photoluminescence properties of copper(i) schiff base...

Chapter IV

Photoluminescence properties of copper(I) Schiff base complexes

Chapter-IV Photoluminescence of copper(I) Schiff ........

4.1. Introduction:

The coordination compounds are fascinating class of molecules that have wide

applications in many areas such as light emitting devices (LED’S), trace metal

analysis, metal signaling, design of optical devices and sensor molecules [1-5].

Amongst green, blue and red, stable blue luminescent complexes, useful for

electroluminescent displays [6, 7], are still rare and very challenging to prepare [8, 9].

In recent years, the design and synthesis of blue luminescent transition metal

complexes has become a very active area of research as a result of the demand for

sensitive and selective chemosensors for in vivo and in vitro purposes.

Luminescent copeer(I) complexes are currently receiving much attention due to their

potential applications in various fields, such as organic light-emitting diodes (OLEDs)

[10], light-emitting electrochemical cells (LECs) [11], supramolecular assemblies

[12], chemical sensors [13], solar-energy conversion schemes [14], biological probing

and oxygen sensing [15, 16]. They are being environmentally friendly, less expensive,

intriguing coordination modes, rich photochemical and photophysical properties.

Many of these complexes have been reported to be luminescent and their emission

behavior varies with structures / steric effects of the coordinated ligands.

4.2. Theory of luminescence:

Luminescence is the emission of light from any substance, and occurs from

electronically excited state. The luminescence is formally divided into two categories

fluorescence and phosphorescence depending upon nature of excited state. The

emission rate of fluorescence is typically 108 s-1 so that the typical fluorescence life

time is near 10 ns. Presently, luminescence is a science closely related or

spectroscopy which is the study of the general laws of absorption and emission

radiation by matter [17]. Both disciplines are based on achievement of quantum

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mechanics. The luminescence is the phenomenon in which all or part of absorbed

energy is reemitted in the form of electromagnetic radiations in the visible region or

near visible region of the spectrum. The luminescence involves at least two steps, the

excitation of the electronic system of the materials and the subsequent emission of

photons. Luminescence contrasts with incandescence which is the production of light

by heated materials [18]. When certain materials absorb various kinds of energy,

some of energy may be emitted as light. This process involves two steps.

I. The incidental energy causes the electrons of the atoms of the absorbing materials

to be excited and jump from the inner orbital of the atoms to the outer orbital.

II. When the electrons fall back to their original state, a photon of light is emitted. The

interval between the steps may be short or long. If the interval is short, the process

is called fluorescence; if the interval is long the process is called phosphorescence.

4.2.1. Photoluminescence:

Generating luminescence through excitation of a molecule by UV light photons is

a phenomenon termed as photoluminescence. It is formally divided into two

categories; fluorescence and phosphorescence depending upon the electronic

configuration of the excited state and the emission pathway [19].

Photochemistry is concerned with reactions, which are initiated by electronically

excited molecules generated by absorption of suitable radiations in visible or near

Ultra-violet region. Since absorption of light produce excited states of atoms and

molecules, photochemistry is really the study of the chemistry of excited states.

Electromagnetic radiation in the visible and ultraviolet region is generally required to

produce chemical reactions because changes in electronic energy levels are required.

More recently absorption of many infrared photons from a high-intensity laser also

causes similar chemical reaction.

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4.2.2. Fluorescence:

Fluorescence is the process of absorbing and reemitting light on a time scale of

about 10-8 seconds. Fluorescence involves emission of previously absorbed light at

different wavelengths. Some of the energy of the absorbed light is channeled into

other things in absorbing substance like vibrational levels, photons, thermal energy,

etc.; the remaining energy is then reemitted as a photon. The process responsible for

the fluorescence of fluorescent probes and other fluorophores is illustrated by the

simple electronic-state diagram (Jablonski diagram) shown in Figure 4.1.

Fig. 4.1: Jablonski diagram and a time scale of photophysical processes

4.2.3. Fluorescence characteristics:

The fluorescence is expressed spectrally in terms of energy of fluorescence as

fluorescence emission (λem) wavelength and also an intensity of emission. The

fluorescence spectra recorded with intensity of fluorescence as a function in nm or Å

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is obtained from the spectrum, where a emission wavelength is maximum. Whereas

the height of emission peak at λem gives the intensity of fluorescence (F).

4.2.4. Fluorescence Intensity:

The intensity of fluorescent light is directly related with the concentration of

fluorescent solute in solution is given by following relation

F = (I0-I) ΦF

If ΦF is constant in the measured excitation range, then the shape of the

fluorescence excitation spectrum is determined slowly by the extinction coefficient ε

of the molecule [20]. The fluorescence intensity of a compound is affected by a

number of factors namely structure of the compound, concentration of fluorescencent

solute, effect of solvent, pH of the solution, temperature and irradiation effect.

