The relationship between molecular structure and electronic properties in dicyanovinyl substituted acceptor-donor-acceptor chromophores

Article abstract of DOI:10.1016/j.tet.2017.04.037

In this contribution we describe a combined experimental and theoretical study of the relation between the molecular structure and the electronic properties of conjugated donor-acceptor type chromophores for light-harvesting applications. A series of model systems was synthesized where a central anthracene (electron donor) is connected to dicyanovinyl units (electron acceptor) through a π-conjugated spacer. The study of the redox and optical properties of these chromophores and of reference compounds without dicyanovinyl units allows us correlate the electronic properties to the presence of the electron withdrawing groups and the molecular conformation. Comparison with calculated electronic structure shows that the construction of chromophores that consist of electron donating and accepting units does not always follow the simple rules that are generally used in the design of such molecules. The results show a subtle relation between the charge transfer character and the geometry of the molecules. In some cases this leads to significant contribution of charge transfer excitation to the absorption spectra of some chromophores while such contributions are completely absent in others.

Full text of DOI:10.1016/j.tet.2017.04.037

Accepted Manuscript
The relationship between molecular structure and electronic properties in dicyanovinyl
substituted acceptor-donor-acceptor chromophores
Simge Tarkuç, Rienk Eelkema, Ferdinand C. Grozema
PII:
S0040-4020(17)30420-9
10.1016/j.tet.2017.04.037
TET 28638
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 2 March 2017
Revised Date: 14 April 2017
Accepted Date: 15 April 2017
Please cite this article as: Tarkuç S, Eelkema R, Grozema FC, The relationship between molecular
structure and electronic properties in dicyanovinyl substituted acceptor-donor-acceptor chromophores,
Tetrahedron (2017), doi: 10.1016/j.tet.2017.04.037.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

Graphical Abstract
ACCEPTED MANUSCRIPT
A systematic study of the relation between the molecular structure and the electronic properties of
new dicyanovinyl (DCV) substituted acceptor-donor-acceptor chromophores is described. A
combination of theoretical and experimental methods shows a subtle relation between the charge
transfer character and the geometry of the molecules.

ACCEPTED MANUSCRIPT
The relationship between molecular structure and electronic
properties in dicyanovinyl substituted acceptor-donor-acceptor
chromophores
Simge Tarkuça,b,*, Rienk Eelkemab,* and Ferdinand C. Grozemab,*
a Arçelik A.Ş R&D Center, Material Technologies, Tuzla/İstanbul 34950 Turkey
bDepartment of Chemical Engineering, Delft University of Technology, Van der Maasweg 9, 2629 HZ
Delft, The Netherlands
e-mail: starkuc09@gmail.com, r.eelkema@tudelft.nl, f.c.grozema@tudelft.nl
*Corresponding author
Abstract
In this contribution we describe a combined experimental and theoretical study of
the relation between the molecular structure and the electronic properties of
conjugated donor-acceptor type chromophores for light-harvesting applications. A
series of model systems was synthesized where a central anthracene (electron
donor) is connected to dicyanovinyl units (electron acceptor) through a π-
conjugated spacer. The study of the redox and optical properties of these
chromophores and of reference compounds without dicyanovinyl units allows us
correlate the electronic properties to the presence of the electron withdrawing
groups and the molecular conformation. Comparison with calculated electronic
structure shows that the construction of chromophores that consist of electron
donating and accepting units does not always follow the simple rules that are
generally used in the design of such molecules. The results show a subtle relation
between the charge transfer character and the geometry of the molecules. In some
cases this leads to significant contribution of charge transfer excitation to the
absorption spectra of some chromophores while such contributions are completely
absent in others.
Keywords: small organic dyes, 2,2’-dicyanovinylene, acceptor-donor-acceptor
type chromophore, electronic structure, geometry deformation, charge transfer
1

ACCEPTED MANUSCRIPT
1. Introduction
Conjugated small molecules and polymers are interesting candidates for
application organic opto-electronic devices.1-5 One of the attractive points of using
such π-conjugated molecules and polymers is that their optical and electronic
properties can be optimized for specific applications by using rational design
approaches.6-11 Therefore building a thorough fundamental understanding of the
relation between the molecular structure and the electronic properties in such
materials is important to provide further guidance for the design of new materials
with optimal properties.12-14 In this respect, small conjugated model compounds
with well-defined geometries prove to be very useful since they allow for a
detailed characterization that combines electrochemistry and spectroscopic
techniques with high-level theoretical methods. Moreover, small organic
molecules are of considerable interest in their own right, for instance for
application in photovoltaic devices due to their high absorption coefficient and the
possibility to tailor the HOMO-LUMO energy gap.15-21 From a synthetic point of
view, small organic molecules are attractive since they are monodisperse in nature
with well-defined chemical structures and a good batch-to-batch reproducibility.
Compared to their polymer counterparts, high-level theoretical studies on small
organic molecules can be performed in a relative straightforward manner.
A variety of molecular design approaches have been used to tune the optical
properties, charge carrier mobility, HOMO and LUMO energy levels and the
structural ordering of small organic molecules in the solid state. One of the most
successful approaches to influence the HOMO and LUMO energies involves
incorporation of alternating electron-rich (Donor, D) and electron-deficient
(acceptor, A) aromatic units into the molecule backbone.22-29 In such combined
polymers and oligomers it is assumed that the HOMO of the donor and the LUMO
of the acceptor are largely responsible for the location of the frontier orbitals in D-
A system.30-31 The HOMO and LUMO can then be tuned individually, resulting in
a straightforward approach to engineer the bandgap of the copolymers/oligomers.
While this approach has been predominantly used in polymers, the combination of
donors and acceptors in small organic molecules, for instance based on a A-π-D-π-
2

ACCEPTED MANUSCRIPT
A configuration, are receiving increasing attention.32-36 This is especially true for
the inclusion of dicyanovinyl (DCV) as an electron-withdrawing unit.37-39
Recently, a series of oligothiophenes end-capped with such DCV acceptor groups
has been developed in the Bäuerle research group.40-43 It was reported that the
introduction of the DCV group leads to a considerably reduced the HOMO-LUMO
energy gap and an intense absorption in the visible region due to the strong
electron withdrawing nature. The DCV units also contribute to the conjugation of
the backbone π-system and can lead to specific intermolecular interactions that
change the packing of molecules in the solid state.44 In most small organic
molecules, acceptor and donor units are linked through a π-spacer to increase the
conjugation length. In addition to the length of the π-conjugated spacer, the
intrinsic electronic properties of the π-spacer may also affect the spectral response
of the molecule.
Scheme 1. Molecular structures of control compounds and DCV-end capped
chromophores.
In this contribution, we explore the relation between molecular structure and the
electronic properties of small conjugated molecules by considering two series of
new model compounds that follow the acceptor-(π-bridge)-donor-(π-bridge)-
acceptor architecture (A-π-D-π-A) (Scheme 1). The effects of the acceptor unit on
3

