Exciton and vibronic effects in the spectroscopy of bianthracene in supersonic beams

Excitation and dispersed fluorescence spectra of 9,9’‐bianthracene in a supersonic expansion are reported. The spectra are anthracene‐like, indicating that the rings are weakly coupled. Exciton effects are considered in the interpretation of the spectra. The torsional potential in S_1 is modeled as a double‐well (Gaussian perturbation on a one‐dimensional harmonic oscillator) with barriers to perpendicularity and planarity of ∼30 and ∼1100 cm^(−1), respectively. The S_0 torsional potential shows negative anharmonicity which is modeled as a quartic perturbation. Anthracenic modes in S_1 and S_0 are also assigned. Finally, measurements of S_1 fluorescence lifetimes up to ∼6000 cm^(−1) excess energy in the excited state show no evidence of charge transfer

Picosecond photofragment spectroscopy. I. Microcanonical state-to-state rates of the reaction NCNO→CN+NO

This paper, the first in a series of three papers, gives a detailed account of our studies on picosecond photofragment spectroscopy. The unimolecular reaction NCNO→CN+NO is examined in detail here. Microcanonical state‐to‐state rates are measured in molecular beams at different energies in the reagent NCNO using pump–probe techniques: one picosecond pulse initiates the reaction from an initial (v,J) state and a second pulse, delayed in time, monitors the CN radical product in a specific rovibrational state, or the reagent NCNO (transient absorption). The threshold energy for reaction is determined to be 17 083 cm^(−1) (bond energy=48.8 kcal/mol). Measured rates are found to be sharply dependent on the total energy of the reagent, but independent of the rotational quantum state of product CN. Results of transient absorption measurements are used to argue that the ground statepotential energy surface dominates the reaction in the range of excess energies studied. The energy dependence of the rates, k_(MC)(E), is compared with that predicted by statistical theories. Both standard RRKM (tight transition state) and phase space theory (loose transition state) fail to reproduce the data over the full range of energies studied, even though nascent product state distributions are known to be in accord with PST at these energies. Furthermore, k_(MC)(E) is not a strictly monotonically increasing function of energy but exhibits some structure which cannot be explained by simple statistical theories. We advance some explanations for this structure and deviations from statistical theories

Weak Temperature Dependence of Electron Transfer Rates in Fixed-Distance Porphyrin-Quinone Model Systems

Electron transfer rate constants of several derivatives of [5-(4'-(4"-(2"',5"'-benzoquinonyl)bicyclo[2.2.2]octyl) phenyl)-2,3,7,8,12,13,17,18-octamethylporphyrinato]zinc(II) have been measured as a function of temperature in 2-methyltetrahydrofuran, toluene, and toluene-d_g. The observed temperature dependencies of the electron transfer rate constants are relatively weak in both the polar and nonpolar solvents. Nonexponential ET dynamics are observed at low temperatures and described in terms of an initial (k_(ET)^0) and an average ET rate constant (k_(av)). The k_(ET)^0 values for the molecules with different driving forces, spanning a range of 0.2 eV, show parallel trends over the range of temperatures studied. The trends in k_(ET)^0 are described in terms of the effects of temperature-dependent changes in solvent dielectric constants on the barrier height. Good agreement is observed for the case of toluene solvent, using a semiclassical model, but poorer quantitative agreement is found for the 2-methyltetrahydrofuran data. The temperature dependence of k_(av) is described using a model incorporating an angle-dependent electronic coupling and interconversion of rotational conformers. A temperature-dependent solvent isotope effect is observed on going from toluene to toluene-d_8, with k_(ET)^0( toluene)/k_(ET)^0( toluene-d_8) being as large at 1.5 over the range of temperatures studied

Weak Temperature Dependence of Electron Transfer Rates in Fixed-Distance Porphyrin -Quinone Model Systems

Electron transfer rate constants of several derivatives of [5-(4'-(4"-(2"',5"'-benzoquinonyl)bicyclo[2.2.2]octyl) phenyl)-2,3,7,8,12,13,17,18-octamethylporphyrinato]zinc(II) have been measured as a function of temperature in 2-methyltetrahydrofuran, toluene, and toluene-d_g. The observed temperature dependencies of the electron transfer rate constants are relatively weak in both the polar and nonpolar solvents. Nonexponential ET dynamics are observed at low temperatures and described in terms of an initial (k_(ET)^0) and an average ET rate constant (k_(av)). The k_(ET)^0 values for the molecules with different driving forces, spanning a range of 0.2 eV, show parallel trends over the range of temperatures studied. The trends in k_(ET)^0 are described in terms of the effects of temperature-dependent changes in solvent dielectric constants on the barrier height. Good agreement is observed for the case of toluene solvent, using a semiclassical model, but poorer quantitative agreement is found for the 2-methyltetrahydrofuran data. The temperature dependence of k_(av) is described using a model incorporating an angle-dependent electronic coupling and interconversion of rotational conformers. A temperature-dependent solvent isotope effect is observed on going from toluene to toluene-d_8, with k_(ET)^0( toluene)/k_(ET)^0( toluene-d_8) being as large at 1.5 over the range of temperatures studied