Vertically π-expanded imidazo[1,2-a]pyridine: The missing link of the puzzle

Article abstract of DOI:10.1002/asia.201402201

The dehydrogenative coupling of imidazo[1,2-a]pyridine derivative has been achieved for the first time. In cases in which the most-electron-rich position of the electron-excessive heterocycle was blocked by a naphthalen-1-yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion-radical coupling reaction, which employed dry O₂ from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2-a]pyridine in 63% yield. Interestingly, whereas the fluorescence quantum yield (Φ_fl) of compound 3, despite the freedom of rotation, was close to 50 the Φ_fl value of flat naphthalene-imidazo[1,2-a]pyridine was only 5 . Detailed analysis of this compound by using DFT calculations and a low-temperature Shpolskii matrix revealed phosphorescence emission, thus indicating that efficient intersystem-crossing from the lowest-excited S ₁ level to the triplet manifold was the competing process with fluorescence.

Full text of DOI:10.1002/asia.201402201

FULL PAPER
DOI: 10.1002/asia.201402201
Vertically p-Expanded ImidazoACHTNUGTRENNUNG
Puzzle
Dikhi Firmansyah,[a] Marzena Banasiewicz,[b] Irena Deperasin´ska,[b] Artur Makarewicz,[b]
Boleslaw Kozankiewicz,*[b] and Daniel T. Gryko*[a, c]
Abstract: The dehydrogenative cou-
pling of imidazo[1,2-a]pyridine deriva-
toluene. Moreover, we discovered new,
excellent conditions for this anion-radi-
cal coupling reaction, which employed
dry O2 from the start in the reaction
mixture. This method afforded vertical-
ly fused imidazoACTHNUTRGNE[NUG 1,2-a]pyridine in 63%
yield. Interestingly, whereas the fluo-
rescence quantum yield (Ffl) of com-
pound 3, despite the freedom of rota-
tion, was close to 50%, the Ffl value of
flat naphthalene-imidazoACHTGNUTERNNU[G 1,2-a]pyridine
ACHTUNGTRENNUNG
tive has been achieved for the first
time. In cases in which the most-elec-
tron-rich position of the electron-exces-
sive heterocycle was blocked by a naph-
thalen-1-yl substituent, neither oxida-
tive aromatic coupling nor reaction
under Scholl conditions enabled the
fusion of the rings. The only method
that converted the substrate into the
was only 5%. Detailed analysis of
this compound by using DFT calcula-
tions and a low-temperature Shpol’skii
matrix revealed phosphorescence emis-
sion, thus indicating that efficient inter-
system-crossing from the lowest-excited
S1 level to the triplet manifold was the
competing process with fluorescence.
Keywords: dehydrogenation
density functional calculations
·
·
corresponding imidazoACHTNUGTREN[NUNG 5,1,2-de]naph-
fluorescence · heterocycles · optical
properties
A
ACHTUNGTRENNUNG
the presence of potassium in anhydrous
Introduction
of considerable current research interest, owing to their fun-
damental optoelectronic properties and potential applica-
tions, such as light-emitting diodes, photovoltaic devices, and
other organic optoelectronic devices.[6,7] Most of the work in
this area has focused on hetero-acenes,[8,9,10] because they
are more accessible than heterocyclic analogues of rylenes.
Although perylene is unique in the family of PAHs and has
been extensively studied, owing to its excellent optical and
electronic properties,[11] incorporating heteroatoms into its
skeleton is a difficult task. Such perylene analogues are
mostly unknown,[12] as a direct consequence of a lack of suit-
able, versatile synthetic methods. For example, the first re-
ported synthesis of fundamental compound 1-azaperylene
was only published as recently as 2010.[13] One possibility for
overcome this intrinsic problem is an intramolecular version
of aromatic dehydrogenative coupling.[14–17] In principle, this
method should lead to the desired structures, from single-
linked precursors, although most of the current procedures
only work for substrates that possess two electron-rich
units.[18–19]
Polycyclic aromatic hydrocarbons (PAHs) have been the
subject of intense investigation in recent years.[1] With multi-
ple applications spanning organic electronics, photonics, bio-
imaging, etc., PAHs have attracted attention from many
vantage points.[2] Although many p-expanded aromatic ar-
chitectures have been synthesized and investigated, most-
prominently by the Mꢀllen group,[3] their heterocyclic coun-
terparts remain elusive.[4,5] Rigid aromatic systems, which
contain heteroatoms in fused aromatic rings, are the subject
[a] D. Firmansyah, Prof. D. T. Gryko
Faculty of Chemistry
Warsaw University of Technology
Noakowskiego 3, 00-664 Warsaw (Poland)
Fax : (+48)22-6326681
E-mail: dtgryko@icho.edu.pl
´
[b] M. Banasiewicz, I. Deperasinska, A. Makarewicz,
Prof. B. Kozankiewicz
Institute of Physics
Imidazo
[1,2-a]pyridine is
a
prototypical heterocyclic
Polish Academy of Sciences
Al. Lotnikꢁw 32/46, 02-668 Warsaw (Poland)
E-mail: kozank@ifpan.edu.pl
system that is known to possess, in addition to medicinal ap-
plications,[20] a very high fluorescence quantum yield.[21]
Over the last five years, numerous papers have described
both advances in its synthesis[22] and its interesting lumines-
cence properties, especially in relation to excited-state intra-
molecular proton transfer (ESIPT).[21,23] Although it is
rather easy to imagine the preparation of analogues of
[c] Prof. D. T. Gryko
Institute of Organic Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, Warsaw (Poland)
Fax : (+48)22-6326681
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402201.
