Synthesis of trifluoroethoxy-coated binuclear phthalocyanines with click spacers and investigation of their clamshell behaviour

Article abstract of DOI:10.1039/b902905b

We disclose here the synthesis of a novel trifluoroethoxy-coated binuclear Pc 1 which is the first example of a never-closing clamshell Pc. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b902905b

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis of trifluoroethoxy-coated binuclear phthalocyanines with click
spacers and investigation of their clamshell behaviour†
Hideyuki Yoshiyama, Norio Shibata,* Takefumi Sato, Shuichi Nakamura and Takeshi Toru
Received 11th February 2009, Accepted 27th March 2009
First published as an Advance Article on the web 14th April 2009
DOI: 10.1039/b902905b
We disclose here the synthesis of a novel trifluoroethoxy-
coated binuclear Pc 1 which is the first example of a never-
closing clamshell Pc.
di-yne spacer and noted its prominent avoidance of intermolec-
ular aggregation.5 Fluorophobic repulsion is responsible for this
behaviour, preventing its aggregation even in the 18 p-conjugation
system. In order to evaluate the strength of the fluorophobic
repulsion, we attempted to impede the intramolecular aggregation.
Indeed, intramolecular aggregation is much more favourable than
intermolecular aggregation for entropic reasons; therefore, the
achievement of freedom from intramolecular aggregation in the
clamshell system was the next challenge. As an extension of our
work on fluorine chemistry,6 we disclose here the synthesis of
a novel trifluoroethoxy-coated binuclear Pc 1 which is the first
example of a never-closing clamshell Pc. The two Pc rings in 1
are covalently connected by a flexible linker using “double-click”
chemistry.5,7 The binuclear Pc 1 is free from intra- and inter-
molecular aggregations independent of solvent and concentration.
It is interesting to note that while sterically demanding tert-Bu
substituents on Pc cannot stop the clamshell closing, fluorophobic
repulsion can stop the clamshell from closing to any great extent
without any external source of help such as coordinating solvents.
Novel mononuclear Pc 2, referred to as “single-click Pc” was also
synthesized for comparison (Fig. 2).
A previously unknown trifluoroethoxy-coated binuclear Pc
1 was synthesized from 23-ethynyl-1,2,3,4,8,9,10,11,15,16,17,18-
dodecakis(2,2,2-trifluoroethoxy)phthalocyaninate zinc(II) (3)5a
and 1,4-bis(azidomethyl)benzene (4a) by our “double-click”
chemistry7 using CuI, Et3N and DMSO in 95% yield. Target 1 is
sufficiently soluble in any organic solvent and was characterized by
reverse-phase HPLC, 1H and 19F NMR, and the isotopic patterns
of Zn in MALDI-TOF MS spectra (see experimental section and
ESI†). The single-click Pc 2 was prepared in high yield by a
procedure similar to that of 1 using (azidomethyl)benzene (4b)
instead of 4a (Scheme 1).
The suppression and control of the aggregation properties of
phthalocyanines (Pcs), both in solvents and in solid films, by only
their inherent specificity without any external assistance is a chal-
lenge in materials science, especially for solar cells, optical filters,
sensors and photodynamic therapy.1 Phthalocyanine (Pc) deriva-
tives offer a very wide choice of molecular physicochemical prop-
erties, including color, photodynamics and catalytic activity by
changing the nature of their aggregation states: monomer, dimer or
highly aggregated. The outstanding optical and photosensitizing
properties of Pcs with both a wide absorbing spectral range
and high luminescence quantum yields are often perturbed in a
randomly aggregated state. Freedom from molecular aggregation
is particularly desirable for solar cells and photodynamic therapy,
because self-association reduces the efficiency of solar cells and
quenches fluorescence and interferes with the formation of singlet
oxygen, respectively.2 Among various Pc derivatives that have been
investigated to add a new aspect to the study of Pc aggregation, the
design and syntheses of novel binuclear Pc derivatives have gained
much attention.3 Intermolecular aggregation can be suppressed by
clamshell behaviour, i.e., cofacial intramolecular self-association
of binuclear Pcs wherein the two Pc rings are bridged by a
spacer with a sufficient linkage length. Since the first report by
Leznoff and coworkers in 1984, a series of binuclear Pcs has been
synthesized to investigate clamshell behaviour with a dynamic
equilibrium existing between open and closed conformations.4
Although intermolecular vs intramolecular aggregations can be
controlled by the choice of bridging linkage and the concentration
of the Pcs, freedom from molecular association is still a challenge
(Fig. 1).
The aggregation behaviour of 1 was explored by 1H and
19F NMR, UV–vis and steady-state fluorescence spectra. The
1H and 19F NMR spectra in d-acetone very clearly indicate
non-aggregation behaviour, while the non-fluorinated, tert-butyl
analogue of 1 shows broad signals due to the intramolecular
closed clamshell formation (see Fig. S1 and S2 in the ESI†).5
In the UV–vis spectra in CHCl3, 1 exhibits strong absorption at
702 nm of the Q-band, and 360 nm of the B-band, which are
indicative of a monomeric state, independent of concentration.
