- Email: [email protected]
Luminescence of mixed-ligand Ru(II) chelates is there any bona fide case of dual emission?
Volume 104. riumber 1
CHEMICAL IWYSICS I.ETTERS
27 January 1984
LUMINESCENCE OF MIXED-LIGAND Ru(Ii) CHELATES. IS THERE ANY BONA FZDE CASE OF DUAL EMISSION? PeteCBELSER, Alex VON ZEUWSKY Institute forlnorghdc cliemktiy, Umber&y of Biboug,
CH-1700 I+i~owg, Switzerland
and Albert0 JURIS, Francesco BARIGELLETTI
and Viucenzo BALZANI
Istituto Cl&n.& ‘G_ CIamician”dell?!hiversit~ and Istituto FMfCMR.
Bologna. Italy
Received I 1 October 1983
The luminescence behavior of Ru(bpy)&) 2*, Ru@w)&iq)*+, Ru@p~)AN0~-bp~)*+, Ru@henfzGx#+, Rufpben);?Cbk#*, Ru(pben)2(DM~)2*, RuCbpy)2(i-biq)2’ and Ru(bpy)(&biq) $’ has been carefully studied. In no case hasbonafideduaIemissionbeenobserved.Thereasonsforthe previouslyreporteddualemissions of R~@py),(NO~-bpy)~+ and Ru@he~&(pq)*+ are discussed.
1. Introduction The photophysical properties of transition-metal complexes [l] are the subject of intense study, particularly in view of the use of these compounds as photosensitizers in cycles for the photochemlcal conversion of solar energy f2$ J. The attention of several research groups [4-201 has been focused recently on complexes of the Ru(diimine)~ family in an attempt to fmd better hotosensitizers than the widely used -54. {Zl] Ru(bpy)3 (bpy = 2,2”-bipyridine).
It has generally been found that, in the mixedIigand Ru(L),(L’)~, complexes (where L and L’ are distinct diimine-type ligands and n is 1 or 2), the absorpti6n spectra show distinct Ru * L and Ru + I.,’ charge-transfer (CT) bands, quite similar to the bands of the parent Ru(L)p and Ru(L’)2;’ complexes, whereas emission only occurs from the lowestenergy
CT excited state [i.e. from the excited state which involves the ligand which is easier to reduce)_ For the Ru(phen),@q)$f, complexes (phen = l,lO-phenanthroline, pq = 2(2-pyridilquinohne), however, luminescence emission from both Ru -+ phen and Ru -+ pq excited states has beeri reported by Cocks et al- [7], an a dual emission was also observed in-our labora100
foxy [I21 from Ru(bpy)z(N0,-bpy)2i trix at 77 K.
in a rigid ma-
1Multiple-state emission is well known to occur for
some Rh(III), Ir(Ii1) and Cr~III)-complexes El] and new exampies concerning complexes of other transition metals have recently appeared [22--241. Since the occurrence of multiplestate emission in the mixed-ligand Ru(I1) chelates would be interesting
from a theoretical point of view [ 1,151 and could also have important implications as to the use of these complexes as photosensitizers, we have re-examined the luminescence behavior of Ru~hen)~~~2~ and Ru(bpy)2(N02-Bpy)2’_ We have also studied the photophysical properties of other mixed-ligand complexes which could be expected to exhibit dual emission. The complexes studied are listed in table f and the structural formulae of the various Iigands are shown in fig. 1.
2. Experimental The complexes listed in table 1 were prepared following described procedures 12253 _ The-products were
purified by recryst@lizatiop and/or chromatography 0.009-2614184/S 03.000 Efsevier Science Publishers R.V. ~o~h-Ho~a~d Physics Publishing Division) _
CHEMICAL
Volume 104, number 1 Table 1 Observed emission maxima and luminesumce CornpIes
PHYSICS LETTERS
decay timesa)
TIGS paper b)
Literature
Ref.
?
