Alexei V. Trofimov

Chemically Initiated Electron Exchange Luminescence of Silyloxyaryl‐Substituted Spiroadamantyl Dioxetanes: Kinetics and Excited State Yields

Abstract— The kinetics of the fluoride‐induced decomposition of the thermally stable silyloxyaryl‐substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated. Two limiting kinetic regimes flash and glow have been identified, which depend on the fluoride concentration, the first at high, the second at low [F‐] triggering, whose detailed kinetic analysis affords the rate constants for the deprotected dioxetanes 2a,b cleavage in acetonitrile and dimethyl sulfoxide and chemiluminescence measurements the CIEEL and phen‐olate 4 (CIEEL emitter) excitation yields. Chloro‐substi‐tution in the spiroadamantyl dioxetane does not affect the deprotection step k2 but leads to a ca five‐fold faster cleavage of the deprotected dioxetane 2, while the chemiexcitation yield is the same for both dioxetanes. The energies of the first excited singlet and triplet states of the emitting phenolate 4 were estimated by AM1 configuration interaction calculations with explicit consideration of acetonitrile as solvent (self‐consistent reaction field approach). The first excited singlet and triplet state of the CIEEL emitter phenolate 4 possess π,π* character, as suggested by the π‐type molecular orbitals and the large singlet‐triplet energy gap. The chemiexcitation of both singlet and triplet states of the excited phenolate 4 is feasible during the dioxetanes 1a,b cleavage, but the experimentally determined high singlet excitation yields suggest that preferentially the phenolate 4 singlet state is populated in the fluoride ion‐triggered CIEEL process.

ENERGY TRANSFER-ENHANCED CHEMILUMINESCENCE OF ADAMANTANONE (n,π*) AND ESTER (π,π*) SINGLET AND TRIPLET EXCITED STATES IN THE THERMOLYSIS OF SILYLOXYARYL-SUBSTITUTED SPIROADAMANTYL DIOXETANES†

Abstract—

Stereoselective formation of inverted housane in the denitrogenation of the diazenyl diradical photolytically derived from a stereolabeled diazabicyclo[2,2,1]hept-2-ene with bridgehead substituents as a function of solvent- and temperature-varied viscosity

Abstract It has been experimentally demonstrated that viscosity differentiates the extent to which inverted and retained housanes are formed in the liquid-phase denitrogenation of a substituted diazabicyclo[2,2,1]hept-2-enes (DBH)-type azoalkane, namely syn -2,3-diaza-4,7-dimethyl-1-ethyl bicyclo[2,2,1]heptene. The fact that the viscosity profiles coincide for the inversion/retention product ratio, assessed for a set of diverse solvents at constant temperature and through the temperature variation in a single solvent ( n -butanol), shows that the temperature dependence of the stereoselectivity is controlled by bulk viscosity effects. The simple free-volume model rationalizes the viscosity behavior of the inversion/retention ratio. Since the viscous medium differentiates between the inversion and retention reaction channels, we conclude that an unsymmetrical, nitrogen-containing intermediate, presumably the singlet diazenyl diradical, intervenes in the liquid-phase deazetation process.

Solvent effects in the photodenitrogenation of the azoalkane diazabicyclo[2.2.1]hept-2-ene: viscosity- and polarity-controlled stereoselectivity in housane formation from the diazenyl diradical

The stereoselectivity in the liquid-phase photolysis of the diazabicyclo[2.2.1]hept-2-ene (DBH), as expressed in terms of the kinv/kret ratio for the formation of the inverted [2(inv)] versus retained [2(ret)] housanes, depends on the viscosity and bulk polarity of the medium, while hydrogen bonding has little effect on the stereodifferentiation. The similar viscosity profiles of the kinv/kret ratio in alcohols and in isooctane/nujol mixtures manifest the same free-volume requirements and thereby reflect similar frictional impositions on the stereoselective inversion for the DBH photolysis in these solvents. The Onsager bulk-polarity parameter (e − 1)/(2e + 1) accounts adequately for the observed solvent influence. This fact implies that stabilization of the dipole moment in the diazenyl diradical 1DZ retards the inversion process during the denitrogenation.

Pressure dependence of the stereoselectivity in the photodenitrogenation of diazabicyclo[2.2.1]hept-2-ene in supercritical fluids: evidence for the diazenyl diradical

Abstract The present study shows that the preference for the stereoselective inversion upon the photolytic nitrogen extrusion from the stereolabeled DBH derivative exo -d 2 -diazabicyclo[2.2.1]heptene ( 1 ) in supercritical media (sc-CO 2 , sc-C 2 H 6 ) depends on pressure. The increase of pressure up to 200 bar leads to ca. 2.3-fold decrease of the stereoselectivity ( k inv / k ret ratio) in sc-CO 2 and sc-C 2 H 6 . Analysis of the observed pressure dependence in terms of collisional (self-diffusion coefficient) and frictional (viscosity) effects discloses that frictional impositions by the fluid media account best for the experimental observations reported herein. This pressure-dependent viscosity effect is consistent with a stepwise denitrogenation mechanism for the photolysis of DBH in the supercritical phase, which proceeds through an unsymmetrical, nitrogen-containing transient, namely the singlet diazenyl diradical.

Structural dependence on the stereoselective formation of inverted housane in the viscosity-controlled photodenitrogenation of DBH-type azoalkanes

The influence of structural changes on the α parameter, which represents the effective volume involved in molecular reorganizations, has been determined through the viscosity dependence of the stereoselectivity in the photodenitrogenation of the DBH-type azoalkanes 1a–c. The structural variation reveals that the stereoselective formation of housane is not only subject to the frictional impositions exerted by the solvent molecules on the flap motion of the methylene bridge, but also on the displacement of the bridgehead substituents during the inversion process.

Viscosity-dependent diastereoselectivity and product selectivity in the photodenitrogenation of a spirocyclopropane-substituted, DBH-type azoalkane

In the photodenitrogenation of the spirocyclopropane-substituted azoalkane 1 in alcoholic solvents of various viscosity (from 0.6 to 89.2 cP) to the diastereomeric housanes 2 (major product) and bicyclo[3.2.0]heptenes 3 (minor product), the same (within experimental error) viscosity dependence is observed in the diastereoselectivity of the housane 2 formation as well as in the product selectivity (2/3). These viscosity effects corroborate the intermediacy of the diazenyl diradical 1DZ as a common branching point for the housane 2 and bicycloheptene 3 formation. In contrast, the diastereoselectivity of the bicycloheptene 3 generation is independent of viscosity, which implies that a nitrogen-free, symmetrical 1,4 diradical serves as precursor to the rearrangement products. The free-volume model of viscosity is employed to rationalize the product selectivity as well as the diastereoselectivity.