Synthesis and characterisation of several di-, tri-, and tetra-radicals linked by flexible or rigid linkers

This paper reports the synthesis of eight new polyradicals (di-, tri-, and tetra-stable radicals) linked by flexible (aliphatic) or rigid (aromatic) units. Their preparation, as well as their electron paramagnetic resonance characteristics are described and discussed. These polyradicals were successfully tested as spin-labels for gold nanoparticles


Introduction
[7][8][9][10][11] Our interest 12 in preparation of new paramagnetic (multispin) compounds prompted us to synthesise several stable di-, tri-, and tetra-radicals, in which the paramagnetic moieties are separated by flexible or rigid linkers.5][6] T There is also an interest in measuring and in the correlation of the distances between spins with the properties of the polyradicals.Measuring molecular scale distances is a topic in some biological systems, in which X-ray spectroscopy cannot be applied.Knowledge of the electronic, redox, and distances between spins is necessary to predict the final properties of the magnetic material, and the electron paramagnetic resonance (EPR or ESR) spectroscopy is a fast and reliable tool, from which the (para)magnetic properties are easily extracted.The distances between spins in a polyradical system greatly affect the shape of ESR spectra (because an exchange interaction appears between two or more vicinal spins). 1,2n ESR spectrum is usually characterised by the hyperfine coupling constants a, the g value, and the exchange interaction J. 1,2 Thus, if the distance between two spins is large, the ESR spectra of the system looks like two superimposed ESR spectrum (J<<a), if the distance is short enough that the two unpaired electrons start to 'feel' each other (because they act as a small magnets), the corresponding spectra have a very complicated shape, with supplementary lines between and outside the normal spectra lines (J~a), or, in the last case, if the distance is very short, a strong interaction appears between the two unpaired electrons, and in the ESR spectra characteristic supplementary lines appears in the middle of the normal ones. 1,2n this paper we report the synthesis and characterisation of several stable polyradicals (Fig. 1).All of them have been tested as spin-labels for gold nanoparticles, as described below.

Experimental Section
General Procedures.All solvents and chemicals were supplied by Aldrich or Chimopar, and used as received.The UV-Vis spectra were measured on a UVD-3500 spectrometer; the NMR spectra were measured on a Varian Gemini 300BB spectrometer; the ESR spectra were measured in DCM at ambient temperature (295-300 K) on a Jeol Jes FA100 spectrometer, using the following general settings: centre field 3330 G, sweep field 100 G, frequency 9.42 GHz, power 1 mW, sweep time 60 s, time constant 0.1 s, modulation frequency 100 kHz, gain 100, and modulation width 1 G. 4-(N,N-diphenylhydrazine)-3,5-dinitrobenzoic acid was synthesised as previously described. 13Phosphine protected gold nanoparticles were synthesised as previously described.Testing polyradicals 1-8 as spin-labels for gold nanoparticles.To a 0.1 mL solution of triphenylphosphine protected gold nanoparticles 3 (10 -4 M, in DCM) has been added 0.1 mL solution of newly synthesised radicals 1-8 (10 -4 M, in DCM), and the mixture analyzed by ESR spectroscopy.

Results and Discussion
Synthesis.All the polyradicals shown in Fig. 1 were easily obtained by standard coupling procedures (Scheme 1), and their purity checked by TLC (single spot).The coupling procedures involved the reactions between an amine with a carboxylic acid in the presence of DCC, or with a carbonyl-chloride in the presence of Py, or with a sulfochloride in the presence of Py as well.Good yields were obtained, usually between 50-80%.The separation of analytical sample from the reaction mixture was done by column chromatography on silica gel, with an appropriate mixture of solvents (see Experimental).The compounds are stable over months at room temperature, and no decomposition products were found after long time.ESR spectra.ESR spectra of polyradicals 1-8 are shown in Fig. 2 (the total length of the recorded ESR spectra is 100 G).These recorded ESR spectra can be divided into three types, according to the general classification. 1In all the spectra, besides compounds 6 and 7, the interaction between the spins is clearly visible, by apparition of the supplementary lines in ESR spectra (un unpaired electron will give three lines in the ESR spectrum, due to the vicinal nitrogen atom splitting, with the usual hyperfine coupling constant of ~15 G).A special case is compound 2, which have two vicinal nitrogen atoms, with similar hyperfine coupling constants (~ 9 G), and hence a different shape of the spectrum is noticed.For all the compounds 1-5 and 8, the presence of the middle low intensity lines between the expected triplets is a strong proves for the interacting spins.A slightly different case is observed also for compound 4, which showed supplementary splitting inside and outside the normal triplet; a careful examination of the ESR spectrum shows two very low intensity peaks outside the triplet.This case is the due to the condition J~a, in which the values for the nitrogen hyperfine coupling constants are of the same order of magnitude as the exchange interaction between the spins.Calculation of the interspin distances.In order to evaluate the distance between the radical moieties (interspin distance), we used the computational method delivered by the CHIMERA free software; 14,15 thus, the average values measured for the compounds 1-8 were 6.69, 17.72, 13.01, 11.83, 8.43, 13.35, 8.50, and 10.61A o , respectively.As we can notice, for the compound 1, which has the smallest interspin distance (6.69A o ), the interaction between spins is the highest, as seen by the ratio between the height of the middle lines (lines second and fourth, which appear only in the case of an interspin interaction) and the other lines (first, third and fifth lines, which characterize the common ESR spectrum).However, for the other compounds, there is no fixed rule regarding the correlation between the average distance provided by the molecular modelling software and the ESR spectra.
Polyradicals 1-8 as spin-labels for gold nanoparticles.The affinity of gold for sulphurcompounds is very well know, and this has been extensively used for the spin-labelling of the gold nanoparticles. 3Thus, the simple mixing of gold nanoparticles with a disulphide diradical lead to the nanoparticles spin-labelling (the radical moiety is strongly attached on the gold surface, Scheme 2).This process is easily monitored by ESR, due to the changes which instantly are revealed in the spectrum shape.For all the compounds (except 6 and 7) the disappearance of the ESR middle lines (attributed to the interactions between adjacent spins) is noticed (Scheme 2).As a remark, even the sulphamido group in compounds 3-5 led to their attachment to the nanoparticles surface.In the case of the compounds 6 and 7, which do not shown the exchange middle lines, it is noticed the decreasing of the intensity of the last line (high-field), which also proved the attachment of the polyradicals on the nanoparticles surface.

Conclusions
ESR spectra demonstrated the synthesis of the stable free (poly)radicals 1-8; for most of them, the apparition of the supplementary lines besides the expected triplets clearly indicates the presence of spin-spin interaction.In the presence of gold nanoparticles, the spin-spin interaction disappears, proving the attachment of the radical moiety on the nanoparticles surface.

Figure 1 .
Figure 1.Chemical structure of the new polyradicals studied in this paper.

Scheme 1 .
Scheme 1.General procedures used for the preparation of the polyradicals 1-8.