hmin
T
(nm)
(IN
(mu)
(M)
6.54
2.6
Ru(bpy)2(biq)*+
728
1.4
Ru @PY)Z (NO2 -bpy)2+ Ru (Phen)z @q)*’
585 =). 6.55 653
4.2ef_ 3.1 3.1
711 =560f). 710 590 540 f), 588
1.8 2-O 4.7 117f)*5.0
-
R@PY)$+ Ru @PY)$+ Ru (pq)$+ Ru
data
AKMX
Ru(bpy)2(pq)*+
Ru (phen)2 (b;q)2+ Ru(phen)2{D~fCH)2~ RU@PY)Z Ci-biq)2* Ru (bpy) (i-biq)s*
27 kfnua.ry 1984
(biq) $+ I -8
Ru(DMCH);+ -
RulNt& -bpy)y
35
667 4720 736 728 585,655 664 x560, 729
1.9 1.4 4-7v3.8 3.8 9.7.3.1 2.1
=650
582
5.2
8)
566
9.8
b)
6.58
3.8
14‘)
718
2.6
12”)
732
-
643
3.8 96
540
Ru (i&q);+
14c) 4d) 14=) itb) Izb) 14 c) 7 c) 1JC)
5 I2b) 13b)
a) Rigid matrix at 77 K unless otherwise noted. b) EtOHjhleOH 1 : 4 (v/v). Uncorrected emission spectra. C) EtOH/hleOH 4 : 1 (v/v)_ Corrected emission spectra. d) EtOH/hfeOH I : 4 (Y~v), room temperature, corrected emission spectra. e, Due to a decomposiotin produet, see text, f, Due to the presence of minor amounts of a parent tris-chelate complex, see text. g) EtOH/MeOH 4 I 1 (v/v), corrected emission spectra 1261.
bm
biq
N02-bpy
i-biq
Pq
DMCH
Fig. 1. Structural formulae and abbreviations of the ligands.
101
Volume 104, number 1
and yielded satisfactory chemical analysis. The emission and excitation spectra were obtained with a Perkin-Elmer 650-40 spectrofluorimeter equipped with a R928 tube. Emission lifetime measurements were carried out with a JK system 2000 neodymiumYAG DLPY 4 laser.
3 _ Results and discussion
The luminescence properties of the mixed-&and complexes studied in this paper have been summarized in table 1, where the previously available literature data as well as data concerning the parent tris-chelates are also reported for comparison. For Ru(bpy)z(pq)2’ and Ru(bpy)2(biq)z’, only one emission was observed, in agreement with the reports of other authors [4,14]. Comparison with the luminescence properties of the parent trischelate complexes clearly shows that emission originates from the lowest metal-to-ligaud charge-transfer (MLCT) excited state (Ru + pq and Ru + biq, respectively). For Ru@py)2(N02-bpy)2+, the two emission bands at 77 K reported previously [12] were again observed with a new sample of the complex. Careful examination of the system, however, showed that the relative intensities of the two bands vary with time. When the solution was cooled to 77 K immediately after dissolution of the complex, the band at 585 nm was small compared with the band at 655 nm. When the solution was maintained for a few minutes at room temperature before cooling, the band at 585 nm was much more intense and the band at 655 nm was much weaker. Such a behavior was also observed when the manipulation of the sample was done in a dark room. These results show that Rt~(bpy)~(NO~bpy)2+ undergoes a rapid thermal reaction which leads to a product emitting at 585 nm. The similarity between the luminescence properties of this product and those of Ru(bpy)p suggests that decomposition involves the loss of the nitro group. In conclusion, the only true emission bf Ru(bpy)2(N02-bpy)2+ is that at 655 run. Comparison with the luminescence properties of the parent tris-chelate complexes clearly indicates that such an emission originates from the lowest excited state, which is Ru 4 (No, -bpy) CT in character_ 102
27 January 1984
CHEMICAL PHYSICS LETTERS
For Ru(phen)2(pq)2’, we have observed only one emission at 653 MI, which can be clearly assigned toa Ru + pq CT excited state. This result is in agreement with the report of Klassen [ 141 but in disagreement with that of Cocks et al. 273 who found two almost equally intense emission bands at ~560 and -650 nm, which were assigned as CT transitions involving Ru + phen and Ru 4 pq excited states. In agreement with Cocks et al. 171, we found that Ru(phen)2(pq)2’ solutions are stable both in the dark and in ambient light. Thus, decomposition can be ruled out as a cause of the observed double emission_ It should be noted, however, that the results obtained by Cocks et al. [7] are also consistent with the occurrence of only one emission from Ru@hen)2@q)2’ and the presence of Ru(phen)F as an emitting impurity. Even the excitation spectra reported by those authors can be interpreted equally well either by a dual emission from Ru@hen)2@q)2’ or by two separate emissions from Ru(phen),(pq)2+and Ru(phen@. To understand whether the presence of Ru(phen): as an impurity can be a plausible explanation for the observed dual emission, we have recorded the ernission spectrum at 77 K of a solution containing 4 X 10”
M Rul$hen)2pq)2* and 4X 10s7 M Ru(phen)y_ As one can see (fig. 2), this spectrum clearly shows two bands at ~560 and ~650 nm and it is quite simi-
s
5
s h ii
k.m ;;I 2
500
600
700
a.nm
830
Fig- 2. Emission spectra at 77 K of (a) Ru(phen)2(pq)2*(3_7 X 10” M); @) Ru(phen)2(pq)**+ 1 mol% Ru@he& [same sasitivity as (a)]; (c) Ru@hen)$+, 3.6 x 10”
(spectralsensitivityreduced by 20 times).
