Phosphorus can also be a photocopy

Article abstract of DOI:10.1021/ja1009426

The syntheses of benzoxaphospholes and new benzobisoxaphospholes that display blue fluorescence are presented. The latter compounds were accessed by the use of a new precursor, 2,5-diphosphinohydroquinone. The new compounds were fully characterized, including a structural study of 2,6- tert -butylbenzo[1,2- d ;4,5- d -]bisoxaphosphole. Quantum yields for photoluminescence were determined for a series of compounds. These materials feature bona fide PC p-p - bonds suitable for conjugated materials having phosphorus as a participatory atom and can thus photocopy the properties of other conjugated organic molecules.

Full text of DOI:10.1021/ja1009426

Published on Web 03/17/2010
Phosphorus Can Also Be a “Photocopy”
Marlena P. Washington,† Vittal B. Gudimetla,† Feng L. Laughlin,† Nihal Deligonul,† Susan He,‡
John L. Payton,† M. Cather Simpson,§ and John D. Protasiewicz*,†
Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106, Department of Chemistry and
Center for Optical Materials Science and Engineering Technologies (COMSET), Clemson UniVersity, Clemson,
South Carolina 29634, and Departments of Chemistry and Physics and the Photon Factory, UniVersity of Auckland,
Auckland, New Zealand
Received February 2, 2010; E-mail: Protasiewicz@case.edu
Organophosphorus compounds featuring p-p π bonds be-
tween phosphorus and carbon display close similarities to related
materials having corresponding carbon-carbon π bonds, and
thus, phosphorus has come to be called a “carbon copy”.1a While
many properties are mirrored between the two sets of com-
pounds, there remain some that are not reflected when CdC units
are replaced by PdC (or PdP) units. The widespread attention
on π-conjugated polymers for applications in areas such as
molecular electronics, sensors, organic light-emitting diodes
(OLEDs), and solar photovoltaics led to a recent Nobel prize
awarded to the three pioneers of these materials.2 It would thus
seem natural to expect that related π-conjugated materials
incorporating other sp2-hybridized elements as participatory units
could be realized. We and others have been seeking to exploit
and extend the analogies between carbon and phosphorus to
create extended conjugated materials,3 such as phospha-PPVs
(Chart 1).
1,3-Benzoxaphospholes constitute a class of heterocycles that
is a subclass of oxaphospholes and have been known for some
time.1b,5 One paper in particular piqued our interest: select
benzoxaphospholes were reported to exhibit intense blue
fluorescence.5a Though this particular facet of the report seems
to have garnered little attention, it led us to examine these
materials in further detail. By means of the described synthesis
(the reaction of 2-phosphinophenols and N-phenyl imidoylchlo-
rides; eq 1), some of these compounds and several new
representatives were prepared. Most of these materials are
distinctly fluorescent (Table 1). Measurement of the excited-
state lifetimes allowed for confident assignment of the emission
as fluorescence. All but the alkyl-substituted derivative 1d
displayed significant photoluminescence (PL) intensity (the PL
intensity of 1d was enhanced in MeOH relative to CH2Cl2).
With an eye toward the eventual synthesis of extended
materials, we set out to synthesize and characterize the previously
These materials, however, show minimal photoluminescence
relative to that displayed by materials such as poly(phenyle-
nevinylene)s (PPVs). Nevertheless, phospholes have been shown
to exhibit significant luminescence and also electrolumines-
cence.4 Because of their pyramidal phosphorus atoms, phosp-
holes do not exhibit significant planarization and n-p π
Scheme 1
Chart 1. Examples of phospha-PPVs
conjugation. Furthermore, many of the materials with interesting
luminescent properties feature tetracoordinate phosphorus in
which the phosphorus is often oxidized in the form of PdE
functionalities (E ) O, S) or by formation of AuCl adducts.
Noting that incorporation of ring structures (and rigidity) into
pendant vinyl units on phenyl rings can bring forth desirable
properties, such as those found in benzodifurans and benzo-
bisoxazoles, we now communicate the synthesis and elaboration
of benzoxaphospholes and benzobisoxaphospholes that display
blue fluorescence and high quantum yields for photolumines-
cence. These materials feature bona fide PdC p-p π bonds
suitable for conjugated materials having phosphorus as a
participatory atom.
unknown benzobisoxaphospholes. The novel precursor 2,5-
diphosphinohydroquinone (4) was readily accessed by reduction
of 3 (Scheme 1), which itself can be readily accessed by
an anionic double Fries rearrangement of 1,4-{OP(dO)-
(OEt)2}C6H4.6 Reaction of 4 with 2 equiv of select imidoyl-
chlorides led to the corresponding 2,6-substituted benzo[1,2-
d;4,5-d′]bisoxaphospholes (Scheme 1, Table 1). Compounds 1
and 5 display greater air and water stability than previously
reported phospha-PPVs.
† Case Western Reserve University.
‡ Clemson University.
The benzobisoxaphospholes were characterized by 31P NMR
spectroscopy, and resonances near those of their simpler
§ University of Auckland.
9
4566 J. AM. CHEM. SOC. 2010, 132, 4566–4567
10.1021/ja1009426  2010 American Chemical Society

