Energy Migration in Dye-Loaded Hexagonal Microporous Crystals

Abstract

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.

Designing Dye–Nanochannel Antenna Hybrid Materials for Light Harvesting, Transport and Trapping

“…The rate equation for FRET by following the arguments given in the original work of Fçrster [24] and the description of exciton coupling based on Davydov's theory [27] have been extremely useful for designing and understanding organized systems based on nanochannel materials. [15,[36][37][38][39][40] We restrict the discussion to one photon processes taking place under ambient temperature conditions. This means that processes in which two or more electronically excited molecules interact with each other-leading to phenomena such as annihilation processes, super radiance, lasing and others; but also very low temperature situations where shallow traps may be important -are not discussed.…”

“…Cross-talking between molecules located in neighboring channels is, of course, possible and has been investigated theoretically and experimentally, to some extent. [36,[39][40][41][42] We do not investigate this further here, despite of the fact that it can be important. In order to realize noncentrosymmetric filling of the channels, one side of the channel opening must be blocked while keeping the other side open.…”

Electronic Excitation Energy Migration in a Photonic Dye–Zeolite Antenna

“…Hence, we compared the rate constants for EnT obtained by the MC simulations with values obtained from Fˆrster theory. [62] The rate constant k EnT for EnT from an excited Py to an unexcited Py is, in the point dipole ± dipole approximation for the intermolecular interactions, given by Equation (32), [63] k EnT 9ln10…”

“…The EnT constant k EnT was calculated using the Markoff chain approach, [63] using the experimental value for the Py ,Py spectral overlap; J % 1.05 ¥ 10 À13 cm 3 mol À1 ; F 1; n rf 1.43; and averaging G over a cone distribution (q half cone 728) for the transition dipole moment of Py in zeolite L. [33] The EnT parameters for different Py loadings, evaluated by MC and by Fˆrster theory, are shown in Figure 14.…”

The Incorporation of Indigo Molecules in Sepiolite Tunnels

“…This leads to the building of artificial antennas. [17][18][19] With reference to dyes and clay minerals, enhanced second-order linearity was evidenced for crystal violet in zirconium phosphate and methylene blue on laponite. In summary, control of the numbers, locations, stability, orientation and interactions of the organic molecules in the internal and external surface of mineral tubes is key in the challenge to develop well-defined unitary bricks with useful properties.…”