Field effects induce bathochromic shifts in xanthene dyes

Article abstract of DOI:10.1021/ja302445w

There is ongoing interest in near-infrared (NIR) absorbing and emitting dyes for a variety of biomedical and materials applications. Simple and efficient synthetic procedures enable the judicious tuning of through-space polar (field) effects as well as low barrier hydrogen bonding to modulate the HOMO-LUMO gap in xanthene dyes. This affords unique NIR-absorbing xanthene chromophores.

Full text of DOI:10.1021/ja302445w

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Article
Field effects induce bathochromic shifts in xanthene dyes
Martha Sibrian-Vazquez, Jorge Omar Escobedo, Mark A.
Lowry, Frank R Fronczek, and Robert Michael Strongin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja302445w • Publication Date (Web): 29 May 2012
Downloaded from http://pubs.acs.org on May 31, 2012
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Field effects induce bathochromic shifts in xanthene dyes
Martha Sibrian-Vazquez,† Jorge O. Escobedo,† Mark Lowry,† Frank R. Fronczek‡ and Robert M.
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Strongin†,*
†Department of Chemistry, Portland State University, 1719 SW 10th Ave., Portland, Oregon 97201, United States
‡Department of Chemistry, Louisiana State University, 232 Choppin Hall, Baton Rouge, Louisiana 70803, United States
Keywords: xanthenes, near-infrared, large Stokes shift
Supporting Information Placeholder
ABSTRACT: There is ongoing interest in near infrared (NIR) absorbing and emitting dyes for a variety of biomedical and
materials applications. Simple and efficient synthetic procedures enable the judicious tuning of through-space polar (field) effects
as well as low barrier hydrogen bonding to modulate the HOMO-LUMO gap in xanthene dyes. This affords unique NIR-absorbing
xanthene chromophores.
the well-known polar effect of heavy atoms on the HOMO-
LUMO energy gap observed in many related classes of
chromophores.11 This useful design concept has been recently
applied to other scaffolds including anthracene,12 fluorescein,13
and a variety rhodamine derivatives.14 Si-rhodamines with
NIR emission as great as 740 nm have been created.
However, their Stokes shifts are typically less than 20 nm and
they are obtained in low overall yields from multistep
procedures.14d
INTRODUCTION
Near infrared (NIR) dyes are of great interest in
biomedicine due to diminished interfering absorption and
fluorescence from biological samples, inexpensive laser diode
excitation, reduced scattering and enhanced tissue penetration
depth.1 However, only one NIR probe, indocyanine green
(ICG), is approved by the FDA.2 ICG is challenging to use for
many applications due to its amphiphilicity and limited
functionalizability. Moreover it displays high affinity for
proteins with concomitant quenching upon binding.3 It
displays a small Stokes shift and low quantum yield and is
known to dimerize in aqueous solution at low concentrations
which further lowers its quantum yield.4 Designing
alternatives to ICG is thus an ongoing goal.
Experimentally observable intramolecular coulombic
interactions resulting from direct action through space rather
than through bonds are known as field effects.15 In 2008 we
reported
that
altering
the
known
regiochemical
functionalization of annulated xanthene dyes led to large (>
100 nm) Stokes shifts and NIR emission.16 This prior study
has led us to propose that by tuning through-space interactions
of polar groups (field effects)15 in the xanthene ring system,
one could also favorably alter the electronic properties of the
chromophore.
Several halogenated and non-halogenated xanthene dyes are
sufficiently non-toxic to be used as food dyes.5 However,
xanthene dyes typically possess small Stokes shifts and are not
active in the NIR. Thus, strategies for red-shifting xanthene
dyes for biodiagnostics and imaging applications have been an
active area of investigation for many years. Strategies
employed have included replacement of the phenyl ring with
electron withdrawing groups such as cyano or
trifluoromehtyl,6 extending the π-conjugation,7 replacement of
the bridging O-atom by tatetrahedral carbon,8 and annulation.9
As a result, there are currently a large number of excellent
long wavelength xanthene-based dyes. They mainly find
utility in cellular imaging applications, as their spectral
properties are outside the useful NIR (ca. 700 nm – 800 nm)
range.
