Dithieno[3,2-c:2′,3′- e ]-2,7-diketophosphepin: A unique building block for multifunctional π-conjugated materials

Article abstract of DOI:10.1021/ja310680x

A series of conjugated materials based on the new dithieno[3,2-c:2′, 3′-e]-2,7-diketophosphepin (DTDKP) building block have been studied for the first time. Theoretical calculations predict DTDKP to be a better electron acceptor than the well-known dithienophosphole and the nitrogen analogue, bithiopheneimide. Cyclic voltammetry studies revealed two reduction processes that support their promising electron-acceptor properties, and modification of the P center with O or gold(I) further reduced the LUMO energy to ca. -3.6 eV. Expansion of the DTDKP core with various aromatic moieties has been realized using the Huisgen alkynyl click reaction, resulting in altered optical and electrochemical properties with compounds showing a high-energy absorption band at ca. 270-290 nm and a low-energy band at ca. 390-460 nm. The acceptor character of the DTDKP core was demonstrated by a red shift following the electron-donating strength of the appended aromatic moiety. Intriguing white-light emission from just a single species with the CIE coordinates of (0.33, 0.34) was observed for some of the extended species as the result of an unexpected dual-emission behavior. The high-energy emission in the blue-to-green region and the low-energy emission in the orange-to-red region are attributed to a π* → π transition of the DTDKP core and charge transfer from the triazole moiety to DTDKP, respectively. Apart from tuning of the molecular properties, this novel building block has also been applied in a self-assembled organogel, which exhibited pronounced luminescence. Scanning electron microscopy confirmed that the gel self-assembled by forming a network of entangled 1D fibrous structures on the micrometer scale.

Full text of DOI:10.1021/ja310680x

Article
pubs.acs.org/JACS
Dithieno[3,2‑c:2′,3′‑e]‑2,7-diketophosphepin: A Unique Building
Block for Multifunctional π‑Conjugated Materials
Xiaoming He, Javier Borau-Garcia, Alva Y. Y. Woo, Simon Trudel, and Thomas Baumgartner*
Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive NW, Calgary,
Alberta, Canada T2N 1N4
S
* Supporting Information
ABSTRACT: A series of conjugated materials based on the
new dithieno[3,2-c:2′,3′-e]-2,7-diketophosphepin (DTDKP)
building block have been studied for the first time. Theoretical
calculations predict DTDKP to be a better electron acceptor
than the well-known dithienophosphole and the nitrogen
analogue, bithiopheneimide. Cyclic voltammetry studies
revealed two reduction processes that support their promising
electron-acceptor properties, and modification of the P center
with O or gold(I) further reduced the LUMO energy to ca.
−3.6 eV. Expansion of the DTDKP core with various aromatic
moieties has been realized using the Huisgen alkynyl click reaction, resulting in altered optical and electrochemical properties
with compounds showing a high-energy absorption band at ca. 270−290 nm and a low-energy band at ca. 390−460 nm. The
acceptor character of the DTDKP core was demonstrated by a red shift following the electron-donating strength of the appended
aromatic moiety. Intriguing white-light emission from just a single species with the CIE coordinates of (0.33, 0.34) was observed
for some of the extended species as the result of an unexpected dual-emission behavior. The high-energy emission in the blue-to-
green region and the low-energy emission in the orange-to-red region are attributed to a π* → π transition of the DTDKP core
and charge transfer from the triazole moiety to DTDKP, respectively. Apart from tuning of the molecular properties, this novel
building block has also been applied in a self-assembled organogel, which exhibited pronounced luminescence. Scanning electron
microscopy confirmed that the gel self-assembled by forming a network of entangled 1D fibrous structures on the micrometer
scale.
materials has recently created new avenues leading to materials
for energy applications that also show some unique properties.
Because of the versatile reactivity and electronic nature of
phosphorus, organophosphorus materials offer considerable
promise for the development of new functional materials with
novel properties and provide a broad range of facile possibilities
for efficient modification of the electronic properties of the
product materials. As a prominent representative of this family,
phosphole-based conjugated systems have attracted significant
attention. Systematic groundbreaking studies by others and our
group revealed that the LUMO energies of the phosphole
derivatives can be stabilized through σ*−π* orbital coupling,
making phosphole-based materials very promising electron-
acceptor materials.13−18 One intriguing example is the
dithienophosphole system (Chart 1), a very strong blue-light
emitter with photoluminescence efficiencies of up to 90% that
was developed by us in 2004.14a Most importantly, the
photophysical properties can be easily tuned by the
introduction of electron-donating or electron-accepting moi-
eties on the scaffold or by simple modification of the
phosphorus center (e.g., E = lone pair, O, S, Me+, BH3, or a
INTRODUCTION
■
Organic conjugated materials have recently drawn increasing
attention because of their utility for important applications in
sustainable and low-cost electronic devices such as organic
photovoltaics (OPVs),1 organic light-emitting diodes
(OLEDs),2 and organic field-effect transistors (OFETs).3 To
function efficiently, these devices require two complementary
semiconductor components: a p-type component that can
conduct positive charges or holes and an n-type component for
negative charge transport. Extensive efforts in the past have
been focused on the development of p-type semiconductor
materials;4 however, the number of n-type materials is quite
limited.5 Therefore, the development of new n-type materials is
a highly rewarding direction for improving the performances of
organic electronics. One of the key parameters for evaluating
the n-type properties of materials is the lowest unoccupied
molecular orbital (LUMO) energy. Materials with low LUMO
energies have high electron affinities and could also enhance the
electron transport. A widely used strategy to lower the LUMO
energy is the introduction of an electron-withdrawing moiety
such as a carbonyl,6 cyano,7 pentafluoro,8 trifluoromethyl,9 or
imide10 group into the organic framework.
