Paradoxical Solvent Effects on the Absorption and Emission Spectra of Amino-Substituted Perylene Monoimides** Peter D. Zoon and Albert M. Brouwer* [a] 1. Introduction Fluorescent dyes that are sensitive to their environment are playing an increasingly important role in ultrasensitive imaging in biology and materials science. [1,2] Our purpose in this work was to design and characterize dyes that 1) respond to medium polarity by solvatochromic shifts of their fluorescence emission band, and 2) meet all the stringent requirements for single-molecule spectroscopy. [3,4] We have found that the amino-substituted perylene imides 5PI and 6PI (Scheme 1) indeed combine excellent detectability at the single-molecule level with substantial solvatochromic sensitivity. On closer inspection, we noted that 5PI shows a very unusu- al solvatochromic behavior: Much as in other push–pull substi- tuted conjugated systems, both absorption and emission bands showed a red-shift upon increasing solvent polarity. The Stokes shift, however, was almost solvent independent. It is commonly (and justifiably!) assumed that solvatochromic shifts of electron donor–acceptor systems are primarily due to the solvent responding to the molecular electrostatic potential, which is different in the ground and excited states, the latter being more polar than the former. The solvatochromic behav- ior of 6PI and 5NI, [5] which we included in this study for com- parison, can be described with this conventional approach; but this fails for 5PI. We will argue below that in 5PI the solva- tochromic shifts are largely caused by solvent-induced changes in the molecular electronic structure, which lead to a decrease in the S 0 –S 1 energy gap and a decrease in the dipole moment difference upon increasing solvent polarity. This unique behav- ior is due to the very large ground-state dipole moment of 5PI, combined with a very large polarizability. 2. Solvatochromism Compounds in which a substantial change of the molecular dipole moment occurs upon electronic excitation usually reveal large spectral shifts with solvent polarity in absorption, in emission, or in both. These solvatochromic shifts are often used to derive the changes of the dipole moment upon excita- tion using dielectric continuum models, formulated in the 1950s by Lippert [6] and Mataga et al., [7] on the basis of the ear- lier work of Ooshika, [8] Onsager, [9] and Kirkwood. [10] Expressions for the solvent-induced changes in the absorption energy and the emission energy relative to their values in vacuum, and the Stokes shift are given in Equations (1)–(3). [11] E abs ¼ E 0 abs  1 1 3  m ! g ð m ! e  m ! g Þðf ðeÞf ðn 2 ÞÞ þ 1 2 ðm 2 e m 2 g Þf ðn 2 Þ  ð1Þ In N-(2,5-di-tert-butylphenyl)-9-pyrrolidinoperylene-3,4-dicarboxi- mide (5PI) the absorption and emission spectra display large sol- vatochromic 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 the solvent polarity, whereas the excited-state dipole moment is less solvent depen- dent. In the corresponding piperidine compound, 6PI, this effect is much less important owing to the weaker coupling between the amino group and the aromatic imide moiety, and in the cor- responding naphthalimide, 5NI, it is absent. The latter shows the conventional solvatochromic behavior of a push–pull substituted conjugated system, that is, minor shifts in absorption and a larger change in the emission energy with solvent polarity. Scheme 1. Structures of 5PI, 6PI, and 5NI. In experiments: R = 2,5-di-tert-bu- tylphenyl; for computational models: R = H. [a] P. D. Zoon, Dr. A. M. Brouwer Van’t Hoff Institute for Molecular Sciences, University of Amsterdam Nieuwe Achtergracht 129, 1018 WS Amsterdam (The Netherlands) Fax (+ 31) 20-525-5670 E-mail: a.m.brouwer@uva.nl [**] Solvatochromism with Solvent-Independent Stokes Shifts. Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author. 1574 # 2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/cphc.200500127 ChemPhysChem 2005, 6, 1574 – 1580