Induction and structural control of chiral nematic phases by the use of photoresponsive tris(?2-diketonato) Co(III) and Ru(III) complexes

Article abstract of DOI:10.1021/jp8011206

Two kinds of tris(?2-diketonato) metal(III) complexes denoted as [M(acac)₂(LC₁₂)] and [M(acac)₂(L _azo)] (M = Ru(III) and Co(III); acac = acetylacetonato; LC ₁₂ = 1,3-didodecyloxyphenyl-1,3-propanedionato; L_azo = 3-[4a?2-(4a?3-(butoxy)phenylazo)phenyl]-pentane-2,4-dionato) were synthesized and optically resolved into ??, ?? isomers. Here, LC ₁₂ and L_azo were designed to be elongated perpendicular to and in parallel with the molecular C₂ axis, respectively. The metal complexes were doped into three kinds of nematic liquid crystals (MBBA, EBBA, and ZLI-1132). Both dopants induced chiral nematic phases with relatively high helical twisting powers. With a purpose to clarify the mechanisms of helical induction, order parameters (S) were determined by means of polarized UV-vis measurements on nematic samples doped with racemic Co(III) complexes. As a result, the long axes of the elongated ligands in both [Co(acac) ₂(LC₁₂)] and [Co(acac)₂(trans-L_azo)] were found to align in the direction of the director vector with S = 0.50 ?± 0.05 and 0.60 ?± 0.05, respectively. Based on the results, the photoresponsive behavior of ??- or ??-[Ru(acac)₂(L _azo)] was interpreted in terms of the variation of S accompanied with the cis-trans isomerization of the azobenzene moiety in L_azo. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp8011206

J. Phys. Chem. B 2008, 112, 9677–9683
9677
Induction and Structural Control of Chiral Nematic Phases by the Use of Photoresponsive
Tris(ꢀ-diketonato) Co(III) and Ru(III) Complexes
Jun Yoshida,†,‡ Hisako Sato,†,§ Naomi Hoshino,|,O and Akihiko Yamagishi*,
Department of Earth and Planetary science, Graduate School of Science, The UniVersity of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, PRESTO, Japan Science and Technology Agency, Tokyo 102-0075,
Japan, and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan
ReceiVed: February 7, 2008; ReVised Manuscript ReceiVed: April 23, 2008
Two kinds of tris(ꢀ-diketonato) metal(III) complexes denoted as [M(acac)2(LC12)] and [M(acac)2(Lazo)] (M
) Ru(III) and Co(III); acac ) acetylacetonato; LC12 ) 1,3-didodecyloxyphenyl-1,3-propanedionato; Lazo
)
3-[4′-(4′′-(butoxy)phenylazo)phenyl]-pentane-2,4-dionato) were synthesized and optically resolved into ∆, Λ
isomers. Here, LC12 and Lazo were designed to be elongated perpendicular to and in parallel with the molecular
C2 axis, respectively. The metal complexes were doped into three kinds of nematic liquid crystals (MBBA,
EBBA, and ZLI-1132). Both dopants induced chiral nematic phases with relatively high helical twisting powers.
With a purpose to clarify the mechanisms of helical induction, order parameters (S) were determined by
means of polarized UV-vis measurements on nematic samples doped with racemic Co(III) complexes. As a
result, the long axes of the elongated ligands in both [Co(acac)2(LC12)] and [Co(acac)2(trans-Lazo)] were found
to align in the direction of the director vector with S ) 0.50 ( 0.05 and 0.60 ( 0.05, respectively. Based on
the results, the photoresponsive behavior of ∆- or Λ-[Ru(acac)2(Lazo)] was interpreted in terms of the variation
of S accompanied with the cis-trans isomerization of the azobenzene moiety in Lazo
.
Introduction
molecules do not seem to be fully understood in terms of
molecular interactions. In many cases, correlation between the
structure of a chiral dopant and induced helix is implicit, and
the magnitude of ꢀM value, even its sign, is difficult to predict.
