Benzo[a]imidazo[5,1,2-cd]indolizines-a new class of molecules displaying excited state intramolecular proton transfer

Article abstract of DOI:10.1039/c3nj00842h

The new class of ESIPT-capable molecules, benzo[a]imidazo[5,1,2-cd] indolizines, bearing the 2-hydroxyphenyl substituent were prepared in a straightforward manner from imidazo[1,2-a]pyridines via a tandem [8+2]cycloaddition-[2+6+2]dehydrogenation reaction. The relationship between the structure and photophysical properties was thoroughly elucidated by comparison with simple analogues i.e. 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines. Compared with parent 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines the new chromophores strongly absorb blue light and emit in the yellow part of the spectrum. In contrast to 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines, emission of π-expanded dyes in protic solvents is usually weaker than in aprotic ones, where they exhibit large Stokes shifts (5500-7000 cm ^-1). Unlike in solution, in the solid state luminescence for benzo[a]imidazo[5,1,2-cd]indolizines is shifted on average by 100 nm relative to the fluorescence of the corresponding imidazo[1,2-a]pyridines, which results in a relatively strong (Φ_fl = 0.18 to 0.27) emission of the red light. Benzo[a]imidazo[5,1,2-cd]indolizines possessing a methoxy substituent have much higher fluorescence quantum yields than analogues bearing fluorine and methyl substituents. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014.

Full text of DOI:10.1039/c3nj00842h

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Benzo[a]imidazo[5,1,2-cd]indolizines – a new
class of molecules displaying excited state
intramolecular proton transfer†
Cite this: New J. Chem., 2014,
38, 189
Anton J. Stasyuk,ab Marzena Banasiewicz,c Barbara Ventura,d Michał K. Cyranski*b
´
and Daniel T. Gryko*a
The new class of ESIPT-capable molecules, benzo[a]imidazo[5,1,2-cd]indolizines, bearing the 2-hydroxyphenyl
substituent were prepared in a straightforward manner from imidazo[1,2-a]pyridines via a tandem
[8+2]cycloaddition–[2+6+2]dehydrogenation reaction. The relationship between the structure and
photophysical properties was thoroughly elucidated by comparison with simple analogues i.e. 2-(20-hydroxy-
phenyl)imidazo[1,2-a]pyridines. Compared with parent 2-(20-hydroxyphenyl)imidazo[1,2-a]pyridines the new
chromophores strongly absorb blue light and emit in the yellow part of the spectrum. In contrast to
2-(20-hydroxyphenyl)imidazo[1,2-a]pyridines, emission of p-expanded dyes in protic solvents is usually weaker
than in aprotic ones, where they exhibit large Stokes shifts (5500–7000 cmꢀ1). Unlike in solution, in the solid
state luminescence for benzo[a]imidazo[5,1,2-cd]indolizines is shifted on average by 100 nm relative to the
fluorescence of the corresponding imidazo[1,2-a]pyridines, which results in a relatively strong (Ffl = 0.18 to 0.27)
emission of the red light. Benzo[a]imidazo[5,1,2-cd]indolizines possessing a methoxy substituent have much
higher fluorescence quantum yields than analogues bearing fluorine and methyl substituents.
Received (in Montpellier, France)
26th July 2013,
Accepted 8th October 2013
DOI: 10.1039/c3nj00842h
www.rsc.org/njc
of this process is an enormous Stokes-shifted luminescence due
to the emission from the keto form. Complexity of ESIPT explains
Introduction
Organic fluorescent molecules undergoing excited state intra- why it depends on many, both internal (e.g., substituents) and
molecular proton transfer (ESIPT) have been important compo- external (e.g. proticity and polarity of the media), factors.
nents in the search for highly efficient organic (opto-)electronic
In spite of a rich body of research that has been performed
devices.1 Their unique photophysical properties are translated into on ESIPT compounds, there are still many unanswered questions
the characteristics of the bulk materials and are easily tailored by related to the proton transfer phenomenon. The vast majority
alteration of the substituents on the periphery of the p-frameworks. of studies performed within last the 60 years focused on the
With multiple applications spanning laser dyes,2 fluorescent solar small heterocyclic systems. This leaves an intriguing question
energy concentrators,3 chemosensors,4 photonics,5 bioimaging,6 unanswered – what is the influence of extension of the p-conjugated
molecular switches7 and UV-photostabilizers,8 they have attracted system on the optical properties of ESIPT dyes? A limited number of
attention from many vantage points. ESIPT is a phototautomer- papers published on this aspect10 revealed that the p-expansion of
ization process that occurs in the electronic excited state of a basic ESIPT-capable chromophores can lead to quenching of excited
molecule and it can be represented by a four-level photocycle state intermolecular proton transfer.10c Our earlier studies of
(En - En* - Ke* - Ke - En).9 The most significant characteristic this aspect led to the conclusion that in the case of p-expanded
benzo[h]quinoline analogues, the energetics of ESIPT depends
on the actual position of p-expansion.11 Park and co-workers1k
proved that the observed behavior concerning ESIPT of p-expanded
a Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3,
00-664 Warsaw, Poland. E-mail: dtgryko@icho.edu.pl; Fax: +48 22 6326681;
derivatives of imidazoles and related molecules can be explained by
Tel: +48 22 3433063
b Warsaw University, Faculty of Chemistry, 00-273 Warsaw, Poland.