M
Volume 104. number 1
27 January 1984
CHEMICAL PHYSICS LElTERS
iar to that reported by Cocks et al. [7] for their Ru(phen)2@q)2’ sample. Since 1% of Ru(phen)F impurity in a sample of Ru(phen)z@q)z+ can hardly be detected by the usual analytical techniques (chemical analysis, column chromatography and NMR spectroscopy [7]), we believe that the 560 MI emission found by Cocks et al. [7] may be due to the presence of some Ru@hen)y and that the only true emission of Ru@hen)2@q)2+ is that at ~650 nm, which involves the lowest-energy Ru + pq CT excited state. The presence of only one emission (from Ru + biq CT excited state) in the quite similar complex substantiates the above Ru@hen)2(biq)2’ conclusion_ Since Anderson et al. [4] suggested that decoupling between excited states localized on different ligands (and thus, dual emission) could be favored by steric rigidity of the ligands, we have also examined the emission spectrum of the Ru(phen), (DMCH)*+ complex, where the flexible pq or biq ligands are replaced by the more rigid DMCH ligand (fig- 1). A very intense emission band at 7 10 nm (at 77 K; 728 nm at room temperature) was observed, which can easily be attributed to the lowest Ru + DMCH CT excited state_ A very weak emission was also observed at ~560 nm, which could originate either from the Ru + phen CT excited state of Ru(phen)2(DMCH)2+ or from some Rubhen)? present as impurity. Addition of 1 mole per cent of Ru@hen)p to our Ru(phen)2(DMCH)2+ caused an increase by a factor of 5 of the weak 560 nm emission. This means that the 560 MI emission of our Ru(phen)2(DMCH)2+ by =0_2 mol% of Ru@“h~$ci~p~~~~~~~ z detectable by analytical techniques_ On the basis of the spectroscopic properties of the parent Ru(bpy)F [21] and Ru(i-biq)? [13] complexes, it can be expected that, in the mixed-ligand Ru(bpy)2(i-biq)2’ and Ru(bpy)(i-biq)y complexes, the lowest excited state is Ru + bpy CT in character and that the lowest excited state involving the i-biq ligand is not a Ru * (i-biq) CT excited state, but a ligandcentered (LC) excited state [ 13,191. In the previously discussed Ru@hen)2(pq)2’ complex, the deactivation of the Ru + phen CT excited state to the Ru + pq CT excited state involves only the transfer of an electron from n*(phen) to a*(pq) orbitals. By contrast, in the Ru(bpy),(i-biq& complexes, the internal conversion of the i-biq LC excited state to
the Ru * bpy CT excited state involves not only the transfer of an electron from n*(i-biq) to n*(bpy) orbitals, but also the transfer of an electron from si(Ru) to n(i-biq). The coupling between the two excited states is thus expected to be less favorable in Ru(bpy),(i-biq):f, than in Ru@hen)2@q)Z+ and dual emission should be more likely for the (bpy)/ (i-biq) mixed-l&and
complexes.
Careful examination
of the emission spectra showed that, while Ru(bpy)z(i-biq)?+ exhibits only emission from the lowest Ru + bpy CT excited state (590 nm), Ru(bpy) (i-biq)j+ also shows another weaker band at 540 nm, where the i-biq localized emission is expected to occur. In exactly the same region, however, emission from the parent Ru(i-biq)$+ comples, present as an impurity, would also be observed_ One can also note that the observed emission lifetime at 540 nm is =Z100 w, i.e. almost the same as that of Ru(i-biq)f+.
From the spectra of solutions containing (Gbiq)_f’ and Ru(i-biq)?,
variable amounts of Ru(bpy)
we were able to estimate that the emission at 540 nm from our “pure” Ru(bpy)(i-biq):+ sample could be accounted for by assuming that i_5% of Ru(i-biq): impurity was present_ Since, as usual, the parent trischelate complexes are obvious impurities in the samples of the mixed-ligand complexes, and since such a low level of impurity is beyond the sensitivity limit of the analytical techniques used to ascertain the purity of the samples, we must conclude that even in this case there is no reason to assume that a dual emission takes place.
