Intersystem-crossing and phosphorescence rates in fac-Ir(III)(ppy)3: a theoretical study involving multi-reference...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Green phosphorescent emitters like fac-tris-(2-phenylpyridine)iridium (fac-Ir(ppy)3) are crucial for organic light-emitting diodes (OLEDs).
• Understanding their photophysical properties, including absorption, emission, and intersystem crossing (ISC), is key to optimizing device performance.

Purpose of the Study:

• To investigate the photophysical properties of fac-Ir(ppy)3 using combined density functional theory (DFT) and multi-reference configuration interaction (MRCI) methods.
• To elucidate the mechanisms governing intersystem crossing (ISC) and phosphorescence in this heavy metal complex.
• To critically evaluate the performance of different quantum chemical approaches for describing the photophysics of fac-Ir(ppy)3.

Main Methods:

• Employed combined DFT and MRCI methods, incorporating spin-orbit coupling (SOC) effects.
• Utilized a perturbational treatment of SOC for UV/Vis spectrum calculation and MRCI for phosphorescence rates.
• Determined ISC rates via Fourier transformation of time correlation functions, including Dushinsky rotations.

Main Results:

• Calculated UV/Vis spectrum shows excellent agreement with experimental data.
• Spin-orbit coupling significantly influences the metal-to-ligand charge transfer (MLCT) band, extending its onset and causing a blue shift.
• Ultrafast ISC (kISC = 6.9 × 10^12 s^-1) and near-unity triplet quantum yield were confirmed.
• Phosphorescence decay times for different fine-structure levels (T1,I, T1,II, T1,III) were computed, agreeing with experimental orders of magnitude.

Conclusions:

• The chosen quantum chemical methods accurately reproduce the photophysical properties of fac-Ir(ppy)3.
• Ultrafast ISC is driven by substantial electronic SOC and a small energy gap.
• Geometric distortion in the triplet state localizes transition densities, influencing phosphorescence characteristics.
• Detailed understanding of ISC and phosphorescence mechanisms provides a basis for designing improved phosphorescent emitters.