Phosphorescent Cationic Iridium(III) Complexes with 1,3,4-Oxadiazole Cyclometalating Ligands: Solvent-Dependent Excited-State Dynamics

Abstract

To elucidate the nature of low-lying triplet states and the effect of ligand modifications on the excited-state properties of functional cationic iridium complexes, the solventdependent excited-state dynamics of two phosphorescent cationic iridium(III) complexes, namely [Ir(dph-oxd) 2 (bpy)]PF 6 (1) and [Ir(dph-oxd) 2 (pzpy)]PF 6 (2), were investigated by femtosecond and nanosecond transient absorption spectroscopy. Upon photoexcitation to the metal-to-ligand charge-transfer (MLCT) states, the excited-state dynamics shows a rapid process (τ =0.7−3 ps) for the formation of solvent stabilized 3 MLCT states, which significantly depends on the solvent polarity for both 1 and 2. Sequentially, a relatively slow process assigned to the vibrational cooling/geometrical relaxation and a long-lived phosphorescent emissive state is identified. Due to the different excited-state electronic structures regulated by ancillary ligands, the solvation-induced stabilization of the 3 MLCT state in 1 is faster than that in 2. The present results provide a better sight of excited-state relaxation dynamics of ligand-related iridium(III) complexes and solvation effects on triplet manifolds.

Aggregation-Induced Enhancement of Molecular Phosphorescence Lifetime: A First-Principle Study

“…Persistent phosphorescent materials have received considerable attention in the bioimaging, , chemical sensors, , photovoltaic devices, , security signs, , and other applications. , The range of luminophores is essentially limited to inorganics or organometallic complexes, − which benefit from various allowed electronic transitions, thanks to abundant d orbitals in transition-metal atoms. In contrast to the expensive and toxic metal-containing phosphorescent materials, pure organic molecules are cheaper, environmentally safer, and offer better molecular design versatility .…”

Transient Absorption Spectroscopy of a Carbazole-Based Room-Temperature Phosphorescent Molecule: Real-Time Monitoring of Singlet–Triplet Transitions

“…Organic room-temperature phosphorescent (RTP) materials present many advantages compared with traditional heavy metal-containing complexes, − including high luminescence efficiency, long lifetimes, good processability, low cost, wide application range, − etc. In particular, the internal quantum yield of phosphorescent materials can theoretically reach 100%, which has led to them playing an increasingly important role in luminescent materials .…”

Investigation of the Relationship between Quantum Yield, Charge-Transfer State, and Structure of the Ligands in Red-Emitting Heteroleptic Iridium(III) Complexes

“…In order to gain insights into the chemical species in the excited state of Ir­(III) complexes, the transient absorption spectra of Red-pq and Red-piq were measured after excitation using a 400 nm pump. The generally accepted photophysics of the iridium­(III) complexes involves ultrafast ISC to the triplet state that occurs on a subpicosecond time scale due to the strong spin–orbit coupling resulting from the iridium metal center. ,, Therefore, the difference spectra in Figure , taken at 5 ps after the photoexcitation, can be safely assumed to originate from the excited-state absorption (ESA) of the triplet transient. The spectra taken at 500 ps after the pump show slight feature changes identified as solvent relaxation …”