Luminescent p-benzyl dithienophospholes - A joint experimental and theoretical investigation

Article abstract of DOI:10.1071/CH13220

A series of P-benzyl functionalised dithieno[3,2-b:2′,3′-b] phospholes with different substitution pattern at the phosphorus as well as the conjugated scaffold was synthesised and characterised via optical spectroscopy. Single crystal X-ray crystallography was performed on one species. The experimentally observed data were solidified with density functional theory calculations. In contrast to related benzylated P-phenyl phospholium species, the new systems show pronounced photoluminescence in solution, with the exception of the phosphole sulfide species. The observed photophysics could be explained with dominating π→π*transitions, despite the presence of the benzyl group that had been found to quench the fluorescence in the predecessor benzyl system with P-phenyl substituent.

Full text of DOI:10.1071/CH13220

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 1171–1178
Full Paper
http://dx.doi.org/10.1071/CH13220
Luminescent P-Benzyl Dithienophospholes A Joint
]
Experimental and Theoretical Investigation
A
A
A B
,
Zisu Wang, Alva Y. Y. Woo, and Thomas Baumgartner
A
Department of Chemistry and Centre for Advanced Solar Materials, University of
Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada.
Corresponding author. Email: thomas.baumgartner@ucalgary.ca
B
A series of P-benzyl functionalised dithieno[3,2-b:20,30-b]phospholes with different substitution pattern at the phosphorus
as well as the conjugated scaffold was synthesised and characterised via optical spectroscopy. Single crystal X-ray
crystallography was performed on one species. The experimentally observed data were solidified with density functional
theory calculations. In contrast to related benzylated P-phenyl phospholium species, the new systems show pronounced
photoluminescence in solution, with the exception of the phosphole sulfide species. The observed photophysics could be
explained with dominating p-p* transitions, despite the presence of the benzyl group that had been found to quench the
fluorescence in the predecessor benzyl system with P-phenyl substituent.
Manuscript received: 30 April 2013.
Manuscript accepted: 20 May 2013.
Published online: 26 June 2013.
Introduction
generally introduces considerable electron-acceptor charac-
ter.[4e,5d,6] Both features are made possible – as a result of the
strongly pyramidal nature of the phosphorus – through an orbital
interaction betweenthe s*-orbital of the exocylic P-substituent(s)
and the p*-system of the main scaffold (Fig. 1b).
In the context of designing dithienophosphole-based self-
organising materials, we were recently able to access the novel
phosphole-lipid system (Fig. 1c) that can form smectic liquid-
crystal (LC) phases (A),[7] or one-dimensional fibrous organo-
gels (B)[8] from hydrocarbon solvents, depending on the nature
of the conjugated scaffold. Next to the self-assembly behaviour,
The development of organic p-conjugated chromophores is an
active area of research, as these species have considerable
potential for practical applications in very diverse areas, such as
organic electronics or the biomedical context.[1] The organic
nature of the chromophores allows for a relatively efficient way
of tuning the electronic properties of the materials at the
molecular level, so that they can be employed in organic light-
emitting diodes (OLED),[1a] organic field-effect transistors
(OFET),[1b] organic photovoltaics (OPV),[1c] but also as sensory
materials,[1d] photoactive switches,[1e,f] and biolabels,[1f,g] to
name but a few important applications.
However, while generally quite successful, the use of genu-
ine i.e. carbon-based, organic materials does have its limitations,
as the modification of the optical and electronic properties of the
materials can require laborious synthetic approaches to achieve
the desired properties. In order to tailor the materials’ properties
in a more substantial fashion, the introduction of main group
elements into the scaffold of organic p-conjugated materials has
recently started to draw an increasing amount of attention.
Particularly boron-,[2] silicon-,[3] and phosphorus-based[4]
conjugated materials have gained considerable popularity in
the field. Due to their intrinsic electronic properties and unique
chemistries, these elements have been found to impart very
desirable properties onto the carbon-based molecular scaffolds.
Along these lines, we have established the highly lumines-
cent dithieno[3,2-b:20,30-d]phosphole system as a powerful
chromophore over the past decade (Fig. 1a).[5] The phosphorus
atom at the core of the molecular scaffold provides a unique set
of opportunities for effectively manipulating the photophysical
and electronic properties of the system. Through simple modifi-
cation of a trivalent P-centre (E ¼ lone pair), we were able to
significantly change the luminescence properties of the scaffold,
but we could also show that incorporation of the P-centre
(a)
(b)
S
S
6
2
R
R
P
P
E
Ph
E ϭ Ip, O, S, Meϩ, etc
(c)
S
π
π
S
S
S
π
π
ϩ
ϩ
P
P
Ph
Ph
OC12H25
C12H25
O
OC12H25
OC12H25
C12H25O
OC12H25
A
B
Fig. 1. (a) Dithienophosphole system (lp ¼ lone pair); (b) s*-p*-
interaction in phospholes; (c) liquid-crystalline (A) and organogel-forming
(B) phosphole-lipids.
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

1172
Z. Wang, A. Y. Y. Woo, and T. Baumgartner
the LC-forming species A also showed some unexpected photo-
physical properties. In contrast to simple dithienophospholes,
these benzylated phospholium species do not exhibit pro-
nounced luminescence in solution at room temperature, but they
rather show aggregation-induced enhanced emission behaviour
in the solid state. Through extensive studies, involving a series
of suitable model compounds,[9] we could establish that the
benzyl group at the phosphorus centre provides several non-
radiative relaxation pathways that quench the luminescence in
solution, which are based on i) the inherent flexibility of this
substituent leading to intramolecular rotation, and ii) its elec-
tronic nature that opens up photoinduced electron-transfer
(PET) processes, particularly for electron-rich benzyl groups.
To further elaborate the effects of a benzyl substituent at the
phosphorus centre on the overall photophysics of dithieno-
phospholes, we now report the structure-property study of a
new family of species that exhibit an exocyclic benzyl sub-
stituent instead of the common phenyl group at phosphorus. This
allowed us to investigate the effects of further functionalisation
of the P-centre using simple modifications according to our
established protocols. It should be noted that due to the
benzylation being the last functionalisation step in the original
phosphole-lipids,[7,9] these modifications are not possible with
the earlier system. In this study, we have further addressed the
flexibility of the system by investigating two scaffold variations
with and without steric bulk that should have an impact on
potential intramolecular rotation. Our experimental studies
involving detailed optical investigations have also been
supported by extensive theoretical calculations.
