Biexponential fluorescence decay of diphenylbutadiene rotational conformers after extreme red edge excitation

Aggregation-Induced Emission in Bridged (E,E)-1,4-Diphenyl-1,3-butadiene Derivatives with Six- and Seven-Membered Rings

Recently, we developed a new aggregation-induced emission (AIE) luminogen (AIEgen), bridged stilbene, by incorporating a propylene group into the C=C bond of the luminescent phenyl stilbene. This bridged structure, featuring a seven-membered ring, induces a significant conformational change, causing the C=C bond to twist in the excited state, thereby enhancing non-radiative decay in solution. In this study, we introduced bridged structures with alkylene groups of varying lengths into (E,E)-1,4-diphenyl-1,3-butadiene (DPB). The variation in the bridged structures of the synthesized DPB derivatives notably influenced the environmental sensitivity of fluorescence. Whereas the compound with two six-membered ring structures exhibited emission in solution and in the polycrystalline state, derivatives with a seven-membered ring exhibited AIE properties. Specifically, BDPB[7,7], featuring two seven-membered ring structures, demonstrated AIE characteristics with solid-state luminescence originating from J-aggregates. However, the fluorescence quantum yield was low in poly(methyl methacrylate) (PMMA) dispersion films, where molecular motion was restricted. These findings open new possibilities for designing unique AIEgens that remain nonluminescent even in highly viscous or confined environments, such as PMMA films.

The Solvent-Dependent Photophysics of Diphenyloctatetraene

Despite many decades of study, the excited state photophysics of polyenes remains controversial. In diphenylpolyenes with conjugated backbones that contain between 2 and 4 double carbon-carbon bonds, the first two excited electronic states are nearly degenerate but of entirely different character, and their energy splitting is strongly dependent on solvent polarizability. To examine the interplay between these different states, steady-state and time-resolved fluorescence spectroscopies were used to undertake a comprehensive investigation of diphenylocatetraene's (DPO) excited state dynamics in 10 solvents of different polarizabilities and polarities, ranging from weakly interacting alkanes to polar hydrogen-bonding alcohols. These data revealed that photopreparation of the optically bright 1Bu state resulted in fast (<170 ps) internal conversion to the lower-lying optically dark 2Ag state. The 2Ag state is responsible for almost all the observed DPO fluorescence and gains oscillator strength via vibronic intensity stealing with the near-degenerate 1Bu state. The fluorescence lifetime associated with the 2Ag state decayed monoexponentially (4.2-7.2 ns) in contrast to prior biexponential decay kinetics reported for similar polyenes, diphenylbutadiene and diphenylhexatriene. An analysis combining the measured fluorescence lifetimes and fluorescence quantum yields (the latter varying between 7 and 21%) allowed for a 190 cm-1 Herzberg-Teller vibronic coupling constant between the 1Bu and 2Ag states to be determined. The analysis also revealed that the ordering of electronic states remains constant in all the solvents studied, with the 2Ag state minimum always lower in energy than that of the 1Bu state, thus making it a relatively simple polyene compared to structurally similar diphenylhexatriene.

Unusually efficient trans-to-cis photoisomerization of diphenylbutadiene dendrimers in water

A diphenylbutadiene-cored dendrimer exhibited a remarkably high quantum yield for trans-to-cis photoisomerization in aqueous solution. Analysis of the fluorescence lifetimes and the wavelength-dependent excitation spectra suggested that the core butadiene adopts multiple conformations, one or several of which is sufficiently distorted to undergo preferential photoisomerization.

The red-edge effects: 30 years of exploration

In 1970, three laboratories independently made a discovery that, for aromatic fluorophores embedded into different rigid and highly viscous media, the spectroscopic properties do not conform to classical rules. The fluorescence spectra can depend on excitation wavelength, and the excited-state energy transfer, if present, fails at the "red" excitation edge. These red-edge effects were related to the existence of excited-state distribution of fluorophores on their interaction energy with the environment and the slow rate of dielectric relaxation of this environment. In these conditions the site-selection can be provided by variation of the energy of illuminating light quanta, and the behaviour of selected species can be followed as a function of time and other variables. These observations found extensive application in different areas of research: colloid and polymer science, molecular biophysics, photochemistry and photobiology. In particular, they led to the development of very productive methods of studying the dynamics of dielectric relaxations in protein and membranes, using the tryptophan emission and the emission of a variety of probes. These studies were extended to the time domain with the observation of new site-selective effects in emission intensity and anisotropy decays. They stimulated the emergence and development of cryogenic energy-selective and single-molecular techniques that became valuable tools in their own right in chemistry and biophysics research. Site-selection effects were discovered for electron-transfer and proton-transfer reactions if they depended on the dynamics of the environment. This review is focused on the progress in the field of red-edge effects, their applications and prospects.