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

Abstract

Multireference quantum chemical calculations were performed in order to investigate the (reverse) intersystem crossing ((R)ISC) mechanisms of 4,5-di(9H-carbazol-9-yl)-phthalonitrile (2CzPN). A combination of density funcional theory (DFT) and multireference configuration interaction methods (MRCI) was used. The excellent agreement of the computed absorption spectrum with available experimental absorption spectra lends confidence to the chosen computational protocol. Vertically, two triplet excited states (T₁ and T₂) are found below the S₁ state. At the excited state minima, the calculated adiabatic energies locate only the T₁ state below the S₁ state. The enhanced charge transfer (CT) character of the geometrically relaxed excited states causes their mutual (direct) spin–orbit coupling (SOC) interaction to be low. Contributions of vibronic SOC to the (R)ISC probability, evaluated by a Herzberg–Teller-like procedure for a temperature of 300 K, are small but not negligible. For ISC, the S₁ → T₁ channel is the fastest (8 × 10⁶ s⁻¹), while the S₁ → T₂ channel is found to be thermally activated (9 × 10⁴ s⁻¹) and less efficient when proceeding from the adiabatic S₁ state. Our calculations also reveal, however, a barrierless S₁ → T₂ ISC pathway near the Franck–Condon region. RISC is found to essentially proceed via the T₁ → S₁ channel, with a rate constant of (3 × 10⁴ s⁻¹) if our adiabatic singlet–triplet energy gap in vacuum (ΔE_ST = 0.12 eV) is employed. Shifting the potentials to match two experimentally reported singlet–triplet energy gaps in toluene (ΔE_ST = 0.21 and 0.31 eV, respectively) leads to a drastic reduction of the computed rate constant by up to 4 orders of magnitude. The T₂ state is not expected to play a major role in mediating triplet–singlet transitions in 2CzPN unless it is directly populated by hot excitons. No indication for a strong vibronic coupling of the T₂ and T₁ potentials is found, which could help overcome the negative exponential dependence of the RISC rate constant on the magnitude of the energy gap.