Organic reactions in liquid crystalline solvents. 5. : Intramolecular phenolic quenching of aromatic ketone triplets as a probe of solute conformational mobility in liquid crystals.

The effects of liquid crystalline order on the confomational motions involved in end-to-end, intramolecular triplet quenching of aromatic ketones by a suitably situated phenolic moiety have been investigated. Triplet state quenching has been monitored using two independent probes: Norrish II fragmentation quantum yields of valeryl-substituted derivatives and direct lifetime measurements in methyl-substituted ketones, using the corresponding anisyl-substituted ketones as models for triplet state behaviour In the absence of phenolic quenching.Quantum yields for Norrish II fragmentation in the liquid crystalline solvents have been estimated using 4-methoxyvalerophenone (MVP) as the actinometer. The ability of liquid crystalline solvents to inhibit intramolecular phenolic quenching is dependent on both phase type and solute length. In one case, Intramolecular quenching Is completely suppressed In smectic phases, indicating that the conformational motions involved in achieving the quenching geometry are slowed by a factor of at least 10³ relative to their rates in non-viscous, isotropic solvents. The Norrish II product ratios from photolysis of MVP in the liquid crystalline solvents are also affected dramatically by liquid crystalline order.     

Photoprocesses and molecular associations

Abstract— The progressive reduction in temperature of hydro-alcoholic mixtures is a way of associating aromatic molecules. It is possible to test the solute association, both by absorption and fluorescence measurements, of ‘polymerizable dyes' and solutes of biological interest. It is also possible to test the solute association by electron spin resonance of stable radicals (Wür-ster's blue). The application of this technique to the study of photoprocesses was performed with flavin mononucleotide (FMN) by electron spin resonance and absorption spectroscopy in order to analyze the mechanism of photoreduction, and the stability of photo-induced free radicals. The procedure of associating molecules by temperature effect in these solvents can be applied to the spectroscopic studies of the triplet state of aromatics in glassy solutions at 77°K.     

Solvent effects on triplet state quenching by tetramethylpiperidcme-N-oxide

Measurements of the triplet state quenching of a series of aromatic hydrocarbons by tetramethylpiperidine-N-oxide in two solvents, acetonitrile and perfluoro-n-hexane, are reported. In both solvents the previously established trend of quenching rate constants with the triplet energy of the aromatic hydrocarbon being quenched was observed. No correlation of quenching efficiency with the charge transfer properties of the aromatic hydrocarbon—free radical collision complex was found.     

Solvent effects in NMR spectra induced by aromatic solvents in 1,4-dioxanes, 1,3-dioxolanes and in sulphur analogues

NMR solvent effects induced by aromatic solvents on some 1,4-dioxanes, 1,3-dioxolanes and on some sulphur analogue derivatives are reported. The shielding effect of the aromatic solvents is examined in respect to the structure of the solute.     

SOLVENT DEPENDENCE OF THE QUANTUM YIELD OF TRIPLET STATE PRODUCTION OF 9-PHENYLANTHRACENE

Abstract— Values of φ T , the quantum yield of triplet state production of 9-phenylanthracene, have been determined in four different solvents by measuring the effects which 'heavy atom quenchers' have on the fluorescence intensity and the triplet state absorption of the solute. A variety of quenchers have been used including organic and inorganic halides and the rare gas, Xenon. There is a converse solvent dependence of φ F and φ T , and for each of the four solvents used φ F +φ T has been found to be unity within experimental error.     

Laser photolysis study of the triplet states of nitroaromatic amides

The spectrokinetic indices of the triplet states of nitroaromatic amides were established. The quenching of triplet states by tertiary aromatic amines in apolar solvents leads to the formation of triplet exciplexes. A charge transfer state makes the major contribution to the structure of these exciplexes. Hydrogen atom transfer proceeds efficiently in the case of secondary aromatic amines, leading to the formation of the corresponding neutral radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1912–1915, August, 1989.     

