Efficient Förster Resonance Energy Transfer in 1,2,3-Triazole Linked BODIPY-Zn(II) Meso-tetraphenylporphyrin Donor–Acceptor Arrays

Cu­(I) catalyzed azide–alkyne cycloaddition (CuAAC)
reactivity
was successfully employed to synthesize three donor–acceptor
energy transfer (EnT) arrays that contain one (Dyad),
three (Tetrad) and four (Pentad) 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(BODIPY) donors connected to a Zn-tetraphenylporphyrin acceptor via
1,2,3-triazole linkages. The photophysical properties of the three
arrays, along with individual donor and acceptor chromophores, were
investigated by UV–vis absorption and emission spectroscopy,
fluorescence lifetimes, and density functional theory (DFT) electronic
structure modeling. Comparison of the UV–vis absorption spectra
and frontier molecular orbitals from DFT calculations of the three
arrays with ZnTPP, ZnTTrzlP, and Trzl-BODIPY shows that the electronic structure of the chromophores is essentially
unperturbed by the 1,2,3-triazole linkage. Time-dependent DFT (TDDFT)
calculations on the Dyad reproduce the absorption spectra
in THF and show no evidence of excited state mixing of the donor and
acceptor. The BODIPY singlet excited state emission is significantly
quenched in all three arrays, consistent with EnT to the porphyrin
core, with efficiencies of 95.8, 97.5, and 97.2% for the Dyad, Tetrad, and Pentad, respectively. Fluorescence
excitation spectra of the three arrays, measured at the porphyrin
emission, mirror the absorption profile of both the porphyrin and
BODIPY chromophores and are consistent with the Förster resonance
energy transfer (FRET) mechanism. Applying Förster theory to
the spectroscopic data of the chromophores gives EnT efficiency estimates
that are in close agreement with experimental values, suggesting that
the through-space mechanism plays a dominant role in the three arrays.