SYNTHESIS AND CHARACTERIZATION OF BIDENTATE LIGANDS WITH RUTHENIUM COMPLEX FOR LUMINESCENT TESTING

SYNTHESIS AND CHARACTERIZATION OF BIDENTATE LIGANDS WITH RUTHENIUM COMPLEX FOR LUMINESCENT TESTING

KHAW YING YING (41687)

Bachelors of Science with Honours (Resource Chemistry)

2016

SYNTHESIS AND CHARACTERIZATION OF BIDENT ATE LIGANDS WITH RUTHENIUM COMPLEX FOR LUMINESCENT TESTING

Khaw Ying Ying (41687)

A dissertation submitted in partial fulfilment of the requirement for the degree of Bachelor of Science (Hons.)

Supervisor: Dr. Tay Meng Guan

RESOURCE CHEMISTRY PROGRAM (WS48) DEPARTMENT OF CHEMISTRY

FACULTY OF RESOURCE SCIENCE AND TECHNOLOGY UNlVERSITI MALAYSIA SARA W AK

Acknowledgment

To my most respected supervisor, Dr. Tay Meng Guan, I would like to express my heartfelt and deepest gratitude for countlessly guiding me and providing steadfast encouragement throughout this project. Without him, I would not be able to learn and master so much of research experiences along this journey. Furthermore, I would also like to convey my sincere appreciation to fellow postgraduates who had put so much effort on leading me in completing this project especially Ms. Chia Ying Ying who managed to uncover the strengths inside me, Mr. Ong Kok Tong who inspired me in looking for new ideas and of course, Ms. Suzie Kuan, Mr. Teo Kien Yung, Ms. Ruwaida Asyikin as well as Ms. Nadia Jasin. Having them during this project was a great experience as they had lighten up and created such a positive environment for me and my fellow undergraduates to work on our projects. I am grateful to my beloved parents who imparted me confidence and belief as well as technical and moral support along this fulfilment of degree course in UNIMAS. Last but not least, a big thank you to my wonderful lab mates who had given their worthful time whenever I needed a help of hand.

I

UNIVERSITI MALAYSIA SARAWAK

Grade: _---JA'--->-.,___

Please tick (V)

Final Year Project Report Masters

PhD

DECLARATION OF ORIGINAL WORK This declaration is made on the ....

-?-.~ .. ...

^{day of...}

~.~

... ... 2016.

Student's Declaration:

I

---!i-~~~-"{----X~~---Y!!:\,.--~~-~~~}-~-~}~~~-tr _ pL_ ~~ ~ ~.1!J!LJ9_~Y)_ ~

^__

K_ I ~_A¥)

^(j

tort

(pLEASE INDICATE STUDENTS NAME, MATRIC NO. AND FACULTY) hereby declare 'tl d SUVljiu.!'(S '- cIA" r~lft.Y\)...;.A.~"" ,i: L··dJ_A...fc>.* U9.~"o/~ /NI-IL . . 1 th t th k t

wo:k. I e

h:~: n:~ ;o:ie~it~~~;ii~~:tfu~~~,or~~~~~~f~~t;~~:~-~::~:: ::c:;~g~nhaere

due reference or acknowledgement is made explicitly in the text, nor has any part been written for me by another person.

;)3 . 06 . 16 kfU+w YIN~ YtNCT (-((Cf! )

Date submitted Name of the student (Matric No.)

Supervisor's Declaration:

I,----~y.-~~~----§.~~-~---

(SUPERVISOR'S

l'jMd~.hef£.hr ~ertify tha~th~

^work

entitled,

~!Pf.&..~~~-~-~ - ct¥.!?f':.#-<;~--;f-A>'c{!-!'1.'i1'1c-lte~~-~-\~-~-'ttfTffEr

.J";S

We~by trt~ ~o~e

named student and was ,\lubmitted to the "FACULl::' as a *partiallfull fulfillment for the conferment of