Unfortunately, the isolation of the trivalent species 2a and 2b
posed some unexpected difficulties, particularly in case of the
oily 2a that precluded its isolation in pure form, whereas 2b
could be recrystallised. In the case of 2a, oxidation of the crude
product and subsequent reduction according to our established
two-step protocol using BH3ꢁSMe2 and NEt3[11] provided a
means of purification. However, both 2a and 2b could
ultimately not be purified to full satisfaction. The identity of
both species, on the other hand, could nevertheless be
established via multinuclear NMR spectroscopy.
Consequently, we opted to oxidise the crude 2a,b according
to our established protocols using H2O2 to provide the corre-
sponding oxides 3a and 3b, which could then be isolated in
appreciable purity in moderate to low yields (3a: 26 %;
3b: 38 %).[5] The same applies to the corresponding phosphole
sulfides that were obtained in 20 % (4a) and 48 % (4b) after
treatment with elemental sulfur and column chromatography,
respectively.[5b] To complete the series, we also targeted the
bis(benzyl)phospholium species 5a,b to provide a suitable link
between the new and the original benzylated systems. In terms
of synthesis, 5a was generated via reduction of the phosphole
oxide 3a, following our reported two-step procedure,[11] and
subsequent treatment with benzyl bromide, providing 5a in
moderate yield (36 %). For the synthesis of the silylated species,
the crude trivalent phosphole 2b was treated with benzyl
bromide, which allowed for the isolation of 5b in 37 % using
preparative TLC.
Multinuclear NMR spectroscopy revealed some interesting
features that provided valuable insights into the molecular
structures of the new species. The 2,6-H-terminated species
3a–5a, showed highly symmetric sets of signals for the organic
scaffold, with 31P NMR shifts of d 26.4 ppm for 3a, 31.3 ppm for
4a, and 25.3 ppm for 5a and a single 1H/13C NMR signal for the
benzyl CH2-group in form of a typical doublet due to coupling
with phosphorus, suggesting no specific interaction of the
benzyl group with the conjugated scaffold in solution. Notably,
the 31P resonances are downfield-shifted from the values for the
available P-phenyl congeners (PO: 19.0 ppm;[5a] PBzþ:
18.8 ppm[7]), likely as a result of the different electronics of
the benzyl group, but they show a similarly small split between
them. The TBDMS-substituted species 3b and 4b also showed
comparable features to those observed for related P-phenyl
congeners as well as 3a and 4a, with 31P NMR shifts of
Results and Discussion
Synthesis
In order to potentially control and limit the degree of rotational
freedom of the benzyl substituent we have targeted two scaffold
variations: one that has no substituents at the 2- and 6-position of
the scaffold, and another that exhibits bulky t-BuMe2Si
(TBDMS) substituents at these positions. The synthesis of the
corresponding 3,30-dibromo-bithiophene precursors 1a,b
followed reported procedures[5b,10] and the incorporation of the
phosphorus centre was adapted from the known synthesis
towards the P-phenyl substituted dithienophospholes[5] using
benzyl dichlorophosphane (BzPCl2) instead of PhPCl2 in Et2O
at ꢀ788C after lithiation of 1a,b with n-BuLi (Scheme 1).
Њ
Ϫ
ϭ
ϭ
ϭ
ϭ
ϭ
ϩ
Ϫ
ϭ
ϭ
ϭ
ϭ
Scheme 1. Synthesis of the benzylated dithienophospholes.

Benzyl-dithienophospholes
1173
d 25.1 ppm for 3b and 28.9 ppm for 4b (c.f.: PO: 14.9 ppm;
PS(SiMe3)2: 23.6 ppm).[5b] In line with earlier observations on
TBDMS-substituted dithienophospholes, the SiMe2 resonances
are split into two signals, due to the restricted rotation resulting
from the bulky nature of the t-Bu group. X-ray crystal structure
analyses of several related P-phenyl dithienophospholes have
revealed an anti arrangement of these groups with one t-Bu
group residing above and the other below the planar
p-conjugated scaffold.[5b,12] From these NMR data, a similar
conformation can thus also be assumed for the new species. The
bis(benzylated) species 5b, however, showed some distinctly
different NMR data from those of the other relatives.
Its 31P NMR shift of d 32.8 ppm is clearly downfield-shifted
from those of the corresponding oxide and sulfide congeners,
opposite to the H-terminated series. In addition, the t-Bu and Me
resonances of the TBDMS group are split into two different sets
of signals in 1 : 1 ratios with a much more pronounced split
between them (Dd(1H) ¼ 0.1 ppm for both t-Bu and Me) sug-
gesting different environments for each of the two groups.
Furthermore, the benzyl CH2-group shows a higher-order split-
Material). A distinctly different molecular structure is further
suggested by the presence of 11 aromatic resonance signals in the
13C NMR spectrum (as opposed to the expected eight signals seen
in the aromatic region for the other congeners). The 1D- as well as
2D-NMR data for 5b thus clearly support an unsymmetrical
molecular scaffold as a result of the ring-opened phosphole unit.
It is well known that phosphole systems (including dithieno-
phospholes) tend to ring-open in the presence of alkoxides or
hydroxides.[13] HRMS data confirms the absence of the bromide
anion that has been formally exchanged for OHꢀ. Consequently,
OHꢀ (likely from water) must have been present during the
synthesis of 5b, whose steric congestion further enabled the
ring-opening process leading to the product shown in Scheme 2.
