Encapsulation Effects on Homogeneous Electron Self-Exchange

Chapter 15

Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch015

Encapsulation Effects on Homogeneous Electron Self-Exchange Dynamics in Tris(Bipyridine) Iron-Core Dendrimers Y o u n g - R a e H o n g a n d C h r i s t o p h e r B. G o r m a n

*

Department of Chemistry, North Carolina State University, Box 8204, Raleigh, N C 27695-8204 Corresponding author: [email protected] *

A series of tris(bipyridine) iron core dendrimers was synthesized up to the second generation. Their homogeneous electron self-exchange dynamics were studied by N M R line– broadening. While the first generation dendrimers showed dramatically attenuated electron self-exchange rate with respect to dimethyl-bipyridine iron complex, little rate change between the first and second generation dendrimers was observed. These results suggest that the effective distance of electron transfer in this homogeneous self-exchange case is not a simple function of dendrimer generation.

© 2006 American Chemical Society

205

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

206

Background Understanding electron transfer is important because it can provide us fundamental knowledge associated with many biological and chemical processes such as those found in metalloproteins, luminescent molecules, and the emerging area of molecular electronics. " The factors that govern the rate and driving force for electron transfer can be investigated by systematically varying the structure of model compounds. To this end, redox-active core dendrimers are attractive molecules because each generation of dendrimer nominally produces a different degree-of-encapsulation of the redox-active core resulting in changes of electron transfer dynamics. We and others have prepared various redox-active core dendrimers and studied their electron transfer dynamics in various scenarios including heterogeneous electron transfer at a metal-solution interface and electron hopping through the dendrimer film. * By observing changes in the rate and/or driving force for electron transfer, several effects of dendrimer generation and primary structure can be rationalized to affect the macromolecular conformation and thus the degree of encapsulation of the redox-active core. One scenario for electron transfer that, to our knowledge has not yet been explored with redox-active core dendrimers is homogeneous, bimolecular electron transfer in solution. Is the rate of this reaction attenuated in the same manner observed previously for heterogeneous electron transfer? Moreover, can this approach allow us to measure a greater degree of rate attenuation? In heterogeneous electron transfer reactions, only a four to five order of magnitude rate decrease can be observed before voltammetry becomes non-quantitative. In contrast, in solution, a variety of spectroscopic techniques are available to measure electron transfer rates, and these can span a much greater range. Although one cannot directly compare heterogeneous and homogeneous electron transfer rates as they represent different kinetic orders, wider detection limits for homogeneous electron transfer rate determination should help us to probe encapsulation effects especially for the slow electron transfer behavior found in redox-active core dendrimers. We recently reported an isostructural series of redox-active, metal tris(bipyridine) core dendrimers that are amenable to study of homogeneous electron transfer kinetics. The central redox units of these dendrimers can be chemically oxidized. Here, we report our first results on homogeneous electron self-exchange kinetics between oxidized and reduced dendrimer species. A s will be shown, the attenuation of the rate of this reaction with dendrimer generation is not the same as the behavior found in heterogeneous, electrochemical rate determinations.


1

6

7

9

10


207

Objectives


In this study, our goals were to understand how the encapsulation of a redox-active, iron tris(bipyridine) core using dendrons affects the homogeneous electron self-exchange rate. This rate was measured by N M R line broadening technique in partially oxidized dendrimer solutions of generations "0" through "2" as defined below.

Experimental Materials 10

A l l synthetic efforts were reported previously except the second generation dendron and dendrimer which were prepared in a similar fashion and characterized as below. G2-Bpy. Yield: 92% (1.79 g); H N M R (CDC1 ) δ (ppm) 1.2 (m, 4H), 1.51.8 (br, 30H), 2.1-2.4 (m, 12H), 2.6 (t, 4H), 3.9 (t, 8H), 5.0 (s,16H), 6.8 (d, 8H), 6.9 (d, 16H), 7.0-7.2 (m, 24H), 7.3-7.6 (m, 42H), 8.2 (s, 2H), 8.6 (d, 2H). Anal. Calcd for C oHi68N 0,2: C, 83.99; H , 6.97; N , 1.15; found: C, 83.75; H , 6.83; N , 1.32. [Fe(G2-Bpy) ](PF ) . Yield: 91% (0.48 g); H N M R (CD C1 ) δ (ppm) 1.2 (m, 4H), 1.4-1.6 (br, 30H), 2.0-2.4 (m, 12H), 2.6 (t, 4H), 3.7 (t, 8H), 5.0 (s,16H), 6.7 (d, 8H), 6.8 (d, 16H), 6.9-7.1 (m, 26H), 7.2-7.4 (m, 42H), 8.2 (s, 2H). Anal. Calcd for C ioH 04F, FeN O P : C , 80.18; H , 6.65; N , 1.10; found: C, 80.13; H , 6.67; N , 1.08. !

