Alkyl-dependent self-assembly of the first red-emitting zwitterionic {Cu4I6} clusters from [alkyl-P(2-Py)3]+ salts and CuI: when size matters

A series of red-emissive {Cu4I6} clusters have been synthesized from alkyl-tris(2-pyridyl)phosphonium halides, [R-PPy3]Hal, and CuI. The size of the alkyl substituent (R) has a dramatic impact on the structure of the clusters assembled. [Me-PPy3]I salt reacts with CuI (1:2) to give the ionic [Cu(Me-PPy3)I](2)Cu2I4 complex consisting of the scorpionate [Cu(N,N,N-Me-PPy3)I](+) cation. Under similar conditions, [Pr-PPy3]I forms the zwitterionic [Cu4I6(Pr-TPP)(2)] complex containing an unusual stepwise [Cu4I6] cluster core. The use of [Bu-PPy3]I or [Bn-PPy3]I in this reaction leads to zwitterionic [Cu4I6(R-TPP)(2)] complexes, in which a linear-shaped [Cu4I6] module appears. Photophysical studies supported by TD-DFT computations have revealed that the title complexes in the solid state at 298 K exhibit a red photoluminescence (emmax = 620-650 nm) with short lifetimes (0.04-2.10 s), which are assigned to the thermally activated delayed fluorescence (TADF) mixed with the cluster centered ((CC)-C-3) phosphorescence. The compounds synthesized are the first red-emitting representatives of the recently discovered family of zwitterionic CuI-based complexes (so-called AIO structures).