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

Abstract

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 (λ^em_max = 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).