Hydroxydialkylamino cruciforms: Amphoteric materials with unique photophysical properties

Article abstract of DOI:10.1002/chem.201002865

Two amphoteric cruciforms 6 and 7 (XF; 4,4′-[(1E,1′E)-(2,5- bis{[4-(dibutylamino)phenyl]ethynyl}-1,4-phenylene)bis(ethene-2,1-diyl)] diphenol, 4,4′-[{2,5-bis[(E)-4-(dibutylamino)styryl]-1,4-phenylene} bis(ethyne-2,1-diyl)]diphenol) were prepared by a Horn

Full text of DOI:10.1002/chem.201002865

DOI: 10.1002/chem.201002865
Hydroxydialkylamino Cruciforms: Amphoteric Materials with Unique
Photophysical Properties
Psaras L. McGrier,[b] Kyril M. Solntsev,[b] Anthony J. Zucchero,[b] Oscar R. Miranda,[c]
Vincent M. Rotello,[c] Laren M. Tolbert,[b] and Uwe H. F. Bunz*[a, b]
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Chem. Eur. J. 2011, 17, 3112 – 3119

FULL PAPER
Abstract: Two amphoteric cruciforms 6
and 7 (XF; 4,4’-[(1E,1’E)-(2,5-bis{[4-
(dibutylamino)phenyl]ethynyl}-1,4-phe-
nylene)bis(ethene-2,1-diyl)]diphenol,
4,4’-[{2,5-bis[(E)-4-(dibutylamino)sty-
nium hydroxide, and metal triflates.
The substitution pattern of 6 and 7
leads to spatial separation of the fron-
tier molecular orbitals, which allows
the HOMO or LUMO of the XF to be
addressed independently by acidic or
basic agents. XF 6, which has hydroxyl
groups on the styryl axis, displays
changes in emission color upon expo-
sure to ten amines in eight different
solvents. The change in fluorescence
upon the addition of amines was ana-
lyzed by linear discriminant analysis.
These XFs may have potential in
sensor applications for metal cations
and amines.
ACHTUNGTRENNUNGryl]-1,4-phenylene}bis(ethyne-2,1-diyl)]-
diphenol) were prepared by a Horner
reaction followed by a Sonogashira
coupling and subsequent deprotection.
The XFs display significant changes in
absorption and emission when exposed
to trifluoroacetic acid, tetrabutylammo-
Keywords: alkynes · amines · fluo-
rescence · sensors · solvatochrom-
ism
Introduction
spatial separation of the frontier molecular orbitals (FMOs)
upon suitable donor–acceptor substitution of the peripheral
aryl units.[1] Similar examples of such disjointed FMO struc-
tures include Haleyꢁs 1,2,4,5-tetrakis(arylethynyl)benzenes,[5]
Scherfꢁs swivel cruciforms,[6] Diederichꢁs tetraethynylethy-
lenes,[7] and Galvinꢁs distyrylbenzene derivatives.[8] By
AHCTUNGTREGpNNNU lacing hydroxyl and amino groups on different axes of the
XFs, as in 6 and 7, we hope to generate spectral responses
upon the addition of various acid–base analytes.
If pyridines or dialkylanilines are incorporated into the p
system of the XF, either a red or blue color change in emis-
sion is observed upon coordination to metal cations. When
both functional groups are present, a two-stage metallores-
ponsive fluorophore results.[9] With hydroxyl groups, spec-
troscopic changes (redshift in absorption and emission) are
observed upon deprotonation, an effect which is useful for
the identification of amines.[10] Herein we incorporate di-
1,4-Distyryl-2,5-bis(arylethynyl)benzenes (cruciforms, XFs)
containing basic nitrogens, pyridines, phenols, and pheno-
thiazines as functional appendages are known.[1] Herein we
have prepared an XF-based fluorescence probe (6) that is
sensitive to both cationic/acidic and basic agents and is po-
tentially useful as a dual function sensor.
