Synthesis and Photophysical Properties of Bipyridine-Extended

Organometallics 2010, 29, 3289–3297 DOI: 10.1021/om100421w

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Synthesis and Photophysical Properties of Bipyridine-Extended Dithienophosphole Chromophores for Transition Metal Complexation Dong Ren Bai and Thomas Baumgartner* Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada Received May 4, 2010

The synthesis and photophysical properties of a series of 5-(2,20 -bipyridyl)acetylene-extended dithieno[3,2-b:20 ,30 -d ]phospholes involving one and two of the latter units are reported. Their molecular scaffolds were found to show limited solubility that could, however, be addressed with the installation of solubilizing groups at the bipyridine unit or the dithienophosphole scaffold, respectively. The photoluminescence features of the new π-conjugated oligomers could be manipulated through complexation to a variety transition metal species (Zn, Pt, and Ru), resulting in polarizable systems with intramolecular charge transfer and/or phosphorescence features, or redox-switching. Introduction The development of organic π-conjugated materials is a flourishing field of research, particularly with a view to utilizing these materials as chromophores and/or light-harvesting building blocks for optoelectronics such as organic light-emitting diodes or organic photovoltaics.1 Since a large variety of purely organic materials have already proven their great value for such applications, the use of organo-main group components has recently provided a new angle toward π-conjugated materials.2-4 The chemical and electronic nature of the main group component provides some intriguing features that create a variety of tuning possibilities that are not accessible in such simplicity with purely organic systems. Particularly organoboron,2 -silicon,3 and -phosphorus4 compounds have thus attracted an increasing amount of attention in this context. The main group functional groups allow for effective overlap of their orbitals (B: p; Si, P: σ*) with the π-conjugated framework (π, π*), thus creating some peculiar electronics and overall enhanced photophysical fea*To whom correspondence should be addressed. E-mail: Thomas. [email protected]. (1) (a) Handbook of Conducting Polymers, 3rd ed.; Skotheim, T. A.; Reynolds, J. R., Eds.; CRC-Press: Boca Raton, 2007. (b) Organic Light Emitting Devices; M€ ullen, K.; Scherf, U., Eds.; Wiley-VCH: Weinheim, 2006. (c) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (d) G€ unes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. (2) (a) Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2009, 87, 8. (b) Elbing, M.; Bazan, G. C. Angew. Chem., Int. Ed. 2008, 47, 834. (c) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574. (d) J€akle, F. Coord. Chem. Rev. 2006, 250, 1107. (e) Hudson, Z. M.; Wang, S. Acc. Chem. Res. 2009, 42, 1584. (3) (a) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc. 2003, 125, 13662. (b) Xu, C.; Wakamiya, A.; Yamaguchi, S. J. Am. Chem. Soc. 2005, 127, 1638. (c) Mouri, K.; Wakamiya, A.; Yamada, H.; Kajiwara, T.; Yamaguchi, S. Org. Lett. 2007, 9, 93. (d) Biaso, F.; Geoffroy, M.; Canadell, E.; Auban-Senzier, P.; Levillain, E.; Fourmigue, M.; Avarvari, M. Chem.Eur. J. 2007, 13, 5394. (e) Shimizu, M.; Tatsumi, H.; Mochida, K.; Oda, K.; Hiyama, T. Chem.-Asian J. 2008, 3, 1238. (4) (a) Baumgartner, T.; Reau, R. Chem. Rev. 2006, 107, 4681 (correction: Chem. Rev. 2007, 108, 303). (b) Hobbs, M. G.; Baumgartner, T. Eur. J. Inorg. Chem. 2007, 3611. (c) Matano, Y.; Imahori, H. Org. Biomol. Chem. 2009, 7, 1258. (d) Crassous, J.; Reau, R. Dalton Trans. 2008, 6865. (e) Fukazawa, A.; Yamaguchi, S. Chem.-Asian J. 2009, 4, 1386. (f ) Fukazawa, A.; Yamada, H.; Sasaki, Y.; Akiyama, S.; Yamaguchi, S. Chem.-Asian J. 2010, 5, 466. (g) Washington, M. P.; Gudimetla, V. B.; Laughlin, F. L.; Deligonul, N.; He, S.; Payton, J. L.; Simpson, M. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2010, 132, 4566. r 2010 American Chemical Society

tures for the materials as a whole. We were able to illustrate the benefits of using organophosphorus components in πconjugated materials by means of the dithieno[3,2-b:20 ,30 -d ]phosphole moiety (I, Chart 1), which we have introduced in this context several years ago.5 This unit exhibits some useful photophysical properties (high quantum efficiency, easy color fine-tuning) and provides flexible options toward derivatization, which makes it an excellent chromophore for a variety of applications.6 To further enhance the electronic properties and the resulting performance of π-conjugated materials in optoelectronic applications, researchers have recently also started to combine the excellent photophysical (light-harvesting as well as emission) properties of organic chromophores with those of transition metal complexes to access materials that exhibit beneficial photoinduced energy/electron transfer processes that may be useful for solar energy conversion or light-emitting processes.7 As part of our ongoing metal chelation-based supramolecular investigations, our initial study combining the excellent photophysical properties of dithienophospholes with transition metal moieties allowed us to furnish the dithienophosphole scaffold with terpyridine (tpy) moieties to create highly luminescent ligands (II) that were complexed with transition (5) (a) Baumgartner, T.; Neumann, T.; Wirges, B. Angew. Chem., Int. Ed. 2004, 43, 6197. (b) Baumgartner, T.; Bergmans, W.; Karpati, T.; Neumann, T.; Nieger, M.; Nyulaszi, L. Chem.-Eur. J. 2005, 11, 4687. (c) Durben, S.; Dienes, Y.; Baumgartner, T. Org. Lett. 2006, 8, 5893. (d) Dienes, Y.; Durben, S.; Karpati, T.; Neumann, T.; Englert, U.; Nyulaszi, L.; Baumgartner, T. Chem.-Eur. J. 2007, 13, 7487. (6) (a) Neumann, T.; Dienes, Y.; Baumgartner, T. Org. Lett. 2006, 8, 495. (b) Dienes, Y.; Eggenstein, M.; Karpati, T.; Sutherland, T. C.; Nyulaszi, L.; Baumgartner, T. Chem.-Eur. J. 2008, 14, 9878. (c) Durben, S.; Nickel, D.; Krueger, R. A.; Baumgartner, T. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 817. (d) Ren, Y.; Dienes, Y.; Hettel, S.; Parvez, M.; Hoge, B.; Baumgartner, T. Organometallics 2009, 28, 734. (e) Romero-Nieto, C.; Merino, S.; RodríguezLopez, J.; Baumgartner, T. Chem.-Eur. J. 2009, 15, 4135. (f ) Romero-Nieto, C.; Durben, S.; Kormos, I. M.; Baumgartner, T. Adv. Funct. Mater. 2009, 19, 3625. (7) (a) Barbieri, A.; Accorsi, G.; Armaroli, N. Chem. Commun. 2008, 2185. (b) Binnemans, K. Chem. Rev. 2009, 109, 4283. (c) Lo, S.-C.; Burn, P. L. Chem. Rev. 2007, 107, 1097. (d) Burn, P. L.; Lo, S.-C.; Samuel, I. D. W. Adv. Mater. 2007, 19, 1675. (e) You, Y.; Park, S. Y. Dalton Trans. 2009, 1267. (f ) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker, J. B.; Bernhard, S.; Abru~na, H. D.; Malliaras, G. G. J. Mater. Chem. 2007, 17, 2976. Published on Web 06/30/2010