~.yf!f.._t?f_~~:!:!Jf._~JYl'!!!_f!c:~~_!~

____

~~~_~!~!'J

(pLEASE INDICATE THE DEGREE), and the aforementioned work, to the best of my knowledge, is the said student's work

Received for examination by:

11t1

^{MMI'7 'UI}N} Date:..2.~ ~ 'O, ~ ..Jo/6 (Name of the supervisor)

II

I declare this Project/Thesis is classified as (please tick (.J»:

o

CONFIDENTIAL (Contains confidential information under the Official Secret Act 1972)*

o

RESTRICTED (Contains restricted information as specified by the organisation where research was done)*

o

OPEN ACCESS

Validation of Project/Thesis

I therefore duly affIrmed with free consent and willingness declared that this said Project/Thesis shall be placed officially in the Centre for Academic Information Services with the abide interest and rights as follows:

• This Project/Thesis is the sole legal property of Universiti Malaysia Sarawak (UNIMAS).

• The Centre for Academic Information Services has the lawful right to make copies for the purpose of academic and research only and not for other purpose.

• The Centre for Academic Information Services has the lawful right to digitise the content to for the Local Content Database.

• The Centre for Academic Information Services has the lawful right to make copies of the ProjecUThesis for academic exchange between Higher Learning Institute.

• No dispute or any claim shall arise from the student himself / herself neither third party on this Project/Thesis once it becomes sole property ofUNIMAS.

• This Project/Thesis or any material, data and information related to it shall not be distributed, published or disclosed to any party by the student except with UNIMAS permISSIOn.

Student's Signature_ _

____

Supervisor', signature,

.#

P

-",i,?(~,,-- ^_ .

(Date) (Date)

.2 ~ - o~ -dol&

Current Address:

a7-

k

"c.t h 1LfC41A ~")-

Notes: * Ifthe Project/Thesis is CONFIDENTIAL or RESTRICTED, please attach together as annexure a letter from the organisation with the period and reasons of confidentiality and restriction.

[The instrument was duly prepared by The Centre for Academic Information Services]

III

Table of Contents

Acknowledgment ... I Declaration ... II Table of Contents ... IV List of Abbreviation ... VII List of Figures ... ... ... IX List of Schemes ... XII List of Table ... XIII

Abstract ... 1

1.0 Introduction... 2

1.1 Luminescence properties ......... .......................................................... 3

1.2 Bidentate Schiff base ligands ............................. ............................. 4

1.2.1 Diimine ..................................................................... .. 4

1.2.2 Diazabutadiene (DAB) ........................... ........ ....... 6

1.2.3 Bis(imino )acenaphthene (BIJIN) ......................... ........ ..... ...... ......... 7

1.3 Ruthenium(I1) diimine complexes ..... ................................................. 8

1.4 Problem statement ... .............................. ............ 1 0 1.5 Objectives ..................................... 1 0 2.0 Literature Review ... 11

2.1 Synthesis of ligands ................................................... ......... ........... 11

2.1.1 Diazabutadiene (DAB) .............................. ............. 11 IV

2.1.2 Bis(imino )acenaphthene (BIAN) ...................... .... ................... 13

2.2 Synthesis of diimine Ruthenium(II) complex ............................ 16

2.3 Luminescence properties ofRuthenium(II) complex ........... ................ 17

3.0 Methodology ... 20

3.1 Materials and reagents .................................................. 20

3.2 Characterizations ..................... ........... ...........20

3.3 Synthesis ofdiimine compounds ............................. ................ ...... 21

3.3.1 Diazabutadiene (DAB) ... ................................ ...... ....... 21

3.3.1.1 Synthesis of 1 ,4-bis(4-methylphenyl)-2,3-dimethyl-1 ,4-diazabutadiene (CH3-DAB) .. .. 21

3.3.1.2 Attempted synthesis of 1 ,4-bis(4-nitrophenyl)-2,3-dimethyl-1 ,4-diazabutadiene (N02­ DAB) ....................................... .....22

3.3.2 Synthesis ofbis(phenylimino)acenaphthene (BIAN) ........................ .....22

3.3.2.l Synthesis ofbis(phenylimino)acenaphthene (BIAN) ... ........ ........ 22

3.3.2.2 Synthesis ofbis(methylphenylimino)acenaphthene (CH3-BIAN) .... ...................23

