Dendrimers made of porphyrin cores and carbazole chromophores as peripheral units. Absorption spectra, luminescence properties, and oxidation behavior

Article abstract of DOI:10.1021/ja0514444

Luminescent and redox-active porphyrin-based dendrimers of first and second generation have been synthesized, and their absorption spectra, photophysical properties, and oxidation behavior have been investigated, together with those of the corresponding a

Full text of DOI:10.1021/ja0514444

Published on Web 07/26/2005
Dendrimers Made of Porphyrin Cores and Carbazole
Chromophores as Peripheral Units. Absorption Spectra,
Luminescence Properties, and Oxidation Behavior
Fre´de´rique Loiseau,† Sebastiano Campagna,*,† Ahmed Hameurlaine,‡ and
Wim Dehaen*,‡
Contribution from the UniVersita` di Messina, Dipartimento di Chimica Inorganica, Chimica
Analitica e Chimica Fisica, Via Sperone 31, I-98166, Messina, Italy, and UniVersity of LeuVen,
Department of Chemistry, Celestijnenlaan 200F, B-3001 LeuVen, Belgium
Received March 7, 2005; E-mail: photochem@chem.unime.it; wim.dehaen@chem.kuleuven.ac.be
Abstract: Luminescent and redox-active porphyrin-based dendrimers of first and second generation have
been synthesized, and their absorption spectra, photophysical properties, and oxidation behavior have
been investigated, together with those of the corresponding aldehyde carbazole precursors. All the
dendrimers contain a porphyrin core and carbazole-based chromophores as branches. The structural
formulas of the new species are represented in Figures 1 and 2, with the corresponding schematizations.
The absorption spectra of the aldehyde carbazole precursors A1-A6 in dichloromethane exhibit intense
transitions in the UV region, centered on the carbazole and benzaldehyde subunits. The lowest-energy
absorption bands receive contribution from charge-transfer transitions. Compounds A1-A6 are luminescent
at room temperature in fluid solution; such a luminescence is attributed to twisted intramolecular charge-
transfer excited states. The luminescence at 77 K in a rigid matrix is blue-shifted with respect to room-
temperature emission and is assigned to locally excited states. Absorption spectra of the porphyrin-cored
dendrimers P1-P6 appear additive as they are constituted by visibile bands due to porphyrin absorption
and bands in the UV region due to transitions centered on the carbazole-based branches. Emission spectra
of P1-P6 both at 77 K and at room temperature are typical of porphyrin species and independent of
excitation wavelength, indicating that the light collected by the peripheral chromophores is quantitatively
transferred to the core. All the compounds exhibit a rich oxidation behavior in 1,2-dichloroethane solution,
with reversible processes centered on the different carbazole subunits. Interaction between the different
carbazole centers depends on the size of the spacer interposed.
level.4 Carbazole molecules are also quite interesting due to their
own photophysical and redox properties: they exhibit relatively
Introduction
Dendrimers built from porphyrin subunits are quite interesting
from many viewpoints: dendrimers containing porphyrins as
cores and different branching units have been studied as
synthetic analogous of biological systems, for example, to
explore and clarify the redox properties of heme species,1 as
well as to investigate long-distance electron transfer at elec-
trodes.2 Dendrimers constructed from multiporphyrin units have
been reported to play the role of light-harvesting antennae3 and,
where properly connected with electron donor and acceptor
groups, as charge-separation species for designing photochemi-
cal solar energy conversion devices operating at the molecular
intense luminescence5 and undergo reversible oxidation pro-
(3) The literature on this topic is too vast to be exhaustively quoted. For some
examples, see: (a) Collin, J.-P.; Harriman, A.; Heitz, V.; Odobel, F.;
Sauvage, J.-P. J. Am. Chem. Soc. 1994, 116, 5679-5690. (b) Burrell, A.
K.; Wasielewski, M. J. Porphyr. Phthalocya. 2000, 4, 401-406. (c) Gust,
D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40-48. (d)
Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem. ReV.
2001, 101, 2751-2796. (e) Campagna, S.; Serroni, S.; Puntoriero, F.; Di
Pietro, C.; Ricevuto, V. In Electron Transfer in Chemistry; Balzani, V.,
Ed.; Wiley-VCH: 2001; Vol. 5, pp 186-214. (f) Holten, D.; Bocian, D.
F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69. (g) Guldi, D. M. Chem.
Soc. ReV. 2002, 31, 22-37. (h) Seth, J.; Palaniappan, V.; Wagner, R. W.;
Johnson, T. E.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118,
11194-11207. (i) Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1998, 120,
10895-10901. (j) Nakano, A.; Yamazaki, T.; Nishimura, Y.; Yamazaki,
I.; Osuka, A. Chem.-Eur. J. 2000, 6, 3254-3271. (k) Prodi, A.; Indelli,
M. T.; Kleverlaan, C. J.; Alessio, E.; Scandola, F. Coord. Chem. ReV. 2002,
229, 48-73. (l) Balzani, V.; Credi, A.; Venturi, M. Molecular DeVices
and Machines. A Journey into the Nanoworld; Wiley- VCH: Weinheim,
2003; Chapter 5.
(4) (a) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn, S. J.;
Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc.
1999, 121, 8604-8614. (b) Nakano, A.; Osuka, A.; Yamazaki, T.;
Nishimura, I.; Akimoto, S.; Yamazaki, I.; Itaya, A.; Murakami, M.;
Miyasaka, H. Chem.-Eur. J. 2001, 7, 3134-3151. (c) Kodis, G.; Liddell,
P. A.; de la Garza, L.; Clausen, P. C.; Lindsey, J. S.; Moore, A. L.; Moore,
T. A.; Gust, D. J. Phys. Chem. A 2002, 106, 2036-2048. (d) Liddell, P.
A.; Kodis, G.; de la Garza, L.; Moore, A. L.; Moore, T. A.; Gust, D. J.
Phys. Chem. B 2004, 108, 10256-10265.
† Universita di Messina`.
‡ University of Leuven.
(1) (a) Dandliker, P. J.; Diederich, F.; Gross, M.; Knobler, C. B.; Louati, A.;
Sanford, E. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1739-1742. (b)
Dandliker, P. J.; Diederich, F.; Zingg, A.; Gisselbrecht, J.-P.; Gross, M.;
Louati, A. HelV. Chim. Acta 1997, 80, 1773-1801. (c) Weyermann, P.;
Gisselbrecht, J.-P.; Boudon, C.; Diederich, F.; Gross, M. Angew. Chem.,
Int. Ed. 1999, 38, 3215-3219.
(2) (a) Pollack, K. W.; Leon, J. W.; Fre´chet, J. M. J.; Maskus, M.; Abrun˜a, H.
D. Chem. Mater. 1998, 10, 30-38. (b) Kimura, M.; Sugihara, Y.; Muto,
T.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Chem.-Eur. J. 1999, 5, 3495-
3500. (c) Juris, A. In Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH: Weinheim, 2001; Vol. 3, pp 655-714.
9
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Porphyrin-carbazole Dendrimers
A R T I C L E S
Figure 1. Structures of the aldehyde carbazole precursors of the first (A1-A3) and second generation (A4-A6) together with their schematic representa-
tions.
cesses which make them suitable as hole carriers.6 The properties
of these carbazole systems prompted us to introduce first and
second generation carbazole branches in dendrimers based on
Ru(II) polypyridine complexes as cores.7 By taking advantage
of the photophysical and redox properties of porphyrin and
carbazole subunits, we have now synthesized new photo- and
redox-active dendrimers of first and second generation incor-
porating porphyrin moieties as cores and various carbazole or
multicarbazole groups as branches (or dendrons). Here we report
the synthesis of these new species together with their absorption
spectra, redox behavior, and luminescence properties. The same
properties of the carbazole precursors, in other words dendrons
containing an aldehyde group, are also reported. The new species
are shown in Figures 1 and 2, which also reports the abbrevia-
tions and symbols used to identify the compounds.
