Theoretical study on photophysical properties of 3′-hydroxyechinenone and the effects of interactions with orange carotenoid protein

- Singlet excited states of a carbonyl carotenoid were studied by RASPT2 calculations.
- The S1 state is 2Ag− and it gains a partial ICT character at twisted geometries.
- The origin of ICT character is mixing with 1Bu+ having a large dipole moment.
- Hydrogen-bonding interactions with the protein enhance the ICT character.