Efficient polymer light-emitting diodes (PLEDs) based on chiral [Pt(C^N)(N^O)] complexes with near-infrared (NIR) luminescence and circularly polarized (CP) light

Article abstract of DOI:10.1039/c9tc04792a

Based on the molecular design of the chiral NIR emitters [Pt(iqbt)(S-Lⁿ)] (1-4), the first examples of CP-NIR-PLEDs with both attractive NIR-emission properties (λ_em = 732 nm; ηMaxEQE = 0.87-0.93%) and high dissymmetry g_EL

Full text of DOI:10.1039/c9tc04792a

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Efficient polymer light-emitting diodes (PLEDs)
based on chiral [Pt(C^N)(N^O)] complexes with
near-infrared (NIR) luminescence and circularly
polarized (CP) light†
Cite this: J. Mater. Chem. C, 2019,
7, 13743
Received 30th August 2019,
Accepted 14th October 2019
a
Guorui Fu,‡a Yani He,‡a Wentao Li,a Baowen Wang,a Xingqiang Lu,
*
DOI: 10.1039/c9tc04792a
¨
cd
Hongshan He *b and Wai-Yeung Wong
*
rsc.li/materials-c
Based on the molecular design of the chiral NIR emitters [Pt(iqbt)-
Pt(II)-complex phosphors have, however, achieved great suc-
(S-Ln)] (1–4), the first examples of CP-NIR-PLEDs with both attrac- cess in the development10 of efficient NIR-OLEDs/PLEDs. In
tive NIR-emission properties (kem = 732 nm; gMEQaEx = 0.87–0.93%) and this context, one elegant strategy toward that restrictive NIR
high dissymmetry gEL factors up to 10À3 are reported.
regime for typical vacuum-deposited NIR-OLEDs is focused on
the significant bathochromatic shift of sublimed Pt(II) complexes
Circularly polarized (CP) light-active materials,1 due to the caused by the unforeseen formation of excimers and strong
tracking of their specific interactions with chiral environments, PtÁÁÁPt intermolecular interactions. However, despite the issues
have recently attracted particular interest in the field of chiral of aggregation-induced quenching or enhancement of emission
optoelectronics.2 CP organic/polymer light-emitting diodes (AIQ or AIE),11 the realization of ZEQE values up to 1%12 or even
(OLEDs/PLEDs)3 show the advantage of being able to directly double figures13 actually makes up for the inevitable high cost of
generate CP light and have achieved high-contrast 3D images the device fabrication and the detrimental aggregation-induced
and true backlight, and are hence significantly superior to traditional efficiency roll-off. Solution-processable NIR-OLEDs/PLEDs14 based
OLEDs/PLEDs. To date, studies of small molecules4 or polymers5 on homogeneous doping of a Pt(II) complex displaying a strong
with 1p–p*-transition fluorescence have been reported. The ability of metal-to-ligand charge-transfer (MLCT) effect into an appropriate
thermally activated delayed fluorescence (TADF)6 systems, transition- host with a deep HOMO–LUMO bandgap have been convincingly
1
metal complexes7 and organo-Ln3+ complexes8 to harvest both S shown to circumvent such problems, and have specifically been
and 1T excitons for vivid visible-light (lem = 400–700 nm) CP-OLEDs/ shown to be cost effective and display desirable device perfor-
PLEDs makes the resulting devices more appealing. In particular, mance (high efficiency with relatively low efficiency roll-off), and
the CP devices that emit in the near-infrared (NIR; lem 4 700 nm) should hence be more seriously considered as an alternative. In
offer numerous advantages for bioassays, information security, and contrast, the development of highly emissive NIR-OLEDs/PLEDs
displays meant to be readable at night.9 However, few such studies without Pt(II)-chromophore interactions remains a great challenge
have been pursued, which can be attributed to several challenges due to the ‘‘energy gap law’’15 limit. Nuzzo et al.16 used a chirally
including apparently unavoidable aggregation of CP-active materials substituted polymer doped with an achiral Pt(II) complex as the
and the lack of NIR-active chiroptical luminogens.
emitting material and fabricated using this material a device
displaying emission of light at a wavelength of about 550 nm.