Results and Discussion
We recently described methods for the preparation of oligo-
carbazole materials.8 The synthesis of the porphyrin-(oligo)-
carbazole conjugates described here involves the preparation
of aldehyde precursors by a multistep procedure via copper-
catalyzed Ullmann coupling reactions or palladium-catalyzed
Suzuki coupling reactions. Successive condensation with pyrrole
is performed under Lindsey conditions:9 acid catalysis at high
dilution followed by addition of oxidant.
(5) (a) Howell, A. G.; Taylor, A. G.; Phillips, D. Chem. Phys. Lett. 1992, 188,
119-125. (b) Yu, H.; Mohd Zain, S.; Eigenbrot, I. V.; Phillips, D. Chem.
Phys. Lett. 1993, 202, 141-147. (c) Mohd Zain, S.; Hashim, R.; Taylor,
A. G.; Phillips, D. THEOCHEM 1997, 401, 287-300.
(6) Kido, J.; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys. Lett. 1993,
63, 2627-2629 and references therein.
(8) (a) Hameurlaine, A.; Dehaen, W. Tetrahedron Lett. 2003, 44, 957-959.
(b) Schaerlaeckens, M.; Hendrickx, E.; Hameurlaine, A.; Dehaen, W.;
Persoons, A. Chem. Phys. 2002, 277, 43-52.
(9) Lindsey, J. S.; Schreyman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz,
A. M. J. Org. Chem. 1987, 52, 827-836.
(7) McClenaghan, N. D.; Passalacqua, R.; Loiseau, F.; Campagna, S.; Verheyde,
B.; Hameurlaine, A.; Dehaen, W. J. Am. Chem. Soc. 2003, 125, 5356-
5365.
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A R T I C L E S
Loiseau et al.
Figure 2. Structures of the porphyrin-cored dendrimers of the first (P1-P3) and second generations (P4-P6) together with their schematic representa-
tions.
The synthetic route to the aldehyde precursors involves a two-
or three-step procedure starting from the corresponding mono-
halogenated (iodo- or bromo-) carbazole species prepared
following the described method8 converted into the function-
alized aldehydes by lithiation in THF and quenching with DMF,
with yields ranging from 25 to 75%. The aldehydes were
converted into the corresponding tetrakis-meso-functionalized
porphyrins in normal to good yields for this type of reaction
(15-30%). All the compounds were purified by column
chromatography over silica gel. Further details are given in the
Experimental Section.
Absorption Spectra. A. Aldehyde Carbazole Precursors.
The absorption spectra of the A1-A6 aldheyde precursors
(Figure 3, Table 1) in dichloromethane exhibit intense transitions
in the UV region. In particular, the broad band at about 230
nm, already present in 3,6-di(tert-butyl)carbazole,7 derives from
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A R T I C L E S
is much higher than the absorption of isolated porphyrin in the
UV region, since strong carbazole-centered absorption bands,
which rival the intense porphyrin Soret bands, are now present.
Oxidation Behavior. A. Aldehyde Carbazole Precursors.
Oxidation behavior of aldehyde carbazoles has been investigated
in 1,2-dichloroethane solution by means of differential pulse
and cyclic voltammetries. Data are collected in Table 3, and
two typical cyclic voltammograms are shown in parts a and b
of Figure 5. In principle, each carbazole subunit of the
compound can undergo a reversible one-electron oxidation at
relatively mild potentials.6,7 This is in fact what has been
observed, with the noticeable exception of A5. The oxidation
potential of 3,6-di(tert-butyl)carbazole (+1.08 V vs standard
calomel electrode (SCE)) is displaced to a more positive
potential in A1 (+1.30 V), as a consequence of the presence of
the phenyl-aldehyde group which drains electron density from
the carbazole center. The positive shift of the carbazole-based
oxidation potential is slightly less pronounced for A2, probably
due to the larger distance between the electron-withdrawing
aldehyde groups and the carbazole units because of the presence
of the additional phenyl rings. Interestingly, the oxidation
process of this compound involves two electrons: the process
is therefore assigned to simultaneous one-electron oxidation of
each carbazole moiety, which are noninteracting one another
as far as the redox behavior is concerned.10 Compound A3
undergoes two one-electron reversible oxidation processes,
separated by 120 mV. We assign such processes to successive
oxidation of the two carbazole subunits. In contrast to the
situation for A2, interaction between the carbazoles in A3 is
sizable and gives arise to oxidation splitting: this is attributed
to the smaller “spacer” between the redox-active groups in A3
(see structural formulas in Figure 1). Compound A4 is the
species exhibiting the richest oxidation behavior within the A1-
A6 series: indeed the voltammograms of this species indicate
that A4 undergoes two successive bielectronic processes,
followed by a third oxidation process involving one electron.
The first process is assigned to simultaneous, one-electron
oxidation of two noninteracting outer carbazole subunits linked
to different phenyls, the second oxidation process is attributed
to simultaneous, one-electron oxidation of the other two outer
carbazoles, and the third process is assigned to the inner
carbazole. Separation between first and second oxidation
processes is related to the interaction between two carbazole
subunits linked to the same phenyl, and is 130 mV, in full
agreement with the separation between the first and second
processes of A3 (see above) assigned to the same effect.
Compound A5 exhibits two successive one-electron oxidation
processes. This compound is somewhat different from the others
in that the carbazole subunits are directly connected to one
another, without the presence of interposed phenyl rings. In fact,
this compound is the only one in the series for which the number
of reversible oxidation processes is less than the number of
carbazole moieties. The two processes here are attributed to
successive one-electron oxidations of the outer carbazoles. The
presence of the directly connected inner carbazole contributes
to keep the oxidation processes at less positive potentials than
Figure 3. Absorption spectra of the aldehyde carbazole precursors A1-
A6 in dichloromethane solutions.
carbazole-centered transitions as well as the narrow band at
about 295 nm, which is present in all the compounds. Interest-
ingly, and in agreement with such an assignment, the molar
absorbance of such bands parallels the number of carbazole
moieties in A1-A6, with the larger values observed for A4
(containing five carbazole subunits, although of two different
types) and A6 (three carbazole subunits) and the smaller value
observed for A1 (one carbazole subunit). Compound A5 does
not follow the rule, in that even though this compound contains
three carbazole subunits, the molar absorbances of its 230 and
295 nm bands are significantly lower than expected (Table 1,
Figure 3): this indicates that the carbazole moieties of this
species strongly perturb one another, because of the direct
connection between outer and inner carbazole moieties (where
for inner carbazoles here we mean units directly connected to
the phenyl-aldehyde group, where outer carbazoles are the units
not directly connected to such a group, which can be seen also
as peripheral ones). To the particular nature of the carbazole
connections in A5 is also linked the broad absorption in the
260-290-nm range (see Figure 3), which is absent in the other
species.
