Charge-transfer properties of lateral triphenylamine-dithienophosphole diads

Article abstract of DOI:10.1021/ol300331f

Installation of an exocyclic triphenylamine group at the phosphorus center provides access to dithienophosphole materials with lateral charge-transfer (CT) ability. The degree of CT can be significantly manipulated not only via oxidation of the P-center b

Full text of DOI:10.1021/ol300331f

ORGANIC
LETTERS
2012
Vol. 14, No. 6
1588–1591
Charge-Transfer Properties of Lateral
TriphenylamineꢀDithienophosphole
Diads
Chris Jansen Chua, Yi Ren, and Thomas Baumgartner*
Department of Chemistry & Centre for Advanced Solar Materials, University of
Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
Thomas.baumgartner@ucalgary.ca
Received February 9, 2012
ABSTRACT
Installation of an exocyclic triphenylamine group at the phosphorus center provides access to dithienophosphole materials with lateral charge-
transfer (CT) ability. The degree of CT can be significantly manipulated not only via oxidation of the P-center but also surprisingly by alkylation of
the 2,6-position of the scaffold.
Due to the steadily growing impact of organic electron-
ics, the development of new π-conjugated chromophores
continues to be an important area of research.1 A vast
array of systems has been developed that, among other
features, quite efficiently addresses not only the light-
emitting and the redox properties of the materials but also
thetransferof energyand charges. Inthe context of charge-
transfer (CT) phenomena, there has been a lot of interest in
the photophysical properties of donorꢀacceptor (DꢀA)
systems linked by a single bond.2 Others and we recently
established that organophosphorus-based conjugated
materials exhibit pronounced electron-accepting features3
that naturally lend themselves to be utilized in such DꢀA
systems. Notably, the acceptor character of phosphorus
can significantly be increased through simple oxidation of
this center.3 While it has been shown that the longitudinal
installation of donor groups at the periphery of phos-
phorus-based conjugated systems can lead to some intrigu-
ing photophysical properties,4 the effect of exocyclic donor
substitutents as a lateral component in a DꢀA architech-
ture is virtually unexplored for these systems.5 However,
the pyramidal geometry of the P-center is particuarly
intriguing in this context, as it could intrinsically lead to
some interesting properties.4 In this contribution we now
report the synthesis and characterization of novel DꢀA
€
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(3) (a) Matano, Y.; Saito, A.; Fukushima, T.; Tokudome, Y.; Suzuki,
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Ichihashi, Y.; Kosaka, Y.; Yamaguchi, S. Chem.;Asian J. 2009, 4,
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Mater. 2011, 21, 4088. (e) Dienes, Y.; Durben, S.; Karpati, T.;
Neumann, T.; Englert, U.; Nyulaszi, L.; Baumgartner, T. Chem.;Eur. J
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2007, 13, 7487. (f) Matano, Y.; Nakashima, M.; Imahori, H. Angew. Chem.,
Int. Ed. 2009, 48, 4002.
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Dienes, Y.; Eggenstein, M.; Karpati, T.; Sutherland, T. C.; Nyulaszi, L.;
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r
10.1021/ol300331f
Published on Web 03/02/2012
2012 American Chemical Society

systems, in which our trademark dithienophosphole unit
constitutes the acceptor component, and an exocyclic
triphenylamine species acts as the donor. Our study ad-
dresses the charge-transfer properties of the system via
modification of the phosphorus center, as well as the 2,6-
position of the dithienophosphole scaffold; the experimen-
tal data are further supported by theoretical calculations.
The synthetic strategy involved the corresponding 3,30-
dibromo-2,20-bithiophenes 1a,b that were lithiated with
nBuLi in situ, followed by the addition 4-N,N-diphenyl-
aminophenylphosphorus dichloride (TPAPCl2) at low
temperature. For purification purposes, the trivalent com-
pounds were immediately oxidized with excess H2O2,
providing 2a and 2b in 48% and 49% yields after column
chromatography, respectively (Scheme 1).
We were also able to obtain single crystals suitable for
X-ray crystallography for 2a from a concentrated CHCl3
solution upon slow evaporation of the solvent (Figure 1).
In general, the bond lengths and angles are in good
agreement with previously reported dithienophosphole
oxides.3b,4b,4e Notably, there is a face-to-face interaction
˚
(d = 3.6 A) between two propeller-shaped TPA units, but
no πꢀπ interactions between two neighboring planar
dithienophosphole scaffolds are found.
