A porphyrin-based molecular tweezer: Guest-induced switching of forward and backward photoinduced energy transfer

Article abstract of DOI:10.1021/ja4124048

A bisindole-bridged-porphyrin tweezer (1), a pair of zinc porphyrins (P_Zn's) connected to bisindole bridge (BB) via the Cu ^I-mediated alkyne-azide click chemistry, exhibited unique switching in forward and backward photoinduced energy transfer by specific guest bindings. The addition of Cu²⁺ caused a change in electronic absorption and fluorescence quenching of 1. MALDI-TOF-MS and FT-IR analyses indicated the formation of stable coordination complex between 1 and Cu²⁺ (1-Cu(II)). Without Cu²⁺ coordination, the excitation energy flows from BB to P_Zn's with significantly high energy transfer efficiency. In contrast, the direction of energy flow in 1 was completely reversed by the coordination of Cu²⁺. The difference in fluorescence quantum yield between 1 and 1-Cu(II) indicates that more than 95% of excitation energy of P_Zn flows into Cu(II)-coordinated BB. The energy transfer efficiency was further controlled by bidentate ligand coordination onto 1-Cu(II). When pyrophosphate ion was added to 1-Cu(II), the recovery of fluorescence emission from P_Zn was observed. The quantum mechanical calculations indicated that the Cu(II)-coordinated BB has square planar geometry, which can be distorted to form octahedral geometry due to the coordination of bidentate ligands.

Full text of DOI:10.1021/ja4124048

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Article
A Porphyrin-based Molecular Tweezer: Guest-induced Switching
of Forward and Backward Photo-induced Energy Transfer
Hongsik Yoon, Jong Min Lim, Hyuk-Chan Gee, Chi-Hwa
Lee, Young-Hwan Jeong, Dongho Kim, and Woo-Dong Jang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4124048 • Publication Date (Web): 08 Jan 2014
Downloaded from http://pubs.acs.org on January 14, 2014
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A Porphyrin-based Molecular Tweezer: Guest-induced Switch-
ing of Forward and Backward Photo-induced Energy Transfer
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Hongsik Yoon, Jong Min Lim, HyukꢀChan Gee, ChiꢀHwa Lee, YoungꢀHwan Jeong, Dongho Kim*, and
WooꢀDong Jang*
Department of Chemistry, Yonsei University. 50 Yonseiꢀro, SeodaemoonꢀGu, Seoul 120ꢀ749, Korea.
Supporting Information
pairs,28ꢀ32 the examples of guest specific control of photoꢀinduced
ABSTRACT: A bisindoleꢀbridgedꢀporphyrin tweezer (1), a pair
energy transfer are still very rare.33 In this study, we have newly
of zinc porphyrins (PZns) connected to bisindole bridge (BB) via
the CuIꢀmediated alkyneꢀazide click chemistry, exhibited unique
designed a bisindoleꢀbridgedꢀporphyrin tweezer (1; chart 1) using
CuIꢀmediated alkyneꢀazide click chemistry, which exhibited
switching in forward and backward photoꢀinduced energy transfer
by specific guest bindings. The addition of Cu2+ caused a change
unique photoluminescence switching phenomena by specific
guest bindings.
in electronic absorption and fluorescence quenching of 1.
MALDIꢀTOFꢀMS and FTꢀIR analyses indicated the formation of
stable coordination complex between 1 and Cu2+ (1ꢀCu(II)).
Without Cu2+ coordination, the excitation energy flows from BB
to PZns with significantly high energy transfer efficiency. In conꢀ
trast, the direction of energy flow in 1 was completely reversed by
the coordination of Cu2+. The difference in fluorescence quantum
yield between 1 and 1ꢀCu(II) indicates that more than 95% of
excitation energy of PZn flows into Cu(II)ꢀcoordinated BB. The
energy transfer efficiency was further controlled by bidentate
ligand coordination onto 1ꢀCu(II). When pyrophosphate ion was
added to 1ꢀCu(II), the recovery of fluorescence emission from PZn
was observed. The quantum mechanical calculations indicated
that the Cu(II)ꢀcoordinated BB has square planar geometry, which
can be distorted to form octahedral geometry due to the coordinaꢀ
tion of bidentate ligands.
Chart 1. Structures of bisindoleꢀbridgedꢀporphyrin tweezer (1)
and bisindoleꢀbridge (2).
EXPERIMENTAL SECTION
INTRODUCTION
Materials and Measurements. All commercially available reaꢀ
gents were reagent grade and used without further purification.
Dichloromethane, nꢀhexane, and tetrahydrofuran (THF) were
freshly distilled before each use. UV/Vis absorption and fluoresꢀ
cence emission spectra were recorded with a JASCO Vꢀ660 specꢀ
trophotometer and JASCO FPꢀ6300 spectrofluorometer, respecꢀ
tively. A timeꢀcorrelated singleꢀphotonꢀcounting (TCSPC) system
was used for measurement of spontaneous fluorescence decay.