considering the nodal pattern of the wave function along with the
delocalization of the electrons in the excited state (the so-called
nodal-plane model developed by Nagaoka).12
Fax: +48 22 6282741; Tel: +48 22 2345801
c
´
Institute of Physics of the Polish Academy of Sciences, Al. Lotnikow 32/46,
02-668 Warsaw, Poland. Fax: +48 22 8430926; Tel: +48 22 8436601
d Istituto per la Sintesi Organica e Fotoreattivita’ (ISOF), CNR, Via P. Gobetti 101,
40129 Bologna, Italy. Fax: +39 (0)51 639 98 44; Tel: +39 (0)51 639 98 12
† Electronic supplementary information (ESI) available: 1H NMR and 13C NMR
spectra of compounds 3c, 3d and 4a–d. See DOI: 10.1039/c3nj00842h
Among various ESIPT-capable molecules, 2-hydroxyphenyl
substituted imidazo[1,2-a]pyridines have attracted significant
˜
interest in recent years. Acuna and co-workers were first to
report the photoinduced intramolecular proton transfer in
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imidazopyridines.13 These authors later proposed and experi-
mentally verified the hypothesis about the switching ‘‘on–off’’
ESIPT phenomenon based on the involvement of molecules of
the polar, protogenic solvent in this process.14 Furthermore,
special attention has been given to the 2-hydroxy derivative of
2-phenylimidazo[1,2-a]pyridines as organic solid-state luminescent
material.15 Araki and co-workers have found that 2-(20-hydroxy-
phenyl)imidazo[1,2-a]pyridine is capable of forming different
polymorphs emitting at different wavelengths with different
quantum yields, which was the demonstration of novel design
concept for tunable organic luminescent solids. Later on deri-
vatives of 2-(20-hydroxyphenyl)imidazo[1,2-a]pyridine were used
Table 1 Results of tandem [8+2]cycloaddition–[2+6+2]dehydrogenation
reaction of 2-(20-hydroxyphenyl)imidazo[1,2-a]pyridine (3a) under various
conditions
Entry
Temperature (1C)
Yield of compound 4aa (%)
1
2
3
4
140b
125c
165c
180c
11
10
23
4
a
b
c
Isolated yields. Thermal activation. All reactions were performed
under the following constant conditions: solvent free, exposition time –
25 min, constant ratio of components.
for fabrication of colorless organic materials exhibiting white significant energy barrier (about 46 kcal/mol), which in turn is
luminescence.9c It should be noted that in the solid state, likely related to the large distortion upon going from the reactant
2-(20-hydroxyphenyl)imidazo[1,2-a]pyridine has
a fluorescence to the transition structure. Consequently, it can be argued that
quantum yield (F) of about 0.37–0.5015 (depending on the poly- the aromatization of the imidazole rings of [8+2] cycloadduct is
morph), which makes this class of compounds interesting not only the main driving force.
from a purely theoretical point of view, but also opens a door for
Our first attempts to perform the reaction between
possible practical applications. On the other hand in solutions in 2-(imidazo[1,2-a]pyridin-2-yl)phenols 3a–d and the benzyne
non-polar solvents, the typical fluorescence quantum yield for the precursor were based on the fact that numerous literature
band corresponding to the ESIPT process is only 0.01–0.06.16
sources indicate that 2-(trimethylsilyl)phenyl trifluoromethane-
One possible way to achieve the goal of expanding the p-electron sulfonate readily forms benzyne molecules at room tempera-
system is tandem [8+2]cycloaddition–[2+6+2]dehydrogenation ture or gentle warming in the presence of CsF in acetonitrile.
reactions.17 Cossio and co-workers reported that in the case of Bearing in mind the fact that starting imidazo[1,2-a]pyridines
imidazo[1,2-a]pyridines (not capable of ESIPT) final products are rather thermostable, we decided to perform this reaction
are aromatic, blue light emitting fluorescent molecules, whose under classical thermal conditions. Thus, substrate 3a was
fluorescence quantum yields are enhanced with respect to the mixed with 2-(trimethylsilyl)phenyl trifluoromethanesulfonate
starting bicyclic molecules.
in the presence of CsF and 18-crown-6 in a sealed tube and
Herein we describe the synthesis of several derivatives of placed in an oil bath, preheated to 140 1C (Tables 1 and 2). After
2-(benzo[a]imidazo[5,1,2-cd]indolizin-1-yl)phenols, capable of overnight stirring at the stated temperature we were able to
undergoing the ESIPT process, based on the tandem [8+2]- isolate the desired product, but the yield of 4a was only 11%
cycloaddition–[2+6+2]-dehydrogenation reactions of 2-(20-hydroxy- (Table 1, entry 1). In addition we isolated another non-polar
phenyl)-imidazo[1,2-a]pyridines with 2-(trimethylsilyl)phenyl compound in comparable quantities, which apparently is a
trifluoromethanesulfonate as a benzyne precursor. Also, in this product of trimerization of benzyne.
paper we present comprehensive optical data for these novel
functional dyes.
