Microporous materials containing linear channels running through
hexagonal microcrystals allow the formation
of very concentrated monomeric and highly anisotropic oriented dye
systems that support extremely fast
energy migration. Energy migration can be described as a
homogeneous Markoffian random walk, in which
each energy transfer step is incoherent and occurs from a thermalized
initial state. The dyes investigated
have an electronic transition dipole moment
μS
1
←
S
0
which coincides with their long axes. The individual
energy transfer steps calculated based on dipole−dipole interactions
occur with rate constants of up to 30
ps-1. This fast energy migration cannot
be described by a diffusive process immediately after irradiation
but
becomes diffusive after several tenths of a picosecond. After this
time a constant diffusion coefficient D can
be defined with values of up to about 0.3 cm2
s-1 for an optimized system based on, for
example, cylindrical
zeolite L microcrystals and oxonine. A main part of this study
refers to excitation trapping on the surface of
cylindrical microcrystals. We distinguish between front
trapping (traps positioned on the front of the
cylinders),
front
−
back trapping (traps on the
front and on the back), coat trapping (traps on the coat),
axial trapping
(traps located in the central channel), and point trapping
(a single trap at the center of the front). In
cylindrical
microcrystals with a size of 50 nm containing about 33 000
chromophores and complete coverage of the
outer surface by traps, a total trapping efficiency of 99.8% can be
obtained. The front−back trapping efficiency
is 60.4% and the coat trapping efficiency is 39.4%. The front
trapping efficiencies reach 99.0% if only the
front is covered by traps. In a microcrystal of 37 nm length,
still containing 12 600 chromophores, point
trapping efficiencies of up to 93.0% have been
calculated.