Thermally and Magnetically Robust Triplet Ground State Diradical

Article abstract of DOI:10.1021/jacs.9b00558

High spin (S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, ?"E_ST ≥ 1.7 kcal mol^-1, leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at a160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J′/k = a14 K, which is associated with the N···N and N···O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains of 2 most isotropic, and therefore an excellent system for studies of low-dimensional magnetism. In polystyrene glass and in frozen benzene or dibutyl phthalate solution, both 1 and 2 are monomeric. Diradical 2 is thermally robust and is evaporated under ultrahigh vacuum to form thin films of intact diradicals on silicon substrate, as demonstrated by X-ray photoelectron spectroscopy. Based on C-K NEXAFS spectra and AFM images of the a1.5 nm thick films, the diradical molecules form islands on the substrate with molecules stacked approximately along the crystallographic a-axis. The films are stable under ultrahigh vacuum for at least 60 h but show signs of decomposition when exposed to ambient conditions for 7 h.

Full text of DOI:10.1021/jacs.9b00558

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Article
Thermally and Magnetically Robust Triplet Ground State Diradical
Nolan Gallagher, Hui Zhang, Tobias Junghoefer, Erika Giangrisostomi, Ruslan
Ovsyannikov, Maren Pink, Suchada Rajca, Maria Benedetta Casu, and Andrzej Rajca
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00558 • Publication Date (Web): 28 Feb 2019
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Thermally and Magnetically Robust Triplet Ground State Diradical.
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Nolan Gallagher†, Hui Zhang†, Tobias Junghoefer§, Erika Giangrisostomi,⊥ Ruslan Ovsyannikov,⊥
Maren Pink‡, Suchada Rajca†, Maria Benedetta Casu*§ and Andrzej Rajca*†
†Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, USA.
§ Institute of Physical and Theoretical Chemistry, University of Tübingen, 72076 Tübingen, Germany
‡Department of Chemistry, Indiana University, Bloomington, Indiana 47405-7102, USA.
⊥Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Albert-Einstein-Str 15, 12489 Berlin, Germany
ABSTRACT: High spin (S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but
typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with
a large singlet-triplet energy gap, EST ≥ 1.7 kcal mol-1, leading to nearly exclusive population of triplet ground state at room
temperature, and good thermal stability with onset of decomposition at ~160 C under inert atmosphere. Magnetic properties of 2 and
the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and
in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain of organic radicals with
intrachain antiferromagnetic coupling of J′/k = −14 K, which is associated with the N···N and N···O intermolecular contacts. The
intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes
1D S = 1 chains of 2 most isotropic, and therefore an excellent system for studies of low-dimensional magnetism. In polystyrene glass
and in frozen benzene or dibutyl phthalate solution, both 1 and 2 are monomeric. Diradical 2 is thermally robust and is evaporated
under ultra-high vacuum to form thin films of intact diradicals on silicon substrate, as demonstrated by X-ray photoelectron
spectroscopy. Based on C-K NEXAFS spectra and AFM images of the ~1.5-nm thick films, the diradical molecules form islands on
the substrate with molecules stacked approximately along the crystallographic a-axis. The films are stable under ultra-high vacuum
for at least 60 h but show signs of decomposition when exposed to ambient conditions for 7 h.
INTRODUCTION
Open-shell organic molecules with high-spin ground
Both TDD and 1 start decomposing at approximately 180 C under
nitrogen atmosphere and neutral diradical 1 could be sublimed
under high vacuum at 140 C without decomposition.
states and large energy gaps between the high-spin ground state and
low-spin excited states possess unique, intriguing characteristics
that are not only of fundamental interest but also have significant
potential for numerous advanced technological applications.
Notably, these molecules have long been considered the holy grail
of purely organic magnets,1-5 and recently have emerged as
promising building blocks for organic spintronics,6 spin filters,7
sensors,8 memory devices,9 and probing quantum interference
effects in molecular conductance.10 Although the design principle
has been well laid out,11,12 such high-spin molecules with strong
ferromagnetic interactions between unpaired electrons and
persistence at room temperature remain highly uncommon,13-16 and
triplet ground state diradicals with robust thermal stabilities are
especially rare.17-19 The advancement in the design and synthesis of
these exotic molecules will be crucial to the development of
advanced organic magnetic materials and devices.
Recently, we reported triplet ground state diradicals with
robust thermal stabilities, such as tetraazacyclophane diradical
dication (TDD) and diradical 1 (Figure 1).17,18 To our knowledge,
these are the only two such diradicals that are well characterized
with respect to stability by thermogravimetric analysis (TGA).
Figure 1. Thermally robust triplet ground state diradicals with
onset of decomposition at T ≥ 160 C based upon TGA.
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While these high-spin diradicals possess remarkable
Synthesis of 2. Our synthetic approach to 2, follows closely the
thermal properties, they are not fully populated in the high-spin
state at room temperature. The singlet triplet energy gap, EST, of
both diradicals is only about 0.5 kcal mol-1, as determined by
quantitative EPR spectroscopy and SQUID magnetometry, which
is similar to thermal energy at room temperature (RT  0.6 kcal mol-
1).17,18 In such a case, there is a significant depopulation of the
triplet ground state at room temperature and above, and thus the
unique magnetic properties of the high-spin state with extra-large
magnetic moment are compromised at ambient temperatures.
To rectify this deficiency, we designed diradical 2 and
preliminary computed its magnetic properties. The DFT
calculations suggested an increased ΔEST by a factor of ~2.5,
compared to 1, and an estimated ~95+% occupancy of the triplet
ground state at room temperature.17 We anticipated diradical 2 to
possess both superior magnetic and excellent thermal properties,
providing a novel high-spin diradical with robust high temperature
stability and near-full occupation of the triplet ground state at
ambient temperatures.
synthesis of diradical 1 and it takes advantage of the unusual
stability of the Blatter radical, such that it may tolerate many
common reaction conditions (Scheme 1).21 Cyano-Blatter radical 3
is synthesized by adopting procedures available in the literature, as
outlined in detail in the Supporting Information.17,22-25 Treatment
of 3 with DIBAL-H followed by hydrolysis of the imine group
provides formyl-Blatter radical 4. Radical 4 is initially reduced to
the corresponding leuco-amine-aldehyde, or alternatively, it is
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directly
condensed
with
2,3-bis(hydroxyamino)-2,3-
dimethylbutane;26 the resultant adduct is oxidized in air to provide
diradical 2, which is purified by normal phase chromatography
(silica gel) at ambient conditions.
