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Topology and spin alignment utilizing the excited molecular field in π-conjugated organic spin systems
Polyhedron 24 (2005) 2299–2308 www.elsevier.com/locate/poly
Topology and spin alignment utilizing the excited molecular field in p-conjugated organic spin systems Yoshio Teki
*
Department of Material Science, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Received 5 October 2004; accepted 5 January 2005 Available online 5 July 2005
Abstract The design and experimental investigations of photo-induced high-spin organic systems (the photo-excited quartet (S = 3/2) and quintet (S = 2) states) is reviewed with focusing p-conjugated organic spin systems. In order to study the photo-induced spin alignments on the excited states, the photo-excited high-spin states of p-conjugated stable radical systems and their p-topological isomers were studied by the time-resolved ESR experiments. The relationship between the p-topology and spin alignment on the photoexcited states is clarified. The mechanism of the photo-induced intramolecular spin alignment and the role of the spin polarization and spin delocalization are revealed with the help of the molecular orbital calculations. One of the key processes for the photo-control of the organic molecular magnetism is established. The guiding principle designing the photo-excited high-spin system and the role of p-topology are clarified. Potential developments toward the functional materials are also proposed utilizing the p-conjugated organic spin systems with the photo-excited high-spin states. 2005 Elsevier Ltd. All rights reserved. Keywords: p-Topology; Photo-induced spin alignment; High-spin excited state; Time-resolved ESR; p-Conjugated organic spin system; Application to functional materials
1. Introduction This review deals with the spin alignment of high-spin organic systems especially with focusing photo-excited high-spin states of p-conjugated stable radicals or biradical systems and their topological isomers [1–7]. We define ‘‘p-topological isomer’’ as a molecule in which differs from others only in the topology of its p electron network, i.e. in the linking positions of its p bonds. p-Conjugated spin systems arising from the aromatic hydrocarbons and the dangling stable radicals are ideal model systems in order to study the relationship between the p-topology and spin alignment on the photo-excited states. The delocalized p orbital network is most important for determining the spin states as shown later. *
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0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.03.115
The spin states on the ground states were determined by the electron spin resonance (ESR). The photo-induced spin alignment and the photo-excited states of p-conjugated spin systems were investigated by the time-resolved ESR (TRESR) technique. The spin density distributions provide the most direct information about the spin correlation among the unpaired electrons and the mechanism of the spin alignment in these organic molecules. The mechanism of the spin alignment was clarified in viewpoint of the spin density distributions obtained by ab initio molecular orbital calculations using the density functional theory (DFT). As well known, the spin polarization effect according to the p-topology plays an important role of the spin alignment on the ground state [8,9]. On the photo-excited state it is unknown whether the similar p-topological rule works or not. In order to clarify the spin alignment in the photo-excited states of the p-conjugated organic spin
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spin systems [1–7], which are generated by a spin alignment through p-conjugation between dangling stable radicals (S = 1/2) and the excited triplet (S = 1) state of aromatic hydrocarbons. The p-conjugation leads to the strong exchange coupling between the triplet photo-excited state of the aromatic hydrocarbons and the dangling radicals. In the biradical systems, 5–8, the stable radicals couple strongly through the spin coupler by p-conjugation making the robust spin alignment possible. p-Conjugated spin systems have the following advantages in view point of the material science and molecular magnetism: (1) p-Conjugation leads to the strong exchange coupling which is one of the important conditions for the balk magnetism at finite temperature. (2) One can well design and synthesize the proper spin systems with the desired spin states using the well-established fine-synthetic chemistry. (3) An enhanced intersystem crossing (ISC) mechanism arising from the attachment of the radical species is available [2,6]. This leads to open a way of the direct ESR detection of the silent photo-excited states with little ISC to the triplet state. Actually, since phenyl or diphenylanthracene has very low efficiency of the ISC toward the triplet state, the triplet states can be not directly detected by TRESR. However, by attaching the radical species through p-conjugation, the excited high-spin states arising from the triplet state of phenyl or diphenylanthracene and the doublet state of radical were detected by TRESR as shown later. The photo-induced intra-molecular spin
systems, we have designed, synthesized the novel p-conjugated stable radical or biradicals, 1–8 shown in Fig. 1 and characterized their spin alignments [1–7]. The knowledge of photo-induced spin alignment on the excited states will give the guiding principles designing photo-induced/switching magnetic spin systems and the photo-control of the molecular magnetism. These studies will open the new strategy for the photomagnetic devices based on the organic materials. Intramolecular spin alignment and exchange interaction through p-conjugation are quite important in the field of the molecular magnetism [10]. However, most studies are limited to ground state systems. Although radical (R) and excited-triplet (T) pairs in solution have been extensively investigated by TRESR technique, the direct detections of the excited high-spin states (S P 3/2) arising from the radical–triplet pair are still limited in the solid phase. The first direct TRESR detection of an excited quartet state of a radical–triplet pair was reported for a fullerene–mononitroxide radical system in solution [11] and for a tetraphenyl-porphilinatozinc (II) (ZnTPP) coordinated by p-pyridyl nitronylnitroxide (nitpy) system in a solid phase [12]. For homologous systems [13–15], excited high-spin states have been so far reported. However, in these systems, a stable radical couples weakly with the excited triplet chromophore through sigma bonds or coordination, in which the spin exchange interactions are small in magnitude. We reported the first observation of excited quartet (S = 3/2) and quintet (S = 2) states in purely organic p-conjugated
CH3
O
N N
O N
CH3
O N
N
1
2
N
N
N
N
N N
O
3
N
4
CH3
O N
N N O
O
N
N N
N
5
N O
6
N N
H3C
O O N
7
N
CH3 N
N
N N
N
N
N N
O H3C
O
8
CH3
Fig. 1. p-Conjugated organic spin systems in which the photo-excited states were investigated.
CH3
Y. Teki / Polyhedron 24 (2005) 2299–2308
alignment through p-conjugation is one of the key processes for the photo-control of the molecular magnetism.
2301 OHC B(OH)2
Br
CH2OH
Br (PPh3)4Pd, K2CO3
HOH2C HO
NHOH NHOH
2. Experimental
N OH N
2.1. Materials
OH O
1. NaIO4
Typical synthetic procedures of monoradical and biradical are shown in Schemes 1 and 2, respectively. 2,3-Bis(hydroxyamino)-2,3-dimethyl-butane was prepared according to the literature method [16]. Other reagents were used as purchased.
The photo-excited states were examined by TRESR. A conventional X-band ESR spectrometer (JEOL TE300) was used in the measurements of TRESR spectra without field modulation. Excitation was carried out at 355 nm light using Nd:YAG pulse laser (Continuum Surelite II-10, pulse width <7 ns). The typical laser power used in the experiments was ca. 2–5 mJ. EPA, 2-MTHF, or isopentane–diethylether mixed glass matrix was used for the TRESR experiments. The measurements were carried out at 30–40 K. The typical microwave power was ca. 10 mW. The spectral simulation was carried out by the eigenfield/exact-diagonalization hybrid method [16,17], taking dynamic electron spin polarization (ESP) into account [2]. The effective-exchange interaction between the triplet state of the aromatic hydrocarbon moiety and the dangling radicals is undoubtedly much larger than the Zeeman and fine-structure interactions in the p-conjugated systems. We have, therefore, used the following ordinary spin Hamiltonian of a pure spin state given in Eq. (1) (negligible quantum mixing of the different spin states) for the analysis: H 0spin ¼ be H.g.S þ S.D.S.
ð1Þ
The resonance field BMs M Ms + 1(h, /) for each transition was directly calculated by solving the following eigenfield equation [17]:
NHOH NHOH
B(OH)2 Br
CHO
(PPh3)4Pd, K2CO3 benzene-EtOH-water
N
O N
2. NaNO2, HCl
N N
Scheme 2. Typical synthetic procedures of biradicals.
A Z ¼ BC Z;
2.2. TRESR measurements and simulation
OHC
N
HO N
HO
THF-MeOH
N N OH
where A and superoperators:
ð2Þ C
are
given
by
the
A ¼ xE E F E þ E F
following ð3Þ
and C ¼ G E E G .
ð4Þ
Here, E is a unit matrix and x is the given microwave frequency. The operators G and F are the field dependent and independent parts of the spin Hamiltonian (1), respectively. By solving the eigenfield equation (Eq. (2)), the resonance field B is obtained. The transition probabilities I(h, /, u) were evaluated by numerically diagonalization of the spin Hamiltonian matrix at the calculated resonance eigenfield [18,19]. Since the resonance field is independent of the third Euler angle, u, the above procedure practically saves the computing time for the simulation. The line-shape function of the TRESR spectrum in the glass matrix is given by Z Z XZ gðBÞ ¼ N du d/ dh sin hP Ms$Msþ1 ðh; /Þ Iðh; /; uÞf ½B BMs$Msþ1 ðh; /Þ.
