Comparative Study of Exciton Relaxation Dynamics in C₆₀ and C₇₀ Solid

Abstract

The exciton dynamics of C70 solid has been investigated by the time-resolved photoluminescence (PL) and transient absorption (TA) technique and compared with the case of C60 solid. Three distinct PL bands were observed at 685, 715, and 740 nm. The high-energy band at 685 nm displayed an almost purely single-exponential decay profile, while the other two exhibited a slow-decaying tail as well, especially at low temperatures. Laser fluence dependence of PL decay profiles shows that the decay in C70 solid, in contrast to the case of C60, is predominantly governed by fast (20 ps) nonradiative exciton-trap annihilation, with the intersystem crossing playing a minor role in lower energy bands at 715 and 740 nm. On the other hand, laser fluence dependence of transient absorption signals shows that the decay in C70 is much faster than PL decay, which indicates the fast component arises from the exciton−exciton annihilation process under extremely high laser fluence condition.

Excited-state Dynamics of Fullerene Nanoparticles Dispersed in Pure Water

“…Indeed, transient absorption studies on solid films of C 60 have shown that the decay curves of the excited state depend on the excitation intensity, and that the singlet exciton annihilation process is observed on a 10-ps-order time scale under excitation intensity conditions similar to those in the present experiment. 20,24,25 Recently, Clements et al reported similar results of transient absorption experiments on nC 60 dispersed in a micellar solution of Triton X-100 and water. 14 They attributed the shortened lifetime to singlet annihilation; however, they did not examine the excitation intensity dependence.…”

“…We measured the fluorescence spectra and decay profile of the same sample of nC 60 in water. A broad emission peak around 750 nm, similar to the fluorescence peak of a thin film, 20 was observed, and a time-correlated single-photon counting experiment revealed the fluorescence lifetime to be about 10 ps. 21 The fluorescence lifetime of nC 60 in water is significantly shorter than that of C 60 in toluene (1.2 ns).…”

“…It has been reported that defects inside solids can easily act as quenchers or trap sites. 14,20 It is notable that the reported intrinsic (monomeric) decay time of the singlet excited state of C 60 thin films is about 100 ps, 24 which is approximately 10 times that of nC 60 in water. It could be considered that the singlet excited state is quenched efficiently at the nanoparticle/ water interface.…”

“…Several species such as ionized states, hot-ground states, trapped sites, and impurities can be candidates for long-lived components. 14,20 It has been reported that C 60 cation and anion radicals have characteristic bands at 980 nm 26 and 1070 nm, 22 respectively. However, the spectrum of nC 60 in water at 500 ps in the near-infrared region did not show such absorption bands.…”

Probing Transient Electric Fields in Photoexcited Organic Semiconductor Thin Films and Interfaces by Time-Resolved Second Harmonic Generation

“…The similar time constants (∼10 ps) at different film thicknesses point to a common origin of the photoinduced dynamic process in C 70 . This process is slower than exciton–exciton annihilation, which features a time constant of 1 ps, but much faster than the nanosecond intersystem crossing process . The photon energy of 2.03 eV is known to excite intermolecular charge-transfer (CT) states, , leading to photocarrier generation.…”