Novel Chiral Pyromellitdiimide (1,2,4,5-Benzenetracarboxydiimide) Dimers and Trimers: Exploring Their Structure, Electronic Transitions, and Exxiton Coupling

摘要

The chiral but highly symmetrical acyclic and cyclic pyrometallitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecuels are linked by a rigid diequatorially 1,2-disubstituted cyclhexane skeleton. The structures of the compounds have been deterined 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 (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic ccircular dichroism (MCD), and circular dichorism (CD) spectroscoy. CD spectrat 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-#pi#~* transitions (I, II). The higher energy bands are due to contributions from up to six #pi#-#pi#~* transitions, these being polarized either along the long (IV-VI, VII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with he experimental data. CD Cotton effects in the region 200-260 nm, which result from excition interactions between electric dipole allowed transitions of two pyromelithdiimide 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.