Synthesis and photodynamics of fluorescent blue bodipy-porphyrin tweezers linked by triazole rings

Article abstract of DOI:10.1021/jp300415a

Novel zinc porphyrin tweezers in which two zinc porphyrins were connected with π-conjugated boron dipyrromethenes (BDP meso-Por₂ and BDP β-Por₂) through triazole rings were synthesized to investigate the photoinduced energy transfer and electron transfer. The UV-vis spectrum of BDP β-Por₂ which has less bulky substituents than BDP meso-Por₂ exhibits splitting of the Soret band as a result of the interaction between porphyrins of BDP β-Por₂ in the excited state. Such interaction between porphyrins of both BDP β-Por₂ and BDP meso-Por₂ is dominant at room temperature, while the coordination of the nitrogen atoms of the triazole rings to the zinc ions of the porphyrins occurs at low temperature. The conformational change of the BDP-porphyrin composites was confirmed by the changes in UV-vis and fluorescence spectra depending on temperature. Photodynamics of BDP meso-Por₂ and BDP β-Por₂ has also been investigated by laser flash photolysis. Efficient singlet-singlet energy transfer from the ZnP to the π-conjugated BDP moiety of both BDP meso-Por₂ and BDP β-Por₂ occurred in opposite direction as compared to energy transfer from conventional BDP to ZnP due to the π-conjugation in nonpolar toluene. In polar benzonitrile, however, additional electron transfer occurred along with energy transfer.

Full text of DOI:10.1021/jp300415a

Article
pubs.acs.org/JPCA
Synthesis and Photodynamics of Fluorescent Blue BODIPY-Porphyrin
Tweezers Linked by Triazole Rings
Antoine Eggenspiller,† Atsuro Takai,‡,∥ Mohamed E. El-Khouly,‡ Kei Ohkubo,‡ Claude P. Gros,†
Claire Bernhard,† Christine Goze,† Franck Denat,† Jean-Michel Barbe,*,† and Shunichi Fukuzumi*,‡,§
†ICMUB, UMR CNRS 6302, Universite
́
de Bourgogne, 9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France
‡Department of Material and Life Science, Graduate School of Engineering, Osaka University, and ALCA, Japan Science and
Technology Agency (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
§Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: Novel zinc porphyrin tweezers in which two
zinc porphyrins were connected with π-conjugated boron
dipyrromethenes (BDP meso-Por2 and BDP β-Por2) through
triazole rings were synthesized to investigate the photoinduced
energy transfer and electron transfer. The UV−vis spectrum of
BDP β-Por2 which has less bulky substituents than BDP meso-
Por2 exhibits splitting of the Soret band as a result of the
interaction between porphyrins of BDP β-Por2 in the excited
state. Such interaction between porphyrins of both BDP β-Por2
and BDP meso-Por2 is dominant at room temperature, while the coordination of the nitrogen atoms of the triazole rings to the
zinc ions of the porphyrins occurs at low temperature. The conformational change of the BDP−porphyrin composites was
confirmed by the changes in UV−vis and fluorescence spectra depending on temperature. Photodynamics of BDP meso-Por2 and
BDP β-Por2 has also been investigated by laser flash photolysis. Efficient singlet−singlet energy transfer from the ZnP to the π-
conjugated BDP moiety of both BDP meso-Por2 and BDP β-Por2 occurred in opposite direction as compared to energy transfer
from conventional BDP to ZnP due to the π-conjugation in nonpolar toluene. In polar benzonitrile, however, additional electron
transfer occurred along with energy transfer.
efficiency and kinetics of energy transfer and electron
transfer.17,18 Thus, in the case of porphyrins covalently linked
with other organic dyes, the linker unit of porphyrin assemblies
plays important roles to control the conformation of each
molecule and photodynamics.14
INTRODUCTION
■
Ensembles of different organic dyes that can undergo efficient
energy transfer and photoinduced electron transfer has
attracted much current interest not only for the development
of solar-energy conversion system1−3 but also for the rational
design of fluorescent sensors and optoelectronic devices.4
Boron dipyrromethene (BDP) fluorophores, which are
derived from 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-
diaza-s-indacene,5 have been widely employed as light-harvest-
ing antenna and electron donor or acceptor as well as laser dyes
in a variety of applications, because a large number of BDP
derivatives are readily obtained by modification at carbon
positions 1, 3, 5, 7, and 8 exhibiting attractive photophysical
and photochemical characteristics with strong absorption bands
in the visible to near-infrared region.6−10
On the other hand, porphyrinoid assemblies act as light-
harvesting antenna and electron donors in the electron-transfer
cascade in the natural photosynthetic system. Porphyrin
assemblies associated with other organic dyes have extensively
been used as light-harvesting and electron donor−acceptor
composites in artificial photosynthetic systems performing
energy transfer and photoinduced electron transfer.11−16 In
such porphyrin assemblies, the distances and the alignment
among porphyrins and associated molecules significantly affect
A five-membered triazole ring, which is formed as a result of
“click chemistry” between azide and alkyne precursors,19 is
regarded as a significant π-conjugated linker unit, because
photodynamics of porphyrin-containing assemblies is signifi-
cantly affected by the position and electronic states of the
triazole ring.20 In addition, it is known that the nitrogen atom
of the triazole ring can coordinate to zinc ion of a zinc
porphyrin to form a stable porphyrin dimer where π−π
interaction between zinc porphyrins is observed.21 Such
coordination of the triazole nitrogen atom to the zinc ion of
porphyrin as well as π−π interaction between zinc porphyrins is
expected to induce dramatic change in conformation, photo-
physical properties, and photodynamics of porphyrin-contain-
ing ensembles.17a,18 The conformation change of porphyrin
assemblies bridged by triazole linker can be induced by change
Received: January 13, 2012
Revised: March 13, 2012
Published: March 21, 2012
© 2012 American Chemical Society
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of temperature. However, the temperature control of the π−π
interaction between porphyrins associated with the coordina-
tion of triazole nitrogen atom to zinc ion of porphyrin has yet
to be reported.
