Effect of 3,5-disubstitution on the optical properties of luminescent 2-(2′-Hydroxyphenyl)benzoxazoles and their borate complexes

Article abstract of DOI:10.1002/ejoc.201300616

This article describes the multistep synthesis and photophysical properties of three highly fluorescent dyes based on the 2-(2′-hydroxyphenyl) benzoxazole (HBO) scaffold and their resulting chelation to a BF₂ fragment. These dyes possess functionalization at the 3,5-positions of the phenol ring with an ethynyl-extended fragment bearing TMS, p-tBuC ₆H₄, or p-NnBu₂C₆H₄ groups. All of the new compounds were characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. The optical properties of the HBO dyes reveal the presence of enol and keto bands as a result of a strong excited-state intramolecular proton transfer (ESIPT). This ESIPT process is highly dependent on the nature of the electronic substituents and the polarity of the solvent. Upon coordination to a BF₂ fragment, typical singlet emission is observed with λ_em ranging from 439 to 553 nm and quantum yields from 0.03 to 0.36. If aromatic amines are involved, strong aggregates are observed that could be dissociated upon protonation of the lone electron pair. A series of luminescent 3,5-disubstituted 2-(2′-hydroxyphenyl)benzoxazoles (HBOs) and their corresponding borate complexes are synthesized and their optical properties investigated. HBO dyes display excited-state intramolecular proton-transfer fluorescence, which can be fine-tuned depending on the substituents. Blueshifted BF₂ complexes exhibit typical S ₀-S₁ fluorescence. Copyright

Full text of DOI:10.1002/ejoc.201300616

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FULL PAPER
HBO Dyes and Borate Complexes
A series of luminescent 3,5-disubstituted 2-
(2Ј-hydroxyphenyl)benzoxazoles (HBOs)
and their corresponding borate complexes
are synthesized and their optical properties
investigated. HBO dyes display excited-
state intramolecular proton-transfer fluo-
rescence, which can be fine-tuned de-
pending on the substituents. Blueshifted
BF2 complexes exhibit typical S0–S1 fluo-
rescence.
J. Massue,* G. Ulrich,*
R. Ziessel* ...................................... 1–10
Effect of 3,5-Disubstitution on the Optical
Properties of Luminescent 2-(2Ј-Hydroxy-
phenyl)benzoxazoles and Their Borate
Complexes
Keywords: Heterocycles / Borates / Dyes/
pigments / Fluorescence / Proton transfer
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J. Massue, G. Ulrich, R. Ziessel
FULL PAPER
DOI: 10.1002/ejoc.201300616
Effect of 3,5-Disubstitution on the Optical Properties of Luminescent 2-(2Ј-
Hydroxyphenyl)benzoxazoles and Their Borate Complexes
Julien Massue,*[a] Gilles Ulrich,*[a] and Raymond Ziessel*[a]
Keywords: Heterocycles / Borates / Dyes/pigments / Fluorescence / Proton transfer
This article describes the multistep synthesis and photophys-
ical properties of three highly fluorescent dyes based on the
2-(2Ј-hydroxyphenyl)benzoxazole (HBO) scaffold and their
resulting chelation to a BF2 fragment. These dyes possess
functionalization at the 3,5-positions of the phenol ring with
an ethynyl-extended fragment bearing TMS, p-tBuC6H4, or
veal the presence of enol and keto bands as a result of a
strong excited-state intramolecular proton transfer (ESIPT).
This ESIPT process is highly dependent on the nature of the
electronic substituents and the polarity of the solvent. Upon
coordination to a BF2 fragment, typical singlet emission is
observed with λem ranging from 439 to 553 nm and quantum
p-NnBu2C6H4 groups. All of the new compounds were char- yields from 0.03 to 0.36. If aromatic amines are involved,
acterized by NMR spectroscopy, mass spectrometry, and ele-
mental analysis. The optical properties of the HBO dyes re-
strong aggregates are observed that could be dissociated
upon protonation of the lone electron pair.
autofluorescence and hence to improve the signal-to-noise
ratio, high chemical modularity enabling good solubility
from common organic solvents to water, high fluorescent
quantum yield, and good chemical and photochemical sta-
bility. Several families of dyes have been found to fulfil these
optimal parameters and have therefore emerged as promis-
ing candidates for biological applications. The chemistry
and photophysics of coumarins,[3] cyanines,[4] squaraines,[5]
and boron dipyrromethene (BODIPY) dyes, to cite a few,
have blossomed in the last decade. In the case of BODIPY
dyes, their outstanding chemical and photophysical proper-
ties have paved the way to a myriad of applications in the
field of chemical biology as well as embedded in optoelec-
tronics devices.[6]
Introduction
Organic luminescence is a useful tool that has been
widely used to report and visualize various physiological
events and processes in tissues or cells (binding of an ana-
lyte, microchanges in polarity, viscosity or pH, internaliza-
tion in vesicles or cells, apoptosis, etc.).[1] Moreover, it
brings fundamental insight about the production, traffick-
ing, localization, and function of important biomolecules
in complex living systems. In the field of medical imaging
and diagnostics, it is of paramount importance to visualize
biological processes occurring in the close environment of
a given fluorophore at very low concentrations, sometimes
reaching the molecular level. Efficient optical methods have
been successfully developed to track fluorescent molecules
with high-spatial accuracy and minimal invasion.[2] To date,
numerous sophisticated molecular systems have been re-
ported as promising candidates for molecular imaging, usu-
ally bioconjugated to biologically relevant molecules (pro-
teins, oligosaccharides, nucleic acids, etc.) through an an-
choring functional group: carboxylic acid, primary amine,
isothiocyanate. Fluorescent dyes targeted by these kinds of
applications must follow specific requirements: strong ab-
sorption in the UV/Vis region of the spectrum, redshifted
emission to minimize UV-centered biological background
More recently, new fluorescent molecular entities bearing
chelating groups for BIII have emerged with the aim to
improve the molecular diversity within the boron-based dye
family while reaching the exceptional properties of
[8]
[9]
BODIPYs.[7] N O and N N π-conjugated chelating
fragments coordinated to boron-containing entities such as
BF2 or B(Ar)2 have also been investigated as original,
highly luminescent synthons. On our part, we have been
interested in exploring the chemistry and chelating ability
of decorated 2-(2Ј-hydroxyphenyl)benzoxazole (HBO) frag-
ments.[10] Outside their affinity to Lewis acids and a wide
range of metallic ions[11] of their corresponding deproton-
ated phenolate form, HBO dyes display interesting fluores-
cent properties as a result of intrinsic excited-state intramo-
lecular proton transfer (ESIPT) leading to improved Stokes’
shifts.[12] Tunable dual emission is usually observed, that is,
a high-energy enol emission (E*) and a lower-energy keto
emission (K*) owing to tautomerization in the excited state.
