Hetero-bischelated complexes

A Comparative Study of Some Luminescence Properties of Homo- and Hetero-Bischelated Complexes of Iridium(III)... 

Several hetero-bischelated complexes of Ir(III) with 1,10-phenanthroline and substituted 1,10-phenanthroline have also been reported to have non-exponential luminescence decay curves (19). Although the individual emission spectra of the non-equilibrated levels of these complexes are again too close to resolve by conventional emission spectroscopy, partial resolution has been accomplished by time-resolved emission spectroscopy via box-car averaging techniques (20). Complete resolution has been accomplished by computer analysis of luminescence decay curves as a function of emission wavelength (20). In these complexes, the luminescent levels appear to arise from both ligand-localized ( tttt ) states and charge-transfer ( ) states. 

In this paper we first review the experimental data which characterizes the luminescence of several complexes formed by the binding of two chloride ions and two bidentate ligands to Ir(III). On the basis of this experimental information, we present a simple molecular orbital model which describes the orbital parentage of the luminescent states of these complexes. This model is used to interpret the non-exponential luminescence of hetero-bischelated complexes of Ir(III). 

The hetero-bischelated complex, [IrCl2,phen) (5,6-mephen)]Cl, displays a non-exponentital luminescence decay curve when excited at 337 nm in ethanol-methanol glass at 77°K (19), Analysis of the decay curves of this complex by a non-linear least squares fit to a function representing the sum of two exponentials indicates that the emission is caused by levels with lifetimes of 65 and 9.5 /msec (20), Both time-resolved spectroscopy and analysis of decay curves as a function of emission wavelength indicate... 

In both hetero-bischelated complexes mentioned above, there is some evidence that there may be three sets of non-equilibrated levels responsible for the luminescence excited by 337-nm excitation (26). The determination of the lifetimes and energies of three levels by least squares analysis of decay curves as a function of emission wavelength is currently being studied and will be reported. For now, we will analyze these complexes based upon the two-level model which was used to interpret our data. 

Here we discuss a simple model for describing the orbital parentage of the luminescent levels of the complexes discussed above. We concentrate on a model which emphasizes the changes which occur in going from a homo- to a hetero-bischelated complex. The model is presented in this way to facilitate a description of the circumstances which lead to the unusual luminescence properties of the hetreo-bischelated complexes. 

Figure 2. Molecular orbital energy level diagram for hetero-bischelated complexes of iridiumflll) with 1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, and 5,6-dimethyl-l,10-phenanthroline...

In Figure 2 we depict the formation of complex molecular 7r-orbitals from the 7r-orbitals of two non-identical phenanthroline ligands. On the left side of the diagram, we treat the hetero-bischelated complex ion, [IrCl2(phen) (5,6-mephen)] The ligand orbtials tta and tta represent molecular orbitals on 1,10-phenanthroline, and the ligand orbitals ttb and... 

In view of these results, it follows that the placement of the orbital labelled 4 in Figures 1 and 2 may have to be altered. This orbital may be, in fact, the lowest unfilled molecular orbital in these complexes. We are currently studying the time resolved spectroscopy of the hetero-bischelated complexes between —196° and 0°C to establish the correct placement of the d-d levels. 

In the simple molecular orbital model for the lowest excited states of these molecules, the primary difference between the homo- and hetero-bischelated complexes is in the localization of the excitation energy in the latter. In this section we associate this localization of energy with the failure of the lowest excited states of the hetero-bischelated complexes to... 

Since the non-equilibrated levels of the hetero-bischelated complexes are separated by only several hundred wavenumbers, there are a limited number of vibrations of the appropriate frequency to enable the molecule to undergo a transition from one of these levels to the other. It is our point of view that in these complexes, there are no appropriate vibrational frequencies of nuclei in the region where the two changing electron distributions overlap to bring about a transition from one to the other. Thus, thermal equilibration of these localized electron distributions does not occur, and emissions from several independent manifolds of different orbital parentage are observed. 

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