Ultrafast Excited-State Intermolecular Proton Transfer of Cyanine Fluorochrome Dyes

Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton, transfer (ESPT) to water, and D2O, from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, k(PT), of QCy7 excited from its protonated form, ROH, is similar to 1.5 x 10(12) s(-1). This is the highest ever reported value in the literature thus far, and it is comparable to the reciprocal of the longest solvation dynamics time component in water, tau(s) = 0.8 ps. We found a kinetic isotope effect (KIE) on the ESPT rate of similar to 1.7. This value is lower than that of, weaker photoacids, which usually have KIE value of similar to 3, but comparable to the KIE on proton diffusion in water of similar to 1.45, for which the average time of proton transfer between adjacent water molecules is similar to that of QCy7.