Simultaneous control of carriers and localized spins with light in organic materials

Article abstract of DOI:10.1002/adma.201203153

An organic insulating crystal reversibly becomes a magnetic conductor under UV irradiation. The rapid and qualitative change in the physical properties is wavelength selective and explained by charge transfer between donor and photochemically active acceptor molecules. The photochemical redox reaction in the crystal produces a partially filled band and localized spins simultaneously.

Full text of DOI:10.1002/adma.201203153

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Simultaneous Control of Carriers and Localized Spins with
Light in Organic Materials
Toshio Naito,* Tomoaki Karasudani, Keishi Ohara, Takahiro Takano, Yukihiro Takahashi,
Tamotsu Inabe, Ko Furukawa, and Toshikazu Nakamura
Photoconduction and its related phenomena have been known
for quite some time, and have attracted a significant amount
of scientific attention.[ Photoconduction is applied in devices,
sensors, detectors, and energy converters, such as charge-cou-
pled devices, complementary metal oxide-semiconductor image
sensors, and photovoltaic cells, in addition to other advanced
applications under investigation.[ In all known photoconduc-
tors, photoexcitation (PE) controls only the carriers. Herein,
a new photoconductor is reported, in which the carriers and
localized spins can be simultaneously controlled with UV irra-
diation. This material transforms from a semiconductor to a
metal under irradiation, distinguishing it from all existing pho-
toconductors. Furthermore, an interaction between the carriers
and localized spins has been discovered, which enables the
localized spins to be controlled and detected by the carriers and
vice versa. In addition, the photoconduction is unique because
it exhibits wavelength selectivity. The selectivity means that the
photoconduction mechanism is different from currently known
mechanism.
No material has been found in which the carriers and local-
ized spins can be simultaneously generated with light. If the
carriers and localized spins are controlled using light (i.e., via
PE), it may be possible to enhance this control over a wider
range, beyond the restrictions of thermodynamic equilibrium.
In order to find such a material, a promising starting point is
to investigate charge-transfer salts consisting of a photochem-
ical redox pair, because, under irradiation, the redox reaction
transfers a larger number of electrons between pairs, and thus
produces, by far, a larger number of carriers and localized spins
than using PE alone. Following a series of examinations of a
1]
variety of charge-transfer salts, focus was placed on salts of
−
[Ni(dmit) ]n (dmit = 1,3-dithiol-2-thione-4,5-dithiolate; 0 ≤ n ≤
2
[
4]
+ [5]
2) and methyl viologen (MVn ) (Figure 1 ). Ni(dmit) mol-
2
2,3]
ecules are known to form conducting solids with multiredox
properties, while the MV is expected to be a spin source under
irradiation because of its characteristic photochemical redox
[
6,7]
reactivity.
Herein, a strategy for realizing photomagnetic-
conductors is proposed and demonstrated based on the elec-
trical and magnetic properties of MV[Ni(dmit) ] .
2
2
Spectroscopic (Supporting Information, Figure S1) and
structural (Supporting Information, Table S1) data indicate
−
that MV[Ni(dmit)2]2 contains radical monoanions [Ni(dmit) ]
2
2+
(with a single unpaired electron) and cations MV (with no
unpaired electrons) when in the dark. (Hereafter, these entities
are designated simply as [Ni] and MV , respectively, and thus
−
2+
MV[Ni(dmit)2]2 = MV[Ni] .) The crystal and molecular struc-
2
tures are shown in Figure 1. There is only one crystallographi-
−
cally independent [Ni] with many S–S interatomic distances
shorter than twice the van der Waals radius (<3.70 Å; denoted
−
−
“short contacts” below) among the [Ni] . The [Ni] stack with
sliding and titling their molecular planes relative to each other
to achieve the closest molecular packing. As a result, the crystal
−
structure consists of several types of [Ni] zigzag columns run-
ning in different directions in the ab-plane, interconnected
−
by the short S–S contacts. Along the c-axis, [Ni] columns are
also interconnected by the short S–S contacts. Therefore, the
Prof. T. Naito, T. Karasudani, Prof. K. Ohara
Graduate School of Science and Engineering
Ehime University
Bunkyo-cho, Matsuyama 790-8577, Japan
E-mail: tnaito@ehime-u.ac.jp
−
[
Ni] arrangement can be better described as a quasi-3D closely
packed network, instead of a columnar structure. In this way
−
the radical anions [Ni] form rather isotropic conduction path-
2
+
ways in MV[Ni] . On the other hand, the MV cations are
2
−
inserted into small spaces in the network, surrounded by [Ni] .
