Controlling molecular assembling by photons: Reversible light-powered monomer-aggregate interconversion of porphyrins

Article abstract of DOI:10.1039/b815168g

Reversible formation and destruction of H-type aggregates of an anionic porphyrin can be controlled exclusively through light inputs of different energy exploiting the different interactions with the two isomeric forms of a cationic photoresponsive azobenzene-based surfactant. The Royal Society of Chemistry.

Full text of DOI:10.1039/b815168g

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COMMUNICATION
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Controlling molecular assembling by photons: reversible light-powered
monomer–aggregate interconversion of porphyrinsw
Fiorella L. Callari and Salvatore Sortino*
Received (in Cambridge, UK) 1st September 2008, Accepted 3rd October 2008
First published as an Advance Article on the web 21st October 2008
DOI: 10.1039/b815168g
Reversible formation and destruction of H-type aggregates of an
anionic porphyrin can be controlled exclusively through light
inputs of different energy exploiting the different interactions
with the two isomeric forms of a cationic photoresponsive
azobenzene-based surfactant.
These aggregates are mainly stabilized by Coulombic interac-
tions between the two oppositely charged species and the
hydrophobic clustering of the alkyl chains of the surfactant.
Inspired by these results, we have explored the possibility of
controlling the aggregation # disaggregation process of TPPS
through a cationic surfactant whose molecular structure, and
consequently its interactions with TPPS, can be reversibly
changed by light stimuli of different energy. To this end, we
have designed and synthesized the amphiphilic molecule 1
which integrates the photochromic azobenzene moiety in its
molecular skeleton. We demonstrate that the trans # cis
photoisomerization of 1 deeply affects the extent of its inter-
action with TPPS allowing repeated H-aggregate # monomer
interconversion by means of alternate UV and visible light
excitation (Fig. 1).
The possibility of reversibly controlling the formation and
destruction of chromogenic assemblies by external inputs
represents one ultimate goal in supramolecular chemistry,
not only for the fundamental interest in controlling molecular
motions at the nanoscale but also in the perspective of future
optical devices.1 Light is a very appealing on/off trigger. Its
ready availability and easy manipulation, associated with the
rapidity and instantaneous initiation/stopping of the photo-
chemical reactions make the photoregulated systems particu-
larly desirable, offering the additional advantage of not
affecting important parameters such as temperature, pH, ionic
strength etc.
Compound 1 is water soluble, undergoes reversible trans #
cis photoisomerization upon UVA-Vis light excitation and,
analogously to similar cationic azobenzene-based amphiphiles,8
forms micelles in aqueous solution at critical micellar concen-
trations (cmc) of ca. 3 mM and 7 mM in the case of the trans
and cis form, respectively (Fig. S1,2w). At neutral pH TPPS is
present as tetraanion exhibiting a Soret absorption at 412 nm
and a double band fluorescence emission with maxima at 640
and 700 nm, respectively.6,7 The spectroscopic properties of
TPPS are profoundly affected by the addition of 1-trans in a
concentration range well below the cmc. As shown in Fig. 2A
the fluorescence emission is significantly reduced and red-
shifted by ca. 22 nm. The excitation spectra show a broadening
of the Soret band which is hypsochromically shifted by ca.
10 nm whereas the Q-bands are all red shifted (Fig. 2B).9
Fig. 2C shows that the maximum decrease of the fluorescence
quantum yield is reached for the ratio [1-trans]/[TPPS] D 20.
Porphyrins constitute one of the most massively investi-
gated families of molecules by virtue of their excellent spectro-
scopic, photochemical and electrochemical properties.2 An
intriguing characteristic of these macrocycles is their tendency
to form aggregates of different order and structure, which have
both scientific and technological importance.3 Of these, those
called J- (edge-to-edge molecular arrangement) or H-type
(face-to-face molecular arrangement) are found to have un-
ique electronic and spectroscopic properties due to their high
order.4 On this basis, the achievement of the light-powered
aggregation–disaggregation of these chromophoric systems
represents an exciting objective to pursue.
As a part of our ongoing project on photocontrolled processes
in organized porphyrin-based systems,5 in this contribution we
report a simple supramolecular approach for reversibly controlling
the monomer–aggregate interconversion of the anionic 5,10,15,
20-tetrakis(4-sulfonatophenyl)-21H,23H-porphyrin (TPPS) in
water medium exploiting exclusively photons of different energy
which are absorbed by a photoswitching center non-covalently
linked to the porphryrin.
The work presented herein has its roots in the elegant
studies carried out in the groups of Periasamy first6 and then
Tabak,7 which pointed out that the cationic surfactant cetyl-
trimethylammonium chloride (CTAC) encourages the forma-
tion of premicellar H-type aggregates of TPPS at neutral pH.
