Stilbene singlet lifetimes

The fluorescence Intensity of substituted stilbenes and stilbene analogues provides a useful indicator of photochemical reactivity. Virtually all of the reported bimolecular photochemical reactions of electronically excited stilbenes involve stilbenes which are fluorescent at room temperature in solution. The absence of fluorescence is indicative of a singlet lifetime too short (< 100 ps) to allow for efficient bimolecular quenching. 

While much has been learned about S near the geometry of tS in solution, there have been no reports on the spectroscopic detection of the twisted excited singlet state of stUbene. Information about the twisted excited singlet state of tetrapheny-lethene will be described in this chapter in Section 2.3. Interesting photochemistry also occurs following the excitation of cw-stilbene however, because of the much shorter lifetime of c -stilbene, femtosecond-pulsed lasers must be used." ... 

The objective of this article is to review critically the bimolecular photochemical reactions of the stilbenes. While the emphasis will be placed upon the unsubstituted stilbenes t-1 and and c-1, comparisons with substituted stilbenes and stilbene analogues will be made in some cases. In most of the reactions that will be described, the reactive excited state is the lowest singlet of t-1, - -t. Because of the short lifetime of It, unimolecular processes (fluorescence and trans-cis isomerization) can compete with bimolecular reactions under normal reaction conditions. While competing unimolecular processes are... 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

Fluorescence Spectra. frons-Stilbene and some derivatives show fluorescence emission in the range from 340 to 400 nm [187], the fluorescence maximum (lf) is sensitive to substituents (Table 10). m-Stilbenes, in contrast, exhibit no fluorescence in solution at ambient temperatures [95]. Only recently, the fluorescence excitation and emission spectra (A = 275 and 409 nm, respectively) of ris-stilbene in n-hexane at room temperature have been reported [240,241], A very short lifetime (<10ps) has been assumed for the excited cis singlet state in fluid solutions (Section A.5). At lower temperatures (below — 120°C) and in rigid media c/s-stilbene shows a broad emission around 450 nm [113, 176, 242 245]. It does not originate from the trans isomer or a cyclization product and is hence assigned to fluorescence [246], Increased viscosity of the solvent likewise results in an increase of the fluorescence quantum yield of the cis isomer ([Pg.39]

Here, /lfexp —(E(/RT) and k0 are rate constants for activated and nonac-tivated decay steps of t, respectively. For stilbene, halogenated, and some other stilbenes (4-R CN, CH3, OCH3) the activated step has been assigned to twisting in the excited singlet state [27, 105, 114, 357, 358], whereas for nitrostilbenes intersystem crossing is involved in the activated step [31]. At low temperatures lifetimes of 1-2 ns have been reported for several trans-stilbenes [314, 337, 342], Below — 150°C an enhanced frans-stilbene was excited by a 532-nm picosecond laser pulse, in addition to a 266-nm pulse [317]. 

Photodimerization, first reported by Ciamician and Silber in 1902 [458], requires a rather high stilbene concentration. The quantum yield increases from <0.01 for 0.01 M frans-stilbene to 0.33 for 0.55 M trans-stilbene [459], The excited t singlet state is the reactive species in this process. Pure ds-stilbene shows no dimerization, a fact that has been explained by the short lifetime of the first excited singlet state [294], As Saltiel and Charlton have pointed out [27] short-lived transients, observed at high frans-stilbene concentrations [460], may be associated with dimerization rather than... 

By monitoring the fluorescence generated by two consecutive picosecond light pulses it has been possible to measure rate constants for the cis trans isomerization of stilbene. The main reaction occurs in the singlet manifold via a perpendicular singlet state with lifetime shorter than a few picoseconds. 

On direct excitation of ds-stilbene, both cyclization to produce dihydro-phenanthrene and double-bond isomerization to p occur competitively in the ratio 30 70. The lifetime of the excited singlet ds-stilbene is estimated as about 1 ps at room temperature in solution as measured via femtosecond spectroscopy by Hochstrasser et al. [9] and Fleming et al. [10]. This... 

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