Indeed such trimerization is described in the literature when
2-(trimethylsilyl)phenyl trifluoromethanesulfonate is heated at
high temperatures, in the presence of cesium fluoride.19 Often
this reaction is carried out in the presence of a catalytic amount
of palladium or copper salts with continued – about 12–15 hours –
stirring or heating (depending on the used catalyst). Since the
Results and discussion
Compounds 3a–d were prepared from appropriate amounts of time of contact in the case of this reaction is crucial, we have
acetophenones 2a–d and 2-aminopyridine (1) via an Ortoleva– decided to revert to the microwave irradiation as described by
King reaction followed by Chichibabin ring closure.16a,18 The Cossio and co-workers. In the following series of experiments,
corresponding imidazo[1,2-a]pyridines were subsequently the reaction between 3a and the benzyne precursor was carried
reacted with a benzyne precursor in the presence of cesium out under microwave-promoted conditions. As major operating
fluoride and an 18-crown-6 ether (Table 1).
parameters, we selected temperature and exposure times as the
It was shown by Aginagalde et al. that the reaction between most easily and accurately controlled (in this case the microwave
unsubstituted imidazo[1,2-a]pyridine and 2-(trimethylsilyl)phenyl oven operated in dynamically adjusted power mode to maintain
trifluoromethanesulfonate can be carried out only under micro- the set temperature). Reactions were carried out without solvent.
wave-promoted conditions. At the same time, the reaction Thus, we have found that at temperatures above 175 1C amount
performed under the classic heating conditions led to a vanishingly of side-product substantially increases and the isolated yield
small amount of the corresponding benzo[a]imidazo[5,1,2-cd]- of target product 4a was only several percent. On the other
indolizine. The mechanism of this reaction by quantum chemistry hand, at temperatures below 130 1C yields of compound 4a
methods was studied in detail. It was shown that despite the fact were about 10%. Eventually, we determined the temperature
that, in general, this process is barrierless in terms of total energy, range of 150–160 1C as the optimal one. The reaction of 2-(20-
the latter reaction (aromatization of [8+2] cycloadduct) has a hydroxyphenyl)imidazo[1,2-a]pyridine with 2-(trimethylsilyl)phenyl
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Table 2 Results of the synthesis of benzo[a]imidazo[5,1,2-cd]indolizines
Keton
R
Imidazo[1,2-a]pyridine
Yield (%)
54
Benzo[a]imidazo[5,1,2-cd]indolizine
Yield (%)
23
2a
3a
4a
2b
3b
61
4b
21
2c
3c
49
47
4c
19
21
2d
3d
4d
trifluoromethanesulfonate at this temperature for 25 minutes from the hydroxyl group – thus enhancing its basicity. The above
resulted in the formation of compound 4a in 23% yield. Having processes should facilitate the ESIPT.
this procedure in hand three analogous benzoimidazopyridines
At the same time, one needs to keep in mind the necessary
4b–d, possessing additional functionalities on phenyl substituents, requirement of the planarity of the system, which provides the
were synthesized in 19–21% yield (Table 2). The choice of sub- requisite spatial arrangement of the groups acting as a donor
stituents was based on the desire to modulate the optical properties and acceptor of the proton. Earlier Araki and co-workers
via the strength of the hydrogen bond between phenolic OH and showed that in the 2-(imidazo[1,2-a]pyridin-2-yl)phenol deviation
basic nitrogen atoms.
of the system from planarity is from 1 to 5.8 degrees depending on
It has been widely accepted that the driving force for the the polymorphic state.15a Later the same authors also showed16b
ESIPT reaction stems from the significant difference between that the quantum chemical calculations carried out at the
the acidity and basicity of the involved groups;20 at the same RHF/6-31G(d) level of theory for the free molecules are in good
time for most of the ESIPT molecules a strong intramolecular agreement with the previously obtained crystallographic data. For
hydrogen bond has been detected. The presence of this hydrogen 4-methoxyphenyl substituted benzo[a]imidazo[5,1,2-cd]indolizine,
bond results in the large downfield shift (d > 10 ppm) of the –OH Aginagalde et al. determined that deviations from planarity were
protons in the 1H NMR spectrum. Thus, comparison of positions about 4 degrees for the mutual arrangement of the methoxy-
of the maxima for protons of the hydroxyl group in the NMR phenyl moiety relative to the benzo[a]imidazo[5,1,2-cd]indolizine
spectrum brings out a significant shift of these bands to a weak and 5.7 degrees for the phenyl part relative to the imidazo[1,2-a]-
field side. Depending on the particular compound this shift pyridine part in the benzo[a]imidazo[5,1,2-cd]indolizine core.
range from 0.58 to 0.79 ppm, which in turn qualitatively indicates The above evidence strongly suggests that in the case of 2-(benzo[a]-
an increase in the strength of the hydrogen bond. The maximum imidazo[5,1,2-cd]indolizin-1-yl)phenol deviations from planar-
value of chemical shift, as well as differences in the chemical ity should be comparable or smaller due to the presence of an
shift values for the structures 3 and 4 was observed for the intramolecular hydrogen bond. That in turn, should provide
methoxy substituted compound. MeO obviously acts as an the possibility to induce an additional red-shift of the absorp-
electron-donating group with a lone pair on the atom adjacent tion spectra via influence of methoxy groups on the nitrogen
to the p-system. The methoxy group through a resonance atom by the resonance donating effect.
