Inorganic and Organometallic Photochemistry - American Chemical

1 Properties and Reactivities of the Luminescent Excited States of Polypyridine Complexes of Downloaded by UNIV OF SOUTH CAROLINA COLUMBIA on August 8, 2013 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0168.ch001

Ruthenium(II) and Osmium(II) NORMAN SUTIN and CAROL CREUTZ Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973

Properties of the luminescent excited states of polypyridineruthenium(II)

and -osmium(II)

complexes (*ML3 ) 2+

cussed in terms of the properties of the ML3 , ML3

+

ground state species. *Ru(bpy)3

are dis-

ML3 ,

2+

and

3+

and *Os(bpy)3

2+

2+

ex-

hibit spectral maxima at ~ 360 nm and 430-460 nm, but the *OsL3

2+

lifetimes

(~

9-85

nsec) are much

shorter

than

those for the corresponding * R u L 3 species (330-1500 nsec). 2+

Excited-state

potentials for the complexes were

estimated

from their spectra and ground state potentials as well as from photocurrent

measurements

trode. The reactivity of *ML3

2+

transfer parallels that of ML3

3+

at an n-type T i O

toward outer-sphere when it undergoes

and that of ML3

when it undergoes

between

and ground

+

*ML3

2+

oxidation.

state

complexes give rise to electron transfer

2

elec-

electron reduction Reactions

polypyridinemetal(II) (+1

and + 3

ions)

or to energy transfer products depending on the pair of complexes used.

The

photochemistry of the polypyridine R u ( I I )

plexes ( M L 3

2 +

and Os(II)

com-

where L is a 2,2'-bipyridine or 1,10-phenanthroline

derivative) has been the subject of m u c h attention i n recent years

(1-8).

Studies of polypyridine complexes are of interest not only i n their o w n right,

but also because these systems might find application i n solar

energy conversion a n d storage (9,10,11,12,13).

T h e polypyridine com

plexes are especially attractive for solar energy applications because of their spectral properties, the long lifetimes of their excited states, and the ease with which they undergo oxidation and reduction. 0-8412-0398-9/78/33-168-001$06.75/0 © 1978 American Chemical Society

In Inorganic and Organometallic Photochemistry; Wrighton, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

INORGANIC AND ORGANOMETALLIC PHOTOCHEMÎSTRY

2

The one-electron oxidation products of tris (2,2'-bipyridine) ruthenium (II) and -osmium ( I I ) ( M L ) have been known for some time, and the one-electron reduction product of the R u ( I I ) complex ( M L ) has been characterized recently (14,15,16,17,18). I n this article we discuss the reactivities of the luminescent excited state of the + 2 oxidation states ( * M L ) i n terms of the properties of the ground state + 2 ion and those of the + 1 and + 3 ions. 3

3 +

3

Downloaded by UNIV OF SOUTH CAROLINA COLUMBIA on August 8, 2013 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0168.ch001

3

+

2 +

Properties of the Excited States of Tris(2>2'-bipyridine)ruthenium(II) and -osmium(II) The absorption spectrum of R u ( b p y ) (bpy — 2,2'-bipyridine) is shown i n Figure 1. The bands below 200 nm and at 285 n m have been assigned to ugand π —> π* transitions b y comparisons with the spectrum of protonated bipyridine (19). T h e two remaining absorption bands at 240 and 450 nm have been assigned to charge-transfer d - » ττ* transitions (19). T h e τ τ - » τ τ * and < 2 - » π * transitions both result i n the formation of the luminescent excited state, and i n fact, the luminescent state can be produced by the absorption of light below ~ 560 n m w i t h close to unit quantum efficiency (20). I n the absence of added reagents the excited state decays largely by nonradiative pathways i n fluid solution at room temperature, with but a few percent of the excitation energy being re leased as the emitted red light (Φ — 0.042) (21). T y p i c a l excited state lifetimes for R u ( I I ) and O s ( I I ) polypyridine complexes are given i n 3

2 +

8.0 h

I

Ε

2.0

0 200

Figure 1.

_J 300

I 400 λ, nm

600

500

Absorption spectrum of Ru(bpy) * in water at 25° C 2

s


1.

suTiN AND CREUTZ

Table I.

