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 {Cu ₄ I ₆ } clusters have been synthesized from alkyl-tris(2-pyridyl)phosphonium halides, [R-PPy ₃ ]Hal, and CuI. The size of the alkyl substituent (R) has a dramatic impact on the structure of the clusters assembled. [Me-PPy ₃ ]I salt reacts with CuI (1 : 2) to give the ionic [Cu(Me-PPy ₃ )I] ₂ Cu ₂ I ₄ complex consisting of the scorpionate [Cu(N,N′,N′′-Me-PPy ₃ )I] ⁺ cation. Under similar conditions, [Pr-PPy ₃ ]I forms the zwitterionic [Cu ₄ I ₆ (Pr-TPP) ₂ ] complex containing an unusual stepwise [Cu ₄ I ₆ ] cluster core. The use of [Bu-PPy ₃ ]I or [Bn-PPy ₃ ]I in this reaction leads to zwitterionic [Cu ₄ I ₆ (R-TPP) ₂ ] complexes, in which a linear-shaped [Cu ₄ I ₆ ] 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 ( ³ CC) phosphorescence. The compounds synthesized are the first red-emitting representatives of the recently discovered family of zwitterionic CuI-based complexes (so-called “AIO” structures).