4.3. Experimental Observable:

Fluorescent molecule has two characteristic spectra viz. excitation spectrum and

emission spectrum. Large molecules with a large number of electron and nuclei have

many vibrational and rotational modes available for a ready distribution of the

absorbed energy and there fluorescence properties are distinctly different from those

of small molecules composed only of several items. Due to this redistribution of the

absorbed energy, their spectra, fluorescence life time and quantum yield are

independent of excitation wavelength. Because of the emission takes place only from

lowest excited state such compound also exhibit only one fluorescence and

phosphorescence bands. Furthermore, due to involvement of vibration and rotational

energy their absorption and luminescence spectra from broad bands instead of the

lines found in simple molecules.

4.3.1. Emission spectra:

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Emission spectrum defines the relative intensity of radiation emitted at various

wavelengths. The fluorescence emission spectrum shows almost mirror-like

symmetry with its absorption. The fluorescence emission spectrum is obtained by

irradiating the sample by a wavelength of maximum absorption as indicated by the

absorption spectrum of sample. Ground state and excited state are associated with the

absorption and emission spectra. It is observed that the absorption spectra give data

about the vibrational level of the ground state. In most of the organic materials the

emission spectrum is the mirror image of the absorption spectrum. This relationship is

an indication of the similarity of the respective vibrational wave functions in the

excited state and ground state.

The independence shape of the fluorescence emission spectrum from the

wavelength of the excitating light arises from the fact that the emission always takes

place from the lowest excited state. If the shape of the emission spectrum changes

with changing wavelengths of the exciting light, then the presence of more than one

fluorescent compound will be suspected [21].

4.3.2. Excitation spectra:

It defines as the relative efficiency of different wavelengths of exciting radiation

to induce fluorescence. The excitation spectrum is obtained by measuring the

fluorescence intensity at fixed emission wavelength, while the excitation wavelength

is scanned. For most large complex molecules, the excitation spectrum is quite stable.

It does not depend on the emission wavelength at which it is monitored.

The excitation spectrum will be identical to the absorption spectrum. The

measurement of quantum efficiency is limited by the sensitivity of the spectro-

fluorometer and that depends upon the intensity of the excitation source. Excitation

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spectroscopy is also used to determine the quantum efficiency of energy transfer

between donor and acceptor molecules [22].

For most of the complexes the quantum yield is independent of excitation

wavelength because of the very efficient process of internal conversion from higher

excited state to the lowest excited singlet state. Even for those molecules excitation

spectra provides a powerful tool for measuring the absorption spectra of the molecules

even if the concentration is too low to be detected by convention absorption

spectrometry. In case of solid crystalline organic material excitation spectrum will be

identical to the absorption spectrum provided. The measurement of quantum intensity,

in case of absorption spectrometry depends on the sensitivity of spectrofluorometer,

which is ultimately governed by the accuracy with which the intensity of the sample

beam is measured with respect to the almost equal reference.

4.3.3. Quantum yield:

The fluorescence quantum yields are perhaps the most important charactertics of

fluorophores. Quantum yields are the number of emitted photons relative to the

number of absorbed photons. Substance with largest quantum yields, approaching

unity, such as rhodomines displays the brightest emissions. A high value of near 1 is

generally observed with molecules having large, planar conjugated systems that are

relatively rigid [23]. More flexible molecules are more likely to have of ΦF, while

molecules whose lowest excited state is achieved by n→π* transition or that contain

heavy atoms such as bromine or iodine are usually non fluorescent. Quantum yield is

characteristics for each fluorescent compound and is independent of the excitation and

emission wavelength. The quantum yield can be close to unity in the radiation less

decay rate is much smaller than the rate of radiate decay.

4.4. Photoluminescence in copper (I) complexes:

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In closed shell systems, the electronic ground states do not experience the ligand

field stabilization effects that are common to d1-9 electronic configurations because of

the completely filled metal d-subshell [24]. Thus neither the coordination number nor

the geometric structure of a d10 complex is imposed by ligand field requirements. In

incompletely filled shells, the electronic transitions between orbitals of the partially

filled d-subshell give rise to ligand field excited states [25]. Since d-d transitions

cannot occur in d10 complexes, the spectroscopy and photochemistry in these systems

are determined solely by the properties of the other types of electronic excited states.

Studies of d10 complexes thus afford the opportunity to obtain information about these

other states under circumstances where ligand field effects are minimized.

