Tải bản đầy đủ - 0 (trang)
II. Luminescent Organometallic Polynuclear Systems and Coordination Polymers Containing a Terminal Isocyanide Ligand

II. Luminescent Organometallic Polynuclear Systems and Coordination Polymers Containing a Terminal Isocyanide Ligand

Tải bản đầy đủ - 0trang

Luminescent Organometallic Polynuclear Systems



΄M΅



C N



΄M΅



Linker



49



N C

n



: Metallic center



FIGURE 3. General structural motif of an organometallic polymer with a bridging

diisocyanide linker.



Interesting examples of assemblies of star-shaped polymetallic systems

with D3h symmetry using 1,3,5-tris[(4-isocyano-3,5-diisopropyl-phenyl)ethynyl]

benzene as the connecting node were provided by Kang and Ko.12 The authors

used the coordination ability of this functional trisisocyanide to attach a broad

variety of transition metal fragments such as AuCl, AuSPh Cr(CO)5, and

[(η5-Cp*)RhCl2]. The coordination of rhenium- and platinum-containing

fragments, such as [Re(bpy)(CO)3(MeCN)]PF6 and [(CNN)PtCl] (CNN 5 o-6phenyl-2,2u-bipyridine), yielded the two luminescent Re(I) and Pt(II)

trisisocyanide complexes A and B (Fig. 4).

The photophysical properties of these two complexes are summarized by

their UV-visible absorption, emission and excitation spectra (Fig. 5).

The absorption bands near 340 nm are assigned to metal to ligand charge

transfer (MLCT). Similarly, the broad emission bands arises from the lowest

energy of a MLCT excited state. In this case, the quantum yields were also

measured, and a strong ligand field effect on the luminescence properties was

quoted. All in all, the existence of these complexes confirms that the isocyanide

ligands are versatile building blocks for the preparation of luminescent oligomers and polynuclear species.

Luminescent copper- and silver-containing coordination polymers were

prepared by Harvey and collaborators using 1,8-diisocyano-p-menthane (dmb)

as the coordinating unit.29,30 This aliphatic diisonitrile ligand can adopt two

conformations as depicted in Figure 6.

The polymer thus obtained, {[Ag(dmb)2Y]}n (Y5counter-anion), is a

one-dimensional (1D) material, in which each silver atom is coordinated in

a tetrahedral fashion and doubly bridged by dmb ligands with the adjacent Ag

neighbors.31 The tetrahedral geometry about the Ag metals is found distorted

where the C-Ag-C angles deviate from the ideal 109.42 . Different counteranions

were investigated, and the structure proved to be variable. The solid-state

arrangement was confirmed by X-ray analysis (Fig. 7). Despite their limited

solubility, it was possible to demonstrate by 13C NMR, T1 (spin-lattice

relaxation time) and by measuring nuclear Overhauser enhancement (NOE)

constants that the polymers dissociate in oligomers of 7À9 units long in

solution. Moreover, these polymers were investigated for their photophysical

properties, because they exhibit a strong luminescence (λemi B 500 nm) upon

UV light irradiation (λexc 5 250 nm). Several investigations were then conducted on these materials to better understand their structures and properties.30,32 Using dmb again as the bridging unit, Ag(I) and Cu(I) as the



50



N



N



Re



N

C

(CO)3

C

Re



N

N



Pt



C



N



N



N



Complex B



FIGURE 4. Luminescent trinuclear complexes bearing terminal isocyanides.



