Design of triads for probing the direct through space energy transfers in closely spaced assemblies

Article abstract of DOI:10.1021/ic3026655

Using a selective stepwise Suzuki cross-coupling reaction, two trimers built on three different chromophores were prepared. These trimers exhibit a D^aA₁-A₂ structure where the donor D (octa-β-alkyl zinc(II)porphyrin either

Full text of DOI:10.1021/ic3026655

Article
pubs.acs.org/IC
Design of Triads for Probing the Direct Through Space Energy
Transfers in Closely Spaced Assemblies
Jean-Michel Camus,† Shawkat M. Aly,‡,§ Daniel Fortin,‡ Roger Guilard,*,† and Pierre D. Harvey*,†,‡
†Institut de Chimie Molec
Techniques, 9 Avenue Alain Savary, BP 47870, 21078 Dijon, France
‡Dep
artement de Chimie, Universite de Sherbrooke, 2500 Boulevard de l’Universite,
́
ulaire de l’Universite
́
de Bourgogne (ICMUB, UMR 6302), Universite
́
de Bourgogne, UFR Sciences et
́
́
́
Sherbrooke, Quebec, Canada J1K 2R1
S
* Supporting Information
ABSTRACT: Using a selective stepwise Suzuki cross-coupling reaction, two
trimers built on three different chromophores were prepared. These trimers
exhibit a D∧A1-A2 structure where the donor D (octa-β-alkyl zinc(II)porphyrin
either as diethylhexamethyl, 10a, or tetraethyltetramethyl, 10b, derivatives)
through space transfers the S1 energy to two different acceptors, di(4-
ethylbenzene) zinc(II)porphyrin (A1; acceptor 1) placed cofacial with D, and
the corresponding free base (A2; acceptor 2), which is meso−meso-linked with
A1. This structure design allows for the possibility of comparing two series of
∧
∧
̂
assemblies, 9a,b (D A1) with 10a,b (D A1-A2), for the evaluation of the S1
energy transfer for the global process D*→A2 in the trimers. From the
comparison of the decays of the fluorescence of D, the rates for through space
energy transfer, kET for 10a,b (kET ≈ 6.4 × 109 (10a), 5.9 × 109 s−1 (10b)),
and those for the corresponding cofacial D∧A1 systems, 9a,b, (kET ≈ 5.0 × 109
(9a), 4.7 × 109 s−1 (9b)), provide an estimate for kET for the direct through
space D*→A2 process (i.e., kET(D∧A1-A2) − kET(D∧A1) = kET(D*→A2) ∼ 1 × 109 s−1). This channel of relaxation represents
∼15% of kET for D*→A1.
Chart 1. Structures of 9a,b and 10a,b Showing the Through
Space S1 Energy Transfers (Ar = 4-Ethylbenzene)
INTRODUCTION
■
Through space singlet, S1, energy transfers are among the key
processes in the antenna effect in the photosynthetic
membranes.1 In the past decades, large efforts have been
devoted to mimic natural photoinduced electron and energy
transfers2,3 and excitation energy migration (exciton).4
However, these molecular-based models, mostly focusing on
multiporphyrin architectures, dyads and multiads, have largely
been based on direct covalent or coordination bonds,2,4 but not
exclusively since H-bonding5 and occasionally electrostatic
interactions2b as means to assemble porphyrins together have
been reported. Consequently in laboratory models, these key
photophysical processes unavoidably occur with chemical or H-
bonds placed between the donor (D) and acceptor (A), hence
drastically contrasting with the natural systems found in the
photosynthesis membranes. These photophysical D−A events
can occur at very short time scale (fs−ps).6 Conversely, several
studies on cofacial D∧A models (∧ = bridge), where only
through space S1 energy transfers occur, were reported.2 To the
best of our knowledge, these models for S1 energy transfers
were exclusively limited to dyads bringing the inconvenience
that relay effect and cascade events could not be studied. This
simplistic situation hinders the understanding of the multiple,
not to say complex, events in the natural systems. This work
reports two trimer D∧A1-A2 systems (10a,b; Chart 1) where D
(octaalkyl zinc(II)porphyrin) can transfer its S1 energy to two
different acceptors, a cofacial diaryl zinc(II)porphyrin (A1;
acceptor 1) and the corresponding free base (A2; acceptor 2)
linked to A1 via a meso−meso-bond (Chart 1). By comparing
the rates of energy transfers, kET, with those for the cofacial
D∧A1 systems (9a,b; Chart 1), kET for the direct through space
S1 energy transfer D*→A2 is deduced (Figure 1). This channel
of relaxation represents ∼15% of kET for D*→A1.
Received: December 5, 2012
Published: July 11, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Article
Figure 1. Diagram showing how the kET for D*→A2 is deduced (blue = D; red = A1; green = A2).
Zn(OAc2)·2H2O was added. The mixture was stirred overnight and
washed with brine. The organic layer was dried over MgSO4 and
evaporated until dryness. Purification by successive silica gel
chromatographic columns (dichloromethane/pentane 1/2 and THF/
pentane 5/100) afforded the product as a brick red powder (0.28 g,
EXPERIMENTAL SECTION
■
Materials. 4,6-Dibromodibenzothiophene (1) and 2,2′-dipyrro-
methane were synthesized according to the literature.7,8 The porphyrin
precursors a,c-biladienes (3a,b) were synthesized as previously
reported.9 The handling of all air/water sensitive materials was carried
out using standard high vacuum techniques. Dried toluene was
obtained by passing through alumina under nitrogen in the solvent
purification systems and then further dried over activated molecular
sieves; extra dry DMF and 4-ethylbenzaldehyde were purchased from
Aldrich. Triethylamine, DCM, and 1,2-dichloroethane were distilled
from CaH2; THF was distilled from sodium benzophenone ketyl.
Unless specified otherwise all other solvents were used as
commercially supplied. Silica gel (Merck; 70−120 mm) was used for
column chromatography. Analytical thin layer chromatography was
performed using Merck 60 F254 silica gel (precoated sheets, 0.2 mm
thick). Reactions were monitored by thin-layer chromatography, UV−
vis spectroscopy, and MALDI/TOF mass spectrometry. Size exclusion
chromatography was carried out under gravity using cross-linked
polystyrene Bio-Beads SX-1 (200 − 400 mesh) in DCM. The 1H
NMR and mass spectra are placed in the Supporting Information (SI).