Scheme 1. Synthesis of the TPA-Functionalized Dithieno[3,2-
b:20,30-d]phospholes
Figure 1. Molecular structure and intermolecular interaction
(inset) of 2a in the solid state (50% probability level, H-atoms
˚
are omitted for clarity). Selected bond lengths [A] and angles
[deg]: P1ꢀC3, 1.811(4); P1ꢀC6, 1.802(4); P1ꢀC11, 1.793(4);
P1ꢀO1, 1.479(3); C1ꢀC2, 1.372(7); C2ꢀC3, 1.422(6); C3ꢀC4,
1.377(6); C4ꢀC5, 1.463(6); C5ꢀC6, 1.373(5); C6ꢀC7, 1.422(5);
C7ꢀC8, 1.360(5); C3ꢀP1ꢀC11, 105.80(17); C6ꢀP1ꢀC11,
104.70(17); C3ꢀP1ꢀO1, 117.80(18); C6ꢀP1ꢀO1, 119.37(17);
C11ꢀP1ꢀO1, 114.23(17).
In order to obtain the trivalent species, the pentavalent
species 2a,b were treated with a slight excess of borane
(from BH3SMe2), followed by the addition of excess NEt3
to provide the corresponding phospholes in good yields
after workup (3a = 68%, 3b = 81%; Scheme 1).4e The
trivalent congeners could thereby be obtained in higher
purity than via the direct approach.
The photophysical properties of the new dithienophos-
pholes were studied in dilute CH2Cl2 solution (∼10ꢀ6 M),
and the results are listed in Table 1. It is interesting to note
that the pentavalent 2a,b show similar absorption prop-
erties in the typical range for dithienophospholes,4 while
the emission of the H-terminated congener 2a shows an
unexpected red-shifted emission compared to the hexyl-
functionalized species at λem = 508 nm in solution and
λem=512 nm in the solid state (cf. 2b:λem=472 and 488 nm).
εmax values for the two compounds range between 16 000
and 18 000 L molꢀ1 cmꢀ1, which is similar to previously
reported dithienophospholes.6 The Stokes shifts on the
other hand range from 8700 to 9900 cmꢀ1 and are larger
than those of the literature-reported dithienophospholes.6
In turn, the photoluminescence quantum yields of 2a,b at
φPL = 0.16 and 0.18, respectively, are comparatively low.4,6
The trivalent compounds 3a,b have similar absorption and
Compound 2a was acquired as a light yellow solid
showing a 31P{1H} NMR chemical shift at δ = 19.1 ppm,
while 2b was obtained as a dark yellow solid showing a
slightly lowfield-shifted 31P NMR resonance at δ= 21.6 ppm,
both in a similar range as known dithienophosphole
1
oxides.4 The compounds showed consistent H and 13C
NMR data, as well as HRMS and elemental analysis,
confirming their identity and purity. The trivalent species
exhibited highfield-shifted 31P NMR resonances at δ =
ꢀ22.4 ppm for the light yellow 3a and δ = ꢀ22.2 ppm for
the bright yellow 3b that clearly confirmed the P-reduced
state.4 Successful formation of the two compounds was
further confirmed by 1H and 13C NMR spectra, as well as
HRMS analysis.
(6) (a) Durben, S.; Linder, T.; Baumgartner, T. New J. Chem. 2010,
34, 1585. (b) Ren, Y.; Baumgartner, T. J. Am. Chem. Soc. 2011, 133,
1328.
Org. Lett., Vol. 14, No. 6, 2012
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Table 1. Photophysical and Electrochemical Properties of the TPA-Functionalized Dithienophospholes
Stokes
shift
λex
(solid)
[nm]
λem
(solid)
[nm]
λabs
[nm]a
εmax
λem
[nm]c
Ered
[V]e
Eox
[V]e
d
compd
[L molꢀ1cmꢀ1 b
]
[cmꢀ1
]
φPL
2a
2b
3a
3b
304, 338
289, 335
319
16 000
18 000
6900
508
472
431
434
9900
8700
8100
8400
0.16
0.18
0.04
0.08
403
441
374
370
512
488
467
469
ꢀ1.35
ꢀ1.31
ꢀ
0.71
0.71
0.60, 1.02
0.57, 1.00
318
7200
ꢀ
a UV/vis absorption maxima in CH2Cl2. b Extinction coefficient from λmax (bold values). c Emission maxima in CH2Cl2, excitation at respective πꢀπ*
transitions. d Photoluminescence quantum yields in CH2Cl2, relative to quinine sulfate; ( 10%. e Peak potentials in CH3CN vs Fc/Fcþ.
emission wavelengths in solution and the solid state.
