Inorganica Chimica Acta 362 (2009) 5073–5079
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Synthesis, characterization, photophysics and intramolecular energy transfer process in bimetallic rhenium(I)–ruthenium(II) complexes Murugesan Velayudham, Seenivasan Rajagopal * Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 1 March 2009 Received in revised form 13 August 2009 Accepted 19 August 2009 Available online 27 August 2009 Keywords: Rhenium(I) Ruthenium(II) Bimetallic complex Photophysics Energy transfer
a b s t r a c t Two new heterobimetallic complexes of rhenium(I) and ruthenium(II) [(CO)3(NN)Re(4,40 -bpy)Ru(NN)2Cl](PF6)2 and already known monometallic complexes [Cl(NN)2Ru(4,40 -bpy)](PF6) and [(CO)3 (NN)Re(4,40 -bpy)](PF6) and bimetallic complexes [Cl(NN)2Ru(4,40 -bpy)Ru(NN)2Cl](PF6)2, [(CO)3(NN)Re(4,40 -bpy)Re(NN)(CO)3](PF6)2 (NN = 2,20 -bipyridine, 1,10-phenanthroline; 4,40 -bpy = 4,40 -bipyridine) are synthesized and characterized by spectral techniques. The photophysical properties of all the complexes are studied. It is found that attachment of rhenium(I) altered the photophysical characteristics of ruthenium(II). Excited state energy transfer from the rhenium(I) chromophore to the ruthenium(II) is observed upon excitation at 355 nm. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Molecular devices capable of performing functions by the stimulation of light have attracted considerable attention in recent years. Of special interest is, metal complexes of ruthenium(II), osmium(II) and rhenium(I) because of their good light harvesting ability, various redox states and light emission in the visible region [1–6]. The photochemical and photophysical behavior is found to be interesting on moving from mononuclear to binuclear system because each metal containing subunit has its own properties and gets altered upon incorporation into a multicomponent system and a number of new processes may take place between different units [6–13]. On account of the aforementioned properties, these complexes are found to be useful for various applications such as molecular wires [14,15], molecular switches [16,17], photocatalysts [18], and LED devices [19,20]. Two component system known as dyads, which contain photoactive and electroactive units like ruthenium(II) and rhenium(I), are suited for the study of photoinduced electron and energy transfer processes. Studies on dyads have greatly contributed to gain knowledge on the effect of basic physical factors such as energy gradient, distance, intervening bonds and medium on the kinetics of electron and energy transfer processes [7,8]. The transition metal complex dyads are attractive systems from several view points. In particular, these systems are flexible in terms of tailoring the energetics. The role
* Corresponding author. E-mail address:
[email protected] (S. Rajagopal). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.08.024
of the spacer, a unit connecting the donor and the acceptor, is crucial. It not only plays a role structural, its chemical nature controls the electronic communication between the selected terminal units. A considerable number of reports are available, in which ruthenium(II) and osmium(II) polypyridyl complexes are connected by different types of bridging ligands [1,7,8,21–25]. Ziessel and co-workers [23–25] have extensively studied the bimetallic polypyridyl complexes of ruthenium(II) and osmium(II) held by different spacers and observed energy transfer from 3MLCT state of ruthenium(II) to osmium(II). However, there are only a few reports that deal with bimetallic complexes, in which ruthenium(II) is connected with rhenium(I) [26–31]. The advantage of using rhenium(I) carbonyl complex is that, it is synthetically very useful [32–34] and shows intense m(CO) stretching frequencies which respond to the electronic distribution at the metal center. The rhenium(I)–ruthenium(II) heterobimetallic complexes find potential applications and recently they have been used as photocatalysts for CO2 fixation [2,18]. We herein, present the synthesis, characterization, electrochemical and photophysical properties of bimetallic complexes of ruthenium(II) and rhenium(I) bridged by 4,40 -bipyridine (4,40 -bpy) ligand, with 2,20 -bipyridine (bpy) and 1,10-phenanthroline (phen) as ancillary ligands. The complexes used in the present study are homobimetallic complexes of ruthenium(II) (IIa, IIb), rhenium(I) (IVa, IVb) and heterobimetallic rhenium(I)–ruthenium(II) (Va, Vb). The monometallic complexes of ruthenium(II) (Ia, Ib) and rhenium(I) (IIIa, IIIb) are also synthesized for comparison purpose (Chart 1).
