Hierarchical assembly of a molecular material through the amorphous phase and the evolution of its fluorescence emission

Article abstract of DOI:10.1039/c3tc30615a

Fabrication of an amorphous phase followed by a hierarchy of forms leading to the crystalline state offers a new dimension in the assembly of small molecule based materials. The morphology and extent of crystallinity of the nanostructures in drop-cast thi

Full text of DOI:10.1039/c3tc30615a

Journal of
Materials Chemistry C
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PAPER
Hierarchical assembly of a molecular material through
the amorphous phase and the evolution of its
fluorescence emission†
Cite this: J. Mater. Chem. C, 2013, 1,
4464
*
Ch. G. Chandaluri and T. P. Radhakrishnan
Fabrication of an amorphous phase followed by a hierarchy of forms leading to the crystalline state offers a
new dimension in the assembly of small molecule based materials. The morphology and extent of
crystallinity of the nanostructures in drop-cast thin films of a diaminodicyanoquinodimethane molecule
are shown to be tunable through the variation in the composition of the solvent mixture used for drop-
casting. The different states of assembly starting from the solvated molecule through amorphous
spherical particles, crystalline nanofibers and nano/microcrystals to bulk crystals are shown to be
accompanied by a smooth variation of the fluorescence emission, the color evolving from green to blue,
and the efficiency increasing steadily with the overall enhancement from the solution to the bulk
crystalline state being ꢀ400 times. Quantum chemical computations on the molecule and its H-bonded
dimer and p-dimers provide insight into the impact of intermolecular interactions in the crystalline
assemblies on the electronic structure. The current study illustrates a significant departure from the
conventional molecules-to-materials transition, opening up new hierarchical pathways of assembly of
molecular materials.
Received 3rd April 2013
Accepted 17th May 2013
DOI: 10.1039/c3tc30615a
www.rsc.org/MaterialsC
the properties of the molecular assembly. It is notable that the
reversible transition between the amorphous and crystalline
Introduction
The fundamental paradigm in the fabrication of molecular
materials is the transition from molecules to materials.
Tailoring the materials' responses and attributes is generally
achieved by tuning the structure and functionalities of the
molecular building blocks and through them their assemblies,
an approach that could be termed as intrinsic design. The
contrasting scenario would be the control of molecular
assembly mediated by extrinsic factors such as template or
substrate effects,1,2 electric/magnetic elds3,4 and mechanical
constraints.5 Organized assembly of molecules into crystalline
lattices makes the elucidation of the materials' structure–
property correlations and further design optimization especially
convenient. However an important aspect that warrants
consideration is the feasibility of realizing a hierarchical
assembly of molecules from the isolated (or solvated) state
through an amorphous solid form to the nal crystalline state,
and its potential implications for the materials' attributes. The
amorphous state opens up a new dimension in the molecules-
to-materials transition and offers the opportunity to ne-tune
states is the basis of commercially important phase-change
materials based on inorganic systems.6 The amorphous state of
molecular nanomaterials is also relevant in the context of
the two-step nucleation theory of fundamental interest in
crystal growth.7–9
The coexistence of amorphous and crystalline regions in
polymers and their critical role in determining the materials'
properties are well recognized; the extent of crystallinity is a
basic characteristic of any polymeric material.
A recent
example of interest for light emitting materials is the tuning of
the crystallinity of poly(3-hexylthiophene) nanoparticles by
varying the solvent composition.10 In the domain of small
molecule based materials, the amorphous phase and its attri-
butes have been less widely realized and exploited. One of the
few cases relates to electroluminescent systems and charge
mobilities promoted by the amorphous nature.11 Thermal and
solvent vapor induced amorphous-to-crystalline trans-
formation (ACT) has been explored in thin lms12,13 and dye
aggregates.14,15 We have developed a simple protocol for the
ACT of nanoparticles of a diaminodicyanoquinodimethane
(DADQ) derivative accompanied by uorescence switching and
enhancement.16 The process involves xing the nanoparticles
(obtained by drop-casting dilute solutions on glass substrates)
in a polymer thin lm and solvent vapor fuming inducing the
ACT of the partially conned particles. We have also investi-
gated the utility of mechanical control of the aggregation of an
School of Chemistry, University of Hyderabad, Hyderabad, 500046, India. E-mail:
tprsc@uohyd.ernet.in; Fax: +91-40-2301-2460; Tel: +91-40-2313-4827
† Electronic supplementary information (ESI) available: Details of synthesis and
characterization, crystal structure, spectroscopy, microscopy and computations.
