Naphthalene Included within All-Silica Zeolites: Influence of the Host on the Naphthalene Photophysics F. Ma ´ rquez,* C. M. Zicovich-Wilson, A. Corma, E. Palomares, and H. Garcı ´a Instituto de Tecnologı ´a Quı ´mica, UniVersidad Polite ´ cnica de Valencia, CSIC AV. De los Naranjos s/n, 46022, Valencia, Spain ReceiVed: June 1, 2001; In Final Form: August 5, 2001 The photophysical properties of naphthalene within pure silica zeolites were studied by means of diffuse reflectance, steady state and time resolved emission spectroscopy, fluorescence polarization, and FT-Raman spectroscopy. The experimental results indicate that naphthalene is strongly affected by the zeolite host. This distortion is reflected in the bathochromic shift of the 0-0 transition, the shortening of the fluorescence lifetimes, the observation of vibronic couplings, the appearance of room temperature phosphorescence, and the shift to lower vibration energy of the Raman peaks due to the weakening of the naphthalene bonds. The electronic structure of naphthalene within different zeolites has been computed on periodic models by using both the Hartree-Fock and the Kohn-Sham theories and the theoretical 0-0 transitions evaluated from the valence and conduction bands. The theoretical results are in agreement with the experimental observation of a red shift of the 0-0 bands indicating that the naphthalene π electrons are affected by confinement effect. 1. Introduction Zeolites are organized assemblies with structures of silica in which aluminum has been substituted into a number of the tetrahedral sites. 1,2 The negative charge carried by the alumi- nosilicate structural unit is balanced by the presence of metal cations that must be in close proximity to the aluminum sites. Simple cations differ substantially in charge, radius, and degree of hydration providing different properties to the zeolite. 3 The framework thus obtained present pores, channels, and cages with different shapes and sizes being able to accommodate guest molecules. Microstructural organization in these systems forms the basis of innovative technologies in petrochemical catalysis and molecular separations, having interesting applications in molecular electronics, integrated optics, and chemical sensing. 4-8 Many investigations have been devoted to the study of the adsorption of organic molecules in zeolites and the modulation of the photochemical and photophysical processes when they are incorporated into the channels or cavities. 9-21 The properties of the organic guest-zeolite supramolecular assembly are clearly different from the molecular properties of the pure organic due to different factors such as the presence of cations, Si/Al atomic ratio, electrostatic field into the zeolite framework, and channel and pore size. The extent to which one or more of these effects can affect to the organic molecule depends on the type of zeolite used. Thus, the singlet-triplet intersystem crossing is a pho- tophysical process that clearly depends on the cation used as counterion, and its quantum yield can be modulated by changing the cation type. 22-24 The presence of Al in the framework is also related to high electrostatic potentials in the zeolite medium that have been claimed to be responsible for the stabilization of charge separated transient species. The pore and channel sizes are also crucial to understand the different photoprocesses involved under incorporation of guest molecules into the hosts. The influence of the cavity dimensions on the electronic structure of some organic molecules when incorporated within zeolites has been related to the quantum confinement concept, 25 and it has been shown that it is to be responsible for significant changes in the chemical and physical properties of the guest molecules. 21,26-28 These changes can be explained as produced by an “electronic confinement” in which the electron density of the guest is constrained to be mainly localized within the zeolite cavity as a result of the strong short-range repulsion with the electrons of the zeolite walls. 21,26,28 Studies on the remarkable effects on various photophysical and photochemical processes occurring upon incorporation of aromatic hydrocarbons in zeolites have been reported. Diffuse reflectance, fluorescence, phosphorescence, and IR spectroscopy measurements 22,29-36 have been explored with the aim to understand the processes involved in these systems. Naphthalene has been very often the probe molecule of choice because its photophysics and photochemistry in various media is thoroughly well understood. In this regard, several papers have dealt with the photophysics of naphthalene included within X and Y zeolites, mainly in an attempt to characterize the modulation on some properties by the host. 36 However, if one wants to study exclusively the influence of the confinement effect avoiding the interference of electrostatic effects due to framework and extraframework charged atoms, it is necessary to prepare samples of pure silica polymorphs of zeolites with different pore dimensions and to use these all- silica zeolites as the host structure. In none of the reports appearing in the literature has this methodology been followed, and discussion of the effect of confinement on the photophysics of naphthalene is flawed by the concurrent contributions of different factors including variations in the framework Si/Al ratio. In this paper, we report the dramatic variations on the photophysical properties of naphthalene incorporated within pure silica zeolitic structures such as Ferrierite, SSZ24, and ZSM48. We have found that the host-guest interaction is responsible for remarkable effects as the changes in the Raman shift, observation of phosphorescence at low temperature without the * To whom correspondence should be addressed. 9973 J. Phys. Chem. B 2001, 105, 9973-9979 10.1021/jp012095c CCC: $20.00 © 2001 American Chemical Society Published on Web 09/25/2001