Multistimuli two-color luminescence switching via different slip-stacking of highly fluorescent molecular sheets

Article abstract of DOI:10.1021/ja1044665

Color tuning and switching of the solid-state luminescence of organic materials are attractive subjects for both the fundamental research and practical applications such as optical recording. We report herein cyanostilbene-based highly luminescent molecular sheets which exhibit two-color fluorescence switching in response to pressure, temperature, and solvent vapor. The origin for the multistimuli luminescence switching is the two-directional shear-sliding capability of molecular sheets, which are formed via intermolecular multiple C-H...N and C-H...O hydrogen bonds. The resulting two distinctive crystal phases are promoted by different modes of local dipole coupling, which cause a substantial alternation of π-π overlap. These changes can be directly correlated with the subsequent intermolecular excitonic and excimeric coupling in both phases, as demonstrated by an in-depth theory-assisted spectroscopic and structural study. Finally, we have prepared a first device demonstrator for rewritable fluorescent optical recording media which showed multistimuli luminescence tuning with fast response. Our multistimuli responsive system is unique in terms of the slip-stacking of molecular sheets and thus provides a novel concept of rewritable fluorescent optical recording media.

Full text of DOI:10.1021/ja1044665

Published on Web 09/14/2010
Multistimuli Two-Color Luminescence Switching via Different
Slip-Stacking of Highly Fluorescent Molecular Sheets
Seong-Jun Yoon,† Jong Won Chung,† Johannes Gierschner,*,‡ Kil Suk Kim,§
Moon-Gun Choi,| Dongho Kim,*,§ and Soo Young Park*,†
Center for Supramolecular Optoelectronic Materials and WCU Hybrid Materials Program,
Department of Materials Science and Engineering, Seoul National UniVersity, ENG 445,
Seoul 151-744, Korea, Madrid Institute for AdVanced Studies, IMDEA Nanoscience, UAM,
Modulo C-IX, AV. Toma´s y Valiente 7, Campus de Cantoblanco, 28049 Madrid, Spain,
Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry,
Yonsei UniVersity, Seoul 120-749, Korea, and Department of Chemistry and Center for
BioactiVe Molecular Hybrids, Yonsei UniVersity, Seoul 120-749, Korea
Received November 11, 2009; E-mail: parksy@snu.ac.kr (S.Y.P.); johannes.gierschner@imdea.org (J.G.);
dongho@yonsei.ac.kr (D.K.)
Abstract: Color tuning and switching of the solid-state luminescence of organic materials are attractive
subjects for both the fundamental research and practical applications such as optical recording. We report
herein cyanostilbene-based highly luminescent molecular sheets which exhibit two-color fluorescence
switching in response to pressure, temperature, and solvent vapor. The origin for the multistimuli
luminescence switching is the two-directional shear-sliding capability of molecular sheets, which are formed
via intermolecular multiple C-H· · ·N and C-H· · ·O hydrogen bonds. The resulting two distinctive crystal
phases are promoted by different modes of local dipole coupling, which cause a substantial alternation of
π-π overlap. These changes can be directly correlated with the subsequent intermolecular excitonic and
excimeric coupling in both phases, as demonstrated by an in-depth theory-assisted spectroscopic and
structural study. Finally, we have prepared a first device demonstrator for rewritable fluorescent optical
recording media which showed multistimuli luminescence tuning with fast response. Our multistimuli
responsive system is unique in terms of the slip-stacking of molecular sheets and thus provides a novel
concept of rewritable fluorescent optical recording media.
and/or stacking have been reported,4-9 although the in-depth
theoretical understanding of the phenomena has rather been
limited.
1. Introduction
In the past years, much attention has been paid on aggregation
phenomena of conjugated organic materials particularly related
to their applications in (opto-)electronic devices.1 To establish
meaningful structure-property relationships in the solid state,
the subtle interplay of molecular and supramolecular level
structures and the envisaged properties has to be understood.2,3
Tuning solid-state properties, in particular (dynamic) solid-state
fluorescence switching, with high efficiency and reproducibility
by controlling the molecular stacking without changing the
chemical structure of the constituting molecules not only
provides fundamental understanding but also has practical
application in optical recording and sensing. Various examples
of fluorescence color change driven by molecular aggregation
Within this line of idea, cyano stilbene derivatives have been
investigated in recent years,10,11 where the extremely large
fluorescence enhancement in the solid state (nanoparticle,
powder, gel, etc.) was attributed not only to the spatial
confinement effect but also to the formation of specific
supramolecular stacking architecture associated with the unique
electronic and geometrical characteristics of the designed
molecules. This cooperative behavior, commonly known as
aggregation-induced enhanced emission (AIEE),10 is of great
practical importance because of its solid-state applications,
particularly in (opto-)electronic devices. Moreover, highly
fluorescent materials in the solid state are expected to offer solid-
state luminescence switching when incorporated with stimuli-
responsive units. We have recently demonstrated various stimuli-
responsive AIEE systems, such as fluorescent photochromic
† Seoul National University.
‡ Madrid Institute for Advanced Studies, IMDEA Nanoscience, UAM.
§ Spectroscopy Laboratory for Functional π-Electronic Systems and
Department of Chemistry, Yonsei University.
| Department of Chemistry and Center for Bioactive Molecular Hybrids,
Yonsei University.
(4) (a) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi,
N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. (b) Sanz, N.;
Baldeck, P. L.; Nicoud, J. F.; Fur, Y. L.; Ibanez, A. Solid State Sci.
2001, 3, 867. (c) Gierschner, J.; Egelhaaf, H. J.; Mack, H. J.; Oelkrug,
D.; Martinez Alvarez, R.; Hanack, M. Synth. Met. 2003, 137, 1449.
(d) Schweikhart, K. H.; Hohloch, M.; Steinhuber, E.; Hanack, M.;
Lu¨er, L.; Gierschner, J.; Egelhaaf, H. J.; Oelkrug, D. Synth. Met. 2001,
121, 1641.
(1) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J.
Chem. ReV. 2005, 105, 1491.
