Bis(4'-dibutylaminostyryl)benzene: Spectroscopic behavior upon protonation or methylation

Article abstract of DOI:10.1002/chem.200900608

We have investigated the UV/Vis absorption and emission of 1,4-bis(4'-dibutylaminostyryl)benzene (1) upon protonation with trifluoroacetic acid in dichloromethane and in acetonitrile. We find that 1 does not display significantly dynamic acidity in the ex

Full text of DOI:10.1002/chem.200900608

FULL PAPER
DOI: 10.1002/chem.200900608
Bis(4’-dibutylaminostyryl)benzene: Spectroscopic Behavior upon Protonation
or Methylation
Anthony J. Zucchero, Juan Tolosa, Laren M. Tolbert, and Uwe H. F. Bunz*[a]
Abstract: We have investigated the
UV/Vis absorption and emission of 1,4-
bis(4’-dibutylaminostyryl)benzene (1)
upon protonation with trifluoroacetic
acid in dichloromethane and in aceto-
nitrile. We find that 1 does not display
significantly dynamic acidity in the ex-
cited state, that is, it is not a photoacid.
Three protonation states of 1 were in-
vestigated, all of which, neutral, singly
protonated, and bis-protonated, are flu-
orescent. As an isolable model for the
mono-protonated species, a methylated
derivative of 1 was prepared by reac-
tion with methyl triflate. This species
displays redshifted emission but similar
absorption as the non-quaternized
parent, 1. The strong observed emis-
sion of all three protonated states of 1
suggests that the kinetic photoacidity
of 1* is low or absent. We assume that
the lifetime of the excited state is too
short to allow solvent reorganization
and therefore we cannot make a state-
ment on the thermodynamic photo-
acidity of the protonated forms of 1.
Keywords: acidochromicity · disty-
AHCTUNGTRENNUNG
·
·
AHCTUNGTRENNUNG
Introduction
ed state ejection of the bound magnesium cations, they
found large blueshifts in absorption but only miniscule shifts
in emission upon complexation with magnesium perchlorate
in acetonitrile. As a consequence of the apparently unsatis-
factory excited state properties of these aminostyryl-based
molecules, they have been relegated to the sidelines and are
not considered as viable cores for advanced sensor design.
In contrast, most 1,4-bis(aminostyryl)-2,5-diarylethynyl-
benzenes (cruciforms, XFs) exhibit spectacular shifts in ab-
sorption and emission upon treatment with trifluoroacetic
acid (TFA) or metal cations.[8] This is a somewhat surprising
result, as XFs such as 5 are in effect distyrylbenzene deriva-
tives; however, XFs as well as their tetraalkynyl analogues
reported by Haley et al.[9] seem to be unhampered by “excit-
ed state decomplexation”, which has thus far plagued other
stilbene- and distyrylbenzene-based fluorophores, requiring
us to reconsider the assumption that excited state decom-
plexation will impede sensory dyes assembled from a distyr-
ylbenzene scaffold.
Surprisingly, and in spite of the intense interest in the
photophysical properties and sensory potential of styryl-de-
rived fluorophores,[10] no study has been performed detailing
the response of simple bis(alkylaminostyryl)benzenes such
as 1 upon protonation. This is of significant interest as it pro-
vides an opportunity for the careful spectroscopic examina-
tion of the cation-induced responses of 1.[11] TFA serves as a
representative model analyte for alkylamino-functionalized
dyes, since it shares a mode of interaction (i.e. nitrogen co-
Fluorophores assembled from stilbene and distyrylbenzene
p-systems have attracted intense interest due to their poten-
tial utility in optoelectronic and sensory applications, includ-
ing poly(phenylenevinylene)s (PPVs) in organic light-emit-
ting diodes.[1] Amino-functionalized stilbenes have been ex-
amined by Tsien et al.,[2] Valeur et al.,[3] and others as po-
tential fluorescent metal ion probes.[4] Though substantial
shifts in the absorption of alkylamino-functionalized stil-
benes were observed upon treatment with Mg, Ca, or Ba
salts, only minor shifts in emission resulted. Subsequent
pump–probe experiments by Valeur et al.[5] and Lapouyade
et al.[6] established light-induced cation ejection as the puta-
tive explanation for the limited excited state sensory re-
sponses observed in these stilbenes.
