Room temperature anthracene excimer emission from self-assembled (aminomethyl)anthracene derivatives in plastic crystalline phase

Article abstract of DOI:10.1016/j.jphotochem.2010.07.003

A series of (aminomethyl)anthracene (AMA) derivatives with single alkyl chain substituent (octyl, dodecyl, octadecyl and p-butylaniline) were synthesized and self-assembly of the compounds was analyzed in dichloromethane and solid thin film. Upon heat treatment of the solid thin films, the AMA derivatives exhibit plastic crystalline phase which can be preserved in a glassy state through rapid cooling of the system to room temperature. In the glassy state, the AMA derivatives are arranged in rectangular discotic columnar fashion, where the face-to-face distance between two anthracene moieties is 3.9 . Steady state and time resolved fluorescence studies were carried out in dichloromethane, which indicate that a fraction of the AMA derivatives form anthracene excimer, upon excitation at 360 nm. Conversely, photoexcitation of the solid thin film at 360 nm leads to the exclusive formation of anthracene excimers due to the close proximity of anthracene units in the discotic columnar arrangement. Furthermore, the self-assembled AMA derivatives were utilized as effective templates for in situ prepared Ag nanoparticles. While the AMA derivatives stabilize the Ag nanoparticles effectively, the excimer emission intensity was quenched in the presence of silver nanoparticles. The experimental results from the present study provide a unique approach for generating anthracene excimer emission from solid thin films at room temperature, which is a rare observation.

Full text of DOI:10.1016/j.jphotochem.2010.07.003

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Room temperature anthracene excimer emission from self-assembled
(aminomethyl)anthracene derivatives in plastic crystalline phase
M. Jaseer, Edamana Prasad∗
Department of Chemistry, Indian Institute of Technology Madras (IITM), Chennai 600 036, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of (aminomethyl)anthracene (AMA) derivatives with single alkyl chain substituent (octyl, dode-
cyl, octadecyl and p-butylaniline) were synthesized and self-assembly of the compounds was analyzed
in dichloromethane and solid thin film. Upon heat treatment of the solid thin films, the AMA deriva-
tives exhibit plastic crystalline phase which can be preserved in a glassy state through rapid cooling of
the system to room temperature. In the glassy state, the AMA derivatives are arranged in rectangular
discotic columnar fashion, where the face-to-face distance between two anthracene moieties is 3.9 Å.
Steady state and time resolved fluorescence studies were carried out in dichloromethane, which indicate
that a fraction of the AMA derivatives form anthracene excimer, upon excitation at 360 nm. Conversely,
photoexcitation of the solid thin film at 360 nm leads to the exclusive formation of anthracene excimers
due to the close proximity of anthracene units in the discotic columnar arrangement. Furthermore, the
self-assembled AMA derivatives were utilized as effective templates for in situ prepared Ag nanoparti-
cles. While the AMA derivatives stabilize the Ag nanoparticles effectively, the excimer emission intensity
was quenched in the presence of silver nanoparticles. The experimental results from the present study
provide a unique approach for generating anthracene excimer emission from solid thin films at room
temperature, which is a rare observation.
Received 24 March 2010
Received in revised form 26 June 2010
Accepted 2 July 2010
Available online 31 July 2010
Keywords:
Excimer emission
Plastic crystal
Self-assembly
Silver nanoparticle
Luminescence
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
with an emission maximum close to 470 nm [22]. The second one
involves the formation of ‘sandwich type’ excimer where the two
Utilizing small organic molecules to generate highly hierarchi-
cal and self-assembled systems is an area of research which is
gaining increasing attention among material chemists [1–3]. Iden-
tifying interesting material properties in self-assembled systems
composed of small organic molecules is largely appealing because:
(a) they are relatively easy to synthesize and characterize unam-
biguously, (b) they are less complex when handling for device
fabrication, and (c) they resemble to many in vivo systems in terms
of numerous aspects of self-assembly [4–7].
