Origin of intense intramolecular charge-transfer interactions in nonplanar push-pull chromophores

Article abstract of DOI:10.1002/chem.200901630

Planar push-pull chromophores which features an intense intramolecular charge-transfer (CT) interactions of the potential applications in molecular electronics and optoelectronic has been reported. Two new series of push-pull chromophores by addition of T

Full text of DOI:10.1002/chem.200901630

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DOI: 10.1002/chem.200901630
Origin of Intense Intramolecular Charge-Transfer Interactions in Nonplanar
Push–Pull Chromophores
Shin-ichiro Kato,[a] Milan Kivala,[a] W. Bernd Schweizer,[a] Corinne Boudon,[b]
Jean-Paul Gisselbrecht,[b] and FranÅois Diederich*[a]
Planar push–pull chromophores featuring intense intramo-
lecular charge-transfer (CT) interactions have been exten-
sively studied in view of their potential applications in mo-
lecular electronics and optoelectronics.[1,2] In contrast, only a
limited number of nonplanar low-molecular-weight donor–
acceptor chromophores has been reported and the impact of
nonplanarity on their p-conjugative and optoelectronic
properties not been systematically investigated.[3,4] Nonpla-
nar CT chromophores tend to feature some desirable physi-
cal properties compared to their planar counterparts: they
are usually more soluble, less aggregating, and more readily
sublimable, forming amorphous, rather than crystalline films
for potential use in optoelectronic devices.[2a,5] We showed
recently that donor-substituted alkynes undergo a formal
[2+2] cycloaddition with electron-accepting olefins, such as
tetracyanoethene (TCNE),[6–8] 7,7,8,8-tetracyanoquinodime-
thanes (TCNQs),[9–11] as well as dicyanovinyl (DCV) and tri-
cyanovinyl derivatives,[12] followed by retroelectrocycliza-
tion, under formation of nonplanar push–pull chromophores
featuring intense intramolecular CT and high third-order op-
tical nonlinearities. Nonplanar, N,N-dimethylanilino (DMA)
key question raised was the origin of the intense intramolec-
ular CT interactions in these push–pull chromophores, in
view of their pronounced nonplanarity which is expected to
lead to disruption of donor–acceptor p-conjugation and con-
comitant reduction in CT efficiency and optical nonlinearity.
To investigate the effects of sterically enforced deconjuga-
tion on optoelectronic properties, we prepared two new
series of push–pull chromophores (1 and 2, and 3–8), by
adding TCNE or TCNQ to tetrathiafulvalene (TTF)- or fer-
rocenyl (Fc)-substituted alkynes, respectively. These strong
electron donors have found numerous applications in inter-
molecular[14,15] and intramolecular CT systems,[16] but have
not seen much use for activating alkynes electronically for
the cycloaddition of TCNE or TCNQ.[17]
TTF-appended TCBD 1 was obtained in 41% yield as a
deep blue solid by reaction of TCNE with TTF-substituted
alkyne 9 in 1,2-dichloroethane at 808C (Scheme 1). The
electronically more activated alkyne 10, with both TTF and
DMA donor groups, reacted at room temperature to give 2
in 79% yield. The ferrocene-substituted expanded TCNQs
3–8 were formed as black metallic-like solids in 41–81%
yield by regioselective cycloaddition between TCNQ and
the acetylenic precursors 11–16. With the exception of 3 and
8, these push–pull chromophores are thermally stable up to
3008C, as revealed by thermal gravimetric analysis (TGA);
compound 6 can actually be sublimed without decomposi-
tion at about 2508C/1ꢀ10À6 Torr.
donor-substituted
1,1,4,4-tetracyanobuta-1,3-dienes
(TCBDs), obtained by [2+2] cycloaddition of TCNE, pro-
duce high-optical quality amorphous films by vapor-phase
deposition[13a] which in the meanwhile have found first appli-
cation in silicon-organic-hybrid (SOH) waveguides.[13b]
A
Single crystals of 1 and 3–5 suitable for X-ray analysis
were grown from CH2Cl2/hexane at À158C (Figure 1 and
Figure S1–S4 in the Supporting Information). Considerable
nonplanarity is observed in the TCBD and expanded TCNQ
acceptor moieties. Thus, the dihedral angle q (C1-C2-C17-
C18) between the two DCV planes in 1 (with two independ-
ent molecules) is À92.7(2)8. In the expanded TCNQ chro-
mophores 3–5, the dihedral angle q between the DCV and
the cyclohexa-2,5-diene-1,4-diylidene moiety changes from
51.9(4)8 (3, torsional angle C1-C2-C3-C4, Figure S2) to
À68.6(2)8 (5, C5-C8-C9-C16, Figure S4), and to À94.0(2)8
(4, C1-C12-C13-C14). Sterically enforced p-deconjugation is
[a] Dr. S.-i. Kato, Dr. M. Kivala, Dr. W. B. Schweizer,
Prof. Dr. F. Diederich
Laboratorium fꢁr Organische Chemie, ETH Zꢁrich
Hçnggerberg, HCI, 8093 Zꢁrich (Switzerland)
Fax : (+41)44-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] Prof. Dr. C. Boudon, Dr. J.-P. Gisselbrecht
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Intstitut de Chimie-UMR 7177, C.N.R.S.
