Extending the porphyrin core: Synthesis and photophysical characterization of porphyrins with π-conjugated β-substituents

Article abstract of DOI:10.1039/b707505g

New free-base and Zn-porphyrins with either vinyl groups or p-phenylene vinylene groups substituted on the β carbon have been synthesized and studied from a photophysical viewpoint in toluene. A bathochromic shift of all absorption bands together with a pronounced decrease in the molar absorption coefficient at the wavelength of maximum absorption (ε_max) and a broadening of the Soret band was observed with increasing conjugation of the substituent. A similar bathochromic shift trend was also observed in the corresponding emission spectra. The results are discussed in terms of delocalization of the frontier molecular orbitals of the porphyrin ring onto the substituent π-orbitals, and semi-empirical calculations confirming this interpretation are presented. High reactivity with oxygen was displayed by the triplet excited states of both free-base and Zn-porphyrins, and sensitized singlet oxygen (¹Δ_g), detected via its luminescence, was formed with high yields. Moreover, the transient absorption spectra of the lowest triplet displayed interesting features in the near-infrared region: a clear enhancement in the molar absorption coefficient in the range 600-900 nm was observed with increasing conjugation of the substituent. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b707505g

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Extending the porphyrin core: synthesis and photophysical
characterization of porphyrins with p-conjugated b-substituents
Barbara Ventura,*a Lucia Flamigni,a Giancarlo Marconi,a Fabio Lodatob and
David L. Officer*bc
Received (in Montpellier, France) 17th May 2007, Accepted 3rd September 2007
First published as an Advance Article on the web 17th September 2007
DOI: 10.1039/b707505g
New free-base and Zn-porphyrins with either vinyl groups or p-phenylene vinylene groups
substituted on the b carbon have been synthesized and studied from a photophysical
viewpoint in toluene. A bathochromic shift of all absorption bands together with a pronounced
decrease in the molar absorption coefficient at the wavelength of maximum absorption (emax) and
a broadening of the Soret band was observed with increasing conjugation of the substituent.
A similar bathochromic shift trend was also observed in the corresponding emission spectra.
The results are discussed in terms of delocalization of the frontier molecular orbitals of the
porphyrin ring onto the substituent p-orbitals, and semi-empirical calculations confirming this
interpretation are presented. High reactivity with oxygen was displayed by the triplet excited
states of both free-base and Zn-porphyrins, and sensitized singlet oxygen (1Dg), detected via its
luminescence, was formed with high yields. Moreover, the transient absorption spectra of the
lowest triplet displayed interesting features in the near-infrared region: a clear enhancement in the
molar absorption coefficient in the range 600–900 nm was observed with increasing conjugation of
the substituent.
the Coulombic interactions between the dipole moments of the
Introduction
adjacent units, that results only in a split of the Soret band. On
the contrary, when the spacer is connected to the porphyrin
rings via a vinyl or an ethynyl group, the p systems can
generally be assumed to be coplanar. This favors the extension
of the p-conjugation over the bridge and the formation of
highly delocalized excited states. The spectral characteristics of
these arrays are a bathochromic shift of all absorption bands
compared to those of the parent porphyrin monomer, a
pronounced broadening and/or splitting of the Soret band,
and an enhancement of the Q-bands.4,7–9 For example, in
fused porphyrin arrays, wherein the individual porphyrins are
triply linked to form long tapes, the extensive p-conjugation
results in extremely red-shifted (up to 3000 cmꢀ1) and intensi-
fied Q-bands.10,11
The search for porphyrin systems linked by p-conjugated
connectors is receiving increasing attention in view of their
potential application in the design of molecular wires for
electronic and photonic applications.1–3 In this context,
the analysis of the effect of p-conjugated substituents on
the photophysical properties of porphyrins is of great
relevance.
The extent of conjugation introduced by a substituent in the
aromatic ring of porphyrins depends on several factors, such
as the type of substituent introduced, the position of the
substituent on the ring and, above all, the degree of coplanar-
ity that can be achieved between the porphyrin plane and the
plane/s of the unsaturated system/s connected to it. In fact,
several studies on multi-porphyrinic arrays have demonstrated
that when the spacer contains or consists of a phenyl group
directly connected to the ring, the orbital interaction between
the porphyrins and the bridge is strongly reduced by the close-
to-perpendicular position preferentially assumed by the
groups in order to minimize steric hindrance.4–6 In these
systems, the through-bond electronic communication between
the porphyrin units is negligible and their spectral properties
are usually characterized by exciton coupling originating from
Since the electronic properties of the frontier orbitals of
porphyrins depend both on the nature and the position of the
substituents,12–14 it is important, in designing substituted
porphyrins characterized by highly delocalized molecular or-
bitals, to understand the effect of varying the point of sub-
stituent attachment. While the electronic properties of meso-
substituted conjugated porphyrins have been well studied,15–19
the effect of conjugated b-substituents on the photochemical
and electrochemical properties of porphyrins has not been
explored in detail, and only a small number of papers reports
on this topic.20–26 We are interested in the use of vinylic
substituents to affect electronic communication between the
units of multi-porphyrin arrays and, therefore, we have
undertaken a detailed photophysical study of a series of
b-vinyl substituted porphyrins commensurate with our inves-
tigations into the photoinduced energy and electron transfer
a Istituto per la Sintesi Organica e la Fotoreattivita` (ISOF), CNR,
Via P. Gobetti 101, 40129 Bologna, Italy
b Nanomaterials Research Centre, Massey University, Palmerston
North, Private Bag, 11222, New Zealand
c MacDiarmid Institute for Advanced Materials and Nanotechnology,
Massey University, Palmerston North, Private Bag, 11222, New
Zealand
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Results and discussion
Synthesis
All the b-substituted porphyrins were designed to contain a
terminal methylene group so that the extension of the con-
jugation was maximized. Okuma et al.34 have shown that
terminal vinyl aromatic compounds can be prepared in very
good yields using the Wittig reaction of methyl phosphonium
salt with activated aromatic aldehydes using DBU as base.
Less activated aldehydes required longer reaction times and
higher temperatures (higher boiling solvents). This seemed
ideally suited to the preparation of the required vinylic por-
phyrins because it has been shown that porphyrin aldehydes
readily undergo Wittig reactions.35 All compounds used in this
study (1–9) were prepared according to this method from the
respective aldehydes (Scheme 1). Because most of the precur-
sor aldehydes are not particularly reactive, all the Wittig
reactions were performed in refluxing dry toluene under argon.
Yields were optimized by dissolving/suspending the porphyrin
aldehyde and three equivalents of the phosphonium salt in
toluene, and adding DBU in excess (10 equivalents) at reflux.
In order to make both free-base and Zn metallated por-
phyrins, Wittig reactions can either be performed on free-base
or on Zn porphyrin aldehydes and the homologue obtained by
Scheme 1 Synthesis of porphyrins 1–8 from precursors 10–15 and
vinylstilbene 9 from 16.
processes in porphyrinic assemblies of different types and
topologies.27–33
We present here the synthesis (Scheme 1) and full photo-
physical characterization in toluene of a series of four free-
base (1, 3, 5, 7) and a series of four Zn-tetraphenylporphyrins
(2, 4, 6, 8) substituted at the b-carbon with either vinyl groups
or p-phenylene vinylene groups (Chart 1). The properties of
the two series of b-substituted porphyrins have been compared
with those of the parent porphyrins, TPP (tetraphenylpor-
phyrin) and ZnTPP (Zn-tetraphenylporphyrin), respectively.
It is shown that increasing delocalization of the porphyrin
p-system occurs as the length of the conjugated substituent
increases, and this has been supported by semiempirical
calculations. Moreover, the triplet states of all the examined
porphyrins show high reactivity with molecular oxygen and
formation of singlet oxygen occurs in high yields.
Chart 1 Porphyrins 1–8, vinylstilbene 9, aldehyde 16 and precursors 10–15.
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Scheme 2 Synthesis of extended porphyrin aldehyde 15.
metallation/demetallation. Both of these latter reactions proceed
under very mild conditions and with almost quantitative yield.
The preparation of 2-vinyl-TPP has already been
proton magnetic resonances typical of b-substituted tetraphenyl-
porphyrins with the characteristic broad NH singlets around
ꢀ2.5 ppm for the free-base porphyrins. As illustrated for 8 in
Fig. 1, the spectra of the vinylporphyrins 1, 2, 5–8 showed the
normal AMX coupling pattern for the protons of a terminal
vinyl group (three doublets of doublets between 5 and 7 ppm,
Fig. 1), with geminal coupling of the terminal protons typically
from 1–2 Hz. The vinylic protons closest to the porphyrin core
typically resonate around 7.0 and 7.3 ppm as can be seen in the
COSY spectrum in Fig. 1. The seven b-pyrollic proton reso-
nances all occur within 0.4 ppm of each other furthest down-
field. LR-MALDI and HR-FAB mass spectra were easily
obtained for all the compounds presented; either molecular
ions (M+) or protonated ions (M + H+) were measured.