(2) Gierschner, J.; Ehni, M.; Egelhaaf, H. J.; Milia´n Medina, B.; Beljonne,
D.; Benmansour, H.; Bazan, G. C. J. Chem. Phys. 2005, 123, 144914.
(3) (a) Curtis, M. D.; Cao, J.; Kampf, J. W. J. Am. Chem. Soc. 2004,
126, 4318. (b) Dong, J.; Solntsev, K. M.; Tolbert, L. M. J. Am. Chem.
Soc. 2009, 131, 662.
9
10.1021/ja1044665  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 13675–13683 13675

A R T I C L E S
Yoon et al.
Scheme 1. Molecular Structure of DBDCS and Its Local Dipoles
(dashed arrows) and Transition Dipole Moment µ(S0fS1) (double
arrow, the length of 12.6 D) and Dihedral Torsional Angles in
Isolated State and Single Crystal State (in brackets)
nanoparticles and polymer films,12 highly sensitive fluorescence
probes for organic vapors,13 array patterning of fluorescent
nanoparticles,14 fluorescent switchable smart gels,15 and fluo-
rescent switchable π-dimer crystals,9e which respond to various
external stimuli including light, organic vapor, acid, temperature,
and pressure. Meanwhile, some cyano distyrylbenzene (DSB)
derivatives have also been shown to exhibit reversible piezo-
chromism by C. Weder group.8c
Herein, we have synthesized a new AIEE-active cyano DSB
derivative, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(4-butoxyphenyl) acry-
lonitrile), DBDCS (see Scheme 1). DBDCS exhibits very high
solid-state fluorescence quantum yield (62%) due to the charac-
teristic AIEE process, as well as multistimuli two-color lumines-
cence switching. DBDCS uniquely forms fluorescent ‘molecular
sheets’ with stacking and shear-sliding capabilities, which are
responsible for stimuli-responsive luminescence switching behavior.
We have utilized this phenomenon to successfully develop rewrit-
able fluorescent optical recording media which showed fast-
responding multistimuli luminescence switching. This multistimuli
responsive system may find practical applications in optical
memory systems. Furthermore, it offers an intriguing example how
to understand and utilize structure-property relationships in the
solid-state fluorescence emission behavior. To gain further insight
into this phenomenon, we have comprehensively explored the
structural, optical and photophysical properties, assisted by quantum-
chemical calculations.
(5) (a) Davis, R.; Rath, N. P.; Das, S. Chem. Commun. 2004, 74. (b) Mutai,
T.; Satou, H.; Araki, K. Nat. Mater. 2005, 4, 685. (c) Zhang, H.; Zhang,
Z.; Ye, K.; Zhang, J.; Wang, Y. AdV. Mater. 2006, 18, 2369. (d) Mutai,
T.; Tomoda, H.; Ohkawa, T.; Yabe, Y.; Araki, K. Angew. Chem., Int.
Ed. 2008, 47, 9522. (e) Zhao, Y.; Gao, H.; Fan, Y.; Zhou, T.; Su, Z.;
Liu, Y.; Wang, Y. AdV. Mater. 2009, 21, 3165.
2. Experimental and Computational Methods
(6) (a) Davis, R.; Kumar, N. S. S.; Abraham, S.; Suresh, C. H.; Rath,
N. P.; Tamaoki, N.; Das, S. J. Phys. Chem. C 2008, 112, 2137. (b)
Kumar, N. S. S.; Varghese, S.; Rath, N. P.; Das, S. J. Phys. Chem. C
2008, 112, 8429. (c) Thomas, R.; Varghese, S.; Kulkarni, G. U. J.
Mater. Chem. 2009, 19, 4401. (d) Kumar, N. S. S.; Varghese, S.;
Suresh, C. H.; Rath, N. P.; Das, S. J. Phys. Chem. C 2009, 113, 11927.
(e) Zhang, Z.; Zhang, Y.; Yao, D.; Bi, H.; Javed, I.; Fan, Y.; Zhang,
H.; Wang, Y. Cryst. Growth Des. 2009, 9, 5069.
2.1. Synthesis and Characterization. (2Z,2′Z)-2,2′-(1,4-phe-
nylene)bis(3-(4-butoxyphenyl) acrylonitrile), DBDCS (see Scheme
1), was synthesized via Knoevenagel reaction of 4-butoxy-benzal-
dehyde and (4-cyanomethyl-phenyl)-acetonitrile. Full synthetic
1
details, H NMR, 13C NMR, mass spectroscopy, and elemental
analysis characterization are found in the Supporting Information
(SI).
(7) (a) Kishimura, A.; Yamashita, T.; Yamaguchi, K.; Aida, T. Nat. Mater.
2005, 4, 546. (b) Dong, Y.; Lam, J. W. Y.; Qin, A.; Li, Z.; Sun, J.;
Sung, H. H. Y.; Williams, I. D.; Tang, B. Z. Chem. Commun. 2007,
40. (c) Dong, Y.; Lam, J. W. Y.; Qin, A.; Sun, J.; Liu, J.; Li, Z.; Sun,
J.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. Chem.
Commun. 2007, 3255. (d) Srinivasan, S.; Babu, P. A.; Mahesh, S.;
Ajayaghosh, A. J. Am. Chem. Soc. 2009, 131, 15122.
2.2. Sample Preparation. Nanoparticle suspensions were ob-
tained by a simple reprecipitation method from DBDCS solution
in THF (2 × 10-5 mol L-1) by injection of water (distilled and
filtered by membrane filter with 0.2 µm pore size) in 1:4 volume
fraction under vigorous stirring.16 Single crystals were obtained
from the ethyl acetate solution at room temperature. Powder samples
were prepared by recrystallization from hot ethanol solutions as
pale whitish sheet-like crystallites. Thin films (thickness of 50 nm)
were fabricated on quartz substrates by vacuum deposition.
