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1. Singlets-Driven Photoredox Catalysts: Transforming Noncatalytic Red Fluorophores to Efficient Catalysts

   https://doi.org/10.1021/jacsau.4c00877  

   Abuhadba, Sara; Fuqua, Charlotte; Maltese, Anthony; Schwinn, Caroline; Lin, Neo; Chen, Angela; Martzloff, Rilee; Esipova, Tatiana V; Mani, Tomoyasu (December 2024, JACS Au)

   Red-light absorbing photoredox catalysts offer potential advantages for large-scale reactions, expanding the range of usable substrates and facilitating bio-orthogonal applications. While many red-light absorbing/emitting fluorophores have been developed recently, functional red-light absorbing photoredox catalysts are scarce. Many photoredox catalysts rely on long-lived triplet excited states (triplets), which can efficiently engage in single electron transfer (SET) reactions with substrates. However, triplets of π-conjugated molecules are often significantly lower in energy than photogenerated singlet excited states (singlets). Combined with the inherent low energy of red light, this could limit the reductive/oxidative powers. Here, we introduce a series of sustainable heavy atom–free photoredox catalysts based on red-light absorbing dibenzo-fused BODIPY. The catalysts consist of two covalently linked units: a dibenzo-fused BODIPY fluorophore and an electron donor, arranged orthogonally. Excitation of the dibenzoBODIPY unit induces charge separation (CS) from the donor to the dibenzo-BODIPY unit, forming a radical pair (RP) state. Unlike the regular BODIPY counterparts, these catalysts do not form triplets. Instead, SET occurs from the high-energy singlet-born RP states, preventing energy loss and effectively utilizing the low-energy red light. The proximity of donor molecules allows efficient charge separation despite the CS being uphill in energy. The molecules demonstrate efficient catalysis of Atom Transfer Radical Addition (ATRA) reaction, yielding products with high yields ranging from 70% to 90%, while the molecule without a donor group does not exhibit catalytic activity. The mechanistic studies by transient absorption and electron paramagnetic resonance (EPR) spectroscopy methods support the proposed mechanism. The study presents a new molecular design strategy for converting noncatalytic fluorophores to effi-cient photoredox catalysts operating in the red spectral region. 

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2. Electron Transfer in Rhodamine–TiO2 Complexes Studied as a Function of Chalcogen and Bridge Substitution

   https://doi.org/10.1021/acs.jpcc.9b11049  

   Zachary Piontkowski, Yu-Chen Wang (January 2020, The journal of physical chemistry)

   Many emerging light-harvesting systems for solar-energy capture depend on absorption of light by molecular dyes and subsequent electron transfer to metal-oxide semiconductors. However, the inhomoge- neous electron-transfer process is often misunderstood when analogies from bimolecular electron transfer are used to explain experimental trends. Here, we develop and apply a theoretical methodology that correctly incorporates the semiconductor density of states and the system reorganization energies to explain observed trends in a series of molecular sensitizers. The effects of chalcogen and bridge substitution on the electron transfer in rhodamine− TiO2 complexes are theoretically investigated by combining density functional theory (DFT)/time-dependent DFT calculations and Fermi’s golden rule for the rate constant. It is shown that all dyes exhibit τeT < 4 ps. Dyes with thiophene bridges exhibit shorter τeT (∼1 ps) than dyes with phenylene bridges (∼4 ps). When the planes of the dye core and bridge are fixed at coplanarity, the dye−TiO2 coupling strength is found to increase by a factor of ∼2 when compared with the Franck− Condon geometry. However, the donor energy level of coplanar dyes falls significantly below the TiO2 conduction band edge so that, despite enhanced coupling, electron transfer is slowed to ∼20 ps. Similar results appear for the excited triplet states of these dyes, showing that the intersystem crossing to low energy triplet states can increase electron-transfer time constants to 60−240 ps. These results are compared to the results of previous photocatalytic hydrogen generation and dye-sensitized solar cell experiments. 