Scheme 1. Synthesis of Diradical 2.
The potential of organic radicals in the development of
organic electronics hinges upon their processability. The capacity
of a molecule to form contacts or to evaporate onto a substrate
without degradation is a critical prerequisite for device fabrication.
In this regard, it is important to test the robustness of diradical 2
toward evaporation. Controlled evaporation of a diradical is
considered very challenging, and to our knowledge, it has not been
reported in literature. Achieving the first thin film of a high-spin
diradical with nearly full-occupation of the triplet ground state at
ambient temperature would be a significant step forward, providing
exciting new avenues for the development of organic electronics.
X-ray crystallography. The structure of diradical 2 consists of
two non-equivalent molecules, A and B, and one molecule of
solvent (CH2Cl2). In molecule A, the nitronyl nitroxide radical
moiety is nearly co-planar with the Blatter radical -system with
the corresponding N-C8A-C1A-C torsions in the –13 – (–15)
range, while in molecule B, there is considerably greater out-of-
plane twisting with the corresponding torsions in the 28 – 30
range (Fig. S1, SI). In the crystal, molecules A and B pack in an
alternating fashion into one-dimensional chains (along the
crystallographic a-axis) with close intermolecular N∙∙∙N and O∙∙∙N
contacts (Figure 2, bottom plot).
Here we report the synthesis and study of high spin
diradical 2 (Figure 1). As predicted, 2 has a large EST of ≥1.7 kcal
mol-1, much larger than the thermal energy at room temperature,
thus possessing a triplet ground state that is nearly exclusively
populated (98+%) at room temperature. For comparison, we
characterize both 1 and 2 by SQUID magnetometry in dilute
matrices and as polycrystalline powders. Notably, polycrystalline
diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain
consisting of close contacts between the heteroatoms (oxygens and
nitrogens) of radical moieties with the largest spin densities. The
1D chain is distinctly different from those previously reported,18,20
and it is the first observed 1D system consisting of nitronyl
nitroxide and Blatter radicals. Importantly, the observed intra-chain
exchange coupling constant of J′/k = −14 K is much larger than the
previously studied 1D S = 1 chains, with the next strongest J′/k = –
5.4 K found in TDD.18 Diradical 2 is thermally robust, with an onset
of decomposition at ~160 °C under inert atmosphere and is
thermally evaporated under ultra-high vacuum to form thin films
on SiO2/Si(111) wafers, with X-ray photoelectron spectroscopy
suggesting the presence of intact 2. The C-K NEXAFS spectra and
AFM images of the films indicates the diradical molecules form
islands on the substrate with molecules stacked approximately
along the crystallographic a-axis. Diradical 2 possesses an
unprecedented combination of a triplet ground state that is nearly
exclusively populated at room temperature and a novel 1D S = 1
antiferromagnetic chain, with remarkable thermal stability and
suitability for thin film deposition via thermal evaporation. The
films are stable under ultra-high vacuum for at least 60 h but show
signs of decomposition when exposed to ambient conditions for 7
h. We present here the preparation and characterization of the first
thin film of high spin organic diradical.
Figure 2. Top: Single crystal X-ray structure of diradical 2 with
molecule A shown only; carbon, nitrogen, and oxygen atoms are
depicted with thermal ellipsoids set at the 50% probability level.
Bottom: Packing of molecules A and B into a one-dimensional S =
1 antiferromagnetic chain; nitrogens and oxygens with large
positive spin densities and forming close intermolecular contacts,
N2A∙∙∙N3B = 3.509 Å and O4B∙∙∙N1A = 3.335 Å, are emphasized
as ball-and-stick. Additional information can be found in the
Supporting Information (Figs. S1-S3 and Table S1).
RESULTS AND DISCUSSION
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SQUID magnetometry: triplet ground states of 1 and 2. We
Within the -conjugated pathway in the diradical, there are two
dihedral angles of 49° and 30 connecting the 1,2,4-
estimate singlet triplet energy gaps in polycrystalline 1 and 2 by
fitting the T vs. T data in the T = 1.8 – 320 K and 70 – 320 K
ranges, respectively, to the diradical model (eq. 1).4a
~
benzotriazinyl and nitronyl nitroxide moieties in 1,17 as opposed to
one dihedral angle (or one torsion) in 2. The radicals in 2 are then
more coplanar than in 1. The DFT geometry optimizations using
UB3LYP/6-31G(d) level of theory27 and starting from either
conformation as observed in molecule A or B give more co-planar
structures that are similar to molecule A, with N-C8-C1-C torsions
in the ±13.7 – (±13.8) ranges (SI, Table S6).
T = (1.118T/H)N{2sinh(a)/[1 + 2cosh(a) + exp((–2J/k)/T)]} (1)
a = 1.345(H/(T – ))
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For 1, an average of three fits (Figure 4 and SI, Fig. S8) gives 2J/k
= 252 ± 10 K, corresponding to EST = 0.50 ± 0.02 kcal mol-1
,
EPR spectroscopy. The EPR spectrum of 2 in glassy matrices
shows exclusively a triplet diradical, without a trace of monoradical
impurities (Figure 3). As expected, the spectral simulation28 reveals
much greater spectral width, 2|D/hc| = 1.616 × 10−2 for 2 vs. 2|D/hc|
= 4.64 × 10−3 for 1,17 because of relative proximity of the unpaired
electrons and the nearly co-planar nitronyl nitroxide and
benzotriazinyl moieties in 2. The B3LYP/EPR-II calculations
estimate the relative values of D as D/hc = +1.20 × 10−2 cm–1 for 2
and D/hc = −5.47 × 10−3 cm–1 for 1.29 The computed D-tensor
components are not only overestimated, as typical for these type of
diradicals, but also the positive sign of D/hc in 2 is inconsistent
with its experimental EPR spectrum (Supporting Information).30,31
which is in good agreement with 2J/k = 234 ± 36 K, obtained
previously by quantitative EPR spectroscopy in dilute
solutions/matrices.17 For 2, a much larger 2J/k = 876 ± 36 K is
obtained as an average of four fits (Figure 5 and SI: Fig. S9 and
Table S2), corresponding to EST = 1.74 ± 0.07 kcal mol-1.