CCDC 931908. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3tc30615a
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amphiphilic DADQ molecule at the air–water interface and its
impact on the optical responses of the resulting monolayer
Langmuir–Blodgett lms.17
Crystal structure
Crystals were grown by slow evaporation of an acetonitrile
solution. A single crystal X-ray diffraction study was carried out
at 100 K on a Bruker SMART APEX CCD area detector system
equipped with a graphite monochromator and a Mo Ka ne-
Aggregation induced enhancement of uorescence,18,19 as
opposed to its quenching, is of considerable interest from the
perspectives of both molecular level interactions/assembly as
well as applications such as displays and sensors. Enhancement
of the uorescence of DADQ molecules from their solution to
the crystalline solid state has been investigated in detail in our
laboratory.20 The size-dependent optical responses of nano/
microcrystals21 and the enhancement of uorescence in the
solution and colloidal states of a DADQ derivative by poly-
electrolyte templating have also been explored.22 Since the
uorescence emission wavelength and efficiency of DADQs are
strongly sensitive to the molecular environment as well as the
rigidication of their structures, it would be interesting to
explore the hierarchical assembly of these molecules through
the amorphous state and monitor the consequence to the
uorescence characteristics.
˚
focus sealed tube (l ¼ 0.71073 A) operated at 1200 W power
(40 kV, 30 mA). The detector was placed at a distance of 6.003
cm from the crystal. A total of 2400 frames were collected with a
scan width of 0.3ꢁ in u mode and an exposure time of 5 s per
frame. The frames were integrated with the Bruker SAINT
soware using a narrow-frame integration algorithm. Analysis
of the data showed negligible decay during data collection. Data
were corrected for absorption effects using the multi-scan
method (SADABS). The structure was solved and rened using
the Bruker SHELXTL (Version 6.14) Soware.23 X-ray diffraction
data of powder samples were recorded on a SMART Bruker
D8 Advance X-ray diffractometer using Cu Ka radiation
˚
(l ¼ 1.5406 A) at 40 kV and 30 mA.
Introduction of aromatic groups through conformationally
labile bonds onto the DADQ framework promotes the formation
of amorphous particles in thin lms formed by drop-casting
their dilute solutions on glass substrates.16 In the present study
we have synthesized and structurally characterized a new DADQ
Spectroscopy
Electronic absorption spectra were recorded on a Varian Model
Cary 100 UV-vis spectrometer. Solid samples were prepared by
rubbing small amounts of the microcrystalline powder onto
quartz plates; the quantity was varied to achieve the desired
optical density at the excitation wavelength. Drop cast lms on
glass/quartz were also studied. Diffuse reectance spectra of the
samples were recorded using the DRA-CA-30I accessory and
converted to absorption spectra using the Kubelka–Munk func-
tion. Steady-state uorescence excitation and emission spectra
were recorded on a Horiba Jobin Yvon Model FL3-22 Fluorolog
spectrouorimeter. The spectra were recorded in right angle
geometry with the samples mounted on a solid sample holder.
The optical density of the samples was maintained below 0.3 to
avoid inner-lter effects. The uorescence quantum yield of the
solution was estimated by comparison with quinine sulfate in
1 N H2SO4 (f ¼ 0.546).24 The absolute value of the uorescence
quantum yield of solid samples was determined using an inte-
grating sphere and the PLQY Calculator v.3 soware.
molecule,
7-pyrrolidino-7-benzylamino-8,8-dicyanoquinodi-
methane (PBEDQ). Drop-cast thin lms obtained from solu-
tions of PBEDQ with varying compositions of a good solvent and
a nonsolvent revealed nanostructures ranging from amorphous
spherical particles, to crystalline bers, to faceted nano/micro-
crystals, and intermediate composite structures. A parallel
evolution of the uorescence emission in terms of color as well
as intensity is observed. The study demonstrates the utility of
extending the hierarchical stages of molecular assembly by
introducing the new dimension of the amorphous state, and its
signicant consequences for the materials' responses.