C O M M U N I C A T I O N S
analogues were found (Scheme 1). The single-crystal structure
of 5b is shown in Figure 1. The PdC bond length of 1.694(1)
Å is somewhat longer than that found in most phosphaalkenes
and shorter than the P-C(2) single-bond distance of 1.782(1)
Å, but consistent with such distances found in benzoxaphosp-
holes. The angle about the phosphorus atom is 88.34(5)°. The
results of a density functional theory (DFT) study of the model
diphenyl-substituted benzobisoxaphosphole 5e showed that the
frontier orbitals consist largely of phosphorus-carbon π and π*
orbitals (Figure 1).
The photoluminescence data for 5a-d are provided in Table
1 and again show that aryl-substituted analogues display much
greater PL intensity than alkyl-substituted materials.
Table 1. Selected Data for Compounds
R
R′
λmax (nm)a
λF,max (nm)a
Φa
τ (ns)b
1a
1b
1c
1d
1e
1f
1g
1h
1i
C6H5
H
H
H
H
H
H
337
343
348
281c
343
343
339
345
351
342
306
304
337
336
425
427
437
426c
426
426
421
424
438
429
367
372
432
436
0.57
0.62
0.63
0.04c
0.62
0.69
0.55
0.58
0.56
0.56
0.03
0.04
0.27
0.22
12.0
11.8
13.5
9.0c
11.9
7.0
Figure 1. (top) X-ray structure of 5b and (bottom) selected molecular
orbitals (DFT B3LYP/6-31+G**) for the minimized structure of 5e.
Selected experimental bond distances (Å) and angles (deg): P(1)-C(2),
1.694(1); P(1)-C(5), 1.782(1); C(2)-P(1)-C(5), 88.34(5).
4-ClC6H4
4-MeOC6H4
Ad
4-BrC6H4
4-MeC6H4
C6H5
iPr
iPr
iPr
iPr
H
H
H
H
5.8
Acknowledgment. We thank the National Science Foundation
for support (CHE-0748982, CHE-0518510, and CHE-0541766)
and Professor Rhett Smith (Clemson University) for helpful
discussions and instrumentation for lifetime measurements.
4-ClC6H4
4-MeOC6H4
4-MeC6H4
Ad
28.0
25.0
4.8
3.9
4.6
1j
5a
5b
5c
5d
tBu
Supporting Information Available: Experimental procedures and
characterization data for all compounds and X-ray analysis data for
5a (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
2,4,6-Me3C6H2
2,6-Me2C6H3
3.7
2.3
a Measurements were performed in CH2Cl2. b Values were determined
in hexanes. c Measurements were performed in CH3OH.
The benzobisoxaphospholes thus mimic the PL characteristics
of their two closest phosphorus-free analogues, benzodifurans
and benzobisoxazoles (Chart 2). While studies of the former are
limited in number despite the fact that they show interesting
References
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Chart 2
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properties,7 benzobisoxazoles, in contrast, have been extensively
studied and found broad use in robust materials having high
mechanical strength and important photophysical and electronic
properties.8 Preliminary electrochemical studies of 1a and 1g
revealed reversible reductions occurring at -1.90 and -1.93 V
versus SCE, respectively (see the Supporting Information), which
are analogous to those reported for other phosphaalkenes and
also suggests possibilities for the use of these materials as
components in n-type conjugated materials.
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Kumar, S. Macromol. Mater. Eng. 2003, 288, 823. (b) So, Y.-H.; Zaleski,
J. M.; Murlick, C.; Ellaboudy, A. Macromolecules 1996, 29, 2783. (c) Mike,
J. F.; Makowski, A. J.; Jeffries-El, M. Org. Lett. 2008, 10, 4915. (d) Klare,
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Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4556.
In summary, we have shown that benzoxaphospholes and
benzobisoxaphospholes are materials that have bona fide PdC
double bonds, do indeed possess interesting and significant PL
properties, and can “photocopy” the properties of other conju-
gated molecules. We are currently preparing extended conjugated
materials for examination in applications such as OLEDs.
JA1009426
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