A structure-property study toward π-extended phosphole chromophores with ambipolar redox properties

Article abstract of DOI:10.1139/cjc-2015-0347

We report the synthesis of a series of π-extended dithienophospholes with phenyl or biphenyl terminal groups via Suzuki Miyaura cross-coupling procedures. The incorporation of the dithienophosphole core into the scaffold on oligophenylenes was found to le

Full text of DOI:10.1139/cjc-2015-0347

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ARTICLE
A structure-property study toward ␲-extended phosphole
chromophores with ambipolar redox properties
Yi Ren and Thomas Baumgartner
Abstract: We report the synthesis of a series of ␲-extended dithienophospholes with phenyl or biphenyl terminal groups via
Suzuki Miyaura cross-coupling procedures. The incorporation of the dithienophosphole core into the scaffold on oligophenyl-
enes was found to lead to pronounced luminescence properties in solution and the solid state, the latter of which also responded
to different solid-state morphologies, i.e., powder versus crystal. More importantly, the investigated molecular architectures also
allowed — for the first time — the observation of ambipolar redox behavior of such species, with the biphenyl-extended species
in particular showing quasi-reversible reduction and oxidation processes; the observed experimental features were correlated
with computational density functional theory studies.
Key words: phosphorus heterocycles, conjugated materials, organic electronics, luminescence, electrochemistry, ambipolar
redox properties, DFT calculations.
Résumé : Nous présentons la synthèse d’une série de dithiénophospholes a` conjugaison ␲ étendue portant des groupes
terminaux phényle ou biphényle que nous avons réalisée par couplage croisé de Suzuki Miyaura. Nous avons observé que
l’incorporation du noyau dithiénophosphole aux oligophénylènes du squelette donnait lieu a` des propriétés de luminescence
marquées, en solution et a` l’état solide. Par ailleurs, ces propriétés se trouvaient modulées selon différentes morphologies de
l’état solide, a` savoir la poudre ou le cristal. Fait important, les architectures moléculaires auxquelles nous nous sommes
intéressés permettaient également — pour la première fois — d’observer le comportement oxydoréducteur ambipolaire de telles
espèces, surtout dans le cas de celles qui étaient étendues grâce aux groupes biphényle, pour lesquelles on observait des processus de
réduction et d’oxydation quasi-réversibles. Ces caractéristiques observées en conditions expérimentales ont été corroborées par
des études de modélisation employant la théorie de la fonctionnelle de la densité. [Traduit par la Rédaction]
Mots-clés : hétérocycles phosphorés, substances conjuguées, électro-organique, luminescence, électrochimie, propriétés
d’oxydoréduction ambipolaires, calculs DFT.
that introducing strongly electronegative fluoro substituents into
well-known hole-transporting materials could induce a strong
Introduction
Organic ␲-conjugated materials can exhibit intriguing photo-
physical properties such as high fluorescence quantum yields,
charge/energy transfer, and tunable absorption/emission as well
as valuable redox behavior and self-organization properties.1–3
These highly diverse features have attracted significant attention
in the area of organic electronics for devices such as organic light-
emitting diodes,1 organic field-effect transistors,2 and organic
photovoltaic cells.3 In addition, low-cost solution processability of
organic ␲-conjugated materials can be further harnessed to fabri-
cate large area and flexible electronic devices for real-world appli-
cations.4 For the majority of practical applications, it is very
important to have a balanced hole and electron injection/trans-
port to achieve good device performance.5 Along with the devel-
opment of organic materials, p-type semiconducting materials (hole
injection/transport materials) have progressed tremendously. How-
ever, n-type semiconducting materials for organic electronics have
remained still relatively limited, even to date.6 Therefore, it is very
desirable to design new organic materials with a low lowest unoccu-
pied molecular orbital (LUMO) energy level that can potentially de-
crease the barrier of electron injection between high work function
electrodes and the LUMO energy levels of organic semiconducting
materials. To lower the LUMO energy level, it has been demonstrated
electron-accepting character and strong intermolecular interactions
with enhanced electron-injection/transport features.5c,7
Compared to thiophenes and pyrroles, on the other hand, both
experimental and theoretical studies revealed that the LUMO energy
level of phosphole derivatives is stabilized through ␴*–␲* orbital cou-
pling.8 This hyperconjugation makes phosphole-based materials
promising electron-acceptor candidates and several articles that
have been published by others and us have now established the
strong electron-acceptor properties of ␲-conjugated phosphole ma-
terials in general.9 However, phosphole-based materials that exhibit
both electron-accepting and electron-donating character evident via
ambipolar redox properties have remained elusive to date. Our early
studies indicated that the dithienophosphole core itself does not
exhibit reversible reduction features.10 Recent efforts by our group
and others involved increasing the ␲-conjugation of the system that
further stabilized the electrochemical reduction features.11 This arti-
cle now deals with the corresponding structure-property studies to-
ward phospholes with ambipolar redox properties, using extended
␲-conjugation to balance the acceptor features of the central
dithienophosphole oxide unit, with (bi)phenyl substituents in the
2,6-positions of the scaffold. Our choice of phenyl terminal groups
was based on their mildly electron-donating character that would
Received 22 July 2015. Accepted 22 August 2015.