donating effect increases the electron density on the benzene
Subsequently, we examined the optical properties of dyes
ring at the ortho- and para-positions, thereby increasing the 4a–d by comparing them with the photophysics of 3a–d
partial positive charge on the carbon atom, which are directly (Table 3, Fig. 1–3).
connected to the hydroxyl group. That eventually leads to an
The photophysical properties of targeted p-expanded chromo-
increase in the acidity of the hydroxyl group. On the other hand phores can be compared to both 2-(2-hydroxyphenyl)imidazo[1,2-a]-
the electron donating effect of the methoxy group via resonance pyridines13–16 as well as to benzo[a]imidazo[5,1,2-cd]indolizines.17
may directly act on the nitrogen atom, which accepts a proton In analogy to data revealed by Cossio and co-workers, the
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Table 3 Spectroscopic properties of compounds 3a–d and 4a–d
Toluene
Dichloromethane
Acetonitrile
Methanol
labs
nm
/
lem
nm
/
Dnss/
labs
nm
/
lem
nm
/
Dnss/
labs
nm
/
lem
nm
/
Dnss/
labs
nm
/
lem
nm
/
Dnss/
Fa
cmꢀ1
Fa
0.005
cmꢀ1
Fa
0.003
0.006
0.001
0.002
0.006
cmꢀ1
Fa
cmꢀ1
3800
3ab
3bb
3c
351
340
337
338
427
431
428
429
573
560
582
601
593
567
604
615
0.011
11 000
11 600
12 500
13 000
6600
345
334
333
333
424
428
425
426
573
572
11 500
12 500
328
330
330
330
420
424
422
422
389
577
385
579
377
4200
13 150
4300
13 000
3800
328
331
329
329
420
424
421
421
375
385
372
375
422
422
422
423
0.084
0.036
0.008
0.003
0.060
0.108
0.037
0.015
0.010
0.124
0.201
0.162
0.017
0.012
0.035
0.023
4300
3500
3700
110
110
60
375
587
376
597
587
0.002
o0.001
o0.001
o0.001
0.041
3400
13 000
3500
13 000
6600
3d
4a
379
0.004
3900
435
588
435
560
430
595
432
603
o0.001
0.003
0.001
0.004
0.002
0.001
0.002
o0.001
800
6800
600
5700
450
6900
600
7100
4b
4c
5600
561
599
612
0.084
0.024
0.009
5600
6900
7200
6800
4d
7100
110
a
b
Determined using quinine sulphate in 0.5 M H2SO4 as a standard. Data taken from ref. 16a.
Fig. 1 Absorption and fluorescence spectra of 3d (dashed line) and 4d
(solid line) in toluene. Excitation at 330 nm (3d) and 400 nm (4d)
respectively.
Fig. 3 Absorption and fluorescence spectra of 4d in MeOH (dashed line)
and in toluene (solid line). Excitation at 400 nm.
maxima are shifted bathochromically only by B20 nm. This
directly results in a decrease of Stokes shift from B13000 cmꢀ1
to 5500–7000 cmꢀ1
.
The emission spectra of all of the studied compounds
strongly depend on the nature of the solvent. Thus, in non-
polar toluene the band shapes are structureless with a large
Stokes shift. We also observed a slight increase in the value of
the Stokes shift in the transition to more polar aprotic solvents.
This partly results from the hypsochromic shift of the absorp-
tion band accompanying increase in the polarity of the solvent.
On the other hand it is associated with the energy dissipation
processes of the excited vibrational states in liquid solution.
It should be noted that the transition from non-polar to polar
aprotic solvents significantly changes fluorescence spectra of all
studied compounds on a qualitative level. In polar aprotic
Fig. 2 Absorption and fluorescence spectra of 4b in MeOH (dashed line)
and in toluene (solid line). Excitation at 400 nm.
absorption maxima of benzo[a]imidazo[5,1,2-cd]indolizines 4a– solution the emission shape consists of two bands: a ‘‘red’’
d are bathochromically shifted by about 100 nm versus parent one which corresponds to the ESIPT luminescence (and can
imidazo[1,2-a]pyridines 3a–d (Table 3, Fig. 1). This is accom- be observed in non-polar solvents), and a ‘‘blue’’ one. In
panied by a strong hyperchromic effect with e reaching 24 000. contrast to toluene solution, the emission spectra in methanol
The data in Table 3 show the presence of a small hypsochromic for 2-(20-hydroxyphenyl)imidazo[1,2-a]pyridines show only a strong
shift of absorption (7–9 nm) for the compounds of both series, non-ESIPT emission band in the blue region accompanied by
with increasing polarity of the solvent. In contrast the emission a moderate Stokes shift – 3500–4200 cmꢀ1. The essentially
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different behavior of imidazo[1,2-a]pyridines in protic solution
versus the non-polar, aprotic one can be explained by taking
into account the hypothesis proposed by Acuna et al.13,14 about
˜
the solvent-induced stabilization of these forms, which are not
able to form intramolecular hydrogen bonds, and therefore are
not capable of proton transfer in the excited state.
