This paper reports on UV-stimulated synthesis of methylcyanotriacetylene carried out in cryogenic rare gas matrixes via coupling of smaller precursors: propyne and cyanodiacetylene. The detection was possible due to the strong visible ã 3A' → X̃ 1A1 phosphorescence of CH3C7N, discovered in the course of this work. The ensuing measurements of electronic spectroscopy revealed the formally forbidden B̃ 1E-X̃ 1A1 system, as well as the allowed one Ẽ 1A1-X̃ 1A1, with origins at approximately 3.32 and 5.4 eV, respectively. It was also possible to revisit the spectroscopic characterization of cyanotriacetylene, HC7N, formed in parallel to the title photoproduct. Spectral assignments were assisted with a density functional theory study.