1
described,36 but in this work it was made from 2-formyl-TPP
10 using the simple Wittig conditions shown in Scheme 1. The
syntheses of the other members of the series 2–8 have not been
previously reported and they were obtained in moderate to
good yields using the Wittig approach. The precursor por-
phyrin aldehydes 10, 11, 12, 13 and 14 have been reported.21
Extended porphyrins 7 and 8, containing two phenylene
groups, were synthesized in four and three steps, respectively,
from 2-formyl-ZnTPP 11 via the extended Zn porphyrin
aldehyde 15 (Scheme 2). Zn insertions were performed accord-
37
ing to the acetate method
and free-base porphyrins were
obtained by treating a dichloromethane solution of the Zn
porphyrin with 3 M HCl.
Absorption spectroscopy
The characterisation of the porphyrins 1–8 was straightfor-
ward. The 1H NMR spectra of all the compounds displayed
The absorption spectra of reference TPP and free-base
porphyrins 1, 3, 5 and 7 are displayed in Fig. 2. From
Fig. 1 1H NMR spectrum (500 MHz, CDCl3) of styryl porphyrin 8, with its COSY spectrum shown in the inset.
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inspection of the spectra it is clear that passing from TPP to 7,
i.e. by increasing the degree of conjugation of the b-substitu-
ent, a dramatic decrease in emax and a red-shift and a broad-
ening of the Soret band occur (Fig. 2 and Table 1). In
particular, there is a strong decrease in emax compared to
TPP, for 1, 3 and 5 where, respectively, one vinyl group,
two vinyl groups and a divinylbenzene group are attached to
the tetrapyrrolic ring. In porphyrin 7, that has a further
phenylene vinylene group in the b-substituent, the Soret emax
remains unchanged compared to 5 but a further broadening of
the band is observed. The red-shift of the Soret band is more
pronounced in passing from TPP to 1 (from 419 to 424 nm)
than to the other porphyrins in the series and in the Q-bands a
general red-shift of all bands along the series occurs with
increasing conjugation.
Table 1 Absorption data for TPP, ZnTPP and porphyrins in aerated
toluene solutions at room temperature
10ꢀ5e/
10ꢀ5e/
l
max/nm Mꢀ1 cmꢀ1
lmax/nm
Mꢀ1 cmꢀ1
TPP
419
514
548
592
648
4.43
0.19
0.08
0.05
0.04
ZnTPP
423
550
589
5.37
0.24
0.05
1
3
5
7
424
519
554
596
653
3.69
0.22
0.08
0.06
0.03
2
4
6
8
425
552
590
3.93
0.22
0.04
426
522
560
600
659
2.57
0.21
0.09
0.07
0.02
430
556
591
2.16
0.18
0.05
In the spectra of the Zn-porphyrins, Table 1, the Soret
bands present a shift toward lower energies similarly to what is
observed in the free-base series; the decrease of emax is evident
for 2 and 4 compared to the model ZnTPP whereas 6 and 8,
with respect to 4, mainly show a broadening of the band
around 470 nm. The Q bands, on the other hand, in addition
to exhibiting the trend in the bathochromic shifts, also show a
variation in the relative intensities: the Q(0,0) band increases
its intensity while the Q(1,0) band is almost unaffected. No
split of the Soret and of the Q bands can be observed in the
absorption spectra of all examined porphyrins, a split that
usually occurs when a significant distorsion of the D4h
symmetry is induced by the presence of a substituent.38
It is quite evident that the absorption spectrum of the free-
base porphyrin 7, that carries the longest substituent, is
different from the plain superposition of the spectra of the
components TPP and vinylstilbene unit 9, that can be con-
sidered a model for the b-substituent present in 7: while the
TPP Soret band has a strongly decreased molar absorption
coefficient and a broadening which extends to 500 nm (Fig. 2),
the bands of 9 around 300–350 nm (e of the order of 30000,
data not shown) shift around 350–400 nm in 7. The same
427
524
564
602
659
1.67
0.17
0.10
0.06
0.01
433
559
594
2.18
0.23
0.11
344 (sh)
373 (sh)
428
0.32
0.34
1.59
0.18
0.13
0.07
0.02
350 (sh)
375 (sh)
433
0.40
0.35
1.96
0.24
0.14
524
568
602
659
560
597
occurs for the Zn homologue 8 compared with the models
ZnTPP and 9.
The observed bathochromic shifts can be explained by
considering that conjugation generally reduces the
HOMO–LUMO gap. In particular, the a1u and a2u HOMO
orbitals of porphyrins have different electron-density on the b
and the meso carbons and substituents with different electron
donating or withdrawing capacity can modulate in different
ways the energetic distribution of the porphyrin frontier
orbitals (i.e. the a2u/a1u energies) according to their posi-
tion.12–14,39 Thus, in the examined b-substituted porphyrins,
the decrease of the HOMO–LUMO gap along the series could
be due both to the stabilization of the LUMO orbitals as a
consequence of the increased delocalization and to the de-
stabilization of the filled HOMO orbitals (a1u in particular) as
a result of mixing with the substituent p-orbitals that lie at
lower energies.
In order to obtain support for the above rationalisations, a
series of simple MO calculations based on the ZINDO/S
Hamiltonian was performed, having first optimized the geo-
40
metries with the MM+ method; the results are reported in
Table 2. Out of the components of the first band Q(0,0) (i.e. Qx
and Qy), we observe a red shift in going from the unsubstituted
models to the substituted porphyrins of both series, while the
energy of the emitting state of compounds 1–8, (Qx), is
calculated to be almost constant. In the case of the
Zn-porphyrins, the oscillator strength of the whole band is
predicted to increase with the extension of conjugation of the
Fig. 2 Molar absorption coefficients of TPP and free-base porphy-
rins in toluene, with an expansion of the 470–700 nm region shown in
the inset.
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Table 2 Calculated data for transition wavelengths (l), oscillator
strengths (f) and dipole moment differences (Dm) between ground and
lowest excited states
Q and Soret bands are much more affected than the y counter-
parts; these components in fact derive, for example in the
series of the Zn-porphyrins, from HOMOꢀ1 (a1u
)
-
00
LUMO+1 (e2g ) promotions by 35 and 55%, respectively.
The destabilization of the a1u orbital is calculated as ꢀ0.39 and
ꢀ0.24 eV going from ZnTPP to 8 and from TPP to 7,
respectively, whereas the difference of energy of a2u is
ꢀ0.02 eV and ꢀ0.03 eV for the same compounds. The increase
in size of the substituent, beyond extending the delocalization
of p electrons with consequent red shift of the fluorescence
bands (see below), also causes a large delocalization of charges
reflected in increased dipole moment differences between S1
and S0 (last column of Table 2). This feature could be
exploited, in principle, to tune the emitting properties of this
class of compounds using the appropriate substituents for
particular functional applications.
l/nm, f (S0 - S1)
Qx Qy
l/nm, f (S0 - S2)
Bx By
Dm(S1 ꢀ S0)/D
TPP
646, 0.16 599, 0.34 355, 3.36 352, 3.14 ꢀ1.37
1
3
5
7
654, 0.14 613, 0.36 359, 2.17 357, 2.27
657, 0.14 616, 0.35 389, 3.09 371, 0.79
658, 0.13 616, 0.38 414, 3.00 357, 1.68
655, 0.15 618, 0.44 449, 3.53 360, 1.78
0.22
0.68
0.65
0.72
ZnTPP 634, 0.26 634, 0.26 362, 2.66 358, 2.32 ꢀ0.65
2
4
6
8
646, 0.32 640, 0.22 375, 2.46 365, 1.94 ꢀ1.79
648, 0.34 641, 0.21 414, 1.79 374, 1.57
650, 0.35 642, 0.17 444, 1.80 381, 1.29
648, 0.39 640, 0.16 468, 2.67 388, 1.49
2.83
2.67
2.79
substituent especially for the Qx component, the Qy compo-
nent being almost unaffected. The largest effect brought about
by the substituent is predicted to be on the B (Soret) bands
(here again with the Bx component much more likely affected
than the By): in agreement with the experimental features, one
observes a large red shift with the increase of the substituent
size and a noticeable intensity drop in going from TPP
(f = 6.50) and ZnTPP (f = 4.98) to the terms with larger
substituent groups. These effects can be understood by exam-
ining the composition of the states in terms of electronic
delocalization.
Our results are in agreement with the observed progressive
broadening and red-shifting of the Q bands with increase in
conjugation chain length reported recently by Walsh et al.25 in
a DFT study on the optical properties of Zn-tetraphenyl-
porphyrins b-substituted with conjugated carboxylic acid side
chains.
