Porphyrin self-assembled monolayers and photodynamic oxidation of tryptophan

Article abstract of DOI:10.1142/S1088424610001696

The zinc and magnesium metalated derivatives of 5,5′-[12,12′- di(thiododecyloxy)-4,4′-phenyl)]-10,10′,15,15′,20, 20′-hexakis(3,3′,4,4′,5,5′-hexakisdecyloxyphenyl) diporphyrin, 1b and 1c, have been synthesised and deposited to form self-assembled monolayer

Full text of DOI:10.1142/S1088424610001696

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 81–88
DOI: 10.1142/S1088424610001696
Published at http://www.worldscinet.com/jpp/
Porphyrin self-assembled monolayers and photodynamic
oxidation of tryptophan
Isabelle Chambriera, David A. Russella, Derek E. Brundishb, William G. Loveb,
Giulio Jori*c◊, M. Magaraggiac and Michael J. Cook*a◊
a Wolfson Materials and Catalysis Center, School of Chemistry, University of East Anglia, Norwich NR4 7TJ, UK
b Destiny Pharma Ltd., Sussex Innovation Center, Science Park Square, Falmer, Brighton BN1 9SB, UK
c Department of Biology, University of Padova, via Trieste 75, 35121 Padova, Italy
Received 23 December 2008
Accepted 27 February 2009
ABSTRACT: The zinc and magnesium metalated derivatives of 5,5′-[12,12′-di(thiododecyloxy)-4,4′-
phenyl)]-10,10′,15,15′,20,20′-hexakis(3,3′,4,4′,5,5′-hexakisdecyloxyphenyl)diporphyrin, 1b and 1c,
have been synthesised and deposited to form self-assembled monolayer (SAM) films on the surface
of gold-coated glass substrates. The SAM films have been characterized by RAIR spectroscopy and
fluorescence spectroscopy. The potential for the porphyrin films to catalyze the oxidation of tryptophan
within human serum albumin upon irradiation with white light has been demonstrated and attributed to
the porphyrins acting as photosensitizers of oxygen to form oxidizing species.
KEYWORDS: porphyrin self-assembled monolayers, SAM films on gold, photodynamic surface.
these studies were extended to the surface coating of gold
INTRODUCTION
nanoparticles with a zinc phthalocyanine derivative in the
The anchoring of molecules onto the surface of a solid
presence of tetraoctylammonium bromide [6]. This con-
substrate to form self-assembled monolayers (SAMs) is
struct has been exploited as a drug delivery system within
now an established and well documented technology [1].
a photodynamic therapy (PDT) protocol. Irradiation of
Molecules designed for particular applications contain a
these nanoparticle conjugates within HeLa cells induced
“functional core”, i.e. a moiety that provides a desired
substantial cell mortality through the photodynamic pro-
effect or response to external stimuli, and a means for
duction of singlet oxygen [7].
attaching it to a solid surface [2]. The latter frequently, but
We now report some parallel work using single-
not invariably, exploits a tether chain that terminates with
component porphyrin SAMs on the surface of gold and
a functional group that reacts or interacts with the surface.
thus contribute to the burgeoning body of research into
A common combination for anchoring molecules to sur-
the characterization and applications of this type of SAM
faces uses a thiol- or disulfide-terminated tether to form
[8–11]. An interesting development has been the appli-
a thiolate linkage to gold; a second approach employs
cation of porphyrin SAMs for heterogeneous oxidation
a silyl function to react with a silica/silicon surface
catalysis and this has exploited, for example, the redox
[1, 2]. In previous work, one of our laboratories exploited
behavior of Mn [9] and Co [10] contained within the
both of these approaches to develop examples of phtha-
macrocycle core. To our knowledge there has been no
locyanine SAMs [3, 4]. SAMs deposited onto the surface
focus on exploiting porphyrin SAM surfaces for photody-
of gold, supported on a glass slide, were investigated for
namic oxidations. The present paper addresses this area,
optically based gas-sensing devices [5]. More recently
describing the synthesis of purpose-designed porphyrin
derivatives and their deposition onto a gold surface sup-
ported on a glass slide. The role of these constructs in
š
SPP full member in good standing
providing a ‘‘photodynamic surface’’ is demonstrated by
the photooxidation of the tryptophyl residue in human
*Correspondence to: Michael J. Cook, email: m.cook@uea.
ac.uk, fax: +44 (0)1603-592015
Copyright © 2010 World Scientific Publishing Company

82
I. CHAMBRIER ET AL.
serum albumin (HSA) when the films are illuminated
with white light.