Nematic liquid crystal (N) phases are known to transform to
chiral nematic liquid crystal (N* or cholesteric liquid crystal)
phases on doping a small amount of chiral molecules (chiral
dopants).1,2 In these phenomena, the chirality of doped molecules
is a main cause in inducing a helical structure. In liquid crystal
systems, the dynamical structure of a molecule is reflected to
the whole system in various ways. When chiral photochromic
molecules are doped into a nematic liquid crystal, for example,
the helical pitch expands or contracts in response to the
photoirradiation.3 In another example, the whole systems rotate
and reorganize when chiral molecular motors are doped into a
nematic liquid crystal.4
Compared with organic dopants with flexible back bones,
octahedral metal complexes are regarded as structurally rigid
chiral dopants.5,6 These complexes are characterized by their
chirality of ∆Λ isomerism with the helical conformation of
ligands. The correlation between ∆Λ isomerism and helicity
was treated theoretically on the assumption that a dopant takes
a single conformation in a host medium.6c–e Calculation was
based on the shape model which is originally proposed for
organic dopants.7 We discovered that the handedness of induced
helix could be controlled by the direction of an elongated ligand
in tris-chelated complexes.6e That is, the ∆ complex, which has
a ligand elongated perpendicular to the C2 axis, induces M helix,
while the ∆ complex, which has a ligand elongated in parallel
with the C2 axis, induces P helix (vice versa for Λ isomer).
The power of inducing an N* phase is measured in terms of
helical twisting power (denoted by HTP or ꢀM) according to
the following equation:
∂p-1
∂x xf0
ꢀ )
(1)
(
)
M
In the present work, two types of tris(ꢀ-diketonato)complexes,
[M(acac)2(LC12)] (M ) Ru(III) and Co(III); LC12 ) 1,3-
didodecyloxyphenyl-1,3-propanedionato) and [M(acac)2(Lazo)]
(M ) Ru(III) and Co(III); Lazo ) 3-[4′-(4′′-(butoxy)phenyla-
zo)phenyl]-pentane-2,4-dionato), were synthesized (Scheme 1).
Here, LC12 and Lazo were designed to be elongated perpendicular
to and in parallel with the molecular C2 axis, respectively. In
order to clarify the mechanisms of helical induction by these
metal complexes, order parameters (S) were determined by
means of polarized UV-vis measurements on nematic samples
doped with racemic Co(III) complexes. Based on the results,
the photoresponsive behavior of ∆- or Λ-[Ru(acac)2(Lazo)] was
interpreted in terms of the variation of S accompanied with the
in which x is the molar fraction of a dopant and p is the pitch
length of an induced helix.1 Although chiral nematic liquid
crystals have been attracting a wide interest from the viewpoint
of dynamical supramolecular systems as stated above, the basic
mechanisms of the transition from N to N* phases by chiral
* Corresonding author. Phone, Fax: +080-3-5978-5575. E-mail:
yamagishi.akihiko@ocha.ac.jp.
† University of Tokyo.
‡ Currently at the Department of Basic Science, Graduate School of Arts
and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo
153-8902, Japan.
§ Japan Science and Technology Agency.
| Hokkaido University.
Currently at the Department of Chemistry, Faculty of Science,
Ochanomizu University, 2-1-1, Otsuka, Bunkyo-ku, Tokyo 112-8610.
O Deceased.
cis-trans isomerization of the azobenzene moiety in Lazo
.
10.1021/jp8011206 CCC: $40.75
2008 American Chemical Society
Published on Web 07/23/2008

9678 J. Phys. Chem. B, Vol. 112, No. 32, 2008
Yoshida et al.
SCHEME 1: Two Types of Studied Complexes with Principal Axial Systems and the Schematic Model of Induced Helix:
(a) ∆-[M(acac)2(LC12)] and Induced M-(left-handed)helix, (b) ∆-[M(acac)2(Lazo)] and Induced P-(right-handed)helix (M
) Co(III) and Ru(III))
Experimental Section
7.31 (d, J ) 8.8 Hz, 2H), 7.02 (d, J ) 8.8 Hz, 2H), 4.06 (t, J
) 6.4 Hz, 2H), 1.94 (s, 6H), 1.81 (m, 2H), 1.52 (m, 2H), 1.00
(t, J ) 7.4 Hz, 3H).
Syntheses of Compounds. (a) 4-[4′-(n-butoxy)phenyla-
zo]iodobenzene (iodo-azo): As a first step, 4-(4′-hydroxyphe-
nylazo)iodobenzene was synthesized according to the literature.8a
4-(4′-hydroxyphenylazo)iodobenzene (5.60 g, 17 mmol) and
potassium t-butoxide (1.93 g, 17 mmol) were dissolved in 30
mL of 2-butanone. To the mixture, 1-bromobutane (2.36 g, 17
mmol) was added and refluxed for 24 h under nitrogen. The
reaction mixture was poured into an ammonium chloride
solution (30 mL) and extracted with dichloromethane (100 mL).
