Non-classical donor-acceptor-donor chromophores. A strategy for high two-photon brightness

Article abstract of DOI:10.1039/c4tc00234b

Rod-like π-expanded bis-imidazo[1,2-a]pyridines were designed and synthesized. Strategic placement of a central electron-poor unit at position C3 of the imidazo[1,2-a]pyridine core resulted in donor-acceptor-donor quadrupolar systems. We further demonstrated the applicability of the Ortoleva-King- Chichibabin tandem process in the preparation of complex imidazo[1,2-a] pyridines. The monomer model heterocycles, differing by substitution at C2 and C3, displayed luminescence properties marginally dependent on the substitution pattern and conjugation extension. The highest luminescence quantum yield (φ_flca. 0.2) is shown by the model compounds without a substitution at C3, whereas lower values are measured for the others. Quantum yields in toluene for the quadrupolar systems varied from φ_fl = 0.7 to φ_fl > 0.9. Apart from bis-imidazo[1,2-a]pyridine possessing two methyl groups, with φ_fl = 0.69 in toluene and 0.35 in dichloromethane, the luminescence properties were only slightly affected by the change in the solvent. Most of the fluorescence lifetimes ranged from 1-2 ns and the radiative rate constants reached values of ca. 6 × 10⁸ s^-1. Intense phosphorescence spectra at 77 K with lifetimes in the second range (τ = 0.6-1.3 s) are recorded for the monomers, whereas for the bis-imidazo[1,2-a]pyridines, phosphorescence spectra are detected only after enhancing intersystem crossing by heavy atoms. Non-linear optical properties of bis-imidazo[1,2-a]pyridines revealed two-photon absorption cross-sections (σ₂) typically of the order of 500-800 GM. The remarkable fluorescence quantum yield, in combination with high a two-photon absorption cross-section, generated a two-photon brightness of ～500 to 750 GM units within the biological window (700-800 nm).

Full text of DOI:10.1039/c4tc00234b

Journal of
Materials Chemistry C
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PAPER
Non-classical donor–acceptor–donor
chromophores. A strategy for high two-photon
Cite this: J. Mater. Chem. C, 2014, 2,
brightness†
4552
a
b
c
c
*
Adina I. Ciuciu, Dikhi Firmansyah, Vincent Hugues, Mireille Blanchard-Desce,
bd
a
*
*
Daniel T. Gryko
and Lucia Flamigni
Rod-like p-expanded bis-imidazo[1,2-a]pyridines were designed and synthesized. Strategic placement of a
central electron-poor unit at position C3 of the imidazo[1,2-a]pyridine core resulted in donor–acceptor–
donor quadrupolar systems. We further demonstrated the applicability of the Ortoleva–King–
Chichibabin tandem process in the preparation of complex imidazo[1,2-a]pyridines. The monomer
model heterocycles, differing by substitution at C2 and C3, displayed luminescence properties marginally
dependent on the substitution pattern and conjugation extension. The highest luminescence quantum
yield (ffl ca. 0.2) is shown by the model compounds without a substitution at C3, whereas lower values
are measured for the others. Quantum yields in toluene for the quadrupolar systems varied from ffl
¼
0.7 to ffl > 0.9. Apart from bis-imidazo[1,2-a]pyridine possessing two methyl groups, with ffl ¼ 0.69 in
toluene and 0.35 in dichloromethane, the luminescence properties were only slightly affected by the
change in the solvent. Most of the fluorescence lifetimes ranged from 1–2 ns and the radiative rate
constants reached values of ca. 6 ꢀ 108
s
ꢁ1. Intense phosphorescence spectra at 77 K with lifetimes in
the second range (s ¼ 0.6–1.3 s) are recorded for the monomers, whereas for the bis-imidazo[1,2-a]-
pyridines, phosphorescence spectra are detected only after enhancing intersystem crossing by heavy
atoms. Non-linear optical properties of bis-imidazo[1,2-a]pyridines revealed two-photon absorption
cross-sections (s2) typically of the order of 500–800 GM. The remarkable fluorescence quantum yield,
in combination with high a two-photon absorption cross-section, generated a two-photon brightness of
ꢂ500 to 750 GM units within the biological window (700–800 nm).
Received 5th February 2014
Accepted 10th March 2014
DOI: 10.1039/c4tc00234b
www.rsc.org/MaterialsC
large values of two-photon brightness (i.e. f ꢀ s2, where f is the
uorescence quantum yield and s2 is the two-photon absorption
Introduction
cross-section). The contribution of dipolar and quadrupolar
architectures to achieve high s2 has driven the efforts to prepare
two photon probes that incorporate various electron-donating
and electron-withdrawing moieties into their structures.2
Since the identication of two-photon excited uorophores
is key to the eld of bioimaging, we turned our attention
towards imidazo[1,2-a]pyridine scaffolds for the construction of
donor–acceptor–donor (D–A–D) quadrupolar systems. These
compounds are particularly attractive because they are electron-
rich aromatic heterocycles possessing inherently high uores-
cence quantum yields. We envisioned the construction of
Two-photon excited uorescence (TPEF) imaging has attracted
much interest for biological applications because of the intrinsic
three-dimensional resolution it offers and since the use of NIR
laser excitation light allows the advantages of minimal back-
ground auto-uorescence, deep light penetration, and minimal
photodamage to living organisms.1 Thus, the development of
high-performance TPEF bio-probes is an expanding eld of
investigation, particularly for the synthesis of compounds with
aIstituto per la Sintesi Organica e Fotoreattivita' (ISOF), CNR, Via P. Gobetti 101,
40129 Bologna, Italy. E-mail: lucia.amigni@isof.cnr.it; Fax: +39 (0)51 639 98 44;
Tel: +39 (0)51 639 98 12
quadrupolar compounds bearing
a central electron-with-
drawing unit combined with two electron-rich imidazo[1,2-a]-
pyridines that would afford materials possessing both high
uorescence quantum yield and large two-photon absorption
cross-section.3 In addition to their interesting luminescence
properties4 and the advances in their synthesis,5 imidazo[1,2-a]-
pyridines show well-known biological activities.6 These drug-
like scaffolds hold great promise for the design of uorescence
probes for in vivo imaging because these molecules can
bWarsaw University of Technology, Noakowskiego 3, 00-664, Warsaw, Poland. Fax: +48
22 628 27 41; Tel: +48 22 234 58 01
c
´
Universite de Bordeaux, ISM (UMR5255 CNRS), F 33400, Bordeaux, France. E-mail:
mireille.blanchard-desce@iu-bordeaux.fr
dInstitute of Organic Chemistry Polish Academy of Sciences, Kasprzaka 44/52 01-224,
Warsaw, Poland. E-mail: dtgryko@icho.edu.pl; Fax: +48 22 632 66 81; Tel: +48 22 343
30 63
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00234b
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potentially retain their molecular recognition ability and signicantly improve the charge transfer character of the
thereby can be used as specic probes to target biologically quadrupolar molecules, resulting in high two-photon absorp-
relevant dedicated sites.
tion cross-section values. Based on our previous experience with
The uorescence properties of imidazo[1,2-a]pyridines may imidazo[1,2-a]pyridines, we chose 2-amino-5-(triuoromethyl)-
be enhanced by altering the emission quantum yield and the pyridine (1) as the backbone. Since the triuoromethyl group is
Stokes shi as well as by tuning their emission wavelength. moderately electron-withdrawing, we expected an improved
Optimization of these factors will improve their usefulness in stability of the nal dye. Unlike nitrile groups, which could
uorescence microscopy and also for optical materials. There- decrease the solubility or nitro and carbonyl groups which may
fore, we have explored the possibility of altering the patterns of quench the uorescence, CF3 groups have limited effects on the
substituents on the imidazo[1,2-a]pyridine scaffold. Several optical properties.
