X-Shaped Multichannel Conjugation Enabled by Excited-State Aromaticity

Abstract Molecular topology governs the photophysical properties of polycyclic π-conjugated systems by defining their effective conjugation pathways. Conventionally, linear π-extension is considered more effective than angular extension in increasing the effective conjugation length and narrowing the optical bandgap. Here we uncover an inverse topology–property relationship in π-extended dihydrophenazines (DHPs): linear extension unexpectedly blueshifts fluorescence, whereas angular extension produces pronounced redshifts. Transient spectroscopy, theoretical calculations and aromaticity analyses reveal that Baird aromaticity–driven planarization of the DHP core redistributes π-delocalization in the S1 state, causing linear fusion to disrupt global π-delocalization and shorten the effective conjugation length, while angular fusion promotes cooperative delocalization across the fused framework and extends it. Successive angular fusion progressively activates four structurally distinct π-extension branches through the shared Baird-aromatic DHP core, generating an X-shaped multichannel conjugation network. These findings reveal how excited-state aromaticity reprograms conjugation topology, offering new insights for controlling π-delocalization and emission.