Fulvenyl-Functionalized Polyisocyanides: Cross-Conjugated Electrochromic Polymers with Variable Optical and Electrochemical Properties

Article abstract of DOI:10.1021/acs.macromol.8b00977

We describe the preparation of arylisocyanide monomers bearing conjugated fulvenyl groups derived from 9-benzylidene-9H-fluorene (Flu), 5-benzylidene-1,2,3,4-tetraphenylcyclopentadiene (TPCp), and 5-benzylidene-5H-dibenzo[a,d]cycloheptene (Dbs). The electrochemical and optical properties of the monomers and their precursors have been characterized and consistently showed the effect of the conjugation of the respective functional group (-NO₂, -NH₂, -NHCHO, and N-C) with the fulvenyl moiety. The isocyanides have been subsequently polymerized to the corresponding polyisocyanides (PICs), which exhibited number-average molecular weights of 124-136 kDa (PDI = 2.0-2.7), as determined by gel permeation chromatography in THF vs polystyrene standards. The thermal, optical, and electrochemical properties of the polymers have been studied in detail. Spectroelectrochemical analyses of polymers equipped with redox-active pentafulvene groups show reversible electrochromism, which allows to lower the optical gap from 2.38 to 1.20 eV (Flu) and from 2.27 to 1.55 eV (TPCp) via chemical or electrochemical reduction.

Full text of DOI:10.1021/acs.macromol.8b00977

Article
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Fulvenyl-Functionalized Polyisocyanides: Cross-Conjugated
Electrochromic Polymers with Variable Optical and Electrochemical
Properties
Sandra Schraff,† Yu Sun,‡ and Frank Pammer*,†
†Ulm University, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
‡
̈
̈
Technische Universitat Kaiserslautern, Erwin-Schrodinger-Strasse 54, D-67663 Kaiserslautern, Germany
S
* Supporting Information
ABSTRACT: We describe the preparation of arylisocyanide
monomers bearing conjugated fulvenyl groups derived from 9-
benzylidene-9H-fluorene (Flu), 5-benzylidene-1,2,3,4-
tetraphenylcyclopentadiene (TPCp), and 5-benzylidene-5H-
dibenzo[a,d]cycloheptene (Dbs). The electrochemical and
optical properties of the monomers and their precursors have
been characterized and consistently showed the effect of the
conjugation of the respective functional group (−NO2, −NH2,
−NHCHO, and −NC) with the fulvenyl moiety. The
isocyanides have been subsequently polymerized to the
corresponding polyisocyanides (PICs), which exhibited
number-average molecular weights of 124−136 kDa (PDI = 2.0−2.7), as determined by gel permeation chromatography in
THF vs polystyrene standards. The thermal, optical, and electrochemical properties of the polymers have been studied in detail.
Spectroelectrochemical analyses of polymers equipped with redox-active pentafulvene groups show reversible electrochromism,
which allows to lower the optical gap from 2.38 to 1.20 eV (Flu) and from 2.27 to 1.55 eV (TPCp) via chemical or
electrochemical reduction.
electron acceptor in organic photovoltaic cells. Applications of
hepta- and triafulvene-derived compounds have been inves-
tigated much less comprehensively. Heptafulvalenes can serve
as chemical sensors that exhibit turn-on fluorescence.37,38
However, the investigation of hepta- and triafulvenes as
electron-donating (p-type) materials seems limited by their
lower redox stability. For instance, dibenzo[a,d]heptafulvenes
(3, Figure 1B) exhibit only irreversible oxidation between +0.1
V (R = NMe2)39 and +0.85 V (R = H).40 A reversible first
INTRODUCTION
■
The chemistry of tria-, penta-, and heptafulvenes1−15 and
fulvalenes4,11,16−21 (Figure 1A), their synthetic preparation,
and the study of their physical properties have been the subject
of continuous interest throughout the past decades. These
nonalternant hydrocarbons do not exhibit classic Huckel
̈
aromaticity but can form ionic species upon reduction
(pentafulvenes) or oxidation (tria- and heptafulvenes) that
experience substantial aromatic stabilization via cyclic deloc-
alization. Pentafulvenes in particular have attracted growing
interest due to their potential use as electron accepting (n-
type) component in organic electronic applications. Extensive
efforts have been made to broaden their synthetic accessi-
bility22−26 and to tailor their optical absorption properties and
accessible redox potentials. For instance, the electron affinities
of the 6,6-dicyanopentafulvenes (2, Figure 1B)27−32 can be
broadly varied via the substitution pattern on the five-
membered ring, while the compounds can be reversibly
reduced at electrochemical potentials in the suitable range for
organic n-type acceptor materials of −3.5 to −4.0 eV.33 The
potentialities of this class of substances were demonstrated by
Wudl,34 who reported the benzannulated pentafulvalene 1
oxidation can be observed for tetraphenyltriafulvene (4, Eox
=
+0.3 V, Figure 1B),41 while donor-stabilized 2,3-diamino-
triafulvenes are much more redox stable, but at the expense of
a significantly lower oxidation potential (Eox ≈ −0.14 V42,43).
