Guest-induced modulation of the energy transfer process in porphyrin-based artificial light harvesting dendrimers

Article abstract of DOI:10.1021/jacs.6b11804

A series of dendritic multiporphyrin arrays (P_ZnTz-nP_FB; n =2, 4, 8) comprising a triazole-bearing focal zinc porphyrin (P_Zn) with a different number of freebase porphyrin (P_FB) wings has been synthesized, and their photoinduced energy transfer process has been evaluated. UV/vis absorption, emission, and time-resolved fluorescence measurements indicated that efficient excitation energy transfer takes place from the focal P_Zn to P_FB wings in P_ZnTz-nP_FB's. The triazole-bearing P_Zn effectively formed host-guest complexes with anionic species by means of axial coordination with the aid of multiple C-H hydrogen bonds. By addition of various anionic guests to P_ZnTz and P_ZnTz-nP_FB's, strong bathochromic shifts of P_Zn absorption were observed, indicating the HOMO-LUMO gap (ΔE_HOMO-LUMO) of P_Zn decreased by anion binding. Time-resolved fluorescence measurements revealed that the fluorescence emission predominantly takes place from P_Zn in P_ZnTz-nP_FB's after the addition of CN^-. This change was reversible because a treatment with a silver strip to remove CN fully recovered the original energy transfer process from the focal P_Zn to P_FB wings.

Full text of DOI:10.1021/jacs.6b11804

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Article
Guest-induced Modulation of Energy Transfer Process in
Porphyrin-based Artificial Light Harvesting Dendrimers
Dajeong Yim, Jooyoung Sung, Serom Kim, Juwon Oh, Hongsik
Yoon, Young Mo Sung, Dongho Kim, and Woo-Dong Jang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b11804 • Publication Date (Web): 15 Dec 2016
Downloaded from http://pubs.acs.org on December 17, 2016
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Dajeong Yim, Jooyoung Sung,† Serom Kim, Juwon Oh, Hongsik Yoon, Young Mo Sung, Dongho
Kim,* and Woo-Dong Jang*
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Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul 120-749, Korea
porphyrin • energy transfer • host-guest chemistry • photoswitching • fluorescence
ABSTRACT: A series of dendritic multi-porphyrin arrays (PZnTz-nPFB; n = 2, 4, 8) comprising a triazole-bearing focal zinc
porphyrin (PZn) with a different number of freebase porphyrin (PFB) wings has been synthesized and their photoinduced
energy transfer process has been evaluated. UV/Vis absorption, emission, and time-resolved fluorescence measurements
indicated that efficient excitation energy transfer takes place from the focal PZn to PFB wings in PZnTz-nPFBs. The triazole-
bearing PZn effectively formed host–guest complexes with anionic species by means of axial coordination with the aid of
multiple C-H hydrogen bonds. By addition of various anionic guest to PZnTz and PZnTz-nPFBs, strong bathochromic shifts
of PZn absorption were observed, indicating the HOMO-LUMO gap (ΔEHOMO–LUMO) of PZn decreasedby anion binding. Time-
resolved fluorescence measurements revealed that the fluorescence emission predominantly takes place from PZn in PZnTz-
nPFBs after the addition of CN-. This change was reversible because a treatment with a silver strip to remove CN- fully
recovered the original energy transfer process from the focal PZn to PFB wings.
for the precise regulation of light–energy conversion pro-
cesses. LHCs possess various molecular accessories for the
regulation of energy transduction by the formation of su-
pramolecular complexes with specific proteins or ionic
species.4,27-29 The binding of molecular accessories to LHCs
can turn on or off the major function of light harvesting.
For example, under strong sun light condition, plants mod-
ulate their energy transfer pathways to protect their light
harvesting system from photo-damage.4
Natural photosynthetic systems convert solar energy to
chemical energy with extremely high energy conversion ef-
ficiency.1-9 Photosynthesis is initiated when light is ab-
sorbed by light-harvesting antenna complexes (LHCs),
which are composed of three-dimensional arrays of a large
number of porphyrin derivatives.6,9 The elegant architec-
ture of LHCs enables effective light absorption as well as
excitation energy transfer to the reaction center.7 Such
structural uniqueness as well as the extraordinary high en-
ergy conversion efficiency offer strong motivations to syn-
thetic chemists upon the molecular designing in photo-
functional materials. For example, as the mimicry of LHCs,
various types of multi-porphyrin arrays have been synthe-
sized to investigate their light energy transduction.10-16
Typically, porphyrin-based dendritic architectures have at-
tracted much attention because their gradient structures
contributed to the directional energy transfer from periph-
ery to the focal core.14,15 Such the directional energy trans-
fer processes would be one of the key factors in the high
energy conversion efficiency of natural light harvesting
systems.1-5 Natural directional energy transfer phenomena
have inspired the designs of artificial photofunctional de-
vices, including photovoltaic,17-19 electroluminescent,9,20,21
and biomedical devices.22-26 Although directed energy
transfer can be achieved by simple conjugation of donor–
acceptor pairs, natural photosynthetic systems involve
much more complicated mechanisms than artificial ones
We recently reported porphyrin-based molecular twee-
zers with a bisindole bridge, which exhibits excellent
switching of energy transfer direction in response to the
coordination of a metal ion with the bisindole bridge.30 Us-
ing this metal–bisindole coordination process, we have
controlled the energy transfer process between two heter-
ogeneous systems, namely a zinc porphyrin and the bisin-
dole bridge.