Synthesis
Aldehydes. The required aromatic aldehyde derivatives
for the synthesis of the desired porphyrins were prepared
from4-hydroxybenzaldehydeand3,4,5-trihydroxybenzal-
dehyde.Yields were improved compared to those reported
in the literature [13] by using a combination of both a
phase-transfer catalyst, tetra-n-butylammonium iodide, to
facilitate the substitution and a high boiling solvent.
Condensation. The mixed cyclotetramerization reac-
tion (Step iv in Scheme 1) was first attempted using BF3-
etherate as the acid and p-chloranil as the oxidant. No
porphyrins were obtained by this method. The reaction
was therefore carried out using propionic acid under aer-
obic conditions. The efficiency of the reaction was further
improved by using an oxidizing co-solvent, nitrobenzene
[14]. Under these conditions there was no evidence of
chlorin formation. However, on using 4-(12-hydroxy-
dodecyloxy)benzaldehyde it was found that the product
was partially esterified to form the propionate leading to
mixtures and non-reproducible preparations. The acetate
derivative of 4-(12-hydroxydodecyloxy)benzaldehyde
(i.e. X = OCOCH3 in Step iv) was therefore preferred
as a precursor. Higher yields for the cyclotetrameriza-
tion were obtained and purification of the product 2a was
straightforward. This compensated for the inclusion of
the extra step required to deprotect 2a to afford 2b.
RESULTS AND DISCUSSION
Design of compounds
Thenon-symmetricallytetra-arylsubstitutedporphyrin
derivatives investigated for deposition as SAM films, 1b
and 1c (Scheme 1), contain two porphyrin moieties linked
through one of their aryl rings via a tether terminated with
a disulfide unit. Three long-chain alkoxy groups on each
of the remaining three aryl rings per porphyrin unit were
incorporated to minimize self-association and the detri-
mental quenching effects this has on the electronically
excited states of the porphyrin. Zinc and magnesium were
chosen as central metals because of their limited adverse
toxic properties so long as these ions are incorporated
into the tetrapyrrolic macrocycle [12]. The route to pre-
pare the compounds 1b and 1c proceeds via the synthesis
of a non-symmetrically substitued metal-free porphyrin
derivative with a tether terminated with a hydroxyl group
2b. This functionality was deemed suitable for subsequent
introduction of the disulfide moiety via the mesylate 2c.
Metalation of the ring with zinc and magnesium provided
the final step of the synthetic work.
CHO
OH
i
OR
OR
CHO
RO
OR
RO
OR
X= OCOCH3
O(CH2)12X
RO
RO
RO
OR
OR
OR
RO
RO
RO
OR
OR
OR
iv
N
N
N
N
N
N
N
N
X= OH
vii
M
ii
M
X= OCOCH3
OR
RO
OR
R= n-C10H21
CHO
O(CH2)12Y
O(CH2)12
S
2
iii
OH
2a
2b
M= H2, Y= OCOCH3
M= H2, Y= OH
1a
1b
M= H2
HO
OH
v
viii
M= Zn
M= Mg
ix
vi
CHO
2c
M= H2, Y= OSO2CH3
1c
Scheme 1. Synthesis of compounds 1b and 1c. In all cases R = C10H21. Reagents: i. Br(CH2)12OH, MEK, K2CO3, KI, tetra-n-
butylammonium iodide; ii. Ac2O; iii. n-bromodecane, MEK, KI, K2CO3, tetra-n-butylammonium iodide; iv. pyrrole, propionic acid,
nitrobenzene; v. NaOH; vi. Methanesulfonylchloride, Et3N; vii. 1) Thiourea, 2) aq. NaOH; viii. Zn(OAc)2; ix. Mg(ClO4)2, dry
pyridine
Copyright © 2010 World Scientific Publishing Company
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PORPHYRIN SELF-ASSEMBLED MONOLAYERS AND PHOTODYNAMIC OXIDATION OF TRYPTOPHAN
83
Metal-free porphyrin disulfide 1a. The free hydroxyl
substrates was undertaken by immersing substrates into a
solution of each compound in cyclohexane at concentra-
tions of ca. 2.5-5 × 10-7 M for 20 hours. The constructs
were then rinsed with cyclohexane and allowed to dry
in air. Reference (blank) substrates, i.e. without a SAM
coating, were similarly immersed in neat cyclohexane for
20 hours and dried as above. Reflection-absorption infra-
red spectroscopy (RAIRS) was used to detect formation
of SAM films on the gold surface.
