Synthesis, characterization and computational studies of luminescent rhenium(I) tricarbonyl diimine complexes with 8-hydroxyquinoline-containing alkynyl ligands

Accepted Manuscript Synthesis, characterization and computational studies of luminescent rhenium(I) tricarbonyl diimine complexes with 8-hydroxyquinoline-containing alkynyl ligands Wai-Kin Chung, Maggie Ng, Nianyong Zhu, Steven Kin-Lok Siu, Vivian Wing-Wah Yam PII:

S0022-328X(17)30243-7

DOI:

10.1016/j.jorganchem.2017.04.016

Reference:

JOM 19903

To appear in:

Journal of Organometallic Chemistry

Received Date: 8 March 2017 Revised Date:

13 April 2017

Accepted Date: 15 April 2017

Please cite this article as: W.-K. Chung, M. Ng, N. Zhu, S.K.-L. Siu, V.W.-W. Yam, Synthesis, characterization and computational studies of luminescent rhenium(I) tricarbonyl diimine complexes with 8-hydroxyquinoline-containing alkynyl ligands, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.04.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT R

R

OC

N

N

N Re

O O CO

O

AC C

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R = H (1), Me (2), tBu (3)

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OC

ACCEPTED MANUSCRIPT

Synthesis, characterization and computational studies of luminescent rhenium(I)

tricarbonyl

diimine

complexes

with

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8-hydroxyquinoline-containing alkynyl ligands

Wai-Kin Chung, Maggie Ng, Nianyong Zhu, Steven Kin-Lok Siu and Vivian Wing-Wah Yam*

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Dedicated to Prof. John A. Gladysz on the occasion of his 65th birthday

Institute of Molecular Functional Materials (Areas of Excellence Scheme, University

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Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China

*Corresponding author: E-mail: [email protected]; Fax: +852 2857 1586; Tel: +852 2859 2153

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Abstract

A series of 8-hydroxyquinoline-containing alkynylrhenium(I) tricarbonyl diimine

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complexes has been designed and synthesized. Their UV-vis absorption, emission as well as electrochemical properties have been studied. Computational studies have also been performed to provide insights into the electronic transitions, excited state origins

1.

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and electrochemical properties of the complexes.

Introduction

The development of novel functional materials using luminescent transition metal complexes has attracted growing attention. For instance, introducing metal centers could promote intersystem crossing and allow access to triplet excited state which is crucial for fabricating OLED devices of high efficiency [1]. Transition metal complexes with metal-to-ligand charge transfer (MLCT) excited states are also known

ACCEPTED MANUSCRIPT to show photoredox reactivities [2]. Their unique and interesting luminescence properties have prompted researchers to gain a more in-depth understanding of their electronic absorption and excited state origins, and to search for alternative approaches to improve their properties. Pioneered by Wrighton and Morse [3], the

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rhenium(I) tricarbonyl diimine complexes, [Re(CO)3(N^N)X]n+ (N^N = diimine; X = halide or pyridine, n = 0 or 1), have been found to possess rich luminescence properties with triplet MLCT excited state and effectively catalyze the reduction of carbon dioxide [4]. There are also various reports demonstrating the versatility of the

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rhenium(I) tricarbonyl diimine system in constructing functional materials such as photoswitchable materials [5], sensors [6], bioanalytical materials [7], OLEDs [8] and

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metallogels [9].

8-Hydroxyquinoline was originally used as a gravimetric reagent to analyze the concentration of metal ions owing to its excellent coordinating abilities [10]. Because of the intense luminescence and good stabilities of the resultant complexes,

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such as aluminum tris(8-hydroxyquinolinate) and zinc bis(8-hydroxyquinolinate), they are incorporated in OLED devices as emissive layers [11]. Since then, there have been emerging studies on metal quinolinate complexes for various practical

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applications such as diagnostics [12], self-assembly aggregates [13] and electron-transporting materials [14]. In addition, the 8-hydroxyquinoline ligand could be structurally modified to achieve fine-tuning of the solubility, steric hindrance as

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well as the luminescence properties of the corresponding complexes [15], which would provide a great opportunity for the development of new classes of metal quinolinate complexes as advanced materials. However, among various luminescent metal quinolinate complexes, examples of alkynylrhenium(I) tricarbonyl diimine systems with 8-hydroxyquinolinate moieties are relatively rare [16]. As demonstrated by early works from Bruce [17], Beck [18] and Gladysz [19], the rhenium(I) alkynyl system has shown great structural diversities and rich electrochemical properties. In addition, incorporation of an alkynyl ligand as a strong σ-donor into the rhenium(I) tricarbonyl diimine could effectively improve the population of the MLCT excited

ACCEPTED MANUSCRIPT state, leading to interesting luminescence properties and different applications [2h,6i,8d,9,16,20]. Herein, we report the design, synthesis, characterization and crystal structure of a new class of 8-hydroxyquinoline-containing alkynylrhenium(I) tricarbonyl diimine complexes. Their photophysical and electrochemical properties

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have also been studied and supported by computational studies, in which the excited states, ground states and molecular orbitals of the complexes have been revealed by density functional theory (DFT) study whereas the electronic transitions of the

2.

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complexes have been interpreted by time-dependent (TD)-DFT study.

Experimental Materials and Reagents

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2.1.

8-Hydroxyquinoline, phenylacetylene and silver trifluoromethanesulfonate were purchased from Aldrich Chemical Co. Triethylamine was purchased from Lancaster Synthesis Ltd and distilled over potassium hydroxide before use.