2.3. X-ray and Thermal Analysis. A single crystal was selected
under ambient condition, coated in epoxy, and mounted on the end
of a glass fiber. Crystal data collection was performed on a Bruker
CCD Apex diffractometer with Mo KR (λ ) 0.71073 Å) radiation
and a collector-to-crystal distance of 5.99 cm. Cell constants were
determined from a list of reflections found by an automated search
routine. Data were collected using the full sphere routine and
corrected for Lorentz and polarization effects. The absorption
corrections were based on fitting a function to the empirical
transmission surface as sampled by multiple equivalent measure-
ments using SADABS software. SAXS measurements were per-
formed on a GADDS (Bruker, Germany) equipped with a 2D area
detector, operating at 3 kW. XRD measurements were performed
on a Powder X-ray Diffractometry (Bruker, Germany), operating
at 3 kW. Differential scanning calorimetry (DSC) was performed
(8) (a) Gawinecki, R.; Viscardi, G.; Barni, E.; Hanna, M. A. Dyes Pigments
1993, 23, 73. (b) Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. J. Am.
Chem. Soc. 2007, 129, 1520. (c) Kunzelman, J.; Kinami, M.;
Crenshaw, B. R.; Protasiewicz, J. D.; Weder, C. AdV. Mater. 2008,
20, 119. (d) Kozhevnikov, V. N.; Donnio, B.; Bruce, D. W. Angew.
Chem., Int. Ed. 2008, 47, 6286. (e) Ito, H.; Saito, T.; Oshima, N.;
Kitamura, N.; Ishizaka, S.; Hinatsu, Y.; Wakeshima, M.; Kato, M.;
Tsuge, K.; Sawamura, M. J. Am. Chem. Soc. 2008, 130, 10044. (f)
Zhang, G.; Lu, J.; Sabat, M.; Fraser, C. L. J. Am. Chem. Soc. 2010,
132, 2160. (g) Tsukuda, T.; Kawase, M.; Dairiki, A.; Matsumoto, K.;
Tsubomura, T. Chem. Commun. 2010, 46, 3255. (h) Zhang, G.; Lu,
J.; Fraser, C. L. Inorg. Chem. 2010, 49, DOI: 10.1021/ic902591s.
(9) (a) Mizukami, S.; Houjou, H.; Sugaya, K.; Koyama, E.; Tokuhisa,
H.; Sasaki, T.; Kanesato, M. Chem. Mater. 2005, 17, 50. (b) Yagai,
S.; Seki, T.; Karatsu, T.; Kitamura, A.; Wu¨rthner, F. Angew. Chem.,
Int. Ed. 2008, 47, 3367. (c) Sagara, Y.; Kato, T. Angew. Chem., Int.
Ed. 2008, 47, 5175. (d) Sagara, Y.; Yamane, S.; Mutai, T.; Araki, K.;
Kato, T. AdV. Funct. Mater. 2009, 19, 1869. (e) Chung, J. W.; You,
Y.; Huh, H. S.; An, B. K.; Yoon, S. J.; Kim, S. H.; Lee, S. W.; Park,
S. Y. J. Am. Chem. Soc. 2009, 131, 8163. (f) Sagara, Y.; Kato, T.
Nat. Chem. 2009, 1, 605.
(10) (a) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem.
Soc. 2002, 124, 14410. (b) An, B. K.; Lee, D. S.; Lee, J. S.; Park,
Y. S.; Song, H. S.; Park, S. Y. J. Am. Chem. Soc. 2004, 126, 10232.
(c) An, B. K.; Gihm, S. H.; Chung, J. W.; Park, C. R.; Kwon, S. K.;
Park, S. Y. J. Am. Chem. Soc. 2009, 131, 3950.
on a Perkin-Elmer DSC7 at a heating rate of 5 °C min-1
.
1
2.4. Spectroscopic Characterization. H NMR spectra were
recorded on a JEOL JNM-LA300 (300 MHz) in CDCl3 solutions.
Mass spectra were measured using a JEOL, JMS AX505WA mass
spectrometer. Elemental analysis was carried out using a CE
instruments EA1110 elemental analyzer. FE-SEM images were
acquired on a JSM-6330F (JEOL). UV-visible absorption spectra
(11) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H. J.; Hanack,
M.; Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902.
(12) (a) Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y.
Angew. Chem., Int. Ed. 2004, 43, 6346. (b) Lim, S. J.; An, B. K.;
Park, S. Y. Macromolecules 2005, 38, 6236.
(13) An, B. K.; Kwon, S. K.; Park, S. Y. Bull. Korean Chem. Soc. 2005,
26, 1555.
(14) An, B. K.; Kwon, S. K.; Park, S. Y. Angew. Chem., Int. Ed. 2007, 46,
1978.
(16) For general preparation methods of organic nanoparticles, see: (a)
Kasai, H.; Nalwa, H. S.; Okada, S.; Oikawa, H.; Nakanish, H.
Handbook of Nanostructured Materials and Nanotechnology; Aca-
demic Press: New York, 2000; Vol. 5, Chapter 8, 433-473. (b) Horn,
D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4330.
(15) (a) Chung, J. W.; An, B. K.; Park, S. Y. Chem. Mater. 2008, 20, 6750.
(b) Chung, J. W.; Yoon, S. J.; Lim, S. J.; An, B. K.; Park, S. Y. Angew.
Chem., Int. Ed. 2009, 48, 7030.
9
13676 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Multistimuli Luminescence Switching of Molecular Sheets
A R T I C L E S
Figure 1. (a) UV-visible absorption spectra of DBDCS in THF solution (c ) 2 × 10-5 mol L-1, black line) and DBDCS nanoparticles in THF/water
mixture (c ) 2 × 10-5 mol L-1, green line). (b) PL spectra of DBDCS in THF solution (black line) and DBDCS nanoparticles in THF/water mixture (green
line). Insets show the fluorescence images of DBDCS in THF solution and DBDCS nanoparticles in THF/water mixture under 365 nm UV light.
