9-Fluorenylidene is an aryl

carbene In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. The term "carbene" ma ...

derived from the bridging methylene group of

fluorene Fluorene , or 9''H''-fluorene is an organic compound with the formula (C6H4)2CH2. It forms white crystals that exhibit a characteristic, aromatic odor similar to that of naphthalene. It has a violet fluorescence, hence its name. For commercial pu ...

. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol (4.6 kJ/mol) lower in energy than the

singlet state In quantum mechanics, a singlet state usually refers to a system in which all electrons are paired. The term 'singlet' originally meant a linked set of particles whose net angular momentum is zero, that is, whose overall spin quantum number s=0. A ...

. For this reason, fluorenylidene has been studied extensively in organic chemistry. Fluorenylidene is a

reactive intermediate In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...

. Reactions involving fluorenylidene proceed through either the triplet or singlet state carbene, and the products formed depend on the relative concentration of spin states in solution, as influenced by experimental conditions. The rate of

intersystem crossing Intersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different spin multiplicity. Excited Singlet and Triplet States When an electron in a molecule with a singlet ground ...

is determined by the temperature and concentration of specific spin-trapping agents.

Structure

The ground state is believed to be a bent triplet, with two orthogonal sp

hybrid orbitals In chemistry, orbital hybridisation (or hybridization) is the concept of mixing atomic orbitals to form new ''hybrid orbitals'' (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to f ...

singly occupied by unpaired spins. One electron occupies an orbital of sigma symmetry in the plane of the rings, while the other occupies an orbital of pi symmetry, which interacts with the pi systems of the adjacent aromatic rings (delocalization into the rings is minimal, since zero-field parameter D is high). The

zero field splitting Zero field splitting (ZFS) describes various interactions of the energy levels of a molecule or ion resulting from the presence of more than one unpaired electron. In quantum mechanics, an energy level is called degenerate if it corresponds to two ...

parameters predict a bond angle greater than 135°, and since the ideal bond geometry for cyclopentane carbons is about 109°, considerable ring strain causes the methylene sigma bonds to be bent. In the singlet state, the spin-paired electrons occupy the sp² hybrid orbital, orthogonal to an empty p-orbital. Conversion of singlet to triplet fluorenylidene is achieved through intersystem crossing (ISC).

Generation of fluorenylidene

Fluorenylidene can be produced by photolysis of 9-diazofluorene (DAF). Ultra-fast (300 fs) time resolved laser-

flash photolysis Flash photolysis is a pump-probe laboratory technique, in which a sample is first excited by a strong pulse of light from a pulsed laser of nanosecond, picosecond, or femtosecond pulse width or by another short-pulse light source such as a fla ...

of DAF implicates a four-step process in the formation of fluorenylidene by irradiation of 9-diazofluorene. # Irradiation of DAF initially yields an excited singlet state diazofluorene molecule ( ¹DAF*) # ¹DAF* decays to form the open shell carbene,¹FL*, as the minor product, and the less energetic closed shell carbene, ¹FL, as the major product. # Any excited singlet ¹FL* in solution relaxes to the lower energy singlet state ¹FL (20.9 ps) # ¹FL equilibrates with the ground state triplet ³FL by intersystem crossing.

Reaction of fluorenylidene in solution

Fluorenylidene reacts with olefins as predicted by the Skell-Woodworth rules. The stereochemistry of cycloaddition products depends on the relative rates of cyclopropanation (or other reactions) and intersystem crossing.{{cite book, author1=CJ Moody , author2=GH Whitham , title=Reactive Intermediates, url=https://archive.org/details/reactiveintermed00mood , url-access=registration , year=1992, publisher=Oxford University Press Stabilization of specific spin states, and, by extension, increased stereospecificity can be achieved by using solvents of different polarities . Fluorenylidene reactions in solution

Triplet fluorenylidene reactivity

Triplet fluorenylidene reacts with olefins in a stepwise fashion to produce a racemic mixture, provided that the rate of spin inversion (intersystem crossing) is not significantly faster than rates of intermediate bond rotation.

Singlet fluorenylidene reactivity

Hexafluorobenzene Solvation of Fluorenylidene

Singlet fluorenylidene reacts with olefins in a concerted fashion, maintaining the stereochemistry of the reactant olefin. Triplet quenchers such as butadiene solvents can be used to increase stereospecific yields. Halogenated solvents also stabilize the singlet state. For example, dibromomethane and hexafluorobenzene deactivate the higher-energy singlet state, decelerating the rate of intersystem crossing in accordance with earlier studies of diphenylcarbene. The mechanism of singlet state deactivation is theorized to occur through halogen-lone pair complexation of empty 1Fl P-orbitals.

References