4-Bromo-6-formyldibenzothiophene (2). A 3.2 g portion of of 4,6-
dibromodibenzothiophene (1) (9.3 mmol) was dissolved in 120 mL of
anhydrous THF, and the solution was cooled down to −78 °C. PhLi
(4.9 mL, 9.8 mmol) was added dropwise, and the reaction mixture was
stirred one hour at low temperature. A 5 mL portion of anhydrous
DMF was added at −78 °C, and the reaction mixture was allowed to
warm to room temperature while stirring for 2 h. The reaction was
quenched with water and diluted with 100 mL of CH2Cl2. The organic
phase was washed with a saturated aqueous NH4Cl solution, washed
with brine, and dried over MgSO4. The solution was concentrated in
vacuo, and crystallization at −30 °C afforded 2 as a light yellow powder
(2.10 g, 78%). 1H NMR (300.16 MHz, CDCl3): δ 10.29 (s, 1H,
CHO), 8.38 (dd, 1H, J = 7.9 Hz, J = 0.8 Hz), 8.15 (dd, 1H, J = 7.9 Hz,
J = 0.8 Hz), 8.02 (dd, 1H, J = 8.0 Hz, J = 1.0 Hz), 7.70 (t, 1H, J = 7.7
Hz), 7.65 (dd, 1H, J = 7.7 Hz, J = 1.0 Hz), 7.39 (t, 1H, J = 7.8 Hz)
ppm. 13C NMR (75.47 MHz, CDCl3): δ 191.3, 143.5, 137.9 (2C),
135.0, 133.4, 130.9, 130.2, 127.7, 126.3, 125.0, 120.4, 116.8 ppm. ESI
MS m/z: 312.9310 [M + Na]+, 312.9293 calcd for C13H7BrNaOS.
Zinc(II) 5-(4-(Bromodibenzothien-6-yl))-(13,17-diethyl-
2,3,7,8,12,18-hexamethyl)porphyrin (4a). A mixture of a,c-biladiene
dihydrobromide (3a) (0.95 g, 1.5 mmol) and 4-bromo-6-formyldi-
benzothiophene (2) (0.44 g, 1.5 mmol) in 150 mL of absolute ethanol
was deoxygenated by bubbling argon for 15 min and heated under
reflux conditions. A solution of p-toluenesulfonic acid (2.0 g, 10.6
mmol) in 100 mL of degassed ethanol was added dropwise over 10 h,
and the reaction mixture was stirred overnight under reflux. After
cooling down the reaction mixture to room temperature, the volatiles
were removed under vacuum, and the residue was dissolved in
CH2Cl2, washed with a saturated NaHCO3 aqueous solution, and
washed with brine. The organic phase was dried over MgSO4 and was
concentrated in vacuo. A saturated methanolic (10 mL) solution of
1
yield 23%). H NMR (300.16 MHz, THF-d8): δ 10.16 (s, 2H, meso),
10.09 (s, 1H, meso), 8.68 (dd, 1H, J = 1.1 Hz, J = 7.8 Hz, thiophene),
8.50 (dd, 1H, J = 1.1 Hz, J = 7.8 Hz, thiophene), 8.18 (dd, 1H, J = 1.1
Hz, J = 7.6 Hz, thiophene), 7.99 (dt, 1H, J = 7.7 Hz, thiophene), 7.58
(dd, 1H, J = 1.1 Hz, J = 7.6 Hz, thiophene), 7.47 (t, 1H, J = 7.7 Hz,
thiophene), 4.14 (q, 2H, J = 7.5 Hz, CH2), 4.13 (q, 2H, J = 7.5 Hz,
CH2), 3.65 (s, 6H, CH3), 3.48 (s, 6H, CH3), 2.27 (s, 6H, CH3), and
1.91 (t, 6H, J = 7.5 Hz, CH3) ppm. ESI HRMS m/z: 772.1208 [M]•+,
772.1160 calcd for C42H37BrN4SZn. UV−vis (THF) λmax (log ε) = 411
(5.53), 541 (4.25), 577 (4.18).
Zinc(II) 5-(4-(Bromodibenzothien-6-yl))-(2,8,13,17-tetraethyl-
3,7,12,18-tetramethyl)porphyrin (4b). This was synthesized by
using a,c-biladiene dihydrobromide (3b) (1.75 g, 2.9 mmol) and
following the above procedure. The compound was isolated as a red
brick powder (0.60 g, yield 27%). 1H NMR (300.16 MHz, THF-d8): δ
10.17 (s, 2H, meso), 10.07 (s, 1H, meso), 8.70 (d, 1H, J = 8.0 Hz,
thiophene), 8.58 (d, 1H, J = 7.2 Hz, thiophene), 8.49 (d, 1H, J = 7.9
Hz, thiophene), 7.97 (t, 1H, J = 7.6 Hz, thiophene), 7.54 (d, 1H, J =
7.9 Hz, thiophene), 7.45 (t, 1H, J = 8.0 Hz, thiophene), 4.14 (q, 2H, J
= 7.5 Hz, CH2), 4.13 (q, 2H, J = 7.5 Hz, CH2), 3.65 (s, 6H, CH3), 3.52
(s, 6H, CH3), 2.88 (q, 2H, J = 7.4 Hz, CH2), 2.32 (m, 2H, CH2), 1.91
(t, 6H, J = 7.4 Hz, CH3), and 1.04 (t, 6H, J = 7.5 Hz, CH3) ppm. ESI
HRMS m/z: 823.1413 [M
+
Na]+, 823.1419 calcd for
C44H41BrN4SZnNa. UV−vis (THF) λmax (log ε) = 412 (5.53), 541
(4.24), 577 (4.18) nm.
10,20-Bis(4-ethylphenyl)porphyrin (5). Following a published
procedure,10 a solution of 2,2′-dipyrromethane (2.0 g, 14 mmol)
and 4-ethylbenzaldehyde (1.7 mL, 14 mmol) in dichloromethane (1.5
L) was flushed with argon for 5 min. The reaction mixture was
protected from the light and vigorously stirred. Trifluoroacetic acid
(0.72 mL, 9.0 mmol) was added dropwise. After stirring overnight,
DDQ (3.18 g, 14.0 mmol) was added in portions, and the mixture was
stirred for another 1 h. Triethylamine (1 mL) was added, and the
entire reaction mixture was concentrated and filtered through a pad of
alumina. The alumina pad was washed with CH2Cl2 until the eluent
was colorless. Removal of solvent and chromatography over silica gel
(CH2Cl2/petroleum ether 2/3) afforded a purple solid which was
precipitated in a CH2Cl2/MeOH medium (2.1 g, 28%). 1H NMR
(300.16 MHz, CDCl3): δ 10.29 (s, 2H, meso), 9.37 (d, J = 4.6 Hz, 4H,
β), 9.10 (d, J = 4.7 Hz, 4H, β), 8.17 (d, J = 7.9 Hz, 4H, Ar), 7.62 (d, J
= 8.0 Hz, 4H, Ar), 3.02 (q, J = 7.6 Hz, 4H, CH2), 1.54 (t, J = 7.6 Hz,
6H, CH3), and −3.10 (s, 2H, NH) ppm. 13C NMR (75.47 MHz,
CDCl3): δ 147.6 (2C), 145.3 (2C), 143.9 (2C), 138.9 (2C), 135.2
(4C), 131.7 (4C), 131.4 (4C), 126.8 (4C), 105.4, 29.1, and 16.1 ppm.