Although the measured values of T > 0.75 emu K mol-1 and fitted
values of EST > 0 would suggest triplet ground states for 1 and 2,
these results are dependent on an accurate weight of the SQUID
sample. Also, the fitting of T vs. T for polycrystalline 1 and 2
require relatively large absolute values of negative mean-field
parameters, 

–6 and –14 K, thus suggesting significant
antiferromagnetic interactions between the S = 1 diradicals.
Figure 4. SQUID magnetometry of polycrystalline (solid) diradical
1: plots of T vs. T and at  vs. T at H = 5000 Oe in the warming
mode. T vs. T data, corrected for diamagnetism, are fit to a
diradical model (eq. 1), using two variable parameters: singlet
triplet energy gap, 2J/k, mean-field correction for intermolecular
interactions between the radicals, . The values of standard error,
SE, and parameter dependence, DEP, are provided; goodness of fit
may be measured by standard error of estimate, SEE. Fitting
parameters: 2J/k = 246 K (SE = 8),  = −5.89 K (SE = 0.08), DEP
= 0.0236, R2 = 0.9941, SEE = 0.0175. Complete set of magnetic
data (and fits) for solid diradical 1 may be found in the SI (Fig. S8
and Eq. S1).
Figure 3. EPR ( = 9.65 GHz) spectrum of 1.2 mM diradical 2 in
4:1 toluene/chloroform glass at 153 K. The Δms = 2 transition is
shown as an inset. Simulation parameters: gxx = 2.0072, gyy
=
2.0026, gzz = 2.0052, |D/hc| = 8.08 × 10−3 cm−1, |E/hc| = 1.17 × 10−3
cm−1; linewidths: LWx = 32.0 MHz, LWy = 100.0 MHz, LWz = 24.9
MHz. For EPR spectra (with simulations) of 2 in polystyrene
matrix at 295 K, see: SI, Fig. S7.
Table 1. Summary of experimental and DFT-computed singlet-
triplet energy gaps and selected EPR parameters for diradicals
1 and 2.
With such large values of | |, magnetization data at low
temperatures could not be fit adequately to Brillouin functions, thus
sample-weight independent evidence for triplet ground state could
not be obtained. Therefore, we prepare dilute diradicals in glassy
matrices by dispersing 1 and 2 in polystyrene.
D/hc
E/hc
EST
(kcal mol-1)
ga
(10–3 cm–1) (10–3 cm–1)
SQUIDb
DFTc EPR DFTd EPR DFTd EPR DFTd
+1.4 2.32 −5.47 0.14 −1.56 2.0044 2.0053
+3.5 8.08 +12.0 1.17 +3.25 2.0050 2.0053
For the dilute sample of 1 and 2 in polystyrene, two-
parameter fits to T vs. T data in the T = 1.8 – 370 and 1.8 – 360 K
ranges give somewhat lower values of 2J/k = 165  18 K and 2J/k
= 838 ± 78 K (vs. polycrystalline diradicals), as an average of two
and three fits, respectively (Figure 6 and SI: Figs. S11 and S12).
Most importantly, these fits indicate that the value of  is very
small and practically negligible.
1
2
+0.50  0.02
+1.74  0.07
a
Isotropic g = (gx + gy + gz)/3. b ΔEST is determined by SQUID of
c
solid diradicals 1 and 2.
BS-DFT-computed ΔEST at the
UB3LYP/6-31G(d,p) level17 Computed at the B3LYP/EPR-II
d
level using ORCA.
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Table 2. Fitting parameters for SQUID data for polycrystalline diradical 2.
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9
 (SE, DEP) b
T
[K]
H
[Oe]
2J/k or J'/k (SE, DEP) a
d
N (SE, DEP) c
SEE
R2 e
Fit
[K]
[K]
30000
5000
844 (14, 0.178)
878 (16, 0.172)
–14.05 (0.08, 0.178)
–13.87 (0.09, 0.172)
0.48 (0.03, 0.701)
NA
NA
0.0028 0.9933
0.0029 0.9926
0.0026 0.9999
70-320
1.8-70
Diradical
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30000
–13.87 (0.06, 0.902)
1.093 (0.002, 0.842)
1D-chain
T
5000
–14.00 (0.07, 0.903)
0.48 (0.04, 0.700)
1.100 (0.003, 0.843)
0.0030 0.9999
30000
5000
5000
500
–9.329 (0.23, 0.767)
–9.590 (0.25, 0.757)
–14.32 (0.11, 0.926)
–14.00 (0.10, 0.927)
–5.49 (0.25, 0.866)
–7.73 (0.59, 0.769)
1.33 (0.03, 0.308)
1.70 (0.01, 0.228)
0.52 (0.01, 0.481)
0.41 (0.01, 0.532)
1.56 (0.02, 0.749)
1.97 (0.01, 0.608)
0.97 (0.01, 0.725)
0.97 (0.2, 0.733)
0.0240 0.9927
0.0269 0.9909
0.0003 0.9871
0.0002 0.989
Dimer
1D-chain
Dimer
1.116 (0.008, 0.918)
1.068 (0.007, 0.917)
0.57 (0.04, 0.676)
0.77 (0.10, 0.562)
1.8-70

5000
500
0.0048
0.0089
-
-
T
[K]
H
[Oe]
J'/k (SE, DEP) a
d
e
Fit
N (SE, DEP)c
Nimp (SE, DEP)f
SEE
R2
[K]
30000
5000
30000
5000
5000
500
–13.82 (0.07, 0.9080)
–13.85 (0.07, 0.8920)
–10.86 (0.07, 0.8817)
–10.42 (0.10, 0.8542)
–14.43 (0.11, 0.9194)
–14.03 (0.08, 0.9182)
–8.75 (0.26, 0.7387)
–8.36 (0.28, 0.7173)
1.086 (0.002, 0.8328)
1.094 (0.003, 0.8333)
0.967 (0.003, 0.7830)
0.974 (0.005, 0.7751)
1.117 (0.007, 0.9079)
1.067 (0.006, 0.9063)
0.861 (0.028, 0.6951)
0.814 (0.030, 0.6845)
0.0110 (0.0010, 0.832) 0.0028 0.9999
0.0082 (0.0009, 0.752) 0.0032 0.9999
0.0696 (0.0014, 0.795) 0.0043 0.9998
0.0537 (0.0018, 0.709) 0.0067 0.9994
0.0108 (0.0002, 0.485) 0.0002 0.9899
0.0077 (0.0001, 0.504) 0.0002 0.9930
1D-chain
1.8-70
1.8-70
T
Dimer
1D-chain
Dimer

5000
500
0.044 (0.001, 0.240)
0.038 (0.001, 0.175)
0.0017 0.4916
0.0018 0.3333
a J/k or J'/k, Heisenberg exchange coupling constant; 2J/k = singlet triplet energy gap in Kelvin; SE, standard error; DEP, parameter
dependence. b , mean-field correction. c N, weight factor. d SEE, standard error of estimate. e R2, coefficient of determination. f Nimp, weight
factor for isolated S = 1 diradical.