ACCEPTED MANUSCRIPT
the optical and electronic properties of the chromophores are investigated and
compared to control compounds that lack DCV-units. Within the first series, the
aromatic π-spacer is varied from benzene through thiophene to dithiophene,
leading to improved optoelectronic properties. In the second series, the changes in
the optical and electrochemical behavior originating from the extension of the
effective conjugation length by insertion of an ethynyl in the π-bridge are reported.
These series of compounds allow us to not only study the effect of combining units
with different electronic properties into a single molecule but also to investigate
geometrical effects, for instance the planarity of the molecules. The electronic
properties have been studied by a combination of experimental methods and
electronic structure calculations.
2. Experimental Section
2.1. Material Synthesis.
All starting materials were purchased from Sigma-Aldrich, Acros or Alfa Aesar
and used as received without further purification. Analytical thin layer
chromatography (TLC) was performed on silica gel 60-F254 (Merck) plates and
detected under a UV lamp. Column chromatography was performed on silica gel
60 (Aldrich). 9,10-dibromoanthracene45, 5-(5-bromothiophen-2-yl)thiophene-2-
carbaldehyde46,
9,10-bis((tri-methylsilyl)ethynyl)-anthracene47,
9,10-
diethynylanthracene (5)48, 2-(10-(thiophen-2-yl)anthracen-9-yl)thiophene (TAT)49,
2-(thiophen-2-yl)-5-(10-(5-(thiophen-2-yl)thiophen-2-yl)anthracen-9-
yl)thiophene(BAB)49, 9,10-bis(2-phenylethynyl)anthracene (EBAB)50, 2-(2-(10-(2-
(thiophen-2-yl)ethynyl)anthracen-9-yl)
ethynyl)thiophene
(ETAT)51
were
synthesized according to literature procedures.
2.2. Characterization.
1H NMR and 13C NMR spectra were performed in the appropriate deuterated
solvents with tetramethylsilane as internal standard on a Bruker Advance
spectrometer at 400 MHz (1H) and 100 MHz (13C); chemical shifts (δ) are reported
4

ACCEPTED MANUSCRIPT
in parts per million. FTIR measurements were carried out on a Perkin Elmer
Spectrum One FTIR spectrometer with ATR. EI-MS measurements were
performed on a Shimadzu GCMS-QP2010S spectrometer. UV-vis absorption
spectra were measured in dichloromethane (CH2Cl2) and recorded on a Perkin
Elmer Lamda 40 UV-Vis spectrometer. Fluorescence studies were carried out on a
QuantaMaster™ 40 UV VIS fluorescence spectrometer.
All electrochemistry experiments were done in a conventional three-electrode cell
using a platinum flag working electrode, a platinum wire counter electrode and a
silver wire pseudoreference electrode. Cyclic voltammograms were obtained in dry
degassed CH2Cl2 using 0.1 M tetra-n-butylammonium hexafluorophosphate (n-
Bu4NPF6) as the electrolyte, scanning at a rate of 50 mV/s. The electrochemical
potential was subsequently calibrated versus the ferrocenium/ferrocene (Fc+/Fc)
using the formal potential of this redox couple as 0.46 V vs SCE in 0.1 M n-
Bu4NPF6 in CH2Cl252-48. The corresponding energy levels are reported relative to
the vacuum level using the following equations HOMO= -(Eonset,ox vs Fc+/Fc+ 5.1)
(eV) and LUMO= -(Eonset,red vs Fc+/Fc + 5.1) (eV).
3. Results and Discussion
3.1. Synthesis.
The route to synthesize the acceptor-(π-bridge)-donor-(π-bridge)-acceptor system
is depicted in Scheme 2 and 3. Boronic ester 1, underwent smooth Suzuki cross-
coupling with brominated aromatic aldehydes in high conversions under the
standard reaction conditions: boronic ester (3 equiv.), sodium hydrogencarbonate
(12 equiv.), Pd(PPh3)4 (5 mol %), THF/H2O, heating to reflux for 24 h. Subsequent
Knoevenagel condensation of the dialdehyde derivatives with malononitrile in the
presence of triethylamine resulted in the DCV end-capped target compounds.
5

ACCEPTED MANUSCRIPT
Scheme 2. Synthetic routes for directly-linked chromophores (DLCs)a
aConditions. (a-c) Pd(PPh3)4, THF, K2CO3 (aq) (a) 4-bromobenzaldehyde, (b) 5-
bromothiophene-2-carbaldehyde, (c) 5-(5-bromothiophen-2-yl)thiophene-2-carbaldehyde,
(d) malononitrile, TEA, CHCl3.
The synthesis of 9,10-bis(ethynylarylaldehyde)anthracene started from anthracene,
which was brominated with bromine in acetic acid to give 9,10-dibrominated
anthracene. The latter compound was coupled with TMS-acetylene in a
Sonogashira coupling reaction to obtain TMS-protected intermediates. The cross-
coupling reaction has been performed in TEA with 5 mol % Pd(PPh3)2Cl2 and 10
mol % cuprous iodide at 60 °C for 3 h. The intermediate product was desilylated
with 5 equiv. of KOH in a THF/methanol mixture at 25oC. To obtain the
dialdehyde precursors for the DCV-substituted systems, brominated aromatic
aldehydes were cross-coupled with deprotectedethynylated intermediates (5) in a
Sonogashira reaction with 5 mol % Pd(PPh3)2Cl2 and 10 mol % cuprous iodide at
25 °C for 16 h. In a final step, the dialdehyde derivatives were transformed into the
corresponding dicyanovinylene compounds by Knoevenagel condensation with
malononitrile in the presence of triethylamine in chloroform. All DCV-substituted
systems could be obtained in high purity by flash chromatography on silica gel.
6

