Volume 71. number 2 CHEhIICAL PHYSICS LETTERS 15 Apnll980

ELECTRONIC ABSORPTION AND EMISSION SPECTRA

OF MlXED-LICAND RU+HENIUM(II) COMPLEXES

[Ru(2,2’-BIPYRIDINE)n (2-(2-PYRIDYL)QUINOLINE}3_,] *+(n = 0, 1,2 OR 3)

Susan ANDERSON, Kenneth R. SEDDON. Raymond D. WRIGHT Oxford-lmpenol Eize!gy Croup. brorgarrrc Chermstr) Laboratory, Urmersrty of Oxford. Oxford OS1 3QR UK

and

Alan T. COCKS

0zet111st~ Du WOII. Celrtral Electrrcrt-! Research Laboratones, Leatherhcad Surrey KT22 7SE, UK

Received 29 January 1980

Room-temperature electromc absorption spectra, and enusslon spectra at room temperature and 77 K, have been mea- sured for the mIxed hgand 7,3’-blpyndme--2~2-pyrldyl)qluno~e complexes of ruthemum(n). The charge-transfer states mltmlly populated by absorption of visible hght are essentially locahsed on mdlndual hgands. and there is httle coupling of these hgand states. Emissions, howerer, occur from a smgle state possessmg the characteristics of the pyndylqumobne hgand alone.

1. Introduction

The photochemistry and Qhotophyncs of the t&.(2,2’-blQyndme)ruthenium(II) cation, [Ro(blQy)3] ?+ (blQy = 2,2’-bipyridme), are currently the foci of much interest [I] _ Ths ongmated with its unique photo-Induced electron-transfer reactions [2], which have led to [Ru(bipy)s] *+ being investigated as a homogeneous photocatalyst for the Qhotodecom- position of water [3,4]. in photogalvanic Cells [s] , and in photoelectrochemical cells for the production of dlhydrogen [6].

However, systematic photophysical studies of ordy one senes of nuxed-hgand ruthemum complexes, the 2,2’-bipyridme-l,lO-phenanthroline [Ru(blpy),(phen)3_,] 2+ system (n = 0, 1,2 or 3), have been made [7,8]. In this system, it was suggested that the couplmg between dissimilar ligands was strong, resulting m single charge-transfer electronic states manifested by single-peak charge-transfer absorp- tion and emlsslon bands. Luminescence decay curve

data provided no evidence for multiple emission at 77 K, and the observed spectral properties of the mlxed- hgand complexes represented an average of the Qroper- ties of [Ru(biQy)3] 2+ and [Ru(phen)3] *+_ We pre- sent here a summary of our fmdmgs upon the novel series [Ru(biQy),r(Qq)3 _,] *+ [Qq = 242-Qyridyl)qumo- hne; PZ = 0, 1,2 or 31 prior to more detailed Qubhca- tlon, and contrast these data wth those for the [Ru(blpy),r(Qhen)3 _n] 2+ system.

2. Experimental

The complexes [Ru(biQy)3] Cl,-xH,O [9], and [Ru(Q&] Cl,-xH,O [lo] have been reported Qre- vlously. For the present work they were prepared b- reaction between a blue methanohc ruthemum(l1) chloride solution [ 1 l] and the free &and, by a tech nique described by us elsewhere [ 12]_ [Ru(blQy)2(Qq)] Cl,-xH20 was prepared by heating an ethanolic solution of cis-[Ru(biQy)2C12] with a

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Volume 7 1, number 2 CHEMICAL PHYSICS LETI’ERS 15 Aprir 1980

shght molar excess of 242-pyndyl)qumoline for 4 h, and [Ru(blpy)(pq)2] Cl*-xH20 was prepared by the reaction between [Ru(b;py)Cl,] with a slight stoichio- metric excess of 2(2-pyridyl)quinohne in 1: 1 ethanol -water at reflux for 24 h. Perchlorace salts were pre- pared by metathesis. All synthetic work was per- formed under dinrtrogen and the products were puri- fied by gel filtration on a column of Sephadex LH-20 m methanol followed by recrystalhsation from ethanol -water.

Room temperature electronic absorption spectra were recorded m ethanolic solutron on a Perkin- Elmer 340 mstrument or in a 4 : 1 (V/V) methanol- ethanol mixture on a Pye-Umcam SP 170.5 spectro- photometer. Emission spectra were obtamed usmg an Apphed Photophysics single photon countmg spec- trometer. They were corrected for emission monochro- mator-photomultrpher response by a correction curve obtained by use of a standard tungsten filament lamp. Excitation spectra and lamp mstability effects m emis- sion spectra were automatically corrected by the use of a rhodamine B quantum counter system. Room temperature emtssion spectra were measured in ethane- nitnle solution (z-10 -4 M) wrth excitation wave- lengths in the range 450-480 nm. Low temperature (77 K) emission and excitation spectra were measured m 4 : 1 (V/V) methanol-ethanol glasses usmg an Oxford Instruments DN 704 hquid nitrogen cooled cryostat_

Solutions of [Ru(pq)j] 2+ were found to be light sensitrve, but there was no evrdence for decompositron dunng the measurements described below.

