UV-induced growth of cyanopolyyne chains in cryogenic solids

Abstract

UV laser excitation of cryogenic solids doped with cyanoethyne, HC(3)N, led to an in situ creation of longer carbon-nitrogen chains, namely HC(5)N, C(4)N(2), and C(6)N(2), heralded by their strong visible luminescence. HC(5)N and C(4)N(2) molecules can form, most probably, within HC(3)N aggregates linked by hydrogen bonds, while the reaction occurring between two isolated, photochemically created C(3)N radicals yields C(6)N(2). This latter species, dicyanobutadiyne, is easily detected in Ar, Kr, N(2), as well as in parahydrogen solids. The C(6)N(2) phosphorescence is identified here for the first time. The reported carbon chain coupling reactions in rigid environments are of interest for astrochemistry of interstellar ices.

Synthesis and spectroscopy of cyanotriacetylene (HC7N) in solid argon

“…18,19,49 The same has been reported for HC 5 N, 50 isoelectronic with NC 4 N. It was therefore reasonable to expect similar characteristics from HC 7 N, for which the number of electrons and the overall structure is reminiscent of NC 6 N, apart from the obvious lack of a symmetry centre.…”

“…Each pulse of the laser has not only served to photodissociate precursor molecules but also to populate the excited electronic states of products. The HC 5 N phosphorescence has indeed appeared; it was accompanied by a stronger emission, easily recognised as originating from C 6 N 2 (this latter product was previously shown to form readily in irradiated Ar/HC 3 N matrices 18,19 ). The sensitivity of detection was much higher than in former IR investigations, 16,17 and the thermal annealing of the sample was not a necessary condition for the appearance of HC 5 N bands.…”

“…Several years ago we have demonstrated, using IR absorption as the detection method, the formation of HC 5 N 16, 17 in far-UV-irradiated solid argon doped with a mixture of either HC 3 N/HC 2 H or NC 4 N/HC 2 H precursors. More recently, taking advantage of a fairly sensitive detection method, that of visible luminescence spectroscopy, we have evidenced the formation of HC 5 N in UV-irradiated rare gas solids doped solely with HC 3 N. 18,19 Formation of long-chain species including C n (n = 3-12), C 2 H, HC 2 H, C 4 H, HC 4 H, C 8 H − , and HC 8 H out of UV-photolysed acetylene molecules isolated in solid neon, was also reported. 20 Our recent study has demonstrated that the cryogenic co-deposition of HC 5 N/HC 2 H 21 or HC 3 N/HC 4 H 22 induced the formation of well-defined complexes.…”

Photochemical activity of Titan’s low-altitude condensed haze

“…Larger unsaturated organics condensed onto aerosols likely have both spin-allowed strong and spin-forbidden (condensed-phase enhanced) weak photoabsorptions and similar solid-state photoprocessing potential. It is known that gas-phase C 4 N 2 could dissociate with 248 nm light 43,44 resulting in the formation of CN and the CCCN The polymer residual spectra (middle and bottom boxes) were referenced to background spectra taken before C 4 N 2 was deposited, whereas all the other spectra show differences between the spectra after irradiation at the specified wavelength and duration and the spectra before irradiation. Negative absorbance means depletion of the spectral feature.…”

Low-temperature phosphorescence of dicyanoacetylene in rare gas solids

“…Its IR and Raman vibrational spectroscopy was thoroughly studied [2][3][4][5], as were the mid-UV [6,7] and vacuum-UV [8] electronic transitions. Electronic luminescence (phosphorescence) has so far been found for several cyanoacetylene-family molecules dispersed in solid rare-gas matrices, namely for NC 4 N [9] and HC 5 N (cyanodiacetylene) [10], for another isoelectronic pair: NC 2 N (cyanogen) [11] and C 3 N -(cyanoacetylide anion) [12], and finally for NC 6 N [13]. Such emission, on the other hand, is not occurring for HC 3 N (cyanoacetylene).…”