Photo-induced transient states

Spiro-oxazine (NOSH) photo-induced ring closure reactions were first described by Bohne et al., who used two-laser two-color excitation in the UV and visible regions [71,72]. In this work, they found that photoexciting the merocyanine in cyclohexane leads to a bleached product which recovers quantitatively to the merocyanine form over 30 xsec. The transient bleach state had an absorbance... 

The next two chapters are devoted to ultrafast radiationless transitions. In Chapter 5, the generalized linear response theory is used to treat the non-equilibrium dynamics of molecular systems. This method, based on the density matrix method, can also be used to calculate the transient spectroscopic signals that are often monitored experimentally. As an application of the method, the authors present the study of the interfadal photo-induced electron transfer in dye-sensitized solar cell as observed by transient absorption spectroscopy. Chapter 6 uses the density matrix method to discuss important processes that occur in the bacterial photosynthetic reaction center, which has congested electronic structure within 200-1500cm 1 and weak interactions between these electronic states. Therefore, this biological system is an ideal system to examine theoretical models (memory effect, coherence effect, vibrational relaxation, etc.) and techniques (generalized linear response theory, Forster-Dexter theory, Marcus theory, internal conversion theory, etc.) for treating ultrafast radiationless transition phenomena. 

Shown in Fig. 2 are examples of two kinetic traces of transient absorbance difference which reflect formation and decarboxylation of carbonyloxy radicals after photo-induced decomposition of DINPO and TBNC. A detailed understanding of the kinetics is obtained from modeling the data. Within the experimental time resolution (150 fs), peroxide primary dissociation produces carbonyloxy radical intermediates, which decay either directly from an electronically excited state within about 500 fs or in a statistical unimolecular reaction on a ps to ps time-scale in the electronic ground state (see Fig. 3). In the case of DINPO photodissociation at 266 nm, the excited state of the 1-naphthylcarbonyloxy radical is too high energetically to be populated to any relevant extent.The reaction on the ground state PES can be treated by statistical unimolecular rate theory. 

Molecular dyads of ruthenium(ii)- or osmium(ii)-bis(terpyridine) chromophores and expanded pyridinium acceptors have been used to demonstrate the effect of the bridge and the metal ions to the photophysical properties of linear systems. In particular, via ultrafast transient absorption spectroscopy, an equilibration between MLCT and photo-induced charge-separated excited states has been observed demonstrating that intramolecular photoinduced electron transfers can occur within multicomponent systems in spite of driving forces virtually approaching zero. ... 

In the first pump probe experiments with picosecond time resolution the real-time spectra of the Nas E) state (Fig. 4.8) reveal a fast exponential decay caused by ultrafast photo-induced dissociation [306]. This behavior could be well explained with the simple fragmentation model described in Sect. 2.2.2. With femtosecond time resolution it was expected to observe the photodissociation process with more detail in the recorded transients. Especially, the energy dependence of the ultrashort fragmentation process should allow deeper insight into the fragmentation dynamics within a photoexcited molecular beam. 

Photo-induced Electron Transfer. Electron transfer is one of the most fundamental and widespread reactions in nature and has been extensively studied. In addition to the optical absorption spectroscopy widely used, TR EPR has become established as an appropriate method to study electron-transfer processes. In most of these investigations CIDEP effects are observed. The spin-polarization effects originate in the spin selectivity of chemical and physical processes involved in free-radical formation and decay, as well as in the spin-state evolution in transient paramagnetic precursors. For this reason, CIDEP constitutes a unique probe of the mechanistic details of electron-transfer processes. 

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