Porphyrin dimers bridged by a platinum-diacetylide unit

Article abstract of DOI:10.1039/b513921j

A series of butadiyne- and platinum diacetylide-bridged conjugated porphyrin dimers have been prepared; electrochemical and UV-vis absorption measurements on these compounds show that the butadiyne-bridge confers stronger porphyrin-porphyrin conjugation t

Full text of DOI:10.1039/b513921j

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Porphyrin dimers bridged by a platinum–diacetylide unit{
Yi-Jen Chen,a Szu-Shuo Chen,a Shang-Shih Lo,a Teng-Hui Huang,a Chen-Chang Wu,a Gene-Hsiang Lee,b
Shie-Ming Pengb and Chen-Yu Yeh*a
Received (in Cambridge, UK) 20th September 2005, Accepted 9th January 2006
First published as an Advance Article on the web 23rd January 2006
DOI: 10.1039/b513921j
A series of butadiyne- and platinum diacetylide-bridged
conjugated porphyrin dimers have been prepared; electroche-
mical and UV-vis absorption measurements on these com-
pounds show that the butadiyne-bridge confers stronger
porphyrin–porphyrin conjugation than the platinum diacety-
lide-bridge.
The synthesis of porphyrin arrays continues to attract attention due
to their peculiar properties. One of the most active subjects in
porphyrin arrays over the last decade focuses on the design and
construction of molecular-scale electronic devices.1 To control the
individual porphyrin units in well-defined shape and dimensions, a
variety of functional groups have been used as the bridges, including
saturated and p-conjugated units.2,3 These multiporphyrin systems
continue to provide valuable information regarding the mechanisms
of electron and energy transfer processes. Numerous metal
bis(acetylide) complexes have been synthesized via s coordination
of two acetylide ligands to a metal center.4 Platinum–acetylide
p-conjugated oligomers and polymers have received much attention
because of their potential applications in electronic and optoelec-
tronic devices.5 Examples of platinum complexes with s-bonded
porphyrinylethynyl are rare.6 Russo et al. reported the synthesis of
this type of porphyrin oligomers in 1998.6 However, some of these
complexes were not well characterized.
It has been shown that the nature of the bridges significantly
influences the extent of the interchromophore electronic and
exciton coupling in multiporphyrins.2,3 However, platinum
diacetylide-bridged bisporphyrins have not been synthesized and
the issue concerning the effects of the platinum bridge that enhance
Scheme 1 Structures of porphyrins studied in this work.
or disrupt the p-conjugation between the porphyrin units has not
been addressed. Here we report the construction of porphyrin
diethylamine. It should be noted that the use of CuI catalyst
dimers in which the porphyrin subunits are connected through a
leads to the formation of butadiyne-bridged porphyrins, thus
platinum bis(s-acetylide) moiety and their electrochemical proper-
ties are described. For comparison, the bisporphyrins linked by a
butadiyne group, which has been shown to promote electronic
coupling between the porphyrin rings, were also synthesized.
As shown in Scheme 1, platinum-bridged bisporphyrins
Ni21 and Zn21 were prepared in good yields by the reaction of
Ni4 and Zn4, respectively, with trans-[Pt(PEt3)2Cl2] in
resulting in a decrease in the yields of the desired platinum-
bridged products. The reference bisporphyrin Zn227 were
synthesized by the coupling reaction of Zn4 catalyzed by CuI
in CH2Cl2 under air.8 Demetallation of Zn22 with aqueous
HCl solution gave the corresponding free base, which was
then treated with Ni(OAc)2 in refluxing DMF to give the
dinickel porphyrin Ni22. Reference compounds Ni5 and Zn5
were synthesized by the reaction of Ni4 and Zn4, respectively,
with trans-[Pt(PEt3)2Cl2], followed by treatment with
trimethylsilylacetylene.
aDepartment of Chemistry, National Chung Hsing University, Taichung
402, Taiwan. E-mail: cyyeh@dragon.nchu.edu.tw; Fax: +886 4 22862547;
Tel: +886 4 22852264
bDepartment of Chemistry, National Taiwan University, Taipei 106,
Taiwan
The absorption spectrum of Ni21 shows peak-broadening and
red shift of the Soret band and slight red shift of Q(0,0) band
with respect to those of the reference compound Ni5 (Table 1).
