CdSe nanocrystals in solution and films can enter a metastable state in which the highly luminescent nanocrystals become dark. This change, which we attribute to a surface transformation, can be caused by heating or by changing the environment of the nanocrystals at room temperature. The metastable transformation is reversed upon illumination of above-band-gap light, at which point the nanocrystals are again highly luminescent.

Photoexcited states in trans polyacetylene lt-(CH), l and in ulkoxy derivatives of poly(puraphenylenevinylene) (PPV) have been studied by the technique of absorption detected magnetic resonance. In addition to the stable charged photoexcitations, which are charged solitons in t-(CH), and bipolarons in PPV, we clearly identify neutral photoexcitations with nonzero spins. In t-(CH), these are spin-, ' neutral solitons S" (S") with an optical transition at 1.35 eV. The neutral photoexcitations in PPV form triplet excitons which cannot dissociate into S "S" pairs due to the confinement caused by the nondegenerate backbone structure.

We report optical-absorption, photoluminescence (PL), and picosecond photoinduced absorption (PA) decay in films of poly-3-hexyl-thiophene at pressures up to 80 kbar. The spectral bands redshift nonlinearly with pressure and the PL intensity decreases markedly. Thermochromic transitions are completely inhibited at pressures as low as 14 kbar. The picosecond recovery of the PA decay at high pressure is similar to that of unpressed polythiophene, but has a power-law exponent consistent with more ordered chains at high pressure. These effects suggest changes with pressure in the chain conformation and in the electronic polarizability; no changes in the interchain transfer integral are observed.

No abstract