ACCEPTED MANUSCRIPT
Scheme 3. Synthetic routes for ethynyl-linked chromophores (ELCs)a
aConditions. (a-c) Pd(PPh3)4, THF, K2CO3 (aq) (a) 4-bromobenzaldehyde, (b) 5-
bromothiophene-2-carbaldehyde, (c) 5-(5-bromothiophen-2-yl)thiophene-2-carbaldehyde,
(d) malononitrile, TEA, CHCl3
3.2. Electrochemistry.
The redox behavior and energies of the frontier molecular orbitals were studied by
cyclic voltammetry. The information that is directly obtained from cyclic
voltammetry is the oxidation and reduction potential. For convenience we have
converted these potentials into approximate HOMO and LUMO energies using the
know oxidation potential and HOMO energy for ferrocene (HOMO = -5.1 eV).
This gives a correlation between the energy required to add (remove) one electron
to (from) a molecule, the standard reduction (oxidation) potential and the energy of
the LUMO (HOMO) have been used to estimate the energy levels of the frontier
orbitals.48 The measurements were performed in CH2Cl2 containing 0.1 M n-
Bu4NPF6 as a supporting electrolyte. As noted above, the molecular orbital
energies are calculated from electrochemical results calibrated with ferrocene with
7

ACCEPTED MANUSCRIPT
a value of 5.1eV (Table 1) and summarized for the two series of compounds in
Figures 2 and 3, respectively.
Figure 1. Cyclic voltammograms of (a) DPA-DPAM and (b) DPAM, TATM, BABM in
0.1 M Bu4NPF6/ CH2Cl2 at a scan rate of 50 mV/s.
In Figure 1a the cyclic voltammograms of DPA and DPAM are shown. The
dicyanovinyl end-capped chromophore (DPAM) has a higher oxidation potential
than that of the unsubstituted analogue (DPA), which translates to a lower HOMO
level for the dicyanovinyl substituted compound. The same trend was observed for
all the model compounds studied in this work. A lower-lying HOMO of the DCV
substituted compounds is consistent with presence of the electron withdrawing
dicyanovinyl units as compared to the control compounds. All of the control
compounds without DCV show well-defined oxidation features; however, no
cathodic reduction processes were recorded, which is different from the
dicyanovinyl-end capped chromophore (see Supporting Information, Figure S1).
The LUMO energies of the control compounds were obtained by subtracting the
optical bandgaps from the electrochemical oxidation data. The LUMO energy
levels show similar values in the range of -3.30 ± 0.10 eV. In Figure 2 the HOMO and
LUMO energy levels are summarized for the directly-linked series of compounds. The
scheme shows that the variation in the HOMO level is very small throughout this series
(~0.4 eV) while for the LUMO level variations exceeding 1eV are observed. This is
consistent with the electron-withdrawing nature of the DCV groups, which mostly affect
the LUMO. In addition, the nature of the π-conjugated spacer between DCV and the
central anthracene unit also affects the LUMO level, while leaving the HOMO energy
8

ACCEPTED MANUSCRIPT
largely unchanged. The trends in the HOMO and LUMO levels can be understood when
considering the results of electronic structure calculations. DFT calculations were
performed using the M062X functional combined with a cc-pVDZ basis set in the
Gaussian’09 program package.54 The effect of the introduction of DCV units in the
directly linked chromophores is clearly illustrated when comparing the shape and
localization of the HOMO and LUMO for DPA and DPAM in Figure 4.
Figure 2. Energy levels of the control compounds and chromophores in directly-linked
chromophores obtained by CV.
9

ACCEPTED MANUSCRIPT
Figure 3: Energy levels of the control compounds and chromophores in ethynyl-linked
chromophores obtained by CV.
For DPA, both the HOMO and LUMO are mostly localized on the anthracene unit and
the electronic properties are almost not affected by the rest of the molecule. While the
HOMO in DPA is largely unchanged when going to DPAM, the LUMO is now almost
completely localized on the DCV units and the π-spacer. More detailed inspection shows
that while the original anthracene LUMO is still present in DPAM (and has the same
energy as in DPA), the DCV units contribute to a higher lying orbital on the outer parts of
the molecule and pull the energy of this orbital below the anthracene LUMO. As is
clearly seen in Figure 4, the LUMO spreads over both the DCVs and the π-spacer, but has
very little density on the anthracene, again due to the large dihedral angle. An interesting
trend can be observed when comparing the HOMO energy of DPAM and TATM.
Thiophene is considered a better donor than benzene, however, the HOMO level of
TATM is lower (more negative) than that of DPAM. This is caused by subtle differences
in the dihedral angles between the central anthracene and the phenyl or thiophene. As can
be see in Table, for the thiophene containing side chains the dihedral angle is very close
to 90 degrees and there is almost no mixing between the HOMO on anthracene and those
on the thiophenes. For the DPAM, the angle is smaller and hence the large (but still
small) mixing between the states on anthracene and the phenyl-containing side chain lead
to slightly less negative HOMO levels.
10

ACCEPTED MANUSCRIPT
Figure 4. Optimized geometries, Frontier molecular orbitals HOMO and LUMO of
DPA, DPAM, EDPA and EDPAM. Calculations were carried out at DFT (M062X/cc-
pVDZ) level; dihedral angles between the planes of donor unit and linker unit are
indicated by red circle.
The breaking of the π-conjugation by the strong deviations from planarity for the
directly linked molecules can be overcome by the introduction of an ethynyl unit in
the π-spacer. This reduces the steric interaction that leads to this conjugation
breaking (Figure 4). Consequently, the ethynyl linked series of compounds have an
almost fully planar conformation. This is reflected in the oxidation and reduction
potentials that are summarized in Figure 3. Now, both the HOMO and LUMO
energies change when structural changes are made in the outer parts of the
molecules, even for the control compounds without DCV units. This indicates that
the DCV units should contribute to both the HOMO and the LUMO. This is
confirmed by the DFT calculations as seen in Figure 4. For EDPA both the HOMO
and LUMO are fully delocalized over the whole molecule. The introduction of
DCV in EDPAM changes this to some extent. The HOMO becomes more
localized towards the anthracene while the LUMO is pulled to the outer parts of
the molecule. This is directly reflected in the oxidation potential, which increases
on introduction of DCV (see Figure 4). The LUMO level is considerably lowered
by the introduction of the DCV units, as expected. In the other ethynyl-linked
11