Universitꢂ de Strasbourg
4, rue Blaise Pascal, 67000 Strasbourg (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901630.
Chem. Eur. J. 2009, 15, 8687 – 8691
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8687

À
teractions. For example, the Cp bonds C21 C22
À
(1.425(3) ꢄ) and C23 C24 (1.417(3) ꢄ) in 4 are significantly
À
À
shorter than the C20 C21 and C20 C24 bonds (both
1.450(3) ꢄ).[18] Also, the C4 C7 bond (1.387(2) ꢄ) is much
À
À
longer than the C13 C14 bond (1.347(3) ꢄ). These data
support a substantial contribution of the fulvene-type reso-
nance structure shown in Figure 1 to the ground-state geom-
etry.[19] Neighboring molecules in 1 and 3–5 interact via mul-
tiple dipolar CN···CN interactions as revealed by the crystal
packings shown in the Supporting Information.[7b,10b]
Cyclic voltammetry (Table 1) and rotating-disk voltamme-
try (RDV, see Table S1 in the Supporting Information) in
CH2Cl2 (+0.1m nBu4NPF6, internal standard Fc+/Fc) show
Table 1. Cyclic voltammetry (CV) data for 1 and 3–6 in CH2Cl2 (+0.1m
nBu4NPF6)[a] and a summary of electrochemical and optical energy gaps.
The complete set of data for 1–8, including those from rotating-disk vol-
tammetry (RDV), are included in the Supporting Information.
Cyclic voltammetry
E8
DEp
Ep
DEredox
DEopt
[V][b]
[mV][c]
[V][d]
[V][e]
[eV][f]
1
3
+0.67
0.87
1.61
+0.24
À0.63
À0.97
+0.36
À0.49
À0.75
+0.36
À0.81
+0.34
À0.74
À0.81
+0.47
+0.33
À0.82
À0.94
75
60
70
75
75
80
85
120
90
60
60
85
85
85
85
0.85
1.55
Scheme 1. Synthesis of push–pull chromophores 1–8. Reagents and condi-
tions: a) TCNE, 1,2-dichloroethane or benzene, 808C (1) or 258C (2). b)
TCNQ, CH2Cl2 or 1,2-dichloroethane, 258C or 70/808C.
4
5
1.17
1.08
1.66
1.61
smallest in 3 with an H-substituent on the DCV moiety, and
this is consistent with the UV/Vis and electrochemical data
described below.
6
1.15
1.63
Most importantly however, the TTF and Fc donors retain
nearly fully planar p-conjugation with one half of the ac-
ceptor moieties. The experimentally observed bond lengths
(Figure 1, Figure S1–S4 in the Supporting Information) sup-
port a significant contribution of polar resonance structures
as a result of intramolecular CT interactions in the ground
state. Thus in 1, the TTF and the neighboring DCV moiety
are coplanar, with a torsional angle C1-C2-C3-S7 of 1.7(3)8.
[a] All potentials are given versus the Fc+/Fc couple used as internal
standard. Working electrode: glassy carbon electrode; counter electrode:
Pt; reference electrode: Ag/AgCl. Scan rate: 0.1 VsÀ1. [b] E8=(Epc
+
Epa)/2, where Epc and Epa correspond to the cathodic and anodic peak po-
tentials, respectively. [c] DEp =EoxÀEred, where the subscripts ox and red
refer to the conjugated oxidation and reduction steps, respectively.
[d] Ep =Irreversible peak potential. [e] Electrochemical gap, DEredox, is
defined as the potential difference between the first oxidative and first
reductive redox potentials. [f] Optical gap, DEopt, is defined as the energy
corresponding to the lmax of the CT absorption.
