ИСТИНА

Aromatic derivatives of 4H-imidazol-4-one locked by the BF2 group and aromatic derivatives of 2H-pyran-2-one are used in bioimaging as fluorescent labels. Several compounds of the first class are synthesized and shown to exhibit high fluorescence quantum yields (FQY) in non-polar solvents, except for compound 1, (Z)-4-(1-(difluoroboryl)-9H-fluoren-9-ylidene)-1,2-dimethyl-imidazol-5(4H)-one. Several compounds of the second class are also synthesized, but only 2-H+, which is the protonated form of compound 2, (E)-4-methoxy-6-(2-(pyridin-4-yl)vinyl)-2H-pyran-2-one, exhibits high FQY of 42% in water; FQYs in non-polar solvents for all compounds of this class, including 2, do not exceed 1.5%. Here, we study the photophysical properties of compounds 1, 2 and 2-H+ to explain their deviation from the fluorescent properties of other characterized compounds of their classes. The ground-state equilibrium geometry of 1 is located at the MP2/(aug)-cc-pVDZ level of theory. By using extended multiconfiguration quasi-degenerate perturbation theory, XMCQDPT2/CASSCF(14,14)/(aug)-cc-pVDZ, we demonstrate that compound 1 exhibits a charge transfer character upon excitation to the S2 state, which is characterized by the highest oscillator strength. According to Kasha's rule, the low FQY can therefore be easily explained, considering an extremely low oscillator strength of the S0-S1 transition and assuming fast internal conversion between the S2 and optically dark S1 states. The calculated vertical excitation energy of the S0-S2 transition (405 nm) is close to the experimental absorption maximum in dioxane (415 nm), while the calculated S0-S1 wavelength is significantly red-shifted (486 nm). The ground-state equilibrium geometries of compounds 2 and 2-H+, as well as their photophysical properties are calculated at the same levels of theory as for 1. The effective fragment potential method is used to account for solvent effects in water solution of 2-H+. The excited-state equilibrium geometries of 2 and 2-H+ in S1 are obtained using the XMCQDPT2/ CASSCF(12,12)/(aug)-cc-pVDZ level of theory. We show that compound 2-H+ exhibits a charge transfer character upon excitation to the S1 state, which is optically bright in both compounds 2 and 2-H+. The experimental absorption maxima are found to be in good agreement with the calculated wavelengths of the S0-S1 transitions. The remarkable difference in the FQYs, which is observed experimentally for compounds 2 and 2-H+, is thought to be related to their different excited-state lifetimes. Internal conversion through a conical intersection coupled to intramolecular rotation is found to be barrier-controlled, thus defining the excited-state decay of compounds 2 and 2-H+. This work is supported by the Russian Science Foundation (grant №17-13-01276). The research is carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University as well as the local resources provided through the Lomonosov Moscow State University Program of Development.