It is demonstrated that chirality of molecules composed of 1,2,4,5‐benzenetetracarboxydiimide (pyromellitic diimide) or 1,4,5,8‐naphthalenetetracarboxydiimide units is reflected by their exciton Cotton effects. The analysis is based on the calculated (ZINDO/S) excited states of the model diimide chromophores 1a and 2a. Rotation of the diimide chromophores around the C‐N bond in diimides 3–5 is evaluated from the dynamic 1H NMR data. A comparison of chiroptical properties of bis‐diimides 3–5 with the CD spectra of bis‐imides 6–8 is also presented. Chirality 12:263–268, 2000. © 2000 Wiley‐Liss, Inc.

The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2–5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2‐disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X‐ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1 a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low‐energy tail (340–360 nm) of the absorption envelope can be assigned to out‐of‐plane polarized n–π* transitions (I, II). The higher energy bands are due to contributions from up to six π–π* transitions, these being polarized either along the long (IV–VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc‐pVDZ and semiempirical INDO/S‐CI calculations have been compared with the experimental data. CD Cotton effects in the region 200–260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2–5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.

Chiroptical properties of a series of 3 and/or 4‐heterosubstituted 2(5H)‐furanones were investigated with respect to correlation with absolute configuration. The n − π* and π − π* Cotton effects have been assigned on the basis of comparison with the UV spectra in solvents of varying polarity. It is demonstrated that the n − π* transition in 4‐amino substituted 2(5H)‐furanones appears at shorter wavelength with respect to the π − π* transition. With the exception of 4‐pyrrolidino and 4‐benzylamino substituted 2(5H)‐furanones, other heterosubstituted 2(5H)‐furanones follow the butenolide configurational rule (Gawronski et al. J. Org. Chem. 61:1513–1515, 1996). Absolute configuration of 2(5H)‐furanone derivatives can also be assigned according to the sign of the Cotton effect (of unknown origin) at 200–230 nm. The structure of four representative sulfur and nitrogen substituted 2(5H)‐furanones has been analyzed by X‐ray diffraction. The results indicate planarity of the furanone ring and extended conjugation in 4‐amino substituted 2(5H)‐furanones. Chirality 9:537–544, 1997. © 1997 Wiley‐Liss, Inc.

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