Two-step photoinduced charge separation and unexpectedly fast one-step charge recombination in a linked donor2-donor1-acceptor system

Dijk, Saskia I. van; Wiering, P. G.; Staveren, Richard van; Ramesdonk, Hendrik J. van; Brouwer, Albert M.; Verhoeven, J. W.

doi:10.1016/0009-2614(93)85673-c

Cited by 13 publications

(10 citation statements)

References 18 publications

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“…However, no effect was observed on the rates for either charge separation or recombination. Unusually fast charge recombination was seen in a triad of the form D 2 −D 1 −A, which undergoes two-step CS to yield the giant-dipole state D 2 + −D 1 −A - . The observed rapid CR reaction was rationalized in the context of a superexchange mechanism involving D−B + −A - virtual states.…”

Section: Introductionmentioning

confidence: 95%

Photoinduced Charge Separation Involving an Unusual Double Electron Transfer Mechanism in a Donor−Bridge−Acceptor Molecule

et al. 2000

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A series of rodlike donor−bridge−acceptor (D−B−A) molecules was synthesized to study the role of bridge energy levels on electron transfer (ET) rates. In these compounds, a 4-aminonaphthalene-1,8-imide (ANI) electron donor is linked to a 1,8:4,5-naphthalenediimide acceptor (NI) via the 1,4 positions on a phenyl bridge. The phenyl bridge is substituted at the 2 and 5 positions with methyl or methoxy groups to yield ANI-diMe-NI and ANI-diMeO-NI. These molecules differ only in the energy levels of the bridge molecular orbitals. Other parameters affecting ET rates such as donor−acceptor distance, orientation, and driving force are constant between the two systems. The rate constants for charge separation (CS) and charge recombination (CR) within ANI-diMeO-NI in toluene are 32 and 1400 times larger, respectively, than the corresponding rate constants for ANI-diMe-NI. Solvents of higher polarity diminish these differences in rate constants, making them comparable to those observed for ANI-diMe-NI. The relative energies of the ion pair states suggest that it is possible for the reaction 1*D−B−A → D−B+−A- to occur via a double electron-transfer process that is somewhat analogous to Dexter energy transfer. The lowest excited singlet state of the donor, 1*ANI, possesses about 70% charge-transfer character, so that significant positive charge is localized on its amine nitrogen, whereas significant negative charge is localized on its naphthalene-1,8-imide ring. Electron transfer from the naphthalene-1,8-imide ring of 1*ANI to NI is concomitant with electron transfer from the p-dimethoxybenzene bridge to the electron-deficient amine nitrogen atom in 1*ANI. A series of reference molecules in which the p-dimethoxybenzene bridge moiety is attached only to ANI or NI alone is used to establish the structural and electronic requirements for this unusual charge separation mechanism.

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Section: Introductionmentioning

confidence: 95%

Photoinduced Charge Separation Involving an Unusual Double Electron Transfer Mechanism in a Donor−Bridge−Acceptor Molecule

et al. 2000

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“…Triads that are based on donors and acceptors that are structurally and electronically simpler than porphyrins provide important insights into the charge separation process. For example, Verhoeven and co-workers have examined a number of interesting triad systems based on aniline electron donors and cyanoarene electron acceptors. − In addition, a number of triads that make use of the metal-to-ligand charge transfer (MLCT) excited states of ruthenium polypyridyl complexes, usually using viologen or diquat as the acceptor, have been studied. − …”

Section: Introductionmentioning

confidence: 99%

Multistep Photochemical Charge Separation in Rod-like Molecules Based on Aromatic Imides and Diimides

