Abstract
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.
Highlights
An aromatic electron donor (D) and acceptor (A) often form a weakly interacting complex in the ground state[1,2,3] with accompanying new absorption at longer wavelengths assignable to the C-T band
The obvious dissimilarities in photochemical behavior observed upon local-band versus C-T excitation of several D–A systems prompted us to further explore spectroscopic investigations
Fluorescence spectra of a mixture of (E)-ST and (R)-F were measured at different excitation wavelengths (Fig. 7, left)
Summary
An aromatic electron donor (D) and acceptor (A) often form a weakly interacting complex in the ground state[1,2,3] with accompanying new absorption at longer wavelengths assignable to the C-T band. Despite the low effective concentration (as a consequence of the generally small association constants), the selective C-T excitation was readily achieved by carefully choosing the irradiation wavelength. The ECT species generated exhibits the photochirogenic and temperature-dependence behaviours significantly different from those displayed by the EX species. We will emphasize with ample examples that such a simple, readily attainable, change as excitation wavelength can dramatically alter the relative contribution of the ground- and excited-state conformer ensembles of a C-T complex, allowing us to favourably modulate the photochemical consequences. As for the longstanding debate on the identity of ECT, we will unambiguously demonstrate that ECT differs from EX both in structure and reactivity
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