As pointed out in numerous examples, the ratio of the emis-
∧
∧
[a] Laboratoire de Chimie Moléculaire et Spectroscopie Avancées
(LCOSA), ICPEES, UMR CNRS 7515, Ecole Européenne de
Chimie, Polymères et Matériaux (ECPM),
25 Rue Becquerel, 67087 Strasbourg Cedex 02, France
E-mail: massue@unistra.fr
gulrich@unistra.fr
ziessel@unistra.fr
Homepage: http://www-lmspc.u-strasbg.fr/lcosa/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300616.
2
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Luminescent 2-(2Ј-Hydroxyphenyl)benzoxazoles
sion intensity of I(E*) versus I(K*) is highly dependent on bonded phenolic proton. Additionally, formation of 1 was
the dipolar moment of the solvent.[13] Taking advantage of confirmed by the appearance of bright green solid-state
this concept, ESIPT emitters have found applications as cat- fluorescence resulting from the ESIPT process.
ion or anion sensors[14] and as pH probes or sensitizers in
molecular dyads for photodynamic therapy (PDT).[15] This
∧
peculiar duality (ESIPT emitters or N O chelates) renders
these HBO dyes very interesting for fine-tuning the fluores-
cence emission. We recently reported the synthesis and op-
tical properties of HBO borate complexes substituted di-
rectly[10a] or through a single ethynyl spacer[10b] that display
highly luminescent emission spanning from the UV region
to the orange region of the spectrum depending on the elec-
tronic substitution and its position on the phenolic side.
These experimental results were rationalized by time-de-
pendent DFT calculations on a selection of decorated HBO
borate dyes.[16] At the molecular level, it is essential to study
and rationalize structure–activity relationships of innov-
ative dyes to be able to optimally design advanced struc-
Scheme 1. Synthesis of HBO 2 and its corresponding borate com-
tures. That is why it seemed interesting to synthesize HBO
dyes possessing additional ethynyl-extended substitutions
on the HBO phenol ring with different electronic substitu-
ents (–CϵC–TMS, –CϵC–C6H4tBu, –CϵC–C6H4NnBu2)
plex 3.
3,5-DiiodoHBO 1 was then subjected to Sonogashira
and to investigate the influence of these substitutions on the cross-coupling reaction conditions, that is, [PdCl2(PPh3)2]
optical properties. Here, we present the synthesis and op- (5 mol-%) catalyst and copper iodide (10 mol-%) in a sol-
tical properties of a series of HBO dyes disubstituted by an vent mixture of toluene/triethylamine with trimethyl-
ethynyl fragment at the 3,5-positions of the phenol ring and silylacetylene (6 equiv.) as the acetylide coreagent. This al-
their subsequent coordination to a BF2 fragment (Figure 1). lowed easy introduction of ethynyltrimethylsilyl fragments
The photophysical properties of the BIII complexes in or- at the 3,5-positions and afforded HBO 2 in 59% yield.
ganic solvents as well as those of some of the phenol ligands Dropwise addition of BF3·OEt2 (6 equiv.) to a solution of
are described.
HBO 2 in dry CH2Cl2 in the presence of N,N-diisopropyl-
amine (DIEA) as a base allowed straightforward boron
∧
complexation at the N O chelating site and led to HBO
borate complex 3 in 76% yield.
When HBO 1 was subjected to similar Sonogashira
cross-coupling conditions (PdII, CuI, NEt3/toluene) with
either a (4-di-n-butylamino)- or (4-tert-butylphenyl)acetyl-
ene coreagent, undesired benzofuran products were ob-
tained as a result of the intramolecular addition of the
phenolate on the triple bond of the unisolated ethynyl inter-
mediate.[17] To prevent this undesired reaction, HBO 1 was
protected with an acetate group by reaction with acetic an-
hydride in CH2Cl2 with a catalytic amount of pyridine.
Compound 4 was obtained in 84% yield after aqueous
workup of the organic solution. Once protected, Sonoga-
shira cross-coupling proceeded smoothly and allowed con-
Figure 1. (a) Substituted HBO borate complexes, (b) ethynyl-ex-
tended HBO borate complexes, and (c) 3,5-bis-ethynyl-extended
HBO borate complexes investigated in this study.
Results and Discussion
The syntheses of disubstituted HBO borate complexes 3, nection of the ethynyl-extended fragments at the 3,5-posi-
9, and 10 bearing ethynyltrimethylsilyl and ethynylphenyl tions in compounds 5 and 6 (74 and 65% yield for the 4-
fragments are described in Schemes 1 and 2, respectively. tert-butylphenyl and 4-(dibutylamino)phenyl moieties,
Commercially available 3,5-diiodosalicylaldehyde and 2- respectively). Removal of the acetate protecting group with
aminophenol were heated at reflux in ethanol to provide a an excess amount of K2CO3 in MeOH/THF (1:1) provided
cyclic carbinolamine, which precipitated from the reaction brightly luminescent HBOs 7 and 8 in nearly quantitative
mixture. After filtration, the precipitate was dissolved in dry yield (97 and 95%, respectively). In a fashion similar to that
CH2Cl2, and the benzoxazole ring was formed by oxidation used for 3, boron complexation was achieved by using an
by using a slight excess amount of 2,3-dichloro-5,6-dicyano- excess amount of BF3·OEt2/DIEA in dry CH2Cl2, and the
1,4-benzoquinone (DDQ) at room temperature; this led to resulting crude complexes were purified by filtration
the isolation of 3,5-diiodoHBO 1 as a white powder in 63% through a basic Al2O3 column eluting with CH2Cl2 to pro-
yield. HBO dye 1 displays a distinctive downfield-shifted vide borate complexes 9 and 10 in 82 and 76% yield, respec-
1
peak in its H NMR spectrum (around 12 ppm) for the H- tively.