imidazoACHTNUTRGNE[UNG 1,2-a]pyridine with p-expanded acene-like struc-
1
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Boleslaw Kozankiewicz, Daniel T. Gryko et al.
tures by using a known method, it would be impossible to
devise suitable substrates for obtaining imidazo[1,2-a]pyri-
AHCTUNGTRENNUNG
dines that are fused with naphthalene at the 3,5 positions by
using the same methods. Therefore, herein, we aimed at de-
veloping an efficient synthesis of this compound through the
intramolecular dehydrogenation of a singly linked precursor.
Results and Discussion
Unsubstituted imidazoACTHNUTRGNE[NUG 1,2-a]pyridine (1) is easily accessible
by Chichibabin condensation between 2-aminopyridine and
chloroacetaldehyde in the presence of sodium bicarbon-
ate.[24] Although, in principle, it would be possible to synthe-
Scheme 1. Synthesis of 3-naphthalen-1-ylimidazo
direct coupling.
ACHTUNGERTN[NUNG 1,2-a]pyridine through
size 3-(naphthalen-1-yl)imidazoACTHNUGTRNEUNG[1,2-a]pyridine (3) in a step-
wise manner through the regioselective bromination of com-
pound 1, followed by Suzuki coupling with naphthalene-1-
boronic acid, we decided to follow an another approach,
that is, direct arylation.[25] Both the transition-metal-cata-
lyzed and transition-metal-free direct arylations of electron-
rich aromatic heterocycles have become powerful tools for
turned our attention to anion-radical coupling reactions.
This rarely studied reaction[9a,34–36] is thought to proceed
through initial electron transfer from a metal to the aromat-
ic compound, which leads to the creation of an anion-radical
species. This reaction implies the formation of two anion
the derivatization of aromatic compounds.[26,27] Imidazo
[1,2-
radicals, which couple to form a new C C bond. Subsequent
ꢀ
a]pyridines have previously been regioselectively arylated at
the 3 position by using various catalytic systems.[22,28] The
most-promising system was Doucet’s low-loading palladium-
catalyzed method,[29] because the synthesis of the desired 3-
(naphthalen-1-yl)imidazoACTHNURTGNEU[GN 1,2-a]pyridine (3) was reported in
that paper. We successfully repeated that procedure to
obtain compound 3 in 87% yield (Scheme 1).
steps are still poorly understood, but a recent fundamental
contribution by Scott and co-workers[35] claimed that the
dianion was a pivotal intermediate, from which H2 was lost,
thereby generating the aromatic product. In practical terms,
this reaction is usually performed by adding potassium to
a solution of the substrates in anhydrous 1,2-dimethoxy-
ethane, THF, or toluene at room temperature, followed by
exposure in air.
It has long been known that the two most electron-rich
positions in imidazo
N
Initially, we repeated this typical procedure, but a rather
low yield of dye 4 was obtained (Table 1, entry 6). Further
optimization with a longer reaction time and a larger
amount of potassium led to a higher yield (Table 1, entry 7).
Work-up with a different oxidant, that is, SO2 (first intro-
duced by Hꢀnig and Wehner[34b]) gave an even higher yield
of compound 4 (31%; Table 1, entries 8 and 9). In accord-
ance with what was previously observed for perylene,[36]
a longer reaction time did not increase the yield (Table 1,
entry 10). Subsequently, we concentrated our efforts on ex-
amining the influence of the solvent and the metal on the
yield of p-expanded imidazopyridine 4. The use of THF did
not give the desired product, whereas xylene gave compara-
ble results (Table 1, entries 11 and 12). Attempts to use lithi-
um did not give the expected product and the starting mate-
rial was recovered after 1 day. It is known that electron-defi-
cient naphthalene-imides undergo intra- and intermolecular
cal oxidative aromatic coupling reagents, such as FeCl3 or
PIFA,[32] did not lead to the desired product (4; Table 1, en-
tries 1–2). Subsequently, we attempted to perform the cycli-
zation reaction with a DDQ/MeSO3H[33] system, as well as
under both original and modified Scholl reaction condi-
tions[15–18] (Table 1, entries 3–5). In light of these failures, we
Abstract in Polish: Po raz pierwszy otrzymano nowy typ
zwia˛zku heterocyklicznego - imidazoACTHUNTGRNEUNG[1,2-a]pirydyne˛ sprze˛z˙o-
na˛ w pozycjach 3 i 5 z naftalenem. Udowodniono, z˙e spos-
rꢁd wszystkich metod wewna˛trzcza˛steczkowego odwodor-
nienia tylko sprze˛ganie z wytworzeniem aniono-rodnikꢁw
prowadzi do oczekiwanego produktu. Opracowano przy tym
nowe warunki prowadzenia tej reakcji, w ktꢁrych suchy tlen
jest obecny od pocza˛tku w układzie reakcyjnym. Dokładne
dehydrogenation in the presence of weaker bases.[37,38] At-
[39]
tempts to replace potassium with K2CO3
or tBuOK/
DBN[40] failed and we also found that the transformation of
compound 3 into compound 4 did not proceed, even in the
presence of a stronger base (CaH2; Table 1, entries 13–18).
