Synthesis, luminescence properties, and theoretical insights of N-alkyl- or N,N-dialkyl-pyrene-1-carboxamide

Article abstract of DOI:10.1016/j.tetlet.2011.07.020

We report the synthesis and photophysical properties of N-alkyl- or N,N-dialkyl-pyrene-1-carboxamide. These derivatives, as well as pyrene, exhibited blue emission. N-Alkyl-type derivatives exhibited strong fluorescence emission (Φ_fl = 0.61 in EtOH) in both nonpolar and polar solvents. On the other hand, N,N-dialkyl-type derivatives showed weak fluorescence emission (Φ_fl <0.01) due to vibrational deactivation. However, in highly viscous solvents such as glycerin, the quantum efficiencies of N-alkyl-type (Φ_fl = 0.91) and N,N-dialkyl-type (Φ_fl = 0.082) derivatives were increased. We also investigated the fluorescence mechanism of these compounds using time-dependent density-functional theory (TD-DFT). From these results, we find that highly fluorescent pyrene-1-carboxamide derivatives can be designed by introducing an appropriate functional group at the nitrogen atom of the amide. Thus, N,N-dialkyl-type pyrene-1-carboxamide has considerable potential for use in applications such as environmental response sensors and probes.

Full text of DOI:10.1016/j.tetlet.2011.07.020

Tetrahedron Letters 52 (2011) 4843–4847
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, luminescence properties, and theoretical insights
of N-alkyl-or N,N-dialkyl-pyrene-1-carboxamide
Yosuke Niko a, Susumu Kawauchi a, Gen-ichi Konishi a,b,
⇑
a Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
b PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and photophysical properties of N-alkyl- or N,N-dialkyl-pyrene-1-carboxamide.
These derivatives, as well as pyrene, exhibited blue emission. N-Alkyl-type derivatives exhibited strong
fluorescence emission (Ufl = 0.61 in EtOH) in both nonpolar and polar solvents. On the other hand,
N,N-dialkyl-type derivatives showed weak fluorescence emission (Ufl <0.01) due to vibrational deactiva-
tion. However, in highly viscous solvents such as glycerin, the quantum efficiencies of N-alkyl-type
Received 2 June 2011
Revised 28 June 2011
Accepted 6 July 2011
Available online 22 July 2011
(
Ufl = 0.91) and N,N-dialkyl-type (Ufl = 0.082) derivatives were increased. We also investigated the fluo-
Keywords:
Pyrene
Pyrenecarboxamide
Fluorescent
Fluorescence quantum yield
DFT theory
rescence mechanism of these compounds using time-dependent density-functional theory (TD-DFT).
From these results, we find that highly fluorescent pyrene-1-carboxamide derivatives can be designed
by introducing an appropriate functional group at the nitrogen atom of the amide. Thus, N,N-dialkyl-type
pyrene-1-carboxamide has considerable potential for use in applications such as environmental response
sensors and probes.
Ó 2011 Elsevier Ltd. All rights reserved.
Pyrene, a well-known chromophore, has a singlet excited state
with a rather high fluorescence quantum yield and long lifetime.1
In addition, it exhibits various unique photophysical properties.
In particular, its fluorescence is sensitive to the polarity of the sur-
rounding medium, and it can form fluorescent excimers in aggre-
gate states; these properties are quite helpful in studying the
local structures or mobilities of multimolecular systems.2–5 There-
fore, pyrene and its derivatives have been widely used as fluores-
cence probes.6–13
Recently, pyrene has also been used as a fluorescence label in
investigations of polypeptides, DNA oligomers, and duplexes in
terms of their distances,14,15 electric field effects,16,17 and confor-
mational dependence18 of their electron transfer rates. In these
investigations, pyrene and its derivatives have usually been
attached to target molecules through alkyl spacer linkages because
the fluorescence of pyrene changes owing to its constituent polar
functional groups.14–19 For example, chromophores having a car-
bonyl group often lose their fluorescence owing to intersystem
crossing from the lowest n–p⁄ singlet state to the triplet state.20,21
In a manner similar to pyrene, formylpyrene also has weak fluores-
cence because of intersystem crossing.22 In contrast, pyrenes hav-
investigations of highly luminescent pyrene carboxamides, and lit-
tle is known of their design. For example, pyrene-1-carboxanilide
exhibited weak fluorescent emission (Ufl = 0.0013).26 In previous
studies,27–29 aromatic anilide derivatives such as benzanilide27,28
and naphthanilide29 were reported to exhibit weak fluorescence.
This phenomenon can be attributed to the formation of twisted
intramolecular charge transfer (TICT) states in benzanilide and
naphthanilide. An anilide group can reduce the luminescence of a
chromophore. However, from a synthetic viewpoint, carboxamides
can be easily prepared and applied to the modification of sub-
strates, such as artificial polymers, biomacromolecules, and other
surfaces. Therefore, from analytical and materials science view-
points, it is very important to design highly luminescent pyrene
carboxamides.30 Toward this end, we focused on N-alkyl- and
N,N-dialkyl-pyrene-1-carboxamides. Previously, N-alkyl-type deriva-
tives were used as fluorescence labels without any explanation of
why their use is preferred.31–33
In this study, we synthesized N-alkyl- and N,N-dialkyl-pyrene-1-
carboxamides (Scheme 1). Our objectives were to investigate how
these functional groups affect the photoluminescence properties
of pyrene and to synthesize highly fluorescent derivatives of pyr-
ene-1-carboxamides. We expected these N-alkyl substituted car-
boxamide derivatives to exhibit fluorescence because, unlike
anilide-type derivatives, they do not form TICT species, owing to
the flexibility of the substituted alkyl chain. Moreover, to the best
of our knowledge (see Supplementary data), this is the first report
that details the photophysical properties of N,N-dialkyl-type
derivatives.
ing
a carboxylic acid group and ester group exhibit strong
fluorescence. Therefore, these compounds have been used as probes
and as sensitizers, respectively.23–25 However, unlike in the case of
these pyrene carbonyl derivatives, there have been few mechanistic
⇑
Corresponding author. Tel.: +81 3 5734 2321; fax: +81 3 5734 2888.
E-mail address: konishi.g.aa@m.titech.ac.jp (G. Konishi).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.020

