Fluorescence of 5-arylvinyl-5′-methyl-2,2′-bipyridyl ligands and their zinc complexes

Article abstract of DOI:10.1021/jo901889y

(Chemical Presented) The photophysical properties of 5-arylvinyl-5′- methyl-2,2′-bipyridyls (AVMBs, 1-9, 11) and their zinc complexes were studied. Similar 2,2′-bipyridyl-based ligands have been applied as optical sensors for metal ions and sensitizers fo

Full text of DOI:10.1021/jo901889y

pubs.acs.org/joc
Fluorescence of 5-Arylvinyl-50-Methyl-2,20-Bipyridyl Ligands and Their
Zinc Complexes
Ali H. Younes, Lu Zhang, Ronald J. Clark, and Lei Zhu*
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390
lzhu@chem.fsu.edu
Received September 1, 2009
The photophysical properties of 5-arylvinyl-50-methyl-2,20-bipyridyls (AVMBs, 1-9, 11) and their
zinc complexes were studied. Similar 2,20-bipyridyl-based ligands have been applied as optical sensors
for metal ions and sensitizers for solar energy conversion. The goal of this investigation is to reveal the
factors that determine the emission band shift and fluorescence quantum yield change of the title
ligand system upon zinc binding. The outcome of this study will not only advance the fundamental
understanding of the coordination-driven photophysical processes embodied in the AVMB platform
but facilitate the rational design of fluorescent probes for metal ions, particularly zinc. The AVMB
ligands were synthesized using the Horner-Wadsworth-Emmons reaction. AVMBs containing
electron-donating aryl groups show absorption and emission in the visible region, which can be
assigned to charge-transfer transitions as supported by solvent-dependency and computational
studies. The binding between AVMB ligands and zinc ion in acetonitrile was studied using isothermal
titration calorimetry (ITC). A multicomponent equilibrium model is suggested that explains the
multiple transitions evidenced in fluorescence titration isotherms. Coordination to zinc ion stabilizes
the charge-transfer excited state of an AVMB ligand with an electron-donating aryl substituent,
consequently results in bathochromic shifts in both absorption and emission. However, unlike the
emission band shift, the fluorescence quantum yield change upon zinc complex formation does not
have an intuitive correlation with the electronic nature of the aryl group. Lifetime measurements
using the Time-Correlated Single Photon Counting method enabled the determination of nonradia-
tive and radiative decay rate constants. Both rates of an AVMB ligand decrease upon zinc binding.
The collective effect gives rise to the change in fluorescence quantum yield with the apparent lack of
correlation with the electronic property of the aryl group.
Introduction
Aryl-substituted 2,20-bipyridyl, 2,2’:60,2’’-terpyridyl, and 1,10-
phenanthrolyl donor-acceptor systems possess an extra di-
mension to their optical properties because metal coordination
significantly alters the photophysical processes that they are
Conjugated donor-acceptor type chromophores have been
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DOI: 10.1021/jo901889y
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J. Org. Chem. 2009, 74, 8761–8772 8761
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capable of.14-24 In most cases, metal coordination enhances the
charge-transfer character of the excited state of the chromo-
phore, provided that the 2,20-bipyridyl metal chelating site is
also the electron acceptor in the metal-free form. The studies of
one particular class of the aryl-substituted 2,20-bipyridyls, the
fluorescent arylvinyl-2,20-bipyridyl (arylvinyl-bipy) ligands,
have been intensified in recent years. Their intriguing metal
coordination-driven photophysical properties have enabled
applications including the development of nonlinear optical
materials,18,25-28 sensitizers for solar cells,29,30 and metal ion
sensors.14,17,22,31-33
Our interest in fluorescent arylvinyl-bipy ligands arises
from their incorporation into a heteroditopic ligand frame-
work (Figure 1A) for quantifying Zn2þ concentration
([Zn(II)]) over large ranges.34,35 The 5- and/or 50-aryl-sub-
stituted 2,20-bipyridyls (bipy), which include the fluoro-
phores in our ditopic ligands, are known to possess
particularly high fluorescence quantum yields comparing
to other substituted bipy derivatives.15,16 The ditopic ligand,
however, is designed to be largely nonfluorescent due to the
photoinduced electron transfer (PET) from the electron-
donating high-affinity Zn2þ binding site (dipicolylamino
site as shown in Figure 1A) to the excited arylvinyl-bipy
fluorophore. The preferential binding of Zn2þ at the dipi-
colylamino site decreases the oxidation potential of the
electron-donating tertiary amino group. Consequently,
the thermodynamic driving force of PET is diminished to
result in a chelation-enhanced fluorescence (CHEF). When
[Zn(II)] is adequately high, the bipy site is also coordinated.
The presumed charge-transfer excited state of arylvinyl-bipy
FIGURE 1. (A) Fluorescent heteroditopic ligand platform con-
tains a 5-arylvinyl-2,20-bipyridyl fluoroionophore (bold). CHEF,
chelation-enhanced fluorescence; ICT, internal (or intramolecular)
charge transfer. The blue and red colors represent two different
emission wavelengths (short and long). They do not correspond to
the true color ranges of blue and red. (B) Structure and numbering
scheme of the title ligand system (AVMB) in this article.
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is consequently stabilized to lead to an emission bathochromic
shift. The relative abundance of the three fluorescence states
(OFF, BLUE, and RED) of a ditopic ligand is dependent
upon [Zn(II)]. By relating the fluorescence intensity at the
BLUE and RED channels to [Zn(II)], the quantification of
[Zn(II)] over a relatively broad range can be achieved.
