Stilbene fluorescence quantum yields

If fluorescence and cis-trans isomerization (9.26)-(9.29) are the main competing reactions upon direct excitation, then inhibition of rotation about the central bond should produce an increase in the fluorescence quantum yield. The rigid systems (3) and (4) both have fluorescence quantum yields of 1.0 at room temperature.<44,52) While the fluorescence of /rmy-stilbene is a... 

The fluorescence quantum yield of trans-stilbene is 0.75 when measured in a rigid glass at 77 K, showing that the rigid medium results in more efficient fluorescence. 

In conclusion, protonation of the donor group of D-A stilbenes leads to a short wavelength shift of both absorption and fluorescence spectra and a decrease of the fluorescence quantum yield while doubly protonated D-D stilbenes exhibit a monodeprotonation in the excited state and emit an additional long-wavelength bond. 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

A surfactant stilbene (76) has been synthesized as a viscosity probe of micellar systems 76). The stilbene surfactant undergoes photochemical trans-cis isomerization with quantum yields very similar to stilbene. Like stilbene, the quantum yield (quantum yield of fluorescence (ft) increases. For example, decreases from 0.5 in CH2C12 to 0.39 in HDTBr, while the f, increases from 0.04 in CH2C12 to 0.18 in HDTBr. This probe has also been used to study bilayer vesicle systems. Cyanine dyes become more photostable and have higher organic solvents 77). This is also due to an increase in viscosity. 

The behavior of unsubstituted stilbene is the opposite Comparison of ps fluorescence decay data [86-88] and fluorescence quantum yields [89] as a function of solvent polarity shows that the nonradiative rates increase in parallel with solvent polarity giving evidence that the quenching state P is more polar than the fluorescing one, E. ... 

Saltiel, J., Waller, A. S., Sears, D. F. Jr., Garrett, C.Z., Fluorescence Quantum Yields of trans stilbene d0 and d2 in n Hexane and n Tetradecane Medium and Deuterium Isotope Effects on Decay Processes, J. Phys. Chem. 1993, 97, 2516 2522. 

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]

Fluorescence Quantum Yields. For tram-stilbcne 4>f is independent of lexc and the concentration (typically < 10 4 M) and only slightly reduced by the presence of c/s-stilbene [13, 81, 99, 240, 241, 294]. The liquid solutions at room temperature f is about 0.04 for trons-stilbene and 4-bromostilbene, and larger for 4-chlorostilbene and several cyanostilbenes. For nitrostilbenes very small, but there are exceptions in the presence of an electron... 

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... 

Table3.1 Sensitivity p) ofthe Stokes shift, fluorescence decay rate constant, fluorescence quantum yield, and radiative deactivation rate constant to intramolecular substituent effects for two different groups of trans-4,4 -disubstituted stilbenes [87].

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 electronic spectroscopy of trans-isomers of 3-(N-phenylamino)stilbene (mlc), 3-(N-methyl-N-phenylamino)stilbene (mid), 3-(N,N-diphenylamino)stilbene (mle), and 3-(N-(2,6-dimethylphenyl)amino)stilbene (mlf) and their double-bond constrained analogues, m2a-m2c and m2e, were studied [71]. When compared with trans-3-aminostilbene (mla), mlc-mle displayed a redshift of the So —> Si absorption and fluorescence spectra, lower oscillator strength and fluorescence rate constants, and more effldent Si —> Ti intersystem crossing. The N-Ph derivatives mlc-mle had lower fluorescence quantum yields and higher photoisomerization quantum yields. The role of Si —> Ti transition in the amino-substituted stilbenes as the predominant nonradiative decay pathway was discussed. The excited triplet (Ti) state formation of stilbene dendrimers (tetramethoxystilbene (generation G) GO, Gl,... 

The effect of solvent polarity on photoisomerization of trans-4,4 -bis(benzoxazolyl) stilbene has been reported [74]. On the basis of fluorescence quantum yield and lifetime measurements, the rate constants of radiative and nomadiative decays were calculated. In a polar solvent, a high fluorescence quantum yield was observed whereas a high rate of photoisomerization occurred in a nonpolar solvent. 

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. 

Photochemical and photophysical properties of a poly(propylene amine) dendri-mer (2) functionalized with -stilbene units have been studied [102]. Z-photoisome-rization and photocyclization of the Z-isomer of the stilbene units were investigated in air-equilibrated acetonitrile solutions. The quantum yields of the E Z photoisomerization reaction and the fluorescence quantum yield of the E were found to be equal to 0.30 and 0.014, respectively. Stilbene dendrimers prepared by coupling 4,4 -dihydroxystilbene with first-, second-, third-, or fourth-generation benzyl ether-type dendrons underwent photoisomerization with the same efficiency as that of 4,4 -dimethoxystilbene [103], The lifetime of the core structure was found to be shorter then 1 ns. According to [104], polyphenylene-based stilbene dendrimers, Gl, G2, and G3, underwent mutual cis-trans isomerization upon direct irradiation with 310 nm light at room temperature. In a solvent glass at 77 K, one-way cis-trans isomerization was observed for G2. 

Water soluble p-sulfonato calbc[w]arenes (w = 8, la-lb and n=6, 2a-2b) was employed as host to control the outcome of photodimerization and photoisomerization of 4-stilbazoles [17]. Novel macrocydic and medium-size stilbenophanes tethered by silyl chains were synthesized, and their photochemical and photophysical properties were examined (Figure 5.5) [18]. Direct irradiation of macrocydic stilbenophanes stereoselectively gave intramolecular photocydoadducts, and the efficiency increased with decreasing distance between the two stilbene units. The triplet-sensitized photoreaction of stilbenophanes caused cis-trans photoisomerization. Photoreactions of as-fixed stilbenophanes under an oxygen atmosphere selectively led to phenanthrenophanes. Fluorescence quantum yields increased with the introduction of silyl substituents, and hence those of silyl-tethered stilbenophanes were larger than that of unsubstituted fraws-stilbene. Intramolecular excimer emis-... 

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