The shift to lower energies, of the order of 0.23 eV, of the
bands of vinylstilbene 9 when it is attached to the porphyrin as
in 7 and 8, can be taken as a further indication of the effective
electronic delocalisation that occurs between the conjugated
substituent and the porphyrin ring. It is well known, in fact,
that the lowest energy absorption peak of p-phenylene viny-
lene oligomers shifts toward lower energies when the length of
the oligomer, i.e. the p-conjugation, increases.42–44
As it is well known from Gouterman’s four orbital theory,38
the Q and B bands derive from destructive and constructive
promotions from the two lowest occupied MOs (a1u and a2u
)
to the degenerate LUMO e2g, in a D4h symmetry. In distorted
symmetries, such as that of TPP, where the central hydrogen
interaction causes a splitting of the e2g orbitals, one observes
two distinct components of the Q band, as it is evident
comparing the calculated energy of TPP and ZnTPP in
Table 2. Moreover, the intensity ratio Q(1,0)/Q(0,0), where
Q(1,0) refers to a vibronic quantum added to Q(0,0), has been
shown to be proportional to the square of the HOMO–LUMO
gap.41 While the substituents in the meso position influence
mainly the a2u (HOMO) orbital, leaving almost unaffected the
Emission spectroscopy
The emission spectra of optically matched solutions of TPP
and free-base porphyrins 1, 3, 5 and 7 in toluene at room
temperature upon excitation at 520 nm are displayed in
Fig. 3(a). The emission of the free-base porphyrins shifts to
lower energy with respect to the emission of TPP when the
conjugation of the b-substituent increases, with a trend that
follows the behavior of the absorption spectra. A shift of
5–7 nm, in fact, occurs in the emission peaks for the first
members of the series (1 and 3, see Table 3) while 5 and 7 are at
the same wavelengths as 3. From inspection of the spectra
a1u (HOMOꢀ1), which presents a node in the correspondence
of the meso carbons of the pyrrole rings, the reversal occurs for
b substitution. This is the reason why the x components of the
Fig. 3 Uncorrected emission spectra of optically matched solutions (A = 0.073) of TPP and free-base porphyrins in toluene excited at 520 nm at
room temperature (a) and normalized spectra at 77 K (b).
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Table 3 Luminescence properties of porphyrins and model 9 in toluene
295 K
77 K
a
Ffl
lmaxa/nm
tb
10ꢀ7kr/sꢀ1
lmaxa/nm
tb/ns
Ec/eV
TPP
1
3
650, 715
655, 721
662, 726
663, 729
662, 730
594, 642
599, 645
605, 654
0.110
0.091
0.091
0.090
0.091
0.047
0.044
0.050
0.063
0.073
0.350
9.3
8.5
8.5
8.1
9.0
1.9
1.7
1.8
1.9
1.8
0.72
1.2
1.1
1.1
1.1
1.0
2.5
2.6
2.8
3.3
4.1
49.0
643, 712
648, 718
652, 721
655, 724
657, 726
598, 654, (780)
602, 656, (777)
607, 663, (785)
611, 667, (789)
613, 669, (792)
357, 376, 398
13.4
12.3
12.4
12.2
13.7
2.7
2.5
2.6
2.6
2.4
1.92
1.91
1.90
1.89
5
7
ZnTPP
1.89
2.07 (1.59)
2.06 (1.59)
2.04 (1.58)
2.03 (1.57)
2.02 (1.57)
3.47
2
4
6
8
9
a
603, 659
603, 660
355, 375, 391 (sh)
2.07
Emission maxima derived from non-corrected emission spectra. Absolute quantum yields were determined by comparing corrected emission
spectra, using TPP in aerated toluene as a standard (Ffl = 0.1112). Excitation at 520 nm for the free-base porphyrin series, at 560 nm for the Zn-
porphyrin series and at 329 nm for model 9. Values in parentheses relate to the triplet state. Excitation at 560 nm for the porphyrin series and at
c
331 nm for model 9. Derived from the emission maxima at 77 K. Values in parentheses relate to the triplet state.
b
obtained in a glassy matrix (Fig. 3(b)) where the emission
bands are better resolved, the trend of the emission maximum
moving to higher wavelengths with the increased conjugation
of the substituent is confirmed and more evident also for the
last two members of the series (see Table 3). This indicates a
decrease of the energies of the lowest singlet excited state in the
free-base porphyrin series, in agreement with the postulated
extension of the p-conjugation of the porphyrin ring over the
substituent. Such an effect was also recently reported for a
series of b-substituted Zn-tetraphenylporphyrins with different
linkers: beyond the effect of p-conjugation, a dependence on
the electron-withdrawing or donating property of the terminal
group was observed.22
also consistent with the constant values extracted for the
radiative constants along this series.
The emission spectra of optically matched solutions of
ZnTPP and Zn porphyrins 2, 4, 6 and 8 in toluene at room
temperature upon excitation at 560 nm are displayed in
Fig. 4(a). All the Zn porphyrin emissions are red-shifted
compared to ZnTPP with 6 and 8 significantly more so
(Fig. 4(a) and Table 3), in a trend that follows the absorption
behavior. Quite interestingly, a clear trend in the inversion of
the relative intensities of the vibronic bands is also observed. It
can be noted that the calculated wavelengths reported in
Table 2 are in satisfactory agreement with the experimental
ones (Table 3), showing a larger stabilization of the Qx band in
both series going from the unsubstituted compounds (TPP
and ZnTPP) to the substituted ones (8–12 nm) than among the
last terms of the series (4 nm at most). The bathochromic shift
of the emission bands of the Zn-porphyrins along the series is
confirmed by the spectra obtained in glassy toluene solutions
at 77 K, reported in Fig. 4(b). The calculated energies of the
lowest singlet excited states are reported in Table 3 and their
values decrease from 2.07 eV for the model ZnTPP to 2.02 eV
for 8. Moreover, measurements at 77 K allowed the detection
of the phosphorescence of all the examined Zn porphyrins that
occurs at 780–790 nm, as can be observed in Fig. 4(b). We
The fluorescence quantum yields Ffl (Table 3), calculated by
comparing emission spectra corrected for the instrumental
response, appear uniform within the series (Ffl
0.090–0.091) and lower than that of reference TPP (Ffl
=
=
0.11). The fluorescence lifetimes remain almost uniform along
the series and similar to that of TPP, with a value that ranges
from 8.1 to 9.0 ns at room temperature and from 12.2 ns to
13.7 ns at 77 K. As a consequence, the radiative rate constant
(kr) of all the free-base porphyrins substantially equals that of
unsubstituted TPP (1.2 ꢂ 107 sꢀ1, see Table 3). The calculated
oscillator strengths, reported in the first column of Table 2, are
Fig. 4 Uncorrected emission spectra of optically matched solutions (A = 0.077) of ZnTPP and Zn porphyrins in toluene excited at 560 nm at
room temperature (a) and normalized spectra at 77 K (b). Expansion of the 750–850 nm region is shown in the inset.
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were unable to detect the phosphorescence in the porphyrins
of the free-base series, but triplet states were identified in both
free-base and Zn-porphyrins by their transient absorption
spectra (see below). The measured energies of the Zn-porphyr-
in triplet states, reported in parentheses in Table 3, indicate a
modest but continuous decrease in energy, indicating that, in
addition to stabilizing the singlet states of porphyrins, an
enhanced delocalization of the molecular orbitals also stabi-
lizes the triplet states.
Transient absorption spectroscopy
The triplet states of the two porphyrin series have been probed
by nanosecond flash photolysis. The transient absorption
spectra obtained upon 532 nm excitation at ambient tempera-
ture in air-free toluene solutions at the end of the laser pulse
are shown in Fig. 5 for the free-base porphyrins TPP, 3, 5 and
7 and in Fig. 6 for Zn-porphyrins ZnTPP, 4, 6 and 8. The
spectral features of 1 and 2, with one vinyl group at the
b-position of the porphyrin ring, are very similar to those of
their analogs with two vinyl groups, 3 and 4, respectively (data
not shown). The main absorption maxima wavelengths are
summarized in Table 4 together with the triplet lifetimes
obtained both in air-free and air-equilibrated toluene. The
spectrum of the model TPP (Fig. 5) is characterized by a strong
absorption band around 440 nm, a weaker band at 780 and a
modest bleaching of the ground state features in the region 500–
700 nm, in agreement with previously reported data.48,49 The
spectra of the other porphyrins of the series show interesting
differences. The first absorption band shifts increasingly along
the series from 440 nm for TPP to 500 nm for 7, and its
maximum absorption value reduces within the series. It must
be borne in mind that the absorption properties in this region
are determined by the ground state absorbance (Fig. 5 and
Fig. 6 reports DA), so the apparent shift to higher wavelengths
and the decrease in absorbance of the band is a consequence of
the increasing absorbance of the ground state in the region
450–500 nm along the series (see Fig. 2). The bands in the near
IR region, on the contrary, where no ground state absorbance
takes place, are genuine: we can see that the band centered at
780 nm in TPP is present almost unchanged in the other
porphyrins, except for 7 where it drastically increases and red-
shifts (see Table 4 and Fig. 5). The existence of this intense
band at 800 nm seems to be related to the presence of two
phenylenevinylene groups attached to the TPP ring, since it is
peculiar to porphyrin 7; this may provide application in the
field of non linear optics in the near IR region.2,50
In contrast to the free-base series, the fluorescence
quantum yields of the Zn-porphyrins 2 and 4 remain almost
unchanged compared to ZnTPP (Ffl = 0.047) and only
6 (Ffl = 0.063) and 8 (Ffl = 0.073, see Table 3) show a
significant increase. The measured lifetimes remain almost
unchanged for all the Zn-porphyrins, varying from 1.7 to
1.9 ns at room temperature and from 2.4 ns to 2.7 ns at
77 K (Table 3). As a consequence an increment of the
calculated radiative rate constant can be observed along the
series, going from 2.5 ꢂ 107 sꢀ1 for ZnTPP to 4.1 ꢂ 107 sꢀ1
for 8. The presence of phenylene vinylene units, therefore, has
a drastic effect on the emitting properties of the Zn-porphyr-
ins, and the calculated oscillator strengths reported for this
series (Table 2) show good agreement with this behaviour,
with an increase of ca. 70% going from the unsubstituted
ZnTPP to compound 8.
