Multifunctional Cationic Iridium(III) Complexes Bearing 2-Aryloxazolo[4,5-f][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption

Article

pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Multifunctional Cationic Iridium(III) Complexes Bearing

2‑Aryloxazolo[4,5‑f][1,10]phenanthroline (N^N) Ligand: Synthesis,

Crystal Structure, Photophysics, Mechanochromic/Vapochromic

Eﬀects, and Reverse Saturable Absorption

Xiaolin Zhu,^†Peng Cui,^†,‡Svetlana Kilina,^†and Wenfang Sun*^,†

^†Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States

^‡Materials and Nanotechnology Program, North Dakota State University, Fargo, North Dakota 58105, United States

S

* Supporting Information

ABSTRACT: A series of 2-aryloxazolo[4,5-f ][1,10]-

phenanthroline ligands (N^N ligands) and their cationic iridium-

(III) complexes (1−11, aryl = 4-NO₂-phenyl (1), 4-Br-phenyl (2),

Ph (3), 4-NPh₂-phenyl (4), 4-NH₂-phenyl (5), pyridin-4-yl (6),

naphthalen-1-yl (7), naphthalen-2-yl (8), phenanthren-9-yl (9),

anthracen-9-yl (10), and pyren-1-yl (11)) were synthesized and

characterized. By introducing diﬀerent electron-donating or

electron-withdrawing substituents at the 4-position of the 2-

phenyl ring (1−5), or diﬀerent aromatic substituents with varied

degrees of π-conjugation (6−11) on oxazolo[4,5-f ][1,10]-

phenanthroline ligand, we aim to understand the eﬀects of

terminal substituents at the N^N ligands on the photophysics of

cationic Ir(III) complexes using both spectroscopic methods and

quantum chemistry calculations. Complexes with the 4-R-phenyl

substituents adopted an almost coplanar structure with the oxazolo[4,5-f ][1,10]phenanthroline motif, while the polycyclic aryl

substituents (except for naphthalen-2-yl) were twisted away from the oxazolo[4,5-f ][1,10]phenanthroline motif. All complexes

1

possessed strong absorption bands below 350 nm that emanated from the ligand-localized π,π*/¹ILCT (intraligand charge

transfer) transitions, mixed with ¹LLCT (ligand-to-ligand charge transfer)/¹MLCT (metal-to-ligand charge transfer) transitions.

At the range of 350−570 nm, all complexes exhibited moderately strong ¹ILCT/¹LLCT/¹MLCT transitions at 350−450 nm, and

broad but very weak ³LLCT/³MLCT absorption at 450−570 nm. Most of the complexes demonstrated moderate to strong room

temperature phosphorescence both in solution and in the solid state. Among them, complex 7 also manifested a drastic

mechanochromic and vapochromic luminescence eﬀect. Except for complexes 1 and 4 that contain NO₂or NPh₂substituent at

the phenyl ring, respectively, all other complexes exhibited moderate to strong triplet excited-state absorption in the spectral

region of 440−750 nm. Moderate to very strong reverse saturable absorption (RSA) of these complexes appeared at 532 nm for

4.1 ns laser pulses. The RSA strength followed the trend of 7 > 11 > 9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. The photophysical

studies revealed that the diﬀerent 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state

characteristics dramatically, which in turn notably inﬂuenced the RSA of these complexes.

requirements for each application, structural modiﬁcations are

needed for ﬁne-tuning of the chemical, electronic, or optical

properties of the Ir(III) complexes.

INTRODUCTION

■

Luminescent octahedral d⁶Ir(III) complexes have been

extensively explored due to the eﬃcient intersystem crossing

induced by the heavy Ir(III) ion, which results in strong room

temperature phosphorescence in solutions. In addition, the

Ir(III) complexes possess broad charge transfer absorption in

the visible spectral region, intriguing electrochemical properties,

and high chemical, thermal, and optical stabilities. These unique

properties make Ir(III) complexes promising candidates for a

variety of optoelectronic and biomedical applications, such as

organic light-emitting devices (OLEDs),¹⁻³light-emitting

electrochemical cells (LEECs),⁴⁻⁶photocatalysis,⁷upconver-

sion,⁸nonlinear optics,⁹⁻²⁵biosensing and bioimaging,²⁶⁻³¹

and photodynamic therapy.^25,32−36To meet the speciﬁc

Cationic heteroleptic Ir(III) complexes are interesting

candidates for the aforementioned applications because the

ground-state and excited-state properties of these complexes

can be readily tuned by substituents on the cyclometalating

(C^N) ligands or the diimine (N^N) ligand,^{9,10,21,37−40}or by

variation of the π-conjugation of the C^N or N^N

ligands.^{8,11,12,15−25,41}One class of frequently used π-conjugated

N^N ligand is the 2-arylimidazole[4,5-f ][1,10]phenanthroline.

Received: June 12, 2017

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Scheme 1. Synthetic Routes of Complexes 1−11

Due to the ease of preparation, and facile tuning of the

photophysical and electronic properties via substitution at the

2-aryl ring or the imidazole ring, or via variations of pH value or

cation/anion, this type of ligand has been widely used in

developing Ir(III) complexes for biomolecular targeting or

labeling, chemosensing, photocatalysis, and photosensitization

applications.^8,42−50Although the photophysical properties of

these complexes are very promising, the presence of acidic N−

H hydrogen on the imidazole ring makes the formed complexes

sensitive to the polarity and nature of the solvent, the pH values

of the solution, and the counterion in the case of cationic Ir(III)

complexes. While such sensitivity to environmental changes can

be beneﬁcial for sensing applications, it would cause potential

stability issues for optoelectronic device fabrication and

application. In addition, the N−H hydrogen would expel the

2-H on the 2-aryl ring, making the 2-aryl substituent non-

coplanar with the imidazole[4,5-f ][1,10]phenanthroline motif

and thus reducing the impact of the substituent at the aryl ring

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on the photophysics and electrochemical properties of the

Ir(III) complexes. It has been reported that the dihedral angle

between the 2-(4-triﬂuoromethyl)phenyl ring and the

imidazole[4,5-f ][1,10]phenanthroline motif was ∼11°.⁴²

the photophysical and RSA studies, we found that complex 7

exhibited interesting mechanochromic and vapochromic

luminescence eﬀects, which showed the potential of these

complexes as smart pressure or organic vapor sensing materials.

To overcome these drawbacks, we focus our investigations

on replacing the imidazole ring by its isoelectronic oxazole ring,

which substitutes the N−H unit by one O atom on the ring and

consequently makes the 2-aryl substituent more coplanar with

the oxazolo[4,5-f][1,10]phenanthroline. Unfortunately, due to

the challenge in synthesizing 2-aryloxazolo[4,5-f ][1,10]-

phenanthroline, the studies on 2-aryloxazolo[4,5-f ][1,10]-

phenanthrolines and their corresponding transition-metal

complexes are quite limited.⁵¹⁻⁶¹To date, the only method

reported for the synthesis of 2-aryloxazolo[4,5-f ][1,10]-

phenanthrolines was the cyclization reaction between 1,10-

phenanthroline-5,6-dione and arylaldehyde in the presence of

NH₄OAc in reﬂuxing acetic acid.^62,63This method is the same

method used for the synthesis of 2-arylimidazole[4,5-f][1,10]-

phenanthrolines, during which 2-aryloxazolo[4,5-f ][1,10]-

phenanthrolines were formed as the byproducts in some

cases, and thus the yield was very low (ca. 10%).^62,63In

addition, only limited types of 2-aryl substituents were

introduced to the oxazolo[4,5-f ][1,10]phenanthroline motif

as reported in the literature.⁵¹⁻⁶³In view of the potentially

better coplanarity between the 2-aryl substituent and the

oxazolo[4,5-f ][1,10]phenanthroline motif, we anticipate a

more pronounced impact of the substituents on the 2-aryl

motif. Therefore, the photophysical properties of the resulting

Ir(III) complexes could be rationally tuned via π-conjugation or

diﬀerent substituents on the 2-aryl group. Moreover, better

coplanarity could potentially improve the emission eﬃciency

and may cause red-shifts of the charge transfer absorption

band(s) in the UV−vis absorption spectra of the Ir(III)

complexes, which could beneﬁt the applications of OLEDs,

photosensitization, or broadband reverse saturable absorption

(RSA, a nonlinear optical phenomenon responsible for

increasing the absorptivity of a material with increased incident

ﬂuence, which has potential applications in optical switching,⁶⁴

laser mode locking,⁶⁵optical pulse shaping,⁶⁶spatial light

modulation,⁶⁷laser beam compression and limiting,^68,69etc.)

In this work, we developed a new synthetic route that can be

used for synthesis of oxazolo[4,5-f][1,10]phenanthroline with

diﬀerent aryl substituents at the 2-position of the oxazole ring.

Eleven 2-aryloxazolo[4,5-f ][1,10]phenanthrolines, i.e., 2-(4-

nitrophenyl) (L1), 2-(4-bromophenyl) (L2), 2-phenyl (L3),

2-(4-diphenylaminophenyl) (L4), 2-(4-aminophenyl) (L5), 2-

(pyridin-4-yl) (L6), 2-(naphthalen-1-yl) (L7), 2-(naphthalen-2-

yl) (L8), 2-(phenanthren-9-yl) (L9), 2-(anthracen-9-yl) (L10),

and 2-(pyren-4-yl) (L11), were synthesized and used as the

N^N ligand for preparation of biscyclometalated monocationic

Ir(III) complexes 1−11 (structures of the complexes are shown

in Scheme 1). By introducing diﬀerent electron-donating or

electron-withdrawing substituents at the 4-position of the 2-

phenyl ring (1−5), and diﬀerent aromatic substituents with

varied degrees of π-conjugation (6−11) on oxazolo[4,5-

f][1,10]phenanthroline, we aim to understand the eﬀects of

terminal substituents at the N^N ligands on the photophysics

of cationic Ir(III) complexes. We also intend to demonstrate

the feasibility of these Ir(III) complexes as reverse saturable

absorbers, which require weak ground-state absorption but

strong excited-state absorption in the visible to the near-IR

region, high quantum yield of the triplet excited-state

formation, and long-lived triplet excited state. In addition to

EXPERIMENTAL SECTION

■

Synthesis and Characterizations. All reagents including p-

bromonitrobenzene, p-dibromobenzene, bromobenzene, 4-bromopyr-

idine, 1-bromonaphthalene, 2-bromonaphthalene, 4-diphenylamino-

benzaldehyde (DABA) were purchased from Sigma-Aldrich or Alfa

Aesar Co. Ltd. Tetrahydrofuran (THF) was distilled under N₂over

sodium benzophenoneketyl. All other reagents were used as received.

