Optical spectra of two families of symmetrical polymethine dyes, bearing a positive and a negative charge, are analyzed based on an essential-state model recently developed for quadrupolar dyes. The model accounts for molecular vibrations and polar solvation and reproduces the anomalous evolution with solvent polarity of experimental absorption band shapes. Fluorescence and excited-state absorption spectra are well-described within the same model, which also quantitatively reproduces the recent observation of an intense two-photon absorption toward the (two-photon forbidden) lowest excited state. An extensive analysis of optical spectra demonstrates that the essential-state model developed for quadrupolar dyes also applies to polymethine dyes and that long polymethine dyes offer the first experimental example of class III quadrupolar chromophores.

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We performed a detailed experimental investigation and quantum-chemical analysis of two-photon absorption (2PA) spectra of a series of symmetrical cationic polymethines and neutral squaraines having similar structures. Degenerate 2PA spectra of these molecules are taken by using two-photon fluorescence spectroscopy and the Z-scan technique. All measurements are made with 150 fs laser pulses of 1 kHz repetition rate in the tuning range of 520-2100 nm ͑0.6-2.4 eV͒. Comparing 2PA spectra of polymethines and squaraines, we find that we can access considerably larger 2PA cross sections (ജ8600ϫ 10 −50 cm 4 s/photon) in the squaraines owing to narrower linear absorption spectra and an increase in the density of unoccupied molecular orbitals introduced by the squaraine acceptor group in the conjugated chain.

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.