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The present work is focused on the study of the spectroscopic and complexometric behavior of different ion-sensitive fl uorescent probes. Three of the four types of probes studied follow a conventional principle of signal generation and their molecular constitution corresponds to either a fl uorescent ligand ( BP(OH)2), so-called intrinsic charge transfer (ICT) fl uorescent probes (donor- acceptor-substituted chalcones), or electron transfer (ET) fl uorescent probes (substituted triaryl-Δ2- pyrazolines). The latter two systems are designed in a modular way and thus, both the signal generating fl uorophore and the complexing receptor can be carefully tuned. Whereas in the case of the ICT probes both units are electronically conjugated, these moieties are more or less electronically decoupled in the ET probes and signal generation is based on the modifi cation of a long range electron transfer process. The fourth type of probe investigated, a boron dipyrromethene derivative (BDP), is also designed in a modular way. But here, due to the highly pretwisted constitution of the molecule, fl uorophore and receptor are only “virtually” decoupled. Besides employing this alternative mechanism of signal generation, control of the selectivity by carefully directed receptor design and improvement of the selectivity and sensitivity of fl uorometric metal ion analysis by recording time resolved emission spectra (TRES) were the main aims of these investigations. Furthermore, with the aid of various model compounds, NMR spectroscopy as well as quantum chemical calculations, a fundamental understanding of the photophysical behavior of the different classes of dyes was obtained. In accordance with its parent compound 2,2‘- bipyridyl, the fl uorescent ligand BP(OH)2 shows complexation induced changes of its absorption and emission behavior in the presence of heavy and transition metal ions. Whereas paramagnetic ions such as CuII or NiII (and also HgII) lead to static fl uorescence quenching, the diamagnetic ions ZnII and CdII form strongly fl uorescent complexes with ion specifi c fl uorescence lifetimes in the ns time range. Both complexes are stable in the excited state which is manifested by a strongly reduced rate constant of non radiative deactivation compared to that of the free ligand. Both the absorption and emission spectra of the complexes largely overlap and a discrimination with steady-state fl uorometry is very limited. Nevertheless, with time resolved fl uorometry a separation of both components is possible and for a series of synthetic ZnII/CdII containing samples this gain in selectivity by global analysis of the time resolved emission spectra (TRES) could be demonstrated. The intrinsic fl uorescent probes of chalcone type contain a cation selective receptor in the donor part of the molecule and show a strong decrease of the intramolecular charge transfer process due to complexation induced weakening of the donor. Depending on molecular probe design and solvent polarity, the fl uorescence of the uncomplexed molecule occurs either from a polar directly exicted (E*) or a highly polar charge transfer state (A*) which is populated via an excited state reaction involving single bond twisting. Here, competing processes include population of a non emissive polar (K*) or weakly polar (P*) transient state. Furthermore, the fl uorescence of these dyes can be quenched by energetically close lying nπ* states (in apolar solvents) or by enhanced internal conversion due to the decreasing energy gap between ground and excited state (in highly polar solvents). As mentioned above, the donor strength is reduced upon cation complexation in polar solvents and a moderate fl uorescence enhancement occurs. Besides increased fl uorescence quantum yields, this effect results in ion specifi c fl uorescence lifetimes of the complexes. Upon exchanging the aza oxa for an aza thia macrocycle in the receptor part of the system, the cation selectivity could be tuned from alkali and alkaline earth metal ions to the thiophilic metal ions AgI and HgII. Here, especially the fl uorescence enhancement in the presence of the well-known quencher HgII is analytically valuable. Upon cation binding the ET probes of the triaryl-Δ2-pyrazoline type show very small spectral shifts but much larger fl uorescence enhancement compared to the ICT probes. Again, by problem specifi c use of different receptors, the selectivities can be controlled. Based on a detailed investigation, for the 5-p-N,N-dialkylanilino-3-benzthiazol-1-phenyl-Δ2-pyrazolines, the intramolecular processes which lead to fl uorescence quencing in the unbound state and “switching on” of the fl uorescence in the complexed state could be identifi ed. For these molecules, the intramolecular charge transfer in the basic chromophore occurs in the sub ps time range and is quenched by an electron transfer process from the largely electronically decoupled receptor to the acceptor part of the basic chromophore on the ps time scale in highly polar solvents. “Virtually” decoupling fl uorophore and receptor in the BDP derivative fi nally makes it possible to yield extremely high complexation induced fl uorescence enhancement signals. For these highly pretwisted donor acceptor biaryls, the directly excited emissive state (LE) forms a highly polar, twisted charge separated charge transfer state (CT) in polar solvents. Emission of the latter is strongly forbidden and, depending on solvent polarity, is largely red shifted compared to the LE fl uorescence. Thus, a largely separated dual fl uorescence is already observed in solvents of medium polarity and in highly polar solvents both fl uorescence bands are nearly completely quenched. Metal ion complexation blocks off the CT process and leads to a “switching on” of the LE emission yielding fl uorescence enhancement factors > 1000. This process is so sensitive that for most of the cation complexes studied, two emissive complex conformers with strongly overlapping absorption and emission spectra occur which could be distinguished by their fl uorescence lifetimes.
