Direct evidence for secondary interactions in planar and nonplanar aromatic π-conjugates and their photophysical characteristics in solid-state assemblies

Article abstract of DOI:10.1021/acs.jpcb.5b01956

Direct evidence for non-covalent secondary interactions in planar and nonplanar aromatic π-conjugates and their solid-state assemblies is established. A series of horizontally, vertically, and radially expanded oligo(phenylenevinylene)s (H-OPVs, V-OPVs, and R-OPVs, respectively) were designed with a fixed π-core and variable alkyl chain lengths on the periphery. Single-crystal structures of the OPVs were resolved to trace the secondary interactions that direct the solid-state self-organization and molecular packing of the chromophores. The H-OPVs were found to be planar, and they did not show any secondary interactions in the crystal lattices. The V-OPVs and R-OPVs were found to be nonplanar and to exhibit multiple CH/π hydrogen-bonding interactions among aryl hydrogen donors and acceptors. The enthalpies of the melting and crystallization transitions revealed that the planar H-OPVs are highly crystalline compared with the nonplanar R-OPVs and V-OPVs. Polarized light microscopy studies revealed the formation of one-dimensional nematic mesophases in H-OPVs. The absolute solid-state photoluminescence quantum yields (PLQYs) of the OPVs were determined using an integrating sphere setup. The highly packed H-OPVs showed low PLQYs compared with those of the weakly packed V-OPVs and R-OPVs. Time-resolved fluorescence decay measurements revealed that the excited-state decay dynamics of highly packed H-OPVs was much faster with respect to their low PLQYs. The decay profiles were found to be relatively slow (with higher life time (τ)) in the V-OPVs and R-OPVs. A field-effect transistor (FET) device was constructed for an OPV sample that showed a hole carrier mobility in the range of 10^-5 cm² V^-1 s^-1. The present investigation thus provides a new opportunity to trace the role of secondary interactions on π-conjugated mesophase self-assemblies and their solid-state emission and FET devices, more specifically based on OPV chromophores.

Full text of DOI:10.1021/acs.jpcb.5b01956

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Article
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Direct Evidence for Secondary Interactions in Planar and Nonplanar
Aromatic π‑Conjugates and Their Photophysical Characteristics in
Solid-State Assemblies
Mahima Goel, Karnati Narasimha, and Manickam Jayakannan*
Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pune 411008,
Maharashtra, India
S
* Supporting Information
ABSTRACT: Direct evidence for non-covalent secondary interactions in
planar and nonplanar aromatic π-conjugates and their solid-state
assemblies is established. A series of horizontally, vertically, and radially
expanded oligo(phenylenevinylene)s (H-OPVs, V-OPVs, and R-OPVs,
respectively) were designed with a fixed π-core and variable alkyl chain
lengths on the periphery. Single-crystal structures of the OPVs were
resolved to trace the secondary interactions that direct the solid-state self-
organization and molecular packing of the chromophores. The H-OPVs
were found to be planar, and they did not show any secondary
interactions in the crystal lattices. The V-OPVs and R-OPVs were found
to be nonplanar and to exhibit multiple CH/π hydrogen-bonding
interactions among aryl hydrogen donors and acceptors. The enthalpies
of the melting and crystallization transitions revealed that the planar H-
OPVs are highly crystalline compared with the nonplanar R-OPVs and V-
OPVs. Polarized light microscopy studies revealed the formation of one-dimensional nematic mesophases in H-OPVs. The
absolute solid-state photoluminescence quantum yields (PLQYs) of the OPVs were determined using an integrating sphere
setup. The highly packed H-OPVs showed low PLQYs compared with those of the weakly packed V-OPVs and R-OPVs. Time-
resolved fluorescence decay measurements revealed that the excited-state decay dynamics of highly packed H-OPVs was much
faster with respect to their low PLQYs. The decay profiles were found to be relatively slow (with higher life time (τ)) in the V-
OPVs and R-OPVs. A field-effect transistor (FET) device was constructed for an OPV sample that showed a hole carrier mobility
in the range of 10−5 cm2 V−1 s−1. The present investigation thus provides a new opportunity to trace the role of secondary
interactions on π-conjugated mesophase self-assemblies and their solid-state emission and FET devices, more specifically based
on OPV chromophores.
