Triplet Reservoir Dynamics and Near-Infrared Electrochemiluminescence of Platinum–Acetylide Oligomers

Abstract Near-infrared (NIR) electrogenerated chemiluminescence (ECL) offers superior tissue penetration and reduced background for bioimaging, yet stable, efficient NIR luminophores remain rare. This study investigates the photophysical dynamics and ECL mechanisms of two donor–acceptor–donor platinum–acetylide oligomers, Pt2DTB and Pt2BTDEDOT. Using femtosecond transient absorption spectroscopy, we identify an ultrafast intersystem crossing process (τISC ≈ 0.5–1.2 ps) driven by strong spin–orbit coupling, which establishes a robust triplet reservoir. Temperature-dependent TRPL and oxygen-quenching measurements support a triplet-mediated delayed-emission channel, while density functional theory calculations reveal large singlet–triplet energy gaps, ΔEST > 0.6 eV, that disfavor efficient TADF. These observations are consistent with a Singlet-Initiated ISC–Triplet pathway involving Triplet–Triplet Annihilation (SIT–TTA). While electrochemical analysis shows that the radical anions of these oligomers are unstable, leading to weak annihilation ECL, the use of tripropylamine (TPrA) as a coreactant bypasses this instability, enhancing ECL intensity by 2–3 orders of magnitude. The resulting emission is significantly red-shifted, indicating a stabilized lower-energy emissive manifold. These findings provide critical design principles for organometallic NIR emitters, demonstrating that coreactant strategies are essential for leveraging triplet reservoirs in high-sensitivity optoelectronics.