Photo-induced excited molecule

Using the first-principles molecular-dynamics simulation, Munejiri, Shimojo and Hoshino studied the structure of liquid sulfur at 400 K, below the polymerization temperature [79]. They found that some of the Ss ring molecules homolytically open up on excitation of one electron from the HOMO to the LUMO. The chain-like diradicals S " thus generated partly recombine intramolecularly with formation of a branched Sy=S species rather than cyclo-Ss- Furthermore, the authors showed that photo-induced polymerization occurs in liquid sulfur when the Ss chains or Sy=S species are close to each other at their end. The mechanism of polymerization of sulfur remains a challenging problem for further theoretical work. 

The chemistry and physics of dendritic compounds started a decade ago [1-5]. Today, this science of uniquely shaped molecules, namely, dendrite-shaped molecules, is one of the most exciting topics of contemporary interdisciphnary research. The dendrimers and their related molecules have been investigated widely not only from the viewpoints of synthetic, physical, and material chemistries but also from that of mathematics. Accompanying the development of the science in this decade, research interest has shifted from the mere challenge of preparing molecules with unique shapes, via their excited state chemistries involving inter- and/or intramolecular photo-induced electron and/or energy transfer, to the nanoscience. 

As described before, the rr-electrons of porphyrin are delocalized over the molecule and the energy levels of the HOMO and the LUMO are high and low, respectively. The resultant narrow intramolecular HOMO-LUMO gap causes absorption of the entire region of visible light. Usually, porphyrins are red to purple and phthalocyanines are blue to green. Furthermore, the long lifetime of their excited states is appHcable to the construction of photo-induced electron and/or energy transfer systems. 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

For most molecules, the energy level reached after internal conversion is the same irrespective of the order of the absorption process used to generate the excited species. As a consequence, all photo-induced processes traditionally activated by IPA can also be accomplished using 2PA. 

A transfer of the excitation energy from the donor to the acceptor will occur when an energy acceptor molecule is placed at the proximity of an excited energy donor molecule. After energy transfer, the donor relaxes to its ground state and the acceptor is promoted to one of its excited states. A photo-induced electron transfer can be initiated after photoexcitation when an excited single electron in the LUMO of the electron donor is transferred to a vacant molecular orbital (LUMO) of the acceptor. 

Photophysical studies on 9(3) and 10(5) revealed that extremely rapid photo-induced ET took place in both systems, from the locally excited methoxybenzene donor to the DCV acceptor.59,60 Particularly noteworthy was the observation of CT fluorescence in 10(5) which was the first documented example of exciplex-like emission from a rigid D-B-A system with a donor-acceptor separation exceeding three C-C bonds. These data clearly point to TB-mediated ET processes in these molecules. 

Nanosecond and picosecond laser-flash photolysis techniques have been used by different groups to elucidate the various intermediate stages involved in the photo-induced reactions of amines. The overall mechanism involving the electron-transfer process in a fluid medium is illustrated in Scheme 13. The dynamics of the process involve the formation of an encoimter complex between the excited-state molecule and the ground-state molecule [117, 140, 141]. The encounter complex can be described as an intermoleculai ensemble of excited- and ground-state molecules, separated by a small distance (ca 7 A) and surroimded by solvent molecules. During... 

An interesting demonstration of the inverted Marcus region has been made with a reaction involving photo-induced back electron transfer within a radical ion pair. Thiophene (diaminophenothiazin-5-ium chloride) is a dye molecule which may be excited to the triplet state using UV light. In the triplet state the... 

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