Why Are Verdazyl Radicals Nonemissive?

Verdazyl radicals are a versatile class of air-stable organic radicals used in various applications. Yet, despite the development of a wide range of verdazyl derivatives, they are all nonemissive. To investigate the reasons behind this and to understand the excited-state dynamics of verdazyls, we combine steady-state and femtosecond pump-probe spectroscopy with quantum chemical calculations. In the carbazole-substituted 2,4,6-triphenylverdazyl (TPV-Cz), we observe ultrafast internal conversion of the first excited state on a time scale of 0.5 ± 0.1 ps, followed by vibrational relaxation with a lifetime of 3.7 ± 0.4 ps. Spin-flip time-dependent density functional theory calculations reveal that the subpicosecond nonradiative decay comes from a low-energy conical intersection between the D₁ and D₀ states, driven by an out-of-plane distortion of the verdazyl ring (VR). This distortion is observed and remains energetically accessible in the isolated VR, in 2,4,6-triphenylverdazyl, and in TPV-Cz. This shows that the conical-intersection geometry is a recurring feature across different types of verdazyl derivatives and explains why all verdazyls are nonemissive despite different functionalization. Our results provide a mechanistic understanding of the photophysical properties of verdazyl radicals and offer a pathway for the future design of emissive verdazyl derivatives.