The first report on the luminescence properties of d10 metal complexes is dated

back to 1970 when Dori and coworkers noted the luminescence behavior of d10 metal

phosphine complexes of copper(I), silver(I), gold(I), nickel(0), palladium(0) and

platinum(0) [26]. Later reports by a number of groups, mainly pioneered by Gray et

al. have established the relevance of metal-metal interactions in the origin of

luminescence behavior [27]. The idea of metal-metal bond establishment in the

excited state of d8-d8 complex [Pt2(POP)4]4- has stimulated interest in photochemistry

of the related polynuclear d10 complexes [28].

As an electron is promoted from an antibonding dσ* to a bonding sσ or pσ orbital,

the bond order between the d10 metal centers increases in the excited state, leading to

stabilization of shorter metal-metal distances in the cluster [27]. However in the

presence of bridging and/or ancillary ligands, the nature of the excited states of

polynuclear d10 metal complexes become more complicated due to ligand

contributions to the frontier orbitals. Accordingly, a number of luminescent metal

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complexes have been found to emit from an excited state that has significant ligand

character.

Numerous examples can be found in the literature on these closed shell systems

exhibiting rich photophysical properties. These include copper(I) clusters with

phosphines [29] with halides [30] and nitrogen based ligands[31], copper(I) acetylide [32]

and chalcogenide complexes [32] silver(I) polynuclear complexes of chalcogenide

and acetylide complexes [32] and gold(I) thiolate complexes with phosphines [33].

(Figure 4.2)

Fig. 4.2 Examples of photoluminescent d10 complexes

Metal-metal interactions and their influence on the nature of the metal-centered

and mixed {XLCT/(d-s)} excited states are thus expected to distinguish the

photophysical features of the polynuclear d10 complexes, relative to their mononuclear

analogues [34]. In case of copper(I), the lowest energy linear combination of Cu 4s

orbitals of a Cun cluster should be metal-metal bonding; thus, the nature of the

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luminoactive excited state should be strongly dependent on the internuclear distance

dCu-Cu. In this context, a cluster-centered excited state having {XMCT/(d-s)} character

should be substantially distorted from the ground state owing to enhance metal-metal

bonding in the excited state. Such distortions are in general indicated by the large

Stokes shifts experienced by the emission bands [34].

McMillin and his coworkers reported a series of copper(I) complexes of the type

[Cu(biL)(PPh3)2]+, biL = 2,2′-bipyridine (bpy), 1,l0-phenanthroline (phen) and 2,9-di

methyl-l,l0-phenanthroline (dmp) [35]. The dmp complex, unlike the others, emits fairly

efficiently in fluid solution. All the complexes exhibit metal-to-ligand charge transfer

(d-π*) transitions in solution, but in solutions the complex give an extra visible

absorption band which is attributable to a second copper complex formed according to

the equilibrium: [Cu(dmp)(PPh)2]+ = [Cu(dmp)]+ + 2PPh3. In rigid glasses at low

temperature all the complexes exhibit broad, structure less emissions with lifetimes in

the microsecond domain which can be assigned to d-π* states. The copper(I) complexes

of the type Cu(PPh3)2(N,N)+ where, (NN) denotes one of the 2,2-bipyridine derivatives

have been reported [36]. It was observed that the nature of substituent on 2,2-

bipyridine have marked effects on the photophysical properties of complexes, especially

with respect to substitution at the 6 and 6′-positions of the bpy moiety. It is possible

that the most significant role of the “front-side” methyl is in inhibiting quenching

processes associated with expansion of the coordination number of the metal.

Rasmussen and his coworkers reported photophysical properties of tetranuclear

copper(l) cluster compounds based on tetradentate N2S2 and bidentate N, S Schiff base

ligands N,N’(2,2’-diphenyl)-bis(1,3-diphenyl-4-iminomethyl-5-thiopyrazole) [37]. The

studies of the Cu4L2 and Cu4L4 complexes showed that each complex displayed a

broad, unstructured emission band in the red with luminescence lifetimes > 1μs. The

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dynamics of the emission from Cu4(L)2 in the presence of several aromatic organic and

several tris(β-dionato)chromium(III) compounds were probed. The second-order

quenching constants displayed a Marcus type relationship with respect to the

calculated potentials for electron transfer from the cluster centered excited states to

the quencher consistent with this process occurring via simple electron transfer.