We report herein synthesis of covalently linked BDP−
porphyrin tweezers bridged by triazole linkers, which are shown
in Scheme 1, to investigate the conformation change by
spectrometry (MALDI/TOF-MS). Absolute dichloromethane
(CH2Cl2, Carlo Erba) for synthesis, CH2Cl2 and toluene
spectroscopic grade (Nacalai Tesque, Inc.) for analysis,
DABCO (Wako Pure Chemical Industries, Ltd.) were obtained
commercially and used without further purification. Benzoni-
trile (PhCN) was purchased from Wako Pure Chemical
Industries, Ltd., and distilled over P2O5 under reduced pressure.
The precursors, 5-p-tolyldipyrromethane25 and 2,3,7,8,12,18-
hexamethyl-5-(4-azidomethylphenyl)-13,17-diethylporphyrina-
to-Zn (II) 3,26 were synthesized as already described.
Scheme 1
Physicochemical and Photophysical Measurements.
1H NMR spectra were recorded on a Bruker Avance II 300
(300 MHz) or on a Bruker Avance DRX 500 (500 MHz)
̂ ́
spectrometers at the “Welience (Pole Chimic Moleculaire)”;
chemical shifts are expressed in parts per million relative to
chloroform (7.26 ppm). UV−vis spectra were recorded on a
Varian Cary 1 spectrophotometer. Mass spectra and accurate
mass measurements (HR−MS) were obtained on a Bruker
Daltonics Ultraflex II spectrometer in the MALDI/TOF
reflectron mode using dithranol as a matrix or on a Bruker
MicroTofQ instrument in electrospray ionization (ESI) mode.
Both measurements were made at the “Welience (Pol
̂
e Chimie
Moleculaire)”. UV−vis−NIR spectra were recorded on a
́
Shimadzu UV−3100PC spectrometer or a Hewlett-Packard
8453 diode array spectrophotometer at various temperatures.
Measurement of fluorescence quantum yields were carried out
on a Hamamatsu C9920−0X(PMA-12) U6039−05 fluores-
cence spectrofluorometer. The fluorescence spectra at various
temperatures were measured on a SPEX Fluorolog τ3
fluorescence spectrophotometer.
Theoretical Calculations. Density-functional theory
(DFT) calculations were performed on a 32-processor
QuantumCube. Geometry optimizations were carried out
using the Becke3LYP functional basis set in Gaussian 09
program, revision A.02.27 The graphics were drawn using the
Gauss View software program (version 5.0) developed by
Semichem, Inc.
Electrochemical Measurements. Cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) were carried
out with a ALS 630B electrochemical analyzer in a deaerated
solvent containing 0.10 M TBAPF6 as a supporting electrolyte
at 298 K. A conventional three-electrode cell was used with a
platinum working electrode and a platinum wire as a counter
electrode. The redox potentials were measured with respect to
the Ag/AgNO3 (1.0 × 10−2 M) reference electrode. The
potential values (vs Ag/AgNO3) are converted to those vs
saturated calomel electrode (SCE) by adding 0.29 V.28
Laser Flash Photolysis. The studied compounds were
excited by a Panther OPO pumped by Nd:YAG laser
(Continuum, SLII-10, 4−6 ns fwhm) with the powers of 1.5
and 3.0 mJ per pulse. The transient absorption measurements
were performed using a continuous xenon lamp (150 W) and
an InGaAs-PIN photodiode (Hamamatsu 2949) as a probe
light and a detector, respectively. The output from the
photodiodes and a photomultiplier tube was recorded with a
digitizing oscilloscope (Tektronix, TDS3032, 300 MHz).
Femtosecond transient absorption spectroscopy experiments
were conducted using an ultrafast source Integra-C (Quan-
tronix Corp.), an optical parametric amplifier TOPAS (Light
Conversion Ltd.), and a commercially available optical
detection system Helios provided by Ultrafast Systems LLC.
The source for the pump and probe pulses were derived from
the fundamental output of Integra-C (780 nm, 2 mJ/pulse, and
temperature and the photophysical properties and photo-
dynamics. The BDP is functionalized to connect two zinc
porphyrin entities through triazole linkers. In these BDP−
porphyrin tweezers, two styryl groups are introduced at the 3-
and 5-positions of the BDP core. The resulting distyryl BDP
Bz2 7, having a more extended π-conjugated system, can absorb
and emit at longer wavelengths (ca. 650 nm) than porphyrin
moieties. Therefore, distyryl BDP works as an energy acceptor
rather than an energy donor in the present system. Although
energy transfer from the BDP to the zinc porphyrin moiety has
been extensively examined in the past decade, energy transfer in
the opposite direction from the porphyrin to π-conjugated BDP
has yet to be scrutinized.16,22−24 We performed femtosecond
laser flash photolysis measurements of BDP−porphyrin
tweezers to reveal the energy transfer and electron-transfer
dynamics in both polar benzonitrile and nonpolar toluene.
EXPERIMENTAL SECTION
■
Chemicals and Reagents. Silica gel (Merck; 70−120 μm)
was used for column chromatography. Analytical thin layer
chromatography was performed using Merck 60 F254 silica gel
(precoated sheets, 0.2 mm thick). Reactions were monitored by
thin-layer chromatography, UV−vis spectroscopy, and matrix-
assisted laser desorption−ionization time of flight mass
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fwhm =130 fs) at a repetition rate of 1 kHz. 75% of the
fundamental output of the laser was introduced into TOPAS,
which has optical frequency mixers resulting in tunable range
from 285 to 1660 nm, while the rest of the output was used for
white light generation. Typically, 2500 excitation pulses were
averaged for 5 s to obtain the transient spectrum at a set delay
time. Kinetic traces at appropriate wavelengths were assembled
from the time-resolved spectral data. All measurements were
conducted at 298 K. The transient spectra were recorded using
fresh solutions in each laser excitation.