À
As a result of efficient intramolecular CT, the S5 C4 bond
(1.694(2) ꢄ) in the TTF moiety is significantly shorter than
À
all other C S bonds (between 1.733(3) and 1.7711(19) ꢄ).
À
Correspondingly, the C3 C4 bond (1.360(3) ꢄ) is elongated
compared to the other TTF double bond C10 C11
(1.327(4) ꢄ). Also, the C1 C2 bond (1.372(3) ꢄ) in the co-
planar DCV is longer than the corresponding C17 C18
bond (1.352(3) ꢄ) in the orthogonally oriented DCV. All
these data support a major contribution of the charge-sepa-
rated resonance structure shown in Figure 1 to the ground-
state geometry of 1.
that TCBDs 1 and 2 exhibit two TTF-type 1eÀ-oxidation
couples at nearly same potentials (+0.24, +0.67 V for 1).
The parent TTF undergoes the oxidation steps at À0.08 and
+0.40 V under the same conditions.[20] Oxidation of 1 and 2
is clearly rendered more difficult by the conjugation with
the strong TCBD acceptor. The two well-resolved, reversi-
ble 1eÀ-reduction steps, centered on the TCBD acceptors,
appear at À0.63 V/À0.97 V (in 1) and at À0.83 V/À1.06 V
(in 2). The cathodic shifts observed for 2 originate from p-
conjugation of the acceptor to the second donor moiety,
namely DMA.
À
À
À
The plane of the cyclopentadienyl (Cp) rings attached to
the acceptor in 3–5 is slightly twisted against the cyclohexa-
2,5-diene-1,4-diylidene plane, with torsional angles between
22.6(4)8 (3, C4-C3-C19-C20), À20.6(3)8 (4, C1-C12-C20-
C24), and À21.6(3)8 (5, C5-C8-C25-C29). Again the experi-
mental bond lengths reveal efficient intramolecular CT in-
In the ferrocene-substituted expanded TCNQs 3–8, the
first oxidation potentials, located on the ferrocene moiety,
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Nonplanar Push–Pull Chromophores
COMMUNICATION
the two DCV moieties are
more efficiently p-conjugated.
In contrast, substitution induces
a large twist between the two
acceptor halves in 4–6 (X-ray)
and first and second reduction
potentials, each centered on
one acceptor half, differ only by
60 mV (4) to 120 mV (6). Inser-
tion of the acetylenic spacer in
7
and 8 reduces the steric
crowding and p-deconjugation,
and the first reduction again be-
comes more favorable—by
about 180 mV—as compared to
4–6 (Table S1).[22]
The UV/Vis spectra of the
two series of push–pull chromo-
phores in CH2Cl2 display broad
CT bands with end-absorptions
reaching into the near infrared
(Figure 2). The two TTF chro-
mophores
exhibit
similar
maxima of their CT bands at
lmax =767 nm (1.61 eV, 1) and
728 nm (1.69 eV, 2), despite the
presence of the DMA donor
moiety in 2 (Figure 2A). This
strongly indicates that the low-
energy transitions in both mole-
cules are predominantly TTF
donor—TCBD acceptor transi-
tions. Time-dependent density
functional theory (TD-DFT) at
Figure 1. ORTEP plots of a) 1 and b) 4 with vibrational ellipsoids at 220 K shown at the 50% probability
level. Hydrogen atoms are omitted for clarity. Arbitrary numbering. The experimental bond lengths support a the
substantial contribution from polar resonance structures as a result of efficient intramolecular CT.
TD-B3LYP/cc-pVDZ//
B3LYP/6-31G*
level
using
Gaussian 03[23] reproduces well
the absorption spectra of 1 and
are nearly independent of the Fc substituent and range from
+0.33 to +0.37 V. In bis-ferrocenyl derivative 6, the two Fc
moieties are in close spatial proximity and are oxidized step-
wise (+0.33 and +0.47 V), while a unique two-electron oxi-
dation is observed for 8 (+0.33 V) in which the two Fc units
are located at greater distance. In 8, the two Fc units act as
independent redox centers,[21] whereas the oxidation of the
second Fc unit in 6 is rendered more difficult by electrostat-
ic repulsion. In the DCV-centered reduction processes, in-
troduction of substituents in the series 3–6 results in large
cathodic shifts of the first reduction potentials (À0.49 V (3),
À0.81 V (4), À0.74 V (5), and À0.82 V (6)). This clearly re-
flects the greater twist and the resulting sterically enforced
p-deconjugation between the two acceptor halves upon sub-
stitution, as evidenced by the X-ray crystal structures
(Figure 1 and Figure S2–S4). The effect of steric deconjuga-
tion is equally visible in the second reduction potentials. A
larger difference between the first and second reduction po-
tentials (260 mV) is observed in unsubstituted 3, in which
2 and confirms that their low-energy electronic transitions
occur from the TTF-located HOMO to the LUMO that is
located mainly on the TCBD moiety (see the Supporting In-
formation). The HOMO density is concentrated on the TTF
moiety, irrespective of the DMA moiety in 2. The overlap of
the HOMO and LUMO in 1 and 2 is relatively small, occur-
ring partially on the TTF moiety.
