Synthesis and multistimuli-responsive behavior of octaethylporphyrindihexylbithiophenedimethylaniline triads connected with diacetylene linkages

Article abstract of DOI:10.1246/cl.2011.944

Octaethylporphyrindihexylbithiophenedimethylaniline triads were synthesized. The triads showed color change from green to yellow upon addition of trifluoroacetic acid. VT-NMR and UVvis measurements revealed acid- and heat-driven multistimuli-response beha

Full text of DOI:10.1246/cl.2011.944

doi:10.1246/cl.2011.944
944
Published on the web September 5, 2011
Synthesis and Multistimuli-responsive Behavior of Octaethylporphyrin-Dihexylbithiophene-
Dimethylaniline Triads Connected with Diacetylene Linkages
Junro Yoshino,* Mizuki Tsujiguchi, Naoto Hayashi, and Hiroyuki Higuchi*
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555
(Received April 20, 2011; CL-110330; E-mail: yoshino@sci.u-toyama.ac.jp, higuchi@sci.u-toyama.ac.jp)
Octaethylporphyrin-dihexylbithiophene-dimethylaniline
triads were synthesized. The triads showed color change from
green to yellow upon addition of trifluoroacetic acid. VT-NMR
and UV-vis measurements revealed acid- and heat-driven
multistimuli-response behaviors. In comparison with pyridine-
connected analogs, a porphyrin-bithiophene diad moiety con-
nected with diacetylene linkage expands the difference in
basicity between dimethylaniline and pyridine.
³-conjugated system. In the triads 2 bearing an N,N-dimethyl-
aminophenyl group instead of the pyridyl group in 1, that of the
aniline nitrogen is anticipated to overlap with the orbitals of the
³-conjugated system (Figure 1) and the effective conjugation
between them would influence their stimuli-response behaviors.
Here we report synthesis, acid- and heat-driven spectral changes,
and interesting effects of ³-conjugated systems on the stimuli-
response behaviors of octaethylporphyrin-dihexylbithiophene-
dimethylaniline triads 2.
Triads 2a and 2b were synthesized by the Eglinton
coupling8 of 4-ethynyl-N,N-dimethylaniline (3) with porphy-
rin-connected terminal alkynes 4a and 4b,7c respectively
(Scheme 1). Structures of 2a and 2b were confirmed by
1H NMR, IR, and MS spectra.9,10
Stimuli-responsive molecules are important because they
can convert nonvisible information, such as chemical stimuli, to
easily usable forms like color change. In particular, multistimuli-
responsive molecules, receiving two or more kinds of stimuli,
are of interest in the field of smart materials.1 For recent
examples, molecular logic gates,2 displaying devices,3 organo-
gelators,4 and responsive micelles5 have been studied. Recently
we reported acid- and heat-driven spectral changes of octa-
ethylporphyrin-dihexylbithiophene-pyridine triads 1a and 1b6
shown in Figure 1 during the course of our study of long one-
dimensional extended ³-conjugated systems connected with
diacetylene linkages.7
To investigate the effect of electronic structure of the proton
acceptor moiety on the stimuli-response behavior, an N,N-
dimethylaminophenyl group was chosen as another proton
acceptor moiety. Conjugate acids of nonsubstituted pyridine and
N,N-dimethylaniline show almost the same pKa values (5.2 and
5.1 in water, respectively), indicating their basicities are not so
different. However, pyridine and aniline show a distinct differ-
ence between their electronic structures around the lone pair of
the nitrogen atom. In the pyridine-connected triads 1, the lone
pair of the pyridine nitrogen is orthogonal to the orbitals of the
In the UV-vis spectra, compounds 2a and 2b showed
characteristic absorptions of porphyrin derivative (Figure 2).
The wavelengths of absorption maxima attributed to the Q and
Soret bands of 2a were 590 and 448 nm, respectively. That
attributed to the Q band of 2b was 596 nm, and the Soret band of
2b showed maxima at 479 and 449 nm. The red shift of
absorption maxima of 2b in comparison with those of 2a
indicates that 2b has more extended ³-conjugated system than
2a due to the higher planarity of bithiophene moiety connected
in a tail-to-tail manner than that in a head-to-head manner. The
presence of splitting of the Soret band of 2b also shows high
magnitude of interaction among the ³-conjugated systems in 2b
through the bithiophene moiety.7a
Scheme 1. Synthesis of triads 2a and 2b. The substituent R
denotes the same substituent shown in Figure 1.
Figure 1. Frameworks of orbital orientations of a lone pair of
the nitrogen atom and ³-conjugated system of pyridine-con-
nected triads 1a and 1b and aniline-connected ones 2a and 2b.
Figure 2. UV-vis spectra of 2a and 2b in CHCl3.
Chem. Lett. 2011, 40, 944-946
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Figure 3. Photographs of 2a in CHCl3 with (a) no TFA,
(b) low TFA concentration, and (c) high TFA concentration.
Figure 5. Change of 1H NMR spectra of 2a in C2D2Cl4 by
addition of TFA-d (300 MHz).11
1a and 1b. Both 2a and 2b required 10 equiv of TFA to
complete the protonation of the aniline moiety, observed in
1H NMR as the first-step change, while 1.1 and 1.2 equiv of TFA
were necessary for 1a and 1b,6b respectively, under the same
condition. Interestingly, these results suggest that the gap of
basicity between dimethylaminophenyl and pyridyl groups is