ESI HRMS m/z: 519.2529 [M + H]+, 519.2543 calcd for C36H31N4.
UV−vis (THF) λmax (log ε) = 405 (5.62), 501 (4.23), 535 (3.80), 576
(3.70), 633 (3.24) nm.
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Inorganic Chemistry
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5-Bromo-10,20-bis(4-ethylphenyl)porphyrin (6). From a standard
procedure,11 a stirred solution of porphyrin 5 (400 mg, 0.77 mmol) in
CHCl3 (250 mL) was cooled in an ice bath for 15 min. NBS (120 mg,
0.70 mmol) was added. After stirring for 10 min the reaction was
quenched with acetone (20 mL), and the crude product was purified
by column chromatography over silica gel (CH2Cl2/petroleum ether
1/5). The solution was concentrated, and the product was precipitated
(s, 2H, NH). ESI HRMS m/z: 1223.4807 [M + H]+, 1223.4856 calcd
for C78H68BN8O2Zn. UV−vis (THF) λmax (log ε) = 418 (5.42), 447
(5.23), 513 (4.49), 559 (4,51), 590 (4.14) 649 (3.47) nm.
Zinc(II) 5-{4-[Zinc(II)(13,17-diethyl-2,3,7,8,12,18-hexamethyl)-
porphyrin-5-yl]-dibenzothien-6-yl}-15-(4,4,5,5-tetramethyl-[1,3,2]-
dioxaborolan-2-yl)-10,20-bis-(4-ethylphenyl)porphyrin (9a). Under
an inert atmosphere, a mixture of 4a (50 mg, 0.064 mmol), 7 (81 mg,
0.1 mmol), cesium carbonate (65 mg, 0.2 mmol), Pd(PPh3)4 (7.4 mg,
0.006 mmol) in 10 mL of distilled toluene, and 5 mL of anhydrous
DMF was stirred at 90 °C for 5 h. The reaction mixture was quenched
with 10 mL of an aqueous NH4Cl (20 mL) solution. The organic layer
was separated, washed with water (2 × 20 mL), and dried over
MgSO4. The solvent was evaporated, and the residue was purified by
column chromatography (silica gel, dichloromethane/petroleum ether
1/2 to 3/2) to afford the product as a red-purple powder (58 mg,
65%). 1H NMR (300.16 MHz, CD2Cl2): δ 9.79 (s, 2H, meso), 9.63 (s,
1H, meso), 9.57 (d, 2H, J = 4.7 Hz, β), 8.88 (d, 2H, J = 7.9 Hz,
thiophene), 8.76 (d, 2H, J = 4.8 Hz, β), 8.74 (d, 2H, J = 4.7 Hz, β),
8.70 (d, 2H, J = 4.6 Hz, β), 8.24 (d, 1H, J = 7.2 Hz, thiophene), 8.06−
7.97 (m, 3H, thiophene), 7.90 (d, 2H, J = 7.6 Hz, C6H4), 7.68 (d, 2H,
J = 7.6 Hz, C6H4), 7.47 (t, 4H, J = 7.6 Hz, C6H4), 3.77 (q, 4H, J = 7.6
Hz, CH2), 3.34 (s, 6H, CH3), 3.31 (s, 6H, CH3), 2.96 (q, 4H, J = 7.6
Hz, CH2), 2.28 (s, 6H, CH3), 1.66 (s, 12H, CH3), 1.60 (t, 6H, J = 7.4
Hz, CH3), and 1.50 (t, 6H, J = 7.4 Hz, CH3) ppm. ESI HRMS m/z:
1420.4328 [M + Na]+, 1420.4338 calcd for C84H75BN8O2SZn2Na.
UV−vis (THF) λmax (log ε) = 410 (5.52), 427 (5.36), 546 (4.26), 577
(4.04), 596 (3.59) nm.
1
as a purple solid upon addition of methanol (0.26 g, 56%). H NMR
(300.16 MHz, CD2Cl2): δ 10.14 (s, 1H, meso), 9.69 (d, J = 5.1 Hz, 2H,
β), 9.26 (d, J = 5.1 Hz, 2H, β), 8.93 (d, J = 4.6 Hz, 4H, β), 8.06 (d, J =
8.0 Hz, 4H, Ar), 7.58 (d, J = 8.0 Hz, 4H, Ar), 2.96 (q, J = 7.6 Hz, 4H,
CH2), 1.49 (t, J = 7.6 Hz, 6H, CH3), and −3.11 (s, 2H, NH) ppm. 13
C
NMR (75.47 MHz, CD2Cl2): δ 143.9 (2C), 138.6 (2C), 134.8 (4C),
132.4 (2C), 132.1, 131.9 (2C), 131.8 (2C), 131.5 (2C), 126.4 (4C),
120.4 (2C), 105.5, 103.5, 28.9 (2C), and 15.8 (2C) ppm. ESI HRMS
m/z: 619.1416 [M + Na]+, 619.1468 calcd for C36H29BrN4Na. UV−vis
(THF) λmax (log ε) = 412 (5.59), 508 (4.20), 542 (3.71), 585 (3.58),
641 (3.54) nm.
Zinc(II) 5,15-Bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
10,20-bis(4-ethyl-phenyl)porphyrin (7). From a standard procedure,10
NBS (0.33 g, 1.90 mmol) was added to an ice bath cooled solution of
porphyrin 5 (0.45 g, 0.86 mmol) in CHCl3 (300 mL). After stirring for
30 min, the reaction was quenched with acetone (20 mL), and the
solution was reduced to a small volume. Methanol was added, and the
precipitate was washed with methanol and dried in vacuo. The
intermediate dibromoporphyrin was suspended in a CHCl3/MeOH
(10/1) mixture, and Zn(OAc2)·2H2O was added. The suspension was
refluxed for 2 h, and the solvent was concentrated in vacuum. Water
was added and the precipitate filtered and washed with water,
methanol, and acetone. The resulting powder was dried to give a
purple solid. The zinc-metalated porphyrin and trans-dichlorobis-
(triphenylphosphine)palladium(II) (38 mg, 0.054 mmol) were placed
in a Schlenk flask, and the flask was pump-filled with argon three times.
Anhydrous 1,2-dichloroethane (80 mL), triethylamine (3.1 mL, 22
mmol), and pinacolborane (2.0 mL, 13.8 mmol) were successively
added, and the resulting mixture was stirred under reflux for 30 min.
