Synergistic steric pairing effects of terfluorenes with ternary side groups on β-conformation transition: Experiments and computations

Article abstract of DOI:10.1039/c7tc04817c

Rational molecular design has led to the achievement of β-conformations of terfluorenes that are usually difficult for oligoalkylfluorenes with less than 5 repeat units. Herein, under the guideline of synergistically molecular attractor-repulsor theory (SMART), we design four models of terfluorenes with the possibility of inducing β-conformation in the terfluorene 2,2′-(spiro[fluorene-9,9′-xanthene]-2,7-diyl) bis(9-(4-(octyloxy)phenyl)-fluoren-9-ol) (2O8-DPFOH-SFX) by manipulating the cooperative effects of steric hindrance, hydrogen bonding (HB) and van der Waals (vdW) forces after thermal annealing. For terfluorene 2O8-DPFOH-SFX, the two isomers, raceme and mesomer, with two side chains residing on the two sides and on the same side of the fluorene backbone, respectively, have different packing styles and hence different optical properties: only the raceme isomer could induce the formation of β-conformation. Due to the steric pairing effect of side chains on both sides and the cooperative interaction of the middle spiro[fluorene-9,9′-xanthene] (SFX) group, the racemes precisely interlace with each other and tend to adopt Δ-shaped stacking, which favors a planar π-conjugated structure for the raceme. This is rationalized by the larger content of planar conformations in the 2O8-DPFOH-SFX raceme, according to molecular dynamics (MD) simulations of the condensed phase. Thus, oligofluorenes with definite molecular structure can be considered as notable materials for gaining insight into the structure-property relationships for optical and electronics applications.

Full text of DOI:10.1039/c7tc04817c

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Table of Contents

Journal of Materials Chemistry C
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Synergistic Steric Pairing Effects of
Terfluorenes with Ternary Side Groups on
β
-Conformation Transition: Experiments
and Computations
，
，
XiangꢀAi Yuan,1 †MengꢀNa Yu2 †, Qiang Zhu,1 WanꢀWan Zhang, 2 LingꢀHai Xie,2,*Wei
Huang,2 Jing Ma1,*
1Key Laboratory of Mesoscopic Chemistry of MOE School of Chemistry & Chemical
Engineering, Nanjing University, 22 Hankou Road, Nanjing, 210093, People’s
Republic of China
2Center for Molecular Systems & Organic Devices (CMSOD), Key Laboratory for
Organic Electronics and Information Displays & Jiangsu Key Laboratory for
Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic
Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts &
Telecommunications, 9 Wenyuan Road, Nanjing 210023, China
† These authors contributed equally to this article.
1

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ABSTRACT
Rational molecular design leads to the achievement of βꢀconformations of
terfluorenes that are usually difficult for oligoalkylfluorenes with less than 5 repeat
units. Herein, under the guideline of synergistically molecular attractorꢀrepulsor
theory (SMART), we design four models of terfluorenes with the possibility of having
the βꢀconformations in the terfluorene, 2,2'ꢀ(spiro[fluoreneꢀ9,9'ꢀxanthene]ꢀ2,7ꢀdiyl)
bis(9ꢀ(4ꢀ(octyloxy)phenyl)ꢀfluorenꢀ9ꢀol) (2O8ꢀDPFOHꢀSFX) through manipulating
the cooperative effect of steric hindrance, hydrogen bonding (HB) and van der Waals
(vdW) force after thermal annealing. For terfluorene 2O8ꢀDPFOHꢀSFX, the two
isomers, raceme and mesomer, with two side chains residing on the two sides and the
same side of fluorene backbone, respectively, have different packing styles and hence
different optical properties: only the raceme isomer could induce the formation of
βꢀconformation. Due to the steric pairing effect of side chains on both sides, and the
cooperative interaction of middle spiro[fluoreneꢀ9,9'ꢀxanthene] (SFX) group, racemes
artfully interlace with each other and tend to adopt ꢀꢀshaped stacking, which is more
favorable for raceme to display planar πꢀconjugated structure. This is rationalized by
the larger content of planar conformations in raceme of 2O8ꢀDPFOHꢀSFX according
to molecular dynamics (MD) simulations of the condensed phase. Oligofluorenes with
definite molecular structure are considered as notable materials to gain insight into the
structureꢀproperty relationships with the potential optical and electronic applications
on demanding.
2

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INTRODUCTION
Conformational transition has a significant influence on the physical and chemical
properties of organic molecules, such as reversible switching, the origin of the glass
transition temperature, plastic crystals, pharmaceutical polymorphs, specific phase
and high performance. As one crucial but subtle item in organic and polymer
semiconductors, conformation transition sometimes has the positive effect on
electronic property or exciton that not only for electrical switching but also for the
mobility and high efficiency, but sometimes disturbs the material functions and
behaviors as defects.
Great efforts have been made to study conformation transition experimentally and
theoretically from the aspects of statistical thermodynamic, kinetics and quantum
mechanism. From the microscopic view of single molecular state, it has been
recognized that the prerequisite to achieve the transition is to overcome the energy
barrier between local minima. In terms of macroscopic thermodynamic theory, the
occurrence of a transition process requires a negative value of the Gibbs free energy
change (ꢁG), which is the resultant from both enthalpy and entropy contributions.
Indeed, from the kinetics standing point, not all the polyfluorenes could trigger the
conformation transition even when heated to a high enough temperature.