Fundamental studies by Hꢀnig and Wehner and by Scott
and co-workers confirmed the influence of the oxidant on
the reaction.[34b,35] On examining various oxidants, we found
that iodine and p-chloranil were not as effective as SO2 in
the second step of the reaction (Table 1, entries 19 and 20).
´
´
zbadanie własciwosci optyczne obu zwia˛zkꢁw zarꢁwno w
roztworze jak i w matrycach Szpolskiego oraz poprzez obli-
czenia kwantowo-mechaniczne wykazały, z˙e za niska˛ wydaj-
´´
nosc kwantowa˛ fluorescencji odpowiedzialne jest przejscie
mie˛dzysystemowe na nisko lez˙a˛cy stan trypletowy.
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Boleslaw Kozankiewicz, Daniel T. Gryko et al.
Table 1. Dehydrogenative coupling 3-naphthalen-1-ylimidazoACHTUGNTRENNNUG
ent NMR spin systems. Three,
well-resolved AB2 patterns
were only possible for com-
pound
4 (Figure 1). Further-
more, NOESY homonuclear
correlations were used to com-
plete the proton assignment.
Entry
Time
1 h
30 min
16 h
30 min
1 h
3 h
1 d
2 h
1 d
5 d
22 h
1 d
1 d
1 d
Reagents (equiv)
Solvent
T [8C]
Oxidant[a]
Yield [%][b]
With both compounds in
hand, we measured their photo-
physical properties under differ-
ent conditions (room-tempera-
ture solutions and in a low-tem-
perature Shpol’skii matrix of n-
nonane), together with the cor-
responding simulated spectra
that were obtained by using
standard quantum chemistry
methods. These calculations
concerned the energies and os-
cillator strengths of the elec-
tronic transitions for the opti-
mized structures of the mole-
cules (in the ground and excited
electronic states). Detailed cal-
culations, such as bond lengths
for the optimized structures, en-
ergies and oscillator strengths
1
2
3
4
5
6
7
8
FeCl3
PIFA, BF3·Et2O
DDQ, MeSO3H
AlCl3, NaCl
AlCl3
CH2Cl2/MeNO2
CH2Cl2
CH2Cl2
neat
nitrobenzene
toluene
toluene
toluene
toluene
toluene
THF
RT
ꢀ78
0!RT
180
0!130
95
–
–
–
–
–
n.d.
n.d.
n.d.
n.d.
n.d.
4
23
10
31
17
K (1.5)
K (3)
K (3)
K (3)
K (3)
K (3)
K (3)
Li (10)
air[c]
air[c]
95
95
95
95
85
130
95
95
190
95
95
95
95
95
95
95
95
[d]
SO2
SO2
SO2
[d]
9
[d]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
air
n.d.
29
[d]
xylenes
toluene
toluene
neat
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
SO2
air[c]
air[c]
–
n.d.
n.d.
n.d.
n.d.
28
23
9
10
52
K2CO3
5 h
1 d
1 d
1 d
1 d
1 d
1 d
1 d
DBN, tBuOK
CaH2 (5)
K (5), K2CO3
K (5), CaH2
K (3)
air[c]
[d]
SO2
air[c]
p-chloranil
I2
K (3)
K (5)
K (10)
K (5)
air[e]
[f]
O2
63
n.d.
[g]
1 d
SO2
[a] Second step. [b] Yield of isolated product. [c] Typical work-up procedure for the anion-radical coupling re- for the electronic transitions,
action: The reaction mixture was quenched with EtOH. [d] SO2 was flowed over the solution for 15 min
before quenching with EtOH. [e] The reaction was performed in air and fitted with an air-condenser to avoid
evaporation of the solvent. [f] The reaction was performed in air or O2, after heating under a flow of argon for
30 min. [g] SO2 was flowed over the solution for 20 min during the reaction. PIFA=(bis(trifluoroacetoxy)io-
and simulations of the spectra,
are reported in Table 2 and in
the Supporting Information, Ta-
bles S1–S4.
do)benzene, DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone, DBN=1,5-diazabicycloACTHNUGRTNEUNG[4.3.0]non-5-ene, p-
chloranil=2,3,5,6-tetrachloro-1,4-benzoquinone.
According to the calculations,
there was a fundamental differ-
ence between the optimized ge-
ometries of compounds 3 and 4.
Finally, we wanted to perform the critical step under
slightly different conditions that would allow air to be pres-
ent from the start. We performed the reaction with a larger
amount of potassium to compensate for its partial decompo-
sition, owing to the competing reaction with O2. Stirring a so-
lution of compound 3 in anhydrous toluene with potassium
in air (rather than argon) resulted in the formation of prod-
uct 4 in astonishingly high yield (52%; Table 1, entry 21).
Replacing air with dry oxygen allowed us to further increase
the yield of compound 4 to 63% (Table 1, entry 22). Finally,
an attempt to replace O2 with SO2 failed, owing to the quick
oxidation of K by SO2, and no conversion was observed
(Table 1, entry 23).
