Is the special pair structure a good strategy for the kinetics during the last step of the energy transfer with the nearest antenna? A chemical model approach

Article abstract of DOI:10.1039/c3cc38740b

A cofacial bis(Mg(ii)porphyrin)-C₆H₄-free base ([Mg₂]-bridge-FB) dyad shows S₁ energy transfer in both directions and much slower rates than similar monoporphyrin systems are observed.

Full text of DOI:10.1039/c3cc38740b

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Is the special pair structure a good strategy for the
kinetics during the last step of the energy transfer
with the nearest antenna? A chemical model
approach†
Cite this: Chem. Commun., 2013,
49, 2228
Received 5th December 2012,
Accepted 26th January 2013
Jean-Michel Camus,a Adam Langlois,b Shawkat M. Aly,b Roger Guilard*a and
Pierre D. Harvey*ab
DOI: 10.1039/c3cc38740b
www.rsc.org/chemcomm
A cofacial bis(Mg(II)porphyrin)–C6H4-free base ([Mg2]–bridge–FB)
dyad shows S1 energy transfer in both directions and much slower
rates than similar monoporphyrin systems are observed.
The photosynthetic membranes in plants and photosynthetic bac-
teria are equipped with reaction center proteins within which a
cofacial slipped dimer of chlorophylls or bacteriochlorophylls called
the special pair is placed.1 It is surrounded by antennas that play a
role in absorbing light and funneling this excitation energy to it
(antenna* - special pair).1 The natural special pair exhibits many
structures depending on the organism (Fig. 1),2 and the S1 energy
transfer, kET(S1), between the nearest antenna and the central special
pair occur in the 25–50 ps time scale.3 Noteworthily, a special pair* 2
antenna* ET equilibrium was also reported to occur at the 100 Æ
50 fs time scale for photo-system II (special pair P680 2 antenna
C670),4a and using selective excitations, at the 25 ps (k1, antenna
B875* - special pair P865) and 8 ps (kÀ1, special pair P865* -
antenna B875) time scales for purple photosynthetic bacteria.4b This
process was shown to regulate the subsequent oxidation of the
Fig. 2 Top: structures of several ET dyads (D: donor; A: acceptor). Bottom: front (A),
top (B) and side (C) views of the optimized geometry of 1. The substituents have
been removed for clarity. The dihedral angles formed by the [Mg2]/C6H4, [Mg2]/FB
and FB/C6H4 and average planes are 69.11, 45.31 and 65.61, respectively.
special pair (i.e. charge separation step).4c,d To the best of our
knowledge, no artificial special pair–antenna model providing
evidence for equilibrium has been reported. We now report chemical
model 1 (Fig. 2, top), built upon a cofacial bis(magnesium(II)-
porphyrin), [Mg2], held by a biphenylene-spacer DPB (CmesoÁÁÁCmeso
distance = 3.80 Å) as the artificial special pair, a 1,4-benzene bridge,
and a free base porphyrin, FB, as an antenna. The kET(S1)’s for
[Mg2]* - FB extracted from the rise time seen in transient absorption
kinetic traces are about one order of magnitude slower than that
reported for dyads strictly built on monoporphyrins (3–5; Fig. 2). This
unexpected result also applies for a dyad bearing no bridge (2 vs. 5).
At first glance, this kET(S1) decrease for the special pair-containing
Fig. 1 Top (blue) and side views (red) of the special pairs included in the purple
photosynthetic bacteria (left) and in photosystems I (middle) and II (right) in
plants. Modified from ref. 3.
a
´
´
Institut de Chimie Moleculaire de l’Universite de Bourgogne (ICMUB,UMR 6302),
´
Universite de Bourgogne, Dijon, France
b
´
´
´
Departement de Chimie, Universite de Sherbrooke, 2550 Boulevard de l’Universite,
´
Sherbrooke, Quebec, Canada J1K 2R1. E-mail: Pierre.Harvey@USherbrooke.ca;
¨
species is counterintuitive, but can be explained by the Forster
Fax: +1-819-821-8017; Tel: +1-819-821-7092
theory.5 Moreover, a [Mg2]* - FB energy transfer was observed
from the fluorescence data indicating the presence of [Mg2]* 2
FB* equilibrium.
