The Impact of Donor Orientation on the Emission Properties of Chlorinated Trityl Radicals

The utilization of stable tris(2,4,6-trichlorophenyl)methyl radical (TTM) doublet emitters in OLEDs is promising, as it overcomes spin-statistical limitations to the efficiency.^[1] Donor-functionalized TTM derivatives show long fluorescence lifetimes, which are in part originating from structural reorganization of the excited state, where the donor moiety becomes more perpendicular with regards to the TTM unit.^[2,3] Suppressing such twisted intramolecular charge transfer is expected to shorten the fluorescence lifetime, potentially opening up other new applications of light-emitting radicals, for example in the field of organic lasers.
First, to suppress rotation and fix the donor in plane with the TTM acceptor, nitrogen atoms are incorporated into the triarylmethyl radical skeleton. This functionalization enforces a planar conformation of carbazole and radical moiety through hydrogen bonding. In fact, rotational confinement leads to shorter fluorescence lifetimes; however, at the cost of diminished photoluminescence quantum yield. While the rate constant for radiative relaxation remains unchanged, non-radiative processes appear accelerated.
Secondly, forcing the donor into a perpendicular orientation towards the radical moiety is achieved by additional chlorine atoms in the meta position. Thus, the electronic communication between donor and acceptor units is drastically diminished. This structural alteration leads to accelerated emission compared to the TTM analogue.^[4] Simultaneously, non-radiative competitive processes are suppressed, increasing the photoluminescence quantum yield.
In general, such knowledge about the impact of the orientation of substituents towards the TTM moiety allows the rational design of radicals with desired optical and electronic properties. This is of particular importance when bridging two TTM units towards a diradical. The geometry of the resulting open-shell symstems is crucial for the communication between the unpaired spins, determining molecular properties such as spin mulitiplicity, diradical character, and emission properties.

[1] C. He, Z. Li, Y. Lei, W. Zou, B. Suo, Journal of Physical Chemistry Letters 2019, 10, 574–580.
[2] V. Gamero, D. Velasco, S. Latorre, F. López-Calahorra, E. Brillas, L. Juliá, Tetrahedron Lett 2006, 47, 2305–2309.
[3] L. Chen, M. Arnold, Y. Kittel, R. Blinder, F. Jelezko, A. J. C. Kuehne, Adv Opt Mater 2022, 10, DOI 10.1002/adom.202102101.
[4] S. Dong, W. Xu, H. Guo, W. Yan, M. Zhang, F. Li, Phys Chem Chem Phys 2018, 20, 18657–18662.