Dual fluorescence

The structure of the benzonitrile 33 is quite typical of ICT systems, which are now described in terms of incorporating an electron donor group and an electron acceptor group in close proximity or linked through a conjugated ti-system. 

3 Excited State Twisted Internal Charge Transfer 

A re-investigation of the anomalous properties of benzonitrile 33 by Grabowski and co-workers revealed that this was not sufficient to account for all the experimental data obtained. The properties of the structural derivatives of benzonitrile 33 with methyl ring substituents meta- and ortho- to the amine group were examined, compounds 34 and 35, respectively. It was found that in the same solvent system, compounds 34 and 35 behaved very differently. The meta-derivative 34 displayed dual fluorescence with a dominant band from the LE state, while the ort o-derivative 35 displayed fluorescence almost exclusively from the ICT state. 

This phenomenon was found to occur in a number of analogous species, the twisted conformation generally representing the most favourable species. This explanation held for over a decade and as such led to the name of excited state twisted internal charge transfer (TICT). In the early 1970s, new experimental evidence revealed that the TICT model failed to provide a consistent interpretation for empirical data in certain cases, nevertheless the concept of a twisted state still provides a valuable model. For a discussion on the range of hypotheses subsequently postulated regarding the structure of the ICT state and the 

4 Fluorescent Internal Charge Transfer Sensory Systems 

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Fig. 46. Scheme of optical transitions, explaining the dual fluorescence resulting from proton transfer in excited electronic state. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

The observed luminescence properties of the copolymer yarns can be easily explained if an energy transfer mechanism is assumed to be operating (Figure 7). Triplet-triplet energy transfer from the terephthalate units to the 4,4 -biphenyl -dicarboxyl ate units explains both the dual fluorescent/phospho-rescent emissions from the 4,4 -biphenyldicarboxyl ate units as well as the quenched phosphorescence from the terephthalate units. 

Yushchenko DA, Shvadchak W, Bilokin MD, Klymchenko AS, Duportail G, Mely Y, Pivovarenko VG (2006) Modulation of dual fluorescence in a 3-hydroxyquinolone dye by perturbation of its intramolecular proton transfer with solvent polarity and basicity. Photochem Photobiol Sci 5 1038-1044... 

Ohuchi Y, Katayama Y, Maeda M (2001) Fluorescence-based sensing of protein kinase A activity using the dual fluorescent-labeled peptide. Anal Sci 17 il465—i 1467... 

Van Benthem MH, Gillispie GD (1984) Intramolecular hydrogen bonding. 4. Dual fluorescence and excited-state proton transfer in 1, 5-dlhydroxyanthraqulnone. J Phys Chem 88 2954-2960... 

The anomalous dual fluorescence emission of p-A V-dimethylamino benzoni-trile (DMABN) in polar solvents was first reported by Ernst Lippert in 1962. Emission spectra of DMABN in solvents of different polarity show a dual emission, where the red-shifted emission is stronger relative to the primary emission when the solvent polarity increases. Furthermore, it can be observed that overall emission intensity is reduced in more polar solvents, but higher solvent viscosity increases the emission intensity. Spectra of DMABN in different solvents are shown in the chapter of Tomin in this book [1]. 

Il ichev YV, Kuhnle W, Zachariasse KA (1998) Intramolecular charge transfer in dual fluorescent 4-(dialkylamino) benzonitriles. Reaction efficiency enhancement by increasing the size of the amino and benzonitrile subunits by alkyl substituents. J Phys Chem A 102(28) 5670-5680... 

Schuddeboom W, Jonker SA, Warman JM, Leinhos U, Kiihnle W, Zachariasse KA (1992) Excited-state dipole moments of dual fluorescent 4-(dialkylamino) benzonitriles. Influence of alkyl chain length and effective solvent polarity. J Phys Chem 96 10809-10819... 

Rettig W (1980) External and internal parameters affecting the dual fluorescence of p-cyano-dialkylanilines. J Lumin 26 21 16... 

Zachariasse KA, von der Haar T, Hebecker A, Leinhos U, Kuhnle W (1993) Intramolecular charge transfer in aminobenzonitriles requirements for dual fluorescence. Pure Appl Chem 65(8) 1745-1750... 