tolyldipyrromethane, and 627 mg (5.21 mmol) of p-
tolualdehyde dissolved in 250 mL of deaerated CHCl3. The
reaction mixture was stirred for 15 min and shielded from light,
before dropwise addition of 500 μL (3.98 mmol) of boron
trifluoride diethyl etherate, BF3·OEt2. Stirring was continued
for 1 h 30 min. p-Chloranil (2.6 g, 10.5 mmol) was then added,
and the reaction mixture was kept under stirring for 2 h. After
evaporation of the solvent under reduced pressure, the residue
was redissolved in a CH2Cl2/MeOH mixture (95:5) and
filtered over a pad of silica to remove any polar materials. The
first fraction was isolated; the solvent was evaporated under
vacuum and redissolved in 250 mL of chloroform. Zinc acetate
dihydrate (2.22 g, 10.0 mmol) and sodium acetate (2.4 g, 30.0
mmol) dissolved in 10 mL of absolute methanol were added,
and the solution was then heated under reflux for 2 h, shielded
from light. The metalation reaction was monitored by UV−vis
and MALDI/TOF-MS spectrometry. The reaction mixture was
then cooled to room temperature and washed three times with
750 mL of deionized water. The organic phase was dried over
magnesium sulfate and evaporated under reduced pressure.
After purification by column chromatography over silica using
CH2Cl2/heptane as eluent (70:30), the titled zinc porphyrin 4
was isolated in 11% yield (910 mg, 1.17 mmol) as a purple
solid. 1H NMR (Pyr D5, 298 K, 300 MHz): δ (ppm) = 2.60 (s,
9H, Ph-CH3), 4.76 (s, 2H, Ar-CH2-N3), 7.56 (d, J = 8.1 Hz, 6H,
Htolyl), 7.72 (d, J = 7.8 Hz, 2H, Ar-CH2−N3), 8.31 (d, J = 8.1
Hz, 6H, Htolyl), 8.47 (d, J = 7.8 Hz, 2H, Ar-CH2−N3), 9.20 (m,
8H, Hβ). MS (MALDI/TOF): m/z = 773.42 [M]+•, 773.22
calcd for C48H35N7Zn. HR−MS (ESI): m/z = 773.2271 [M]•+,
773.2240 calcd for C48H35N7Zn. UV−vis (CH2Cl2): λmax (nm)
(ε, 10−3 M−1 cm−1) = 420 (416), 550 (15), 592 (5).
2,6-Diethyl-1,3,5,7-tetramethyl-8-(p-methylbenzoyl)-
BDP (1). 2,4-Dimethyl-3-ethylpyrrole (1.72 g, 14 mmol) and
methyl 4-formylbenzoate (1.16 g, 7 mmol) were dissolved in
700 mL of CH2Cl2. Trifluoroacetic acid (0.03 mL) was added,
and the mixture was stirred at room temperature for 24 h. A
solution of DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone,
1.58 g, 7 mmol) in CH2Cl2 was added to the mixture. Stirring
was continued for 50 min, followed by the addition of 17 mL of
triethylamine and 17 mL of BF3·OEt2, and the reaction mixture
turned purple. After stirring for 1 h, the reaction mixture was
washed with water and dried over sodium sulfate, and the
solvent was evaporated. The residue was purified by silica gel
column chromatography using CH2Cl2/hexane (5:1) as eluent
and recrystallized in a mixture of CH2Cl2 and hexane to give
1
compound 1 as a red solid in 40% yield (1.2 g, 2.7 mmol). H
NMR (CDCl3, 298 K, 300 MHz): δ (ppm) = 0.95 (t, J = 7.5
Hz, 6H, −CH2−CH3), 1.23 (s, 6H, CH3), 2.26 (q, J = 7.5 Hz,
4H, −CH2-CH3), 2.51 (s, 6H, −CH3), 3.95 (s, 3H, -OCH3),
7.38 (d, J = 8.3 Hz, 2H, Ar-COOMe), 8.15 (d, J = 8.3 Hz, 2H,
Ar-COOMe). 11B NMR (CDCl3, 298 K, 128 MHz,): δ (ppm)
= 0.78 ppm (t, J = 33.2 Hz). MS (ESI): m/z = 439.23 [M+H]+,
461.22 [M+Na]+, 899.44 [2M+Na]+, 439.24 calcd for
C25H30BF2N2O2. UV−vis (CH3CN): λmax (nm) (ε, 10−3 M−1
cm−1) = 524 (86.1), 496 (30.4), 379 (9.1). Anal. Calcd for
C24H29BF2N2O2 + 0.1 CH2Cl2: C, 67.47; H, 6.59; N, 6.27;
Found: C, 67.95; H, 6.90; N, 6.15.
1,9-(4-((4-Vinylphenoxy)methyl)-ethynyl)-2,6-diethyl-
1,7-dimethyl-8-(p-methylbenzoyl)-BDP (2). Compound 1
(1.0 g, 2.32 mmol) and 4-(prop-2-yn-1-yloxy)-benzaldehyde
(1.1 g, 6.96 mmol) were refluxed in a mixture of toluene (50
mL), glacial acetic acid (1 mL), and piperidine (1.3 mL). Any
water formed during the reaction was removed by heating
overnight in a Dean−Stark apparatus. The solvent was
evaporated, and the crude solid was purified by silica gel
column chromatography (first EtOAc/Hexane (90:10) and
then CH2Cl2/MeOH (99:1)). The blue fraction was collected,
and the solvent was evaporated. The solid was washed three
times with 50 mL of diethyl ether to give 2 as a solid (350 mg,
21%). 1H NMR (CDCl3, 298 K, 500 MHz): δ (ppm) = 1.13 (t,
J = 7.5 Hz, 6H, −CH2−CH3), 1.27 (s, 6H, −CH3), 2.53 (t, J =
2.4 Hz, 1H, −CCH), 2.57 (q, J = 7.5 Hz, 4H, −CH2-CH3), 3.97
(s, 3H, -OCH3), 4.73 (d, J = 2.4 Hz, 2H, −CH2-CCH), 7.00 (d,
J = 8.8 Hz, 4H, −CHCH-Ar-O), 7.20 (d, J = 17.2 Hz, 2H,
−CHCH-Ar), 7.42 (d, J = 8.2 Hz, 2H, Ar-COOMe), 7.57 (d,
J = 8.8 Hz, 4H, −CHCH-Ar-O), 7.65 (d, J = 17.2 Hz, 2H,
−CHCH-Ar), 8.17 (d, J = 8.2 Hz, 2H, Ar-COOMe). HR−
MS (ESI): m/z = 745.2984 [M+Na]+, 745.3019 calcd for
C45H41BF2N2NaO4. UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1
cm−1) = 652 (80), 422 (10), 363 (6.1).