MgSO4 (5.0 g) was added to the solution and filtered. After
evaporating the filtrate, the crude mixture was purified on a silica
gel column using a hexane/dichloromethane (3:1 (v/v)) as an
eluent. Yield: 3.75 g (58%). Elemental analyses: (calcd) C,
50.54; H, 4.51; N, 7.37; (found) C, 50.40; H, 4.57; N, 7.12. 1H
NMR (400 MHz, CDCl3): 7.89 (d, J ) 8.8 Hz, 2H), 7.83 (d, J
) 8.8 Hz, 2H), 7.60 (d, J ) 8.8 Hz, 2H), 6.99 (d, J ) 8.8 Hz,
2H), 4.05 (t, J ) 6.4 Hz, 2H), 1.81 (m, 2H), 1.53 (m, 2H), 1.00
(t, J ) 7.4 Hz, 3H).
(b) 3-[4′-(4′′-(n-butoxy)phenylazo)phenyl]pentane-2,4-dione
(Lazo):8b–d Iodo-azo (13.7 g, 36 mmol) was dissolved in 40 mL
of DMF. To this mixture, CuI (1.43 g, 7.5 mmol), pentane-2,4-
dione (7.20 g, 72 mmol) and K2CO3 (15.1 g, 109 mmol) were
added successively under nitrogen, and heated at 120 °C for
4 h. After the reaction, HCl (1.2 N, 10 mL) was added to the
mixture and extracted with dichloromethane (100 mL). MgSO4
(5.0 g) was added to the solution and filtered. Evaporating the
filtrate, the crude mixture was purified on a silica gel column
using a hexane/dichloromethane (3.5:1 (v/v)) as an eluent. Yield:
1.70 g (13%). Elemental analyses: (calcd) C, 71.57; H, 6.86;
N, 7.95; (found) C, 71.00; H, 7.12; N, 7.75. FAB-MS (positive
FAB in m-nitrobenzyl alcohol, calcd for C21H24N2O3); [M +
H]+ ) 353.2; (found) 353. 1H NMR (400 MHz, CDCl3): 16.72
(s, 1H), 7.92 (d, J ) 8.8 Hz, 2H), 7.89 (d, J ) 8.8 Hz, 2H),
(c) 1,3-di(4-n-dodecyloxyphenyl)propane-1,3-dione (LC12):
This compound was synthesized according to the previous
reports and references therein.6c Yield 45%. 1H NMR (400 MHz,
CDCl3): 17.15 (s, 1H), 7.94 (d, J ) 8.8 Hz, 4H), 6.95 (d, J )
9.0 Hz, 4H), 6.72 (s, 1H), 4.02 (t, J ) 6.6 Hz, 4H), 1.80 (m,
4H), 1.45 (m, 4H), 1.27 (m, 32H), 0.87 (t, J ) 6.8 Hz, 6H).
(d) [Ru(acac)2(LC12)]:9 Yield 30%. Elemental analyses:
(calcd) C, 59.03; H, 7.19; (found) C, 58.19; H, 7.31. FAB-MS
(positive FAB in m-nitrobenzyl alcohol, calcd for C21H24N2O3);
[M + H]+, [M - acac]+ ) 891.2, 792.1; (found) 891.3, 792.3.
1H NMR (400 MHz, CDCl3): 12.72 (s, 4H), 6.21 (s, 4H), 4.04
(m, 4H), 2.16 (m, 4H), 1.30∼1.58 (m, 36H), 0.90 (t, J ) 6.4
Hz, 6H), -2.73 (s, 6H), -4.91 (s, 6H), -27.63 (s, 2H), -38.42
(s, 1H).
(e) [Co(acac)2(LC12)]:10 [Co(acac)3] (1.93 g, 5.4 mmol) and
LC12 (3.2 g, 5.4 mmol) were dissolved in dichloromethane under
nitrogen. To the mixture, BF3-CH3COOH (7.5 mmol) was
added slowly and stirred at room temperature for 24 h. After
the reaction, a 10% CH3COOK solution (10 mL) was added
and extracted with ether (200 mL). The solution was evaporated
and a crude product was purified on a silica gel column using
a mixture of benzene/acetonitrile (10:1) as an eluent. Yield:
1.50 g (36%). Elemental analyses: (calcd) C, 69.32; H, 8.67;
1
(found) C, 69.12; H, 8.78. H NMR (400 MHz, CDCl3): 7.90
(d, J ) 8.8 Hz, 4H), 6.85 (d, J ) 8.8 Hz, 4H), 6.68 (s, 1H),
5.59 (s, 2H), 3.99 (t, J ) 6.6 Hz, 4H), 2.28 (s, 6H), 2.13 (s,
6H), 1.79 (m, 4H), 1.46 (m, 4H), 1.27 (m, 32H), 0.88 (t, J )
7.0 Hz, 6H)
(f) [Ru(acac)2(Lazo)]:9 [Ru(acac)2(CH3CN)2](ClO4)2 (1.6 g, 3.3
mmol) and Lazo (1.17 g, 3.3 mmol) were dissolved in EtOH
(75 mL) and KHCO3 (0.35 g, 3.5 mmol) was added under a

Chiral Nematic Phases
J. Phys. Chem. B, Vol. 112, No. 32, 2008 9679
nitrogen flow. The mixture was refluxed for 24 h. The crude
product was extracted with dichloromethane and dried. After
evaporation, the residue was purified on a silica gel column
using a mixture of benzene and acetonitrile (10:1) as an eluent.
