Catenates luminescence

Figure 15. Luminescence spectra of some M.5"" catenates in CH2CI2. From left- to right-hand-side, maxima correspond to M = Li, Zn, Ag, H, Pd"+, Cu+ (alternate full and dashed line, for the sake of clarity). The spectra of the silver) 1) and palladium(II) catenates are recorded at 77 K, all the others at 298 K.

The wide tuning of the electronic properties of [2]catenates, as observed from electrochemistry and UV-visible absorption, causes a nice modulation of the luminescence properties in CH2CI2 solution. On changing the metal ion, the luminescence bands of this catenate family are tuned throughout the whole visible spectral region [60] (Figure 15). The luminescence data at 298 and 77 K in CH2CI2 are reported in Table 9. 

In detail, the luminescence bands of the Li+, Cd ", Zn +, H+ catenates M.5 are of LC nn ) character (fluorescence and phosphorescence) for Ag.S" " only the 77 K LC phosphorescence is observed due to the rapid singlet-triplet intersystem crossing induced by the heavy metal ion the luminescence band shown (only) at 77 K by Pd.5+ is difficult to attribute (see above), but a substantial MLCT character is indicated by the spectral red-shift and shorter lifetime compared to, for instance, Ag.S" finally, the Cu.5 shows the typical MLCT band (see above). No luminescence is observed for Ni.S" and Co.5 owing to the presence of low-lying non-... 

The absorption spectra of the free ligand 10 and its mono- and dinuclear complexes Cu.lO, CuCu.lO, CuAg.lO, CuZn-lO " ", CuZn.lO, are reported in Ref. [64] these spectra are well-matched with the sum of the spectra of the two chromophoric (M.5" -type) component units. The luminescence properties of all [3]catenates at 298 and 77 K in CH2CI2 are reported in Table 10. 

The emission behavior of the free ligand 10 and of the homodinuclear CuCu.lO catenate is quite comparable to that of the simpler corresponding [2]catenand 2 and [2]catenate Cu.5"". By contrast, a dramatic luminescence quenching of one of the two moieties is observed in all the heterodinuclear compounds. 

A similar behavior is found for CuZn.lO, where the Cu-based moiety shows luminescence properties quite similar to those of CuAg-lO " " and CuCu.lO + instead, the Zn-based moiety is quenched, with /tq = 1.1 x 10 s at 298 K. Quite peculiarly, for this [3]catenate a broad emission band with maximum at 560 nm is detected, that can hardly be assigned to an impurity of some sort [64], Tentatively, it was attributed to an intercomponent CT interaction which could play a role in the quenching of the LC fluorescence in CuZn.l0 +. 

Finally, the CuCo.l0 + catenate does not luminesce under any condition, showing that the potentially luminescent MLCT level of the Cu-complexed moiety is quenched by the Co-based one. A possible quenching mechanism is energy transfer, since the d Co + metal ion has low-energy d-d levels (<10 000 cm ) [65], but also electron transfer is thermodynamically allowed (AG = -0.54 eV). 

For the [2]catenand 5, two clearly distinct protonation reactions are found, and it was demonstrated unambiguously that both protonated forms have a catenate structure [60, 67], i.e., the two dap fragments are arranged in an entwined structure identical to that involved for metal catenates. The driving force for this unexpected behavior is likely to be the nn electronic donor-acceptor interactions between phenathroline and anisyl fragments that can be established only in the catenate-type arrangement, and not in a open form. The luminescence properties of 5 and its protonated forms H-S" " and (H2).5 are reported in Table 9. 

For the [3]catenand 10, four successive protonation reactions are observed via absorption and luminescence spectroscopy [66]. Interestingly, after the first protonation step a nonsymmetrical proton catenate is formed, where one moiety is protonated and the other is free. In this case, luminescence quenching of the unproto-nated subunit by the protonated one is found, most likely by energy transfer. 

The absorption spectra of the RuM.8" catenates [68] eorrespond to the sum of the spectra of the pertinent model compounds, as observed for the [3]catenates described in Seetion 8.3.3. This indicates that ground-state intercomponent interactions are weak, and is consistent with the fact that the two moieties are linked by aliphatic chains. The luminescence properties are reported in Table 11 and compared to those of the proper model compounds they show that in all cases there is always luminescence quenching of one of the two moieties, suggesting that strong intercomponent excited state interactions occur. 

Photophysical Properties of Catenate. and Knots 617 Table 13. Luminescence data for the CuIVI.14" knots in CH2Cl2 ... 

The luminescence properties of a 3-catenand (20) and related 3-catenates containing a copper(I) center [Cu 20]+ were reported in 1991 [96]. The photophysical data are summarized in Table 5. It is interesting to note that while the mononuclear [Cu 20]+ shows ligand-centered tctc, and MLCT emission, the dinuclear complex [Cu2 20] + only reveals a single MLCT phosphorescence band at 700 nm in CH2CI2. The mixed-metal complex [CuCo 20] + is not lumi-... 

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