Stilbenes in Solution

Fluorescence properties of two series of trinuclear diimine Re(I) tricarbonyl complexes [Re(CO)3(NN)]3(p -Ll,2) (PF6)3 (NN = ethylenediamine or substituted bipyridine (A) and phenanthroline l,3,5-tris(4-ethenylpyridyl)benzene = LI, 1,3,5-tris(4-ethynylpyridyl)benzene = L2) (B) linked by an isomerizable stilbene-like 

NN = polypyridyl ligands and L = stilbene-like ligands, in acetonitrile solution and in poly(methyl methacrylate) (PMMA) polymer film exhibited hypsochromic shifts as the medium rigidity increases due to the luminescence rigidochromic effect. Time-resolved IR (TRIR) spectroscopy, in combination with other techniques, characterized the excited-state electronic properties of the fac-[Re(CO)3(phen) (bpe)]PFa complex, where bpe is l,2-bis(4-pyridyl)ethylene. 

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. 

The unique fluorescence properties of trons-3,5-dimethoxystilbene were attributed to the formation of a highly polarized charge transfer excited state (pe = 13.2 D). The fluorescence of all five trons-isomers was quenched by 2,2,2-trifluoroethanol. 

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, pftc, to the cis-stilbene fluorescence 

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The direct photolysis of cis- or ftmr-stilbene in solution gives rise to photo-isomerization. In addition, dihydrophenanthrene is formed from the cis isomer ... 

H.-O. Hamaguchi and K. Iwata, Physical chemistry of the lowest excited singlet state of trans-stilbene in solution as studied by time-resolved Raman spectroscopy, Bull. Chem. Soc. Jpn. 2002, 75(5), 883. 

Figure 16 Photosensitized oxidation of frans-stilbene in solution and within ZSM-5 zeolite.

Rao et al. observed that photoirradiation of tran.v-stilbene in crystalline y-cyclodextrin inclusion complexes yields a single isomer of. svn-tctraphenylcy-clobutane (119) [109] stereoselectively in high yield (70%). In contrast, the photodimerization of stilbene in solution is very inefficient, and no photodimer was observed even after prolonged irradiation of pure stilbene crystals. 

Figure 2.20 Potential energy diagram illustrating the room-temperature photoinduced isomerization of stilbene in solution wavy lines represent non-radiative transitions, curved arrows isomerizations and straight lines radiative transitions...

The observed and calculated ts values of rrans-stilbenes at room temperature (Tables 14 and 13, respectively) are (with a few exceptions) smaller than 0.1ns. In particular, it has been shown that the fluorescence decay of trans-stilbene in solution is single exponential. This contradicts a proposal postulating an equilibrium between configurations of the trans ( t ) and the twisted ( p ) excited singlet states [376, cf. 338 and 360]. However, Charlton and Saltiel [377] have analyzed the results from Birch and Birks [376] and revised the model of a 11 1 p equilibrium for stilbene. 

Picosecond laser measurements with trans-stilbene in solution (and even in the gas phase) agree in the respect that the fluorescence decay is single exponential (Table 14). Earlier results assuming a slow and a fast fluorescence decay component [376] have not been confirmed in at least four different laboratories [314, 321-323, 342, 349]. Because of the monoexponential decay, the back reaction 1 p —> 11 has to be much slower than the forward reaction t -. Elowever, the adiabatic formation of t from c indicates that the rate constants of both processes in solution have the same order of magnitude, independent of temperature. The low efficiency, %0.3%, of the p -> t process reflects very rapid p - p decay rather than a low p energy [81]. 

The mesoporous character of MCM-41 overcomes the size limitations imposed by the use of zeolites and it is possible to prepare the complex by refluxing the chiral ligand in the presence of Mn +-exchanged Al-MCM-41 [34-36]. However, this method only gives 10% of Mn in the form of the complex, as shown by elemental analysis, and good results are only possible due to the very low catalytic activity of the uncomplexed Mn sites. The immobihzed catalyst was used in the epoxidation of (Z)-stilbene with iodosylbenzene and this led to a mixture of cis (meso) and trans (chiral) epoxides. Enantioselectivity in the trans epoxides was up to 70%, which is close to the value obtained in solution (78% ee). However, this value was much lower when (E)-stilbene was used (25% ee). As occurred with other immobilized catalysts, reuse of the catalyst led to a significant loss in activity and, to a greater extent, in enantioselectivity. 

