Effect of the C(3)-Substituent in Verdazyl Radicals on their Profluorescent Behavior

Abstract

Methods for the detection of reactive intermediates such as transient radicals are important in organic chemistry, polymer chemistry, biology or medicine. Along these lines we recently reported that 1,5-diphenyl-6-oxo verdazyl radicals can be used as fluorescent spin sensors. In situ generated C-centered radicals are efficiently trapped by the verdazyls, which in turn undergo transformation from a paramagnetic non-fluorescent state to a diamagnetic fluorescent state. Whereas the N-phenyl substituent in the spin probes is of high importance for obtaining profluorescent behavior, the effect of the C(3)-substituent has not been investigated to date. We herein present the synthesis and characterization of various 1,5-diphenyl-6-oxo-verdazyl radicals bearing differently hybridized C-substituents at the C(3) position. Steady-state and time-resolved fluorescence spectroscopy in solution and in the solid state along with time-dependent density functional theory (TDDFT) calculations reveal that a C(3)-aryl substituent is crucial for obtaining fluorescence after spin trapping. In addition, it is shown that the emission wavelength of the C(3)-aryl substituted verdazyl derivatives can be tuned by selective destabilization of the HOMO and the LUMO.

Cooperative Magnetism in Crystalline N‐Aryl‐Substituted Verdazyl Radicals: First‐Principles Predictions and Experimental Results

“…Although verdazyls are generally considered to be hard to obtain in the form of single crystals, we were able to solve and refine the structure of a 1,5‐diphenyl‐6‐oxo‐verdazyl radical bearing a tert ‐butyl group at the C(3) position. Verdazyl radical 3 a (for the synthesis, see Scheme ) crystallizes in the monoclinic crystal system with space group P 2 1 / c and unit cell parameters a= 15.8770(9), b= 5.9373(3), c= 17.0945(5) Å and four molecules within the unit cell (for more detailed crystallographic data, see Table S1 in the Supporting Information). As can be seen from the CNNC dihedral angle of 2.8°, the verdazyl heterocycle adopts an almost planar 7π‐system (Figure ).…”

“…a) Pivalaldehyde (1.0 equiv), MeOH, 40–60 °C, 2–3 h; b) 1,4‐benzoquinone (1.7 equiv), CH 2 Cl 2 , sealed tube, 60 °C, 3–4 h. [a] See ref. .…”

Neutral Stable Nitrogen‐Centered Radicals: Structure, Properties, and Recent Functional Application Progress

“…Besides, various 6-oxo-verdazyl radicals with profluorescent behavior are reported to be applied as profluorescent spin probes to detect radical intermediates. [122,123] The non-emissive verdazyl radicals can become fluorescent after reacting with carboncentered radicals.…”

Black-box determination of temperature-dependent susceptibilities for crystalline organic radicals with complex magnetic topologies

“…Verdazyl radicals 2 28 and 3 were prepared according to a known literature procedure. 27,28,40 In a one pot reaction, first 2,4diphenylcarbohydrazide was reacted with an aldehyde in MeOH or EtOH to give the corresponding cyclic tetrazinan-3one, which was subsequently oxidized with para-quinone to afford verdazyl radical 2 in 30% yield and verdazyl radical 3 in 33% yield. For verdazyl radical 4, the intermediate tetrazinan-3one was isolated prior to oxidation by simple filtration of the methanolic reaction mixture.…”

“…22-26. One class of stable radicals, which is getting increasing attention, is the family of verdazyl radicals. 21,27,28 They are common magnetic building blocks and are thus often used to create metal-free or hybrid metal-organic materials with magnetic properties. 22,23 The main reason for the stability of verdazyl radicals is the delocalization of the unpaired electron over the verdazyl ring itself, which is often further extended to aromatic substituents.…”