Photoinduced Electron Transfer between a Carotenoid and TiO₂Nanoparticle

Abstract

The dynamics of photoinduced electron injection and recombination between all-trans-8'-apo-beta-caroten-8'-oic acid (ACOA) and a TiO(2) colloidal nanoparticle have been studied by means of transient absorption spectroscopy. We observed an ultrafast ( approximately 360 fs) electron injection from the initially excited S(2) state of ACOA into the TiO(2) conduction band with a quantum yield of approximately 40%. As a result, the ACOA(*)(+) radical cation was formed, as demonstrated by its intense absorption band centered at 840 nm. Because of the competing S(2)-S(1) internal conversion, approximately 60% of the S(2)-state population relaxes to the S(1) state. Although the S(1) state is thermodynamically favorable to donate electrons to the TiO(2), no evidence was found for electron injection from the ACOA S(1) state, most likely as a result of a complicated electronic nature of the S(1) state, which decays with a approximately 18 ps time constant to the ground state. The charge recombination between the injected electrons and the ACOA(*)(+) was found to be a highly nonexponential process extending from picoseconds to microseconds. Besides the usual pathway of charge recombination forming the ACOA ground state, about half of the ACOA(*)(+) recombines via the ACOA triplet state, which was monitored by its absorption band at 530 nm. This second channel of recombination proceeds on the nanosecond time scale, and the formed triplet state decays to the ground state with a lifetime of approximately 7.3 micros. By examination of the process of photoinduced electron transfer in a carotenoid-semiconductor system, the results provide an insight into the photophysical properties of carotenoids, as well as evidence that the interfacial electron injection occurs from the initially populated excited state prior to electronic and nuclear relaxation of the carotenoid molecule.

Investigation of optoelectronic properties of N3 dye-sensitized TiO2 nano-crystals by hybrid methods: ONIOM (QM/MM) calculations

“…A visual comparison of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of N3 and some of its derivatives in the gas, acetonitrile, and ethanol phases shows an increasing and obvious shifting in absolute energy and reveal a clear difference in the Figure 2 The types of attaching anchor groups on the adsorbent layer [4].…”

“…Electron transfer (ET) at organic/inorganic interfaces plays a key role in a variety of applications including photocatalysis, photoelectrolysis, and solar cell and nano-scale electronics [1][2][3][4][5]. The dyesensitized nano-crystalline solar cell, or Grätzel cell, is a popular alternative in comparison to the costly traditional solar cell [3,6].…”

“…The dye is adsorbed in a prototype binding of the two carboxylic acid anchor groups of one of the bipyridine ligands in a bridging bi-dentate adsorption mode. Figure 2 shows the types of attachment anchor groups on the adsorbent layer [4].…”

<title>Beyond vibrationally mediated electron transfer: interfacial charge injection on a sub-10-fs time scale</title>

“…The back reaction and charge recombination to the ground state of the dye depends on the solvent, the presence/absence of electrolytes, the morphology of the TiO 2 and also on the amount of adsorbed cations on the semiconductor surface [32]. The time scales for the back reaction vary from sub-picosecond up to milliseconds [5,8,10,11,30,[33][34][35][36][37][38][39][40][41][42][43][44]. A detailed theoretical treatment of interfacial electron transfer is complicated, since reliable molecular models for the interface are needed and the presence of surface states or structural inhomogeneities have to be considered.…”

Fotocolheita em interface híbrida de molécula orgânica e óxido de titânio

“…As nanopartículas mais usadas são de óxido de titânio [11,12,13,14,15] mas também existem propostas para partículas de óxido de zinco ou sulfeto cádmio [16].…”