Molecular Packing and Solid-State Photophysical Properties of 1,3,6,8-Tetraalkylpyrenes

Article abstract of DOI:10.1002/chem.201903224

The relationship between the photophysical properties and molecular orientation of 1,3,6,8-tetraalkylpyrenes in the solid state is described herein. The introduction of alkyl groups with different chain structures (in terms of length and branching) did not affect the photophysical properties in solution, but significantly shifted the emission wavelengths and fluorescence quantum yields in the solid state for some samples. Pyrenes bearing ethyl, isobutyl, or neopentyl groups at the 1-, 3-, 6-, and 8-positions showed similar emission profiles in both the solution and solid states. In contrast, pyrenes bearing other alkyl groups exhibited an excimer emission in the solid state, similar to that of the parent pyrene. On studying the photophysical properties in the solid state with respect to the obtained crystal structures, the observed solid-state photophysical properties were found to depend on the relative position of the pyrene chromophores. The solid-state photophysical properties can be controlled by the alkyl groups, which provide changing crystal packing. Among the pyrenes tested, 1,3,6,8-tetraethylpyrene showed the highest fluorescence quantum yield of 0.88 in the solid state.

Full text of DOI:10.1002/chem.201903224

A Journal of
Accepted Article
Title: Molecular Packing and Solid-state Photophysical Properties of
1,3,6,8-Tetraalkylpyrenes
Authors: Takanori Iwasaki, Shin Murakami, Youhei Takeda, Gaku
Fukuhara, Norimitsu Tohnai, Yumi Yakiyama, Hidehiro
Sakurai, and Nobuaki Kambe
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Molecular Packing and Solid-state Photophysical Properties of
,3,6,8-Tetraalkylpyrenes
1
Takanori Iwasaki,*[a][b] Shin Murakami, Youhei Takeda, Gaku Fukuhara, Norimitsu Tohnai, Yumi
[a]
[a]
[c]
[d]
[
a]
[a]
[a]
Yakiyama, Hidehiro Sakurai, Nobuaki Kambe*
Abstract: The relationship between the photophysical properties and
molecular orientation of 1,3,6,8-tetraalkylpyrenes in the solid state is
described herein. The introduction of alkyl groups with different chain
structures (in terms of length and branching) did not affect the
photophysical properties in solution, but significantly shifted the
emission wavelengths and fluorescence quantum yields in the solid
state for some samples. Pyrenes bearing Et, isoBu, or neoPen groups
at the 1-, 3-, 6-, and 8-positions showed similar emission profiles in
both the solution and solid states. In contrast, pyrenes bearing other
alkyl groups exhibited an excimer emission in the solid state, similar
to that of the parent pyrene. On studying the photophysical properties
in the solid state with respect to the obtained crystal structures, the
observed solid-state photophysical properties were found to depend
on the relative position of the pyrene chromophores. The solid-state
photophysical properties can be controlled by the alkyl groups, which
provide changing crystal packing. Among the pyrenes tested, 1,3,6,8-
tetraethylpyrene showed the highest fluorescence quantum yield of
characterized.[1] To modulate the photophysical properties of
pyrene, various pyrene derivatives have been developed by
incorporating (hetero)aryl,[2] alkenyl,[3] alkynyl,[4] and heteroatom
groups[5] into the pyrene core. Regulating the photophysical
properties such as fluorescence wavelength, quantum yields, and
lifetimes of pyrenes in solution can be achieved by introducing the
aforementioned substituents. However, the solid-state
photophysical behaviors are responsible for the intrinsic single-
molecular properties and self-assembled molecular orientations
originating from their unique stacking characteristics (Figure 1).
To date, various approaches for controlling the photophysical
properties of pyrene derivatives in the solid state, as well as in
films and micelles, have been reported.[6] However, rational and
reliable strategies to predict their photophysical properties in the
solid state have not yet been established. Therefore, the
systematic examination of the effects of molecular orientation of
substituted pyrenes on their solid-state photophysical properties
remains as a significant knowledge gap.
0.88 in the solid state.
Introduction
Pyrene is a polycyclic aromatic hydrocarbon with high prospects
to be used in the fields of physical chemistry and material
sciences, and its photophysical properties have been well-
[a]
[b]
[c]
[d]
Prof. Dr. T. Iwasaki, S. Murakami, Prof. Dr. Y. Takeda, Prof. Dr. Y.
