Slow and fast singlet energy transfers in BODIPY-gallium(III)corrole dyads linked by flexible chains

Article abstract of DOI:10.1021/ic402798f

Red (no styryl), green (monostyryl), and blue (distyryl) BODIPY-gallium(III) (BODIPY = boron-dipyrromethene) corrole dyads have been prepared in high yields using click chemistry, and their photophysical properties are reported. An original and efficient control of the direction of the singlet energy transfers is reported, going either from BODIPY to the gallium-corrole units or from gallium-corroles to BODIPY, depending upon the nature of the substitution on BODIPY. In one case (green), both directions are possible. The mechanism for the energy transfers is interpreted by means of through-space Foerster resonance energy transfer (FRET).

Full text of DOI:10.1021/ic402798f

Article
pubs.acs.org/IC
Slow and Fast Singlet Energy Transfers in BODIPY-gallium(III)corrole
Dyads Linked by Flexible Chains
Bertrand Brizet,†,‡ Nicolas Desbois,† Antoine Bonnot,‡ Adam Langlois,‡ Adrien Dubois,†
Jean-Michel Barbe,† Claude P. Gros,*,† Christine Goze,*,† Franck Denat,† and Pierre D. Harvey*,‡
†ICMUB (UMR 6302), Universite
́
de Bourgogne, Dijon, France
de Sherbrooke, Sherbrooke, Queb
‡Dep
́
artement de Chimie, Universite
́
́
ec, Canada
S
* Supporting Information
ABSTRACT: Red (no styryl), green (monostyryl), and blue
(distyryl) BODIPY-gallium(III) (BODIPY = boron-dipyrromethene)
corrole dyads have been prepared in high yields using click chemistry,
and their photophysical properties are reported. An original and
efficient control of the direction of the singlet energy transfers is
reported, going either from BODIPY to the gallium-corrole units or
from gallium-corroles to BODIPY, depending upon the nature of the
substitution on BODIPY. In one case (green), both directions are
possible. The mechanism for the energy transfers is interpreted by
means of through-space Forster resonance energy transfer (FRET).
̈
transfer occurs from BODIPY to the porphyrin,8 whereas, in
the case of blue π-conjugated BODIPY−zinc porphyrin
tweezers, the energy transfer occurs in the opposite direction,
for example, from the photoexcited singlet zinc porphyrin to
the π-conjugated BODIPY moiety of the composites as
compared to energy transfer from the singlet excited state of
conventional BODIPY to zinc porphyrins.9
We now report the synthesis and spectroscopic character-
ization of new covalently linked red (no styryl), green (styryl-
monosubstituted), and blue (styryl-disubstituted)-BODIPY-
gallium(III) corrole derivatives bridged by triazole linkers
(Chart 1; see the real color of the solutions in Supporting
Information, Figure S1). To the best of our knowledge, no
previous example of a corrole macrocycle covalently linked to
BODIPY has been reported. It is noteworthy that an original
and efficient control of the direction of the singlet energy
transfers is herein reported, for example, from BODIPY to the
gallium-corrole units or from gallium-corroles to BODIPY,
depending upon the nature of the BODIPY (blue, green, or
red).
INTRODUCTION
■
Efforts have recently been devoted for the design of
porphyrin−BODIPY (BODIPY = boron-dipyrromethene)
dyads acting mainly as light-harvesting antennas for the
photochemical conversion of solar energy.1 In almost all
reported examples, the role of the BODIPY unit was to
essentially fill the blue-green region of the visible spectrum
where porphyrins absorb weakly (i.e., between the Soret and Q-
bands). Indeed, increasing the number of chromophore
molecules can contribute to cover absorption in the entire
visible spectrum resulting in, for example, a panchromatic light
capture. In this respect, other recent developments on
porphyrin−BODIPY dyad indeed focused on a novel broad-
band capturing and emitting system useful for solar energy
harvesting.2
Concurrently, our group explored over a few years the
chemistry of corrole macrocycles, akin to porphyrins but with
one less meso carbon atom in their outer periphery.3 Literature
investigations show that the insertion of gallium(III) into the
corrole macrocycle can efficiently enhance corrole fluorescence
intensity.4 This property is of prime importance for the
potential use of corroles in medicinal applications5 and
photophysics6 particularly as fluorescent probes. This feature
becomes even more appealing if these corrole macrocycles are
incorporated into polychromophoric arrays. The first corrole
gallium(III) complex as well as its X-ray structure were
reported by Gross et al.7 The metalation of the corrole free base
by GaCl3 was shown to be convenient and preceded in almost
quantitative yields.
EXPERIMENTAL SECTION
■
Materials. Unless otherwise stated, all chemicals and solvents were
of analytical reagent grade and were used as received. Absolute
dichloromethane was obtained from Carlo Erba. Silica gel (Merck;
70−120 mm) was used for column chromatography. Analytical thin-
layer chromatography (TLC) was performed with Merck 60 F254 silica
gel (precoated sheets, 0.2 mm thick). Reactions were monitored by
thin-layer chromatography, ultraviolet−visible (UV−vis) spectroscopy,
Recently, we reported the synthesis and spectroscopic
characterization of new zinc porphyrin−BODIPY dyads that
exhibit efficient singlet energy transfers. In one case, energy
Received: November 7, 2013
Published: March 24, 2014
© 2014 American Chemical Society
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rescence emission and excitation spectra were obtained on a Fluorolog
SPEX 1680 0.22 m double monochromator spectrometer using quartz
cuvettes (1 cm, 3 mL). All fluorescence spectra were corrected for
apparatus response. Fluorescence lifetimes were measured on an
apparatus that incorporated a nitrogen laser as the source and a high-
resolution dye laser (full width at half-maximum (fwhm) = 1.4 ns).
Fluorescence lifetimes were obtained from high-quality decays and
deconvolution or distribution lifetime analysis. The uncertainties
ranged from 20 to 40 ps on the basis of multiple measurements.
Quantum Yield Measurements. Measurements were performed
in distillated 2-methyltetrahydrofuran (2-MeTHF), and spectrophoto-
metric grade methanol (Aldrich) was used for references. Quartz
cuvettes of 3 mL with path length of 1 cm equipped with a septum
were used, and all solutions were Ar-degassed prior to measurements.
Three different measurements (i.e., different solutions) were
performed for each quantum yield. The sample concentrations were
chosen to obtain an absorbance of about 0.05. The fluorescence
quantum yield (ΦF) measurements were performed with the slit width
of 0.5−1.5 nm for both excitation and emission. Relative quantum
efficiencies were obtained by comparing the areas under the corrected
emission spectra of the sample relative to a known standard, and the
following equation was used to calculate quantum yield: ΦF(sample) =
ΦF(standard) (Isample/Istandard) (Astandard/Asample) (ηsample2/ηstandard2),
where ΦF(standard) is the reported quantum yield of the standard, I
is the integrated emission spectrum, A is the absorbance at the
excitation wavelength, and η is the refractive index of the solvents used.
Rhodamine 6G (ΦF = 0.94 in methanol),10 cresyl violet (ΦF = 0.54 in
methanol),11 and rhodamine 101 (ΦF = 1.00 in methanol)12 were used
as standards.13 In all ΦF determinations, correction for the solvent
refractive index (η) was applied (in 2-MeTHF, η = 1.406; in methanol,
η = 1.328).14
Chart 1. Structure of the Different BODIPY−Corrole Dyads
DFT Calculations. All computer modeling was performed using
the density functional theory (DFT) and Gaussian 0915 at the
Universite
supported by Le Res
́
de Sherbrooke with the Mammouth supercomputer
́
́ ́
eau Quebecois De Calculs Hautes Performances.
