Staircase-form assembly with 5,15-bis(imidazol-4-yl)porphinatogallium steps

Article abstract of DOI:10.1039/b211839d

Large supramolecular assemblies of 5,15-bis(imidazol-4-yl)porphinatogallium 2Ga have been synthesized by hexacoordination of two imidazolyl groups to Ga(III) in a staircase arrangement of porphyrin planes as each step . The structure of the supramolecular assembly was characterized by UV, X-ray diffraction, and AFM. The conductivity of 2Ga films was enhanced 8 times by photoirradiation.

Full text of DOI:10.1039/b211839d

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Staircase-form assembly with 5,15-bis(imidazol-4-yl)porphinatogallium
steps
Naoto Nagata, Shin-ichi Kugimiya, Ei-ichi Fujiwara and Yoshiaki Kobuke*
Graduate School of Materials Science, Nara Institute of Science and Technology, CREST,
Japan Science and Technology Corporation (JST), 8916-5 Takayama, Ikoma City 630-0101,
Japan. E-mail: kobuke@ms.aist-nara.ac.jp
Received (in London, UK) 28th November 2002, Accepted 24th February 2003
First published as an Advance Article on the web 6th March 2003
Large supramolecular assemblies of 5,15-bis(imidazol-4-yl)porphinatogallium 2Ga have been synthesized
by hexacoordination of two imidazolyl groups to Ga(III) in a staircase arrangement of porphyrin planes as
each ‘‘step’’. The structure of the supramolecular assembly was characterized by UV, X-ray diffraction, and
AFM. The conductivity of 2Ga films was enhanced 8 times by photoirradiation.
Introduction
Results and discussion
Synthesis of molecular wires and switches is the first target to
opening the world of molecular electronics.1 Many attempts to
construct supramolecular wires for efficient transport of elec-
trons/holes and excitons have been reported by the use of
non-covalent2–4 linkages of porphyrins. Most of these systems
were constructed in the solid state or in highly concentrated
solution such as under NMR conditions (mmol dmꢀ3). Only
a few systems3a–d,4 have been reported for the preparation of
supramolecular wires in solution by the use of the large bind-
ing constants (K > 108 dm3 molꢀ1). In the mid-1990s, we suc-
ceeded in preparing the special pair model3a by slipped cofacial
coordination of meso-substituted bis(N-methylimidazol-
2-yl)porphinatozinc 1Zn.3a The binding constant of 1Zn in
CHCl3 is more than 109 dm3 molꢀ1. This large binding con-
stant is a result not only of complementary imidazolyl coordi-
nation to the zinc ion, but also of p–p stacking interactions
between two porphyrin rings. Therefore, imidazolylporphyrin
zinc complexes may be one of the most ideal molecular units
for constructing supramolecular wires. We have recently suc-
ceeded in constructing a giant supramolecular porphyrin array
(ꢁ800 porphyrin units based on GPC) by extending this stra-
tegy.3c We report here the preparation of a novel coordination
polymer of a bis(imidazol-4-yl)porphyrin gallium complex by
the use of hexacoordinating gallium(III) as the central metal
ion (Scheme 1).
Thec choice of the gallium(III) ion is based on the following
two reasons. Firstly, hexacoordination of Ga ions is well estab-
lished in coordination chemistry and actually Ga porphyrin
forms a complex with two nitrogen ligands.5 Moreover, coor-
dination of imidazolate anion to gallium(III) ion will be stron-
ger than that of neutral imidazolyl nitrogen to gallium(III) ion.
Secondly, the d10 electronic configuration is free from excita-
tion energy quenching and appropriate for constructing photo-
controllable supramolecular switches.6 Hexacoordinated Co
porphyrin complexes4 are certainly one of the candidates for
supramolecular wires. However Co porphyrins are not suitable
for a photo-controllable wire, because of the fast radiationless
decay of the excitation energy.6
Synthesis of Ga complex 2Ga
Bis(imidazol-4-yl)porphinatogallium complex 2Ga was pre-
pared by mixing the free base 2 with excess GaCl3 in
CHCl3–MeOH followed by washing the CHCl3 layer with
saturated aqueous sodium bicarbonate.3c The insertion of
Ga(III) ion was confirmed by MALDI-TOF mass, UV and
fluorescence spectra. Significant line broadening in the 1H
NMR spectrum of 2Ga precluded unfortunately the use of
NMR spectroscopy. Fig. 1 shows the MALDI-TOF mass spec-
trum of bis(imidazolyl)porphinatogallium complex 2Ga (using
Scheme 1
DOI: 10.1039/b211839d
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N-methylimidazolylporphinatozinc.3a Since 2Zn dimer has
imidazolyl N–H groups, they can interact with each other
through hydrogen-bonds in CDCl3 as shown in Scheme 2a.
