A stronger acceptor decreases the rates of charge transfer: Ultrafast dynamics and on/off switching of charge separation in organometallic donor-bridge-acceptor systems

To unravel the role of driving force and structural changes in directing the photoinduced pathways in donor-bridge-acceptor (DBA) systems, we compared the ultrafast dynamics in novel DBAs which share a phenothiazine (PTZ) electron donor and a Pt(ii) trans-acetylide bridge (-C = C-Pt-C C-), but bear different acceptors conjugated into the bridge (naphthalene-diimide, NDI; or naphthalene-monoimide, NAP). The excited state dynamics were elucidated by transient absorption, time-resolved infrared (TRIR, directly following electron density changes on the bridge/acceptor), and broadband fluorescence-upconversion (FLUP, directly following sub-picosecond intersystem crossing) spectroscopies, supported by TDDFT calculations. Direct conjugation of a strong acceptor into the bridge leads to switching of the lowest excited state from the intraligand ³IL state to the desired charge-separated ³CSS state. We observe two surprising effects of an increased strength of the acceptor in NDI vs. NAP: a ca. 70-fold slow-down of the ³CSS formation—(971 ps) ⁻¹vs. (14 ps) ⁻¹, and a longer lifetime of the ³CSS (5.9 vs. 1 ns); these are attributed to differences in the driving force ΔG _et, and to distance dependence. The 100-fold increase in the rate of intersystem crossing—to sub-500 fs—by the stronger acceptor highlights the role of delocalisation across the heavy-atom containing bridge in this process. The close proximity of several excited states allows one to control the yield of ³CSS from ∼100% to 0% by solvent polarity. The new DBAs offer a versatile platform for investigating the role of bridge vibrations as a tool to control excited state dynamics.