The spectroscopic behavior of amphoteric photoacids/
photobases can be complicated by multiple acid–base equili-
bria between different species in the ground state, including
the formation of zwitterions. However, a more serious draw-
back for such materials can result from their excited-state
dynamics because they may undergo phototautomerization
when the two functions are not vicinal. Protons can be shut-
tled along hydrogen-bond wires formed in protic solvents[2]
as the result of a potentially dramatic increase in the acidity
or basicity of the corresponding functional groups upon ex-
citation.[2a,3,4]
AHCTUNGTREGaNNNU lkylaniline and hydroxyl substituents into one XF to study
the photophysics of amphoteric compounds by creating a
two-stage probe that is responsive to protons, metal cations,
bases, and amines.
An attractive way to simplify the prototropic behavior of
chromophores in the ground and excited states and to pre-
vent tautomerization is to separate the acidic and basic moi-
eties spatially and electronically. XFs with styryl and aryle-
thynyl branches attached to a central benzene ring allow the
Results and Discussion
Synthesis of hydroxydialkylamino XFs: The synthesis of
XFs 6 and 7 started with a Horner[11] reaction of 2a or 2b to
give distyrylbenzene derivatives 3a and 3b in 77 and 71%
yield, respectively, after recrystallization (Scheme 1).[9,10]
Subsequently, a Sonogashira coupling[12] with either 4 or 5
gave intermediates that were deprotected at ꢀ788C by tri-
fluoroacetic acid (TFA) to afford XFs 6 and 7 in 84 and
82% yield, respectively.
[a] Prof. U. H. F. Bunz
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢂt
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax : (+49)6221548404
E-mail: uwe.bunz@oci.uni-heidelberg.de
[b] Dr. P. L. McGrier, Dr. K. M. Solntsev, Dr. A. J. Zucchero,
Prof. L. M. Tolbert, Prof. U. H. F. Bunz
School of Chemistry and Biochemistry
Georgia Institute of Technology
901 Atlantic Drive, Atlanta, GA 30332 (USA)
Spectroscopic properties of hydroxydialkylamino XFs:
Figure 1 shows the absorption and emission spectra of XFs 6
and 7 in four representative solvents. The absorbance spec-
trum of 6 displays broad absorption maxima ranging from
l=359 to 372 nm. XF 7 exhibits a significant charge-transfer
band in all solvents at around l=423 to 445 nm and a single
more intense absorption at lꢁ338 nm. The absorbance
[c] O. R. Miranda, Prof. V. M. Rotello
Department of Chemistry
710 North Pleasant St., University of Massachusetts Amherst
MA 01003 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002865.
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U. H. F. Bunz et al.
sions (990 and 1180 cmꢀ1) when
investigated in diethyl ether or
toluene. To analyze the solvato-
chromic behavior of XFs 6 and
7
in emission, we used the
Kamlet–Taft multivariate ap-
proach,[13] which correlates the
spectral shift n of the XF with
the solvent parameters that are
responsible for its acidic (a),
basic (b), and polar solvating
(p*) properties.[14,15,10b] It allows
a straightforward separation of
selective (hydrogen bonding)
and
non-selective
(dipole–
dipole interaction) solvation.
*
nð1000 cmÞ ¼ n0 þ pp þ aa þ bb
ð1Þ
The solvatochromic behavior
of XFs 6 and 7 is governed by
the polar and acid–base proper-
ties of the solvents (Table 3); a,
b, and p are fitted parameters.
The index p* denotes the abili-
Scheme 1. Synthesis of the XFs 6 and 7. OTHP=tetrahydropyranyl. Conditions: i) NaH, THF; ii) [PdCl2-
(Ph3P)2], CuI/THF, piperidine, 258C then TFA, CH2Cl2, ꢀ788C.