Planar chain conformation, also called βꢀconformation in polyfluorenes, becomes
a unique model to investigate the picture of molecular phase change.1ꢀ19 The
formation of βꢀconformation is usually characterized by the appearance of shoulder
peak at around 430 nm in UVꢀvis absorption spectra and redꢀshifted emission peak at
about 447 nm.20,21 To date, planar conformation has been studied intensively in
consideration of both the inherently molecular design and external condition
regulation. On the one hand, many efforts have been made to investigate the effects of
molecular structure22, such as molecular weight23,24, alkyl substituent,1,11 and
copolymerization9, on the formation of
βꢀconformation in polyfluorenes. On the other
hand, one resorted to some convenient strategies focused on external environment
regulation, including solubility in certain solvent22, solvation temperature variation25,
thermal annealing18, blending26 and so on. Numerous progresses have been reported,
3

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yet a few questions still exist, including the roles of the molecular segments; the
thickness effects on ꢀconformation; the reason why some polyfluorenes with
ꢀconformation while others without; the ways to incorporate between molecular
structures and external conditions, and the difference between solvent crystallization
induced ꢀconformation and thermal induced process. What’s more, how to realize
β
β
β
the electrically induced crystallization and phase change has become one thorny issue
in optoelectronic semiconducting memory field.
Therefore, the stateꢀofꢀtheꢀart model is particularly important to understand the
singleꢀelement node to map the whole picture. According to the attractorꢀrepulsor
molecular design,27ꢀ31 Xie and coworkers sophisticatedly tuned the positions of bulky
groups with steric hindrance and alkoxy side chains with vdW forces, and
unprecedentedly obtained the
β
ꢀconformation in a bulky polydiarylfluorene
ꢀconformation has also been observed in some
(PODPF).32 Except for polyfluorene,
β
oligofluorenes. Oligofluorenes with wellꢀdefined chain lengths and definite molecular
structure are considered as notable materials for understanding the structureꢀproperty
relationship, in sharp contrast to polyfluorenes which have difficulty in precise
characterization due to polydispersity and conformation complexity. Nakashina et al.
reported that at least 15 or more repeating fluorene units are necessary to form
β
β
ꢀconformation structure in oligo(9ꢀ9’ꢀdioctylfluorenes).13 Moreover, the
ꢀconformation of shorter mainꢀchain oligmeric fluorene consisted of 5 monomer
units was achieved by Lidzey et al.8 Recently, diarylfluorene dimers exhibit
conformational planarization with a torsion angle of 180o, indicating the ability of
very short oligofluorenes forming coplanar conformation.33 We hence wonder that
with the cooperative effects of the hydroxyl group (acting as the HB donor/acceptor),
bulky spirofluorene group (with steric hinderance), and octyl sideꢀchains (with
favorable vdW forces) to tune the interꢀchain arrangement, is it possible to achieve
planar backbone of short oligofluorenes in film or solution with proper chemical
modification?
In fact, planar conformation is the metastable state with the nature of the
pervasive highꢀenergy conformations, but can be stabilized by intermolecular
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nonꢀcovalent interactions in some cases. Secondary interactions, such as van der
Waals force, πꢀπ interaction and hydrogen bonding interaction, have been proposed as
a promising tool to modulate possible planarization when they are rationally
incorporated into a molecular design. In this work, we attempted to achieve
β
ꢀconformation in the fluorenol trimer through the collective effects of HB
interaction, bulky steric hindrance, and steric pairing effects of side chains. Herein,
four terfluorenes were designed and synthesized:
2,2'ꢀ(spiro[fluoreneꢀ9,9'ꢀxanthene]ꢀ2,7ꢀdiyl)bis(9ꢀ(4ꢀ(octyloxy)phenyl)ꢀfluorenꢀ9ꢀol)
(2O8ꢀDPFOHꢀSFX),
2,2'ꢀ(spiro[fluoreneꢀ9,9'ꢀxanthene]ꢀ2,7ꢀdiyl)bis(9ꢀphenylꢀfluorenꢀ9ꢀol)
(DPFOHꢀSFX),
9,9''ꢀbis(4ꢀ(octyloxy)phenyl)ꢀ9'ꢀphenylꢀ[2,2':7',2''ꢀterfluorene]ꢀ9,9',9''ꢀtriol
(2O8ꢀTPFOH) and 9,9',9''ꢀtriphenylꢀ[2,2':7',2''ꢀterfluorene]ꢀ9,9',9''ꢀtriol (TPFOH).
Unexpectedly, 2O8ꢀDPFOHꢀSFX displays planar conformation after thermal
annealing. An inꢀdepth research reveals that 2O8ꢀDPFOHꢀSFX exhibits the
isomerꢀdependent conformation transition behavior and only the racemeꢀisomer
favors the formation of
steric hindrance SFX group and two side chains on both sides. The molecular level
insight into the origin of isomerꢀdependence formation of ꢀconformation and the HB
βꢀconformation through the steric pairing effects between
β
promoted π stacking modes will be gained by the joint efforts of molecular dynamics
(MD) simulations on packing structures, density functional theory (DFT) calculations
on electronic structures, and NMR experiments of chemical shifts. The observation of
β
ꢀconformation in short backboneꢀchain of oligofluorene and the insight into their
packing modes provides a new opportunity for preparing novel systems and regulating
the high electron transport properties in oligofluorene materials.