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Organometallics, Vol. 29, No. 15, 2010 Chart 1

Bai and Baumgartner Scheme 2. Synthesis of the Solubilized 50 -Bipyridine Acetylene 7

Scheme 3. Synthesis of the Solubilized Chromophores 8 and 10 Scheme 1. Synthesis of Bipyridine-Functionalized Dithienophospholes 2 and 3

metal species.8 In this contribution we now report our structureproperty studies of dithienophosphole-functionalized 2,20 -bipyridines (bpy) and their behavior upon metal complexation. Our studies include the synthesis and photophysical characterization of bipyridines with one or two dithienophosphole moieties, as well as complexation of these π-conjugated oligomers to Zn2þ, Pt2þ, and Ru2þ species in order to investigate the electronic effects of the transition metal species.

Results and Discussion Synthesis and Characterization of Bipyridine-Functionalized Dithienophospholes. Following our previously published procedures for the attachment of the tpy functionality to the 2(6)-position of the dithienophosphole scaffold,8 the (bpy)-capped dithienophosphole (2) and the bis(dithienophosphole)-capped bpy derivative (3) were successfully synthesized via catalytic Heck-Cassar-Sonogashira cross-coupling protocol between 2-iodo-dithienophosphole oxide (1) and 5-bpyacetylene/5,50 -bpy-diacetylene in moderate yields (Scheme 1). However, these bpy derivatives were found to possess very poor solubility in common organic solvents; the products actually precipitated from the THF reaction mixture, which prevented any further functionalization, such as metal complexation, and limited their comprehensive characterization to photophysical studies. (8) Bai, D. R.; Romero-Nieto, C.; Baumgartner, T. Dalton Trans. 2010, 39, 1250.

In order to address the solubility issue, we then decided to furnish the 50 -position of the bpy-acetylene with a trimethylsilyl group for the monodithienophoshole system and the 6-position of the dithienophosphole scaffold with an octyl moiety for the bis(dithienophosphole) system. As shown in Scheme 2, the access to the 50 -trimethylsilyl-bpy-acetylene (7) involves a four-step process. First, 2-bromo-5-trimethylsilylpyridine (4) was converted into its 2-tributyltin counterpart (5) before undergoing Stille coupling with 2,5-dibromopyridine. The resulting 5-bromo50 -trimethylsilyl-2,20 -bipyridine (6) was then reacted with trimethylsilylacetylene to afford 5-ethynyl-50 -trimethylsilyl-2,20 -bipyridine (7) after deprotection; the choice of deprotection agent was found to have a strong influence on the conversion efficiency with K2CO3, giving 91% versus 30% by KF. The necessary 2-bromo-6-octyl-dithienophosphole (9) for the synthesis of the bis(dithienophosphole) oligomer was reported as part of our initial study involving the tpy-functionalized systems.8 The solubility-enhanced bpy-functionalized dithienophospholes 8 and 10 (Scheme 3) were then conveniently accessed after 48 h via Heck-Cassar-Sonogashira cross-coupling, in a typical protocol involving [Pd(PPh3)2Cl2], CuI, and diisopropylamine in THF at room temperature. The similar 31P NMR resonances of δ = 19.4 and 19.7 ppm for 8 and 10, respectively, correlate well with that of the tpy analogue IIb (Chart 1)8 and the nonsolubilized species 2 (δ=18.7 ppm) and 3 (δ=19.1 ppm), supporting a “pyridinyl”-acetylene-functionalized dithienophosphole core. The 1H and 13C NMR data are also consistent

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Table 1. Photophysical and Computational Data of the Ligands 2, 3, 8, and 10 and the Complexes 11-1510 compound

λmax [nm]a

λabs [nm]b

λem [nm]c

φPLd

HOMO [eV]e

LUMO [eV]e

HOMO-LUMO gap [eV]e

opt band gap [eV] f

2 3 8 10 11 12 13 14 15

404 417, 440(sh) 402 424 422 460 425, 460(sh) 477, 503(sh) 448

401 424 405 433 424 460

483 474 483 491 506 600

0.06 0.20 0.08 0.12

-5.49 -5.38 -5.47 -5.31

-2.33 -2.55 -2.32 -2.45

3.16 2.83 3.15 2.86

-6.03

-3.53

2.50

-5.89

-3.53

2.36

2.72 2.59 2.67 2.61 2.59 2.30 2.53 2.27 2.21

a UV-vis in CH2Cl2. b λmax for excitation in CH2Cl2. c λmax for emission in CH2Cl2. d Relative to fluorescein (2 M H2SO4 soultion). e B3LYP/6-31G(d) or B3LYP/LanL2DZ level of theory. f Obtained from the low-energy absorption onset of the UV-vis spectra.