Like other imidazolylporphinatozinc complexes, 2Zn has
two split Soret bands at 418 and 430 nm in the UV-Vis spec-
trum in CHCl3 as shown in Fig. 3. These splittings are caused
by the exciton coupling of the corresponding B transitions of
porphyrins in a slipped cofacial arrangement as shown in
Scheme 3.3,7 There are two in-plane transition moments, My
and Mz , in monomeric porphyrin. In the case of 2Zn, the Soret
band is split into two peaks: blue shifted 418 nm for the Mz
component of face to face orientation and red shifted 430
nm for the My component with head to tail excitons. The
hydrogen-bonds in 2Zn do not seem to have an effect on the
split, since free base porphyrin gave only broadening of
the peak.3e The supramolecular array 2Ga showed very large
split Soret bands compared to Zn complex 2Zn (Fig. 3). In
the case of 2Ga, the Soret band was split into two peaks:
394 nm and 438 nm as more blue shifted Mz and more red
shifted My components, respectively as shown in Scheme 3.
The splitting energy DE of 2Ga (2550 cmꢀ1) is far greater than
that of 2Zn (670 cmꢀ1). Since the splitting energies of Soret
bands depend on the number of interacting porphyrins,8
supramolecular array 2Ga must have a larger degree of poly-
merization than 2Zn.
Fig. 1 MALDI-TOF mass spectrum of bis(imidazolyl)porphinato-
gallium complex, 2Ga.
In spite of the above estimation, we could not determine the
average molecular weight of supramolecular array 2Ga by
GPC due to the serious adsorption. Low solubility of 2Ga in
CHCl3 also prevented us from determining the average mole-
cular weight of 2Ga by VPO or DLS measurement.
Fig. 2 Fluorescence spectrum of 2Ga in CHCl3 at 6 mmol dmꢀ3
lEX ¼ 438 nm.
.
Molecular modeling
dithranol as a matrix, Mw ¼ 1029.84). As expected, the peak
with the highest intensity was observed at 1030 mass units.
In addition to this signal, there were observed three additional
peaks at 2061.64, 3090.01, 4120.1 mass units, corresponding to
dimers, trimers, and tetramers of 2Ga, respectively. Fluores-
cence peak intensities of 2Ga were relatively strong, as
expected (Fig. 2).6
Supramolecular array 2Ga was constructed using a SGI pro-
gram Cerius2. Geometry optimization was carried out using
the universal 1.02 force field. The geometry obtained for supra-
molecular array 2Ga by molecular mechanics calculations
(Fig. 4) shows a porphyrin plane like staircase steps.9 The cal-
culated distance between the porphyrin planes is 0.35 nm.
The Ga–Ga distance is 0.6 nm, and the height (Im–Ga–Im)
and the width (o-CH3–o-CH3) of 2Ga are 1.4 and 4.8 nm,
respectively.
Split Soret band of porphyrin arrays
The bis(imidazolyl)porphinatozinc complex, 2Zn, was also
prepared as a reference compound from the corresponding free
base porphyrin 2 and zinc acetate. The structure was con-
firmed by 1H-NMR, MALDI-TOF mass, and UV-Vis spectra.
The 1H-NMR spectrum of 2Zn was broad in CDCl3 due
to the formation of hydrogen-bond networks. In a solution
containing 2.2 mol dmꢀ3 d4-methanol, the peaks of the
1H-NMR spectrum of 2Zn were sharpened and were identified
as the zinc dimer complex by comparison with the spectrum of
X-Ray diffraction study
The size of supramolecular array 2Ga was also confirmed by
X-ray diffraction studies in the solid state.10 As shown in
Fig. 5, clear diffractions were observed at around 31.3ꢂ and
45.4ꢂ (after 2y–I conversion). Two peaks appearing close
together may indicate the presence of two types of Ga–ligand
coordination bonds with similar structures. The lattice con-
stant d ¼ 0.57–0.6 nm was obtained by using Bragg’s equation
Scheme 2 Hydrogen-bond network through imidazolyl groups in 2Zn supramolecular assembly in CDCl3 was cleaved by d4-MeOD.