group of 2b was converted into a mesylate, to afford 2c,
which was reacted with thiourea under aerobic condi-
tions. The porphyrin disulfide derivative 1a was obtained
after hydrolysis using aqueous sodium hydroxide. The
use of an aqueous solution of NaOH proved to be a satis-
factory alternative to an NaOH solution in ethanol which
1
led to a product whose H NMR spectrum contained an
unexplained extra set of signals.
Metalated porphyrin disulfides 1b and 1c. Finally,
the metalated derivatives 1b and 1c were obtained by
refluxing the metal-free compound 1a in refluxing THF
and dry pyridine containing zinc acetate and magnesium
perchlorate respectively. Expected changes in the UV-vis
spectra were observed upon the introduction of the metal
into the macrocycle. The four weak absorption bands at
516.0, 552.5, 592.5 and 650.5 nm present in the metal-
free porphyrin 1a are replaced by two weak bands at
557.0 and 598.0 nm, and 569.0 and 615.0 nm for the Zn
and Mg derivatives, 1b and 1c, respectively. Similarly,
the Soret band was shifted from 423.0 nm in the metal-
free compound to 427.5 nm for the zinc derivative and
432.0 nm for the magnesium derivative.
Figure 1 shows the RAIR spectra (2600–3200 cm-1
region) of an example of a SAM film of 1b. Comparable
spectra were obtained for SAMs of 1c. The absorption
bands shown in Fig. 1 are assigned to the aliphatic C-H
stretches associated with the peripheral alkyloxy chains
viz. CH3 (νAS) at 2959 cm-1, CH2 (νAS) at 2926 cm-1, and
CH2 (νS) at 2857 cm-1. Assignments of other bands in the
spectra were not attempted. Intensities of the absorption
bands showed good reproducibility over different regions
of the film and from one film to another prepared in the
same batch. The nature of the central metal ion does not
seem to have any effect on SAM formation.
The thickness of the gold coating precluded acquisi-
tion of UV-vis absorption spectra. However, fluorescence
emission spectra of the films and fluorescence excitation
spectra were recorded. Examples of these are shown in
Figs 2 and 3, respectively. The emission spectrum (Fig. 2)
shows the fluorescence spectrum obtained upon irradia-
tion at 415 nm. The emission is weak and gives rise to
a broad band spectrum, not unexpected and comparable
to that observed for phthalocyanine SAMs [5]. There is
a narrower peak centered at ca. 620 nm which we tenta-
tively assign to a Raman band. Evidence in support of
this is that the band position varies with the irradiating
wavelength. Its intensity can be ascribed to the effect of
the gold surface, as exploited in surface enhanced reso-
nance Raman scattering (SERRS) measurements. Esti-
mation of the fluorescence quantum yields was attempted
by comparing band intensities assigned to fluorescence
1
Both products were characterized by H NMR spec-
troscopy which showed the signal corresponding to
the protons α to the sulfur atom as a triplet at 2.71 and
2.6 ppm for 1b and 1c, respectively. MALDI-TOF mass
spectra showed isotopic clusters at 4601 [M+] for 1b and
4518 [M+] for 1c. Satisfactory C, H, N elemental analysis
data confirmed the structures and attested to the purity of
the compounds.
