Dicyano and pyridine derivatives of retinal: Synthesis, vibronic, electronic and photophysical properties

Abstract

Two new derivatives of all-trans retinal, one containing a pyridine and a cyanide substituent (15,15’-cyanopyridyl retinal, 2) and the other containing two cyanide substituents (15,15’-dicyano-retinal, 3) have been synthesized by the Knoevenagel reaction. Compound 2 crystallizes in the space group P-1; compound 3 crystallizes in the space group P21/n. The polyene chains are bowed. The compounds exhibit C≡N vibrations in the infra-red and Raman regions and undergo irreversible oxidations. Compounds 2 and 3 display electronic absorptions with maxima located at 442 nm and 487 nm, and emission peaks located at 583 nm and 500 nm, respectively. Density Functional Theory and Time Dependent Density Functional Theory calculations reveal that the HOMO and LUMO are located primarily on the polyene chain and the energy gap between them is consistent with the observed optical spectrum.