Substituents engineered deep-red to near-infrared phosphorescence from tris-heteroleptic iridium(iii) complexes for solution processable red-NIR organic light-emitting diodes

View Article Online

View Journal

Journal of

Materials Chemistry C

Accepted Manuscript

This article can be cited before page numbers have been issued, to do this please use: H. U. Kim, S.

Sohn, W. Choi, M. Kim, S. U. Ryu, T. Park, S. Jung and B. M. D. K S, J. Mater. Chem. C, 2018, DOI:

10.1039/C8TC04321C.

This is an Accepted Manuscript, which has been through the

Volume

4 Number 1 7 January 2016 Pages 1–224

Royal Society of Chemistry peer review process and has been

accepted for publication.

Journal of

Materials Chemistry C

Accepted Manuscripts are published online shortly after

Materials for optical, magnetic and electronic devices

www.rsc.org/MaterialsC

acceptance, before technical editing, formatting and proof reading.

Using this free service, authors can make their results available

to the community, in citable form, before we publish the edited

article. We will replace this Accepted Manuscript with the edited

and formatted Advance Article as soon as it is available.

You can find more information about Accepted Manuscripts in the

author guidelines .

Please note that technical editing may introduce minor changes

to the text and/or graphics, which may alter content. The journal’s

standard T erms & Conditions and the ethical guidelines, outlined

in our author and reviewer resource centre, still apply. In no

event shall the Royal Society of Chemistry be held responsible

for any errors or omissions in this Accepted Manuscript or any

consequences arising from the use of any information it contains.

ISSN 2050-7526

PAPER

~

Nguyên T. K. Thanh, Xiaodi Su et al.

Fine-tuning of gold nanorod dimensions and plasmonic properties using

the Hofmeister eﬀects

rsc.li/materials-c

Please do not adjust margins

Page 1 of 21

Journal of Materials Chemistry C

View Article Online

DOI: 10.1039/C8TC04321C

Journal Name

ARTICLE

Substituents engineered deep-red to near-infrared

phosphorescence from tris-heteroleptic iridium(III) complexes for

solution processable Red-NIR organic light-emitting diodes

Received 00th January 20xx,

Accepted 00th January 20xx

Hae Un Kim,^a‡Sunyoung Sohn,^b‡Wanuk Choi,^cMinjun Kim,^aSeung Un Ryu,^aTaiho Park,^a*

DOI: 10.1039/x0xx00000x

Sungjune Jung,^b* K. S. Bejoymohandas^a*

www.rsc.org/

Research on near-infrared- (NIR-) emitting materials and devices has been propelled by fundamental and practical

application demands surrounding information-secured devices and night-vision displays to phototherapy and civilian

medical diagnostics. However, the development of stable, highly efficient, low-cost NIR-emitting luminophores is still a

formidable challenge owing to the vulnerability of the small emissive bandgap toward several nonradiative decay

pathways, including the overlapping of ground- and excited-state vibrational energies and high-frequency oscillators.

Suitable structural designs are mandatory for producing an intense NIR emission. Herein, we developed a series of deep-

red to NIR emissive iridium(III) complexes (Ir1–Ir4) to explore the effects of electron-donating and electron-withdrawing

substituents anchored on the quinoline moiety of (benzo[b]thiophen-2-yl)quinoline cyclometalating ligands. These

substituents help engineer the emission bandgap systematically from the deep-red to the NIR region while altering the

emission efficiencies drastically. Single-crystal X-ray structures authenticated the exact coordination geometry and

intermolecular interactions in these new compounds. We also performed an in-depth and comparative photophysical

study in the solution, neat powder, doped polymer film, and freeze matrix at 77ꢀK states to investigate the effects of

substitution on the excited-state properties. These studies were conducted in conjunction with density functional theory

(DFT) and time-dependent density functional theory (TD-DFT) calculations. Most importantly, the –CH₃substituted Ir1,

unsubstituted Ir2, and –CF₃substituted complex (Ir4) were promising novel compounds with bright phosphorescence

quantum efficiency in doped polymer films. Using these novel molecules, deep-red to NIR emissive organic light-emitting

diodes (OLEDs) were fabricated using a solution-processable method. The unoptimized device exhibited maximum

external quantum efficiency (EQE) values of 2.05% and 2.11% for Ir1 and Ir2, respectively.

NIR LEDs have unique applications in military photonic devices,

telecommunications, night-vision displays, photodynamic

therapy, and photobiomodulation therapy, among others.^20-25

Introduction

A surplus of research has already been carried out on blue,

green, yellow, and red emissive dyes for highly efficient

organic light-emitting diodes (OLEDs) with respect to emission

efficiency, chromaticity, and stability.^1-18Owing to the need for

distinguishable visible light emission and high tissue

penetration, the development of deep-red to near-infrared

(NIR) (650–2,500ꢀnm) emissive dyes is of utmost importance.¹⁹

For practical applications, a long operational lifetime is crucial

for the device. This issue must be successfully addressed by

organic luminogens.²⁶Until now, the organic NIR emitters

desirable for NIR OLEDs primarily include small organic

molecules,^{23, 27-30}conjugated polymers,^31-34lanthanide

complexes, and transition-metal complexes.^{23-25, 35, 36}From the

transition-metal complex list, phosphorescent transition-metal

complexes based on osmium(II), platinum(II), and iridium(III)

are more advantageous owing to the effective harvesting of

both singlet (25%) and triplet (75%) excitons in a device,

enabling the theoretical internal quantum efficiency of 100%

to be attained.^37-39Currently, platinum(II) phosphors stand out

as the best NIR OLEDs, with a recorded external quantum

efficiency (EQE) value of 24% at 740ꢀnm.^40-42Unfortunately,

platinum-based NIR OLEDs typically endure from severe

efficiency roll-off at higher current density owing to their

inherent long phosphorescence lifetimes. This mainly arises

from the facile aggregation of the dyes because of their

^a.Department of Chemical Engineering, Pohang University of Science and

Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, Korea, Fax:

+82-54-279-8298; Tel: 82-54-279-2394, *E-mail: taihopark@postech.ac.kr;

bejoymohan@postech.ac.kr

^b.Department of Creative IT Engineering, Pohang University of Science and

Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, Korea, *E-

mail: sjjung@postech.ac.kr

^c.Division of Environmental Science and Engineering, Pohang University of Science

and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, Korea

‡ These authors contributed equally to this work.

† Electronic Supplementary Information (ESI) available: [Materials and physical

measurement methods, NMR spectra, MALDI-TOF MASS spectra of ligands and

complexes, Crystal packing structure, Thermal stability analysis, Optimized

geometries, Electroluminescence spectra]. See DOI: 10.1039/x0xx00000x

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 1

Please do not adjust margins

Journal of Materials Chemistry C

Page 2 of 21

ARTICLE

Journal Name

homoleptic complexes using thenoyltrifluoro_Va_ic_ewet_Ao_rtn_ice_{le On}(_li_ne-

DOI: 10.1039/C8TC04321C

diketonate) as an ancillary ligand. The effects of substituents

on the electrochemical energy levels, excitation transitions,

and geometry of ground and excited states were further

analyzed using density functional theory (DFT) and time-

dependent density functional theory (TD-DFT) calculations. An

in-depth and comparative photophysical study was conducted

in the solution, freeze matrix at 77ꢀK, doped polymer films, and

neat powder forms to understand the effects of the

substitution on the associated excited-state properties. Finally,

the newly designed compounds were successfully employed as

phosphorescent dopants in a cohost material based on 1,3-

bis(N-carbazolyl)benzene

and

tris(4-carbazoyl-9-

ylphenyl)amine (mCP:TCTA) to fabricate a multilayer solution-

processable PhOLED device to investigate electroluminescence

properties.

Chart 1 Structure of Ir1-Ir4 and relative electronic effects of the substituents

represented as Hammett constants toward the ortho and para positions.

square-planar geometry. Thanks to the typical octahedral

coordination geometry in iridium(III) complexes, which favor

short triplet lifetimes, this has significantly mitigated the

efficiency roll-off in iridium-based OLEDs.^43-47Moreover,

replacing platinum(II) with iridium(III) can considerably reduce

the percentage of noble metal content in the phosphorescent

dopants.^{48, 49}Iridium(III) dyes exhibit spectacular performance

in visible light emissive OLEDs and are expected to be

promising candidates for NIR OLEDs.^{37, 50-62}However, just a

fistful of deep-red and NIR-emitting iridium(III) complexes

have been reported to date. Qiao et al. have made outstanding

efforts on phosphorescent NIR OLEDs, achieving a maximum

EQE of 5.7%.^{45-47, 63}Considering visible light emitters, deep-red

to NIR emitters suffer from severe photoluminescent quantum

efficiency drops, which are directly reflected in their low

device performance efficiencies. The NIR emissive bandgap is

always prone to several nonradiative decay pathways,

including the overlapping of ground- and excited-state

vibrational energies and high-frequency oscillators. A suitable

molecular design is essential to address these issues effectively

for producing an intense emission. State-of-the-art research on

NIR emissive iridium(III) complexes has mostly focused on

emission color control or attaching additional hole or electron

transporting moieties for improving the device’s efficiency.^44,

Experimental

Synthesis of ligand precursors

Synthesis of ethyl 2-chloroquinoline-4-carboxylate (L3a).

A

mixture of 2-hydroxyquinoline-4-carboxylic acid (1.7 g, 9.0

mmol) in POCl₃(10 mL) was heated at reflux for 3 h under

argon. Then the cooled reaction mixture was poured into ice-

water (Caution!, highly exothermic!), and the precipitate was

extracted with CHCl₃. The combined extracts were dried over

sodium sulfate and evaporated under reduced pressure. To

the residue was added thionyl chloride (10 mL), and the

resulting mixture was heated at reflux under argon for 2 h. The

thionyl chloride excess was then removed under reduced

pressure, and 20 mL of ethanol and 6 mL of triethylamine were

added. The resulting reaction mixture was heated at reflux for

30 min, reduced the volume under low pressure, and

separated between CHCl₃and water. The organic partition was

dried over magnesium sulfate and concentrated under

reduced pressure. The crude product was purified by flash

chromatography (ethyl acetate 20 v/v%: hexane v/v 80% as

the eluent) (1.5 g, 67% pale yellow powder). ¹H NMR (400

MHz, Chloroform-d) δ 8.73 (dd, J = 8.6, 1.4 Hz, 1H), 8.08 (d, J =

8.5 Hz, 1H), 7.89 (s, 1H), 7.79 (ddd, J = 8.5, 6.8, 1.4 Hz, 1H),

7.66 (ddd, J = 8.3, 6.9, 1.3 Hz, 1H), 4.52 (q, J = 7.1 Hz, 2H), 1.48

(t, J = 7.1 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 164.92, 150.14,

148.91, 138.17, 130.85, 129.13, 128.32, 125.73, 123.99,

123.68, 62.27, 14.24.

63-88

In order to control the nonradiative pathways in an NIR

emissive molecule, the placement of low-frequency oscillating

functional groups at the frontier molecular orbital (MO)

localized areas is crucial. Few reports exist on the effect of

structural modifications using substituent-induced electronic

effects on the excited-state properties of deep-red to NIR

emissive iridium(III) complexes. For the first time, we have

developed a series of deep-red to NIR emissive iridium(III)

Synthesis of 2-chloro-4-(trifluoromethyl)quinoline (L4a).

A

round bottom flask was charged with 4-trifluoromethyl-2 (1H)

-quinolinone(1g, 4.69 mmol) and POCl₃(1o mL, 107.28 mmol).

