Twisted-internal charge transfer (TICT) state mechanisms may be less common than expected

We performed a combined experimental and ab initio study of the excited state dynamics of a carbazole-bromobenzothiadiazole (CBB) fluorophore, a molecule that was designed to exhibit strong solvatochromic shifts due to large charge separation in the minimum of the excited state potential energy surface. While the experimental Stokes shifts - obtained here through Excitation Emission Matrix spectroscopy - showed the expected large solvatochromic shift, we found no evidence that this shift is induced by a twisted-internal charge transfer (TICT) state as had been previously (and reasonably) predicted. Instead, ab initio calculations using TD-DFT and the CAM-B3LYP/6-31G+(d,p) (IEF-PCM) model explained the shift semi-quantitatively using a moderate charge separation in the excited state combined with small contributions of solvent-induced dipole moments in CBB that depend on the solvent polarity. While a TICT state could be identified as a local minimum on the S1 potential energy surface through its large dipole along the donor-acceptor axis and by examination of the natural transition orbitals, the global minimum on the S1 surface is close to the Franck-Condon region of excitation and much more accessible. This study highlights some of the complexities in identifying TICT states from experimental observations and frontier orbitals, alone. A combined experimental and ab initio study of the excited state dynamics of an organic fluorophore shows the complexities in identifying twisted-internal charge transfer (TICT) states from solvatochromic shifts and frontier orbitals, alone.