Development and investigation of the photophysical process of photo-energy harvesting and transfer using σ-π and π-π interactions.

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The photophysical properties of α, ω-diaryloligosilanes : (1-naphthyl)-(SiMe_2)_n-(1-naphthyl) (1-5) and (9-anthryl)-(SiMe_2)_n-(9-anthryl) (6-10), (9-anthryl)-(SiMe_2)_n-(1-naphthyl) (11-15), n=1,2,3,4 and 6 were investigated. In these compounds, three types of interactions, a σ-σ interaction between silicon atoms in the chain unit, a π-π interaction between two aromatic groups, and a σ-π interaction between aromatics and a silicon chain unit were observed. As a result, intramolecular excimer/exciplex emissions and charge-transfer (CT) emission were observed. Intramolecular excimer emission was observed in cyclohexane when n【greater than or equal】2. The strongest excimer emission of 2 and 7 is in contrast to the Hirayama-rule (n=3) proposed for carbon analogues and also shows that intramolecular cycloaddition is minor. The time resolved fluorescence (TR-FL) spectra of an anthryl series revealed that the time constant of the excimer formation varied depending on the chain length (82-152 ps). In the cases of 4,5,and 10,charge-transfer (CT) emission was observed in acetonitrile or THF. The time constant of the CT state formation for 10 was relatively slow (45 ps), which may indicate a conformational change. A monomer emission from the locally excited state was observed for 1 and 6 in both cyclohexane and acetonitrile. For 12-15,the intramolecular exciplex emission was observed in cyclohexane. For the stationary and the TR-FL measurements, the excitation of the naphthyl moiety showed a very efficient intramolecular energy transfer to the anthryl moiety, thus only fluorescence from the anthryl moiety was observed. In the case of 14 and 15,a charge-transfer (CT) emission was observed in acetonitrile. Then TR-FL of 15 revealed that the intramolecular-energy transfer from the naphthyl to anthryl moieties is faster than the charge transfer.