T. Takano, Dr. Y. Takahashi, Prof. T. Inabe
Graduate School of Science
Hokkaido University
Kita 10, Nishi 8, Kita-ku, Sapporo, 060-0810
T. Takano, Dr. Y. Takahashi, Prof. T. Inabe
Japan, Graduate School of Chemical Sciences and Engineering
Hokkaido University
Kita 10, Nishi 8, Kita-ku, Sapporo 060-0810, Japan
Dr. K. Furukawa, Prof. T. Nakamura
Institute for Molecular Science
−
2+
The [Ni] anions surrounding an MV cation prevent it from
directly interacting with any ions other than its immediate [Ni]−
neighbors. Thus, assuming that the crystal structure remains
unchanged under irradiation, an unpaired electron on a pho-
toexcited MV2 (MV *) is expected to behave almost wholly as
+
2+
2+
a localized spin on MV *. In addition, this crystal structure
favors interactions between unpaired electrons on the pho-
toexcited [Ni] ([Ni] *) (the carriers) and those on MV * (the
localized spins). This is because [Ni] and MV are similar in
size and planar in shape, but have opposite charges, so that the
closest molecular packing is achieved in a face-to-face arrange-
ment. This maximizes the overlap between their π-molecular
orbitals, in which the carriers and localized spins are accom-
modated upon irradiation. In the dark, the calculated band
−
−
2+
−
2+
Myodiaji, Okazaki 444-8585, Japan
Dr. K. Furukawa, Prof. T. Nakamura
The Graduate University for Advanced Studies
Myodiaji, Okazaki 444-8585, Japan
DOI: 10.1002/adma.201203153
Adv. Mater. 2012,
DOI: 10.1002/adma.201203153
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Figure 2. a) Photocurrent Iph of MV[Ni] under UV irradiation (375 ± 5 nm;
2
−2
≈
3.3 eV) in vacuo: Iph response to UV irradiation (11.6 mW cm ) at 300 K.
b) Light-intensity dependence of the (photo)current ratio (Iph0/Idark): ᭺ at
00 K; • at 300 K.
2
thermally activated carriers are responsible for the conduc-
tion. At room temperature (RT) and in the dark, the resistivity
5
is 2.0 × 10 Ω cm and the activation energy is 0.28 ± 0.03 eV
(
≈3250 ± 350 K). Such highly insulating properties are desirable
for photoconductors, as they impart a high on/off ratio (i.e., the
ratio between the dark conductivity and the photoconductivity).
Figure 2a shows the Iph response to UV irradiation (375 nm;
3
.3 eV) at RT, where Iph denotes the photocurrent at a given
time/light intensity. A rapid, clear, and stable response was
observed. Upon UV irradiation, the photocurrent Iph rapidly
increases and saturates. Figure 2b shows the light-intensity
dependence of the ratio of the saturated to dark photocurrent
0
(
Iph /Idark). This ratio increases rapidly with increasing light
−
2
intensity, and reaches 1000 at 200 K and 15.7 mW cm . If
the photoconduction is a result of the UV-irradiation heating
effect, then the dark resistivity behavior (Supporting Informa-
tion, Figure S4) requires a temperature jump (ΔT) of ≈130 K,
which a single crystal of MV[Ni] cannot endure, as it would
2
break apart. In addition, the temperature dependence of the
conduction under UV irradiation is qualitatively different from
that under dark conditions (see below). This indicates that the
photoconduction originates from a qualitatively different elec-
tronic state: semiconducting in the dark vs. metallic under
Figure 1. a) Molecular structures of Ni(dmit) and MV. The colored spheres
indicate sulfur (yellow), nickel (pale green), carbon (blue), hydrogen
2
(white), and nitrogen (pink) atoms. b–d) Crystal structures of MV[Ni]2.
