Solvent effect on two-photon absorption and fluorescence of rhodamine dyes

Abstract

For a series of rhodamine dyes, two-photon absorption (TPA) and two-photon fluorescence (TPF) have been performed in different solvents. Solvent-dependent TPA spectra of these dyes were measured with open aperture z-scan method and compared to their respective single-photon spectra at equivalent energies. In the TPA spectra, relative peak intensities and positions are highly solvent dependent, which could be a result of vibronic couplings that depend on solvent environment. Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents. Certain complementary and similarity between TPA and TPF are also elucidated. Finally, a two-photon figure-of-merit is presented for these dyes in different solvents as a function of wavelength.

Fig. 1

(a) Femtosecond experimental setup. (b) Open aperture z-scan trace of Rh6G in MeOH. (c) Closed aperture z-scan plot of Rh6G in MeOH at 806 nm by MHz oscillator showing negative n₂. (d) Closed aperture z-scan plot of Rh6G in MeOH at 810 nm by kHz laser Odin, showing positive n₂.

Fig. 2

Linear absorption spectra of the rhodamine dyes in different solvent medium: (a) Rh6G, (c) RhB and (d) Rh101 indicating the S₁ and S₂ transition peaks. A concentration of 10⁻⁵ M is used for measuring the spectra. Corresponding dye structures are also given in the inset. (b) The Jablonski diagram of rhodamine dyes.

Fig. 3

Comparison of one-photon (solid lines) and two-photon (lines + symbol) absorption spectra of rhodamine 6G, rhodamine B and rhodamine 101. The y-axis values represent two-photon absorption cross-sections for the dye. The one-photon intensities are plotted in arbitrary scale at the same y-axis. At the x-axis, the TPA spectra are plotted against half the excitation wavelength value to allow comparison of the transition wavelengths for the one-photon and two-photon allowed electronic transitions.

Fig. 4

(a) Maximum TPA cross-sections among all measured wavelengths for the different dyes as a function of solvent polarity. (b) Maximum TPA cross-sections of the dyes as a function of solvent polarity at 780 nm excitation. In both the graphs TPA cross-sections of Rh101 are plotted in a different Y-scale in the right side.

Fig. 5

(a) Plot of two-photon fluorescence (TPF) intensity for Rh6G in MeOH versus excitation intensity of the laser showing quadratic dependence. Inset shows the linear dependence of TPF intensity on the square of the excitation intensity. (b and c) TPF spectra of Rh6G and RhB, respectively in different solvents at 780 nm excitation. (d) Calculated TPEACS spectra of different dyes in different solvents. (e) Comparison of TPA and TPEACS spectra of Rh6G in DMF measured from z-scan and TPF technique, respectively.