The photophysical behavior of bis-bidentate Schiff base copper (I) complexes with

formulae [CuL(CH3CN)2]ClO4 and [CuL(PPh3)2]ClO4, L= butane-2,3-dione bis

(salycilahydrazone) have been reported [38]. At room temperature ligand L is weakly

fluorescent in CH2Cl2, however in both the complexes the emission is quenched. This

emission can be tentatively assigned to the intra-ligand fluorescent emission, which

related to the energy gap between π-π* molecular orbital of the π conjugation of the

ligand system. The quenching of the fluorescence by paramagnetic and diamagnetic

metal ions which might be due to the electron transfer by heavy atom of the ligand.

The structural variations and photoluminescence properties of copper(I) complexes

with bis(Schiff base) ligands and their copper(I) mixed ligand complexes with PPh3

have been reported [39]. The copper(I) bis(Schiff base) complexes show

photoluminescence at room temperature, which is ascribed to be from intraligand π–π*

transition mixed with MLCT characters.

J. Lu reported the copper(I) complex having formula [(PPh3)2Cu(dmp)-O-(dmp)

Cu(PPh3)2]2+ (dmp = 2,9-dimethyl-1,10-phenanthroline) [40]. The complex displays

luminescence from an MLCT band in fluid solution and emission from both a metal

centered charge-transfer and intraligand (phenanthroline) state at 77 K in chloroform.

The photoluminescence properties of flavonalato bis(triphenylphosphine)copper (I)

complex [Cu(fla)(PPh3)2]+X-, shows luminescence at room temperature which

originate from flavonolate [41]. The emission consists of fluorescence (λ max, 595

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nm, τ- 9.8 μs) and phosphorescence (λ max, 810 nm, τ-1.2 μs). This conclusion is

based upon the observation that the 810 nm emission decays much slower that the 595

nm luminescence intensity at lower temperature is also consistent with these

assignment. The simultaneous appearance of intraligand fluorescence and phosphor-

rescence under ambient conditions is not restricted to [Cu(fla)(PPh3)2 ]+

Chen et al. [42] synthesized mixed ligand complexes having formula [Cu(H-Norf)

(PPh3)2]+ClO4 which shows strong fluorescence and red shifted to about 20 nm in

comparison to that of the free Norfloxacin. This behavior is probably due to the

formation of supramolecular hydrogen bonding interactions in the solid state. The

emission of complex is neither MLCT nor LMCT in nature, and can probably be

assigned to intraligand fluorescent emission.

Yam and his coworkers have investigated luminescence behavior of tetranuclear

copper(I) diynyl complexes[43]. These copper(I) diynyl complexes [Cu4Par3)4(μ3-ή1–

C≡C-C≡CR’)4] (Ar = Ph, R’ = Ph, C6H4CH3-p, C6H4OCH3-p, Ar = C6H4CH3-p,

C6H4F-p, R’ = Ph) show interesting spectroscopic and photophysical properties. The

origin of low energy emission in the tetranuclear copper(I) diynyl complexes is

assigned as derived from a metal centered 3d94S1 state, mixed with LMCT

[diynyl→Cu4] and intraligand [π-π*(diynyl)] states. The mononuclear copper(I) complexes

of the type Cu(PPh3)2(oxine)(BF4) (1) and Cu(PPh3)2(Quin)(BF4) (2) exhibit

photoluminescence at room temperature in the solid state originated from the

intraligand (oxine or quinoline) triplet enhanced by copper coordination [44].

Similarly, the binuclear copper(I) complex containing mixed-ligands [Cu(dppm)

(phen)]2(NO3)2.6H2O (dppm = Ph2PCH2PPh2, phen = 1,10-phenan throline) exhibit

intense photoluminescent behavior at room temperature [45].

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Araki and his coworkers synthesized copper(I) complexes of the type

[Cu2(X)2(PPh3) (L)n] with various mono and bidentate N-heteroaromatic ligands

where X = Br, I; L = 4,4-bipyridine, pyrazine, pyrimidine, 1,5-naphthyridine, 1,6-

naphthyridine, quinazoline, N,N-dimethyl-4-aminopyridine, 3-benzoylpyridine, 4-

benzoylpyridine; n = 1, 2 [46]. The complexes showed strong emission at room

temperature as well as at 80 K in the solid state. The emissions of the complexes

varied from red to blue by the systematic selection of the N-heteroaromatic ligands

(λmax: 450 nm, N,N-dimethyl-4-aminopyridine to 707 nm, pyrazine), and were assigned

to metal-to-ligand charge-transfer (MLCT) excited states with some mixing of the

halide-to-ligand (XLCT) characters. The emission energies were successfully

correlated with the reduction potentials of the coordinated N-heteroaromatic ligands,

which were estimated by applying a simple modification based on the calculated

stabilization energies of the ligands by protonation.

