Stable and Luminescent Open-Shell Singlet Diradicals using TTM-PhTTM

In a recent study published in the prestigious journal Angewandte Chemie International Edition, Professor Qiming Peng from Nanjing Tech University in collaboration with Professor Alim Abdurahman, Jingmin Wang, Yihan Zhao, Dr. Ping Li, Dr. Li Shen have made significant strides in the field of open-shell singlet (OS) diradicals. These   compounds have attractive extensive research lately due to their unique electronic, optical, and magnetic properties. However, the practical application of OS diradicals has been limited by several challenges, for instance, one of the primary challenges in working with OS diradicals is their poor stability. Researchers previously have employed both thermodynamic and kinetic approaches to improve stability, but the number of stable carbon-centered OS diradicals remains limited. Secondly the complex molecular structure and strict purification requirements of OS diradicals, such as Zethrene and Heptazethrene-like diradicals, have made their synthesis and characterization arduous tasks. Thirdly, significant drawback of most OS diradicals is their non-emissive nature, primarily due to efficient internal conversion processes. This limits their usefulness in various applications.

To overcome these challenges and unlock the full potential of OS diradicals, the authors proposed innovative molecular design strategies: First, reducing Internal Conversion: By minimizing the spin-spin interaction between the radical centers, researchers can reduce the internal conversion of OS diradicals, making them more suitable for luminescent applications. Second, choosing Carbon-Centered Radicals: Selecting carbon-centered radical materials that offer steric hindrance and spin delocalization simultaneously is crucial for enhancing stability and achieving luminescent properties.

Following these design strategies, the research team successfully synthesized TTM-PhTTM, a stable Kekulé diradical derived from Müller’s hydrocarbon. Experimental results confirmed an OS ground state with thermally accessible triplet states at room temperature. This innovation not only addresses the stability issue but also opens the door to luminescent OS diradicals.

Density functional theory calculations provided the authors with valuable insights into the electronic and spin characteristics of TTM-PhTTM. A high diradical character (y0) value of 0.90, calculated using Yamaguchi’s equation, signifies the compound’s open-shell nature. The spin density distribution revealed extensive electron delocalization throughout the molecule, contributing to its stability. The small singlet-triplet energy gap (ΔES-T) of approximately -1.51 kcal/mol further supports its suitability for luminescence. Moreover, the authors conducted cyclic voltammetry analysis and demonstrated the reversible oxidations and reductions of TTM-PhTTM, providing insights into its electrochemical behavior. The matching of SOMO energy levels between TTM-PhTTM and its mono-radical counterparts TTM and TTM-Ph confirmed the theoretical predictions. Furthermore, TTM-PhTTM exhibited remarkable luminescent properties, including a fluorescence emission at 671 nm with a photoluminescence quantum yield of 0.4% and a fluorescent lifetime (τ) of 1.0 ns in cyclohexane solution. These properties were further enhanced when TTM-PhTTM was incorporated into a PMMA film, with a PLQY of 1.3% and a τ of 4.3 ns. The enhanced luminescence in the polymer matrix was attributed to the suppression of vibrational deactivation.

When they research team performed temperature-dependent studies they showed that the spin statistics of TTM-PhTTM could be controlled by temperature, allowing for the modulation of its optical properties. This unique feature distinguishes it from mono-radicals like TTM-Ph. They also ran femtosecond transient absorption spectroscopy which gives important information on the excited-state dynamics of TTM-PhTTM. The ultrafast excited-state lifetimes of TTM-PhTTM, while shorter than mono-radicals, are significantly longer than previously reported diradicals, explaining its luminescent properties. Another important studies the authors conducted was thermogravimetric analysis which demonstrated TTM-PhTTM’s high thermal stability, with a decomposition temperature of up to 320°C. Furthermore, the compound exhibited superior photo-stability compared to mono-radicals TTM and TTM-Ph, making it a promising candidate for various applications.

In conclusion, the new study led by Professor Qiming Peng and his colleagues has developed for the first time TTM-PhTTM, a stable and luminescent Kekulé diradical. This achievement overcomes the key challenges faced by OS diradicals, including stability and non-emissive properties, and opens up exciting possibilities for applications in optoelectronics, magnetism, and beyond. The thorough characterization and innovative design strategies presented in this study provide a solid foundation for future research in the field of open-shell diradicals.