Braun D, Rettig W, Delmond S, Letard J-F, Lapouyade R (1997) Amide derivatives of DMABN a new class of dual fluorescent compounds. J Phys Chem A 101 6836-6841... 

Herbich J, Dobkowski J, Rulliere C, Nowacki J (1989) Low-temperature dual fluorescence in 9-morpholinoacridine picosecond TICT state formation J Lumin 44 87-95... 

Wu FY, Li Z, Wen ZC et al (2002) A novel thiourea-based dual fluorescent anion receptor with a rigid hydrazine spacer. Org Lett 4 3203-3205... 

Frigoli M, Ouadahi K, Larpent C (2009) A cascade FRET-mediated ratiometric sensor for Cu2+ ions based on dual fluorescent ligand-coated polymer nanoparticles. Chemistry 15 8319-30... 

Baleizao C, Nagl S, Schaeferling M, Berberan-Santos MN, Wolfbeis OS (2008) Dual fluorescence sensor for trace oxygen and temperature with unmatched range and sensitivity. Anal Chem 16 6449-6457... 

Internal rotation, accompanying solvent relaxation or not, can occur in many fluorophores, but dual fluorescence and right angle twist (required for full charge... 

TICT probes can be used as polarity probes (Rettig and Lapouyade, 1994). The classical TICT compound exhibiting dual fluorescence, DMABN (see Section 3.4.4),... 

Intramolecular charge transfer in conjugated donor-acceptor molecules may be accompanied by internal rotation leading to TICT (twisted intramolecular charge transfer) states. A dual fluorescence may be observed as in PCT-5 (Letard et al., 1994) (which resembles the well-known DMABN (see section 3.4.4) containing a dimethylamino group instead of the monoaza-15-crown-5) the short-wavelength... 

The formation of a TICT state is often invoked even if no dual fluorescence is observed. For donor-acceptor stilbenes (PCT-2 and PCT-3), the proposed kinetic scheme contains three states the planar state E reached upon excitation can lead to state P (non-fluorescent) by double-bond twist, and to TICT state A by singlebond twist, the latter being responsible for most of the emission. 

Figure 5.1. Various adiabatic photochemical reaction mechanisms (see text for details), (a) Simple case of dual fluorescence (b) illumination changes sample (i.e., photochemistry) (c) strong fluorescence quenching (photochemical funnel) (d) competitively coupled product species (e) consecutively coupled product species.

The TICT model was put forward to account for the observation that the dual fluorescence of DMABN with its normal band (B band) at around 350 nm and its anomalous one (A band, around 450 nm in medium polar solvents) depends on the... 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

Often all three approaches ((1) to (3)) are simultaneously active, e.g., in the dual fluorescence of the classical HCT compounds like DMABN where both the energy of the long-wavelength emission (factor (1)) as well as the ratio between the two bands (factors (2) and (3) above) varies. The simultaneous presence of two bands allows very accurate ratio measuring, and small changes in this ratio can be detected more accurately as if only changes in absolute quantum yields have to be observed. These probes with dual fluorescence will be called type-3. ... 

Figure 5.7, (a) Widely used biological probes of the TICT type. 4, s4 s7> (b) TICT compounds with dual fluorescence and interesting properties amenable to tailor-making of fluorescence probes. Functionalization and inclusion in larger molecular assemblies is easily possible, e.g., at the positions indicated by R. This holds especially for esters and amides which have been used as probes in large polymers 97-1011 or in TICT-labeled cyclodextrins.(l49)... 

The dual fluorescing type 2 compounds DMABN and its ester derivatives have been used to extract detailed free volume parameters, especially for the case where the probes are covalently linked to the polymeric backbone, e.g., by an oxymethylene chain of variable length/97-101 As the TICT reaction requires a comparatively large reaction volume, the measurable effects are especially strong in solutions of these labeled polymers but they tend to disappear for the pure polymer due to its too-large... 

DMABN suffers from the fact that dual fluorescence is only observable for polar media. Therefore, the pretwisted ester DMPYRBEE has been developed which shows dual fluorescence also in alkane solvents. 9 This probe allowed measurement of nonpolar polymeric siloxane oils and a comparison with the corresponding measurements using an EXCIMER probe. As expected from the decreased reaction volume necessary for the TICT photoreaction, the latter is usable down to much lower temperatures (higher viscosities) and probes a larger fraction of free volume. 26 ... 

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