We performed a comprehensive experimental investigation of two-photon absorption (2PA) spectra of a series of 12 symmetrical and asymmetrical cationic polymethine dyes, including complete one-and two-photon excitation anisotropy measurements. Quantum-chemical calculations were performed with the goal of understanding the nature of 2PA bands and of uncovering structure-property relations. We found that there are 2PA bands in the spectral region between the first absorption band and that for twice its energy. A weakly allowed 2PA band within the short-wavelength shoulder of the first absorption band was observed owing to the effects of vibrational and charge distribution symmetry breaking. The nature of the strongest 2PA band is connected to the electron transition from the molecular orbital localized at the benzene rings of the terminal groups to the lowest unoccupied molecular orbital (LUMO). Structure-property relations revealed that the 2PA cross section tends to be enhanced by either an increase in the length of the polymethine chromophore or an increase in the donor strengths in the terminal groups.
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