N



(CO)3



N



N



C



N



Pt



N



N



C

Pt



(CIO4Ϫ)3



N



(PF6Ϫ)3



N

Re

(CO)3

C



Complex A



N











N



Luminescent Organometallic Polynuclear Systems



51



(a)



Normalized Absorbance



1.2



Abs

Ex



1.0



Em



0.8

0.6

0.4

0.2

0.0

250 300 350 400 450 500 550 600 650 700

Wavelength (nm)



(b)



Normalized Absorbance



1.2



Abs

Ex



1.0



Em

ϫ 10 Abs



0.8

0.6

0.4

0.2

0.0

500

400

Wavelength (nm)



300



600



700



FIGURE 5. UV-visible (Abs), excitation (Ex), and emission (Em) spectra of complex A

(a) and complex B (b) as 1.1025 M solutions in CH2Cl2/DMF at 298 K. (Modified from

Ref. 12.)



NC

NC



NC



U-conformation



NC

Z-conformation



FIGURE 6. Both conformations of the 1,8-diisocyano-p-menthane (dmb) ligand.



52



Luminescent Organometallic Coordination Polymers Built



Agϩ



Agϩ



Agϩ



Agϩ

n



ϭ

NC



NC



Z

Y



X



FIGURE 7. The {[Ag(dmb)21]}n polymers (top) and the crystal structure of

{[Ag(dmb)2]BF4}n (bottom). (Counteranions are omitted.)



metallic centers, and different counteranions (BF42, PF62, NO32, ClO42,

CH3COO2), researchers were able to obtain a broad range of novel metallopolymers. The 1D structure of the polymers was again confirmed by X-ray

analysis. It turned out that regardless of the metal ion (Ag or Cu), the same

1D arrangement was always observed. Calorimetric properties were also

studied and the glass transition temperatures (Tg) were noted.

Thermogravimetric analysis (TGA) demonstrated that these polymers

showed limited thermal stability for these materials, with decomposition

occurring at temperatures .130 C. This stability is thought to be related to the

M-CNR bond. The Tg data ranged from 37 to 96 C. The X-ray powder

diffraction (XRD) pattern data provided complementary information concerning morphology of the polymers, as summarized in Table 1.

Different approaches were used to determine the molecular weight for the

{[M(dmb)2]Y}n polymers. First, the average molecular weights in weight (Mw)

were measured using light-scattering techniques. But an innovative way to

determine Mn (average molecular weight in number) was the use of NMR

techniques, namely T1 and NOE constant measurements.33 From these

experiments, it appeared that some polymers, especially those with Ag (I), are

rather oligomeric in solution with the number of repetitive units ranging

between 7 and 9 (Table 2).



Luminescent Organometallic Polynuclear Systems



53



TABLE 1. Calorimetric Properties and Morphologies of {[M(dmb)2]Y}n Polymers



Polymers



Tg ( C)



Tdec ( C)



Morphology



38À44

None

37

58

40

None

57

45

79

96



165À166

170À171

180

133À134

168À170

176

178

165

148À150

140



Highly crystalline

Highly crystalline

Highly crystalline

Highly crystalline

Highly crystalline

Highly crystalline

Semicrystalline

Amorphous

Amorphous

Semicrystalline



{[Ag(dmb)2]BF4}n (1)

{[Ag(dmb)2]PF6}n (2)

{[Ag(dmb)2]CH3COO}n (3)

{[Ag(dmb)2]NO3}n (4)

{[Ag(dmb)2]ClO4}n (5)

{[Cu(dmb)2]BF4}n (6)

{[Cu(dmb)2]BF4}n (7)

{[Cu(dmb)2]BF4}n (8)

{[Cu(dmb)2]NO3}n (9)

{[Cu(dmb)2]ClO4}n (10)



TABLE 2. Molecular Weight Determination using Light Scattering and NMR



Polymers



Mw (light scattering) Mw (NMR)



{[Ag(dmb)2]BF4}n (1)

{[Ag(dmb)2]NO3}n (4)

{[Ag(dmb)2]ClO4}n (5)

{[Cu(dmb)2]BF4}n (8)

{[Cu(dmb)2]ClO4}n (10)



#10 000





160 000

#10 000



4026

4400

5291







Number of Units

7

8

9

301





N



11 : M ϭ Agϩ

12 : M ϭ Cuϩ



C

C



M

C

N



N



C

N



FIGURE 8. Structural motif of reference complex [M(CN-tBu)4]1.