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2. DETAILS of EXPERIMENTS AND COMPUTATIONS
2.1 Experimental Details
2.1.1 Chemicals
All the solvents and reagents were purchased from commercial suppliers and used
without further purification unless otherwise noted. All products were purified by
flash column chromatography using Kanto Silica Gel (Kanto Chemical Co., Inc.,
Xuhui District, Shanghai, China) 60N (40–63 ꢂm). Spectrochemicalꢀgrade solvents
were
tetra(triphenylphosphine)palladium, 1,1'ꢀbis(diphenylphosphino)ferrocene (dppf),
2,7ꢀdibromoꢀ9ꢀfluorenone, 2ꢀbromoꢀ9ꢀfluorenone, bromobenzene,
used
for
optical
measurements.
Palladium(II)
acetate,
1ꢀbromoꢀ4ꢀ(octyloxy) benzene, and 2,2'ꢀbithiophene were obtained from Aldrich
Chemical Co. (Xuhui District, Shanghai, China). Borontrifluorideetherate, potassium
carbonate, magnesium sulfate, chloroform and toluene were purchased from
Sinopharm Chemical Reagent Co, Ltd (Nanjing, Jiangsu, China) and were used
without further purification. Dichloromethane was dried using anhydrous sodium at
room temperature. THF and toluene were dried over the sodium using benzophenone
as indicator and distilled under dry nitrogen atmosphere immediately prior to use. The
following
2,7ꢀdibromoꢀspiro[fluoreneꢀ9,9′ꢀxanthene]
(DBrSFX),
2,7ꢀdibromoꢀ9ꢀphenylꢀfluorenꢀ9ꢀol (DBrPFOH), 2ꢀbromoꢀ9ꢀphenylꢀfluorenꢀ9ꢀol
(BrPFOH),
9ꢀ(4ꢀ(octyloxy)phenyl)ꢀ2ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀfluorenꢀ9ꢀol
(TMBꢀPFOHO8)
and
2ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ9ꢀphenylꢀfluorenꢀ9ꢀol (TMBꢀPFOH)
were obtained according to previously published protocols.2,34ꢀ36
2.1.2 Characterization
1
The H and 13CꢀNMR spectra were recorded in CDCl3 with tetramethylsilane (TMS)
as the internal standard reference, using a Bruker 400 MHz spectrometer. The mass
spectra were recorded on a Shimadzu GCꢀMS 2010 PLUS. The MALDIꢀTOF MS
spectra were recorded in the reflective mode using substrates. The absorption spectra
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were measured using a Shimadzu UVꢀ3600 spectrometer (Shimadzu Corporation,
Nanjing, Jiangsu, China) at room temperature of the pristine films before and after
annealing at 220 °C, and the emission spectra were recorded using a Shimadzu
RFꢀ530XPC luminescence spectrometer (Shimadzu Corporation, Nanjing, Jiangsu,
China) after excitation at the absorption maxima. The thin films were prepared by
spinꢀcoating on a quartz plate at spinꢀcoating speed of 1500 rpm from 10 mg/mL in
toluene solution.
2.1.3. Synthesis of 2O8-DPFOH-SFX, DPFOH-SFX, 2O8-TPFOH and TPFOH
via Suzuki Coupling Reaction
The fluorene tertiary alcohol monomer was synthesized via a Grignard reaction as
previously reported.35 The synthetic routes for 2O8ꢀDPFOHꢀSFX, DPFOHꢀSFX,
2O8ꢀTPFOH and TPFOH are shown in Figure S1.36 The target oligofluorenols were
synthesized via a palladiumꢀcatalyzed carbonꢀcarbon Suzuki crossꢀcoupling reaction.
A typical preparation procedure is that under N2 atmosphere, the reactants were added
to a threeꢀnecked Schlenk flask (150 mL). The flask was evacuated and backꢀfilled
with nitrogen atmosphere more than three times before injecting with degassed
toluene/THF (30 mL, 1:1 v/v) and an aqueous K2CO3 solution (5 mL, 10 eq) through
a syringe. The reactant mixture was heated to 90 °C and stirred for 48 h. The mixture
was poured into water and extracted with dichloromethane. The combined extracts
were dried by anhydrous MgSO4, and then the solvent was removed under vacuum
condition. The product was purified by silica gel chromatography to afford powder
1
product. The detailed information for synthetic routes was shown in Figures S1. H,
13CꢀNMR, MALDIꢀTOFꢀMS, UVꢀvis absorption, photoluminescence spectrum, TLC
photographs of 2O8ꢀDPFOHꢀSFX, and MALTIꢀTOFꢀMS spectra of two isomers of
1
2O8ꢀDPFOHꢀSFX were shown in Figures S2ꢀS7. The calculated HꢀNMR spectra
were also given in Figures S4a.