ð5Þ
In the simulation, the dynamic electron polarization (ESP), PMs M Ms + 1(h, /), on each spin sublevel in zero magnetic field was given as parameters. Thus, the relative populations of the Ms sublevels was taken into account as parameters. The simulation was carried out using a program written by the author on a personal computer. The details of the simulation procedures for the high-spin TRESR spectra were described in our previous paper [2].
73 %
O
1, NaIO4 / MeOH-water
3. Results and discussion N N
2, NaNO2, HCl / CH2Cl2-water
Scheme 1. Typical synthetic procedures of monoradical.
3.1. Photo-excited quartet high-spin states Typical TRESR spectrum of 1 is shown in Fig. 2(a) [1,2]. The observed TRESR spectrum with well resolved
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Y. Teki / Polyhedron 24 (2005) 2299–2308
doublet states. The g and D values for the quartet state (Q) are given by the following relationship [20]:
a
gðQÞ ¼ ð2=3ÞgðTÞ þ ð1=3ÞgðRÞ
Abs.
ð7Þ
and XY1
Emi. Abs.
DðQÞ ¼ ð1=3ÞfDðTÞ þ DðRTÞg.
b
Here, D(T) is the D tensor of the excited triplet moiety, D(RT) is the dipolar interaction between the dangling radical spin and the triplet moiety. Using these equations g and D values were calculated as shown in Table 1. The experimentally determined g, D and E values for the excited quartet state are in excellent agreement with the calculations.
Z1
Z3
Emi. XYZ2 A
250
XY3
300 350 400 Magnetic Field / mT
Fig. 2. Typical TRESR spectra of 1 at 30 K in EPA rigid glass matrix. (a) The observed spectrum of 1 at 0.8 ls after laser excitation. (b) The simulated spectrum of the excited quartet state.
fine structure splitting has been unambiguously analyzed to be a excited quartet (S = 3/2) state (Q) by the spectral simulation shown in Fig. 2(b). The determined g value, fine-structure parameters, and relative populations of the Ms sublevels are listed in Table 1. Similar excited quartet states were observed for 3 [5] and 4 [6]. The typical TRESR spectrum observed for the pyrene–verdazyl radical, 4, is also shown in Fig. 3. In these quartet systems the triplet state of the aromatic hydrocarbon moiety couples with the doublet state of the dangling stable radical by the spin-exchange interaction. In such a exchange-coupled system the wavefunction of the whole molecule |W(S, M)æ is approximately given by the direct product of the wavefunctions of the two isolated moieties, |T(SA, mA)æ and |R(SB, mB)æ, as jWðS;MÞi ¼
X
ð8Þ
CðS A S B S;mA mB MÞjTðS A ;mA Þij
RðS B ;mB Þi; for S A ¼ 1 and S B ¼ 1=2; ð6Þ where C(SASBS; mAmBM) is the Clebsch–Gordan coefficient. The radical–triplet pair has one quartet and one
3.2. Excited spin states of the p-topological isomers of 1 In the ground state p-topology plays very important role in the spin alignment of the alternant hydrocarbons. Spin polarization effect governs the spin alignment and the spin multiplicity of the ground state depends drastically on the p-topology. In order to test the role of the p-topology on the spin alignment in the photo-excited state, TRESR experiments were performed for 2 [2], which is the p-topological isomers of 1. In contrast to the result of 1, no TRESR signal attributable to the high-spin excited state was observed for 2 and only a very weak absorptive signal due to the doublet state was observed as shown in Fig. 4. This agrees with the p-topological rule, indicating that the lowest excited state of 2 is a low-spin doublet (S = 1/2) spin state and the sign of the intramolecular exchange between the dangling radical spin and the excited triplet spin of the anthracene moiety is antiferromagnetic according to the spin polarization pathway through the dangling phenyl group. This finding shows that the sign of the intramolecular exchange interaction and the spin multiplicity of the lowest photo-excited spin state change drastically depending on the topological nature of p-electron network. Thus, the p-topological rule of the spin polarization established for the ground states of the aromatic hydrocarbons also plays an important role in the spin alignment on the photo-excited states.
Table 1 Spin Hamiltonian parameters (S, g, D and E), relative populations of the Ms sub levels of the zero-field eigenfunctions, and the estimated D values Molecule 1 3 4 5 6 8
S 3/2 3/2 3/2 2 1 2 2
g
D (cm1)
|E| (cm1)
Relative population
gest
Dest (cm1)
Ref.
2.0043 2.0040 2.0035 2.0043 2.0045 2.0035 2.0035
0.0235 0.0230 0.0290 0.0130 0.0360 0.0125 0.0128
0.0 0.0 0.0090 0.0 0.0 0.0 0.0
P 03=2 ¼ 0.0; P 01=2 ¼ 0.5 P 03=2 ¼ 0.0; P 01=2 ¼ 0.5 P 03=2 ¼ 0.0; P 01=2 ¼ 0.5 P 02 ¼ 0.0; P 01 ¼ 0.35; P 00 ¼ 0.30 PX = PY = 0.0, PZ = 1.0 P 02 ¼ 0.0; P 01 ¼ 0.0; P 00 ¼ 1.0 P 02 ¼ 0.0; P 01 ¼ 0.37; P 0 ¼ 0.26
2.0040 2.0032 2.0033 2.0045 2.0045 2.0050 2.0035
0.0256 0.0248 0.0310 0.0137
[1,2] [3,5] [6] [1,2] [7]
0.0120 0.0130
[5]
Here, PMs 0 is the relative population of the spin sublevels in zero magnetic field, which magnetic quantum number corresponds to the value, Ms, in the high field limit.
Y. Teki / Polyhedron 24 (2005) 2299–2308
2303
Stable Radical
a
R
R
Abs.
J1
Laser on off
Ground-State Singlet Spin Coupler
Emi.
J2
R
R
Photo-Excited Triplet Spin Coupler
Fig. 5. Photo-induced intramolecular spin alignment.
D
b Abs.
Emi.
250
300 350 Magnetic Field / mT
400
Fig. 3. Typical TRESR spectrum of pyrene–verdazyl radical, 4.
3.3. Photo-induced spin alignment in diphenylanthracenebis(radical) systems In order to clarify the photo-induced spin alignment using the photo-excited molecular field, we investigated the photo-excited spin states of diphenyanthracenebis(radical) systems which are illustrated in Fig. 5. In this spin system, the diphenylanthracene spin coupler between two dangling stable radical moieties becomes the photo-excited triplet state with two unpaired electrons upon the photo-excitation. Two dangling radicals are interacted with each other through the triplet spin coupler. The total spin Hamiltonian of the four spin system is given by H spin ¼H ex þ be H g T S T þ be H gR1 S R1 þ be H gR2 S R2 þ S T DðTÞ S T þ S R1 DðR1 TÞ S T þ S R2 DðR2 TÞ S T ; H ex ¼ 2J 1 S
R1
T
T
R2
S 2J 2 S S .
ð9Þ ð10Þ
Here, we omitted the dipolar interactions between two radicals moieties. The p-conjugated spin systems, the exchange interaction is much larger than other interactions in Eq. (9). The energies of each spin state in zero magnetic field were obtained to zero-th order by diagonalizing Hex. The expectation values of the fine-structure terms of each spin states were calculated by using the eigenfunctions. There are two kinds of the triplet states (T1 and T2), one quintet state (Qu) and one singlet state (S). The energy versus J2/J1 diagram for each spin states are shown in Fig. 6 [7]. The fine-structure terms (D) of each spin states are related to the fine-structure tensor (D(T)) of the triplet state of anthracene by a reduction factor (f) (D = f Æ D(T)). The reduction factor of each spin states are shown in Fig. 7 [7]. In this calculation, we omitted D(R1T) and D(R2T) in Eq. (10), because D(T) is much larger than them. As already shown in Sections 3.1 and 3.2, para-joint configuration between the iminonitroxide radical and the triplet excited state of the anthracene moiety leads to the ferromagnetic coupling and meta-joint configuration is antiferromagnetic coupling between them. Therefore, para-joint biradical 5 is expected to have a quintet state (S = 2) as the lowest photo-excited state and meta-joint biradical 7 has a singlet state (S = 0). An interesting lowest photo-excited triplet state arising from four unpaired electrons will be realized in 6 by the antiferromagnetic spin exchange coupling of the two dangling radical spins through the photo-excited triplet spin coupler (diphenylanthracene moiety). In order to prove this prediction, TRESR experiments were carried out for the p-topological isomers, 5–8 as shown later.
