SPECTRAL SIMILARITY AND DIFFERENCE OF NAPHTHALENETETRACARBOXYLIC DIANHYDRIDE, PERYLENETETRACARBOXYLIC DIANHYDRIDE, AND THEIR DERIVATIVES

Although the sizes of their pi-conjugation systems are different, the absorption spectra of 1,4,5,8-naphthalenetetracarboxylic dianhydride, its diimide (NTCAI), 3,4,9,10-perylenetetracarboxylic dianhydride, and its diimide (PTCAI) show similar experimental features consisting of the main band with some subordinational bands. However, the origins of the transitions in the observed bands are reported to be quite different. In order to understand the relationship between the conjugation size and the derails of the transition properties, we studied the absorption spectra of naphthalene, 1,8-naphthalenedicarboxylic anhydride imide (NDCAI), NTCAI, and PTCAI by the semiempirical MO INDO/S method. Comparison of the observed and calculated spectra showed that NDCAI and NTCAI have two pi-pi* transitions that are perpendicular and close in energy. Thus the observed bands can be described by the mixing of two electronic transitions with vibronic progressions. For PTCAI the observed bands consist of one electronic transition and its vibronic progressions. The mechanism of the absorption wavelength shift from naphthalene, NDCAI, and NTCAI to PTCAI was analyzed. The pi-pi* transitions with high intensity in the direction of the long axis show a remarkable bathochromic shift, due to the high content of HOMO --> LUMO excitation. The bathochromic shift is explained by the decrease of the orbital energy gap. The pi-pi* transitions with low intensity in the direction of the short axis show only a small bathochromic shift, and this shift can be explained by the small change of the orbital energy gaps. The n-pi* transitions in NDCAI, NTCAI, and PTCAI were also analyzed; the absorption wavelengths of the n-pi* transitions are kept at almost the same levels for NDCAI, NTCAI, and PTCAI.