Interplay between Through-Space and Through-Bond Electronic Coupling in Singlet Fission

Area of Science:

• Photophysical processes
• Materials science
• Organic electronics

Background:

• Singlet fission (SF) is a process where one absorbed photon generates two triplet excited states, offering a pathway to exceed the theoretical efficiency limit of solar cells.
• Understanding the electronic coupling mechanisms, both through-space and through-bond, is crucial for developing efficient SF materials.
• Existing SF materials often have complex solid-state packing, making it challenging to isolate the effects of electronic coupling.

Purpose of the Study:

• To investigate the role of electronic coupling in controlling intramolecular singlet fission (i-SF) in designed pentacene dimer systems.
• To correlate structure-property relationships with excited-state dynamics in novel pentacene-subphthalocyanine (SubPc) dimers.
• To provide model systems that elucidate the opposing effects of electronic coupling on triplet-pair formation and decoherence.

Main Methods:

• Design and synthesis of three complex pentacene dimers, two incorporating subphthalocyanines (SubPcs).
• Utilized quantum chemical calculations and molecular dynamics simulations to analyze electronic coupling and structure-property relationships.
• Employed steady-state absorption/emission spectroscopy and transient absorption pump-probe experiments to study excited-state dynamics.

Main Results:

• SubPcs act as efficient light-harvesting antennae, funneling energy to pentacene dimers via intramolecular Förster resonance energy transfer (i-FRET) for panchromatic absorption.
• The formation rate and yield of the correlated triplet-pair state, 1(T1T1), are directly proportional to interpentacene electronic coupling.
• The yield of uncorrelated triplet excited states (T1 + T1) is inversely proportional to interpentacene electronic coupling, demonstrating control over i-SF.

Conclusions:

• The designed pentacene dimers serve as excellent model systems for studying intramolecular singlet fission.
• Electronic coupling plays a decisive role in i-SF, influencing both the formation of correlated triplet pairs and their subsequent decoherence.
• These findings offer insights into controlling SF processes for potential applications in advanced solar energy conversion.