Benzannulated xanthenes typically possess polar groups on
C-3 and C-11, as in 1 or C-3 and C-10 as in 3 (Figure 1).
Positioning a polar group on C-1 (as in 4) or C-1 and C-3 (as
in 2) can potentially modulate the HOMO-LUMO gap. This
embodies a relatively facile means to reduce the energy gap
without the need to replace the internal oxygen with heavy
atoms.
In 2008 it was demonstrated that when an internal xanthene
ether oxygen is replaced with a dimethylsilyl moiety, a
bathochromic shift of of 90 nm is obtained.10 This is due to
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O
R2
O
11
R2
Table 1. Experimental and calculated spectral properties of
1b-4b in MeOH.
13
1
O
O
2
3
4
5
6
7
8
9
λmax
em
λmax abs
Comp. λmax abs
HOMO LUMO
Gap
(eV)
calc
(nm)
R1
R1
(nm)
(nm)
688
816
(eV)
(eV)
1b
2b
3b
4b
598
701
-5.011
-4.781
5.442
5.606
-2.719
-2.688
2.367
2.667
-2.292
-2.093
3.075
2.939
541
592
403
422
R3
2a R1=CO2H, R2=OH, R3=H
2b
1a R1=CO2H, R2=OH
1b R1=CO2Me, R2=OH
R1=CO2Me, R2=OH, R3=H
487, 521 550
501, 536 582
-
2c R1=CO2Me, R2=O-, R3=H
1c
R1=CO2Me, R2=O
2d R1=Me, R2=OH, R3=Me
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1
3
R2
R2
Figure 2a displays the absorption spectra and a room light
photograph of 1b-4b in MeOH, summary of the
O
O
O
10
O
a
experimental and calculated absorption and emission maxima
values are given in Table 1. The placement of R2 proximal to
the xanthene internal oxygen in 2b and 4b affords
bathochromic shifts of 103 and 15 nm as compared to 1b and
3b, respectively. The formal transposition of R2 also imparts
similar corresponding shifts in the excitation spectra (Figure
2b) and large bathochromic shifts in the fluorescence emission
(Figure 2c) of 2b and 4b (128 nm and 32 nm as compared to
1b and 3b, respectively). Importantly, 2b absorbs and emits in
the near infra-red with absorption and emission maxima of
701 nm and 816 nm.19 Analogous trends are observed in other
solvent systems (see below and SI, Figure S34). Spectral
properties of these and other compounds from families 1-4 in
other solvents are tabulated in the supporting information (SI,
Table S1).
R1
R1
4a R1=CO2H, R2=OH
4b R1=CO2Me, R2=OH
4c R1=CO2Me, R2=O-
4d R1 =CO2Me, R2=OMe
3a
R1=CO2H, R2=OH
3b R1=CO2Me, R2 = OH
3c R1=CO2Me, R2 = O-
Figure 1. Structures of benzannulated xanthenes.
RESULTS AND DISCUSSION
Synthesis
of
symmetric
and
asymmetric
naphthoxanthenes. In order to test this hypothesis, a series of
symmetric and asymmetric naphthoxanthenes 1-4 were
synthesized and evaluated. Naphthofluorescein 1a is
commercially available. Known SNAFL-1 (herein 3a) was
prepared as previously reported.9b The rest of the symmetric
and asymmetric naphthoxantene fluorophores were
synthesized by condensation of 1,8-dihydroxynaphthalene
with phthalic anhydride, 3,5-dimethylbenzaldehyde, and 2-
(2,4-dihydroxybenzoyl)benzoic acid respectively; using a
mixture of CH3SO3H:TFA 1:1 at 80 ºC for 16-24 h. The
corresponding methyl ester derivatives were obtained by
Fischer esterification in MeOH catalyzed by H2SO4; further
methyl alkylation was furnished by treatment of either the
carboxylate or methyl ester intermediate with methyl iodide in
the presence of K2CO3 in DMF.
