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 D ₂O 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 ∼1.5 × 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, τ _S = 0.8 ps. We found a kinetic isotope effect (KIE) on the ESPT rate of ∼1.7. This value is lower than that of weaker photoacids, which usually have KIE value of ∼3, but comparable to the KIE on proton diffusion in water of ∼1.45, for which the average time of proton transfer between adjacent water molecules is similar to that of QCy7.