Unusual triplet-triplet annihilation in a 3D copper(i) chloride coordination polymer

A new coordination polymer (CP) defined as [Cu2Cl2(EtS(CH2)(4)SEt)(4)](n) (CP2) was prepared by reacting EtS(CH2)(4)SEt with CuCl in acetonitrile in a 1 : 2 stoichiometric ratio. The X-ray structure reveals formation of non-porous 3D material composed of parallel 2D-[Cu2Cl2S2](n) layers of Cl-bridged Cu-2(mu-Cl)(2) rhomboids assembled by EtS(CH2)(4)SEt ligands. A weak triplet emission (phi(e) < 0.0001) is observed in the 400-500 nm range with tau(e) of 0.93 (298 K) and 3.5 ns (77 K) as major components. CP2 is the only 2nd example of emissive thioether/CuCl-containing material and combined DFT/TDDFT computations suggest the presence of lowest energy M/XLCT excited states. Upon increasing the photon flux (i.e. laser power), a triplet-triplet annihilation (TTA) is induced with quenching time constants of 72 ps (k(Q) = 1.3 x 10(10) s(-1)) and 1.0 ns (k(Q) = 7.1 x 10(8) s(-1)) at 298 and 77 K, respectively, proceeding through an excitation energy migration operating via a Dexter process. Two distinct (I-o)(1/2) (I-o = laser power) dependences of the emission intensity are depicted, indicating saturation as the observed emission increases with the excitation flux. These findings differ from that previously reported isomorphous CP [Cu2Br2(mu-EtS(CH2)(4)SEt)(4)](n) (CP1), which exhibits no TTA behaviour at 77 K, and only one (laser power)(2) dependence at 298 K. The similar to 18-fold increase in k(Q) upon warming CP2 from 77 to 298 K indicates a temperature-aided TTA process. The significant difference between the presence (slower, CP2) and absence (CP1) of TTA at 77 K is explained by the larger unit cell contraction of the former upon cooling. This is noticeable by the larger change in inter-rhomboid CuMIDLINE HORIZONTAL ELLIPSISCu separation for CP2.