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Chart 1. Structure of the complexes.
2. Experimental section 2.1. Materials Re2(CO)10 (Alfa), RuCl3xH2O (Aldrich), 2,20 -bipyridine (Merck), 4,4 -bipyridine (Fluka), AgCF3SO3 (Alfa), and NH4PF6 (Alfa) were used as received. All solvents used for the synthesis were of reagent grade and for spectral measurements spectral grade was used. The monometallic and homobimetallic complexes of ruthenium and rhenium were prepared by literature procedures [35– 40]. 0
2.2. Syntheses 2.2.1. Preparation of [(CO)3(bpy)Re(4,40 -bpy)Ru(bpy)2Cl](PF6)2 (Va) The compound [Re(CO)3(bpy)(CH3CN)](CF3SO3) (0.31 g, 0.5 mmol) was dissolved in degassed EtOH (30 mL) and to this was added [(bpy)2ClRu(4,40 -bpy)](PF6) (0.37 g, 0.5 mmol) and refluxed for 6 h under inert atmosphere. The reaction mixture was evaporated to dryness and the solid redissolved in minimum amount of ethanol and loaded onto the alumina column. Elution was performed with a mixture of toluene/EtOH. Initially 2:1 toluene/EtOH was used to remove the unreacted fac. [Re(bpy) (CO)3(CH3CN)](TFMS) and then the composition of the EtOH was increased to collect the product. The product was then metathesized to PF6 salt by dissolving in a minimum volume of acetone and then an aqueous solution of NH4PF6 was added to get the precipitate. The solid was filtered and dried in vacuum. Yield = 73%. IR (KBr, cm1): 2030, 1938, 1920; ESIMS (m/z): 1177. 2.2.2. Preparation of [(CO)3(phen)Re(4,40 -bpy)Ru(phen)2Cl](PF6)2 (Vb) The compound [Re(CO)3(phen)(CH3CN)](CF3SO3) (0.32 g, 0.5 mmol) was dissolved in degassed EtOH (30 mL) and to this was added [(phen)2ClRu(4,40 -bpy)](PF6) (0.5 g, 0.5 mmol) and refluxed for 8 h under inert atmosphere. The reaction mixture was evaporated to dryness and the solid redissolved in ethanol and put into the column packed with alumina. Elution was first performed with
2:1 toluene/EtOH mixture to remove the unreacted [Re(phen)(CO)3(CH3CN)](TFMS) and then the composition of the EtOH was increased gradually to 1:1 toluene/EtOH mixture to collect the product. The product was then metathesized to PF6 salt using NH4PF6. Yield = 69%. IR (KBr, cm1): 2025, 1928, 1905; ESIMS (m/ z): 1249. 2.3. Physical measurements UV–Vis spectra were recorded on Analtikjena Specord S100 spectrophotometer and emission spectra were measured in JASCO FP6300 spectrofluorimeter. All the samples used for emission were purged with dry nitrogen gas for about 20 min. IR spectra were measured using JASCO FT-IR spectrophotometer. The samples were recorded in the form of KBr pellets. All the mass spectra were recorded using a Quattro LC triple-quadrupole mass spectrometer (Micromass, Manchester, UK) interfaced to an (electro spray ionization) ESI source; data acquisition was done under the control of Masslynx software (version 3.2). The ESI capillary voltage was maintained between 4.0 and 4.2 kV and the cone voltage kept at 25 V. Nitrogen was used as desolvation and nebulization gas. The source and desolvation temperatures were 100 °C. 2.4. Electrochemistry The redox potentials of the complexes were determined by cyclic voltammetric and differential pulse voltammetric techniques using EG & G Princeton Applied Research Potentiostat/Galvanostat Model 273 A. Measurements were performed by purging the solution in acetonitrile (spectroscopic grade) by dry nitrogen gas for 30 min. The supporting electrolyte was 0.1 M TBAP, Pt was the working electrode, and the Ag/AgCl was the reference electrode. 2.5. Lifetime and transient measurements The excited state lifetime measurements for all the complexes were made, after degassing the solutions with argon for 30 min
M. Velayudham, S. Rajagopal / Inorganica Chimica Acta 362 (2009) 5073–5079