ACCEPTED MANUSCRIPT
compounds the nature of the π-spacer also affects both the HOMO and LUMO
because it contributes to both of these orbitals that are now delocalized along the
whole backbone of the molecule (See Table S1).
Table 1. Optical and electrochemical properties of chromophores.
Chromophore
Absmax
(nm)a
390
εmax
PLmax
(nm)b
605
Egop
(eV)c
2.54
2.51
2.44
2.17
2.02
1.98
EHOMO ELUMO EgEC
(M-1cm-1)
16 x 103
60 x 103
23 x 103
20 x 104
36 x 104
47 x 104
(eV)
-6.68
-6.77
-6.53
-6.35
-5.94
-5.78
(eV)
-4.42
-4.52
-4.64
-4.18
-4.28
-4.16
(eV)d
2.26
2.25
1.89
2.17
1.66
1.62
DPAM
TATM
381
587
BABM
453
633
EDPAM
ETATM
EBABM
537
515
521
587
620
634
a
First real maximum in spectrum, this does not directly correspond to the lowest
c
electronic state. b From photoluminescence spectra in supporting information. Energy
gap determined from the onset of the optical absorption. d Energy gap from CV data.
12

ACCEPTED MANUSCRIPT
Figure 5. The optical absorption spectra of (a) control compounds and (b) DLCs
chromophores (DPAM, TATM, BABM) in CH2Cl2 solution (c) Theoretical
simulation of the UV-vis absorption spectra for DLCs chromophores (DPAM,
TATM, BABM).
3.3. Optical Properties.
In Figure 5, the absorption spectra of the directly linked chromophore series are
shown, together with those of the control compounds without DCV. The spectral
data obtained from these spectra and the fluorescence emission spectra in the
supporting information are summarized in Table 1.
In the unsubstituted analogues multiple absorption maxima are observed in the
absorption spectra which are associated with the π-π* transition of anthracene core.
The characteristic vibrational fine structure of the isolated anthracene group
confirms the results from the calculations described above that the introduction of
bulky substituents to the 9 and 10 positions on the anthracene results in a
nonplanar conformation with restricted π-electron delocalization. Therefore, the
absorption spectrum is largely determined by the anthracene unit. Introduction of
the electron withdrawing DCV groups leads to considerable red shifts in the
absorption spectra for all chromophores. At the same time, the absorption spectra
lose the characteristic vibrational fine structure that was observed in the
unsubstituted analogues. This shows that the nature of the excited state changes,
from a localized excitation on the anthracene to a delocalized excitation to which
the anthracene, π-spacer and the DCV all contribute. The optical band gaps, taken
as the onset of the absorption, are found to be 2.48 eV for DPAM with the benzene
linker, 2.45 eV for TATM with the thiophene linker, 2.42 eV for BABM with the
dithiophene linker. The emergence of a shoulder in the absorption spectra of
DPAM and less pronounced in TATM could be an indication of an intramolecular
charge transfer between the electron rich anthracene core and the electron poor
dicyanovinyl end groups. In order to obtain more information on the nature of the
different features in the UV/Vis spectra of the chromophores studied in this work
13

ACCEPTED MANUSCRIPT
we have performed time-dependent DFT calculations.56-57 A variety of density
functionals and basis sets (GGA (BP86), Hybrid (B3LYP) and meta-GGA (M06-
2X)) were compared in this study to establish the best functional for the calculation
of the optical properties of the DCV substituted compounds as described in the
supporting information. In agreement with extensive benchmarks of DFT
functionals for the calculation of excitation energies it is found that M06-2X
performs very well.58-59
14

ACCEPTED MANUSCRIPT
Table 2. Calculated and experimental transition energies (ꢀE in eV), calculated oscillator
strengths (f) and composition of excited state.
Compound
∆E
∆E
f
Composition of excited state
(exp.) (calc.) (calc.)
in eV in eV
DPAM
2.92
3.23
3.99
2.77
3.33
3.60
4.31
3.42
0.38
0.12
1.60
0.67
0.61(H
-0.32(H
0.49(H-3
→
→
→
L) + 0.33 (H
L) + 0.61 (H→
L) – 0.48(H-2
→
L+2)
L+2)
L+1)
→
TATM
0.14(H-2
→
L) – 0.14(H-1
(H L+2)
L) + 0.48(H-1
0.19(H L+2)
L+2) + 0.45(H-3
0.50(H L+3)
L) + 0.40(H-1
0.39(H L+2)
L+1) + 0.49(H-1
L) + 0.28(H L) + 0.58(H
L+1) +0.68(H L) +0.15(H
-0.16(H-2 L) + 0.12(H-1 L+1) +
0.65(H L+2)
L) – 0.22(H-1
+0.18(H L+2)
L) - 0.13(H-1 L+1) + 0.11(H
0.62(H L+2) + 0.24(H-2
L+1)
L) - 0.27(H
L+1)
L+1) + 0.64(H
→
L+1) + 0.68
→
3.17
3.56
2.72
3.00
3.95
5.31
3.26
1.31
1.31
1.91
-0.48(H-2
→
→
L+1) +
→
-0.21(H-5→
→
L+2) +
→
BABM
0.41(H-2→
→L+1) –
→
3.43
3.54
2.54
3.65
0.59
0.10
1.75
0.32
0.49(H-2
0.28(H-2
-0.11(H-1
→
→L)
→
→
→
→
→
L)
L+2)
EDPAM
2.30
3.25
→
→
→
→
4.00
0.82
0.61(H-2
→
→L+1)
→
ETATM
EBABM
2.54
3.75
2.43
3.53
1.94
0.36
0.68(H→
→
→
L+2)
→
→
L) + 0.18(H-
1
→
4.72
3.80
0.50
0.59(H-2→
→
L+2) + 0.19(H-
1
→
2.40
2.92
2.39
3.24
2.69
0.39
-0.18(H-1
→
→
L) -0.18(H
L) – 0.28(H-1 L+1 +
0.56(H L+2))
L) -0.28(H-1
0.33(H L+2)
→L+2)
0.27(H-2
→
→
→
3.59
0.45
0.48(H-2
→
→L+1) –
→
In Table 2, the calculated transition energies of the lowest three excited states with
an oscillator strength (f) larger than 0.1 are listed, together with the composition of
the excited states. The experimental values for the low-lying transition were
15

