Two-photon absorption spectrum and characterization of the upper electronic states of the dye IR780

In this work, the full two-photon absorption (2PA) spectrum of cyanine dye IR780 in methanol was measured and some important properties of the upper excited electronic states were investigated. Specifically, two IR780 2PA bands of intensities nearing 140 and 2800 Goeppert-Mayer (GM) were found. In order to determine the optical properties of the upper electronic singlet states, a deconvolution of the absorption peaks in the UV region of the spectrum was made. Based on this, properties such as transition dipole moments, oscillator strengths, absorption maxima in the UV-vis spectra, S₂-S₁ vibrational couplings and predictions of the lifetime of the second excited state were calculated. Moreover, by combining experimental and computational results, the 2PA transitions were assigned to the upper excited states S₂ and S₄. Cross-section magnitudes, positions and shapes of the 2PA bands have been satisfactorily explained with a four-state model that comprises the singlet states S₁, S₂ and S₄. From these results, the cyanine investigated in the present work could be used as a novel and interesting moiety for more complex systems that respond to two-photon excitation.