As discussed in the previous section, the spectral changes
observed both in the absorption and in the emission properties
along the two series indicate a progressively increased deloca-
lization of the frontier molecular orbitals of the porphyrin
unit. The extension of the p-conjugation over the substituent is
possible if the molecule can assume a conformation where the
porphyrin ring and the plane/s of the aromatic unit/s of the
substituent are co-planar. For a b-substituted free-base tetra-
phenylporphyrin similar to porphyrin 5 (with an aldehyde
terminal group attached to the phenyl instead of a vinyl one),
the X-ray characterization demonstrated the co-planarity of
the porphine ring and the phenyl substituent, with no partici-
pation in the porphine conjugation by the twisted meso-phenyl
substituents.45 We observed the same feature in the calculated
geometries of compounds 1–8, where an almost planar geo-
metry of the porphine-substituent system is obtained even for
the porphyrins with the longest conjugated substituents such
as 7 and 8, supporting our interpretation of the experimental
data. The meso-phenyls are twisted by ca. 421 with respect to
the plane defined by the pyrroles and this deformation remains
constant along the series.
The ZnTPP spectrum (Fig. 6 and Table 4) has a strong band
at 480 nm and a less pronounced band at 840 nm, in agreement
with previous reports.49,51 Inspection of the spectra of the Zn
series reveals a trend similar to the free-base series in the
apparent decrease and bathochromic shift of the first absorp-
tion band. The small band at 840 nm present in ZnTPP shifts
to longer wavelengths in the other porphyrins, although in
porphyrin 8 there is no clear indication of a band in the range
explored (up to 900 nm). A clear increase in the molar
absorption coefficient in the range 600–900 nm is evident with
increasing conjugation along the series. It should be noted that
the observed spectra result from transitions from the lowest
triplet to the upper levels of the same multiplicity and they
cannot be a source of information on the energy of the triplet
states, which are derived from the 77 K phosphorescence
spectra, as reported above. They are, however, interesting
since the substituted porphyrins exhibit clear spectral features
that are quite different from the parent porphyrins (Table 4),
indicating the involvement in the transitions of excited triplet
states at energies different from TPP and ZnTPP. It is
interesting to note that, in agreement with our data, in a study
where the p-conjugation of tetraphenylmetalloporphyrins is
As
a
model of the appended substituent in the
has been
most conjugated porphyrins, compound
9
characterized and the luminescence data are summarized in
Table 3. Vinylstilbene 9 is a good emitter (Ffl = 0.35) with a
lifetime of 0.72 ns at room temperature and 2.07 ns at 77 K,
and the emitting excited state is localized at 3.47 eV, in
agreement with previously reported oligo(p-phenyleneviny-
lene)s with two phenyl rings.42–44 Steady-state experiments
with excitation on the vinylstilbene band evidenced the com-
plete quenching of the luminescence of unit 9, indicative of a
very efficient energy transfer to the porphyrin, as previously
reported for oligo(p-phenylenevinylene)s meso-substituted
porphyrins.46,47
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Fig. 5 Transient absorption difference spectra in air-free toluene at the end of the pulse after excitation at 532 nm (18 ns pulse, 0.6 mJ pulseꢀ1) for
free-base porphyrins TPP, 3, 5, 7 (A532 = 0.53).
increased through aromatic ring condensation at the pyrroles,
together with an overall red-shift in the ground state absorp-
tion spectra, a red-shift of the T1–Tn absorption spectra and an
increase of the triplet molar extinction coefficient through the
near-infrared region with annulation was found.52
The triplet lifetimes of the free-base porphyrins are similar
and range from 110 to 130 ms in air-free solutions whereas in
Zn-porphyrins the effect of the substituents on the triplet
lifetimes is evident; the lifetimes decrease from 136 ms for
ZnTPP to 73 ms for 8 in air-free solutions. The oxygen
Fig. 6 Transient absorption difference spectra in air-free toluene at the end of the pulse after excitation at 532 nm (18 ns pulse, 0.6 mJ pulseꢀ1) for
Zn-porphyrins ZnTPP, 4, 6, 8 (A532 = 0.55).
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Table 4 Triplet excited state spectral features, lifetimes, quenching
rate constants and singlet oxygen (1Dg) luminescence yields in toluene
solutions at 295 K
described homogeneity in the emission quantum yields of the
free-base series (Table 3) where Ffl are all moderately reduced
compared to that of TPP and an increased intersystem cross-
ing, with a consequent higher yield of triplet formation,
probably occurs. For the Zn series a more inhomogeneous
behavior is observed but the values are similar to that of
ZnTPP (Table 4). These data indicate that the yield of singlet
oxygen production is independent on the type of substitution
in the examined Zn-porphyrins. However, the high values
obtained in both series, confirm the strong reactivity of these
b-substituted porphyrins with molecular oxygen and their
possible use as photosensitizers.
c
FD
lmax/nm
ta/ms
tb/ns
10ꢀ9kq/Mꢀ1 sꢀ1
TPP
1
3
440, 780
450, 780
460, 780
490, 780
500, 800
480, 840
480, 850
490, 860
510, 870
510, —
115
110
122
110
130
136
126
85
279
272
259
260
203
497
554
503
433
275
2.0
2.0
2.1
2.1
2.7
1.1
1.0
1.1
1.3
2.0
c
0.70
0.79
0.80
0.79
0.77
0.64
0.80
0.68
0.79
0.76
5
7
ZnTPP
2
4
6
8
a
90
73
b
Air-free solutions. Air-equilibrated solutions. The standard used
is TPP in aerated toluene at room temperature (FD = 0.7058).
Excitation at 520 nm for the free-base porphyrin series and at
560 nm for the Zn-porphyrin series.
Conclusions
A series of four free-base porphyrins and a series of four Zn-
porphyrins b-substituted with either vinyl groups or p-pheny-
lene vinylene groups have been synthesized and photophysi-
cally characterized in toluene solutions. The bathochromic
shift of both absorption and emission bands of the examined
porphyrins is consistent with an increasing delocalization of
the frontier molecular orbitals of the porphyrin unit when the
p-conjugation of the substituent increases, delocalization that
was confirmed by semiempirical calculations. The singlet
excited state lifetimes and fluorescence quantum yields were
similar to those of the unsubstituted porphyrin references for
both series and no clear trend was observed along the series,
except for an increase of fluorescence quantum yield in the Zn-
porphyrins in case of phenylene–vinylenic substitution. The
triplet states, with lifetimes of the order of one hundred
microseconds, displayed an interesting increase in the molar
absorption coefficient of their transient absorption spectra
above 700 nm along both series, with possible applications
in the field of non linear optics in the near IR. These results
suggest that conjugated b-substituents can be considered as
useful interporphyrin linkers in the design of multiporphyrinic
arrays with enhanced electronic communication. The reaction
of the triplet states with oxygen, moreover, is fast and the yield
of singlet oxygen sensitization is high for all the examined
porphyrins; this could open promising applications in the use
of conjugated porphyrins as therapeutic agents in photody-
namic therapy.
quenching reaction rate constant, kq, can be derived from the
lifetimes in air-equilibrated solutions (see Experimental
Section and Table 4). In agreement with previously reported
data, 48–49,51,53 most of the free base porphyrins have kq values
of about 2 ꢂ 109 Mꢀ1 sꢀ1and most of the Zn-porphyrins have
rate constants of the order of 1 ꢂ 109 Mꢀ1 sꢀ1, but within the
series an increased reactivity of the last member, either 7 or 8,
is observed.