We were able to obtain single crystals of 4a by slow diffusion
of pentane into an acetone solution at room temperature, that were
suitable for an X-ray diffraction study (Fig. 2, Table 1). The bond
lengths and angles of the main scaffold are completely in line
with those of other crystallographically characterised dithieno-
phospholes, suggesting excellent p-conjugation.[5–7,11,12]
Compared with a related P-phenyl species, the exocyclic
1
˚
ting pattern in the H NMR spectrum, which clearly deviates
from the doublet seen for the other relatives (see Supplementary
P1–C11 is slightly elongated at 1.8277(16) A (c.f.: 1.810
˚
(3) A),[5b] which can be attributed to the sp3-hybridised nature
˚
of the benzyl CH2-group. The P1–S3 bond length of 1.9436(5) A
is also slightly elongated with respect to the P-phenyl congener
(c.f.: 1.9267(13) A).[5b] The most pronounced structural feature
˚
Ϫ
ϩ
ϩ
Ϫ
Ϫ
is the fact that the benzyl group does not reside above the
p-conjugated scaffold, but is rotated away from it. This is
surprising, since all of the originally reported benzylated
P-phenyl dithienophospholes exhibit the former conforma-
tion.[7,9] However, intermolecular packing involving the benzyl
group that could lead to the observed conformation could not be
discerned from the X-ray crystallography; the only ‘strong’
intermolecular interaction arises from partial p–p stacking
of the conjugated scaffold.
5b
Ϫ
Scheme 2. Ring-opening reaction in the presence of OHꢀ leading to 5b.
S1
C1
S2
C4
C2
C3
Photophysical Properties
C5
To our surprise, almost all of the new P-benzyl species 3–5
turned out to be highly fluorescent in solution and the solid state.
C8
C6
C7
P1
S3
Table 1. Crystal data and refinement
details for 4a
C17
C11
C16
C15
C12
Temperature [K]
Formula
173(2)
C15H11PS3
318.42
C14
Formula weight
Crystal system
Space group
C13
Monoclinic
P21/n
˚
a [A]
˚
b [A]
˚
c [A]
10.59110(10)
6.4870(10)
21.9877(2)
90
a [8]
b [8]
g [8]
94.0391(2)
90
3
˚
V [A ]
1445.15(3)
4
Z
F000
656
Dcalc [g cmꢀ3
Rfln col/ind
R(int)/Prms
]
1.464
26451/2838
0.0290/216
0.0290
Fig. 2. Molecular structure of 4a (top, 50 % probability level) and inter-
˚
molecular interaction (bottom) in the solid state. Selected bond lengths (A):
R1 [I .2s(I)]
wR2 [I .2s(I)]
GoF
0.0775
C1–C2: 1.361(2); C2–C3: 1.416(2); C3–C4: 1.378(2); C4–C5: 1.456(2);
C5–C6: 1.374(2); C6–C7: 1.412(2); C7–C8: 1.362(2); P1–C3A: 1.8064(15);
P1–C6: 1.8027(14); P1–C31: 1.8277(16); P1–S3: 1.9436(5).
1.093
Peak/Hole
0.494/–0.198

1174
Z. Wang, A. Y. Y. Woo, and T. Baumgartner
As exception, the phosphole sulfides 4a,b are only weakly
fluorescent. Given the fact that both the oxides 3a,b as well as
the bis(benzyl)phospholium species 5a,b are indeed highly
fluorescent, the weak luminescence of 4a,b could likely be
attributed to the heavy atom effect of the sulfur, rather than
intramolecular rotation. A similar feature was observed in the
recently reported dithiazolophosphole system.[14] The photo-
physical data of 3a,b–5a,b are summarised in Table 2 and the
UV/Vis spectra are shown in Fig. 3.
deviation reflects the observed NMR data and confirms the
disruption of the conjugation due to ring-opening (see
above).[13b] The photoluminescence quantum yields of 5a and
5b are also quite different with fPL ¼ 0.16 for 5a, and an
impressive value near unity for 5b, which is likely the result
of a very rigid molecular scaffold in 5b due to steric congestion.
Theoretical Calculations
To gain some deeper insight into the observed experimental
features, we have optimised the molecular structures of
compounds 2a–5a (as well as the originally targeted ring-closed
5b9) using density functional theory (DFT) calculations at the
B3LYP/6–31þG(d) level of theory[15] that has provided satis-
factory results for the dithienophosphole system before.[5–7,11]
In all cases, the minimisation resulted in a molecular structure
similar to that observed with 4a in the solid state. As can be seen
in Fig. 4, the benzyl group is rotated away from the conjugated
scaffold, the latter of which also dominates the frontier orbitals.
Similar to typical dithienophospholes, the HOMO represents the
p-system, whereas the LUMO represents the p*-system with
the phosphole-typical s*–p* interaction.[5] Depending on the
P-functionalisation, the p-system of the benzyl group is part of
the HOMO-1. However, in most cases, the HOMO-1 energies
are significantly lower than those of the corresponding HOMO,
which indicates only a remote possibility for these orbitals to be
involved in optical transitions. It is consequently plausible that
the optical transitions are dominated by p-p* transitions
leading to the observed strong photoluminescence of the system
(see below). The calculations further confirm that the sulfur lone
pairs of 4a are indeed involved in the optical transitions (as part
of the HOMO, but also the HOMO-1, which is nearly degenerate
to the HOMO as a result of the heavy atom effect) and clearly
explain the quenched fluorescence of 4a. The second, high-
energy absorption band at labs ¼ 310 nm in the UV spectra of 4a
(Fig. 3), which is absent for the other species (except 4b), also
supports this conclusion.
In order to verify that the conformation of the benzyl group
does not have a considerable impact in the photophysics (other
than the previously established shift in absorption and
emission),[7,9] we have also calculated rotational conformers
of 2a, 3a, and 5a, in which the benzyl group resides above the
conjugated scaffold. It should be noted that, when the benzyl
group was placed above the scaffold as input, the optimisation
also provided a local energy minimum with this conformation.
However, no discernable energy difference between the two
conformers could be detected, suggesting that while free
rotation of the benzyl group should be possible, it does not lead
to noticeable differences in the optical transitions. In fact, most
of the orbital distributions, as well as energies remain fairly
close to those of the corresponding ‘anti’-conformers (Fig. 5).