parameters: singlet triplet energy gap, 2J/k and mean-field
correction for intermolecular interactions between the radicals, .
Numerical fits to 1D chain (eq. 2) or dimer (eqs. S2A&B, SI)
models for T = 1.8 – 70 K are carried out with three variable
parameters: intermolecular Heisenberg exchange coupling
constant, J'/k, weight factor, N, and weight factor for isolated S = 1
diradical, Nimp. The results for numerical fits are summarized in
Table 2 and in the Supporting Information (Table S3 and Figs. S9
and S10).
The magnetization (M) versus magnetic field (H) data,
that is M versus H/(T − θ), at low temperatures (T = 1.8 – 5 K)
provide excellent fits to the Brillouin functions with a small
negative mean-field parameter, |θ| ≤ 0.05 K. Such fits have two
variable parameters: total spin (S) and magnetization at saturation
(Msat); the mean-field parameter θ is adjusted until the M/Msat
versus H/(T − θ) plots overlap at all temperatures. The values of S
 1.0 (0.978−1.012), determined from the curvature of the Brillouin
plots, unequivocally indicate the triplet (S = 1) ground state for both
diradicals (Figure 6 and SI: Figs. S11 and S12). Since the values of
Msat = 0.74 and 0.925 µB for 1 and 2, respectively (µB = Bohr
magneton) match well the corresponding values of N = 0.73 and
0.93 obtained from fits of T vs. T data to the diradical model, this
implies that both diradicals are pure and N < 1.00 is obtained
because of a “weighing error” of sub-milligram amounts of
diradicals. The values of θ < 0 K and |θ|  0 K imply nearly
Figure 5. SQUID magnetometry of polycrystalline (solid)
diradical 2: plots of T vs. T and at  vs. T at various applied
magnetic fields, H = 300000 Oe and 5000 Oe in the warming mode
and H = 500 Oe in the cooling mode. Numerical fits to the diradical
model (eq. 1) for T = 70 – 320 K are carried out with two variable
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negligible, and almost certainly intermolecular, antiferromagnetic
coupling.
maximum at about 19 K in  vs. T data is observed, thus suggesting
relatively strong intermolecular coupling between S =1 diradicals 2
(Figures 4 and 5). Two limiting models for such antiferromagnetic
coupling are considered: (1) one-dimensional (1-D) Heisenberg
chains of S = 1 diradicals (spin-1 chain) (eq. 2) and (2) pairs of S =
1 diradicals (dimer) (eq. S2, SI).32,33 The numerical fits to these
models are obtained at low temperatures, T = 1.8–70 K, to ensure
that diradical 2 is almost completely in its S = 1 ground state at the
highest temperature (70 K), which is significantly below 2J/k  880
K.
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Initially, we fit T vs. T data in the low temperature
range, T = 1.8 – 70 K, using Eq. 2 and Eq. S2B (Table 2, Figure 5).
3-Parameter fits with the following variable parameters,
intermolecular Heisenberg exchange coupling constant, J'/k,
weight factor, N, and weight factor for isolated S = 1 diradical, Nimp
,
are in excellent agreement with 1D-chain model (coefficient of
determination, R2 = 0.9999 and standard error of estimate, SEE =
0.0028 or 0.0032); fits to an S = 1 dimer model are less satisfactory
(R2 = 0.9998 or 0.9994 and SEE = 0.0043 or 0.0067). Even larger
differences in fit quality between the two models are observed
when the variable parameter Nimp is replaced with the mean-field
parameter θ > 0 (Table 2).
The χ versus T data provide a more sensitive measure of
fit quality for different models. The fits to a 1D-chain (eq. 2)
provide SEE = 0.0002 – 0.0003, while the S = 1 dimer fits (SI, eq.
S2A) are much less satisfactory, with much larger SEE = 0.0017 –
0.0089, (Table 2, Figure 5).
1D = N{[3/(2k(T – ))][(2 + a1K + a2K2)/(3 + b1K + b2K2 +
b3K3)]} + Nimp(1.118/H){2sinh(a)/[1 + 2cosh(a)]}
(2)
a1 = 0.0194, a2 = 0.777, b1 = 4.346, b2 = 3.232, b3 = 5.634,
K = –J’/kT and a = 1.345(H/(T – ))
The 1D antiferromagnetic chain of weakly coupled S = 1
diradicals in polycrystalline 2 is consistent with the crystal packing,
as discussed above (Figure 2). Two types of short N2A∙∙∙N3B =
3.509 Å (dimer I) and O4B∙∙∙N1A = 3.335 (dimer II) contacts
between molecules A and B are identified. Because all nitrogens
and oxygens in diradical 2 bear large positive spin densities, such
contacts are anticipated to give rise to intermolecular
antiferromagnetic coupling between pairs of molecules A and B
within 1D chain
Figure 6. SQUID magnetometry of 30 – 40 mM diradicals 1 (A &
B) and 2 (C & D) in polystyrene matrix. Plots A and C: M/Msat vs.
