Assembly of twisted luminescent architectures based on acenaphtho[1,2-k]fluoranthene derivatives

Article abstract of DOI:10.1039/c5cc00476d

Acenaphtho[1,2-k]fluoranthene derivatives DPAF-n as new building blocks for one-dimensional (1D) structure assembly have been developed and employed to fabricate luminescent twisted nano/micro-wires; and the DPAF rigid core attached via flexible alkyl cha

Full text of DOI:10.1039/c5cc00476d

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Assembly of twisted luminescent architectures
based on acenaphtho[1,2-k]fluoranthene
derivatives†
Cite this:DOI: 10.1039/c5cc00476d
Received 17th January 2015,
Accepted 4th February 2015
Liang Han, Yuewei Zhang, Weiping Chen, Xiao Cheng, Kaiqing Ye, Jingying Zhang*
and Yue Wang*
DOI: 10.1039/c5cc00476d
www.rsc.org/chemcomm
Acenaphtho[1,2-k]fluoranthene derivatives DPAF-n as new building six-membered p-conjugation systems. The special structural char-
blocks for one-dimensional (1D) structure assembly have been acteristic suggested that DPAF based molecules have promising
developed and employed to fabricate luminescent twisted nano/ luminescence and semiconducting properties.13 Therefore,
micro-wires; and the DPAF rigid core attached via flexible alkyl chains DPAF is considered to be a desirable building block to construct
with suitable lengths is critical for the formation of twisted architectures. functional molecules for supramolecular assembly. In this commu-
nication three molecules DPAF-n (n = 8, 12, 16) (Fig. 1) were designed
Among the attractive one-dimensional (1D) architectures, twisted and synthesized by introducing two phenyl groups with alkyl chains
and helical nano/micro-wires and ribbons based on organic to the DPAF skeleton. DPAF-n molecules possess unique molecular
functional molecules are the most representative candidates for structure and functional characteristics which are as follows:
a better understanding of some life substances and fabrication of (i) the cross type geometry constructed by a p-quinquephenyl
optoelectronic devices.1 Generally, the twisted and helical nano/ and acenaphtho[1,2-k]fluoranthene moieties that form the
micro-structures were constructed by assembling the molecules rigid core; (ii) the distinctive and large p-conjugated system
with chiral groups such as peptides,2 sugar-based derivatives,3 suggesting promising emission and semiconducting properties;
p-conjugant oligomers4 and so on.5 Fabrication based on chiral (iii) the DPAF-n structure features a rigid central core and flexible
building blocks is a popular and efficient approach to achieve side alkyl chains allowing formation by supramolecular assembly.
twisted and helical wires and ribbons. Nevertheless, assembling DPAF-n molecules, which are remarkably different from the
achiral organic molecules into twisted and helical nano/micro- reported organic molecules for the assembly of twisted or helical
structures has become a hot topic in the area of supramolecular structures,6–9 represent a new classical building block for the
chemistry recently. Achiral molecules such as phthalocyanines,6 fabrication of functional nano- and micro-materials.
perylene bisimides,7 quinacridones8 and so on9 have been employed
DPAF-n were synthesized using Suzuki cross-coupling reactions
as building blocks to fabricate twisted and helical structures by between 7,14-bis[4-bromophenyl]acenaphtho[1,2-k]-fluoranthene
intermolecular hydrogen bonding and p–p stacking interactions. and 3,4,5-trialkyloxyphenylboronic acid (Scheme S1, ESI†). To
These organic functional molecules with extended p-conjugated investigate the assembly properties of DPAF-n molecules, the phase
aromatic systems endow their assembly systems with unique transfer method based on a variety of binary solvents systems was
catalysis,10 biomaterials11 and optoelectronic12 properties. employed. DPAF-n samples were dissolved in good solvents such
Therefore, it is still an important issue to develop new building as dioxane, chloroform, toluene and tetrahydrofuran. Then the
blocks for the construction of twisted or helical structures with
desired characteristics.
7,14-Diphenylacenaphtho[1,2-k]fluoranthene (DPAF) is an
extended and fused-ring molecule composed of five- and
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun 130012, P. R. China.
E-mail: zhangjingy@jlu.edu.cn, yuewang@jlu.edu.cn; Fax: +86-431-85193421;
Tel: +86-431-85168484
† Electronic supplementary information (ESI) available: Synthetic procedures and
characterization data; additional emission spectra, XRD patterns, crystal data,
fluorescence microscopy, FESEM images. CCDC 1043332. For ESI and crystallographic
Fig. 1 Molecular structures of DPAF-n.
data in CIF or other electronic format see DOI: 10.1039/c5cc00476d
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DPAF-n solution was added into test tubes, and sequentially a poor achieved through slow evaporation of DPAF-12 solution (Fig. S2
solvent such as methanol, ethanol and isopropanol was carefully and S3, ESI†).
