Catch-and-release of porphyrins by photoswitchable self-assembled monolayers

Article abstract of DOI:10.1039/b708851e

A novel cationic, photoresponsive azobenzene derivative with the characteristic of self-assembly, 2-{4-[(E)-(4-{[11-(dodecylthio)undecyl]oxy} phenyl)diazenyl]phenoxy}-N,N,N-trimethylethanaminium bromide (1), has been synthesized. Self-assembled monolayers (SAMs) of 1 on ultrathin, transparent platinum electrodes have been fabricated and the reversible trans cis interconversion has been demonstrated by absorption spectroscopy in transmission mode and contact angle measurements. It is shown that such photoresponsive SAMs promote the binding of anionic porphyrins at the metal surface and trigger their release upon light excitation exploiting the different light-controlled electrostatic interactions with the two isomeric forms of 1. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708851e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
‘‘Catch-and-release’’ of porphyrins by photoswitchable self-assembled
monolayers
Fiorella L. Callari and Salvatore Sortino*
Received 11th June 2007, Accepted 19th July 2007
First published as an Advance Article on the web 6th August 2007
DOI: 10.1039/b708851e
A novel cationic, photoresponsive azobenzene derivative with the characteristic of self-assembly,
2-{4-[(E)-(4-{[11-(dodecylthio)undecyl]oxy}phenyl)diazenyl]phenoxy}-N,N,N-trimethyl-
ethanaminium bromide (1), has been synthesized. Self-assembled monolayers (SAMs) of 1 on
ultrathin, transparent platinum electrodes have been fabricated and the reversible trans O cis
interconversion has been demonstrated by absorption spectroscopy in transmission mode and
contact angle measurements. It is shown that such photoresponsive SAMs promote the binding of
anionic porphyrins at the metal surface and trigger their release upon light excitation exploiting
the different light-controlled electrostatic interactions with the two isomeric forms of 1.
suitable prerequisites for the achievement of the above goal.
Introduction
In fact: i) the thioether group allows facile chemisorption on
The fabrication of self-assembled monolayers (SAMs) on
noble metal surfaces, ii) the quaternary ammonium group
noble metal substrates constitutes one of the most active areas
terminal group should encourage binding with the anionic
of modern materials science.1 In this widely explored arena,
TPPS through favorable electrostatic interaction, iii) the photo-
the design of SAMs performing specific functions under the
responsive azobenzene moiety is expected to change the extent
input of external stimuli is one of the most exciting goals in the
of such interactions, in view of its reversible trans O cis
perspective of ‘‘smart’’ nanocomposite devices.2 Light is a
interconversion controlled by UV-Vis light,7 iv) the unsub-
very appealing trigger. Its easy availability and manipulation
stituted alkyl chain provides the sweep volume for feasible
photoisomerization within the SAM.8
associated with the fast response of many photochemical
reactions make light-controlled systems relatively simple and
particularly attractive.
Experimental
Porphyrins and their metal complexes represent a family of
massively investigated molecules by virtue of their excellent
spectroscopic, photochemical and electrochemical properties.3
Organization of these macrocycles onto solid substrates has
been extensively studied aiming to develop a variety of
optoelectronic, photonic, photosynthetic, and sensing mate-
rials.4 On these grounds, the design of SAMs able to control
the porphyrin assembly and disassembly on two-dimensional
(2D) surfaces under the input of photons represents a valuable
objective to pursue. In this context, Cook et al. have recently
reported the immobilization and release of zinc tetraphenyl-
porphyrin on 2D surfaces exploiting the ‘‘on–off’’ metal–
ligand coordination of an arylazopyridine self-assembled on
gold surfaces.5 Inspired by this work and stimulated by the
recent results concerning the ability of cationic amphiphile
monolayers to trap anionic hydrophilic 5,10,15,20-tetrakis(4-
sulfonatophenyl)-21H,23H-porphyrin (TPPS) mainly via
electrostatic interactions,6 we have explored the possibility to
achieve the ‘‘catch-and-release’’ of porphyrins on 2D surfaces
simply by exploiting the Coulombic interactions between a
cationic, photoresponsive SAM and the anionic TPPS. To
this end, we have designed and synthesized the azobenzene
derivative 1 (Fig. 1). This compound offers a combination of
Materials and general procedures
4-Aminophenol, phenol, 11-bromo-1-undecene, 1-dodeca-
nethiol, 9-borabicyclo[3.3.1]nonane (9-BBN), trimethylamine,
sodium nitrite and sodium carbonate were purchased from
Sigma-Aldrich (Milan) and used as received. All solvents used
(from Carlo Erba, Milan) were analytical grade. Syntheses
were carried out under a low intensity level of light.
