Electron photochemistry

Various alternatives to the conventional one-electron photochemistry were reviewed recently [116,122], A rational framework for advancing the multi-electron photochemistry of new metal complexes has been developed using two-electron mixed-valence complexes as the redox platform. Two-electron mixed valence is a useful design concept for hydrogen and oxygen photocatalysis. As single-electron... 

A multimeric transition metal complex (Mn4OsCa cluster) is employed to transiently store equivalents generated by fast electron photochemistry that are subsequently used to perform slow four-electron water-splitting chemistry. 

The stoichiometries of the two reactions above raise an interesting mechanistic question how is the one-electron photochemistry in PS II coupled to the four-electron water-splitting chemistry The now-classical single turnover flash experiments of Joliot and co-workers [2], which showed period four oscillation in the O2 yield with flash number, provided a clearcut answer the PS II units function independently in accumulating the four oxidizing equivalents required to split water. This observation was quickly confirmed [3,4] and Kok provided the S-state model which is now widely used to summarize the situation ... 

Some of the problems associated with selective vibrational photochemistry can be avoided in electronic photochemistry, and dye lasers are available to provide tunable radiation. To trap the selectively excited species, reactions involving molecules as distinct from radicals are preferred, since then the opportunities for isotopic scrambling are minimized. This approach to isotope... 

Heterogeneous photochemical reactions fall in the general category of photochemistry—often specific adsorbate excited states are involved (see, e.g.. Ref. 318.) Photodissociation processes may lead to reactive radical or other species electronic excited states may be produced that have their own chemistry so that there is specificity of reaction. The term photocatalysis has been used but can be stigmatized as an oxymoron light cannot be a catalyst—it is not recovered unchanged. 

Surface photochemistry can drive a surface chemical reaction in the presence of laser irradiation that would not otherwise occur. The types of excitations that initiate surface photochemistry can be roughly divided into those that occur due to direct excitations of the adsorbates and those that are mediated by the substrate. In a direct excitation, the adsorbed molecules are excited by the laser light, and will directly convert into products, much as they would in the gas phase. In substrate-mediated processes, however, the laser light acts to excite electrons from the substrate, which are often referred to as hot electrons . These hot electrons then interact with the adsorbates to initiate a chemical reaction. 

In this chapter, we look at the techniques known as direct, or on-the-fly, molecular dynamics and their application to non-adiabatic processes in photochemistry. In contrast to standard techniques that require a predefined potential energy surface (PES) over which the nuclei move, the PES is provided here by explicit evaluation of the electronic wave function for the states of interest. This makes the method very general and powerful, particularly for the study of polyatomic systems where the calculation of a multidimensional potential function is an impossible task. For a recent review of standard non-adiabatic dynamics methods using analytical PES functions see [1]. 

Interaction with light changes the quantum state a molecule is in, and in photochemistry this is an electronic excitation. As a result, the system will no longer be in an eigenstate of the Hamiltonian and this nonstationaiy state evolves, governed by the time-dependent Schrddinger equation... 

The majority of photochemistry of course deals with nondegenerate states, and here vibronic coupling effects aie also found. A classic example of non-Jahn-Teller vibronic coupling is found in the photoelection spectrum of butatiiene, formed by ejection of electrons from the electronic eigenfunctions [approximately the molecular orbitals). Bands due to the ground and first... 

To use direct dynamics for the study of non-adiabatic systems it is necessary to be able to efficiently and accurately calculate electronic wave functions for excited states. In recent years, density functional theory (DFT) has been gaining ground over traditional Hartree-Fock based SCF calculations for the treatment of the ground state of large molecules. Recent advances mean that so-called time-dependent DFT methods are now also being applied to excited states. Even so, at present, the best general methods for the treatment of the photochemistry of polyatomic organic molecules are MCSCF methods, of which the CASSCF method is particularly powerful. 

Quantum chemical methods, exemplified by CASSCF and other MCSCF methods, have now evolved to an extent where it is possible to routinely treat accurately the excited electronic states of molecules containing a number of atoms. Mixed nuclear dynamics, such as swarm of trajectory based surface hopping or Ehrenfest dynamics, or the Gaussian wavepacket based multiple spawning method, use an approximate representation of the nuclear wavepacket based on classical trajectories. They are thus able to use the infoiination from quantum chemistry calculations required for the propagation of the nuclei in the form of forces. These methods seem able to reproduce, at least qualitatively, the dynamics of non-adiabatic systems. Test calculations have now been run using duect dynamics, and these show that even a small number of trajectories is able to produce useful mechanistic infomiation about the photochemistry of a system. In some cases it is even possible to extract some quantitative information. 

J. Michl and Bonacic-Koutecky, Electronic aspects of organic photochemistry, John Wiley. Sons, Tnc., New York, 1990. 

Although this reaction appears to involve only two electrons, it was shown by Mulder [57] that in fact two jc and two ct elections are required to account for this system. The three possible spin pairings become clear when it is realized that a pair of carbene radicals are formally involved. Figure 14. In practice, the conical intersection defined by the loop in Figme 14 is high-lying, so that often other conical intersections are more important in ethylene photochemistry. Flydrogen-atom shift products are observed [58]. This topic is further detailed in Section VI. 

We illustrate the method for the relatively complex photochemistry of 1,4-cyclohexadiene (CHDN), a molecule that has been extensively studied [60-64]. There are four it electrons in this system. They may be paired in three different ways, leading to the anchors shown in Figure 17. The loop is phase inverting (type i ), as every reaction is phase inverting), and therefore contains a conical intersection Since the products are highly strained, the energy of this conical intersection is expected to be high. Indeed, neither of the two expected products was observed experimentally so far. 

The exchange of two pairs of a electrons is expected to lead to a high-lying conical intersection that is not likely to be important in the UV photochemistry of CHDN. This winds up the possibilities of loops involving two-election pair exchanges only. 

The physical properties of the xanthene type dye stmcture in general have been considered. For example, the aggregation phenomena of xanthene dyes has been reviewed (3), as has then photochemistry (4), electron transfer (5), triplet absorption spectra (6), and photodegradation (7). For the fluoresceins in particular, spectral properties and photochemistry have been reviewed (8), and the photochemistry of rhodamines has been investigated (9). 

Photochemical technology has been developed so as to increasingly exploit inorganic and organometaUic photochemistries (2,7), recognizing the importance of photoinduced electron transfer as the phenomenological basis of a majority of commercially successful photochemical technologies (5,8). 

The years from 1923 to 1938 were relatively unproductive for G. N. Lewis insofar as his own research was concerned. The applications of the electron-pair bond came largely in the areas of organic and quantum chemistry in neither of these fields did Lewis feel at home. In the early 1930s. he published a series of relatively minor papers dealing with the properties of deuterium. Then in 1939 he began to publish in the field of photochemistry. Of approximately 20 papers in this area, several were of fundamental importance, comparable in quality to the best work of his early years. Retired officially in 1945, Lewis died a year later while carrying out an experiment on fluorescence. 

Lamola, A. A. (1969). Electronic energy transfer in solutions theory and application. In Leermakers, P. A., and Weissberger, A. (eds.), Energy Transfer and Organic Photochemistry, Technique of Organic Chemistry 14 17-132. Interscience Publishers, New York. 

Electronic spectra and photochemistry of complexes containing quadruple metal-metal bonds. W. C. Trogler and H. B. Gray, Acc. Chem. Res., 1978,11,232-239 (43). 

---