single crystals. This leads to a larger ambiguity in establishing
the correlation among various factors such as molecular
planarity, types of non-covalent secondary interactions, packing
abilities, and photophysical and charge mobility characteristics
of π-conjugated materials in general.4
Liquid-crystalline (LC) mesophase assemblies of π-con-
jugated chromophores represent an emerging approach for
optoelectronic applications.21−23 In this approach, the
chromophores can be easily aligned thermotropically on
desired substrates (e.g., a glass plate) by means of a solvent-
free melt crystallization process. Optimization of the LC active
temperature window to get the right fluid states of the
mesogens, LC morphologies, and structure−property relation-
ships are some of the issues yet to be addressed in π-conjugated
LC materials.23 For the past few years, our group has devoted
effort to the study of π-conjugated LC mesophase assemblies of
INTRODUCTION
■
Secondary interactions are important driving forces in the
precise three-dimensional molecular arrangements of organic π-
conjugated chromophores in electronic devices such as light-
emitting diodes,1 photovoltaics,2 field-effect transistors
(FETs),3 etc. Thin-film FETs materials such as pentacene
and acenes,4−6 oligothiophenes,7−9 fused thiophene aro-
matics,10,11 and tetrathiafulvalenes12,13 were reported to exhibit
either a face-to-face or face-to-edge orientation as the most
preferred configuration in the solid state. Surprisingly, these
planar π-conjugates did not show any π−π stacking as expected
in the crystal lattices.6,10 Recent studies revealed that
nonplanarity was observed in the aromatic cores of phenylene,
naphthalene, and anthracene rings irrespective of the topology,
size, and nature of the substitution.14,15 Non-covalent
interactions such as CH/π16,17 and RCH2O/π18−20 (R = H
or alkyl) were reported to stabilize the packing of π-conjugated
chromophores. Large numbers of π-conjugated materials
belonging to phenylenevinylenes, fluorenes, and low-band-gap
materials were found to be sluggish to produce good-quality
Received: February 27, 2015
Revised: March 23, 2015
Published: March 24, 2015
© 2015 American Chemical Society
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DOI: 10.1021/acs.jpcb.5b01956
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Figure 1. (a) Spatial expansion of π-conjugated OPVs horizontally (along the X axis), vertically (along the Y axis) and radially (along both the X and
Y axes). The red solid part represents the rigid aromatic OPV π-core, and the gray lines and fillings around the π-core represent the aliphatic content.
(b) Role of planarity in the CH/π interactions and LC mesophase of OPVs.
Scheme 1. Synthesis of Horizontally, Vertically, and Radially Expanded OPVs
hydrocarbon skeleton oligo(phenylenevinylene) (OPV) chro-
mophores.24,25 Preliminary investigations of the single-crystal
structures of these OPVs revealed that multiarm CH/π
hydrogen-bonding interactions facilitate the formation of chiral
nematic ring banded self-assemblies.26,27 Efforts were also made
to trace the roles of hydrocarbon- and fluorocarbon-tailed
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Figure 2. (a) Aromatic ring distortion in the OPV backbone. (b−d) Single-crystal X-ray structures of (b) OPV1, (c) V-OPV4, and (d) V-OPV11
and their three-dimensional packing. In (d), the long alkyl tails in the middle aromatic rings have been omitted for better representations of ψA and
ψC.
OPVs in their smectic LC mesophases.28 These studies
facilitated the making of helical segmented OPV polymers29
and their organogel photonic switches.30 A few other groups
have also reported the single-crystal structures of OPVs to
understand their emission characteristics.31−35Ajayaghosh and
co-workers reported the conversion of OPV chromophores into
organogels and supramolecular helical self-assemblies.36−39
Meijer and co-workers developed unique quadruple-hydro-
gen-bonded OPV assemblies for making supramolecular
polymers and organogels.40−43These studies emphasized the
importance of the development of aromatic π-conjugated
assemblies. However, until now there has been no effort to
trace the secondary forces that control the mesophase
assemblies of OPV π-conjugates and their solid-state
luminescence properties. Addressing this multitask problem is
highly dependent on the following criteria: (i) the appropriate
design of π-conjugates with sufficient structural diversity; (ii)
the ability of the π-conjugates to produce good-quality single
crystals to allow the study of the secondary interactions; and
(iii) the ability of the π-conjugates to produce self-assembled
mesophases and so on.
horizontally, vertically, and radially expanded OPVs (H-OPVs,
V-OPVs, and R-OPVs, respectively) were synthesized by
varying the aliphatic content along the X axis, the Y axis, and
both the X and Y axes, respectively (see Figure 1). Large
numbers of single-crystal structures of these OPVs were
resolved, allowing us to trace the non-covalent forces that
dictate the chromophore packing in the 3D crystal lattices.
