Dithiafulvenyl-substituted phenylacetylene derivatives: Synthesis and structure-property-reactivity relationships

Article abstract of DOI:10.1039/c5ob01651g

A series of regioisomers for dithiafulvenyl-substituted phenylacetylene derivatives was synthesized and characterized to show structure-dependent electronic properties and different reactivities in their oxidized states.

Full text of DOI:10.1039/c5ob01651g

Organic &
Biomolecular Chemistry
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Dithiafulvenyl-substituted phenylacetylene
derivatives: synthesis and structure–property–
reactivity relationships†
Cite this: DOI: 10.1039/c5ob01651g
Received 6th August 2015,
Accepted 21st August 2015
DOI: 10.1039/c5ob01651g
Yunfei Wang and Yuming Zhao*
www.rsc.org/obc
A series of regioisomers for dithiafulvenyl-substituted phenyl-
acetylene derivatives was synthesized and characterized to show
structure-dependent electronic properties and different reactivities
in their oxidized states.
The application of π-conjugated materials has achieved tre-
mendous advancement in the cutting-edge fields such as
molecular electronics, chemo-/bio-sensors, light-emitting
Fig. 1 Structures of acetylenic DTFs 1a–c.
1
–3
devices, and organic photovoltaics.
From the chemistry
perspective, continued efforts to explore π-molecular building
blocks with novel properties and functions are still imperative
and highly needed. For decades, tetrathiafulvalene (TTF)—the
first organic conductor—has played an indispensable and
1
8–21,23
alkynyl group.
Our previous study on 1a naturally led us
to ponder on its two other regioisomers, 1b and 1c (Fig. 1). As
building blocks, acetylenic DTF isomers 1a–c would give rise
to π-conjugated materials with different structures and pro-
perties as a result of their different molecular shapes and
π-conjugation patterns. In this context, it is of fundamental
importance to conduct comparative studies on the properties
and reactivities of 1a–c as well as on the π-extended systems
derived from them.
The synthesis of DTFs 1a–c was executed through a phos-
phite-promoted olefination method,
silylethynyl-substituted benzaldehydes 2a–c and thione 3 as
the starting materials (Scheme 1). Experimentally, the three
olefination reactions were completed in 3 hours at 105 °C,
giving the desired products 1a–c with similar yields
4
central role in the area of molecular electronics. In modern
TTF chemistry, a myriad of π-extended TTF analogues (i.e.,
exTTF) have been synthesized and many of them were found
to show intriguing redox, optoelectronic, and supramolecular
5
–8
properties.
Among numerous types of exTTF, the so-called
9
,10
tetrathiafulvalene vinylogues (TTFVs)
pursued by us and others
have been actively
over the past few years. A par-
1
1
12–14
ticularly appealing aspect of TTFVs, especially aryl-substituted
TTFVs, is its redox-controllable conformational switching be-
2
4,25
using trimethyl-
1
1,14,15
haviour.
cular systems have been prepared and reported recently,
Built upon aryl-TTFVs, various functional mole-
1
6,17
including stimuli-responsive conjugated polymers,
rescent molecular tweezers,
persistent macrocycles,
our previous study on certain alkynylated TTFV systems
derived from phenylacetylene-substituted dithiafulvene
DTF) precursor 1a (Fig. 1).
fluo-
1
8,19
12
redox-active ligands, shape-
and molecular rotors. Of note is
(
ca. 75–79%). Compounds 1a–c were then subjected to an oxi-
dative coupling in CH Cl at room temperature using iodine
2 2
2
0,21
22
2
6
as an oxidant (Scheme 1). The reactions yielded acetylenic
diphenyl-TTFVs 3a–c in very good yields. Note that in our pre-
vious work, certain ortho-substituents (Br and Me) retarded the
deprotonation step in the oxidative dimerization mechanism,
leading to the formation of a unique bis-spiro structure other
than the typical TTFV product.
not happen in the oxidative dimerization of ortho-alkynyl DTF
c, because the alkynyl group is linear-shaped and much less
bulky.
Besides the oxidative dimerization reactions, the acetylenic
a
2
0,21
(
Herein the inclusion of the
alkynyl group in this DTF compound greatly enhances its syn-
thetic versatility by enabling diverse efficient C–C bond
forming reactions to take place on both the DTF unit and the
2
7,28
This scenario however did
1
Department of Chemistry, Memorial University, St. John’s, NL A1B 3X7, Canada.