Fuchter et al.17 incorporated a chiral Pt(II) complex into a
conventional light-emitting polymer and the CP electrolumi-
nescence was observed at a wavelength of about 600 nm. In
light of the promising NIR (lem = 707 nm) emission18 of
[Pt(iqbt)(dpm)] (Scheme 1), we developed in the current work
the first example of CP-NIR-PLEDs by introducing simple
chirality into the N^O-Schiff-base ancillary ligands S-HLn
(n = 1–4) for the purpose of synthesizing their corresponding
chiral [Pt(iqbt)(S-Ln)]-heteroleptic complexes (1–4; see also
Scheme 1). Both CP-light activity and NIR electroluminescence
(732 nm) were observed. This result demonstrated the potential
application of chiral NIR-light-emitting Pt(II) complexes in
solution-processable CP-NIR-PLEDs.
a School of Chemical Engineering, Shaanxi Key Laboratory of Degradable Medical
Material, Northwest University, Xi’an 710069, Shaanxi, China.
E-mail: lvxq@nwu.edu.cn; Fax: +86-29-88302312; Tel: +86-29-88302312
b School of Chemistry, Eastern Illinois University, Charleston, IL 61920, USA.
E-mail: hhe@eiu.edu
c Department of Applied Biology and Chemical Technology, The Hong Kong
Polytechnic University, Hung Hom, Hong Kong, P. R. China.
E-mail: wai-yeung.wong@polyu.edu.hk
d Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057,
China. E-mail: wai-yeung.wong@polyu.edu.hk
† Electronic supplementary information (ESI) available: Starting materials and
characterization; and XRD, UV, and PL data. CCDC 1941079. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c9tc04792a
‡ These authors contributed equally and should be considered co-first authors.
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Fig. 1 A perspective drawing of the mononuclear framework of the chiral
[Pt(iqbt)(S-L3)] (3); H atoms are omitted for clarity.
Scheme 1 Synthetic scheme for the chiral Schiff-base ligands S-HLn
(n = 1–4) and their Pt(II) complexes [Pt(iqbt)(S-Ln)] (n = 1–4).
atom for [Pt(iqbt)(S-L3)] (3) was well retained. Interestingly, com-
plementary intermolecular interactions (3.566(2) Å, 3.791(2) Å,
and 3.710(30) Å for a pÁÁÁp, pÁÁÁPt, and C18–H18ÁÁÁp interactions,
respectively; Fig. S4, ESI†) but no valid PtÁÁÁPt interactions were
observed between every adjacent two mononuclear units, giving
rise to the 1D polymeric chain. The crystallographic data for this
compound are provided in Tables S2 and S3 (ESI†).
Based on the reaction (also Scheme 1) of K2PtCl4 with the
C^N-cyclometalated ligand Hiqbt synthesized from the Suzuki
coupling of cost-effective 2-Cl-isoquinoline19 with benzo[b]thien-2-y
boronic acid, the m-chloro-bridged dimer intermediate [Pt(iqbt)-
(l-Cl)]2 was obtained in 89% yield. The chiral Schiff-base-type
ligands S-HLn (n = 1–4) were prepared from the rational
condensation20 of S-(À)-1-phenylethylamine with the appropriate
salicylaldehyde derivatives, respectively. Furthermore, through the
treatment of [Pt(iqbt)(l-Cl)]2 with each of the chiral ligands S-HLn
(n = 1–4) in the presence of t-BuOK, the targeted chiral Pt(II)
complexes [Pt(iqbt)(S-Ln)] (1–4), each soluble in common organic
solvents, were afforded, respectively.