Because of the variable electronic properties cited above,
polyfulvenes linked via carbon atoms of the ring have been
proposed as a versatile class of electronic materials.44 However,
none of the suggested polymers could so far be prepared. In
this paper, we report the first examples of compounds that
formally represent polyfulvenes linked via the exocyclic fulvene
double bond. To achieve this goal, we aimed at combining the
electronic properties of fulvenes with those polyisocya-
nides45−48 (PIC, Figure 1C,D).
(Figure 1B). 1 can be reversibly reduced to its radical anion
red1
1−• at E
≈ −1.2 V, relative to the ferrocene/ferrocenium
Received: May 8, 2018
Revised: June 26, 2018
1/2
redox couple (FcH/FcH+),35,36 corresponding to an electron
affinity of ca. −3.9 eV, and could be successfully employed as
© XXXX American Chemical Society
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Figure 2. Examples for the redox response in a (A) pentafulvenyl- and
(B) heptafulvenyl-functionalized PIC.
of the cross-conjugated polymer backbone into a more
effectively conjugated polyacetylene-like system, which should
exhibit a reduced optical gap.62−65 This concept would be
applicable to electron-accepting pentafulvenes (Figure 2A) and
electron donating tria- or heptafulvenes (Figure 2B). This
would make a class of electrochromic polymers available that
cover a broad range of redox potentials and may be employed
as cathode/anode materials in organic polymer batteries
(OPBs)66,67 and electrochromic applications.68−72
Figure 1. (A) Basic structures of fulvenes and fulvalenes. (B)
Examples for fulvalenes (1) and fulvenes (2−4) and their
corresponding reduction and oxidation potentials relative to FcH/
FcH+. (r)/(i): reversible/irreversible process. (C) Isocyanides and
polyisocyanides. (D) Polymer structures targeted herein.
PICs73−75 and polyacetylenes62 equipped with redox-active
functional groups, which showed reversible electrochromic
behavior, have been reported in the past. In these cases,
however, the redox-active functional groups were not
conjugated to the polymer backbone, and the accessible
redox potentials were limited to those of the introduced
ferrocenyl73,62 and tetrathiafulvalenyl groups.74,75
PICs have been investigated extensively since the 1970s45−48
but have attracted renewed interest with the recent develop-
ment of new polymerization catalysts that allow the living
polymerization of isocyanides.49−52 This has opened an access
to the preparation of a variety of block copolymers including
PICs as coblocks to conjugated polymers.53−59 Still, reports of
PICs as electronic materials are comparatively rare and focus
on the introduction of functional side chains, rather on taking
advantage of the electronic properties of the polymer
backbone. The backbone of PICs consists of consecutive sp2-
hybridized imine-carbon atoms. However, the polymer
structure is such that the carbon atoms within the main
chain are linked by single bonds and are therefore cross-
conjugated. As a consequence, electronic effects typically
associated with extended conjugation are not observed, as
evidenced by comparatively large optical bandgaps of ca. 2.5
eV for poly(arylisocyanide)s.60,61 However, reduction or
oxidation of PICs bearing conjugated fulvenyl side-chains
should lead to charged rings in the side-chains stabilized by
aromatic delocalization and to concurrent redistribution of the
π-electrons in the conjugated linker and the polymer backbone
(Figure 2A,B). This redistribution constitutes a transformation
The chemistry of isocyanides is well-developed, since alkyl
and aryl isocyanides are commonly employed in organic
syntheses76−79 and as ligands in organometallic coordination
chemistry.80−83 The latter field is attracting growing attention
due to emerging new applications of isocyano complexes as
catalysts for hydration84 and hydroarylation85,86 and as
luminophores and photosensitizers in photocatalysis87−95 and
materials chemistry.96
However, comparatively few examples are known, wherein
isocyanide groups are conjugated to more extended π-systems.
Examples of α,β-unsaturated isocyanides97−104 and very
sensitive isocyanoimines105,106 have been reported as well as
diisocyano-oligophenylenes.87−95,107,108 The limited number of
examples prompted the questions of whether the targeted
monomers would be synthetically accessible and whether
subsequent polymerization would leave the fulvene moieties
intact. In the following, we summarize our results on the
preparation and study of a series of isocyanide monomers and
the corresponding polymers.
B
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a
Scheme 1. (A) Synthetic Route for the Preparation of Isocyanide Monomers and Polyisocyanides and (B) Introduced
Fulvenyl Groups
a
Reagents and conditions: (i) base, toluene/THF, −35 °C → RT;(ii) SnCl2, HCl, 70 °C; (iii) CHCl3, KOH, EtOH, [Et3NBz]+Cl, DCM, RT; (iv)
4-nitrophenylformate, THF, RT; (v) POCl3, DIPA, DCM; (vi) Ni[o-anisyl(dppe)]Br, toluene, RT.
