J. Phys. Chem. 1994,98, 3295-3299
3295
Solvation Dynamics of Coumarin 153 in Molten Salts E. Bart, A. Meltsin, and D. Huppert' Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Ramat Aviv 69978, Israel Received: October 18, 1993; I n Final Form: January 11, 1994"
Static and time-resolved fluorescence techniques are employed to measure the solvation statics and dynamics of electronically excited coumarin 153 in several molten tetraalkylammonium salts. The solvation of coumarin 153 occurs on two time scales which are both dependent on the cation size. The larger the cation size, the longer the relaxation time. For coumarin 153 in molten tetrabutylammonium hydrogen sulfate, the Stokes shift relaxation times are -40 and 220 ps for the short and long component respectively. For coumarin 153 in molten tetradodecylammonium perchlorate, the relaxation times are 200 and 2000 ps. The steady-state emission band position is strongly dependent on the molten salt cation size. The band maximum position for coumarin 153 in molten tetrapropylammonium hydrogen sulfate is at 18 250 cm-I (548 nm), while for tetradodecylammonium perchlorate the band maximum is at 20 120 cm-l (497 nm).
Introduction In recent works'-'we have studied the static and dynamic "salt effect" on solvation of fluorescent probe molecules in electrolyte solutions. These probe molecules are used to measure the relaxation times of the solvation energy after a change in the probe charge distribution due to electronic excitation. These and other measurementsby Chapman and Maroncelli5 and recent molecular dynamic simulations by Neria and Nitzan6 lead to several conclusionsconcerning the solvation statics and dynamics of probe molecules in solutions of organic liquids containing electrolytes. The limiting case of an aqueous or a nonaqueous ionic solution from which all of the solvent is removed is a molten salt. In other words, the molten salts are the logical extremity of the following series of media: protic solvents, dipolar aprotic solvents, and molten salts. This new medium is becoming increasingly important as solvents for salts, metals, and gases.' One of the distinguishing features of molten salts is their relatively high electricalconductivity. The conductivity of molten inorganic salts is high compared to that of water, but it is some 10 OOO times lower than that of a liquid metal and is roughly a factor 10times larger than thoseof concentrated aqueous solutions of strong electrolytes. There is a general trend toward lower conductivity in going from ionic to covalent salts. For a given anion the equivalent conductance of the molten salt decreases with increasing size of the cation. Some molten salts conduct by ionic drift, the ions being free or associated. Viscosities of simple inorganic molten salts are in the range 1-5 cP. Good electrical conductivity, low viscosity, wide liquid range together with their excellent ability todissolve salts and metals make them extremely useful reaction media. Physical-chemical studies of molten organic salts have made a considerable contribution to molten salt chemistry.8.9 There are several distinct advantages of organic molten salts over inorganic ones. First of all, their low melting points, which lie in the range 50-250 OC, while most inorganic salts melt above 500 OC. Also, as solvents, they generally havea greater solubility range, e.g., some solvatochromic probe molecules (coumarin 153, coumarin 102, LDS-750)show complete liquid miscibility with moltenquatemaryorganicsalts,while are not miscible with molten inorganic salts. Since both ion association10 and anion solvation are absent in such melts, they can be regarded as important supplement to the nonaqueous and waterlike solvents. Recently we employed static and time-resolved fluorescence techniques to measure the Stokes shift of coumarin 153 in molten Abstract published in Advance ACS Abstracts, March 15, 1994.
0022-3654/94/2098-3295$04.50/0
tetrabutylammonium hydrogen sulfate at 450 K.l1 Thesolvation of excited coumarin 153 occurs on two time scales: (1) at about 40 ps and (2) at about 220 ps. Molten tetraalkylammonium salts possess low melting points and polarities,'* they are ionic yet less polar than other molten salts because of their alkyl groups, they cannot interact with solutes and they permit only electrostatic ion-dipole, ion-induced dipole, and dispersion forces. They are able to mix with a wide variety of organic materials. Potential obstacles to their use as solvents might be their instability in acids and possible thermal decomposition. It has been shown" that many of the quaternary ammonium salts may be used for short periods (about 3 h) at temperatures just above their melting points without decomposition. There are two modes of possible thermal quaternary ammonium salt decomposition: Ft,N+X-
I I H ' I
(-C-C-),N%-
-
-
RX+ 5 N
>c=c
