Strong donor-acceptor couplings in a special pair-antenna model

Article abstract of DOI:10.1039/c0cc03665j

A special pair model composed of two cofacial zinc porphyrins (acceptor) linked to a free base (donor) acts as an energy transfer dyad. Despite the absence of conjugation, ππ*/charge transfer excited states and ultrafast energy transfer (～5 ps) are noted.

Full text of DOI:10.1039/c0cc03665j

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Strong donor–acceptor couplings in a special pair-antenna modelw
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Mikhail A. Filatov, Frederic Laquai, Daniel Fortin, Roger Guilard* and
Pierre D. Harvey*ac
Received 4th September 2010, Accepted 12th October 2010
DOI: 10.1039/c0cc03665j
A special pair model composed of two cofacial zinc porphyrins
(acceptor) linked to a free base (donor) acts as an energy
transfer dyad. Despite the absence of conjugation, pp*/charge
transfer excited states and ultrafast energy transfer (B5 ps)
are noted.
system exhibits a spectacular role reversal with respect to
triad C where the S1 ET now occurs from the free base to
the cofacial unit. The very close bite distance (B3.8 A) of the
bridge squishes the cofacial dimer to form an entity acting
as a single unit. The three components are heavily coupled due
to the close proximity and the ET time scale is ultrafast
(B5 ps).
The mimicry of the special pair or the special pair flanked with
electron and energy transfer accessories of photosystems PS I
and II is still a topic of current interest.1 In most cases, zinc
porphyrins play the role of chlorophyll derivatives and the
accessories are mostly changed for quinones, C60, pyromellit-
diimide or other acceptors.2 In the purple photo-synthetic
bacteria, bacteriochlorophylls and pheophytins are the elec-
tron acceptors and the electron transfer (et) occurs on the
0.9–2.3 ps time scale.2 A model containing a special pair and
electron acceptor groups was reported by Osuka et al. using
benzene (triad A) and biphenyl (triad B) spacers to separate the
special pair from the monoporphyrin residue and the quinone
acceptor (Scheme 1).3 The et takes place within 26–40 ps after
excitation, which is B10 times slower than those cited above.
We recently reported the anthracenyl-containing triad C
(Scheme 1). The octamethylzinc(II)-, diarylzinc(II)- and free
base porphyrins act as S1 energy transfer (ET) donor 1,
donor 2 and acceptor, respectively.4 The noted ET between
donor 1 and donor 2 raises the question whether the anthra-
cenyl bite distance is short enough to adequately mimic the
special pair. We now report a biphenylene-containing special
pair directly linked to an ET accessory (1; Scheme 2). This
Trimer 1 was prepared from three sequential Suzuki cross-
coupling reactions (steps A, B and C in Scheme 2), applying
our recently developed approach.4 The coupling between 2
and 3 being performed with Pd2(dba)3 and KOH in the
presence of Buchwald’s SPhos ligand, known to suppress
hydrodebromination in cross-coupling reactions,5 led to the
formation of the undesired product 4. Alternatively, the use of
a larger excess of 2 (2 eq.) in the presence of Pd(PPh3)4 and
Cs2CO3 in DMF–toluene selectively provided product 5 in
94% yield. The latter was then reacted with the porphyrin
diboronate 6 under optimized conditions with Ba(OH)2 and
Pd(PPh3)4 in DMF and gave the borylated face-to-face
porphyrin dimer 7 in 48% yield. The coupling between 7
and 8 was attempted using different catalysts (Pd(PPh3)4,
Pd2(dba)3, PdCl2(Ph3P)2) and bases (KOH, Cs2CO3, Ba(OH)2)
in DMF or toluene–DMF (2 : 1) mixtures at 90–100 1C. The
yield of the target trimer 1 was found to be critically dependent
on the reagents ratio rather than on the catalytic system used.