In the present study, we attempted to develop a new ar-
tificial light harvesting dendritic multiporphyrin arrays in
which guest binding to the center of dendritic architecture
can modulate the energy transfer process. A series of multi-
porphyrin dendrimers (PZnTz-nPFB; n = 2, 4, 8, Chart 1),
composed of a triazole-bearing focal zinc porphyrin (PZn)
with a different number of freebase porphyrin (PFB) wings,
was synthesized and their photoinduced energy transfer
process was evaluated. Structural fragments of PZnTz-nPFB,
namely, PZnTz and PFB-OH, were also prepared for control
experiments (Chart 1).
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Materials and Measurements. All commercially avail- Time Correlated Single Photon Counting Measure-
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able reagents were reagent grade and used without further
purification. Dichloromethane (CH2Cl2) and tetrahydrofu-
ran (THF) were freshly distilled before each use. All anions
were used as a form of tetrabutylammonium salt, which
have used as received from Sigma-Aldrich. Steady-state
electronic absorption spectra and fluorescence emission
spectra were on a JASCO V-660 and JASCO FP-6300 spec-
trometers, respectively. All steady-state measurements
were carried out by using a quartz cuvette with a path
length of 1 cm at ambient temperatures. H and C NMR
spectra were recorded on a Bruker Advance DPX 250, DPX
400 spectrometer at 25 °C in CDCl3, THF-d8, and DMSO-d6.
MALDI-TOF-MS was performed on Bruker Daltonics
LRF20 with dithranol (1,8,9-trihydroxyanthracene) as the
matrix. Recycling SEC was performed on JAI model LC9021
equipped with JAIGEL-1H, JAIGEL-2H and JAIGEL-3H col-
umns using CHCl3 (J. T. Baker) as the eluent. Analytical
SEC was performed on a JASCO HPLC equipped with HF-
403HQ and HF-404HQ columns (Shodex, Tokyo, Japan)
using THF as the eluent.
ment. Time-resolved fluorescence lifetime experiments
were performed by the time-correlated single-photon-
counting (TCSPC) technique. As an excitation light source,
we used a Ti:sapphire laser (Mai Tai BB, Spectra-Physics)
which provides a repetition rate of 800 kHz with ~ 100 fs
pulses generated by a homemade pulse-picker. The output
pulse of the laser was frequency-doubled by a 1 mm thick-
ness of a second harmonic crystal (-barium borate, BBO,
CASIX). The fluorescence was collected by a microchannel
plate photomultiplier (MCP-PMT, Hamamatsu, R3809U-51)
with a thermoelectric cooler (Hamamatsu, C4878) con-
nected to a TCSPC board (Becker & Hickel SPC-130). The
overall instrumental response function was about 25 ps
(the full width at half maximum (fwhm)). A vertically po-
larized pump pulse by a Glan-laser polarizer was irradiated
to samples, and a sheet polarizer, set at an angle comple-
mentary to the magic angle (54.7°), was placed in the fluo-
rescence collection path to obtain polarization-independ-
ent fluorescence decays.
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Computational Method. Quantum mechanical calcu-
lation was performed with the Gaussian 09 program suite.31
All calculations were carried out by the density functional
theory (DFT) method with Becke’s three-parameter hybrid
exchange functional and the Lee-Yang-Parr correlation
functional (B3LYP), employing the 6-31G(d,p) basis set.32,33
The oscillator strength was calculated by performing time
dependent (TD)-DFT calculation.
CHART 1. Chemical structures of multi-porphyrin
dendrimers (PZnTz-nPFB) and their structural frag-
ments (PZnTz and PFB-OH).
C8H17
O
N
NH
N
N
N
N
M eO 2
C
O H
HN
N
N
N
N
C8H17
O
M eO
Zn
O M e
PFB-O H
CO 2M e
N
Synthesis. The synthetic procedure of PZnTz and PZnTz-
nPFB are outlined in Scheme 1. Hydroxy-group-bearing por-
phyrin (PFB-OH), bromide-bearing porphyrin (PFB-Br), di-
pyrromethane, and 2-((trimethylsilyl)ethynyl)benzalde-
hyde (1) were synthesized according to the literature pro-
cedure.34-36 The synthetic procedures of compounds 2-11
are shown in supporting information.
N
N
N
N
Tz
=
CO 2M e
N
PZnTz
Tz
O C8H17
HN
N
N
N
N
N
C8H17
O
O
HN
N
N
O
Zn
O
NH
N
O C8H17
NH
C8H17
Tz
O C8H17
O C8H17
.
