David A. Blank

Upon binding 4-(perylen-3-yl)benzoic acid 1, to oleate-capped ZnO nanocrystals (NCs), significant broadening of the perylene absorption spectrum and fluorescence quenching were observed. In contrast, no broadening of the absorption spectrum or fluorescence quenching was observed when 1 was bound to oleylamine-capped ZrO2 NCs. Measurement of emission quenching by ZnO NCs as a function of dye concentration quantified binding and established that approximately 150 dyes attached to the surface of 3.7 nm diameter ZnO NCs, which correlated roughly to the number of zinc ions on the NC surface. A combination of steady-state spectroscopy, spectroelectrochemistry, and ultrafast pump-probe spectroscopy established that fluorescence quenching occurred via a fast excited-state electron transfer mechanism with a lifetime of less than 100 fs. The singlet excited state of 1 was also generated in solution and on the surface of ZrO2 NCs via triplet–triplet annihilation upconversion (TTA-UC) using a bodipy sensitizer that was present in solution or bound to the ZrO2 surface. Out of a possible maximum of 0.5, the TTA-UC quantum yields in CH2Cl2 were 0.0037 with and 0.0034 without ZrO2 NCs. Following selective excitation of the bodipy dye, dispersions containing both dyes and ZnO NCs showed no upconverted fluorescence from the perylene, demonstrating the ability of ZnO NCs to harvest two low-energy photons through TTA-UC.

Strong photoreductants based on first-row transition metals are rare. We report an anionic heterobimetallic Co(-I)-Ga complex, [(H₂)CoGa(N(o-(NCH₂PⁱPr₂)C₆H₄)₃)]^- ([2-H₂]^-), that catalyzes the hydrodefluorination of electron-rich fluoroarenes under violet light irradiation (390 nm), in the presence of stoichiometric alkoxide base, substoichiometric crown ether, and 1 atm H₂. The photodissociation of H₂ from [2-H₂]^- to form the 'naked' Co(-I) species [2]^- was observed by ultrafast transient absorption spectroscopy. In combination with its reactivity profile, [2]^- is proposed to be photoexcited to form an active photoreductant that reduces fluoroarenes via outer-sphere single-electron transfer. Detailed mechanistic studies suggest that the resultant Co(0) complex (2^ox-H₂), which is characterized by EPR spectroscopy, re-enters the catalytic cycle via a putative anionic Co(0) hydride intermediate [2^ox-H]^-. To the best of our knowledge, this is the first example of a 3d metal photoredox catalyst for C-F bond activation.

Ionic liquids (ILs) used in solar, electrochemical, nuclear, or even space exploration applications will necessarily be exposed to conditions in which electron detachment may occur and transient charge-depleted radical species may form. In a recent article (J. Am. Chem. Soc. 2025, 147, 23395–23398), we studied the different possible kinetic fates of an excess electron in the popular 1-butyl-1-methylpyrrolidinium dicyanamide IL but did not address the nature of electron-deficient transient radical species these leaving electrons produce, the so-called holes. Using pulse radiolysis measurements and first-principles calculations in the bulk and gas phases to interpret these, we show that the most likely hole species in this IL is a dimer radical anion that, somewhat surprisingly, has similar spectral features to the excess electron, including a broad near-infrared absorption band.

Ionic liquids (ILs) for electrochemical, nuclear, and solar energy applications operate under harsh conditions, where electrons and transient radical species can form. This communication discusses why anions such as bis(trifluoromethylsulfonyl)imide (Tf₂N^-) are reduced at the electron-rich electrode whereas in laser photoionization or pulse radiolysis studies, where electrons are ejected from species in the bulk, we often detect long-lived electrons in cavities that interact with IL cations instead. This work argues that bulk excess electrons generated photolytically or radiolytically follow kinetically favored pathways. As such, cavity electrons may not be the most energetically favorable states, but when they form, and they do form, they are kinetically stable. Reduction reactions of anions or electron localization in cavities and subsequent reactions are all expected outcomes. Here we focus on a pyrrolidinium-based IL of the dicyanamide (N(CN)₂^-) anion because of its large electrochemical window and very low viscosity, which are ideal for energy applications.

We have prepared and characterized two diradicaloid systems 5a and 5b that originated from the oxidation of a 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPY core. The aza-BODIPY diradicaloids were characterized by a large array of experimental and computational methods. The diamagnetic closed-shell state was postulated as the ground state in solution and a solid-state with the substantial thermal population originating from both open-shell diradical and open-shell triplet states observed at room temperature. Transient absorption spectroscopy indicates fast (1H NMR spectra indicate the solvent dependency of the diradical character in 5a and 5b. The diradicaloids could be stepwise reduced to the mixed-valence radical-anion and dianion states upon consequent single-electron reductions. Similarly, deprotonated 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPYs can be oxidized to the diradicaloid form. Both mixed-valence and dianionic forms exhibit an intense absorption in the NIR region. Density functional theory (DFT) and time-dependent DFT calculations were used to explain the transformations in the UV-Vis-NIR spectra of all target compounds.

