Directly coupled vs. spectator linkers on diimine Pt(II) acetylides - change the structure, keep the function?

A series of [Donor-Acceptor-Anchor] Platinum(II) bipy (bis)acetylides has been developed in order to systematically compare the effect of conjugated vs. electronically decoupled modes of attachment of protected anchoring groups on the photophysical properties of these light-harvesting units. This first example of "decoupled" phosphonate-diimine Pt(II) complexes presented here are compared and contrasted to their carboxylate analogs. Ultrafast TRIR and femtosecond transient absorption spectroscopies revealed that all complexes possess a charge transfer lowest excited state, with lifetimes between 2 and 14 ns. Vibrational signatures and dynamics of charge-transfer states have been identified; the assignment of electronic states and their vibrational origin was aided by TDDFT calculations. Ultrafast energy re-distribution accompanied by structural changes was detected, indicating, unexpectedly, a significant difference between the structures of the electronic ground and charge-transfer excited states, as well as the differences in the structural reorganisation in the complexes bearing directly attached vs. decoupled anchoring groups. This work demonstrates that decoupling of the anchoring group from the diimine via a saturated spacer is an easy approach that permits combining higher reduction potential and 10-fold longer charge-transfer excited state lifetime with the possibility of surface attachment, whilst retaining essential charge transfer properties.