Solution-based photoinduced electron transfer

The formation of the transient reduced complex can also be observed by flash photolysis in the presence of CT-DNA [1(X)]. This shows clearly the existence of a photoinduced electron transfer from a base of the polynucleotide to the excited complex. However, the relative amount of reduced complex which is photoproduced, is smaller in the presence of CT-DNA than in the presence of GMP this may be attributed to a more important back electron transfer process in the ion pair produced on the polynucleotide compared to that in solution with the mononucleotide. 

Lastly, electron transfer in D—[H]—A assemblies is not a perquisite of the excited states of metal complexes. Organic ensembles 38 and 39 (R = SiMe2 Bu), containing a dimethylaniline-anthracene redox pair, have been synthesized recently [124]. Preliminary time-resolved and steady-state fluorescence experiments indicate the occurrence of photoinduced electron transfer. In work related to Watson Crick base-paired systems, the excited state of the fluorescent pyrene derivative 40 is efficiently quenched (94-99 %) by 2 -deoxyguanosine (dG), 2 -deoxycytidine (dC), or 2 -deoxythymidine (dT) in aqueous solution [125]. A PCET mechanism is thought to be responsible for this process, as the thermodynamics of electron transfer are unfavorable unless coupled to a rapid proton-transfer step. The quenched lifetime of 40 in the presence of dC and dT in H2O is significantly extended by a factor of 1.5-2.0 in D2O this isotope effect is similar to that observed in the kinetics studies of 1 [70]. The invoked PCET reaction mechanism also accounts for the inability of dC and dT to quench the fluorescence of 40 in the aprotic organic solvent DMSO. 

A special situation is encountered with metal-porphyrin or metal-phthalo-cyanine molecules that can be either deposited by sublimation under UHV conditions or in solution environments. For these macrocyclic compounds, free-base species exist, i.e., the metal centers are not required per se as a construction unit. The building of supramolecular structures that incorporate porphyrin subunits is of great interest to many research groups. The rich photochemistry and redox properties (e.g., photoinduced electron transfer, luminescence, and light harvesting) of porphyrins have driven this interest. Porphyrins or phtalocyanines have a rich coordination chemistry that allows the inclusion of many different metal centers at their macrocycle. They serve in many respects as a model system since this constitutes a low-coordination complex. Recent STM studies report on the organization of metal-coordinated or free-base porphyrins as well as phthalocyanines on... 

Pcs have also been widely employed as electron donors in donor-acceptor (D-A) systems in which photoinduced electron transfer (PET) from the donor to the acceptor unit can efficiently take place as demonstrated by solution experiments. Moreover, significant changes in the photophysical properties (e.g., Ufetime of the charge-separated state) have been demonstrated in organized Pc-based D-A systems with respect to the monomeric species. 

The results of photoinduced absorption in polymer solutions and oligothiophene films and solutions (see Sections 3 and 4) show a profound effect of the surrounding medium on the character of the photo-excited species. Based on these results, we inferred that for pristine conjugated polymers embedded in a nonpolar medium, neutral photoexcitations are favored, whereas in polar media, charged and neutral photoexcitations co-exist. In conjugated polymer/C6o mixtures, embedded in a non-polar medium, energy transfer and triplet sensitization are favored, whereas in polar media, photoinduced electron transfer phenomena clearly dominate. 

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