Alkyl-dependent self-assembly of the first red-emitting zwitterionic {Cu 4 I 6 } 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]2Cu2I4 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 (λemmax = 620-650 nm) with short lifetimes (0.04-2.10 μs), which are assigned to the thermally activated delayed fluorescence (TADF) mixed with the cluster centered (3CC) phosphorescence. The compounds synthesized are the first red-emitting representatives of the recently discovered family of zwitterionic CuI-based complexes (so-called "AIO" structures).