Biomimetic self-assembling acylphthalocyanines

Article abstract of DOI:10.1039/c5cc04602e

We synthesized a series of biomimetic self-assembling phthalocyanines equipped with carbonyl groups as recognition motifs, a central zinc atom and diverse solubilizing alkyl chains mimicking for the first time with these robust pigments the natural chloro

Full text of DOI:10.1039/c5cc04602e

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Biomimetic self-assembling acylphthalocyanines†
Hong-Guang Jin, Mihaela Carmen Balaban, Sabine Chevallier-Michaud,
Michel Righezza and Teodor Silviu Balaban*
Cite this:DOI: 10.1039/c5cc04602e
Received 4th June 2015,
Accepted 22nd June 2015
DOI: 10.1039/c5cc04602e
www.rsc.org/chemcomm
We synthesized a series of biomimetic self-assembling phthalocya- red-shifted to B740 nm and increases by up five times its
nines equipped with carbonyl groups as recognition motifs, a central full-width-at-half-maximum mainly due to inhomogeneous
zinc atom and diverse solubilizing alkyl chains mimicking for the first broadening.7 Our laboratory has mimicked8 and extended the
time with these robust pigments the natural chlorosomal bacterio- self-assembly algorithm9 (Fig. 1b) by employing different recog-
chlorophylls. Upon self-assembly a very broad and red-shifted Q-band nition motifs as well as various solubilizing groups grafted onto
absorption extending to over 900 nm is put into evidence.
porphyrins or chlorins which self-assemble in the same way as
the natural BChl’s. We report here on equipping Pc’s with acyl
Highly ordered self-assembling functional dyes with unique photo- recognition groups which will then induce self-assembly under
electric properties as a consequence of excitonic interactions appropriate conditions in a similar manner as the natural
between neighboring dye monomers are desirable for materials BChl’s and our previously reported 3,13-diacetyl-Zn-porphyrins for
science.1 A well-defined manner connecting these dye monomers which by means of single crystal X-ray structures we could unravel
are crucial for a device to transfer the excitation energy to a their supramolecular architecture, essential for light-harvesting.10
designated acceptor for efficient energy utilization.2 Compared to
Herein we present a series of self-assembling tetra-acylated
traditional covalent bonds, the supramolecular self-assembly based ZnPc’s with long alkyl chains as solubilizing groups and carbonyl
on noncovalent interactions is an useful approach to reach this goal, groups as the recognition groups mimicking the self-assembly
including (i) relative easy synthesis of monomers, (ii) the self-healing algorithm of BChl’s. When the self-assembly operates, a broad
and thermodynamically stable self-assemblies. Phthalocyanines
(Pc’s),3 are the most stable and robust light-harvesting system with
high absorption coefficients in the visible and near-infrared region.
However, Pc’s based devices have been rather limited due to their
poor solubility in common organic solvents and their disordered
aggregation on the corresponding surface,4 meanwhile, in contrast
with other dyes,5 incorporating specific recognition sites into Pc’s to
induce self-assembly, is still synthetically challenging.
Natural light-harvesting systems function with high efficiencies,
also under low-light illumination conditions. Green photosynthetic
bacteria can scavenge photons even at one hundred meters under
water surface6 thanks to their special organelles, called chloro- Fig. 1 (a) Natural self-assembling BChl’s where R8 and R12 can be homo-
logous groups such as methyl, ethyl, n-propyl or isobutyl. Farnesol is
somes, which agglomerate homologues of bacteriochlorophylls
(BChl’s) c, d, and e as functional pigments (Fig. 1a). In polar
solvents, the monomeric forms prevail having an intense and
depicted as the long-chain alcohol esterifying the 17-propionic acid but
also other fatty alcohols such as stearol, cetol, phytol, geranyl-geraniol, etc.
can be encountered in various bacteria. Asterisks denote stereocenters.
sharp Qy-band at B670 nm. Upon self-assembly, this band is
(b) Single crystal structure of a BChl mimic: 3,13-diacetylporphyrin having Zn
replacing the central Mg atom self-assembled by ZnÁÁÁOQC o ligations.10
A tetramer is shown with the partially disordered 3,5-di-tert-butyl groups
truncated at their ipso carbon for clarity. Broad and red-shifted absorption
bands characterize this stair-case-like architecture, typical for J-aggregates.11
(c) Targeted constructs in this work whereby a diacylated ZnPc is statistically
monoreduced to a ketol with alkyl chains of various lengths for tuning
the solubility.
´
Aix-Marseille Universite, CNRS UMR 7313, Centrale Marseille, Chirosciences,
Service 442, Avenue Escadrille Normandie-Niemen, F-13013 Marseille, France.
