Intersystem crossing processes in the 2CzPN emitter: a DFT/MRCI study including vibrational spin-orbit interactions

Area of Science:

• Computational Chemistry
• Photophysics
• Organic Electronics

Background:

• Understanding intersystem crossing (ISC) and reverse intersystem crossing (RISC) is crucial for designing efficient organic optoelectronic materials.
• 4,5-di(9H-carbazol-9-yl)-phthalonitrile (2CzPN) is a molecule of interest due to its photophysical properties.
• Investigating the mechanisms governing these transitions is key to controlling excited-state dynamics.

Purpose of the Study:

• To elucidate the detailed mechanisms of (reverse) intersystem crossing in 2CzPN.
• To determine the relative contributions of different excited states and coupling mechanisms to ISC and RISC.
• To provide theoretical insights that can guide the development of advanced materials.

Main Methods:

• Multireference quantum chemical calculations, including density functional theory (DFT) and multireference configuration interaction (MRCI).
• Calculation of excited state energies, absorption spectra, and spin-orbit coupling (SOC) matrix elements.
• Analysis of vibronic contributions to SOC and temperature-dependent rate constants.

Main Results:

• Computed absorption spectrum shows excellent agreement with experimental data, validating the computational approach.
• Identified two triplet excited states (T1 and T2) below the S1 state vertically, with only T1 at the minimum.
• ISC proceeds fastest via S1→T1 (8 × 10^6 s^-1), while S1→T2 is thermally activated. A barrierless S1→T2 pathway exists near the Franck-Condon region.
• RISC primarily occurs via T1→S1, with rates highly dependent on the singlet-triplet energy gap (ΔE_ST), decreasing significantly with larger gaps.

Conclusions:

• The T2 state plays a minor role in triplet-singlet transitions unless directly populated by hot excitons.
• RISC rates are strongly modulated by the ΔE_ST, highlighting the importance of energy gap engineering.
• Vibronic coupling between T1 and T2 potentials is not significant enough to overcome the energy gap dependence of RISC.