CHIRAL DISCRIMINATION IN ELECTRONIC ENERGY-TRANSFER PROCESSES IN SOLUTION - EFFECTS OF TEMPERATURE AND SOLUTION PROPERTIES ON CHIRALITY-DEPENDENT RATE PARAMETERS
Dp. Gloverfischer et al., CHIRAL DISCRIMINATION IN ELECTRONIC ENERGY-TRANSFER PROCESSES IN SOLUTION - EFFECTS OF TEMPERATURE AND SOLUTION PROPERTIES ON CHIRALITY-DEPENDENT RATE PARAMETERS, Chemical physics, 198(1-2), 1995, pp. 207-234
Time-resolved chiroptical luminescence (TR-CL) measurements are used t
o study the kinetics of chirality-dependent excited-state quenching pr
ocesses in solution. Solution samples contain a racemic mixture of chi
ral luminophore molecules (L) and a small, optically-resolved concentr
ation of chiral quencher molecules (Q) in an achiral solvent (H2O, D2O
, or an H2O-ethylene glycol mixture). The luminophores are excited wit
h a pulse of unpolarized light to create an initially racemic excited-
state population of Delta L and Delta L* enantiomers, and TR-CL measu
rements are then used to monitor the differential decay kinetics of th
e Delta L and Delta L* subpopulations. Observed differences between t
he Delta L and Delta L* decay kinetics reflect differential rate proc
esses and efficiencies for Delta L - Q versus Delta L* - Q quenching
actions, and they are diagnostic of chiral discriminatory interactions
between the luminophore and quencher molecules. In each of the system
s examined here, quenching occurs via electronic energy-transfer proce
sses in transient (Delta L - Q) and (Delta L* - Q) encounter complexe
s, and the chiral discriminatory rate parameters reflect the relative
stabilities and lifetimes of these complexes as well as their structur
es and internal (electronic and nuclear) dynamics. Both quenching rate
s and enantioselectivity are observed to increase with increasing temp
erature over the temperature ranges represented in this study (253-353
K for H2O-ethylene glycol solutions and 273-353 K for H2O and D2O sol
utions), and differences between the free-energies of activation for D
elta L - Q versus Delta L* - Q quenching processes increase from appr
oximate to 2.5 to 4.8 kJ/mol over the 273-353 K temperature range. Bot
h the luminophore and quencher molecules examined in this study have t
hree-bladed propeller-like structures with very similar shapes and siz
es, and differential steric constraints in (Delta L - Q) versus (Delt
a L - Q) contact pairs are small. However, at room temperature and ab
ove, the relative efficiencies of Delta L - Q versus Delta L* - Q que
nching processes differ by factors > 3.2, indicating an extraordinary
degree of chiral discrimination in the underlying interaction processe
s.