Kaltzoglou et al. reported a series of mixed ligand copper(I) bromide complexes

containing 1,2-bis(diphenylphosphino)benzene (dppbz) and some heterocyclic thione

(L) [47]. At room temperature, the complexes in the solid state exhibit strong

emission assigned to a metal-ligand charge transfer of type copper(I)→(PPh2). The

strong fluorescence observed in 1D coordination polymer of copper(I) complex

[Cu(PPh3)(L)](ClO4) (PPh3 = triphenylphosphine, L= 1,2-bis(3′-pyridylmethylene

amino)ethane and [Cu(PPh3)2(CH3CN)2]ClO4 at room temperature [48].

Tronic et al. [49] reported the metallorganic networks of CuCN with diimines (L)

pyrazine (Pyz), 2-aminopyrazine (PyzNH2), quinoxaline (Qox), phenazine (Phz), 4,4-

bipyridyl (Bpy), pyrimidine (Pym), 2-aminopyrimidine (PymNH2), 2,4-diamino

pyrimidine (Pym-(NH2)2), 2,4,6-triaminopyrimidine (Pym(NH2)3), quinazoline (Qnz),

pyridazine (Pdz), and phthalazine (Ptz). The products are weakly or nonluminescent,

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presumably as a result of the quenching of the CuCN activity by the aromatic spacer

ligands. The copper(I) cyanide coordination polymers based on 2-(n-pyridyl)

benzimidazole ligands (n = 2, 3, 4) of the type [Cu2(CN)(2-PyBIm)]n (1),

[Cu2(CN)2(3-PyHBIm)]n (2), {[Cu3(CN)3(4-PyHBIm)4]2H2O}n (3) and [Cu5(CN)3(4-

PyBIm)2]n (4) possess one dimensional linear chain, ladder-like double chain, saddle-

shaped helical chain, and two-dimensional layer polymeric structures. These

complexes are thermally stable and display luminescence in the solid states. The ratio

of Cu/CN, the position of pyridine N atom in three 2-(n-pyridyl)benzimidazole (n = 2, 3,

4) ligands, and the deprotonated or neutral ligands influence the coordination structures,

thermal stabilities and luminescent properties.

Aslanidis et al. [50] synthesized mixed-ligand complexes of the formula

[CuX(rac-binap)(thione)] by the reactions of copper(I) halides with racemic 2,2’-

bis(diphenylphosphano)-1,10-binaphthyl(rac-binap) in 1:1 molar ratio followed by the

addition of 1 equiv. of pyridine-2-thione (py2SH), pyrimidine-2-thione (pymtH) or

4,6-dimethyl-pyrimidine-2-thione (dmpymtH). The complexes are strongly luminescent

in the solid state at ambient temperature. The origin of the luminescence assigned as

having mixed character between metal–ligand charge transfer (MLCT) of the type

Cu(I)→(PPh2) and ligand–ligand charge transfer (LLCT) of the type thione→(PPh2)

or/and halide→(PPh2), with an unambiguous prevalence of the MLCT emission.

Z. Li and his coworkers studied cluster-based coordination polymers

[(CuStBu)4(dppe)]n (1) and [(CuStBu)6(bix)]n (2) by the solvothermal reactions of

copper(I) tert-butylthiolate (CuStBu) with 1/3 equiv. of dppe (bis(diphenylphosphino)

ethane) or bis(1,4-bis(imidalzole-1-ylmethyl)benzene) in CH3CN [51]. The presence

of organic linkers in these complexes can enhance the photoluminescent performance

of CuStBu compounds. Compound 1 and 2 exhibit strong photoluminescence with

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peaks maximum at 603 and 629 nm, respectively. The mononuclear neutral Cu(I)

complexes, Cu(L1)PPh3 and Cu(L2)(PPh3)2 [L1 = [{N(C6H3Pr2-2,6)C(H)}2CPh]- L2 =

[{N(C6H5Pr2-2,6)C(H)}2CPh]- exhibit green emission in a solid state at room

temperature [52].

Shia and coworkers reported a novel copper(I) complex [Cu(POP)(PTZ)]BF4

(POP = bis[2-(diphenyl-phosphino)phenyl]-ether, PTZ = 5-(2-pyridyl)tetrazole) [53].

The complex shows bright bluish-green emission in solid-state at room temperature,

arising from metal to ligand charge-transfer (MLCT) transition. The photoluminescence

quantum yield measured in air is 0.12. The excited state lifetime of [Cu(POP)(PTZ)]

BF4 is on the order of microsecond indicating the presence of phosphorescent emission.

The emission intensity of [Cu(POP)(PTZ)]BF4 appears to be highly sensitive to the O2

concentration, meaning it possesses oxygen sensing property. The luminescence of

[Cu(POP)(PTZ)]BF4 is easily quenched by oxygen and exhibits strong oxygen

dependent characteristics.

