Immobilization of chlorophyll derivatives possessing ethoxysilyl groups into mesoporous silica and energy transfer between chromophores in mesopores

Abstract

Chlorophyll derivatives possessing ethoxysilyl groups were synthesized and grafted onto FSM-type mesoporous silica (FSM) to construct an energy transfer system between chromophores. Chlorophyllous pigments (M-APTS-Chl; M = Zn and Cu) were obtained by reacting metallo-substituted pyrochlorophyllide a with 3-aminopropyltriethoxysilane (APTS). FSM-type mesoporous silica was silylated with M-APTS-Chls in a dichloromethane solution. Unreacted silanol groups were silylated with APTS to prevent chlorophyll denaturation. The XRD pattern of grafted FSM, which is characteristic of a typical hexagonal array of mesopores, was quite similar to that observed for unmodified FSM, indicating that the internal structure was well retained after silylation. The green-color FSM powders were scarcely bleached by washing with acetone. This indicates that M-APTS-Chl formed Si¿O¿Si bondings with Si¿OH groups on FSM. Silylation was also confirmed by IR and solid-state 29Si NMR spectroscopies. The Qy absorption peaks of Zn-APTS-Chl and Cu-APTS-Chl grafted onto FSM were located at 660 and 655 nm, respectively. The visible absorption spectra of these FSM/M-APTS-Chl powders were similar to those of metallo-substituted Chl a in acetone. In the FSM powders containing both Zn-APTS-Chl and Cu-APTS-Chl, the absorption spectrum of co-adsorbed M-APTS-Chls was interpreted as a simple sum of the two pigments. However, the emission intensity of the co-adsorbed compound (Zn-APTS-Chl/Cu-APTS-Chl = 1/2) was decreased to one-fifth in comparison to that of FSM/Zn-APTS-Chl. The finding indicates that an efficient energy transfer occurs from Zn-APTS-Chl to Cu-APTS-Chl in the FSM-type mesoporous silica.