Singlet excitation, processes occurring

The photolysis of dimethyl sulphoxide (at 253.7 nm) in a wide range of solvents has been studied in detail176. Three primary reactions occur, namely (i) fragmentation into methyl radicals and methanesulphinyl radicals, equation (60), (ii) disproportionation into dimethyl sulphone and dimethyl sulphide, equation (61) and (iii) deactivation of the excited state to ground state dimethyl sulphoxide. All chemical processes occur through the singlet state. Further chemical reactions of the initial photochemical products produce species that have been oxidized relative to dimethyl sulphoxide. 

The primary photophysical processes occuring in a conjugated molecule can be represented most easily in the Jablonski diagram (Fig. 1). Absorption of a photon by the singlet state So produces an excited singlet state S . In condensed media a very fast relaxation occurs and within several picoseconds the first excited singlet state Si is reached, having a thermal population of its vibrational levels. The radiative lifetime of Si is in the order of nanoseconds. Three main routes are open for deactivation ... 

To secure the efficiency of the one electron transfer, electron acceptors like 9,10-dicyanoanthracene (DCA) or quinones in conjunction with UV or visible light irradiation are commonly employed. Although photosensitized reactions are usually associated with a singlet excited state (equations 4 and 5), some processes of this type can occur thermally in the dark. 

In the second case, the HEI is formed in one or several reaction steps which are rate-limiting and the interaction of the ACT with this intermediate occurs in a rapid, kineticaUy unobservable, reaction step. Therefore, kinetic data cannot provide evidence of the occurrence of this interaction (Scheme 5). The classical example of this type of activated CL is the peroxyoxalate reaction , for which an electron transfer between a HEI and the ACT has already been postulated by Rauhut (see Section VI.C). In an elegant experiment, Lechtken and Turro confirmed the occurrence of an activated CL process in this reaction, using energy acceptors with different singlet excitation energies (Eg)... 

Collisional deactivation and energy transfer play important roles in tropospheric chemistry. For example, electronically excited S02 in the 3B, state can be deactivated by 02 (as well as by N2 and H20) to the ground ( A,) state, with part of this process occurring via triplet-triplet energy transfer to generate singlet electronically excited states of 02 ... 

As sensitization has come into widespread use, it has become apparent that energy transfer is by no means the only process which can occur when a sensitizer is irradiated in the presence of an acceptor. Other unwanted and often totally unexpected interactions between the sensitizer and itself, the substrate, or the solvent can produce confusing or misleading results. These interactions, which include chemical reaction of sensitizers, involvement of singlet excited states, and numerous other complications, will constitute the... 

The present work comprises of a first step towards this goal and involves measured photoelectron spectra of trans-Stilbene following the excitation with tuneable femtosecond UV laser pulses. The ionization process occurs in a (1+1) REMPI scheme, using the first excited singlet state (Si) of trans-Stilbene as an intermediate resonance. 

The initial or primary processes of photosynthesis occur in the reaction centers in which chlorophyll or bacteriochlorophyll absorbs a photon.3183 Then, the chlorophyll, in its singlet excited state (Chi ), donates an electron to some acceptor A to form a radical A-and to leave an oxidized chlorophyll Chl+ radical (Eq. 23-31). 

The presence of a base is also essential for the efficient reductive dehalogenation of RX by 1-benzyl-1,4-dihydronicotinamide (BNAH) via photoinduced electron transfer [121,122], Since the one-electron oxidation potential of the singlet excited state of BNAH ( BNAH ) is —3.1 V (vs. SCE) [50], which is more negative than the one-electron reduction potential of benzyl bromide (PhCH2Br), photoinduced electron transfer from BNAH to PhCH2Br occurs efficiently with the diffusion-limited rate [122]. This fast process needs no base catalyst to accelerate the electron transfer rate further. However, the photoinduced electron transfer results in... 

Recently, the primary processes were investigated using pulse radiolysis with two extractant-alkane systems (182, 292). Transient optical absorption spectra proved that in the presence of ligands like TODGA, the excited species of -dodecane (singlet excited state and radical cation) disappeared immediately. Results showed that an energy transfer occurred from the excited alkane to the extractant molecule (TBP, TOPO, or amide), which constituted an additional decomposition route, as described in the following set of reactions ... 

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