As already reported for carbazole-phenanthroline com-
pounds,7 charge-transfer (CT) transitions, in which the phenyl-
aldehyde groups play the role of the acceptor and the carbazole
subunits act as the electron donor, can also contribute to the
absorption feature at wavelengths longer than 300 nm. Actually,
the band between 300 and 400 nm which is present in A1-A6
is solvent dependent, exhibiting a long tail in dichloromethane
and THF (probably due to the CT contribution), which is lost
in pentane, where the band also becomes structured (see
Supporting Information). Detailed assignment of the various CT
bands, anyway, is quite complicated, due to the different
carbazoles involved.
B. Porphyrin-Cored Dendrimers. The absorption spectra
of the porphyrin-cored P1-P6 dendrimers in dichloromethane
are shown in Figure 4, and the relevant data are gathered in
Table 2. The absorption spectra of the dendrimers appear
additive as they are made of visible bands which can be assigned
to the porphyrin centers, namely, the intense Soret band at about
425 nm and the weaker Q-bands at longer wavelengths, and of
bands which are mainly attributed to carbazole subunits in the
UV region. Interaction of porphyrin and carbazoles seems
negligible as far as the electronic absorption viewpoint is
concerned. As expected, the absorption of the dendritic species
(10) For a through discussion of the redox behavior of noninteracting centers,
see: Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem.
Soc. 1978, 100, 4248-4253. For recent applications, see: Venturi, M.;
Serroni, S.; Juris, A.; Campagna, S.; Balzani, V. Top. Curr. Chem. 1998,
197, 193-202, and refs. therein.
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Loiseau et al.
Table 1. Spectroscopic and Photophysical Data of the Aldehyde Carbazole Precursors A1-A6 in Deaerated Dichloromethane Solution
(298 K) or in butyronitrile Rigid Matrix (77 K)
a CZ is for 3,6-di(tert-butyl)carbazole. b PhCZ is for (N-phenyl)3,6-di(tert-butyl)carbazole.
with two oxidized carbazoles so its highest-occupied molecular
orbital (HOMO) is strongly stabilized.
Finally, compound A6 exhibits a first process, bielectronic
in nature, followed by a monoelectronic process. The first
process is assigned to simultaneous oxidation of the two outer,
noninteracting carbazole units (in this case the “spacer” is
roughly identical to the one contained in A4, which “separates”
the redox-active units by the electrochemical viewpoint), and
the second one is attributed to the inner carbazole group.
All the compounds exhibit an irreversible oxidation process
at about +1.65 V, which has not been investigated. This process,
anyway, is probably due to the well-known electropolymeriza-
tion of carbazole derivatives,11 as suggested by one of the
reviewers.
B. Porphyrin-Cored Dendrimers. The porphyrin-cored
dendrimers P1-P6 contain a large number of redox-active
subunits, including all the carbazole centers and the porphyrin
Figure 4. Absorption spectra of the porphyrin-cored dendrimers P1-P6
in dichloromethane solutions.
the other species. The inner carbazole is not oxidized within
the potential window (or, better to say, before irreversible
oxidation takes place, see later) because it is strongly connected
(11) Mengoli, G.; Musiani, M. M.; Schreck, B.; Zecchin, S. J. Electroanal. Chem.
1988, 73-86.
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Table 2. Spectroscopic and Photophysical Data of the Porphyrin-Cored Dendrimers P1-P6 in Deaerated Dichloromethane Solution (298 K)
or in butyronitrile Rigid Matrix (77 K)
a TPP is for meso-tetraphenylporphyrin.
core itself. For example, in principle up to 21 redox-active
groups are present in P4: 16 outer carbazoles, 4 inner
carbazoles, and 1 porphyrin subunit (here, for inner carbazoles
we mean those directly connected with the porphyrin phenyl
rings and for outer carbazoles we mean the peripheral ones).
So the oxidation behavior of these systems is expected to be
very rich (see Table 3). Actually, P1 undergoes a single
reversible oxidation process at +1.28 V, which involves four
electrons. Compound P1 is derived from A1, and indeed the
oxidation potential of the process is quite close to that exhibited
by its parent aldehyde precursor (+1.30 V), so the oxidation of
P1 can be straightforwardly assigned to simultaneous one-
electron oxidation of the four noninteracting carbazole units.
The situation is similar for P2, which is derived from A2:P2
indeed undergoes one oxidation process at +1.20 V (compare
with the oxidation potential of A2, +1.21 V), which involves
eight electrons; the process is therefore assigned to simultaneous
one-electron oxidation of the eight carbazole subunits. Carbazole
subunits belonging to the same “dendron” are oxidized simul-
taneously since their spacer is large enough to separate them
electrochemically, as is the case for A2. P2 also undergoes an
irreversible process at about +1.37 V, which is attributed to
porphyrin oxidation. Quite surprisingly, P2 is the only com-
pound of the series in which porphyrin oxidation is evidenced
(see Table 3). A possible explaination could be linked to the
presence in P2 of a larger aromatic spacer (the four phenyl
system, see Figure 2) interposed between the redox-active
carbazole groups and the porphyrin core. Such a larger spacer
can better isolate branching and core sites, allowing the
occurring of the porphyrin oxidation at mild potentials.12
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Loiseau et al.
Table 3. Oxidation Potentials of the Compounds in Argon-Purged
1,2-Dichloroethane Solutiona
Figure 5. Cyclic voltammograms of A4 (a, scan rate ) 200 mV/s) and
A6 (b, scan rate, 50 mV/s) in argon-purged 1,2-dichloroethane solutions.
Figure 6. Cyclic voltammograms of P4 (a, scan rate ) 20 mV/s) and P5
(b, scan rate ) 50 mV/s) in argon-purged 1,2-dichloroethane solutions.
a Number of exchanged electrons is given in brackets. b Irreversible
processes.
belonging to different branching units. The separation of the
oxidation processes is 120 mV (Table 3), in agreement with
the splitting of the oxidation processes of the parent A3 species.
Compound P4 undergoes two successive eight-electrons
processes, separated by 180 mV (Table 3, Figure 6a). The first
oxidation is assigned to simultaneous one-electron oxidation
processes involving eight outer carbazole subunits, each of them
linked to different outer phenyls, and the second oxidation
process is attributed to simultaneous one-electron oxidation of
the other, not yet oxidized, eight outer carbazole subunits. The
inner carbazoles are not oxidized at a potential less negative
than 1.66 V, at which an irreversible process takes place that
obscures any other process.
The splitting of the two carbazole oxidations showed by A3,
for which there is a sizable carbazole-carbazole interaction (see
above) is reflected in the oxidation behavior of P3. Such a
compound actually exhibits two reversible processes, each one
involving four electrons; the oxidations are here attributed to
two series of one-electron oxidations involving a carbazole
(12) Interaction between the oxidized carbazole branches and the porphyrin core
(in the P1-P6 compounds, the carbazoles, or at least some of them, are
the first to be oxidized, so the interaction involves porphyrin core and
oxidized branches) is expected to move porphyrin-centered oxidation to
more positive potentials. Such an interaction is depending on the spacer
between core and branching subunits.
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The absence of the inner carbazole oxidation, surprising as
it was for P4, is expected for P5, since inner carbazole oxidation
is not observed in A5 (see above). Compound P5 undergoes
two four-electron oxidation processes (see Figure 6b), assigned
to sequential processes involving simultaneous one-electron
oxidation of the outer carbazoles, following the scheme operat-
ing in P3 and P4, while inner carbazole oxidation is indeed not
observed.
Compound P6 exhibits an ill-behaved oxidation, in that it
undergoes two irreversible processes at mild potentials (Table
3). The reason of this behavior, which represents an exception
within the series, has not been investigated.
