Abnormal Enhancement of the Photoisomerization Process in a trans-Nitroalkoxystilbene Dimer Sequestered in β-Cyclodextrin Cavities

Deciphering the unusual fluorescence in weakly coupled bis-nitro-pyrrolo[3,2-b]pyrroles

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it also leads to fluorescence quenching. Here, we demonstrate how such electronic coupling affects the photophysics of acceptor-donor-acceptor fluorescent dyes, with nitrophenyl acceptors and a pyrrolo[3,2-b]pyrrole donor. The position of the nitro groups and the donor-acceptor distance strongly affect the fluorescence properties of the bis-nitrotetraphenylpyrrolopyrroles. Concurrently, increasing solvent polarity quenches the emission that recovers upon solidifying the media. Intramolecular charge transfer (CT) and molecular dynamics, therefore, govern the fluorescence of these nitro-aromatics. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for large fluorescence quantum yields, vibronic borrowing accounts for medium dependent IC via back CT. These mechanistic paradigms set important design principles for molecular photonics and electronics.

Circularly Polarized Luminescence of Langmuir-Schaefer Films of Amphiphilic Stilbene Enhanced via Interfacial Reaction with Cyclodextrins

Two enantiomeric amphiphiles containing the stilbene moiety (l-StG and d-StG) were assembled into ordered Langmuir-Schaefer (LS) films through the air/water interface and their circularly polarized luminescence (CPL) was investigated. When the molecules were spread at the air/water interface, a monolayer with nanofiber structures was formed, which could be subsequently transferred onto solid substrates by the LS method. The LS films showed both circular dichroism (CD) and CPL, whose handedness was determined by the molecular chirality of the amphiphiles. When the amphiphilic molecules were spread on the aqueous subphases containing cyclodextrins (including α-CyD, β-CyD, or γ-CyD), similar nanofiber-featured films were formed. However, the CD and CPL showed different changes. When l-StG was spread on the cyclodextrins, both CD and CPL were enhanced. When d-StG was reacted with cyclodextrins, the CD signal decreased while the CPL was enhanced. It was suggested that the chirality cooperation and conflict between the point chirality from the amphiphilic stilbene and the cavity chirality of cyclodextrin led to the phenomenon. However, in any case, the immobilization of the stilbene by the cyclodextrins caused the enhancement of CPL.

Two-Photon Initiating Efficiency of a Ditopic Alkoxynitrostilbene Reacting through a Self-Regenerative Mechanism

The photophysical properties and the photoinitiating reactivity of a ditopic alkoxynitrostilbene were compared to those of its single branch chromophore used as a reference. Whereas a trivial additive effect is observed when considering the one- and two-photon absorption properties, a clear and very significant amplification has been highlighted for the photoreactivity of this free radical photoinitiator which was used as a hydrogen abstractor in presence of an aliphatic amine co-reactant. We indeed demonstrate that the proximity of two nitroaromatics moieties within the same molecular architecture gives rise to an original cycling mechanism based on a stepwise photo triggering of each photoredox center followed by a subsequent regenerative process. The combination of a high two-photon absorption cross-section (δ_780nm ≈330 GM) with a strong enhancement in photoreactivity makes this nitrostilbene bichromophore a very suitable candidate for two-photon polymerization applications.

Relay proton transfer triggered twisted intramolecular charge transfer

The mechanism for the dual emission of 2-(4′-N,N-dimethylaminophenyl)imidazo[4,5-c]pyridine (DMAPIP-c) in protic solvents was investigated by synthesizing and studying its analogues.