Doktorarbeit / Dissertation, 2016
41 Seiten, Note: 9.0
1.1 Introduction
1.1.1 Phenomena of fluorescence and phosphorescence
1.1.2 Design of fluorescent molecular sensors
1.1.3 Photophysical mechanisms of fluorescent sensors
1.1.3.1 Photoinduced electron transfer (PET)
1.1.3.2 Energy transfer quenching (ET)
1.1.4 Fluorescent chemosensors based on rhodamine
1.1.5 Aim and outline of the current work
1.2 Experimental work
1.2.1 Materials and physical measurements
1.2.2 Synthesis of chemosensors SAR-31 and SAR-
1.2.3 Ion bonding study
1.3 Result and discussion
1.3.1 Synthesis and characterizations
1.3.2 Stoichiometry and binding mode study
1.4 Conclusion
1.5 References
Fluorescence spectroscopy and ultraviolet techniques have been applied to various analytical, bio-analytical, environmental, medical and forensic investigations. Several analytical methods that are offered for recognition of target concerned such that flame photometry, AAS, HPLC, mass spectrometry, ion sensitive electrode, microprobe analysis, neutron activation analysis, have been developed [1-4]. But these methods are expensive and time uncontrollable process that involves complicated instrumentation and do not allow permanent monitoring. When compared to absorption techniques, flourimetric method is more sensitive and selective and rapidly performed. In nature, any compound analysed by using a suitable analytical technique which basically depends on the nature and properties of the target compound. If the target compound exhibit phenomenon called as Luminescence where the emission of electromagnetic radiation of longer wavelength to that of absorbed radiation can be seen are analysed by using the modern spectroscopic technique called as ‘flourimetry’[5].
Hence, significant hard works are life form complete to develop selective fluorescent sensor for recognition of targeted species. To blind date different fluorescent molecular sensors with different excitation and emission wavelengths comprise be employed such like coumarin, 1,8-naphthamide, pyrene, xanthenes, cynine, squaraine, boron dipyrromethene difuoride, nitrobenzofurazan… etc [1-3].
A large number of substance are recognized which absorb take up ultraviolet or visible light energy. But these substances are unable to find overload energy as heat through collisions with near atom or molecules. But a number of significant substances are too known which lose only fraction of this overload energy absorbed. This method of emitting radiation is cooperatively known as luminescence. Luminescence is an emission of UV/visible or infrared photons from an electronically excited species. The word luminescence was first introduced as luminescenz by Eilhardt Wiedemann in 1888, to explain the phenomena of light which are not solely dependent on the rise in temperature. Luminescence light created at low temperatures. Therefore, the light shaped by this procedure is regarded as “light without heat” or “cold light”. Luminescence is cold light but incandescence is hot light [6].
Fluorescence and phosphorescence are types of luminescence. In 1852 the factual science of fluorescence was bring to light by sir George stokes. He apply the scientific method to fluorescence and developed the ‘stokes law ‘of fluorescence. The Greek word phosphorescence means ‘which bear light’. The word phosphors have definitely been assigning since the middle age to resources that shine in dark following introduction to light. The jablonski diagram is suitable for visualizing in an easy way to recognize the procedure of fluorescence and phosphorescence.
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Figure 1.1. Jablonski diagram.
A representative jablonski diagram is depicted in figure 1.1. The singlet ground, first and second electronic state are represent by S0, S1, and S2, in the same way. The transition between the states are depict as vertical lines to show the direct nature of light. Absorption of UV/ Visible radiation causes transition from singlet ground state to singlet excited state. As this excited state is not stable, it emits the excess energy and returns back to ground state. The transition arise in with reference to 10-15s, a time too short for importance displacement of nuclei. The following possible de-excitation process is as follows;
Internal conversion (IC)
Internal conversion is a non-radiative transition connecting two electronic states of the similar spin variety. Internal conversion is applied to refer an intermolecular method in which a molecular passes to a lower electronic state without emission of radiation. This is capable when two electronic energy levels are sufficiently close for overlap of vibrational level to exist.
Internal conversion due to overlapping of vibration levels is additional possible than the loss of energy with fluorescence from higher excited state. Hence, internal conversion from s0 to s1 occurs by emission of protons (fluorescence) and intersystem crossing T1 from which emission of protons (phosphorescence) can be pragmatic.
Fluorescence
Emission of radiation while there is transition of electron from singlet excited state to singlet ground state is known as fluorescence. The wavelength of absorbed radiation is called as excitation wavelength and that of emitted radiation is called as emission wavelength [7]. The emission rates of fluorescence are typically 10-8 s, so that characteristic fluorescence life time near 10 ns. Due to the short timescale of fluorescence, capacity of the time-resolved emission needs complicated optics and electronics.
The difference between place of the band maxima of the absorption and emission spectra of equal electronic transition is called stokes shift. It is named after Irish physicist George G. stokes. All useful fluorescent schemes are complex organic molecules containing one or more aromatic functional groups. Transitions observed in organic compound are mostly n՜π and π՜π* types. It is the π՜π*type of excitation which leads to important fluorescence, the n՜π*transition produce merely feeble fluorescence. The electronic transitions equivalent to charge-transfer band also guide to a burly fluorescence.
Intersystem crossing
At favourable conditions similar to low temperature and absence of oxygen, there is transition from excited singlet state to triplet state which is called as inter system crossing. Here this method multiplicity of the molecule changes. The probability of this mechanism is also enhanced if the vibrational level of the two states overlaps. This system is most ordinary in deep atoms such as bromine or iodine. The possibility of spin and orbital motions is more in serious atoms so the change in spin is most likely. The existence of paramagnetic type also enhances ISC resulting into decreased fluorescent intensity.