RESULTS AND DISCUSSION
■
Synthesis. To explore this chemistry, we endeavored to
prepare the polyisocyanides 12a−e depicted in Scheme 1B (a:
Rm = 9-fluorenyl, Flu; b: Rm = 5-(1,2,3,4-tetraphenyl)-
cyclopentadienyl, TPCp; c: Rm = 5-dibenzo[a,d]cycloheptenyl
= dibenzo[a,d]suberenyl, Dbs; d: Rm = (2,5-dimethyl-3,4-
diphenyl)cyclopentadienyl, MPCp; e: Rm = 2,3-dimesityl-
cyclopropenyl, Cyp) that feature redox-active fulvenyl groups
conjugated to the polymer backbone via a p-phenylene linker.
The corresponding isocyanide monomers (11) were
synthesized from p-nitrophenyl-substituted fulvenes (Scheme
1, 8a−d), as outlined in Scheme 1. Fulvene 8a was prepared by
Wittig coupling of the 9-fluorenyltriphenylphosphonium salt
Figure 3. (A) Crystal structure and valence structure of dimer 6d2,
2R,3R,3aS,4S,7R,7aR-diastereomer shown. (B1, B2) Crystal structure
of 2,3-dimesitylcyclopropenone (top-down and side view). Ellipsoids
7a with p-nitrobenzaldehyde. This approach proved imprac-
tical, however, for the preparation of the fulvenes 8b−d, which
were instead synthesized by Wittig coupling of a tri(n-
butyl)phosphonium salt (5) to 2,3,4,5-tetraphenyl-
cyclopentadienone (6b), dibenzo[a,d]cyclohepten-5-one
(6c), and 2,5-dimethyl-3,4-diphenylcyclopentadienone (6d),
respectively. The use of electron-rich trialkylphosphonium salt
5 proved essential to effect conversion of these less reactive
and sterically hindered ketones.109 Interestingly, 6d readily
dimerizes to 6d2 (Figure 3A) via [4 + 2]-cycloaddition but is in
equilibrium with its monomer31,110 and can therefore still be
converted into the fulvene 8d. However, subsequent reduction
of 8d to the corresponding aniline 9d failed under various
conditions, and the synthesis of the corresponding isocyanide
11d was therefore not completed. Similarly, Wittig coupling of
2,3-dimesitylcyclopropenone (6e) with 5 furnished only trace
amounts of the triafulvene 8e, which could not be isolated.
Nevertheless, single crystals suitable for X-ray diffraction could
be obtained of both the dimerization product 6d2 (Figure 3A)
and of the cyclopropenone 6e (Figure 3B1,B2). Crystal
drawn at 55% probability level. Hydrogen atoms have been omitted
for clarity.
structures were also obtained for the successfully prepared p-
nitrophenylfulvenes 8a−d and will be discussed in a
subsequent section. The corresponding structural data has
been enclosed in the Supporting Information.
In the next step, the p-nitro groups in 8a−c were reduced
with SnCl2/HCl to give the corresponding anilines, from
which the isocyanides were then synthesized via two different
methods: Direct carbamylation77 of the anilines with chloro-
form and concentrated sodium hydroxide solution was possible
under phase transfer conditions. However, while this
procedure provided high yields it proved difficult to reproduce
and to scale up. Alternatively, formylation of the anilines with
the p-nitrophenylformate111 allowed to isolate the correspond-
ing N-formamides 10a−c in satisfying yields of 60−70%.
Subsequent dehydration with POCl3 and diisopropyl-
C
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ethylamine (DIPA) then readily furnished the isocyanides
11a−c in yields of 75−95%.
absorption compared to analogous but mismatched donor−
donor (D−D) or acceptor−acceptor (A−A) systems.
The intermediates and the isocyanide monomers 11a−c
were characterized by 1H NMR, 13C NMR, and high-resolution
mass spectrometry. The isocyanides 11a, 11b, and 11c were
unambiguously identified by their 13C NMR resonances of the
isocyanide carbon atom at 164.9, 165.3, and 162.8 ppm and
their characteristic infrared absorption bands at 2119, 2118,
and 2121 cm−1, respectively (see Figures S36, S39, and S41).
Furthermore, reaction of the monomers with chloro(dimethyl
sulfide)gold(I) (Me2S·Au(I)Cl) led to a signal shift of the
isocyano bands in the infrared spectra to 2213, 2212, and 2214
cm−1 (see Figures S36, S39, and S41), respectively, within the
typical range for gold(I) complexes of isocyanides.112,113
Structure and Properties of Conjugated Fulvenes. In
order to further elucidate the effects the variation of the
fulvenyl moiety and the conjugated functional group (−NO2,
−NH2, −NHCHO, and −NC) on the electronic properties
of the conjugated systems, the precursors and monomers have
been investigated by UV−vis spectroscopy and electrochemical
methods.
