Fullerene-bridge-aniline dyad and the model fulleropyrrolidine compound form stable, optically transparent clusters in mixtures (3:1) of acetonitrile and toluene. Ground-and excited-state properties of the clusters of the dyad and the model compound are compared with their corresponding monomeric forms. Clustering of the dyad as well as the model compound exhibits a red-shifted emission maximum (λ max ∼ 738 nm) compared to their monomeric forms (λ max ∼ 714 nm). The electron transfer from the appended electron donor moiety to the parent fullerene core in the dyad cluster is evident from the decreased (∼80%) fluorescence yield. The formation of fullerene radical anion (absorption maximum at 1010 nm) with a lifetime of several hundreds of microseconds was further confirmed using nanosecond laser (337 nm) flash photolysis experiments. In contrast, the dyad molecules in their monomeric form did not yield any detectable yield of C 60 radical anion following laser pulse excitation. The failure to observe any charge-transfer intermediates following laser pulse excitation, even in polar solvents such as benzonitrile or nitromethane, suggested that fast back-electrontransfer process must be operative in the monomeric dyad system. On the other hand, clustering of the fullerenebased dyads in a mixed-solvent system can provide a unique way to decrease the rate of back electron transfer, thus stabilizing the electron-transfer products.

The photoisomerization of a few substituted dibenzodihydropentalenofurans to the corresponding dibenzosemibullvalenes is reported. Steady-state photolysis of the dibenzodihydropentalenofurans 3a--d gave the corresponding dibenzosemibullvalenes 2a--d in nearly quantitative yields. The quantum yields of this photoisomerization were found to be in the range 0.17--0.26. Laser flash photolysis studies of the dibenzodihydropentalenofurans 3a-e showed transients, with absorption maxima around 410 nm and decaying by first-order kinetics. The lifetimes were in the range 14--30 micros in degassed benzene at 25 degrees C. These transients were readily quenched (trapped) by molecular oxygen, and the Stern-Völmer quenching constants were found to be in the range (2.45--3.17) x 10(9) M(-1) s(-1). As a representative example, the 1,3-biradical intermediate from 3e was trapped by molecular oxygen to give the corresponding endoperoxide 11e. The transients were weakly quenched by triplet/radical quenchers such as 2,2,6,6-tetramethylpiperinyl-1-oxy (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperinyl-1-oxy (HTEMPO), and the quenching constants are found to be in the range (1.09--3.19) x 10(6) M(-1) s(-1). The decay rates of the transients were found to be temperature dependent and obeyed the Arrhenius equation. For example, the activation energy of the transient from 3a was approximately 4.5 kcal mol(-1) and the Arrhenius preexponential factor log(A/s(-1)) for the decay of the transients was approximately 7.5. On the basis of our studies, these transients were assigned as the ground-state triplet biradicals, generated by the cleavage of the C--O bond of the starting dibenzodihydropentalenofurans.