ACHTUNGTRENNUNG
ty of
a solvent to stabilize
dipole or internal charge trans-
fer, whereas a and b reflect the
acidity and basicity of the cor-
responding solvent. The compo-
nent p dominates the solvato-
chromic behavior of 6 and 7,
but both acidity and basicity of
the solvent also induce smaller
bathochromic shifts. The excep-
tion is the anion of 6, which is
formed upon reaction of 6 with
diethylamine. In this case the a
parameter is +0.8, that is, the
emission is blueshifted on in-
creasing the acidity of the sol-
vent, whereas in the neutral
and cationic species a small red-
shift is observed on increasing
the basicity or acidity of the
solvent. The large positive a
value for the conjugate base of
6 would be expected as increas-
ing interaction of the phenolate
Figure 1. Absorption (left) and emission (right) spectra of 6 and 7 in diethyl ether (c), THF (a), MeOH
(c), and DMSO (c).
with
a good proton donor
would lead to a blueshift in the
spectra for both compounds depend weakly on solvent po-
larity indicating a small ground-state dipole moment.
The emission spectra of the XFs range from l=461 to
540 nm (see Tables 1 and 2). XF 7 displays vibronic progres-
optical features through increasing neutralization of the
anion by hydrogen bonding. The fluorescence quantum
yield F in MeOH was ꢁ14% for 6 and 7. The fluorescence
decays in MeOH (l=550 nm excitation) was 5.6 ns for 6
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Hydroxydialkylamino Cruciforms
FULL PAPER
Table 1. Absorption and emission maxima for 6 in various solvents.
Acid–base studies and quantum chemical calculations of hy-
droxydialkylamino XFs: The effect of adding an excess of
TFA or tetrabutylammonium hydroxide (TBA-OH) to 6 or
7 in different solvents is shown in Figures 2 and 3. The addi-
tion of acid to 6 leads to a blueshift in absorption and emis-
sion, regardless of the solvent. The addition of tetrabutylam-
monium hydroxide (TBA-OH) induces a red shift in both
absorption and emission of 6. The spectral responses can be
easily rationalized by assuming that the HOMO is stabilized
Solvent
lmax,abs [nm]
lmax,em [nm]
Stokes shift [cmꢀ1
]
MeOH
MeCN
DMF
DMSO
THF
CH2Cl2
diethyl ether
toluene
363
364
369
372
364
364
359
363
508
537
521
538
485
501
461
464
7860
8850
7910
8290
6850
7510
6160
6000
Table 2. Absorption and emission maxima for 7 in various solvents.
Solvent
lmax,abs
lmax,em
Stokes shift
[cmꢀ1
Vibronic progression
[nm]
[nm]
]
[cmꢀ1
]
MeOH
MeCN
DMF
DMSO
THF
CH2Cl2
diethyl
ether
339, 431 523
340, 440 535
339, 434 533
340, 445 540
337, 430 499
339, 433 517
10400, 4080
10700, 4040
10700, 4280
10900, 3950
9630, 3220
–
–
–
–
–
–
990
10200, 3750
337, 423 480, 504 8840, 2810
toluene
338, 433 482, 511 8840, 2350
1180
Table 3. Solvatochromic parameters [103 cm] used in multivariate regres-
sion fits of emission data for XFs 6 (with and without diethylamine) and
7 according to Equation (1).
Compound n0
p
a
b
R[a]
6
23ꢂ0.9 ꢀ4.0ꢂ1.4
ꢀ0.5ꢂ0.8 ꢀ0.7ꢂ1.2 0.87
0.8ꢂ0.5 ꢀ1.0ꢂ0.8 0.90
ꢀ0.6ꢂ0.9 ꢀ0.8ꢂ0.8 0.91
6+DEA[b]
19.6ꢂ0.6
ꢀ2.5ꢂ0.9
7
22.0ꢂ0.6 ꢀ3.1ꢂ0.9
[a] Correlation coefficient. [b] DEA=diethylamine.