Y. Ren* and T. Baumgartner. Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive NW,
Calgary, AB T2N 1N4, Canada.
Corresponding author: Thomas Baumgartner (e-mail: thomas.baumgartner@ucalgary.ca).
*Present address: Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544, USA.
Can. J. Chem. 94: 1–8 (2016) dx.doi.org/10.1139/cjc-2015-0347
Published at www.nrcresearchpress.com/cjc on xx xxx xxxx.

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Can. J. Chem. Vol. 94, 2016
Scheme 1. Synthesis of the ␲-extended phospholes.
not lead to a strongly pronounced charge-transfer state for the
overall conjugated scaffold. Moreover, the presence of torsion
angles between the terminal biphenyl rings and the dithienophos-
phole that potentially limits the conjugation throughout the ex-
tended backbone was expected to support a balancing point
where it is not only possible to stabilize the reduction process of
dithienophosphole core but also to maintain an easy oxidation
process of phenyl/biphenyl groups in the (bi)phenyl-substituted
phospholes.
To gain a deeper understanding of the evolution of potentially
ambipolar redox behavior, we have focused on the systematic exten-
sion of the dithienophosphole scaffold via introduction of two
(symmetric and asymmetric) and four phenyl rings (symmetric) in
the context of this combined experimental and computational
study.
group10c followed by the reaction between the trivalent phos-
phole and trimethoxybenzyl bromide to afford the cationic ben-
zylated derivative 6 (Scheme 1).14
Photophysical properties
The photophysical properties of the extended phosphole oxides
are summarized in Table 1. These phosphole species show the
expected red-shifted absorption and emission upon extension of
the central dithienophosphole core in the order 4 < 3 < 5 (Fig. 2).
Notably, upon changing the solvent polarity, the compounds
show more pronounced red-shifts in their emission (3: ␭
=
em
522 nm in EtOH, ␭em = 507 nm in CH3CN, ␭em = 515 nm in CHCl3,
= 498 nm in cyclohexane; 4: ␭ = 499 nm in EtOH, ␭
␭
=
em
em
em
487 nm in CH3CN, ␭em = 492 in CHCl3; 5: ␭em = 533 nm in EtOH,
em = 522 nm in CH3CN, ␭em = 528 nm in CHCl3) compared to their
absorption maxima (3: ␭ = 422 nm in EtOH, ␭ = 423 nm in
␭
abs
abs
Results and discussion
CHCl3, ␭abs = 418 nm in CH3CN, ␭abs = 415 nm in cyclohexane; 4:
abs = 402 nm in EtOH, ␭abs = 398 nm in CH3CN, ␭abs = 401 in CHCl3;
5: ␭abs = 440 nm in EtOH, ␭abs = 434 nm in CH3CN, ␭abs = 438 nm in
CHCl3), which can be attributed to a more polarized structure in
the excited state.