Several dehydrogenative coupling reactions have led to
benzo[j]fluoranthene-type structures, rather than products
that contain a rylene moiety.[41] Therefore, scrupulous struc-
tural identification and assignment were achieved by using
1D NMR (1H and 13C NMR) and 2D NMR spectroscopy
(gCOSY, NOESY, HSQCAD, and IMPACT_gHMBCAD).
In particular, COSY analysis allowed us to distinguish be-
tween two possible structures, because they possessed differ-
Figure 1. COSY spectra: AB2-system assignment.
3
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Table 2. Experimental and calculated data for the S0!S1 and S1!S0 transitions for compounds 4 and 3; E is the calculated transition energy, f is the os-
cal
exp
cillator strength, n00 and n00 are the calculated and experimentally determined frequencies of the (0,0) transition, respectively, and m(S0) and m(S1) are
the calculated dipole moments in the S0 and S1 states, respectively.
cal
exp
Molecule
S0!S1
S1!S0
n00
[cmꢀ1
n00
[cmꢀ1
m(S0)
[D]
m(S1)
[D]
E [cmꢀ1
]
f
E [cmꢀ1
]
f
G
]
G
]
4
21800
0.1399
17277
0.0887
18955
19200 (red site)
19460 (blue site)
2.99
3.11
3a
3b
28506
28326
23343
34258
0.1737
0.0864
0.3610
0.0555
24814
23958
20781
28130
0.1917
0.1059
0.3576
0.0400
25641
25204
21310
30114
27200
(ꢁ1000)
3.19
3.45
0
6.85
7.01
0
perylene
1-azaindolizine
22533
3.30
2.95
Whereas the molecule of compound 4 was planar in both
the ground (S0) and S1 excited electronic states, the molecule
of compound 3 was not planar in any of these states. More-
over, this molecule may exist as two isomers, 3a and 3b
(Figure 2). These isomers differ in the angle between the
Compound 3 can be considered as a member of the
family of bichromophoric molecules.[42–47] Thus, the mole-
cules were studied in the context of energy transfer between
two chromophores under various conditions, including sol-
vents of different polarity and viscosity. In general, the dif-
ference between the spectra of isomers of bichromophoric
molecules is small and extraction of information about the
properties of a particular isomer requires low-temperature,
planes of the naphthalene and imidazoACTHNUTRGNEUNG[1,2-a]pyridine moiet-
ies (368 and 2168, respectively). Isomer 3a is associated
with the open structure, whereas isomer 3b is associated
with the folded structure. In practice, these two isomers are
energy equivalent in the S0 ground state, because the calcu-
lated energy difference between them is only 5 cmꢀ1. Thus,
at room temperature, there are equal concentrations of both
isomers. Optimization of the energy in the lowest-excited
electronic state (S1) predicts that isomer 3b is more stable,
by 536 cmꢀ1. The calculated height of the energy barrier sep-
arating isomers 3a and 3b in their S1 states is 4320 cmꢀ1.
high-resolution
measurements
under
a
supersonic
beam.[42–45] Our calculations (cf. Table 2 and the Supporting
Information, Tables S1–S3) revealed that compound 3 also
showed similar properties. Under our experimental condi-
tions, we observed a spectrum that was sum of the spectra
of the two isomers.
Room-temperature absorption and fluorescence spectra
of compound 3 in three solvents of different polarity are
shown in Figure 3a. The onset of the absorption, defined as
the wavelength at which the extinction coefficient dropped
to 100mꢀ1 cmꢀ1 (from 4000mꢀ1 cmꢀ1 at 300 nm in cyclohex-
ane, 11000mꢀ1 cmꢀ1 in MeOH, and 8000mꢀ1 cmꢀ1 in MeCN),
was located at 375(ꢁ5) nm. The onset of fluorescence, de-
fined as the wavelength at which the intensity reached about
1% of the intensity of the band maximum, was located at
365(ꢁ5) nm. From the mean of these values, we concluded
that the energy separation between the ground (S0) and
lowest-excited (S1) singlet states (energy of the (0,0) transi-
tion) in compound 3 was close to 27030 cmꢀ1 (370 nm).
A shift of the maximum of the observed fluorescence
bands to lower energy with increasing polarity of the solvent
(in the series cyclohexane, MeOH, and MeCN, in which the
band maxima were observed at 24900, 24450, and
24050 cmꢀ1, respectively) indicated that the dipole moment
of dye 3 was very different in the lowest-excited singlet state
(S1) to that in the ground state (S0). This conclusion was
confirmed by the calculation results (Table 1), which gave
values of 3.19 and 3.45 D for the dipole moments of isomers
3a and 3b, respectively, in the S0 ground state and 6.85 and
7.01 D, respectively, in the S1 excited state.
The fluorescence quantum yield of compound 3, versus
perylene as a reference,[48] was 52, 38, and 42% in cyclohex-
ane, MeCN, and MeOH, respectively. Absorption and fluo-
rescence spectra of compound 3 in an n-nonane matrix at
5 K are shown in Figure 3b. Both spectra are broad and sim-
ilar to those at room temperature, with the onset of absorp-
tion at lower energy than the onset of fluorescence. This ob-
Figure 2. Projections of two isomers of 3-(naphthalen-1-yl)imidazolACTHNUTRGNE[NUG 1,2-
a]pyridine (3a,b), viewed in the plane of the naphthalene moiety and per-
pendicular to the planes of both moieties.