Yield: 73 mg (3.4%). Elemental analyses: (calcd) C, 57.22; H,
5.73; N, 4.31; (found) C, 56.99; H, 5.76; N, 4.14. FAB-MS
(positive FAB in m-nitrobenzyl alcohol): (calcd for
C31H37N2O7Ru) [M + H]+, [M - acac]+ ) 651.7, 551.6;
(found) 652, 552. 1H NMR (400 MHz, CDCl3): 10.36 (s, 2H),
7.84 (d, J ) 8.8 Hz, 2H), 7.04 (d, 2H), 4.58 (s, 2H), 4.08 (t, J
) 6.0 Hz, 2H), 1.82 (m, 2H), 1.57 (m, 2H), 1.00 (t, J ) 6.8
Hz, 3H), -3.76 (s, 6H), -6.89 (s, 6H), -10.04 (s, 6H), -31.32
(s, 2H).
(g) [Co(acac)2(Lazo)]:10 [Co(acac)3] (1.4 g, 3.9 mmol) and Lazo
(1.2 g, 3.4 mmol) were dissolved in dichloromethane under
nitrogen. To the mixture, BF3-CH3COOH (3.9 mmol) was
added slowly and stirred at room temperature for 24 h. After
the reaction, 10% CH3COOK solution (10 mL) was added and
extracted with ether (200 mL). The solution was evaporated
and the crude product was purified on a silica gel column using
a mixture of benzene/acetonitrile (99:1 (v/v)) as an eluent. Yield:
59 mg (3%). Elemental analyses: (calcd) C, 61.18; H, 6.13; N,
Figure 1. CD spectra of the enantiomers of [Co(acac)2(LC12)] in a
chloroform solution: (a) ∆ isomer and (b) Λ isomer.
as a model compound of [Co(acac)2(LC12)]. The molecule was
assumed to belong to C2 symmetry. After the geometry optimiza-
tion, excited states were calculated using TDDFT. The detailed
results are shown in Supporting Information.
1
4.60; (found) C, 61.30; H, 6.05; N, 4.77. H NMR (400 MHz,
Results
CDCl3): 7.91 (d, J ) 9.2 Hz, 2H), 7.86 (d, J ) 8.4 Hz, 2H),
7.31 (d, J ) 8.4 Hz, 2H), 7.00 (d, J ) 9.2 Hz, 2H), 5.57 (s,
2H), 4.05 (t, J ) 6.6 Hz, 2H), 2.26 (s, 6H), 2.21 (s, 6H), 1.99
(s, 6H), 1.82 (m, 2H), 1.55 (m, 2H), 1.00 (t, J ) 7.4 Hz, 3H).
Liquid Crystal Materials. N-methoxybenzylidene-4-n-bu-
tylaniline (MBBA, Tokyo Kasei, Japan), N-ethoxybenzylidene-
4-n-butylaniline (EBBA, Aldrich), and ZLI-1132 (Merck, Japan)
were used as received. ZLI-1132 was a mixture of 4-(4-
alkylcyclochexyl)benzonitrile and 4-(4-alkylcyclohexyl)-4′-cy-
anobiphenyl derivatives (TNI ) 72.3 °C, average molecular
weight ) 263.2).