The excitation and probe wavelengths were 400 and 650 nm for 1,
and 370 and 400 nm for 2, respectively. As an excitation light
source, we used a modeꢀlocked Ti:sapphire laser (Spectra Physics,
MaiTai BB) which provides ultrashort pulse (80 fs at full width
half maximum, fwhm) of 800 nm. The output pulse was frequenꢀ
cyꢀdoubled by a 1 mm thickness of a BBO crystal (EKSMA). The
fluorescence was collected by a microchannel plate photomultiꢀ
plier with a thermoelectric cooler connected to a TCSPC board
(Becker & Hickel SPCꢀ130).34 Femtosecond (fs) timeꢀresolved
transient absorption (TA) spectra were recorded using a specꢀ
trometer consisting of a homemade noncollinear optical parametꢀ
ric amplifier (NOPA) pumped by a Ti:sapphire regenerative amꢀ
plifier system (Quantronix, IntegraꢀC) operating at 1 kHz repetiꢀ
tion rate coupled with an optical detection system. The generated
visible NOPA pulses had a pulse width of ~100 fs and an average
The photoꢀinduced energy transfer is a decisive process in the
lightꢀenergy conversion of natural light harvesting system.1ꢀ3 The
photochemical reaction in the light harvesting complex is usually
initiated by the light absorption by pigment array and successive
vectorial energy transfer to the reaction center.4 The vectorial
energy transfer phenomena gave great inspiration for the design in
artificial photoꢀfunctional devices, including photovoltaic, electroꢀ
luminescent, and biomedical devices.5ꢀ15 The natural light harvestꢀ
ing involves obviously much more complicated mechanisms than
artificial ones for the precise regulation of lightꢀenergy conversion.
For example, plants and bacteria have evolved photoꢀprotective
mechanisms to avoid an overꢀexpose to strong light because the
reactive oxygen species generated during photosynthesis are
harmful to damage the photosynthetic system itself.3,16ꢀ18 The
natural photosynthetic systems are composed of various molecular
accessories which are utilized for the regulation of light harvestꢀ
ing process by the supramolecular complex formation with specifꢀ
ic proteins or ionic species.19ꢀ23 Typically, the metal ion bindings
to the light harvesting antenna give significant influences on the
efficiency and dynamics of photosynthesis.24ꢀ27 Although the conꢀ
tinuous efforts by synthetic chemists enable to establish the direcꢀ
tional energy transfer through the conjugation of donorꢀacceptor
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power of 1 mW in the range 480ꢀ700 nm, which were used as
1: CuSO4•5H2O (7 mg, 0.03 mmol) and sodium ascorbate (11 mg,
1
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4
5
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7
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pump pulses. White light continuum (WLC) probe pulses were
generated using a sapphire window (2 mm of thickness) by focusꢀ
ing of small portion of the fundamental 800 nm pulses that were
picked off by a quartz plate before entering into the NOPA. The
time delay between pump and probe beams was carefully conꢀ
trolled by causing the pump beam to travel along a variable optiꢀ
cal delay (Newport, ILS250).35 1H and 13C NMR spectra were
recorded on a Bruker Avance DPX 250 and DPX 400 spectromeꢀ
ter, respectively, at 25 °C in [D8]THF, or CDCl3. MALDIꢀTOFꢀ
MS was performed on Bruker Daltonics LRF20 with Dithranol
(1,8,9ꢀtrihydroxyanthracene) as the matrix. FTꢀIR was performed
on Bruker Vertax 70 (The specimens were prepared by dropꢀ
casting of a THF solution onto CaF2 window).
0.06 mmol) were added to a mixture of Ethynyl-BB (20 mg, 0.05
mmol) and PZn-N3 (259 mg, 0.29 mmol) in 7 mL THF/H2O (1:1).
