Very fast single-step photoinduced charge separation in zinc porphyrin bridged to a gold porphyrin by a bisethynyl quaterthiophene

Article abstract of DOI:10.1021/ic800727e

A new heterometallic dyad composed of a zinc porphyrin linked by bisethynyl quaterthiophene to a gold porphyrin was synthesized according to a stepwise modular approach. The latter dyad and the parent reference compounds (porphyrin-ethynylquaterthiophene)

Full text of DOI:10.1021/ic800727e

Inorg. Chem. 2009, 48, 518-526
Very Fast Single-Step Photoinduced Charge Separation in Zinc
Porphyrin Bridged to a Gold Porphyrin by a Bisethynyl Quaterthiophene
Je´roˆme Fortage,† Julien Boixel,† Errol Blart,† Hans Christian Becker,*,‡ and Fabrice Odobel*,†
UniVersite´ de Nantes, CEISAM, Chimie Et Interdisciplinarite´, Synthe`se, Analyse, Mode´lisation,
Faculte´ des Sciences et des Techniques, 2, rue de la Houssinie`re, BP 92208, 44322 NANTES
Cedex 3, France, and CNRS, UMR 6230, Department of Photochemistry and Molecular Science,
The Ångstro¨m Laboratories, Uppsala UniVersity, RegementsVa¨gen 1, 752 37 Uppsala, Sweden
Received April 23, 2008
A new heterometallic dyad composed of a zinc porphyrin linked by bisethynyl quaterthiophene to a gold porphyrin
was synthesized according to a stepwise modular approach. The latter dyad and the parent reference compounds
(porphyrin-ethynylquaterthiophene) were characterized by electrochemistry, spectroelectrochemistry, and femtosecond
transient absorption spectrocopy. We showed that light excitation of the zinc or the gold porphyrin induces a very
fast and quantitative charge separation over a distance of 25 Å which occurs through a superexchange mechanism.
The lifetime of the charge-separated state is 3.3 ns in toluene and 100 ps in dichloromethane, and it recombines
to the ground state in both solvents.
Introduction
spacer directly on a meso position of a porphyrin via an
ethynyl moiety represents an effective strategy to make
effective long-range photoinduced processes.4,6-8 This strat-
egy contrasts with the usual approach of connecting the
spacer on a meso aryl substituent of the porphyrin.3 The large
dihedral angle between the porphyrin macrocycle and the
meso aryl group prevents efficient mixing of the electronic
wave functions of the linked elements and thereby limits the
electronic coupling, whereas the ethynyl linker increases the
distance between the porphyrin and the spacer and permits
an extensive π-conjugation and therefore electronic com-
munication.4,6,8,9 In the donor-spacer-acceptor model
systems, it has been recognized theoretically and then
experimentally that the spacer controls the rate of the forward
and backward charge separation between the donor and the
Linear multichromophoric arrays for photoinduced charge
separation represent extremely important targets, owing to
their potential applications for solar energy conversion and
optoelectronics.1-3 We have been particularly interested in
the design of electronically coupled arrays to achieve long-
range charge separations in a single step with porphyrin
derivatives.4-7 To reach this goal, the attachment of the
* Authors to whom correspondence should be addressed. Tel.: +46-18-
471 3648(H.C.B.), +33 2 51 12 54 29(F.O.). Fax: +46-18-471 6844
(H.C.B.), +33 2 51 12 54 02 (F.O.). E-mail: hcb@fotomol.uu.se (H.C.B.),
Fabrice.Odobel@univ-nantes.fr (F.O.).
†
Universite´ de Nantes.
Uppsala University.
‡
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Rodgers, M. A. J.; Mattay, J.; Astruc, D.; Gray, H. B.; Winkler, J.;
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Eds.; Wiley-VCH: Weinheim, 2001. (b) Supramolecular Photochem-
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518 Inorganic Chemistry, Vol. 48, No. 2, 2009
10.1021/ic800727e CCC: $40.75  2009 American Chemical Society
Published on Web 12/12/2008

Photoinduced Charge Separation in Zinc Porphyrin
acceptor.1,10,11 Wasielewski and co-workers12 as well as
Guldi and co-workers13 have recently developed dyads,
linked by a long π-conjugated spacer, capable of efficient
electron transfer (ET) over very long distances. Oligoth-
iophenes represent an attractive class of π-conjugated
molecules that can be used as spacers, or as the photoactive
unit in molecular devices for electron transfer, especially for
photovoltaic applications.14 We have previously shown that
bisethynyl quaterthiophene was suitable to efficiently mediate
energy transfer between a zinc and a free base porphyrin.9
The linking of a zinc porphyrin to a gold porphyrin is
particularly amenable to photoinduced charge separation, as
already shown by Sauvage and co-workers,15,16 Albinsson
and co-workers,17 and Fukuzumi and co-workers.18,19
We now report the stepwise preparation and the photo-
physical properties of a new heterometallic bisporphyrin
system composed of a zinc porphyrin as the electron donor
and a gold porphyrin as the electron acceptor. This dyad
demonstrates the suitability of the bisethynyl quaterthiophene
spacer to promote very fast single-step charge separation
upon excitation of either porphyrin component.
Experimental Section
Materials and Methods. 1H and 13C NMR spectra were recorded
on a Bruker ARX 300 MHz or a Bruker AMX 400 MHz
1
spectrometer. Chemical shifts for H NMR spectra are referenced
relative to residual protium in the deuterated solvent (CDCl3 δ )
7.26 ppm). High-resolution electro-spray mass spectra were col-
lected in the positive mode on a Micromass MS/MS ZABSpec TOF
apparatus equipped with a geometry EBE TOF. The samples were
injected into a mixture of 9:1 CHCl3/MeOH. Electron-impact mass
spectra were recorded on a HP 5989A spectrometer. Matrix-assisted
laser desorption mass spectrometry analyses were performed on
Bruker biflex III MALDI-TOF spectrometer using cyano 4-hy-
droxycinnamic acid (CHCA) or dithranol as the matrix. The
instrument is equipped with a nitrogen laser emitting at 337 nm, a
2-GHz sampling rate digitizer, a pulsed ion extraction source, and
a reflectron.
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Schuster, D. I. Chem.sEur. J. 2005, 11, 3375. (c) De la Torre, G.;
Giacalone, F.; Segura, J. L.; Martin, N.; Guldi, D. M. Chem.sEur. J.
2005, 11, 1267. (d) Giacalone, F.; Segura, J. L.; Martin, N.; Guldi,
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Filippone, S.; Martin, N. Chem. Commun. 2007, 5164.
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T.; Aso, Y. J. Phys. Chem. A 2006, 110, 3471. (b) Otsubo, T.; Aso,
Y.; Takimiya, K. Pure Appl. Chem. 2005, 77, 2003. (c) Nakamura,
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Takimiya, K.; Otsubo, T.; Aso, Y.; Nakamura, T.; Araki, Y.; Ito, O.