Silica gels (230−400 mesh) used for chromatography were purchased

from Sorbent Technology. The intermediate compounds and ligands

were characterized by ¹H NMR spectroscopy, while the Ir(III)

complexes were characterized by ¹H NMR, HRMS, and elemental

analysis. ¹H NMR spectra were measured on a Bruker 400 MHz

spectrometer using CDCl₃as the solvent, with tetramethylsilane

(TMS) as internal standard. ESI-HRMS analyses were performed on a

Waters Synapt G2-Si mass spectrometer. Elemental analyses were

carried out by NuMega Resonance Laboratories, Inc., in San Diego,

California. Powder X-ray diﬀraction (XRD) patterns of the samples

were collected on a Phillips X’Pert MPD powder X-ray diﬀractometer.

Synthetic routes for the synthesis of complexes 1−11 are provided

in Scheme 1. Compound 1,10-phenanthroline-5,6-dione (PhenDO)

was synthesized according to the literature procedure.⁷⁰The synthetic

scheme, experimental details, and ¹H NMR data for the other

precursors, i.e., 9-bromomethylphenanthrene, 9-bromomethylanthra-

cene, and 1-bromomethylpyrene, are given in Scheme S1. The Ir(III)

dimer [Ir(ppy)₂]₂Cl₂(ppy refers to 2-phenylpyridine (C^N) ligand)

was synthesized following the Nonoyama method.⁷¹The synthetic

details and characterization data for 1,10-phenanthroline-5,6-dione-6-

oxime (PhenNO), ligands L1−L4 and L6−L11, and complexes 1−11

are provided below.

PhenNO. Compounds 1,10-phenanthroline-5,6-dione (PhenDO)

(1.89 g, 9.0 mmol), NH₂OH·HCl (657 mg, 9.45 mmol), and 45 mL of

EtOH were added to a ﬂask. The mixture was heated to reﬂux for 3 h.

After cooling to rt, the solid was collected by ﬁltration. The crude

product was puriﬁed by recrystallization in ethanol to obtain yellow

powder (1.46 g, yield: 73%). ¹H NMR (400 MHz, d₆-DMSO): δ 9.51

(d, J = 8.0 Hz, 1H), 9.12 (dd, J₁= 4.8 Hz, J₂= 1.6 Hz, 1H), 8.92 (dd, J₁

= 4.8 Hz, J₂= 1.6 Hz, 1H), 8.64 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H),

7.92−7.88 (m, 2H).

General Procedure for the Synthesis of L1−L3 and L6−L11.

To a Schlenk tube were added PhenNO (135 mg, 0.6 mmol),

ArCH₂Br (1.0 mmol), triethylamine (TEA) (182 mg, 1.8 mmol), dry

THF (6 mL), and dry DMF (3 mL), and the mixture was degassed

with N₂for 30 min. Then the reaction mixture was stirred at 50 °C for

24 h. The mixture was poured into water and extracted with ether. The

organic layer was dried over Na₂SO₄. After removal of the solvent, the

residue was puriﬁed by column chromatography (silica gel, CH₂Cl₂/

MeOH = 100/1, v/v) twice to obtain the crude product. The crude

product was recrystallized with CH₂Cl₂/ether to obtain the pure

product.

Ligand L1 (crude): light yellow solid (82 mg, 40% yield). Because

of the poor solubility of the crude product, the light yellow solid

obtained was washed with CH₂Cl₂and ether and used for the next

step reaction without further puriﬁcation

1

Ligand L2: white solid (79 mg, 35% yield). H NMR (400 MHz,

CDCl₃): δ 9.23−9.21 (m, 2H), 8.92 (d, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H),

8.66 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H), 8.24−8.21 (m, 2H), 7.78−7.70

(m, 4H).

1

Ligand L3: white solid (70 mg, 39% yield). H NMR (400 MHz,

CDCl₃): δ 9.22−9.20 (m, 2H), 8.92 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H),

8.66 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H), 8.36−8.33 (m, 2H), 7.76−7.72

(m, 2H), 7.58−7.55 (m, 3H).

1

Ligand L6: white solid (57 mg, 32% yield). H NMR (400 MHz,

CDCl₃): δ 9.31−9.28 (m, 2H), 8.96 (dd, J₁= 8.0 Hz, J₂= 1.6 Hz, 1H),

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8.91 (dd, J₁= 4.4 Hz, J₂= 1.6 Hz, 2H), 8.74 (dd, J₁= 8.0 Hz, J₂= 1.6

Hz, 1H), 8.23 (dd, J₁= 4.4 Hz, J₂= 1.6 Hz, 2H), 7.85−7.81 (m, 2H).

(m, 3H), 6.90 (t, J = 7.6 Hz, 1H), 6.43 (d, J = 8.0 Hz, 2H). ESI-HRMS

(m/z): calcd for [C₄₁H₂₇IrN₅O]⁺, 798.1847; found, 798.1847.

Elemental analysis calcd (%) for C₄₁H₂₇IrN₅OPF₆·0.2CH₂Cl₂: C,

51.55; H, 2.88; N, 7.30. Found: C, 51.25; H, 2.54; N, 7.00.

1

Ligand L7: white solid (68 mg, 33% yield). H NMR (400 MHz,

CDCl₃): δ 9.65 (d, J = 8.4 Hz, 1H), 9.31−9.29 (m, 2H), 9.11 (dd, J₁=

8.0 Hz, J₂= 1.6 Hz, 1H), 8.78 (dd, J₁= 8.0 Hz, J₂= 1.6 Hz, 1H), 8.60

(dd, J₁= 7.2 Hz, J₂= 1.2 Hz, 1H), 8.12 (d, J = 8.4 Hz, 1H), 8.02 (d, J =

8.4 Hz, 1H), 7.87−7.79 (m, 3H), 7.73−7.66 (m, 2H).

1

Complex 4: orange-red solid (57 mg, 51% yield). H NMR (400

MHz, CDCl₃): δ 9.16 (d, J = 8.0 Hz, 1H), 8.99 (d, J = 8.0 Hz, 1H),

8.31 (dd, J₁= 13.6 Hz, J₂= 4.0 Hz, 2H), 8.19 (d, J = 8.8 Hz, 2H),

7.98−7.87 (m, 4H), 7.75−7.71 (m, 4H), 7.51 (d, J = 5.6 Hz, 1H), 7.38

(t, J = 7.6 Hz, 5H), 7.25−6.98 (m, 13H), 6.90 (t, J = 6.0 Hz, 1H), 6.43

(d, J = 7.6 Hz, 2H). ESI-HRMS (m/z): calcd for [C₅₃H₃₆IrN₆O]⁺,

965.2584; found, 965.2579. Elemental analysis calcd (%) for

C₅₃H₃₆IrN₆OPF₆·2.5H₂O·0.5C₆H₁₄: C, 56.13; H, 4.04; N, 7.01.

Found: C, 56.25; H, 4.41; N, 7.14.

1

Ligand L8: white solid (73 mg, 35% yield). H NMR (400 MHz,

CDCl₃): δ 9.29−9.27 (m, 2H), 9.03 (dd, J₁= 8.0 Hz, J₂= 1.6 Hz, 1H),

8.92 (s, 1H), 8.80 (dd, J₁= 8.0 Hz, J₂= 1.6 Hz, 1H), 8.47 (dd, J₁= 8.8

Hz, J₂= 2.0 Hz, 1H), 8.10−8.07 (m, 2H), 7.98−7.96 (m, 1H), 7.85−

7.81 (m, 2H), 7.66−7.63 (m, 2H).

1

Ligand L9: white solid (62 mg, 26% isolated yield). H NMR (400

Complex 6: orange solid (56 mg, 59% yield). ¹H NMR (400 MHz,

CDCl₃): δ 9.20 (d, J₁= 8.4 Hz, J₂= 1.6 Hz, 1H), 9.09 (d, J = 8.0 Hz,

1H), 8.93 (d, J = 6.4 Hz, 2H), 8.38 (t, J = 6.0 Hz, 2H), 8.26 (d, J = 6.0

Hz, 2H), 8.02−7.92 (m, 4H), 7.76−7.71 (m, 4H), 7.52 (d, J = 6.0 Hz,

1H), 7.41 (d, J = 6.0 Hz, 1H), 7.12 (t, J = 7.2 Hz, 2H), 7.04−7.00 (m,

3H), 6.91 (t, J = 7.2 Hz, 1H), 6.42 (dd, J₁= 7.6 Hz, J₂= 3.6 Hz, 2H).

ESI-HRMS (m/z): calcd for [C₄₀H₂₆IrN₆O]⁺, 799.1799; found,

799.1815. Elemental analysis calcd (%) for C₄₀H₂₆IrN₆OPF₆: C,

50.90; H, 2.78; N, 8.90. Found: C, 50.73; H, 2.90; N, 8.79.

MHz, CDCl₃): δ 9.65 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H), 9.26−9.23

(m, 2H), 9.04 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H), 8.81−8.72 (m, 4H),

8.07 (d, J = 8.4 Hz, 1H), 7.82−7.68 (m, 6H).

1

Ligand L10: light yellow solid (31 mg, 13% yield). H NMR (400

MHz, CDCl₃): δ 9.30−9.26 (m, 2H), 9.03 (dd, 1H), 8.71 (s, 1H), 8.66

(dd, 1H), 8.15−8.10 (m, 4H), 7.82−7.73 (m, 2H), 7.55−7.51 (m,

4H).

1

Ligand L11: greenish-yellow solid (56 mg, 22% yield). H NMR

(400 MHz, CDCl₃): δ 9.94 (d, J = 9.6 Hz, 1H), 9.28−9.25 (m, 2H),

9.12 (d, J = 9.6 Hz, 1H), 9.01 (d, J = 8.4 Hz, 1H), 8.82−8.79 (m, 1H),

8.36 (d, J = 8.4 Hz, 1H), 8.35−8.28 (m, 3H), 8.21−8.08 (m, 3H),

7.85−7.81 (m, 2H).