E-mail: ts.balaban@univ-amu.fr
† Electronic supplementary information (ESI) available: Synthetic procedures,
product characterization, spectral data. See DOI: 10.1039/c5cc04602e
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and red-shift near-infrared absorption onset at over 900 nm can Finally, deprotection of the carbonyl groups proved to be possible
be seen in the electronic absorption spectra. The temperature with TiCl4 furnishing the desired tetraacylated ZnPc-6a–d. The
dependence of the equilibrium between the monomer and the synthetic intermediates, excluding 2a, are also novel compounds
aggregated species was also studied.
which could have interesting applications as they can be easily
Prior to our studies,12 no acyl-Pc’s had been described in the obtained cheaply in large quantities.
literature. This is probably due to the reactivity of the carbonyl
After succeeding in equipping the series of ZnPc-6a–d with
groups under the strongly basic and high temperature conditions carbonyl groups and a central zinc atom as recognition groups, we
required for the templated cyclo-tetramerization by divalent metal then performed self-assembly experiments of these compounds in
salts of the Pc synthons. In a shot-gun approach we tried in vain to nonpolar solvents. With the increasing solubility brought about
acylate under various Friedel–Crafts conditions the unsubstituted by the longer alkyl chains, the ZnPc-6b–d could self-assemble in
CuPc. Due to its insolubility in various solvents, unreacted material solvents of low polarity such as n-heptane, while ZnPc-6a could
was always recovered. In concentrated sulfuric acid, apparently also self-assemble in moderately polar solvents such as dichloro-
a good solvent for CuPc, neither anhydrides nor acyl chlorides methane and chloroform (Fig. S3, ESI†). Below, we present ZnPc-6d
reacted with or without preformed complexes of AlCl3.
We then turned our attention to a multistep synthetic sequence
as an example of the self-assembly process.
We performed UV-Vis absorption measurements in dry
which was optimized as shown in Scheme 1. Friedel–Crafts acylation n-heptane at room temperature for ZnPc-6d (Fig. 2). The pure
of ortho-dichlorobenzene, which is available in bulk quantities, with sample powder and the cuvettes were thoroughly vacuum dried
various acyl chlorides, also available on an industrial scale, provided and the prepared solution was also sonicated for 2 min before the
the 4-acyl-1,2-dichlorobenzene derivatives 2a–d with R = CnH2n+1 measurement. From the absorption spectra, a very broad and red-
(n = 1, 5, 11, 15). Direct conversion to the phthalodinitriles occurred shifted Q-band absorption with an onset at over 900 nm could be
unproblematically by Pd-catalysis but the cyclotetramerization reac- seen, which is rare among the phthalocyanines. When two drops
tion to the corresponding Pc’s, although it occurred in good yields, of methanol were added to the 3 mL cuvette, only a partial
was accompanied by a Meerwein–Pondorf–Verley type reaction. disassembly occurred because methanol can ligate and compete
Thereby some of the carbonyl groups became reduced to the to the coordination of the central zinc ion. The Pc assemblies are
secondary alcohols, giving much too complex reaction mixtures, thus much more robust than our previous porphyrinic BChl
albeit their self-assembly could be undoubtedly proven.12
mimics where stoichiometric amounts of Zn-coordinating alcohols
Therefore we restored to protect at the early 4-acyl-1,2- or even adventitious water were sufficient to monomerize the
dichlorobenzene stage the carbonyl groups with neopentyl glycol, aggregates.8,9 Further additions of five drops of methanol led to
which can eventually be easily removed. Cyanation involving phase separation which could be prevented by further adding five
palladium catalysis of these derivatives led to the precursor drops of dichloromethane thus ensuring a homogeneous solution.
phthalodinitriles 4a–d in moderate to high yields. Their cyclo- Thereby, the aggregates disassemble completely, giving intense
tetramerization gave the tetracarbonyl-protected ZnPc-5a–d. monomeric Q-bands, with no aggregates absorbing above 750 nm.
Fig. 2 Absorption spectra of ZnPc-6d in dry n-heptane (3.3 Â 10À5 M,
black line), after addition of two drops of methanol (red line) and followed
by five more drops of methanol and five drops dichloromethane (the green
line). A 3 mL cuvette with 1 cm pathlength is shown before (at left) and after
Scheme 1 Synthetic route of the desired tetraacylated ZnPc-6a–d. Yields
for procedures (a)–(c) performed several times on a multigram scale are
detailed in the ESI.† The overall yields for ZnPc-6a–d are based on the
procedures (d) and (e) having 4a–d as starting material.
addition of methanol and gentle shaking (at right). A microscopic image of
the fluffs characteristic of these macro aggregates is provided in the ESI†
(Fig. S7). Instead of methanol, pyridine or THF can be used to disassemble
the aggregates (Fig. S9, ESI†).