The photophysical properties were also investigated in detail. To fully

understand the photophysics of these polymers, these were compared with

monomeric complexes, namely [M(CN-tBu)4]BF4, as model compounds

(M 5 Cu (I), Ag(I)) (Fig. 8).

The absorption spectra of these polymers were very similar to that of the

model compounds (Fig. 9). Calculations (DFT, B3LYP, 3-21G*) performed on

the model compounds, [M(CN-tBu)4]1, predicted that the lowest energy



54



Luminescent Organometallic Coordination Polymers Built



Absorbance



0.6



0.4



0.2



0.0



200



220

240

260

Wavelength (nm)



280



Emission Intensity (10^5 cps)



FIGURE 9. Absorption spectra of polymer 1 (solid line) and the model compound 11

(dotted line). (Modified from Ref. 30.)



12



8



4



0



400



500



600



700



Wavelength (nm)



FIGURE 10. Emission spectrum of polymer 2 in KBr pellets at 77 K (λexc 5 250 nm).

The sharp peak (*) located at 500 nm is due to the first harmonic of the excitation line.

(Modified from Ref. 30.)



electronic transition is a MLCT, consistent with the d10 electronic configuration of the metal and the presence of empty π* orbitals on the CN fragments.

The materials are weakly luminescent at room temperature. However,

intense blue emissions are observed at 77 K, both in the solid state and in

solution. The emission bands are very broad, and generally centered around

500 nm (Fig. 10; Table 3).



Luminescent Organometallic Polynuclear Systems



55



TABLE 3. Emission Data of the Model Compounds and Polymeric Materials at 77 K



λemi (nm)

Compound

[Cu(CN-tBu)4]BF4

{[Cu(dmb)2]BF4}n

[Ag(CN-tBu)4]BF4

{[Ag(dmb)2]BF4}n

{[Ag(dmb)2]PF6}n

{[Ag(dmb)2]NO3}n

{[Ag(dmb)2]CH3CO2}n



EtOH Solution



Solid State



KBr Pellets



510

548

435

502

484

435

454



490

517

474

486

467

489

492



514

550

467

510

467

499

492



A large difference in λmax emission between the monomeric and polymeric species is noted. This observation suggests that M?M interactions play

a role in the stabilization of the emissive state. Emission lifetimes range from 7

to 600 μs. Such long lifetimes along with the large Stokes shift indicate the

presence of a phosphorescence and allow one to assign the luminescence to

a 3MLCT state, the metal being in its d10 electronic configuration. The emission

decays for the polymeric materials unexpectedly revealed a polyexponential

kinetic. In addition, comparison of the decay curve in the solid state or in

solution showed only settled differences, which were attributed to slightly

different deactivation pathways. Time-resolved emission spectroscopy in the μs

time scale demonstrated that the broad emission is composed of an infinite

number of emission bands. In addition, it was demonstrated that the counteranions play a role in the deactivation mechanism because the emission

lifetime decays were not the same from one counterion to another. A typical

emission decay trace is presented in Figure 11 as an example.

The time-resolved emission spectra at early delay times exhibit emission

bands that are blue shifted with respect to the steady-state spectrum. The

comparison of both the approximated lifetimes and the emission maxima of the

short-lined species closely resembles that observed for the mononuclear model

complexes ML41. On the other hand, the longer-lived species (i.e., longer decay

times) exhibit red-shifted emission bands. These emission features are due to an

exciton process consistent with an energy delocalization within the backbone

similar to eximer emissions. This process is particularly useful for energy

migration across a material. To corroborate the existence of this phenomenon,

the polarization ratios of these emissions were also measured. The polarization

ratio, N, turned out to be oscillating around N 5 1 (in a scale that can vary

between 0.5 and 3) all along the emission band, indicating that the emitted light

was depolarized. The result is entirely consistent with the exciton process

because the information about the transition moment is lost in the reversible

energy migration across the polymer chain.

