The fluorescence properties of the BODIPY dye and its two meso-substituted derivatives, tert-butyl-and phenyl-BODIPY, are rationalized. The non-emissive behavior of the latter two are attributed to the energetically accessible low-lying conical intersection between the ground state and the lowest excited singlet state. Both intramolecular non-covalent interactions and excited state charge transfer character are identified as being crucial for 'stabilizing' the intersection and prompting the nonradiative decay. Similar crossing was located in the bare BODIPY dye, however, being energetically less accessible, which correlates well with the high fluorescence quantum yields of the parent dye.

The photoinduced hydrogen evolution reaction (HER) by decamethylruthenocene,C p 2 *Ru II (Cp* = C 5 Me 5 ), is reported. The use of am etallocene to photoproduce hydrogen is presented as an alternative strategy to reduce protons without involving an additional photosensitizer.T he mechanism was investigated by (spectro)electrochemical and spectroscopic (UV/Vis and 1 HNMR) measurements.T he photoactivated hydride involved was characterized spectroscopically and the resulting [Cp 2 *Ru III ] + species was electrochemically regenerated in situ on af luorinated tin oxide electrode surface.Apromising internal quantum yield of 25 %w as obtained. Optimal experimental conditionsespecially the use of weakly coordinating solvent and counterions-are discussed.Thedevelopmentofsimpleandefficientmethodstoproduce molecular hydrogen (H 2 )i st he focus of intense research. Va rious state-of-the-art multicomponent artificial photosystems for H 2 generation are currently under heavy scrutiny and generally consist of ah ighly engineered catalyst, photosensitizer,electron mediator or relay combinations, [1] and are often fueled by sacrificial electron donors (for example, triethylamine, [2] triethanolamine, [2b] benzyl-dihydronicotinamide, [3] and so forth). Thel atter irreversibly oxidizes upon charge transfer and provides protons and electrons to the catalyst. Consequently,s acrificial systems consume af uel to produce H 2 while electrochemical systems only consume electricity (that is now being increasingly produced in as ustainable manner). Indeed, the electrode can both accept and donate electrons.N oi rreversible reactions take place at this step,a nd the protons are supplied from the solution.Metallocenes appear as an attractive class of molecules capable of achieving the complex photogeneration of H 2 by themselves.Indeed, they are able to both reduce protons and undergo photoactivation. Therefore,t hese "all-in-one" molecules would offer an interesting alternative to state-ofthe-art multicomponent photosystems as fewer electron transfer steps are involved. Moreover,they are simple,easily synthesized molecules,w ith ligands and metal centers that may be tuned to obtain certain desired properties,s uch as tailored solubility,a bsorbance wavelength, or redox potentials.Recently,w ed emonstrated the possibility to produce H 2 in the dark using decamethylferrocene (Cp 2 *Fe II ;C p* = C 5 Me 5 )asanelectron donor in abiphasic system.[4] Motivated by these early findings,weset out to explore the reactivity of other metallocenes as suitable electron donors.I nterestingly, both osmocene (Cp 2 Os II ;C p = C 5 H 5 ) [5] and decamethylosmocene (Cp 2 *Os II ) [6] demonstrated the capability to produce H 2 upon light irradiation. Other works have proposed the use of asingle molecule to achieve photogeneration of H 2 . Fore xample,C ole-Hamilton [7] reported ap latinum phosphine compound, while both Miller [8] and Gray [9] used iridium chloride complexes.Herein, we report Cp 2 *Ru II as the first metallocene capable of perfo...

The optoelectronic properties of various carbon allotropes and nanomaterials have been well established, while the purely sp-hybridized carbyne remains synthetically inaccessible. Its properties have therefore frequently been extrapolated from those of defined oligomers. Most analyses have, however, focused on the main optical transitions in UV-Vis spectroscopy, neglecting the frequently observed weaker optical bands at significantly lower energies. Here, we report a systematic photophysical analysis as well as computations on two homologous series of oligoynes that allow us to elucidate the nature of these weaker transitions and the intrinsic photophysical properties of oligoynes. Based on these results, we reassess the estimates for both the optical and fundamental gap of carbyne to below 1.6 eV, significantly lower than previously suggested by experimental studies of oligoynes.

We report here the photophysical properties of a water-soluble conjugated polythiophene with cationic side-chains. When dissolved in aqueous buffer solution (PBS, phosphate buffered saline), there is ordering of the polymer chains due to the presence of the salts, in contrast to pure water, where a random-coil conformation is adopted at room temperature. The ordering leads to a pronounced colour change of the solution (the absorption maximum shifts from 400 nm to 525 nm). Combining resonance Raman spectroscopy with density functional theory computations, we show a significant backbone planarization in the ordered phase. Moreover, the ratio of ordered phase to random-coil phase in PBS solution, as well as the extent of intermolecular interactions in the ordered phase, can be tuned by varying the temperature. Femtosecond transient absorption spectroscopy reveals that the excited-state behaviour of the polyelectrolyte is strongly affected by the degree of ordering. While triplet state formation is favoured in the random-coil chains, the ordered chains show a weak yield of polarons, related to interchain interactions. The investigated polyelectrolyte has been previously used as a biological DNA sensor, based on optical transduction when the conformation of the polyelectrolyte changes during assembly with the biomolecule. Therefore, our results, by correlating the photophysical properties of the polyelectrolyte to backbone and intermolecular conformation in a biologically relevant buffer, provide a significant step forward in understanding the mechanism of the biological sensing.

The synthesis and characterization of a new class of neutral aminyl radicals is reported. Monoradicals were obtained by reduction of azoimidazolium dyes with potassium. Structural, spectroscopic, and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation of a biradical with largely independent unpaired electrons. Both the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.