0
structure (Supporting Information, Figure S2), the magnetic
susceptibility (Supporting Information, Figure S3), and the
electrical resistivity (Supporting Information, Figure S4) indi-
UV irradiation. This on/off ratio (i.e., the maximum Iph /Idark)
is comparable to those currently used in practical applica-
tions. Interestingly, MV[Ni]2 exhibits photoconductivity exclu-
sively when irradiated by light with a wavelength in the range
cate that the ground state of MV[Ni] has no carriers, and that
2
2
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≈
250–450 nm. In other words, unlike existing photoconductors,
the photoconductivity of MV[Ni]2 exhibits wavelength selec-
tivity. Known photoconductors generally exhibit photoconduc-
tivity whenever they are irradiated with light having a photon
energy which exceeds the band gap. The absorption bands of
2
+
−
both MV and [Ni] lie in the UV region (Supporting Informa-
2
+
tion, Figure S1). The PE of MV considerably enhances redox
reactions[ such as (MV + X)* → MV+ + X . Some of the
6]
2+
+
optical transitions in the UV region in MV[Ni] are assigned
2
to intermolecular LUMO → (LUMO + n) (n ≥ 2) transitions
−
between the [Ni] species. However, the (LUMO + 2) and higher
levels of Ni(dmit) are practically localized levels in this mate-
2
rial, unable to provide the high electrical conduction observed.
Accordingly, UV irradiation does not appear to produce such
an evident Iph in this material unless an active role of the MV
is taken into consideration. The UV irradiation brings about
2
+
−
the charge transfer (CT) transitions between MV and [Ni] , as
−
well as those between the [Ni] themselves (Supporting Infor-
mation, Figure S1a). The latter CT transitions are also possible
upon UV irradiation of (n-C H ) N[Ni(dmit) ], which, however,
4
9 4
2
hardly produces any photoconductivity (Supporting Informa-
tion, Figure S5). Therefore, the discussion thus far indicates a
strong connection between a large Iph and cation-anion inter-
actions (i.e., the importance of the CT transition between the
cation and the anion). The CT transitions in this material have
the character of a solid-state (reversible) photochemical redox
reaction. Because many bands around the Fermi level (EF)
Figure 3. ESR spectra during UV irradiation measured on aligned single
crystals of MV[Ni]2 at RT. a) First derivative of the spectrum after sub-
tractingthespectrummeasuredunderdarkconditionsonthesamesample
prior to irradiation. b) Absorption spectrum obtained by integrating the
contain contributions of both the MV and Ni(dmit) molecular
2
orbitals with different mixing ratios, the interband transitions
of a few eV (UV region) involve some charge transfer between
the donor and acceptor molecules (Supporting Information,
Figure S6). Since the unit cell contains as many as two MV and
spectrum in (a) (
); deconvolution of the two main peaks (
).
,
);
total contribution of the two deconvoluted peaks (
four Ni(dmit) molecules (Z = 2), and since there are signifi-
MV[Ni] is electron-spin-resonance (ESR) silent in the dark
2
2
cant interactions among them, there are many bands around
EF that are nearly degenerate to each other (Supporting Infor-
mation, Figure S2). This enables various CT absorption bands,
which are observed as an envelope of a series of broad bands
extended in the entire UV region (Supporting Information,
Figure S1a). The large sum of the oscillator strengths (≈0.24)
in the UV region suggests a large number of electrons trans-
at all temperatures (1.2–300 K). However, under UV irradia-
tion, a complex signal is clearly observed in the ESR spectrum
(Figure 3 ), which is consistent with an electrical response to
UV irradiation. The line shape indicates that the spectrum con-
tains two contributions and some additional fine structures.
This is consistent with the UV spectrum (Supporting Informa-
tion, Figure S1), which contains several absorption peaks due
ferred between MV2 and [Ni] . The amount of charge transfer
under UV irradiated conditions can be roughly estimated from
a band calculation and the UV spectra: δ ≈ 0.02 by applying
Equation S1 (see Supporting Information, below Figure S6) to
+
−
−
2+
to both [Ni] and MV . To reveal the essential characteristics of
the ESR spectrum, it was deconvoluted into two contributions.
By a curve-fitting analysis, the g-values were found to be g =
1
1.9923–2.0043 and g = 2.0677–2.0687. Generally, the unpaired
2
−
+
the bands located at ≈2.75–5 eV (250–450 nm) above E . This
electrons on [Ni(dmit)2] and MV exhibit ESR signals at g =
F
−
indicates a decrease of ≈2% in the charge on the [Ni] under
2.04–2.07 (Supporting Information, Figure S7) and g ≈ 2.003,
(
1–δ)–
[8,9]
UV irradiation (δ ≈ 0.02 in [Ni(dmit)2]
). This type of CT
respectively.