The full width at half maximum (FWHM) is 1421 cm21 for Ni21,
which is about 30% larger than that for Ni5 (FWHM =
1026 cm21). These spectral features suggest electronic
{ Electronic supplementary information (ESI) available: Synthesis of
porphyrins 1, 2 and 5; absorption spectra of 1, 3 and 5; cyclic
voltammograms and differential pulse voltammograms of 1 and 2;
absorption spectra of the neutral forms, monocations and dications of
Zn21 and Zn22. See DOI: 10.1039/b513921j
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Chem. Commun., 2006, 1015–1017 | 1015

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Table 1 Redox and absorption data in CH2Cl2
Redox dataa
Absorption (cm21
E1/2(red)Neutral form
)
E1/2(ox)
Monocationc
Dicationc
Ni21 +0.70, +0.83, +1.1921.37b 22272, 18116, 16807
Zn21+0.50, +0.56, +1.0121.47 22523, 17422, 16234
Ni22 +0.93, +1.10, +1.3021.14b 22321, 21186, 18215, 15773
22883, 18382, 17123, 15385, 11173, 3964 23148, 21645, 18904, 15385, 10471
22676, 18939, 17637, 16340, 15129, 1058223041, 21739, 18939, 15129, 10417
24096, 19608, 12034, 3957
24096, 13793, 12034
Zn22+0.69, +0.78, +1.2421.22 22222, 20790, 17699, 15873, 1474923640, 21551, 15873, 14749, 11123, 4026 23981, 21786, 10363
Ni3 +0.97, +1.21
Zn3 +0.78, +1.16
Ni5 +0.70, +1.19
Zn5 +0.48, +0.95
a
21.26 23419, 18484, 17422
21.31 23202, 17794, 16529
21.44b 22624, 18116, 17007
21.53b 22421, 17361, 16181
The electrochemical reactions were carried out in CH2Cl2 containing 0.1 M TBAPF6 at a scan rate of 100 mV s21. Potentials (V) are
reported vs. Ag/AgCl and referenced to the Fc/Fc+ couple which occurs at E1/2 = +0.43 V vs. Ag/AgCl. Irreversible reactions. The mono-
and dications were generated in situ by reacting the neutral molecules with 1 and 2 eq. [(p-BrC6H4)3N][SbCl6], respectively.
b
c
interactions between the porphyrin units via the bridge. It should
be pointed out that the red shift of the Q band on comparing
Ni21 with Ni5 of 200 cm21 is very much less than the red shift of
nonplanar conformation and the two individual porphyrin mean
planes are nearly coplanar. The nitrogen and nickel atoms only
slightly deviate from the porphyrin mean plane. The average
the Q band on comparing Ni22 with Ni3 of 1649 cm21
,
˚
deviations from the mean plane are 0.561, 0.322 and 0.218 A for
meso-, a- and b-carbon atoms, respectively. It should be noted that
both complexes have crystallographically imposed inversion
symmetry.
suggesting that the butadiyne-bridge confers stronger
porphyrin–porphyrin conjugation than the platinum diacety-
lide-bridge. In contrast, the absorption spectrum of Zn21 show
slight blue shift of the Soret and Q bands as compared to Zn5,
and the FWHM of the Soret band is 593 cm21 for both Zn21 and
Zn5. This indicates that the platinum bridge disrupts the
electronic interaction between the two porphyrin units in Zn21.
The identities and conformations of bisporphyrins Zn22 and
Ni21 were further confirmed by their crystal structures.{ As
expected for Zn22, the individual porphyrin rings are in a coplanar
arrangement (Fig. 1(a)). This is similar to the structure of a meso-
butadiyne-linked porphyrin dimer reported by Anderson et al.8
The ORTEP drawing of compound Ni21 is shown in Fig. 1(b). To
our knowledge, this is the first crystal structure of a platinum
diacetylide-bridged bisporphyrin. The geometry of the platinum
ion does not exhibit significant deviation from that expected for a
square-planar arrangement. Notably, the porphyrin rings adopt a
The electronic coupling of the porphyrin units in these
porphyrin dimers were investigated by electrochemical methods.