3.3.3 Synthesis of Ruthenium(II) complex ............... ............... 24

3.3.3.1 Synthesis ofRuthenium(I~omplex with CH3-DAB (Ru-CH3-DAB) ...........24

3.3.3.2 Synthesis of the precursor RuCb(PPh3)3 ...... .............. ................ 24

3.3.3.3 Attempted synthesis of Ruthenium(II) complex with BIAN (Ru-BIAN) .......... 24

3.3.3.4 Synthesis of Ruthenium(II) complex with CH3-BIAN (Ru-CH3-BIAN) ...... .....25

3.3.4 Luminescent testing on Ruthenium(II) complex ........................ 25

4.0 Results and discussion ... ... 26

4.1 Synthesis and characterization of diimine compounds and Ruthenium(II) complexes .... ...26 V

4.2 Spectroscopic studies of diimine compounds and Ruthenium(H) complexes ... 30

4.2.1 UV -Visible spectroscopy ................... 30

4.2.2 Infrared spectroscopy ........... .33

4.2.3 Gas Chromatography-Mass Spectrometry (GC-MS) ........... 36

4.2.4 NMR Spectroscopy ............. ... 38

4.3 Luminescent testing on diimine compounds and the Ruthenium(IT) complexes ..... .45

4.4 Synthesis, characterization and spectroscopic studies of attempted diimine compound and Ruthenium(IT) complex ................. 47

4.4.1 Attempted synthesis of 1 ,4-bis( 4- nitrophenyl)-2,3-dimethyl-1 ,4-diazabutadiene (N02-DAB) .............. ... ...... 47

4.4.2 Attempted synthesis ofRuthenium(II) complex with BIAN (Ru-BIAN) ... .48

5.0 Conclusion ... ... 50

6.0 Suggestion for future research ... 51

7.0 References ... 52

8.0 Appendix ... ... ... ... 58

VI

II

I'

;:t

[Ru(bpy)3f+

~s

IH

IMLCT 31p 3IL 3MLCT

A

acac AcOH BIAN Bpy DAB DFT DMSO DNA Dppe Dppz FTIR G GC-MS LUMO

List of Abbreviation Tris(bipyridine )ruthenium (II) ion Microsecond

Hydrogen-l NMR

Singlet metal-to-ligand charge transfer Phosphorus-31 NMR

Intraligand triplet state

Triplet metal-to-ligand charge transfer Angstrom

Acetylacetone Acetic acid

B is( imino )acenaphthene Bipyridine

Diazabutadiene

Density-functional theory Dimethyl sulfoxide DeoxYribonucleic acid

I ,2-bis( diphenylphosphino )ethane Dipyrido [3, 2-a: 2', 3'-c] phenazine Fourier transform infrared spectroscopy Gram

Gas Chromatography-Mass Spectrometry Lowest unoccupied molecular orbital

VII

MeCN mesBIAN

Min mL

MMLL'CT mmol

ms

nm NMR ns phen

RuCh(PPh3)3 SOC

TFA TLC TMS T-T UV

(l

~G

~H 1t

Acetonitrile

bis( mesitylimino )-acenaphthene Minute

Milliliter

Mixed-metal-ligand-to-ligand-charge-transfer Millimole

Millisecond Nanometer

Nuclear magnetic resonance Nanosecond

phenanthroline

Precursor dichlorotris(triphenylphosphino) ruthenium (II) Spin orbital coupling

T rifluoroacetate

Thin Layer Chromatography Tetramethysilane

Triplet1tJiplet Ultraviolet Alpha

Change in Gibbs free energy Change in enthalpy

pi Anti pi

VIII

II

It

Figure 1.1

1.2 1.3 1.4 1.5 2.1 2.2 2.3 2.4 2.5

4.1 4.2 4.3 4.4 4.5 4.6 4.7

List of Figures

Title Page

Simplified Jablonski diagram representing energy levels and spectra 4 of absorption and emission as fluorescence and phosphorescence

General structure of diimine 5

Structure of diazabutadiene (DAB) 6

Structure ofbis(imino )acenaphthene (BIAN) 7

Splitting of the d orbitals in octahedral geometry 8

N.N'-bis{(-)-(cis)-myrtanyl} butylene-2,3-diimine (BMDI) 12

1,4-bis(2,6-diisopropylphenyl)acenaphthenediimine 13

Bis( 4-methoxyphenylimino )acenaphthene 14

Bis( 4-fluorophenylimino )acenaphthene 15

Simplified diagram ofrelative energies of the HOMO and LUMO 18 with the decay pathways of excited species

UV-Vis spectrum ofCH3-BIAN 31

UV-Vis spectrum of Ru-M'h-BIAN 32

IR spectra of (a) CH3-BIAN and (b) Ru-CH3-BIAN 35

GC-MS mass spectrum ofCH3-DAB compound 37

TLC ofCH3-DAB compound compared to the starting materials 39

Postulated structure ofRu(dppe)2(CH3-DAB) complex 40

Comparison IH NMR spectra of (a) CH3-BIAN and (b) Ru-CH3- 42 BIAN

IX I'

4.8 Initial postulated structure ofRu-CH3-BIAN complex 42

4.9 Final postulated structure ofRu-CH3-BIAN complex 43

4.10 31p NMR spectrum ofRu-CH3-BIAN complex 44

4.11 Luminescence testing of Ru-CH3-DAB on TLC with (a) short wave 45 and (b) long wave

4.12 Luminescence testing ofRu-CH3-BIAN on TLC with (a) short wave 46 and (b) long wave

4.13 GC-MS mass spectrum ofN02-DAB compound 47

4.14 31 P NMR spectrum of Ru-BIAN complex 49

8.1 UV-Vis spectrum ofCH3-DAB 58

8.2 UV-Vis spectrum ofBIAN 58

8.3 IR spectrum ofCH3-DAB 59

8.4 IR spectrum of BIAN 59