1

Complex 7: light yellow solid (52 mg, 52% yield). H NMR (400

MHz, CDCl₃): δ 9.56 (d, J = 8.4 Hz, 1H), 9.32 (dd, J₁= 8.0 Hz, J₂=

1.2 Hz, 1H), 9.11 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H), 8.65 (dd, J₁= 7.2

Hz, J₂= 1.2 Hz, 1H), 8.39−8.35 (m, 2H), 8.15 (d, J = 8.0 Hz, 1H),

8.04−7.93 (m, 5H), 7.82−7.65 (m, 7H), 7.54 (d, J = 4.8 Hz, 1H), 7.43

(d, J = 4.8 Hz, 1H), 7.13 (t, J = 7.6 Hz, 2H), 7.05−7.70 (m, 3H), 6.91

(td, J₁= 7.6 Hz, J₂= 1.2 Hz, 1H), 6.45 (d, J = 7.6 Hz, 2H). ESI-HRMS

(m/z): calcd for [C₄₅H₂₉IrN₅O]⁺, 848.2004; found, 848.2000.

Elemental analysis calcd (%) for C₄₅H₂₉IrN₅OPF₆: C, 54.43; H,

2.94; N, 7.05. Found: C, 54.04; H, 2.81; N, 7.06.

Synthesis of Ligand L4. Compounds 4-diphenylaminobenzalde-

hyde (164 mg, 0.6 mmol), phenanthrolinedione (126 mg, 0.6 mmol),

ammonium acetate (462 mg, 6 mmol), and glacial acetic acid (15 mL)

were added to a Schlenk tube. The mixture was degassed with nitrogen

for 30 min and then reacted at 110 °C overnight. After cooling to rt,

the mixture was poured into water and neutralized with NH₃·H₂O.

The mixture was extracted with CH₂Cl₂, and the organic layer was

dried over MgSO₄. The crude product was puriﬁed by column

chromatography (silica gel, CH₂Cl₂/MeOH = 50:1, v/v) twice to

aﬀord orange solid 27 mg, yield: 10%. ¹H NMR (400 MHz, CDCl₃): δ

9.28−9.21 (m, 2H), 8.93−8.90 (d, J = 8.4 Hz, 1H), 8.63 (d, J = 8.4 Hz,

1H), 8.17−8.13 (m, 2H), 7.76−7.70 (m, 2H), 7.34−7.30 (m, 4H),

7.20−7.11 (m, 8H).

General Procedure for the Synthesis of Complexes 1−4 and

6−11. To a stirred solution of L1−L11 (except for L5) (0.1 mmol)

and [Ir(ppy)₂]₂Cl₂(54 mg, 0.05 mmol) in degassed CH₂Cl₂(30 mL)

and methanol (15 mL) was added AgSO₃CF₃(26 mg, 0.1 mmol). The

mixture was reﬂuxed overnight under nitrogen. After cooling to rt,

NH₄PF₆(163 mg, 1.0 mmol) was added. The mixture was stirred for

another 2 h. After removal of the solvent, the residue was puriﬁed by

column chromatography (silica gel, CH₂Cl₂/MeOH = 100:1−50:1, v/

v) twice to aﬀord the pure product.

1

Complex 8: light yellow solid (57 mg, 57% yield). H NMR (400

MHz, CDCl₃): δ 9.17 (dd, J₁= 8.4 Hz, J₂= 1.2 Hz, 1H), 9.09 (dd, J₁=

8.4 Hz, J₂= 1.2 Hz, 1H), 8.93 (s, 1H), 8.40 (dd, J₁= 8.4 Hz, J₂= 1.6

Hz, 1H), 8.33−8.29 (m, 2H), 8.07−7.96 (m, 3H), 7.92−7.86 (m, 4H),

7.72−7.67 (m, 4H), 7.61−7.57 (m, 2H), 7.50−7.49 (m, 1H), 7.37−

7.35 (m, 1H), 7.10−7.06 (m, 2H), 7.00−6.94 (m, 3H), 6.86 (td, J₁=

8.0 Hz, J₂= 1.2 Hz, 1H), 6.39 (d, J = 7.6 Hz, 2H). ESI-HRMS (m/z):

calcd for [C₄₅H₂₉IrN₅O]⁺, 848.2004; found, 848.1999. Elemental

analysis calcd (%) for C₄₅H₂₉IrN₅OPF₆·0.5H₂O: C, 53.94; H, 3.02; N,

6.99. Found: C, 53.79; H, 3.08; N, 7.04.

1

Complex 9: light yellow solid (51 mg, 49% yield). H NMR (400

MHz, CDCl₃): δ 9.63 (d, J = 8.0 Hz, 1H), 9.33 (d, J = 8.4 Hz, 1H),

9.21 (d, J = 8.4 Hz, 1H), 8.98 (s, 1H), 8.85 (d, J = 8.0 Hz, 1H), 8.77

(d, J = 8.0 Hz, 1H), 8.38 (t, J = 6.0 Hz, 2H), 8.19 (d, J = 8.0 Hz, 1H),

8.07−8.04 (m, 1H), 7.98−7.93 (m, 3H), 7.86−7.70 (m, 8H), 7.57 (d, J

= 5.6 Hz, 1H), 7.44 (d, J = 5.6 Hz, 1H), 7.14 (t, J = 7.6 Hz, 2H),

7.06−7.00 (m, 3H), 6.92 (t, J = 6.8 Hz, 1H), 6.45 (d, J = 7.2 Hz, 2H).

ESI-HRMS (m/z): calcd for [C₄₉H₃₁IrN₅O]⁺, 898.2161; found,

898.2180. Elemental analysis calcd (%) for C₄₉H₃₁IrN₅OPF₆·

1.8H₂O: C, 54.73; H, 3.24; N, 6.51. Found: C, 54.60; H, 3.10; N, 6.90.

Complex 1: orange solid (34 mg, 35% yield). ¹H NMR (400 MHz,

CDCl₃): δ 9.18 (dd, J₁= 8.3 Hz, J₂= 1.4 Hz, 1H), 9.12 (dd, J₁= 8.3

Hz, J₂= 1.4 Hz, 1H), 8.65−8.55 (m, 2H), 8.48−8.30 (m, 4H), 8.01−

7.91 (m, 4H), 7.81−7.69 (m, 4H), 7.52 (d, J = 5.2 Hz, 1H), 7.43 (d, J

= 5.2 Hz, 1H), 7.17−7.08 (m, 2H), 7.08−6.98 (m, 3H), 6.96−6.88 (m,

1H), 6.43 (dd, J₁= 8.2 Hz, J₂= 1.6 Hz, 2H). ESI-HRMS (m/z): calcd

for [C₄₁H₂₆IrN₆O₃]⁺, 843.1698; found, 843.1699. Elemental analysis

calcd (%) for C₄₁H₂₆IrN₆O₃PF₆: C, 49.85; H, 2.65; N, 8.51. Found: C,

49.88; H, 3.00; N, 8.56.

1

Complex 10: light yellow solid (44 mg, 42% yield). H NMR (400

MHz, d₆-DMSO): δ 9.35 (d, J = 8.8 Hz, 1H), 9.19 (d, J = 8.0 Hz, 1H),

9.09 (s, 1H), 8.36−8.31 (m, 5H), 8.25−8.16 (m, 2H), 8.10 (d, J = 8.0

Hz, 2H), 8.02−7.92 (m, 5H), 7.70−7.59 (m, 6H), 7.12−6.98 (m, 6H),

6.34 (d, J = 5.6 Hz, 2H). ESI-HRMS (m/z): calcd for [C₄₉H₃₁IrN₅O]⁺,

898.2161; found, 898.2182. Elemental analysis calcd (%) for

C₄₉H₃₁IrN₅OPF₆·0.45CH₂Cl₂·1.5H₂O: C, 53.59; H, 3.17; N, 6.32.

Found: C, 53.92; H, 3.57; N, 6.65.

Complex 2: orange solid (58 mg, 57% yield). ¹H NMR (400 MHz,

CDCl₃): δ 9.17 (dd, J₁= 8.4 Hz, J₂= 1.2 Hz, 1H), 9.06 (dd, J₁= 8.4

Hz, J₂= 1.2 Hz, 1H), 8.36−8.28 (m, 4H), 8.01−7.90 (m, 4H), 7.78−

7.71 (m, 6H), 7.51 (d, J = 5.6 Hz, 1H), 7.40 (d, J = 5.6 Hz, 1H), 7.12

(t, J = 7.6 Hz, 2H), 7.04−6.98 (m, 3H), 6.89 (t, J = 7.6 Hz, 1H), 6.43

(d, J = 8.0 Hz, 2H). ESI-HRMS (m/z): calcd for [C₄₁H₂₆BrIrN₅O]⁺,

876.0936; found, 876.0939. Elemental analysis calcd (%) for

C₄₁H₂₆BrIrN₅OPF₆: C, 48.20; H, 2.56; N, 6.85. Found: C, 48.32; H,

2.43; N, 6.62.

Complex 11: yellow solid (49 mg, 45% yield). ¹H NMR (400 MHz,

CDCl₃): δ 9.87 (d, J = 9.2 Hz, 1H), 9.35 (d, J = 8.4 Hz, 1H), 9.14 (d, J

= 8.0 Hz, 1H), 9.03 (d, J = 8.4 Hz, 1H), 8.43−7.95 (m, 13H), 7.78−

7.73 (m, 4H), 7.57 (d, J = 5.6 Hz, 1H), 7.46 (d, J = 5.6 Hz, 1H), 7.14

(t, J = 7.6 Hz, 2H), 7.06−7.01 (m, 3H), 6.94 (t, J = 7.6 Hz, 1H), 6.46

(d, J = 7.6 Hz, 2H). ESI-HRMS (m/z): calcd for [C₅₁H₃₁IrN₅O]⁺,

922.2161; found, 922.2183. Elemental analysis calcd (%) for

C₅₁H₃₁IrN₅OPF₆·0.2CH₂Cl₂: C, 56.73; H, 2.92; N, 6.46. Found: C,

56.64; H, 2.85; N, 6.31.

