Pathways to fluorescence via restriction of intramolecular motion in substituted tetraphenylethylenes

Intramolecular O···H Hydrogen Bonding of Salicylic Acid: Further Insights from O 1s XPS and 1H NMR Spectra Using DFT Calculations

Intramolecular hydrogen bonding (HB) is a complex phenomenon that extends beyond a simple valence event, affecting the core electrons of a molecule. Salicylic acid (SA) and its conformers provide an excellent model compound for studying intramolecular HB as the proton donor (H) and acceptor (O) can be toggled by rotating the C-O and C-C bonds to form up to seven potential conformers through various HB. In this study, we computationally investigated intramolecular interactions in SA conformers with and without such HB, by examining their calculated O 1s core electron-binding energy (CEBE) and ¹H NMR chemical shifts validated using recent measurements. The quantum mechanically stable SA conformers are fully defined by three rotatable bonds in the compound, which are abstracted as SA(η₁η₂η₃) digital structures, where η_i = 0 if the η_i angles match the most stable SA conformer (000) and η_i = 1 otherwise. Our findings suggest that the stability is dominated by the appearance of the intergroup intramolecular HB of H_p···O (where O is in the carboxylic acid functional group and H_p is the phenolic proton in -OH_p), and η₃ serves as a switch of such HB. As a result, the (η₁η₂0) SA conformers containing such H_p···O HB are more stable than other SA conformers (η₁η₂1) without such the H_p···O HB. The present density functional theory calculations reveal that this H_p···O HB results in splitting of the O 1s CEBEs of two hydroxyl groups (-OH) by up to 1 eV and deshielding the H_p proton ¹H NMR (δH_p) up to 11.68 ppm for the (η₁η₂0) conformers. Without such H_p···O HB, the O 1s XPS binding energies of two -OH groups will be closely located in the same band, and the ¹H NMR chemical shift of the H_p atom will be as small as an 4.09 ppm SA conformer [SA-G(101)]. The present study indicates that the O 1s CEBE splitting between two -OH groups serves as an indicator of the presence of the H_p···O HB in SA conformers, which is also supported by the ¹H NMR results.

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.

Deciphering the ultrafast dynamics of a new tetraphenylethylene derivative in solutions: charge separation, phenyl ring rotation and CC bond twisting

Tetraphenylethylene (TPE) derivatives are one of the fundamental units for developing aggregation induced emission (AIE) scaffolds. However, the underlying mechanisms implicated in the relaxation of the excited TPE remain a topic of ongoing discussion, while the effect of bulky substituents on its photobehaviour is still under scrutiny. Here, we report a detailed study of the photophysical properties of a new symmetrical and bulky TPE derivative with terphenyl groups (TTECOOBu) in solvents of different polarities and viscosities. Using femto- to nanosecond (fs-ns) time-resolved absorption and emission techniques, we elucidated the role of the phenyl group rotations and core ethylene bond twisting in its behaviour. We demonstrate that TTECOOBu in DCM solutions undergoes a 600 fs charge separation along the ethylene bond leading to a resonance structure with a lifetime of ∼1 ns. The latter relaxes via two consecutive events: a twisting of the ethylene bond (∼ 9 ps) and a rotation of the phenyl rings (∼ 30 ps) leading to conformationally-relaxed species with a largely Stokes-shifted emission (∼ 12 500 cm^-1). The formation of the red-emitting species clearly depends on the solvent viscosity and rigidity of the medium. Contrary to the photobehavior in the highly viscous triacetin or rigid polymer matrix of PMMA, a reversible mechanism was observed in DCM and DMF solutions. These results provide new findings on the ultrafast mechanisms of excited TPE derivatives and should help in the development of new molecular rotors with interesting AIE properties for photonic applications.

α-Cyanostilbene: A Multifunctional Spectral Engineering Motif

The remarkable photophysical phenomenon of aggregation-induced emission offers excellent strategies to obtain the molecular materials possessing unique spectral signatures such as high fluorescence intensity, excellent quantum yield, large Stokes shift...