Multi-wavelength top-hat nanosecond Z scans to determine excited-state absorption cross sections of a platinum bipyridyl complex in the visible

We report the results of measurements performed on a particular platinum(II) 4,4'-bis[3-ethyl-1-(2-ethylhexyl)heptyl]-2,2'-bipyridyl complex bearing 2-(benzothiazol-2'-yl)-7-ethynyl-9,9-dihexadecyl-fluorenyl units. A similar complex, identical except for the presence of ethyl groups at the 9-position of the fluorenes and tert-butyl groups at the 4- and 4'- positions of the bipyridine, was recently reported to possess a very high ratio of triplet excited-state absorption to ground-state absorption, a quantity that has long been used as a figure of merit for reverse saturable absorbers; in addition, femtosecond transient difference absorption experiments and picosecond open-aperture Z-scans have shown it to display broad nonlinear absorption throughout the visible spectrum. In this work, we measured the triplet excitedstate absorption cross section at several representative wavelengths between 450 nanometers and 660 nanometers in an open-aperture top-hat Z-scan experiment employing a nanosecond-pulsed tunable optical parametric oscillator (OPO). The open-aperture Z-scan is a highly sensitive single-beam experiment used to measure nonlinear absorption. Since the spatial profile of the OPO beam resembled a cross-pattern, we closed an adjustable iris on the beam to create a top-hat profile. A dynamic five-level model was used to fit the Z-scan data.