Anthracene derivatives play a pivotal role in the design of lumi-
nescent materials, with wide applications in lasers, phosphors,
and light emitting devices [8–12]. Anthracene derivatives are also
known for their [4+4] cycloaddition reactions as well as excimer
formation [13–21]. It has been reported in the literature that
anthracene generally form two types of excimers with slightly
different geometry in the excited state [22]. The first one, where
two anthracene units overlap at an angle of 55◦ to each other,
is relatively stable and has a short excited state lifetime (<10 ns)
anthracene units are symmetrically ␲-stacked and such excimers
exhibit longer lifetimes at lower temperatures with an emission
maxima at 560 nm [22]. A recent report suggests the formation
of another type of anthracene excimer (T-shaped excimer) with
an emission maximum at 510 nm and an excited state lifetime of
∼20 ns [23]. Although the planar structure of anthracene is quite
favorable for excimer formation on photoexcitation, the excimer
emission from anthracene-based systems is rarely reported, espe-
cially at room temperature [11]. Herein we report the generation
of highly emitting anthracene excimers at room temperature from
a series of amino(methylanthracene) (AMA) derivatives.
(Aminomethyl)anthracene derivatives have been well studied
for their luminescence and charge transfer properties [24]. How-
ever, the self-assembling property of these small organic molecules
has not been explored in detail. We have identified that, upon heat
treatment, the AMA derivatives (compounds C8-I, C12-II, C18-III
and PhC4-IV shown in Chart 1) self-assemble to rectangular dis-
cotic columnar structures, where the anthracene units are placed
in close proximity to each other. We have also identified that the
self-assembly at elevated temperatures renders plastic crystalline
behavior to the AMA derivatives. The plastic crystalline proper-
ties are originated from the free rotation of lattice points, often
∗
Corresponding author. Tel.: +91 44 2257 4232; fax: +91 44 2257 4202.
E-mail address: pre@iitm.ac.in (E. Prasad).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.07.003

M. Jaseer, E. Prasad / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
249
2.3. In situ generation of silver nanoparticles
In a typical procedure, solid AgNO3 (0.002 g, 0.01 mmol) was
added in portions to a solution of C18-III (0.03 g, 0.05 mmol) in dry
methanol (10 mL), and the mixture was stirred in a dark cham-
ber for 24 h at room temperature, to get a clear solution, which is
slightly reddish brown in color. The solution was drop cast on a
glass slide and another glass slide was gently pressed on the top of
it to form a thin film in between the glass plates. The heat-treated
glass plate was then examined through POM.
2.4. Instruments
UV–visible absorption spectra were recorded on a Perkin Elmer
Lambda 25 UV–visible spectrophotometer. Luminescence experi-
ments were carried out using HORIBA JOBIN YVON Fluoromax 4
spectrometer. The lifetime studies were conducted using the Time
Correlated Single Photon Counting technique (TCSPC) with micro-
channel plate photomultiplier tube (MCP-PMT) as detector and
nanosecond LED as the excitation source (model Fluorocube, Horiba
Jobin Yvon). The optical textures of the mesophases were stud-
ied using Nikon Eclipse LV 100 POL polarizing optical microscope
(POM) attached with a heating and cooling stage. The transition
temperatures and change in enthalpy for phase transitions were
determined by differential scanning calorimetry (DSC) with NET-
ZSCH DSC 204 instrument operated at a scanning rate of 5 ◦C/min.
Transmission electron microscopy (TEM) images of the nano-silver
were taken using a FEI Tecnai G2 30 S-TWIN, operated at 300 kV and
JEM 3010 JEOL, operated at 200 kV. Scanning electron microscopy
(SEM) images were taken using FEI; QUANTA scanning electron
microscope. The X-ray measurements were performed using Cu K␣
radiations (l = 1.5418 Å) from a fine focus sealed-tube generator in
conjunction with double mirror focusing optics.
Chart 1. Structure of compounds used in the present study.
at elevated temperatures, keeping their centre of mass at ordered
sites in the crystalline lattice [25]. The plastic crystalline phase of
the compounds was characterized through X-ray diffraction and
OPM studies. Since the plastic crystalline phase was preserved in a
glassy state upon rapid cooling of the system from the isotropiza-
tion point to room temperature, anthracene excimers were readily
formed in the glassy state of the AMA derivatives. The AMA deriva-
tives were also utilized for in situ generation and stabilization of
silver nanoparticles and the effect of Ag nano-guest molecules on
the excimer emission intensity was investigated in the solid thin
films. The mechanistic studies suggest that controlling the self-
assembling property of the AMA derivatives provides a unique
approach for generating intense anthracene excimer emission at
room temperature.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Identifying plasticity in the AMA derivatives
9-Anthraldehyde and 4-butylaniline were purchased from
Aldrich chemical company, USA. Octadecylamine, dodecylamine,
octylamine and silver nitrate were purchased from Sd Fine Chemi-
cals, India and the amines were purified through distillation under
reduced pressure.
