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

## TL;DR

This paper studies how electrons move in a complex molecule with a ferrocene donor and naphthalimide acceptor, revealing how structural flexibility affects electron transfer dynamics.

## Contribution

The study reveals the role of diabatic state coupling and torsional flexibility in controlling ultrafast electron transfer in ferrocene-based donor-bridge-acceptor systems.

## Key findings

- Femtosecond TA measurements identified three relaxation times (0.3–0.5 ps, ∼2.6 ps, and 17–20 ps) in Fc-C4-NAP.
- TD-DFT computations showed strong coupling (200–500 cm–1) between acceptor and ferrocene states.
- Torsional angle between NAP and cyclopentadienyl ring strongly influences state mixing and energy transfer.

## Abstract

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.