Yakiyama, Prof. Dr. H. Sakurai, Prof. Dr. N. Kambe
Department of Applied Chemistry, Graduate School of Engineering
Osaka University
Suita, Osaka 565-0871 (Japan)
E-mail: kambe@chem.eng.osaka-u.ac.jp
Prof. Dr. T. Iwasaki
Department of Chemistry and Biotechnology, Graduate School of
Engineering
The University of Tokyo
Figure 1. Photophysical properties of 1,3,6,8-tetraalkylpyrenes 1 and pyrene 2
in the solution and solid.
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
In general, alkyl substituents in -systems do not significantly
change the optical properties of the resulting derivative but greatly
affect the molecular orientation through steric hindrance in the
solid state. Therefore, introduction of alkyl groups into -
conjugated molecules can provide useful information regarding
E-mail: iwasaki@chembio.t.u-tokyo.ac.jp
Prof. Dr. G. Fukuhara
Department of Chemistry, Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551 (Japan)
and
JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012
(Japan)
the relationship between molecular orientations and solid-state
photophysical properties.[7-9] Kitamura et al. reported that, in the
Prof. Dr. N. Tohnai
Department of Material and Life Science, Graduate School of
Engineering
Osaka University
Suita, Osaka 565-0871 (Japan)
solid state, the UV/vis spectra of 1,4,7,10-tetraalkyltetracenes
strongly depended on the length of the alkyl substituents.[
Similarly, UV/vis and fluorescence spectral changes by varying
the alkyl chain lengths have been reported for a number of -
conjugated systems.[ Konishi et al. prepared pyrene derivatives
10]
Supporting information for this article is given via a link at the end of
the document.
11]
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with mono- to tetraalkyl substituents and investigated their
photophysical properties, demonstrating the alkyl substituent
effects on the pyrene chromophores, but the solid-state
photophysical properties were not examined. Undoubtedly, the
systematic incorporation of alkyl groups is necessary for better
utilization of the unique photophysical properties of pyrenes.[12]
Herein, we determined the solid-state photophysical properties
and molecular orientation of 1,3,6,8-tetraalkylpyrenes bearing
linear alkyl or branched alkyl groups. In dilute solutions, 1,3,6,8-
tetraalkylpyrenes showed an unchanged luminescence color
summarized in Table 1. The introduction of alkyl groups at the 1-,
3-, 6-, and 8-positions of pyrene caused ca. 2060 cm-1
bathochromic shifts of the overall absorption profile (indicated by
the blue lines in Figure 2a) when compared with that of the non-
substituted pyrene 2 (orange line in Figure 2a). This bathochromic
shift reflects the narrower HOMO-LUMO bandgaps of 1 compared
[
9]
to those of 2, which were likely caused by increasing the HOMO
energy levels via C-H - hyper conjugation.[
9,14]
Regardless of
the length and branching pattern of the alkyl groups, no significant
differences in absorption wavelength and spectral shape among
all tetraalkylpyrenes were observed, as shown in the blue trace
(
1
Figure 1a) in sharp contrast to that of the crystalline samples of
,3,6,8-tetraalkylpyrenes depending on the length of the alkyl
2 2
(Figure 2a). The fluorescence spectra of 1 in CH Cl also showed
groups (Figure 1b). The systematic results obtained herein
provide a mechanistic understanding of photophysical properties
by comparing solution- and solid-state behaviors.
a similar tendency (i.e. bathochromic shift when compared with 2,
no significant difference in the series of 1), with emissions
observed at 377, 410, and 430 nm (Figure 2b). The time-resolved
fluorescence spectrum of 1-Dod in a dilute CH
indicated that vibronic-structured emissions at 377, 410, and 430
nm correspond to the monomer fluorescence (S to S ), while the
2 2
Cl solution
Results and Discussion
1
0
long-lived emission at >450 nm is derived from an excimer (Figure
S17), which is typical for non-substituted pyrene in solution.[
These observations in the absorption and emission spectra
clearly show that the alkyl group structures do not significantly
affect the photophysical behavior of the pyrene chromophore in
15]
Synthesis of 1,3,6,8-tetraalkylpyrenes
Twelve pyrenes with four linear alkyl groups at the 1-, 3-, 6-, and
8-positions (Alkyl = Me, Et, Pr, Bu, Pen, Hex, Hep, Oct, Non, Dec,
Und, or Dod) were synthesized via Kumada-Tamao-Corriu cross-
coupling reaction of 1,3,6,8-tetrabromopyrene with corresponding
dilute CH
2
Cl
2
solution.