Laser photolysis study of the spectral-kinetic characteristics of short-lived triplet exciplexes and the mechanism of atomic hydrogen transfer in the course of 2,6-diphenyl-1,4-benzoquinone triplet quenching with aromatic amines

The spectral-kinetic characteristics of short-lived triplet exciplexes arising in the quenching of 2,6-diphenyl-1,4-benzoquinone triplet with aromatic amines: N,N,N′,N′-tetramethyl-p-phenylenediamine, triphenylamine and diphenylamine have been studied by means of the nanosecond laser photolysis technique. The absorption spectra of triplet exciplexes exhibit distinct maxima characteristic of the absorption spectra of corresponding amine radical cations. The state with complete charge transfer gives the basic contribution to the exciplex structures. A detailed analysis is presented of the kinetic and thermodynamic deactivation characteristics of triplet exciplexes in low-polar solvents.     

Benzophenone triplet exciplexes with aromatic amines

Conclusions Laser photolysis was used for the direct detection of the spectral and kinetic characteristics of the contact radical-ion pair triplet exciplexes generated upon the interaction of triplet benzophenone with tertiary aromatic amines in nonpolar solvents. The exciplexes disappear as a result of reverse electron transfer and proton transfer.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 928–930, April, 1988.     

Phosphorescence characteristics of acetophenone, benzophenone, p-aminobenzophenone and Michler's ketone in various environments

The phosphorescence characteristics (excitation and emission spectra and lifetimes) of acetophenone (AP), benzophenone (BP), p-aminobenzophenone (PABP) and Michler's ketone (MK) adsorbed on Whatman No. 1 filter paper were measured at various temperatures, and compared with the phosphorescence characteristics in different solvent glasses at 77 K. Both AP and BP phosphoresce on filter paper only at low temperature (208 K). The phosphorescence lifetimes of AP and BP are < 1 msec, indicating a (3)(n,pi(*)) lower triplet level for paper substrates. With PABP, the low lying triplet state in polar solvents is (3)(CT) and in non-polar solvents is (3)(n, pi(*)); PABP on filter paper results in spectral characteristics similar to those of PABP in polar solvents at 77 K. The lifetime of PABP is longer than that of BP, indicating a (3)(CT) low-lying triplet state. MK, like PABP, has strongly environment-dependent photophysical properties. MK, when adsorbed on filter paper, has an intense long-lived luminescence at room temperature, resulting in a limit of detection of 3 ng ml or 3 pg, and a linear dynamic range of over 3 orders of magnitude. MK appears to be strongly hydrogen-bonded to the filter paper. In studies in ethanol and other solvents, MK adsorbed on filter paper shows a dramatic change in its phosphorescence spectrum when the temperature is lowered from 298 K to 208 K; the phosphorescence peak moves to longer wavelengths and the intensity decreases. The temperature effect could arise from the presence of several conformers of MK or be due to different environmental sites or E-type delayed fluorescence. The low-lying triplet state of MK on filter paper is most likely a (3)(CT) state. Lowering the temperature appears to increase the phosphorescence intensity for ketones which phosphoresce in the (3)(n,pi(*)) triplet state, but affects it only slightly for analytes which phosphoresce in the (3)(pi,pi(*)) triplet state. Room-temperature phosphorescence seems to arise for aromatic ketones and aldehydes with low-lying (3)(pi, pi(*)) or (3)(CT) triplet states.     

RESONANCE RAMAN and ABSORPTION SPECTRA OF ISOMERIC RETINALS IN THEIR LOWEST EXCITED TRIPLET STATES

Abstract— The triplet-triplet absorption spectra of 9-cis-, 13-cis- and all-fraw-retinal as well as the time-resolved resonance Raman spectra of the lowest electronically excited triplet states of 9-cis-, 11-cis, 13-ciy and all-trans-retinal in aromatic solvents at room temperature have been obtained under conditions ensuring the isomeric purity of the starting materials. The triplet states were produced by triplet energy transfer from a sensitizer in pulse-radiolysis experiments. The overall results suggest that the isomeric retinals form either different relaxed triplet species or different mixtures of relaxed triplet species. The possible implications about the size of the energy barriers separating the various triplet species are discussed. The resonance Raman spectra obtained by using either anthracene (E_T= 177.7 kJ mol^-1) or naphthalene (E_T= 254.8 kJ mol^-1) as sensitizers were virtually identical for the corresponding triplet states from each of the isomers 11-cis-, 13-a's- and all-tams-retinal, suggesting that the relaxed triplet species or the mixture of relaxed triplet species formed from each isomer is independent of the energy of the sensitizer.     

