Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores

Abstract

Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups. The addition of sulfonate groups enables solubility in pure water, and it also induces assembly with the dicationic electron acceptor methyl viologen (MV2+) via bimolecular, dynamic electrostatic interactions. The effect of the sulfonate groups on the ground- and excited-state properties was evaluated by comparison with the unsulfonated analogues in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated complexes are similar to what we expect from previous literature, with the exception of the sulfonated heteroleptic complex whose metal-to-ligand charge-transfer (MLCT) lifetime in water has two components that are fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced MV+• in both solvent environments following MLCT excitation, but control measurements in 1:1 acetonitrile/water with the unsulfonated analogues showed no PET to MV2+, indicating that electrostatically driven supramolecular assemblies of the sulfonated complexes with MV2+ facilitate the observed PET. Additionally, the strength of the intermolecular interactions driving the formation of these assemblies changes drastically with the solvent environment. In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes with quantum yields (ΦET) of 2–3% but increased to a remarkable 98% for the sulfonated heteroleptic complex with a 3 μs CS-state lifetime in water.

Better Covalent Connection in a Molecular Triad Enables More Efficient Photochemical Energy Storage

“…The primary photoproduct is a charge-separated state, in which one reducing equivalent is separated spatially from an oxidizing equivalent, in the form of a quinone radical anion and a chlorophyll radical cation. The formation and decay of charge-separated states by photoinduced electron transfer have therefore received enormous attention, and many donor–acceptor compounds have been investigated over the past five decades. , In fully integrated molecular compounds with all reaction components linked covalently to one another, the overall photochemical reactivity is not limited by diffusion, , and therefore, direct insight into the photoinduced electron (or energy) transfer events is obtainable. In the simplest case, this approach focuses on molecular dyads comprising merely the donor and the acceptor, but often molecular triads containing a separate photosensitizer in addition to the donor and the acceptor are explored. − In rarer cases, even tetrads or pentads, for example involving secondary donor and acceptor moieties, become of interest. − …”

Towards Electrochemiluminescence Microscopy Exploration of Plasmonic‐Mediated Phenomena at the Single‐Nanoparticle Level

“…Overall, the identification of two different enhancement mechanisms showcases the potential of ECLM for the exploration of several plasmon‐mediated phenomena such as plasmon‐induced resonant energy transfer, plasmon‐mediated (photo)electrocatalysis, or strong‐coupling interactions. This could be especially relevant for the study of plasmonic electrochemical interfaces, as a more straightforward alternative to electrochemical transient absorption [12] . The proposed ECLM setup can also be effortlessly implemented for the characterization of more complex plasmonic metasurfaces, expanding the study of localized plasmon resonances to chiral plasmons and collective lattice resonances [13] .…”

“…This could be especially relevant for the study of plasmonic electrochemical interfaces, as a more straightforward alternative to electrochemical transient absorption. [12] The proposed ECLM setup can also be effortlessly implemented for the characterization of more complex plasmonic metasurfaces, expanding the study of localized plasmon resonances to chiral plasmons and collective lattice resonances. [13] In this context, its combination with scalable and affordable fabrication methodologies such as colloidal self-assembly or in situ patterned nanoparticle growth can potentially result in the rational design of more efficient catalysts, optoelectronics and sensors.…”

Enhanced Visible Light Absorption in Heteroleptic Cuprous Phenanthrolines

“…This platform facilitates a facile pathway for obtaining heteroleptic Cu­(I) complexes with the generic formula [Cu­(mesPhen)­(L)] + , where L is a second and distinct phenanthroline ligand (Chart ). The secondary phenanthroline moiety can be systematically varied to impart desired photophysical and electrochemical properties. ,− Optically and electronically tuned HETPHEN Cu­(I) complexes have been designed to feature bulky substituents to limit the distortion similar to numerous homoleptic complexes , and have incorporated phenyl groups at the 4,7-positions to increase the absorbance cross-section in the visible MLCT region by promoting π-delocalization of the charge-transfer excited state to increase oscillator strength. ,,− …”