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The far red region of the spectrum is increasingly being utilised in many applications in the biosciences. However, apart from the cyanine group of dyes, there are relatively few far red fluorescent probes available which are of practical use. We have synthesised and characterised a new class of far red fluorescent probes based on the perylene dione chromophore. The 2,10 di-substituted perylene diones possess broad absorption spectra (>90 nm bandwidths), large Stokes shifts (>60 nm) and quantum yields of up to 0.5 with a maximum absorption at 610–640 nm in organic solvents or in solutions of non-ionic detergents. A number of derivatives have been synthesised that can be used as membrane probes, as chromogenic substrates for alkaline phosphatase, and for the labelling of macromolecules such as proteins and DNA. These novel far red fluorophores have potential applications in diagnostic and research applications.