The required 1,8-
dihydroxynaphthalene and 2-(2,4-dihydroxybenzoyl)-benzoic
acid derivatives were synthesized according to described or
modified literature protocols.9a,16,17 All compounds were
isolated by flash column chromatography and characterized by
NMR and MS. The structure of compound 4a as the lactone
form (Figure S1, SI) was also confirmed by X-ray single
crystal structure. The target compounds were obtained in
overall yields ranging from 17 to 86% as described in the
supporting information.
Spectroscopic Characterization
The study of 1a-4a in MeOH, DMSO and aqueous acid is
complicated by the carboxylate-lactone equilibrium. (SI
Figures S27-S30, Scheme S2) This prompted us to focus our
initial investigations on esters 1b-4b and their corresponding
anions1c-4c. It is noteworthy, however, that 2a exists as the
corresponding colorless lactone in 1:9 DMSO:buffer below
physiological pH of 7.4. It thus embodies a unique pH probe,
in stark contrast to known intracellular pH sensors18 such as 1a
and congeners that generally function as turn-on indicators >
pH = 5 (SI, Figure S29).
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Figure 2. Absorption and fluorescence spectra of fully annulated
1b, 2b, and semiannulated 3b and 4b in MeOH. (a) Absorption.
OH
10.05 ppm
1
2
3
4
5
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7
8
1b
2b
Inset: Room light Photograph of solutions in MeOH (left to right:
3b, 4b, 1b and 2b). (b) Normalized fluorescence excitation
spectra. (c) Normalized fluorescence emission spectra. Formal
transposition of R2 (Figure 1) modulates the HOMO-LUMO gap
and shifts the absorption and fluorescence in both the symmetric
(1 and 2) and asymmetric (3 and 4) pairs. Additional spectra in
MeOH and DMSO are provided (SI, Figures S31-S33).
OH
14.45 ppm
OH
10.57 ppm
3b
4b
Induced through-space interactions and intramolecular
hydrogen bonding. Although the red shift in the absorption of
4b in MeOH appears modest, its corresponding anion 4c in
DMSO:aqueous base 1:9 is shifted more substantially by 50
nm to 599 nm as compared to anion 3c under the same
condition.20 Its fluorescence emission is shifted by an even
larger 130 nm, emitting at 760 nm (see below). The
comparatively large shift of the anion 4c further supports a
polar effect as playing a key role in modulating the HOMO-
LUMO gap. The oxoanion of asymmetric 4c should have a
more significant field effect on the polarity of the proximal
ether oxygen as compared to the corresponding oxoanion 3c.
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OH
10.78 ppm
15.5
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11.5
10.5
9.5
8.5
f1 (ppm)
1
Figure 3. H NMR spectra in DMSO-d6 of compounds 1b, 2b,
3b, and 4b showing the different chemical shifts observed for the
hydroxyl protons.
As further evidence of the effect of the hydrogen bonding
network on the characteristics of 2, removal of the shared
proton (under harsh conditions, 12.5 mM NaOH, to form 2c)
does not result in a large bathochromic shift in the absorption
as is observed when 1b is deprotonated to 1c (SI, Figure S36).
In addition, unlike conventional naphthofluorescein
compounds in which the fluorescence is minimal for the protic
species in aqueous solvents9a,9f the protic species of family 2 is
more fluorescent than the corresponding ionized chromophore.
For example, NIR fluorescence was observed in family 2
compounds up to pH 9; while removal of the shared proton
was observed to quench the fluorescence (SI, Figure S37-38).
Molecular simulations demonstrate that the hydrogen bonding
network enhances coplanarity of the fused ring system of
compounds in family 2 (Figure 4 and SI, Figure S39).