Table 4. Photophysical data of 6 and 7 in MeOH.
[a]
lmax,abs [nm]
lmax,em [nm]
Ffl
tav [ns]
6
7
363
339, 431
508
523
0.14
0.13
5.6
1.5
Figure 3. Absorption (left) and emission (right) spectra of 6 (c) upon
addition of TFA (c) and TBA-OH (c) in a) MeOH (lmax,abs =330
(TFA), 363 (6), 377 nm (TBA-OH); lmax,em =481 (TFA), 507 (6), 545 nm
(TBA-OH)), b) MeCN (lmax,abs =329 (TFA), 364 (6), 383 nm (TBA-OH);
lmax,em =480 (TFA), 537 (6), 593 nm (TBA-OH)), and c) CH2Cl2
(lmax,abs =332 (TFA), 364 (6), 391 nm (TBA-OH); lmax,em =472 (TFA),
500 (6), 584 nm (TBA-OH)).
[a] Quantum yield.
and 1.5 ns for 7 (Table 4). The reason for this difference is
unknown.
by protonation and destabilized
by deprotonation. In the neu-
tral compound (Figure 4), the
HOMO and LUMO are only
slightly disjointed because both
the OH and the NBu2 substitu-
ents are electron donors. Upon
deprotonation, the HOMO lo-
calizes on the phenolate that
contains the distyrylbenzene
axis, because more electron
density is now in the system.
Upon addition of TFA, the
NBu2 groups are protonated
and the conjugation to the p
Figure 2. Exposure of 6 and 7 to trifluoroacetic acid (TFA) and tetrabutylammonium hydroxide (TBA-OH).
The samples were excited by using UV radiation at lem =366 nm.
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U. H. F. Bunz et al.
Figure 5. Absorption (left) and emission (right) spectra of 6 in MeOH/
water (2:1 v/v) at different pH values. Top: Acidic titrations, bottom: ba-
sic titrations.
and emission as expected. A new band emerges at l=
330 nm along with a shoulder at lꢁ370 nm. In the emission
spectrum, a new distinct band emerges at pH 0.80 (l=
479 nm), which disappears at pH 6.08 (l=531 nm) due to
full ground-state protonation of the dialkylanilines. Upon
addition of aqueous KOH, no significant change in the ab-
sorption spectrum of 6 is observed. The small bathochromic
shift in absorption is surprising and persists upon addition of
excess KOH. Similar behavior is observed in the emission
spectrum, with only a small lꢁ20 nm shift from l=529
(pH 4.80) to 549 nm (pH 13.7). This band is distinct and
does not change upon addition of excess KOH. It is not
clear why only small bathochromic shifts are observed for 6
at high pH.
Figure 4. Molecular orbital plots of neutral (center), bisprotonated
(bottom), and bisdeprotonated (top) XF 6. The calculations were per-
formed at the B3LYP/6-31G*//6-31G* level on
QuantumCube.
a QS8-2800C-O64D
system is turned off. The LUMO is localized on this axis. In
the protonated species, the HOMO is then localized on the
(relatively) electron-rich di(hydroxyACTHNUTRGNEUNGstyrylbenzene) axis.
The quantum chemical calculations (B3LYP/6-31G*//6-
31G*) of the neutral, bisprotonated, and bisdeprotonated
forms of 6 qualitatively explain the features of the observed
spectroscopic changes, but fall short for the numerical
values for the HOMO–LUMO gaps. Upon deprotonation,
the expected decrease in the HOMO–LUMO gap is predict-
ed, but upon double protonation the calculations incorrectly
predict a decreased HOMO–LUMO gap.[17] This failure is
not too surprising because DFT calculations are not very re-
liable when applied to ions because the effects of the solvent
are not included.