expanded and made observable by substitution with a long
extended ³-conjugated system. The ³-conjugated system is
believed to reduce basicity of the lone pair of the aniline
nitrogen of 2 by effective conjugation.
Heat-driven responses of 2a and 2b were also different from
those of 1a and 1b. In the VT-1H NMR spectra of 2a and 2b with
25 equiv of TFA-d in tetrachloroethane-d2, elevation of the
temperature afforded upfield shifts of the signals assigned to
aromatic protons of the dimethylaminophenyl moiety and
sharpening of the signals assigned to meso-position protons
(Figures 6 and S59). Those changes suggest that deprotonation
of 2-TFA complex and regeneration of nonprotonated 2 occur at
high temperature (Scheme 2). The UV-vis spectra supported
this assumption because reappearance of absorption maxima of
nonprotonated 2a was observed at high temperature despite
the presence of excess TFA (Figure 7). In contrast, pyridine
derivative 1a showed only sharpening of the meso-proton and no
shift of aromatic proton, indicating its TFA complex was stable
even at high temperature and no deprotonation occurred.6 The
basicity of 2 is believed to be weak enough to dissociate the TFA
complex upon escape of the TFA from the solution phase as a
vapor at high temperature.
Figure 4. Change of UV-vis spectra of 2a in CHCl3 by addi-
tion of TFA. Isosbestic points are denoted by blue block arrows.
Addition of trifluoroacetic acid (TFA) to a chloroform
solution of 2a and 2b gave color change. The solution color of
2a and 2b turned from green to yellow with increasing amount
of TFA (Figure 3). In the UV-vis spectra of 2a (3.4 ¯M) in
CHCl3, slight red shift of absorption occurred upon addition of
TFA until the amount of acid reached 2 © 104 equiv (Figure 4).
In the region of this amount of acid, an isosbestic point was
found at 600 nm. Upon addition of more acid the spectra was
drastically changed. In the region from 6 © 104 to 1 © 106 equiv
of acid, initial Soret and Q bands decreased and new absorptions
appeared at 700 and 500 nm. Another isosbestic point appeared
at 475 nm during this change. These changes suggest that at
least two equilibria exist in the reaction of 2a with TFA. In
the 1H NMR spectra of 2a (1.8 mM) in tetrachloroethane-d2,
two different changes were observed by addition of TFA-d
(Figure 5). One change was downfield shift of signals at 6.6 and
7.4 ppm assigned to aromatic protons of the dimethylamino-
phenyl moiety, which was completed upon addition of 10 equiv
of TFA-d. The other change was broadening of the signals at
9.4 ppm assigned to the meso-protons of the porphyrin moiety,
which showed more sluggish change than the former. The
former indicates protonation of the dimethylamino group, and
the latter is thought to be caused by interaction between
porphyrin moiety and the acid (Scheme 2).12 Addition of tri-
ethylamine to the mixture of 2a and trifluoroacetic acid gave
In conclusion, porphyrin derivatives 2 bearing a long-one-
dimensional-extended ³-conjugated system and dimethyl-
aminophenyl group as a proton acceptor were synthesized by
utilizing transition-metal-catalyzed cross-coupling reactions.
Drastic color changes and multistimuli-responsive behavior of
2 in response to both acid and temperature were observed.
Through comparison of acid-base reaction behaviors of 2 with
those of pyridine-connected analog 1, the long-one-dimensional-
1
recovery of the initial peaks of 2a in the UV-vis and H NMR
1
spectra. 2b showed similar behavior to 2a in the H NMR and
UV-vis spectra (Figures S39 and S49).13
To investigate the effect of electronic structure of the proton
acceptor moiety on the basicity, protonation behaviors of 2a and
2b were compared by 1H NMR with pyridine-connected analogs
Chem. Lett. 2011, 40, 944-946
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946
Scheme 2. Hypothetical states of equlibria of mixture of 2 and TFA.
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
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Figure 7. VT-UV-vis spectra of 2a with and without TFA in
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8
9
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extended ³-conjugated system was found to expand and make
observable the gap of basicity among proton acceptor moieties.
Further tuning of multistimuli-responsive behaviors such as
colors and detecting target molecules based on the study of these
unique porphyrin derivatives are now underway.
10 1H NMR spectra of 2a and 2b are shown in Figures S19 and
S2.9
11 A signal observed at 6.0 ppm is attributed to C2HDCl4.
12 An unsubstituted octaethylporphyrin nickel(II) complex also
shows reversible broadening of a signal assigned to the meso-
1
Financial support by a Grant-in-Aid for Scientific Research-
es from the Ministry of Education, Culture, Sports, Science
and Technology, Japan (Integrated Organic Synthesis;
No. 22106512), is gratefully acknowledged.
proton in the H NMR spectra upon addition of excess TFA.
13 Evaluation of effects of the difference at the bithiophene
moiety between 2a and 2b on the acid-response behavior is
now underway.
Chem. Lett. 2011, 40, 944-946
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/