Excited States of Ru(II) and Os(II)

Excited-State Lifetimes for Polypyridineruthenium(II) and -osmium (II) Complexes in Water at 25°C RuL, * To (usee) (32)

OsL, *

2

Ligand, L 4,4'-(CH ) bpy bpy bpy-dg 5,6-(CH ) phen 5-(CH )phen phen 5-Cl(phen) 5-N0 phen 3


3

3

2

2

3

2

T (Ru) To(Os)

- 9 19 32 63 69 84 78

~ 37 31 22 29 19 11 12

0

0.33 0.60 0.69 1.81 1.33 0.92 0.94 ^ 0.005

2

(nsec)

—

—

Table I. It is evident that the osmium lifetimes are shorter and vary more with the nature of the ligands than do the ruthenium lifetimes. Excited State Spectra. The spectra of *Ru(bpy) and *Os(bpy) determined by a laser flash-photolysis technique are presented in Figure 2. (The difference spectrum previously reported (22) for *Ru(bpy) is in good accord with that used in calculating the spectrum shown in Figure 2.) For *Ru(bpy) maxima at 360 nm (c 1.3 Χ 1 0 M " cm" ) and 430 nm (c 0.6 Χ 1 0 M " cm' ) are observed. The spectrum of *Os(bpy) is very similar with maxima at 360 nm (e 1.8 X 1 0 M cm" ) and 460nm (c 0.5 Χ 1 0 M " cm" ). Both are similar to that of the 2,2'bipyridine anion (23) which is shown in the insert. The radical anion has peaks in this spectral region at 558 and 527 nm (c 0.48 Χ 1 0 and 0.50 Χ 1 0 M " cm" , respectively), ~ 420 nm (shoulder, e ^ 1.1 Χ 1 0 NT cm" ), and at 386 nm (c 2.5 Χ 1 0 M cm ). The similarity of the spectra of the excited states to that of the bipyridine anion suggests that the observed excited state transitions are ligand localized. 3

2+

3

3

3

2+

4

4

3

1

1

2+

1

1

2+

4

4

2+

1

1

1

1

4

4

1

1

1

4

4

1

1

-1

The luminescent state arises from the net transfer of an initially metal-localized d electron to a ττ* orbital of the ligand system. Harrigan and Crosby (24) have proposed that the acceptor π* orbital extends over all three bipyridine ligands. The similarity of the spectra of the excited states and of the bipyridine radical ion is somewhat unexpected if, as proposed, the charge-transfer excited state consists of a Ru(III) center with the additional electron delocalized over the three bipyridine rings (25,26) rather than as a metal complex of a single bipyridine radical anion. Moreover, since other low energy transitions (for example, bpy -> Ru(III) charge-transfer seen at ~ 640 nm in Ru(bpy) , and bpy anion - » Ru(III) charge-transfer) might occur in this spectral region, the rela tive simplicity of the spectra is somewhat surprising. Possibly such transitions are present but either he outside the spectral range studied or 3

3+


4

INORGANIC AND ORGANOMETALLIC PHOTOCHEMISTRY

3.0

*0s (bpy)

2+ 3

«360 ~ '-β * Ό

2.0

4

~'

c m

No* (bpy)"


MAHON and REYNOLDS

i.o «

~0.5χΙ0 4Τ αη" 4

4 β 0

,

1

*Ru(bpy) " 2

3

i.o

«—J^xIO^'cm"

1

360

0.5 \ —

Figure 2. Absorption spectra of the luminescent excited state of Ru(bpy) (lower spectrum) and Os(bpy) + (upper spec trum) in water at 25°C. The insert shows the spectrum of Na bpy" (23). The errors on the extinction coefficients are estimated to be ±15%. 2+

3

2

s

+

01 300

400 X,nm

500

are obscured b y the low resolution and considerable experimental errors associated w i t h these measurements. Excited State Potentials. I n a physical model the formation of the charge-transfer excited state can be regarded as the creation of a sepa-


1.

