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

TL;DR

This study explores how the concentration of a platinum–acetylide chromophore in ormosil glasses affects its excited-state behavior, revealing changes in absorption, emission, and exciton coupling.

Contribution

The paper introduces a quantitative relationship between chromophore concentration, aggregation, and exciton dynamics in ormosil glasses.

Findings

At low concentrations, the chromophore behaves similarly to in solution, with no aggregation effects.

Higher concentrations lead to aggregation, causing changes in absorption and emission characteristics.

Exciton coupling and triplet lifetimes are modulated by concentration and aggregation.

Abstract

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.

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