Enhancement of photodriven charge separation by conformational and intermolecular adaptations of an anthracene-perylenediimide-anthracene triad to an aqueous environment

Photoinduced electron-transfer dynamics of an electron donor-acceptor-donor triad, consisting of anthracenes and perylenediimide (An₂PDI), were investigated in different media by using time-resolved laser spectroscopic techniques. The aromatic components are attached by flexible linkers containing hydrophilic quaternary ammonium joints in the triad. In MeOH, in which An ₂PDI dissolves completely, no electron-transfer products were observed in the transient absorption measurements after the excitation of anthracenes and PDI because of the rapid back-electron transfer. The charge-separation rate of the triad in MeOH was estimated as 1.2 × 10 ¹⁰ s^-1 from the quenching of the singlet-excited state of PDI. In contrast, the formation of electron-transfer products was evident in water, and the electron-transfer rate was 200 times faster than the rate in MeOH in the course of the excitations of the selected components of An ₂PDI (2.5 × 10¹² s^-1). It is concluded from the time-resolved data that the conformational disposition of the hydrophilic joints due to hydrophilic-lipophilic interactions and the facile π-stacking of hydrophobic PDI cores in water results in the contraction and the relative rigidity of the electron-transfer distance and the intermolecular stabilization of electron-transfer species within the polymeric self-assemblies of An₂PDI, enabling an efficient photodriven electron-transfer process to occur.