On the other hand, the incorporation of inorganic main-
Received: November 1, 2012
group elements, particularly B,11 Si,12 and P,13 in π-conjugated
© XXXX American Chemical Society
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of the two carbonyl groups, which have successfully been
established to induce n-type features.
Chart 1. (a) Molecular Design of the Target Dithieno[3,2-
c:2′,3′-e]-2,7-diketophosphepin (DTDKP) by Hybridization
of Dithienophosphole and Bithiopheneimide (BTI); (b)
Examples of Cyclic Diketophosphanyl Compounds
In this contribution, we now present the synthesis and
characterization of DTDKP and demonstrate its successful
application in the construction of conjugated materials.
DTDKP’s photophysical properties can be further tuned by
the use of combinations of various aromatics with different
electronic properties and simple modification of the phospho-
rus center. In addition, the attachment of long alkoxy chains
provides the potential for intermolecular hydrophobic−hydro-
phobic interactions as a driving force for aggregation at the
microscale, with the goal of improving DTDKP’s materials
properties via a self-assembly approach.
RESULTS AND DISCUSSION
■
Synthesis. Literature reports on cyclic diketophosphanyl
systems are very limited, and the first few examples (P1−P3 in
Chart 1) were reported by Cowley and co-workers back in
1987.20a However, it was not until 2004 that the first X-ray
structure of this class of cyclic compounds (P1) was
reported.20b Very recently, Crossley and co-workers reported
another interesting example that deals with the self-assembly of
diphosphametacyclophane P4.20c To the best of our know-
ledge, these four species represent the comprehensive list of
cyclic systems with a diketophosphanyl moiety.
metal).14 Very recently, our group reported a series of strongly
electron-accepting organophospholes formed by introducing
nitrogen atoms into the dibenzophosphole framework, some of
which exhibited remarkably low reduction potentials of ca.
−0.51 V, demonstrating the exceptional benefits derived from
the electron-withdrawing nature of the phosphole materials.14h,i
In 2008, Facchetti, Marks, and co-workers reported an
excellent n-type building block, bithiopheneimide (BTI, Chart
1), which has successfully been applied in conjugated
homopolymers and donor−acceptor (D−A) copolymers for
high-performance OFETs and OPVs.19 Replacement of the
nitrogen atom in BTI with phosphorus would provide the
novel phosphorus-containing building block dithieno[3,2-
c:2′,3′-e]-2,7-diketophosphepin (DTDKP) (Chart 1). In
contrast to BTI, the energy levels of the novel DTDKP should
be further tunable by simple modification of the phosphorus
center (Chart 1), potentially enhancing the value of this
building block over its N analogue. As a guide for experimental
studies, preliminary theoretical calculations (vide infra) showed
that these P species indeed exhibit lower LUMO energies and
that subsequent oxidation of the P center further decreases the
LUMO energy and consequently further increases its acceptor
character. The novel compounds can also be deemed “next-
generation” dithienophospholes as a result of the introduction
The synthetic routes toward two basic diketophosphepins 1
and 2 are shown in Scheme 1. The key intermediate
PhP(SiMe3)2 was synthesized according to a modification of
a reported procedure via the reaction of PhPH2 with Me3SiOTf
in the presence of Et3N at room temperature overnight.21
Because of its highly air- and moisture-sensitive nature,
PhP(SiMe3)2 was prepared in situ for the next step. The
synthesis of 3,3′-dicarboxylic acid-5,5′-bisalkynyl-2,2′-bithio-
phene (S2) was adapted from the known procedure leading
to S1 (Scheme 1).19a Alkynylation of 3,3′,5,5′-tetrabromo-2,2′-
bithiophene with TMS−CCH in the presence of
PdCl2(PPh3)2, CuI, and PPh3 in i-Pr2NH gave 3,3′-dibromo-
5,5′-bis(trimethylsilylacetylenyl)-2,2′-bithiophene (S3). To ob-
tain the corresponding dicarboxylic acid, slow addition of nBuLi
to the THF solution of S3 followed by quenching of the
dilithiated salt with dry CO2 and subsequent desilylation using
KOH in one pot afforded the dilithiated salt of 3,3′-
dicarboxylate-5,5′-bisalkynyl-2,2′-bithiophene; S2 was finally
obtained by acidification using dilute HCl. The corresponding
diacid chlorides of S1 and S2 were synthesized by reacting the
diacid derivatives with oxalyl chloride at room temperature
Scheme 1. Synthesis of DTDKPs
B
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Scheme 2. Synthesis of Compounds 3−6 by the Huisgen Alkynyl−Azide Click Reaction
without further purification. Finally, formation of the cyclic
DTDKPs was achieved by reaction of the diacid chlorides with
PhP(SiMe3)2 in Et2O at −78 °C according to a literature report
on a somewhat related system (Scheme 1).20a Both 1 and 2
were fully characterized by 1H, 13C, and 31P NMR spectroscopy,
high-resolution mass spectrometry (HRMS), and elemental
analysis. The structures of 1 and 2 were furthermore confirmed
by single-crystal X-ray crystallography (vide infra).
modifications were carried out using 1 as representative
example (Scheme 3). The conversion to 1-AuCl was readily
Scheme 3. Modification of the Phosphorus Center in 1
For further extension of the conjugation of the basic
framework, our initial efforts focused on the synthesis of a
dibrominated DTDKP for subsequent cross-coupling reactions
using N-bromosuccinimide under various conditions (e.g., in
CHCl3/AcOH, N,N-dimethylformamide, or trifluoroacetic
acid/H2SO4); however, all attempts were met with failure.