Synthesis. 2-{4-[(E)-(4-{[11-(Dodecylthio)undecyl]oxy}phenyl)
diazenyl]phenoxy}-N,N,N-trimethylethanaminium bromide (1)
was synthesized according to the steps reported in Scheme 1.
4,49-Dihydroxyazobenzene (A) and 1-(11-bromo-undecyl-
sulfanyl)dodecane (C). Compounds A and C were prepared
according to the methods described in the literature.9,10
Dipartimento di Scienze Chimiche, Universita` di Catania, Viale Andrea
Doria 8, I-95125, Catania, Italy. E-mail: ssortino@unict.it
Fig. 1 The chemical structure of 1.
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mixture was concentrated under reduced pressure to afford 1
(yield 95%) as a yellowish powder. Anal. Calcd (%) for
C40H68Br1N3O2S1: C, 65.37, H, 9.33, N, 5.72, S, 4.36; found:
C, 66.33, H, 8.88, N, 6.05, S, 4.64. ESI-MS m/z: [M]+ 654.5
(100%). 1H NMR (CDCl3): d 7.8 (dd, 4H, J = 5 Hz, 4 Hz), 7.0
(dd, 4H, J = 7 Hz, 5 Hz), 4.6 (t, 2H, J = 6.2 Hz), 3.8 (t, 2H, J =
5.5 Hz), 3.3 (s, 9H), 4.1 (2H, t, J = 7.3 Hz), 2.5 (4H, t, J =
7.4 Hz), 1.8 (2H, t, J = 7.5 Hz), 1.5 (4H, q, J = 7.4 Hz), 1.2
(34H, broad s), 0.8 (3H, t, J = 7.5 Hz).
Instrumentation
1H NMR spectra were recorded on a VARIAN INOVA 200
spectrometer, using TMS as internal standard. ESI-MS spectra
were recorded on an Agilent 1100 Series ESI/MSD spectro-
meter. Experimental conditions were as follows: capillary
voltage, 3.5 kV; fragmentor, 100 V; source temperature,
350 uC; drying gas, N2 (10 l min21), carrier solvent, methanol
(0.4 ml min21). The samples were dissolved in methanol
containing trifluoroacetic acid. UV/vis absorption spectra were
recorded with a Jasco V-560 spectrophotometer. Aqueous
contact angles were measured using a goniometer (KERNCO)
under ambient conditions.
Scheme 1
UV-Vis irradiation was performed, under ambient condi-
tions, by using the monochromatic radiation of a fluorimeter
Fluorolog-2 (mod. F-111).
