Photophysical studies of neutral aromatic species confined in zeolite L: Comparison with cationic dyes

Abstract

The photophysical properties of a few aromatic molecules incorporated into the straight channels of zeolite L were investigated, mainly by luminescence spectroscopy. Two significant observations were made. (1) Intense room temperature phosphorescence (RTP) was observed for 9-ethylcarbazole and other aromatics included in the dehydrated K⁺-form of the zeolite L (KL) whereas RTP was very weak in 47 atom % Na⁺-exchanged KL (NaKL) and the K⁺- and Na⁺-forms of zeolite Y (KY and NaY). Previously, observation of RTP was only made in zeolites exchanged with heavy atom cations such as Rb⁺ and Cs⁺. For 9-ethylcarbazole, the tight fit into the KL channels and resultant increased structural rigidity are largely responsible for the remarkable reduction in the rate of nonradiative intersystem crossing from T₁ to S₀ , leading to the enhanced phosphorescence lifetime even at room temperature. (2) Anthracene, intercalated in the channels, formed dimers that can be detected by the excimer emission. It was found that the dimer formed in NaKL has less overlap than that formed in KL. Moreover, the dimer must have a remarkably short separation between the two rings, comparable to that of anthracenophane, judging from its distorted absorption spectrum. Formation of naphthalene dimers with partial overlap in KL was identified from the characteristic fluorescence spectrum of the second excimer. No naphthalene dimer was formed in NaKL even at high loadings. Thus the photophysics of anthracene and naphthalene in zeolite L is remarkably different from that in solutions and large-pore faujasite zeolites where the framework exerts only weak conformational control over the guest molecules. Additionally, the charge compensating cations in zeolite L were found to have the ability to control the distribution and conformation of the guest species within the channels. The present findings show that the zeolite L is a better host matrix than the large pore faujasite zeolites for manipulating the photophysics of neutral guest species.