E-mail: yuming@mun.ca; Fax: +1 709 864 3702; Tel: +1 709 864 8747
†
Electronic supplementary information (ESI) available: Detailed synthetic pro-
DTF synthons could also be synthetically elaborated to attain
π-extension through alkynyl coupling reactions. Scheme 2 out-
cedures and characterization data for new compounds. See DOI: 10.1039/
c5ob01651g
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Scheme 1 Synthesis of acetylenic DTFs 1a–c and related TTFV deriva-
tives 3a–c.
Fig. 2 UV-Vis absorption spectra of compounds 1a–c, 3a–c, and 4a–c
2 2
measured in CH Cl at room temperature.
Scheme 2 Synthesis of bis(DTF)-endcapped butadiynes 4a–c via Hay
coupling. (a) (i) K
rt, 5 h.
2 3 2 2
CO , THF/MeOH, 30 min, (ii) CuI, TMEDA, air, CH Cl ,
in the three isomers can be reasonably correlated with its
ortho-substitution structure which engenders the most signifi-
cant steric interactions between the alkynyl group and the DTF
moiety to attenuate the π-conjugation.
2
9
lines the Hay coupling reactions of desilylated 1a–c. The syn-
The π → π* transition bands of TTFV isomers 3a–c show the
thesis began with protiodesilylation with K CO to first gen- same trend of λmax shift (i.e., para > meta > ortho) as that in
2
3
erate free terminal alkyne intermediates, which after a brief 1a–c, but the extent of variation is much greater (Fig. 2B). For
workup were immediately subjected to alkynyl homocoupling para-isomer 3a the λmax value is observed at 380 nm, which is
catalyzed by CuI/TMEDA, affording bis(DTF)-endcapped conju- nearly identical to that of its DTF precursor 1a and indicates
gated butadiynes 4a–c in satisfactory yields. Diynes 4a and 4b that the central TTFV moiety in 3a does not contribute to
are yellow coloured solids, whereas 4c shows an intense increasing π-delocalisation due to its twisted cis-like confor-
1
1,15,30
orange color, suggesting that it has very different electronic mation.
Compared with 3a, the absorption peak of meta-
absorption properties than the other two isomers (vide infra).
isomer 3b shows a significant blueshift to 350 nm, which is
The electronic absorption properties of acetylenic DTFs and suggestive that the π-electron delocalisation in 3b is more dis-
their related π-derivatives were studied by UV-Vis spectroscopy. rupted than 3a. The absorption spectrum of ortho-isomer 3c
As can been from Fig. 2, the three DTF isomers 1a–c show π → differs pronouncedly from its other two isomers, wherein the
π* transition bands with similar molar absorptivities. The
maximum absorption wavelengths (λmax) shift in the order of the spectrum of 3c features a broad absorption slope in the
a (384 nm) > 1b (376 nm) > 1c (357 nm), giving a qualitative range of ca. 350 to 500 nm. Worth noting is that such a spec-
comparison of the degree of π-delocalisation among the three tral profile bears great resemblance to that of bis-
which we had reported
λmax value at 329 nm is considerably blueshifted. In addition,
1
a
3
1
isomers. The observation of 1c showing the lowest λmax value (1-naphthyl)-TTFV compound
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previously. Given that the central TTFV moiety of the bis(1- through electrochemical oxidative dimerization. For ortho-
naphthyl)-TTFV has been confirmed to assume a completely isomer 1c, the reverse CV scan reveals two cathodic peaks at
3
1
planar trans conformation by X-ray analysis, the molecular +0.59 and +0.33 V respectively in the positive potential
shape of 3c is thus deemed as adopting a similar trans motif. window. These two peaks are ascribed to the stepwise single-
2
+
Further evidence of this conformational argument can be eli- electron transfers occurring on the [TTFV ] product, which is
cited from cyclic voltammetric analysis (vide infra). evidenced by the nearly identical reduction features observed
The UV-Vis spectral profiles of bis(DTF)-endcapped buta- in the voltammogram of ortho-alkynyl TTFV 3c. In the negative
diynes 4a–c varied significantly from one to another (Fig. 2C). potential window, all the voltammograms of 1a–c show a
The spectrum of diyne 4a shows a monotonous absorption noticeable cathodic peak around −0.47 V. The intensity of this
peak centred at 423 nm, with absorptivity in the range of peak shows dependence on scan rate; that is, at relatively slow
2
70–350 nm being very low. The spectral envelope of 4b is sub- rate this peak just diminishes. It is therefore reasoned that
stantially blueshifted relative to 4a, showing three well-resolved this particular peak is associated with some intermediary
bands at 339, 316, and 295 nm and a shoulder peak at species formed electrochemically which would readily diffuse
3
61 nm, respectively. The spectrum of 4c gives two intense π → away from the working electrode surface.