The calculations for the ring-closed 5b9 (polarisablecontinuum
model, PCM; solvation, CH2Cl2) suggest a conformation
similar to that of 5a-rot (see Supplementary Material) with
electron-density distributions and energies close to those obtained
for the frontier orbitals of the non-silylated species 5a-rot. The
only difference is the s-donor/p-acceptor character of the
TDBMS group, which is reflected in slightly different energies
for 5b9 (HOMO-1: E ¼ ꢀ7.34 eV; HOMO: E ¼ ꢀ6.58 eV;
LUMO: E ¼ ꢀ2.94 eV). This further indirectly supports the
ring-opened nature of the experimentally isolated 5b.
In line with our earlier observations on the P-phenyl
congeners,[5b] the absorption and emission wavelengths of the
phosphole oxide and sulfide species are nearly identical
(3a: labs ¼ 356 nm; lem ¼ 437 nm, 4a: labs ¼ 357 nm; lem
¼
444 nm). In addition, the TBDMS groups lead to a small red
shift in absorption of 3b and 4b, as well as increased extinction
coefficients, that are also similar to earlier observations and can
be attributed to the s-donor/p-acceptor character of this sub-
stituent (3b: labs ¼ 369 nm; 4b: labs ¼ 369 nm), while the
emission of 4b does not show a similar shift (lem ¼ 443 nm;
c.f. 3b: lem ¼ 444 nm). Moreover, there is a distinct difference in
the photoluminescence quantum yields of the oxide and sulfide
species, respectively (3a,b fPL E 69 %; 4a,b fPL E 1 %). It is
also worth mentioning that while the bis(benzyl) species 5a
shows a similar behaviour as related P-phenyl phospholium
species with further red-shifted absorption and emission wave-
lengths (labs ¼ 366 nm; lem ¼ 452 nm),[7,9] the absorption and
emission wavelengths of 5b are blue-shifted to all the other
species in this series (labs ¼ 305 nm; lem ¼ 406 nm); this
Table 2. Photophysical properties of compounds 3a,b, 4a,b, and 5a,b
in solution
Compound
lAabs [nm]
e [L molꢀ1 cmꢀ1
]
lAem [nm]
fBPL
3a
3b
4a
4b
5a
5b
356
369
7370
12100
437
444
444
443
452
406
69 %
69 %
0.7 %
1.6 %
16 %
99 %
310, 357
315, 369
366
5910, 6600
5970, 13100
5450
305
10500
AIn dichloromethane, c E 10ꢀ6 M.
BVersus aqueous quinine sulfate 0.1 M.
3a
4a
5a
3b
4b
5b
2ϫ104
1.5ϫ104
1ϫ104
5ϫ103
0
250
300
350
400
450
Wavelength [nm]
To finally verify the nature of the observed optical transi-
tions, we have also performed time-dependent (TD)-DFT cal-
culations using both rotational isomers of 3a as well as anti-4a
Fig. 3. UV-Vis spectra of the P-benzyl dithienophospholes in
dichloromethane.

Benzyl-dithienophospholes
1175
Compd
2a
HOMO-1
HOMO
LUMO
Ϫ6.36 eV
Ϫ5.70 eV
Ϫ1.68 eV
3a
4a
Ϫ6.77 eV
Ϫ6.06 eV
Ϫ2.16 eV
Ϫ6.17 eV
Ϫ6.02 eV
Ϫ2.19 eV
5a
Ϫ7.30 eV
Ϫ6.67 eV
Ϫ2.91 eV
Fig. 4. DFT-calculated orbitals (HOMO-1, HOMO, and LUMO) and energies of minimised structures
of 2a–5a in anti-conformation. A PCM solvation model (CH2Cl2) was applied to 5a to account for the
omission of the counter anion.
Compd
2a-rot
HOMO-1
HOMO
LUMO
Ϫ6.22 eV
Ϫ5.67 eV
Ϫ1.65 eV
3a-rot
Ϫ6.77 eV
Ϫ6.04 eV
Ϫ2.14 eV
5a-rot
Ϫ7.31 eV
Ϫ6.68 eV
Ϫ2.90 eV
Fig. 5. DFT-calculated orbitals (HOMO-1, HOMO, and LUMO) and energies of minimised structures of 2a-rot,
3a-rot, and 5a-rot in syn-conformation. A PCM solvation model (CH2Cl2) was applied to 5a-rot to account for the
omission of the counter anion.

1176
Z. Wang, A. Y. Y. Woo, and T. Baumgartner
as representative examples. For both isomers of 3a, the lowest-
energy absorption, which also happens to be the most intense (as
derived from its oscillator strength), corresponds to HOMO-
LUMO (100 %; 3a: f ¼ 0.1677, 3a-rot: f ¼ 0.1533), with the
absorption wavelength showing a slight shift as a function of the
conformation (3a: 360 nm; 3a-rot: 362 nm), in line with our
earlier observations and the optical spectroscopy data. By
contrast, the most intense absorption for 4a at 342 nm (f ¼
0.1681) is a combination of HOMO-2-LUMO (73.2 %) and
HOMO-LUMO (11.7 %). The HOMO-2 (E ¼ ꢀ6.35 eV) has a
similar distribution to that of the HOMO, however with a
bonding combination between the sulfur lone pair and the
p-system of the scaffold. The TD-DFT data thus clearly support
that the observed optical spectroscopy data indeed involve
strong p–p* transitions, regardless of the conformation and
consequently explain the significant photoluminescence of the
new system. Moreover, the mixing of the sulfur lone pair with
the p–p* transitions also explains the origin of the reduced
luminescence of 4a. The calculations further eliminate the
possibility of PET and thus explain the distinctly different
behaviour of this system compared with the originally reported
P-phenyl benzyl-phospholium species.
MBraun solvent purification system before use. 3,30-Dibromo-
2,20-bithiophene (1a),[10] and 5,50-bis(t-butyldimethylsilyl)-
3,30-dibromo-2,20-bithiophene (1b)[5b] were prepared according
to reported procedures. All reactions and manipulations were
carried out under a dry nitrogen atmosphere employing standard
1
Schlenk techniques. H, 13C, and 31P NMR were recorded in
CDCl3 on Bruker DRX400 and Avance (-II,-III) 400 MHz
spectrometers. Chemical shifts were referenced to external 85 %
H3PO4 (31P) or residual non-deuterated solvent peaks (1H, 13C).