and the details are given in previous reports [41]. Transient absorption measurements were made with laser flash photolysis technique as described elsewhere [41]. 3. Results and discussion 3.1. Syntheses The schematic structures of the complexes that are used for the present investigation are illustrated in Chart 1. The ruthenium(II) complexes [35,36] Ia, Ib, IIa and IIb and rhenium(I) complexes [37–40] IIIa, IIIb, IVa and IVb were prepared by reported procedures. The rhenium(I)–ruthenium(II) heterobimetallic complexes Va and Vb were prepared by complex as metal/complex as ligand approach [42]. The heterobinuclear complexes were synthesized by reacting monometallic ruthenium complex carrying bridging ligand 4,40 -bipyridine with rhenium precursor complex [(CO)3(NN)Re(CH3CN)](CF3SO3). These complexes were characterized by ESIMS, FT-IR, cyclic voltammetry, UV–Vis and luminescence spectroscopy. 3.2. Infra-red spectra The infra-red spectra of rhenium(I) complexes are characterized by intense carbonyl stretching frequencies m(CO) [40]. Three m(CO) stretching frequencies are in general observed between 1900 and 2050 cm1 corresponding to the facial orientation of Re(CO)3 unit. The IR spectral data are gathered in Tables 1 and 2. The highest energy transition appears as a very sharp peak around 2030 cm1 and the other two bands are relatively broad and overlap considerably. 3.3. ESI-MS The complexes synthesized were characterized by ESI-MS technique. They show m/z values corresponding to the loss of one PF6 ion to give a monocharged species. Some of the monocharged species are given here, [(CO)3(phen)Re(4,40 -bpy)Re(phen) (CO)3](PF6) m/z 1203, [Cl(phen)2Ru(4,40 -bpy)Ru(phen)2Cl](PF6) m/z 1297 and [(CO)3(bpy)Re(4,40 -bpy)Ru(bpy)2Cl](PF6) m/z 1177
Table 1 IR spectral and electrochemical dataa of monometallic and bimetallic complexes. Complex
Ia IIa IIIa IVa Va a
E1/2 (V) Ru3+/2+
E1/2 (V) Re2+/+
Re
+/0
0.75 0.89
0.83
+1.71 +1.68 +1.80
mCO (cm1)
E1/2 (V)
1.23 1.22 1.30
0/
0
bpy
4,4 -bpy
1.59 1.61 1.05 1.06 1.06
0.96 0.97 0.92 0.85
0/
2034, 1936, 1920 2032, 1940, 1919 2030, 1938, 1920
[(CO)3(phen)Re(4,40 -bpy)Ru(phen)2Cl](PF6) m/z 1249. The ESIMS spectrum of the complexes IIb, IVb and Vb are shown in Figs. S1–S3, respectively. In the ESIMS spectrum of IIb (Fig. S1), the peaks with m/z values of 497 and 538 are assigned to [Ru (phen)2Cl]+ and [Ru(phen)2Cl(CH3CN)]+ species, respectively. The peak with m/z value of 653 is assigned to [Ru(phen)2Cl(4,40 bpy)]+ fragment. In the case of IVb and Vb (Figs. S2 and S3) the peaks with m/z values of 451, 492 and 607 correspond to [Re (phen)(CO)3]+, [Re(phen)(CO)3(CH3CN)]+ and [Re(phen)(CO)3(4,40 bpy)]+ fragments. 3.4. Electrochemistry The electrochemical behavior of the monometallic and bimetallic complexes was studied in acetonitrile solution, and the relevant redox potentials are listed in Tables 1 and 2. The ruthenium complexes Ia and IIa show oxidation waves with the E1/2 value 0.75 and 0.89 V, respectively and are assigned to Ru2+/3+ couple [35,36]. In the cathodic region reduction occurs at considerably lower potential (0.95 V) which can be assigned to the reduction of bridging ligand and other two reductions at large negative potentials are due to the reduction of ancillary 2,20 -bipyridine ligand. Similarly ruthenium(II) complexes with phenanthroline as the ancillary ligand, Ib and IIb, show oxidation peak corresponding to Ru2+/3+. The cyclic voltammogram of IIb is shown in Fig. S4. For the rhenium(I) complexes IIIa and IVa, an irreversible oxidation peak occurs at 1.71 and 1.68 V, respectively, and is assigned to Re+/2+. Similarly for the rhenium(I) complexes with phenanthroline ligand, IIIb