2. 2 COMPUTATIONAL DETAILS
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Molecular Dynamics Simulations. To study the packing structures of fluorenols
(TPFOH, 2O8ꢀTPFOH, DPFOHꢀSFX, 2O8ꢀDPFOHꢀSFX) in amorphous state,
several MD simulations were carried out in the canonical (NVT) ensemble at 298 K
and the annealing at 493 K, respectively, by using Andersen thermostat.37 The heating
or cooling process in annealing was carried out by gradually raising or lowering the
temperature 50 °C in each MD simulation, as illustrated by Figure S9 of SI. At each
temperature, a 300 ps MD simulation was implemented to reach the equilibrium for
the studied systems. In the last temperature raising step, the system was heated from
448 K by 45 °C to 493 K, which is the same condition used in experiments. After 300
ps MD simulation for reaching equilibrium, the cooling process was carried out
according to the inverse process of the heating process (Figure S9). When temperature
was cooled to 298 K, the 10ꢀnsꢀMD simulation was subsequently carried out. The
polymer consistent force field (PCFF)38ꢀ41 is employed in this work to describe the
packing structures of substituted and unsubstituted oligofluorenols in amorphous
states. The performance of PCFF has been tested in our previous works of
πꢀconjugated fiveꢀmembered heterocyclic oligomers systems in amorphous phase.42ꢀ43
For the homologue of our studied TPFOH system, RꢀTFOH with one OC8H17
sideꢀchain at 9ꢀpositon of the middle of fluorenol unit was designed, PCFF was
validated to be effective by quantum chemical calculations.44 It would be useful to
compare the PCFF optimized geometry with the crystal structure of the studied
terfluorene. Unfortunately, we couldn’t obtain suitable single crystals of terfluorene
for single crystal Xꢀray diffraction after many trails via solvent diffusion method with
different
mixed
solvent
systems
(such
as
dichloromethane/ethanol,
tetrahydrofuran/isopropanol and chloroform/hexane systems et al.). Instead, the
crystal structures of cis/trans DFOH (being substituted with OH group) and BSFX
(with SFX group) were studied by using PCFF. Detailed results are shown in Table S1.
One can find that PCFF can reproduce the experimental crystal structures of the
selected fluorene oligomers.
To model amorphous solids, 20 oligofluorenols have been randomly distributed
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in the box with periodic boundary condition (PBC). The density, ρ, of amorphous is
set to 1.03 g/cm3, which is close to experimental density of glassy α phase of PFO
films (~1.04 g/cm3).5 In order to gain a reasonable prediction of packing structures,
some independent MD simulations were run with various different initial
conformations. The cutoff of van der Waals interactions was set to be 15.5 Å. The
electrostatic interaction was evaluated by the Ewald summation. Motion equations
were integrated by using the velocity Verlet algorithm with the time step of 1 fs. The
10ꢀns simulation was subsequently carried out after the equilibrium state has been
reached at 298 K. We sampled the conformations from 10ꢀnsꢀtrajectories after
annealing at 493 K for the subsequent absorption spectra calculations. All MD
simulations were performed with the discover module in Materials Studio package.45
Geometry and Vertical Excitation Energy. All DFT, vertical excitation energies and
1HꢀNMR signals were performed using the Gaussian 09 program package.46 The
influence of various DFT functionals including BLYP, PBE, B3LYP, CAMꢀB3LYP,
wB97XD, and M06ꢀ2X, on the maximum absorption peak, λmax, have been tested for
fluoreneol monomer MFOH, DFOH with two fluorenol units, RꢀPFOH with three
fluorenol units and one octyloxy sideꢀchain at the middle of the fluorenol unit at
9ꢀpositon,44 as well as other πꢀconjugated syetems47. Accordingly, the theoretical level
of CAMꢀB3LYP and 6ꢀ31+G(d) basis set was selected to calculate the absorption
spectra and 1HꢀNMR signals in this work. The ground state geometries of terfluorenes
were optimized using the B3LYP functional and 6ꢀ31+G(d) basis set for all the C, H,
and O atoms. The selfꢀconsistent field (SCF) convergence criteria for geometry
optimization and vertical excited energy and 1HꢀNMR signals were set to 10ꢀ6 a.u. The
1
DFTꢀbased HꢀNMR chemical shifts (δ) were computed by using gauge invariant
atomic orbital (GIAO) method.
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3. RESULTS AND DISCUSSION
3.1 Synthesis and Optical Properties of Four Terfluorenes
In order to achieve conformation transition of terfluorenes by supramolecular steric
hindrance (SSH) principle, we designed and synthesized the model terfluorene,
2O8ꢀDPFOHꢀSFX that contains SFX as bulky groups with repulsion force, hydroxyls
with hydrogen bonding interaction and alkoxy chains with vdW force. We also
synthesized model molecules named DPFOHꢀSFX without alkoxy side chains at the
phenylfluorenes, 2O8ꢀTPFOH without SFX core and TPFOH for comparison to
carefully clarify the exact roles of each segment (Scheme 1). The synthetic routes of
intermediates and terfluorenes are shown in Figure S1. The terfluorenes display the
high decomposition temperatures (Td) of 340, 379, 272, 330 °C for 2O8ꢀDPFOHꢀSFX,
DPFOHꢀSFX, 2O8ꢀTPFOH and TPFOH (Figure S8), respectively, indicating that the
four terfluorenes have high thermal stability.
Scheme 1. The synthetic route and chemical structures of 2O8ꢀDPFOHꢀSFX,
DPFOHꢀSFX, 2O8ꢀTPFOH, and TPFOH .
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Figure 1. Optical properties of the synthesized terfluorenes. The experimental UVꢀvis
absorption and photoluminescence spectra of (a) toluene dilute solution and (b) films
spinꢀcoated from toluene solution after thermal annealing at 220 °C in N2
environments.
