Stilbenes fluorescence lifetimes

Figure 7.4. Temperature dependence of the quantum yield for trans-cis isomerization (A), of the quantum yield p of fluorescence (o), and of the fluorescence lifetime ( ) of stilbene (by permission from Saltiel and Charlton, 1980).

Fluorescence Lifetimes and Picosecond Dynamics. The fluorescence lifetime (ts) of fran.s -stilbenes at room temperature is rather short. A value of approximately lOOps may be estimated from radiative rate constant ( r) using ts = f//rr [295]. Direct lifetime measurements with picosecond laser pulses confirm this finding (Table 14). Important progress in understanding of the excited state dynamics was achieved in several laboratories [314-375], especially by Yoshihara [314 319], Hochstrasser [262, 320 335], Fleming [343-348], Troe [352 355], Zewail [361-364] and their co-workers. Saltiel and Sun [28] extensively discussed the literature con-... 

Absorption and emission spectra of 20 trans-4,4 -disubstituted stilbenes have been measured in four solvents cyclohexane (CH), chlorobenzene (CB), 2-butanone (methylethylketone, MEK), anddimethylsulfoxide (DMSO) at room temperature [24]. Fluorescence quantum yields (4>f) and fluorescence lifetimes (Xf) have been measured for these stilbenes. Substituent effects on the Stokes shift were described by a spectroscopic Hammett equation... 

A series of aminostilbenes (lA-C, 2A-C) have been synthesized to test the effect of substitution of the amino group upon the photophysics and photochemistry of stilbenes [45]. This study indicated that the photophysics properties of trans-2-aminostilbene, lA, and trans-3-aminostilbene, IB, were similar. as-2-Aminostilbene, 2A, and ds-3-aminostilbene, 2B, showed similar fluorescent lifetimes. Anomalous behavior of emission anisotropy for short-living derivatives of stilbenes was described [46]. The absorption, fluorescence and polarization excitation, and emission spectra... 

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... 

Temperature dependence of the fluorescence quantum yields and fluorescence lifetimes of frans-4,4 -di-fert-butylstilbene in n-hexane and n-tetradecane allowed to define the index of refraction dependence of the radiative rate constants, kf= (3.9 — 1.8) X 10 s, and fluorescence lifetime [78]. This relationship was used to calculate torsional relaxation rate constants ktp> for traws-4,4 -dimethyl- and frans-4,4 -di-ferf-butylstilbene in the n-alkane solvent series. It was found that activation parameters for ktp, based on Eyring s transition state theory, adhered to the medium-enhanced thermodynamic barrier model relationship, AHtp = AHt + aEr, and to the isokinetic relationship. The isokinetic relationship between the activation parameters for the parent frans-stilbene led to an isokinetic temperature of P = 600K and brings it into agreement with the isokinetic temperature for activation parameters based on estimated microviscosities, qp, experienced by stilbene in its torsional motion. The authors concluded that only microviscosities raflier than shear viscosities, q, can be employed in the expression ktp = ktSq — b, when a = b. These data clearly indicated the important role of the media dynamics in the stilbene cis-trans photoisomerization. 

As seen in Figure 10.7, the fluorescence-photochrome method allows to measure the probe motion as a whole in temporal scale dose to the fluorescence lifetime and in the same time and the same space to detect twisting of stilbene fragment in the singlet excited states for 180° in the microsecond scale. A separation of effects of a probe rotation frequency and angle of twisting appears to be a serious problem in fluorescence and spin probing. 

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. 

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... 

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. 

Adams and Cherry (78) have investigated the effects of stilbene substitution on the behavior of their excited complexes with fumaronitrile and find that the rate constants for fluorescence and nonradiative decay are insensitive to substitution, but that the rate constant for intersystem crossing is increased by electron-donating substituents (lower stilbene oxidation potential). This trend is attributed to a decrease in the energy gap between the excited complex and locally excited 3t (Fig. 4). The observed energy gap dependence of the exciplex lifetime could also account for the absence of fluorescence (or cycloadduct formation, see Section IV-B) from the excited charge-transfer complexes of t-1 with stronger electron acceptors such as maleic anhydride (76) or tetracyanoethylene (85). 

In non-viscous organic solvents the fluorescence efficiency of most SNA derivatives is comparable to that of TS as are the yields of trans to ois photoisomerization (33,3A). The double chain derivatives, MSNA and MSN, show substantially higher fluorescence yields (0.2 0.02) in non-viscous solvents with a corresponding decrease in the trans to ois isomerization efficiency (3A). In the sensitized isomerization, the MSN derivatives investigated thus far show only photoisomerization but there is an increase in the triplet lifetime by a factor of 2 however there is no change in the decay ratio of the isomerization precursor and the sum of the benzophenone sensitized isomerization efficiencies of ASA is approximately unity. When water insoluble SNA or MSNA molecules are spread as a film, either pure or in mixtures with insoluble fatty acids, at the air-water interface, it is found that the films show comparable behavior on compression to those of pure fatty acid with an indicated area per molecule of the surfactant stilbenes very close to that of a linear fatty acid (oa. 202 /molecule) (35). This suggests strongly that the TS chromophore in these molecules offers suitable molecular dimensions to pack into a crystalline-like array of linear parafin chains. 

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 . ... 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

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