4-{(E)-[4-(2-Bromoethoxy)phenyl]diazenyl}phenol (B). A
mixture of A (2.6 g, 12 mmol), 1,2-dibromoethane (1 mL,
12 mmol) and sodium carbonate (18 g, 170 mmol) was refluxed
in 100 ml of acetonitrile for 5 days. After cooling to ambient
temperature the resulting suspension was filtered. The organic
solution was concentrated under reduced pressure and purified
by column chromatography (dichloromethane 100%) to give B
(yield 20%) as a yellowish powder. Anal. Calcd (%) for
C14H13Br1N2O2: C, 52.36; H, 4.08; N, 8.72; found: C, 49.21; H,
3.95; N, 8.37. ESI-MS m/z: [M]+ 321 (100%). H1 NMR CDCl3
(200 MHz) d 7.9 (dd, 4H, J = 5 Hz, 4 Hz), 7 (dd, 4H, J = 7 Hz,
5 Hz), 5.6 (br, 1H), 4.4 (t, 2H, J = 6.2 Hz), 3.7 (t, 2H, J =
5.5 Hz).
Results
Firstly, we tested the photobehavior of 1 in solution. As shown
in Fig. 2A, irradiation of a CH2Cl2 solution of 1 with UV light
(E)-1-[4-(2-Bromoethoxy)phenyl]-2-(4-{[11-(dodecylthio)
undecyl]oxy}phenyl)diazene (D). A mixture of B (0.124 g,
0.4 mmol), C (0.8 g, 1.9 mmol) and sodium hydride (0.02 g,
0.8 mM) was refluxed in 50 ml of THF for 5 days. After
cooling to ambient temperature, the resulting suspension was
filtered and washed with THF. The organic residues were dried
under reduced pressure and purified by column chromato-
graphy (dichloromethane–cyclohexane 6 : 4) to give
D
(yield 18%) as a yellowish powder. Anal. Calcd (%) for
C37H59Br1N2O2S1: C, 65.75, H, 8.80, N, 4.14, S, 4.74; found:
C, 67.85; H, 9.23; N, 3.97, S, 4.55. ESI-MS m/z: [M]+ 675.4
(100%). 1H NMR (CDCl3): d 7.9 (dd, 4H, J = 5 Hz, 4 Hz), 7.0
(dd, 4H, J = 7 Hz, 5 Hz), 4.4 (t, 2H, J = 6.2 Hz), 3.7 (t, 2H, J =
5.5 Hz). 4.0 (2H, t, J = 7.3 Hz), 2.4 (4H, t, J = 7.4 Hz), 1.7 (2H,
t, J = 7.5 Hz), 1.5 (4H, q, J = 7.4 Hz), 1.2 (34H, broad s), 0.8
(3H, t, J = 7.5 Hz).
2-{4-[(E)-(4-{[11-(dodecylthio)undecyl]oxy}phenyl)diaze-
nyl]phenoxy}-N,N,N-trimethylethanaminium bromide (1).
Compound D (0.02 g, 0.03 mmol) was dissolved in ethanol
and was allowed to react with an excess of trimethylamine at
60 uC for one week. After cooling to room temperature, the
Fig. 2 Absorption spectra of a 18 mM CH2Cl2 solution of 1 recorded
after intervals of irradiation with 360 nm light (A) and afterwards with
450 nm light. The spectra were recorded under ambient conditions
using a quartz cell with an optical pathlength of 10 mm.
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results in bleaching of the 356 nm band accompanied by
the formation of new absorptions at 310 and 450 nm. A
photostationary state (ca. 90% conversion of the trans isomer)
was reached after 15 min irradiation.{ Further irradiation of
this solution with visible light led to the almost complete
recovery (up to 95% of the initial value) of the original band at
356 nm (Fig. 2B). These results are in full agreement with the
reversible trans O cis photoisomerization of the azobenzene
moiety, suggesting that the functionalization of both the
phenyl rings does not affect its photochromic properties.
In order to achieve SAMs of 1 (SAMs-1), we used our
recently developed ultrathin Pt films11 as suitable platforms.
These metal substrates (thickness ca. 20 nm, roughness ca.
2 nm) exhibit a unique combination of excellent optical trans-
parency, homogeneity, robustness, and conductivity features.
As a result they offer the advantage to probe the spectroscopic
properties of adsorbed chromophores simply by conventional
spectrophotometry in transmission mode.11 SAMs-1 were
obtained by immersion of the Pt substrates in a CH2Cl2
solution of 1 (0.5 6 1023 M) for 20 h, at room temperature in
the dark. The substrates were then rinsed several times with
CH2Cl2 first and then methanol to remove any physisorbed
material, and dried at room temperature.{
Fig. 3A shows the absorption spectra of a typical SAM-1
recorded before and after irradiation with UV light. The
absorption spectrum of SAM-1 before irradiation shows a
main absorption band peaked at ca. 360 nm and safely
assignable to the p–p* electronic transition of the trans-
azobenzene moiety. The related absorbance value allows the
2 12
Fig. 3 (A) Absorption spectrum of SAM-1 recorded before (—) and
after (- - -) irradiation with UV (360 nm) light. (B) Switching of the
absorbance maximum response ($) and aqueous contact angle (#)
observed after alternate cycles of UV (360 nm, 5 min) and visible
(450 nm, 20 min) light irradiation. The absorption spectra were
recorded using as a reference sample the same ultrathin Pt substrate
before chemisorption of 1.
˚
estimation of a surface coverage of ca. 1 molecule per 100 A .
