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

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 (T-1 and T-2) are found below the S-1 state. At the excited state minima, the calculated adiabatic energies locate only the T-1 state below the S-1 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 S-1 -> T-1 channel is the fastest (8 x 10(6) s(-1)), while the S-1 -> T-2 channel is found to be thermally activated (9 x 10(4) s(-1)) and less efficient when proceeding from the adiabatic S-1 state. Our calculations also reveal, however, a barrierless S-1 -> T-2 ISC pathway near the Franck-Condon region. RISC is found to essentially proceed via the T-1 -> S-1 channel, with a rate constant of (3 x 10(4) s(-1)) if our adiabatic singlet-triplet energy gap in vacuum (Delta E-ST = 0.12 eV) is employed. Shifting the potentials to match two experimentally reported singlet-triplet energy gaps in toluene (Delta E-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 T-2 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 T-2 and T-1 potentials is found, which could help overcome the negative exponential dependence of the RISC rate constant on the magnitude of the energy gap.

Automated summary

Multireference quantum chemical calculations were used to investigate the (reverse) intersystem crossing mechanisms of 4,5-di(9H-carbazol-9-yl)-phthalonitrile. The results indicate that the enhanced charge transfer character and low spin-orbit coupling interaction between excited states play important roles in the (R)ISC probability. Additionally, vibronic spin-orbit coupling has a small but non-negligible impact on the intersystem crossing process.