covered on the top surface of the DPAF-n solution. Slow diffusion of
The morphologies of the obtained microstructures are sensitive
the poor solvent into the DPAF-n solution resulted in 1D nano- to the polarity of the chosen ‘‘poor’’ solvent for the phase transfer
or micro-materials with the luminescence characteristic in the methodology. In dioxane solution of the DPAF-12–methanol system,
interlayer between the good and the poor solvent phases.
although the twisted wires were prepared, their morphologies were
Well distributed microwires with a twisted morphology based not well-defined (Fig. S4, ESI†). When isopropanol was employed
on DPAF-12 were obtained in the interlayer between dioxane as the ‘‘poor’’ solvent, whose polarity is lower, large numbers
solution of DPAF-12 (1 mg mLÀ1) and ethanol (Fig. 2a and c). The of crystalline straight ribbons were generated in the interlayer
twisted microwires were as long as several micrometres. Their between dioxane solution and isopropanol solvent (Fig. S5, ESI†).
width and pitch were 2–3 mm and 30–40 mm, respectively, as The fluorescence of straight needle-like ribbons showed
demonstrated using fluorescence microscopy and FESEM (field significant blue-shifts compared with the twisted microwires that
emission scanning electron microscopy) images. Both right- and formed in dioxane solution–ethanol system. For the same organic
left-handed twisted wires were obtained and twisted samples compound that may adopt flexible molecular conformation or
were racemic mixtures in this study, which was attributed to the configuration, the different solvent media may result in various types
absence of a chiral center in the molecules. The morphologies of of molecular packing and crystalline phases (namely polymorphs).14
the twisted wires displayed obviously concentration dependent The aggregated structures of organic molecules are strongly
characteristics. Upon decreasing the concentration of the DPAF-12 dependent on the intermolecular interactions.15 The solubility
dioxane solution, the width and the pitch of the generated of DPAF-12 in various solvents is different, which may induce
twisted microwires reduced successively (Fig. 2b, d and e). When the generation of aggregated states based on distinct inter-
the concentration reached 0.05 mg mLÀ1, the twisted microwires molecular interactions. Therefore, the DPAF-12 molecules with six
formed accompanied by a few straight needle-like crystalline long alkyl chains should take different configurations and inter-
wires (Fig. 2f).
molecular interaction in different solvent systems, which can result
Similar DPAF-12 based twisted microstructures could be in the formation of the nano- or micro-structures with different
also generated from other double layer solvent systems such morphologies and luminescence properties. So far, a precise
as chloroform solution–methanol, chloroform solution–ethanol, mechanism for the result in this study remains unclear and
THF solution–ethanol and chloroform solution–isopropanol with some detailed investigation should be performed in the future.
different morphologies, lengths, widths and pitches (Fig. S1, ESI†).
To check the assembly properties of DPAF derivatives with
On the other hand, by precisely regulating the composition and different lengths of alkyl chains, DPAF-16 and DPAF-8 were
ratio of the mixture solvent, the twisted architectures could be used as building blocks to fabricate nano/micro-structures.
Indeed, the lengths of the alkyl chains have a dramatic effect
on the morphologies of the resulted architectures composed of
DPAF derivatives. When compound DPAF-16 with longer alkyl
chains was employed as building blocks for molecular assem-
bly, the individual twisted wires and twisted wire bundles were
generated (Fig. 3a) in the interlayer of the chloroform solution
(1 mg mLÀ1)–methanol binary solvent system. FESEM micro-
scopy images illustrated the width and pitch of the single
twisted wires was about 100 nm and several tens of nanometers,
respectively, which were much smaller than those of DPAF-12
microwires prepared using the same procedure. Under identical
conditions except that methanol was replaced by ethanol, most
of the resulting 1D ribbons presented wider distribution in
dimension and only a small amount of twisted wires were
observed (Fig. 3b). Carefully regulating the concentration and
solvent media of DPAF-16 in solution has not led to the
formation of well-defined twisted wires (Fig. S6–S9, ESI†).
For compound DPAF-8 with shorter alkyl chains, non-twisted
architectures were dominant in the resulting samples (Fig. 3c
and d). The twisted structure was only obtained in the phase
transfer system of dioxane solution (0.05 mg mLÀ1)–methanol
(Fig. S10, ESI†). It is worth noting that DPAF-8 molecules easily
assembled into flat thin emissive crystals with a smooth surface
Fig. 2 Fluorescence microscopy and FESEM images of DPAF-12 micro-
(Fig. S11, ESI†) by phase transfer (chloroform solution (1 mg mLÀ1)–
structures prepared by phase transfer method from ethanol and dioxane
methanol) or slow evaporation of dioxane solution (1 mg mLÀ1
)
solutions at different concentrations: (a, c) 1 mg mLÀ1; (d) 0.5 mg mLÀ1
;
(b, e) 0.1 mg mLÀ1; (f) 0.05 mg mLÀ1
.
(Fig. S11, ESI†).