were recorded on a Shimazu, UV-1650 PC spectrometer. Photo-
luminescence spectra were obtained using a Shimazu, RF 5301 PC
spectrometer. The relative fluorescence quantum yield was measured
using 9,10-diphenylanthracence (DPA) in benzene as a standard
reference (1 × 10-4 mol L-1, Φf ) 83%). Time-resolved fluores-
cence lifetime experiments were performed by the time-correlated
single-photon-counting (TCSPC) technique. As an excitation light
source, we used a Ti:sapphire laser (Mai Tai BB, Spectra-Physics)
which provides a repetition rate of 800 kHz with ∼100 fs pulses
generated by a homemade pulse-picker. The output pulse of the
laser was frequency-doubled by a 1 mm thickness of a second
harmonic crystal (ꢀ-barium borate, BBO, CASIX). The fluorescence
was collected by a microchannel plate photomultiplier (MCP-PMT,
Hamamatsu, R3809U-51) with a thermoelectric cooler (Hamamatsu,
C4878) connected to a TCSPC board (Becker and Hickel SPC-
130). The overall instrumental response function was about 25 ps
(the full width at half-maximum (fwhm)). A vertically polarized
pump pulse by a Glan-laser polarizer was irradiated to samples,
and a sheet polarizer, set at an angle complementary to the magic
angle (54.7°), was placed in the fluorescence collection path to
obtain polarization-independent fluorescence decays. The femto-
second time-resolved transient absorption (TA) spectrometer was
pumped by a Ti:sapphire regenerative amplifier system (Quantronix,
Integra-C) operating at 1 kHz repetition rate and an optical detection
system. The frequency doubled 400 nm pulses had a pulse width
of ∼100 fs and an average power of 1 mW which were used as
pump pulses. White light continuum (WLC) probe pulses were
generated using a sapphire window (2 mm of thickness) by focusing
of small portion of the fundamental 800 nm pulses. The time delay
between pump and probe beams was carefully controlled by making
the pump beam travel along a variable optical delay (Newport,
ILS250). Intensities of the spectrally dispersed WLC probe pulses
are monitored by miniature spectrograph (OceanOptics, USB2000+).
To obtain the time-resolved transient absorption difference signal
(∆A) at a specific time, the pump pulses were chopped at 25 Hz
and absorption spectra intensities were saved alternately with or
without pump pulse. Typically, 6000 pulses excite samples to obtain
the TA spectra at a particular delay time. The polarization angle
between pump and probe beam was set at the magic angle (54.7°)
in order to prevent polarization-dependent signals. Cross-correlation
fwhm in pump-probe experiments was less than 200 fs and chirp
of WLC probe pulses was measured to be 800 fs in the 400-800
nm region. To minimize chirp, all reflection optics in probe beam
path and 2 mm path length of quartz cell were used. The three-
dimensional data sets of ∆A versus time and wavelength were
subjected to singular value decomposition and global fitting to
obtain the kinetic time constants and their associated spectra using
Surface Xplorer software.
B3LYP functional and 6-31+G** basis set. The dipole moment
was calculated on the basis of the molecular geometry from single-
crystal X-ray analysis. The transition dipole moment was calculated
on the basis of the molecular geometry from single-crystal X-ray
analysis at the (time-dependent) DFT B3LYP level, using the
6-31+G** basis set. For a first qualitative picture of the excitonic
interactions in the system, exciton splitting calculation was
performed by a supramolecular approach on a coplanar molecular
dimer pair, using ZINDO/S (Zerner’s spectroscopic parametrization
for intermediate neglect of differential overlap for spectroscopy)
method, taking into account full single configuration interaction
(SCI) over the occupied- and unoccupied-types of molecular orbitals
(MOs).
3. Results
Like structurally similar compounds,10-15 DBDCS exhibited
intense solid-state fluorescence due to the characteristic AIEE
process. To easily examine the AIEE process of DBDCS free
from artificial effects induced by optically thick samples
typically observed for deposited films or powders and also to
conveniently measure solid-state fluorescence quantum yields,11,18
DBDCS nanoparticle suspensions were used. The nanoparticles
were prepared by a simple precipitation method, which generates
stable suspensions without surfactants by (rather kinetically
controlled) spontaneous self-assembly. Field emission scanning
electron microscopy (FE-SEM) of nanoparticles which were
deposited on a substrate revealed small spheres with a mean
diameter of 20-30 nm (see Figure S2 in SI). Parts a and b of
Figure 1 show the absorption and emission spectra of DBDCS
in THF solution19 and its nanoparticle suspension (THF/water
mixture), respectively. DBDCS exhibits a drastic change of
fluorescence intensity from the nonfluorescent THF solution
(quantum yield ΦF ) 2.6 × 10-3) to the strongly fluorescent
nanoparticle suspension (ΦF ) 0.62) as demonstrated in the
inset photograph of Figure 1b. The fluorescence enhancement
upon aggregation thus spans 2 orders of magnitude. At the same
time, the fluorescence lifetime τF increases from 4.2 ps in
solution to 11.9 ns in the nanoparticle suspension (Vide infra).
In the absorption spectra, the peak maximum of the nanoparticle
suspension is blue-shifted from 381 to 349 nm with respect to
(17) Frisch, M. J.; et al. Gaussian 03; Gaussian Inc.: Wallingford, CT,
2004.
(18) Gierschner, J. PhD Dissertation, 2000, Verlag Ko¨hler: Tu¨bingen,
ISBN: 3-932694-76-7.
(19) DBDCS solutions show very similar absorption and emission spectra
in a series of solvents with a broad range of polarity. The solvent
polarity independence in the absorption and emission maxima of
DBDCS solution is summarized in Figure S3 in SI.
2.5. Quantum Chemical Calculations. Single-molecule cal-
culations in the gas phase were performed at the density functional
theory (DFT) level of theory with the Gaussian 03 software.17
Herein, the ground-state geometry was fully optimized using the
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13677

A R T I C L E S
Yoon et al.
