Photoinduced hole transfer in QD-phthalocyanine hybrids.

A series of CdSe quantum dot (QD)-phthalocyanine (Pc) hybrids were synthesized and their photophysics was studied using steady state and time-resolved spectroscopic methods. Emission of QDs was progressively quenched upon increasing the concentration of Pc in the hybrids. A detailed transient absorption study of the hybrids revealed that the mechanism of quenching is charge separation, resulting in the formation of hybrids with negatively charged QDs and the Pc cation. Direct photo-excitation of Pc did not show any detectable interaction between the excited state of Pc and the QD to which it is attached. An explanation is proposed, based on the suggestion that the energy of the lowest unoccupied molecular orbital (LUMO) of Pc is lower than the lower edge of the QD conduction band, while the energy of the highest occupied molecular orbital (HOMO) of Pc is sufficiently higher than the high energy edge of the QD valence band (VB), thus permitting hole transfer from the QD VB to the Pc HOMO after photo-excitation of QDs.

Reaction dynamics of CN radicals with tetrahydrofuran in liquid solutions.

Transient, broadband infra-red absorption spectroscopy with picosecond time resolution has been used to study the dynamics of reactions of CN radicals with tetrahydrofuran (THF) and d(8)-THF in liquid solutions ranging from neat THF to 0.5 M THF in chlorinated solvents (CDCl(3) and CD(2)Cl(2)). HCN and DCN products were monitored via their v(1) (C≡N stretching) and v(3) (C-H(D) stretching) vibrational absorption bands. Transient spectral features indicate formation of vibrationally excited HCN and DCN, and the onsets of absorption via the fundamental bands of HCN and DCN show short (5-15 ps) delays consistent with vibrational relaxation within the nascent reaction products. This interpretation is confirmed by non-equilibrium molecular dynamics simulations employing a newly derived analytic potential energy surface for the reaction in explicit THF solvent. The rate coefficient for reactive formation of HCN (as determined from measurements on both the 1(1)(0) and 3(1)(0) fundamental bands) decreases with increasing dilution of the THF in CDCl(3) or CD(2)Cl(2), showing pseudo-first order kinetic behaviour for THF concentrations in the range 0.5-4.5 M, and a bimolecular rate coefficient of (1.57 ± 0.12) × 10(10) M(-1) s(-1) is derived. Simultaneous analysis of time-dependent HCN 1(1)(0) and 3(1)(0) band intensities following reaction of CN with THF (3.0 M) in CD(2)Cl(2) suggests that C-H stretching mode excitation is favoured, and this deduction is supported by the computer simulations. The results extend our recent demonstration of nascent vibrational excitation of the products of bimolecular reactions in liquid solution to a different, and more strongly interacting class of organic solvents. They serve to reinforce the finding that dynamics (and thus the topology of the reactive potential energy surface) play an important role in determining the nascent product state distributions in condensed phase reactions.

Quasi-classical trajectory study of the dynamics of the Cl + CH4→ HCl + CH3 reaction.

We present an on-the-fly classical trajectory study of the Cl + CH(4)→ HCl + CH(3) reaction using a specific reaction parameter (SRP) AM1 Hamiltonian that was previously optimized for the Cl + ethane reaction [S. J. Greaves et al., J. Phys Chem A, 2008, 112, 9387]. The SRP-AM1 Hamiltonian is shown to be a good model for the potential energy surface of the title reaction. Calculated differential cross sections, obtained from trajectories propagated with the SRP-AM1 Hamiltonian compare favourably with experimental results for this system. Analysis of the vibrational modes of the methyl radical shows different scattering distributions for ground and vibrationally excited products.