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Y. Niko et al. / Tetrahedron Letters 52 (2011) 4843–4847
Scheme 1. Synthesis of N-alkyl type (secondary amide, 1) and N,N-dialkyl-type (tertiary amide, 2) pyrene-1-carboxamide derivatives.
N-Alkyl-type (secondary, 1) and N,N-dialkyl-type (tertiary, 2)
pyrene-1-carboxamide derivatives were prepared from pyrene-1-
carboxylic acid chloride and the corresponding amines with good
yields. Their structures were confirmed by 1H NMR, 13C NMR, FT-
IR, and mass spectrometry as well as by elemental analysis.
In the case of N,N-dialkyl-type (tertiary) derivatives, it was spec-
ulated that cis and trans conformations may exist. In photophysical
investigations, mixing these conformations is unsuitable because it
then becomes difficult to determine which conformation generates
given spectra and parameters. Therefore, because it appears difficult
to synthesize N,N-dialkyl-type derivatives with a fixed trans
structure, we synthesized 2a with the objective of producing N,N-
dialkyl-type derivatives with a fixed cis structure via the introduc-
tion of a bulky substituent onto the nitrogen atom. On the other
hand, N-alkyl-type (secondary) derivatives are considered to have
a trans conformation based on the large size difference between
the hydrogen and the alkyl substituents.
N,N-dialkyl-type compound 2a was very low (Ufl <0.01). However,
that of the N-alkyl type compound 1a was high (Ufl = 0.61). The
fluorescence quantum yield of 1a was higher than that of pyrene
(
Ufl = 0.39) measured in our laboratory under the same condi-
tions.34 Similar results were obtained for the other N-alkyl-type
compounds 1b and 1c, indicating that N-alkyl-type carboxamide
groups can enhance the fluorescent performance of pyrene dyes
(see Supplementary data).
In order to examine the fluorescence mechanism in greater
detail, the electronic structure and the singlet and triplet ener-
gies of 1a and 2a in the ground state were investigated by the
time-dependent density-functional theory (TD-DFT) method at
the B3LYP/6-31G level of theory. Compounds 1a and 2a each
have a large dihedral angle between the pyrene and the carbonyl
group (41.3° and 56.8°, respectively). In contrast, other pyrene-1-
carbonyl compounds have a planar structure (Chart 1). This
structure results in weaker stabilizing interactions between the
p-orbitals on the pyrene and those on the carbonyl group, which
probably caused the blue-shift in the UV–vis absorption and
fluorescence wavelength relative to other pyrene-1-carbonyl
compounds.
From Chart 1b and c, N,N-dialkyl-type 2a was expected to have
two conformations. The results of DFT calculations indicated that
the cis structure is much more stable than the trans structure in
the ground state (by ca. 40.2 kcal/mol). The results of 1H NMR spec-
troscopy suggested the same. N,N-Dialkyl-type 2b contained a
symmetric amine and showed split peaks for the protons on the
two carbons adjacent to the nitrogen. N,N-Dialkyl-type 2a, how-
ever, contained an asymmetric amine and did not show split peaks
for the protons on the tertiary butyl group and the benzyl position
(see Supplementary data).
The UV–vis absorption spectra (Fig. 1a), fluorescence spectra
(Fig. 1b), absolute quantum yields (Ufl), and fluorescence lifetimes
(s) of 1a and 2a were obtained in a deaerated ethanol solution. In
this study, we performed the fluorescence measurements of 1a
and 2a in dilute solution (<10À5 M); therefore, excimer emissions
were not observed. The spectroscopic parameters of the pyrene
derivatives are summarized in Table 1. As shown in Figure 1a,
the wavelengths of the absorption maxima (kabs) were similar
(337–339 nm). As compared to the kabs value of other pyrene-1-
carbonyl derivatives such as 1-formylpyrene (359 nm)22 and ben-
zyl-1-pyrenoate (350 nm),25 those of pyrene-1-carboxamides 1a
and 2a were blue-shifted by ca. 10–20 nm. The molar absorption
coefficients (e) of 1a and 2a at the maxima did not differ signifi-
cantly from each other.
As shown in Figure 1b, the fluorescence spectra and emission
wavelength (kem) of 1a and 2a were also similar. In the fluorescence
spectrum of the tertiary carboxamide 2a, a vibronic fine structure
was observed. The absolute fluorescence quantum yield of the
As shown in Table 2, HOMO–LUMO transitions in S0?S1 had the
highest oscillator strengths, and these transitions were assigned as
p–
p⁄ transitions in both 1a and 2a. Moreover, all triplet states near
S1 were also assigned as
p–
p⁄ transitions. From these results and
Figure 1. Normalized (a) UV–vis absorption and (b) fluorescence spectra of 1a and 2a (ethanol solvent, kex = kabs, room temperature).

