Tuning Photophysical Properties and Improving Nonlinear Absorption of Pt(II) Diimine Complexes with Extended π-Conjugation in the Acetylide Ligands

Two new Pt(II) 4,4'-di(5,9-diethyltridecan-7-yl)-2,2'-bipyridine complexes (1 and 2,) bearing 9,9-diethyl-2-ethynyl-7-(2-(4-nitrophenyl)ethynyl)-9H-fluorene ligand and N-(4-(2-(9,9-diethyl-7-ethynyl-9H-fluoren-2-yl)ethynyl)phenyl)-N-phenylbenzeneamine ligand, respectively, were synthesized and characterized. Their photophysical properties were investigated systematically by UV vis absorption, emission, and transient absorption (TA) spectroscopy, and the nonlinear absorption was studied by nonlinear transmission technique. Theoretical TD-DFT calculations using the CAM-B3LYP functional were carried out to determine the nature of the singlet excited electronic states and to assist in the assignment of significant transitions observed in experiments. Complex 1 exhibits an intense, structureless absorption band at ca. 397 nm in CH2Cl2 solution, which is attributed to mixed metal-to-ligand charge transfer ((MLCT)-M-1)/ligand-to-ligand charge transfer ((LLCT)-L-1)/intraligand charge transfer ((ILCT)-I-1)/(1)pi,pi* transitions, and two (MLCT)-M-1/(LLCT)-L-1 transitions in the 300-350 nm spectral region. Complex 2 possesses an intense acetylide ligand localized (1)pi,pi* absorption band at ca. 373 nm and a moderately intense (MLCT)-M-1/(LLCT)-L-1 tail above 425 nm in CH2Cl2. Both complexes are emissive in solution at room temperature, with the emitting state being tentatively assigned to the predominant (3)pi,pi* state for 1, whereas the emitting state of 2 exhibits a switch from (3)pi,pi* state in high-polarity solvents to (MLCT)-M-3/(LLCT)-L-3 state in low-polarity solvents. Both 1 and 2 exhibit strong singlet excited-state TA in the visible to NIR region, where reverse saturable absorption (RSA) is feasible. The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these Pt complexes can be tuned drastically by extending the pi-conjugation of the acetylide ligands. In addition, strong RSA was observed at 532 nm for nanosecond (ns) laser pulses from 1 and 2, demonstrating that the RSA of the Pt(II) diimine complexes can be improved by extending the pi-conjugation of the acetylide ligands.