Trans-stilbene fluorescence quantum yield

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. 

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. 

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]

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].

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-... 

The effect of temperature on the quantum yields of isomerization and fluorescence was determined for frons-stilbene and for certain halogen-substituted frans-stilbenes (Dyck and McClure, 1962 Malkin and Fischer, 1962). It was found that as the temperature is decreased (below about 200°K) the trans cis quantum yield decreases but that the fluorescence quantum yield increases. These results suggest that an activated process (i.e., internal conversion or intersystem crossing) competes with fluorescence from S - and that this process is an essential step in the mechanism for isomerization. The results for intersystem crossing into T. ... 

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. 

FIGURE 10. Relative quantum yields for exciplex fluorescence (filled symbols) and addition product formation (open symbols) versus solvent dielectric constant for trans-stilbene with di isopropyl methyl amine (0)> ethyldiisopropylamine (A), and triethylamine ( ) in hexane-ethyl acetate and ethyl acetate-acetonitrile mixed solvents. From ref. (114) with permission of the American Chemical Society. 

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).

Vesicles are ordered fluids or liquid crystals, a fact which reflects well in those photoreactivities that are particular to vesicle solutions. Fatty acid derivatives 24 and 25, for example, show low quantum yields of fluorescence ((j)f) and high quantum yields of cis-trans isomerization (< )c) as well as short fluorescence life times (tf) in both methylcyclohexane and micellar SDS solutions (Table 1). In DPPC vesicles, on the other hand, cis-trans isomerizations are cumbersome and much slower, and the fluorescence yield and lifetime rise considerably (Table 1). For those stilbene derivatives which are embedded in the middle of a fatty acid backbone, isomerization is virtually eliminated in the low-temperature or gel phase of the bilayer. The vesicle thus plays the role of stabilizing trans configurations which fit into the frozen oligomethylene chain matrix. 

Table 1 Photochemical quantum yields of fluorescence ( f), cis-trans isomerization (i c) ndfluorescence lifetimes (if) for stilbene-fatty acid derivatives in free solution, micelles and vesicles.

Arylethenes carrying bis-stilbene units undergo not only one- but also twofold adiabatic cis - trans isomerization. Thus 4-styrylstilbene (22) undergoes adiabatic Z E isomerization from the ZZ isomer to the ZE and EE isomers in the excited singlet state as revealed by fluorescence spectroscopy [97]. The fluorescence from the ZZ isomer decays with components of 0.19 ps, 0.6S ps, and 1.25 ns at 298 K, assigned to the fluorescence of ZZ, ZE, and EE isomers. The quantum yields of the adiabatic isomerization from ZZ to ZE and to EE are 0.038 and 0.019, respectively. 

The fluorescence of Irons-stilbene and four methoxy-substituted stUbene derivatives has been detected in a variety of solvents [37]. Compared to other stUbene derivatives, trons-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. 

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