M. I. Knyazhanskii

Luminescence and photochemistry of azoles (review)

A review of the luminescence and photochemical properties of azoles and their relationship to their structures is given. The mechanism of the deactivation of the excited states of the molecules is discussed.

SYNTHESIS, STRUCTURE, AND SPECTRAL AND PHOTOCHEMICAL PROPERTIES OF FULGIDES OF THE INDOLE SERIES WITH AN ADAMANTYLIDENE FRAGMENT

New fulgides of the indole series with adamantylidene fragments were synthesized: 3-(adamant-2-ylidene)-2-[1′-(3′-indolylethylidene)]succinic anhydride (I) and 3-(adamant-2-ylidene)-2-(3′-indolylmethylene)]succinic anhydride (2). By means of electronic absorption and1H NMR spectroscopy and X-ray diffraction analysis, fulgides1 and2 were shown to haveZ- andE-configurations, respectively. The photochemical properties of1 and2 were studied. It was shown that along withZ-E photoisomerization of these compounds, in solution they undergo a cyclization reaction to yield a colored photoproduct. Specific structural features of fulgide1 affecting its photochemical properties were discussed. The introduction of a bulky adamantylidene group decreases the extent of the conversion of compound1 into cyclic isomerC.

Structures and photo- and thermochromic properties of spiropyrans of the 2-oxaindan series with polycondensed chromene fragments

Spiropyrans of the 2-oxaindan series with 2H-chromene fragments benzo- and 2,3-naphthoannelated at the 7–8 bond were synthesized. The molecular structures of four of the spiropyrans obtained were determined by x-ray diffraction studies, and their photo- and thermochromic properties were studied. Annelation of arene rings significantly increases the relative stabilities fo the photocolored forms of the spiropyrans. A correlation between the length of the Cspiro-O bond and the capacity of spiropyrans for photocoloration was discovered. In addition, the free energies of activation of the thermochromic transformations of spiropyrans determined by dynamic NMR spectroscopy are insensitive to variations in the length of the indicated bond.

Luminescence-spectral characteristics and structure of 1-hydroxyphenyl-substituted 2,4,6-triphenylpyridinium perchlorates

Conclusions1.A significant difference was found in the luminescence characteristics of 1-(2-hydroxyphenyl)- and 1-(4-hydroxyphenyl)-2,4,6-triphenylpyridinium perchlorates at various temperatures. At 293°K the luminescence of both compounds in the crystalline state and in solutions is fluorescence with an anomalously large Stokes shift; at 77°K it is fluorescence for the second compound and long-lived phosphorescence for the first.2.Comparison of the luminescence-spectral characteristics of these compounds with their structure and with the luminescence characteristics of model compounds made it possible to attribute the appearance of the anomalously large Stokes shift of the fluorescence to changes in the molecular structure in the excited state, due to flattening of the molecule as a result of rotation of the N-aryl fragment about the N-C bond.

Photochemical properties and structures ofN-amino andN-azomethine derivatives of 2,4,6-triphenylpyridinium perchlorates

The molecular structures of the initial compounds and the products of photocyclization involving amino and azomethine groups in perchlorates ofN-amino (1) andN-azomethine (2) derivatives of 2,4,6-triphenylpyridinium cations were studied. Cations1 and2 have an essentially non-coplanar arrangement of the α-Ph rings, and the product of photocyclization of2, cation3, is characterized by a flattened structure. It was found that the orientation of the lone electron pair of the N atom with respect to one of the planes of the α-Ph rings in molecule1 is more favorable for monocyclization involving the amino group. The structure of molecule2 allows two routes of the photoreaction,viz., O → N proton transfer and monocyclization with an α-Ph ring. However, the structural features of molecule2 are more favorable for photocyclization.

Photochemistry of 2-amino-2H-benzochromenes

Diabatic photoinduced ring opening of the chromene ring of 2-amino-5,6-benzo-2H-chromenes and 2-amino-6-methyl-7,8-benzo-2H-chromenes leads, in the same way as a thermally induced reaction, to the establishment of ring-chain tautomer equilibrium in the ground electronic state, the position of this equilibrium depending on the polarity of the solvent, the temperature, and structural factors. o-Quinoid tautomers exist as several stable isomeric forms: an acoplanar cis-S-cis-trans-form absorbing in the shortwave region, and S-trans-isomers absorbing in the longwave region of the spectrum. Photoexcitation of the o-quinoid forms in the temperature range 125–190 K initiates mutual conversions of these and the initial 2H-chromene structure. The relative stability of the conformers of the o-quinoid form depends on steric and electronic factors: benzanellation in positions 5,6 leads to stabilization of the cis-S-cis-trans-isomer and in positions 7,8 the S-trans-isomers are more preferred: π-acceptor substituents on the amine component increase the stability of the cis-S-cis-trans isomer and electron-donor substituents stabilize the S-trans-isomer.

Mechanism of the photoreactions of fulgides based on 1,2-dimethyl-3-formylindole

Only E ⇄ Z isomerization is observed for the steady irradiation of fulgides based on 1,2-dimethyl-3-formylindole in contrast to the analogs based on 3-acetylindole. The formation of cyclic photoisomers is registered by the method of impulse photolysis. The presence of a methyl substituent in the position 2 of the indole portion leads to a significant increase in the stability of the photochromic system. The introduction of alternative reaction centers of cyclization in the form of bulky phenyl groups leads to the suppression of electrocyclization at the position 2 of the indole portion and the formation of dihydronaphthalene structures.

Energetics and structural mechanisms of photochemical processes in molecules of aldonitrone vinylogs

Depending on the multiplicity of the excited state, alternative photoreactions are realized in methoxy and acetoxy derivatives of aldonitrone vinylogs: diabatic nitrone-oxazypyridone rearrangement in the first excited singlet state and E-Z isomerization involving the C=N bond in the triplet state. In the case of aldonitrone vinylogs with an o-hydroxy group in the α-aryl ring one observed adiabatic proton transfer from the hydroxy group to the azotoxido group with a subsequent diabatic reaction involving cyclization to the 2H-chromene structure. The same transfer is also realized in the triplet state. The quantum yields were determined, and the energies of activation of the photoreactions and the reverse thermal processes were evaluated.

Structure and photochromic transformations of fulgides of the indole series

Previously undescribed fulgides of the indole series, viz., (1-methyl-3-indolylmethylene)- and (1-methyl-3-indolylethylidene) (isopropylidene)succinic anhydride, were synthesized. A Z conformation for (1-methyl-3-indolylmethylene) (isopropylidene)succinic anhydride and an E configuration for (1-methyl-3-indolylethylidene)-(isopropylidene) succinic anhydride were established by electronic and PMR spectroscopy and x-ray diffraction analysis. The photochromic properties of fulgides were observed and investigated. A scheme of photoinduced and thermal transformations of fulgides that includes a step involving Z → E isomerization in the excited state is proposed. An additional photoisomer was recorded by pulse photolysis and fluorescence.

Synthesis and spectral properties of furyl-substituted pyridines and pyrylium and pyridinium salts

Abstract2,6-Diphenyl-4-(5-R-2-furyl) pyrylium and 2,4-diphenyl-6-(5-bromo-2-furyl) pyrylium perchlorates and the corresponding pyridines and 1-methylpyridinium perchlorates were synthesized. On the basis of the UV and PMR spectra it was concluded that there is significant electronic interaction of the furylium substituent with the pyrylium and pyridinium cations.