Synthesis, photophysics and excited state structure of 1,8-di(p-tolyl)-1,3,5,7-octatetrayne

The Cadiot-Chodkiewicz type C2-elongation of p-tolylacetylene followed by dimerization of the resulted diyne p-CH3C6H4(CC)2TMS (1) gave thermally stable octatetrayne p-CH3C6H4(CC)4-p-C6H4CH3 (2) as light yellow powder in 75% yield. Compound 2 was characterized by spectroscopic methods and X-ray crystallography. Careful analysis of the crystal data revealed high degree of chain linearity with a potential for 1,n-topochemical polymerization. Next, photophysical properties of 2 were studied in details by experimental and advanced theoretical methods (ab initio HF as well as DFT calculations involving both the ground and excited state geometry optimization). These properties are similar to the properties of the parent polyyne C6H5(CC)4C6H5 previously described in literature (in particular, the lowest electronic excited singlet state (S1) of 2 is a dark state). This was confirmed by the experimental facts, namely, few progressions of the stretching mode (ag) of the polyyne chain, starting from different “false origins” lying below the second excited (bright) state (S2), were observed in the fluorescence excitation spectrum. The red shift of the electronic spectra (155 cm−1 and 300 cm−1 for absorption and fluorescence, respectively) and an increase of an energy gap between S1 and S2 states by 450 cm−1 are the effects of methylation of C6H5(CC)4C6H5 to 2. Theoretical results showed that in the excited state of 2 a shortening of single bonds and elongation of triple bonds occurred. This is in accordance with an observation of a long progression of the stretching vibration mode in the experimental absorption spectrum of 2. Besides that, stretching vibration of the polyyne chain is also active in the fluorescence spectra.