Luminescence Properties of PhosphineâIsocyanide Cu(I)- and Ag(I

Chapter 33

Downloaded by PENNSYLVANIA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch033

Luminescence Properties of Phosphine-Isocyanide Cu(I)- and Ag(I)-Containing Oligomers in the Solid State Pierre D. Harvey and É r i c Fournier Département de Chimie, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada

The luminescence properties of the binuclear complexes M (dmpm) ( M = Cu, A g ; dmpm = bis(dimethylphosphino)methane), and Cu (dmpm) (CN-t-Bu) (as B F salts), as well as the oligomers described as {Cu (dmpm) (dmb) } and {Ag (dmpm) (dmb) } (dmb = 1,8-diisocyano-p-menthane), were investigated and compared to the well-known {M(dmb) } polymers ( M = Cu, Ag). These compounds exhibit emission maxima ranging from 445 to 485 nm with emission lifetimes found in the μs regime in the solid state at 298 K . The time-resolved emission spec tra for the oligomers and polymers exhibit blue-shifted emission ban ds at the early stage of the photophysical event after the excitation pulse, which red-shift with delay times. The decay traces are non -exponential, and their analysis according to the Exponential Series Method (ESM) exhibit a distribution of lifetimes that is fairly broad, consistent with an exciton phenomenon. A qualitative correlation between the number of units and the distribution width is reported. 2+

2

3

-

2+

2

3

2

4

2+

2

3

1.33

3

2+

2

3

1.33

3

+

2

472

n

© 2006 American Chemical Society

In Metal-Containing and Metallosupramolecular Polymers and Materials; Schubert, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

473

Background The syntheses and the applications of metal-containing polymers where the metal is located in the backbone are becoming the subject of more and more recent research projects. While these polymers are built with various assembling ligands, the nature of the M - L coordinations is largely dominated by M - N and M - C bonding. On the other hand, organometallic/coordination polymers built upon diphosphines and diisocyanides are more rare. Among the sought after properties for these new and original materials, luminescence has become more predominant, " since these materials may find applications for digital displays, light-emitting diodes, and sensors. This group recently reported a series of works on luminescent Cu(I) and Ag(I)-containing polymers built with assembling diisocyanides and diphosphines, as well as mixed-ligand species. * Examples include the doubly bridged homoand mixed-ligand polymers {M(dmb) } " and {M(dmb)(dppm) }„ ( M = Cu, A g ; dmb = 1,8-diisocyano-p-menthane; dppm = 6/£(diphenylphosphino)methane), and the recently reported oligomers {Cu (dmpm)3(dmb)u3 }3 and {Ag (dmpm) (dmb)i. 3 }3 (dmpm 6«(diphenylphosphino)methane; Schemes 1 and2). 1

1


2

11

12

20

+

2

+

16

20

n

13

2+

2+

2

2

2

3

12

(^-conformation

{M(dmb) *}„;M = Cu.Ag 2

Ν HI

C

{M (dppm) (dmb) *} ; M = Cu, Ag 2

2

2 2

n

Z-conformation

Scheme 1


474 +

One important particularity is that the various M ( C N R ) / , M ( C N R ) P , C u ( C N R ) P and A g ( C N R ) P chromophores ( M = Cu, A g ; Ρ = phosphine) are separated by saturated chains such as methylene groups, included within the assembling ligands. Consequently, no electron délocalisation is possible between the chromophores, and so interactions or "communications" between these, i f any, must occurs through space. During the earlier investigations, the presence of an intra-chain excitonic process (Scheme 3) was reported for the {M(dmb) } polymers ( M = Cu, A g ) . While the exciton phenomena is well known for organic materials, ' it is less so for organometallic and coordination polymers due, in part, to the recent development of this field. 2