Mass spectra were run on a Finnigan SSQ 7000 spectrometer or
a Bruker Daltonics AutoFlex III system. All photophysical
experiments were carried out on a Jasco FP-6600 spectrofluoro-
meter and UV-Vis-NIR Cary 5000 spectrophotometer. Quan-
tum yields were referenced to aqueous quinine sulfate in 0.1 M
sulfuric acid (excitation at 366 nm; fPL ¼ 0.54). Theoretical
calculations have been carried out at the B3LYP/6–31þG(d)
level by using the GAUSSIAN 03 suite of programs using a PCM
solvation model (solvent ¼ CH2Cl2) for the benzylated
species.[14]
Crystal Structure Determination and Refinement
A single crystal of 4a was mounted on MiTeGen micro-loops
and then placed under an Oxford Cryosystems Cryostream Plus
for temperature control. The crystal structure determination was
carried out on a Bruker APEX II diffractometer with graphite-
Conclusion
We have synthesised two sets of dithienophospholes with
P-benzyl groups that were functionalised with either hydrogen
or TBDMS groups at the 2,6-positions of the molecular scaffold.
Utilising standard phosphorus-modification protocols, we have
further modified this centre with a variety of substituents
including oxygen, sulfur, and benzyl. The 2,6-H-terminated
phosphole sulfide could be characterised crystallographically.
Its structure showed a conformation in which the benzyl group
was rotated away from the conjugated scaffold, in contrast to the
P-phenyl relatives. Moreover, most of the new species unex-
pectedly showed pronounced photoluminescence in solution,
with the exception of the phosphole sulfides, which is also in
contrast to the P-phenyl relatives and the result of dominant
p-p* transitions. Due to steric congestion and the resulting
ring-opening of the central phosphole unit, the TBDMS-
terminated bis(benzylated) relative of this series showed unex-
pected blue-shifted photophysics, however, with an impressive
quantum yield near unity that is distinctly different from the
other relative in this series.
˚
monochromated CuKa radiation (1.5418 A) at 173 K. The
intensity data collection was performed in the v-f scanning
mode with the goniometer and detector angular settings opti-
mised. The unit cells and the orientation matrices were refined
using the entire dataset of reflections. The diffraction spots were
measured in full, scaled with SAINT, corrected for Lorentz-
polarisation correction, and integrated using SAINT (Bruker).[16]
All structures were solved using direct methods with SHELXS-
97 software and least-squares refinement against F2 was per-
formed with SHELXL-97 software.[17] Anisotropic temperature
factors were used for refinement of all atoms excluding hydro-
gen; hydrogen atoms were refined using isotropic temperature
factors in the riding mode. All crystal structure figures were
created using Mercury 3 software. CCDC-935961 contains the
supplementary crystallographic data for compound 4a. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
The experimental data could be corroborated by TD-DFT
calculations at the B3LYP/6–31þG(d) level of theory that
suggest that while the benzyl group is indeed somewhat flexible
in the present system, intramolecular rotation does not affect its
pronounced luminescence in solution. The same is true for
photo-induced electron transfer that can be excluded to occur
from the calculations. This deviating behaviour to the original
phosphole-lipids can thus be explained with the different elec-
tronics of the benzyl group (compared with phenyl), which
seems to make non-radiative relaxation pathways energetically
unfavourable. Future investigations will include the substitution
of the benzyl group with moieties that exhibit stronger electron-
donating character to further elucidate the boundaries of this
group’s effect on the photophysics of the scaffold.
Synthetic Procedures
P-Benzyl-dithieno[3,2-b:20,30-d]phosphole Oxide (3a)
3,30-Dibromo-2,20-bithiophene (2.01 g, 6.2 mmol) was dis-
solved in 250 mL dry Et2O. The solution was cooled to ꢀ788C.
n-Butylithium (2.5 M in Et2O, 5.1 mL, 12.75 mmol) was added
dropwise at ꢀ788C. The reaction was kept at this temperature for
1.5 h. Dichlorobenzylphosphine (1.2 g, 6.2 mmol) was dissolved
in 15 mL of Et2O then added slowly to the reaction at ꢀ788C.
After the addition, the suspension was warmed up to room
temperature with a water bath. Solvent was removed under
vacuum. The residue was taken up in pentane then filtered
through a neutral alumina plug. Pentane was then replaced with
40 mL of CH2Cl2. H2O2 (6 mL) was added and the reaction was
stirred overnight. The solution was dried by MgSO4 then
concentrated. Column chromatography (CH2Cl2 : MeCN¼ 10 : 1)
afforded the product as an off-white solid (0.48 g, 26 %).
Experimental
General
All chemical reagents were purchased from commercial sources
(Aldrich, Alfa Aesar) and were, unless otherwise noted, used
without further purification. Solvents were dried using an
l
max/nm(e/Mꢀ1 cmꢀ1) 356 (7370); dH (CDCl3) 7.28–7.19 (m,
1
5H, Ph), 7.13–7.06 (m, 2H, thieno), 7.04 (dd, JH-H ¼ 4.8 Hz,
3JH-P ¼ 2.1 Hz, 2H, thieno), 3.48 (d, JH-P ¼ 15.5 Hz, 2H,
2

Benzyl-dithienophospholes
1177
P-CH2-Ph); dP ꢀ27.49; dC 145.28 (d, JC-P ¼ 23.0 Hz), 137.03 (d,
Afterwards, a bright yellow precipitate had formed and was
filtered off. The crude product was washed with acetone then
crystallised from a mixture of methanol and acetone to afford the
product as light yellow crystals (0.4 g, 36 %). lmax/nm(e/Mꢀ1
cmꢀ1) 366 (5450); dH 8.04 (dd, 1JH-H ¼ 5.0 Hz, 3JH-P ¼ 1.7 Hz,
2H, thieno), 7.43 (dd, 3JH-H ¼ 5.0, 2JH-P ¼ 4.0 Hz, 2H, thieno),
7.23 – 7.16 (m, 4H, Ph), 7.16–7.04 (m, 6H, Ph), 5.03 (d,
JC-P ¼ 107.1 Hz), 131.29 (d, JC-P ¼ 8.4 Hz), 129.66 (d, JC-P
¼
5.6 Hz), 128.40 (d, JC-P ¼ 3.2 Hz), 127.83 (d, JC-P ¼ 14.4 Hz),
127.06 (d, J ¼ 3.7 Hz), 126.22 (d, JC-P ¼ 13.7 Hz), 37.77 (d, JC-P
¼
67.6 Hz); m/z (HR-MS ESI) 303.0065; [MþH]þ requires
303.0062.