Both εmax and Stokes shifts for the trivalent species
are lower than those of the pentavalent derivatives at
6900ꢀ7200 L molꢀ1 cmꢀ1 and 8100ꢀ8400 cmꢀ1, respec-
tively. The same is true for their quantum yields at
φPL = 0.04 for 3a and φPL = 0.08 for 3b. Judging by
the observed absorption and emission wavelengths, the
dithienophosphole appears to remain the main chromo-
phore in these diads, but the low quantum yields and
large Stokes shifts indicate that the origin of the lowest
energy transitions are indeed CT processes.2,4
To further gauge the suitability of the system for the
desired CT processes, the electrochemical properties of the
TPA-containing dithienophospholes were investigated
through cyclic voltammetry (Table 1). Both pentavalent
compounds show a similar trend having a quasireversible
reduction at Ered,peak = ꢀ1.35 and ꢀ1.31 V for 2a and 2b,
respectively, in the same range as observed for related
dithienophospholes before.3e Moreover, the two systems
also show a reversible process at Eox,peak = 0.71 V for both
species, due to the expected oxidation of the TPA unit.7a,8
As for the trivalent congeners, no reduction was observed,
but both systems exhibit two oxidation peaks: a quasire-
versible oxidation for the TPA units at Eox,peak = 0.60 V
for 3a, Eox,peak = 0.57 V for 3b, and an irreversible oxi-
dation at Eox,peak = 1.02 V for 3a, Eox,peak = 1.00 V for 3b,
due to dithienophosphole oxidation.
emission for all four species. Emission of 2b varies from
em = 457 nm for the nonpolar CyH to λem = 480 nm for
the polar EtOH. The photoluminescence quantum yields
decrease as the polarity increases (CyH: φPL = 0.28;
CH3CN: φPL = 0.06), but the quantum yield in EtOH is
realtively high (φPL = 0.15).4c,10
Remarkably, 2a shows a much stronger solvatochromic
effect than the alkylated variety. The emission ranges from
λem = 435 nm in CyH all the way to λem = 560 nm in
CH3CN. The 125 nm differential clearly supports very
efficient CT. The quantum yields of 2a also drastically
decrease upon increasing solvent polarity (φPL = 0.23 for
CyH; φPL = 0.01 for CH3CN).
λ
A simple but effective way to study CT phenomena is via
solvatochromism.7 The solvents chosen for the study of all
four species ranging from the least polar to the most polar
with their respective ET(30)9 values include cyclohexane
(CyH: 30.9), diethylether (Et2O: 34.6), dichloromethane
(CH2Cl2: 40.7), acetonitrile (CH3CN: 46.7), and ethanol
(EtOH: 51.9). The latter was also used to study the
potential effects of hydrogen bonding within the system.
A positive solvatochromism was observed for all species
and the effect is more pronounced in the emission spectra,
as expected.7,9 Figure 2 summarizes the CT effects on the
Figure 2. Normalized emission spectra for the solvatochromism
of 2a,b and 3a,b.
Surprisingly, the red shift in EtOH (λem = 525 nm) is not
as pronounced as would be expected for its high polarity.
This observation can be attributed to the hydrogen-bond-
ing capability of EtOH, which evidently impacts the
acceptor ability of the dithienophosphole unit, likely
(7) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006. (b) Valeur, B. Molecular Fluorescence, Principles
and Applications; Wiley-VCH: Weinheim, 2002.
(8) Koech, P. K.; Polikarpov, E.; Rainbolt, J. E.; Cosimbescu, L.;
Swensen, J. S.; Von Ruden, A. L.; Padmaperuma, A. B. Org. Lett. 2010,
12, 5534.
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3rd ed.; University Science Books: Sausalito, CA, 2006.
(10) Sung, J.; Kim, P.; Lee, Y. O.; Kim, J. S.; Kim, D. J. Phys. Chem.
Lett. 2011, 2, 818.