Zhang and coworkers synthesized copper(I) complexes of 4,5-diazafluoren-9-one-

derived (Dafo-derived) from diimine ligands with bis(2-(diphenylphosphanyl)phenyl)

ether as the auxiliary ligand [54]. The introduction of an electron-donor moiety into

diimine ligand leads to a dramatic red shift of the absorption of corresponding copper

(I) complex, while, an electron-acceptor moiety demonstrates no obvious effect on

copper (I) complex absorption when introduced into diimine ligand. The intraligand

charge transfer of Dafo-derived ligands acts as an efficient luminescence quencher

within their corresponding copper(I) complexes, leading to luminescence absence

from metal-to-ligand charge-transfer (MLCT) excited state.

Dairiki et al. reported photoluminescence in mixed-ligand copper(I) complexes

containing phosphinesulfide ligands [55]. The emissive excited states are assigned to

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MLCT transitions involving of the π* orbital of dmp, as deduced from the

luminescent behavior of [Cu(dmp)2]+, dmp = 2,9-dimethyl-1,10-phenanthroline;

DPEPhos = bis[2-(diphenylphosphino)phenyl]ether), 2,9-diphenyl-1,10-phenanthroline

(dpp) and other mixed-ligand complexes. A relatively large stokes shift was observed

in copper(I) complexes results from flattening distortion which occurs in the MLCT

excited state. For both complexes [Cu(dmp)(dppe)]+ and [Cu(dmp)(dppp)]+ no

emission is observed in solution. In contrasts with the fact that [Cu(dmp)(dppe)]+ and

[Cu(dmp)(dppp)]+ emit in solution. It may be caused by the fact that coordination

sphere around the copper atom of the diphosphine sulfide complexes is less crowded

than the diphosphine complexes. The less crowded structure allows the metal center

to facilitate tetragonal flattening distortion of the excited luminophore and the solvent

molecules to attack the metal center in the excited states. These quenching

mechanisms effectively affect the photophysical properties in solution. Mixed-ligand

copper(I) complexes containing diimine and chelating phosphinesulfide ligands are

reported. These complexes shows strong emission in the solid state, and their bands

are assigned to MLCT. The copper(I) complexes containing 8-(diphenylphosphino)

quinoline (PPh2qn) or 8-diphenylphosphinoquinaldine (PPh2qna) ligands shows

luminescence behavior which is assigned to a MLCT transition involving π* of the

quinoline group [56].

Chen et al. reported the luminescent homonuclear copper(I) halide complexes

based on the 3,5-bis{6-(2,2’-dipyridyl)}pyrazole ligand [{Cu(PPh3)X}2(μ-HL)] (HL =

3,5-bis{6- (2,2’-dipyridyl)}pyrazole, X = I(1), Br(2), by straightforward double-layer

diffusion approach [57]. The complexes exhibit photoluminescence in the solid state

at ambient temperature, ascribed to a combination of intraligand charge transfer

(ILCT) and metal-to-ligand charge transfer (MLCT) transitions. The emission of

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complex (2) is red shifted 18 nm relative to that of (1) due to the substitution of the

iodides of (1) by the weaker electron donating bromides. The copper(I) complexes of

the type [CuL2]ClO4 (2) were obtained by the reaction of 2,6-diamino-3-[(2-

carboxymethyl) phenylazo]pyridine(L) and Cu(ClO4)2.6H2O exhibit photoluminescence

at 557 nm [58]. The copper(I) cluster with triazenide ligand [Cu3L3].THF.CH3OH (1)

formed by the reaction of 1[(2-carboxymethyl)benzene]-3[benzothiazole]triazene

(HL) and CuCl in THF/methanol in the presence of Et3N also shows strong

photoluminescence at room temperature [59].

Zhang et al. synthesized mononuclear copper (I) complexes of the type [Cu(bop)

(PPh3)2]BF4, [Cu(fop)(PPh3)2]BF4 and [Cu(pop)(PPh3)2]BF4 where 2(5-ter-butyl-1,3,

4-oxadiazol-2-yl)pyridine (bop), 2(5-(trifluoromethyl)-1,3,4-oxadiazol-2-yl)pyridine (fop)

or 2-(5-phenyl-1,3,4-oxadiazol-2-yl)pyridine (pop), used as N,N chelate ligands and

triphenylphosphine as ancillary ligand [60]. Impact of substituents in the N, N ligand

on the emission wavelength of copper(I) complexes was investigated. The complex

[Cu(fop)(PPh3)2]BF4 with a CF3 group in the N,N chelate ligand exhibited the lowest

energy absorption and emission band. The electrochemical analyses combined with

density functional theory (DFT) calculations showed that the introduction of electron

withdrawing group (CF3) decreases the HOMO-LUMO energy gap and the introduction

of electron donating group (t-Bu) into 1,3,4-oxadiazole moiety has a similar effect on the

emission wavelength as that of the introduction of phenyl group with π-conjugation.

