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Over the last years, carbon nano-onions (CNOs) have been in focus in material science research. Their red-shifted absorption allows utilizing CNOs as promising photosensitizers. We report here a systematic study of excited state properties of six double-layered Li+ doped fullerenes of Ih symmetry: [Li@C60@C240]+, [Li@C60@C540]+, [Li@C60@C960]+, [Li@C240@C540]+, [Li@C240@C960]+ and [Li@C540@C960]+. On the basis of TDDFT calculations, we show that the long-wave absorption by the Li+ doped species leads to charge transfer (CT) between the inner and the outer shells unlike their neutral double-layered precursors. The CT energy depends strongly on the size of the concentric fullerenes and it can easily be tuned by varying both the encapsulated metal ion and the size of the shells. Two types of low-lying excited states are identified (1) capacitor-like structures, as Li+@C60-@C240+, with alternating positive and negative charges, and (2) states, where the positive charge is delocalized over the outer shell, as in Li@C240@C540+. We suggest a simple expression to estimate the energy difference of these excited states and to predict the type of the lowest CT state in nested fullerenes. The effect of nature of the encapsulated ion on the charge transfer state energies is considered. The HOMO - LUMO transition energy is found to vary significantly when going from [Li@C60@C240]+ to [Li@C540@C960]+

Photoinduced electron transfer reactions play an important role in chemistry, biochemistry, and material sciences. Electronic coupling of donor and acceptor is a key parameter that controls the rate of charge separation and charge recombination processes. The fragment charge difference (FCD) method is extended to calculate the electronic couplings and diabatic energies for the photoinduced reactions. It is shown that FCD provides consistent values of the ET parameters for any 3-state model system. We compare the matrix elements obtained within the 2- and 3-state treatment for different situations and suggest how to check adiabatic states included in the diabatization procedure. Two examples demonstrate the use of the FCD method in combination with MS-CASPT2 calculations to derive the ET parameters.

The possibility to optimize optoelectronic devices, such as organic light-emitting diodes or solar cells, by exploiting the special characteristics of triplet electronic states and their migration ability is attracting increased attention. In this study, we analyze how an intervening solvent modifies the distance dependence of triplet electronic energy transfer (TEET) processes by combining molecular dynamics simulations with quantum chemical calculations of the transfer matrix elements using the Fragment Excitation Difference (FED) method. We determine the β parameter characterizing the exponential distance decay of TEET rates in a stacked perylene dimer in water, chloroform, and benzene solutions. Our results indicate that the solvent dependence of β (β vacuum = 5.14 Å -1 > β water = 3.77 Å -1 > β chloroform = 3.61 Å -1 > β benzene = 3.44 Å -1) can be rationalized adopting the McConnell model of superexchange, where smaller triplet energy differences between the donor and the solvent lead to smaller β constants. We also estimate the decay of hole transfer (HT) and excess electron transfer (EET) processes in the system using the Fragment Charge Difference (FCD) method and find that β TEET can be reasonably well approximated by the sum of β EET and β HT constants C.C. acknowledges support from the Spanish Ministry of Economy and Competitivity through the Ramon y Cajal program (ref. RYC-2011-08918). A.A.V. acknowledges financial support from MICINN (Ministry of Science and Innovation, Spain) and the FEDER fund (European Fund for Regional Development) provided by grants CTQ2011-26573, UNGI08-4E-003, and UNGI10-4E-801

Recently synthesized π-stacked polyfluorenes with cofacially oriented fluorene moieties are promising materials for molecular electronics. In the paper, I consider triplet excitation energy transfe...

To decide whether counterions can affect the rate of charge recombination in DNA hairpins, the influence of the salt concentration on the free energy of this process was studied computationally using molecular dynamics simulations. The calculations were performed using the implicit treatment of the surroundings (the MM-PBSA and MM-GBSA techniques) and the explicit description of counterions and water molecules (thermodynamic integration). The data obtained with these distinct approaches were found to be in good agreement. The effect of counterions on the free energy of the charge recombination process was found to be small. For the hairpins with six A/T base pairs between the donor and acceptor chromophores, the difference in the value of the free energy does not exceed 0.5 kcal/mol when the environment changes from pure water to the 0.1 M NaCl solution typical for kinetic measurements of charge recombination in DNA hairpins. Even a smaller salt effect has numerically been obtained for a shorter hairpin c...

This contribution deals with a computational study of hole transfer (HT) and excess electron transfer (EET) in complexes of a buckyball and a molecular catcher having two concave buckybowls as described by Sygula et al. (J. Am. Chem. Soc. 2007, 129, 3842). Three systems are considered: (1) the inclusion complex consisting of the fullerene and catcher subunits, (2) a system formed by two catchers, and (3) a supermolecular system consisting of two fullerene and two catcher molecules. Such a type of organic system is of potential interest in the design of nanoelectronic devices. HT and EET electronic couplings of the subunits in the systems are calculated using the HF/6-31G* method and the semiempirical INDO/S scheme; the calculation of the reorganization energy is carried out within the B3LYP/6-31G* method. On the basis of the computed values of electronic couplings and reorganization energies, we predict that excess charge (hole or electron) should be strongly confined to a single subunit in the complexes. Then, we suggest that excess charge propagation within the fullerene-catcher system in the solid state occurs as superexchange mediated hopping; in this process, fullerene molecules function as stepping stones, while the catcher effectively mediates the charge transport between buckyballs. The properties of the buckycatcher, acting as a charge-transfer mediator from and to fullerenes, are found to be quite different for electron- and hole-transfer processes. We estimate the absolute rate of the hole and the excess electron hopping between fullerenes, kHT = 1.28 1012 s-1, while kEET = 1.07 109 s-1 and find that the HT is much faster than the EET because of (1) the stronger HT coupling between neighboring buckybowls of the catchers and (2) the smaller HT reorganization energy relative to that for EET This work has been supported by Project No. CTQ2005-04563 of the Spanish Ministerio de Educación y Ciencia (MEC)

Photoinduced hole injection by an acridine derivative X + within the complex 5'-TG-2TX+TG2T-3/ has been studied using quantum chemical calculations. The probability of charge shift from the excited state of the intercalated chromophore, (X + )*, to the guanine acceptors, G -2 and G 2 , is computed for 24 different conformations of the complex generated by rotation of the chromophore about the DNA axis. Because the free energies and the reorganization energies for the charge shift in both direction are very similar, the probability of G -2 or G 2 oxidation is determined by the ratio of squared electronic couplings between (X + )* and the diabatic states with a hole on the guanine bases. To estimate the electronic couplings we employ the multistate generalized Mulliken-Hush method and electronic adiabatic states of the system calculated within the multiconfigurational INDO/S approach. We show that the rate of the charge shift reaction critically depends on the chromophore position and that there are well-separated regions in the conformational space, where the hole charge is mainly injected either into G -2 or into G 2 or is unlikely. Thus, the direction of the hole injection can be switched by rotation of the intercalated chromophore and the photocleavage efficiency of the acridine derivative and related species can be modulated by the conformational changes.