Tuning the Optoelectronic Properties of Stannoles by the Judicious Choice of the Organic Substituents

Stannoles
are organometallic rings in which the heteroatom is involved in a
form of conjugation that is called σ*−π* conjugation.
Only very little is known about how the substituents on the Sn atom or
substituents on the stannole ring determine the optoelectronic properties
of these heterocycles. In this work, this question has been studied experimentally
and theoretically. Calculations of optimized equilibrium geometries,
energy gaps between the highest occupied molecular orbitals (HOMOs)
and lowest unoccupied molecular orbitals (LUMOs), and of the absorption
spectra of a wide range of compounds were performed. The computational
data showed that the substituents on the Sn atom influence the optoelectronic
properties to a lower extent than the substituents in the 2 and 5
positions of the ring. These substituents in the 2 and 5 positions of the stannole
ring can also have a strong influence on the overall planarity of
the structure, in which mesomeric effects can play a substantial role
only if the structure is planar. Thus, only structures with a planar
backbone are of interest in the context of tuning the optoelectronic
properties. These were selected for the experimental studies. On the
basis of this information, a series of six novel stannoles was synthesized
by the formation of a zirconium intermediate and subsequent transmetalation
to obtain the tin compound. The calculated electronic HOMO–LUMO
energy gaps varied between 2.94 and 2.68 eV. The measured absorption
maxima were located between 415 and 448 nm compared to theoretically
calculated values ranging from 447 nm (2.77 eV) to 482 nm (2.57 eV).
In addition to these optical measurements, cyclic voltammetry data
could be obtained, which show two reversible oxidation processes for
three of the six stannoles. With this study, it could be demonstrated
how the judicious choice of the substituents can lead to large and
predictable bathochromic shifts in the absorption spectra.