Abstract
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, σ2 = (1−3) × 103 GM, are obtained in the excitation wavelength region λex = 815−850 nm and are explained by (1) a new charge transfer state appearing in the one-photon absorption (1PA) 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, λex = 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.
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