Direct photoexcitation processes

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

Photochemistry is the branch of chemistry that deals with the causes and courses of chemical deactivation processes of electronically excited particles, usually with the participation of ultraviolet, visible, or near-infrared radiation [1]. The photochemist is interested in both the modes of excited-state formation processes (direct photoexcitation, energy transfer, etc.) and the deactivation pathways of excited atoms, molecules, and ions. 

In a molecular system which clearly possess a ct LUMO the photoexcitation process may involve promotion of an electron from a n HOMO to the ct LUMO. This process may or may not produce bond cleavage. Direct observation of a 7t — ct electronic transition is often difficult due to the localized nature of the ct molecular orbitals resulting in a low probability for the transition [91]. More likely ait->it electronic transition takes place initially and ET, i.e., a - ct, is required to eventually populate the lower energy ct antibonding molecular orbital. Onium salts are examples of chemical structures that possess a ct LUMO and are expected to behave in this manner (see in Sect. 3.3). 

The lack of the triplet-triplet absorption of carbocyanine dyes is due to low values of the intersystem crossing rate constants as compared with the rate constants of competing processes [5, 9]. The dye-DNA interactions lead to an increase in the quantum yield of the triplet state of the dye molecules, since the complexation impedes the processes of photoisomerization and vibrational relaxation (nonradiative deactivation), thus permitting the detection of T-T absorption spectra of the bound dyes upon direct photoexcitation. In the presence of DNA in the solutions, the triplet lifetimes of the dyes comprise himdreds of microseconds [10]. 

Returning to the effect of solvents, two reasons for the frequently observed dominance of [2 + 2] cycloadditions in nonpolar solvents can be proposed. Less polar solvents will be less effective in stabilizing the radical cation species, thus giving rise to formation of CIP with the reduced sensitizer and more likely and more exothermic BET. Additionally, forward electron transfer might not favorable in all cases. The thermodynamically disfavored ET in nonpolar solvents increases the probability of a direct photoexcitation of the substrate, leading to the cyclobutane adduct via the normal Woodward-Hoffman dictated excited-state process. ... 

However, when the photochemist is concerned mainly about the excited states of species which accept the energy, it is reasonable to refer to the subsequent transformations of the newly excited molecule as primary steps in the mechanism of that acceptor molecule. In this sense, energy transfer is used as an alternate to direct photoexcitation for the production of excited molecules. Then any continuous sequence of one or more steps which begins with the energy transfer step and ends with any one of the primary steps of the acceptor molecule is defined as an acceptor primary process. In the acceptor primary process, an excited donor molecule, such as Eg in Figure 6, plays the same role in the formation of A2 as do the photons in the direct excitation of A to A2 in step 02. 

In the sense that acceptor primary processes complement the primary processes of direct photoexcitation, investigations of mechanisms and their relation to the experimentally measured quantities is facilitated. This is especially so as acceptor primary processes can be examined with a variety of different donor molecules. 

In addition to surface-enhanced exciton dissociation and geminate recombination, direct photoexcitation (Northrop and Simpson, 1956 Heilmeier et al., 1963 Harima et al., 1989) and exciton-exciton annihilation (Silver et al., 1963 Jortner et al., 1963 Braun, 1968 Johnston and Lyons, 1968 Foumy et al., 1968 Swenberg, 1969 Braun and Dobbs, 1970 Orlowski and Scher, 1983) arguments have been proposed. In direct photoexcitation, a free electron and free hole are created without the involvement of intermediate states. With the exception of the work of Harima et al. and Orlowski and Scher, there have been few references to direct or exciton-exciton photogeneration processes in the past one and a half decades. 

There are two types of photopolymerization processes (1) photopolymerization with photocatalytic systems and (2) direct photoexcitation of the monomer. The former has been demonstrated by the formation of polythiophene and polypyrrole either by n-type... 

The distinction between direct dissociation processes discussed in the present section and indirect dissociation or predissociation processes discussed in Section 7.3 to Section 7.14 is that in a direct process photoexcitation occurs from a bound state (typically v = 0 of the electronic ground state) directly to a repulsive state (or to an energy region above the dissociation asymptote of a bound state) whereas in an indirect process the photoexcitation is to a nominally bound vibration-rotation level of one electronically excited state which in turn is predissociated by perturbative interaction with the continuum of another electronic state. Direct dissociation, often termed a half collision is much faster and dynamically simpler than indirect dissociation. In a direct dissociation process the distance between atoms increases monotonically and the time required for the two atoms to separate is shorter than a typical vibrational or rotational period (Beswick and Jortner, 1990). 

The direct photoexcitation of water molecules by ultrashort laser pulses is used for the investigation of primary events occurring from 10 s (thermal orientation of water molecules and ultrafast proton transfer) to 10" s (primary reactions of a solvated electron with protic species) (57,58,61-65). The nonlinear interaction of ultrashort UV pulses (typically less than 100 fs in duration and having a power of 10 W cm" ) with water molecules triggers multiple electron photodetachment channels within a hydrogen bond network (see equations 4-7). An initial energy deposition via a two-photon absorption process (2 X 4 eV) leads to the formation of nonequilibrium states of an excess electron... 

The lowest-lying triplet states of Cgg and C70 play dominating roles in their photochemical processes. Upon direct photoexcitation, undergoes intersys-tem crossing quantitatively, with a yield of imity [3]. The intersystem crossing yield of C70 is also very high (0.86) [4,42]. 

The electronic modes provide a direct real-space link between the structure of complex molecules such as organic oligomers with a delocalized r-electronic system and their optical properties. They clearly show how specific variations in molecular design, such as chain length or donor/acceptor substitutions, can impact their optical response. In the remainder of the paper, we apply this approach to various classes of molecules and to different types of optical response. The two-dimensional real space analysis of the transition densities (slices or two-dimensional plots ) provides an attractive alternative to the traditional molecular orbital based quantum-chemical analysis of photoexcitation processes. 

Polystyrene Direct photoexcitation/photoxidation of phenyl rings. Charge-transfer complex with oxygen. -C=0 and -OOH groups from processing or from oxidation. Interactions with singlet oxygen. [32,95-101]... 

The emission process in conjugated polymers occurs from neutral excited states, which are intrachain singlet excitons in most cases. They are generated either by direct photoexcitation or by recombination processes from other excited (band) states. The emission process is determined by the probability for the radiative transition from the excited state to the ground state. The emission color then depends on the shape and the energetic position of the emission spectrum. The same excited species are responsible for EL and PL emission therefore, the intrinsic components of their spectra are identical (see previous sections). 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

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