Antioxidant-substituted tetrapyrazinoporphyrazine as a fluorescent sensor for basic anions

Article abstract of DOI:10.1039/c2cc30712j

Tetrapyrazinoporphyrazine substituted at its periphery with eight antioxidant 3,5-di-t-butyl-4-hydroxyphenyl groups behaves as a turn-on fluorescent sensor for fluoride anions. Conversely, the precursor antioxidant-substituted 1,2-phthalonitrile was found

Full text of DOI:10.1039/c2cc30712j

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COMMUNICATION
Antioxidant-substituted tetrapyrazinoporphyrazine as a fluorescent sensor
for basic anionswzy
Jonathan P. Hill,*ab Navaneetha K. Subbaiyan,c Francis D’Souza,*c Yongshu Xie,*d
Satyajit Sahu,e Noelia M. Sanchez-Ballester,a Gary J. Richards,af Toshiyuki Morif and
Katsuhiko Arigaab
Received 1st February 2012, Accepted 22nd February 2012
DOI: 10.1039/c2cc30712j
Tetrapyrazinoporphyrazine substituted at its periphery with
eight antioxidant 3,5-di-t-butyl-4-hydroxyphenyl groups behaves
as a turn-on fluorescent sensor for fluoride anions. Conversely,
the precursor antioxidant-substituted 1,2-phthalonitrile was
found to act in turn-off mode suggesting that the origin of the
phenomenon lies at the phenolate-substituted 1,4-pyrazinyl
moiety.
one of the best studied classes of organic compounds.4 On the
other hand, fluoride and other anions are important analytes
for a variety of reasons including potential toxicities. Detection
of fluoride anions may be facilitated due to its differing
reactivity from the other halides and several promising chemical
sensors of fluoride anions have been reported.5 Usually their
sensing capabilities are based on variation of electronic absorption
(or fluorescence) spectra in the presence of fluoride anions
or modulation of electrochemical potentials.6 Additionally,
substantial changes in colour can be used as a qualitative
indicator of the presence of fluoride (or other) anions in
analyte solutions.7
In this work we have prepared a tetrapyrazinoporphyrazine8
substituted with antioxidant 2,6-di-t-butylphenol groups with
the initial aim of investigating any structural changes that
might be promoted by oxidation of the compound in a
way analogous with other similarly substituted molecules.9
Unexpectedly, our new tetrapyrazinoporphyrazine derivative
was found to operate as a fluorescent sensor especially for
fluoride anions due to the phenolic substituents whose acidity
is such that deprotonation occurs in the presence of
basic anions.10 That is, the compound 2,3,9,10,16,17,23,24-
octakis(3,5-di-t-butyl-4-hydroxyphenyl) tetrapyrazinoporphyrazine,
1 (Fig. 1), operates as a chromogenic and fluorogenic indicator
for the presence of fluoride anions (and other basic anions) in
solutions. 1 precursor 211,12 was also found to be sensitive to
the presence of basic anions. Control analogues of 1 and 2
bearing no phenol substituents were also prepared.12
Chromophoric molecular systems for sensing of analyte species
are receiving increasing attention1 due to their ease of preparation,
high sensitivities and tunable specificities as well as for their
simple implementation. Several dye species have been developed
for this purpose especially the oligo-pyrromethanes2 and
porphyrinogens.3 Phthalocyanines, although possessing several
of the requirements of facile syntheses, well-defined electro-
chemistry and intense optical absorptions, which might be
strongly perturbed by analyte species, have been less well developed
for use as sensing elements despite the phthalocyanines being
a WPI-Centre for Materials Nanoarchitectonics, National Institute of
Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki,
305-0044, Japan. E-mail: Jonathan.Hill@nims.go.jp;
Tel: +81-(0)-29-8604832
b JST-CREST, Namiki 1-1, Tsukuba, Japan
c Department of Chemistry, University of North Texas, 1155,
Union Circle, Denton, TX 76203-5017, USA.