3

i7

2

l

3

6

5

2

2

5

2

6

36

2

2

N M R kinetic measurements A l l deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc, dried following standard procedures, and stored in a dry box under nitrogen atmosphere. N M R spectra were obtained using Varian 300 M H z N M R spectrometer at 25 °C unless otherwise noted. Sample solutions for N M R measurement were prepared freshly in the dry box prior to use. A solution of Iron(II) core dendrimer in C D C I : C D C N (5:1 v/v) were prepared and transferred to 5 mm Kontes brand threaded N M R tube in the dry box. Aliquots (typically 10 pL) of the oxidants (NOPF or Fe(bpy) ) in C D C N were added to the N M R tube between each N M R measurement. Line widths were 2

2

3

3+

6

3

3


208 determined by fitting the experimental signals to a Lorenzian function using commercially available N U T S software. Temperatures were maintained by temperature controller available in Varian spectrometers. The second order electron self-exchange rate constants were measured by N M R line broadening techniques. The proton N M R spectra of the partially oxidized dendrimer solutions were interpreted by use of the two-site exchange model between diamagnetic iron(II) and paramagnetic iron(III) cores. The approximate Bloch-McConnell equations for the electron self-exchange rate constants in the fast exchange and slow exchange limits were used to calculate the rate constants.


,M3

2

Anf f (ôv) p d

k

=

for the fast exchange limit

W -f w -f w )c DP

k

OY

=

d

—

D

p

P

lol

—

for the slow exchange limit

[P]

In the equation, ÔV is the contact shift in H z (chemical shift movements caused by paramagnetic electrons, that is the chemical shift difference between pure diamagnetic and pure paramagnetic species), f and U are the mole fractions of paramagnetic and diamagnetic species respectively, W , W , and W are the peak width (full line width at half-maximum) for paramagnetic species only, diamagnetic species only, and the mixture o f two species respectively, C , is total molar concentration and [P] is molar concentration of paramagnetic species. Values for f for the fast exchange system were more precisely determined using the relationship p

P

D

D P

to

v

δν assuming that the chemical shifts vary linearly with mole fraction. v and v d

dp

are the resonance frequency o f the diamagnetic species and the mixture respectively.


209

Results & Discussion Synthesis We previously reported the synthesis and preliminary electrochemical characterization for a series of metal tris(bipyridine) core dendrimers. Second generation of the dendron (G2-Bpy) was prepared by similar procedures with high yields. This dendron ligand was then successfully attached to the iron(II) metal core and characterized by *H-NMR spectroscopy. Figure 1 shows the dendrimer structures employed in this study and defines, in this case, what is regarded as the zeroth, first and second generation of this dendrimer.


10

Determination of homogeneous electron self-exchange rate constants The homogeneous electron self-exchange rate of dimethyl-bipyridine iron complex [Fe(G0-Bpy) ] in C D C 1 / C D C N (5:1 v/v) was measured by N M R line broadening techniques. A solvent mixture was used in this study to ensure the 3

2

2

3


210 solubility of the oxidants and dendrimers. The rate of electron self-exchange for Fe(G0-Bpy) was found to be in the fast exchange limit, indicated by the linear relationship between the mole fraction,/ and chemical shift. ' The singlet corresponding to the methyl protons at 2.5 ppm on the methyl-bipyridine ligand was used in the calculation of the rate constant as it showed sufficient chemical shift movements and line broadening during the experiments (Figure 2). 2+/3+

3

12

1 3


p

GO

= 0.124

f

v

f = 0.092 p

f - 0.060 9

A

f = 0.029 p

^ , = 0.000 —

« —

-

.1

-rn -,,,,Π,,,Γ-,,-, „•,„•,

411

45

35

«

,.À

2.S

10

2

« M

/3

Figure 2. Ή NMR spectra ofFe(G0-Bpy) * * systems at 20 °C in CD CyCD CN (5:1 v/v) mixture. The counter anion was PF \ 3

2

s

6

2

Although NOPF was an acceptable oxidant for Fe(G0-bpy) *, oxidation of the core of Fe(Gl-bpy) by N O P F was unsuccessful because the dendron itself was oxidized to some extent by N O P F . As the dendron is intended only as an encapsulating moiety, an alternate, milder oxidant was required. Fortunately, it was determined that unsubstitued iron tris(bipyridine) [Fe(bpy) ] had slightly higher redox potential compared to the methyl substituted iron complex [Fe(G0bpy)3 ] and, upon addition of Fe(bpy) to Fe(Gn-bpy) (n = 0, 1 2) the former oxidized the latter. The redox potential difference between these two species (ca. 160 mV in C H C N ) provided a very mild route to prepare the oxidized dendrimers. Moreover, the resulting Fe(bpy) did not interfere with the spectroscopic observation of the desired, line broadened signals. 6

