Fluorescence stilbene

The characterization of the laser pulse widths can be done with commercial autocorrelators or by a variety of other methods that can be found in the ultrafast laser literature. " For example, we have found it convenient to find time zero delay between the pump and probe laser beams in picosecond TR experiments by using fluorescence depletion of trans-stilbene. In this method, the time zero was ascertained by varying the optical delay between the pump and probe beams to a position where the depletion of the stilbene fluorescence was halfway to the maximum fluorescence depletion by the probe laser. The accuracy of the time zero measurement was estimated to be +0.5ps for 1.5ps laser pulses. A typical cross correlation time between the pump and probe pulses can also be measured by the fluorescence depletion method. 

This mechanism is consistent with all of the experimental and theoretical information presented. The fact that no Pms-stilbene fluorescence is observed when riir-stilbene is irradiated would indicate that the following reaction is not important, that is, the twist singlet is much lower in energy than is the trans singlet state ... 

The only head-to-head polymer which has been examined for excimer fluorescence is polystyrene 25). Unfortunately, the synthetic route to this polymer leaves a number of stilbene-based structures in the sample, which have a lower-energy singlet state than either PS monomer (285 nm) or excimer (330 nm). Thus, fluorescence from these intrinsic stilbene traps was seen in the spectra of head-to-head PS in pure films and, to a lesser extent, in fluid solution. In the latter, the fluorescence of PS monomer was predominant, and the small amount of stilbene fluorescence was increased when a nonsolvent (methanol or cyclohexane) was added to the 2-methyl-tetrahydrofuran solution. In films of the polymer, stilbene fluorescence was the major spectral band, although some PS excimer fluorescence was also present in the spectrum. No monomer fluorescence at 285 nm was detected from films. Given the impure nature of the head-to-head PS sample, no conclusions on excimer formation in these systems could be drawn. 

FIGURE 6. trans-Stilbene fluorescence quenching rate constants as a function of diene (O) and vinyl ether (A) ionization potential. Dashed line is that obtained by Rehm and Weller (83a) for electron-transfer quenching. 

TABLE 11. trans-Stilbene Fluorescence Quenching and Exciplex Fluorescence Data for Amine Quenchers3... 

Dorr, Lewis, and co-workers found evidence through quenching experiments and flash spectroscopy for a triplex in the system trans-stilbene — amine — benzene — [105]. They quenched singlet excited trans-stilbene with various mono- and diamines and found a steric effect on the quenching constant The a, co-diamines (dabco, diaminoethane, -propane and -butane) quenched the stilbene fluorescence more efficiently than the monoamines, depending on the chain length between the amino groups. This was ascribed to the formation of cyclic radical cations, with a N-N three electron a-bond. In this case, an exciplex between diamine and stilbene is formed. 

This Synchroscan [68] streak camera system has been used to study the time resolved fluorescence of trans-stilbene in the picosecond time regime. The experimental arrangement [69] is shown in Fig. 20. An acousto-optically mode-locked argon ion laser (Spectra Physics 164), modulated at 69.55 MHz was used to pump a dye laser. The fundamental of this dye laser, formed by mirrors M, M2, M3 and M4, was tunable from 565 to 630 nm using Rhodamine 6G and second harmonic output was available by doubling in an ADP crystal placed intracavity at the focal point of mirrors M5 and M6. The peak output power of this laser in the ultraviolet was 0.35W for a 2ps pulse which, when focused into the quartz sample cell of lens L, produced a typical power density of 10 KW cm-2. Fluorescence was collected at 90° to the incident beam and focused onto the streak camera photocathode with lens L3. The fluorescence was also passed through a polarizer and a bandpass filter whose maximum transmission corresponded to the peak of the trans-stilbene fluorescence. 

Fig. 28 Photoinduced electron transfer studies carried out on 6-mer (not shown) and 7-mer hairpin DNA duplexes capped by a stilbene acceptor chromophore.31 167 168 The duplexes contain A-T base pairs and a single G-C base pair, whose position in the duplex is varied, (a) The photoinduced ET process is illustrated for 3G C, in which the G-C base pair is third removed from the stilbene (St) group. The stilbene fluorescence is quenched by electron transfer from the G donor, (b) Damping factors for charge separation, /3(CS), and subsequent charge recombination, /3(CR), for n G C, in which the G base is connected to the T-bearing strand, (c) Damping factors, /3(CS) and /3(CR), for n C G, in which the G base is connected to the A-bearing strand.