with the targeted structures for 8 and 10 (2 and 3). All four oligomers show maximum wavelengths of absorption at the blue edge of the optical spectrum (λmax = 402-424 nm) with emission in the blue-green region (λem = 474-491 nm). The photoluminescence data reported in Table 1 of all four ligands in solution reflect the extent of π-conjugation, with the bis(dithienophosphole) systems showing absorption values redshifted from those of the monodithienophosphole systems (see Figure 1). Although the occurrence of a vibronic sideband/ shoulder implies a more rigid molecular scaffold in 3, its emission features do not follow the same trend, showing a surprising blue shift of emission when compared to 2 and 8. The observed values of the extended systems 3 and 10 are, however, comparable to those of somewhat related, π-conjugated oligomers with two dithienophosphole end-caps that we have reported recently.9 The octyl group in 10 has a similar effect on the photophysical properties to that in the tpyfunctionalized monodithienophosphole (IIa,b), showing a red shift of Δλmax=8 nm and Δλem=17 nm upon alkylation (cf. IIa, b: Δλmax=13 nm and Δλem=17 nm). The photophysical data of 8, on the other hand, do not show any notable shift compared to those of 2. Surprisingly however, the photoluminescence quantum yields for all four extended π-conjugated materials are only low to moderate (φPL = 0.06-0.20), which is in stark contrast to the corresponding tpy-system, with quantum yields between φPL=0.55 and 0.79.8 The values are, however, in line with those of a bis(dithienophosphole)-capped oligomer with 4,40 -biphenyl linker (cf. φPL = 0.21).9 For a better understanding of the electronic and luminescence properties of the π-conjugated oligomers 2, 3, 8, and 100 ,10 DFT calculations were carried out at the B3LYP/631G(d) level of theory.11 As shown in the frontier orbitals (9) Durben, S.; Linder, T.; Baumgartner, T. New J. Chem. 2010, 34, doi: 10.1039/c0nj00026d. (10) The octyl group has been replaced with a methyl substituent to shorten the DFT calculation times. To account for this truncation, corresponding molecules are labeled 100 , 120 , and 140 . (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01; Gaussian Inc.: Wallingford, CT, 2007.

Figure 1. Optical spectra (top: absorption; middle: excitation; bottom: emission) of the bpy-dithienophospholes 2, 3, 8, and 10.

(HOMO and LUMO) of 2 (Figure 2), the bpy group adopts a twisted syn-conformation. Although obviously in conjugation with the main framework, the peripheral pyridyl ring

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Figure 2. Frontier orbitals of compounds 2, 3, 8, and 100 (B3LYP/6-31G(d) level of theory).

contributes only to a minor extent to the π-conjugation, resulting in a HOMO-LUMO gap of E = 3.16 eV. By comparison, 3 exhibits a similar frontier orbital profile to that of 2 (Figure 2). However, the HOMO-LUMO gap is reduced by 0.35 eV (E = 2.83 eV), due to the greater degree of conjugation involving the second dithienophosphole unit. In either case, the lowest electronic transition can be described as a π-π* transition extended over the terminal dithienophosphole(s), the triple bond(s), and the central bipyridine ring(s). The DFT calculation results also confirmed the inclusion of a triple bond as a good π-spacer to ensure effective electronic coupling between the dithienophosphole and bpy units. Attachment of a trimethylsilyl group to the bpy unit in 8 (E = 3.15 eV) and octyl functionalization10 of a dithienophosphole ring in 100 (E = 2.86 eV), respectively, revealed an only minimal effect on the HOMO/LUMO energy levels (ΔE(2,8) = 0.01 eV, ΔE(3,100 ) = 0.03 eV; Table 1). Although the absolute values for the calculated HOMO-LUMO gaps for 2, 3, 8, and 100 are slightly different from the optical band gaps obtained through UV spectroscopy (low-energy absorption edge), their trends are in excellent agreement with the experimental values. It should be noted in this context that only 3 shows a fully coplanar anti-conformation of the bipyridine unit, whereas the other three compounds were minimized to structures with twisted syn-arrangement. Due to the similar absolute HOMO-LUMO gap values for 3 and 100 , it is therefore plausible that the activation barrier for rotation is low and both conformations represent equivalent minima. Consequently, the twisting of the bipyridine unit could explain the

observed low quantum yields, by increasing the probability for nonradiative relaxation pathways.12 Synthesis and Characterization of Metal Complexes. To investigate if the photophysical properties of the new bpyfunctionalized dithienophospholes can be further tuned through metal coordination, we have synthesized a series of transition metal complexes with the solubilized ligands 8 and 10. To account for mainly geometric parameter changes to the scaffold, we have targeted the corresponding Zn2+ complexes, whereas the targeted Pt2+ and Ru2+ complexes were expected to allow for the introduction of additional features, such as energy transfer, phosphorescence, or redox switching.7 With the two solubilized bpy-dithienophospholes in hand, the synthesis of the corresponding complexes was relatively straightforward (Scheme 4). The Zn2+ complexes 11 and 12 with tetrahedral geometry around Zn2+ were readily synthesized by the reaction of ZnCl2 and the corresponding bpy ligand at room temperature in dichloromethane. The reactions proceeded very fast, as evidenced by the immediate color and fluorescence change upon addition of ZnCl2. The observed red shifts in the absorption (see Figure 3) and emission features of 11 and 12 can generally be rationalized in terms of planarization of the molecular scaffold upon binding to ZnCl2.8 Whereas the effect is less pronounced in the monodithienophosphole complex 11 (Δλmax = 20 nm; (12) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006.

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Scheme 4. Synthesis of Complexes 11-15

Figure 3. Normalized absorption spectra of the complexes 11-15 in CH2Cl2.