One possible reason for this behavior could be the strong
electron-accepting character of DTDKP, which likely inhibits
electrophilic bromination of the scaffold. Consequently,
compound 2 with two terminal alkynyl groups was identified
as a suitable target species for the Huisgen alkynyl−azide “click”
reaction, which has also been widely used in the construction of
organic conjugated materials because of its ease of synthesis,
high functional group tolerance, and excellent reaction yields.22
Subsequently, click reactions were run using Cu(PPh3)3Br as
the catalyst in the presence of N,N-diisopropylethylamine
(DIPEA) as the base in tetrahydrofuran (THF) solution at
room temperature for 48 h (Scheme 2).23 The organic-soluble
and air-stable Cu(PPh3)3Br catalyst not only allowed for
homogeneous reaction conditions, thereby increasing the
efficiency, but also avoided Cu coordination of the P center
in the trivalent diketophosphepin, which potentially could
poison the catalyst and hamper the reaction process.
achieved by reaction with Au(tht)Cl (tht = tetrahydrothio-
phene) in CH2Cl2 at room temperature, according to our
established procedures for the dithienophosphole system.14
The successful conversion was confirmed by a distinct upfield
shift in the 31P NMR signal in going from 1 (64.6 ppm) to 1-
AuCl (49.5 ppm) as well as elemental analysis. This inverse
chemical shift trend is contrary to what we expected14 but can
likely be attributed to conformation changes in the seven-
membered ring induced by the modification of the P center.
In contrast to the dithienophosphole system, the synthesis of
the oxide 1-O occurred successfully only when m-chloroper-
oxybenzoic acid (mCPBA) was used as the oxidant. The
application of conventional oxidation conditions, such as H2O2,
t-BuOOH, or NaIO4, led to quick decomposition into S1, as
confirmed by NMR analysis. Remarkably, the reaction with
mCPBA is very fast and evident by the immediate color change
of the solution from yellow to red. A similarly abnormal upfield
shift of the 31P NMR signal for 1-O at 10.5 ppm was also
observed for this transformation.14−18 Overall, while somewhat
limited in this case, the reactivity of the P center nevertheless
confirmed the important advantage of organophosphorus
materials over other genuine organic systems and also their
nitrogen counterparts.
X-ray Crystallography. Single crystals of 1 and 2 suitable
for X-ray crystallography were obtained from slow evaporation
of concentrated acetone solutions at room temperature. The
molecular structures of 1 and 2 in the solid state are depicted in
Figures 1 and 2, respectively. It should be noted that the
structure of 2 showed some disorder arising from two out-of-
plane positions of the P atom that consequently affected the
position of the O atoms and the phenyl ring as well; Figure 2
shows only one of the conformations. The crystal data and
structure refinement details are included in the Supporting
Information (SI). In both cases, the two thiophene rings of the
main scaffold are arranged in almost coplanar fashion, with
small C3−C4−C5−C6 torsion angles of 1.4 and 7.5° for 1 and
2, respectively. This highly planar conjugated character was also
observed in the only X-ray-crystallographically characterized N
analogue, which has a torsion angle of 4.3°.19a However, a
distinct difference of DTDKP is that the P center is pyramidal,
as would be expected.14a−c Moreover, the P atom is significantly
twisted out of the plane, with interplanar angles (between the
Notably, all of the trivalent compounds 1−6 are air- and
moisture-stable, supporting a reduced Lewis basicity of the P
center as well as reduced electrophilic character of the carbonyl
groups. The thermal stability of the compounds was evaluated
by thermogravimetric analysis (TGA) using four representative
examples. The decomposition temperatures (defined as the
temperatures of 5% mass loss) for 1, 2, 4, and 5 were
determined to be 245, 430, 300, and 220 °C, respectively,
indicating modest-to-high thermal stability.
The 31P NMR resonance signals of the compounds are
located in the range 64−67 ppm, which are shifted significantly
downfield from those of trivalent dithienophospholes (−20
3
ppm).14 On the basis of the limited available information, the
strongly low-field-shifted 31P NMR resonances of this new class
of cyclic phosphorus compounds can be attributed to the ring
size. The ring size of related cyclic organophosphanes has been
found to lead to quite large shift variances between −28 and 74
ppm.20a A similar ring effect on the 31P NMR resonance has
also been observed in chelated phosphane−metal complexes.24
To elaborate on the potential reactivity of the trivalent
phosphorus center of the diketophosphepin system, two typical
C
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bonds are slightly shorter than the exocyclic one, and can be
attributed to the geometric strains resulting from the different
ring sizes rather than to a lack of the conjugation.14a The
lengths of endocyclic P−C bonds in 1 and 2 are comparable to
those reported for P1 (ca. 1.85 Å)20b and P4 (ca. 1.89 Å)20c
and clearly longer than the average PC bond in
phosphaalkenes (1.61−1.71 Å), suggesting the absence of full
delocalization of the phosphorus lone pair toward the carbonyl
groups.25 For compound 2, two terminal alkynyl groups are
clearly present, showing CC bond lengths of 1.175(9) and
1.178(8) Å, and the bond angles of the sp-hybridized carbon
are close to the ideal value of 180°.
Both compounds 1 and 2 show weak intermolecular π−π
stacking interactions at a distance of ca. 3.6 Å (Figures 1 and 2),
but their respective molecular packing modes are different.
Compound 1 exhibits obvious inverse face-to-face interactions
between the bithiophene planes with almost complete overlap
of the scaffolds of neighboring molecules. For 2, neighboring
molecules show the same orientation of the P centers, with only
partial overlap of bithiophene planes as a consequence of the
steric bulk of the phosphanyl groups. However, this slipped-
stacking mode does not preclude the formation of a well-
ordered one-dimensional (1D) organization (Figure 2c).
