Intermolecular energy transfer across nanocrystalline semiconductor surfaces

Area of Science:

• Photochemistry and Materials Science
• Nanomaterials and Energy Transfer Dynamics

Background:

• Metal-to-ligand charge-transfer (MLCT) excited states of ruthenium (Ru) and osmium (Os) complexes are crucial for energy transfer studies.
• Anchoring these complexes to mesoporous titanium dioxide (TiO2) thin films enables investigation of their photophysical properties in a solid-state environment.

Purpose of the Study:

• To quantify energy transfer yields and dynamics from Ru(2+) and Os(2+) excited states anchored to TiO2.
• To investigate the influence of surface coverage and solvent environment on lateral energy transfer between Ru(2+)* and Os(2+).

Main Methods:

• Quantification of energy transfer yields and excited-state decay kinetics for Ru(2+) and Os(2+) complexes on TiO2.
• Measurement of energy transfer as a function of relative surface coverage and solvent environment (acetonitrile, THF, CCl4, hexanes).
• Application of parallel first- and second-order kinetic models and Monte Carlo simulations to analyze decay pathways and estimate rate constants.

Main Results:

• Observed lateral energy transfer from excited Ru(2+)* to Os(2+), with yields dependent on surface coverage and solvent.
• Ru(2+)*/TiO2 exhibited parallel first- and second-order decay kinetics, while Os(2+)*/TiO2 showed primarily first-order decay.
• The second-order decay pathway for Ru(2+)*/TiO2, attributed to intermolecular energy transfer and triplet-triplet annihilation, increased with surface coverage and excitation intensity.

Conclusions:

• Established a kinetic model to differentiate between radiative/nonradiative decay and intermolecular energy transfer pathways.
• Determined the Ru(2+)* --> Ru(2+) intermolecular energy transfer rate constant to be approximately (30 ns)(-1).
• Demonstrated the significant role of surface coverage and excitation intensity in modulating energy transfer dynamics in Ru/Os-TiO2 systems.