Solvent effects on photocycloadditions of dibenzoylmethanatoboron difluoride with enol ethers

Dibenzomethanatoboron difluoride (DBMBF2) interacted with alkylbenzenes from its singlet excited state to form exciplexes ranging from weak polarity up to contact radical ion pairs (CRIP); this exciplex series shows the characteristics in the Marcus "normal" region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (phi(infinity)ex). The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums (vmax) was used to estimate the coefficient ("a" and "c") of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and phi(infinity)ex of these exciplexes, the radiative (k(ex)f) and nonradiative (k(ex)NR) rate constants were calculated. The former k(ex)f were also computed from a semi-empirical approach based on the assumption that the exciplex wavefunction could be adequately described by CT and LE states, and that *DBMBF2 primarily contributes to the probability of exciplex emission. Two results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k(ex)r and k(ex)NR (or their logarithmic value) against the LE contribution (c2) and transition energy gaps (hvmax) afford better correlation than those against -deltaG(-et). This indicates the role played by the LE contribution in generating the stabilization energy (U(s)) in these exciplexes through the A-D+)<==> *AD) resonance interaction; U(s), in turn, modifies -deltaG(-et) to afford the decay driving force hvmax. Also, those plots from k(ex)f values (being determined directly from experiments) show better correlation than those from k(ex)NR. In contrast to the CRIP type exciplexes in the Marcus "inverted" region, these k(ex)f and k(ex)NR increase in the common trend with increasing transition energy gaps. The k(ex)NR plots show less steep slopes and attains more quickly a minimum toward the CRIP region; the latter is identified as the turning point from the "normal" to "inverted" region. Both the attenuation and reversal of the k(ex)NR value with increasing polarity are believed to be generated by the emerging contribution of the intersystem crossing process as an additional nonradiative process, which is induced by the increased spin-orbit coupling in highly polar exciplexes.

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The Structure and Decay Dynamics of Exciplexes Derived from Dibenzoylmethanatoboron Difluoride and Alkylbenzenes in Cyclohexane

Chow

Liu

Johansson

et al. 2000

Chemistry A European J

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1,3,2‐Dioxaborines as potential components in advanced materials—a theoretical study on electron affinity

Fabian

Hartmann

2004

J of Physical Organic Chem

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Abstract1,3,2‐Dioxaborines with fluorine and other substituents at boron were calculated by first‐principles methods based on density functional theory (DFT: B3‐LYP) or many‐body perturbation theory at the second order [MBPT(2): MP2]. 1,3,2‐Dioxaborines are derivatives of 1,3‐dicarbonyl compounds also known as 1,3‐diketoborates. According to quantum chemical calculations, 2,2‐difluoro‐1,3,2‐dioxaborines are of puckered structure with a low barrier to inversion. The calculated charge distribution does not reflect well the traditional formula description. The boron atom carries a positive rather than a negative charge. Some compounds have a pronounced zwitterionic character accompanied by relatively high dipole moments. The calculated positive electron affinities (EAs) classify 1,3,2‐dioxaborines as organic electron‐acceptor compounds. Depending on the type of substitution, the EAs vary between about 0.5 and 3.5 eV (DFT calculations). The EAs of various substituted 1,3,2‐dioxaborines are of the same order of magnitude as those of quinones and close to electron affinities of strong organic acceptor compounds such as tetracyanoethylene (TCNE). Numerical predictions are verified by comparison with EAs of a series of well‐known medium‐sized organic compounds studied experimentally and theoretically at the same level of theory. In good agreement with results reported for other series of compounds, the average absolute error between theoretical and experimental EAs was 0.19 eV. Because of the ability of 3,5‐diaryl‐2,2‐difluoro‐1,3,2‐dioxaboranes to fluoresce efficiently and to accept electrons easily, these compounds are potential candidates for future applications. Copyright © 2004 John Wiley & Sons, Ltd.

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A novel photorearrangement of 1,3-diketonatoborinate: a photoinduced intramolecular alkylation on a conjugated carbonyl system

Ishiyama¹,

Chow²

1999

Can. J. Chem.

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Irradiation of borinates derived from 9-borabicyclo[3,3,1]nonane and 1,3-diketones at their π-π* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the BC bonds followed by a 1,3- or 1,5-migration and the formation of a more stable BO bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4-addition of an alkyl-boron group to an α,β-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the BC bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.Key words: borinate photochemistry, charge transfer absorption, alkylborane addition, boronate intermediates, BC bond homolytic scission

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Solvent effects on photocycloadditions of dibenzoylmethanatoboron difluoride with enol ethers

Cited by 4 publications

References 12 publications

The Structure and Decay Dynamics of Exciplexes Derived from Dibenzoylmethanatoboron Difluoride and Alkylbenzenes in Cyclohexane

The Structure and Decay Dynamics of Exciplexes Derived from Dibenzoylmethanatoboron Difluoride and Alkylbenzenes in Cyclohexane

1,3,2‐Dioxaborines as potential components in advanced materials—a theoretical study on electron affinity

A novel photorearrangement of 1,3-diketonatoborinate: a photoinduced intramolecular alkylation on a conjugated carbonyl system

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