Click Reaction Synthesis and Photophysical Studies of Dendritic Metalloporphyrins

Several dendritic zinc(II)-porphyrins bearing carbazole units at the terminals have been prepared through click reaction of azide-substituted Zn-porphyrin precursors and carbazole-based alkynes under [Cu(NCCH3)(4)][PF6] catalysis. This family of new dendritic metalloporphyrins shows dual luminescence from both the upper S-2 and the lowest S-1 singlet states. The observed trends in the spectroscopic data and the photophysical properties of the dendrimers have been rationalized in terms of the type of meso-spacer between the macrocycle and dendritic shell. The key feature of the meso-spacer is proposed to be the degree of hindrance towards aryl ring rotation relative to the mean porphyrin plane. Based on the observed differences in the S1S0 fluorescence quantum yield on going from four- to five-coordinate dendrimers, it was shown that dendrimer architectures with all four meso-aryl spacers sterically hindered, are most appropriate to monitor processes related to axial ligation of the dendrimer core, since they provide larger luminescence response differences between the four-coordinate and five-coordinate forms. The blue S2S0 fluorescence quantum yield has been measured and the observed trend has been rationalized in terms of the S2S1 energy gap law. No significant differences were observed between the compounds either with different rotational degree of freedom of the meso-spacers or with different ligation states of the dendrimer core.