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A donor–chromophore complex containing the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine
Inorganic Chemistry Communications 10 (2007) 1159–1163 www.elsevier.com/locate/inoche
A donor–chromophore complex containing the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine Shamindri M. Arachchige, Karen J. Brewer
*
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212, United States Received 27 April 2007; accepted 19 June 2007 Available online 29 June 2007
Abstract The complex [(PTZpbpy)2Ru(dpp)](PF6)2 coupling a phenothiazine derived electron donor to a ruthenium complex with a polyazine bridging ligand dpp (PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) has been synthesized and the redox, spectroscopic, and excited state properties elucidated. In the coupled electron donor system, the emission intensity from the Ru(dp) ! dpp(p*) 3MLCT state is quenched by 94% of that of the analogous system [(bpy)2Ru(dpp)](PF6)2 (bpy = 2,2 0 -bipyridine) which lacks a coupled electron donor. Coupling a donor–chromophore unit to a bridging ligand will allow the expansion of the molecular architecture to form complex molecular assemblies which contain covalently attached electron donors. 2007 Elsevier B.V. All rights reserved. Keywords: Donor–chromophore complexes; MLCT; Ru light absorbers; Phenothiazine; Electron donor; Polyazine ligand; Bridging ligand; Emission quenching
Photoinduced charge separation is a key component in many schemes for conversion of solar energy to chemical energy. In applying charge transfer states, extension of the excited state lifetime of the charge separated state is desired for efficient chemical energy transformations. Molecular diads and triads have been designed to covalently attach light absorbing chromophores, electron donors, and/or electron acceptors into a single molecular architecture to generate relatively long lived photoinduced charge separated states [1–6]. Triads are constructed to systematically promote sequential electron transfer events affording charge separated states which store light energy as chemical energy [1–12]. Diad systems support the study of electron transfer events leading to the charge separated states [11–15]. Transition metal–polyazine complexes are often included in these multicomponent molecular architectures as light absorbers [16,17]. In these systems, the photochemistry is initiated by metal-to-ligand charge *
Corresponding author. Tel.: +1 540 231 6579; fax: +1 540 231 3255. E-mail address: [email protected] (K.J. Brewer).
1387-7003/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2007.06.019
transfer (MLCT) excitation. Many of the electron transfer studies have focused on triad systems which incorporate covalently attached phenothiazine (PTZ) electron donors and various electron acceptors to ruthenium polypyridyl based chromophores [3–6]. In these systems, the MLCT state is initially generated and then quenched by electron transfer from the PTZ to the metal to form charge separated states. Molecular architectures that couple donor– chromophore systems to bridging ligands have fundamental significance in the construction of complex molecular devices for applications in solar energy conversion schemes. The bridging ligand provides a point of attachment between the donor–chromophore unit and an electron acceptor, allowing the expansion of the molecular assembly. Despite the potential significance, this area of research remains largely unexplored. In the bimetallic system, [(PTZmbpy)(CO)3ReI(4,4 0 -bpy)ReI(CO)3(bpz)](PF6)2 (4,4 0 -bpy = 4,4 0 -bipyridine; bpz = 2,2 0 -bipyrazine, PTZmbpy = 4-phenothiazinyl-4 0 -methyl-2,2 0 -bipyridine; m in PTZmbpy denotes a methylene unit), a long-range photoinduced charge separation across the ligand bridge afforded
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S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163
N
N
S N N
N
N
N
PTZpbpy
dpp
Fig. 1. Structures of two polyazine ligands.
½ðPTZþ mbpyÞðCOÞ3 ReI ð4; 40 -bpyÞReI ðCOÞ3 ðbpz-ÞðPF6 Þ2 [18]. In the triad system, PTZmbpy–RuII–BQ (BQ = benzoquinone acceptor with a distal bipyridyl unit that can act as a bridge), a fairly long lived charge separated state between the donor and acceptor was observed [19]. However, this triad system proved to be photolabile, discouraging further expansion of the molecular assembly through the bridge. In this study, we report the synthesis and characterization of a donor–chromophore complex containing a bridging ligand, of the form [(PTZpbpy)2Ru(dpp)](PF6)2(PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine; p in PTZpbpy denotes a pentyl unit, dpp = 2,3-bis(2-pyridyl)pyrazine), Fig. 1. The terminal ligand used is PTZpbpy in which the electron donor PTZ