Photophysical Properties. The UV−vis spectra of
compounds 1−5 in CH2Cl2 are depicted in Figure 3, and the
Figure 1. (left) Molecular structure of 1 (50% probability level) and
(right) intermolecular interactions in the solid state. Selected bond
lengths (Å) and angles (deg): P1−C9, 1.843(3); P1−C10, 1.848(2);
P1−C11, 1.811(2); C3−C9, 1.469(3); C6−C10, 1.471(3); C9−O1,
1.219(3); C10−O2, 1.216(2); C1−C2, 1.351(4); C2−C3, 1.427(4);
C3−C4, 1.381(4); C4−C5, 1.455(3); C5−C6, 1.381(3); C6−C7,
1.423(4); C7−C8, 1.342(4); C9−P1−O10, 107.9(1); C9−P1−C11,
100.7(1); C10−P1−C11, 102.1(1); C4−C3−C9, 129.0(2); C5−C6−
C10, 128.9(3); C3−C9−P1, 121.8(2); C6−C10−P1, 120.6(2).
Figure 3. UV−vis spectra of 1−5 in CH2Cl2.
photophysical data are shown in Table 1. All of the compounds
show a high-energy absorption band at ca. 270−290 nm and a
low-energy band at ca. 390−460 nm. Compared with 1, the
extended conjugated species 2−5 show lower-energy absorp-
tion bands and higher extinction coefficients. The absorption
data indicate narrower HOMO−LUMO energy gaps for the
extended species, which is in line with the results of theoretical
calculations (vide infra).
Attachment of the various aromatic moieties with different
electronic properties in 3−5 was found to show the expected
effect on the absorption maximum (λmax) and onset of the UV−
vis absorption spectra. The trend in λmax, 4 (461 nm) > 3 (458
nm) > 5 (440 nm), is consistent with the electron-donating
abilities of the aromatic moieties, 4 (p-C6H4-OMe) > 3 (Ph) >
5 (C6F5), clearly supporting the electron-accepting ability of the
diketophosphepin moiety and consequently a donor−accept-
or−donor architecture.26
While species 1 and 2 show blue-to-green emissions with
maxima at λem = 468 and 492 nm that can be attributed to the
π* → π transition of the DTDKP core (see the SI), some
intriguing unexpected excitation-dependent luminescence be-
havior was observed for 3−5. In addition, molecules 3−5 with
more extended π systems have larger quantum yields than 1
and 2. Figure 4a shows the UV−vis absorption spectrum and
Figure 2. (a) Molecular structure of 2 in the solid state (50%
probability level) and (b, c) its intermolecular packing. Only one of the
disordered conformations is shown. Selected bond lengths (Å) and
angles (deg): P1−C9, 1.756 (7); P1−C10, 1.946 (7); P1−C11, 1.798
(7); C3−C9, 1.502 (8); C6−C10, 1.478 (8); C9−O1, 1.309(1); C10−
O2, 1.189(7); C1−C2, 1.352(8); C2−C3, 1.408(8); C3−C4,
1.363(8); C4−C5, 1.446(8); C5−C6, 1.388(8); C6−C7, 1.406(8);
C7−C8, 1.358(9); C8−C19, 1.413(9); C19−C20, 1.175(9); C1−C17,
1.432(8); C17−C18, 1.178(8); C9−P1−O10, 101.3(3); C9−P1−
C11, 105.9(5); C10−P1−C11, 101.2(5); C4−C3−C9, 128.7(5); C5−
C6−C10, 129.1(5); C3−C9−P1, 120.0(5).
C9−P1−C10 and C3−C4−C5−C6 planes) of 47.9 and 59.6°
for 1 and 2, respectively; this is in contrast to the in-plane P
atom of the dithienophosphole system.14a−c The rotation about
the exocyclic P−C bond results in an almost orthogonal
orientation of the phenyl ring with respect to the bithiophene
moiety. The two endocyclic P−C bonds were determined to be
1.843(3) and 1.848(2) Å in 1 and 1.756(7) and 1.946(8) Å in
2, while the exocyclic P−C bond lengths for 1 and 2 are
1.811(2) and 1.798(7) Å, respectively. This trend is inverse to
the one in the dithienophosphole system, where the endocyclic
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Table 1. Photophysical, Electrochemical, and Calculated Data for 1−6
electrochemistry
calculated/eV
a
d
e
f
compd
λabs/nm (ε/dm3 mol−1 cm−1
)
λem/nm (ΦF) (CH2Cl2) λem/nm (solid)
Ered,1/2/V
ELUMO/eV
−3.14
ELUMO
EHOMO
Eg
b
1
276 (27870), 390 (6650)
468 (0.02)
NA
NA
578
Ered1 = −1.66
Ered2 = −1.95
Ered1 = −1.17
Ered2 = −1.65
Ered1 = −1.15
Ered2 = −1.59
Ered3 = −1.99
Ered1 = −1.42
Ered2 = −1.73
NA
−2.66
−6.06
3.40
1-O
283 (17152), 422 (4859)
470
−3.63
−3.65
−3.21
−3.74
−6.18
−6.74
2.97
3.00
c
1-AuCl
294 (15800), 411 (4475), 460sh (3000)
285 (35053), 410 (10945)
586 (0.10)
b
2
492 (0.03)
NA
−3.38
−2.93
−5.98
3.05
c
c
c
3
4
5
289 (33315), 458 (11172)
592 (0.08)
599 (0.10)
485 (0.15)
NA
617
630
NA
−2.79
−2.78
−2.85
−5.66
−5.60
−5.77
2.87
2.82
2.92
285 (44178), 461 (13528)
NA
NA
286 (33033), 411 (9717), 440 (9623)
Ered1 = −1.45
Ered2 = −1.72
Ered1 = −1.53
Ered2 = −1.86
NA
−3.35
c
6
285 (35012), 461 (11454)
598 (0.09)
NA
582, 635 (sh)
−3.27
NA
NA
NA
BTI
NA
b
NA
NA
−2.32
−6.18
3.86
a
c
In CH2Cl2. Photoluminescence quantum yield in CH2Cl2 relative to quinine sulfate. Photoluminescence quantum yield in CH2Cl2 relative to
d
e
f
fluorescein. Potentials vs ferrocene/ferrocenium (Fc/Fc+). Calculated using the Fc HOMO level at −(4.8 + Ered,1/2) eV. Eg = ELUMO − EHOMO
.
excitation and emission spectra (excited at different wave-
lengths) of 4 in CH2Cl2.