Individual solvent/solute interactions through social isomerism

Reversible coencapsulation of a solute molecule and a single solvent molecule takes place in solution at ambient temperature. Two isomeric complexes are formed (social isomers), and their relative energies are assessed by NMR methods. Intermolecular interactions between 3 aromatic solutes and 15 common solvents are evaluated.     

Aromatic solvent induced shifts (ASIS) in 1H N.M.R. spectra; A graphical analysis

By plotting ASIS values determine in the conventional way against ASIS values for a polyprotic solute in C₆D₆ approximately straight lines are obtained the slopes of which represent ASIS effects for solvents in comparison with that of benzene. These values are independent of effects introduced by choice of reference solvent and standard, whether internal or external.Any modification of benzene that introduces an electronegative atom or group reduces the positive ASIS slope of benzene. The effect of several modifications is incremental and eventually produces a marked negative slope. By themselves, ASIS values are not a measure of ring current ir of aromaticity.It is only the presence of electronegative atoms or groups that affects ASIS slopes; orientation is immaterial showinf that dipoles in aromatic solvents are not concerned in the ASIS phenomenon.Mixtures (1:1) of benzene and hexafluorobenzene shield every proton in a solute; the resulting ASIS slopes are nearly zero as expected, but this is not a consequence of competition between the solvents but of a roughly additive property in shieldings. No evidence was obtained in support of the view that ASIS deshielding exists, or that there is any strong tendency of solvent molecules to cluster about a dipolar solute.The following solvents were studied: benzene, eleven fluorobenzenes, nitrobenzene, 1,2-fluoronitrobenzene, 1,2,4-fluorodinitrobenzene, pyridine, pyrimidine, pyridazine, 3-methylpyridine oxide, pyrrole, thiophene, furan, 3,5-dimethylisoxazole, 1,2-dimethylimidazole, methoxybenzene, 1,3-dimethoxybenzene, methylenedioxybenzene. The chief solute was ethyl 4-dimethylaminobenzoate.     

The role of weak hydrogen-bonds on the formation of short-lived molecular associations

Weak molecular homo- and hetero-associations among some ethene derivatives and several common solvents are studied using (1)H NMR spectroscopy. In connection with the aromatic-solvent induced shifts which is a special case of molecular association between aromatic solvents and polar molecules a model for association was suggested that involves the interaction of the positive end of the solute dipole with the aromatic pi-electrons. This model met with certain difficulties. An alternative model for configuration of associated molecules that explains the literature results and the results obtained in this paper is presented. The model is based on interaction of protons of the acceptor molecules with the lone-pair electrons or pi-electrons of the donor molecules. The present model removes the difficulties met with the former model and extends the concept of aromatic-solvent induced shifts to include non-polar molecules besides polar ones. In all cases, the interaction of protons of acceptor molecules with lone-pair electrons of donor molecules gives rise to a deshielding effect, while interaction with aromatic pi-electrons leads to shielding.     

Concerning the mechanism of laser photoionization of solutions of aromatic hydrocarbons in alkane solvents

It is suggested that recent results of optical absorption and conductivity studies of the laser flash photoionization of solutions of aromatic compounds in alkane solvents may be more readily explained by formation of the solvent radical action than by direct ionization of the absorbing solute.     

