Synthesis and Electronic Phosphorescence of Dicyanooctatetrayne (NC₁₀N) in Cryogenic Matrixes

Abstract

The rodlike 1,8-dicyano-octa-1,3,5,7-tetrayne (NCN) molecule was synthesized with UV-assisted coupling of rare-gas matrix-isolated cyanobutadiyne (HCN) molecules. Detection of NCN molecule was possible due to its strong orange-red (origin at 618 nm) electronic luminescence. Excitation spectra of this emission (ã Σ-X̃ Σ phosphorescence) gave access to studying the fully allowed H̃ Σ-X̃ Σ UV system of NCN. The identification of observed spectral features was assisted with quantum chemical computations. Certain regularities shaping the electronic spectroscopy of NCN molecules have been discussed.

Phosphorescence of C5N− in Rare Gas Solids

“…The most intense vibronic bands in C 5 N − phosphorescence are those due to the fundamental modes associated with triple bonds, namely ν 1 , ν 2 , and ν 3 (it should be noted that the shorter chain of C 3 N − has just two alike modes with frequencies around 2000 cm −1 ; both were shown to dominate in its triplet-singlet emission [19]). In phosphorescence of other previously studied mono- [21,[23][24][25] and dicyanopolyyne [22,[26][27][28] molecules, the vibronic bands produced by the triple bonds were also prominent. The mode contributing especially strongly to the vibronic structure of these emissions was a stretching, either a quasi-centrosymmetric one for the C 2n+1 N backbone [24] or fully symmetric for NC 2n N [22], which caused, along the chain, shrinking of the consecutive interatomic distances alternated with their expansion (i.e., an in-phase oscillation of the three triple bonds) [29].…”

“…In phosphorescence of other previously studied mono- [21,[23][24][25] and dicyanopolyyne [22,[26][27][28] molecules, the vibronic bands produced by the triple bonds were also prominent. The mode contributing especially strongly to the vibronic structure of these emissions was a stretching, either a quasi-centrosymmetric one for the C 2n+1 N backbone [24] or fully symmetric for NC 2n N [22], which caused, along the chain, shrinking of the consecutive interatomic distances alternated with their expansion (i.e., an in-phase oscillation of the three triple bonds) [29]. Distortion patterns of that type qualitatively resembled the geometry changes predicted to accompany the a 3 Σ + -X 1 Σ + transitions of cyanopolyyne-family molecules, including C 5 N − [29].…”

“…Their frequencies are listed in Table 1. This vibrationally structured emission was distinctly different from the phosphorescence of either the precursor molecule [21] or of NC 10 N [22]. It is recognized as the phosphorescence of C 5 N − based on the following arguments: (i) analysis of its vibronic structure reveals three stretching fundamental frequencies of C 5 N − that were independently measured via IR absorption (see Table 1); (ii) it is long-lived, in the millisecond time range; and (iii) its origin (2.68-2.69 eV) conforms to the vertical singlet-triplet excitation energy (3.03 eV) theoretically predicted for C 5 N − [18].…”

“…We therefore sought to find phosphorescence of C 5 N − by photolyzing HC 5 N. The ArF laser radiation (193 nm) was applied. The choice of optimal conditions to photoexcite the anion was crucial, given that the parent species phosphoresces and that the presence of an intensely emitting photoproduct (dicyanooctatetrayne, NC 10 N) in irradiated HC 5 N/NG samples was previously reported [22].…”

“…Time synchronization between excitation pulses and luminescence detection was provided by a home-made triggering device. Tunable excitation permitted us to separate the new emission features from those of the already known phosphorescence of the parent molecule [21] or of the chain-growth products [22].…”