P-Benzyl-dithieno[3,2-b:20,30-d]phosphole Sulfide (4a)
2JH-P ¼ 15.7 Hz, 4H, P-CH2-Ph); dp 26.35; dC 149.66 (d, JC-P
¼
3,30-Dibromo-2,20-bithiophene (0.32 g, 1 mmol) was dis-
solved in 50 mL dry Et2O. The solution was cooled to ꢀ788C.
n-Butylithium (2.5M in Et2O, 0.8 mL, 2 mmol) was added
dropwise at ꢀ788C. The reaction was kept at this temperature
for 1.5 h. Dichlorobenzylphosphine (0.19 g, 1 mmol) was dis-
solved in 5 mL of Et2O then added slowly to the reaction at
ꢀ788C. After the addition, the suspension was warmed up with a
room temperature water bath. Solvent was removed under
vacuum. The residue was taken up in pentane then filtered
through a neutral alumina plug. Pentane was then replaced with
10 mL of CH2Cl2. S8 powder (0.13 g, 4 mmol) was added. The
reaction was stirred overnight, after which the solution was
concentrated. Column chromatography (CH2Cl2 : hexane ¼
1 : 1) afforded the product as a light yellow powder (0.065 g,
17.0 Hz), 130.11 (d, JC-P ¼ 5.9 Hz), 129.87 (d, JC-P ¼ 13.6 Hz),
129.73 (d, JC-P ¼ 14.0 Hz), 128.82 (d, JC-P ¼ 3.8 Hz), 128.43 (d,
JC-P ¼ 4.4 Hz), 127.39 (d, JC-P ¼ 9.9 Hz), 125.43 (d, JC-P
377.0583; [M-Br]þ requires 377.0583.
¼
93.5 Hz), 29.21 (d, JC-P ¼ 41.2 Hz); m/z (HR-MS ESI)
P-Benzyl-2,6-bis(t-butyldimethylsilyl)-dithieno[3,2-
b:20,30-d]phosphole (2b)
3,30-Dibromo-5,50-bis(tert-butyldimethylsilyl)-2,20-bithio-
phene (1.6 g, 2.9 mmol) and N,N,N0N0-tetramethylethylene-
diamine (2.25 mL, 15 mmol) were dissolved in 72 mL Et2O.
The solution was cooled to ꢀ788C. n-Butyllithium (2.5 M in
Et2O, 2.3 mL, 5.75 mmol) was added dropwise to the solution.
The reaction was kept at this temperature for 1 h. Dichloroben-
zylphosphine (0.56 g, 2.9 mmol) was dissolved in 15 mL of Et2O
then added to the reaction slowly. After the addition, the
reaction was warmed up with a warm water bath. Subsequently,
the solvent was removed under vacuum. The residue was
taken up in pentane then filtered through a neutral alumina
plug. The product was obtained as a beige amorphous powder
(0.56 g, 22 %).
20 %). lmax/nm(e/Mꢀ1 cmꢀ1) 357 (6600); dH 7.24 (dd, 3JH-H
¼
2
4.7 Hz, JH-P ¼ 3.8 Hz, 2H, Ph), 7.13 (m, 3H, Ph), 7.07 (dd,
1JH-H ¼ 4.9 Hz, 3JH-P ¼ 2.0 Hz, 2H, thieno), 6.93–6.85 (m, 2H,
2
thieno), 3.57 (d, JH-P ¼ 14.7 Hz, 2H, P-CH2-Ph); dp 32.35;
dC 144.03 (d, JC-P ¼ 18.9 Hz), 139.65 (s), 138.77 (s), 131.05
(d, JC-P ¼ 9.2 Hz), 129.66 (d, JC-P ¼ 5.8 Hz), 128.09 (d,
JC-P ¼ 10.0 Hz), 127.27 (d, JC-P ¼ 4.3 Hz), 125.66 (d, JC-P
¼
14.8 Hz), 42.92 (d, JC-P ¼ 49.0 Hz). m/z (HR-MS ESI)
dH 7.09–7.08 (m, 3H, Ph), 7.03 (s, 2H, thieno), 6.88–6.83 (m,
318.9831; [MþH]þ requires 318.9834.
2
2H, Ph), 3.10 (d, JH-P ¼ 3.5 Hz, 2H, P-CH2-Ph) 0.92 (s, 18H,
tBu-Si), 0.28 (d, JH-P ¼ 1.2 Hz, 12H, Me-Si); dp ꢀ24.78;
P-Benzyl-dithieno[3,2-b:20,30-d]phosphole (2a)