The reaction mixture was stirred for 7 h at 50 °C, and then the
organic layer was separated, dried over MgSO4 and filtered. After
evaporation of the solvent under reduced pressure, the residue was
purified using column chromatography with 70% CH2Cl2/hexane,
as an eluent and evaporated to dryness. The residue was recrystalꢀ
lized from CH2Cl2/hexane to produce 1 as a purple solid (90 mg,
1
83%): H NMR(400 MHz, [D8]THF, 25 °C) δ = 11.15 (s, 2 H),
9
10.23 (s, 4 H), 9.38ꢀ9.36 (t, J = 4.8 Hz, 8 H), 9.15ꢀ9.14 (d, J = 4.4
Hz, 4 H), 9.03ꢀ9.02 (d, J = 4.4 Hz, 4 H), 8.80 (s, 2 H), 8.31ꢀ8.29
(d, J = 7.6 Hz, 4 H), 7.87ꢀ7.85 (d, J = 7.6 Hz, 4 H), 7.74 (s, 2 H),
7.70 (s, 2 H), 7.40 (s, 4 H), 7.07 (s, 2 H), 6.94 (s, 2 H), 6.13 (s, 4
H), 4.18ꢀ4.15 (t, J = 6.4 Hz, 8 H), 1.89ꢀ1.28 (m, 32 H), 0.88ꢀ0.85
ppm (t, J = 6.4 Hz, 12 H); 13C NMR (100 MHz, [D8]THF, 25 °C)
δ = 159.60, 151.00, 150.81, 150.78, 150.62, 149.48, 146.04,
144.69, 143.55, 136.44, 136.25, 133.90, 133.25, 133.03, 132.64,
132.44, 132.31, 131.43, 127.04, 121.36, 120.68, 119.61, 118.10,
117.29, 115.63, 114.57, 106.61, 101.26, 100.32, 69.03, 54.79,
35.52, 33.03, 32.63, 30.78, 30.72, 30.62, 30.46, 27.32, 23.72,
14.60 ppm; MALDIꢀTOFꢀMS: m/z: calcd. for C134H150N16O4Zn2:
2179.55 [M+]; found 2180.42; ε412.5 nm = 559,000.
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Estimation of Quantum Yield. The quantum yield can be calcuꢀ
lated from followed equation:
ꢄꢂꢅ ꢇꢈ
ꢆ
ꢂ
ꢀꢂꢁ ꢃ
ꢀꢁꢆ
ꢄꢆꢅꢇꢆꢈ
ꢂ
where ꢀꢂꢁ and ꢀꢆꢁ are the photoluminescence quantum yields of
the sample and standard, respectively. The subscript f is used
because in most cases one is dealing with fluorescence. The ꢄꢂ
and ꢄꢆ are the integrated intensities (areas) of sample and standꢀ
2: CuSO4•5H2O (7 mg, 0.03 mmol) and sodium ascorbate (11 mg,
0.06 mmol) were added to a mixture of Ethynyl-BB (20 mg, 0.05
mmol) and benzyl azide (38.61 mg, 0.29 mmol) in 7 mL
THF/H2O (1:1). The reaction mixture was stirred for 7 h at 50 °C,
and then the organic layer was separated, dried over MgSO4 and
filtered. After evaporation of the solvent under reduced pressure,
the residue was purified using column chromatography with
CH2Cl2, and evaporated to dryness. The residue was recrystallized
from CH2Cl2/hexane to produce 2 as a white solid (30 mg, 91%):
1H NMR(400 MHz, CDCl3, 25 °C) δ = 10.69 (s, 2 H), 7.92 (s, 2
H), 7.64 (s, 2 H), 7.52ꢀ7.36 (m, 12 H), 6.93 (s, 2 H), 5.67 (s, 4 H),
1.41 ppm (s, 18 H); 13C NMR (100 MHz, CDCl3, 25 °C) δ =
148.26, 141.95, 135.94, 135.09, 129.38, 128.95, 128.33, 124.03,
120.47, 119.36, 116.62, 112.86, 54.56, 34.93, 32.30 ppm;
MALDIꢀTOFꢀMS: m/z: calcd. for C42H42N8: 658.84 [M+]; found
659.39.
ard spectra, respectively (in units of photons). The ꢅ is the abꢀ
ꢉ
sorption factor (also known under the obsolete term “absorpꢀ
tance”), the fraction of the light impinging on the sample that is
absorbed (ꢅ ꢃ 1 ꢊ 10ꢋꢌ , where A = absorbance). The ni and ns
ꢍ
ꢉ
are the reflective indexes of the solvents for sample and standard,
respectively.
As
reference
compounds,
5,10,15,20ꢀ
tetraphenylporphyrin (λex = 420 nm,
Φ
= 0.12 in toluene) and
anthracene (λex = 342 nm,
Φ = 0.27 in ethanol) were utilized for
the determination of quantum yields of 1 and 2, respectively.36
Molecular Modeling Study. Quantum mechanical calculations
were performed using the Gaussian09 program suite. All calculaꢀ
tions were carried out using density functional theory (DFT) with
the unrestricted Becke’s threeꢀparameter hybrid exchange funcꢀ
tional and the LeeꢀYangꢀParr correlation functional (UB3LYP),
employing a basis set of lanl2dz for all atoms. Molecular dynamic
calculations were carried out by CAChe7.5 using MM2 force field.
Synthesis. The synthetic procedure of 1 and 2 are outlined in
Scheme 1. Hydroxyꢀgroupꢀbearing porphyrin (PZn-OH), bromideꢀ
bearing porphyrin (PZn-Br) and ethynylꢀbearing bisindole
(Ethynyl-BB) were synthesized according to the literature proceꢀ
dure.8,9,37 Azideꢀbearing porphyrin (PZn-N3) was prepared by
azidation of PZnꢀBr. The CuIꢀmediated click reaction between
PZnꢀN3 and Ethynyl-BB resulted in 1, which was unambiguously
1
characterized by H, 13C NMR, and MALDIꢀTOFꢀMS analyses.