Luminescence Properties. A. Aldehyde Carbazole Precur-
sors. The carbazole-containing aldehyde species here investi-
gated are compounds in which strong electron-donor groups (the
carbazole moieties) are interfaced with a good electron-acceptor
moiety (the benzaldehyde). As such, CT transitions and excited
states at relatively low energies are expected, as evidenced by
the absorption spectra (see above). Moreover, the similitude of
A1-A6 with 4-dimethylaminobenzonitrile and similar com-
pounds, which can give arise to twisted intramolecular charge-
transfer (TICT) excited state and luminescence,13 is evident.
TICT excited states are quite common in species containing
donor and acceptor subunits and in which rotation of part of
the molecule with respect to the other is allowed. We start the
discussion by assuming that TICT states are involved in the
luminescence properties of the present systems. To briefly
summarize, within this assumption upon light excitation in the
CT bands an electron from the donor carbazole-centered orbital
is initially transferred to a benzaldehyde-centered orbital, with
formation of an excited state with partial CT character, although
delocalized over all the molecule. This state maintains the
conformation of the ground state (having roughly coplanar
HOMOs and lowest-unoccupied molecular orbital (LUMO)s)
and can be called locally excited (LE), analogously to N,N-
dimethylaminobenzonitrile and similar compounds.13,14 Within
the lifetime of the LE excited state and depending on several
factors including solvent effects, HOMO and LUMO orbitals
can be decoupled by internal (intramolecular) rotation of the
donor part of the molecule compared to the acceptor part. In
this way, a net charge separation is obtained and the relaxed
excited state is termed TICT. TICT emission can then take place,
at energies lower than the LE emission, and usually also with
a broad spectral shape.13,14 In the A1-A6 compounds here
studied, TICT emission at room temperature in fluid solution
is supported by the following results: (i) the large energy
difference between lower-energy absorption bands and emission
bands (Table 1), which rules out a singlet π-π* emission; (ii)
the broad shape of the emission spectra (Figure 7); the lifetimes
of the emission, which rule out a triplet assignment (Table 1).
The large polar character of the emitting excited state, in good
agreement with a TICT assignment, is also supported by the
Figure 7. Room-temperature uncorrected emission spectra of the aldehyde
carbazole precursors A1-A6 in dichloromethane solutions. Corrected values
of the emission maxima are given in Table 1.
Figure 8. Room-temperature emission spectra of A1 in solvents of different
polarities.
solvent dependence of the emission, as shown in Figure 8 for
compound A1. For such a species, as well as for all the other
species (not shown), the emission band strongly moves to the
blue on decreasing the polar nature of the solvent, reaching a
maximum at 384 nm in pentane: in this solvent the emission
spectrum also becomes structured, similar to the carbazole-
centered emission.7 We suggest that the excited state responsible
for the emission in pentane is the LE state, with the TICT state
highly destabilized in such a solvent. A further indication for a
large conformational movement (as in TICT formation) on the
excited-state surface prior to emission comes from plotting the
emission maximum energy as a function of the Lippert solvent
polarity.15 In this type of “Lippert-Mataga” plots,16 the slope
is a measure of the variation in dipole moment upon excitation
and a break in the linear relationship suggests the presence of
two different excited states. Indeed, the emission maximum
energy in pentane deviates from the linear relationship followed
by the emission energy in the other solvents.17,18
(13) (a) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.;
Baumann, W. NouV. J. Chim. 1979, 3, 443-447. (b) Grabowski, Z. R.;
Dobkowski, J. Pure Appl. Chem. 1983, 55, 245-247. (c) Bonacic-Koutecky,
V.; Michl, J. J. Am. Chem. Soc. 1985, 107, 1765-1766. (d) Rettig, W.
Angew. Chem., Int. Ed. Engl. 1986, 25, 971-988. (e) Lippert, E.; Rettig,
W.; Bonacic-Koutecky, V.; Heisel, F.; Miehe´, J. A. AdV. Chem. Phys. 1987,
68, 1-13. (f) (e) Klessinger, M.; Michl, M. Excited States and Photo-
chemistry of Organic Molecules; VCH: New York, 1995.
(14) (a) Rettig, W.; Lippert, E. J. Mol. Struct. 1980, 61, 17-24. (b) Rettig, W.;
Gleiter, R. J. Phys. Chem. 1985, 89, 4676-4680. (c) Rettig, W. In
Supramolecular Photochemistry; Balzani, V., Ed.; Reidel: Dordrecht, The
Netherlands, 1987; p 329 and references therein.
To discuss in detail the differences in emission energy within
the series of compounds (Table 1) is not an easy task: the donor
orbital is not necessarily localized on the subunits corresponding
(15) Reichardt, C. Chem. ReV. 1994, 94, 2319-2358.
(16) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra;
Dekker: New York, 1970.
(17) We would like to thank one of the reviewer for suggesting us to use the
“Lippert-Mataga” plots.
9
J. AM. CHEM. SOC. VOL. 127, NO. 32, 2005 11359

A R T I C L E S
Loiseau et al.
Figure 9. Uncorrected emission spectra of the aldehyde carbazole
precursors A3-A6 in butyronitrile rigid matrix at 77 K (emission spectra
of A1 and A2 are not shown, in that they are very similar to the one of
A4.). Corrected values of the emission maxima are given in Table 1.
Figure 10. Room-temperature uncorrected emission spectra of the por-
phyrin-cored dendrimers P1-P6 in dichloromethane solutions. Corrected
values of the emission maxima are given in Table 2.
matrix and at room temperature in a fluid solution (Table 2,
Figure 10). The spectra are typical of porphyrin emission,19 so
they are assigned in all the cases to the porphyrin core subunits.
Interestingly, the emission spectra are independent of excitation
wavelength (within the range 320-630 nm) and excitation
spectra fairly overlap the absorption spectra. This indicates that
the light collected by the peripheral carbazole chromophores is
transferred quantitatively to the porphyrin core. Therefore, P1-
P6 are efficient light-harvesting antennae, with the porphyrins
acting as the energy trap.
Compound P3 warrants additional comments, since its
luminescence quantum yield is significantly lower and its
emission spectrum exhibits a different ratio between first and
second vibrational maxima than the other species (Table 2,
Figure 10). An increased intensity of the second vibrational band
compared to the first one could suggest a larger difference
between ground state and emitting state nuclear coordinates.
This also would take into account for the reduced quantum yield,
since large distortion of the emitting state translates into more
favorable Franck-Condon factors for nonradiative decay pro-
cesses. However, this should be reflected in a shorter lifetime,
and this is not the case. So, reasons for the slightly anomalous
photophysical properties of P3 should possibly lie in the
radiative rate constant of the emitting state. Indeed, smaller
molar absorbance of the low-energy bands in the absorption
spectrum of P3 compared to the other species of the series
(namely, the 645- and 550-nm features) can be noted, which
points out to a smaller oscillator strength for the radiative
transitions between the ground and the emitting state in P3.
Steric crowding around the phenyl rings bearing the carbazole
substituents, as already mentioned for A3, could be responsible
for this behavior, leading to a somewhat distorted porphyrin
plane.
to the first oxidation process, since inner carbazoles, when inner
and outer subunits are present, are expected to be more strongly
involved in the spectroscopic and photophysical processes for
Coulombic reasons (larger separation between positive and
negative charges in TICT states, as the case when outer
carbazole are involved, would lead to destabilization), so
correlation between redox and spectroscopic data is less useful.
Different carbazole positions with respect to the benzaldehyde
group (i.e., meta or para substitution) also add complications.