4.5. Experimental:

The photoluminescence properties of Schiff base ligands L1-3 and their copper(I)

complexes were recorded in 10-3 M dichloromethane solution at room temperature.

Luminescence properties were measured using a JASCO F.P.750 fluorescence

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spectrophotometer equipped with quartz cuvett of 1 cm3 path length at room temperature.

Quinine sulfate is used as references material for the quantum yield calculations. The

quantum yields were estimated with references to quinine sulfate with known φR of

0.52. The area of emission spectrum was integrated using the software available in the

instrument and quantum yield was calculated according to the following equation.

φS = AS/AR X (Abs)R / (Abs)S X φR (4.1)

Here φS and φR are the fluorescence quantum yield of the sample and reference,

respectively. AS and AR are the area under the fluorescence spectra of the sample and

reference, respectively. (Abs)S and (Abs)R are the respective optical densities of the

sample and the reference solution at the wavelength of excitation.

4.6. Results:

The photoluminescence properties of Schiff base ligands L1-3 and their copper(I)

complexes 1a-d, 2a-d, 3a-d, 4a-d, 5a-d and 6a-d were investigated in CH2Cl2

solution (10-3 M) at room temperature and are shown in Figs. 4.3-4.9. The results of

excitation wavelength, emission wavelength and quantum yield investigations are

summarized in Table 4.1-4.3.

4.6.1. Schiff base ligands L1-3:

The excitation spectra of Schiff base ligands L1-3 display maximum excitation

wavelength at 303 nm in L1, 310 nm in L2 and 325 nm in L3, respectively. Upon

excitation at 303 nm (within the charge transfer envelope) at room temperature in

dichloromethane ligand L1 shows weak emission band with maximum at 413 nm. The

ligand L2 exhibit fluorescence signal with maxima at 405 nm excited upon 310 nm.

However, the ligand L3 shows emission band at 425 nm upon excitation at 325 nm.

The emission observed in these ligands can be tentatively assigned to the intra-ligand

131


fluorescent emission, which is related to the energy gap between the π→π* molecular

orbital of the π–conjugation of the ligand system [61]. The increase in the

fluorescence efficiency was observed between the Schiff base ligand L3 as a result of

electron withdrawing (p-NO2) group in Schiff base moiety. The emission quantum

yield (φ) is determined and it is found to be 0.014-0.045 against quinine sulphate in

0.1 N H2SO4.

Table 4.1: Photophysical data of ligands L1-L3

Compound Excitation (nm) Emission (nm) Quantum yield (φ)

L1 303 413 0.014

L2 310 405 0.045

L3 325 425 0.029

Fig. 4.3: Emission spectra of the Schiff base ligand (L1-L3)

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4.6.2 Copper(I) complexes containing PPh3 ligand

The copper(I) complexes containing PPh3 ligand 1a-d, 2a-d and 3a-d display

maximum excitation wavelength at 386-392, 381-390 and 382-386 nm respectively.

Upon excitation at 386-392 nm the complexes 1a-d show intense broad emission band

with maxima at 420-504 nm (Fig. 4.4). The complexes 2a-d exhibit fluorescence

signal with maxima at 418-498 nm excited upon 381-390 nm (Fig. 4.5). However, the

complexes 3a-d shows emission band at 447-525 nm upon excitation at 382-386 nm.

(Fig. 4.6). The fluorescence emission observed in all these complexes at room

temperature which is ascribed to from intra-ligand π→π* transition mixed with metal-

ligand charge transfer (MLCT) transitions and are red shifted by 25-100 nm as

compared the corresponding free ligands L1-3 [62]. The fluorescence quantum yield

(φ) of the complexes was determined using the quinine sulfate in 0.1 N H2SO4 as a

reference with known φR of 0.52 and appeared at 0.032-0.072, 0.0.038-0.064 and

0.045-0.075 for complexes 1a-d, 2a-d and 3a-d, respectively. The area of emission

spectrum was integrated using the software available in the instrument and quantum

yield was calculated by using equation 4.12 given in experimental section. It is

Table 4.2: Photophysical data of copper(I) complexes containing PPh3 ligand


1a 386 434 0.038

1b 386 420 0.032

1c 392 499 0.072

1d 389 504 0.050

2a 390 426 0.039

133


2b 381 418 0.036

2c 385 467 0.041

2d 389 498 0.044

3a 384 475 0.048

3b 382 447 0.044

3c 386 512 0.046

3d 385 525 0.052

Fig. 4.4: Emission spectra of the copper(I) complexes (1a-d)

134


Fig. 4.5: Emission spectra of copper (I) complexes (2a-d)

Fig. 4.6: Emission spectra of copper(I) complexes (3a-d)

135


observed that all the complexes show blue green emission with high quantum yield in

dichloromethane solution.