The effect of the internal reference on ASIS correlations: 2—The intrinsic ASIS of TMS protons in aromatic hydrocarbon solvents

A method is developed to calculate the intrinsic ASIS of TMS protons used as internal reference, based on linear correlation of the ASIS of a group of sensor protons in two fixed aromatic solvents. Several solute systems (p-X-benzaldehydes, camphor, α-Br-camphor, 5-X-furfurals, p-X-acetophenones and methyl ketones) are used to calculate a series of intrinsic ASIS values of TMS in 12 aromatic hydrocarbon solvents. The Δ^α _TMS values are used, in turn, to calculate the intrinsic ASIS of CHO protons in dimethylformamide and diphenylformamide, and of the CH₃ protons in diethyl ether and diisopropyl ether.     

Photosensitive decomposition of organic solvents at 77 degrees K by an aromatic amine, diphenylparaphenylenediamine

Abstract— –By e.s.r. we have studied the photoexcitation of an aromatic amine to its triplet state at 77°K, its photoionization to a radical cation and the simultaneous formation of solvent radicals proceeding from the photosensitization of the organic glassy matrix. In the case of methanol and ethanol matrix we observe approximately one solvent radical per solute radical cation. In the case of isopropanol and methyltetrahydrofuran we find respectively three and two solvent radicals per solute radical cation. The results suggest two possible processes of photosensitization. By successive absorption of two photons, the amine reaches an excited triplet state which is able either to dissociate giving one electron and one cation radical or to transfer its energy to the solvent, this last being decomposed. It is assumed that in the case of methanol and ethanol, the radicals from the solvent are only formed by reaction on the matrix by the released electron, whereas in the case of isopropanol and methyltetrahydrofuran, the second process is prevalent or exclusive.     

Triplet state dynamics in polycyclic aromatic molecules

Utilizing the technique of triplet absorption detection of magnetic resonance (TADMR), we have determined the rates of intersystem crossing for the individual spin levels of the anthracene-h₁₀ and -d₁₀ and 3,4-benzpyrene triplet states in Shpolskii solvents. The anthracene results are in agreement with other experimental and theoretical work on intersystem crossing in aromatic hydrocarbons. The benzpyrene results, however, are unique, showing a large contribution from the out-of-plane spin level. Possible explanations of this result are discussed.     

Polar bonded phase with the zwitterionic sulfobetaine functional group. Comparison to silica.

A new bonded phase has been prepared by the reaction of n-propylsultone with dimethylaminopropylsilane-modified silica. The resulting functional group is the zwitterionic ammonium propane sulfonate. Chromatographic solvents based on three strong solvents, methylene chloride, diisopropyl ether and acetonitrile, were prepared in hexane. Solutes, including substituted benzenes, polycyclic aromatic hydrocarbons and low-molecular-weight species commonly used as solvents, were chromatographed on the zwitterionic phase and on silica from the same supplier of the dimethylamino phase. The zwitterionic phase is a weaker adsorbent than silica and retentions are less influenced by the type of strong solvent employed, compared to silica. The retention (log k') of solutes on the zwitterionic phase is highly correlated with the free energy of transfer of solute from the vapor phase to water.     

Effect of the internal reference on ASIS correlations: 3—Intrinsic ASIS of formyl protons

The intrinsic ASIS of formyl protons in p-X-benzaldehydes, dimethylformamide and diphenylformamide in a series of aromatic hydrocarbon solvents have been calculated. These values of Δ¯_x^α of the formyl protons present excellent correlations in aromatic hydrocarbon solvents, and are factorizable into solute and solvent factors U_x and V_α, respectively. The U_x values are linearly related to the value of σ_p of the substituent in the solute. The V_α values depend on the substitution pattern of the benzene ring of the solvent molecules.     

Etude du Comportement Photochimique de quelques Diaryl-1,3-triazènes

Study of the Photochemical Behaviour of Some Diaryl-1,3-triazenes The photolysis of Diaryl-1,3-triazenes gives products whose structures are consistent with a cage recombination process of homolytically formed radicals and subsequent abstraction of hydrogen from the solvent molecules by arylamino radicals. In aromatic solvents, a free-radical chain process leads to the formation of products resulting from the homolytic substitution on the solvent. Quenching experiments show that singlet and triplet excited states are reactive but that intersystem crossing efficiency is low.