The UV−vis spectra of the fulvenes show absorption
maxima in the range of 325−406 nm that largely reflect the
properties of the respective combination of functional groups
(Table 1 and Figure 5; see also Figures S1−S11). The trend is
most pronounced in the amino-substituted derivatives 9a−c
and in the amides 10a−c. The aniline 9b, featuring the
strongest pentafulvene acceptor, exhibits the longest wave-
length absorption with a maximum at λmax = 406 nm, and the
lowest optical gap of Egopt = 2.56 eV, followed by 9a (λmax
=
372 nm, Egopt = 2.84 eV). 9c exhibits a wider optical gap and a
blue-shifted λmax (λmax = 325 nm (sh), Egopt = 3.35 eV; for
deconvolution see Figure S9), which can be attributed to the
donor−donor combination in this system. The amides show an
analogous trend, with the TPCp derivative 10b (λmax = 364
nm, Egopt = 2.95 eV) exhibiting the most red-shifted absorption
and the lowest optical gap, followed by the fluorenyl derivative
10a (λmax = 344 nm, Egopt = 3.07 eV). The Dbs-functionalized
10c (λmax = 318 nm (sh), Egopt = 3.48 eV, for deconvolution
see Figure S10) again shows a widened optical gap. The blue-
shifted absorption and the wider optical gaps of the amides
directly reflect the reduced donor strength of the N-
formylamido compared to the amino groups.114,115 The same
trend of the optical gaps and longest wavelength absorption
maxima is observed for the monomers 11a−c (11a: λmax = 336
nm, Egopt = 3.04 eV; 11b: λmax = 348 nm, Egopt = 2.82 eV; 11c:
λmax = 320 nm (sh), and Egopt = 3.30 eV; for deconvolution see
Figure S11). The low optical gap of 11b and the large gap of
11c are somewhat curious, however, since the isocyano group
is a relatively strong acceptor, comparable to an acyl
group,114,115 and 11b and 11c therefore constitute A−A and
D−A systems, respectively. We tentatively attribute this
observation to the optical absorption in 11b being dominated
by the properties of the fulvene moiety, while steric strain
hinders effective conjugation in 11c. The strain in Dbs
derivatives like 11c is evident from the crystal structure of 8c,
wherein the nitrophenyl moiety is twisted out of the ring plane,
even though an even stronger acceptor is present in this
compound than in 11c (see Figure 8C1,C2 and associated
As exemplified in Figure 4, conjugation in the neutral
fulvene structure without formal charges (R0, Figure 4, center)
Figure 4. Exemplary resonance structures. FG = functional group:
−NO2, −NH2, −NHCHO, and −NC.
can lead to charge redistribution toward the fulvene moiety (R0
↔ R−) or toward the functional group (R0 ↔ R+). This effect
is expected to be most pronounced in donor−acceptor (D−A)
systems and should lead to a more red-shifted optical
Table 1. Physicochemical Data of Substituted Fulvenes
UV−vis absorption
electrochemical properties
b
c
d
mp [°C]
λ2 [nm]
λmax
λonset/Eogpt [nm]/[eV]
E1/2 [V]
Ep [V]
8a
8b
8c
8d
9a
9b
9c
10a
10b
10c
11a
11b
11c
167.6−168.8
227.7−229.3
152.3−153.1
185.9−187.4
oil
116.7−119.6
58.8−60.5
182.6−186.5
190.7−193.5
115.4−117.2
123.4−125.2
259.8−269.7 (dec)
152.2−160.4 (dec)
359
352
427
415
422
415
436
484
370
404
420
356
408
440
376
2.90
2.99
2.94
2.99
2.84
2.56
3.35
3.07
2.95
3.48
3.04
2.82
3.30
−1.43, −1.68
−1.52
−1.75
267
289
269
−1.51, −1.59
−1.79
−1.52, −1.68
a
a
360
341
372
406
325
344
364
318
336
348
320
−1.50, −1.70
265
298
a
a
a
a
e
+0.37, +0.52
261
292
a
−1.93, −2.17
−1.85, −2.28
+0.84, +0.97, +1.14
−1.90, −2.29
+0.80, +0.91, +1.11
a
e
e
292
a
b
Shoulder band, λmax derived via Gaussian deconvolution of the absorption spectrum. Derived from the absorption onset of spectra recorded in
c
CH2Cl2 solution. Half-potentials of reversible redox processes determined via cyclic voltammetry (CV) in THF (reduction) or CH2Cl2
(oxidation) solution with 0.1 M [NnBu4][PF6] as supporting electrolyte. Peak potentials determined via square-wave voltammetry (SWV) in THF
(reduction) or CH2Cl2 (oxidation) solution with 0.1 M [NnBu4][PF6]. Peak potentials of irreversible oxidation.
d
e
D
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Figure 5. UV−vis absorption spectra of isocyanide monomers and the corresponding precursors. Recorded in CH2Cl2 solution.