Similarly, the click reaction between Ethynyl-BB and benzyl
azide resulted in bisindoleꢀbridge (BB) without porphyrin units (2)
as a structural fragment.
PZn-N3: NaN3 (0.50 g, 7.70 mmol) was added to a solution of PZn-
Br (1.20 g, 1.28 mmol) in a 15 mL THF/H2O mixture (4:1). The
resulting suspension was stirred for 9 h at 25 °C, and the reaction
mixture was evaporated to dryness. The residue was purified usꢀ
ing column chromatography with 50% CH2Cl2/hexane as an eluꢀ
1
ent to produce PZn-N3 as a purple solid (1.00 g, 87%): H NMR
Scheme 1. Synthesis of porphyrinꢀbased molecular tweezer.
(400 MHz, CDCl3, 25 °C) δ = 10.07 (s, 2 H), 9.35ꢀ9.34 (d, J = 4.4
Hz, 2 H), 9.29ꢀ9.28 (d, J = 4.8 Hz, 2 H), 9.22ꢀ9.21 (d, J = 4.4 Hz,
2 H), 8.86ꢀ8.85 (d, J = 4.4 Hz, 2 H), 7.99ꢀ7.97 (d, J = 7.6 Hz, 2H),
7.48 (s, 2 H), 7.02ꢀ7.00 (d, J = 8 Hz, 2 H), 6.91 (s, 1 H), 4.15ꢀ4.12
(t, J = 6.4 Hz, 4 H), 3.50 (s, 2 H), 1.90ꢀ1.29 (m, 32 H), 0.94ꢀ0.90
ppm (t, 6.4 Hz, 6 H); MALDIꢀTOFꢀMS: m/z: calcd. for
C53H61N7O2Zn: 893.51 [M+]; found 891.85.
Metal Coordination and Ligand Binding Experiments. Soluꢀ
tions of various metal salts in THF (2.0 mM, 10 ꢀL) were added
to THF
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Figure 3. Color change of a) 1, and fluorescence change of b) 1
and c) 2 by addition of various metal ions as form of following
salts, Pb(ClO4)2ꢁ2H2O, Mn(OAc)2, Hg(OAc)2, Cd(NO3)2ꢁ4H2O,
Zn(OAc)2, PtCl2,: Ni(OAc)2 4H2O, Cu(ClO4)2ꢁ6H2O, Pd(OAc)2.
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Figure 1. Absorption (solid line) and fluorescence (dotted line)
spectra of 1 (red, 2.0 ꢀM) and 2 (blue, 20 ꢀM), where λex = 342
and 350 nm for 1 and 2, respectively.
Figure 2. Fluorescence spectra of 1 (blue; 2.0 ꢀM, λex = 350 nm)
and nonꢀcovalent reference (red; 1:2 mixture of 2 and PZn-OH,
λex = 350 nm).
solution of 1 and 2 (2.0 ꢀM, 2.0 mL), and visual changes and
fluorescence emission changes were observed to test the capabilꢀ
ity of metal coordination. The fluorescence emission change was
observed upon 350 nm irradiation by UV handy lamp (ex: 360 nm,
VILBER LOURMAT, VLꢀ4LC). For the UV/Vis and fluorescence
titration studies, 10 and 1 x 102 ꢀM solution of Cu(ClO4)2 in THF
was successively added to THF solution of 1 (2.0 ꢀM) and 2 (20
ꢀM), respectively, and monitored spectral changes using spectroꢀ
photometer. THF solutions of several anionic species, 2,2ꢀ
bipyridine, and 1,3ꢀbis(diphenylphosphino)propane were added to
a mixture solution of 1 (2.0 ꢀM) and Cu(ClO4)2 (4.0 ꢀM), and the
fluorescence and absorption changes were monitored for the evalꢀ
uation of additional ligand bindings. The same experiment was
carried out to the mixture solution of 2 (20 ꢀM) and Cu(ClO4)2
(40 ꢀM).
Figure 4. Absorption changes of a) 1 (2.0 ꢀM) and b) 2 (20 ꢀM)
by successive addition of Cu(ClO4)2 (0 ꢀ2.0 eq., 0.25 eq. intervals).
negligible in 1, but strong emissions around 580 and 640 nm apꢀ
peared upon 350 nm excitation. Moreover, the excitation spectra
of 1 monitored at 637 nm, fluorescence from PZn, showed the
overlap of PZn and BB absorption (Figure S1). As control experiꢀ
ments, the absorption and fluorescence emission of 1 was comꢀ
pared with a nonꢀcovalent reference system, a 1:2 mixture of 2
and PZn-OH in THF. Although both 1 and the nonꢀcovalent referꢀ
ence exhibited similar absorption (Figure S2), the nonꢀcovalent
reference predominantly emitted strong fluorescence around 400
nm (Figure 2), indicating the energy transfer does not take place
in the nonꢀcovalent reference system. When the relative intensity
of fluorescence emission around 400 nm was compared, 1 exhibꢀ
ited less than 1% of fluorescence intensity in comparison with the
nonꢀcovalent reference, indicating the excitation energy transfer
efficiency from BB to PZn units is significantly high.