There are through-space polar interactions between R2 and
the internal bridge oxygen in compounds 4a-d. In the case of
symmetric compounds 2a-d the field effect is further enhanced
by the oxygen on C-13 (Figure 1). The relatively larger
difference in absorption maxima of 2b as compared to 1b
(Figure 2a) is also attributable at least in part to the unique
intramolecular hydrogen bonding network present in 2b
(Figures 3 and 4). This network results in enhanced negative
charge density on the ionizable oxygen atoms, thereby
promoting a relatively large bathochromic shift, as in the
analogous case of anionic 4c described above.
1
Hydrogen bonding is evidenced in the H NMR in DMSO-
d6 of 2b (Figure 3) and absorption spectra (SI, Figure S35).
The shared proton resonates significantly downfield (14.45
ppm) as compared to the corresponding phenolic proton of 1b
(10.05 ppm). This strong deshielding can be attributed to a
redistribution of the electronic density around the hydrogen
atom that occurs upon hydrogen bond formation between the
hydroxyl proton, the carbonyl and the central oxygen of 2b.
Evidence of hydrogen bonding is also observed in 4b, but to a
lesser degree. The hydrogen bonded proton is slightly
downfield (10.75 ppm) as compared to the phenolic proton of
3b (10.58 ppm). The strong deshielding observed for the
hydroxyl proton of 2b indicates that it is involved in the
formation of a low barrier hydrogen bond, (LBHB).21 Low
barrier hydrogen bonds play an important role in enzymatic
processes 21a,22 as well as in supramolecular assemblies.23
In general, low barrier hydrogen bonds have been observed
in systems were the donor acceptor distances are shorter than
normal hydrogen bonds.22c,24 In 2b, simulations show that the
total hydrogen bond distance O···H-O between the carbonyl
and the hydroxyl at the C-1 position is 2.52 Å, which is lower
than the typical hydrogen bonding distance O···H-O (≥ 2.8 Å)
and close to the average distance observed for low barrier
hydrogen bonds (2.55 Å). In 4b, the total hydrogen bond
distance O···H-O between the hydroxyl at the C-1 position and
the xanthene central oxygen is 2.95 Å, which falls in range of
common hydrogen bonds.
Figure 4. Hydrogen bond network of 2 and energy-minimized
structure of 2b showing how this enhances coplanarity of the
fused ring system.
Molecular modeling. The bathochromic shifts in the
absorption spectra induced through formal transposition
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(Figure 1, from C3 to C1) of the polar ionizable oxygen
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groups were evaluated by calculating the HOMO-LUMO
energies of both the protic and anionic species and comparing
the calculated absorbance maxima with experimental data in
MeOH or DMSO:aqueous 1:9, respectively.25 Molecular
surfaces and energies are provided in the supporting
information (SI, Figures S40-S50) and included for
comparison with the spectral data in Table S1. Modeling
generally systematically underestimated the HOMO-LUMO
gap (i.e. overestimated the absorption wavelength) of
deprotonated species and overestimated the gap (i.e.
underestimated the absorption wavelength) for protic species.
However, in each case, bathochomic shifts were both
predicted and observed for compounds with formally
transposed R groups (i.e. families 2 and 4 as compared to 1
and 3 respectively). Figure 5 shows the calculated molecular
surfaces anions 1c and 2c. The results indicate that formal
transposition effectively modulates the HOMO-LUMO gap.
Molecular modeling most adequately predicts the magnitude
of transposition-induced shifts for the deprotonated fully
annulated compounds. When comparing 1c and 2c, the 73 nm
calculated bathochromic shift is in agreement with the 79 nm
observed experimental shift of 2c as compared to 1c. An
experimental bathochromic shift of ~50 nm (calculated shift of
~90 nm) was observed for semiannulated anion 4c as
compared to 3c. The modeling also reveals an unusual
through-space overlap of the C-1 and C-13 oxygen orbitals in
the HOMO of 2c that result in homoconjugation of the π-
system in the chromophore. For 2c, the distance between the
oxygens of C-1 and C-13 is 3.2 Å. Since the typical distance
for a O-O bond is about 1.5 Å, this interaction could be
involving variations of the molecular dipole moment through
space. The five-membered fused-ring system in the 2 family
is less planar in the anionic form due to the electrostatic
repulsion of the oxygens of C-1 and C-13, and this allows the
through-space overlapping of the oxygens’ orbitals of the
HOMO that would not be possible if the anionic form was
planar (Figure 6).