Important factors that determine the sensitivity of [Zn(II)]
quantification using our heteroditopic system include (1) the
extent of CHEF upon Zn2þ binding at the dipicolylamino
group, (2) the magnitude of shift that the emission band of
the AVMB fluorophore undergoes (i.e., from BLUE to RED
in Figure 1A) upon Zn2þ coordination at the bipy site, and
(3) the fluorescence quantum yields of the mono- and
dicoordinated complexes. The first factor is related to the
oxidation potential of the fluorophore. A fluorophore with a
low oxidation potential would afford a large thermodynamic
driving force of PET from the dipicolylamino donor group to
the excited fluorophore in the free ditopic ligand shown in
Figure 1A, which amplifies the CHEF effect when Zn2þ is
coordinated.35 For factor #2, larger emission band shift
would result in smaller overlap between BLUE and RED
channels, which would lead to better sensitivity. In this
article, we primarily describe the studies on the factors (#2
and #3) that determine the extent of emission band shift and
fluorescence quantum yield change of AVMB fluorophores
(Figure 1B) upon Zn2þ coordination. In addition to aiding
the rational designs of fluorescent probes for metal ions that
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SCHEME 1
are effective over large concentration ranges34,36 and molec-
ular logic functions,37 the conclusion of this study is expected
to advance our understanding of fundamental coordination-
driven photophysical processes embodied in this class of
fluorescent compounds.
angles between the two pyridyl rings,34,39-41 the bipy moiety
in 6 is completely planar along with the double bond
(Figure 2B). The N-methylpyrrole group has a slight twist
off the plane of the vinyl-bipy moiety to minimize the steric
Results and Discussion
Synthesis. Compounds 1-9 were prepared via Horner-
Wadsworth-Emmons reactions using, in most cases, NaH
as the base between various arylaldehydes and phosphonate
10 (Scheme 1). When arylaldehydes with electron-withdraw-
ing substituents were used (3, 8, and 9), t-BuOK which is
unable to reduce the arylaldehydes, was employed as the base
instead. All of the products were isolated as trans isomers
after purification using silica gel chromatography in good
yields (70-80%). Some of the products were further purified
via recrystallization in either dichloromethane (DCM) or
ethyl acetate (EtOAc).
Compound 11 was prepared by direct bromination of 6
with Br2 (Scheme 2). The electron-rich N-methylpyrrole ring
FIGURE 2. Two views of the X-ray crystal structure of 6 (50%
probability ellipsoids). Black, carbon; blue, nitrogen; yellow,
hydrogen. The dipole moment vectors of the individual aromatic
rings in 6 are shown in (A).
SCHEME 2
congestion between the N-methyl group and the vinyl hydro-
gen. The dihedral angle (C2-C3-C4-C5) is 12.11ꢀ.
Cyclic Voltammetry Studies. The motivation for the cyclic
voltammetry studies was to determine the HOMO energy
levels (which are related to the oxidation potentials) of
the AVMB ligands so that the relative efficiency of PET in
the heteroditopic framework shown in Figure 1 can be
predicted. All the ligands undergo irreversible electrochemi-
cal oxidation as shown in their cyclic voltammograms
(Supporting Information, SI). The first anodic peak poten-
tials are listed in Table 1. The increased electron-donating
ability of an aryl group results in a higher oxidation potential
(or oxidizability). As shown in a previous study,35 the
heteroditopic ligand containing 1 (Epa = 1.57 V vs Ag/AgCl)
as the fluorophore demonstrates high efficiency of PET,
whereas the incorporation of the electron-richer ligand
5 (Epa = 1.36 V) results in much lower PET efficiency. Based
on the Epa data shown in Table 1, compounds 2, 3, 8, and 9
which have larger anodic peak potentials (lower oxidation
potentials) than that of 1 are expected to be efficient electron-
acceptors when excited in the heteroditopic framework.
Consequently, a sensitive CHEF of the ditopic ligand may
was brominated selectively over the alkene in 6. The 1-bromo-
N-methylpyrrole group with strengthened electron-withdraw-
ing ability slows down the dibromination of the alkene which
allowed the isolation of 11 in 48%.
X-Ray Crystal Structure of 6. The single crystals of
compound 6 were grown via slow evaporation of a solution
of 6 in a mixture of hexanes and DCM. In addition to a
transoid conformation in the 2,20-bipy unit,38 only one
conformer in respect to the rotations of the C1-C2 and
C3-C4 bonds was observed (Figure 2A). The dipole mo-
ments of neighboring aromatic rings are aligned opposite
each other to minimize the overall dipole moment of the
molecule. In contrast to the bipy-containing structures with-
out arylvinyl caps which show 6-7ꢀ (or larger) dihedral
(36) Zhu, L.; Zhang, L.; Younes, A. H. Supramol. Chem. 2009, 21, 268–
283.
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1882.
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J. Org. Chem. Vol. 74, No. 22, 2009 8763

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TABLE 1. Computed HOMO and LUMO Molecular Orbitals at the B3LYP/6-31þþG(d,p) level, and the First Anodic Peak Potentials (vs Ag/AgCl)
with the Calculated HOMO-LUMO Gap of 1-9 and 11
occur when Zn2þ is present. On the other hand, com-
pounds 4, 6, 7, and 11 which are more oxidizable than
4 might not perform well with dipicolylamino group as
a PET pair.42
connecting the nitro or the cyano substituent and the aryl
ring in the LUMOs of 8 and 9 appears to be higher than those
in other ligands.
The structure-dependent trend of the calculated
HOMO/LUMO gaps (in nanometers, Table 1) correlates
well with the trend of the experimentally determined
wavelengths at maximum absorption (λmax in Table 3).
It appears that the electron density of the aryl group in an
AVMB determines the extent of charge transfer in its
excited state, the oxidizability (Epa), and the HOMO/
LUMO gap (or λmax of absorption) of the compound.
The enhancement of the electron-donating ability of
the aryl group leads to more pronounced charge-transfer
in the excited state, a greater oxidizability, and a
smaller HOMO/LUMO gap (which results in a bath-
ochromic shift in absorption).43 This correlation may be
unique in chromophores capable of internal charge
transfer (ICT), which may pose a dilemma in engineering
fluorescent heteroditopic ligands with large con-
trast between the three fluorescence states shown in
Figure 1A.
Computational Studies. The frontier molecular orbitals of
all the AVMBs were computed at the B3LYP/6-31þþG(d,p)
level. For AVMBs with electron-donating aryl groups (4, 5,
6, 7, and 11), and to a lesser extent compounds 2 and 3, the
electron density of each molecule shows a net shift from the
aryl side of the highest occupied molecular orbital (HOMO)
to the pyridyl side of the lowest unoccupied molecular orbital
(LUMO, Table 1), which is consistent with a charge-transfer
transition to the S1 state. Although the chloro (2) and the
fluoro (3) substituents are electron-withdrawing, they are
considered to be electron donors through resonance. There-
fore, charge-transfer from the aryl moiety to the pyridyl side
is also expected for 2 and 3 upon excitation. Intramolecular
charge transfer in 8 and 9, if there is any, appears to occur
in the opposite direction to the nitro and cyano substi-
tuents, respectively. The electron density resides on the bond
(42) Ueno, T.; Urano, Y.; Setsukinai, K.-i.; Takakusa, H.; Kojima, H.;
Kikuchi, K.; Ohkubo, K.; Fukuzumi, S.; Nagano, T. J. Am. Chem. Soc. 2004,
126, 14079–14085.