´
Dipartimento di Scienze Chimiche, Universita di Catania, Viale
Andrea Doria 8, I-95125 Catania, Italy. E-mail: ssortino@unict.it
w Electronic supplementary information (ESI) available: Synthesis
details, conductivity measurements, additional spectroscopic and
photochemical experiments. See DOI: 10.1039/b815168g
Fig. 1 A schematic view of the light-powered assembling–disassem-
bling of TPPS mediated by 1.
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into cis which, at the above concentration, does not interact
significantly with the porphyrins leading to their disassembling
into monomers. The validity of our design is illustrated in
Fig. 3 where absorption and fluorescence measurements after
UVA and Vis excitation are reported together. As displayed in
Fig. 3A irradiation with 360 nm light induces trans to cis
isomerization of 1. Beside, a fluorescence revival of TPPS
whose intensity and maxima position are virtually identical to
those of the monomeric form, is observed (see, Fig. 3B and 2A,
for comparison). Furthermore, the RLS intensity drops dra-
matically (inset Fig. 3B) confirming the destruction of the
H-type aggregates. Irradiation of the system with 450 nm light
reverts back the cis isomer of 1 into the trans form, as
confirmed by the almost complete recovery of the UV absorp-
tion band (Fig. 3A). Besides, we observe that the fluorescence
spectrum decreases to the original value and the RLS signal
increases again as consequence of the reformation of the
H-type aggregates. The H-aggregate # monomer intercon-
version can be switched repeatedly several times by means of
alternate UV-Vis light excitation (inset Fig. 3B).
Fig. 2 (A) Fluorescence emission (lexc = 530 nm) and (B) excitation
(lem = 640 nm) spectra of TPPS (1.5 mM) upon addition of increasing
amounts of 1-trans in the range 0–52 mM, in phosphate buffer pH =
7.4, T = 25 1C. (C) Relative fluorescence quantum yield of TPPS
observed in the presence of different amounts of 1-trans (K) and 1-cis
(’). (D) RLS spectra of the mixture 1:TPPS at molar ratio 20:1.
In summary, we have devised a simple strategy for control-
ling the aggregation–disaggregation of porphyrins, based on
two independent components whose interactions can be ex-
clusively controlled by photons of different energy without
affecting important parameters such as temperature, pH, ionic
strength etc. To the best of our knowledge, this represents the
first supramolecular approach for reversibly controlling the
monomer–aggregate interconversion of porphyrins in an aqu-
eous medium by exploiting exclusively light inputs absorbed
by a secondary photoswitching unit non-covalently linked to
the porphyrin center.15 In view of the considerably different
spectroscopic and photochemical properties exhibited by the
monomeric and aggregate species and their potential in dif-
ferent fields ranging from optical to biological, the strategy
illustrated herein represents an appealing starting point for
further development of dynamic chemical devices in these
research areas.
This spectroscopic behavior has much in common with that
previously observed with the CTAC surfactant and is typical for
a face-to-face arrangement of the TPPS units in H-type aggre-
gates.6,7 However, the low but not negligible fluorescence
emission observed in the presence of the azo-surfactant, sug-
gests a deviation from the strict parallel arrangement of the
dipole moments of TPPS units.10 Further evidence for the
formation of aggregates is given by resonance light scattering
(RLS) measurements, a powerful tool for testing aggregate
formation.11 As shown in Fig. 2D, an intense signal in the
correspondence of the Soret absorption band confirms that the
aggregate is large enough to scatter the light, providing clear
evidence for the involvement of multiple TPPS molecules per
aggregate.12 This hypothesis is in good agreement with the
reduced fluorescence quantum yield of the aggregate compared
to the monomer because of self-quenching phenomena among
TPPS molecules in close proximity.
Financial support by MIUR, Rome, Italy (PRIN 2006,
n. 2006031909) and by University of Catania is greatly
appreciated.
