Ultrafast Electron Dynamics of a Ferrocene-Based Butadiyne-Bridged Complex

Photoinduced electron transfer (ET) in alkyne-linked donor-bridge-acceptor (DBA) compounds is strongly influenced by torsional flexibility, allowing control over ET without altering the donor-acceptor distance. Here, we investigate excited-state dynamics in Fc-C4-NAP, a DBA compound featuring a ferrocene (Fc) donor, a butadiyne bridge (C4), and a 1,8-naphthalimide (NAP) acceptor. Unlike analogues DBA compounds with fully organic planar donors, Fc-C4-NAP exhibits a complex excited-state manifold. Femtosecond transient absorption (TA) measurements in the visible and mid-IR regions found three characteristic relaxation times (0.3-0.5 ps, ∼2.6 ps, and 17-20 ps) following its excitation at 402 nm, which prepares NAP-centered excited states.TD-DFT computations indicate that the acceptor-based locally excited (LE) and the charge separated (CS) diabatic states are well coupled to the Fc states associated with d-states of Fe. This bridge-mediated coupling, estimated at 200-500 cm^-1, is strong enough to induce significant mixing of the diabatic states, which also depends strongly on the torsional angle between the NAP and the C4-bonded cyclopentadienyl ring. The spectral changes observed in the TA experiments suggest that the fast component of 0.3-0.5 ps reflects the lifetime of the bright, dominantly NAP-centered state, which relaxes predominantly to the Fc-based states. The middle component of 2.6 ps could have multiple contributions, including relaxation of the nominal CS state, vibrational cooling, and solvation. The slow decay component of ca. 20 ps corresponds to the lifetime of the lowest-energy Fc states; two Fc states of similar energies but perpendicular polarizations. The complex nature of the eigenstates, unraveled by TD-DFT analysis, results in efficient competition of the energy transfer process to the Fc-based excited states with the CS process. These results highlight the key role played by diabatic state coupling, conformational dynamics, and Fc d-orbitals in shaping the ultrafast dynamics of Fc-based DBA systems, guiding the future design of photoactive materials for solar energy and molecular electronics applications.