Efficient production of methyl viologen cation radicals using RuII systems

Journal

of Photochemistry

and Photobiology,

A:

Chernisby,

54 (1990)

283-292

Efficient production of methyl viologen cation radicals using Ru” systems Taial

S. Akasheh+ and Nathir A. F. ALRawashdeh

Chemisny

Department,

Yumouk

Univemity,

I&d

(Jordan)

(Received March 2, 1990)

Abstract A number of RuU-diimine complexes were used to photosensitize methyl viologen cation

radical (h+fWc’) production in the presence of ethylenediaminetetraacetic acid (EDTA). An unusually high quantum yield (~(W+c’)=1.88) was obtained for [Ru(bpz)ZdpqJ2’ (bpz - 2’,2-bipyrazine; dpq = 2,3-di-(2’-pyridyX)-quinoxaline) at pH 11. This value is higher than any previously known for Ru” complexes. The quantum yield is discussed in terms of the electron transfer quenching process and the cage escape effects which affect geminate recombination of the redox species. Time-resolved transient absorption spectra were used to study the formation of MV+’ and Ru’ in the case of [Ru@pz)2(dpq)]2+.

1. Introduction Light-induced electron transfer reactions present an important means for the utilization of solar energy. Of the many known systems which undergo such reactions, Ru”-diimine complexes have been extensively studied [l-6] and may eventually offer an alternative viable energy conversion solution. In such complexes, the Iuminescent metal-to-ligand charge transfer state (3MLCT) [7-91 is known to be responsible for the photosensitization of a large number of electron transfer reactions. As an example, oxidative quenching of triplet [R~(bpy)~]‘+ (bpy -Z&2’-bipyridine) by Fem has been used to produce a photopotential at a platinum electrode [lo]. Oxidative quenching by @+ and reductive quenching by disodium ethylenediaminetetraacetic acid (EDTA) can produce MV+’ (cation radical), which in the presence of a suitable catalyst can produce fuel HZ from water [ll-231. Two mechanisms are known to exist for the production of MV+’ in the presence of EDTA, depending on whether oxidative or reductive quenching of the “MLm state occurs. The oxidative mechanism involves oxidation of this state by w+ to form Rum and MY+’ This mechanism is typical of [R~(bpy)~]*’ [14,15,17]. In the reductive quenching mechanism, typical of [Ru(bp~)~]*+(bp z - 2,2’-bipyrazine), EDTA reduces the excited state to produce Ru’ which in turn reduces MY*+ to hN+* [18]. The two mechanisms differ in three main reaction steps. The oxidative mechanism under basic conditions [15] is given below Run &+

*Run

*R,$’ 5

R,,”

‘Author

Excitation

(1)

Radiative and non-radiative decay

(2)

to whom correspondence

should be addressed.

Q Ekvier

Sequoia/Printed

in The Netherlands

284

+ LR,$n+MV+’

*Rurl + @ Ru’n+MV+‘-

h

Run+w+

Rum + EDTA

5

EDTA+ k6-MV2+

MV’-‘+EDTA+ EDTA’

-%

Quenching (oxidative)

(3)

Back electron transfer

(4)

+ Run

(5)

+EDTA

(6)

EDTA’ + H’

(7)

EDTA’ + MV2+ - ka MV+‘+ products

(8)

2EDTA’ *products

(9)

where EDTA+ is th,e oxidized form of EDTA with an electron removed from one of the nitrogens ON-CH;,-) and EDTA’ is formed by proton loss (>N-CH-)

WI-

In the reductive mechanism,

steps (3), (4) and (5) are replaced by [18]

- k3 Ru’+ EDTA+

(31)

Run + EDTA - ” Ru’ + products Ru’+ MV2+ kjRUn+MV+-

(4’)

*Run + EDTA

(5’)

