Chemiluminescence of lanthanide beta-diketonates in the reaction with 1,2-dioxetane
INTRODUCTION
Chemiluminescence of lanthanide ions was observed from their inorganic, organic and organometallic compounds in the following reactions:
1. Lanthanide accepts the energy of excited products of chemiluminescent reaction and re-emits chemiluminescence, but does not alter the rate/pathway of reaction. Highly luminescent lanthanide chelates are used to enhance the light intensity in those CL reactions where the primary excited product formed is non-luminescent but can transfer its energy to the Ln(III) chelate, e.g. in the case of triplet excited species.
2. Lanthanide accelerates the rate of chemiluminescent reaction, accepts the energy of excited products formed, and emits the CL, e.g. in the chemiluminescent decomposition of 1,2-dioxetanes catalysed by lanthanide shift-reagents Ln(FOD)3 and Ln(DPM)3. The higher rate of reaction results in a higher CL intensity.
3. Lanthanide initiates CL reaction without being emitter in it, e.g. in the case of CL observed during the oxidation of organic substances by Ce(IV).
4. Lanthanide chemiluminescence is excited in red/ox transitions from less-common Ln oxidation states, e.g. in the reduction of Tb(IV) and Pr(IV) or oxidation of Eu(II) and Yb(II).
References on lanthanide chemiluminescence and triboluminescence
A recent review: "Chemiluminescence of systems containing lanthanide ions" M. Elbanowski, B. Makowska, K. Staninski, M. Kaczmarek J. Photochem. Photobiol. A: Chem. 2000, 130, 75-81.
For information on chemi- and bioluminescence
visit "The Society for Bioluminescence and Chemiluminescence" http://www.unibo.it/isbc/
1,2-Dioxetanes are four-membered cyclic peroxides that easily decompose upon thermolysis to give ketones partially formed in the triplet and singlet excited states. The decomposition of 1,2-dioxetanes is often accompanied by chemiluminescence due to the radiative deactivation of ketone.
The1,2-dioxetanes were studied because they were found to be key intermediates in some bioluminescent reactions and they were shown to be excellent chemiluminescent labels in analytical biochemistry.
The chemiluminescence intensity of 1,2-dioxetanes can be substantially increased with the use of activators. The ideal activator should accept electronic energy of both singlet and triplet excited ketone products and posess high luminescence quantum yield to re-emit chemiluminescence in the desirable spectral range. The organic dyes and ruthenium and lanthanide complexes were used as activators of 1,2-dioxetane chemiluminescence.
We studied the system Lanthanide Beta-Diketonate + Adamantylideneadamantane-1,2-Dioxetane (AAD).
Lanthanide beta-diketonates were chosen as activators because they show efficient, line-like luminescence of lanthanide ion in visible and infrared range of spectrum. The 1,2-dioxetane AAD was chosen as a model because it is unusually stable. Its decomposition is accompanied by blue chemiluminescence at 420 nm due to fluorescence of adamantanone.
1. Chemiluminescence of weakly emissive lanthanide ions, e.g. praseodymium, neodymium and ytterbium
Previously, the lanthanide chemiluminescence was mainly studied with europium and terbium compounds that show efficient visible luminescence (red and green respectively).
A We have observed chemiluminescence of praseodymium(III) ion in visible and infra-red region. The striking feature of the spectrum shown below is that praseodymium(III) emits with comparable efficiency from three excited ff-states, e.g. Vavilov's law is not applicable to this ion.
J. Photochem. Photobiol. A: Chem. 1998, 119, 177-186
Mendeleev Commun. 1998, 110-112
Figures
Left. The chemiluminescence spectrum of Pr(FOD)3 in the reaction with 1,2-dioxetane AAD at 90 C in toluene. Note that ff-emission bands of Pr(III) are line-like and cover a wide spectral range. The blue, yellow and red emission bands are due to transitions from upper excited states of Pr(3+) while red and infrared emission bands originate from the lower excited state of Pr(3+).
