Experimental and theoretical approaches to understanding two-photon absorption spectra in polymethine and squaraine molecules

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 eV). Comparing 2PA spectra of polymethines and squaraines, we find that we can access considerably larger 2PA cross sections (>= 8600 x 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. (c) 2006 Optical Society of America.