E-mail: francis.dsouza@unt.edu; Tel: 940-369-8832
d Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology,
Meilong Rd. 130, Shanghai 200237, P. R. China.
E-mail: yshxie@ecust.edu.cn
Compound
1 was synthesized according to methods
e Surface Characterization Group, Advanced Key Technologies
Division/Nano Characterization Unit, National Institute of Materials
Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan
f Fuel Cell Materials Center, National Institute for Materials Science
(NIMS), 1-1 Namiki, Tsukuba, 305-0044, Japan
reported by Makhseed, McKeown and coworkers.12,13
McKeown and coworkers have previously prepared phthalo-
cyanines peripherally substituted with up to four 3,5-di-t-
butyl-4-hydroxyphenyl groups.14 A subphthalocyanine analogue
of 1 was also recently reported.15 Although metal complexes
(e.g. CoII) of 1 can be obtained with good purity,16 samples of the
free base tetrapyrazinoporphyrazine 1 always contained B5% of
a singly hexyloxy-substituted molecule due to the in situ sub-
stitution of a 3,5-di-t-butyl-4-hydroxyphenyl group during the
macrocyclisation (Li/hexanol; reflux) as reported previously.13
Fig. 1(a,b) respectively show the chemical structure and a
scanning tunneling microscopy image of 1 while Fig. 1(c,d)
respectively show the structure of 1 precursor 2 as well as a
w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’
web themed issue.
z Electronic Supplementary Information (ESI) available: Details of
syntheses, additional experimental procedures and cif files for 2 and its
semi-substituted precursor 5-chloro-6-(3,5-di-t-butyl-4-hydroxyphenyl)
pyrazine-2,3-dicarbonitrile, 3. See DOI: 10.1039/c2cc30712j
y X-ray Crystallography data for 2: C34H42N4O2, M = 538.72 g molꢀ1
,
space group P 43 21 2, a = 15.9922(12), b = 15.9922(12), c =
12.4393(19) A, a = 90.001, U = 3181.36(59) A3, Z = 4, rcalcd
=
1.125 mg mꢀ3, m(Mo-Ka) = 0.070 mmꢀ1, 25867 reflections measured,
1997 were unique (Rint. 0.0271); refinement against F2 to wR2: 0.1360,
R1 = 0.0491 (1855 reflections with I 4 2s(I)), 193 parameters.
c
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Chem. Commun., 2012, 48, 3951–3953 3951

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Fig. 1 (a) Chemical structure of 1. (b) Scanning tunneling micro-
scopy (STM) image of 1 (sample bias = 1.5 V). (c) Chemical structure
of 2 and (d) its X-ray crystal structure.
view of the X-ray crystal structure of the latter. The STM
image clearly shows the lobes of the molecule and a two-tier
arrangement of the bulky t-butyl groups12 caused by their
twisting due to steric requirements and surface interactions.
The molecular structure of 2 reveals the twisting of phenyl
substituents and an angle of 39.81 is subtended between the
1,4-pyrazinyl ring and phenyl rings of the phenol substituents.
Such an arrangement may persist in tetramerised 2.
Fig. 2 Electronic absorption spectra (UV-vis) and fluorescence
emission (Fl) spectra of 1 and 2 in the presence of increasing quantities
of fluoride anions in benzonitrile solution. (a) 1; UV-vis. (b) 1; Fl. (c =
5 ꢁ 10ꢀ6 M) (c) Photographs of solutions of 1 (upper) and 2 (lower) in
benzonitrile: (i) No anions present; (ii) no anions present with irradia-
tion (365 nm UV); (iii) just following addition of a drop of a solution
of tetrabutyl-ammonium fluoride; (iv) mixed with an excess of Fꢀ
anions; (v) with Fꢀ under 365 nm UV. (d) Change in fluorescence
emission intensity from 1 in the presence of increasing quantities of the
noted anions. (e) 2; UV-vis. (f) 2; Fl. (c = 8 ꢁ 10ꢀ6 M).