is covalently attached to the terminal ligand 4,4 0 dimethyl-2,2 0 -bipyridine (DMB) via a methylene linkage consisting of five methylene units, Fig. 1. Phenothiazine and its derivates are efficient intramolecular reductive quenchers [3–6]. The bridging ligand used in this study is dpp, Fig. 1. This bridging ligand has been extensively studied in the development of molecular assemblies that have emissive MLCT states capable of undergoing intramolecular energy and electron transfer [17,20,21]. Recent studies of a new tetranuclear complex, [Os{(l-2,3-dpp)Ru(PTZmbpy)2}3]8+, in which the [Os{(l-2,3-dpp)Ru(bpy)2}3]8+ chromophore is coupled to PTZ electron donors, demonstrated efficient electron transfer by the peripheral PTZ donors to the Os(II) core, despite the presence of the intermediate Ru(II) centers [22]. This system attached [(PTZmbpy)2RuCl2] subunit to the [Os(dpp)3]2+ core with the preparation of the [Os{(l-2,3dpp)Ru(PTZmbpy)2}3]8+ electron donor–chromophore assembly. In the present system under investigation, the arrangement of PTZpbpy and dpp on a single ruthenium chromophore to form [(PTZpbpy)2Ru(dpp)](PF6)2 would allow the exploitation of the unique properties of both ligands and the preparation of the bridging ligand complex would allow the incorporation of this assembly in a wide variety of polymetallic architectures. The presence of an electron donor and the bridging ligand within the same chromophore unit would allow the study of complex
Scheme 1. Synthetic scheme outlining the building block approach to synthesizing [(PTZpbpy)2Ru(dpp)](PF6)2 (PTZpbpy = 4-methyl-4 0 -(4-(Nphenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163
photochemical molecular devices with long-lived charge separated states. The synthesis of [(PTZpbpy)2Ru(dpp)](PF6)2 used a building block approach, Scheme 1. The terminal ligand, PTZpbpy, was synthesized as previously described [6]. The precursor to PTZpbpy, which is 4-(bromopentyl)-4 0 methyl-2,2 0 -bipyridine was purified using a slight modification to the literature procedure. The precursor, 4-(bromopentyl)-4 0 -methyl-2,2 0 -bipyridine, was easily separated from the excess 1,4-dibromobutane in the reaction medium in the form of the acid chloride salt. The acid chloride salt was neutralized by the dropwise addition to a stirring biphasic solution of dichloromethane/Na2CO3(aq) (0.5 M). Neutral 4-(bromopentyl)-4 0 -methyl-2,2 0 -bipyridine was extracted into the organic layer. The organic extract was dried over Na2SO4 and the volatile components were removed by rotary evaporation. The product 4-(bromopentyl)-4 0 -methyl-2,2 0 -bipyridine was purified by column chromatography on silica gel prior to use for the synthesis of PTZpbpy [6]. The monometallic precursor [(PTZpbpy)2RuCl2] was synthesized using previously published methods [6]. Refluxing [(PTZpbpy)2RuCl2] and excess dpp in a mixture of EtOH/H2O in the dark for 3 h followed by purification by column chromatography using methanol-deactivated adsorption alumina afforded [(PTZpbpy)2Ru(dpp)](PF6)2 as an orange–brown solid in moderate yield (40%) [23]. Fast atom bombardment-mass spectrometry analysis of [(PTZpbpy)2Ru(dpp)](PF6)2 was consistent with its formulation, (m/z; relative abundance): [MPF6]+ (1355, 65); [M2PF6]+ (1210, 100); [MPF6PTZH]+ (1156, 65); [M2PF6PTZH]+ (1011, 90). The electrochemical properties of [(PTZpbpy)2Ru(dpp)](PF6)2 were assigned by comparison to model systems: the terminal ligand, PTZpbpy, and the bpy analog,
Fig. 2. Cyclic voltammograms of (a) PTZpbpy and (b) [(PTZpbpy)2Ru(dpp)](PF6)2 in 0.1 M n-Bu4NPF6 in CH3CN using glassy carbon working electrode and a Ag/AgCl reference electrode (PTZpbpy = 4-methyl-4 0 -(4(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
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Table 1 Electrochemical data for PTZpbpy, [(PTZpbpy)2Ru(dpp)](PF6)2, and the related model system [(bpy)2Ru(dpp)](PF6)2 (PTZpbpy = 4-methyl4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) Complex
E1/2a (V)
Assignments
PTZpbpy
0.73 1.49 0.74 1.47b 1.03 1.50 1.31 1.06 1.50
PTZ0/+ PTZ+/2+ PTZ0/+ RuII/III, PTZ+/2+ dpp0/ bpy0/ RuII/III dpp0/ bpy0/
[(PTZpbpy)2Ru(dpp)](PF6)2
[(bpy)2Ru(dpp)](PF6)2c
a Potentials reported versus the Ag/AgCl (0.29 V) reference electrode in 0.1 M Bu4NPF6 in CH3CN. b Eap value. c Ref. [24]. Reported in V vs. SCE.
