Broadband Nonlinear Absorbing Platinum 2,2′-Bipyridine Complex Bearing 2-(Benzothiazol-2′-yl)-9,9-diethyl-7-ethynylfluorene Ligands

A platinum 2,2'-bipyridine complex (1) bearing 2-(benzothiazol-2'-yl)-9,9-diethyl-7-ethynylfluorene ligands was synthesized and characterized. Its photophysical properties and nonlinear absorption characteristics were systematically investigated by UV vis absorption, emission, and transient difference absorption spectroscopy, as well as Z-scan and nonlinear transmission techniques. Complex 1 exhibits a strong structureless (1)pi,pi* absorption band at 374 nm and a broad, weak metalto-ligand charge transfer ((MLCT)-M-1) transition in the visible region in CH2Cl2 solution. It emits at approximately 565 nm with vibronic structures at room temperature in polar solvents, attributed to the acetylide ligand (3)pi,pi* excited state. In low-polarity solvents such as hexane and toluene, the emission band becomes structureless and red-shifted, which is assigned to the (MLCT)-M-3 state. The emission spectrum becomes more structured and slightly blue-shifted at 77 K in butyronitrile glassy matrix. In femtosecond and nanosecond transient absorption measurements, 1 exhibits both singlet and triplet excited-state absorption from 450 to 800 nm, which are tentatively attributed to the (1)pi,pi*/(MLCT)-M-1 and (3)pi,pi*/(MLCT)-M-3, respectively. Z scan experiments were carried out using nanosecond and picosecond pulses at 532 nm, and picosecond pulses at a variety of other wavelengths in the visible and near-IR, and the experimental data were fitted by a five-level model using the excited-state lifetimes and estimated cross-section values from the photophysical study. In this way, values were obtained for the first and second singlet excited-state absorption cross sections and the triplet excitedstate absorption cross section throughout the visible and near-IR and for the two-photon absorption (TPA) cross section in the near-IR region. Our results demonstrate that 1 possesses extremely large ratios of the excited-state absorption cross sections to the ground-state absorption in the visible spectral region and, compared to the other two-photon absorbing platinum complexes, the largest two-photon absorption cross sections in the near-IR region. This makes complex 1 a very promising candidate for photonic devices that require large and broadband nonlinear absorption. Reverse saturable absorption of 1 in CH2Cl2 solution at 532 nm for a nanosecond laser pulse was demonstrated. A remarkable transmission decrease was observed when the incident fluence increased.