(1) Syntheses and Optical Resolution. [M(acac)2(LC12)] and
[M(acac)2(Lazo)] (M(III) ) Ru(III) and Co(III); Scheme 1) were
prepared and resolved into pure enantiomers. In the cases of
[Ru(acac)2(LC12)], [Co(acac)2(LC12)], and [Ru(acac)2(Lazo)], two
well-separated major peaks were observed to the baseline separation
in chromatogram. Figure 1 shows the CD spectra of chloroform
solutions of ∆- and Λ-[Co(acac)2(LC12)]. The assignment of ∆-
or Λ- isomer was possible by comparing the CD spectra with that
of ∆- or Λ-[M(acac)3] (M ) Co and Ru).14,15 For these complexes,
the less retained fractions were assigned to the Λ isomers, while
the more retained fractions were assigned to the ∆ isomers. In the
case of [Co(acac)2(Lazo)], only the Λ isomer, which was colored
dark green, was isolated from a less retained fraction. The more
retained fraction, which was colored orange, exhibited no CD
absorbance. Most probably the ∆ isomer was decomposed stereo-
selectively to the orange colored species during the elution.16 The
CD spectra of enantiomers of [Co(acac)2(Lazo)] and
[Ru(acac)2(Lazo)] are shown in Supporting Information. Optically
resolved complexes, [M(acac)2(LC12)] (M ) Co, Ru), were found
to racemize to less than 0.5% in one month. [Ru(acac)2(Lazo)] did
not racemize at less than 120 °C or under photoirradiation, while
[Co(acac)2(Lazo)] racemized gradually at higher temperature (>50
°C) or under the irradiation of a light.
(2) HTP Measurements. The enantiomers of
[M(acac)2(LC12)] and [M(acac)2(Lazo)] (M(III) ) Co(III) and
Ru(III)) were dissolved as chiral dopants in three kinds of
nematic liquid crystals, N-methoxybenzylidene-4-n-butylaniline
(MBBA), N-ethoxybenzylidene-4-n-butylaniline (EBBA), and
ZLI-1132. Molecular HTP (ꢀM; eq 1) was evaluated by the Cano
method as described in the experimental section. The inverse
of pitch (p-1) was plotted against the molar fraction (x) of the
dopant. Examples of such plots are shown in Figure 2 in case
of ∆-[Co(acac)2(LC12)].
Optical Resolution. Optical resolution was performed by
high performance liquid chromatography (HPLC) on a chiral
column (Ceramosphere RU-1, Shiseido, Japan). The column was
packed with an ion-exchanged material of synthetic hectorite
and ∆-[Ru(phen)3]2+ 11
methanol.
.
All compounds were eluted with
Physical Measurements. NMR spectra were measured with
a JEOL JNM-AL400. In addition, FAB-MS measurements were
carried out with a JEOL JMS-700P, a JEOL JMS-HX110, or a
JMS-700TZ for the identification of the synthesized compounds.
UV-vis spectra were recorded with a spectrophotomer (Hitachi
U-2810). Polarized UV-vis spectra were recorded with a
spectrophotomer (Shimadzu UV 2000). Circular dichroism (CD)
spectra were recorded with a JASCO J-720 spectropolarimeter.
The CD or UV-vis spectra of liquid crystal materials doped
with metal complexes were measured using a homogeneous
alignment cell of 25 µm thickness (RP type, EHC, Japan). HTP
measurements were performed by the Cano method using a
wedge cell (EHC, Japan).6b,12 HTP measurements were carried
out at 30, 60, and 35 °C for MBBA, EBBA, and ZLI-1132,
respectively. Temperature was controlled to (0.1 °C by a
Mettler FP82HT hot stage and an FP90 control unit. Photoir-
radiation measurements were carried out using a xenon lamp
with optical filters (UV λ ) 200-400 nm, λmax ) 360 nm;
visible λ ) 400-700 nm).
The sign of HTP was determined by measuring the induced
circular dichroism (ICD) spectra in the range of 350∼400 nm.
Figure 3 shows the ICD spectra of [Co(acac)2(LC12)] in EBBA.
Circular dichroism was induced because of the helical arrange-
ment of host molecules in a chiral nematic phase.17 The positive
or negative ICD spectra was related to the formation of P (right-
handed) or M (left-handed) helix, respectively. The results of
HTP measurements are summarized in Table 1.
Calculation of Transition Moments. Electronic transition
moments were calculated by use of the Gaussian 03 program at
the DFT level (B3LYP functional with LANL2DZ for transition
metals and 6-31G(d) for other atoms).13 The electronic properties
of [Co(acac)2(dbm)] (dbm ) dibenzoylmethanato) were studied

9680 J. Phys. Chem. B, Vol. 112, No. 32, 2008
Yoshida et al.
Figure 2. Plots of the inverse of helical pitch (p-1) versus the molar
fraction of ∆-[Co(acac)2(LC12)] doped in (a) MBBA, (b) EBBA, and
(c) ZLI-1132.
Figure 4. UV-vis spectral change of rac-[Ru(acac)2(Lazo)] (1.4 ×
10-5 M) in a methanol solution under the irradiation of UV or visible
light. The UV-vis spectra were recorded in the following order; (a)
the initial state, (b) after the visible light irradiation, (c) after the UV
light irradiation, and (d) after the reirradiation of visible light.