Experimental and computational
Synthesis
PBEDQ was synthesized in two steps starting from 7,7,8,8-tet-
racyanoquinodimethane (TCNQ) (Scheme 1). Pyrrolidine was
reacted with TCNQ in ꢀ1 : 1 mol ratio to yield 7-pyrrolidino-
7,8,8-tricyanoquinodimethane (PTCNQ) which on reacting with
benzylamine yielded PBEDQ.23
Microscopy
A Carl Zeiss model Ultra 55 FESEM and Philips Model XL30 SEM
were used to observe directly the morphology of the nano-
structures in the drop-cast thin lms prepared on the glass
substrate; the samples were coated by a thin layer of sputtered
gold and 20 kV beam voltage was used. An FEI model TECNAI G2
20 S-Twin TEM operated at 200 kV was used to image the
nanostructures and record the electron diffraction; the sample
preparation procedure is discussed later. A Carl Zeiss LSM 710
NLO ConfoCor 3 laser scanning confocal microscope was used
to record the uorescence images of the drop-cast lms on
glass/quartz; lexc ¼ 365 nm, lem ¼ 417–600 nm. Fluorescence
lifetimes in solution drops and drop-cast lms were probed
using the time-domain technique with a MicroTime 200
Instrument (PicoQuant) coupled to an Olympus IX71 Micro-
scope (PicoQuant). Excitation was achieved using a 405 nm
Scheme 1
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Table 1 Crystallographic data for PBEDQ
pulsed laser diode and the uorescence observed through a
430 nm long pass lter. The FWHM of the pulse response
function was 176 ps. The intensity decays were analyzed with a
multi-exponential model using SymPhoTime v. 5.0 soware.23
Empirical formula
Crystal system
Space group
C21H20N4
Monoclinic
P21/n
˚
a/A
9.4123 (7)
12.9289 (10)
14.5929 (11)
99.2920 (10)
1752.5 (2)
4
1.245
0.76
100 (2)
0.71073
3087
240
0.976, 0.967
1.060
0.0416
0.1090
˚
b/A
Computations
˚
c/A
Ab initio quantum chemical computations were carried out
using the density functional method at the B3LYP/6-31G* level;
the Gaussian03 program was used.25 Geometry optimization
was carried out for the isolated molecule as well as for the
molecule with different solvent environments using the self-
consistent reaction eld (SCRF) approach. The solvation model
provides a useful approximation of the microscopic environ-
ment of the molecule in the crystal lattice;26 the appropriate
solvent was chosen based on the agreement of the computed
structure with that observed in the crystal, specically in terms
of the characteristic dihedral twist angle of the DADQ frame-
work that is sensitive to the dielectric environment. Electronic
excitation proles of the molecule and its H-bonded dimer and
p-dimers extracted from the crystal lattice were calculated using
the time-dependent density functional (TD-DFT) method; the
impact of the lattice environment was incorporated using the
solvation model.
b/deg.
V/A
3
˚
Z
rcalc./g cmꢂ3
m/cmꢂ1
Temperature/K
˚
l/A
No. of reections
No. of parameters
Max., min. transmission
GOF
R [for I $ 2sI]
wR2
ꢂ3
˚
Largest difference peak and hole/e A
0.376/ꢂ0.309
˚
p–p contact with an interplanar distance of 2.772 A and a
˚
centroid–centroid distance of 5.121 A; these contacts link the
H-bonded chains into sheet structures in the ac plane (Fig. 2).