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Scheme 2. Synthesis of HBOs 7 and 8 and their corresponding borate complexes 9 and 10.
1
All new compounds, including HBOs 1, 2, 7, and 8 and spectra). The H NMR spectra of HBO dye 8 and corre-
HBO borate complexes 3, 9, and 10, were unambiguously sponding borate complex 10 are presented in Figure 2. For
1
characterized by H NMR and 13C NMR spectroscopy in HBO dye 8, a distinctive downfield shift of the peak attrib-
CDCl3, as well as by MS and elemental analysis (see the uted to the strongly H-bonded phenolic proton (δ =
1
Supporting Information for the H NMR and 13C NMR 12.17 ppm) is observed. Chelation to the BF2 fragment can
Figure 2. (a) 1H NMR spectra of HBO 8 and HBO borate complex 10 in CDCl3 at room temperature and (b) expansion of the aromatic
regions of these spectra.
4
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Luminescent 2-(2Ј-Hydroxyphenyl)benzoxazoles
be readily monitored by assessing the disappearance of the low 300 nm, which are characteristic of the π–π* transitions
peak characteristic for this proton. Boron complexation is of the phenyl rings.
also correlated to significant splitting of the peaks in the
For HBO dyes 7 and 8, excitation in the lower-energy
aromatic region, as exemplified in the expansions shown in absorption band leads to one or two main emission bands.
Figure 2 (b). The phenyl rings on the phenolate exhibit two For HBO dye 7, tautomerization in the excited state, which
distinctive AAЈBBЈ systems at δ = 7.45/6.59 ppm (J = depends on the substitution pattern and the solvent (Fig-
8.7 Hz) and at δ = 7.37/6.59 ppm (J = 9 Hz), which evidence ure 3, a,b), leads to the emission of only the keto form (K*)
the influence of the electron-withdrawing BF2 group on the in toluene and CH2Cl2 (λem = 553 and 549 nm with Φ =
resonances of the aromatic protons. Finally, the 13C NMR 0.27 and 0.32, respectively). Notably, this band is not solva-
spectra are in good accord with the signals expected for tochromic. The ESIPT process is responsible for the ob-
tertiary and quaternary aromatic carbon atoms, as well as served large Stokes’ shift, ΔSS (over 8000 cm–1) (see Figure 4
for the methylene carbon atoms present in the butyl chains. for a schematic explanation). Interestingly, in EtOH, in ad-
The photophysical data for HBO dyes 7 and 8 and HBO dition to K* emission, a weak shoulder appears at higher
borate complexes 3, 9, and 10 in various solvents are energy (λem = 465 nm), which is attributed to the enol form
gathered in Table 1. The absorption and emission spectra (E*). This feature has been observed in related compounds
of dyes 7 and 8 are presented in Figure 3, whereas those of in solvents with stronger dipole moments or in protic sol-
HBO borate complexes 3, 9, and 10 are displayed in Fig- vents in which E* emission is usually enhanced.[12f,13] For
ures 4, 5, and 6, respectively. As a general trend, all HBO HBO dye 8, the photophysical behavior strongly differs
dyes and HBO borate complexes exhibit rather similar ab- from that of 7 (Figure 3, b).
sorption spectra with unstructured bands located in the UV
In all of the solvents, the emission bands of both E* and
part of the spectrum between 376 and 410 nm and extinc- K* are observed, but their relative intensities seem to be
tion coefficients ranging from 9400 to 22700 m–1 cm–1. Note highly dependent on the dipolar moment of the solvent
that all of these bands are UV centered and do not show used. In toluene, CH2Cl2, and EtOH, the relative intensities
drastic changes upon delocalization, that is, modification of of the two bands are in the same range (λem = 439/586, 501/
the substitution pattern of the phenol core (–CϵC–TMS, 579, 520/572 nm with Φ = 0.09/0.08, 0.14, and 0.32, overall
–CϵC–C6H4tBu, –CϵC–C6H4NnBu2). Moreover, they are yield respectively). Moreover, the K* band does not appear
not solvatochromic. More intense bands are observed be- to be solvatochromic unlike the E* band, which spans from
Figure 3. (a) Absorption (grey) and emission (black) spectra of HBO 7 in toluene, CH2Cl2, and EtOH; (b) absorption (grey) and emission
(black) spectra of HBO 8 in toluene, CH2Cl2 (without and with bubbling HCl gas into the solution), and EtOH.
Table 1. Photophysical data measured in aerated solution at room temperature.
Dye
λabs [nm]
ε [m–1 cm–1]
λem [nm]
ΔSS [cm–1]
Φf[a]
τ [ns]
Solvent
3
7
384
377
376
376
394
392
362
396
403
400
410
386
388
393
10400
12400
13300
12000
18000
22700
17300
6600
12800
12300
14900
15400
17500
9400
439
553
549
465, 543
439, 586
501, 579
398, 535
520, 572
470
470
502
448
482
3300
8400
8400
8200
2600, 8300
5600, 8200
2500, 8900
7800
3500
3700
4500
3600
5000
7400
0.21
0.27
0.32
0.21
0.09/0.08
0.14
0.33
0.32
0.30
0.27
0.03
0.36
0.23
0.11
1.7
3.2
3.5
2.6
2.5/1.5
5.1/1.5
2.6/4.3
3.2
2.6
2.5
5.2
1.5
4.0
4.5
CH2Cl2
toluene
CH2Cl2
EtOH
toluene
CH2Cl2
CH2Cl2 + HCl(g)
EtOH
toluene
CH2Cl2
CH2Cl2
CH2Cl2 + HCl(g)
THF
CH3CN
8
9
10
553
[a] Quantum yield determined in solution by using quinine sulfate as a reference Φ = 0.55 in 1 n H2SO4, λex = 366 nm for dyes emitting
below 480 nm or Rhodamine 6G as a reference Φ = 0.88 in ethanol, λex = 488 nm for dyes emitting between 480 and 570 nm.
