Photo-induced electron transfer of carotenoids in mesoporous sieves (MCM-41) and surface modified MCM-41: The role of hydrogen bonds on the electron transfer

Abstract Two types of hydrogen bonds (H-bond) can be formed when a carotenoid containing a hydroxy group, such as retinol, interacts with a silanol group on the surface of MCM-41. H-bond 1 , the oxygen atom of the carotenoid interacts with the proton of the SiOH group on the surface of MCM-41, and H-bond 2 , the proton of the OH group of the carotenoid interacts with the oxygen atom of SiOH group on the surface of MCM-41. DFT calculations show that the formation of the H-bond 1 decreases the LUMO of the carotenoid and stabilizes the neutral species more than the radical cation, and thus disfavors the photo-induced electron transfer (ET) from the carotenoid to MCM-41. The opposite is true for the formation of H-bond 2 . This conclusion is confirmed by the EPR study of the photo-induced ET of retinol imbedded in MCM-41 and the surface modified MCM-41. Although the formation of H-bond 1 results in low ET efficiency, the efficiency is much higher than that of the same carotenoid physisorbed on the surface of MCM-41. These results are relevant for improving the low solar-light-to-energy conversion efficiency in dye-sensitized solar cells.