Acidochromicity of bisarylethynylbenzenes: Hydroxy versus dialkylamino substituents

Article abstract of DOI:10.1021/jo901811a

(Chemical Equation Presented) This contribution investigates the electronic difference of dibutylamino/dibutylammonium and phenol/phenolate groups in four simple distyrylbenzenes and bisarylethynylbenzenes. The compounds and their protonated and deprotona

Full text of DOI:10.1021/jo901811a

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Acidochromicity of Bisarylethynylbenzenes: Hydroxy versus
Dialkylamino Substituents
Scott M. Brombosz, Anthony J. Zucchero, Psaras L. McGrier, and Uwe H. F. Bunz*
School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta,
Georgia 30332
uwe.bunz@chemistry.gatech.edu
Received August 26, 2009
This contribution investigates the electronic difference of dibutylamino/dibutylammonium and
phenol/phenolate groups in four simple distyrylbenzenes and bisarylethynylbenzenes. The com-
pounds and their protonated and deprotonated species, respectively, were investigated by UV-vis
and emission spectroscopies. While the anilinium and phenol compounds displayed similar spectro-
scopic properties, this was less so the case for the comparison of the dialkylanilines with the
corresponding phenolates. In this case, the hydrogen bonding ability of the phenolates distorted
the results as the hydrogen donating/accepting properties of the investigated solvents have a
disproportional influence on the electronic properties of the phenolates when compared to the
dialkylamines. If one uses acetonitrile as solvent, these effects disappear, as acetonitrile is neither a
good hydrogen bond acceptor nor a hydrogen bond donor. The results are in line with the
para-Hammet constant for OH (σ = -0.37) being significantly smaller than that for NMe2
substituents (σ = -0.83) and reinforces the notion that the lone pairs in these phenols are not
readily available for interaction with the π-system, as they are perhaps energetically too low lying.
However, in the case of the phenolates, the lone pairs do interact significantly.
Introduction
molecular orbital (FMO) structure, geometry, and electron
count display similar reactivity and properties. It is a quali-
tative model that guides the understanding of properties and
reactivities of analogous molecules. One should, therefore,
be able to use the isolobal principle to predict;at least
qualitatively;the expected responses of classes of consan-
guine fluorophores toward change of pH or metal coordina-
tion. Superficially, one might expect hydroxy substituents to
be isolobal to amino groups. However, a simple application
of the isolobal principle will not always suffice in such
organic systems, as the relative orbital ordering results in
systems where (in a formal sense) free electron pairs interact
predominately with either the σ- or the π-system. If the free
electron pairs are energetically low lying, we expect them to
interact predominately with the σ-system, while energetically
higher lying electron pairs should have a larger interaction
with the π-system.
Reactive chromophores are fluorophores that change
color, emission wavelength, and/or emission intensity upon
exposure to analytes and are potentially useful as sensors.
They contain a chromophoric π-conjugated core with an
embedded functionality possessing free electron pairs before
or after addition of an analyte.1 The interaction of the free
electron pairs of functional fluorophores with suitable ana-
lytes or stimuli influences the position of the HOMO, the
LUMO, or both and elicits changes in absorption and
emission.
The concept of isolobality of molecules was set forth by
Hoffmann2 and asserts that molecules of similar frontier
*To whom correspondence should be addressed. Phone: 404-510-2443.
Fax: 404-385-1795.
(1) Zucchero, A. J.; Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2006,
128, 11872–11881.
(2) (a) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711–724.
(b) Evans, D. G.; Mingos, D. M. P. J. Organomet. Chem. 1982, 232, 171–191.
A simple test bed for this hypothesis would be compounds
3a and 4a, bis(arylethynyl)benzenes functionalized with
dibutylamino and hydroxy groups, respectively. Though
DOI: 10.1021/jo901811a
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Published on Web 11/02/2009
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Brombosz et al.