Complex 3: orange solid (64 mg, 68% yield). ¹H NMR (400 MHz,

CDCl₃): δ 9.19 (dd, J₁= 8.0 Hz, J₂= 1.2 Hz, 1H), 9.05 (dd, J₁= 8.0

Hz, J₂= 1.2 Hz, 1H), 8.43−8.41 (m, 2H), 8.36−8.32 (m, 2H), 8.02−

7.90 (m, 4H), 7.76−7.73 (m, 4H), 7.64−7.62 (m, 3H), 7.52 (d, J = 5.2

Hz, 1H), 7.40 (d, J = 5.2 Hz, 1H), 7.12 (t, J = 7.6 Hz, 2H), 7.02−7.00

D

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Complex 5. To a Schlenk tube were added complex 1 (39 mg,

0.039 mmol), Pd/C (10%, 10 mg), and absolute ethanol (6 mL)under

N₂atmosphere. Then hydrazine monohydrate (1 mL) was added

dropwise and the mixture was reﬂuxed overnight under N₂

atmosphere. After cooling to rt, the solvent was removed and the

residue was puriﬁed by column chromatography (Al₂O₃, EtOAc) to

Both DFT and TDDFT calculations were performed in solvent

utilizing the conductor-like polarizable continuum model

(CPCM)^101−103implemented in Gaussian09. Phosphorescence

calculations were performed using the COSMO continuum solvation

model^104,105applying two-time-scale model of solvent polarization,¹⁰⁶

which accounts for the nonequilibrium eﬀect of slow component of

reaction ﬁeld of solvent, as implemented in the NWChem software

package. Acetonitrile (CH₃CN, ε_r= 37.5) was chosen as the solvent

medium for consistency with the experiments. A considerable

agreement was observed in the proﬁles of simulated spectra and the

experimental UV−vis spectra, especially at the low-energy absorption

region, indicating the validity of our computational approach for

simulating optical spectra of the considered complexes.

1

aﬀord pale yellow solid (17 mg, yield: 45%). H NMR (400 MHz,

CDCl₃): δ 9.14 (dd, J₁= 8.3 Hz, J₂= 1.5 Hz, 1H), 8.97 (dd, J₁= 8.4

Hz, J₂= 1.4 Hz, 1H), 8.30 (ddd, J₁= 15.1 Hz, J₂= 5.0 Hz, J₃= 1.4 Hz,

2H), 8.17 (d, J = 8.7 Hz, 2H), 7.95−7.87 (m, 4H), 7.74−7.71 (m,

4H), 7.49 (d, J = 4.8 Hz, 1H), 7.38 (d, J = 6.0 Hz, 1H), 7.11 (t, J = 7.6

Hz, 2H), 7.00−6.91 (m, 3H), 6.89 (t, J = 6.6 Hz, 1H), 6.81 (d, J = 8.8

Hz, 2H), 6.41 (d, J = 6.7 Hz, 2H), 4.20 (s, 2H). ESI-HRMS (m/z):

calcd for [C₄₁H₂₈IrN₆O]⁺, 813.1956; found, 813.1954. Elemental

analysis calcd (%) for C₄₁H₂₈IrN₆OPF₆·0.5CH₂Cl₂: C, 49.83; H, 2.92;

N, 8.40. Found: C, 49.92; H, 2.73; N, 8.46.

X-ray Crystallographic Analysis. Single crystals of complexes 3

and 7 were obtained by slow diﬀusion of diethyl ether into their dilute

dichloromethane solutions. Single crystal X-ray diﬀraction data of 3

(CCDC 1457721) and 7 (CCDC 1509624) were collected on a

Bruker Apex Duo diﬀractometer with an Apex 2 CCD area detector at

T = 296 and 101 K, respectively. Mo radiation was used. All structures

were processed with an Apex 2 v2010.9-1 software package (SAINT

v.7.68A, XSHELL v.6.3.1).⁷²⁻⁷⁴A direct method was used to solve the

structure after multiscan absorption corrections. Details of data

collection and reﬁnement are given in Table S1.

Natural transition orbital (NTO)¹⁰⁷analysis was performed using

Gaussian09 software to obtain the hole−electron pairs that correspond

to each absorption frequency, where a hole−electron transition from a

ground state to an excited state could be realized through unitary

transformation of transition density matrix of a speciﬁc excited state.¹⁰⁷

For visualizing the lowest-energy emitting state, we plotted the

dominant molecular orbitals by performing the eigenvector analysis on

the lowest excited state. Chemcraft-1.7 software¹⁰⁸was used for

plotting the ground- and excited-state charge densities by setting the

isovalue as 0.02.

Nonlinear Transmission Experiment. The nonlinear trans-

mission experiments at 532 nm for complexes 1−11 were carried

out in acetonitrile solutions in a 2 mm cuvette using 4.1 ns laser pulses.

The linear transmission of 1−11 in acetonitrile was adjusted to 90% in

the 2 mm cuvette at 532 nm. A Quantel Brilliant ns laser with a

repetition rate of 10 Hz was used as the light source. The experimental

setup and details were described previously.¹⁰⁹The beam radius at the

focal point was approximately 96 μm.

Photophysical Measurements. All the solvents used for

photophysical studies were spectrophotometric grade and purchased

from Alfa Aesar Co. Ltd. A Cary 50 spectrophotometer and an

HORIBA Fluoro-Max 4 ﬂuorescence spectrophotometer were used to

measure the UV−vis absorption and emission spectra, respectively.

The emission quantum yields were determined by the relative

actinometry method⁷⁵in degassed solutions, in which a degassed

CH₃CN solution of [Ru(bpy)₃]Cl₂(Φ_em= 0.097, λ_ex= 436 nm)⁷⁶was

used as the reference for 1−11. The nanosecond transient diﬀerence

absorption (TA) spectra and decays were measured in degassed

CH₃CN solutions on an Edinburgh LP920 laser ﬂash photolysis

spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser

(Quantel Brilliant, pulse width = 4.1 ns, repetition rate = 1 Hz) was

used as the excitation source. Each sample was purged with argon for

45 min prior to measurement.

RESULTS AND DISCUSSION

■

Synthesis and Characterization. Although many meth-

ods have been developed for synthesizing 2-arylbenzoxazoles,

the synthetic methods for 2-aryloxazolo[4,5-f ][1,10]-

phenanthrolines are extremely rare due to the diﬃculties in

synthesizing the required phenanthroline precursors.¹¹⁰The

classical methods utilized for the synthesis of benzoxazoles are

the condensation of 2-aminophenol with either aldehyde or

carboxylic acid followed by oxidative cyclization.¹¹¹Thus,

obtaining 6-amino-1,10-phenanthrolin-5-ol becomes the key for

the synthesis of 2-aryloxazolo[4,5-f ][1,10]phenanthrolines

based on these methods. Unfortunately, despite many attempts,

it has not proven possible to obtain 6-amino-1,10-phenan-

throlin-5-ol via any of the methods depicted in Scheme S2.

Therefore, considerable eﬀorts had been switched to synthesize

the 2-aryloxazolo[4,5-f ][1,10]phenanthrolines via cross-cou-

pling reaction between 5,6-dibromophenanthroline and benza-

mide, followed by intramolecular cyclization (see route 5 in

Scheme S2) in the presence of transition metal catalysts, such

as Pd(OAc)₂, Pd₂(dba)₃, CuI, CuCl₂, and CuO. However, none

of these eﬀorts led to the desired compound after dozens of

experiments. Finally, by modiﬁcation of the procedure reported

by Katritzky for synthesizing 2-substituted benzoxazoles via

cyclization of α-oxo-oximes,¹¹²we succeeded in obtaining 2-

phenyloxazolo[4,5-f][1,10]phenanthroline (ligand L3) via a

condensation reaction between 1,10-phenanthroline-5-oxo-6-

oxime and phenylmethyl bromide in dry THF/DMF in the

presence of triethylamine under N₂atmosphere. The same

procedure was then followed to synthesize the other 2-

aryloxazolo[4,5-f ][1,10]phenanthroline ligands (except for L5)

with the corresponding arylmethyl bromides and 1,10-

phenanthroline-5-oxo-6-oxime. The yields of the reactions

were in the range of 13−39%, due to the formation of probable

ring-opening byproducts 6-(arylmethyl)imino-1,10-phenan-

Singlet depletion method⁷⁷was used to determine the triplet

excited-state molar extinction coeﬃcients (ε_T1−Tn) at the TA band

maximum. After the ε_T1−Tnvalue was obtained, the triplet excited-state

quantum yield was calculated by the relative actinometry,⁷⁸in which

SiNc in benzene was used as the reference (ε₅₉₀= 70 000 M⁻¹cm⁻¹,

Φ_T= 0.20).⁷⁹

Computational Methods. The ground-state geometry optimiza-

tion and absorption spectra calculations were performed using the

Gaussian09 software package.⁸⁰The ideal octahedral geometries of

complexes 1−11 were optimized in their singlet and triplet spin

conﬁgurations by density functional theory (DFT) using the hybrid

Perdew, Burke, and Ernzerhof functional (PBE1PBE)⁸¹⁻⁸³and the

LANL2DZ basis set⁸⁴⁻⁸⁶assigned for Ir and Br atoms and 6-31G*

basis set⁸⁷⁻⁹¹for all the remaining atoms. The absorption spectra were

calculated using linear-response time dependent DFT (TDDFT)^92,93

by iteratively solving the eigenvalue equation using the Davidson

algorithm.⁹⁴⁻⁹⁷For excited-state calculations, the density functional

and basis sets were chosen being the same as those for the ground-

state calculations. A total of 80 excited states were calculated with their

corresponding transition energies and oscillator strengths. The proﬁle

of absorption spectra was plotted based on inhomogeneous Gaussian

line broadening of 0.1 eV to ﬁt the proﬁle of the experimental spectra

at room temperature.

The phosphorescence energies were calculated using the ΔSCF

approach,⁹⁸where the lowest triplet ground-state geometry was

optimized using an unrestricted DFT method, and then the lowest

triplet excitation energy was calculated through a combined scalar

relativistic ZORA⁹⁹and TDDFT approach using NWChem

software.¹⁰⁰

E

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Figure 1. (a) Perspective view of 3a with the thermal ellipsoids drawn at the 50% probability level and the H atoms omitted for clarity. (b)

Schematic diagram of the asymmetric unit for 3. (c) Molecular stacking in crystals of 3: green dotted lines, π···π interactions; red dotted lines,

intermolecular C−H···F interactions.

Figure 2. (a) Schematic diagram of the overlay of the two diastereomers of 7 in the unit cell. PF₆anions are omitted for clarity. (b) Perspective view

of 7 with the thermal ellipsoids drawn at the 50% probability level and the H atoms removed for clarity. (c) Molecular stacking in crystals of 7: green

dotted lines indicate the π···π interactions.

throlin-5-ones. The structures of L2−L4 and L6−L11 were

characterized by ¹H NMR. Because of the poor solubility of L1,

it was unable to be puriﬁed via column chromatography or

recrystallization. Therefore, the crude product of L1 was

washed with CH₂Cl₂and ether and used for the complexation

reaction without further puriﬁcation.

