Regiospecific Formation and Unusual Optical Properties of 2,5‐Bis(arylethynyl)rhodacyclopentadienes: A New Class of Luminescent Organometallics

Abstract

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, CO2Me, CN, NO2, -C≡C-(p-C6H4-NHex2), -C≡C-(p-C6H4-CO2Oct)) 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-C6H4 -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 S1 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.

Convenient synthetic access to fluorescent rhodacyclopentadienes via ligand exchange reactions

“…The CO 2 Mesubstituted compound 4 shows a bathochromic shift in the absorption and emission spectrum in comparison to its SMesubstituted congener 5, the influence of the acceptor substituent exceeding that of the donor. The effect of donors and acceptors can be explained by acceptors having a greater influence on the lowest unoccupied molecular orbital (LUMO) than on the highest unoccupied molecular orbital (HOMO), and donors showing an opposite effect, therefore both decreasing the HOMOeLUMO gap, as previously reported for related 2,5-bis(arylethynyl)thiophenes [14g] and 2,5-bis(arylethynyl)rhodacyclopentadienes [19,20].…”

“…with different linked a,u-bis(arylethynyl)alkanes (rigidification of the rhodacycle backbone) yields quantitatively, in all cases, only the 2,5-bis(arylethynyl)rhodacyclopentadiene isomer (Scheme 1) [19,20].…”

“…Such longlived triplet excited states of the rhodium biphenyl complexes (B), on a timescale of tens to hundreds of microseconds, can be very useful for applications in color-tunable OLEDs, in which, at high Scheme 1. Synthesis of 2,5-bis(arylethynyl)rhodacyclopentadienes [19,21]. voltages, the emissive excited state of a low-energy emitter must have a long lifetime in order to become saturated by energy transfer from a short-lived high-energy luminophore.…”

Completely Unexpected Coordination Selectivity of Copper Iodide for Thioether Over Ethynyl

“…2), because of the template effect discovered (below). Å; γ = 51.78°) at 120 K [22]. This weak compressibility indicates that the stacking is rather compact.…”

“…Upon the incorporation two methyl thioethers onto this motif, an emissive multidentate ligand is obtained (L2; Fig. 1) [21], which is also known to be prone to generate luminescent (rhodium) organometallic complexes [22], but the SMe groups do not coordinate the metal at all.…”