Rodriguez-Serrano Angela, Dinkelbach Fabian, Marian Christel M
Institut für Theoretische Chemie und Computerchemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
Phys Chem Chem Phys. 2021 Feb 7;23(5):3668-3678. doi: 10.1039/d0cp06011a. Epub 2021 Feb 2.
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 and T) are found below the S state. At the excited state minima, the calculated adiabatic energies locate only the T state below the S 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→ T channel is the fastest (8 × 10 s), while the S→ T channel is found to be thermally activated (9 × 10 s) and less efficient when proceeding from the adiabatic S state. Our calculations also reveal, however, a barrierless S→ T ISC pathway near the Franck-Condon region. RISC is found to essentially proceed via the T→ S channel, with a rate constant of (3 × 10 s) if our adiabatic singlet-triplet energy gap in vacuum (ΔE = 0.12 eV) is employed. Shifting the potentials to match two experimentally reported singlet-triplet energy gaps in toluene (ΔE = 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 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 and T potentials is found, which could help overcome the negative exponential dependence of the RISC rate constant on the magnitude of the energy gap.
为了研究4,5-二(9H-咔唑-9-基) - 邻苯二甲腈(2CzPN)的（反向）系间窜越((R)ISC)机制，进行了多参考量子化学计算。采用了密度泛函理论(DFT)和多参考组态相互作用方法(MRCI)相结合的方式。计算得到的吸收光谱与现有实验吸收光谱的高度吻合为所选的计算方法提供了可信度。垂直方向上，在S态下方发现了两个三重激发态(T₁和T₂)。在激发态最小值处，计算得到的绝热能量表明只有T₁态在S态下方。几何弛豫激发态增强的电荷转移(CT)特性导致它们之间的（直接）自旋 - 轨道耦合(SOC)相互作用较低。通过类似Herzberg - Teller方法在300 K温度下评估的振动SOC对(R)ISC概率的贡献较小但不可忽略。对于ISC，S→T₁通道最快(8×10⁶ s⁻¹)，而S→T₂通道被发现是热激活的(9×10³ s⁻¹)，并且从绝热S态开始时效率较低。然而，我们的计算还揭示了在弗兰克 - 康登区域附近存在无势垒的S→T₂ ISC途径。发现RISC基本上通过T₂→S通道进行，如果采用我们在真空中的绝热单重态 - 三重态能隙(ΔE = 0.12 eV)，其速率常数为(3×10⁴ s⁻¹)。将势能进行调整以匹配甲苯中两个实验报道的单重态 - 三重态能隙(分别为ΔE = 0.21和0.31 eV)会导致计算得到的速率常数急剧降低多达4个数量级。除非T₂态直接被热激子占据，否则预计它在介导2CzPN中的三重态 - 单重态跃迁中不会起主要作用。未发现T₁和T₂势能存在强振动耦合的迹象，而这种耦合可能有助于克服RISC速率常数对能隙大小的负指数依赖性。