The mixture was heated to reflux for three hours under argon

atmosphere and followed by concentrating the reaction

mixture under reduced pressure to remove excess POCl₃. The

mixture was the based with 5N NaOH (Caution!, highly

exothermic!) and extracted with dichloromethane. The

concentrated crude mixture was then purified by column

chromatography (ethyl acetate 5 v/v%: hexane v/v 95% as the

complexes

(Ir1–Ir4) by anchoring electron-donating and

electron-withdrawing substituents on the quinoline moiety of

(benzo[b]thiophen-2-yl)quinoline cyclometalating ligands

(Scheme 1). The choice of the largely π-extended 1-

(benzo[b]thiophen-2-yl)quinolone cyclometalated ligand was

purely based on the impressive emission efficiency (Φ_PLꢀ=ꢀ20%)

exhibited by its homoleptic derivative.⁷⁰From a synthetic

perspective, heteroleptic complexes are preferred over

eluent)

to

give

the

product

2-chloro-4-

1

(trifluoromethyl)quinoline (0.9 g, white powder). H NMR (400

2 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 3 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

MHz, Chloroform-d) δ 8.19 – 8.07 (m, 2H), 7.85 (ddd, J = 8.4, Synthesis of 2-(benzo[b]thiophen-2-yl)-4-(trifluoromethyl)-

View Article Online

7.0, 1.3 Hz, 1H), 7.77 – 7.65 (m, 2H). ¹³C NMR (151 MHz, CDCl₃) quinoline [bttmq] (L4). The crude product was then purified by

DOI: 10.1039/C8TC04321C

δ 149.84, 148.76, 131.39, 129.56, 128.57, 124.10, 123.52, silica column chromatography (ethyl acetate 10 v/v%: hexane

121.82, 119.94, 119.90. ¹⁹F NMR (471 MHz, CDCl₃) δ -61.70.

v/v 90% as the eluent) (yield 80% pale yellow powder) ¹H NMR

(400 MHz, Chloroform-d) δ 8.25 (d, J = 8.5 Hz, 1H), 8.21 (s, 1H),

8.12 (d, J = 8.6 Hz, 1H), 8.06 (s, 1H), 7.93 – 7.85 (m, 2H), 7.82 (t,

General synthesis procedure for cyclometalating ligands

To

a

suspension of one equivalent of substituted 2- J = 7.7 Hz, 1H), 7.65 (t, J = 7.7 Hz, 1H), 7.46 – 7.37 (m, 2H). ¹³C

[2-bromoquinoline or 2-chloro-4- NMR (151 MHz, CDCl₃) δ 151.76, 148.98, 144.28, 141.41,

haloquinoline

methylquinoline or ethyl 2-chloroquinoline-4-carboxylate or 2- 140.28, 135.02, 134.80, 130.65, 130.39, 128.06, 125.84,

chloro-4-(trifluoromethyl)quinoline] with 1.2 equivalents of 124.76, 124.57, 123.95, 123.43, 122.66, 122.08, 115.02. ¹⁹F

benzo[b]thiophene-2-boronic acid was dissolved in 30 mL of NMR (471 MHz, CDCl₃) δ -61.60.

dry THF and 0.06 equivalent of tetrakis(triphenylphosphine)

General snthesis procedure for iridium(III) dimer complexes

palladium(0) was added. A solution of air free 5% sodium

carbonate (10 mL) was added and the refluxed the reaction A mixture of IrCl₃·xH₂O (0.57 mmol) and the corresponding

mixture with stirring for one day, at nitrogen atmosphere. ligand (L1-L4) (1.27 mmol) in 30 mL of water and 2-

After being cooled, the mixture was poured into water, and ethoxyethanol (1:3) was refluxed at 140 °C for one day. Upon

the organic part was extracted with ethyl acetate. The pouring 20 mL of water in to the cooled reaction mixture

collected organic layer was dried over magnesium sulphate. resulted in a dark red precipitate in the case of L1 and L2 and

The crude product obtained after vacuum evaporation was black precipitate for L3 and L4. The precipitate was collected

then purified by column chromatography using silica gel with by filtration and washed with diethyl ether. Without any

ethyl acetate: n-hexane (2: 8) as the eluent to give the final further purification, the crude product was utilized for the next

product.

reaction.

Synthesis of 2-(benzo[b]thiophen-2-yl)-4-methylquinoline

[btmq] (L1). The crude product was then purified by silica

General procedure for the synthesis of complexes Ir1 – Ir4

column chromatography (ethyl acetate 20 v/v%: hexane v/v One equivalent of the corresponding dimer, 2.6 equivalents of

1

80% as the eluent) (yield 92%, white powder). H NMR (400 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione

and

11

MHz, Chloroform-d) δ 8.15 (d, J = 8.4 Hz, 1H), 8.01 – 7.94 (m, equivalents of sodium carbonate were mixed together and

2H), 7.92 – 7.88 (m, 1H), 7.87 – 7.82 (m, 1H), 7.79 (d, J = 1.1 Hz, stirred overnight in 1:3 mixture of ethanol and

a

1H), 7.72 (ddd, J = 8.4, 6.8, 1.4 Hz, 1H), 7.54 (ddd, J = 8.3, 6.9, dichloromethane (50 mL) at 65 °C under inert atmosphere

1.3 Hz, 1H), 7.37 (dt, J = 7.3, 3.5 Hz, 2H), 2.77 (d, J = 0.9 Hz, (Argon). The progress of the reaction was monitored by TLC.

3H). ¹³C NMR (151 MHz, CDCl₃) δ 151.96, 148.00, 145.70, Upon completion of the reaction, the solvent was removed by

144.67, 141.16, 140.52, 130.08, 129.58, 127.64, 126.29, vacuum evaporation. The obtained crude product was poured

125.21, 124.80, 124.46, 123.65, 122.60, 122.20, 118.45, 18.95. into water and extracted with ethyl acetate (3 × 50 mL). All the

Synthesis of 2-(benzo[b]thiophen-2-yl)quinoline [btq] (L2). organic layers were combined and dried over magnesium

The crude product was then purified by silica column sulphate. The solvent was removed under reduced pressure to

chromatography (ethyl acetate 15 v/v%: hexane v/v 85% as give a crude residue. The crude product was subjected to

1

the eluent) (yield 90%, off white powder) H NMR (400 MHz, column chromatography purification using silica gel with

Chloroform-d) δ 8.18 (dd, J = 12.9, 8.5 Hz, 2H), 8.00 (s, 1H), CH₂Cl₂: methanol (9:1) mixture solvent as eluent, giving the

7.94 (d, J = 8.6 Hz, 1H), 7.90 (dd, J = 5.9, 3.2 Hz, 1H), 7.85 (dd, J desired compound as dark red powder (for Ir1 and Ir1) or black

= 6.0, 3.2 Hz, 1H), 7.81 (d, J = 8.1 Hz, 1H), 7.73 (ddd, J = 8.5, powder (for Ir3 and Ir2) with the following yields: Ir1 (77%), Ir2

6.8, 1.5 Hz, 1H), 7.53 (ddd, J = 8.1, 6.9, 1.2 Hz, 1H), 7.38 (dt, J = (76%), Ir3 (68%), Ir4 (75%). All purified compounds were

7.2, 3.5 Hz, 2H). ¹³C NMR (151 MHz, CDCl₃) δ 152.31, 148.16, recrystallized from dichloromethane and n-hexane mixture,

145.54, 141.22, 140.49, 136.57, 129.94, 129.52, 127.50, followed by vacuum drying before conducting all analysis.

126.55, 125.32, 124.52, 124.29, 122.62, 122.43, 117.83.

Spectral data of bis[2-(benzo[b]thiophen-2-yl)-4-methyl

Synthesis of ethyl 2-(benzo[b]thiophen-2-yl)quinoline-4- quinoline]iridium(III) (thenoyltrifluoroacetonate): (btmq)₂

carboxylate [btecq] (L3). The crude product was then purified Ir(tta) = Ir1. Yield: 77%. Anal. Calcd (%) for C₄₄H₂₈F₃IrN₂O₂S₃: C,

by silica column chromatography (ethyl acetate 15 v/v%: 54.93; H, 2.93; N, 2.91, S, 10.00, Found: C, 55.05; H, 2.71; N,

1

hexane v/v 85% as the eluent) (yield 73%, pale yellow powder) 3.11, S, 9.87. H NMR (400 MHz, Chloroform-d) δ 7.98 (d, J =

¹H NMR (400 MHz, Chloroform-d) δ 8.72 (d, J = 8.5 Hz, 1H), 8.8 Hz, 1H), 7.92 – 7.85 (m, 2H), 7.81 – 7.73 (m, 5H), 7.45 (dd, J

8.43 (s, 1H), 8.24 (d, J = 8.5 Hz, 1H), 8.11 (s, 1H), 7.89 (dq, J = = 5.0, 1.2 Hz, 1H), 7.40 (t, J = 7.6 Hz, 1H), 7.32 (dd, J = 3.8, 1.2

12.6, 4.2, 3.8 Hz, 2H), 7.78 (t, J = 7.7 Hz, 1H), 7.63 (t, J = 7.7 Hz, Hz, 2H), 7.26 (d, J = 8.2 Hz, 1H), 7.10 – 7.01 (m, 2H), 6.98 – 6.90

1H), 7.39 (dt, J = 7.2, 3.8 Hz, 2H), 4.58 (q, J = 7.1 Hz, 2H), 1.54 (m, 2H), 6.60 (td, J = 7.5, 4.8 Hz, 2H), 6.42 (dd, J = 16.2, 8.1 Hz,

(t, J = 7.1 Hz, 3H). ¹³C NMR (151 MHz, CDCl₃) δ 166.18, 151.86, 2H), 5.58 (s, 1H), 2.94 (d, J = 15.2 Hz, 6H). ¹³C NMR (151 MHz,

149.11, 144.79, 141.32, 140.40, 135.94, 130.13, 130.07, CDCl₃) δ 177.06, 166.32, 166.25, 149.33, 149.16, 148.62,

127.93, 125.58, 125.54, 124.64, 124.43, 124.30, 123.01, 147.46, 147.36, 147.21, 147.09, 145.44, 142.00, 138.93,

122.65, 119.16, 62.03, 14.38.

131.71, 131.01, 130.39, 128.47, 127.92, 126.53, 126.41,

125.71, 125.45, 125.41, 125.35, 125.06, 124.73, 123.98,

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 3

Please do not adjust margins

Journal of Materials Chemistry C

Page 4 of 21

ARTICLE

Journal Name

View Article Online

DOI: 10.1039/C8TC04321C

Table 1 Crystallographic and refinement data for complexes Ir1-Ir4

Ir1

Ir2

C₄₂H₂₄F₃Ir N₂O₂S₃

934.01

Ir3

C₄₈H₃₂F₃Ir N₂O₆S₃

1163.06

100

Ir4

C₄₄H₂₂F₉Ir N₂O₂S₃

1154.94

100

Formula

Formula weight

Temp (K)

C₄₄H₂₈F₃Ir N₂O₂S₃

1057.10

100

Wavelength (Å)

Crystal system

Space group

Hall group

Crystal size (mm³)

a [Å]

0.90000

monoclinic

P2₁/n

0.62000

monoclinic

P2₁/c

0.71073

triclinic

P-1

0.71073

monoclinic

P2₁/c

-P 2yn

-P 2ybc

-P 1

-P 2ybc

0.020 × 0.020 × 0.020

13.243 (3)

16.245 (3)

20.886 (4)

90.00

0.070 × 0.060 × 0.030

11.387 (2)

17.324 (3)

18.358 (4)

90.00

0.160 × 0.080 × 0.020

13.947 (6)

16.406 (7)

20.411 (8)

86.55

0.250 × 0.250 × 0.040

11.613 (4)

18.187 (7)

20.859 (7)

90.00

b [Å]

c [Å]

α [°]

β [°]

108.37

92.59

82.30

103.32

γ [°]

90.00

75.73

90.00

V [Å³]

Z

4264.3 (16)

4

3617.8 (12)

4

4484 (3)

4

4287 (3)

4

ρ_calc[g/cm³]

μ (Mo Kα) [mm^-1]

h , k, l max

Total reflections

Unique reflections

R_F,R_w(F²) [I > 2σ(I)]

GOF on F²

CCDC

1.645

3.472

14, 17, 23

20693

6068

0.0921, 0.2617

1.079

1853783

1.715

3.920

17, 27, 29

50634

14463

0.1379, 0.2834

1.078

1853785

1.723

3.165

17, 20, 25

18563

11797

0.1256, 0.1505

1.053

1859422

1.790

3.467

16, 25, 29

12876

10460

0.0475, 0.0475

1.031

1859423

123.83, 123.68, 123.63, 122.42, 122.39, 118.23, 118.03, 91.57, d) δ 8.62 (t, J = 9.5 Hz, 2H), 8.34 (d, J = 3.5 Hz, 2H), 7.97 (d, J =

19.19, 19.15. ¹⁹F NMR (471 MHz, CDCl₃) δ -75.59. (MALDI-TOF- 8.9 Hz, 1H), 7.78 (t, J = 8.9 Hz, 3H), 7.50 – 7.41 (m, 2H), 7.38 –

MS) [C₄₄H₂₈F₃IrN₂O₂S₃]: calcd, m/z = 962.11; found, m/z = 7.28 (m, 3H), 7.10 (q, J = 6.8 Hz, 2H), 7.02 – 6.91 (m, 2H), 6.64

962.11 [M+].

Spectral

(td, J = 7.7, 3.8 Hz, 2H), 6.38 (dd, J = 13.0, 8.1 Hz, 2H), 5.59 (s,

bis[2-(benzo[b]thiophen-2- 1H), 4.71 – 4.59 (m, 4H), 1.61 (dt, J = 9.9, 7.1 Hz, 6H). ¹³C NMR

data

of

yl)quinoline]iridium(III) (thenoyltrifluoroacetonate): (btq)₂Ir (151 MHz, CDCl₃) δ 177.52, 166.51, 165.63, 151.65, 150.85,

(tta) = Ir2. Yield: 76%. Anal. Calcd (%) for C₄₂H₂₄F₃IrN₂O₂S₃: 150.42, 150.21, 146.85, 146.73, 144.87, 142.30, 139.07,

C,54.01; H, 2.59; N, 3.00, S, 10.30, Found: C, 54.21; H, 2.32; N, 138.24, 138.05, 132.34, 131.68, 131.04, 129.08, 128.11,

2.89, S, 10.41. ¹H NMR (400 MHz, Chloroform-d) δ 8.19 (dd, J = 126.64, 126.53, 126.42, 126.27, 126.08, 125.85, 125.63,

15.7, 8.6 Hz, 2H), 8.00 – 7.92 (m, 2H), 7.89 (d, J = 8.5 Hz, 2H), 125.58, 125.17, 124.11, 124.06, 122.61, 122.26, 118.91,

7.76 (dq, J = 14.6, 7.6, 6.9 Hz, 5H), 7.49 – 7.42 (m, 1H), 7.38 (t, J 118.64, 91.61, 62.38, 14.39. ¹⁹F NMR (471 MHz, CDCl₃) δ -

= 7.4 Hz, 1H), 7.34 – 7.31 (m, 1H), 7.25 (d, J = 5.1 Hz, 1H), 7.06 75.58. (MALDI-TOF-MS) [C₄₈H₃₂F₃IrN₂O₆S₃]: calcd, m/z

(q, J = 6.7 Hz, 2H), 7.00 – 6.90 (m, 2H), 6.60 (q, J = 6.8 Hz, 2H), 1078.18; found, m/z = 1078.10 [M+].