The structure of the four (aminomethyl)anthracene derivatives
used in the present study is given in Chart 1. The compounds
C8-I and C12-II were reported in the literature [26,27]. However,
no mesophase properties were investigated for these compounds.
Compounds C18-III and PhC4-IV were synthesized adopting the
procedure for C8-I with necessary modification.
Polarization optical microscopy is a powerful tool to understand
the morphology of molecular assembly in the solid state as well as
liquid crystalline state. The POM image of C12-II is given in Fig. 1
which showed the characteristic features of spherulitic texture,
indicating the presence of discotic columnar arrangement in the
system [28–30].
The POM image was taken after the heat treatment of the sample
on a glass slide above the melting point (78.2 ◦C) followed by sud-
den cooling to room temperature. The POM images of C8-I, C18-III
and PhC4-IV also exhibited similar discotic columnar arrange-
ments in the respective systems (please see Supporting information
Figs. S1–S3).
Differential scanning calorimetry (DSC) experiments were per-
formed in order to understand the thermodynamic properties of
the phase transitions in the systems upon heat treatment. Fig. 2
shows the DSC thermograms of the compounds C8-I, C12-II, C18-
III and PhC4-IV, all of which show two thermodynamic transitions
during the heating scan. The first peak indicates transition from the
crystal phase to mesophase and the second peak corresponds to the
melting of the mesophases to isotropic liquid.
2.2. Synthesis of 9-(p-butylphenylaminomethyl) anthracene
(PhC4-IV)
In a typical procedure, a solution of 9-anthraldehyde (0.1 g,
0.48 mmol) in ethanol (20 mL) was added drop-by-drop to a solu-
tion of p-butylaniline (0.076 g, 0.48 mmol) dissolved in ethanol
(5 mL). The mixture was stirred under reflux condition for 12 h. The
reaction mixture was allowed to cool to room temperature. To the
imine formed, solid NaBH4 (0.02 g, 0.58 mmol) was added in por-
tions and the stirring was continued for another 12 h. The reaction
mixture was evaporated to dryness, extracted with ether and puri-
fied by column chromatography (silica gel, hexane/ethyl acetate
10:1) to give 9-(p-butylphenylaminomethyl)anthracene (PhC4-IV)
(0.10 g, 62%) as a pale yellow solid.
1H NMR (400 MHz, CDCl3) ı: 8.4 (1H, s), 8.22 (2H, d), 7.96 (2H,
d), 7.55 (2H, t), 7.43 (5H, q), 7.04 (2H, d), 6.69 (3H, t), 5.07 (1H, s),
2.51 (2H, t), 2.1 (2H, s), 1.54 (2H, q), 1.31 (2H, q), 0.88 (3H, t). Mass
(ESI): calculated for C25H25N. 339; found 338 (M–H), 191 (Ar–CH2).
The procedures for synthesizing other compounds (C8-I, C12-II,
and C18-III) as well as the spectral data are given in Supporting
information.

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M. Jaseer, E. Prasad / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
Fig. 3. XRD pattern of C18-III at 36.7 ◦C after melting and cooling in a similar pro-
cedure adopted for POM studies.
Fig. 1. POM image of C12-II (magnification 20×). Sample for POM was prepared by
making thin film of C12-II (0.010 g, 0.03 mmol) in between two glass slides. The film
was made uniform by heating the sample to melting followed by rapid cooling to
room temperature.
While the POM images and DSC diagrams clearly suggest the
presence of mesophase with texture properties similar to discotic
columnar arrangement, X-ray diffraction (XRD) studies were per-
formed to confirm the plastic crystalline property of the AMA
derivatives. The XRD pattern for the compound C18-III is given in
Fig. 3. The sample was heated above the melting point (73.23 ◦C)
mounted in a variable temperature XRD facility and the XRD pattern
was recorded after cooling of the sample close to room tempera-
tures (36.7 ◦C).
