plethora of recently proposed molecular sensors, those belonging to the class of fluorescent molecular rotors (FMRs) have attracted much attention owing to their peculiar photophysical properties that enable an unprecedented sensitivity towards environmental microviscosity. The usual FMR synthetic design prescribes chromophores characterized by an intramolecular rotation between two well-defined excited states, a locally excited state and a twisted internal charge transfer (TICT) state, where the sensing capabilities arise from a dual competition of the corresponding radiative/non-radiative decay processes. However, we have recently demonstrated a different modus operandi of a new subclass of solvatochromic FMRs, which exploit a solvent-independent, barrier-free intramolecular rotation of the excited dye. The rotational dynamics is modulated by local viscosity and, in turn, manifested through a variable spectral signal. In order to translate the same rotational mechanism in a versatile sensor of polarity and viscosity, we designed and thoroughly characterized a novel FMR, namely 4-(triphenylamino)-phthalonitrile (TPAP). Remarkably, in addition to a high sensitivity versus solvent polarity and viscosity, TPAP is also able to form stable fluorescent nanoparticles characterized by aggregation-induced emission, via a simple sonochemical treatment. Such peculiar features are tested in different applications aiming at illustrating its capability to report on solvatochromic and vapochromic effects, as well as to provide detailed intracellular information through bioimaging studies.

Aggregation‐Induced Emission: Lighting up Cells, Revealing Life!

“…Detailed information about the detection of lipid droplets will be presented in Section . Another highly sensitive intracellular viscosity probe with AIE properties, named TPAP, was reported by Bottari and co‐workers (Figure g–h) . As the solvent viscosity increased, from methanol to glycerol, the rotation of TPAP was gradually constrained and the fluorescence intensity and lifetimes were enhanced.…”

“…Scale bars: i) 10 μm, (j) 5 μm. Reproduced with permission . Copyright 2016, Royal Society of Chemistry.…”

Strategies to Overcome the Limitations of AIEgens in Biomedical Applications

“…So far, various AIEgens have been synthesized to image bacteria, cell membranes, mitochondria, lysosomes, nuclei, tissue, blood vessels, and even tumors . In addition, different AIE probes have also been developed to monitor biological parameters within intracellular microenvironments like pH, biothiol levels, and viscosity through fluorescence, as well as lifetime imaging . Recently, AIEgens have been successfully used to light‐up cellular processes including autophagy, apoptosis, protein fibrillation, and others …”

Simultaneous Detection of Local Polarizability and Viscosity by a Single Fluorescent Probe in Cells

“…Indeed, the characteristic long lifetime of electronic excited states (from a few picoseconds to nanoseconds) makes fluorescence exquisitely sensitive to changes in the local (nano) environments. Accordingly, many fluorescent polarity and viscosity probes have been employed to image the cell environment and have contributed to the elucidation of biochemical mechanisms (8)(9)(10)(11)(12)(13)(14)(15). Most probes, however, are characterized by photophysical processes of their excited states (e.g., twisted intramolecular charge transfer) that lead to complex and intertwined responses to polarity and viscosity (16)(17)(18).…”