Phospholium salt 6 exhibits a further red-shifted absorption
and emission due to the more polarized phosphonium center
arising from its increased acceptor character when compared to
its oxide congener 5. Moreover, the emission spectrum of 6 is also
very broad (full width at half-maximum (fwhm) = 94 nm) relative
to the oxide derivatives (cf. fwhm = 81 nm for 5, fwhm = 81 nm for
3, fwhm = 79 nm for 4), suggesting a more flexible structure of 6
similar to that found in the related phosphole-lipids and their
model compounds.13,14 Remarkably, the extended phosphole ox-
ide and phospholium compounds show very high quantum yields
in CHCl3 (Table 1), with 3 showing the highest quantum yield of
␭
The required mono-/dibromodithienophosphole starting mate-
rials 1 and 2 were synthesized according to the procedures re-
ported by our group earlier10c,12 and subsequently reacted with
aryl boronic acid species via Suzuki Miyaura-based protocols to
give the target compounds 3,10c 4, and 5 (Scheme 1). The identity
of all new compounds was confirmed by H, 13C, and 31P NMR
1
spectroscopy and high-resolution mass spectrometry. Interestingly,
the extended phospholes 3, 4, and 5 were found to form 1D
needle-shaped crystals during slow evaporation of the solvent dur-
ing the crystallization that were, however, unsuited for single-
crystal X-ray crystallography (Fig. 1).
This suggested some self-assembly potential for the new molec-
ular materials, a feature that we have developed in our recent
studies via the novel “phosphole-lipids”, which form intriguing
liquid-crystal phases as well as stimuli-responsive organogels from
hydrocarbon solvents.13 In the latter, we have utilized the pres-
ence of long dodecyloxy chains as well as a charged phosphorus
center, which were balanced with the ␲-stacking interactions of
the main conjugated scaffold to give rise to self-assembled nano-/
microstructures. We have thus included the phospholium species
6 in this study as well, particularly since the cationic phosphorus
center should exhibit a more pronounced electron-acceptor char-
acter than the corresponding oxide.11b For the synthesis of the
cationic congener 6, oxide species 5 was first reduced to the tri-
valent phosphole species via procedures also reported by our
␾
= 0.89, which is in line with previously reported ␲-extended
FL
phosphole oxides10c,10f and related phosphole-lipids.13b
The emission maxima of the thin films of 3, 4, 5, and 6 are
red-shifted compared with those of their solutions, probably due
to the restricted rotation of the terminal aryl groups and ␲–␲
interaction between neighboring molecules in the solid state.13b,14
The decreased but still considerable fluorescence quantum yields
of 3, 4, 5, and 6 in the solid state also indicate that ␲–␲ interactions
between the molecules quench the fluorescence to some extent. It is
worth mentioning that 6 with the bulky trimethoxybenzyl group
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Ren and Baumgartner
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Fig. 1. Polarized optical micrographs of the 1D needle-shaped crystals of (a) 3, (b) 4, and (c) 5.
Table 1. Photophysical data of the extended phospholes.
abs (nm)a,
Compound CHCl3
␭
em (nm)a,
␾
,
Band
␭
em (nm)d,
␭
em (nm),
␾
,
FL
b
b
␭
FL
3 (L mol−1 cm−1
)
CHCl3
CHCl3 gap (eV)c film
solid
solid
3
4
5
6
423
401
438
456
19 740
14 700
29 580
515
491
527
0.89
0.51
0.83
0.75
2.58
2.72
2.49
2.38
537
508
558
577
555e
508e
579f
571g
0.15
0.28
0.12
0.36
Not determined 558
aMeasured at 10−5 mol/L.
bDetermined by a calibrated integrating sphere system.
cDetermined from the onset of the absorption.
dDrop cast film from CHCl3 solution (10−3 mol/L).
e1D needle-shaped crystals.
fSolid powders from CHCl3 solution.
gCrystals.