RESULTS AND DISCUSSION
Figure 1 shows the electronic absorption and fluorescence emisꢀ
sion spectra of 1 and 2 (2.0 and 20 ꢀM in THF, respectively).
Compound 1 exhibits strong absorption around 412 nm, correꢀ
sponding to the B band of zinc porphyrin (PZn). Compound 2
exhibits absorption around 350 nm with some vibronic bands.
Upon excitation at 342 nm, 2 emits strong fluorescence around
Because the indolic and triazole nitrogen atoms are potential metꢀ
al coordination sites, we have tested the capability of metal bindꢀ
ing into BB. To THF solutions of 1 and 2 (2.0 ꢀM), 5 equivalents
of several metal cations were added and visual changes of color
and emission were monitored. The addition of Cu(ClO4)2 caused a
slight color change of the solution (Figure 3a). The same changes
400 nm (τ = 1.2 ns,
Φ = 89.9%). The emission band of 2 partially
overlaps with the B band absorption of PZn (see shaded area in
Figure 1). Therefore, effective Förster type energy transfer from
BB to PZns can be expected. In fact, the fluorescence emission
from BB was almost
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Figure 5. MALDIꢀTOFꢀMS spectra of 1 and 1ꢀCu(II) complexes
obtained by addition of Cu(OAc)2 and Cu(ClO4)2.
Figure 7. Spectral changes of 1 and 2 by 2 equivalents of various
metal ion addition. Absorption changes of a) 1 (2.0 ꢀM) and b) 2
(20 ꢀM), and fluorescence changes of c) 1 (2.0 ꢀM) and d) 2 (2.0
ꢀM). All spectra were measured after 10 min of metal additions.
Figure 6. Fluorescence emission changes of a) 1 (2.0 ꢀM; λex
=
542 nm) and b) 2 (2.0 ꢀM; λex = 350 nm) by successive addition
of Cu(ClO4)2 (0 ꢀ2.0 eq., 0.25 eq. intervals).
same m/z values at 2241.56 and 2241.44, respectively. Notably,
the observed m/z value for 1ꢀCu(II) was less than 0.02% deviated
from the calculated value for [C134H148CuN16O4Zn2]+, which is
corresponding to the molecular formula for [1 – 2H + Cu]+, indiꢀ
cating the deprotonation of indolic protons at BB to provide the
coordination sites for Cu2+ (Figure 5). The MALDIꢀTOFꢀMS
analysis for Cu(II)ꢀcoordinated complexes of 2 (2ꢀCu(II)) also
showed the same aspect (Figure S3). FTꢀIR measurement of 1 and
2 also exhibited the disappearance of NH stretching band at 3400
cmꢀ1 by the formation of Cu(II)ꢀcoordinated complexes (Figure
S4), reassuring the deprotonation of indolic protons. Due to the
were observed when Cu(OAc)2 was added instead of Cu(ClO4)2,
indicating that the color change of the solution was caused by the
interaction between 1 and Cu2+ ion. Therefore, the electronic abꢀ
sorption and fluorescence spectra of 1 in THF were recorded upon
successive addition of Cu(ClO4)2. Upon the addition of Cu(ClO4)2,
1 exhibited gradual decrease in the absorption around 350 nm and
concomitant increase in the absorption around 495 nm (Figure 4a).
The same spectral changes with isosbestic points at 302 and 380
nm have also been observed for 2 (20.0 ꢀM) by addition of
Cu(ClO4)2, indicating the binding site of Cu2+ is not related with
PZns (Figure 4b). The continuous variation method (Job’s plot)38
indicated 1:1 complex formation for both 1 and 2 with Cu2+ ion.
The formation of Cu(II) coordinated complexes has been directly
confirmed by MALDIꢀTOFꢀMS analysis. MALDIꢀTOFꢀMS specꢀ
trum of 1 exhibited a peak at m/z = 2180.42, corresponding to [1 +
H]+. The Cu(II)ꢀcoordinated complexes of 1 (1ꢀCu(II)) formed by
the additions of Cu(OAc)2 and Cu(ClO4)2 exhibited nearly the
1
paramagnetic nature of Cu2+ ion, the H NMR spectrum of 1 was
also greatly broadened by the formation of 1ꢀCu(II) (Figure S5).
