Platinum(ii) Schiff base as versatile phosphorescent core component in conjugated oligo(phenylene-ethynylene)s

Article abstract of DOI:10.1039/b715495j

Novel phosphorescent conjugated oligo(phenylene-ethynylene)s featuring a central tunable platinum(ii) Schiff base signalling unit with promising photophysical properties have been investigated to pave their development towards polymeric congeners for sensing applications.

Full text of DOI:10.1039/b715495j

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Platinum(II) Schiff base as versatile phosphorescent core component in
conjugated oligo(phenylene–ethynylene)s†‡
Wah-Leung Tong, Lo-Ming Lai and Michael C. W. Chan*
Received 9th October 2007, Accepted 22nd November 2007
First published as an Advance Article on the web 4th December 2007
DOI: 10.1039/b715495j
Novel phosphorescent conjugated oligo(phenylene–ethy-
nylene)s featuring a central tunable platinum(II) Schiff base
signalling unit with promising photophysical properties have
been investigated to pave their development towards poly-
meric congeners for sensing applications.
becomes feasible; (4) reports depicting metal–salen derivatives in
sensor applications remain sparse in the literature,7 and develop-
ment of luminescent versions are very limited.8 We herein describe
novel phosphorescent oligo(phenylene–ethynylene)s (OPEs) con-
jugated with a Pt(II) Schiff base core module, which constitute
the initial step towards the development of phosphorescent salen-
based conjugated polymers as tunable sensory materials. Reports
on the photophysical studies of luminescent OPEs incorporating
octahedral Re–, Ru–, and Ir–bpy moieties have appeared.9
The rapid advancement of poly(phenylene–ethynylene)s (PPEs)
as novel functional materials for fluorescent sensing (and op-
toelectronic) applications has been impressive, and their salient
appeal is the enhanced sensitivity arising from the conjugation-
mediated amplified quenching effect.1 The pursuit of phospho-
rescent relatives of PPEs and other conjugated organic materials,
which can potentially transmit the sensing process by visible light
without interference from organic/background fluorescence, is a
more recent development, and the incorporation of conjugated
metal–organic motifs can engender tunability and diverse excited
states.2 The phosphorescent properties and quenching of a con-
jugated PPE–platinum(II) acetylide polyelectrolyte material3 and
cyclometallated Pt(II) poly(phenylene)s4 were recently reported,
although the anticipated quenching amplification was not detected
and design modification of the Pt–acetylide backbone in the
former is non-trivial.
The synthetic routes utilized in this work are summarized
in Scheme 1. The Pt–diiodosalphen precursor complex 1 was
obtained in moderate yield (40–50%) from the gently heated
reaction10 of the tetradentate bis-phenol (generated from typi-
cal diamine/salicylaldehyde bis-condensation) with K2PtCl4.‡The
aldehyde-capped mono- and di(phenylene–ethynylene) ligands
MPE and DPE have been assembled in a stepwise fashion
from 2,5-diheptoxy-1,4-diiodobenzene using established proce-
dures or modifications thereof.‡ The desilylation reaction to give
DPE using KOH followed by HCl was originally unsuccessful
(13C NMR characterization indicated conversion of the internal
ethynylene group to vinylene), but changing the base to KF in
THF/methanol cleanly afforded DPE in virtually quantitative
yield. The C–C formation step between the terminal phenylene–
ethynylenes and Pt–Schiff base 1 was accomplished using con-
ventional Pd-catalysed coupling reactions to afford the mono-
dispersed conjugated oligomers Pt-n (n = 2, 4 = no. of PE units)
bearing a versatile luminescent Pt–Schiff base interior (ca. 55
and 30% yields respectively). The characterisation data11 for Pt-
2 and Pt-4, including the corresponding number of 13C NMR
C≡C resonances and excellent agreement between the ES-MS and
isotopic distribution simulation for M+,‡ are entirely consistent
with the proposed molecular structures.