The photophysical properties of terfluorenes in solution and film states are shown in
Figures 1 and S5. All the terfluorenes show only one absorption band in both solution
and spinꢀcoated film. The maximum absorption peaks are at 354 nm in dilute solution
attributed to mainꢀchain backbones of oligofluorenes, and only show a redꢀshift with
~3 nm in solid films (Figure 1a and S5a). The emission spectra of four terfluorenes
exhibit three similar wellꢀresolved emission bands at 396, 416, 442 nm, corresponding
to 0ꢀ0, 0ꢀ1, and 0ꢀ2 vibronic transitions, respectively, and show a slight difference in
solution. In terms of films, the PL spectra show the similar emission behavior (Figure
S5b). Above results indicate that the alkoxy and bulky substituents have a little
influence on the electronic structures of terfluorene backbones. Interestingly,
2O8ꢀDPFOHꢀSFX exhibited dramatic differences in absorption and emission spectra
of films after annealing process compared to other molecules (Figure 1b). Apart from
the maximum absorption peak at 364 nm, the appearance of the shoulder peak at 402
nm, which was similar to the characteristic absorption peak of PFO
βꢀconformation at
436 nm,20,21 confirms the formation of
β
ꢀconformation. Moreover, the redꢀshifted PL
spectrum about 16 nm further supports this point. While DPFOHꢀSFX and
2O8ꢀTPFOH only showed a redꢀshift of 5 nm in absorption spectra with the absence
of shoulder peak. It is worth noting that, with the introduction of two octyloxy
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sideꢀchains and absence of SFX group in 2O8ꢀTPFOH, there was a broad peak at
500ꢀ600 nm (green band emission) in the photoluminescence spectra.
1
Figure 2. H NMR spectra of two isomers of 2O8ꢀDPFOHꢀSFX with different
molarity in CDCl3, (a) two side chains at the different sides, namely raceme including
(R,R) or (S,S)ꢀconfigurations; (b) two side chains at the same side, namely mesomer
including (R,S) –configuration. The DFT optimized (calc.) geometries of two isomers
are also presented.
During the separation and purification process, surprisingly, two isomers of
1
2O8ꢀDPFOHꢀSFX were observed in TLC and confirmed via H NMR. As seen in
Figure S6, owing to the polarity difference, spots of the two isomers can be observed
from TLC under 254 nm UV lamp. We purified the isomers separately, and
determined their molecular weight of 1099.5 by the MALDIꢀTOFꢀMS spectra (Figure
S7). Theoretical calculations indicate that the ground state energy of raceme with the
side chain groups lying at the different sides is close to that mesomer with the energy
difference of less than 2 kcal/mol, supporting the concurrence of two isomers in
experiment. As shown in Figure 2, the two isomers show the similar 1H NMR spectra,
and the most notable peak is at 1’ and 8’ position of the spiro[fluoreneꢀ9,9'ꢀxanthene].
As for the hydroxyl groups at the same side, the chemical shifts of xanthene are
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nonequivalent, leading to the split signals shown in Figure 2. The calculated NMR
spectra of two different isomers are in good agreement with experiments (Figure S4a).
The optimized structures and relative energies of the different isomers were presented
in Table S2.
Figure 3. (a) The experimental UVꢀvis absorption and photoluminescence spectra of
isomers in toluene dilute solution. (b) Absorption and emission spectra of raceme of
o
2O8ꢀDPFOHꢀSFX films before and after thermal treatment under 220 C in N2. (c)
Absorption and emission spectra of mesomer of 2O8ꢀDPFOHꢀSFX films before and
after thermal treatment under 220 oC in N2. (d) PL and excitation spectra of raceme of
2O8ꢀDPFOHꢀSFX in dilute toluene and CHCl3 solutions (10ꢀ3 mg/mL) at low
temperature ambient (liquid nitrogen), respectively.
The photophysical properties of isomers in solution and film states are shown in
Figure 3. Both the isomers have only one absorption band at 355 nm and three
wellꢀresolved emission bands at 397, 419 and 444 nm in dilute solution (Figure 3a).
Interestingly, the two isomers of 2O8ꢀDPFOHꢀSFX exhibit dramatic differences in
absorption and emission spectra of spinꢀcoated films after thermal treatment. For the
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pristine raceme of 2O8ꢀDPFOHꢀSFX film, the maximum peak is observed at 358 nm
for UVꢀvis spectra, and the corresponding PL spectra consisted of four emission bands
at 405, 425, 452 and 484 nm in Figure 3b. And for the mesomer, the maximum
absorption peak appears at 360 nm, and PL spectra consisted of two emission bands at
418 and 484 nm in Figure 3c. However, dramatic differences appeared after thermal
annealing. Besides the 360 nm main peak, a shoulder peak at 402 nm in Figure 3b was
found in the UVꢀvis spectra of racemeꢀterfluorene, indicating the existence of planar
conformation. Moreover, the obvious redꢀshifts occur in the PL spectrum of
racemeꢀterfluorene. In addition, the fluorescence quantum efficiency of planar
conformation (54%) is lower than amorphous state (66%). But for the
mesomerꢀterfluorene, the maximum absorption peak in Figure 3c is bathochromic 13
nm from 360 nm to 373 nm without the appearance of shoulder peak after annealing
process, reflecting that this isomer only has the trend of crystalline but not of planarity.
To the best of our knowledge, the isomerꢀdependent conformation transition in
shortꢀbackbone chain terfluorene has never been reported in previous literatures.
In order to probe whether βꢀconformation of racemeꢀterfluorene could be generated
in dilute solution, lowꢀtemperature optical analysis was employed here (Figure 3d).