Figure 2. (a) Photo of a single crystal: before annealing, under room light (i), and UV light (ii), and after annealing, under room light (iii), and UV light
(iv) (scale bar ) 0.2 mm). (b) Photo of the pristine powder under room (left) and UV light (right). (c) Photo of the ground powder under room light (left)
and UV light (right). (d) SEM image of the surface morphology of DBDCS single crystal. (e) SEM images of the surface morphology of DBDCS annealed
crystal. (f) Normalized PL spectra of DBDCS single crystal (green solid line), annealed crystal (blue solid line), pristine powder (blue dashed line), ground
powder (green dashed line), reannealed powder (sky-blue dashed line), solution (black dashed line).
solution, and shows a pronounced asymmetric band shape (note
that the excitation PL spectrum of the nanoparticle suspension
is almost identical to the absorption spectrum of it, see Figure
S4 in SI). Both observations can be attributed to H-type
aggregation,2,20 which is consistent with a substantial decrease
of the rate constant kF ) ΦF/τF from 6.2 × 108 s-1 in solution
to 5.2 × 107 s-1 in the nanoparticles. The observed H-type
aggregation in DBDCS is quite different from previous reports
of other cyano stilbene derivatives.10 The shape of the strongly
red-shifted, unstructured emission spectrum of the DBDCS
nanoparticle suspension, on the other hand, is reminiscent of
excimer emission.2
Figure 2a shows the images of DBDCS single crystals as
grown from ethyl acetate solution. As prepared, they are yellow
in color and emit green light (λmax ) 533 nm) under UV
illumination (Figure 2f), which is to be called the G-phase of
DBDCS crystal. Heating the sample to 125 °C for an hour,
however, changes the color to pale whitish green and the
emission to blue (B-phase, 458 nm), while the crystal appearance
changes from transparent to opaque (Figure 2, a and f). The
crystal surface after annealing, as investigated by FE-SEM, was
rough and showed sheetlike regular projections and cracks,
indicating a rearrangement of the molecules in the crystal lattice
(Figure 2, d and e). In fact, this crystalline-state transition with
characteristic fluorescence change upon 125 °C annealing is
associated with the first-order endothermic peak in the dif-
ferential scanning calorimetry (DSC) thermogram (Figure 3a).
Figure 3. (a) DSC trace (first heating) of single crystal (green line) and
annealed crystal (blue line). (b) SAXS pattern of annealed DBDCS crystal
(inset: d-spacing of the reflection peaks, Miller indices).
Figure 4 shows the molecular packing structures of the
j
G-phase DBDCS single crystal (space group P1, one molecule
per unit cell) in different perspectives as determined by single-
crystal X-ray analysis (see Table S1 and Figure S1 in SI). Most
uniquely, DBDCS crystallizes to form the planar ‘molecular
sheets’ assisted by the multiple C-H· · ·N and C-H· · ·O
hydrogen bonds combined with the appropriate length of alkyl
substituents (Figure 4c and Figure S5 in SI). It is shown that
the molecular sheets in this G-phase crystal are arranged in slip-
stacks along the long molecular axis with a pitch angle of 26.6°,
whereas only a very small slip along the short axis is observed
(corresponding to a roll angle of 83.3°), as shown in Figure 4,
a and b. The interlayer distance between the adjacent molecular
sheets is 3.7 Å, consistent with other π-π stacking distances
reported earlier for substituted DSBs.2 The driving force for
this specific slip-stack formation is the antiparallel coupling
between the local dipoles21,22 in adjacent molecular sheets (see
Scheme 2 for the coupling scheme, and Figure 4a and Figure
S6b in SI for the crystallographic data). Since the outer phenyl
rings are electron-rich with butoxy-substituents while the central
(21) (a) Cacelli, I.; Feretti, A.; Girlanda, M.; Macucci, M. Chem. Phys.
2006, 320, 84. (b) Haino, T.; Tanaka, M.; Fukazawa, Y. Chem.
Commun. 2008, 468.
(20) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965,
11, 371.
9
13678 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Multistimuli Luminescence Switching of Molecular Sheets
A R T I C L E S
Figure 4. G-phase DBDCS molecules in the crystal structure: (a) Side view and illustration of pitch angle. (b) Front view and illustration of roll angle. (c)
Top view and illustration of hydrogen-bonded molecular sheet.
Scheme 2. Illustration of Two Different Modes of Slip-Stacking in
DBDCS Molecular Sheets, Dictated by Different Ways of
reasons to be discussed below (Vide infra), we suggest that the
Antiparallel/Head-to-Tail Coupling of Local Dipoles
of 62.8°; however, the exact pitch/roll angles are unknown. For
principal slip direction of the molecular sheets is along the
shorter axis of DBDCS molecule to effectively offset the ar-
omatic rings and establishing efficient head-to-tail coupling of
the local dipoles in adjacent molecular sheets (see Scheme 2
for the coupling situation).
Blue-emitting DBDCS powder could alternatively be prepared
by recrystallization from hot ethanol solutions as pale whitish
sheet-like crystallites, as shown in Figure 2b. Interestingly, when
grinding the blue powder in a mortar or rubbing it with a spatula,
it was observed that the fluorescence color changed immediately
to green (Figure 2c). This piezochromic transition8,9 thus
represents the reversal of the annealing process and is likely to
phenyl ring is electron-poor with cyano groups, DBDCS is a
be driven by the stacking mode change as well. As a matter of
D-A-D molecule comprising two local dipoles (Scheme 1)
fact, the mechanically ground green-emitting powder of DBDCS
which add to a zero net dipole moment. As illustrated in Figure
returns to the original blue-emitting one by brief thermal
4a, antiparallel dipole coupling places the central ‘A’ ring of
annealing at 125 °C for 5 min (see Figure S8 in SI). It is
the upper sheet just above the ‘D’ ring of the lower sheet,
important to note that the emission spectra of the reversible
bringing about efficient excitonic and excimeric coupling
piezochromism process exactly matches those of the B- and
between DBDCS molecules (Vide infra).
G-phases of the single crystal, as shown in Figure 2f and
extensively summarized in Table 1. Namely, the structured blue
emission spectra of both the pristine and the reannealed powders
are centered at 458 nm, i.e. the same as the B-phase of the
annealed crystal, and the unstructured green emission spectrum
of the ground powder, centered at 536 nm, corresponds to the
spectrum of the G-phase single crystal. Concomitantly, the
Thermal annealing of the G-phase crystal to the B-phase must
accompany specific stacking changes, which unfortunately could
not be monitored by single crystal X-ray analysis due to the
poor crystal quality of B-phase DBDCS as mentioned above.
