New Two-Photon Absorbing BODIPY-Based Fluorescent Probe: Linear Photophysics, Stimulated Emission, and Ultrafast Spectroscopy

The synthesis and comprehensive linear spectroscopic and nonlinear optical properties of a new BODIPY-based fluorene-containing derivative 4,4-difluoro-8-(4-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}phenyl)-1,3,5,7-tetramethyl-2,6-di[(9,9-di{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-9H-fluoren-2-yl)ethynyl]-4-bora-3a,4a-diaza-s-indacene (1) are reported. The steady-state absorption, emission, and excitation anisotropy spectra along with fluorescence lifetimes of 1 were analyzed in organic solvents of different polarity. The degenerate two-photon absorption (2PA) spectrum of 1 was obtained over a broad spectral range by the open aperture Z-scan method using 1 kHz femtosecond excitation, and the 2PA cross section had a maximum value of ∼400 GM. The one-photon stimulated emission depletion spectrum of 1 was determined by a fluorescence quenching method with values of stimulated emission cross sections close to the corresponding ones of the ground state linear absorption. The nature of ultrafast relaxation processes in 1 was analyzed by a transient absorption femtosecond pump-probe technique, and the characteristic times of intramolecular relaxations between the excited electronic states (<150 fs) and solvation dynamics (4-6 ps) were determined. Efficient superluminescence emission of 1 was observed in solution under one- and two-photon femtosecond pumping. Detailed analysis of the singlet excitations in 1 was performed by a time-dependent density functional theory (TD-DFT) method. Three one-photon and two two-photon absorbing states were predicted in the wavelength range investigated. A reactive handle was included in the meso-position of the BODIPY chromophore to facilitate future bioconjugation or functionalization for bioimaging applications.