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3. Impact of molecular structure on singlet and triplet exciton diffusion in phenanthroline derivatives

   https://doi.org/10.1039/D0TC00716A  

   Rai, Deepesh; Bangsund, John S.; Barriocanal, Javier Garcia; Holmes, Russell J. (May 2020, Journal of Materials Chemistry C)

   We demonstrate the impact of subtle changes in molecular structure on the singlet and triplet exciton diffusion lengths ( L D ) for derivatives of the archetypical electron-transport material 4,7-diphenyl-1,10-phenanthroline (BPhen). Specifically, this work offers a systematic characterization of singlet and triplet exciton transport in identically prepared thin films, highlighting the differing dependence on molecular photophysics and intermolecular spacing. For luminescent singlet excitons, photoluminescence quenching measurements yield an L D from <1 nm for BPhen, increasing to (5.4 ± 1.2) nm for 2,9-dichloro-4,7-diphenyl-1,10-phenanthroline (BPhen-Cl 2 ) and (3.9 ± 1.1) nm for 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). The diffusion of dark triplet excitons is probed using a phosphorescent sensitizer-based method where triplets are selectively injected into the material of interest, with those migrating through the material detected via energy transfer to an adjacent, phosphorescent sensitizer. Interestingly, the triplet exciton L D decreases from (15.4 ± 0.4) nm for BPhen to (8.0 ± 0.7) nm for BPhen-Cl 2 and (4.0 ± 0.5) nm for BCP. The stark difference in behavior observed for singlets and triplets with functionalization is explicitly understood using long-range Förster and short-range Dexter energy transfer mechanisms, respectively. 

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4. Open-shell donor–π–acceptor conjugated metal-free dyes for dye-sensitized solar cells

   https://doi.org/10.1039/D0ME00091D  

   Sabuj, Md Abdus; Rai, Neeraj (November 2020, Molecular Systems Design & Engineering)

   null (Ed.)

   Dye-sensitized solar cells (DSCs) have drawn a significant interest due to their low production cost, design flexibility, and the tunability of the sensitizer. However, the power conversion efficiency (PCE) of the metal-free organic dyes is limited due to the inability of the dye to absorb light in the near-infrared (NIR) region, leaving a large amount of energy unused. Herein, we have designed new DSC dyes with open-shell character, which significantly red-shifts the absorption spectra from their counterpart closed-shell structure. A small diradical character ( y < 0.10) is found to be beneficial in red-shifting the absorption maxima into the NIR region and broadening up to 2500 nm. Also, the open-shell dyes significantly reduce the singlet–triplet energy gaps (Δ E ST ), increase the total amount of charge-transfer to the semiconductor surface, reduce the exciton binding energy, and significantly increase the excited-state lifetimes compared to the closed-shell systems. However, the closed-shell dyes have higher injection efficiency with increased intramolecular charge transfer (ICT) character. Our study reveals the design rule for open-shell DSC dyes to be able to absorb photons in the NIR region, which can increase the efficiency of the solar cell device. 

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5. Exciton Basis Description of Ultrafast Triplet Separation in Pentacene-(Tetracene)2-Pentacene Intramolecular Singlet Fission Chromophore

   https://doi.org/10.1021/acs.jpca.5c03774  

   Nazir, Arifa; Shukla, Alok; Mazumdar, Sumit (October 2025, The Journal of Physical Chemistry A)

   Shea, Joan E; Dreuw, Andreas (Ed.)

   Precise understanding of the electronic structures of optically dark triplet–triplet multiexcitons of π-conjugated carbon-based systems requires computations incorporating configuration interaction (CI) with up to quadruple excitations from the single-particle ground state of many-electron Hamiltonians. This continues to be a challenge in the context of intramolecular singlet fission (iSF), where the systems of interest are oligomers of large monomer molecules, and CI matrices have dimensions of several million. We have performed many-body calculations of the complete set of excited states relevant to iSF in Pentacene-(Tetracene)2-Pentacene oligomers, consisting of two terminal pentacene monomers linked by two tetracene monomers. Our computations within the Pariser–Parr–Pople Hamiltonian use an exciton basis that gives physical pictorial descriptions of all of the eigenstates. They are performed over an active space of twenty-eight monomer molecular orbitals and include configuration interaction with all relevant quadruple excitations within the active space, thereby ensuring high accuracy. We discuss the many-electron structures of the lowest optical predominantly intramonomer spin-singlets, intermonomer charge-transfer excitations, and most importantly, the complete set of low-energy covalent triplet–triplet multiexcitons. We can explain the weak binding energy of the pentacene-tetracene triplet–triplet eigenstate that is generated immediately following photoexcitation as opposed to the large binding energy of the triplet–triplet in polypentacene. Our approach allows calculations of excited state absorptions (ESAs) from the optical singlet as well as the triplet–triplet, thereby making comparisons with experimental transient absorption measurements in Pentacene-(Tetracene)2-Pentacene feasible. We explain the increase in lifetime with increasing numbers of tetracene monomers of the transient absorption associated with contiguous pentacene-tetracene triplet–triplet oligomers in this family of oligomers. We are able to give a pictorial description of the triplet separation following generation of the initial triplet–triplet, leading to a state with individual triplets occupying only the two pentacene monomers. We expect many applications of our theoretical approach to triplet separation in the iSF. 

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