Instead, the small-angle X-ray scattering (SAXS) pattern of the
blue-emitting crystal could be measured to show (100) and (200)
reflection peaks of lamellar structure (Figure 3b). Comparing
SAXS pattern of blue-emitting powder shows the same d-
the d-spacing (27.9 Å) with the molecular van der Waals length
spacing as that of the annealed crystal (B-phase) (see Figure
of 31.4 Å, we could infer a different slip-stack featuring an angle
S7 and Table S2 in SI). The mechanically ground green-emitting
powder gave a d-spacing of 21.9 Å and thus a slip angle of
44.2°. However it should be noted that the piezo-process is not
a complete conversion process since the shear force applies
unevenly to the bulk pristine powder. The remaining peaks
corresponding to the pristine powder are always included in
(22) (a) Gill, R. E.; vanHutten, P. F.; Meetsma, A.; Hadziioannou, G. Chem.
Mater. 1996, 8, 1341. (b) Kishikawa, K.; Furusawa, S.; Yamaki, T.;
Kohmoto, S.; Yamamoto, M.; Yamaguchi, K. J. Am. Chem. Soc. 2002,
124, 1597. (c) Yasuda, T.; Imase, T.; Nakamura, Y.; Yamamoto, T.
Macromolecules 2005, 38, 4687.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13679

A R T I C L E S
Yoon et al.
Table 1. Spectroscopic and Structural Data of the Observed Phases for the Different Systems Prepared in This Work
B-phase
G-phase
λabs (nm)
λem (nm)
slip angle (deg)
λabs (nm)
λem (nm)
slip angle (deg)
nanoparticles
single crystal
s
pristinea
pristine
349
s
507
533
-
annealed
s
458
62.8
pitch: 26.6
roll: 83.3
44.2
powder
pristine
s
s
303
302
458
458
457
457
62.8
62.8
60.4
s
pressure
s
536
reannealed
TS ) 100 °C
annealed
VD film
PMMA film
TS ) 25 °C
pristinea
pressure
vapora
340
339
361
339
538
510
523
510
46.0
s
s
s
a Probably (slightly) different crystal structure from G-phase with rather loose crystalline order due to the spherical particle shape.
Figure 5. DBDCS vacuum-deposited thin film with substrate temperature of 100 °C (B-phase VD film, blue line) and 25 °C (G-phase VD film, green line):
(a) UV-visible absorption spectra (b) PL spectra (filled symbols) and excitation PL spectra (open symbols). Insets show the fluorescence images of DBDCS
thin film with substrate temperature of 100 and 25 °C under 365 nm UV light. (c) XRD pattern of DBDCS thin film with substrate temperature of 100 °C.
(d) XRD pattern of DBDCS thin film with substrate temperature of 25 °C.
the SAXS patterns of the ground powder. However, reannealing
of the ground green-emitting powder completely recovers the
initial SAXS pattern, which confirms the same nature of blue
emission in the pristine and reannealed powder. Thus, in all,
we could identify two distinctively different phasess‘green’ (G)
and ‘blue’ (B)sin both single crystal and powders, which can
be reversibly switched, from G to B by thermal annealing and
from B to G by pressure through a specific phase transition
induced by two-directional shear-sliding capability of the
molecular sheets (see Table 1 for details).
G- and B-phases can also be observed in thin films (thickness
of 50 nm) prepared by vacuum deposition (VD) on quartz
substrates, where the phase formation depends sensitively on
the substrate temperature TS during deposition. The B-phase
was obtained at TS ) 100 °C, with a similar emission spectrum
(Figure 5b) as compared to the annealed powder and single
crystal spectra discussed above. Accordingly, the XRD pattern
of the B-phase VD film (Figure 5c) (here of a rather thick
sample, i.e. suitable for X-ray investigation) showed similar
d-spacing (27.3 Å) and slip angle (60.4°) to those of the annealed
powder. On the other hand, the G-phase VD film was obtained
at TS ) 25 °C with the emission spectrum (Figure 5b) again
coinciding with the ground powder and the single crystal
spectrum (see Figure 2). XRD pattern of G-phase VD film
(Figure 5d) showed similar d-spacing (22.6 Å) and slip angle
(46.0°) to those of the ground powder sample as expected (see
Table 1).
Different from the optically thick samples (crystals, powder)
described above, the absorption spectra of thin VD films could
be conveniently recorded. As already described for the nano-
particle suspension, the G-phase of the VD film also shows
H-type aggregation in the absorption spectrum,2,20 shifting the
maximum from 381 nm in solution to 340 nm in film (Figure
5a). In the B-phase, H-aggregation is obviously stronger, with
the absorption maximum observed at 303 nm. The photolumi-
nescence excitation (PLE) spectra in Figure 5b confirm the
conclusions made from the absorption spectra. We want to stress
at this point that the measurements on thicker samples, e.g. thick
9
13680 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Multistimuli Luminescence Switching of Molecular Sheets
A R T I C L E S
Figure 7. Fluorescence decay profiles of DBDCS nanoparticles in THF/
water mixture (dark green line), G-phase VD film (green line), B-phase
VD film (blue line) under 400 nm excitation. Red lines are the best fitting
curves.
Figure 6. Femtosecond transient absorption spectra of DBDCS under 400
nm excitation in THF solution. Inset shows a decay profile and fitting line
at 488 nm.
clearly associated with the nonradiative decay channel, with an
VD and spin-coated films or smeared powders are highly
problematic to make conclusive judgments on H- or J-aggrega-
tion since the large optical thickness often leads to saturation
phenomena, which might mask the intrinsic optical properties
in both absorption and PLE spectra to generate artifact in their
spectral shape.11,18 In this regard, optically thin samples (e.g.,
thin VD films, or dilute nanoparticle suspensions) are crucial
for a correct assignment.