=

6.40 (dd, J = 15.7, 8.1 Hz, 2H), 5.59 (s, 1H). ¹³C NMR (151 MHz, Spectral

data

of

bis[2-(benzo[b]thiophen-2-yl)-4-

CDCl₃) δ 177.27, 167.35, 167.08, 167.01, 150.04, 149.85, (trifluoromethyl)quinoline]iridium(III) (thenoyltrifluoroaceto-

149.67, 149.26, 147.11, 146.98, 145.23, 142.14, 139.28, nate): (bttmq)₂Ir(tta) = Ir4. Yield: 75%. Anal. Calcd (%) for

139.12, 139.00, 135.71, 134.28, 133.56, 131.94, 131.48, C₄₄H₂₂F₉IrN₂O₂S₃: C, 49.39; H, 2.07; N, 2.62, S, 8.99, Found: C,

130.89, 128.66, 128.62, 128.00, 127.98, 127.85, 126.50, 49.60; H, 2.16; N, 2.47, S, 8.85. ¹H NMR (400 MHz, Chloroform-

126.39, 125.58, 125.37, 125.35, 125.32, 125.15, 125.04, d) δ 8.15 (s, 2H), 8.04 (d, J = 8.4 Hz, 2H), 7.98 (d, J = 8.8 Hz, 1H),

124.79, 123.80, 123.74, 122.49, 122.47, 117.44, 117.17, 91.59. 7.85 – 7.75 (m, 3H), 7.54 – 7.45 (m, 2H), 7.42 – 7.31 (m, 3H),

¹⁹F NMR (471 MHz, CDCl₃)

[C₄₂H₂₄F₃IrN₂O₂S₃]: calcd, m/z = 934.06; found, m/z = 934.08 3.8 Hz, 1H), 6.68 (q, J = 7.9 Hz, 2H), 6.43 (d, J = 8.1 Hz, 1H), 6.36

[M+].

(d, J = 8.1 Hz, 1H), 5.60 (s, 1H). ¹³C NMR (151 MHz, CDCl₃) δ

Spectral data of bis[ethyl 2-(benzo[b]thiophen-2-yl)quinoline- 177.90, 167.37, 166.53, 166.45, 153.32, 152.46, 150.31,

4-carboxylate]iridium(III) (thenoyltrifluoroacetonate): 150.07, 146.69, 146.55, 144.60, 142.53, 139.09, 139.04,

(btecq)₂Ir(tta) Ir3. Yield: 61%. Anal. Calcd (%) for 132.71, 132.26, 131.56, 130.89, 129.30, 128.84, 128.22,

δ -75.66. (MALDI-TOF-MS) 7.14 (q, J = 7.4 Hz, 2H), 7.01 (t, J = 7.9 Hz, 1H), 6.96 (dd, J = 5.0,

=

C₄₈H₃₂F₃IrN₂O₆S₃: C, 53.47; H, 2.99; N, 2.60, S, 8.92, Found: C, 126.82, 126.56, 126.54, 126.45, 126.39, 125.79, 125.40,

53.55; H, 3.18; N, 2.73, S, 9.06. ¹H NMR (400 MHz, Chloroform- 124.42, 124.32, 124.25, 122.79, 122.76, 119.91, 119.84,

4 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 5 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

View Article Online

DOI: 10.1039/C8TC04321C

Scheme 1 General synthetic procedure for the cyclometalating ligands (L1-L4) and iridium(III) complexes (Ir1–Ir4).

118.58, 115.21, 114.99, 91.71. ¹⁹F NMR (471 MHz, CDCl₃) δ -

61.41, -61.48, -75.77. (MALDI-TOF-MS) [C₄₄H₂₂F₉IrN₂O₂S₃]:

calcd, m/z = 1070.05; found, m/z = 1070.00 [M+].

Results and discussion

X-Ray crystallographic analysis. Synchrotron single crystal X-

ray diffraction (XRD) data for four different tris-Heteroleptic

Iridium complex (Ir1, Ir2, Ir3, and Ir4) were measured at -173

Synthesis and characterization

Scheme 1 outlines the synthetic pathway employed for the

new iridium complexes (Ir1–Ir4). We synthesized two ligand

precursors for the first time, namely, ethyl 2-chloroquinoline-

4-carboxylate (L3a) and 2-chloro-4-(trifluoromethyl)quinoline

^oC on the 2D beamline of the Pohang Acceleration Laboratory

(PAL; Pohang, Korea) using MX225-HS CCD area detector with

monochromated X-ray in a scan width of 1° around omega axis

by collecting 360 frames. The raw data were processed and

scaled using the programs HKL3000 and DENZO.⁸⁹Corrections

for the Lorentz and polarization effects, along with an

empirical absorption correction, were applied. The space

group was determined by the program XPREP,⁹⁰and the

structure was solved by direct methods using SHELXS.⁹¹The

structure refinement was carried out with full-matrix least-

squares refinements on F²using the program SHELXL2018.⁹¹

The data collection conditions and crystallographic parameters

are summarized in Table 1.

(L4a). These were required for the synthesis of the new

cyclometalating ligands ethyl 2-(benzo[b]thiophen-2-

yl)quinoline-4-carboxylate (L3) and 2-(benzo[b]thiophen-2-yl)-

4-(trifluoromethyl)quinoline (L4), respectively. The syntheses

were carried out according to the literature.⁹²L3a was

synthesized initially by chlorination of 2-hydroxyquinoline-4-

carboxylic acid using POCl₃in the first step, followed by thionyl

chloride treatment. Finally, a base-catalyzed esterification

reaction was carried out using triethylamine in ethanol. The

other precursor compound, L4a

, was synthesized by

chlorination using POCl₃, followed by a slow, extremely

exothermic, basification reaction using 5ꢀN NaOH. The new

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 5

Please do not adjust margins

Journal of Materials Chemistry C

Page 6 of 21

ARTICLE

Journal Name

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 1 Crystal structures of the complexes Ir1

–

Ir4. The general structure shows the labels for selected atoms as used in Table 2

cyclometalating ligands (L1–L4) used in the current study were Ir1–Ir4 suitable for X-ray diffraction (XRD) measurements were

synthesized by well-established Suzuki coupling of the grown by slow diffusion of hexane in dichloromethane

corresponding substituted 2-bromo or 2-chloro quinoline with solutions at ambient temperature.

benzo[b]thiophene-2-boronic acid in the presence of sodium

X-Ray single-crystal structures

carbonate

and

catalytic

tetrakis(triphenylphosphine)

palladium(0). Pure ligands were obtained after flash column Structural authentication of the compounds Ir1–Ir4 was

chromatography on silica. The detailed synthetic procedures completed using single-crystal XRD. The corresponding single-

can be found in the Supporting Information. The dimer crystal structures are depicted in Fig. 1. The structure

precursors to the iridium(III) complexes

(Ir1–Ir4) were refinement parameters and selected crystallographic data can

prepared by the standard procedure proposed by Watts and be found in Table 1. All four iridium(III) complexes had a

coworkers.⁹³A µ-chloro bridged dimer complex was obtained distorted octahedral arrangement of ligands around the Ir³⁺

through the reaction of a cyclometalated ligand with IrCl₃·H₂O center with Ir–N coordinated quinolines in trans positions.

in a mixture of water and 2-ethoxyethanol. The new iridium Overall, the coordination geometry around the iridium metal

complexes (Ir1–Ir4) were formed in the presence of Na₂CO₃was not significantly altered by substituent changes on the

through reaction of the ancillary ligand: 4,4,4-trifluoro-1-(2- cyclometalated ligands. The Ir–C, Ir–N, and Ir–O bond lengths

thienyl)-1,3-butanedione (TTA) and µ-chloro bridged dimer.^94,for Ir1

–

Ir4 can be found within the range previously known for

95

After purification by flash column chromatography and those of similar class of compounds (Table 2).^96-98The

1

recrystallization, detailed characterization by H, ¹⁹F, and ¹³C coordinating bonds from the ancillary ligands to iridium were

NMR, MALDI-TOF mass spectrometry, and elemental analysis longer than those observed with the cyclometalating ligand.

was performed (see details in the Experimental Section given This indicated low ligand field strength, which is known for -

in the supporting information, Figs. S1–S30). Single crystals of diketonate-type ligands.⁹⁹In the –CH₃substituted Ir1, there

6 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 7 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

unsubstituted Ir2, the decomposition temper_Va_iet_wur_Ae_rts_iclew_One_lir_ne_e

333°C and 340°C, respectively. The –CO₂C₂H₅substituted Ir3

Table 2 Selected average bond lengths (Å) and angles (°) for the complexes Ir1–Ir4

DOI: 10.1039/C8TC04321C

had the lowest thermal stability at 318°C. This may be due to

the ethyl ester substituent being susceptible to

decarboxylative decomposition. The –CF₃substituted Ir4

displayed a lower decomposition temperature (326°C) than

that of –CH₃substituted Ir1 and unsubstituted Ir2. This

reduced stability might be related to the presence of a large

number of fluorine atoms (a total of nine F atoms) in the

molecule. At elevated temperatures, fluorine atoms are more

susceptible to cleavage, as evident from the previous reports

by Bryce et al.¹⁰⁰When compared to many of the

cyclometalated phenyl-pyridine-based derivatives, the thermal

stability of the current complexes was found to be lower.

Furthermore, we encountered difficulties in sublimation

purification by vacuum evaporation below 300°C due to the

low decomposition temperature and high molecular weight of

the complexes. Furthermore, these compounds showed a

good morphological stability, as can be seen from the glass

transition temperatures (T_g) recorded from the second heating

Ir1

Ir2

Ir3

Ir4

Bond length (Å)

Ir-O1

2.145

2.136

2.101

2.086

1.970

1.975

1.455

1.447

2.159

2.113

2.065

2.060

1.960

1.973

1.413

1.433

2.128

2.144

2.074

2.089

1.977

1.964

1.439

1.434

2.148

2.150

2.094

2.082

1.979

1.980

1.424

1.432

Ir-O2

Ir-N1

Ir-N2

Ir-C1

Ir-C5

C2-C3

C6-C7

Bond angle (º)

N1-Ir-C1

N2-Ir-C5

O1-Ir-O2

N1-Ir-N2

C5-Ir-O1

C1-Ir-O2

N1-Ir-O1

N1-Ir-C5

Dihedral angle (º)

C1-C2-C3-C4

N1-C2-C3-S

C5-C6-C7-C8

N2-C6-C7-S

80.18

80.13

86.55

179.23

174.87

173.63

97.11

80.18

79.58

80.45

86.09

179.85

175.92

176.40

79.44

99.42

80.55

101.58

86.11

79.93

79.77

86.56

177.87

174.54

175.82

99.39

178.60

176.04

175.37

77.98

101.58

101.01

cycle of the DSC curve (range: 180–225

interesting to note that the introduction of a trifluoromethyl

group in Ir4 reduced the glass transition temperature (T_g

180 C) to a certain extent. However, the observed T_gvalues

were sufficient to guarantee morphological stability for Ir1 Ir4

°C, Fig. S33). It is also

179.20

169.51

177.46

171.93

179.44

169.30

178.66

172.69

178.37

174.43

176.20

171.08

179.89

168.96

178.23

167.40

=

°

–

within the operating temperature of the device (~110°

C).¹⁰¹

was a slight elongation in the C₁–C₂or C₆–C₇bond that

connects both the Ir–C bonded benzothiophene and Ir–N

coordinating quinolone part of the cyclometalating ligand. This

change in bond length could be due to the electron-donating

Theoretical calculations

In order to gain an insight into the substituent effect on the

electronic structure and the optical properties of Ir1 Ir4, we

performed DFT/TD-DFT calculations on both the excited and

the ground states. The structures of Ir1 Ir4 were optimized in

–

effect (Hammett constant: Cσ_mꢀ=ꢀ−0.07) from the meta-C2

position. The electron-withdrawing effects in Ir3 and Ir4 were

not reflected on the C₁–C₂or C₆–C₇bond lengths. Both the

quinoline and the benzothiophene moieties of the

cyclometalating ligands were not coplanar, as evident from the

observed torsion angles (N1–C2–C3–S or N2–C6–C7–S). π–π

interactions between adjacent molecular entities rarely exist,

possibly due to the loss in coplanarity, as can be seen from the

packing structure. The C(5)–Ir–O(1) and C(1)–Ir–O(2) bond

angles in Ir1 (174.87 and 173.67, respectively) and Ir2 (175.92

and 176.40, respectively) were found to be reasonably

distorted from linearity, this may be induced by intermolecular

interactions. Even in the absence of any classical hydrogen

bonding interactions, the crystal packing structure revealed

–

their electronic (singlet) ground and triplet excited states using

the Perdew–Burke–Ernzerhof hybrid functional (PBE1PBE) with

the LANL2DZ basis set for the Ir atom and 6-31G* basis set for

all other atoms (for computational details, see the Supporting

Information).¹⁰²The geometrical structures of the iridium(III)

complexes obtained from the DFT optimization are presented

in Fig. S34. The modeled structures shows C1 point group

symmetry from a distorted octahedral geometry around the

iridium metal center. The Ir−C (mean value: 1.980ꢀÅ) and Ir−N

(mean value: 2.075ꢀÅ) bond lengths in Ir1 and Ir2 were in good

accordance with the single-crystal XRD data (Ir−C, mean value

= 1.975ꢀÅ; Ir–N, mean value = 2.073ꢀÅ) (Table S4). The

isodensity surfaces for the Highest Occupied Molecular

Orbitals (HOMOs) and the Lowest Unoccupied Molecular

several weak intermolecular hydrogen bonds via C(π)–H···

between the cyclometalating ligands and the ancillary TTA in

the crystal lattices of Ir1 and Ir4 (Fig. S31) For other key

structural parameters, such as distances and angles, please see

the Supporting Information (Tables S1–S3).