4.0
S
J1> 0
T2
Energy / J1
2.0 Abs.
T1
0.0
Emi.
Qu
-2.0 250
300
350
400
Magnetic Field /mT Fig. 4. Typical TRESR spectrum of 2 at 30 K in EPA rigid glass matrix.
-1.0
-0.50
0.0 J /J 2
0.50
1
Fig. 6. Energy vs. J2/J1 diagram.
1.0
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Y. Teki / Polyhedron 24 (2005) 2299–2308
Reduction Factor (f)
D(T1)
D(Qu)
D(T2) -1.0
-0.50
0.0
0.50
1.0
J2 / J1
Fig. 7. J2/J1 Dependence of the reduction factor.
3.3.1. Photo-excited quintet high-spin states Typical TRESR spectrum of the photo-excited quintet (S = 2) state (Qu) of the biradical system, 5 [1,2], is given in Fig. 8 together with the spectral simulation. The spin Hamiltonian parameters are also listed in Table 1. Similar excited quintet state was observed for a verdazyl biradical, 8 [5]. All the determined spin Hamiltonian parameters and relative population of the Ms sublevels of these excited high-spin states are summarized in Table 1. According to a similar manner described in the case of the quartet state, the g, D and E values of the quintet state (Qu) have been estimated using the following equations, which are listed in Table 1: gðQuÞ ¼ f2gðTÞ þ gðR1 Þ þ gðR2 Þg=4
ð11Þ
and DðQuÞ ¼ ð1=6ÞðDðTÞ þ DðR1 TÞ þ DðR2 TÞ þ DðR1 R2 ÞÞ. ð12Þ
Abs. a XY2
Emi. XY1
Abs.
Z2 Z1
b
Z4
Emi.
Z3 XY4
XY3
250
300 350 Magnetic Field /mT
400
Fig. 8. Typical TRESR spectra of 5 at 30 K in 2-MTHF rigid glass matrix. (a) The observed spectrum of 5 at 0.8 ls after laser excitation. (b) The simulated spectrum of the excited quintet state.
Here, D(R1R2) is the dipolar interaction between the two radical spins. This was actually neglected in the estimation because of the small magnitude. The estimated values of the fine-structure parameters are quite well in agreement with the observed values, showing that the observed quintet state is a high-spin species originated from the triplet spin coupler and two dangling radicals. The direct observations of the excited quintet states show that the photoinduced intramolecular spin alignment is realized between the excited triplet state (S = 1) of the diphenylanthracene moiety and the doublet spins (S = 1/2) of the dangling radicals as illustrated in Fig. 9. In Qu the photo-induced intramolecular ferromagnetic spin alignment between two dangling radicals is achieved through the photo-excited triplet (S = 1) spin coupler of the diphenylanthracene moiety [1,2]. 3.3.2. Excited spin states and photo-induced spin alignment of p-topological isomers of 5 In order to test the role of the p-topology on the spin alignment in the photo-excited state, TRESR experiments were performed for 6 [7] and 7 [2], which are the p-topological isomers of 5. A drastic change in the TRESR spectrum was observed for 6 and 7. No TRESR signal for the excited spin state was observed in 7, indicating that the lowest excited state is a low-spin singlet (S = 0) spin state, which is ESR silent. Therefore, the intramolecular spin exchange between two dangling radical spins and the excited triplet spin of the spin coupler is antiferromagnetic, similar to the case of 2. For the molecule 6, TRESR signals arising from the lowest triplet photo-excited state (T1) and the low-lying quintet spin state (Qu) closely above the triplet state were observed as shown in Fig. 10 [7]. The spin-Hamiltonian parameters for both spin states are also listed in Table 1. The magnitude of D of the triplet state is ca. 50% of that of anthracene. The unique triplet state has an interesting electronic structure, which D value is reduced by the antiferromagnetic spin alignment between two radical spins through the excited spin coupler. This small D value is characteristic nature of the unique triplet state, which is constructed from four unpaired electrons and explained well theoretically as already shown in Fig. 7 [7]. From the ratio D(T1)/D(Qu), determined in this experiment, the ratio of the exchange interactions, can be estimated that J2/J1 = 0.05. The sign and magnitude of the exchange coupling between the radical moiety and the triplet spin coupler is proportional to the product of the spin densities at linking positions. The sign of the spin densities of the spin coupler linked by two radical moieties is opposite and the spin density for R2 is expected to be small. Therefore, the small negative value of J2/J1 is reasonable. The quintet state is located very closely above the triplet state in this case. The co-existence of the triplet and the quintet signals in the spectrum is well understood from the nearly
Y. Teki / Polyhedron 24 (2005) 2299–2308
2305
T1 (S = 1) *
S0 (S = 0) S = 1/2
Laser (hn) N N O
O N
on
N
N
off
N O
O N N
S = 1/2
S = 1/2
Spin Polarization
S = 1/2
weak antiferromagnetic interaction
ferromagnetic interaction
Spin Delocalization Fig. 9. Photo-induced intramolecular spin alignment and sign inversion of the effective exchange between two dangling radical spins.
a
Abs.
b
Emi.
c
d
280
300 320 340 Magnetic Field / mT
360
Fig. 10. Typical TRESR spectrum of 6. (a) Observed spectrum at 0.5 ls after the laser excitation at 30 K in the glass matrix. (b) Simulated spectrum (super-imposed T1 and Qu). (c) Simulation of T1. (d) Simulation of Qu.
degenerated energy locations as predicted for the small negative J2/J1 value in Fig. 6 [7]. These findings agree with the spin prediction shown in Fig. 6. Thus, the two dangling radical spins can be manipulated through the photo-excited triplet spin coupler (diphenylanthracene moiety) by the photo-excitation. The p-topological rule of the photo-induced spin alignment is established on the excited states based on these findings. 3.4. Physical picture of photoinduced intra-molecular spin alignment in viewpoint of spin density distribution In order to obtain the physical picture of the photoinduced intramolecular spin alignment, the ab initio molecular orbital calculations were carried out based on the density functional theory (DFT) using GAUSSIAN 98 [21]. In these calculations, an unrestricted Hartree–
Fock method (Ubecke 3LYP/6-31G) was employed in order to clarify the role of the spin polarization effect. Fig. 11 shows the calculated total spin densities on the atomic sites in the excited quartet state of 1 [4]. The mechanism of the spin alignment was clarified based on the spin distribution. In the excited quartet state the spin density distribution of the anthracene moiety is nearly symmetrical as shown in Fig. 11. The almost two net unpaired spins exist on the anthracene moiety and the remained one unpaired spin exists on the dangling radical. This shows that the excited quartet state is generated by the one-electron transition from the b-HOMO (p) to the a-LUMO (p*). These results are consistent with our experimental results, in which the magnitude of the observed fine-structure splitting has been well interpreted as the exchange coupled system constructed from the pp* excited triplet state of anthracene and the dangling radicals. In the anthracene moiety, positive spin densities overcome the negative spin densities. This spin distribution shows that spin delocalization overcomes the spin polarization effect within the anthracene moiety. In contrast, the alternating sign of the spin density is realized on the dangling phenyl groups as shown in Fig. 11. This sign alternation of the spin densities shows that the spin polarization mechanism overcomes the spin delocalization within the dangling phenyl groups even in the excited state. Thus, through the spin polarization pathway in the dangling phenyl group, the unpaired spin on the dangling radical 0.080 0.099 0.237
0.243 -0.138 0.656 -0.138 0.243
0.080
C-
-0.122
0.526 O
0.349 0.029 - -0.020 C N 0.662 -0.116 C- -0.019 C- -0.074 -0.014 C-0.122 CN 0.029 -0.020 0.235 0.237 0.025 0.099
Fig. 11. The spin density distribution in the excited quartet state of 1.
