Effective exchange coupling in alternating-chains of a π-extended 1,2,4-benzotriazin-4-yl

Article abstract of DOI:10.1039/c3nj01235b

Air stable 1,3,7-triphenyl-1,4-dihydrothiazolo[5′,4′:4,5] benzo[1,2-e][1,2,4]triazin-4-yl packs in 1D π stacks made of radical pairs with alternate short and long interplanar distances. The magnetic susceptibility exhibits a broad maximum at 73 ± 5 K and

Full text of DOI:10.1039/c3nj01235b

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Effective Exchange Coupling in AlternatingꢀChains
of a πꢀExtended 1,2,4ꢀBenzotriazinꢀ4ꢀyl†‡
Christos P. Constantinides,a Andrey A. Berezin,a Maria Manoli,a Gregory M. Leitus,b Michael
Bendikov,b Jeremy M. Rawsonc and Panayiotis A. Koutentisa,*
aDepartment of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus. Eꢀmail:
koutenti@ucy.ac.cy; Fax: +357 22892809; Tel: +357 22892783
bDepartment of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot, Israel
cDepartment of Chemistry & Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor,
ON, Canada N9B 3P4
† This manuscript is dedicated to the memory of Prof. Michael Bendikov.
‡
Electronic supplementary information (ESI) available: Cyclic voltammetry, solidꢀstate and
solution EPR, crystal structure geometry, table of energies for the triplet and broken symmetry
singlet states along with spin contaminations, crystallographic experimental details and
crystallographic information file (CIF) for radical 1 (CCDC 953369). For ESI and
crystallographic data in CIF or other electronic format see DOI:
ABSTRACT: Air stable 1,3,7ꢀtriphenylꢀ1,4ꢀdihydrothiazolo[5′,4′:4,5]benzo[1,2ꢀe][1,2,4]triaꢀ
zinꢀ4ꢀyl pack in 1D π stacks made of radical pairs with alternate short and long interplanar
distances. The magnetic susceptibility exhibits a broad maximum at 73 ± 5 K and is interpreted
1

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in terms of an alternating antiferromagnetic Heisenberg linear chain model with an average
exchange interaction of J = ꢀ43.8 cmꢀ1 and an alternation parameter α = 0.3 (gsolid = 2.0028). The
enhanced overlap between the πꢀextended SOMO orbitals leads to strong antiferromagnetic
interactions along the chains (J1 = ꢀ87.6 cmꢀ1 and J2 = ꢀ26.3 cmꢀ1).
INTRODUCTION
The search of organic multifunctional materials has attracted considerable attention in recent
years.1 Persistent organic radicals have been intensively pursued as promising building blocks
for such materials.2 The presence of unpaired electrons in these molecules offers the potential to
combine magnetic, charge transport and optical properties. Customizing and tuning these
properties requires the development of “structureꢀproperty relationships” that help to understand
the effects of molecular structure on solidꢀstate packing and their resultant properties.
We recently engaged in a magnetoꢀstructural study of the 1,2,4ꢀbenzotriazinyl family of
radicals which, despite their exceptional air and moisture stability, have received little attention.3
As part of our studies we have explored the chemistry of 1,2,4ꢀbenzotriazine4 and developed
high yielding synthetic routes to a range of 1,2,4ꢀbenzotriazinyl derivatives functionalized at the
C7 position of the benzoꢀfused ring.5 A range of oneꢀdimensional magnetic properties have been
found,6 however, in an effort to increase the dimensionality of the magnetic interactions we have
also recently prepared imidazoloꢀ, oxazoloꢀ and thiazoloꢀfused benzotriazinyls.5c,d We anticipate
that the extension of the π acene core will provide an effective pathway for the exchange
interactions. Herein we present the solidꢀstate characterization of 1,3,7ꢀtriphenylꢀ1,4ꢀdihydroꢀ
thiazolo[5′,4′:4,5]benzo[1,2ꢀe][1,2,4]triazinꢀ4ꢀyl (1) and provide magnetostructural correlation
2

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based on variableꢀtemperature (VT) magnetic susceptibility data, the Xꢀray structure and DFT
calculations.