4. Conclusions In no case has a bona fide dual emission been observed for the miued-ligand Ru(II) chelates examined in this paper. For the complexes which exhibit two emission bands, the highenergy emission band can be accounted for by a decomposition reaction or by the presence of very small amounts (of the order of 1%) of a parent tris-chelate complex as an impurity. As recently discussed by Baggott et al. [ 151. the absence of dual emission means that the coupling between dissimilar ligands is strong enough to produce kinetic effects, i.e. to cause relaxation rates which are much faster than the radiative decay of the upper excited states. Thus, the only excited state which is suf-
103
Volume 104, number 1
ficiently long-lived tb give rise to luminesckce emission and to-be involved in bimolecular processes is the
Iowest excited state of the complexAny attempt to obtain mixed-&and lL
[IO] R-R. Ruminski and J.D. Peterseri,Inog. (1982) 3706.
Tu~~~dV.Ea~,~hern.
1161 [17]
References
[19]
36 (1981) 325. [2] N. GrBzel. ed, Energy resources by photochemistry and catalysis (Academic Press, New York, 1983). 131 J. Rabani. ed., Photochemical conversion and storage of solar energy (Weizmann Science Press, Jerusalem, 1982). (41 S. Anderson, K.R. Seddon, R-D-Wright and A.T.Cocks,
[ 381
1201 [21] f22] [23]
Chem. Phys. lxM.ers 71 (1980) 220. 15 ] A. Juris, V. Baizani, P. Belser and A. von Zelewsky, Helv.CMn. Acta 64 (198X) 2175. [6] SF. Agnew, H.L. Stone and G.A. Crosby, Chem. Phys. Letters 85 (1982) 57. [7] A.T. Cocks, R. Wright and K.R. Seddon, Chem. Phys. Letters 85 (1982) 369. IS J A. Basu, M.A. Weiner, T-C. Streaks and H-D_ Gafney,
104
J. Phys. Chem. 86
Chem. 21
Phys. Letters 89 (1982)
101.
[ 141 D.M. Klassen. Chem. Phys. Letters 93 (1982) 383. [IS] I.E. Baggott, GX. Gregory, MJ. Filling, S. Anderson,
Financial support from the Swiss National Science Foundation and from the Minister0 della Pubblica Istruzione is gratefully acknowledged_
[l ] M.K. DeArmond and CM. CarEn, Coord. Chem. Rev.
1984
f 1 j ] D-S_ RiIIema an+ K&t Mack, lnorg. Chem. 21 (1982) 3849. [ 121 A. Juris; F. Barigellctti, V- Balzani, P. Belser and li von Zelewsky, Israel J. Chem. 22 (I 982) 87_ (131 P. Belser, A. YOIIZelewsky. A. Juris, F. Bari&lctti, A.
Adcnowledgement
Inorg. Chem. 21 (1982) 1085. [9] D.P. Segers and M.K. De+rmond, (1982) 3768.
27 Jan-
CHEhflCAL PHYSICS LE-fTERs
j24f
1251 1261
K-R. Seddon and J E. Turp, J. Chem. Sot. Faraday Trans. II79 (1983) 195. RJ. CrutchIey,M. Kress and A.B.P. Lever, J. Am. Chem. Sot. 105 (1983) 1170. PJ. Steel, F. Lahonsse, D. Lerner and C. Mar&t, Inorg. Chem.22 (1983) 1488. L J. Fritzpatrick, ll.A. Goodwin, A. Launikonis, A.W_-H. Mau and W.H.F. !&se. Chem. Phys. Jxtters, to be published. P. Be&r. A. von Zelewsky. A. Juris, F. Barigelletti and V_ BaImni, Gazz. Chim. ItaI, to be published. F_ BarigeIIetti. A. Juris, V- BaIzani, P. BeIser and A. von Zelewsky, Inorg. Chem., to be published. K. Kalyanasundaram, Coord. Chem. Rev.46 (1982) 159. D.M. Manuta and A.J. Lees, Inorg. Chem. 22 (1983) 572. hl. h&tin, M.B.Krogh-Jespexsen, hl.Hsu, J.Tewksbury, M. Laurent, K. Viswanath and H. Patterson, Inorg. Cbem. 22 (1983) 647_ R.A.Rader,D.R.h~cMiiIin,M.T.Buckner,T.G.Matthews, DJ.Casadonte, RX. Lengel, S.B.Whittaker, L.M. Darmon and FE. Lytle, J. Am. Chem. Sot. IO3 (1981) 5906. P_ BeIser and A. von Zelewsky, Helv. Chim. Acta 63 (1980) 1675. J_N. Edemas and G.A. Crosby, J. Am. Chem. Sot. 93 (1971) 2841.
