Regiospecific formation and unusual optical properties of 2,5-bis(arylethynyl)rhodacyclopentadienes: a new class of luminescent organometallics

A series of 2,5-bis(arylethynyl)rhodacyclopentadienes has been prepared by a rare example of regiospecific reductive coupling of 1,4-(p-R-phenyl)-1,3- butadiynes (R=H, Me, OMe, SMe, NMe₂, CF₃, CO ₂Me, CN, NO₂, -C≡C-(p-C₆H ₄-NHex₂), -C≡C-(p-C₆H₄-CO ₂Oct)) at [RhX(PMe₃)₄] (1) (X=-C≡C- SiMe₃ (a), -C≡C-(p-C₆H₄-NMe₂) (b), -C≡C-C≡C-(p-C₆H₄-NPh₂) (c) or -C≡C-{p-C₆H₄-C≡C-(p-C₆H ₄-N(C₆H₁₃)₂)} (d) or Me (e)), giving the 2,5-bis(arylethynyl) isomer exclusively. The rhodacyclopentadienes bearing a methyl ligand in the equatorial plane (compound 1 e) have been converted into their chloro analogues by reaction with HCl etherate. The rhodacycles thus obtained are stable to air and moisture in the solid state and the acceptor-substituted compounds are even stable to air and moisture in solution. The photophysical properties of the rhodacyclopentadienes are highly unusual in that they exhibit, exclusively, fluorescence between 500-800 nm from the S ₁ state, with quantum yields of Φ=0.01-0.18 and short lifetimes (τ=0.45-8.20 ns). The triplet state formation (Φ_ISC=0.57 for 2 a) is exceptionally slow, occurring on the nanosecond timescale. This is unexpected, because the Rh atom should normally facilitate intersystem crossing within femto- to picoseconds, leading to phosphorescence from the T1 state. This work therefore highlights that in some transition-metal complexes, the heavy atom can play a more subtle role in controlling the photophysical behavior than is commonly appreciated.