Photochemical probes of conformational mobility in liquid crystals. The photochemistry of β-phenyl-4-methoxypropiophenone in the nematic and smectic B phases of CCH-n and OS-nm liquid crystals

Abstract

. Can. J. Chem. 69, 1435 (1991).The photochemistry of P-phenyl-4-methoxypropiophenone (1) in the isotropic and liquid crystalline (nematic and smectic B) phases of trans,trans-4'-alkyl-(l,l'-bicyclohexyl)-4-carbonitrile (CCH-n) and trans-4-alkylcyclohexyl-trans-4'-alkylcyclohexylcarboxylate (0s-nm) mesogens has been investigated using nanosecond laser flash photolysis techniques. Solubility limits of 1 as a function of temperature in the smectic phases have been rigorously determined by 'H NMR spectroscopy, using a-, methoxy-, and P-phenyl-deuterated analogs of the ketone. Triplet decay of 1 in the smectic (Crystal B) phase of CCH-4 is multiexponential, suggesting that the ketone is solubilized in a distribution of conformations in the highly-ordered liquid crystalline lattice. The average triplet lifetime of 1 at various temperatures throughout the smectic phase is about a factor of three longer than that in homologous nematic and isotropic phases at the same temperature. The differences in the Arrhenius parameters for triplet decay in the smectic and nematiclisotropic phases are very small, however. The present results for 1 in the smectic phase of the CCH-n mesogens correct the interpretation of previously reported data obtained with higher concentration mixtures, and afford an accurate indication of the true effects of smectic B liquid crystalline order on the conformation-dependent triplet state behavior of this molecule. In 0s-nm liquid crystals, triplet decay follows clean first order kinetics in all phases, but the variations in Arrhenius dependence with solvent phase are similar to those observed in the CCH-n liquid crystals.Key words: smectic, liquid crystals, 'H NMR, nanosecond laser flash photolysis, intramolecular triplet quenching, photochemistry. On a CtudiC la photochimie de la P-phknyl-4-mCthoxypropiophCnone (1) dans les phases isotropes et cristalline liquide (nkmatique et smectique) des mCsogbnes trans,trans-4'-alkyl-(1,1-bicyclohexyl)-4-carbonitrile (CCH-n) et trans-4-alkylcyclohexyl-trans-4'-alkyl-cyclohexanecarboxylate (0s-nm) en utilisant la technique de photolyse Cclair avec un laser ?i nanoseconde. On a dCterminC rigoureusement par la rmn du 'H, la limite de solubilitC du composC 1 en fonction de la tempkrature dans la phase smectique, en utilisant les analogues a-mCthoxy et P-phCnyl deutCriCs de la cCtone. La dtcroissance du triplet du compost 1 dans la phase smectique (cristal B) du CCH-4 est multiexponentielle, suggkrant ainsi que la cCtone est solubiliske avec une distribution de conformations dans le rCseau cristallin liquide fortement ordond. La durCe de vie moyenne du triplet du compost 1 ?i diffkrentes temptratures ?i travers la phase smectique est environ trois fois plus longue que celle du triplet dans les phases homologues ntmatique et isotrope i la m&me temptrature. Cependant la diffkrence dans les parambtres d'ArrhCnius pour la dCcroissance du triplet dans les phases smectique et nCmatiquelisotrope, est trks faible. Les rksultats actuels relatifs au compost 1 dans la phase s...

Modulation of Lifetimes and Diastereomeric Discrimination in Triplet-Excited Substituted Butane-1,4-diones through Intramolecular Charge-Transfer Quenching

“…12,13 Consequently, they have been employed as excellent probes of supramolecular systems. [14][15][16][17][18][19][20][21] The involvement of charge transfer in the intermolecular quenching of triplet-excited ketones by arenes has been long recognized. [22][23][24][25][26] Detailed studies by Wagner and co-workers on the photoreduction of substituted acetophenones and benzophenones with alkylbenzenes showed that both n,π* and π,π* triplets react with alkylbenzenes predominantly by a ratedetermining charge-transfer complexation.…”

“…Compounds for which photophysical properties can be conformationally controlled have the potential to probe how supramolecular structures affect the intramolecular mobility of an incorporated molecule. In this context, β-phenylpropiophenones are particularly interesting in that their triplet lifetimes are limited by the conformation, in which the triplet-excited carbonyl group and the β-phenyl ring are in close proximity. , Consequently, they have been employed as excellent probes of supramolecular systems. − …”

Remarkable Discrimination in the Triplet Lifetimes of the Diastereomers of 1,4-Bis(p-methoxyphenyl)- 2,3-diphenylbutan-1,4-dione

“…In addition, studies with derivatives substituted on the β-phenyl ring have established the importance of charge transfer − It is noteworthy that novel diastereomeric discrimination in the triplet lifetimes of p -methoxy-β-phenylbutyrophenone has been observed in the solid state (racemic vs pure R or pure S crystals), while no discrimination was observed in solution …”

Liquid Crystals as Solvents for Spectroscopic, Chemical Reaction, and Gas Chromatographic Applications

“…Examples of these effects are found in time-resolved studies of the dynamics of triplet excited state ketones [354,355,361,362] and in kinetic studies of the allyl-p-dimethylaminobenzenesulphonate quaternization discussed above [357,359,3631. Prior to the development of a full understanding of the ramifications of phase separation phenomena, such complex behaviour was often misinterpreted and ascribed to the reaction of a homogeneously solubilized solute in the smectic phase [359, 361 -3631. Examples of SmB -SmA biphasic reactions are also known [364].…”

“…Generally, the presence of the less-ordered phase will result in a 'dilution' of the effect of the higher ordered phase. This has been illustrated succinctly in studies of the Norrish/Yang Type I1 reaction of aryl alkyl ketones in highly ordered smectic phases [352][353][354][355]3601, where the maximumeffect of liquid crystalline order on product distribution (this is always gauged in relation to results obtained for the isotropic phase of the same mesogen, or a reasonable model solvent) was shown to occur at concentrations at or below the solubility limit of the solute. The magnitude of the effect decreases as the bulk solute concentration increases beyond the solubility limit in the mesophase; this is because an increasing fraction of solute resides (and reacts) in a coexisting, solute-rich isotropic or nematic phase.…”