studies on imidazo[1,2-a]pyridines have taken advantage of the
Three imidazo[1,2-a]pyridines were synthesized as the
excited state intramolecular proton transfer (ESIPT)4b,c process, starting materials (Schemes 1 and 2). Compound 3a was
both in solution and in the solid state.4d,4g,7 Although interesting synthesized following known procedures for condensation of
luminescence properties of imidazo[1,2-a]pyridine derivatives amine 1 and chloroacetone (2a).13 Using the tandem Ortoleva–
have been presented, quantitative data of uorescence King–Chichibabin method,7a we obtained compound 3b in
quantum yields have not been reported.8
almost 90% yield, while analogous 3c was obtained in a
To date, only limited success has been achieved in optimizing moderate yield of 44%. The triethylene glycol group was intro-
the luminescence properties of imidazo[1,2-a]pyridines, particu- duced to improve the solubility of the nal dyes in more polar
larly in polar solvents. A recent report based on combinatorial solvents.
synthesis and screening showed that a derivative of the imidazo
Imidazo[1,2-a]pyridines 3a–c were easily converted into
[1,2-a]pyridine drug Zolpidem can lead to luminescence quantum bromo-derivatives 4a–c using well established general proce-
yields ranging from f ¼ 0.11 to f ¼ 0.89 in dimethylsulfoxide dures.12b,d,g The bromination took place at room temperature
solutions.9 Substitution at the C2 position proved to be a valid and the product precipitated upon completion of the reaction.
strategy for increasing the luminescence quantum yield. A f ¼ Consequently, products 4a–c, were pure enough and were
0.83 in ethanol solutions was obtained by substitution with long obtained in high yields. The terminal alkynes 6a–c were
chain arylpiperazines.10 We3 and others4c have reported that prepared following a typical pathway, i.e., the Sonogashira
introduction of various aryl substituents at C2 of imidazo[1,2-a]- reaction with 2-methylbut-3-yn-2-ol, to obtain the alkynols 5a–c
pyridines improved their luminescence properties up to f ¼ 0.7–
0.8. Substitution on the six membered ring was less effective on the
uorescence properties, whereas extension of the delocalization
increased the luminescence quantum yields in toluene solutions.3
Some preliminary experimental results on non-linear second
harmonic generation properties of chromophores involving
imidazo[1,2-a]pyridines have been described in the literature.11
Later, we reported3 the rst quadrupolar systems which con-
tained two imidazo[1,2-a]pyridines moieties connected by one
electron withdrawing unit. These compounds displayed
reasonable uorescence yields of f ¼ 0.3 in toluene and s2 of
the order of 102 GM.
The aim of this study was to increase both f and s2 in
Scheme 1 Synthesis of imidazo[1,2-a]pyridine 3a.
quadrupolar systems based on imidazo[1,2-a]pyridines. Imidazo-
[1,2-a]pyridines possess the highest electron-density at C3 and
this is usually the site of attack in electrophilic aromatic sub-
stitution.4b,12 We envisioned that attaching the acceptor core at
the C3 position would lead to compounds with favorable optical
properties. Herein, we report the synthesis, spectroscopic and
photophysical properties, as well as the two-photon absorption
characteristics of a series of compounds comprising ve quad-
rupolar structures formed by two differently substituted imidazo
[1,2-a]pyridine moieties (D) connected by either a pyrazine or a
1,4-dicyano benzene group (A) to construct a D–A–D structure.
Results and discussion
Design and synthesis
We hypothesized that attachment of an electron-acceptor to the
most electron-rich position of the imidazo[1,2-a]pyridine would Scheme 2 Synthesis of imidazo[1,2-a]pyridines.
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Scheme 3 Preparation of terminal alkynes.
followed by deprotection under basic conditions (Scheme 3).
Although the removal of the protecting group required
extremely dry toluene and harsher conditions compared to silyl-
based protecting groups, the clear advantage was the substan-
tial polarity difference between products and substrates.14 This
polarity difference facilitated the removal of the unreacted
starting material during the purication process.
Scheme
thalonitrile core.
4
Synthesis of D–A–D systems possessing the tereph-
Initially, 2,5-dibromoterephthalonitrile (7) was used as the
starting material for the nal double Sonogashira coupling
(Scheme 4). Compound 7 was a very reactive starting material reactant was transformed into the mono-substituted product.
for Sonogashira coupling, and the addition of CuI (as co-cata- In the case of bis-imidazo[1,2-a]pyridines, the product is usually
lyst) could oen be omitted. It is worth noting that Sonogashira well soluble in dichloromethane and CHCl3 and poorly soluble
coupling without CuI was very favorable, since the formation of in hexane/EtOAc. Purication was based on the latter's solu-
the Glaser coupling by-product had ceased. Following this bility property and impurities were removed by trituration.
procedure, D–A–D quadrupoles 8b and 8c were synthesized in
Pyrazine was chosen as an alternative electron-withdrawing
high yields.
core. 2-Bromo-5-iodopyrazine (9), the key building block, was
The phenyl substituted imidazo[1,2-a]pyridine 6b was synthesized via a known procedure.15 Electron-deciency of
transformed into product 8b in 75% yield, making the overall compound 9 facilitated the Sonogashira coupling without CuI.
yield for this ve steps process ꢂ40%. Because dye 8b was We observed a decrease in yield as the size of the substituent
poorly soluble, we undertook the synthesis of its analogue 8c. increased. This result was consistent with the previous results
The double Sonogashira reaction starting from substrate 6c for compounds 8b and 8c. Steric hindrance also affected the
yielded quadrupole 8c in only a moderate yield. The presence of temperature required for reactivity of imidazo[1,2-a]pyridines.
a larger substituent on alkynes tends to make the coupling more Reactions with smaller substituents usually required lower
difficult due to steric effects. On the other hand, dye 8c temperatures to reach completion than did reactions with
possessed far better solubility compared to 8b, including in larger substituents (Scheme 5). Pyrazine derivatives 10a–c dis-
DMSO.
played better solubility in CHCl3 compared to 8b–c, which can
Pd-catalyzed coupling at high temperature is known to lead be attributed to the two electron pairs present in their cores. All
to debromination of products. Attempts to improve the yield by synthesized quadrupoles demonstrated high thermal resistance
reducing the temperature and prolonging the reaction time despite their low solubility. Decomposition only occurred at a
were not successful for 8c. As veried by 1H NMR, a part of the temperature above 240 C.
ꢃ
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The absorption spectra determined in TOL for the samples
are illustrated in Fig. 1 (spectra determined in DCM are pre-
sented in Fig. S1, ESI section†) and data are collected in Table 1.
Compounds 3a–c, without substitution at C3, displayed a single
broad band with a maximum at 320–340 nm. The intensity of
this band increased as the units intended to link the monomer
to the core structure were extended and the lmax become slightly
more red-shied (from 320 nm to 340 nm). The monomers 5a–c
and 6a–c displayed similar absorption spectra with a structured
band around 300 nm and a broader one at 340–360 nm. The
absorption spectra of the bis-imidazo[1,2-a]pyridines were
characterized by an intense band containing a shallow vibra-
tional structure at 480–490 nm for the 1,4-dicyanobenzene-
containing bis-imidazo[1,2-a]pyridines 8b,c and at 420–460 nm
for the structures possessing a pyrazine bridge, 10a–c. The
absorption spectra of the bis-imidazo[1,2-a]pyridines were
distinctly different from those of the component monomers,
which is a clear indication of an extended electronic delocal-
ization in the quadrupolar structures. The molar absorption
coefficients determined in TOL and DCM were almost identical
(Table 1). The low solubility of 8b prevented a careful determi-
nation of the molar absorption coefficient. Therefore the
absorption spectrum of 8b was normalized to that of the similar
compound 8c. For the spectra acquired in the more polar DCM,
we observed a hypsochromic shi of a few nanometers (2–4 for
monomers, 6–9 for bis-imidazo[1,2-a]pyridines) as compared to
spectra in TOL (Table 1).
Emission. The emission spectra of monomers 3a–c, 5a–c and
6a–c were acquired using 325 nm as the excitation wavelength.
