Rigidochromism by imide functionalisation of an aminomaleimide fluorophore

Novel Rigidochromic and Anti-Kasha Dual Emission Fluorophores Based on D-π-A Dyads as the Promising Materials for Potential Applications Ranging from Optoelectronics and Optical Sensing to Biophotonics and Medicine

Today we see an increasing demand for new fluorescent materials exhibiting various sensory abilities due to their broad applicability ranging from the construction of flexible devices to bioimaging. In this paper, we report on the new fluorescent pigments AntTCNE, PyrTCNE, and PerTCNE which consist of 3-5 fused aromatic rings substituted with tricyanoethylene fragments forming D-π-A diad. Our studies reveal that all three compounds exhibit pronounced rigidochromic properties, i.e., strong sensitivity of their fluorescence to the viscosity of the local environment. We also demonstrate that our new pigments belong to a very rare type of organic fluorophores which do not obey the well-known empirical Kasha'rule stating that photoluminescence transition always occurs from the lowest excited state of an emitting molecule. This rare spectral feature of our pigments is accompanied by an even rarer capability of spectrally and temporally well-resolved anti-Kasha dual emission (DE) from both higher and lowest electronic states in non-polar solvents. We show that among three new pigments, PerTCNE has significant potential as the medium-bandgap non-fullerene electron acceptor. Such materials are now highly demanded for indoor low-power electronics and portable devices for the Internet-of-Things. Additionally, we demonstrate that PyrTCNE has been successfully used as a structural unit in template assembling of the new cyanoarylporphyrazine framework with 4 D-π-A dyads framing this macrocycle (Pyr₄CN₄Pz). Similarly to its structural unit, Pyr₄CN₄Pz is also the anti-Kasha fluorophore, exhibiting intensive DE in viscous non-polar medium and polymer films, which strongly depends on the polarity of the local environment. Moreover, our studies showed high photodynamic activity of this new tetrapyrrole macrocycle which is combined with its unique sensory capacities (strong sensitivity of its fluorescent properties to the local environmental stimuli such as viscosity and polarity. Thus, Pyr₄CN₄Pz can be considered the first unique photosensitizer that potentially enables the real-time combination of photodynamic therapy and double-sensory approaches which is very important for modern biomedicine.

Restriction of photoinduced electron transfer as a mechanism for the aggregation-induced emission of a trityl-functionalised maleimide fluorophore

The restriction of intramolecular motion (RIM) and restricted access to a conical intersection (RACI) have been accepted as general working mechanisms for aggregation-induced emission (AIE) phenomena. However, as the family of AIE molecules grows, the RIM and RACl mechanisms cannot be used to fully understand some AIE phenomena. Herein, the restriction of the photoinduced electron transfer (RPET) state is proposed to rationalize the AIE phenomena of trityl-functionalised maleimide molecule based on density functional theory calculations. The "state-crossing from a locally excited to an electron transfer state" (SLEET) model was employed to predict the ON/OFF molecular PET in solution and solid states. According to the SLEET model, we showed that a non-emissive electron transfer excited state leads to the fluorescence quenching of trityl-functionalised maleimide in solution. However, due to the reduced polarity of the environment in aggregates, the electron transfer state is thermodynamically inaccessible, and a low-lying locally excited state exhibits intense emission. These findings provide a theoretical foundation to understand the working mechanisms of AIE molecules and the design of new AIEgens. We expect that the RPET mechanism can be used to screen potential AIEgens using the SLEET model.

Enhancing Dual-State Emission in Maleimide Fluorophores through Fluorocarbon Functionalisation

Herein, a library of trifluoroethyl substituted aminomaleimide derivatives are reported with small size and enhanced emissions in both solution and solid-state. A diCH₂ CF₃ substituted aminochloromaleimide exhibits the most efficient dual-state emission (Φ_f >50 % in solution and solid-state), with reduced quenching from protic solvents. This is attributed to the reduction of electron density on the maleimide ring and suppressed π-π stacking in the solid-state. This mechanism was explored in-depth by crystallographic analysis, and modelling of the electronic distribution of HOMO-LUMO isosurfaces and NCI plots. Hence, these dual-state dyes overcome the limitations of single-state luminescence and will serve as an important step forward for this rapidly developing nascent field.

Multicolour Fluorescence Based on Excitation-Dependent Electron Transfer Processes in o-Carborane Dyads

Organic materials with excitation wavelength-dependent (Ex-de) emission are highly attractive for anticounterfeiting, optoelectronics and bioassay applications; however, the realization of Ex-de fluorescence, independent of aggregation states, remains a challenge. We herein report a photoinduced electron transfer (PeT) strategy to design Ex-de fluorescence materials by manipulating the relaxation pathways of multiple excited states. As expected, the o-carborane dyad presents a clear Ex-de fluorescence colour in the aggregated states, resulting from the tunable relative intensity of the dual-fluorescence spectra. Taking TP[1]B as an example, the amorphous powders emitted bright blue-violet, white and yellow colours under 390 nm, 365 nm and 254 nm UV illumination, respectively. Importantly, multicolour, flexible and transparent films as well as an anticounterfeiting application using this o-carborane dyad are demonstrated.