3,5-(8,12-Diethyl-2,3,7,13,17,18-hexamethyl-20-
para((4-((4-vinylphenoxy)methyl)-triazole)-N-benzyl)-
zinc bisporphyrin)-2,6-diethyl-1,7-dimethyl-8-(p-methyl
benzoyl)-BDP (5) (BDP β-Por2). Under argon and shielded
from light, BDP dialkyne 2 (39.9 mg, 0.051 mmol) and zinc
porphyrin 3 (86.1 mg, 0.133 mmol) were dissolved in 20 mL of
dichloromethane. Copper iodide (10 mg, 0.053 mmol) and
diisopropylethylamine (DIPEA, 20 μL, 0.116 mmol) were then
added, and the mixture was stirred at room temperature for 7 h.
The reaction mixture was diluted by addition of 100 mL of
dichloromethane. The organic phase was washed 3 times with
100 mL of distilled water, dried over magnesium sulfate, and
evaporated under reduced pressure. The crude product was
dissolved in CH2Cl2 and purified by column chromatography
over silica (CH2Cl2/CH3CN (90:10)). After evaporation of the
solvent, the title compound 5 was isolated as a blue
1
microcrystalline solid in 46% (47 mg, 0.023 mmol). H NMR
(Pyridine-D5, 298 K, 300 MHz): δ (ppm) = 1.01 (t, J = 7.5 Hz,
6H, −CH2−CH3 bodipy), 1.33 (s, 6H, −CH3 bodipy), 1.89 (t, J = 7.5
Hz, 12H, −CH2−CH3 porph), 2.36 (s, 12H, −CH3 porph), 2.58 (q,
J = 7.5 Hz, 4H, −CH2-CH3 bodipy), 3.57 ppm (s, 12H,
−CH3 porph), 3.64 (s, 12H, −CH3 porph), 3.95 (s, 3H, −CO−
O−CH3), 4.15 (q, J = 7.5 Hz, 8H, −CH2-CH3 porph), 5.50 (s,
4H, Ar−CH2-triazole), 5.78 (s, 4H, −O−CH2-triazole), 7.26
(d, J = 8.6 Hz, 4H, triazole-CH2-Ar-porph), 7.40 (d, J = 7.9 Hz,
4H, −CHCH-Ar-O), 7.52 (d, J = 8.5 Hz, 2H, bodipy-Ar-
COOMe), 7.61 (d, J = 16.4 Hz, 2H, bodipy-CHCH-Ar),
7.86 (d, J = 8.6 Hz, 4H, triazole-CH2-Ar-porph), 7.93 (d, J = 7.9
Hz, 4H, −CHCH-Ar-O), 8.10 (s, 2H, Htriazole), 8.35 (d, J =
8.5 Hz, 2H, bodipy-Ar-COOMe), 8.41 (d, J = 16.4 Hz, 2H,
bodipy-CHCH-Ar), 10.41 (s, 2H, Hmeso), 10.48 ppm (s, 4H,
5-(4-(Azidomethyl)phenyl)-10,15,20-tritolylporphyrin
Zinc (4). In a 500-mL three-neck flask equipped with a
condenser and N2 bubbling were added 2.56 g (15.9 mmol) of
4-azidomethylbenzaldehyde, 5.01 g (21.2 mmol) of 5-p-
1
Hmeso). See Figure S1 in Supporting Information for H NMR
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spectrum.29 MS (MALDI/TOF): m/z = 2008.65 [M]•+,
2008.79 calcd for C119H115BF2N16O4Zn2. HR−MS (ESI): m/
distyryl derivatives corresponding to one and two condensa-
tions. Interestingly, this condensation reaction provides an easy
way to functionalize the BDP unit.7
z
= , 1004. 3969 calcd for
1004. 3966 [M] 2 +
C
119H115BF2N16O4Zn2 (see Figure S2 in Supporting Informa-
Styryl-BDP diporphyrins were easily prepared from starting
BDP 1 in only two steps involving a Knoevenagel
condensation8b,31 followed by a Huisgen reaction (also well-
known as “Click Chemistry”)32 as described in Scheme 2. We
recently reported the efficiency of click chemistry for the
elaboration of new porphyrin-tripods.26
tion). UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1 cm−1) = 406
(380), 416 (400), 547 (70), 584 (68), 665 (110).
3,5-(5,10,15-Tritolyl-20-para((4-((4-vinylphenoxy)-
methyl)-triazole)-N-benzyl) zinc bisporphyrin)-2,6-di-
ethyl-1,7-dimethyl-8-(p-methylbenzoyl)-BDP (6). (BDP
meso-Por2). The title compound was obtained as a blue
solid in 96% yield (65 mg, 0.028 mmol), as described for 5,
starting from BDP dialkyne 2 (41 mg, 0.029 mmol) and zinc
porphyrin 4 (50 mg, 0.064 mmol). For this coupling reaction,
15 mg (0.078 mmol) of copper iodide and 23 μL (0.133 mmol)
Scheme 2
1
of DIPEA were used. H NMR (Pyridine-D5, 298 K, 300
MHz): δ (ppm) = 1.09 (t, J = 7.5 Hz, 6H, −CH2−CH3 bodipy),
1.33 (s, 6H, −CH3 bodipy), 2.58 (s, 18H, Ar−CH3 tolyl), 2.58 (4H,
−CH2-CH3 bodipy), 3.94 (s, 3H, −CO−O−CH3), 5.46 (s, 4H,
Ar−CH2-triazole), 5.93 (s, 4H, −O−CH2-triazole), 7.19 (d, J =
8.9 Hz, 4H, triazole-CH2-Ar-porph), 7.52 (m, 18H, HAr), 7.64
(d, J = 8.0 Hz, 4H, −CHCH-Ar-O), 7.77 (d, J = 8.9 Hz, 4H,
triazole-CH2-Ar-porph), 8.31 (m, 18H, HAr), 8.39 (s, 2H,
H
triazole), 9.00−9.25 (m, 16H, Hbeta). MS (MALDI/TOF): m/z
= 2249.73 [M-F]+, 2249.76 calcd for C141H111BFN16O4Zn2.