The uorescence quantum yields were determined using
quinine sulfate as a standard. The emission spectra in TOL,
presented in Fig. 2, showed emission maxima in the 380–
420 nm region and displayed good mirror images with the
absorption spectra. The a series, with a methyl substituent at
position 2 of imidazo[1,2-a]pyridine, manifested the lowest
emission quantum yield (f ¼ 0.06–0.17) whereas the c series,
Scheme 5 Synthesis of D–A–D systems possessing a pyrazine core.
Linear optical properties
characterized by
a
4-(oligoethyleneglycol)phenyl (OTEG)
Absorption. Chart 1 shows the structures of the investigated substituent in the same position produced the highest yield
compounds. Prior to construction of the dimeric structures, we (0.17–0.23), Table 1. Within each a, b, and c series, the 3 family
prepared 3 monomers for every dimer, each with a different with no substituent at C3 exhibited the highest emission
substituent chosen to maximize the properties of solubility and quantum yields, ranging from 0.17 to 0.23. As expected,
photo or thermal stability. Family 3a–c had no substituent at compounds 5 and 6 of each series, differing only for the pres-
C3, family 5a–c had a tertiary alcohol linked to an alkyne group, ence of a tertiary alcohol at the alkyne substituent, have almost
and family 6a–c displayed an alkyne group.
the same properties (Table 1). The emission spectra of mono-
Monomers 6a–c exhibited excellent stability in toluene both mers in DCM are displayed in Fig. S2, ESI† section and the
under light and in the dark, whereas monomers 3a–c and in corresponding data are collected on Table 1. The band maxima
particular 5a–c were less stable under prolonged (3–20 hours) and emission quantum yield of each dye were unaffected by the
irradiation under laboratory light. Bis-imidazo[1,2-a]pyridines solvent polarity, within experimental error.
10a–c and 8b,c in toluene proved non perfectly stable aer a few
Fluorescence lifetimes of 1–2 ns were determined for the
hours of light exposure. In dichloromethane, all monomeric entire library of compounds, (with the exception of sample 3a
samples as well as the corresponding bis-imidazo[1,2-a]pyri- which had a s > 6 ns) and found to be independent of solvent
dines 10a–c and 8b,c showed excellent photo and thermal polarity (Table 1). A krad (krad ¼ f/s) in the order of 107 to 108
stability over hours or days. The photophysical properties of the sꢁ1 was calculated for the monomers. The measured lifetimes
samples 8b,c and 10a–c were examined in two solvents of are in agreement with those of similar imidazole-based
different polarities, toluene (TOL, 3 ¼ 2.38) and dichloro- compounds reported in the literature.3,7b,16 The emitting singlet
methane (DCM, 3 ¼ 8.9) and compared to those of the indi- states were tentatively identied as p–p* in nature, as based on
vidual imidazopyridine components 3a–c, 5a–c and 6a–c.
the molar absorption coefficients of the corresponding
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Chart 1 Structures of the investigated compounds.
Fig. 1 Absorption spectra of the studied compounds in TOL solutions.
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Table 1 Photophysical parameters in TOL and DCM measured at 295 K. Monomers are in italics
TOL
max/Mꢁ1 cmꢁ1
DCM
max/Mꢁ1 cmꢁ1
3
3
(lmax/nm)
lmemax/nm
f
s/ns
(lmax/nm)
lmemax/nm
f
s/ns
3a
5a
2000 (320)
8800 (306)
2300 (333)
11 000 (304)
3100 (331)
11 800 (335)
44 600 (412)
46 600 (431)
5700 (327)
6000 (338)
3700 (354)
9000 (313)
4900 (343)
5100 (356)
3400 (374)
6800 (311)
4300 (338)
4600 (351)
3000 (368)
n.d.
402
423
0.17
0.06
6.33
3.20
2400 (317)
9500 (304)
2500 (330)
8700 (302)
2500 (327)
13 000 (330)
46 700 (410)
47 000 (424)
6300 (324)
6500 (333)
4000 (350)
9500 (311)
5000 (340)
5100 (352)
3100 (371)
7200 (309)
4800 (336)
5000 (348)
3000 (367)
n.d.
400
422
0.20
0.07
6.56
3.16
6a
419
0.06
0.69
2.91
1.38
418
0.07
0.35
2.80
1.35
10a
455; 477
470; 490
3b
5b
379; 401; 416
401; 423; 444
0.18
0.13
2.40
2.63
374; 394; 415
400; 420; 442
0.19
0.13
2.15
2.48
6b
396; 416; 440
0.15
2.86
394; 413; 433
0.15
2.67
8b
10b
507; 541
473; 500
0.92
0.93
2.31
1.49
507; 540
480; 503
0.83
0.88
2.66
1.81
44 700 (428)
46 700 (448)
8000 (329)
9000 (342)
6200 (358)
10 700 (305)
5900 (345)
6400 (359)
3900 (379)
12 000 (305)
6300 (342)
6900 (355)
4700 (371)
32 000 (339)
21 800 (384)
38 100 (462)
56 700 (490)
38 100 (318)
47 300 (439)
51 300 (459)
48 400 (423)
47 800 (440)
8300 (324)
9500 (336)
6400 (354)
10 900 (303)
6400 (343)
6700 (355)
3900 (376)
11 800 (303)
6900 (338)
7300 (351)
4800 (370)
32 800 (333)
23 200 (381)
40 100 (459)
55 700 (484)
40 000 (310)
45 800 (432)
46 800 (450)
3c
5c
385; 402; 417
408; 426; 448
0.23
0.17
2.24
2.55
384; 403; 416
405; 424; 446
0.24
0.18
1.98
2.41
6c
399; 420; 438
525; 562
0.19
0.84
0.87
2.75
2.67
1.73
398; 419; 436
529; 563
0.19
0.80
0.82
2.54
3.25
2.27
8c
10c
489; 517
498; 525
absorption bands, on the lack of effect of solvent polarity on the assignment for the monomers, the uorescence was ascribed to
energy and quantum yields of the uorescence emission, and the transition from the lowest p–p* singlet excited state. A
on the large emission values.
closer examination of the spectra in more polar solvents
The bis-imidazo[1,2-a]pyridines 10a–c and 8b,c were excited revealed the effects of charge transfer (CT). The spectra
at 424 nm and the emission quantum yields were measured measured in the more polar DCM were broader and the lumi-
using coumarin 153 as the reference compound. The results are nescence quantum yield lower, but generally by less than 10%.
summarized in Table 1 for both solvents and the spectra in TOL The exception is for the less substituted 10a structure, which
are shown in Fig. 3. Fig. S3 (ESI†) shows the spectra of the displayed a 50% decrease in the emission quantum yield from
dimeric structures in DCM. The emission bands were located at TOL to DCM (Table 1). In the case of samples 10a–c (containing
470–520 nm for the bis-imidazo[1,2-a]pyridines containing the pyrazine) a slight bathochromic shi occurred in DCM, a
pyrazine unit and at 510–570 for bis-imidazo[1,2-a]pyridines conrmation of some intramolecular CT character. The
containing the dicyanobenzene unit. The emission quantum extended conjugation in the other bis-imidazo[1,2-a]pyridines
yields were extremely high, up to 0.93 (Table 1). The emission 8b,c stabilized the p–p* excited state and weakened the CT
spectra displayed a good mirror image with the absorption character, making their luminescence less affected by the
spectra, reproducing the vibrational structures and, as in the solvent polarity (Table 1). Within the same solvent,
a
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Fig. 2 Fluorescence spectra of optically matched solutions of reference monomers in TOL at room temperature. Excitation wavelength was 325
nm and A325 nm ¼ 0.07.
Fig. 3 Fluorescence spectra of optically matched solutions of the bis-imidazo[1,2-a]pyridines 10a–c and 8b,c in TOL at room temperature.
