J. Phys. Chem. 1995,99, 17558-17565
17558
Exciplexes of (Dibenzoy1methanato)borodBenzenes: The Control of Exciplex Electronic Structure Yuan L. Chow* and Carl I. Johansson Department of Chemistry, Simon Fraser University, Bumaby, BC, Canada V5A IS6 Received: July 11, I995@ Singlet excited (dibenzoy1methanato)boron difluoride ((DBM)BFz) interacted with a series of substituted benzenes (SB) to give strongly fluorescent exciplexes, which exhibited a wide range of emission maxima, hvmax(5500 cm-I), in cyclohexane depending on SB electron-donating ability. Exciplex dipole moments were determined by solvatochromic shifts, which ranged from the limiting value of 13.8 for the contact radical ion pair (CRIP) to 1.1 D; these were correlated with the oxidation potential difference between (DBM)BF2 and SB. These observations were explained in terms of relative contributions of charge transfer (IA-D')) and locally excited (I*AD)) states. The formation of these exciplexes are shown to be driven by an I*AD) resonance electrochemical force from charge transfer as well as the stabilization energy from 1A-D') interaction. The latter is shown to be the highest at the vicinity of a lowest HOMO-HOMO energy gap and becomes negligible at values greater than 0.7 eV, which can be shown from calculations. A plot of fluorescence maxima against the redox potential differences reveals electrochemical driving force, stabilization energy, and I *AD)-dominated emission. The exciplex formation pattern is also rationalized with the frontier orbital theory.
Introduction The boron difluoride complexes of 1,3-diketones have been shown to react by dual pathways with a variety of olefins and arenes from their singlet excited state; in general, they tend to react with poor electon donating substrates by photocycloaddition, but by electron transfer with good electron donors.' Among them, (dibenzoy1methanato)boron difluoride ((DBM)BF2) shows weak addition reactivity toward substituted benzene (SB), but forms a fluorescent excimer with itself and exciplexes with these arenes in acetonitrile.',2 Evidence exists that these exciplexes are key transients that control the fate of photoreactions as proposed in recent publications.2 A critical question is whether the exciplex is the intermediate for both pathways. To unravel the driving force controlling the dual photoreactivity, it is necessary to clarify the nature of these *(DBM)BFz/SB exciplexes. Taking advantage of the strong *(DBM)BFz/SB exciplex fluorescence, we have investigated their electronic structure. These results will be eventually used to rationalize the dual photochemical reactions. In this paper, we wish to report the first part of a larger study on the electronic properties of *(DBM)BFdSB ex~iplexes.~ On the basis of Mulliken's valence bond description of donor-acceptor interaction: the investigation of charge transfer (CT) exciplexes was carried out by Weller et al.,5 Mataga et al.,6 and other group^,^.^ where the exciplex electronic structure was described as a linear combination of CT (IA-D')) and locally excited (LE) states (I*AD)) as shown in eq 1 ignoring insignificant contributions of other statesS9 The coefficients a and c are the key indicators of the exciplex properties and are related by the normalization condition: a2 c2 2acS = 1. The state overlap integral, S = (*ADIA-Df), in exciplexes is fairly small and can be neglected, i.e., a2 c2 = l.9a,c
+ + +
@
Abstract published in Advance ACS Abstracts, November 15, 1995.
The coefficients a and c can be estimated theoretically and compared to experimentally determined values from exciplex dipole moments be,)by eq lb, where ~ C R I Pis the hypothetical contact radical ion pair (CRIP) from full one-electron transfer (Le.,a = 1 and c = O).9c The present (DBM)BF2 exciplex series is characterized by a wide variation in the coefficients and provides a model for the study. For high-CT exciplexes (a >> c), in nonpolar solvents, Weller's group has shown empirical relationshipsof the exciplex emission maximum (hv,,,) and enthalpy of dissociation (AHex) with respect to the donor-acceptor's redox potentials (E,, and &d), as in eq 2.5k These equations are used in demonstrating high-CT exciplexes close to CRIP characters and estimating their enthalpy values. For exciplex dipole moments much lower than PCRIP, their hv,,, is smaller than the CRIP correlation by the amount of U S ,owing to the stabilization arising from IA-D+) I*AD) resonance interaction^.^^^^
-
AHe, =E,, - Ered- E, hv,,,
+ 0.13 eV
(24
= E,, - Ere*- 0.15 - Us eV
where hv,,,(CRIP)
= E,, - Ered- 0.15 eV
(2b)
Experimental Section Materials. (DBM)BFz was prepared using a published procedure and recrystallized twice from acetonitrile, giving yellow needles: mp 191-193 "C (uncorrected; lit. 193-194 "C, 197 oC).2a Solvents were of spectrograde quality and used without further purification: acetonitrile, carbon tetrachloride (BDH), chloroform (Fisher), dichloromethane, and cyclohexane (Mallicrodkt). Benzene donors were either distilled, sublimed, or recrystallized from acetonitrile: cyanobenzene, methyl benzoate, chlorobenzene, xylenes, mesitylene (MCB), benzene, toluene (Fisher), pentamethylbenzene (PMB), 1,2,4-trimethylbenzene (1,2,4-TriMB), 1,2,3,4-tetramethylbenzene(1,2,3,4-
0022-365419512099-17558$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 49, 1995 17559
