Push–pull dyes containing malononitrile dimer as acceptor: synthesis, spectroscopy and quantum chemical calculations

Abstract

The effect of structural variations of either the donor (number and position of methoxy groups, replacement of the aryl by Fischer's base) or the acceptor moiety (bisacylation of the amino group, extension of the conjugated system by condensation with dimethylformamide–dimethylacetale) of a series of dyes containing malononitrile dimer as acceptor is investigated both experimentally and computationally. Semiempirical (AM1) and ab initio (HF/6-31G∗ and HF/3-21G) calculations are used for a conformational analysis of the acceptor groups and model systems thereof. Electronic excitation energies are obtained by ZINDO, ab initio CIS/6-31+G∗ and time-dependent density functional theory (TDDFT(B3LYP)/6-31+G∗) computations. Inclusion of solvent effects is important for a reliable description of the trends observed in the experimental absorption spectra by ZINDO. Methoxyaryl, even polysubstituted, derivatives absorb at significantly shorter wavelengths than the corresponding p-dimethylamino compounds. Replacement of the p-dimethylaminoaryl donor by Fischer's base leads to a bathochromic shift, as does the bisacylation of malononitrile dimer's amino group. Functionalization by condensation with dimethylformamide–dimethylacetale generally has only a small and irregular effect on absorption spectra.