Dual Fluorescence from Two Polar Excited States in One Molecule. Structurally Additive Photophysics of Crystal Violet Lactone

Crystal violet lactone (CVL) emits in aprotic solvents at room temperature single (A band, low polar solvents) or dual (A and B bands, medium and highly polar solvents) fluorescence. Strong solvatochromic shifts of both fluorescence bands prove significant charge redistribution in both emitting states ( 1 CT A and 1 CT B ). Comparison with model compounds mimicking structural subunits of CVL, 6-dimethylaminophthalide (6-DMAPd), and malachite green lactone (MGL) shows that the A band is displayed from a polar excited state localized within the 6-DMAPd subunit ( 1 CT A, μ e ≅ 10.7 D), and the B band, from a highly polar excited state ( 1 CT B, μ e ≅ 25.2 D) formed after electron transfer from one of the dimethylaniline groups to 6-DMAPd moiety. The 1 CT B state becomes accessible and is populated during solvation by a sufficiently polar environment. CVL phosphorescence strictly matches that of 6-DMAPd, indicating ISC to a triplet state localized on 6-DMAPd moiety in low polar solvents. In polar solvents, transient absorption measurements indicate spin flip and relaxation to a charge-transfer triplet state, as evidenced by identical S 1 → S n and T 1 → T n spectra in acetonitrile indicating presence of dimethylaniline cation radical. In protic media, CVL undergoes fast photodissociation of the C−O bond in lactone ring and diabatic formation of a zwitterion stabilized by hydrogen bonding with solvent molecule.