Strong two-photon absorption in push-pull phthalocyanines: Role of resonance enhancement and permanent dipole moment change upon excitation

We study near-IR two-photon absorption (2PA) spectra of nonsymmetric push-pull metal-free phthalocyanines, which are present in solution at room temperature as a mixture of two spectrally nonequivalent tautomers T1 and T2. Large 2PA cross sections, sigma 2 = (1-3) x 10(3) GM, are obtained in the excitation wavelength region lambda(ex) = 815-850 nm and are explained by (1) a new charge transfer state appearing in the one-photon absorption (IPA) spectrum of these compounds at 420-430 nm and (2) a strong resonance enhancement within the quasi-three-level system when the Q-state(s) serve as the real intermediate state(s). In the Q-region, lambda(ex) = 1100-1500 nm, 2PA and 1PA transitions coincide, and the 2PA strength of any pure electronic or vibronic band is quantitatively described in terms of a two-level approximation.