4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) are fascinating dyes with great potentials for various applications. To establish the design principle for the modification of the BODIPY skeleton, we now investigate the electronic impacts of the introduction of ring-fused structures. The DFT calculations revealed that while the benzene-fusing at the a bond in the BODIPY skeleton increases the HOMO level, the benzene-fusing at the b bond leads to a decreased LUMO level. Based on these results, a benzo[b]-fused BODIPY 1 and a fully-fused BODIPY dimer 2 were synthesized. X-ray crystal structure analysis demonstrated that the benzo[b]-fused structure significantly perturbs the π-conjugation with enhancement of an electron-accepting azafulvene character. In the ring-fused BODIPY dimer 2, the central benzene ring is largely deviated from the aromatic benzene geometry. As a consequence, 2 has a significantly low-lying LUMO delocalized along the periphery. In cyclic voltammograms, the benzene-fused BODIPY 1 and fully-fused dimer 2 indeed showed reversible reduction waves at much less negative potentials (E_1/2 = −1.05 V vs. Fc/Fc⁺ for 1, −0.85 V for 2), relative to a non-fused BODIPY 3 (E_1/2 = −1.40 V). In addition, the benzene-fused BODIPYs showed broad and intense absorption bands in the Vis-NIR region. In particular, the BODIPY dimer 2 showed an intense absorption at 629 nm with a broad shoulder band reaching 900 nm. Corresponding to the red-shifted absorption, compounds 1 and 2 showed weak fluorescence in the deep red region (λ_em = 690 nm, Φ_F < 0.01) and in the near-infrared region (940 nm, Φ_F < 0.01), respectively.

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