Charge-transfer excitation: unconventional yet practical means for controlling stereoselectivity in asymmetric photoreactions

In chiral donor-acceptor (D-A) systems, irradiation wavelength plays vital roles in determining the photochemical consequences. Selective excitation of a D-A complex at the charge-transfer (C-T) band affords an excited C-T complex (ECT), while the local-band excitation of D or A may lead to the formation of a conventional exciplex (EX) upon subsequent interaction with the D-A partner. These two excited species, generated from the same D-A pair, may be categorized formally as excited complexes or exciplexes, but should be distinguished, provided that they significantly differ in structure and reactivity. Indeed, ECT and EX exhibit distinctly different temperature-dependent photophysical and photochemical behaviours, which are assignable to the differences in relative stability, conformational flexibility and/or solvation properties. Fine-tuning excitation wavelength further enabled us to discriminate stereoisomeric intramolecular C-T complexes through preferential excitation, as C-T complexes are generally composed of an ensemble of various geometries. Besides temperature and solvent polarity, the excitation wavelength was shown to be employed as an unconventional yet practical tool for critically controlling the chemo-, regio- and stereoselectivities in molecular and supramolecular photochemistry. This review article summarizes recent advances in the asymmetric photoreactions of C-T complexes with an emphasis on the effect of excitation wavelength.