In later experiments, Perry et al. explored the metal-bind-
ing capabilities of donor–acceptor substituted distyrylben-
zenes as two-photon absorbing fluorophores for the detec-
tion of metal ions.[7] While these authors did not claim excit-
[a] A. J. Zucchero, Dr. J. Tolosa, Prof. L. M. Tolbert, Prof. U. H. F. Bunz
School of Chemistry and Biochemistry
Georgia Institute of Technology
901 Atlantic Drive, Atlanta, GA 30332ACTHNUTRGNEUG(N USA)
Fax : (+1)404-385-1795
E-mail: uwe.bunz@chemistry.gatech.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900608.
Chem. Eur. J. 2009, 15, 13075 – 13081
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Scheme 1. Summary of ground- and excited-state equilibria following
protonation of 1.
addition of TFA. In this fashion, TFA can function as a
simple proxy for metal cations, should ꢁexcited state decom-
plexationꢂ inhibit the sensory response of a dye. In the
event, we would observe a rich and complex photophysical
behavior for these fluorescent dyes upon protonation.
Results and Discussion
Target 1 was synthesized by a Horner reaction of 4-dibutyl-
AHCTUNGTREGaNNUN minobenzaldehyde with tetraethyl para-xylenediphospho-
nate and obtained as a yellow crystalline solid (see the Sup-
porting Information).[14] The distyrylbenzene derivative 1
ordination) with metal cations. This permits simulation of
the response observed upon exposure of distyrylbenzenes to
metal ions. However, the higher binding constant, greater
solubility, and lack of coordination-induced assembly in pro-
tonation experiments are advantageous; simplicity matters.
To a first approximation, the basicity of the chromophore
might be expected to mirror the binding affinity, both in the
ground and excited states. Thus, a detailed study of the
displays a vibrant blue emission in toluene solution (lmax =
457 nm). While limited solvatochromic shifts are observed
in the absorption of 1, substantial solvent dependence is ob-
served in emission, ranging from a structured emission of
457 nm in toluene to a broadened emission of 517 nm in a
2:1 mixture of methanol and water (Figure 1).
acidoACHTUNGTRENNUNGchromicity in both states should shed light on whether
ꢁexcited state decomplexationꢂ expressed as enhanced pho-
toacidity might be a favorable process in 1 and related com-
pounds. If excited state effects do not diminish analyte inter-
actions in the excited state, one would expect concomitant
spectral changes in absorption and emission upon addition
of TFA. However, if these distyrylbenzenes are prone to ex-
cited state deprotonation,[12] one might anticipate excited-
state decomplexation as well.[13] The thermodynamics of
proton transfer in the excited state is provided by the Fçr-
ster cycle [Eq. (1)], where R is the gas constant, T is the ab-
solute temperature, N is Avogadroꢂs number, h is Planckꢂs
constant, c is the speed of light, and nF is the respective
emission in wavenumbers.
EHA ꢀ EAꢀ
DpKa ¼ pKa ꢀ pKaꢁ ¼
Ei ¼ NhcðnF Þ
Figure 1. Normalized emission spectra of 1 in different solvents.
2:3RT
ð1Þ
Thus the comparison of the ground and excited-state acid-
ities of 1H+ and 1(H+)2 (see Scheme 1), may provide clues
to their metallochromic behavior. In this case, we would
expect to observe either shifts exclusively in the absorption
or a lagging change in emission relative to absorption upon
We chose TFA in acetonitrile to probe the excited state
acidochromicity (i.e. photoacidity) of 1 as a proxy for its
metallochromicity. Although acetonitrile does not support
fast proton transfer,[15] the solubility of our dyes is limited in
water and the proton transfer kinetics are complex, having
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Chem. Eur. J. 2009, 15, 13075 – 13081

Spectroscopic Behavior of Bis(4-dibutylaminostyryl)benzene
FULL PAPER
been the subject of numerous investigaACTHNUTRGNEUNG
tions.[13a] Addition of
Table 1. Binding data obtained from a SPECFIT analysis of the absorp-
tion and emission spectra.
an excess (50 equivalents) of TFA to 1 elicits a blueshift in
both absorption (411 ! 343 nm) and emission (496 !
414 nm). This result is significant, as related aminostyryl flu-
orophores have not been shown to exhibit large emission re-
sponses upon cation binding. To understand this effect, we
Absorption
Emission
log (b1)
3.4
3.7
s (b1)
log (b2)
0.039
7.0
0.032
7.4
s (b2)
s (fit)
relative error of fit
0.063
3.4ꢃ10ꢀ3
2.1%
0.031
3.5ꢃ10ꢀ3
3.6%
performed
a titration of 1 with TFA in acetonitrile
(Figure 2); we observe a decrease in the absorption of 1 at
411 nm accompanied by the development of a blueshifted
independently from the absorption and emission spectra
suggest a similar solution composition as the titration pro-
ceeds. This offers further confirmation that excited state ef-
fects do not significantly inhibit the fluorescent response of
1 upon protonation.
