From Energy Matching to Lifetime Control: Tunable Excited-State Equilibria in an Iron-Pyrene Dyad

Area of Science:

• Photoredox Catalysis
• Inorganic Chemistry
• Materials Science

Background:

• Noble-metal photosensitizers are limited by cost and availability.
• Iron complexes offer earth-abundant alternatives but suffer from short excited-state lifetimes due to rapid deactivation.
• Extending excited-state lifetimes is crucial for efficient energy storage and catalytic applications.

Purpose of the Study:

• To design and synthesize a novel iron(III)-polyaromatic hydrocarbon dyad for enhanced photoredox catalysis.
• To investigate the excited-state dynamics and energy transfer pathways in the designed dyad.
• To explore the use of external stimuli for controlling excited-state properties and improving triplet state population.

Main Methods:

• Synthesis of a rational iron(III)-polyaromatic hydrocarbon dyad ([Fe(LPhPy)2]+) incorporating a pyrene acceptor.
• Femtosecond to nanosecond spectroscopic analysis to probe excited-state evolution.
• Temperature-dependent kinetic analysis to elucidate reaction mechanisms and thermodynamic parameters.

Main Results:

• The dyad exhibits a sequential population of the pyrene triplet state (3*PhPy) via a charge-separated state (CSS) intermediate.
• Excited-state manifolds with energies of the 2LMCT state, CSS, and 3*PhPy within ~0.2 eV were observed.
• A >50-fold increase in excited-state lifetime compared to the parent iron complex was achieved, with tunable lifetimes via solvent polarity and ionic strength.

Conclusions:

• The designed iron-based molecular dyad effectively extends excited-state lifetimes by populating a long-lived triplet state.
• External stimuli (solvent, salt) provide powerful control over excited-state dynamics and triplet yield.
• This work presents a rational design strategy for next-generation iron photosensitizers with enhanced efficiency for energy storage and catalysis.