Room temperature liquid porphyrins

Article abstract of DOI:10.1246/cl.2010.714

The syntheses of 5,10,15,20-tetrakis(3,4,5-trialkoxyphenyl)porphyrins, which exhibit fluid state behavior at room temperature, are reported. The thermal and rheological measurements indicate that they behave like liquid at 25 °C (298 K).

Full text of DOI:10.1246/cl.2010.714

doi:10.1246/cl.2010.714
714
Published on the web June 5, 2010
Room Temperature Liquid Porphyrins
Sumio Maruyama,* Kenta Sato, and Hiroyuki Iwahashi
Nano Science Research Center, Dai Nippon Printing Co., Ltd. (DNP),
1-1-3 Midorigahara, Tsukuba, Ibaraki 300-2646
(Received February 10, 2010; CL-100144; E-mail: Maruyama-S3@mail.dnp.co.jp)
OR
OR
The syntheses of 5,10,15,20-tetrakis(3,4,5-trialkoxyphen-
RO
RO
OR
OR
yl)porphyrins, which exhibit fluid state behavior at room
temperature, are reported. The thermal and rheological measure-
ments indicate that they behave like liquid at 25 °C (298 K).
N
H
N
N
H
N
Porphyrin derivatives are widely applied in functional
electronic materials such as sensors,1a organic electrolumines-
cent materials,1b organic transistors,1c,1d solar cells,1c,1e nonlinear
optics,1f and liquid crystals.1g ³-Expanded porphyrins are used
in organic electronic applications due to their large ³-conjuga-
tion.1g,2 Porphyrins are also used with functional carbon
molecules such as fullerene dyads3a and carbon nanotubes
(CNTs) for organic electronics.3b Although porphyrins are useful
for electronic applications, their low solubility in organic
solvents makes their handling difficult in solution-process
techniques such as spin-coating and printing for device
fabrications. Only a few reports are available that describe the
use of simple-modified porphyrins with high solubility in
organic solvents, and these use polymers. One simple method
for increasing porphyrin solubility is introducing long alkyl
chains with porphyrins,1c incorporating these within polymers,
and using sol-gel technique.4 Liquids are for easy handling and
processing. Furthermore, bucky-gels,5 developed from ionic
liquid and CNTs, and liquid fullerenes6 are reported to be easily
processed, and therefore can be used for electronic applications.
During our research on simple-modified solution-processible
porphyrins, we believed that room temperature (RT) liquid
porphyrins might be obtained by an appropriate molecular
design. Here, we report the syntheses, thermal properties, and
rheological properties of liquid porphyrins at RT. They are
single-component molecules with fluidity, which may have
potential use in organic electronic applications such as organic
solar cells and organic electrochemical cells.
The most general porphyrins are 5,10,15,20-tetraphenylpor-
phyrin (TPP), which are synthesized from pyrrole with benzal-
dehyde using Lewis acids as catalysts following oxidation with
an oxidant such as 2,3-dichloro-5,6-dicyano-p-benzoquinone
(DDQ).7 The benzaldehyde units are easily modified by
introducing alkyl chains6 connecting the dendrons to dendri-
mers,8 therefore we chose TPP. For our molecular design, to
introduce more alkyl chains, prevent stereoisomers, and because
we expect the molecule to interact with fullerenes and CNTs, we
elected to synthesize 5,10,15,20-tetrakis(3,4,5-trialkoxyphen-
yl)porphyrins (1-Cn), in which n denotes alkyl chain length
(Figure 1). 1-Cn molecules were synthesized as shown in
Scheme 1. 3,4,5-Trialkoxybenzaldehydes were synthesized ac-
cording to a modification of a previously reported procedure.9
Ethyl 3,4,5-trihydroxybenzoate (2) reacted with alkyl bromide
and potassium carbonate in DMF at 90 °C to give ethyl 3,4,5-
trialkoxybenzoate 3. 3 was reduced with lithium aluminum
RO
RO
OR
OR
OR
OR
1-Cn
Figure 1. Structure for porphyrins. R and n denote n-alkyl
group and alkyl chain length, respectively.
OH
OH
OR
OR
O
O
O
O
i
OH
OR
2
3
iii
ii
OR
OR
OR
OR
OR
OR
O
iv
HO
HO
4
5
v
OR
OR
OR
O
H
vi
1-Cn
6
Scheme 1. Reagents and conditions: i) alkyl bromide, K2CO3,
DMF, 90 °C; ii) LAH, THF, reflux; iii) KOH, THF-MeOH-
water, reflux; iv) LAH, THF, reflux; v) MnO2, CHCl3, rt; vi)
pyrrole, TFA, CHCl3, rt, then DDQ.
hydride (LAH) in refluxing THF to give 3,4,5-trialkoxybenzyl-
alcohol 4. In this case, it seemed that the reaction resulted in
little decomposition and low yields when the preliminary
material was oil. To prevent decomposition, the reduction
processes were performed in two steps: first, deprotection of
ester refluxing in THF-methanol-water with potassium hydrox-
ide gave 3,4,5-trialkoxybenzoic acid 5, following the reaction
Chem. Lett. 2010, 39, 714-716
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

