Linear photophysics, stimulated emission, and ultrafast spectroscopy of new two-photon absorbing diketopyrrolopyrrole derivatives

The synthesis and comprehensive linear photophysical and nonlinear optical characterization of new diketopyrrolopyrrole derivatives, 2-(2-ethylhexyl)-3,6-di(pyridin-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1) and 2-(10-azidodecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2), are reported. The nature of the main linear absorption bands of 1 and 2 was ascertained based on experimental excitation anisotropy spectra and quantum chemical calculations. Three-dimensional fluorescence maps, emission lifetimes, and photochemical decomposition quantum yields of the new derivatives were obtained in a number of organic solvents at room temperature. Two-photon absorption (2PA) spectra of 1 and 2 were measured over a broad spectral range using an open aperture Z-scan technique, revealing a maximum 2PA cross section of ∼100 GM. Transient absorption and stimulated emission pump-probe measurements with femtosecond time resolution revealed two different types of fast relaxations in the excited states of 1 and 2, and one-photon stimulated emission depletion (STED) spectra were determined. Efficient superfluorescence emission of 1 was observed under femtosecond transverse pumping conditions. A quantum-mechanical investigation of the electronic structure of the new compounds was performed using TD-DFT methodology. Simulated linear absorption spectra, including Franck-Condon band structure of the first excited states, were found in good agreement with experimental data, while 2PA cross sections obtained without vibronic contributions were considerably underestimated.