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.

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

The ground state conformational, UV absorption and fluorescence spectral as well as decay time data for a series of polymethylene- and one polyoxyethylene-linked di(9-anthracenes) in solution are compared, for chainlengths varying from n=0 (directly linked system A0A, 9,9′-bianthryl) to n=∞ (A∞, reference system 9-decylanthracene). 1H NMR spectroscopy of A2A to A11A shows that there is no clearly preferred ground state conformation of the anthracene moieties. The crystal structure of A2A was solved and the anti conformation of the anthracene units was established. The UV spectra show a redshift interpreted within the Forster exciton model (predominance of extended conformations). Fluorescence spectra for (CH2)n-bridged systems show no clear excimer band for short-chain systems but redshifted excimer emission for A4A to A9A. Loss of structure and fluorescence band shape changes as well as biexponential decay times allow the conclusion that excimer formation occurs in all chained systems. For a related polyoxyethylene-chained system, the excimer band is substantial, probably a consequence of the increased flexibility of the chain and moderate photoreactivity. Solvent polarity changes are strong for A0A [formation of a twisted intramolecular charge transfer (TICT) state] but also significant in the systems A1A and A2A. This is rationalized by the possiblity for competitive formation of an excimer-type (parallel but tilted chromophore arrangement) and a TICT-type (near perpendicular) conformation. For AnA (n>2) the solvent polarity was found to have virtually no influence.

https://pubs.rsc.org/en/content/articlepdf/1999/NJ10.1039/A809363F

Intramolecular Excimer Formation in Short- and Long-chainlength di(9-anthryl) Bichromophoric Compounds and Relation to Ground State Properties

The emission properties of styrene oligomers from dimer to tridecamer have been investigated by picosecond pulse radiolysis at room temperature. The oligomers are precisely fractionated and are considered to be models of atactic polystyrene. The rate constant for excimer formation kDM of the order of 108 s−1, increases with increasing number of monomer units n for 2≤n≤8, then levels off at the value for polystyrene. The rate constant for singlet energy migration between adjacent chromophores in polystyrene was determined to be 3×1010 s−1 (∼30 ps). An average time of 7.2 ns was found to be necessary for two adjacent chromophores to assume the excimer conformation (g−t or tg− in a racemo dyad; tt in a meso dyad) from their initial equilibrium distribution of conformations. The present result is an indication that one may investigate the dynamics of internal rotation by photophysical techniques. Moreover, from a comparison of the results of photostationary measurements with those of transient methods, it is found that there is a discrepancy between them. This discrepancy is assumed to be caused by a molecular weight dependence either of the rate constant for the radiative deactivation from the excited monomer singlet kFM or of intramolecular self‐quenching.

Intramolecular excimer formation of oligostyrenes from dimer to tridecamer: The measurements of rate constants for excimer formation, singlet energy migration, and relaxation of internal rotation

Kinetic study of photophysical and photochemical processes in α,ω-(bis-9-anthryl)-n-alkanes (ethane to decane) at room temperature

Normal and second intramolecular excimer fluorescence of 1,3-bis(4-methoxy-1-naphthyl)propane and 1,3-bis-(4-hydroxy-1-naphthyl)propane

The observation of intramolecular anthracene excimer in 1,3,‐dianthrylpropane by coincidental line constant of the excimer rise with fluorescence decay of anthryl moiety is reported. (AIP)

Sachiko Kobayashi

Papers

Direct observation of intramolecular anthracene excimer in 1,3‐dianthrylpropane