Complexes 1−4 and 6−11 were obtained via reaction of the

corresponding 2-aryloxazolo[4,5-f ][1,10]phenanthroline ligand

with the [Ir(ppy)₂Cl]₂dimer in reﬂuxing CH₃OH/CH₂Cl₂as

we reported before for the synthesis of other heteroleptic

cationic Ir(III) complexes,^21,24whereas complex 5 was obtained

by reduction of the NO₂group in complex 1. The structures of

1−11 were characterized by ¹H NMR spectroscopy (see

Figures S1 −S11), elemental analyses, and high resolution mass

spectrometry. The structures of the complexes were further

conﬁrmed by X-ray single crystal structural analysis of

complexes 3 and 7 (Figures 1 and 2, vide infra).

Geometry Optimization. The geometries of 1−11 were

optimized in CH₃CN using DFT (see Figures S12 and S13).

The optimized geometries of complexes 1−3, 5, 6, and 8

displayed almost coplanar orientation between the 2-aryl

substituents and the oxazolo[4,5-f ][1,10]phenanthroline

motif. For complex 4, while the 2-phenyl ring was coplanar

with the oxazolo[4,5-f ][1,10]phenanthroline motif, the pro-

peller shaped diphenylamino substituent was twisted from the

2-phenyl ring with dihedral angles of ca. 47°. In contrast, upon

geometry optimization of complexes 7 and 9−11, the 2-aryl

substituents (i.e., naphthalen-1-yl, phenanthren-9-yl, anthracen-

9-yl, and pyren-1-yl) were twisted from oxazolo[4,5-f][1,10]-

phenanthroline by angles of ca. 16−50°. The dihedral angle in

7 (25.2°) is smaller than that obtained from the single crystal

structure (50.9°, vide infra). This diﬀerence could be attributed

to the diﬀerent temperatures used for the calculation (0 K) and

for the crystallography (100 K) and the diﬀerent forms (i.e.,

calculation was done in CH₃CN solution while the single

crystal structure was obtained in the solid state). Additionally,

intermolecular interactions in crystals, which are absent in

calculations of an isolated molecule, may also be responsible for

this diﬀerence in dihedral angle. Overall, calculations show that

phenyl and its derivatives adopt an almost coplanar structure

with the oxazolo[4,5-f ][1,10]phenanthroline motif, while the

polycyclic arene (except for naphthalen-2-yl) substituents are

twisted away from the oxazolo[4,5-f ][1,10]phenanthroline

motif.

X-ray Crystallography. Single crystals suitable for X-ray

crystallographic analysis of complexes 3 and 7 were obtained by

diﬀusing diethyl ether into dilute CH₂Cl₂solutions of the

complexes, and their crystal structures were determined at 100

K. Complexes 3 and 7 crystallized in the triclinic P-1 and

monoclinic C1 2/c 1 space group, respectively. The details of

the crystallographic data are summarized in Table S1. The

obtained crystal structures and packing modes of 3 and 7 are

shown in Figures 1 and 2, respectively. The selected bond

lengths and angles are compiled in Tables S2 −S4.

Crystal structural analysis of complex 3 revealed that the unit

cell contained two independent host molecules with distinct

conformations (referred to as 3a and 3b, see Figures 1b and 1c

and Table S2). Each of the iridium ions is octahedrally

coordinated by one N^N ligand and two C^N ligands, with

dihedral angles of ca. 77.6−97.3°. However, in conformer 3a,

F

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Figure 3. Experimental UV−vis absorption spectra of 1−6 (a) and 7−11 (b) in CH₃CN. The insets in panels a and b show the expanded

experimental spectra beyond 500 nm.

Table 1. Experimental Photophysical Data for 1−11

b

e

λ_em/nm

(τ_em/μs);

_em, rt

λ_em/nm

λ

nm ,

rt

/

^emf

g

(τ_em/μs),

λ_T1−Tn/nm (ε_T1−Tn/10⁴

a

c

d

λ_abs/nm (ε/10⁴L mol⁻¹cm⁻¹

280 (5.66, sh), 334 (3.76), 387 (1.97, sh), 432 (0.26, sh) 530 (−) ; −

)

Φ

k_r/s⁻¹

k_nr/s⁻¹

77 K

L mol⁻¹cm⁻¹, τ_T/μs); Φ_T

h

i

h

1

2

3

4

5

6

7

8

9

−

522 (306),

566 (250)

2.3 × 10⁶531 (5.59)

−

j

272 (5.25), 280 (5.19), 340 (2.00, sh), 374 (0.80),

390 (7.86), 438 (0.15, sh)

600 (0.36);

0.16

4.4 × 10⁵

5.7 × 10⁵

598 425 (− , 0.34), 590 (1.64,

0.34); 0.59

270 (4.59), 336 (1.43, sh), 377 (0.60), 392 (0.57), 432

(0.12, sh)

600 (0.37);

0.21

2.1 × 10⁶524 (6.37)

584 425 (−, 0.35), 600 (1.45,

0.36); 0.79

h

i

j

285 (3.88, sh), 390 (3.18), 460 (0.25, sh)

597 (−) ;

−

535 (220),

574 (188)

605

−

0.005

289 (4.61), 335 (3.89), 363 (3.24, sh), 450 (0.26, sh)

590 (0.03);

0.008

2.7 × 10⁵

5.0 × 10⁵

7.7 × 10⁴

4.9 × 10⁵

8.1 × 10⁴

1.8 × 10⁴

3.3 × 10⁷526 (94),

574 457 (2.94, 0.03); 0.23

564 (74)

2.8 × 10⁶527 (5.13)

j

270 (6.82), 331 (2.31, sh), 368 (0.94), 384 (1.00), 441

(0.14, sh)

602 (0.30),

0.15

608 420 (− , 0.34), 609 (2.95,

0.34); 0.45

285 (3.34), 336 (2.37), 349 (2.28), 394 (0.88), 432 (0.18, 600 (2.47);

3.3 × 10⁵550 (362),

568 539 (4.40, 2.59); 0.39

sh)

0.19

594 (280)

j

267 (8.26), 281 (6.25, sh), 316 (3.84), 334 (3.56), 352

(2.69, sh), 369 (1.33, sh), 395 (1.09), 426 (0.30, sh)

600 (0.37);

0.18

2.2 × 10⁶520 (334),

598 432 (− , 0.39), 590 (2.61,

560 (235)

0.39); 0.31

261 (8.45), 286 (4.58, sh), 336 (2.85), 372 (1.94, sh), 395 600 (2.47);

3.2 × 10⁵550 (2580),

601 435 (5.09, 2.38), 531

(4.37, 2.38); 0.37

(1.07, sh), 430 (0.18, sh)

0.20

597 (2110)

h

j

10 253 (10.38), 368 (1.15), 387 (1.27), 464 (0.18, sh)

590 (0.39);

0.007

2.5 × 10⁶555 (4.62),

−

432 (− , 24.4), 513 (3.57,

600 (3.89)

25.0); 0.58

i

h

658 (− ),

h

11 283 (6.80, sh), 382 (4.68), 398 (4.49), 415 (4.02), 448

548 (0.60);

−

483 (3.39, 28.5); 0.68

h

(0.30, sh)

−

721 (− )

a

b

Room temperature electronic absorption band maxima and molar extinction coeﬃcients in CH₃CN. Room temperature emission energy, lifetime,

c

d

and quantum yield in CH₃CN. Radiative decay rate in CH₃CN at room temperature. Nonradiative decay rate in CH₃CN at room temperature.

e

f

Emission energies and lifetimes at 77 K measured in 2-methyltetrahydrofuran glassy matrix, c = 1 × 10⁻⁵mol/L. Room temperature emission

g

energy in the solid state. ns TA band maxima, triplet extinction coeﬃcients, triplet excited-state lifetimes and quantum yield in toluene. The

h

i

j

reference used was SiNc in C₆H₆(ε₅₉₀= 70,000 L mol⁻¹cm⁻¹, Φ_T= 0.20). Signal was too weak to be detected. Cannot be calculated. The ground-

state absorption at these wavelengths could not be neglected; thus the singlet depletion method could not be used to estimate the ε_T1‑Tnvalues at

these wavelengths.

the 2-phenyl ring was almost coplanar (with a dihedral angle of

0.81°) with the oxazolo[4,5-f ][1,10]phenanthroline motif,

which is much smaller than the 11° dihedral angle reported

for the Ir(III) complex with 2-(4-triﬂuoromethyl)phenyl-

imidazole[4,5-f ][1,10]phenanthroline ligand.⁴²The length of

the bridging C−C bond (C61−C67) between the 2-phenyl

substituent and the oxazolo[4,5-f ][1,10]phenanthroline motif

was 1.465 Å. However, in conformer 3b, the 2-phenyloxazolo

part was slightly bent from the phenanthroline (the dihedral

angle is ca. 10.1°), with a shorter bond length of 1.430 Å for the

bridging C−C bond (C31−C32). The interactions between the

adjacent 3a and 3b molecules led to the formation of cross-

stacked dimers with C···O distance of 3.089 Å, C···N distance

of 3.205 Å, π···π distance of 3.290 Å, and C···C distance of

3.265 Å. Meanwhile, the relatively strong intermolecular C−

H···F interactions (3.060−3.350 Å) between the cationic

complex and its PF₆counteranion also played a very important

role in the crystal formation, especially for the formation of the

distorted conformer 3b. It is relatively rare that an organo-

metallic complex possesses two distinct conformations in one

crystal structure.

Figure 2a presents the cocrystal (1:1) of two diastereomers

of complex 7 in one unit cell. The coordination core

Ir(phen)(ppy)₂⁺was the same in both diastereomers. However,

the 2-(naphthalen-1-yl)oxazolo parts were mirror image to each

other in the diastereomers. In both isomers, the 2-(naphthalen-

1-yl) rings were twisted out of the oxazolo[4,5-f ][1,10]-

phenanthroline plane with a dihedral angle of 50.9°. The planar

oxazolo[4,5-f ][1,10]phenanthroline motifs in two adjacent

complexes were packed by π···π (3.400 Å) and C−H···O

interactions in a head-to-head fashion, while the PF₆anion

connected two face-to-face complexes by C−H···F interactions.

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Table 2. Natural Transition Orbitals (NTOs) Representing the Lowest Energy Transitions

All these interactions led to a tight diamond array along the b

axis. To the best of our knowledge, cocrystallization of two

diastereomers in one lattice is also rare in organometallic

complexes.