+

2

+

3

2

+

2

n

19


21 24

{Cu (dmpm) (dmb) 33 *h 2

3

1

2

Scheme

+

Numerous striking features were apparent for the {M(dmb) } polymers. For example, the luminescence decay traces were non-exponential for all investigated media (solid state, crystal, solution), the decay traces were also super-imposable for both solution and solid state (indicating an intra-chain phenomenon), and the emission light was depolarized. Finally and more importantly, the time-resolved emission spectroscopy exhibited a continuous redshift of the luminescence band with delay time after the excitation pulse, all strongly contrasting with the typical linear logarithm decay traces, polarized emission light and uniqueness of the emission maximum with delay times for 2

n


475 non-interacting chromophores. Between these two extreme series of features, there is no data, so one cannot tell how these properties change with the polymer length.

Mi h v

M2 2

M3

?hv

3


)

?hv

etc M5 Mg

M4 §hV5

4

)

)

(* = exciton)

Scheme 3

Objective The objective of this study was to provide some information on a possible correlation between the chain length and the photophysical properties. In this respect, the newly synthesized coordination oligomers {Cu2(dmpm)3(dmb)i. 3 }3 and {Ag2(dmpm) (dmb)i 33 } were investigated in some detail in the solid state. The solid state was selected to avoid dissociation phenomena in solution often encountered in d metallic species. 2+

3

2+

2

3

1 0

Experimental Materials. [Cu (dmpm) ](BF )2 (1), [Ag (dmpm) ](BF ) (2), [Cu (dmpm) (CN-/-Bu) ](BF ) (3), {[Cu (dmpm)3(dmb) .33](BF ) }3 (4), {[Ag (dmpm) (dmb) 3](BF ) }3 (5) and {[M(dmb) ]BF } ( M = Cu (6, crystalline form), A g (7)) were synthesized according to literature procedures. 2

2

4

13

4

3

2

4

2

2

2

2

3

1

4

4

2

2

2

3

2

2

4

12,19

Apparatus: The continuous wave emission and excitation spectra were obtained using a S P E X Fluorolog II spectrometer. The emission lifetimes were measured with a nanosecond N laser system from PTI model GL-3300. The time-resolved emission spectra were acquired on the same instrument used for the lifetime measurements. The excitation wavelength was 311 nm for all experiments. Procedures. The average molecular weight in number (M ) of the crystalline {[Cu(dmb) ]BF } polymer was obtained from the measurements of the intrinsic viscosity using polymethyl methacrylate standards from Aldrich (M = 12000, 2

n

2

4

n

n


476 15000, 120000, and 320000). The evaluated M is 24000 (i.e. about 45 units). The emission lifetimes were analyzed using the E S M (Exponential Series Method), which consisted of calculating a decay curve composed of 200 exponentials. The results were checked to ensure that they did not depend on input parameters. Typically, the results were presented as a distribution of lifetimes. For single exponential decays, the traditional deconvolution (1-4 components) and E S M methods gave the same results, and the distribution of lifetimes was narrow. For data giving a large distribution with E S M , the deconvolution method failed to find satisfactory fits with 2, 3, or 4 decays. The quality of the fit between the experimental and calculated curves was addressed using the parameter χ (goodness of fit), which approached the diagnostic value of 1 for an acceptable fit for all reported data, and from the analysis of the residual. n


25,26

Results & Discussion The photophysical data of the two targeted oligomers 4 and 5 are compared with the binuclear complexes [M (dmpm) ](BF ) ( M = Cu (1), A g (2)) and [M (dmpm) (CN-/-Bu) ](BF ) (3; Scheme 4), and polymers {[M(dmb) ]BF }„ ( M = C u (6), A g (7)). The nature of the excited states of the closely related lumophores M ( C N R ) , C u ( d p p m ) ( 0 C C H ) \ M P (P = phosphine), ' and M (dmpm) ( M = Cu, A g ) " was established to be triplet metal-to-ligandcharge-transfer (MLCT) for the first two, and metal-ligand [1 ^σ*)] [ί (π*,ρσ*)] and metal-metal da*pa for the last two, respectively, based upon experiments and D F T calculations. 2

