Résumé (John Grey)

My research project studies the interplay between structural and electronic properties in luminescent inorganic complexes. The systems chosen for this objective are square-planar platinum(II) and palladium(II) [d⁸] complexes and trans-dioxo rhenium(V) and osmium(VI) [d²]complexes. These molecules have idealized D_4h symmetry with the former group representing the tetragonally elongated limit of the D_4h point group and the latter representing the tetragonally compressed limit. The observed luminescence in these systems has been assigned as a formal ³E_g to ¹A_1g transition.^1-4 However, the large spin-orbit coupling inherent to second- and third-row transition metal ions splits the emitting state into six spin-orbit components and luminescence actually occurs from the lowest energy component.² Multiple spectroscopic techniques are used to characterize the ground- and emitting states as well as excited state relaxation dynamics and measurements are recorded as a function of temperature and pressure. All systems exhibit well-resolved vibronic structure at low temperature that allows for an analysis of emitting state displacements along all vibrational modes coupled to the electronic transition. Pressure-dependent luminescence, Raman, and lifetime measurements then provide a valuable insight into the dependence of emissive properties on the molecular geometries of the complexes.

In the trans-dioxo complexes, low temperature luminescence bandshapes reveal that the ground electronic states are anharmonic. This anharmonicity arises from avoided crossings between the ground state and excited states of the same symmetry and is most evident in systems that luminesce at lower energies, i.e., closer energetic proximity of coupled states.⁵ A series of trans-[ReO2]+ complexes with substituted N,N,NŐ,NŐ-ethylenediamine ligands were studied that, despite the structural similarity, show markedly different luminescence energies and bandshapes. Pressure-dependent luminescence spectra at room temperature show resolved structure in the high-frequency (~900 cm^-1) O=Re=O symmetric stretching mode. As pressure increases, the intensity distribution of the vibronic members changes across the bands. This variable vibronic structure allows for a detailed analysis of the luminescence bands using one-dimensional potential energy surfaces and the time-dependent theory of spectroscopy to calculate spectra. Initially, the ground and emitting states were approximated as simple harmonic potentials. This led to unsatisfactory fits, namely on the low-energy side of the luminescence bands, and therefore necessitated a more sophisticated model.6 The coupled states model, previously used to analyze low temperature spectra, was employed to rationalize the pressure-dependent luminescence bandshapes. The successful adaptation of this model to the pressure-dependent case further validates its use in trans-dioxo complexes.⁷

Square planar d⁸ complexes have also proven to be excellent systems to study the interdependence between molecular structure and electronic properties. Thiocyanate and selenocyanate complexes of platinum(II) and palladium(II) were studied and found to exhibit significant variations of luminescence intensities and lifetimes with temperature and pressure. The luminescence intensities at ambient temperature and pressure are very weak and almost undetectable. By lowering temperature, the luminescence intensities and lifetimes increase dramatically. Low temperature (ca. 5 K) luminescence spectra show highly resolved vibronic structure revealing large displacements along multiple vibrational coordinates, including Jahn-Teller active non-totally symmetric stretching and bending modes. Upon increasing pressure on single crystal samples at room temperature, large increases are observed for both luminescence intensities and lifetimes of up to three orders of magnitude.^8,9 Remarkably, the lower energy emitting palladium(II) complexes show a much larger pressure effect than the platinum(II) complexes. The rich vibronic structure is then analyzed and the emitting state offsets are obtained for each displaced normal coordinate. The temperature- and pressure-dependent luminescence decay behavior can be rationalized using an analytical expression for the nonradiative decay rate constant in the strong-coupling limit of radiationless decay theory, in which the nonradiative transition probability is a Gaussian function of the energy of activation. This simple model uses one effective accepting vibrational mode and offset while neglecting interactions with other excited states. The main advantage of this expression is that all of the important ingredients can be calculated in terms of experimental quantities and values used in the fits of the high-resolution spectra, such as individual normal coordinate offsets. There remains only one adjustable parameter and the experimental trends of the temperature dependent luminescence decay rates are modeled within the temperature range studied (5 to 300 K). To simulate the effect of pressure on the nonradiative rate constant, the effective emitting state offset is varied while keeping all other parameters constant. The simulations predict a larger decrease of the nonradiative rate constant for the palladium(II) complexes than the platinum(II) complexes within a similar pressure range, which gives rise to a larger increase in luminescence lifetimes for the former systems and in excellent agreement with experiment. The unprecedented behavior observed in these square-planar complexes appears to be restricted to systems having emitting states of lower symmetry than of the ground electronic states, i.e., significant displacements of non-totally symmetric vibrational modes. It is now of interest to determine how different ligands and environments may influence the luminescence the luminescence properties of these, and related, systems.

          (1)      Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 1373.

          (2)      Winkler, J. R.; Gray, H. B. Inorg. Chem. 1985, 24, 346.

          (3)      Pelletier, Y.; Reber, C. Inorg. Chem. 2000, 39, 4535-4541.

          (4)      Tuszynski, W.; Gliemann, G. Z. Naturforsch. 1979, 34a, 211.

          (5)      Savoie, C.; Reber, C. J. Am. Chem. Soc. 2000, 122, 844.

          (6)      Grey, J. K.; Triest, M.; Butler, I. S.; Reber, C. J. Phys. Chem. A 2001, 105, 6269.

          (7)      Grey, J. K.; Butler, I. S.; Reber, C. J. Am. Chem. Soc. 2002, 124, 11699.

          (8)      Grey, J. K.; Butler, I. S.; Reber, C. J. Am. Chem. Soc. 2002, 124, 9384.

          (9)      Grey, J. K.; Butler, I. S.; Reber, C. Inorg. Chem. 2003, Submitted.