Toward Broadband Reverse Saturable Absorption: Investigating the Impact of Cyclometalating Ligand pi-Conjugation on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium Complexes

摘要

The synthesis, photophysics and reverse saturable absorption of a series of bis-cyclometalated Ir(III) complexes Ir((CN)-N-boolean AND)(2)L center dot PF6, where L = 3,8-bis[9,9-di(2-ethylhexyl)-9H-fluoren-2-y1]-1,10-phenanthroline and (CN)-N-boolean AND = 2-phenylpyridine (ppy, 1), 2-phenylquinoline (pqu, 2), 1-phenylisoquinoline (piq, 3), 2-phenylbenzo[g]quinoline (pbq, 4), and 2,3-diphenylbenzo[g]-quinoxaline (dpbq, 5), are reported. By gradually increasing the pi-conjugation along the pyridine or pyrazine ring of the (CN)-N-boolean AND ligands, the energies of the lowest singlet (S-1) and triplet (T-1) excited states are significantly reduced, as reflected by the pronouncedly red-shifted charge transfer absorption bands at >450 nm and the emission band(s) in their UV-vis absorption and emission spectra, respectively. Additionally, our density functional theory (DFT) calculations confirm that the natures of the S-1 and T-1 states vary with the increased pi-conjugation of the (CN)-N-boolean AND ligands, with the S-1 state changing from the exclusive (LLCT)-L-1 (ligand-to-ligand charge transfer)/(MLCT)-M-1 (metal-to-ligand charge transfer) transitions in 1-3 to the predominant (ILCT)-I-1 (intraligand charge transfer)/(1)pi,pi*/(MLCT)-M-1/ILLCT transitions in 4 and 5, and with the T-1 state being altered from the predominant ligand L based (ILCT)-I-3 or (ILCT)-I-3/(3)pi,pi* nature in 1 and 2, respectively, to the (CN)-N-boolean AND ligand-localized (3)pi,pi*/(MLCT)-M-3/(ILCT)-I-3 parentage in 3-5. All complexes exhibit broad and positive transient absorption (TA) in the visible to the near-IR region (ca. 430-800 nm) upon nanosecond laser excitation at 355 nm. However, the TA spectral features and the triplet lifetimes vary dramatically from 1 to 5, reflecting the different natures of the T-1 states when the degree of pi-conjugation of the (CN)-N-boolean AND ligands increases. Our nonlinear transmission experiments demonstrate moderate to strong reverse saturable absorption (RSA) for 1-5 for nanosecond laser pulses at 532 nm. The relative strength of the RSA follows the trend 1 > 3 > 2 > 4 > 5. Our joined experimental and computational studies manifest that judicious choice of the CAN ligand with appropriate pi-conjugation is an effective approach to obtain Ir(III) complexes with desired photophysical properties for reverse saturable absorbers.