PZnTz: CuSO4 5H2O (479 mg, 1.9 mmol) and sodium
PZnTz-2PFB
N
N
C8H17
O
O C8H17
ascorbate (373 mg, 1.9 mmol) were added to a mixture of 6
(148 mg, 0.19 mmol) and methyl 4-(azidomethyl)benzoate
(310 mg, 0.94 mmol) in 20 mL THF/H2O (1:1). The reaction
mixture was stirred for 5 h at 50 °C, and then poured into
CH2Cl2 (100 mL). The organic layer was separated. After
evaporation of the solvent under reduced pressure, the res-
idue was purified using column chromatography with 1%
MeOH/CH2Cl2 as the eluent to give PZnTz as a purple pow-
der (147 mg, 67%): 1H NMR (400 MHz, DMSO-d6, 25 °C)
= 8.75-8.74 (d, 4 H, J = 4.0 Hz), 8.63-8.62 (d, 4 H, J = 4.0
Hz), 8.59-8.57 (d, 2 H, J = 8.0 Hz), 8.13-8.11 (d, 2 H, J = 8.0
Hz), 8.01-7.99 (d, 4 H, J = 8.0 Hz), 7.97-7.95 (t, 2 H, J = 8.0
Hz), 7.80-7.78 (t, 2 H, J = 8.0 Hz), 7.33-7.31 (d, 4 H, J = 8.0
Hz), 7.03-7.01 (d, 4 H, J = 8.4 Hz), 5.72-5.70 (d, 4 H, J = 8.4
Hz), 4.71 (s, 2 H), 4.40 (s, 4 H), 4.04 (s, 6 H), 3.78 (s, 6 H).
13C NMR (100 MHz, CDCl3, 25 °C)  = 165.82, 159.52, 150.90,
149.88, 147.52, 139.94, 138.60, 135.64, 134.93, 134.60, 132.93,
131.57, 129.14, 129.03, 128.54, 127.47, 126.68, 125.76, 122.07,
121.17, 119.40, 112.43, 55.77, 52.22, 51.98, 0.21. MALDI-TOF-
HN
NH
NH
HN
Tz
N
N
N
N
O
O
O
O
N
N
N
N
Zn
Tz
NH
HN
HN
NH
C8H17
O
O C8H17
N
N
PZnTz-4PFB
O C8H17
O C8H17
O C8H17
C8H17
O
C8H17
O
O C8H17
N
N
NH
HN
HN
NH
N
N
C8H17
O
O C8H17
NH
HN
N
N
O
O
C8H17
O
O C8H17
N
N
HN
NH
Tz
O
O
O
O
O
O
O
O
N
N
N
N
Zn
Tz
HN
NH
N
N
C8H17
O
O C8H17
O
O
N
N
NH
HN
PZnTz-8PFB
C8H17
O
O C8H17
N
N
HN
N
NH
N
+
NH
HN
MS: m/z: calcd. for C68H50N10O6Zn: 1168.57 [M] ; found
1166.60.
C8H17
O
O C8H17
O C8H17
C8H17
O
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Scheme 1. Synthesis of PZnTz-nPFB (n = 2, 4, 8)
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2
3
4
5
HO
Y
TMS
X
B
OTBDMS
CO2Me
HO
NH HN
+
N
N
N
N
N
N
N3
N
N
PZnTz
ZO
Zn
OZ
X
Zn
TMS
Y
6
7
8
9
TMS
O
Me
TMS =
H
Si Me
Me
1
2: X = H
3: X = Br
4: Y = TMS, Z = TBDMS
5: Y = H, Z = H
Me
Si
TBDMS =
6: Y = H, Z = Me
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11
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Me
N
Tz =
N
N
CO2Me
Tz
N
CO2Me
N
N
N3
PFB-Br
HO
Zn
OH
5
PZnTz-2PFB
N
Tz
OC8H17
OC8H17
HN
N
N
7
R
NH
PFB-Br : R = Br
PFB-OH: R = OH
OTBDMS
OTBDMS
HO
Tz
B
TMS
HO
CO2Me
ZO
ZO
OZ
OZ
HO
HO
OH
N
N
N
N
8
N
N
N
N
N3
PFB-OH
PZnTz-4PFB
Zn
Zn
3
OH
TMS
Tz
11
9: Y = TMS, Z = TBDMS
10: Y = H, Z = H
OC8H17
HN
N
N
O
O
NH
OH
OH
OC8H17
OC8H17
PFB-Br
11
PZnTz-8PFB
HO
HO
NH
N
N
HN
OC8H17
12
PZnTz-2PFB: A dry THF solution (6 mL) of 7 (60 mg,
H, J = 8.0 Hz), 8.21-8.19 (d, 4 H, J = 8.4 Hz), 8.19-8.17 (d, 2
H, J = 8.0 Hz), 8.11-8.09 (d, 4 H, J = 8.0 Hz), 8.00-7.96 (t, 2
H, J = 8.0 Hz), 7.79-7.75 (t, 2 H, J = 8.0 Hz), 7.63-7.60 (d, 4
H, J = 8.4 Hz), 7.44 (s, 4 H), 6.94-6.92 (m, 6 H), 5.76-5.74
(m, 8 H), 4.55 (s, 2 H), 4.30 (s, 4 H), 4.19-4.15 (t, 8 H, J = 8.0
Hz), 3.78 (s, 6 H), 1.93-1.86 (m, 8 H), 1.42-1.25 (m, 40 H),
