Embedded amide in π-conjugation as a tunable double-bond motif for controlling optical properties

Replacing C[double bond, length as m-dash]C bonds in polycyclic aromatic hydrocarbons with heteroatom-containing double-bond motifs (e.g. C[double bond, length as m-dash]N, B-N, C[double bond, length as m-dash]P) is an effective approach for their electronic modulation. However, structural modification of these motifs is difficult, making the tunability of their electronic properties challenging. To overcome this limitation, we propose using the amide C-N bond as a tunable double-bond unit. We synthesized a series of amide-embedded pyrenes (AmPys) incorporating cis-amide bonds within the π-framework. The degree of double-bond character of the amide C-N bond was intrinsically modulated through covalent modifications (O- and N-methylation), non-covalent interactions (hydrogen bonding), and protonation. Spectroscopic studies revealed a unique relationship; species with shorter C-N lengths (more double-bond character) gave lower T₁ state energies, whereas those with longer C-N lengths (less double-bond character) gave higher T₁ state energies. The S₁ state energies for all AmPy derivatives remain relatively constant. Therefore, the S₁-T₁ energy gap (ΔE _ST) narrows as the C-N bond lengthens. Computed triplet spin densities and nucleus-independent chemical shifts clarified that short C-N bonds promote localized π-conjugation within the two fused K-region rings, acting as an electronic insulator that spatially separates unpaired electrons to lower the T₁ state energies. In contrast, long C-N bonds weaken such π-conjugation, leading to more delocalized spin densities across the π-system, which raises the T₁ state energies. This work establishes the amide bond as a versatile double-bond unit where the C-N bond length serves as a design parameter for tuning electronic properties.