ACCEPTED MANUSCRIPT
obtained from fitting the absorption spectra with multiple Gaussian peaks since
they typically consist of multiple close lying transitions (see Supporting
Information Figure S7). For DPAM the lowest excited state consists mostly of
excitation of an electron from HOMO to LUMO. From the discussion above on the
localization of the HOMO and LUMO on the anthracene and dicyanovinyl parts of
the molecule it is clear that the lowest allowed transition for DPAM
(corresponding to the shoulder in the absorption spectrum at 425 nm) has a
substantial charge transfer character. For the other two directly linked
chromophores, TATM and BABM the observation is rather different. In this case
the lowest excited state does not contain any HOMO-LUMO character but
excitations from other orbitals are dominant. From a detailed inspection of the
different excited states it was found that in both TATM and BABM a charge
transfer excited state similar to that in DPAM exists but with negligible oscillator
strength so it does not contribute to the observed absorption spectrum. The large
contribution of HOMO-LUMO transition in the lowest excited state for DPAM
leads to considerable CT character in this case. The other transitions involved in
the allowed excited states for the directly-linked compounds involve orbitals that
are localized on the same unit, as can be inferred from Table 2 and the orbital
pictures that are shown in Figure S5 in the supporting information. As an example,
for TATM in the (H->L+2) contribution both orbitals are localized on the
anthracene units, while for the other two contributions (H-2->L and H-1->L+1)
both orbitals are localized on the DCV and the pi-spacer. The same trend is also
observed for all other transitions in these three compounds. From the data for the
lowest three calculated transitions in Table 2 it also becomes clear that the
appearance of the experimental absorption spectra for the directly linked
chromophores results from several close lying transitions. This is also true for
instance for BABM where contributions from different electronic states to the
experimental spectrum are less obvious than in DPAM and TATM. In order to
directly compare the experimental spectra we have used the 30 lowest calculated
transitions from the TDDFT calculations to construct simulated absorption spectra,
using a Gaussian broadening of the absorption bands of 0.25 eV. The result of this
16

ACCEPTED MANUSCRIPT
simulation is shown in Figure 5c. Overall, the agreement of the general shape of
the spectra is rather good, with a consistent blue shift of the spectra compared to
the experimental data. The latter is attributed to the absence of any environment
effect in the calculations, while the experiments were done in CH2Cl2 solution. In
the second series of chromophores, ethynyl groups containing carbon-carbon triple
bonds were introduced in the π-spacer to enhance the effective conjugation over
the molecule by removing the steric interaction between the anthracene core and
the aromatic bridges. As discussed above, the viability of this approach is
confirmed by geometry optimizations that show that the ethynyl-inserted systems
are planar (see Figure 4). The absorption spectra of the control compounds without
DCV, shown in Figure 6a, clearly show the effect of the insertion of the ethynyl
units.
Figure 6. The optical absorption spectra of (a) control compounds and (b) ELCs
chromophores (EDPAM, ETATM, EBABM) in CH2Cl2 solution (c) Theoretical
simulation of the UV-vis absorption spectra for ELCs chromophores (EDPAM, ETATM,
EBABM).
17

ACCEPTED MANUSCRIPT
The absorption bands shift to lower energy, become broadened and lose the vibrational
structure as compared to the directly linked control compounds (Figure 5a). Inclusion of
the DCV units leads to a pronounced red-shift in the absorption spectrum in all three
compounds, similar as observed for the directly linked chromophores. The optical band
gaps calculated from the onset of the π-π* transition were found to be 2.17, 2.02 and 1.98
eV for EDPAM, ETATM and EBABM, respectively. This trend reflects the more
electron-rich nature of the thiophene group, as compared to the phenyl group, leading to a
higher lying HOMO and hence a decreased energy gap. Addition of a second thiophene
ring in EBABM further increases the HOMO (see Figure 3) but apparently also moves
the LUMO level to higher energy, resulting in only a minor decrease in the energy gap.
DFT calculations for the ethynyl linked series show that the lowest allowed transition is
in all cases dominated by HOMO-LUMO excitation. As was discussed above, both the
HOMO and the LUMO for the ethynyl linked chromophores are rather delocalized, but
the HOMO still has more density on the anthracene while the LUMO is localized more
towards the dicyanovinyl. Therefore, the lowest transitions in EDPAM, ETATM and
EBABM exhibit small charge transfer character (see below). In the case of the ethynyl-
linked chromophores the DFT calculations show that the different electronic transitions
are well-separated, with roughly 1 eV difference between the lowest two excited states in
all cases. The shape simulated absorption spectra are also in good agreement with the
experimental spectra (Figure 6c), although again the relative intensities for the different
compounds are not reproduced.
3.4. Charge transfer character in the excited state
The charge transfer character in the excited states can be more clearly identified by
comparing the electron density in the ground state and the excited states.44 In Figure 7 the
change in the charge distribution upon excitation to the lowest (allowed) excited state is
shown for all six chromophores. These changes in charge distribution were obtained by
taking the difference in the sum of the Mulliken charges for a specific fragment
(anthracene, ethynyl, phenyl, thiophene and dicyanovinyl) of the molecule in the ground
and excited state. The figure clearly illustrates the significant charge transfer character for
18

ACCEPTED MANUSCRIPT
the lowest excited state in DPAM; a significant amount of negative charge moves from
the anthracene unit towards the dicyanovinyl units in the outer parts of the molecule. For
TATM and BABM there is hardly any shift in charge upon excitation, which clearly
shows that there is very little charge transfer character in the lowest allowed excited state.
This is consistent with the absence of HOMO-LUMO character in this excited state. For
all ethynyl-linked chromophores (EDPAM, ETATM and EBABM) the picture is similar.
There is a clear shift of electrons from the anthracene towards the dicyanovinyl units but
the amount of charge is considerably less than for DPAM (0.1-0.2 e vs. ~0.4 e for
DPAM). This can be understood when the shape of the HOMO and LUMO orbitals are
considered. Both the HOMO and the LUMO are delocalized, but with some extra density
on the anthracene for the HOMO and some extra density on the dicyanovinyl for the
LUMO. Since the lowest excited state is dominated by a transition from the HOMO to
the LUMO in all of these chromophores, excitation leads to a small shift in charge upon
excitation.
19

