CTTS state

The population transfer between the excited La and Lb states of 6.5 2 ps is determined from indole dissolved in ethanol and cyclohexane. In water the appearance of presolvated elections within the time resolution of our experiment and the fluorescence quantum yield of 0.09 indicate an ultrafast branching between the fluorescing state and the CTTS state immediately after photoexcitation. The solvation of the generated electrons shows the same initial dynamics of 350 fs for solvated indole and for pure water but differs on longer timescales. 

This simple oxidoreduction reaction involves complex OH - water molecules interactions whose the spectral signatures are assigned to Charge-Transfer-To-Solvent states (CTTS states). Indeed, the anionic precursor of the hydrated OH radical represents an interesting system for the direct investigation of elementary redox events in a protic molecular solution. 

Computed fits of experimental signals probed at different wavelengths allow for the careful investigation of ultrafast electronic pathways (Fig.2). The transient signal at 400 nm is assigned to a very short-lived CTTS state of aqueous hydroxyl ions (OH), . This excited state is instantaneously populated, typically in less than 50 fs and follows a pseudo first order dynamics with a frequency rate of 5 x 1012 s. Semi-quantum MD simulations emphasize that transient excited CTTS states play a crucial role in photoinduced electron transfers [4-6]. 

Femtosecond spectroscopic investigations in the spectral range 400-880 ran have permitted to discriminate specific OH effects on the dynamics of short lived UV excited CTTS states and transient near-IR (HO e )H20 pairs. The complex nature of ultrafast prehydration elementary redox reactions with nascent OH radical (strong acid) must be contemplated in the framework of ion-pairs dynamics, ion-solvent correlation function, short-range ordering water molecules, solvent screening or anisotropic electric field effects and short-time vibronic couplings. 

Examples of such systems include alkali metal atoms solvated by ammonia and by water four solvent molecules appear to fill the first solvation layer. Thus, in Li(H20) , when > 4, the valence electron appears to move out into the second solvation layer, forming Li+(H20) . Another example is solvated iodine anions [such as I (CH3CN) photoexcitation leads to the transfer of an electron to the solvent even for n = 2. The nature of the excited state is determined by the entire arrangement of the solvent molecules, as expected for a CTTS state. 

The behavior of CTTS states is dependent on energy levels of the ion-solvent molecular couphngs. These levels can lead to internal relaxation and/or complete electron detachment via adiabatic or nonadiabatic electron transfer. The ultrafast spectroscopic investigations of electronic dynamics in ionic solutions would permit us to learn more about the primary steps of an electron-transfer reaction within a cationic atmosphere. The influence of counterions on early electron photodetachment trajectories from a hahde ion can be considered as prereactive steps of an electron transfer. 

Figure 5. Set of time-resolved UV-near-IR spectroscopic data (3.44-0.99 eV) following the femtosecond UV excitation of an aqueous sodium chloride solution ([H20]/[NaCl] = 55). An instrumental response of the pump-probe configuration at 1.77 eV (n-heptane) is also shown in the middle part of the figure. The ultra-short-lived components discriminated by UV and IR spectroscopy correspond to low or high excited CTTS states (CTTS, CTTS ), electron-atom pairs (Che pairs), and excited hydrated electrons (ehyd )- The spectral signature of relaxed electronic states (ground state of a hydrated electron, (ehyd) electron-cation pairs, a e hyd) observed in the red spectral region.

Figure 8. Energy-level diagram of ultrafast electron-transfer processes in aqueous sodium chloride solution. Transitions (eV) correspond to experimental spectroscopic data obtained for different test wavelengths. The abscissa represents the appearance and relaxation dynamics of nonequilibrium electronic populations (CTTS ", CTTS, (e hyd) fCl e pairs). The two channels involved in the formation of an s-like ground hydrated electron state (e hyd, c hyd ) (dso reported in the figure. From these data, it is clear that the high excited CTTS state (CTTS ) corresponds to an ultrashort-lived excited state of aqueous chloride ions preceding an electron photodetachment process.

Photo-oxidation of ferrocene by CCI4 in solution can normally only be effected by u.v. irradiation. However it has been observed that the reaction may be carried out with visible light in cetyltrimethylammonium chloride micelles, albeit with low quantum yield.It is suggested that the main effect of micellization may be an increase in the oxidation potential of ferrocene or alternatively that a CTTS state of ferrocene is involved under these conditions. The ring substitution of ruthenocene by irradiation in 1 1 (v/v) solutions of ethanol with CCI4, CHCI3, or CH2CI2 proceeds by a mechanism similar to that previously found for ferrocene. Other reports consider the synthesis of ferrocenyl thioesters and the photooxidation of ferrocene. ... 

The quantum yield for pyridine photo-aquation (24) from [Ru(NH3)6(py)]2+ varies only slightly with wavelength in the range 436—254 nm, and photooxidation to give Ru,n is only important at < 334nm.82e Similarly, only photo-aquation of [Ru(NH3)5(MeCN)]2+ is found on irradiation at 366 nm while photoredox reactions are very important at shorter wavelengths ( = 0.51 at 214 nm).889 For both complexes a CTTS state is implicated in the photoredox reactions. As H2 is a significant product for [Ru(NH3)6(MeCN)]2+ photolysis at 254 nm, particularly in the presence of propan-2-ol, steps (25) and (26) are probably important. 

The electron-lattice relaxation is described by the quantity Sfico where hoo is a phonon energy, and 5, is a Huang-Rhys parameter, i = X, IT, CTT. In the case of the IT and CTT states, S is much larger than 1 (usually >10), whereas for internal... 

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