EXPERIMENTAL
General
Synthesis of radical 1 was previous reported.5c Melting points were determined using a TA
Instruments DSC Q1000 with samples hermetically sealed in aluminum pans under an argon
atmosphere, using heating rates of 5 °C/min. Solvents used for recrystallization are indicated
after the melting point. UV spectra were obtained using a PerkinꢀElmer Lambdaꢀ25 UV/vis
spectrophotometer and inflections are identified by the abbreviation “inf”. IR spectra were
recorded on a Shimidazu FTIRꢀNIR Prestigeꢀ21 spectrometer with Pike Miracle Ge ATR
accessory and strong, medium and weak peaks are represented by s, m and w, respectively. Low
resolution (EI) mass spectra were recorded on a Shimadzu Q2010 GCMS with direct inlet probe.
MALDIꢀTOF MS were conducted on a Bruker BIFLEX III timeꢀofꢀflight (TOF) mass
spectrometer. Cyclic voltammetry (CV) measurements were performed on a Princeton Applied
Research Potentiostat/Galvanostat 263A apparatus. The concentration of the benzotriazinyl
radical used was 1 mM in DCM. A 0.1 M DCM solution of tetraꢀnꢀbutylammonium
tetrafluoroborate (TBABF4) was used as electrolyte. The reference electrode was Ag/AgCl and
the scan rate was 50 mV/s. Ferrocene was used as an internal reference; the E1/2(ox) of ferrocene
in this system was 0.352 V.6 EPR spectra were recorded on a Bruker EMXplus Xꢀband EPR
spectrometer at room temperature on solid state sample of the benzotriazinyl 1 and on dilute
solutions in DCM. For the dilute solution spectra, the microwave power was in the region 5 – 70
mW with modulation frequencies of 50 or 100 kHz and modulation amplitudes of 0.5 – 1.0 Gpp.
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Simulations of the solution and solidꢀstate spectra were made using Winsim.7 Magnetic
properties were studied using a Quantum Design SQUID MPMS2 fieldꢀshielded magnetometer.
The DC (direct current) magnetic moment was measured on a 62.2 mg sample of radical 1,
which was placed in a gelatin capsule held by polyethylene straw. The magnetic susceptibility
was measured in the temperature region 5 – 300 K in an applied field of 0.4 T. Data were
collected in both warming and cooling modes with no significant differences in sample
susceptibility. Data were corrected using a diamagnetic contribution of χdia = ꢀ150 × 10ꢀ3 emu
molꢀ1. Xꢀray data of radical 1 (CCDC 953369) were collected on an OxfordꢀDiffraction
Supernova diffractometer, equipped with a CCD area detector utilizing MoꢀKα radiation (λ =
1.5418 Å). A suitable crystal was attached to glass fibers using paratoneꢀN oil and transferred to
a goniostat where they were cooled for data collection. Unit cell dimensions were determined
and refined by using 2746 (4.06 ≤ θ ≤ 72.44º) reflections. Empirical absorption corrections
(multiꢀscan based on symmetryꢀrelated measurements) were applied using CrysAlis RED
software.8 The structures were solved by direct method and refined on F2 using fullꢀmatrix least
squares using SHELXL97.9 Software packages used: CrysAlis CCD8 for data collection,
CrysAlis RED8 for cell refinement and data reduction, WINGX for geometric calculations,10 and
Mercury 3.111 for molecular graphics. The nonꢀH atoms were treated anisotropically. The
hydrogen atoms were placed in calculated, ideal positions and refined as riding on their
respective carbon atoms. DFT singleꢀpoint calculations were carried out on the Xꢀray
crystallographically determined geometry to calculate the energies of the triplet (ET) and broken