The DFT geometry optimizations16 were carried out using the B3LYP
method. A 6-31g* basis set was used for C, H, N, and F atoms. VDZ
(valence double ζ) with SBKJC effective core potential was used for B
and Ga atoms.17
Synthesis of 10-(4-azidomethylphenyl)-5,15-dimesitylcor-
role gallium(III)(pyridine) (1). 10-(4-Azidomethylphenyl)-5,15-
dimesitylcorrole3a (50.0 mg, 0.075 mmol) was added to a 0.114 M
solution of GaCl3 in dry pyridine (6.6 mL, 0.75 mmol). The reaction
mixture was stirred to reflux for 1 h and 30 min, shielded from light.
The solvent was removed, and the crude product was purified by
column chromatography over silica (pentane−dichloromethane−
pyridine, 100:30:0.5). The title compound 1 was isolated as a purple
1
microcrystalline solid in 69% yield (42.0 mg, 0.052 mmol). H NMR
(300 MHz, CDCl3) δ (ppm): 1.88 (s, 12H, CH3), 2.61 (s, 6H, CH3),
3.58 (m, 2H, HPy), 4.66 (s, 2H, CH2), 5.97 (m, 2H, HPy), 6.76 (t, 1H, J
= 7.8 Hz, HPy), 7.26 (s, 4H, HMes), 7.63 (d, 2H, J = 8.1 Hz, HPh), 8.16
(d, 2H, J = 8.1 Hz, HPh), 8.55 (m, 4H, Hβ), 8.62 (d, 2H, J = 4.5 Hz,
Hβ), 9.01 (d, 2H, J = 3.9 Hz, Hβ). UV−vis (2-MeTHF): λmax (nm) (ε
× 10−3 L mol−1 cm−1) = 418 (206), 527 (7), 566 (13), 599 (26). MS
(MALDI-TOF) m/z = 731.03 [M-pyridine]+, 731.23 calcd for
C44H36GaN7. HR-MS (ESI) m/z = 731.2263 [M-pyridine]+,
731.2283 calcd for C44H36GaN7.
Compound 2. N-Hydroxybenzotriazole (959 mg, 7.10 mmol),
diisopropylamine (995 μL, 7.10 mmol), 1-(3-dimethylaminopropyl)-3-
ethylcarbodiimide hydrochloride (1.35 g, 7.10 mmol), and propargyl-
amine (227 μL, 3.50 mmol) were successively added to a solution of
BODIPY acid18 (1.53 g, 3.50 mmol) in 100 mL of dimethylformamide
(DMF), and the solution was stirred at RT. After total consumption of
starting material (2 h) monitored by TLC, the solvent was evaporated.
The resulting solid was washed with water (3 × 100 mL) and extracted
with dichloromethane. The organic layer was dried over magnesium
sulfate, and the solvent was evaporated to give a red oil. The crude
product was purified by column chromatography on silica gel
(dichloromethane/heptane 60:40) followed by a recrystallization in a
mixture of dichloromethane and hexane to give a reddish solid in 72%
and matrix-assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF MS).
Instrumentation. The 1H, 11B, and 13C NMR spectra were
recorded at room temperature (RT) on a Bruker Avance II 300 (300
MHz) or on a Bruker Avance DRX 600 (600 MHz) spectrometer at
̂ ́ ́
the Welience, Pole Chimie Moleculaire de l’Universite de Bourgogne
(WPCM). Chemical shifts (1H NMR spectra) are expressed in ppm
relative to chloroform (7.26 ppm) or pyridine (7.22, 7.58, 8.74). UV−
vis spectra were recorded on a Varian Cary 300 Bio spectropho-
tometer. Mass spectra and accurate mass measurements (high-
resolution (HRMS)) were obtained on a Bruker Daltonics Ultraflex
II spectrometer in the MALDI-TOF reflectron mode using dithranol
as a matrix or on a linear trap quadrupole (LTQ) Orbitrap XL
(THERMO) instrument in electrospray ionization (ESI) mode. Both
measurements were registered at WPCM. The steady-state fluo-
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1
yield (1.15 g, 2.49 mmol). H NMR (300 MHz, CDCl3) δ (ppm):
0.96 (t, J = 7.5 Hz, 6H), 1.22 (s, 6H), 2.29 (q, J = 7.5 Hz, 4H), 2.30 (t,
J = 2.5 Hz, 1H), 2.51 (s, 6H), 3.78 (dd, J = 2.5 Hz, J = 5.2 Hz, 2H),
6.27 (t, J = 5.2 Hz, 1H), 7.38 (d, J = 8.4 Hz, 2H), 7.91 (d, J = 8.4 Hz,
2H). 13C NMR (75 MHz, CDCl3) δ (ppm): 11.9, 12.5, 14.6, 17.1,
30.9, 72.1, 79.3, 127.8, 128.9, 130.4, 133.1, 134.0, 138.1, 138.5, 139.7,
154.3, 166.2. 11B NMR (192.5 MHz, CDCl3) δ (ppm): 0.78 (t, J =
33.4 Hz). UV−vis (2-MeTHF) λmax (nm) (ε × 10−3 L mol−1 cm−1):
238 (39), 494 (27), 525 (90). MS (ESI) m/z = 460.39 [M + H]+,
484.34 [M + Na]+. HRMS (ESI) 484.235 59 calcd for C27H30B-
F2N3ONa 484.234 70.