Formation and spectral characterization of self-
assembled monolayer films of 1b and 1c
Conditions for the preparation of the gold-coated glass
substrates are described in the Experimental section.
The deposition of 1b and 1c to form SAM films on the
Fig. 1. RAIR spectrum of a SAM film of 1b showing the 3100–2700 cm-1 region
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 83–88

84
I. CHAMBRIER ET AL.
errors of the order of +/- 0.01. The low
values may reflect a degree of quenching
of the first excited singlet state of the mol-
ecules within the films through a variety
of possible pathways, arising for example
from their proximity to the gold surface or
self-quenching through close association
of the molecules despite the presence of
the long alkoxy chains substituents [11,
15]. The occurrence of ground state inter-
actions between porphyrin molecules in
the SAMs is suggested by the broadness
of the Soret band in the excitation spec-
trum and the presence of at least two
maxima for such bands (Fig. 3); moreover,
the poorly resolved emission bands above
700 nm (Fig. 2) is suggestive of the for-
mation of weakly fluorescent side-by-side
porphyrin dimers [12].
Photodynamic oxidation studies
The photosensitizing efficiency of the
two types of porphyrin SAMs was tested
against a tryptophan model, because the
indole side chain of this amino acid is
susceptible to photooxidative attack by
both type I (radical-involving) and type II
(singlet oxygen-involving) pathways [12,
16–18]. Thus, a kinetic study of its pho-
tosensitized modification provides reliable
information on the photoefficiency of the
porphyrin derivatives that is independent
of the specific mechanism of the photo-
process. In particular, we selected a widely
diffused protein, albumin, as a substrate
since the three-dimensional structure of
the polypeptide chain provides a “bio-
logical” microenvironment for the trypto-
phan (human serum albumin contains one
tryptophyl residue per protein molecule).
Most conveniently, the time-course of the
photosensitized modification of tryptophyl
residues can be readily followed by spec-
trophotofluorimetric analysis, namely by
following the decrease in the intensity of
the 290 nm-excited fluorescence emission
in the 360 nm wavelength interval. Trypto-
phan is known to be largely converted into
photooxidation products that are kynure-
Fig. 2. Fluorescence emission spectrum from a SAM of 1b upon irradiation at
415 nm
Fig. 3. Excitation spectrum of a film of 1b (emission at 620 nm)
emission and band intensities in the excitation spectra
with spectral data from solutions of haematoporphy-
rin IX used as a fluorescence standard (quantum yield
0.11 in a non-associated state in EtOH). This indirect
approach, using data for seven films of 1b and six films
of 1c, gave consistent data which indicated that for both
types of SAM the quantum efficiency was ca. 0.02 with
nine or hydroxyindole based [18]. These do not interfere
with the determination of tryptophan fluorescence, since
their absorption and emission are shifted to the 400–500
nm spectral region [19]. For both SAMs, the photooxida-
tion (white light irradiation) of the albumin-bound tryp-
tophan sensitized by the porphyrin occurred according
to first-order kinetics, as represented by semilogarithmic
Copyright © 2010 World Scientific Publishing Company
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PORPHYRIN SELF-ASSEMBLED MONOLAYERS AND PHOTODYNAMIC OXIDATION OF TRYPTOPHAN
85
silica gel was then eluted with acetone. A pale yellow
oil was obtained after evaporation of the solvent which
crystallized upon cooling. The title product was recrys-
tallized from petroleum ether (bp 40–60°C). Yield 43.1 g
1
(65%). H NMR (60 MHz): δ, ppm 3.62 (t, 2H), 3.42
(t, 2H), 1.4 (brs, 21H).
4-[12-hydroxydodecyloxy]benzaldehyde. A mixture
of 12-bromododecanol (15 g, 57 mmol), 4-hydroxy-
benzaldehyde (6.9 g, 57 mmol), potassium carbonate
(excess), catalytic amounts of potassium iodide and tetra-
n-butylammonium iodide was refluxed in methyl ethyl
ketone (50 ml) for 16 h. After cooling, the solid was fil-
tered off and washed with acetone. The combined organ-
ics were concentrated under reduced pressure. Diethyl
ether was added to the oil and the flask placed in the
fridge. The precipitate was filtered. Yield 12.5 g (72%).
