Light-activated theranostics offer promising opportunities for disease diagnosis, image-guided surgery, and site-specific personalized therapy. However, current fluorescent dyes are limited by low brightness, high cytotoxicity, poor tissue penetration, and unwanted side effects. To overcome these limitations, we demonstrate a platform for optoelectronic tuning, which allows independent control of the optical properties from the electronic properties of fluorescent organic salts. This is achieved through cation-anion pairing of organic salts that can modulate the frontier molecular orbital without impacting the bandgap. Optoelectronic tuning enables decoupled control over the cytotoxicity and phototoxicity of fluorescent organic salts by selective generation of mitochondrial reactive oxygen species that control cell viability. We show that through counterion pairing, organic salt nanoparticles can be tuned to be either nontoxic for enhanced imaging, or phototoxic for improved photodynamic therapy.

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new lowspin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from −200 to −430 mV versus Fc +/0 . The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron selfexchange rate constants of 2.2−4.2 M −1 s −1 compared to the highspin to low-spin redox couple [Co(bpy) 3 ] 3+/2+ at 0.27 M −1 s −1 , which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy) 3 ] 3+/2+ redox shuttle (19.8 vs 18.0 mA/cm 2 ). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

Copper complexes have recently shown remarkable performance upon conversion from liquid-based to solid-state hole transport materials (HTMs) in mesoscopic solar cells; however, the diffusion mechanism is not clear. In this work, we apply an in situ solidification analysis of the charge diffusion and find that the dominant mechanism of [Cu­(dmbpy)2]2+/+ (dmbpy = 6,6′-dimethyl-2,2′-bipyridine) changes from ionic to electronic diffusion. Through use of the modified Dahms–Luff equation, a fast self-exchange rate constant of hole-hopping in the HTM of 8.3 × 108 (±5 × 107) M–1 s–1 is calculated, which indicates a small reorganization energy of 0.47 eV. These results introduce a new methodology to analyze the transport mechanism of solids, reveal the mechanism of charge transport in molecular-based HTMs, and offer insight into ways to control the flow of charge in optoelectronic systems.

A new low-spin (LS) cobalt(II) outer-sphere redox shuttle (OSRS) [Co(PY5Me 2 )(CN)] + , where PY5Me 2 represents the pentadentate ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, has been synthesized and characterized for its potential application in dye-sensitized solar cells (DSSCs). Introduction of the strong field CN − ligand into the open axial coordination site forced the cobalt(II) complex, [Co-(PY5Me 2 )(CN)] + , to become LS based upon the complex's magnetic susceptibility (1.91 ± 0.02 μ B ), determined by the Evans method. Interestingly, dimerization and subsequent cobalt hexacyanide cluster formation of the [Co(PY5Me 2 )(CN)] + monomer was observed upon long-term solvent exposure or addition of a supporting electrolyte for electrochemical characterization. Although long-term stability of the [Co(PY5Me 2 )-(CN)] + complex made it difficult to fabricate liquid electrolytes for DSSC applications, short-term stability in neat solvent afforded the opportunity to isolate the self-exchange kinetics of [Co(PY5Me 2 )(CN)] 2+/+ via stopped-flow spectroscopy. Use of Marcus theory provided a smaller than expected self-exchange rate constant of 20 ± 5.5 M −1 s −1 for [Co(PY5Me 2 )(CN)] 2+/+ , which we attribute to a Jahn−Teller effect observed from the collected monomer crystallographic data. When compared sideby-side to cobalt tris(2,2′-bipyridine), [Co(bpy) 3 ] 3+ , DSSCs employing [Co(PY5Me 2 )(CN)] 2+ are expected to achieve superior charge collection, which result from a smaller rate constant, k et , for recombination based upon simple dark J−E measurements of the two redox shuttles. Given the negative redox potential (0.254 V vs NHE) of [Co(PY5Me 2 )(CN)] 2+/+ and the slow recombination kinetics, [Co(PY5Me 2 )(CN)] 2+/+ becomes an attractive OSRS to regenerate near IR absorbing sensitizers in solid-state DSSC devices.

The electron transfer kinetics in the DSSCs were tuned dramatically by coordination of the strong-field ligand 2,6-dimethyl isocyanide to induce a low-spin Co(ii) redox shuttle.