Metal-Containg and Metallosupramilecular Polymers and Materials

Chapter 37

Coordination Compounds for Functional Nonlinear Optics: Enhancing and Switching the Second-Order Nonlinear Optical Responses 1

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4

Downloaded by COLUMBIA UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch037

3

IngeAsselberghs ,Michael J.Therien ,Benjamin J. Coe, JonA.McCleverty ,and Koen Clays 1

1,*

Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B3001 Leuven, Belgium Department of Chemistry, University of Pennsylvania, Philadelphia, P A 19104-6323 2

3

Department of Chemistry, University of Manchester, Oxford Road, Manchester M 1 3 9PI, United Kingdom School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom 4

In this work, we describe the second-order nonlinear optical (NLO) properties of a number of chromophores that feature transition metal ions in classic coordination environments. We focused our attention on the advantages of these species over standard hyperpolarizable chromophores based on conventional all-organic frameworks. For example, studies of Ruthenium(II)-based electron donor-acceptor (D-A) polyenes illustrate that transition metal-based compounds can show atypical conjugation-length dependences of the observed hyperpolarizability relative to closely related organic NLO chromophores. Likewise, the second-order N L O responses observed for highly conjugated (polypyridyl)metal(phorphinato)zinc(II) chromophores can be extraordinarily large, illustrating how coupled oscillator photophysics can be exploited to design materials with record hyperpolarizabilities at telecommunication-relevant wavelengths. Finally, our work demonstrates that the presence of a transition metal ion in N L O chromophores makes possible new strategies to switch and gate N L O responses voltammetrically. © 2006 American Chemical Society

527

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

528

Introduction The linear optical properties of coordination compounds of transition-metal ions have been widely investigated. These properties are strongly influenced by the electronic nature of the metal, the ligands and the remaining organic moiety that together constitute the chromophore. In coordination compounds, transition metal ions can serve as electron-releasing or -withdrawing agents, or as a constituent of the molecular bridge that provides electron coupling between other electron donating or accepting groups. As such, coordination compounds are often highly colored, featuring optical absorptions in the UV-Vis-NIR spectral regions that are either intra-valent (IVCT), metal-to-metal ( M M C T ) , ligand-to-metal ( L M C T ) , or metal-to-ligand ( M L C T ) charge transfer in origin. Further, i f these transitions are of appropriate oscillator strength and the chromophores noncentrosymmetric, such species are also ideally suited for characterization using second-order N L O spectroscopic methods. According to the two-level model the static hyperpolarizability β can be given by:


1,2,3

4,5

0

η

_3A/4 (/i 2

(Ε V

| 2

)

2

Ϋ max /

where μη is the transition dipole moment, Δμ is the dipole moment change, and E is the maximal energy of the C T absorption. β is a measure of the intrinsic N L O response under non-resonant conditions. Therefore, such complexes can form the molecular basis of active photonic components, such as optical frequency-doublers or electro-optic modulators. Taking advantage of properties inherent to chromophoric coordination compounds has led to the design of supermolecules that exhibit extremely large first hyperpolarizabilities at telecommunication-relevant wavelengths, as well as systems that offer the capability for molecular-level switching of the first hyperpolarizability. ί2

max

0

Experimental - Materials and Techniques Materials. The syntheses of the compounds discussed in this review have been previously reported. For all the optical measurements - linear and nonlinear - the products were dissolved in the appropriate solvents. A l l solvents were used as obtained from A C R O S . Linear absorption spectra were taken before and after the nonlinear optical measurements to ensure temporal and optical stability of the chromophores. For the (spectro)electrochemical experiments, an optically


529 passive (transparent) electrolyte B u N P F was used to ensure conductivity. In this case the solvent (dichloromethane) was dried (P2O5) and distilled. Techniques. The linear optical spectra were taken on a Perkin-Elmer Lambda 900 spectrometer. The first hyperpolarizabilities β (= second-order nonlinear molecular polarizability) for these compounds were determined by the hyper-Rayleigh scattering (HRS) method. This incoherent scattering technique is used because of the ionic nature of the compounds. The measurements are performed at 1064 nm. The external reference method was used to determine the /lvalues of the compounds and p-nitroaniline (PNA) was used as standard. However, it was sometimes necessary to be able to discriminate between scattered (HRS) and emitted (by multi-photon fluorescence, M P F ) photons. Therefore femtosecond HRS was used. With such short femtosecond pulses, temporal resolution between immediate (nonlinear) scattering and the timedelayed (multi-photon) fluorescence is possible. Our instrument is the Fouriertransform implementation in the frequency-domain of this principle in the timedomain. These measurements are performed at 800 nm and/or 1300 nm. The standard was Crystal Violet (800 nm) and Disperse Red 1 (1300 nm). The (spectro)electrochemistry was performed using a Perkin-Elmer model 263A potentiostat. In-situ electrochemical switching of the molecular hyperpolarizability in combination with real-time hyper-Rayleigh scattering is performed in specially designed electrochemical optical cells. 4