JOCFeatured Article
synthetically simple, their sensory responses have not been
examined.3 Comparison of 3a and 4a with their analogous
distyrylbenzenes4 5a and 6a permits the expansion of this
study to investigate differences that arise when alkenyl
groups are exchanged for alkynyl groups. Probing the acido-
chromicity and photophysical properties of 3a-6a should
offer insight into the application of the isolobal principle and
provide an understanding of fundamental physical-organic
issues in these systems.
SCHEME 1. Synthesis of Compounds 3a and 4a from 1 via
Sonogashira Coupling of Substituted p-Iodobenzenes 2a,b
classes (i.e., nonpolar, polar protic, and polar aprotic
solvents). The photophysical responses of all herein investi-
gated species were well-behaved yet interesting in these
solvents. In the case of sets C and D, with the exception of
4a in the very polar solvent DMSO, the absorption spectra
for both compounds are nearly each superimposable in a
range of solvents. The absorption spectra of 3b and 4a are
only ∼10 nm apart and display similar vibronic features.
Similarities are also observed in the emission spectra of set D;
3b displays nearly overlapping, structured emissions in dif-
ferent solvents. 4a exhibits a similarly featured emission in
diethyl ether; however, as solvent polarity increases, the
vibronic features give way to a broadened, smooth line
shape. Once again, the emission λmax of 4a is similar to that
of 3b. Set C behaves in a nearly identical fashion to D;
however, the absorption and emission spectra are red-shifted
by approximately 30 and 40 nm, respectively.
In sets C and D, the chromophores lack available lone
pairs; as a result, we would expect little solvent dependence in
their absorption or emission λmax. Furthermore, the isolobal
principle suggests all four chromophores should exhibit
similar photophysical properties. Indeed, this is what is
observed. Surprising differences were observed in sets A
and B, where the chromophores possess available lone pairs.
The isolobal principle predicts that pairs 5a and 6b and 3a
and 4b should exhibit similar photophysical properties;
furthermore, we expect sets A and B to behave in a similar
fashion. While sets A and B are similar, differences appear in
the pairs 5a and 6b and 3a and 4b. In the case of dibutyl-
amino-functionalized 5a and 3a, the absorption spectra in a
variety of solvents are similarly featured and exhibit a
minimal (∼25 nm) solvent dependence. Greater solvent
dependence is observed in the emission spectra. The emission
of 5a and 3a in ether is featured; as solvent polarity increases,
the emission is red-shifted (∼60 nm) and vibronic definition
disappears.
Results and Discussion
Distyrylbenzene compounds 5a and 6a were synthesized
according to literature procedures.5,6 Surprisingly, 4a7 has
been reported only once and 3a is unreported, although the
dimethyl-8 and dihexyl-substituted9 compounds are known.
Heck-Cassar-Sonogashira-Hagihara (HCSH) coupling
of 2a to 1 furnishes 3a. Similarly, 4a was synthesized from
the HCSH coupling of 2b with 1 (Scheme 1).10 Upon
protonation with trifluoroacetic acid or deprotonation with
tetrabutylammonium hydroxide, compounds 3b, 4b, 5b, and
6b are obtained.
For ease of discussion, isolobal pairs have been placed
into sets (A-D, Figures 1 and 2). These compounds were
examined through UV-vis and fluorescence spectroscopies
(dilute solutions in ethyl ether, 1,4-dioxane, chloroform,
dichloromethane, methanol, ethanol, isopropanol, tert-bu-
tyl alcohol, acetonitrile, dimethylformamide, and dimethyl-
sulfoxide; see Supporting Information and Figure 2).
Figure 2 displays the absorption and emission of sets A-D
in four representative solvents to permit a qualitative exam-
ination of solvent effects upon each compound. For simpli-
city, ethyl ether, methanol, acetonitrile, and dimethyl-
sulfoxide were chosen because they represent different
(3) For the photophysics of hydroxystilbenes and aminostilbenes, see:
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In 6b and 4b, methanol exhibits the highest energy absorp-
tion, and dramatic solvent dependence (∼80 nm) is observed
in the absorption maxima. Divergence is also observed in the
emission spectra. The emission of 6b and 4b in diethyl ether is
considerably red-shifted relative to their alkylamino coun-
terparts (∼80-100 nm). Little solvent dependence is ob-
served in the emission of 6b (∼20 nm), while in the case of 4b,
a large solvent effect is seen. Here, the emission of 4b varies
by more than 150 nm, ranging from MeOH at highest energy
to ether at lowest energy.