Another intermolecular slipped p–p contact between the
phenyl rings of the benzylamine group is also observed in the
crystal structure.23
Results and discussion
Crystals of PBEDQ grown from acetonitrile solution are found
to belong to the P21/n space group with one molecule in the
asymmetric unit. The molecular structure (with one of the
disordered carbon atoms23 placed in the average position) and
the unit cell packing are shown in Fig. 1; the signicant crys-
tallographic data are collected in Table 1. The molecular
structure shows the prominent twist between the diamino-
methylene moiety and the benzenoid ring plane (bearing the
dicyanomethylene group) characteristic of DADQ mole-
cules;26,27 the relevant dihedral angles are sN9–C7–C1–C2 ¼ 54.7ꢁ,
and sN10–C7–C1–C6 ¼ 58.3ꢁ. Intermolecular H-bonds between the
Electronic absorption and uorescence emission spectra of
PBEDQ in acetonitrile solution and as a microcrystalline solid
are presented in Fig. 3. The solution shows an absorption with
lmax at 370 nm. This arises due to the intramolecular charge
transfer, a characteristic feature of the zwitterionic benzenoid
framework of DADQ molecules.20,28 The solid shows a broad
(320–400 nm) absorption in the same region. The uorescence
emission shows lmax at 512 and 456 nm in the solution and
solid respectively. The signicant blue shi of the peak in the
solid is accompanied by an enhancement of intensity of ꢀ400.
Quantum chemical computations were carried out to gain
insight into these spectral shis. The solution absorption was
computed using the fully optimized structure of PBEDQ; the
˚
0
amine H and one of the cyano Ns [rH10/N13 ¼ 2.001 A,
qN10-H10/N13 ¼ 160.2ꢁ] lead to extended supramolecular chains
0
ꢀ
along the [1 0 1] direction. The benzenoid rings show a slipped
Fig. 2 H-bonding (cyan broken line) and short intermolecular contacts (pink
broken line) involved in the p–p interaction leading to layer structures in crystals
of PBEDQ. C (grey), N (blue), and H (white) atoms are indicated, the disordered
C17 is shown at the averaged position and the H atoms (except those involved in
H-bonding) are omitted for clarity.
Fig. 1 (a) Molecular and (b) crystal structures of PBEDQ from X-ray diffraction
analysis. C (grey), N (blue), and H (white) atoms are indicated, and the disordered
C17 is shown at the averaged position and the H atoms (except the amine ones)
are omitted in (b) for clarity.
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Table 3 Spectroscopic features of PBEDQ dimers from the crystal obtained from
the B3LYP/6-31G* and B3LYP/6-31G*/SCRF computations (see the ESI for the
structures). The absorption with lmax > 250 nm and the oscillator strength (f) > 0.1
are listed. The specific data directly relevant to the discussion in the text are shown
in italics
lmax (nm) [f]
Structure
Vacuum
Water
H-bonded dimer
480.8 [0.40]
470.9 [0.25]
292.4 [0.30]
291.0 [0.26]
414.0 [0.55]
277.8 [0.48]
364.3 [0.52]
359.6 [0.46]
260.1 [0.26]
259.6 [0.21]
354.2 [0.76]
264.1 [0.17]
257.3 [0.25]
366.4 [0.93]
264.0 [0.38]
260.7 [0.10]
p-dimer 1
p-dimer 2
495.8 [0.63]
300.7 [0.75]
structure of DADQs is sensitive to the dielectric constant of
the environment imposed during the optimization.26 In the
case of PBEDQ, an environment equivalent to that of water is
required to reproduce the dihedral twist of the molecule in the
crystal (Table 2). This indicates that within this simple model,
the dielectric environment of the molecule in the crystal can
be mimicked using water solvation. The absorption features
computed for the PBEDQ monomer and different dimers are
collected in Tables 2 and 3 respectively. From the computa-
tions including water solvation, it is seen that the lmax (with
high oscillator strength) decreases down to ꢀ354 nm. These
results indicate that the molecular packing in the crystal
lattice and the electronic interaction between the chromo-
phores lead to the blue shi in the optical absorption. This is
likely to be the cause for the blue shi in the emission peak as
well. The uorescence enhancement in the solid state can be
attributed to the prevention of excited state geometry relaxa-
tion which, in the solution state, leads to non-radiative
decay.20,22
Fig. 3 (a) Optical absorption spectra (with concentrations adjusted to match
peak absorbances) and (b) normalized fluorescence emission spectra of aceto-
nitrile solution and microcrystalline solid of PBEDQ (inset: relative emission
intensity of samples with matched optical density).
molecular structure from the crystal was used as the starting
point for the optimization and the acetonitrile solvent envi-
ronment was included in the computation. The signicant
absorption lmax (with high oscillator strength) is found at
ꢀ386 nm (Table 2), in good agreement with the solution
spectrum. The dihedral twist angle in the optimized molecular
In a conventional reprecipitation process, injection of small
amounts of an acetonitrile solution of PBEDQ into toluene
(nonsolvent for PBEDQ) produced a colloid. The colloid shows a
blue emission characteristic of crystalline PBEDQ. It was ltered
Table 2 Geometric and spectroscopic features of the PBEDQ molecule from the B3LYP/6-31G* and B3LYP/6-31G*/SCRF computations (see the ESI for the structures).