The electrochemical data are listed in Table 1. In the case of the
reference compound Zn22, the cyclic voltammogram shows two
overlapping 1-e2 couples at E1/2 = 0.69 and 0.78 V, which can be
resolved by differential pulse voltammetry, and one 2-e2 process at
E1/2 = 1.24 V. The first and second 1-e2 processes can be assigned
to the first electron abstraction from the two porphyrin units,
suggesting interporphyrin communication in the molecule.
Similarly, compound Zn21 undergoes two overlapping 1-e2
reversible oxidations at E1/2 = 0.50 and 0.56 V. Comparing with
Zn22, compound Zn21 exhibits smaller separation of the first and
second oxidation processes (DE1/2 = 60 mV). This may be ascribed
to the longer distance between porphyrin units or/and to inclusion
of a platinum ion. In the nickel porphyrin dimers, the electronic
coupling between the porphyrin units is more pronounced. Three
oxidation processes can be resolved for compound Ni21, with the
respective redox potentials being at E1/2 = 0.70 (1-e2), 0.83 (1-e2)
and 1.19 (2-e2) V. The stepwise oxidations demonstrate that the
porphyrin units are electrochemically coupled with each other via
the platinum–diacetylide bridge and that insertion of a platinum
atom into an alkynyl–alkynyl unit does not completely disrupt the
electronic delocalization of the p-conjugation. The separation of
170 mV for the first two successive oxidations in Ni22 is larger by
˚
40 mV than that in Ni21 because the bridge length of 9.253 A in
˚
Ni21 is much longer (by about 2.6 A) than that in Ni22. In a
conjugated dimer of nickel octaethylporphyrin linked by an
ethyne–phenylene–ethyne unit,9 with an estimated bridge length of
˚
about 10.9 A, the redox potentials for the first electron abstraction
from the porphyrin rings were observed at 0.91 and 0.96 V vs. Ag/
AgCl at 293 K. The separation of these two successive oxidations,
about 50 mV, is much smaller than that observed in Ni21. These
results suggest that the platinum–diacetylide can be an alternative
bridge, which provides a conductive pathway for charge-transport,
for the construction of conjugated multiporphyrins, although
platinum–diacetylide is less conjugated as compared with
butadiyne. Comparing with Zn21, compound Ni21 shows electro-
nic communication between the chromophores.
Fig. 1 Crystal structure of (a) Zn22 and (a) Ni21 drawn at 50%
probability ellipsoids. Additional ‘‘A’’ letters in the atom labels denote
atoms at equivalent position (2x, 2y, 2z). Aryl groups and H atoms were
˚
omitted for clarity. Selected distances (A) and angles (u): Zn22: C20–C21
1.427(3), C21–C22 1.202(4), C22–C22A 1.371(5), Zn–O1 2.158(2),
…
O1 O2 2.664; /C20–C21–C22 176.0(3), /C21–C22–C22A 179.2(4);
Ni21: C20–C21 1.432(7), C21–C22 1.204(7), Pt–C22 1.995(5), Pt–P1
2.286(1); /C22–Pt–P1 92.08(14).
1016 | Chem. Commun., 2006, 1015–1017
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Extensive studies of the strongly coupled butadiyne-bridged
bisporphyrin systems showed that the bisporphyrin monoanion
and monocation exhibit an intense intervalence charge transfer
(IVCT) band in the near-IR region.9 It has been pointed out that
the term ‘‘IVCT’’ is not suitable for a class III system (Robin and
Day’s classification).9,10 The absorption in the near-IR region
might be termed a ‘‘charge resonance band’’ for a class III
system.10 Since the term ‘‘charge resonance band’’ is not widely
used, in keeping with convention, we use the term IVCT to
describe this near-IR band. In all cases except dimer Zn21, a broad
band in the near-IR region for their corresponding monocations
was observed. Fig. 2 shows absorption spectra of the neutral forms
and mono- and dications of the dinickel porphyrins Ni21 and Ni22.