HR−MS (ESI): m/z = 2291.7399 [M+Na]+, 2291.7529 calcd
for C141H111BF2N16O4Zn2Na, 1157.3613 [M+2Na]2+,
1157.3711 calcd for C141H111BF2N16O4Zn2Na2 (see Figure S3
in Supporting Information). UV−vis (CH2Cl2): λmax (nm) (ε,
10−3 M−1 cm−1) = 366 (50), 430 (578), 562 (32), 603 (34),
658 (46).
3,5-(4-((4-Vinylphenoxy)methyl)-triazole)-N-benzyl-
2,6-diethyl-1,7-dimethyl-8-(p-methylbenzoyl)-BDP (7)
(BDP Bz2). The title compound was obtained as a blue solid
in 63% yield (20 mg, 0.020 mmol), as described for 5, starting
from 23 mg of BDP dialkyne 2 (0.032 mmol) and 9.5 mg of
benzyl azide (0.071 mmol). For this coupling reaction, 18 mg
(0.094 mmol) of copper iodide and 25 μL (0.145 mmol) of
DIPEA were used. 1H NMR (Pyridine-D5, 298 K, 300 MHz): δ
(ppm) = 1.10 (t, J = 7.5 Hz, 6H, −CH2−CH3 bodipy), 1.35 (s,
6H, −CH3 bodipy), 2.58 (q, J = 7.5 Hz, 4H, −CH2-CH3 bodipy),
3.94 (s, 3H, −CO−O−CH3), 5.39 (s, 4H, Ar−CH2-triazole),
5.70 (s, 4H, −O−CH2-triazole), 7.12 (d, J = 8.7 Hz, 4H,
−CHCH-Ar-O), 7.35 (m, 10H, Hbenzyl), 7.53 (d, J = 15.8 Hz,
2H, bodipy-CHCH-Ar), 7.54 (d, J = 8.0 Hz, 2H, bodipy-Ar-
COOMe), 7.72 (d, J = 8.7 Hz, 4H, −CHCH-Ar-O), 8.18 (s,
2H, Htriazole), 8.31 (d, J = 15.8 Hz, 2H, bodipy-CHCH-Ar),
8.34 (d, J = 8.0 Hz, 2H, bodipy-Ar-COOMe). MS (MALDI/
TOF): m/z = 988.21 [M]•+, 988.44 calcd for C59H55BF2N8O4,
969.23 [M-F]+, 969.44 calcd for C59H55BFN8O4. HR−MS
(ESI): m/z = 988.4530 [M+H]+, 988.4516 calcd for
C59H56BF2N8O4 (see Figure S4 in Supporting Information).
UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1 cm−1) = 368 (51),
658 (66).
The first coupling step, the Knoevenagel condensation, was
driven by the continuous removal of the water formed during
the reaction, using a Dean−Stark apparatus. The key precursor
2 was obtained in 21% yield after purification by silica gel
column chromatography. The two porphyrin subunits were
further attached to the resulting blue BDP by CuI-catalyzed
cycloaddition. The resulting biszinc β- and meso-substituted
diporphyrins, 5 and 6, were easily obtained in 47 and 96% yield,
respectively.
HR-MS ESI-QTOF measurements were carried out to fully
characterize the blue-BDP porphyrin derivatives. In both cases,
the perfect match between experimental and simulated ionic
patterns undoubtedly confirmed the structure of diporphyrins 5
and 6 (see Figures S2 and S3 in Supporting Information for
HR-MS spectra). In the case of the β-substituted diporphyrin 5,
the calculated mass for the dication [M]2+ is equal to 1004.3969
Da (C119H115BF2N16O4Zn2), agreeing well with the exper-
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Styryl-BDP derivatives
have been prepared by direct condensation of 3,5-dimethyl-
BDPs with aromatic aldehydes to yield red emitting
fluorophores.6−8 Indeed, it has been shown that 3- and 5-
methyl BDP-substituents are acidic enough to participate in
Knoevenagel reactions.7 With p-dialkylaminobenzaldehyde, the
reaction can be restricted to one condensation,8d,30 but 4-
alkoxybenzaldehydes tend to give a mixture of mono- and
1
imental value found at 1004.3966 Da, [M]2+. The H NMR
spectrum of the diporphyrin 5 is informative (see Figure S1 in
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Supporting Information). Indeed, the absence of any signals
(1H) in the 2−3 ppm range corresponding to the −CCH
ethynyl proton and the presence of a singlet at 8.10 ppm (1H)
are both in agreement with the formation of the triazolic linking
unit. In the case of diporphyrin 5, the α-methylene protons of
the azido group are shifted downfield from 4.76 ppm (in
CDCl3) to 5.50 ppm (in pyridine-D5) due to the formation of
the triazole macrocycle and the same trend is also observed for
the singlet signal of the -CH2-CCH protons which are shifted
from 4.73 ppm (in CDCl3) to 5.78 ppm (in pyridine-D5).
A reference compound 7 (BDP Bz2) was also prepared for
comparison in 63% yields by the reaction of BDP dialkyne 2
and benzyl azide (see Experimental Section).
Energy Transfer from Porphyrin to distyryl BDP.