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Figure 5. Calculated molecular surfaces of the HOMO and
LUMO of 1c and 2c (basis set DFT B3LYP/6-31G).29 Additional
molecular surfaces for other compounds are provided in the
supporting information.
Figure 6. Overlapping of the molecular orbitals of the oxygens at
C-1 and C-13 in compound 2c.
N
N
OH
OH
Examples of other molecular architectures (restricted
coplanarity featuring oxygen atoms relatively close in space)
similar to 2 include 1,1’-binaphthalene-2.2’-diol (binol)26 and
more recently, HelMe, a [4]heterohelicene cation.27 In binol
the positions of the oxygens are not restricted and therefore a
direct comparison with 2c is not possible. In the case of the
[4]heterohelicene cation, HelMe (Figure 7) the oxygen atoms
seem to be forced to be close but the calculated molecular
surfaces of the published HOMO do not show a through-space
overlapping of the oxygens’ orbitals as in the case of 2c.28
O O
HelMe
binol
Figure 7. Known architectures positioning oxygen atoms
relatively close in space.
The NIR absorbing properties of 2 render these compounds
attractive. NIR absorbing compounds are widely used in areas
such as optical recording, thermal-writing displays, laser
printers, laser filters, infrared photography, photodynamic
therapy and many other applications including solar cells and
heat absorbers.30 A current limitation in the field of solar cells
is the mismatch between the absorbing dye and the solar
emission spectrum.30b Compound 2b is NIR active in the
solid state (SI, Figure S51) and the absorption spectra of 2b
and 2c match well the 690 nm maximum photon flux of
sunlight.30b
The long-wavelength absorptions of the dyes in the 2 family
suggest that they could be useful as dyes for solar cells, given
that a major goal for improvement of organic solar cells is
improved absorption in the far red and NIR region.31 In
addition, the best dyes for dye-sensitized solar cells (DSSCs)
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use phenylcarboxylates as anchoring groups and have a donor- SNAFL-1 including an additional carboxylate group on the
1
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4
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acceptor structure in which the HOMO is localized on the
donor portion while the LUMO is localized on the acceptor
portion; this facilitates injection of the excited electron into the
semiconductor (e.g., TiO2) on which the dye is adsorbed.32
lower ring). The emission enhancing properties afforded
through the combination of formal transposition with
methylation is in contrast to previously reported methyl ethers
of seminaphthofluorescein compounds lacking such
enhancement.9b Interestingly, methylation of 4b results in only
4d (exhaustive attempts to synthesize the corresponding
isomer of 4d, where the substituents on C-1 and C-10 are
interchanged were unsuccessful, see Scheme S1). The
quantum yield of 4d is ~1.5 times greater than the short
wavelength emitting methyl ether of carboxy SNAFL-1 in
which the C-3 oxygen was methylated, and ~15 times greater
than the analogous red-emitting C-10 methyl ether.9b Thus,
methylation of 4b affords pH independent and relatively
The electron density of the HOMO of 2c (Figure 5) and 4c
is located nearly exclusively on the chromophore, whereas in
the LUMO nearly all of the electron density is located on the
lower ring containing the ester moiety. These striking
electronic properties are unique to chromophores 2 and 4,
except, as expected, for 2d, which contains electron donating
groups on the lower arene. Enhanced electron density in the
LUMO at the sight of attachment to the metal surface of a
solar cell is critical for efficient electron transfer.
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brighter
red
fluorescence
compared
to
known
Spectroscopic Characterization in Aqueous Media. The
unique absorption and fluorescence properties of protic
species 2b that arise from through space field effects and
intramolecular hydrogen bonding, involving all three oxygens
of the xanthene chromophore, were described above.
