Indenofluorene

Most IF synthesis and research work is done on these derivatives Despite being first synthesized in the late 19th century [3] by Dr. S. Gabriel when he synthesized the substituted indeno [1,2-a] fluorene (shown right), the lack of robust synthesis routes left this family of molecules unexplored until the mid 20th century.

[1] This is likely to change, however, as improvements on synthesis and expansion of IF examples continues to be an active area of research.

The preferred version uses a [1,2] or [2,1] to describe if the orientation of the methylene bridges on the 5 member rings are anti ([1,2]) or syn ([2,1]).

Similarly, the scope of indenofluorenes have been increasing over the decades to include heteroatoms, such as sulfur,[4][5] within the ring system.

If triflic acid, trifluoroacetic anhydride, and zinc bromide were added in a sealed reaction vessel to the carboxylic acid then heated, the result would be the desired dione in a 90 + percent yield [11] The first account of this scaffold was disclosed in 1951 by Deuschel et al.[1] Chardonnens and Ritter, in 1955, offered the better route (shown above) in 79% yield [1]

The [2,1-c] IF isomer was the last to be discovered and was published in 1961 by Ginsburg and Altman with an alternate route presented by Chardonnens and Ritter.

In crystal form, [1,2- b] IFs generally show one dimensional column stacking.,[1] however, the stacking can be tuned based on the substitution of the molecule.

[1][14] No matter what substitution is present, though, the molecules pack fairly closely with distances of about 3.30 Å [1] Kamatsu et al., in their work with [1,2-b] IFs, have shown that they behave as n-type semiconductors.

[1][15] The best n-type behavior was shown in the dione of the [1,2-b] IF in which the para positions were substituted with fluorine, which was presented by Yamahita et al. to be 0.17 cm2/Vċs.

[1][14] Cyclic voltammetry data has shown that various [1,2-b] IFs, can reversibly accept two electrons with the first reduction occurring at -0.8 V. The parent dione has a first reduction potential of -1.19 V, and halogenated versions reduce around -0.6 V.[1] The fully conjugated version, first postulated by Deuschel in the 1950s, is believed to be even better as an electron carrier when compared to the analogus fullerene.

[1][16][17] Confirmation of these low lying LUMOs wis provided by a tips acetylene appended, on the methyl bridges and the center ring, fully conjugated [1,2-b] IF had a first reduction of -0.62 V. The improvement in reduction potential is linked to the fact that the addition of 2 electrons yields an aromatic molecule with increased stability.

[1][16][7] This biradical nature is both an advantage, in that it is believed that [2,1-a] IFs will make excellent organic electron carriers, and a curse, owing to the decreased stability of the molecule.

Overall the applications for IFs are anticipated to be as replacement for fullerenes in organic electronic systems such as OLEDs, OFETs, and OPVCs.

Not all of the IF regioisomers are suited to incorporation into organic electronics mostly owing to difficult synthesis and instability.