alkyl Grignard reagents and were obtained in moderate yields
Scheme 1).[
9,12c,13]
The prepared 1,3,6,8-tetraalkylpyrenes were
(
purified by successive silica gel column chromatography, size
exclusion chromatography, and recrystallization prior to
photophysical analyses. Branched alkyl groups such as isobutyl
1
-Me
-Et
1
1-Pr
1
1
1
-Bu
(
1-isoBu) and neopentyl (1-neoPen) were also introduced to the
pyrene ring by following the same procedure. Recrystallization of
the 1,3,6,8-tetraalkylpyrenes from CHCl /MeOH afforded
-Pen
-Hex
1-Hep
3
1-Oct
1-Non
1-Dec
1-Und
1-Dod
2
colorless needle crystals, except for 1-Me, 1-Et, and 1-isoBu,
which afforded colorless plate crystals.
2
50
300
350
400
450
1
1
1
1
1
1
1
1
1
1
1
-Me
-Et
-Pr
-Bu
-Pen
-Hex
-Hep
-Oct
-Non
-Dec
-Und
Scheme 1. Synthesis of the 1,3,6,8-tetraalkylpyrenes.
1-Dod
2
3
50
400
450
500
Photophysical properties in solution
The normalized steady-state UV/vis and fluorescence spectra of
the prepared 1,3,6,8-tetraalkylpyrenes in CH
2
Cl
2
(1.0 x 10–5 M)
Figure 2. Normalized steady-state absorption (a) and emission (b) spectra of
the 1,3,6,8-tetraalkylpyrenes and pyrene in CH Cl
(1.0 x 10–5 M).
along with those of the unsubstituted pyrene 2 are shown in
Figure 2 (for the UV/vis and fluorescence spectra of each pyrene
derivative, see Figures S1–S15), and spectral data are
2
2
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Table 1. UV/vis and fluorescence spectral data of the 1,3,6,8-
Table 2. Steady-state photophysical properties of the 1,3,6,8-
tetraalkylpyrenes and pyrene in the solid state
tetraalkylpyrenes and pyrene in the dilute solution[
a]
[a]
4
–
em (nm)[c]
em (nm)[a]

max
 (10 L mol

Stokes shift
 (cm )
max (nm)


F
1
–1 [b]
-1
(nm)
cm
)
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
–
467
445
415
433
418
418[b]
419
409
419
418
417
417
417
408
411
0.68[16]
0.23
0.88
0.51
0.79
0.26
0.75
0.79
0.65
0.44
0.57
0.63
0.25
0.26
0.32
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
337
363
362
362
363
363
363
363
363
363
363
363
363
368
363
3.7
4.1
4.0
4.2
4.0
4.4
4.0
4.4
4.2
4.4
4.0
4.2
4.1
4.1
4.0
395, 373
2864
1841
1811
1838
1788
1788
1788
1802
1788
1802
1788
1788
1841
1414
1788
-Me
391
390
392
373
373
378
370
379
379
376
378
377
373
368
-Me
431.8, 409.6, 389.0
430.8, 408.4, 387.4
431.2, 408.4, 387.8
432.4, 409.2, 388.2
431.6, 408.8, 388.2
431.8, 409.0, 388.2
432.2, 409.4, 388.4
432.2, 409.2, 388.2
432.2, 409.4, 388.4
432.2, 409.2, 388.2
432.2, 409.2, 388.2
432.2, 409.4, 389.0
432.4, 409.2, 388.2
432.4, 409.2, 388.2
-Et
-Et
-Pr
-Pr
-Bu
-Bu
-Pen
-Hex
-Hep
-Oct
-Non
-Dec
-Und
-Dod
-isoBu
-neoPen
-Pen
-Hex
-Hep
-Oct
-Non
-Dec
-Und
-Dod
-isoBu
-neoPen
[
a] ex = 320 nm except for 1-Me (ex = 340 nm). [b] One of the two
[
3
a] 1.0 x 10–5 M in CH
20 nm.
2 2
Cl . [b] Molar absorbance coefficient at max. [c] ex =
polymorphism (vide infra).
