Conjugated polycyclic hydrocarbons [1] have attracted much attention because of their novel fundamental properties and potential application in electronic materials. [2] Recently, thanks to the development of excellent synthetic methods, hydrocarbons with unique electronic structures such as dibenzopentalenes 1[3] and zethrenes 2 [4] have been subjects of extensive research. In particular, polycyclic hydrocarbons containing a quinodimethane (QDM) structure [5, 6] have been studied because of their low-energy bandgaps and excellent electrochemical behavior, which make them potential candidates for optoelectronic materials.Very recently, Haley et al. have reported the synthesis and properties of a stable p-quinodimethane (pQDM) derivative, indeno [1,2-b]fluorene 3 b (TIPS = triisopropylsilyl) [5] which was regarded as a fully conjugated 20p-electron hydrocarbon with fused s-trans diene linkages across the top and bottom parts of the carbon framework. [7,8] In connection with our own interest in the transannular cyclization of dehydrobenzoannulenes, [9] we became interested in a structural isomer of 3 a, indeno[2,1-a]fluorene (6 a), which possesses an o-quinodimethane (oQDM) structure instead of the pQDM moiety of 3 a, for two reasons: 1) Since oQDM [10] has a smaller HOMO-LUMO energy gap than pQDM, [11] the HOMO-LUMO gap of 6 a would be smaller than that of 3 a. 2) In contrast to the pQDM derivatives, studies on compounds containing an oQDM structure are scarce because of their high reactivity due to the s-cis diene unit. [12] In fact, studies on pQDM-type hydrocarbons such as Thieles hydrocarbon 7[13] and Chichibabins hydrocarbon 8, [14] which are stable enough to be isolated as crystals, [15] were performed over a century ago. On the other hand, molecules containing the oQDM structure, such as tetraphenyl-oQDM 9 [16] and pleiadene (10) [17] are reported to be highly reactive. These compounds were generated and detected in rigid glass matrices but could not be isolated. Though tetraaryl-oQDM 11 [18] and a highly reactive oQDM derivative 12 [19] were recently synthesized and isolated, the fundamental properties originating from the oQDM moiety remain to be clarified. In this regard, indeno-[2,1-a]fluorene (6 a) is an intriguing molecule because the benzo bridge to the oQDM structure would not only extend the length of the p-conjugated carbon framework but also prevent one of the typical reactions of oQDMs, cyclization to form benzocyclobutenes. Additionally, the fact that 6 a contains an antiaromatic as-indacene moiety, which has not been isolated to date, [20] also makes 6 a a fascinating hydrocarbon.Le Berre et al. synthesized the 11,12-diphenyl derivative 6 b [21] which exhibited an absorption maximum at 556 nm. However, 6 b turned out to be highly reactive toward oxygen [22] and therefore its molecular structure and detailed electronic structures, such as electrochemical behavior and ring-current effect, have not been investigated. In addition, a series of reports by Le Berre et al. in the mid-1950s are the