F

.

Orbitals (LUMOs) of Ir1–Ir4 from the DFT calculations are

presented in Fig. 2. A complete list of all orbital populations is

shown in Table S5 in the Supporting Information. MO analysis

indicated that the HOMO for all four iridium(III) complexes

Thermal properties

(

Ir1

–

Ir4

)

contained

a mixture of Ir(III) d-orbitals and

In order to measure the thermal stability of the developed

complexes (Ir1–Ir4), a thermogravimetric analysis (TGA) and

differential scanning calorimetry (DSC) were conducted under

nitrogen. As shown in Fig. S32 in the Supporting Information,

the TGA curves for all complexes exhibited a good thermal

stability up to 300°C. For –CH₃substituted Ir1 and

benzothiophene

π-orbitals from both of the cyclometalating

ligands. The percentage-of-contribution analysis revealed that

the HOMO was mainly localized over the π-orbitals of the

benzothiophene moiety of the cyclometalated ligand (~54%),

5d-orbitals of the Ir atom (27%), and a small contribution from

the substituted quinoline (17%) (Table S6). This trend differed

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 7

Please do not adjust margins

Journal of Materials Chemistry C

Page 8 of 21

ARTICLE

Journal Name

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 2 Selected MO diagrams portraying the front-heavy isodensity HOMO and LUMO surfaces for complexes Ir1

–

Ir4. All the MO surfaces correspond to an isocontour

value of |Ψ| = 0.03.

slightly from the commonly observed picture for iridium(III) with the TTA ancillary compared with the electron-donating –

complexes, where the HOMO lies primarily on the Ir atom CH₃substituted or unsubstituted quinolinate moieties of the

(>50%). Thelow metal contribution to the HOMO eventually cyclometalating ligand in Ir1 and Ir2. Thus, the percentage of

reduces the metal-to-ligand charge-transfer (MLCT) character contribution to the frontier MOs from the various parts of the

of the transition. This might be due to the presence of the ligands suggests the possible dominance of intraligand charge-

electron-rich sulfur atom on the benzothiophene moiety, transfer (ILCT) transitions over MLCT in all complexes.

because sulfur increases the electron-donating ability; thus, However, in the case of Ir1 and Ir2, a predominant ligand-to-

the ligand-centered MO energy levels above the 5d-orbitals of ligand charge-transfer (LLCT) transition originating from the

the iridium(III) center may become destabilized. It is also benzothiophene of the cyclometalating ligand to the ancillary

noteworthy that the HOMO-1 orbital was found to be highly TTA is expected.

localized over the cyclometalating ligands with a very low

From the optimized excited- and ground-state structures,

metal contribution (3%). For the LUMO of Ir3 and Ir4, a major the absorption spectral properties in CH₂Cl₂were calculated

component (80% for Ir3 and 69% for Ir4) of the frontier using a TD-DFT approach at the B3LYP/6-31G* level of theory.

orbitals was localized on the substituted N-coordinated Since the solvent effects are crucial in predicting the

quinoline along with considerable involvement (11% for Ir3 absorption and emission spectra, the PCM framework was

and 15% for Ir4) from the benzothiophene moiety of the incorporated in the TD-DFT calculations. The first vertical

cyclometalating ligand and a minimum contribution (3%) from triplet and singlet excitations were calculated to reproduce the

the TTA ancillary ligand. This observation confirmed the mere experimental spectra for validating this methodology. The

spectator role of ancillary ligands in these complexes. It is primary contributions of the different theoretical peaks with

interesting to note that, in complexes Ir1 and Ir2, the TTA substantial oscillator strengths are listed in Table S7. The TD-

ancillary ligand played

a key role in the excited-state DFT excitations were analyzed with respect to the

properties, with a large portion of the LUMO localized on the corresponding electronic transitions between occupied and

TTA ligand (90%). This observation highlights the fact that unoccupied frontier MOs, and only the dominant character of

much of the stabilized LUMO energy levels were associated each excited state is tabulated. However, in the most general

8 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 9 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 3 (a) Calculated and experimental MO energy level diagram for complexes Ir1

–

Ir4. The arrows are intended to highlight the calculated HOMO–LUMO energy gaps.

Correlation diagram for the (b) reduction potential and (c) oxidation potential versus the Hammett constants of the unique substituent on Ir1–Ir4.

case, several MO transitions will characterize a simple TD-DFT benzothiophene/Ir (HOMO-2) → TTA ancillary (LUMO+2).

excitation. In Fig. S35, TD-DFT excitation transitions are Regarding singlet-to-triplet transitions, the initial transitions

compared with the experimental observations. For an easy with very weak oscillator strengths occurred at 657 and

comparison, transitions were broadened using Gaussians of 650ꢀnm (Ir1), 664 and 656ꢀnm (Ir2), 797 and 771ꢀnm (Ir3), and

width 0.20ꢀeV and the same was applied to all spectra. The 745 and 729ꢀnm (Ir4). These were mainly composed of

agreement between experiment and theory was good in HOMOꢀ→ꢀLUMO

and

HOMOꢀ→ꢀLUMO+1

transitions,

general. Even if a minimal mismatch has been seen from the respectively. Thus, they can be considered an admixture of

calculated oscillator strengths and experimentally observed LLCT- and MLCT-type transitions. Earlier investigations have

absorption peaks, the positions of the different absorption suggested that, the strong SOC of iridium complexes could

bands and their shoulder peaks were rather well described. facilitate the singlet-to-triplet transitions and the same will

The initial experimental absorption peaks for Ir1 and Ir2 were contribute to the absorption spectra in the form of a low-

assigned to MLCT transitions at 420 to 580ꢀnm, which were in energy tail.¹⁰³The calculated dipole moments of Ir1

, Ir2, Ir3,

good agreement with the theoretical predictions (Table S7) and Ir4 are 5.71, 5.62, 8.68, and 5.49 D, respectively (Fig. S36).

from the large oscillator strength for the transition from The ethyl ester (–CO₂C₂H₅) substituted complex Ir3 had the

benzothiophene/iridium (HOMO and HOMO-1) to TTA highest dipole moment value, which was ascribed to the high

ancillary/quinolinate (LUMO, LUMO+1, and LUMO+2). delocalization of the

Similarly, for Ir3 and Ir4, the experimental absorption range of along with the polarized LUMO localizations. A similar increase

480–680ꢀnm was found to be in good agreement with the of the dipole moment was observed in earlier studies with

 electron cloud through the ester group

redicted transitions between the benzothiophene/iridium iridium(III) derivatives containing a phenyl group at the 5-

(HOMO and HOMO-1) and substituted quinolinate (LUMO, position of the phenyl ring of phenylpyridine cyclometalated

LUMO+1, and LUMO+2). The experimentally observed ligands .¹⁰⁴

shoulder peaks in Ir1 and Ir2 (359–410 nm) could be assigned

to transitions S₁₆to S₁₉(395–415 nm, Table S7), as the LLCT or

Electrochemical properties

ILCT transitions involving HOMO-1 (benzothiophene)

LUMO+1/LUMO+2 (quinolinate). For Ir3, the shoulder peak substitution on the cyclometalated ligand, cyclic voltammetry

observed at 420 nm could be ascribed to LLCT/MLCT mixed (CV) experiments were carried out on Ir1 Ir4 using ferrocene

nature of transitions S₁₉and S₂₀, which correspond to as the standard (Fig. S37). The calculated HOMO and LUMO

absorption from benzothiophene/iridium (HOMO-2) to TTA energies for complexes Ir1 Ir4 are tabulated (Table 3). The

→

In order to understand the electronic effects induced by

–

ancillary (LUMO+2). Similarly, for Ir4, the peak observed at E_onset(oxd) values of the respective iridium(III) complexes were

400ꢀnm matched S₁₉well, whereas transitions originated as determined using CV with reference to a

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 9

Please do not adjust margins

Journal of Materials Chemistry C

Page 10 of 21

ARTICLE

Journal Name

View Article Online

DOI: 10.1039/C8TC04321C

Table 3 Electrochemical properties, thermal properties, and substituent constants for Ir1–Ir4

Triplet

energy^f

(eV)

a

d

e

g

h

E^1/2

(V)

HOMO^b

(eV)

LUMO^c

(eV)

E_g(elec)

(eV)

E_g(opt)

(eV)

T_g

T_d

oxd

E^{1/2 a}(V)

Substituent

ⁱσ_m

ⁱσ_p

^jF

^jR

red

(˚C)

Ir1

Ir2

Ir3

Ir4

0.72

0.76

0.86

0.96

−1.64

−1.61

−1.29

−1.41

−4.83

−4.87

−4.97

−5.07

−2.68

−2.75

−3.11

−3.16

2.37

2.38

2.16

2.37

2.15

2.12

1.86

1.91

1.99

1.96

1.85

1.82

225

220

201

180

333

340

318

326

–CH₃

–H

–CO₂C₂H

–CF₃

−0.07

0.00

0.37

0.43

−0.17 0.01 −0.18

0.00

0.45

0.54

0.03 0.00

0.34 0.11

0.38 0.16

5

^abElectrochemical data versus FeCp₂⁺/FeCp₂⁰(FeCp₂= ferrocene) were collected in CH₂Cl₂/0.1ꢀM TBAH (tetrabutylammonium hexafluorophosphate). ^bHOMO = −[4.8 −

f

(0.69) + E_oxd]. ^cLUMO = −[4.8 − (0.69) + E_red]. ^dElectrochemical bandgap. ^eOptical bandgap and triplet energy calculated from emission spectra at 77ꢀK. ^gT_g: glass

transition temperature from the DSC curve. ^hT_d: decomposition temperature (5% weight loss) from the TGA curve. ⁱHammett constants. ^jSwain–Lupton constants.

ferrocene/ferrocenium redox potential. All iridium(III) indicates more inductive effect (σ-electron-inductive nature),

complexes showed quasi-reversible dual oxidation peaks and a smaller R was more resonance effect (π-electron-

within the scan limits. The first oxidation potential could donative nature).³⁹Among the electron-withdrawing

originate from the oxidation of the electron-rich substituents, the –CF₃has the highest F-value of 0.38; hence,

benzothiophene moiety, followed by a second oxidation from Ir4 was expected to exhibit a higher positive shift in the

the iridium(III) center (Ir³⁺/Ir⁴⁺). These assignments are in reduction potential than Ir3 (F = 0.34 for the ethyl ester).

accordance with the increased percent of HOMO, HOMO-1, However, the observed reduction potential of Ir3 was much

and HOMO-2 orbital localizations over benzothiophene lower than what was anticipated (−1.29ꢀV versus Fc⁺/Fc). This

compared with iridium in the DFT calculations. As depicted in could be attributed to the extension of -conjugation into the

Fig. 3(a), there was a stepwise increase in the HOMO energy quinolinate moiety over the substituted ester group, which

level along with the corresponding increase in oxidation peak eventually increased the possibility of reduction from the

toward the positive potential. Since the HOMO-localized carbonyl moiety instead of the expected reduction solely from

benzothiophene was connected meta to the substituted C in the N-coordinated quinolinate ring of the cyclometalating

the N-coordinated quinolinate moiety, the observed change in ligand. This finding correlated well with the considerable

the oxidation potential could be linearly correlated with the amount of LUMO localization over the ester substituent in Ir3

Hammett parameter (σ_m) [Figs. 3(b) and 3(c)]. Compared to (Fig. 2).

the unsubstituted Ir2, the oxidation potential of Ir1 was shifted

−40ꢀmV. This could be ascribed to the increase of the electron

Electronic spectroscopy

density in the Ir–C bonded benzothiophene moiety by the The room temperature UV-Vis absorption spectra of Ir1–Ir4

electron-donating –CH₃(σ_m= −0.07) substituent. For the measured in a degassed CH₂Cl₂(c = 2 × 10⁻⁵ꢀM) solution are

electron-withdrawing –CO₂C₂H₅(σ_m

= 0.37) and –CF₃shown in Fig. 4(a), and the corresponding electronic

(σ_mꢀ=ꢀ0.43) substituents, the loss of the electron density from absorption data are summarized in Table 4. The absorption

the benzothiophene moiety resulted in a more difficult spectra contained intense bands with extinction coefficients in

oxidation, as evidenced from the more positive shift in the the order of 4.2ꢀ×ꢀ10⁴to 4.6ꢀ×ꢀ10⁴M⁻¹ꢀcm⁻¹in the 230–325ꢀnm

oxidation potentials for Ir3 (0.86ꢀV) and Ir4 (0.96ꢀV). The extent range. These bands could be assigned to the spin-allowed

of the observed positive shift in the potential was found to be intraligand ¹LC (¹π→π*) transition of substituted 2-

in accordance with the Hammett parameter (σ_m). All the (benzo[b]thiophen-2-yl)quinoline

cyclometalating

ligand

iridium(III) complexes, except Ir3 displayed irreversible derivatives and the TTA ancillary ligands. The peaks around