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Fig. 3 UV-Vis spectra of bisimidazolylporphyrin complexes in
CHCl3 . Solid line: 2Ga (30 mmol dmꢀ3), dotted line: 2Zn (5 mmol
dmꢀ3).
(1).10 Using the full-width at half-maximum (fwhm) of the
Bragg peak, 2.4 ꢃ 10ꢀ3–3.1 ꢃ 10ꢀ3 radians in the X-ray data
and the Debye–Scherrer equation (2),10 the size of supramole-
cular assembly 2Ga in the solid state was estimated as 23–28
nm. The evaluated aggregation number of 2Ga was 66–80 by
the use of 0.35 nm as the porphyrin plane–plane distance. Such
a diffraction pattern pattern was not observed in the corre-
sponding free base 2. Coordination of Ga(III) to imidazolyl
groups is assumed to be essential to form the staircase-type
supramolecular array 2Ga.
Fig. 4 Minimum energy structure of 2Ga calculated by Cerius2 with
universal force field. Side chains were omitted for the sake of clarity. a)
Top view and b) side view with a stick model.
(gap ¼ ca. 100 mm) with/without light irradiation (l ¼ 532
nm, a continuous wave YAG laser, 160 mW cmꢀ2). A sche-
matic representation of cells for the measurement of conduc-
tivity is shown in Scheme 5. After the sample was dried
completely, the currents between the electrodes were measured
as shown in Fig. 7. Rough estimates of specific resistances of
2Ga multilayered films in the dark and under continuous light
irradiation are about 1.5 ꢃ 106 and 1.8 ꢃ 105 O m, respectively,
assuming that the 100 mm gap and 25 nm depth of the cell were
filled with 2Ga. The current level is still low mainly because
of the presence of boundaries between assemblies. Even so,
the conductivity between the gap electrodes was increased by
800% upon light irradiation. This large enhancement of the
conductivity is particularly noteworthy. The photocurrent of
2Ga did not change during the measurements with continuous
light irradiation for 5 min. The light irradiation may facilitate
electron/hole transfers in the 2Ga array.
AFM imaging
Fig. 6 displays a tapping mode AFM image of the sample pre-
pared from a solution of 2Ga in CHCl3 (ca. 5 mmol dmꢀ3, 1
mL) on a fresh HOPG substrate surface. Some long plates
(150 ꢃ 50 nm) were observed on the substrate along with
shorter plates (50 ꢃ 60 nm). The image of the HOPG substrate
prior to the sample application was featureless. The average
height of 2Ga in the topographic AFM image shown in Fig.
6b was 1.8–2.2 nm, which was a little higher than 1.4 nm
and shorter than 4.8 nm, the calculated height and width of
2Ga as shown in Scheme 4. Based on this result, the monomers
in the supramolecular assembly 2Ga possibly spread their alkyl
chains in the horizontal direction.
Photo-induced current measurement
The current–voltage characteristics of the 2Ga supramolecular
assembly were measured between gold micro-gap electrodes
Scheme 3 Exciton coupling of bisimidazolylporphyrins 2Zn or 2Ga
in a slipped cofacial arrangement.
Fig. 5 X-Ray diffraction patterns of 2Ga self-assembled array. a)
Large y region, b) small y region.
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Fig. 6 Topographic image of 2Ga acquired by AFM on a HOPG
substrate. a) Top view, b) section along the line.
Concluding remarks
We have synthesized supramolecular assemblies of 5,15-bis-
(imidazol-4-yl)porphinatogallium 2Ga by axial coordination
of two imidazolyl groups to Ga(III) in a staircase arrangement.
These results indicate that 2Ga transports electrons under light
irradiation as a photo-supramolecular wire. The calculated dis-
tance between adjacent porphyrin planes of the staircase is
0.35 nm, which is similar to the distance (0.34 nm) between
adjacent base pairs in B-form of DNA11 and the distance
(0.39 nm) between adjacent porphyrins in discotic liquid
crystalline porphyrins.12a In these discotic liquid crystalline
porphyrins, similar enhancement (ca. 5 times) of the con-
ductivity with light irradiation has been observed.12b These
materials have been studied intensively for new electronic and
optoelectronic applications. We believe that the imidazolylpor-
phyrin arrays are one of the suitable candidates for construct-
ing supramolecular light switches or wires.