Other functional polymers were prepared for which the counteranions

(BF42, PF62, . . .) were replaced by the tetracyano-p-quinodimethane radical



56



Luminescent Organometallic Coordination Polymers Built



In (Emission Intensity)



8

7

6



([Ag(dmb)2]BF4)n



5

4

3

0.2



0.4



0.6



0.8



1.0



1.2



1.4



1.6



Time (ms)



FIGURE 11. Typical emission decay trace for the polymer {[Ag(dmb)2]BF4}n in the

solid state at 77 K. (Modified from Ref. 30.)



NC



CN



NC



CN



NC



CN



Ϫ



NC



CN

À



FIGURE 12. Structure TCNQ (left) and TCNQ (right)



anion (TCNQ2) (Fig. 12), thus providing materials of the type {[M(dmb)2]

TCNQ}n (M 5 Cu, Ag).34 These new materials were readily prepared by

counteranion metathesis with TCNQ2. They are electrically insulating materials because of the coupling of the radicals and the lack of appropriate

structure for the creation of a valence and conducting bands of the materials.

However, conductive materials were obtained by doping the polymers with

neutral TCNQ (Fig. 12) in acetonitrile solution and subsequent evaporation of

the solvent. The isolated materials exhibited semiconducting properties based

on the measurements of the resistivity as a function of the temperature (i.e., the

graph of the conductivity as a function of the reverse of the temperature gave a

negative slope).

X-ray analysis reveals a sandwich structure for these semi-conducting

materials of formula {[M(dmb)2]TCNQnTCNQ }n (N 5 1, 1.5), the polymeric

chains of {M(dmb)2}nn1 are separated by layers of TCNQ2 and neutral TCNQ

(i.e., mixed-valence TCNQ2 layers). The latter layer is responsible for the

conductivity. Since TCNQ is an electron acceptor, electron transfer from

the polymer to the mixed valence TCNQ layer is also possible from the Cu(I)

center to the TCNQÀ nTCNQ layer. This photoinduced electron transfer from



Luminescent Organometallic Polynuclear Systems



57



the electron-rich d10 Cu(I) center in {Cu(dmb)21}n to the TCNQ neutral species

was indeed demonstrated. This process is indicated in equations 1 and 2:

nỵ*

fCudmbị2 gnỵ

n ỵ hv-fCudmbị2 gn



1ị



fCudmbị2 gnỵ*

ỵ TCNQ-fCudmbị2 gnỵ1ịỵ

ỵ TCNQ2

n

n



2ị



This photoprocess creates a hole defined by a d9 Cu(II) center within the

d Cu(I) chain, which can be stabilized by hole delocalization along

the polymer backbone (exciton). The presence of π-stacked TCNQn2 in the

lattice generates a valence and conducting band, which lead to photoconducting materials in these cases and photovoltaic cells.35 Such devices were

indeed constructed by spin-coating an acetonitrile solution of {[Cu(dmb)2]

TCNQ}n and neutral TCNQ onto a semitransparent and semiconducting SnO2

glass. A counter electrode was then prepared by evaporating Al.31 The typical

setup for such a photovoltaic cell is presented in Figure 13.

Upon irradiation, a charge separation state is created, as indicated by

equations 1 and 2, in which TCNQn2 is now TCNQ(n11)2 in the {[Cu(dmb)2]

TCNQ.nTCNQ }n materials. The reaction {Cu(dmb)2}n(n11)111 e2-{Cu

(dmb)2}nn1 occurs at the cathode surface, whereas the process TCNQ(n11)2-1

TCNQn211 e2, occurs at the anode, hence completing the circuit. Unfortunately, the resulting photocurrents were modest in comparison with other

organic-based solar cells described in the literature.35 However, no precautions

were taken against impurities included in the air (dust) and solvent. Better

photocurrent can still be obtained with more rigourous handling conditions.