8.5 Secondary GC-MS mass spectrum ofCH3-DAB compound 60

8.6 GC-MS mass spectrum ofBIAN compound 60

8.7 GC-MS chromatogram ofCH3-DAB compound 61

8.8 GC-MS chromatogram o~,IAN compound 62

8.9 GC-MS chromatogram ofN02-DAB compound 63

8.10 IH NMR spectrum ofCH3-DAB compound 64

8.11 IH NMR spectrum ofBIAN compound 65

8.12 IH NMR spectrum ofCH3-BIAN compound 66

8.13 31p NMR spectrum ofRu-CH3-DAB complex 67

8.14 IH NMR spectrum ofRu-CH3-DAB complex 68

x

8.15 31p NMR spectrum of precursor Ru(PPh3)3Ch 69

8.16 31p NMR spectrum of Ru-CH3-BIAN complex in solution mixture 70

8.17 IH NMR spectrum of Ru-CH3-BIAN complex 71

II

~

II

XI

I'

List of Schemes

Scheme Title Page

2.1 General method for synthesizing aromatic DAB 11

2.2 Formation ofN,N -dimesityldiazabutadiene 12

2.3 Synthesis of 1 ,4-bis( 4-carboxylphenyl)-2,3-dimethyl-l ,4- 12 diazabutadiene

2.4 Synthesis ofbis(imino )acenaphthene 14

2.5 Synthesis ofbis( I-naphthylimino )acenaphthene 15 2.6 General synthesis of ruthenium complex with dppe and diimine ¹⁷

ligands

XII

List of Table

Table Title Page

4.1 Physical data of diimine compounds and ruthenium complexes 26 4.2 Solubility data of diimine compounds and ruthenium complexes 28 4.3 Summary of absorption bands of diimine compounds and 30

ruthenium complex

4.4 The IR band of diimine compounds and ruthenium complex 33

4.5 Purity analysis of diimine compounds 36

4.6 The lH NMR data of diimine compounds and ruthenium 38 complexes

II

~

II

XIII

.. ^{--~ ~ ~} ...

SYNTHESIS AND CHARACTERIZATION OF BIDENTATE LIGANDS WITH RUTHENIUM COMPLEX FOR LUMINESCENT TESTING

Khaw Ying Ying

Resource Chemistry

Faculty of Resource Science and Technology Universiti Malaysia Sarawak

ABSTRACT

a-Diimine such as diazabutadiene was a type of bidentate Schiff base ligand used for the synthesis ofruthenium complex as it mimicked the aromatic bipyridine ruthenium complex that emits luminescence. Many diimine ligands structures had been modified to increase the emission quantum yield and luminescence lifetime. DAB-containing complexes had been widely studied while BIAN compound itself had only been recently explored for the photophysical properties due to its electron delocalization from the naphthalene unit. Their unique properties allowed further research on the binding to metal especially ruthenium(I1) complex to test on the luminescence properties. Thus, the objective of this project was to synthesize and characterize the a-<iiimine ruthenium(I1) complex for luminescent testing. In this study, the ligands were synthesized by using a diketone and substituted aniline through condensation process. The ruthenium(I1) complexes containing phosphorus type ligands were synthesized by the substitution of DAB into RuCh·xH20 while BIAN substituents into RU(PPhJ)3Ch under reflux condition in ethyl acetate and dichloromethane respectively. The complexes were confirmed to have binding with the diimine and P-containing ligands in an octahedral geometry. Ruthenium(II) complex with CH3-BIAN ligand was found to produce luminescence under UV lamp.

Keywords: a-<iiimine, DAB, BIAN, ruthenium(I1) complex, luminescence ABSTRAK

a-Diimina seperti diazabutadiene adalah sejenis ligan Schiff bes bidentat yang digunakan untuk sintesis komplek rutenium kerana ia merupai komplek rutenium bipyridina aromatik yang mengeluarkan pendarkilau. Banyak struktur ligan bidentat telah diubahsuai untuk meningkatkan pengeluaran kuantum hasi! dan jangka hidup pendarkilauan. Komplek yang mengandungi DAB telah dikaji denla"n meluas tetapi kajiaan kompaun BlAN masih lagi di tahap permulaan untuk kegunaan perdarki!au disebabkan oleh penyelerakan elektron dari unit naftalena. Keistimewaan penggunaannya membolehkan penyelidikan yang selanjutnya dalam pengikatan pada komplek logam terutamanya komplek rutenium(II) untuk mengaji perdarkilauannya. Oleh itu, objektif projek ini adalah untuk mensintesiskan dan mencirikan a­

diimina komplek rutenium(II) untuk pendarkilauan. Ligan telah disintesiskan menggunakan diketon dan tukar ganti anilin melalui proses kondensasi. Komplek rutenium(II) yang mengalldungi jenis ligan fosforus telah disintesiskan menggunakan penukargantian DAB pada RuCh·xH20 manakala penukar ganti BlAN pada Ru(PPh3)JCI2 dijalankan dalam keadaan refluks dengan menggunakan eti! asetat dan diklorometana masing-masing. Kompleks tersebut lelah disahkan mempunyai pengikatan ligan fosforus dan diimina dalam susunan octahedron.

