1.

(a) Structures of the molecules under study. (b) Peak normalized steady-state absorption and emission spectra for SAA and DHAQ in THF. Dotted blue and dotted maroon lines show emission when exciting at 280 nm; this is the S₄ ← S₀ transition for SAA and S₆ ← S₀ transition for DHAQ, confirmed from TD-DFT and well matched FCHT progressions. Peak normalized emission and excitation spectra recorded at 77 K in glassy 2-MeTHF for (c) SAA and (d) DHAQ (λ_ex = 380 nm). Room temperature absorption in solid blue (SAA) and maroon (DHAQ) in all panels. Computed absorption spectra for the indicated transitions are overlaid, together with the emission(s) from the monoketo tautomer (shaded areas) in (c) and (d). Note that spectra have been shifted to match the experimental data to account for the small energetic shifts in transitions typically observed in TD-DFT.