† Electronic supplementary information (ESI) available: Complete experimental
section, all photophysical spectra and data, and graphs showing the total energy
(DFT) against the dihedral angles. See DOI: 10.1039/c3cc38740b
c
2228 Chem. Commun., 2013, 49, 2228--2230
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Scheme 1 Synthesis of 1. (i) 1,4-BrC6H4I, Pd(PPh3)4, Cs2CO3; (ii) HCl; (iii) zinc(II)
5,15-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10,20-bis-(nonyl)porphyrin
10, Pd(PPh3)4 Cs2CO3; (iv) MgBr2(OEt2), NEt3; (v) 8, Pd2(dba)3, S-PHOS, Cs2CO3.
(Ar = 3,5-di(t-butyl)phenyl).
The strategy to prepare polyads exhibiting a rigidly held cofacial
unit flanked with a side arm using stepwise Suzuki couplings was
recently reviewed by us.6 It allows tuning of the synthesis of
mimicking models by controlling the spacer and metallation in
the cofacial dimer and the anchoring of a suitable monomer. This
approach is also used for the synthesis of 1 (Scheme 1 and ESI†). It
consists of two converging paths to first prepare precursors 8 and 11
from the known macrocycles 6, 9, and 10.7 The anchoring of the
benzene bridge on 6 proceeds via a Suzuki coupling with BrC6H4I
forming 7. The latter is demetallated using HCl providing precursor
8 (92% yield). Concurrently, 9 and 10 are coupled in the same
manner to afford the borane-containing bisporphyrin 11. The
coupling of 11 with 8 gives a bis(zinc(II)porphyrin)–C6H4-(free base)
dyad, analogous to 1, but could not be obtained pure enough for
reliable photophysical analyses. 1 is then obtained by the demetalla-
tion of 11 with HCl (affording 12 in 62% yield), followed by the
metallation with MgBr2(OEt2) (forming 13 in 80% yield),8 and by a
Fig. 3 (A) Absorption spectrum of 13 (e: 7080 MÀ1 cmÀ1) normalized with the
fluorescence band of 8. (B) Absorption spectrum of 8 (e: 1950 MÀ1 cmÀ1
)
normalized with the fluorescence band of 13. (C) Absorption, fluorescence and
excitation spectra of 1. (D) Transient absorption spectra of 1. (E) Kinetic trace of
the 450 nm transient peak of 1. lexc = 388 nm. The component of 5.2 Æ 1.0 ns is
due to the [Mg2] unit in air as checked with 13 and 8 in air by fluorescence. The
instrument time scale is limited to 3 ns so the longer component was not
accessible. All frames are in 2 MeTHF at 298 K in air.
Suzuki coupling with 8. No intra-molecular transmetallation or indicates quenching. The good match of the excitation and absorp-
demetallation was detected by mass spectrometry and 1 was isolated tion spectra indicates efficient ET. This double ET role should
with a satisfactory yield after repetitive SEC purifications (28%). A generate two unquenched and two quenched emissions (graphic
resonance at À2.6 ppm characteristic of the FB flanked to the art). The fluorescence lifetimes, tF (quantum yields, FF), for 13 and 8
cofacial dimer was detected (1H NMR). 1, 8, and 11–13 were are 7.3 Æ 0.1 (0.019), and 13.9 Æ 0.1 (0.067) ns, respectively,
1
characterized by H NMR, ESI-TOF mass and UV-vis spectroscopy consistent with those reported for magnesium(II)tetraphenyl-
(ESI†). In the absence of X-ray data, the geometry of 1 was optimized porphyrin (8.1 ns) and 10,20-bis(3,5-di(t-butyl)phenyl)porphyrin
by DFT (B3LYP; Fig. 2) to extract the center-to-center [Mg2]–FB (11.3 ns) at 298 K.10
distance, r. Rotations about the C6H4–[Mg2] and C6H4–FB single
The decay for 1 (FF = 0.023) monitored at 670 nm, where both
bonds lead to dissymmetric double potential wells for which the units emit (using lexc = 510 nm, where both absorb), exhibits three
deepest minima are placed 3.0–3.5 kJ molÀ1 below the activation components (tF = 0.21 Æ 0.06, 7.6 Æ 0.5 and 11.2 Æ 1.2 ns; ESI†).
barriers (ESI†).