In this case, moderate Stokes shifts are satisfactorily described
in the framework of relaxation theory: explaining the shift of
the emission band (energy loss) based on the reorientation of
solvent dipoles, and the redistribution of electron density in the
molecules of the solvent. Both the spectral evidence and
structural similarity allows us to extend the applicability of
that hypothesis to the class of 2-(benzo[a]imidazo[5,1,2-cd]-
indolizin-1-yl)phenols. In a polar aprotic solvent the value of
Stokes losses is comparable to the value of losses of the vibra-
tional energy due to relaxation processes and was on the order of
500–800 cmꢀ1. Similar values of the Stokes shift were reported
for a series of ESIPT-silent benzo[a]imidazo[5,1,2-cd]indolizine
derivatives.17 It must be emphasized that for all the studied
benzo[a]imidazo[5,1,2-cd]indolizine derivatives extremely small
Stokes shifts (100 cmꢀ1) were observed in methanol (Fig. 2
and 3). Fluorescence quantum yield for all the studied imidazo-
[1,2-a]pyridines, for the band corresponding to the ESIPT process is
0.5–4% (in toluene) while emission from the locally excited state is
stronger (8–20% in methanol). These values of Ffl are in good
agreement with the results of earlier reports.16b By increasing the
effective p-electron system we have been able to not only shift the
absorption band to the violet-blue region (B430 nm), but also
significantly increase Ffl, in non-polar solvents. Although emission
in MeOH comes only from the local excited state, it is significantly
weaker for dyes 4a–d than for compounds 3a–d (Table 2).
The key difference between derivatives of imidazo[1,2-a]-
pyridines and their novel p-expanded analogues is that in the
latter case the ESIPT-emission (observed exclusively in toluene)
is more intense (1.5–11% vs. 0.3–1.1%). The highest value of the
fluorescence quantum yield was recorded for 2-(benzo[a]-
imidazo[5,1,2-cd]indolizin-1-yl)-5-methoxyphenol (4b) which
amounted to 11%.
The quantum chemical study of 2-(20-hydroxyphenyl)imidazo-
[1,2-a]pyridines published by Araki and co-workers15b showed
that the ESIPT state (S1) smoothly approached the S0–S1 conical
intersection, coupled with twisting motion around the central
C–C single bond connecting the phenyl and imidazo[1,2-a]pyridine
rings. In other words the S0–S1 energy gap was sufficiently small
at a dihedral angle of 601 for rapid radiationless decay in
solution. It seems therefore that in the case of p-expanded
compounds 4a–d the planar conformation is more preferred in
fluid media, most probably because of limited rotation of the
2-hydroxyphenyl substituent imparted by the presence of
hydrogen atoms at the fused benzene ring.
Fig. 4 Corrected emission spectra of compounds 4a–d in the solid state.
Excitation at 440 nm. The spectral areas are scaled proportionally to the
emission quantum yields.
an additional benzene ring were characterized by moderately red-
shifted absorption (B50 nm), slightly bathochromically shifted
emission (B10 to 20 nm) and decreased Ffl versus the parent
molecule.11 We previously found that specific types of extending
of conjugated aromatic system can result in prevention of ESIPT,
in spite of the presence of strong intramolecular hydrogen
bonds.11a,d,21 In contrast to all these previous examples, fusion
of 2-(20-hydroxyphenyl)imidazo[1,2-a]pyridines with benzene
molecule results in ESIPT-capable compounds with strongly
(B100 nm) bathochromically shifted absorption, non-shifted
emission (in solution) and increased fluorescence quantum
yield (from the ESIPT-state, in solution). These differences are
probably associated with crucial function of free rotation and
unusual mode of p-system expansion in heterocycles 4a–d.
Finally we also carried luminescence measurements for
investigated compounds in the solid state (Fig. 4, Table 4).
The solid samples have been gently milled with a mortar and
placed inside two quartz slides. The absorption spectra of the
four samples are shown in Fig. S1 (ESI†).
Although fluorescence is often significantly quenched in a
solid state due to intermolecular interactions, Park and co-workers
as well as other research groups introduced new classes of com-
pounds displaying aggregation-induced emission enhancement
(AIEE).22 AIEE was also found in the case of parent 2-(20-hydroxy-
phenyl)imidazo[1,2-a]pyridines, where strong fluorescence has
Table 4 Solid state absorption and emission data for compounds 3a–d
and 4a–d
c
Compound
labsa/nm
lemb/nm
Fem
3ad
3bd
3cd
3dd
4a
4b
4c
4d
331
336
332
332
410, 435
416, 443
410, 435
414, 439
494
520
522
529
612
568
626
626
0.39
0.41
0.37
0.31
0.267 ꢁ 0.002
0.242 ꢁ 0.013
0.184 ꢁ 0.018
0.211 ꢁ 0.006
The photophysical behaviour of dyes 4a–d is different than
previously reported p-expanded ESIPT-capable chromophores.
