Intramolecular charge separation

M. Zander and W. Rettig, Fluorescence studies on solvent-induced intramolecular charge separation in symmetric biaryls, Chem. Phys. Lett. 110, (Ml (1984). 

Electrooptical emission measurements in the gas phase12 show that the TICT state with its anomalous emission can be reached independently of any solvent environment (thus its formation is a true intramolecular phenomenon), and that the observed excited state dipole moment is consistent with a large degree of intramolecular charge separation. 

The lifetimes of the charge separated state in degassed toluene for 1 through 4 are 5, 104, 150, and 300 ns, respectively, as determined by time-resolved transient absorption experiments [60-62], As Table 3 illustrates, this trend is preserved in the liquid crystalline environment, with the caveat that the ion pair lifetimes are increased by at least one order of magnitude. This variation allows for the study of photorefractivity as a function of the lifetime of the intramolecular charge separated state. 

They undergo intramolecular charge separation with nearly 100% quantum yield. 

Of particular interest is System 45 described by Seta et al. [101] in which the possibility of PET across lipid membrane by virtue of intramolecular charge separation process has been demonstrated (see Fig. 4e). Photoexcitation of porphyrin in a Car—P—Q covalently-linked triad (see the structure of Car—P—Q in Fig. 21 a of the Chapter by Zamaraev and Khairutdinov) leads to a two-step transfer of an electron from the Car fragment to the Q fragment ... 

Upon photoexcitation fast intramolecular charge-separation is observed similar in rate and in temperature dependence to the early steps of photosynthesis. It is proposed that the bridges play an active role in mediating electron transfer via through-bond interaction, thereby enhancing the rate of charge separation significantly as compared to other model systems. 

Intramolecular charge separation is energetically favoured over intermolecular charge separation because the spatial separation of charges, and hence the Coulomb term, is limited by the fixed distance between donor... 

Replacing the weak electron donor DMA with a 7V,7V,A, Ar -tetramethyl-/j-phenylenediamine (TMPD) donor increases the energy gap between the fullerene singlet excited state and the charge-separated state [323]. As a consequence of a more exergonic electron transfer, the Ceo-TMPD dyad (8) gives rise to fast intramolecular charge separation, irrespective of the solvent polarity (i.e., non-polar methylcyclohexane and polar benzonitrile) [323]. 

It follows from the discussion in the Section 5.4.1 that excited state redox properties can be controlled through ground-state redox potentials (Sections 5.3.2. and 5.3.3.) and excited state energy bo- The excited state energy is dependent on the molecular structure, but in a way that is interrelated with the structural dependence of the redox potentials. In Section 5.3.1 it was mentioned that the MLCT excitation energy, hv, (Figme 2) depends linearly on the difference between oxidation and reduction potentials. This dependence stems from the intramolecular charge separation that occurs in MLCT excited states ... 

Figure 6. Schematic molecular level structure for electron transfer processes in an isolated molecule. Excitation So(D-A) — S2[(D-A) ] selects the vibronic level(s), which undergo(es) intramolecular charge separation (denoted by horizontal arrow) to the Si(D+-A ) vibronic manifold quasidegenerate with it. Excitation So — Si selects the vibronic levels of the charge-transfer singlet state, which undergo intramolecular charge recombination (denoted by a horizontal arrow) to the ground-state vibronic manifold. Radiative electron transfer exemphfied by the CT fluorescence is labeled with a broken arrow. Adapted from Refs. [103a-d].

A laser flash photolytic study of the reaction between 2,2 -dipyridyl and tryptophan has been described. The primary photochemical step has been demonstrated to be pH independent and involves an electron transfer from the tryptophan to the dipyridyl triplet state. The triplet excited state of some peptide conjugates is produced on irradiation by a nanosecond laser flash. C-C Bond cleavage is the result of irradiation of the pinacols (214) in chloroform. This yields the corresponding aldehydes. The mechanism of the cleavage process has been shown to involve single electron transfer with chloroform as the electron acceptor. A study of intramolecular charge separation in aminophenyl(phenyl)acetylene and A, A-dimethylaminophenyl(phenyl)-acetylene has been reported. ... 

Figure 7.2.5 Light-induced electron transfers in amide- and ester-linked porphyrin-quinone pairs. The resulting long-lived ESR spectrum shows porphyrin as well as semi-quinone anion radical signals, (a) n = 2, x = VH (h) n = 3, x = NH observed signals for intramolecular charge transfer (c, d) calculated spectra for the sum of porphyrin and quinone radicals (e) asymmetrical signal for intramolecular charge separation. (From McIntosh et al., 1983.)...

A mechanism involving intramolecular charge separation after photoexcitation serves to explain the UVA properties of (non-phenolic) cyanoacrylates (see Scheme 9.10). 

The figure illustrates the mechanism of intramolecular charge separation. The transition and possible charge recombination might be controlled by either electrostatic charge stabilization or trans-cis-isomerization of the C-13 bond. Obviously, a combination of both principles is realized. 

Hviid L, Brouwer AM, Paddon-Row MN and Verhoeven JW. Long-Lived Short-Distance Intramolecular Charge Separation Via Intermolecular Triplet Sensitization. ChemPhysChem 2001 2 232-235. 

Mataga, N., Chosrowjan, H., Shibata, Y., Yoshida, N., Osuka, A., et al. First unequivocal observation of the whole bell-shaped energy gap law in intramolecular charge separation from S2 excited state of directly linked porphyiin-imide dyads and its solvent-polarity dependencies. J. Am. Chem. Soc. 123, 12422-12423 (2001)... 

In the latter reaction involving a multiply-charged reactant ion, intramolecular charge separation is expected to occur and, in this case, may even act to direct the polymerization away from the surface of Ceo-... 

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