(0.53, 0.46). These results are intriguing insofar as white-light
emission is generally obtained using strategies based on additive
color mixing by combining different luminophores through
covalent linkages or via self-assembly approaches;27a single
molecules that exhibit white-light emission remain rare to
date.27b,c
Detailed excitation studies revealed that the high- and the
low-energy emissions originate from different states (Figure 4).
The higher-energy emission, derived from a featureless
excitation at ca. 399−404 nm, is attributed to the π* → π
transition of the core similar to those of 1 and 2. The low-
energy emission band, derived from an excitation band at ca.
453−495 with a shape similar to the low-energy absorption
band, is tentatively assigned to charge transfer from the
peripheral aromatic part to the DTDKP core.28 The dual
emissions are attributed mainly due to a twisted structure in
solution.28 Similar dual-emission behavior was also observed for
3, for which the ratio of the intensities of the high- and low-
energy emissions (IH/IL) is 1.28 (λex = 400 nm; see the SI). For
compound 5, however, the high-energy emission is predom-
inant (IH/IL = 4.46, λex = 400 nm). It is plausible that the
electron-accepting C6F5 group stabilizes the energy levels,
making the charge-transfer emission in the low-energy region
less favorable (see the SI).28
In the solid state, photoexcitation at either high (λex = 365
nm) or low energy (λex = 450−520 nm) produced only a single
low-energy emission at λem = 617 and 630 nm for 4 and 5,
respectively, suggesting the suppression of the twisted structure,
which allows the charge-transfer process to be more dominant
(see the SI). The red-shifted emissions in the solid state also
suggest more efficient conjugation due to the planarity of the
molecules. In contrast to 4, however, 5 with the electron-
accepting C6F5 moiety exhibits a more pronounced low-energy
emission, which can be attributed to the presence of
intramolecular F···H hydrogen-bonding interactions (see the
SI). Although weak in solution, such intramolecular six-
membered-ring F···H hydrogen-bonding interactions have
very recently been reported to be an important non-covalent
solid-state force for triazole systems.29
Figure 4. (a) UV−vis absorption spectrum (solid blue), emission
spectra (solid black and red), and excitation spectra (dashed black and
red) of 4 in CH2Cl2. (b) CIE coordinates for photoluminescence of 4
excited at different wavelengths [a: λex = 365 nm, CIE (0.33, 0.34); b:
λex = 400 nm, CIE (0.33, 0.36); c: λex = 460 nm, CIE (0.53, 0.46)].
Upon excitation at λex = 365 nm, white photoluminescence
with dual emission bands at λem = 475 and 600 nm with an
intensity ratio (I475/I600) of 1.30 was observed. The
complementary colors of blue-green (475 nm) and orange
(600 nm) in one single molecule contribute to the intriguing
white-light emission. To characterize the white-light emission,
the commonly used Commission International de l’Eclairage
(CIE) coordinates were determined to be (0.33, 0.34), which is
very close to those of pure white light, (0.33, 0.33) (Figure
4b).14d Remarkably, excitation at longer wavelength (λex = 460
nm) led to exclusively orange emission with CIE coordinates of
E
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To support the presence of a charge-transfer transition in
compound 4, solvatochromic absorption and emission studies
in representative solvents of different polarity (hexanes,
CH2Cl2, and CH3OH) were carried out. As shown in Figure
5, the low-energy emission was clearly red-shifted from 600 to
nm and the concomitant formation of new bands at 283 and
422 nm. Three isosbestic points at ca. 287, 341, and 411 nm
were observed, suggesting the clean transformation of 1 into 1-
O. The conversion was very fast and complete upon addition of
1 equiv of mCPBA (see the SI), which is in line with the NMR
spectroscopic studies (vide supra). Similar red shifts were also
observed in the UV−vis absorption spectra of 2−5 upon
oxidation (see the SI). Moreover, red shifts in the emission
bands by about Δλem = 70 nm were also observed for the more
conjugated molecules 3−5 (see the SI), indicating the high
synthetic value of P modification for narrowing the HOMO−
LUMO energy gap of conjugated materials.
Apart from phosphorus oxidation, the formation of the
gold(I) complex of 1 resulted in a similar red shift of the UV−
vis absorption (see the SI). Strikingly, the gold(I) complex 1-
AuCl also showed dual-emission behavior. Excitation at λex
=
365 or 380 nm produced a high-energy emission at λem = 450
nm and a low-energy emission at λem = 585 nm with IH/IL =
0.73 or 0.29, respectively (Figure 7). However, excitation at
Figure 5. Effect of the solvent on the emission spectra of 4. Inset:
photograph of emission in different solvents (from left to right:
hexane, CH2Cl2, and MeOH).
628 nm, as the solvent was changed from dichloromethane to
methanol. Most interestingly, three distinct emission colors
(red, yellow, and green) were observed in different solvents
with increasing polarity (hexanes, CH2Cl2, and MeOH,
respectively) as a result of the IH/IL ratios (hexane: IH/IL =
0.86; CH2Cl2: IH/IL = 1.29; MeOH: IH/IL = 6.60). The polar
methanol likely forms hydrogen bonds with a N atom of the
triazole moiety, thereby interrupting the coplanarity of the
conjugated backbone, which minimizes the low-energy
emission band in favor of the dominant high-energy band.