The appearance of the shoulder peaks at 409 nm at liquid nitrogen ambient confirms
the formation of planar conformation in dilute toluene and CHCl3 solid solution
(solvents show solid state at such low temperature). PL spectra of raceme of
2O8ꢀDPFOHꢀSFX in dilute solid solution exhibited three wellꢀresolved emission
bands at 388, 409, and 436 nm, respectively, and no redꢀshifts happened (Figure 3d).
In our previous work, the content of the
completely energy transfer from amorphous state to
ꢀconformation emission needed more content of planar chains compared with PFO
β
ꢀconformation was not sufficient to realize
β
ꢀconformation. Here, this
β
(~1% for PFO), which can be ascribed to a lower efficiency of energy transfer. The
lowꢀtemperature induced conformation transition might be the result of molecular
aggregation and lower molecular rotation. At 77 K, interꢀchain interactions are
maximized and sufficient to overcome the steric repulsion, and finally facilitated the
conformation planarization.1 The gaseous structure of raceme is slightly energetically
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lower than the mesomerꢀstructure, as predicted at both B3LYP/6ꢀ31+G(d) and PCFF
levels (Table S2 of SI). The optimized packing structures of 20 chains in a cubic box
with the periodic boundary condition show that the packed raceme solid has larger
amount of planar segments than that in mesomer one, correlating with the appearance
of shoulder peak at ~402 nm in UVꢀvis absorption spectra.
The simulated packing structures of isomers, raceme and mesomer, of
2O8ꢀDPFOHꢀSFX in amorphous state are schematically shown in Figure 4. For
raceme the two side chains were on both sides of fluorene backbone, leading to the
evenly aligned ꢀꢀshaped stacking with the interꢀchain distance of about 12ꢀ14 Å in
each side of a triangle (called ‘ꢀ’ here). However, for mesomer, whose side chains
were on the same side of fluorene backbone, the two neighboring backbones are
bundled together in an Xꢀshape with a smaller interꢀchain distance of about 9 Å. Such
an interlace arrangement was also called steric pairing effect in previous works done
by Yao and coworkers [48,49]. The steric pairing effects in ꢀꢀshaped stacking of raceme
led to the increase of planar conformations. In contrast, the Xꢀshaped stacking
mesomers formed some unevenly distributed clusters in aggregate with their side
chains stretching outside, just like pine needles.
Figure 4. Schematic structure and representative packing pattern of the amorphous
state of the (a) raceme and (b) mesomer of 2O8ꢀDPFOHꢀSFX system. The other
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chains are not shown for clarity.
3.2 Theoretical Study on Packing Structures and Optical Properties of Four
Terfluorenes
In order to give a deeper understanding on the role of SFX and sideꢀchain groups
played in photophysics properties and the origin of the appearance of βꢀphase in the
2O8ꢀDPFOHꢀSFX, we further made the joint efforts of molecular dynamics (MD)
simulations on packing structures and density functional theory calculations on
electronic structures. The molecular level insight would be gained in the aggregation
effects on the optical properties in the condensed phase.
3.2.1 Molecular Aggregation and the Effect of HB Interaction on the 1H-NMR.
The existence of intermolecular hydrogen bonding (HB) interaction between the
adjacent TPFOH molecules significantly affects the aggregation modes. In fact, the
selfꢀassemble behavior of Hꢀbond donor and acceptor sites has been utilized to
regulate the fold and duplex via intraꢀ or intermolecular Hꢀbonds of oligomers.50 It
was also found in zinc porphyrin−pyridine ligand complexes that the formation of
intramolecular Hꢀbonding interactions enabled the evaluation of the e ect of
conformational flexibility on cooperativity in supramolecular complexes.51 As shown
in Scheme 2, there are two kinds of stacking modes, tailꢀtoꢀtail (TT) and tailꢀtoꢀmid
(TM), named as the relative position of anchor sites (i.e., hydroxyl groups in the
terminal and middle units) for HB interactions. For a trimeric system, the number of
hydroxyl groups in terminal units is twice that in the center group. It can be thus
anticipated that the TT stacking, with the HB formed between two terminal groups, is
more popular than the TM packing style. In the TT stacking, the conjugated segments
in two different chains are usually parallel displaced (PD) stacked. Different from TT
stacking, TM stacking mainly adopted two modes, parallel displaced (PD) and
Tꢀshaped stacking, depending on the relative orientation of two neighboring TPFOH
chains.
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Scheme 2. Effects of hydrogen bonding on packing modes of oligofluorenols.
Figure 5 depicts the difference in packing structures caused by the addition of side
chains or steric SFX group. Without chemical modification, unsubstituted TPFOH
mainly adopted TT packing with the percentage of about 85%. The typical snapshot
selected from MD simulation shows the allꢀtwist structure in TPFOH backbone. The
introduction of OC8H17 sideꢀchains in the terminal unit seems to cut down the
superiority of TT stacking and leads to a balanced distribution of two packing modes
in 2O8ꢀTPFOH with 51% TT and 49% TM stacking. The planar segments appeared in
the selected snapshots, probably contributing to the broad peak at 500ꢀ600 nm (green
band emission) in the photoluminescence spectra of 2O8ꢀTPFOH (Figure 1). The
sideꢀchain effect can be also reflected by the intermolecular distance, which was
measured by the distance between the centroids of two fluorenol units of adjacent
TPFOH molecules. The addition of the OC8H17 long sideꢀchain in 2O8ꢀTPFOH
increases the intermolecular distance to 6 Å, which is longer than that (5 Å) in
TPFOH and DPFOHꢀSFX. The stacking distances in TPFOH, DPFOHꢀSFX and
2O8ꢀTPFOH oligomers were longer than that (3.7 Å) reported for
poly(9,9’ꢀdioctylfluorene), but shorter than that (7.3 Å) that reported in 2,7ꢀPFs in
amorphous states52 and that (11.8 Å) of cis DFOH crystal in our previous work.44 For
both DPFOHꢀSFX and 2O8ꢀDPFOHꢀSFX, the replacement of SFX group in the
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middle unit does not alter much the predominance of TT aggregation modes in
comparison with TPFOH. However, the planarity is enhanced and the percentage of
twistꢀplanar or allꢀplanar conformations is increased in 2O8ꢀDPFOHꢀSFX.