,

reduction potentials. The onset of reduction peak potential 325–400ꢀnm, with extinction coefficients in the order of

was observed at −1.64, −1.61, −1.29, and −1.41ꢀV, respectively, 3.2ꢀ×ꢀ10⁴ꢀM⁻¹ꢀcm⁻¹, could be assigned to spin-allowed

for Ir1–Ir4. Since the LUMO orbitals were localized on the TTA intraligand

charge

transfer

(¹ILCT)

between

the

ancillary ligand in Ir1, the electron-donating substituents on cyclometalating ligands. Shoulder peaks depicting the

the quinolinate moiety of the cyclometalated ligand had barely cyclometalating ligand to ancillary ligand charge-transfer

affected the reduction potential (~20ꢀmV). Nevertheless, the transitions (¹LLCT) appeared for Ir1 and Ir2 at 359–361ꢀnm (

 =

substitution of electron-withdrawing groups, such as –CO₂C₂H₅2.8ꢀ×ꢀ10⁴M⁻¹cm⁻¹) and for Ir3 and Ir4 at 415–419ꢀnm

(σ_p= 0.45) and –CF₃(σ_p= 0.54) in Ir3 and Ir4, induced the

ꢀ=ꢀ2.8ꢀ×ꢀ10⁴M⁻¹cm⁻¹). A broad band appeared around 450–

stabilization of the LUMO, which has reflected in a positive 575ꢀnm (

= 0.8ꢀ×ꢀ10⁴M⁻¹cm⁻¹) for Ir1 and Ir2 and around

shift of the reduction potential. However, the correlation 500–680ꢀnm (

= 0.6ꢀ×ꢀ10⁴M⁻¹cm⁻¹) for Ir3 and Ir4. This was

(



between the Hammett constant (σ_p) and the experimental attributed to the spin-allowed metal-to-ligand charge- transfer

reduction potential was not linear. Of special note was the (¹MLCT) transitions. In addition, spin-forbidden ³MLCT

deviation associated with Ir3 [Figs. 3(b) and 3(c)]. The Swain– transition bands, with negligible molar extinction coefficients

Lupton constant, as a modification of the Hammett rule, could

be used in explaining the observed reduction behavior of these 688ꢀnm for Ir1

complexes. As can be seen from Table 3, a larger F-value efficient SOC, which is a prerequisite for phosphorescent

(

~ 0.3ꢀ×ꢀ10⁴M⁻¹cm⁻¹), were noted at 640, 649, 704, and

Ir4 respectively. These peaks indicated

–

,

10 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 11 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 4 (a) UV-Vis absorption spectra of Ir1

= 2 × 10−5 M) at 298 K.

–Ir4 in dichloromethane (inset: magnified absorption in the 680–800ꢀnm region). (b) Emission spectra in dichloromethane (c

Table 4 Solution-state photophysical properties of Ir1–Ir4

λ_max

(nm)

Powder

a

Emission at 298 K in CH₂Cl₂

Absorption^aλ_max(nm) (ԑꢀ×ꢀ10³M⁻¹cm⁻

)

1

a

λ_max

τ_obs

b

1

Φ_PL

k_r^c10⁵(S⁻

)

k_nr^d10⁵(S⁻

)

(nm)

655

665

758

722

(µs)

0.11

0.67

0.05

0.94

Ir1

Ir2

Ir3

Ir4

230 (45), 294 (47), 336 (32), 359 (28), 522 (9), 640 (0.2)

0.020

0.081

0.011

0.106

0.18

0.12

0.22

0.11

8.90

1.44

655

665

746

730

231 (42), 295 (46), 338 (32), 361 (27), 531 (8), 649 (0.2)

232 (47), 304 (43), 353 (32), 415 (13), 564 (6), 704 (0.4)

232 (43), 303 (42), 351 (31), 419 (13), 576 (7), 688 (0.3)

19.78

0.95

^aAbsorption and emission spectra measured in CH₂Cl₂: [M] = 2.0 × 10^{−5 b}Phosphorescence quantum efficiency measured in degassed CH₂Cl₂by the

.

absolute method using a Hamamatsu Quantaurus-QY. ^c,dRadiative and nonradiative rate constants were deduced by Φ_PLfrom the solution state and τ_obs

e

according to two equations: k_r= Φ_PL/τ_obsand k_nr= (1 − Φ_PL)/τ_obs. Emission spectrum was measured in a frozen 2-methyltetrahydrofuran glass matrix at 77

K; [M] = 2.0 × 10⁻⁵

.

emission [inset of Fig. 4(a)]. These assignments were endorsed The unsubstituted Ir2 shows a vibronically resolved deep-red

by theoretical calculations (vide supra). The nature of the emission in the 610–850ꢀnm region with a maximum intensity

substituents at the para position to the coordinating nitrogen at 665ꢀnm. Meanwhile, Ir1 (an electron-donating –CH₃

atom in the substituted quinoline moiety of the substituent) shows an identical emission profile with a 10ꢀnm

cyclometalated ligand had significant effects on the absorption (230ꢀcm⁻¹) blue-shifted emission maximum at 655ꢀnm. The

profiles as well as on the extinction coefficient values of the strong electron-withdrawing –CF₃substituted Ir4 displayed a

intraligand (π→π*) transition band. The absorption band at bright NIR emission in the 675–850ꢀnm region with an

230ꢀnm expected from the benzothiophene part of the emission maximum located at 722 nm (Φ_PL= 0.10), 57 nm

cyclometalating ligand remained unaffected by the (1,416ꢀcm⁻¹) red-shifted from the unsubstituted Ir2. As per

1

substituents. However, the ILCT, LLCT, and MLCT transitions Hammett’s parameter, the electron-withdrawing power of an

involving contributions from the substituted quinolinate ethyl ester (σ_m= 0.37, σ_p= 0.45) is considered stronger than

moiety clearly exhibited red shifts when substituted with that of the –CF₃substituent (σ_m= 0.43, σ_p= 0.54), and the

electron-withdrawing groups. The indirect influence (σ_meffect) emission intensity of Ir3 was largely red-shifted (93 nm,

of electron-withdrawing substituents on the HOMO 2,074ꢀcm⁻¹). The –CF₃substituted Ir4 had a broad emission

(benzothiophene moiety and Ir center) was expected to profile with a maximum at 758 nm (Φ_PL= 0.01). This unusual

reduce the electron density and transition probability. This was red shift in the emission spectrum of Ir3 was attributed to the

clearly evidenced by the reduced ¹MLCT absorption

-conjugation of the LUMO over the entire quinolinate moiety

coefficients of Ir3 and Ir4 when compared to unsubstituted Ir2

.

through the ethyl ester substituent. This delocalization effect

dominates the anticipated electron-withdrawing effect and

assists strongly in stabilizing the aromatic quinolinate ring.

Solution-state emission properties

Fig. 4(b) displays the room temperature emission spectra Additionally, a predominant n→π^*transition from the carbonyl

(normalized) of the investigated iridium(III) complexes (Ir1

–

Ir4

)

group can be expected to cause a small emissive bandgap.

recorded in degassed CH₂Cl₂(c = 5.0ꢀ×ꢀ10⁻⁵ꢀM), and a summary Here, it is noteworthy to mention that both Ir1 and Ir2

of the corresponding photophysical data is given in Table 4. exhibited inferior emission efficiencies (Φ_PL= 0.02 and 0.08 for

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 11

Please do not adjust margins

Journal of Materials Chemistry C

Page 12 of 21

ARTICLE

Journal Name

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 5 Emission spectra of Ir1–

Ir4 in (a) a frozen 2-methyltetrahydrofuran glass matrix (c = 5 × 10⁻⁵M) at 77 K and (b) 5 wt.% doped PMMA film.

nm (294 cm⁻¹) for Ir4 compared to their corresponding room

temperature emission maxima (Fig. 6(c) vide infra). The

moderate blue shift noted for Ir1, Ir2, and Ir4 corroborates the

dominance of the ^1/3LC character over the ^1/3MLCT character of

the excited-state transitions in these complexes. Conversely,

Ir3 shows a large blue shift in the frozen medium, revealing the

extinguished charge-transfer contribution (n→π^*transition of

the ester substituent) resulting from the rigidochromic effects

that result to the destabilization of the charge-transfer states,

in contrast to the relatively unaffected ^1/3LC π→π* excited

Table 5 Excited-state properties of Ir1–Ir4.

^aE_em(0-0)

(nm)

^bΔυ_1/2

(cm⁻

^cћω_M

^dS_M

1

)

(cm⁻

1408

1439

1304

1406

)

Ir1

Ir2

Ir3

Ir4

638

645

705

707

411

456

832

545

0.20

0.18

0.14

^aObtained from the peak emission wavelength in 2-methyltetrahydrofuran

at 77ꢀK. ^bFWHM for the (0-0) band obtained from the emission spectra at

77 K. ^cFrom the energy difference of the (0-0) and (0-1) at 77ꢀK. ^dThe

Huang–Rhys factor S_Mwas calculated from the peak heights and energies of

states in Ir1, Ir2, and Ir4. In addition, a larger difference from

the 298 K and 77ꢀK emission indicates more structural

distortion in the excited state, which is supported by the high

dipole moment observed for Ir3 (Fig. S36). The 77ꢀK emission

profile of Ir3 show a more unresolved vibrational progression,

which may arise from the increase in the low-frequency

metal–ligand bond vibrations (~400ꢀcm⁻¹) in the excited state

than the high-frequency ligand-based vibrations.^{104, 107}The full-

width at half-maximum (FWHM, Δυ_1/2) values (Table 5) of the

the (0-0) and (0-1) peaks of the emission spectra at 77

(I_0,1/I_0,0)(υ_0,0υ_0,1)].

K [S_M=

͞

/͞

Ir1 and Ir2, respectively) compared with many of the

previously reported deep-red emissive iridium complexes.⁸⁴

Emission properties in a frozen solvent matrix

The emission spectra recorded in frozen 2-methyl-

tetrahydrofuran at 77ꢀK are displayed in Fig. 5(a). The

vibrationally resolved emission progressions and rigidochromic

effect are clearly visible. However, the phosphorescence bands

show a blue shift upon the transition from room temperature

to 77ꢀK. The rigidochromic effect on selected transition-metal

complexes is known.³⁹When a molecule exhibits a polar

excited state, a rapid reorientation of solvent molecules in the

vicinity of the excited occurs through dipole–dipole

interactions within the excited state lifetime, mainly due to the

low viscosity of the medium at 298ꢀK. This results in the

formation of a fully relaxed excited state.¹⁰⁵Therefore, the

room temperature emission always occurs from a fully relaxed

excited state. Conversely, emission at 77ꢀK occurs before

solvent relaxation occurs, leaving in rigidochromic effects on

the emission spectra. Generally, the extent of metal

contribution (MLCT) on the emissive excited state can be

evidenced from the observed rigidochromism at 77ꢀK.¹⁰⁶The

E_0-0emission peaks were blue-shifted by 17 nm (407ꢀcm⁻¹) for

Ir1, 20 nm (466 cm⁻¹) for Ir2, 53 nm (992 cm⁻¹) for Ir3, and 15

resolved highest energy vibronic bands of Ir1–Ir4 are 411, 456,

832, and 545 cm⁻¹, respectively. The large FWHM value of Ir3

indicated that it had an excited state with the highest

reorganizational energy. The difference in energy between the

E_0-0and E_0-1peaks provides the measure of quantum spacing

for the ground-state vibrational modes (ћω_Mvalue) of these

complexes, which lies in the range of 1,304–1,439ꢀcm⁻¹. This

reveals the existence of

a dominant vibrational mode

associated with the excited distortion that may arise from the

aromatic out-of-plane and in-plane ring bending and stretching

vibrations (ring breathing modes) of the cyclometalating

ligands.^{39, 104}The degree of vibrational nonradiative decay can

be estimated by the Huang–Rhys factor (S_M). The S_Mvalues of

complexes Ir1–Ir4 were found to be 0.20, 0.20, 0.18, and 0.14,

respectively (Table 5). The larger S_Mvalue shows a stronger

coupling between the dominant ligand-localized vibrations in

the ground and excited states.^{108, 109}Thus, a large Huang-Rhys

factor guides to enhanced vibrational nonradiative decay,

resulting in a small Φ_PLfor Ir1–Ir3. Additionally, a parallel

increase in the S_Mvalues with emission energies indicates

12 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 13 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 6 Correlation diagram of the emission energy versus Hammett constants σ_p(a) and σ_m(b). (c) Comparison of the normalized emission spectra recorded in the

CH₂Cl₂, 5 wt.% doped PMMA film, 77 K, and powder form at room temperature.

highly localized triplet states and a decreased MLCT character

of the excited state in the sequence of Ir4 > Ir3 > Ir2 ≥ Ir1.

Table 6 Photophysical properties of Ir1–Ir4 in a PMMA polymer film.

Emission at 298 K in 5 wt.% doped PMMA

Emission properties in a polymer matrix

a

d

λ_max

τ_obs

(µs)

k_r^c

10⁵S^-1

k_nr

b

Φ_PL

Fig. 5(b) depicts the normalized emission spectra of 5 wt.% of

Ir1 Ir4 doped in a poly(methyl methacrylate) (PMMA) polymer

film at room temperature. The emission profiles of Ir1 Ir2, and

(nm)

655

665

735

722

10⁵S^-1

0.41

0.29

4.37

0.77

–

Ir1

Ir2

Ir3

Ir4

0.240

0.305

0.038

0.130

1.85

2.38

0.22

1.12

0.13

0.17

0.12

,

Ir4 in spin-coated thin films are quiet similar to their

corresponding emissions in liquid state. This indicated the

absence of any intermolecular interactions in the amorphous

state. The lack of rigidochromism in these compounds further

supports the predominant LC nature of the excited state over

MLCT. However, a strong blue-shifted emission profile was

observed at 735 nm for Ir3. The 23ꢀnm (413 cm⁻¹) blue shift

and the unresolved nature of the emission profile for Ir3

indicated the polar nature in the excited state mostly

originated from the n→π* nature of the ester substituent,

which encompass a large contribution to the LUMO energy

levels (Fig. 2). Irrespective of the rigidochromism exhibited by

Ir3, a linear relationship can be seen from the correlation

diagram of the emission energies versus the corresponding

Hammett constants for the substituents [Figs. 6(a) and 6(b)].