Excitation wavelength was 424 nm and A424 nm ¼ 0.07 for 10a–c and A424 nm ¼ 0.03 for 8b,c.
bathochromic shi as the size of the substituent increased from
series a to b and c, was evident. The uorescence lifetimes
ranged from 1 to 3 ns (Fig. 4) leading to a calculated krad > 108
sꢁ1. To the best of our knowledge, the present samples are the
rst to exhibit such remarkable luminescence properties for
quadrupolar systems based on imidazo[1,2-a]pyridine. Excita-
tion spectra in TOL and DCM measured at the emission maxima
of both monomers and bis-imidazo[1,2-a]pyridines were
perfectly superimposable on the corresponding absorption
spectra, Fig. S4,† indicating a genuine emission from the lowest
excited state and the absence of contaminants.
Fig. 4 Fluorescence decay data of 8b and 10b together with the
corresponding fitting curve (grey) and the excitation profile, in TOL.
Compounds 8c and 10c could also be dissolved in dimethyl
sulfoxide (DMSO) and pure water thanks to the presence of the
OTEG side chain. Interestingly, in these environments, the
emission is signicantly red-shied (with emission maxima
located at 655 nm and 556 nm in H2O for compounds 8c and Consequently, their uorescence quantum yield decreases in
10c respectively). This is indicative of signicant symmetry those polar environments (see Table S1†), leading to a uores-
breaking and related charge transfer in the excited state (as cence quantum yield of 13% for the more hydrophilic dye 10c in
already reported in more conventional quadrupoles).17 pure water and of 4% for 8c.
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Emission in solid matrices at 77 K. For insight into the
Journal of Materials Chemistry C
Table 2 Photophysical parameters in TOL measured at 77 K. Delayed
luminescence maxima of bis-imidazo[1,2-a]pyridines derived from
spectra in TOL with 50% EtI. Monomers are in italics
behavior of the samples in solid matrices, we performed steady
state and time resolved experiments in a solid TOL matrix at 77 K.
This experiment also provided useful information of the triplet
excited state of the examined samples. The registered uorescence
spectra for the monomers 3a–c, 5a–c and 6a–c in TOL are reported
in Fig. 5 and S5 (ESI section†) and the derived parameters are
collected in Table 2. The monomers maintained the spectral
features of the uorescence observed at room temperature, but
their maxima underwent slight hypsochromic shis. Fluorescence
lifetimes of a few nanoseconds, almost identical to those at room
temperature, were measured. Intense and structured phospho-
rescence spectra were recorded between 450 and 650 nm when
delayed spectra were registered (see the Experimental section for
details) Fig. 5, S5† and Table 2. The phosphorescence lifetime was
of the order of seconds, see the ESI section, Fig. S6.†
In some cases, (5a, 3c, 5c, 6c etc.), delayed uorescence was
also detected, with an identical spectral shape to normal uo-
rescence, but a much longer lifetime. We ascribed the delayed
uorescence as a P-type, based on the lifetime. P-type delayed
uorescence is the product of a triplet–triplet reaction and, as
such, its lifetime is half of that of phosphorescence, Fig. S6.†
At 77 K, the bis-imidazo[1,2-a]pyridines displayed a uores-
cence either more resolved than at room temperature or with a
slight hypsochromic shi. The delayed spectra indicated no
clear trace of phosphorescence (data not shown). This result
was expected since the triplet yield formation in bis-imidazo
[1,2-a]pyridines is very low due to the extremely high radiative
decay of the singlet excited state, krad > 108 sꢁ1. Therefore, in
order to detect the phosphorescence and unambiguously derive
the triplet energy level, we utilized the heavy atom effect with
50% ethyl iodide (EtI), which enhances spin–orbit coupling and
hence increases the triplet yield (Fig. 6 and S7†). With the
addition of EtI, well-dened structured phosphorescence
spectra above 550 nm were recorded. The relevant photo-
physical parameters are presented in Table 2. Notably, the
presence of EtI not only enhanced the excited singlet to excited
triplet conversion, but also helped the transition of the excited
triplet to the ground state singlet. Hence, the phosphorescence
lifetimes in the presence of EtI do not represent the normal
TOL
State
lmemax/nm
s/ns
E/eVa
3a
5a
6a
10a
3b
5b
6b
8b
10b
3c
13a
33a
15a
35a
16a
36a
110a
310a
13b
33b
15b
35b
16b
36b
18b
38b
110b
310b
13c
381
5.50
3.25
2.64
3.14
2.59
3.14
2.64
2.72
2.02
3.38
2.60
3.17
2.59
3.25
2.60
2.52
2.23
2.62
2.12
3.37
2.60
3.14
2.57
3.25
2.65
2.43
2.09
2.54
2.24
469; 500; 527
395; 422; 452
479; 509; 549
394
469; 500; 540
455; 485
0.98 ꢀ 109
3.61
1.08 ꢀ 109
3.10
1.31 ꢀ 109
2.36
612; 669
—
366; 384; 404
476; 508; 541
391; 413; 432
479; 511; 550
381; 400; 420
476; 505; 538
492; 524
2.58
1.02 ꢀ 109
4.08
1.09 ꢀ 109
4.27
1.29 ꢀ 109
2.30
554; 595; 638
473; 504
—
1.42
—
585; 626; 680
367; 385; 404
476; 507; 533
394; 418; 442
481; 510; 549
381; 401; 424
467; 501; 543
509; 541
2.31
33c
0.85 ꢀ 109
3.40
5c
15c
35c
0.96 ꢀ 109
3.65
6c
16c
36c
1.11 ꢀ 109
8c
18c
2.60
38c
592; 650; 707
487; 517
553; 592; 635
—
1.70
—
10c
110c
310c
a
E0–0 from the maxima of the luminescence spectra at 77 K.
Table 2. Again, as observed for the monomers, a delayed uo-
rescence was detected in almost all dimers. The 77 K experi-
ments in DCM solutions produced results identical to those
reported for TOL (data not shown).
Two-photon absorption
triplet lifetimes and therefore, for these cases, the triplet life- The two-photon absorption (2PA) cross-sections of bis-imidazo-
times of the bis-imidazo[1,2-a]pyridines are not reported in [1,2-a]pyridines 8b,c and 10a–c were determined in solution
Fig. 5 Prompt and delayed luminescence of 3b and 6c in TOL at room temperature and 77 K, upon excitation at 325 nm. The scale of lumi-
nescence intensity is arbitrary. Other data can be found in the ESI section.†
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Fig. 6 RT luminescence in TOL and prompt and delayed luminescence at 77 K of 8b and 10b in TOL with 50% EtI, upon excitation at 424 nm. The
scale of luminescence intensity is arbitrary. Other data can be found in the ESI section.†
using the two-photon induced uorescence (TPE) method-
ology.18 The measurements were conducted in the 700–1000 nm
range (corresponding to the relevant biological spectral
window) (Table 3). As illustrated in Fig. 7 for compound 8b, the
lowest (strongly one-photon allowed) excited state is almost
silent, a higher (only weakly one-photon allowed) state is
responsible for the large 2PA response. This feature, common to
all bis-imidazo[1,2-a]pyridines compounds 8b,c and 10a–c
(Fig. S8, ESI section†), was consistent with the behavior of
symmetrical quadrupolar derivatives.1f,19
Fig. 8 Two-photon absorption spectra of bis-imidazo[1,2-a]pyridines
8b,c.
Dimeric symmetrical compounds 8b,c constructed from a
(electron-withdrawing) dicyanobenzene core exhibited an
intense 2PA band located around 800 nm (Fig. 8). The 2PA
maxima were almost unaffected by the presence of the solubi-
lizing OTEG side chains on the phenyl substituents of the
imidazo[1,2-a]pyridine terminal moieties whereas a red-shi of
the 2PA band was observed (30 nm). These factors contributed
to a maximum 2PA cross-section of about 800 GM at 800 nm,
resulting in a large two-photon brightness (s2f ¼ 640 GM) for
the unconventional symmetrical “quadrupolar-like” derivative
8c.