Photophysical properties of the chiral Pt(II) complexes 1–4
in solution and the solid state were explored at RT and 77 K
(Table S4 and Fig. 2, 3 and Fig. S5–S9, ESI†). As shown in Fig. 2,
The chiral Schiff-base-type ligands S-HLn (n = 1–4) and their
Pt(II) complexes 1–4 were well characterized using EA, FT-IR,
1H NMR (Fig. S1 and S2, ESI†) and ESI-MS. All of the Pt(II)
complexes exhibited relatively high thermal stabilities. TGA
results (Fig. S3, ESI†) of the chiral Pt(II) complexes 1–4 showed
them to display high decomposition temperatures (Td values
corresponding to 5% weight loss of 230 1C for 1, 267 1C for 2,
239 1C for 3 and 214 1C for 4). The relatively favourable thermal
stability of 2 with its electron-withdrawing Br atom, and the
relatively poor thermal stability of 4 with its electron-donating
tert-butyl groups, can be reasonably explained by the relatively
shorter Pt–C (1.99421 Å), Pt–N (2.05599–2.06234 Å) and Pt–O
(2.11856 Å) bond lengths (Table S1, ESI†) for 2 than for 4 (Pt–C
(2.00438 Å), Pt–N (2.05585–2.06706 Å) and Pt–O (2.12124 Å)),
determined from the TD-DFT calculations described below. The
molecular structure of [Pt(iqbt)(S-L3)] (3) was confirmed using
X-ray single-crystal diffraction analysis. The complex crystallized
in the triclinic space group P1, and its non-centrosymmetric
nature originated from the (S-L3)À-induced chirality. As shown
in Fig. 1, one C11^N1-chelating (iqbt)À ligand and one ancillary
ligand (S-L3)À with the N2^O2-chelate mode coordinated to the
central Pt2+ ion (Pt1) in a square planar geometry, leading to the
formation of a typical asymmetric mononuclear framework.18
Noticeably, in contrast to the co-planar conformation of the
cyclometalated (iqbt)À ligand, the terminal phenyl ring of the
chiral (S-L3)À ligand was observed to be almost perpendicular to
its N2^O2-chelation plane with a dihedral angle of 84.1(2)1, for
which the S-chiroptical character centered at the chiral –C*24-H
Fig. 2 The acquired normalized UV-visible absorption and emission
spectra of the chiral Pt(II) complexes 1–4 in degassed solution and of
PVK-PBD in the solid state, all at RT.
Fig. 3 The CP light spectra of the chiral Pt(II) complexes 1–4 in degassed
solution at RT.
13744 | J. Mater. Chem. C, 2019, 7, 13743--13747
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chiral Pt(II) complexes 1–4 in solution showed significantly of corresponding NIR light emission peak, probably due to
broader UV-visible absorption spectra peaks (lab o 400 nm; different levels of electron cloud sharing22 especially within the
Fig. S5, ESI†) than did the Hiqbt and S-HLn ligands: the high- chiral (S-L4)À portion (see also Table S4 and Fig. S10, ESI†).
energy-state intense absorption in the 200–400 nm range was
Theoretical studies were also carried out, and they provided
assigned to the ligand-based 1p–p* transitions, and the further insights into the photophysical properties of these
relatively weak absorptions at lab 4 430 nm were probably due chiral complexes [Pt(iqbt)(S-Ln)] (1–4). TD-DFT calculations
to a mixture of 1,3LLCT and 1,3MLCT transitions. The substituents (Table S5 and Fig. S10, ESI†) indicated that the chiral Pt(II)
on the (S-Ln)À ligands displayed little effect on the absorption and complexes 1–4 displayed similar LUMO+1 as well as LUMO
CD spectra (Fig. S6, ESI†) of their Pt(II) complexes. Importantly, in electron density distribution patterns, with most (90.20–
contrast to the ligand-related visible-light emissions (Fig. S7, ESI†) 93.04%) of the LUMO+1 electron density contributed by the
from both the Hiqbt and S-HLn ligands, all four chiral Pt(II) (S-Ln)À ligand and most (90.40–90.68%) of the LUMO electron
complexes 1–4 in solution at RT emitted, uniquely, NIR light, as density from the (iqbt)À ligand. The HOMO electron density