The reaction was cooled to room temperature and quenched with 30%
aq. KCl (50 mL). The organic phase was separated, washed with water
(100 mL), and dried over MgSO4. The solvent was removed by rotary
evaporation, and the residue was then purified by column
chromatography (silica gel, CH2Cl2/petroleum ether 4/1) to deliver
the product as a purple powder (0.55 g, 77% yield). 1H NMR (300.16
MHz, CDCl3): δ 9.93 (d, J = 4.7 Hz, 4H, β), 9.13 (d, J = 4.7 Hz, 4H,
β), 8.15 (d, J = 7.9 Hz, 4H, Ar), 7.61 (d, J = 7.9 Hz, 4H, Ar), 3.04 (q, J
= 7.6 Hz, 4H, CH2), 1.86 (s, 24H, CH3), 1.57 (t, J = 7.6 Hz, 6H, CH3)
ppm. 13C NMR (75.47 MHz, CDCl3): δ 153.4 (4C), 150.4 (4C),
143.5, 140.5, 134.8 (4C), 133.0 (4C), 132.7 (4C), 126.2 (4C), 121.0
(2C), 85.4 (2C), 29.1 (2C), 25.6 (4C), and 16.1 (2C) ppm. HR-
Zinc(II) 5-{4-[Zinc(II)(2,8,13,17-tetraethyl-3,7,12,18-tetramethyl)-
porphyrin-5-yl]-dibenzothien-6-yl}-15-(4,4,5,5-tetramethyl-[1,3,2]-
dioxaborolan-2-yl)-10,20-bis-(4-ethylphenyl)porphyrin (9b). Follow-
ing the above procedure, a mixture of 4b (56 mg, 0.070 mmol), 7 (70
mg, 0.084 mmol), cesium carbonate (55 mg, 0.17 mmol), and
Pd(PPh3)4 (7.4 mg, 0.006 mmol) in 15 mL of anhydrous toluene/
DMF solvents (2/1) was stirred at 90 °C for 5 h. The reaction mixture
was quenched with 10 mL of water. After a workup similar as
1
described above, a red-purple solid (59 mg, 59%) was obtained. H
NMR (both isomers, 300.16 MHz, CD2Cl2): δ 9.87 (s, 1H, meso), 9.85
(s, 3H, meso), 9.69 (s, 1H, meso), 9.63 (s, 1H, meso), 9.58 (d, 1H, J =
4.9 Hz, β), 9.57 (d, 3H, J = 4.9 Hz, β), 8.92 (dd, 1H, J = 0.9 Hz, J = 4.4
Hz, thiophene), 8.89−8.87 (m, 3H, thiophene), 8.77−8.66 (m, 8H, β),
8.64 (d, 4H, J = 0.8 Hz, β), 8.30 (dd, 1H, J = 0.8 Hz, J = 7.5 Hz,
thiophene), 8.20 (dd, 1H, J = 1.0 Hz, J = 7.5 Hz, thiophene), 8.18 (dd,
1H, J = 0.8 Hz, J = 7.5 Hz, thiophene), 8.13 (dd, 1H, J = 0.9 Hz, J =
7.5 Hz, thiophene), 8.05−7.97 (m, 4H, thiophene), 7.89 (m, 4H,
C6H4), 7.67 (d, 4H, J = 7.1 Hz, C6H4), 7.47 (m, 8H, C6H4), 3.78 (m,
8H, CH2), 3.41 (s, 3H, CH3), 3.39 (s, 3H, CH3), 3.37 (s, 6H, CH3),
3.36 (s, 6H, CH3), 2.97 (q, 8H, J = 7.8 Hz, CH2), 2.72 (q, 4H, J = 7.8
Hz, CH2), 2.54 (m, 4H, CH2), 2.26 (s, 3H, CH3), 1.67 (s, 24H, CH3),
1.63 (s, 3H, CH3), 1.60 (t, 9H, J = 7.5 Hz, CH3), 1.56−1.48 (m, 18H,
CH3), and 1.02 (t, 9H, J = 7.5 Hz, CH3) ppm. HR-MALDI-TOF m/z:
1426.4745 [M]•+, 1426.4730 calcd for C86H79BN8O2SZn2. UV−vis
(CH2Cl2) λmax (log ε) = 405 (5.66), 422 (5.45), 543 (5.45), 573
(5.29) nm. The presence of two conformational isomers was noted,
VT 1H NMR measurements were performed to demonstrate this, and
molecular modeling was used to find the origin of this effect (SI).
Zinc(II) 5-{4-[Zinc(II)(13,17-diethyl-2,3,7,8,12,18-hexamethyl)-
porphyrin-5-yl]-dibenzothien-6-yl}-[10,20-bis-(4-ethylphenyl)-por-
phyrin-5-yl]-10,20-bis-(4-ethyl-phenyl)porphyrin (10a). Under an
inert atmosphere, a mixture of 9a (35 mg, 0.025 mmol), 6 (22 mg,
0.037 mmol), cesium carbonate (16 mg, 0.050 mmol), and Pd(PPh3)4
(3.4 mg, 0.003 mmol) in 15 mL of anhydrous toluene/DMF solvents
(2/1) was stirred at 90 °C for 14 h. The reaction mixture was
quenched with 10 mL of an aqueous NH4Cl (20 mL) solution. The
organic layer was separated, washed with water (2 × 20 mL), and dried
over MgSO4. Purification by column chromatography (silica gel,
chloroform/petroleum ether 1/1) followed by preparative size
exclusion chromatography (BioRad Bio-Beads SX-1 packed in
CH2Cl2) afforded the product as a brownish purple solid (22 mg,
MALDI-TOF m/z: 832.3388 [M]•+
, 832.3326 calcd for
C48H50B2N4O4Zn. UV−vis (THF) λmax (log ε) = 420 (5.55), 546
(4.13), 581 (3.63) nm.
Zinc(II) 5-[10,20-Bis-(4-ethylphenyl)porphyrin-5-yl]-15-(4,4,5,5-
tetramethyl-[1,3,2]-dioxaborol-an-2-yl)-10,20-(4-ethylphenyl)-
porphyrin (8). Under an inert atmosphere, a mixture of 6 (72 mg, 0.12
mmol), 7 (0.14 g, 0.17 mmol), cesium carbonate (0.11 g, 0.34 mmol),
Pd(PPh3)4 (14 mg, 0.012 mmol) in 10 mL of anhydrous toluene, and
5 mL of DMF was stirred at 90 °C for 5 h. The solvents were removed,
and the residue was redissolved in toluene (50 mL) and stirred with an
aqueous saturated NH4Cl solution (50 mL). The organic layer was
separated and washed with water (2 × 60 mL) and dried over MgSO4.