Fig. 2. (a and b) Absorption and emission spectra of the extended phospholes in CHCl3. (c and d) Emission spectra of 5 (left) and 6 (right) at
different states.
exhibits a relatively higher fluorescence quantum yield compared
to the oxide congeners, which is probably due to its propensity to
suppress strong ␲–␲ stacking.13b,14
Interestingly, the extended phospholes also show different
emission features in different states. The 1D needle-shaped crys-
crystal-state emission (␭
= 571 nm) (Fig. 2d). Particularly, the
em
broad thin-film emission of 6 strongly suggests the presence of
several different isomers (i.e., conformers arising from twisted
terminal phenyl groups as well as rotation of the p-benzyl sub-
stituent),13,14 which is not the case for its crystal-state emission,
resulting from a single ‘locked’ conformation for the latter.
tals (␭ = 555 nm) of 3 show a red-shifted emission compared
em
with that of the thin film (␭em = 537 nm). The powder of 5 (␭
=
em
Electrochemical properties
579 nm) obtained from slow evaporation of CHCl3 exhibits a
20 nm red-shifted emission maximum compared with that of the
thin film emission (␭em = 558 nm) (Fig. 2c). The thin film emission
of 6 (␭em = 577 nm) with the bulky trimethoxybenzyl group is very
broad, with a red-shifted shoulder (␭em = 613 nm) compared to its
The electrochemical data are summarized in Table 2. Generally,
the extended phosphole oxides show quasi-reversible reduction
features (Fig. 3) whose potentials are decreasing upon extension of
the central dithienophosphole core (Ered = –2.18 V for 4, –2.08 V for
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Can. J. Chem. Vol. 94, 2016
Table 2. Electrochemistry data of the extended phospholes.
cationic in the case of 6) species (Table 3) as well as their oxidized
and reduced congeners (Table 4), including the spin densities of
the latter.
Compound
E
red (V)a
E
ox (V)a
Band gap (eV)d EA (eV)e IP (eV)f
3
4
−2.08b
−2.18b
0.87b
0.77b
1.09c
0.75b
1.08b
2.59
2.75
−2.52
−2.42
6.01
5.91
The relevant frontier orbitals for all species show the typical
distributions for (extended) dithienophospholes (see Fig. 4 for re-
presentative species and the Supplementary material section for
the complete sets), with the highest occupied molecular orbitals
(HOMOs) comprising the ␲-systems, while the LUMO orbitals rep-
resent the corresponding quinoidal ␲*-systems. HOMO–1 and
LUMO+1 also correspond to the ␲- and ␲*-systems (with the excep-
tion of 6 for which the LUMO+1 largely corresponds to the exocy-
clic P-phenyl group) but have larger contributions from the
appended (bi)phenyl substituents. Importantly, the HOMO–LUMO
gaps match the trend observed for the species by optical spectros-
copy as well as CV (4: 3.39 eV > 3: 3.22 eV > 5: 3.07 eV > 6: 2.84 eV)
but overestimate the band gaps between 0.5 and 0.7 eV (1.2 eV for
6; from CV). It should be pointed out in this context that this
discrepancy could be attributed to the fact that the DFT-derived
values represent the gas phase for 3, 4, and 5, whereas the exper-
imental values have been obtained from solution, which is known
to considerably affect the electronics of polar conjugated systems.
Moreover, it is now also well established that DFT calculations on
conjugated molecules with charge-transfer properties fail to re-
produce the energies of the relevant frontier orbitals.18
The frontier orbitals for 6 support the increased polarity of the
molecular scaffold, suggesting a much more pronounced charge-
transfer character for the HOMO ¡ LUMO transition, evident in
the considerably more localized LUMO on the dithienophosphole
core. Moreover, the LUMO energy level of the cationic 6 is nearly
1 eV lower than that of the other oxide congeners, in line with the
experimentally determined photophysics, the reduction poten-
tials, and the ionic nature of the phosphorus center (vide supra).
Other important parameters are the torsion angles between the
dithienophosphole core and the appended (bi)phenyl termini. It is
important to note that each species shows a considerable twist
between these elements with torsion angles ␶␣ being roughly be-
tween 24° and 27° (Chart 1), while ␶␤ is relatively fixed at 36.5° in
either direction. It should be noted that the calculations also sug-
gest no preference in direction for the twisting; both possible
rotational isomers seem to represent minima on the electronic
hypersurface.