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protonated species (Figure 3, b). Moreover, the quantum
yields calculated for the fluorescence emission of HBO dyes
7 and 8 resulting from the ESIPT process are among the
highest reported (up to 0.32). This is presumably due to
the improved electronic delocalization brought by the two
ethynyl fragments, which in return minimize nonradiative
deactivations. Finally, the lifetimes of the K* emission for
dyes 7 and 8 are in the nanosecond regime, which is consis-
tent with the literature.[19]
Excitation in the lower-energy band of HBO borate com-
plex 3 (λabs = 384 nm) leads to one emission band centered
at 439 nm (Φ = 0.21, Figure 5). This is a slight redshift rela-
tive to the emission band of the HBO borate monoethynyl-
trimethylsilyl-extended at the 5-position (λabs = 366 nm, λem
= 427 nm, Φ = 0.31).[10b] Finally, the excellent match be-
tween absorption and excitation spectra excludes the pres-
ence of aggregates in solution.
Figure 4. Schematic representation of the photophysical ESIPT
process in HBO dye 8.
439 nm in toluene to around 520 nm in EtOH. These inter-
esting observations could pave the way to the ratiometric
determination of polarity in the nearby environment of the
probes in membrane mimics such as micellar or vesicular
nano-objects.[18] To assess the internal charge-transfer
(ICT) character of the emission band in HBO dye 8, which
features a p-di-n-butylaminophenylacetylene moiety that
readily undergoes protonation, the absorption and emission
spectra were recorded in CH2Cl2 after HCl gas was bubbled
into the solution. A pronounced hypsochromic shift was
observed both in the absorption (λabs = 362 vs. 392 nm) and
in the emission (λem = 398/535 vs. 501, 579 nm) along with
a significant increase in the quantum yield (Φ = 0.33 vs.
0.14). Furthermore, the ratio of intensities I(K*)/I(E*)
seems to be strongly affected by the protonation of the lone
pair of electron on the nitrogen atoms in favor of K* emis-
sion at 535 nm. The emission of E* is blueshifted, which
highlights the strong ICT character of the emission in 8.
This photophysical behavior is in strong contrast with that
of HBO 7, which is fairly similar in molecular structure.
The presence of the strongly electron-donating p-di-n-
butylamino group presumably increases the pKa of the phe-
nolate and therefore lowers its acidity. This diminished
acidic character is not in favor of pronounced ESIPT be-
havior (Figure 4), as highlighted by the presence of the
emission bands of both E* and K*. As a result of proton-
ation, the pKa of the phenolate decreases, which enhances
the ESIPT process, as observed by the almost-complete dis-
Figure 5. Absorption (dashed), emission (plain), and excitation
(dotted) spectra of HBO borate 3 in CH2Cl2 at room temperature.
Similar photophysical behavior was observed for HBO
borate complex 9 in which two p-tert-butylphenylacetylene
units are connected at the 3,5-positions of the phenol ring
of the HBO core (Figure 6). Excitation in the maximum of
the absorption band (λabs = 403 and 400 nm in toluene and
CH2Cl2, respectively) engenders the formation of a blue-
centered unique emission (λem = 470 nm for toluene and
CH2Cl2). This purely singlet-emitting band is not solva-
tochromic, a feature reminiscent to that observed for corre-
Figure 6. Absorption and emission of HBO borate complex 9 in
appearance of the E* band in the emission spectrum of the toluene (grey) and CH2Cl2 (black) at room temperature.
6
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Luminescent 2-(2Ј-Hydroxyphenyl)benzoxazoles
Figure 7. (a) Absorption (grey) and emission (black) spectra of HBO borate complex 10 in CH2Cl2, THF, and CH3CN; (b) absorption
(grey) and emission (black) spectra of HBO borate complex 10 in CH2Cl2 (without and with bubbling HCl gas into the solution).
sponding ligand 7. Connecting a second ethynyl spacer at served around 650 nm in CH2Cl2, only minor amounts of
the 3-position led to a significant redshift in the emission aggregates were detected in THF. Furthermore, these aggre-
by nearly 40 nm relative to that of the corresponding mono- gates can dissociate upon protonation of the lone pair of
ethynyl-extended borate complex (λabs = 372 nm, λem
=
electrons of the nitrogen atom by using HCl gas. Indeed, in
431 nm, Φ = 0.30 in toluene).[10b] Unexpectedly, different addition to the observed blueshift, protonation of the lone
photophysical data were collected for HBO borate complex pair of electrons of the nitrogen atoms leads to the total
10.
disappearance of the aggregation band in the lower-energy
Excitation in the main absorption band leads to an in- window. This is further confirmed by the excellent match
tense emission band that is fairly solvatochromic owing to between the absorption and excitation spectra.