Exciplexes of (Dibenzoy1methanato)borodBenzenes TetraMB), 1,2,3,5-tetramethylbenzene(1,2,3,5-tetrW), durene (Aldrich), and hexamethylbenzene (HMB, Kodak). Instrumentation and Procedures. Fluorescence emission and excitation spectra were recorded at ambient temperatures (23 f 1 "C) on either a PTI LS-100 fluorimeter (corrected) or a Perkin-Elmer MPF-44B (uncorrected); reported emission A,,,= were obtained from spectra recorded on the LS-100. The PTI LS-100 was corrected according to a procedure recommended by the manufacturer. Solutions of (DBM)BF2, (1-3) x M, were purged with argon for 7 min in a 1 cm path length fluorimeter cell. Poorly resolved *(DBM)BF2 and exciplex emission profiles were separated from the total emission by a procedure discussed by Walker et al.IO and detailed in the thesis3 The selfconsistency of this method was checked by dividing the subtracted *(DBM)BF2 emission (IA) by *(DBM)BF2 emission in the absence of quencher (PA); the I/PAratio must be constant over the entire wavelength. An incomplete resolution will be evident from a wavelength dependence on the IA/PAratio. A second method of Walker et al. utilized the exciplex emission profile obtained at high donor concentrations (negligible parent emission) as the basis to resolve the total emission profile at low quencher concentrations. This method was used previously?b This method cannot be used since the (DBM)BF2/SB exciplex emission profile shows bathochromatic shift at higher benzene concentrations.3 (DBM)BF2's fluorescence quantum yield in cyclohexane was determined according to the dilute method and is described elsewhere. I The fluorescence standard was anthracene (Eastman-Kodak), which was chosen on the basis that (i) its fluorescence emission occurred in the same spectral region as that of (DBM)BF2's, (ii) its fluorescence yield in cyclohexane (0.25, aerated solution) is known, and (iii) the same excitation wavelength could be used (366 nm) for both.I2 Cyclic Voltametry. Cyclic voltammetry was performed using a three-electrode arrangement with a SCE reference electrode (Fisher), a stationary Pt working electrode (surface area 0.010 cm2), and a R counter electrode. The electronics included a Princeton Applied Research (PAR) Model 173 programmer, a PAR Model 178 electrometer, and a PAR Model 173 potentiostat/galvanostat equipped with a PAR Model 179 digital coulometer that provided feedback compensation for ohmic drop between working and reference electrodes. Cyclic voltammograms were recorded on a Allen recorder. The electrochemical cell was based on the design of Kissinger and Heinemann, but adapted with a Luggin-Haber tube to reduce the ohmic drop. Acetonitrile was refluxed and distilled over CaH2 under argon. Concentration of the electroactive species was 2-3 mM. The electrolyte was tetraethylammonium perchlorate (GFS Chemicals, support electrolyte grade; 0.1 M). The reported redox potentials were calibrated with respect to the ferrocene/ferrocenium couple. The experimental range of the apparatus was determined, in the absence of an electroactive species, to be -2 to +3 V (100 mV/s).
Results Singlet excited (DBM)BF2 interacts with SB to form exciplexes that fluoresce with fair quantum efficiency even in polar acetonitrile. In less polar solvents *(DBM)BF2 exhibited a typical red-shifted broad exciplex fluorescence band with higher intensities. The exciplex emission band with SB of lower oxidation potentials was better separated from the parent *(DBM)BF2 emission. For poor electron donors (D = SB), these two fluorescence bands were extensively overlapped, as shown in Figure 1, and the exciplex emission intensities had to
370
I
I
I
I
I
470
570
370
470
570
Wavelength (nm)
um" m
M) fluorescence spectra by SB: (top left) by o-xylene at the concentrations of (a) 0, (b) 0.05, (c) 0.1 1, (d) 0.15, and (e) 0.21 M; (top right) by 1,2,3,4-tetramethylbenzene at (a) 0, (b) 0.04, (c) 0.09, (d) 0.13, and (e) 0.20 M; (bottom) by cyanobenzene at 0.285 M.
Figure 1. Quenching of (DBM)BF2 (1 x
be calculated by a proportionality technique (see the Experimental Section). It should be noted that these exciplex fluorescence spectra from poor electron donors (cyanobenzene) displayed vibrational structures similar to that of *(DBM)BF2 (see Figure 1). The spectra in Figure 1 showed concentration dependence and isoemissive points that were typical of exciplex formations. The excitation spectra monitored at the (DBM)BF2 fluorescence peak at 398 nm and that of the exciplex band at 550 nm were nearly identical, indicating the latter emission was indeed derived from *(DBM)BF2. The study3by W-visible spectroscopy showed that (DBM)BF2 and SB formed weak electron donor-acceptor complexes (EDA) with association constants Ka < 0.15 M-' (in acetonitrile), as determined by the Benesi-Hildebrand method; with xylenes, toluene, and benzene, their Ka's were too small to be measured. At the low concentrations of (DBM)BF2 (< M) and with SB used for exciplex studies, the percentage of EDA complex formed relative to the initial (DBM)BF2 concentration was less than 1%, hence too small to affect the kinetics of dynamic exciplex formations. It was also obvious that at [(DBM)BF2] < M, its excimeSa could not come into play in mechanistic considerations. In short, the *(DBM)BF2 quenching by SB was the sole source of the exciplex formation in this investigation. The exciplex fluorescence intensity was surprisingly strong in cyclohexane and became weaker in more polar solvents; in acetonitrile, exciplex emission could only be detected for those derived from weak electron donating benzene^.^ The fluorescence quantum yield of *(DBM)BF2/SB exciplexes (QP,",) in cyclohexane at the total quenching of *(DBM)BF2 was determined according to eq 3:
@ : =