The fluorescence-photochrome technique was first applied to studying molecular dynamics of a stilbene fluorescence-photochrome molecule, SITC, attached covalently... 

Weber, K.. Liechti, P., Meyer, H.R., and Siegrist, A.E., SZ -stilbene fluorescent whiteners, Ciba-Geigy A.-G., Swiss Patent Appl. CH 560736, 1975 Chem. Abstr. 83, 61745, 1975. 

Table 7.6 Cw-stilbene Fluorescence Decay Times in Organic Solvents for Emission at 475 nm [23]...

In trans-stilbene, fluorescence and rotation around the CH = CH bond are known to be the main photophysical processes. Assuming that the photophysical behavior of DAS and tt-DAS is similar to that of trans-stilbene, the increase in fluorescence of tt-DAS,... 

Olson was the first to postulate that optical excitation of the ethylenic double bond involves rotation around a double bond in its excited state and that this rotation leads to an observable photoisomerization process [8-10]. Olson dealt with this aspect in terms of potential energy curves and mentioned the possibility of adiabatic photoisomerization process. Later, Lewis and co-workers [11] studied the photoisomerization process of tronj-stilbene with great interest but could not detect the cw-stilbene fluorescence. More recently, more detailed fluorescence studies carried out by Saltiel and co-workers [12-15] revealed that cw-stilbene fluoresces very weakly (Oa 0.(X)01) and shows an inefficient adiabatic isomerization process. The singlet mechanism currently proposed by Saltiel [16] is supported by quenching studies [17-20]. The extensive studies carried out on stil-bene and its analogs have already been reviewed [21-23]. Here the nature of the singlet excited state involved in the trans-cis isomerization process is dealt with. 

Detailed and careful fluorescence analysis carried out by Saltiel and coworkers [12-15] on cw-stilbene demonstrated that cis-to-trans isomerization is indeed adiabatic in nature. c -Stilbene fluorescence is found to be composed of emission from cis- and rran -stilbene-excited singlet states. 

Olson [1] was the first to postulate that electronic excitation of compounds carrying ethylenic double bonds leads to free internal rotation in excited states. Several years later Lewis [95] observed that frans-stilbene fluoresces whereas the cis form does not. He concluded that excited cis-stilbene is not converted to the fluorescing state of trans-stilbene, thus excluding free internal rotation from cis to trans in the excited singlet state. More detailed fluorescence studies by Saltiel et al. [81, 240, 241] have recently uncovered both a weak c fluorescence, [Pg.79]

The stilbenes are typically extracted from peanuts with 95% ethanol, and following partial purification the samples are subjected to either thin-layer chromatographic (TLC) or liquid chromatographic (LC) analyses. On TLC plates, the stilbenes fluoresce blue under 254 nm UV light or they can be detected in LC analysis by UV detection ranging from 290-335 nm (5-7. 21. 22). 

Fluorescence quantum yields of cis-stilbene-do and -d2 were measured as a function of temperature in n-hexane and n-tetradecane [38]. Emission contributions from c, (pfcc, and adiabatically formed H, [Pg.81]

The effect of Si02 and Si02-Ti02 surfaces (in the powder form) on the fluorescence and lifetimes of the adsorbed two push-pull stilbenes, , -l-(4-cyanophenyl)-4-(4-N,N-dimethylaminophenyl)-l,3-butadiene and -9-(4-cyanostyryl)-2,3,6,7-tetrahy-dro-lH,5H-pyrido[3,2,l-i,j]-quinoline was observed [56]. The stilbene fluorescence was quenched effectively by mechanism of electron injection from excited stilbene... 

The isomerization dynamics of cis-stilbene in the first ecdted singlet state was studied by fluorescence upconversion [30]. Lifetime measurements were made with subpicosecond resolution in 2-propanol, decanol, hexane, and tetradecane. The cis-stilbene fluorescence decay curves were single exponential in all solvents except for decanol, where they were adequately described by a double exponential. A picosecond component in the fluorescence anisotropy decay measurements made in 2-propanol suggests that motion along the reaction coordinate was directly measured. 

Rigidification of 40 by disaccharide binding increases the stilbene fluorescence. In particular, the disaccharide D-melibiose shows higher selectivity for 40 than other common disaccharides. 

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