Figure 3. The induced circular dichroism spectra of EBBA* samples
doped with (a) ∆-[Co(acac)2(LC12)] (solid line) or (b)
Λ-[Co(acac)2(LC12)] (dotted line). Pitch length was 5.5 µm for the ∆
isomer and 4.4 µm for the Λ isomer, respectively. A monitoring light
was incident in the normal direction or along the helical axis. The optical
length of a cell was 25 µm.
Figure 5. Reversible change of HTP in a ZLI-1132 sample doped
with (a) ∆-[Ru(acac)2(Lazo)] (0.15 mol %) and (b) Λ-[Ru(acac)2(Lazo)]
(0.15 mol %) under photoirradiation.
TABLE 1: Values for HTP (ꢀM/µm-1) of ∆, Λ-[M(acac)2(LC12)]
and ∆, Λ-[M(acac)2(Lazo)] in MBBA (30 °C), EBBA (60 °C),
and ZLI-1132 (35 °C)
occurred in the plane perpendicular to the vicinal acac- ligand
(Scheme 1). Accordingly, the azobenzene moiety was electroni-
cally separated from the chiral metal(III) centers.
dopant
MBBA
EBBA
ZLI-1132
In order to see the effect of light-induced isomerization in
liquid crystalline media, the HTP of a ZLI-1132 sample doped
with ∆- or Λ-[Ru(acac)2(Lazo)] was measured under the irradia-
tion of either UV or visible light. As shown in Figure 5, HTP
(µm-1) changed reversibly during photoirradiation from 27 to
16 for the ∆ isomer and from -31 to -18 for the Λ isomer.
(4) Order Parameters of Dopants in ZLI-1132. The order
parameter (S) of a dopant was determined by measuring the
polarized UV-vis spectra of nematic samples. In the case that
an achiral molecule is doped into a nematic liquid crystal, S is
calculated according to the following equation:19
∆-Ru(acac)2(LC12)]
Λ-Ru(acac)2(LC12)]
∆-[Co(acac)2(LC12)]
Λ-[Co(acac)2(LC12)]
∆-[Ru(acac)2(Lazo)]
Λ-[Ru(acac)2(Lazo)]
Λ-[Co(acac)2(Lazo)]
-127
120
-144
135
32
-58
-13
-66
72
-64
68
26
-46
-2
-55
58
-69
71
27
-30
(3) Photoresponsive Behavior of [M(acac)2(Lazo)] in Solu-
tions and Liquid Crystals. Figure 4 shows the change of the
UV-vis spectrum of [Ru(acac)2(Lazo)] in a methanol solution
when the solution was irradiated by UV (<400 nm) or visible
light (>400 nm). Under the illumination of a UV light shorter
than 400 nm, the absorption peak at 356 nm decreased. The
intensity of the peak recovered almost reversibly when the
solution was illuminated by a visible light longer than 400 nm.
The results were ascribed to the cis-trans isomerization of the
azobenzene moiety in [Ru(acac)2(Lazo)].18 The similar change
was observed for [Co(acac)2(Lazo)] except for the slight decrease
of reversibility. The irreversibility might be caused by the
photodecomposition of [Co(acac)2(Lazo)]. In contrast to the
changes in the absorption spectra, no change was induced in
CD spectra under the irradiation of either UV or visible light
for both of enantiomeric [Ru(acac)2(Lazo)] and [Co(acac)2(Lazo)]
(Supporting Information). This was rationalized in terms of the
assumption that the isomerization of the azobenzene moiety
DR - 1
DR + 2
S )
(2)
Here, DR is the dichroic ratio of parallel absorbance to vertical
absorbance (or Apara/Aper). N* phase is thought to have a similar
S value to that in N. ZLI-1132 was chosen as a host medium
because it was transparent at the wavelength longer than 300
nm. Thus, a dopant molecule was desired to have a characteristic
absorption band in this wavelength region. If the transition
moment was correlated to the molecular axis, it was possible
to obtain information on the molecular orientation of a dopant
in liquid crystal phases.