The two dihedral angles (see the text for details) and the average value for each structure are shown and the absorption with lmax > 250 nm and oscillator strength
(f) > 0.1 are listed. The specific data directly relevant to the discussion in the text are shown in italics
lmax (nm) [f]
Molecular geometry
From crystal structure
Optimized (vacuum)
Optimized (acetonitrile)
Optimized (water)
Twist angles, s [average] (deg.)
58.3, 54.7 [56.5]
Vacuum
Acetonitrile
Water
512.4 [0.33]
303.4 [0.37]
430.0 [0.80]
272.2 [0.11]
—
372.7 [0.47]
264.1 [0.27]
—
367.6 [0.48]
262.4 [0.26]
—
33.6, 36.4 [35.0]
53.4, 53.6 [53.5]
386.0 [0.56]
262.3 [0.23]
—
—
55.1, 54.8 [55.0]
—
378.3 [0.54]
262.4 [0.23]
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through a nanoporous membrane and observed in an FESEM; carbon-coated copper grid and dissolving the polystyrene lm
the formation of brous nanostructures could be discerned in toluene, TEM images of the nanostructures could be
clearly.23 The spectroscopy and microscopy observations recorded. Images of the structures formed in the drop-cast
suggest that PBEDQ molecules assemble into crystalline bers lms obtained using the 100 : 0, 40 : 60 and 10 : 90 acetoni-
in the presence of toluene. This prompted us to explore, in trile–toluene mixtures are shown in Fig. 5; the morphologies
detail, thin lms of PBEDQ fabricated by drop-casting solutions are similar to those in the FESEM images (Fig. 4). The spher-
with different acetonitrile–toluene compositions.
ical particles obtained in the rst case (Fig. 5a) gave no elec-
Typically 0.3–0.6 mM solutions of PBEDQ in acetonitrile– tron diffraction pattern; this observation is consistent with its
toluene mixtures with varying compositions (100 : 0 to 10 : 90 amorphous nature. The nanobers (Fig. 5b) produced a well-
v/v) were drop-cast on glass/quartz substrates and the resulting dened ring pattern typical of polycrystalline materials; the
lms dried quickly in ambient atmosphere. FESEM images lattice spacing is consistent with one of the reections in the
illustrating the morphology of the thin lms formed are PBEDQ crystal, indicating a preferred growth direction.23 The
collected in Fig. 4. The shape of the particles is spherical in the nanocrystals (Fig. 5c) gave clear electron diffraction spots
case of pure acetonitrile. The morphology is characteristic of which could also be indexed to the lattice structure of
amorphous nanoparticles formed by DADQ derivatives pos- PBEDQ.23 In a separate experiment, we have enriched the
sessing aromatic substituent groups connected through con- content in the acetonitrile–toluene solution (10 : 90) by
formationally labile bonds.16 With the introduction of small keeping it for several hours and ltered through a nanoporous
amounts of toluene in the solvent, the drop-cast lms show membrane. The powder X-ray diffraction pattern of the parti-
spherical particles together with brous nanostructures. The cles obtained23 is fully consistent with the crystal structure of
presence of toluene not only reduces the solubility of PBEDQ, PBEDQ, conrming the crystalline nature and ruling out the
but also slows down the drying of the drop-cast lm. When the formation of any polymorphic structures.
