Influence of a Naphthaldiimide Substituent at the Diimine Ligand on the Photophysics and Reverse Saturable Absorption of Pt^II Diimine Complexes and Cationic Ir^III Complexes

Abstract

Two PtII complexes containing benzothiazolylfluorenyl (BTF) acetylide ligands and different diimine (NN) ligands [NN = 1,10‐phenanthroline (1), 5‐naphthaldiimide–1,10‐phenanthroline (2)] and two heteroleptic cationic IrIII complexes containing the 5‐naphthaldiimide–1,10‐phenanthroline ligand and different phenylpyridine (CN) ligands [CN = 2‐phenylpyridine (3), 2‐{3‐[7‐(benzothiazol‐2‐yl)fluoren‐2‐yl]phenyl}pyridine (4)] were synthesized and characterized. The influence of naphthaldiimide (NDI) attached to the NN ligands and benzothiazolylfluorene attached to the CN ligands on the photophysical properties of these PtII/IrIII complexes has been investigated spectroscopically and theoretically. All complexes exhibit ligand‐localized 1π,π* transitions below 400 nm, and broad and structureless metal‐to‐ligand/ligand‐to‐ligand charge transfer (1MLCT/1LLCT) transitions between 400 and 550 nm. Complexes 1 and 4 show red phosphorescence in toluene solutions at room temperature, where the emitting state is assigned predominantly to the 3π,π* state for complex 1 and 3LLCT/3MLCT states for complex 4. Incorporation of the NDI substituent to the NN ligand completely or partially quenches the emission of complexes 2–4. Complexes 1–4 all possess broad singlet transient absorption from 400 to 800 nm, which is ascribed to 1π,π*/1MLCT/1LLCT states for complex 1, and to 1NDI–· for complexes 2–4. The ns transient absorption study reveals that complex 1 exhibits a broad triplet excited‐state absorption from the 3π,π*/3MLCT states, while complexes 2–4 give moderately strong triplet excited‐state absorption from 3NDI–·. Nonlinear transmission experiments at 532 nm using nanosecond laser pulses demonstrate that these complexes all exhibit strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. Introduction of the NDI substituent to the NN ligand causes charge transfer to the NDI component, which in turn increases the RSA of the PtII complex 2 in comparison with that of the PtII complex 1; while attaching the BTF component to the CN ligands enhances the RSA of the IrIII complex 4 in comparison with that of complex 3.

Monocationic Iridium(III) Complexes with Far‐Red Charge‐Transfer Absorption and Near‐IR Emission: Synthesis, Photophysics, and Reverse Saturable Absorption

“…The chosen DFT‐based methodology represents one of the currently most popular density functionals and basis sets, which previously has shown good agreement with UV/Vis experimental data for various Pt(II), Ru(II) and Ir(III) complexes . The validity of various hybrid DFT functionals with different portions of Hartree–Fock (HF) exchange has been widely tested for description of geometries and optical properties of various transition metal complexes , .…”

“…In order to facilitate RSA of nanosecond laser pulses, an absorber should have a weak ground‐state absorption to populate the excited states, but long–lived triplet excited states, large triplet‐triplet excited‐state absorption coefficients, and a high quantum yield for triplet excited‐state formation. In recent years, the RSA of heavy transition‐metal complexes, such as octahedral Ir(III) complexes, have been extensively explored by our group, , , , and other groups, because these complexes displayed the aforementioned characteristics that well match the requirements for RSA. In addition, by structural modifications, both the ground‐ and excited‐state properties can be readily tuned in these complexes for optimization of the RSA.…”

“…Among which the lack of considerable ground‐state absorption in the red to the near‐infrared (NIR) regions remains to be one of the obstacles. In seeking solutions to red‐shift the ground‐state absorption but meanwhile keep a long–lived and strongly absorbing triplet excited state, immense effects have been put by our group on exploring N ^ N or C ^ N ligands suitable for reverse saturable absorbers , , , , . We discovered that incorporation of quinoxaline or benzo[ g ]quinoxaline unit into either the N ^ N or C ^ N ligand red–shifted the spin‐forbidden 3 π ,π* / 3 CT (charge transfer) absorption bands into longer wavelengths , , .…”

Syntheses, Crystal Structures, and Photoluminescence of a Series of Iridium(III) Complexes Containing the Pentafluorosulfanyl Group

“…The strong spin–orbit coupling (SOC) caused by the central Ir­(III) atom can promote the triplet to ground state radiative transition, leading to high phosphorescence quantum yields at room temperature. This unique character makes Ir­(III) complexes promising candidates for applications in organic light-emitting devices, organic light-emitting electrochemical transistors, low-power upconversion, photocatalysts, luminescent biological labeling, nonlinear optics, and ion detection …”

Novel Fullerene Platinum Alkynyl Complexes with High Second-Order Nonlinear Optical Properties as a Springboard for NLO-Active Polymer Films

“…Also, recently, some of us reported that a “phenylalkynyl-Ru” moiety behaves as a good donor in various NLO-efficient push–pull architectures bearing an acceptor group such as methyl cyanoacrylate or 2,1,3-benzothiadiazole . In addition, the photophysical properties of various Pt­(II) acetylide complexes have been investigated and some of them are characterized by interesting third-order NLO properties such as two-photon absorption cross sections or strong reverse saturable absorption . Also, some terpyridyl and cyclometalated (dipyridyl)­benzene platinum­(II) complexes with a phenylalkynyl ancillary ligand are characterized by a high, second-order NLO response, as determined by the hyper-Rayleigh scattering and EFISH techniques, where the “phenylalkynyl-Pt” moiety and the tridentate ligand act as a donor and an acceptor, respectively …”