Influence of a Naphthaldiimide Substituent at the Diimine Ligand on the Photophysics and Reverse Saturable Absorption of PtII Diimine Complexes and Cationic IrIII Complexes

Abstract

AbstractTwo 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.