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Solvent-Dependent Quenching of the Lowest Excited Singlet State of 9,10-Dibromoanthracene by Ground-State 2,5-Dimethylhexa-2,4-Diene Yielding 9-Bromoanthracene or the [4+2] Adduct

100%

Nagahara T., Nakayama T., Hamanoue K.

Acta Physica Polonica A

1998

vol. 94

issue 4

701-713

Irrespective of the dielectric constant (ε) or the hydrogen-bonding ability of a pure solvent, the lowest excited singlet state (DBA*) of 9,10-dibromoanth racene (DBA) is quenched by ground-state 2,5-dimethylhexa-2,4-diene (DMHD) giving rise to the appearance of an exciplex emission. By means of sub-picosecond and nanosecond laser photolysis as well as steady-state photolysis, however, the following solvent-dependent results are obtained; (1) in acetonitrile (ε=37.5) and acetone (ε=20.7), an exciplex (DBA-DMHD)* formed between DBA* and DMHD generates the DBA radical anion (DBA^{•-+}) as an intermediate for formation of 9-bromoanthracene (BA) from DBA; (2) in ethanol (ε=24.6), 2-propanol (ε=19.9), 1-octanol (ε=10.3), diethyl ether (ε=4.34), and heptane (ε=1.92), a neutral radical species (NR^{•}) generated by decomposition of (DBA-DMHD)* (or by a reaction of DBA* with DMHD) is an intermediate for formation of a dibenzobicyclo[2.2.2]octadiene-type compound (a [4+2] adduct); (3) both DBA^{•-} and NR^{•} are generated in methanol (ε=32.7) but the rate (v) of BA formation upon steady-state photolysis of DBA in the presence of 1 M DMHD decreases in the order of v(acetonitrile, ε=37.5) > v(acetone, ε=20.7) > v(methanol, ε=32.7). It thus can be concluded that the mechanism of exciplex decomposition and the reaction of DBA* with DMHD are affected by not only the dielectric constant of a pure solvent but also its hydrogen-bonding ability.

Formation of a Complete Electron Transfer State of 9,9'-Bianthryl in a Polar Solvent

80%

Nishiyama K., Honda T., Okada T.

Acta Physica Polonica A

1998

vol. 94

issue 5-6

847-856

It has been generally recognized that excited 9,9'-bianthryl in polar solvents achieves a partial charge-separation state, instead of a complete electron transfer state, even in high-polarity solvents like acetonitrile at room temperature. However, here we firstly found that 9,9'-bianthryl reaches the complete electron transfer state in a polar alkanenitrile solvent at lower temperature as elucidated by means of transient absorption and steady-state fluorescence spectroscopy. A stepwise increase in the fluorescence Stokes shift observed when temperature was reduced implies a formation of a state more polar than that dominated at the room temperature. At low temperature (165 K), the transient absorption spectra of 9,9'-bianthryl in butyronitrile gradually changed from the spectrum ascribed to the partial change-separation state toward that of the complete electron transfer state. We suggest that at low temperature, due to the increase in the solvent polarity, the energetically stabilized electron transfer state should be predominant.

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2 Acta Physica Polonica A

2 1998

1 Hamanoue K.

1 Honda T.

1 Nagahara T.

1 Nakayama T.

1 Nishiyama K.

1 Okada T.

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Solvent-Dependent Quenching of the Lowest Excited Singlet State of 9,10-Dibromoanthracene by Ground-State 2,5-Dimethylhexa-2,4-Diene Yielding 9-Bromoanthracene or the [4+2] Adduct

Formation of a Complete Electron Transfer State of 9,9'-Bianthryl in a Polar Solvent