Excited-state Proton Transfer Dye Research Articles

Excited state proton transfer dye with an emission quantum yield up to 60% upon Zn2+ coordination

Intramolecular excited state proton transfer (ESIPT) dyes are appealing for metal ion sensing because of their large apparent Stokes-shift, environmental sensitivity, and turn-on fluorescence capabilities. Here we introduce 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole (2) as a new ESIPT sensing motif. The 2,5-bis(2-pyridylimino)pyrrole portion of 2 readily coordinates Hg2+, Zn2+, Co2+, Cu2+, and Ni2+ ions and results in a visible red-shift in absorption. While coordination of Hg2+, Co2+, Ni2+, and Cu2+ completely quenches emission, coordination to Zn2+ increases the ESIPT emission quantum yield from 25% to 60%. Unlike traditional ESIPT sensors, the metal ion binding site of 2 is intrinsic to the chromophore but is not the ESIPT site, thus enabling the selective modification of each functionality. As proof of concept, we use bromobutyrate as an alcohol protecting group whose cleavage by hydrazine results in turn-on fluorescence response. Collectively these results demonstrate the 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole motif is sensitive to several stimuli (i.e. metal ion, base, solvent, and N2H4) and is a promising scaffolding for sensing applications.

Enantioselective Protonation of Silyl Enol Ether Using Excited State Proton Transfer Dyes.

Enantiopure excited state proton transfer (ESPT) dyes were used for the asymmetric protonation of silyl enol ether. Under 365 nm irradiation, with 3,3'-dibromo-VANOL as the ESPT dye, up to 49% enantioselectivity with a 68% yield of product was observed at room temperature. The reaction is effective with a range of silyl enol ethers and can also be achieved with visible light upon the addition of triplet sensitizer. The relatively low ee of the protonated product is due to the racemization/decomposition of the ESPT dye in the excited state as indicated by circular dichroism, HPLC, and UV-vis spectroscopy.