Electron vs Energy Transfer in Arrays Featuring Two Bodipy Chromophores Axially Bound to a Sn(IV) Porphyrin via a Phenolate or Benzoate Bridge

In this report we describe the synthesis of multichromophore
arrays
consisting of two Bodipy units axially bound to a Sn­(IV) porphyrin
center either via a phenolate (3) or via a carboxylate
(6) functionality. Absorption spectra and electrochemical
studies show that the Bodipy and porphyrin chromophores interact weakly
in the ground state. However, steady-state emission and excitation
spectra at room temperature reveal that fluorescence from both the
Bodipy and the porphyrin of 3 are strongly quenched suggesting
that, in the excited state, energy and/or electron transfer might
occur. Indeed, as transient absorption experiments show, selective
excitation of Bodipy in 3 results in a rapid decay (τ
≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant
rise of a charge-separated state evolving from the porphyrin-based
singlet excited state. In contrast, room-temperature emission studies
on 6 show strong quenching of the Bodipy-based fluorescence
leading to sensitized emission from the porphyrin moiety due to a
transduction of the singlet excited state energy from Bodipy to the
porphyrin. Emission experiments at 77 K in frozen toluene reveal that
the room-temperature electron transfer pathway observed in 3 is suppressed. Instead, Bodipy excitation in 3 and 6 results in population of the first singlet excited state
of the porphyrin chromophore. Subsequently, intersystem crossing leads
to the porphyrin-based triplet excited state.