Photochemical reactions internal conversion

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

If a quantum efficiency less than unity is observed, even in the absence of a photochemical reaction, a direct internal conversion from St -> S0 via higher vibrational level of the ground state is proposed. 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

The above considerations lead us naturally to the question of the nature of the one or more intermediate states involved directly in the photochemical reaction. We have displayed in Figure 27 the simplest scheme consistent with the above, admittedly preliminary, experimental results. The prime superscript refers to the frans-isomer, while the processes denoted 1, 2, 3,4, and 5 describe fluorescence, Sx S0 internal conversion, Si -> X intersystem crossing, isomerization, and deactivation to the ground state, respectively. Again, X and X may represent a common state, as, for example, is the case with the twisted triplet in ethylene (cf. Mulliken and Roothaan190 and Kaldor and Shavitt191). As late as 1962, investigators were still unable to determine the source of the temperature... 

It is a commonly accepted belief that the planar excited states of simple alkenes, formed by light absorption or energy transfer, decay very rapidly to more stable nonplanar species.6 The perpendicular geometry is believed to be the stablest configuration of both the lowest singlet and triplet states. In this particular case it is obvious that reactions (10) and (11) are virtually the same process, although, in chemical parlance, reaction (10) is part of an internal conversion and (11) is the last step in a photochemical reaction. 

BM) forms with a quantum yield of 0.017 0.005 when Re2Cl " is irradiated at 313 nm. Cleavage of the quadruple bond to form ReCl4(CH3CN)2 is followed by rapid conversion to the neutral monomer by means of acetonitrile replacement of chloride. No reaction occurs in the absence of irradiation at reflux, and the 88 excited state is not responsible for the observed photochemistry, since 632.8 nm radiation does not lead to cleavage of the rhenium dimer (135). Flash-photolysis studies suggest that the dominant photochemical process involves internal conversion to a transient 65 excited... 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

Figure 4.25 Jablonski diagram of the heavy atom effect on photochemical reactivity. If excitation to S2 (hv2) is followed by intersystem crossing (isc) to T2, the quantum yield of reaction R decreases at higher excitation energies, ic = internal conversion, a = absorption, f = fluorescence, p = phosphorescence...

Hammond has suggested that normally A is converted into B by the mechanism of Equation 13.29 at about the same rate as ground-state A is converted into ground-state B and that most photochemical reactions are driven by internal conversion of electronic energy as in Equation 13.3O.40... 

Secondary unimolecular reactions in these systems usually result either from production of hot energized species by chemical reaction or from conventional thermal activation. In a few systems, residual excitation from the original photochemical process may be of importance. An interesting and potentially valuable example, due to Srinivasan, is the production of highly vibrationally excited, ground electronic state diene molecules by internal conversion which follows photoexcitation. 

---