refluxed during 2 h in a Dean−Stark apparatus, and the solvent was
removed in situ. Dry toluene (50 mL) and piperidine (3 mL) were
added, and the mixture was refluxed for another 2 h. After
consumption of approximately a third of the starting material
(monitored by UV−vis), the solvent was evaporated. The resulting
solid was washed with water (3 × 100 mL) and extracted with
dichloromethane. The organic layer was dried over magnesium sulfate,
and the solvent was evaporated to give a blue solid. The crude product
was purified by column chromatography on silica gel (dichloro-
methane/heptane 40:60), followed by a recrystallization in a mixture
of dichloromethane and hexane to give 7 (73.0 mg, 125 μmol) in 5%
1
Compound 3. Compound 2 (100 mg, 0.22 mmol) and p-
anisaldehyde (106 μL, 0.870 mmol) were dissolved in a mixture of
dry toluene (40 mL), p-toluenesulfonic acid (PTSA, 6.0 mg, 0.035
mmol), and piperidine (752 μL, 7.60 mmol). The mixture was refluxed
during 2 h in a Dean−Stark apparatus, and the solvent was removed in
situ. Dry toluene (40 mL) and 750 μL of piperidine were added, and
the mixture was refluxed for another 2 h. After total consumption of
the starting material (monitored by UV−vis), the solvent was
evaporated. The resulting solid was washed with water (3 × 100
mL) and extracted with dichloromethane. The organic layer was dried
over magnesium sulfate, and the solvent was evaporated to give a blue
solid. The crude product was purified by column chromatography on
silica gel (heptane/CH2Cl2 80:20), followed by a recrystallization in a
mixture of dichloromethane and hexane to give a blue solid in 36%
yield. Compound 6 was also obtained as subproduct in 3% yield. H
NMR (300 MHz, CDCl3) δ (ppm): 0.97 (t, J = 7.5 Hz, 3H), 1.12 (t, J
= 7.5 Hz, 3H), 1.25 (s, 3H), 1.27 (s, 3H), 2.29 (q, J = 7.5 Hz, 2H),
2.52 (t, J = 2.4 Hz, 1H), 2.56 (s, 3H), 2.56 (q, J = 7.5 Hz, 2H), 3.97 (s,
3H), 4.71 (d, J = 2.4 Hz, 2H), 6.97 (d, J = 8.8 Hz, 2H), 7.16 (d, J =
16.8 Hz, 1H), 7.41 (d, J = 8.3 Hz, 2H), 7.54 (d, J = 8.8 Hz, 2H), 7.60
(d, J = 16.8 Hz, 1H), 8.16 (d, J = 8.3 Hz, 2H). 13C NMR (75 MHz,
CDCl3) δ (ppm): 11.8, 12.1, 13.0, 14.3, 14.7, 17.3, 17.5, 18.5, 52.6,
56.1, 75.9, 78.6, 115.4, 118.6, 128.8, 129.1, 130.5, 130.9, 131.3, 131.6,
133.4, 134.0, 135.1, 138.0, 141.1, 150.0, 155.8, 158.2, 166.8. 11B NMR
(192.5 MHz, CDCl3) δ (ppm): 0.98 (t, J = 33.8 Hz). UV−vis (2-
MeTHF): λmax (nm) (ε × 10−3 L mol−1 cm−1) = 238 (19), 319 (17),
339 (29), 389 (8), 448 (27), 587 (81). MS (ESI): m/z = 603.4 [M +
Na]+. HRMS (ESI) 603.262 33 calcd for C35H35BF2N2O3Na 603.260
71.
1
yield (55.0 mg, 78.8 μmol). H NMR (300 MHz, CDCl3) δ (ppm):
1.14 (t, J = 7.5 Hz, 6H), 1.28 (s, 6H), 2.32 (t, J = 2.5 Hz, 1H), 2.58 (q,
J = 7.5 Hz, 4H), 3.84 (s, 6H), 4.30 (dd, J = 2.5 Hz, J = 5.2 Hz, 2H),
6.36 (t, J = 5.2 Hz, 1H), 6.93 (d, J = 8.7 Hz, 4H), 7.20 (d, J = 16.7 Hz,
2H), 7.43, (d, J = 8.3 Hz, 2H), 7.56 (d, J = 8.7 Hz, 4H), 7.65 (d, J =
16.7 Hz, 2H), 7.93 (d, J = 8.3 Hz, 2H). 13C NMR (75 MHz, CDCl3) δ
(ppm): 11.8, 14.1, 18.4, 55.4, 72.2, 79.3, 114.3, 127.8, 128.9, 129.4,
130.3, 132.5, 133.9, 134.1, 135.8, 136.1, 138.3, 140.1, 150.9, 160.3,
166.3. 11B NMR (192.5 MHz, CDCl3) δ (ppm): 1.23 (t, J = 34.2 Hz).
UV−vis (2-MeTHF): λmax (nm) (ε × 10−3 L mol−1 cm−1): 250 (21),
334 (31), 368 (68), 608 (37), 654 (93). MS (ESI): m/z = 720.35 [M
+ Na]+. HRMS (ESI) 720.320 39 calcd for C43H42BF2N3O2Na
720.318 69.
General Procedure for the Huisgen Reaction. 10-(4-
Azidomethylphenyl)-5,15-dimesityl-corrole gallium(III)(pyridine)
(30.0 mg, 37.0 μmol or 60.0 mg, 74.0 μmol for preparation of
compound 8) and BODIPY alkyne (37.0 μmol) were solubilized in 5
mL of THF. CuI (10.5 mg, 55.1 μmol or 21.0 mg, 110 μmol for
preparation of compound 8) and N,N-diisopropylethylamine (DIPEA)
(14.0 μL, 148 μmol) were added. The mixture was stirred under N2 at
RT for 3 h. The crude mixture was evaporated to dryness. Then 20 mL
of water was added. The compound was extracted with dichloro-
methane (3 × 20 mL). The collected organic solution was dried over
MgSO4, and the solvent was removed under reduced pressure. The
product was recrystallized from CH2Cl2−hexane.
Compound 6. BODIPY ester18 (200 mg, 0.46 mmol) and 4-(prop-
2-yn-1-yloxy)benzaldehyde (73.0 mg, 0.460 mmol) were dissolved in a
mixture of dry toluene (20 mL), p-toluenesulfonic acid (PTSA, 50.0
mg, 0.290 mmol), and piperidine (989 μL, 10.0 mmol). The mixture
was refluxed during 2 h in a Dean−Stark apparatus. The solvent was
removed in situ, and 10 mL of dry toluene and 73 mg of 4-(prop-2-yn-
1-yloxy)benzaldehyde were added. The mixture was refluxed for
another 1 h, and solvent was removed. This step was repeated five
times. After total consumption of starting material (monitored by
UV−visible), the solvent was evaporated. The resulting solid was
washed with water (3 × 50 mL) and extracted with dichloromethane.
The organic layer was dried over magnesium sulfate, and the solvent
was evaporated to give a blue solid. The crude product was purified by
column chromatography on silica gel (dichloromethane/heptane
50:50), followed by a recrystallization in a mixture of dichloromethane
and hexane to give 6 (133 mg, 184 μmol) in 40% yield as blue solid.
1H NMR (300 MHz, CDCl3) δ (ppm): 1.13 (t, J = 7.6 Hz, 6H), 1.28
(s, 6H), 2.53 (t, J = 2.4 Hz, 2H), 2,58 (q, J = 7.6 Hz, 4H), 3.97 (s,
3H), 4.73 (d, J = 2.4 Hz, 4H), 7.01 (d, J = 8.8 Hz, 4H), 7.20 (d, J =
16.6 Hz, 2H), 7.42 (d, J = 8.2 Hz, 2H), 7.56 (d, J = 8.8 Hz, 4H), 7.66
(d, J = 16.6 Hz, 2H), 8.20 (d, J = 8.2 Hz, 2H). 13C NMR (75 MHz,
CDCl3) δ (ppm): 10.7, 13.0, 17.4, 51.4, 54.9, 74.7, 77.3, 114.2, 117.5,
127.8, 128.1, 129.3, 129.6, 130.1, 131.5, 133.0, 134.6, 137.4, 140.8,
149.8, 151.2, 157.1, 165.6. UV−vis (2-MeTHF): λmax (nm) (ε × 10−3
L mol−1 cm−1) = 255 (21), 332 (31), 366 (63), 428 (11), 604 (36),
654 (85). 11B NMR (192.5 MHz, CDCl3) δ (ppm): 1.20 (t, J = 34.6
Hz). MS (ESI): m/z = 745.30 [M + Na]+. HRMS (ESI) 745.303 08
calcd for C45H41BF2N2O4Na 745.302 73.