The solvent was evaporated, and purification by column chromatog-
raphy (silica gel, chloroform/petroleum ether 1/1 to 3/1) afforded the
1
product as a purple solid (90 mg, 62%). H NMR (300.16 MHz,
CD2Cl2): δ 10.37 (s, 1H, meso), 9.98 (d, 2H, J = 4.8 Hz, β), 9.46 (d,
2H, J = 4.6 Hz, β), 9.15 (d, 2H, J = 4.7 Hz, β), 9.10 (d, 2H, J = 4.7 Hz,
β), 8.69 (d, 2H, J = 4.6 Hz, β), 8.64 (d, 2H, J = 4.8 Hz, β), 8.14 (m,
8H, Ar), 8.09 (d, 2H, J = 4.7 Hz, β), 8.02 (d, 2H, J = 4.8 Hz, β), 7.55
(d, 4H, J = 8.1 Hz, C6H4), 7.52 (d, 4H, J = 8.1 Hz, C6H4), 2.92 (q, 4H,
J = 7.4 Hz, CH2), 2.91 (q, 4H, J = 7.4 Hz, CH2), 1.91 (s, 12H, CH3),
1.44 (t, 6H, J = 7.4 Hz, CH3), 1.43 (t, 6H, J = 7.4 Hz, CH3), and −2.42
1
48%). H NMR (300.16 MHz, CD2Cl2): δ 10.29 (s, 1H, meso), 9.93
(s, 2H, meso), 9.87 (s, 1H, meso), 9.40 (d, 1H, J = 4.9 Hz, β), 9.39 (d,
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a
Scheme 1. Synthesis of 10a,b (Ar = 4-C6H4Et)
a
(i) (1) PhLi, (2) DMF, (3) H2O; (ii) p-TSA, (3a,b), (2) Zn(OAc)2; (iii) (1) NBS; (iv) (1) NBS, (2) Zn(OAc)2, (3) pinacolborane, PdCl2(PPh3)2,
NEt3; (v) 6, Pd(PPh3)4, Cs2CO3, toluene/DMF, 90 °C (5 h); (vi) 4a,b, Pd(PPh3)4, Cs2CO3, toluene/DMF, 90 °C (5 h); (vii) 4b, Pd(PPh3)4,
Cs2CO3, toluene/DMF, 90 °C (14 h); (viii) 6, Pd(PPh3)4, Cs2CO3, toluene/DMF, 90 °C (14 h).
1H, J = 4.9 Hz, β), 9.03 (d, 1H, J = 4.6 Hz, β), 9.00 (d, 1H, J = 4.7 Hz,
β), 8.92 (m, 2H, thiophene), 8.82 (d, 2H, J = 4.7 Hz, β), 8.75 (d, 2H, J
= 4.6 Hz, β), 8.52 (d, 1H, J = 4.8 Hz, β), 8.34 (m, 3H, 1 thiophene +
2β), 8.22 (d, 1H, J = 4.8 Hz, β), 8.12−8.01 (m, 5H, 3 thiophene +
2C6H4), 7.95 (d, 2H, J = 8.1 Hz, C6H4), 7.89 (d, 2H, J = 8.2 Hz,
C6H4), 7.84 (d, 1H, J = 4.9 Hz, β), 7.75 (d, 2H, J = 4.7 Hz, β), 7.66 (d,
2H, J = 8.2 Hz, C6H4), 7.48 (d, 2H, J = 8.3 Hz, C6H4), 7.43 (d, 2H, J =
7.6 Hz, C6H4), 7.30 (m, 5H, 1β+C6H4), 3.94 (q, J = 7.7 Hz, 4H, CH2),
3.46 (s, 6H, CH3), 3.36 (s, 6H, CH3), 2.93 (q, J = 7.7 Hz, 2H, CH2),
2.86 (q, J = 7.7 Hz, 6H, CH2), 2.33 (s, 6H, CH3), 1.74 (t, J = 7.6 Hz,
6H, CH3), 1.47 (m, masked by water), 1.40 (t, J = 7.6 Hz, 3H, CH3),
1.39 (t, J = 7.4 Hz, 6H, CH3), and −2.60 (s, 2H, NH) ppm. ESI
HRMS m/z: 1789.5944 (M + H+), 1789.5856 calcd for
C114H93N12SZn2, 1811.5764 [M + Na]+, 1811.5764 calcd for
C114H92N12SNaZn2. UV−vis (THF) λmax (log ε) = 411 (5.65), 432
sh (5,12), 455 (5.17), 513 (4.48), 546 (4.42), 562 (4.53), 577 (4.36),
600 (4.05), 647 (3.49) nm.
solid was obtained starting from 9b (43 mg, 0.030 mmol) and 6 (24
mg, 0.040 mmol). For route B, in a Schlenk tube, 4 mL of anhydrous
toluene was added under argon on a mixture of 4b (30 mg, 0.037
mmol), 8 (35 mg, 0.028 mmol), cesium carbonate (22 mg, 0.067
mmol), and Pd(PPh3)4 (3.3 mg, 0.0030 mmol). A 2 mL portion of
anhydrous DMF was added, and the reaction mixture was stirred at 90
°C for 14 h. The solvents were removed, and the residue was
redissolved in toluene (50 mL) and stirred with an aqueous saturated
NH4Cl solution (50 mL). The organic layer was separated and washed
with water (2 × 60 mL). The solvent was evaporated and purification
by column chromatography (silica gel, chloroform/petroleum ether 1/
1) followed by preparative size exclusion chromatography (BioRad
Bio-Beads SX-1 packed in CH2Cl2) afforded the product (28 mg,
55%). 1H NMR (both isomers, 300.16 MHz, CD2Cl2): δ 10.30 (s, 2H,
meso), 9.95 (s, 1H, meso), 9.93 (s, 2H, meso), 9.92 (s, 1H, meso), 9.86
(s, 1H, meso), 9.83 (s, 1H, meso), 9.46 (m, 1H, β), 9.40 (m, 3H, β),
9.10 (1H, m, β), 9.03 (m, 3H, β), 8.93 (m, 4H, thiophene), 8.76−8.66
(m, 8H, thiophene + β), 8.51 (d, 1H, J = 4.8 Hz, β), 8.50 (d, 1H, J =
4.8 Hz, β), 8.38−8.23 (m, 8H, thiophene + β), 8.08−7.99 (m, 13H,
thiophene + β + C6H4), 7.88 (m, 4H, C6H4), 7.83 (t, 2H, J = 4.8 Hz,
β), 7.75 (d, 3H, J = 5.2 Hz, β), 7.64 (m, 4H, C6H4), 7.53−7.48 (m,
Zinc(II) 5-{4-[Zinc(II)(2,8,13,17-tetraethyl-3,7,12,18-tetramethyl)-
porphyrin-5-yl]-dibenzothien-6-yl}-[10,20-bis-(4-ethylphenyl)-por-
phyrin-5-yl]-10,20-bis-(4-ethyl-phenyl)porphyrin (10b). For route A,
following the above procedure, 23 mg (44%) of a brownish purple
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10H, C6H4), 7.37 (m, 10H, β + C6H4), 3.93 (m, 8H, CH2), 3.47 (s,
3H, CH3), 3.44 (s, 6H, CH3), 3.40 (s, 3H, CH3), 3.39 (s, 6H, CH3),
3.00−2.72 (m, 23H, CH2+ CH3), 2.57 (m, 4H, CH2), 2.30 (s, 3H,
CH3), 1.74 (t, J = 6.9 Hz, 6H, CH3), 1.72 (t, J = 7.3 Hz, 6H, CH3),
1.61−1.44 (m, masked by water, CH3), 1.42−1.36 (m, 21H, CH3),
1.06 (t, J = 7.4 Hz, 12H, CH3), −3.42 (s, 1H, NH), and −2.59 (s, 3H,
NH) ppm. HR-MALDI-TOF m/z: 1816. 6177 [M]•+, 1816.6185 calcd
for C116H96N12SZn2. UV−vis (THF) λmax (log ε) = 412 (5.46), 456
(5.01), 513 (4.36), 566 (4.39), 600 (3.96), 648 (3.49) nm.
