A STEADY-STATE AND TIME-RESOLVED FLUORESCENCE STUDY OF QUENCHING REACTIONS OF ANTHRACENE AND 1,2-BENZANTHRACENE BY CARBON TETRABROMIDE AND BROMOETHANE IN SUPERCRITICAL CARBON-DIOXIDE
Jw. Zhang et al., A STEADY-STATE AND TIME-RESOLVED FLUORESCENCE STUDY OF QUENCHING REACTIONS OF ANTHRACENE AND 1,2-BENZANTHRACENE BY CARBON TETRABROMIDE AND BROMOETHANE IN SUPERCRITICAL CARBON-DIOXIDE, Journal of the American Chemical Society, 119(42), 1997, pp. 9980-9991
This paper reports on the solvent effect on energy transfer reactions
in supercritical CO2. The energy transfer reactions are studied by ste
ady-state and time-resolved fluorescence spectroscopy and the fluoroph
or/quencher reaction pairs are chosen to vary the reactions from diffu
sion-controlled to kinetically-controlled. In particular, the fluoresc
ence quenching of anthracene by CBr4, 1,2-benzanthracene by CBr4, and
anthracene by C2H5Br in supercritical CO2 at 35 degrees C has been rep
orted. Experimental rate constants for the first two reaction pairs, a
nthracene/CBr4 and 1,2-benzanthracene/CBr4, follow the predicted diffu
sion control Limit at all pressures from 77.9 to 160.6 bar, indicating
that local solvation does not enhance the reaction rate nor substanti
ally impede the diffusion process in supercritical CO2. The rate const
ants for the third reaction, the quenching of anthracene by C2H5Br, ar
e several orders of magnitude below the diffusion control limit, indic
ating that the reaction is kinetically controlled, as it is in liquids
. Ln supercritical CO2 the apparent rate constants (i.e., those based
on bulk concentrations of the reactants) for the anthracene/C2H5Br rea
ction decrease dramatically with increasing pressure. We believe that
this apparently large pressure effect on the reaction rate is primaril
y due to the local composition enhancement of the quencher molecules a
round the dilute anthracene solute. This analysis is supported by fluo
rescence spectra and solvatochromic shift data of anthracene in pure C
O2, and in mixtures of CO2 with C2H5Br at 35 degrees C that indicate b
oth local density augmentation and local composition increases around
the anthracene.