Δλem = 21 nm), a surprisingly dramatic change is observed for the extended system 12, particularly for its emission (Δλmax =37 nm; Δλem =109 nm). In addition to the simple planarization, Zn complexation also appears to introduce additional electronic features in this latter case. Considering the electronics of the Zn-bpy moiety and the dithienophosphole oxides, a donor-acceptor-donor (DAD) architecture likely applies to 12 that allows for ligand-based intramolecular charge transfer (ICT) processes upon excitation/ relaxation.12,13 Typical ICT consists of charge redistribution from the donor to acceptor unit upon excitation. The inherent increase of the Stokes shift upon complexation of 10 to 12 (Δν=2728 f 3241 cm-1) supports a polarized structure in the excited state of 12. On the contrary, the excited state of complex 11 appears to be of lower polarity than that of the free ligand 8 (Δν = 4073 f 3822 cm-1), likely due to the peripheral location of the Zn-acceptor species, reducing the (13) Valeur, B. Molecular Fluorescence, Principles and Applications; Wiley-VCH: Weinheim, 2002.

probability of ICT. The latter data are comparable with those of the related tpy-monodithienophosphole (IIb) and its Zn complex (Δλabs =29 nm; Δλem =25 nm; Δν=3935 f 3422 cm-1) and support the planarization/rigidification as the main contributor to the shifted photophysics in 11.8 To confirm the ICT and the DAD architecture for 12, we have performed DFT calculations10 at the B3LYP/LanL2DZ level of theory11 that support the Zn-bpy subunit as an acceptor moiety. The LUMO has an increased electron density at this central component (Figure 4), whereas the HOMO evenly stretches over the whole molecular scaffold of the ligand, however, without notable participation of the ZnCl2 moiety. An ICT in dichloromethane was further supported by the solvatochromism that 12 exhibits. In contrast to the photoluminescence data in dichloromethane, excitation and emission wavelengths experience a dramatic blue shift in polar donor solvents such as acetone, dioxane, and DMSO (Figure 5). This can be rationalized in terms of dielectric solvent interactions with the Zn center,12,14 effectively reducing its acceptor character within the molecular scaffold of 12, and therefore, suppressing the ICT. In fact, the values observed in these donor solvents (cf. λabs=434-449 nm; λem=496-511 nm) correlate very well with those of the other related Zn complexes of dithienophospholes reported herein and earlier.8 Remarkably, the Stokes shifts of Δν=2700-2880 cm-1 also reflect the donor strength of the solvent used, with the strongest donor (DMSO) resulting in the smallest Stokes shift (Table 2). The blue shift is also observed in the UV spectrum of 12 (cf. CH2Cl2: λmax = 460 nm; acetone: λmax = 423 nm). Pt complexation was successfully performed by stirring 8 or 10 with K2PtCl4 in a mixture of dichloromethane/H2O/ DMSO at room temperature, respectively, and could be followed by the gradual loss of fluorescence upon excitation with a hand-held UV lamp (at 366 nm). This solvent combination is the key to the successful Pt complex formation, as (14) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583.

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Figure 4. Frontier orbitals (B3LYP/LanL2DZ level of theory) for complexes 120 (left) and 140 (right).10,11 Table 2. Fluorescence Data of Complex 12 in Different Solvents solvent

λabs [nm]a

λem [nm]b

Δν [cm-1]

dichloromethane acetone dioxane DMSO

503 434 446 449

601 496 508 511

3241 2880 2737 2702

a

Figure 5. Normalized optical spectra of 12 in different solvents (top: excitation, bottom: emission).

the commonly used EtOH/H2O combination,15 even at refluxing temperatures, was found to be ineffective, with strong fluorescence remaining after refluxing overnight; most of the respective free ligand was recovered after workup. By using (15) (a) Sun, Y.; Wang, S. Inorg. Chem. 2009, 48, 3755. (b) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro, M. L.; Rajapakse, N. Coord. Chem. Rev. 2006, 250, 1819. (c) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Che, C.-M.; Zhu, N.; Lee, S.-T. J. Am. Chem. Soc. 2004, 126, 4958. (d) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg. Chem. 2001, 40, 4053.

λmax for excitation. b λmax for emission.

the appropriate solvent mixture, both square-planar complexes 13 and 14 could be isolated in moderate to good yields as yellow and orange powders after aqueous workup, respectively. The UV-vis absorption spectra of both complexes (Figure 3) exhibit red-shifted values for the π-π* transitions at λmax =425 nm for 13 and λmax =477 nm for 14 (cf. 8: λmax=402 nm, 10: λmax=424 nm), which are in line with the planarization of the main scaffold similar to the Zn species 11 and 12. In addition to the π-π* transition, both Pt complexes also exhibit shoulders at λmax = 460 nm for 13 and λmax = 503 nm for 14, respectively, that can be assigned as metal-to-ligand charge transfer (MLCT) bands.15 The latter is comparable to the observations with the related tpy-dithienophosphole PtCl2 complex of IIb (cf. λmax(MCLT) = 490 nm).8 As mentioned, the fluorescence of the complexation reaction mixtures gradually decreases upon formation of 13 and 14, suggesting the potential occurrence of phosphorescence. We have therefore representatively looked into time-dependent photoluminescence measurements of the extended complex 14. The observed relaxation decay for 14 in solution at room temperature could satisfactorily be fitted for a dual lifetime at τ1 = 0.2 μs (74%) and τ2 = 15.43 μs (26%). The absolute values and probabilities suggest that the shorter component arises from a phosphorescence process involving a DAD architecture, whereas the long component appears to involve a Pt-based phosphorescence process that possibly arises from aggregates.15 The ICT process is also supported by the frontier orbitals of 14, which confirm the contribution from the Pt center in the mostly ligand-centered HOMO and LUMO (Figure 4). Reaction between the extended bis(dithienophosphole) 10 and Ru(bpy)2Cl2 in refluxing ethanol produced a mixture of the desired ruthenium complex together with a significant amount of unreacted 10, as monitored by 1H and 31P NMR. Replacing ethanol with 2-methoxyethanol was found to result in a more complete formation of the [Ru(bpy)2(10)] Cl2 complex, which is believed to be due to the higher boiling

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Scheme 5. Synthesis of the bpy-Functionalized Pt Complex 16

point of 2-methoxyethanol over ethanol. Complex 15 was obtained through anion exchange with KPF6 in aqueous solution. The dark red complex 15 could be characterized by means of 1H and 31P NMR spectroscopy, as well as mass spectrometry. Complex 15 was also subjected to cyclic voltammetry studies. The characteristic reversible Ru2+/3+ redox couple at E1/2 = 1.43 V (vs Ag/AgCl) in acetonitrile clearly confirmed the presence of a (bpy)3Ru2+ environment for 15 and the successful complexation of 10.16 The observed UV-vis absorption wavelength of λmax = 448 nm is in line with the other complexes of 10 reported herein (Figure 3). A corresponding MLCT process, on the other hand, could not unambiguously be identified, due to the broad absorption profile of 15. Further Modification of a Pt Complex. Complex 14 was envisioned to be an excellent building block for heteropolynuclear arrays with potentially dual luminescence features. It is well known in the literature that the chlorides at Pt can easily be replaced with acetylides.15,17 When the acetylides are appended with additional bipyridine units, the whole molecular scaffold could potentially be further complexed with, for example, lanthanide moieties, allowing the extended (bpy)Pt-bisacetylide moieties to act as sensitizers for lanthanide luminescence.17 In an attempt to access a similar scaffold, functionalization of 14 with bpy-4-acetylene was carried out by Cu(I)-catalyzed substitution reaction in refluxing dichloromethane solvent in the presence of diisopropylamine. Although 16 could be obtained as dark orange powder in modest yield that allowed for characterization by heteronuclear NMR spectroscopy and highresolution mass spectrometry, the bpy-functionalized Pt complex was found to be unstable in solution. It decomposes into the uncomplexed 10, which could be confirmed by the occurrence of yellow fluorescence, as well as NMR spectroscopy.18 The instability of 16 unfortunately precluded any further characterization and complexation.

