Near-IR Emissive Chlorin–Bacteriochlorin Energy-Transfer Dyads with a Common Donor and Acceptors with Tunable Emission Wavelength

Design,
synthesis, and optical properties of a series of novel
chlorin–bacteriochlorin energy transfer dyads are described.
Each dyad is composed of a common red-absorbing (645–646 nm)
chlorin, as an energy donor, and a different near-IR emitting bacteriochlorin,
as an energy acceptor. Each bacteriochlorin acceptor is equipped with
a different set of auxochromes, so that each of them emits at a different
wavelength. Dyads exhibit an efficient energy transfer (≥0.77)
even for chlorin–bacteriochlorin pairs with large (up to 122
nm) separation between donor emission and acceptor absorption. Excitation
of the chlorin donor results in relatively strong emission of the
bacteriochlorin acceptor, with a quantum yield Φf range of 0.155–0.23 in toluene and 0.12–0.185 in DMF.
The narrow, tunable emission band of bacteriochlorins enables the
selection of a series of three dyads with well-resolved emissions
at 732, 760, and 788 nm, and common excitation at 645 nm. Selected
dyads have been also converted into bioconjugatable N-succinamide ester derivatives. The optical properties of the described
dyads make them promising candidates for development of a family of
near-IR fluorophores for simultaneous imaging of multiple targets,
where the whole set of fluorophores can be excited with the common
wavelength, and fluorescence from each can be independently detected.