We have embarked upon a modular design approach for
sensor development based on the controllable integration of
functionalised phosphorescent signalling units into well-defined
conjugated materials. Our interest in Schiff base–Pt(II) modules
stems from the following: (1) phosphorescent multidentate Pt(II)
Schiff base materials have recently been examined for OLED
applications,5 and their exceptionally high quantum efficiencies
and robustness would facilitate sensing applications in polar
(aqueous) media; (2) compared to d6 octahedral Ru(bpy)3
2+
luminophores which exhibit sphere-like coordination, the ‘open’
unsaturated geometry of square planar d8 Pt(II) can afford excited
states that are sensitive to the micro-environment;6 (3) the pro-
found impact of Schiff base (salen) complexes upon research areas
such as enantioselective organic oxidation has led to tremendous
molecular diversity with well-established synthetic methodologies,
so that structural modification is readily available and modularity
The photophysical properties of Pt-2 and Pt-4 have been
investigated (Table 1). Their UV-vis absorption spectra in CH2Cl2
1
(Fig. 1) feature two highly intense (pp*) absorption bands at
around kmax 300 (e > 4.8 × 104 mol−1 dm3 cm−1) and 400
(e > 1 × 105 mol−1 dm3 cm−1) nm. In addition, a moderately
intense band that is absent for the ligand DPE is observed at
kmax 509 and 501 nm (e = (1.6 and 1.9) × 104 mol−1 dm3 cm−1
respectively) for Pt-2 and Pt-4 respectively, with a shoulder at
around 550 nm (e ≈1 × 104 mol−1 dm3 cm−1). The influence of
solvent polarity upon the UV-vis absorptions of Pt-4 was probed.
While the energies of the absorption maxima at k < 420 nm remain
relatively unchanged, the low-energy absorption band (and the
shoulder in particular) undergoes noticeable red shifts in low-
polarity solvents (kmax): 496 (554 sh) nm in THF; 501 (555 sh)
nm in CH2Cl2; 509 (571 sh) nm in toluene (inset of Fig. 1). Such
Department of Biology and Chemistry, City University of Hong Kong, Tat
Chee Avenue, Kowloon, Hong Kong, China. E-mail: mcwchan@cityu.edu.hk
† Dedicated to Prof. Ken Wade for his teaching and guidance on the
occasion of his 75th birthday.
‡ Electronic supplementary information (ESI) available: Experimental de-
tails and characterisation, including mono- and di(phenylene-ethynylene)
synthesis; ES-MS and calculated isotopic distribution for Pt-2 and Pt-4;
solvent effects upon emission of Pt-4; 77 K glassy emission and transient
absorption–difference spectra. See DOI: 10.1039/b715495j
1412 | Dalton Trans., 2008, 1412–1414
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Scheme 1
Table 1 Photophysical dataa,b
em
max
em
max
em
77 K solid k (s)
max
Complex
UV-vis kmax (e × 104)
298 K fluid k (s; U)
77 K glass k (s)e
Pt-2
Pt-4
292 (4.80), 398 (10.2), 509 (1.56), 553 (sh, 0.87)
312 (6.67), 404 (12.8), 501 (1.88), 555 (sh, 1.01)
c310 (7.20), 399 (13.9), 496 (2.04), 554 (sh, 1.05)
d304 (6.63), 403 (13.6), 509 (1.80), 571 (sh, 0.93)
304 (2.19), 314 (2.12), 397 (2.99)
596, 658 (max), 727 (1.2; 2.5 × 10−3
)
)
651 (max), 716 (4.2)
646 (max), 714 (2.3)
671 (max), 745 (1.6)
685 (max), 758 (1.1)
604, 658 (max), 725 (1.3; 3.1 × 10−3
c
601, 653 (max), 720 (1.6; 7.4 × 10−3
)
)
d608, 659 (max), 727 (2.2; 2.6 × 10−2
DPE
457
a k in nm; e in dm3 mol−1 cm−1; lifetimes (s) in ls. b In CH2Cl2 unless specified. c In THF. d In toluene. e In 2-Me-THF.
1[l(phenoxide) → p*(imine)], mixed with 1[Pt(5d) → p*(Schiff
base)] MLCT character.
Complexes Pt-2 and Pt-4 display structured phosphorescence
in fluid solution (kmax 658 [sh 596 and 604 respectively] nm in
CH2Cl2; Fig. 2) and the solid state (kmax 660 [sh 725] and 668 [sh
730] nm respectively) at room temperature. Compared with the
Fig. 1 UV-vis absorption spectra of Pt-2, Pt-4 and DPE in CH2Cl2 at
298 K. Inset: negative solvatochromic shifts for low-energy absorptions of
Pt-4.
negative solvatochromic shifts indicate that the ground state is
more polar than the excited state and have been previously detected
for related Pt–Schiff base derivatives5,12 such as Ptq2.13 We therefore
similarly assign this absorption to a charge transfer transition
involving the phenoxide lone pair (l) and the imine p* orbital, i.e.