0.0525 mmol), PFB-Br (90 mg, 0.110 mmol), K2CO3 (29 mg,
0.210 mmol), and 18-crown-6 ether (1.4 mg, 0.00525 mmol)
was refluxed under N2 for 14 h, and subsequently evapo-
rated. The residue was dissolved in CH2Cl2 (100 mL) and
washed with water (100 mL x 3), and evaporated. The resi-
due was chromatographed on silica gel using 1%
MeOH/CH2Cl2 as the eluent, and the product was collected,
evaporated to dryness, and freeze-dried with benzene to
give PZnTz-2PFB as a purple powder in 87% yield (119.1 mg):
1H NMR (400 MHz, CDCl3, 25 °C) = 10.35 (s, 4 H), 9.45-
9.41 (m, 8 H), 9.23-9.22 (d, 4 H, J = 4.8 Hz), 9.19-9.18 (d, 4
H, J = 4.8 Hz), 9.06-9.05 (d, 4 H, J = 4.0 Hz), 8.93-8.92 (d, 4
H, J = 4.0 Hz), 8.75-8.73 (d, 2 H, J = 8.0 Hz), 8.43-8.41 (d, 4
13
0.89-0.86 (t, 12 H, J = 4.0 Hz), -3.11 (s, 4 H). C NMR (100
MHz, CDCl3, 25 °C) = 165.94, 159.06, 158.80, 151.05, 150.10,
147.73, 147.26, 145.55, 145.39, 143.24, 141.57, 140.01, 138.73,
136.65, 135.90, 135.40, 135.07, 133.11, 131.93, 131.84, 131.47,
131.20, 129.30, 127.70, 126.83, 126.71, 125.94, 122.17, 121.31,
119.70, 119.32, 118.80, 114.75, 113.63, 105.55, 101.10, 70.71, 52.40,
52.22, 32.04, 29.64, 29.48, 26.36, 22.89, 14.34, 0.22. MALDI-
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+
TOF-MS: m/z: calcd. for C164H154N18O10Zn: 2602.47 [M] ;
found 2599.77.
of DIAD was added to the reaction mixture at 0 °C. Then
1
2
after stirring for 12 h, the mixture solution was washed with
water (100 mL) and the organic layer was evaporated in
vacuo. The residue was purified by column chromatog-
raphy with EtOAc/CH2Cl2 and recycling SEC with CHCl3 as
the eluent. PZnTz-8PFB was obtained as a purple solid: 1H
NMR (400 MHz, CDCl3, 25 °C)  = 9.98 (s, 16 H), 9.22-9.20
(d, 4 H, J = 4.0 Hz), 9.17-9.16 (d, 16 H, J = 4.0 Hz), 9.12-9.11
(d, 16 H, J = 4.0 Hz), 9.05-9.04 (d, 16 H, J = 4.0 Hz), 8.91-
8.90 (d, 16 H, J = 4.0 Hz), 8.87-8.86 (d, 4 H, J = 4.0 Hz),
8.43-8.41 (d, 2 H, J = 8.0 Hz), 8.10-8.08 (d, 16 H, J = 8.0 Hz),
7.94-7.92 (d, 2 H, J = 8.0 Hz), 7.77-7.73 (t, 2 H, J = 8.0 Hz),
7.72 (s, 4 H), 7.69-7.67 (d, 16 H, J = 8.0 Hz), 7.61-7.57 (t, 2 H,
J = 8.0 Hz), 7.38 (s, 4 H), 7.37 (s, 20 H), 6.97 (s, 8 H), 6.93
(s, 4 H), 6.88 (s, 10 H), 6.34-5.32 (d, 4 H, J = 8.0 Hz), 5.34 (s,
16 H), 5.01-4.99 (d, 4 H, J = 8.0 Hz), 4.63 (s, 2 H), 4.09-4.05
(t, 32 H, J = 6.4 Hz), 3.52 (s, 6 H), 3.41 (s, 4 H), 1.82-1.79 (m,
32 H), 1.48-1.40 (m, 32 H), 1.25-1.23 (m, 128 H), 0.84-0.81 (m,
48 H), -3.27 (s, 16 H). MALDI-TOF-MS: m/z: calcd. for
C486H502N42O32Zn: 7508.83 [M] +; found 7513.75.
PZnTz-4PFB: A dry THF solution (1 mL) in a schlenk tube
of 11 (57 mg, 0.049 mmol), PFB-OH (154 mg, 0.21 mmol),
and PPh3 (54 mg, 0.21 mmol) was stirred at 0 °C under N2
for 30 min, and 0.11 mL (40% in toluene 1.9 M, 0.21 mmol)
of DEAD was added to the reaction mixture at 0 °C. After
stirring for 12 h, the mixture solution was washed with wa-
ter (100 mL) and the organic layer was evaporated in vacuo.