Photophysical properties in the solid state
Next, we focused on the solid-state photophysical properties of
,3,6,8-tetraalkylpyrenes, because they exhibited different
1-Me
1
-Et
-Pr
1
1
1-Bu
1
1
1
-Pen
-Hex
-Hep
luminescence colors in the solid state upon irradiation with UV
light (Figure 1b).[16] Unsubstituted pyrene 2 showed a light-blue
luminescence in the solid state, in sharp contrast to its violet
1-Oct
1
-Non
-Dec
2 2
luminescence in the dilute CH Cl solution. On the other hand, the
1
crystalline sample of 1,3,6,8-tetraethylpyrene 1-Et and many
alkylated pyrenes furnished with longer chains showed violet
luminescence (em 408–419 nm) similar to that observed in
solution, indicating the monomer emission is the main decay path
of the excited states of the tetraalkylated pyrenes. Pyrenes
containing propyl (1-Pr) and dodecyl (1-Dod) groups showed a
similar luminescence color as pyrene due to the mixture of the
monomer and excimer emissions.
1-Und
1-Dod
2
3
80
430
480
530
580
Figure 3. Normalized steady-state fluorescence spectra of 1,3,6,8-
tetraalkylpyrenes and pyrene 2 in the solid state.
Figure 3 shows the steady-state fluorescence spectra of
pyrene 2 and the prepared 1,3,6,8-tetraalkylpyrenes in the solid
state. Pyrene 2 exhibited a broad excimer emission peak centered
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at 467 nm in the solid state (Figure 3, orange line).[17] When Me
groups were introduced to the pyrene core, an emission peak was
observed at 445 nm along with shoulders at 472 and 430 nm,
implying the additional contribution of excimers. Interestingly, 1-
Et showed a sharp emission band at 415 nm, comparable to that
0~4 ns
4
8
1
~8 ns
~12 ns
2~16 ns
16~22 ns
2
2
3
2~28 ns
8~38 ns
8~44 ns
2 2
observed in the CH Cl solution (408 nm). Although the other
1,3,6,8-tetraalkylpyrenes bearing longer linear alkyl chains,
44~56 ns
except 1-Pr and 1-Hep, showed emission bands at ca. 417–419
nm similar to that of 1-Et, they were accompanied by broad
emission bands at approximately 440 nm. In contrast, 1-Pr
showed em at 433 nm and the introduction of heptyl, isobutyl
3
80
430
480
530
580
630
(
Figure S14), and neopentyl (Figure S15) groups hypochromically
shifted the maxima to 409, 408, and 411 nm, respectively.
Representative time-resolved fluorescence spectra of 1-Et
and 1-Dod are shown in Figures 4 and 5, respectively. The time-
resolved fluorescence spectrum of the solid-state sample of 1-Et
clearly showed that the observed emissions originated from the
monomer (Figure 4), suggesting that the pyrene cores are
separated in the solid state by Et groups, suppressing excimer
formation. In contrast, broad emission bands at approximately
Figure 5. Time-resolved fluorescence spectra of 1-Dod in the solid state; brown:
-4 ns, red: 4-8 ns, orange: 8-12 ns, yellow: 12-16 ns, light green 16-22 ns,
green: 22-28 ns, cyan: 28-38 ns, blue: 38-44 ns, navy blue: 44-56 ns.
0
We estimated the solid-state fluorescence lifetimes of several
pyrene derivatives and found that 1-Et have two representative
lifetimes of ca. 40 and 80 ns (Table S1). In addition, the rate
constants for the solid-state fluorescence radiative and
nonradiative decays of 1-Bu were five times and one-fiftieth
compared to those in diluted solution, respectively.
These results indicate that the decay pathways, i. e., F, can
be controlled using the alkyl group by varying the molecular
orientations in solid state (vide infra).
450 nm appeared over several tens of nanoseconds for the 1-Bu
(
Figure S18) and 1-Dod (Figure 5) samples, which were attributed
to long-lived excited species compared to the monomer species.
The time-resolved fluorescence spectrum of 1-isoBu (em = 408
nm) indicated monomer fluorescence rather than that arising from
an excimer (Figure S19).