Figure 3.17 presents ps-TR spectra of the olehnic C=C Raman band region (a) and the low wavenumber anti-Stokes and Stokes region (b) of Si-rra i-stilbene in chloroform solution obtained at selected time delays upto 100 ps. Inspection of Figure 3.17 (a) shows that the Raman bandwidths narrow and the band positions up-shift for the olehnic C=C stretch Raman band over the hrst 20-30 ps. Similarly, the ratios of the Raman intensity in the anti-Stokes and Stokes Raman bands in the low frequency region also vary noticeably in the hrst 20-30 ps. In order to better understand the time-dependent changes in the Raman band positions and anti-Stokes/Stokes intensity ratios, a least squares htting of Lorentzian band shapes to the spectral bands of interest was performed to determine the Raman band positions for the olehnic... 

Figure 3.18. Time dependence of the peak position of the 1570 cm Raman band of Sj trans-stilbene in chloroform solution (filled triangle). The time dependence of the anti-Stokes/Stokes intensity ratio is also shown with open circles. The best fit of the peak position change with a single-exponential function is shown with a solid curve, while the best fit of the anti-Stokes/Stokes intensity ratio is shown with a dotted curve. The obtained lifetime for both single-exponential decay functions was 12ps. (Reprinted with permission from reference [78]. Copyright (1997) American Chemical Society.)...

Kleinschmidt J, Rentsch S, Tottleben W, Wilhelmi B (1974) Measurement of strong nonlinear absorption in stilbene-chloroform solutions, explained by the superposition of two-photon absorption and one-photon absorption from the excited state. Chem Phys Lett 24 133-135... 

Emission spectra have been recorded for electron injection into Au and Ag spherical electrodes and hole injection into Au(lll) planar electrodes. These processes were brought about in solutions of acetonitrile containing tetrabutylammonium hexafluoro-phosphate (TBAHP), using the trans-stilbene radical anion as the electron injector and the thianthrene radical cation as hole injector. The spectrum for the hole injection process into planar Au(lll) electrodes has been resolved into the P S-polarised components of the emitted light. A comparison of the spectral distribution of emitted light for the above electron injection process, occurring at both Au and Ag... 

Figure 2. Current-time and photon-time data for the electron injection process by t-stilbene into a Au sphere electrode. These data were obtained using electronic compensation for the residual IR drop in solution, with a modulation amplitude of 3.1V (ie. -2.6V to 0.5V).

Fang and Yamamoto [351] reported on postpolymerization functionalization of triarylamine-fluorene copolymer 253, resulting in copolymers 254a,b with stilbene pendant groups. Whereas in the solid-state absorption and PL maxima of both polymers are essentially the same, PL in solution is strongly influenced by solvent (from 433 nm in toluene to 466 nm in jY-methylpyrrolidone). Copolymer 254a showed d>PL in the solid state of 51%, comparable to that of poly(9,9-dialkylfluorenes) (Chart 2.66). 

For stilbenes, bearing moderately electron-donating substituents, the reaction occurs through open intermediates, present in solution as ion sandwich, 4. Formation of 4 is the rate determining step ... 

Stilbeneamines. The functionalization of stilbenes with arylamino groups leads to materials that emit in the green-to-yellow spectral region. For example, 9,10-bis(4-(7V,/V-diphenylamino)styryl-anthracene (BSA, 21) absorbs at429nm and emits at 585 nm [141]. Compound 21 and other derivatives of bistyrylanthra-cene have been successfully applied in yellow emitting OLEDs [64]. Tetra(tri-phenylamino)ethylene (TTPAE, 20) emits at 539 nm [109]. The latter compound exhibits a large quantum yield of 25% in the amorphous film, but does not show fluorescence in solution. 

The ii-stilbene molecule is planar in crystalline form, in gas phase, and presumably in solutions, although the phenyl group may be rotated as much as 32° to reduce nonbonded repulsions between hydrogen atoms (Waldek 1991, Meier 1992). This modest twisting still allows sufficient... 

Stilbene derivatives are reduced in dimethylformamide to the dihydrocompound and in a number of cases [6,18] the mechanism is known to involve accumulation of the radical-anion in solution. Ihis disproportionates to the starting material and the dianion, which is protonated. 

The hydroxylation of n-hexane and epoxidation of stilbens were conducted at 300K in the suspended solutions with PhIO for each powdered sample such as FePc(t-Bu)4/NaY, FePc(t-Bu)4+NaY, FePc/NaY and FePc+NaY. The products in solution were filtered and analyzed by FID gc using a capillary column(PEG-25M). 

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." ... 

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. 

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

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