To explore the singlet excited state dynamics of DBDCS in
THF solution, nanoparticle suspension, and G/B-phase VD
films, we have carried out the femtosecond transient absorption
and time-correlated single photon counting (TCSPC) measure-
ments by 400 nm femtosecond laser excitation. First, the
transient absorption spectra of DBDCS in THF solution exhibit
stimulated emission (SE) corresponding to the steady state
emission band and broad excited-state absorption (ESA) signals
in the range of 550-800 nm. And the decay profile at SE (488
nm) band can be fitted to a single exponential function with
the time constant of 4.2 ps (Figure 6). This ultrafast excited
state dynamics of DBDCS in THF solution is well correlated
with the nonfluorescence behavior. On the other hand, the
emission lifetimes of DBDCS nanoparticle suspension (11.9 ns),
G-phase VD film (23.9 ns), and B-phase VD film (6.1 ns) are
almost several thousand times longer than that of solution
(Figure 7).
estimated rate constant of knr ) (1 - ΦF)/τF ) 2.4 × 1011 s-1
,
which is mainly a consequence of the nonplanarity of DBDCS
in both the ground (S0) and excited state (S1), which enables
torsional-induced nonradiative deactivation as discussed ear-
lier.11 Indeed, the optimized ground state of DBDCS at the DFT
level calculation shows a strong twisting of the outer phenyl
rings with dihedral torsional angles of 28.0° and 4.9° (for
notation see Scheme 1 and Figure S9a [SI]), in agreement with
the previous investigations.11,24
Our X-ray analysis in the solid state reveals a substantial
planarization in the solid state. The core phenyl ring is much
less twisted to the outer phenyl ring with both dihedral
torsional angles of 5.6° (Scheme 1 and Figure S9b [SI]). Such
pronounced planarization in the crystalline state is not always
observed for CN-substitution in the vinylene unit,11,24 where
the dihedral torsion angles became larger with additional
substituents in the central phenyl ring. In our case, the
planarity is now promoted by the presence of multiple
hydrogen-bonds in the DBDCS crystal (see Figure 4c), which
gives a substantial planarization and thus effectively locks
their conformations in the crystal lattice. This aggregation-
induced planarization generates the AIEE phenomenon of
DBDCS, which significantly reduces the nonradiative deac-
tivation pathways compared to solution. With Φf ) 0.62 and
τF ) 11.9 ns, the latter is now knr ) 3.2 × 107 s-1, thus 4
orders of magnitudes smaller than that in solution.
4. Discussion
The substantial reduction of the radiative rate constant (kF
) 5.2 × 107 s-1) in the nanoparticle suspension by 1 order
of magnitude compared to the solution (kF,solution ) 6.2 ×
108 s-1) can be unambiguously described as the H-type
aggregation due to the excitonic coupling between the
transition dipoles of adjacent molecules, which leads to a
blue shift in the absorption and forbidden (or only slightly
allowed) lowest excited state, concomitantly a small oscillator
strength and thus a small kF.
The detailed structural information on the observed reversible
G/B phase transition of DBDCS and the comprehensive optical
and photophysical characterization allow for a unique insight
into the structure-property relationships in π-conjugated organic
materials, which will be elucidated in the following with the
help of molecular modeling.
In solution, the molecule is practically nonfluorescent, which
can be in principle attributed to a small radiative or a large
nonradiative rate constant. The radiative rate constant (kF ) ΦF/
τF) in solution is high, kF ) 6.2 × 108 s-1, which is similar to
the parent highly fluorescent distyrylbenzene (DSB) in solution
and to other (non)fluorescent substituted DSB systems.23 It also
agrees well with the expectation according to the Strickler-Berg
formula for the strongly allowed S0fS1 optical transition in
DBDCS, which is responsible for the emission process,23 and
thus cannot be the source for the low ΦF. Hence, the latter is
(23) (a) Oelkrug, D.; Tompert, A.; Egelhaaf, H.; Hanack, M.; Steinhuber,
E.; Hohloch, M.; Meier, H.; Stalmach, U. Synth. Met. 1996, 83, 231.
(b) Gierschner, J.; Oelkrug, D.; Nalwa, H. Encyclopedia of Nano-
science and Nanotechnology; American Scientific Publishers: Steven-
son Ranch, CA, 2004; Vol. 8, pp 219-238.
(24) (a) Souza, M. M.; Rumbles, G.; Gould, I. R.; Amer, H.; Samuel,
I. D. W.; Moratti, S. C.; Holmes, A. B. Synth. Met. 2000, 111, 539.
(b) Lange, F.; Hohnholz, D.; Leuze, M.; Ryu, H.; Hohloch, M.;
Freudenmann, R.; Hanack, M. Synth. Met. 1999, 101, 652.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13681

A R T I C L E S
Yoon et al.
nm),25 which amounts to 5000 cm-1. A simple dimer calculation
with the single crystal X-ray structure gives an excitonic splitting
of 800 cm-1 (see Figure 8), which should be reasonable, keeping
in mind the three-dimensional character of the crystal and the
expected breakdown of the nearest-neighbor approximation.25
The small displacement along the short axis (x) of adjacent
molecules in the G-phase leads to a substantial overlap of the
π-systems and, thus considerable excited state delocalization.
The latter allows for efficient vibronic coupling of interchro-
mophore breathing modes, which is responsible for the signifi-
cantly red-shifted and unstructured ‘excimer-like’ emission.2,26,27
Upon thermal treatment, the metastable G-phase transforms
into the thermodynamically favored B-phase, aided by energy
efficient slip along the molecular sheets. Both the absorption
and emission spectra point to a pronounced different coupling
situation; the substantially larger hypsochromic shift of the
absorption spectrum suggests a stronger coupling with an exciton
splitting of 8600 cm-1 as estimated from the absorption spectra.
On the other hand, the emission spectrum of the B-phase gains
vibronic structure and shows a pronounced hypsochromic shift
(see Figure 5b), indicating a loss of excited state delocalization
between adjacent molecules by a substantial reduction of π-π
overlap. As a result, the emission lifetime becomes also shorter
(6.1 ns) than that of the G-phase (Figure 7). In order to reduce
such overlap, the slip in the B-phase must be essentially along
the short x-axis, and not like in the G-phase along y. Indeed, as
seen in Figure 8, an x-slip leads to a substantial increase of
Figure 8. Color contour map of calculated exciton splittings for a dimer
pair at a different displacements x, y (in Å); the separation in z amounts
3.7 Å. A slip of 4 Å (14) corresponds to a translation by half a molecular
length in x (y).