2

3

2

+

4

4

2

2

27

2

2

30

2

+

3

4

28 29

4

35

3

5

2

4

19

2+

2

3

2

1

2

M (dmpm) * (M = Cu (1), Ag (2)) 2

2

Cu (dmpm) (CN-f-Bu)

3

2

3

2+

2

(3)

Scheme 4 +

As stated, the photophysical properties of the {M(dmb) }„ polymers were previously reported by us, and were compared to the tetrahedral mononuclear model complexes M(CN-t-Bu) ( M = Cu, A g ) . The spectroscopic and 2

+

19

4


477 photophysical properties were found to be drastically different between the mononuclear species and polymers. The key features are as follows: the X for the M ( C N - t - B u ) / species are blue-shifted with respect to the corresponding polymers (up to 40 nm), and the fwhm of the emission bands are smaller as well under continuous wave excitation. The decay traces are rigorously monoexponential for the mononuclear complexes, while they are non-exponential in the polymers. Time-resolved emission spectra indicate that at the early event after the light pulse, both A ^ and slope of the emission decay traces (equivalent of a lifetime) compare favorably with those of the corresponding mononuclear chromophores M(CN-t-Bu) ( M = Cu, Ag). At longer delay times, the recorded emission bands are significantly red-shifted (up to 63 nm). Finally, another notable difference is that the emission arising from the polymers is depolarized. A l l these features are typical of energy transfer exciton phenomena (Scheme 3). One of the interesting features is that the decay traces in the polymers are found to be independent of the medium (solution vs solid), indicating that the process is primarily intramolecular. max

ax

+


4

This latter feature is related to the long intermolecular Ν " N distances (8.23 and 8.67 A ) " which indicate that the chains are relatively isolated from one another in the solid state, while the intramolecular N " ' N separations are in the order of 4.5 À (dmb in its [/-conformation). Despite the fact that no X-ray data is available for 4 and 5, it is possible to anticipate what the approximate interchromophore distances in the solid state are, by examining the X-ray data for the building blocks 1 and 2. For these species, the closest intermolecular Ρ Ρ distances are 6.683 À for 1, and 6.328, 6.585 and 6.982 A for 2. These distances are greater than the intramolecular Ν Ν separation in the linking dmb ligand (Z-conformation) in the recently reported computed model compound Cu (dmpm) (dmb) (CN-/-Bu) (-5.8 A ; PC-Model; Figure l ) and in the polymer {[Pd (dmb) (dmb)] } (5.549 A ; X-ray). If one accepts that the rate for exciton hopping (or energy transfer; k ^ ) varies as k T °c 1/r (r = interchromophore distance; for a i.e. Fôrster mechanism), then the contribution of the intermolecular process is minor. 1 6

2 0

12

6+

6

9

2

1 2

4

2+

4

4

36

n

6

E

37

Figure 2 shows the time resolved emission spectra (20-2000 μβ) for 4 and 5 in the solid state. 4 and 5 exhibit blue emission maxima at 482 and 447 nm, respectively, when submitted to continuous wave excitation. At the early event after the light pulse, the recorded emission band is blue-shifted with respect to the emission band measured in continuous wave mode. As the delay time increases, the observed emission band red-shifts constantly and the intensity decreases. A l l in all, the emission bands measured with continuous wave light are composed of a number of blue- and red-shifted components. The maximum band shifts are -10 and -34 nm for 4 and 5, respectively, which are smaller than those observed for the longer {M(dmb) } polymers (up to 50 nm). The 298 Κ decay traces are found to be linear for 1-3 with emission lifetimes of 251 (472), +

2

19

n



478

U

9.1 A

• •!

Cu2(dmpm)3

2+

Figure 1. Representation of a fragment of the {[Cu (dmpm))(dmb)u3](BF oligomer (4), where the computed intrachain interchromophore distan indicated (as 5.8 and 9.1 À are for the N"'N and Cu'Cu separations, respectively). 2


479 41 (445) and 291 (476 nm), respectively. These results indicate for the first time that qualitatively the amplitude of red-shift increases as the polymer length increases. The shorter lifetimes normally encountered for the A g species with respect to the Cu homologues is due to the larger spin-orbit coupling of the heavier element, commonly called "heavy atom effect".