Compound 4. The title compound was isolated as a purple
1
microcrystalline solid in 96% yield (45.0 mg, 35.4 μmol). H NMR
(300 MHz, pyridine-d5) δ (ppm): 0.84 (m, 6H, CH3), 1.20 (s, 6H,
CH3), 2.13 (m, 16H, CH2, CH3), 2.62 (s, 6H, CH3), 2.70 (s, 6H,
CH3), 5.25 (d, 2H, J = 5.1 Hz, CH2N), 6.05 (s, 2H, CH2), 7.38 (m,
6H, Hmes, HPh), 7.49 (s, 1H, Htriazole), 7.76 (d, 2H, J = 7.8 Hz, HPh),
8.20 (d, 2H, J = 8.1 Hz, HPh), 8.46 (d, 2H, J = 8.1 Hz, HPh), 8.79 (m,
4H, Hβ), 8.88 (d, 2H, J = 4.5 Hz, Hβ), 9.25 (d, 2H, J = 3.9 Hz, Hβ),
10.07 (m, 1H, NH). UV−vis (2-MeTHF): λmax (nm) (ε × 10−3
L
mol−1 cm−1) = 419 (150), 525 (80), 567 (13), 600 (21). MS
(MALDI-TOF) m/z = 1173.46 [M-pyridine-F]+, 1173.47 calcd for
C71H66BFGaN10O. HR-MS (ESI) m/z = 1192.4762 [M-pyridine]+,
1192.4744 calcd for C71H66BF2GaN10O.
Compound 5. The title compound was isolated as a purple
1
microcrystalline solid in 97% yield (54.0 mg, 35.8 μmol). H NMR
(300 MHz, pyridine-d5) δ (ppm): 1.04 (t, 6H, J = 7.2 Hz, CH3), 1.24
(s, 6H, CH3), 2.11 (s, 12H, CH3), 2.56 (m, 10H, CH2, CH3), 3.65 (s,
6H, OCH3), 5.25 (d, 2H, J = 5.1 Hz, CH2N), 6.04 (s, 2H, CH2), 6.91
(d, 2H, J = 8.7 Hz, HPh), 7.37 (m, 6H, Hmes, HPh), 7.50 (m, 3H, Halkene
,
Htriazole), 7.71 (m, 6H, HPh), 8.19 (d, 2H, J = 8.1 Hz, HPh), 8.29 (d, 2H,
J = 16.5 Hz, Halkene), 8.47 (d, 2H, J = 8.1 Hz, HPh), 8.76 (m, 4H, Hβ),
8.86 (d, 2H, J = 4.5 Hz, Hβ), 9.23 (d, 2H, J = 3.9 Hz, Hβ), 10.08 (m,
1H, NH). UV−vis (2-MeTHF): λmax (nm) (ε × 10−3 L mol−1 cm−1) =
368 (83), 419 (164), 605 (58), 654 (96). MS (MALDI-TOF) m/z =
1428.46 [M-pyridine]·+, 1428.56 calcd for C87H78BF2GaN10O3. HR-
MS (ESI) m/z = 1428.5610 [M-pyridine]+, 1428.5583 calcd for
C87H78BF2GaN10O3.
Compound 8. The title compound was isolated as a purple
microcrystalline solid in 97% yield (84 mg, 35.8 μmol). 1H NMR (300
MHz, pyridine-d5) δ (ppm): 1.09 (m, 6H, CH3), 1.29 (2s, 6H, CH3),
2.13 (s, 24H, CH3), 2.61 (m, 16H, CH2, CH3), 3.94 (s, 3H, OCH3),
5.48 (s, 4H, CH2), 6.07 (s, 4H, CH2), 7.29 (m, 4H, HPh), 7.37 (m, 8H,
Compound 7. BODIPY ester18 (1.10 g, 2.51 mmol) and 4-(prop-2-
yn-1-yloxy)benzaldehyde (442 mg, 2.76 mmol) were dissolved in a
mixture of dry toluene (50 mL), p-toluenesulfonic acid (PTSA, 53.0
mg, 0.308 mmol), and piperidine (3.3 mL, 33 mmol). The mixture was
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Hmes), 7.50 (m, 6H, Htriazole, Halkene, HPh), 7.76 (m, 6H, HPh, HPh), 8.34
(m, 10H, HPh, Halkene), 8.79 (m, 8H, Hβ), 8.88 (d, 4H, J = 4.5 Hz, Hβ),
9.24 (d, 4H, J = 3.9 Hz, Hβ). UV−vis (2-MeTHF): λmax (nm) (ε ×
10−3 L mol−1 cm−1) = 372 (63), 419 (244), 568 (32), 602 (60), 658
(63). MS (MALDI-TOF) m/z = 2165.76 [M-pyridine-F]+, 2165.77
calcd for C133H113BFGa2N16O4. HR-MS (ESI) m/z = 1092.8895 [M-2-
pyridine]2+, 1092.8832 calcd for C133H113BF2Ga2N16O4.
consists in a Knoevenagel condensation between compound 2
and 4 equiv of p-anisaldehyde with p-toluenesulfonic acid
(PTSA) and piperidine, in a Dean−Stark apparatus, using dry
toluene as the solvent (Scheme 2).9
Similarly, the di- and monostyryl precursors 6 and 7 were
respectively obtained using the same reaction, starting from
carbomethoxy-meso-substituted BODIPY derivative and 4-
(prop-2-yn-1-yloxy)benzaldehyde (Scheme 2). The reaction
was monitored by UV−vis spectroscopy to obtain the maximal
quantity of monocondensation product 7. Difficulties in
purification and formation of dicondensation subproduct 6
explain the low yield of reaction for 7 (5%). Nevertheless
compound 6 can be obtained in good yield (40%) by using a
larger quantity of 4-(prop-2-yn-1-yloxy)benzaldehyde, as
described in the Experimental Section.
Compound 9. The title compound was isolated as a purple
1
microcrystalline solid in 96% yield (49.5 mg, 35.6 μmol). H NMR
(300 MHz, pyridine-d5) δ (ppm): 0.89 (t, 3H, J = 7.2 Hz, CH3), 1.08
(t, 3H, J = 7.2 Hz, CH3), 1.32 (2s, 6H, CH3), 2.14 (m, 14H, CH2,
CH3), 2.63 (m, 8H, CH2, CH3), 2.77 (s, 3H, CH3), 3.93 (s, 3H,
OCH3), 5.52 (s, 2H, CH2), 6.12 (s, 2H, CH2), 7.35 (d, 2H, J = 8.4 Hz,
HPh), 7.38 (m, 4H, Hmes), 7.50 (m, 4H, HPh, Halkene, Htriazole), 7.76 (d,
2H, J = 8.4 Hz, HPh), 7.81 (d, 2H, J = 8.1 Hz, HPh), 8.33 (m, 5H,
Halkene, HPh), 8.80 (m, 4H, Hβ), 8.89 (d, 2H, J = 4.5 Hz, Hβ), 9.26 (d,
2H, J = 3.9 Hz, Hβ). UV−vis (2-MeTHF): λmax (nm) (ε × 10−3
L
mol−1 cm−1) = 340 (34), 418 (145), 591 (84). MS (MALDI-TOF) m/
z = 1292.51 [M-pyridine-F]+, 1292.50 calcd for C79H71BFGaN9O3.
HR-MS (ESI) m/z = 1311.5018 [M-pyridine]+, 1311.5004 calcd for
C79H71BF2GaN9O3.