The residue was purified by column chromatography with
1% MeOH/CH2Cl2 and recycling SEC with CHCl3 as the el-
uent. Then, after short column chromatography, PZnTz-
4PFB was obtained by recrystallization as a reddish powder:
1H NMR (400 MHz, CDCl3, 25 °C)  = 10.24 (s, 8 H), 9.37-
9.36 (d, 8 H, J = 4.4 Hz), 9.28-9.27 (d, 8 H, J = 4.4 Hz), 9.25-
9.24 (d, 4 H, J = 4.8 Hz), 9.21-9.20 (d, 8 H, J = 4.8 Hz), 9.09-
9.08 (d, 8 H, J = 4.4 Hz), 8.97-8.95 (d, 4 H, J = 4.4 Hz), 8.31-
8.29 (m, 10 H), 8.09-8.07 (d, 2 H, J = 7.6 Hz), 7.97-7.95 (d,
8 H, J = 7.6 Hz), 7.80 (s, 4 H), 7.51 (s, 2 H), 7.44-7.43 (d, 8
H, J = 2.0 Hz), 7.43-7.41 (d, 2 H, J = 7.6 Hz), 7.39-7.35 (t, 2
H, J = 6.8 Hz), 6.93-6.92 (t, 4 H, J = 2.0 Hz), 6.75-6.73 (d, 4
H, J = 8.0 Hz), 5.67-5.63 (m, 8 H), 4.69 (s, 2 H), 4.16-4.10 (m,
24 H), 3.64 (s, 6 H), 1.91-1.84 (m, 16 H), 1.53-1.46 (m, 16 H),
1.39-1.25 (m, 64 H), 0.88-0.85 (t, 24 H, J = 6.8 Hz), -3.13 (s,
8 H). 13C NMR (100 MHz, CDCl3, 25 °C)  = 170.99, 165.74,
158.82, 158.52, 150.50, 150.38, 147.77, 147.31, 147.18, 145.50,
145.36, 144.69, 143.25, 141.47, 140.03, 138.48, 136.59, 135.39,
133.11, 133.08, 133.03, 132.15, 132.08, 132.05, 131.85, 131.43, 131.10,
129.44, 129.21, 126.60, 126.16, 122.09, 121.20, 119.86, 119.32,
118.74, 115.80, 115.75, 115.69, 114.88, 114.85, 114.79, 114.72,
105.50, 101.19, 77.44, 70.70, 68.63, 52.26, 32.03, 29.93, 29.62,
29.46, 26.35, 22.87, 14.31, 0.22. MALDI-TOF-MS: m/z: calcd.
for C262H262N26O16Zn: 4096.43 [M] +; found 4096.76.
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Absorption and emission spectra of all the compounds
studied here were acquired in CH2Cl2: PZnTz, PFB-OH,
PZnTz-nPFBs, and 1:n mixtures of PZnTz and PFB-OH
(PZnTz/nPFB-OH) as noncovalent references for PZnTz-
nPFB (Figure 1). PZnTz exhibited a strong Soret absorption
peak at 423.5 nm and two Q-band absorption peaks at 550.5
and 589.5 nm. The Soret absorption peak of PFB-OH ap-
peared at 407.5 nm, a slightly shorter wavelength region
than that of PZnTz. The Q-band absorptions of PFB-OH ap-
peared at 502.5, 536.5, 575.0, and 630.0 nm. The lowest ex-
citation energy of PFB-OH was slightly smaller than that of
PZnTz. Because the emission band of PZnTz overlaps with
the absorption band of PFB, the excitation energy of the fo-
cal PZn unit is expected to transfer efficiently to the PFB
wings.
12: 3,5-Dihydroxy benzyl alcohol (83.8 mg, 0.59 mmol),
PFB-Br (990 mg, 1.2 mmol), K2CO3 (411 mg, 3.0 mmol), and
18-crown-6-ether were mixed in a 100 mL two-neck round
bottomed flask. The flask was degassed under high vacuum
and back-filled with N2. Acetone (9 mL) and dry DMF (3
mL) were added under nitrogen. The reaction mixture was
refluxed for 12 h. The solution was quenched with water
and removed acetone. The mixture was dissolved in CH2Cl2
and washed with water. The organic layer was separated.
After evaporation of the solvent under reduced pressure,
the residue was purified using column chromatography
with CH2Cl2 to give 12 (860.4 mg, 90%): 1H NMR (400 MHz,
CDCl3, 25 °C)  = 10.30 (s, 4 H), 9.40-9.39 (d, 4 H, J = 4.8
Hz), 9.38-9.37 (d, 4 H, J = 4.4 Hz), 9.20-9.19 (d, 4 H, J = 4.8
Hz), 9.14-9.13 (d, 4 H, J = 4.4 Hz), 8.37-8.35 (d, 4 H, J = 8.0
Hz), 7.97-7.95 (d, 4 H, J = 8.0 Hz), 7.44-7.43 (d, 4 H, J = 2.0
Hz), 7.03-7.02 (d, 1 H, J = 2.0 Hz), 6.99-6.98 (d, 2 H, J = 2.0
Hz), 6.93-6.92 (t, 2 H, J = 2.0 Hz), 5.55 (s, 4 H), 4.89-4.87 (d,
2 H, J = 6.0 Hz), 4.18-4.14 (t, 8 H, J = 8.0 Hz), 1.89-1.83 (m,
8 H), 1,54-1.48 (m, 8 H), 1.40-1.28 (m, 32 H), 0.88-0.85 (t, 12
H, J = 6.4 Hz), -3.11 (d, 4 H). MALDI-TOF-MS: m/z: calcd.
for C105H116N8O7: 1602.09 [M] +; found 1602.28.
As expected, the fluorescence emissions of PZnTz-nPFB
and PZnTz/nPFB-OH differed considerably. Upon 400 nm
excitation, corresponding to the absorption of PFB,
PZnTz/nPFB-OH exhibited emission peaks at 634.0 and
696.0 nm, consistent with the emission of PFB-OH alone.