2-(20-Hydroxyphenyl)imidazoles with extended effective conjuga-
tion length reported by Park and co-workers1k displayed proper-
ties similar to parent molecules, unless p-expansion was along the
a
Absorption maxima from reflectance spectra analyzed using
b
Kubelka–Munk conversion. Emission maxima from corrected spectra.
c
d
Absolute emission quantum yields; excitation at 440 nm. Data taken
nodal plane.12 Analogues of 10-hydroxybenzo[h]quinoline bearing from ref. 16b.
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been observed with the band corresponding to the excited state their emission in a protic solvent leads to radiationless deactiva-
intramolecular proton transfer.9c,15a The solid state optical tion rather than luminescence. Aggregation-induced emission
properties for compounds 3a–d perfectly correlate with data enhancement is responsible for relatively strong red lumines-
published earlier by Araki and co-workers (and therefore the cence of benzo[a]imidazo[5,1,2-cd]indolizines in the solid
latter ones were placed in Table 4).16b
state. These results are not only of theoretical significance in
The solid state absorption of compounds 4a–d is bathochro- that they provide the first study of optical properties of this
mically shifted versus their smaller counterparts by B100 nm. heterocyclic scaffold, but they may also open the door to
Unlike in solution, in the solid state emission bands for benzo- practical applications.
[a]imidazo[5,1,2-cd]indolizines 4a–d are shifted on average, by
100 nm relative to the fluorescence of the corresponding
Experimental section
General
imidazo[1,2-a]pyridines. The emission measurements showed
that all compounds, except 4b, have a main emission centered
at 610–630 nm and a shoulder at 530 nm. Excitation spectra of
All chemicals were used as received unless noted otherwise.
the two bands revealed that the main emission is coming from
Reagent grade solvents (CH2Cl2, hexanes) were distilled prior to
a species with the absorption maximum of around 440 nm
use. Spectrophotometric grade solvents were used without
(overlapping with the absorption spectrum), whereas the
further purification. All reported 1H NMR and 13C NMR spectra
shoulder comes from a species absorbing mainly in the region
were collected using 500 or 600 MHz spectrometers. Chemical
490–500 nm. The emission spectrum of this second species,
shifts (d ppm) were determined using TMS as an internal
probably an aggregate or a form with a different crystal struc-
standard; J values are given in Hz. Melting points were deter-
ture, is, in fact, well resolved upon excitation at 490 nm. The
mined using a capillary-type apparatus. Dry column vacuum
contribution of this secondary emission to the whole emission
chromatography (DCVC) was performed on preparative thin-
of the sample upon excitation at 440 nm can be estimated. The
layer chromatography using silica gel. Mass spectra were
calculation leads to the following percentages: ca. 6% for 4a,
obtained via electron impact mass spectrometry using a double
ca. 7% for 4c and ca. 4% for 4d. Fig. S2 (ESI†) shows the
focusing sector mass analyzer (EI-MS) or using a time-of-
contribution of the two forms and the relative excitation
flight spectrometer in positive electrospray ionization mode
spectra. In the case of 4b, the emission spectral features are
(TOF MS ES+).
very different with respect to the other compounds and there is
no strong evidence of emission from secondary species.
Interestingly fluorescence quantum yields in the solid state
are roughly twice lower for p-expanded imidazo[1,2-a]pyridines
4a–d when compared with compounds 3a–d (Table 4). Still, they
are rather high (B20%), which suggests that AIEE also occurs in
these cases. The anomalous emission of dye 4b is most probably
related to the fact that an additional electron-donating MeO group
Absorption and fluorescence spectroscopy
The UV/Vis absorption spectra were recorded on a Perkin Elmer
LAMBDA 35 spectrometer in toluene, DCM, acetonitrile and
methanol. This apparatus has a 190–1100 nm wavelength range
with a bandwidth of 1 nm. It is equipped with deuterium and
tungsten pre-aligned sources with automatic switch-over. The
optical system is a sealed double-beam scheme. Emission
acts as an additional hydrogen-bond acceptor, altering crystal
packing in the solid state. The polymorphs differing in solid-state
spectra were recorded on a fluorescence spectrophotometer
luminescence, among 2-(20-hydroxyphenyl)imidazo[1,2-a]pyridines
were observed in 2008 by Araki and co-workers and later thoroughly
investigated by Shigemitsu et al.15b
Perkin Elmer 512 toluene, DCM, acetonitrile and methanol.
The units are equipped with a Xe lamp (150 W), monochromator
and photomultiplier tube HAMAMATSU R446. It has
a
200–900 nm wavelength range with 3 nm resolution. Spectra
are presented in photon units and have been corrected for
instrumental factors.
Conclusions
Solid-state determination made use of powder samples
gently milled with a mortar and placed inside two quartz slides.
Reflectance spectra were acquired with a Perkin–Elmer Lambda 9
UV/Vis/NIR spectrophotometer equipped with a 60 mm integrating
sphere. Absolute photoluminescence quantum yields were
measured on a Edinburgh FLS920 fluorimeter equipped with
a 6 inches Labsphere integrating sphere, according to the method
of deMello.23 Each measurement was repeated from three to five
times. The limit of detection of the system is 2%.