Experiments performed with the cis isomer of 1 show a
remarkably different behavior. In fact, the spectroscopic pro-
perties of TPPS remain basically unaffected when this com-
pound is added in the same range of concentration of the trans
species (Fig. 2C) and no significant RLS signal is observed
(Fig. 2D). These findings reflect the more polar structure of the
cis isomer for which the relevance of the hydrophobic con-
tribution to the stabilization of the aggregate is expected to be
much smaller than that of the parent trans.13 Short chain
surfactants have been in fact demonstrated to be able to
induce aggregation only at higher concentrations due to their
reduced hydrophobic contributions.14
This drastically different behavior between the two isomeric
forms of 1 can thus be exploited to reversibly control the
destruction and formation of the porphyrin aggregates by the
photoisomerization of 1 if its concentration is appropriately
chosen. For instance, if the concentration of 1-trans is ca. 30 mM,
TPPS will be present as H-aggregate (see Fig. 2C). UVA
irradiation will induce conversion of the trans azo-surfactant
Fig. 3 (A) Absorption spectra of the mixture 1-trans:TPPS (20:1)
before (TT), after 10 min irradiation with 360 nm light (-Á-) and
subsequent 20 min irradiation with 450 nm light (Á Á Á), in phosphate
buffer pH = 7.4, T = 25 1C. (B) Fluorescence emission spectra
(lexc = 530 nm) of the same sample of (A). The inset shows the
RLS intensity upon alternate cycles of UV and Vis light irradiation.
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with the absorption of the azobenzene chromophore. In contrast,
fluorescence excitation spectra allow the clear observation of the
spectral changes of the TPPS without the interference of the azo-
chromophore due to fact that the azobenzene does not show any
luminescence. The absorption changes observed in the Q-band
region (data not shown) reproduced well those noted in the
excitation spectra in the same spectral region.
Notes and references
1 (a) J. M. Lehn, Science, 2002, 295, 2400; (b) V. Balzani, A. Credi,
F. M. Raymo and J. F. Stoddart, Angew. Chem., Int. Ed., 2000, 39,
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2 The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, Boston, 2000.
3 See, for example: (a) M. C. Balaban, A. Eichhoefer, G. Buth,
R. Hauschild, J. Szmytkowski, H. Kalt and T. S. Balaban, J. Phys.
Chem. B, 2008, 112, 5512; (b) H. Onouchi, T. Miyagawa,
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2381; (c) A. Satake, A. Akiharu, M. Yamamura, M. Oda and
Y. Kobuke, J. Am. Chem. Soc., 2008, 130, 6314; (d) A. Mammana,
A. D’Urso, R. Laceri and R. Purrello, J. Am. Chem. Soc., 2007,
129, 8062.
10 Ideal H-type aggregates are non emissive as a result of the forbidden
transition from the fluorescent S1 state:10a (a) E. G. McRae and
M. Kasha, J. Chem. Phys., 1958, 28, 271.
11 (a) R. F. Pasternack and P. J. Collings, Science, 1995, 269, 935;
(b) R. F. Pasternack, C. Bustamante, P. J. Collings, A. Giannetto
and E. J. Gibbs, J. Am. Chem. Soc., 1993, 115, 5393.
12 An association constant 1-trans/TPPS of ca. 1 Â 105 MÀ1 was
estimated through the fluorescence spectral changes.
13 We cannot rule out that p–p stacking interaction between the
aromatic rings of the 1-trans can probably play a key role in
the stabilization of the H-aggregates. Such type of interactions
are expected to be disfavored in the 1-cis due to its non-
flat structure, accounting for the weak affinity of this isomer
for TPPS.
4 M. Kasha, Radiat. Res., 1963, 20, 55.
5 (a) E. B. Caruso, E. Cicciarella and S. Sortino, Chem. Commun.,
2007, 47, 5028; (b) F. L. Callari, A. Mazzaglia, L. Monsu Scolaro,
L. Valli and S. Sortino, J. Mater. Chem., 2008, 18, 802;
(c) F. L. Callari and S. Sortino, J. Mater. Chem., 2007, 17, 4184;
(d) L. Valli, G. Giancane, A. Mazzaglia, L. Monsu Scolaro,
S. Conoci and S. Sortino, J. Mater. Chem., 2007, 17, 1660.
6 N. C. Maiti, S. Mazumdar and N. J. Periasamy, J. Phys. Chem. B,
1998, 102, 1528.
14 N. C. Maiti, S. Mazumdar and N. Periasamy, Curr. Sci., 1996, 70,
997.
15 An example of aggregate–monomer interconversion of porphyrins
in organic solvents modulated by a photochromic center has been
recently reported by Yao and coworkers (Y. Liu, M. Fan, S. Zhang
and J. Yao, J. Phys. Org. Chem., 2007, 20, 884). This approach
exploits the solvent polarity changes induced by the ring opening of
a spiropyran.
7 S. C. M. Gandini, V. E. Yushmanov, I. E. Borissevitch and
M. Tabak, Langmuir, 1999, 15, 6233.
8 Y. Orihara, A. Matsumura, Y. Saito, N. Ogawa, T. Saji,
A. Yamaguchi, H. Sakai and M. Abe, Langmuir, 2001, 17, 6072.
9 Absorption spectroscopy does not allow clear observation of the
changes in the Soret absorption spectral profile due to the overlap
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