Figure 6. Cyclic voltammograms of fulvenes and polyisocyanides. (A) 8a, 11a, and 12a. (B) 8b, 11b, and 12b. (C) 8c, 11c, and 12c. Black line:
−NO2; red line: −NC; blue line: PIC.
comments). The optical properties of the nitro derivatives are
similarly ambiguous. A−A-substituted fulvenes 8b and 8d
exhibit comparatively large optical gaps of Egopt = 2.99 eV each
(λonset = 415 nm), whereas for 8a (λonset = 427 nm, Egopt = 2.90
eV) and 8c (λonset = 422 nm, Egopt = 2.94 eV) lower optical
gaps are observed. Seemingly, the properties of 8a are
dominated by the nitro group, while the formally electron-
donating Dbs group is again not effectively conjugated to the
acceptor.
Overall, we can say that the optical properties reflect
electronic communication between the fulvenyl moiety and the
peripheral functional groups, particularly when electron donors
are present and donor−acceptor systems are created.
All compounds were nonfluorescent with the exception of
10b and 11b, which showed weak fluorescence in solution with
emission maxima at 431 and 470 nm, respectively (see Figure
S5).
Analyses by cyclic voltammetry shows the p-nitrophenyl-
fulvenes 8a−d to undergo 2-fold reversible or quasi-reversible
electrochemical reduction to the radical anion and dianion,
respectively (Figure 6). The first reduction of 8a occurs with a
half-potential of E = −1.43 V vs FcH/FcH+, followed by a
red1
1/2
red2
1/2
second reduction at E
= −1.68 V. For 8b one broad
red1
reduction wave centered on E
= −1.52 V was observed,
1/2
which could be resolved by square wave voltammetry (SWV)
into two individual reductions at Erped1 = −1.51 V and Erped2
=
−1.59 V (see Figure S27). 8d exhibited similar properties with
red1
1/2
red2
1/2
E
= −1.50 V and E
= −1.70 V (see Figures S30 and
S31). The donor-substituted 8c showed only a broad, quasi-
reversible reduction wave with an onset at ca. −1.45 V, which
E
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Figure 7. (A) Resonance in p-nitrophenylfulvenes 8a−d upon electrochemical reduction. (B) Numbering of solid-state structures of 8a−d.
Figure 8. Crystal structures of p-nitrophenylfulvenes. Top-down and side views of 8a (A1, A2), 8b (B1, B2), 8c (C1, C2), and 8d (D1, D2).
Ellipsoids drawn at 55% probability level. Hydrogen atoms have been omitted for clarity.
Table 2. Structural Data for p-Nitrophenylfulvenes (Numbering According to Figure 7B)
XRD
C5−C6 [Å] C4−C5 [Å] C1−N [Å] C3/C3′−C4−C5−C6 [deg] C4−C5−C6−C7/C7′ [deg] C5−C6/ring plane [deg]
8a
8b
8c
8d
P21/c, Z = 4
P21/c, Z = 4
P21/c, Z = 4
P21/c, Z = 4
1.349(1)
1.350(3)
1.340(2)
1.356(1)
1.517(1)
1.476(3)
1.474(2)
1.468(1)
1.511(1)
1.462(3)
1.468(2)
1.469(1)
59.9(1)/−122.8(1)
52.1(2)/−128.5(2)
30.9(2)/−151.3(1)
34.4(2)/−149.8(1)
5.2(2)/−175.7(1)
7.8(3)/−173.9(2)
5.9(2)/−178.6(1)
5.4(2)/−179.6(1)
1.4(1)
0.4(1)
50.6(1)(1)
4.1(1)
could not be resolved further by SWV (Erped = −1.79 V, see
Figure S28). No further reduction processes were observed
down to −2.5 V. The first reductions generally occur close to
the reduction potential of common nitroarenes (4-nitro-
biphenyl: −1.43 V/−2.00 V; 4-nitro-1,2,3,6,7,8-hexahydropyr-
ene: −1.62 V/−2.13 V116), while the potential of second
reduction scales with the acceptor strength of the fulvene
moiety. We can therefore conclude that the first electron
reduction mainly involves the nitrophenyl ring, while the
second reduction is governed by the fulvene moiety (Figure
7A).
For the isocyanides 11a and 11b two steps of irreversible
electrochemical reduction could also be observed (Figure 6).
In agreement with the weaker electron-withdrawing effect of
the isocyanide group,114,115 the first and second reduction
potentials each appeared shifted by 0.40−0.45 V to lower
potential compared to the nitro compounds.
Electrochemical oxidation potentials of 9c and 11c were also
measured. For the electron-rich aniline 9c two oxidations with
peak potentials at +0.37 and +0.52 V vs FcH/FcH+ were
observed, while monomer 11c exhibited three irreversible
oxidation processes at higher potentials (Eopx = +0.84, +0.97,
and +1.14 V vs FcH/FcH+), in agreement with the electron-
withdrawing nature of the isocyano group.
