Unconventional Synthesis of a Cu^I Rotaxane with a Superacceptor Stopper: Ultrafast Excited-State Dynamics and...

A Cu^I bis-phenanthroline rotaxane was prepared by using the [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction to graft a bulky dicyanoquinodimethane (DCNQ) stopper. The electronic properties were investigated with electrochemical and photophysical techniques, in parallel with three reference compounds, namely, the DCNQ derivative alone, the DCNQ-based phenanthroline ligand, and an analogue Cu^I complex lacking the DCNQ moiety. In all the systems containing the DCNQ unit, the lowest electronic excited states are centered thereon, with the singlet level (S₁ ) located at about 1.0 eV, as suggested by TDDFT calculations. Accordingly, in the DCNQ-equipped rotaxane, the typical metal-to-ligand charge-transfer luminescence of the Cu^I center is totally quenched. Ultrafast transient absorption and emission studies show that, in the rotaxane, the final sink of photoinduced processes is the lowest singlet state of the DCNQ moiety (S₁ ), which exhibits strong charge-transfer character and a lifetime of 0.4 ps. Its deactivation leads to population of another excited state with a lifetime of 1.3 ps, which can be the related triplet state (T₁ ) or a vibrationally hot level (hot-S₀ ). Notably, S₁ also shows stimulated fluorescence in the near-infrared (NIR) region between 1100 and 1500 nm, corroborating the TDDFT prediction. This unusual finding opens up the study of ultrashort-lived NIR luminescence in organic donor-acceptor systems.