Fast Energy Migration in Pyronine-Loaded Zeolite L Microcrystals

Abstract

The stacking of pyronine and oxonine in the channels of zeolite L microcrystals is possibly due to their high affinity for entering the channels and to the narrowness of inside the channels, which prevents the dyes from gliding past each other. This allowed us to invent experiments for observing energy migration in pyronine-loaded zeolite L microcrystals of cylinder morphology. Organic dyes have the tendency to form aggregates at relatively low concentrations which cause fast thermal relaxation of electronic excitation energy. The role of the zeolite is to prevent this aggregation even at very high concentrations and to superimpose a specific organization. Light is absorbed by a pyronine molecule located somewhere in one of the zeolite channels. The excitation energy migrates preferentially in both directions along the axis of the cylinder because of the pronounced anisotropy of the system. It is eventually trapped by an oxonine located at the front or at the back of the microcrystal. This process is called front−back trapping. The electronically excited oxonine then emits the excitation with a quantum yield of approximately one. The pronounced anisotropy of the electronic transition moments of both pyronine and oxonine can be observed in an optical fluorescence microscope by means of a polarizer. Maximum luminescence appears parallel to the longitudinal axis of the cylindrical microcrystals, extinction appears perpendicular to it, and their base always appears dark. We report experimental results for the front−back trapping efficiency of pyronine-loaded zeolite L microcrystals of different average lengths, namely 700, 1100, and 1500 nm, different pyronine occupation probability, ranging from 0.03 to 0.48; and modification at the base with oxonine as luminescent traps. Extremely fast electronic excitation energy migration along the axis of cylindrical crystals has been observed, supported by the increase of the effective excitation lifetime caused by self-absorption and re-emission of the pyronine vertical to the cylinder axis. Effective energy migration lengths of up to 166 nm upon pyronine excitation have been observed, which thus leads to the remarkable properties of this material.

Energy transfer in nanostructured oligothiophene inclusion compounds

“…Recently much interest has been placed on supramolecular architectures ± obtained by spontaneous self assembly or by layer by layer growth ± which realize controlled spatial organization of molecules. Fabrication of ordered arrays of conjugated molecules can also be obtained by their incorporation in host systems capable of forming nanochannels of suitable dimensions for containing the desired active molecules [1,2]. This approach has attracted considerable interest since it demonstrates the possibility to manifest macroscopic physical properties based on embedded microscopic structures.…”

“…By this process the energy is transported (spatially) and converted (energetically) by the following sequence: absorption of energy from a chromophore present in the system (indicated hereafter with D, donor); excitation transfer to another chromophore (A, acceptor); emission of radiation by the latter [4]. Energy transfer can also take place within the same molecular species (the D molecules), thus producing a fast migration of the excitation throughout the crystal [2,5]. Organic lasers [6] based on both¯uorescent and phosphorescent dopants have been fabricated using this cascade process with the advantage of reducing unwanted absorption loss [7±9].…”

“…The host±guest system investigated in the present work provides the unique opportunity to better understand the role played by the long-range homotransfer processes among linear conjugated molecules since the peculiar disposition of the molecules in the channels of PHTP prevents the short-range hopping processes. It is worth stressing that a quantitative understanding, and thus a control, of homotransfer along the channel is important for a directional and, possibly fast,¯ow of energy [2,5]. 2.…”

Supramolecular photochemistry in zeolites: From catalysts to sunscreens

“…For example, when pyronine (Scheme 2), a green-emitting dye, is encapsulated in the channels of zeolite L crystals, it is prevented from stacking by the molecular restriction in the channels and instead forced into an end-to-end configuration. If each cylindrical zeolite microcrystal is capped with oxonine, a red-emitting dye, selective excitation of the pyronine results in the observation of red emission as the excitation energy migrates along the column of pyronine molecules via energy transfer until it is eventually trapped by oxonine [66][67][68].…”

Directional Energy Transfer in Nanocrystals of [Ru(2,2′‐bipyridine)3]­[NaCr(oxalate)3]

“…[1][2][3][4] Excitation-energy transfer processes play an important role for these applications, and materials that show efficient long-range directional energy transfer are of particular interest in this respect. [4] Light is a convenient tool to control the physical and photophysical properties of a material. [5] To improve the efficiency of photophysical processes in nanomaterials, full control and optimization of tunable optical properties are required.…”