The situation is similar in the G-phase crystal, whose spectral
features are akin to those of the nanoparticle suspension. In
addition, the emission lifetime is quite long (23.9 ns) (see Figure
7), and clear H-aggregation behavior is observed in the
absorption spectrum (Figure 5b). The overall exciton splitting
can be estimated from the energy difference between the
absorption maximum (340 nm) and the onset of absorption (410
Figure 9. DBDCS/PMMA film: (a) Photos of the luminescence writing/erasing cycle. (b) UV-visible absorption spectra under vapor- (dark green line),
thermo- (blue line), piezo- (green line) stimulus. (c) PL spectra under vapor- (dark green line), thermo- (blue line), piezo- (green line) stimulus.
9
13682 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Multistimuli Luminescence Switching of Molecular Sheets
A R T I C L E S
excitonic coupling roughly by a factor of 2, in a good agreement
with the experimental result.
Under all these conditions, a reproducible and fully reversible
two-color fluorescence writing/erasing process was achieved and
both emission colors of blue and green were found to be
persistent over one year of observation under ambient conditions.
In this way, a rewritable fluorescent optical recording media
was newly demonstrated.
At this point, we stress that the phase transition observed
here is a quite unique example to study separately exciton and
excimer coupling in molecular crystals; while the G-phase shows
rather weak excitonic coupling, excimer formation is favored
by pronounced overlap of the π-systems. In the B-phase,
excimer formation is diminished, while excitonic interaction
substantially increases. The driving force for the phase transition
is clearly provided by the local dipoles as introduced through
the cyano group. While in the metastable G-phase antiparallel
coupling of the local dipoles kinetically stabilizes the structure,
a smooth slip of the molecular sheets with a very low activation
barrier leads to the formation of the B-phase with the energeti-
cally favored formation of a head-to-tail arrangement of the local
dipoles (see Scheme 2).
6. Conclusion
We have synthesized and fully characterized a novel
multistimuli luminescence switching material (DBDCS) with
highly enhanced fluorescence emission in the solid state.
DBDCS uniquely forms highly fluorescent (ΦF ) 0.62)
‘molecular sheets’ assisted by the multiple C-H · · · N and
C-H · · · O hydrogen-bonds with stacking and shear-sliding
capabilities via external stimuli (temperature, pressure,
solvent vapor). On the basis of structural, optical, photo-
physical, and computational studies, we identified two
different phases, i.e. the metastable green-emitting G-phase
and the thermodynamically stable blue-emitting B-phase, and
elucidated the phase transition pathways as well as their
spectroscopic implications. In the G-phase, antiparallel
coupling of the local dipoles kinetically stabilizes a structure
with rather moderate excitonic coupling, but efficient excimer
formation. Upon annealing, a smooth slip of the molecular
sheets with a low activation barrier forms the B-phase with
a head-to-tail arrangement of the local dipoles. Here the
excimer formation is diminished, while excitonic interaction
substantially increases. It was found that the application of
pressure to the B-phase crystal could restore the original
G-phase. With this concept at hand, we have successfully
demonstrated rewritable fluorescent optical recording media
which showed fast-responding and reversible multistimuli
luminescence switching.
5. Multistimuli Device Demonstrator
As a practical fast-responding demonstrator for multistimuli
luminescence tuning, we prepared a green-emitting (G-phase)
composite film of DBDCS in PMMA (20 wt %) by spin
coating on glass. The surface morphology of the film indicates
a phase separation with small grains of about 10 nm size
(see Figure S11 in SI). When thermal stimulus (125 °C for
10 s with hot letter stamp) was applied, the green fluorescence
changed to blue (Figure 9), in accordance with the G- to
B-phase transition. The thermally annealed B-phase film
showed high pressure-sensitivity, changing the emission color
immediately to green even with a very small shear force,
thus allowing for sensitive piezo-writing as shown in Figure
9. Upon exposing the whole films to the organic vapor
(CH2Cl2 for 30 s), we were able to erase this piezo-writing,
changing the entire emission color to green (Figure 9a). The
possibility of vapor adsorption in altering the fluorescence
color is excluded by vacuum treatment after vapor exposure.
In any case, it should be noted that G-phase induced by the
exposure to vapor is basically same with but slightly different
from the G-phase induced by shear force, as evident from
the comparison of the emission spectra in Figure 9c as well
as the absorption spectra in Figure 9b. This implies that vapor
exposure also promotes an alternation of the slip-stacking,
although to a somewhat different degree compared to the
case of pressure application.
Acknowledgment. This work was supported by the Basic
Science Research Program through the National Research Founda-
tion of Korea (NRF) funded by the Ministry of Education, Science,
and Technology (CRI; RIAMIAM0209(0417-20090011)). J.G. is
a Ramo´n y Cajal research fellow of the Spanish Ministry for Science
and Innovation. The work at Yonsei University was supported by
the Star Faculty and World Class University (R32-2008-000-10217-
0) Programs from the Ministry of Education, Science, and Technol-
ogy (MEST) of Korea and the AFSOR/AOARD Grant (FA2386-
09-1-4092).
Supporting Information Available: Complete ref 17, synthe-
sis, characterization, crystallographic data (CIF file - CCDC
778284), experimental details, and a movie of the piezo-writing
process. This material is available free of charge via the Internet
at http://pubs.acs.org.
(25) Gierschner, J.; Huang, Y. S.; Van Averbeke, B.; Cornil, J.; Friend,
R. H.; Beljonne, D. J. Chem. Phys. 2009, 130, 044105.
(26) Hayer, A.; de Halleux, V.; Ko¨hler, A.; El-Garoughy, A.; Meijer, E. W.;
Barbera´, J.; Tant, J.; Levin, J.; Lehmann, M.; Gierschner, J.; Cornil,
J.; Geerts, Y. H. J. Phys. Chem. B 2006, 110, 7653.
(27) Gierschner, J.; Mack, H. G.; Oelkrug, D.; Waldner, I.; Rau, H. J. Phys.
Chem. A 2004, 108, 257.
JA1044665
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13683