As shown in Figure 3, the spectra of all complexes possessed

intense absorption bands below 350 nm, moderately strong

absorption bands between 350 and 450 nm, and broad but

weak tails at ca. 450−570 nm. Based on the molar extinction

coeﬃcients, the minor solvatochromic eﬀect in diﬀerent

solvents (Figure S15), and the calculated natural transition

orbitals (NTOs, Table 2 and Tables S5 and S6), we attribute

the highest-energy absorption bands at <300 nm to the ppy

Electronic Absorption. The UV−vis absorption character-

istics of complexes 1−11 were studied in CH₃CN at diﬀerent

concentrations (5 × 10⁻⁶to 1 × 10⁻⁴mol L⁻¹). The absorption

obeyed the Beer’s law in the studied concentration range,

suggesting the absence of ground-state aggregation in this

concentration range. This phenomenon is consistent with that

reported for other Ir(III) complexes¹⁵⁻²⁵and is attributed to

the octahedral geometry around the Ir(III) ion, which reduces

the intermolecular interactions. The experimental UV−vis

absorption spectra of 1−11 in CH₃CN are presented in Figure

3, and the absorption band maxima and extinction coeﬃcients

are compiled in Table 1. The simulated UV−vis absorption

spectra via TDDFT calculations and the comparison to those of

the experimental spectra are provided in Figure S14. The

theoretical spectra overall reproduced the experimental spectral

features very well.

1

ligand localized π,π* transitions, and the strong absorption

bands between 300 and 350 nm predominantly to the N^N

and/or ppy ligand localized ¹π,π* transitions mixed with

signiﬁcant contributions from the ¹LLCT (π(ppy) →

π*(phen))/¹MLCT (d(Ir) → π*(phen))/¹ILCT (π(Ar) →

π*(phen)) transitions. For the absorption bands between 350

1

and 450 nm, ILCT/¹LLCT/¹MLCT transitions played the

major role, admixing with contributions from the ppy ligand

1

localized π,π* transitions. The ILCT transition became the

dominant contributor in complexes 3, 4, 5, and 7−11 that

either bear strongly electron-donating substituents at the 4-

position of the 2-aryl group or have the aromatic rings with

H

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Figure 4. Normalized emission spectra of (a) 1−11 in CH₃CN solutions and (b) 2−9 in the solid state at rt.

good π-donating ability. On the contrary, complexes 1, 2, and 6

containing electron-withdrawing substituents/group (NO₂, Br,

and pyridyl) had the ¹LLCT/¹MLCT as the major contributors

to their 350−450 nm bands. It should be noted that, for all

complexes, the lowest-energy singlet transition (S₁) is

Table 3. Emission Data for Complexes 1−11 in Diﬀerent

a

Solvents at Room Temperature

b

λ_em/nm (τ_em/ns, Φ_em

)

toluene/

10%

CH₂Cl₂

1

exclusively LLCT/¹MLCT (see NTOs in Table 2) in nature.

THF

CH₂Cl₂

acetone

acetonitrile

Both electron-donating substituents (NPh₂and NH₂in 4 and

5, respectively) and electron-withdrawing substituents (NO₂

and Br in 1 and 2, respectively) at the 4-position of the 2-aryl

group signiﬁcantly increased the molar extinction coeﬃcients of

the absorption bands compared to those in 3. Comparison of

the spectra of 3 to 6 revealed that the spectra of these two

complexes had similar shapes; however, the electron-deﬁcient

pyridyl ring caused a slight blue-shift of the absorption bands in

6 but increased the extinction coeﬃcients of the corresponding

bands.

In addition to the aforementioned absorption bands, all

complexes possessed very weak absorption tails at ca. 450−570

nm. In view of the small molar extinction coeﬃcients and with

reference to the other reported Ir(III) complexes,^{20,22,24,25,41,113}

this tail can be ascribed to the spin-forbidden transitions from

³LLCT/³MLCT.

1

2

3

4

5

6

7

8

9

601 (310, 604 (290,

595 (150, 532 (−,

530 (−,

c

0.16)

0.097)

0.084)

−)

600 (370, 600 (380, 0.22) 587 (670, 600 (340, 600 (360,

0.22) 0.45) 0.17) 0.16)

600 (400, 600 (410, 0.24) 584 (700, 600 (360, 600 (370,

0.30)

0.62)

582 (820, 595 (50,

0.47) 0.008)

589 (40, 0.016) 579 (780, 593 (10,

0.77) 0.007)

0.21)

597 (100, 592 (180,

0.054)

597 (−,

c

0.088)

0.005)

581 (10,

0.019)

590 (30,

0.008)

601 (330, 605 (330, 0.16) 592 (600, 605 (290, 602 (300,

0.19)

0.34)

585 (2900, 600 (1540, 600 (2470,

0.50) 0.18) 0.19)

0.11)

0.15)

600 (3770, 600 (1330,

0.32)

0.25)

600 (410, 600 (420, 0.21) 586 (720, 600 (367, 600 (370,

0.24)

0.48)

584 (3000, 600 (1570, 600 (2470,

0.55) 0.17) 0.20)

0.17)

0.18)

589 (4090, 596 (1590,

0.24)

0.32)

10 588 (390, 589 (600, 0.13) 580 (690, 590 (390, 590 (390,

Photoluminescence. The emission spectra of 1−11 were

investigated in diﬀerent solvents and in the solid state at rt, and

in MTHF glassy matrix at 77 K. Their normalized emission

spectra in CH₃CN at rt are presented in Figure 4, and the

emission data are summarized in Table 1. As shown in Figure

4a and Table 1, complexes 2−10 exhibited red emission with

the band maxima at ca. 600 nm in degassed CH₃CN at rt, along

with lifetimes spanning from tens of nanoseconds to several

microseconds (see Table 1). In contrast, the emission of

complexes 1 and 11 is blue-shifted. Considering the proneness

of emission toward oxygen quenching, the large Stokes shifts,

and the relatively long emission lifetimes, we assign the

emission of these complexes to phosphorescence. The emission

quantum yields of these complexes show that strongly electron-

withdrawing or -donating substituents in complexes 1, 4, and 5

and more π-expansive aryl substituents in complexes 10 and 11

either signiﬁcantly decreased the radiative decay rates (k_r), such

as in 10, or increased the nonradiative decay rates (k_nr), such as

in 5, which consequently decreased the emission quantum

yields in these complexes. The diﬀerent solvents also inﬂuenced

the emission characteristics pronouncedly (Table 3 and Figure

S16). The data presented in Table 3 clearly demonstrated that

the emission was much stronger in less polar solvents such as

toluene, CH₂Cl₂, and THF compared to that in more polar

solvents, such as acetone and acetonitrile. Especially the

0.004)

0.015)

0.008)

0.007)

11 541 (−,

585, (−, −),

665 (−, −),

577 (−,

548 (−,

548 (600,

c

−), 664

c

−), 664

c

−), 659

c

−)

c

(−, −)

730 (−, −)

(−, −)

a

b

The excitation wavelength was 436 nm. The reference used for the

quantum yield measurement was [Ru(bpy)₃]Cl₂in degassed CH₃CN

solution (Φ_em= 0.097, λ_ex= 436 nm). The emission signals were too

weak to allow for the lifetimes and quantum yields to be accurately

measured.

c

emission became the strongest in CH₂Cl₂. Similar to their

very weak emission in solutions, the emission of complexes 1,

10, and 11 in the solid state (Figure 2b) was nondetectable,

while the other complexes exhibited bright yellow to orange

emission.

It is worth noting that, in comparison to the reported Ir(III)

complex bearing the 2-phenylimidazole[4,5-f ][1,10]-

phenanthroline (N^N) ligand (i.e., complex 1 in ref 43), the

corresponding oxazole based complex 3 in this work possessed

a similar emission lifetime (ca. 700 ns in CH₂Cl₂) but much

stronger emission intensity, which is reﬂected by the much

larger emission quantum yield of 0.62 for complex 3 in this

work vs 0.24 for complex 1 in ref 43 in CH₂Cl₂solutions. In

addition, the emission energy of 3 (584 nm in CH₂Cl₂and 598

nm in CH₃CN) in this work is lower than that of 1 in ref 43

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Figure 5. (a) Normalized emission spectra of complex 7 in the solid state, after grinding, and after fuming with CH₂Cl₂. The inset shows the

emission photograph of the initial sample and ground sample of 7 under irradiation of 365 nm UV light. (b) XRD patterns of the corresponding

samples.

(568 nm in CH₂Cl₂and 587 nm in CH₃CN). The almost

identical emission lifetime for these two complexes suggests

that the emission of 3 likely emanates from the same emitting

state for 1 in ref 43, i.e., the ³MLCT/³LLCT state, which will be

discussed further in the following paragraphs. The red-shifted

emission band and enhanced emission of complex 3 in this

work should be attributed to the almost coplanar structure of

the phenyl ring with respect to the oxazolo[4,5-f][1,10]-

phenanthroline motif in complex 3, which increased the k_rin 3

(i.e., 8.9 × 10⁵s⁻¹for 3 vs 3.4 × 10⁵s⁻¹for 1 in ref 43) while

decreasing the k_nr(5.4 × 10⁵s⁻¹for 3 vs 1.1 × 10⁶s⁻¹for 1 in

ref 43). Complexes 1 and 11 exhibited anomalous solvent-

dependent emission energy compared to the minor solvato-

chromic eﬀects observed in complexes 2−10 (Figure S16). The

emission energies were drastically blue-shifted in more polar

solvents, such as CH₃CN and acetone, compared to those in

THF, CH₂Cl₂, and toluene. The excitation spectra of 1 in

CH₃CN and THF were diﬀerent (Figure S17). Thus, we

speculate that the emitting state of 1 in high polarity solvents is

diﬀerent from that in low to medium polarity solvents. This

speculation is partially conﬁrmed by the low temperature study

at 77 K (vide infra, Figures S18 and S19) and is fully supported

by the NTOs of the emitting states. Diﬀering from 1, which has

the emitting state switched in diﬀerent solvents, 11 appeared to

show dual emission in all of the solvents tested except for

CH₃CN, in which the emission was too weak to observe the

longer wavelength emission bands. The high-energy emission

band at <600 nm was structureless, while the low-energy

emission possessed somewhat vibronic structures with a clear

progression of 1340 cm⁻¹in THF. The long-wavelength

emission band(s) became the dominant one in THF. In view of

the diﬀerent features for these bands, we tentatively ascribe the

origin of the high-energy emission to a charge transfer excited

stabilized due to the lack of solvent reorganization in the rigid

frozen matrix. In contrast to these complexes, the ΔE_thvalue for

complex 1 is only ∼320 cm⁻¹, implying the ³π,π* nature of the

emitting state for this complex in CH₃CN. However, the ΔE_th

value was approximately 2600 cm⁻¹in MTHF for 1 (Figure

S19), suggesting a charge transfer nature of the emitting state in

MTHF. This ﬁnding supports our aforementioned assumption

that the dramatically diﬀerent emission energy of 1 in solvents

with diﬀerent polarities indeed emanates from diﬀerent

emitting states. Such a notion is further conﬁrmed by the

molecular orbitals corresponding to the emitting triplet excited

states of 1 in CH₃CN and THF (Table S7). For complex 11,

the ΔE_thvalue in MTHF was ∼180 cm⁻¹for the low-energy

emission band(s), which is in accordance with our presumed

³π,π* nature of this emission band. Interestingly, the emission

of 11 in BuCN glassy matrix at 77 K manifested a similar

energy and shape to that in MTHF glassy matrix, implying that,

3

at low temperature, only the low-energy π,π* state emit.