[(bpy)2Ru(dpp)](PF6)2, which lacks the PTZ donors. Fig. 2 illustrates the cyclic voltammograms of [(PTZpbpy)2Ru(dpp)](PF6)2 and PTZpbpy. Table 1 summarizes the electrochemical data for PTZpbpy, [(PTZpbpy)2Ru(dpp)](PF6)2, and [(bpy)2Ru(dpp)](PF6)2. The oxidative couples of [(PTZpbpy)2Ru(dpp)](PF6)2 were examined using a sequential increase of the switching potential. The oxidative electrochemistry of [(PTZpbpy)2Ru(dpp)](PF6)2 is dominated by PTZ based processes. The first reversible oxidation at E1/2 = 0.74 V corresponds to PTZ0/+ and occurs at the same potential as the free PTZpbpy PTZ0/+ couple. Increasing the switching potential to 2 V shows an oxidation at 1.47 V. The oxidation at 1.47 V represents the reversible RuII/III couple as a shoulder on the larger irreversible PTZ+/2+ couple. The potential of this PTZ+/2+ couple also corresponds to that of the PTZ+/2+ couple in free PTZpbpy, indicating coordination to the metal does not dramatically impact PTZ redox potentials. The reversible reductive couples at E1/2 = 1.03 and 1.50 V, corresponds to the reduction of dpp0/ and bpy0/, respectively. The dpp0/ and bpy0/ couples are comparable to the reductive couples observed for [(bpy)2Ru(dpp)](PF6)2. The facile oxidation of PTZ and reduction and of dpp suggest the feasibility of the formation of [(PTZ+pbpy)2Ru(dpp)](PF6)2 charge separated state in this complex. The electronic absorption spectrum of [(PTZpbpy)2Ru(dpp)](PF6)2 is illustrated in Fig. 3. The UV region of the spectrum is dominated by PTZpbpy and dpp ligand based p–p* transitions, with the dpp based transitions occurring as low energy shoulders. Two overlapping 1MLCT absorption bands appear in the visible region at 430 nm (e = 1.3 · 104 M1 cm1) and 480 nm (1.2 · 104 M1 cm1). The lowest energy transition is dpp based, Ru(dp) ! dpp(p*) charge transfer (CT), characteristic of dpp systems and in agreement with the electrochemical properties. For comparison, the electronic absorption spectrum of [(bpy)2Ru(dpp)](PF6)2 is characterized by intense bpy (290 nm) and dpp (320 nm (sh)) based p–p* transitions in
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S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163 Electronic Absorption Spectrum RoomTemperature Emission 77 K emission
10
-4
8
Intensity
-1
-1
ε x 10 (M cm )
12
6 4 2 0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 3. Spectroscopic data for [(PTZpbpy)2Ru(dpp)](PF6). The electronic absorption and RT emission data collected in deoxygenated acetonitrile solution at room temperature. The 77 K emission spectrum was collected in 4:1 EtOH/MeOH glass. Emission spectra are normalized and correct for PMT response (PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
the UV region and 1MLCT transitions in the visible region [25]. The [(PTZpbpy)2Ru(dpp)](PF6)2 displays more intensity at 258 nm (e = 7.8 · 104 M1 cm1), characteristic of the PTZ unit. Similar to [(PTZpbpy)2Ru(dpp)](PF6)2, the visible region of [(bpy)2Ru(dpp)](PF6)2 consists of overlapping absorption bands at 430 nm and 470 nm (e = 1.2 · 104 M1 cm1) [25]. The lowest energy Ru(dp) ! dpp(p*) CT transition at 480 nm for [(PTZpbpy)2Ru(dpp)](PF6)2 is slightly red shifted compared to the corresponding 470 nm transition for [(bpy)2Ru(dpp)](PF6)2. Emission spectroscopy was used to probe the excited state properties of [(PTZpbpy)2Ru(dpp)](PF6)2. This complex displays a weak emission from the Ru(dp) ! dpp(p*) 3 MLCT at 694 nm at room temperature in acetonitrile solution, Fig. 3. Compared to [(bpy)2Ru(dpp)](PF6)2, the em kem = 0.010 max ¼ 675 nm [25], emission quantum yield U 1 [26], excited state lifetime s = 380 ns [27], the emission from the 3MLCT state of [(PTZpbpy)2Ru(dpp)](PF6)2 is reduced by 94%, Uem = 5.5 · 104 with a concomitant reduction in the excited state lifetime (s = 32 ns) in a deoxygenated acetonitrile solution. The emission from the Ru(dp) ! dpp(p*) 3MLCT state of [(PTZpbpy)2Ru(dpp)](PF6)2 is quenched by intramolecular electron transfer affording [(PTZ+pbpy)2Ru(dpp)](PF6)2 leading to the population of the lower-lying PTZ ! dpp charge separated state (PTZ-dpp 3CS), Fig. 4. In addition, a slight shift to lower energy was observed in the Ru(dp) ! dpp(p*) 3 MLCT emission profile of [(PTZpbpy)2Ru(dpp)](PF6)2, suggesting a stabilization of the 3MLCT with respect to the model complex. Assuming kr and knr are the same for [(bpy)2Ru(dpp)](PF6)2 and [(PTZpbpy)2Ru(dpp)](PF6)2, efficient electron transfer is predicted for population of the 3CS state of [(PTZpbpy)2Ru(dpp)](PF6)2, ket = 2.9 · 107 s1. This rate of electron transfer from PTZ to the excited ruthenium chromophore is comparable to that seen for electron transfer from the PTZ to the osmium core in [Os{(l-2,3-dpp)Ru(PTZmbpy)2}3]8+ reported recently 1 Uem of [(bpy)2Ru(dpp)](PF6)2 was measured vs. [Ru(bpy)3](PF6)2 Uem = 0.062, using our conditions.