1H NMR (60 MHz): δ, ppm 9.9 (s, 1H), 7.9 (d, 2H), 7.0
(d, 2H), 4.1 (t, 2H), 3.62 (t, 2H), 1.3 (brs, 21H).
4-[12-acetyloxydodecyloxy]benzaldehyde. 4-[12-hyd-
roxydodecyloxy]benzaldehyde (1.1 g, 3.6 mmol) was
refluxed in acetic anhydride (15 ml) for 90 min. After
cooling, the solution was poured into water with stirring.
A precipitate formed which was filtered, washed with
water and dried. Yield 1.1 g (88%). 1H NMR (60 MHz):
δ, ppm 9.82 (s, 1H), 7.8 (d, 2H), 7.0 (d, 2H), 4.0 (t, 4H),
2.02 (s, 3H), 1.3 (brs, 20H).
Fig. 4. Typical example of photokinetic plot describing the photo-
sensitized oxidation of the tryptophyl residue in albumin for 1c
plots (see Fig. 4). The slope of the plots allows the calcu-
lation of the rate constant k for the photoprocess; hence, k
can be taken as a parameter defining the photosensitizing
efficiency of the porphyrin SAMs. Under our experimen-
tal conditions, the rate constant for the photoprocesses
is of the order of 2 × 105 s-l; this value is about 20-fold
smaller than those found for the photosensitized modi-
fication of tryptophan by free porphyrins [12, 20]. The
decrease in the efficiency of a photosensitizer molecule,
once it is immobilized on a solid matrix, is to be expected
[17]. However, it is worth emphasizing that the photo-
efficiency of the immobilized porphyrins 1b and 1c is
unusually large when compared with that observed for
other matrix-bound photosensitizing agents [17].
3,4,5-trisdecyloxybenzaldehyde. A mixture of 3,4,5-
trihydroxybenzaldehyde (2 g, 13 mmol), 10-bromod-
ecane (15 ml, excess), potassium carbonate (excess),
catalytic amounts of potassium iodide and tetra-n-bu-
tylammonium iodide was refluxed in methyl ethyl ketone
(25 ml) for 16 h. After cooling, the solution was filtered
and the solid washed with acetone. The solvent was
evaporated leaving an oil. This was filtered through silica
gel (eluent: petroleum ether (bp 40–60°C)). Excess bro-
modecane was obtained after evaporation of the solvent.
The silica gel was then eluted with acetone. The solvent
was evaporated leaving a pale yellow oil which was used
EXPERIMENTAL
Equipment
1H NMR spectra were measured in CDCl3 at 270 MHz
on a Jeol EX 270 or at 300 MHz on a Varian Gemini-300
using TMS as the internal reference. Mass spectra were
obtained using the MALDI technique in Dithranol matrix
or the FAB technique in Noba matrix. UV-vis spectra of
solutions were measured in THF (unless otherwise speci-
fied) using a Hitachi U-3000 spectrophotometer. RAIRS
experiments were performed using a Bio-Rad FTS40 Fou-
rier transform infrared spectrometer. Fluorescence emis-
sion and fluorescence excitation spectra were obtained
using a Perkin Elmer MPF4 spectrophotofluorimeter.
1
without further purification. Yield 7 g (93%). H NMR
(60 MHz): δ, ppm 9.8 (s, 1H), 7.1 (s, 2H), 4.1 (brt, 6H),
1.3 (brs, 48H), 0.9 (t, 9H).
5-(12-acetyloxydodecyloxyphenyl)-10,15,20-tri-
(3,4,5-trisdecyloxyphenyl)porphyrin (2a). 3,4,5-tris-
decyloxybenzaldehyde (3.51 g, 6 mmol) and 4-(12-acety-
loxydodecyloxy)benzaldehyde (0.71 g, 2 mmol) were
heated in propionic acid (40 ml) containing nitrobenzene
(10 ml) to 130°C. Pyrrole (0.55 g, 8 mmol) was added
at once and the temperature maintained for 3 h. The
solution was then cooled and excess methanol added.