In the case of C8-I, the crystalline phase transforms into
mesophase upon heating at 72.39 ◦C and the system isotropizes on
further heating up to 88.4 ◦C. The temperature range where com-
pounds exhibit the mesogenic property (phase II) was very narrow
for C12-II and relatively wide for PhC4-IV. The entropy change for
the phase I–II transition increases by an order of magnitude along
the series from C8-I to C18-III (0.004–0.04 J K−1 mol−1). Interest-
ingly, the transition temperature from phase II to III decreases as
the alkyl chain length increases (from 88.4 to 73.2 ◦C), except for
PhC4-IV. In the case of both transitions (from phase I to II as well as
from II to III) the compound PhC4-IV shows relatively less entropy
change, due to the strong hydrophobic interactions between the
aromatic units in the molecular packing, which is consistent with
the increased transition temperature from phase II to phase III
(115.3 ◦C). The phase transition temperatures as well as thermody-
namic parameters associated with the transition are summarized
in Table S1 in Supporting information.
The presence of very sharp peaks in wide-angle region of
intensity vs. 2ꢀ profile typically confirms the presence of plas-
tic crystalline nature of the material [30]. It is noteworthy that
the entropy of fusion for the compounds obtained from DSC
analysis is also consistent with Timmerman’s criteria for plas-
tic crystalline system (i.e., considerably less than 20 J K−1 mol−1
)
[31]. Since POM images clearly indicate the columnar discotic
type arrangement in the system, XRD pattern was further ana-
lyzed to understand whether rectangular or hexagonal type discotic
arrangement is present in the plastic crystal. In the case of rect-
angular symmetry, the lattice parameter ‘a’ should satisfy the
relation
a = 2d(20)
,
whereas for hexagonally symmetric systems, the lattice parameter
should be in consistent with the relation
2
a =
d(10),
√
3
where d is the inter planar distance [32]. For rectangular columnar
phases, d can be calculated using the expression
ꢀ
1
d(hk)
h2
a2
k2
b2
=
+
where h and k are the Miller indices and a and b are lattice
parameters [32]. The XRD pattern of C18-III strictly satisfied the
requirement for rectangular symmetry (a = 68.11 and d = 34.05,
from experiment) (please see Supporting information, Table S2 for
XRD details). Thus, based on the XRD results for C18-III, it can
be concluded that the AMA derivatives exhibit plastic crystalline
behavior with rectangular columnar arrangement.
While organic molecules form discotic type arrangement when
they contain usually 3 or 4 alkyl or alkoxy groups, the present case
provides an exceptional example, most likely due to the presence
of anthracene moiety, which occupies most of the volume of the
‘disc’, which otherwise is occupied by the multialkyl chains present
in the system [30]. The initial report on the crystal structure of
Fig. 2. DSC thermograms of the plastic crystalline compounds (all second heat-
ing cycle with a heating rate of 5◦/min). PI = crystalline phase, PII = mesophase and
PIII = isotropic phase.

M. Jaseer, E. Prasad / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
251
Fig. 4. UV–visible absorption and steady state emission spectra of C8-I (1 × 10−3 M)
in dichloromethane. The excitation wavelength was 360 nm.
Chart 2. Cartoon representation showing the rectangular columnar arrangement in
C8-I.
3.2. Photophysical properties of the plastic crystalline molecules
in dichloromethane
Since the anthracene units are arranged in close proximity in the
self-assembled AMA derivatives, we explore the likelihood of the
anthracene excimer formation in solution, upon photoexcitation.
The photophysical properties of compounds C8-I, C12-II, C18-III
and PhC4-IV were initially examined in dichloromethane, since the
compounds were highly soluble in dichloromethane.
C8-II described a ‘zig-zag’ arrangement of the molecules in the
crystal lattice with orthorhombic unit cell arrangement [27]. How-
ever, upon heat treatment the self-assembling propensity of the
molecules is changed, which is not uncommon for systems con-
taining anthracene units. For example, the recent report by Ziessel
and co-workers describe the self-assembly of ionic liquid crystals
containing anthracene core, where the zig-zag arrangement of the
crystal lattice is altered to hexagonal columnar phase upon heat
treatment [33].
The ␲–␲ interaction between the aromatic moieties as well as
the hydrogen bonding between the amine groups could lead to
columnar disc like arrangements of the compounds, which easily
stack in to three-dimensional rectangular geometry as schemat-
ically shown in Chart 2. The POM images of all the compounds
exhibit identical type textures, indicating that rectangular colum-
nar arrangement is present in all the systems examined in this
study.
The minimum energy configuration for a pair of C8-I molecules
in the face-to-face geometry was obtained through the MM2 com-
putations of the Chem. 3D pro., which suggests the stacking of the
two aromatic units one over the other, with the alkyl chains stretch-
ing towards either sides. The distance between two anthracene
units, which are placed one over the other was 3.87 Å in the mini-
mum energy configuration. This was consistent with the XRD data
obtained for the compound (entry 15 in Table S2 in Supporting
information).