symmetry singlet (EBS) states. Calculations were performed using the Gaussian 03 suite of
programs, Rev C.02 for the UB3LYP functional and Gaussian 09 suite of programs, Rev. D.01
for the UMO6ꢀ2X and UX3LYP functionals.12,13
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Spectral analysis for radical 1:
1,3,7ꢀTriphenylꢀ1,4ꢀdihydrothiazolo[5',4':4,5]benzo[1,2ꢀe][1,2,4]triazinꢀ4ꢀyl (1)5c
o
o
Black needles (74 mg, 74%), mp (DSC) onset: 253.1 C, peak max: 260.3 C, decomp. onset:
o
o
298.2 C, peak max: 327.1 C (from PhH), Rf 0.36 (Et2O/nꢀhexane, 1:1); (found: C, 74.90; H,
4.01; N, 13.36. C26H17N4S requires C, 74.80; H, 4.10; N, 13.42%); λmax(DCM)/nm 259 inf (log ε
3.52), 282 inf (3.67), 304 (3.72), 365 (2.86), 402 inf (3.34), 423 (3.48), 470 (2.70), 514 (2.65),
561 (2.66), 660 inf (2.42); νmax/cmꢀ1 3067w, 3057w and 3019w (Ar CH), 1589w, 1530w, 1506w,
1491m, 1479m, 1458s, 1439s, 1391s, 1346m, 1317w, 1287w, 1258m, 1225m, 1204w, 1182w,
1169w, 1157w, 1136w, 1067w, 1045w, 1024m, 1001w, 982w, 955m, 926w, 912w, 854s, 826m,
797m, 787m, 779m, 770w, 756s; m/z (EI) 417 (M+, 100%), 401 (3), 341 (4), 314 (9), 209 (30),
180 (7), 177 (3), 167 (4), 165 (3), 139 (4), 106 (8), 77 (27), 69 (3), 62 (6), 51 (8).
RESULTS AND DISCUSSION
Synthesis
Radical 1 was prepared in three steps beginning from the benzotriazinꢀ7(H)ꢀone 2 in an overall
yield of 50%:5c The benzotriazinone 2 is directly aminated using Oꢀbenzylhydroxylamine
hydrochloride and Hünig’s base in refluxing ethanol to give the 6ꢀaminobenzotriazinone 4 in
79% on a multigram scale (Scheme 1). Subsequent reaction of the 6ꢀaminobenzotriazinone 4
o
with benzoyl chloride and NaH (1.2 equiv.) in DMF at ca. 20 C for 12 h gave the Nꢀ(benzoꢀ
triazinꢀ6ꢀyl)carboxamide 5 in 86% yield. Treating the carboxamide 5 with P2S5 (1 equiv.) in
refluxing PhMe gave the 1,3,7ꢀtriphenylthiazolobenzotriazinyl 1 in 74% (Scheme 1).
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Scheme 1. Synthetic Route to the 1,3,7ꢀtriphenylthiazolobenzotriazinyl 1
EPR and Cyclic Voltammetry
The redox behaviour of radical 1 is similar to that of other benzotriazinyl radicals,3,14
exhibiting two fully reversible waves which correspond to the ꢀ1/0 and 0/+1 processes (Figure S1
0/+1
in the ESI). The oxidation potential occurred at E1/2
= 0.21 V vs ferrocene/ferrocenium
ꢀ1/0
couple, whereas the reduction potential occurred at E1/2 = ꢀ0.87 V. Room temperature solidꢀ
state EPR spectra on radical 1 revealed essentially isotropic singlet EPR spectrum (gsolid
=
2.0028) consistent with an organic radical with little spinꢀorbit coupling (Figure S2 in the ESI).
In solution the gꢀfactor of the radical (gsoln = 2.0024) was similar to the one observed in the solidꢀ
14
state but the lower concentration permitted the resolution of the N hyperfine coupling to the
three chemically distinct N atoms of the triazinyl ring (Figure S3 in the ESI). The similarity in aN
values led to the spectrum typically appearing as a nonꢀbinomial 7ꢀline multiplet, though full
6

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simulation revealed subtle differences in the hyperfine coupling constants (hfcc). Previous EPR
15
and ENDOR studies by Neugebauer on Nꢀlabelled benzotriazinyl radicals showed that aN1 >>
aN4 > aN2.3d,15 Assignment of hfcc for radical 1, extracted from simulated firstꢀderivative spectra,
are given according to Neugebauer’s reported trend; aN1 = 6.48, aN2 = 4.80 and aN4 = 5.25 G.