6

6,7


8,9

10

Results Enhancing the value of the first hyperpolarizability. The first successful strategy for enhancing the first hyperpolarizability for this material type was to change from the historically used metallocenes with an out-of-plane chargetransfer to the compounds with an in-plane charge-transfer. Extension of the conjugated π-electron bridge between donor and acceptor moieties has been shown, both theoretically and experimentally to lead to a maximum hyperpolarizability in metalorganic complexes for relatively short lengths. A study of/mw5-[Ru"(NH ) (L )(L )] ( L - electron-rich ligand, L = pyridyl pyridinium ligand) (see Figure. 1) on the effects of the polyene chain extension on the linear absorption and N L O properties has revealed the phenomenon of blue-shifting the M L C T band with the addition of each CH=CH unit (Table 1). A decrease in the static hyperpolarizability is noticed with the extension beyond 2 double bonds (Figure 2). The /rara-l,3-butadienyl bridge is optimal according to our measurements. This is in contrast to related purely organic dyes, for which β increases up to at least 8 double bonds. 11,12

13,14

D

3

A

3+

D

A

4

15

0


530

+

[PF"b


6

η = 0,1,2,3; L = NH3, l-methylimidazole(mim) D

D

A

3+

Figure 1. Structures oftnms-[Ru"(NH^(L )(L )] investigated

Table 1. Linear Absorption Data for Compound L NH NH NH NH mim mim mim mim D

3

3

3

3

η 0 1 2 3 0 1 2 3

D

A

3+

fre/is-lRu"(NH,)4(L )(L )l 3

1

λΐΜχ/nm

ε / dm mol" cm"'

590 595 584 568 602 604 592 570

15800 16100 18700 17500 16200 16200 21400 21900



Figure 2. Plot of the Static Hyperpolarizability β as a Function of the Number of CH-CH units Present in the Molecule 0

Enhancement of β can also be achieved by changing the co-ligands, in order to increase the electron donating strength of the Ru(II) center. For example, replacement of the neutral ammine group in the trans position by the thiocyanato anion results in a bathochromic shift of the M L C T absorption (see Table 2). This trend is obviously consistent with the expectation that the thiocyanate is a stronger electron donor when compared with N H and therefore causes the largest destabilization of the Ru(II)-based H O M O . The HRS measurements were performed at 1064 nm and the estimated static hyperpolarizabilities β were obtained by application of the two-state model. The data show larger β values for the compounds with the thiocyanato ligands than for their counterparts with the ammine ligands. These increases can be traced to lower transition energies E (or a larger value) and higher transition dipoles μ (or a larger extinction coefficient ε). 0

16

3

0

0

max

12


532 Table 2. Example of the Influence of the Extension of the π-bridge and Donor Ligand in fra«s-|Ru (NH ) (L )(L )r (n - 2 or 3) Complexes ,I

D

3

Compound L NH

+

3 0

D

A

L * MeQ

3

SCW

MeQ

NH

PhQ

+

PhQ

+

3

SCN* Downloaded by COLUMBIA UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch037

A

4

3

1

ε / dm mol' cm' 590 15800 628 20000 628 19300 672 21300

+

+

/W10- esu /? /10- esu 750 123 963 247 858 220 957 343

1

30

0

+

* MeQ* = N-methyM^'-bipyridinium; PhQ = N-phenyl-4,4'-bipyridinium

Coupled-oscillators for Unusual Frequency Dispersion Effects and Large-Magnitude Dynamic Hyperpolarizabilities. A n alternative strategy shows that appropriate coupling of multiple charge transfer oscillators can give rise to supramolecular systems which feature substantial dynamic hyperpolarizabilities {βA). Examples of such supermolecular N L O chromophores are shown in Figure 3. These systems exploit an ethyne-elaborated, highly polarizable porphyrinic component and a metal polypyridyl complex that serves as an integral donor (D) and acceptor (A) element The rigid cylindrically π-symmetric connectivity between the [porphinato]zinc(II) and the metal(II)polypyridyl units in these systems align the C T transition dipoles of the chromophoric components in a headto-tail arrangement. Thus, coupled oscillator photophysics and metal-mediated cross-coupling can be exploited to elaborate high βχ supermolecules (Table 3). For example, die βι value determined for Ru-PZn is 2.5 times larger than that determined for any other chromophore at this wavelength, and highlights that this design strategy enables fabrication of new classes of chromophores that possess extraordinarily large βχ values at telecommunication-relevant wavelengths. It is also important to appreciate that coupling of multiple oscillators does not necessarily give rise to supermolecules in which every C T transition possesses a Δμ value of identical sign. Such spectroscopic characteristics can give rise to chromophores that manifest an oscillatory dependence of the dynamic hyper polarizability as the incident irradiation wavelength is varied. RuPZnOs (Table 3) features porphyrin B-and Q-state-derived transitions that have Δμ values of opposite sign; thus in addition to the frequency dependent sign of the resonance enhancement factor, appropriate engineering of the wavelength-specific relative contribution and sign of Δμ for a given C T transition constitutes an additional tool to tune the magnitude of the dynamic hyperpolarizability, which can potentially enable the development of novel materials with enhanced and more selective N L O properties. 11

