Abstract: Lanthanide complexes with organic ligands have been of interest due to the possibility of sensitising lanthanide ions through the antenna effect. We have recently shown that the zinc(II) salt of 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)-4,5,6,7-tetrafluorobenzothiazole, Zn(FBTZ)2, can provide very high levels of sensitisation to organic erbium complexes whilst maintaining very high quantum yields for the erbium near-infra-red luminescence. We now report the spectroscopic properties of the zinc(II) salts of 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)-6-chloro-4,5,7-trifluorobenzothiazole and 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)-6-bromo-4,5,7-trifluorobenzothiazole. The substitution of a single chlorine atom into the chromophore is sufficient to double the sensitisation efficiency compared to the fully fluorinated chromophore. This result implies a 50% reduction in the pump intensity needed to achieve population inversion in the erbium ions.

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