Photoexcitation of 11-Z-cis-7,8-dihydro retinal and 11-Z-cis retinal: A comparative computational study

We present vertical excitation properties calculated using Time-Dependent Density Functional Theory (TD-DFT) and Resolution of Identity second-order coupled cluster (RI-CC2) methods for the lowest energy states of 11-Z-cis retinal and the recently synthesized 11-Z-cis-7,8-dihydro retinal. Within 11-Z-cis retinal the lowest expected energy transition is from the HOMO (π) to LUMO (π∗), which results in the known isomerization. Contrary to 11-Z-cis retinal, the lowest energy transition within 11-Z-cis-7,8-dihydro retinal possesses some charge transfer character, and it is in fact the second lowest energy excitation from HOMO−1 (π) to LUMO (π∗) which could lead to the experimentally observed isomerization.