Organic Ion-Pair Charge-Transfer (IPCT) Nanoparticles: Synthesis and Photoinduced Electrochromism

We report fabrication of photoinduced-electrochromic (or photochromic) organic nanoparticles that can be regarded as a new class of photoswitchable nanomaterials. Ion association between dicationic benzyl viologen (BzV²⁺) and tetraphenylborate anion (TPB^-) in water produces organic ion-pair charge-transfer-type (IPCT) nanoparticles with a mean diameter of 15-20 nm. Electrostatic binding of BzV²⁺ with TPB^- results in the prominent appearance of a new broad absorption band at 400-550 nm, which can be assign to the CT transition between these species. The IPCT nanoparticles show an interesting photoinduced electrochromic behavior caused by the electron transfer (ET) from TPB^- to BzV²⁺, giving an intense blue-color dispersion. The feed charge stoichiometry (= [TPB^-]/(2 × [BzV²⁺])) does not influence the kinetics of the photoinduced electrochromism, but it remarkably controls the reverse ET reaction (or retention time) in the nanoparticles; in the case of neutral stoichiometry, a very long-lived ET state can be observed, which is mainly due to decomposition of the oxidized TPB (sacrificial donor) and the resultant absence of the counterpart of benzyl viologen radical cations (BzV⁺•) for back ET reaction. On the other hand, the excess presence of TPB^- does not cause the decomposition of oxidized TPB in the nanoparticle, making the reverse ET reaction faster, but a moderately long-lived ET state is still observable, which is probably originated from efficient electron diffusion or transport between molecules inside the nanoparticle.