Tuning Electronic Structure, Redox, and Photophysical
Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs
- Publication date
- Publisher
Abstract
Stepwise modification
of the methyl groups at the α positions of BODIPY <b>1</b> was used for preparation of a series of mono- (<b>2</b>, <b>4</b>, and <b>6</b>) and diferrocene (<b>3</b>) substituted
donor–acceptor dyads in which the organometallic substituents
are fully conjugated with the BODIPY π system. All donor–acceptor
complexes have strong absorption in the NIR region and quenched steady-state
fluorescence, which can be partially restored upon oxidation of organometallic
group(s). X-ray crystallography of complexes <b>2</b>–<b>4</b> and <b>6</b> confirms the nearly coplanar arrangement
of the ferrocene groups and the BODIPY π system. Redox properties
of the target systems were studied using cyclic voltammetry (CV) and
differential pulse voltammetry (DPV). It was found that the first
oxidation process in all dyads is ferrocene centered, while the separation
between the first and the second ferrocene-centered oxidation potentials
in diferrocenyl-containing dyad <b>3</b> is ∼150 mV.
The density functional theory-polarized continuum model (DFT-PCM)
and time-dependent (TD) DFT-PCM methods were used to investigate the
electronic structure as well as explain the UV–vis and redox
properties of organometallic compounds <b>2</b>–<b>4</b> and <b>6</b>. TDDFT calculations allow for assignment
of the charge-transfer and π → π* transitions in
the target compounds. The excited state dynamics of the parent BODIPY <b>1</b> and dyads <b>2</b>–<b>4</b> and <b>6</b> were investigated using time-resolved transient spectroscopy.
In all organometallic dyads <b>2</b>–<b>4</b> and <b>6</b> the initially excited state is rapidly quenched by electron
transfer from the ferrocene ligand. The lifetime of the charge-separated
state was found to be between 136 and 260 ps and demonstrates a systematic
dependence on the electronic structure and geometry of BODIPYs <b>2</b>–<b>4</b> and <b>6</b>