Platinum(II) Complexes Bearing 2-(9,9-Dihexadecyl-7-R-fluoren-2-yl)-1,10-phenanthroline Ligands: Synthesis, Photophysics and Reverse Saturable Absorption

Six platinum(II) complexes bearing 2-(9,9-dihexadecyl-7-R-fluoren-2-yl)-1,10-phenanthroline ligands (R = NO2, CN, I, H, benzothiazole, thiophene for ligands 1-6, respectively) were synthesized and their photophysics and reverse saturable absorption (RSA) were systematically investigated. All complexes exhibit intense absorption bands between 350 and 450 nm, which are attributed to (1),*/(ILCT)-I-1 (intraligand charge transfer) transitions, and broad (MLCT)-M-1 (metal to ligand charge transfer) transitions between 475 and 550 nm. All complexes are emissive at room temperature in a variety of solvents and at 77 K in butyronitrile glassy matrix, with the long-lived, structured emission appearing between 600 and 750 nm. The emitting states are assigned to predominantly the (3),* state with an admixture of (MLCT)-M-3 characters. Upon ns laser excitation at 355 nm, all complexes display broad, moderately strong transient absorption from the visible to the near-IR region. As a result, RSA is observed from all complexes at 532 nm for ns laser pulses. The strength of the RSA for these complexes follows the trend: Pt-2 > Pt-3 > Pt-4 > Pt-1 >> Pt-5 > Pt-6, which correlates well with the trend of the estimated phi(T)sigma(ex)/sigma(0) values (where phi(T) is the triplet excited state quantum yield, and sigma(ex) and sigma(0) are the excited-state and ground-state absorption cross-section, respectively) at 532 nm. It is revealed that substitution at the 7-position of the fluorenyl component exerts a distinct effect on the photophysics and RSA of the complexes. Aromatic substituents such as benzothiazol-2-yl and 2-thionyl cause pronounced red-shifts of the absorption bands, the emission bands, and the transient difference absorption bands, but a decrease of the RSA at 532 nm.