H/(T – ) plots, where  = –0.04 or –0.05 K, at T = 1.8 – 5 K
(symbols) and the Brillouin curves corresponding to S = ½ - 3/2
(lines). Plots B and D: T vs. T data at H = 5000 Oe in the warming
mode were fit to a diradical model (eq. 1), using two variable
parameters: singlet-triplet energy gap, 2J/k, mean-field correction
for intermolecular interactions between the radicals, . The values
of standard error, SE, and parameter dependence, DEP, coefficient
of determination, R2, and standard error of estimate, SEE. Fitting
parameters: diradical 1: 2J/k = 182 K (SE = 5), N = 0.733 (SE =
0.001), DEP = 0.5311, R2 = 0.9441, SEE = 0.0109; diradical 2: 2J/k
= 800 K (SE = 20), N = 0.9278 (SE = 0.0008), DEP = 0.3336, R2 =
0.746, SEE = 0.0066. Further details are reported in the SI: Table
S3, Figs. S11 and S12.
We carried out DFT calculations, based upon the broken
symmetry approach, at the fixed X-ray geometry for molecules A
and B forming dimers I (AB) and II (BA) as well as trimers (ABA
and BAB),18,34 to determine the values of J'/k. The computed J'/k
are summarized in Table 3.
Table 3. B3LYP-computed values of intrachain J’/k for 2.a
Dimer
Trimer
We also investigate dilute diradicals in benzene and
dibutylphthalate (DBP) matrices. Similar results are obtained for 1
and 2 in benzene and 2 in DBP; however, the fits to the Brillouin
functions are less satisfactory because of much larger |θ| = 0.6 – 0.8
K observed in these matrices (SI, Table S3, Figs. S13 – S15).
I (AB) II (BA)
ABA
BAB
J’/k, 6-31G(d)
–14.1
–19.8
–12.7
–9.2
–7.2
–12.0
[K]
J’/k, 6-311++G(d,p)
[K]
–
–
a Broken symmetry triplets and quintets for dimers and trimers are
used, respectively. Further details may be found in the SI (Table
S8).
SQUID magnetometry: 1D antiferromagnetic S = 1 chain for
polycrystalline diradical 2. For polycrystalline 1, the  vs. T plot
shows continuously increasing  with decreasing T. For 2, a broad
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The agreement between the computed values of J’/k and
the experimental J’/k = –14 K is reasonable, especially when
accuracy of the DFT computations is considered.35 Also, while the
alternating chain of J’/k may not be excluded, it is predicted by
computation that the degree of alternation is relatively small and in
agreement with the experiment.
N
2
Multilayer
C 1s
Multilayer
N 1s
N
4
C-C
N
NN
C-H
N
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Stability. Diradical 2 possesses excellent stability at ambient
conditions, not only in air-saturated solution at ambient conditions
(Fig. S4, SI) but also on silica gel. Notably, solid 2 shows no signs
of decomposition after storing on air at –20 C for more than 2
years. In addition, solid 2 shows remarkable stability under vacuum
annealing at 100 C for 24 h, and we found that such condition
effectively removes the solvent of crystallization (CH2Cl2) from the
crystal lattice (SI, Fig. S6). Thus, the annealed polycrystalline 2 is
obtained for the EPR spectroscopy (Figure 3) and other studies,
including TGA (Figure 7), as well as for thin films (Figure 8).
Thermogravimetric analysis data suggest that thermal
decomposition of 2 starts at 160 C, at a temperature that is about
15 °C lower than that for diradical 1 (Figure 7).
C-N
S
1
S
2
S
1
S
2
292 288 284
Binding Energy (eV)
408
402
396
Binding Energy (eV)
Powder
N 1s
Powder
C 1s
292 288 284
Binding energy (eV)
408 404 400 396
Binding energy (eV)
Figure 7. Thermogravimetric analysis (TGA) of diradicals 1 and 2
under N2; heating rate = 5 °C min–1. Further details may be found
in the SI (Figs. S4-S6).
Figure 8. C 1s and N 1s core level XPS spectra of a multilayer of
diradical 2 deposited on SiO2/Si(111) wafers (top plots), compared
to the powder spectra (bottom plots).
We focus on the C 1s and N 1s spectra, because the O 1s
spectrum is a convolution of the substrate and the molecule signal
making the analysis less reliable. The film C 1s spectrum is
characterized by a main line at around 285 eV due to photoelectrons
emitted from the atoms in the aromatic ring and the carbon atoms
bound to hydrogen atoms (C-C, C-H and CH3). The shoulder at
higher binding energy is due to contributions from the electrons
emitted from carbon atoms bound also to nitrogen (C-N). Nitrogen
atoms, because of their higher electronegativity, shift the electronic
cloud. Thus, the carbon atoms bound to nitrogen atoms have
smaller electron density and, consequently, the electrons are
emitted with lower kinetic energy, i.e., higher binding energy. The
N 1s core level spectrum shows contributions due to five nitrogen
atoms: the three nitrogen atoms belonging to the Blatter radical
have different chemical environment,40,41 while the two nitrogen
atoms belonging to the nitronyl nitroxide (NN) radical have an
equivalent chemical environment. These differences are mirrored
in the spectrum by the presence of two broad features, showing the
highest intensity at around 402 eV. This binding energy
corresponds to the line expected in the NN radical N 1s core level
spectrum.42
Thin films of 2 on SiO2/Si(111) substrate. We test the robustness
of diradical 2 toward evaporation. This aspect is important in view
of the potential applications of this diradical in electronics. The
ability to attach a molecule to a contact or to evaporate it onto a
substrate, without degradation, is a requirement for device
fabrication. Controlled evaporation of diradicals is considered very
challenging, and not yet reported in literature. The presence of two
radical sites, in particular the cross-conjugated diradicals with
significant spin density within the π-system connecting two radical
sites, such as in 2, could potentially increase their instability during
evaporation.36
We prepare thin films of diradical 2 on SiO2/Si(111)
wafers by using organic molecular beam deposition (OMBD) that
allows for controlled evaporation and the consequent deposition of
molecules onto a substrate, tuning the preparation conditions in
ultra-high vacuum (UHV).37 We investigate the films by using X-
ray photoelectron spectroscopy (XPS). XPS is an effective and
powerful tool for investigation of organic and organic radical thin
films.38 Besides providing insight into the occupied states, it is
element-sensitive, and it can also deliver quantitative information
on the stoichiometric composition of the films,39 due to the high
sensitivity of the signal to the concentration of the emitting atoms.