Singlet oxygen photosensitization
Since both series showed a high reactivity with ground state
molecular oxygen, in order to get information on the cap-
ability of conjugated porphyrins to generate singlet oxygen, we
1
explored their photosensitizing ability by determining the Dg
quantum yield (FD) via the detection of its luminescence at
1270 nm. There are, in fact, reports on the use of vinyl-
substituted porphyrins in photodynamic therapy.54,55 The
quantum yields of 1Dg production of the free-base and Zn-
porphyrin series are collected in Table 4 and representative
cases of 1Dg luminescence are reported in Fig. 7. It can be seen
that for the free-base porphyrins the yields are uniform within
the series, around a value of 0.80, and higher than that of the
reference TPP. This result can be related to the previously
Experimental
Synthesis
High-resolution FAB mass spectra were recorded on a VG-
70SE instrument at University of Auckland, New Zealand and
on a VG ZAB 2 SEQ VG-Micromass at the Australian
National University, Australia. 1H NMR spectrometry experi-
ments were performed using 400 and 500 MHz Bruker Avance
instruments running TOPSPIN 1.3 software. Proton chemical
shifts in CDCl3 are relative to TMS. Chemical shifts in other
solvents are relative to residual protons (tetrahydrofuran-d8,
3.58 ppm; dioxane-d8, 3.53 ppm). Data are expressed as
position (in ppm), multiplicity (s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet, br = broad, app =
apparent), relative integral, coupling constant (JP indicating
Fig. 7 Singlet oxygen luminescence detected in air-equilibrated to-
luene for optically matched solutions of 3, 7 and TPP after excitation
at 520 nm.
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(20-(50,100,150,200-Tetraphenylporphyrinato zinc(II))yl)ethene
1H–31P coupling) and assignment. Coupling constants were
not reported when smaller than 1 Hz.
2. Free-base vinylporphyrin 1 (10 mg, 15.6 mmol) was metal-
lated as described in the general procedure above providing
10.5 mg (96%) of Zn porphyrin 2. 1H NMR (500 MHz,
Column chromatography employed silica gel (0.032–0.063
mm, Merck Kieselgel 60) or equivalent. Thin layer chromato-
graphy (TLC) was performed using pre-coated silica gel plates
(Merck Kieselgel 60F254). Flash chromatography was used for
compound purification. AR grade solvents and reagents were
used for synthesis, and distilled laboratory grade solvents were
used for chromatography. DBU (diazabicycloundecene),
tBuOK and methyltriphenylphosphonium bromide (Me–ps)
were purchased from Acros. All reagents were used as
purchased. Porphyrin aldehydes 10–14 were prepared as
previously reported.21 The preparation of toluene(diethylphos-
phonate)56 and 1,4-xylenebis(diethylphosphonate)57 were car-
ried out according to literature procedures from the respective
commercial bromides.
CDCl3):
d 8.98 (s, 1H, Hb-pyrrolic), 8.96–8.87 (m, 5H,
Hb-pyrrolic), 8.81 (d, J = 4.6 Hz , 1H, Hb-pyrrolic), 8.23–8.17
(m, 6H, Ho-Ph), 8.07 (app d, J = 7.5 Hz, 2H, Ho-Ph), 7.81–7.67
(m, 12H, Hm,p-Ph), 6.44 (dd, Jtrans = 17 Hz, Jcis = 10 Hz , 1H,
H
alkene), 5.85 (dd, Jtrans = 17 Hz, Jgeminal = 1.8 Hz, 1H,
H
alkene), 5.11 (dd, Jcis = 10 Hz, Jgeminal = 1.8 Hz, 1H, Halkene).
UV-Vis (toluene): lmax/nm (10ꢀ3e) 425 (393), 552 (22), 590 (4).
FAB-HRMS for M+ (C46H30N4Zn): 702.1756, calc.:
702.1762.
1-Trans,trans-(20-(50,100,150,200-tetraphenylporphyrinato
zinc(II))yl)butadiene 4. Zn porphyrin 13 (32 mg, 44 mmol) and
Me-ps (24 mg, 66.5 mmol) were dissolved/suspended in dry
toluene (25 mL) under argon. The solution was heated to
reflux andexcess of DBU (60 mg, 0.4 mmol) was added. After
24 h, the solution was evaporated to dryness and the product
isolated using flash chromatography (silica gel, DCM–hexane
= 1 : 1). The first band was collected and evaporated to
Porphyrin Zn metallations. The acetate method37 was used
for all Zn insertions into free-base porphyrins. The porphyrin
was dissolved in dichloromethane (AR grade) and 1.2 equiva-
lents of Zn(OAc)2 ꢃ H2O (BDH), dissolved in the same amount
of MeOH–water (10 : 1), were added. The mixture was stirred
for 2 h, then washed with water and the porphyrin precipitated
with MeOH. The solid collected was the pure Zn porphyrin.
Yields were always higher than 95%.
1
dryness to give 9 mg (28%) of pure 4 as purple crystals. H
NMR (assignments aided by COSY spectra) (500 MHz,
CDCl3):
d 8.96 (s, 1H, Hb-pyrrolic), 8.90–8.82 (m, 5H,
Hb-pyrrolic), 8.80 (d, J = 4.7 Hz, 1H, Hb-pyrrolic), 8.27–8.14
(m, 6H, Ho-Ph), 8.19 (d, J = 7 Hz, 2H, Ho-Ph), 7.81–7.68 (m,
12H, Hm,p-Ph), 6.91 (dd, Jtrans = 15.2 Hz, Jcis = 10.4 Hz, 1H,
Zn demetallations. A general procedure was used for the
removal of Zn to give free-base porphyrins. The porphyrin
was dissolved in dichloromethane (Laboratory Grade) and the
same volume of 3 M HCl was added. The mixture was stirred
for 20 min after which the organic layer was separated, washed
twice with 3 M HCl, bicarbonate solution and water.
The product was finally precipitated (generally using MeOH,
or water when methanol soluble) affording yields higher
than 90%.
0 0
alkene ), 6.23–6.13 (m, 2H, 1Halkene + 1Halkene ), 5.32 (app d,
H
Jtrans = 17 Hz, 1H, Halkene), 5.10 (app d, Jcis = 10.4 Hz, 1H,
alkene ). UV-Vis (toluene): lmax/nm (10ꢀ3e) 430 (216), 556
0
H
(18), 554 (5). FAB-HRMS for MH+ (C48H33N4Zn): 729.1990,
calc.: 729.1997.
1-Trans,trans-(20-(50,100,150,200-tetraphenylporphyrin)yl)bu-
tadiene 3. Free-base porphyrin 3 (8.7 mg, 95%) was obtained
by demetallation of 4 (10 mg, 13.7 mmol) according to the
general procedure described above. 1H NMR (500 MHz,
(20-(50,100,150,200-Tetraphenylporphyrin)yl)ethene 1. 2-For-
myl-TPP 10 (43.3 mg, 67 mmol) and Me-ps (70 mg, 194 mmol)
were dissolved/suspended in dry toluene (25 mL) under argon.
The solution was heated to reflux and excess of DBU (50 mg,
329 mmol) was added. After 20 h, TLC (DCM–hexane = 1 : 1)
showed the reagent as the main component with a less polar
product. More DBU was added (40 mg, 263 mmol) and the
reflux continued for further 24 h. The solution was then
evaporated to dryness and separated through flash chromato-
graphy (silica gel, DCM–hexane = 1 : 1). The first band was
collected and recrystallized from DCM–acetonitrile to yield 11
mg (26%) of pure 1 as purple crystals. 1H NMR (assignments
aided by COSY spectra) (500 MHz, CDCl3): d 8.88 (s, 1H,
Hb-pyrrolic), 8.85–8.74 (m, 5H, Hb-pyrrolic), 8.69 (d, J = 4.7 Hz,
1H, Hb-pyrrolic), 8.25–8.17 (m, 6H, Ho-Ph), 8.07 (app d, J = 7.5
CDCl3):
Hb-pyrrolic), 8.24–8.18 (m, 6H, Ho-Ph), 8.10 (app d, J = 7.4
Hz, 2H, Ho-Ph), 7.83–7.67 (m, 12H, Hm,p-Ph), 6.96 (dd, Jtrans
d 8.88 (s, 1H, Hb-pyrrolic), 8.84–8.72 (m, 6H,
=
0
15.2 Hz, Jcis = 10.6 Hz, 1H, Halkene ), 6.23–6.13 (m, 2H,
0
1Halkene + 1Halkene ), 5.35 (app d, Jtrans = 17 Hz, 1H, Halkene),
0
5.14 (app d, Jcis = 10.6 Hz, 1H, Halkene ), ꢀ2.64 (s, 2H,
H
N-pyrrolic). UV-Vis (toluene): lmax/nm (10ꢀ3e) 426 (257), 522
(21), 560 (9), 600 (7), 659 (2). FAB-HRMS for M+
(C48H34N4): 666.2807, calc.: 666.2783.