1
kisc
MLCT
hv E 1
GS
kr
3
MLCT
ket
PTZ-dpp 3CS
knr knr'
Fig. 4. Energy state diagram for [(PTZpbpy)2Ru(dpp)](PF6)2 (PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
(>5 · 108 s1) [22] and the rate of electron transfer from PTZ to an excited ruthenium chromophore of the tris-heteroleptic diad, [(PTZmbpy)Ru(bpy)(DMB)]2+, (2.5 · 107 s1) [19]. In contrast to [(PTZpbpy)2Ru(dpp)](PF6)2, in [Os{(l-2,3-dpp)Ru(PTZmbpy)2}3]8+ and [(PTZmbpy)Ru(bpy)(DMB)]2+, the PTZ is covalently attached to the terminal ligand via a methylene linkage consisting of only a single methylene unit. The ket in [(PTZpbpy)2Ru(dpp)](PF6)2 also compares well with ket = 4.5 · 106 s1 reported for [(PTZpbpy)2Ru(DMB)](PF6)2 [13]. At 77 K, an intense structured emission centered at 637 nm was observed for [(PTZpbpy)2Ru(dpp)](PF6)2 in an ethanol/methanol glass displaying a increased excited state lifetime (s = 5 ls), comparable to the s = 4.2 ls reported for [(bpy)2Ru(dpp)](PF6)2 at 77 K [27] consistent with intramolecular electron transfer being responsible for the decreased 3MLCT emission and lifetime at room temperature. The donor–chromophore complex, [(PTZpbpy)2Ru(dpp)](PF6)2, incorporating a phenothiazine electron donor and the polyazine bridging ligand, dpp, has been synthesized and characterized. In the coupled electron donor system, the emission intensity was quenched by 94% of that of the analogous system which lacks a coupled electron donor. The reduced emission intensity suggests that this donor–chromophore complex is very efficient in generating a charge separated state that can be exploited in solar energy conversion schemes. The dpp ligand of these new chromophores provides a means to expand this molecular
S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163
architecture generating supramolecular complexes with coupled electron donors [28]. Acknowledgements Acknowledgment is made to the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, US Department of Energy for their generous support of our research. Appendix A. Supplementary material Detailed experimental information are provided. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2007.06.019. References [1] D. Gust, T.A. Moore, Science 244 (1989) 35. [2] M.R. Wasielewski, M.P. Niemczyk, W.A. Svec, E.B. Pewitt, J. Am. Chem. Soc. 107 (1985) 5562. [3] E. Danielson, C.M. Elliott, J.W. Merkert, T.J. Meyer, J. Am. Chem. Soc. 109 (1987) 2519. [4] L.F. Cooley, S.L. Larson, C.M. Elliott, D.F. Kelley, J. Phys. Chem. 95 (1991) 10694. [5] K.A. Opperman, S.L. Mecklenburg, T.J. Meyer, Inorg. Chem. 33 (1994) 5295. [6] S.L. Larson, C.M. Elliott, D.F. Kelley, J. Phys. Chem. 99 (1995) 6530. [7] J.A. Treadway, P. Chen, T.J. Rutherford, F.R. Keene, T.J. Meyer, J. Phys. Chem. A 101 (1997) 6824. [8] C.A. Slate, D.R. Striplin, J.A. Moss, P. Chen, B.W. Erickson, T.J. Meyer, J. Am. Chem. Soc. 120 (1998) 4885. [9] T. Klumpp, M. Linsenmann, S.L. Larson, B.R. Limoges, D. Bu1rssner, E.B. Krissinel, C.M. Elliott, U.E. Steiner, J. Am. Chem. Soc. 121 (1999) 1076. [10] K.A. Maxwell, M. Sykora, J.M. DeSimone, T.J. Meyer, Inorg. Chem. 39 (2000) 71. [11] S. Chakraborty, T.J. Wadas, H. Hester, R. Schmehl, R. Eisenberg, Inorg. Chem. 44 (2005) 6865. [12] J.-P. Co1lin, S. Guillerez, J.-P. Sauvage, F. Barigelletti, L. De Cola, L. Flamigni, V. Balzani, Inorg. Chem. 30 (1991) 4230.
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[13] S.L. Larson, C.M. Elliott, D.F. Kelley, Inorg. Chem. 35 (1996) 2070. [14] S. Fanni, T.E. Keyes, S. Campagna, J.G. Vos, Inorg. Chem. 37 (1998) 5933. [15] J.E. McGarrah, R. Eisenberg, Inorg. Chem. 42 (2003) 4355. [16] V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, Chem. Rev. 96 (1996) 759–834. [17] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. Von Zelewesky, Coord. Chem. Rev. 84 (1988) 85. [18] R. Duesing, G. Tapolsky, T.J. Meyer, J. Am. Chem. Soc. 112 (1990) 5378. [19] M. Borgstro¨m, O. Johansson, R. Lomoth, H.B. Baudin, S. Wallin, L. ˚ kermark, L. Hammarstro¨m, Inorg. Chem. 42 (2003) 5173. Sun, B. A [20] S. Campagna, C. Di Pietro, F. Loiseau, B. Maubert, N. McClenaghan, R. Passalacqua, F. Puntoriero, V. Ricevuto, S. Serroni, Coord. Chem. Rev. 229 (2002) 67. [21] M. Elvington, K.J. Brewer, Inorg. Chem. 45 (2006) 5242. [22] F. Puntoriero, F. Nastasia, M. Cavazzini, S. Quici, S. Campagna, Coord. Chem. Rev. 251 (2007) 536. [23] [(PTZpbpy)2Ru(dpp)](PF6)2: A 100 mL round bottom flask was charged with [(PTZpbpy)2RuCl2] (0.214 g, 0.204 mmol), dpp (0.070 g, 0.30 mmol), and a 2:1 mixture of ethanol/water (36 mL). The resultant mixture was refluxed for 3 h in the dark. The solution turned from dark purple to brown during this time. Upon cooling, the brown solution was added to a saturated aqueous solution of KPF6. The brown crude product was filtered and washed with water and ethanol and dried in a vacuum desiccator. The