Related dimeric symmetrical compounds 10a–c constructed
from a pyrazine core showed an intense 2PA band located in the
700–750 nm spectral region (Fig. 9). As observed with
compounds 8b,c, the nature of the substituents on the imidazo
[1,2-a]pyridine terminal moieties affected the position of the
2PA band (as the electron donating strength increased, the peak
became more red shied) whereas the peak 2PA cross-sections
were similar with a two-photon absorption maximum cross-
section in the 550–650 GM range, resulting in two-photon
brightness values of 450–550 GM (Table 3). Comparison of the
2PA responses of derivatives 8b,c and 10b,c indicated that the
dicyanobenzene-core derivatives displayed 2PA bands that were
more intense and more red-shied (allowing deeper
Table 3 Two-photon absorption properties of bis-imidazo[1,2-a]-
pyridines in DCM
2lmOPaAx/nm
lmTPaAx/nm
sm2 ax/GM
sm2 axf/GM
8b
8c
10a
10b
10c
942, 748
968, 762
848
880
900, 620
770
800
#700
730
880
800
$500
640
730
640
>180
560
750
550
450
Fig. 7 Two-photon absorption spectra of bis-imidazo[1,2-a]pyridine
8b (full circles) compared to rescaled one-photon absorption spectra Fig. 9 Two-photon absorption spectra of bis-imidazo[1,2-a]pyridines
(solid line).
10a–c.
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Journal of Materials Chemistry C
penetration depth in scattering media) than their pyrazine- was done by means of column chromatography with silica ash
cored analogues. However, the hydrophilic pyrazine core P60 (40–63 mm, SiliCycle) or aluminum oxide 90 (neutral, 70–
structures may prove more promising for bioimaging purposes. 230 mesh, Merck), and dry column vacuum chromatography
The poor solubility of the OTEG substituted samples in water (DCVC) with silica (MN-Kieselgel P/UV254) or aluminum oxide
prevents the 2PA absorption spectra determination. However, (MN-aluminiumoxid G).
taking into account the uorescence quantum yield, a two-
Identity and purity of prepared compounds. Compound
photon brightness of about 80 GM can be estimated for the structures were proved by H NMR and 13C NMR (Varian 500/
hydrophilic two-photon dye 10c in this solvent, whereas 200 MHz). High-resolution mass spectra (EI HRMS, ESI HRMS,
compound 8c shows a two-photon brightness of about 30 GM. and FD HRMS) were obtained on a MaldiSYNAPT G2-S HDMS/
When compared with other D–A–D systems possessing a 1,4- GCT Premier, Waters. All melting points were measured with an
dicyanobenzene core,20 dyes 8b,c exhibited comparably high Ez-Melt, SRS and were given without correction.
uorescence quantum yields but higher two-photon absorption
1
cross-section values. They also showed bathochromically shif-
ted emission. Since the pyrazine core had never been utilized
Synthesis
before in the construction of two-photon active dyes, no direct
1-(4-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)phenyl)ethanone
comparisons of the optical behavior of compounds 10a–c were (2c).
1-(4-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)phenyl)etha-
possible. Still, they compare very favorably with quadrupolar none (2c) was synthesized via a typical protection procedure
dyes of similar complexity1i,m,o and are expected to be more from 40-hydroxyacetophenone. Most of the product was recrys-
biocompatible.
tallized from hexane : EtOAc prior to use for analysis, except for
bis-imidazopyridines (recrystallized or isolated by trituration
with CHCl3).
Conclusions
2-Methyl-6-(triuoromethyl)imidazo[1,2-a]pyridine (3a).
A
We have provided a synthesis route for intensely emitting two- mixture of 2-amino-5-(triuoromethyl)pyridine 1 (3.9 g, 24
photon uorophores and discovered that by utilizing the elec- mmol) and chloroacetone (1.7 mL, 20 mmol) in ethanol (20 mL)
tron-rich nature of imidazo[1,2-a]pyridines, it is possible to was heated under reux for 48 h. The ethanol was removed by
obtain a p-expanded ring system with superb two-photon evaporation to obtain a yellow oil. 5% Na2CO3(aq) was then
brightness. The p-expansion of the imidazo[1,2-a]pyridine added to the residual material, followed by direct extraction
skeleton at position C3 increased the uorescence quantum with water and dichloromethane. The oil was dried under
yield from <0.2 to ꢂ1.0. These D–A–D type bis-imidazo[1,2-a]- reduced pressure and cooled to obtain crystals which were then
pyridines are characterized by high absorbance in the visible recrystallized from hexane to afford 3a (0.89 g, 22%) as a off-
1
ꢃ
region (400–550 nm) with a molar absorption coefficient of 5 ꢀ white crystal, mp. 112.5 C. H NMR (500 MHz, CDCl3): d 8.40
ꢁ1
ꢁ1
104
M
cm
and intense uorescence in the wavelength (d, J ¼ 1 Hz, 1H), 7.59 (d, J ¼ 9.0 Hz, 1H), 7.43 (s, 1H), 7.26 (dd, J1
range 450–600 nm. In addition, the bis-imidazo[1,2-a]pyridines ¼ 9.5 Hz, J2 ¼ 1.5 Hz, 1H), 2.48 (s, 3H). Physicochemical prop-
showed signicant 2PA responses in the biological spectral erties concur with existing data.21
window. The dimers with the dicyanobenzene-core showed red-
Other imidazo[1,2-a]pyridine derivatives were prepared via
shied and enhanced 2PA bands as compared to their pyrazine- the Ortoleva–King–Chichibabin procedure7a with slight
core analogues. Linking two imidazo[1,2-a]pyridine moieties at modications.
position C3 offers signicant advantages over similar D–A–D
2-Phenyl-6-(triuoromethyl)imidazo[1,2-a]pyridine (3b). A
architectures linked at position C5.3 In particular, the uores- round-bottom pressure ask (250 mL) charged with 1 (1.6 g,
cence quantum yield is 2.5 times higher and the s2 is ꢂ10 times 10 mmol), acetophenone (0.6 mL, 5 mmol), and I2 (2.0 g, 8 mmol)
ꢃ
larger, permitting these quadrupolar compounds to display was heated at 100 C for 19 h. NaOH(aq) (50%, 50 mL) was then
signicant two-photon brightness (500–750 GM). Among these added and the mixture was reuxed for 1.5 h (the mixture must be
compounds, the dye which combines a hydrophilic core (pyr- homogeneous). Aer cooling to r.t. the mixture was dissolved in
azine) and substituents (OTEG) holds great promise as a two- CH2Cl2 and subsequently 300 mL H2O was added and the mixture
photon probe for bioimaging.
was extracted with CH2Cl2. The organic phase was dried over
anhydrous Na2SO4, ltered, and the solvent was removed. The
extract was used without further purication (purity proved by
NMR) and afforded 3b (1.17 g, 89%), as an off-white crystal, mp.