Figure 3. SPECFIT plots derived from the absorption (solid traces) and
emission (dashed traces) describing the solution composition as the TFA
titration progresses. Here, the percent composition of 1 (light gray), mo-
noprotonated 1H+ (dark gray), and diprotonated 1(H+)2 (black) are plot-
ted as a function of TFA concentration. Similar profiles are obtained in-
dependently from the absorption and emission spectra.
Figure 2. Spectrophotometric titration of 1 with TFA in CH3CN. Selected
absorption (top) and emission (bottom) traces are shown here for clarity.
To ensure constant absorptivity during the course of the titration, excita-
tion occurred near the isosbestic point (367 nm). Complete data sets used
for SPECFIT analysis are included in the Supporting Information.
The term b1 represents the equilibrium K1 for proton
transfer between TFA and 1 in MeCN (see Scheme 1),
which means that 1H+ is less acidic than TFA by ꢀlog K1,
or 3.4 pKa units. TFA has a pKa of 0.52 in water[17] and 3.45
in DMSO;[18] thus, the pKa of 1H+ is roughly 3.9 anchored
to water and 6.9 anchored to DMSO. To our knowledge, the
pKa of TFA in CH3CN has not been reported; as a rough
approximation for the purposes of discussion, we will
assume the pKa of TFA in CH3CN is similar to its pKa in
DMSO. We note that the pKa of dibutylaniline is about 6 in
DMSO,[19] which should be slightly less basic than 1. Similar-
ly, b2 represents K1 K2, so ꢀlog K2 =7-3.4=3.6, making
1(H+)2, pKa =3.6+3.5=7.1 slightly less acidic than 1H+.
Thus the binding affinity of the second nitrogen is relatively
little affected by protonation of the first.
The quality of this fit and the residual analysis performed
confirm two sequential protonation events. Because of the
closeness of the two complexation constants, a superficial
evaluation of the acetonitrile titration does not reveal an in-
termediate species which could be unambiguously assigned
to the monoprotonated intermediate. Deconvolution of the
titration using SPECFIT was unable to adequately resolve
the emission spectra of 1H+; however, it does suggest the
absorption at 343 nm. As TFA is added, a decrease in the in-
tensity of the emission at 496 nm is observed. A strong blue
emission at 414 nm develops; in addition, a small shoulder
at approximately 585 nm is visible. Significantly, a qualita-
tive assessment of the shifts in absorption and emission ob-
served upon treatment of 1 with TFA suggests that the re-
sponses occur concomitantly.
We employed SPECFIT[16] to deconvolute the binding
data independently from the absorption and emission spec-
tra using a complexation model which assumed two distinct,
spectroscopically distinguishable binding events. Table 1
highlights the results of this analysis. The b values obtained
from the absorption and emission spectra are in close agree-
ment, suggesting the absence of excited-state prototropism.
Through SPECFIT, we obtained the relative abundance of
1, 1H+, and diprotonated 1(H+)2 as the titration progresses
(Figure 3). Plots detailing the solution composition obtained
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13077

U. H. F. Bunz et al.
presence of an emission at lower energy (ca. 585 nm). We
suspected that the shoulder appearing in the emission spec-
tra at 580 nm during the course of the titration might be the
emission of 1H+. The minimal intensity of this feature is
readily explained by the low abundance of monoprotonated
1H+ during the course of the titration in acetonitrile
(<30% at maximum). However, addition of TFA per-
formed in dichloromethane reveals a yellow-emissive inter-
mediate visible both by the naked eye and by visible spec-
troscopy (Figure 4).
Figure 4. Fluorescence color of 1 after addition of TFA in dichlorome-
thane. Samples contain increasing amounts of TFA and are illuminated
under blacklight (lex =365 nm). Solutions were photographed using a
Canon EOS Digital Camera equipped with an EFS 18-55 mm lens. Spec-
tra of solutions 0–7 are available in the Supporting Information.