Conclusion In conclusion, we were able to successfully functionalize the dithienophosphole unit with bipyridine mono- and bisacetylene groups, which allowed for the additional incorporation of (16) Stange, A. F.; Tokura, S.; Kira, M. J. Organomet. Chem. 2000, 612, 117. (17) (a) Chen, Z.-N.; Fan, Y.; Ni, J. Dalton Trans. 2008, 573. (b) Muro, M. L.; Rachford, A. A.; Wang, X.; Castellano, F. N. Top. Organomet. Chem. 2010, 29, 159. (18) A pure sample of 10 could be isolated from the reaction mixture by means of preparative thin-layer chromatography.

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transition metal centers in the molecular scaffold. Our studies have indicated that the resulting mono- and bis(dithienophosphole) bipyridines exhibit reduced solubility. However, satisfactory solubility features can efficiently be achieved by functionalizing either the bipyridine moiety with a trimethylsilyl group in the 50 -positon or the dithienophosphole scaffold with an octyl group in the 6-position. The resulting extended bipyridine ligands show moderate photoluminescence features that can be tuned further by the incorporation of a Zn, Pt, or Ru metal center. The bis(dithienophosphole) complexes of Zn2þ and Pt2þ, in particular, show some intriguing photophysical features that arise (a) from donor-acceptor-donor architectures and (b) from the access to triplet excited states (for Pt) that open up phosphorescence relaxation pathways; the Ru2þ complex 15 was found to provide for stable redox switching between Ru2þ/3þ. The extended Pt-bis(dithienophosphole) complex 14 was used to access a further extended molecular scaffold for multimetallic arrays, which was, however, found to be unstable in solution over extended periods of time, releasing the uncomplexed bis(dithienophosphole) ligand 10, likely due to steric overcrowding. Building upon the observations presented herein, we are currently addressing further bis(dithienophosphole) systems that involve cyclometalating complexation sites for the central transition metal unit. In addition, we are also exploring the possibility of generating supramolecular assemblies that involve more than one bis(dithienophosphole) ligand. The results of these studies will be reported elsewhere in due course.

Experimental Section General Procedures. Reactions were carried out in dry glassware and under an inert atmosphere of purified nitrogen using Schlenk techniques. Solvents were dried using an MBraun solvent purification system. n-BuLi (2.5 M in hexane), 2,5-dibromopyridine, K2PtCl4, ZnCl2 (0.1 M in THF), [Pd(PPh3)4] CuI, and [Pd(PPh3)2Cl2] were used as received. Diisopropylamine, tributyltin chloride, and trimethylsilyl acetylene were freshly distilled prior to use. 2-Iododithieno[3,2-b:20 ,30 -d ]phosphole oxide (1),8 5-ethynyl2,20 -bipyridine,19 5,50 -diethynyl-2,20 -bipyridine,19 2-bromo-5-trimethylsilylpyridine (4),16 2-bromo-6-octyldithieno[3,2-b:20 ,30 -d ]phosphole oxide (9),8 and Ru(bpy)2Cl220 were prepared by literature methods. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on a Bruker Avance-II 400 spectrometer. Chemical shifts were referenced to external 85% H3PO4 (31P) and TMS (1H, 13C). It should be mentioned in this context that due to their reduced solubility, no satisfactory 13C NMR data could be obtained for the transition metal complexes 11-16. Mass spectra were run on a Finnigan SSQ 7000 spectrometer, or a Bruker Daltonics AutoFlex III system. Optical spectroscopy experiments were recorded in a dichloromethane solution using a Jasco FP-6600 spectrofluorometer and UV-vis NIR Cary 5000 spectrophotometer. Electrochemical studies were performed on an Autolab PGSTAT302 instrument with a Pt rod as working electrode, Pt wire as counter electrode, and AG/AgCl/KCl3M as reference electrode; supporting electrolyte was NBu4PF6; scan rate was 100 mV s-1. Theoretical calculations have been carried out at the B3LYP/6-31G(d) (2, 3, 8, 100 ) and B3LYP/LanL2DZ (120 , 140 ) level by using the GAUSSIAN 03 suite of programs.13 The geometries were fully optimized, and at the resulting structures second derivatives were calculated. Synthesis of Compound 2. 2-Iododithieno[3,2-b:20 ,30 -d ]phosphole oxide (0.36 mmol, 150 mg), 5-ethynyl-2,20 -bipyridine (19) Grosshenny, V.; Romero, F. M.; Ziessel, R. J. Org. Chem. 1997, 62, 1491. (20) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17, 3334.