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8-Tert-butyloxycarbonyloxy-5-ethynylquinoline [11b−d], [Re(CO)3(Me2bpy)Br] [3], and [Re(CO)3(tBu2bpy)Br] [3] were synthesized according to the literature procedures. All other reagents were of analytical grade and were used as received. All reactions

techniques.

Syntheses

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2.2.

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were carried out under an inert atmosphere of nitrogen using standard Schlenk

2.2.1. [Re(CO)3(bpy)(C≡C−Quin−8−OBoc)] (1)

A

mixture

of

[Re(CO)3(bpy)Cl]

(93

mg,

0.18

mmol),

8-tert-butyloxycarbonyloxy-5-ethynylquinoline (74 mg, 0.28 mmol), AgOTf (52 mg, 0.20 mmol) and NEt3 (2 ml) in THF (70 ml) was allowed to reflux under an inert atmosphere of nitrogen in the dark for 24 hours. After cooling to room temperature, the dark brown suspension was filtered and the orange color filtrate was reduced in

ACCEPTED MANUSCRIPT volume under reduced pressure. The residue was then purified by column chromatography on silica gel using dichloromethane as the eluent. The first band, which contained the unreacted acetylene was discarded, and the second band gave the desired product. Subsequent recrystallization from vapour diffusion of diethyl ether into a dichloromethane solution of 1 gave orange crystals. Yield: 25 mg, 20%. 1H

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NMR (acetone-d6, 298 K, 400 MHz): δ 1.46 (s, 9H, tBu), 7.15 (d, 1H, J = 7.8 Hz, 7-quinoline H’s), 7.24 (q, 2H, J = 4.2 Hz, 3- and 6-quinoline H’s), 7.79 (s, 1H, 4-quinoline H’s), 7.82 (d, 2H, J = 6.2 Hz, 4- and 4’-bipyridyl H’s), 8.38 (d, 2H, J =

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8.0 Hz, 5- and 5’-bipyridyl H’s), 8.71 (d, 1H, J = 4.2 Hz, 2-quinoline H’s), 8.76 (d, 2H, J = 8.0 Hz, 3- and 3’-bipyridyl H’s), 9.23 (d, 2H, J = 3.3 Hz, 6- and 6’-bipyridyl

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H’s). Positive-FAB MS: ion clusters at m/z 696 {M}+, 640 {M − 2CO}+, 612 {M − 3CO}+, 595 {M − CO2tBu}+. IR (KBr disc, ν/cm−1): 2086 (w) ν(C≡C); 2006 (s), 1915 (s), 1891(s) ν(C≡O). Elemental analyses: Found (%): C 48.93, H 3.60, N 5.90. Calcd for [Re(CO)3(bpy)(C≡C−Quin−8−OBoc)]·H2O: C 48.87, H 3.39, N 5.90.

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2.2.2. [Re(CO)3(Me2bpy)(C≡C−Quin−8−OBoc)] (2)

The procedure was similar to that described for the preparation of 1, except

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[Re(CO)3(Me2bpy)Br] (66 mg, 0.12 mmol) was used in place of [Re(CO)3(bpy)Br]. Subsequent recrystallization from vapour diffusion of diethyl ether into a dichloromethane solution of 2 gave orange crystals. Yield: 10 mg, 11%. 1H NMR

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(acetone-d6, 298 K, 300 MHz): δ 1.47 (s, 9H, tBu), 2.64 (s, 6H, Me), 7.21 (m, 3H, 3-, 4- and 7-quinoline H’s), 7.63 (d, 2H, J = 4.8 Hz, 5- and 5’-bipyridyl H’s), 7.86 (dd, 1H, J = 2.0 and 6.6 Hz, 6-quinoline H’s), 8.60 (s, 2H, 3- and 3’-bipyridyl H’s), 8.73 (q, 1H, J = 2.0 Hz, 2-quinoline H’s), 9.03 (d, 2H, J = 5.8 Hz, 6- and 6’-bipyridyl H’s). Positive FAB-MS: ion clusters at m/z 724 {M}+, 668 {M – 2CO}+. IR (KBr disc, ν/cm−1): 2082 (w) ν(C≡C); 2006 (s), 1900 (s), 1885 (s) ν(C≡O). Elemental analyses: Found

(%):

C

49.69,

H

3.64,

N

5.69.

Calcd

[Re(CO)3(Me2bpy)(C≡C−Quin−8−OBoc)]·1½H2O: C 49.66, H 3.90, N 5.60.

for

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2.2.3. [Re(CO)3(tBu2bpy)(C≡C−Quin−8−OBoc)] (3)

The procedure was similar to that described for the preparation of 1, except

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[Re(CO)3(tBu2bpy)Br] (78 mg, 0.13 mmol) was used in place of [Re(CO)3(bpy)Br]. Subsequent recrystallization from vapour diffusion of diethyl ether into a dichloromethane solution of 3 gave dark red crystals. Yield: 25 mg, 24%. 1H NMR (CDCl3, 298 K, 300 MHz): δ 1.47 (s, 18H, tBu), 1.53 (s, 9H, tBu), 7.11 (q, 1H, J = 4.3

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Hz, 7-quinoline H’s), 7.18 (s, 2H, 5- and 5’-bipyridyl H’s), 7.50 (dd, 2H, J = 2.0 and 4.3 Hz, 3- and 6-quinoline H’s), 8.09 (d, 2H, J = 1.8 Hz, 3- and 3’-bipyridyl H’s), 8.13

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(d, 1H, J = 1.7 Hz, 4-quinoline H’s), 8.74 (dd, 1H, J = 1.7 and 4.3 Hz, 2-quinoline H’s), 9.05 (d, 2H, J = 5.9 Hz, 6- and 6’-bipyridyl H’s). Positive FAB-MS: ion clusters at m/z 809 {M}+, 780 {M – CO}+, 753 {M – 2 CO}+, 725 {M – 3CO}+. IR (KBr disc, ν/cm−1): 2086 (w) ν(C≡C); 2003 (s), 1914 (s), 1891 (s) ν(C≡O). Elemental analyses: Found

(%):

C

54.71,

H

4.67,

N

5.15.

