Intermolecular Electronic Excitation Transfer in a Confined Space: A First-Principles Study Ettore Fois,* Aldo Gamba, Cinzia Medici, and Gloria Tabacchi [a] Dedicated to Professor Michele Parrinello on the occasion of his 60th birthday. 1. Introduction Many properties of a molecular assembly in the solid state may be influenced by the packing characteristics. Supramolec- ular organization is a key factor in the design of materials with desirable properties. [1] Confinement of guest molecules inside the channels of zeolite molecular sieves is a viable approach for obtaining particular molecular assemblies embedded in a regular (crystalline) matrix. [2] Depending on the zeolite frame- work structure, [3] there are many possible supramolecular archi- tectures that guest molecules may achieve. [4] For example, the encapsulation of conjugated linear oligomers inside the one- dimensional channels of zeolites may allow the fabrication of host/guest composite materials whose properties are different from those of the oligomers’ bulk phase. Such particular one- dimensional architecture might be exploited in molecular- based-device production. By photoexcitation, intermolecular electronic excitation energy transfer (EET) is more efficient in an architecture where the electronic transition dipole moments are parallel to the channel axis than for other orientations. [4b,5] An arrangement confined inside one-dimensional pores of a suitable zeolite therefore can be devised as a supramolecular energy transfer apparatus, which can absorb a photon at one end of the channel and transfer the excitation to the other end of the pore, realizing a synthetic, light-harvesting anten- na. [4] Such host/guest systems have been fabricated [4] and their properties and applicability are currently being investigated. [4,6] Excitation energy transfer may occur according to two main mechanisms: Fçrster long-range EET [7] and the short-range Dexter scheme. [8] In the former, the molecular excited dipole localized on a photoexcited molecule is transferred via dipole– dipole coupling to a neighbouring unexcited molecule that can be  50  apart in a tail-to-head geometry. The Dexter mechanism, on the other hand, is short-range, as the overlap of wavefunctions is mandatory in such a case. Herein, the advantages of a first-principles molecular dynam- ics method [9] are exploited to study short-range EET in a supra- molecular system in confined space. The confining host matrix is the zeolite bikitaite; [10] the guest system is composed of chlorine molecule wires. At the outset, it should be stressed that the simulated system has not been synthesized yet, and should be considered as a model. On the other hand, as halo- gens are easily sorbed in anhydrous zeolites, [11,12] the practical realization of our model system, a chlorine-filled bikitaite, should be possible. Bikitaite [10] is a naturally occurring phase characterised by a non-centro-symmetric space group and one-dimensional noncrossing channels of about 3  diameter, in which lithium cations and water are hosted. Water mole- cules are arranged in a very peculiar architecture: They form chains of hydrogen-bonded H 2 O running along the noncross- ing channels, parallel to the b axis (one-dimensional ice). [10c] Each water oxygen is coordinated to a Li + and each H 2 O is hy- drogen-bonded to the two adjacent molecules. Bikitaite, like other zeolites, can be dehydrated at high temperature, [13] thus leaving the channels ready for other species to be sorbed. Molecular chlorine is a green gas, its colour is due to a sin- glet–triplet transition. [14] Such a transition, that should general- ly be forbidden, is however allowed owing to rotational cou- pling. When Cl 2 molecular rotations are hindered, for example by adsorption on a surface, [15] the ground-state singlet–excited triplet transition is dumped and the sample turns colourless. In the Cl 2 Franck–Condon region, the triplet excited state is rela- tively close to an unbound singlet state. [16] If excitation occurs to such a state, a Cl 2 molecule decomposes in two atoms. However, in a rare gas matrix, complete dissociation is prevent- ed by cage effects and recombination occurs. [16] A supramolec- ular organization of Cl 2 molecules inside the host channels [a] Prof. Dr. E. Fois, Prof. Dr. A. Gamba, C. Medici, Dr. G. Tabacchi DSCA, Universita’ dell’Insubria and INSTM UdR Como Via Lucini 3, 22100 Como (Italy) Fax: (+ 39) 031-326-230 E-mail: fois@fis.unico.it Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author. The process of intermolecular electronic excitation transfer (EET) in a monodimensional supramolecular arrangement of molecules in confined space has been modelled and investigated by means of first-principles molecular dynamics simulations. The chosen model system consists of a wire of chlorine molecules hosted in the noncrossing channels of the zeolite bikitaite. The time evolu- tion of the system in its first excited singlet state has been de- scribed by the restricted open shell Kohn–Sham formalism. Simu- lation results have highlighted that excitation, initially localized on a guest molecule, is transferred to an adjacent moiety in the molecular wire on the picosecond scale via a collision-induced Dexter-type short range EET. Analysis of the modifications of the electronic structure of the system brought about by EET has given insight into the microscopic details of the process. ChemPhysChem 2005, 6, 1917–1922 DOI: 10.1002/cphc.200400561 # 2005 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim 1917