The effect of H-bonding solvents on the electronically exited states properties of  7-dimethylamino-4-methyl-coumarin and psoralens

L.A. Brueva, N.E. Voropaeva, I.V. Sokolova, J.P. Morozova.     One of the most important problems of optics and spectroscopy of poly atomic molecules is the determination of how the spectral-luminescent properties are connected with the features of the electronic structure of the molecule. Knowing of such regularities is that allows one to create compounds with the efficient radiating rate. In the case of fluorescent probes or biological labels the sensitive reaction of the fluorophore on the smaller changing of the environment properties is the dominant. The derivatives of coumarin are those satisfying such requirements. A particularity of the structure of the coumarins is the high polarizability of such molecules and their high charge transition rate and high dipole moment values. Representatives of this class take one of the first places among the most efficient green-blue spectral region laser dyes.

    Almost all photochemical and chemical reactions elapse in the liquid medium. Hence, a complete analysis of the mechanism, kinetics and thermodynamics of the reaction should include medium influence consideration, i.e. consideration of the solvatation effects. It is necessary to have a look on the solvent influence into parameters being measured. A location intensity and shape of the absorption and emission bands use to depend on the solvent. This dependence is the result of the molecular interaction forces leading to changing of energy difference between the ground and the first exited states.
    The goal of this investigation is to begin building of the solvation shells of the 7-dimethylamino-4-methyl-coumarin (coumarin 1) molecule and psoralen molecule being relative to the coumarin 1 molecule. Psoralens are widely used in medicine. being combined with the UV-radiation they are successfully used for the cure of skin maladies.
    We regard interaction of these molecules the hydroxil-containing solvent - the methanol molecule for studying of the solvatation process give a possibility of prognosis of the molecular spectral properties in the solvent mixtures. Together with the experimental investigation a quantum-chemical calculation of the 1:1 complexes of coumarin 1 + methanol and psoralen + methanol, i.e. the first stage of the specific solvatation was investigates by the means of super molecules method. The theoretical consideration of electronic structure and spectral properties of aminocoumarin molecules where carried out by INDO method  using the special spectroscopic parametrization [1]. The result of the quantum-chemical calculations are the level scheme given in Table 1.

Table 1.

From the data of these tables one can see, that the blue shift of the pp*-levels and  of the np*-levels occur when approaching of methanol molecule. But in experiment the red shift of  the pp*-levels occur. Also the inversion of 4 and 5 levels is observed in the case of psoralen and that of 5 and 6 - in the case of coumarin 1. Dipole moment increases relative to one of the isolated molecule. However, the shifts in spectra are not large and wrong direction of shift for pp* band, allowing to conclude that a more complicated complex than of the 1:1 , really arises consequently, it is necessary to increase an amount of molecules of methanol and to consider the interaction of nitrogen of diethylamino-group with methanol. The results of experimental investigation are presented in the graph of the absorption and emission band shift dependence on the alcohol concentration in the solvent. One sees from the experimental data that a shift to the lager wave length region when increasing the alcohol concentration. It seems also, that the shift does occur by the addition of alcohol. It leads to conclusion that the structure of solvation shell does not fit the structure of the solvent.
    The experiment showed that the absorption band had a complicated form. Two singlet  pp* and one np* transition were observed.
    The capability for intermolecular interaction and the most probable paths for the approach of the proton or proton-donor solvent are determined with the help of the method of molecular electrostatic potential (MESP) using wave function obtained by INDO method [2,3]. the results of this work are present in Table 2 and Table 3.
 

From the data of  Table 2 and Table 3 one can see that proton-accepting ability of the carbonyl group increases in these compounds in Spp*, Tpp*  states and decreases in Spp*, Tpp*  states.

    Also, the excitation of the coumarin 1 and psoralen to the pp* state of any multiplication leads to increasing of the probability of proton-transfer reaction and solvation at the C=O group, and the exitation to the np* state leads to decreasing of this channel.

References.

1. V.Ya. Artykhov and A.I. Galeeva, Izv. Vyssh Uchebn. Zaved., Ser. Fizika, No. 11, 96(1986).
2. G.N. Jones, W.R.Jackson, Choi Chol-yao, W.R.Bergmark, J. Phys. Chem., 89(1985).
3. K.M.Degtyarenko, T.N. Kopylova, R.T. Kuznetsova, G.V. Mayer, E.N. Tel'minov, Atmos. Oceanic Opt., 6(1993)421.