The excited state dynamics in two fully characterized pyridoneBODIPY-fullerene complexes were investigated using time-resolved spectroscopy. Photoexcitation was initially localized on the pyridoneBODIPY chromophore. The energy was rapidly transferred to the fullerene, which subsequently underwent ISC to form a triplet state and returned the energy to the pyridoneBODIPY via triplet-triplet energy transfer. This ping-pong energy transfer mechanism resulted in efficient (>85%) overall conversion of the excited state pyridoneBODIPY constituent despite a complete lack of ISC in the pyridoneBODIPY in the absence of the fullerene partner. The small difference in attachment chemistry for the fullerene did not impact the initial singlet energy transfer. However, the N-methylpyrrolidine bridge did slow both the triplet-triplet energy transfer and the ultimate relaxation rate of the final triplet state when compared to an isoxazole-based bridge. The rates of each step were quantified, and computational predictions were used to complement the proposed mechanism and energetics. The result demonstrated efficient triplet sensitization of a strong chromophore that lacks significant spin-orbit coupling.

Several pyrene–boron-dipyrromethene (BODIPY) and pyrene–BODIPY–ferrocene derivatives with a fully conjugated pyrene fragment appended to the α-position(s) of the BODIPY core have been prepared by Knoevenagel condensation reaction and characterized by one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR), UV–vis, fluorescence spectroscopy, high-resolution mass spectrometry as well as X-ray crystallography. The redox properties of new donor–acceptor BODIPY dyads and triads were studied by electrochemical (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)) and spectroelectrochemical approaches. Formation of weakly bonded noncovalent complexes between the new pyrene–BODIPYs and nanocarbon materials (C₆₀, C₇₀, single-walled carbon nanotube (SWCNT), and graphene) was studied by UV–vis, steady-state fluorescent, and time-resolved transient absorption spectroscopy. UV–vis and fluorescent spectroscopy are indicative of the much stronger and selective interaction between new dyes and (6,5)-SWCNT as well as graphene compared to that of C₆₀ and C₇₀ fullerenes. In agreement with these data, transient absorption spectroscopy provided no evidence for any significant change in excited-state lifetime or photoinduced charge transfer between pyrene–BODIPYs and C₆₀ or C₇₀ fullerenes when the pyrene–BODIPY chromophores were excited into the lowest-energy singlet excited state. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations suggest that the pyrene fragments are fully conjugated into the π-system of BODIPY core, which correlates well with the experimental data.

The electronic communication between two ferrocene groups in the electron-deficient expanded aza-BODIPY analogue of zinc manitoba-dipyrromethene (MB-DIPY) was probed by spectroscopic, electrochemical, spectroelectrochemical, and theoretical methods. The excited-state dynamics involved sub-ps formation of the charge-separated state in the organometallic zinc MB-DIPYs, followed by recovery of the ground state via charge recombination in 12 ps. The excited-state behavior was contrasted with that observed in the parent complex that lacked the ferrocene electron donors and has a much longer excited-state lifetime (670 ps for the singlet state). Much longer decay times observed for the parent complex without ferrocene confirm that the main quenching mechanism in the ferrocene-containing 4 is reflective of the ultrafast ferrocene-to-MB-DIPY core charge transfer (CT).

The ultrafast transient absorption spectrum of 1-butyl-1-methyl-pyrrolidinium dicyanamide, [Pyr_1,4⁺][DCA^-], was measured in the visible and near-IR spectral regions. Excitation of the liquid at 4.6 eV created initially delocalized and highly reactive electrons that either geminately recombined (69%) or localized onto a cavity with a time constant of ~300 fs. Electron localization was reflected in the evolution of the TA spectrum and the time dependent loss of reactivity with a dichloromethane quencher. The delocalized initial state and spectrum of the free electrons were consistent with computational predictions by Xu and Margulis (J. Phys. Chem. B, 119, 532--542 (2015)) on excess electrons in [Pyr_1,4⁺][DCA^-]. The computational study considered two possible localization mechanisms for excess electrons, localization on ions and localization on cavities. In the case of photo-generated electron-hole pairs the results presented here demonstrate localization to cavities as the dominant channel. Following localization onto a cavity, the free electrons underwent solvation and loss of reactivity with the quencher with rates that slowed in time. The dynamics were similar to an analogous prior study on the related liquid [Pyr_1,4⁺][NTf₂^-]. One significant difference was the larger yield of free electrons from photo-excitation of [Pyr_1,4⁺][DCA^-]. This was found to primarily reflect more efficient localization onto cavities rather than a slower geminate recombination rate.