An indenofluorene (IF) is any of five hydrocarbons with formula , whose carbon skeleton is a sequence of five fused rings with 6, 5, 6, 5, and 6 carbon atoms; an arrangement that can be described as the fusion of an indene core and a fluorene core (hence the common name).
The five structural isomers (regioisomers) differ in the way the rings are connected. They have unique properties, applications, and research interests.
The name "indenofluorene" is also commonly used for any derivative of those five compounds (which are then specifically called "parent" or "unsubstituted" IFs), conceptually obtained by substituting other functional groups for some hydrogen atoms, and/or by hydrogenating the methylidene bridges (thus turning them into methylene bridges). Most IF synthesis and research work is done on these derivatives
History
Despite being first synthesized in the late 19th century 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. After Gabriel, the next major step was the synthesis of the indeno [2,1-a] fluorene in 1939 by Weizmann et al. Next major advances came from Chardonens and Ritter with the synthesis the indeno [1,2-b] fluorene and indeno [2,1-b] fluorene in 1951. Continuing with their work, Chardonens and Ritter, synthesized indeno [1,2-a] fluorene in 1955. The final regioisomer, the indeno [2,1-c] fluorene was synthesized in 1961 by Ginsburg and Altman. Since the fifties and sixties, when these molecules were first being synthesized and discovered, improved synthetic routes and instrumentation have allowed IFs to be explored for uses in organic electronics including organic photovoltaics, organic light emitting diodes, and organic field effect transistors. Even with these advancements, the properties of IFs have remained largely unexplored. This is likely to change, however, as improvements on synthesis and expansion of IF examples continues to be an active area of research.
Structure and nomenclature
There are several conventions for naming IFs currently in use. 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]). The a, b, c designation indicates connectivity. a indicates there are no carbons between the indene and the starred fluorene carbon. Similarly, b indicates one carbon and c indicates two carbons.
While indenofluorenes are members of the polycyclic hydrocarbon family, they are not necessarily members of the polycyclic aromatic hydrocarbon family. For example, the fully conjugated versions, shown below, have 20π electrons making them formally anti-aromatic.
Stability
Because of the instability of parent compounds, most synthesis work and research on indenofluorenes tend to focus on the dione substituted IFs, the fully conjugated IF, or the hydrogenated (methylene bridged) IFs. Even in these molecules, though, stability remains a problem so it is not uncommon to stabilize the core indenofluorene with aromatic or bulky substituents, such as mesityl or triisopropyl silyl. Similarly, the scope of indenofluorenes have been increasing over the decades to include heteroatoms, such as sulfur, within the ring system. Other structural expansions include addition of rings to the outer edges, off the center and expanding the center core.
Synthesis
There is no one way to synthesize each of the regioisomers and new routes are being discovered. Presented here are some of the published ways to get to synthetically useful versions of each IF. Preference was given to the most efficient way to get to the minimally substituted IF.
[1,2-a] IF
The first [1,2-a] IF scaffold was synthesized by Chardonens and Ritter in 1955. In their publication, they showed two ways to get to the dione of the [1,2-a] IF. The first utilizes an oxidative cleavege followed by a ring closure utilizing concentrated sulfuric acid. Later they developed a route in which they condensed an indenyl ketone with quinolone base. This intermediate was then reacted sodium dichromate to produce the final dione in decent yield. Despite having multiple routes to the dione, the synthesis of the fully conjugated IF remained elusive until 2017.
Using the synthesis presented by Chardonens and Ritter, Dressler et al. appended various R- groups at the carbonyl to produce the diol product which was then reduced using a tin chloride catalyst to get to the desired product (below). The pure IF, with no substitution, was not synthesized due to instability.