Conjugated ligand-based tribochromic luminescence

Article abstract of DOI:10.1039/b915089g

A Au(i) complex containing a terthienyl diphosphine ligand is non-emissive in the crystalline form, but exhibits intense ligand-based emission upon grinding attributed to increased planarization in the terthienyl ligand.

Full text of DOI:10.1039/b915089g

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Conjugated ligand-based tribochromic luminescencew
Angela M. Kuchison, Michael O. Wolf* and Brian O. Patrick
Received (in Berkeley, CA, USA) 27th July 2009, Accepted 20th October 2009
First published as an Advance Article on the web 5th November 2009
DOI: 10.1039/b915089g
A Au(I) complex containing a terthienyl diphosphine ligand is
non-emissive in the crystalline form, but exhibits intense ligand-
based emission upon grinding attributed to increased planarization
in the terthienyl ligand.
centers have been shown to primarily affect structure, while
having only minimal electronic effects, with other conjugated
ligands.12 Here we report the first example of a metal
complex which exhibits conjugated ligand-based tribochromic
luminescence.
The diphosphine 3,300-bis(diphenylphosphino)-2,2:50,200-
terthiophene (P2T3) was selected as a suitable conjugated
ligand. Conjugated phosphines13–16 have been previously used
as ligands, including in gold(I) complexes.12,17 P2T3 was
synthesized via lithiation of Br2T3,18 followed by reaction with
Ph2PCl (Scheme 1). After warming to room temperature,
addition of water immediately precipitated P2T3 as a yellow
solid. Reaction of AuCl(tht) (tht = tetrahydrothiophene) with
P2T3 resulted in the formation of (AuCl)2P2T3 which was
isolated as an off-white solid.z
Materials that show changes in photoluminescence with
applied pressure are of interest due to their potential application
in data recording, forensics (i.e. fingerprinting) and
stress–strain sensors. This phenomenon, known either as
tribochromic1,2 or mechanochromic3 luminescence, is typically
a result of either alterations in intermolecular interactions
resulting in phase transformations,3,4 chemical reactions which
result in changes in the solid-state structure,1 or solid-state
defects.5 Most examples of pressure dependent luminescence
are attributed to metal–metal interactions, particularly in
Au(I) and Pt(II) complexes.6 In an effort to identify new
structural motifs designed to elicit a luminescence response
with applied pressure, we became interested in the possibility
of pressure-induced conjugation changes in conjugated ligands
attached to metals. A few organic molecules such as sterically
congested dicyanomethylene derivatives7 are known to undergo
changes in conformation under mild applied pressure
(scratching of a solid sample) resulting in tribochromic behaviour.
The p–p* absorption bands of poly(3-alkylthiophenes)
red-shift at high applied pressure due to more extended
conjugation resulting from packing of the alkyl substituents
under pressure.8,9
Crystals of (AuCl)2P2T3 were grown from a CH2Cl2–
hexanes solution. The solid-state structure contains two,
crystallographically unique, molecules of (AuCl)2P2T3. The
solid-state molecular structure of one of the two molecules is
shown in Fig. 1. The Au centers are linearly coordinated, and
the Au–P and Au–Cl bond lengths are similar to those in
19
ClAuPPh3 and other related Au complexes.12,17 The central
thiophene is twisted significantly out of plane with the
adjacent, terminal thiophene rings. The bond lengths and
angles in the second (AuCl)2P2T3 molecule in the structure
are very similar. No intermolecular or intramolecular
gold–gold interactions are present in the solid state.
Crystalline (AuCl)2P2T3 has an absorption band with
In order to evoke tribochromic luminescence under mild
pressure, conjugated molecules must undergo structural
transformations resulting in a change in conjugation when
ground or pressed. Many conjugated oligomers pack
preferentially as planar structures in the solid state,10 for
example, oligothiophenes tend to adopt planar conformations
in the solid state despite a low energy barrier for rotation
between thiophene rings11 and the reduction in steric stress
that arises when adjacent rings are twisted out of plane relative
to one another. Judiciously chosen substituents on the oligomer
are needed to allow for more than one stable solid structure to
exist at near ambient pressures. One strategy is to incorporate
conjugated groups as ligands on metal complexes. This
introduces a powerful structural organization element (the
metal) which may be used to influence ligand conjugation.12,13
Au(I) complexes are a particularly good choice as these metal
l
max = 350 nm (see ESIw), attributed to a terthienyl ligand-based
p–p* transition. This is blue-shifted relative to the absorption
of yellow P2T3 in the solid state (lmax = 400 nm). The higher
energy of the p–p* absorption in (AuCl)2P2T3 is consistent
with the reduced coplanarity between thiophene rings in the
structure of (AuCl)2P2T3 (interannular torsion angles B45–501)
compared with those in the single crystal X-ray structure of
P2T3 (interannular torsion angles B17–301, see ESIw). When a
solid sample of (AuCl)2P2T3 (dropcast on a quartz slide from a
hexanes slurry) is photoexcited with a 365 nm hand-held
UV-lamp, no emission is observed by eye. When a second
quartz slide was pressed by hand onto the sample and twisted
several times to grind the (AuCl)2P2T3, the sample began to
emit blue light under 365 nm excitation. This emission remains
when the pressure is released from the slide. The solid-state
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver BC, Canada V6T 1Z1. E-mail: mwolf@chem.ubc.ca
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data, absorption and emission
spectra, PXRD, Raman spectra and selected crystallographic data.
CCDC 742148 and 749960. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/b915089g
Scheme 1
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Chem. Commun., 2009, 7387–7389 | 7387