The rates of the individual steps in the above mechanism vary considerably with pH and ionic strength. It has already been established that in alkaline media steps (6), (7) and (8) are fast [17, 18, 241. However, step (9), although fast in acidic media, is slowed down as the pH increases [18, 241. Thus it is clear that the mechanism is more complicated in acidic media due to reaction (9) as well as the formation of unknown Run products [18]. Hence, the treatment to follow concentrates on alkaline media. On steady state irradiation of the Run-MV*+ -EDTA system in basic media, the initial formation (MV+‘= 0) of MV +* is linear with time [15]; the steady state treatment results in a quantum yield of MV+’ formation under oxidative quenching conditions [15, 171 of

404~+‘)

=

k3fMV”+l

ki[Mv”l k,[MV+*] +k,[EDTA]

k2+k3[w+]

k,(k, - &[MV+*])[EDTA] + (k,[MV+-] +k,[EDTA])(k,+k,[MV+*]) When MV+’ is small and the rate constants are kS[EDTA] * k,[MYC’] the quantum yield becomes

I k,%- k6[MV”]

and

and the theoretical maximum yield will be two (k3[w+] *k2) as expected, two steps of MV+’ formation are stipulated for each photon absorbed. With reductive quenching

since

&X(Mv+‘)

+r(MV+.)

=

=

such

that

2k,[WCl k2+k3[MV+]

1 + k, -kdM’V+‘l

k3[ED574

k2 f k;[EDTA]

I

k, + k,[MV+‘]

When

k,a k@fV+‘]

this leads to

2k;[EDTA] *@‘lV+-)

= k2 +k;[EDTA]

It has been proposed

[17] that another way of writing the initial quantum yield

is

where A, is the intersystem crossing efficiency for formation of *Run following excitation (step (1)) and is usually unity for Run-diimine complexes [25-291, #9 is the quenching efficiency of quencher Q (i.e. k,[Q]/kz +s[Q]) and 4, is the efficiency of secondary MV+’ formation from EDTA (as opposed to EDTA degradative reactions) and is unity in alkaline media [17]. If the details of reaction (3) or (3’) are written as [Rut or Rum . . . Q’ or Q-1

*Run + Q -

[Ru’ or Run1 . . . Q’ or Q-1 [Ru’ or Rum

-

. . . Q’ or Q-1 -

Ru’ or Rum +Q’ Run + Q

(3a, 3a’) or Q-

(3b, 3b’)

(3c, 3c’)

then 4, represents the efficiency of the release of the redox products after formation within the solvent cage (3b or 3b’) in competition with the geminate pair back electron and & both equal to unity, the expression for transfer (3c or 3~‘). Thus with & thus affording a simple means of calculating c&. +(MV+.) becomes 2&#=, MV+* production using PWvWpd2+ 7 and [Ru(bpy)(dpp)2]2’ (dpp - 2,3-di-(2’-Py[Ru(?py);::bp$r’, [::(bp;;;;)2]2+ dmbpy = 4,4’-dimethyl-2,2’-bipyridine; pyq = 2-(2’-pyridyl)-quinoline; ridyl)-pyrazine; dpq=2,3-di-(2’-pyridyl)-quinoxaline) (Fig. 1) in acidic and basic media. Quenching rate constants, MV+’ formation efficiencies and & (in basic media) were determined using steady state experiments. The highly efficient sensitizer [Ru(bpz)2(dpq)2]2+ was further studied using laser flash photolysis to follow the time-dependent formation of Ru’ and MV++‘.