The minimal change in the absorptive nature of this mo-
noprotonated intermediate 1H+ when compared to 1 make
its study problematic. To better understand the system, we
prepared the mono- (2) and dimethylated (3) analogues of
1, which should serve as stable, isolable, and isoelectronic
models of the protonated compounds formed upon treat-
ment of 1 with TFA. Reaction of 1 with 0.3 equivalents of
methyl triflate in dichloromethane results in a mixture of 1–
3 separable by column chromatography (see the Supporting
Information). The absorption and emission spectra of 2 and
3 are shown in Figure 5 (see also Table 2).
As expected, 3 closely resembles the diprotonated end-
point of the titration of 1 with TFA, displaying an absorp-
tion of 340 nm and a structured blue emission at 416 nm in
acetonitrile (see Table 2). Cation 2, our model for 1H+, pos-
sesses an absorption at 406 nm as well as at 317 nm and a
broad emission at 582 nm. The emission of 2 corresponds to
the shoulder visible upon titration of 1 with TFA. The simi-
larity of the long wavelength absorptions of 1 and 2 and the
short wavelength absorption of 2 and 3 suggests that this is
a localized p–p* absorption, with components in the aniline
end and the anilinium end, and explains the lack of a visible
monoprotonated intermediate in the titration absorption
spectra. The absorption and emission of 2 are similar to that
of an asymmetrically donor–acceptor substituted distyryl-
benzene 4, resulting in a charge-transfer excited state.
Figure 5. Normalized absorption and emission spectra of 1 (A), 2 (B), 3
(C), and 4 (D) in acetonitrile.
Table 2. Selected spectroscopic data for compounds 1–4, 1H+, and
1(H+)2. pKa values were calculated by using SPECFIT and anchored to
DMSO as an approximation for CH3CN; pKa* values were calculated by
using the Fçrster equation. For the purposes of Fçrster cycle calculations,
lmax values for 1H+ were estimated by using the values for mono-methy-
lated 2, whereas lmax values for 1(H+)2 were obtained from the titration
endpoint (Figure 2).
lmax
lmax
pKa
pKa*
F
t
absorption
in CH3CN
[nm]
emission
in CH3CN
[nm]
in
in
CH3CN
CH3CN
[ns]
The absorption and emission spectra of 1–3 were recorded
in a variety of solvents (see the Supporting Information) to
construct Lippert–Mataga plots, where Df, the Lippert–
Mataga solvent parameter, is defined by Equation (2).[20]
The slope of the best fit provides a relative assessment of
the degree of charge transfer present in 1–3 (Figure 6). Di-
1
411
406
343
406
340
415
496
582
414
582
416
558
–
–
0.60
–
0.73
0.13
0.48
–
1.3
–
1.1
1.0
1.3
–
1H+
6.9
7.1
–
–
–
13.2
ꢀ7.5
–
–
–
1(H+)2
2
3
4
ACHTUNGTRENNUNGstyrylbenzene 1, which is donor substituted and possesses
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Chem. Eur. J. 2009, 15, 13075 – 13081

Spectroscopic Behavior of Bis(4-dibutylaminostyryl)benzene
FULL PAPER
creased electron density at the ionic end relative to the
ground state. This is in contrast to the previously reported
metal sensors,[5,6] for which excitation produces a charge-
transfer state with decreased electron density at the binding
site. Conversely, the diprotonated 1(H+)2 is a stronger photo-
acid by about 14 pKa units. The fact that no dynamic behav-
ior from 1(H+)2 is observed upon irradiation is not surpris-
ing, in keeping with numerous observations that excited-
state proton transfer is relatively slow in the absence of a
preorganized proton-accepting network.[13] However, the de-
creased photoacidity of 1H+, as predicted by the corre-
sponding hypsochromic absorption of 2, should make prop-
erly designed non-symmetrical aminodistyrylbenzenes very
powerful sensory cores for the detection and quantitation of
transition-metal cations, for which solvent reorganization
should play a more modest role, and for which the differen-
ces in solvation energies are less dramatic.
We have cursorily investigated the interaction of l with
metal salts, to see if the observed trends for protonation are
mirrored by the addition of metal triflates in acetonitrile
(Figure 8). Addition of zinc triflate or magnesium triflate to
the solutions of 1 in acetonitrile lead to emission responses
that are qualitatively identical to those observed upon pro-
tonation of 1. If calcium triflate is added, the situation is
more complex as apparently only partial binding is ob-
served. Interestingly, a mix of the uncomplexed and the
doubly complexed species are seen as envisioned by the
emissions at 414 and 596 nm. In
Figure 6. Lippert–Mataga plot of compounds 1 (light gray trace), 2 (dark
gray trace), and 3 (black trace). Spectral details of 1–3 in a variety of sol-
vents as well as details of solvent factors for the solvents used are includ-
ed in the Supporting Information.
two free electron pairs, displays a modest degree of charge
transfer.