Most likely this topic would require a theoretical approach,
which is out of the aims of this work.
Compounds A1-A6 also exhibit luminescence at 77 K (Table
1, Figure 9), which is strongly blue-shifted with respect to room-
temperature emission in all cases except A3. Moreover, the
spectra become structured and look like the mirror image of
the lowest-energy absorption band. We assign the 77 K emission
of all the compounds but A3 to the LE excited states. The
internal twist necessary to form the TICT state is indeed
inhibited in a rigid glass, so emission can happen from the
initially prepared LE states. A3, as mentioned above, is the
exception, as the blue-shift of the emission on passing to low
temperature in a rigid matrix is much smaller than for the other
species and the 77 K spectrum does not exhibit a structure. It
should be noted that this species is particularly crowded around
the phenyl ring, so the carbazole subunits cannot be coplanar
with the phenyl. In other words, the ground state should have
a structure quite similar to the one typical for the TICT state,
so that TICT-type emission is also probable at 77 K. However,
a contribution from the LE emission, as suggested by the double
exponential decay, cannot be excluded.
B. Porphyrin-Cored Dendrimers. All the compounds P1-
P6 exhibit relatively intense emission, both at 77 K in a rigid
Conclusions
(18) In principle, most of the experimental results that lead to a TICT assignment
for the room-temperature emission of A1-A6 could also support a twisted
ground state that becomes more planar upon CT excitation. However such
an alternative interpretation can hardly justify the complete charge-transfer
calculated from the Lippert-Mataga plot (in fact, assuming a charge
separation over a distance of 3 Å in the excited emitting state and a variation
in the dipole moment between ground and emitting states of ca. 12 D (from
the slope of the Lippert plot), about 1 electron is calculated to be involved
in the CT process, i.e., complete charge transfer) and, even more important,
cannot explain the 77 K luminescence properties of A3 (see later in the
main text).
Six novel dendrimers constituted by a porphyrin core and
four repetitive carbazole-based branching subunits have been
(19) (a) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin
Complexes; Academic Press: London, 1991. (b) Gust, D.; Moore, T. A.
In The Porphyrin Handbook, Volume 8, Electron Transfer; Kadish, K. M.,
Smith, K. M., Guillard, R., Eds.; Academic Press: San Diego, 2000; p
153 and references therein.
9
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Porphyrin-carbazole Dendrimers
A R T I C L E S
20 mV/s, a pulse height of 75 mV, and a duration of 40 ms and by
taking advantage of the presence of ferrocene used as the internal
reference.
prepared, along with their corresponding benzaldehyde carbazole
precursors. The absorption spectra, luminescence properties at
room temperature in fluid solution and at 77 K in rigid matrix,
and oxidation behavior of all the compounds have been
investigated. Absorption spectra of the dendrimers are additive,
covering both UV and visible regions of the spectrum. Oxidation
behavior of the compounds is quite rich, with reversible
processes mostly centered on the carbazole moieties. Differences
in electronic communication between the subunits have been
highlighted according to the structure of the dendrons. All the
aldehyde carbazole precursors are luminescent at room tem-
perature and at 77 K. In fluid solution, their emission is
attributed to TICT excited states, whereas in rigid matrix, where
internal twist is inhibited, emission is assigned to locally excited
states. With concern for the porphyrin-cored dendrimers, the
luminescence both at room temperature and at 77 K is typical
of the porphyrin moieties, whatever the excitation wavelength
is. Moreover, excitation spectra fairly overlap the absorption
spectra, showing that all the light absorbed by the peripheral
carbazole chromophores is transferred to the porphyrin core.
The new dendrimers can therefore be regarded as efficient light-
harvesting antenna systems, with the porphyrin subunits acting
as the energy trap.
Experimental uncertainties were as follows: absorption maxima, (2
nm; molar absorption coefficients, 10%; emission maxima, (5 nm;
excited-state lifetimes, 10%; luminescence quantum yields, 20%; redox
potentials, (10 mV.
Synthesis. General Notes. All manipulations were performed under
a dry nitrogen atmosphere using standard techniques. THF was distilled
over sodium/benzophenone immediately before use. 3,6-Di(tert-butyl)-
carbazole21 was prepared according to literature procedures.
4-[3,6-Bis(tert-butyl)carbazol-9-yl]benzaldehyde (A1). Under a dry
atmosphere of nitrogen, a solution of n-BuLi in hexane 2.5 M (0.4 g,
6.2 mmol) was added dropwise to 3,6-bis(tert-butyl)-9-(4-iodophenyl)-
carbazole (2 g, 4.15 mmol) in anhydrous THF (10 mL) cooled at -78
°C, and the mixture was allowed to stir for 15 min. Then, anhydrous
DMF (0.6 g, 8.3 mmol) was added, and the mixture was vigorously
stirred for additional 15 min. The cooling bath was removed, and the
temperature was allowed to rise to room temperature. After the mixture
was stirred for 2 h at room temperature, water was added. After
extraction with dichloromethane, the organic layer was dried over
anhydrous magnesium sulfate and the solvent was removed under
reduced pressure. The residue was purified by column chromatography
over silica gel with a mixture of hexane/dichloromethane (7:3) as eluent
to give 0.85 g of A1 as a yellow solid in 55% yield. mp 154-156 °C.
4
1H NMR (300 MHz, CDCl3, ppm): δ 10.1 (s, 1 H, CHO), 8.13 (d, J
) 1.8 Hz, 2H), 8.10 (d, 3J ) 8.4 Hz, 2 H), 7.78 (d, 3J ) 8.4 Hz, 2H),
7.46 (m, 4H), 1.48 (s, 18H). 13C NMR (75 MHz, CDCl3, ppm): δ
191.4, 144.3, 138.8, 134.6, 131.8, 126.6, 124.5, 124.3, 116.9, 109.7,
Experimental Section
Materials and Methods. NMR spectra were acquired on commercial
instruments (Bruker Avance 300 MHz or Bruker AMX 400 MHz), and
chemical shifts (δ) are reported in parts per million referenced to internal
residual solvent protons (1H) or the carbon signal of deuterated solvents
(13C). Mass spectrometry data were obtained with an HP MS apparatus
5989A (chemical ionization (CI), CH4) or a Micromass Quattro II
apparatus (electrospray ionization (ESI), solvent mixture: CH2Cl2/
MeOH + NH4OAc). Melting points were recorded on a Reichert
Thermovar instrument; values are uncorrected.
35.2, 32.4. MS (CI) m/z: 384 (MH+), 368 (M+ - CH3), 328 (M+
-
t-Bu).
3,5-Bis[3,6-bis(tert-butyl)carbazol-9-yl)phenyl]biphenyl-4-carbal-
dehyde (A2). The synthesis was performed according to the synthetic
procedure described for A1 using 1-(p-bromophenyl)-3,5-bis[p-[3,6-
bis(tert-butyl)carbazol-9-yl]phenyl]benzene (0.47 g, 0.5 mmol), n-BuLi
(0.05 g, 0.75 mmol), and DMF (0.07 g, 1 mmol) in THF (10 mL).