In the case of 7, deprotonation leads to full quenching of
the fluorescence. The reason for the markedly different be-
havior of 6 and 7 is not well understood. It has a parallel in
the bis(4-hydroxyarylethynyl)benzenes, which also display
only weak fluorescence upon deprotonation.[16,17]
Interaction of hydroxydialkylamino XFs with metal salts:
Figure 6 shows the absorbance and emission spectra of XFs
6 and 7 in the presence of different metal ions. Figure 7 dis-
plays photographs of both XFs before and after addition of
an excess of ten different metal triflates. The experiments
were conducted in MeCN and CH2Cl2. Although the addi-
tion of Mg2+, Ca2+, or Li+ does not lead to changes in fluo-
rescence for 6, the other metal cations produce changes in
emission. For XF 7, metal cations lead to either quenching
or changes in emission color with the exception of Li+ in
MeCN. The fluorescence changes are qualitatively similar to
those observed upon protonation, but do not occur for each
XF with every metal.
Titration studies of hydroxydialkylamino XFs: XFs 6 and 7
are insoluble in pure water at neutral pH. To investigate
their acid–base behavior, we performed titrations of 6 (see
Figure 5), which is somewhat soluble in a 2:1 v/v mixture
MeOH/water, but not XF 7 because it is poorly soluble even
under these conditions. This data might not only reflect pro-
tonation and deprotonation, but also the dissolution of ag-
gregates. Upon protonation with aqueous hydrochloric acid
(HCl), XF 6 experiences a hypsochromic shift in absorption
In MeCN, Zn2+, Mn2+, Sn2+, Ba2+, and Ag+ all result in
a blueshift in the emission of 6. The addition of Hg2+
quenches the emission of 6 due to a heavy atom effect, and
paramagnetic Cu2+ is also an efficient quencher. Similar
spectroscopic properties are observed in CH2Cl2, but Cu2+
induces a blueshift in the emission of 6. The fluorescence
changes are slightly different for 7 (Figure 6). In MeCN,
Zn2+, Mn2+, Sn2+, or Ba2+ result in a blueshift in emission,
whereas the addition Mg2+, Ca2+, Hg2+, and Cu2+ salts
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Hydroxydialkylamino Cruciforms
FULL PAPER
with ten different amines
(0.1 mL per sample, 0.7–7.2 mm
concentration range). Figure 8
shows real-color photographs of
the amine-spiked samples upon
UV illumination.
In the ground state, 6 forms
weakly hydrogen-bonded com-
plexes to the amines and the
UV/Vis spectra of these sam-
ples do not show much change.
The effect of the addition of
amines upon the fluorescence
of 6 can be dramatic and de-
pends upon the amine–solvent
combination. Emission colors
of blue, green, yellow, or red
are observed (Figure 8). De-
pending upon the solvent, we
see proton transfer in various
stages from a weak hydrogen-
bonded complex to perhaps the
formation of ion pairs (Fig-
ures 8 and 9).[18] If the amine
under consideration is not very
basic, such as morpholine and
piperazine, there is little change
in the fluorescence color and
intensity, even though some
changes are observed in the
basic solvents DMF, DMSO,
and acetonitrile. Putrescine, 1,3-
diaminopropane,
ethylenedi-
AHCTUNGERTGaNNUN mine, and DBU give the most
distinct changes in fluorescence
regardless of solvent. However,
the pKa of the ammonium ions
is not the only determinant of a
strong response, given that pu-
tresceine and piperazine have
quite similar pKa values (9.9
and 9.8, respectively) but dis-
play different responses.
Figure 6. Normalized absorption (left) and emission (right) of 6 and 7 in MeCN (top) and CH2Cl2 (bottom) in
the presence of different metal cations.