The available crystal data on the series of p-nitro-
phenylfulvenes 8a−d (Figure 8 and Table 2; see Figure 7B
for numbering), enabled us to evaluate the structural effects of
the conjugation between the nitro group and the fulvene
moieties. The most relevant parameter in this respect is the
fulvene bond (C5−C6). For 8a, 8b, and 8d these bonds are of
comparable length (8a: 1.349(1) Å; 8b: 1.350(3) Å; 8d:
1.356(1) Å) and indicate no significant weakening of the bond
through resonance, in agreement with the acceptor−acceptor
character of these systems. However, the C5−C6 bond in the
Dbs-containing fulvene 8c (1.340(2) Å) is marginally shorter
than in the pentafulveneseven though resonance should be
more effective in this donor−acceptor system. The reason lies
in the deformation of the central ring in 8c into a boat
conformation that leads to the C5−C6 fulvene bond being
twisted out of the mean plane of the seven-membered ring by
50.6(1)° (C2, Figure 8). Similar deformations have been
previously observed in 4-methylenedibenzo[a,d]-
cycloheptenes,117−119 even in the presence of strong acceptors
in the periphery,120 which indicates that the fulvene bond is
not effectively conjugated to the Dbs unit. In contrast, the
fulvene moieties in 8a, 8b, and 8d are close to coplanar
(<4.1(1)°), thus enabling effective conjugation with peripheral
substituents. Unfortunately, the lack of effective conjugation in
Dbs-based systems indicates that this molecular structure is not
ideal for the applications targeted herein. Instead, triafulvenes
or non-benzannulated heptafulvenes may be better suited as p-
type materials.
Polymer Syntheses and Properties. Polymers 12a−c
were prepared from solutions of the monomers in toluene
F
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(11b,c) or in a toluene/THF solvent mixture (11a) by stirring
overnight at room temperature in the presence of (o-
anisyl)Ni(II)(dppe) bromide (dppe: 1,2-bisdiphenyl-
phosphinoethane).121 The polymers were isolated and purified
by precipitation into hexanes, followed by centrifugation,
which furnished orange (12a), brown (12b), and yellow (12c)
powders. The polymers dissolved well in polar, halogenated,
and aromatic organic solvents such as THF, dichloromethane,
and toluene but remained insoluble e.g. in diethyl ether and
acetonitrile.
Thermal analysis via thermal gravimetry (TGA) showed
them to be thermally robust, with decomposition temperatures
(Td) of 345 °C for 12c and 365 and 370 °C for 12b and 12a,
respectively (see Figure S44). Differential scanning calorimetry
showed no melting or phase transitions prior to decom-
position.
the molecular weights derived from GPC. We attribute this
difference to fragmentation of the high-molecular-weight
chains upon ionization, which is not unusual in MALDI-
MS,126,127 and more facile ionization of these lower molecular
weight fragments. This assumption is corroborated by data of a
reference sample of poly(p-methoxyphenylisocyanide) (Mn =
5.3 kDa, PDI = 1.45), which showed a signal distribution that
matched the GPC data (see Figure S35 and associated
comments in the Supporting Information).
Information about the polymer chain ends could not be
derived from the available data due to the lower resolution of
linear mode MALDI-TOF-MS and also due to the large
number of possible end-groups that are generated by different
initiation pathways, fragmentation, and partial hydrolyses of
individual imine bonds.
Optical and Electrochemical Properties. Compared to
the monomers (11a: λmax = 336 nm; 11b: λmax = 348 nm; 11c:
λmax = 400 nm, shoulder band), the polymers 12a−c exhibit
moderately red-shifted absorption maxima, with λmax values in
solution of 344 nm (12a), 368 nm (12b), and 400 nm (12c,
shoulder band) (Figure 9) and also show only a slight
The formation of polyisocyanides could be confirmed by IR
spectroscopy, which showed the typical CN stretching
bands around v
̃
= 1600 cm−1 (see Figures S37, S40, and
S42).122−124 The polymers exhibited number-average molecule
weights of Mn = 136−124 kDa, as determined via gel
permeation chromatography in THF, relative to polystyrene
standards, and showed polydispersities of 2.0−2.7 (see Figure
S32). The polymers were found to be stable in air both as
solids and as spin-coated films as well as in dry solvents under
an inert atmosphere. Slow decomposition occurred, however,
in solutions exposed to air and moisture. We attribute this to
partial hydrolyses of the imine groups on the polyisocyanides
backbone, as indicated in Scheme 2, which is signified by the
emergence of a carbonyl stretching vibration in IR spectra (see
Figure S38 and associated comments in the Supporting
Information).
Scheme 2. Partial Hydrolysis of a Polyisocyanide
Figure 9. UV−vis absorption spectra of polyisocyanides in CH2Cl2
solution and as thin films.
Mass Spectrometry. Given that polyisocyanides are
rodlike polymers, the molecular weights derived from GPC
most likely overestimate the actual molecular weights of the
material. To further corroborate the polymer formation, 12a
and 12b were therefore also analyzed by matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass
spectrometry (see Figures S33 and S34), which has only
rarely been employed to characterize polyisocyanides.125 In
both cases, signal series up to ca. 8 kDa, which correspond to
more than 25 and 15 repeat units for 12a and 12b,
respectively, could be observed via linear mode MALDI-
TOF MS. The observed mass range is significantly lower than
bathochromic shift, when moving from solution to thin films.