The absorption and emission spectra of the AMA derivatives
were identical to that of anthracene when the concentration of
the solution was 10−5 M. The quantum yields of emission from
C8-I, C12-II, C18-III to PhC4-IV in dichloromethane were deter-
mined using coumarin as standard and the values are summarized
in Table 1. The AMA derivatives exhibit emission quantum yield
slightly less than that of pure anthracene. However, increase in the
concentration by two orders of magnitude resulted in interesting
changes in the photophysical properties. Fig. 4 shows the absorp-
tion and steady state emission spectra of C8-I in dichloromethane,
where the concentration of C8-I was 10−3 M. The steady state emis-
sion spectrum of the C8-I was similar to that of anthracene emission
spectrum at this concentration also. However, a less intense,
but broad shoulder band on the longer wavelength region was
observed, which indicates that a fraction of monomer anthracene
units in the AMA derivative forms excimers upon excitation.
The excimer formation from the AMA derivatives in
dichloromethane at higher concentration was further con-
firmed thorough time resolved experiments. The time resolved
fluorescence decays of compounds C8-I, C18-III and PhC4-IV
were biexponential whereas that of C12-II was triexponential in
dichloromethane. Fig. 5 contains the fluorescence decay of C8-I
(please see Supporting information Figs. S4–S6 for the fluores-
Table 1
The photophysical properties of C8-I, C12-II, C18-III and PhC4-IV in dichloromethane.
Compound
Absorption maxima, nm
Emission maxima, nm
420, 442, 472
Emission quantum yielda
0.26
ꢁ, ns (relative amplitude, %)
C8-I
262, 357
365, 384
ꢁ1 = 3.52 (22.34)
ꢁ2 = 7.46 (77.66)
C12-II
264, 355
366, 384
420, 444, 474
0.22
ꢁ1 = 2.78 (33.79)
ꢁ2 = 5.56 (04.76)
ꢁ3 = 11.10 (61.44)
C18-III
264, 355
365, 382
418, 442, 471
420, 443, 473
0.16
0.33
ꢁ1 = 2.72 (19.12)
ꢁ2 = 7.28 (80.88)
PhC4-IV
263, 356
364, 384
ꢁ1 = 2.31 (32.89)
ꢁ2 = 7.07 (67.11)
a
Emission quantum yield of plastic crystals were determined using Coumarin as standard. Error bar is 5%.

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M. Jaseer, E. Prasad / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
Fig. 5. Biexponential fluorescence decay of C8-I (1 × 10−3 M) in dichloromethane.
Fig. 6. Excitation (left) and emission (right) spectra of anthracene (dot) and C18-III
(solid) at solid thin film, after heat treatment. ꢂexc. = 360 nm. Emission spectra are
normalized to unity. Inset shows the UV illuminated photo of C18-III in solid thin
film on a round glass slide, at room temperature, providing anthracene excimer
emission.
Excitation wavelength was 340 nm.
cence decay curves for the other compounds). The shortest lifetime
component of the fluorescence decays in C8-I, C12-II, C18-III and
PhC4-IV possess a value of 3 ns. This decay is assigned to the
emission of anthracene monomer, which is quenched by solvent
collision or photoinduced electron transfer (PET) from amine
to the excited state anthracene unit. The excited state lifetime
component of ∼7–11 ns, with relative amplitude >60–80%, was
assigned to one particular type of anthracene excimer emission
formed by the angular overlap of the individual anthracene units
in the excited state with an angle of approximately 55◦, based on
literature reports [22].
The fact that more than 60% of the fluorescence decay com-
ponent corresponds to the excimer emission suggests that the
propensity of anthracene units to align in a columnar fashion is high
even in solution phase, at concentration above 10−4 M. In the case of
C12-II, an additional excited state decay component was observed
with a lifetime value of ∼5 ns, which is identical to the excited state
decay of unquenched anthracene. Nonetheless, the relative ampli-
tude of this lifetime component was only ∼5% indicating that PET
exists as a prevailing fluorescence quenching mechanism in all the
compounds. Table 1 summarizes the photophysical properties of
the four plastic crystals examined in the present study. Examination
of Table 1 suggests that as the chain length increases, the fluores-
cence quantum yield decreases. This could be, presumably due to
the increased flexibility of the system in the solution, introduced by
the presence of long hydrocarbon chains. While the fluorescence
quantum yield varies according to the chain length, the relative
amplitudes of the monomer–excimer ratio remain identical within
experimental error, through out the system.
the closely packed anthracene units in C18-III. Structure-less, broad
emission from anthracene containing compounds around 470 nm
indicates the presence of an excimer with angular geometry in the
excited state. Identical emission spectra were obtained for other
AMA derivatives used in the present study. The inset of Fig. 4 con-
tains the photograph of C18-III under UV illumination exhibiting
the anthracene excimer at ambient conditions.
