Ground- and Excited-State Electron Donor−Acceptor (EDA) Complexes from (Dibenzoylmethanato)boron Difluoride and Substituted Benzenes:  Their Relation to the Reaction Mechanism

Abstract

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.

Exciplexes or ground state complexes of (dibenzoylmethanato)boron difluoride and benzene derivatives? A study of their optical properties revisited via liquid state investigations and structure calculations

“…With trans-anethole, the absorption spectrum of DBMBF2 develops a new absorption which extends beyond the longest wavelength edge from 420 to 520 nm [4]. This new absorption also appears, though in a lesser extent over 420-435 nm (~820 cm -1 ), in ternary systems with DBMBF2, p-xylene and ethylene diamine (EDTA) in acetonitrile [22] and was assigned to a ground state charge transfer complex of DBMBF2:p-xylene stabilized in the presence of EDTA. In this last case, the red extension of the DBMBF2 absorption band appears to us very similar to the one we presently observe over 390-400 nm (~640 cm -1 ) for DBMBF2 in cyclohexane upon addition of benzene.…”

Vibronic bandshape of the absorption spectra of dibenzoylmethanatoboron difluoride derivatives: analysis based on ab initio calculations

“…Recently, both time-dependent and time-independent approaches to calculations of vibronic absorption and emission band shapes have been developed. [37][38][39][40][41][42][43] A typical representative of this class of compounds, dibenzoylmethanatoboron difluoride (DBMBF 2 ), its derivatives, and complexes with some aromatic compounds were studied experimentally in detail 35,37,42 with the aim of using DBMBF 2 in optical chemical sensors for various aromatic compounds. In optical chemical sensors, the absorption or emission bandshape can serve as an analytical signal responding to the interaction of the chromophore with analytes or environment.…”

X-ray crystallographic study of three (N→B)-borinates prepared from 8-hydroxyquinoline and 2-hydroxypyridine

“…Complexes with an extended re-electron system in the coordinated ligand are often colored and permit the quantitative determination of the chelate components [17][18][19][20]. They have interesting physical properties such as fluorescence [21,22], barrier crossing [23,24], photoconductive charge transfer [25][26][27], electron acceptance in the photoexcited state [21,22], and second hamaonic generation [28]. $ [2] 6a [7]: R = (C~)3NI-13*OAc" 7a 171: R' = H. R" = H 6b [7]: R = (CH2)aN=C(C~) 2 7bPl: R' = CI~COOH So far little is known about the structure and stability of borinic esters obtained with azaarene alcohol derivatives.…”