Homo-transfer

Collective effects in multiple transfers can be seen when the donor and acceptor are the same molecules and display the so-called homo-transfer. The homo-transfer is reversible, so the excitation energy can travel within the ensemble of closely located dyes until the act of emission. Commonly, since the dyes are oriented randomly in space, the emission of such an ensemble becomes totally depolarized. This case is depicted in Fig. 4a. 

Electron donor molecules are oxidized in solution easily. Eor example, for TTE is 0.33V vs SCE in acetonitrile. Similarly, electron acceptors such as TCNQ are reduced easily. TCNQ exhibits a reduction wave at — 0.06V vs SCE in acetonitrile. The redox potentials can be adjusted by derivatizing the donor and acceptor molecules, and this tuning of HOMO and LUMO levels can be used to tailor charge-transfer and conductivity properties of the material. Knowledge of HOMO and LUMO levels can also be used to choose materials for efficient charge injection from metallic electrodes. 

Fig. 11. Mechanism of electron transfer from an excited dye molecule to a silver haUde crystal where HOMO and LUMO are highest occupied and lowest...

Another common loss process results from electron—hole recombination. In this process, the photoexcited electron in the LUMO falls back into the HOMO rather than transferring into the conduction band. This inefficiency can be mitigated by using supersensitizing molecules which donate an electron to the HOMO of the excited sensitizing dye, thereby precluding electron—hole recombination. In optimally sensitized commercial products, dyes... 

After an additional 10 min, a 1 % solution of hydrochloric acid (100 ml) is slowly added to the stirred reaction mixture and the resultant mixture is transferred to a separatory funnel. The ether layer is separated and washed sequentially with water, 5 % sodium bicarbonate solution, water and saturated salt solution. The washed ether solution is dried over anhydrous sodium sulfate, filtered, and evaporated to give an oily residue (0.45 g). Chromatography of the crude product on silica gel (50 g) followed by crystallization of the solid thus obtained (0.18 g) from ethanol gives 3 -hydroxy-B-homo-cholest-5-en-7a-one acetate (67 0.14 g) mp 90-91° [a]o 99° (CHCI3). 

The lowest excited states in molecular crystals are singlet and triplet excitons [3]. Since it costs coulombic energy to transfer an electron that has been excited optically from the HOMO (highest occupied molecular orbital) to the LUMC)... 

S-adenosyl-L-methionine (AdoMet, SAM) is a cofactor and the most important donor of the methyl (CH3-) group for methyltransferases, including COMT. When the methyl-group has been transferred, the remaining demethylated compound is called S-adenosyl-L-homo-cysteine. 

The Stale Correlation Diagram (SCD) approach introduced by Shaik and Pross96 appears similar in some respects. However, the LUMO, HOMO and the first two exciled stales are considered, (refer Figure 1.5)4,53 Thus, if we consider the interaction of the radical with the olefin in its ground (singlet) state (R + C=C ) and excited (triplet) state (R + C=CJ) and two charge transfer... 

Molecules have some occupied and some unoccupied orbitals. There occur diverse interactions (Scheme 1) when molecules undergo reactions. According to the frontier orbital theory (Sect 3 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume), the HOMO d) of an electron donor (D) and the LUMO (fl ) of an electron acceptor (A) play a predominant role in the chemical reactions (delocalization band in Scheme 2). The electron configuration D A where one electron transfers from dio a significantly mixes into the ground configuration DA where... 

Strong donor-acceptor interaction shifts the reaction from the pseudoexcitation band to the transfer band. Electrons delocalize from the HOMO of propene to the LUMO of X=Y too much to form a bond between the double bonds. One electron transfers and a radical ion pair forms. The negatively charged X=Y... 

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