the presence of two electron-donating p-di-n-butylamino-
phenylacetylene fragments on the core of the dye (Figure 7,
a). Although weakly luminescent in CH2Cl2 (λabs = 410 nm,
Conclusions
λem = 502 nm, Φ = 0.03), HBO borate complex 10 lightens
We report herein the synthesis and full spectroscopic
up in THF and CH3CN (λabs = 388 nm, λem = 482 nm, Φ characterization of three original HBO dyes and their sub-
= 0.23 and λabs = 393 nm, λem = 553 nm, Φ = 0.11). This sequent coordination to a BF2 entity. These dyes are all
behavior is reminiscent to that observed in the correspond- functionalized at the 3,5-positions of the phenol ring of the
ing monoethynyl-extended borate complex substituted by a HBO core with ethynyl-extended fragments bearing TMS,
single p-di-n-butylaminophenylacetylene fragment in the 3- p-tBuC6H4, or p-NnBu2C6H4 groups. The optical data for
position; the luminescence of this species was fully the HBO dyes reveal a strong influence of the electronic
quenched in CH2Cl2 owing to strong ICT, but this species character of the substituents on the ratio of the intensities
was emissive in other solvents.[10b] To obtain more insight of the emission bands of the K* and E* forms. This behav-
into this ICT process, we recorded optical data in CH2Cl2 ior could be rationalized in terms of acidity of the corre-
after bubbling HCl into the solution, which resulted in a sponding phenolate; the higher the acidity, the more pro-
significant blueshift in the absorption and emission and a nounced the ESIPT, which in return enhances the K* emis-
major increase in the quantum yield (λabs = 386 nm, λem
=
sion band. These interesting features make these dyes opti-
448 nm, Φ = 0.36; Figure 7, b). In apolar solvents, such as mal candidates as ratiometric detectors of environment po-
toluene, the low-energy absorption band is significantly larity in membranes mimics. In addition, the dipolar mo-
larger and spans above 480 nm. Irradiation at 395 nm re- ment of the solvent and the protic character is of para-
sulted in dual emission with a strong emission band cen- mount importance in the ESIPT emission of the dyes. After
tered at 550 nm. The weaker emission at 448 nm is reminis- coordination to a BF2 fragment, the resulting HBO borate
cent of the emission found in more polar solvents and is complexes all emit in the visible region of the spectrum with
characteristic of the nonaggregated dye. The strong emis- quantum yields up to 0.36. The presence of the di-n-
sion around 550 nm is specific to aggregates, as previously butylamino groups leads to a fairly solvatochromic absorp-
observed with related compounds (Figure S27a, Supporting tion and emission as evidenced by the blueshift observed
Information).[20] Notably, these aggregates are observed upon protonation of the lone pair of electrons of the nitro-
even after dilution of the solution down to around 10–8 m. gen atom. Furthermore, strong aggregates are observed in
The presence of these aggregates in solution is further con- apolar solvents to CH2Cl2, but they can be easily dissoci-
firmed by a very poor match between the absorption and ated in the presence of acid, as evidenced by an excellent
excitation spectra. As would be expected on the basis of the match between the absorption and excitation spectra. These
apolarity of 10, we noticed that by increasing the dipolar nascent aggregates formed in apolar solvents require further
moment of the solvent, the formation of aggregates de- characterization in particular by dynamic light scattering
creased. Indeed, although some aggregates were still ob- (DLS). Work along these lines is currently in progress.
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J. Massue, G. Ulrich, R. Ziessel
FULL PAPER
added PdCl2(PPh3)2 (18 mg, 0.0025 mmol) and triethylamine
(2 mL). The resulting suspension was degassed with argon for
30 min before trimethylsilylacetylene (0.43 mL, 3 mmol) and CuI
(10 mg, 0.05 mmol) were added. The medium, which turned rapidly
black, was stirred overnight at room temperature. The dark solu-
tion was then taken up in dichloromethane (30 mL), washed with
water, dried (MgSO4), and concentrated in vacuo. The crude resi-
due was purified by silica gel chromatography (CH2Cl2/petroleum
ether, 1:4) to afford HBO 2 as an off-white powder (120 mg, 59%).
1H NMR (300 MHz, CDCl3): δ = 12.30 (s, 1 H, OH), 8.12 (d, J =
2.1 Hz, 1 H, CH Ar), 7.72–7.75 (m, 1 H, CH Ar), 7.71 (d, J =
2.1 Hz, 1 H, CH Ar), 7.58–7.64 (m, 1 H, CH Ar), 7.38–7.44 (m, 2
H, CH Ar), 0.30 (s, 9 H, CH3), 0.26 (s, 9 H, CH3) ppm. 13C NMR
(75 MHz, CDCl3): δ = 161.7, 159.5, 149.2, 140.6, 139.6, 130.7,
126.0, 125.3, 119.5, 114.6, 113.0, 110.8, 110.8, 103.2, 100.3, 99.0,
93.7, 0.0 ppm. C23H25NO2Si2 (403.63): calcd. C 68.44, H 6.24, N
3.47; found C 68.24, H 5.92, N 3.17. MS (EI): m/z (%) = 403.0
(100).
Experimental Section
General Methods and Equipment: All reactions were performed un-
der a dry atmosphere of argon by using standard Schlenk tech-
niques. All chemicals were received from commercial sources (Ald-
rich, Alfa Aesar) and used without further purification. Dichloro-
methane was distilled from P2O5 under an argon atmosphere. Thin-
layer chromatography (TLC) was performed on silica gel or alumi-
num oxide (Al2O3) plates coated with fluorescent indicator. Chro-
matographic purifications were conducted by using 40–63 μm silica
gel or basic aluminum oxide. All mixtures of solvents are given in
1
v/v ratio. H NMR and 13C NMR spectra were recorded at room
temperature with perdeuterated solvents with residual protonated
solvent signals as internal references. Mass spectra were measured
with an ESI-MS mass spectrometer. Electronic absorption and
emission spectra were measured under ambient conditions by using
commercial instruments. UV/Vis spectra were recorded by using
a dual-beam grating spectrophotometer with a 1 cm quartz cell.
Fluorescence spectra were recorded with a spectrofluorimeter. Sol-
vents for spectroscopy were spectroscopic grade and were used as
received. All fluorescence spectra were corrected. Luminescence
lifetimes were measured with a spectrofluorimeter by using soft-
ware with time-correlated single photon mode coupled to a Strobo-
scopic system. The excitation source was a laser diode (λ =
320 nm). The instrument response function was determined by
using a light-scattering solution (LUDOX). The fluorescence quan-
tum yield (Φexp) was calculated from Equation (1).