As per the correlation diagram, all the compounds at 77ꢀK

strictly followed the Hammett parameter in defining the

emissive bandgap. However, the room temperature emission

energies in solution and PMMA deviated, mostly owing to the

polar nature of the excited state associated with Ir3. The

photoluminescence quantum efficiencies of the doped PMMA

films were 0.24 (Ir1), 0.30 (Ir2), 0.03 (Ir3), and 0.13 (Ir4) (Table

6). Compared to the solution state, a 16-fold increase in the

lifetime of Ir1 (1.85ꢀµs) was observed (Fig. S38). This likely

arose from the suppression of vibrational motions associated

with –C–H oscillations from the –CH₃substituent and more

^aEmission spectrum was measured for the 5 wt.% doped PMMA film. ^bQuantum

efficiency measured by the absolute method using a Hamamatsu Quantaurus-QY.

^c,dRadiative and nonradiative rate constants were deduced from Φ_PLof the solid

state and τ_obsaccording to two equations: k_r= Φ_PL/τ_obsand k_nr= (1 − Φ_PL)/τ_obs

.

flexible TTA ancillary ligand. While a fourfold increase was

observed in Ir2 (2.38ꢀµs), Ir3 (0.22ꢀµs) indicated the arrest of

the TTA ancillary ligand based distortions

(Ir2) and

nonradiative vibrational channels associated with ethyl ester

substituents (Ir3). Moreover, the suppression of nonradiative

decay pathways was also evidenced from the reduced

nonradiative rate constants (0.41ꢀ×ꢀ10⁵S⁻¹, 0.29ꢀ×ꢀ10⁵S⁻¹, and

4.37ꢀ×ꢀ10⁵ꢀS⁻¹for Ir1

, Ir2, and Ir3, respectively). It is also

interesting to note that the emission efficiency (0.13), lifetime

(1.12ꢀµs), radiative (0.12ꢀ×ꢀ10⁵S⁻¹), and nonradiative

(0.77ꢀ×ꢀ10⁵S⁻¹) rate constants for the –CF₃substituted Ir4

remained unchanged in both solution and solid states. This

indicated the judicious suitability of the low-frequency

oscillating –CF₃group in red-shifting the emission to the NIR

region without affecting the emission efficiencies.

Emission profiles as a neat powder

In order to gain more insights into the effect of intermolecular

interactions on the excited state, we recorded the emission

J. Name., 2013, 00, 1-3 | 13

Please do not adjust margins

Journal of Materials Chemistry C

Page 14 of 21

ARTICLE

Journal Name

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 7 (a) Correlation diagram for the emission peak wavelength of Ir1

–

Ir4 versus the quantum yield, lifetime, radiative, and nonradiative decay rate constants. (b)

Schematic representation of the ground- and excited-state PESs for the loss of excitation energy in Ir3 and Ir4. (c) Spin densities of the triplet excited states for

complexes Ir1–Ir4 obtained at the B3LYP/LANL2DZ/6-31G(d,p) level of theory (isosurface value = 0.003, blue for positive and yellow for negative).

spectrum of all the complexes in the neat powder form. Fig. quantum efficiency, lifetime, radiative, and nonradiative decay

6(c) displays a comparison of the emission profiles in the neat rate constants in both the solution and film states. In general,

powder, solution, PMMA films, and frozen matrix forms. Ir1 all the compounds exhibited impressive quantum efficiencies

and Ir2 showed identical emission maxima in the powder when imbedded in a PMMA thin film than their solution state

state, indicating that intermolecular interactions had no due to the suppression of nonradiative pathways in the rigid

effects on the excited state. Ir4 exhibits an 8ꢀnm (152ꢀcm⁻¹) polymer matrix. However, the observed k_nrvalues of the

bathochromic shift in the emission maximum, ascribable to the iridium(III) complexes can be correlated to the nature of the

intermolecular π–π interactions from the LUMO localized substituents on the cyclometalated ligands. The k_nrvalues of

cyclometalating ligands aggregated in the neat solid state. Ir1 and Ir2 in the solution state were much higher than in the

However, such an interaction did not produce much effect in film state, indicative of nonradiative channels associated with

Ir1 and Ir2, probably because the LUMO was localized over the the lack of rigidity in these complexes. Meanwhile, a nearly

TTA ancillary ligand. It is also interesting to note the blue shift identical k_nrvalue was noted for Ir4, revealing the effect of

(12ꢀnm, 212ꢀcm⁻¹) associated with Ir3, strongly indicating low-frequency –CF oscillators in restricting nonemissive

possible rigidochromism due to the highly polarized excited- pathways. Among the substituents investigated, only the

state and arresting ground-state vibrational relaxations. The electron-withdrawing ester group from Ir3 yielded a high k_nr,

small difference between the E_0-0and E_0-1peak heights of Ir1

,

indicating that the reduced bandgap and most vulnerable –

Ir2, and Ir4 compared to their respective solution or film state C=O and –O–C vibrational distortions from the ester group

emission profiles indicated a large Huang–Rhys factor, which caused large coupling of the lowest excited-state potential

facilitates strong coupling of the ligand localized vibrations energy surfaces (PESs) with the ground-state vibrational

between the ground and excited states, leading to very low energy levels, ultimately leading to robust nonradiative

emission efficiencies.

pathways [Fig. 7(b)]. To understand the unexpected loss in the

It is also significant to discuss the entirely different emission solution-state emission efficiency of Ir1 and Ir2, we analyzed

efficiency exhibited by these compounds in the solution and the geometry of the singlet and triplet optimized excited states

doped film states. Fig. 7(a) clearly demonstrates the and population of triplet spin density states. As can be seen

correlation between the emission energies toward the from Fig. 7(c), the triplet spin density in Ir1 and Ir2 was

14 | J. Name., 2012, 00, 1-3

Please do not adjust margins

Page 15 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 8 (a) Chemical structures for the materials used during device preparation. (b) Energy level diagram of the device with doped Ir1

device. (d) EL plots. (e) CIE 1931 chromaticity diagram.

–Ir4. (c) A schematic of the EL

localized over the TTA ancillary ligand. For Ir3 and Ir4, this was with the emissive excited-state energy of Ir3 observed at

localized on the cyclometalating ligand. The corresponding 13,192 cm⁻¹. Additionally, the localization of the LUMO over

triplet optimized geometry of Ir1 and Ir2 revealed a twist in the ester substituent could increase the oscillation frequency

the position of the TTA from the coordinating plane, which of the carbonyl group. The elongation of the bond distance

caused a serious geometrical change in the excited-state through the repulsive PES of the excited state (T₁) finally

structure, as revealed by elongation of the Ir–O bond lengths touches the PES of the ground state (S₀), causing radiationless

in the triplet excited states (Table S4). Thus, in the solution deactivation. The presence of prominent nonradiative decay

state, the observed twist in the position of the ancillary ligand rate constants (k_nr= 19.92 for Ir3) further supports our

and associated metal–ligand bond vibrations will be more hypothesis. It is important to note the high efficiency for the –

active, which can easily facilitate the nonradiative decay CF₃substituted Ir4 (Φ_PL= 0.10, τ_obs= 0.94ꢀµs). The observed

channels, resulting in a poor quantum efficiency. In addition, a efficiency was almost ten times higher than that of Ir3 and

poor quantum efficiency was also noted for Ir3, along with a comparable to many other highly efficient NIR emissive iridium

68, 72, 75, 78, 87

low photoluminescence lifetime (Φ_PL= 0.011, τ_obs= 0.05ꢀµs). complexes in the 700–840ꢀnm region.^63,

The

The lowest efficiencies could be associated with the bandgap vibrational satellites from the low-frequency –CF₃oscillators

law: the ground-state vibrational satellites from –C=O do not interfere with the emissive process, as evidenced from

oscillators associated with the ethyl ester substituent may the very low nonradiative decay rate constants (k_nr= 0.95) for

overlap with the emissive energy levels due to the minimal Ir4 [Fig. 7(b)].

emissive bandgap [Fig. 7(b)]. To better understand the

vibrational oscillators associated with the complex, we

OLED characteristics

measured the Fourier-transform infrared spectroscopy The energy band diagram (Fig. 8) was obtained for a simple

spectrum and compared the experimental data to the PhOLED fabricated from the TCTA and mCP cohost for newly

calculated vibrational transitions from the optimized singlet synthesized iridium(III) complex dopants in the emitting layers.

ground-state and triplet excited-state geometries (Figs. S39 The HOMO and LUMO values of Ir1

, Ir2, Ir3, and Ir4 were

and S40). Compared to Ir1 Ir2, and Ir4, the symmetric and determined using DFT calculations and CV measurements

,

asymmetric stretching frequencies associated with the (Table 3). The HOMO-LUMO levels of the Ir1 and Ir2 dopants

carbonyl group of the ester substituent were dominant in Ir3 were suitable with mCP:TCTA in high-efficiency devices.

at 1,728ꢀcm⁻¹. High-frequency aliphatic –C–H vibrations were Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

noted around 3,010ꢀcm⁻¹[Fig. 7(b)] and the fourth or fifth (PEDOT: PSS), mCP, TCTA, and 1,3,5-tris(1-phenyl-1H-

overtone of these high-frequency oscillators matched well benzimidazol-2-yl)benzene (TPBi) materials were obtained

J. Name., 2013, 00, 1-3 | 15

Please do not adjust margins

Journal of Materials Chemistry C

Page 16 of 21

ARTICLE

Journal Name

View Article Online

DOI: 10.1039/C8TC04321C

Fig. 9 (a) Current efficiency versus voltage (b) Luminance versus voltage (c) EQE versus Luminance. Current-density characteristics versus applied voltages for (d)

HODs and (e) EODs. Inset: energy level diagram for HOD or EOD demonstration.

from Ossila and Lumtec Co. Details of PhOLED fabrication and recombination from efficient electron injection. This is because

measurements are described in the Supporting Information. the LUMO levels of the Ir1 (−2.68ꢀeV) and Ir2 (−2.75ꢀeV)

The normalized electroluminescent (EL) spectra were obtained dopants were similar to that of the TPBi (−2.7ꢀeV) electron

for devices with mCP:TCTA cohost and iridium(III) complex injection layer. The Commission Internationale de I'Eclairage

dopants. Fig. 8(d) shows the maximum EL emission peak of the (CIE) coordinates of Ir1 and Ir2 doped devices were obtained

Ir1, Ir2, Ir3, and Ir4 doped devices to be 657, 667, 747, and 730 with (0.71, 0.28) in CIE 1931 chromaticity diagram [Fig. 8(e)].

nm, respectively. The EL spectra were similar to the PL spectra Ir1 and Ir2 doped devices showed deep-red emission. On the

(Table 6). The EL peaks of Ir1 and Ir2 doped devices showed a other hand, the low luminance versus high current flow in the

shoulder peak in addition to the main peak: 718 nm for Ir3 and Ir4 doped devices means that electrons and holes

mCP:TCTA:Ir1 and 727 nm for mCP:TCTA:Ir2 at high voltages remained unbalanced in the emitting layers. This results in a

(Fig. S41). These peaks implied that other electronic transitions high current loss as well as low EQE; however, this problem

occurred in the host:dopant system in addition to the can be solved by introducing an additional electron transport

intermolecular transitions associated with a long-wavelength layer for high-efficiency devices. The CIE coordinate of NIR

electromer emission. Two- and three-dimensional AFM images emissive Ir3 and Ir4 doped devices was (0.53, 0.42) and (0.59,

were measured with 3ꢀ×ꢀ3ꢀµm scan size using noncontact mode 0.37), respectively.