192.5 ꢃC. 1H NMR (200 MHz, CDCl3): d 8.48 (m, 1H), 7.98–7.93 (m,
Experimental section
General
Materials. All commercially available compounds were used 3H), 7.72 (dd, J1 ¼ 8 Hz, J2 ¼ 1.5 Hz, 1H), 7.50–7.29 (m, 4H).
as received. All solvents were dried and distilled prior to use. 2-(4-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)phenyl)-6-(tri-
Transformations and handling of oxygen sensitive compounds uoromethyl)imidazo[1,2-a]pyridine (3c). Prepared following
were performed under an argon atmosphere. The reaction the procedure for 3b from 1-(4-(2-(2-(2-methoxyethoxy)ethoxy)
progress was monitored by means of thin layer chromatography ethoxy)phenyl)ethanone 2c (2.82 g, 10 mmol). The mixture was
(TLC) which was performed on aluminium sheets, coated with puried by DCVC on SiO2 (CHCl3) to afford 3c (1.88 g, 44%), as
silica gel 60 F254 (Merck) or aluminium oxide 60 F254 (neutral an off-white amorphous solid, mp. 68–70 ꢃC. 1H NMR (500 MHz,
Merck) with detection by use of a UV-lamp. Product purication CDCl3): d 8.47 (s, 1H), 7.88–7.85 (m, 3H), 7.68 (d, J ¼ 9.5 Hz, 1H),
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7.29 (dd, J1 ¼ 9 Hz, J2 ¼ 2 Hz, 1H), 7.01–6.98 (m, 2H), 4.18 (t, J ¼ 140–142 ꢃC. 1H NMR (500 MHz, CDCl3): d 8.42 (s, 1H), 7.65 (d, J
5 Hz, 2H), 3.88 (t, J ¼ 5 Hz, 2H), 3.75 (m, 2H), 3.69 (m, 2H), 3.66 ¼ 9 Hz, 1H), 7.33 (dd, J1 ¼ 9 Hz, J2 ¼ 2 Hz, 1H), 2.52 (s, 3H), 1.72
(m, 2H), 3.55 (m, 2H), 3.37 (s, 3H). 13C NMR (126 MHz, CDCl3) d (s, 6H). 13C NMR (126 MHz, CDCl3) d 123.7, 121.4, 117.6, 107.0,
159.4, 147.8, 145.4, 127.6, 125.9, 124.5, 124.5, 120.5, 117.9, 66.1, 31.7, 14.3; HRMS (EI): m/z (M+) calcd for C14H13F3N2O:
115.1, 108.4, 72.1, 71.0, 70.8, 70.7, 69.9, 67.6, 59.2; HRMS (ESI): 282.0980; found: 282.0981.
m/z ([M + H]+) calcd for C21H23F3N2O4: 425.1688; found:
425.1685.
2-Methyl-4-(2-phenyl-6-(triuoromethyl)imidazo[1,2-a]pyri-
din-3-yl)but-3-yn-2-ol (5b). Compound 4b (1.36 g, 4 mmol),
3-Bromo-2-methyl-6-(triuoromethyl)imidazo[1,2-a]pyridine puried by DCVC on SiO2 (1% MeOH in CH2Cl2) afforded 5b
1
(4a). NBS (0.53 g, 3 mmol) was added to 3a (0.6 g, 3 mmol) in dry (1.26 g, 91%), as colorless crystals, mp. 120–121 ꢃC. H NMR
acetonitrile (1 mL). The mixture was stirred for 1 h at r.t., and (500 MHz, CDCl3): d 8.43 (s, 1H), 8.23–8.21 (m, 2H), 7.73 (d, J ¼ 9
the resulting solid was ltered, dried, and used without further Hz, 1H), 7.43–7.35 (m, 4H). 13C NMR (126 MHz, CDCl3) d 149.9,
purication to afford 4a (0.69 g, 83%), as an off-white amor- 144.7, 132.6, 129.3, 128.6, 127.3, 123.9, 123.9, 122.2, 118.1,
phous solid, mp. 86 ꢃC. 1H NMR (500 MHz, CDCl3): d 8.39 (dd, J1 107.5, 70.6, 66.1, 31.5; HRMS (EI): m/z (M+) calcd for
¼ 1 Hz, J2 ¼ 2 Hz, 1H), 7.63 (d, J1 ¼ 9.5 Hz, J2 ¼ 1 Hz, 1H), 7.35
C
19H15F3N2O: 344.1136; found: 344.1138.
4-(2-(4-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)phenyl)-6-(tri-
(dd, J1 ¼ 9.5 Hz, J2 ¼ 1.5 Hz, 1H), 7.28 (d, J ¼ 7.0 Hz, 1H), 2.49 (s,
3H). 13C NMR (126 MHz, CDCl3) d 145.1, 144.4, 122.8, 122.8, uoromethyl)imidazo[1,2-a]pyridin-3-yl)-2-methylbut-3-yn-2-ol
120.4, 117.8, 95.0, 13.8; HRMS (EI): m/z (M+) calcd for (5c). Compound 4c (0.76 g, 1.5 mmol), puried by DCVC on SiO2
C9H6BrF3N2: 277.9666; found: 277.9665.
(75% Et2O in hexanes) afforded 5c (0.75 g, 99%), as colorless
3-Bromo-2-phenyl-6-(triuoromethyl)imidazo[1,2-a]pyridine crystals, mp. 78–79 ꢃC. 1H NMR (500 MHz, CDCl3): d 8.46 (d, J ¼
(4b). 3-Bromo-2-phenyl-6-(triuoromethyl)imidazo[1,2-a]pyri- 1 Hz, 1H), 8.19–8.17 (m, 2H), 7.69 (d, J ¼ 9 Hz, 1H), 7.36 (dd, J1 ¼
dine (4b) was prepared following the procedure for 4a. 9.5 Hz, J2 ¼ 1.5 Hz, 1H), 6.99–6.96 (m, 2H), 4.18 (t, J ¼ 5 Hz, 2H),
Compound 3b (1.57 g, 6 mmol) afforded 4b (1.69 g, 82%) as an 3.88 (t, J ¼ 5 Hz, 2H), 3.75 (m, 2H), 3.69 (m, 2H), 3.66 (m, 2H),
off-white amorphous solid, mp. 154 ꢃC. 1H NMR (500 MHz, 3.55 (m, 2H), 3.38 (s, 3H), 1.74 (s, 6H). 13C NMR (126 MHz,
CDCl3): d 8.55 (s, 1H), 8.14–8.13 (m, 2H), 7.75 (d, J ¼ 9.5 Hz, 1H), CDCl3) d 159.7, 149.6, 144.7, 128.7, 125.5, 123.8, 123.7, 122.0,
7.52–7.49 (m, 2H), 7.45–7.41 (m, 2H). 13C NMR (126 MHz, 117.8, 114.7, 107.4, 72.1, 71.0, 70.8, 70.7, 69.8, 67.6, 66.1, 59.2,
CDCl3) d 145.4, 144.8, 132.2, 129.0, 128.8, 128.1, 123.3, 123.2, 31.5; HRMS (ESI): m/z ([M + H]+) calcd for C26H30F3N2O5:
121.2, 118.5, 117.9, 117.6, 93.5; HRMS (EI): m/z (M+) calcd for 507.2107; found: 507.2108.
C
14H8F3N2: 339.9823; found: 339.9830.
3-Bromo-2-(4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)-
6-(triuoromethyl)imidazo[1,2-a]pyridine (4c). 3-Bromo-2-(4-(2-
(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)-6-(triuoromethyl)-
General procedure for the synthesis of 3-ethynylimidazo[1,2-a]-
pyridine
imidazo[1,2-a]pyridine (4c) was prepared following the proce- A solution of alkynol (1 mmol) and NaOH (30 mg) in tolueneanh
ꢃ
dure for 4a. Compound 3c (1.06 g, 2.5 mmol) afforded 4c (1.09 g, (2 mL) was heated at 120 C under argon in an ACE pressure
86%) as an off-white amorphous solid, mp. 125–126 ꢃC. 1H NMR tube for 2 h. Then the mixture was evaporated with silica and
(500 MHz, CDCl3): d 8.52 (s, 1H), 8.01 (d, J ¼ 9 Hz, 2H), 7.71 (d, J puried as follows:
¼ 9.5 Hz, 1H), 7.40 (d, J ¼ 9 Hz, 1H), 7.04 (d, J ¼ 8.5 Hz, 2H), 4.20
3-Ethynyl-2-methyl-6-(triuoromethyl)imidazo[1,2-a]pyridine
(s, 2H), 3.95 (s, 2H), 3.76 (m, 2H), 3.70 (m, 2H), 3.66 (m, 2H), (6a). Compound 5a (283 mg, 1 mmol) and KOH (60 mg, 1.1
3.55 (s, 2H), 3.38 (s, 3H). 13C NMR (126 MHz, CDCl3) d 159.5, mmol) instead of NaOH, puried by DCVC on neutral
145.3, 144.6, 129.6, 124.9, 123.1, 121.0, 118.2, 117.4, 114.9, 92.7, aluminium oxide (1% MeOH in CH2Cl2) afforded 6a (145 mg,
72.1, 71.0, 70.8, 70.7, 69.8, 67.6, 59.2; HRMS (ESI): m/z ([M + H]+) 65%), as colorless crystals, mp. 109.5 ꢃC. H NMR (500 MHz,
1
calcd for C21H24BrF3N2O4: 504.0793; found: 504.0794.