37

I

2+

120 {Ag(dmpm)3(dmb)i .33 } 2

400

3

450 500 550 600 Wavelength (nm)

650

Figure 2. Time resolved emission spectra for 4 and 5 in the solid state at 298 K. The measurements were made in the following time frames: for 4: 474 nm, 2070; 478, 500-600; 481, 1000-1300; 484; 2000-2500μ5; for 5: 444 nm, 20-70; 453, 300-400; 472, 500-600; 478, 1000-1300

As stated, the decay traces are non-exponential for 4-7. A typical example is shown in Figure 3, where a straight line is observed for the model compound 3 and a curve is measured for the polymer 6 in the log plot of the emission intensity vs delay time after the excitation pulse.


480


10000

500

1000

1500

2000

Time (μβ) Figure 3. Solid state decay tracefor the emission of 3 (—) versus 6(~~)at298K.

In these cases, the data are analyzed using the E S M , and the results are plotted as population distribution (or relative amplitude) vs lifetimes in Figures 4 and 5. The maximum of probability represents an average lifetime or the most probable lifetime, and these are 257 (4) and 31 μβ (5). The width of the distribution is related to the curvature of the decay traces (log plot); as the width increases, the curvature increases. The data are not dependent on the excitation intensity (using neutral density filters), indicating that local heating has little or no effect on the results.

120

100

150 200 250

300

350 400

Lifetimes (με) Figure 4. Comparison of the distribution of lifetimes as a function of lifetime fitting the emission decay traces for 1 (—), 4 ( —) and 6 (—-) in the solid state at 298 K.


481


2+

2+

A

Ag (dmpm) {Ag (dmpm)3(dmb) 3 }3 { 9 ( 2

3

2

13

d m b

+

) }n 2

120 100 c ο

'•§ 80 η •55 •5 60

I

Q. Ο

40

α. 20 J 0

50 100 150 0

50 100 150 200 250 300 350 400 Lifetimes (με)

Figure 5. Comparison of the distribution of lifetimes as a function of lifetimes fitting emission decay traces for 2, 5 and 7 in the solid state at 298 K.


482


However one interesting question arises. The number of units is 1, 3 and ~ 45 (here as evaluated by the measurement of the intrinsic viscosity), for 1, 4 and 6, and 1, 3, and "very large" for 2, 5, and 7, respectively. The width of the population distribution plots in Figures 4 and 5 does not follow the trend proportionally, particularly for 4 and 6 as a large change in the number of units should be accompagnied by a large change in width. This observation cannot be explained straightforwardly. First, one has to consider that the exciton process is reversible (Scheme 5). So a small chain can exhibit the same spectroscopic feature as a long chain depending on the efficiency of exciton hopping.

Μι hv

M2 2

I

hv

(* = exciton)

M 3

I

3

hv

4

Scheme S

Secondly, it is not necessarily true that the extent of exciton migration is as extensive for one chromophore vs another, simply because the atomic distribution of the H O M O and L U M O is not identical for all chromophores. Therefore, the probability of energy transfer cannot be the same. In addition, the distance between the chromophores measured as M C s N " * N s C M or M*"M or P " P are different as well. One interesting and important remark concerning this point is the comparison of the photophysical properties of the Cu(I) and Ag(I) species described above with Pd-Pd and Pt-Pt bond-containing oligomers and polymers also built with dmb and diphosphine bridging ligands, such as {Pd (dmb) (diphos) +} , {Pd (dmb) (dmb) } and {Pt (dmb) (diphos) +} (Scheme 5 ) . While the former polymers exhibit non-exponetial decay traces and depolarized broad emissions, the latter ones exhibit mono-exponetial luminescence decays, indicating the presence of non-interacting chromopohores within the chain, and polarized emissions, strongly suggesting that the exciton process is either very weak or absent in these cases. A close examination o f the atomic contributions of the H O M O and L U M O reveals that a large contribution of the C and Ν ρ orbitals (η and π* orbitals) are computed for the Cu(I) and Ag(I) chromophores, while these are minor or non-existent for the Pd and Pt species. In other words, the electronic density is more spread out in the former 2