The excitation and emission spectra of the AMA derivatives
before and after the heat treatment were recorded in order to ver-
ify whether the formation of plastic crystalline phase is essential to
generate the excimer emission. Fig. 7 shows the overlaid spectra of
excitation and emission of C8-I in solid thin film before and after
heat treatment. It is evident from the figure that generating C8-I in
the plastic crystalline phase through the heat treatment facilitates
the anthracene excimer formation. This is attributed to the close
proximity as well as face-to-face arrangement of anthracene units
in the plastic crystalline phase, which is absent in the orthorhombic
crystalline phase of C8-I. Upon rapid cooling, the plastic crys-
talline phase of the AMA derivatives can be preserved in the
glassy state, where the molecular arrangement of anthracene units
remains in the face-to-face geometry. Since the intermolecular dis-
tance between the anthracene units remains at 3.9 Å in the glassy
phase, excimer formation is highly feasible. Such readily formed
excimers are termed as ‘static excimers’ since the monomer units
3.3. Anthracene excimer emission from the AMA derivatives in
the plastic crystalline phase
Next, the steady state luminescence properties of the AMA
derivatives in the plastic crystalline phase were examined and com-
pared with that in the crystalline phase. The plastic crystalline
phase of the AMA derivatives was prepared through heat treatment
at identical conditions adopted for POM experiments. Fig. 6 shows
the excitation and emission spectra of C18-III, along with that of
pure anthracene at identical conditions. The significant differences
between the pure anthracene and C18-III spectra in the solid thin
film are the following: (i) an additional peak appeared in the exci-
tation spectra of C18-III at 412 nm, (ii) the emission maximum of
C18-III is red-shifted to 467 nm and (iii) the emission spectra of
C18-III is broad and structure-less. The data clearly indicates that
the emission is originated purely from anthracene excimer due to
Fig. 7. Excitation (left) and emission (right) spectra of C8-I before (solid) and
after (dash) heat treatment in solid thin film at identical experimental conditions.
ꢂexc. = 360 nm. Intensities are normalized to unity.

M. Jaseer, E. Prasad / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
253
are pre-arranged in the ground state in a face-to-face geometry.
The static type of excimer formation is further evident from the
bathochromic shift observed in the excitation spectrum shown in
Fig. 7, [34]. Moreover, the absorption spectra of the AMA derivatives
in dichloromethane (10−3 M) also exhibited absorbance of aggre-
gated AMA derivatives at the longer wavelength of the spectrum
(Supporting information Fig. S7), indicating that anthracene units
in the compounds are pre-arranged in the ground state, favoring
the static type excimer upon excitation [35]. The results from the
photophysical studiescarriedout in dichloromethaneandsolid thin
film taken together suggest that self-assembly of the AMA deriva-
tives provides a simple method to generate anthracene excimer
emission at room temperature, by generating static excimer of
anthracene.
3.4. Preparation of in situ silver nano-clusters utilizing the AMA
derivatives
Fig. 8. UV–visible spectra of C18-III in the presence (dash) and absence (solid) of
AgNO3 in methanol.
It has been reported that stable silver nanoparticles can be
generated using amphiphilic triblock copolymers arranged in
hexagonal columnar phase [36]. Since the compounds C8-I, C12-
II, C18-III and PhC4-IV self-assemble in rectangular columnar
fashion; we have utilized them as templates for stabilizing silver
nanoparticles. In general, three different methods are adopted in
the literature for the preparation of silver nanoparticles from sil-
ver ions. The first one involves the radiation reduction of silver
ions with ␥-ray [37], ultraviolet or visible light [38], microwave
[39], or ultrasound irradiation. Because of the ease of the control-
ling the ‘on’ or ‘off’ state of radiation, this method has attracted
considerable attention. The second preparation method involves
relatively strong reducing agents such as sodium borohydride [40],
hydrazine, and tetrabutylammonium borohydride [41]. In the third
method, silver ions were reduced by weak reducing agents, such
as alcohols, through prolonged refluxing or stirring at room tem-
perature. While reduction of silver by methanol, ethanol, glycerol
etc. is feasible, these solvents alone were not able to stabilize sil-
ver nanoparticles [42,43]. Consequently, these processes have been
performed in the presence of a capping agent or in microemulsions.