(43) Anslyn, E. V.; Dougherty, D. A. In Modern Physical Organic
Chemistry; University Science Books: Sausalito, CA, 2006; Chapter 3, p 186.
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TABLE 2. Photophysical Parameters of 6 and its Zn2þ Complex in Various Solventsa
HX
BZ
DX
TL
CHCl3
EtOH
MeOH
MeCN
DMF
DMSO
ε
λ
λ
2.02
372
436
2.3
382
464
2.3
381
464
2.4
382
459
4.8
380
476
25
33
36.6
378
488
5963
0.087
0.51
0.47
0.17
1.79
421
38.3
384
493
5758
0.19
0.65
0.70
0.28
1.25
419
47
abs/nm
em/nm
383
501
6150
0.20
0.92
0.69
0.22
0.86
426
589
382
507
6454
0.18
1.00
0.85
0.18
0.82
426
586
388
500
5773
0.19
0.64
0.72
0.30
1.26
423
585
SS/cm-1
φf
3946
0.36
0.86
0.42
0.42
0.75
NDd
NDd
4626
0.24
0.51
0.60
0.46
1.50
NDd
NDd
4695
0.24
0.59
0.56
0.41
1.29
428b
587b
4392
0.25
0.56
0.40
0.44
1.34
NDd
NDd
5307
0.25
0.62
0.95
0.40
1.22
447c
583c
τ/ns
χ2
kr/ns-1
knr/ns-1
λ
λ
abs/nm (Zn2þ complex)
em/nm (Zn2þ complex)
607
587
aAbbreviations: HX, hexanes; BZ, benzene; DX, dioxane; TL, toluene; CHCl3, chloroform; EtOH, ethanol; MeOH, methanol; MeCN, acetonitrile;
DMF, dimethylformamide;DMSO, dimethylsulfoxide;ε, relative permittivity of a solvent; SS, Stokes Shift; τ, lifetime;χ2, reduced goodness-of-fit parameter;
kr, radiative decay rate constant; knr: non-radiative decay rate constant. bData were collected in THF. Compound 6shows incompletecoordination with Zn2þ
in dioxane. cData were collected in DCM. Compound 6 shows incomplete coordination with Zn2þ in CHCl3. dND, Not Determined.
FIGURE 3. Normalized absorption (dashed lines) and emission (solid lines) spectra of 6 in different solvents. (A) Spectra collected in
“nonpolar” solvents; (B) spectra collected in “polar” solvents.
Solvent Dependency Studies. 1. Solvent Effect on Absorp-
tion Spectra. The absorption spectra of compound 6, which
was selected as an example of AVMBs containing electron-
donating aryl groups, were collected in solvents (Figure 3,
dashed lines) of different polarities and hydrogen bond
donating abilities. Two absorption bands of high molar
absorptivities were observed at ∼260 and ∼380 nm, respec-
tively. Both bands are rather insensitive to solvent except in
hexanes where the band of the lowest energy centered at 372
nm and is relatively structured. The wavelengths at maximal
absorption and related molar absorptivities as well as other
photophysical parameters are listed in Table 2.
solvatofluorochromism12 is evident which suggests the for-
mation of an emissive polar excited state.44 The spectrum in
hexanes is clearly structured with vibronic bands. As solvent
polarity increases, the emission band gradually loses its
vibronic structure to become rounded, which can be assigned
to the emission from a charge-transfer excited state. The
hydrogen bond donating solvents such as MeOH (ε = 33)
and EtOH (ε = 25) enable larger emission bathochromic
shifts (from hexanes) than polar aprotic solvents such as
DMSO (ε = 47). The specific solvent effect shown in MeOH
and EtOH can be attributed to the strengthened hydrogen
bond between the solvent molecule and the charge-transfer
excited state (Figure 4).45 The solvent effect on the emission
of 6 (and 7 in SI) is similar to what was observed in the
2. Solvent Effect on Emission Spectra. The emission spectra
of 6 are shown in Figure 3 (solid lines). The positive
(44) Suppan, P.; Ghoneim, N. Solvatochromism; The Royal Society of
Chemistry: London, 1997.
(45) Lakowicz, J. R. In Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006; Chapter 7.
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FIGURE 4. Schematic representation of hydrogen bonding between MeOH and excited 6 (the charge-separated resonance structure).36
dimethylaminostyryl terpyridine derivative reported by
Schmehl et al.20
The magnitude of solvatofluorochromism is related to the
electron-donating ability of the aryl substituent. The elec-
tron-deficient 4-cyanophenyl group enables very little sol-
vent-induced spectral shift of 9 (Figure S10, SI) whereas
compound 7 containing a more electron-donating 4-di-
methylaminophenyl group shows more sensitive solvent
effect (Figure S9, SI) than that of 6. The Stokes shifts of 6
and 7 were plotted against the orientation polarizabilities of
the solvents to afford the Lippert plots (Figure 5).45 The
radius of the cavity (a) where the fluorophore resides is
assumed to be similar in all solvents so that the slope of the
plot represents the degree of polarization upon excitation
based on the Lippert-Magata equation (eq 1). The linearity
of both plots is only moderate due to the presence of (1)
transitions of different natures (i.e., locally excited and
charge-transfer), and (2) specific solvent effects, which are
not considered in the theoretical model for the Lippert-
Magata equation. Specifically, the relatively structured emis-
sion spectra collected in nonpolar solvents, in particular
hexanes (Figure 3A, solid lines), and the smooth spectra
obtained in polar solvents (Figure 3B, solid lines) are likely
originated from different emissive states. In EtOH and
MeOH, the hydrogen bond donating ability (not represented
by the orientation polarizability) in addition to their general
polarity determines the magnitude of the Stokes shift. The
qualitative conclusion drawn from the Lippert plots is that
ligand 7 which contains a more electron-donating aryl group
undergoes a higher degree of polarization upon photoexcita-
tion than that of 6.
FIGURE 5. Lippert plots of compounds 6 (blue) and 7 (red). Data
derived from hexanes are grouped in the circle on the left, and the
data derived from EtOH and MeOH are grouped in the two circles
on the right. Equation 2 was used to calculate the orientation
polarizability.
fluorescence intensity decay curve from which the lifetime
value(s) can be extracted.46,47 As evidenced by the small
reduced χ2 values, single exponentials were adequate to fit
the fluorescence decay traces collected in various solvents.48
Combining with the fluorescence quantum yield (φf) of 6,
which was found to decrease slightly with increasing solvent
polarity, the radiative and nonradiative decay rates (kr and
knr, Table 2) were calculated using eqs 3 and 4.
