Potential energy surfaces (PESs) for Cz dimer
To gain insight into the relative stabilities of different dimer arrangements, the PESs for the Cz dimers in the ground and excited states were first calculated without taking geometric relaxation into account (Figure 2). On the one hand, the Stg dimers (θ = 90~105°) are the most stable both in the ground and excited states. In particular in the ground state, the syn(θ = 0°) and anti (θ = 180°) dimers are significantly higher in energy by ca. 0.32 and 0.23 eV, respectively, and do not correspond to local minima, either. Interestingly, however, such large energy differences between the most stable Stg dimer and the others significantly decreases in the excited states. In the T1 state, for instance, the energy gap between theStg and syn dimers reduces to 0.14 eV. The antidimer becomes even more stable and such an energy gap is calculated to be only ca. 30 meV. In the S1 state, the syndimer is in a deep local minimum and only ca. 70 meV higher in energy than the Stg dimer, whereas the anti dimer becomes ca. 0.16 eV higher than the Stg dimer. These results suggest that the dimer can be arranged in the syn and anti conformations in the S1 and T1 states, respectively, if the Cz dimers are pre-arranged in such conformations. In particular, given the considerable energy barrier (ca. 0.18 eV) between thesyn and Stg dimers, the syn dimer can be easily isolated and the interconversion between these two dimers unlikely occurs in the S1 state.
However, these results were obtained without taking structural relaxations into account. Because such relaxations obviously depend on both the dimer conformation and the electronic state, the relative stabilities of Cz dimers can be altered when considering such relaxations.[12, 36] Thus, we now turn to the adiabatic energy differences among Cz dimers including geometric relaxations.