The Huisgen copper-catalyzed azide−alkyne cycloaddition,
known as “click reaction,”21 was used to efficiently link the
BODIPY and the gallium-corrole subunits. Indeed, many
reports have already mentioned the usefulness of the click
chemistry for the elaboration of sophisticated structures
involving mainly porphyrin as the backbone.9,22 We applied a
recent methodology described for the preparation of tripod
porphyrins.22 The reaction, catalyzed by CuI/di(isopropyl)-
ethylamine (DIPEA), proceeds quantitatively in THF at RT for
3 h (Schemes 3 and 4). All BODIPY−Ga(III)-corroles were
RESULTS AND DISCUSSION
Synthesis. The azide-containing free base corrole (see
starting product in Scheme 1) was prepared using the common
■
Scheme 1. Synthesis of Gallium-Corrole 1
1
fully characterized by H NMR spectroscopy in pyridine-d5.
HR-MS measurements were performed using an ESI-Orbitrap
instrument to further confirm the successful formation of
compounds 4, 5, 8, and 9. For example for compound 8, the
molecular peak is observed at m/z = 2165.76 [M-pyridine-F]+
in the MALDI-TOF spectra, (2165.77 calcd for C133H113B-
FGa2N16O4), whereas by ESI HR-MS, the molecular peak
appears at m/z = 1092.8895 [M-2-pyridine]2+ (1092.8832 calcd
for C133H113BF2Ga2N16O4).
Computer Modeling. To address whether the possible
conformations due to the flexibility of the chain are
energetically accessible, DFT geometry optimizations were
performed for compounds 4 and 9. The results from these
computations are presented in Figure 1. Only representative
examples are necessary to illustrate that many conformations of
various energies coexist. The most stable conformations for 4
(and 5 due to their strong structural similarity) are the unfolded
conformations C−E, differing by only the relative orientation of
the C6H4CH2 group in the chain. Concurrently, the unfolded
and semifolded conformations D−F are noted to be the most
stable ones for compound 9, but the semifolded conformers B
and C are only 1 kJ mol−1 destabilized with respect to the most
stable ones. The folded conformer A is either too high in
energy (4) or unstable (9). The conclusion is that many
unfolded and semifolded conformers coexist in solution, but
the folded conformer A is simply not accessible for 4 and 9.
Electronic Absorption Spectra. Table 1 summarizes the
UV−vis data for all compounds in 2-MeTHF. Compound 1
exhibits one Soret band at 418 nm and three Q bands at 527,
566, and 599 nm of the corrole chromophore. Compounds 2,
3, 6, and 7 exhibit bands characteristic of the BODIPY
chromophore; the S0−S1 band at 525, 654, 587, and 658 nm is
assigned to spin-allowed π−π* transitions. Dyads 4, 5, 8, and 9
display the expected corrole Soret band and Q bands in
addition to the BODIPY π−π* transitions. The absorption
spectra of the dyads are simply the sum of the individual
chromophores (gallium-corrole and BODIPYs) and do not
show any new bands or broadening of the base peaks,
“2 + 1” method.19 The preparation involves the condensation
of an azidobenzyl aldehyde (1 equiv) with a mesityldipyrro-
methane (2 equiv) in the presence of a catalytic amount of
trifluoroacetic acid (TFA, 0.08 equiv) using dichloromethane as
solvent. Sterically hindered mesityldipyrromethane was used to
avoid any acidolysis that could occur during the cyclization
reaction.
Gallium(III) was inserted in the corrole cavity according to a
literature procedure.7,19 The inorganic salts were separated
from the desired product by column chromatography on silica
gel, affording the (pyridine)gallium(III) corrole 1 in 69% yield
(Scheme 1). The formation of the gallium-corrole 1 was
monitored by MALDI-TOF MS using the molecular peak at m/
z = 731.2263 (731.2283 calcd for C44H36GaN7), which shows a
characteristic isotopic pattern corresponding to [M-pyridine]·+.
The coordination of one pyridine molecule to the gallium metal
1
center was confirmed by H NMR spectroscopy.
Ethynyl-functionalized BODIPY 2 was easily obtained by
peptidic coupling between propargylamine and a BODIPY
bearing a carboxylic group (generated from the methyl ester) in
meso position as previously described in the literature.20 The
coupling reagent we used (Scheme 1) was based on a
carbodiimide (e.g., 1-ethyl-3-(3-dimethylaminopropyl)-
carbodiimide, EDCI·HCl) in the presence of 1-hydroxy-1H-
benzotriazole (HOBt) and N,N-diisopropylethylamine
(DIPEA). The ethylenic proton exhibits a triplet (δ = 2.30,
4JA = 2.5 Hz) in CDCl3 1H NMR spectroscopy. The distyryl
BODIPY 3 was synthetized using a reported method, which
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a
Scheme 2. Synthesis of Compounds 2, 3, 6, and 7
a
(i) HOBT, EDCI·HCl, DIPEA, propargylamine, DMF, RT; (ii) p-anisaldehyde, PTSA, piperidine, dry toluene, reflux; (iii) 4-(prop-2-yn-1-yloxy)-
benzaldehyde, PTSA, piperidine, dry toluene, reflux.
Scheme 3. Synthesis of Compounds 4 and 5
suggesting that electronic mixing between chromophores is
minimal or nonexistent. The absorption spectra of gallium-
corrole lack any significant absorption intensity in the region
from 450 to 520 nm, thus allowing for selective excitation of the
BODIPY donor chromophore for the examination of energy
transfer (ET) processes from red BODIPY to gallium-corrole.
Inversely, the absorption spectra of BODIPYs 3 and 6 lack any
significant absorption intensity in the region from 460 to 550
nm, thus allowing for selective excitation of the gallium-corrole
donor chromophore for the examination of ET processes from
gallium-corrole to blue BODIPY.
precursors 2, 7, and 3. These spectra show that the
introduction of conjugated styryl substituent on BODIPY
induces a bathochromic shift of the absorption and fluorescence
bands. Gallium-corrole 1 exhibits a strong fluorescence band at
612 nm and a weaker vibronic shoulder at 670 nm. The
fluorescence quantum yield is 0.11. The BODIPYs 2, 3, 6, and
7 display an intense emission band at 543, 681, 684, and 603
nm, respectively.
Figure 3 shows the absorption, fluorescence, and excitation
spectra of the bichromophoric species 4, 5, 8, and 9. Each dyad
displays a fluorescence band near 612 nm readily attributable to
the fluorescence of the gallium-corrole unit and a luminescence
band due to the BODIPY moiety placed either at higher energy
(for 4, 540 nm) or at lower energies (for 5 and 8, 675 and 676
nm, respectively), depending on the number of styryl groups
Steady-State Fluorescence Spectroscopy and Life-
times. Figure 2 shows the absorption, fluorescence, and
excitation spectra of gallium-corrole 1 and of the (red)
unfunctionalized, (green) mono-, and (blue) distyryl-BODIPY
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a
Scheme 4. Synthesis of Compounds 8 and 9
a
(i) CuI, DIPEA, THF, N2, RT.