F
The fluorescence quantum yield ( ) for each compound is
listed in Table 2 (For fluorescence lifetime, see: Table S1). It
should be noted that, among the prepared 1,3,6,8-
tetraalkylpyrenes, pyrenes bearing linear alkyl groups with even-
Crystal structures
To understand the effects of the alkyl chains on the solid-state
photophysical properties, X-ray crystallographic analyses were
performed. Fine crystals suitable for X-ray crystallography were
obtained by recrystallization from CHCl
Et, 1-Pr, 1-Bu, and 1-Pen. The molecular structures of the
number carbons exhibited high 
contrast, pyrenes containing linear alkyl groups with odd-number
carbons showed smaller  values for 1-Me, 1-Pen, and 1-Non,
and larger  values for 1-Pr, 1-Hep, and 1-Und. This
F
values except for 1-Dod. In
3
and MeOH for 1-Me, 1-
F
F
1
,3,6,8-tetraalkylpyrenes determined at 123 K are shown in
phenomenon may arise from the packing mode of alkyl chains in
the solid state, and the more effective interaction of even-
numbered alkyl chains between adjacent columns (vide infra)
suppresses vibrational relaxation to enhance the quantum
Figure 6.[ It should be noted that the conformation of the alkyl
chains was not disordered in all samples and 1-Et and 1-Pr
adopted similar geometries. The ethyl and propyl groups were
oriented orthogonally to the pyrene ring, where two pairs of alkyl
group on each side were oriented in the opposite direction (C2h
symmetry).[ In contrast, 1-Bu and 1-Pen contained two alkyl
groups at the cater-corner positions in the same plane as the
pyrene core, and the other two alkyl chains were orthogonal to the
19]
yields.[18] It should be noted that 1-Et exhibited the highest 
0
of
F
.88, higher than that of unsubstituted pyrene 2 (
F
= 0.68).[
8,9,17]
20]
0
6
1
5
9
1
1
~6 ns
i
pyrene ring and oriented in the opposite direction (C symmetry).
~16 ns
The conformation of the alkyl chains was an anti-conformation
except for the C–C bond between the - and -carbons of the
orthogonally oriented Bu and Pen groups (gauche-conformation).
From theoretical calculations, it was proposed that the Bu groups
in the same plane as the pyrene core participated in C-H -
conjugation (vide infra).[
6~52 ns
2~94 ns
4~140 ns
40~190 ns
90~278 ns
9]
Although polymorphism was observed for 1-Pen, em and the
molecular structure of the pyrene core in the crystals were
essentially the same for both forms. In the Figure 6, the molecular
structures of the polymorphism are shown, where two asymmetric
units of the isolated pair reside in the crystal. Differences in the
asymmetric units manifested in the orientation of the terminal
methyl groups of the pentyl groups that were in the same plane
as the pyrene ring and the terminal propyl groups of the
3
80
430
480
530
580
630
Figure 4. Time-resolved fluorescence spectra of 1-Et in the solid state; brown:
-6 ns, red: 6-16 ns, orange: 16-52 ns, light green 52-94 ns, green: 94-140 ns,
cyan: 140-190 ns, blue: 190-278 ns.
0
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Figure 6. Molecular structures of the 1,3,6,8-tetraakylpyrenes bearing linear alkyl groups in the crystal.
For 1-Me, 1-Et, and 1-isoBu, the molecular orientation in the
crystal was classified as herringbone, and the vertical distances
(
7
v) between the two parallel pyrene cores were 3.524, 4.982, and
.153 Å, respectively.[ The face-to-face pair of 1-Me and 1-Et
25]
slipped toward the short axis (s) of pyrene by 4.316 and 5.320 Å,
respectively, whereas that of 1-isoBu slipped towards the long
axis (l) of pyrene by 5.342 Å. As the distance of the two parallel
Table 3. Distances between the two pyrene cores.
Figure 7. Molecular structures of the 1,3,6,8-tetraakylpyrenes bearing branched
alkyl groups in the crystal.
orthogonally oriented pentyl groups. All pentyl groups of one of
the asymmetric units adopted an anti-conformation, whereas the
two pentyl groups in the same plane as the pyrene ring in the other
asymmetric unit adopted a gauche-conformation for the terminal
methyl group. The polymorphism likely arose from conformational
differences of the alkyl chains, which affected the interaction
between adjacent columns (vide infra).