dC 148.69 (d, JC-P ¼ 14.3 Hz), 147.65 (s), 139.15 (d, JC-P
3.7 Hz), 136.57 (s), 134.79 (d, JC-P ¼ 17.1 Hz), 128.87 (d, JC-P
¼
¼
P-Benzyl-dithieno[3,2-b:20,30-d]phosphole oxide 3a (0.3 g,
1 mmol) was dissolved in 20 mL CH2Cl2. BH3ꢁSMe2 (2 M in
Et2O, 2.5 mL, 5 mmol) was added. The reaction was stirred for
3 h at room temperature. All volatile material was removed
under vacuum. 20 mL CH2Cl2 was used to dissolve the residue,
NEt (1 mL, 7.1 mmol) was added. The reaction was stirred at
room temperature for 3 h. All volatile material was removed
under vacuum. The residue was washed several times with
pentane and then dissolved in CH2Cl2. The solution was passed
through a neutral alumina plug. Evaporation of solvent under
vacuum afforded the crude product as a thick yellow oil (0.2 g,
70%).dH 7.48–7.11(m, 5H, Ph), 7.02 (m, 4H, thieno), 3.15(s, 2H,
P-CH2-Ph); dP ꢀ21.04; dC 146.77 (d, JC-P ¼ 13.4 Hz), 136.79
4.6 Hz), 128.00 (d, JC-P ¼ 1.7 Hz), 126.07 (d, JC-P ¼ 2.5 Hz),
35.62 (d, JC-P ¼ 19.7 Hz), 26.52 (s), 17.15 (s), ꢀ4.77 (d, JC-P
¼
2.7 Hz); m/z (HR-MS EI) 514.1764; [M]þ requires 514.1769.
P-Benzyl-2,6-bis(t-butyldimethylsilyl)-dithieno
[3,2-b:20,30-d]phosphole Oxide (3b)
P-Benzyl-2,6-bis(tert-butyldimethylsilyl)-dithieno[3,2-b:
20,30-d]phosphole (0.09 g, 0.17 mmol) was dissolved in 10 mL
CH2Cl2. H2O2 (30 % solution, 1 mL) was added. The reaction
was stirred in open air overnight. TLC (CH2Cl2 : MeCN ¼ 5 : 1)
afforded the product as a white solid (0.035 g, 38 %). lmax/nm
(e/Mꢀ1 cmꢀ1) 369 (12100); dH 7.20–7.14 (m, 3H, Ph), 7.07 (d,
(d, JC-P ¼ 1.9 Hz), 128.98 (d, JC-P ¼ 5.1 Hz), 128.30 (d, JC-P
¼
3JH-P ¼ 1.8 Hz, 2H, thieno), 7.02 (m, 2H, Ph), 3.46 (d, 2JH-P
¼
1.3 Hz), 127.08 (d, JC-P ¼ 18.4 Hz), 126.34 (d, JC-P ¼ 2.5 Hz),
125.83 (d, JC-P ¼ 5.7 Hz), 124.30 (d, JC-P ¼ 59.0 Hz), 35.56 (d,
JC-P ¼ 19.4 Hz); m/z (HR-MS ESI) 286.0035; [M]þ requires
286.0040.
15.6 Hz, 2H, P-CH2-Ph), 0.93 (s, 18H, tBu-Si), 0.28 (s, 12H, Me-
Si); dp 26.12; dC 150.21 (d, JC-P ¼ 23.6 Hz), 142.08 (d, JC-P
¼
10.5 Hz), 138.86 (d, JC-P ¼ 103.9 Hz), 133.59 (d, JC-P ¼ 12.4 Hz),
131.58 (d, JC-P ¼ 8.4 Hz), 129.70 (d, JC-P ¼ 5.5 Hz), 128.28 (d,
P,P-Dibenzyl-dithieno[3,2-b:20,30-d]phospholium
Bromide (5a)
JC-P ¼ 3.2 Hz), 126.95 (d, JC-P ¼ 3.7 Hz), 37.92 (d, JC-P
¼
66.8 Hz), 26.23 (s), 16.88 (s), ꢀ5.04 (d, JC-P ¼ 3.1 Hz); m/z
(HR-MS ESI) 531.1783; [MþH]þ requires 531.1792.
P-Benzyl-dithieno[3,2-b:20,30-d]phosphole oxide (0.49 g,
1.6 mmol) was dissolved in 50 mL CH2Cl2. BH3ꢁSMe2 (2 M
in Et2O, 4.1 mL, 8.2 mmol) was added. The reaction was stirred
at room temperature for 3 h. All volatile materials were then
pumped off. The residue was dissolved in CH2Cl2. NEt3
(1.2 mL, 8.6 mmol) was added. The reaction was again stirred
at room temperature for 3 h. All volatile materials were, again
pumped off. Toluene (30 mL) and THF (15 mL) were used to
dissolve the residue. Benzylbromide (0.5 g, 2.9 mmol) was
added. The reaction was heated and stirred overnight at reflux.
P-Benzyl-2,6-bis(t-butyldimethylsilyl)-dithieno
[3,2-b:20,30-d]phosphole Sulfide (4b)
P-Benzyl-2,6-bis(tert-butyldimethylsilyl)-dithieno[3,2-b:20,
30-d]phosphole (0.105 g, 0.20 mmol) was dissolved in 5 mL
CH2Cl2. S8 powder (0.013 g, 0.4 mmol) was added. The reaction
was stirred overnight. Preparative TLC (CH2Cl2 : hexane ¼
1 : 1) afforded the product as a yellow solid (0.053 g, 48 %).
l
max/nm(e/Mꢀ1 cmꢀ1) 369 (13100); dH 7.21 (d, J ¼ 1.9 Hz, 2H,

1178
Z. Wang, A. Y. Y. Woo, and T. Baumgartner
thieno), 7.18–7.14 (m, 1H, p-Ph), 7.13–7.06 (m, 2H, Ph),
6.91–6.83 (m, 2H, Ph), 3.63 (d, 2JH-P ¼ 14.7 Hz, 2H, P-CH2-Ph),
0.97 (d, J ¼ 2.9 Hz, 18H, tBu-Si), 0.33 (d, J ¼ 5.1 Hz, 12H, Me-Si);
[3] (a) T.-Y. Chu, J. Lu, S. Beaupre´, Y. Zhang, J.-R. Pouliot, S. Wakim,
J. Zhou, M. Leclerc, Z. Li, J. Ding, Y. Tao, J. Am. Chem. Soc. 2011,
133, 4250. doi:10.1021/JA200314M
(b) J. Hou, H.-Y. Chen, S. Zhang, G. Li, Y. Yang, J. Am. Chem. Soc.