ACCEPTED MANUSCRIPT
Figure 7. Change in the charge density on the different sub-units in the chromophores
upon excitation to the first excited state as obtained from a Mulliken population analysis.
3.5. Stokes shift and geometric structure reorganization.
Excitation of a chromophore to an excited state generally leads to changes in the
geometry of the molecule. Such geometry changes are reflected in the Stokes shift; the
difference between the absorption and emission maxima. These shifts can be obtained
from experimental data but can also be calculated by optimization of the geometry of the
excited state using time-dependent density functional theory. The experimental and
calculated absorption and emission energies are listed in Table 3 together with the Stokes
shift obtained from these data. Both in the experimental and the calculated data there is a
clear distinction in the Stokes shift for the DLCs and ELCs series. Although the
difference is more pronounced in the calculations than in the experimental data, it is clear
that the Stokes shift is larger for the directly linked series than it is for the ethynyl linked
series. This points to larger geometric structure reorganization upon excitation for the
directly linked series than for the ethynyl linked chromophores. In the directly linked
chromophores the geometric structure reorganization can roughly be divided in two parts;
planarization and a transition from aromatic and quinoidal bonding patterns. From the
geometry optimization it becomes clear that relaxation of the geometry in the directly
linked chromophores leads to a significant decrease in the dihedral angle between the
anthracene and the neighboring phenyl or thiophene groups (see Table 3). This partial
planarization results in an increased degree of conjugation in the excited state, and hence
a significant Stokes shift. Apart from the planarization there are many other degrees of
freedom by which geometry relaxation can take place after excitation. Most importantly
for conjugated molecules, deformations in the carbon-carbon bond lengths occur upon
excitation. In the ground state a sequence of (formally) single and multiple bonds can be
followed through the molecule where there are distinct differences in the bond length; i.e.
a bond-length alternation can be observed. This bond-length alternation generally
becomes less upon transition to the excited state due to the quinoidal bonding patterns.
This geometry deformation leads to a Stokes shift. The calculated Stokes shift due to all
geometry deformations is roughly 1 eV for the directly linked chromophores.
20

ACCEPTED MANUSCRIPT
Table 3. Calculated and experimental absorption and emission energies (∆E in eV).
Compound
∆Eabs ∆Eem Stokes ∆Eabs ∆Eem Stokes
(exp. (exp. shift (calc. (calc. shift
(exp.) in eV) in eV) (calc.)
φgs
φes
(deg.) (deg.)
in
in
eV)
eV)
DPAM
TATM
BABM
EDPAM
ETATM
EBABM
2.92
2.77
2.72
2.30
2.54
2.40
2.15
2.16
2.04
2.13
1.98
1.89
0.77
0.61
0.68
0.17
0.56
0.51
3.33
3.42
3.26
2.54
2.43
2.39
2.51
2.34
2.32
2.19
2.08
2.01
0.82
1.08
0.94
0.35
0.35
0.38
80
90
90
0
0
0
51
45
48
0
0
0
Figure 8. Change in the C-C bond lengths along the chromophore backbone upon
excitation to the lowest excited state. The numbering of the bonds is indicated in the
scheme.
For the ethynyl-linked chromophores the Stokes shift is overall significantly smaller, both
in the experiments and in the calculations. The main difference between the directly
linked series and the ethynyl linked series is that the ethynyl linked chromophores are
fully planar in the ground state. Therefore no planarization can occur in the excited state.
Therefore, we conclude that in the directly linked series, the larger Stokes shift is due to
21

ACCEPTED MANUSCRIPT
planarization of the non-planar molecule. In the ethynyl-linked series both the ground
state and the excited state are fully planar and the only geometry deformation that can
occur is in the bond length. As can be seen in Figure 8 and Figure S4, the changes in the
C-C bond lengths are similar to those in the directly-linked chromophores. In all cases the
largest change in bond length (0.035Å) is found in the bond connecting the central
anthracene unit with the rest of the molecule. Generally, the deformations become less
pronounced in the outer parts of the chromophores, which is consistent with the changes
in charge distribution on excitation.
4. Summary and Conclusions
In this work, we have presented a combined experimental and theoretical characterization
of the structural and electronic properties of Acceptor-Donor-Acceptor (A-D-A) type
chromophores, incorporating dicyanovinyl (DCV) units as the terminal acceptor moieties.
DCV units are connected to the donor unit through different π-spacers. Specific emphasis
is placed on the structure of the benzene- and thiophene-based π- spacer and the effect of
adding an additional “ethynyl” unit that leads to more planar geometries. A comparison
between the experimental and theoretical work leads to detailed insight in the relationship
between the molecular structure and electronic structure of A-D-A type chromophores. It
is shown that for compounds that exhibit large deviations from planarity, the orbitals are
rather localized on different subunits. However, the charge transfer character in most
allowed transitions remains small because generally, only transitions between orbitals on
the same units are involved. The more planar structures obtained after inserting ethynyl
units exhibit more delocalized orbitals, leading to a significant red-shift in the absorption
spectra.
While A-D-A molecular design approach allowed us to systematically lower the LUMO
energy levels of the target chromophores, HOMO-LUMO gap is not directly related to
the absorption properties. This is due to the subtle effect that the molecular geometry has
on the shape and delocalization of the orbitals, especially for non-planar compounds. In
the case of planar pi-conjugated compounds the HOMO-LUMO transition is dominating
the optical absorption but due to the more delocalized nature of the frontier orbitals the
energy levels of the HOMO and LUMO cannot be controlled individually.
22

ACCEPTED MANUSCRIPT
In order to establish the suitability of the new materials presented here for photovoltaic
devices we have performed preliminary photoconductivity experiments on drop-casted
blends with C60-PCBM. While clear evidence of photoinduced charge separation was
observed, the experiments are complicated by the high tendency of these materials to
form crystalline domains. This could be resolved by using physical vapor deposition and
we are currently working in this direction.
Acknowledgements
This work is supported by the Netherlands Organization for Scientific Research (NWO)
through an VIDI grant to FCG and by the European Research Council FP7 ERC grant
agreement no. 240299.
References and Notes
1. Nielsen, C.; Turbiez, M.; McCulloch I. Adv. Mater. 2013 ,25, 1859-1880.
2. Zhang J.; Deng D.; He C.; He Y.; Zhang M.; Zhang Z. G.; Zhang Z.; Li Y. Chem.
Mater. 2011, 23, 817-822.
3 . Kulkarni A. P.; Tonzola C. J.; Babel A.; Jenekhe S. A. Chem. Mater. 2004, 16, 4556-
4573.
4. Takacs C. J.; Sun Y.; Welch G. C.; Perez L. A.; Liu X.; Wen, W.; Bazan G. C.; Heeger
A. J. Am. Chem. Soc. 2012,134, 16597-16606.
5. Estrada L. A.; Liu D. Y.; Salazar D. H.; Dyer A. L.; Reynolds J. R. Macromolecules
2012, 45, 8211-8220.
6. Beaujuge P. M.; Frechet J. M. J. J. Am. Chem. Soc. 2011, 133, 20009-20029.
7. Lin Y.; Li Y.; Zhan X. Chem. Soc. Rev. 2012, 41, 4245-4272.
8. Wen L.; Heth C. L.; Rasmussen S. C. Phys. Chem. Chem. Phys. 2014, 16, 7231-7240.
9. Tlach B. C.; Tomlinson A. L.; Bhuwalka A.; Jeffries-El M. J. Org. Chem. 2011, 76,
8670-8681.
10. Mishra A.; Bäuerle P. Angew. Chem. Int. Ed. 2012, 51, 2020-2067.
11. Yao H.; Ye L.; Zhang H.; Li S.; Zhang S.; Hou J. Chem. Rev. 2016, 116, 7397-7457.
12. Guide M.; Pla S.; Sharenko A.; Zalar P.; Fernández-Lázaro F.; Sastre-Santos Á. ;
Nguyen T. Q. Phys. Chem. Chem. Phys. 2013, 15, 18894-18899.
23