Freedom from inter- and intramolecular aggregations, as well as
the closed clamshell formation, was strictly ascertained by the
addition of a metal-coordinating molecule, pyridine.8 Namely, the
shape of the spectrum did not change even after the addition
of 1% pyridine (Fig. 3a). The behaviour was next investigated
thoroughly in a variety of solvents (see Fig. S3–S11 in the
ESI†). The same phenomenon was observed with common
Fig. 1 Aggregation and clamshell behaviour of binuclear Pcs.
We recently synthesized trifluoroethoxy-coated binuclear Pc
wherein the two Pcs are covalently linked with a conjugated rigid
Department of Frontier Materials, Graduate School of Engineering, Nagoya
Institute of Technology, Gokiso, Showa-ku, Nagoya, 466-8555, Japan.
E-mail: nozshiba@nitech.ac.jp; Fax: +81-52-735-5442; Tel: +81-52-735-
7543
† Electronic supplementary information (ESI) available: Experimental
details and Fig. S1–21. See DOI: 10.1039/b902905b
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Fig. 2 Binuclear Pc 1 and mononuclear Pc 2 with click spacer.
Scheme 1 Synthesis of Pcs 1 and 2.
organic solvents such as dioxane, THF, pyridine, DMF, PhCN
(Fig. S4–S7 and S9). The non-aggregated state was also indicated
by strong and sharp absorption bands of the UV–vis spectra
in less-polar solvents as PhCF3, CH2Cl2, m-difluorobenzene or
for the single-click Pc 2 (Fig. 3b). The absorption spectra of the
binuclear Pc 1 can be superimposed on the corresponding figures
of single-click Pc 2 despite its homodimeric structure. These results
indicate that the binuclear Pc 1 behaves like mononuclear Pc 2,
and neither the intermolecular aggregated nor closed clamshell
formation of Pc 1 are detected in organic solvents.
Steady-state fluorescence spectra in CHCl3 were next investi-
gated to prove the aggregation state in 1. While the emission of
closed clamshell binuclear Pcs is well-known to be self-quenched
between the coupled halves of the binuclear species, a single
strong lem was observed for Pc 1 at 715 nm both in a non-
coordinating solvent, CHCl3 (fluorescence quantum yield, Uf =
0.24), and a coordinating solvent, dioxane (Uf = 0.26). A reference,
non-aggregated single-click Pc 2 also showed similarly strong
R
Solkaneꢀ (CF3CH2CF2CH3), but the Q-bands in these solvents
were slightly more sharpened with the addition of pyridine
(Fig. S3, S8, and S10–S11†). The results implied that the effect
of fluorophobic repulsion on impeding aggregation in less polar
solvents, especially fluorinated solvents, is somewhat decreased
compared to that in polar solvents. Although minor differences are
observed in these solvents, all the UV–vis absorbances are very dif-
ferent from those of previously reported sterically demanding non-
fluorinated binuclear Pcs in the closed clamshell comformation.4,7
It should also be noted that similar UV–vis spectra were observed
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Table 2 DPV peak potentials of 1 and 2 (V vs Ag/AgNO3) in THF
containing 0.1 M of TBAPF
ox1
red1
red2
Pc
E1/2
E1/2
E1/2
1
2
0.640
0.635
-1.160
-1.155
-1.435
-1.595
Fig. 5 Differential pulse voltammograms of 1 (top) and 2 (bottom).
attributed to the mononuclear Pc moiety. The voltammograms of
mono-click Pc 2 do not differ considerably from those of Pc 1, and
lead to the following explanations: the two Pc units in binuclear
1 are oxidized at the same potential, and no splitting of the Pc-
based oxidation process is observed. This indicates that the two
Pc moieties in the dyad 1 are electrochemically equivalent. In the
cathodic scan, dyad 1 showed two reduction processes at -1.199
and -1.434 V, and Pc 2 showed the same processes at -1.152 and
-1.534 V. A similar speculation was done in DPV.
Finally, regioisomeric binuclear Pcs 6 and 7 with ortho- and
meta-double click spacers were synthesized in a manner similar to
that of Pc 1 from 3 using 1,2- or 1,3-bis(azidomethyl)benzene (5a or
5b) instead of 4a (Scheme 2). UV–vis and steady-state fluorescence
spectra similar to those of Pc 1 were observed for binuclear Pcs 6
and 7, which clearly revealed their non-aggregation behaviour (see
Fig. S14–S21 in the ESI†).
Fig. 3 UV–vis spectra of (a) Pc 1 in CHCl3 (bold black to grey solid
lines: 2.6 ¥ 10-6 to 10-7 M, dotted: with 1 vol% of pyridine) and (b) Pc 2 in
CHCl3 (bold black to gray solid lines: 1.0 ¥ 10-4 to 10-6 M, dotted line: with
1 vol% of pyridine).
fluorescence at 715 nm (Uf = 0.27 in CHCl3; Uf = 0.30 in dioxane).
These results strongly support the fact that Pc 1 exists free of
aggregations (see Fig. S12–S13 in the ESI†).
