Implications of Intramolecular Interactions on the Fluorescence of a Highly Flexible Ethane-Bridged Porphyrin Dimer

Understanding the role of conformational flexibility in multichromophoric systems is crucial for elucidating excited-state processes such as electronic coupling, energy transfer, and exciton-exciton annihilation, which are central to the rational design of artificial light-harvesting assemblies. In the present work, the excited-state dynamics of a highly flexible free-base porphyrin dimer has been investigated and compared with its monomer using femtosecond transient absorption spectroscopy and femtosecond optical gating. While the excited-state kinetics of the dimer closely resemble those of the monomer at low pump fluence, pump-fluence-dependent transient absorption and femtosecond optical gating measurements reveal additional ultrafast components of 1 ps and 18-28 ps. Potential energy surface scan reveals a strong dependence of electronic coupling on the dihedral angle between the two porphyrin units. Comparison of the estimated electronic coupling with the reorganization energy indicates that the dimer lies in the weak-coupling regime. Correlation of femtosecond optical gating measurements with transition dipole moments calculated at different dihedral angles suggests that conformers with more favorable transition-dipole orientations predominantly contribute to the blue side of the emission spectrum. These components are attributed to Förster-mediated singlet-singlet annihilation, arising from the significant spectral overlap between the S ₀ ← S ₁ emission and the S _n ← S ₁ excited-state absorption. Overall, these results demonstrate that conformational flexibility modulates Förster-mediated annihilation dynamics in a porphyrin dimer.