Efficient harvesting of solar energy, without interference from electron transfer, is reported for a series of bichromophoric light-harvesting antenna molecules.

We report here the synthesis and photophysical study of a series of electron donor–acceptor molecules, in which electron-donating 4-methoxyphenoxy groups are attached to the 1,7-bay positions of four different perylene tetracarboxylic acid derivatives, namely, perylene tetraesters 1, perylene monoimide diesters 2, perylene bisimides 3, and perylene monobenzimidazole monoimides 4. These perylene derivatives are used because of their increasing order of electron-accepting capability upon moving from 1 to 4. Two additional donor–acceptor molecules are synthesized by linking electron-donating 4-methoxyphenyl groups to the imide position of perylene monoimide diester 2 and perylene bisimide 3. The motivation for this study is to achieve a good control over the photoinduced charge-transfer (CT) process in perylene-based systems by altering the position of electron donors and tuning the electron deficiency of perylene core. A comprehensive study of the photophysical properties of these molecules has shown a highly systematic trend in the magnitude of CT as a function of increased electron deficiency of the perylene core and solvent polarity. Importantly, just by changing the attachment of electron-donating group from “bay” to “imide” position, we are able to block the CT process. This implies that the positioning of the electron donor at the perylene core strongly influences the kinetics of the photoinduced CT process. In these compounds, the CT process is characterized by the quenching of fluorescence and singlet excited-state lifetimes as compared to model compounds bearing non-electron-donating 4-tert-butylphenoxy groups. Transient absorption spectroscopy did not reveal spectra of CT states. This most likely implies that the CT state is not accumulated, because of the faster charge recombination.

The excited-state dynamics of perylene-based bichromophoric light harvesting antenna systems has been tailored by systematic modification of the molecular structure and by using solvents of increasing polarity in the series toluene, chloroform, and benzonitrile. The antenna systems consist of blue light absorbing naphthalene monoimide (NMI) energy donors (D1, D2, and D3) and the perylene derived green light absorbing energy acceptor moieties, 1,7-perylene-3,4,9,10-tetracarboxylic tetrabutylester (A1), 1,7-perylene-3,4,9,10-tetracarboxylic monoimide dibutylester (A2), and 1,7-perylene-3,4,9,10-tetracarboxylic bisimide (A3). The design of these antenna systems is such that all exhibit ultrafast excitation energy transfer (EET) from the excited donor to the acceptor, due to the effective matching of optical properties of the constituent chromophores. At the same time, electron transfer from the donor to the excited acceptor unit has been limited by the use of a rigid and nonconjugated phenoxy bridge to link the donor and acceptor components. The antenna molecules D1A1, D1A2, and D1A3, which bear the least electron-rich energy donor, isopentylthio-substituted NMI D1, exhibited ultrafast EET (τEET ∼ 1 ps) but no charge transfer and, resultantly, emitted a strong yellow-orange acceptor fluorescence upon excitation of the donor. The other antenna molecules D2A2, D2A3, and D3A3, which bear electron-rich energy donors, the amino-substituted NMIs D2 and D3, exhibited ultrafast energy transfer that was followed by a slower (ca. 20–2000 ps) electron transfer from the donor to the excited acceptor. This charge transfer quenched the acceptor fluorescence to an extent determined by molecular structure and solvent polarity. These antenna systems mimic the primary events occurring in the natural photosynthesis, i.e., energy capture, efficient energy funneling toward the central chromophore, and finally charge separation, and are suitable building blocks for achieving artificial photosynthesis, because of their robustness and favorable and tunable photophysical properties.

The synthesis and characterization of different conjugated phthalocyanine–perylenemonoimidebenzimidazole [ZnPc-PBIm­(OR)4] and nonconjugated phthalocyanine–perylenediimide [ZnPc-PDI­(OR)4] dyads are carried out. UV–vis, 1H NMR, and electrochemistry measurements reveal the interaction between perylene and phthalocyanine moieties in the ground state in the conjugated hybrid and the lack of interaction in the nonconjugated one. Ultrafast transient absorption measurements show that a state with substantial charge-transfer character is formed in both compounds, but the rates for the formation and recombination from this state are much faster for the conjugated compound.

A new design for the perylene dye-based light-harvesting antenna systems is presented, which enables complete suppression of parasitic intramolecular charge-transfer.

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