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Osaka University 100 Papers : 24 Selected Graphics

ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2004-2005

In a classical organic reaction involving (closed-shell)ground states of reactants and products, the electron spin

is a negligible quantity, neither influencing the efficiency northe selectivity of a transformation. The number of excep-tions is, however, constantly increasing and calls the formergeneral statement into question. The best known historicalexceptions from this general statement in organic synthesisare 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 areinvolved 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 knownwhich are initiated by triplet excited molecules in carbonyl,enone, alkene or arene photochemistry. Traditional mecha-nistic analyses of photochemical reactions involving tripletexcited substrates have partly ignored the spin state prob-lem and focussed on the primary interactions on the tripletsurface.

We have made significant progress in elucidating therelation between regio- as well as diastereoselectivityand the spin inversion process in triplet 2-oxatetramethyl-ene-biradicals through a combination of experimentaland theoretical studies. Significant and sometimes sur-prising temperature, solvent, isotope, and substituenteffects 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 ProcessesABE Manabu(Graduate School of Engineering)

Accounts of Chemical Research, 37, 919-928 (2004)sNo.28 in “100 Papers Selection” (p.69)

Novel organic solar cells have been prepared using qua-ternary self-organization of porphyrin (donor) and

fullerene (acceptor) moieties units by clusterization with goldnanoparticles on nanostructured SnO2 electrodes. Thehighly colored composite clusters of porphyrin goldnanoparticles and fullerenes have been assembled asthree-dimensional arrays onto nanostructured SnO2 filmsusing an electrophoretic deposition method. The compositecluster 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 conversionefficiency of the OTE/SnO2/ (H2PC15MPC+C60)m compos-ite electrode reaches as high as 1.5%, which is 45 timeshigher than that of the reference system consisting of thesingle component of porphyrin or fullerene.

Photovoltaic Cells Using Composite Nanoclusters of Porphyrins and Fullerenes with Gold NanoparticlesHASOBE 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-statereactivity is that very weak forces like spin-orbit coupling(SOC, some cal/mol) - in systems carrying no heavyatoms - or hyperfine coupling (HFC, some mcal/mol) - in theabsence of strong external magnetic fields - can stronglyparticipate in influencing the reaction diastereoselectivity.

In the corresponding singlet reactions, diastereoselec-tion is born out of the enthalpy differences between the twoterminal 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-shellproducts, however, requires spin inversion and with that, theselectivity 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 stericand stereoelectronic interactions in the specific conformersthat are prone to undergo intersystem-crossing. Thus,very weak forces connected with spin inversion processescan lead to substantial control of regio- and diastereose-lectivity.