A novel fluorescent dye bis-(pyridin-2-yl-methyl)-(1,3,4-triphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amine (P1) has been synthesized and investigated by means of steady state and time-resolved fluorescence techniques. This compound acts as sensor for fluorescence detection of small inorganic cations (lithium, sodium, barium, magnesium, calcium, and zinc) in highly polar solvents such as acetonitrile. The mechanism which allows application of this compound as sensor is an electron transfer from the electron-donative part of molecule (amine) to the acceptor part (pyrazoloquinoline derivative), which is retarded upon complexation of the electro-donative part by inorganic cations. The binding constants are strongly dependent on the charge density of the analyzed cations. The 2/1 complexes of P1 with Zn⁺⁺ and Mg⁺⁺ cations posses large binding constants. Moreover, in the presence of these cations a significant bathochromic shift of fluorescence is observed. The most probable explanation of such behaviour is the formation of intramolecular excimer. This is partially supported by the quantum chemical calculations.

Geometry and vibrational modes of the anthranilic acid molecule in the S0 and S1 states were computed using ab initio methods: Hartree–Fock (HF) and configuration interaction of singly excited configurations (CIS) as well as the density functional theory with time-dependent perturbation (TD-DFT). The intensity distribution in the laser-induced fluorescence excitation spectra was modeled in two ways: using displacement parameters for independent modes and using multidimensional Franck–Condon integrals. The change in the molecular geometry upon excitation was calculated from the band intensities within the above two models. Displacement parameters of eight in-plane modes active in the excitation spectrum were optimized to reproduce the experimental intensities of about 40 most intensive and well-separated vibrational bands, while displacement parameters of other in-plane modes were kept frozen at the values resulting from the quantum chemical calculations. The intramolecular hydrogen bond is significantly stronger in the S1 state than in the ground state. Additionally, bond lengths and angles in the aromatic ring, within the substituents and between the ring and the substituents undergo significant changes and they induce the presence of strong fundamentals in the excitation spectrum.