toluene content is increased, the nanobers become
Absorption and emission spectra of the drop-cast lms are
the dominant structures, and the lm is exclusively made of collected in Fig. 6a and b respectively. The relatively broad
the bers when the acetonitrile–toluene composition is 40 : 60. absorption spectrum with higher baseline of the lm with the
It may be noted that crystalline organic bers have been spherical particles can be attributed to stronger scattering from
fabricated earlier by the fast evaporation of capillary lms.29 the amorphous particles; the impact of different environments
The emergence of faceted nano/microcrystals of PBEDQ is of the chromophores cannot be ruled out. The emission spectra
observed when the solvent composition reaches 10 : 90. By are recorded by exciting at the maximum of the uorescence
coating the drop-cast lms with a thin layer of polystyrene, excitation spectrum in each case;23 they reveal the evolution
drying and peeling off, we have fabricated free-standing lms from amorphous to crystalline nanostructures, consistent with
containing the nanostructures xed in it. Placing these on a the microscopy observations above. The lm with spherical
particles obtained from the acetonitrile solution shows
Fig. 4 FESEM images of thin films fabricated by drop-casting PBEDQ in aceto-
nitrile–toluene mixtures with varying compositions: (a) 100 : 0, (b) 80 : 20, (c)
40 : 60, (d) and (e) 10 : 90. Scale bar is 1 mm in (a) to (d) and 5 mm in (e).
Fig. 5 TEM images (along with the SAED) of nanoparticles, nanofibers and
nano/microcrystals fabricated by drop-casting PBEDQ in acetonitrile–toluene
mixtures with the compositions: (a) 100 : 0, (b) 40 : 60 and (c) 10 : 90.
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Fig. 6 (a) Absorption and (b) emission spectra of thin films fabricated by drop-casting (on quartz) PBEDQ in acetonitrile–toluene mixtures with varying compositions.
(c) CIE chromaticity plot of the thin films fabricated by drop-casting acetonitrile–toluene mixtures of different compositions [100 : 0 (spherical particles); 80 : 20
(spherical particles + fibers); 40 : 60 (fibers); 10 : 90 (fibers + nano/microcrystals)] together with those of the solution and microcrystalline solid.
emission (lmax ꢀ 488 nm) close to green, reminiscent of the
Subtle variations in the drop-casting conditions and treat-
solution. With increasing toluene content in the solvent used ment of the drop-cast lms lead to the formation of a wide
for drop-casting, the peak shis steadily to the blue; emission of range of morphologies with varying amorphous/crystalline
the lms with nanobers and nano/microcrystals resemble that structures. The following typical examples illustrate this.
of the bulk microcrystalline solid (lmax ꢀ 460 nm). The emission Enclosing the lms drop-cast from an acetonitrile solution
spectrum of the lms containing both spherical particles and (3 mM) inside a covered Petri dish slows down the evaporation
bers is slightly sensitive to the excitation wavelength, as it of the solvent and drying of the lm, leading to the formation of
depends on the relative content of the two morphologies.23 nanobers (Fig. 8a) instead of the spherical particles. Films
Tuning of the emission across the series of drop-cast thin lms obtained by quick drying, made up of the spherical particles
can be visualized by the confocal uorescence images taking (Fig. 4a), when exposed again to acetonitrile vapors show the
into account also the spectral responses from the individual formation of nanobers emanating from the spherical seeds
nanostructures (Fig. 7). Fine-tuning of the emission color across (Fig. 8b), a vivid illustration of the crystalline structures
the series (including also the solution and bulk solid) is clearly emerging smoothly from the amorphous ones. Slow evapora-
demonstrated by the chromaticity plot in Fig. 6c. Signicantly, tion of the solvent in the lms drop-cast from methanol solu-
the quantum yield for the emission is also found to increase tions of PBEDQ reveal spherical particles enveloped in a ber
across the series (Table 4), in parallel to the extent of crystal- network (Fig. 8c). These observations point to the rich variety of
linity. This is consistent with the general concept of lattice morphologies and more signicantly, extents of crystallinity of
rigidication inhibiting structural (rotational) relaxation of the the PBEDQ nanostructures that can be realized by optimization
excited state of DADQ molecules leading to uorescence of the conditions for fabrication of the drop-cast thin lms. An
enhancement.20,22 FLIM images of the drop-cast lms23 illus- earlier study of some interest in this context is the control of
trate the progression of excited state lifetimes across the series self-assembly and long-range orientational order in H-bonded
(Table 4) consistent with the uorescence emission oligothiophene polymorphs through variation of solvent evap-
enhancement.
oration and crystal growth conditions.30
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Fig. 8 FESEM images of thin films fabricated by drop-casting PBEDQ in (a)
acetonitrile and evaporating the solvent slowly, (b) acetonitrile, evaporating the
solvent quickly and then exposing to acetonitrile vapors, and (c) methanol and
evaporating the solvent slowly. Scale bar ¼ 1 mm.