Calcd

for

Physical measurements and instrumentation

1

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2.3.

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[Re(CO)3(tBu2bpy)(C≡C−Quin−8−OBoc)]·½H2O: C 54.47, H 4.82, N 5.15.

H NMR spectra were recorded on a Bruker DPX 300 (300 MHz) or Bruker

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DPX 400 (400 MHz) Fourier transform NMR spectrophotometers with chemical shifts reported relative to tetramethylsilane, (CH3)4Si. All positive-ion fast atom bombardment (FAB) and electron impact (EI) mass spectra were recorded on a Finnigan MAT95 mass spectrometer. IR spectra were obtained as KBr disks on a Bio-Rad FTS-7 Fourier transform infrared spectrophotometer (4000‒400 cm‒1). Elemental analyses were performed on a Flash 1112 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences in Beijing, P. R. China.

The electronic absorption spectra were obtained using a Hewlett-Packard

ACCEPTED MANUSCRIPT 8452A diode array spectrophotometer. Steady state excitation and emission spectra at room temperature and 77 K were recorded on a Spex Fluorolog-2 Model F111 fluorescence spectrofluorometer. Solid-state photophysical studies were carried out with solid samples contained in a quartz tube inside a quartz-walled Dewar flask.

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Measurements of the ethanol-methanol (4:1 v/v) glass or solid-state samples at 77 K were similarly conducted with liquid nitrogen filled in the optical Dewar flask. All solutions for photophysical studies were degassed on a high-vacuum line in a two-compartment cell consisting of a 10-ml Pyrex bulb and a 1-cm path length quartz

solutions

were

rigorously

degassed

with

no

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cuvette and sealed from the atmosphere by a Bibby Rotaflo HP6 Teflon stopper. The less

than

four

successive

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freeze-pump-thaw cycles. Emission lifetime measurements were performed using a conventional laser system. The excitation source used was a 355-nm output (third harmonic) of a Spectra-Physics Quanta-Ray Q-switched GCR-150-10 pulsed Nd-YAG laser. Luminescence decay signals were detected by a Hamamatsu R928 photomultplier tube and recorded on a Tektronix Model TDS-620A (500 MHz, 2GS/s)

computer.

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digital oscilloscope and analyzed using a program for exponential fits on a PC

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Cyclic voltammetric measurements were performed by using a CH Instruments, Inc. model CHI 620A electrochemical analyzer. Electrochemical measurements were performed in acetonitrile solutions with 0.1 M nBu4NPF6 (TBAH)

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as supporting electrolyte at room temperature. The reference electrode was a Ag/AgNO3 (0.1 M in acetonitrile) electrode and the working electrode was a glassy carbon electrode (CH Instruments, Inc.) with a platinum wire as the counter electrode. The working electrode surface was first polished with 1 µm alumina slurry (Linde), followed by 0.3 µm alumina slurry (Linde) on a microcloth (Buehler Co.). It was then rinsed with ultra-pure deionized water and sonicated in a beaker containing ultra-pure water for five minutes. The polishing and sonicating steps were repeated twice and then the working electrode was finally rinsed under a stream of ultra-pure deionized water. The ferrocenium/ferrocene couple (FeCp2+/0) was used as the internal reference

ACCEPTED MANUSCRIPT [21]. All solutions for electrochemical studies were deareated with pre-purified argon gas prior to measurements.

Crystal structure determination

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2.4.

Single crystals of 3 suitable for X-ray diffraction studies were grown by vapour diffusion of diethyl ether into a concentrated dichloromethane solution of 3. A crystal of dimensions 0.4 mm × 0.25 mm × 0.2 mm mounted in a glass capillary was

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used for data collection at 28oC on a MAR diffractometer with a 300 mm image plate detector using graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å). The

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experimental details are given in Table 1. The structure was solved by direct methods employing SHELXS-97 program [22] on PC. Re and many non-H atoms were located according to the direct methods. The positions of the other non-hydrogen atoms were found after successful refinement by full-matrix least-squares using program

2.5.

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SHELXL-97 [23] on PC.

Computational details

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All calculations were carried out using Gaussian 09 software package [24]. The ground-state geometries of complexes 1–3 were fully optimized in dichloromethane with density functional theory (DFT) at the PBE0 level [25] in

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conjunction with the conductor-like polarizable continuum model (CPCM) using dichloromethane as the solvent [26]. Vibrational frequency calculations were then performed on all stationary points to verify that each was a minimum (NIMAG = 0) on the potential energy surface. Based on the ground state optimized geometries, time-dependent density functional theory (TDDFT) method [27] at the same level associated with CPCM (dichloromethane) was employed to compute the singlet–singlet transitions in the electronic absorption spectra of complexes 1–3. The unrestricted UPBE0 method was used for the geometry optimization of triplet states. For all the calculations, the Stuttgart effective core potentials (ECPs) and the

ACCEPTED MANUSCRIPT associated basis set were utilized to describe Re [28] with f-type polarization functions (ζ= 0.869) [29], while the 6-31G(d,p) basis set was employed to describe all other atoms [30]. The DFT and TDDFT calculations were performed with a pruned (99,590) grid for numerical integration. The unit for the isovalues of the spatial plots

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of complexes 1−3 is (e/Bohr3)1/2 while the unit for the isovalue of the spin density of complex 3 is e/Bohr3. The cartesian coordinates for the optimized geometries of all the complexes have been included in the supplementary material.