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Thin-layer chromatography (TLC) was performed on aluminum
sheets precoated with Merck 5735 Kieselgel 60F254. Column
chromatography was carried out either with Merck 5735 Kieselgel
60F (0.040-0.063 mm mesh) or with SDS neutral alumina
(0.05-0.2 mm mesh). Air-sensitive reactions were carried out under
argon in dry solvents and glassware. Chemicals were purchased
from Aldrich or Acros and used as received. The solvents for the
reactions were distillated prior to use following the standard
purification procedures. The compounds 3,3′′′-dioctyl-[2,2′;5′,2′′;
5′′,2′′′] quaterthiophene (1),20 zinc(II)-5,15-bis-(3,5-diterbutyl phen-
yl)-10-phenyl-20-iodoporphyrin (7),4 and the mixture of gold(III)-
5,15-bis-(3,5-diterbutylphenyl)-10-phenyl-20-bromoporphyrin and
gold(III)-5,15-bis-(3,5-diterbutylphenyl)-10-phenyl-20-chloropor-
phyrin (8)4 were prepared according to literature methods.
Electrochemistry and Spectroelectrochemistry. The electro-
chemical measurements were performed with a potentiostat-
galvanostat MacLab model ML160 controlled by resident software
(Echem v1.5.2 for Windows) using a conventional single-compart-
ment three-electrode cell. The working electrode was a platinum
disk of 4 mm2 area. The auxiliary electrode was a Pt wire, and the
reference electrode was a saturated potassium chloride calomel
electrode (SCE). The supporting electrolyte was 0.15 M Bu4NPF6
in dichloromethane, and the solutions were purged with argon before
the measurements. All potentials are quoted relative to SCE. In all
of the experiments, the scan rate was 100 mV/s for the cyclic
voltammetry and 15 Hz for pulse voltammetry. Spectroelectro-
chemical spectra were recorded in a thin quartz UV-visible cell
(0.5 mm) equipped with a reference electrode (SCE) and a platinum
wire as the counter electrode. The working electrode was a platinum
grid inserted within the quartz cell, and the other conditions were
the same as those for the cyclic voltammetry.
Spectroscopy. UV-visible absorption spectra were recorded on
a UV-2401PC Shimadzu spectrophotometer. Fluorescence spectra
(19) Ohkubo, K.; Sintic, P. J.; Tkachenko, N. V.; Lemmetyinen, H. E. W.;
Ou, Z.; Shao, J.; Kadish, K. M.; Crossley, M. J.; Fukuzumi, S. Chem.
Phys. 2006, 326, 3.
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63, 8632.
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Hutchison, J. A.; Ghiggino, K. P.; Sintic, P. J.; Crossley, M. J. J. Am.
Chem. Soc. 2003, 125, 14984.
Inorganic Chemistry, Vol. 48, No. 2, 2009 519

Fortage et al.
were recorded on a SPEX Fluoromax fluorimeter and were correct-
ed for the wavelength-dependent response of the detector system
(Hamamatsu R928).
Time-correlated single-photon counting (fwhm ≈ 80 ps) was
performed with a Hamamatsu MCP as the detector. Excitation was
done with 400 nm, 120 fs pulses from a Coherent RegA Ti:sapphire
amplifier.
product was purified by column chromatography over flash silica
gel (petroleum ether) to afford 3 as a yellow oil (114 mg, 91%).
1H NMR (300 MHz, CDCl3, 25 °C): δ 0.87 (m, 6H), 1.12 (s, 21H),
1.27 (m, 20H), 1.63 (m, 4H), 2.69-2.80 (m, 4H), 6.92 (d, 3J ) 5.1
3
Hz, 1H), 7.01 (d, J ) 3.6 Hz, 2H), 7.06 (s, 1H), 7.10-7.12 (dd,
3
3
3J ) 3.6 Hz, J ) 3.6 Hz, 2H), 7.17 (d, J ) 5.1 Hz, 1H). 13C
NMR (75 MHz, CDCl3, 25 °C): δ 11.3, 14.1, 18.7, 22.7, 29.2, 29.4,
30.5, 30.7, 31.9, 99.29, 123.9, 126.5, 126.8, 130.1, 131.9, 135.3,
136.5, 139.4, 139.9. MS (EI), m/z calcd for C43H62S4Si: 734.35.
Found: 734 (100) [M]+, 649 (21), 554 (23), 318 (99), 268 (38),
212 (34), 45 (93).
Transient absorption was performed using a previously described
system.21 Briefly, the output (80 fs, 800 nm) from a Coherent
Legend-HE amplifier was used to pump an optical parametric
amplifier (TOPAS, Light Conversion) to produce 620 nm light (for
ZnP excitation) or 690 nm light (for AuP excitation). Pump
intensities were kept below 500 nJ/pulse to avoid nonlinear effects
and to minimize sample degradation. Probe light, generated on a
CaF2 plate from a fraction of the 800 nm output, was split into
signal and reference beams and detected in a 512-pixel-wide diode
array (Pascher Instruments, Lund, Sweden). Data were averaged
with a total of 5000-10 000 measurements per time point. The
angle between the polarization of the pump and probe was kept at
55°. The sample cell was continously moved up and down to avoid
photobleaching and accumulation of the photoproducts. An analysis
of transient absorption data was done using the Igor Pro software
package (Wavemetrics Inc., Lake Oswego, OR) by iterative
reconvolution with a Gaussian response function. The time resolu-
tion was, in all cases, better than 300 fs, and the fits were robust
with respect to varying initial guesses.
5-(Triisopropylsilyl)ethynyl-5′′′-iodo-3,3′′′-dioctyl-[2,2′;5′,2′′;
5′′,2′′′]quaterthiophene, 4. Triisopropylsilyl)ethynyl quaterthio-
phene 3 (220 mg, 0.3 mmol), I2 (84 mg, 0.32 mmol), and
bis(trifluoroacetoxy)iodobenzene (168 mg, 0.39 mmol) were mixed
in CHCl3 (35 mL) and stirred at room temperature overnight. The
reaction mixture was quenched with water, diluted with dichlo-
romethane, and washed with water. The organic phase was dried
over MgSO4, and the solvents were rotary evaporated. The residue
was purified by column chromatography over flash silica gel
1
(petroleum ether) to afford 4 as a yellow oil (171 mg, 89%). H
NMR (300 MHz, CDCl3, 25 °C): δ 0.87 (m, 6H), 1.12 (s, 21H),
1.27 (m, 20H), 1.58-1.65 (m, 4H), 2.69-2.74 (m, 4H), 6.95 (d, 3J
3
) 3.6 Hz, 1H), 7.01 (d, J ) 3.6 Hz, 1H), 7.06 (s, 1H), 7.07 (s,
1H), 7.10-7.11 (dd, 3J ) 3.6 Hz, 3J ) 3.6 Hz, 2H). 13C NMR (75
MHz, CDCl3, 25 °C): δ 11.3, 14.1, 18.7, 22.7, 28.9, 29.2, 29.4,
30.4, 30.6, 31.9, 71.9, 96.6, 99.3, 121.4, 124.0, 124.1, 126.8, 127.0,
131.8, 134.0, 134.6, 135.4, 136.2, 136.9, 137.1, 139.5, 139.8, 141.7.
