Synthesis and emission studies of polypyridyl complexes of platinum (II).

Abstract

Chapter One serves as an introduction to the photophysical properties of square-planar 2,2':6',2/1-terpyridyl and 6-phenyl-2,2'-bipyridylligand complexes of platinum(II). A brief description is provided of the absorption and emission spectra of each complex, the latter generally both in solution and the solid state. The assignments made to the absorption and emission bands are reported, together with the lifetimes ofthe emitting states where these have been measured. The relationship between the molecular and/or crystal structure of the complex and its photophysical properties is discussed. Given this background, the overall aims of the work in this dissertation are presented.

In Chapter Two the synthesis and characterisation of nine novelligands are described. These are 4'-(O-naphthyl)-2,2':6',2/1-terpyridine (4'-fJNp-terpy), 4'-(meta-biphenyl)-2,2':6',2/1-terpyridine (4'-mBiph-terpy), 4'-(para-biphenyl)-2,2':6',2"-terpyridine (4'-pBiph-terpy), 4-(fJnaphthyl)-

6-pheny1-2,2'-bipyridine (H4-fJNp-phbipy), 4-(para-biphenyl)-6-pheny1-2,2'bipyridine (H4-pBiph-phbipy), 4-phenyl-6-(2'-pyrazinyl)-2,2'-bipyridine (4-Ph-pzbipy), 4(ortho-CH3-phenyl)-6-(2'-pyrazinyl)-2,2'-bipyridine (4-oMePh-pzbipy), 4-(ortho-CF3-phenyl)6-pyrazinyl-2,2'-bipyridine (4-oCF3Ph-pzbipy) and 4-phenyl-2,6-bis(2'-pyrazinyl)-pyridine (4-Ph-pybipz).

In Chapter Three the synthesis, characterisation and photophysical properties of platinum(II) complexes ofthe ligands 4'-jlNp-terpy, 4'-mBiph-terpy and 4'-pBiph-terpy are described. The complexes are salts prepared of the following composition: [Pt(4'-jlNp-terpy)CI]SbF6 (1),

[Pt(4'-jlNp-terpy)Cl]BF4 (~), [Pt(4'-jlNp-terpy)Cl]CF3S03 (J), [Pt(4'-mBiph-terpy)Cl]SbF6 (1), [Pt(4'-mBiph-terpy)Cl]BF4 (~), [Pt(4'-mBiph-terpy)Cl]CF3S03(~), [Pt(4'-pBiph-terpy)CI]SbF6 (1), [Pt(4'-pBiph-terpy)Cl]BF4 (~) and [Pt(4'-pBiph-terpy)Cl]CF3S03 (2). Each ligand is

characterised by a substituent in the 4'-position that has been designed to permit free rotation of that substituent about the interannular bond to the terpyridyl fragment. The rotational freedom allows the 4'-substituent to adopt an orientation nearly coplanar with that of the

terpyridyl fragment. As a result ofthe extensive n-conjugation, there is a strong ligand-centred influence in their excited state emission spectra. Evidence is also presented for 3MLCT (metal 111 to ligand charge transfer) character in the emitting state, in particular that the emission is

quenched in coordinating solvents. In addition, an 3ILCT (intraligand charge transfer) contribution has been proposed to account for the exceptionally long lifetimes measured in fluid solution for [Pt(4'-j3Np-terpy)C!t (12 jJs), [Pt(4'-pBiph-terpy)Cq+ (4 jJs) and [Pt(4'-aNpterpy) C!t (17 jJs). We conclude that large easily ionisable substituents in the 4'-position of the terpyridyl moiety ensure long lifetimes in fluid solution for platinum(II) complexes of these ligands. The same assignments apply to the solid state spectra of the yellow or pale orange salts that exhibit monomeric emission in the solid state, these being: [Pt(4'-j3Np-terpy)CI]SbFu (l), [Pt(4'-j3Np-terpy)CI]BF4 (I), [Pt(4'-j3Np-terpy)CI]CF3S03(J.), [Pt(4'-pBiph-terpy)CI]SbFu

(1) and [Pt(4'-pBiph-terpy)CI]BF4 (~). In the solid state two additional types of solid state emission have been identified in cases where intermolecular interactions are present. The first type is typical of red or dark orange salts viz., [Pt(4'-mBiph-terpy)Cl]SbFu(1), [Pt(4'-pBiphterpy)

Cl]BF4 (red form) (~) and [Pt(4'-pBiph-terpy)Cl]CF3S03 (.2). All three salts exhibit MMLCT (metal-metal to ligand charge transfer) emission brought about by close intermolecular Pt···Pt interactions in the solid. Finally, the three orange compounds [Pt(4'mBiph-terpy)Cl]SbFu.CH3CN (4a), [Pt(4'-mBiph-terpy)Cl]BF4 (~) and [Pt(4'-pBiphterpy) Cl]CF3S03(.2) exhibit simultaneous emission from IL (intraligand) and MMLCT states that are similar in energy. In the case of [Pt(4'-mBiph-terpy)Cl]SbF6.CH3CN (4a) the

assigmnent ofMMLCT emission is supported by a X-ray crystal structure determination that shows the cations interacting in dimeric pairs which have a platinum-platinum separation of 3.356(2) A.

