Temporal Dynamics of Localized Exciton–Polaritons in Composite Organic–Plasmonic Metasurfaces

We use femtosecond transient absorption spectroscopy to study the temporal dynamics of strongly coupled exciton–plasmon polaritons in metasurfaces of aluminum nanoantennas coated with J-aggregate molecules. Compared with the thermal nonlinearities of aluminum nanoantennas, the exciton–plasmon hybridization introduces strong ultrafast nonlinearities in the composite metasurfaces. Within femtoseconds after the pump excitation, the plasmonic resonance is broadened and shifted, showcasing its high sensitivity to excited-state modification of the molecular surroundings. In addition, we observe temporal oscillations due to the deep subangstrom acoustic breathing modes of the nanoantennas in both bare and hybrid metasurfaces. Finally, unlike the dynamics of hybrid states in optical microcavities, here, ground-state bleaching is observed with a significantly longer relaxation time at the upper polariton band

Two Roads Converged in a Yellow Dye: Unusual Spectral Broadening in the Emission of Auramine‑O Possibly Caused by Low-Friction Intramolecular Rotation

Steady-state and time-resolved optical techniques were employed to study the rather complex relaxation of excited states of Auramine-O (AuO) in several liquids at room temperature. We found three relaxation times in the decay of the pump–probe signals of the excited states of AuO. We focused our study on the short time decay, with a duration of within about 150–300 fs. We found that the temporal changes of the emission band of AuO could be divided into three behaviors, depending on the solvent characteristics. In dimethyl sulfoxide (DMSO), a hydrogen-bond-accepting solvent, AuO, shows, at short times, a relatively broad emission band with small changes in its peak position and width as a function of time. In acetonitrile and in acetic acid, both hydrogen-bond-donating solvents, we found large changes in the band peak and width as a function of time. Dichloromethane is a solvent lacking strong solvent interactions, it is apolar and is neither a strong hydrogen-bond-donator nor a strong hydrogen-bond-acceptor. For AuO in dichloromethane we found an oscillation with a time constant of 200 fs in the time-resolved emission signal. We attribute the rather large changes of the emission band with time in the short time window to the twist of the dimethylamino groups of the aniline groups of AuO

Comprehensive Study of Ultrafast Excited-State Proton Transfer in Water and D₂O Providing the Missing RO^–···H⁺ Ion-Pair Fingerprint

Steady-state and time-resolved optical techniques were employed to study the photoprotolytic mechanism of a general photoacid. Previously, a general scheme was suggested that includes an intermediate product that, up until now, had not been clearly observed experimentally. For our study, we used quinone cyanine 7 (QCy7) and QCy9, the strongest photoacids synthesized so far, to look for the missing intermediate product of an excited-state proton transfer to the solvent. Low-temperature steady-state emission spectra of both QCy7 and QCy9 clearly show an emission band at <i>T</i> < 165 K in H₂O ice that could be assigned to ion-pair RO^–*···H₃O⁺, the missing intermediate. Room-temperature femtosecond pump–probe spectroscopy transient spectra at short times (<i>t</i> < 4 ps) also shows the existence of transient absorption and emission bands that we assigned to the RO^–*···H₃O⁺ ion pair. The intermediate dissociates on a time scale of 1 ps and about 1.5 ps in H₂O and D₂O samples, respectively