HBO Borate Complex 3: To a stirred solution of HBO 2 (20 mg,
50 mmol) in dry dichloromethane was added BF3·OEt2 (48 μL,
300 mmol) by syringe under an argon atmosphere. After 5 min,
DIEA (52 μL, 300 mmol) was added, and the resulting mixture was
stirred at room temperature for 1 h. The crude solution was filtered
through a column of basic Al2O3 (CH2Cl2) to afford clean HBO
borate complex 3 (17 mg, 76%) as a yellow powder after evapora-
tion of the solvents in vacuo. 1H NMR (300 MHz, CDCl3): δ =
8.03 (d, J = 2.1 Hz, 1 H, CH Ar), 7.98–8.02 (m, 1 H, CH Ar), 7.88
(d, J = 2.1 Hz, 1 H, CH Ar), 7.71–7.78 (m, 1 H, CH Ar), 7.58–
7.65 (m, 2 H, CH Ar), 0.29 (s, 9 H, CH3), 0.26 (s, 9 H, CH3) ppm.
13C NMR (75 MHz, CDCl3): δ = 160.9, 159.8, 149.1, 144.6, 130.7,
128.8, 128.1, 127.9, 117.5, 116.3, 115.5, 111.9, 106.4, 102.3, 102.0,
98.3, 95.1, 0.0 ppm. C23H24BF2NO2Si2 (451.43): calcd. C 61.19, H
5.36, N 3.10; found C 60.92, H 5.07, N 2.83. MS (EI): m/z (%) =
451.1 (100), 432.1 (30).
(1)
I denotes the integral of the corrected emission spectrum, OD is
the optical density at the excitation wavelength, and η is the refrac-
tive index of the medium. The reference systems used were: Quinine
Φ = 55% in 1 n H2SO4, λexc = 366 nm for dyes emitting below
480 nm; Rhodamine 6G, Φ = 88% in ethanol λexc = 488 nm for
dyes emitting between 480 and 570 nm; and cresyl violet, Φ = 55%
λexc = 546 nm in ethanol for dyes emitting above 570 nm.
2-(Benzoxazol-2-yl)-4,6-diiodophenyl Acetate (4): HBO 1 (800 mg,
1.73 mmol) was dissolved in dichloromethane (10 mL). Acetic an-
hydride (4 mL) and pyridine (2 mL) were then added, and the mix-
ture was stirred at room temperature for 2 h. The crude solution
was diluted with dichloromethane (30 mL), washed with 1 m HCl
and water, dried (MgSO4), and concentrated in vacuo to yield pro-
tected HBO 4 (733 mg, 84%) as a beige powder. 1H NMR
(300 MHz, CDCl3): δ = 8.41 (d, J = 2.1 Hz, 1 H, CH Ar), 8.15 (d,
J = 2.1 Hz, 1 H, CH Ar), 7.58–7.64 (m, 1 H, CH Ar), 7.41–7.47
(m, 1 H, CH Ar), 7.19–7.30 (m, 2 H, CH Ar), 2.39 (s, 3 H,
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 168.2, 166.4, 157.1,
149.1, 148.8, 141.5, 138.6, 126.1, 124.9, 122.8, 120.5, 110.6, 95.2,
90.6, 22.0 ppm. C15H9I2NO3 (505.05): calcd. C 35.67, H 1.80, N
2.77; found C 35.51, H 1.52, N 2.62. MS (EI): m/z (%) = 505.0
(100).
Synthesis: All reagents were used directly as obtained commercially
unless otherwise noted. p-(Di-n-butylamino)phenylacetylene was
synthesized according to a reported procedure.[21]
2-(Benzoxazol-2-yl)-4,6-diiodophenol (1): To a solution of 2-amino-
phenol (58 mg, 0.53 mmol) in absolute ethanol was added 3,5-di-
iodo-2-hydroxybenzaldehyde (200 mg, 0.53 mmol), and the re-
sulting suspension was heated at reflux for 1 hour before an orange
precipitate formed. Upon cooling, the precipitate was filtered and
washed with ethanol. It was then redissolved in dry dichlorometh-
ane and DDQ (146 mg, 0.64 mmol) was added as a concentrated
dichloromethane solution. The resulting dark mixture was stirred
at room temperature overnight. After solvent evaporation, the
crude residue was purified by silica gel chromatography (CH2Cl2/
petroleum ether, 1:1). HBO 1 (156 mg, 63%) was obtained as a
white powder after recrystallization in pentane. 1H NMR
(300 MHz, CDCl3): δ = 12.43 (s, 1 H, OH), 8.29 (d, J = 2.1 Hz, 1
H, CH Ar), 8.16 (d, J = 2.1 Hz, 1 H, CH Ar), 7.71–7.76 (m, 1 H,
CH Ar), 7.60–7.65 (m, 1 H, CH Ar), 7.39–7.45 (m, 2 H, CH
Ar) ppm. 13C NMR (75 MHz, CDCl3): δ = 160.5, 157.3, 149.8,
149.3, 139.3, 135.4, 126.2, 125.5, 119.6, 112.3, 110.9, 86.7,
81.0 ppm. C13H7I2NO2 (463.01): calcd. C 33.72, H 1.52, N 3.03;
found C 33.54, H 1.38, N 2.84. MS (EI): m/z (%) = 462.5 (100).
