Creation and Function of Novel Open-Shell π-Conjugated Systemswith a Weak π-Bond

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An indeno[2,1-b]fluorene derivative, a 20π electron hydrocarbon, was synthesized to clarify novel properties associated with singlet diradical characters. The moderate amplitude diradical character of the indeno[2,1-b]fluorene derivative was theoretically and experimentally proved. The indeno[2,1-b]fluorene derivative exhibits an extremely low-energy light absorption band whose absorption end extends to 2000 nm despite its small conjugation space consisting of only 20 π electrons. The present knowledge is useful for the design of new classes of materials based on small molecules with low-energy light absorption.