Why Are Verdazyl Radicals Non-Emissive? An Experimental and Computational Study

Abstract

Verdazyl radicals are a versatile class of air-stable organic radicals used in various applications, especially for their magnetic properties. Despite the development of a wide range of verdazyl derivatives, however, they are all non-emissive. 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 timescale of 0.5 $\pm$ 0.1 ps, followed by vibrational relaxation with a lifetime of 3.7 $\pm$ 0.4 ps. Spin-flip time-dependent density functional theory calculations reveal that the sub-picosecond non-radiative 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. This distortion is observed and remains energetically accessible in the isolated verdazyl ring 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 non-emissive despite different functionalization. Our results provide a mechanistic understanding of the photophysical properties of verdazyl radicals and offers a pathway for the future design of emissive verdazyl derivatives.