Ultrafast Dynamics of a Fluorescent Tetrazolium Compound in Solution

Tetrazolium salts are exploited in various fields of research by virtue of their low reduction potentials. Increasingly, associated applications also attend to the photochemical and luminescence properties of these systems. Here, we investigate the photoinduced dynamics of phenyl-benzo[c]tetrazolo-cinnolinium chloride (PTC), one of the very few known fluorescent tetrazolium compounds, by using time-correlated single-photon counting, femtosecond fluorescence upconversion, and ultrafast transient absorption spectroscopy. PTC is generated photochemically by ultraviolet illumination of 2,3,5-triphenyl-tetrazolium chloride (TTC) in various alcohols. Time-resolved fluorescence measurements on PTC with different excitation wavelengths disclose biphasic solvation and vibrational relaxation dynamics. Depending on the solvent, the emission behavior of PTC is characterized by quantum yields on the order of several tens of percent and corresponding excited-state lifetimes of several hundreds of picoseconds. The radiative rate is basically constant for the studied alcohols, whereas the rate of the competing non-radiative process is sensitive to the solvent polarity. Hence, we discuss the possible involvement of intermediate radicals and further presumptive reaction pathways pursued after photoexcitation of PTC.