Electronic absorption and phosphorescence of cyanodiacetylene

Electronic absorption and emission spectra have been investigated for cyanodiacetylene, $HC5N$⁠, an astrophysically relevant molecule. The analysis of gas-phase absorption was assisted with the parallel rare gas matrix isolation experiments and with density functional theory (DFT) predictions concerning the excited electronic states. Mid-UV systems $B̃Δ1←X̃Σ1+$ (origin at 282.5 nm) and $ÃΣ1−←X̃Σ1+$ (306.8 nm) were observed. Vibronic assignments have been facilitated by the discovery of the visible phosphorescence $ãΣ3+←X̃Σ1+$ in solid Ar, Kr, and Xe. Phosphorescence excitation spectra, as well as UV absorption measurements in rare gas matrices, revealed the enhancement of $Ã←X̃$ transitions. The vibronic structure of dispersed phosphorescence spectra supplied new data concerning the ground state bending fundamentals of matrix-isolated $HC5N$⁠. The experimental singlet-triplet splitting, 2.92 eV in Ar, closely matches the value of 3.0 eV predicted by DFT.