CDCl3): d 8.54 (s, 1H), 7.64 (d, J ¼ 9 Hz, 1H), 7.38 (dd, J1 ¼ 9 Hz, J2
¼ 1.5 Hz, 1H), 3.97 (s, 1H), 2.52 (s, 3H). 13C NMR (126 MHz,
CDCl3) d 151.4, 144.6, 124.0, 124.0, 121.7, 117.7, 90.3, 70.9, 14.5;
HRMS (EI): m/z (M+) calcd for C11H7F3N2: 224.0561; found:
224.0566.
General procedure for the synthesis of 4-(imidazo[1,2-a]-
pyridin-3-yl)-2-methylbut-3-yn-2-ols (alkynols)
Pd(PPh3)2Cl2 (28 mg, 4 mol%) was added to the mixture of
3-Ethynyl-2-phenyl-6-(triuoromethyl)imidazo[1,2-a]pyridine
bromo-imidazopyridine (1 mmol) and 2-methylbut-3-yn-2-ol (6b). Compound 5b (345 mg, 1 mmol), puried by DCVC on SiO2
(0.2 mL, 2 mmol) in DMFanh (3 mL) and TEAanh (10 mL) under a (CH2Cl2) (228 mg, 79%) afforded 6b, as colorless crystals, mp.
continuous ow of argon. Aer bubbling through with argon for 160 ꢃC. 1H NMR (500 MHz, CDCl3): d 8.65 (m, 1H), 8.32–8.30 (m,
10–30 minutes, CuI (10 mg, 5 mol%) was added and the mixture 2H), 7.75 (d, J ¼ 9.5 Hz, 1H), 7.51–7.42 (m, 4H). 13C NMR (126
was heated at 80 ꢃC for 17 hours. Upon completion, the MHz, CDCl3) d 150.5, 144.9, 132.5, 129.4, 128.8, 127.5, 124.3,
resulting mixture was evaporated under reduced pressure with 124.2, 122.5, 118.3, 91.2, 72.3; HRMS (EI): m/z (M+) calcd for
celite/silica and puried as follows.
C16H9F3N2: 286.0718; found: 286.0726.
2-Methyl-4-(2-methyl-6-(triuoromethyl)imidazo[1,2-a]pyri-
3-Ethynyl-2-(4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)-
din-3-yl)but-3-yn-2-ol (5a). Compound 4a (0.56 g, 2 mmol), 6-(triuoromethyl)imidazo[1,2-a]pyridine (6c). Compound 5c
puried by DCVC on neutral aluminium oxide (1% MeOH in (634 mg, 1.25 mmol), puried by DCVC on SiO2 (1% MeOH in
CH2Cl2) afforded 5a (0.42 g, 74%), as colorless crystals, mp. CHCl3) afforded 6c (399 mg, 71%), as colorless crystals, mp.
4562 | J. Mater. Chem. C, 2014, 2, 4552–4565
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1
ꢃ
136.5–137 C. H NMR (500 MHz, CDCl3): d 8.62 (s, 1H), 8.26–
8.24 (m, 2H), 7.72 (d, J ¼ 9.5 Hz, 1H), 7.42 (dd, J1 ¼ 8 Hz, J2 ¼ 2
Hz, 1H), 7.04–7.02 (m, 2H), 4.21 (t, J ¼ 5 Hz, 2H), 4.14 (s, 1H),
3.89 (t, J ¼ 5 Hz, 2H), 3.76 (m, 2H), 3.70 (m, 2H), 3.67 (m, 2H),
3.55 (m, 2H), 3.38 (s, 3H). 13C NMR (126 MHz, CDCl3) d 159.9,
150.5, 144.9, 128.9, 125.4, 124.1, 124.1, 122.3, 117.9, 114.9,
104.3, 91.1, 72.6, 72.1, 71.0, 70.8, 70.7, 69.8, 67.6, 59.2; HRMS
(ESI): m/z ([M + H]+) calcd for C23H24F3N2O4: 449.1688; found:
449.1685.
2,5-Bis((2-phenyl-6-(triuoromethyl)imidazo[1,2-a]pyridin-3-yl)-
ethynyl)pyrazine (10b). Compound 6b (43 mg, 0.15 mmol) and 2-
bromo-5-iodopyrazine 9 (21.4 mg, 0.075 mmol) in 1.5 mL of TEA
were heated at 80 ꢃC for 16 h. The mixture was puried by DCVC
on silica (CHCl3) to afford 10b (30 mg, 56%), as a yellow amor-
phous solid; the compound decomposed at 315 ꢃC. 1H NMR (500
MHz, TFA): d 9.42 (s, 2H), 9.39 (s, 2H), 8.46 (d, J ¼ 9 Hz, 2H), 8.41
(d, J ¼ 9 Hz, 2H), 8.23 (dd, J1 ¼ 8 Hz, J2 ¼ 1 Hz, 4H), 7.88–7.83 (m,
6H). 13C NMR (126 MHz, CDCl3) d 147.0, 143.5, 140.3, 136.4, 133.4,
132.7, 129.9, 127.4, 123.1, 113.6, 106.0, 97.0, 81.4; HRMS (ESI): m/z
([M + H]+) calcd for C36H19F6N6: 649.1575; found: 649.1581.
2,5-Bis((2-(4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)-
6-(triuoromethyl)imidazo[1,2-a]pyridin-3-yl)ethynyl)pyrazine
(10c). Compound 6c (68 mg, 0.15 mmol) and 2-bromo-5-iodo-
pyrazine 9 (21.4 mg, 0.075 mmol) in 1.5 mL of TEA were heated
General procedure for the synthesis of bis-imidazo[1,2-a]-
pyridines
Pd(PPh3)2Cl2 (28 mg, 4 mol%) was added to a mixture of the
corresponding alkyne (1 mmol), dihaloarene (0.5 mmol), in
anhydrous DMF and anhydrous TEA under a continuous ow of
argon gas in a Schlenk ask. The resulting mixture was bubbled
through with argon gas for 30 min, closed, and heated. Then,
the mixture was evaporated with Celite and puried as follows:
2,5-Bis((2-phenyl-6-(triuoromethyl)imidazo[1,2-a]pyridin-3-
yl)ethynyl)terephthalonitrile (8b). Compound 6b (172 mg,
0.6 mmol) and 2,5-dibromoterephthalonitrile (83 mg, 0.29
mmol) in 8 mL of DMF : TEA (1 : 3) were heated at 50 ꢃC for 4 h.
The mixture was puried by DCVC on silica (1% MeOH in
CHCl3) to afford 8b (151 mg, 75%), as an orange amorphous
solid; the compound decomposed at 280 ꢃC. 1H NMR (500 MHz,
TFA): d 9.27 (s, 2H), 8.39 (d, J ¼ 9 Hz, 2H), 8.34 (s, 2H), 8.28 (d, J
¼ 9 Hz, 2H), 8.17 (d, J ¼ 6.5 Hz, 4H), 7.71–7.66 (m, 6H). 13C NMR
(126 MHz, CDCl3) d 144.8, 142.4, 138.5, 135.4, 134.4, 131.9,
ꢃ
at 80 C for 4.5 h. The mixture was puried by DCVC on silica
(CHCl3) to afford 10c (37 mg, 51%), as a yellow amorphous
solid; the compound decomposed at 246 ꢃC. 1H NMR (500 MHz,
CDCl3): d 8.84 (s, 2H), 8.74 (s, 2H), 8.31–8.29 (m, 4H), 7.78 (d, J ¼
9 Hz, 2H), 7.50 (dd, J1 ¼ 9 Hz, J2 ¼ 2 Hz, 2H), 7.09–7.07 (m, 4H),
4.23 (t, J ¼ 5 Hz, 4H), 3.91 (s, 4H), 3.77 (m, 4H), 3.71 (m, 4H),
3.67 (m, 4H), 3.56 (m, 4H), 3.38 (s, 6H). 13C NMR (126 MHz,
CDCl3) d 160.3, 152.4, 146.9, 145.9, 137.5, 129.2, 125.2, 124.4,
124.4, 123.3, 122.4, 118.3, 117.9, 104.0, 99.9, 84.1, 72.1, 71.1,
70.8, 70.7, 69.8, 67.7, 59.2; HRMS (ESI): m/z ([M + Na]+) calcd for
C
50H46F6N6O8Na: 995.3179; found: 995.3187.