Table 1. UV−Vis Absorption Data in 2-MeTHF at 298 K
λabs (max) (nm) (ε × 10−3 M−1 cm−1
)
298 K
77 K
corrole
dye Soret band Q bands
BODIPY
S1 band
corrole
BODIPY
S1 band
Q bands
1
418 (206)
532 (8.69)
566 (12.7)
599 (27.1)
529
557, 570
605, 619
2
3
4
525 (89.5)
654 (92.8)
525 (59.2)
492, 526
614, 674
419 (104)
419 (164)
566 (7.60)
599 (21.9)
566 (16.8)
607 (40.7)
5
653 (66.6)
6
7
8
654 (85.1)
587 (81.2)
657 (47.2)
612, 670
550, 597
419 (109)
419 (137)
567 (32.2)
606 (34.5)
9
591 (81.5)
Figure 1. Some optimized geometries of compounds 4 and 9 in
various conformations, notably folded and unfolded, indicating the
relative stabilization energies with respect to the lowest-energy
conformations arbitrarily set at 0. Conformation A for compound 9
does not lead to any stable conformation upon geometry optimization.
This conformation simply unfolds.
However, the presence of two emission bands for the arrays
(one for the corrole and one for the BODIPY) indicates that a
part of the energy of the donor is not transferred to the
acceptor, generating a residual emission attributed to the donor
(cf. Figure 3).
present. For compound 9, only one band is observed at 603
nm, which is attributed to both the BODIPY and gallium-
corrole units (i.e., strong fluorescence superposition). On the
basis of the fluorescence positions, the BODIPY chromophore
can act as an energy donor (such as in 4) and the energy
acceptor (such as in 5 and 8). In compound 9, this role is not
well-defined when only based on these spectra.
The excitation spectra of the arrays exhibit a perfect match
with the absorption, confirming the efficient energy between
the two chromophores, following an excitation of the donor.
The fluorescence data of all compounds at 298 and 77 K are
summarized in Tables 2 and 3. The model BODIPY
compounds 2, 3, 6, and 7 exhibit high fluorescence quantum
yields ΦF ranging between 0.42 for compound 3 and 0.76 for
compound 7. The fluorescence lifetimes τF range from 4.64 ns
for compound 2 to 7.03 ns for compound 3. Gallium-corrole 1
displays a lower ΦF (i.e., ΦF = 0.11, τF = 2.17 ns) with respect
to BODIPY, consistent with the literature.23 The ΦF values of
the dyads were determined using an excitation wavelength
where the donor absorbs more, bearing in mind that singlet−
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Figure 2. (black) Absorption, (red) emission, and (blue) excitation spectra of (a) gallium-corrole 1 (λex = 540 nm, λem = 650 nm), (b) BODIPY 2
(λex = 470 nm, λem = 590 nm), (c) BODIPY 3 (λex = 550 nm, λem = 750 nm), and (d) BODIPY 7 (λex = 510 nm, λem = 670 nm) in 2-MeTHF at 298
K.
Figure 3. (black) Absorption, (red) emission, and (blue) excitation spectra of (a) dyad 4 (λex = 470 nm, λem = 670 nm) and compounds (b) 5 (λex
550 nm, λem = 740 nm), (c) 8 (λex = 550 nm, λem = 670 nm), and (d) 9 (λex = 510 nm, λem = 670 nm) in 2-MeTHF at 298 K.
=
singlet energy transfer may occur and contribute to the
observed intensity of the acceptor. However, these ΦF values
turn out to be lower than that of their BODIPY (3) and
gallium-corrole (1) precursors (i.e., 0.016 for compound 8 and
0.093 for compound 5), indicating that some additional
nonradiative processes occur, namely, internal conversion
(kic) S1 ∼> S0.
The fluorescence lifetimes (τF) of each dyad were measured
separately at the fluorescence maxima of the gallium-corrole
and the BODIPY chromophores at 298 and 77 K. Dyad 4
exhibits two emissions from the BODIPY (donor, 540 nm) and
the gallium-corrole unit (acceptor, 613 nm; τF = 1.9 0.1 (298
as the acceptor, the slight decrease in τF indicates the presence
of a small nonradiative process associated with the incorpo-
ration of the flexible chain also called “loose bolt” effect (which
takes part in the internal conversion rate, kic).25 Its rate can be
approximated by kLB = (1/τF(4-“Ga”) − (1/τF(1)) ≈ 7.2 × 107
s−1, where τF(4-“Ga”) is the fluorescence lifetime of the
acceptor chromophore (i.e., gallium-corrole) and τF(1) is the
lifetime of the gallium-corrole model compound. This
estimation is relevant with regard to the added uncertainties
in the evaluation of the slower rates of energy transfers below.
Concurrently, the fluorescence decay of the BODIPY residue is
biphasic in dyad 4 (τF (in ns) = 0.7 0.3 (52%) and 3.0 0.1
(48%) at 298 K; 1.7 0.4 (71%) and 4.5 0.6 (29%) at 77 K)
and is shorter than that measured for the BODIPY-containing
K), 2.6
0.1 (77 K)). In comparison with the model
compound 1 (τF = 2.2 0.1 (298 K), 3.2 0.1 (77 K)) acting
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Table 2. Fluorescence Quantum Yields (ΦF) and Emission
Table 4. Energy Transfer Rates (kET) Calculated with eq 1
(2-MeTHF)
Maxima
λem (max) (nm)
kET (108 s−1
)
298 K
77 K
D*→A
298 K
77 K
a
a
4
5
(red-BODIPY* → Cor)
12 (85%)
1.2 (35%)
45 (91%)
0.7 (13%)
1.7 (27%)
0.8 (30%)
1.0 (18%)
4.9 (82%)
1.2 (54%)
17 (84%)
dye
ΦF
corrole
BODIPY
corrole
BODIPY
b
b
c
1
2
3
4
5
6
7
8
9
0.11
0.59
0.42
612, 670
616, 671
b
(Cor* → blue-BODIPY)
543
681
540
675
684
603
676
539, 569
696, 750
534, 565
680, 744
689, 744
611, 655
678, 742
651
b
b
b
b
d
no transfer
b
be
,
8
9
(Cor* → blue-BODIPY)
(green-BODIPY*→Cor)
(Cor*→green-BODIPY)
0.4 (13%)
1.0 (40%)
0.4 (12%)
0.036
0.093
613, 667
613
616, 671
608
b
b
de
,
d
0.44
a
b
c
Cor = gallium-corrole chromophore. This value is most likely mixed
with the loose bolt effect, and thus no reliable value for kET can be
0.76
de
,
0.016
0.082
611
610
ce
,
extracted.
613, 667
611, 668
a
The quantum yields, ΦF, were measured in 2-MeTHF at 298 K. All Φ
b
are corrected for changes in refractive index. Reference = rhodamine
c
0.1 (77 K). These values are also shorter than that for the
model compound 3 (τF (in ns) = 6.3 0.1 and 12.1 0.3).
The estimated kLB values are 1.2 × 108 (298 K) and 0.9 × 108
s−1 (77 K). This qualitative analysis indicates that the
contribution of kLB in the dyads ranges from ∼0.7 × 108 to
∼1.2 × 108 s−1, and the conclusion is that the apparent
deactivation rates evaluated for slow processes are most likely
composed of two contributions, namely, kET (rate of energy
transfer) and kLB, which may be of comparable sizes. Table 3
summarizes all the τF data and can be separated into two
categories: the flexible chain is placed near (2−5) or far (6−9)
from the BF2 unit. The main reason for this separation is that
the fluorescence decays of the donor exhibit a double
exponential for compounds 4 and 5 (presumably due to a
chain folding) but not in dyads 8 and 9. The flexible chains in
these two series slightly differ by the presence of a (CO)NH
in the former sets instead of an −O− link used in the second
series.