2

2+

2

n

4

4

2

n

4

4

36,38,39

19

39


n

483 two series, and more localized around the M - M bond in the latter compounds. In this respect, the distance for energy transfer is greater for the Pd-Pd- and Pt-Ptcontaining chromophores.

V^^rf-NgC-Pd—Pd—Pd^Pd--- 1

III

Ph

Ph

PI 1

η 4

Ph ι

Ph

Ph

Ph

4

2

2

n

^

{Pt (dmb) (d«phos) } 4

2+

z+

2

=

η 4

Jn

{Pd (dmb) (diphos) }

{Pd (dmb) (dmb) *} Downloaded by PENNSYLVANIA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch033

Ph

I

Ν

Ç

Ph

β c

n

β c

m = 4-6

n

Scheme 6

Conclusion It is arguable that the investigated systems are not ideal for rigourous analysis to fine probe the exciton migration across the chain, but the tight control of chain length for coordination polymers is not yet achieved. This work has, however, unquestionably demonstrated that a qualitative relationship between the number of chromophores in the oligomer/polymer chain and the extent of exciton migration measured as the width of the population distribution of emission lifetimes, or the curvature of the decay traces in the log scale, exists. Further research in this area are in progress, including the use of the {M(dmpm)(dmb)}n polymers ( M = Cu, Ag) which represent other suitable materials for comparison purposes with the materials described in this work.

Acknowledgment. This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). P D H also thanks the students that contributed to the various aspects of the research on organometallic/coordination polymers over the years. Their names are listed within the references below.


484

References 1.

2. 3.


4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Macromolecule Containing Metal and Metal-like Elements, Eds. Abd-ElAziz, A . S . , Carraher, C. E., Jr., Pittman, C. U., Jr., and Zeldin, M.; eds. Wiley Intersceince, John Wiley and Sons Inc., New York, Vol. 5, 2004. Wu, C.-D.; Ngo, H . L . ; Lin, W., Chem. Comm. 2004, 1588. Dong, Y . - B . ; Wang, P.; Huang, R.-Q.; Smith, M. D. Inorg. Chem. 2004, 43, 4727. Fernandez, E. J.; Lopez-de-Luzuriaga, J. M.; Monge, M.; Montiel, M.; Olmos, M. E.; Perez, J.; Laguna, A.; Mendizabal, F.; Mohamed, Α. A.; Fackler, J. P., Jr. Inorg. Chem. 2004, 43, 3573. Miller, Τ. Α.; Jeffery, J. C.; Ward, M. D.; Adams, H . ; Pope, S. J. Α.; Faulkner, S. Dalton Trans. 2004, 1524. Song, J.-L.; Zhao, H.-H.; Mao, J.-G.; Dunbar, K . R. Chem. Materials, 2004, 16, 1884. Fleming, C. N.; Jang, P.; Meyer, T. J.; Papanikolas, J. M. J. Phys. Chem. Β 2004, 108, 2205. Dong, Y.-B.; Zhao, X.; Tang, B.; Wang, H.-Y.; Huang, R.-Q.; Smith, M. D.; zur Loye, H.-C. Chem. Comm. 2004, 220. Song, D.; Wang, S. Eur. J. Inorg. Chem. 2003, 3774. Dong, W.; Zhu, L.-N.; Sun, Y.-Q.; Liang, M.; Liu, Z.-Q.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P.; Cheng, P. Chem. Comm. 2003, 2544. Seward, C.; Chan, J.; Song, D.; Wang, S. Inorg. Chem. 2003, 42, 1112. Founder, É.; Decken, Α.; Harvey, P. D. Eur. J. Inorg. Chem. 2005, 1011. Fournier, É; Lebrun, F.; Drouin, M.; Decken, Α.; Harvey, P. D. Inorg. Chem. 2004, 43, 3127. Fournier, É.; Sicard, S.; Decken, Α.; Harvey, P. D. Inorg. Chem. 2004; 43, 1491. Mongrain, P.; Harvey, P. D. Can. J. Chem. 2003, 81, 1246. Turcotte, M.; Harvey, P. D. Inorg. Chem. 2002 41, 2971. Fortin, D.; Drouin, M.; Harvey, P. D. Inorg. Chem. 2000, 39, 2758. Fortin, D.; Drouin, M.; Harvey, P. D. J. Am. Chem. Soc. 1998, 120, 5351. Fortin, D.; Drouin, M.; Turcotte, M.; Harvey, P. D. J. Am. Chem. Soc., 1997, 119, 531. Perreault, D.; Drouin, M.; Michel, Α.; Harvey, P. D. Inorg. Chem. 1992, 31, 3688. Martini, I. B . ; Smith, A . D.; Schwartz, B . J. Physical Rev. 2004, 69, 035204-2. List, E . J. W.; Leising, G. Synth. Metals 2004, 141, 211. Gunaratne, T.; Kennedy, V . O.; Kenney, M. E.; Rogers, M. A . J. J. Phys. Chem. A 2004, in press. Mirzov, O.; Cichos, F.; von Borczyskoswky, C.; Scheblykin, I. G . Chem. Phys. Lett. 2004, 386, 286.