Subsequently, structural variations on these polymers were made by

replacing one dmb ligand with a diphosphine assembling ligand, diphenylphosphinomethane (dppm).36 A scheme of such a 1D polymer is presented in Figure 14.

This structure was elucidated upon X-ray analysis of a single crystal for M 5 Ag (I).

10



AI electrode

Photoactive

{Cu(dmb)2TCNQ}n.TCNQ layer

SnO2



Glass substrate



FIGURE 13. Structure of a solar cell designed with {[Cu(dmb)2]TCNQ Á TCNQ }n.



57



58



P



Ph



P



P







P



Ph

P

Ph



13 M ϭ Ag (I)

14 M ϭ Cu (I)







P



P



Ph







P







P



P







P



n

ϭ

NC



P



NC







P

P



P







Ph



Ph

P







P

P



P



15 M ϭ Ag (I)

16 M ϭ Cu (I)







P

P



Ph



Ph







P

P







FIGURE 14. The mixed-ligand coordination polymers (M 5 Cu, Ag). The conteranion is BF42.







P

n



Luminescent Organometallic Polynuclear Systems



59



It is interesting to note that in case of bisdiphenylphosphinoethane (compounds 15

and 16), the diphos ligand is no longer bridging but chelating.

A better description of the photonic properties of the polymers was made

by comparison with model compounds 17À20 (Fig. 15).

The electronic spectra for the new mixed-ligand 1D polymers exhibit an

absorption band at %272 nm, assigned to a MLCT (rather than ππ* arising

from the phenyl group). Solid-state emission spectra were recorded for both

the model compounds and the polymers. The luminescence appears as broad

bands located between 480 and 550 nm. Large Stokes shifts were observed,

and the corresponding emission lifetimes (τe) ranged between 18 and 48 μs

(Table 4). These experimental data indicate that these emissions are in fact

phosphorescence.

Palladium(II)- and platinum(II)-containing polymers assembled by diisocyanide ligands are generally not luminescent.37 Because photoinduced labilization of the ligand in MÀCNÀR systems is possible, absence of luminescence

can occur when this photochemical process happens, and consequently the light

energy is “wasted.” As a result, luminescence is often not observed at room

temperature in solution for such coordination polymers. Occasionally, only weak

emissions at low temperature in the solid state can be detected.

Luminescence properties were reported for polymers that exhibit Pt(I) or

Pd(I) in the main chain. A typical example is the ring-stressed complex

Pd2(dmb)2X2 (where X 5 Cl or Br).38 The latter exists as a binuclear complex in

solution but forms a polymer in the solid state (Fig. 16).

While the binuclear complexes are emissive in glass solutions at 77 K,

only a weak luminescence was observed for the polymers in a solid state. A



Ph

17 M ϭ Ag x ϭ 1

18 M ϭ Ag x ϭ 2

19 M ϭ Cu x ϭ 1

20 M ϭ Cu x ϭ 2



Ph

P

X



M

C



P

Ph



N

C



N



Ph



FIGURE 15. Structure of model compounds 17À20. The environment about the metal

is tetrahedral, based on X-ray structures.



TABLE 4. Emission Data for the Model Compounds

Coordination Polymers in the Solid State at 298 K



Compounds

{[Ag(dppe)(dmb)](BF4)}n (15)

{[Cu(dppe)(dmb)](BF4)}n (16)

[Ag(dppe)(CN-tBu)2](BF4) (17)

[Cu(dppe)(CN-tBu)2](BF4) (20)



and



Corresponding



λem (nm)



Emission Lifetimes (τe in μs)



548

480

515

540



27

38

21

42



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

II. Luminescent Organometallic Polynuclear Systems and Coordination Polymers Containing a Terminal Isocyanide Ligand

Tải bản đầy đủ ngay(0 tr)

×