Charge transfer intramolecular

This chapter briefly reviews recent progress in the investigation of fluorescent and phosphorescent properties of stilbenes as well as such phenomena as triplet-triplet and singlet-singlet energy transfer and Raman scattering. The trends in this area include the use of a wide arsenal of stilbenes, employment of elaborated experimental methods such as nano and picosecond time-resolved absorption and fluorescent spectroscopy, and the use of modern theoretical calculations, for example, density function theory. The importance of these research endeavors for further basic and applied applications of stilbenes cannot be overestimated. 

Likhtenshtein, G.I. (1988) Chemical Physics of Redox MetaUoenzymes, Springer-Verlag, 

Lalevee, J., Allonas, X., and Fouassier, J.P. (2005) Chemical Physics Letters, 401, 483-486. 

8 Glnsac, K.D. and Schanze, K.S. (2002) Polymer Preprints (American Chemical Society, Division of Polymer Chemistry), 

12 Muszkat, KA., Castel, N., Jakob, A., Fischer, E., Luettke, W., and Rauch, K (1991) Journal of Photochemistry and Photobiology A Chemistry, 56, 219-226. 

The rate of ET was measured. The main factor that influences the rate is the distance through the spacer and the chemical structure of the bridge. The notion of through space interaction could be ruled out. In this case, we are dealing with ET through a orbitals. 

Verhoeven was one of the first to show that electron interactions and ET can occur through several a bonded spacers. He showed that electron coupling is an experimental reality. Earlier, it was believed that ET is only possible if there is a contact between two metallic electrodes, or possibly between two )t-systems. 

FIGURE 13.6 Possible PES for charge recombination to the ground state. 

ET through aliphatic bridges with weak exponential decrease of the rate with distance, is possible provided the chain is all-trans. The dihedral of four carbon atoms is then 180°. If there are cw-turns, the coupling decreases fast with distance, roughly proportional to the number of CM-tums. The Verhoeven molecules are built in a way that closely resembles the all-trans structure. 

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So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

An intramolecular charge transfer toward C-5 has been proposed (77) to rationalize the ultraviolet spectra observed for 2-amino-5-R-thiazoles where R is a strong electron attractor. Ultraviolet spectra of a series of 2-amino-4-p-R-phenylthiazoles (12) and 2-amino-5-p-R-phenylthiazoles (13) were recorded in alcoholic solution (73), but, reported in an article on pK studies, remained undiscussed. Solvent effects on absorption spectra of 2-acetamido and 2-aminothiazoles have been studied (92). 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

Copolymers composed of electron-donating and electron-withdrawing units are considered to have intramolecular charge transfer structure.41 These copolymers show absorption bands at wavelengths longer than those of the corresponding homopolymers (Fig. 9.2). 

Kndr, G. (2001) Intramolecular charge transfer excitation of meso-tetrakis (1-pyrenyl) porphyrinatogold(lll) acetate. Photosensitized oxidation of guanine. Inorganic Chemistry Communications,... 

Rettig, W. Photoinduced Charge Separation via Twisted Intramolecular Charge Transfer States. 169, 253-300 (1994). 

Kopczynski, M., F. Ehlers, T. Lenzer, and K. Oum. 2007. Evidence for an intramolecular charge transfer state in 12 -apo-beta-caroten- l 2 -al and 8 -apo-beta-caroten-8 -al Influence of solvent polarity and temperature. J. Phys. Chem. A 111 5370-5381. 

Boratastilbene has recently been synthesized and structurally characterized.26 A study of the consequence of the isoelectronic B for C substitution on the pho-tophysics/photochemistry led to the conclusion that, due to intramolecular charge transfer arising from the inequivalent charge density of the two aromatic rings, nonaggregated boratastilbene is highly emissive relative to stilbene. 

Grabowski ZR, Rotkiewicz K, Rettig W (2003) Structural changes accompanying intramolecular electron transfer Focus on twisted intramolecular charge-transfer states and structures. Chem Rev 103 3899 4031... 

Gormin D, Kasha M (1988) Triple fluorescence in aminosalicylates. Modulation of normal, proton-transfer, and twisted intramolecular charge-transfer (TICT) fluorescence by physical and chemical perturbations. Chem Phys Lett 153 574-576... 

Seo J, Kim S, Park SY (2004) Strong solvatochromic fluorescence from the intramolecular charge-transfer state created by excited-state intramolecular proton transfer. J Am Chem Soc 126 11154-11155... 

Keywords Biofluids Chemosensors Emission spectroscopy Mechanosensors Optical properties Polarity Rheology Twisted intramolecular charge transfer Viscosity... 

Fig. 1 Mesomeric structures of a para-substituted benzene intramolecular charge transfer (ICT) complex in the ground state and in the dipolar excited state...

Rettig W, Lapouyade R (1994) Fluorescence probes based on twisted intramolecular charge transfer (TICT) states and other adiabatic photoreactions. Topics in fluorescence spectroscopy 4 109-149... 

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Grabowski ZG, Dobkowski J (1983) Twisted intramolecular charge transfer (TICT) excited states energy and molecular structure. Pure Appl Chem 55(2) 245-252... 

Bulgarevich DS, Kajimoto O, Hara K (1995) High-pressure studies of the viscosity effects on the formation of the twisted intramolecular charge-transfer (TICT) state in 4,4 -diaminodi-phenyl sulfone (DAPS). J Phys Chem 99(36) 13356—13361... 

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... 

Lukac S (1984) Thermally induced variations in polarity and microviscosity of phospholipid and surfactant vesicles monitored with a probe forming an intramolecular charge-transfer complex. J Am Chem Soc 106 4386 -392... 

Loutfy RO, Law KY (1980) Electrochemistry and spectroscopy of intramolecular charge-transfer complexes. p-N, N-dialkylaminobenzylidenemanononitriles. J Phys Chem 84 2803-2808... 

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