Phosphorescence
After ISC to a triplet excited state, more deactivation can arise either by internal or external conversion or by phosphorescence. A singlet conversation is more likely than a triplet-singlet transition. The average lifetime of the excited triplet state with deference to emission ranges from 104 to several seconds. It is, hence, observed that emission from this transition continues for a short time subsequent to the irradiation has been discontinued. Phosphorescence is usually observed in rigid media at low temperature. External and internal conversations compete with phosphorescence.
The decompose form the triplet to the ground state singlet is forbidden by spin equilibrium and is therefore slow. Thus, the life-time of phosphorescence is much longer than fluorescence. The above system of phosphorescence involving singlet-triplet rot scheme has been confirmed by the magnetic susceptibility and ESR measurements. According to Hund’s rule, the triplet level for all time lies lower than the corresponding singlet level and for this reason phosphorescence spectrum is not the mirror image of the absorption spectrum and it always occurs at longer wavelengths compared with the absorption and fluorescence spectrum.
Fluorescent sensors consists of a fluorophore (fluorescent molecule) covalently connected to an ionophore and is therefore called a fluroionophore which selectively attach alkali, alkaline and transition metal ions. The ionophore is necessary for careful binding of the substrate, at the same time as the fluorophore provides the means of
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Figure 1.2. Main aspects of fluorescent molecular sensors for cation recognition.
The signaling moiety act like a signal transducer, i.e. it transforms the information (detection) into a visual signal expressed as the change in the photophysical individuality of the fluorophore (see Figure 1.2). These changes are unpaid to the perturbation (by the bound cation) of photoinduced process such as e- -transfer, charge transfer, energy transfer, excimer or exciplex arrangement or disappearance, etc.
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Figure 1.3. (a) The thermodynamic classify for PET is that the excited state energy of the fluorophore must be improved than to require to oxidation the ionophore and to reduction the fluorophore. (b) Through introduce a target, i.e. metal ion, into the ionophore opening its oxidation potential is raise, because the thermodynamic classify for PET to be removed.
The theory of Luminescence PET sensing is summarizing in Fig. 1.3. We note down the three-module ‘fluorophore–Spacer–ionophore’ design [8] which permit electron move starting the ionophore to the fluorophore (or vice versa) but the system is thermodynamically and kinetically possible [9]. Here the majority order subsequently distant, kinetic limits are minor although exceptions are coming to light [10-11]. Importantly, the electron transfer rate in lots of favourable cases is much more quickly than the luminescence time when PET is thermodynamically deplorable. Luminescence and electron transfer be the two most important competitors, which switch off the photo excited state of these designed system. Binding of direct to metal ion to the ionophore can completely modify PET thermodynamics to an endergonic state. At the simplest level, this position is cause by electrostatic exchanges between the ionophore–intention pair. Luminescence is now the winner of the challenge. Luminescence can thus be switched between ‘on’ and ‘off’ states by introduction and removal, in that order of the intention species, which give us with the quick to take action quenching mechanism we requisite. Selected examples of PET based fluorescent sensors are depict in Figure 1.4. The first and simplest coronand PET sensor 1 was reported by de Silva et al [12]. Upon binding with K+ in methanol fluorescence quantum yield of PET sensor 1 increase from 0.003 to 0.14 in to the sensor 2 [13]. The methoxy groups are ortho position to the nitrogen atom of the crown join in the complexation to complete strong binding with Na+ and accompany with switching ‘on’ the fluorescence. This was a balanced result from the proton signaling structure 3 [14-15]. The uncomplicated amine group within 2 was elaborate into azacrown ether in 1 for the purpose of signaling alkali cation. The podand- based sensor 4 [16] have polyamine chain and used for the binding of Zn+2, on the other hand it also shows strong binding with Cu+2. Two PET dynamic receptors were also present within 5 [17], which be one more product from 1. The anthryl-9, 10-dimethyl turn again bring a degree of length detection. α,ω-alkanediammonium ions be the targets and outstanding fluorescence enhancement were found. As expected, monoammonium ions give related answer only at a lot high concentrations. Captivatingly, 5 respond best to putrescine and cadaverine dication which are as expected shaped within rotting biomaterials. The anthryl-9, 10-dimethyl turn has also feature in Fabbrizzi’s propose for the fluorescent sensing of imidazole derivatives [18] though the anthryl-8,-dimethyl back has been oppressed by Vance and Czarnik for sensing pyrophosphate [19].
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Figure 1.4. Structures of PET based fluorescent sensors.
Most of the PET fluorescent sensors are designed on the basis of mechanism shown in Figure 3.5, however in some cases; other PET-based processes can also be possible upon complexation of transition metal ions. A transition metal ion can quench an excited fluorophore via an electron transfer mechanism, either by a bimolecular or an intramolecular process, if it possesses empty or half-filled d orbital’s of appropriate energy. The transition metals exhibit redox activity and electron transfer can occur from the fluorophore to the bound metal ion, or vice versa, which results in quenching of the fluorophore by non-radiative energy transfer according to the Dexter mechanism in which a metal ion can quench the fluorescence of the excited state of the fluorophore by an energy transfer mechanism [20, 21].
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Figure 1.5. Dexter Mechanism for electron transfer [ET] in system containing anexcited fluorophore and metal ion.
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