Fluorescence in Rhoda- and Iridacyclopentadienes Neglecting the Spin–Orbit Coupling of the Heavy Atom: The Ligand Dominates

We present a detailed photophysical study and theoretical analysis of 2,5-bis­(arylethynyl)­rhodacyclopenta-2,4-dienes (1a–c and 2a–c) and a 2,5-bis­(arylethynyl)­iridacyclopenta-2,4-diene (3). Despite the presence of heavy atoms, these systems display unusually intense fluorescence from the S₁ excited state and no phosphorescence from T₁. The S₁ → T₁ intersystem crossing (ISC) is remarkably slow with a rate constant of 10⁸ s^–1 (i.e., on the nanosecond time scale). Traditionally, for organometallic systems bearing 4d or 5d metals, ISC is 2–3 orders of magnitude faster. Emission lifetime measurements suggest that the title compounds undergo S₁ → T₁ interconversion mainly via a thermally activated ISC channel above 233 K. The associated experimental activation energy is found to be ΔH_ISC^⧧ = 28 kJ mol^–1 (2340 cm^–1) for 1a, which is supported by density functional theory (DFT) and time-dependent DFT calculations [ΔH_ISC^⧧(calc.) = 11 kJ mol^–1 (920 cm^–1) for 1a-H]. However, below 233 K a second, temperature-independent ISC process via spin–orbit coupling occurs. The calculated lifetime for this S₁ → T₁ ISC process is 1.1 s, indicating that although this is the main path for triplet state formation upon photoexcitation in common organometallic luminophores, it plays a minor role in our Rh compounds. Thus, the organic π-chromophore ligand seems to neglect the presence of the heavy rhodium or iridium atom, winning control over the excited-state photophysical behavior. This is attributed to a large energy separation of the ligand-centered highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO) from the metal-centered orbitals. The lowest excited states S₁ and T₁ arise exclusively from a HOMO-to-LUMO transition. The weak metal participation and the cumulenic distortion of the T₁ state associated with a large S₁–T₁ energy separation favor an “organic-like” photophysical behavior.