and IVb, oxidation peak is assigned to Re+/2+ couple. The DPV of IVb is shown in Fig. S5. For the rhenium(I) complexes, in the cathodic region of the cyclic voltammogram, reduction at less negative potential (0.9 V) can be assigned to that of bridging ligand. An irreversible wave at 1.23 V can be assigned to the Re+/0 reduction and another reduction peak is assigned to ancillary ligand reduction. These assignments were made following the earlier reports [37–40]. In the case of rhenium(I)–ruthenium(II) heterobimetallic complexes Va and Vb, two oxidation peaks are observed. Based on the comparison of ruthenium(II) and rhenium(I) monometallic complexes, oxidation at less positive value is assigned to Ru2+/3+ couple and at higher potential is assigned to Re+/2+ oxidation. The anodic part of the DPV of Vb is shown in Fig. S6. The oxidation potential of RuII/III couple is slightly shifted to a more positive region compared to Ia and Ib and this is due to the attachment of electron-withdrawing Re(I) unit to the ruthenium(II). In the negative part of the cyclic voltammogram, the first reduction is assigned to the bridging ligand and subsequently rhenium(I) and ancillary ligand reductions occur at higher negative potentials (Tables 1 and 2). 3.5. Absorption spectra
Ag/AgCl reference electrode; 0.1 M TBAP supporting electrolyte.
Table 2 IR spectral and electrochemical dataa of monometallic and bimetallic complexes (Ib– Vb). Complex E1/2 (V) E1/2 (V) E1/2 (V) mCO (cm1) Ru3+/2+ Re2+/+ Re+/0 phen0/ 4,40 -bpy0/ Ib IIb IIIb IVb Vb a
5075
0.81 0.85
0.89
+1.41 +1.46 +1.50
1.61 1.63 0.91 1.24 0.88 1.22 0.84 1.16
0.97 0.99 0.67 0.72
2031, 1946, 1915 2032, 1946, 1917 2025, 1928, 1905
Ag/AgCl reference electrode; 0.1 M TBAP supporting electrolyte.
The UV–Vis absorption spectra for all monometallic and bimetallic complexes were recorded in CH3CN and the spectral data are collected in Tables 3 and 4. Ruthenium(II) complexes Ia and IIa display MLCT transitions in the region 325–375 nm and 470– 500 nm with maximum absorption occurring at 487 and 489 nm, respectively The absorption spectrum of IIa is shown in Fig. S7. For the known complexes the absorption spectral data observed here are very similar to the reported values [35,36]. On comparing these values with the parent complex [Ru(bpy)3]2+, which has the absorption maximum at 452 nm in CH3CN, a red shift to a tune of 35 and 37 nm are observed for Ia and IIa, respectively. This red shift can be attributed to the presence of Cl ligand in Ia and IIa. Similarly the ruthenium(II) complex with phenanthroline ancillary ligand IIb show intense transitions in the UV region due to ligand
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Table 3 Absorption and photophysical properties of monometallic and bimetallic complexes Ia–Va in CH3CN at 298 K. Complex
kmax
kem,
max
s (ns)
kr
Uem
10
Ru(bpy)32+ Ia IIa IIIa IVa Va
290,452 292,377,487 291,354,489 306, 348 268,306,350 273,297,341 412
615 722 726 580 580 589
1100 28 28 240 270 330
0.064 0.0015 0.0023 0.090 0.029 0.035
knr 4
5.8 1.5 7.8 37 12 11
6 10
0.85 36 35 38 3.6 2.9
Table 4 Absorption and photophysical properties of monometallic and bimetallic complexes Ib–Vb in CH3CN at 298 K. Complex
kmax
kem,
max
s (ns)
Uem
kr 10
Ru(phen)32+ Ib IIb IIIb IVb Vb
266, 265, 264, 271, 272, 268,
442 451 463 332 331 417
604 680 684 584 586 591
460 33 72 1418 1373 1607
0.0280 0.0035 0.0040 0.0100 0.0090 0.0110
knr 4
6.08 10.6 5.55 7.05 6.55 6.84
tion maximum at 412 nm, a blue shift to a magnitude of 75 nm is observed. This may be attributed to the coordination of electronwithdrawing Re(I) center with ruthenium(II), which increases the ligand-field splitting energy of ruthenium(II) unit [22,43–45].