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dilute solution. To understand the above emission phenomena,
powder X-ray diffraction (XRD) measurements were performed
(Fig. S12, ESI†). The twisted and flexible wires showed similar XRD
patterns to the crystal powder sample, suggesting that these three
samples belong to the same crystalline phase. The patterns were
indexed (part of indexing peaks are marked in Fig. S12c, ESI†) and
the calculated packing period distance is around 15.2 Å reflecting
the (100) crystal plane. For straight ribbons, the calculated packing
period distance is as large as 22.213 Å. The longer packing period
distance means relatively loose molecular packing, which can
enhance the emission. In contrast, the dense molecular packing
can lead to self-absorption, which induced emission quenching.15
The XRD analysis results provide rational explanation for the
emission behavior of the DPAF-12 solid samples.
To figure out molecular packing and assembly characteristics
of DPAF-n series building blocks, the single crystal structure of
DPAF-8 was investigated. In DPAF-8 crystal the molecules adopt
two different conformations, namely S and I type conformations
(Fig. 5a and Fig. S13a, ESI†). For S type molecules, the side alkyl
chains show a sharp bend feature, while for I type molecules, the
alkyl chains adopt a relatively slight bend orientation. The S and I
type molecules array alternately to form the molecular chains,
which pack together to result in the architecture of crystal.
Fig. 3 FESEM images: (a) DPAF-16 twisted nanowires prepared from the
chloroform solution (1 mg mLÀ1)–methanol system; (b) DPAF-16 microribbons
prepared from chloroform solution (1 mg mLÀ1)–ethanol system; (c) DPAF-8
branched structures prepared from THF solution (1 mg mLÀ1)–methanol
system; (d) DPAF-8 flat micro-ribbons prepared from dioxane solution
(1 mg mLÀ1)–methanol system using the phase transfer method.
The emission spectra of DPAF-12 samples in different states
are illustrated in Fig. 4. The twisted and flexible wires prepared
from dioxane solution of DPAF-12–methanol or ethanol systems
showed very similar luminescence properties. The emission
maxima (lem) at 526 nm and photoluminescence quantum yields
(FF) of 0.18 were recorded for the two samples. The crystal
powder sample of DPAF-12 precipitated from chloroform
%
Along the [101] direction, the molecules were linked together by
intermolecular C–HÁ Á Áp interactions between alkyl chains and
aromatic cores (Fig. 5b and Fig. S13c, ESI†). Along the [101]
direction, the molecules stack each other based on the inter-
molecular C–HÁ Á Áp interactions between adjacent molecular
aromatic cores that adopt a nearly vertically packing mode
(Fig. 5c and Fig. S13d, ESI†). From the supramolecular point
view, the DPAF-8 molecular arrangement in the crystal has
obviously helical features. Therefore, the molecular chains in
Fig. 5b and c may pack into wires with twisted or helical
characteristic. For molecules DPAF-12 and DPAF-16 with longer
flexible alkyl chains, the twisted or helical architectures should
be more easily generated based on supramolecular assembly.16
Single crystal analysis demonstrated a possible explanation for
the formation mechanism of twisted wires in this study.
In conclusion, three DPAF derivatives DPAF-n (n = 8, 12, 16)
with identical central aromatic cores and different lengths of
flexible alkyl chains have been synthesized and employed as
building blocks to fabricate 1D nano/micro-structures. It was
solution displayed very similar emission characteristics (lem
=
527 nm, FF = 0.19) to twisted wires. The DPAF-12 straight
ribbons generated in the interlayer between dioxane solution
and isopropanol demonstrated a much shorter emission maximum
(504 nm) and higher photoluminescence quantum yield (0.62)
compared with twisted and flexible wires. The emission properties
of the straight ribbons were quite similar to those of DPAF-12
Fig. 4 Fluorescence spectra of DPAF-12 based samples: (a) dilute solution
in dioxane (0.1 mg mLÀ1); (b) microwires prepared from dioxane solution
(1 mg mLÀ1)Àmethanol system by phase transfer method; (c) twisted
microwires prepared from dioxane solution (1 mg mLÀ1)–ethanol system
using the phase transfer method; (d) needle-like crystalline ribbons prepared
from dioxane solution (1 mg mLÀ1)–isopropanol by phase transfer method;
(e) solids in the powder state.
Fig. 5 Molecular packing in DPAF-8 crystal structures: (a) view of the
molecules arrangement in twisted form along the ac axial plane; (b) the
%
molecular chains observed along [101] direction under space fill style;
(c) the molecular chains observed along [101] direction under space fill style.
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7 (a) D. Ke, A. Tang, C. Zhan and J. Yao, Chem. Commun., 2013, 49,
demonstrated that 1D luminescent twisted wires were achieved
based on DPAF-n. Compared with DPAF-8 and DPAF-16, DPAF-12
molecules displayed stronger twisted aggregation property and
could assemble into a 1D twisted structure under wide phase
transfer conditions. The DPAF-12 based wires with twisted and
straight morphologies presented different emission characteristics,
which were attributed to the different molecular packing in the 1D
structures. Single crystal X-ray studies revealed supramolecular
helical structural features in a DPAF-8 crystal, which should be
the driving force to induce the formation of a twisted structure
in this study.
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