The spectral data and photo-
graphs give some discrimination
between the amines. We con-
quench the fluorescence of 7. In CH2Cl2, Zn2+, Mn2+, and
Cu2+ are the only cations that result in blueshifts in the
emission of 7, whereas Mg2+ and Li+ induce orange and
yellow emissions, respectively. This is due to a dual emission
that occurs between the complexed and uncomplexed forms
of cations coordinated to the lone pairs of the dibutylani-
lines (see Figure 6).
verted the color from the amine panel into RGB values and
subtracted these from the reference (Contrast Analyzer).[19]
This data was subjected to a linear discriminant analysis
(LDA, program SYSTAT),[20] reducing the data into a 2D
LDA plot that contains two factors by which all ten amines
are separated (Figure 10). The plot shows the diamines
grouped together in the top left corner, whereas the secon-
dary amines, such as diethylamine and diisopropylamine, are
grouped together in the bottom right corner.
Amine sensing with hydroxydialkylamino XFs: To observe
the ability to utilize 6 as a potential amine sensor, a 10 mm
stock solution of 6 was prepared in eight solvents and tested
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U. H. F. Bunz et al.
Conclusion
We have prepared XFs 6 and 7.
XF 6 displays red- and blue-
shifted absorption and emission
upon protonation and deproto-
nation, whereas XF 7 displays
similar properties, but its fluo-
rescence is quenched in the
presence of bases. Both XFs are
cation-reactive, and 6 is a viable
amine-reactive fluorophore that
Figure 7. Exposure of 6 and 7 to different metal cations in MeCN and CH2Cl2. Photographs were taken under displays color shifts in different
UV radiation at lem =366 nm.
solvents. It would be useful to
have amphoteric XF fluoro-
phores, such as 6, attached to
polystyrene or glass beads. Ex-
posure to acid or amine vapor
would then lead to dramatic
color changes, making these
chromophores interesting as po-
tential dosimeter-type amine-
responsive materials. Such in-
vestigations are underway and
will be reported elsewhere.
Experimental Section
The experimental details for the syn-
thesis of XFs 6, 7, and their intermedi-
ates, and details of the photophysical
experiments and the absorption and
emission spectra for all systems stud-
ied are available in the Supporting In-
formation.
Figure 8. Exposure of 6 to different amines in various solvents. a) XF 6 only (reference), b) 6+morpholine
(8.33), c) 6+piperazine (9.83), d) 6+putrescine (9.90), e) 6+1,3-diaminopropane (10.47), f) 6+ethylenedi-
Acknowledgements
ACHTUNGTRENNUNGamine (10.70), g) 6+piperidine (10.80), h) 6+triethylamine (10.80), i) 6+diethylamine (11.00), j) 6+diisopro-
pylamine (11.10), k) 6+1,8-diazabicycloACHTUNTRGNEUNG[5.4.0]undec-7-ene (DBU~12). The numbers in parentheses are the
We thank the National Science Foun-
dation (CHE-0809179 for K.M.S. and
L.M.T. and CHE-0750275 for U.B. and
P.M.) and the Department of Energy
(DE-FG02-04ER46141 for V.R. and
O.M.) for generous financial support.
P.M. thanks the Georgia Institute of
Technology for a FACES fellowship.
We thank Katya Kouznetsova for the
frontispiece design.
pKa values of the corresponding ammonium ions. Illumination was performed by using UV radiation at lem
=
366 nm.
Figure 9. Absorption and emission of XF 6 on addition of different amines in MeCN.
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Hydroxydialkylamino Cruciforms
FULL PAPER
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Figure 10. LDA analysis of the differential RGB values of 6 obtained
from Figure 8; ꢃ: DBU, ꢃ: morpholine, ꢃ: piperazine, ꢃ: putrescine,
1
2
3
4
ꢃ: 1,3-diaminopropane, ꢃ: ethylenediamine, ꢃ: pipiridine, ꢃ: triethyl-
5
6
7
8
amine, ꢃ: diethylamine, ꢃ: diisopropylamine.
9
10
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Received: October 5, 2010
Published online: February 21, 2011
Chem. Eur. J. 2011, 17, 3112 – 3119
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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