Intermolecular Charge-Transfer-Induced Strong Optical Emission from Herringbone H-Aggregates

Area of Science:

• Molecular aggregate luminescence
• Organic electronics
• Theoretical chemistry

Background:

• Luminescence quenching in molecular aggregates is a key challenge.
• Intermolecular charge transfer and exciton formation are known quenching pathways.
• H-aggregates typically exhibit dipole-forbidden lower Frenkel excitons, leading to weak luminescence.

Purpose of the Study:

• To theoretically explain the strong luminescence observed in DPA and dNaAnt herringbone aggregates within organic light-emitting transistors (OLETs).
• To identify the conditions necessary for achieving bright luminescence in molecular aggregates.
• To reconcile efficient luminescence with high charge mobility for advanced electronic applications.

Main Methods:

• Development of a three-state model incorporating intermolecular charge transfer and excitonic coupling.
• Utilizing localized diabatization to analyze state interactions.
• Theoretical analysis of electron and hole transfer integrals (t_e, t_h) and excitonic coupling (J).

Main Results:

• A low-lying intermolecular charge-transfer state can couple with an upper bright Frenkel exciton.
• This coupling forms a dipole-allowed S1 state below the dark state, explaining strong luminescence in OLETs.
• The condition for this bright state formation is t_e × t_h > 2J^2, where transfer integrals are of the same sign and larger than excitonic coupling.

Conclusions:

• The theoretical model successfully rationalizes recent experimental findings on strong luminescence in specific organic aggregates.
• This work reveals a pathway to achieve both strong luminescence and high charge mobility simultaneously.
• The findings are significant for the development of efficient organic light-emitting transistors (OLETs) and electrically pumped lasers.