Upon excitation at 430 nm, corresponding to the absorp-
tion of PZn, PZnTz/nPFB-OH exhibited emission peaks at
600.5 and 648.0 nm, consistent with the emission of PZn
alone. In sharp contrast, PZnTz-nPFB exhibited emission
peaks at 634.0 and 696.0 nm, being well matched with the
emission of PFB-OH (Figure 1 and Table 2). Notably, the
shape of the emission spectrum was not varied by changing
the excitation wavelength, reflecting efficient energy trans-
fer from PZn to PFB. Therefore, when we excite the focal PZn
unit in PZnTz-nPFB, the emission takes place predomi-
nantly from PFB wings.
PZnTz can effectively form host–guest complexes with
anionic species by means of axial coordination with the aid
of multiple C-H hydrogen bonds provided by triazole
groups.35-38 Table 1 summarizes the association constants
for associations between various anionic species and PZnTz.
Binding of F−, N3−, and CN− to PZnTz was typically strong.
PZnTz-8PFB: A dry THF solution (2 mL) in a schlenk tube
of 11 (29 mg, 0.025 mmol), 12 (158.2 mg, 0.099 mmol), and
PPh3 (25.9 mg, 0.099 mmol) was stirred at 0 °C under N2
for 30 min, and 0.05 mL (40% in toluene 1.9 M, 0.099 mmol)
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Axial coordination of guest species onto the porphyrin in-
duces bathochromic shifts in the absorption spectra, the
1
2
degree of which depends on the electron-donating ability
of the guest species: CN− > N3− > Cl− > F− > AcO− (Figure 2).
The addition of I− and Br− led to an appearance of shoulders
in the absorption spectra, but saturated binding by these
weakly binding guest species would require that they
should be present in large excess. Absorption spectrum of
PZnTz (2.0 M in CH2Cl2) exhibited strong bathochromic
shifts with clear isosbestic points by addition of CN- in the
form of tetrabutylammonium salt (Figure S1).39 1H NMR
spectrum of PZnTz showed downfield shift of triazole pro-
tons by addition of CN- (Figure S2). The continuous varia-
tion method (Job’s plot) indicated the formation of 1:1
host–guest complexes (Figure S1). The bathochromic shift
again confirmed the CN- binding in PZnTz-nPFB (S4, and
S5). The addition of anionic guests caused the absorption
originating from the focal PZn to shift to long-wavelength
regions, whereas the peaks originating from the PFB wings
did not change at all.
3
4
5
6
7
8
9
Figure 2. UV-Vis response of PZnTz (2 M) to various anions
(10 eq.) in CH2Cl2.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
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35
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37
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39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Fluorescence spectra changes without CN- (solid line)
and with CN- (dotted line) in CH2Cl2. a) PZnTz (2 M); b) PFB-
OH (2 M); c) PZnTz-2PFB (2 M); d) PZnTz/2PFB-OH (1:2 mix-
ture) (2 M); e) PZnTz-4PFB (1 M); f) PZnTz/4PFB-OH (1:4
mixture) (1 M); g) PZnTz-8PFB (0.5 M); h) PZnTz/8PFB-OH
(1:8 mixture) (0.5 M). ex = 400, 430 and 440 nm. [CN-]/[Host]
= 10 eq.
Table 1. Association constants K for the complex for-
mation of PZnTz[a] with various anionic guests[b] at 298
K in CH2Cl2.
Figure 1. Absorption (solid line) and fluorescence (dotted line)
spectra of a) PFB-OH (red, 2.0 M) and PZnTz (black, 2.0 M),
b) PZnTz-2PFB (2.0 M), c) PZnTz/2PFB-OH (2.0 M of PZnTz),
d) PZnTz-4PFB (1.0 M), e) PZnTz/4PFB-OH (1.0 M of PZnTz),
f) PZnTz-8PFB (0.5 M), g) PZnTz/8PFB-OH (0.5 M of PZnTz)
in CH2Cl2, where ex = 400 (red) and 430 nm (blue).
-
Anion
F-
Cl-
Br-
I-
OAc-
6.46
N3
CN-
15.3
K/105
10.2
4.06
0.21
0.02
7.46
[a] The concentration of PZnTz was 2.0 M. [b] All guests were in the form of
tetrabutylammonium salts.
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Because absorption shifts indicate changes in the
PZnTz, indicating that the addition of CN− causes energy
transfer from PFB to PZn to take place in PZnTz-nPFB.
1
2
3
4
5
6
7
8
HOMO–LUMO gap (ΔEHOMO–LUMO), the binding of anionic
guests may influence the energy transfer process in PZnTz-
nPFB. The bathochromic shift of the focal PZn unit in PZnTz-
nPFB indicates decreased EHOMO–LUMO. If the EHOMO–LUMO
of the focal PZn unit becomes smaller than that of the PFB
wings, the energy transfer should take place from PFB wings
to the focal PZnTz unit. Because the greatest bathochromic
shift of PZnTz occurred by CN− addition, we investigated
the influence of CN− coordination to the focal PZn unit on
the vertical transition energy and EHOMO–LUMO by using
DFT calculations at the B3LYP/6-31g(d,p) level based on
geometry-optimized structures (Figure S6). The calculated
EHOMO–LUMO of PZnTz (2.82 eV) was slightly larger than
that of PFB (2.76 eV). However, the EHOMO–LUMO of PZnTz
became smaller than that of PFB upon coordination with
CN− (2.64 eV), reinforcing our hypothesis that the energy
transfer direction can be reversed by addition of CN−.