To verify the electrochemical features observed via CV, we then
proceeded to calculate the minimized structures, orbital energies,
and spin density distributions of the oxidized and reduced species
of 3–5, respectively.19 In the case of 5, we also included the dicat-
ionic relative arising from a second oxidation, which was also
observed to be quasi-reversible from the CV. The resulting spin
densities for the radical cations and anions are shown in Fig. 5 and
suggest full delocalization of the respective radicals across the
entire ␲-conjugated scaffold. Also noteworthy is the fact that the
spin densities correspond to the respective singly occupied molec-
ular orbitals (SOMOs) of the oxidized/reduced species as well as
the related HOMOs (oxidation) and LUMOs (reduction) of the neu-
tral species. While this supports the stability of the radicals and
hence the largely quasi-reversible oxidation behavior, it is diffi-
cult to derive the deviating reduction behavior between the spe-
cies, as observed by CV. However, when comparing the energy
levels of the corresponding SOMOs, it is evident that the SOMO of
4-red is significantly higher in energy (–1.78 eV) than those of
3-red and 5-red at approximately –2.0 eV, indicating that it may
be more prone to subsequent decomposition reactions.
5
6
−2.04b
−1.56c
2.47
−2.56
na
5.89
na
0.39c/0.62c 1.66
1.02c/1.16d
1.30c
Note: Versus Fc/Fc+, measured in CH2Cl2 with a platinum rod as the working
electrode, platinum wire as a counterelectrode, and Ag/AgCl/KCl (3M) as the
reference electrode; the supporting electrolyte was NBu4PF6, standard scan rate
was 100 mV/s.
aEred(Eox) = 1/2(Epc + Epa) for reversible or quasi-reversible process; otherwise,
E
red(Eox) = Epc(Epa).
bQuasi-reversible process.
cIrreversible process.
dDetermined from onset of oxidation/reduction.
eElectron affinity.
fIonization potential.
3, –2.04 V for 5 versus Fc/Fc+), in line with the obtained photo-
physical data. Despite being irreversible, 6, with the more positive
phospholium center, shows a more positive reduction potential at
–1.56 V due to a further lowered LUMO energy level, which is
consistent with theoretical studies (vide infra). Interestingly, all
extended phosphole oxides also exhibit quasi-reversible oxida-
tion processes, which is rarely observed in phosphole-based small
conjugated molecules.11b It was found that compounds 4 and 5
with the biphenyl terminus usually display similar oxidation pro-
cesses (first and second oxidation), which are more positive than
the oxidation potentials of the phenyl-substituted compound
(Eox = 0.87 V for 3, 0.77/1.09 V for 4, 0.75/1.08 V for 5 versus Fc/Fc+).
This observation indicates that the quasi-reversible oxidation is
likely due to the oxidation of terminal aryl groups (phenyl and
biphenyl) instead of the phosphole core.
Notably, the electrochemically determined band gaps for the
neutral species 3–5 (3: 2.59 eV; 4: 2.72 eV; 5: 2.49 eV) are near
perfect matches with the optical band gaps from UV-vis spectros-
copy (Table 1). There is, however, a discrepancy between the elec-
trochemically determined and optical band gap for 6, which can
be ascribed to the cationic nature of the species. This may lead to
a more complex situation during the cyclic voltammetry (CV),
potentially involving the electron-donating trimethoxyphenyl
moiety as well, which may also be responsible for the observed
irreversible features.
The solid-state electron affinities and ionization potentials of
the phosphole oxides were estimated using the electrochemical
data (Table 2).15 The estimated solid-state electron affinity values
of 3 and 5 are –2.52 and –2.56 eV, respectively, which are more
exothermic than that of aluminum trisquinolate, Alq3, a popular
electron-transport material (approximately –2.3 eV) and closer to
C60 (approximately –3.7 eV), while the solid-state ionization po-
tential of 5 (5.9 eV) is somewhat higher than that of pentacene, a
benchmark p-type semiconductor material at approximately
5.1 eV.16 Therefore, the current electrochemistry studies suggest
that the ␲-extended phosphole oxides, and particularly 5, may
potentially be suitable candidates for ambipolar semiconducting
materials.