However, both in solution and in the thin film, the absorption
falls off gradually toward longer wavelengths, leading to
significantly lowered optical gaps of 2.38 eV (12a), 2.27 eV
(12b), and 2.40 eV (12c). In the case of 12c only a single
broad absorption band centered on 400 nm can be discerned.
However, for 12a and 12b the origin of these tailings can be
linked to weaker underlying absorption bands centered on 420
nm (12a) and 500 nm (12b), respectively, which have been
Table 3. Physicochemical Properties of Polyisocyanides 12a−c
c
UV−vis absorption
electrochemical properties
a
d
e
sol
max
sol
onset
film
max
film
Mn [kDa]
DPn
PDI
Td
λ
[nm]
λ
[nm]
λ
[nm]
λ [nm]
onset
Eogpt [nm]
ECp V [V]
ESpWV [V]
b
12a
12b
12c
136
132
124
487
273
407
2.7
2.0
2.7
370
365
345
344
368
400
494
520
498
347 (420 )
520
560
517
2.38
2.27
2.40
−2.44
−1.55, −2.03
+1.02
b
381 (500 )
−1.50, −1.91
b
b
400
a
b
Decomposition temperature determined by TGA. Longest wavelength absorption band derived via Gaussian deconvolution of the absorption
c
d
spectra. Recorded as dip-coated film on a platinum electrode vs 0.1 M [NnBu4][PF6] in NCMe solution. Peak potentials observed in cyclic
voltammetry. Peak potentials observed in square-wave voltammetry.
e
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Figure 10. Spectroelectrochemical response of polyisocyanides to electrochemical reduction: (A) 12a and (B) 12b.
determined by Gaussian deconvolution of the thin film
absorption spectra (see Figures S12−S17). Overall, the optical
gaps of the polymers are smaller than in typical poly-
(arylisocyanide)s, while the absorption maxima are observed
in a similar range (e.g., poly(4-methoxyisocyanide): λmax = 360
nm, λonset = 490 nm, and Eogpt = 2.53 eV;128 see Figure S18).
Since the absorption maxima of the polymers lie in the same
range as those of the molecular precursors, we attribute the
observed main absorption bands to individual repeat units and
functional groups. The significantly lowered optical gaps of the
polymers compared to the monomers indicate extended
conjugation in the polymers. However, analyses of polymer
batches with different molecular weights showed no correlation
between the degree of polymerization and the absorption
spectra of the polymers, even for low-MW batches with DPn <
10 (see Figures S2, S4, and S7). Conjugation along the
polymer backbone therefore does not seem to substantially
contribute to the overall absorption, likely because steric
crowding of the PIC backbone leads to helical torsion angles
between adjacent imine-groups (>35°).45−47 Notably, PICs
can also adopt a syndio-conformation129 wherein imine dyads
assume a coplanar geometry (R−NC−CN−R: ≈180°),
while being rotated by ca. 90° relative to neighboring imine
groups. We therefore tentatively attribute the lowered optical
gaps to conjugation that largely involves adjacent repeat units,
rather than extended conjugation along the polymer backbone.
Electrochemical characterization of the polymers also
reflected the different peripheral substituents (Figure 6):
Thin films of 12a showed irreversible reduction with an onset
at −2.20 V vs FcH/FcH+. The reduction does not result in
decomposition of the polymer, however, since the film remains
redox-active and only gradually degrades over multiple redox
cycles (see Figure S29). 12b showed a broad quasi-reversible
reduction with an onset at −1.30 V vs FcH/FcH+, while for
12c only irreversible oxidation was observed to set in at +0.65
V vs FcH/FcH+. The cathodic shift of the reduction potentials
of 12a and 12b, relative to the monomers, may be attributable
to the lower electron-withdrawing character of the poly(imine)
backbone compared to the isocyano group.114,115 The
irreversibility of the oxidation of 12c may be due to
detachment of the polymer film from the electrode or due to
degradation of the polymer. The latter is more likely, however,
since its precursors also did not exhibit chemically reversible
redox behavior. Notably, the reduction potential of 12b
corresponds to a LUMO level of ca. −3.8 eV, which renders
this polymer suitable for use e.g. n-type acceptor in organic
photovoltaics.35,36
Spectroelectrochemical Analyses. Nevertheless, the
quasi-reversible redox behavior of 12a and 12b showed that
these materials can serve as electrochromic materials. We
therefore also studied the changes in their absorption spectra
upon chemical (12aredC/12bredC) and electrochemical
(12aredE/12bredE) reduction (see the Supporting Information
for experimental details).
12a showed a strong electrochromic response with the
emergence of a broad band centered on 700 nm, accompanied
by a shift of the absorption onset from 550 nm (Eogpt(12a) =
2.25 eV)130 to ca. 1030 nm (Egopt(12aredE) = 1.20 eV, Figure
10A). The absorption change sets in at −1.50 V vs FcH/FcH+
and was found to be largely reversible, since reversal of the
applied voltage lead to disappearance of the long wavelength
band (see Figure S20). Partial degradation of the materials
occurred, however, since the original baseline could not be
fully recovered. We attribute this to the gradual degradation of
the polymer, as observed in cyclic voltammetry experiments
(see Figure S29). Virtually full reversibility was observed for
polymer 12b (Figure 10B; see also Figures S21 and S22).