Spin densities estimated based on hfcc using McConnell’s equation are ρN1 = 0.259, ρN2 = 0.226
and ρN4 = 0.248.16,17
Crystal Structure
Suitable single crystals of 1 for Xꢀray diffraction studies were obtained by slow diffusion of nꢀ
pentane into a benzene solution of the radical. Radical 1 crystallizes in the triclinic Pꢀ1 space
group with one molecule in the asymmetric unit (see Figure S4 in the ESI for atom numbering
used in the Xꢀray discussion). The intramolecular geometrical parameters are similar to
previously studied benzotriazinyl radicals.3,14 The phenyl substituents at C1 (3ꢀPh) and C8 (7ꢀPh)
are close to planarity with dihedral angles of 0.4(4)o and 7.1(4)o, respectively. The thiazole ring
is also planar with interatomic distances and angles typical of other benzothiazoles; S1ꢀC6,
1.736(2) Å; N4ꢀC7, 1.387(3) Å and C8ꢀS1ꢀC6, 89.1(2)o; C8ꢀN4ꢀC7, 110.8(2)o.
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Figure 1. Packing diagram of radical 1 along the c axis (hydrogens are omitted for clarity).
…
Thermal ellipsoid are at 50% probability. Shortest intermolecular contacts: pair IꢀII [dC1
=
C2
…
…
3.373(4) Å, (ꢀx, 1ꢀy, ꢀz)] and pair IIꢀIII [dC1 C12 = dC3 C14 = 3.367(4) Å, (ꢀx, 1ꢀy, 1ꢀz)].
It is anticipated that the πꢀextended acene core of radical 1 will favour πꢀstacking and the
enhanced orbital overlap will lead to stronger exchange interactions. Molecules of the 1,3,7ꢀ
triphenylthiazolobenzotriazinyl 1 form 1D πꢀslipped stacked columns, with the mean interplanar
distance between radicals alternating down the stack. This gives rise to a centrosymmetric pair
(radicals IꢀII in Fig. 1) where the spinꢀbearing triazinyl rings are related through a centre of
…
inversion and overlap via a pair of short faceꢀtoꢀface interactions [dC1 C2 = 3.373(4) Å, (ꢀx, 1ꢀy, ꢀ
z)]. The structure of pair IꢀII is further supported by a pair of two crystallographically equivalent,
…
weak CꢀH N contacts between the triazinyl N3 and an orthoꢀhydrogen atom of the Nꢀphenyl
o
…
…
ring [dC26
= 3.860(3) Å, (ꢀx, 1ꢀy, ꢀz), CꢀH N angle = 143 ]. The centrosymmetric pairs IꢀII
N3
pack parallel to the crystallographic c axis and relate to each other via a centre of inversion
running in the middle of the stacked thiazole rings; this places the triazinyl ring above the 7ꢀPh
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…
…
and links pairs IꢀII via a set of equidistant C C interactions [radical pair IIꢀIII in Fig. 1; dC1
C12
…
= dC3 C14 = 3.367(4) Å, (ꢀx, 1ꢀy, 1ꢀz)].
…
Neighbouring stacks, pack along the a axis via a net of CꢀH N contacts between the thiazolo
…
N4 and the paraꢀhydrogen atom of the Nꢀphenyl ring [dC24 N4 = 3.337(3) Å, (ꢀ1+x, ꢀ1+y, z), Cꢀ
o
…
H N angle = 131 ; Fig. 2]. A tight packing with no voids is completed with edgeꢀtoꢀface πꢀ
contacts among the Nꢀphenyl and the 3ꢀ/7ꢀphenyl substituents.
Figure 2. Packing of radical 1 along the a axis showing the interstack interactions (hydrogens
are omitted for clarity and thermal ellipsoid are at 50% probability).
