We have successfully achieved the electron-transfer (ET) state of 9-mesityl-10-methylacridinium ion, produced by a single step photoinduced electron transfer, which has a much longer lifetime (e.g., 2 h at 203 K) and higher energy (2.37 eV) than that of the natural system without loss of energy due to multistep electron-transfer processes.

The crystal structures of the pure, unsubstituted firefly emitter oxyluciferin (OxyLH(2)) and its 5-methyl analogue (MOxyLH(2)) were determined for the first time to reveal that both molecules exist as pure trans-enol forms, enol-OxyLH(2) and enol-MOxyLH(2), assembled as head-to-tail hydrogen-bonded dimers. Their steady-state absorption and emission spectra (in solution and in the solid state) and nanosecond time-resolved fluorescence decays (in solution) were recorded and assigned to the six possible trans chemical forms of the emitter and its anions. The spectra of the pure emitter were compared to its bioluminescence and fluorescence spectra when it is complexed with luciferase from the Japanese firefly (Luciola cruciata) and interpreted in terms of the intermolecular interactions based on the structure of the emitter in the luciferase active site. The wavelengths of the emission spectral maxima of the six chemical forms of OxyLH(2) are generally in good agreement with the theoretically predicted energies of the S(0)-S(1) transitions and range from the blue to the red regions, while the respective absorption maxima range from the ultraviolet to the green regions. It was confirmed that both neutral forms, phenol-enol and phenol-keto, are blue emitters, whereas the phenolate-enol form is yellow-green emitter. The phenol-enolate form, which probably only exists as a mixture with other species, and the phenolate-enolate dianion are yellow or orange emitters with close position of their emission bands. The phenolate-keto form always emits in the red region. The concentration ratio of the different chemical species in solutions of OxyLH(2) is determined by several factors which affect the intricate triple chemical equilibrium, most notably the pH, solvent polarity, hydrogen bonding, presence of additional ions, and pi-pi stacking. Due to the stabilization of the enol group of the 4-hydroxythiazole ring by hydrogen bonding to the proximate adenosine monophosphate, which according to the density functional calculations is similar to that due to the dimerization of two enol molecules observed in the crystal, the phenolate ion of the enol tautomer, which is the predominant ground-state species within the narrow pH interval 7.44-8.14 in buffered aqueous solutions, is the most probable emitter of the yellow-green bioluminescence common for most wild-type luciferases. This conclusion is supported by the bioluminescence/fluorescence spectra and the NMR data, as well the crystal structures of OxyLH(2) and MOxyLH(2), where the conjugated acid (phenol) of the emitter exists as pure enol tautomer.

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.