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 Si excited state and no phosphorescence from T-1. The S-1 -> T-1 intersystem crossing (ISC) is remarkably slow with a rate constant of 10(8) 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 S-1 -> T-1 interconversion mainly via a thermally activated ISC channel above 233 K. The associated experimental activation energy is found to be Delta H-ISC(double dagger) = 28 kJ mol(-1) (2340 cm(-1)) for 1a, which is supported by density functional theory (DFT) and time-dependent DFT calculations [Delta H-ISC(double dagger)(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 S-1 -> T-1 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 pi-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 S-1 and T-1 arise exclusively from a HOMO-to-LUMO transition. The weak metal participation and the cumulenic distortion of the T-1 state associated with a large S-1-T-1 energy separation favor an "organic-like" photophysical behavior.