J. Phys. Chem. 1996, 100, 13381-13385
13381
Ground- and Excited-State Electron Donor-Acceptor (EDA) Complexes from (Dibenzoylmethanato)boron Difluoride and Substituted Benzenes: Their Relation to the Reaction Mechanism Yuan L. Chow,* Carl I. Johansson, and Zhong-Li Liu† Department of Chemistry, Simon Fraser UniVersity, Burnaby, British Columbia, Canada, V5A 1S6 ReceiVed: April 3, 1996X
The ground-state EDA complex of (dibenzoylmethanato)boron difluoride (DBMBF2) with substituted benzenes (SB) was studied by absorption spectroscopy to determine the equilibrium constant and enthalpy of the molecular association process in acetonitrile and cyclohexane. Excitation of the EDA complexes showed the corresponding exciplex fluorescence due to excitation of DBMBF2 in the presence of SB. Integration of these data with the previously published kinetics and thermodynamics of the exciplex formation and decay showed that both types of excitations converged on the exciplex energy well and that the former excitation goes through the Franck-Condon state, *(A-D+)v, which is different in molecular distance and electron and energy distributions. In acetonitrile, the discrepancy of fluorescence intensities from two excitations is taken as proof for electron transfer at the encounter pair on the energy surface to generate radical ions. This also implies that excitation of EDA complexes in acetonitrile forms the corresponding exciplexes directly (without the intermediacy of the encounter pair), which undergo solvolysis to give free ions. A mechanism is proposed to represent the observed photochemistry and photophysics.
Introduction Boron difluorides derived from 1,3-diketones interact from their singlet excited state with olefins and substituted benzenes (SB) in divergent ways. With simple olefins, enones, and unsaturated esters they undergo cycloaddition to give (after rearrangement) 1,5-diketones,1,2 but with electron-rich dienes they undergo electron transfer to generate transient diene cation radicals1a,2a that dimerize or rearrange; in both cases the quantum efficiency ranges from fair to very good except in reactions involving hindered and electron-poor π-bonds.1c These singletexcited BF2 compounds also interact with SB(dD) to give primarily fluorescent exciplexes3,4 and slow cycloadditions.1b,d As these photoreactions are synthetically useful, (dibenzoylmethanato)boron difluoride5 (DBMBF2) was used as a model to study the reaction mechanism, which was facilitated by its fluorescence. We have described the exciplex electronic structure,4a the formation kinetics,4b and the solvent effects on the product pattern.2b These works have lead to a scheme to explain the reaction dichotomy of *DBMBF2(d*A) with substrates (e.g., photocycloaddition and cation radical reactions) in acetonitrile. The partition occurs4 at the encounter pair *A(S)D, which collapses to the exciplex *(AD) or undergoes electron transfer to give a solvent-separated radical ion pair A-(S)D+. Since substrate concentrations are relatively high in preparative photochemistry, the presence of ground-state electron donor-acceptor (EDA) complexes5a,6 has to be considered in an overall mechanism. In this paper we wish to describe the ground-state EDA complex between DBMBF2 and SB and to relate their excitation processes to the dynamics of the exciplex formation and decay in solution. For clarity, exciplexes and excited EDA complexes refer to different species here. Results On admixing colorless acetonitrile solutions of DBMBF2 (9 mM) and benzene ( 0.05 M and [DBMBF2] ) 0.009 M; DA is the molar absorption coefficient of EDA at the monitoring wavelength (430 and/or 450 nm in the present cases). Substrate concentrations were adjusted to yield an EDA absorbance of 0.2-0.8. Typical examples of plots according to eq 1 are shown in Figure 2; Ka (dy-intercept/slope) determined at two different wavelengths was averaged and is listed in Table 1. The saturation range, i.e., the fraction of DBMBF2 tied up in EDA, was 0.01-0.07. These Ka-values should be regarded only as order of magnitude values for reasons to be discussed later.
[A]/Abs. ) (KaDA[DA])-1 + DA-1
(1)
ln ∆Abs. ) -∆Ha/RT + constant
(2)
The heat of EDA formation (enthalpy) representing binding energy of the association6,9 was determined from the change of absorbance as a function of temperature, as expressed in eq 2; the slope of such a plot (Figure 3) gave the enthalpy of EDA formation, -∆Ha. This method had the advantage of using a single solution without the need to known the exact concentrations. The concentration was corrected for solvent density changes with temperature. The measurement in cyclohexane was limited to a few cases here by a low DBMBF2 solubility (
