Material design for piezochromic luminescence: Hydrogen-bond-directed assemblies of a pyrene derivative

Article abstract of DOI:10.1021/ja0677362

A novel design principle for materials showing piezochromic luminescence is proposed. Amide-substituted tetraphenylpyrene C6TPPy, composed of a planar fluorescent core and multiple hydrogen-binding sites, displays a remarkable change and restoration of its luminescence color by pressing with a spatula and heating, respectively. Copyright

Full text of DOI:10.1021/ja0677362

Published on Web 01/24/2007
Material Design for Piezochromic Luminescence: Hydrogen-Bond-Directed
Assemblies of a Pyrene Derivative
Yoshimitsu Sagara, Toshiki Mutai, Isao Yoshikawa, and Koji Araki*
Institute of Industrial Science, UniVersity of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan
Received October 30, 2006; E-mail: araki@iis.u-tokyo.ac.jp
For more than a decade, tunable solid-state organic luminescent
materials have been attracting considerable interest in various fields
of application.1 The tuning and switching of the luminescence of
organic solids by controlling the mode of molecular packing instead
of alteration of the chemical structure are attractive targets for both
fundamental research and practical applications, but successful
examples are quite limited and have been reported only recently.2,3
In this letter, we will present a novel material design for piezo-
chromic luminescence based on a pressure-dependent mode of
molecular packing. Piezochromism4,5 is the phenomenon of color
change caused by mechanical grinding and reversion to the original
color by heating or recrystallization, in which the presence of two
different pressure-dependent stable or metastable states is essential.4
Figure 1. (a) Molecular structure of C6TPPy. (b) The B-form under room
light (left) and under UV irradiation at 365 nm (right). (c) The G-form
under room light (left) and under UV irradiation at 365 nm (right). (d)
Luminescent image of the B-form solid cast on a glass plate after pressed
Up to now, organic piezochromic substances that show a color
change by their luminescence are quite limited.6 Since luminescence
can be detected with high sensitivity, materials that show piezo-
chromic luminescence can find a wide variety of applications such
as optical recording and strain- or pressure-sensing systems.
For a better understanding and prediction of molecular packing
in organic crystals, Kitaigorodskii’s close-packing principle7 and
Etter’s first hydrogen-bond rule8 are regarded as useful concepts
and have been discussed extensively.9 Using these concepts as
competing factors, we adopted a novel design principle to incor-
porate two strongly demanding factors for molecular packing, a
planar aromatic core and multiple hydrogen-bonding sites, into a
single molecular structure. The crystals of disk-shaped aromatics
tend to be closely packed with an interplane distance of 0.34-
0.36 nm,4 while hydrogen-bond-directed columnar assemblies have
a wider interplane distance of 0.47-0.48 nm.10 The designed
molecule, an amide-substituted tetraphenylpyrene derivative C6TPPy,
successfully showed a piezochromic luminescence by pressure-
induced alteration of the molecular packing and subsequent heating-
induced restoration. This design principle is simple and applicable
to developing a broad range of materials showing piezochromic
luminescence.
1,3,6,8-Tetraphenylpyrene (TPPy) has a planar aromatic core and
is a highly efficient fluorophore showing strong blue luminescence
in solution (quantum yield Φ ) 0.9 in cyclohexane).11 To the para
position of the phenyl groups of this parent molecule, four hexyl
amide units were introduced as the multiple hydrogen-bonding sites
(C6TPPy, Figure 1a). The addition of methanol to a chloroform
solution of C6TPPy resulted in precipitation of a white powder
(B-form), which exhibited strong blue luminescence upon irradiation
of UV-light (Figure 1b). Once the precipitate was formed, this
B-form solid did not dissolve again in chloroform or other common
organic solvents but dissolved only slowly in warm 1-methyl-2-
pyrrolidinone. Interestingly, this blue-emitting white solid (B-form)
was converted to a yellowish solid showing a strong greenish
luminescence (G-form, Figure 1c) simply by pressing it with a
spatula, and this change occurred only at the pressed area (Figure
1d). Grinding in a mortar thoroughly converted the B-form powder
with a metal block shaped ‘IIS’.
Figure 2. (a) Absorption and luminescence spectra of the B-form (blue),
the G-form (green), and a chloroform solution (black; 1.0 × 10-5 M) of
C6TPPy. (b) Infrared absorption spectra of the B-form (solid line) and the
G-form (dotted line); normalized to the CH stretching peak at 2922 cm-1
.
to the G-from. This G-form solid after grinding readily dissolved
in chloroform and reprecipitation from this solution gave the B-from
powder again. This is a typical example of piezochromic lumines-
cence.
A chloroform solution of C6TPPy showed structureless broad
absorption and fluorescence bands (Φ ) 0.7, life time τ ) 1.3 ns)
