Tuning the Förster overlap integral: energy transfer over 20 Ångstroms from a pyrene-based donor to borondipyrromethene (Bodipy).

A linear molecular dyad was synthesised comprising a pyrene-thiophene energy donor linked via a triazole unit to a borondipyrromethene (Bodipy) energy acceptor. The donor to acceptor separation distance is around 20 Å. Emission from the donor originates from a mixed π-π* and partial charge-transfer state and overlaps favourably with the absorption profile for the acceptor. The level of spectral overlap is dependent on the solvent polarity. Rates for electronic energy transfer were measured by transient absorption spectroscopy in solvents of varying polarity and refractive index. The measured rates for Förster energy transfer (k(EET)) correlate fairly well with the calculated overlap integrals (J(F)). A sigmoidal relationship is observed between k(EET) and the solvent polarity function ΔF.