The presence of free hydroxyl groups does not interfere with
tandem [8+2]cycloaddition–[2+6+2]dehydrogenation reaction
in the imidazo[1,2-a]pyridine series. The fusion of imidazo-
[1,2-a]pyridine to a benzene ring at positions 3 and 5 leads to
unusual heterocycles comprising of a four-ring core. We have
demonstrated that by extension of the chromophore it is
possible to easily manipulate both absorption and emission
characteristics of novel yellow dyes with substantial batho-
chromic shifts. An expanding p-system alters the energy levels
resulting in strikingly different characteristics of these novel
ESIPT-capable compounds. While they still undergo ESIPT,
General procedure for the preparation of
imidazo[1,2-a]pyridines starting from 2-aminopyridine
their Ffl in non-polar solvents increases from 1–3% to 2–11% A sealed tube was charged with the appropriate amounts of
and the Stokes shift decreases to 6000 cmꢀ1. On the other hand ketone (1.20 mmol), 2-aminopyridine (2.76 mmol, 2.3 eq.) and
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iodine (1.44 mmol, 1.2 eq.). The reaction mixture was stirred at J2 = 7.2 Hz, J3 = 7.7 Hz), 7.61 (ddd, 1H, J1 = 0.9 Hz, J2 = 7.4 Hz,
110 1C. After 4 h, the mixture was cooled to 70 1C and stirred J3 = 7.6 Hz), 7.79 (ddd, 1H, J1 = 1.1 Hz, J2 = 7.4 Hz, J3 = 7.8 Hz),
overnight. The residue was diluted with 5 mL of distilled water, 7.94–7.99 (m, 2H), 8.04 (dd, 1H, J1 = 1.7 Hz, J2 = 7.9 Hz), 8.28
and an excess of aqueous sodium hydroxide (45%) was added. (dd, 1H, J1 = 1.6 Hz, J2 = 7.7 Hz), 8.36 (d, 1H, J = 7.9 Hz), 8.46
The reaction mixture was stirred at 100 1C for 1 h. After cooling (d, 1H, J = 8.2) 13.40 (broad s, 1H); 13C NMR (125 MHz,
to room temperature, the reaction mixture was diluted with CDCl3, d) 109.05, 111.94, 117.20, 118.03, 119.15, 119.83,
25 mL of CH2Cl2. 10% aqueous HCl was added to the water– 121.19, 123.15, 125.00, 126.93, 128.63, 128.81, 129.42, 130.11,
organic mixture until a neutral pH was obtained. The mixture 130.89, 131.13, 137.23, 146.38, 158.59; EI-HR obsd 284.0948,
was then extracted with suitable solvents. The organic layer was calcd exact mass 284.0950 (C19H12N2O).
washed with water, dried over Na2SO4, and concentrated under
2-(Benzo[a]imidazo[5,1,2-cd]indolizin-1-yl)-5-methoxyphenol (4b)
reduced pressure. The solid was isolated by column chromato-
graphy. Imidazo[1,2-a]pyridines 3a and 3b were prepared as
described previously.
The compound was prepared as described above. The pure
product was obtained as a yellow solid (27 mg, 21%). Data: mp
192–194 1C; 1H NMR (500 MHz, CDCl3, d) 3.91 (s, 3H), 6.70–6.74
(m, 2H), 7.59 (ddd, 1H, J1 = 0.9 Hz, J2 = 7.3 Hz, J3 = 7.6 Hz), 7.77
(ddd, 1H, J1 = 1.1 Hz, J2 = 7.2 Hz, J3 = 7.7 Hz), 7.93–7.94 (m, 1H),
8.02 (dd, 1H, J1 = 2.4 Hz, J2 = 5.5 Hz), 8.19 (d, 1H, J = 8.4 Hz),
8.36 (d, 1H, J = 8.0 Hz), 8.42 (d, 1H, J = 8.3 Hz), 13.64 (broad s,
1H); 13C NMR (125 MHz, CDCl3, d) 55.42, 101.97, 106.89,
108.73, 110.44, 111.29, 120.93, 123.11, 124.63, 126.61, 128.71,
129.24, 129.66, 129.79, 130.09, 130.84, 137.31, 146.89, 160.53,
162.04; EI-HR obsd 314.1056, calcd exact mass 314.1055
(C20H14N2O2).
General procedure for the preparation of
2-(benzo[a]imidazo[5,1,2-cd]indolizin-1-yl)phenol
2-(Trimethylsilyl)phenyl trifluoromethanesulfonate (120 mg,
0.40 mmol) was added to a mixture of appropriate amounts
of imidazo[1,2-a]pyridine (1.20 mmol,
3 eq.), 18-crown-6
(211 mg, 0.80 mmol) and CsF (122 mg, 0.80 mmol) in a glass
reactor. The reaction mixture was irradiated in a monomode
microwave reactor at 160 1C for 25 min. The resulting mixture
was dissolved in ethyl acetate and evaporated. The product was
isolated by DCVC chromatography on silica gel and obtained as
a solid.