4-(Trans-20-(200-(500,1000,1500,2000-tetraphenylporphyrinato
zinc(II))yl)ethen-10-yl)styrene 6. Zn porphyrin aldehyde 14
(45 mg, 56 mmol) and Me-ps (60 mg, 166 mmol) were dis-
solved/suspended in 30 mL of dry toluene, under argon. The
solution was heated to reflux and excess of DBU (60 mg, 0.4
mmol) was added. After 24 h, the solution was evaporated to
dryness and the product mixture separated through flash
chromatography (silica gel, DCM–hexane = 1 : 1). The first
band was collected, evaporated to dryness and the product
recrystallized from DCM/acetonitrile yielding 29 mg (64%) of
pure 6 as purple crystals. 1H NMR (500 MHz, CDCl3): d 9.11
(s, 1H, Hb-pyrrolic), 8.97–8.88 (m, 5H, Hb-pyrrolic), 8.82 (d, J =
Hz, 2H, Ho-Ph), 7.81–7.67 (m, 12H, Hm,p-Ph), 6.43 (dd, Jtrans
=
17 Hz, Jcis = 10.7 Hz , 1H, Halkene), 5.89 (dd, Jtrans = 17 Hz,
Jgeminal = 1.4 Hz, 1H, Halkene), 5.15 (dd, Jcis = 10.7 Hz,
J
geminal = 1.4 Hz, 1H, Halkene), ꢀ2.68 (s, 2H, HN-pyrrolic). UV-
Vis (toluene): lmax/nm (10ꢀ3e) 424 (369), 519 (22), 554 (8), 596
(6), 653 (3). FAB-HRMS for M+ (C46H32N4): 640.2647, calc.:
640.2627.
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yield was 113 mg (68%). 1H NMR (assignments aided by
4.7 Hz, 1H, Hb-pyrrolic), 8.32–8.20 (m, 8H, Ho-Ph), 7.92–7.70
0
(m, 12H, Hm,p-Ph), 7.38 (d, J = 8 Hz, 2H, HPh ), 7.27–7.19 (m,
COSY spectra) (500 MHz, CDCl3): d 9.96 (s, 1H, Haldehyde),
9.11 (s, 1H, Hb-pyrrolic), 8.96–8.88 (m, 5H, Hb-pyrrolic), 8.82 (d, J
= 4.7 Hz, 1H, Hb-pyrrolic), 8.28–8.18 (m, 8H, Ho-Ph), 7.87–7.72
0
3H, 2HPh + 1Halkene), 7.03 (d, J = 16 Hz, 1H, Halkene), 6.75
0
(dd, Jtrans = 17.6 Hz, Jcis = 10.8 Hz, 1H, Halkene ), 5.80 (app d,
0
trans = 17.6 Hz, 1H, Halkene ), 5.26 (app d, Jcis = 10 Hz, 1H,
0
0
J
(m, 14H, 12Hm,p-Ph + 2HPh )0, 7.64 (d, J = 8 Hz, 2H, HPh ),
H
alkene ). UV-Vis (toluene): lmax/nm (10ꢀ3e) 433 (218), 559
7.51 (d, J = 8 Hz, 2H, HPh ), 7.28–7.22 (m, 4H, 2HPh
+
00
0
(23), 594 (11). FAB-HRMS for MH+ (C54H37N4Zn):
1Halkene + 1Halkene0), 7.17 (d, J = 16.3 Hz, 1H, Halkene), 7.03
+
0
(d, J = 16.1 Hz, 1H, Halkene ). FAB-HRMS for M
805.2270, calc.: 805.2310.
(C61H40N4OZn): 908.2479, calc.: 908.2494.
4-(Trans-20-(200-(500,1000,1500,2000-tetraphenylporphyrin)yl)-
ethen-10-yl)styrene 5. Free-base porphyrin 5(12.5 mg, 90%)
was obtained by demetallation of 6 (15 mg, 18.6 mmol)
according to the general procedure described above. 1H
4-(Trans-20-(400-(Trans-200 0-(20000-(50000,100000,150000,200000-tetra-
phenylporphyrinato zinc(II))yl)ethen-1000-yl)phenyl)ethen-10-yl)-
styrene 8. Porphyrin aldehyde 15 (35 mg, 38.5 mmol) and
Me-ps (40 mg, 111 mmol) were dissolved/suspended in dry
toluene (25 mL) under argon. The solution was heated to
reflux and an excess of DBU (70 mg, 0.46 mmol) was added.
After 24 h, the solution was evaporated to dryness and the
product mixture separated by flash chromatography (silica gel,
DCM–hexane = 1 : 1). The first band was collected, evapo-
rated to dryness and recrystallized from DCM–methanol,
yielding 27.6 mg (79%) of pure 8 as purple crystals. 1H
NMR (assignments aided by COSY spectra) (500 MHz,
NMR (500 MHz, CDCl3): d 8.99 (s, 1H, Hb-pyrrolic),
8.84–8.74 (m, 5H, Hb-pyrrolic), 8.71 (d, J = 4.7 Hz, 1H,
Hb-pyrrolic), 8.26–8.18 (m, 8H, Ho-Ph), 7.86–7.72 (m, 12H,
H
m,p-Ph), 7.39 (d, J = 8 Hz, 2H, HPh), 7.28 (d, J = 16 Hz,
H, Halkene), 7.22 (d, J = 8 Hz, 2H, HPh), 6.99 (d, J = 16 Hz,
H, Halkene), 6.75 (dd, Jtrans = 17.6 Hz, Jcis = 10.9 Hz , 1H,
0
alkene ), 5.81 (app d, Jtrans = 17.6 Hz, 1H, Halkene ), 5.28
0
H
0
(app d, Jcis = 10 Hz, 1H, Halkene ) ꢀ2.59 (s, 2H, HN-pyrrolic).
UV-Vis (toluene): lmax/nm (10ꢀ3e) 427 (167), 524 (17), 564
(10), 602 (6), 659 (1). FAB-HRMS for M+ (C54H38N4):
742.3113, calc.: 742.3097.
CDCl3): d 9.11 (s, 1H, Hb-pyrrolic), 8.96–8.87 (m, 5H,
Hb-pyrrolic), 8.83 (d, J = 4.7 Hz, 1H, Hb-pyrrolic), 8.27–8.18
(m, 8H, Ho-Ph), 7.88–7.71 (m, 12H, Hm,p-Ph), 7.45 (m, 4H,
Diethyl 4-(trans-20-(200-(500,1000,1500,2000-tetraphenylporphyri-
nato zinc(II))yl)ethen-10-yl)benzylphosphonate 17. 2-Formyl-
ZnTPP 11 (1 g, 1.43 mmol) and 1,4-xylenebis(diethylpho-
sphonate) (715 mg, 1.89 mmol) were dissolved in 100 mL of
dry THF, under argon atmosphere. To the stirred solution,
solid tBuOK (225 mg, 2 mmol) was added at three intervals in
30 min during which the product began precipitating. After a
further 10 min, the solution was evaporated to dryness and the
crude product was purified using flash chromatography on
silica gel: side-products were eluted with DCM after which the
desired product was eluted with DCM–MeOH = 100 : 1. This
solution was then evaporated to dryness, redissolved in DCM/
THF and reprecipitated by adding MeOH. Purple crystals of
pure 17 (620 mg, 47%) were obtained by recrystallization from
0
2HPh + 2HPh ), 7.40 (d, J = 8 Hz, 2H, HPh ), 7.28–7.22
00
0
00
0
(m, 3H, 2HPh + 1Halkene), 7.12 (br s, 2H, Halkene ), 7.03 (d, J
= 15.9 Hz, 1H, Halkene), 6.72 (dd, Jtrans = 17.5 Hz, Jcis = 10
00
Hz, 1H, Halkene ), 5.78 (app d, Jtrans = 17.5 Hz, 1H, Halkene ),
00
00
5.25 (app d, Jcis = 10 Hz, 1H, Halkene ). UV-Vis (toluene):
l
max/nm (10ꢀ3e) 433 (196), 560 (24), 597 (14). FAB-HRMS for
M+ (C62H42N4Zn): 906.2694, calc.: 906.2700.
4-(Trans-20-(400-(Trans-200’-(20000 0-(50000,1000 0’,150000,2000’0-tetra-
phenylporphyrin)yl)ethen-100 0-yl)phenyl)ethen-10-yl)styrene 7.
Free-base porphyrin 7 (7.8 mg, 98%) was obtained by deme-
tallation of 8 (8.5 mg, 9.4 mmol) according to the general
1
procedure described above. H NMR (500 MHz, CDCl3): d
9.13–8.70 (m, 7H, Hb-pyrrolic), 8.28–8.17 (m, 8H, Ho-Ph),
1
0
DCM/MeOH. H NMR (assignments aided by COSY spec-
7.88–7.72 (m, 12H, Hm,p-Ph), 7.55–7.48 (m, 4H, 2HPh
+
00 0
2HPh ), 7.44 (d, J = 8 Hz, 2H, HPh ), 7.28–7.22 (m, 3H, 2HPh
00
tra) (500 MHz, THF-d8,): d 9.03 (s, 1H, Hb-pyrrolic), 8.84–8.73
(m, 5H, Hb-pyrrolic), 8.70 (d, J = 4.5 Hz , 1H, Hb-pyrrolic),
8.28–8.14 (m, 8H, Ho-Ph), 7.93–7.69 (m, 12H, Hm,p-Ph),
7.30–7.18 (m, 5H, 4HPh + 1Halkene), 7.03 (d, J = 16.1 Hz,
0
+ Halkene), 7.16–7.14 (m, 2H, 2Halkene ), 7.04 (d, J = 17 Hz,
1H, Halkene), 6.72 (dd, Jtrans = 17.5 Hz, Jcis = 10.8 Hz, 1H,
H
alkene), 5.78 (app d, Jtrans = 17.5 Hz, 1H, Halkene), 5.25 (app
1H, Halkene), 4.03–3.95 (m, 4H, HCH -ethyl), 3.13 (d, JP = 21.8
d, Jcis = 10.8 Hz, 1H, Halkene), ꢀ2.58 (s, 2H, HN-pyrrolic). FAB-
2
Hz, 2H, HCH P), 1.23 (t, J = 7 Hz, 6H, HCH -ethyl). FAB-
HRMS for MH+ (C62H45N4): 845.3660, calc.: 845.3644.