Scheme 4 Schematic illustration of 2Ga assembly. a) Typical size of
2Ga observed in AFM, b) unit size calculated by Cerius2, c) estimation
of the assembly.
dissolved in 5 mL of methanol was added. The solution was
refluxed for 1.5 h. After removal of the solvents, the residue
was dissolved in 30 mL of CHCl3 and washed with distilled
water. The CHCl3 layer was concentrated to dryness. The pro-
duct was purified by silica gel (CHCl3:CH3OH 5:1 v/v) chro-
matography to give dark red crystals 2Zn (4.2 mg, 4.1 mmol,
1
yield 79%). H NMR (400 MHz, CDCl3 0.5 ml, CD3OD 80
ml) d: 9.04 (4 H, brs, pyrrolebH), 8.95 (4 H, brs, pyrrolebH),
8.51 (4 H, brs, phenoxy), 8.22 (4 H, brs, pyrrolebH), 7.88 (4
H, brs, phenoxy), 7.60 (2 H, s, imidazole), 7.45 (2 H, s, imida-
zole), 7.40 (4 H, brs, phenoxy), 7.19 (4 H, brs, phenoxy), 5.82
(4 H, brs, pyrrolebH), 4.30 (8 H, brs, OCH2), 2.00 (8 H, m,
–CH2–), 1.65–1.16 (72 H, m, –(CH2)9–), 1.00 (12H, m, –CH3),
UV-Vis (CHCl3) lmax/nm: 418, 431, 568, 619. Fluorescence
(CHCl3 , lEX ¼ 431 nm) lEM/nm: 626. MALDI-TOF mass
Experimental
Preparation of porphyrin Ga complex, 2Ga
Into a solution of 6.5 mg (6.75 mmol) of free base porphyrin 2
dissolved in 5 mL of CHCl3 , 118 mg (675 mmol) of gallium
chloride dissolved in 2 mL of ethanol was added. The solution
was refluxed for 3 h under Ar. After removal of the solvents,
the residue was dissolved in 20 mL of CHCl3 and washed with
saturated aqueous sodium bicarbonate solution. The CHCl3
layer was concentrated to dryness. The product was purified
by silica gel (CHCl3:CH3OH 5:1 v/v) chromatography to give
dark green crystals 2Ga (5 mg, 4.86 mmol, yield 72%). UV-Vis
(CHCl3) lmax/nm: 394, 438, 566, 623.5. Fluorescence (CHCl3 ,
lEX ¼ 438 nm) lEM/nm: 613, 661. MALDI-TOF mass (dithra-
nol): m/z ¼ 1029.84 (calc. M+: 1029.50 for C62H7269GaN8O2)+.
Preparation of porphyrin Zn complex, 2Zn
Into a solution of 5 mg (5.2 mmol) of free base porphyrin 23c
dissolved in 6 mL of CHCl3 , 95 mg (520 mmol) of zinc acetate
Scheme 5 Dimensions of gold electrodes for photocurrent measure-
ment and 2Ga film.
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room temperature for 3 h, the dark current between the elec-
trodes was measured. The conductivity of the 2Ga film was
measured at 25 ꢂC by a Manual Prober Model705B, Micro-
nics, Japan and a Semiconductor Parameter Analyzer 4156C,
Agilent Technologies. The temperature of the sample was con-
trolled at 25 ꢂC by a Peltier effect device. The photocurrent of
the 2Ga film was measured under irradiation with a CW YAG
laser (532 nm, 160 mW cmꢀ2). One scan from ꢀ20 V to 20 V
took about 90 seconds. Three scans were recorded sucessively.
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˚
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lK
cos yDð2yÞ
L ¼
ð2Þ
7
8
Conductivity measurements
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˚
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˚
rate of approximately 2–3 A per second. The dimensions of the
cell are shown in Scheme 5. The cast film of 2Ga was obtained
by casting a CHCl3 (ca. 0.1 mmol dmꢀ3, 1 mL) suspension on
the gold electrodes. The 2Ga film covered the electrodes com-
pletely. After the sample was dried under vacuum (0.1 Torr) at
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