Results and discussion

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3.

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3.1. Synthesis and characterization

The alkyne with quinolinate moiety was prepared by adopting the literature procedure

[11b−d].

The

hydroxy

group

in

the

starting

material,

5-bromo-8-hydroxyquinoline was first protected by tert-butyloxycarbonyl (Boc) protecting group prior to the Sonogashira coupling reaction. The alkyne synthesized

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was used as a ligand to undergo reaction in a mixture of [Re(CO)3(N^N)Br], AgOTf and NEt3 in THF under reflux condition in an inert atmosphere of nitrogen for 24 hours. The crude product was purified by column chromatography on silica gel using

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dichloromethane as eluent. Subsequent recrystallization from vapour diffusion of diethyl ether into dichloromethane solutions of complexes 1–3 afforded the pure forms of the desired products. Although complex 3 could also be prepared by the

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reaction of [Re(CO)3(tBu2bpy)Cl] with LiC≡CR in THF, the same approach would only result in intractable solids when [Re(CO)3(bpy)Cl] and [Re(CO)3(Me2bpy)Cl] were used. The current synthetic route involving the use of AgOTf as halogen abstracting agent is an improved protocol which could be applied for rhenium(I) tricarbonyl

systems

with

different

diimine

ligands

other

than

4,4’-di-tert-butyl-2.2’-bipyridine. The relatively low percentage yield may be attributed to the slightly oxidizing behavior of Ag(I). Replacement of AgOTf with TlPF6 has been reported to give slightly better yields [9b,31]. The IR spectra of complexes 1–3 show three intense ν(C≡O) bands at ca. 1890, 1910 and 2000 cm−1.

ACCEPTED MANUSCRIPT With reference to previous studies on related rhenium(I) tricarbonyl diimine complexes [3,4a−d,5e−o,6c,g,j,20], this confirms that the three carbonyl ligands are arranged in a facial fashion [3,4a−d,5e−o,6c,g,j,20]. Moreover, all the complexes show a weak band at ca. 2090 cm−1, typical of the ν(C≡C) mode in terminal metal

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alkynyl complexes [17−20].

3.2. X-Ray crystal structure

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The perspective drawing of complex 3 with atomic numbering is depicted in Fig. 1. The coordination geometry at the rhenium atom of complex 3 is found to be

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distorted octahedral with three carbonyl ligands arranged in a facial fashion. The crystal and structure determination data are collected in Table 1 while selected bond distances and angles are tabulated in Table 2. The N−Re−N bond angle for complex 3 is 74.4(1)o, which is found to be less than 90o, as required by the bite distance exerted

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by the steric demand of the chelating bipyridine ligand [5i−l,6c,e,f,h,j,20].

The C≡C bond length of complex 3 is 1.2(7) Å, typical of that found in other σ-bonded metal alkynyl systems [17,18c,e,f,19b,c,e,i,k,20,32]. The Re−C≡C unit is

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essentially linear with a Re−C≡C bond angle of 172.3(4)o, which is similar to that found in other related rhenium(I) alkynyl systems [17,18c,e,f,19b,c,e,i,k,20].

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3.3. Electronic absorption and emission properties

Dissolution of complexes 1–3 in dichloromethane gives yellow solutions and

their electronic absorption spectral data at ambient temperature are shown in Table 3. The electronic absorption spectrum of 1 recorded at 298 K is illustrated in Fig. 2. In general, complexes 1–3 show intense high-energy absorption bands at ca. 258–372 nm in CH2Cl2, with extinction coefficients in the order of 104 dm3 mol−1 cm−1. These high-energy absorption bands are also found in the free bipyridine and alkynyl ligands, and can be assigned as the π → π* transitions of the bipyridine and alkynyl ligands.

ACCEPTED MANUSCRIPT Low-energy absorption bands at ca. 432–440 nm with extinction coefficients of the order of 103 dm3 mol−1 cm−1 are also observed for complexes 1–3. A trend in these low-energy absorption bands is observed upon varying the nature of the diimine ligands. The energy of the low-energy absorption bands for complexes 1–3 is found to

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follow the order: 1 (440 nm) < 2 (436 nm) < 3 (432 nm). This trend could be rationalized by the presence of electron-donating methyl and tert-butyl substituents on the bipyridine ligands in 2 and 3, which raise their π* orbital energies, making them poorer π-accepting ligands than the unsubstituted bpy ligands. This, together with spectroscopic

works

on

rhenium(I)

diimine

systems

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related

[3,4a−d,5,6a,c,f,h,j,8d,e,9,16,20], suggests that the low-energy absorption bands are

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assigned as the dπ(Re) → π*(diimine) MLCT transition, with some mixing of the alkynyl-to-diimine ligand-to-ligand charge transfer (LLCT) character.

In solution state, complexes 1–3 show luminescence behavior with emission maxima at ca. 632 to 643 nm upon excitation at λ > 440 nm while in the solid state,

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these complexes are found to emit at ca. 575 to 595 nm. The emission data of complexes 1–3 are summarized in Table 3 and the representative emission spectrum of 2 in dichloromethane is depicted in Fig. 3. The relatively long luminescence

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lifetimes, in the sub-microsecond to microsecond range, measured for all the complexes at low and ambient temperatures suggest the triplet parentage of the

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emissive states. With reference to previous studies on rhenium(I) diimine alkynyl complexes [3,4a−d,5,6a,c,f,h,j,8d,e,9,16,20], the emission origin is assigned as derived from states of 3MLCT [dπ(Re) → π*(diimine)] origin mixed with certain 3

LLCT [π(C≡C−Quin−8−OBoc) → π*(diimine)] character.