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respective absorption spectra of the two compounds.
Time-resolved emission decays for both P2T3 and (AuCl)2P2T3
are multiexponential with lifetimes of B250 ps (B51%) for
P2T3 and B90 ps (B90–95%) for (AuCl)2P2T3. The similarity
of the emission spectra and lifetimes, and their insensitivity to
the presence of oxygen, support the conclusion that the
emissive excited state in solution is a terthiophene-localized
p–p* singlet state for both (AuCl)2P2T3 and P2T3. The
sterically hindered gold centers in (AuCl)2P2T3 prevent
aurophilic interactions, which have previously led to emission
from low-lying Au–Au excited states,20,21 and mechanochromic
luminescence.1,3 Thin films of (AuCl)2P2T3 dropcast from
CHCl3 solution also emit strongly at 465 nm, suggesting that
the conformation of the molecules in these films is similar to
the solution structure.
Fig. 1 Solid-state molecular structure of one of the two (AuCl)2P2T3
molecules. Hydrogen atoms and occluded solvent are omitted for
clarity and thermal ellipsoids are drawn at 50% probability.
The solid-state emission of (AuCl)2P2T3 (lmax = 475 nm)
after grinding is very similar to the emission in solution
(lmax = 465 nm), suggesting that emission arises from the
same state in both cases. The emission of solid P2T3 is also
influenced by grinding. A new emission band with lmax = 490 nm
appears when a sample of P2T3 is ground, in addition to the
original broad emission band at 560 nm, however this new
emission feature is temporary and disappears when the sample
rests overnight. Ground (AuCl)2P2T3 shows no decrease or
change in luminescence after one week.
emission spectra of the sample before and after grinding are
shown in Fig. 2. The ground sample shows a strong emission
band with a maximum at 475 nm.
The powder X-ray diffractogram (PXRD) of the micro-
crystalline material deposited on the quartz slide matches the
diffractogram calculated from the single crystal structure
confirming that the structure of (AuCl)2P2T3 prior to grinding
is the same as in the single crystal structure. When the sample
is ground, the sharp reflections disappear in the PXRD and are
replaced with a broad reflection with a maximum at 2y = 201.
An additional broad reflection is observed at 131, with
underlying sharp reflections at 91 and 131 which are coincident
with the peaks observed for as-prepared (AuCl)2P2T3. These
sharper reflections may be due to remaining unground
(AuCl)2P2T3. Overall, the PXRD results indicate that grinding
reduces the crystallinity of the sample.
Grinding of the (AuCl)2P2T3 sample results in a small
(B10 nm) red-shift in the solid-state absorption spectrum
(see ESIw). This red-shift is consistent with an increase in the
interannular conjugation in the terthienyl group in the ground
sample relative to what is observed in the crystal structure.
This is similar to the red-shift in absorption observed for
poly(3-dodecylthiophene) under pressure which has been
postulated to result from the uncoiling and flattening of the
polythiophene backbone.9 It is likely that weak inter- or
intramolecular interactions result in the observed large inter-
annular torsion angle in the microcrystalline (AuCl)2P2T3
sample, but these are disrupted upon grinding, bringing the
thiophene rings into greater conjugation.
To elucidate the origin of the tribochromic luminescence,
absorption and emission data of (AuCl)2P2T3 and P2T3 were
compared. In CH2Cl2 solution, both (AuCl)2P2T3 and P2T3
emit with lmax = 465 nm (see ESIw), however the excitation
spectra differ with lmax = 336 nm for (AuCl)2P2T3 and
lmax = 362 nm for P2T3. Both excitation spectra match the
Both crystalline and ground (AuCl)2P2T3 have Raman
spectra characteristic of oligo- and polythiophenes with strong
bands at 1510, 1459, and 998 cmꢁ1 assigned to the asymmetric
n(CQC), symmetric n(CQC), and n(C–S) stretches,
respectively (see ESIw).22 Between 400 and 800 cmꢁ1 the
spectra of the crystalline and ground samples show significant
differences. Notably, the crystalline (AuCl)2P2T3 sample has a
band at 670 cmꢁ1 which is absent in the spectrum of the
ground material. Analysis of the Raman spectra of 2,20:50,200-
terthiophene in solution and the solid state has shown that
ring bending vibrations between 650 and 700 cmꢁ1 are
dependent on the interannular torsion angles.23 Solid
