On the aromaticity and photophysics of 1-arylbenzo[a]imidazo[5,1,2-cd]indolizines as bicolor fluorescent molecules for barium tagging in the study of double-beta decay of 136Xe.

In this paper, the behavior of a bicolor fluorescent indicator for the detection of barium cations formed by double-beta decay of ¹³⁶Xe is analyzed by means of computational tools. Both DFT and TDDFT permit to understand the origin of the bicolor fluorescent signal emitted by 1-arylbenzo[a]imidazo[5,1,2-cd]indolizines in the free and Ba²⁺-bound states. The aromatic character of the fluorophore is analyzed by means of energetic (hyperhomodesmotic equations), structural (harmonic oscillator model of aromaticity, HOMA) and magnetic (nucleus independent chemical shifts, NICS) criteria. It is concluded that the aromatic character of the fluorophore is better described as the combination of two aromatic subunits integrated in the polycyclic system. Different DFT functional are used to analyze the photochemical behavior of this family of sensors. It is concluded that PBE0 and M06 functionals describe better the excitation process in the free state, whereas interaction of the sensor with Ba²⁺ requires the M06L functional. TDDFT analysis of the emission spectra shows larger errors, which have been corrected by means of a structural model. The bicolor behavior is rationalized based on the decoupling between the para-phenylene and benzo[a]imidazo[5,1,2-cd]indolizine components that results in a blue shift upon Ba²⁺ coordination.