S₂ Fluorescence from [26]Hexaphyrin Dianion.

Area of Science:

• Photochemistry and Photophysics
• Supramolecular Chemistry
• Organic Electronics

Background:

• Expanded porphyrins are large, highly conjugated macrocycles with unique electronic properties.
• Observation of S2 fluorescence is rare in highly conjugated systems due to rapid decay pathways.
• Understanding excited-state dynamics is crucial for optoelectronic applications.

Purpose of the Study:

• To report the first observation of S2 fluorescence in an expanded porphyrin system.
• To investigate the population kinetics and decay mechanisms from excited states (S2, S1, S0).
• To explore the potential of these molecules in advanced optical and electronic devices.

Main Methods:

• Femtosecond time-resolved absorption spectroscopy.
• Broad-band fluorescence upconversion spectroscopy.
• Transient absorption spectroscopy.

Main Results:

• S2 fluorescence was observed from a meso-hexakis(pentafluorophenyl)-substituted [26]hexaphyrin dianion.
• Simultaneous observation of S2 fluorescence decay (visible) and S1 fluorescence rise (NIR) with a 0.22 ps time constant.
• Identification of a direct decay pathway from the S2 state to the S0 state.
• S2 fluorescence beyond 700 nm was detected, which is uncommon.

Conclusions:

• The study demonstrates S2 fluorescence in a large expanded porphyrin, challenging previous notions about decay pathways.
• Femtosecond spectroscopy revealed rapid population transfer between excited states and a direct S2-S0 decay route.
• These findings open new avenues for designing novel photophysical properties in extended π-conjugated systems.