crude product was then dissolved in minimal acetonitrile and flash precipitated into excess diethyl ether. The product was recovered by vacuum filtration and chromatographed on a methanol deactivated adsorption alumina column with 3:2 acetonitrile/toluene eluent. The combined fractions (tested by TLC) were rotary evaporated, taken into a minimal amount of acetonitrile, flash precipitated into diethyl ether, and collected by vacuum filtration to give (0.122 g, 40%) of pure [(PTZpbpy)2Ru(dpp)](PF6)2 as an orange–brown solid. FAB-MS ion (m/z; relative abundance): [MPF6]+ (1355, 65); [M2PF6]+ (1210, 100); [MPF6PTZH]+ (1156, 65); [M2PF6PTZH]+ (1011, 90). [24] K.J. Brewer, W.R. Murphy, S.R. Spurlin, J.D. Petersen, Inorg. Chem. 25 (1986) 882. [25] C.H. Braunstein, A.D. Baker, T.C. Strekas, H.D. Gafney, Inorg. Chem. 23 (1984) 857. [26] J.V. Caspar, T.J. Meyer, J. Am. Chem. Soc. 105 (1983) 5583. [27] G. Denti, S. Campagna, L. Sabatino, S. Serroni, M. Ciano, V. Balzani, Inorg. Chem. 29 (1990) 4750. [28] S.M. Arachchige, K.J. Brewer, in preparation.
A donor–chromophore complex containing the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine Shamindri M. Arachchige, Karen J. Brewer
*
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212, United States Received 27 April 2007; accepted 19 June 2007 Available online 29 June 2007
Abstract The complex [(PTZpbpy)2Ru(dpp)](PF6)2 coupling a phenothiazine derived electron donor to a ruthenium complex with a polyazine bridging ligand dpp (PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) has been synthesized and the redox, spectroscopic, and excited state properties elucidated. In the coupled electron donor system, the emission intensity from the Ru(dp) ! dpp(p*) 3MLCT state is quenched by 94% of that of the analogous system [(bpy)2Ru(dpp)](PF6)2 (bpy = 2,2 0 -bipyridine) which lacks a coupled electron donor. Coupling a donor–chromophore unit to a bridging ligand will allow the expansion of the molecular architecture to form complex molecular assemblies which contain covalently attached electron donors. 2007 Elsevier B.V. All rights reserved. Keywords: Donor–chromophore complexes; MLCT; Ru light absorbers; Phenothiazine; Electron donor; Polyazine ligand; Bridging ligand; Emission quenching
Photoinduced charge separation is a key component in many schemes for conversion of solar energy to chemical energy. In applying charge transfer states, extension of the excited state lifetime of the charge separated state is desired for efficient chemical energy transformations. Molecular diads and triads have been designed to covalently attach light absorbing chromophores, electron donors, and/or electron acceptors into a single molecular architecture to generate relatively long lived photoinduced charge separated states [1–6]. Triads are constructed to systematically promote sequential electron transfer events affording charge separated states which store light energy as chemical energy [1–12]. Diad systems support the study of electron transfer events leading to the charge separated states [11–15]. Transition metal–polyazine complexes are often included in these multicomponent molecular architectures as light absorbers [16,17]. In these systems, the photochemistry is initiated by metal-to-ligand charge *
Corresponding author. Tel.: +1 540 231 6579; fax: +1 540 231 3255. E-mail address: [email protected] (K.J. Brewer).
1387-7003/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2007.06.019
transfer (MLCT) excitation. Many of the electron transfer studies have focused on triad systems which incorporate covalently attached phenothiazine (PTZ) electron donors and various electron acceptors to ruthenium polypyridyl based chromophores [3–6]. In these systems, the MLCT state is initially generated and then quenched by electron transfer from the PTZ to the metal to form charge separated states. Molecular architectures that couple donor– chromophore systems to bridging ligands have fundamental significance in the construction of complex molecular devices for applications in solar energy conversion schemes. The bridging ligand provides a point of attachment between the donor–chromophore unit and an electron acceptor, allowing the expansion of the molecular assembly. Despite the potential significance, this area of research remains largely unexplored. In the bimetallic system, [(PTZmbpy)(CO)3ReI(4,4 0 -bpy)ReI(CO)3(bpz)](PF6)2 (4,4 0 -bpy = 4,4 0 -bipyridine; bpz = 2,2 0 -bipyrazine, PTZmbpy = 4-phenothiazinyl-4 0 -methyl-2,2 0 -bipyridine; m in PTZmbpy denotes a methylene unit), a long-range photoinduced charge separation across the ligand bridge afforded
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S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163
N
N
S N N
N
N
N
PTZpbpy
dpp
Fig. 1. Structures of two polyazine ligands.