Here, in agreement with the high electron affinity of the TPCp
group, the electrochromic response already sets in at ca. −1.20
V vs FcH/FcH+. A weakening of the main absorption band at
390 nm was observed, while two weaker bands centered on
480 and 540 nm emerge. The absorption onset shifts
accordingly from 560 nm (Eogpt(12b) = 2.21 eV)130 to ca.
800 nm (Eogpt(12bredE) = 1.55 eV). Chemical reduction lead to
similar results (Figures S23−S26): The absorption onset of
12a shifted to ca. 900 nm (Eogpt(12aredC) = 1.38 eV), and the
one of 12b shifted to ≈1100 nm (Eogpt(12bredC) = 1.13 eV). We
attribute the different absorption shifts compared to electro-
chemical reduction to presence of stronger coordinating metal
cations, instead of the noncoordinating nBu4N+ counterions in
the electrochemical experiments. Exposure of the chemically
reduced polymers to air then resulted in dedoping (Figures
S24 and S26) but lead to only partial recovery of the baseline,
likely due to the harsher reaction conditions in these
experiments and concurrent decomposition of the polymers.
The decrease of the optical gaps in electrochemical and
chemical reductions of −1.05 eV (12a → 12aredE) and −0.87
eV (12a → 12aredC), respectively, for 12a and of −0.66 eV
H
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(12b → 12bredE) and −1.08 eV (12b → 12bredC), respectively,
for 12b corroborates the concept laid out in the Introduction
(Figure 2). However, whether newly emerging absorption
bands originate from the switch of the electronic configuration
in the main chain or from the charged side-chains could not be
ascertained at this point.
ACKNOWLEDGMENTS
■
We thank Bernhard Mueller for support with X-ray data
collection.
ABBREVIATIONS
■
Flu, 9-fluorenyl; TPCp, 5-(1,2,3,4-tetraphenyl)-
cyclopentadienyl; Dbs, 5-(dibenzo[a,d])cycloheptenyl, also
referred to as 5-dibenzo[a,d]suberenyl; MPCp, 5-(1,4-
dimethyl-2,3-diphenyl)cyclopentadienyl; Cyp, 2,3-dimesityl-
cyclopropenyl; GPC, gel permeation chromatography; TGA,
thermal gravimetry; CV, cyclic voltammetry; SWV, square-
wave voltammetry.
CONCLUSION
■
In summary, we have prepared a series of penta- and
heptafulvene derivatives bearing conjugated nitro, amino, N-
formylamino, and isocyanophenyl groups and characterized
their optical, electrochemical, and structural properties. These
analyses showed a clear correlation of the electronic properties
between the introduced (electron-donating or -accepting)
fulvene moiety and the respective functional group. Fur-
thermore, we polymerized the respective fluorenyl-,
tetraphenylcyclopentadienyl-, and dibenzo[a,d]cycloheptenyl-
functionalized isocyanides (11a−c) to the corresponding
polyisocyanides (12a−c). The polymers 12a−c were shown
to be chemically robust materials with optical, electrochemical,
and electrochromic properties governed by the redox-active
fulvenyl groups. Furthermore, the tetraphenylcyclopentadienyl-
functionalized polymer 12b exhibited a LUMO level of ca.
−3.8 eV and may therefore serve as n-type material in a range
of organic electronic applications. Most importantly, however,
the large-bandgap polymers 12a and 12b (Eogpt = 2.38 and 2.27
eV) exhibit a largely reversible electrochromic response that
allows the reduction of their optical gap by ΔEogpt ≈ −0.7 to
−1.1 eV, depending on the experimental conditions. The
absorption properties of these polymers can therefore be
switched from largely transparent to opaque across the whole
visible range.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b00977.
Additional analytical data, detailed experimental proce-
dures, copies of spectra, and crystallographic data (PDF)
(10) Jayamurugan, G.; Dumele, O.; Gisselbrecht, J.-P.; Boudon, C.;
Schweizer, W. B.; Bernet, B.; Diederich, F. Expanding the Chemical
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Tetracyanofulvene with Electroch-Rich Alkynes. J. Am. Chem. Soc.
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AUTHOR INFORMATION
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Corresponding Author
*(F.P.) E-mail: frank.pammer@uni-ulm.de.
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ORCID
Frank Pammer: 0000-0002-3869-0196
Funding
The authors thank the German Chemical Industry Fund (FCI,
Liebig Scholarship to F.P.) and the German Science
Foundation (DFG) for financial support.
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Notes
The authors declare no competing financial interest.
I
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and is evident from the nonzero baseline at wavelength > 600 nm.
Further baseline correction was not possible without severe distortion
of the spectra.
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