6G (ΦF = 0.94 in methanol). Reference = cresyl violet (ΦF = 0.54 in
d
methanol). Reference = rhodamine 101 (ΦF = 1.00 in meth-
anol).10,11,24 Total quantum yield (BODIPY and corrole).
e
model compound 2 (τF = 4.6 0.1 (298 K) and 9.7 0.3 (77
K)). The short component is clearly faster than that for
compound 2 by ∼6−7 fold and is consistent with the presence
of an efficient energy transfer process (BODIPY*→gallium-
corrole). This efficiency of ET is obtained from ETeff = ((1/τF)
− (1/τF°))/(1/τF) (where τF° and τF are, respectively, the
fluorescence lifetimes of donor in the absence and presence of
an acceptor), and the data are placed in Table 4. This value is in
the order of 85 and 82% at 298 and 77 K, respectively, in this
case. The presence of a second component suggests that at least
two conformations, associated with the possible folding and
unfolding of the flexible chain, exist in solution. The possibility
that the loose bolt effect contributes to the decrease in τF exists,
but the calculated rate (1/τF(4-“BODIPY”)) − (1/τF(2)),
where τF(4-“BODIPY”) is the lifetime of the slow component
of the BODIPY fluorescence decay, gives a value (∼1.2 × 108
s−1) that is only ∼2-fold larger than the kLB evaluated for the
gallium-corrole chromophore in compound 4 (i.e., ∼7.2 × 107
s−1). Similarly, dyad 5 exhibits a blue BODIPY acting as the
energy acceptor with τF (in ns) = 3.6 0.1 (298 K) and 5.8
Singlet Energy Transfer Rates. The rate of energy
transfer (i.e., kET) for each dyad was evaluated using the τF data
placed in Table 3. The kET values were calculated using eq 1:
⎛
⎞
1
1
kET
=
−
⎜
⎝
⎟
⎠
τ
τFo
(1)
F
Table 3. Fluorescence Lifetimes (τF) in 2-MeTHF
298 K
77 K
τF1 (ns)
dye
λexc
λem
τF1 (ns)
τF2 (ns)
λem
τF2 (ns)
(nm)
(nm)
(rel. int.)
(rel. int.)
(nm)
(rel. int.)
(rel. int.)
1
2
3
4
527
470
550
470
612
543
681
540
2.2 0.1
4.6 0.1
6.3 0.1
0.7 0.3
(0.52)
619
540
695
534
3.2 0.1
9.7 0.3
12.1 0.3
1.7 0.4
(0.71)
3.0 0.1
(0.48)
4.5 0.6
(0.29)
613
611
1.9 0.1
0.2 0.1
(0.87)
615
611
2.6 0.1
0.5 0.2
(0.68)
5
550
1.9 0.1
(0.13)
3.3 0.1
(0.32)
675
684
603
611
676
613
667
3.6 0.1
7.0 0.1
5.4 0.1
1.6 0.1
3.7 0.2
1.8 0.1
3.8 0.2
679
690
611
610
678
611
666
5.8 0.1
11.9 0.4
6.8 0.1
2.8 0.1
5.0 0.1
2.6 0.1
4.1 0.1
6
7
8
510
510
550
9
510
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Figure 4. Superposition of the normalized absorption and fluorescence spectra in 2-MeTHF at 298K of (A) 1 (blue) (ε: 27 100 M−1 cm−1) with 2
(red), (B) 3 (red) (ε: 92 800 M−1 cm−1) with 1 (blue), (C) 1 (blue) (ε: 27 100 M−1 cm−1) with 7 (red), and (D) 7 (red) (ε: 81 200 M−1 cm−1) with
1 (blue). The absorption and fluorescence spectra of the BODIPYs and gallium-corroles are in red and blue, respectively. The overlaps are shaded in
gray. Note that the folded conformation drawn for the dyads is only to make them fit within the graphs.
Table 5. Values of J, kF°(D), kET(cal), and kET(exp) for dyads 4, 5, and 9 (Both Directions) at 298 K
a
κ2
a
ET direction
θA (deg)
θD (deg)
ϕ (deg)
r
(Å)
kET(cal) (108 s−1
)
kET(exp) (108 s−1
)
kET(cal)/kET(exp)
b
kF° = 1.28 × 108 s−1
c
J = 2.7 × 10−14
4B
4C
4D
4E
red BODIPY*→Cor
red-BODIPY*→Cor
red-BODIPY*→Cor
red-BODIPY*→Cor
kF° = 0.50 × 108 s−1
J = 34 × 10−14
129.6
55.7
88.0
57.4
134.3
120.5
97.3
19.6
67.7
58.1
56.8
0.2
1.5
0.3
1.8
17.9
47
272
26
12
12
12
12
3.9
23
18.7
21.1
19.0
2.2
25
119.0
298
5B
5C
5D
5E
Cor*→blue-BODIPY
Cor*→blue-BODIPY
Cor*→blue-BODIPY
Cor*→blue-BODIPY
kF° = 0.67 × 108 s−1
J = 5.2 × 10−14
green-BODIPY*→Cor
green-BODIPY*→Cor
green-BODIPY*→Cor
green-BODIPY*→Cor
green-BODIPY*→Cor
kF° = 0.50 × 108 s−1
J = 4.1 × 10−14
129.6
55.7
88.0
57.4
134.3
120.5
97.3
19.6
67.7
58.1
56.8
0.2
1.5
0.3
1.8
17.9
18.7
21.1
19.0
231
1338
128
45
45
45
45
5.1
30
2.8
32
119.0
1465
d
d
9B
9C
9D
9E
9F
82.7
98.9
95.0
148.0
113.4
149.9
129.6
81.5
50.3
26.2
32.6
27.0
0.03
0.1
2.4
0.4
0.3
16.5
17.5
22.3
17.1
20.7
12
27
0.8
15
d
d
0.8
34
d
d
56.9
153
125
30
0.8
190
d
d
94.2
0.8
156
d
d
100.0
0.8
37
d
d
9B
9C
9D
9E
9F
Cor*→green-BODIPY
Cor*→green-BODIPY
Cor*→green-BODIPY
Cor*→green-BODIPY
Cor*→green-BODIPY
82.7
98.9
95.0
148.0
113.4
149.9
129.6
81.5
50.3
26.2
32.6
27.0
0.03
0.1
2.4
0.4
0.3
16.5
17.5
22.3
17.1
20.7
7
16
90
74
18
1.0
7
d
d
1.0
16
d
d
56.9
1.0
90
d
d
94.2
1.0
74
d
d
100.0
1.0
18
a
b
The r and κ2 values result from the DFT computations (optimized geometries; Figure 1). kF°(D) = ΦF(D)/τF(D); these data are from Tables 2
c
d
and 3, respectively. Cor = gallium-corrole chromophore. J is in mmol−1 cm6. This value is unreliable due to the loose bolt effect (see text).
