Discovery of HCCCH_2CCH in TMC-1 with the QUIJOTE line survey

We present the first detection in space of 1,4-pentadiyne. It has been found towards TMC-1 with the QUIJOTE line survey in the 31-50 GHz range. We observed a total of 17 transitions with $J$ = 2 up to 13 and $K_a$ = 0,1 and 2. The observed transitions allowed us to derive a rotational temperature of 9.5 pm 0.5 K and a column density of (5.0 pm 0.5)times 1012 cm$^ $. This molecule was the last non-cyclic isomer of the C$_5$H$_4$ family that could be detected via radio astronomy. A computational chemistry study was performed to determine the energies of the five most stable isomers. The isomer ($c$-C$_3$H$_3$CCH) has a considerably higher energy than the others, and it has not yet been detected. To better understand the chemical reactions involving these species, we compared the ethynyl and cyano derivatives. The observed abundances of these species are in good agreement with the branching ratios of the formation reactions studied with our chemical model of TMC-1.