Ultrafast Excited-State Dynamics and Dispersion Studies of Third-Order Optical Nonlinearities in Novel Corroles

Ultrafast nonlinear optical (NLO) properties of four novel Corroles in the visible spectral range (660-800 rim) were evaluated using picosecond Z-scan technique. Ultrafast excited state dynamics have also been appraised with picosecond (ps) and femtosecond (fs) degenerate pump-probe techniques using excitation wavelengths of 800 and 600 nm, respectively. The excitation by 800 nm photons resulted in two-photon absorption at adequately high peak intensities, thereby facilitating the access to higher excited states (S-n). The nonradiative relaxation mechanisms from these states, reflected in the pump-probe data, consisted of double-exponential decay with a slow component in the range of 54-277 ps and faster component in the range of 2.0 to 2.5 ps. When excited with 600 run photons (unfocused), photoinduced absorption was observed with the first excited state S-1 being populated, and as a consequence single decay was observed in the data of all molecules studied. These retrieved lifetimes were analogous to those obtained with picosecond pump-probe data. The long lifetime is attributed to nonradiative decay from the S-1 state with possible contribution from triplet states, whereas the shorter lifetime is attributed to the internal conversion (S-2 to S-1*), followed by vibrational relaxation (S-1* to S-1) processes. Time-resolved fluorescence lifetime measurements revealed the magnitude of radiative lifetimes to be in the nanosecond regime. NLO coefficients were evaluated from the Z-scan data at wavelengths of 660, 680, 700, 740, and 800 nm. Large two-photon absorption coefficients (beta)/cross-sections (sigma(2)) at 740 nm/680 nm were recorded for these molecules, making them apposite for applications such as two-photon induced photodynamic therapy and lithography. Figure of merit, T, was <1 for all molecules at 740 and 800 nm, suggesting that these molecules find use in photonic device applications.