ИСТИНА

Derivatives of coumarin with withdrawing substituents in 2-H-pyran ring have attracted a great interest because of their unique photophysical and photochemical properties. The intensity of fluorescence of 3-cyano-7-hydroxycoumarin (Fig 1b) increases in basic solvents, whereas intensity of fluorescence of 3-cyano-6-hydroxycoumarin (Fig 1a) on the contrary significantly decreases in basic solvents [1]. Ab initio calculations of the equilibrium geometry and absorption spectra in the gas-phase and in the water for anion and neutral forms of compounds (1) and (2) were carried out. The optimized ground-state geometries were obtained using DFT with PBE0/(aug)-cc-pVDZ. In the case of water solution the structure of global minimum was preliminarily estimated by means of molecular dynamics, annealing protocol, with OPLSAA force field. Initial temperature 300 K, final temperature 0 K, 1 ps of equilibration, 20 ps of cooling (sigmoidal protocol), step length 1 fs. The model consists of chromophore (atoms are inactive) and ~3000 water molecules. Next was performed optimization of ~200 water molecules whereof ~10 molecules were presented as QM part (DFT, PBE0/(aug)-cc-pVDZ) and other were described as EFP. Transition energies (vertical) and oscillator strengths were calculated with multiconfigurational, quasidegenerate perturbation theory in the version XMCQDPT2 [2]. Nonperturbed wave functions were obtained from CASSCF (16/14) method. All values of transition energies are in good agreement with experimental data, oscillator strengths also correlate with intensities. S0-S1 and S0-S2 transition energies in anion of (1) were 674 nm and 336 nm in the gas-phase and 474 nm and 324 nm in water solution (experimental data ~450 nm and 325 nm) that shows solvatochromic shift 200 nm (0.7 eV). Was shown that in the case of (1) takes place a transition of electronic density from one ring to another by the S0-S1 and S0-S2 excitation. Nearby overlap of wave functions of ground and first excited states is rather small which causes small intensity of this transition (oscillator strength 0,11) in absorption and emission spectra. This effect does not appear for other model compounds, so this is explanation of such difference in photophysical properties. This work is supported by RFBR (project № 08-03-00914-а.).