10
6
2.10 30.1 13.8 0.63 0.66 0.55
centered p?p* transition and low energy 1MLCT transition in the region 450–475 nm (Fig. 1). When we compare the absorption due to the 1MLCT transition to that of the parent complex [Ru(phen)3]2+, which has an absorption maximum at 442 nm in CH3CN, a red shift to a tune of 11 nm is observed. The rhenium(I) complexes IIIa and IVa show a peak at 306 nm due to ligand centered p?p* transition and another peak at ca. 350 nm because of 1MLCT transition. The absorption spectrum of IVa is shown in Fig. S7. Similarly rhenium(I) complexes with phenanthroline ancillary ligand show p?p* transition at 271 nm and 1MLCT transition at ca. 332 nm. The absorption spectrum of IVb is shown in Fig. 1. The heterobimetallic complex Va shows a peak at 297 nm due to ligand centered p?p* transition. The peaks at ca. 341 and 412 nm are assigned to Re(dp)?p* and Ru(dp)?p* transitions, respectively (Fig. S7). The complex Ia has absorption maximum at 487 nm and when it is compared with Va, which has the absorp-
Fig. 1. Absorption spectra of IIb (—), IVb (. . ..) and Vb (——) in CH3CN at 298 K.
3.6. Emission spectra The emission spectra for all the complexes were recorded in CH3CN and the values are given in Tables 3 and 4. The ruthenium(II) complexes Ia and IIa display emission at 722 and 726 nm, respectively from 3MLCT state upon excitation at the wavelength corresponding to 1MLCT absorption and the emission maximum is independent of excitation wavelength. The emission spectrum of IIa is shown in Fig. S8. The parent complex [Ru(bpy)3]2+ has emission maximum at 612 nm in CH3CN. When we compare this value with that of Ia, a substantial red shift is observed to a magnitude of 110 nm. The emission spectrum of IIb is shown in Fig. 2. The parent complex [Ru(phen)3]2+ has the emission maximum at 604 nm in CH3CN and a red shift to a magnitude of 76 and 80 nm are observed for Ib and IIb, respectively. This substantial red shift can be attributed to the presence of Cl ligand in Ia and Ib. Upon excitation at the wavelength corresponding to 1MLCT absorption, rhenium(I) complexes IIIa and IVa show emission around 580 nm and the emission profile suggests that it comes from 3MLCT state. For the known complexes emission maxima observed here are in accordance with the reported values [37,39]. The heterobimetallic complex Va has emission maximum at 590 nm and irrespective of excitation wavelength it displays same emission maximum. A substantial blue shift to a magnitude of 132 nm is observed for the complex Va when compared to IIa. These results are consistent with the results obtained in the UV– Vis absorption spectra indicating the attachment of electron-withdrawing ReI(CO)3 unit with ruthenium(II) leading to enhancement of ligand-field splitting energy of ruthenium(II) unit. Similar observation is noted in heterobimetallic complex Vb (Fig. 2). 3.7. Excited state lifetime and quantum yield The excited state lifetime and quantum yield of all the complexes are given in Tables 3 and 4. The ruthenium(II) complexes Ia and IIa have excited state lifetime of 28 ns each. The lifetime decreased substantially for complex Ia when compared to [Ru
Fig. 2. Emission spectra of IIb (—) (kex = 464 nm), IVb (——) (kex = 332 nm) and Vb (. . ..) (kex = 350 nm) recorded in degassed CH3CN at 298 K.
