Picosecond excitation and selective intramolecular rates in supersonic molecular beams III: Photochemistry and rates of a charge transfer reaction

The picosecond state-selective dynamics and photochemistry of the molecule A–(CH₂)₃– φ, where A and φ are aromatic chromophores, was studied under collision-free conditions in a supersonic beam. Time-resolved fluorescence measurements of the reactant and the charge transfer (exciplex) product were undertaken as a function of specific vibrational energy above the zero point level of S₁. From these studies along with an analysis of the excitation spectra, dispersed flourescence, and quantum yields, the following results and conclusions were reached: (i) IVR is much faster than reaction at all excess energies studied. (ii) The energy threshold for product formation is E₀≃900 cm⁻¹ (2.6 kcal/mol). The analysis of the rates using an effective temperature model gives a frequency factor of A₀ ≈ 1.2 × 10¹⁰ s⁻¹. Four torsions were identified as critical to the reaction dynamics which were modeled according to a multidimensional reaction coordinate using an RRKM scheme. (iii) The thermodynamics of the isolated charge transfer product indicates strong stabilization ΔH = − 9.2 kcal/mol and extensive charge transfer, the static dipole moment is 13 D, and the charge transfer contribution to the total electronic wave function |c₂|² is 0.86. (iv) A comparison of the present work to solution phase studies of A–(CH₂)₃– φ indicates similar static properties but different dynamics. The calculated thermal (room temperature) reaction time for exciplex formation in the vapor (540 ps) was compared to the shortest observed value in solution (1.4 ns) to assess the role of the solvent on the chain motions which lead to product formation.