Photoisomerization quantum yields, stilbene

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 effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

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. 

Diphenyl-1,3,5-hexatriene. The quantum yield for photoisomerization of fllTfrani-l,6-diphenyl-l,3,5-hexatriene (DPH) is much lower than that of fra 5-stilbene. ° The DPH compound is the first in the series of vinylogous stil-benes for which the 2 A state is lower than the 1 B state. Early picosecond fluor-... 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

The photostationary Z/E ratio of stilbene, (Z/E)pss, is known to be significantly dependent on the excitation wavelength. Thus, the Z/E ratio is almost unity (48 52) upon irradiation at 254 nm, but is remarkably enhanced up to 93 7 upon irradiation at 313 nm. This apparently surprising change is readily interpreted in terms of the following equation, which relates the (Z/E)pss ratio with the relative extinction coefficient of the two isomers at the excitation wavelength and the relative efficiency (quantum yield) of the forward and reverse reactions (Z/E)pss = ee/ezx z E/E z [201]. Preparative-scale direct irradiation should be done at low stilbene concentrations, since photodimerization of (fi)-stilbene may compete with the photoisomerization as the concentration increases [202-206]. 

Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and polarization239 Coupling of solute and solvent decreases as the size of the solvent molecules increases. The applicability of currently favoured models for the activated barrier crossing in the photoisomerization of stilbene is discussed, A method for measuring quantum yields in the photoisomerization of trans-stilbene gives high accuracy without use of a chemical actinometer . Evidence has been found for dynamic solvent effects on the photoisomerization of 4,4 -dimethoxystilbene in which the effects of temperature and hydrostatic pressure were made in n-alkane and n-alkyl alcohol. A ps laser time-resolved study fits frequency dependent solvent shifts but gives results inconsistent with the free volume model. Photophysical and theoretical studies of trans and 9-... 

Isomerization can be induced by light in both directions or by heat in the Z — E direction. The reverse thermal reaction is not observed at normal temperatures. Any one of the elementary reactions can be missing. Z-azobenzene in solution has a thermal Z E activation enthalpy AH 96 kJ moT and a half life time of 2 to 3 days at room temperature. Thus, the thermal reaction is irrelevant for the photoisomerization at usual irradiation intensities (for comparison Z-stilbene has Eg 180 kJ moT is liquid, and is kinetically stable). On the other hand, one of the photoreactions may not be active (e.g., when an irradiation wavelength is selected where one form does not absorb or when the quantum yield is too small). Inspection of Figure I.IB shows that E- and Z-azobenzene have virtually no spectral region without overlapping absorption. 

The photoisomerization mechanism in aminoazobenzene- and pseudo-stilbene-type compounds has attracted far less attention than the mechanism for azobenzenes. In pseudo-stilbenes, the (n,7t ) state is buried under the intense it —> Jt band and cannot be populated selectively. No state-specific quantum yields are available because the yields are independent of the exciting wavelength.There is only a very narrow experimental basis for a discussion of these two mechanisms. However, this may change when pseudo-stilbenes are subjected to ultrashort-time experiments. 

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. 

Arylalkenes, such as stilbene derivatives, are important model compounds for the study of the E Z photoisomerization.529 559 560 The compounds absorb significantly over 250 nm, therefore direct irradiation is technically simple. Photolysis of unsubstituted stilbene in aliphatic hydrocarbons at 313 nm affords a photostationary state (PSS) consisting of 93% of (Z)-stilbene and 7% of ( )-stilbene (Scheme 6.4).112 In addition, a photoinduced 67t-electrocyclic (Section 6.1.2) formation of dihydrophenanthrene with a quantum yield of < > 0.10 competes with (Z)-stilbene isomerization ([Pg.232]

Sandros and Sundahl [127] have synthesized all six isomers of 4,4-bis (3,5-di-fert-butylstyryl) stilbene (Exhibit 16) to study photoisomerization. Isomerization is preferred from cis-to-trans conversion and single isomerizations are more dominant. The increase in the quantum yield of isomerization resulting from an increase in the solvent polarity is interpreted to mean that a twisted singlet excited (p ) state is polar in nature and its interaction with polar medium lowers its energy, thereby facilitating the isomerization process. They have found that 103 with central double bond in cis configuration displayed adiabatic cis-to-trans isomerization. 

TABLE 5a Quantum Yields of cis trans Photoisomerization and Photostationary State for Stilbene"... 

A list of quantum yields and the cis/trans ratio in the photostationary state is presented in Tables 9a-c. The entries for stilbene show that a great variety of sensitizers, including some polymeric arylketones [218] promote cis/trans photoisomerization rather efficiently (Table 9a). For high-energy sensitizers typical values are <6 -, = 0.4, = 0.5, and ([c]/[t])sens = 1.3. 

TABLE 9a Quantum Yields of Sensitized cis trans Photoisomerization for Stilbene"... 

In conclusion, the cis trans photoisomerization of stilbene at low concentrations in fluid media is readily explained by the singlet mechanism (Scheme I). In addition to this major route for cis - trans photoisomerization, a minor adiabatic route ( c - p - H ) with a quantum yield of 2 x 10 3 has been reported [240,241]. The situation can change on substitution or at higher viscosities and/or lower temperatures. To a minor extent a contribution of an upper excited triplet mechanism for trans - cis photoisomerization is possible (Section VI.C.l). 

A related system uses photoisomerization of a stilbene-4,4 -disulfonyl cap covalently bound to P-CD. The trans-AD regioisomer (for nomenclature, see Scheme 10) converts completely and with high quantum efficiencies to the cis-AD regioisomer. No photostationary state is established, because the cis isomer photocyclizes to the dihydrophenanthrene-capped p-CD with a quantum yield of 0.048, which is much higher than that for conversion to the trans isomer (< 0.001). The relative yield of the two processes is reversed for the model of the cis cap, when free in solution [358]. 

G. Gauglitz, K. Goes, W. StooB, R. Raue, Determination of partial photochemical quantum yields of reversible photoisomerizations of stilbene-1 derivatives, Z. Natur-forschg, 40A (1985) 317. 

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. 

---