3

2+

3

6

6

2+/3+

3

3+

2+/3+

2+

3

3

3

2+

3


211

2.45 mM 2.18 mM 1.90 mM 1.60 mM


1.30 mM 0.99 mM 0.67 mM 0.00 mM 10

10

2.45 mM 2.18 mM 1.90 mM 60 mM 1.30 mM 0.00 mM

2+/3+

2+/3

Figure 3. Ή NMR spectra ofFe(Gl-Bpy) andFe(G2-Bpy) systems at 25 °C in CD CyCDiCN (5/1 v/v) mixture. The counter anion was PF \ 3

3

2

6

The electron self-exchange rate for the first and second generation dendrimers were found to be in the slow exchange limit as determined by a lack of noticeable change in chemical shift over the concentration range examined (Figure 3). Data were fit to the slow exchange equation and average k values were determined from slopes of plots 7t(W -W ) vs [Fe(Gn-Bpy) ] (Figure 4). The spin-spin coupling of the observed proton peaks around 2.7 ppm with the methylene protons next to it hampered the determination of the line width. Thus, triplet peaks in pure Fe(Gn-Bpy) (n = 1 and 2) were deconvoluted to determine W values. W values of the sample mixture were measured after line broadening was corrected for this spin-spin coupling. i%

3+

DP

D

3

2+

3

D

D P



212

Table 1. Electron self-exchange rate constants Systems

C (mM) tot

Fe(G0-Bpy)

3

Fe(Gl-Bpy)

3

Fe(G2-Bpy)

3

1

k

ex

(M'V )'

2+/3+

11.13

2.59 (0.19) χ 10

7

fast exchange

2+/3+

8.56

4.71 (0.27) χ 10

4

slow exchange

0

2+/3+

8.51

3.00 (0.21) χ 10

4

slow exchange

0

a. values in parentheses represent the magnitude of the 90% confidence interval b. fast exchange equation was applied. c. slow exchange equation was applied.


b

213 This treatment of data gave more acceptable results for both molecules, giving an intercept much closer to the expected value of zero (Figure 4). Observed electron self-exchange rate constants for all generations were tabulated in Table 1. As can be seen from the data in Table 1, the electron self-exchange rate for the first generation dendrimer was retarded dramatically relative to Fe(G0bpy) system. In contrast, the attenuation of the electron self-exchange rate between the first and the second generation dendrimer was insignificant compared to that between Fe(G0-Bpy) and F e ( G l - B p y ) . This result was unexpected as it does not correlate with the "encapsulation" behaviors illustrated in heterogeneous electron transfers in dendrimers previously. This result is perhaps even more surprising i f one regards homogeneous electron transfers between the redox centers in Fe(G2-Bpy) and Fe(G2-Bpy) must occur effectively through four "layers" of repeat units (e.g. two for each molecule). We rationalize these results by considering the possibility of relatively rapid core motion within the dendrimer architecture. Previously, the effects of the core motion were argued to be important in governing the rate of heterogeneous electron transfer iron-sulfiir cluster core dendrimers containing alternately flexible and rigid repeat units and in a series of iron-sulfur cluster core dendrimer constitutional isomers. * Most notably, in films of these molecules, the rate of the electron hopping through these films was observed to be in the slow exchange realm and no significant variation in the electron self-exchange rate constants was observed with generation. Similar explanations used to rationalize the behavior in these systems may be applicable here. Over the timescale of the slow exchange limit, the redox core unit within the dendrimer can move so as to achieve a relatively close approach with another redox core in a neighboring dendrimer. Thus, the effective distance for electron self-exchange appears to be not strongly affected by dendrimer size in the slow electron exchange limit. While these results do not span the range of dendrimer size (generation), core and repeat unit required to make this conclusion in a general sense, these preliminary data do represent a result that is difficult to interpret without core mobility playing an important role in the kinetics of this process. 2+/3+

3

2+/3+

2+/3+


3

3

7

2+

3

7

3+

3

9

8

Conclusions As dendrimers are studied in a greater variety of ways, the idea of dendritic encapsulation is less and less well described by a static dendrimer model. Here, in exploring electron transfer between dendrimers in solution, the simple generational dependence of encapsulation on rate attenuation does not suffice. To understand the effects of encapsulation for the electron transfer kinetics, the


214 time-scale of the electron transfer as well as the structure and conformational mobility of the dendrimers are important factors. In this case, a simple rationale for the behaviors observed is that the redox core within these dendrimers in the slow electron self-exchange limit can move in a non-rate limiting fashion toward a neighboring redox core with the result that the structural effect of the dendrimer is reduced and electron transfer is facilitated in larger dendrimers.


Acknowledgements The National Science Foundation (CHE-0315311) is gratefully acknowledged for the support of this research. We thank Dr. Sabapathy Sankar and Dr. Hanna Gracz for assistance with N M R measurements.

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Encapsulation Effects on Homogeneous Electron Self-Exchange

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