½ðPTZþ mbpyÞðCOÞ3 ReI ð4; 40 -bpyÞReI ðCOÞ3 ðbpz-ÞðPF6 Þ2 [18]. In the triad system, PTZmbpy–RuII–BQ (BQ = benzoquinone acceptor with a distal bipyridyl unit that can act as a bridge), a fairly long lived charge separated state between the donor and acceptor was observed [19]. However, this triad system proved to be photolabile, discouraging further expansion of the molecular assembly through the bridge. In this study, we report the synthesis and characterization of a donor–chromophore complex containing a bridging ligand, of the form [(PTZpbpy)2Ru(dpp)](PF6)2(PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine; p in PTZpbpy denotes a pentyl unit, dpp = 2,3-bis(2-pyridyl)pyrazine), Fig. 1. The terminal ligand used is PTZpbpy in which the electron donor PTZ
is covalently attached to the terminal ligand 4,4 0 dimethyl-2,2 0 -bipyridine (DMB) via a methylene linkage consisting of five methylene units, Fig. 1. Phenothiazine and its derivates are efficient intramolecular reductive quenchers [3–6]. The bridging ligand used in this study is dpp, Fig. 1. This bridging ligand has been extensively studied in the development of molecular assemblies that have emissive MLCT states capable of undergoing intramolecular energy and electron transfer [17,20,21]. Recent studies of a new tetranuclear complex, [Os{(l-2,3-dpp)Ru(PTZmbpy)2}3]8+, in which the [Os{(l-2,3-dpp)Ru(bpy)2}3]8+ chromophore is coupled to PTZ electron donors, demonstrated efficient electron transfer by the peripheral PTZ donors to the Os(II) core, despite the presence of the intermediate Ru(II) centers [22]. This system attached [(PTZmbpy)2RuCl2] subunit to the [Os(dpp)3]2+ core with the preparation of the [Os{(l-2,3dpp)Ru(PTZmbpy)2}3]8+ electron donor–chromophore assembly. In the present system under investigation, the arrangement of PTZpbpy and dpp on a single ruthenium chromophore to form [(PTZpbpy)2Ru(dpp)](PF6)2 would allow the exploitation of the unique properties of both ligands and the preparation of the bridging ligand complex would allow the incorporation of this assembly in a wide variety of polymetallic architectures. The presence of an electron donor and the bridging ligand within the same chromophore unit would allow the study of complex
Scheme 1. Synthetic scheme outlining the building block approach to synthesizing [(PTZpbpy)2Ru(dpp)](PF6)2 (PTZpbpy = 4-methyl-4 0 -(4-(Nphenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163
photochemical molecular devices with long-lived charge separated states. The synthesis of [(PTZpbpy)2Ru(dpp)](PF6)2 used a building block approach, Scheme 1. The terminal ligand, PTZpbpy, was synthesized as previously described [6]. The precursor to PTZpbpy, which is 4-(bromopentyl)-4 0 methyl-2,2 0 -bipyridine was purified using a slight modification to the literature procedure. The precursor, 4-(bromopentyl)-4 0 -methyl-2,2 0 -bipyridine, was easily separated from the excess 1,4-dibromobutane in the reaction medium in the form of the acid chloride salt. The acid chloride salt was neutralized by the dropwise addition to a stirring biphasic solution of dichloromethane/Na2CO3(aq) (0.5 M). Neutral 4-(bromopentyl)-4 0 -methyl-2,2 0 -bipyridine was extracted into the organic layer. The organic extract was dried over Na2SO4 and the volatile components were removed by rotary evaporation. The product 4-(bromopentyl)-4 0 -methyl-2,2 0 -bipyridine was purified by column chromatography on silica gel prior to use for the synthesis of PTZpbpy [6]. The monometallic precursor [(PTZpbpy)2RuCl2] was synthesized using previously published methods [6]. Refluxing [(PTZpbpy)2RuCl2] and excess dpp in a mixture of EtOH/H2O in the dark for 3 h followed by purification by column chromatography using methanol-deactivated adsorption alumina afforded [(PTZpbpy)2Ru(dpp)](PF6)2 as an orange–brown solid in moderate yield (40%) [23]. Fast atom bombardment-mass spectrometry analysis of [(PTZpbpy)2Ru(dpp)](PF6)2 was consistent with its formulation, (m/z; relative abundance): [MPF6]+ (1355, 65); [M2PF6]+ (1210, 100); [MPF6PTZH]+ (1156, 65); [M2PF6PTZH]+ (1011, 90). The electrochemical properties of [(PTZpbpy)2Ru(dpp)](PF6)2 were assigned by comparison to model systems: the terminal ligand, PTZpbpy, and the bpy analog,
Fig. 2. Cyclic voltammograms of (a) PTZpbpy and (b) [(PTZpbpy)2Ru(dpp)](PF6)2 in 0.1 M n-Bu4NPF6 in CH3CN using glassy carbon working electrode and a Ag/AgCl reference electrode (PTZpbpy = 4-methyl-4 0 -(4(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
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Table 1 Electrochemical data for PTZpbpy, [(PTZpbpy)2Ru(dpp)](PF6)2, and the related model system [(bpy)2Ru(dpp)](PF6)2 (PTZpbpy = 4-methyl4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) Complex
E1/2a (V)
Assignments
PTZpbpy
0.73 1.49 0.74 1.47b 1.03 1.50 1.31 1.06 1.50
PTZ0/+ PTZ+/2+ PTZ0/+ RuII/III, PTZ+/2+ dpp0/ bpy0/ RuII/III dpp0/ bpy0/
[(PTZpbpy)2Ru(dpp)](PF6)2
[(bpy)2Ru(dpp)](PF6)2c
a Potentials reported versus the Ag/AgCl (0.29 V) reference electrode in 0.1 M Bu4NPF6 in CH3CN. b Eap value. c Ref. [24]. Reported in V vs. SCE.