Photophysics
129.7, 128.2, 125.4, 108.3, 99.2, 88.8; HRMS (EI): m/z (M+) calcd The solvents used were spectroscopic grade toluene and
for C40H18F6N6: 696.1497; found: 696.1507. dichloromethane (C. Erba). All samples were freshly prepared
2,5-Bis((2-(4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)- and carefully stored in the dark before the experiments. Ethyl
6-(triuoromethyl)imidazo[1,2-a]pyridin-3-yl)ethynyl)tereph- iodide was from Aldrich. Absorption spectra were recorded with
thalonitrile (8c). Compound 6c (68 mg, 0.15 mmol) and 2,5- a Perkin-Elmer Lambda 650 spectrophotometer and the emis-
dibromoterephthalonitrile 7 (21.5 mg, 0.075 mmol) in 1.5 mL of sion spectra, corrected for the photomultiplier (PMT) response
TEA were heated at 80 ꢃC for 4.5 h. The mixture was puried by if not otherwise stated, were detected by using an Edinburgh
DCVC on silica (1% MeOH in CHCl3) to afford 8c (20 mg, 26%), as FLSP920 uorometer equipped with a R928P Hamamatsu
an orange amorphous solid; the compound decomposed at 260 photomultiplier. Luminescence quantum yields of the samples,
ꢃC. 1H NMR (500 MHz, CDCl3): d 8.91 (s, 2H), 8.30–8.27 (m, 4H), fs, were evaluated against a standard with known emission
7.94 (s, 2H), 7.78 (d, J ¼ 9 Hz, 2H), 7.53 (dd, J1 ¼ 9.5 Hz, J2 ¼ 2 Hz, quantum yield fr by comparing areas under the corrected
2
2H), 7.11–7.09 (m, 4H), 4.25 (t, J ¼ 5 Hz, 4H), 3.92 (s, 4H), 3.78 (m, luminescence spectra by using the equation: fs/fr ¼ Arns
4H), 3.71 (m, 4H), 3.68 (m, 4H), 3.57 (m, 4H), 3.38 (s, 6H). 13C (area)s/Asnr2 (area)r, where A is the absorbance, n is the refractive
NMR (126 MHz, CDCl3) d 160.5, 152.9, 146.3, 134.7, 129.3, 125.8, index of the solvent employed and s and r stand for the sample
124.9, 124.7, 123.8, 118.2, 117.5, 116.3, 115.2, 103.8, 97.9, 89.5, and reference, respectively. The standard used for monomers
72.1, 71.1, 70.8, 70.7, 69.8, 67.8, 59.2; HRMS (ESI): m/z ([M + Na]+) was air-equilibrated quinine sulfate in 1 N H2SO4 with an
calcd for C54H46F6N6O8Na: 1043.3179; found: 1043.3180.
emission quantum yield f ¼ 0.546,22 whereas for bis-imidazo
2,5-Bis((2-methyl-6-(triuoromethyl)imidazo[1,2-a]pyridin- [1,2-a]pyridines, air-equilibrated coumarin 153 in ethanol with
3-yl)ethynyl)pyrazine (10a). Compound 6a (34 mg, 0.15 mmol) an emission quantum yield f ¼ 0.544 was used.23 Determina-
and 2-bromo-5-iodopyrazine (9, 21.4 mg, 0.075 mmol) in 1.5 mL tions at 77 K on glassy solutions made use of capillary quartz
of TEA were heated at 50 ꢃC for 16 h. The mixture was puried tubes dipped in a home-made quartz Dewar lled with liquid
by ash chromatography (CHCl3) to afford 10a (30 mg, 77%), as nitrogen and made use of the same Edinburg FLSP920 appa-
a yellow amorphous solid; the compound decomposed at 295 ratus. Phosphorescence spectra, corrected for the PMT
ꢃC. 1H NMR (500 MHz, CDCl3): d 8.82 (s, 2H), 8.67 (s, 2H), 7.71 response, were registered upon excitation with a microsecond
(d, J ¼ 9.5 Hz, 2H), 7.47 (d, J ¼ 9 Hz, 2H), 2.68 (s, 6H). 13C NMR Xenon ash lamp mF920H, aer a 1 ms delay and with a 50 ms
(126 MHz, CDCl3) d 153.4, 146.8, 145.6, 137.5, 124.3, 122.7, gate. Phosphorescence spectra of bis-imidazo[1,2-a]pyridines in
118.2, 118.0, 106.8, 99.4, 82.2, 14.9; HRMS (EI): m/z (M+) calcd glassy solvents with 50% ethyl iodide were registered upon
for C26H14F6N6: 524.1184; found 524.1187.
excitation with the same lamp, aer a 0.550 ms delay and with a
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 4552–4565 | 4563

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Journal of Materials Chemistry C
Paper
10 ms gate. The phosphorescence lifetimes in the millisecond–
second range were measured with the same instrument and
elaborated with standard curve tting soware procedures.
Fluorescence lifetimes in the nanosecond region were detected
by using a Time Correlated Single Photon Counting apparatus
(IBH) with excitation at 331 or 373 nm.
Estimated errors are 10% on molar extinction coefficients
and quantum yields and 2 nm on emission and absorption
maxima. The working temperature, if not otherwise specied, is
295 ꢄ 2 K.
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Two-photon absorption
Two-photon absorption cross-sections (s2) were determined
from the two-photon uorescence (TPEF) action cross-sections
(two-photon brightness, s2f) and the uorescence emission
quantum yield (f). TPEF action cross-sections were measured
relative to uorescein in 0.01 M aqueous NaOH in the 715–980
nm range,18,24 using the method described by Xu and Webb and
the appropriate solvent-related refractive index corrections.25
Reference values between 700 and 715 nm for uorescein were
taken from the literature.26 The quadratic dependence of the
uorescence intensity on the excitation power was checked at all
wavelengths. Measurements were conducted using an excitation
source delivering fs pulses. This procedure prevents excited-state
absorption during the pulse duration, a phenomenon which has
been shown to lead to overestimated two-photon absorption
cross-section values. To scan the 680–1080 nm range, a
Nd:YVO4-pumped Ti:sapphire oscillator was used generating
140 fs pulses at a 80 MHz rate. The excitation was focused into
the cuvette through a microscope objective (10ꢀ, NA 0.25). The
uorescence was detected in epiuorescence mode via a
dichroic mirror (Chroma 675dcxru) and a barrier lter (Chroma
e650sp-2p) by a using a compact CCD spectrometer module
BWTek BTC112E. Total uorescence intensities were obtained
by integrating the corrected emission. The experimental uncer-
tainty of the two-photon action cross-section values determined
from this method has been estimated to be ꢄ10%.
Acknowledgements
LF thanks the CNR of Italy (DP.04.010, Project MACOL) and
´
MBD gratefully acknowledges Conseil Regional d'Aquitaine for
nancial support (Chaire d'Accueil grant and fellowship to VH).
The research leading to these results has received partial
funding from the European Community's Seventh Framework
Programme under TOPBIO project-grant agreement no. 264362.
We thank Jillian Larsen (UC Riverside) for a preliminary reading
of the text.
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