On the other hand, the solubility of 4 is diminished in hexanes,
and the corresponding photophysics likely the result of
aggregation. Consequently, the observed low-energy red
emission results from extended π conjugation due to the
planarized scaffold in the aggregates. Notably, aggregation is
further supported by the observation that when the hexane
solution of 4 was allowed to sit for ca. 1 h, a precipitate formed
that showed the same red emission in the solid state as was
observed for its solution.
As mentioned previously, one unique advantage of organo-
phosphorus materials is that their photophysics is tunable by
simple modification of the P center. Indeed, modification of
phosphorus with O or Au(I) produced considerable UV−vis
absorption and emission changes. Generally, oxidation of the
trivalent compounds results in a red-shifted absorption due to
the lowered HOMO−LUMO gap. A representative example of
UV−vis changes upon oxidation is shown in Figure 6. Upon
addition of mCPBA, the spectrum of 1 overall underwent a red
shift, accompanied by decreases in the bands at ca. 276 and 390
Figure 7. Emission spectra of 1-AuCl excited at different wavelengths
in CH2Cl2.
longer wavelength (λex = 400 nm) produced only one emission
at λem = 585 nm. On the other hand, no significant solvent-
dependent emission was observed for 1-AuCl, and unlike 4 and
5, the solid-state emission of 1-AuCl did not show any obvious
red shift either. This indicates that next to the absence of π−π
interactions, aurophilic interactions that usually lead to lower-
energy luminescence do not exist because of the bulky
phosphorus center.30 On the basis of the above facts and the
results of theoretical calculations (vide infra), the two emissions
are assigned to the metal-perturbed π* → π transition of the
DTDKP core and charge transfer from the lone pair of the Cl
to DTDKP, respectively.
Electrochemistry. The electrochemical features of the new
DTDKPs were determined by cyclic voltammetry to solidify
further the electron-accepting character of the new system. To
our satisfaction, each trivalent compound showed two quasi-
reversible reduction peaks, and no oxidation peak was observed.
The electrochemical data are summarized in Table 1, and
representative cyclic voltammograms of compounds, 1, 2, and 5
are shown in Figure 8a. The first reduction peaks of 1, 2, and 5
were determined to be Ered1 = −1.66, −1.42, and −1.45 V,
respectively. With ferrocene as an internal reference, the
LUMO energies (ELUMO) of 1, 2, and 5 were determined from
the first reduction peaks to be −3.14, −3.38, and −3.35 eV
[ELUMO = −(Ered1 + 4.8) eV].31 Because of the relatively low
solubility of 3 and 4 in CH2Cl2, the cyclic voltammograms of
these two compounds were not accessible.
Interestingly, simple modification of 1 and 2 with gold(I) or
O significantly decreased the reduction potentials, and two
irreversible (or quasi-reversible) reduction peaks can be seen
Figure 6. UV−vis spectral changes of 1 upon addition of mCPBA in
CH2Cl2.
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09 suite of programs at the B3LYP level of theory with the 6-
31G+(d) basis set for 1−5 and 1-O and the LANL2DZ basis
set for 1-AuCl.33 The coordinates for the optimized geometries
of these compounds are included in the SI. The HOMO and
LUMO orbital diagrams and energy levels are shown in Figures
9 and 10 and Table 1.
Figure 8. Cyclic voltammograms of (a) the trivalent compounds 1, 2,
5, and 6 and (b) 1-O and 1-AuCl obtained from 1 by modification of
the P center, recorded in CH2Cl2 solution containing 0.1 M
tetrabutylammonium hexafluorophosphate as the supporting electro-
lyte. Scan rate = 100 mV/s. Ferrocene was added as an internal
standard and referenced to 0 V.
Figure 10. Frontier molecular orbital diagrams and the electronic
transitions calculated using TD-DFT for 3−5.
The HOMO of 1 mainly represents the π system of the
bithiophene section of the DTDKP core. With further
extension of the core, the electron density of the HOMO is
considerably delocalized over the extended conjugated system
in compounds 2−5. The electron-donating methoxyphenyl
group in 4 shows a larger contribution to the HOMO than the
electron-accepting C6F5 group in 5, supporting the expectedly
stronger donor−acceptor interaction in 3. The LUMOs of 1−5
mainly reside on the π* orbital of the DTDKP core, with
hyperconjugation between the σ* orbital of the exocyclic P−C
bond. Such a σ*−π* interaction is typical for organo-
(Figure 8b). The LUMO energies of 1-AuCl, 1-O, and 2-O are
ELUMO = −3.65, −3.63, and −3.85 eV, respectively, suggesting
that they have a considerably stronger electron-accepting
character than their trivalent counterparts. These low-lying
LUMOs are comparable to that of PCBM-C60 (ELUMO = −3.7
eV),32 which is a common electron acceptor in organic
electronics.
DFT Calculations. To obtain a better understanding of the
photophysical and electronic properties, density functional
theory (DFT) calculations were performed using the Gaussian
Figure 9. Frontier molecular orbitals for 1, 1-O, and 1-AuCl.