The branched alkyl groups in 1-isoBu and 1-neoPen adopted
the same orientations, in which the two pairs of alkyl groups
occupied the orthogonal position on opposite sides of the pyrene
ring (C2h symmetry; Figure 7).
v (Å)
l (Å)
s (Å)
c (Å)
1
-Me
-Et
3.543
4.982
7.153
5.929
5.764
3.472
0.022
0.086
5.342
4.584
6.692
1.668
4.316
5.320
0.534
4.966
7.792
0.084
6.276
6.283
7.109
7.207
6.611
7.913
1
1-isoBu
2
2
2
(Form I)[22]
(Form II)[23]
(Form III)[24]
Crystal packing
Herein, we obtained three types of packing systems, which were
categorized as herringbone (1-Me, 1-Et, and 1-isoBu), slipped-
parallel (1-Pr, 1-Bu, 1-Pen, and 1-neoPen), or isolated-pair types
(
1-Pen).[21] For 1-Pen, polymorphism, including slipped-parallel
and isolated-pair types, was observed (vide supra).
Herringbone packing
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pyrene cores (v) decreased, the emission wavelength exhibited a
bathochromic shift (1-Me = 445 nm, 1-Et = 415 nm, and 1-isoBu
Among the 1,3,6,8-tetraalkylpyrenes examined, 1-Et with
herringbone-type packing showed the highest quantum yield (
of 0.88, higher than that of the parent pyrene 2 as well as 1-Me
and 1-isoBu, which have shorter and longer values,
F
)
=
408 nm). The theoretical calculations suggested that pyrene
formed an excimer with distances between the two parallel pyrene
rings of 3.410 Å for v and 1.524 Å for l.[26] These calculations
support the model where 1-Me forms an excimer in the crystal,
resulting in an emission band at 445 nm. In contrast, for 1-Et and
v
respectively. This is likely due to the short contacts between the
orthogonally oriented pyrenes via aromatic and aliphatic C–H-
interactions that immobilize excitons (Figure 8). Indeed, the Et
groups infilled the space between the pyrene cores and formed
multiple C-H- interactions, preventing excimer formation and
facilitating non-emitting vibrational relaxation. Although the
orientation of the two orthogonally oriented pyrene cores in 1-Me
1-isoBu, pyrene chromophores were separated by alkyl groups,
suppressing excimer formation. These molecular orientations in
the crystals are consistent with the observed emission spectra (1-
Me = 445 nm, 1-Et = 415 nm, and 1-isoBu = 408 nm).
It is well-known that pyrene 2 also forms herringbone-type
packing, and three polymorphic forms were reported, as
summarized in Table 3. In forms I and II, the distance between
the two pyrene cores in the face-to-face orientation is >5.7 Å,
indicating that the two pyrene cores shift toward both the long and
short axes, resulting in a large void between them. Therefore, an
exciton could couple with an adjacent pyrene even in the solid
state, resulting in a broad excimer emission at approximately 467
nm. However, for 1-Et and 1-isoBu, orthogonally oriented alkyl
groups filled the void between the two parallel pyrene rings,
suppressing excimer formation.
was quite similar to that of 1-Et (Figure 8, top and middle), its 
was significantly lower compared to that of 1-Et.
F
In contrast, the contact between the orthogonally oriented
pyrene cores of 1-isoBu occurs via a four-point C-H- interaction
between the benzylic methylene and one methyl group (Figure 8,
bottom). This suggests that the predominant C-H- interaction of
the sterically hindered alkyl group occurs in aliphatic C-H bonds
rather than in aromatic C-H bonds.
Figure 9. Interactions between the orthogonally oriented pyrene cores in
slipped-parallel-type packing.
Slipped-parallel
The solid-state photophysical properties of 1,3,6,8-
tetraalkylpyrenes exhibiting slipped-parallel-type packing
structure can be discussed with respect to the relative position of
the two parallel pyrene rings. Figure 9 shows the two pyrene rings
of 1-Pr, 1-Bu, 1-Pen (slipped-parallel), and 1-neoPen, where
alkyl groups on one of the two 1,3,6,8-tetraalkylpyrenes were
omitted for clarity, and face-to-face distances (v) and slipped
distances between the centers of the pyrene rings in the long (l)
and short (s) axes are shown. It should be noted that the face-to-
face distance between the two pyrene rings (v) was similar for all
samples (3.417–3.518 Å). In contrast, a significant difference was
Figure 8. Packing mode and interactions between the orthogonally oriented
pyrene cores in herringbone-type packing. Alkyl groups on one of the two
1,3,6,8-tetraalkylpyrenes were omitted for clarity,
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observed in slipping of the two pyrene cores (s and l), where 1-Pr
largely shifted toward the short axis (s) by 3.045 Å rather than the
long axis (l = 1.318 Å). These parameters (s and l) of 1-Bu were
isolated-pair-type crystals, respectively (Figure 10, side view).
