Electronic structure, photophysics, and relaxation dynamics of charge transfer excited states in boron-nitrogen-bridged ferrocene-donor organic-acceptor compounds

We present a study of the electronic, photophysical, and picosecond excited-state relaxation characteristics of a class of derivatives comprised of multiple bipyridylboronium acceptors covalently linked to a ferrocene donor. These compounds exhibit a broad visible absorption band, which we attribute to a metal-to-ligand charge transfer transition between the donor and the acceptor. A comparison of optical absorption, spectroelectrochemical, and theoretical results confirms the assignment of the band and provides information on the degree of electron delocalization between the donor and the acceptor. Picosecond transient absorption measurements reveal that the back-electron transfer relaxation is critically dependent on the structural flexibility of the bridging bonds between the donor and the acceptor. In the case where the acceptor substituents are free to rotate about the bridging bonds between the boron and the cyclopentadienyl rings of the ferrocene, a significant portion of the excited state decays directly back to the ground state on a time scale of ∼18 ps, whereas in the case where an additional ansa-bridge that connects acceptor substituents enforces a more rigid conformation, the ground-state recovery proceeds only on a ∼800-ps time scale. This demonstrates the importance of conformational degrees of freedom for the internal conversion and back-electron transfer in these systems.