Molecules designed to carry out photochemical energy conversion typically employ several sequential electron transfers, as do photosynthetic proteins. Yet, these molecules typically do not achieve the extensive charge transport characteristic of semiconductor devices. We have prepared a large molecule in which four perylene-3,4:9,10-tetracarboxydiimide (PDI) molecules that both collect photons and accept electrons are attached to a central zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP) electron donor. This molecule self-assembles into ordered nanoparticles both in solution and in the solid-state, driven by van der Waals stacking of the PDI molecules. Photoexcitation of the nanoparticles results in quantitative charge separation in 3.2 ps to form ZnTPP(+)PDI(-) radical ion pairs, in which the radical anion rapidly migrates to PDI molecules that are, on average, 21 A away, as evidenced by magnetic field effects on the yield of the PDI triplet state that results from radical ion pair recombination. These nanoparticles exhibit charge transport properties that combine important features from both photosynthetic and semiconductor photoconversion systems.

We have prepared a green chromophore, 1,7-bis(pyrrolidin-1-yl)-3,4:9,10-perylene-bis(dicarboximide) (5PDI), that exhibits photophysical and redox properties similar to those of chlorophyll a (Chl a), yet unlike Chl a, it can be easily functionalized and incorporated into a wide variety of biomimetic electron donor−acceptor systems. The N,N ‘-dicyclohexyl derivative (5PDI) absorbs strongly (ε = 46 000 M-1 cm-1) at 686 nm in toluene and fluoresces at 721 nm with a 35% quantum yield. Additionally, 5PDI is both oxidized and reduced in CH2Cl2 at 0.57 V and −0.76 V vs SCE, respectively, making it a facile electron donor or acceptor. Rodlike covalent electron donor−acceptor molecules 5PDI−PI, 5PDI−NI, and 5PDI−PDI were prepared by linking the imide group of the 5PDI donor to pyromellitimide (PI), 1,8:4,5-naphthalenebis(dicarboximide) (NI), and 1,7-bis(3,5-di-tert-butylphenoxy)-3,4:9,10-perylene-bis(dicarboximide) (PDI) acceptors via an N−N bond. Following femtosecond laser excitation of 5PDI, 5PDI−PI, 5PDI−NI, and 5PDI−PDI in both toluene and 2-methyltetrahydrofuran, the formation and decay of their excited and radical ion pair states were monitored directly by transient absorption spectroscopy. We also report steady state emission and spectro-electrochemistry data for these molecules, which aid in elucidation of the transient spectra and the mechanisms of photoinduced charge separation. In toluene, charge separation occurs with high yield only in 5PDI−NI and 5PDI−PDI, whereas for 5PDI−PI charge separation is slow relative to excited-state decay of 1*5PDI−PI indicating that ΔG CS ≅ 0. This fact provides a means of estimating the ionic radii of the photogenerated ions, which for perylene chromophores 5PDI and PDI are 7.6 ± 0.5 Å, whereas those of the PI and NI electron acceptors are 5.6 ± 0.5 Å. These ionic radii are used in turn to determine the free energies of reaction of the remaining molecules with the series. Electroabsorption measurements are used to show that the change in dipole moment, Δμ that occurs upon formation of 1*5PDI is 3.5 D. The rates of charge separation in 5PDI−NI, 5PDI−PI, and 5PDI−PDI are compared to those of related donor−acceptor molecules having a 9-(pyrrolidin-1-yl)-perylene-3,4-dicarboximide (5PMI) donor. The 5PMI donor with Δμ = 15.4 D has a lowest excited singlet state with significantly higher charge-transfer character than does 5PDI, and has greater electron density near the imide group to which the acceptor is attached. The rate constants for charge separation from 1*5PMI are greater than those from 1*5PDI, which suggests that the rates of electron transfer from donors with CT excited states to an attached acceptor depend on the charge distribution in the CT excited state.

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The equilibrium acidities in DMSO for phenylhydrazine, five of its p-substituted derivatives, 1,2-diphenylhydrazine, and 1,1,2-triphenylhydrazine were measured and the BDEs of their acidic N−H bonds were estimated by using the following equation:  BDE = 1.37pK HA + 23.06E ox(A-) + 73.3 kcal/mol. The α-N−H bonds in the hydrazides CH3CONHNH2, PhCONHNH2, NH2NHCO2Et, and PhSO2NHNH2 were found to be 2 to 4 pK HA units more acidic than the α-N−H bonds in the corresponding amides, and the BDEs were estimated to be 23−27 kcal/mol weaker. Similarly, the BDE of a N−H bond in hydrazine was estimated to be 26 kcal/mol weaker than that of an N−H bond in NH3. Introduction of a RCO group into hydrazine had little or no effect on the BDE, but introduction of RCO into the β-position of PhCONHNH2 caused about an 8 kcal/mol increase in BDE. An increase in BDE was also observed for introduction of an RCO group into aniline. Here the carbonyl group is effectively destabilizing a nitrogen-centered radical by virtue of its strong electron-withdrawing effect. Incorporation of an open-chain carbohydrazide into a ring structure tends to strengthen the acidity of the N−H bond and weaken its BDEs.