2-(Benzoxazol-2-yl)-4,6-bis{[4-(tert-butyl)phenyl]ethynyl}phenyl
Acetate (5): To a solution of compound 4 (270 mg, 0.53 mmol) in
toluene (8 mL) was added PdCl2(PPh3)2 (37 mg, 0.053 mmol) and
triethylamine (4 mL). The resulting suspension was degassed with
argon for 30 min before 4-(tert-butyl)phenylacetylene (0.58 mL,
3.18 mmol) and CuI (20 mg, 0.11 mmol) were added. The medium
was stirred overnight at room temperature. The dark solution was
then taken up in dichloromethane (30 mL), washed with water,
dried (MgSO4), and then concentrated in vacuo. The crude residue
was purified by silica gel chromatography (CH2Cl2/petroleum ether,
2:1) to afford compound 5 as a beige powder (0.225 g, 74%). 1H
NMR (300 MHz, CDCl3): δ = 8.42 (d, J = 1.5 Hz, 1 H, CH Ar),
7.88 (d, J = 1.8 Hz, 1 H, CH Ar), 7.77–7.82 (m, 1 H, CH Ar),
2-(Benzoxazol-2-yl)-4,6-bis[(trimethylsilyl)ethynyl]phenol (2): To a
solution of HBO 1 (232 mg, 0.50 mmol) in toluene (5 mL) was
8
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

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Date: 15-07-13 12:31:59
Pages: 10
Luminescent 2-(2Ј-Hydroxyphenyl)benzoxazoles
7.56–7.60 (m, 1 H, CH Ar), 7.52 (d, J = 6.9 Hz, 2 H, CH Ar), 7.49
(d, J = 6.6 Hz, 2 H, CH Ar), 7.41 (d, J = 12.6 Hz, 4 H, CH Ar),
7.33–7.39 (m, 2 H, CH Ar), 2.57 (s, 3 H, CH3), 1.36 (s, 18 H,
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 168.9, 158.9, 152.4,
152.1, 150.3, 149.3, 141.9, 137.9, 132.5, 131.5, 125.8, 125.5, 125.5,
124.8, 122.3, 121.4, 120.7, 120.6, 120.6, 119.7, 119.5, 110.6, 95.7,
91.1, 86.5, 82.5, 34.9, 31.2, 21.1 ppm. C39H35NO3 (565.71): calcd.
C 82.80, H 6.24, N 2.48; found C 82.69, H 5.84, N 2.13. MS (EI):
m/z (%) = 565.1 (100).
(m, 8 H, CH2), 0.97 (t, J = 7.2 Hz, 12 H, CH3) ppm. 13C NMR
(75 MHz, CDCl3): δ = 162.2, 158.1, 149.2, 148.1, 148.0, 139.8,
139.0, 133.1, 132.8, 128.8, 125.7, 125.1, 119.4, 115.8, 114.2, 111.3,
111.2, 110.8, 110.7, 108.7, 108.7, 96.4, 90.0, 85.6, 81.9, 50.7, 29.4,
20.4, 14.0 ppm. C46H55NO2 (653.95): calcd. C 81.02, H 8.13, N
6.16; found C 80.87, H 7.44, N 6.04. MS (EI): m/z (%) = 665.2
(100).
HBO Borate Complex 9: To a stirred solution of HBO 7 (100 mg,
0.19 mmol) in freshly distilled dichloromethane (10 mL) was added
BF3·OEt2 (0.14 mL, 1.1 mmol) by syringe under an argon atmo-
sphere. After 5 min, DIEA (0.19 mL, 1.1 mmol) was added, and
the resulting mixture was stirred at room temperature for 1 h. The
crude solution was then filtered through a column of basic Al2O3
(CH2Cl2), and the solvent was evaporated in vacuo. Pure HBO bor-
2-(Benzoxazol-2-yl)-4,6-bis{[4-(dibutylamino)phenyl]ethynyl}phenyl
Acetate (6): To a solution of p-(di-n-butylamino)phenylacetylene
(500 mg, 2.18 mmol) in toluene (8 mL) was added compound 4
(275 mg, 0.54 mmol), PdCl2(PPh3)2 (38 mg, 0.054 mmol), and tri-
ethylamine (3 mL). The resulting suspension was degassed with ar-
gon for 30 min before CuI (21 mg, 0.108 mmol) was added. The
medium was stirred overnight at room temperature. The dark solu-
tion was then extracted with dichloromethane (30 mL), washed
with water, dried (MgSO4), and concentrated in vacuo. The crude
residue was purified by silica gel chromatography (CH2Cl2/petro-
leum ether, 3:1) to afford compound 6 as a yellow powder (0.182 g,
1
ate complex 9 was obtained as a yellow powder (87 mg, 82%). H
NMR (300 MHz, CDCl3): δ = 8.05 (d, J = 2.4 Hz, 1 H, CH Ar),
7.96–7.99 (m, 1 H, CH Ar), 7.94 (d, J = 2.1 Hz, 1 H, CH Ar),
7.71–7.77 (m, 1 H, CH Ar), 7.54–7.58 (m, 4 H, CH Ar), 7.48 (d, J
= 8.1 Hz, 2 H, CH Ar), 7.39 (d, J = 8.1 Hz, 4 H, CH Ar), 1.35 (s,
18 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 160.9, 159.1,
152.0, 149.0, 143.2, 131.7, 131.4, 130.6, 127.9, 127.6, 125.5, 125.3,
119.9, 119.7, 117.2, 116.6, 115.8, 111.9, 106.4, 96.2, 90.0, 86.3, 82.9,
34.9, 31.2 ppm. C37H32BF2NO2 (571.47): calcd. C 77.76, H 5.64,
N 2.45; found C 77.57, H 5.29, N 2.28. MS (EI): m/z (%) = 571.1
(100), 552.1 (35).
1
65%). H NMR (300 MHz, CDCl3): δ = 8.32 (d, J = 2.1 Hz, 1 H,
CH Ar), 7.81 (d, J = 2.4 Hz, 1 H, CH Ar), 7.76–7.81 (m, 1 H, CH
Ar), 7.56–7.62 (m, 1 H, CH Ar), 7.37–7.42 (m, 6 H, CH Ar), 6.61
(d, J = 8.1 Hz, 4 H, CH Ar), 3.31 (t, J = 7.8 Hz, 8 H, CH2), 2.57
(s, 3 H, CH3), 1.55–1.65 (m, 8 H, CH2), 1.32–1.44 (m, 8 H, CH2),
0.99 (t, J = 7.2 Hz, 12 H, CH3) ppm. 13C NMR (100 MHz, CDCl3):
δ = 169.0, 159.3, 150.3, 148.4, 148.2, 141.9, 137.1, 133.1, 133.1,
131.3, 125.6, 124.7, 123.1, 121.2, 121.1, 120.5, 111.2, 111.2, 110.6,
108.1, 108.0, 97.2, 92.4, 85.3, 81.2, 50.7, 29.4, 21.1, 20.4, 14.0 ppm.
