Effect of the Locally Excited State on Symmetry-Breaking Charge Transfer and Transition Dipole Moment in Quadrupolar Molecules with Inverse Level Order

The symmetry-breaking charge transfer (SBCT) in quadrupolar molecules with normal and inverse level orders (LOs) (the lowest excited state is odd or even) is theoretically investigated. The effect of the locally excited state (LES) on SBCT and the transition dipole moment (TDM) is addressed in detail. The degree of symmetry breaking and the TDM between the lowest excited and ground states are examined in relation to the LO and solvent polarity. The parities of the lowest excited state and the LES are shown to have a profound effect on the degree of SBCT and the TDM magnitude. This opens up wide possibilities for managing the extent of SBCT and TDM. The key finding of this study is that the extent of SBCT is a critical factor governing the photophysics of molecules with inverse LO. A large SBCT magnitude is shown to lead to a pronounced enhancement in the oscillator strength of the lowest even excited state, resulting in a radiative efficiency that rivals that of systems with a normal LO. Furthermore, this work provides simple, practical formulas for quantifying the extent of symmetry breaking and the magnitudes of the key transition dipole moments. These analytical tools offer significant utility for experimentalists and facilitate the design of new studies and the interpretation of spectroscopic data. Together, these insights provide a foundation for advancing the rational design of optoelectronic materials, where precise control over symmetry-breaking processes is paramount.