Aggregates and Excitons: Excited-State Behavior of Platinum–Acetylide Two-Photon Chromophore-Doped Ormosil Glasses

We probe the excited-state dynamics of a platinum-acetylide chromophore dissolved in ormosil glasses in the concentration range of 0.1-400 mM to gain a better understanding of how the environment of the dye reflects upon the overall kinetics observed. At 0.1 mM, ground-state absorption, fluorescence, excited-state absorption (ESA), and triplet ESA reproduce solution behavior. Above ≥10 mM, a weak 485 nm ground-state band appears, consistent with a nominally forbidden S₀ → T₁ transition, and steady-state emission shows quenched fluorescence with enhanced phosphorescence. Following 355 nm flash photolysis, high-concentration samples initially exhibit triplet ESA identical to the 0.1 mM case, but a blue-shifted triplet ESA develops at longer delays; direct excitation of the 485 nm band yields the same blue-shifted spectrum, confirming aggregation effects. Kinetically, the 0.1 mM sample displays a single triplet lifetime, whereas ≥10 mM samples require two. The shorter lifetime at all loadings follows a Freundlich adsorption dependence, consistent with monomer binding to ormosil sites, while the longer lifetime is attributed to aggregation. Ultrafast transient absorption (TA) resolves two ESA bands whose energy separation and relative areas suggest intramolecular exciton coupling between ligand-localized transitions. Fitting the data with exciton theory gives the interligand transition-dipole angle and the excitonic splitting; both evolve with concentration and pump-probe delay, reflecting symmetry breaking, intersystem crossing, and charge-transfer reorganization. At ∼1 mM, the time-dependent band separation is consistent with excimer formation, whereas no excimer signatures are observed at ≥10 mM. These results establish a quantitative structure-dynamics-concentration relationship: aggregation and ormosil-induced microphase separation create coexisting free and aggregated populations that modulate exciton coupling (dipole geometry and splitting) and govern the triplet photophysics.