2306
Y. Teki / Polyhedron 24 (2005) 2299–2308
0.020
0.129
0.503 O 0.345
-0.064 N 0.103
-0.020
0.152
0.236
-0.152
-0.074
0.088 0.670
0.322
0.085
C
0.668
-0.122
C-
-
-0.124
-0.021
-0.062
0.322
-0.075 0.063
-
-0.157
C
0.090 -0.067 0.235
0.152 0.128
-
C -0.151
N
0.107
-0.157 N
-0.065
N
-0.021 C-
-0.020
C-
0.344 O 0.505
0.021
Fig. 12. Spin density distribution in the excited quintet state of 5.
couples ferromagnetically to the excited triplet spins delocalized on the anthracene moiety, leading to the excited quartet high-spin state. The calculated spin distribution of the excited quintet state of 5 [4] is shown in Fig. 12. The spin delocalization and spin polarization effects similar to the case of 1 occurred in the quintet state. The large positive spin densities at the 9 and 10 positions of the anthracene moiety are realized by the spin delocalization. Each dangling radical spin (S = 1/2) couples ferromagnetically to the large positive spin of the excited triplet spins on the anthracene moiety, through the spin polarization pathway in the phenyl groups. This mechanism leads to the ferromagnetic exchange between excited triplet state and two radical spins in 5. As the results, the sign of the effective spin coupling of the two dangling radicals in 5 can be changed from antiferromagnetic to ferromagnetic as illustrated in Fig. 9 by the exchange coupling through the excited triplet moiety generated by
A
hv A
D
D
A
D
+*
3.5. Potential developments toward functional materials using photo-excited high-spin organic systems The observations of the high-spin photo-excited states mean that the intersystem crossing (ISC) from low-spin state to high-spin states occurs efficiently. As previously reported in our paper [2,6], this effective ISC are generated by the attachment of radical species. The enhanced ISC mechanism is concerned with the p-conjugation between the pp orbital of radical moiety and aromatic hydrocarbon moiety. The enhanced ISC to the high-spin photo-excited states will open a new channel to relax from the high-spin photo-excited state toward the ground/low-lying high-spin states. Moreover, since the high-spin photo-excited states have fairly long life-time (10–20 ls at 77 K), the energy
*
Exciton Transfer
R
R -
the photo-excitation. Similar results were obtained for the quintet states of 8.
Photo-Induced Electron Transfer
R
A
High-Spin Excited State Spin Alignment
A
High-Spin Excited State
Spin Alignment High-Spin Excited State
Spin Alignment using Photo-Induced ET from Excited High-Spin State
Spin Exchange Interaction Coupled with Exciton Transfer
Photo-Excited HighSpin Organic Systems
D1 Q1
R
R
R
R
A D+ A D+A D+ A D + CT Complex using Excited High-Spin Organic Systems (a Kind of Spin Polarized Donor Proposed by Prof.T.Sugawara)
Q0 D0 Coupled System with Spin-Cross Over Complex
Fig. 13. Potential developments toward functionality materials using the p-conjugated organic spin system with photo-excited high-spin states.
Y. Teki / Polyhedron 24 (2005) 2299–2308
transfer and the photo-induced electron transfer processes will be expected from such high-spin photo-excited states. These processes also channel a new pathway using the photo-excited state toward the new functionality materials. There are some potential developments of the photo-excited high-spin organic systems toward the functionality materials shown in Fig. 13. In order to realize such functionality materials, it is necessary to develop the systems to superstructure materials. One of the ways is to make the charge-transfer complexes using the excited high-spin organic spin systems as the electron donor and to use the photo-induced electron transfer from the donor to the acceptor. In the intramolecular donor-acceptor linked systems using the photo-excited high-spin molecule are also potential candidates. Moreover, the cation of the photo-excited high-spin system is expected to have interesting electronic structure, in which the triplet state becomes the electronic ground state. Therefore, this system is a kind of the spin-polarized donor proposed by Sugawara et al. [22]. Since the photo-excited high-spin state has fairly long life-time, the exciton transfer is possible to occur in their crystals. The intermolecular photo-induced spin alignment of the dangling radical spins may be achieved by the exciton transfer. The combination with the metal complex will be one of the potential developments. It is special interest in the construction of the spin crossover complex using the photo-excited high-spin organic molecules as their ligands. In the case, the photo-excitation of the ligand leads to open the new pathway to relax the high-spin ground state of the whole system. There will be also other potential applications of the p-conjugated organic spin systems with the photo-excited high-spin states.
4. Summary The spin alignment in the photo-excited states of the purely organic p-conjugated spin systems and their topological isomers are reviewed. In the photo-excited states the photo-induced spin alignment is investigated by TRESR experiments and ab initio MO calculations. Role of the spin delocalization and the spin polarization effects are revealed in the spin alignment on the photoexcited states. The photo-induced spin alignment was achieved through the photo-excited triplet spin coupler. p-Topology plays very important role on the spin alignment on the photo-excited states as well as the ground states. One of the key processes for the photo-control of the organic molecular magnetism is established. The guiding principle designing the photo-excited high-spin system and the role of p-topology are clarified. Some ideas of the potential developments toward the function-
2307
ality materials utilizing the p-conjugated organic spin systems with the photo-excited high-spin states are also proposed.
Acknowledgements I express my gratitude to all collaborators to the work described here. A particular thank to Prof. Y. Miura for the syntheses of some anthracene derivatives. This work was supported by PRESTO of the Japan Science and Technology Corporation and the Grant-in-Aid for Scientific Research on the general (No. 13440211, 16350079) and Priority Area ‘‘Application of Molecular Spins’’ (Area No. 769, Proposal No. 15087208) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References [1] Y. Teki, S. Miyamoto, K. Iimura, M. Nakatsuji, Y. Miura, J. Am. Chem. Soc. 122 (2000) 984. [2] Y. Teki, S. Miyamoto, M. Nakatsuji, Y. Miura, J. Am. Chem. Soc. 123 (2001) 294. [3] Y. Teki, M. Nakatsuji, Y. Miura, Int. J. Mod. Phys. B 15 (2001) 4029. [4] Y. Teki, Polyhedron 20 (2001) 1163. [5] Y. Teki, M. Nakatsuji, Y. Miura, Mol. Phys. 100 (2002) 1385. [6] Y. Teki, M. Kimura, S. Narimatsu, K. Ohara, K. Mukai, Bull. Chem. Soc. Jpn. 77 (2004) 95. [7] Y. Teki, S. Nakajima, Chem. Lett. 33 (2004) 1500. [8] K. Itoh, Pure Appl. Chem. 50 (1978) 1251. [9] Y. Teki, K. Itoh, in: P.M. Lahti (Ed.), Magnetic Properties of Organic Materials, Marcel-Dekker, 1999, p. 237, and references cited therein. [10] Most recent overviews of the molecule-based magnetism. See: G. Christou (Ed.), Polyhedron 22 (2002) 1725, Proceedings of the VIIIth International Conference on Molecule-based Magnets. [11] (a) C. Corvaja, M. Maggini, M. Prato, G. Scorrano, M. Venzin, J. Am. Chem. Soc. 117 (1995) 8857; (b) N. Mizouchi, Y. Ohba, S. Yamauchi, J. Chem. Phys. 101 (1997) 5966. [12] (a) K. Ishii, J. Fujiwara, Y. Ohba, S. Yamauchi, J. Am. Chem. Soc. 118 (1996) 13079; (b) K. Ishii, J. Fujisawa, A. Adachi, S. Yamauchi, N. Kobayashi, J. Am. Chem. Soc. 120 (1998) 3152. [13] P. Ceroni, F. Conti, C. Corvaja, M. Maggini, F. Paolucci, S. Roffia, G. Scorrano, A. Toffoletti, J. Phys. Chem. A 104 (2000) 156. [14] (a) N. Mizouchi, Y. Ohba, S. Yamauchi, J. Phys. Chem. 103 (1999) 7749; (b) F. Conti, C. Corvaja, A. Toffoletti, N. Mizouchi, Y. Ohba, S. Yamauchi, M. Maggini, J. Phys. Chem. 104 (2000) 4962; (c) J. Fujiwara, Y. Iwasaki, Y. Ohba, S. Yamauchi, N. Koga, S. Karasawa, M. Fuhs, K. Mo¨bius, S. Weber, Appl. Magn. Reson. 21 (2001) 483. [15] K. Ishii, Y. Hirose, N. Kobayashi, J. Phys. Chem. 103 (1999) 1986. [16] (a) M. Lamchen, T.W. Mittag, J. Chem. Soc. C 2 (1966) 300; (b) C. Hirel, K.E. Vostrikova, J. Pecaut, V.I. Ovcharenko, P. Rey, Chem. Eur. J. 7 (2001) 2007.
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[17] G.G. Belford, R.L. Belford, J.F. Burkhalter, J. Magn. Reson. 11 (1973) 251. [18] Y. Teki, I. Fujita, T. Takui, T. Kinoshita, K. Itoh, J. Am. Chem. Soc. 116 (1994) 11499. [19] In the spectral simulation of the TRESR spectra, the calculation of ESP has been added as a subroutine to the computer program. More detailed procedures of the spectral simulation for high-spin organic molecules are similar to those described in
our previous paper for the perturbation approach: Y. Teki T. Takui, H. Yagi, K. Itoh, H. Iwamura, J. Chem. Phys. 83 (1985) 539. [20] A. Bencini, D. Gatteschi, EPR of Exchange Coupled Systems, Springer-Verlag, Berlin, 1990. [21] The DFT calculation was carried out using GAUSSIAN 98, Revision A.11, Gaussian, Inc., Pittsburgh, PA, 1998. [22] T. Sugawara, Mol. Cryst. Liq. Cryst. 334 (1999) 257.
