pH Effect on Two-Photon Cross Section of Highly Fluorescent Dyes Using Femtosecond Two-Photon Induced Fluorescence

Effect of solution pH on two-photon absorption cross-section of highly fluorescent Coumarin and Rhodamine dyes with high repetition rate femtosecond laser pulses at 780 nm is presented using two-photon induced fluorescence technique. A correspondence in the measured two-photon and single-photon cross-section values is seen when the pH changes from acidic to basic conditions (pH = 2-10) for solutions in 1:1 water-ethanol binary mixture. By plotting changes in the single-photon and two-photon fluorescence in this pH range, the excited state pK_a values are found. The ground state pK_a values are also affected by the protonation deprotonation equilibrium as a result of variation in pH from acidic to basic, which are characterized by changes in absorbance spectra. Most of these single-photon and two-photon induced fluorescence spectra show characteristic blue shifts. Different fluorescence quantum yields calculated at each pH reflect a change in structure corresponding to their associated properties as a result of acid base equilibrium.