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(bpy)3]2+ which has an excited state lifetime of 1100 ns in CH3CN. Similarly the lifetime of complexes Ib and IIb decreased tremendously compared to [Ru(phen)3]2+ which has an excited state lifetime of 460 ns and quantum yield of 0.028. The decrease in the lifetime value of Ia, IIa and Ib, IIb can be explained in terms of energy gap law [3], which says that the non-radiative decay rate increases exponentially with decrease in the energy gap between ground and excited states. Since the energy gap between ground and excited state is less for these complexes, it facilitates faster non-radiative relaxation which leads to low excited state lifetime. An alternative explanation for the decreased lifetime in these complexes is, the presence of weak-field Cl ligand keeps the ligandfield state close to 3MLCT state, which facilitates non-radiative decay by thermal deactivation [45]. The rhenium(I) complexes IIIa and IVa have the excited state lifetime of 240 and 270 ns, respectively. The excited state lifetime data observed here are in accordance with the reported values [37,39] The excited state lifetime of heterobimetallic complex Va is 330 ns and it is significantly higher than ruthenium(II) mononuclear complex Ia. The increase in the lifetime value of Va correlates well with blue shift observed in emission and also supports that, complexation of Re(I) unit with ruthenium(II) unit increases the 10Dq value of ruthenium(II) center leading to decreased non-radiative relaxation (vide supra). In addition to this, the higher lifetime observed from Re–Ru complex compared to the value observed from monometallic and homobimetallic Ru(II) complexes may be due to the emission after energy transfer from 1Re(I) to 1Ru(II) as well as from 3Re(I) to 3Ru(II). Similar explanation has been offered for the observation of emission from Os(II) component in the case of Ru(II)–Os(II) [23–25,46] complexes and Re(I)–Os(II) [40,47] heterobimetallic complexes. From the excited state lifetime (s) and quantum yield (Uem), the radiative and non-radiative rate constants can be calculated by the following expressions (1) and (2)
kr ¼ Uem =s
ð1Þ
1=s ¼ kr þ knr
ð2Þ
The radiative (kr) and non-radiative (knr) rate constants for all the complexes are given in Tables 3 and 4. As expected the knr values are higher for complexes Ia and IIa. On the other hand the rate of non-radiative relaxation is lesser for the heterobimetallic complex Va indicating increased energy gap between ground state and lowest excited state.
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Fig. 3. Transient absorption spectra of IIa recorded in argon degassed CH3CN upon excitation at 355 nm at RT. Recorded at 400 ns after excitation.
The transient spectra of complexes Va and Vb are shown in Figs. 5 and 6, respectively. At shorter time scale (33 ns), a broad absorption is seen from near UV to mid-visible region, which is indicative of excitation of Re(I) 1MLCT state. The transient spectra recorded at 500 ns after excitation shows bleaching in the region 400–525 nm. This bleach is due to the disappearance of ground state 1MLCT of ruthenium(II). A broad and less intense absorption is seen in the region 560–600 nm which can be attributed to the formation of bridge localized 4,40 -bpy radical anion [40]. These observations reveal that shortly after excitation Re(I) complex absorbs energy and goes to the excited state with the formation 4,40 bpy radical anion. The excited *Re(I) chromophore transfers energy to the ruthenium(II) 1MLCT state which in turn collects the energy and goes to the excited state. The decay of ground state 1 MLCT of ruthenium(II) can be understood from the bleach seen in the region 400–525 nm in transient spectrum. Similarly for complex Vb, the transient spectrum recorded at 51 ns after excitation shows a broad absorption in the region 370–475 nm and a broad and less intense absorption in the region
3.8. Transient spectra The transient absorption spectra of the complexes were recorded in acetonitrile solution at room temperature following excitation at 355 nm. The transient spectrum of ruthenium(II) complex IIa (Fig. 3) shows a bleach around 490 nm due to the decay of the ground state 1MLCT absorption. The transient spectrum of parent complex [Ru(bpy)3]2+ shows bleaching around 450 nm due to the decay of ground state 1MLCT absorption and formation in the region 370–390 nm due to bpy radical anion absorption. Similar decay around 490 nm observed for Ia and IIa indicates the decay of ground state 1MLCT, and a broad absorption seen ca.590 nm is due to 4,40 -bipyridine anion radical absorption [40] which indicates that 1MLCT state is bridge localized. This happens because the p* level of 4,40 -bpy is lower than that of 2,20 -bpy. This is supported by the reduction potential values and it is 0.9 V for 4,40 -bpy and 1.1 V for 2,20 -bpy. Rhenium(I) complexes IIIa and IVa have broad absorption from near UV to mid-visible region with higher intensity in the region 370–390 nm and the results are consistent with previous reports [37]. The transient absorption spectrum of IIIa is shown in Fig. 4.
