Nonlinear absorption in a series of Donor-pi-Acceptor cyanines with different conjugation lengths

A detailed experimental and theoretical study of the linear and nonlinear absorption of a series of asymmetrical D-pi-A cyanine dyes with the same trimethylindolin donor (D) and diethylaminocoumarin-dioxaborine acceptor (A) terminal groups and different conjugation lengths, is presented. Strong solvatochromic behavior affecting the fluorescence quantum yields, lifetimes, and the linear and nonlinear absorption properties is observed due to the presence of permanent ground state dipole moments. Detailed experimental studies of lifetime dynamics are performed by direct time-correlated single photon counting and pump-probe techniques. We find that an increase in p-conjugation in the investigated series of dyes leads to an enhancement of the excited-state absorption and two-photon absorption (2PA) cross-sections (delta(2PA)). The 2PA spectra for all of the investigated dyes consist of two well-separated bands. The first band occurs at two-photon excitation into the vibrational levels and not into the absorption peak of the main transition, S-0 -> S-1, which is more typical of that observed for symmetrical cyanines. The position of the second 2PA band for all the molecules remains unchanged in solvents of different polarity contrary to the large solvatochromic shift of the S-0 -> S-1 band, resulting in a large intermediate state resonance enhancement and, therefore, a larger 2PA in acetronitrile (delta(2PA) approximate to 10000 GM) compared to toluene (delta(2PA) approximate to 4700 GM).