715
104
103
102
101
100
cooling
G' (r=10)
G" (r=10)
G' (r=100)
G" (r=100)
G' (r=1000)
G" (r=1000)
heating
10-1
10-2
10-3
1st peak-heating
1st peak-cooling
2nd peak-heating
2nd peak-cooling
10-1
100
101
ω/rad s-1
102
-100
-50
0
50
100
Temperature/°C
Figure 3. Rheological behavior of 1-C12 at 25 °C. Angular
frequency dependence on storage and loss moduli at a £ of 10,
100, and 1000.
Figure 2. DSC chart of 1-C12 at a heating and cooling rate of
10 °C min under nitrogen: 1st scan (red line) and 2nd scan
(blue line), respectively.
¹1
100: 0 w/w
99: 1 w/w
98: 2 w/w
95: 5 w/w
90: 10 w/w
430 nm
with LAH in refluxing THF to give 4. 4 was reacted with
manganese(IV) oxide in chloroform at RT to give 3,4,5-
trialkoxybenzaldehyde 6. 1-Cns were obtained by a reaction
with pyrrole and 6 in chloroform using trifluoroacetic acid (TFA)
as a catalyst at RT followed by oxidation with DDQ.7 The
structures of 1-Cn molecules were assigned by 1H NMR and
MALDI-TOF MS (Electronic Supporting Information (ESI)).10
The purities of 1-Cns were estimated from careful integration
of the 1H NMR spectrum, thermogravimetry, and differential
thermal analysis (TG-DTA). We synthesized 1-Cn with the
alkyl chain lengths from pentyl (C5) to pentadecyl (C15)
derivatives.
655 nm
200
300
400
500
600
700
800
Wavelength/nm
The thermal properties for the synthesized porphyrins were
measured by TG-DTA and differential scanning calorimetry
Figure 4. Normalized absorption spectra based on 655 nm for
1-C12/C60 (w/w) on spin-coated film in air.
(DSC). From TG-DTA analyses (temperature range 30-600 °C,
¹1
with a heating rate of 10 °C min
under nitrogen), the
decomposition temperature (2 wt % of initial weight loss) was
more than 230 °C, and DTA peaks (corresponding to melting
points) of the porphyrins were observed at less than 100 °C, so
that the temperature range for DSC analyses was from ¹100 to
in the same manner. Before solidification, we observed the
liquid-like behavior of the porphyrins and handled them as
liquids. Upon heating, the solidified 1-C14 melted and the liquid
phase reappeared (ESI).
¹1
100 °C, with heating and cooling rates of 10 °C min under
To obtain verification of liquid properties, we measured
rheological properties for fluid 1-C12 at 25 °C. Over the
measured angular frequency (½) range and the strain amplitude
(£) of 10, 100, and 1000, the loss modulus (G¤¤) was higher than
the storage modulus (G¤), thus suggesting liquid-like behavior
(Figure 3). When £ used was 0.01, 0.1, and 1, G¤ was not stable,
also suggesting liquid-like behavior. The frequency dependence
also indicated that there exist a number of components with a
long relaxation time that might reflect cluster formation and
complex dynamics. The complex viscosity (©*) of 1-C12 was ca.
16.5 Pa s as shown in Figure S-1 (ESI). The viscosity (©: Pa s) of
1-C12 is also shown in Figure S-2.
We measured absorption and emission spectra to investigate
the interaction of 1-C12 with [60]fullerene (C60) on spin-coated
film. Absorption spectra for 1-C12 with C60 are shown in
Figures S-3 (normalized based on 430 nm as Soret band) and
S-4 (normalized based on 655 nm as Q band). In Figure S-3, the
absorbance of Q band for 1-C12 increased with addition of C60.
On the other hand, the absorbance of Soret band changed with
addition of C60, as shown in Figure 4. The emission spectra for
nitrogen. For example, the DSC chart of 1-C12 is shown in
Figure 2; the first and second heating scans of the molecule
exhibited a sharp endothermic peak at ca. ¹6 °C, and a
corresponding exothermic peak at ca. ¹21 °C, indicating that
this molecule behaves like a liquid below RT. 1-C5 and 1-C6
melted at 78.6 and 83.9 °C, respectively, and did not show
corresponding exothermic peaks, indicating that they were
amorphous. For 1-C7, 1-C8, and 1-C9, no peaks were observed
from ¹100 to 100 °C. The endothermic peaks of 1-C10 and 1-C11
were observed at ¹40 and ¹30 °C in the first and second heating
scans, respectively. The corresponding exothermic peak of 1-C10
was very weak and that of 1-C11, although very weak or broad,
was observed at ¹23 °C. For 1-C15, the endothermic peak of the
second scan was observed above 25 °C, indicating that it did not
behave as a liquid at RT; therefore, the liquid range of the alkyl
chain lengths are from C10 to C14 derivatives. It seems that 1-Cns
have two phases, the liquid and solid phases, similar to the liquid
fullerenes.6 For example, 1-C14 was obtained in the form of oil,
but slowly solidified. 1-C12 or the other molecules may behave
Chem. Lett. 2010, 39, 714-716
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

716
References and Notes
100: 0 (w/w)
99: 1 (w/w)
98: 2 (w/w)
95: 5 (w/w)
90: 10 (w/w)
85: 15 (w/w)
80: 20 (w/w)
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-20
-30
-10
0
10
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Figure 5. DSC chart of 1-C12/C60 (w/w) at a heating and
¹1
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Chem. Lett. 2010, 39, 714-716
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/