[Ru(acac)2(LC12)] and [Ru(acac)2(Lazo)] have a strong absorp-
tion band around 350 nm. However, the transition moments of
these absorption bands were not uniquely related to molecular
axes, because [Ru(acac)3] itself showed similar strong absorp-

Chiral Nematic Phases
J. Phys. Chem. B, Vol. 112, No. 32, 2008 9681
[M(acac)2(Lazo)] (M(III) ) Co(III) and Ru(III)) as dopants. In
rationalizing the results, it was previously assumed that the metal
complexes oriented with their elongated ligands in the direction
of director of a medium.6e In order to verify these assumptions,
we attempted to determine the order parameter of dopants by
means of polarized UV-vis measurements. The used com-
plexes, [Co(acac)2(LC12)] and [Co(acac)2(trans-Lazo)], possessed
characteristic absorption bands in the range of 350-400 nm,
which were uniquely correlated with the molecular axes. From
the observed dichroic ratios, S was determined with respect to
the director (n) of a host medium as S ) 0.50 ( 0.05 for
[Co(acac)2(LC12)] and 0.60 ( 0.05 for [Co(acac)2(Lazo)]. The
concerned transition moment lied approximately perpendicular
to or in parallel with the molecular C2 axis of [Co(acac)2(LC12)]
or [Co(acac)2(Lazo)], respectively. Thus, for both complexes, the
elongated ligands were concluded to orient in the direction of
n. The same situations were expected to hold for the corre-
sponding Ru(III) complexes.
Figure 6. UV-vis spectra of (a) [Co(acac)2(LC12)] (3.8 × 10-5 M)
and (b) [Co(acac)2(trans-Lazo)] (2.3 × 10-5 M) in a chloroform solution.
tion. In contrast, [Co(acac)2(LC12)] exhibited a strong absorption
peak around 385 nm (ꢀ ) 18 000; Figure 6), which was absent
for [Co(acac)3]. Molecular orbital calculation was carried out
in order to determine the transition moment of the concerned
band. In the calculation, [Co(acac)2(dbm)] (dbm ) 1,3-diphenyl-
According to the shape model, the value of HTP (ꢀM) is
expressed by the following equation:7
RTꢁ
2πK22υm
ꢀM )
Q
(3)
(
)
1,3-propanedionato) was chosen as
a
model of
Here R is the gas constant, T is temperature, K22 is the twist
elastic constant, the interaction energy, and υm is the molar
volume of the solution. By using ordering matrix (Sij) and
helicity tensor Qij, Q is expressed by the following equation:
[Co(acac)2(LC12)]. As a result, among the electronic transitions
with the oscillator strength larger than 0.1, the strong absorption
band was predicted to locate at 343 nm (Supporting Informa-
tion). The band was assigned mainly to the d-π transition.20
Although the experimentally observed peak was located at 40
nm longer wavelength (385 nm), the discrepancy was thought
to be tolerable on the present calculation level. The transition
moment of the absorption lies nearly in the direction of the long
axis of the LC12 ligand or perpendicular to the molecular C2
axis (Scheme 1a). As shown in the same figure, [Co(acac)2(Lazo)]
showed the strong absorbance at 356 nm (ꢀ ) 26 000), which
is assigned to the π-π* transition of the azobenzene moiety.
Its transition moment is thought to be aligned along the long
axis of the azo-group or in parallel with the molecular C2 axis
(Scheme 1b).
Figure 7a,b shows the polarized UV-vis spectra of a ZLI-
1132 sample doped with rac-[Co(acac)2(LC12)] in a glass cell
with homogeneous alignment. A monitoring light was polarized
either in parallel with or perpendicular to the ordering direction
of the cell. Distinct dichroic effect was observed in the
wavelength range of 350-400 nm. Inserting the observed DR
value of the absorption at 385 nm to eq 2, S was calculated to
be 0.50 ( 0.05.
Figure 8a-d shows the polarized spectra of a ZLI-1132
sample doped with a racemic mixture of [Co(acac)2(Lazo)]. A
clear dichroic effect was observed around 370 nm for the initial
sample (curves a and d). From the DR value of the initial
absorbance due to [Co(acac)2(Lazo)], S was estimated to be 0.60
( 0.05. The results corresponded to the trans-form of Lazo in
the complex. Polarized spectra were also measured on the same
sample after the irradiation of UV light (curves b and e).
Although the dichroic effects were still observed at the peak at
370 nm, which was ascribed to the residual fraction of the trans-
form of Lazo, no clear dichroic effect was observed around the
2
Q ) -
(QxxSxx + QyySyy + QzzSzz)
(4)
ꢀ3
On the basis of the results of order parameter as stated above,
the z axis was taken to be perpendicular to or in parallel with
the molecular C2 axis for [M(acac)2(LC12)] or [M(acac)2(Lazo)],
respectively. Three components of helicity tensor Qij are
calculated for the Λ enantiomers of [M(acac)2(LC12)] and
[M(acac)2(Lazo)], respectively, as
3
Qxx ) 2
a3 Qyy ) 0 Qzz )
ꢀ8
3
-2
a3 (for Λ-[M(acac)2(LC12)]) (5)
ꢀ8
3
Qxx ) 0 Qyy ) -2
a3 Qzz )
ꢀ8
3
2
a3 (for Λ-[M(acac)2(Lazo)]) (6)
ꢀ8
Here a represents the effective radius of an acetylacetonato
ligand, assuming that the ligand takes a semispherical form.
