The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five-membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible-light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.

Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon matrices by IR, UV-vis, and X-band EPR spectroscopy and in MTHF glasses by W-band EPR and Q-band ENDOR spectroscopy. UV (365 nm) irradiation of the system results in formation of predominantly the triplet carbene, whereas visible (450 nm) light shifts the photostationary equilibrium toward the singlet state. Upon annealing at higher temperatures (>10 K), the triplet is converted to the singlet; however, cooling back to 3 K does not restore the triplet. Therefore, depending on matrix temperature and irradiation conditions, matrices containing predominantly the triplet or singlet carbene can be generated. Controlling the magnetic and chemical properties of carbenes by using light of different wavelengths might be of general interest for applications such as information storage and radical-initiated polymerization processes.

We have observed the inversion of the solvation environment of ao ne-dimensional solid by low-temperature scanning tunneling microscopy. Adsorption of 3-methoxy-9diazofluorene on Ag(111) yields highly oriented supramolecular chains,w hich are then exposed to water molecules.T he annealing of dry and water-decorated chains results in diametrically opposed outcomes.While the former simply leads to an increase in chain length and number,t he latter results in acomplete loss of order and produces water clusters decorated with the organic molecule.

Metal carbenes are key intermediates in a plethora of homogeneous and heterogeneous catalytic processes. However, despite their importance to heterogeneous catalysis, the influence of surface attachment on carbene reactivity has not yet been explored. Here, we reveal the interactions of fluorenylidene (FY), an archetypical aromatic carbene of extreme reactivity, with a Ag(111) surface. For the first time, the interaction of a highly reactive carbene with a metal surface could be studied by scanning tunneling microscopy (STM). FY chemisorbs on Ag(111) with an estimated desorption energy of 3 eV, forming a surface bound silver−carbene complex. The surface interaction leads to a switching of the electronic ground state of FY from triplet to singlet, and to controlled chemical reactivity. This atomistic understanding of the interplay between carbenes and metal surfaces opens the way for the development of novel classes of catalytic systems based on surface metal carbenes.

The reaction of triplet tetrachlorocyclopentadienylidene with BF in rare gas matrices yields a zwitterion consisting of a cyclopentadienyl cation bearing a positive charge and a negatively charged BF unit. IR and UV-vis spectra as well as the absence of EPR signals demonstrate a singlet ground state of the zwitterion, and its calculated geometry and magnetic properties clearly reveal a strong antiaromatic character. The zwitterion is highly labile and by visible or IR irradiation rearranges via a 1,2-fluorine migration from boron to carbon. Interaction with a second molecule of BF stabilizes the zwitterion and suppresses the fluorine migration, thus providing a convenient and efficient synthesis of an antiaromatic molecule under very mild conditions.

The control of the spin states of molecules opens the path to tuning selectivity in chemicalr eactions and to developing novel magnetically switchable materials.3-Methoxy-9fluorenylidene is ac arbene that is generated in cryogenic matrices both in its lowest energy singlet and triplet states,and the ratio of these states can be shifted by selective irradiation. The interconversion of the nearly degenerate spin states is induced by ac onformational change of the methoxy group: switching the methoxy group into the "up" position results in the singlet state and switching into the "down" position in the triplet state.T he spin control via ar emote functional group makes this carbene unique for the study of spin-specific reactions,w hich is demonstrated for the hydrogenation reaction. Spin switching by switching the conformation of aremote functional group is an ovel phenomenon with potential applications in the design of functional materials.

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five‐membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible‐light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CFI has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CFI at 3 K. Bimolecular reactions between the carbenes and CFI were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25-30 K.