terthiophene, where the rings are coplanar, has a peak at
691 cmꢁ1, while in solution the presence of non-coplanar
conformers gives rise to four peaks in this region. The Raman
spectrum of ClAu(PPh3)24 also has phenyl vibrations in this
region, less intense bands in the spectra of as-prepared and
ground (AuCl)2P2T3 are assigned to these vibrations. The
enhanced intensity of the Raman band at 670 cmꢁ1 in crystal-
line (AuCl)2P2T3 is therefore attributed to a Raman mode in
the less planar terthienyl group which, upon application of
Fig. 2 Emission spectra of solid (AuCl)2P2T3 (a) as prepared and
(b) after grinding. lex = 400 nm.
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pressure, weakens in intensity as the terthiophene group
becomes more planar. The other intensity differences in the
Raman spectra are also attributed to conformational changes
upon grinding. These Raman spectral differences provide
additional support for the conclusion that grinding results in
increased coplanarity of the terthienyl group. This structural
change apparently results in deactivation of a non-radiative
decay pathway present in the crystalline solid, causing the
observed increase in luminescence.
(I 4 2.0 s(I)) = 0.0715, wR2 (I 4 2.0 s(I)) = 0.0669. CCDC 742148.
Crystal data for P2T3: C36H26P2S3, M = 616.69, triclinic space group
ꢀ
P1 (#2), Z = 2, a = 9.3829(7), b = 11.5838(9), c = 14.4954(12) A,
a = 89.673(4)1, b = 72.855(4)1, g = 80.512(4)1, V = 1483.4(2) A3,
T = 173(2) K, 24 4601 reflections measured, 7094 unique (Rint = 0.050)
final R1 (I 4 2.0 s(I)) = 0.0462, wR2 (I 4 2.0 s(I)) = 0.0920. CCDC
749960.
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In summary, we report tribochromic luminescence resulting
from planarization of a conjugated terthienyl group attached
to Au(I) centers. This is the first example of conjugated ligand-
based emission tribochromism in a metal complex. Further
explorations of this new type of tribochromic luminescence are
underway.
This research was supported by the Natural Sciences and
Engineering Research Council of Canada. We thank Dr
Michael J. Katz (Simon Fraser University) for determining
the crystal structure of (AuCl)2P2T3. A. M. K. thanks UBC
and MEC for funding.
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z Synthesis of P2T3. An ether solution (50 mL) of Br2T3 (1.00 g,
2.46 mmol) was cooled to ꢁ78 1C and 1.6 M n-BuLi (3.84 mL, 6.15 mmol)
in hexanes was added dropwise. The reaction was slowly warmed to
ꢁ30 1C and PClPh2 (1.15 mL, 6.40 mmol) was added. The reaction
was allowed to warm to room temperature and stirred overnight.
The reaction was quenched by addition of water (50 mL). P2T3
immediately precipitated as a bright yellow solid, and was collected
by vacuum filtration. Yield = 1.01 g (67%). Crystals suitable for
single crystal X-ray diffraction were grown from a CDCl3–hexanes–
acetone solution. 1H NMR (300 MHz, CDCl3): d 6.59 (d, 2H, J = 5 Hz),
7.06 (s, 2H), 7.16 (d, 2H, J = 5 Hz), 7.33 (m, 20H). 31P{1H} (121 MHz,
CDCl3): d ꢁ24.50 (s). EI-MS m/z 616 (100%, [M]+). Anal. calcd for
C36H26P2S3: C, 70.11; H, 4.25%. Found: C, 69.58; H, 4.30%. Synthesis
of(AuCl) P2T3ꢂCH2Cl2.
A solution (10 mL) of P2T3 (125 mg,
2
0.203 mmol) in CH2Cl2 was added to a stirring CH2Cl2 solution (20 mL)
of AuCl(tht) (130 mg, 0.405 mmol). After one hour, the CH2Cl2 was
removed in vacuo, leaving a yellow residue. The residue was dissolved
in a minimal amount of CH2Cl2 and an equal amount of hexanes was
added. The mixture was left undisturbed overnight, and (AuCl)2P2T3
was collected as a white crystalline solid by vacuum filtration. Yield =
97 mg (44%). Crystals suitable for X-ray diffraction were grown from
CH2Cl2–hexanes solution. 1H NMR (300 MHz, CDCl3): d 6.61
(m, 2H), 6.81 (s, 2H), 7.38 (d, 2H, J = 6.6 Hz), 7.48 (m, 20H).
31P{1H} (121 MHz, CDCl3): d 15.08 (s). TOF-MS m/z 1045 ([M ꢁ Cl]+).
Anal. calcd for C36H26Au2Cl2P2S3ꢂCH2Cl2: C, 38.10; H, 2.42%.
Found: C, 38.05; H, 2.49%. Crystal data for (AuCl)2P2T3ꢂCH2Cl2:
C37H28Au2Cl4P2S3, M = 1166.51, monoclinic, space group P21/c
(#14), Z = 8, a = 9.6976(2), b = 28.8439(6), c = 27.6767(6) A,
a = 901, b = 97.3710(10)1, g = 901, V = 7677.7(3) A3, T = 113(2) K,
55 083 reflections measured, 24 495 unique (Rint = 0.0619) final R1
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Chem. Commun., 2009, 7387–7389 | 7389