Electronic structure of the polydiacetylenes: effect of bond length variations in a model system

• Published in: Chemical physics letters Vol. 51; no. 2; pp. 273 - 278
• Main Authors: Boudreaux, D.S., Chance, R.R.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.1977
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/0009-2614(77)80401-6

SCF Xα calculations are reported for a cyclic molecule whose bonding sequence is a model for the backbone bonding sequence in the polydiacetylenes. The electronic structure of the model molecule, with particular emphasis on the two lowest energy optical transitions, is determined in conjunction with a systematic variation in bonding sequence. Transition energies are in fair agreement with experiment for polydiacetylenes with the acetylene linkage, ▪, and for the model molecule in solution. The same analysis predicts a substantial red shift on going to the butatriene linkage, ▪, whereas a small blue-shift is observed experimentally. This qualitative disagreement between theory and experiment is attributed to the inability of the present scheme to represent excitonic excitations and/or the neglect of interchain interactions. The transition energies from the SCF Xα treatment can be satisfactorily represented in a simple Hückel model with the following resonance integral-bond length relationship:β ij = 67.28 exp (- d ij /0.424) eV.