et al. 1996

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A series of intramolecular triads with linear, rod-like structures has been developed that undergo very efficient two-step electron transfer following direct excitation of a chromophore possessing a charge transfer (CT) excited state. The CT state of 4-aminonaphthalene-1,8-imide (ANI), produced by direct excitation of the chromophore, has about 70% of a negative charge transferred from the amine to the imide. Attachment of aniline (An) and p-methoxyaniline (MeOAn) donors to ANI by means of a piperazine bridge results in linear dyads, An-ANI and MeOAn- ANI, that undergo rapid electron transfer in about 10-11 s to give a >99% yield of the ion pairs, An + -ANI - and MeOAn + -ANI - , in which the charges are separated by 7.7 Å. The formation and decay of these ion pairs can be monitored directly by transient absorption spectroscopy. Further attachment of a 1,8:4,5-naphthalenediimide (NI) electron acceptor to the imide group of ANI using a 2,5-dimethylphenyl spacer results in triads An-ANI- NI and MeOAn-ANI-NI. Excitation of the CT state of ANI within these triads results in the same high yield charge separation step observed in the corresponding dyads followed by a subnanosecond charge shift reaction to yield the giant dipole states An + -ANI-NI - and MeOAn + -ANI-NI - in 72% and 92% yield, respectively, in toluene. The lifetime of MeOAn + -ANI-NI - is 310 ns. These triad molecules make explicit use of a CT excited state to initiate a multistep electron transfer process. The excited singlet CT state and the two ion pair states are all spectroscopically distinct, and all states are unambiguously spectrally resolved by transient absorption measurements. In addition, the ion pair states An + -ANI - and MeOAn + -ANI - undergo radiative recombination, thereby allowing a more detailed analysis of the energetics of charge separation and the influence of the CT excited state on the rates of subsequent longer distance charge shift reactions in these molecules.

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“…Photoinduced electron transfer processes in artificial bridged electron-donor (D)−electron-acceptor (A) systems are widely studied in order to understand factors that affect charge transfer (CT) rates and efficiencies. − One of the central goals in such research has been to mimic natural photosynthetic systems where the photoexcitation process is followed by multiple electron transfer steps, which lead to a long-lived trans-membrane charge-separated state in high yield. , Indeed, quite a number of multichromophoric systems, triads, − tetrads, , and even pentads 15 have recently been realized in which multistep charge transfer leads to a charge separation between the terminal chromophores. This, however, did not always 8 lead to a very significant increase in the lifetime of the charge separation, probably because the energy gap between the fully charge-separated state and the ground state in such systems tends to be small, which implies that charge recombination occurs under close-to-“optimal” conditions.…”

Section: Introductionmentioning

confidence: 99%

Long-Lived Triplet State Charge Separation in Novel Piperidine-Bridged Donor−Acceptor Systems

et al. 1996

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Two bichromophoric systems are presented that contain an N-alkylnaphthalimide electron acceptor and a 4-methoxyaniline (3a) or an aniline (3b) electron donor, respectively. Upon photoexcitation of 3a in cyclohexane electron transfer occurs in the singlet manifold to afford the short-lived (τ f) 0.75 ns) 1 (D +-A-) state in ca. 70% yield. An important decay pathway of this D +-A-state consists of intersystem crossing (ISC) to yield a triplet state localized on the naphthalimide moiety (D-3 A). In a slightly more polar solvent like din -butyl ether, an equilibrium between D-3 A and 3 (D +-A-) is observed by means of transient absorption spectroscopy. Both species decay with an overall decay time of ca. 1 µs. Thus, upon changing the spin multiplicity of the D +-A-state from singlet to triplet, an increase of its lifetime by three orders of magnitude is observed. In more polar solvents like dioxane, THF, and acetonitrile the 3 (D +-A-) state is the only species observed in the transient absorption spectrum, with decay times of ca. 1, 0.5, and 0.1 µs, respectively. The D-3 A state is the precursor state for the 3 (D +-A-) state in these solvents. It is proposed that, upon increasing solvent polarity, the singlet charge-separation process is retarded as a result of the large driving force (-∆G S°> 1 eV), which allows the triplet pathway (D-1 A f D-3 A f 3 (D +-A-)) to compete effectively. Compound 3b possesses a somewhat weaker donor chromophore than 3a resulting in a smaller driving force. The decay of the locally excited singlet state of 3b occurs mainly Via charge separation in the singlet manifold (D-1 A f 1 (D +-A-)). Only in the very polar solvent acetonitrile does the triplet pathway become competitive, and evidence is found for the formation of 3 (D +-A-).

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Two-step photoinduced charge separation and unexpectedly fast one-step charge recombination in a linked donor2-donor1-acceptor system

Cited by 13 publications

References 18 publications

Photoinduced Charge Separation Involving an Unusual Double Electron Transfer Mechanism in a Donor−Bridge−Acceptor Molecule

Photoinduced Charge Separation Involving an Unusual Double Electron Transfer Mechanism in a Donor−Bridge−Acceptor Molecule

Multistep Photochemical Charge Separation in Rod-like Molecules Based on Aromatic Imides and Diimides

Long-Lived Triplet State Charge Separation in Novel Piperidine-Bridged Donor−Acceptor Systems

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