Density functional theory (DFT) calculations
It is also important to note that that oxidation of the ␲-systems
resulted in a considerable flattening of the scaffold, evident in
torsion angles ␶␣ ranging from approximately 1° in 4-ox and 3° in
5-ox to 11.5° in 3-ox. In combination with the least reversible
oxidation behavior of 3, this suggests that a biphenyl group with
its extended ␲-system indeed stabilizes the oxidized species much
To gain some deeper insight into the observed electronic features
of the ␲-extended dithienophospholes, and particularly their elec-
trochemical properties, we have performed DFT calculations at
the B3LYP/6-31+G(d) level of theory using the Gaussian03 suite of
programs.17 In this context, we have determined the frontier
orbital energies and minimized geometries for the neutral (and
more effectively than a single phenyl substituent. Moreover, ␶
␣
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Fig. 3. Cyclic voltammograms of (a) 3, (b) 4, (c) 5, and (d) 6 in CH2Cl2 versus Fc/Fc+; NBu4PF6 as supporting electrolyte, scan rate 100 mV/s.
Table 3. Frontier orbital energies (B3LYP/6-31+G(d)) and twist angles
Chart 1. Definition of torsion angles ␶␣ and ␶␤.
of the terminal phenyl substituents of the extended phospholes.
LUMO+1 LUMO HOMO HOMO−1
Compound (eV)
(eV)
(eV)
(eV)
␶
␣ (°)b
␶
␤ (°)b
3
−0.79
−2.08 −5.30
−6.54
27.0 (1) na
−27.0 (2)
4
5
−1.00
−1.15
−2.02 −5.41
−2.14 −5.21
−6.40
−6.08
26.0 (1) −36.7 (1)
25.8 (1) −36.5 (1)
24.8 (2) −36.6 (2)
27.6 (1) 36.5 (1)
24.0 (2) −36.5 (2)
6a
−2.02
−3.11
−5.95
−6.46
much less pronounced with angles ranging from 24° to 32°, sug-
gesting only a minor contribution from the outermost rings for
stabilization of the oxidized congeners.
Similarly affected torsion angles are also observed for the cor-
responding reduced species, with 4-red (with its irreversible re-
duction) showing the largest value for ␶␣ at 2.9°.
aCalculated using the PCM solvation model (in CH2Cl2) to account for omis-
sion of anions.
bFor definition, see Chart 1.
Table 4. Frontier orbital energies (B3LYP/6-31+G(d)) and twist angles
of the terminal phenyl substituents of the extended phospholes in
their oxidized and reduced states.
Conclusions
LUMO
(eV)a
SOMO
(eV)a
HOMO−1
(eV)a
In conclusion, we have synthesized a series of (bi)phenyl ex-
tended dithienophospholes that are highly luminescent in solu-
tion and that also show considerable emissive properties in the
solid state. Depending on the molecular geometry and available
degrees of rotational freedom in the solid state that determine the
morphologies (i.e., crystal versus amorphous film), the solid-state
emission can be further tuned over wavelengths of up to 20 nm
(for 5) that suggest high utility of this species as stimuli-responsive
solid-state emitters. Moreover, we were able to show that the
␲-extension of the system triggers electrochemically ambipolar
behavior, allowing access to quasi-reversibly stable radical cations
and anions, particularly in the case of the symmetrically biphen-
ylated congener 5. The computationally observed structural changes
upon oxidation and reduction of the scaffold as well as the elec-
tronic distribution within the HOMO and LUMO orbitals lend sup-
port for the phenyl groups being the primary site of oxidation,
while the dithienophosphole core is primarily reduced. Once
oxidized/reduced, the extended ␲-system of the scaffold is able to
largely stabilize the resulting radicals via delocalization. This is
the first example of a highly luminescent dithienophosphole spe-
cies that shows this amipolar redox behavior. Consequently, this
Compound
␶␣ (°)
␶␤ (°)
3-ox
−3.31
−5.63
−7.03
−11.5 (1)
11.5 (2)
0.79 (1)
−3.5 (1)
−2.9 (2)
−1.6 (1)
0.6 (2)
−0.7 (1)
0.7 (2)
2.9 (1)
na
4-ox
5-ox
−3.22
−3.26
−5.60
−5.41
−7.03
−6.48
−28.1 (1)
−31.2 (1)
−31.1 (2)
−23.1 (1)
−22.9 (2)
na
5-ox2
3-red
−5.64
−0.18
−6.02b
−1.92
−7.54
−4.14
4-red
5-red
−0.43
−0.66
−1.78
−2.04
−4.19
−4.16
−28.0 (1)
−30.5 (1)
−30.7 (2)
1.5 (1)
2.4 (2)
aAverage of ␣ and ␤ series orbital energies; PCM solvation model (CH2Cl2).
