Towards Efficient and Stable Donor-Acceptor Luminescent Radicals

Area of Science:

• Organic Chemistry
• Materials Science
• Photophysics

Background:

• Organic luminescent radicals feature spin-allowed transitions due to doublet ground and excited states.
• High photoluminescent quantum efficiency (PLQE) is often observed in donor-acceptor (D-A•) radical structures.
• Factors influencing D-A• luminescent radical efficiency and stability are not fully understood.

Purpose of the Study:

• To investigate how substituent conjugation and imine nitrogen content affect the luminescence efficiency and stability of tri(2,4,6-trichlorophenyl)methyl (TTM) radical derivatives.
• To correlate experimental findings with theoretical calculations to elucidate structure-property relationships.

Main Methods:

• Synthesis of a series of TTM radical derivatives with systematically varied donor substituents.
• Experimental characterization of photoluminescence quantum efficiency (PLQE) and radical stability.
• Density functional theory (DFT) calculations to model electronic structures and energy states.

Main Results:

• Luminescence efficiency and radical stability increase with the degree of conjugation in the donor substituents.
• Luminescence efficiency and radical stability decrease with an increasing number of imine nitrogen atoms in the substituents.
• Experimental trends were accurately reproduced by DFT calculations.

Conclusions:

• The luminescence efficiency and stability of D-A• luminescent radicals are governed by substituent conjugation length and imine nitrogen content.
• The energy difference between charge transfer (CT) and local-excitation (LE) states is a key factor, decreasing with increased imine nitrogens or decreased conjugation.