As aforementioned, the fluorescence emission of PZn completely
disappeared by the formation of 1ꢀCu(II) complex (Figure 3b and
6a), where the fluorescence quantum yields of 1 and 1ꢀCu(II)
were estimated to be 7.3 and 0.4%, respectively. Similarly, the
fluorescence emission of 2 around 398 nm (τ = 1.2 ns,
Φ = 89.9%)
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Figure 8. Energy diagram and frontier molecular orbitals of 2ꢀ
Cu(II) and 2ꢀHg(II) estimated by DFT calculation.
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Figure 9. Comparison of metalꢀcoordinated BB and typical metalꢀ
loporphyrin. Distances among metal and coordinated nitrogen
atoms are summaries in table.
completely disappeared by the addition of Cu2+ (Figure 3c and 6b),
where the fluorescence quantum yield for 2ꢀCu(II) was estimated
to be less than 0.1%. Such fluorescence quenching phenomena of
2 was also observed by the addition of Hg2+, indicating that Hg2+
can be coordinated into the BB (Figures 3c and 7). However, all
other metal ion additions, Pb2+, Mn2+, Cd2+, Zn2+, Pt2+,: Ni2+, and
Pd2+, did not influence on the absorption as well as the emission
of both 1 and 2, indicating those metal ions cannot bind to the BB
(Figure 7). Although the coordination of Hg2+ to 2 caused a comꢀ
plete fluorescence quenching, the addition of Hg2+ to 1 did not
affect the fluorescence emission upon Qꢀband excitation of PZn
(Figures 3c and 7). When we consider the molecular structure of 1,
the fluorescence emission of PZn cannot be directly affected by
metal coordination because there is no πꢀconjugation between BB,
the metal coordination site, and PZns. Therefore, we need to conꢀ
sider a new mechanism to explain the fluorescence quenching of
PZns in 1-Cu(II).
.
Figure 10. Femtosecondꢀtransient absorption spectra and decay
profiles of a) 1, b) 1ꢀCu(II), c) 2, and d) 2ꢀCu(II) in THF.
complexes with frontier molecular orbitals. The nonꢀfluorescent
characteristics of 2ꢀCu(II) and 2ꢀHg(II) complexes can be exꢀ
plained by the ligand to metal charge transfer (LMCT)39,40 and the
heavy atom effect,41 respectively. The broad absorption band
around 500 nm of 2ꢀCu(II) complex is assignable to the LMCT
band of square planar Cu(II) complex. The structural optimization
of 2ꢀCu(II) complex also exhibited the square planar geometry,
where the central metal ion is surrounded by 4 nitrogen atoms
belonging to 5ꢀmemberd aromatic rings. The HOMO of 2ꢀCu(II)
complex is
The structural optimization of metalꢀcoordinated BB was carried
out by the DFT calculations using the uB3LYP/Lanl2dz parameꢀ
ters. Figure 8 shows the optimized structure of 2ꢀCu(II) and 2ꢀ
Hg(II)
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Figure 11. Florescence change of 1ꢀCu(II) by addition of PPi.
predominantly delocalized on the BB moiety, whereas the LUMO
of beta spin state is delocalized around the metal center, reinforcꢀ
ing the fluorescence quenching by LMCT. From the geometrical
viewpoint, this square planar 2ꢀCu(II) complex might be the
pseudoꢀporphyrin structure that has partially expanded aromatic
characters. The bond lengths between metal ion and nitrogen atꢀ
oms estimated by the DFT calculations are also very close to that
of typical porphyrin structure (Figure 9). Although the structural
optimization of 2ꢀHg(II) complex showed the square planar geꢀ
ometry, it exhibited relatively longer metal to ligand bond lengths
than those in 2ꢀCu(II) complex. The energy level and electronic
distribution of HOMO of 2ꢀHg(II) complex were very close to
those of 2ꢀCu(II) complex, whereas the electronic distribution of
LUMO is predominantly delocalized over triazole rings in 2ꢀ
Hg(II).
Figure 12. Proposed mechanism of fluorescence changes in 1
with energy minimized model calculated by MM2 force field.
the values were estimated to be 0.8 (65%) and 8.0 ps (35%). Such
fast decay dynamics was again observed in 1ꢀCu(II) complex, but
the major component of S1 lifetime was estimated to be 63 ps,
which is greatly reduced value compare with that of 1. Because
the transient absorption spectra of 1ꢀCu(II) complex didn’t show
any characteristic absorption bands of cationic radical species of
PZn, the reduced S1 lifetime reinforces the excitation energy transꢀ
fer from PZns to the nonꢀfluorescent Cu(II)ꢀcoordinated BB comꢀ
plex. As aforementioned, the square planar Cu(II)ꢀcoordinated BB
resembles the porphyrin structure that has partially expanded
aromatic characteristics, which can be an excellent energy accepꢀ
tor. The reduced S1 lifetime as well as fluorescence quantum yield
changes of 1 by the formation 1ꢀCu(II) complex indicate that
more than 95% of excitation energy of PZn flows into Cu(II)ꢀ
coordinated BB. Because 1 originally exhibited directional energy
flow from BB to PZns with significantly high energy transfer effiꢀ
ciency, we can conclude here that the coordination of Cu2+ to BB
almost exclusively changes the energy transfer direction from
forward to backward direction.
