Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S1 and T1) and a locally excited (LE) triplet state (T3), can be associated with the (Me → N) conformer, the other two CT-type states (S2 and T2) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin–orbit interactions involving the LE T3 state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S1 and T3 states (likewise S2 and T3) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin–orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

: Vibrational mode 1 at the S 0 (Me→N) geometry in vacuo. (c) (d) Figure S2: Scan along vibrational mode 1: (a) Relative DFT/MRCI ground state energy differences w.r.t. the undistorted S 0 (Me→N) geometry, (b) DFT/MRCI excitation energies w.r.t. the ground state energy at the undistorted S 0 (Me→N) geometry, (c) oscillator strengths for the S 0 → S 1 and S 0 → S 2 absorption processes, (d) sum of the squared SOCMEs for transitions discussed in this study.     Figure S6: BH-LYP molecular orbitals (cutoff 0.03) at the S 0 (Me→Ph) geometry in vacuo. Figure S7: Difference densities (± 0.001) of the excited states at the S 0 (Me→Ph) geometry in vacuo. Areas losing electron density in comparison to the electronic ground state are shown in red, areas gaining electron density in yellow.

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Figure S10: Difference densities (± 0.001) of the excited states at the S 1 geometry in vacuo. Areas losing electron density in comparison to the electronic ground state are shown in red, areas gaining electron density in yellow.    Figure S12: BH-LYP molecular orbitals (cutoff 0.03) at the S 2 geometry in vacuo.
Figure S13: Difference densities (± 0.001) of the excited states at the S 2 geometry in vacuo. Areas losing electron density in comparison to the electronic ground state are shown in red, areas gaining electron density in yellow.
Figure S14: Fragment-based analysis for the vertical singlet and triplet state DFT/MRCI-R2016 wavefunctions at the S 2 geometry in vacuo.
Figure S16: Difference densities (± 0.001) of the excited states at the T 1 geometry in vacuo. Areas losing electron density in comparison to the electronic ground state are shown in red, areas gaining electron density in yellow.
Figure S17: Fragment-based analysis for the vertical singlet and triplet state DFT/MRCI-R2016 wavefunctions at the T 1 geometry in vacuo.          (c) (d) Figure S29: Scan along vibrational mode 9: (a) Relative DFT/MRCI ground state energy differences w.r.t. the undistorted S 0 (Me→N) geometry, (b) DFT/MRCI excitation energies w.r.t. the ground state energy at the undistorted S 0 (Me→N) geometry, (c) oscillator strengths for the S 0 → S 1 and S 0 → S 2 absorption processes, (d) sum of the squared SOCMEs for transitions discussed in this study. Figure S30: Scan along vibrational mode 12: (a) Relative DFT/MRCI ground state energy differences w.r.t. the undistorted S 0 (Me→N) geometry, (b) DFT/MRCI excitation energies w.r.t. the ground state energy at the undistorted S 0 (Me→N) geometry, (c) oscillator strengths for the S 0 → S 1 and S 0 → S 2 absorption processes, (d) sum of the squared SOCMEs for transitions discussed in this study.         Figure S35: Important coupling vibrational normal modes calculated at the S 1 minimum of TpAT-tFFO. Figure S36: Important coupling vibrational normal modes calculated at the S 2 minimum of TpAT-tFFO. Figure S37: Important coupling vibrational normal modes calculated at the T 1 minimum of TpAT-tFFO.