MS (EI), m/z calcd for C43H61IS4Si: 861.2. Found: 861 (33) [M]+,
735 (25), 380 (100), 59 (50), 41 (69).
Experimental Procedures for the Synthesis of the Com-
pounds. 5-Bromo-3,3′′′-dioctyl-[2,2′;5′,2′′;5′′,2′′′]quaterthiophene
(2) and 3,3′′′-dioctyl-[2,2′;5′,2′′;5′′,2′′′]quaterthiophene (1; 240 mg,
0.4 mmol) were dissolved in degassed CS2 (20 mL) in a three-
necked flask. The mixture was cooled to -20 °C, and a solution of
NBS (61 mg, 0.34 mmol dissolved in 10 mL of CHCl3) was added
dropwise under an argon atmosphere for 1 h. The mixture was
stirred at this temperature for half an hour and then brought to 0
°C and stirred for another 30 min.
5-(Triisopropylsilyl)ethynyl-5′′′-(trimethylsilyl)ethynyl-3,3′′′-
dioctyl-[2,2′;5′,2′′;5′′,2′′′]quaterthiophene, 5. Iodo quaterthiophene
4 (171 mg, 0.2 mmol), trimethylsilylacetylene (85 µL, 0.6 mmol),
Pd2dba3 ·CHCl3 (21 mg, 0.02 mmol), PPh3 (42 mg, 0.16 mmol),
CuI (17 mg, 0.09 mmol), and diisopropylamine (84 µL) were
dissolved in triethylamine (25 mL). The solution was purged with
argon through three freeze-pump-thaw cycles and was heated at
70 °C overnight. The reaction mixture was quenched with water,
extracted with dichloromethane, and washed with water. The
organic phase was dried over MgSO4, and the solvents were
removed. The product was purified by column chromatography over
silica gel (petroleum ether), to yield 5 as a yellow fluorescent oil
The mixture was diluted with CH2Cl2, and the organic phase
was washed with an aqueous saturated solution of ammonium
chloride, then with water. The organic phase was dried over MgSO4,
and the solvents were evaporated to dryness. The residue was
purified by column chromatography over flash silica (petroleum
1
ether) to afford 2 as a yellow oil (164 mg, 65%). H NMR (300
MHz, CDCl3, 25 °C): δ 0.87 (m, 6H), 1.31 (m, 20H), 1.58 (m,
1
3
3
(129 mg, 79%). H NMR (300 MHz, CDCl3, 25 °C): δ 0.24 (s,
4H), 2.71 (t, J ) 7.8 Hz, 2H), 2.77 (t, J ) 7.8 Hz, 2H), 6.90 (s,
3
3
3
9H, TMS), 0.87 (m, 6H), 1.12 (s, 21H), 1.27 (m, 20H), 1.62 (m,
4H), 2.69-2.74 (m, 4H), 7.02-7.03 (m, 2H), 7.06 (s, 1H), 7.07
(s, 1H), 7.11-7.13 (m, 2H). 13C NMR (75 MHz, CDCl3, 25 °C):
δ 0.0, 11.4, 14.2, 18.8, 22.8, 29.4, 29.5, 30.59, 32.02, 121.0, 121.6,
124.2, 127.1, 134.8, 135.5, 135.8, 137.0, 139.6. UV-vis (CH2Cl2):
λ/ε (nm/×104 M-1 cm-1): 256 (0.86), 289 (0.58), 404 (2.06).
MALDI-TOF, m/z calcd for C48H70S4Si2: 830.39 [M]+. Found:
830.62 [M]+.
1H), 6.94 (d, J ) 5.2 Hz), 6.96 (d, J ) 4.2 Hz, 1H), 7.02 (d, J
3
3
) 3.8 Hz, 1H), 7.11 (d, J ) 4.2 Hz), 7.12 (d, J ) 3.8 Hz), 7.18
(d, 3J ) 5.2 Hz, 1H). 13C NMR (75 MHz, CDCl3, 25 °C): δ 14.1,
22.7, 29.3, 29.4, 29.5, 30.5, 30.7, 31.9, 110.6, 123.8, 123.9, 124.0,
126.5, 127.0, 127.8, 130.1, 131.8, 132.7, 133.8, 135.6, 136.5, 137.4,
140.0, 140.5. MS (EI), m/z calcd for C32H41BrS4: 633.8. Found:
634 (83) [M]+, 554 (55), 368 (100).
5-(Triisopropylsilyl)ethynyl-3,3′′′-dioctyl-[2,2′;5′,2′′;5′′,2′′′]qua-
terthiophene, 3. Bromo quaterthiophene 2 (110 mg, 0.17 mmol),
triisopropylsilylacetylene (0.23 mL, 1.0 mmol), Pd2dba3 ·CHCl3 (5
mg, 0.0048 mmol), PPh3 (10 mg, 0.038 mmol), CuI (4 mg, 0.021
mmol), and diisopropylamine (0.07 mL) were dissolved in triethy-
lamine (15 mL). The solution was purged with argon through three
freeze-pump-thaw cycles and was heated at 70 °C overnight. The
reaction mixture was quenched with water, and dichloromethane
was added. The resulting organic phase was washed with water
and dried over MgSO4. The solvents were removed, and the crude
5-(Triisopropylsilyl)ethynyl-5′′′-ethynyl-3,3′′′-dioctyl-[2,2′;5′,
2′′;5′′,2′′′]quaterthiophene, 6. Quaterthiophene 5 (127 mg, 0.15
mmol) and K2CO3 (211 mg, 1.53 mmol) were dissolved in
dichloromethane (2 mL) and methanol (4 mL). The solution was
stirred at room temperature for 3 h. Water was added, and the
mixture was extracted with dichloromethane. The organic phase
was dried over MgSO4, and the solvents was removed under a
1
vacuum, affording 6 as a yellow oil (116 mg, 100%). H NMR
(300 MHz, CDCl3, 25 °C): δ 0.88 (m, 6H), 1.14 (s, 21H), 1.28 (m,
20H), 1.59-1.66 (m, 4H), 2.70-2.75 (m, 4H), 3.39 (s, 1H),
7.02-7.04 (m, 2H), 7.07 (s, 1H), 7.11 (s, 1H), 7.11-7.13 (m, 2H).
