Long-Lived π‑Shape Platinum(II) Diimine Complexes Bearing 7‑Benzothiazolylfluoren-2-yl Motif on the Bipyridine and Acetylide Ligands: Admixing π,π and Charge-Transfer Configurations

Two π-shape platinum(II) diimine complexes (1 and 2) bearing 7-benzothiazolylfluoren-2-yl motif on the bipyridine and acetylide ligands were synthesized, and their photophysics and reverse saturable absorption were investigated systematically. Both complexes exhibit low-energy, broad, and weak 1MLCT (metal-to-ligand charge transfer)/1LLCT (ligand-to-ligand charge transfer) transitions and intense high-energy bipyridine ligand or acetylide ligand localized 1 π,π* transitions mixed with some 1MLCT/1LLCT/1ILCT (intraligand charge transfer) characters in their UV–vis absorption spectra. The emission of 1 and 2 includes a weak fluorescence band and a broad, structureless phosphorescence band emanating from the 3MLCT/3LLCT/3 π,π* states for 1 and predominantly from 3 π,π* state for 2. Insertion of CC bond between the bipyridine and fluorene components induces pronounced red-shifts of the absorption and emission bands because of the extended π-conjugation in the bipyridine ligand. Because of the admixture of the 3 π,π* excited state with the charge-transfer excited states, both complexes possess extremely long-lived, broadband (visible to the near-IR region) triplet excited-state absorption, resulting in remarkably strong reverse saturable absorption at 532 nm for nanosecond laser pulses. Therefore, complexes 1 and 2 are excellent candidates for devices that require strong reverse saturable absorption.