[(bpy)2Ru(dpp)](PF6)2, which lacks the PTZ donors. Fig. 2 illustrates the cyclic voltammograms of [(PTZpbpy)2Ru(dpp)](PF6)2 and PTZpbpy. Table 1 summarizes the electrochemical data for PTZpbpy, [(PTZpbpy)2Ru(dpp)](PF6)2, and [(bpy)2Ru(dpp)](PF6)2. The oxidative couples of [(PTZpbpy)2Ru(dpp)](PF6)2 were examined using a sequential increase of the switching potential. The oxidative electrochemistry of [(PTZpbpy)2Ru(dpp)](PF6)2 is dominated by PTZ based processes. The first reversible oxidation at E1/2 = 0.74 V corresponds to PTZ0/+ and occurs at the same potential as the free PTZpbpy PTZ0/+ couple. Increasing the switching potential to 2 V shows an oxidation at 1.47 V. The oxidation at 1.47 V represents the reversible RuII/III couple as a shoulder on the larger irreversible PTZ+/2+ couple. The potential of this PTZ+/2+ couple also corresponds to that of the PTZ+/2+ couple in free PTZpbpy, indicating coordination to the metal does not dramatically impact PTZ redox potentials. The reversible reductive couples at E1/2 = 1.03 and 1.50 V, corresponds to the reduction of dpp0/ and bpy0/, respectively. The dpp0/ and bpy0/ couples are comparable to the reductive couples observed for [(bpy)2Ru(dpp)](PF6)2. The facile oxidation of PTZ and reduction and of dpp suggest the feasibility of the formation of [(PTZ+pbpy)2Ru(dpp)](PF6)2 charge separated state in this complex. The electronic absorption spectrum of [(PTZpbpy)2Ru(dpp)](PF6)2 is illustrated in Fig. 3. The UV region of the spectrum is dominated by PTZpbpy and dpp ligand based p–p* transitions, with the dpp based transitions occurring as low energy shoulders. Two overlapping 1MLCT absorption bands appear in the visible region at 430 nm (e = 1.3 · 104 M1 cm1) and 480 nm (1.2 · 104 M1 cm1). The lowest energy transition is dpp based, Ru(dp) ! dpp(p*) charge transfer (CT), characteristic of dpp systems and in agreement with the electrochemical properties. For comparison, the electronic absorption spectrum of [(bpy)2Ru(dpp)](PF6)2 is characterized by intense bpy (290 nm) and dpp (320 nm (sh)) based p–p* transitions in
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S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163 Electronic Absorption Spectrum RoomTemperature Emission 77 K emission
10
-4
8
Intensity
-1
-1
ε x 10 (M cm )
12
6 4 2 0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 3. Spectroscopic data for [(PTZpbpy)2Ru(dpp)](PF6). The electronic absorption and RT emission data collected in deoxygenated acetonitrile solution at room temperature. The 77 K emission spectrum was collected in 4:1 EtOH/MeOH glass. Emission spectra are normalized and correct for PMT response (PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
the UV region and 1MLCT transitions in the visible region [25]. The [(PTZpbpy)2Ru(dpp)](PF6)2 displays more intensity at 258 nm (e = 7.8 · 104 M1 cm1), characteristic of the PTZ unit. Similar to [(PTZpbpy)2Ru(dpp)](PF6)2, the visible region of [(bpy)2Ru(dpp)](PF6)2 consists of overlapping absorption bands at 430 nm and 470 nm (e = 1.2 · 104 M1 cm1) [25]. The lowest energy Ru(dp) ! dpp(p*) CT transition at 480 nm for [(PTZpbpy)2Ru(dpp)](PF6)2 is slightly red shifted compared to the corresponding 470 nm transition for [(bpy)2Ru(dpp)](PF6)2. Emission spectroscopy was used to probe the excited state properties of [(PTZpbpy)2Ru(dpp)](PF6)2. This complex displays a weak emission from the Ru(dp) ! dpp(p*) 3 MLCT at 694 nm at room temperature in acetonitrile solution, Fig. 3. Compared to [(bpy)2Ru(dpp)](PF6)2, the em kem = 0.010 max ¼ 675 nm [25], emission quantum yield U 1 [26], excited state lifetime s = 380 ns [27], the emission from the 3MLCT state of [(PTZpbpy)2Ru(dpp)](PF6)2 is reduced by 94%, Uem = 5.5 · 104 with a concomitant reduction in the excited state lifetime (s = 32 ns) in a deoxygenated acetonitrile solution. The emission from the Ru(dp) ! dpp(p*) 3MLCT state of [(PTZpbpy)2Ru(dpp)](PF6)2 is quenched by intramolecular electron transfer affording [(PTZ+pbpy)2Ru(dpp)](PF6)2 leading to the population of the lower-lying PTZ ! dpp charge separated state (PTZ-dpp 3CS), Fig. 4. In addition, a slight shift to lower energy was observed in the Ru(dp) ! dpp(p*) 3 MLCT emission profile of [(PTZpbpy)2Ru(dpp)](PF6)2, suggesting a stabilization of the 3MLCT with respect to the model complex. Assuming kr and knr are the same for [(bpy)2Ru(dpp)](PF6)2 and [(PTZpbpy)2Ru(dpp)](PF6)2, efficient electron transfer is predicted for population of the 3CS state of [(PTZpbpy)2Ru(dpp)](PF6)2, ket = 2.9 · 107 s1. This rate of electron transfer from PTZ to the excited ruthenium chromophore is comparable to that seen for electron transfer from the PTZ to the osmium core in [Os{(l-2,3-dpp)Ru(PTZmbpy)2}3]8+ reported recently 1 Uem of [(bpy)2Ru(dpp)](PF6)2 was measured vs. [Ru(bpy)3](PF6)2 Uem = 0.062, using our conditions.
