Facilitating Electron Transfer by Resizing Cyclocarbon Acceptor from C18 to C16.

Recent advances in synthetic methods, combined with tip-induced on-surface chemistry, have enabled the formation of numerous cyclocarbon molecules. Here, we investigate computationally the experimentally studied C16 and C18 molecules as well as their van der Waals (vdW) complexes with several typical donor and acceptor molecules. Our results demonstrate a remarkable electron-withdrawing ability of cyclocarbon molecules. The vdW complexes of C16 and C18 exhibit a thermodynamically favorable photoinduced electron transfer (ET) from the donor partner to the cyclocarbons that occurs on a picosecond time scale. The lower reorganization energy of C16 compared to C18 leads to a significant acceleration of the ET reactions.

Aromaticity controls the excited-state properties of host-guest complexes of nanohoops.

π-Conjugated organic molecules have exciting applications as materials for batteries, solar cells, light-emitting diodes, etc. Among these systems, antiaromatic compounds are of particular interest because of their smaller HOMO-LUMO energy gap compared to aromatic compounds. A small HOMO-LUMO gap is expected to facilitate charge transfer in the systems. Here we report the ground and excited-state properties of two model nanohoops that are nitrogen-doped analogs of recently synthesized [4]cyclodibenzopentalenes - tetramers of benzene-fused aromatic 1,4-dihydropyrrolo[3,2-b]pyrrole ([4]DHPP) and antiaromatic pyrrolo[3,2-b]pyrrole ([4]PP). Their complexes with C60 fullerene show different behavior upon photoexcitation, depending on the degree of aromaticity. [4]DHPP acts as an electron donor, whereas [4]PP is a stronger electron acceptor than C60. The ultrafast charge separation combined with the slow charge recombination that we found for [4]PP⊃C60 indicates a long lifetime of the charge transfer state.

Nitrogen-doped molecular bowls as electron donors in photoinduced electron transfer reactions.

In recent years, the chemistry of curved π-conjugated molecules has experienced a sharp rise. The inclusion of a heteroatom in the carbon network significantly affects its semiconducting properties. In this work, we computationally study the photoinduced electron transfer in a series of C60 fullerene complexes with experimentally established nitrogen-doped molecular bowls. Our results demonstrate that introducing nitrogen into pentagonal rings of the bowl-shaped π-conjugated molecules and extending the π-conjugation can modulate their electron-transfer properties. Among the studied complexes, the hub-NCor⊃C60 complex exhibits the most desirable combination of ultrafast charge separation and slow charge recombination, suggesting its potential use in photovoltaics.

Photoinduced electron transfer in non-covalent complexes of C60 and phosphangulene oxide derivatives.

Investigation of photoinduced electron transfer (PET) in a series of experimentally reported complexes of fullerene with phosphangulene oxides shows that the replacement of O atoms in the bridge of phosphangulene with S atoms promotes efficient and ultrafast ET from phosphangulene oxide to fullerene in PGOOSS⊃C60 and PGOSSS⊃C60 complexes. The results obtained can be useful for the development of photovoltaic devices based on phosphangulenes.

Photoinduced electron transfer in nano-Saturn complexes of fullerene.

The photoinduced electron transfer is studied computationally in several Saturn-shaped inclusion complexes of carbo-aromatic rings and C60 fullerene - C72⊃C60, C96⊃C60, C120⊃C60, and C168⊃C60. Analysis of their structural and electronic properties shows that the charge separation process is efficient in C120⊃C60 and C168⊃C60 where the host molecule resembles the conjugated [24]circulene unit. In contrast, the electron transfer is not feasible in the complexes of the oligophenylene-based rings C72⊃C60 and C96⊃C60.

Electron Transfer in a Li+-Doped Zn-Porphyrin-[10]CPP⊃Fullerene Junction and Charge-Separated Bands with Opposite Response to Polar Environments.

Recently synthesized porphyrin-cycloparaphenylene (ZnP-[10]CPP) junction is a powerful platform to develop useful organic photovoltaic devices. In this work, we computationally study photoinduced electron transfer processes in the supramolecular complex ZnP-[10]CPP⊃C60 and its Li+-doped derivative. The most striking finding is charge-separated (CS) bands in ZnP-[10]CPP⊃Li+@C60 with opposite response to solvent polarity. Besides CS bands that demonstrate a bathochromic shift, there exist CS transitions showing a rarely observed hypsochromic shift. The rates of energy transfer, charge separation, and charge recombination in the supramolecular complexes are computed by using the semiclassical approach. These estimates suggest that the both types of CS states can be efficiently populated in polar media by decay of locally excited states.

Photoinduced electron transfer and unusual environmental effects in fullerene-Zn-porphyrin-BODIPY triads.

Molecular arrays containing donor-acceptor sites and antenna molecules are promising candidates for organic photovoltaic devices. Photoinduced electron transfer (PET) in multi-chromophore systems is controlled by a subtle interplay of donor and acceptor properties and solvent effects. In the present study, we explore how PET of fullerene [C60]-Zn-porphyrin-BODIPY triads can be modulated by passing from non-polar to polar media. To this end we perform a computational study of this complex using the DFT/TDDFT method. We find that the stabilization energy of charge transfer states by a polar medium depends significantly on whether the BODIPY moiety acts as an electron donor or an electron acceptor. To understand this effect of the environment, a detailed analysis of the initial and final states of the ET reactions is performed. We show that additional deactivation channels of the porphyrin excited state may come into play as solvent polarity increases.

Reliable charge assessment on encapsulated fragment for endohedral systems.

A simple scheme to determine charge distribution in endohedral complexes is suggested. It is based on comparison of inner-shell atomic orbital energies of the encapsulated species to the corresponding energies in reference systems with unambiguously defined charges on X. This robust approach is applied to endohedral borospherenes X@B39, for which the conventional schemes provide in some cases quite different results. Efficiency of proposed scheme also has been proven for typical fullerene based Sc3N@C80 endohedral complex.

On the performance of the Kohn-Sham orbital approach in the calculation of electron transfer parameters. The three state model.

We have tested the performance of the Kohn-Sham orbital approach to obtain the electronic coupling and the energetics for hole transfer (HT) in the guanine-indole pair, using a three-state model. The parameters are derived from the simple DFT calculations with 10 different functionals, and compared with benchmark MS-CASPT2 calculations. The guanine-indole pair is a simple model for HT in DNA-protein complexes, which has been postulated as a protection mechanism for DNA against oxidative damage. In this pair, the first excited state of the indole radical cation has low energy (less than 0.3 eV relative to the ground state of the cation), which requires the application of very accurate quantum chemical methods and the invocation of a 3-state model. The Kohn-Sham orbital approach has been tested on six π stacked and three T-shaped conformers. It has been shown to provide quite accurate results for all ten tested functionals, compared to the reference MS-CASPT2 values. The best performance has been found for the long-range corrected CAM-B3LYP functional. Our results suggest that the Kohn-Sham orbital method can be used to estimate the excited state properties of radical cation systems studied using transient spectroscopy. Because of its accuracy and its low computational cost, the approach allows one to calculate relatively large models and to account for the effects of conformational dynamics on HT between DNA and a protein environment.