Considering that Sij ) 0 and Szz > Sxx, Syy, the following
conclusions are derived:
Q > 0 (for ∧-[M(acac)2(LC12)])
Q < 0 (for ∧-[M(acac)2(Lazo)])
(7)
(8)
Comparing these calculated results with the experimentally
observed HTP values (Table 1), the theory was concluded to
predict correctly the observed sign inversion of ꢀM between the
same enantiomers of [M(acac)2(LC12)] and [M(acac)2(Lazo)].
We observed the reversible change of HTP in a ZLI-1132
sample doped with ∆- or Λ-[M(acac)2(Lazo)]. The results could
be explained by the change of S before and after photoirradia-
tion. From the measurements of the polarized UV-vis measure-
ments of the ZLI-1132 sample doped with rac-[Co(acac)2(Lazo)],
absorption band around 450 nm, which corresponded to the cis-
18
form of Lazo
.
Discussion
In the present work, the principle that helical sense could be
controlled by the elongated direction of a tris-chelated metal
complex was confirmed in cases of [M(acac)2(LC12)] and

9682 J. Phys. Chem. B, Vol. 112, No. 32, 2008
Yoshida et al.
Figure 7. Polarized UV-vis spectra for a ZLI-1132 sample doped with rac-[Co(acac)2(LC12)] (25 µm cell). A monitoring light was polarized (a)
in parallel with or (b) perpendicular to the ordering direction of the cell. The molar concentration of the dopant was 0.25 mol %. Schematic image
of a mixture of ZLI-1132 and Co(III) complex in a homogeneous alignment cell was inserted.
Figure 8. Polarized UV-vis spectra of a ZLI-1132 sample doped with rac-[Co(acac)2(Lazo)] (25 µm optical length). The spectra were measured
with a polarized light in the following order: (a) the initial state (parallel), (b) after UV irradiation (parallel), (c) after visible light irradiation
(parallel), (d) the initial state (perpendicular), (e) after UV irradiation (perpendicular) and (f) after visible light irradiation (perpendicular). The
molar concentration was 0.45 mol% and 0.25 mol%. Schematic image of a mixture of ZLI-1132 and Co(III) complex in a homogeneous alignment
cell was inserted.
[Co(acac)2(trans-Lazo)] showed high S value in the initial state,
while [Co(acac)2(cis-Lazo)] did not show good ordering after
the UV light irradiation. The isomerization of the azobenzene
moiety might occur in an x-z plane (Scheme 1), which was
consistent with the fact that no CD spectral change occurred
before and after UV irradiation (supporting data). On the basis
of these facts, Lazo in [M(acac)2(cis-Lazo)] (M ) Co(III) and
Ru(III)) is no longer aligned with the C2 axis, leading to the
decrease of S value. These situations might lead to the effects
of photoirradiation on HTP as shown in Figure 5.
meric [Ru(acac)2(Lazo)] after the irradiation of UV light; the
calculated transition moments of [Co(acac)2(dbm)]; the deriva-
tion of eqs 5 and 6. This material is available free of charge via
the Internet at http://pubs.acs.org.
References and Notes
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[M(acac)2(LC12)] and [M(acac)2(Lazo)] (M ) Ru(III) and
Co(III)) were newly synthesized and optically resolved. Chiral
complexes were doped in three kinds of nematic liquid crystals
(MBBA, EBBA, and ZLI-1132) and their HTPs were measured
by the Cano method. The sign of HTPs was opposite between
the same enantiomers of [M(acac)2(LC12)] and [M(acac)2(Lazo)].
The order parameter (S) of racemic Co(III) dopants in ZLI-
1132 was determined by means of polarized absorption spectra.
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handedness of an induced helix. HTP change by photoirradiation
was accomplished in the case of the ZLI-1132 sample doped
with ∆- or Λ-[Ru(acac)2(Lazo)]. The change was interpreted in
terms of the S change triggered by photoisomerization.
Supporting Information Available: The CD spectrum of
Λ-[Co(acac)2(Lazo)]; the change of the CD spectra of enantio-

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