Tuning of the crystallinity and optical responses of PBEDQ
nanostructures has interesting parallels and signicant
contrasts with the case of conjugated polymer nanoparticles of
poly(3-hexylthiophene) mentioned in the Introduction.10 In both
cases, the extent of crystallinity is controlled by the solvent
composition, and the increasing content of the nonsolvent
enhances the crystallinity. The present study is a novel illustra-
tion of such phenomena with small molecule based materials.
While the extent of crystalline assembly primarily inuences the
electronic absorption features and photoluminescence decays in
the polymer nanoparticles, the case of PBEDQ involves prom-
inent changes in the emission characteristics in terms of the
color and more signicantly the quantum yields, aspects of
considerable interest in practical applications.
Fig. 7 Confocal fluorescence images of thin films fabricated by drop-casting
PBEDQ in acetonitrile–toluene mixtures with varying compositions: (a) 100 : 0, (b)
80 : 20, (c) 40 : 60, (d) 10 : 90. Scale bar ¼ 5 mm; false colors are used in the
images based on the spectral response of the full sample.23 (e) Confocal fluores-
cence spectral responses from the individual nanostructures in (b).
Conclusions
A new diaminodicyanoquinodimethane derivative PBEDQ is
synthesized, structurally characterized and demonstrated to be
an interesting system to develop strongly uorescent nano-
structures. The nanostructures of PBEDQ in thin lms fabri-
cated by drop-casting solvent mixtures of varying compositions
exhibit a smooth progression of morphologies and crystallinity.
While the solution and the crystalline solid state represent the
two extremes of molecular materials commonly studied, the
nanostructures of PBEDQ including the amorphous phase
represent logical intermediates in the course of supramolecular
assembly and the evolution of uorescence characteristics in
terms of energies and efficiencies. The current explorations
establish a fundamentally interesting approach to new lumi-
nescent molecular materials based on tuned assembly.
Table 4 Fluorescence quantum yield and average excited state lifetime of
PBEDQ in different states, solution to drop cast films of varying amorphous/
crystalline nature to microcrystalline solid
Acetonitrile–
toluene
Quantum
yield (%)
Average life
time (ns)
Structure
—
100 : 0
Solution
Film
0.1
1.6
<0.18
0.64
(spherical particles)
Film
80 : 20
2.2
0.89
(spherical particles +
bers)
Film (bers)
Film
Acknowledgements
40 : 60
10 : 90
4.4
6.5
1.02
1.93
Financial support from the Department of Science and Tech-
nology, New Delhi and infrastructure support from the Centre
for Nanotechnology, the National Single Crystal X-ray
(bers + nano/
microcrystals)
Microcrystalline solid
—
36.4
2.63
4470 | J. Mater. Chem. C, 2013, 1, 4464–4471
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Paper
Journal of Materials Chemistry C
Diffractometer Facility (School of Chemistry) and the FESEM 19 A. Patra, C. G. Chandaluri and T. P. Radhakrishnan,
facility (School of Physics) at the University of Hyderabad are Nanoscale, 2012, 4, 343.
gratefully acknowledged. CGC thanks the CSIR, New Delhi for a 20 S. Jayanty and T. P. Radhakrishnan, Chem.–Eur. J., 2004, 10,
Senior Research Fellowship. We thank M. Durgaprasad, M. 791.
Laxminaryana, M. Nalini and S. Patra for the TEM, FESEM, 21 A. Patra, N. Hebalkar, B. Sreedhar, M. Sarkar, A. Samanta
confocal microscope and FLIM imaging respectively.
and T. P. Radhakrishnan, Small, 2006, 2, 650.
22 C. G. Chandaluri, A. Patra and T. P. Radhakrishnan, Chem.–
Eur. J., 2010, 16, 8699.
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