Excited States of the Phthalimide Chromophore and Their Exciton Couplings:  A Tool for Stereochemical Assignments

Abstract

The electronically excited states of the phthalimide chromophore have been studied by means of linear dichroism (LD) of samples partially oriented in poly(vinyl alcohol) films, magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. On the basis of the LD measurements, the low-energy tail (340−320 nm) of the first absorption band is assigned to an out-of-plane polarized n→π* transition (I). At higher energy, the electronic spectrum is resolved into contributions from five π→π* 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 (∼210 nm, short-axis polarized). The results from semiempirical (INDO/S-CI) and ab initio (CIS/6-31+G(d)) MO calculations compare well with the proposed assignments of the excited states. Degenerate exciton interaction between electric-dipole-allowed transitions of two phthalimide chromophores is observed in the electronic absorption spectra of the achiral bis-phthalimides 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 computed CD spectra confirms that the coupled-oscillator exciton model provides 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 chromophores is born out in the CD spectra of homochiral molecules 3c and 3d with the rigid cyclohexane skeleton. Finally, the exciton coupling method is used to make stereochemical assignments for the acyclic, conformationally flexible derivatives 4a−c and 5b.

Local dipole interactions induce helicity of (S)-2-butyl-N-(1,8-naphthaloyl)-2-aminobenzoate molecules containing 1,8-naphthalimide rings utilized as a column building blocks: X-ray and quantum mechanical studies

“…Monoimides, like phtalimides and 1,8-naphtalimides, have been used successfully in crystal engineering [8] and as chromophores in stereochemical studies [9][10][11]. Most recently, it has been demonstrated that bichromophore consisting of the benzoate substituted in the ortho position with the naphthalimide group (the NAB chromophore) has the ability to act as a sensor of chirality of aliphatic secondary alcohols [12].…”

Chirality of aromatic bis-imides from their circular dichroism spectra

“…These include use as acceptors for rigid electron-transfer model systems, [1][2][3][4] as linkers in electron donor-acceptor molecules, [5][6][7] as constituents of threading intercalators for DNA, 8 molecules stabilizing oligonucleotide bulged duplex, 9 rigid molecular clefts, 10 macrocyclic receptor molecules, 11 electrically conducting dendrimers, 12 as well as UV absorbing agents in polymers. 13 Recently, we reported the study of electronic states and exciton coupling of aromatic monoimides phthalimides 14,15 and 1,8-naphthalimides. 16 These imides are considered stereochemical probes for nonchromophoric amino groups and have been used successfully for the determination of absolute configuration 14,15,17 and conformation 18 of chiral amines.…”

“…13 Recently, we reported the study of electronic states and exciton coupling of aromatic monoimides phthalimides 14,15 and 1,8-naphthalimides. 16 These imides are considered stereochemical probes for nonchromophoric amino groups and have been used successfully for the determination of absolute configuration 14,15,17 and conformation 18 of chiral amines.…”

Exciton coupling between enones: Quassinoids revisited

“…Our numerical results demonstrate how much less efficient may the NDEC mechanism be with respect to the DEC mechanism on similar chromophores. Of course, several successful applications of nondegenerate exciton chirality are known . However, the combination of NDEC and twisted chromophores, or chromophores with possibly large second sphere chirality, as in the case of quassinoids and chlorahololides, is very likely to produce ECD effects surmounting those due to exciton coupling.…”