The lifetimes of 6 in all solvents are short (<1 ns). Similarly
solvent-dependent short lifetimes have been reported for ICT-
type compounds such as 4-methylamino-40-cyanostilbene49 and
N-alkyl-4-(p-N,N-dimethylaminostyryl)pyridinium bromide.50
Relatively large values were recorded in hydrogen bonding
solvents (0.92 ns in EtOH and 1.0 ns in MeOH). The radiative
rate constant (kr) decreases with increasing solvent polarity (e.g.,
comparing the rates in solvents listed in Figure 3B vs those in
Figure 3A). Such a solvent dependency of kr can be explained
using eq 5 derived by Stricker and Berg, by which a shorter
maximum absorption wavelength leads to a larger radiative
decay rate.51,52 In hexanes and two hydrogen bonding solvents,
low nonradiative decay rates (knr) were observed. The low knr in
2Δf
pca3
2
Stokes ShiftðSSÞ ¼ νA - νF ¼
ðμE -μGÞ þ const ð1Þ
νA, absorption in wavenumber; νF, emission in wavenum-
ber; p, Planck’s constant; c, speed of light; a, radius of the
cavity in which the fluorophore resides; μE, excited state
dipole; μG, ground state dipole.
ε -1
n2 -1
Δf ¼
-
ð2Þ
2ε þ 1 2n2 þ 1
(46) Valeur, B. In Molecular Fluorescence. Principles and Applications;
WILEY-VCH: New York, 2002; pp 173-176.
ε, relative permittivity or dielectric constant of a solvent; n,
refractive index.
(47) Lakowicz, J. R. In Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006; pp 103-107.
3. Solvent Effect on Lifetimes and Fluorescence Quantum
Yields. Lifetimes (τ) of 6 (Table 2) were determined using the
time-correlated single photon counting (TCSPC) method. In
a TCSPC experiment, the detector is adjusted to capture only
a single photon emitted by the sample after each excitation
pulse. This photon is recorded in a time channel correlated
to the time lapse between the excitation and the detection.
After repetitive pulsed excitations, the numbers of photons
collected in all the channels are used to reconstruct the
(48) Lakowicz, J. R. In Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006; pp 129-133.
ꢀ
ꢁ
(49) Abraham, E.; Oberle, J.; Jonusauskas, G.; Lapouyade, R.; Rulliere,
C. Chem. Phys. 1997, 214, 409–423.
(50) Mishra, A.; Behera, G. B.; Krishna, M. M. G.; Periasamy, N. J.
Lumin. 2001, 92, 175–188.
(51) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. In Handbooks of
Photochemistry, 3rd ed.; CRC Taylor and Francis: London, 2006; pp 25-28.
(52) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. In Principles of
Molecular Photochemistry, An Introduction; University Science Books: Sau-
salito, CA, 2009; pp 195-196.
8766 J. Org. Chem. Vol. 74, No. 22, 2009

Younes et al.
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FIGURE 6. ITC titrations of (A) 1 (0.40 mM) with Zn(ClO4)2 (0.93 mM), n (molar ratio) = 0.31, ΔHꢀ = -33 ( 0.05 kcal/mol, K = (3.3 (
0.16) ꢁ 107 M-1, ΔSo = -76 eu; (B) 3 (0.14 mM) with Zn(ClO4)2 (0.93 mM), n = 0.35, ΔHꢀ = -33 ( 0.2 kcal/mol, K = (2.6 ( 0.51) ꢁ 107 M-1
0.05 kcal/mol, K = (1.9 ( 0.36) ꢁ 107 M-1, ΔSo = -2.1 eu.
,
ΔSo = -75 eu. The small endothermic transition is marked by the red arrow; (C) 1 (0.40 mM) with ZnCl2 (3.4 mM), n = 0.93, ΔHꢀ = -11 (
hexanes may result from the relatively weak interactions be-
tween solvent and the excited solute. In MeOH and EtOH, the
hydrogen bonds between the solvent molecules and the pyridyl
groups may have restricted the rotation of the C2-C20 bond of
bipy, which results in low knr. Other than these three solvents, no
obvious dependency of knr on solvent polarity was observed. It
should be noted, however, the explanations accounting for the
trends of knr and kr of compound 6 may not be entirely
applicable to other ligands because specific functional groups
contained in other structures may open or close related relaxa-
tion pathways.
indicated by a red arrow) which was tentatively ascribed to
the conversion of the 3:1 complex (L3Zn) to the 2:1 and/or
1:1 complexes (L2Zn, LZn) at elevated [Zn(II)]. Based on the
ITC data, we postulate that the formation of the 3:1 complex
is strongly exothermic due to the net gain of the pyridyl-Zn2þ
dative bonds (Scheme 3). However, the conversion from the
3:1 to 2:1 and/or 1:1 complex at high [Zn(II)] is not as
thermally sensitive because only exchanges but no net gain
of pyridyl-Zn2þ dative bonds are involved (Scheme 4). In
the case of ligand 3, a weakly endothermic transition was
observed (Figure 6B).
1
τ ¼
ð3Þ
SCHEME 3
kr þ knr
kr
φf ¼
ð4Þ
ð5Þ
kr þ knr
krꢁν2f
ν, the wavenumber corresponding to the maximum absorption
wavelength; f, oscillator strength (eq 6).
SCHEME 4
Z
f ꢀ 4:3 ꢁ 10-9 εdνꢁÆΨ1jΨ2æ2
ð6Þ
Oscillator strength (f ) is proportional to the experimen-
tally determined molar absorptivity (ε). ÆΨ1|Ψ2æ is the
transition dipole moment.