Efforts were made to establish the correlation between
molecular planarity, aromatic π-stacking, and CH/π hydro-
gen-bonding interactions in π-conjugated OPV systems.
Furthermore, the roles of these secondary interactions on the
LC mesophase assemblies and lamellar packing were also
studied by absolute solid-state quantum yield determinations
and time-resolved fluorescence decay dynamics studies.
Preliminary efforts were also made to employ an OPV
molecule as the active layer in an FET. The present approach
provides new insight into the precise molecular arrangements
of planar and nonplanar geometries, mesophase alignment, and
solid-state characteristics more specifically based on OPV
chromophores.
RESULTS AND DISCUSSION
The present work is one of the first attempts to trace the
secondary forces in π-conjugated chromophores with planar or
nonplanar geometries in a single system, and it also aims to
establish the correlation between the π-conjugated chemical
structures and the mesophase self-assemblies. For this purpose,
spatially expandable OPV molecules having an identical
aromatic π-core and variable aliphatic content were developed
through tailor-made approaches (see Figure 1). Three series of
■
Synthesis and Crystal Structures of OPVs. Three series
of horizontally, vertically, and radially expanded OPVs were
synthesized by varying the aliphatic content along the X axis,
the Y axis, and both the X and Y axes, respectively (see Figure
1). The H-OPVs, V-OPVs, and R-OPVs were synthesized as
shown in Scheme 1. The length and position of the alkyl chains
were varied from C1 to C11 in both R1 and R2 in the OPV core.
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In the first series, R1 was fixed as CH3 and R2 was varied as
CH3, C4H9, C8H17, and C11H23 to make H-OPVs (see Scheme
1). In the second series, R2 was fixed as CH3 and R1 was varied
as CH3, C4H9, C8H17, and C11H23 to make V-OPVs. Retaining
R1 = R2 and varying the number of carbon atoms as 1, 4, 8, and
11 (CH3, C4H9, C8H17, and C11H23) afforded the series of R-
OPVs. The basic OPV structure having all four sides with
methoxy substitution is named as OPV1 (R1 = R2 = CH3). The
other OPVs are named as H-OPVn, V-OPVn, and R-OPVn,
where n represents the number of carbons in the alkyl chains
varied in that particular series. All of these OPVs were
characterized by 1H and 13C NMR, FT-IR, MALDI-TOF-TOF,
and elemental analysis, and these details are provided in the
Supporting Information. H-OPV4 and V-OPV4; H-OPV8 and
V-OPV8; and H-OPV11 and V-OPV11 are structural isomers
that differ only at the point of alkyl chain attachment in the π-
core.
The crystallographic data for the OPV molecules are
provided in the Supporting Information (see Tables ST1 and
ST2). OPV structures are broadly classified into two types with
respect to their planarity. The angles ψA and ψC subtended by
two terminal rings on the central aromatic ring are shown in
Figure 2a. The three phenyl rings have been labeled as A, B,
and C for easy identification. For a planar molecule ψA = ψC ≈
0, and for the nonplanar OPVs ψA ≠ ψc ≠ 0 or ψA = ψC ≠ 0.
The unit cell for OPV1 is shown in Figure 2b (see SF-1 and SF-
2 in the Supporting Information for more information). In
OPV1, the two terminal aromatic rings A and C have moved
out of the plane of the central aromatic ring B in opposite
directions, giving rise to a nonplanar structure (see Figure 2b).
The torsional angles of the central aromatic ring with the
terminal aromatic rings were measured to be ψA = 34.02° and
ψC = 62.59°. Intermolecular close contacts revealed that a total
of four CH/π interactions are present between neighboring
molecules (see SF-3). The CH/π interactions are validated by
the four parameters, dc−x, θ, ϕ, and dHp−x (see Table ST3), as
reported earlier.27,28,42,43 The CH/π interactions are involved
between aryl C−H donors and aryl rings as π-acceptors in a
perpendicular orientation in the edge-to-face arrangement (see
SF-1). The three-dimensional packing of these molecules shows
head-to-tail arrangements that are interlocked by four CH/π
interactions in a square-planar fashion (shown by the arrows in
Figure 2b). The interlocking in a head-to-tail fashion among
rings A and C produces long one-dimensional molecular stacks.
