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

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Abstract

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 1π,π* absorption band at 374 nm and a broad, weak metal-to-ligand charge transfer (1MLCT) 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 3π,π* excited state. In low-polarity solvents such as hexane and toluene, the emission band becomes structureless and red-shifted, which is assigned to the 3MLCT 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 1π,π*/1MLCT and 3π,π*/3MLCT, 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 excited-state 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.

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