In addition, the features contributing to the spectroscopic lines are
sensitive to the different chemical environment of the atoms of the
same element. These assets are the basis for our analysis of core
level spectra of a multilayer of 2 (Figure 8).
A best fit procedure allows identifying the contributions
from different atomic sites having slightly different binding
energies due to variations in the chemical environment.43 In
calculating the best fit, we applied several constraints based on
electronegativity, and bond strength42,43 (see also Supporting
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Information for details). We used Voigt profiles, with fixed
with an average orientation of the molecules in the film (note that
constant Lorentzian width (0.08 and 0.10 eV, for C 1s and N 1s
curves, respectively).42-44 The Voigt profile takes into account both
the finite core-hole lifetime (Lorentzian profile) and the broadening
due to the finite experimental resolution and various
inhomogeneities, e.g., molecular packing and local morphology43,45
(Gaussian profile). To calculate the stoichiometry of the films, we
also took into account the intensity of the satellites43,44 typical
features in photoemission that appear as an effect of the relaxation
processes due to the creation of a core-hole.46 Based on comparison
of the film fit results (SI: Tables S4 – S6) and the molecule
stoichiometry, we can conclude that there was no degradation of
the diradical molecules under our controlled evaporation condition.
This result is further supported by the XPS investigations
performed on the powder samples, i.e., on molecules that did not
undergo evaporation (Figure 8, bottom plots). Apart from a
broadening of the lines and small energy shifts, due to typical
charging effects occurring in organic crystals,47 the film spectra are
fully concomitant with the powder spectra. Thus, it is evident that
diradical 2 is stable and robust to be evaporated under controlled
conditions to form films of intact diradical molecules.
the NEXAFS signal is averaged on the area spotted by the photon
beam) similar to the one adopted in the single crystals and with the
crystallographic a-axis of the unit cell almost perpendicular to the
substrate (see Figure 2).
1.0
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0.6
0.4
0.2
0.0
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0.0
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Nominal Thickness (nm)
To shed light on the growth mode of diradical 2 under
this preparation conditions, we follow the XPS core level signal of
the substrate (Si 2p) by looking at its attenuation upon film
deposition (Figure 9, top panel). The curve is characterised by a
very slow decay. This intensity trend hints at a Volmer-Weber
(VW) growth mode, i.e., island growth.48 This result is consistent
with the atomic force microscopy (AFM) ex-situ images obtained
on diradical 2 films (Figure 9 middle panel) clearly showing a film
morphology dominated by islands. The VW growth mode occurs
when the the interaction between the deposited molecules is much
stronger than between the molecules and the substrate. The VW
growth of diradical 2 signifies the interacting molecules of diradical
2, possibly resembling the 1D spin-1 chain in the solid state, on the
inert SiO2/Si(111) surfaces. The line profile (Figure 9, bottom
panel), obtained averaging the AFM signal over all rows, evidences
the formation of islands of different lateral size that for big
assemblies ranges between 80 and 300 nm.
1.4
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1.0
0.8
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0.4
0.2
0.0
Near edge X-ray absorption fine structure (NEXAFS)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
spectroscopy offers the advantage to investigate in-situ not only the
electronic structure of a material, namely the unoccupied states, but
also the structural properties of very thin films. Thus, we
investigate diradical 2 thin films to determine the molecular
orientation with respect to the substrate, by using two different
polarization directions of the incident light, giving rise to NEXAFS
dichroism (Figure 10).
Lenght (mm)
Figure 9. Attenuation of the Si 2p XPS signal, normalized to the
corresponding saturation signal, as a function of film nominal
thickness, deposition at room temperature (top panel). A typical
AFM image of a 1.4-nm nominally thick film (middle panel) and
its averaged height profile (bottom panel).
We focus on the C-K edge spectra. In analogy with the
NEXAFS spectra of carbon-based molecules, the spectra in Figure
10 are characterized by two main regions: the * region up to
around 290 eV and the * region in the photon energy range above
290 eV.50 Several features are typically expected in the 286.0-287.4
eV photon range in the spectra of N-substituted aromatic carbon:51
indeed, a strong resonance is visible at 286.2 eV. This resonance is
due to transitions from C 1s levels, belonging to carbon atoms
bound to nitrogen.52-54 We also observe a very pronounced shoulder
with two small knees at 284.4 and 285.2 eV. They have a C=C
character, with the first shoulder mainly due to excitations along
the molecular backbone and the intensity at around 285 eV within
the phenyl groups.50,55-57 The features show a strong dichroic
behavior with the signal intensity at around 285 eV quenched and
the peak at 286.2 eV losing intensity in normal incidence. First, this
clearly indicate that the island aggregation is not amorphous
because in that case the signal for the two polarization directions
would overlap. Second, this kind of NEXAFS dichroism agrees
C-K NEXAFS

E
Grazing Incidence (30°)
Normal Incidence

280
285
290
295
300
305
Photon Energy (eV)
Figure 10. C-K NEXAFS spectra obtained from a 1.5-nm
nominally thick film (left panel). The spectra were taken in grazing
incidence and in normal incidence as indicated. Geometry of the
experiment (right panel).