In the present case, the third possibility (i.e., solvent induced reduc-
tion) is more likely, as the solvent (methanol) is present in bulk
amount and no other reducing agents have been used for reduc-
tion. The silver nanoparticles can be stabilized by the coordination
with the amine groups present in the system. While amines can also
reduce silver ions [44], FT-IR studies of the system before and after
nanoparticle formation clearly indicate that amine groups have not
participated in any redox reactions in the present case (Supporting
information S8).
All the compounds, except PhC4-IV, shown in Chart 1 gen-
erate nano-silver upon mixing and stirring with AgNO3 for 24 h
in methanol at room temperature (vide infra). Fig. 8 contains
the UV–visible absorption spectrum of C18-III in the presence
and absence of AgNO3 in methanol. The absorption band around
500 nm, which was absent for C18-III alone, clearly suggests the
formation of nano-silver. The plasmon absorption of nano-silver
was also evident in the cases of C8-I and C12-II.
The wavelength of plasmon absorption of silver nanoparticles
has been found to be strongly dependant on the size and shape of
the nanoparticles formed [45]. A bathochromic shift from the blue
region of the absorption spectra generally indicates the formation
of larger sized metal nanoparticles [46]. Another equally important
possibility is the interparticle coupling between silver nanopar-
ticles due to the proximity effect, which increases the dielectric
constant of the medium, causing the plasmon resonance maximum
to shift to lower energies [47]. A third possibility is due to the for-
mation of anisotropic metal nanoparticles in the system. According
to Mie’s theory, anisotropic particles could give rise to multiple
absorption peaks in the UV–visible spectrum [48]. It is reported that
absorption bands at 341 nm and 509 nm from nano-silver could be
due to the formation of hexagonal nanoplates [49]. In the present
cases, the absorption peaks from anthracene units mask the region
below 400 nm in the spectrum, preventing further inference on the
nanoparticle shape from the UV spectrum.
The silver nanosystems were further examined by scanning
and transmission electron microscopic studies (SEM and TEM).
Fig. 9 contains the SEM and TEM images of nano-silver gener-
ated in situ by C18-III. The size distribution of silver nanoparticles
in C18-III and C12-II was almost similar, indicating that tem-
plate effect of the AMA derivatives does not depend on the alkyl
chain length (Supporting information Fig. S9). The nano-silver is
likely to be formed in between the layers of the discotic columnar
arrangements, which is stabilized through the coordination with
the amines in the alkyl chains.
Since excess amount of the AMA derivative is present in the
system, all the silver ions will likely be complexed to the amines
present in the compounds. It has been reported in the literature
that the reduction potential of the silver–amine complex is much
lower than that of the corresponding silver ion [50]. Thus, the sol-
vent (methanol) induced reduction of silver ions will be facilitated
due to the silver–amine complex formation. The importance of the
silver–amine complex formation in the reduction process of silver
ions is further corroborated by the observation that no plasmon
absorption was formed for the system C8-I/AgNO3 at acidic condi-
tions, where the amines in the system are protonated and unable
to complex with the silver ions. NMR and FT-IR spectral analysis
of the AMA derivatives after the nanoparticle formation did not
exhibit any difference from the spectra taken before mixing with
AgNO3, indicating that amines are not participating in the reduc-
tion process. Among the four different (aminomethyl)anthracene
derivatives, compound PhC4-IV did not generate nano-silver even
after stirring with AgNO3 for extended period of time. Since the
nitrogen is directly attached to the electron deficient benzene ring,
PhC4-IV is expected to be less basic compared to the other com-
pounds used in the present study. Subsequently, the formation of
the corresponding silver–amine complex would be considerably
less. The poor yield of silver nanoparticles is, thus, attributed to
the absence of the appreciable amount of silver–amine complex
formation in the system.