Isothermal Titration Calorimetry Studies of Zinc Coordi-
nation. 1. Exothermicity of Binding. Isothermal titration
calorimetry (ITC) was used to determine the binding stoi-
chiometry of AVMBs to Zn2þ in MeCN. In our previous
study,35 a 3:1 (ligand to Zn(ClO4)2) ratio was reported for
5,50-dimethyl-2,20-bipyridyl in MeCN. Herein, the fluores-
cent analogs 1 and 3 also show a binding stoichiometry of 3:1
(ligand to Zn(ClO4)2, Figure 6A and B). However, a second,
small endothermic transition was also observed (Figure 6B,
2. Counter Ion Effect on Binding Stoichiometry. The bind-
ing stoichiometry of AVMBs with Zn2þ salts in MeCN
shows a sensitive dependence on counterion. As the more
strongly coordinating chloride replaces perchlorate as the
counterion, the binding stoichiometry of ligand 1 to Zn2þ
switches to 1:1 (Figure 6C, Scheme 5). The counterion
dependence of binding stoichiometry between Zn2þ and
the bipy moiety in solution is echoed by the X-ray crystal
structures obtained under different conditions. In most
J. Org. Chem. Vol. 74, No. 22, 2009 8767

Younes et al.
JOCArticle
FIGURE 7. (A) Absorption spectra of 4 (6.0 μM) in MeCN upon addition of Zn(ClO4)2 (0-110 μM). (B) Fluorescence spectra of 4 (λex
=
362 nm, 6.0 μM) in MeCN upon addition of Zn(ClO4)2 (0-26 μM). Blue, spectra of the free ligand; red, spectra at the end of the titration,
presumably of the 1:1 complex.
cases, octahedral coordinated Zn2þ involving three biden-
tate ligands were observed.53-56 However, when ZnCl2 was
used as the Zn2þ source, the X-ray crystal structures of the
charge-transfer character of the Franck-Condon S1 excited
state, which results in bathochromic shifts of the absorption
spectra (Table 3). Long absorption wavelengths are desired
in zinc sensing applications, especially when live cellular
samples are used, because autofluorescence-causing and
physiologically damaging UV excitation can be avoided.
On the other hand, the absorption spectra of all compounds
but 8 undergo bathochromic shifts upon Zn2þ-coordination
(e.g., spectra of 4 in Figure 7), which strengthens the elec-
tron-accepting ability of the pyridyl group. The extent of the
shift (Δνabs/cm-1) represents the stabilization of the
Franck-Condon S1 state of the AVMB by the Zn2þ ion at
the negative end of its dipole moment. The more electron-
donating aryl group (Ar in Figure 1B) results in a larger Δνabs
value, due to a more favorable electrostatic interaction
between Zn2þ and the negative end of a more polarized
excited state dipole. Compound 8, which has a strongly
electron-withdrawing 4-nitrophenyl cap, undergoes a hyp-
sochromic shift instead upon Zn2þ coordination, which
corroborates the prior computed results (Table 1) that
electron density of 8 shifts from the bipy side to the aryl side
upon excitation.
In the compounds featuring weakly electron-donating
(1, 2, 3) or weakly electron-withdrawing (9) groups, the molar
absorptivities of the lowest energy bands in free ligands are
high (∼50 000). As stronger electron-donating groups are
incorporated (4, 5, 6, 7, 11), the molar absorptivities drop to
∼30000. Similarreductioninmolar absorptivityof anAVMB
was observed upon Zn2þ-coordination (9 and 11 are the
exceptions). The reduced molar absorptivities at wavelengths
of maximum absorption as stronger electron-donating groups
are incorporated and/or Zn2þ is bound (which enhances
electron-accepting ability of the pyridyl) may be ascribed to
the increasing degree of charge-transfer character of the
transition. The small overlap density of the two molecular
orbitals involved in a charge-transfer transition leads to a
small transition moment, when results in a low probability of
transition (eq 6).59
SCHEME 5
1:1 complexes were obtained by Kozhevnikov et al.57,58
where Zn2þ adopts a tetrahedral geometry with two chloride
ions coordinated.
TABLE 3. Position (λmax) and Molar Absorptivities (ε) of the Lowest
Energy Absorption Bands of Both Free Ligands and Their Zn2þ Com-
plexes in MeCNa
free ligand
complex
ligand
λmax/nm ε (L/mol/cm)
λ
max/nm ε (L/mol/cm) Δνabs/cm-1
1
2
3
4
5
6
7
8
9
11
335
337
334
345
348
372
383
349
343
373
50500
48766
55055
34285
43300
36835
33600
16181
50996
35404
356
357
355
374
375
417
434
345
354
416
41920
42652
44414
28642
36050
30867
32700
12681
52804
35548
1761
1662
1771
2248
2069
2901
3068
-332
906
2771
aZn2þ-induced absorption band shifts (Δνabs/cm-1) are shown in the
rightmost column.
Absorption Titration Studies. As supported by the
computational studies (Table 1), increasing the electron
density of the aryl group in AVMB leads to the enhanced
(53) Bilyk, A.; Harding, M. M.; Turner, P.; Hambley, T. W. J. Chem.
Soc., Dalton Trans. 1994, 2783–2790.
(54) Murphy, B.; Aljabri, M.; Light, M.; Hursthouse, M. B. J. Chem.
Cryst. 2003, 33, 195–202.
(55) Lemoine, P.; Bendada, K.; Viossat, B. Acta Crystallogr. 2004, C60,
m489–m491.
(56) Juric, M. S.; Planinic, P.; Giester, G. Acta Crystallogr. 2006, E62,
m2826–m2829.
Fluorescence Titration Studies. The emission bands of
most AVMBs (except 8) also undergo bathochromic shifts
upon Zn2þ binding (see spectra of 4 in Figure 7B). Increasing
(57) Kozhevnikov, D. N.; Shabunina, O. V.; Kopchuk, D. S.; Slepukhin,
P. A.; Kozhevnikov, V. N. Tetrahedron Lett. 2006, 47, 7025–7029.
(58) Kozhevnikov, V. N.; Shabunina, O. V.; Kopchuk, D. S.; Ustinova,
(59) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. In Handbooks of
Photochemistry; 3rd ed.; CRC Taylor and Francis: London, 2006; pp 28-30.
€
M. M.; Konig, B.; Kozhevnikov, D. N. Tetrahedron 2008, 64, 8963–8973.
8768 J. Org. Chem. Vol. 74, No. 22, 2009

Younes et al.
JOCArticle
TABLE 4. Positions (λmax) of the Lowest Energy Emission Bands and
Stokes Shifts (SS) of Both Free Ligands and Their Zn2þ Complexes in
MeCNa
the aryl group. In order to gain more insight in the change
of excited state decay processes upon Zn2þ coordina-
tion, lifetimes of both AVMB ligands and their Zn2þ
complexes were measured and the rate constants of radia-
tive and nonradiative decays were calculated (Table 5).
