Photoinduced process

The first estimations of for photoinduced processes were reported by Dvorak et al. for the photoreaction in Eq. (40) [157,158]. In this work, the authors proposed that the impedance under illumination could be estimated from the ratio between the AC photopotential under chopped illumination and the AC photocurrent responses. Subsequently, the faradaic impedance was calculated following a treatment similar to that described in Eqs. (22) to (26), i.e., subtracting the impedance under illumination and in the dark. From this analysis, a pseudo-first-order photoinduced ET rate constant of the order of 10 to 10 ms was estimated, corresponding to a rather unrealistic ket > 10 M cms . Considering the nonactivated limit for adiabatic outer sphere heterogeneous ET at liquid-liquid interfaces given by Eq. (17) [5], the maximum bimolecular rate constant is approximately 1000 smaller than the values reported by these authors. 

Ritze H-H, Lippert H, Samoylova E, Smith VR, Hertel IV, Radloff W, Schultz T (2005) Relevance of jra states in the photoinduced processes of adenine, adenine dimer, and adeninewater complexes. J Chem Phys 122 224320... 

Over the last 20 years, supramolecular photochemistry has made considerable advances and photoinduced processes involving supramolecular arrays have been developed that may be exploited for practical purposes, such as ... 

S. E. Braslavsky, G. E. Heibel. Time-Resolved Photothermal and Photoacoustic Methods Applied to Photoinduced Processes in Solution. Chem. Rev. 1992,92,1381-1410. 

Class 3 fluorophores linked, via a spacer or not, to a receptor. The design of such sensors, which are based on molecule or ion recognition by a receptor, requires special care in order to fulfil the criteria of affinity and selectivity. These aspects are relevant to the field of supramolecular chemistry. The changes in photophysical properties of the fluorophore upon interaction with the bound analyte are due to the perturbation by the latter of photoinduced processes such as electron transfer, charge transfer, energy transfer, excimer or exciplex formation or disappearance, etc. These aspects are relevant to the field of photophysics. In the case of ion recognition, the receptor is called an ionophore, and the whole molecular sensor is... 

The fluorescent molecular sensors will be presented with a classification according to the nature of the photoinduced process (mainly photoinduced electron or charge transfer, and excimer formation) that is responsible for photophysical changes upon cation binding. Such a classification should help the reader to understand the various effects of cation binding on the fluorescence characteristics reported in many papers. In most of these papers, little attention is often paid to the origin of cation-induced photophysical changes. 

Figure 5.18. Schematic representation of photoinduced processes in a fluor-spacer-receptor signalling system (a) when ion free, (b) when ion bound, and (c) one example.

Megiatto JD, Schuster DI, Abwandner S et al (2010) [2]Catenanes decorated with porphyrin and [60]fullerene groups design, convergent synthesis, and photoinduced processes. J Am Chem Soc 132 3847-3861... 

Given the availability of a suitable chromophore, the use of light offers the mildest possible thermal environment in which to effect a synthetic transformation insofar as photoinduced processes may be activated at temperatures well below 0 °C. Not unexpectedly then, one finds the photosynthetic procedure to be, thus far, the only one which has been successfully applied to the construction of the relatively sensitive parent (unrestricted) 77- excessive heteroannulenes as shown in Scheme 9a. Similarly, exposure of the tetracyclic azides depicted in Scheme 9b to low-pressure mercury irradiation leads to deazotation and the formation of the pyridine-like 14- and 18-membered parents (49) and (75). 

Comparison of Ultrafast Photoinduced Processes in Indole(NH3)n and Indole(H20)n Clusters... 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

Femtosecond pump-probe investigation of primary photoinduced processes in C60/Sn nanostructures... 

Investigation of the photoinduced processes in different C6o-Sn nanostructures excited by 150... 

The separation between inorganic and organic chemistry is rather artificial and there are many areas of overlap especially in the field of metal complexes. Within a narrow definition, inorganic photoinduced processes involve a metal which has its own electronically excited states. We consider... 

Photoinduced unimolecular reactions often have kinetics of the order of ps. One example of isomerization in ps times is shown in Figure 8.9. This is the photochromic reaction of a spiropyran. The photoinduced process takes place... 

PSLCs exhibit a weak dc signal when one beam is incident on the sample that decays over a much longer time scale (a few hours). No such lifetime increase is observed for photorefractive gratings in unpolymerized LCs, which is illustrated by the inset of Fig. 17. These experiments show that a photoinduced process occurs in the PSLCs that encourages charge separation, but also discourages charge recombination. 

In another study, [Re(bpy)(CO)3(py)]+ was encapsulated into microp-orous NaY and mesoporous A1MCM-41 molecular sieves. Irradiation of the molecular sieve encapsulated rhenium complex in the presence of C02 and H20 (either gaseous or adsorbed onto the matrix) led to formation of CO and carbonate species. The proposed photoinduced process is outlined in Scheme 5 [95,96]. 

Additional work by the Forster group, making use of transient emission spectroscopy, probed the rate of photoinduced electron transfer between metal centers within a novel trimeric complex [Os(II)(bpy)2(bpe)2 ] [Os(II) (bpy)2Cl]2 4+, where bpy is 2,2/-bipyridyl and bpe is fra s-l,2-bis-(4-pyridyl) ethylene. Transient emission experiments on the trimer, and on [Os(bpy)2(bpe)2]2+ in which the [Os(bpy)2Cl]+ quenching moieties are absent, reveal that the rate of photoinduced electron transfer (PET) across the bpe bridge is 1.3 0.1 x 108s-1. The electron transfer is believed to be from the peripheral Os(II)Cl metal centers to the [Os(bpy)2(bpe)2]2+ chro-mophore. Comparison to rate constants for reduction of monolayers at a Pt electrode reveals that the photoinduced process is significantly faster [39]. 

Keywords Iridium complexes Cyclometalating ligands Phosphorescence Electrochemistry Multicomponent arrays OLEDs Photoinduced processes... 

The study of photoinduced processes in Ir(III)-based arrays has been exploited since facile synthetic methods for the preparation of Ir(III)-polyimine complexes became available. Polynuclear complexes containing, in addition to Ir(III) centers, either Os(II)-, Ru(II)-, Re(II) or Cu(I)-polyimine units linked by different bridging ligands, displayed a rich variety of photoinduced energy transfer processes. In Fig. 22 are reported some representative cases of early studies. 

An extension of this study revealed that following selective excitation of the Ir(III)-based unit at 355 nm, rather than that of the porphyrin units at 532 nm, i.e., when the Ir center acts as a photosensitizer, the type of ensuing photoinduced process can be switched in the case of array (a), Fig. 27, from the electron transfer discussed above, to energy transfer [87]. 

The 17r<7 states also dominate the photoinduced processes in hydrogen-bonded chromophore-solvent clusters. The photoinduced hydrogen transfer reaction is experimentally and computationally well documented in clusters of phenol and indole with ammonia [14,16,32], There is no clear evidence for the existence of an excited-state proton transfer process in these systems [14], The same conclusion applies to bi functional chromophores solvated in finite clusters, such as 7HQ-ammonia and 7HQ-water clusters [15]. In future work, the photochemistry of larger and biologically relevant chromophores (such as tyrosine, tryptophan, or the DNA bases) should be investigated in a finite solvent environment. 

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