Im Rahmen dieses Projekts wurden Fluoreszenzfarbstoffe und neue intrinsische und konjugierte Fluoreszenzmarker mit Sauerstoff, Schwefel, Stickstoff-Heteromakrozyklen als kationenspezifische Rezeptoren für analytische Applikationen der zeitaufgelösten Fluorometrie von der Kiewer Gruppe synthetisiert. Die in Berlin an diesen Verbindungen durchgeführten fluorometrischen Charakterisierungen und spektroskopischen Untersuchungen zum Kationenkomplexierungsverfahren ermöglichten die Aufstellung von ersten Designprinzipien für Fluoreszenzmarker für Übergangs- und Schwermetallionen.
The complexation behaviour of 2,2-bipyridyl-3,3-diol and 2,2-bipyridyl-3-ol, BP(OH)2 and BPOH, with various heavy and transition metal
ions has been investigated in aqueous, alcoholic, and acetonitrile solutions. Whereas the complexes with paramagnetic ions and HgII
are non-fluorescent, ZnII and CdII form highly fluorescent complexes, their coordination geometries depending on the
solvent proticity and hydrogen-bond donating ability. Through a comparative study with the corresponding chelates of 2,2-bipyridyl (bipy),
N,O-coordination in a six-membered ring chelate has been found to be the dominant binding mode in both the ZnII and CdII
complexes in the protic solvents water and ethanol. Only for ZnII and BPOH is exclusive N,N-chelation found in acetonitrile. NMR
measurements on BP(OH)2, BPOH, and bipy in the presence of ZnII and CdII in acetonitrile confirmed these
findings.
The ability and structural requirements of 4-dimethylamino-4-cyano-substituted biphenyls showing photoinduced intramolecular charge transfer (CT) to serve as hydrogen bond- or pH-sensitive fluorescent probes is investigated. The donoracceptor (DA) biphenyls I and II being planar in the CT excited state are most suitable as pH-sensitive fluorescent probes. The two compounds show analytically valuable features such as well-separated absorption and emission bands and signal changes spanning four orders of magnitude and can be employed in ratiometric, self-calibrating, and highly sensitive pH fluorosensing in the range 0<pH<4. Because of an enhanced charge separation in the excited state, the highly twisted DA biphenyl III shows an increased proton sensitivity and can therefore be used to probe solvent proticity ia hydrogen bond formation. The molecular and electronic characteristics of the two types of probes are discussed for the different neutral and acidic alcoholic and/or aqueous solvent mixtures investigated.
The spectroscopic properties and the photophysical behaviour of difluoroboradiaza-s-indacene 1, especially designed for the near infrared (NIR) spectral region and equipped with a p-dimethylaminophenyl group at the meso-position, were studied by steady-state and time-resolved optical spectroscopy. Solvent-dependent measurements revealed that for 1, excited state deactivation is governed by population of a non-emissive charge transfer excited state (1CT) as the solvent polarity increases, whereas reference compound 2 shows strong fluorescence from a locally excited state (1LE) in all the solvents employed. Accordingly, protonation of 1 completely suppresses the quenching excited state charge transfer process and leads to strong enhancement of fluorescence in the NIR, distinguishing 1 as a very sensitive fluorescent sensor molecule for pH or solvent acidity in this favourable wavelength region.
The acceptor strength of the boron-dipyrromethene chromophore in directly linked donoracceptor compounds can be tuned by substituents in such a way that a fast excited state charge transfer takes place even for the comparatively weak benzo crown electron donor. This leads to strong fluorescence quenching. Upon binding of cations (Na+ and K+) to the benzo crown receptor, the donor properties of the latter are further reduced, partly suppressing charge transfer. Large fluorescence enhancement factors and cation-selective fluorescence decay times result which are the basis for improved analytical application of these dyes as highly sensitive fluorescent probes.