Rational Design of Conjugated Phenylpropanoid-Polyene Hybrids: Density Functional Theory Insights into Antiradical and Optical Properties.

A structural analysis of phenylpropanoids demonstrates that the benzene ring and the propenoic fragment act as two largely independent π-electron systems. This distinctive feature provides a theoretical basis for the rational design of novel compounds obtained through the structural integration of phenylpropanoids with polyene aldehydes and acids. These classes may be combined by elongating the carbon backbone via iterative vinyl group extension, thereby generating an expanded conjugated double-bond system. Alternatively, the structure of polyene aldehydes may be modified by replacing the unreactive methyl group with a benzene ring bearing suitable functional substituents. DFT computational studies performed at the B3LYP/QZVP level of theory indicate that the designed analogs predominantly scavenge radicals through the sequential proton loss electron transfer (SPLET) mechanism in aqueous environments. This pathway involves the initial deprotonation of carboxyl, aldehyde, or phenolic groups, with the hydroxyl moiety exhibiting the greatest propensity for proton dissociation. Carbon chain extension exerts only a minor influence on proton affinity (PA) values but significantly affects electron transfer enthalpy (ETE) parameters. Consequently, increasing the number of conjugated double bonds enhances activation of the second step of the SPLET mechanism, thereby improving overall radical-scavenging activity. Comparison of the calculated chemical reactivity parameters substantiates the conclusions drawn from the thermodynamic analysis. A pronounced enhancement in the reactivity of the modeled compounds, relative to the parent constituents, is observed. Time-dependent density functional theory (TD-DFT) calculations further predict absorption in the visible region, indicating potential applications of the modeled compounds as radical-scavenging dyes in food, pharmaceutical, cosmetic, and dietary supplement formulations.