Komplek rutenium(ll) dengan ligan CH3-BlAN telah disahkan mengeluarkan pendarkilau m nt'rusi lampu UV

Kala kunci: a-diimina, DAB, BlAN, rutenium(II) komplek, pendarkilau I

1.0 Introduction

Bipyridine (bpy) is the fIrst diimine that shows the presence of luminescent properties and has been actively further studied since the past few decades (White et al., 2010). Luminescence is an emission oflight by the complex due to the relaxation of the excited state to the ground state.

[RU(bpY)3f+ is well known to be a luminophore for producing redox-active and luminescent supramo1ecular metal complex. The popularity for the research is because the fundamental mononuclear [Ru(bpY)3f+ has stable ground and excited state properties that can be easily modified and synthesized by selection of their heterocyclic ligands (Nag et al., 2011). The modification is to support the complex with possible higher absorption and emission energies across the UV, visible and near IR regions. Other modification includes the transformation from typical [Ru(bpY)3]2+ to tridentate, [Ru(C"NI\C)(bpy)(C=N)]2+ that has successfully increased the quantum yield that can be used for endocytosis luminescent imaging (Tsui et al., 2015). The advancement also includes the fact that the structure contains non-labile auxilIary ligand, C=N and it is nontoxic for both human breast carcinoma cell and immortalized noncancerous human retinal pigmented epithelium cell. From time to time, researchers have further explored the theories and reasons behind the luminescence properties. Not all late transition metal coordinated to diimine ligands will ha,~,luminescence emission. However, ruthenium complex synthesized with modified diimine such as phenanthroline (phen) which is a modification of bipyridine structure with more aromatic rings can contribute to luminescence properties due to its highly rigid structure. Rajendiran and coworkers (2012) have reported that phen and dipyrido [3, 2-a: 2', 3'-c] phenazine (dppz) ruthenium complex exhibit intense luminescence in the presence of double helical DNA. The dppz moiety is sealed from solvent quenching upon int calation into DNA. Other than probes, ruthenium complex is also popular in luminescent

2

l

^I

oxygen sensing application (Roth, 2000; Ji et al., 2010). Later on, aromatic diimine such as bpy and phen have been modified to a-diimine such as diazabutadiene (DAB) and bis(imino)acenaphthene (BIAN). According to Evans et al. (20 t 5), BIAN is found to be a good example showing luminescent properties in his findings of Zn(II) BIAN complex due to the ^1[­

conjugated naphthalene backbone, rigid structure and the presence of tunable flanking aryl substituents.

1.1 Luminescence properties

Luminescence can be explained as an emission of light that happens at low temperature which can be generated from chemical reactions, electrical energy, subatomic motions and crystal stress (Cummins, 2011). In luminescence theory of transition metal complexes, absorption of light before the emission may excite electrons to different orbitals such as electrons from metal d-orbital to unoccupied ligand orbital causing metal to ligand charge transfer (MLCT) (Mulhern, 2003). Basically, MLCT means electron from metal-centered 7[ orbital is promoted to ligand­

centered 7[* orbital which results in low oxidation number of metal center and reduction of ligands (Roth, 2000). Transition metal complexes that are promoted to MLCT excited state are most likely to exhibit luminescence. A more specific field of luminescence is called photoluminescence which is a result from absorption of photon. Fluorescence and phosphorescence are types of photoluminescence in which they have a lifetime of nanoseconds and milliseconds respectively. Fluorescence emission of [Ru(bpyh]2+ was first observed in 1959 by Paris and Brandt (Cummins, 2011). Fluorescence can only be observed when the excited state of the compound produced by ¹MLCT absorption is relaxed to ground state while for p horescence emission, excited electrons undergoes intersystem crossing (lSC) from

.'

3

IMLCT to 3MLCT and the relaxation produces long-lived phosphorescence (Ma et al., 2013).

The excitation and relaxation ofthe electron are illustrated in Jablonski diagram Figure 1.1 (Li, 2006).

S2