For instance, by using a stoichiometric amount or 2-fold
excess of bromoporphyrin 8 with respect to boronate 7,
similarly to step A and other reported couplings,4 only trace
amounts of desired product 1 along with unreacted 7 and 8
were isolated despite increasing the reaction time or adding the
catalyst. Such a result can be explained by the presence of
steric hindrance, here caused by the cofacial dimer moiety,
known to decrease the efficiency of the Suzuki reaction.6
However, the use of 2 eq. of dimer 7 with respect to 1 eq. of
8 provided full conversion of 8 within 6–8 h and gave trimer 1
in up to 50% preparative yield when carried out with
Pd(PPh3)4/Cs2CO3 in DMF/toluene. The excess of 7 was
recovered from the reaction mixture. The product was purified
by chromatography on silica and size-exclusion chromato-
graphy and was characterized by mass spectrometry and
NMR spectroscopy. The profound effect of the reagents ratio
on the coupling of 7 and 8 is not clear with respect to other
sterically hindered Suzuki couplings, which are usually
achieved by the use of specific ligands on Pd.6 In an attempt
to develop an alternative route towards 1 we prepared the
borylated meso–meso dimer 9 which was subjected to coupling
with 5 (Scheme 2, steps D and E) using the synthetic protocol
developed for step C. However, regardless of the reaction
conditions, no trimer was observed, while the starting material
was converted to the corresponding meso–meso dimers 10,
Scheme 1
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Institut de Chimie Moleculaire de l’Universite de Bourgogne
(ICMUB), UMR 5260 CNRS, 9 Avenue Alain Savary,
BP 47870-21078 Dijon, France.
E-mail: roger.guilard@u-bourgogne.fr, pierre.harvey@u-bourgogne.fr;
Fax: +33380396117; Tel: +33380396111
b Max Planck Research Group for Organic Optoelectronics,
Max Planck Institute for Polymer Research, Ackermannweg 10,
D-55128 Mainz, Germany
c
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Departement de Chimie, Universite de Sherbrooke, Sherbrooke, PQ,
Canada J1K 2R. Fax: +819 821-8017; Tel: +819 821-7092
w Electronic supplementary information (ESI) available: Experimental
section, UV-vis, fluorescence, transient absorption spectra, and
computed electronic transitions of trimer 1 (TDDFT). See DOI:
10.1039/c0cc03665j
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Fig. 1 (A) Comparison of the UV-vis spectra of 1, 4 and 12 in THF.
(B) Fluorescence spectra of 1 in various solvents (298 K).
680 nm. The excitation spectra superpose well the absorption
spectra. In benzene, the 0–0 peak is visible at 663 nm. Because
this value matches best that observed for 4 (667) and 7
(679 nm) with respect to 12 (635 nm; ESIw), this emission is
due to the bis(zinc porphyrin) unit. This value also compares
well with the low-energy Q-band in 1. The lack of emission at
635 nm suggests an efficient S1 ET from the free base (donor)
to the cofacial unit (acceptor). The fluorescence lifetime of 1 in
THF (2.27 ns) compares well to that for 4 (1.95 ns) but not
with 12 (10.1 ns). One of the salient features is the change in
emission intensity as a function of the medium polarity. The
0–0 peak is visible for 1 in benzene but the fluorescence band is
unstructured and weak in DMF. At a first glance such
behaviour may be assigned to a charge transfer (CT) excited
state, however due to the lack of a red shift with increasing
medium polarity, the exact nature of this state needs analysis.
The HOMO and LUMO for 1 are composed of p-systems
primarily centered on the cofacial dimer indicating that the
lowest energy excited state is dimer-centered (Fig. 2; DFT;
ESIw). Moreover the LUMOs show atomic contributions
arising from p-systems that spread over both the dimer and
the free base, meaning that all these pp* electronic transitions
have some minor bis(zinc porphyrin) - free base CT
character. This is consistent with the lack of a red shift of
the fluorescence upon increasing the medium polarity. The
computed UV-vis spectra of 1 (TDDFT, ESIw) compare well
with the experimental one in Fig. 1. The individual calculated
transitions and their major contributions (ESIw) also support
the assignment of the S1 states.