Lifetime values generally increase upon Zn2þ complex for-
mation. The ligands that show charge-transfer characters
(1-6, 11) have lifetimes of their Zn2þ complexes g2 ns. For 8
and 9 that do not exhibit a charge-transfer character, the
lifetimes of Zn2þ complexes are shorter (1.20 and 1.60 ns,
respectively). Both kr and knr of a ligand drop upon Zn2þ
coordination. The reduction of kr upon Zn2þ binding can be
explained using eq 5. The decrease in knr, on the other hand,
may be partly due to the reduction of rotational freedom
upon Zn2þ binding. The rigidifications of Haley’s cross-
conjugated phenylacetylenes5 and Finney’s biphenyls60 and
biarylacetylenes61 were shown to enhance fluorescence quan-
tum yields over their unlocked precursors, presumably via
reducing knr. As both kr and knr decrease upon Zn2þ binding,
the collective effect which contributes to the change in
fluorescence quantum yields does not have a simple correla-
tion with the electronic nature of the aryl group. Some
interesting observations are described in the next paragraph.
Compounds 1-3 contain electronically neutral (or weakly
donating) aryl groups. The fluorescence quantum yields of
1-3 in MeCN are high (0.6-0.8) in both free ligands and
complexes. The emission band shifts upon Zn2þ coordina-
tion are moderate (3500-3800 cm-1). For compounds 4 and
5 with electron-donating groups of intermediate strengths,
the fluorescence quantum yields undergo significant en-
hancement upon Zn2þ binding. The emission band shifts
are larger than those of 1-3. For compounds featuring even
stronger electron-donating groups (6, 7, 11), large emission
band shifts (>4000 cm-1) were also observed. However,
their fluorescence quantum yields either remain unchanged
or decrease upon Zn2þ coordination. In the case of 7, highly
efficient fluorescence quenching was observed upon bind-
ing Zn2þ in MeCN. A similar observation was reported
by Tor et al. where 3,8-di(4-dimethylaminophenylethynyl)-
1,10-phenanthroline underwent fluorescence quenching
upon Zn2þ complex formation in MeCN.15 For compounds
8-9 containing electron-withdrawing groups, the emission
band shifts are much smaller. The S1 excited states of nitro-
containing compounds are often resulted from the n-π*
transitions,62 which might explain the low φf’s of 8 and its
complex. On the other hand, compound 9 featuring a much
weaker electron-withdrawing cyano group undergoes a no-
ticeable fluorescence enhancement upon Zn2þ coordination.
More intricacies in the fluorescence properties of different
species during Zn2þ titration experiments were revealed by
the titration isotherms. For example, the absorption spec-
trum of compound 6 undergoes a bathochromic shift upon
Zn(ClO4)2 addition in MeCN with an apparent isosbestic
point at 395 nm (Figure 8A). The binding isotherm
(Figure 8B) suggested a 2:1 (ligand to Zn2þ) or 3:1 com-
plexation. The fluorescence titration, on the other hand,
ligand
complex
ligand
λmax/nm
SS/cm-1
λmax/nm
SS/cm-1
Δνem/cm-1
1
2
3
4
5
6
7
8
9
11
392
392
391
431
422
480
532
551
403
480
4341
4163
4365
5784
5039
6048
7313
10504
4341
5976
459
455
460
533
504
604
712
493
419
600
6303
6033
6430
7976
6825
7425
8997
8702
4382
7372
3724
3532
3836
4440
3855
4277
4752
-2135
948
4167
aZn2þ-induced emission band shifts (Δνem/cm-1) are shown in the
rightmost column.
electron density in the aryl moiety leads to a larger Zn2þ
induced shift (Δνem, Table 4). The extent of shift (Δνem
-
)
represents the combined effect of Franck-Condon excited
state stabilization by Zn2þ coordination (Δνabs) and differ-
ential solvent relaxation, which is the difference between the
Stokes shifts of free ligand and Zn2þ complex (ΔSS, eq 7).
The Stokes shifts of the complexes are larger than those of
their respective free ligands (except 8 and 11), indicating that
the Franck-Condon excited state of the complex is more
polar than that of the free ligand. On the other hand, the
emission of the Zn2þ complex is less solvent-sensitive than
that of the free ligand (Table 2), which suggests that the
primary stabilization effect of the emissive S1 state is resulted
from the presence of Zn2þ rather than solvent reorganization
(limited solubilities of the complexes in hexanes, benzene,
and toluene prevented the collection of the emission spectra
of in these solvents).
Δνem ¼ Δνabs þ ΔSS
ð7Þ
Δνem and Δνabs, emission and absorption band shifts upon
Zn2þ coordination, respectively, in wavenumbers; ΔSS, the
difference between Stokes shifts of the Zn2þ complex and the
free ligand.
The structural dependence of the emission band shifts
) and the fluorescence quantum yield changes
(Δνem
(Table 5) upon zinc binding provides a foundation for
developing ratiometric indicators for zinc effective within
various emission windows. On the other hand, the correlation
between the electron density of the aryl group and the Zn2þ
-
effected emission band shift potentially poses a dilemma
in fluorescent heteroditopic ligand design for achieving
high contrast between three fluorescence states shown in
Figure 1A. An electron-rich aryl group would lead to a large
Zn2þ-effected emission shift which results in a large contrast
between the mono- and dizinc complexes. However, increas-
ing electron density in the aryl group would also increase the
oxidation potential of the AVMB ligand which attenuates
the CHEF effect, thus reducing the contrast between the
fluorescence of the free and monozinc-bound ligands. This
dilemma will be examined and addressed in a future study.
Lifetime and Fluorescence Quantum Yields of Ligands and
Zn2þ Complexes. The fluorescence quantum yield change
upon Zn2þ coordination in MeCN (Table 5) does not
follow a simple correlation with the electronic nature of
(60) McFarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2001, 123, 1260–
1261.
(61) McFarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2002, 124, 1178–
1179.
(62) Suppan, P.; Ghoneim, N. In Solvatochromism; The Royal Society of
Chemistry: London, 1997; pp 105.
J. Org. Chem. Vol. 74, No. 22, 2009 8769

Younes et al.