3.5. Photophysical properties of the AMA derivatives in presence
of silver nanoparticles
In order to understand whether the nano-guest particles affect
the excimer formation of anthracene units in the AMA derivatives
through reducing the proximity between the anthracene units,

254
M. Jaseer, E. Prasad / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
Fig. 9. (A) TEM and (B) SEM images of nano-silver generated and stabilized by the plastic crystal C18-III (scale = 50 ␮m for SEM, 50 nm for TEM).
excitation and emission spectra of the AMA derivatives were taken
in the solid thin film after the heat treatment. Fig. 10 shows the
excitation and emission spectra of C8-I and C18-III in the glassy
state, in the presence and absence of silver nanoparticles.
It is clear from the figure that the presence of nanoparticles
reduces the propensity of C8-I to generate anthracene excimer, as
the emission at 470 nm is substantially decreased. This could be
due to the distribution of the nano-guests in between the discotic
arrangement of the molecules, creating larger separation distance
between the anthracene units. POM analysis of the AMA deriva-
tives in the presence of silver nanoparticles suggests a flower type
arrangement (Fig. 11) for the molecular system, which also rep-
resent discotic arrangement [28]. The variations in the POM image
shown in Fig. 11 compared to that in Fig. 1 indicates that anthracene
units are slightly displaced from the initial arrangements. Iden-
tical behavior was observed for compound C12-II (Supporting
information Fig. S10).
Interestingly, the presence of silver nanoparticles does not
quench the excimer emission from C18-III, completely. The inset
of Fig. 10 contains the normalized excitation and emission spec-
tra of C18-III in the presence of Ag nanoparticles. This indicates
that nano-guests are not able to separate the self-assembled C18-
III molecules in the solid thin film to an extent where the excimer
formation is prevented. The POM image (Supporting information
Fig. 11. Flower like texture from C8-I (magnification 20×). Sample for POM was pre-
pared by making thin film of a mixture of stirred solution of C8-I (0.01 g, 0.03 mmol)
and AgNO3 (0.001 g, 0.006 mmol) in between two glass slides. The film was made
uniform by heating the sample to melting followed by rapid cooling to room tem-
perature.
Fig. S11) also corroborates this hypothesis, where the spherulitic
structural pattern was clearly visible even in the presence of sil-
ver nanoparticles, indicating that structural changes in C18-III are
marginal compared to C8-I and C12-II.
The results obtained from the present study clearly suggest
that the AMA derivatives are promising candidates for generating
anthracene excimer emission at room temperature, which is a rare
observation. Attempts to generate conducting molecular materi-
als utilizing the plastic crystalline phase of the AMA derivatives
are currently undergoing in our laboratory and the results will be
published in due course.
4. Conclusion
A
series
of
organic
plastic
crystals
based
on
(aminomethyl)anthracene (AMA) derivatives was synthesized
and the phase transitions are characterized utilizing XRD, DSC
and POM. These organic plastic crystals with discotic rectangular
columnar arrangement exhibit anthracene excimer emission in
the solid thin film, upon exciting at 360 nm. The excimer forma-
tion is attributed to the close proximity of the anthracene units
in the plastic crystalline phase, which provides the necessary
condition to overlap between two anthracene units in an angular
Fig. 10. Excitation (left) and emission (right) spectra of C8-I in the presence (dot)
and absence (solid) of silver nanoparticle after heat treatment. Inset shows the same
for C18-III, where the emission intensity is normalized. ꢂexc. = 360 nm.

M. Jaseer, E. Prasad / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 248–256
255
fashion upon photoexcitation. Further investigation has shown
that the silver nanoparticles can be generated and stabilized by
the AMA derivatives, at the expense of the excimer emission
intensity. Since the plastic crystalline phase can be preserved
in a glassy state by rapid cooling of the system from elevated
temperatures to room temperature, the experiment described
here provides an easy and straight forward methodology to
generate anthracene excimer emission in solid thin film at room
temperature.
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Acknowledgements
We thank Department of Science and Technology (SR/S1/PC-
26/2007), Govt. of India for the financial support. M.J. thanks
Council of Scientific and Industrial Research (CSIR), India for the
fellowship. We sincerely thank Dr. Krishna Prasad, CLCR Bangalore,
India for providing the XRD facility as well as fruitful discus-
sions. Dr. T. Pradeep, IITM is acknowledged for TEM images and
Department of Metallurgy, IIT M is thanked for SEM images. We
thank Dr. P.K. Sudhadevi Antharjanam, Fast Track Scientist, Depart-
ment of Chemistry, IIT Madras for her useful comments on the
manuscript.
Appendix A. Supplementary data
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