The CH/π interactions were found to be present only along
the molecular axis in each stack, and there are no interactions
between the adjacent stacks.
expansion, the V-OPV11 molecules were found as molecular
dimers (see Figure 2d). Each molecular dimer consists of two
molecules with different aromatic geometries (see SF-5). The
two terminal aromatic rings in the first molecule have been
moved out of the plane of the middle aromatic ring equally in
the same direction, giving rise to a nonplanar molecule. The
torsional angles were measured to be 29.50° for one such
aromatic system (top molecule in Figure 2d). In the second
molecule, the torsional angles between the terminal and central
rings were measured to be 38.63° (bottom molecules in Figure
2d). The three-dimensional packing revealed that the molecules
are arranged right on the top of each other, giving rise to
lamellar packing (see SF-5). Each green molecule present in the
stack is locked by two blue molecules (top and bottom) at
either side (see Figure 2d, shown by dotted lines). The CH/π
interactions are directed in a linear fashion (see SF-6) rather
than a square-planar or angular fashion, as observed in their
shorter counterpart V-OPV4. These interactions reveal that
each central molecule exhibits CH/π interactions with an
aromatic ring as well as CC double bonds (see SF-6 and
Table ST3).
The radially extended OPV molecule R-OPV8 is shown in
SF-7. The middle aromatic ring in R-OPV8 has been moved
out of the plane constituted by the terminal rings. Both
torsional angles of the central aromatic ring with respect to the
terminal aromatic rings were found to be 45.71°. One of the
terminal C8 alkyl tails has two carbons in a gauche
conformation. The octyl chains attached to the terminal rings
are laid along the molecular axis, whereas those attached to the
central aromatic rings protrude above and below the molecular
plane. CH/π interactions between H22 (donor) and the central
aryl ring (acceptor), between aryl H2 (donor) and the C12C15
double bond (acceptor), and between H35A (donor) and C4
C7 (acceptor) were observed (see SF-8). The three-dimen-
sional structure of these molecules shows lamellar-type packing
in which the molecules are interdigitized in a parallel fashion
along the long molecular axis. The R-OPV11 molecule was
found to be planar, and there are no CH/π interactions
between aryl H atoms and aryl rings. However, weak CH/π
interactions between the R−CH2O hydrogens and aryl rings
were observed.27 Among the horizontal OPVs, only the H-
OPV11 molecule produced a single-crystal, and its structure
was found to be planar with no C−H/π interactions.27 OPVs
such as H-OPV4, H-OPV-8, V-OPV8, and R-OPV4 did not
produce good-quality crystals.
CH/π Hydrogen Bonding and Planarity of the π-Core.
Single-crystal X-ray diffraction (XRD) analysis revealed that
both the length of the alkyl chains and their point of
attachment in the aromatic core as well as the planarity of
the π-backbone play important roles in determining the solid-
state packing of the chromophores. Furthermore, it was
observed that the planar molecules are devoid of CH/π
interactions. On the other hand, the nonplanar molecules
exhibited strong CH/π interactions. In all cases except in R-
OPV11, the CH/π interactions occurred between aryl C−H
donors and aryl rings as acceptors. In order to establish a direct
correlation between the planarity of the molecules and the CH/
π interactions, the angles ψA and ψC were plotted, as shown in
Figure 3. In OPV1, the two terminal aromatic rings have been
moved out of the plane of the middle aromatic ring in opposite
directions, giving rise to a nonplanar molecule. In contrast to
OPV1, the two terminal aromatic rings in V-OPV4 have been
moved out of the plane of the central ring in the same direction,
Vertical expansion of the alkyl carbon atoms in V-OPV4
produces an orthorhombic cystal system, and the molecule
belongs to the centrosymmetric space group Pbca (see Figure
2c and SF-4). In V-OPV4, the two terminal aromatic rings have
been moved out of the plane in the same direction (unlike in
OPV-1; see Figure 2b), giving rise to a nonplanar geometry.
The angles subtended by the terminal aromatic rings on the
central aromatic ring are equal and were measured to be ψA =
ψC = 37.68°. The three-dimensional packing revealed an
extended network of CH/π interactions in the zigzag chains
(see SF-4). The CH/π interactions are directed in an angular
fashion, subtending an angle of 104.8° relative to each other.
These interactions are locked through four identical CH/π
interactions involving Ar−C−H6 as the donor and the terminal
phenyl ring as the π-acceptor system (see Figure 2c). As the
alkyl chain length further was increased in the vertical
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DOI: 10.1021/acs.jpcb.5b01956
J. Phys. Chem. B 2015, 119, 5102−5112