Spectroscopic properties of macrocyclic oligo(phenyldiacetylenes)-II. Synthesis and theoretical study of diacetylenic dehydrobenzoannulene derivatives with weak electron-donor and -acceptor groups

Diacetylenic dehydrobenzo[12]annulene and dehydrobenzo[18]annulene derivatives with electron-donor and -acceptor groups were synthesized (including push–pull Eglinton–Galbraith dimer derivative 1c) via an oxidative coupling reaction, and spectroscopically and structurally characterized. The solid–solid phase transition in 1b has been revealed at 45 °C by DSC measurements. Its room temperature crystal structure has been solved by X-ray diffraction measurements. The 1H and 13C NMR chemical shifts, UV/vis and infrared absorption spectra and Raman scattering spectra have been analyzed by using ground-state DFT calculations. The strongest absorptions in the UV/vis spectra of 1 and 2 most probably are not due to the HOMO→LUMO excitations but due to the (HOMO-1)→LUMO and HOMO→(LUMO+1) excitations. The substitution effects on the electronic charge distribution of the all-carbon annulenic cores can be particularly well observed in the distribution of IR intensities in the region of acetylenic stretching vibrations. IR intensities are thus useful in studying the extent of resonance interactions also in acetylenic macrocycles.