bSecond oxidation occurs from SOMO and the resulting orbital is HOMO.
flattens even more when 5-ox is oxidized into 5-ox2 (from 3° to 1°),
suggesting an even better delocalization of the ␲-system within
the core scaffold in the latter. It should be noted that ␶ is also
␤
reduced upon oxidation of the main scaffold, but the effect is
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Can. J. Chem. Vol. 94, 2016
Fig. 4. Relevant frontier molecular orbitals of compounds 4–6 (B3LYP/6-31+G(d)).
Fig. 5. Spin density distributions (B3LPY/6-31+G(d)) for the oxidized (left) and reduced (right) varieties of compounds 3–5.
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Ren and Baumgartner
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study has provided important insights for the further design of
stable ambipolar dithienophospholes for optoelectronic applica-
tions and corresponding studies now underway in our laboratory.
14.3 Hz) ppm. HRMS (MALDI-TOF): m/z = 593.1166 ([M+H]+; calcd.
593.1157).
6
Compound 6 was obtained by refluxing the reaction mixture of
the corresponding trivalent phosphole derivative of 5, obtained
via in situ reduction of 5 (0.34 mmol, 200 mg) via the established
procedure,10c,14 and 3,4,5-trimethoxybenzyl bromide (1.2 equiv.,
106 mg) in a toluene THF mixture (30 mL, 40:20) overnight. All
solvents were subsequently removed under vacuum and the phos-
pholium bromide salts were purified by column chromatography
(silica, hexane ethyl acetate: from 9:1 to 1:9). Yield 131 mg, 51%.
1H NMR (400 MHz, CD2Cl2): ␦ = 8.59 – 8.54 (m, 2H), 8.39 (d, J = 2.00,
2H), 7.75 – 7.73 (m, 4H), 7.69 – 7.67 (m, 9H), 7.65 – 7.62 (m, 9H),
7.50 – 7.46 (m, 4H), 7.42 – 7.37 (m, 2H), 6.56 (d, J = 3.20 Hz, 2H), 5.28
(d, J = 14.8 Hz, 2H), 3.76 (s, 3H), 3.58 (s, 6H) ppm. 31P{1H} NMR
(162 MHz, CD2Cl2): 20.0 ppm. 13C{1H} NMR (100 MHz, CDCl3): ␦ =
153.1 (d, J = 4.5 Hz), 150.3 (d, d, J = 14.9 Hz), 147.5 (d, d, J = 12.6 Hz),
142.2 (s), 139.8 (s), 135.2 (d, J = 3.2 Hz), 134.3 (d, J = 11.8 Hz), 131.1 (s),
130.2 (d, J = 13.6 Hz), 129.30 (s), 128.0 (s), 127.9 (s), 127.0 (s), 126.6,6
126.2 (d, J = 96.7 Hz), 124.1 (d, J = 14.0 Hz), 122.1 (d, J = 10.3 Hz), 115.5
(d, J = 83.4 Hz), 107.7 (d, J = 6.5 Hz), 60.9 (s), 56.2 (s), 29.7 (s) ppm.
HRMS (MALDI-TOF): m/z = 757.1971 ([M–Br–]+; calcd. 757.1994).