suTiN AND CREUTZ


5

rated electron-hole pair. The excited state thus is expected to be both an electron acceptor (hole on the ruthenium center) and an electron donor (excess electron on the bipyridine). A molecular orbital description of the excited state (adapted from Ref. 15) is shown i n Figure 3. T h e excited state has a d metal center characteristic of R u ( b p y ) and an electron i n a ligand π* orbital characteristic of R u ( b p y ) . F o r these reasons, * R u ( b p y ) is expected to exhibit some properties of both Ru(bpy) and R u ( b p y ) . 5

3

3

3


3

3 +

+

2 +

3 +

3

+

e(7T*) /_,

t (Trd)
R u L

2 *AG/nF >

3

3 +

(#3,2°

+ RuL -

+

(25a)

#2,i°)

(25b)

3

thermodynamically favored if Equation 25b is satisfied, a condition which is certainly met for a l l the ruthenium and osmium complexes. F o r * R u ( b p y ) , evidence for excited state disproportionation has been sought from both absorbance and emission measurements but Reaction 25a is evidently slow with k ^ 10 M " sec" (44). The disproportionation can, however, be catalyzed by a pair of redox quenchers, e.g. R u ( N H ) 3

2+

7

1

1

3


e

2 +

1.

suTiN AND CREUTZ


23

and R u ( N H ) ; the reductant generates R u ( b p y ) from the excited state while the oxidant generates R u ( b p y ) . (This particular pair of quenchers would not give high yields of the disproportionation products since the back-reaction rate constants are diffusion controlled. ) The reaction of one excited molecule with its parent ground-state molecule (Reaction 26a) proceeds spontaneously only when condition 26b 3

e

3 +

3

3


#

ML

3

+ ML

2 +

3

-> M L

2 +

*AG/nF 5> (E °

3

+ ML

3 +

-

3t2

+

3+

3

(26a)

+

(26b)

E ,i°) 2

is satisfied. Inspection of Table III indicates that ( E , ° - E , i ° ) is about 2.5 V for R u L complexes and about 2.0 V for the OsL* * complexes. These potential differences substantially exceed the excitation free ener gies of ~ 2.1 and ~ 1.8 V for R u L and O s L , respectively; thus, E q u a tion 26b is not satisfied for these systems. ( In fact, the reverse of Reaction 26a occurs readily and has been observed i n electrochemical experiments from its striking chemiluminescence in acetonitrile (15) and i n water (56). The reaction of * R u ( b p y ) with M L also is very rapid (44). A lightinduced disproportionation analogous to Reaction 26a however is feasible if the excited and ground state molecules are different (Reaction 27). 3

3

2

2

2 +

2

3

3

2 +

2 +

3

3

ML«

3 +

^

MI«

Reaction 27a can occur when * E , ° < 3

2 +

+

+ M'L ' 3

while Reaction 27b is

(Ε )' Ό

2Λ

2

(27b)

3 +

favored when *E ,i° > ( £ 3 , 2 ° ) ' . A complicating but interesting feature of reactions between the metal (II) polypyridine complexes is that energy transfer processes can occur in competition with electron transfer processes. Energy transfer certainly must be taken into account when M and M ' are identical ( AG ~ 0), while, when M is R u but NT is Os, exergonic energy transfer can occur. O n the other hand, energy transfer from * O s L to R u L ' should be uphill and is expected to be slow. These predictions are borne out by the quenching rate constants for the following reactions (in all of which electron transfer Reactions 27a and 27b are thermodynamically u n favorable). 2

3

*Ru(4/7-(CH ) phen) 3

2

3

2+

+

Os(bpy)

3

k = 1.9 Χ 10 M " q

9

1

sec"

2

3

2 +

2+

> R u (4,7- ( C H ) phen) 3

2 +

3

2 +

+ O s (bpy) #

3

2 +

1


(28)

24

INORGANIC AND ORGANOMETALLIC PHOTOCHEMISTRY

*Ru(bpy)

3

2+

+ Ru(terpy)

2

2+

> R u (bpy)

3

+ * R u (terpy)

2 +

Κ ~l fc = 1.5X 1 0 M " i sec" 9

q

*Os(5-Clphen)

3

2+

+ Ru(terpy)


