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Abstract: The structure and electronic spectra of 2-methylamino-6-methyl-4-nitropyridine N-oxide (2M6M) were investigated. The X-ray structure of 2M6M molecule shows that it crystallizes in the monoclinic system of the P21/n space group and forms internally hydrogen-bonded [NO=2.563(3)Ǻ] monomers linked by the same type of intermolecular [2.799(3)Ǻ] hydrogen bonds in linear oligomers. Due to weak intermolecular Ceractions such oligomers are arranged along the z-axis into a helical-shaped zigzag. The results of quantum chemical DFT/B3LYP/6-31G(d,p) calculations reproduce the absorption spectrum of 2M6M well. The 2M6M molecule shows complex photophysical and photochemical behavior. It exhibits dual fluorescence (with emission maxima at 476 and 536nm) and decays bi-exponentially (with lifetimes of 2.13 and 0.1ns). The temperature dependence of the fluorescence intensity in the two bands has a slope of 1450cm−1 in the temperature ranges of 200–300K. These effects are interpreted in terms of excited state intramolecular proton transfer (ESIPT) and creation of tautomeric form of 2M6M. Below 200K we see the appearance of an emission which can be attributed to a dimeric form. [Copyright &y& Elsevier]

The Cadiot–Chodkiewicz type C2-elongation of p-tolylacetylene followed by dimerization of the resulted diyne p-CH3C6H4(C≡C2TMS (1) gave thermally stable octatetrayne p-CH3C6H4(C≡C)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(C≡C)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 (a g) 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 (155cm−1 and 300cm−1 for absorption and fluorescence, respectively) and an increase of an energy gap between S1 and S2 states by 450cm−1 are the effects of methylation of C6H5(C≡C)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. [ABSTRACT FROM AUTHOR]