2. Experimental

details

The complexes were prepared using standard procedures [30,31]. EDTA (Aldrich AR) was used as received. Methyl viologen dihydrochloride (w+) (Aldrich) was purified by precipitating from methanol using ether. All solutions were made up in deionized distilled water which was further fractionaliy distilled from KMnOd. Fluorescence quenching data were obtained from the emission intensity at the maximum emission wavelength on a Perkin-Elmer MPF44B instrument. W-visible measurements were recorded using a Varian Gary 2300 series or DMSlOO spectrophotometer. Quantum yields of MV+’ production were determined against a potassium ferrioxalate actinometer [32] with about a 15% experimental error. [MV+‘] was determined from the absorbance at 605 nm (e = 1.37 X lo4 M-’ cm-‘) [33]. Lifetimes were measured using an Edinburgh Instruments model 199M photon counting system. Laser flash photolysis experiments were performed using a K347 Applied Photophysics system with a Spectron Nd:YAG laser (g-10 ns). Pulse energies were kept to about 6-g mJ. Laser and steady state photolysis experiments were performed after degassing the aqueous solutions with argon for at least 20 min.

2,2’-bipyridine (bpy)

2,2’-bipyrazine (bpz)

4,4’-dimethyl-2,2’-bipyridine (dmbpy)

2-(2’-pyridyl)-quinoline (pyq)

2,3-di-(2’-pyridyl)-pyrazine (dpp)

2,3-di-(2’-pyridyl)-quinoxaline (dpq)

Fig. 1. Structures of the ligands.

r

0.8

I 0.00

0.02

0.04

an6

0.0 8

bv=+l

Fig. 2. Stem-Volmer plot of the quenching of [Ru(bpy)(dpp)2]2f by m+ at pH 4. (&,/I in this and other figures is the ratio of the unquenched to quenched intensity of luminescence at the maximum emission wavelength.) Concentration of complex, 5 X lo-’ M. Fig. 3. Stem-Volmer plot of the quenching of [Ru@py)(dpp)2]2+ by EDTA at pH 4. Concentration of compIex, 5~ 10s5 M. 3. Results and discussion Air-saturated quenching data were obtained on steady state irradiation at pH 4 and 11. Figures N show typical Stem-Volmer plots. Although many cases show a linear behaviour, deviations from linearity sometimes occur (e.g. Fig. 4). This can be

attributed

to aggregation

between

m’

and Cl-

at high Mvz+

concentrations

and

287

3.t

22

1.4

0.0 * 0.00

OJl 0.02 0.53 0.04 bv=+l Fig. 4. Stem-Volmer plot of the quenching of [Ru(bpy)(pyq)2]2’ of complex, SX lop5 M.

by WC

at pH 4. Concentration

to aggregation between EDTA and the sensitizer [14, 16-18, 341. The Stern-Volmer constant was evaluated in the early linear region at low quencher concentrations. Table 1 summarizes these results for EDTA and w+ in acidic and basic media, together with the maximum emission wavelength at which the observation was made, the lifetimes in the absence of quencher and the excited state redox potentials (in occurs exclusively by EDTA and acetonitrile) [31, 35, 361. Quenching of [Ru(bpz)J*+ occurs exclusively by w+. It can be seen that quenching of [Ru(bpy)3]2f *+ behaves in a similar manner to the bipyrazine complex (quenching [Ru(bpz),(dpq)l occurs only reductively by EDTA at pH 11). The behaviour of the remaining complexes is similar to [Ru@py)$+ (oxidative quenching by MY*’ is more important). Correlation of the results with A?Z*(2+/1+) shows that this value is highest for [Ru(bpz)g]2+ indicating that complexes with hE*(2 +/l + ) = 1.03 V followed by [Ru(bpz)2(dpq)]2+, and below are less susceptible to reductive quenching by EDTA. It is expected that kq should be lower for [Ru(bpz),(dpq)]*+; however, k2 was measured in water, whereas AJZ* was calculated from acetonitrile data [36]. It should also’be noted that kg at pH was estimated from three points only as rapid early curvature 11 for [Ru(bpz)2(dpq)]2+ was observed in the Stern-Vohner plot. Thus there is a large error in this kg vaiue at pH 11, although the more reliable value at pH 4 is similar. Although kg for and EDTA seems to be diffusion limited, all other values are less [Ru(b~zL(d~qN*+ than this limit. Oxidative quenching by MV*+ is roughly predicted by hE*(2+ /3 + ). with A.!?*(2 + /3 -t ) = - 0.09 V only negligible w+ quenThus for [Ru(bpz)z(dpq)“+ ching is observed at pH 11 (the results in acidic media do not follow this criterion). The other complexes all have positive AE*(2+/3 +) values (0.48 V or higher), but regardless of the absolute value, all values of kg at pH 4 and 11 are lower than those obtained for the quenching of [Ru(bpy)#+ by @+. To ascertain the production of MV+’ as a result of the quenching processes described above, the W-visible spectra of argon-degassed solutions of the complexes in the presence of EDTA and w+ were recorded at pH 11 and 4 on steady state irradiation. A typical run is shown in Fig. 5 in which the increasing peaks at 395 nm (~=4.21 X 1U4 M-l cm-l) and 605 nm (e= 1.37X lo4 M-l cm-‘) indicate the formation of MV+’ 1331. The peak at 605 nm was used to follow the rate of MV+’ formation