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Boleslaw Kozankiewicz, Daniel T. Gryko et al.
Figure 3. a) Absorption and fluorescence spectra (normalized to the in-
tensity of the band maximum, lexc =250 nm) of compound 3 at RT in cy-
clohexane (solid black line), MeOH (dotted blue line), and MeCN
(dashed red line). b) Absorption and fluorescence spectra (lexc =308 nm)
of compound 3 in an n-nonane matrix at 5 K.
Figure 4. a) Absorption and fluorescence spectra of compound 4 at RT in
cyclohexane (solid black line), MeOH (dotted blue line), and MeCN
(dashed red line). b) Absorption and fluorescence (lexc =450, 489.5 nm)
spectra of compound 4 in an n-nonane matrix at 5 K. Fluorescence spec-
tra are shifted upwards for clarity.
servation indicated that molecules of compound 3 were em-
bedded into the polycrystalline matrix in various cavities
that were characterized by different microscopic solvent
shifts. That is, the inhomogeneous broadening was large and
different molecules of compound 3 did not occupy well-de-
fined sites in the polycrystalline matrix. The average energy
ane, MeCN, and MeOH, respectively. These values were
considerably lower than those for compound 3. Absorption
and fluorescence spectra of compound 4 in a low-tempera-
ture (5 K) n-nonane matrix are shown in Figure 4b. Both
spectra were structured, and their inspection clearly indicat-
ed contributions from two main sites in the matrix that were
occupied by molecules of compound 4. The “red” sites had
their origin (0,0) line at 19200 cmꢀ1 (520.7 nm), whereas the
“blue” sites has their origin (0,0) transition at 19460 cmꢀ1
(513.9 nm). Both sites contributed to the absorption spec-
trum. By using an appropriate excitation wavelength, we ex-
cited molecules of compound 4 that only occupied one site.
Figure 4b shows the fluorescence spectrum of the “red” site
(excitation with the 489.5 nm laser line) and the fluores-
cence spectrum with contributions from both sites (excita-
tion with the 450 nm line).
The low-temperature experimental fluorescence spectrum
of compound 4 in the “red” sites (the spectrum of which is
shown in Figure 4b), together with the calculated vibronic
structure of the fluorescence of the isolated molecule, are
shown in Figure 5. Good agreement was observed and
almost all of the features of the experimentally observed vi-
bronic structure were sufficiently well-reproduced by the
calculated spectrum. The frequencies of the characteristic vi-
brations and their theoretical assignations are listed in the
of the (0,0) transition was 27200ACHTNUGTRENNUNG
estimated from the room-temperature spectra. This value
was slightly higher than the calculated values, 25641 and
25203 cmꢀ1.
Absorption and fluorescence spectra of solutions of com-
pound 4 in cyclohexane, MeOH, and MeCN are shown in
Figure 4a. The onsets of the absorption spectra were located
between 520 and 530 nm (19050ꢁ200 cmꢀ1), whereas the
onsets of fluorescence were located between 485 and
495 nm (20400
sition at 19700
ACHTUNGTRENNUNG
N
rescence spectra were slightly structured, thus reflecting mo-
lecular vibrations. A shift of the spectrum in different sol-
vents was less visible than in the case of compound 3, which
may suggest that the dipole moment did not change much
under the electronic transition from the ground state into
the lowest-excited singlet state. This conclusion was in
agreement with the calculated values of 2.99 and 3.11 D for
the ground and excited states, respectively (Table 2).
The fluorescence quantum yield of compound 4, versus
perylene as a reference, was 4.9, 4.6, and 4.3% in cyclohex-
5
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Boleslaw Kozankiewicz, Daniel T. Gryko et al.
tions are shown in Figure 6a,b. These diagrams are an ex-
tension of the characteristic data of compounds 3 and 4 in
Table 2.
Both the HOMO and LUMO of compound 4 are local-
ized over the whole molecule, thereby retaining the features
of the perylene orbitals, although they are partially de-
formed by the replacement of one naphthalene moiety by
an imidazoACTHNUTRGNEUG[N 1,2-a]pyridine moiety. The corresponding orbi-
tals are shown in Figure 7. The consequence of the loss of
symmetry in molecule 4 compared with perylene is a de-
crease in its transition energy and oscillator strength
(Table 2) and a much richer vibronic structure of the fluo-
rescence compared to that for perylene.
The S0!S1 transition energies for both isomers of com-
pound 3 are smaller than that in imidazoACTHNUTRGNEUNG[1,2-a]pyridine, but
Figure 5. Calculated (red) and experimental fluorescence spectra (black)
of compound 4 in an n-nonane matrix at 5 K (red site). The calculated
spectrum is presented with fwhm=15 cmꢀ1. The measured and calculated
frequencies of the main vibrations are listed in the table. The strongest
vibrations are indicated in bold type.
larger than those in perylene and compound 4 (Table 2).
The oscillator strength for the transitions between the S0
and S1 states for both isomers of compound 3 are relatively
large and larger than that in imidazoACTHNUTRGNE[NUG 1,2-a]pyridine.