Upon incorporation of the methyl and tert-butyl substituents on the bipyridine ligand, a shift to higher emission energy is observed, with complex 1 (643 nm) < 2 (633 nm) ≤ 3 (632 nm). This trend is also in line with the electronic absorption result and can be ascribed to the presence of electron-donating substituents on the diimine ligand that raise the π* orbital energy, confirming the origin for the

ACCEPTED MANUSCRIPT emission as MLCT phosphorescence.

For the emission studies in EtOH-MeOH (4:1 v/v) glass at 77 K, complexes 1–3 exhibit highly structured emission bands at ca. 563–613 nm with vibrational progressional spacings of ca. 1310–1450 cm−1, corresponding to the ground state

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ν(C C) and ν(C N) vibrational modes of the quinolinate moiety. This is supportive of an involvement of the ethynyl-8-tert-butylcarbonyloxyquinolinate unit in the emissive state, and hence the emissive origin is suggested to be an admixture of MLCT and 3IL [π(C≡C−quinolinate) → π*(C≡C−quinolinate)] characters.

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3

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3.4. Electrochemical studies

All the complexes display an irreversible oxidation wave at ca. +0.96 to +1.04 V and one quasi-reversible reduction couple at ca. −1.42 to −1.54 V vs. SCE in acetonitrile (0.1 moldm−3 nBu4NPF6). Complex 2 also shows a quasi-reversible oxidation wave at +1.72 V vs. SCE. The electrochemical data of complexes 1–3 are

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summarized in Table 4.

With reference to the previous electrochemical studies on the related

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alkynylrhenium(I) diimine complexes [8d,19f−k,20g,h], the oxidation wave of complexes 1−3 which occurrs at ca. +1.0 V vs. SCE is tentatively assigned as the alkynyl ligand-centered oxidation. However, the involvement of the oxidation of Re(I)

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to Re(II) should not be excluded owing to the fact that all complexes showed a less positive potential than their chloro counterpart, [Re(CO)3(N^N)Cl] (ca. +1.22 V) [30b], which is consistent with the better σ- and π-donating ability of the alkynyl ligand than the chloro ligand, rendering the rhenium(I) metal center more electron-rich and hence an increase in its ease of oxidation.

The quasi-reversible oxidation couple observed at +1.72 V vs. SCE for complex 2 is tentatively assigned as the oxidation process of Re(I) to Re(II) of the corresponding solvento analogue, [Re(CO)3(N^N)(MeCN)]+, which is formed by a

ACCEPTED MANUSCRIPT proposed EC mechanism, similar to those observed from previous studies of other related rhenium(I) alkynyl systems [8d,19f−k,20g,h]. This mechanism proposed that upon the first oxidation, the metal-carbon bond between the rhenium center and the alkynyl ligand would be weakened, resulting in the loss of an alkynyl radical and the

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formation of a coordinative-unsaturated intermediate. The intermediate would then readily pick up an acetonitrile molecule from the solvent to form the solvento complex. The lack of observation of the second oxidative wave in 1 and 3 may arise from the fact that 1 and 3 have more positive first oxidative wave than 2, such that the

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second oxidative wave for 1 and 3 may also occur at more positive potential than

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+1.72 V, which probably lies beyond the solvent window for them to be observed.

The quasi-reversible reduction couple at ca. −1.42 V to −1.54 V vs. SCE is tentatively assigned as the diimine-based ligand-centered reduction. The potential value is found to be varied with the nature of the diimine ligands. A more negative reduction potential for complex 2 (−1.54 V vs. SCE) and 3 (−1.52 V vs. SCE) than 1

and tBu2bpy than bpy.

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(−1.42 V vs. SCE) can be rationalized by the poorer π-accepting ability of Me2bpy

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3.5. Computational studies

Density functional theory (DFT) and time-dependent density functional theory

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(TDDFT) calculations have been performed to provide further insight into the respective electrochemical, and electronic absorption and emission properties of the alkynylrhenium(I) diimine complexes 1−3. The optimized structure of 3 is shown in Fig. 4 and the selected structural parameters of all the complexes are listed in Table 5. As shown in the optimized structure, complex 3 adopt a slightly distorted octahedral structure with three carbonyl groups in a facial arrangement. The calculated corresponding bond lengths of the three complexes are very close. For instance, the calculated C≡C bond lengths of the three complexes are the same (1.232 Å). The M−CO bonds for all the complexes at trans position to the alkynyl ligand are found to

ACCEPTED MANUSCRIPT be longer than other M−CO bonds. This can be rationalized by the stronger trans-influence of the alkynyl ligand relative to the pyridine groups of the diimine ligands. The calculated bond distances are also in good agreement with those found in the X-ray crystal structure of complex 3.

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The frontier molecular orbitals for complexes 1− −3 are similar and their corresponding frontier molecular orbitals are shown in Fig. 5−7. The HOMO mainly consists of the π orbital of the alkynyl ligands with certain degree of contribution

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from the metal dπ orbital. The HOMO−1 is mainly contributed from the metal dπ orbital mixed with an in-plane π orbital of the alkynyl ligand of proper symmetry in

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an antibonding fashion and the HOMO−2 is the metal dπ orbital. On the other hand, the LUMO is on the π* orbitals of the diimine ligands while the LUMO+1 is the π* orbitals of the 8-hydroxyquinolinate moieties. The energy level diagram of the frontier orbitals of complexes 1−3 are shown in Fig. 8 for comparison. The HOMO energies of the complexes are very similar whereas the LUMO energy increases from

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complex 1 to 2 to 3. This is consistent with the electron-donating abilities of the substituents, where t-butyl ≥ methyl > H, and further confirm the assignment of the

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first oxidation and reduction processes in the electrochemical studies.

