EXCITED-STATES OF THE PHTHALIMIDE CHROMOPHORE AND THEIR EXCITON COUPLINGS - A TOOL FOR STEREOCHEMICAL ASSIGNMENTS

The electronically excited states of the phthalimide chromophore have been studied by means of linear dichroism (LD) of samples partially or iented in poly(vinyl alcohol) films, magnetic circular dichroism (MCD) , and circular dichroism (CD) spectroscopy. On the basis of the LD mea surements, the low-energy tail (340-320 nm) of the first absorption ba nd is assigned to an out-of-plane polarized pi-->pi transition (I). A t higher energy, the electronic spectrum is resolved into contribution s from five pi-->pi transitions: II(300 nm, long-axis polarized), III (275 nm, short-axis polarized), IV (235 nm, short-axis polarized), V (220 nm, long-axis polarized), and VI (similar to 210 nm, short-axis p olarized). The results from semiempirical (INDO/S-CI) and ab initio (C IS/6-31+G(d)) MO calculations compare well with the proposed assignmen ts of the excited states. Degenerate exciton interaction between elect ric-dipole-allowed transitions of two phthalimide chromophores is obse rved in the electronic absorption spectra of the achiral bis-phthalimi des 2a-c and in the CD spectrum of the chiral bis-phthalimide 3a. For the latter compound, the solid-state geometry has been determined by X -ray diffraction analysis. Good agreement between experimental and com puted CD spectra confirms that the coupled-oscillator exciton model pr ovides the basis for a reliable nonempirical method for the assignment of absolute configuration for this class of compounds. Nondegenerate exciton coupling between phthalimide and benzoate or phenyl chromophor es is born out in the CD spectra of homochiral molecules 3c and 3d wit h the rigid cyclohexane skeleton. Finally, the exciton coupling method is used to make stereochemical assignments for the acyclic, conformat ionally flexible derivatives 4a-c and 5b.