Cyclic voltammograms (CV) and differential-pulse voltammo-
grams (DPV) of binuclear Pc 1 and single-click Pc 2 in THF
are summarized in Fig. 4/Table 1 and Fig. 5/Table 2. The
voltammograms of 1 and 2 show quite similar features that may be
In summary, we synthesized trifluoroethoxy-coated binuclear
Pcs showing a powerful non-aggregation property. The 1H and 19
F
NMR, UV–vis and steady-state fluorescence spectra, CV and DPV
data strongly support that, for the first time, the trifluoroethoxy-
coating can ideally prevent binuclear Pc clamshells from closing,
independent of the solvent concentration and substitution pattern
of the spacers (ortho-, meta- and para-). Although the fluorophobic
repulsion in the binuclear Pcs slightly diminishes in fluorinated
solvents as well as CH2Cl2, the non-aggregation effect is clearly
superior to that of sterically demanding tert-Bu substituents on
Pcs. The net effect of fluorophobic repulsion in the dyad under a
variety of solvents is now under investigations.5a
Experimental section
Fig. 4 Cyclic voltammograms of 1 (top) and 2 (bottom).
Synthesis of 1: a mixture of 23-ethynyl-1,2,3,4,8,9,10,11,15,16,17,
18-dodecakis(2,2,2-trifluoroethoxy)phthalocyaninate zinc(II) (3)
(44.9 mg, 0.025 mmol), 1,4-bis(azidomethyl)benzene (4a) (1.9 mg,
0.010 mmol), CuI (1.0 mg, 0.005 mmol), Et3N (0.05 ml) and
DMSO (1.0 ml) was freeze dried to remove oxygen. Then the
solution was filled with Ar and stirred at 60 ◦C for 2 h.
After cooling it to rt, water and dil. sulfuric acid were added.
Table 1 Half-wave redox potentials of 1 and 2 (V vs Ag/AgNO3) in THF
containing 0.1 M of TBAPF
ox1
red1
red2
Pc
E1/2
E1/2
E1/2
1
2
0.605
0.610
-1.199
-1.152
-1.434
-1.534
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Scheme 2 Synthesis of regioisomeric Pcs 6 and 7 with ortho- and meta-double click spacers.
The precipitates were filtered. Purification by silica gel column
(5.11) nm; (0.5 ¥ 10-6 M in Solkane): lmax (log e) = 692 (5.27),
640 (4.73), 361 (4.91) nm; Fluorescence (CHCl3): lem = 715 nm,
Uf = 0.24, (dioxane): lem = 715 nm, Uf = 0.26.; MALDI-TOF MS
(dithranol): m/z = 3740.8–3749.8 ([M+], isotopic pattern); HPLC:
(H2O:MeCN:THF = 8:42:50, 0.3 ml/min), tR = 14.0 min.
chromatography (hexane : acetone = 70:30 to 60 : 40) gave green
1
solids (36.0 mg, 95%). C124H64F72N22O24Zn2; M.W.: 3744.63, H-
NMR (acetone-d6, 400 MHz): d 5.06–5.19 (m, 24H), 5.72–5.85
(m, 24H), 5.96 (s, 4H), 7.72 (s, 4H), 8.82 (d, J = 8.4 Hz, 2H),
8.87 (s, 2H), 9.45 (d, J = 8.4 Hz, 2H), 9.87 (s, 2H); 19F-NMR
(acetone-d6, 376 MHz): d -74.1– -74.0, -73.7, -73.1, -72.8; IR
(KBr): 2975, 1488, 1456, 1275, 1159, 1068, 1008, 970, 860, 762,
670 cm-1; UV–vis (2.6 ¥ 10-6 M in CHCl3): lmax (log e) = 702
(5.61), 631 (4.89), 360 (5.11) nm; (0.5 ¥ 10-6 M in PhCF3): lmax
Acknowledgements
Support was provided by KAKENHI (19390029), by a Grant-in-
Aid for Scientific Research on Priority Areas “Advanced Molec-
ular Transformations of Carbon Resources” from the Ministry
of Education, Culture, Sports, Science and Technology Japan
(19020024), and Nagase Science and Technology Foundation.
(log e) = 695 (5.44), 644 (4.92), 358 (4.99) nm; (0.5 ¥ 10-6
M
in dioxane): lmax (log e) = 703 (5.62), 632 (4.90), 368 (5.10) nm;
(0.5 ¥ 10-6 M in THF): lmax (log e) = 700 (5.68), 630 (4.95), 358
(5.16) nm; (0.5 ¥ 10-6 M in pyridine): lmax (log e) = 704 (5.66),
632 (4.99), 370 (5.18) nm; (0.5 ¥ 10-6 M in DMF): lmax (log e) =
699 (5.62), 630 (4.93), 368 (5.12) nm; (0.5 ¥ 10-6 M in CH2Cl2):
Notes and references
lmax (log e) = 702 (5.46), 643 (4.85), 365 (5.02) nm; (0.5 ¥ 10-6
M
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(0.5 ¥ 10-6 M in m-PhF2): lmax (log e) = 699 (5.62), 630 (4.92), 364
2268 | Org. Biomol. Chem., 2009, 7, 2265–2269
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The Royal Society of Chemistry 2009
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