Pyrromethene dyes (BODIPY) can form ground state homo and hetero dimers: photophysics and spectral properties.

Homo and hetero dimerisation of two spectroscopically different BODIPY chromophores was studied, namely, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene and its 5-styryl-derivative. These exhibit very similar absorption and fluorescence spectral shape, but are mutually shifted by ca. 70 nm. For this reason the former and the latter are referred to as the green and red BODIPY, which here are denoted gB and rB, respectively. Various spectroscopic properties of the rB in different common solvents were determined. The calculated and experimental fluorescence quantum yield is found to be close to 100%, the fluorescence relaxation has a single exponential decay with a lifetime of about 4.5 ns, and the Förster radius for donor-donor energy migration is 67+/-1A. The dimerisation in different solvents was examined by using custom synthesised; mono and bis BODIPY-labelled forms of 1,2-cis-diaminocyclohexane. It is shown that gB and rB can form ground state homo- as well as hetero dimers. The dimers are non-fluorescent, compatible with H-dimers and may act as excitation traps or as acceptors to the corresponding excited monomers.