CHEMICAL IWYSICS I.ETTERS
27 January 1984
LUMINESCENCE OF MIXED-LIGAND Ru(Ii) CHELATES. IS THERE ANY BONA FZDE CASE OF DUAL EMISSION? PeteCBELSER, Alex VON ZEUWSKY Institute forlnorghdc cliemktiy, Umber&y of Biboug,
CH-1700 I+i~owg, Switzerland
and Albert0 JURIS, Francesco BARIGELLETTI
and Viucenzo BALZANI
Istituto Cl&n.& ‘G_ CIamician”dell?!hiversit~ and Istituto FMfCMR.
Bologna. Italy
Received I 1 October 1983
The luminescence behavior of Ru(bpy)&) 2*, Ru@w)&iq)*+, Ru@p~)AN0~-bp~)*+, Ru@henfzGx#+, Rufpben);?Cbk#*, Ru(pben)2(DM~)2*, RuCbpy)2(i-biq)2’ and Ru(bpy)(&biq) $’ has been carefully studied. In no case hasbonafideduaIemissionbeenobserved.Thereasonsforthe previouslyreporteddualemissions of R~@py),(NO~-bpy)~+ and Ru@he~&(pq)*+ are discussed.
1. Introduction The photophysical properties of transition-metal complexes [l] are the subject of intense study, particularly in view of the use of these compounds as photosensitizers in cycles for the photochemlcal conversion of solar energy f2$ J. The attention of several research groups [4-201 has been focused recently on complexes of the Ru(diimine)~ family in an attempt to fmd better hotosensitizers than the widely used -54. {Zl] Ru(bpy)3 (bpy = 2,2”-bipyridine).
It has generally been found that, in the mixedIigand Ru(L),(L’)~, complexes (where L and L’ are distinct diimine-type ligands and n is 1 or 2), the absorpti6n spectra show distinct Ru * L and Ru + I.,’ charge-transfer (CT) bands, quite similar to the bands of the parent Ru(L)p and Ru(L’)2;’ complexes, whereas emission only occurs from the lowestenergy
CT excited state [i.e. from the excited state which involves the ligand which is easier to reduce)_ For the Ru(phen),@q)$f, complexes (phen = l,lO-phenanthroline, pq = 2(2-pyridilquinohne), however, luminescence emission from both Ru -+ phen and Ru -+ pq excited states has beeri reported by Cocks et al- [7], an a dual emission was also observed in-our labora100
foxy [I21 from Ru(bpy)z(N0,-bpy)2i trix at 77 K.
in a rigid ma-
1Multiple-state emission is well known to occur for
some Rh(III), Ir(Ii1) and Cr~III)-complexes El] and new exampies concerning complexes of other transition metals have recently appeared [22--241. Since the occurrence of multiplestate emission in the mixed-ligand Ru(I1) chelates would be interesting
from a theoretical point of view [ 1,151 and could also have important implications as to the use of these complexes as photosensitizers, we have re-examined the luminescence behavior of Ru~hen)~~~2~ and Ru(bpy)2(N02-Bpy)2’_ We have also studied the photophysical properties of other mixed-ligand complexes which could be expected to exhibit dual emission. The complexes studied are listed in table f and the structural formulae of the various Iigands are shown in fig. 1.
2. Experimental The complexes listed in table 1 were prepared following described procedures 12253 _ The-products were
purified by recryst@lizatiop and/or chromatography 0.009-2614184/S 03.000 Efsevier Science Publishers R.V. ~o~h-Ho~a~d Physics Publishing Division) _
CHEMICAL
Volume 104, number 1 Table 1 Observed emission maxima and luminesumce CornpIes
PHYSICS LETTERS
decay timesa)
TIGS paper b)
Literature
Ref.
?