Figure 1 shows UV−vis spectra of BDP, BDP Bz2, BDP meso-
Figure 2. Fluorescence spectra of (a) BDP β-Por2 and (b) BDP meso-
Por2 in CH2Cl2 at 293 K. The excitation wavelengths of BDP β-Por2
and BDP meso-Por2 are 540 and 550 nm, respectively.
energy transfer from the BDP to the porphyrin moiety is
observed in BDP−porphyrin dyad, because the fluorescence
emission of typical BDP is overlapped with the Q-bands of
porphyrin.16,22,23 The inverse energy transfer shown in this
study results from the π-elongation of BDP at 3,5-positions.
The fluorescence emission of porphyrin moieties is overlapped
with the absorption of π-conjugated BDP, particularly in the
case of BDP meso-Por2 (meso-substituted porphyrin).
Figure 1. UV−vis spectra of BDP (black), BDP Bz2 (green), BDP
meso-Por2 (red), and BDP β-Por2 (blue) at 3.0 × 10−6 M in CH2Cl2 at
298 K.
UV−vis spectra of BDP β-Por2 and BDP meso-Por2 in
CH2Cl2 at different temperatures are shown in Figure 3. The
split Soret band of BDP β-Por2 is combined into a single band
at 417 nm upon lowering the temperature from 298 to 223 K.
The spectrum at 223 K is similar to what is observed in
benzonitrile (see Figure S6 in Supporting Information) where
little interaction between porphyrins is expected. The
absorbance of both the Soret and Q bands of BDP meso-Por2
increases with decreasing temperature. These results indicate
that the two porphyrins of BDP β-Por2 and BDP meso-Por2
behave independently at low temperature, which may result
from coordination of the N atoms of triazole rings to the Zn
ions of porphyrins.21 The sharper absorption band of the π-
conjugated BDP unit (λmax = 660 nm) at lower temperature
also suggests such coordination mode to reduce the conforma-
tional fluctuations of π-conjugated BDP. Similar UV−vis
spectral changes of BDP β-Por2 were observed by the addition
of 1,4-diazabicyclo[2.2.2]octane (DABCO) as shown in Figure
S9 in Supporting Information, which again indicates that the
coordination of N atoms of DABCO to two Zn ions of
porphyrin tweezers affects the conformational flexibility of BDP
units in addition to the porphyrin units.
Por2, and BDP β-Por2 in CH2Cl2 at 298 K. Comparison of each
spectrum reveals that the absorption bands at around 660 nm
of BDP Bz2, BDP meso-Por2, and BDP β-Por2 are attributed to
π-conjugated BDP moieties. These characteristic absorption
bands at longer wavelength than those typically observed for
BDP (∼530 nm) result in green or blue color of BDP Bz2, BDP
meso-Por2, and BDP β-Por2 (see a picture in Figure S5 in
Supporting Information). Splitting of the Soret band of BDP β-
Por2 in CH2Cl2 indicates the π-electron overlap between
porphyrins in slipped cofacial conformation.14 Such splitting
was also observed in other noncoordinating solvents such as
CHCl3 and toluene as shown in Figure S6 in Supporting
Information. No splitting of the Soret band of BDP meso-Por2
was observed (Figure 1), indicating that there is less π-electron
overlap between porphyrin rings in BDP meso-Por2 as
compared to that in BDP β-Por2 because of the bulkiness of
the meso substituents.
Fluorescence spectra and absolute fluorescence quantum
yields (Φ) of a series of BDPs and porphyrins are shown in
Figure 2 and Figure S7 in Supporting Information. Even when
porphyrin units of BDP meso-Por2 and BDP β-Por2 are excited,
the fluorescence emission due to BDP moieties are mainly
observed. In addition, the excitation spectrum of BDP β-Por2
clearly shows the contribution of porphyrin units to the
emission of BDP at 680 nm (Figure S8 in Supporting
Information). These observations highlight that energy transfer
from the porphyrin to the BDP moiety occurs.33 Generally,
The optimized structure of BDP meso-Por2 obtained by DFT
calculations is shown in Figure 4. The N atoms of triazole rings
coordinate to Zn ions of porphyrins. This intramolecular
coordination mode is similar to intermolecular coordination
mode of two triazole−porphyrins in a crystal.21 Such closed
conformation of BDP meso-Por2 is more stable by 23 kcal mol−1
than the open form where no intramolecular interaction is
expected (see Figure S10 in Supporting Information).
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Figure 4. Optimized structure of BDP meso-Por2 based on DFT
calculations at the B3LYP/6-31G(d) level: (a) top view and (b) side
view. Hydrogen atoms are not shown for clarity.
Once the fluorescence emission decreases upon lowering the
temperature from 293 to 258 K, the fluorescence due to the
BDP unit is significantly enhanced from 258 to 198 K. The plot
of fluorescence intensity at 680 nm corresponds to the
Figure 3. Temperature dependence of UV−vis spectra of (a) BDP β-
Por2 and (b) BDP meso-Por2 in CH2Cl2.
Temperature dependence of fluorescence spectra of BDP β-
Por2 and BDP meso-Por2 was also investigated as shown in
Figures 5 and 6. Each isosbestic point in the temperature
dependence of absorption spectra (Figure 3) was chosen as an
excitation wavelength (540 and 550 nm for BDP β-Por2 and
BDP meso-Por2, respectively) to excite the porphyrin units
selectively. In the case of BDP β-Por2, the fluorescence due to
the porphyrin unit at 605 nm increases, while the fluorescence
due to the BDP unit at 676 nm decreases with lowering
temperature (Figure 5a). The increase in the fluorescence
emission of the porphyrin may result from (i) the increase in
the viscosity of CH2Cl2 to depress thermal fluctuation34 and
(ii) the coordination of the triazole nitrogen atom to the zinc
ion of porphyrin to decrease forbidden transition caused by the
face-to-face porphyrin interaction as illustrated in Scheme
3.14,35 The decrease in fluorescence emission of BDP indicates
that the efficiency of energy transfer from porphyrin to BDP
decreases because the conformational rigidity at low temper-
ature makes porphyrin and BDP units difficult to come close to
each other. The plot of fluorescence intensities at 604 and 676
nm corresponds to the absorbance change at 667 nm (parts b
and c of Figures 5).