2-(Benzo[a]imidazo[5,1,2-cd]indolizin-1-yl)-4-fluorophenol (4c)
The compound was prepared as described above. The pure
product was obtained as a yellow solid (23 mg, 19%). Data: mp
233–234 1C; 1H NMR (500 MHz, CDCl3, d) 7.11–7.12 (m, 2H),
7.65 (ddd, 1H, J1 = 0.9 Hz, J2 = 7.4 Hz, J3 = 7.7 Hz), 7.83 (dd, 1H,
J1 = 1.1 Hz, J2 = 8.3 Hz), 7.94 (dd, 1H, J1 = 2.5 Hz, J2 = 9.3 Hz),
7.99–8.01 (m, 2H), 8.08 (dd, 1H, J1 = 2.8 Hz, J2 = 5.0 Hz), 8.40
(d, 1H, J = 7.8 Hz), 8.46 (d, 1H, J = 8.2 Hz), 13.12 (broad s, 1H);
13C NMR (125 MHz, CDCl3, d) 109.30, 112.28, 113.93, 114.12,
117.67, 118.91, 120.06, 121.19, 123.27, 125.33, 127.26, 128.61,
129.70, 130.33, 131.30, 137.23, 145.11, 154.68, 156.82; EI-HR
obsd 302.0851, calcd exact mass 302.0855 (C19H11N2OF).
4-Fluoro-2-(imidazo[1,2-a]pyridine-2-yl)phenol (3c)
The compound was prepared as described above using CH2Cl2
for the extraction. The pure product was obtained as a
yellow solid (180 mg, 49%). Data: mp 155–156 1C (lit.16b mp
1
161–162 1C); H NMR (500 MHz, CDCl3, d) 6.86 (t, 1H, J = 6.8),
6.93–6.98 (m, 2H), 7.23–7.25 (m, 2H), 7.82 (s, 1H), 8.14 (d, 1H,
J = 6.8), 12.48 (broad s, 1H); 13C NMR (125 MHz, CDCl3, d)
107.1, 111.3, 111.5, 113.4, 116.2, 116.4, 116.9, 118.4, 118.5,
125.5, 153.4, 155.0, 156.8; HR – TOF MS(ES+) obsd 229.0776,
calcd exact mass 229.0777 C13H10FN2O – [M–H]+.
2-(Imidazo[1,2-a]pyridine-2-yl)-4-methylphenol (3d)
2-(Benzo[a]imidazo[5,1,2-cd]indolizin-1-yl)-4-methylphenol (4d)
The compound was prepared as described above using CH2Cl2
for the extraction. The product was isolated by DCVC chroma-
tography (silica, CH2Cl2/hexane 1 : 2 - CH2Cl2). The pure
product was obtained as a yellow solid (175 mg, 47%). Data:
mp 148–149 1C (lit.16b mp 150.9–151.2 1C); 1H NMR (500 MHz,
CDCl3, d) 6.75 (t, 1H, J = 6.8 Hz), 6.86 (d, 1H, J = 8.3 Hz), 6.95
(dd, 1H, J1 = 1.6 Hz, J2 = 8.2 Hz), 7.12 (t, 1H, J = 7.9 Hz), 7.31
(s, 1H), 7.49 (d, 1H, J = 9.1 Hz), 7.76 (s, 1H), 8.05 (d, 1H, J =
6.6 Hz) 12.41 (broad s, 1H); 13C NMR (125 MHz, CDCl3, d) 20.7,
106.7, 113.2, 115.9, 116.9, 117.6, 125.2, 125.5, 126.1, 128.0,
130.6, 143.6, 145.6, 155.3; HR – TOF MS(ES+) obsd 225.1029,
calcd exact mass 225.1028 C14H13N2O – [M–H]+.
The compound was prepared as described above. The pure
product was obtained as a yellow solid (25 mg, 21%). Data: mp
174–175 1C; 1H NMR (500 MHz, CDCl3, d) 2.51 (s, 3H), 7.07
(d, 1H, J = 8.3 Hz), 7.18 (d, 1H, J = 8.2 Hz), 7.57 (t, 1H, J = 7.5 Hz),
7.76 (t, 1H, J = 7.6 Hz), 7.92–7.94 (m, 2H), 7.99–8.01 (m, 1H),
8.04 (s, 1H), 8.32 (d, 1H, J = 8.0 Hz), 8.38 (d, 1H, J = 8.2 Hz),
13.09 (broad s, 1H); 13C NMR (125 MHz, CDCl3, d) 20.86, 108.90,
111.84, 116.75, 117.72, 119.68, 121.01, 123.10, 124.85, 126.76,
128.02, 128.75, 128.81, 129.35, 129.98, 131.08, 131.68, 137.28,
146.64, 156.38; EI-HR obsd 298.1097, calcd exact mass 298.1106
(C20H14N2O).
2-(Benzo[a]imidazo[5,1,2-cd]indolizin-1-yl)phenol (4a)
Acknowledgements
The compound was prepared as described above. The pure
product was obtained as a yellow solid (26 mg, 23%). Data: mp A. J. S., M. K. C. and D. T. G. gratefully acknowledge the
177–178 1C; 1H NMR (500 MHz, CDCl3, d) 7.12 (t, 1H, J = 7.5 Hz), Foundation for Polish Science for supporting this work
7.18 (dd, 1H, J1 = 0.9 Hz, J2 = 8.2 Hz), 7.38 (ddd, 1H, J1 = 1.6 Hz, under the MPD/2010/4 project ‘‘Towards Advanced Functional
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