hmin
T
(nm)
(IN
(mu)
(M)
6.54
2.6
Ru(bpy)2(biq)*+
728
1.4
Ru @PY)Z (NO2 -bpy)2+ Ru (Phen)z @q)*’
585 =). 6.55 653
4.2ef_ 3.1 3.1
711 =560f). 710 590 540 f), 588
1.8 2-O 4.7 117f)*5.0
-
R@PY)$+ Ru @PY)$+ Ru (pq)$+ Ru
data
AKMX
Ru(bpy)2(pq)*+
Ru (phen)2 (b;q)2+ Ru(phen)2{D~fCH)2~ RU@PY)Z Ci-biq)2* Ru (bpy) (i-biq)s*
27 kfnua.ry 1984
(biq) $+ I -8
Ru(DMCH);+ -
RulNt& -bpy)y
35
667 4720 736 728 585,655 664 x560, 729
1.9 1.4 4-7v3.8 3.8 9.7.3.1 2.1
=650
582
5.2
8)
566
9.8
b)
6.58
3.8
14‘)
718
2.6
12”)
732
-
643
3.8 96
540
Ru (i&q);+
14c) 4d) 14=) itb) Izb) 14 c) 7 c) 1JC)
5 I2b) 13b)
a) Rigid matrix at 77 K unless otherwise noted. b) EtOHjhleOH 1 : 4 (v/v). Uncorrected emission spectra. C) EtOH/hleOH 4 : 1 (v/v)_ Corrected emission spectra. d) EtOH/hfeOH I : 4 (Y~v), room temperature, corrected emission spectra. e, Due to a decomposiotin produet, see text, f, Due to the presence of minor amounts of a parent tris-chelate complex, see text. g) EtOH/MeOH 4 I 1 (v/v), corrected emission spectra 1261.
bm
biq
N02-bpy
i-biq
Pq
DMCH
Fig. 1. Structural formulae and abbreviations of the ligands.
101
Volume 104, number 1
and yielded satisfactory chemical analysis. The emission and excitation spectra were obtained with a Perkin-Elmer 650-40 spectrofluorimeter equipped with a R928 tube. Emission lifetime measurements were carried out with a JK system 2000 neodymiumYAG DLPY 4 laser.
3 _ Results and discussion
The luminescence properties of the mixed-&and complexes studied in this paper have been summarized in table 1, where the previously available literature data as well as data concerning the parent tris-chelates are also reported for comparison. For Ru(bpy)z(pq)2’ and Ru(bpy)2(biq)z’, only one emission was observed, in agreement with the reports of other authors [4,14]. Comparison with the luminescence properties of the parent trischelate complexes clearly shows that emission originates from the lowest metal-to-ligaud charge-transfer (MLCT) excited state (Ru + pq and Ru + biq, respectively). For Ru@py)2(N02-bpy)2+, the two emission bands at 77 K reported previously [12] were again observed with a new sample of the complex. Careful examination of the system, however, showed that the relative intensities of the two bands vary with time. When the solution was cooled to 77 K immediately after dissolution of the complex, the band at 585 nm was small compared with the band at 655 nm. When the solution was maintained for a few minutes at room temperature before cooling, the band at 585 nm was much more intense and the band at 655 nm was much weaker. Such a behavior was also observed when the manipulation of the sample was done in a dark room. These results show that Rt~(bpy)~(NO~bpy)2+ undergoes a rapid thermal reaction which leads to a product emitting at 585 nm. The similarity between the luminescence properties of this product and those of Ru(bpy)p suggests that decomposition involves the loss of the nitro group. In conclusion, the only true emission bf Ru(bpy)2(N02-bpy)2+ is that at 655 run. Comparison with the luminescence properties of the parent tris-chelate complexes clearly indicates that such an emission originates from the lowest excited state, which is Ru 4 (No, -bpy) CT in character_ 102
27 January 1984
CHEMICAL PHYSICS LETTERS
For Ru(phen)2(pq)2’, we have observed only one emission at 653 MI, which can be clearly assigned toa Ru + pq CT excited state. This result is in agreement with the report of Klassen [ 141 but in disagreement with that of Cocks et al. 273 who found two almost equally intense emission bands at ~560 and -650 nm, which were assigned as CT transitions involving Ru + phen and Ru 4 pq excited states. In agreement with Cocks et al. 171, we found that Ru(phen)2(pq)2’ solutions are stable both in the dark and in ambient light. Thus, decomposition can be ruled out as a cause of the observed double emission_ It should be noted, however, that the results obtained by Cocks et al. [7] are also consistent with the occurrence of only one emission from Ru@hen)2@q)2’ and the presence of Ru(phen)F as an emitting impurity. Even the excitation spectra reported by those authors can be interpreted equally well either by a dual emission from Ru@hen)2@q)2’ or by two separate emissions from Ru(phen),(pq)2+and Ru(phen@. To understand whether the presence of Ru(phen): as an impurity can be a plausible explanation for the observed dual emission, we have recorded the ernission spectrum at 77 K of a solution containing 4 X 10”
M Rul$hen)2pq)2* and 4X 10s7 M Ru(phen)y_ As one can see (fig. 2), this spectrum clearly shows two bands at ~560 and ~650 nm and it is quite simi-
s
5
s h ii
k.m ;;I 2
500
600
700
a.nm
830
Fig- 2. Emission spectra at 77 K of (a) Ru(phen)2(pq)2*(3_7 X 10” M); @) Ru(phen)2(pq)**+ 1 mol% Ru@he& [same sasitivity as (a)]; (c) Ru@hen)$+, 3.6 x 10”
(spectralsensitivityreduced by 20 times).