Figure 5. (a) Temperature dependence of fluorescence spectra of BDP
β-Por2 in CH2Cl2 from 293 to 218 K. The excitation wavelength is 540
nm. (b) Plot of fluorescence changes at 604 and 676 nm. (c) Plot of
absorbance change at 667 nm.
On the other hand, the fluorescence spectra of BDP meso-
Por2 at different temperatures change in two steps (Figure 6).
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Figure 7. Plot of absorbance change at 563 nm and fluorescence
change at 680 nm of BDP meso-Por2 in CH2Cl2.
Table 1. One-Electron Redox Potentials (V vs SCE), Energy
Levels of the Charge-Separated States (ΔGCR), and Free
Energy Changes for Charge Separation (ΔGCS) for BDP,
BDP Bz2, BDP β-Por2, and BDP meso-Por2 in PhCN
Figure 6. Temperature dependence of fluorescence spectra of BDP
meso-Por2 in CH2Cl2: (a) from 293 to 258 K and (b) from 258 to 198
K. The excitation wavelength is 550 nm.
Scheme 3
redox potential, V (vs SCE)
Por/
BDP/
BDP/
−ΔGCR
,
−ΔGCS
,
a
b
compound
BDP
Por•+
BDP•+
BDP•−
eV
eV
0.98
0.69
0.70
0.68
−1.31
−1.04
−1.04
−1.05
BDP Bz2
BDP β-Por2
0.50
0.58
1.54
1.63
0.54
0.40
BDP meso-
Por2
a
ΔGCR = e(Eox − Ered). The electrostatic term can be neglected in a
b
polar solvent (PhCN). −ΔGCS = ΔE0−0 − ΔGCR, where ΔE0−0 is the
energy of the lowest excited states of BDP β-Por2* (2.08 eV) and
BDP meso-Por2* (2.03 eV).
1
1
compounds in benzonitrile (PhCN). The redox data are
summarized in Table 1. The BDP unit exhibits both one-
electron oxidation and reduction processes. The elongation of
π-conjugation of BDP unit (BDP Bz2, BDP β-Por2, and BDP
meso-Por2) makes their one-electron oxidation potentials lower
and their one-electron reduction potentials higher than those of
BDP. The first one-electron oxidation potential of porphyrin
unit is lower than that of BDP unit. Thus, the first one-electron
oxidation of BDP-Por2 5 and 6 occurs at the porphyrin unit,
while the first one-electron reduction occurs at the BDP unit.
These results collectively suggest that zinc(II) porphyrins being
electron rich and BDP being electron deficient in these
molecular systems. The molecular orbitals of BDP meso-Por2
based on DFT calculations (B3LYP/6-31G(d) basis set) are
shown in Figure S13 in Supporting Information. The highest
occupied molecular orbital (HOMO) and the HOMO−1 are
delocalized over the porphyrin moieties, while the lowest
unoccupied molecular orbital (LUMO) is localized to the BDP
framework. This result is consistent with the electrochemical
data of BDP meso-Por2 shown in Table 1 and Figure S12 in
Supporting Information.
absorbance change at 563 nm (Figure 7). Only the increase in
fluorescence emission is observed for BDP and BDP Bz2 with
decreasing the temperature (see Figure S11 in Supporting
Information). These results indicate that the conformational
change of BDP meso-Por2 causes decreased fluorescence
emission in the same manner as BDP β-Por2, while restriction
of the free rotation of phenyl rings around the BDP unit causes
increased fluorescence emission in the same manner as BDP
and BDP Bz2.34 The latter is expected to be dominant below
245 K to increase the fluorescence intensity of BDP unit. Such
absorption/fluorescence spectral changes can be observed only
in this type of BDP−porphyrin tweezer composites where
energy transfer from the porphyrin to the BDP unit occurs
efficiently.
Electrochemistry and Energy Levels. To establish the
energy levels of BDP−porphyrin tweezers, electrochemical
studies using cyclic voltammetry were performed. Figure S12 in
Supporting Information shows CV voltammograms of the BDP
β-Por2 5 and BDP meso-Por2 6 as well as the reference
The energy levels of the charge-separated states are also
listed in Table 1. By comparing these energy levels of the
charge-separated states with the energy levels of the excited
states, the driving forces of charge separation (ΔGCS) were also
evaluated.
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Photodynamics of BDP meso-Por2 and BDP β-Por2.
Photodynamics of BDP meso-Por2 and BDP β-Por2 were then
investigated. Time-resolved transient absorption spectra of
BDP meso-Por2 and BDP β-Por2 were recorded by femtosecond
and nanosecond laser flash photolysis in deaerated solvents.
The femtosecond transient absorption spectra of the ZnP
references (see Figures S14 and S15 in Supporting
Information) revealed the instantaneous formation of the
ZnP singlet-excited state features. Here, the transient
absorption spectra exhibited the absorption bands in the visible
region with a maximum at 460 nm, which decayed slowly (4.0
× 108 s−1) to populate the corresponding triplet manifold with
a maximum absorption at 480 nm. Upon excitation of BDP
meso-Por2 at 426 nm in toluene, the absorption spectrum of
BDP β-Por2 at 1 ps after the laser pulse shows characteristic
bands assigned to the singlet excited state of the porphyrin
(Figure 8). With time, the ZnP singlet emission peak at 460 nm
ZnP unit to the singlet ground state of BDP in a polar media
(kENT = 6.2 × 1011 s−1). Interestingly in benzonitrile, the energy
transfer product of the BDP meso-Por2 revealed faster decay
than that observed for the BDP control compound (Figure 9
Figure 9. Differential absorption spectra obtained upon femtosecond
flash photolysis (426 nm) of BDP β-Por2 6 in benzonitrile at 2 and 55
ps. Inset: Time profile of differential absorbance at 670 nm.
and Figure S16 in Supporting Information). These results
suggest occurrence of subsequent electron transfer from the
1
ZnP to the BDP* moiety to produce the charge-separated
product (ZnP•+−BDP•−), because an alternative pathway, i.e.,
energy transfer from 1BDP* to ZnP is energetically not feasible.