(Fig. S42). As a surface morphological factor, the root mean

square (R_q) and arithmetic average of absolute (R_a) values of fabricated hole-only devices (HODs) or electron-only devices

the emitting layers containing the substituted complexes (Ir1 (EODs) to compare the carrier mobility of the emitting

To determine the cause of the device properties, we

–

Ir4) were evaluated to be 0.47 and 0.28 nm (Ir1), 0.91 and 0.50 materials [Figs. 9(d) and 9(e)]. To investigate charge carrier

nm (Ir2), 1.33 and 0.63 nm (Ir3), and 1.28 and 0.63 nm (Ir4), transport, we used PEDOT:PSS or LiF buffer layers for ITO, Al,

respectively. The Ir1 and Ir2 doped emitting layers showed a and Ag electrodes, as described in the inset. Using Ir1 and Ir2

surface roughness lower than 1.0 nm, which was a relatively in the EODs was more than 10 times different compared to Ir3

uniform surface structure compared with the Ir3 and Ir4 and Ir4. The hole current density of Ir1, Ir2, and Ir3 in HODs

doped films. The surface morphology of solution-processed showed similar levels. We can expect that the devices using Ir1

films could be related to dopant solubility because an or Ir2 (electron-donating ability) will generate high electron

inhomogeneous solution would not result in good film current density, whereas the devices using Ir3 or Ir4 (electron-

morphology. The solutions of Ir1 and Ir2 were clear and withdrawing ability) will generate high hole current in the

resulted in well-melted films during casting. The surface devices. Although the mCP:TCTA cohost materials have wide

morphological property can affect device performance.¹¹⁰In bandgaps suitable for applying various dopants, the cohost

the current density and luminance graphs versus voltages materials doped by Ir3 or Ir4 showed a low device efficiency;

[Figs. 9(a) and 9(b)], the high EQE [Fig. 9(c)] of the Ir1 (2.05%) that is, the corresponding devices provided relatively poor

and Ir2 (2.11%) devices, compared to those of the Ir3 (0.6%) electron-hole balance because hole carriers in the cohost were

and Ir4 (0.15%) devices, caused

a

balanced carrier in excess in the emitting layer. On the other hand, the Ir1 or

16 | J. Name., 2012, 00, 1-3

Please do not adjust margins

Page 17 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

Ir2 doped devices had relatively good electron current density EQE in the series (EQE of Ir1 = 2.05%; EQE of Ir2 _V=_iew2._A1_rt1_ic%_le)_O._nO_linn_e

DOI: 10.1039/C8TC04321C

and showed a high device efficiency (~fourfold) as well as high the other hand, unbalanced electrons and holes in the

luminance (~fivefold) due to the balanced electron-hole carrier emitting layers resulted in a high current loss as well as low

combination in the emitting layer.

EQE for the Ir3 and Ir4 doped devices. Further device

optimizations to attain higher efficiencies suitable for

photobiomodulation studies are currently underway in our

laboratory. Since the device efficiency can be linearly

correlated with the phosphorescence efficiency, future

perspective should be more focused on improving the

luminescence quantum yield of the molecules in the NIR

emissive region. The present work highlights the detrimental

effects of high frequency oscillating substituents or flexible

ligands towards vibrational nonradiative pathways, when

excited states are populated in their vicinity. Thus, it is highly

recommended to use the low frequency oscillating

substituents (eg. -F or –CF₃) and rigid ancillary ligand frame

works having high triplet energy which are more resistible to

band gap law in an NIR emissive iridium(III) complex.

Conclusions

A series of deep-red to NIR emissive iridium(III) complexes

(Ir1–Ir4) with different electron-withdrawing (–CF₃and –

CO₂C₂H₅) and electron-donating (–CH₃) substituents on the

para position on the N-coordinating quinoline moiety on a 2-

(benzo[b]thiophen-2-yl)quinoline cyclometalating ligand were

synthesized,

photophysically. For the electron-donating –CH₃substituted

Ir1) and unsubstituted (Ir2) complexes, a faint deep-red

well

characterized,

and

investigated

(

emission occurred at 655 and 660ꢀnm, respectively. The

corresponding LUMO energy levels in Ir1 and Ir2 were found to

be localized over the flexible TTA ancillary ligand. The triplet

optimized geometry and density maps revealed an excited-

state geometrical change associated with TTA, which accounts

for the inferior emission efficiency in the solution state (0.02

and 0.08 for Ir1 and Ir2, respectively). However, in a PMMA

polymer matrix, these geometrical changes were arrested, as

evidenced from the bright-red phosphorescence (0.24 and

0.30 for Ir1 and Ir2, respectively). While the electron-

withdrawing (–CF₃) substituted iridium(III) complex Ir4

displayed intense NIR emissions around 722ꢀnm at room

temperature in dichloromethane (Φ_PL= 0.10), the quantum

efficiency was further improved to 0.13 in the doped polymer

films. On the other hand, the electron-withdrawing –CO₂C₂H₅

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Synchrotron single crystal X-ray diffraction (XRD) were

performed on the 2D beamline of the Pohang Acceleration

Laboratory (PAL; Pohang, Korea). This work was supported by

grants from the Center for Advanced Soft Electronics under

the Global Frontier Research Program (2012M3A6A5055225),

and the Technology Development Program to Solve Climate

Changes of the National Research Foundation (NRF) of Korea

(2015M1A2A2056216 and 2016M1A2A2940912).

substituted iridium(III) complex

(Ir3) exhibited inferior

emission efficiencies, which could be attributed to the

bandgap law. A considerable overlap between the ground- and

excited-state vibrational levels associated with the ester

carbonyl group prevented radiative decay, as evidenced from

the massive nonradiative rate constant (19.78ꢀ×ꢀ10⁵ꢀs⁻¹). The

observed emission efficiency of Ir1-Ir4 helps to derive the

relationship between molecule orbital, vibrational motions and

quantum efficiency. The electron donating –CH₃substituted

Ir1 and unsubstituted Ir2 destabilizes the excited energy level

of the cyclometalating ligands, resulting in the LUMO

population over a more flexible TTA ancillary ligand. The

twisting motions associated with TTA upon excitation has

resulted in the inferior efficiency of Ir1 and Ir2. Meantime the

electron-withdrawing –CO₂C₂H₅and –CF₃has stabilized the

energy of cyclometalating ligand than ancillary TTA, allowing

LUMO population over the substituted quinolinate moiety. The

presence of high frequency oscillating –CO₂C₂H₅in the LUMO

localized quinolinate of Ir3 has considerably reduced the

emission efficiency, while the low frequency oscillating –CF₃in

Ir4 resulted in a respectable quantum efficiency among NIR

emitters. A preliminary electroluminescence study on the

newly developed complexes was conducted by fabricating

unoptimized PhOLEDs using solution-processable methods.

The electroluminescence profiles appeared to be identical to

the corresponding photoluminescence profiles. The Ir1 and Ir2

doped devices showed deep-red emissions with the highest

J. Name., 2013, 00, 1-3 | 17

Please do not adjust margins

Journal of Materials Chemistry C

Page 18 of 21

ARTICLE

Journal Name

28. S. Wang, X. Yan, Z. Cheng, H. Zhang, Y. Liu and Y. Wang, Angew.

Notes and references

1. M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S.

R. Forrest, Appl. Phys. Lett., 1999, 75, 4-6.

2. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E.

Lee, C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson,

J. Am. Chem. Soc., 2001, 123, 4304-4312.

3. Z. W. Liu, M. Guan, Z. Q. Bian, D. B. Nie, Z. L. Gong, Z. B. Li and C.

H. Huang, Adv. Funct. Mater., 2006, 16, 1441-1448.

4. W. Y. Wong, G. J. Zhou, X. M. Yu, H. S. Kwok and B. Z. Tang, Adv.

Funct. Mater., 2006, 16, 838-846.

5. C. H. Yang, W. L. Su, K. H. Fang, S. P. Wang and I. W. Sun,

Organometallics, 2006, 25, 4514-4519.

6. W. Y. Wong, G. J. Zhou, X. M. Yu, H. S. Kwok and Z. Y. Lin, Adv.

Funct. Mater., 2007, 17, 315-323.

7. G. J. Zhou, W. Y. Wong, B. Yao, Z. Y. Xie and L. X. Wang, Angew.

Chem. Int. Ed., 2007, 46, 1149-1151.

8. Y. Chi and P. T. Chou, Chem. Soc. Rev., 2010, 39, 638-655.

9. P. S. Wagenknecht and P. C. Ford, Coord. Chem. Rev., 2011, 255,

591-616.

10. D. Tanaka, H. Sasabe, Y. J. Li, S. J. Su, T. Takeda and J. Kido,

Japanese Journal of Applied Physics Part 2-Letters & Express

Letters, 2007, 46, L10-L12.

11. C. H. Yang, Y. M. Cheng, Y. Chi, C. J. Hsu, F. C. Fang, K. T. Wong,

P. T. Chou, C. H. Chang, M. H. Tsai and C. C. Wu, Angew. Chem.

Int. Ed., 2007, 46, 2418-2421.

12. Y. S. Park, J. W. Kang, D. M. Kang, J. W. Park, Y. H. Kim, S. K.

Kwon and J. J. Kim, Adv. Mater., 2008, 20, 1957-+.

13. D. S. Leem, S. O. Jung, S. O. Kim, J. W. Park, J. W. Kim, Y. S. Park,

Y. H. Kim, S. K. Kwon and J. J. Kim, J. Mater. Chem., 2009, 19,

8824-8828.

14. M. G. Helander, Z. B. Wang, J. Qiu, M. T. Greiner, D. P. Puzzo, Z.

W. Liu and Z. H. Lu, Science, 2011, 332, 944-947.

15. S. Y. Kim, W. I. Jeong, C. Mayr, Y. S. Park, K. H. Kim, J. H. Lee, C.

K. Moon, W. Brutting and J. J. Kim, Adv. Funct. Mater., 2013, 23,

3896-3900.

16. S. Lee, K. H. Kim, D. Limbach, Y. S. Park and J. J. Kim, Adv. Funct.

Mater., 2013, 23, 4105-4110.

17. S. Lee, D. Limbach, K. H. Kim, S. J. Yoo, Y. S. Park and J. J. Kim,

Org. Electron., 2013, 14, 1856-1860.

View Article Online

Chem., Int. Ed., 2015, 54, 13068.

DOI: 10.1039/C8TC04321C

29. P. Data, P. Pander, M. Okazaki, Y. Takeda, S. Minakata and A. P.

Monkman, Angew. Chem., Int. Ed., 2016, 55, 5739.

30. J. Xue, C. Li, L. Xin, L. Duan and J. Qiao, Chem. Sci., 2016, 7,

2888.

31. S. C. Moratti, R. Cervini, A. B. Holmes, D. R. Baigent, R. H. Friend,

N. C. Greenham, J. Grüner and P. J. Hamer, Synth. Met., 1995,

71, 2117.

32. X. Li, W. Zeng, Y. Zhang, Q. Hou, W. Yang and Y. Cao, Eur. Polym.

J., 2005, 41, 2923.

33. R. Yang, R. Tian, J. Yan, Y. Zhang, J. Yang, Q. Hou, W. Yang, C.

Zhang and Y. Cao, Macromolecules, 2005, 38, 244.

34. A. C. Grimsdale, K. Leok Chan, R. E. Martin, P. G. Jokisz and A. B.

Holmes, Chem. Rev., 2009, 109, 897.

35. Y. Kawamura, Y. Wada, Y. Hasegawa, M. Iwamuro, T. Kitamura

and S. Yanagida, Appl. Phys. Lett., 1999, 74, 3245.

36. S. V. Eliseeva and J. C. G. Bunzli, Chem. Soc. Rev., 2010, 39, 189.

37. C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J Appl

Phys, 2001, 90, 5048-5051.

38. M. L. P. Reddy and K. S. Bejoymohandas, J Photoch Photobio C,

2016, 29, 29-47.

39. K. S. Bejoymohandas, A. Kumar, S. Varughese, E. Varathan, V.

Subramanian and M. L. P. Reddy, J Mater Chem C, 2015, 3,

7405-7420.

40. K. Tuong Ly, R. W. Chen-Cheng, H. W. Lin, Y. J. Shiau, S. H. Liu, P.

T. Chou, C. S. Tsao, Y. C. Huang and Y. Chi, Nat. Photonics, 2017,

11, 63.

41. K. R. Graham, Y. Yang, J. R. Sommer, A. H. Shelton, K. S. Schanze,

J. Xue and J. R. Reynolds, Chem. Mater., 2011, 23, 5305.

42. J. R. Sommer, R. T. Farley, K. R. Graham, Y. Yang, J. R. Reynolds,

J. Xue and K. S. Schanze, ACS Appl. Mater. Interfaces, 2009, 1,

274.

43. E. L. Williams, J. Li and G. E. Jabbour, Appl. Phys. Lett., 2006, 89,

083506.

44. J. Qiao, L. Duan, L. Tang, L. He, L. Wang and Y. Qiu, J. Mater.

Chem., 2009, 19, 6573.

45. R. Tao, J. Qiao, G. Zhang, L. Duan, C. Chen, L. Wang and Y. Qiu, J.

Mater. Chem. C, 2013, 1, 6446.

46. X. Cao, J. Miao, M. Zhu, C. Zhong, C. Yang, H. Wu, J. Qin and Y.

Cao, Chem. Mater., 2015, 27, 96.

47. S. Kesarkar, W. Mróz, M. Penconi, M. Pasini, S. Destri, M.

Cazzaniga, D. Ceresoli, P. R. Mussini, C. Baldoli, U. Giovanella

and A. Bossi, Angew. Chem., Int. Ed., 2016, 55, 2714.

48. H. Fukagawa, T. Shimizu, T. Kamada, Y. Kiribayashi, Y. Osada, M.

Hasegawa, K. Morii and T. Yamamoto, Adv. Opt. Mater., 2014,

2, 1070.

18. Y. S. Park, S. Lee, K. H. Kim, S. Y. Kim, J. H. Lee and J. J. Kim, Adv.

Funct. Mater., 2013, 23, 4914-4920.

19. Y. Zhang, Y. Wang, J. Song, J. Qu, B. Li, W. Zhu and W.-Y. Wong,

Advanced

10.1002/adom.201800466.

20. R. J. Curry and W. P. Gillin, Appl. Phys. Lett., 1999, 75, 1380.

Optical

Materials,

2018,

DOI:

21. H. T. Whelan, R. L. Smits, E. V. Buchman, N. T. Whelan, S. G.

Turner, D. A. Margolis, V. Cevenini, H. Stinson, R. Ignatius and T.

Martin, J. Clin. Laser Med. Surg., 2001, 19, 305.

22. A. Rogalski and K. Chrzanowski, Opto-Electron. Rev., 2002, 10,

111.

