Polyhedron 20 (2001) 1643 – 1646
EPR study of the trans and cis isomers of a ferrocenyl
Schiff-based polychlorotriphenylmethyl radical
I. Ratera, D. Ruiz-Molina, J. Vidal-Gancedo, C. Rovira, J. Veciana *
Institut de Cie `ncia de Materials de Barcelona (CSIC), Campus Uniersitari, 08193 Bellaterra, Catalonia, Spain
Received 18 September 2000; accepted 13 October 2000
Abstract
The isomeric trans -1 and cis -1 radicals have been studied by EPR spectroscopy. At 220 K, the EPR spectrum of the trans -1
isomer shows a hyperfine structure corresponding to the coupling of the unpaired electron with the different nuclei with non-zero
magnetic moments. In contrast, the EPR spectrum of the cis -1 isomer at similar temperatures shows the characteristic structure
of a triplet species. The differences in the EPR spectra of both isomers have been attributed to the fact that the cis -1 isomer
aggregates in solution originating the thermodynamically stabilized hydrogen-bonded diradical species, in which strong antiferro-
magnetic interactions are developed. EPR frozen solution experiments, down to liquid helium temperature, showed that the
magnetic ground state of the resulting dimeric species is the singlet state and the triplet should be associated with a thermally
accessible excited state. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: cis ; trans ; Supramolecular; Hydrogen-bond; Diradical; EPR spectroscopy
www.elsevier.nl/locate/poly
1. Introduction
Obtaining hydrogen-bonded supramolecular mag-
netic materials whose properties may be systematically
tuned and/or controlled by external stimuli remains an
elusive field despite its enormous interest. Very recently
[1] we reported the synthesis and characterization of the
ferrocenyl Schiff-based polychlorotriphenylmethyl radi-
cal (1), which exhibits a photoinduced trans -1 to cis -1
isomerization (see Scheme 1). Interestingly, the cis -1
isomer aggregates in solution originating the thermody-
namically stabilized hydrogen-bonded diradical species,
in which strong antiferromagnetic interactions are de-
veloped. This was the first example, so far reported, of
a hydrogen-bonded supramolecular magnetic material
whose properties can be systematically controlled by
light.
In this communication, we report a detailed EPR
study of the two radicals, trans -1 and cis -1. We will
show that EPR spectroscopy not only provides detailed
and definitive information about the electronic struc-
ture of the two isomeric forms but also allows a struc-
tural characterization of the supramolecular diradical
species.
2. Results and discussion
X-band EPR spectra of a toluene/CH
2
Cl
2
(1:1) solu-
tion of radical trans -1 were obtained in the temperature
range of 220–300 K. EPR spectra measured at 300 K
do not provide any specific information about the fine
structure of the radical. It is at low temperatures that
the spectra show lines corresponding to the coupling of
the unpaired electron with the different nuclei with
non-zero magnetic moments; i.e. with
1
H,
14
N and
naturally abundant
13
C isotope at the and aromatic
positions. The experimental and simulated spectra of
radical trans -1 at 220 K are compared in Fig. 1.
Computed simulation gave the isotropic g -values (g
iso
)
and the isotropic hyperfine coupling constants (a
i
) of
the unpaired electrons with the different nuclei with
non-zero magnetic moments. The g
iso
value for the
trans -1 isomer is 2.0030 and the values for the coupling
constant are: a (
14
N) =1.2 G; a
1
(
1
H) =1.1 G(6H
meta
);
a
2
(
1
H)=0.3 G(
1
H
imino
), a
1
(
13
C)=28.5 G(1C
); a
2
(
13
C) =
* Corresponding author. Tel.: +34-93-5801853; fax: +34-93-
5805729.
E-mail address: vecianaj@icmab.es (J. Veciana).
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
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