These differences in the relative orientation of adjacent columns
likely arise from the weak van der Walls interactions between the
alkyl groups.
1.946 and 3.595 Å, respectively, and 1-Pen and 1-neoPen
showed a similar slipping mode as that of 1-Bu. In these cases,
the two methylenes at the - and -positions of one alkyl group
contributed to the C-H- interaction, which is in sharp contrast
with 1-Pr, where two C-H bonds of the -methylene formed a C-
H- interaction. The 1-Pr compound showed a significant
bathochromic shift (em = 433 nm) in its solid-state fluorescence
spectrum, where the slipping distance along the long axis (l) was
short (1.318 Å). For 1-Bu and 1-Pen, the two pyrene rings slipped
by 3.595 and 3.540 Å, respectively, along the long axis, and em
was observed at 418 nm. Slipping in the long axis direction
The polymorphic samples of 1-Pen showed a slightly different
solid-state emission. The crystals of slipped-parallel and isolated-
pair 1-Pen exhibited em at 421 and 418 nm, respectively. The
slipped-parallel crystals also showed relatively high intensities at
ca. 440 and 470 nm than isolated-pair crystals. The drop-casted
sample of 1-Pen showed a broad emission at 471 nm, which is in
sharp contrast to the same fluorescence profile of 1-Et in crystal
and film forms (Figures S22 and S23).
increased (l = 4.428 Å) when neoPen was introduced, and the Conclusions
fluorescence spectrum was not largely changed compared to
spectrum of the compound in the dilute solution. These results
indicate that the slipping toward the long axis rather than the short
axis significantly contributed to the suppression of excimer
formation in the solid state.
We demonstrated the systematic synthesis of 1,3,6,8-
tetraalkylpyrenes with 14 different alkyl groups and the
subsequent determination of their solid-state photophysical
properties. The introduction of alkyl groups at the 1-, 3-, 6-, and
Among the four abovementioned pyrene derivatives, 1-Bu
8-positions caused bathochromic shifts in the emission
F
showed the best quantum yield ( ) of 0.79. The formation of C–
wavelength from the parent pyrene, but emission peaks did not
vary based on the structures of the alkyl groups in the dilute
solution. However, in the solid state, emission wavelengths and
quantum yields varied depending on the structures of the
introduced alkyl chains. Among the pyrene derivatives examined,
tetraethylpyrene 1-Et showed the highest fluorescence quantum
yield of 0.88, higher than the parent pyrene. The fluorescence
maximum of 1-Et in the solid state did not shift from that observed
H- interactions between the pyrene core and Bu group in-plane
likely contributed to the suppression of vibrational relaxation.[27]
Isolated-pair
The polymorphism was observed in the X-ray crystallographic
analysis of 1-Pen, involving slipped-parallel and isolated-pair-type
packings. Both packing modes contained independent columns
(
Figure 10), including a slipped-parallel stacking in the same
manner as that of 1-Bu. Therefore, the face-to-face interaction
between pyrene cores was essentially unchanged, and the
distances between the two pyrene cores in the face-to-face
orientation ranged from v = 3.499 to 3.604, l = 3.412 to 3.561, and
s = 1.983 to 1.959 Å.
-1
in solution, in contrast to the large bathochromic shift (5396 cm )
of pyrene in the solid state, which is a result of excimer formation.
The molecular orientation of the 1,3,6,8-tetraalkylpyrenes in
crystal form was revealed by X-ray crystallography and
categorized into three types, namely herringbone, slipped-parallel,
or isolated-pair-type. Comparison of the relative orientation of
pyrene cores in the crystals indicated that the face-to-face and
slipping distances along the long axis of the pyrene ring exerted
strong effects on photophysical properties likely due to excimer
formation. The alkyl groups affected the emission color and
quantum yield. Interactions between the pyrene core and alkyl
groups through C-H- interaction improved the fluorescence
quantum yield by suppressing the vibrational relaxation of
excitons.
Although alkyl groups are often introduced to functional
materials and/or -molecules to improve their solubility and
stability, the effects of these groups on the properties of the
materials have not been systematically examined in many cases.