On the basis of the decay rates of the 1BDP* of BDP meso-Por2
and the control BDP, the rate constant of the charge-separation
process (kCS) for BDP β-Por2 was determined to be 9.2 × 109
s−1. Similar observations were also made for BDP meso-Por2 in
benzonitrile as shown in Figure S18 in Supporting Information,
Figure 8. Differential absorption spectra obtained upon femtosecond
flash photolysis (426 nm) of BDP β-Por2 in toluene at indicated time
intervals. Inset: Time profile of the singlet-excited state (1BDP* β-
Por2) at 670 nm.
1
where energy transfer from ZnP* to BDP (kENT) followed by
electron transfer from ZnP to 1BDP* (kCS) was clearly
observed. The kENT and kCS values were determined to be 1.3
× 1011 and 1.3 × 109 s−1, respectively.
diminished in intensity with concomitant increase of the BDP
emission at 670 nm providing direct evidence of excitation
transfer from the photoexcited singlet ZnP unit to the singlet
ground state of BDP. The rate constant of energy transfer
The energy diagram of BDP β-Por2 and BDP meso-Por2 are
shown in Scheme 4. It should be noted that the strong overlap
of the ZnP radical cation with the strong bleaching of the BDP
entity has precluded the accurate determination of the rates of
1
charge recombination of BDP•− β-Por2 and BDP•− meso-
•+
(kENT) from ZnP* to the BDP moiety was determined by
exponential fitting of the decay of 1ZnP* and the rise of 1BDP*
to be 8.5 × 1011 s−1. Further, the decay rate constant of 1BDP*
was determined to be 4.4 × 109 s−1, which is close to the decay
of the singlet excited state of BDP Bz2 control compound (see
Figure 16 in Supporting Information). This indicates that the
1BDP* is not quenched by the attached ZnP in toluene. Similar
observations were also made for the investigated BDP meso-
Por2 in toluene (Figure 17 in Supporting Information). The
energy transfer from 1ZnP* to BDP (kENT) was clearly
observed with a rate constant of 3.1 × 1011 s−1, which is
considerably smaller than that of BDP β-Por2.
•+
Por2 in polar benzonitrile.
The complementary nanosecond transient absorption
measurements of BDP meso-Por2 6 and BDP β-Por2 5 in
polar benzonitrile with 550-nm laser excitation, which
selectively excited the ZnP moiety, exhibited no absorption
bands of the radical species suggesting that the charge
recombination process via the singlet porphyrin is too fast to
be detected in the nanosecond region (Figures S19−S21 in
Supporting Information). On the basis of the energy levels of
the CS states and the triplet states of BDP 7 and ZnP control
compounds, one can expect that the radical species decay
rapidly to populate the low-lying triplet BDP 7 (1.15 eV) as
well as the ground states.36,37 Indeed, the transient absorption
spectra of BDP meso-Por2 6 in benzonitrile exhibited weak
absorption bands in the visible region with maxima at around
490 nm (Figure S21 in Supporting Information). These
By changing the solvent from toluene to benzonitrile, the
transient absorption spectra of BDP β-Por2 exhibited
diminished ZnP singlet emission peak at 458 nm with the
concomitant increase of the BDP emission at 669 nm providing
direct evidence of energy transfer from the photoexcited singlet
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Scheme 4
absorption bands can be assigned to the triplet-excited state of
BDP 2, which may populate from the charge recombination in
BDP•− meso-Por2•+. The decay rate constant was determined to
be 2.70 × 104 s−1. It should be noted that the electron transfer
from the triplet porphyrin to BDP is not expected due to the
extremely fast and efficient energy transfer from the singlet ZnP
to the attached π-conjugated BDP entity.
temperature dependence of fluorescence spectra of BDP and
BDP Bz2 (S11), cyclic voltammograms (S12), molecular
orbitals of BDP meso-Por2 based on DFT calculations (S13),
transient absorption spectra upon femtosecond laser excitation
(S14−S18), and transient absorption spectra upon nanosecond
laser excitation (S19−S21). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp (S.F.); Jean-Michel.
Barbe@u-bourgogne.fr (J.-M.B.).
CONCLUSIONS
■
■
The present study has revealed spectroscopic properties and
photodynamics of a new class of blue π-conjugated BDP−zinc
porphyrin tweezer composites. Efficient energy transfer from
the photoexcited singlet zinc porphyrin to the π-conjugated
BDP moiety of the composites occurred in opposite direction
due to the π-conjugation as compared to energy transfer from
the singlet excited state of conventional BDP to zinc porphyrins
as indicated by the fluorescence spectra. The efficiency of
energy transfer was changed by the conformation change due to
coordination of the N atoms of triazole rings to the Zn ions of
porphyrins at low temperature. The dynamics of energy
transfer from the photoexcited singlet porphyrin to the BDP
Present Address
∥Organic Materials Group, Polymer Materials Unit, National
Institute for Materials Science (NIMS), 1−2−1 Sengen,
Tsukuba 305−0047, Japan
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Grants-in-Aid (Nos. 20108010
and 23750014) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, and KOSEF/MEST
through WCU project (R31-2008-000-10010-0) of Korea. A.T.
appreciates a support from the Global COE program ″Global
Education and Research Center for Bio-Environmental
Chemistry″ of Osaka University (S.F.) and JSPS fellowship
for young scientists. The Centre National de Recherche
1
moiety of 5 and 6 in nonpolar toluene to populate the BDP*
was revealed by femtosecond laser flash photolysis measure-
ments. When the solvent was changed to more polar
benzonitrile, the deactivation pathway of the energy transfer
product, 1BDP* was changed to electron transfer involving ZnP
to generate the charge separation product, BDP•−−Por2
.
•+
These results provide new strategy for the design toward
photoswitching devices, which can be controlled by the
conformation change depending on temperature.
́
Scientifique (CNRS, UMR 6302) and “Region Bourgogne”
are also acknowledged for funding.
REFERENCES
ASSOCIATED CONTENT
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