To investigate quantitatively the energy transfer dynam-
ics in PZnTz-nPFB, time-resolved fluorescence decay pro-
files were obtained by time-correlated single photon
counting technique. The fluorescence decay profile of
PZnTz was fitted well by the time constant of 1.7 ns, a simi-
lar singlet state lifetime of 5,10,15,20-tetrakis aryl zinc por-
phyrin. PFB-OH exhibited a single exponential decay pro-
file with a time constant of 8.2 ns, a slightly shorter singlet
state lifetime than that of 5,10,15,20-tetrakis aryl freebase
porphyrin.40 Interestingly, whether the PFB wings or the fo-
cal PZn moieties were excited selectively, the fluorescence
decay profiles of PZnTz-2PFB showed the same singlet state
lifetimes of 8.2 ns (Figure 4 and Table 3). No short decay
component arising from the singlet state of the PZn moie-
ties was observed, indicating efficient intramolecular en-
ergy transfer from PZn to PFB moieties in PZnTz-2PFB. Simi-
larly, PZnTz-4PFB also showed only the singlet state lifetime
of 8.3 ns indicating efficient energy transfer from PZn to PFB
moieties (Figure 5). The fluorescence decay curve of PZnTz-
8PFB was fitted by two time components 7.4 (71%) and 1.3
(29%) ns, indicating the energy transfer efficiency is lower
than those of PZnTz-2PFB and PZnTz-4PFB due to the in-
creased distance between PZn and PFBs by the additional
hydroxymethyl benzyl linker in PZnTz-8PFB (Figure 5 and
Table 4).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
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49
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Table 2. Wavelengths of fluorescent emission of PZnTz,
PFB-OH, PZnTz-nPFB, and PZnTz/nPFB-OH.
Without CN-
With CN-
440
400[a]
430
400
-
600.5,[b]
648.5
628.5,
685.0
PZnTz
-
634.0,
696.0
634.0,
696.0
PFB-OH
-
-
634.0,
696.0
634.0,
696.0
631.5,
689.5
631.5,
689.5
PZnTz-2PFB
PZnTz-4PFB
PZnTz-8PFB
PZnTz/2PFB-OH
PZnTz/4PFB-OH
PZnTz/8PFB-OH
634.0,
696.0
634.0,
696.0
632.0,
690.5
632.0,
690.5
635.0,
697.0
635.0,
697.0
634.0,
696.0
634.0,
696.0
634.0,
696.0
600.5,
648.0
634.0,
696.0
628.5,
685.0
633.5,
695.5
600.5,
649.0
633.5,
695.5
628.0,
685.0
Figure 4. Time-resolved fluorescence decay profiles of PZnTz-
2PFB (2.0 M) in CH2Cl2 upon the addition of CN- (0, 1, 3, 5 eq.).
Each decay profiles were monitored at 700 nm with photoex-
citation at a) 400 nm and b) 445nm.
633.5,
695.5
600.5,
647.0
634.5,
696.5
629.0,
686.5
[a] ex / nm. [b] em / nm.
Table 3. Best-fit parameters of the fluorescence decay
[a]
Fluorescence spectra of PZnTz, PFB-OH, PZnTz-nPFB, and
PZnTz/nPFB-OH were measured again in the presence of
CN− (10.0 eq) (Figure 3). Figure 3 shows the spectral
changes of PZnTz-2PFB and PZnTz/2PFB-OH emission by the
addition of CN−. The emission peaks of PZnTz, originally
observed at 600.5 and 648.0 nm, were shifted to 628.5 and
685.0 nm by the addition of CN−, again indicating the re-
duced EHOMO–LUMO. In contrast, the emission bands of PFB-
OH, observed at 634.0 and 696.0 nm, were not changed at
all regardless of the addition of CN− (Table 2). For the case
of PZnTz-2PFB, while its emission spectrum coincided with
that of PFB-OH without CN− addition, the emission bands
of PZnTz-2PFB were slightly hypsochromically shifted and
decreased in intensity upon the addition of CN-. Similar
features were observed for PZnTz-4PFB and PZnTz-8PFB
(Figure 3). Here, the emission spectra of PZnTz-nPFB with
CN- addition corresponded to that of the CN− complex of
profiles of PZnTz-2PFB with various concentration of
titrated CN-[b] in CH2Cl2.
400[c]
445
[CN-][b]
1 / ns
8.20
2 / ns
1 / ns
7.80
2 / ns
0
1
n.d.[c]
n.d.
8.19 (51%)
7.45 (2%)
7.50 (1%)
2.04 (49%)
1.70 (98%)
1.73 (99%)
6.8 (11%)
n.d.[c]
n.d.
1.80 (89%)
1.73
3
5
1.75
[a] The concentration was 2.0 M. [b] [CN-]/[PZnTz-2PFB] = 0, 1, 3, 5. [c] ex
nm. [d] Not detected. Relative errors of  values are less than ± 0.06 ps.