13C NMR (300 MHz, CDCl3, 25 °C): δ 10.3, 13.1, 17.6, 21.6, 28.2,
(21) Chaignon, F.; Torroba, J.; Blart, E.; Borgstro¨m, M.; Hammarstro¨m,
L.; Odobel, F. New J. Chem. 2005, 29, 1272.
520 Inorganic Chemistry, Vol. 48, No. 2, 2009

Photoinduced Charge Separation in Zinc Porphyrin
Scheme 1. Synthesis of the Unsymmetrical Quaterthiophene Spacer 6a
28.4, 29.4, 30.9, 81.1, 95.6, 98.2, 118.7, 120.4, 123.0, 123.1, 125.8,
126.0, 130.8, 131.4, 133.2, 133.6, 134.3, 135.0, 135.8, 136.2, 138.5.
MALDI-TOF, m/z calcd for C45H62S4Si: 758.35 [M]+. Found:
758.64 [M]+.
Compound 9. Quaterthiophene 6 (85 mg, 0.11 mmol), the zinc
iodoporphyrin 7 (99 mg, 0.10 mmol), Pd2dba3 ·CHCl3 (24 mg, 0.023
mmol), and AsPh3 (29 mg, 0.093 mmol) were dissolved in dry Et3N
(6 mL) and dry toluene (25 mL). The solution was purged with
argon through three freeze-pump-thaw cycles and was heated at
60 °C for 17 h. The solvents were removed, and the product was
purified by column chromatography over flash silica (elution with
an 8:2 mixture of petroleum ether/CH2Cl2) to afford 9 as a green
1
solid (50 mg, 30%). H NMR (300 MHz, CDCl3, 25 °C): δ 0.92
(m, 6H), 1.17 (m, 21H), 1.33 (m, 20H), 1.58 (m, 36H), 1.6-1.8
(m, 4H), 2.71-2.77 (m, 4H), 7.03-7.04 (m, 2H), 7.11-7.13 (m,
2H), 7.20-7.23 (m, 2H), 7.29-7.33 (m, 2H), 7.73-7.77 (m, 3H),
7.85 (m, 2H), 8.13 (m, 4H), 8.18-8.21 (m, 2H), 8.87-8.95 (m,
4H), 9.08-9.10 (m, 2H), 9.77-9.79 (m, 2H). UV-vis (CH2Cl2),
λ/ε (nm/×104 M-1 cm-1): 314 (2.23), 379 (3.23), 440 (16.4), 449
(17.8), 560 (1.33), 630 (3.93). MALDI-TOF, m/z calcd for
C99H116N4S4SiZn: 1582.7 [M]+. Found: 1582.8 [M]+.
Compound 10. The compound 9 (40 mg, 0.025 mmol) was
dissolved in THF (2.2 mL), and Bu4NF (60 µL, 1 M in THF) was
added. The solution was stirred at room temperature for 1.5 h. The
THF was removed, and CH2Cl2 was added. The organic phase was
washed with water, dried over MgSO4, and evaporated to dryness
to afford 10 as a green solid (36 mg, 100%). 1H NMR (300 MHz,
CDCl3, 25 °C): δ 0.90 (m, 6H), 1.25 (m, 20 H), 1.55 (m, 36H),
1.62-1.70 (m, 2H), 1.77-1.82 (m, 2H), 2.72-2.76 (m, 2H),
2.85-2.90 (m, 2H), 3.40 (s, 1H), 6.98 (s, 1H), 7.06 (m, 1H), 7.12
(s, 1H), 7.15-7.20 (m, 2H), 7.49 (s, 1H), 7.71-7.73 (m, 3H), 7.80
(m, 2H), 8.08 (m, 4H), 8.17-8.19 (m, 2H), 8.83-8.87 (m, 4H),
9.03-9.04 (m, 2H), 9.74 (m, 2H). MALDI-TOF, m/z calcd for
C90H96S4Zn: 1426.58 [M]+. Found: 1426.40 [M]+.
a
Reagents and conditions: (a) NBS, CS2, -20°C (65%). (b) HC2TIPS,
CuI, Pd2dba3-CHCl3, PPh3, iPr2NH, Et3N (91%). (c) (CF3CO2)2PhI, I2, CCl4,
RT, (89%). (d) HC2TMS, CuI, Pd2dba3-CHCl3, PPh3, iPr2NH, Et3N, 70°C
(82%). (e) K2CO3, CH2Cl2, MeOH, RT (100%).
(m, 2H), 1.79-1.82 (m, 2H), 2.52 (m, 2H), 3.12 (m, 2H), 6.98 (s,
1H), 7.10 (m, 1H), 7.12 (s, 1H), 7.15-7.20 (m, 2H), 7.49 (s, 1H),
7.60-7.80 (m, 3H), 7.80-8.00 (m, 5H, 2H), 8.03 (m, 2H), 8.05
(m, 4H), 8.19 (m, 4H), 8.37 (m, 2H), 8.48 (m, 2H), 8.70-8.9 (m,
8H), 9.04 (m, 2H), 9.42 (m, 4H), 10.03 (m, 2H). UV-vis (CH2Cl2),
λ/ε (nm/×104 M-1 cm-1): 305 (2.87), 427 (17.1), 1.59 (447), 574
(1.74), 630 (4.31). MALDI-TOF, m/z calcd for C144H150AuN8S4Zn:
2379.98 [M]+. Found: 2380.00 [M]+.
Results
Synthesis. The preparation of dyad 11 calls for the
synthesis of the bisethynyl quaterthiophene spacer 6, which
is illustrated in Scheme 1. The known dioctyl quater-
thiophene22 was initially monobrominated with NBS in a
cold mixture of CS2/DMF in 65% yield.23 The tris(isopro-
pyl)silylacetylene group was first introduced according to a
Sonogashira cross-coupling reaction to give 3. It was
subsequently iodinated with a mixture of iodine/bis(trifluo-
roacetoxy)iodobenzene in 89% yield.24 Finally, 4 was sub-
jected to a second Sonogashira cross-coupling reaction,
affording 5 with an 82% yield. The trimethylsilyl protecting
group was then cleaved with potassium carbonate in a
quantitative yield.
The synthesis of dyad 11 is illustrated in Scheme 2.