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(0.4 mmol, 72 mg), [Pd(PPh3)4] (22 mg), diisopropylamine (3 mL), and THF (15 mL) were heated at 80 °C for 72 h under nitrogen. After cooling to ambient temperature, the precipitate was filtered and dissolved in an excess amount of CHCl3. The solution was then washed with aqueous KOH solution and water. The organic layer was separated and dried over MgSO4, and the solvents were evaporated under reduced pressure to afford a yellow solid of 2 in 45% yield (0.076 g). 31P{1H} NMR (162 MHz, CD2Cl2): δ 18.7 ppm. 1H NMR (400 MHz, CDCl3): δ 8.79 (dd, J = 2.0, 0.8 Hz, 1H), 8.70 (d, J = 4.0 Hz, 1H), 8.44 (d, J = 8.0 Hz, 2H), 7.92 (dd, J = 8.0, 2.0 Hz, 1H), 7.84 (dt, J = 2.0 Hz, 1H), 7.79-7.73 (m, 2H), 7.60-7.58 (m, 1H), 7.49-7.46 (m, 2H), 7.39-7.34 (m, 3H), 7.23 (dd, J = 4.8, 2.4 Hz, 1H) ppm. 13 C{1H} NMR (100 MHz, CDCl3): δ 155.50 (J = 8.0 Hz), 151.39, 149.34, 146.45 (J = 23 Hz), 145.45 (J = 25 Hz), 139.60 (J = 93 Hz), 139.34, 138.60 (J = 93 Hz), 136.99, 132.80 (J = 3.0 Hz), 130.95 (J = 14 Hz), 130.87 (J = 12 Hz), 129.33 (J = 35 Hz), 128.43, 126.32 (J = 14 Hz), 125.98 (J = 17 Hz), 92.83, 85.81 ppm. HRMS: m/z 466.0365 ([M]þ, calcd 466.0363). Synthesis of Compound 3. 2-Iododithieno[3,2-b:20 ,30 -d ]phosphole oxide (0.9 mmol, 350 mg), 5,50 -diethynyl-2,20 -bipyridine (0.36 mmol, 73 mg), [Pd(PPh3)4] (70 mg), diisopropylamine (10 mL), and THF (25 mL) were heated at 80 °C for 90 h under nitrogen. After cooling to ambient temperature, the precipitate was filtered and washed with plenty of ether and dried over MgSO4, and the solvents were evaporated under reduced pressure to afford a reddish solid of 3 in 15% yield (0.042 g). 31P{1H} NMR (162 MHz, CD2Cl2): δ 19.1 ppm. 1H NMR (400 MHz, CDCl3): δ 8.79 (d, J = 2.0 Hz, 1H, bpy), 8.47 (dd, J = 8.0, 3.2 Hz, 1H, bpy), 7.93 (dd, J = 8.0, 2.0 Hz, 1H,bpy), 7.79 (m, 2H, bpy), 7.58 (m, 1H), 7.53 (m, 2H), 7.37 (m, 2H), 7.22 (dd, J = 4.8, 2.4 Hz, 2H) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ 154.34, 151.45, 146.52 (J = 22 Hz), 145.37 (J = 24 Hz), 139.95 (J = 75 Hz), 138.83 (J = 73 Hz), 138.47, 132.72, 131.07 (J = 14 Hz), 130.84 (J = 22 Hz), 129.53 (J = 15 Hz), 129.09 (J = 109 Hz), 129.03 (J = 13 Hz), 126.26 (J = 14 Hz), 125.77 (J = 17 Hz), 120.72, 119.64, 92.72, 86.28 ppm. HRMS: m/z 776.0012 ([M]þ, calcd 776.0039). Synthesis of 2-(Tributylstannyl)-5-(trimethylsilyl)pyridine (5). To a solution of n-BuLi (2.5 M, 5.5 mL) diluted with dry Et2O (6 mL) was added a solution of 2-bromo-5-(trimethylsilyl)pyridine (4) in dry ether (13 mL) at -78 °C. The mixture was stirred for 1 h at -78 °C before a solution of tributyltin chloride was added dropwise. The reaction was allowed to reach room temperature and left stirring overnight. The reaction was quenched with saturated NH4Cl solution, and the organic layer was washed with H2O. The organic phase was then dried with MgSO4 and evaporated to afford a yellow oil in 95% yield (5.6 g). 1H NMR (400 MHz, CDCl3): δ 8.82 (s, 1H, py), 7.59 (dd, J = 7.2, 2.0 Hz, 1H, py), 7.38 (dd, J = 7.2, 1.2 Hz, 1H, py), 1.57 (m, 6H), 1.34 (m, 6H), 1.12 (m, 6H), 0.89 (m, 9H), 0.29 (s, 9H, TMS) ppm. 13 C{1H} NMR (100 MHz, CDCl3): δ 174.24, 154.36, 138.18, 132.58, 131.96, 29.18, 27.08, 13.69. 9.72, -1.50 ppm. HRMS: m/z 426.1630 ([M - CH3]þ, calcd 426.1639). Synthesis of 5-Bromo-50 -(trimethylsilyl)-2,20 -bipyridine (6). Compound 5 (6.3 mmol, 780 mg), 2,5-dibromopyridine (6.0 mmol, 1.466 g), [Pd(PPh3)4] (80 mg), and toluene (50 mL) were heated at 110 °C for 72 h under nitrogen. After cooling to ambient temperature, the solvent was removed and the residue was subjected to column chromatography (ethyl acetate/hexane, 1:4). The first fraction was collected and run through a KF/ Celite mixture to afford a light yellow solid (1.5 g, 77%). 1H NMR (400 MHz, CDCl3): δ 8.74 (m, 2H, bpy), 8.33 (m, 2H, bpy), 7.93 (m, 2H,bpy), 0.35 (s, 9H) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ 155.16, 154.79, 153.39, 150.19, 142.14, 139.42, 135.71, 122.27, 121.08, 120.09, -1.34 ppm. HRMS: m/z 306.0181 ([M]þ, calcd 306.0188). Synthesis of 5-(Trimethylsilyl)-50 -((trimethylsilyl)ethynyl)2,20 -bipyridine. Compound 6 (2.5 mmol, 770 mg), trimethylsilyl acetylene (2.7 mmol, 0.83 mL), [Pd(PPh3)2Cl2] (186 mg), CuI