~~~~~~~o. oo _o .=m

ilntemal 00 ^_ _ ' . 0 0 _ 00 _ . _ ~ . . . ._ _ _ _ ~_ _

i Converlion ==:;:::=.==~·~*c==^T2

• '" -= ⁱ

... . t Intersystem

i

^Internal

51 :::jQ::;!==::::}:/.AJ~~~.o o_ooooo_o~.~ssing ! Conversion

;

Vi::'1i~al ·ooo~.~§Y~§~ ."'~~TI

RelDII1ion :

Heal

~

Fluorescence

r{~PhCMIphorescence

Figure 1.1: Simplified Jablonski diagram representing energy levels and spectra of absorption and emission as fluorescence and phosphorescence CLi, 2006).

1.2 Bidentate Schiff base ligands 1.2.1 Diimine

Imines (Figure 1.2) or more commonly called Schiff bases, are compounds that contain the functional group of azomethine, (-C=N-) (Aliyu & Isyaku, 2010). The organic compound is developed by a German chemist, Hugo Schiff in 1864 (Sebastian, 2010). The synthetic pathway of producing Schiff base is via condensation of aldehyde or ketone with primary amine.

4

R R

K

R-N N-R

R=H, alkyl, aryl

Figure 1.2: General structure of diimine

Diimine is a bidentate nitrogen ligand as well as a derivative of Schiff base carrying a pair of azomethine group. R-substituted-diimine is known as a good N-coordinated electron-acceptor ligand that exhibits low-lying 7rorbitals (Zalis et

at.,

2003). There are aromatic and nonaromatic a-diimines. The most common example of diimine used in low oxidation state of late transition metal complexes are 1,10-phenanthroline, pyridine-carbaldimine (Pyca) derivatives, N,N'­

disubstituted-l,4-diazabutadiene (R-DAB) and bis(arylimino)acenaphthene (Ar-BIAN) (Tromp et

at.,

2002). In general, diimine metal complexes are popular for the simple synthesis pathway, ligand stability and good chelating properties (Sebastian, 2010). The application has made diimine metal complexes to be unique because they are able to possess high molar absorptivity, solvatochromism and luminescence properties as compared to the almost similar structure of

1,2-ethylenediamine that does not have metal-to-ligand charge transfer (MLCT) transition (Zalis et

at.,

2003). This is due to I,2-ethy~ediamine ligand lacking low lying 1t orbitals. In fact, diimine ligand structure has been modified from time to time to increase its luminescence properties to meet its wide application demand.

5

1.2.2 Diazabutadiene (DAB)

DAB (Figure 1.3) is a common parent system for the a-diimine synthesis due to the structure that enable the phenyl to be substituted for various applications.