Preparation of the porphyrin building blocks 7 and 8 has
been reported elsewhere.4 Zinc iodo porphyrin 7 was
assembled with spacer 6 using a copper-free Sonogashira
cross-coupling reaction reported by Lindsey and co-work-
ers.25 The deprotection of the triisopropylsilyl group was
carried out with tetrabutylammonium fluoride, and the
resulting acetylenic derivatives were subjected to the final
Sonogashira cross-coupling reaction with the mixture of
halogeno gold porphyrin 8. As described previously,4 we
found that the Pd(dppf)Cl2 catalyst (dppf ) diphenylphos-
Compound 12. The quaterthiophene 6 (38 mg, 0.05 mmol), the
equimolar mixture of gold bromo and chloro porphyrins 8 (39 mg,
0.033 mmol), Pd(dppf)Cl2 (5 mg, 0.007 mmol), and CuI (1 mg,
0.007 mmol) were dissolved in dry Et3N (0.5 mL) and dry DMF
(3.1 mL). The solution was purged with argon through three
freeze-pump-thaw cycles and was heated at 45 °C for 20 h.
CH2Cl2 was added, and the organic phase was washed with water
and dried over MgSO4. The solvents were removed, and the residue
was purified by column chromatography over flash silica (CH2Cl2)
1
to afford 12 as a green solid (17 mg, 28%). H NMR (300 MHz,
CDCl3, 25 °C): δ 0.89 (m, 6H), 1.14 (m, 21H), 1.25 (m, 20H),
1.56 (m, 36H), 1.7-1.8 (m, 4H), 2.68-2.73 (m, 4H), 7.001-7.024
(m, 1H), 7.07 (s, 1H), 7.12 -7.14 (m, 1H), 7.14 -7.19 (m, 2H),
7.67 (s, 1H), 7.83-7.88 (m, 3H), 7.94 (m, 2H), 8.09 (m, 4H),
8.19-8.21 (m, 2H), 9.18-9.24 (m, 4H), 9.38-9.40 (m, 2H),
10.06-10.08 (m, 2H). UV-vis (CH2Cl2), λ/ε (nm/×104 M-1 cm-1):
422 (4.77), 524 (1.44), 624 (1.44). MALDI-TOF, m/z calcd for
C99H116AuN4S4Si: 1713.75 [M]+. Found: 1713.20 [M]+.
Dyad 11. Compound 10 (36 mg, 0.025 mmol), the equimolar
mixture of gold bromo and chloro porphyrins 8 (20 mg, 0.017
mmol), Pd(dppf)Cl2 (2.5 mg, 0.003 mmol), and CuI (1 mg,
0.003 mmol) were dissolved in dry Et3N (0.75 mL) and dry DMF
(3.2 mL). The solution was purged with argon through three freeze-
pump-thaw cycles and was heated at 45 °C for 20 h. CH2Cl2 was
added, and the organic phase was washed with water and dried
over MgSO4. The solvents were evaporated, and the residue was
purified over preparative TLC (98:2 CH2Cl2/MeOH) to afford the
dyad 11 as a green solid (21 mg, 50%). 1H NMR (300 MHz, CDCl3,
25 °C): δ 0.90 (m, 6H), 1.25 (m, 20 H), 1.55 (m, 72H), 1.62-1.71
(22) Nakanishi, H.; Sumi, N.; Aso, Y.; Otsubo, T. J. Org. Chem. 1998,
63, 8632.
(23) Ba¨uerle, P.; Wu¨rthner, F.; Go¨tz, G.; Effenberger, F. Synthesis 1993,
1993, 1099.
(24) D’Auria, M.; Mauriello, G. Tetrahedron Lett. 1995, 36, 4883.
(25) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem.
1995, 60, 5266.
Inorganic Chemistry, Vol. 48, No. 2, 2009 521

Fortage et al.
Scheme 2. Preparation of Dyad 11a
Figure 2. UV-vis absorption spectra of 11 along with those of the
reference porphyrins 9 and 12. The spectra were recorded in dichlo-
romethane.
Table 1. UV-Vis Absorption Data of the Compounds Recorded in
Dichloromethanea
a
Reagents and conditions: (a) AsPh3, Pd2(dba)3 ·CHCl3, toluene, Et3N,
compound
λ )
max/ε (nm/M-1 cm-1
60°C (30%). (b) Bu4NF, THF, r.t. (100%). (c) 8, CuI, Pd(dppf)Cl2, Et3N,
DMF, 40-60°C (50%).
5
9
256 (8.6 × 103); 289 (5.8 × 103); 404 (2.06 × 104)
314 (2.23 × 104); 379 (3.23 × 104; 440 (1.64 × 105);
449 (1.78 × 105); 560 (1.33 × 104); 630 (3.93 × 104)
422 (4.77 × 104); 524 (1.44 × 104); 624 (1.44 × 104)
305 (2.87 × 04); 427 (1.71 × 105); 447 (1.59 × 105);
574 (1.74 × 04); 630 (4.31 × 104)
Scheme 3. Photophysical Behavior of Dyad 11
12
11
AuTdTBuP+ 416 (2.57 × 105); 524 (1.26 × 104); 564 (shoulder)
ZnTdTBuP
423 (3.13 × 105); 550 (1.32 × 104); 590 (4.3 × 103)
a ZnTdTBuP: tetra(3,5-diterbutylphenyl) zinc(II). AuTdTBuP+: tetra(3,5-
diterbutylphenyl) porphyrin gold(III).
Table 2. Excited State Energies Determined from 0-0 Absorption and
3
Fluorescence Band Maxima, Except for AuP, Where the Energy Is
Taken as the Phosphorescence Maximum at 77 K in
2-Methyltetrahydrofuran
E00 (1ZnP*)/eV
E00 (1S*)/eV
E00 (3AuP)/eV
11
5
1.92
1.66
2.67
suggest the existence of noticeable electronic interactions
between the units in the ground state. The attachment of the
electron-rich quaterthiophene spacer to the gold porphyrin
induces a broadening and a tailing of the Q bands. We
tentatively attribute this extra absorbance to charge transfer
transitions from the quaterthiophene to the gold porphyrin.
This assumption is supported by the observation that this
residual absorbance is absent in compound 9, which only
contains ZnP.
Excited-state energies (Table 2) used to calculate driving
forces for charge separation were determined using the
spectroscopic properties.
Electrochemistry. The redox potentials of the porphyrin
units were determined by cyclic voltammetry in dichlo-
romethane (Figure S2, Supporting Information), and the half-
wave potentials recorded for the compounds are gathered in
Table 3.
In the dyad, the first reduction occurs on the gold
porphyrin, which is reported to be a metal-based process.
Conversely, the second reduction, also assigned to the gold
porphyrin, is known to be localized on the porphyrin
π-macrocycle. The half-wave potential of this second reduc-
tion appears to be anodically shifted by about 200 mV in
the dyad compared to the reference gold tetraarylporphyrin
(AuP+). This probably results from the stabilization of the
phoferrocene) with copper iodide in a mixture of triethy-
lamine and dimethylformamide proved to be suitable for this
coupling, forming dyad 11 with a 50% yield (Scheme 2).26
The reference gold prophyrin 12 was also prepared
following a Sonogashira cross-coupling reaction between the
gold porphyrin 8 and spacer 6 using Pd(dppf)Cl2-CuI as a
catalytic system (Figure 1).
