Chromophore state dissociation

The signature of proton transfer in solution is the red-shifted fluorescence of the deprotonated chromophore [4-7], From the transition frequencies of absorption and emission and the ground-state dissociation constant, the dissociation constant in the excited state can be calculated [4-7], The time scales of proton transfer processes generally are very short, of the order of picoseconds or below [7], Only recently has it become possible to detect the photoinduced proton transfer dynamics in solution in real time [9,10],... 

The most important conclusions of these dynamical studies is that van der Waals clusters behave in a statistical manner and that IVR/VP kinetics are given by standard vibrational relaxation theories (Beswick and Jortner 1981 Jortner et al. 1988 Lin 1980 Mukamel and Jortner 1977) and unimolecular dissociation theories (Forst 1973 Gilbert and Smith 1990 Kelley and Bernstein 1986 Levine and Bernstein 1987 Pritchard 1984 Robinson and Holbrook 1972 Steinfeld et al. 1989). One can even arrive at a prediction for final chromophore product state distributions based on low energy chromophore modes. If rIVR tvp [4EA(Ar)i], a statistical distribution of cluster states is not achieved and vibrational population of the cluster does not reflect an internal equilibrium distribution of vibrational energy between vdW and chromophore states. If tvp rIVR, and internal vibrational equilibrium between the vibrational modes is established, and the relative intensities of the Ar = 0 torsional sequence bands of the bare chromophore following IVR/VP can be accurately calculated. A statisticsl sequential IVR/VP model readily explains the data set (i.e., rates, intensities, final product state distributions) for these clusters. 

Photoinduced unimolecular decomposition reactions are among the simplest reactions which can be studied experimentally and theoretically. One such reaction which has received considerable attention is the vibrational predissociation of small isolated van der Waals (vdW) clusters for which one molecule is a chromophore and the other is a small "solvent" molecule. Two dynamical events may transpire in such a system following the initial photoexcitation to Si vibronic levels vibrational energy may be redistributed to modes other than the optically accessed zero order chromophore states and at sufficient energies the cluster may dissociate. The fundamental theoretical understanding of these two kinetic processes should be accessible in terms of Fermi s golden rulel and unimolecular reaction rate2 concepts. 

Sulfur forms a series of homoatomic dianions catena-S (x = 2-8), which, without exception, have unbranched chain structures in the solid state.The electrochemical reduction of cyclo-Sg in aprotic solvents occurs via an initial two-electron process to produce catenaS P In solution, catena- and other long-chain polysulfides, e.g. catena- and catenaSi, dissociate via an entropy-driven process to give radical anions S (x = 2-4), including the ubiquitous trisulfur radical anion (x = 3). This intensely blue species is the chromophore in the mineral lapis lazuli, which is used in the manufacture of jewellery. 

Because of the rapidity with which the first-formed excited states are usually converted to the Sx or Tx state, most photochemical reactions start from these states. There are exceptions. An obvious one is when an upper dissociative excited state, in which the molecule immediately fragments, is populated.49 Other exceptions occur when a molecule contains two different chromophores. For example, 20 reacts analogously to franj-stilbene (21) from its state but also undergoes photoreduction, a process typical of an n,n state (see p. 719).60... 

In some cases excited state chromophores form supramolecular complexes either with ground state chromophores on the same molecule or one nearby resulting in the formation of an excimer (excited state dimer) or, if the two chromophores are different to one another an exciplex. Formally an excimer as defined as a dimer which is associated in an electronic excited state and which is dissociative in its ground state.1 Formation of the pyrene excimer is illustrated in Figure 11.3. 

The RRKM theory of unimolecular reactions predicts that the rate constant for dissociation will be given by eq. (5-3). The probability of populating a state with energy Ev restricted into the chromophore vibrations is proportional to the ratio of the density of van der Waals states at E — Ev to that at ... 

To complete the RRKM calculations for the cluster dissociation rates and final bare 4EA molecule product distributions, the cluster binding energy E0 and the energy v of the chromophore vibrational state to be populated must be found. These can be estimated from selected fits to the experimental rates and intensities (Hineman et al. 1993a). The results of the rate and product distribution calculations are presented in Table 5-4. The predictions of the model are quite good—less than 30% error for all observations for the 4EA(N2)1 and 4EA(CH4), clusters. 

The first law of photochemistry [the Grotthus-Drapper principle (30)] states that for a photochemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry [the Stark-Einstein principle (3J)j states that a molecule can only absorb one quantum of radiation. The absorbed energy causes the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to formation of chromophoric groups such as carbonyls, carboxyls, qui-nones, peroxides, hydroperoxides, and conjugated double bonds. 

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