M
Volume 104. number 1
27 January 1984
CHEMICAL PHYSICS LElTERS
iar to that reported by Cocks et al. [7] for their Ru(phen)2@q)2’ sample. Since 1% of Ru(phen)F impurity in a sample of Ru(phen)z@q)z+ can hardly be detected by the usual analytical techniques (chemical analysis, column chromatography and NMR spectroscopy [7]), we believe that the 560 MI emission found by Cocks et al. [7] may be due to the presence of some Ru@hen)y and that the only true emission of Ru@hen)2@q)2+ is that at ~650 nm, which involves the lowest-energy Ru + pq CT excited state. The presence of only one emission (from Ru + biq CT excited state) in the quite similar complex substantiates the above Ru@hen)2(biq)2’ conclusion_ Since Anderson et al. [4] suggested that decoupling between excited states localized on different ligands (and thus, dual emission) could be favored by steric rigidity of the ligands, we have also examined the emission spectrum of the Ru(phen), (DMCH)*+ complex, where the flexible pq or biq ligands are replaced by the more rigid DMCH ligand (fig- 1). A very intense emission band at 7 10 nm (at 77 K; 728 nm at room temperature) was observed, which can easily be attributed to the lowest Ru + DMCH CT excited state_ A very weak emission was also observed at ~560 nm, which could originate either from the Ru + phen CT excited state of Ru(phen)2(DMCH)2+ or from some Rubhen)? present as impurity. Addition of 1 mole per cent of Ru@hen)p to our Ru(phen)2(DMCH)2+ caused an increase by a factor of 5 of the weak 560 nm emission. This means that the 560 MI emission of our Ru(phen)2(DMCH)2+ by =0_2 mol% of Ru@“h~$ci~p~~~~~~~ z detectable by analytical techniques_ On the basis of the spectroscopic properties of the parent Ru(bpy)F [21] and Ru(i-biq)? [13] complexes, it can be expected that, in the mixed-ligand Ru(bpy)2(i-biq)2’ and Ru(bpy)(i-biq)y complexes, the lowest excited state is Ru + bpy CT in character and that the lowest excited state involving the i-biq ligand is not a Ru * (i-biq) CT excited state, but a ligandcentered (LC) excited state [ 13,191. In the previously discussed Ru@hen)2(pq)2’ complex, the deactivation of the Ru + phen CT excited state to the Ru + pq CT excited state involves only the transfer of an electron from n*(phen) to a*(pq) orbitals. By contrast, in the Ru(bpy),(i-biq& complexes, the internal conversion of the i-biq LC excited state to
the Ru * bpy CT excited state involves not only the transfer of an electron from n*(i-biq) to n*(bpy) orbitals, but also the transfer of an electron from si(Ru) to n(i-biq). The coupling between the two excited states is thus expected to be less favorable in Ru(bpy),(i-biq):f, than in Ru@hen)2@q)Z+ and dual emission should be more likely for the (bpy)/ (i-biq) mixed-l&and
complexes.
Careful examination
of the emission spectra showed that, while Ru(bpy)z(i-biq)?+ exhibits only emission from the lowest Ru + bpy CT excited state (590 nm), Ru(bpy) (i-biq)j+ also shows another weaker band at 540 nm, where the i-biq localized emission is expected to occur. In exactly the same region, however, emission from the parent Ru(i-biq)$+ comples, present as an impurity, would also be observed_ One can also note that the observed emission lifetime at 540 nm is =Z100 w, i.e. almost the same as that of Ru(i-biq)f+.
From the spectra of solutions containing (Gbiq)_f’ and Ru(i-biq)?,
variable amounts of Ru(bpy)
we were able to estimate that the emission at 540 nm from our “pure” Ru(bpy)(i-biq):+ sample could be accounted for by assuming that i_5% of Ru(i-biq): impurity was present_ Since, as usual, the parent trischelate complexes are obvious impurities in the samples of the mixed-ligand complexes, and since such a low level of impurity is beyond the sensitivity limit of the analytical techniques used to ascertain the purity of the samples, we must conclude that even in this case there is no reason to assume that a dual emission takes place.