It was demonstrated that the alkyl groups exerted no significant
electronic effects on the chromophores, but largely contributed to
the emission color and quantum yields in the solid state by
modifying the molecular orientation of the crystals. Both the length
and branching of the alkyl substituents were important for
controlling the solid-state photophysical properties, which may be
expandable to other chromophoric materials and -systems.
Figure 10. Two polymorphism of 1-Pen and its packings. H atoms were omitted
for clarity
The difference in the packing of the two polymorphic forms is
the orientation of columns, where columns are directed in a
parallel and staggered manner in the slipped-parallel and
Experimental Section
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General: All reactions using air- and moisture-sensitive compounds were
carried out by the standard Schlenk techniques under a nitrogen
atmosphere unless otherwise noted. Dehydrated THF were purchased
from Kanto Chemical Company and purified by SPS[28] prior to use.
Dehydrated 1,4-dioxane was purchased from Wako Pure Chemical
Industries and used as received. Grignard reagents were prepared by a
standard procedure or purchased from Aldrich or TCI. These Grignard
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scientific
Research (A) (JP25248025, to NK), a Grant-in-Aid for Young
Scientists (A) (JP16H06039, to TI), and Grant-in-Aid for Scientific
Research on Innovative Areas “Middle Molecular Strategy”
reagents were used after titration using I
2
. 1,3,6,8-Tetrabromopyrene was
(
(
JP16H01150 and JP18H04410, to TI) and “-System Figuration”
JP17H05155, to YT). The computations were performed using
prepared according to literature.[29] Analytical grade CH
2
Cl for UV-Vis and
2
fluorescence measurements was purchased from Kishida and was used
as received.
the Research Center for Computational Science, Okazaki, Japan.
Synthesis of 1,3,6,8-tetraalkylpyrenes:[12c] To a 100 mL three necked
flask containing 1,3,6,8-tetrabromopyrene (414 mg, 0.80 mmol) and
Keywords: alkyl • crystal engineering • fluorescence • pyrene •
solid-state structure
NiCl
RMgCl (in THF, 9.6 mmol). After refluxing for 48 h, 1 M HCl aq. was
carefully added to the reaction mixture and then diluted by 50 mL of H O.
The aqueous layer was extracted by Et O (50 mL x 2). The organic layer
was dried over Na SO , concentrated, and purified by silica gel column
chromatography (eluent: hexane or hexane/EtOAc 9/1). Further
purification by GPC (eluent: CHCl ) followed by recrystallization from
CH Cl/MeOH (twice) gave a pure sample for the analysis of photophysical
properties.
2
(dppe) (21 mg, 0.04 mmol) was added 1,4-dioxane (24 mL) and
[
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a
FP-8500
2
2
Solid-state UV/Vis absorption and fluorescence spectroscopy and
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SHELXS97, SHELXT,[31] or SIR2008[32]). The structure was refined on
[30]
2
by the full-matrix least-squares method using SHELXL97[30] or
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[
33]
CCDC 1939308 (1-Me at 123 K), 1939309 (1-Et at 123 K), 1939310 (1-Pr
at 123 K), 1939311 (1-Bu at 123 K), 1939312 (1-Pen (Slipped-parallel)
at 123 K), 1939313 (1-Pen (Isolated-pair) at 123 K), 1939314 (1-isoBu
at 123 K), 1939315 (1-neoPen at 123 K), 1939316 (1-Et at 293 K),
1939317 (1-Pr at 293 K), and 1939318 (1-isoBu at 293 K) contain the
supplementary crystallographic data for this paper. These data are
provided free of charge by The Cambridge Crystallographic Data Center.
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Chemistry - A European Journal
10.1002/chem.201903224
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Entry for the Table of Contents (Please choose one layout)
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This study details systematical
evaluation of the fluorescent
T. Iwasaki,* S. Murakami, Y. Takeda, G.
Fukuhara, N. Tohnai, Y. Yakiyama, H.
Sakurai, N. Kambe*
properties of 1,3,6,8-tetraalkylpyrenes
with various alkyl substituents in both
the solution and solid states. Although
bathochromic shifts were observed for
unsubstituted pyrene in the emission
spectrum of the solid-state from those
of solution state, tetraalkylpyrenes
bearing appropriate alkyl groups
showed violet luminescence in the
solid state similar to that observed in
dilute solutions.
Page No. – Page No.
Molecular Packing and Solid-state
Photophysical Properties of 1,3,6,8-
Tetraalkylpyrenes
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