π-Conjugated Zwitterions as Paradigm of Donor–Acceptor Building Blocks in Organic-Based Materials

Abstract

The very peculiar characteristics of zwitterions, as well as a clearand unambiguous definition, have been overlooked in past literature. However, these compounds are particularly important in view of the impact they have had in the recent past and will likely continue to have in the future as components of performing functional organic and hybrid materials. In this Account, we primarily aim to define critically important organic concepts of zwitterions regarding both their design and nomenclature. We will particularly focus on a specific kind of zwitterions we define as π-conjugated zwitterions. These types of zwitterions are systems pertaining to the class ofdonor-acceptor (push-pull) molecules. In the ground state, they are preferentially represented in terms of an electron donor moiety bearing a negative net charge, and electron acceptor one bearing a positive net charge connected by a conjugated bridge. As such, they are possibly the most effective example of push-pull structure, possessing relevant features for applications like nonlinear optics, photovoltaics, imaging, and high capacitance dielectrics. In addition, the interaction between these dipolar compounds and the environment is highly specific and can be exploited in the construction of well-organized nanostructures, both in solution and in the solid state. According to the Gold Book of IUPAC for nomenclature, the distinction between zwitterions and the charged molecule called a betaine is subtle. The betaine is a particular class of zwitterion possessing an onium atom not bearing a hydrogen. The two terms are often considered equivalent, thus generating confusion while retrieving literature. In this Account, we define and describe π-conjugated zwitterions systems that are dipolar in the ground state, admitting resonance limiting structures that are neutral and chargeless. For the purpose of this Account and to the benefit of researchers striving to retrieve materials-related zwitterion literature data, we suggest to use the term π-zwitterions instead of the commonly used plain term "zwitterions". We show that this definition enables the clear identification of a class of compounds having unique properties distinct from "dipolar conjugated compounds." We describe the most common donor and acceptor groups in π-zwitterions. In particular, we focus our attention on the special case of the nitrile functionality, which tends to be contiguous to a negative charge. We also address special emphasis to benzenoid components that are substituted by heteroaromatic units in π-zwitterions, because the HOMO-LUMO energetic consequences are specifically involved in these cases. We make reference to the paradigmatic case of π-zwitterions second order nonlinear optical properties. Here, the value of the first hyperpolarizability β versus the alternation in bond length turns out to be a measure of the balance of the chargeless and the dipolar contribution to the description of the zwitterion ground state. We also report literature data, collected both from our group and others, concerning π-zwitterions containing heteroaromatic and/or nitrile groups, those based on the most performing acceptors so far described, and merocyanines. With particular reference to merocyanines, we show how π-zwitterions can play a fundamental role in the fast growing field of organic photovoltaics. Finally, we present π-zwitterions made up of heteroaromatic groups that open new scenarios in heteroaromatic chemistry.