Dicyano and Pyridine Derivatives of Retinal: Synthesis and Vibronic, Electronic, and Photophysical Properties

Two new derivatives of all-trans retinal, one containing a pyridine and a cyanide substituent, all-trans-15-cyano-15′-pyridylretinal (2), and the other containing two cyanide substituents, all-trans-15,15′-dicyanoretinal (3), have been synthesized from all-trans-retinal (1) by the Knoevenagel reaction. Compound 2 crystallizes in the space group P1̅; compound 3 crystallizes in the space group P2₁/n. The polyene chains are bowed. The compounds exhibit CN vibrations in the infrared and Raman regions and undergo irreversible oxidations. Compounds 2 and 3 display electronic absorptions with maxima located at 442 and 487 nm and emission peaks located at 583 and 500 nm, respectively. Density Functional Theory and Time Dependent Density Functional Theory calculations reveal that the electronic transition from the HOMO to the LUMO can be associated with the A_g → B_u electronic transition and a peak corresponding to the HOMO → LUMO+1 transition can be associated with the “cis-peak”.