Paul Suppan

Invited review solvatochromic shifts: The influence of the medium on the energy of electronic states

Abstract 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.

Excited-state dipole moments from absorption/fluorescence solvatochromic ratios

Abstract The permanent dipole moments of excited molecules can be obtained from the ratio of the solvent shifts of absorption and fluorescence spectra. This ratio method eliminates the uncertain solute cavity radius parameter, as well as the solvent polarity function. In the case of the first excited singlet state of aniline the dipole moment is 5 D (versus 1.57 D in the ground state).

Local polarity of solvent mixtures in the field of electronically excited molecules and exciplexes

In mixtures of solvents of different dielectric polarities a process of preferential solvation described as ‘dielectric enrichment’ occurs in the solvation shell of dipolar solute molecules. Dielectric enrichment requires the diffusion of solvent molecules, and its extent depends on solvent viscosity and on time; in the case of electronically excited molecules it may be limited by their lifetime. Measurements on short-lived ( 10 ns) fluorescers show that in non-viscous liquid media dielectric enrichment reaches equilibrium in times of 1 ns to over 20 ns depending on the polar solvent mole fraction in the mixture. In the case of exciplexes the process of dielectric enrichment is complicated by the quenching action of polar solvents, which reduces the exciplex lifetime.

Photophysics of aminophthalimides in solution I. Steady-state spectroscopy

Abstract In non-protic solvents the dipole moment of the first excited singlet state of 4-aminophthalimide (4AP) increases to 7 D from the ground state dipole moment of 3.5 D. In protic solvents such as alcohols and water, hydrogen bonding results in a further increase in dipole moment to about 18 D, corresponding to nearly total charge transfer (CT) from the aromatic ring to the acceptor imide system. Quenching of the fluorescence in the CT state takes place by protonation in protic solvents and acid solutions. In mixtures of non-polar/protic solvents, the processes of relaxation and of preferential solvation are complicated by the two sequential increases in the dipole moments as well as by the quenching of hydrogen bonded CT state emitters. The 3AP isomer behaves differently, and does not show such large excited state dipole moment and quenching effects. This may be related to the presence of an intramolecular hydrogen bond which involves one of the carbonyl groups.

The marcus inverted region

The “Marcus Inverted Region” (MIR) is that part of the function of rate constant versus free energy where a chemical reaction becomes slower as it becomes more exothermic. It has been observed in many thermal electron transfer processes such as neutralization of ion pairs, but not for photoinduced charge separation between neutral molecules. The reasons for this discrepancy have been the object of much controversy in recent years, and the present article gives a critical summary of the theoretical basis of the MIR as well as of the explanations proposed for its absence in photoinduced electron transfer. The role of the solvent receives special attention, notably in view of the possible effects of dielectric saturation in the field of ions. The relationship between the MIR and the theories of radiationless transitions is a topic of current development, although in the Marcus-Hush Model electron transfer is treated as a thermally activated process.

Hydrogen bonding and dielectric effects in solvatochromic shifts

When a dipolar molecule is dissolved in a mixture of two solvents N and P of different static relative permittivity, the solvent shell becomes enriched in the more polar solvent (P). This effect of dielectric enrichment is described quantitatively by the preferential solvation index Z which relates the bulk and local mole ratios X and Y, respectively, according to Y=X exp Z where X=xN/xP and Y=yN/yP.If the solute molecule can form a hydrogen-bonded complex with the polar solvent P, it is necessary to use two preferential solvation indices Z1 and Z2; Z1 is then related to the dielectric enrichment and Z2 to the energy balance of hydrogen-bond formation. This H-bond preferential solvation index is in general negative, because the energy spent in breaking the solvent structure exceeds the solute–solvent hydrogen-bond energy. The two preferential solvation indices Z1 and Z2 explain the peculiar solvatochromic shifts observed in mixtures of protic and non-protic solvents.

Solvatochromic shifts of non-dipolar molecules in polar solvents

Abstract 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.

Thermochromic shifts of some molecular and exciplex fluorescence spectra

Abstract The displacement of the fluorescence band of several dipolar molecules and of the pyrene/N,N-dimethylaniline exciplex have been measured as a function of temperature in various solvents. The excited-state dipole moments calculated from these thermochromic shifts agree with those derived by other methods and this confirms the general validity of the thermochromic shift method for the estimation of excited-state dipole moments.

The importance of the electrostatic interaction in condensed-phase photoinduced electron transfer

The electrostatic interaction energy between two ions created by electron transfer between two neutral molecules can be much larger than estimated by the usual Coulomb term C=–e2/Iµr, because the space (r) between the charge centres in not filled with the solvent of static dielectric constant Iµ. When charge transfer occurs between molecules in contact the Coulomb term provides stabilization in > 1 eV even in highly polar solvents.

Quenching of triplet benzophenone by methyl and methoxy benzenes: are triplet exciplexes involved?

The quenching of triplet benzophenone by methyl and methoxy substituted benzenes in acetonitrile was investigated. For both series of donors the quenching rate constants are considerably higher than expected from the Rehm-Weller relationship, which could be explained by a mechanism involving a (triplet) exciplex intermediate. Two corresponding models from the literature were chosen for simulating the experimental data. The results are in accord with the exciplex proposal. For a given driving force the quenching rate constants differ substantially for the two kinds of donor employed. Together with the free ion yields, obtained by transient photoconductivity measurements, this confirms the important role of the chemical structure of the donors which determines the stability of the exciplexes.