HBO Borate Complex 10: To a stirred solution of HBO 8 (100 mg,
0.15 mmol) in freshly distilled dichloromethane (10 mL) was added
BF3·OEt2 (0.11 mL, 0.9 mmol) by syringe under an argon atmo-
sphere. After 5 min, DIEA (0.16 mL, 0.9 mmol) was added, and
the resulting mixture was stirred at room temperature for 1 h. The
crude solution was then filtered through a column of basic Al2O3
(CH2Cl2), and the solvents were evaporated in vacuo. Pure HBO
borate complex 10 was obtained as an orange powder (80 mg,
76%). 1H NMR (300 MHz, CDCl3): δ = 7.99–8.02 (m, 1 H, CH
Ar), 7.97 (d, J = 2.1 Hz, 1 H, CH Ar), 7.91 (d, J = 2.1 Hz, 1 H,
CH Ar), 7.72–7.78 (m, 1 H, CH Ar), 7.56–7.63 (m, 2 H, CH Ar),
7.45 (d, J = 8.7 Hz, 2 H, CH Ar), 7.37 (d, J = 9 Hz, 2 H, CH Ar),
6.59 (d, J = 9 Hz, 4 H, CH Ar), 3.30 (t, J = 7.8 Hz, 8 H, CH2),
1.54–1.64 (m, 8 H, CH2), 1.31–1.43 (m, 8 H, CH2), 0.98 (t, J =
7.2 Hz, 12 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 161.3,
158.6, 149.0, 148.1, 142.7, 133.3, 132.9, 130.8, 127.7, 127.6, 126.3,
117.4, 117.2, 116.7, 111.8, 111.3, 108.6, 108.2, 106.2, 97.7, 91.1,
84.9, 81.7 ppm. C37H32BF2NO2 (571.47): calcd. C 75.73, H 7.06,
N 5.89; found C 75.54, H 6.67, N 5.61. MS (EI): m/z (%) = 713.3
(100), 694.3 (45).
C
47H53N3O3 (707.95): calcd. C 79.74, H 7.55, N 5.94; found C
79.51, H 7.38, N 5.69. MS (EI): m/z (%) = 707.2 (100).
2-(Benzoxazol-2-yl)-4,6-bis{[4-(tert-butyl)phenyl]ethynyl}phenol (7):
Compound 5 (100 mg, 0.18 mmol) was dissolved in a mixture
of MeOH/THF (50:50, 8 mL). Potassium carbonate (245 mg,
18 mmol) was added, and the resulting mixture was stirred at room
temperature for 1 h. The crude solution was extracted with di-
chloromethane (15 mL), washed with water, dried (MgSO4), and
concentrated in vacuo. The crude solid was washed with pentane
to yield HBO 7 as a bright yellow powder (93 mg, 97%). 1H NMR
(300 MHz, CDCl3): δ = 12.29 (s, 1 H, OH), 8.17 (br. s, 1 H, CH
Ar), 7.76 (br. s, 1 H, CH Ar), 7.65–7.71 (m, 1 H, CH Ar), 7.60–
7.65 (m, 1 H, CH Ar), 7.57 (d, J = 8.7 Hz, 2 H, CH Ar), 7.50 (d,
J = 8.1 Hz, 2 H, CH Ar), 7.41 (dd, J = 9 Hz, 6 H, CH Ar), 1.36
(s, 18 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 161.9, 158.9,
151.8, 151.6, 149.2, 139.7, 139.5, 131.6, 131.3, 129.9, 125.9, 125.4,
125.4, 125.3, 120.1, 119.5, 115.0, 113.5, 110.9, 110.8, 95.0, 89.0,
87.2, 83.4, 34.9, 31.2 ppm. C37H33NO2 (523.67): calcd. C 84.86, H
6.35, N 2.67; found C 84.64, H 6.04, N 2.44. MS (EI): m/z (%) =
523.1 (100).
Supporting Information (see footnote on the first page of this arti-
cle): Copies of the 1H NMR and 13C NMR spectra for all com-
pounds and spectroscopic data.
2-(Benzoxazol-2-yl)-4,6-bis{[4-(dibutylamino)phenyl]ethynyl}phenol
(8): Compound 6 (100 mg, 0.14 mmol) was dissolved in a mixture
of MeOH/THF (50:50, 8 mL). Potassium carbonate (195 mg,
14 mmol) was added, and the resulting mixture was stirred at room
temperature for 1 h. The crude solution was extracted with dichlo-
romethane (15 mL), washed with water, dried (MgSO4), and con-
centrated in vacuo. The crude solid was washed with pentane to
yield HBO 8 as a bright yellow powder (92 mg, 95%). 1H NMR
(300 MHz, CDCl3): δ = 12.17 (s, 1 H, OH), 8.11 (d, J = 2.1 Hz, 1
H, CH Ar), 7.76 (d, J = 2.4 Hz, 1 H, CH Ar), 7.72–7.75 (m, 1 H,
CH Ar), 7.60–7.66 (m, 1 H, CH Ar), 7.44 (d, J = 9 Hz, 2 H, CH
Ar), 7.33–7.42 (m, 4 H, CH Ar), 6.60 (d, J = 8.7 Hz, 4 H, CH Ar),
3.30 (t, J = 7.8 Hz, 8 H, CH2), 1.54–1.64 (m, 8 H, CH2), 1.31–1.43
Acknowledgments
Centre National de la Recherche Scientifique (CNRS) is gratefully
acknowledged for providing financial support and research facili-
ties.
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J. Massue, G. Ulrich, R. Ziessel
FULL PAPER
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