ACCEPTED MANUSCRIPT
13. Chen Y. H.; Lin L. Y.; Lu C. W.; Lin F.; Huang Z. Y.; Lin H. W.; Wang P. H.; Liu Y.
H.; Wong K. T.; Wen J.; Miller D. J.; Darling S. B. J. Am. Chem. Soc. 2012, 134, 13616-
13623.
14. Wrackmeyer M. S.; Hummert M.; Hartmann H.; Riede M. K.; Leo K. Tetrahedron
2010, 66, 8729-8733.
15. Zhang S.; Wang X.; Tang A.; Huang J.; Zhan C.; Yao J. Phys. Chem. Chem. Phys
2014, 16, 4664-4671.
.
16. Liu Y.; Wan X.; Wang F.; Zhou J.; Long G.; Tian J.; You J.; Yang Y.; Chen Y. Adv.
Energy Mater. 2011, 1, 771-775.
17. Bürckstümmer H.; Kronenberg N. M.; Gsänger M.; Stolte M.; Meerholz K.;
Würthner F. J. Mater. Chem. 2010, 20, 240-243.
18. Roquet S.; Cravino A.; Leriche P.; Alévêque O.; Frère P.; Roncali J. J. Am. Chem.
Soc. 2006, 128, 3459-3466.
19. Bagnis D.; Beverina L.; Huang H.; Silvestri F.; Yao Y.; Yan H., Pagani G. A.; Marks
T. J.; Facchetti A. J. Am. Chem. Soc. 2010, 132, 4074-4075.
20. Luponosov Y. N.; Min J.; Solodukhin A. N.; Kozlov O. V.; Obrezkova M. A.;
Peregudova S. M.; Ameri T.; Chvalun S. N.; Pshenichnikov M. S.; Brabec C. J.;
Ponomarenko S. A. Organic Electronics 2016, 32, 157-168.
21. Du J.; Fortney A. ; Washington K. E. ; Bulumulla C.; Huang P.; Dissanayake D.;
Biewer M.C.; Kowalewski T.; Stefan M. C. ACS Appl. Mater. Interfaces 2016, 8,
33025-33033.
22. Osaka I.; Shimawaki M.; Mori H.; Doi I.; Miyazaki E.; Koganezawa T.;
Takimiya K. J. Am. Chem. Soc. 2012, 134, 3498-3507.
23. Ying W.; Guo F.; Li J.; Zhang Q.; Wu W.; Tian H.; Hua J. ACS Appl. Mater.
Interfaces 2012, 4, 4215-4224.
24. Pappenfus T. M.; Schneiderman D. K.; Casado J.; Navarrete J. T. L.; Ruiz
Delgado M. C.; Zotti G.; Vercelli B.; Lovander M. D.; Hinkle L. M.; Bohnsack J.
N.; Mann K. R. Chem. Mater. 2011, 23, 823-831.
25. Leliège A.; Blanchard P.; Rousseau T.; Roncali J. Org. Lett. 2011,13, 3098-3101.
26. Gupta A.; Ali A.; Bilic A.; Gao M.; Hegedus K.; Singh B.; Watkins S. E.; Wilson
G. J.; Bach U.; Evans R. A. Chem. Commun. 2012, 48, 1889-1891.
24

ACCEPTED MANUSCRIPT
27. Chiu S. W.; Lin L. Y.; Lin H. W.; Chen Y. H.; Huang Z. Y.; Lin Y. T.; Lin F.;
Liu Y. H.; Wong K. T. Chem. Commun. 2012, 48, 1857-1859.
28. Sonar P.; Singh S. P.; Leclère P.; Surin M.; Lazzaroni R.; Lin T. T.; Dodabalapur
A.; Sellinger A. J. Mater. Chem. 2009
29. He G.; Li Z.; Wan X.; Liu Y.; Zhou J.; Long G.; Zhang M.; Chen Y. J. Mater.
Chem 2012, 22, 9173-9180.
, 19, 3228-3237.
.
30. Salzner U. J. Phys. Chem. B 2002, 106, 9214-9220.
31. Salzner U.; Kose M. E. J. Phys. Chem. B 2002, 106, 9221-9226.
32. Singh M.; Kurchania R.; Mikroyannidis J. A.; Sharma S. S.; Sharma G. D. J.
Mater. Chem. A, 2013, 1, 2297-2306.
33. Li C.; Chen Y.; Zhao Y.; Wang H.; Zhang W.; Li Y.; Yang X.; Ma C.; Chen L.;
Zhu X.; Tu Y. Nanoscale 2013, 5, 9536-9540.
34. Lai Y.Y.; Yeh J. M.; Tsai C. E.; Cheng Y. J. Eur. J. Org. Chem. 2013, 5076-
5084.
35. Calbo J.; Arago J.; Oton F.; Lloveras V.; Mas-Torrent M.; Vidal-Gancedo J.;
Veciana J.; Rovira C.; Orti E. Chem. Eur. J. 2013, 19, 16656-16664.
36. Steinberger S.; Mishra A.; Reinold E.; Levichkov J.; Uhrich C.; Pfeiffer M. ;
Bäuerle P. Chem. Commun. 2011, 47, 1982-1984.
37. Qi T.; Liu Y.; Qiu W.; Zhang H.; Gao X.; Liu Y.; Lu K.; Du C.; Yu G.; Zhu D. J.
Mater. Chem. 2008, 18, 1131-1138.
38. Yang L.; Zhang S.; He C.; Zhang J.; Yao H.; Yang Y.; Zhang Y.; Zhao W.; Hou
J. J.Am.Chem.Soc. 2017, 139, 1958-1966.
39. Wu J.; Ma Y.; Wu N.; Lin Y.; Lin J.; Wang L.; Ma C. Organic Electronics 2015
,
23, 28-38.
40. Fitzner R.; Mena-Osteritz E.; Mishra A.; Schulz G.; Reinold E.; Weil M.; Körner
C.; Ziehlke H.; Elschner C.; Leo K.; Riede M.; Pfeiffer M.; Uhrich C.; Bäuerle P.
J. Am. Chem. Soc. 2012,134,11064-11067.
41. Haid S.; Mishra A.; Uhrich C.; Pfeiffer M.; Bäuerle P. Chem. Mater. 2011, 23,
4435-4444.
25