Magnetic Properties
The 1D πꢀslipped stacking of the 1,3,7ꢀtriphenylthiazolobenzotriazinyl 1 suggests uniꢀ
dimensional magnetic interactions that propagate parallel to the stacking direction, along which,
the orbital overlap between the singlyꢀoccupied molecular orbitals is maximized. Variableꢀ
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temperature magneticꢀsusceptibility measurements on radical 1 were obtained by using a SQUID
magnetometer in the temperature region 5 – 300 K in an applied field of 0.4 T. Data were
collected in both warming and cooling modes with no significant differences in sample
susceptibility. Data were corrected using a diamagnetic contribution of χdia = ꢀ150 × 10ꢀ3 emu
molꢀ1. The temperature dependence of χ and χT for radical 1 is presented in Fig. 3. On cooling
from 300 K, the molar susceptibility χ increases gradually and reaches a broad maximum at Tmax
= 73 ± 5 K indicating strong antiferromagnetic interactions. After passing through the maximum,
χ falls off gradually down to 20 K before it increases again at lower temperatures. The increase
of χ below 20 K is attributed to uncoupled radicals at defect sites in the lattice. The alternating
interplanar distances along the stacking direction (dIꢀII = 3.350 Å, dII III = 3.393 Å) indicates that
ꢀ
the appropriate model to fit the magnetic data is Hatfield’s expression18 for the magnetic
susceptibility of Heisenberg alternatingꢀchains of S = ½ spins. The spin Hamiltonian for this
model is:
n / 2
H = −2J
[
S S
+
α
S2i S2i+1
]
∑
2i 2i−1
i=1
Where J and αJ are the exchange integrals along the stacking direction and α is a parameter
indicating the degree of alternation. At the extremes, when α = 0 the model corresponds to an
isolated dimer system and when α = 1 the model reduces to the regular linearꢀchain model.
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Figure 3. Temperature dependence of χ (top) and χT (bottom) for radical 1. The red line
represents the best fit to the Heisenberg alternating linearꢀchain model18 with J = ꢀ43.8 cmꢀ1, α =
0.3, ρ = 0.95, gsolid = 2.0028.
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The best fit to the magnetic susceptibility data was obtained using the set of polynomial
parameters for 0 ≤ α ≤ 0.4. A parameter ρ was included to account for the susceptibility arising
from uncoupled radicals at the lattice defect sides. The fit reproduces well both the position and
the maximum in χ when J = ꢀ43.8 cmꢀ1, α = 0.3, ρ = 0.95, gsolid = 2.0028. By applying the ratio of
α = J2 / J1, where J1 = 2J and J2 = 2αJ, we determined estimated J1 and J2 values along the chain
direction of ꢀ87.6 and ꢀ26.3 cmꢀ1, respectively.
Calculation of exchange interactions
Exchange coupling within radical pair IꢀII is expected to be stronger since the radicals overlap
predominantly over the spinꢀbearing triazinyl moieties maximizing the SOMOꢀSOMO
interaction. Conversely the molecular arrangement in radical pair IIꢀIII brings the 7ꢀphenyl ring,
bearing little spin density (SOMO orbital coefficients are smaller, Fig. 4), on top of the adjacent
triazinyl ring. Therefore, in the absence of other dominating exchange coupling interactions, J1
and the weaker J2 could be assigned to the interactions within radical pairs IꢀII and IIꢀIII,
respectively.
Figure 4. The calculated SOMO orbital of radical 1 at the UB3LYP/6ꢀ311G++(d,p) level of
theory.
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Direct estimates of the magnetic interactions within radical pairs can be obtained from
quantum chemistry methods.19 Although the macroscopic magnetic behaviour is the outcome of
all existing magnetic interactions working together, for radicals with low dimensional magnetic
topology, e.g., 1,3,7ꢀtriphenylthiazolobenzotriazinyl 1, computed exchange interactions can
correlate in a qualitative way with experimentally determined exchange couplings.
