Experimental and Theoretical Insights into Molecular and Solid-State Properties of Isomeric Bis(salicylaldehydes)

A series
of five bis­(salicylaldehydes), including four isomeric
compounds based on a benzene scaffold and a closely related naphthalene
derivative, were investigated in order to elucidate the impact of
resonance effects and intramolecular hydrogen bonds (HBs) on the macroscopic
properties of these systems. Density functional theory (DFT) computations
revealed important differences between isomers on the molecular level,
which was reflected in different charge distributions, aromatic C–C
bond orders, and aromaticity characters. The consequences of these
features were evidenced by the UV–vis absorption spectra: for
1,3-diformyl-4,6-dihydroxybenzene (2), the longest wavelength
absorption band is observed at 285 nm, while its isomers 1,4-diformyl-2,5-dihydroxybenzene
(1), 1,4-diformyl-2,3-dihydroxybenzene (3), and 1,2-diformyl-3,6-dihydroxybenzene (4) are characterized
by absorption in the visible range (379–407 nm). The specificity
of 2 results from simultaneous lowering and elevation
of HOMO and LUMO energy levels, respectively. We have found that the
HOMO/LUMO energy variations follow trends observed in isomeric dihydroxybenzenes
(HOMO) and phthalaldehydes (LUMO), and these effects operate separately
to some extent. Furthermore, theoretical calculations indicate that
the UV–vis spectral properties of bis­(salicylaldehydes) are
directly transferable to the corresponding bis­(salicylaldimines) and
their boron complexes. Finally, the influence of structural and molecular
stabilization effects was analyzed by means of X-ray structural analysis
and periodic DFT computations.