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Finally, we monitor the stability of the films in UHV
P-1 was determined based on intensity statistics and the lack of
(base pressure 2 × 10-10 mbar) by using XPS, focusing on the N 1s
core level spectra that represent nitronyl nitroxide and Blatter
radicals.40-42 We observe no major changes in the spectra of the
films after their exposure to UHV at room temperature for 17 and
60 h (Fig. S20, SI). However, the films are much less robust in air,
as we observed major changes in their XPS after 7 h of air exposure
(Fig. S21, SI). We note that the previously studied films of nitronyl
nitroxide and the Blatter monoradical derivatives showed similar
changes in their XPS after the films were kept for several weeks
and several months at ambient conditions, respectively.40-42,58
While we have demonstrated that it is possible to evaporate
diradicals and deposit their thin films under controlled conditions
without degradation, our results indicate that the diradical films are
less stable when compared to the films of their monoradical
analogues.
systematic absences. The structure was solved and refined using the
SHELX suite of programs.64 All non-hydrogen atoms were refined
with anisotropic displacement parameters. The hydrogen atoms
were placed in ideal positions and refined as riding atoms with
relative isotropic displacement parameters. Crystal and structure
refinement data for 2 are in the Supporting Information and the
accompanying file in CIF format.
Synthesis of 2. Standard techniques for synthesis under inert
atmosphere (argon or nitrogen), using custom-made Schlenk
glassware, custom-made double manifold high vacuum lines,
argon-filled Vacuum Atmospheres gloveboxes, and nitrogen-filled
glovebags. Chromatographic separations were carried out using
normal phase silica gel. Multi-step, efficient synthesis and
characterization of the starting Blatter radical 3 is outlined in the
Supporting Information (Scheme S1).
Blatter radical 4. Starting 7-cyano Blatter radical 3 (1.605 g, 5.19
mmol) was dissolved in dichloromethane (100 mL) and cooled to
–78 C under a light N2 flow. DIBAL-H (1 M in hexane, 12.0 mL,
12.0 mmol) was then added to the solution at –78 C. The reaction
was stirred at –78 C for one hour and then warmed to room
temperature with stirring for one hour. Then, 1 M HCl (100 mL)
was added and the bilayer was stirred at room temperature for about
20 min. This caused a sizeable amount of precipitate to collect on
the walls of the round bottom flask. The bilayer was decanted,
separated, and the organic layer was shaken vigorously with
aqueous KOH. The organic layer was then dried over Na2SO4 and
evaporated (0.634 g); TLC indicated this solid to be radical 4 with
only very minor impurities. The precipitate that was formed after
HCl addition was then exposed to concentrated KOH and
dichloromethane, causing it to dissolve in the organic layer upon
mixing. TLC indicated a sizeable amount of target material 4, but
with significantly more impurities. The solvent was evaporated and
this residue purified on silica (dichloromethane eluent) to yield an
additional 0.199 g of pure radical, to provide total of 0.833 g of 4
(52% yield). IR (powder, cm-1): 3072, 3018, 2825, 2756, 2727,
1682, 1572, 1487, 1386, 1311, 1184, 1114, 1026, 829, 768. EPR
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CONCLUSION
We have synthesized an organic diradical 2, which, at
room temperature, exists nearly exclusively in its high-spin, S = 1,
ground state and it possesses a remarkable thermal stability to
permit fabrication of intact diradical thin films on silicon substrate
via evaporation under ultra-high vacuum. The diradical molecules
form islands on the substrate with molecules stacked approximately
along the crystallographic a-axis. The diradical films were found to
stable under ultra-high vacuum for at least 60 h, however, within
few hours of exposure to air, XPS of the films showed major
changes. While we have demonstrated that it is possible to
evaporate diradicals and deposit their thin films under controlled
conditions without degradation, our results indicate that the
diradical films are less stable when compared to the films of
nitronyl nitroxide or Blatter monoradicals. Polycrystalline diradical
2 consists of nearly isotropic 1-D antiferromagnetic S = 1
Heisenberg chains at low temperature. Notably, 2 possesses record
intra-chain antiferromagnetic coupling, J’/k = –14 K, among all to
date studied S = 1 chains of organic radicals,18,20 with a Haldane
gap of 0.41 × 2|J′/k|  11.5 K. The 1D chain of 2 is also most
isotropic, with very weak local anisotropy, |D/2J′|  4 × 10–4,59 and
thus is potentially an excellent system for studies of low
dimensional magnetism.60 Such diradical with an unprecedented
combination of novel magnetic and thermal properties, suitable for
thin film fabrication under ultra-high vacuum, could facilitate the
development of purely organic magnetic and electronic materials.
(X-band, 9.65 GHz, benzene): g = 2.0035, aN1 = 0.77 mT, aN2
=
0.48 mT, aN3 = 0.46 mT. HR-ESI: 313.1229, 100%, [M+H]+,
calculated for [M+H]+: 313.1215, also: 312.1136, 77%, M+,
calculated for M+: 312.1137. M.p. (DSC, 5 C/min): 212-217 C.
To further characterize radical
4 by diamagnetic NMR
spectroscopy, 4 (~4 mg) was dissolved in DMSO-d6 (~0.5 mL),
and then an excess of sodium dithionite was added to the NMR
sample. Gently heating the NMR tube (to dissolve enough sodium
dithionite to reduce the radical to leuco-triazine) caused a color
change to the characteristic yellow of the reduced radical. This
EXPERIMENTAL SECTION
Frozen solution EPR spectra were obtained using a Bruker EMX or
EMX-plus X-band spectrometer and simulated with the EasySpin
software.28 The TGA/DSC or TGA instrument (TA Instruments
TGA 550) was run either without or with IR attachment (Thermo
NICOLET Is50 NIR). Variable temperature (from 1.8 K to up to
370 K) magnetic susceptibility measurements of 1 and 2 were
performed using a Quantum Design SQUID magnetometer with
applied magnetic fields of 30 000, 5000, and 500 Oe. Variable field
(0 – 50,000 Oe) magnetization studies were carried out at
temperatures of 1.8 – 5 K. Sample tubes for SQUID studies in dilute
matrices are described in the SI.61
1
allowed for acquisition of H NMR and 13C NMR spectra for the
leuco-triazine. 1H NMR (400 MHz, DMSO-d6): 9.56 (s, 1H), 9.34
(s, 1H), 7.82 (dd, 2H, J1 = 7.8 Hz, J2 = 1.4 Hz), 7.43-7.51 (m, 7H),
7.33 (dd, J1 = 7.8 Hz J2 = 1.4 Hz), 7.22-7.18 (m, 1H), 6.86 (d, 1H,
7.6 Hz), 6.68 (s, 1H). 13C NMR (DMSO-d6): δ = 190.6, 146.4,
143.3, 140.5, 135.1, 132.2, 130.7, 130.4, 129.53, 129.34, 128.5,
125.9, 124.6, 122.2, 113.1, 107.9
Diradical 2. Note: in this procedure, the first step of reduction of
radical 4 with Na2S2O4 was omitted, that is, the second step
(condensation with bis-hydroxyamine) was run directly on the
radical. Blatter radical 4 (0.626 g, 0.46 mmol) was added to a
Schlenk vessel followed by 2,3-bis(hydroxyamino)-2,3-
dimethylbutane (0.503 g, 3.34 mmol). After purging the Schlenk
vessel with nitrogen gas, MeOH (20 mL) was added. The
suspension was heated to 70 °C in the Schlenk vessel overnight,
during which time the mixture became homogenous. Then the
X-ray crystallography. Crystals of 2 for X-ray studies was
prepared by slow evaporation from solution in DCM/cyclohexane.
