Cyanopolyynes are a distinct class of small organic molecules. Only the smallest of these CN-capped carbon chains are available via standard preparatory synthesis methods, while comprehensive characterization is desirable also for the longer ones, considering their astrophysical significance. The cryogenic rare-gas matrix isolation technique can be employed for both the creation and spectroscopic description of such unstable chemical species.
Targeted compounds may be formed in UV-stimulated cryogenic elongation of unsaturated carbon-nitrogen chain precursors trapped in solidified rare gases. Photochemical experiments starting with molecules like HC5N, C2H2, CH3C5N, and leading to tri- (CH3C7N) or tetraynic (HC9N, C10N2) nitriles isolated in solid Kr will be presented, together with a general scheme elucidated for the relevant chain-growth processes.
Strong phosphorescence has been discovered for the three above mentioned products. Respective vibronic bands were measured not only in dispersed emission but also in luminescence excitation spectra. That latter approach, carried out with a tuneable UV laser system, gave access to the characterisation of selected singlet excited electronic states. Similarities/differences in selection rules, transition energies, vibronic patterns, and phosphorescence decay rates will be discussed for the electronic spectroscopy of molecules belonging to HC2n+1N, NC2nN, and CH3C2n+1N homologous series. In particular, the observed linear correlation of - vibrationless origin wavelength with the size of the conjugated carbon-nitrogen backbone allows for the extrapolation into the realm of as yet unexplored cyanopolyynes.
This work was financially supported by:
– the French Government scholarship Bourse Eiffel, managed by Campus France;
– PICS (Programmes Internationaux de la Coopération Scientifique): PAN-CNRS (2014-2016);
– French–Polish Partenariat Hubert-Curien Polonium project (2012-2013 and 2015-2016) of scientific cooperation;
– the project “Scholarships for PhD students of Podlaskie Voivodeship”; the project was co-financed by the European Social Fund, Polish Government and Podlaskie Voivodeship.