The calculated spinꢀspin exchange interactions J ꢀ and JII III were determined by the
ꢀ
I II
unprojected equation, 2JDFT = 2(EBS – ET)/2.20 The energies of the triplet (ET) and broken
symmetry singlet (EBS) states were determined by single point calculations on
crystallographically determined geometries of radical pairs IꢀII and IIꢀIII (Table T1 in the ESI).
Table 1. Calculated Exchange Interactionsa of Pair IꢀII (JIꢀII) and Pair IIꢀII (JIIꢀIII) on their XꢀRay
Determined Geometries at the UB3LYP Level of Theory.
b
b
Basis Set
JIꢀII
JIIꢀIII
J1
J2
6ꢀ311++G(d,p)
6ꢀ311++G(2d,p)
6ꢀ311++G(d,p)
6ꢀ311++G(2d,p)
ꢀ46.6c
ꢀ103.8c
ꢀ92.4d
ꢀ205.6d
+6.6c
ꢀ6.2c
ꢀ87.6
ꢀ26.3
+13.1 d
ꢀ12.4 d
a Exchange interactions are given in cmꢀ1.
b Experimentally determined exchange interactions in cmꢀ1.
c Calculated by the unprojected method.
d Calculated by the approximate projected method.
Although advanced and computationally expensive quantum chemical methods such as
CASSCF and CASPT2 provide good results, DFT functionals such as the B3LYP perform well
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in the computation of exchange interactions.19 The latter method was employed for the
calculations of JIꢀII and JII III using the 6ꢀ311++G(d,p) and 6ꢀ311++G(2d,p) basis sets (Table 1).
ꢀ
Adding 2d functions on heavy atoms to take in account the presence of the S atom, at the
UB3LYP/6ꢀ311++G(2d,p) level of theory using the unprojected method, successfully reproduced
both the sign and magnitude of the magnetic exchange interactions (JIꢀII = ꢀ103.8 cmꢀ1 and JII III
=
ꢀ
ꢀ6.2 cmꢀ1 vs J1 = ꢀ87.6 cmꢀ1 and J2 = ꢀ26.3 cmꢀ1). Calculations confirm the qualitative assignment
of J to pair IꢀII and J to pair IIꢀIII, as JIꢀII > JII III and J1 > J2. Although B3LYP combined with
ꢀ
1
2
the unprojected method provides reasonable values for the exchange interactions within radical
pairs, we screened two more hybrid metaꢀGGA functionals, X3LYP and MO6ꢀ2X, using the
6ꢀ311++G(2d,p) basis set. These two functionals provide better descriptions of nonꢀbond
interactions such as van der Waals, hydrogen bonding, πꢀπ stacking, chargeꢀtransfer complexes
and dipole interaction complexes.21 The values of the calculated exchange interactions at the
UMO6ꢀ2X/6ꢀ311++G(2d,p) and UX3LYP/6ꢀ311++G(2d,p) level of theory were similar to the
values obtained from the UB3LYP/6ꢀ311++G(d,p) method (Table 2). Applying the approximate
projection method, 2JDFT = 2(EBS – ET)/(<Ŝ2>T – <Ŝ2>BS), which takes in account the overlap
between the magnetic orbitals via the calculated eigenvalues of the Ŝ2 operator, the magnitude of
the computationally and experimentally determined exchange interactions are in excellent
agreement for the UB3LYP/6ꢀ311++G(d,p), UM06ꢀ2X/6ꢀ311++G(2d,p) and UX3LYP/6ꢀ
311++G(2d,p) level of theories. Nevertheless, these functionals in combination with the
approximate projected method, failed to reproduce the sign of the JII III
.
ꢀ
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Table 2 Calculated Exchange Interactionsa of Pair IꢀII (JIꢀII) and Pair IIꢀII (JIIꢀIII) on their XꢀRay
Determined Geometries Using the UMO6ꢀ2X and UX3LYP Functionals.b
c
c
Functional
UMO6ꢀ2X
UX3LYP
UMO6ꢀ2X
UX3LYP
JIꢀII
JIIꢀIII
J1
J2
ꢀ47.4d
ꢀ46.9d
ꢀ94.8e
ꢀ92.9e
+11.2d
+10.6d
+22.4e
+21.1e
ꢀ87.6
ꢀ26.3
a Exchange interactions are given in cmꢀ1.
b Employing the 6ꢀ311++G(2d,p) basis set.
c Experimentally determined exchange interactions in cmꢀ1.
d Calculated by the unprojected method.
e Calculated by the approximate projected method.
