Electronic Excited‐State Engineering

Abstract

The immense flow of sun power that hits the atmosphere, approximately 170000 trillion watts (170 PW), drives large scale natural processes such as photosynthesis, which are key to sustaining life in the biosphere. The conversion of solar light energy in the photosynthetic process is done by complex supramolecular architectures which convert the photon flux into chemical fuel. This operation is enabled by electronic excited states residing in the molecular subunits of photosynthetic systems. These systems accept, store and transfer the excitation energy in a concerted temporal and spatial sequence. In the last 20 years we have witnessed the development of supramolecular photochemistry. The assembly by design of synthetic molecular fragments having specific basic functions (e.g. light harvesting) to build up a supramolecular architecture capable of capturing photons and delivering a superior action (e.g. energy conversion, mechanical motion) is the basic concept of the discipline. This research can ultimately be viewed as a kind of electronic excited state engineering, in which chemists make and test supramolecular systems which deliver a specific action, but only when they are properly designed in terms of the excited state interactions among their components—as found in nature. Thus, a huge number of supramolecular photoactive systems, for example light-powered nanomachines or prototype artificial photosynthetic modules, have been prepared over the years. When engineering artificial photoactive supramolecular systems researchers are often puzzled by problems related to the improvement and/or tuning of the electronic properties of the molecules they wish to assemble. This challenging work may have to do, for instance, with the increase of absorption in a given spectral window, the tuning of the luminescence colour, or the prolongation of an excited-state lifetime. The last issue is particularly important because the duration of an electronic excited state largely dictates its final destiny, especially in supramolecular systems. In a seminal paper from 1992, Ford and Rodgers first reported that the luminescent metal-to-ligand-charge transfer triplet excited state (MLCT*) of a Rupolypyridine complex exhibits a prolonged lifetime (11-fold) when covalently linked to a pyrene fragment via a hydrocarbon chain. The reason for this unusual and potentially interesting behaviour is that the p-p triplet excited state of the pyrene unit (pp*) is lower-lying and almost isoenergetic to the MLCT* level of the Ru-complex moiety and, in solution at room temperature, they may undergo thermal equilibration upon MLCT*!pp* energy transfer followed by back-transfer of energy, as depicted in Figure 1. The lifetime prolongation relies on the temporary storage of the excitation energy on the long-lived pyrene triplet level, which practically acts as an energy reservoir, a role reminiscent of a hydroelectric basin. Eventually, this gives back the excess energy to the metal complex which undergoes delayed deactivation. The engineering concept of this photoactive dyad encompasses two crucial features: a) the reservoir lifetime (pyrene triplet) must be significantly longer than that of the Ru-based MLCT* level ; b) the thermal rate of energy back-transfer (kb in Figure 1) must be significantly faster