The first fifteen singlet excited states of complexes 1−3 in CH2Cl2 have been

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calculated using the TDDFT/CPCM method on the basis of the optimized ground state geometries. Selected singlet excited states of complexes 1−3 are listed in Table 6. The singlet−singlet transition at λ > 400 nm with significant oscillator strength corresponds to an excitation from the HOMO−1 to LUMO. These intense transitions are computed at 422−431 nm and contain [dπ(Re) → π*(diimine)] MLCT character mixed with [π(C≡C−Quin−8−OBoc) → π*(diimine)] LLCT character. Complex 2 also shows another singlet−singlet transition at λ > 400 nm with comparable oscillator strengths. The transition is related to excitation from the HOMO to the LUMO, which also contains [dπ(Re) → π*(diimine)] MLCT character and [π(C≡C−Quin−8−OBoc) → π*(diimine)] LLCT character. In addition, among the singlet−singlet transitions at

ACCEPTED MANUSCRIPT λ < 400 nm, the most intense transition for 1−3 is computed at 372−374 nm, which is contributed by an admixture of IL π → π* transition of the 8-hydroxyquinolinate substituted alkynyl ligand mixed with MLCT [dπ(Re) → π*(C≡C−Quin−8−OBoc)] transition. The computed MLCT/IL transition is also in reasonable agreement with the

assignment of the transition of these absorption bands.

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experimental λmax of the high-energy absorption bands at 370−372 nm and support the

The nature of the excited state and its difference from the ground state have

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been studied by using the unrestricted UPBE0 functional. Fig. 9 reveals the optimized structure of the triplet state of complex 3 with selected changes in the structural

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parameters relative to that of the ground state. Geometry optimization of the triplet state starting from the ground state structure has led to an excited state structure with the distortion mainly occurring in the metal−ligand core, diimine ligand as well as the C≡C moiety. A plot of spin density of the 3MLCT/LLCT state of complex 3 has been shown in Fig. 10, in which the spin density is mainly localized on the diimine ligand,

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metal center and the alkynyl unit. The lower- and higher-energy singly occupied molecular orbitals (SOMOs) of the triplet excited state are mainly contributed from an admixture of metal dπ orbital, π orbital of the C≡C−Quin−8−OBoc unit and the π* orbital of the diimine ligand, suggesting that the triplet excited state contains an

4.

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admixture of 3MLCT and 3LLCT character.

Conclusion

A new series of luminescent alkynylrhenium(I) tricarbonyl diimine complexes

has been designed, synthesized and characterized. The luminescence and the electrochemical properties of the complexes have been studied. This class of complexes are found to exhibit low-energy transition bands, which are assigned as the dπ(Re) →

π*(diimine) (MLCT) transition

mixed

with

alkynyl-to-diimine

ligand-to-ligand charge transfer (LLCT) character. Upon photo-excitation, the complexes are found to show emission at 298 and 77 K in various media. The origins

ACCEPTED MANUSCRIPT of the emission bands measured in CH2Cl2 at 298 K are assigned as derived from 3

MLCT [dπ(Re) → π*(diimine)] excited state with some mixing of

3

LLCT

[π(C≡C−Quin−8−OBoc) → π*(diimine)] character. It has also been revealed that the luminescence behavior of the complexes could be readily modified by attaching

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substituents of different electron-donating abilities on the diimine ligands. DFT and TDDFT calculation results are also consistent with the experimental observations on X-ray crystallography, UV-vis absorption, emission and electrochemical studies, further confirming their corresponding assignments. This study provides further

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understanding on the photoluminescence properties of the alkynylrhenium(I) tricarbonyl diimine system and suggests that these complexes could be exploited for

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further application as functional materials.

Acknowledgments

V.W.-W.Y. acknowledges support from The University of Hong Kong under

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the URC Strategic Research Theme on New Materials. This work has been supported by the University Grants Committee Areas of Excellence Scheme (AoE/P-03/08), and a General Research Fund (GRF) (HKU 17305614) grant from the Research Grants

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Council of Hong Kong Special Administrative Region, PR China. W.-K.C. and S.K.-L.S. acknowledge the receipt of Postgraduate Studentship from The University

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of Hong Kong. References [1]

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Scheme 1.

Structure

of

the

alkynylrhenium(I)

tricarbonyl

diimine

Fig. 1.

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complexes 1−3.

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Perspective view of complex 3 with atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are

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shown at the 30% probability level.

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2.0

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−1 −1

2.5

1.5

−4

ε x 10 / dm mol cm

3.0

3

3.5

0.5 0.0 250

350

400

450

500

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300

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1.0

550

Wavelength / nm

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Electronic absorption spectrum of [Re(CO)3(bpy)(C≡C− Quin−8−OBoc)] (1) in CH2Cl2 at 298 K.

Emission Intensity

Fig. 2.

550

600

650

700

750

800

Wavelength / nm Fig. 3.

Emission spectrum of [Re(CO)3(Me2bpy)(C≡C−Quin− 8−OBoc)] (2) in CH2Cl2 at 298 K.

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Optimized ground-state geometry of complex 3.

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Fig. 4.

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LUMO+1

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LUMO

HOMO

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HOMO−1

Fig. 5.