DISCUSSION
The magnetic behaviour of 1,2,4ꢀbenzotriazinyls can be rationalized by using the molecular
2
orbital (MO) model.22 In the MO model the exchange coupling interaction (JAB = 2Kij + 4cijSij )
is proportional to the size of the overlap between SOMO orbitals (Sij). The sign of the exchange
interactions depends primarily on the distance and orientation of the coupled radicals. In related
π systems, Oakley demonstrated that there is a fine balance between ferroꢀ and antiferromagnetic
interactions by adjusting the degree of slippage through chemical or mechanical pressure.23 For
1,2,4ꢀbenzotriazinyls the latitudinal and longitudinal slippage angles as well as the distance
between the radicals determine the nature of the interactions along the πꢀstacking direction.14 A
direct comparison with other 1,2,4ꢀbenzotriazinyls is not straight forward as these radicals pack
in a variety of ways owing to a complex set of geometrical parameters that give rise to a range of
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magnetic interactions. However, the magnitude of these magnetic exchange interactions is
directly related to the degree of overlap between the SOMO orbitals, irrespective of their nature
(either ferroꢀ or antiferromagnetic). The magnitude of the exchange interactions of 1,3,7ꢀ
triphenylthiazolobenzotriazinyl 1 (J1 = ꢀ87.6 cmꢀ1 and J2 = ꢀ26.3 cmꢀ1) is considerably larger than
the exchange interactions of nonꢀfused benzotriazinyls such as the 1D regular chains of the 1,3,7ꢀ
triphenylꢀ and 1,3ꢀdiphenylꢀ7ꢀ(4ꢀfluorophenyl)ꢀ1,4ꢀdihydroꢀ1,2,4ꢀbenzotriazinꢀ4ꢀyls with 2J = ꢀ
25.8 and ꢀ23.6 cmꢀ1.14c The πꢀextended acene core of thiazoloꢀfused radical 1 leads to an
enhanced orbital overlap and therefore to stronger exchange interactions. While sideꢀon
intermolecular contacts connect neighbouring chains, they fail to increase the dimensionality of
the magnetic interactions. The low dimensionality of the magnetic topology can be explained by
the presence of phenyl groups around the periphery which isolate the spinꢀbearing units and
prohibit interactions over the other two dimensions. If longꢀrange ordering is to be realized then
fine tuning of the substituents’ steric effects is required to make the 1,2,4ꢀbenzotriazinyl core
more available to the surrounding environment.
CONCLUSIONS
In conclusion, the crystal structure and magnetic properties of 1,3,7ꢀtriphenylꢀ1,4ꢀdihydroꢀ
thiazolo[5′,4′:4,5]benzo[1,2ꢀe][1,2,4]triazinꢀ4ꢀyl (1) have been investigated. The radicals π stack
in 1D columns along the c axis and comprise of radical pairs (IꢀII and IIꢀIII) with alternate short
and long interplanar distances. Magnetic susceptibility measurements along with quantum
chemical calculations reveal the existence of strong antiferromagnetic interactions within the 1D
stacks, as a result of an enhanced overlap between the πꢀextended SOMO orbitals.
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ACKNOWLEDGEMENT
The authors wish to thank the University of Cyprus (mediumꢀsize grants) and the Cyprus
Research Promotion Foundation [grants no. ΥΓΕΙΑ/ΒΙΟΣ/0308(ΒΙΕ)/13, ΝΕΚΥΠ/0308/02 and
ΑΝΑΒΑΘΜΙΣΗ/0308/32]. JMR would like to thank the CFI and ORF for financial support. The
work in Israel was supported by the Israel Science Foundation.
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249x116mm (96 x 96 DPI)