Metallacyclopentadienes are used in a variety of applications, including as catalysts, as photosensitizers for dye-sensitized solar cells, and as luminescent materials for organic light-emitting diodes (OLEDs). Synthetic access to conjugated 2,5-bis(R-ethynyl)metallacyclopentadienes should, in principle, be possible by reductive coupling of 1,3-butadiynes at a suitable transition-metal precursor complex. However, examples of such a coupling are scarce, and most of the reactions do not occur with the desired regioselectivity.

Todd Marder, Julius Maximilians-Universität Würzburg, Germany, and colleagues have shown that reductive coupling of 1,4-bis(p-R-arylethynyl)-1,3-butadiynes is a general synthetic approach to rhodacyclopentadienes. They have used this method to create a series of 2,5-bis(arylethynyl)rhodacyclopentadienes with a variety of substituents of differing electronic donor or acceptor strength and different ligand combinations at the Rh center. The reactions occur regiospecifically, yielding the 2,5-isomer exclusively.
The photophysical properties of the rhodacyclopentadienes are highly unusual in that they exhibit, exclusively, fluorescence between 500–800 nm from the S₁ state, with low to moderate quantum yields (Φ = 0.01–0.18) and short lifetimes (τ = 0.45–8.20 ns). The triplet state formation 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 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.

• Regiospecific Formation and Unusual Optical Properties of 2,5-Bis(arylethynyl)rhodacyclopentadienes: A New Class of Luminescent Organometallics,
  Andreas Steffen, Richard Ward, Meng Guan Tay, Robert Edkins, Fabian Seeler, Magda van Leeuwen, Lars-Olof Pålsson, Andrew Beeby, Andrei Batsanov, Judith Howard, Todd Marder,
  Chem. Eur. J. 2014.
  DOI: 10.1002/chem.201304068