The compounds in sets C and D behave as isolobal pairs;
however, the suprising lack of “isolobality” in the case of A
and B requires an explanation. Previously, we have analyzed
solvent-dependent absorption and emission spectra of simi-
lar compounds utilizing the Lippert-Mataga equation:6a A
solvent’s dielectric constant and refractive index are used to
calculate an orientation polarizability value (Δf) for a given
solvent; Δf is then plotted against the energy of the Stokes
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FIGURE 1. Acid/base equilibrium relationships of 3a-6b are shown. Diagonal isolobal relationships are indicated.
FIGURE 2. Absorption (left) and emission (right) spectra of 3a-6b in ethyl ether (blue), methanol (green), acetonitrile (orange), and
dimethylsulfoxide (gray). Compounds are grouped by electronic structure into isolobal sets A-D (far right).
shift for each measured solvent.11 Generally, a linear plot is
obtained with the magnitude of the slope reflecting the
change in a fluorophore’s dipole moment upon excitation.
A Lippert-Mataga analysis of 3a-6b proved difficult;
whereas the dibutylamino compounds (3a, 3b, 5a, and 5b)
were well correlated, the phenolic compounds (4a, 4b, 6a,
and 6b) showed no meaningful relationship. The Lip-
pert-Mataga equation only considers nonspecific effects
related to solvent reorganization. Solvent-fluorophore in-
teractions may, however, play a critical role in understanding
the behavior of the phenolates.
hydrogen bonding or acid/base reactions.12 KT relies on a
multivariate linear regression analysis of the absorption λmax
of a chromophore in a variety of solvents (eq 1).
Kamlet-Taft multivariate approach :
ꢀ
νð1000=cmÞ ¼ ν0 þ sπ þ aR þ bβ
ð1Þ
The KT approach correlates the solvent-dependent spec-
tral shifts observed (ν) for a chromophore with three
solvent-dependent parameters (R, β, and π*). Here, ν0
We subjected 3a-6b to a Kamlet-Taft (KT) solvent
analysis accounting for solvent-specific interactions due to
(12) (a) Kamlet, M. J.; Abboud, J.-L.; Abraham, M. H.; Taft, R. W.
J. Org. Chem. 1983, 48, 2877–2887. (b) Dong, J.; Solntsev, K. M.; Tolbert,
L. M. J. Am. Chem. Soc. 2006, 128, 12038–12039. (c) Wu, Y.; Lawson, P. V.;
ꢀ
Henary, M. M.; Schmidt, K.; Bredas, J.-L.; Fahrni, C. J. J. Phys. Chem. A
2007, 111, 4584–4595.
(11) (a) Von Lippert, E. Z. Electrochem. 1957, 61, 962–975. (b) Mataga,
N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465–470.
J. Org. Chem. Vol. 74, No. 23, 2009 8911

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TABLE 1. Coefficient Values Obtained from Kamlet-Taft Analysis
and 5a? One might attribute this differential behavior to the
increased basicity of a phenolate (pKa ∼ 10)13 as compared to
a dialkylamino group (pKa ∼ 6.6).1 A look into the Hammett
σ values14 is instructive, as here the σ values of -O-,
-N(C3H7)2, -OH, and -NMe2Hþ are -0.81, -0.93, -0.37,
and 0.70, respectively.15 The Hammett values testify to the
apparent electronic similarity of the phenolate to the dialky-
lamino groups but of course do not take into account the
hydrogen bonding contributions that will be stronger in the
case of a phenolate than in a neutral amine. More surprising
is the similarity of the spectroscopic properties of the phenols
and the ammonium salts (where hydrogen bonding appar-
ently does not play a significant role), given the larger
differences in their respective Hammett parameters. While
the correlation with Hammet σp parameters is appealing and
grossly correct, they clearly cannot explain the subtleties in
this interesting system.