This gave, after purification by column chromatography over silica gel
with a mixture of heptane/chloroform (8:2 to 5:5) as eluent, 0.1 g of
A2 as a white solid in 23% yield. mp > 300 °C. 1H NMR (400 MHz,
CDCl3, ppm): δ 10.11 (s, 1 H, CHO), 8.16 (s, 4 H), 8.05 (t, 1 H), 8.03
(s, 2 H), 7.96 (s, 4 H), 7.94 (d, 3J ) 8.4 Hz, 4 H), 7.71 (d, 3J ) 8.4 Hz,
Electronic absorption spectra were recorded on a Jasco V-560
spectrophotometer. For steady-state luminescence measurements, a
Jobin Yvon-Spex Fluoromax 2 spectrofluorimeter was used, equipped
with a Hamamatsu R3896 photomultiplier, and the spectra were
corrected for photomultiplier response using a program purchased with
the fluorimeter. For the luminescence lifetimes, an Edinburgh OB 900
time-correlated single-photon-counting spectrometer was used in the
nanosecond range. As excitation sources, a Hamamatsu PLP 2 laser
diode (59-ps pulse width at 408 nm) and the nitrogen discharge (pulse
width ) 2 ns at 337 nm) were employed. Luminescence quantum yields
have been calculated by the optically diluted method.20
4
3
3
4 H), 7.50 (dd, J ) 1.6 Hz, J ) 8.7 Hz, 4 H), 7.44 (d, J ) 8.7 Hz,
4 H), 1.48 (s, 36 H). 13C NMR (100 MHz, CDCl3, ppm): δ 191.8,
146.9, 143.1, 142.1, 141.3, 139.2, 138.1, 135.6 130.4, 128.7, 128.0,
127.1, 126.2, 125.4, 123.7, 123.6, 116.3, 109.2, 34.8, 32.0. MS (EI)
-
m/z: 888 (M+•), 873 (M+• CH3), 429 (M+• - Ph-cbz - Ph-CHO).
3,5-Bis[3,6-bis(tert-butyl)carbazol-9-yl]benzaldehyde (A3). The
synthesis was performed according to the synthetic procedure described
for A1 using 1-bromo-3,5-di[3,6-bis(tert-butyl)carbazol-9-yl]benzene
(0.71 g, 1 mmol), n-BuLi (0.1 g, 1.5 mmol), and DMF (0.15 g, 2 mmol)
in THF (10 mL). This gave, after purification by column chromatog-
raphy over silica gel with a mixture of heptane/chloroform (6:4) as
Electrochemical measurements were carried out in argon-purged 1,2-
dichloroethane at room temperature with a PAR 273 multipurpose
equipment interfaced to a PC. The working electrode was a glassy
carbon (8 mm2, Amel) electrode. The counterelectrode was a Pt wire,
and the reference electrode was an SCE separated with a fine glass
frit. The concentration of the complexes was about 5 × 10-4 M.
Tetrabutylammonium hexafluorophosphate was used as supporting
electrolyte, and its concentration was 0.05 M. Cyclic voltammograms
were obtained at scan rates of 20, 50, 200, and 500 mV/s. For reversible
processes, half-wave potentials (vs SCE) were calculated as the average
of the cathodic and anodic peaks. The criteria for reversibility were
the separation of 60 mV between cathodic and anodic peaks, the close-
to-unity ratio of the intensities of the cathodic and anodic currents,
and the constancy of the peak potential on changing scan rate. The
number of exchanged electrons was measured with differential
pulse voltammetry (DPV) experiments performed with a scan rate of
1
eluent, 0.5 g of A3 as a yellow solid in 75% yield. mp > 300 °C. H
NMR (300 MHz, CDCl3, ppm): δ 10.18 (s, 1, H, CHO), 8.15 (d, 4J )
1.83 Hz, 4 H, cbz + 2 H, Ph), 8.09 (t, 1 H, Ph), 7.50 (dd, 4J ) 1.5 Hz,
3J ) 8.7 Hz, 4 H, cbz), 7.46 (d, 3J ) 8.7 Hz, 4 H, cbz), 1.47 (s, 36 H,
t-Bu). 13C NMR (75 MHz, CDCl3, ppm): δ 190.6, 143.9, 140.9, 139.3,
138.6, 129.4, 125.0, 124.0, 123.9, 116.6, 108.9, 34.8, 32.0. MS (CI)
m/z: 661 (MH+), 606 (M+ - t-Bu).
4-[3′,6′-Bis[3,5-bis(3,6-bis(tert-butyl)carbazol-9-yl)phenyl]carba-
zol-9-yl]benzaldehyde (A4). A mixture of 4-[3,6-diiodocarbazol-9-yl]-
benzaldehyde (0.174 g, 0.33 mmol), 4-[3,5-[3,6-bis(tert-butyl)carbazol-
9-yl]phenylboronic acid (0.45 g, 0.66 mmol), and an aqueous solution
(21) Neugebauer, F. A.; Fisher, H.; Bamberger, S.; Smith, H. O. Chem. Ber.
1972, 105, 2686-2691.
(20) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
9
J. AM. CHEM. SOC. VOL. 127, NO. 32, 2005 11361

A R T I C L E S
Loiseau et al.
of sodium carbonate (2M, 0.88 g, 0.83 mmol) in THF (10 mL) was
placed in a flask and stirred. Nitrogen was bubbled through the reaction
mixture for 5 min to remove dissolved air. Then, tetrakis(triphenylphos-
phine)palladium(0) (0.08 g, 0.066 mmol) was added, and the mixture
was heated at 70 °C overnight. The reaction mixture was then poured
in methanol, and the precipitate was collected by filtration and purified
over silica gel with a mixture of heptane/chloroform (8:2) to give 0.75
CDCl3, ppm): δ 143.7, 140.9, 139.7, 138.5, 136.4, 125.1, 124.3, 124.2,
120.1, 116.9, 109.9, 35.2, 32.5.
5,10,15,20-Tetrakis[3,5-bis[4-(3,6-bis(tert-butyl)carbazol-9-yl)-
phenyl]terphenyl]porphyrin (P2). The synthesis was performed
according to the synthetic procedure described for P1 using 3,5-bis-
[3,6-bis(tert-butyl)carbazol-9-yl)phenyl]biphenyl-4-carbaldehyde A2
(0.5 g, 0.56 mmol), pyrrole (0.038 g, 0.56 mmol), BF3‚Et2O (0.02 g,
0.15 mmol), p-chloranil (0.25 g, 1.0 mmol), and dichloromethane (200
mL). This afforded, after purification by column chromatography over
silica gel using a mixture of heptane/dichloromethane (7:3) as eluent,
1
g of A4 as a yellow solid in 50% yield. mp > 300 °C. H NMR (300
MHz, CDCl3, ppm): δ 10.1 (s, 1 H), 8.47 (d, 4J ) 1.1 Hz, 2 H), 8.18
(d, 3J ) 8.4 Hz, 2 H), 8.13 (d, 4J ) 1.8 Hz, 8 H), 7.99 (d, 4J ) 1.8 Hz,
3
4
3
1
4 H), 7.85 (d, J ) 8.4 Hz, 2 H), 7.81 (dd, J ) 1.5 Hz, J ) 8.4 Hz,
0.13 g of P2 as a purple solid in 25% yield. mp > 300 °C. H NMR
3
3
3
2 H), 7.75 (t, 2 H), 7.61 (d, J ) 8.8 Hz, 2 H), 7.53 (d, J ) 8.8 Hz,
(400 MHz, CDCl3, ppm): δ 9.07 (s, 8 H, pyrrole), 8.45 (d, J ) 8.1
8 H), 7.45 (dd, J ) 1.8 Hz, J ) 8.8 Hz, 8 H), 1.44 (s, 72 H). 13C
NMR (75 MHz, CDCl3, ppm): δ 190.8, 144.6, 143.2, 142.8, 140.6,
140.1, 139.0, 135.0, 132.8, 131.6, 126.8, 126.0, 124.6, 123.8, 123.6,
122.6, 119.4, 116.4, 110.6, 109.2, 34.7, 32.0. MS (ES) m/z: 1532
(MH+).