Df ¼ ½ðeꢀ1Þ=ð2e þ 1Þꢀðn2ꢀ1Þ=2ðn2ꢀ1Þꢂ
ð2Þ
Upon methylation, 2 displays an increased charge transfer
character relative to 1, as indicated by the increased slope.
This is readily rationalized as 2 becomes a donor-acceptor
substituted distyrylbenzene; the result is a redshifted and
broadened emission centered at 582 nm (Figure 5 and
Figure 7). The second methylation produces 3, which dis-
addition, a redshifted tail is ap-
parent, perhaps due to the
mono-metallated species. Quali-
tatively, the changes in emission
and absorption go hand-in-
hand, suggesting that excited
state demetallation does not
play a significant role in the
chosen
fluorophore/solvent
system; however, when going
into water, the situation might
Figure 7. Serial protonation or methylation of 1 and change in its emission color.
plays a decreased charge transfer character; this is expected
as 3 no longer possesses available free electron pairs on the
alkylamino nitrogen atoms. The resulting dimethylated di-
ACHTUNGTRENNUNG
The bathochromic shift upon protonation of 1, as modeled
by the spectra of 2, suggests that 1H+ exhibits increased ba-
sicity in the excited state, according to the Fçrster equa-
tion.[21] Using an approximate pKa of 6.9 for 1H+, we pre-
+
dict the excited state acidity pKa* of 1(H)2 to be 13.2!
Thus the shift in emission maximum from 494 nm to 586 nm
corresponds to an increased binding affinity for protons in
the excited state. This suggests that 1 should, in fact, bind
more strongly to metals in the excited state rather than less,
since the charge-transfer quality of 2* would imply in-
Figure 8. Normalized emission of 1 in CH3CN (dashed black trace) upon
exposure to Mg
ACHUTGTNERN(NUG OTf)2 (solid black trace), ZnACHTUNGTREN(NUNG OTf)2 (dark gray trace),
and Ca(OTf)2 (light gray trace).
AHCTUNGTRENNUNG
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the dialkylanilines to metal cations is weaker than to pro-
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chloromethane but also acetonitrile.
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Conclusions
The results suggest a two-stage response in the absorption
and emission of 1 upon exposure to TFA; TFA as a model
cation provides a uniquely valuable opportunity to quantita-
tively assess the sensory responses of 1. Changes in absorp-
tion are accompanied by concomitant changes in emission,
suggesting that excited state decomplexation will not hinder
the performance of all amino-styryl based fluorophores if
the charge-transfer nature of the excited-state is properly
designed. Surprising is the finding of the enhanced photoba-
sicity (or decreased photoacidity) of the monoprotonated
species 1H+, which flies in the face of common chemical
reasoning and accentuates the dramatic differences between
stilbenes, where according to Valeur et al.[3,5] cation ejection
is observed, and distyrylbenzenes, where other rules seem to
apply.
Future studies will more carefully explore the metallo-
chromicity of 1 and related alkylamino-substituted distyryl-
benzenes. The proton-induced shifts in 1 are reminiscent of
the protonation of the dianions of bis(hydroxystyryl)ben-
zene 6.[12a] There, the first protonation of the bisphenolate of
6 results in a significant redshift in emission but only a very
small redshift in absorption. Upon full protonation, then
into 6, both a blueshift in absorption as well as a large blue-
shift in emission is reported. Consequently, 1 and 6 are con-
sanguine and their common distyrylbenzene core dictates
their emissive behavior upon protonation or deprotonation.
Both 1 and 6 are fundamentally different from stilbene-de-
rived fluorophores as well as the laterally donor–acceptor
substituted distyrylbenzenes which have been previously ex-
amined.[6] A number of interesting detection schemes for
cationic analytes is therefore envisioned by using either 1
and 2, attaching attractive sensory appendages to this pow-
erful core.
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Acknowledgements
We thank the Georgia Institute of Technology and the National Science
Foundation (U.H.F.B.: NSF-CHE 0750275, L.M.T.: NSF-CHE 00809179)
for financial support. A.J.Z. acknowledges the Center for Organic Pho-
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for a graduate research fellowship.
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FULL PAPER
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Received: March 6, 2009
Revised: September 3, 2009
Published online: October 26, 2009
Chem. Eur. J. 2009, 15, 13075 – 13081
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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