Bai and Baumgartner (80 mg), diisopropylamine (9 mL), and THF (50 mL) were stirred at room temperature for 48 h under nitrogen. The solvent was removed, and the residue was dissolved in pentane. The precipitate was filtered, the mother liquid was dried over MgSO4, and the solvents were evaporated under reduced pressure to afford the product as a pale white solid in 86% yield (0.70 g). 1 H NMR (400 MHz, CDCl3): δ 8.75 (m, 2H, bpy), 8.37 (t, J = 8.0 Hz, 2H, bpy), 7.93 (dd, J = 8.0, 2.0 Hz, 1H,bpy), 7.87 (dd, J = 8.0, 2.0 Hz, 1H, bpy), 0.35 (s, 9H), 0.30 (s, 9H) ppm. 13 C{1H} NMR (100 MHz, CDCl3): δ 155.45, 155.13, 153.42, 152.05, 142.09, 139.74, 135.68, 120.54, 120.12, 101.91, 99.08, -0.13, -1.32 ppm. HRMS: m/z 324.1463 ([M]þ, calcd 324.1478). Synthesis of 5-Ethynyl-50 -(trimethylsilyl)-2,20 -bipyridine (7). To a stirred solution of 5-(trimethylsilyl)-50 -((trimethylsilyl)ethynyl)-2,20 -bipyridine (0.3 mmol, 100 mg) in MeOH/THF (10/10 mL) was added KF (60 mg) as a solid. After overnight stirring, the solvent was removed to give a crude product, which was further purified by flash chromatography (pentane). Yield: 70 mg (95%). 1H NMR (400 MHz, CDCl3): δ 8.78 (m, 2H, bpy), 8.38 (dd, J = 20, 8.0 Hz, 2H, bpy), 7.93 (m, 2H, bpy), 3.30 (s, 1H), 0.35 (s, 9H). 13C{1H} NMR (100 MHz, CDCl3): δ 155.52, 155.26, 153.38, 152.17, 142.06, 139.88, 135.73, 120.51, 120.18, 119.05, 81.34, 80.75, -1.35 ppm. HRMS: m/z 252.1077 ([M]þ, calcd 252.1083). Synthesis of Compound 8. 2-Iododithieno[3,2-b:20 ,30 -d ]phosphole oxide (1) (0.79 mmol, 328 mg), compound 7 (0.8 mmol, 200 mg), [Pd(PPh3)4] (80 mg), diisopropylamine (12 mL), and THF (40 mL) were heated at 80 °C for 90 h under nitrogen. After cooling to ambient temperature, the solvent was removed under vacuum and extracted with CH2Cl2. The organic phase was combined and dried over MgSO4. The crude was subjected to preparative TLC (ethyl acetate) to afford a reddish solid of 8 in 62% yield (0.260 g). 31P{1H} NMR (162 MHz, CD2Cl2): δ 19.4 ppm. 1H NMR (400 MHz, CDCl3): δ 8.77 (d, J = 2.0 Hz, 1H), 8.44 (d, J = 8.0 Hz, 1H), 7.91 (dd, J = 8.0, 2.0 Hz, 1H), 7.84 (dt, J = 2.0 Hz, 1H), 7.79-7.73 (m, 2H), 7.60-7.58 (m, 2H), 7.497.46 (m, 2H), 7.39-7.34 (m, 3H), 7.23 (dd, J = 4.8, 2.4 Hz, 1H), 0.35 (s, 9H) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ 155.07, 154.97, 153.16, 151.42, 146.42 (J = 23 Hz), 145.40 (J = 24 Hz), 142.49, 139.90 (J=74 Hz), 139.14, 138.43, 136.12, 132.71 (J=2.8 Hz), 132.09 (J=10 Hz), 130.84 (J = 12 Hz), 129.46 (J= 15 Hz), 129.11 (J=108 Hz), 129.01 (J=13 Hz), 128.48 (J=12 Hz), 126.25 (J=15 Hz), 125.87 (J = 17 Hz), 120.59 (J = 28 Hz), 119.41, 92.76, 85.96, -1.35 ppm. HRMS: m/z 538.0764 ([M]þ, calcd 538.0759). Synthesis of Compound 10. Dithieno[3,2-b:20 ,30 -d ]phosphole oxide (9) (0.9 mmol, 350 mg), 5,50 -diethynyl-2,20 -bipyridine (0.36 mmol, 73 mg), [Pd(PPh3)4] (70 mg), diisopropylamine (10 mL), and THF (25 mL) were heated at 80 °C for 72 h under nitrogen. After cooling to ambient temperature, the solvent was removed under vacuum, and the residue was extracted with CH2Cl2. The organic phase was collected and dried over MgSO4. After evaporation of the solvent, the product was further purified by washing with cold diethyl ether to afford a reddish solid of 10 in 79% yield (0.340 g). 31P{1H} NMR (162 MHz, CD2Cl2): δ 19.7 ppm. 1H NMR (400 MHz, CDCl3): δ 8.78 (d, J = 1.6 Hz, 2H, bpy), 8.44 (d, J = 8.0 Hz, 2H, bpy), 7.91 (dd, J = 8.0, 2.0 Hz, 2H, bpy), 7.76 (m, 4H), 7.55 (m, 2H), 7.47 (m, 4H), 7.33 (d, J = 2.8 Hz, 2H), 6.88 (s, 2H), 2.82 (t, J = 7.2 Hz, 4H), 1.68 (m, 4H), 1.28 (m, 20H), 0,.89 (t, J = 4.4 Hz, 6H) ppm. 13 C{1H} NMR (100 MHz, CDCl3): δ 154.33, 152.06 (J = 16 Hz), 151.45, 147.39 (J=25 Hz), 142.64 (J=28 Hz), 139.47 (J= 112 Hz), 139.06, 137.69 (J = 113 Hz), 132.65, 131.03 (J = 19 Hz), 130.88 (J = 11 Hz), 129.53 (J = 92 Hz), 129.00 (J = 14 Hz), 124.85 (J = 19 Hz), 122.92 (J = 15 Hz), 120.70, 119.74, 92.44, 86.52, 31.81, 31.47, 30.51, 29.24, 29.15, 28.96, 22.65, 14.09 ppm. HRMS: m/z 1001.2622 ([M]þ, calcd 1001.2638). Synthesis of Zn(8)Cl2 (11). To a solution of compound 8 (25 mg, 0.025 mmol) in CH2Cl2 (6 mL) was added a THF solution of ZnCl2 (0.5M, 0.06 mL). The solution immediately turned