I

MS@+

EDTA

from refs. 35 and 36.

tWbzW2+ PI

‘Data in volts in acetonitrile

MV2+

EDTA

Mvz+

EDTA

tWwM2’ WI

Pb4ddps)12+

PWwNh.%12+

MV2+

EDTA

692 46 43

199 193 -

-

266 258

628 -

[Wbw>WbwM2 +

MV’+

151 145 -

688

11 4 11 4 11 4 11 4 11 4 11 4 11 4 11 4 11 4.7 11 4.7

EDTA

Pu@m9(ws>~12 +

at 468 nm) 7 (ns)

excitation Acm

solutions; PH

data (air-saturated

Quencher

quenching

Complex

Stern-Volmer

TABLE

6.71 x 10” 6.49 x lo8 Negligible 7.2 x 10’ 2.3 x 10’ 9.42 x 10’ 1.54 x log 1.68 x 109 Negligible 1.73 x 108 1.1 x log 1.0x log 7.7 x lo8 2.0x 1oB

4.06 x l@ 3.34 x 108 -

-

kg (M-’ s-l)

1.36

0.76 -

1.03

0.83

AJF*(2+/1+)”

-0.30

- 0.48

- 0.85

-

- 0.49

AS!z*(3+/2+)

289

0.0 700

600

500 Wavelength InsI

400

x(-m) 0

7

14

21

Fig. 5. W-visible absorbance spectrum of the [Ru(bpy)(pyq)#+-m+-EDTA function of irradiation time (maximum irradiation time, 55 min). Experimental complex, 1.0 x 10m4 M; [MV’*] = 0.02 M, [EDTA] = 0.1 M; pH 4.0.

28

min.

system as a concentrations:

Fig. 6. Plot of MV” formation as a function of time of irradiation of the system [Ru(bpy)(dmbpy)#+ (1.0x 10T4 M). MV*+ (0.02 M) and EDTA (0.1 M) at pH 4.0.

Fig. 7. Plot of MV*’ formation Fig. 6 at pH 11.0.

as a function of time of irradiation

for the same system as in

with time. Typical plots of MV+’ as a function of time are shown in Figs. 6 and 7. Initially, the increase is linear with time, but eventually the plots start to reach a plateau as the rate of consumption of MY+* starts to increase. Since MV+ is less stable in acidic media, this behaviour is reached more quickly at pH 4 (e.g. Fig. 6). By employing actinometric techniques, the linear region can be used to calculate the initial quantum yields of MY+’ formation. Table 2 shows the values of 4(hIV+) and A. at pH 11. Since the mechanism in acidic media is too complicated [14-231 to warrant a calculation of &,, only cj@N+‘) is given at pH 4. It can be seen that &(MV+‘) closely follows +,, indicating that cage release effects which prevent back electron transfer between quencher and sensitizer are dominant in enhancing the formation of MV+-. This is irrespective of whether reductive or oxidative quenching

26C Havelength b~rnl

-.OL

-.o:

E

.03

.06

-09

.12

, I 300 340

I

420

I

380

I

460

I

500

I

I

580

HavelengthInn]

540

i/

Fig. 9. Absorbance changes of an aqueous solution (pH 11) of [Ru(bpz)t(dpq)]2+ alone (0), the same complex with EDTA (0) and the complex with EDTA and ML@’ (A) at 200 ns after the 355 nm laser pulse. Concentrations: complex, 2.0X lo-’ M, [EDTA]=O.l M; [MV*+] =0.02 M.