The flask was placed in the fridge overnight. The dark
oil at the bottom of the flask was separated and washed
with methanol. The product was separated by column
chromatography over silica gel (eluent: petroleum ether
(bp 40–60°C)/THF 10:1) twice. The sample was used
Materials
12-bromododecanol. 1,12-dodecanediol (50 g, 0.25
mol) in hydrogen bromide 48% (220 ml) was continu-
ously extracted with petroleum ether (bp 80–100°C)
(300 ml) for 18 h. The solvent was evaporated and the
crude oil was filtered through silica gel (eluent: petroleum
ether (bp 40–60°C)). A colorless fraction was obtained
containing 1,12-dibromododecane (10.2 g, 13%). The
1
in the next stage without further purification. H NMR
(270 MHz): δ, ppm 8.86–8.94 (m, 8H), 8.1 (d, J = 8 Hz,
2H), 7.42 (s, 6H), 7.27 (d, J = 8 Hz, 2H), 4.29 (brt, 8H),
Copyright © 2010 World Scientific Publishing Company
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86
I. CHAMBRIER ET AL.
4.08 (t, 14H), 2.26 (s, 3H), 1.2–2.0 (m, 164H), 0.91 (t,
9H), 0.83 (t, 18H), -2.79 (s, 2H).
diporphyrinatozinc (1b). 5,5′-(12,12′-dithiodidode-
cyloxyphenyl)-10,10′,15,15′,20,20′-hexa(3,3′,4,4′,5,5′-
hexakisdecyloxyphenyl)diporphyrin (114 mg, 25 µmol)
was refluxed in THF (5 ml). Zinc acetate dihydrate
(20 mg, 90 µmol) was added and reflux continued for
30 min. The solvent was evaporated and the residue
chromatographed through a short column over silica gel
(eluent: petroleum ether (bp 40–60°C)/THF 4:1). Yield
116 mg (99%). 1H NMR (270 MHz): δ, ppm 8.9–9.1 (m,
8H), 8.1 (d, J = 8 Hz, 2H), 7.42 (s, 6H), 7.27 (d, J =
8 Hz, 2H), 4.29 (t, 8H), 4.24 (t, 2H), 4.09 (t, 12H), 2.67
(t, 2H), 1.2–2.05 (m, 164H), 0.91 (t, 9H), 0.83 (t, 18H).
UV-vis (THF): λ, nm (ε × 105 M-1.cm-1) 598.0 (0.12),
557.0 (0.38), 427.5 (12.6), 406.5 (0.86). MALDI-MS:
isotopic clusters at m/z 4601 [M]+ and 2300 ([M]2+,
100%). Anal. calcd. for C292H462N8O20S2Zn2: C, 76.25; H,
10.12; N, 2.44%. Found: C, 76.39; H, 10.39; N, 2.12.
5,5′-(12,12′-dithiodidodecyloxyphenyl)-10,10′,
15,15′,20,20′-hexa(3,3′,4,4′,5,5′-hexakisdecyloxy-
phenyl)diporphyrinatomagnesium (1c). 5,5′-(12,12′-
dithiodidodecyloxyphenyl)-10,10′,15,15′,20,20′-hexa
(3,3′,4,4′,5,5′-hexakisdecyloxyphenyl)diporphyrin (100
mg, 22 µmol) was refluxed in pyridine (10 ml, dried over
KOH) under N2 atmosphere. Magnesium perchlorate
(10 mg, 80 µmol) was added and reflux continued for
4 h. After cooling, the mixture was poured onto water and
extracted with diethyl ether. The organics were washed
twice with dil. HCl (20%), brine, dried (MgSO4), filtered
and the solvent evaporated. The product was purified by
column chromatography over silica gel (eluent: petro-
leum ether (bp 40–60°C)/THF 10:1).Yield 92 mg (93%).
1H NMR (270 MHz): δ, ppm 8.8–9.1 (m, 8H), 8.1 (d, 2H),
7.25–7.5 (m, 8H), 3.9–4.4 (m, 22H), 2.7 (t, 2H), 1.1–2.1
(m, 164H), 0.8–1.0 (m, 27H). UV-vis (THF): λmax, nm
(ε×105 M-1.cm-1)615.0(0.23),569.0(0.26),432.0(10.77).