ACCEPTED MANUSCRIPT
42. Fitzner R.; Reinold E.; Mishra A.; Mena-Osteritz E.; Ziehlke H.; Körner C.; Leo
K.; Riede M.; Weil M.; Tsaryova O.; Weis A.; Uhrich C.; Pfeiffer M.; Bäuerle P.
Adv. Func. Mater. 2011, 27, 897-910.
43. Steinberger S.; Mishra A.; Reinold E.; Mena-Osteritz E.; Müller H.; Uhrich C.;
Pfeiffer M.; Bäuerle P. J. Mater. Chem. 2012, 22, 2701-2712.
44. Elschner C.; Schrader M.; Fitzner R.; Levin A. A.; Bäuerle P.; Andrienko D.; Leo
K. ; Riede M. RSC. Adv. 2013, 3, 12117-12123.
45. Simon J.; Atherton J.C.C. Synth. Commun. 2001, 31, 1799-1802.
46. Chen R.; Yang X.; Tian H.; Wang X.; Hagfeldt A.; Sun L. Chem. Mater. 2007
,
19 ,4007-4015.
47. Colella S.; Mazzeo M.; Grisorio R.; Fabiano E.; Melcarne G.; Carallo S.;
Angione M. D.; Torsi L.; Suranna G.P.; della Sala F.; Mastrorilli P.; Gigli G.
Chem. Commun. 2010, 46, 6273-6275.
48. Khan M. S.; Al-Mandhary M. R. A.; Al-Suti M. K.; Al-Battashi F. R.; Al-Saadi
S.; Ahrens B., Bjernemose J.K.; Mahon M. F.; Raithby P.R.; Younus M.;
Chawdhury N.; Köhler A.; Marseglia E. A.; Tedesco E.; Feederd N.; Teate S.
Dalton. Trans. 2004, 2377-2385.
49. Fraind A. M.; Tovar J. D. J. Phys. Chem. B. 2010, 114, 3104-3116.
50. Swager T. M.; Gil C. J.; Wrighton M.S. J. Phys. Chem. 1995, 99, 4886-4893.
51. Wang C.; Liu Y.; Ji Z.; Wang E.; Li R.; Jiang H.; Tang Q.; Li H.; Hu W. Chem.
Mater. 2009, 21, 2840-2845.
52. Cardona C. M.; Li W.; Kaifer A. E.; Stockdale D.; Bazan G.C. Adv. Mater. 2011
,
23, 2367-2371.
53. Beaujuge P. M.; Subbiah J.; Choudhury K. R.; Ellinger S.; McCarley T. D.; So
F.; Reynolds J. R. Chem. Mater. 2010, 22, 2093-2106.
54. Frisch M. J.; Trucks G. W.; Schlegel H. B.; Scuseria G. E.; Robb M.A.;
Cheeseman J. R.; Scalmani G.; Barone V.; Mennucci B.; Petersson G. A.;
Nakatsuji H.; Caricato M.; Li X.; Hratchian H.P.; Izmaylov A.F.; Bloino J.; Zheng
G.; Sonnenberg J. L.; Hada M.; Ehara M.; Toyota K.; Fukuda R.; Hasegawa J.;
Ishida M.; Nakajima T.; Honda Y.; Kitao O.; Nakai H.; Vreven T.; Jr.
Montgomery J.A.; Peralta J. E.; Ogliaro F.; Bearpark M.; Heyd J.J.; Brothers E.;
26

ACCEPTED MANUSCRIPT
Kudin K. N.; Staroverov V.N.; Kobayashi R.; Normand J.; Raghavachari K.;
Rendell A.,. Burant J. C; Iyengar S.S.; Tomasi J.; Cossi M.; Rega N.; Millam N.J.;
Klene M.; Knox J. E.; Cross J. B.; Bakken V.; Adamo C.; Jaramillo J.; Gomperts
R.; Stratmann R. E.; Yazyev O.; Austin A. J., Cammi R.; Pomelli C.; Ochterski
J.W.; Martin R. L.; Morokuma K.; Zakrzewski V. G.; Voth G. A.; Salvador P.;
Dannenberg J. J.; Dapprich S.; Daniels A. D.; Farkas Ö.; Foresman J. B.; Ortiz
J.V.; Cioslowski J.; Fox D. J. Gaussian, Inc.: Wallingford, CT, 2009.
55. Fraind A. M.; Sini G.; Risko C.; Ryzhkov L. R.; Brédas J. L; Tovar J. D. J. Phys.
Chem. B 2013, 117, 6304-6317.
56. Estrada L. A.; Stalder R.; Abboud K. A.; Risko C.; Brédas J. L.; Reynolds J. R.
Macromolecules 2013, 46, 8832-8844.
57. Salman S.; Brédas J. L.; Marder S. R.; Coropceanu V.; Barlow S.
Organometallics 2013, 32, 6061-6068.
58. Leang S. S.; Zahariev F.; Gordon M. S. J. Chem. Phys. 2012, 136, 104101-
104101-11.
59. Zhao Y.; Truhlar D. G. Theor. Chem. Account 2008, 120, 215–241.
Supplementary Materials
Electronic Supplementary Information (ESI) available: [Methods and materials, synthetic
procedures and characterization details. DFT data for the optimize geometries, frontier
orbitals, electronic structure.]
27