Short communication

Solvent effect on eximer emission from pyrene

Centro-symmetric molecules such as anthracene and its 9,10-disubstituted derivatives often show solvatochromic shifts of their absorption and fluorescence spectra in a series of solvents of different static dielectric constants. These shifts have been explained by the existence of small dipole moments of such molecules in excited states or by the “solvent Stark effect” which results from fluctuations of the electric field produced by the polar solvent. It is suggested that the origin of these solvatochromic shifts is the solute quadrupole-solvent dipole interaction which is not considered in the simple electrostatic models.

Solvatochromic shifts of pyrene (Py) and naphthalene (Np) excimers were obtained in polar and non-polar solvents. The observed shifts for both excimers are explained by changes in the polarizability between the excimer and the dissociative ground state. The magnitudes of the shifts in the pyrene excimer are larger, indicating that the pyrene excimer is more polarizable than the naphthalene excimer.

Thermochromic shifts of pyrene excimer were obtained in cyclohexane (25–60°C) and ethanol (25–70°C). The observed shifts are due to changes in polarizability between the states involved in the electronic transition. The parameters obtained from thermochromic shifts agree with the ones calculated from solvatochromic shifts. This implies that the difference in polarizability between the electronic states does not change with solvent and temperature in the range of temperatures covered.

Solvatochromic shifts of pyrene excimer fluorescence were obtained in both polar and non-polar solvents. The observed shifts were explained in terms of solute—solvent dispersion interactions, a solute transition dipole moment term and the solvent Stark effect (for polar solvents). These results show that pyrene excimer is non-polar and its polarizability differs between the first singlet excited state and the dissociative ground state.

The displacement of electronic absorption and luminescence spectra (solvatochromic shifts) are related to the solute—medium interactions. These interactions can be non-specific (dielectric interactions) when they depend only on multiple and polarizability properties of the solute and solvent molecules; but specific associations such as hydrogen bonding can also be important.

A number of examples of solvatochromic shifts are shown and discussed according to the various solute—medium interactions. The properties of solvent mixtures and those of rigid media are considered, as well as the “thermochromic shifts” which result from the change in the temperature of the medium.

The use of solvatochromic shifts for the determination of the dipole moment and of the polarizability of electronically excited molecules has been important for an understanding of electron distribution changes in such states; examples of such determinations are given, together with references to the original literature.

In the final section some limitations of the theories of solvent shifts and possible improvements are discussed.