23. G. Qian and Z. Y. Wang, Chem. - Asian J., 2010, 5, 1006.

24. H. Xiang, J. Cheng, X. Ma, X. Zhou and J. J. Chruma, Chem. Soc.

Rev., 2013, 42, 6128.

25. H. Xu, R. Chen, Q. Sun, W. Lai, Q. Su, W. Huang and X. Liu, Chem.

Soc. Rev., 2014, 43, 3259.

26. C. Borek, K. Hanson, P. I. Djurovich, M. E. Thompson, K.

Aznavour, R. Bau, Y. Sun, S. R. Forrest, J. Brooks, L. Michalski

and J. Brown, Angew. Chem., Int. Ed., 2007, 46, 1109.

27. L. Yao, S. Zhang, R. Wang, W. Li, F. Shen, B. Yang and Y. Ma,

Angew. Chem., 2014, 126, 2151.

49. D. Zhang, L. Duan, D. Zhang and Y. Qiu, J. Mater. Chem. C, 2014,

2, 8983.

50. C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo,

M. E. Thompson and S. R. Forrest, Appl Phys Lett, 2001, 79,

2082-2084.

51. M. A. Baldo, C. Adachi and S. R. Forrest, Phys Rev B, 2000, 62,

10967-10977.

52. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E.

Thompson and S. R. Forrest, Nature, 1998, 395, 151-154.

53. Y. Y. Chang, J. Y. Hung, Y. Chi, J. P. Chyn, M. W. Chung, C. L. Lin,

P. T. Chou, G. H. Lee, C. H. Chang and W. C. Lin, Inorg Chem,

2011, 50, 5075-5084.

54. S. M. Chen, G. P. Tan, W. Y. Wong and H. S. Kwok, Adv Funct

Mater, 2011, 21, 3785-3793.

18 | J. Name., 2012, 00, 1-3

Please do not adjust margins

Page 19 of 21

Journal of Materials Chemistry C

Journal Name

ARTICLE

55. Z. Q. Chen, Z. Q. Bian and C. H. Huang, Adv Mater, 2010, 22, 85. G. Li, P. Li, X. Zhuang, K. Ye, Y. Liu and Y. Wang, ACS Appl Mater

View Article Online

1534-1539.

Interfaces, 2017, 9, 11749-11758.

DOI: 10.1039/C8TC04321C

56. Y. Chi and P. T. Chou, Chem Soc Rev, 2007, 36, 1421-1431.

57. P. T. Chou and Y. Chi, Chem-Eur J, 2007, 13, 380-395.

58. B. S. Du, C. H. Lin, Y. Chi, J. Y. Hung, M. W. Chung, T. Y. Lin, G. H.

86. A. K. Pal, D. B. Cordes, A. M. Z. Slawin, C. Momblona, A.

Pertegás, E. Ortí, H. J. Bolink and E. Zysman-Colman, RSC

Advances, 2017, 7, 31833-31837.

Lee, K. T. Wong, P. T. Chou, W. Y. Hung and H. C. Chiu, Inorg 87. Z. Hao, M. Li, Y. Liu, Y. Wang, G. Xie and Y. Liu, Dyes and

Chem, 2010, 49, 8713-8723. Pigments, 2018, 149, 315-322.

59. J. M. Fernandez-Hernandez, C. H. Yang, J. I. Beltran, V. Lemaur, 88. Y.-I. Kim, S.-J. Yun, D. Kim and S. K. Kang, Bulletin of the Korean

F. Polo, R. Frohlich, J. Cornil and L. De Cola, J Am Chem Soc,

2011, 133, 10543-10558.

60. C. L. Ho and W. Y. Wong, New J Chem, 2013, 37, 1665-1683.

Chemical Society, 2018, 39, 133-136.

89. W. Minor, M. Cymborowski, Z. Otwinowski and M. Chruszcz,

Acta Crystallogr D, 2006, 62, 859-866.

61. E. Holder, B. M. W. Langeveld and U. S. Schubert, Adv Mater, 90. G. M. Sheldrick, Acta Crystallogr A, 2008, 64, 112-122.

2005, 17, 1109-1121. 91. A. L. Spek, Acta Crystallogr D, 2009, 65, 148-155.

62. Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem Soc Rev, 2011, 40, 92. A. Cappelli, A. Gallelli, M. Manini, M. Anzini, L. Mennuni, F.

5361-5388.

Makovec, M. C. Menziani, S. Alcaro, F. Ortuso and S. Vomero, J

Med Chem, 2005, 48, 3564-3575.

93. F. O. Garces, K. A. King and R. J. Watts, Inorg. Chem., 1988, 27,

3464-3471.

63. J. Xue, L. Xin, J. Hou, L. Duan, R. Wang, Y. Wei and J. Qiao,

Chemistry of Materials, 2017, 29, 4775-4782.

64. H.-Y. Chen, C.-H. Yang, Y. Chi, Y.-M. Cheng, Y.-S. Yeh, P.-T. Chou,

H.-Y. Hsieh, C.-S. Liu, S.-M. Peng and G.-H. Lee, Canadian Journal 94. K. S. Bejoymohandas, T. M. George, S. Bhattacharya, S.

of Chemistry, 2006, 84, 309-318. Natarajan and M. L. P. Reddy, J Mater Chem C, 2014, 2, 515-523.

65. H. K. Dahule, S. J. Dhoble, J. S. Ahn and R. Pode, Journal of 95. K. S. Bejoymohandas, A. Kumar, S. Sreenadh, E. Varathan, S.

Physics and Chemistry of Solids, 2011, 72, 1524-1528.

66. G. Li, Journal of Luminescence, 2011, 131, 184-189.

Varughese, V. Subramanian and M. L. P. Reddy, Inorg Chem,

2016, 55, 3448-3461.

67. M. Song, J. S. Park, Y.-S. Gal, S. Kang, J. Y. Lee, J. W. Lee and S.-H. 96. S. J. Lee, K. M. Park, K. Yang and Y. Kang, Inorg. Chem., 2009, 48,

Jin, The Journal of Physical Chemistry C, 2012, 116, 7526-7533. 1030-1037.

68. R. Tao, J. Qiao, G. Zhang, L. Duan, L. Wang and Y. Qiu, J. Phys. 97. M. L. Xu, G. B. Che, X. Y. Li and Q. Xiao, Acta. Crystallogr. E. ,

Chem. C, 2012, 116, 11658. 2009, 65, M28-U379.

69. C.-L. Ho, B. Yao, B. Zhang, K.-L. Wong, W.-Y. Wong, Z. Xie, L. 98. Y. J. Cho, S. Y. Kim, J. H. Kim, D. W. Crandell, M. H. Baik, J. Lee, C.

Wang and Z. Lin, Journal of Organometallic Chemistry, 2013,

730, 144-155.

H. Kim, H. J. Son, W. S. Han and S. O. Kang, J Mater Chem C,

2017, 5, 4480-4487.

70. S. Ikawa, S. Yagi, T. Maeda, H. Nakazumi, H. Fujiwara, S. Koseki 99. J.-H. Kim, S.-Y. Kim, Y.-J. Cho, H.-J. Son, D. W. Cho and S. O.

and Y. Sakurai, Inorg. Chem. Commun., 2013, 38, 14.

71. H. J. Bae, J. Chung, H. Kim, J. Park, K. M. Lee, T. W. Koh, Y. S. Lee,

S. Yoo, Y. Do and M. H. Lee, Inorg Chem, 2014, 53, 128-138.

72. X. Cao, J. Miao, M. Zhu, C. Zhong, C. Yang, H. Wu, J. Qin and Y.

Cao, Chemistry of Materials, 2014, 27, 96-104.

73. X. Liu, S. Wang, B. Yao, B. Zhang, C.-L. Ho, W.-Y. Wong, Y. Cheng

and Z. Xie, Org Electron, 2015, 21, 1-8.

Kang, The Journal of Physical Chemistry C, 2018, 122, 4029-

4036.

100. Y. Zheng, A. S. Batsanov, R. M. Edkins, A. Beeby and M. R.

Bryce, Inorg Chem, 2012, 51, 290-297.

101. N.-R. Jung, E.-J. Lee, J.-H. Kim, H.-K. Park, K.-M. Park and Y.-J.

Kang, Bulletin of the Korean Chemical Society, 2012, 33, 183-

188.

74. M. Schulze, A. Steffen and F. Wurthner, Angew Chem Int Ed 102.M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.

Engl, 2015, 54, 1570-1573.

Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson,

H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G.

Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz,

A. F. Izmaylov, J. L. Sonnenberg, Williams, F. Ding, F. Lipparini, F.

Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D.

Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W.

Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.

Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K.

Throssell, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J.

Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N.

Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K.

Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi,

M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W.

Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman

and D. J. Fox, Journal, 2016.

75. L. Xin, J. Xue, G. Lei and J. Qiao, RSC Adv., 2015, 5, 42354.

76. B. Jiang, Y. Gu, J. Qin, X. Ning, S. Gong, G. Xie and C. Yang, J

Mater Chem C, 2016, 4, 3492-3498.

77. J.-H. Jou, Y.-T. Su, M.-T. Hsiao, H.-H. Yu, Z.-K. He, S.-C. Fu, C.-H.

Chiang, C.-T. Chen, C.-h. Chou and J.-J. Shyue, The Journal of

Physical Chemistry C, 2016, 120, 18794-18802.

78. S. Kesarkar, W. Mroz, M. Penconi, M. Pasini, S. Destri, M.

Cazzaniga, D. Ceresoli, P. R. Mussini, C. Baldoli, U. Giovanella

and A. Bossi, Angew Chem Int Ed Engl, 2016, 55, 2714-2718.

79. H. U. Kim, J.-H. Jang, F. Xu, J. Y. Lee and D.-H. Hwang, Journal of

Nanoscience and Nanotechnology, 2016, 16, 2773-2778.

80. A. Liang, G. Huang, Z. Wang, W. Wu, Y. Zhong and S. Zhao,

Electronic Materials Letters, 2016, 12, 615-621.

81. W. Cho, H. Kim, Y.-S. Gal and S.-H. Jin, Molecular Crystals and

Liquid Crystals, 2017, 654, 62-72.

82. W. Cho, G. Sarada, A. Maheshwaran, Y.-S. Gal, Y. Nam, J. Yong

Lee and S.-H. Jin, J Mater Chem C, 2017, 5, 10029-10038.

103.E. Baranoff, B. F. Curchod, F. Monti, F. Steimer, G. Accorsi, I.

Tavernelli, U. Rothlisberger, R. Scopelliti, M. Gratzel and M. K.

Nazeeruddin, Inorg Chem, 2012, 51, 799-811.

83. M. Huang, B. Jiang, G. Xie and C. Yang, J Phys Chem Lett, 2017, 104.Y. Byun, W. S. Jeon, T. W. Lee, Y. Y. Lyu, S. Chang, O. Kwon, E.

8, 4967-4973.

Han, H. Kim, M. Kim, H. J. Lee and R. R. Das, Dalton Trans, 2008,

84. Y.-M. Jing, F.-Z. Wang, Y.-X. Zheng and J.-L. Zuo, J Mater Chem C,

2017, 5, 3714-3724.

DOI: 10.1039/b719205c, 4732-4741.

J. Name., 2013, 00, 1-3 | 19

Please do not adjust margins

Journal of Materials Chemistry C

Page 20 of 21

ARTICLE

Journal Name

105.J. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer

Academic/Plenum Publishers, New York, Boston, Dordrecht,

London, Moscow, 1999.

View Article Online

DOI: 10.1039/C8TC04321C

106.Y. You and S. Y. Park, Dalton T, 2009, DOI: 10.1039/b812281d,

1267-1282.

107.R. J. Watts, Comments on Inorganic Chemistry, 1991, 11, 303-

337.

108.J. V. Caspar, T.D.Westmoreland, G. H. Allen, P. G. Bradley, T. J.

Meyer and W. H. Woodruff, J. Am. Chem. Soc, 1984, 106, 3492-

3501.

109.R. J. Watts, Comments Inorg. Chem., 1991, 11, 303-308.

110.Y. R. Cho, H. S. Kim, Y.-J. Yu and M. C. Suh, Scientific Reports,

2015, 5, 15903.

20 | J. Name., 2012, 00, 1-3

Please do not adjust margins

Page 21 of 21

Journal of Materials Chemistry C

View Article Online

DOI: 10.1039/C8TC04321C

39./03)3 4/#

{ꢀꢁꢂꢃꢄꢃꢀꢅꢆꢃ ꢅꢇꢇꢅꢈꢃꢂ ꢉꢆ ꢃꢊꢅ ꢋꢊꢉꢃꢉꢋꢊꢌꢂꢄꢈꢂ ꢉꢇ ꢍꢅꢅꢋꢎꢏꢅꢍ ꢃꢉ ꢆꢅꢐꢏꢎꢄꢆꢇꢏꢐꢏꢅꢍ ꢅꢑꢄꢂꢂꢄꢒꢅ ꢄꢏꢄꢍꢄꢀꢑꢓLLLꢔ ꢈꢉꢑꢋꢕꢅꢖꢅꢂ

Article Doi

Substituents engineered deep-red to near-infrared phosphorescence from tris-heteroleptic iridium(iii) complexes for solution processable red-NIR organic light-emitting diodes

DOI: 10.1039/c8tc04321c

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c8tc04321c

Full text of DOI:10.1039/c8tc04321c

Products guided by the article

R&D Labs maybe for 2806-29-3

Relevant to this article

Hot Product