8

1

sec"

(29)

2+

1

2

2+

> Os (5-Clphen)

Κ ~ 10-5 fc^lX 10 M "

2

3

+ * R u (terpy )

2 +

2

(30)

2 +

1

(It is worth mentioning that F e ( p h e n ) quenching of both * R u ( b p y ) and * O s ( 5 - C l p h e n ) proceeds with k > 2 Χ 10 M sec" . As redox quenching is uphill i n both cases, an energy transfer quenching process yielding * F e ( p h e n ) seems likely. The results suggest that the excitation energy of the acceptor F e ( I I ) state lies below that of both * R u L and *OsL so that * E < 1.8 V for * F e L . Additional evidence for this (or perhaps a different) F e L excited state comes from flash-photolysis experiments i n which transient bleaching of the F e L ground state absorption ( L = bpy, d -bpy, 4 , 4 - ( C H ) b p y , phen, 5-Clphen, 4,7( C H ) p h e n ) was observed. F r o m these experiments the lifetimes of the F e L excited states i n water at 25°C are estimated to lie i n the range 0.1-2 nsec. ) Because energy transfer quenching of * O s L by ruthenium ( I I ) polypyridine complexes is expected to be slow (that is, considerably slower than diffusion controlled, cf Reaction 30), it seems reasonable that quenching of * O s ( 5 - C l p h e n ) by R u ( T P T Z ) for which k is 2.6 Χ 10 M " sec" does occur by oxidation of the excited state as shown in Reaction 31. 3

3

3

2+

2+

3

9

q

1

2+

3

3

2 +

3

3

2 +

2 +

2 +

3

8

3

2 +

1

/

3

2 +

2

2

3

2 +

3

3

9

1

*Os(5-Clphen)

2 +

2+

2

2 +

q

1

3

2+

+ Ru(TPTZ)

2

2 +

> Os(5-Clphen)

3

3+

+ Ru(TPTZ)

2

+

(31)

K~10

Further studies of the above and related systems are being made to confirm these preliminary conclusions, but i n any case it is clear that Reactions 27a or 27b must occur for certain pairs of complexes. W h e n these electron transfer reactions are thermodynamically favored, they should be nearly diffusion controlled because the self-exchange rates for all the ground- and excited-state polypyridine couples are very high. Energy conversion ( and temporary storage ) according to Reaction 27 is highly attractive because the driving force for the excited-state capture is small so that little of the excited-state free energy is lost on conversion to the electron transfer products. W i t h this energy conversion scheme (as with most others involving solution reactions of excited molecules),


1.

AND CREUTZ

suTiN


problems in long term energy storage arise because of "back reactions;" the newly formed products (for example M L and M ' L / ) can undergo very rapid reaction, leading to regeneration of the species present before irradiation and the production of heat. Schemes devised for the defeat of back reactions include the use of semiconductor electrodes, membranes, micelles, and the exploitation of the inverted region (44). Some of these should be applicable to the present problem. (Reactions analogous to Reaction 27 may be responsible for the observations reported by Sprintschnik, Sprintschnik, Kirsch, and Whitten (13). These authors initially reported the formation of hydrogen and oxygen from the photodecomposition of water mediated by monolayers of R u L complexes but were unable to reproduce these observations using purified materials. The presence of R u L ' impurities in the original sample could have given rise to +1 and +3 complexes which might have produced hydrogen and oxygen in subsequent reactions.) The important point is that reactions such as Reaction 27 convert one ground-state and one excited-state mole cule into a powerful oxidant and a powerful reductant, each of which lives longer than the excited state and which can, as a consequence, undergo a larger variety of reactions. In this chapter, we have attempted to describe the luminescent charge-transfer excited state of Ru(bpy) as a novel oxidation state of a ruthenium complex. Its lifetime is so long that it equilibrates with the surrounding solution, and consequently, some of its thermodynamic prop erties (namely its oxidation-reduction potentials) can be calculated. In fact it is even possible to determine its electrochemical properties di rectly. With regard to substitution reactions, *Ru(bpy) is inert like Ru(bpy) and Ru(bpy) . In its electron-transfer reactions, its reac tivity can be related to that of Ru(bpy) and Ru(bpy) . In summary, the physical and chemical properties of the excited state are presently as well characterized as those of many ground state complexes. 3