Following the electronic HOMO!LUMO excitation in
both isomers 3a and 3b, the characteristic feature was nota-
table in Figure 5. The dominant vibrations in the fluores-
cence spectrum are shown in the Supporting Information,
Figure S1. Notably, two of the vibrations, that is, those at
246 cmꢀ1 (calcd 241 cmꢀ1) and 353 cmꢀ1 (calcd 353 cmꢀ1), re-
semble the corresponding vibrations in perylene (251 and
350 cmꢀ1, respectively), whereas the vibration at 1501 cmꢀ1
(calcd 1509 cmꢀ1) resembles the calculated vibration at
ble charge transfer from the imidazoACTHNGUTERNNU[G 1,2-a]pyridine moiety
to the naphthalene moiety (Figure 6). The increase in the
dipole moment of the molecule in the S1 excited state was
experimentally manifested by the above-described large
shift in the fluorescence maximum with increasing solvent
polarity. Notably, despite a partial change in the nature of
the electronic transitions in compound 3 as compared to
1523 cmꢀ1 for imidazo
[1,2-a]pyridine.
imidazoACTHNGUTER[NNUG 1,2-a]pyridine, it retained the important property of
As demonstrated above, the experimental spectra were
well-reproduced by the calculated ones. For example, the ex-
perimental frequencies (n00exp) were well-reproduced by the
calculated values (n00cal; Table 2). A similar correlation was
observed in terms of the shape
imidazoACTHNGUTER[NNUG 1,2-a]pyridine, that is, the high fluorescence quan-
tum yield.[49]
Perylene and imidazoACHTNUGRTENUNG[1,2-a]pyridine are well-known for
their large fluorescence quantum yields.[48,49] Therefore, the
of the absorption spectrum
within the region 200–400 nm,
with several S0!Si electronic
transitions (cf. Figure 3 and
Figure 4 with the data in the
Supporting Information, Ta-
bles S2–S4). Moreover, there
was good agreement between
the experimental and calculated
vibronic structures in the fluo-
rescence spectrum of compound
4. Thus, we will discuss the
properties of these molecules in
more detail.
In both compounds 3 and 4,
the lowest-excited singlet states
had p* nature (although, in
compound 3, the p-electron
system was not completely
planar). The electronic states of
both molecules, the shapes of
the HOMOs and LUMOs, and
Figure 6. HOMO and LUMO levels that contribute to the transitions between the S0 and S1 states in molecules
4 and 3. Calculated transition energies in the absorption (S0!S1FC) and fluorescence (S1!S0FC) of the isolated
molecules; FC indicates the appropriate Franck-Condon configuration. Energetic positions of the lowest-excit-
the S0!S1 and S1!S0 transi- ed triplet states (Ti) of the compounds are also shown.
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Boleslaw Kozankiewicz, Daniel T. Gryko et al.
Table 3. Calculated energy gaps between the first excited singlet state
(S1) and lower-lying triplet states T1 and T2.
DE
(S1–T1) [cmꢀ1
]
(S1–T2) [cmꢀ1
DEACHTUNRGTNEGNU ]
4
3a
3b
perylene
5778
6349
5156
9648
8083
ꢀ201
3873
3241
ꢀ2831
imidazoACHTUNTRGNEU[GN 1,2-a]pyridine
ꢀ508
(and the T3 state is far above them). Thus, relaxation
through intersystem crossing is also expected be inefficient,
as confirmed by a high fluorescence quantum yield. Howev-
er, in compound 4, the calculated energy gap DEACTHNUTRGNE(UNG S1–T2) is
relatively small (based on the theoretical calculations, both
states can be considered as isoenergetic) and intersystem
crossing to the T2 state, followed by energy relaxation
within the triplet states to the T1 state, may open up an effi-
cient depopulation pathway.
To experimentally confirm our theory that intersystem
crossing was responsible for the lower fluorescence quantum
yield of compound 4, we investigated its phosphorescence
(Figure 8). The obtained phosphorescence spectrum, which
was some orders of magnitude weaker than the fluorescence
spectrum (Figure 4b and Figure 5), was obtained when the
emission (fluorescence) was promptly removed by mechani-
cal “choppers” and the spectrum monitored with a sensitive,
cooled EMCCD camera. The spectrum, which was obtained
with a relatively broad excitation line, originated from at
least four sites, with (0,0) lines at 13564 and 13541 cmꢀ1
(sites a1 and a2), 13389 cmꢀ1 (site b), and 13288 cmꢀ1
(site c). Each of these lines had a vibrational partner about
354 cmꢀ1 lower in energy. This shape (vibrational component
with similar frequency) resembled the phosphorescence
spectrum of perylene in a polycrystalline anthracene.[50] The
vibrational component with the same frequency was active
in the fluorescence spectrum of compound 4. A slightly
higher energy of the phosphorescence (0,0) lines in com-
pound 4 compared with the energy for perylene in anthra-
Figure 7. Comparison of the HOMO and LUMO levels of compound 4
with the orbitals of perylene and imidazo[1,2-a]pyridine.
AHCTUNGTRENNUNG
very low quantum yield of fluorescence for compound 4 sur-
prised us at first. Reduction of the fluorescence quantum
yield indicated opening of a non-radiative channel for relax-
ation of the excitation energy, which could either be through
internal conversion within the single states or through inter-
system crossing to the triplet state. Internal conversion is
usually governed by the energy-gap law and, because the
energy separation between the S0 and S1 states in perylene
and compound 4 are not very different, we expect similar ef-
ficiencies in terms of energy relaxation through this pathway
in both compounds. Therefore, we looked more carefully at
the possibility of more efficient relaxation by intersystem
crossing, which may be efficient if the energy gap between
the S1 state and a lower-lying triplet state is small.