Scheme 2 (i) KOH, Pd2(dba)3, SPhos, PhMe, 90 1C, 15 h; (ii) Pd(Ph3P)4,
Cs2CO3, PhMe/DMF, 90 1C, 15 h; (iii) Ba(OH)2, Pd(Ph3P)4, DMF,
100 1C, 24 h; (iv) 5-bromo-5,15-di-(3,5-di-tert-butylphenyl)porphyrin
(0.5 eq.), Pd(Ph3P)4, Cs2CO3, PhMe/DMF, 90 1C, 8 h; (v) Pd(Ph3P)4,
Cs2CO3, PhMe/DMF, 90 1C, 20 h.
resulting from a deborylation process, and 11 arising from an
aryl–aryl interchange reaction.7
The comparison of the UV-vis spectra of 1 with those of 4
and 12 (Fig. 1A) showed broader Soret and Q-bands for 1
indicating interporphyrin interactions. Attempts to recon-
struct this spectrum with the individual spectra of 4 and 12
failed to find a good fit (ESIw) hence providing evidence for
interactions. Such interactions have also been previously
demonstrated for a cofacial biphenylene–bisporphyrin,8 but
interactions for all three is new. In this respect, the absorption
spectra of 9 were studied (ESIw) and the UV-vis bands were
found to be broader than those observed for the mono-
porphyrins. The fluorescence spectrum of 1 measured in three
different solvents (Fig. 1B) exhibited a band centered at
Transient absorption spectroscopy (TAS) was used to
resolve the ultrafast dynamics of the excited states in 1 by
tracking their photoinduced absorption (PA) profiles. The TA
spectra of 1 (Fig. 3 top) are similar to that of the porphyrin
dimer 4 (ESIw) but not the free base 12 (ESIw) indicating that
the photo-induced absorption stems mainly from the acceptor
unit, i.e. the cofacial dimer. A clear assignment of the spectral
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time of B3165 ps (Æ35 ps). The latter is due to the excited
state lifetime of the cofacial dimer (tF(1)= 2.27 ns). Consider-
ing that neither the PA signal of the free base 12 nor of the
cofacial dimer 4 showed changes on the ps time scale (ESIw), it
appears likely that the much shorter component seen for 1 is
associated with an excited state ET process free base -
cofacial dimer in 1 accompanied by a change in the excited
state cross-section in the observed spectral region leading to
the rise of the PA when going from an excited state on the
freebase to an excited state on the dimer.
This ET is extremely fast and comparable to those seen in
natural systems.2 The very fast ET rate here is certainly aided
by the short donor and acceptor distance, but the lack of large
transition moments in porphyrins with respect to chlorophyll
derivatives must slow down the rate. So these CT interactions
appear to counter balance this effect. One of the key features is
the biphenylene bridge which compresses the two cofacial
aromatics. The resulting strong pp-interactions form a true
dimer with red-shifted absorptions, hence reversing the role of
the energy donor and acceptor moieties. However, we find that
the coupling between the frontier HOMOs and LUMOs is
large with regard to special pair models, so models for et based
on this spacer should also exhibit very fast rates. New systems
with decoupled donors and acceptors are on the way.
Fig. 2 MO drawings of selected frontier MOs of 1.
This research was supported by the Centre National de la
Recherche Scientifique (CNRS) and by the Agence Nationale
de la Recherche (ANR) for a Research Chair to P.D.H.
Notes and references
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2 P. D. Harvey, C. Stern and R. Guilard, in Handbook of Porphyrin
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Engineering, Biology and Medicine, ed. K. M. Kadish, K. M. Smith,
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press.
3 A. Osuka, T. Nagata and K. Maruyama, Chem. Lett., 1991, 481;
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Fig. 3 Top: TA spectra of 1 in deaerated THF at delay times of 1, 10,
100 and 1000 ps. Note the increase of the photoinduced absorption
between 1 ps (’) and 10 ps (red circles). Bottom: Kinetic traces of the
photoinduced TA integrated between 700–750 nm. The red lines
represent exponential fits to the experimental data with a rise
(5.1 ps) and a decay time (B3.2 ns).
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compounds 4 and 12 overlaps in this spectral region.
Moreover, the experiments are further complicated due to
the overlap of the ground state absorption of 4 and 12. In fact,
it is not possible to exclusively excite one moiety in 1.
Consequently, a superposition of the signals from 4 and 12
can be expected for 1 and was indeed noted. Nevertheless, it is
possible to extract information on the ultrafast ET processes
occurring in 1. The kinetic trace of the integrated spectral
region 700–750 nm (Fig. 3 bottom) exhibits an exponential rise
time of B5 ps (Æ0.1 ps) and a significantly longer relaxation
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c
9178 Chem. Commun., 2010, 46, 9176–9178
This journal is The Royal Society of Chemistry 2010