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HOMO−2

Spatial plots (isovalue = 0.03) of selected frontier molecular orbitals of

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complex 1 obtained from PBE0/CPCM calculation.

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LUMO+1

HOMO

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HOMO−1

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LUMO

Fig. 6.

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HOMO−2

Spatial plots (isovalue = 0.03) of selected frontier molecular orbitals of

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complex 2 obtained from PBE0/CPCM calculation.

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LUMO+1

HOMO

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HOMO−1

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LUMO

Spatial plots (isovalue = 0.03) of selected frontier molecular orbitals of complex 3 obtained from PBE0/CPCM calculation.

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Fig. 7.

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HOMO−2

Fig. 8.

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Orbital energy diagram of the frontier molecular orbitals (H = HOMO

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and L = LUMO) of complexes 1–3.

Fig. 9.

The change in bond lengths (in Å) in the triplet excited state of

complex 3 relative to the ground state. Only differences in bond lengths of larger than 0.013 Å are shown. The positive and negative values indicate bond elongation and contraction respectively. All hydrogen atoms are omitted for clarity.

Fig. 10.

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Plots of the spin density (isovalue = 0.002) of the first triplet excited

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EP

TE D

M AN U

state (T1) of complex 3.

ACCEPTED MANUSCRIPT Table 1

Crystal and structure determination data for complex 3 C37H38N3O6Re

Formula weight

806.90

Temperature

301(2) K

Wavelength

0.71073 Å

Crystal system

Monoclinic

Space group

P21/n

Unit cell dimensions

a = 10.672(2) Å

α = 90o

b = 14.766(3) Å

β = 101.26(3)o

Z

4

M AN U

3605.0(13) Å3

γ = 90o

SC

c = 23.326(5) Å Volume

RI PT

Empirical formula

1.487 g cm‒3

Density (calculated)

3.418 mm‒1

Absorption coefficient F(000)

1616

0.4 mm × 0.25 mm × 0.2 mm

Crystal size

θ range for data collection

2.25 to 25.66o

Reflections collected

TE D

‒12 ≤ h ≤ 12, ‒17 ≤ k ≤ 16, ‒27 ≤ l ≤ 28

Index ranges

25474

6741 [Rint = 0.0593]

Completeness to theta = 25.66o

99.1%

EP

Independent reflections

None

Refinement method

Full-matrix least-squares on F2

Data/ restraints/ parameters

6741 / 0 / 427

Goodness-of-fit on F2

0.742

Final R indices [I>2σ(I)]

R1 = 0.0343, wR2 = 0.0876a

R indices (all data)

R1 = 0.0453, wR2 = 0.0909a

Largest diff. peak and hole

1.135 and ‒1.347 eÅ‒3

AC C

Absorption correction

a

w = 1/[σ2(Fo2)], where P is [2Fc2 + Max(Fo2,0)]/3.

ACCEPTED MANUSCRIPT Table 2

Selected bond distances (Å) and bond angles (o) with estimated standard deviations (e.s.d.s) in parentheses for complex 3

Selected bond distances (Å) 2.2 (3)

Re(1)-N(2)

2.2 (3)

Re(1)-C(1)

1.9 (8)

Re(1)-C(2)

Re(1)-C(3)

1.9 (6)

Re(1)-C(4)

C(4)-C(5)

1.2 (7)

C(1)-O(1)

C(2)-O(2)

1.2 (6)

C(3)-O(3)

1.2 (6)

C(10)-N(3)

1.4 (6)

C(14)-N(3)

1.3 (6)

C(9)-O(6)

1.4 (6)

C(15)-O(4)

1.2 (5)

C(15)-O(5)

1.3 (6)

C(15)-O(6)

1.4 (5)

C(16)-O(5)

1.5 (6)

RI PT

Re(1)-N(1)

1.9 (5) 2.1 (6)

TE D

M AN U

SC

1.2 (8)

Selected bond angles (o) N(1)-Re(1)-N(2)

74.4 (12)

C(1)-Re(1)-C(4)

172.0 (4)

173.2 (18)

C(3)-Re(1)-N(1)

172.49 (18)

C(4)-C(5)-C(6)

172.6 (5)

C(5)-C(4)-Re(1)

172.3 (4)

C(14)-N(3)-C(10)

117.3 (4)

O(4)-C(15)-O(5)

129.4 (5)

O(4)-C(15)-O(6)

123.9 (5)

O(5)-C(15)-O(6)

106.6 (4)

C(8)-C(9)-O(6)

120.4 (4)

O(6)-C(9)-C(10)

118.1 (4)

O(5)-C(16)-C(18)

102.7 (4)

O(5)-C(16)-C(19) 109.6 (5)

AC C

EP

C(2)-Re(1)-N(2)

ACCEPTED MANUSCRIPT Table 3

Photophysical data of alkynylrhenium(I) tricarbonyl diimine complexes 1−3 Absorption λmax / nm (εmax / dm3mol−1cm−1)

Complex

Emission Medium

λmax/nm (τo/µs)

(T/K) 258 (34180), 297 (16370), 370 (10580), 440 (1770)

CH2Cl2 (298) Solid (298) Solid (77) Solid (77)a

643 (<0.1) 595 (0.22) 575 (0.51) 563, 613 (4.62)

[Re(CO)3(Me2bpy)−

260 (42520), 294sh (20750), CH2Cl2 (298) 370 (13710), 436 (2040) Solid (298) Solid (77) Solid (77)a

633 (<0.1) 593 (0.23) 590 (0.21) 564, 612 (4.56)

[Re(CO)3(tBu2bpy)− (C≡C−Quin−8− OBoc)] (3)

