Roles of the Rydberg transitions in fast excitation transfer studied in cyclohexane and n-heptane using synchrotron radiation

Excitation spectra of PPO (2,5-diphenyl oxazole) in cyclohexane solution show peaks at 63 000 and 78 000 cm−1, while those in n-heptane solution show a single peak near 66 000 cm−1. The 63 000 cm−1 peak for cyclohexane and 66 000 cm−1 peak for n heptane are ascribed to excitation transfer at respective valence-band absorption peaks. An extra peak at 78 000 cm−1 for cyclohexane solution is tentatively ascribable to the Rydberg transitions and it is situated near the eighth peak of the Rydberg series. The expanded reaction radius is, therefore, estimated to be about 30 Å. Taking account of the Rydberg series can explain the known fact that excitation transfer in cyclohexane is extraordinarily fast compared with that in n heptane. Kinetic analysis shows that these upper excited states at 63 000 and 78 000 cm−1 of cyclohexane make direct excitation transfer, competing with relaxation to the lowest excited state at 49 750 cm−1. It is briefly discussed that the Rydberg series, which can be observed only in cyclic (or round) alkanes, may also be one of the key processes that explain molecular structural dependence observed in the excitation transfer and electron mobilities in alkanes.