485 25. Siemiarczuk, A.; Wagner, Β. D.; Ware, W. R. J. Phys. Chem. 1990, 94, 1661. 26. Siemiarczuk, Α.; Ware, W. R. Chem. Phys. Lett. 1989, 160, 285. 27. Harvey, P. D.; Drouin, M.; Zhang, T. Inorg. Chem. 1997, 36, 4998. 28. Harvey, P. D.; Schaefer, W. P.; Gray, H . B . Inorg. Chem. 1988, 27, 1101. 29. Orio, Α. Α.; Chastain, B.B. ; Gray, H . B . Inorg. Chim. Acta 1969, 3, 8. 30. Leung Κ. H.; Phillips, D. L.; Mao, Z.; Che, C.-M.; Miskowski, V . M.; Chan, C . - M . Inorg. Chem. 2002, 41, 2054. 31. Zhang, H.-X.; Che, C . - M . Chem. Eur. 2001, 7, 4887. 32. Fu, W.-F.; Chan, K-C.; Cheung, K.-K.; Che, C . - M . Chem. Eur. J. 2001, 7, 4656. 33. Leung, Κ. H . ; Phillips, D. L . ; Tse, M.-C.; Che, C.-M.; Miskowski, V.M. J. Am. Chem. Soc. 1999, 121, 4799. 34. Fu, W.-Fu; Chan, K.-C.; Miskowski, V . M.; Che, C . - M . Angew. Chem. Int. Ed. 1999, 38, 2783. 35. Piché, D.; Harvey, P. D. Can. J. Chem. 1994, 72, 705. 36. Zhang, T.; Drouin, M.; Harvey, P. D. Inorg. Chem. 1999, 38, 1305. 37. Turro, N. J., Modem Molecular Photochemistry, Benjamen / Cummings Pub. Co., Menlo Park, 1978. 38. Zhang, T.; Drouin, M.; Harvey, P. D. Inorg. Chem. 1999, 38, 957. 39. Sicard, S.; Berubé, J.-F.; Samar, D.; Messaoudi, Α.; Fortin, D.; Lebrun, F.; Fortin, J.-F.; Decken, Α.; Harvey, P. D. Inorg. Chem. 2004, 43, 5321.


Luminescence Properties of PhosphineâIsocyanide Cu(I)- and Ag(I

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