260 (50310), 296sh (24670), CH2Cl2 (298) 372 (16770), 432 (2740) Solid (298) Solid (77) Solid (77)a

EP

TE D

Measured in EtOH–MeOH (4:1 v/v)

AC C

a

SC

OBoc)] (2)

M AN U

(C≡C−Quin−8−

RI PT

[Re(CO)3(bpy)− (C≡C−Quin−8− OBoc)] (1)

632 (<0.1) 587 (0.19) 578 (0.16) 566, 612 (4.98)

ACCEPTED MANUSCRIPT Table 4

Electrochemical data for alkynylrhenium(I) tricarbonyl diimine complexesa

Oxidation

Reduction

E½b / V vs. SCE

E½b / V vs. SCE

(∆Ep / mV) +1.03c

[Re(CO)3(Me2bpy)(C≡C−Quin−8−

+0.96c

OBoc)] (3)

M AN U

[Re(CO)3(tBu2bpy)(C≡C−Quin−8−

−1.54 (65)

+1.04c

−1.52 (55)

TE D

In acetonitrile (0.1 mol dm−3 nBu4NPF6). Working electrode: glassy carbon; Scan rate: 100 mV s−1.

b

−1.42 (53)

+1.72 (91)

OBoc)] (2)

a

(∆Ep / mV)

SC

[Re(CO)3(bpy)(C≡C−Quin−8−OBoc)] (1)

RI PT

Complex

E½ = (Epa + Epc) /2; Epa and Epc are the anodic and cathodic peak potentials,

Irreversible oxidation wave. The potential refers to Epa which is the anodic peak potential.

AC C

c

EP

respectively. ∆Ep = |Epa – Epc|.

ACCEPTED MANUSCRIPT Table 5

Selected structural parameters of the PBE0 optimized geometries of complexes 1–3a

2

3b

Re–C(1)

2.113

2.114

2.114 (2.130)

Re–N(1)

2.190

2.190

Re–N(2)

2.191

2.190

Re–COtrans to C(1)

1.961

1.960

Re–COtrans to N(1)

1.918

1.917

1.917 (1.908)

Re–COtrans to N(2)

1.918

1.917

1.918 (1.910)

C(1)–C(2)

1.232

1.232

1.232 (1.203)

C(2)–C(3)

1.423

1.423

1.423 (1.441)

RI PT

1

2.188 (2.185)

2.189 (2.187)

M AN U

SC

1.960 (1.922)

The atomic numbering is shown in Fig. 4. Bond lengths are in Å.

b

The structural parameters in parentheses are from X-ray crystallographic data.

AC C

EP

TE D

a

ACCEPTED MANUSCRIPT Table 6

Selected singlet excited states (Sn) of complexes 1–3 computed by TDDFT/CPCM (CH2Cl2) at the optimized ground-state geometries

(Coefficient)b

wavelength/nm

S1

H → L (0.70)

499

S2

H−1 → L (0.70)

429

S3

H → L+1 (0.69)

374

S4

H−2 → L (0.62)

364

Sn

H−3 → L (0.32) 353

S6

H−1 → L+1 (0.67)

S7

Characterd

0.005

MLCT/LLCT

0.045

MLCT/LLCT

0.442

MLCT/IL

0.001

MLCT MLCT/LLCT MLCT/LLCT

347

0.000

MLCT/IL

H−3 → L (0.58)

344

0.010

MLCT/LLCT

S1

H → L (0.70)

475

0.021

MLCT/LLCT

S2

H−1 → L (0.70)

431

0.020

MLCT/LLCT

S3

H → L+1 (0.70)

372

0.407

MLCT/IL

S4

H−2 → L (0.67)

356

0.000

MLCT

S5

H−1 → L+1 (0.68)

348

0.000

MLCT/IL

S6

H → L+2 (0.70)

338

0.007

MLCT/LLCT

S7

H−3 → L (0.65)

333

0.057

MLCT/LLCT

S1

H → L (0.70)

469

0.019

MLCT/LLCT

S2

H−1 → L (0.70)

422

0.028

MLCT/LLCT

TE D

M AN U

0.004

AC C

3

H → L+2 (0.69)

EP

2

S5

fc

RI PT

1

Vertical excitation

SC

Complex

Excitationa

S3

H → L+1 (0.70)

373

0.399

MLCT/IL

S4

H−2 → L (0.66)

353

0.001

MLCT

S5

H−1 → L+1 (0.67)

349

0.000

MLCT/IL

S6

H → L+2 (0.70)

342

0.012

MLCT/LLCT

S7

H−3 → L (0.55)

331

0.033

MLCT/LLCT

H → L+3 (-0.37)

MLCT/LLCT

a

The orbitals involved in the excitation (H = HOMO and L = LUMO).

b

The coefficients in the configuration interaction (CI) expansion that are less than 0.3

ACCEPTED MANUSCRIPT are not listed. c

Oscillator strengths.

d

Character of the transition. electrochemistry, luminescence, metal alkynyl, rhenium(I) diimine, UV-vis absorption

AC C

EP

TE D

M AN U

SC

RI PT

Keywords

ACCEPTED MANUSCRIPT Highlights A new series of luminescent alkynylrhenium(I) tricarbonyl diimine complexes has been designed, synthesized and characterized.

structure of one complex have been studied.

RI PT

The photophysical and the electrochemical properties of the complexes and the crystal

The luminescence behavior of the complexes could be readily modified by attaching substituents of different electron-donating abilities on the diimine ligands.

AC C

EP

TE D

M AN U

SC

DFT and TDDFT calculation results are consistent with the experimental observations on X-ray crystallography, UV-vis absorption, emission and electrochemical studies.