Figure 5. Representative snapshots and packing pattern of the MDꢀsimulated
amorphous of substituted and unsubstituted TPFOH type systems. In amorphous
phase, 20 oligofluorenols have been initially randomly distributed in the PBC box.
The density of oligofluorenols is set to 1.03 g/cm3, which is close to experimental
density of glassy α phase of PFO films (~1.04 g/cm3).5 the sizes of simulation cells
were 2.9×2.9×2.9 nm3 for (a) TPFOH, 3.2×3.2×3.2 nm3 for (b) 2O8ꢀTPFOH,
3.0×3.0×3.0 nm3 for (c) DPFOHꢀSFX, 3.3×3.3×6.6 nm3 for (d) 2O8ꢀDPFOHꢀSFX.
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1
The formation of intermolecular HB can be reflected in the change of HꢀNMR from
the isolated TPFOH molecule to the molecular aggregates. Shown in Figure S10 are
the calculated NMR spectra of isolated molecule and a pair of stacked molecules,
along with the experimental spectra for comparison. It clearly indicated that the
chemical shift of H proton of hydroxyl groups in TPFOH aggregates downshifted
obviously, relative to that predicted for an isolated molecule at the same theoretical
level. The calculated 1HꢀNMR spectra of oligofluorenol aggregates are consistent with
the experimental results.
3.2.2 β-Phase: UV-vis Absorption and Emission Spectra
Chemical modification of TPFOH not only affects their molecular packing but also
allows the appearance of various conformations, which may be energetically
unfavorable in vacuum. For an isolated molecule, there is little difference in gaseous
geometry of backbone and rotation barrier of interꢀunit bond caused by the
introduction of sideꢀchain or/and SFX group in TPFOH (Figure S11). The torsion
angles between adjacent fluorenol units of the studied TPFOH family were around
35°ꢀ39°. Such highly twisted backbones were planarized to some extent in amorphous
state or film due to aggregation. Compared with the isolated molecule in vacuum, the
torsion angles of both substituted and unsubstituted TPFOH in amorphous distributed
in a very broad range including some small torsion angles of less than 15°, indicating
the presence of planar conformations. The percentage of planar conformations, β%, is
analyzed with the criterion of D1 < 15° and D2 < 15° (D1 and D2 was the torsion
angle between two adjacent fluorenol units as shown in Figure S11) in trimeric
fluorenols. In the molecular aggregates, the content of βꢀphase of 2O8ꢀDPFOHꢀSFX
was 0.42%, higher than that (0.076%) in DPFOHꢀSFX and (0.13%) in 2O8ꢀTPFOH.
The maximum absorption peaks of four typical planar conformations (which were
sampled from MD trajectories) were redꢀshifted to about 415ꢀ469 nm relative to that
(324ꢀ328 nm) predicted in vacuum, as shown in Figure S12.
As discussed above, the simultaneous adding of sideꢀchain and SFX group in
2O8ꢀDPFOHꢀSFX brought out the most evident changes in the relative percentage of
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various possible conformations. The 2O8ꢀDPFOHꢀSFX was then selected to study the
relationship between the three typical conformations (allꢀtwist, twistꢀplanar, allꢀplanar)
and UVꢀvis absorptions spectra in Figure S13. The planarity of allꢀplanar
conformation of 2O8ꢀDPFOHꢀSFX renders the maximum absorption peak, λmax, in
UVꢀvis absorption spectra redꢀshifted to 412 nm, in comparison with that λmax=370
nm for allꢀtwist conformation and λmax=405 nm for the twistꢀplanar one. The major
orbital contributions for λmax of these three kinds of conformations were involved with
the transition from the highest occupied molecular orbital (HOMO, called H for short)
to the lowest unoccupied molecular orbital (LUMO, called L). For allꢀtwist
conformations the HOMO and LUMO are mainly located in two fluorenol repeat
units. But with the enhancement in planarity, the orbital are delocalized in the whole
fluorenol backbone for twistꢀplanar and allꢀplanar conformations.
We further studied the calculated and experimental absorption spectra of substituted
and unsubstituted TPFOH after annealing at 493 K. As shown in Figure S14, the
calculated absorption spectra are in consistent with the experimental absorption
spectra. The maximum absorption peak of TPFOH with two OC8H17 sideꢀchains
(2O8ꢀTPFOH) or with one SFX group (DPFOHꢀSFX) occurs to slight redꢀshift
relative to that of unsubstituted oligomer TPFOH(TFOH: 360 nm; 2O8ꢀTPFOH: 367
nm; DPFOHꢀSFX: 363 nm). The cooperative effect of two OC8H17 sideꢀchains and
SFX group caused the absorption spectra of 2O8ꢀDPFOHꢀSFX exhibiting a shoulder
peak at ~405 nm, which was the characteristic peak of β
ꢀphase in PFs.20,21
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Scheme 3. Chemical modifications on oligofluorenols and their possible
conformations.