Instrumentation. H NMR spectra were recorded on a Bruker
Avance II 300 (300 MHz) or on a Bruker Avance DRX 500 (500
MHz) spectrometer at the “Plateforme d’Analyse Chimique et de
Synthese Moleculaire de l’Universite de Bourgogne (PACSMUB)”;
̀ ́ ́
chemical shifts are expressed in ppm relative to chloroform (7.26
ppm), methylene chloride (5.32 ppm), or THF-d8 (1.73 and 3.58
ppm). Mass spectra and accurate mass measurements (HR-MS) were
obtained on a Bruker Daltonics Ultraflex II spectrometer in the
MALDI/TOF reflectron mode using dithranol as a matrix or on a
Bruker MicroQTof instrument in ESI mode. Both measurements were
nation of 5 at the meso-position using N-bromosuccinimide
(NBS) yields compound 6, meanwhile using 2 equiv of NBS
generates the intermediate dibromoporphyrin.10,11 The latter is
then metalated with Zn(OAc)2·2H2O and converted into the
bisborylated zinc(II)porphyrin 7 with pinacolborane under
Masuda conditions.24 These three successive steps provide 7 in
77% overall yield. This versatile metalloporphyrin is then
suitable for Suzuki reactions and can be used for the synthesis
of DPS-based trimers 10a,b in two pathways depending on the
sequence of the reactants 4a,b versus 6. The metal-catalyzed
stepwise functionalization of porphyrins at meso and β positions
has recently emerged as a promising tool for the direct carbon−
carbon bond formation between porphyrinic units.25 After a
rapid survey of catalysis conditions using Pd(PPh3)4 as the
catalyst, the Suzuki couplings are performed in a toluene−DMF
medium and Cs2CO3 as base and the reagent ratios and
reaction times were optimized.25c,h,26 The cross coupling
between 7 and 4a,b provides the cofacial bisporphyrins 9a,b
in yields of ∼65% similar to that reported by Nocera and co-
workers for DPS-based cofacial bis(porphyrins).27 Dimer 8 was
isolated in 62% yield from the reaction between 7 and 6. The
subsequent Suzuki reaction between dimers 9a,b and 6 yields
triads 10a,b in 44−48% but requires a longer reaction time, and
higher ratio free base/dimer lowers the byproduct formation.
Compound 10b was also alternatively synthesized from the
cross coupling of 8 with 4b. Traces of a fully metalated triad
were also detected with 10b after silica gel chromatography.
This mixture was submitted to size exclusion chromatography,
and a very low amount of pure triad 10b was obtained after
several purification steps. The presence of the fully metalated
trimer makes this route unfavorable. The latter compound was
not investigated.
1
performed at the “Plateforme d’Analyse Chimique et de Synthes
̀
e
Moleculaire de l’Universite de Bourgogne (PACSMUB)”. UV−vis
́
́
absorption spectra were recorded on a Hewlett-Packard, model
8452A, diode array, or on a Varian Cary model 500 or on a Varian
Cary 1 spectrophotometer. The emission spectra were obtained with a
Spex Fluorolog 2. Nanosecond temporal decay profiles were measured
using a PTI TimeMaster Model TM-3 instrument fitted with a
nitrogen laser-pumped dye laser (fwhm ∼1.4 ns). Fluorescence
lifetimes were obtained by iterative reconvolution of the instrument
response functions with trial mono-, bi-, or triexponential functions
and by distribution lifetimes analysis (exponential series method;
ESM), using goodness-of-fit metrics to obtain the best decay
parameters. These measurements were performed several times in
order to have an uncertainty based on the maximum and minimum
obtained values. With high quality data, this technique allows reliable
and reproducible data down to 100 ps. Nanosecond fluorescence
lifetimes at 77 K were obtained with the PTI instrument by placing the
sample in 2-MeTHF (2-methyltetrahydrofuran) in an NMR tube and
flash freezing it in liquid N2.
In the absence of X-ray data, DFT (B3LYP) was used to
address the structures of 10a,b and 9a,b. It is assumed that 10a
and 9a are representative examples. The optimized structure of
10a (Figure 2) exhibits the expected slipped cofacial geometry
for the octaalkyl zinc(II)porphyrin/diaryl zinc(II)porphyrin
Quantum Yields. S1 fluorescence quantum yields were measured
in triplicate in 2-MeTHF at room temperature on samples prepared
under an inert atmosphere (PO2 < 25 ppm) in a glovebox. The
reference fluorophore was the free base tetraphenylporphyrin, H2TPP,
a common standard for porphyrin spectroscopy,12 for which Φf =
0.1112a as different from Φf = 0.048 (in deoxygenated benzene)
reported by Finikova et al. (See refs 40, 41, 48, and 49 of reference
12b.) The sample and standard concentrations were adjusted to obtain
the same absorbance, A ≤ 0.05, at the 550 nm excitation wavelength,
and each absorbance was measured five times for improved precision.
The quantum yields listed represent the total emission yields from the
integrated areas under the whole of the emission spectra; no attempt
was made to try to separate the total emission into separate
contributions from the excited free base and zinc porphyrins.