To unambiguously assign the nature of the emitting states in

1−11 and understand the aforementioned diﬀerent emission

characteristics of these complexes, TDDFT calculations of the

emission of 1−11 in CH₃CN were conducted, and the obtained

emission energies and molecular orbitals (MOs) corresponding

to the phosphorescent emitting states are provided in Table S7.

The emission of 1 in THF was also calculated to understand

the diﬀerent natures of the emitting state in diﬀerent solvents.

The calculated emission energies reproduced the experimental

emission energies very well, with 1 and 11 having higher

emission energies and the other complexes possessing similar

emission energies. Meanwhile, the MOs in Table S7 clearly

show the diﬀerent natures of the emitting states for these

complexes in CH₃CN and for 1 in diﬀerent solvents. In

CH₃CN, the electrons of 1 are delocalized on the ppy ligands,

the d-orbitals of the Ir(III) ion, and the 2-phenyloxazolo[4,5-

f ][1,10]phenanthroline, while the holes are exclusively on the

2-(4-NO₂-phenyl)oxazole component. Thus, the emitting state

of 1 in CH₃CN has the ³π,π*/³ILCT/³LLCT/³MLCT

parentage and led to emission at high energy (i.e., 530 nm).

In contrast, the emitting state became the ³LLCT/³MLCT state

in THF, resulting in emission at ∼600 nm. In complexes 2 and

6 that either have a weak electron-withdrawing Br substituent at

the phenyl ring or bear the electron deﬁcient pyridyl ring, the

3

state and the low-energy emission to the ligand-localized π,π*

state. The dual emission phenomenon has been reported for

Ru(II) complexes that contain the pyrenyl substituent on the

diimine ligand.¹¹⁴This notion is also supported by the 77 K

emission study discussed in the following paragraph.

The emission of all complexes except for 1 and 11 at 77 K in

butyronitrile glassy matrix was blue-shifted in comparison to

those in CH₃CN solutions at rt (see Figure S18) due to the

rigidochromic eﬀect.^115,116The thermally induced Stokes shifts

(ΔE_th) for 2−6 and 8 are ca. 2200 cm⁻¹, while for 7, 9, and 10

they are around 1300 cm⁻¹. The large ΔE_thvalues for these

complexes indicate charge transfer nature of the emitting states

for these complexes because charge transfer states are less

3

emitting states all have the LLCT/³MLCT nature mixed with

3

some π,π* and minor ILCT (in 6) character. The emitting

state of 3 has pure ³LLCT/³MLCT character; while in 4 and 5,

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Figure 6. Nanosecond TA spectra of (a) 2, 3, and 5 and (b) 6−11 in CH₃CN immediately after 355 nm excitation. A₃₅₅= 0.4 in a 1 cm cuvette.

which contain strongly electron-donating NPh₂or NH₂

substituents, the emitting states are exclusively ILCT within

crystallization−amorphization conversion upon external stim-

uli.

3

To rationalize the unique mechanochromic behavior that was

only observed in complex 7, but not in the other complexes, we

compared the crystal packing patterns of 7 and 3. The single

crystal packing pattern of 7 shown in Figure 2c revealed that

the molecules of 7 adopted a slipped head-to-head arrangement

with an intermolecular π−π distance of 3.40 Å between the two

phenanthroline rings on the two closest neighboring molecules,

resulting in a relatively weaker packing. Considering the

presence of two diastereomers in one unit cell, and the twisted

naphthalen-1-yl ring from the oxazolo[4,5-f ][1,10]-

phenanthroline motif, we infer that the molecular packing is

relatively loose with low lattice energy. Thus, the packing can

be readily modiﬁed by external mechanical grinding, resulting

in increased coplanarity between the naphthalen-1-yl ring and

the oxazolo[4,5-f ][1,10]phenanthroline motif. This improves

the π-conjugation and intermolecular interactions; both of

them can cause a bathochromic shift of the emission. In

contrast, complex 3 adopted a face-to-face packing in its single

crystal with a closer π−π distance of 3.29 Å between the two

phenanthroline rings on the two adjacent molecules. Con-

sequently, the packing in 3 was more stable due to stronger

intermolecular interactions. Therefore, unlike what was

observed in 7, the emission of 3 cannot be readily modiﬁed

upon applying external stimuli.

Transient Absorption. To further understand the

inﬂuence of the substituent at the oxazolo[4,5-f ][1,10]-

phenanthroline ligand on the triplet excited-state character-

istics, nanosecond transient absorption (TA) studies of

complexes 1−11 were carried out. The TA spectra of 2, 3,

and 5−11 recorded upon excitation at 355 nm in degassed

acetonitrile solutions are shown in Figure 6 and Figure S20.

The TA of 1 and 4 were too weak to be detected. The triplet

excited-state absorption band maxima, molar extinction

coeﬃcients, lifetimes, and quantum yields of 2, 3, and 5−11

in CH₃CN are compiled in Table 1.

the N^N ligands. For complexes 7−11, which contain π-

donating polycyclic aromatic rings, their emitting states have

predominant ³ILCT/³π,π* characters; while the contribution of

the pyrene-based ³π,π* character increased in 11. In complex 8,

³MLCT state appears to make important contribution to the

emission as well. Overall, electron-donating substituents or

polycyclic aromatic rings signiﬁcantly increased the ³ILCT

(π(donor) → π*(phen)) character in the emitting states; while

3

electron-withdrawing substituents induced more π,π*/³ILCT

(π(phen) → π*(acceptor)) character. Variations of the

emitting state parentage account for the observed diﬀerences

in emission characteristics in diﬀerent solvents and the varied

ΔE_thvalues.

Mechanochromic and Vapochromic Luminescence.

The mechanochromic eﬀect is a phenomenon by which the

color and/or the luminescence of a solid-state material changes

upon applying mechanical forces. The change in luminescence

could be energy or intensity. Such a phenomenon has potential

applications in mechanosensors, security printing, or optical

storage.^117−124Interestingly, the as-prepared pale yellow

powder of complex 7 exhibited bright yellow phosphorescence

(λ_max= 568 nm) under UV light irradiation. After grinding, an

orange powder with orange emission (λ_max= 609 nm) was

obtained (Figure 5a). The ground orange powder was readily

converted back to the initial yellow emission by exposure to

CH₂Cl₂, acetone, diethyl ether, methanol, or THF vapors for

several seconds to minutes, with CH₂Cl₂vapor giving the

fastest change. Evidently, complex 7 manifested a pronounced

mechanochromic eﬀect and vapochromic eﬀect, which can be

easily observed by naked eyes. Moreover, the convertibility of

the emission color change for complex 7 in CH₂Cl₂can be

repeated for many cycles, which is an important feature for

applications in sensing and security printing.

To understand the mechanochromic behavior of 7, a powder

X-ray diﬀraction (PXRD) study was carried out. The PXRD

patterns (Figure 5b) were consistent with the single-crystal

diﬀraction data of 7. The sharp and intense reﬂection peaks in

the simulated XRD patterns based on the single crystal X-ray

data represented the characteristic diﬀraction peaks of 7 in the

crystalline state. In contrast, the ground sample exhibited

weaker and broader diﬀraction signals, indicating an amorphous

state. After fuming the ground sample with CH₂Cl₂vapor, the

intense and sharp diﬀraction peaks were recovered. This implies

that the mechanochromic eﬀect of 7 emanates from the

Figure 6 shows that the TA spectra of 2, 3, and 5−10 possess

broad positive absorption bands from ca. 410 to 800 nm, while

the positive absorption band appears at 435−740 nm for 11.

The TA spectra of 2, 3, 6, and 8 exhibited similar shape and

intensity, and the triplet lifetimes deduced from the decay of

TA were similar to those obtained from the decay of emission.

These features suggest that the TA of 2, 3, 6, and 8 likely

originates from the same excited state that emits. Thus, the

transient absorbing excited states for 2, 3, 6, and 8 can be

ascribed predominantly to ³MLCT/³LLCT states for these four

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complexes. The undetectable TA of complexes 1 and 4 are

likely attributed to the rapid decay of the triplet excited states

(<10 ns), which exceeds the time resolution of our nanosecond

TA Instruments. Complex 5 exhibited positive absorption

bands in the same wavelength range of 410−800 nm as those

observed in complexes 2, 3, 6, and 8, but with quite diﬀerent

spectral feature(s). The TA decay proﬁle of 5 resembled that of

its emission decay proﬁle, implying that the TA of 5 arises from

its ³ILCT state. In contrast, the TA features of 7, 9, 10, and 11

are all distinct from each other and diﬀer from those of 2, 3, 5,

6, and 8 with respect to spectral shape, lifetime, and TA

intensity. The TA of 7, 9, 10, and 11 are all much stronger and

longer-lived than those of 2, 3, 5, 6, and 8. The TA lifetimes of

7 and 9 were similar to those obtained from emission, while the

TA lifetimes of 10 and 11 were one order of magnitude longer

than their corresponding emission lifetimes. Considering the

nature of the emitting states for these complexes, we attribute

material increases with increased incident ﬂuence. To achieve

RSA, the material is required to have a large ratio (>1) of the

excited-state absorption cross section (σ_ex) relative to that of

the ground-state absorption (σ₀) at the interested wavelengths.

Meanwhile, the excited-state lifetime needs to be longer than

the laser pulse width. For RSA of ns laser pulses, a high

quantum yield of triplet excited-state formation is also crucial.

Based on the broad and positive TA signals for these

complexes in the region of ca. 425−800 nm as discussed in the

TA section, we anticipate that these complexes exhibit RSA in

the visible spectral region upon ns laser irradiation. Therefore,

nonlinear transmission experiments at 532 nm using 4.1 ns

laser pulses were carried out for 1−11 in acetonitrile solutions

with a 90% linear transmission in a 2 mm cuvette at 532 nm.