Fig. 8. Absorbance changes of an aqueous solution of {Ru(bpz),(dpq)]*+ (2.0X lo-’ M) and EDTA (0.1 M) at pH 11 following laser pulse excitation at 355 nm at 0 (0), 100 (0) and 200 ns (A). The 200 ns spectrum is the same as that at 100 ns at wavelengths above 400 nm and is not completely shown.

-.2

-.17

-.I4

-.ll

-.OS

-.OS

-:02

s

E

z3

IU

a

291

TABLE 2 Quantum

yield of MT’

Complex

and cage release effect ( f 15%) pH

FWW9(ws)~12+ FWbw~(dmbm9d2+ [Wbpy)(dpp)21z+

PWbpWdpq)12+ [WbwM2+

production

WI

[Wbpz>sl*+ WI

II

pH4

40.

&Cm

0.18 0.23 0.095 1.07

0.19 0.35 0.091 1.88 0.21 1.3

-

‘3

NW+-) 0.011 0.012 0.0023 0.0021 0.14 0.27

occurs. [Ru(bpy)(dmbpy)z]‘+, which undergoes oxidative quenching, is more efficient than [Ru(bpy)3]2+ in forming MV+*. However, the most efficient Ruh-diimine sensitizer is FW~p~M+q)l 2+- It has a +(MV+‘) value of close to the theoretical limit of two and a cage release efficiency of unity. Laser flash photolysis studies of [Ru(bpz)z(dpq)]‘+ were performed. At pH 11 EDTA quenching should yield Ru’. Transient absorption decays were measured at various wavelengths and the signals at 0, 100 and 200 ns after the laser pulse were recorded. The change in absorbance of the sensitizer (in the presence of 0.1 M EDTA at pH 11) ws. wavelength is shown in Fig. 8. At time zero the spectrum is similar to the absorbance changes in an EDTA-free solution. At times greater than 100 ns a long-lived species persists and the absorbance changes associated with it are shown at 100 and 200 ns. &ince reductive quenching occurs, it is expected that the spectral changes are due to [Ru(bpz),(dpq)]+. The peak at 515 nm has a corresponding peak at 510 run in [Ru(bpy)s]+ [37-391. Figure 9 shows the spectral changes that occur 200 ns after the laser pulse for the sensitizer alone, the sensitizer and EDTA and, finally, the sensitizer in the presence of EDTA and MV+. At 200 ns, the sensitizer signal has all but disappeared. In the presence of EDTA, the Ru’ spectral changes (in Fig. 8) are shown. Finally, in the presence of MV’+, the characteristic spectrum of the methyl viologen cation radical (Mv+) is observed with a maximum at 395 nm and a rising edge in the range 520-580 nm. 4.

Conclusions

The oxidative and reductive quenching by MVz4 and EDTA can be roughly correlated with the excited state redox potentials for the series of sensitizers studied here. The most efficient formation of MV+’ is observed for [Ru@pz)2(dpq)]2+. The laser flash photolysis experiments strongly indicate Ru’ production on quenching of EDTA. These results warrant further investigation of the kinetic rate constants of the reaction steps and the effect of ionic strength, pH, etc. We are currently undertaking these studies. Acknowledgment T.S.A. wishes to acknowledge the research grant from the Ministry of Planning, Jordan, the Kuwait Development Fund and Yarmouk University. This work is part of the M.Sc. thesis of N.A.F.A.

292

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