1
kisc
MLCT
hv E 1
GS
kr
3
MLCT
ket
PTZ-dpp 3CS
knr knr'
Fig. 4. Energy state diagram for [(PTZpbpy)2Ru(dpp)](PF6)2 (PTZpbpy = 4-methyl-4 0 -(4-(N-phenothiazinato)pentyl)-2,2 0 -bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).
(>5 · 108 s1) [22] and the rate of electron transfer from PTZ to an excited ruthenium chromophore of the tris-heteroleptic diad, [(PTZmbpy)Ru(bpy)(DMB)]2+, (2.5 · 107 s1) [19]. In contrast to [(PTZpbpy)2Ru(dpp)](PF6)2, in [Os{(l-2,3-dpp)Ru(PTZmbpy)2}3]8+ and [(PTZmbpy)Ru(bpy)(DMB)]2+, the PTZ is covalently attached to the terminal ligand via a methylene linkage consisting of only a single methylene unit. The ket in [(PTZpbpy)2Ru(dpp)](PF6)2 also compares well with ket = 4.5 · 106 s1 reported for [(PTZpbpy)2Ru(DMB)](PF6)2 [13]. At 77 K, an intense structured emission centered at 637 nm was observed for [(PTZpbpy)2Ru(dpp)](PF6)2 in an ethanol/methanol glass displaying a increased excited state lifetime (s = 5 ls), comparable to the s = 4.2 ls reported for [(bpy)2Ru(dpp)](PF6)2 at 77 K [27] consistent with intramolecular electron transfer being responsible for the decreased 3MLCT emission and lifetime at room temperature. The donor–chromophore complex, [(PTZpbpy)2Ru(dpp)](PF6)2, incorporating a phenothiazine electron donor and the polyazine bridging ligand, dpp, has been synthesized and characterized. In the coupled electron donor system, the emission intensity was quenched by 94% of that of the analogous system which lacks a coupled electron donor. The reduced emission intensity suggests that this donor–chromophore complex is very efficient in generating a charge separated state that can be exploited in solar energy conversion schemes. The dpp ligand of these new chromophores provides a means to expand this molecular
S.M. Arachchige, K.J. Brewer / Inorganic Chemistry Communications 10 (2007) 1159–1163
architecture generating supramolecular complexes with coupled electron donors [28]. Acknowledgements Acknowledgment is made to the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, US Department of Energy for their generous support of our research. Appendix A. Supplementary material Detailed experimental information are provided. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2007.06.019. References [1] D. Gust, T.A. Moore, Science 244 (1989) 35. [2] M.R. Wasielewski, M.P. Niemczyk, W.A. Svec, E.B. Pewitt, J. Am. Chem. Soc. 107 (1985) 5562. [3] E. Danielson, C.M. Elliott, J.W. Merkert, T.J. Meyer, J. Am. Chem. Soc. 109 (1987) 2519. [4] L.F. Cooley, S.L. Larson, C.M. Elliott, D.F. Kelley, J. Phys. Chem. 95 (1991) 10694. [5] K.A. Opperman, S.L. Mecklenburg, T.J. Meyer, Inorg. Chem. 33 (1994) 5295. [6] S.L. Larson, C.M. Elliott, D.F. Kelley, J. Phys. Chem. 99 (1995) 6530. [7] J.A. Treadway, P. Chen, T.J. Rutherford, F.R. Keene, T.J. Meyer, J. Phys. Chem. A 101 (1997) 6824. [8] C.A. Slate, D.R. Striplin, J.A. Moss, P. Chen, B.W. Erickson, T.J. Meyer, J. Am. Chem. Soc. 120 (1998) 4885. [9] T. Klumpp, M. Linsenmann, S.L. Larson, B.R. Limoges, D. Bu1rssner, E.B. Krissinel, C.M. Elliott, U.E. Steiner, J. Am. Chem. Soc. 121 (1999) 1076. [10] K.A. Maxwell, M. Sykora, J.M. DeSimone, T.J. Meyer, Inorg. Chem. 39 (2000) 71. [11] S. Chakraborty, T.J. Wadas, H. Hester, R. Schmehl, R. Eisenberg, Inorg. Chem. 44 (2005) 6865. [12] J.-P. Co1lin, S. Guillerez, J.-P. Sauvage, F. Barigelletti, L. De Cola, L. Flamigni, V. Balzani, Inorg. Chem. 30 (1991) 4230.
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