Compounds 4a-d possess enhanced spectral properties for
applications including biological imaging and diagnostics
wherein long wavelength activity is advantageous. Positioning
the hydroxyl group on C-1, to afford a field effect between
two polar atoms, the bridging xanthene oxygen and the C-1
oxygen in the case of 4c, was found to have a greater effect on
the emission wavelength of 4c than on the absorption as
compared to 3c (~130 nm bathochromic shift in emission vs.
~50 nm shift in absorption, described above). The difference
between these shifts results in NIR emission at 760 nm with a
significantly enhanced Stokes shift of 160 nm. For
comparison, the analogous long wavelength anionic form of
commercial SNAFL-1 (3a) is reported to absorb and emit at
537 nm and 624 nm, respectively with a comparatively modest
Stokes shift of 87 nm and a quantum yield of 0.093.9b The
Stokes shift of other xanthene dyes, i.e. fluorescein and
rhodamine derivatives, is typically ~ 20 nm. Note that in the 1
and 3 series, field effects are not promoted due to the distal
positioning of all three oxygens of the chromophore moiety.
seminaphthofluorescein probes.
Figure 8. Fluorescence emission spectra of 3b, 3c and methyl
ether 4d in 10:90 DMSO:aqueous. Solutions were prepared such
that the absorbances at the 500 nm excitation wavelength were
equal for all solutions. Inset: Fluorescence emission in a darkened
room of 7.5 ꢀM solutions (3b, 4d, and 3c from left to right)
excited from below with 3-watt megaMAX 505 nm ALS system.
Neutral compound 3b is in pH 6 phosphate buffer, 12.5 mM while
anion 3c is in pH 12 NaOH. Compound 4d is in pH 7.4 phosphate
buffer, 12.5 mM.
CONCLUSIONS
In summary, the results described herein demonstrate that
xanthene dye absorption and emission can be shifted to the
NIR via relatively simple structural (e.g., regioisomeric)
changes involving field effects or low barrier hydrogen bonds,
depending on the ionization of the chromphore. This design
principle, involving the environment of the bridging ether
oxygen, apparently allows for favorable modulation of the
HOMO-LUMO gap. Large Stokes shifts are observed as well
as an enhancement in quantum yield and brightness as
compared to analogous known long wavelength emitting dyes.
Ongoing efforts in our laboratory include the study of related
field effects on rhodols, rhodamines, and other xanthenes, as
well as other chromophores.
Like many other NIR emitting compounds, 4c displays a
relatively low quantum yield (less than 1%) in aqueous
solution. The fluorescence quantum yield of ICG is limited by
internal conversion to only a few percent.4 The brightness of
4c is comparable to that of other novel NIR probes including
carbon nanotubes33 which exhibit extinction coefficient34 of
approximately 4,400 and low fluorescence efficiencies
typically < 0.1%.35 It is well known that the absence of
interfering background increases the sensitivity of NIR
fluorophores far beyond other visible dyes with much higher
quantum yields.36 Efforts to increase the quantum yield
beyond that of other NIR emitting probes are underway.