2008, 130, 16144. doi:10.1021/JA806687U
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[4] (a) T. Baumgartner, R. Re´au, Chem. Rev. 2006, 106, 4681.
doi:10.1021/CR040179M
dp 29.89; dC 149.01 (d, JC-P ¼ 19.6 Hz), 142.41 (d, JC-P
¼
10.7 Hz), 140.70 (d, JC-P ¼ 86.0 Hz), 132.94 (d, JC-P ¼ 13.4 Hz),
131.28 (d, JC-P ¼ 9.2 Hz), 129.62 (d, JC-P ¼ 5.7 Hz), 127.87 (d,
JC-P ¼ 3.8 Hz), 127.13 (d, JC-P ¼ 4.3 Hz), 43.34 (d, JC-P
(HR-MS ESI) 547.1558; [MþH]þ requires 547.1563.
¼
48.5 Hz), 26.28 (s), 16.90 (s), ꢀ4.99 (d, JC-P ¼ 3.9 Hz);m/z
(b) J. Crassous, R. Re´au, Dalton Trans. 2008, 6865. doi:10.1039/
B810976A
(c) Y. Matano, H. Imahori, Org. Biomol. Chem. 2009, 7, 1258.
doi:10.1039/B819255N
P,P-Dibenzyl-3-(5,50-bis(t-butyldimethylsilyl)-
2,20-dithienyl) Phosphane Oxide (5b)
P-Benzyl-2,6-bis(tert-butyldimethylsilyl)-dithieno[3,2-b:20,
30-d]phosphole (0.101 g, 0.2 mmol) was dissolved in 4 mL
toluene and 2 mL THF before addition of benzylbromide
(0.045 g, 0.20 mmol). The reaction was stirred overnight at
reflux. After work-up, preparative TLC (CH2Cl2 : MeCN ¼
1 : 1) afforded the product as a white solid (0.05 g, 37 %).
(d) A. Fukazawa, S. Yamaguchi, Chem. – Asian J. 2009, 4, 1386.
doi:10.1002/ASIA.200900179
(e) Y. Ren, T. Baumgartner, Dalton Trans. 2012, 41, 7792.
doi:10.1039/C2DT00024E
[5] (a) T. Baumgartner, T. Neumann, B. Wirges, Angew. Chem. Int. Ed.
2004, 43, 6197. doi:10.1002/ANIE.200461301
(b) T. Baumgartner, W. Bergmans, T. Ka´rpa´ti, T. Neumann,
M. Nieger, L. Nyula´szi, Chem. – Eur. J. 2005, 11, 4687. doi:10.
1002/CHEM.200500152
(c) M. G. Hobbs, T. Baumgartner, Eur. J. Inorg. Chem. 2007, 3611.
doi:10.1002/EJIC.200700236
l
max/nm(e/Mꢀ1 cmꢀ1) 305 (10300); dH 7.54 (d, 2JH-P ¼ 3.4 Hz,
1H, thieno), 7.27–7.19 (m, 6H, Ph), 7.16 (d, 2JH-P ¼ 3.1 Hz, 1H,
thieno), 7.12 (m, 4H, Ph), 3.24 (dt, JH-H ¼ 36.1, 2JH-P ¼ 14.5 Hz,
4H, P-CH2-Ph), 0.98 (s, 9H, tBu-Si), 0.87 (s, 9H, tBu-Si), 0.36 (s,
(d) C. Romero-Nieto, T. Baumgartner, Synlett 2013, 24, 920.
doi:10.1055/S-0032-1317804
[6] Y. Ren, T. Baumgartner, J. Am. Chem. Soc. 2011, 133, 1328.
doi:10.1021/JA108081B
[7] Y. Ren, W. H. Kan, M. A. Henderson, P. G. Bomben, C. P. Berlinguette,
V. Thangadurai, T. Baumgartner, J. Am. Chem. Soc. 2011, 133, 17014.
doi:10.1021/JA206784F
[8] Y. Ren, W. H. Kan, V. Thangadurai, T. Baumgartner, Angew. Chem.
Int. Ed. 2012, 51, 3964. doi:10.1002/ANIE.201109205
[9] Y. Ren, T. Baumgartner, Inorg. Chem. 2012, 51, 2669. doi:10.1021/
IC2026335
6H, Me-Si), 0.24 (s, 6H, Me-Si); dp 32.84; dC 147.62 (d, JC-P
¼
12.1 Hz), 140.70 (s), 139.70 (d, JC-P ¼ 12.0 Hz), 139.09 (s),
138.36 (d, JC-P ¼ 10.6 Hz), 135.87 (s), 131.78 (d, JC-P ¼ 7.4 Hz),
131.09 (s), 129.97 (d, JC-P ¼ 5.3 Hz), 128.48 (d, JC-P ¼ 1.9 Hz),
126.78 (d, JC-P ¼ 2.4 Hz), 37.27 (d, JC-P ¼ 65.4 Hz), 26.30
(d, JC-P ¼ 10.4 Hz), 16.82 (d, JC-P ¼ 12.8 Hz), ꢀ4.99 (d, JC-P
¼
14.0 Hz); m/z (HR-MS CI) 622.2356; [M]þ requires 622.2345.
Supplementary Material
Fluorescence spectra of 3a,b–5a,b, frontier orbitals for 5b9, and
final coordinates of DFT-calculated species, as well as copies of
the NMR spectra of 2a,b–5a,b are available on the Journal’s
website.
[10] H. Usta, G. Lu, A. Facchetti, T. J. Marks, J. Am. Chem. Soc. 2006, 128,
9034. doi:10.1021/JA062908G
[11] Y. Dienes, S. Durben, T. Ka´rpa´ti, T. Neumann, U. Englert,
L. Nyula´szi, T. Baumgartner, Chem. – Eur. J. 2007, 13, 7487.
doi:10.1002/CHEM.200700399
Acknowledgements
[12] Y. Dienes, M. Eggenstein, T. Neumann, U. Englert, T. Baumgartner,
Financial support by NSERC of Canada and the Canada Foundation for
Innovation (CFI) is gratefully acknowledged. Z.W. thanks Alberta
Innovates – Technology Futures for a graduate scholarship, and A.Y.Y.W.
the University of Calgary for a Queen Elizabeth (II) graduate scholarship.
Dalton Trans. 2006, 1424. doi:10.1039/B509277A
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V78-406
(b) S. Durben, Y. Dienes, T. Baumgartner, Org. Lett. 2006, 8, 5893.
doi:10.1021/OL062580W
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