Computations. The structure optimizations were calculated with
the Gaussian 09 software,13 at the Universite
́
de Sherbrooke’s
́ ́ ́
Mammouth super computer, supported by the Reseau Quebecois de
Calcul de Haute Performance. The DFT14−17 was calculated by the
B3LYP18−20 method with 3-21G* basis sets on all atoms.21−23
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of 10a,b proceeds in several steps
from the dibenzothiophene, which is first brominated in the
known 4,6-dibromodibenzothiophene (1)7 (Scheme 1).
Selective monolithiation of 1 with PhLi in the presence of
DMF, followed by hydrolysis of the resulting imidate salt,
provides the 4-bromo-6-formyldibenzothiophene compound 2
in 78% yield. The acid-catalyzed condensation of 2 with 1 equiv
of a,c-biladienes 3a,b9 followed by metalation gives the zincated
porphyrins 4a,b in modest yields (∼25%). The monobromi-
Figure 2. Views of the optimized geometry (DFT: B3LYP) of 10a
(top) and 9a (bottom).
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unit, blocked by the free base macrocycle. Steric hindrance
prevents the free base from being placed between the two
central Et groups. The dihedral angle formed by the diaryl
zinc(II)porphyrin and free base average planes is 84.2°. This
angle is calculated to be similar to 82.3° for 8. The angle
formed by the atom surrounding the Cmeso−Cmeso bond (i.e.,
C2Cmeso−CmesoC2) is 88.3° resembling that of dimer 8 (89.2°).
The latter feature indicates that electronic communication
between A1 and A2 via conjugation is minimal. The other
Figure 3. Absorption (left) and fluorescence (right) of 4a (black), 9a
(red), and 10a (green) in 2-MeTHF at 298 K. Inset shows expansion
of the Q-region.
...
characteristic features, such as the computed Cmeso
C
meso
separations between D and A1 (6.12 and 6.05 Å), the angles
formed by the average planes of D and A1 (12.9° and 6.9°), and
the angle formed by the average Cmeso···Z···Cmeso axis of D and
A1 (22.4° and 25.5° for 10a and 9a, respectively), indicate a
strong structure similarity making 9a,b appropriate for
comparison with 10a,b.
Table 1. Absorption Data for 4a,b−10a,b in 2-MeTHF
λmax nm (at 298 K)
λmax /nm (at 77 K)
Soret
Soret
region
410
compd region
Q bands
Q bands
532, 572
From a photophysical standpoint, the choice of these systems
is based on several important considerations. First, the
dibenzothiophene bridge (DPS) is selected because of its
long resulting Cmeso···Cmeso separation in the cofacial bis
porphyrin skeleton (6.30 Å) so that the fluorescence lifetime,
τF, of D (octaalkyl zinc(II)porphyrin) in the presence of an
acceptor is still long enough to obtain the maximum precision
possible on the measurements.28 Second, the fluorescence
lifetime in the absence of the acceptor, τF(D)o, is the same for
the bromo-DPS-octaalkyl zinc(II)porphyrin (4a,b, Scheme 1)
as for the DPS-bis(octaalkyl zinc(II)porphyrin).28 This means
that the rates for intersystem crossing, kISC, and internal
conversion, kIC, are not affected when a group is placed the
position of chromophore A1. This is important because this
allows us to use 4a,b as an adequate standard for τF(D)o for
accurate extraction of the rate for energy transfer (kET)
described below. Third, the use of octaalkyl zinc(II)porphyrin,
diaryl zinc(II)porphyrin, and free base diarylporphyrin
promotes only energy transfer and no electron transfer occurs.2
Finally, because of the possible dynamic rotation about the
DPS-(diaryl zinc(II)porphyrin) single bond, this study was also
performed in frozen media at 77 K to ensure that only the
lowest energy conformation is analyzed (see section on the
fluxionality of 9b and 10b). This effect was recently
demonstrated for a series of cofacial dyads similar to 9a,b
where a linear relationship is observed at 77 K between kET and
the gap of the 0−0 peak positions of the absorption or
fluorescence of the donor and acceptor but not at 298 K.29
The indicated roles D, A1, and A2 in Figure 1 for 4a, 9a,b,
and 10a,b are assigned from the position of the 0−0 peaks
observed in the absorption and fluorescence spectra, associated
with the chromophores octaalkyl zinc(II)porphyrin (577 and
∼580 nm, higher energy), diaryl zinc(II)porphyrin (∼600 and
606 nm, intermediate energy), and diarylporphyrin free base,
respectively (648 and 654 nm, lower energy; Figure 3 for 4a,
9a, and 10a at 298 K, see SI for 8, 9b, and 10b at 298 K and all
77 K spectra; Figures S32−S45). Tables 1 and 2 provide the
absorption and fluorescence data.
4a
4b
5
410
412
405
541, 577
541, 577
501, 535, 576, 633
513, 559, 590, 649
546, 577, 596
8
418, 447
410, 427
424, 450 516, 560, 588, 644
9a
412, 430
542, 555, 562,
574, 600
9b
411, 426
411, 455
412, 456
546, 577, 602
414, 430
542, 555, 562,
575, 599
10a
10b
513, 546, 562, 577,
600, 647
412, 430,
458
514, 540, 566,
574, 582
513, 566, 600, 648
414, 429, 514, 542, 562, 583
458
Table 2. Fluorescence Data for 4a,b−10a,b (2-MeTHF,
Uncertainties of the λmax 1 nm)
298K
77 K
λexc
nm
/
λmax/
nm
a
compd
τF/ns
ΦF
λmax/nm
τF/ns
4a
505
580,
634
1.7
0.021
576, 632, 702, 785
1.92
4b
525
581,
633,
696
b
b
b
b
5
8
505
505
505
644,
712
10.1
10.4
0.075
655,
718
0.093 599, 643, 655 sh, 684,
702 sh, 713
2.34
c
9a
584,
607,
657
2.2
0.027
575, 603, 661, 768
0.181,
3.02
9b
505
584,
606,
655
0.27,
2.1
0.031
576, 603, 662, 770
0.195,
3.39
10a
10b
495
495
654,
716
b
b
b
b
576, 615, 644, 655 sh,
683, 705 sh, 713
0.145,
11.6
d
654,
716
574, 618, 643, 655 sh,
683, 703 sh, 713
0.155,
d
11.6
a
The ΦF values were measured against H2TPP (ΦF = 0.11), but a later
report indicated that the value of H2TPP is 0.048 (in deoxygenated
benzene).12b These values should be taken with caution. Not
b
c
measured. A value of 0.25 ns was observed, but the relative intensity is
very weak. A ∼2 ns component is also noted but could not be
accurately evaluated. It is likely due to the zinc(II)porphyrin
d
The excitation spectra monitored all across the fluorescence
bands superpose well the absorption (see Figures S32−S45 in
the SI) meaning that the S1 energy transfers occur efficiently for
all chromophores. As a result of the efficient energy transfers,
the fluorescence intensity of D and A1 is expectedly weak as
clearly illustrated in Figure 3. The kET(S1) values are extracted
chromophore A1.