JOCArticle
TABLE 5. Fluorescence Quantum Yields (φf), Lifetime (τ),a and Rates of Radiative (kr) and Nonradiative (knr) Decays of Ligands and Their Zn2þ
Complexes in MeCN
ligand
φf
complex
τ/ns
χ2
kr/ns-1
knr/ns-1
τ/ns
χ2
φZn
kr/ns-1
knr/ns-1
1
2
3
4
5
6
7
8
9
11
1.06
1.01
0.99
0.58
0.23
0.51
1.22
0.31
0.60
0.66
1.31
0.73
0.50
0.57
0.64
0.47
1.02
0.99
0.94
1.30
0.67
0.76
0.69
0.28
0.042
0.087
0.34
0.045
0.27
0.13
0.63
0.75
0.70
0.48
0.18
0.17
0.28
0.15
0.45
0.20
0.31
0.24
0.31
1.24
4.17
1.79
0.54
3.08
1.22
1.32
2.41
2.14
2.30
3.14
1.91
2.76
NDb
1.20
1.60
2.91
1.19
0.64
0.52
1.28
1.30
0.78
NDb
0.93
0.41
1.77
0.77
0.82
0.79
0.46
0.25
0.10
0.0002
0.019
0.65
0.13
0.32
0.38
0.34
0.15
0.13
0.04
NDb
0.02
0.41
0.04
0.10
0.08
0.09
0.17
0.39
0.33
NDb
0.82
0.22
0.30
aIn the TCSPC experiments, all samples were excited using a 370 nm LED. The fluorescence decays were observed at the respective emission λmax
shown in Table 4. bND, Not Determined due to very low fluorescence quantum yield of the Zn2þ complex of 7.
FIGURE 8. (A) Spectrophotometric titration of 6 (17 μM) with Zn(ClO4)2 (0 - 34 μM) in MeCN. Blue spectrum, [Zn(II)] = 0; red spectrum,
[Zn(II)] = 34 μM. Arrows show the spectral change upon Zn2þ addition. (B) Absorbance of 6 (17 μM) at 413 nm vs [Zn(II)]/[L]. (C)
Fluorescence titration of 6 (5.3 μM, λex = 397 nm) with Zn(ClO4)2 (0-13 μM) in MeCN at 25 ꢀC. Blue spectrum, [Zn(II)] = 0; red spectrum,
[Zn(II)] = 13 μM. Arrows show the spectral change upon Zn2þ addition. (D) Fluorescence intensity of 6 (5.3 μM) at 483 nm (() and 607 nm (0)
vs [Zn(II)]/[L].
produced much richer information. Overall, with addition of
Zn2þ the fluorescence at 483 nm decreases and the band at
607 nm increases (Figure 8C). However, the fluorescence
attenuation at 483 nm and enhancement at 607 nm, that is,
the emission “bathochromic shift”, are not synchronized
during the entire titration process. Three transitions were
identified in the emission isotherm (Figure 8D). At the very
earliest stage, the “bathochromic shift” is synchronized63
followed by a period during which the monotonous reduction
of the free ligand emission was observed. As the fluorescence of
the free ligand all but disappears, the emission at 607 nm starts
to increase. Clearly, the enhancement at 607 nm during the
third leg of the titration is not due to the conversion of the free
ligand to the Zn2þ complex because the free ligand has already
been consumed judged by the disappearance of the band at 483
nm. The current hypothesis, corroborated by the ITC study, is
that the Zn2þ titration first affords weakly emissive 3:1 and 2:1
(ligand to Zn2þ) complexes. The fluorescence quenching ob-
served at this stage may be attributed to the interfluorophore
interactions in the 3:1 or 2:1 complexes. When all the free
(63) See Figure S25, SI, for data collected from another independently
prepared sample.
8770 J. Org. Chem. Vol. 74, No. 22, 2009

Younes et al.
JOCArticle
ligands are consumed, further addition of Zn2þ converts the
3:1 or 2:1 complex to the much more emissive 1:1 complex (The
absorption spectra of all the Zn2þ complexes with different
association stoichiometries, however, are similar in this
hypothesis). Therefore, the delayed fluorescence enhancement
at 607 nm was observed. The unsynchronized emission “bath-
ochromic shifts” are not unique to compound 6. All ligands
investigated in this study display this behavior to various
degrees.
before brine (2 mL) was added. The reaction mixture was stirred
for another 5 min, and was partitioned between EtOAc and
water. The aqueous layer was washed with EtOAc (50 mL ꢁ 3)
and the organic portions were combined. The organic portions
were dried over Na2SO4 followed by solvent removal under
vacuum. Compound 2 was isolated using silica column chroma-
tography eluted by EtOAc in DCM (gradient 0-30%). The
1
isolated yield was 70% (54 mg). H NMR (300 MHz, CDCl3):
δ/ppm 8.74 (s, 1H), 8.52 (s, 1H), 8.36 (d, J = 8.2 Hz, 1H), 8.30 (d,
J = 8.2 Hz, 1H), 7.96 (dd, J = 2.4, 8.6 Hz, 1H), 7.64 (dd, J = 2.1,
8.1 Hz, 1H), 7.48 (d, J = 8.6 Hz, 2H), 7.36 (d, J = 8.5 Hz, 2H),
7.18 (d, J = 16.2 Hz, 1H), 7.10 (d, J = 16.5 Hz, 1H), 2.40 (s, 3H);
13C NMR (75 MHz, CDCl3): δ/ppm 155.6, 152.5, 149.9, 148.3,
137.7, 137.6, 135.5, 134.1, 133.6, 132.6, 129.6, 129.2, 128.1, 125.7,
120.9, 120.8, 18.6; HRMS (ESIþ): calcd. (C19H15ClN2þHþ)
307.1002, found 307.1005.
Compound 3. Compound 10 (102 mg, 0.32 mmol) was dis-
solved in anhydrous THF. t-BuOK (82 mg, 0.72 mmol) and
4-fluorobenzaldehyde (90 mg, 0.72 mmol) was added subse-
quently. The reaction mixture was allowed to stir for overnight
after which the reaction was quenched carefully with water and
extracted by EtOAc. The aqueous layer was washed with EtOAc
(50 mL ꢁ 3). The combined organic layers were dried over
Na2SO4 and the solvent was removed under reduced pressure.
