Theoretical investigation of steric effects on the S1 potential energy surface of o-carborane-anthracene derivatives

TDDFT scan calculations were performed for s-carborane-anthracene derivatives (o-CB-X-Ant where X=-H, -CH(3), -C(2)H(5) and tert-butyl or -tBu) in order to understand the interplay between the steric effects, S(1) potential energy surface (PES) and photophysical properties. The results show that all systems exhibit three local minima on the S(1) PES, which correspond to the emissive LE and TICT state, along with the nonemissive CT state respectively. In the case of the unsubstituted system (o-CB-H-Ant), and -CH(3) and -C(2)H(5) substituted cases, S(1) PES is predicted to be quite flat for certain conformations indicating that it is possible for these systems to reach the nonemissive CT state without a large energy penalty. In comparison, conformational pathways for the nonemissive CT state are predicted to be energetically unfavorable for o-CB-tBu-Ant as a result of both steric and electronic effects. These results provide a mechanism for the enhanced emission of σ-CB-fluorophore molecules with bulky ligands.