MALDI-MS: isotopic clusters at m/z 4518 ([M]+, 100%)
and 2260 [M]2+. Anal. calcd. for C292H462N8O20S2Mg2: C,
77.63; H, 10.31; N, 2.48%. Found: C, 77.39; H, 10.28;
N, 2.42.
5-(12-hydroxydodecyloxyphenyl)-10,15,20-tri(3,4,5-
trisdecyloxyphenyl)porphyrin (2b). 5-(12-acetyloxydo-
decyl-oxyphenyl)-10,15,20-tri(3,4,5-trisdecyloxyphenyl)
porphyrin obtained above was refluxed in THF
(10 ml). Ethanolic NaOH (excess) was added. The
reaction was followed by tlc (eluent: petroleum ether
(bp 40–60°C)/THF 10:1) until complete (rf starting mate-
rial = 0.8, rf product = 0.2). The solvent was evaporated
and the residue was chromatographed over silica gel
(eluent: petroleum ether (bp 40–60°C)/THF 4:1). Yield
0.49 g (11% from benzaldehydes). 1H NMR (270 MHz):
δ, ppm 8.85–9.0 (m, 8H), 8.1 (d, J = 8 Hz, 2H), 7.43 (s,
6H), 7.3 (d, J = 8 Hz, 2H), 4.3 (t, 8H), 4.09 (t, 12H), 3.65
(brt, 2H), 1.2–2.05 (m, 164H), 0.91 (t, 9H), 0.84 (t, 18H),
-2.77 (s, 2H).
5-(12-methanesulfonyloxydodecyloxyphenyl)-10,
15,20-tri(3,4,5-trisdecyloxyphenyl)porphyrin (2c). 5-
(12-hydroxydodecyloxyphenyl)-10,15,20-tri(3,4,5-
trisdecyloxyphenyl)porphyrin (0.193 g, 86 µmol) was
dissolved in dichloromethane (8 ml) and the solution
cooled with a water bath. Et3N (1 ml) was added, fol-
lowed by methanesulfonylchloride (20 drops). The reac-
tion was stirred at rt for 1 h. The organics were washed
with a dilute HCl solution, brine, dried (MgSO4), filtered
and the solvent removed under reduced pressure. The
residue was purified by column chromatography over
silica gel (eluent: petroleum ether (bp 40–60°C)/THF
4:1). Yield 0.159 g (80%). 1H NMR (270 MHz): δ, ppm
8.85–9.0 (m, 8H), 8.1 (d, J = 8 Hz, 2H), 7.43 (s, 6H), 7.28
(d, J = 8 Hz, 2H), 4.2–4.35 (m, 10H), 4.09 (t, 12H), 2.97
(s, 3H), 1.1–2.0 (m, 192H), 0.91 (t,9H), 0.84 (t, 18H),
-2.77 (s, 2H).
5,5′-(12,12′-dithiodidodecyloxyphenyl)-10,10′,
15,15′,20,20′-hexa(3,3′,4,4′,5,5′-hexakisdecyloxy-
phenyl)diporphyrin(1a).5-(12-methanesulfonyloxydo-
decyloxyphenyl)-10,15,20-tri(3,4,5-trisdecylox-
yphenyl)-porphyrin (159 mg, 0.07 mmol) and thiourea
(excess) were refluxed in 1-pentanol (5 ml) until tlc (elu-
ent: petroleum ether (bp 40–60°C)/THF 4:1) indicated no
starting material was left (approx. 1 h) (rf product=0). Eth-
anol was added (2 ml) followed by aqueous NaOH (10%)
(2 ml) and reflux continued for 5 min. The solution was
left to cool and then added to dil. HCl (10%) and the
organics were extracted with dichloromethane, washed
with brine, dried (MgSO4), filtered and the solvent evapo-
rated. The residue was chromatographed over silica gel
(eluent: petroleum ether (bp 40–60°C)/THF 4:1). Yield
Deposition of SAM films
Glass microscope slides (BDH Ltd.) were used as the
substrates for the SAMs. The glass surface was wiped
clean with a soft, detergent-free tissue and then washed
with a stream of methanol in order to remove any bulk
surface contamination. The glass substrates were then
immersed into a solution of potassium hydroxide in
aqueous methanol (100 g of potassium hydroxide was
dissolved in 100 ml of Millipore water, then diluted to
250 ml with methanol) for 12 hours. The substrates were
rinsed with fresh Millipore water and then dried in a
stream of refluxing propan-2-ol. The resultant clean sub-
strates were stored in sample jars, with air-tight lids.