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Noteworthy is that during the disassembly process, the absorp- red-shifts or band broadening (Fig. S8, ESI†). However, the
tion bands and fluorescent emission attributed to the mono- ketals ZnPc-5, due to steric hindrance, prevent the formation of
mer were also intensified gradually (Fig. S6, ESI†). Meanwhile, face-to-face H-aggregates11d,e which could translate into the blue-
in the absence of Zn-coordinating solvents, slow growth of shifted shoulders and peaks of the Q-bands encountered in the
aggregates from a few hundred nanometres to visible fluffs spectra of the deprotected ZnPc-6 after addition of Zn-coordinating
which flocculate in the n-heptane solution could also be seen by solvents such as methanol, THF or pyridine (Fig. S9, ESI†).
naked eye or microscopy (Fig. S7, ESI†). Gentle shaking or short
sonication restores homogeneous solutions which were stable tions have been performed on different columns in order to
for various spectroscopic studies. achieve baseline separation of the regioisomers of ZnPc-5a–d
High-performance liquid chromatography (HPLC) optimiza-
All of the above phenomena resembled both that of the natural and ZnPc-6a–d. Since the pioneering studies of Michael Han-
BChl’s and of our previous mimics. In order to further study nack14 and Clifford Leznoff,15 very few authors report efficient
the self-assembly and disassembly process, variable-temperature resolutions of such regioisomers and usually a statistical mixture
UV-Vis absorption measurements were performed from T = 20 to is implicitly assumed when a nonsymmetrically substituted
95 1C. As shown in Fig. 3, an equilibrium could be put into evidence. phthalodinitrile is cyclotetramerized. In our hands, we often
During the temperature-rise period, the aggregates gradually encountered serious deviations from the 50%, 25%, 12.5% and
disassemble to monomers while in the temperature-fall period, 12.5% for our Cs, C2v, C4h, and D2h regioisomers, respectively
the monomers again self-assemble almost fully reversibly, to the (A4 in Scheme 2).
aggregates. Upon heating the aggregates gradually disassemble
We did had previous experience with reverse phase HPLC
in a fashion resembling the melting of DNA with a monotonous separations of similarly substituted water-soluble Pc’s.16 An
variation (red curves in the insets of Fig. 3). Evidence for a elegant study of water soluble Pc’s, with a view on photodynamic
cooperative and autocatalytic aggregate growth process is pro- therapy applications, stands out by the very careful experimenta-
vided by the sigmoidal shape of the absorbance upon cooling tions and the efficient HPLC separations.17
(blue traces in the insets of Fig. 3). These show an inflexion point
Even more complex mixtures resulted in our hands when the
at 65 1C (thin black lines) at both monitored wavelengths. Very protected phthalodinitriles 4a–d (A) were submitted to a mixed
similar trends were earlier encountered for the natural BChl c, condensation with the unsubstituted phthalodinitrile (B in
where a detailed kinetic modeling provided the size of a critical Scheme 2). In these cases, beside the four regioisomers A4
nucleus of about 14 molecules.13 Assemblies larger than this of type ZnPc-5, highly colored Pc’s having one, two, or three
critical nucleus grow to macroscopic sizes, while smaller oligo- dioxolane (D) protected acyl residues were formed, beside some
mers disassemble.
unsubstituted ZnPc (B4 in Scheme 2) which could be easily
In order to verify that the biomimetic self-assembly is indeed separated due to its limited solubility. HPLC coupled with mass-
operating, we used the same conditions for diluting into dry spectrometric and diode-array UV-Vis detectors allow analytical
n-heptane the carbonyl protected compound ZnPc-5d. In this
case only monomeric absorption spectra were observed with no
Fig. 3 Temperature-variable UV-Vis absorption of ZnPc-6d (3.3 Â 10À5 M) in
dry n-heptane containing B2% dry dichloromethane. Above is the temperature-
rise period and below is the temperature-fall period. The solid arrows reflect the
Scheme 2 Mixed condensation of two different phthalodinitriles A and B.
D stands for the 1,3-dioxolane protecting group which after removal gives
the acylated phthalocyanines with RCQO being either acetyl, hexanoyl,
lauroyl or palmitoyl residues.
electronic absorption’s trend. Insets show the variable temperature evolution of
the aggregate maxima monitored at 780 nm and that of the monomeric
building blocks monitored at 620 nm. Black dotted traces show the almost
perfect reversibility of the absorbance after a heating/cooling cycle.
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¨
¨
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Chem. Commun.
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