An important additional point is the quantum yields of
these eight compounds, which we determined in acetonitrile.
Generally, in the pairs A and B, the aniline always has a
significantly higher quantum yield than the phenolate. In the
case of 4b, the quantum yield is below 0.01. For the pairs C
and D, the differences are much smaller and the quantum
yields are generally quite substantial. In both cases, the
ammonium species display a higher quantum yield than the
phenols. The reason for the differences in the quantum yields
is somewhat obscure, and the only rough trend is that the
higher the emission wavelength, the lower the emission
quantum yield is; a notable exception is 4b with its vanishing
emission. Generally, the amines do better with respect to
emission quantum yield than the phenols and phenolates, for
subtle reasons that are not easily divined.
A
B
C
D
compound 5a
6b
3a
4b
5b
6a
3b
4a
a
ν0
s
a
25.1 25.4 27.4 26.8 28.8 27.9 31.2 30.7
-1.2 -1.4 -1.5 -2.5 -0.76 -0.76 -0.26 -0.52
0.29 2.7
0.17 2.9
0.60 0.52 -0.07 0.32
b
0.16 -2.7 0.14 -1.5 -0.63 -0.31 0.30 -0.55
0.95 0.90 0.95 0.80 0.83 0.77 0.58 0.80
Rb
aUnits of ν0 are in 103 cm-1. bR is the correlation coefficient.
corresponds to the projected absorption or emission energy
of the chromophore in a vacuum, while s, a, and b are fitted
coefficients obtained from the linear regression analysis (see
Supporting Information). The index π* expresses the ability
of the solvent to stabilize the chromophore’s charge and/or
dipole via nonspecific dielectric interactions, while R and β
incorporate solvent-solute interactions; β describes the
proton accepting character of the solvent, while R corre-
sponds to the hydrogen donating character of the solvent. By
analyzing the coefficients, it is possible to determine the
degree to which each mode of interaction (R, β, and π*)
affects the absorption λmax of a chromophore.
Table 1 shows the results of the Kamlet-Taft analysis.
The calculated ν0 values range from 25.1 to 31.2 ꢁ 103 cm-1
;
the compounds within isolobal set A have similar ν0 values as
do those in sets B, C, and D. As one would expect, the values
of ν0 for the styryl isolobal set A are slightly lower, indicating
a red shift in the gas phase absorption relative to their
arylethynyl congeners in set B. The red shift is a consequence
of the hybridization change (sp f sp2) in the bridge carbons
when going from alkynes to alkenes. This more electron-rich
system allows the phenyl groups to interact somewhat more
strongly through the conjugative bridge. The same relation-
ship holds true for the styryl compounds in C relative to their
arylethynyl analogues in D.
Conclusions
We examined the photophysical properties and acidochro-
micity of hydroxy- and dibutylamino-functionalized distyryl-
benzenes and arylethynylbenzenes. While sets C and D
(Figure 2) exhibit similar photophysical behavior as expected
and do not possess effective lone pairs, sets A and B
The s coefficient of the π* term reflects the contribution of
nonspecific dielectric interactions of the solvent with the
fluorophore and is somewhat analogous to the slope ob-
tained from a Lippert-Mataga analysis; it is related to the
fluorophore’s dipole. In all cases, this term is negative,
inducing a spectral red shift. Isolobal pairs behave similarly
and as we would expect. In sets C and D, electron pairs are
involved in proton bonding. As a consequence, s is less
significant, suggesting a smaller dipole. In sets A and B,
where free electron pairs are more available, s is larger,
suggesting a greater dipole.