Topology and spin alignment utilizing the excited molecular field in p-conjugated organic spin systems Yoshio Teki
*
Department of Material Science, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Received 5 October 2004; accepted 5 January 2005 Available online 5 July 2005
Abstract The design and experimental investigations of photo-induced high-spin organic systems (the photo-excited quartet (S = 3/2) and quintet (S = 2) states) is reviewed with focusing p-conjugated organic spin systems. In order to study the photo-induced spin alignments on the excited states, the photo-excited high-spin states of p-conjugated stable radical systems and their p-topological isomers were studied by the time-resolved ESR experiments. The relationship between the p-topology and spin alignment on the photoexcited states is clarified. The mechanism of the photo-induced intramolecular spin alignment and the role of the spin polarization and spin delocalization are revealed with the help of the molecular orbital calculations. One of the key processes for the photo-control of the organic molecular magnetism is established. The guiding principle designing the photo-excited high-spin system and the role of p-topology are clarified. Potential developments toward the functional materials are also proposed utilizing the p-conjugated organic spin systems with the photo-excited high-spin states. 2005 Elsevier Ltd. All rights reserved. Keywords: p-Topology; Photo-induced spin alignment; High-spin excited state; Time-resolved ESR; p-Conjugated organic spin system; Application to functional materials
1. Introduction This review deals with the spin alignment of high-spin organic systems especially with focusing photo-excited high-spin states of p-conjugated stable radicals or biradical systems and their topological isomers [1–7]. We define ‘‘p-topological isomer’’ as a molecule in which differs from others only in the topology of its p electron network, i.e. in the linking positions of its p bonds. p-Conjugated spin systems arising from the aromatic hydrocarbons and the dangling stable radicals are ideal model systems in order to study the relationship between the p-topology and spin alignment on the photo-excited states. The delocalized p orbital network is most important for determining the spin states as shown later. *
Tel./fax: +81 666052559. E-mail address: [email protected].
0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.03.115
The spin states on the ground states were determined by the electron spin resonance (ESR). The photo-induced spin alignment and the photo-excited states of p-conjugated spin systems were investigated by the time-resolved ESR (TRESR) technique. The spin density distributions provide the most direct information about the spin correlation among the unpaired electrons and the mechanism of the spin alignment in these organic molecules. The mechanism of the spin alignment was clarified in viewpoint of the spin density distributions obtained by ab initio molecular orbital calculations using the density functional theory (DFT). As well known, the spin polarization effect according to the p-topology plays an important role of the spin alignment on the ground state [8,9]. On the photo-excited state it is unknown whether the similar p-topological rule works or not. In order to clarify the spin alignment in the photo-excited states of the p-conjugated organic spin
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Y. Teki / Polyhedron 24 (2005) 2299–2308
spin systems [1–7], which are generated by a spin alignment through p-conjugation between dangling stable radicals (S = 1/2) and the excited triplet (S = 1) state of aromatic hydrocarbons. The p-conjugation leads to the strong exchange coupling between the triplet photo-excited state of the aromatic hydrocarbons and the dangling radicals. In the biradical systems, 5–8, the stable radicals couple strongly through the spin coupler by p-conjugation making the robust spin alignment possible. p-Conjugated spin systems have the following advantages in view point of the material science and molecular magnetism: (1) p-Conjugation leads to the strong exchange coupling which is one of the important conditions for the balk magnetism at finite temperature. (2) One can well design and synthesize the proper spin systems with the desired spin states using the well-established fine-synthetic chemistry. (3) An enhanced intersystem crossing (ISC) mechanism arising from the attachment of the radical species is available [2,6]. This leads to open a way of the direct ESR detection of the silent photo-excited states with little ISC to the triplet state. Actually, since phenyl or diphenylanthracene has very low efficiency of the ISC toward the triplet state, the triplet states can be not directly detected by TRESR. However, by attaching the radical species through p-conjugation, the excited high-spin states arising from the triplet state of phenyl or diphenylanthracene and the doublet state of radical were detected by TRESR as shown later. The photo-induced intra-molecular spin
systems, we have designed, synthesized the novel p-conjugated stable radical or biradicals, 1–8 shown in Fig. 1 and characterized their spin alignments [1–7]. The knowledge of photo-induced spin alignment on the excited states will give the guiding principles designing photo-induced/switching magnetic spin systems and the photo-control of the molecular magnetism. These studies will open the new strategy for the photomagnetic devices based on the organic materials. Intramolecular spin alignment and exchange interaction through p-conjugation are quite important in the field of the molecular magnetism [10]. However, most studies are limited to ground state systems. Although radical (R) and excited-triplet (T) pairs in solution have been extensively investigated by TRESR technique, the direct detections of the excited high-spin states (S P 3/2) arising from the radical–triplet pair are still limited in the solid phase. The first direct TRESR detection of an excited quartet state of a radical–triplet pair was reported for a fullerene–mononitroxide radical system in solution [11] and for a tetraphenyl-porphilinatozinc (II) (ZnTPP) coordinated by p-pyridyl nitronylnitroxide (nitpy) system in a solid phase [12]. For homologous systems [13–15], excited high-spin states have been so far reported. However, in these systems, a stable radical couples weakly with the excited triplet chromophore through sigma bonds or coordination, in which the spin exchange interactions are small in magnitude. We reported the first observation of excited quartet (S = 3/2) and quintet (S = 2) states in purely organic p-conjugated
CH3
O
N N
O N
CH3
O N
N
1
2
N
N
N
N
N N
O
3
N
4
CH3
O N
N N O
O
N
N N
N
5
N O
6
N N
H3C
O O N
7
N
CH3 N
N
N N
N
N
N N
O H3C
O
8
CH3
Fig. 1. p-Conjugated organic spin systems in which the photo-excited states were investigated.
CH3
Y. Teki / Polyhedron 24 (2005) 2299–2308
alignment through p-conjugation is one of the key processes for the photo-control of the molecular magnetism.
2301 OHC B(OH)2
Br
CH2OH
Br (PPh3)4Pd, K2CO3
HOH2C HO
NHOH NHOH
2. Experimental
N OH N
2.1. Materials
OH O
1. NaIO4
Typical synthetic procedures of monoradical and biradical are shown in Schemes 1 and 2, respectively. 2,3-Bis(hydroxyamino)-2,3-dimethyl-butane was prepared according to the literature method [16]. Other reagents were used as purchased.
The photo-excited states were examined by TRESR. A conventional X-band ESR spectrometer (JEOL TE300) was used in the measurements of TRESR spectra without field modulation. Excitation was carried out at 355 nm light using Nd:YAG pulse laser (Continuum Surelite II-10, pulse width <7 ns). The typical laser power used in the experiments was ca. 2–5 mJ. EPA, 2-MTHF, or isopentane–diethylether mixed glass matrix was used for the TRESR experiments. The measurements were carried out at 30–40 K. The typical microwave power was ca. 10 mW. The spectral simulation was carried out by the eigenfield/exact-diagonalization hybrid method [16,17], taking dynamic electron spin polarization (ESP) into account [2]. The effective-exchange interaction between the triplet state of the aromatic hydrocarbon moiety and the dangling radicals is undoubtedly much larger than the Zeeman and fine-structure interactions in the p-conjugated systems. We have, therefore, used the following ordinary spin Hamiltonian of a pure spin state given in Eq. (1) (negligible quantum mixing of the different spin states) for the analysis: H 0spin ¼ be H.g.S þ S.D.S.
ð1Þ
The resonance field BMs M Ms + 1(h, /) for each transition was directly calculated by solving the following eigenfield equation [17]:
NHOH NHOH
B(OH)2 Br
CHO
(PPh3)4Pd, K2CO3 benzene-EtOH-water
N
O N
2. NaNO2, HCl
N N
Scheme 2. Typical synthetic procedures of biradicals.
A Z ¼ BC Z;
2.2. TRESR measurements and simulation
OHC
N
HO N
HO
THF-MeOH
N N OH
where A and superoperators:
ð2Þ C
are
given
by
the
A ¼ xE E F E þ E F
following ð3Þ
and C ¼ G E E G .
ð4Þ
Here, E is a unit matrix and x is the given microwave frequency. The operators G and F are the field dependent and independent parts of the spin Hamiltonian (1), respectively. By solving the eigenfield equation (Eq. (2)), the resonance field B is obtained. The transition probabilities I(h, /, u) were evaluated by numerically diagonalization of the spin Hamiltonian matrix at the calculated resonance eigenfield [18,19]. Since the resonance field is independent of the third Euler angle, u, the above procedure practically saves the computing time for the simulation. The line-shape function of the TRESR spectrum in the glass matrix is given by Z Z XZ gðBÞ ¼ N du d/ dh sin hP Ms$Msþ1 ðh; /Þ Iðh; /; uÞf ½B BMs$Msþ1 ðh; /Þ.