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where τF° and τF are, respectively, the fluorescence lifetimes of
donor in the absence and presence of an acceptor. Thus, these
values are extracted from the model compounds (2, 3, 6, and 7)
and dyads (4, 5, 8, and 9), respectively. This method is more
accurate than measuring the ΦF values for strongly overlapping
fluorescence and absorption spectra, as both the area under the
corrected fluorescence spectrum and the exact absorbance of
the donor chromophore are not accessible with enough
precision. The kET values are summarized in Table 4; the
values marked with footnote-b indicate that the value is most
likely mixed with the loose bolt effect kLB, and consequently the
only reliable conclusion is that the energy transfer process is
either nonexistent or too slow to reliably access kET.
encountered before where a ratio of kET(cal)/kET(exp) between
11 to 31 was noted.28 The main reason for this is that the
Forster approach is considered an approximation.29 Nonethe-
̈
less, comparisons are still possible.
One important issue is that the relative size of both kET(exp)
and kET(cal) between compounds 4 and 5 (same flexible chain)
and 9 (two directions, so same flexible chain) follow the same
relative trend. Indeed, the values of both kET(cal) and kET(exp)
are ∼5 times larger for compound 5 than are those for 4. This
comparison indicates that the dominant parameter for this
effect is the J integral, which is ∼10 times larger for compound
5 by virtue of the larger absorptivity of the acceptor (for
compound 3, ε = 92 800 M−1 cm−1). For compound 9, the
values of both kET(cal) and kET(exp) are larger for the green-
BODIPY*→Cor process than they are for the Cor*→green-
BODIPY process. In this case, both kF°(D) and J contribute to
the larger size of kET. It is also interesting to note that the
kET(cal) values for 4 and 5 falls into two groups, slow and fast
rates, where ∼1 order of magnitude separates the two groups.
This observation corroborates well with the presence of two τF
values (i.e., two kET values) in these cases, suggesting that two
families of conformations provide similar rates, namely, directed
by κ2.
Singlet Energy Transfer Analysis. The Forster theory26 is
̈
used to interpret the experimental kET values of the dyads 4, 5,
8, and 9. The calculated rates of energy transfer are obtained
using eq 2:
8.8 × 10−25κ2kF°(D)
kET
=
× J
n4r6
(2)
where kF°(D) = ΦF°/τF°, with ΦF° being the fluorescence
quantum yield of the donor in the absence of the acceptor, r is
the center-to-center donor−acceptor separation, n is the
refractive index of the solvent (here 1.406), κ2 is an orientation
factor describing the relative orientation of the electronic dipole
moments of the associated transitions of the donor and
acceptor (κ2 = (sin(θD)sin(θA)cos(ϕ) − 2 cos(θD)cos(θA)2,
where θD and θA are the angles made by the donor−acceptor
vector with the transition moment vector of the donor and
acceptor, respectively). For both BODIPY and gallium-corrole
units, the transition moment is oriented along the center of the
chromophore toward Cmeso. J is the spectral overlap (∫ FD(λ)-
εA(λ)λ4dλ/∫ FD(λ)dλ, in mmol−1 cm6, also called the J integral)
of the donor fluorescence spectra (FD(λ)) and the acceptor
absorption profile (εA(λ)). Figure 4 overlays the fluorescence
spectra of the donors with the absorption spectra of the
acceptors of the necessary model compounds building the
dyads. For 9, the proximity of the absorption and fluorescence
bands for both units makes J non-nil for both directions and
should be considered.
The calculated (cal) values of J, kF°(D), structural parameters
(r, ϕ, θD, θA), kET(cal), and kET(exp) for dyads 4, 5, and 9 (both
directions) are placed in Table 5. In the absence of X-ray
structures, structural parameters were extracted from DFT
calculations (B3LYP; geometry optimizations placed in Figure
1). The J values compare favorably to that recently reported for
a pyrene−BODIPY dyad (Figure 5).27 The calculated kET
values are ∼2 to ∼30 times larger than the experimental ones
for compounds 4 and 5. For compound 9, this ratio is larger
but unreliable. This situation is not uncommon as it has been
However, one issue needs to be addressed. The kET values for
the recently reported pyrene−BODIPY dyad (Figure 5) are
faster by 1 to 2 orders of magnitude than those reported here.27
The authors assumed (based on the Forster theory) that the
̈
center-to-center distance is 20 Å (similar to the unfolded
conformations investigated here), which is the distance
between the center of pyrene and the center of BODIPY.
However, the pyrene unit is conjugated all the way to the
triazole linker (i.e., N-phenyl position), and so the effective
distance is much shorter than 20 Å. Indeed, by changing 20 by
9.6 Å in eq 2, the calculated kET increases by 2 orders of
magnitude. Moreover, a relationship between the Cmeso−Cmeso
distance (not center-to-center) and kET was demonstrated for a
series of cofacial bisporphyrins,30 and also demonstrated was
that any atom or group placed exactly between the closest
positions (i.e., closest carbons) between the donor and the
acceptor slowed down the energy transfer process.31 The
selection of the correct r value turns out to be a difficult task. In
addition, the Dexter mechanism can also contribute to the
overall process for this pyrene−BODIPY (Figure 5).
Consequently, kET(total) can be the sum of both contributions,
that is, the Forster and the Dexter, and this despite unfavorable
̈
dihedral angles between the phenyl group and the donor and
acceptor moieties. This phenomenon, where unfavorable
dihedral angles are present and yet efficient energy transfers
occur, was recently demonstrated by us for truxene-containing
polyporphyrin dyads.32 The obvious absence of conjugation in
the compounds investigated in this work and the large donor−
acceptor separations (avoiding any orbital overlap) preclude
this mechanism. Considering these parameters, the kET values
for dyads 4, 5, 8, and 9 are bound to be significantly slower
than that reported for the pyrene−BODIPY dyad of Figure 5.
CONCLUSION
■
Dyads incorporating various types of BODIPY chromophores,
here called red, green, and blue, and gallium-corroles have been
synthesized in good yields using the click chemistry. The
direction of the singlet energy transfer, gallium-corrole*→
BODIPY or BODIPY*→ gallium-corrole, can be modulated
depending on whether zero, one, or two styryl groups are
Figure 5. Structure of the recently investigated dyad pyrene−
BODIPY.
3401
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Inorganic Chemistry
Article
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the Forster theory). The qualitative analysis of the k data
̈
ET
using FRET clearly corroborates the presence of conformers in
solution; the folded conformers are more stable for compounds
4 and 5, and the unfolded one is more stable for compound 9,
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integral plays a key role on the size of the kET value.
ASSOCIATED CONTENT
■
S
* Supporting Information
Comparison of absorbance and emission spectra of compounds
1
2, 7, and 3. H NMR spectra, mass spectra, and superposition
of absorption and excitation spectra of each compound. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: claude.gros@u-bourgogne.fr. Phone: 33 (0)3 80 39 61
12. Fax: 33 (0)3 80 39 61 17. (C.P.G.)
■
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Phys. Chem. A 2012, 116 (15), 3889−3898.
*E-mail: christine.goze@u-bourgogne.fr. Phone: 33 (0)3 80 39
90 43. Fax: 33 (0)3 80 39 61 17. (C.G.)
*E-mail: pierre.harvey@USherbrooke.ca. Phone: 819-821-7092.
Fax: 819-821-8017. (P.D.H.)
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The Centre National de la Recherche Scientifique (ICMUB,
UMR CNRS 6302) is gratefully thanked for financial support.
■
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the 3MIM integrated project (Marquage de Molecules par les
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Ministry of Research for Ph.D. grants. P.D.H. thanks the
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