Replacing conventional dialkylamino substituents with a three-membered aziridine ring in naphthalimide leads to significantly enhanced brightness and photostability by effectively suppressing twisted intramolecular charge transfer formation. This replacement is generalizable in other chemical families of fluorophores, such as coumarin, phthalimide, and nitrobenzoxadiazole dyes. In highly polar fluorophores, we show that aziridinyl dyes even outperform their azetidinyl analogues in aqueous solution. We also proposed one simple mechanism that can explain the vulnerability of quantum yield to hydrogen bond interactions in protonic solvents in various fluorophore families. Such knowledge is a critical step toward developing high-performance fluorophores for advanced fluorescence imaging.

The twisted intramolecular charge transfer (TICT) mechanism and twists beyond TICT have guided the creation of numerous bright and sensitive fluorophores. We reviewed the structure–property relationships of these dyes with representative examples.

The fluorescence intensity of N,N-dimethyl-4-((2-methylquinolin-6-yl)ethynyl)aniline exhibits an unusual intensification with increasing temperature, by activating more vibrational bands and leading to stronger TICT emissions upon heating in dimethyl sulfoxide. Based on the different temperature dependence at various wavelengths, as shown in the TICT fluorescence spectrum, this dye can be employed to ratiometrically detect temperature.

Inhibition of TICT can significantly increase the brightness of fluorescent materials. Accurate prediction of TICT is thus critical for the quantitative design of high‐performance fluorophores and AIEgens. TICT of 14 types of popular organic fluorophores were modeled with time‐dependent density functional theory (TD‐DFT). A reliable and generalizable computational approach for modeling TICT formations was established. To demonstrate the prediction power of our approach, we quantitatively designed a boron dipyrromethene (BODIPY)‐based AIEgen which exhibits (almost) barrierless TICT rotations in monomers. Subsequent experiments validated our molecular design and showed that the aggregation of this compound turns on bright emissions with ca. 27‐fold fluorescence enhancement, as TICT formation is inhibited in molecular aggregates.

Viscosity in the intracellular microenvironment shows a significant difference in various organelles and is closely related to cellular processes. Such microviscosity in live cells is often mapped and quantified with fluorescent molecular rotors. To enable the rational design of viscosity-sensitive molecular rotors, it is critical to understand their working mechanisms. Herein, we systematically synthesized and investigated two sets of BODIPY-based molecular rotors to study the relationship between intramolecular motions and viscosity sensitivity. Through experimental and computational studies, two conformations (i.e., the planar and butterfly conformations) are found to commonly exist in BODIPY rotors. We demonstrate that the transformation energy barrier from the planar conformation to the butterfly conformation is strongly affected by the molecular structures of BODIPY rotors and plays a critical role in viscosity sensitivity. These findings enable rational structure modifications of BODIPY molecular rotors for highly effective protein detection and recognition.

Photoinduced electron transfer (PET) is one of the most important mechanisms for developing fluorescent probes and biosensors. Quantitative prediction of the quantum yields of these probes and sensors is crucial to accelerate the rational development of novel PET-based functional materials. Herein, we developed a general descriptor (ΔE) for predicting the quantum yield of PET probes, with a threshold value of ∼0.6 eV. When ΔE < ∼0.6 eV, the quantum yield is low (mostly <2%) due to the substantial activation of PET in polar environments; when ΔE > ∼0.6 eV, the quantum yield is high because of the inhibition of PET. This simple yet effective descriptor is applicable to a wide range of fluorophores, such as BODIPY, fluorescein, rhodamine, and Si-rhodamine. This ΔE descriptor enables us not only to establish new applications for existing PET probes but also to quantitatively design novel PET-based fluorophores for wash-free bioimaging and AIEgen development.

Rhodamine spirolactams with adjacent amino groups work as acid-resistant and photoswitchable fluorophores in single-molecule localization super-resolution imaging.

Charge transfer and separation are important processes governing numerous chemicalreactions.Fundamental understanding of these processes and the underlying mechanisms is critical for photochemistry.H erein, we report the discovery of anew charge-transfer and separation process, namely the twisted intramolecular charge shuttle (TICS). In TICS systems,t he donor and acceptor moieties dynamically switch roles in the excited state because of an approximately 908 8 intramolecular rotation. TICS systems thus exhibit charge shuttling.T ICSs exist in several chemical families of fluorophores (sucha sc oumarin, BODIPY,a nd oxygen/carbon/ silicon-rhodamine), and could be utilized to construct functional fluorescent probes (i.e., viscosity-or biomoleculesensing probes). The discovery of the TICS process expands the current perspectives of charge-transfer processes and will inspire future applications.Photo-induced charge transfer and separation is af undamental process, [1] responsible for photosynthesis, [2] and has applications in solar cells, [3] photocatalysis, [4] and fluorescence probes. [5] Greater understanding of charge-transfer and separation processes is thus important to aid in improving photochemistry.H owever,o wing to fast photon-absorption rates and short excited state lifetime,u nderstanding charge transfer and separation at amolecular level remains asignificant challenge.This challenge could be overcome if the model systems under study emit fluorescence (or other forms of luminescence). [1b] Thec hanges in fluorescence output (for example, intensity,l ifetime,a nd wavelength) provide critical information on the thermodynamics and kinetics of the chargetransfer and separation processes in the excited state,a long with ap lethora of other important photo-physical and -chemical information. Accordingly,s tudies have been con-ducted to investigate charge-transfer and separation mechanisms in fluorescent compounds. [6] It has been widely known that the intramolecular charge transfer (ICT) could be modulated via adjusting the "push-pull" effect in acompound consisting of an electron-donating group (EDG) and/or an electron-withdrawing group (EWG). [7] In such compounds, ICT is further enhanced upon photoexcitation. In particular, in al andmark study,G rabowski and co-workers proposed that quasi-rigid donor-acceptor (D-A) compounds could undergo approximately 908 8 intramolecular twisting in the excited state,greatly reinforcing charge transfer and resulting in ac harge-separated state (D + -A À ). [8] They named this mechanism twisted intramolecular charge transfer (TICT; Figure 1a). Rationalization of the TICT mechanism [9] has greatly facilitated the development of many functional materials and devices,s uch as bright and photostable fluorophores, [10] dark quenchers, [11] viscosity sensors, [12] and polarity sensors. [6b] Notably,c harge-transfer and separation processes in both the ICT and TICT states are unidirectional, i.e., from the donor (D) to the acceptor (A) upon photoexcitation.In this study,w er...