Deactivating unproductive pathways in multichromophoric sensitizers

Area of Science:

• Photochemistry and Photophysics
• Materials Science
• Supramolecular Chemistry

Background:

• Multichromophoric complexes containing boron-dipyrromethene (Bodipy) and platinum offer potential for solar energy applications.
• Unfavorable triplet energy transfer (TEnT) from the platinum complex's (3)MMLL'CT state to the Bodipy (3)ππ* state limits efficiency.
• Understanding solvent and substituent effects is crucial for controlling excited-state dynamics.

Purpose of the Study:

• To investigate methods for lowering the energy of the (3)MMLL'CT state in a Bodipy-platinum dyad.
• To make TEnT unfavorable and improve the complex's suitability for solar energy applications.
• To explore the impact of solvent polarity and electron-withdrawing substituents on excited-state lifetimes.

Main Methods:

• Steady-state absorption and emission spectroscopy.
• Ultrafast transient absorption spectroscopy.
• Synthesis and characterization of modified Bodipy-platinum complexes.

Main Results:

• In acetonitrile, excitation leads to TEnT from the (3)MMLL'CT state to the Bodipy (3)ππ* state in 8-160 ps.
• Using a low dielectric constant solvent extends the (3)MMLL'CT state lifetime to over 1 ns.
• Electron-withdrawing groups (carboxylate, phosphonate esters) on the bpy ligand lower the (3)MMLL'CT state energy, preventing TEnT.
• A single methylene spacer between bpy and phosphonate ester raises the (3)MMLL'CT state energy, inducing relaxation to the (3)ππ* state.

Conclusions:

• Solvent polarity and substituents significantly influence excited-state dynamics in Bodipy-platinum dyads.
• Low dielectric solvents and electron-withdrawing groups on the bpy ligand effectively suppress detrimental TEnT.
• These strategies enhance the potential of such complexes for efficient solar energy conversion by preventing energy loss pathways.