Based on these g-values, the primary contribu-
transition is essentially similar to the optical transitions across
band gaps in standard photoconduction. However, the number
of electrons transferred is characteristically larger than any
other type of optical transition, and might account for the larger
tions to the observed signals are assigned to unpaired electrons
on MV2 (g ) and on [Ni] * (g ). The line widths support this
+*
−
1
2
assignment, since delocalized (conduction) electrons gener-
ally exhibit larger ESR line widths than localized spins. Both
−
*
2+*
photoconduction than other Ni(dmit) complexes without such
[Ni] and MV have much-larger ESR line-widths than their
related compounds (Supporting Information, Figure S7 and
2
photoredox-active cations. This is consistent with a band calcu-
lation (Supporting Information, Figure S2), which shows that
a decrease in the number of electrons partially fills the lowest
unoccupied molecular orbital (LUMO) band of [Ni]. Accord-
[
5]
work by Bird and Kuhn ), so they overlap. These facts suggest
that the unpaired electrons on MV * and those on [Ni] * may
interact. To confirm this interpretation, the ESR spectra were
simulated by assuming one-electron spin (S = 1/2) per MV *
and [Ni] *. The simulated ESR spectra reproduce the primary
2+
−
−
2+
2+
ingly, photoinduced electron transfer between [Ni] and MV
may be responsible for the large Iph 0, as well as the wavelength
−
selectivity.
features of the observed spectra (Figure 3) well, supporting this
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DOI: 10.1002/adma.201203153
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Yield: 2.30 g (52%); IR (KBr): ν = 3016 (vs), 1638 (vs), 815 cm− (vs)
1
(
for details, see Supporting Information, Figure S1); UV–vis (CH CN):
3
λmax (ε) = 206 (sh; ≈37 000), 248.5 nm (35 500); Anal. calcd for
C H N I : C 32.40, H 3.16, N 6.31, I 57.62; found: C 32.00, H 2.68, N
1
2
14 2 2
6
.26, I 58.35.
Single crystals of MV[Ni(dmit)2]2 were obtained from a diffusion
method of MV·I2 (30 mg) and [n-(C H ) N][Ni(dmit) ] (80 mg) in dry
4
9 4
2
acetonitrile (Wako; 60 mL) solution under a N atmosphere in an H-tube
2
with fine glass frits (G4) for separation of each compartment. The
solution stood in the dark for 10 d at room temperature (RT) to yield
diamond-shaped shiny black platelets of 0.5–1.0 mm in size.
−1
IR (KBr): ν = 1638 (w), 1344 (vs), 1056 (vs) cm (for details, see
Supporting Information, Figure S1); Anal. Calcd for C H N Ni S20:
24
14
2
2
Figure 4. Temperature (T) dependence of resistance (R) of MV[Ni] under
C 26.46, H 1.30, N 2.57, S 58.89; found: C 26.17, H 1.52, N 2.52, S 58.92.
In the following measurements on the single crystals, except for the
magnetic susceptibility, all of the single crystals were briefly checked
by X-ray oscillation photographs to identify the crystal quality and the
directions of the crystallographic axes.
2
UV irradiation showing the linear variation of R vs. logT at low T, which
is characteristic of carriers interacting with localized spins (the Kondo
effect).
interpretation. Additionally, a calculation of the overlap integrals
of the molecular orbitals (Supporting Information, Table S2,S3)
Irradiation Method: Various light sources with or without band-pass
filters (Δλ = 40–60 nm) and/or optical fibers were used to examine
the photoresponses in the physical properties and their wavelength
dependence: a high-pressure Hg/Xe lamp (250–450 nm) (SAN-EI
Electric, SUPERCURE-203S; 200 W), a Xe lamp (200–400 nm) (Asahi
2+
suggests that the localized spins on MV * and the carriers on
−
[
Ni] * interact. To obtain further convincing evidence for the
2+
interaction between the unpaired electrons on MV * (localized
spins) and on [Ni] * (carriers), the temperature-dependence of
Spectra, LAX-Cute; 100 W), a D /tungsten-halogen lamp (200–1100 nm)
2
−
(Hamamatsu Photonics K. K., L7893; 30 W (D ) & 5 W (halogen)), a
2
the resistivity under UV irradiation (Figure 4 ) was measured.