4.6.3. Copper(I) complexes containing dppe ligand:

The copper(I) complexes containing dppe ligand 4a-d, 5a-d and 6a-d display

maximum excitation wavelength at 385-392, 391-398 and 382-390 nm, respectively.

Upon excitation at 385-392 nm the complexes 4a-d show intense broad emission band

with maxima at 448-532 nm (Fig. 4.4). The complexes 5a-d exhibit fluorescence

signal with maxima at 434-506 nm excited upon 395-398 nm (Fig. 4.5). However, the

complexes 6a-d shows emission band at 465-548 nm upon excitation at 379-390 nm.

(Fig.4.6). The fluorescence emission observed in all these complexes at room

temperature which is ascribed to from intra-ligand π→π* transition in combination in

the instrument and quantum yield was calculated by using equation 4.12. All the

Table 4.3: Photophysical data of copper(I) complexes containing dppe ligand


4a 385 464 0.030

4b 385 448 0.032

4c 387 525 0.069

4d 389 532 0.050

5a 395 445 0.031

5b 397 434 0.025

5c 398 496 0.027

5d 395 506 0.023

6a 385 492 0.035

6b 379 465 0.042

6c 382 532 0.040

136


6d 390 548 0.043



137



complexes shows blue green emission with high quantum yield at 0.030-0.069, 0.038-

0.072 and 0.031-0.054 nm in the complexes 4a-d, 5a-d and 6a-d, respectively. 4.7.

Discussion:

The photophysical properties of the Schiff base ligands (L1-3) and their copper(I)

complexes were investigated in 10-3 M dichlomethane and it was found that the

copper(I) complexes (1a-6a, 1b-6b, 1c-6c and 1d-6d) show a significant enhancement

in their fluorescence as compared to the corresponding free ligands L1-3. It has been

observed that metal ions can enhance or quench the fluorescence emission of nitrogen

containing compounds. In the absence of metal ions the fluorescence of the ligands is

probably quenched by the appearance of a photo induced electron transfer (PET)

process due to the presence of a lone pair of electron on the nitrogen atoms. Such PET

process is prevented by complexation of the ligand with metal ions; thus the

138


fluorescence intensity may be greatly enhanced by coordination to metal ion which is

defined as chelation enhanced fluorescence intensity [21]. The complexation with

metal ions effectively increases the rigidity of the ligands and reduces the loss of

energy via radiation less thermal vibrational decay [63, 64].

Interestingly, it is observed that the steric, electronic and conformational effects

imparted by the coordinating ligands play an important role in improving the

photophysical properties of the complexes. It is observed that the introduction of the

substituents on Schiff base ligands (L1-3) has marked effect on the emission spectra of

the complexes. The copper(I) complexes 2a-d and 5a-d containing electron-donating

group (p-OCH3) on Schiff base ligand (L2) display an emission at 415-498 and 434-

506 nm, respectively However, the complexes 3a-d and 6a-d containing electron

withdrawing group (p-NO2) on Schiff base ligand (L3) show maximum emission

wavelength at 447-525 and 465-548 nm, respectively. These variations of fluorescence

efficiency observed in all these complexes support the conclusion that substituents with

different electronic effects on Schiff base ligands can evidently affect the fluorescence

efficiency of the complexes.

It has been also observed that the photoluminescence efficiency of the complexes

containing dppe ligand (4a-d, 5a-d and 6a-d) appeared at longer wavelength as

compared to the complexes containing PPh3 ligand (1a-d, 2a-d and 3a-d). It has been

found that the copper(I) complexes containing PPh3 ligand (1a-d, 2a-d and 3a-d)

display an emission at 434-504, 418-498 and 447-525 nm, respectively. However, the

complexes containing dppe ligand (4a-d, 5a-d and 6a-d) show maximum emission

wavelength at 448-532, 434-506 and 465-548 nm, respectively. This anomaly could

be attributed to the fact that the complexes 4a-d, 5a-d and 6a-d are least sterically

139


crowed molecules, and hence the most susceptible to structural relaxation resulting in

a relatively longer wavelength in solution [65].

140


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