/
By addition of CN- to PZnTz-nPFB, intriguing features
were observed in a series of fluorescence decay profiles. An
additional short decay component of about 1.7 ns was ob-
served upon the addition of CN− to PZnTz-2PFB (Figure 4
and Table 3). Because this lifetime constant was consistent
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would also be lower than those of PZnTz-2PFB and PZnTz-
1
2
3
4
5
6
4PFB. Although the energy transfer efficiency of PZnTz-8PFB
was not as greatly high as that of PZnTz-2PFB and PZnTz-
4PFB, the directional control of energy transfer was ob-
served within porphyrin-based artificial light harvesting
dendrimer.
7
8
9
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41
42
43
44
45
46
47
48
49
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51
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60
Figure 5. Time-resolved fluorescence decay profiles of a-b)
PZnTz-2PFB (2.0 M), c-d) PZnTz-4PFB (1.0 M), and e-f) PZnTz-
8PFB (0.5 M) with (blue circle) and without (black circle) 10
eq. CN- addition in CH2Cl2. Each decay profiles were moni-
tored at 630 nm with photoexcitation at a, c, e) 400 nm and b,
d, f) 445nm.
Table 4. Best-fit parameters of the fluorescence decay
profiles of PZnTz-nPFB with and without 10 eq. CN- ad-
dition in CH2Cl2.
400[a]
445
1 / ns
2 / ns
n.d.[b]
1.7 (99%)
n.d.
1 / ns
8.2
2 / ns
n.d.
1.7
PZnTz-2PFB
8.2
PZnTz-2PFB + CN-
PZnTz-4PFB
6.5 (1%)
8.3
n.d.
8.3
n.d.
2.0
(86%)
PZnTz-4PFB + CN-
6.5 (14%)
7.4 (71%)
4.8 (8%) 1.9 (92%)
Figure 6. Absorption changes of PZnTz-2PFB by treatment
with CN- and silver strip. a) UV/Vis spectral changes, b) ab-
sorption monitored at 424 nm (black, solid line) and 440 nm
(blue, dotted line).
PZnTz-8PFB
1.9 (29%)
7.4 (72%) 1.9 (28%)
4.3 (51%). 1.2 (49%)
PZnTz-8PFB + CN-
4.7 (40%) 1.6 (60%)
[a]ex / nm. [b] not detected.
The energy transfer direction in PZnTz-nPFB can be con-
trolled in a reversible manner. Initially, the excitation en-
ergy in PZnTz-nPFB flows from the focal PZn to the PFB wings.
After the addition of CN−, the excitation energy of PFB
wings flows to the focal PZn. Owing to the high affinity of
silver to CN−, we could remove CN− from the solution by
treating with a silver strip. The changes in the absorption
spectra of PZnTz-2PFB observed after CN− addition were re-
versed after subsequent treatment with silver (Figure 6);
the effect on fluorescence emission was similar (Figure S7).
Therefore, the energy transfer direction in PZnTz-2PFB can
be reversibly controlled by treatment with CN− and silver.
with that of PZnTz, we presumed that this newly observed
fast decay component originated from the focal PZn moiety.
Therefore, it can be again inferred that anionic binding of
the PZn moiety with CN− lowers the lowest energy state of
PZn, leading to a reversal in the direction of intramolecular
energy transfer. Moreover, as the concentration of CN− was
increased, the short-lifetime component became dominant,
eventually reaching 99% for the addition of 5 eq CN−. Sim-
ilarly, the fluorescence decay component of 1.9 ns became
predominant with 10 eq. of CN- addition to PZnTz-4PFB
(Figure 5 and Table 4). Consequently, we concluded that
the coordination of CN− to the focal PZn almost completely
reversed the energy transfer direction. In the case of PZnTz-
8PFB, the ratio of short lifetime component increased by ad-
dition of CN- (Figure 5 and Table 4). As aforementioned,
the energy transfer efficiency from PZn to PFB moieties in
PZnTz-8PFB was relatively lower than those of PZnTz-2PFB
and PZnTz-4PFB due to the increased distance from Pzn to
PFBs in PZnTz-8PFB. Therefore, the efficiency of reverse di-
rectional energy transfer from PFB moieties to the focal PZn
As a mimicry of light harvesting antenna, we have de-
signed multi-porphyrin dendrimers comprising a triazole-
bearing focal zinc porphyrin with a different number of
freebase porphyrin wings. By addition of various anionic
guest, we could control HOMO–LUMO gap of the focal
zinc porphyrin. Therefore, the energy transfer pathway in
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Jeong, Y.-H.; Son, M.; Yoon, H.; Kim, P.; Lee, D.-H.; Kim, D.;
the artificial light harvesting dendrimer could be modu-
lated by guest bindings. Such a guest-induced modulation
of energy transfer pathway possibly provides a new strat-
egy toward molecular-based photonic switch or molecular
machinery.
Jang, W.-D. Angew. Chem. Int. Ed. 2014, 53, 6925.
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dongho@yonsei.ac.kr
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The authors declare no competing financial interest.
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This work was supported by the Mid-Career Researcher Pro-
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Program (2013K1A1A2A02050183) funded by the National Re-
search Foundation (NRF) of Korea, Ministry of Science, ICT &
Future, and the Korea Institute of Energy Technology Evalua-
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