The a and b coefficients for the isolobal sets C and D are
modest. The lack of availabile free electron pairs results in
minimal solvent-specific interaction. Similarly, in the case of
dibutylamino compounds 3a and 5a, the a and b values are
also relatively small. The s term is the predominant influence
on the observed absorption. However, in the case of the
deprotonated phenols 4b and 6b, a and b become significant,
with a inducing a hypsochromic shift and b resulting in a
bathochromic shift. This results in the divergent photophy-
sical behavior observed in 4b and 6b relative to their isolobal
counterparts.
(Figure 2)
with the π-system of the fluorophore
;
possessing lone pairs that interact effectively
show different beha-
;
vior in absorption and emission. These differences stem from
fluorophore-solvent interactions, which disproportionally
affect the phenolate-substituted dyes.
The true electronic similarity of 3a-6b can be appreciated
when viewing their absorption and emission in aceto-
nitrile
;
a solvent possessing small and similar R and β
parameters (Figure 3 and Table 2). The contribution of
solute-specific effects is minimized; the isolobal similarity
of A and B as well as C and D becomes readily apparent.
The phenolate and dibutylamino groups are isolobal despite
the difference in their pKa values and the presence of the ionic
phenolate. However, they behave quite differently in prac-
tice, particularly in hydrogen bonding solvents.
Interesting and somewhat unexpected is the finding that
free electron pairs in the hydroxy compounds 4a and 6a are
not available for conjugation with the π-system. Apparently,
Why is this pronounced solvent effect observed exclu-
sively in 4b and 6b and not in their isolobal counterparts 3a
(14) (a) Hammett, L. P. Chem. Rev. 1935, 17, 125–136. (b) Hansch, C.;
Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165–195.
(13) For comparison, the pKa values of methanol and ethanol are 15.5
and 16.0, respectively. Additional pKa values for the solvents used in this
study are listed in the Supporting Information.
(15) Interpolated from the σp values for -NH3þ (0.60) and NMe3þ (0.82);
the other values are directly from Table 1 in ref 14b.
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TABLE 2. Selected Photophysical Data of Compounds 3a-6b in
CH3CN
A
B
C
D
compound
5a
6b
3a
4b
5b
6a
3b
4a
λ
max absorption (nm) 410 431 378 408 353 364 321 328
λ
max emission (nm) 494 542 466 496 414 426 351 380
ε (M-1 cm-1
)
7774 17515 6799 9632 4712 24191 6089 10230
0.60a 0.13 0.51 <0.01 0.73a 0.43 0.54 0.43
3
Φ
aPreviously reported in ref 4a.
to which a solvent effects the absorption of a molecule is
nearly identical among an alkene-alkyne pair as can be seen
through similar values of s, a, and b. From the above, we
therefore would recommend acetonitrile as the preferred
solvent for the comparison of a series of consanguine fluoro-
phores. In addition, our study gives design guidelines as how
to engineer absorption and emission wavelengths in bisstyryl-
benzene and bisarylethynylbenzene-like dyes.
FIGURE 3. Absorption (left) and emission (right) spectra of 3a-6b
in acetonitrile. Top: 3a (blue), 4b (green), 5a (orange), 6b (gray).
Bottom: 3b (blue), 4a (green), 5b (orange), 6a (gray).
these electrons are too low in energy to permit efficient
interaction. The other, somewhat expected trend is that dyes
containing alkene bridges display red-shifted spectral fea-
tures when compared to analogous fluorophores featuring
alkyne groups. We note that the change hybridization (sp f
sp2) increases the intrinsic electron donating character of
the distyryl compounds as compared to the bisarylethynyl
compounds. A red shift in ν0 results. While the gas phase
absorption, ν0, is red-shifted in all of the alkene compounds
relative to the corresponding alkyne compounds, the degree
Acknowledgment. We thank the Georgia Institute of
Technology and the National Science Foundation (NSF-
CHE 0750275) for financial support. A.J.Z. acknowledges
the Center for Organic Photonics and Electronics (COPE) at
Georgia Tech for a graduate research fellowship.
Supporting Information Available: Additional figures and
methods. This material is available free of charge via the
Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 23, 2009 8913