This value is basically the same as we have obtained for a
similar azo derivative not bearing the quaternary ammonium
group and self-assembled on the same Pt substrate,8f,11c
indicating that such chemical modification does not affect
the packing process. UV irradiation of SAM-1 results in
significant bleaching of the 360 nm absorption band as well as
a slight blue shift of lmax, reaching a photostationary state
after 5 min (Fig. 3A). This behavior fully agrees with that
observed upon UV irradiation of 1 in CH2Cl2 solution (see
Fig. 2A) and provides unambiguous, direct evidence for the
trans to cis photoisomerization of SAM-1. Aqueous contact
angle (ha) measurements give ha # 42u, a value reflecting well
the presence of both hydrophobic and hydrophilic end groups
in the monolayer. In fact, such a value is around the average
expected for typically hydrophobic (.100u) and hydrophilic
(,10u) monolayers.1a,b
The reversible trans O cis photoswitching of the azoben-
zene-based SAMs was proven by alternate cycles of UV and
visible irradiation in which the decrease and recovery of
the 360 nm absorption band were monitored (Fig. 3B).
Accordingly, similar behavior is also observed in ha changes
upon UV-Vis irradiation. In particular, the increase of ca. 10u
in the ha value after UV irradiation accounts for a reduced
hydrophilicity of the cis isomer of 1, probably due to reduced
accessibility of the quaternary group to the water surface.
The suitability of SAM-1 to immobilize TPPS was tested by
soaking the film in an aqueous solution of porphyrin (0.1 mM)
for 3 h.§ The substrate was then washed several times with
water to remove the free TPPS and dried at room temperature.
The absorption spectrum (Fig. 4A) shows clearly the presence
of the typical Soret absorption of the TPPS at ca. 424 nm
beside the absorption band of SAM-1 at 360 nm." By
assuming that no significant differences in the molar absorp-
tivity values of SAM-1 and TPPS occur upon binding, we can
roughly estimate the ratio 1 : TPPS to be ca. 7 : 1.
The sample was then placed in a spectrophotometric cell
(10 mm pathlength) containing 3 ml of water and irradiated
with 360 nm light at several interval times. After each step
of irradiation the substrate was withdrawn, dried and
§ Longer reaction times and higher TPPS concentrations did not affect
the amount of TPPS bound to the monolayer, as determined from the
absorbance values at 424 nm (vide infra).
{ The percentage of the photostationary state was estimated on the
basis of the extinction coefficients of the pure trans and cis isomers.7b
{ Longer and shorter reaction times led to lower degrees of
chemisorption of 1, as determined from absorbance values at 360 nm
(vide infra)
" That the immobilization of TPPS is strongly encouraged by
electrostatic interactions was well-supported by control experiments
carried out using the cationic meso-tetrakis(N-methylpyridyl)por-
phyrin, which did not show any significant porphyrin absorption in
the SAM.
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Scheme 2 Idealized representation for the ‘‘catch and release’’ of
TPPS by the photoswitchable SAM-1.
surface can be achieved by exploiting the light-controlled
electrostatic interactions with the two isomeric forms of a
cationic azobenzene SAM. In view of the non-specific nature
of the Coulombic interactions, we believe that such an appro-
ach is not limited to the example presented herein but may also
be suitable for controlling the immobilization and release
of molecules rich in negative charges such as polynucleotides
and/or other biologically relevant targets. These studies are
currently in progress in our laboratory and the results will
appear in forthcoming papers.
Fig. 4 (A) Absorption spectra of SAM-1 after immersion for 3 h in a
solution of TPPS and washing with water before (m) and after (n) 2 h
irradiation with UV (360 nm) light. (B) Absorbance changes of the
Soret band in the case of the films non-irradiated (&), irradiated with
424 nm light (n) and irradiated with 360 nm light (#). The solid line
represents the fitting of the data according to a first-order process.
Acknowledgements
This work was supported by MIUR, Rome, Italy (PRIN 2006,
no. 2006031909).
References
investigated by UV-Vis spectroscopy. A photostationary state
was reached after 2 hours of irradiation. As shown in Fig. 4A,
the corresponding absorption spectrum reveals the occurrence
of the trans–cis photoisomerization of SAM-1 accompanied
by ca. 80% decrease of the TPPS absorbance. According to
what was expected for the photoisomerization process, the
porphyrin photorelease resembles first-order kinetics (Fig. 4B)
and is in line with the changes of the surface properties
occurring upon UV irradiation of the SAM. In fact, the
cationic end-groups switch to a conformation probably less
favorable for effective electrostatic interactions with the
anionic porphyrin. The reduced hydrophilicity observed in
the case of the cis form of SAM-1 (vide supra) supports well
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the photoisomerization of the azo chromophore is further
corroborated by two key control experiments in which similar
samples were i) kept in the dark and ii) irradiated at the
wavelength of the Soret absorption for the same interval times.
As shown in Fig. 4B no significant release of porphyrin was
observed in both cases.
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