55 Osaka University 100 Papers : 24 Selected Graphics ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2004-2005 I n a classical organic reaction involving (closed-shell) ground states of reactants and products, the electron spin is a negligible quantity, neither influencing the efficiency nor the selectivity of a transformation. The number of excep- tions is, however, constantly increasing and calls the former general statement into question. The best known historical exceptions from this general statement in organic synthesis are carbene cycloadditions (singlet versus triplet reactivity) and the pronounced reactivity difference between ground- state triplet and the first excited singlet state of oxygen. In organic photochemistry, different spin states are involved by nature of the excited states which are often dif- ferent from their ground-state precursors as well as closed- shell products. A plentiful number of reactions are known which are initiated by triplet excited molecules in carbonyl, enone, alkene or arene photochemistry. Traditional mecha- nistic analyses of photochemical reactions involving triplet excited substrates have partly ignored the spin state prob- lem and focussed on the primary interactions on the triplet surface. We have made significant progress in elucidating the relation between regio- as well as diastereoselectivity and the spin inversion process in triplet 2-oxatetramethyl- ene-biradicals through a combination of experimental and theoretical studies. Significant and sometimes sur- prising temperature, solvent, isotope, and substituent effects were observed. Most surprising for our classical Selectivity Control in Electron Spin Inversion Process- es: Regio- and Stereochemistry of Paternò-Büchi Pho- tocycloadditions as a Powerful Tool for Mapping Inter- system Crossing Processes ABE Manabu (Graduate School of Engineering) Accounts of Chemical Research, 37, 919-928 (2004) sNo.28 in “100 Papers Selection” (p.69) N ovel organic solar cells have been prepared using qua- ternary self-organization of porphyrin (donor) and fullerene (acceptor) moieties units by clusterization with gold nanoparticles on nanostructured SnO 2 electrodes. The highly colored composite clusters of porphyrin gold nanoparticles and fullerenes have been assembled as three-dimensional arrays onto nanostructured SnO 2 films using an electrophoretic deposition method. The composite cluster electrode exhibits an incident photon-to-photocur- rent efficiency (IPCE) as high as 54% and broad photocur- rent action spectra (up to 1000 nm). The power conversion efficiency of the OTE/SnO 2 / (H 2 PC15MPC+C 60 ) m compos- ite electrode reaches as high as 1.5%, which is 45 times higher than that of the reference system consisting of the single component of porphyrin or fullerene. Photovoltaic Cells Using Composite Nanoclusters of Porphyrins and Fullerenes with Gold Nanoparticles HASOBE Taku and FUKUZUMI Shunichi (Graduate School of Engineering) Journal of the American Chemical Society, 127, 1216-1228 (2005) sNo.29 in “100 Papers Selection” (p.69) Reprinted with permission from C. Arleen Courtney. Copyright (2005) American Chemical Society Engineering understanding of organic reactions involving single-state reactivity is that very weak forces like spin-orbit coupling (SOC, some cal/mol) - in systems carrying no heavy atoms - or hyperfine coupling (HFC, some mcal/mol) - in the absence of strong external magnetic fields - can strongly participate in influencing the reaction diastereoselectivity. In the corresponding singlet reactions, diastereoselec- tion is born out of the enthalpy differences between the two terminal C– C bond forming trajectories (if at all a two- step process is involved). Starting with the fully equili- brated triplet biradicals, the formation of closed-shell products, however, requires spin inversion and with that, the selectivity of the product formation is spin-related. The ∆∆G ‡ for the formation of diastereoisomers in the Paternò- Büchi reaction involves several kcal/mol due to steric and stereoelectronic interactions in the specific conformers that are prone to undergo intersystem-crossing. Thus, very weak forces connected with spin inversion processes can lead to substantial control of regio- and diastereose- lectivity.