The fluorescence quenching of PZns in 1ꢀCu(II) can be explained
by energy transfer from PZns to the Cu2+ꢀcoordinated BB. The
DFT geometry optimization indicated that the Cu2+ꢀcoordinated
BB adopted pseudoꢀporphyrin structure, which may have partially
expanded aromatic characters. In fact, the absorption spectrum of
2ꢀCu(II) was widely spread over long wavelength region. In conꢀ
trast, the absorption spectrum of 2ꢀHg(II) exhibited only a small
shoulder around 420 nm. The HOMOꢀLUMO band gaps estimatꢀ
ed by the DFT calculations are also consistent with the electronic
absorptions of both 2ꢀ Cu(II) and 2ꢀHg(II), where the HOMOꢀ
LUMO band gaps for 2ꢀCu(II) and 2ꢀHg(II) were estimated to be
2.29 (543 nm) and 2.98 eV (420 nm), respectively. Therefore, the
effective overlap between the emission band of PZn and the abꢀ
sorption of Cu(II)ꢀcoordinated BB may facilitate the effective
excitation energy transfer. In contrast, the excitation energy of
PZns in 1ꢀHg(II) is difficult to be transferred to the core Hg2+ꢀ
coordinated BB due to its higher energy level. Because the
LUMO of beta spin state in the Cu2+ꢀcoordinated BB is delocalꢀ
ized around the metal center, the transferred energy from PZns
would directly flow to the central Cu2+ ion and dissipate via nonꢀ
radiative pathways.
Another interesting aspect was observed when pyrophosphate
(PPi) was added to 1ꢀCu(II) complex (Figures 11 and S7). The
spectral changes of 1ꢀCu(II) complex were recorded upon addiꢀ
tion of several anionic species as potential ancillary ligands. As a
result, the fluorescence emission from PZn in 1ꢀCu(II) was graduꢀ
ally recovered by the successive addition of PPi (Figure 11). The
fluorescence lifetime of 1ꢀCu(II) was also recovered to 2.1 ns
(Figure S6). In contrast, no fluorescence emission from 2ꢀCu(II)
has been observed even after excess addition of PPi, indicating
that the fluorescence enhancement of PZns in 1ꢀCu(II) is not
caused by demetallation (Figure S8). Although 2ꢀCu(II) didn’t
show fluorescence enhancement by addition of PPi, the electronic
absorption was gradually changed with isosbestic points at 245
and 385 nm, indicating that the PPi successfully works as an anꢀ
cillary ligand to the Cu2+ (Figure S9). Because the maximum coꢀ
ordination number of Cu2+ is six, additional ligands can be bound
to the square planar 1ꢀCu(II) complex to form the octahedral geꢀ
ometry. The square planar Cu(II) complexes can accommodate
monodentate ligands without distortion of the square planar geꢀ
ometry by means of axial coordination. However, the square plaꢀ
nar geometry should be distorted to form the octahedral geometry
when bidentate ligand was coordinated to the Cu(II) complex
(Figure 12). Unlike other anionic species, PPi can coordinate to
metal ions as bidentate ligand. Therefore, the square planar geomꢀ
etry of Cu(II)ꢀcoordinated BB should be distorted by the coordiꢀ
nation of PPi, which eventually reduces the aromaticity of Cu(II)ꢀ
coordinated BB. In fact, the electronic absorption of 2ꢀCu(II) in
long wavelength region (λ > 400 nm) was gradually reduced by
Femtosecond timeꢀresolved fluorescence and transient absorption
measurements were conducted for 1 and 2. The excitation and
probe wavelengths for the timeꢀresolved florescence measureꢀ
ments are 400 and 650 nm for 1, and 370 and 400 nm for 2, reꢀ
spectively (Figure S6). The fluorescence lifetime of 1 was deterꢀ
mined to be 2.5 ns as single exponential decay, which is correꢀ
sponding to that of typical PZn derivatives. The fluorescence lifeꢀ
time of 2 was estimated to be 1.2 ns. Figure 10 shows transient
absorption spectra and decay profiles of 1, 2, 1ꢀCu(II), and 2ꢀ
Cu(II). The excitation wavelength of 1 and 1ꢀCu(II) was 540 nm,
which is in resonance with the Qꢀband absorption of PZn. The
transient absorption spectra of 2 and 2ꢀCu(II) were obtained upon
excitation at 380 and 500 nm, respectively. The singlet excited
state (S1) lifetimes of both 1 and 2, estimated from femtosecondꢀ
transient absorption measurements, are consistent with the fluoꢀ
rescence lifetimes. By the coordination of Cu2+, the S1 lifetime of
2 was greatly shortened where
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(5)
Choi, M. S.; Yamazaki, T.; Yamazaki, I.; Aida, T. Angew.
the successive addition of PPi, indicating the increase of HOMOꢀ
Chem. Int. Ed. 2004, 43, 150.