Fig. 2 Normalised emission spectra of Pt-2, Pt-4 and DPE in CH2Cl2 at
298 K.
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absorption data, the fluid emission of Pt-4 displays less prominent
solvatochromic effects (e.g. kmax 651 nm in THF is red-shifted by ca.
190 cm−1 to 659 nm in toluene).‡ Emission lifetimes in the 1–2 ls
regime and luminescent quantum yields of up to 2.6 × 10−2 for Pt-4
in toluene are recorded. The 77 K glassy emission of Pt-2 and Pt-4
in 2-Me-THF are partially blue-shifted and highly structured;‡
for example, Pt-4 exhibits peak maxima at 646 and 714 nm
(vibronic progression = 1470 cm−1). As expected, all excitation
spectra (monitored at the emission energy) are comparable in
shape to the corresponding UV-vis absorption spectra. Upon
consideration of the large Stokes shifts, negative solvatochromism
and previous assignments,5,12,13 plus the vibronic progressions of
ca. 1400–1500 cm−1 in the 77 K solid-state and glassy emission
spectra, we assign the excited states of Pt-2 and Pt-4 to mixed
3MLCT and 3[l → p*(imine)] (l = lone pair/phenoxide) parentage.
The transient absorption–difference (TA) spectrum‡ for the
excited state of Pt-4 (900 ns delay) in CH2Cl2 at 298 K is
characterized by bleaching in the pp* region (k < 470 nm), an
excited-state absorption band at ca. kmax 505 nm, and additional
absorptions extending and rising in the visible region (k >
600 nm) towards the near-IR. While the latter far-red absorptions
are not detected in the TA spectra of previously reported Pt–
Schiff base analogues5a,b and are reminiscent of the OPE pp*
state,9b,c,14 we suggest further investigations to resolve the detailed
contribution and impact of the OPE units upon the excited state.
The decay lifetime (sTA ) of 1.4 ls approaches the emission lifetime
(1.3 ls), indicating that the observed excited-state absorption can
be ascribed to the excited state of Pt-4.
Significantly, with regards to sensing applications, the phos-
phorescent characteristics of the oligomeric Pt-2 and Pt-4
derivatives (prominent MLCT component in the excited state;
higher quantum yields; low-energy red emission where the hu-
man eye is more perceptive) are promising and nevertheless
appealing compared with the previously reported Zn/Ni/V-based
poly(salphenylene–ethynylene)s8 (weakly or non-emissive) and Pt–
oligo/poly(phenylene)s4 (dominated by 3LC/pp* excited state). It
is anticipated that the prominent spin–orbit coupling and MLCT
character evident in the excited states of Pt-2 and Pt-4 can be
exploited in due course. Needless to say, we have attempted the
Pd-catalysed polymerisation of the diiodo complex 1 with 2,5-
diheptyloxy-1,4-diethynylbenzene, but the resultant deep red solid
proved intractable and insoluble in organic solvents. Longer alkyl
chains are apparently required, and studies dedicated to the fabri-
cation of phosphorescent conjugated polymers featuring Pt–salen
moieties are in progress. In conclusion, monodispersed phenylene–
ethynylene oligomers linked by a phosphorescent Pt(II) Schiff
base unit have been designed and synthesised, and interesting
emissive properties have been observed. Based on this work, we
envisage that new classes of Pt(II)–Schiff base conjugated materials
can be developed for luminescent sensing applications.
Acknowledgements
We acknowledge financial support from City University of Hong
Kong (SRG 7001787) and Research Grants Council of the Hong
Kong SAR, China (CityU 100405).
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13
=
11 Selected C NMR data for Pt-2 (126 MHz, CDCl3): 188.9 (C O),
=
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1666 (C O), 1594 (C N) cm−1; for Pt-4 (thin film): v = 2200 (C≡C),
=
=
−1
=
=
1679 (C O), 1595 (C N) cm
.
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1414 | Dalton Trans., 2008, 1412–1414
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The Royal Society of Chemistry 2008
©