Electronic Absorption Spectra. The spectrum of 11
differs significantly from that of a regular tetraaryl zinc or
gold porphyrin (Figure S1, Supporting Information). The
UV-vis absorption spectrum of dyad 11 along with its
components is shown in Figure 2. As observed previously
in ethynyl substituted porphyrins, the spectrum of 11 shows
a bathochromic shift of all of the visible absorption transi-
tions, a broadening of the Soret band, and an inversion of
the intensities of the Q bands (Figure 2).4,9 These features
Figure 1. Structure of the reference compound 12.
522 Inorganic Chemistry, Vol. 48, No. 2, 2009

Photoinduced Charge Separation in Zinc Porphyrin
Table 3. Redox Potentials of the Compoundsa
ZnP+/ZnP
S+/S
AuP+/AuP•
AuP•/AuP-
5
9
12
11
0.94 V
0.86 V
0.96 V
0.89 V
0.64 V
0.64 V
0.74 V
-0.57 V
-0.58 V
-0.68 V
-1.04 V
-1.04 V
-1.23 V
AuTdTBuP+
ZnTdTBuP
a The electrochemistry was recorded in dichloromethane with 0.15 M
Bu4NPF6 as the supporting electrolyte, and all of the potentials are reported
versus the saturated calomel electrode (SCE). ZnTdTBuP: tetra(3,5-
diterbutylphenyl) zinc(II). AuTdTBuP+: tetra(3,5-diterbutylphenyl) por-
phyrin gold(III). S ) bisethynylquaterthiophene.
Figure 4. Spectra monitored in the course of the oxidation of 11 in CH2Cl2
containing 0.15 M Bu4NPF6. The black bold trace is the initial spectrum,
and the red bold trace is the final one. The arrows indicate the changes
upon oxidation.
the gold porphyrin remain quite stable, since the spectrum
of the oxidized dyad shares some similarities with the
spectrum of the reference AuP-S 12 (Figure 4).
Upon reduction of the gold porphyrin, the net absorption
of the Soret region is less broad since one of the underlying
bands is decreased in intensity (Figure 3). The Q-band region
is also red-shifted to leave a spectrum that features some
characteristics of the reference ZnP-S 9.
Figure 3. Spectra monitored in the course of the reduction of 11 in CH2Cl2
containing 0.15 M Bu4NPF6. The black bold trace is the initial spectrum,
and the red bold trace is the final one. The arrows indicate the changes
upon reduction.
Summarily, the oxidation of ZnP and the reduction of
AuP+ in the dyad induce changes that mostly occur in the
Soret band region due to a decrease of the absorption of this
transition, as already observed in tetraaryl porphyrins.
Fluorescence Spectroscopy. The steady-state fluorescence
spectra of the dyad and the parent compound Spacer-ZnP 9
were recorded in toluene and dichloromethane and show that
the singlet excited state is strongly quenched in both solvents
(Figure S3, Supporting Information). Using time-correlated
single-photon counting and streak camera measurements, the
lifetime of the ZnP singlet in 11 was determined to be 70 ps
in toluene, while that of the reference 9 is 1.8 ns. The
lifetimes are in good agreement with the strong quenching
observed in the steady-state experiments. Irradiation of the
ZnP moiety in dichloromethane leads to rapid photodegra-
dation in the dyad as well as the reference. Excitation of the
AuP moiety leads to the formation of the nonemissive 3AuP
species within a few hundred femtoseconds, which precludes
using emission to determine excited-state lifetimes.27
Femtosecond Transient Absorption Spectroscopy. Tran-
sient absorption spectroscopy was then undertaken to elu-
cidate the deactivation processes occurring in the dyad. First,
the transient absorption spectra of the references ZnP-S 9
and AuP-S 12 were recorded in toluene and dichloromethane
(Figure S4, Supporting Information). Excitation of ZnP leads
to the singlet excited state, which decays with a time constant
of 1800 ps to form the characteristic spectral signature of
the ZnP triplet. In CH2Cl2, the ZnP decomposes rapidly,
π-molecular lowest unoccupied molecular orbitals (LUMOs),
owing to their extension on the spacer. In the anodic region,
the
first
oxidation
is
assigned
to
the formation of the radical cation on the zinc porphyrin.
The lower oxidation potential of ZnP in 11 relative to the
ZnTdTBuP is certainly due to the stabilization of the radical
cation by the quaterthiophene spacer. Finally, the quater-
thiophene oxidation is a reversible process since the spacer
is substituted on both extremities. Importantly, the most
oxidizable unit in the dyad is the zinc porphyrin moiety,
which prevents the charge-separated state from decaying by
a stepwise hole shift passing intermediately on the spacer.14
The electrochemical data confirm the existence of elec-
tronic interactions between the porphyrins and the spacer,
deduced previously by the inspection of the electronic ab-
sorption spectra.
Spectroelectrochemistry. The dyad was investigated by
spectroelectrochemistry to determine the exact absorption
maxima of the reduced gold porphyrin and of the oxidized
zinc porphyrin in this electronically coupled dyad. Indeed,
due to the existence of substantial ground-state electronic
interactions in this system, we could anticipate that the exact
absorption spectra of these species could differ from those
published in regular tetraaryl porphyrins. Figures 3 and 4
respectively show the UV-vis spectra of the radical of gold
porphyrin and of the radical cation of the zinc porphyrin
generated by the in situ electrolysis of dyad 11. The oxidation
of zinc porphyrin is accompanied by a strong bleaching of
the Soret and Q bands of this porphyrin, whereas those of
(26) Ljungdahl, T.; Pettersson, K.; Albinsson, B.; Mårtensson, J. J. Org.
Chem. 2006, 71, 1677.
(27) Andre´asson, J.; Kodis, G.; Lin, S.; Moore, A. L.; Moore, T. A.; Gust,
D.; Martensson, J.; Albinsson, B. Photochem. Photobiol. 2002, 76,
47.
Inorganic Chemistry, Vol. 48, No. 2, 2009 523

Fortage et al.
Figure 5. Transient absorption spectra recorded in toluene upon femto-
second excitation of the AuP moiety in dyad 11. Blue ) -10 ps; green )
40 ps; red ) 2 ns. Dashed line: sum of difference spectra for the
electrochemical reduction of AuP-S and oxidation of ZnP-S. The final
spectrum of the photoproduct is similar to the expected spectrum for the
CS product.
presumably undergoing electron transfer with the solvent.
For 12, the situation is more interesting. Despite the fact
3
that oxidation of the bridge by AuP is thermodynamically
possible, we observed no indication of formation of AuP•
and S+ radicals, either in toluene or in dichloromethane.