1 responds to the presence of fluoride anions by changing
colour of its solutions in organic solvents from gray-blue to
green. Variation of its UV/vis spectrum upon addition of
fluoride anions is shown in Fig. 2(a). Essentially, bands
around 700 nm coalesce while the broad band at 540 nm
disappears. Additionally, a large fluorescence response was
found (Fig. 2b) with a very substantial increase in emission
intensity at 680 nm with a decrease in intensity of the
fluorescence emission band at 500 nm.17 Phthalonitrile pre-
cursor 2 also responds to the presence of fluoride anions.
A new intense absorbance band appears at 550 nm in its UV/vis
spectrum with concurrent attenuation of the band at 400 nm
(Fig. 2e). Also, in contrast to 1, fluorescence emission intensity
of 2 is completely attenuated by addition of fluoride anions
(Fig. 2f). Analogues of 1 and 2 containing no phenol groups12
underwent no similar variation in their electronic absorption
or fluorescence spectra indicating strongly that the phenol
groups of 1 and 2 are interacting with fluoride anions, most
likely through deprotonation.10
anions causes formation of a deep purple almost non-emissive
solution [Fig. 2c(v); lmax = 550 nm].19 In contrast, 1 contains
bands at 500 nm and a weak fluorescence band at 680 nm.12,17
When Fꢀ anions are added emission at 500 nm is attenuated
while emission at 680 nm due to deprotonated species is
enhanced. We are currently investigating these phenomena
from experimental and theoretical viewpoints.
Fig. 3 shows the cyclic voltammograms of 1 and 2 measured in
benzonitrile solution. 1 exhibits two reversible and three quasi-
reversible reduction processes and, interestingly, these reduction
waves of 1 are anodically shifted by up to 100 mV depending on
the quantity of fluoride anions added. On the other hand, 2 does
not undergo well-defined redox processes in its reduction wave.
The redox behaviour of 1 is due to the conversion of phenolate
groups to easily reducable hemiquinonoid groups, a feature
which is observed in other similarly substituted compounds
including porphyrins20 and several other species.9c,d,e 1 appears
not to be significantly affected by addition of Fꢀ anions in terms
of UV/vis (i.e. it still exhibits a typical phthalocyanine-like
spectrum) so that it is not surprising that only observable shifts
in reduction potentials can be observed. On the other hand, 2 is
strongly affected apparently experiencing strong delocalization of
its electron system upon deprotonation. This results in a new
UV/Vis band19 and also to two reversible oxidations presumably
due to oxidation to a quinonoid state.12
To assess the degree of deprotonation of 1 necessary for
anion detection the dependency of the fluorescence emission
intensity on the amount of fluoride or other anions present
was investigated (see Fig. 2d). This data suggests that approxi-
mately 4 equivs of the respective anions suffice to saturate the
fluorescence response of 1. Photographs of solutions of 1 and 2
in benzonitrile18 in the absence and presence of Fꢀ anions
(Fig. 2c) show its colour variations suggesting use of 1 as a
sensor for Fꢀ anions. 1 and 2 exhibit a similar yellow-green
fluorescence [around 500 nm; Fig. 2c(ii)], which originates at
their substituted pyrazinyl moieties. In 2, addition of Fꢀ
c
3952 Chem. Commun., 2012, 48, 3951–3953
This journal is The Royal Society of Chemistry 2012

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2 (a) P. Anzenbacher, Top. Heterocycl. Chem., 2010, 24, 205;
(b) J. L. Sessler, N. M. Barkey, G. D. Pantos and V. M. Lynch,
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Chem., 2010, 24, 237; (d) H. Maeda, Top. Heterocycl. Chem., 2010,
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3 (a) J. L. Sessler and P. A. Gale, in The Porphyrin Handbook,
ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press,
San Diego, CA and Burlington, MA, 2000, vol. 6, pp. 257–278;
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J. W. Steed, John Wiley & Sons, 2012, pp. 1713–1730.
