Regioisomeric π-Extended Nanographene with Long-Lived Phosphorescence Afterglow

Area of Science:

• Materials Science
• Organic Chemistry
• Photophysics

Background:

• Nonplanar graphene structures, especially twisted nanographenes, offer unique (chiro)optical properties.
• Tuning molecular structure is key to controlling photophysical behavior in advanced materials.

Purpose of the Study:

• To synthesize and characterize two regioisomeric π-extended nanographenes with distinct structures.
• To investigate the excited-state characteristics and photophysical properties, including delayed fluorescence and phosphorescence.
• To explore the structure-property relationship, particularly the influence of nonplanar topology on optical features.

Main Methods:

• Synthesis of regioisomeric π-extended nanographenes by varying the carbazole connection points (2,7- and 3,6-).
• Single-crystal X-ray diffraction for structural confirmation (bent and helical conformations).
• Photophysical measurements (thermally activated delayed fluorescence, phosphorescence) and Density Functional Theory (DFT) calculations.

Main Results:

• Both nanographene derivatives exhibited thermally activated delayed fluorescence at room temperature and phosphorescence at low temperatures.
• The bent nanographene displayed a significantly longer red afterglow (>30 seconds) compared to the helical structure.
• DFT calculations indicated an isoenergetic higher triplet state (T8) and weak spin-orbit coupling (T1-S0) in the bent nanographene, explaining its long-lived afterglow.

Conclusions:

• Regioisomerism in nanographenes significantly impacts their excited-state characteristics and optical properties.
• Nonplanar topology, specifically a bent conformation, can lead to exceptional long-lived afterglow phenomena.
• These findings provide crucial insights into structure-property relationships for designing advanced nanographene-based materials.