Tuning the optoelectronic properties of cyanine and ketocyanine dyes by incorporation of 9,9-di-n-propylfluorenylindolenine

•Cyanine and ketocyanine dyes containing 9,9-di-n-propylfluorenyl moieties are synthesized.•The 9,9-di-n-propylfluorenyl units enable tuning of the optoelectronic properties of the dyes.•The absorption properties of the dyes follow the Dewar–Knott colour rule.

The synthesis and properties of novel cyanine and ketocyanine dyes incorporating 9,9-di-n-propylfluorenyl units is reported. While bathochromic shifts of order 100 nm are observed in cyanines for addition of each vinylene unit (lengthening the methine chain) it was observed that addition of the fluorenyl unit enabled fine tuning of the UV–visible absorption spectrum. The hypsochromic shift in the UV–visible absorption spectrum of the ketocyanines relative to the corresponding cyanine dyes was found to be consistent with the Dewar–Knott colour rule. The oxidation (‘Highest Occupied Molecular Orbital energy’) and reduction (‘Lowest Unoccupied Molecular Orbital energy’) potentials were also qualitatively consistent with the Dewar–Knott colour rule, with the ‘HOMO energies’ of the comparative dyes being the same and the LUMO energies of the ketocyanines being destabilized relative to the cyanine dyes.

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