4. Conclusions In no case has a bona fide dual emission been observed for the miued-ligand Ru(II) chelates examined in this paper. For the complexes which exhibit two emission bands, the highenergy emission band can be accounted for by a decomposition reaction or by the presence of very small amounts (of the order of 1%) of a parent tris-chelate complex as an impurity. As recently discussed by Baggott et al. [ 151. the absence of dual emission means that the coupling between dissimilar ligands is strong enough to produce kinetic effects, i.e. to cause relaxation rates which are much faster than the radiative decay of the upper excited states. Thus, the only excited state which is suf-
103
Volume 104, number 1
ficiently long-lived tb give rise to luminesckce emission and to-be involved in bimolecular processes is the
Iowest excited state of the complexAny attempt to obtain mixed-&and lL
[IO] R-R. Ruminski and J.D. Peterseri,Inog. (1982) 3706.
Tu~~~dV.Ea~,~hern.
1161 [17]
References
[19]
36 (1981) 325. [2] N. GrBzel. ed, Energy resources by photochemistry and catalysis (Academic Press, New York, 1983). 131 J. Rabani. ed., Photochemical conversion and storage of solar energy (Weizmann Science Press, Jerusalem, 1982). (41 S. Anderson, K.R. Seddon, R-D-Wright and A.T.Cocks,
[ 381
1201 [21] f22] [23]
Chem. Phys. lxM.ers 71 (1980) 220. 15 ] A. Juris, V. Baizani, P. Belser and A. von Zelewsky, Helv.CMn. Acta 64 (198X) 2175. [6] SF. Agnew, H.L. Stone and G.A. Crosby, Chem. Phys. Letters 85 (1982) 57. [7] A.T. Cocks, R. Wright and K.R. Seddon, Chem. Phys. Letters 85 (1982) 369. IS J A. Basu, M.A. Weiner, T-C. Streaks and H-D_ Gafney,
104
J. Phys. Chem. 86
Chem. 21
Phys. Letters 89 (1982)
101.
[ 141 D.M. Klassen. Chem. Phys. Letters 93 (1982) 383. [IS] I.E. Baggott, GX. Gregory, MJ. Filling, S. Anderson,
Financial support from the Swiss National Science Foundation and from the Minister0 della Pubblica Istruzione is gratefully acknowledged_
[l ] M.K. DeArmond and CM. CarEn, Coord. Chem. Rev.
1984
f 1 j ] D-S_ RiIIema an+ K&t Mack, lnorg. Chem. 21 (1982) 3849. [ 121 A. Juris; F. Barigellctti, V- Balzani, P. Belser and li von Zelewsky, Israel J. Chem. 22 (I 982) 87_ (131 P. Belser, A. YOIIZelewsky. A. Juris, F. Bari&lctti, A.
Adcnowledgement
Inorg. Chem. 21 (1982) 1085. [9] D.P. Segers and M.K. De+rmond, (1982) 3768.
27 Jan-
CHEhflCAL PHYSICS LE-fTERs
j24f
1251 1261
K-R. Seddon and J E. Turp, J. Chem. Sot. Faraday Trans. II79 (1983) 195. RJ. CrutchIey,M. Kress and A.B.P. Lever, J. Am. Chem. Sot. 105 (1983) 1170. PJ. Steel, F. Lahonsse, D. Lerner and C. Mar&t, Inorg. Chem.22 (1983) 1488. L J. Fritzpatrick, ll.A. Goodwin, A. Launikonis, A.W_-H. Mau and W.H.F. !&se. Chem. Phys. Jxtters, to be published. P. Be&r. A. von Zelewsky. A. Juris, F. Barigelletti and V_ BaImni, Gazz. Chim. ItaI, to be published. F_ BarigeIIetti. A. Juris, V- BaIzani, P. BeIser and A. von Zelewsky, Inorg. Chem., to be published. K. Kalyanasundaram, Coord. Chem. Rev.46 (1982) 159. D.M. Manuta and A.J. Lees, Inorg. Chem. 22 (1983) 572. hl. h&tin, M.B.Krogh-Jespexsen, hl.Hsu, J.Tewksbury, M. Laurent, K. Viswanath and H. Patterson, Inorg. Cbem. 22 (1983) 647_ R.A.Rader,D.R.h~cMiiIin,M.T.Buckner,T.G.Matthews, DJ.Casadonte, RX. Lengel, S.B.Whittaker, L.M. Darmon and FE. Lytle, J. Am. Chem. Sot. IO3 (1981) 5906. P_ BeIser and A. von Zelewsky, Helv. Chim. Acta 63 (1980) 1675. J_N. Edemas and G.A. Crosby, J. Am. Chem. Sot. 93 (1971) 2841.