ð5Þ
In the simulation, the dynamic electron polarization (ESP), PMs M Ms + 1(h, /), on each spin sublevel in zero magnetic field was given as parameters. Thus, the relative populations of the Ms sublevels was taken into account as parameters. The simulation was carried out using a program written by the author on a personal computer. The details of the simulation procedures for the high-spin TRESR spectra were described in our previous paper [2].
73 %
O
1, NaIO4 / MeOH-water
3. Results and discussion N N
2, NaNO2, HCl / CH2Cl2-water
Scheme 1. Typical synthetic procedures of monoradical.
3.1. Photo-excited quartet high-spin states Typical TRESR spectrum of 1 is shown in Fig. 2(a) [1,2]. The observed TRESR spectrum with well resolved
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Y. Teki / Polyhedron 24 (2005) 2299–2308
doublet states. The g and D values for the quartet state (Q) are given by the following relationship [20]:
a
gðQÞ ¼ ð2=3ÞgðTÞ þ ð1=3ÞgðRÞ
Abs.
ð7Þ
and XY1
Emi. Abs.
DðQÞ ¼ ð1=3ÞfDðTÞ þ DðRTÞg.
b
Here, D(T) is the D tensor of the excited triplet moiety, D(RT) is the dipolar interaction between the dangling radical spin and the triplet moiety. Using these equations g and D values were calculated as shown in Table 1. The experimentally determined g, D and E values for the excited quartet state are in excellent agreement with the calculations.
Z1
Z3
Emi. XYZ2 A
250
XY3
300 350 400 Magnetic Field / mT
Fig. 2. Typical TRESR spectra of 1 at 30 K in EPA rigid glass matrix. (a) The observed spectrum of 1 at 0.8 ls after laser excitation. (b) The simulated spectrum of the excited quartet state.
fine structure splitting has been unambiguously analyzed to be a excited quartet (S = 3/2) state (Q) by the spectral simulation shown in Fig. 2(b). The determined g value, fine-structure parameters, and relative populations of the Ms sublevels are listed in Table 1. Similar excited quartet states were observed for 3 [5] and 4 [6]. The typical TRESR spectrum observed for the pyrene–verdazyl radical, 4, is also shown in Fig. 3. In these quartet systems the triplet state of the aromatic hydrocarbon moiety couples with the doublet state of the dangling stable radical by the spin-exchange interaction. In such a exchange-coupled system the wavefunction of the whole molecule |W(S, M)æ is approximately given by the direct product of the wavefunctions of the two isolated moieties, |T(SA, mA)æ and |R(SB, mB)æ, as jWðS;MÞi ¼
X
ð8Þ
CðS A S B S;mA mB MÞjTðS A ;mA Þij
RðS B ;mB Þi; for S A ¼ 1 and S B ¼ 1=2; ð6Þ where C(SASBS; mAmBM) is the Clebsch–Gordan coefficient. The radical–triplet pair has one quartet and one
3.2. Excited spin states of the p-topological isomers of 1 In the ground state p-topology plays very important role in the spin alignment of the alternant hydrocarbons. Spin polarization effect governs the spin alignment and the spin multiplicity of the ground state depends drastically on the p-topology. In order to test the role of the p-topology on the spin alignment in the photo-excited state, TRESR experiments were performed for 2 [2], which is the p-topological isomers of 1. In contrast to the result of 1, no TRESR signal attributable to the high-spin excited state was observed for 2 and only a very weak absorptive signal due to the doublet state was observed as shown in Fig. 4. This agrees with the p-topological rule, indicating that the lowest excited state of 2 is a low-spin doublet (S = 1/2) spin state and the sign of the intramolecular exchange between the dangling radical spin and the excited triplet spin of the anthracene moiety is antiferromagnetic according to the spin polarization pathway through the dangling phenyl group. This finding shows that the sign of the intramolecular exchange interaction and the spin multiplicity of the lowest photo-excited spin state change drastically depending on the topological nature of p-electron network. Thus, the p-topological rule of the spin polarization established for the ground states of the aromatic hydrocarbons also plays an important role in the spin alignment on the photo-excited states.
Table 1 Spin Hamiltonian parameters (S, g, D and E), relative populations of the Ms sub levels of the zero-field eigenfunctions, and the estimated D values Molecule 1 3 4 5 6 8
S 3/2 3/2 3/2 2 1 2 2
g
D (cm1)
|E| (cm1)
Relative population
gest
Dest (cm1)
Ref.
2.0043 2.0040 2.0035 2.0043 2.0045 2.0035 2.0035
0.0235 0.0230 0.0290 0.0130 0.0360 0.0125 0.0128
0.0 0.0 0.0090 0.0 0.0 0.0 0.0
P 03=2 ¼ 0.0; P 01=2 ¼ 0.5 P 03=2 ¼ 0.0; P 01=2 ¼ 0.5 P 03=2 ¼ 0.0; P 01=2 ¼ 0.5 P 02 ¼ 0.0; P 01 ¼ 0.35; P 00 ¼ 0.30 PX = PY = 0.0, PZ = 1.0 P 02 ¼ 0.0; P 01 ¼ 0.0; P 00 ¼ 1.0 P 02 ¼ 0.0; P 01 ¼ 0.37; P 0 ¼ 0.26
2.0040 2.0032 2.0033 2.0045 2.0045 2.0050 2.0035
0.0256 0.0248 0.0310 0.0137
[1,2] [3,5] [6] [1,2] [7]
0.0120 0.0130
[5]
Here, PMs 0 is the relative population of the spin sublevels in zero magnetic field, which magnetic quantum number corresponds to the value, Ms, in the high field limit.
Y. Teki / Polyhedron 24 (2005) 2299–2308
2303
Stable Radical
a
R
R
Abs.
J1
Laser on off
Ground-State Singlet Spin Coupler
Emi.
J2
R
R
Photo-Excited Triplet Spin Coupler
Fig. 5. Photo-induced intramolecular spin alignment.
D
b Abs.
Emi.
250
300 350 Magnetic Field / mT
400
Fig. 3. Typical TRESR spectrum of pyrene–verdazyl radical, 4.
3.3. Photo-induced spin alignment in diphenylanthracenebis(radical) systems In order to clarify the photo-induced spin alignment using the photo-excited molecular field, we investigated the photo-excited spin states of diphenyanthracenebis(radical) systems which are illustrated in Fig. 5. In this spin system, the diphenylanthracene spin coupler between two dangling stable radical moieties becomes the photo-excited triplet state with two unpaired electrons upon the photo-excitation. Two dangling radicals are interacted with each other through the triplet spin coupler. The total spin Hamiltonian of the four spin system is given by H spin ¼H ex þ be H g T S T þ be H gR1 S R1 þ be H gR2 S R2 þ S T DðTÞ S T þ S R1 DðR1 TÞ S T þ S R2 DðR2 TÞ S T ; H ex ¼ 2J 1 S
R1
T
T
R2
S 2J 2 S S .
ð9Þ ð10Þ
Here, we omitted the dipolar interactions between two radicals moieties. The p-conjugated spin systems, the exchange interaction is much larger than other interactions in Eq. (9). The energies of each spin state in zero magnetic field were obtained to zero-th order by diagonalizing Hex. The expectation values of the fine-structure terms of each spin states were calculated by using the eigenfunctions. There are two kinds of the triplet states (T1 and T2), one quintet state (Qu) and one singlet state (S). The energy versus J2/J1 diagram for each spin states are shown in Fig. 6 [7]. The fine-structure terms (D) of each spin states are related to the fine-structure tensor (D(T)) of the triplet state of anthracene by a reduction factor (f) (D = f Æ D(T)). The reduction factor of each spin states are shown in Fig. 7 [7]. In this calculation, we omitted D(R1T) and D(R2T) in Eq. (10), because D(T) is much larger than them. As already shown in Sections 3.1 and 3.2, para-joint configuration between the iminonitroxide radical and the triplet excited state of the anthracene moiety leads to the ferromagnetic coupling and meta-joint configuration is antiferromagnetic coupling between them. Therefore, para-joint biradical 5 is expected to have a quintet state (S = 2) as the lowest photo-excited state and meta-joint biradical 7 has a singlet state (S = 0). An interesting lowest photo-excited triplet state arising from four unpaired electrons will be realized in 6 by the antiferromagnetic spin exchange coupling of the two dangling radical spins through the photo-excited triplet spin coupler (diphenylanthracene moiety). In order to prove this prediction, TRESR experiments were carried out for the p-topological isomers, 5–8 as shown later.
4.0
S
J1> 0
T2
Energy / J1
2.0 Abs.
T1
0.0
Emi.