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a
Table 2. TD-DFT-Calculated Transition Data for 1−5
1
1-O
1-AuCl
2
3
4
5
b
λ/nm (f)
440 (0.11)
496 (0.10)
507 (0.11)
484 (0.31)
519 (0.51)
527 (0.52)
507 (0.48)
c
transitions
H → L (75%)
H → L (51%)
H−1 → L (35%)
321 (0.12)
H → L (89%)
H → L (80%)
H → L (84%)
H → L (85%)
H → L (84%)
b
λ/nm (f)
286 (0.25)
314 (0.21)
302 (0.30)
323 (0.57)
325 (0.67)
329 (0.41)
c
transitions
H → L+2 (17%)
H−6 → L (65%)
H → L+1 (8%)
H−6 → L (71%)
H → L+1 (32%)
H−9 → L (29%)
H−8 → L (29%)
H → L+2 (73%)
H → L+2 (69%)
H → L+2 (67%)
H → L+1 (77%)
a
b
c
H = HOMO; L = LUMO. f = oscillator strength. The coefficient percentages of the orbitals involved in the transitions are given in parentheses.
phosphorus molecules and contributes to lowering the LUMO
energy.13
emission is due to ligand-to-ligand charge transfer from the
lone pair of the Cl ligand that is involved in the HOMO−1
orbital to the π* orbital of the DTDKP.
The HOMOs and LUMOs of 1-O and 1-AuCl are quite
similar to those of 1 and represent the π and π* systems,
respectively. It should be noted in this context that one lone
pair of the chloride in 1-AuCl contributes considerably to the
HOMO−1 level, which is an important feature for the observed
photophysics of 1-AuCl. The HOMO and LUMO energies of 1
are EHOMO = −6.06 eV and ELUMO = −2.66 eV, respectively. It
is noteworthy that 1 has a much lower LUMO energy than its
nitrogen analogue BTI (EHOMO = −6.18 eV, ELUMO = −2.32
eV),34 which can be attributed to the σ*−π* interaction
discussed above, highlighting the promising advantage of
organophosphorus materials. Incorporation of the π-conjugated
moieties in the scaffolds of 2−5 stabilizes the LUMO (ΔELUMO
for 2−5 = −0.27, −0.13, −0.12, and −0.19 eV, respectively)
and destabilizes the HOMO (ΔEHOMO for 2−5 = 0.08, 0.40,
0.46, and 0.29 eV, respectively), resulting in an overall decrease
in the energy gap (Eg), as is commonly observed for conjugated
materials. Eg follows the trend 4 (2.82 eV) < 3 (2.87 eV) < 5
(2.92 eV) < 2 (3.05 eV) < 1 (3.40 eV), which is consistent with
the trend observed by UV−vis spectroscopy. Among the
trivalent compounds, 2 exhibits the lowest LUMO energy
because of the electron-accepting features of the alkynyl groups.
In line with the electrochemistry results, modification of the
phosphorus center significantly lowers the LUMO energy of 1
(ΔELUMO = −0.55 eV for 1-O and ΔELUMO = −1.08 eV for 1-
AuCl), further confirming the strong electron-accepting nature
of the modified DTDKP.
The TD-DFT calculations for the trivalent compounds 1−5
indeed predict low- and high-energy absorption bands for each
compound (see Table 2). The low-energy absorptions mainly
involve the HOMO → LUMO (π → π*) transition. The high-
energy absorptions involve the HOMO → LUMO+2 transition
for 1−4 and the HOMO → LUMO+1 transition for 5,
composed of the π* orbital of the phenyl and triazole moieties.
Overall, the theoretical calculations fully support the electro-
chemical and photophysical properties of these novel
molecules.
Gelation Study. Next to the successful design of small
molecules based on 2, further attachment of self-assembly
groups was applied in hopes of accessing features of self-
assembly materials.36 To our satisfaction, compound 6 indeed
formed an organogel in hydrocarbon solvents (hexane or
heptanes), as shown in Figure 11.
Figure 11. Photographs of (a) the organogels of 6 in (left) hexane and
(right) heptane and (b) the orange photoluminescence of the
organogel.
To gain a better understanding of the optical absorption
properties, time-dependent DFT (TD-DFT) calculations were
carried out subsequently.35 The contributions of the molecular
orbitals involved in the UV−vis transitions were determined on
the basis of their oscillator strengths (f). The first 20 singlet and
triplet states were calculated. The calculated transitions were
consistent with the experimental data. The assignments of the
transitions and the corresponding molecular orbital diagrams
are shown in Table 2 and Figure 10.
Importantly, the composition of the frontier orbitals and the
narrower energy gaps provide theoretical proof for the
intriguing dual emissions of compounds 3−5 and 1-AuCl.
The reduced HOMO−LUMO energy gap due to the extension
of the DTDKP core and modification of the P center results in
lower-energy emissions at λem = 577−599 nm that can be
attributed to charge transfer from the peripheral moieties
(particularly the triazole unit) to the DTDKP core in 3−5 and
the π → π* transition of the DTDKP core in 1-AuCl. As a
result of the ring twist in solution, high-energy photoexcitation
Gel formation required the heating of the heterogeneous
mixture of 6 in the solvent to access a homogeneous solution
phase that upon cooling formed an organogel. The critical gel
concentration (CGC) was determined to be ca. 10 mg/mL in
hexane and 8.5 mg/mL in heptane. Interestingly, upon cooling
to room temperature, and in contrast to other traditional gels,34
where the gelation process traps all of the solvent to form an
inversion-stable soft material, a “gel island” of 6 in the solvent
was observed that resulted from simple shrinking of the gel
without any other morphological changes, akin to a sponge
(Figure 11). The following gelation process of 6 involving two
steps as shown in Scheme 4 is thus proposed: First, the solution
of 6 forms a transparent metastable gel phase by trapping the
hydrocarbon solvent in the interstitial network made up of 6.
Subsequently, this transparent gel network contracts upon
cooling and expels some of the solvent molecules, likely as a
result of stronger or more compatible hydrophobic interactions
between the long alkoxy chains as well as π−π interactions
between the rigid cores of the molecules of 6, forming the final
shrunken, stable gel phase.
of 3−5 also produces short-wavelength emissions at λem
=
473−485 exclusively from the DTDKP core, which are very
likely suppressed in the solid state. For 1-AuCl, the high-energy
H
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higher potentials than those of 5, likely because of the
interruption of the electronic connection between the triazole
and phenyl groups.
