Vibronic and spin-orbit coupling effects in the absorption spectra of pyrazine: A quantum chemical approach

Abstract

Derivatives of dipole transition moments between spin-orbit coupled (SOC) multireference configuration interaction wave functions have been used in conjunction with vibrational frequencies from density functional theories to compute vibronic S 1 ←S 0 (1 1 B 3u ←1 1 A g) and T 1 ←S 0 (1 3 B 3u ← 1 1 A g) absorption spectra in Herzberg-Teller approximation. The experimentally known spectra are well reproduced. The calculations reveal unexpectedly small spin-orbit couplings between the 1 3 B 3u (3 nπ*) state and nearby optically bright 1 B 2u (1 ππ*) states, thus explaining the absence of the 1 1g0 1b (1 10a0 ν) fundamental in the vibrational fine-structure of the T 1 ←S 0 transition. Adiabatically, two triplet states are found below the S 1 state. The out-of-plane distorted T 2 minimum results from a pseudo Jahn-Teller interaction between two 3 ππ* states of B 1u and B 2u symmetry. At the D 2h-symmetric S 0 and S 1 minimum geometries, the latter states are located well above S 1. The S 1 and T 2 potentials intersect at geometries far away from the Franck-Condon region. This explains the apparently contradictory results that the linewidth in the higher energy regime above the T 1 ←S 0 origin suddenly broadens while no trace of a second triplet state, located energetically below the S 1 origin, could be identified in phosphorescence excitation spectra of the ultracold isolated pyrazine molecule.

The Photophysics of Dibenzo[a,j]phenazine

“…LUMO and HOMO‐4 orbitals for the T 1 state resemble the shape of the ones for the singlet states. A similar arrangement was found in a recent computational investigation of pyrazine, where the aromatic core showed additional out‐of‐plane distortions in the structure of the T 2 state [48] …”

“…A similar arrangement was found in a recent computational investigation of pyrazine, where the aromatic core showed additional out-ofplane distortions in the structure of the T 2 state. [48] For the states mentioned so far, structures were optimized and vertical excitation energies with respect to the ground state were computed. The resulting energy profiles are depicted in Figure 11.…”

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

“…Then, the phases of the wave functions were adjusted in such a way that the largest coefficient of each wave function is positive. 73 Temperature effects on the rate constants were accounted for by assuming a Boltzmann distribution of the harmonic vibrational state populations in the initial electronic state. 74 Zero-point vibrational energy (ZPVE) corrections are automatically included in the VIBES calculations.…”

Large Inverted Singlet–Triplet Energy Gaps Are Not Always Favorable for Triplet Harvesting: Vibronic Coupling Drives the (Reverse) Intersystem Crossing in Heptazine Derivatives

“…For IC, only the linear coupling terms of the nonadiabatic corrections were taken into account. The gradients of the matrix elements and the derivative couplings were obtained by distorting the nuclear framework along dimensionless normal modes using a step size of 0.5 units, utilizing averaged two-point finite difference techniques. , The phases of these gradients are arbitrary and need to be aligned properly by relating the phases of the molecular orbitals and of the DFT/MRCI wavefunctions to a reference calculation as performed in earlier work . Evaluation of nonradiative rate constants in the energy domain according to is not practicable for molecules as large as those investigated here because the density of vibrational states is too high.…”