shown in Fig. 2, with specifically a strong peak located at lem
=
was also predominantly from the ligands, with more from the
730 nm and a shoulder at 806 nm. Of these two emission features (iqbt)À ligand (43.90%) than from the (S-L1)À ligand (38.36%)
(which were attributed to 0–0 and 0–1 transitions, respectively), as for 1, but more from the (S-Ln)À (n = 2–4) ligand for 2–4 (47.81%
a result of different degrees of vibrational coupling, the lower- for 2, 59.87% for 3 and 60.48% for 4). The introduction of the
energy emission feature was probably made up of high-frequency 3-position MeO/tert-butyl and/or 5-position Br/tert-butyl groups
normal modes with larger Huang–Rhys factors.21 Their phosphor- in 2–4 appeared to have resulted in the observed preferred
escences showed a short lifetime of 0.88 ms for 1, and significantly contribution (53.47% for 2, 62.32% for 3 or 63.88% for 4) of
longer lifetimes of 1.02 ms for 2 and 0.96 ms for 3, the latter two of the (iqbt)À ligand to their HOMOÀ1 electron densities. The
which were probably due to Br-facilitated intersystem-crossing. introduction of the Br ligand appeared to have substantially
The shortest lifetime, 0.85 ms, was found for 4 and was probably destabilized both the HOMOs and LUMOs while the introduc-
due at least in part to the electron-donating effects of two tert- tion of MeO- and tert-butyl groups appeared to have stabilized
butyl groups. It is worth noting that the NIR-light-emission them. Hence, relatively narrow HOMO–LUMO bandgaps were
efficiency (FPL) values of all four chiral Pt(II) complexes were also observed for 2–4 (2.86 eV for 2; 2.83 eV for 3 or 2.80 eV for 4)
controlled by the (S-Ln)À-involved electronic effects – and that relative to that of 1 (2.91 eV). Even though the calculated
their FPL values centered at 731 nm, more attractive than those of HOMO - LUMO transition (at 514 nm for 1; 524 nm for 2;
the reported NIR-light-emitting Pt(II) complexes10 just with the 526 nm for 3 and 532 nm for 4) came mainly (over 97%) from
MLCT impact, were attributed to their large radiative rate constant the S0 - S1 transition, the experimentally determined lower-
(kr = 105 sÀ1). The Br-containing chiral Pt2+ complex 2 gave rise to energy (lab 4 430 nm; 518–521 nm for 1–4) absorptions
the best NIR-emission FPL of 21%, which was due to its displaying probably also included the LLCT/MLCT transitions. NTO calcu-
the highest kr (2.10 Â 105 sÀ1) and lowest knr (7.75 Â 105 sÀ1). The lations (Table S6 and Fig. S11, ESI†) for S0 - T1 excitations
effective radiative transition with the weak vibrational coupling indicated that about 80% of the hole orbital was contributed by
for the four chiral Pt2+ complexes 1–4 was further confirmed by (S-Ln)À and 92% of the particle orbital was from the (iqbt)À
the results of their rigidochromic shifts (Fig. S8, ESI†) at 77 K, ligand for each of the chiral Pt(II) complexes 1–4, forming
whereas the (S-Ln)À-incorporation benefited from effectively side- almost the entire (ca. 99%) hole - particle transition, and
stepping the intramolecular rotation/distortion for more of a hence suggesting that the 3LLCT-dominated transitions and the
chance to realize the T1-preferred radiative decay with longer less prevalent 3MLCT transitions were responsible for their
phosphorescence lifetimes. Slight blue shifts (lem = 725–727 nm) NIR-light-emission phosphorescences. These bandgaps were
for 1–2 and red shifts (lem = 741–751 nm) for 3–4 were observed in found to be quite similar to those determined electrochemically
the solid state (Fig. S9, ESI†) relative to that (lem = 732 nm) in (Fig. S12 and also Table S4, ESI†).
solution at RT, and these results were probably due to strong
intermolecular interactions (see also Fig. S4, ESI†) as in the case of flexible CP-NIR-PLEDs based on these chiral Pt(II) complexes
traditional Pt2+ complexes.
