Electronic structure of neutral and charged C60 clusters

Contribution of thermal energy to initial ion production in matrix-assisted laser desorption/ionization observed with 2,4,6-trihydroxyacetophenone

RATIONALE

Although several reaction models have been proposed in the literature to explain matrix-assisted laser desorption/ionization (MALDI), further study is still necessary to explore the important ionization pathways that occur under the high-temperature environment of MALDI. 2,4,6-Trihydroxyacetophenone (THAP) is an ideal compound for evaluating the contribution of thermal energy to an initial reaction with minimum side reactions.

METHODS

Desorbed neutral THAP and ions were measured using a crossed-molecular beam machine and commercial MALDI-TOF instrument, respectively. A quantitative model incorporating an Arrhenius-type desorption rate derived from transition state theory was proposed. Reaction enthalpy was calculated using GAUSSIAN 03 software with dielectric effect. Additional evidence of thermal-induced proton disproportionation was given by the indirect ionization of THAP embedded in excess fullerene molecules excited by a 450 nm laser.

RESULTS

The quantitative model predicted that proton disproportionation of THAP would be achieved by thermal energy converted from a commonly used single UV laser photon. The dielectric effect reduced the reaction Gibbs free energy considerably even when the dielectric constant was reduced under high-temperature MALDI conditions. With minimum fitting parameters, observations of pure THAP and THAP mixed with fullerene both agreed with predictions.

CONCLUSIONS

Proton disproportionation of solid THAP was energetically favorable with a single UV laser photon. The quantitative model revealed an important initial ionization pathway induced by the abrupt heating of matrix crystals. In the matrix crystals, the dielectric effect reduced reaction Gibbs free energy under typical MALDI conditions. The result suggested that thermal energy plays an important role in the initial ionization reaction of THAP.

Molecular Structure and Dynamics at the Interfaces within Bulk Heterojunction Materials for Solar Cells

The molecular structures within the interfaces of the bulk heterojunction material comprising regioregular-poly(3-hexylthiophene-2,5-diyl), rrP3HT, and C(60) or its soluble derivative, [6,6]-phenyl-C(61)butyric acid methyl ester, PCBM, have been studied by one- and two-dimensional nuclear magnetic resonance (NMR). The local structure within the interface was inferred from chemical shift (CS) data obtained from composite films (CFs) fabricated at room temperature (PCBMCF-RT and C(60)CF-RT) and from CFs that had been subsequently annealed at 150 degrees C for 30 min (PCBMCF-A150 and C(60)CF-150A). In PCBMCF-RT, the alkyl side chains of rrP3HT are close to the C(60) ball; C(60) is essentially 'wrapped' by the alkyl side chains. In PCBMCF-A150, the alkyl side chains self-assemble such that rrP3HT and PCBM are separated. The observation of well-defined splittings in the CS spectrum of the (13)C of C(60) in C(60)CF-A150 indicates a distortion from spherical symmetry. Measurements of the spin-lattice relaxation rate, 1/T(1), of C(60) imply local magnetic field fluctuations that arise from the dynamics of the C(60) distortion.

Direct experimental probe of the on-site Coulomb repulsion in the doubly charged fullerene anion C70 2-

Vibrationally resolved photoelectron spectra were obtained for cold C(70)- and C(70)2-. Accurate values for the first and second electron affinities (EA's) of C(70) were measured as 2.765 +/- 0.010 and 0.002 (+0.01/-0.03)eV, respectively, establishing that C(70)-2 is an electronically stable dianion in the gas phase. The difference between the first and second EA (2.75 eV) provides a direct experimental measure for the on-site Coulomb and exchange interactions between the two excess electrons in C(70)-2. Strong electron correlation effects were also observed between the two excess electrons in C(70)-2.