Controlling through-space and through-bond intramolecular charge transfer in bridged D-D′-A TADF emitters

Abstract

Donor-donor′-acceptor molecules where the donor′ bridges the donor and acceptor have different possible interaction pathways for charge transfer. Here we study a series of donor-donor′-acceptor molecules, having the same acceptor and donor′ but different donors, and donor′-phenyl spacer-acceptor to change the spatial separation and overlap between potential through-space donor-acceptor charge transfer (CT) in competition with donor′-acceptor through-bond CT. We determine that the charge transfer driving force plays a large role in dictating which charge transfer channel is favoured. Strong donors and acceptors with large driving force favour through-space CT. We also find that solid state host packing plays an important role, with small molecule hosts that pack tightly, distorting the guest molecules, reducing D-A separation to stabilise the through-space CT state over the through-bond state. Only the through-space CT states give fast reverse intersystem crossing and efficient TADF. These results give the first insight into the photophysics of through-space CT compared to through-bond states on the same molecule.