1–4. PVK-PBD was used as the host due to its good hole-electron
Of particular interest are the cost-effective and large-area
Their CD spectra (see also Fig. S6, ESI†) showed the Cotton transport, and the significant spectral overlap (also Fig. 2)
effects with two positive signals at 259–268 and 328–331 nm between the emission of light from PVK-PBD and the LLCT/
and a negative one at 286–313 nm as has been observed for the MLCT absorption of light by the chiral Pt(II) complexes 1–4,
23
free S-HLn ligands, but also showed additional broad Cotton- features beneficial for effective Forster energy transfer from
¨
effect bands arising from the (S-Ln)À-controlled exciton-coupled PVK-PBD to the chiral Pt(II)-complex species. Since the LUMO
1,3LLCT/1,3MLCT transitions with three negative signals and (À2.95 to À2.86 eV) and HOMO (À5.66 to À5.61 eV) levels of the
two positive signals between 355 nm and 600 nm. The emission chiral Pt(II) complexes 1–4 fit perfectly within those (À2.50 to
dissymmetry factor (gPL) values (Fig. 3) of the four (S-Ln)À- À2.00 eV of LUMOs and À6.10 to À5.50 eV of HOMOs) of PVK
involved chiral Pt(II) complexes were all on the order of and PBD, respectively, the injected holes from PEDOT:PSS and
10À3 (specifically À2.14 Â 10À3 (731 nm) for 1; À2.07 Â 10À3 the injected electrons from TPBi would pass through the PVK-
(729 nm) for 2; À1.76 Â 10À3 (730 nm) for 3, and À2.75 Â 10À3 PBD host to be trapped by the chiral Pt(II)-complex species, and
(731 nm) for 4), indicative of an (S-Ln)À control of the intensity subsequently direct charge carrier trapping and recombination
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effect of the 5-position Br resulted in CP-NIR-PLED-A-2
displaying a significantly higher efficiency roll-off (48%) than
did of the other three devices (20–37%). For optimization,
CP-NIR-PLED-B-2 (see also Fig. 4(a)) with an additional hole-
blocking BCP layer was fabricated. Delightfully, due in part to a
greater number of excitons confined within the broadened
recombination zone,24 a distinctively improved NIR-emission
electroluminescence (see also Fig. 4(b–e)) was achieved. First,
despite using an illumination voltage of only 6.0 V, the LMe ax was
almost three times higher than that of the CP-NIR-PLED-A-2.
Moreover, the reformed carrier balance within the CP-NIR-
Max
EQE
PLED-B-2 also endowed it with an Z
of up to 0.93%,
attractive relative to those of previously described Pt(II)-
complex-based NIR-PLEDs.10,14 Interestingly enough, its effi-
ciency roll-off (23%) was also relieved. Worthy of notice, as
shown in Fig. 4(f), the dissymmetry factors (gEL, see also Table S7,
ESI†) of all of the CP-NIR-PLEDs were also on the order of 10À3
(determined by (S-Ln)), similar to those of the previously reported
Pt(II)-complex-based visible CP-OLEDs.7
Fig. 4 (a) Device structures and energy level diagrams. (b) Normalized
electroluminescence spectra. (c) J–V, (d) Le–V, (e) ZEQE–Le, and (f) gEL
curves based on NIR-CP-PLEDs-A/B from the chiral Pt(II) complexes 1–4.
In conclusion, through the involvement of (S-Ln)À-N^O
ancillary ligands with different electronic effects, a series of
chiral [Pt(iqbt)(S-Ln)] complexes (1–4) displaying emission of
NIR light (lem = 729–731 nm) and CPL-active properties were
should take place. Through the solution-processable procedure obtained. Moreover, as a result of their being doped into the
with the same configuration shown in Fig. 4(a), a series of PVK-PBD host, the resulting CP-NIR-PLEDs showed quite
Max
CP-NIR-PLEDs-A/B using the chiral Pt(II) complexes 1–4 as the attractive device performance measures (lem = 732 nm; Z
EQE
dopants with an intended doping level of 8 wt% were fabri- up to 0.87–0.93%) and high gEL of up to 10À3, features observed
cated. The difference between the A and B series was that a for the first time to the best of our knowledge. This result
hole-blocking BCP layer was added in the B series.
As shown in Table S7 (ESI†) and Fig. 4(b)/Fig. S13 (ESI†), all plexes are promising candidates for future CP-NIR-PLEDs.
of the normalized electroluminescence NIR-emission (lem
suggested that the (S-Ln)À-involved chiral [Pt(iqbt)(S-Ln)] com-
=
732 nm) spectra for the CP-NIR-PLEDs-A series well resembled
those of the chiral Pt(II) complexes 1–4 in solution at RT, with
this similarity due to the low-concentration dispersion of the
dopants 1–4 into the corresponding PVK-PBD host together
with the absence of the above-described strong intermolecular
interactions (see also Fig. S4, ESI†). The spectra also showed
featureless structures and independence of the applied bias
voltage. Fig. 4(c–e) show the J–V, Le–V and ZEQE–Le curves for
CP-NIR-PLEDs-A with the different chiral Pt(II) complexes 1–4.
In contrast to the monotonously increasing values of J and Le
with increasing applied bias voltage, ZEQE in each case
increased initially and then instantly decreased throughout
the corresponding illumination. The highest device efficiency
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the NNSF (21373160, 21173165),
the Wisteria Scientific Research Cooperation Special Project of
Northwest University, and the NSF (1507871) of the USA.
Notes and references
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EQE
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