Y. Niko et al. / Tetrahedron Letters 52 (2011) 4843–4847
4845
Table 1
Spectroscopic parameters of compounds 1a and 2a (ethanol, room temperature)
b
Entry
kabs (nm)
e
(MÀ1 cmÀ1
)
kem (nm)
s
(ns)
Ufl
kra (SÀ1
)
knr (SÀ1
)
1a
2a
337
339
30,800
30,600
383, 401
378, 387, 398, 420
27.1
2.9
0.61
0.0076
2.3 Â 107
1.4 Â 107
2.8 Â 106
3.2 Â 108
a
kr = Ufl/s.
kn = (1 À Ufl)/
b
s.
Chart 1. Optimized structures of (a) 1a, (b) cis structure of 2a and (c) trans structure of 2a.
Table 2
Excitation wavelength, oscillator strength, orbital, and contribution calculated for 1a and 2a using TD-DFT (B3LYP/6-31G (d))
Entry
State
Transition wavelength (nm)
Oscillator strength
Main transition orbital
Contribution
1a
T1
S1
T2
602
370
365
—
0.754
—
HOMO–LUMO
HOMO–LUMO
HOMOÀ1–LUMO
HOMO–LUMO+1
HOMO–LUMO
HOMO–LUMO
HOMOÀ1–LUMO
HOMO–LUMO+1
0.94
0.98
0.63
0.37
0.94
1
2a
T1
S1
T2
599
365
364
—
0.737
—
0.39
0.55
Chart 2. Frontier orbitals of (a) 1a and (b) 2a.
El-Sayed’s rule,35 it is suggested that the intersystem crossing pro-
cess was inefficiently quenched in both 1a and 2a, while that
quenching was often observed in formylpyrene (Chart 2).22
In order to evaluate the solvent dependence of the UV–vis
absorption and fluorescence spectra of 1a and 2a, they were mea-
sured in several polar solvents (acetonitrile and ethanol), non-polar

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Y. Niko et al. / Tetrahedron Letters 52 (2011) 4843–4847
Figure 2. Fluorescence spectra of (a) 1a and (b) 2a in various solvents (
e
= 10,000, ethanol solvent, kex = kabs, room temperature).
solvents (cyclohexane and dichloromethane), and a high-viscosity
solvent (glycerin)30 (Fig. 2).
is expected that new functions and applications of such compounds
will be proposed. We are currently investigating whether other
fluorophores such as naphthalene, anthracene, and polycyclic aro-
matic hydrocarbons can exhibit the same effects when N-alkyl-type
or N,N-dialkyl-type amide groups are incorporated into them.
N-Alkyl-type 1
As shown in Figure 2a, the shape of the N-alkyl-type 1a fluores-
cence spectra exhibited no dependence on the polarity of the sol-
vent. Although solvent-sensitive changes in the vibronic structure,
one of the advantageous features of pyrene, were not observed, com-
pound 1a showed strong emission in all solvents, especially viscous
glycerin media (Ufl = 0.91), as well as a stable emission color. The
fact that 1a shows a similar wavelength in both the absorption
and the fluorescence emissions, along with the higher fluorescence
quantum yields as compared to those of pyrene, indicates that such
compounds are useful as fluorescence probes, labels, and other
luminescence materials.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.07.020.
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