Article orange and was stirred overnight. The solvent was removed under vacuum and then washed with cold ether to afford pure 11 in 59% yield (10 mg). 31P{1H} NMR (162 MHz, DMSO-d6): δ 17.15 ppm. 1H NMR (400 MHz, DMSO-d6): δ 8.89 (1H), 8.83 (1H), 8.52 (1H), 8.45 (1H), 8.19 (m, 2H), 7.81 (m, 2H), 7.72 (m, 2H), 7.63 (m, 1H), 7.53 (m, 2H), 7.44 (m, 1H), 0.34 (s, 9H) ppm. HRMS: m/z 638.97092 ([M - Cl]þ, calcd 638.97071). Synthesis of Zn(10)Cl2 (12). To a solution of 10 (25 mg, 0.025 mmol) in CH2Cl2 (6 mL) was added a THF solution of ZnCl2 (0.5 M, 0.06 mL). The solution immediately turned orange and was stirred overnight. The solvent was removed under vacuum and then washed with cold ether to afford pure 12 in 54% yield (15 mg). 31P{1H} NMR (162 MHz, DMSO-d6): δ 17.64 ppm. 1H NMR (400 MHz, DMSO-d6): δ 8.88 (s, 2H), 8,44 (d, 2H), 8.15 (d, 2H), 7.79 (2H), 7.68 (m, 4H), 7.63 (m, 2H), 7.54 (m, 4H), 7.19 (s, 2H), 2.85 (m, 4H), 1.65 (m, 4H), 1.23 (m, 20H), 0.84 (m, 6H) ppm. HRMS: m/z 1101.15417 ([M - Cl]þ, calcd 1101.15019). Synthesis of Pt(8)Cl2 (13). To a stirred solution of K2PtCl4 (38 mg, 0.09 mmol) in a mixed solvent of H2O (10 mL) and DMSO (0.5 mL) was added 8 (40 mg, 0.07 mmol) in CH2Cl2 (5 mL). The mixture was stirred at room temperature overnight. The organic phase was separated and dried over MgSO4. After the evaporation of solvent, the residue was washed by cold diethyl ether to afford a light yellow powder of 13 in 50% yield (28 mg). 31 P{1H} NMR (162 MHz, CD2Cl2): δ 18.85 ppm. 1H NMR (400 MHz, CDCl3): δ 9.66 (m, 2H), 8.22 (dd, J = 8.0, 1.6 Hz, 1H), 8.16 (dd, J = 8.0, 1.6 Hz, 1H), 8.40 (d, J = 8.0 Hz, 2H, bpy), 7.95 (dt, J = 8.0, 2.0 Hz, 4H,bpy), 7.78 (m, 2H), 7.61 (m, 1H), 7.50 (m, 2H), 7.40 (d, J = 2.4 Hz, 2H), 7.18 (dd, J = 2.4, 4.8 Hz, 1H), 0.40 (s, 9H) ppm. HRMS: m/z 826.96737 ([M þ Na]þ, calcd 826.966263). Synthesis of Pt(10)Cl2 (14). To a stirred solution of K2PtCl4 (25 mg, 0.06 mmol) in a mixed solvent of H2O (10 mL) and DMSO (0.5 mL) was added 10 (50 mg, 0.05 mmol) in CH2Cl2 (10 mL). The mixture was stirred at room temperature overnight. The organic phase was separated and dried over MgSO4. After the evaporation of solvent, the residue was washed by cold diethyl ether to afford a dark orange powder of 14 in 66% yield (50 mg). 31P{1H} NMR (162 MHz, CD2Cl2): δ 19.25 ppm. 1H NMR (400 MHz, CD2Cl2): 9.69 (s, 2H), 8.17 (d, J = 9.2 Hz,


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2H), 8.00 (d, J=7.6 Hz, 2H), 7.72 (m, 4H), 7.57 (m, 2H), 7.47 (m, 4H), 7.37 (d, J = 2.4 Hz, 2H), 6.82 (s, 1H), 2.80 (m, 4H), 1.60 (m, 4H), 1.28 (m, 20H), 0.89 (m, 6H) ppm. HRMS: m/z 1289.14061 ([M þ Na]þ, calcd 1289.14555). Synthesis of Ru(10)bpy2(PF6)2 (15). Compound 10 (50 mg, 0.05 mmol) and Ru(bpy)2Cl2 (60 mg, 0.12 mmol) were heated to 125 °C overnight. After the mixture cooled, an aqueous solution of KPF6 (200 mg) was added. The solution was stirred for 2 days. The solvent was removed under vacuum, and the residue was extracted with CH2Cl2 and dried over MgSO4. After the evaporation of the solvent, the residue was washed with diethyl ether to afford a dark red powder of 15 in 47% yield (40 mg). 31 P{1H} NMR (162 MHz, CD3CN): δ 17.30, -143.68 (sept) ppm. 1H NMR (400 MHz, CD3CN): δ 8.49 (m, 6H), 8.11 (m, 8H), 7.79 (m, 4H), 7.69 (m, 8H), 7.43 (m, 8H), 6.97 (s, 2H), 2.86 (m, 4H), 1.68 (m, 4H), 1.28 (m, 20H), 0.89 (m, 6H) ppm. HRMS: m/z 1414.29816 ([M - 2PF6]þ, calcd 1414.29747). Synthesis of Pt(10)(4-ethynyl-bpy)2 (16). Compound 14 (145 mg, 0.141 mmol), 5-ethynyl-2,20 -bipyridine (83 mg, 0.461 mmol), and CuI (30 mg) were dissolved in CH2Cl2 (12 mL) and diisopropylamine (3 mL) and refluxed for 4 days. The organic phase was separated and dried over MgSO4. After the evaporation of solvent, the residue was washed by cold diethyl ether first and then cold ethyl acetate to afford a dark orange powder of 16 in 14% yield (30 mg). 31P{1H} NMR (162 MHz, CD2Cl2): δ 19.21 ppm. 1H NMR (400 MHz, CD2Cl2): 9.88 (1H), 8.91 (2H), 8.68 (2H), 8.45 (2H), 8.39 (2H), 8.17 (2H), 7.97 (4H), 7.86 (4H), 7.72 (4H), 7.53 (2H), 7.41 (6H), 6.86 (s, 1H), 2.81 (m, 4H), 1.60 (m, 4H), 1.26 (m, 20H), 0.89 (m, 6H) ppm. HRMS: m/z 1555.34479 ([M þ H]þ, calcd 1555.34984).

Acknowledgment. Financial support by the Natural Sciences and Engineering Research Council (NSERC) of Canada and by the Canada Foundation for Innovation (CFI) is gratefully acknowledged. T.B. also thanks Alberta Ingenuity now Part of Alberta Innovates - Technology Futures for a New Faculty Award. We thank Paolo Bomben for his support with the phosphorescence lifetime measurements.

Synthesis and Photophysical Properties of Bipyridine-Extended

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