Temperature dependence of the fluorescence emission of substituted 3-hydroxychromones

THOMAS M. JOVIN; LUCIANA GIORDANO; ALAN M. SZALAI; PEDRO F. ARAMENDÍA

Santiago

Conferencia; Inter-American Photochemistry Society Conference; 2016

3-Hydroxychromone (3HC) compounds exhibit a large sensitivity of their spectroscopic properties to environment due to the existence of tautomeric forms both in the ground and in the first excited singlet state.1 The reversible transformation of the tautomers involves a proton transfer, which can take place either intramolecularly or mediated by the solvent. We synthesized a variety of 3HC derivatives with different substituent pattern.The emission properties were studied by steady state fluorescence excitation and emission spectroscopy, by fluorescence anisotropy, and by time resolved emission spectroscopy. These studies were performed as a function of temperature in solvents of different polarity and hydrogen bond ability. All three compounds display emission from the two tautomeric forms (normal, N*, and tautomer, T*, emission attributed to the higher and lower energy bands, respectively). Thisemission is solvent, excitation wavelength, and temperature dependent. In dichloromethane, taken as solvent of intermediate polarity, the emission ratio of N* compared to T* increases with temperature for 1, whereas it decreases for 2. The total emission decreases with temperature increase for 1 and 3, indicating the existence of an activated non radiative process. The tendency of the total emission of 2 with temperature is solvent dependent. The emission decay dynamics displays a fast component in the 100 ps time range, which isattributed to the relaxation of the excited state tautomeric equilibrium, and a slow component in the 2 - 3 ns time range. Whereas the fast component is accelerated with the increase in temperature for both 1 and 2, the slow component is accelerated for 1 but it is slowed down for 2. The temperature dependence of the system is interpreted by a four state scheme involving a tautomeric equilibrium in the ground and in the excited state. The temperaturedependence of the rate constants is interpreted on the basis of the enthalpy difference of the tautomeric equilibrium and the activation energy of the nonradiative processes.