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.