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Fig. 3 Cyclic voltammograms in benzonitrile/(0.1 M n-Bu4N ClO4).
(a) 1. (b) 2. (c) 1 in the presence of increasing amounts of fluoride
anions (up to 10 eq.). (d) 2 in the presence of increasing amounts of Fꢀ
(up to 10 eq.).
6 A. L. Schumacher, J. P. Hill, K. Ariga and F. D’Souza, Electrochem.
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In conclusion, we have synthesized for the first time a
phthalocyanine-type molecule, 1, symmetrically substituted at its
periphery with eight antioxidant 3,5-di-t-butyl-4-hydroxyphenyl
groups. The phenol groups of 1 confer on it sensing abilities
in terms of its increased fluorescence emission when it is
deprotonated by basic anions especially fluoride, acetate or
phosphate. In addition, 1 precursor 2 is also active as a
fluorescent sensor for basic anions although it contrasts with
1 in that its fluorescence is attenuated. 1 and 2 also exhibit
electrochemical responses which are modified in the presence
of fluoride anions making these compounds also interesting as
potential redox anion sensing elements. Apart from these
features, 1 is an unusual molecule bearing multiple phenol
groups that can exist in several stable forms including as
phenoxyl radicals. This implies potential applications for 1
as a molecular memory element since it can in principle possess
many redox states although these may be subject to delocali-
zation. We are currently investigating the redox properties of 1
and some unsymmetrically substituted derivatives and metal
complexes at the single molecule level.
7 J. P. Hill, A. L. Schumacher, F. D’Souza, J. Labuta, C. Redshaw,
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11 2 was prepared from 5,6-dichloropyrazine-2,3-dicarbonitrile by
nucleophilic substitution using 2,6-di-t-butylphenol.
substituted derivative 5-chloro-6-(3,5-di-t-butyl-4-hydroxyphenyl)
pyrazine-2,3-dicarbonitrile, 3, was also isolated and characterised12
A mono-
.
12 See Electronic Supplementary Information.
13 S. Makhseed, F. Ibrahim, C. G. Bezzu and N. B. McKeown,
Tetrahedron Lett., 2007, 48, 7358.
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16 Synthesis and properties of the metal complexes of 1 will be
reported elsewhere.
17 Weak fluorescence bands at 680 nm may be due to a small degree
of deprotonation of 1 caused by the basicity of the solvent used.
18 Benzonitrile was used as solvent since 1,2-dichlorobenzene under-
goes substitution reactions with Fꢀ anions when irradiated with
UV light. It remains unclear whether this process is mediated by 1.
19 This colour is characteristic of galvinoxyl-type radicals. See
E. L. Altwicker, Chem. Rev., 1967, 67, 475.
This research was partly supported by the World Premier
International Research Center Initiative on Materials Nano-
architectonics from MEXT, Japan, and by the Core Research
for Evolutional Science and Technology (CREST) program of
JST, Japan, and the National Science Foundation (grant no.
1110942 to FD). Y. X. thanks NSFC (21072060), China, Oriental
Scholarship, SRFDP (20100074110015), NCET, and the Funda-
mental Research Funds for the Central Universities (WK1013002).
N. M. S.-B. thanks the JSPS for a research fellowship.
Notes and references
1 (a) J. L. Sessler, P. A. Gale and W.-S. Cho, Anion Receptor
Chemistry, RSC Publishing, Cambridge, 2006; (b) P. D. Beer and
P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486.
20 (a) L. Milgrom, Tetrahedron Lett., 1983, 39, 3895; (b) L. Milgrom,
J. P. Hill and P. F. Dempsey, Tetrahedron, 1994, 50, 13477.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3951–3953 3953