25

3 +

3

3

3

+

2+

2+

3

2+

r

3

3

2+

3

2+

3+

3

+

3

3+

Acknowledgment This research was carried out at Brookhaven National Laboratory under contract with the U . S. Department of Energy and was supported by its Division of Basic Energy Sciences. Literature Cited 1. Gafney, H. D., Adamson, A. W.,J.Am. Chem. Soc. (1972) 94, 8238. 2. Bock, C. R., Meyer, T. J., Whitten, D. G., J. Am. Chem. Soc. (1974) 96, 4710. 3. Navon, G., Sutin, N., Inorg. Chem. (1974) 13, 2159. 4. Laurence, G. Α., Balzani, V., Inorg. Chem. ( 1974) 13, 2976. 5. Balzani, V., Moggi, L., Manfrin, M. F., Bolletta, F., Laurence, G. S., Coord. Chem. Rev. (1975) 15, 321.



26

INORGANIC A N D O R G A N O M E T A L L I C PHOTOCHEMISTRY

6. Van Houten, J., Watts, R. J., J. Am. Chem. Soc. (1976) 98, 4853. 7. Winterle, J. S., Kliger, D . S., Hammond, G. S., J. Am. Chem. Soc. (1976) 98, 3719. 8. Demas, J. N . , Harris, E. W . , McBride, E. P., J. Am. Chem. Soc. (1977) 99, 3547. 9. Creutz, C., Sutin, N., Proc. Nat. Acad. Sci., U.S.A. (1975) 72, 2858. 10. Young, R. C., Meyer, T . J., Whitten, D . G., J. Am. Chem. Soc. (1975) 97, 4781. 11. Lin, C.-T., Sutin, N., J. Phys. Chem. (1976) 80, 97. 12. Clark, W . D . K., Sutin, N . , J. Am. Chem. Soc. (1977) 99, 4676. 13. Sprintschnik, G . , Sprintschnik, H . W., Kirsch, P. P., Whitten, D . G . , J. Am. Chem. Soc. (1977) 99, 9747. 14. Baxendale, J. H . , Fiti, M . , J. Chem. Soc., Dalton Trans. (1972) 1995. 15. Tokel-Takvoryan, Ν. E., Hemingway, R. E., Bard, A . J., J. Am. Chem. Soc. (1973) 95, 6582. 16. Saji, T., Aoyagui, S., J. Electroanal. Chem. Interfacial Electrochem. (1975) 58, 401. 17. Creutz, C., Sutin, N . , J. Am. Chem. Soc. (1976) 98, 6384. 18. Anderson, C. P., Salmon, D . J., Meyer, T . J., Young, R. J., J. Am. Chem. Soc. (1977) 99, 1980. 19. Lytle, F . E., Hercules, D . M . , J. Am. Chem. Soc. (1969) 91, 253. 20. Demas, J. N . , Crosby, G . Α., J. Am. Chem. Soc. (1971) 93, 2841. 21. Van Houten, J., Watts, R. J., J. Am. Chem. Soc. (1975) 97, 3843. 22. Bensasson, R., Salet, C., Balzani, V . , J. Am. Chem. Soc. (1976) 98, 3722. 23. Mahon, C., Reynolds, W . L . , Inorg. Chem. (1967) 6, 1927. 24. Harrigan, R. W . , Crosby, G. Α., J. Chem. Phys. (1973) 59, 3468. 25. Hipps, K. W . , Crosby, G . Α., J. Am. Chem. Soc. (1975) 97, 7042. 26. Bock, C. R., Meyer, T . J., Whitten, D . G . , J. Am. Chem. Soc. (1975) 97, 2909. 27. Creutz, C., Sutin, N . , Inorg. Chem. (1976) 15, 496. 28. Memming, R., Photochem. Photobiol. (1972) 16, 325. 29. Memming, R., Kursten, G., Ber. Bunsenges. Phys. Chem. (1972) 76, 4. 30. Forster, L . S., in "Concepts of Inorganic Photochemistry," A . W . Adamson, P. D . Fleischauer, Eds., p. 1, Wiley-Interscience, New York, 1975. 31. Hager, G . D., Watts, R. J., Crosby, G . Α., J. Am. Chem. Soc. (1974) 97, 7037. 32. See footnote 47 in L i n , C.-T., Böttcher, W., Chou, M . , Creutz, C., Sutin, N., J. Am. Chem. Soc. (1976) 98, 6536. 33. Saji, T., Aoyagui, S., J. Electroanal. Chem. (1975) 63, 31. 34. Young, R. C., Keene, F . R., Meyer, T . J., J. Am. Chem. Soc. (1977) 99, 2468. 35. Marcus, R. A., J. Chem. Phys. (1965) 43, 679. 36. Marcus, R. Α., J. Chem. Phys. (1965) 43, 2654. 37. Biedermann, G., Silber, Η. B., Acta Chem. Scand. (1973) 27, 3761. 38. Latimer, W . M . , "Oxidation Potentials," Prentice-Hall, Englewood Cliffs, NJ, 1952. 39. Swift, Ε. H . , " A System of Chemical Analysis," p. 542, Prentice-Hall, New York, 1949. 40. Hoselton, Μ. Α., Lin, C.-T., Schwarz, H . , Sutin, N . , J. Am. Chem. Soc. (1978) 100, 2383. 41. Toma, Η. E., Creutz, C., Inorg. Chem. (1977) 16, 545. 42. Creutz, C., Inorg. Chem. (1978) 17, 1036. 43. Maestri, M . , Grätzel, M . , Ber. Bunsenges. Phys. Chem. (1977) 81, 504. 44. Creutz, C., Sutin, N., J. Am. Chem. Soc. (1977) 99, 241. 45. Baker, B. R., Orhanovic, M . , Sutin, N . , J. Am. Chem. Soc. (1967) 89, 722. 46. Meyer, T . J., Taube, H . , Inorg. Chem. (1968) 7, 2369. 47. Brown, G . , Krentzien, H . , Taube, H . , cited by H . Taube in "Bioinorganic Chemistry—II," Adv. Chem. Ser. (1977) 162, 127.