1
2
3
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5
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7
8
LUMO band gap (Figure S9). Such changes may influence the
energy transfer process from PZns to the Cu(II)ꢀcoordinated BB.
Because the excitation energy of Cu(II)ꢀcoordinated BB becomes
higher by the PPi coordination, the energy transfer efficiency
would eventually be decreased. To confirm this hypothesis, 2,2ꢀ
bipyridine, and 1,3ꢀbis(diphenylphosphino)propane, as other poꢀ
tential bidentate ligands, have been added to 1ꢀCu(II) and 2ꢀCu(II)
complexes (Figure S10). Although the degree of fluorescence
enhancement showed a large deviation, those molecules also exꢀ
hibited similar fluorescence enhancement and decrease of absorpꢀ
tion in long wavelength region, indicating the distortion of square
planar geometry is caused by coordination of bidentate ligands.
Therefore, we can conclude here that the energy transfer process
from PZns to BB can be further controlled by the coordination of
bidentate ligands into the Cu2+ꢀcoordinated BB.
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Hasobe, T.; Kashiwagi, Y.; Absalom, M. A.; Sly, J.;
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CONCLUSIONS
A bisindoleꢀbridgedꢀporphyrin tweezer exhibited a unique switchꢀ
ing behavior in forward and backward photoꢀinduced energy
transfer by specific guest bindings. Because the fluorescence
emission of BB well overlaps with the B band absorption of PZn,
effective Förster type energy transfer takes place from BB to PZns.
However, the direction of energy flow is completely reversed by
the coordination of Cu2+ into BB. The square planar Cu(II) comꢀ
plex of BB becomes a new energy acceptor, and PZns become
energy donors Moreover, the binding of bidentate ligands to
Cu(II)ꢀcoordinated BB decreases the energy flow from PZns to BB.
Therefore, we can demonstrate that the energy flow of the bisinꢀ
doleꢀbridgedꢀporphyrin tweezer was successfully controlled by a
guest specific manner. This active control of energy flow would
be the best biomimetic model as well as a strong motif for the
design of photofunctional nanoꢀdevices.
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ASSOCIATED CONTENT
Supporting Information
Ghanotakis, D. F.; Topper, J. N.; Babcock, G. T.; Yocum, C. F.
Additional spectral data (Figure S1ꢀ10). This material is available
free of charge via the Internet at http://pubs.acs.org.
Kruse, O.; Zheleva, D.; Barber, J. FEBS Lett. 1997, 408, 276.
Vassiliev, I. R.; Kolber, Z.; Wyman, K. D.; Mauzerall, D.;
AUTHOR INFORMATION
Corresponding Author
Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992,
wdjang@yonsei.ac.kr, dongho@yonsei.ac.kr
Priyadarshy, S.; Therien, M. J.; Beratan, D. N. J. Am. Chem.
Notes
Soc. 1996, 118, 1504.
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26, 198.
(31)
(32)
(33)
Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993,
The authors declare no competing financial interests.
Kurreck, H.; Huber, M. Angew. Chem. Int. Ed. 1995, 34, 849.
Ward, M. D. Chem. Soc. Rev. 1997, 26, 365.
Marchi, E.; Baroncini, M.; Bergamini, G.; Van Heyst, J.;
ACKNOWLEDGMENT
This work was supported by the Midꢀcareer Researcher (No.
2012005565) Program of National Research Foundation (NRF)
grant funded by Korea government (MEST). The quantum calcuꢀ
lations were performed by using the supercomputing resources of
the Supercomputing Center/Korea Institute of Science and Techꢀ
nology Information (KISTI).
Vo gtle, F.; Ceroni, P. J. Am. Chem. Soc. 2012, 134, 15277.
(34) Kim, P.; Sung, J.; Uoyama, H.; Okujima, T.; Uno, H.; Kim, D.
J. Phys. Chem. B 2011, 115, 3784.
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Lim, J. M.; Inoue, M.; Sung, Y. M.; Suzuki, M.; Higashino, T.;
Osuka, A.; Kim, D. Chem. Commun. 2011, 47, 3960.
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FeryꢀForgues, S.; Lavabre, D. J. Chem. Educ. 1999, 76, 1260.
Chang, K. J.; Moon, D.; Lah, M. S.; Jeong, K. S. Angew.
Connors, K. A. Binding constants: the measurement of
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