Fig. 4. Transient absorption spectrum of IIIa recorded in argon degassed CH3CN upon excitation at 355 nm at RT. Recorded at 106 ns after excitation.
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Fig. 5. Transient absorption spectra of Va recorded in argon degassed CH3CN upon excitation at 355 nm at RT. Recorded at 33 ns (-j-) and 503 ns (-d-) after excitation.
Fig. 7. Energy level diagram depicting energy transfer from *Re(I) to Ru(II).
one molecular center to the other. The electron exchange mechanism is exponentially dependent on distance between the donor and acceptor and becomes negligible at distances greater than 10 Å. The distance of rhenium(I) to ruthenium(II) bridged by 4,40 bpy is 10.6 Å [40] and hence Dexter mechanism is favorable. When the heterobimetallic complex is excited, the Re(I) chromophore goes to the excited state with a creation of hole in the ground state, which can be filled by an electron in the ground state of Ru(II) unit. Simultaneously the electron in the excited state of *Re(I) is transferred to Ru(II) with a net release of energy. However, at this stage it is not possible to rule out operation of Förster mechanism here and the system will be further studied in order to understand the mechanism clearly. 4. Conclusion
Fig. 6. Transient absorption spectra of Vb recorded in argon degassed CH3CN upon excitation at 355 nm at RT. Recorded at 51 ns (-j-) and 701 ns (-.-) after excitation.
550–600 nm (Fig. 6). This observation indicates excitation of 1 MLCT state of rhenium(I). At 701 ns, after excitation the transient spectrum shows a pronounced bleaching from 425 to 525 nm followed by an intense absorption at 370 nm and a broad and less intense absorption in the region 570–600 nm show the decay of ground state 1MLCT of ruthenium(II) and formation of 4,40 -bpy radical anion. These observations clearly show energy transfer from 3MLCT state of rhenium(I) to 3MLCT state of ruthenium(II). This behavior is consistent with the results observed in a number of analogous systems [29–31,40,47]. The energy level diagram depicting the energy transfer from *Re(I) to Ru(II) is shown in Fig. 7. Since we used nano-second transient absorption technique, we have only measured triplet–triplet energy transfer, though singlet–singlet energy transfer is also possible. The mechanism of energy transfer can be described by either Förster [48] or Dexter [49] mechanisms. Förster mechanism involves through space coulombic interaction and is operative up to 100 Å. Dexter mechanism consists of electron exchange from
A series of monometallic and bimetallic complexes of rhenium(I) and ruthenium(II) were synthesized and characterized. Also two new heterobimetallic complexes of rhenium(I) and ruthenium(II) were synthesized, characterized and the electrochemical and photophysical behavior of these complexes were studied. The coordination of rhenium(I) to ruthenium(II) is reflected by the blue shift seen in absorption and emission spectra. The excited state lifetime and quantum yield also get increased in complexes Va and Vb. Energy transfer from *Re(I) to Ru(II) is evident from the transient spectra which show a bleach in the region 420– 520 nm corresponding to the loss of 1MLCT state of ruthenium(II). These heterobimetallic complexes may be potentially useful as photocatalyst and light harvesting antenna system. Acknowledgement M.V. and S.R. thank Department of Science and Technology (DST), New Delhi, India for sanctioning a project to carry out this research work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.08.024. References [1] V. Balzani, G. Bergamini, P. Ceroni, Coord. Chem. Rev. 252 (2008) 2456.
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