For the Fc-substituted expanded TCNQs 3–8, clear steric
effects on their electronic transitions are observed, in full
agreement with the electrochemical properties (Figure 2B).
Thus, 3 with the H-substituent displays the lowest-energy
CT band at 801 nm (1.55 eV) and an end-absorption near
1400 nm (0.89 eV). Methyl-substitution in 4 results in a hyp-
sochromic shift to lmax =749 nm (1.66 eV), reflecting steri-
cally enforced deconjugation between the two DCV accept-
or halves. This deconjugation elevates the LUMO rather
than the HOMO level, as confirmed by CV (Table 1) and
theoretical calculations (see the Supporting information),
thereby resulting in a larger optical gap. The CT band maxi-
Chem. Eur. J. 2009, 15, 8687 – 8691
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F. Diederich et al.
tained by cycloaddition of strong acceptors (TCNE, TCNQ)
to donor-substituted alkynes emerges. While the resulting
TCBD and expanded TCNQ acceptor moieties are highly
nonplanar and the twist between the planes of the two DCV
acceptor halves can be further increased by substitution, one
DCV moiety in 1 and 2 as well as the cyclohexa-2,5-diene-
1,4-diylidene-spaced DCV moiety in 3–8 remain in nearly
planar p-conjugation with the donor, as evidenced by the X-
ray crystal structures. The intramolecular CT mainly in-
volves this part of the molecule. The second DCV moiety
however, even if twisted out of this plane up to near ortho-
gonality, also contributes to the acceptor potency by its
strong electron-accepting s-induction. This picture explains
that the energetic effects of steric p-deconjugation in the ac-
ceptor are remarkably small, with lmax =801 (1.55 eV) in H-
substituted 3 (twist angle between the two acceptor halves
of 51.9(4)8) and 749 (1.66 eV) in Me-substituted 4 (twist
angle: À94.0(2)8). Thus, the energy of the CT bands and the
electrochemical HOMO–LUMO gaps of these nonplanar
push–pull systems are comparable to the lowest values ob-
tained for fully planar TTF- and ferrocene-based push–pull
chromophores.[16] On the other hand, the nonplanar chromo-
phores add distinct advantages in physical properties, favor-
ing their use in optoelectronic devices.[13] These include
higher solubility, less propensity to undergo aggregation
which affects the optical properties, and frequently sublima-
bility without decomposition, allowing amorphous high-
quality optical film formation by vapor-phase deposition.
Another specific advantage of the reported class of nonpla-
nar push–pull chromophores remains undoubtedly their
short and efficient one-step synthesis, which allows rapid es-
tablishment of comprehensive structure–property relation-
ships in the development of advanced functional materials.
Acknowledgements
Figure 2. Electronic absorption spectra of TTF conjugates 1 and 2 (A)
and Fc derivatives 3–8 (B) in CH2Cl2 at 258C.
This work was supported by a grant from the ETH research council and
a JSPS Postdoctoral Fellowships for Research Abroad Science to S.-i.
Kato.
mum shifts hypsochromically (801 nm (3), 772 nm (5),
749 nm (4)) as the twist in the acceptor increases (see
above). The introduction of the substituent groups on the
acceptor moiety affords an enhancement of the molar ex-
Keywords: charge transfer · cycloaddition · ferrocene ·
optoelectronic properties
tetrathiafulvalene
· push–pull chromophores ·
tinction coefficient
e in the shorter-wavelength region
around 400–550 nm, which would be assignable to p-p* tran-
sitions,[17b] relative to the intensity of the CT band around
650–1000 nm. There exists a good linear correlation between
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Nonplanar Push–Pull Chromophores
COMMUNICATION
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Received: May 18, 2009
Published online: July 20, 2009
Chem. Eur. J. 2009, 15, 8687 – 8691
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8691