1.

SUTIN AND CREUTZ


27

48. Gordon, B. M . , Williams, L . L., Sutin, N . , J. Am. Chem. Soc. (1963) 83, 2061. 49. Ford-Smith, M . H . , Sutin, N . ,J.Am. Chem. Soc. (1961) 83, 1830. 50. Brunschwig, B., unpublished data. 51. Przystas, T . J., Sutin, N.,J.Am. Chem. Soc. (1973) 95, 5545. 52. Ballardini, R . , Varani, G . , Scandola, F . , Balzani, V., J. Am. Chem. Soc. (1976) 98, 7432. 53. Chou, M . , Creutz, C., Sutin, N . ,J.Am. Chem. Soc. (1977) 99, 5615. 54. Candlin, J. P., Halpern, J., Trimm, D . L . , J. Am. Chem. Soc. (1964) 86, 1019. 55. Faraggi, M . , Feder, Α., Inorg. Chem. (1973) 12, 236. 56. Carlyle, D. W., Espenson, J. H . ,J.Am. Chem. Soc. (1968) 90, 2272. 57. Brown, G. M . , Clark, W . D . K., unpublished data. 58. Zwickel, Α., Taube, H . ,J.Am. Chem. Soc. (1961) 83, 793. 59. Doyle, J., Sykes, A. G.,J.Chem. Soc., A (1968) 2836. 60. Nordmeyer, F . , Taube, H . ,J.Am. Chem. Soc. (1968) 90, 1163. 61. Dockal, E . R . , Gould, E . S.,J.Am. Chem. Soc. (1972) 94, 6673. 62. Endicott, J. F . , Taube, H . ,J.Am. Chem. Soc. (1964) 86, 1686. 63. Faraggi, M . , Feder, Α., Inorg. Chem. (1973) 12, 236. 64. Gaunder, R . G., Taube, H . , Inorg. Chem. (1970) 9, 2627. R E C E I V E D September 20, 1977.


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