Experimental data
General procedures
All manipulations were carried out under a dry nitrogen at-
mosphere employing standard Schlenk techniques. Solvents
were dried using an MBraun solvent purification system. Com-
mercially available chemicals were purchased from Sigma-
Aldrich and Alfa-Aesar and were, if not otherwise noted, used as
received. 2,6-Dibromodithieno[3,2-b:2=,3=-d]phosphole oxide 110c
and 2-bromodithieno[3,2-b:2=,3=-d]phosphole oxide 212 were pre-
pared according to literature procedures. NMR solvents were pur-
chased from Cambridge Isotope Laboratories. 31P{1H} NMR, 1H NMR,
and 13C{1H} NMR were recorded on Bruker Avance (-II,-III) 400 MHz
spectrometers. Chemical shifts were referenced to external 85%
H3PO4 (31P) and external TMS (13C and 1H). MALDI/TOF and EI mass
spectra were run on a Bruker Daltonics AutoFlex III system and
Finnigan SSQ 7000 instrument, respectively. All solution fluores-
cence and UV-vis experiments were recorded on a Jasco FP-6600
spectrofluorometer and UV-Vis-NIR Cary 5000 spectrophotome-
ter. Fluorescence quantum yield and lifetimes were measured by
using an Edinburgh Instruments Ltd FLS920P fluorescence spec-
trometer equipped with an integrating sphere. CV analyses were
performed on an Autolab PGSTAT302 instrument with a polished
platinum electrode as the working electrode, a platinum wire as
counterelectrode, and an Ag/AgCl/KCl3M reference electrode
using ferrocene/ferrocenium (Fc/Fc+) as internal standard. If not
otherwise noted, CV experiments were performed in DCM with
tetrabutylammonium hexafluorophosphate (0.1 mol/L) as the
supporting electrolyte. Theoretical calculations have been carried
out at the B3LYP/6-31+G(d) level using the Gaussian03 suite of
programs.17
Supplementary material
Supplementary material is available with the article through
the journal Web site at http://nrcresearchpress.com/doi/suppl/
10.1139/cjc-2015-0347.
Acknowledgements
Financial support by NSERC of Canada and the Canada Foundation
for Innovation is gratefully acknowledged. Y.R. thanks Alberta Inge-
nuity now part of Alberta Innovates - Technology Futures as well as
Talisman Energy for graduate scholarships.
General synthetic procedure for 3,10c 4, and 5
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4
1H NMR (400 MHz, CD2Cl2): ␦ = 7.82 – 7.76 (m, 2H), 7.62 – 7.57 (m,
6H), 7.57 – 7.53 (m, 1H), 7.47 – 7.43 (m, 4H), 7.39 – 7.34 (m, 2H), 7.31
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ppm. 31P{1H} NMR (162 MHz, CD2Cl2): 20.3 ppm. 13C{1H} NMR
(100 MHz, CDCl3): ␦ = 148.0 (d, J = 14.3 Hz), 146.1 (d, J = 42.2 Hz), 144.1
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111.7 Hz), 132.5 (d, J = 2.9 Hz), 132.3 (s), 130.9 (d, J = 11.3 Hz), 129.7 (d,
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(EI-TOF): m/z = 440.0448 ([M]; calcd. 440.0458).
5
1H NMR (400 MHz, CD2Cl2): ␦ = 7.87 – 7.82 (m, 2H), 7.63 – 7.60 (m,
12H), 7.59 – 7.54 (m, 1H), 7.50 – 7.44 (m, 6H), 7.41 (d, J = 2.40 Hz, 2H),
7.37 (tt, J = 6.40 Hz, J = 1.2 Hz, 1H) ppm. 31P{1H} NMR (162 MHz,
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(s), 144.5 (s), 144.3 (s), 140.7 (d, J = 104.0 Hz), 139.4 (d, J = 110.9 Hz),
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13.1 Hz), 128.9 (s), 127.8 (s), 127.7 (s), 126.9 (s), 126.1 (s), 121.5 (d, J =
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(19) Unfortunately, all attempts to obtain meaningful results for the oxidized
and reduced varieties of 6 failed, as neither calculation successfully con-
verged to a minimum.
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