Data collection was performed at the Advanced Photon Source,
Argonne National Laboratory using  = 0.41328 Å synchrotron
radiation (silicon monochromators). Final cell constants were
calculated from the xyz centroids of 9989 strong reflections from
the actual data collection after integration (SAINT).62 The intensity
data were corrected for absorption (SADABS).63 The space group
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solution was cooled and diluted approximately fourfold with ethyl
acetate, and subsequently washed twice with brine, dried, and
evaporated. The solid obtained was dissolved in dichloromethane
(300 mL), and then triethylamine (2.0 mL, 14.28 mmol) was added.
The solution was stirred overnight with a light air flow bubbling
through the reaction. The resulatant purple/red colored solution was
evaporated. The diradical was purified on silica eluting with 4:1
dichloromethane/EtOAc. Then the solid diradical was washed
sequentially with 10 mL pentane, 10 mL Et2O, and finally 10 mL
MeOH; the solid was then dried under high vacuum at 100 °C in a
chamber overnight, to remove any co-crystallized dichloromethane
and other residual solvents (0.332 g, 38% yield). IR (powder, cm-
1): 3101, 3049, 2982, 2914, 1585, 1483, 1388, 1361, 1315, 1269,
1136, 823, 777, 733. HR-ESI: 439.2010, 100%, M +, calculated for
M +: 439.2008.
ASSOCIATED CONTENT
Supporting Information
General procedures and materials, additional experimental details,
X-ray crystallographic files for 2 in CIF format, stoichiometry and
integrated XPS experimental signal intensities for the thin films of
radical, fit results for the energy positions and relative intensities of
the photoemission lines in the C 1s and N 1s spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
* Corresponding Author
arajca1@unl.edu
benedetta.casu@uni-tuebingen.de
Computational details. All geometry optimizations were carried
out at the UB3LYP/6-31G(d) level of theory, with obtained minima
confirmed by frequency calculations. The broken-symmetry
approach was applied for open-shell singlet calculations and spin
contamination errors were corrected by approximate spin-
projection method.65 Computations of an intra-dimer coupling
constant J’/k were carried out using dimers and trimers of 2 at X-
ray geometry, using broken symmetry approach at the UB3LYP/6-
31G(d) or UB3LYP/6-311++G(d,p) levels of theory.34,66 All
calculations were performed with the Gaussian 09 program suite.27
Thin film growth and XPS measurements. Thin film growth and
XPS measurements were performed in an UHV system comprising
a substrate preparation chamber and a dedicated OMDB chamber
connected to an analysis chamber (base pressure 2 x 10-10 mbar)
equipped with a monochromatic Al K source (SPECS Focus 500)
and a SPECS Phoibos 150 hemispherical electron analyser. As a
substrate, native SiO2 grown on single-side-polished n-Si(111)
wafers was used. The substrate was cleaned in an ultrasonic bath in
acetone and ethanol (one hour each consecutive bath) and then
annealed at around 500 K for 15 hours. The cleanness was verified
by XPS. Thin films of 2 were grown in-situ by OMBD using a
Knudsen cell keeping the substrate at room temperature. Powder
samples were obtained embedding the powder in a passivated
indium foil. The nominal thickness was determined by using the
attenuation of the Si 2p XPS signal of the substrate. The spectra
were measured at 20 eV pass energy, and the binding energy
calibrated to the Si 2p signal at 99.8 eV. Because of the radiation-
sensitivity of the diradical, beam exposure was minimized and a
freshly prepared film was used for each set of spectra to prevent
radiation damage. For the XPS measurements probing stability, the
set of spectra was measured on the same films upon UHV or air
exposure minimizing the acquisition time. The spectra have a
slightly worse signal-to noise ratio to preserve the intactness of the
molecules in the films.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank the National Science Foundation (NSF), Chemistry
Division for support of this research under Grants No. CHE-
1362454 (AR) and CHE-1665256 (AR) and the National Institutes
of Health (NIGMS #R01GM124310-01 to S.R. and A.R.) for the
upgrade of EPR spectrometer. NSF’s ChemMatCARS Sector 15 is
principally supported by the NSF/Department of Energy under
grant number NSF/CHE-1346572. Use of the Advanced Photon
Source was supported by the U. S. Department of Energy, Office
of Science, Office of Basic Energy Sciences, under Contract No.
DE-AC02-06CH11357. We also would like to thank Thomas
Chassé for accessing the photoelectron laboratory at the University
of Tübingen, Hilmar Adler for technical support, and Helmholtz-
Zentrum Berlin (HZB) for providing beamtime at BESSY II.
Financial support from the German Research Foundation (DFG)
under the contract CA852/11-1 and from Helmholtz-Zentrum
Berlin is gratefully acknowledged. We thank Chan Shu for
thermogravimetric studies of compounds 1 and 2, and for
determining stability of 2 in solution on air.
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