1590
Org. Lett., Vol. 14, No. 6, 2012

through interaction with the P-center.11 The solvatochro-
mic effect for the trivalent system 3b is smaller than that for
its pentavalent counterpart 2b, in line with the generally
reduced acceptor character of the trivalent P-center. Simi-
lar to 2a, the positive solvatochromism of the emission for
3a from CyH at λem = 407 nm to CH3CN at λem = 453 nm
is more pronounced than in the hexyl-substituted conge-
ner, but not as pronounced as for its pentavalent relative.
Again, the emission in EtOH does not fit the expected
trend, as 3a shows the most blue-shifted emission max-
imum in this solvent, likely for similar reasons as men-
tioned for its pentavalent counterpart. In general, the
nonalkylated species 2a and 3a show more pronounced
CT that is probably the result of a more stable charge
separated state. The hexyl units apparently impart signifi-
cant electron density onto the dithienophosphole scaffold
making it a much weaker acceptor than the H-substituted
relatives.
To support the experimental results, DFT calculations
(B3LYP/6-31G(d)) have been performed on the TPA-
containing phospholes, their frontier orbital energies and
distributions were determined,12 and TD-DFT was per-
formed to explain dynamic excitation processes (see Sup-
porting Information (SI) for full details). Indeed, the
theorectical calculations confirm the observations from
the optical spectroscopy studies. The LUMOs of all four
species are localized at the dithienophosphole-acceptor
end, asexpected; howevertheirenergiesdiffer substantially
as a function of the P-oxidation state and, more impor-
tantly, 2,6-substitution, with 2a showing the lowest energy
at EL = ꢀ1.77 eV and 3b0 (hexyl truncated with Me) the
highest energy at EL = ꢀ1.13 eV (cf.: EL = ꢀ1.65 eV (2b0);
EL = ꢀ1.21 eV (3a)). Furthermore, the HOMOs of the
four species mainly show contributions from not only the
TPA units but also the dithienophosphole scaffold. How-
ever, the latter contribution drops considerably upon
moving from 3b0 to 2a, confirming a concomitant increase
in charge separation and CT ability in these systems. Due
to the fact that the HOMOs are dominated by the TPA
unit, the HOMO energy levels are quite comparable in the
four species ranging from EH = ꢀ4.99 eV for 3b0 to EH =
ꢀ5.19 eV for 2a. TD-DFT confirmed that in all cases the
S0fS1 transitions indeed involve HOMO and LUMO and
show relatively low oscillator strengths from f = 0.006 to
0.103, supporting their charge-transfer character (see SI).
It should be mentioned in this context that while the
theoretically obtained wavelengths do not match the ex-
perimental values, they nevertheless reproduce a similar
trend.
In conclusion, we have developed a synthetic pathway
toward laterally functionalized DꢀA dithienophospholes
that show efficient charge transfer from the triphenyla-
mine-donor group to the dithienophosphole-acceptor
group, as verified through the study of their solvatochro-
mism and theoretical calculations. Remarkably, the CT
character in these species can significantly be enhanced by
increasing the acceptor character of the dithienophosphole
unit utilizing phosphorus-chemistry approaches, i.e. oxi-
dation from a trivalent to a pentavalent species. More
surprisingly however, CT can in turn be noticeably sup-
pressed by installation of simple alkyl substituents at the
2,6-positions of the dithienophosphole scaffold. Based on
these initial studies, further development of laterally do-
nor-functionalized dithienophospholes is now underway.
Acknowledgment. Financial support by NSERC of
Canada and the Canada Foundation for Innovation (CFI)
is gratefully acknowledged. C.J.C. thanks the University of
Calgary for an Open scholarship. Y.R. thanks Alberta
Ingenuity now part of Alberta Innovates - Technology
Futures, and Talisman Energy for graduate scholarships.
Supporting Information Available. Synthetic proce-
dures, NMR spectra, absorption spectra, detailed solva-
tochromism data, (TD)DFT calculation details, and CV
plots for 2a,b and 3a,b; CIF file for the X-ray crystal-
lographic data of 2a (CCDC no.: 866194). This material is
available free of charge via the Internet at http://pubs.acs.
org.
(11) Ren, Y.; Baumgartner, T. Chem.;Asian J. 2010, 5, 1918.
(12) Frisch, M. J. T., et al. . Gaussian 03, revision E.01; Gaussian Inc.:
Wallingford, CT, 2007. See Supporting Information for full reference.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 6, 2012
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