Figure 1.3: Structure of diazabutadiene (DAB)

The reason for this is that DAB has been long proven to be a better 7l'-acceptor than bpy (Guillon et al., 2010). The author discovered that DAB has shorter Ru-N bond with the DAB ligand

(2.04-2.06

A)

than to the bpy ligand (2.10

A)

that results in stronger metal-to-ligand bond. DAB metal complex is able to luminesce strongly in the near-infrared region, even in solution at room temperature (Klein et ai., 2002). This is the reason why DAB served as a higher potential ligand as compared to other ligands such as phosphine ligands for the production of photoactive materials such as diodes and probes due to its absorption in the visible region (Yempally et ai., 2014). Other than that, Ru-DAB complex is mostly reported to possess solvatochromism properties called marginal solvatochromic effect on range of solvents from nonpolar, hexane to polar, acetonitrile (Grupp et ai., 2014).

6

1.2.3 Bis(imino)acenaphthene (BIAN)

Diazabutadiene (Figure 1.4) is the precursor for the BIAN synthesis in which DAB undergoes fusion with naphthalene moiety (Vasudevan, 2009).

~-o

N \ ;)

Figure 1.4: Structure ofhis(imino)acenaphthene (BIAN)

BIAN is a better electron acceptor as compared to DAB as it functions as electron sink that takes up twice the electrons into the ligand system as compared to DAB which has only two electrons from the diimine. This is because the two extra electrons of BIAN come from the naphthalene unit. Besides, this additional exocyclic naphthalene ring is able to stabilize the higher or lower oxidation state of metal center in the complex due to the good a-donating and n-accepting properties (EI-Ayaan & Abdel-Aziz, 2005). Many past researches have reported the experimental observations on the relative energies of LUMO and LUMO+ 1 as obtained on the basis of Density-Functional Theory ~FT) calculations which is a method to analyze the molecular and electronic structures of transition metal complexes (Guillon et ai., 2010). For the research application, BIAN ligands have been widely tested for the polymerization catalyst.

However, it is seldom being tested on photophysical application. In 2007, Fei reported the photoluminescent properties on platinum(II) complex and discovered that the non-heterocyclic identate diimine ligand, bis(mesitylimino )-acenaphthene (mesBIAN) showed a strong a tion along the visible region at 600 nm and emit longer than 750 nm wavelength.

7

Similarly, the same complex synthesized by Adams, Fey, & Weinstein (2006) showed that the emission is allowed by the existence of low-lying ^1(* orbitals on the mesBIAN. The rigid structure allows this weak-field ligands to exhibit lowest charge transfer excited state that has long-lived red luminescence in fluid solution at ambient temperature. As from the studies of Rosa (2008), heteroaromatic ring system from BIAN contributes the a-donating and 7t­

accepting properties that enable it to posses better photophysical properties as compared to bpy or phenanthroline.

1.3 Ruthenium(II) diimine complexes

Theoretically ruthenium(II) diimine octahedral complex is a '" transition metal complexes and has a strong ligand field (Roth, 2000). The strong ligand field formed by imine ligands splits the 5d orbitals into three low-lying t2g orbitals and two high-lying eg orbitals (Figure 1.5) causing it to distribute into the pairing the low-lying t2g orbitals instead of non-pairing.

Orbital 9:',

Energy

dXy dxz

Figure 1.5: Splitting of the d orbitals in octahedral geometry

Upon so many types of Schiff base ligands that have been tested on ruthenium complex for this strong field ligand theory, researchers has discovered that ruthenium(II) complex with ligand ification can alter the luminescence lifetime (Ji et aI., 2010; White et at., 2010). This can

8

be done by establishing an equilibrium triplet-triplet state or switching 3MLCT emission state

to intraligand triplet state elL). Other than ruthenium, there are many types of late transition metals being tested as well. However, ruthenium is still considered as an ideal metal complex to test on luminescence because it experiences facile spin-orbital coupling (SOC) with almost 100% high efficiency to produce triplet state photosensitizer with visible-light activation (Reichardt et

at.,

2015). Due to that, diimine ruthenium(II) complex has been reported to produce the longest lifetime in deoxygenated acetonitrile and alcoholic glass with 270 IlS and 3.44 ms respectively. In fact, transition metal complexes have been developed much on its luminescence properties for chemosensors and biological sensors designing especially ruthenium(lI) polypyridine complex as it shows high photoluminescence properties apart from having high photostability and luminescence quantum yield (Yeung & Yam, 2015). The discovered properties are results from large SOC contributed by heavy metal center and 3MLCT excited state.

9