In Chapter Four the synthesis, characterisation and photophysical properties of the neutral complexes [Pt(4-j3Np-phbipy)Cl] (10) and [Pt(4-pBiph-phbipy)Cl] (11) are described. The ligands, 4-j3Np-phbipy and 4-pBiph-phbipy are closely related to 4'-j3Np-terpy and 4'-pBiphterpy,

the only difference being that the terpyridyl fragment is replaced by a phenyl-bipyridyl moiety that binds to the platinum via a deprotonated carbon atom in the ortho-position of the phenyl ring. The stronger a-donor strength of the anionic carbon atom is shown to result in:

(i) a marked shift to the red in the MLCT absorption bands for the two complexes and (ii) a significant lowering of the energy ofthe emitting state, as compared to that observed for the cationic terpyridylligand derivatives. The assignment of the emission as 3ILPMLCTin origin remains the same, however, both in solution and in the solid state.

In Chapter Five we report the synthesis, characterisation and photophysical properties of a series of platinum(II) complexes of the ligands 4-Ph-pzbipy, 4-oMePh-pzbipy, 4-oCFJPhpzbipy and 4-Ph-pybipz viz., [Pt(4-Ph-pzbipy)CI]BF4 (12), [Pt(4-oMePh-pzbipy)CI]SbF6 (14), [Pt(4-oMePh-pzbipy)CI]BF4 (15), [Pt(4-oMePh-pzbipy)CI]CFJSOJ (16), [Pt(4-oCFJPhpzbipy)

CI]SbF6 (17), [Pt(4-oCFJPh-pzbipy)CI]BF4 (18) and [Pt(4-oCFJPh-pzbipy)CI]CFJSOJ (19). These ligands coordinate to platinum via a binding domain analogous to the terpyridyl fragment except that one (or both in the case of 4-Ph-pybipz) of the outer pyridine rings is replaced with a pyrazine ring. The ligands 4-oMePh-pzbipy and 4-oCFJPh-pzbipy are distinguished from 4-Ph-pzbipy by the presence of bulky methyl and trifluoromethyl substituents, respectively, in the ortho position of the peripheral phenyl ring. Monomer

emission measured in room temperature fluid solution in a 77 K rigid glass for the three luminophores viz. [Pt(4-Ph-pzbipy)CIt, [Pt(4-oMePh-pzbipy)CIt and [Pt(4-oCFJPhpzbipy) Clr derives from an excited state of JILPMLCT mixed orbital parentage. Their emission energies are stabilised compared to their terpyridyl analogues but emission lifetimes are essentially the same. Solid state emission spectra have been recorded for salts ofthe [Pt(4oMePh-

pzbipy)CW and [Pt(4-oCFJPh-pzbipy)CW cations. In every case emission is derived from an excited state brought about by intermolecular interactions, either in the form of excimeric or of MMLCT emission. In general orange compounds ([Pt(4-oCFJPhpzbipy) CI]SbF6 (orangeform ) (17) and [Pt(4-oCFJPh-pzbipy)CI]BF4 (18)} displayexcimeric emission and dark red compounds {[Pt(4-oMePh-pzbipy)CI]SbF6 (14), [Pt(4-oMePhpzbipy)

CI]BF4 (15), [Pt(4-oMePh-pzbipy)CI]CFJSOJ(16), [Pt(4-oCFJPh-pzbipy)CI]SbF6 (red form) (17) and [Pt(4-oCFJPh-pzbipy)CI]CFJSOJ (19)} display emission from a MMLCT excited state. The [Pt(4-oCFJPh-pzbipy)CI]SbF6 (17) salt displays polymorphism, existing in two formS, one red and the other orange. In all cases the lower energy of the n-acceptor orbitals of the pyrazinyl-bipyridyl moiety causes emission to be at lower energies compared to that recorded for closely related terpyridyl analogues. Finally, attempts to coordinate the bis-pyrazinylligand, 4-Ph-pybipz, to platinum to form [Pt(4-Ph-pybipz)CI]SbF6 (13) were unsuccessful.