Qu
-2.0 250
300
350
400
Magnetic Field /mT Fig. 4. Typical TRESR spectrum of 2 at 30 K in EPA rigid glass matrix.
-1.0
-0.50
0.0 J /J 2
0.50
1
Fig. 6. Energy vs. J2/J1 diagram.
1.0
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Y. Teki / Polyhedron 24 (2005) 2299–2308
Reduction Factor (f)
D(T1)
D(Qu)
D(T2) -1.0
-0.50
0.0
0.50
1.0
J2 / J1
Fig. 7. J2/J1 Dependence of the reduction factor.
3.3.1. Photo-excited quintet high-spin states Typical TRESR spectrum of the photo-excited quintet (S = 2) state (Qu) of the biradical system, 5 [1,2], is given in Fig. 8 together with the spectral simulation. The spin Hamiltonian parameters are also listed in Table 1. Similar excited quintet state was observed for a verdazyl biradical, 8 [5]. All the determined spin Hamiltonian parameters and relative population of the Ms sublevels of these excited high-spin states are summarized in Table 1. According to a similar manner described in the case of the quartet state, the g, D and E values of the quintet state (Qu) have been estimated using the following equations, which are listed in Table 1: gðQuÞ ¼ f2gðTÞ þ gðR1 Þ þ gðR2 Þg=4
ð11Þ
and DðQuÞ ¼ ð1=6ÞðDðTÞ þ DðR1 TÞ þ DðR2 TÞ þ DðR1 R2 ÞÞ. ð12Þ
Abs. a XY2
Emi. XY1
Abs.
Z2 Z1
b
Z4
Emi.
Z3 XY4
XY3
250
300 350 Magnetic Field /mT
400
Fig. 8. Typical TRESR spectra of 5 at 30 K in 2-MTHF rigid glass matrix. (a) The observed spectrum of 5 at 0.8 ls after laser excitation. (b) The simulated spectrum of the excited quintet state.
Here, D(R1R2) is the dipolar interaction between the two radical spins. This was actually neglected in the estimation because of the small magnitude. The estimated values of the fine-structure parameters are quite well in agreement with the observed values, showing that the observed quintet state is a high-spin species originated from the triplet spin coupler and two dangling radicals. The direct observations of the excited quintet states show that the photoinduced intramolecular spin alignment is realized between the excited triplet state (S = 1) of the diphenylanthracene moiety and the doublet spins (S = 1/2) of the dangling radicals as illustrated in Fig. 9. In Qu the photo-induced intramolecular ferromagnetic spin alignment between two dangling radicals is achieved through the photo-excited triplet (S = 1) spin coupler of the diphenylanthracene moiety [1,2]. 3.3.2. Excited spin states and photo-induced spin alignment of p-topological isomers of 5 In order to test the role of the p-topology on the spin alignment in the photo-excited state, TRESR experiments were performed for 6 [7] and 7 [2], which are the p-topological isomers of 5. A drastic change in the TRESR spectrum was observed for 6 and 7. No TRESR signal for the excited spin state was observed in 7, indicating that the lowest excited state is a low-spin singlet (S = 0) spin state, which is ESR silent. Therefore, the intramolecular spin exchange between two dangling radical spins and the excited triplet spin of the spin coupler is antiferromagnetic, similar to the case of 2. For the molecule 6, TRESR signals arising from the lowest triplet photo-excited state (T1) and the low-lying quintet spin state (Qu) closely above the triplet state were observed as shown in Fig. 10 [7]. The spin-Hamiltonian parameters for both spin states are also listed in Table 1. The magnitude of D of the triplet state is ca. 50% of that of anthracene. The unique triplet state has an interesting electronic structure, which D value is reduced by the antiferromagnetic spin alignment between two radical spins through the excited spin coupler. This small D value is characteristic nature of the unique triplet state, which is constructed from four unpaired electrons and explained well theoretically as already shown in Fig. 7 [7]. From the ratio D(T1)/D(Qu), determined in this experiment, the ratio of the exchange interactions, can be estimated that J2/J1 = 0.05. The sign and magnitude of the exchange coupling between the radical moiety and the triplet spin coupler is proportional to the product of the spin densities at linking positions. The sign of the spin densities of the spin coupler linked by two radical moieties is opposite and the spin density for R2 is expected to be small. Therefore, the small negative value of J2/J1 is reasonable. The quintet state is located very closely above the triplet state in this case. The co-existence of the triplet and the quintet signals in the spectrum is well understood from the nearly
Y. Teki / Polyhedron 24 (2005) 2299–2308
2305
T1 (S = 1) *
S0 (S = 0) S = 1/2
Laser (hn) N N O
O N
on
N
N
off
N O
O N N
S = 1/2
S = 1/2
Spin Polarization
S = 1/2
weak antiferromagnetic interaction
ferromagnetic interaction
Spin Delocalization Fig. 9. Photo-induced intramolecular spin alignment and sign inversion of the effective exchange between two dangling radical spins.
a
Abs.
b
Emi.
c
d
280
300 320 340 Magnetic Field / mT
360
Fig. 10. Typical TRESR spectrum of 6. (a) Observed spectrum at 0.5 ls after the laser excitation at 30 K in the glass matrix. (b) Simulated spectrum (super-imposed T1 and Qu). (c) Simulation of T1. (d) Simulation of Qu.
degenerated energy locations as predicted for the small negative J2/J1 value in Fig. 6 [7]. These findings agree with the spin prediction shown in Fig. 6. Thus, the two dangling radical spins can be manipulated through the photo-excited triplet spin coupler (diphenylanthracene moiety) by the photo-excitation. The p-topological rule of the photo-induced spin alignment is established on the excited states based on these findings. 3.4. Physical picture of photoinduced intra-molecular spin alignment in viewpoint of spin density distribution In order to obtain the physical picture of the photoinduced intramolecular spin alignment, the ab initio molecular orbital calculations were carried out based on the density functional theory (DFT) using GAUSSIAN 98 [21]. In these calculations, an unrestricted Hartree–
Fock method (Ubecke 3LYP/6-31G) was employed in order to clarify the role of the spin polarization effect. Fig. 11 shows the calculated total spin densities on the atomic sites in the excited quartet state of 1 [4]. The mechanism of the spin alignment was clarified based on the spin distribution. In the excited quartet state the spin density distribution of the anthracene moiety is nearly symmetrical as shown in Fig. 11. The almost two net unpaired spins exist on the anthracene moiety and the remained one unpaired spin exists on the dangling radical. This shows that the excited quartet state is generated by the one-electron transition from the b-HOMO (p) to the a-LUMO (p*). These results are consistent with our experimental results, in which the magnitude of the observed fine-structure splitting has been well interpreted as the exchange coupled system constructed from the pp* excited triplet state of anthracene and the dangling radicals. In the anthracene moiety, positive spin densities overcome the negative spin densities. This spin distribution shows that spin delocalization overcomes the spin polarization effect within the anthracene moiety. In contrast, the alternating sign of the spin density is realized on the dangling phenyl groups as shown in Fig. 11. This sign alternation of the spin densities shows that the spin polarization mechanism overcomes the spin delocalization within the dangling phenyl groups even in the excited state. Thus, through the spin polarization pathway in the dangling phenyl group, the unpaired spin on the dangling radical 0.080 0.099 0.237
0.243 -0.138 0.656 -0.138 0.243
0.080
C-
-0.122
0.526 O
0.349 0.029 - -0.020 C N 0.662 -0.116 C- -0.019 C- -0.074 -0.014 C-0.122 CN 0.029 -0.020 0.235 0.237 0.025 0.099
Fig. 11. The spin density distribution in the excited quartet state of 1.