We find that straightforward functionalization through
methylation of the C-3 oxygen to afford methyl ether 4d
dramatically improves the quantum yield. The quantum yield
of yellow-orange emitting 4d is 0.4649, 40 times greater than
the corresponding neutral 4b. It is slightly higher than yellow-
green emitting neutral 3b and > 2 times higher than orange-red
emitting anion 3c (Figure 8). Formal transposition of the
hydroxyl moiety (Figure 1) shifts the spectral properties such
that the short wavelength forms of these compounds (4b and
4d) are redder than then short wavelength forms and approach
the long wavelength forms of known seminaphthofluorescein
probes (for example 3a and its analogues, i.e. carboxy
EXPERIMENTAL
Chemistry. Unless otherwise indicated, all commercially
available starting materials were used directly without further
purification. Silica gel Sorbent Technologies 32-63 ꢀm was
used for flash column chromatography. 1H- and 13C NMR
were obtained on either a ARX-400 or ARX 600 Advance
Bruker spectrometer. Chemical shifts (δ) are given in ppm
relative to DMSO-d6 (2.50 ppm, 1H, 39.52 13C) unless
otherwise indicated. MS (HRMS, ESI) spectra were obtained
at the PSU Bioanalytical Mass Spectrometry Facility on a
ThermoElectron LTQ-Orbitrap high resolution mass
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indotricarbocyanine perchlorate; HelMe, 1,13-dimethoxy-5,9-
dimethyl-5,9-dihydroquinolino[2,3,4-kl]acridin-13b-ylium;
HPDITCP, 1,10,3,3,30,30-hexamethyl-3,5-propylene-4-
spectrometer with a dedicated Accela HPLC system. 1,8-
dihydroxynaphthalene and 2-(2,4-dihydroxybenzoyl)-benzoic
acid were synthesized as described in the literature.16 9a,17
1
2
(dimethylamino)-2,20-indotricarbocyanine perchlorate HOMO,
highest occupied molecular orbital; HRMS, high resolution mass
spectrometry; ICG, indocyanine green; LBHB, low-barrier
hydrogen bond; LUMO, lowest unoccupied molecular orbital;
NIR, near-infrared; SNAFL, seminaphthofluorescein; TFA,
trifluoroacetic acid.
3
4
5
6
7
8
Crystallographic data for compound 4a: C24H14O5,
monoclinic space group C2/c,
a = 19.5457(15), b =
12.2406(10), c = 15.4763(14) Å, β = 114.233(5)˚, V =
3376.5(5) Å3, Z = 8, ρcalcd= 1.504 g cm-3, ꢀ(CuKα) = 0.87 mm-
1, R = 0.033 for 2667 data with Fo2>2σ(Fo2) of 3070 unique
data and 269 refined parameters. A total of 13,499 data (Rint
=
9
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0.13 x 0.17 mm. Refinement was by full-matrix least squares
using SHELXL, with H atoms in idealized positions, except
for OH hydrogen atoms, which were located by difference
maps and their positions refined.
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spectrophotometer. Fluorescence spectra were collected on a
Cary Eclipse fluorescence spectrophotometer (Agilent
Technologies).
All absorbance spectra were reference
corrected. Fluorescence spectra were corrected for the
wavelength dependent response of the R928 photomultiplier
tube using a manufacturer generated correction file. Quantum
yields are reported as the average of multiple measurements
using multiple references. Red emitting probes were
referenced to rhodamine 101 (0.913)37 and sulforhodamine
101 (0.900)38 in EtOH. NIR emitting probes were referenced
to Nile Blue (0.27)39, IR-125 (0.132)37, and HEDITCP
(0.195)37 in EtOH. HEDITCP was prepared as reported by
Descalzo.40
Concentrations were adjusted such that
absorbance values were ~0.05 or lower. Differences in
refractive index between sample and solution were accounted
for.
Molecular Modeling. molecular orbital calculations were
performed using the Density functional theory (DFT) at
B3LYP/6-31G level using Gaussian 03W.
ASSOCIATED CONTENT
Supporting Information. Detailed experimental procedures,
synthesis, spectral and characterization data. Crystallographic data
for compound 4a (as the lactone form). This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Email: strongin@pdx.edu
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENT
The authors thank Dr. Carl Wamser for valuable discussions. This
work was supported by the National Institutes of Health (RO1
EB002044) and the NSF (Grant 0741993) for the purchase of the
LTQ-Orbitrap Discovery.
ABBREVIATIONS
DMF, dimethyl formamide; DMSO, dimethylsulfoxide; DSSCs,
dye-sensitized solar cells; ESI, electrospray ionization; FDA,
Food and Drug Administration; HEDITCP, 1,10,3,3,30,30-
hexamethyl-3,5-ethylene-4-(dimethylamino)-2,20-
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