4
3
Hz, 8 H, Ph), 8.29 (d, 4J ) 1.08 Hz, 8 H, Ph), 8.17 (s, 16 H, cbz), 8.12
(t, 4 H, Ph), 8.09 (d, 3J ) 8.4 Hz, 16 H, Ph), 7.76 (d, 3J ) 8.4 Hz, 16
4
3
3
H, Ph), 7.51 (dd, J ) 1.4 Hz, J ) 8.6 Hz, 16 H, cbz), 7.48 (d, J )
8.6 Hz, 16 H, cbz), 1.48 (s, 72 H, t-Bu), -2.56 (br s, 2 H, NH). 13C
NMR (100 MHz, CDCl3, ppm): δ 143.0, 142.3, 142.0, 141.7, 140.3,
139.6, 139.3, 138.0, 135.3, 128.8, 127.1, 125.8, 125.6, 125.4, 123.7,
123.6, 116.3, 109.3, 34.8, 32.0.
4-[3,6-Bis(3,6-bis(tert-butyl)carbazol-9-yl)carbazol-9-yl]benzalde-
hyde (A5). The synthesis was performed according to the synthetic
procedure described for A1 using N-(4-bromophenyl)-3′,6′-[3,6-bis-
(tert-butyl)carbazol-9-yl]carbazole (0.2 g, 0.23 mmol), n-BuLi (0.023
g, 0.34 mmol), and DMF (0.034 g, 0.46 mmol) in THF (10 mL). This
gave, after purification by column chromatography over silica gel with
a mixture of heptane/chloroform (7:3) as eluent, 0.027 g of A5 as a
5,10,15,20-Tetrakis[3,5-di[3,6-bis(tert-butyl)carbazol-9-yl]phenyl]-
porphyrin (P3). The synthesis was performed according to the synthetic
procedure described for P1 using 3,5-di[3,6-bis(tert-butyl)carbazol-9-
yl]benzaldehyde A3 (0.5 g, 0.76 mmol), pyrrole (0.051 g, 0.76 mmol),
BF3‚Et2O (0.02 g, 0.16 mmol), p-chloranil (0.25 g, 1.0 mmol), and
dichloromethane (200 mL). This afforded, after purification by column
chromatography over silica gel using a mixture heptane/chloroform
(9:1) as eluent, 0.08 g of P3 as a purple solid in 15% yield. mp > 300
1
white solid in 30% yield. mp > 300 °C. H NMR (300 MHz, CDCl3,
4
3
ppm): δ 10.17 (s, 1 H), 8.25 (d, J ) 1.8 Hz, 2 H), 8.23 (d, J ) 8.4
Hz, 2 H), 8.16 (d, 4J ) 1.8 Hz, 4 H), 7.96 (d, 3J ) 8.4 Hz, 2 H), 7.72
3
4
3
1
(d, J ) 8.8 Hz, 2 H), 7.64 (dd, J ) 1.8 Hz, J ) 8.8 Hz, 2 H), 7.46
°C. H NMR (300 MHz, CDCl3, ppm): δ 9.12 (s, 8 H, pyrrole), 8.54
(dd, J ) 1.8 Hz, J ) 8.8 Hz, 4 H), 1.46 (s, 36 H). 13C NMR (75
MHz, CDCl3, ppm): δ 190.9, 142.8, 142.7, 140.0, 139.6, 135.2, 131.7,
131.6, 127.0, 126.2, 124.6, 123.6, 123.2, 119.4, 116.3, 111.1, 109.0.
MS (ES) m/z: 826 (MH+).
4
3
4
4
(d, J ) 1.5 Hz, 8 H, Ph), 8.32 (t, 4 H, Ph), 8.13 (d, J ) 1.8 Hz, 16
H, cbz), 7.81 (d, 3J ) 8.7 Hz, 16 H, cbz), 7.50 (dd, 4J ) 1.8 Hz, 3J )
8.7 Hz, 16 H, cbz), 1.40 (s, 72 H, t-Bu), -2.71 (br s, 2 H, NH). 13C
NMR (75 MHz, CDCl3, ppm): δ 143.4, 141.9, 140.7, 139.5, 136.4,
125.1, 124.3, 124.2, 120.1, 116.6, 109.0, 34.6, 32.0.
4-[3′,6′-Bis[4-(3,6-bis(tert-butyl)carbazol-9-yl)phenyl]carbazol-9-
yl]benzaldehyde (A6). The synthesis was performed according to the
synthetic procedure described for A4 using 4-[3,6-diiodocarbazol-9-
yl]benzaldehyde 5.66 (0.52 g, 1 mmol), 4-[3,6-bis(tert-butyl)carbazol-
9-yl]phenyl boronic acid (0.8 g, 2 mmol), an aqueous solution of sodium
carbonate (2M, 0.26 g, 2.5 mmol), and tetrakis(triphenylphosphine)-
palladium(0) (0.07 g, 0.06 mmol) in THF (10 mL). The reaction mixture
was poured in methanol, and the precipitate was collected by filtration,
purified over silica gel with a mixture of heptane/chloroform (9:1),
and recrystallized over methanol to give 0.6 g of A6 as a white solid
5,10,15,20-Tetrakis[4-[3′,6′-bis[3,5-bis(3,6-bis(tert-butyl)carbazol-
9-yl)]phenyl]carbazol-9-yl]phenyl]porphyrin (P4). The synthesis was
performed according to the synthetic procedure described for P1 using
4-[3′,6′-Bis[3,5-bis(3,6-bis(tert-butyl)carbazol-9-yl)phenyl]carbazol-9-
yl]benzalde hyde A4 (0.5 g, 0.33 mmol), pyrrole (0.02 g, 0.33 mmol),
BF3‚Et2O (0.02 g, 0.15 mmol), p-chloranil (0.25 g, 1.0 mmol), and
dichloromethane (200 mL). This afforded, after purification by column
chromatography over silica gel using a mixture heptane/chloroform
(5:4) as eluent, 0.04 g of P4 as a purple solid in 8% yield. mp > 300
1
1
in 61% yield. mp > 300 °C. H NMR (400 MHz, CDCl3, ppm): δ
°C. H NMR (400 MHz, CDCl3, ppm): δ 9.15 (s, 8 H, pyrrole), 8.60
10.17 (s, 1 H), 8.55 (d, 4J ) 1.8 Hz, 2 H), 8.22 (d, 3J ) 8.4 Hz, 2 H),
(s, 8 H, cbz), 8.59 (d, 3J ) 8.0 Hz, 8 H, Ph), 8.13 (d, 4J ) 1.6 Hz, 32
4
3
3
3
4
8.16 (d, J ) 1.5 Hz, 4 H), 7.96 (d, J ) 8.4 Hz, 4 H), 7.92 (d, J )
H, cbz), 8.12 (d, J ) 8.0 Hz, 8 H, Ph), 8.06 (d, J ) 1.7 Hz, 16 H,
Ph), 8.0 (d, 3J ) 8.6 Hz, 8 H, cbz), 7.96 (dd, 4J ) 1.8 Hz, 3J ) 8.6 Hz,
8 H, cbz), 7.77 (t, 8 H, Ph), 7.56 (d, 3J ) 8.6 Hz, 32 H, cbz), 7.46 (dd,
4J ) 1.8 Hz, 3J ) 8.6 Hz, 32 H, cbz), 143 (s, 288 H, t-Bu), -2.55 (br
s, 2 H, NH). 13C NMR (100 MHz, CDCl3, ppm): δ 143.2, 141.3, 140.2,
139.06, 132.5, 126.0, 124.5, 123.8, 123.7, 119.5, 116.4, 109.3, 34.7,
32.0.
