Unexpected Azulene-to-Naphthalene Rearrangement Enables the Synthesis of Nanographenes with Tunable Two-Photon Absorption.

Azulene, a constitutional isomer of naphthalene, has long been regarded as a stable building block for constructing defect-containing nanographenes and azulene-to-naphthalene rearrangements generally require harsh conditions. In this study, we report an unexpected azulene-to-phenanthrene rearrangement during the Scholl reaction of tetraarylated biazulene precursors. Combined experimental and computational studies suggest that this transformation proceeds through a proton-promoted, arenium-ion-mediated rearrangement pathway. A key intermediate product 1a-i was isolated and fully characterized, providing direct insight into the rearrangement process. This transformation enables efficient access to a series of substituted nanographenes. Comprehensive photophysical and electrochemical characterization, together with density functional theory calculations, demonstrate that the resulting nanographenes 1a-1d possess tunable optical and electrochemical properties that can be modulated by peripheral substituents. Moreover, single-crystal x-ray diffraction confirmed the structures of nanographenes 1a, 1c, 1d, and 1a-i. Their flapping-wing conformation and globally aromatic π-conjugated framework contribute to the pronounced two-photon absorption (TPA) responses of these nanographenes. Their TPA cross sections are tunable and reach up to 1239 GM, representing remarkable TPA performance among molecular nanographenes. This work reveals an unusual rearrangement pathway of azulene motifs under a mild condition and provides a useful strategy for accessing nanographenes with tunable electronic structures and nonlinear optical properties.