Instead, the AuP+ triplet decays back to the ground state
without undergoing any photochemistry, with a lifetime of
2.4 ns in toluene and 500 ns in dichloromethane. Energy
transfer to form the triplet of quaterthiophene can be ruled
out, first, because the level of a triplet of quaterthiophene
(1.8 eV) is uphill from that of the gold porphyrin triplet
excited state (1.6 eV). Second, the absorption spectrum of
the triplet of quaterthiophene is long-lived (microsecond) and
is sufficiently different from that of the gold porphyrin to
be detectable during the transient absorption spectroscopy
study, which was not the case.28
Figure 6. Transient spectra (bottom) and decays (top) of dyad 11 in toluene
for ZnP excitation at 590 nm. (Top) Dark blue, 425 nm; light blue, 450
nm; green, 530 nm; red, 690 nm. (Bottom) Dark blue, -10 ps; light blue,
5 ps; orange, 50 ps; red, 250 ps; dark red, 2 ns. Black trace: 3ZnP spectrum
recorded for the ZnP-S reference compound 9 scaled to equal Soret band
bleaching.
The spectroelectrochemistry allows us to construct the
expected spectrum for the CS product by adding the re-
duction and oxidation difference spectra (dashed line trace
in Figure 5). For the dyad in toluene, following ZnP
excitation, we observe initially the same spectrum as for the
reference 9 (Figure 6). However, this spectrum rapidly decays
to form an intermediate species which we, on the basis
of spectroelectrochemical measurements, identify as the
ZnP+-S-AuP• charge-separated state (Figure 5).
The electron transfer occurs with the same time constant
as the fluorescence lifetime (70 ps), which is consistent with
electron transfer. The charge-separated state then returns to
the ground state with a time constant of 3.3 ns. A small
amount of 3ZnP is formed in the back reaction, as evidenced
by a long-lived fraction of the ZnP triplet seen in the transient
absorption spectra, but the overwhelming majority of the
recombination occurs in the ground state.
Upon excitation of the gold porphyrin, the rapidly formed
triplet27 oxidizes the ZnP moiety through hole transfer (HT;
Figure 7). The kinetics show that the red part of the Soret
band, where ZnP dominates, is bleached with the same rate
as the absorption goes less negative in the red edge of the
Figure 7. Transient spectra (bottom) and decays (top) of dyad 11 in toluene
for AuP excitation at 690 nm. (Top) Dark blue, 430 nm; light blue, 440
nm; green, 580 nm; red, 660 nm. (Bottom) Dark blue, -10 ps; light blue,
5 ps; green, 70 ps; orange, 250 ps; red, 2 ns. The double peak in the Soret
band is likely not associated with the CS process (see text).
spectrum, as we expect from spectroelectrochemistry. This
process is slightly slower than the ET process, in line with
its slightly lower driving force. Again, we would like to stress
the fact that we see no sign of the oxidized bridge, despite
(28) Matsumoto, K.; Fujitsuka, M.; Sato, T.; Onodera, S.; Ito, O. J. Phys.
Chem. B 2000, 104, 11632.
524 Inorganic Chemistry, Vol. 48, No. 2, 2009

Photoinduced Charge Separation in Zinc Porphyrin
Table 4. Time Constants of Photophysical Processes in psa
Table 5. Driving Forces for Photoinduced Charge Separation in Dyad
11
toluene
CH2Cl2
-∆G°HT
(3AuP/ZnP)/
eV
-∆G°HT
(3AuP/S)/
eV
-∆G°CR
(3AuP/ZnP)/
eV
kET
kHT
kCR
kET
kHT
kCR
-∆G°ET
/
eV
11
70 ( 5 140 ( 10 3300 ( 100
b
2
2
20 ( 3 100 ( 10
13c
14c
7
8
130*
130*
130
240
b
2
90
11
0.70
0.44
0.19
1.22
170
a *Energy transfer process. kET ) Electron transfer rate from singlet ZnP*
to AuP+. kHT ) Hole transfer rate from triplet AuP* to ZnP. kCR ) Rate of
charge recombination reaction. b Rate not measurable. c From ref 4.
solvent polarity. In a polar solvent, such as dichloromethane,
the counteranion is certainly partly or fully dissociated from
AuP+; therefore, the photoinduced reaction can be considered
as a real hole shift. In toluene, the PF6- is certainly strongly
associated with the AuP+, and the reaction could be rather
viewed as an electron transfer. It is also noteworthy that the
ratio of forward to back charge transfer is lower in polar
solvents than in nonpolar toluene.
In addition to the formation of the charge-shifted state,
we observe ultrafast relaxation upon AuP excitation in the
dyad in both solvents. The initial spectrum is characterized
by a double peak in the Soret band bleach. This feature
disappears on a 10-20 ps time scale to be replaced by the
3AuP signature, which then converts into the spectrum of
the charge-shifted species. The origin of this relaxation
process is unknown, but we think it is likely related to
relaxation of the ZnP-S-AuP* state, where the electronic
communication between D, S, and A is changed in the
excited state and may also be related to the ground-state
charge transfer we observe.
The excitation of ZnP in 11 is not as selective as for AuP+
excitation, and some 3AuP is formed in addition to the 1ZnP
initially. Although, the formation of an intermediate charge-
shifted state is clearly resolvable in the kinetic analysis. For
Figure 8. Transient spectra of dyad 11 in dichloromethane for AuP
excitation at 690 nm. (Top) Dark blue, 420 nm; light blue, 450 nm; green,
545 nm; red, 630 nm. (Bottom) Dark blue, -10 ps; light blue, 5 ps; green;
20 ps; orange, 100 ps; red, 2000 ps. The double-peak feature in the Soret
band disappears within about 10 ps and is likely not related to the charge-
shift process (see text). Note that the 5 ps spectrum has been scaled by a
factor of 0.3.
3
comparison, the ZnP spectrum (black trace in Figure 6)
recorded for ZnP-S reference 9 is included. We can notice
3
the difference between the ZnP spectrum and that of the
CS intermediate.
Discussion
the favorable thermodynamics. The back charge shift occurs
with the same rate as in the ET pathway (3.3 ns), something
that is commonly observed in zinc/gold porphyrin dyads.15,16
Just as for ET, recombination occurs in the ground state.
We cannot monitor the complete decay to the ground state
with the femtosecond laser, but the spectrum and lifetime
of the CS intermediate is clearly different from that of 3ZnP-S
and 3AuP-S references. Besides, supporting this conclusion
is the observation that the ZnP Soret band bleach of 11 in
toluene upon ZnP excitation measured by nanosecond flash
photolysis is within the response function of the instrument
(≈10 ns), in clear contrast to the ZnP-S reference (Figure
S5, Supporting Information).