Figure 7 shows a “ladder” of the calculated triplet states
for the studied compounds. In compound 4, the T2 triplet
state lies not far from the S1 state; however, this is not the
case for both isomers of compound 3. Table 3 lists the calcu-
lated energy separations between the S1 state and the lower-
lying T1 and T2 triplet states for these compounds. In the
cases of perylene and imidazoACHTNUTRGENNUG[1,2-a]pyridine, the T1 state is
located about 9000 cmꢀ1 below the S1 state, whereas the T2
triplet states of these compounds are located far above the
S1 states. Thus, intersystem crossing is ineffective and the
fluorescence quantum yield of both of these compounds is
high.[48,49] In the case of isomers 3a and 3b, triplet states T1
and T2 lie thousands of wavenumbers below the S1 states
Figure 8. Phosphorescence spectrum of compound
4 in an n-nonane
matrix at 5 K; the sample was excited by a broad 445 nm line from
a 100 mW cw diode laser. The numbers in parenthesis denote the energy
separation of vibrational lines with respect to their corresponding (0,0)
origin lines. Lines attributed to different sites are indicated by letters a–c.
7
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Boleslaw Kozankiewicz, Daniel T. Gryko et al.
cene (12844 cmꢀ1),[50] was in agreement with our calcula-
tions. According to the calculations, the energy separation
between the S0 and T1 states of isolated perylene was
12392 cmꢀ1, whereas, in compound 4, it was 13780 cmꢀ1.
By using the same experimental setup, we were not able
to monitor the phosphorescence of perylene in an n-nonane
matrix; this result was not surprising, because, in n-nonane,
we were not dealing with intermolecular intersystem cross-
ing, which was operative and allowed phosphorescence in an
anthracene matrix.[51] In summary, we experimentally ob-
served that the rate of intersystem crossing was much faster
in compound 4 than in perylene and that it was responsible
for the lower fluorescence quantum yield in the former com-
pound.
S1 state and be responsible for an unexpectedly low fluores-
cence quantum yield of compound 4 (4–5%). Such a theoret-
ical guess was confirmed experimentally by the observation
of phosphorescence emission, which is usually undetectable
in perylene derivatives.
Experimental Section
Materials
All commercially available compounds were used as received. All sol-
vents were dried and distilled prior to use. Transformations of oxygen-
sensitive compounds were performed under an argon atmosphere. The
reaction progress was monitored by thin layer chromatography (TLC) on
aluminum sheets coated with silica gel 60 F254 (Merck) and detected by
using a UV lamp. Purification was performed by dry column vacuum
chromatography (DCVC) on silica gel (Merck-Silica gel 60 PF254 for
preparative TLC). The identity and purity of the compounds were con-
firmed by 1D NMR (1H and 13C NMR) and 2D NMR spectroscopy
(gCOSY, NOESY, HSQCAD, and IMPACT_gHMBCAD) on a Varian
600 MHz spectrometer. HRMS (ESI) was performed on a MaldiSYNAPT
G2-S HDMS/GCT Premier, Waters. Melting points were recorded on an
Ez-Melt, SRS, and are reported without correction.
Conclusions
We have shown that 3-(naphthalen-1-yl)imidazoACHTNUTRGNE[NUG 1,2-a]pyri-
dine, which contains an imidazopyridine moiety with the
most-electron-rich position occupied by an aryl substituent,
cannot be obtained through dehydrogenative coupling
under oxidative aromatic coupling or Scholl reaction condi-
tions. On the other hand, it can be oxidatively fused in the
presence of potassium with continuous oxidation by oxygen.
These conditions are probably generally applicable for all
cases in which two non-electron-rich units are linked at their
1,1’ positions. Although imidazoACTHNUGTRNEUNG[1,2-a]pyridines are known
to be strongly fluorescent, fusion with the naphthalene
moiety led to a lower fluorescence quantum yield. In con-
trast, 3-(naphthalen-1-yl)imidazoACHTUNTRGNEUNG[1,2-a]pyridine, which, in
principle, possesses a non-radiative deactivation channel
Synthesis
ImidazoACTHNUGTRENNUG[1,2-a]pyridine (1) and 3-(naphthalen-1-yl)imidazoACHTUGNTREN[NUGN 1,2-a]pyridine
(3) were synthesized according to literature procedures and the photo-
physical data concurred with literature data.[24,28a]
ImidazoACTHNUGTRENNUG[5,1,2-de]naphthoACHTUNGTNER[NUGN 1,8-ab]quinolizine (4): In a Schlenk flask, com-
pound 3 (92.5 mg, 0.38 mmol) was dissolved in dry toluene (6 mL) under
an argon atmosphere. Then, potassium (150 mg, 3.8 mmol) was added
and the mixture was degassed and backfilled with argon. The reaction
mixture was stirred at 958C for 30 min under a flow of argon with fitted
a dephlegmator. Subsequently, oxygen (balloon) was introduced and the
mixture was stirred at the same temperature for 1 day, quenched with
EtOH under an argon atmosphere, and directly absorbed onto celite. The
product was purified by DCVC on silica (1% MeOH in CH2Cl2) and tri-
turated from hexanes/EtOAc to afford the desired product as a yellow
solid (58 mg, 63% yield). M.p. 2048C; 1H NMR (600 MHz, CDCl3): d=
7.85 (s, 1H), 7.66 (d, J=7.3 Hz, 1H), 7.52 (d, J=8.1 Hz, 1H), 7.41 (d, J=
7.3 Hz, 1H), 7.35 (d*, 1H), 7.33 (d*, 1H), 7.30 (t*, 1H), 7.28 (t*, 1H),
7.17 (d, J=7.1 Hz, 1H), 7.12 ppm (dd, J1 =8.8 Hz, J2 =7.5 Hz, 1H);
13C NMR (151 MHz, CDCl3): d=146.3, 135.5, 135.2, 129.6, 129.0, 126.7,
127.4, 126.9, 126.7, 126.6, 125.8, 124.7, 123.2, 118.7, 116.9, 116.3 ppm;
HRMS (ESI): m/z calcd for C17H11N2: 243.0922 [M+H]+; found:
243.0924. * Multiplicity determined by COSY.
