Optimization of the Cyclic Cross-Hyperconjugation in 1,4-Ditetrelcyclohexa-2,5-dienes

Cyclic cross-hyperconjugation can
exist to variable extents in 1,4-ditetrelcyclohexa-2,5-dienes, i.e.,
all-carbon cyclohexa-1,4-dienes and 1,4-disila/digerma/distanna/diplumbacyclohexa-2,5-dienes.
In this study we first use density functional theory (DFT) computations
to optimize the conjugation strength by seeking the optimal atom E
and substituent group E′Me3 in the two saturated
E­(E′Me3)2 moieties (E and E′ as
the same or different tetrel (group 14) elements). We reveal that
the all-carbon cyclohexadienes with gradually heavier E′Me3 substituents at the two saturated carbon atoms display significant
cross-hyperconjugation. The first electronic excitations in these
compounds, which formally have two isolated CC bonds, are
calculated to reach wavelengths as long as 400 nm (excitation energies
of 3.1 eV). These transitions are mostly forbidden, and the lowest
allowed transitions are found at 387 nm (3.2 eV). The silicon analogues
are also cross-hyperconjugated, while a decline is observed in the
1,4-digerma/distanna/diplumbacyclohexa-2,5-diene. Experiments on two
substituted 1,4-disilacyclohexa-2,5-dienes confirm the effect of the
E′Me3 substituents, with regard to both electronic
excitations and geometries as determined by UV absorption spectroscopy
and X-ray crystallography, respectively. At the end, we reveal through
computations how electron-donating and electron-withdrawing substituents
at the CC double bonds influence the electronic properties
of the all-carbon ring. We find that the first calculated excitation,
which is forbidden, can be shifted to 440 nm (2.83 eV). This shows
to what extent cyclic cross-hyperconjugation can affect the electronic
and optical properties of a compound with two formally isolated CC
double bonds.