2
3
HRMS for M+ (C57H45N4PO3Zn): 928.2486, calc.: 928.2521.
Trans-4-stilbenecarbaldehyde 16. Toluene(diethylphospho-
nate) (138 mg, 539 mmol) and terephthaldicarbaldehyde
(Merck), 283 mg, 2 mmol) were dissolved in dry THF (15
mL) under argon. To the stirred solution, solid tBuOK (70 mg,
624 mmol) was added in two portions over 1 h. After a further
10 min, the solution was evaporated to dryness and the
product mixture separated by flash chromatography (silica
gel, DCM–hexane = 1 : 1). The first band afforded 84 mg
(75%) of pure trans product 16 as a colourless solid. 1H NMR
(assignments aided by COSY spectra) (400 MHz, CDCl3): d
10.0 (s, 1H, Haldehyde), 7.88 (d, J = 8.2 Hz, 2H, HPhꢀCHO),
7.66 (d, J = 8.2 Hz, 2H, HPhꢀCHO), 7.55 (d, J = 7.5 Hz, 2H,
4-(Trans-20-(400-(Trans-200 0-(20000-(50000,100000,150000,200000-tetra-
phenylporphyrinato zinc(II))yl)ethen-1000-yl)phenyl)ethen-10-yl)-
benzaldehyde 15. Zn porphyrin phosphonate 17 (170 mg, 183
mmol) and terephthaldicarbaldehyde (Merck), 35 mg, 261
mmol) were dissolved in dry THF (30 mL) under argon. To
the stirred solution, solid tBuOK (30 mg, 260 mmol) was added
in three portions over 1 h. After a further 10 min, the solution
was evaporated to dryness and the product mixture separated
through flash chromatography (silica gel, DCM–hexane =
3 : 1). The first band contained the disubstituted by-product,
followed by the desired product 15, which was collected and
precipitated by hexane addition as purple-red crystals. The
H
Ph), 7.39 (app t, J = 7.4 Hz, 2H, HPh), 7.33 (d, J = 7.2 Hz,
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176 | New J. Chem., 2008, 32, 166–178

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1H, HPh), 7.27 (d, J = 16.2 Hz, 1H, Halkene), 7.15 (d, J = 16.2
Estimated uncertainties are 10% for transient absorbance
Hz, 1H, Halkene).
lifetimes, 8% for luminescence lifetimes, 10% for molar ab-
sorption coefficients and 20% for quantum yields, and the
working temperature, if not otherwise specified, was 295 ꢄ 2 K.
The excited state properties of the molecules were calculated
using the ZINDO/S (singly excited configurations in a space of
14 ꢂ 14 promotions), having optimized the ground state
geometries with the Molecular Mechanics (MM + force field)
program included in the Hyperchem package.40
Trans-4-vinylstilbene 9. Trans-4-stilbenecarbaldehyde 16 (20
mg, 96 mmol) and Me-ps (100 mg, 279 mmol) were dissolved/
suspended in dry toluene (25 mL) under argon. The solution
was heated to reflux and an excess of DBU (80 mg, 0.5 mmol)
was added. After 18 h, the solution was evaporated to dryness
and the product mixture separated using flash chromatogra-
phy (silica gel, DCM–hexane = 1 : 3). The first band was
collected, evaporated to dryness and recrystallized from
DCM–methanol, yielding 14.5 mg (73%) of pure 9 as pale
pink crystals. 1H NMR (assignments aided by COSY spectra)
(400 MHz, CDCl3): d 7.52 (d, J = 7.6 Hz, 2H, HPh), 7.48 (d, J
= 8 Hz, 2H, Hstylbene), 7.41 (d, J = 8 Hz, 2H, Hstylbene), 7.36
(app t, J = 7.6 Hz, 2H, HPh), 7.26 (m, 1H, HPh), 7.11 (s, 2H,
Acknowledgements
We would like to acknowledge financial support from Italian
CNR, (PM.P04.010 MACOL) and Ministero dell’Istruzione,
dell’Universita e della Ricerca, the New Zealand New Econ-
omy Research Fund (MAUX0202) and the New Zealand
Marsden Fund (MAUX0407).
H
H
alkene), 6.72 (dd, Jtrans = 17.6 Hz, Jcis = 10.8 Hz , 1H,
0
alkene ), 5.77 (app d, Jtrans = 17.6 Hz, 1H, Halkene ), 5.25 (app
0
+
0
d, Jcis = 10.8 Hz, 1H, Halkene ). FAB-HRMS for M
(C16H14): 206.1103, calc.: 206.1096.
References
1 H. L. Anderson, Chem. Commun., 1999, 2323–2330.
2 M. J. Frampton, H. Akdas, A. R. Cowley, J. E. Rogers, J. E.
Slagle, P. A. Fleitz, M. Drobizhev, A. Rebane and H. L. Anderson,
Org. Lett., 2005, 7, 5365–5368.
3 K. Susumu, T. V. Duncan and M. J. Therien, J. Am. Chem. Soc.,
2005, 127, 5186–5195.
4 J. J. Piet, P. N. Taylor, B. R. Wegewijs, H. L. Anderson, A. Osuka
and J. M. Warman, J. Phys. Chem. B, 2001, 105, 97–104.
5 A. Osuka, N. Tanabe, S. Kawabata, I. Yamazaki and Y. Nishi-
mura, J. Org. Chem., 1995, 60, 7177–7185.
6 K. Pettersson, A. Kyrychenko, E. Ronnow, T. Ljungdahl, J.
Martensson and B. Albinsson, J. Phys. Chem. A, 2006, 110,
310–318.
Spectroscopy and photophysics
Spectroscopic grade toluene (C. Erba) was used without
further purification. Absorption spectra were recorded with
a Perkin-Elmer Lambda 9 spectrophotometer and emission
spectra were detected by a Spex Fluorolog II spectrofluori-
meter equipped with a Hamamatsu R928 photomultiplier.
Standard 10 mm fluorescence cells were used at 295 K whereas
experiments at 77 K made use of capillary tubes in a home-
made quartz dewar filled with liquid nitrogen. Emission
quantum yields were determined after correction for the
photomultiplier response, with reference to TPP in aerated
toluene (Ffl = 0.1112). An FLS920 spectrofluorimeter (Edin-
burgh) equipped with an Hamamatsu R5509-72 supercooled
photomultiplier tube at 193 K and a TM300 emission mono-
chromator with a NIR grating blazed at 1000 nm was used to
measure the singlet oxygen luminescence quantum yield FD
using that for TPP in toluene as reference (FD = 0.7058). The
excitation was at 520 nm for the free-base porphyrins and at
560 nm for the Zn-porphyrins. Luminescence lifetimes in the
nanosecond range were obtained with an IBH single photon
counting equipment with excitation at 560 nm and 331 nm
from a pulsed diode source (resolution 0.5 ns).
7 D. P. Arnold and D. A. James, J. Org. Chem., 1997, 62, 3460–3469.
8 F. Odobel, S. Suresh, E. Blart, Y. Nicolas, J. P. Quintard, P.
Janvier, J. Y. Le Questel, B. Illien, D. Rondeau, P. Richomme, T.
Haupl, S. Wallin and L. Hammarstrom, Chem.–Eur. J., 2002, 8,
3027–3046.
¨
9 T. H. Huang, Y. J. Chen, S. S. Lo, W. N. Yen, C. L. Mai, M. C.
Kuo and C. Y. Yeh, Dalton Trans., 2006, 2207–2213.
10 A. Tsuda and A. Osuka, Science, 2001, 293, 79–82.
11 D. F. Perepichka and M. R. Bryce, Angew. Chem., Int. Ed., 2005,
44, 5370–5373.
12 P. G. Seybold and M. Gouterman, J. Mol. Spectrosc., 1969, 31,
1–13.
13 R. A. Binstead, M. J. Crossley and N. S. Hush, Inorg. Chem., 1991,
30, 1259–1264.
14 J. A. Shelnutt and V. Ortiz, J. Phys. Chem., 1985, 89, 4733–4739.
15 D. P. Arnold, Synlett, 2000, 12, 296–305.
16 S. M. LeCours, S. G. DiMagno and M. J. Therien, J. Am. Chem.
Soc., 1996, 118, 11854–11864.
