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–[cursive phi], where A and [cursive phi] 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[approximately-equal-to]900 cm^−1 (2.6 kcal/mol). The analysis of the rates using an effective temperature model gives a frequency factor of A0[approximately-equal-to]1.2×10^10 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 DeltaH=−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–[cursive phi] 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

Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments

In this and the following paper, we present a full account of our earlier report [Syage et al., Chem. Phys. Lett. 88, 268 (1982)] on the spectra and picosecond dynamics of stilbene isomerization in supersonic jets. The jet-cooled excitation and dispersed fluorescence spectra of t-stilbene-h12 and -d12 are reported and assigned for the Bu 00 wavelengths for h12 and d12 (in excitation) are 3101.4 and 3092.5 Å, respectively. Previously unidentified low frequency modes (as low as 20 cm^−1 in S0 for -h12) have been observed and tentatively assigned as out-of-plane modes of au symmetry in C2h. This indicates that t-stilbene has a propeller-like geometry involving phenyl rotation (i.e., C2 symmetry). A Franck–Condon analysis of the low frequency modes and particularly the ag, nu25 in-plane symmetric bend mode indicates that a large geometry change takes place upon electronic excitation possibly due to a delocalization of double bond character from the Ce–Ce bond to Ce–[cursive phi] bond. The geometry change of the in-plane Ce–Ce–[cursive phi] between S1 and S0 was determined from the Franck–Condon and a normal mode analysis to be 1.3°±0.3°. The rms amplitude of this bend motion for the symmetric nu25 bend mode (for one quanta in S0) is |^2|^1/2=1.0±0.2°. Most ag modes involving benzene-type vibrations (other than C–H stretch modes) have been assigned. Dispersed fluorescence spectra exhibited a broad background indicative of IVR which increased rapidly with S1 vibrational energy. The spectra were completely diffuse above 1200 cm^−1 which is consistent with the barrier for isomerization being at about 1100–1200 cm^−1. The excitation spectra show a rapid decline in intensity at higher energies due to the process of isomerization which competes with radiative decay. However, sharp (albeit weak) structure could still be discerned at energies well in excess of 2000 cm^−1. In the accompanying paper, we present results on the dynamics of isomerization and its dependence on mode mixing and the nature of the reactive surface (adiabatic vs diabatic)

Picosecond dynamics and photoisomerization of stilbene in supersonic beams. II. Reaction rates and potential energy surface

Using picosecond excitation in a supersonic jet, we present a full account of our earlier report on the dynamics of state-selective photoisomerization of t-stilbene. Collisionless isomerization in this case indicates the twisting of the molecule about the ethylene bond away from the trans configuration Central to this reaction is the question of vibrational energy redistribution or IVR. From direct (single vibronic level) time-resolved measurements, relative fluorescence quantum yields from relaxed and unrelaxed states, and a thorough vibrational analysis from excitation and dispersed fluorescence spectra (previous paper), the following conclusions are reached: (i) The IVR yield is state selective being more extensive from combination modes than from fundamental modes of similar energy. The IVR yield becomes very significant above [approximately-equal-to]900–1000 cm^−1. The rate is much faster than the reaction at all energies studies. (ii) The barrier to isomerization is observed at 3.3±0.2 kcal/mol (1100–1200 cm^−1). The radiative lifetimes, measured from the 0° level fluorescence decays, are 2.7±0.1 ns (h12) and 2.5±0.1 ns (d12). (iii) The observed isomerization rates in the isolated molecule are approximately an order of magnitude less than the calculated RRKM rates and observed solution phase rates. (iv) The apparent non-RRKM behavior in the isolated behavior is explained by considering the nature of IVR and by adopting a diabatic representation of the reactive surface (i.e., an allowed surface) using a Landau–Zener–Stueckelberg model. (v) Finally, we compare t-stilbene with other related isolated molecules and to solution phase t-stilbene results in order to assess the role of mode mixing and the nature of the reactive surface

Picosecond excitation and selective intramolecular rates in supersonic molecular beams. IV. Alkylanthracenes

To assess the role of alkylation on IVR, the dynamics of jet cooled 9-methyl and 9-hexylanthracene excited to single vibronic levels (SVL) in S1 are investigated and compared with the parent molecule, anthracene, whose picosecond IVR dynamics are now well characterized. Vibrations in S1 and S0 are analyzed. Decay rates and SVL fluorescence spectra are also presented. The decay rates as a function of excess vibrational energy increase rapidly at low energy but become relatively constant at high energy. The approximate energy threshold at which the decay rate "saturates" is dependent on the substitutent; anthracene ([approximately-equal-to]1800 cm^−1), 9-methylanthracene ([approximately-equal-to]1000 cm^−1), 9-hexylanthracene ([approximately-equal-to]400 cm^−1), and A–(CH2)3–[cursive phi] (<=400 cm^−1). These identified thresholds are discussed and related to IVR processes. Finally, some comments on the importance of low frequency modes to IVR are given

Jet spectroscopy of anthracene and deuterated anthracenes

Fluorescence excitation and SVL fluorescence spectra of jet-cooled h10-, 9d1-, 9,10d2-, and d10-anthracene are reported. Ground state vibrational assignments are presented for all these species and are compared with literature values. In addition, assignments for the first excited singlet state of h10-anthracene are made using SVL spectra and rotational band contours as guides. The work presented herein serves as an essential reference for other work from this research group concerning the dynamics of excited anthracene (see accompanying papers), and completes the spectroscopy of the polyacene series