3. Results and discussion

The electronic absorption spectra of the series

[Ru(bipy),(pa)3_,] 2+ are presented :br fig. I_ The mixed-&and complexes clearly show two independent charge-transfer transitions, the intensities of which ap- proximate to a werghted superimposition of the spec- tra of the terminal members of the series. Gel filtra- tron studies upon the solutions revealed that there is only one ruthenium-contaming species present, thus eliminatmg the possrbihty that the followmg drspro- portronation may have occurred

a

w Y S 0” I c

(Jy 600 550 500 450 400 350

Fig. I. Absorption spectra: (a) [Ru@ipy)&q)l 2*, (3) [Ru@ipy)(pq)21*+, (cl IRu(~)sl*+. W IRu(bipY)sl*C.

+ n FWbit-w)3 I 2+ I- (3-n) [Ran ] 2t.

The spectra are in sharp contrast to the results ob- tained previously for [Ru(bipy),(phen)3_,]2t ]7,8], in which there is no clear resolution into two bands, in which the general shape of the absorption envelope is the same for each member of the series, and in which the positions of the absorption maxima for the inner members of the series is spaced fairly regularly between the terminal members.

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Volume 7 1, number 2 CHEhIlCAL PHYSICS LEIl-ERS 15 April 1980

Frg 2. Room-temperature emlsslon spectra (a) [Ru(bipy)3] ‘*, cb) [RuCbipy)z(pq)l 2V, Cc) [Ru(br~~)Wl)zl 2+. (d) [Ru@s)312+.

The corrected room-temperature emission spectra of the complexes [Ru(blpy),(pq)3 _,I 2+ in room- temperature solution are shown m fig. 2 and lt IS clear that the spectra of [Ru(blpy)a(pq)] z+, [Ru(blpy)(pq)z] 2+ and [Rug] 2+ are essentially similar, and distinct from that of [Ru(bipy)3] *+_ The same effect was observed for rigid 77 K glasses Emis- son spectra are independent of excltatlon wavelength in the range 450-480 nm and the luminescence exclta- tion profiles are very similar to the absorption spectra of the complexes (fig. 3).

It would appear, therefore, that m the mixed

400 450 500

x nm

b

400 450 500 550 x/m

Fig. 3. 77 K ekatatlon spectra- (a) [Ru(bipy)2(pq)] **, (b) [Ru(blpy) (pq)z I 2+-

bipy-pq complexes of ruthenium(H), the charge- transfer states populated by the initial absorption of radiation are essentially locahsed on the individual hgands. However, the similaty of the emission spec- tra of all the complexes containing pq suggests that little mixing of the hgand emitting states occurs and that both the initial excited states of pq and blpy rap- idly internally convert to the emitting state of pq alone.

The precise nature of the states involved in the ab- sorption and emission of visible radiation in the parent ion [Ru(bipy)3] 2+ has not yet been resolved [ 131, and discussion of the states and mode of electron trans- fer in the mlxed hgand complexes will be deferred to a

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more detailed pubhcation. It is of interest to note that 2 prehmmary study of the mixed phenanthroline- pyridylquinobne complexes of ruthenium(II) in our laboratories [ 141 indicates that both absorption to and

emission from the individual ligand states occurs. It would appear, therefore, that substituting bipyridine by larger, more sterically ngid but electronically simi- lar groups brings about increasing decoupling of both absorption and emission states. Further investigatrons of the photophysical properties of the complexes of ruthemum(II) to explore the nature of state couplmg are currently being undertaken.

Acknowledgement

KRS and SA thank the Central Electricity Generat- mg Board for financial support and for the awards of a research fellowship at St. Catherine’s College (KRS) and a research studentship (SA). RDW thanks the SRC. for a C.A.S.E. studentship. We also thank Drs. P. Day and M-J. Pdhng for useful discussions.

References

[ I] K-R. Seddon and E.A. Seddon, The chew- oP ruthenium (Elsewer. Amsterdam. 1980). and references therein.

[2] N. Sutin, J. Photochem. IO (1979) 19. and references therein.

[3] J.-M. Lehn and J.-P. Sawage. Now. J. Chim. 1 (1977) 449.

141 J.-M. Lehn. J.-P. Sauvage and R. Ziessel, Now. J. Chiz. 3 (1979) 423.

[S] W.J. Albery and A.W. Fauhls. J. Photochem 10 (1979)

[6] ~burham, W.J. Dressick and TJ. Meyer. I. Chea Sot. Chem. Commun. (1979) 381.

[7] B. Bosmch, lnorg. Chem. 7 (1968) 2379. 181 C.A. Crosby and W.H. Elfring Jr., J. Pbys. Chem. 80

(1976) 2206. [91 F.H. &staU. J. Chem. Sot. (1936) 173.

[lo] D-M. Klassen, Inorg. Chem. 15 (1976) 3166. [l l] D. Rose and C. Wilkkson, J. Chem. Sot. A (1970) 1791. [ 121 S. Anderson azd K.R. Seddon, J. Chem. Res. S ($979)

74. [13] F. Febx, J. Ferguson, H.U. Giidel and A. Ludi. (hem.

Phys Letters 62 (1979) 153. [ 141 S. Anderson, I. Nicholas, K-R. Seddon and A.T. Cocks,

unpubbshed results.

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