Absorption and magnetic circular dichroism (MCD) studies of 1,4,5,8-naphthalenetetracarboxy diimides in terms of CASSCF method and FC theory

Abstract

Abstract The electronic and geometrical structures of the low-energy states of 1,4,5,8-naphthalenetetracarboxylic dianhydride parent diimide ( 1 ) are studied in terms of the complete active space self-consistent field (CASSCF) method employed at different level with respect to the size and the quality of the active space. In the framework of the vibronic model based on the Franck–Condon (FC) effect the absorption and magnetic circular dichroism (MCD) spectra are studied in the excitation region corresponding to two low-energy 1 1 A g → 1 1 B 2u and 1 1 A g → 1 1 B 3u electronic transitions in diimides. In that (visible) excitation region the CASSCF computations with the 5π[4n]5π active space (i.e., the naphthalene-like π orbitals enriched by the four lone pair orbitals of the oxygen atoms) were found to reproduce very well the empirical absorption and the MCD spectra measured for the dicyclohexyl- N , N ′ -substituted diimide ( 2 ). At the same CASSCF/5π[4n]5π level, the electronic absorption of diimides in the near UV excitation region were attributed to the 1 1 A g → 2 1 B 1u , 1 1 A g → 2 1 B 3u and 1 1 A g → 2 1 B 2u electronic transitions; the latter two are mostly localized on the “diimide chromophore”. For these transitions the calculated magneto-optical characteristics, such as sign pattern and intensity distribution in the MCD spectrum, were found to be consistent with that experimentally observed for the diimide 2 compound.