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Y. Teki / Polyhedron 24 (2005) 2299–2308
0.020
0.129
0.503 O 0.345
-0.064 N 0.103
-0.020
0.152
0.236
-0.152
-0.074
0.088 0.670
0.322
0.085
C
0.668
-0.122
C-
-
-0.124
-0.021
-0.062
0.322
-0.075 0.063
-
-0.157
C
0.090 -0.067 0.235
0.152 0.128
-
C -0.151
N
0.107
-0.157 N
-0.065
N
-0.021 C-
-0.020
C-
0.344 O 0.505
0.021
Fig. 12. Spin density distribution in the excited quintet state of 5.
couples ferromagnetically to the excited triplet spins delocalized on the anthracene moiety, leading to the excited quartet high-spin state. The calculated spin distribution of the excited quintet state of 5 [4] is shown in Fig. 12. The spin delocalization and spin polarization effects similar to the case of 1 occurred in the quintet state. The large positive spin densities at the 9 and 10 positions of the anthracene moiety are realized by the spin delocalization. Each dangling radical spin (S = 1/2) couples ferromagnetically to the large positive spin of the excited triplet spins on the anthracene moiety, through the spin polarization pathway in the phenyl groups. This mechanism leads to the ferromagnetic exchange between excited triplet state and two radical spins in 5. As the results, the sign of the effective spin coupling of the two dangling radicals in 5 can be changed from antiferromagnetic to ferromagnetic as illustrated in Fig. 9 by the exchange coupling through the excited triplet moiety generated by
A
hv A
D
D
A
D
+*
3.5. Potential developments toward functional materials using photo-excited high-spin organic systems The observations of the high-spin photo-excited states mean that the intersystem crossing (ISC) from low-spin state to high-spin states occurs efficiently. As previously reported in our paper [2,6], this effective ISC are generated by the attachment of radical species. The enhanced ISC mechanism is concerned with the p-conjugation between the pp orbital of radical moiety and aromatic hydrocarbon moiety. The enhanced ISC to the high-spin photo-excited states will open a new channel to relax from the high-spin photo-excited state toward the ground/low-lying high-spin states. Moreover, since the high-spin photo-excited states have fairly long life-time (10–20 ls at 77 K), the energy
*
Exciton Transfer
R
R -
the photo-excitation. Similar results were obtained for the quintet states of 8.
Photo-Induced Electron Transfer
R
A
High-Spin Excited State Spin Alignment
A
High-Spin Excited State
Spin Alignment High-Spin Excited State
Spin Alignment using Photo-Induced ET from Excited High-Spin State
Spin Exchange Interaction Coupled with Exciton Transfer
Photo-Excited HighSpin Organic Systems
D1 Q1
R
R
R
R
A D+ A D+A D+ A D + CT Complex using Excited High-Spin Organic Systems (a Kind of Spin Polarized Donor Proposed by Prof.T.Sugawara)
Q0 D0 Coupled System with Spin-Cross Over Complex
Fig. 13. Potential developments toward functionality materials using the p-conjugated organic spin system with photo-excited high-spin states.
Y. Teki / Polyhedron 24 (2005) 2299–2308
transfer and the photo-induced electron transfer processes will be expected from such high-spin photo-excited states. These processes also channel a new pathway using the photo-excited state toward the new functionality materials. There are some potential developments of the photo-excited high-spin organic systems toward the functionality materials shown in Fig. 13. In order to realize such functionality materials, it is necessary to develop the systems to superstructure materials. One of the ways is to make the charge-transfer complexes using the excited high-spin organic spin systems as the electron donor and to use the photo-induced electron transfer from the donor to the acceptor. In the intramolecular donor-acceptor linked systems using the photo-excited high-spin molecule are also potential candidates. Moreover, the cation of the photo-excited high-spin system is expected to have interesting electronic structure, in which the triplet state becomes the electronic ground state. Therefore, this system is a kind of the spin-polarized donor proposed by Sugawara et al. [22]. Since the photo-excited high-spin state has fairly long life-time, the exciton transfer is possible to occur in their crystals. The intermolecular photo-induced spin alignment of the dangling radical spins may be achieved by the exciton transfer. The combination with the metal complex will be one of the potential developments. It is special interest in the construction of the spin crossover complex using the photo-excited high-spin organic molecules as their ligands. In the case, the photo-excitation of the ligand leads to open the new pathway to relax the high-spin ground state of the whole system. There will be also other potential applications of the p-conjugated organic spin systems with the photo-excited high-spin states.
4. Summary The spin alignment in the photo-excited states of the purely organic p-conjugated spin systems and their topological isomers are reviewed. In the photo-excited states the photo-induced spin alignment is investigated by TRESR experiments and ab initio MO calculations. Role of the spin delocalization and the spin polarization effects are revealed in the spin alignment on the photoexcited states. The photo-induced spin alignment was achieved through the photo-excited triplet spin coupler. p-Topology plays very important role on the spin alignment on the photo-excited states as well as the ground states. One of the key processes for the photo-control of the organic molecular magnetism is established. The guiding principle designing the photo-excited high-spin system and the role of p-topology are clarified. Some ideas of the potential developments toward the function-
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ality materials utilizing the p-conjugated organic spin systems with the photo-excited high-spin states are also proposed.
Acknowledgements I express my gratitude to all collaborators to the work described here. A particular thank to Prof. Y. Miura for the syntheses of some anthracene derivatives. This work was supported by PRESTO of the Japan Science and Technology Corporation and the Grant-in-Aid for Scientific Research on the general (No. 13440211, 16350079) and Priority Area ‘‘Application of Molecular Spins’’ (Area No. 769, Proposal No. 15087208) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References [1] Y. Teki, S. Miyamoto, K. Iimura, M. Nakatsuji, Y. Miura, J. Am. Chem. Soc. 122 (2000) 984. [2] Y. Teki, S. Miyamoto, M. Nakatsuji, Y. Miura, J. Am. Chem. Soc. 123 (2001) 294. [3] Y. Teki, M. Nakatsuji, Y. Miura, Int. J. Mod. Phys. B 15 (2001) 4029. [4] Y. Teki, Polyhedron 20 (2001) 1163. [5] Y. Teki, M. Nakatsuji, Y. Miura, Mol. Phys. 100 (2002) 1385. [6] Y. Teki, M. Kimura, S. Narimatsu, K. Ohara, K. Mukai, Bull. Chem. Soc. Jpn. 77 (2004) 95. [7] Y. Teki, S. Nakajima, Chem. Lett. 33 (2004) 1500. [8] K. Itoh, Pure Appl. Chem. 50 (1978) 1251. [9] Y. Teki, K. Itoh, in: P.M. Lahti (Ed.), Magnetic Properties of Organic Materials, Marcel-Dekker, 1999, p. 237, and references cited therein. [10] Most recent overviews of the molecule-based magnetism. See: G. Christou (Ed.), Polyhedron 22 (2002) 1725, Proceedings of the VIIIth International Conference on Molecule-based Magnets. [11] (a) C. Corvaja, M. Maggini, M. Prato, G. Scorrano, M. Venzin, J. Am. Chem. Soc. 117 (1995) 8857; (b) N. Mizouchi, Y. Ohba, S. Yamauchi, J. Chem. Phys. 101 (1997) 5966. [12] (a) K. Ishii, J. Fujiwara, Y. Ohba, S. Yamauchi, J. Am. Chem. Soc. 118 (1996) 13079; (b) K. Ishii, J. Fujisawa, A. Adachi, S. Yamauchi, N. Kobayashi, J. Am. Chem. Soc. 120 (1998) 3152. [13] P. Ceroni, F. Conti, C. Corvaja, M. Maggini, F. Paolucci, S. Roffia, G. Scorrano, A. Toffoletti, J. Phys. Chem. A 104 (2000) 156. [14] (a) N. Mizouchi, Y. Ohba, S. Yamauchi, J. Phys. Chem. 103 (1999) 7749; (b) F. Conti, C. Corvaja, A. Toffoletti, N. Mizouchi, Y. Ohba, S. Yamauchi, M. Maggini, J. Phys. Chem. 104 (2000) 4962; (c) J. Fujiwara, Y. Iwasaki, Y. Ohba, S. Yamauchi, N. Koga, S. Karasawa, M. Fuhs, K. Mo¨bius, S. Weber, Appl. Magn. Reson. 21 (2001) 483. [15] K. Ishii, Y. Hirose, N. Kobayashi, J. Phys. Chem. 103 (1999) 1986. [16] (a) M. Lamchen, T.W. Mittag, J. Chem. Soc. C 2 (1966) 300; (b) C. Hirel, K.E. Vostrikova, J. Pecaut, V.I. Ovcharenko, P. Rey, Chem. Eur. J. 7 (2001) 2007.
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[17] G.G. Belford, R.L. Belford, J.F. Burkhalter, J. Magn. Reson. 11 (1973) 251. [18] Y. Teki, I. Fujita, T. Takui, T. Kinoshita, K. Itoh, J. Am. Chem. Soc. 116 (1994) 11499. [19] In the spectral simulation of the TRESR spectra, the calculation of ESP has been added as a subroutine to the computer program. More detailed procedures of the spectral simulation for high-spin organic molecules are similar to those described in
our previous paper for the perturbation approach: Y. Teki T. Takui, H. Yagi, K. Itoh, H. Iwamura, J. Chem. Phys. 83 (1985) 539. [20] A. Bencini, D. Gatteschi, EPR of Exchange Coupled Systems, Springer-Verlag, Berlin, 1990. [21] The DFT calculation was carried out using GAUSSIAN 98, Revision A.11, Gaussian, Inc., Pittsburgh, PA, 1998. [22] T. Sugawara, Mol. Cryst. Liq. Cryst. 334 (1999) 257.