Right. The excited ff-states of Pr(3+) ion. The emission of praseodymium is observed from the three marked states. According to Vavilov's law the Pr(3+) was expected to emit only from the lower marked level, however this is not the case.
B We have studied infra-red chemiluminescence of ytterbium (III) and neodymium (III) ions. Organic chelates of these near infrared emitting ions provide new opportunities to prepare (chemi)luminescent lanthanide probes for analytical biochemistry.
J. Photochem. Photobiol. A: Chem. 2000, 131, 61-65
Fig. The chemiluminescence spectra of Yb(3+) and Nd(3+) beta-diketonates excited from 1,2-dioxetane AAD decomposition at 90 C in toluene.
2. Singlet-singlet energy transfer from ketone (adamantanone) to the singlet state of the beta-diketone ligand in lanthanide beta-diketonates studied by chemiluminescence
J. Photochem. Photobiol. A: Chem. 2000, 131, 61-65
We have observed that the intensity of blue chemiluminescence of adamantanone is quenched when lanthanide beta-diketonate is added to the 1,2-dioxetane AAD solution. The efficiency of quenching by chelate depends only on the type of ligand (and is independent of the lanthanide ion) and is proportional to the spectral overlap between the ligand-centered absorption of the chelate and the chemiluminescence spectrum of adamantanone.
Figures
Left. The chemiluminescence spectrum of adamantanone at the decomposition of 1,2-dioxetane in MeCN. The ligand-centered absorption spectra of ytterbium beta-diketonates Yb(TTA)3.2H2O and Yb(BTFA)3.2H2O in MeCN. The LC-absorption spectrum of Yb(AA)3.3H2O (not shown) is situated above 30000 cm(-1). The spectral overlap for the chelates decreases in the order TTA > BTFA >> AA.
Right. The quenching of adamantanone chemiluminescence by ytterbium beta-diketonates Yb(TTA)3.2H2O, Yb(BTFA)3.2H2O and Yb(AA)3.3H2O. Note that the efficiency of quenching by the chelates TTA > BTFA >> AA parallels their spectral overlap.
In this system the first excited singlet state of adamantanone serves as energy donor and the first excited singlet state of the ligand (responsible for the intense LC-absorption of chelates) serves as energy acceptor. Thus, the observed quenching is due to the singlet-singlet energy transfer from adamantanone to the ligand levels in the chelate. Accordingly, no quenching is observed with AA chelates while with TTA chelates the kq approaches diffusion limit.
Fig. First excited singlet states of the adamantanone and of the beta-diketone ligand in the lanthanide chelates.
3. Activation of 1,2-dioxetane chemiluminescence by various lanthanide ions
J. Photochem. Photobiol. A: Chem. 2000, 136, 203-208
Figures
The chemiluminescence spectra of samarium(III), dysprosium(III), terbium(III) and europium(III) chelates excited by decomposition of 1,2-dioxetane AAD in toluene at 90 C. All emission peaks are due to the ff-transitions of lanthanide(III) ions. Note that for Eu(FOD)3 the emission is observed from two excited states - the resonant (5)D(0) and the upper-lying (5)D(1).
4. The catalytic decomposition of 1,2-dioxetane in the presence of lanthanide beta-diketonates
J. Photochem. Photobiol. A: Chem. 2000, 136, 203-208
J. Photochem. Photobiol. A: Chem. 1998, 119, 177-186
The coordination-unsaturated lanthanide beta-diketonates catalyze decomposition of 1,2-dioxetanes through the formation of complex in which the peroxide is coordinated to lanthanide ion. The Ln(3+) ion is excited by intramolecular energy transfer from the carbonyl products formed when 1,2-dioxetane decomposes in the inner-coordination sphere of Ln(3+).
We have shown that the lanthanide chelates that catalyse decomposition of 1,2-dioxetane show higher intensity of chemiluminescence when compared to those chelates that act only as a passive energy acceptors.