300

ί2

Ι2

ί2



533

Ru-PZn

Ru-PZn-Os

Figure 3. Structures of Molecules Showing Coupled-Oscillator Photophysics

Table 3. Dynamic Hyperpolarizabilities (βχ) of complexes compound Ru-PZn Ru-PZn-Os

30

& /10- esu A charge-transfer transitions. In the oxidized Fe(III) complex the CT transitions are replaced by a characteristic weak L M C T transition of the octamethylferrocenium unit at 851 nm, resulting in a yellow color (Figure 7). The switching effect is illustrated in Figure 7b which shows the alternation in the HRS signal by alternately oxidizing and re-reducing the Fe(II) form (solid lines). The dotted lines are the results obtained starting with the Fe(III) form and alternately reducing and oxidizing it. 19

20

4

3


535

(a)


2 10

4

r

Wavelength (nm) (b) 1000 -

1

2

3

4

N o . o f cycles

Figure 5. (a) Linear absorption spectra ofRu(II) andRu(III) form; (b) redo switching between Ru(II) and Ru(lII) measured by HRS


536

-NO

s'

'2

Fefll)

-e +e

Figure 6. Structures of Molecules usedfor Combined Electrochemistry and Hyper-Rayleigh Scattering Experiments.


(a)

1.4 1.2 1

8

h

i

0.8

1

0.6 A

•e

< 0.4 0.2 0 300

400

500

600

700

800

900

Wavelength (nm)

(b)

C/5

1 No. of cycles m

F/gwre 7. (ty Absorption Spectra of the Fe" and Fe Compound (cone. 4.00 x 10' M, optical path 6 cm) (b) redox switching between Fe andFe" (cone. 4.00 χ W M). 5

n

1

S


537 Due to their readily accessible redox potentials and the stability of the compound in both reduced and oxidized form, this system is very attractive for performing in-situ electrochemistry and hyper-Rayleigh scattering. Some restraints are imposed on the design of a combined electrochemistry and H R S cell. The optical path should remain free (x-axis) as to be able to focus a high power beam into the cell. At the same time, the optical path should be free also in the perpendicular direction of the incoming beam (y-axis). Evenso, it is important that the necessary electrodes (rotating Pt-working electrode, a Ptcounter electrode and a A g / A g reference electrode) do not induce additional scattering. Therefore, a custom cell was designed as shown in Figure 8. Note that the large volume size (IS cm ) is necessary to meet all the previously mentioned conditions. This large volume size is also responsible for the long operating times required for complete oxidation or reduction. 21

+


3

A g£À g+refer ene e eleforo de ί Ρt-gauze working electrode Ρ t-wire counter electrode

Figure 8. Schematic View of the Combined Electrochemistry/HRS cell

The oxidation was performed at a potential of 0.8 V for about 30 min. and the reduction at -0.7 V for 25 min. The switching of the second-order nonlinear response was in accordance with the variation of the linear optical data (Figure 9). The M L C T band disappears together with the hyper-Rayleigh signal and both signals re-appear upon reduction of the compound. Taking into account the difference in resonance enhancement, the dispersion-free value for the first hyperpolarizability can be switched between 100 and 10 χ 10" esu. 30



(a)

1.4

r

400

500

600

700

800

900

W a v e l e n g t h (nm)

(b)

250

-

200

-

No. of cycles Figure 9. (a) Linear Absorption Data for Electrochemical Oxidation ofFe(U) to F(1II) (cone. 4.00 χ JOT M; Optical Path Length 6 cm) (b) Electrochemical Switching of the HRS response between Fe(II) and Fe(III). 3


539

Conclusion and Outlook High-stability coordination compounds of transition metal ions offer an attractive alternative to conventional N L O chromophores based on traditional all-organic frameworks. Indeed, comparative studies have shown that Ru(II) pyridyl ammine complexes have larger β responses than their 4(dimethylamino)phenyl counterparts. Additionally, appropriate coordination compounds can replace both the electron releasing dialkylamino or electronwithdrawing nitro groups of classic N L O chromophores to provide hyperpolarizable structures featuring augmented thermal stability. While both noncentrosymmetric organic and coordination compounds can manifest low-lying C T transitions that feature large transition dipole moments, coordination compound-derived supermolecular chromophores possess electronic structural features that make possible new opportunities to both enhance and modulate the magnitude of the molecular first hyperpolarizability. Such insights have enabled the design of new classes of chromophores that possess the largest hyper polarizabilities yet determined at practical telecommunication wavelengths, and make possible new strategies to switch and gate N L O responses voltammetrically. 0


22,23

Acknowledgments This research was supported by the National Fund for Scientific Research (FWO-V, G.0297.04), the Belgium government (GOA/2000/03) and the KULeuven (IUAP P51/03) (IA and K C ) . IA is a Postdoctoral fellow of the FWO-Vlaanderen. M J T thanks the Office of Naval Research for generous financial support. Studies carried out by the group of Coe were supported by the U K Engineering and Physical Sciences Research Council (grants GR/L56213, GR/M93864 and GR/R54293). The EPSRC ( U K ) and the COST organization, through Action D-14-WG0011, is also thanked for support (JAM).

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Metal-Containg and Metallosupramilecular Polymers and Materials

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