are compared in Table 3. The decays along with the ESM
analyses for 9a,b and 10a,b are presented in Figures S46−S48
(SI). Because the fluorescence lifetime of A1 could not be
evaluated in this work (see footnote in Table 3), the energy
o
o
from kET = (1/τF) − (1/τF ) where τF and τF are the
fluorescence lifetimes of the donor, respectively, in the presence
and absence of an energy acceptor. The τF , τF, and kET data
2
o
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compares well with that for 13 (Figure 4; 4.6 × 109 s−1).28 The
similarity of the photophysical data between 4a and 4b, and 9a
and 9b indicates that the R group (Me vs Et) has a minimal
effect, if any, on τF and kET(S1). It is noteworthy that τF(A2)
increases only very slightly going from 8 (10.9 0.1 ns) to
10a,b, (both 11.6 0.2 ns) indicating that the rate for internal
conversion, kic, is not increased upon the incorporation of A2 in
the trimers and has no major effect. In other words, kic has no
role on the change in the τF upon the incorporation of the A2
chromophore in the 10a,b trimers.
The kET(S1) values for 10a,b are systematically larger than
that for 9a,b indicating the presence of an extra deactivation
pathway, and again cannot be associated with any additional
nonradiative deactivation processes. Their amplitude (∼6.2 ×
109 s−1) is similar to that recently reported for compound 14
(11 × 109 s−1; Figure 4) using time-resolved femtosecond
transient absorption spectroscopy.30 The larger global kET(S1)
value for 14 over 10a,b is fully consistent with the shorter
D···A1 separation for the former (distance Cmeso···Cmeso = 4.98 Å
for anthracenyl, 6.30 Å for DPS).28 Consequently, D is closer to
o
Table 3. τF and τF Data in 2-MeTHF at 77 K
a
b
compd chromophore
τF/ns
λ/nm
kET(S1) × 10−9/s−1
4a
8
D
1.9 0.1
575
584
640
565
593
575
600
575
600
640
575
605
654
A1
A2
D
10.9 0.1
0.181 0.013
3.02 0.04
0.195 0.025
3.39 0.33
0.145 0.050
c
9a
5.0
4.7
6.4
A1
D
9b
A1
D
10a
A1
A2
D
11.6 0.2
0.155 0.050
c
10b
5.9
A1
A2
11.6 0.2
a
The τF values are measured several times, and the uncertainties are
based on three different measurements using highly accurate ESM
analyses. The data were highly reproducible. The uncertainties are
from the largest and smallest measured values. Monitoring wave-
b
c
length. The decays exhibit components of both D and A2, but a
both A and A . Indeed, Forster’s theory that states k (S ) ∝
̈
component of ∼2 ns was also necessary to obtain the best fits, hence
accurately extracting the short-lived components at low wavelength
and the long-lived one at high wavelengths. This component is most
likely due to A1, but its exact value is unsure.
1
2
ET
1
(1/r6).32
The subtraction of kET(S1) of 9a,b from 10a,b gives kET(S1)
the through space process D*→A2 (Figure 1). The resulting
kET values are 1.4 × 109 and 1.2 × 109 s−1, with 39% and 54%
uncertainties, for 10a and 10b, respectively. Assuming kET(S1)
is still independent of R (R = Me or Et), then kET(S1) is ∼1 ×
109 s−1, which represents ∼15% of the total value found for D
in 10a,b (i.e., ∼6 × 109). This lower value is an expected
consequence of the longer center-to-center D···A2 distance over
transfer process A1*→A2 is not investigated (i.e., the kET(S1)
could not be measured reliably).
o
The τF and τF data (Table 3) were tested for consistencies
by comparing the τFo data for 4a with those previously reported
for 11 and 12 (Figure 4). Indeed, the data compare favorably.
Similarly, kET(S1) for 9a,b (5.0 × 109 and 4.7 × 109 s−1) also
that for D···A1,28 as the Forster theory predicts.31 Moreover,
̈
Figure 4. Structures of compounds 11−15. The arrows indicate the directions of the S1 energy transfer.
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Article
Porphyrins Phthalocyanines 2010, 14, 55−63. (f) Otsuki, J. J Porphyrins
Phthalocyanines 2009, 13, 1069−1081. (g) Boyle, N. M.; Rochford, J.;
Pryce, M. T. Coord. Chem. Rev. 2010, 254, 77−102. (h) Aratani, N.;
Kim, D.; Osuka, A. Acc. Chem. Res. 2009, 42, 1922−1934.
(i) Albinsson, B.; Martensson, J. J. Photochem. Photobiol., C 2008, 9,
138−155.
the relative orientation of the transition moment vectors are
also unfavorable for efficient energy transfer (near the right
angle).32 Despite the structural resemblance between 10a,b and
1531 (Figure 4), the kET values differ greatly (∼6 × 109 vs 200
× 109 s−1). A previously proposed and reasonable explanation
for this is the presence of a large MO mixing occurring in 15
notably between D and A2 due to very close proximity.33
Although the A1*→A2 energy transfer process could not be
reliably investigated in this work, ultimately the A2 acceptor is
the final site prior to most of the observed fluorescence with
only a very weak signal associated with A1 (green trace in
Figure 3; right). Because the kET(S1) is the same for 9a,b and
13, the kET(S1) for this A1*→A2 process in 10a,b must be larger
than that for the D*→A1 process as it is not the limiting factor.
To the best of our knowledge, there is no other work that
reports a donor−acceptor system, here D and A2 (i.e., not
identical chromophores), linked by conjugated bonds only,
normally favored through bond energy transfers, but occurring
exclusively via through space energy transfer.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: roger.guilard@u-bourgogne.fr (R.G.); Pierre.Harvey@
USherbrooke.ca (P.D.H.). Phone: 33 380 39 61 11 (R.G.);
001-819-821-7092 (P.D.H.). Fax: 33 380 39 61 17 (R.G.); 001-
819-821-8017 (P.D.H.).
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Notes
The authors declare no competing financial interest.
§On leave from the Chemistry Department, Assiut University,
Assiut, Egypt.
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ACKNOWLEDGMENTS
P.D.H. thanks the Natural Sciences and Engineering Research
Council of Canada, NSERC, and the Fonds Queb
Recherche sur la Nature et les Technologies, FQRNT, and the
Agence Nationale de la Recherche, ANR, for the grant of a
Research Chair of Excellence for P.D.H.. R.G. thanks the CNRS
(Centre National de la Recherche Scientifique) and the
■
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