2. Paradoxical solvent effects on the absorption and emission spectra of amino-substituted perylene monoimides: solvatochromism with solvent independent Stokes Shifts

In N-(2,5-di-tert-butylphenyl)-9-pyrrolidinoperylene-3,4-dicarboximide (5PI) the absorption and emission spectra display large solvatochromic shifts, but, remarkably, the Stokes shift is practically independent of solvent polarity. This unique behavior is caused by the extraordinarily large ground state dipole moment of 5PI, which further increases upon increasing solvent polarity, while the excited state dipole moment is less solvent dependent. In the corresponding piperidine compound 6PI, this effect is much less important due to the weaker coupling between the amino group and the aromatic imide moiety, and in the corresponding naphthalimide 5NI it is absent. The latter shows the conventional solvatochromic behavior of a push-pull substituted conjugated system, i.e. minor shifts in absorption and a larger change in the emission energy with solvent polarity. ‡ Published in part in P. D. Zoon, A. M. Brouwer, ChemPhysChem 2005, 6, 1574-1580 Paradoxical Solvent Effects