Scheme 4. Proposed Process for the Self-Assembly of the
Organogel of 6
CONCLUSIONS
■
A series of multifunctional conjugated materials 1−6 based on
the novel DTDKP building block have been designed and
synthesized, and their intriguing photophysical, electrochem-
ical, and self-assembly properties have been established. The
design of this new building block was inspired by our extensive
research on the dithienophosphole system as well as the
recently introduced electron acceptor BTI.
To confirm the self-assembly process and morphology
further, scanning electron microscopy (SEM) was performed
on xerogels (i.e., networks formed by the removal of solvent) of
6 obtained from hexane and heptanes and a solid sample
obtained from evaporation of the dichloromethane solution.
Interestingly, as shown in Figure 12, a network of microscopi-
cally entangled 1D fibers was observed for the xerogels (Figure
12 a−c), while no such fibril morphology was observed for the
solid sample (Figure 12d), clearly supporting the self-assembly
behavior of 6 in the gel phase. On the basis of the structure of 2
in the solid state determined by X-ray crystallography, it is
inherently plausible that the conjugated backbone in 6 could
form well-organized 1D π−π stacks with the help of
intermolecular hydrophobic interactions between the long
alkoxy chains, resulting in the observed fibrous structures.
The photoluminescence properties of 6 in different states
(solutions in hexane and CH2Cl2, gel from hexane, and solid)
were also investigated (see the SI). In line with the other
extended congeners 3−5, a similar excitation-dependent
emission was also observed for 6 in CH2Cl2. When excited at
λex > 450 nm, the emissions of 6 from hexane and CH2Cl2
solution and the gel state were determined to be λem = 574,
598, and 579 nm, respectively. The obvious red shift in CH2Cl2
relative to the other states suggests the charge-transfer character
of the emission. Emission from the solid exhibited a lower-
energy shoulder at 635 nm, suggesting the presence of
intermolecular π−π interactions. Compound 6 also showed
two reduction processes (Ered1 = −1.53, Ered2 = −1.86 eV), in
line with its relatives 3−5 (Figure 8), but they occurred at
The new building block, DTDKP, exhibits promising
electron-accepting character, as confirmed by theoretical
calculations and electrochemistry, where two reduction
processes were observed. By attaching various terminal
aromatic groups with different electronic properties to the
DTDKP core, conjugated compounds 2−6 were purposely
designed. The extended structures feature red-shifted UV−vis
absorption and emission spectra and consequently narrower
HOMO−LUMO energy gaps. Because of twisting of the
appended triazole units and the core in molecules 3−5,
photoexcitation at high energy leads to an intriguing dual-
emission behavior, where the high-energy and low-energy
emissions are attributed to the π* → π transition of the
DTDKP core and charge transfer from the triazole moiety to
DTDKP, thereby producing white-light emission. As a
synthetic advantage of organophosphorus materials, the
phosphorus center of these molecules can easily be modified
by oxidation and gold(I) complexation, leading significantly
altered photophysics and electronics. In addition to tuning the
intrinsic molecular properties of the building block, we could
also successfully address extrinsic materials properties by
incorporating self-assembly groups into the DTDKP core,
leading to an interesting luminescent organogel of 6 with a
microscopic 1D fiber structure.
Figure 12. Variable-pressure SEM images of (a, b) a xerogel of 6 obtained from heptane, (c) a xerogel of 6 obtained from hexanes, and (d) solid 6
obtained from evaporation of CH2Cl2 (landing energy = 20 kV, ambient pressure = 30 Pa for all images).
I
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Journal of the American Chemical Society
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(10) (a) Oh, J. H.; Suraru, S. L.; Lee, W.-Y.; Konemann, M.; Hoffken,
̈
̈
In summary, a novel organophosphorus building block,
DTDKP, has been designed and demonstrated to be valuable
for applications involving multifunctional conjugated materials.
Further detailed studies on the intriguing properties and
potential applications of this new system are currently
underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
Detailed experimental procedures; CIF files; crystal data and
structure refinement for 1 and 2; additional photophysical data;
full characterization (1H, 31P, 13C, and 19F NMR) of 1−6;
optimized structure coordinates of 1−5, 1-O, and 1-AuCl; and
complete ref 33. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
thomas.baumgartner@ucalgary.ca
́
1714−1742. (c) Chu, T.-Y.; Lu, J.; Beaupre, S.; Zhang, Y.; Pouliot, J.-
■
R.; Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. J. Am.
Chem. Soc. 2011, 133, 4250−4253. (d) Schroeder, B. C.; Huang, Z.;
Ashraf, R. S.; Smith, J.; D’Angelo, P.; Watkins, S. E.; Anthopoulos, T.
D.; Durrant, J. R.; McCulloch, I. Adv. Funct. Mater. 2012, 22, 1663−
1670. (e) Hou, J.; Chen, H.-Y.; Zhang, S.; Li, G.; Yang, Y. J. Am. Chem.
Soc. 2008, 130, 16144−16145. (f) Coffin, R. C.; Peet, J.; Rogers, J.;
Bazan, G. C. Nat. Chem. 2009, 1, 657−661.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(13) (a) Baumgartner, T.; Rea
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u, R. Dalton
Financial support by NSERC of Canada, the Canada School for
Energy and Environment, and the Canada Foundation for
Innovation (CFI) is gratefully acknowledged. We thank Prof.
Todd Sutherland for his kind help with the electrochemistry, J.
B. Lin for helpful discussions regarding the X-ray crystal
structure of 2, and M. Stolar for her help with the TGA
measurements.
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