In contrast with the observed semiconducting behavior (∂T < 0;
high-power tungsten-halogen lamp (300–2600 nm) (Spectral Products,
∂
R
model ASBN-W100F-L; 100 W), and a laser (375 ± 5 nm) (NEOARK;
2
0 mW). The actual light power of a particular wavelength under
R = resistance, T = temperature) in the dark, MV[Ni] exhibits
2
ꢀ
ꢁ
each experimental condition was measured using a Si-diode power
meter (OPHIR, NOVA). Similarly, some of the lamps with filters and a
fiberscope above were used to examine the spectral changes in the ESR.
The details of the specifications of the light sources used in this work are
∂
∂
R
T
clear metallic behavior
> 0 above ≈200 K under UV irra-
diation. However, R increases linearly with logT when T is
reduced below ≈100 K. This behavior can be explained by the
Kondo effect.[ Electronic systems composed of carriers and
localized spins characteristically exhibit this effect at low tem-
perature due to the interaction between carriers and localized
spins. The Kondo effect is typically observed in metallic elec-
tronic systems containing dilute localized spin systems. Since
UV irradiation excites only the MV2 cations at the crystal sur-
10]
[23,24]
described in our previous papers.
Physical-Property Measurements: Single-crystal X-ray structural analysis
and magnetic susceptibility measurements were performed under dark
conditions. The details of the structural analysis are described in the cif
file deposited to the Cambridge Crystallographic Data Centre (CCDC).
Electrical resistivity and ESR measurements were performed both
under irradiation and in the dark. During the resistivity measurements
performed under irradiation, the sample temperature was monitored
using a Si-diode thermometer on which the sample was mounted;
both the sample and the thermometer were simultaneously irradiated.
Because of the high electrical resistivity under dark conditions, and for
the sake of ensuring an effective irradiation area, as wide as possible,
on the crystal surface, we applied a two-probe method (suitable for
samples with high resistance) instead of the standard four-probe
method (suitable for samples with low resistance). Because the accurate
penetration depth of the UV light was difficult to measure, and because
one could not discuss the absolute values of resistivity by the two-probe
method in the case of metallic samples, the resistivity under irradiation
is shown by an arbitrary unit in Figure 4. In the ESR measurements, the
background signal (originating from the cavity, the quartz sample tube,
and a polytetrafluoroethylene (Teflon) piece used to mount the sample)
was measured under both dark and irradiated conditions at RT and 77 K.
During UV irradiation in the ESR measurements, the temperature near
the sample was monitored using a Si-diode thermometer in the same
sample tube in an independent measurement (Supporting Information,
Figure S8). To estimate the effect of heating during UV irradiation,
we also measured the ESR spectra at 313 K, which was approximately
the temperature of the sample during irradiation at RT; no signal was
observed in this measurement. Further details of the experiments are
described in the Supporting Information. The Supporting Information
also describes the parameters (Supporting Information, Table S2) and
other details of the band dispersion and related calculations.
+
face to produce a dilute localized spin system in MV[Ni] , the
2
observed behavior of R is consistent with the Kondo effect, as
well as all of the data presented above.
In conclusion, it has been established that PE reversibly gen-
erates a metallic state with localized spins, and that the carriers
and localized spins interact in the photoexcited state of MV[Ni]2.
This is related to charge transfer, corresponding to a photochem-
ical partial redox reaction between MV2 and [Ni] ions, resulting
+
−
−
in doping (carrier injection) in the Ni(dmit) band; [Ni]
→
2
(1–δ)−
[Ni]
(δ ≈ 0.02). Organic materials are currently considered to
be the most-promising materials for realizing spintronics. Recent
advances in related studies have been remarkable, yet there still
remains much to be explored about the process of spin injection
into organic materials.[
11–21]
This novel type of photoconductor
provides an alternative pathway to organic spintronics.
Experimental Section
Sample Preparation: All of the chemicals were purchased as the
highest grade and used as received. [n-(C H ) N][Ni(dmit) ] was
4
9 4
2
synthesized according to the literature.[
22]
MV·I2 was prepared by
treating 4,4′-bipyridyl (Tokyo Chemical Industry, 1.56 g: 0.01 mol)
with a large excess of methyl iodide (Wako Pure Chemical Industries,
CCDC 881155 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2
8.0 g: 0.20 mol) in refluxing ethanol (Wako, 50 mL) for 3 h, followed
by recrystallization from hot methanol (Wako). Reddish orange needles
were produced.
4
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DOI: 10.1002/adma.201203153

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www.MaterialsViews.com
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