These are assigned to the quenched and unquenched fluorescence
The energy donor and acceptor cannot be directly assigned from of [Mg2] and unquenched emission of the FB, respectively. The
the 0–0 peak positions of 8 and 13 as the absorption and emission relative intensity of the two slowest components decreases as the
data suggest opposite roles (ESI†). Moreover, the comparison of monitoring wavelength is shifted to the blue where the FB emits
the absorption trace of 13 with the fluorescence spectra of 8 and more. Then tF for the unquenched FB is accurately extracted (11.0 Æ
vice versa show clear overlaps (see grey area in Fig. 3, frames A and B) 0.1 ns). The quenched component of the FB was not accessible from
indicating that both can act as donors and acceptors based on the emission decays due to the large pulse width of the laser (fwhm B
J-integral of the Forster’s theory.5 This is caused by the broad signals 1.6 ns; detection limit B 100–150 ps). Therefore, the S1 lifetime of
of the DPB-[Mg2] unit, well-known for DPB-containing bispor- the FB donor was obtained from fs transient absorption spectro-
phyrins, which is due to the cofacial p-systems’ interactions.9 scopy (frames D and E; Fig. 3). The spectra show the typical
The fluorescence spectrum of 1 shows a shoulder at B645 nm porphyrin S1 - Sn signal,11 which evolves between 0.3 and
matching that of the 0–0 peak of 8 but its rather weak intensity 100 ps. The transient absorption trace, DA vs. time, monitored at
c
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13 and 8 in MÀ1 cmÀ1 nmÀ1 units (Fig. 3; frames A and B), the ratio
koF  grey area (FB* - [Mg2])/kFo  grey area ([Mg2]* - FB) is 6.5,
which is consistent with 6.3.
This work shows that 1 and 2 exhibit no kinetic benefit in
using a cofacial bisporphyrin structure. However, the presence
of an artificial special pair* 2 antenna* process is observed
(25 Â 109/4.6 Â 109 sÀ1). To the best of our knowledge, evidence
for an artificial special pair* 2 antenna* equilibrium is
provided for the first time using chemical models. Not only
Table 1 Comparison of the kET(S1) data for 1–5
Compd
kET(S1) (Â 1010 sÀ1
2.5 (FB* - [Mg2])
)
Time scale (ps)
Reference
1
40
This work
This work
7b
12a
12a
0.46 ([Mg2]* - FB)
20
29
10
210
5.1
3.4
10
2
3
4
5
180
0.55
12b
450 nm exhibits two clear rises; the first one due to the direct are these observations consistent with literature findings of
excitation of both chromophores by the laser pulse (o1 ps), and the natural special pair-antenna’s,4 but more importantly open the
second one of 40 ps, associated with a clear ET process feeding door for further model designs in order to establish structure–
the acceptor Sn level. Attempts to extract the 210 ps rise time of the property relationships for this behaviour relevant for the reg-
[Mg2]* - FB process at 450 and 480 nm failed (ESI†). This is due ulation of the charge separation process in natural systems.
to the strong overlap of the S1 - Sn bands of both units and its
PDH thanks the Agence National de la Recherche (ANR) for
likely weaker intensity relative to the noise level. kET(S1) is the grant of a Research Chair of Excellence. JMC and RG thank
obtained from kET(S1) = (1/tS1) À (1/toS1) with tS1 and tSo1 being the Centre National de la Recherche Scientifique.
the S1 lifetimes of the donor, respectively, in the absence and the
presence of an acceptor.1b kET(S1) (FB* - [Mg2]) is 2.5 Â 1010 sÀ1
References
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for 2 is smaller than that for 5 (species exhibiting no bridge). Second,
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kET for ZnTTP-type donors are prone to being 3.33 faster than that
for the FB. Moreover, the rÀ6 zinc(II)porphyrin/rÀ6 [Mg2] ratio is 2.33/
1.73 = 1.35. Combined with the above parameter, 3 and 5 (i.e. ZnTTP-
type donors) are prone to being 4.5 faster than the FB (1). The kET(S1)
4/kET(S1) 1 ratio is 4 and matches this prediction. Finally, it was
recently shown that the modification of the substituents and metal
on the porphyrin rings could change kET(S1) up to 4-fold.15 However,
the J-integrals for 3–5 are not available. Nonetheless, the product
of 4.5 Â 4-fold is 18-fold, so the kET(S1) decrease of about an order of
magnitude (from FB such as in 8 to ZnTTP-type donors) is not
unreasonable.
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rate relative to FB* - [Mg2] (25 Â 109
s
À1) is again explained
using the Forster theory. Using the surface of the grey area of
15 J.-M. Camus, S. M. Aly, C. Stern, R. Guilard and P. D. Harvey, Chem.
Commun., 2011, 47, 8817.
c
2230 Chem. Commun., 2013, 49, 2228--2230
This journal is The Royal Society of Chemistry 2013