17 T. E. O. Screen, I. M. Blake, L. H. Rees, W. Clegg, S. J. Borwick
and H. L. Anderson, J. Chem. Soc., Perkin Trans. 1, 2002,
320–329.
18 M. Drobizhev, F. Q. Meng, A. Rebane, Y. Stepanenko, E. Nickel
and C. W. Spangler, J. Phys. Chem. B, 2006, 110, 9802–9814.
19 W. N. Yen, S. S. Lo, M. C. Kuo, C. L. Mai, G. H. Lee, S. M. Peng
and C. Y. Yeh, Org. Lett., 2006, 8, 4239–4242.
20 C.-T. Chen, H.-C. Yeh, X. Zhang and J. Yu, Org. Lett., 1999, 1,
1767–1770.
21 Q. Wang, W. M. Campbell, E. E. Bonfantani, K. W. Jolley, D. L.
Officer, P. J. Walsh, K. Gordon, R. Humphry-Baker, M. K.
Nazeeruddin and M. Gratzel, J. Phys. Chem. B, 2005, 109,
15397–15409.
Transient absorbance in the nanosecond range made use of
a laser flash photolysis apparatus based on a Nd:YAG laser
(JK Lasers) delivering pulses of 18 ns.59 The second harmonic
(532 nm) was used for excitation. The absorbance of the
solutions at the exciting wavelength was about 0.5 and the
pulse energy used was 0.6 mJ pulseꢀ1 for the determination of
the spectra and of the order of 0.1 mJ pulseꢀ1 for the triplet
lifetime determination. Experiments were conducted in home-
made, 10 mm optical path cuvettes, the solution bubbled with
argon for 10 minutes to remove dissolved oxygen and sealed.
The oxygen quenching reaction rate constant, kq, was deter-
mined from the equation: k = k0 + kq[O2], by measuring the
rate constant, k, for the air-free and the air-saturated cases.
The oxygen solubility in air-equilibrated toluene was taken to
be 1.8 ꢂ 10ꢀ3 M.60
22 E. Annoni, M. Pizzotti, R. Ugo, S. Quici, T. Morotti, M. Bruschi
and P. Mussini, Eur. J. Inorg. Chem., 2005, 3857–3874.
23 A. Lembo, P. Tagliatesta and D. M. Guldi, J. Phys. Chem. A, 2006,
110, 11424–11434.
ꢁc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
New J. Chem., 2008, 32, 166–178 | 177

View Online
24 M. Morone, L. Beverina, A. Abbotto, F. Silvestri, E. Collini, C.
Ferrante, R. Bozio and G. A. Pagani, Org. Lett., 2006, 8,
2719–2722.
25 P. J. Walsh, K. C. Gordon, D. L. Officer and W. M. Campbell, J.
Mol. Struct. (THEOCHEM), 2006, 759, 17–24.
26 P. J. Walsh, K. C. Gordon, P. Wagner and D. L. Officer,
ChemPhysChem, 2006, 7, 2358–2365.
27 L. Flamigni, A. M. Talarico, J. C. Chambron, V. Heitz, M. Linke,
N. Fujita and J. P. Sauvage, Chem.–Eur. J., 2004, 10,
2689–2699.
28 I. M. Dixon, J. P. Collin, J. P. Sauvage and L. Flamigni, Inorg.
Chem., 2001, 40, 5507–5517.
44 H. C. Lin, C. M. Tsai, G. H. Huang and J. M. Lin, J. Polym. Sci.
Polym. Chem., 2006, 44, 783–800.
45 A. K. Burrell, D. L. Officer and D. C. W. Reid, Angew. Chem., Int.
Ed. Engl., 1995, 34, 900–902.
46 M. Wolffs, F. J. M. Hoeben, E. H. A. Beckers, A. Schenning and
E. W. Meijer, J. Am. Chem. Soc., 2005, 127, 13484–13485.
47 T. G. Jiu, Y. J. Li, H. Y. Gan, Y. L. Li, H. B. Liu, S. Wang, W. D.
Zhou, C. R. Wang, X. F. Li, X. F. Liu and D. B. Zhu, Tetrahedron,
2007, 63, 232–240.
48 L. Flamigni, N. Armaroli, F. Barigelletti, V. Balzani, J. P. Collin,
J. O. Dalbavie, V. Heitz and J. P. Sauvage, J. Phys. Chem. B, 1997,
101, 5936–5943.
29 L. Flamigni, M. R. Johnston and L. Giribabu, Chem.–Eur. J.,
2002, 8, 3938–3947.
49 L. Flamigni, A. M. Talarico, B. Ventura, C. Sooambar and N.
Solladie, Eur. J. Inorg. Chem., 2006, 11, 2155–2165.
´
30 L. Flamigni, A. M. Talarico, S. Serroni, F. Puntoriero, M. J.
Gunter, M. R. Johnston and T. P. Jeynes, Chem.–Eur. J., 2003, 9,
2649–2659.
31 L. Flamigni, A. M. Talarico, B. Ventura, G. Marconi, C. Sooam-
bar and N. Solladie, Eur. J. Inorg. Chem., 2004, 2557–2569.
32 L. Flamigni, A. M. Talarico, B. Ventura, R. Rein and N. Solladie,
Chem.–Eur. J., 2006, 12, 701–712.
33 A. K. Burrell, D. L. Officer, P. G. Plieger and D. C. W. Reid,
Chem. Rev., 2001, 101, 2751–2796.
34 K. Okuma, O. Sakai and K. Shioji, Bull. Chem. Soc. Jpn., 2003, 76,
1675–1676.
35 A. K. Burrell and D. L. Officer, Synlett, 1998, 1297–1307.
36 H. J. Callot, Tetrahedron, 1973, 29, 899–901.
37 J. W. Buchler, in Porphyrins and Metalloporphyrins, ed. K. M.
Smith, Elsevier, Amsterdam, 1975, ch. 5.
38 M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138–163.
39 C. Djerassi, Y. Lu, A. Waleh, A. Y. L. Shu, R. A. Goldbeck, L. A.
Kehres, C. W. Crandell, A. G. H. Wee, A. Knierzinger, R.
Gaeteholmes, G. H. Loew, P. S. Clezy and E. Bunnenberg, J.
Am. Chem. Soc., 1984, 106, 4241–4258.
50 D. Beljonne, G. E. Okeefe, P. J. Hamer, R. H. Friend, H. L.
Anderson and J. L. Bredas, J. Chem. Phys., 1997, 106, 9439–9460.
51 L. Flamigni, A. M. Talarico, M. J. Gunter, M. R. Johnston and T.
P. Jeynes, New J. Chem., 2003, 27, 551–559.
52 J. E. Rogers, K. A. Nguyen, D. C. Hufnagle, D. G. McLean, W. J.
Su, K. M. Gossett, A. R. Burke, S. A. Vinogradov, R. Pachter and
P. A. Fleitz, J. Phys. Chem. A, 2003, 107, 11331–11339.
53 S. L. Murov, I. Carmichael and G. L. Hug, in Handbook of
Photochemistry, ed. Marcel Dekker Inc., New York, 2nd edn,
1993, p. 250.
54 B. Aveline, O. Delgado and D. Brault, J. Chem. Soc., Faraday
Trans., 1992, 88, 1971–1976.
55 E. M. P. Silva, F. Giuntini, M. A. F. Faustino, J. P. C. Tome, M.
Neves, A. C. Tome, A. M. S. Silva, M. G. Santana-Marques, A. J.
Ferrer-Correia, J. A. S. Cavaleiro, M. F. Caeiro, R. R. Duarte, S.
A. P. Tavares, I. N. Pegado, B. d’Almeida, A. P. A. De Matos and
M. L. Valdeira, Bioorg. Med. Chem. Lett., 2005, 15, 3333–3337.
56 A. Michaelis and R. Haehne, Chem. Dtsch. Ber. Ges., 1898, 31,
1048.
57 J. K. Lee, R. R. Schrock, D. R. Baigent and R. H. Friend,
Macromolecules, 1995, 28, 1966–1971.
58 F. Wilkinson, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref.
Data, 1993, 22, 113–262.
40 Hyperchem 6.02, Hypercube Inc., Gainesville, FL, 2000.
41 P. J. Spellane, M. Gouterman, A. Antipas, S. Kim and Y. C. Liu,
Inorg. Chem., 1980, 19, 386–391.
42 O. Narwark, A. Gerhard, S. C. J. Meskers, S. Brocke, E. Thorn-
Csanyi and H. Bassler, Chem. Phys., 2003, 294, 17–30.
43 W. B. Davis, W. A. Svec, M. A. Ratner and M. R. Wasielewski,
Nature, 1998, 396, 60–63.
59 L. Flamigni, J. Phys. Chem., 1992, 96, 3331–3337.
60 S. L. Murov, I. Carmichael and G. L. Hug, in Handbook of
Photochemistry, ed. Marcel Dekker Inc., New York, 2nd edn,
1993, p. 293.
ꢁc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
178 | New J. Chem., 2008, 32, 166–178