Solvent and Structural Fluctuations Induced Symmetry-Breaking Charge Transfer in a Porphyrin Triad

This study aims to show that the symmetry-breaking charge transfer (SBCT) in a donor-acceptor-donor (DAD) porphyrin triad occurs via solvent and structural fluctuations measured by using femtosecond broadband fluorescence upconversion spectroscopy, which can directly trace the origin of the emitting state by monitoring its emission dipole moment as a function of time. While the emission dipole moment of the triad in the excited state remains nearly unchanged in nonpolar solvents such as cyclohexane and toluene, it is significantly reduced in polar solvents such as benzonitrile due to a change in the emitting state from quadrupolar (the exciton coupled state) to dipolar symmetry (the relaxed S₁ state). The latter state is formed by the SBCT process of DAD via a combination of solvent and structural fluctuations.