Polychlorotriphenylmethyl Radicals: Towards Multifunctional Molecular Materials - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds

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isomers and are isolable as stable solids that show outstanding stabilities since they do not show any sign

of decomposition up to 250 ◦ C, even in air. 40

(M,M,P )-1

C 2 Symmetry

( M,M,M )-1

D 3 Symmetry

A linear solvation free-energy study was performed with quartet 3 to study the influence of the molecular

characteristics of the two diastereoisomers on their physicochemical properties. It was found that the

different surface areas and fractal dimensionalities of the two diastereomeric forms of this rigid molecule

are among the most important molecular parameters controlling some of their physicochemical properties.

Their ability to interact with the surrounding media is a result mainly of cavitational effects. 37 Moreover,

the two isolated solids have been characterized by single crystal X-ray diffraction. In this respect, the two

solid state structures, each corresponding to the two different diastereomers, constituted the first reported

examples of crystal structures of carbon-based high-spin polyradicals. 37

Cyclic voltammetry of quartet 3 shows three reversible reduction waves at

0.75 V

versus SCE (Figure 2.4), corresponding to its reduction to the mono-, di- and trianionic species respectively,

while diradical 2 revealed only two reversible waves at

−

0.19,

−

0.49 and

−

0.22 and

−

0.54 V, corresponding to the reduction

to the mono- and dianionic species respectively.

X-band Electron Paramagnetic Resonance (EPR) spectra of high-spin radicals 2 and 3 in dilute toluene

solutions at room temperature show symmetrical signals centered at g iso =

0026. In frozen media the EPR

spectrum of triradical 3 showed a fine structure characteristic of two quartet species with distinct popula-

tions: a major quartet ( 3a , 83 %) with

0036 cm − 1 ,

0001 cm − 1 , g xx

| /

g yy

0029

0050 cm − 1 ,

0cm − 1 ,and

and g zz

0012, and a minor quartet ( 3b , 17 %) with

| /

g xx

0024. The observed rigid media EPR spectra of pure 3a and 3b quartets, as well

as those spectra obtained by a simulation procedure, are shown in Figure 2.5. EPR data permit the

assignment of the 3a quartet to the enantiomeric pair with C 2

g yy =

g zz

symmetry, that is to ( M,M,P )- 3

( P,P,M )- 3 , and the 3b quartet to that with D 3 symmetry, that is to (M,M,M)- 3

(P,P,P)- 3 . Rigid

media EPR spectra of quartets C 2 -3 and D 3 -3 also showed weak signals corresponding to forbidden

m s

=±

transitions

(Figure

2.5,

top).

Variable

temperature

EPR

experiments

with

pure C 2 -3

and D 3 -3 quartets

were

performed

the

temperature

range

130 - 200 K.

Total

intensities

their

1 signals increase linearly with 1/T, indicating that quartets are the ground states of both

stereoisomers and that the low-spin excited states of both isomers are thermally inaccessible in this

temperature range.

m s

=±

Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds

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