π* bands at 398 and 332 nm. Overall, the λmax values of the The voltammograms of TTFVs 3a and 3b both show a
bis(DTF)–butadiyne isomers 4a–c follow an order of para > similar reversible redox wave pair, which can be unambigu-
ortho > meta, which is different from those of DTFs 1a–c and ously assigned to the simultaneous two-electron redox pro-
TTFVs 3a–c. An empirical rationalization for such spectro- cesses on the diphenyl-substituted TTFV moiety which
11,15,30
scopic behaviour can be made based on the π-delocalisation normally would take a twisted cis-like conformation.
Sig-
degree argument; that is, the DTF–phenylene–diyne skeletons nificantly different are the patterns in the voltammogram of
of compounds 4a and 4c are in linear π-conjugation, whereas ortho-isomer 3c, in which the redox processes of TTFV
that of compound 4b is cross-conjugated due to its meta-sub- undergo two distinct single-electron steps. Such CV behaviour,
3
0
31
stitution pattern.
according to the previous studies by Yamashita and us,
The redox properties of compounds 1a–c, 3a–c, and 4a–c alludes to a scenario that the TTFV moiety in 3c prefers a
were characterized by cyclic voltammetry (CV), and the detailed planar trans conformation. The ortho-alkynyl groups in 3c are
voltammograms are shown in Fig. 3. When scanned in the believed to play a key role in directing the central TTFV
positive direction, DTFs 1a–c all show a similar anodic peak at conformation.
•
+
ca. +0.8 V which can be assigned to the formation of a [DTF ]
Likewise, the CV analysis of bis(DTF)-endcapped butadiynes
1
5,32
radical cation via a single-electron oxidation step.
In the 4a–c also discloses notable substitution effects on their redox
reverse scan, a cathodic peak at +0.48 V is seen in the voltam- properties. The voltammograms of para- and meta-isomers 4a
mograms of para- and meta-isomers 1a and 1b. Compared with and 4b show the typical DTF oxidation peak at ca. +0.8 V along
the voltammograms of TTFVs 3a and 3b, the origin of this with a steadily growing redox wave pair due to the formation of
current peak can be assigned to the two-electron reduction of TTFV during multicycle CV scans. The CV results confirm that
2
+
the [TTFV ] dication formed on the working electrode surface 4a and 4b can readily undergo electrochemical polymerization
2 2
Fig. 3 Cyclic voltammograms of compounds 1a–c, 3a–c, and 4a–c measured in CH Cl at room temperature. Experimental conditions: supporting
electrolyte: Bu NBF (0.1 M), working electrode: glassy carbon, counter electrode: Pt wire, reference electrode: Ag/AgCl (3 M NaCl), scan rate:
4
4
−
1
2
00 mV s .
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Fig. 4 Optimized structure of compound 5 at the B3LYP/6-31G(d) level.
(A) Front view, (B) side view.
Scheme 3 Formation of compound 5 via an intramolecular cyclo-
addition pathway.
derivatives and conjugated butadiyne systems behave in a dra-
matically different way in terms of electronic absorption and
redox reactivity compared with their para- and meta-isomers.
The structure–property–reactivity relationships established
herein offer a useful guidance to the ongoing research of DTF
and alkyne based conjugated materials.
1
5,20,33,34
through the oxidative DTF coupling.
Interestingly,
such reactivity appears to be disfavoured in the case of ortho-
isomer 4c, for there is no observation of the characteristic
TTFV redox wave pair in its voltammogram. Worth noting is
that the CV profile of 4c exhibits an irreversible pattern with a
few weak cathodic current peaks discernible in the reverse
scans (see the inset in Fig. 3), suggesting that the oxidation of
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