The cleaned, dried glass substrates were then coated
with a layer of chromium (99.999% purity, Johnson Mat-
they Ltd., 1 nm) followed by a layer of gold (99.999%
1
114 mg (74%). H NMR (270 MHz): δ, ppm 8.87–9.0
(m, 8H), 8.11 (d, J = 8 Hz, 2H), 7.44 (s, 6H), 7.3 (d, J =
8 Hz, 2H), 4.31 (t, 8H), 4.27 (t, 2H), 4.1 (t, 12H), 2.71
(t, 2H), 1.2–2.05 (m, 164H), 0.92 (t, 9H), 0.85 (t, 18H),
-2.76 (s, 2H). UV-vis (THF): λ, nm 650.5, 592.5, 552.5,
516.0, 423.0.
5,5′-(12,12′-dithiodidodecyloxyphenyl)-10,10′,15,
15′,20,20′-hexa(3,3′,4,4′,5,5′-hexakisdecyloxyphenyl)-
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 86–88

PORPHYRIN SELF-ASSEMBLED MONOLAYERS AND PHOTODYNAMIC OXIDATION OF TRYPTOPHAN
87
purity, Johnson Matthey Ltd., 45 nm). The chromium
each of three of the benzenoid rings, with a tether chain
on the fourth, can serve as photodynamic surfaces. The
choice of highly substituted TPP derivatives was made
to disrupt or at least minimize close porphyrin-porphyrin
ring interactions and this, together with a long tether to
maintain a significant distance of the core from the gold
surface, has ensured that the photoreactive electronically
excited states of the molecules are not fully quenched.
The latter has been demonstrated by the observation of
fluorescence from the film and further by the photooxi-
dation of the tryptophan moiety within human serum
albumin. The efficiency of the porphyrin SAMs as het-
erogeneous catalysts for photooxidations has potential
applications within chemical synthesis and also within
medical therapies. Thus other porphyrin derivatives,
deposited onto appropriate surfaces, could in principle be
incorporated within medical devices or implants where a
photodynamic effect could be advantageous.
layer was deposited to ensure a good adhesion of the gold
onto the glass surface. Both the chromium and gold lay-
ers were deposited by thermal evaporation under vacuum
using an Edwards Auto 306 vacuum evaporator.
The SAMs were formed by immersing the freshly
prepared gold-coated substrates into a solution of known
concentration of porphyrin, typically between 2.5 × 10-7
and 5 × 10-7 M in cyclohexane at room temperature for
20 h.
Reference substrates for the SAMs were fabricated by
immersing freshly prepared gold-coated substrates into
cyclohexane for 20 h. The SAMs and the references were
then washed in fresh cyclohexane, dried in air and subse-
quently stored in clean amber jars with air-tight lids.
Spectroscopic characterization of SAM films
The BioRad RAIR spectrometer housed a liquid
nitrogen cooled MCT detector and was coupled with a
Spectra-Tech FT85 reflectance unit. RAIR spectra of the
films were acquired using p-polarized light at an inci-
dence angle of 85°. For further details see reference 3c.
The spectra were obtained from the co-addition of 1024
scans at a resolution of 4 cm-1. Measurement of the fluo-
rescence emission and fluorescence excitation spectra of
the films was achieved using a Perkin Elmer MPF4 spec-
trophotofluorimeter equipped with accessories for front-
surface fluorescence detection.
Acknowledgements
We thank the EPSRC for access to the mass spectrom-
etry service at Swansea.
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