As shown in Scheme 3, the transitions from the most favorable allꢀtwist to the
metastable twistꢀplanar and allꢀplanar conformations need to overcome rotation
barriers of about 2.3 and 4.5 kcal/mol, respectively. The values were close to the
energy (3.7±0.6 kcal/mol) required to planarize the fluorene backbone reported by
Monkman experimentally.1 In the center of trimeric fluorenol,
a
spiro
[fluoreneꢀ9,9′ꢀxanthene] (SFX) group is introduced as a pin to fix the middle point of
terfluorene. It is shown that with the steric pairing effect of two octyloxy sideꢀchains
on both sides and the cooperative interplay with SFX group in raceme of
2O8ꢀDPFOHꢀSFX, the backbone torsion was largely restricted, resulting in the
increase of planar segments in such a short chain.
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4. CONCLUSIONS
In conclusion, we demonstrated the rational molecular design using the cooperative
effect of hydrogen bonding, steric hindrance and van der Waals forces to achieve the
β
ꢀphase in shortꢀbackbone chain terfluorene 2O8ꢀDPFOHꢀSFX by thermal annealing.
For the first time, we observed the isomer dependence of the βꢀphase conformation
and only the racemeꢀisomer of 2O8ꢀDPFOHꢀSFX could induce βꢀconformation.
Theoretical calculations were further carried out to study the origin of
isomerꢀdependence and the effect of octyloxy side chains, steric hindrance group, HB
interaction on their stacking structure and intramolecular torsion. With the cooperation
effect of steric hindrance group and supramolecular weak interaction, the content of
planar fragments was greatly increased in 2O8ꢀDPFOHꢀSFX. Molecular dynamics
simulations revealed that the largerꢀcontent planar conformations in racemeꢀstructures
of 2O8ꢀDPFOHꢀSFX make it induce
βꢀconformation. With the cooperative steric
interaction of the middle spirofluorene SFX, the octyloxy sideꢀchains on different
sides of 2O8ꢀDPFOHꢀSFX backbone artfully manipulated the intramolecular structure
and staggered well into ꢀꢀlike array to facilitate the appearance of planar
conformations in raceme. Due to the steric pairing effect, mesomers, whose two side
chains are on the same side of backbones, take Xꢀshaped stacking into some unevenly
distributed clusters, and the long sideꢀchains in the two ends of 2O8ꢀTPFOH resulted
in two competitive packing models. Tailꢀtoꢀtail (TT) and tailꢀtoꢀmid (TM) through
weakening the intermolecular hydrogen bonding between the terminal groups, are
predominant in other oligomers. The obvious lowꢀfrequency shift of chemical shift of
1
1
proton H in HꢀNMR spectrum of molecular aggregates relative to the isolated
molecule indicated the presence of hydrogen bonding interaction in TPFOH and its
derivatives. The modulation of packing structures through the tunable steric pairing
and hydrogen bonding networks gives rise to a new strategy in rational design of
novel materials with the desired optical and electronic properties.
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ASSOCIATED CONTENT
Supporting Information Available: This material is available free of charge via the
Internet. Experimental Section: preparation of thin films for optical analysis;
synthesis of DPFOHꢀSFX, 2O8ꢀTPFOH and 2O8ꢀDPFOHꢀSFX, and their 1H,
13CꢀNMR and MALDIꢀTOFꢀMS; UVꢀvis absorption and photoluminescence spectrum;
TLC photographs of 2O8ꢀDPFOHꢀSFX; MALTIꢀTOFꢀMS spectra of two isomers of
2O8ꢀDPFOHꢀSFX. Computational Section: the heating and cooling process in
simulation annealing; the optimized ground state geometries, packing structures and
relative energies of the different isomers of 2O8ꢀDPFOHꢀSFX; the experimental and
1
calculated HꢀNMR and UV/Vis absorption spectra in vacuum and amorphous state;
typical planar conformations and their populations according to 10 ns MDꢀsimulation
for four terfluorenes; The major molecular orbital contributions for λmax in UVꢀvis
absorption spectra of different conformations in 2O8ꢀDPFOHꢀSFX; the coordination
of optimized structures of 2O8ꢀDPFOHꢀSFX isomers.
AUTHOR INFORMATION
Corresponding Authors
*(J. Ma) Eꢀmail: majing@nju.edu.cn Tel: (86)25ꢀ89687408.
*(L. Xie) Email: iamlhxie@njupt.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEGEMENTS
This work was supported by the National Natural Science Foundation of China (Grant
Nos. 21290192, 21673111, U1301243, 21274064 and 21703118), the Priority
Academic Program Development of Jiangsu Higher Education Institutions, PAPD
(YX03002), the Six Peak Talents Foundation of Jiangsu Province (XCLꢀCXTDꢀ009),
Open Project from State Key Laboratory of Supramolecular Structure and Materials at
Jilin University (sklssm201710), Shandong Provincial Natural Science Foundation,
China (No.ZR2017MB038). Project was funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions. We are grateful to the High
Performance Computing Center of Nanjing University for doing the quantum
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chemical calculations in this paper on its IBM Blade cluster system.
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