Figure 7 displays the nonlinear transmission vs incident energy

3

the transient absorbing species in 7 and 9 to ILCT/³π,π* and

those for complexes 10 and 11 to the respective anthracenyl or

3

pyrenyl substituted N^N ligand based π,π* states due to their

much longer TA lifetimes. Assigning the nature of the long-

lived transient absorbing states in these two complexes to the

anthracene and pyrene based ³π,π* states was excluded because

the observed TA spectra of 10 and 11 are quite diﬀerent from

those reported for anthracene¹²⁵and pyrene.¹²⁶The diﬀerent

TA lifetimes and emission lifetimes in 10 and 11 also clearly

indicate the diﬀerent origins of the emitting and TA states.

Although this phenomenon is uncommon, it has been reported

in Ru(II),^127−131Pt(II),^132,133and other Ir(III)²⁵complexes

that contain large π-conjugated aromatic substituents like

pyrenyl group on the ligand(s), in which the complexes

demonstrated a high-lying, short-lived charge transfer emitting

state and a low-lying, long-lived nonemissive but strongly

absorbing ³π,π* state.^25,127−133This phenomenon suggests that

the anthracenyl or pyrenyl substituted N^N ligand based ³π,π*

states in complexes 10 and 11 are substantially lower than their

emitting states, thus thermal equilibration between these two

states is impeded, leading to the rapid population of the long-

Figure 7. Nonlinear transmission curves for complexes 1−11 in

acetonitrile at 532 nm in a 2 mm cuvette. The pulse duration was 4.1

ns, and the linear transmission of all solutions was 90% at 532 nm in a

2 mm cuvette. The radius of the beam waist at the linear focal plane

was approximately 96 μm.

curves for complexes 1−11. The transmission of the solutions

kept decreasing with the increase of the incident energy,

conﬁrming the occurrence of RSA in all complexes. The

strength of the RSA for these complexes followed this trend: 7

> 11 > 9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. Complexes 7, 9,

and 11 that possess larger π-conjugated aromatic substituents

displayed much stronger RSA than the complexes possessing

the phenyl substituent; and complex 1 with strongly electron-

withdrawing NO₂substituent exhibited the weakest RSA.

It appears that the RSA trend of 1−11 parallels the intensity

of the TA signal at zero decay at 532 nm, but opposes the

strength of the ground-state absorption at 532 nm (see the

inset in Figure 3). This is in accordance with the key parameter

determining the strength of RSA, i.e., the ratio of the excited-

state absorption cross section to that of the ground state (σ_ex/

σ₀). As presented in Table 4, complexes 7, 9, and 11 possess

very much larger excited-state absorption cross sections at 532

nm, but small ground-state absorption cross sections. Thus, the

σ_ex/σ₀ratios are much larger in these three complexes in

comparison to the other complexes. These ratios are also

among the largest values for Ir(III) complexes reported to

date.^15−21,25In complex 10, although its σ_exvalue at 532 nm is

the third largest, its much stronger ground-state absorption at

532 nm (actually the largest σ₀value among all complexes)

counteracts the strong excited-state absorption. Consequently,

the σ_ex/σ₀ratio for 10 is signiﬁcantly reduced and the RSA

became weaker. Among these complexes, complex 1 bearing

3

lived dark π,π* state instead.

Similar to the trend observed for emission, both the strongly

electron-withdrawing NO₂group and electron-donating NPh₂

and NH₂substituents on the oxazolo[4,5-f ][1,10]-

phenanthroline ligand reduced the triplet excited-state lifetime

of the complexes, likely due to the stronger charge transfer

character in the lowest triplet excited states of these two

3

complexes, i.e., stronger LLCT character in 1 and ILCT

character in 4 and 5 (see the molecular orbitals (MOs)

corresponding to the triplet emitting states of 1, 4, and 5 in

CH₃CN in Table S7). The reduced triplet lifetime and weak

phosphorescent emission are common features for Pt(II) and

Ir(III) complexes studied before that bear strongly electron-

donating NPh₂substituent on the ligands.^21,134,135On the

contrary, extending π-conjugation of the polycyclic aromatic

substituents via benzannulation at the 2,3-position of phenyl

ring in complexes 7, 9, 10, and 11 increased the triplet lifetimes

and the TA intensity. These variations could be related to the

increased steric hindrance between the aromatic substituents

and the oxazolo[4,5-f ][1,10]phenanthroline motif, which

changed the parentage of the lowest triplet excited states

from ³MLCT/³LLCT states to the N^N ligand based ³ILCT or

³π,π* state.

Reverse Saturable Absorption (RSA). RSA is one of the

nonlinear optical phenomena in which the absorptivity of the

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a

Table 4. Ground-State (σ₀) and Excited-State (σ_ex) Absorption Cross Sections of Complexes 1−11 in Acetonitrile at 532 nm

1

2

3

4

5

6

7

8

9

10

11

σ₀/10⁻¹⁹cm²

σ_ex/10⁻¹⁷cm²

σ_ex/σ₀

3.51

2.79

5.75

206

123

2.02

5.08

250

199

3.51

2.52

7.35

292

66

2.60

7.06

272

122

1.45

17.3

1193

470

3.55

8.67

244

75

2.82

16.7

590

218

8.60

12.5

145

85

3.32

11.2

337

228

b

Φ_Tσ_ex/σ₀

a

σ = 2303ε/N_A, where N_Ais Avogadro’s constant. ε₀is obtained from the UV−vis absorption spectra, ε_exis estimated from the ΔOD values at 532

nm and at the bleaching band minimum immediately after the laser excitation using the singlet depletion method.⁷⁷Due to the undetectable TA

b

signals, these values were unable to be estimated or calculated.

the strongly electron-withdrawing NO₂substituent exhibited

the weakest RSA, likely due to the short triplet lifetime and

stronger ground-state absorption. A general trend appeared that

the complexes possessing larger π-conjugated aromatic

substituents, such as those in 7, 9, and 11, displayed much

stronger RSA than the complexes possessing the phenyl

substituent. Considering the strong RSA of 7, 9, and 11 at

532 nm and their long triplet lifetimes, they are good

candidates for applications that require strong RSA at 532

nm. For complex 10, although its RSA is not the strongest at

532 nm, its broader weak ground-state absorption to 700 nm

and broad excited-state absorption in the visible to the near-IR

region, and its long-lived triplet excited state, make it a

potential good candidate as broadband optical limiting material.

strong reverse saturable absorption (RSA) at 532 nm for ns

laser pulses. The strength of RSA followed this trend: 7 > 11 >

9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. Again, complexes with π-

expansive substituents on the oxazolo[4,5-f ][1,10]-

phenanthroline ligand manifested much stronger RSA

compared to the complexes with phenyl or substituted phenyl

on the oxazolo[4,5-f ][1,10]phenanthroline ligand mainly due

to their much stronger excited-state absorption at 532 nm.

Therefore, these complexes could be good candidates for

photonic applications that require strong RSA at 532 nm, while

complex 10 with weak ground-state absorption extending to

700 nm but broad and strong excited-state absorption in the

visible to the near-IR region, and long-lived triplet excited state,

has the potential to be developed into a broadband optical

limiting material. In addition, the drastic mechanochromic and

vapochromic eﬀects revealed in complex 7 make it a promising

candidate for security printing or pressure sensing.

CONCLUSIONS

■

We have developed a new synthetic route for the synthesis of 2-

aryl-substituted oxazolo[4,5-f ][1,10]phenanthroline. A series of

cationic Ir(III) complexes utilizing these 2-aryloxazolo[4,5-

f][1,10]phenanthrolines (1−11) as the diimine ligands were

ASSOCIATED CONTENT

* Supporting Information

■

S

1

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.7b01472.

synthesized and characterized by H NMR, HRMS, elemental

analysis, and X-ray crystallography. Single crystal X-ray analysis

revealed that the 2-phenyl substituent adopted an almost

coplanar orientation with the oxazolo[4,5-f ][1,10]-

phenanthroline motif in complex 3. However, the polycyclic

aryl substituents, such as naphthalen-1-yl (7), phenanthren-9-yl

(9), anthracen-9-yl (10), and pyren-1-yl (11), still twisted away

from the oxazolo[4,5-f][1,10]phenanthroline plane based on

the single crystal structure of 7 and the DFT optimized

geometries of 9−11. The increased coplanarity did not exert

pronounced impact on the energies of the electronic transitions

in their UV−vis absorption spectra in comparison to their

corresponding complexes bearing the 2-phenylimidazole[4,5-

f][1,10]phenanthroline ligand. However, the increased copla-

narity signiﬁcantly improved the emission intensity both in

solution and in the solid state. Among all complexes, complex 7

showed not only intense emission in solutions but also drastic

mechanochromic and vapochromic eﬀects in the solid state.

Both the electron-withdrawing (NO₂) and electron-donating

(NPh₂and NH₂) substituents at the 4-position of the phenyl

ring dramatically decreased the emission intensity and reduced

the emission lifetimes in polar solvents like CH₃CN. In line

with this trend, all complexes except for 1 and 4, which contain

NO₂and NPh₂substituent, respectively, at the 4-position of the

phenyl ring, exhibited moderate to strong triplet excited-state

absorption in the visible to the near-IR spectral region; and

complexes bearing π-expansive substituents at the 2-position of

the oxazolo[4,5-f ][1,10]phenanthroline ligand possessed

strong excited-state absorption and longer lifetimes. Especially

the anthracen-9-yl substituent in 10 and pyren-1-yl substituent

in 11 made these two complexes possessing very long-lived

transient absorbing T₁excited states. All complexes exhibited

¹H NMR, UV−vis, emission, excitation, and TA spectra,

X-ray crystallographic data, optimized ground-state

geometries, NTOs, MOs, and the full author list for ref

80 (PDF)

Accession Codes

CCDC 1457721 and 1509624 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

*E-mail: Wenfang.Sun@ndsu.edu. Tel: 701-231-6254. Fax:

701-231-8831.

■

ORCID

Svetlana Kilina: 0000-0003-1350-2790

Wenfang Sun: 0000-0003-3608-611X

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

We would like to thank Dr. A. Ugrinov at North Dakota State

University for his help in the X-ray crystallographic measure-

ments. We acknowledge the ﬁnancial support from the

National Science Foundation (DMR-1411086 and CNS-

M

DOI: 10.1021/acs.inorgchem.7b01472

Inorg. Chem. XXXX, XXX, XXX−XXX

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Multifunctional Cationic Iridium(III) Complexes Bearing 2-Aryloxazolo[4,5-f][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption

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