at 392 and 439 nm (Figure 2a), respectively, which are not much
different from those of TPPy.12 In the solid state, the emission band
of the B-form (Φ ) 0.3, τ ) 3.1 ns) appeared at a position similar
to that in solution, but the G-form solid showed considerable red-
shifted emission at 472 nm (Φ ) 0.3, τ ) 3.2 ns). Tetraphen-
ylpyrene derivatives are generally yellowish in color in the solid
state showing blue-green to green luminescence, and the G-form
solid falls in this category.13 It is noteworthy that the absorption
band of the B-form solid appeared at the higher energy side,
resulting in a white appearance. Since it is well documented that
H-type aggregation in the columnar stacks induces a blue shift of
the absorption, it is likely that the B-form solid has a columnar
molecular assembly (see Supporting Information).
To clarify the different spectroscopic properties of these two
solids, their solid-state structures were studied. The IR spectra of
C6TPPy in the B- and G-forms were essentially the same, and the
lower-shifted peak of the amide NH stretching at 3282 cm-1
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C O M M U N I C A T I O N S
green after thermal annealing, suggesting the importance of the
solid-state arrangement of the luminophores. In our study, we
showed the critical role of the quadruple amide hydrogen bonds
for construction of the well-defined columnar assembly in the blue-
emitting B-form solid. Intermolecular amide hydrogen bonds
contributed to the stability of this form under an ambient condition,
showing that the B-form has the hydrogen bond-directed structure.
Applied pressure to the B-form solid caused disruption of the
columnar structure and yielded the G-form solid with the poorly
ordered molecular packing. Since disruption of the columnar
packing resulted in a different luminophore arrangement within the
solid, it is not surprising that the G-form solid showed greenish
luminescence. Thus, the design principle of introducing the planar
fluorescent aromatic core and the multiple hydrogen-bonding sites
within the same molecular structure was shown to be effective for
designing materials displaying piezochromic luminescence. Our
preliminary study on a perylene derivative having two amide side
chains also showed change in luminescence upon grinding, though
thermal reversion was not observed in this case. Therefore, the
design principle we adopted here could be widely applicable to
other molecular systems.
Figure 3. Powder X-ray diffraction patterns of C6TPPy: (a) the B-form
obtained by reprecipitation from chloroform-methanol; (b) the G-form
prepared by grinding the B-form solid.
indicated the formation of strong hydrogen bonds. Since no free
amide NH stretching was observed at all, the amide units were
fully hydrogen-bonded both in the B- and G-form solids. However,
closer examination of the spectra revealed that the NH stretching
peak of the G-form solid was apparently broader and extended to
the higher wavenumber side (Figure 2b), indicating the presence
of weakly hydrogen-bonded amide units. Therefore, the hydrogen
bonds between the amide groups in the G-form were not in a
uniform pattern but suffered a different extent of deformation by
the applied pressure. A powder X-ray diffraction (XRD) pattern of
the B-form solid (Figure 3a) showed clear reflection peaks,
indicating that C6TPPy molecules in the B-form were packed in a
relatively well-defined microcrystalline-like structure. The observed
diffraction pattern can be interpreted by assuming a monoclinic or
triclinic unit cell with a ) 1.90 nm, b ) 1.57 nm, and γ ) 93°,
though the lattice parameter c is difficult to read from the diffraction
pattern. The lattice parameters are well reproduced by a compu-
tational study on a columnar packing model of C6TPPy (see
Supporting Information). Therefore, it is quite likely that the B-form
solid has a columnar molecular assembly due to the quadruple
hydrogen bonds, and the observed blue shift of the absorption band
supports this conclusion. The estimated energy of the transition
dipole interaction showed that the inter-ring distance of 0.48 nm is
sufficient for inducing the observed blue shift of the absorption
band if the rings are fixed by the quadruple hydrogen bonds (see
Supporting Information). On the other hand, the G-form solid did
not show any noticeable diffraction in the XRD profile (Figure 3b).
The result indicated that the crystalline-like ordered structure of
the B-form was disrupted in the G-form solid, agreeing well with
the IR results.
Acknowledgment. This work was partly supported by a Grant-
in-Aid for Scientific Research (B) (No. 18310076) from the Ministry
of Education, Culture, Sports, Science and Technology (MEXT)
of Japan.
Supporting Information Available: Synthesis, characterization,
DSC analysis, molecular modeling of C6TPPy, and an effect of
transition dipole interaction in the solid state. This material is available
free of charge via the Internet at http://pubs.acs.org.
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