4
3
3
8.4 Hz, 2 H), 7.83 (dd, J ) 1.8 Hz, J ) 8.4 Hz, 2 H), 7.69 (d, J )
3
4
3
8.4 Hz, 4 H), 7.66 (d, J ) 8.4 Hz, 2 H), 7.50 (dd, J ) 1.8 Hz, J )
8.4 Hz, 4 H), 7.45 (d, 3J ) 8.7 Hz, 4 H). 13C NMR (100 MHz, CDCl3,
ppm): δ 190.9, 143.2, 142.9, 140.2, 140.1, 139.3, 137.0, 134.9, 133.8,
131.6, 128.5, 127.1, 126.8, 126.0, 124.8, 123.6, 123.4, 119.1, 116.3,
110.5, 109.3, 34.8, 32.0. MS (ES) m/z: 978 (MH+).
5,10,15,20-Tetrakis[p-[3,6-bis(tert-butyl)carbazol-9-yl]phenyl]por-
phyrin (P1). To a stirred mixture of aryl aldehyde A1 (0.5 g, 1.3 mmol)
and pyrrole (0.087 g, 1.3 mmol) in dichloromethane (200 mL) at room
temperature was added boron trifluoride etherate (0.03 g, 0.2 mmol).
After the mixture was stirred for an additional 2 h, p-chloranil (0.3 g,
1.2 mmol) was added and the reaction mixture was allowed to stir for
30 min at room temperature. The solvent was removed under reduced
pressure, and the residue was chromatographed over silica gel using
dichloromethane as eluent. The obtained product was redissolved in
dichloromethane and precipitated with methanol, yielding 0.17 g of
5,10,15,20-Tetrakis[4-[3′,6′-di[(3,6-bis(tert-butyl)carbazol-9-yl)]-
carbazol-9-yl]phenyl]porphyrin (P5). The synthesis was performed
according to the synthetic procedure described for P1 using 4-[3,6-
bis(3,6-bis(tert-butyl)carbazol-9-yl)carbazol-9-yl]benzaldehyde A5 (0.5
g, 0.6 mmol), pyrrole (0.04 g, 0.6 mmol), BF3‚Et2O (0.02 g, 0.15 mmol),
p-chloranil (0.3 g, 1.2 mmol), and dichloromethane (200 mL). This
afforded, after purification by column chromatography over silica gel
using dichloromethane as eluent, 0.1 g of P5 as a purple solid in 20%
1
yield. mp > 300 °C. H NMR (400 MHz, CDCl3, ppm): δ 9.23 (s, 8
H, pyrrole), 8.68 (d, 3J ) 8.2 Hz, 8 H, Ph), 8.37 (d, 4J ) 1.5 Hz, 8 H,
1
P1 as purple solid in 30% yield. mp > 300 °C. H NMR (300 MHz,
cbz), 8.24 (d, 3J ) 8.2 Hz, 8 H, Ph), 8.18 (d, 4J ) 1.2 Hz, 16 H, cbz),
CDCl3, ppm): δ 9.13 (s, 8 H, pyrrole), 8.51 (d, 3J ) 8.4 Hz, 8 H, Ph),
8.08 (d, J ) 8.6 Hz, 8 H, cbz), 7.79 (dd, J ) 1.6 Hz, J ) 8.6 Hz,
3
4
3
4
3
4
3
3
8.27 (d, J ) 1.5 Hz, 8 H, cbz), 8.04 (d, J ) 8.4 Hz, 8 H, Ph), 7.82
8 H, cbz), 7.49 (dd, J ) 1.6 Hz, J ) 8.7 Hz, 16 H, cbz), 7.42 (d, J
) 8.6 Hz, 16 H, cbz), 1.48 (s, 144 H, t-Bu), -2.45 (br s, 2 H, NH).
13C NMR (100 MHz, CDCl3, ppm): δ 142.7, 141.8, 140.5, 140.3, 137.4,
3
4
3
(d, J ) 8.4 Hz, 8 H, cbz), 7.64 (dd, J ) 1.8 Hz, J ) 8.7 Hz, 8 H,
cbz), 1.55 (s, 72 H, t-Bu), -2.56 (br s, 2 H, NH). 13C NMR (75 MHz,
9
11362 J. AM. CHEM. SOC. VOL. 127, NO. 32, 2005

Porphyrin-carbazole Dendrimers
A R T I C L E S
136.3, 131.5, 126.3, 125.4, 124.5, 123.6, 123.3, 119.6, 116.3, 111.4,
109.2, 34.8, 32.1.
CDCl3, ppm): δ 142.9, 141.1, 140.5, 139.4, 137.1, 136.2, 133.4, 131.7,
128.6, 127.2, 126.0, 125.2, 124.7, 123.6, 123.5, 119.3, 116.3, 110.7,
109.3, 34.8, 32.0.
5,10,15,20-Tetrakis[4-[3′,6′-bis[4-(3,6-bis(tert-butyl)carbazol-9-yl)-
phenyl]carbazol-9-yl]phenyl]porphyrin (P6). The synthesis was
performed according to the synthetic procedure described for P1 using
4-[-3′,6′-bis[4-(3,6-bis(tert-butyl)carbazol-9-yl)phenyl]carbazol-9-yl]-
benzaldehyde A6 (0.49 g, 0.5 mmol), pyrrole (0.034 g, 0.5 mmol),
BF3‚Et2O (0.02 g, 0.15 mmol), p-chloranil (0.25 g, 1.0 mmol), and
dichloromethane (200 mL). This afforded, after purification by column
chromatography over silica gel using a dichloromethane as eluent, 0.1
Acknowledgment. We thank the University of Leuven
(postdoctoral fellowship to A.H.), the Ministerie Voor Weten-
schapsbeleid (Project IAP-V-3), MIUR (PRIN and FIRB-
RBNE019H9K projects), and the University of Messina (PRA
projects) for financial support.
1
g of P6 as a purple solid in 20% yield. mp > 300 °C. H NMR (400
MHz, CDCl3, ppm): δ 9.25 (s, 1 H, pyrrole), 8.69 (s, 8 H, cbz), 8.66
Supporting Information Available: Absorption spectra of A1
in various solvents and plot of the emission maximum of A1
as a function of the Lippert solvent polarity. This information
is available free of charge via the Internet at http://pubs.acs.org.
3
3
(d, J ) 7.9 Hz, 8 H, Ph), 8.20 (d, J ) 8.2 Hz, 8 H, Ph), 8.18 (s, 16
3
4
H, cbz), 8.5 (d, J ) 8.1 Hz, 16 H, Ph + 8 H, cbz), 8.0 (dd, J ) 1.8
Hz, 3J ) 8.9 Hz, 8 H, cbz), 7.73 (d, 3J ) 8.2 Hz, 16 H, Ph), 7.51 (dd,
4J ) 1.4 Hz, 3J ) 8.7 Hz, 16 H, cbz), 7.48 (d, 3J ) 8.7 Hz, 16 H, cbz),
1.49 (s, 144 H, cbz), -2.50 (br s, 2 H, NH). 13C NMR (100 MHz,
JA0514444
9
J. AM. CHEM. SOC. VOL. 127, NO. 32, 2005 11363