Compound 3 was isolated using silica column chromatography
using EtOAc in DCM (0-30%). The isolated yield was 80% (71
mg). 1H NMR (300 MHz, CDCl3) δ/ppm 8.73 (d, J = 2.1 Hz,
1H), 8.50 (s, 1H), 8.35 (d, J = 8.3 Hz, 1H), 8.28 (d, J = 8.1 Hz,
1H), 7.93 (dd, J = 2.3, 8.1 Hz, 1H), 7.61 (dd, J = 1.6, 8.1 Hz,
1H), 7.50 (dd, J = 5.4, 8.7 Hz, 2H), 7.16 (d, J = 16.4 Hz,
1H), 7.04 (m, 3H), 2.38 (s, 3H); 13C NMR (75 MHz, CDCl3):
δ/ppm 164.5, 161.2, 155.4, 153.5, 149.8, 148.1, 137.6, 133.5,
133.2, 132.7, 129.6, 128.5, 124.8, 120.7, 116.1, 115.8, 18.6;
HRMS (ESIþ): calcd (C19H15FN2þHþ) 291.1298, found
291.1302.
Conclusion
The photophysical properties and Zn2þ coordination
chemistry in MeCN of fluorescent 5-arylvinyl-50-methyl-
2,20-bipyridyl (AVMB) ligands are described. The primary
objective is to reveal the factors that control the emission
band shift and fluorescence quantum yield change of the
AVMB ligands upon Zn2þ coordination. The solvent depen-
dency studies and computational analysis of free ligands
support the charge-transfer nature of the S1 excited states of
most ligands, as we have invoked in previous studies.34,35
The Zn2þ coordination in MeCN was investigated by,
in addition to spectroscopic means, isothermal titration
calorimetry (ITC). A multiple species equilibrium involving
the formation of 3:1 and/or 2:1 (ligand to Zn2þ) com-
plexes en route to the eventual 1:1 complex best
explains the observed ITC traces in which Zn(ClO4)2 was
used as the Zn2þ source. When ZnCl2 was used in the ITC
experiment, a 1:1 stoichiometry was observed instead,
demonstrating the counterion-dependence of binding stoi-
chiometry.
The electron-donating ability of the aryl group is a major
determinant of the magnitude of absorption and emission
band shifts of an AVMB ligand upon Zn2þ coordination in
MeCN. The coordination of Zn2þ at the negative end of the
excited dipole greatly attenuates the energy of the excited
state, leading to a bathochromic shift. The fluorescence
quantum yield change, however, does not carry a similar
correlation with the electronic nature of the aryl group. The
Zn2þ coordination was found to reduce both radiative (kr)
and nonradiative (knr) decay rates, which collectively con-
tribute to the change in fluorescence quantum yields. The
decrease of kr can be explained using eq 5 where kr is
proportional to the square of transition energy. The drop
in knr may be attributed, in part, to the reduced degrees of
rotational freedom of the molecule upon Zn2þ coordination.
This study in the fundamental coordination chemistry and
photophysics of the AVMB ligands, which is garnering
increasing attention as the molecular optical components
in various applications, is expected to aid the rational design
of molecular sensors and possibly materials for energy
conversion.
Compound 11. Compound 6 (73 mg, 0.26 mmol) was dissolved
in dry DCM (2.6 mL) in a round-bottom flask containing a
magnetic stirr bar. The solution was cooled in an ice bath. A
solution of Br2 (13 μL, 0.26 mmol) in DCM (1.3 mL) was added
dropwise to the reaction mixture through an addition funnel.
After the addition, the stirring was continued for 2 h while the
temperature was allowed to rise to rt. The solvent was carefully
removed on a rotary evaporator with an aqueous Na2S2O3
solution (∼ 10 mL, 1 M) in the reservoir to absorb the residual
Br2. The crude product was partitioned between EtOAc (50 mL)
and brine. After separation, the brine layer was washed with
EtOAc (50 mL
ꢁ
2) before all the organic fractions
were combined and dried over anhydrous Na2SO4. TLC
(silica plate, eluted with EtOAc) showed a light yellow spot
which was later verified as the product. Compound 11 was
purified on a silica column (eluted with 0-10% EtOAc in
DCM). The yield was 48% (45 mg). 1H NMR (300 MHz,
CDCl3): δ/ppm 8.68 (s, 1H), 8.51 (s, 1H), 8.33 (d, J = 8.4 Hz,
1H), 8.31 (d, J = 7.5 Hz, 1H), 7.91 (d, J = 2.4 Hz, 1H), 7.63
(d, J = 6.6 Hz, 1H), 7.05 (d, J = 16.2 Hz, 1H), 6.87 (d, J =
16.2 Hz, 1H), 6.55 (d, J = 3.6 Hz, 1H), 6.24 (d, J = 4.2 Hz,
1H), 3.69 (s, 3H), 2.41 (s, 3H); 13C NMR (75 MHz, CDCl3):
δ/ppm 154.8, 153.5, 149.8, 147.7, 137.6, 133.4, 133.1, 132.9,
132.8, 122.4, 120.7, 120.6, 119.1, 111.5, 108.1, 105.0, 32.6, 18.5;
HRMS (ESIþ): calcd. (C18H16BrN3þHþ) 354.0558, found
354.0578.
Experimental Section
Representative Procedures. Compound 2. NaH (160 mg, 60%
in mineral oil, 4 mmol) was added at 0 ꢀC to a solution of
4-chlorobenzaldehyde (39 mg, 0.28 mmol) in dry dimethox-
yethane (4.0 mL) in a flame-dried round-bottom flask. The
suspension was stirred for 5 min. A solution of 10 (90 mg,
0.28 mmol) indry dimethoxyethane (4.0 mL) was added dropwise
to the flask with stirring at 0 ꢀC. The stirring was continued for
overnight at rt. The reaction mixture was then cooled to 0 ꢀC
Acknowledgment. This work was supported by the Flor-
ida State University through a start-up fund, a New Inves-
tigator Research (NIR) grant from the James and Esther
King Biomedical Research Program administered by the
Florida Department of Health, and the National Science
J. Org. Chem. Vol. 74, No. 22, 2009 8771

Younes et al.
JOCArticle
Foundation (CHE 0809201). The effort by Professor Jack
Saltiel and Dr. Bert van de Burgt in securing fund for and
setting up the TCSPC accessory is acknowledged. The
authors also thank Professor Kenneth Knappenberger for
enlightening discussions, Dr. Mariappan Manoharan for
initial guidance in using the Gaussian software, the Institute
of Molecular Biophysics at FSU for providing access to a
VP-ITC microcalorimeter (MicroCal), and Dr. Claudius
Mundoma for technical assistance.
Supporting Information Available: Synthetic procedures
and characterization of new compounds, additional spectra,
fluorescence decay traces, and cyclic voltammograms. This
material is available free of charge via the Internet at http://
pubs.acs.org.
8772 J. Org. Chem. Vol. 74, No. 22, 2009