In the more polar solvent dichloromethane, the rates for
both forward and backward CS are increased (Table 4 and
Figure 8). Charge separation through hole transfer occurs
with a time constant of 20 ps, while recombination occurs
in the ground state with a time constant of 100 ps. These
differences in rates certainly reflect that the energetics of
the charge-transfer reaction differ in these two solvents,
although they are difficult to estimate from the continuum
dielectric model. We suspect that there is a different degree
of ion pairing with the gold porphyrin according to the
The emission data and the redox potentials were used to
build an energy diagram of the different states of interest of
dyad 11 (Scheme 3). From the energies, we conclude that
electron transfer from the zinc porphyrin singlet excited state
to the gold porphyrin and hole transfer from gold porphyrin
triplet excited state to the quaterthiophene or to the zinc
porphyrin are all exergonic processes (Table 5). If the
solvations of the zinc and the gold porphyrin radical cation
are similar, we can consider that the free energies for charge
separation are independent of the solvent polarity,18 since
the Columbic terms are zero for a charge shift reaction. From
the redox potentials, we can estimate the position of
ZnP+-S--AuP+. This state lies 0.5 eV above the singlet
excited state of ZnP*; therefore, a forward electron transfer
via a hopping mechanism is unlikely from ZnP*. On the other
hand, the hole transfer from the triplet excited state of AuP+
to the quaterthiophene spacer is an exergonic reaction, but
we have not detected the formation of X-S+-AuP• by
transient absorption spectroscopy either in dyad 11 or in the
reference compound 12. We are not certain at this point why
the hole transfer does not take place, but it is entirely clear
from the transient spectroscopy that the charge-separation
Inorganic Chemistry, Vol. 48, No. 2, 2009 525

Fortage et al.
(Figure 10). Indeed, according to the superexchange theory,
the electron transfer rate is inversely proportional to the
energy difference between these two levels.1,11
The bisethynyl quaterthiophene modifies significantly the
rates and the nature of the deactivation processes occurring
upon excitation of the zinc or gold porphyrin. First, in
toluene, the latter spacer affords an almost quantitative charge
separation yield irrespective of which porphyrin is photo-
excited, whereas in dyads 13 or 14, excitation of AuP+ is
accompanied by energy transfer to form the ZnP triplet state
(Table 3). Another valuable distinction of the bisethynyl
quaterthiophene is that the charge-separated state recombines
Figure 9. Structures of the dyads 13 and 14.
3
with the ground state instead of ZnP, as observed in 13 or
14 in toluene. This is certainly a consequence of the lower-
lying charge shifted state in the new dyad 11, owing to the
lower oxidation potential of the ZnP unit. As a result, the
triplet excited state of ZnP might be located slightly above
that of ZnP+-S-AuP• even in toluene. Most importantly,
dyad 11 gives the longest-lived charge-separated state and
the largest kCS/kCR ratio in toluene. This is consistent with a
charge recombination leading directly to the ground state,
instead of to the triplet state of ZnP for 13 and 14. As a
consequence, the driving force of this process is larger and
pushes it further away in the inverted Marcus region, hence
resulting in a slower rate.1,11
Figure 10. Orbital energies calculated from electrochemical potentials and
excited-state energies. Dashed lines: HOMO and LUMO of the OPE spacers
in 13 and 14. Straight lines: HOMO and LUMO of the quaterthiophene in
11.
reactions in dyad 11 essentially occur according to a
superexchange mechanism and that the hole never resides
on the bridge.
The rate of formation and the lifetime of the charge-shifted
state is strongly solvent-dependent, as we have also observed
for ZnP-AuP dyads 13 and 14 linked by OPE spacers (Table
4 and Figure 9). Similarly, we also observe virtually no
difference in the lifetime of the CS state formed via hole
transfer and that formed via electron transfer. Our explanation
for the behavior in the present dyad 11 is similar to that
given for the OPE dyads, namely, that the heavy gold atom
blurs the spin identity of the CS state.4
Given that the chromophores are identical, and the D-A
distances in 11, 13, and 14 are very similar, we can compare
the photophysical behavior of the new dyad 11 with those
of the previously studied dyads 13 and 14 (Figure 9 and
Table 4).4
Figure 10 shows the orbital energy diagram of the dyads
11, 13, and 14 calculated from spectroscopic and electro-
chemical data. In comparison with the OPE bridges in dyads
13 and 14, the highest occupied molecular orbital (HOMO)
of the quaterthiophene bridge lies just between that of ZnP
and that of the AuP+ HOMO, while the energy gap between
the LUMO of ZnP and that of quaterthiophene is a bit larger
than that of the OPE bridges (Figure 10). Thus, one would
expect a faster rate for the hole transfer reaction upon AuP+
excitation and a slower rate of electron transfer upon ZnP
excitation in 11 versus those in 13 and 14. Indeed, with the
quaterthiophene bridge, we get hole transfer in both CH2Cl2
and toluene instead of triplet energy transfer (CH2Cl2) or no
reaction (toluene) observed for the OPE bridges in dyads
13 and 14. The stronger hole transfer coupling expected from
the closer-lying HOMOs thus favors HT over triplet energy
transfer in 11. Conversely, the electron transfer rate measured
in toluene with 11 is slower than in dyads 13 and 14.
Superexchange by electron transfer is unlikely in the case
of AuP+ excitation, due to the large energy spacing between
the LUMO of the AuP+ and that of the quaterthiophene
Conclusion
We have described the synthesis of a novel bis porphyrin
dyad performing a very fast single-step photoinduced electron
transfer over a long distance via a bridge-mediated super-
exchange mechanism. This new dyad shows that the attach-
ment directly on the meso position of a π-conjugated bridge
via an ethynyl linker is a well-suited strategy to perform
efficient and long-range photoinduced charge separation
between two distant porphyrins. Furthermore, the bisethynyl
quaterthiophene is a better suited spacer than oligophenylene-
ethylenes, because it promotes charge separation over energy
transfer, it prohibits the decay of the charge-separated state
to the ZnP triplet state, and finally it permits reaching a
longer-lived charge-separated state. It also belongs to the rare
cases in which photoinduced charge separation can be
brought about by excitation of any of the sensitizers of a
bichromophoric dyad.16 This new dyad represents, therefore,
an attractive photochemical system to build more complex
systems for solar energy conversion or molecular electronic
applications.
Acknowledgment. The French Research Ministry is
gratefully acknowledged for the financial support of these
researches through the programs “ACI Jeune Chercheur” and
the ANR blanc “PhotoCumElec” and the European Com-
munity for COST D35 program.
Supporting Information Available: 1H NMR of the new
compounds and extra physical charaterizations. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC800727E
526 Inorganic Chemistry, Vol. 48, No. 2, 2009