The monocations and dications, generated in situ by reacting the
In summary, we have successfully synthesized novel platinum
diacetylide-bridged porphyrin dimers. The two porphyrin units are
essentially independent in Zn21. In contrast, the chromophores
exhibit electronic coupling via the bridge in dimer Ni21. Theoretical
calculations would be helpful to provide some insights into the
dxz–p orbital interaction in this new type of porphyrin dimers.
Efforts along this line are in progress.
We thank the National Science Council of Taiwan for financial
support of this work. We are grateful to Professor Kuan-Jiuh Lin
for the access of UV-Vis-Near IR spectrometer. Special apprecia-
tion is acknowledged to Professors Fung-E Hong and Feng-Yin Li
for insightful discussions.
Notes and references
neutral
molecules
with
stoichiometric
amounts
of
[(p-BrC6H4)3N][SbCl6], are stable under the chemical oxidation
{ Crystal data: Ni21, crystals were grown from CHCl3–MeOH.
Ni21?6CHCl3, C146H178Cl18N8Ni2P2Pt, M = 3057.51, triclinic, space group
conditions. Upon one-electron oxidation, the broad IVCT bands
for Ni21+ and Ni22+ were observed at 3964 and 3957 cm21
¯
˚
P1, T = 150(1) K, a = 9.7645(1), b = 19.8365(2), c = 22.0587(2) A, a =
,
3
˚
68.4657(6), b = 80.3541(6), c = 89.0879(6)u, V = 3913.36(7) A , Z = 1, m =
1.505 mm21, 69469 reflections collected, 17970 independent, Rint = 0.0579,
final residuals R1 = 0.0671, wR2 = 0.1922 [I > 2s(I)]; R1 = 0.0807, wR2 =
0.2037 (all data). Zn22, crystals were grown from CH2Cl2–MeOH.
Zn22?4CH3OH?2CH2Cl2, C134H162Cl4N8O4Zn2, M = 2221.26, triclinic,
respectively. The IVCT band disappears upon further oxidation by
[(p-BrC6H4)3N][SbCl6] to generate the corresponding dication. In
monocations Ni21+ and Ni22+, the existence of the IVCT bands
confirms the interporphyrin electronic communication. Based on
the fact that the IVCT band is narrow and asymmetric, the radical
cation and anion of the butadiyne bisporphyrin has been described
by a fully delocalized class III system.9 For the platinum
bisacetylide-bridged porphyrin dimers, the shape of the IVCT
band for Ni21+ cannot be completely determined since the low-
energy side of this band is out of the limit of our instrument.
Analysis of this IVCT band shows that the band width is broad
and the intensity is relatively weak, suggesting that the monocation
Ni21+ can be described by an intermediate coupling system (class
II). The monocation Zn21+ can be classified by a localized system
(class I), since such a IVCT band in the near-IR region was not
observed for this monocation. Based on the electrochemical
measurements and analysis of absorption spectra of monocations
Ni21+ and Zn21+, we proposed that the HOMOs for Ni21 and
Zn21 are mainly contributed from the mixing of a2u orbitals, based
on Gouterman’s four-orbital model and the p orbitals of the
ethynyl groups. However, the dxz orbital of the platinum ion more
or less involves in the HOMO for Ni21, whereas the d orbitals of
the platinum bridge do not involve in the HOMO for Zn21 (z-axis
= P1A–Pt–P axis; x-axis = C22–Pt–C22A axis).
¯
space group P1, T = 150(1) K, a = 13.8674(2), b = 14.9588(2), c =
˚
16.0677(2) A, a = 85.2430(8), b = 87.9895(9), c = 68.9350(9)u, V =
3
3099.58(7) A , Z = 1, m = 0.528 mm21, 64819 reflections collected, 14190
˚
independent, Rint = 0.0539, final residuals R1 = 0.0574, wR2 = 0.1551 [I >
2s(I)]; R1 = 0.0885, wR2 = 0.1760 (all data). CCDC 285537 and 285538.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b513921j
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Fig. 2 Theabsorptionspectraof(a)Ni21(solid),Ni21+(dashed)andNi212+
(dotted) and (b) Ni22 (solid), Ni22+ (dashed) and Ni222+ (dotted) in CH2Cl2.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1015–1017 | 1017