A series of novel Re(i)(CO)-NHC complexes bearing unsubstituted benzimidazol-2-ylidene ligands as well as a variety of bisimine ligands has been prepared and comprehensively characterised. The complexes were found to exhibit potent antimicrobial activity on Gram-positive bacterial strains in the low micromolar concentration range, rendering these compounds interesting lead structures for the development of novel metal-based antibiotic agents. Further, the complexes exhibit pronounced luminescence with large Stokes shifts in acetonitrile and water at ambient temperature. The photophysical properties including luminescence lifetimes and quantum yields are consistent with emission from MLCT (d(Re) → π*(bisimine)) states.

Super‐photoacids, that is, photoacids with a negative pKnormala value in the electronically excited state, can trigger an excited‐state proton transfer (ESPT) to the solvent. For the neutral pyranine‐derived super‐photoacid studied here, even indications for ESPT in acetoneous solution are reported. The characteristics of ESPT in this environment, that is, which intermediates exist and what the impact of cosolvents is, remain unsettled though. In this work, we study ESPT in acetone‐water mixtures by steady‐state and time‐resolved fluorescence spectroscopy. Various effects are observed: First, the addition of water supports the formation of a hydrogen‐bonded ground‐state complex comprising one water molecule and the photoacid, whose excitation triggers the formation of a hydrogen‐bonded ion pair on a sub‐ns time scale. Second, water has an overall accelerating effect on the fluorescence dynamics of the involved emitting species, whose contributions are disentangled in a global analysis scheme, enabling the identification of emission from the free photoacid, a photoacid‐water complex, a hydrogen‐bonded ion pair, and the deprotonated photoacid. At least two water molecules are necessary for ESPT in the environment. Third, additional acidification thwarts an efficient ground‐state complex formation of the photoacid and water. However, upon excitation, complexation may occur on a timescale faster than the photoacid's excited‐state lifetime, so that emission from a nascent complex emerges.

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatochromic effects. To disclose the underlying mechanisms contributing to excited-state proton transfer (ESPT) and the temporal characteristics of solvation and ESPT, we scrutinize the associated ultrafast dynamics of the strongest photoacid of this class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)8-hydroxypyrene-1,3,6-trisulfonate, in acetoneous environment, thereby finding experimental evidence for ESPT even under these adverse conditions for proton transfer. Juxtaposing results from time-correlated single-photon counting and femtosecond transient absorption measurements combined with a complete decomposition of all signal components, i.e., absorption of ground and excited states as well as stimulated emission, we disclose dynamics of solvation, rotational diffusion, and radiative relaxation processes in acetone and identify the relevant steps of ESPT along with the associated time scales. Graphical abstract

We investigate how the absorption and fluorescence of halogenated imidazolium compounds in acetonitrile solution is influenced by the presence of counterions and the ability to act as halogen-bond donors. Experimental...