Environment-sensitive behavior of fluorescent molecular rotors

The term molecular rotor is commonly used to describe a fluorescent molecule that has the ability to undergo an intramolecular twisting motion in the fluorescent excited state. Typically, a molecular rotor consists of three subunits, an electron donor unit, an electron acceptor unit, and an electron-rich spacer unit that is composed of a network of alternative single and double bonds. This network brings the donor and acceptor units in conjugation, thus facilitating electron movement between this pair, but it ensures minimum overlap of the electron donor and electron acceptor orbitals [1]. In this configuration, the molecule responds to photoexcitation with an intramolecular charge transfer from the donor to the acceptor unit. Whereas the three subgroups assume a planar or near-planar configuration in the ground state, electrostatic forces induce an intramolecular twisting motion of the sub-groups relative to each other [2]. The molecule enters a nonplanar (twisted) state with a lower excited-state energy, and relaxation from the twisted state is associated with either a red-shifted fluorescence emission or nonfluorescent relaxation, depending on the molecular structure [3,4]. The basic structure of a molecular rotor, together with several typical examples, is shown in Figure 1. Moreover, several chemical classes of molecular rotors exist [5], which are listed in Table 1 together with photophysical characteristics of typical representatives [6-9]. Other fluorophores exist which show predominantly polarity-sensitive behavior that has been attributed to formation of twisted states, but they are much less well-explored than the classes listed in Table 1. Examples are Nile Red [10,11] and 8-(phenylamino)-1-naphthalenesulfonate (ANS) [12]. A meso-substituted form of dipyrrometheneboron difluoride (BODIPY) has also been hypothesized to form twisted states [13]. However, chemical computations by Kee et al. [13] present a very atypical picture where a planar configuration (