ꢀ
through rotation around a single C C bond, is strongly fluo-
rescent.
Quantum chemical calculations revealed different effects
of the extension of imidazoACTHNURGTNE[UNG 1,2-a]pyridine p-system with
ꢀ
one or two C C single bonds: Whereas molecule 4 is planar,
molecule 3 can exist in two nonplanar, isomeric forms. The-
oretical calculations nicely predicted the experimentally ob-
served energies and the vibrational structure of the absorp-
tion (S0!S1) and fluorescence spectra (S1!S0) of com-
pounds 3 and 4. In the ground electronic state (S0), com-
pound 4, as well as two isomers of compound 3, had similar
Optical Properties
Absorption and fluorescence spectra of solutions of compounds 3 and 4
in cyclohexane, MeOH, and MeCN (spectroscopic grade) were recorded
at RT on a PerkinElmer UV/Vis Spectrometer Lambda 35 and a Perki-
nElmer 512 Fluorescence Spectrometer, respectively. Fluorescence quan-
tum yields (Ffl) were determined by using a solution of perylene in cyclo-
hexane as a standard (Ffl =0.96).[48] We estimated that the inherent error
in the estimation of Ffl was less than 10%.
dipole moments to that of imidazoACTHNUGTRNEUNG[1,2-a]pyridine (approxi-
mately 3 D). Electronic excitation of compound 4 into the
S1 state did not lead to a significant increase in the dipole
moment of the molecule, like that observed in the excitation
of imidazoACHTUNGTRENNUNG[1,2-a]pyridine and perylene. In contrast, elec-
Absorption spectra of compounds 4 and 3 were measured at low temper-
ature (5 K) in a single-beam configuration with the aid of a homemade
cuvette. An n-nonane Shpol’skii matrix was frozen between two quartz
windows that were separated by a 1.5 mm Teflon ring. The light from
a Xe lamp was transmitted through the sample, dispersed with a McPher-
son 207 monochromator, and detected by an EMI96659 photomultiplier
that was operated in photon-counting mode.
tronic excitation of both isomers of compound 3 into the S1
state led to charge-transfer characteristics and resulted in an
increase in the dipole moment of the molecule.
Most importantly, quantum chemical calculations indicat-
ed that the T2 triplet state of compound 4 was located very
close in energy to the S1 state, which was not the case for
Fluorescence spectra of compounds 4 and 3 in an n-nonane matrix at 5 K
were measured by using the photon-sampling technique on a McPherson
207 monochromator, an EMI96659 photomultiplier, and a Stanford Re-
search SR259 boxcar averager. The excitation source was either
perylene and imidazoACHTNUGRTENUNG[1,2-a]pyridine, as well as for com-
pound 3. Thus, efficient S1!T2 intersystem crossing, fol-
lowed by T2!T1 internal conversion, could depopulate the
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Boleslaw Kozankiewicz, Daniel T. Gryko et al.
a Lambda Physics LPX100 excimer laser (lexc =308 nm) or a Lambda
physics FL1001 dye laser (pumped by the LPX100 excimer laser) be-
tween 470 and 500 nm with Coumarin 102 and between 440 and 480 nm
with Coumarin 47.
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Received: March 7, 2014
Published online: && &&, 0000
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FULL PAPER
Fluorescence
Flat beat: The fusion of imidazoACHTUNGTRENNUNG[1,2-
a]pyridine with naphthalene can only
be achieved under anion-radical cou-
pling conditions. The resulting, flat
Dikhi Firmansyah,
Marzena Banasiewicz,
´
Irena Deperasinska, Artur Makarewicz,
imidazoACTHNUGTERNNU[G 1,2-a]pyridine, which was
Boleslaw Kozankiewicz,*
Daniel T. Gryko*
fused with naphthalene at the 3,5 posi-
tions, was weakly fluorescent. Shpol’
skii matrix and DFT calculations
revealed that relaxation by intersystem
crossing between the S1 state and
a lower-lying triplet state was responsi-
ble for the low fluorescent quantum
yield.
&&&& — &&&&
Vertically p-Expanded ImidazoACTHNUTRGENUG[N 1,2-
a]pyridine: The Missing Link of the
Puzzle
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ÝÝ These are not the final page numbers!