Room Temperature Emission

1. Hemilabile Coordination Complexes as a Tool for Small Molecule Sensing Anthony Tomcykoski, Wayne E. Jones Jr.* Department of Chemistry, SUNY at Binghamton, Binghamton, NY 13902 Hemilabile Complexes Room Temperature Emission Introduction Since Demas and Adamson’s introduction of tris(2,2’-bipyrdine)ruthenium(II) as a photosensitizer, numerous applications have been developed to take advantage of its rich photophysical properties.1 In the ground state, [Ru(bpy)3]+2 is a chiral molecule with Δ and L isomers. Upon absorption of electromagnetic radiation in the solar spectrum, various electronic excited states are observed. Of particular interest is the spin-forbidden transition state (3MLCT) having a long excited state lifetime in solution (5μs) that radiatively decays in high yield. Utilizing this complex as a luminescent model plays a significant role in developing fluorescent chemosensors. Hemilabile ligands have been of great interest to chemists working toward the development of molecular sensors. Hemilabile coordination is found to occur amongst polydentate ligands that contain both chemically inert and labile sites bound to a metal center. In the presence of molecules with a strong affinity to the metal center, an exchange reaction can occur in which dissociative-associative and interchange mechanisms have been proposed. After reacting, the hemilabile ligand will remain tethered in close proximity to the metal center due to the inert binding position. Upon coordination by a competing molecule, the photophysical properties of the complex as a whole will change resulting in a signal that can be monitored. Of the three classes of chemosensors, chromophoric, potentiometric, and fluorescent, this research aims at the latter as the means by which the signal is obtained. We have been investigating a series of hemilabile coordination complexes which contain polypyridyl chromophoric ligands centered on ruthenium. These systems show promise as chemosensors due to electronic transitions to the chromophoric polypyridyl ligand. In terms of hemilabile coordination, various phosphine-ether ligands have been explored as ancillary ligands due to the inert phosphine binding site and labile ether binding site. Phosphine-ether ligands previously have been shown to exhibit reversible binding in the presence of small polar molecules such as acetonitrile, acetone, and water. With many possible applications, the use of these complexes as humidity sensors is the driving force within the scope of this research. In addition, the tridentate complexes are designed in a manner to serve as receptor units in conjugated polymer systems. An application of this type would allow for the construction of thin film sensors suitable for practical devices. [RuPOMe] RuPOMe [2.85 x 10-3M] RuPOMe 601nm RuP(OMe)2 438nm RuP(OMe)6 [3.0 x 10-3M] [RuP(OMe)6] [RuP(OMe)2] 1.11 x 10-3M (424nm) 2.22 x 10-4M (413nm) 4.44 x 10-5M (402nm) Absorbance Spectra ppm Conclusions The blue shift in emission spectra implies that the energy gap between the excited state and ground state increases. This can be described based on destabilization of the LUMO by introduction of electron donating groups. The presence of methoxy groups on the ancillary ligand shifts the electron density towards the metal center resulting in an increasing frequency in transitions. 5.7 x 10-4M 2.3 x 10-4M 9.2 x 10-5M 3.7 x 10-5M The photophysics of hemilabile coordination complexes are dependent upon concentration. The complexes RuPOMe, RuP(OMe)2, and RuP(OMe)6 show blue shifting emission spectra with lmax of 601, 438, and 424nm respectively. RuP(OMe)6 shows a blue shift in the emission band with decreasing solution concentrations. The main differences between the bidentate and tridentate ligands are seen as a blue shift in emission spectra. Although the complexes studied are luminescent at room temperature, terpyridyl complexes in general have a much shorter excited state lifetime. This photophysical property makes bipyridyl chromophores slightly more appealing for practical use. Finally, the 31P NMR spectra show unique complex resonances alluring to the fact of coordination through phosphorous. MLCT 450nm Synthesis 31P NMR Studies Other means must be employed to convincingly show the relative amount of each specie present in solution. Quantitative NMR techniques may be used to show abundance of a specific nuclei of interest. Given that phosphorous nuclei are spin-½ with 100% natural abundance, 31P NMR can be used to show various complex forms in a given sample solution. The primary interest for acquiring 31P NMR spectra is to show coordination through phosphorous. Further tests need to be performed to assign all peaks observed, but the resonance furthest upfield can be labeled as the ether-bound complex. The least chemically shielded nuclei resonate at a higher frequency, and this is the case for the bound phosphine ligand with coordinate covalent bonds to ruthenium through methoxy groups. The synthesis of bis(2,2’-bipyridyl){diphenyl(2-methoxyphenyl)phosphine} ruthenium(II) hexafluorophosphate [RuPOMe] has been reported by Rogers and Wolf.2 4’-Tolylterpyridylbis(2-methoxyphenyl)phenylphosphineruthenium(II) tetrafluoroborate [RuP(OMe)2] and 4’-tolylterpyridyltris(2,6-methoxyphenyl)phosphineruthenium(II) tetrafluoroborate [RuP(OMe)6] are prepared by reacting one equivalent of the terpyridyl ligand and LiCl with RuCl3.nH2O in N,N-dimethylformamide at reflux for 48 hours. The solution is cooled to room temperature to which is added an equal volume of acetone. The reaction mixture is refrigerated overnight and then filtered through fritted glass. The crystals are stored and used as the starting material for coordination by the tridentate hemilabile ligand. Ru(ttpy)Cl3 is dissolved in acetone and to the mixture is added three equivalents of AgBF4 to stir overnight under a steady stream of nitrogen. Upon filtration of solid AgCl, one equivalent of the appropriate phosphine-ether ligand is added to the acetonated complex and allowed to react at reflux for 48 hours under N2 atmosphere. The mixture is cooled with any solid impurities being separated by filtration. The reaction mixture is evaporated to dryness and the crystals stored in a desiccator. Acknowledgements MLCT 21.7kK A.T. and W.E.J. thank The Research Foundation and The Chemistry Department for financial support. A special thanks goes to Dr. Jürgen Schulte and Dr. Justin Martin for instrumental support and useful discussions. References 31P NMR Chemical Shifts (ppm) • Demas, J.N.; Adamson, A.W. J. Am. Chem. Soc.1971, 93, 1800-1801 • Rogers, C.W.; Wolf, M.O. Chem. Commun. 1999, 2297-2298 • Angell, S.E.; Zhang, Y.; Rogers, C.W.; Wolf, M.O.; Jones, W.E.; Inorg. Chem.2005, 44, 7377-7384. For RuP(OMe)6, the band at 2.17 x 104 cm-1 (460nm) can be identified as a metal-to-ligand charge transfer (MLCT) with a molar absorptivity of 1.52 x 103 cm-1.M-1. The bottom spectrum is displayed in in terms of wavenumber (cm-1) to show a linear relationship to energy. …………………………..