Structural factors controlling the spin-spin exchange coupling: EPR spectroscopic studies of highly asymmetric trityl-nitroxide biradicals

Abstract

Highly asymmetric exchange-coupled biradicals, e.g., the trityl-nitroxides (TNs), possess particular magnetic properties that have opened new possibilities for their application in biophysical, physicochemical, and biological studies. In the present work, we investigated the effect of the linker length on the spin-spin coupling interaction (J) in TN biradicals using the newly synthesized biradicals CT02-GT, CT02-AT, CT02-VT, and CT02-PPT as well as the previously reported biradicals TNN14 and TN1. The results show that the magnitude of J can be easily tuned from ~4 G (conformer 1 in CT02-PPT) to >1200 G (in TNN14) by varying the linker separating the two radical moieties and changing the temperature. Computer simulations of EPR spectra were carried out to estimate J values of the TN biradicals directly. In addition to the spin-spin coupling interaction of TN biradicals, their g, hyperfine-splitting, and zero-field-splitting interactions were explored at low temperature (220 K). Our present study clearly shows that varying the spin-spin interaction as a function of linker distance and temperature provides an effective strategy for the development of new TN biradicals that can find wide applications in relevant fields.

Figure 1

Experimental and simulated EPR spectra of TN biradicals at 357K.

Figure 2

Two possible stable conformations of CT02-PPT showing different interspin distances. The structures were obtained on the basis of the most stable and unique conformers using Spartan 04 at the MMFF level.

Figure 3

Simulated EPR spectra of TN biradicals with various J values. The simulation input standard deviation of J is zero, and no relaxation effect is considered. T and N indicate the trityl and nitroxide signal, respectively; (*) shows forbidden transitions of biradicals.

Figure 4

J values of (A) trityl-nitroxide biradicals TN1 and TNN14 as well as (B) CT02-GT, CT02-AT and CT02-GT as a function of temperature obtained by simulating the corresponding EPR spectra at various temperatures.

Figure 5

(A) J values of two conformers, (B) hyperfine splitting (A_N) and (C) population of Conformer 2 of CT02-VT as a function of temperature which were obtained by simulating EPR spectra at various temperatures.

Figure 6

Experimental and simulated EPR spectra of TNN14 and TN1 at 220 K in ethylene glycol/H₂O(1:1, v/v) glass-forming solution. #, the signal of trityl monoradical.

Chart 1

Molecular structure of TN biradicals