The Role of Adenine in Fast Excited-State Deactivation of FAD: a Femtosecond Mid-IR Transient Absorption Study

We present a study of excited-state dynamics of two flavin cofactors: flavin−adenine dinucleotide (FAD) and flavin−mononucleotide (FMN). We used femtosecond mid-R transient absorption spectroscopy to study the effect of FAD conformation on its excited-state behavior. The conformation of FAD was modulated by changing the solvent polarity: in D2O, FAD is present predominantly in the “stacked” conformation, in which flavin and adenine moieties are in close proximity to each other, whereas the increased amount of DMSO led to an increased amount of the “open” conformer. FMN served as a model system which lacks adenine. We found that the “stacked” conformer undergoes an intramolecular photoinduced electron transfer from adenine to flavin with the forward electron transfer rate of kf = 1.9·1011 s−1 and the geminate recombination rate of kb = 1.1·1011 s−1. In the case of the “open” conformer, no intramolecular electron transfer was observed

Photoinduced Charge Separation in Platinum Acetylide Oligomers

The series of three donor-spacer-acceptor complexes, DPAF-Ptn-NDI, has been synthesized and characterized using time-resolved absorption spectroscopy. In these complexes, the donor is a (diphenylamino)-2,7-fluorenylene (DPAF) unit, the acceptor is a naphthalene diimide (NDI), and the spacers are a series of platinum acetylides of varying lengths, [−Pt(PBu3)2CCPhCC−]n (where Bu = n-butyl, Ph = 1,4-phenylene and n = 1, 2, and 3). Electrochemistry indicates that the DPAF-Ptn-NDI system has a charge transfer state at ca. 1.5 eV above the ground state that is based on one electron transfer from the DPAF donor to the NDI acceptor. Transient absorption spectroscopy on time scales ranging from 0.2 ps to 1 μs reveals that excitation of all of the complexes leads to production of the charge transfer state with nearly unit quantum efficiency. The rates for charge separation and charge recombination are not strongly dependent upon the length of the platinum acetylide spacer, suggesting that the spacer is actively involved in the electron (hole) transport processes. Analysis of the experimental results leads to a model in which charge separation and charge recombination occur by hole-hopping via states localized on the [−Pt(PBu3)2CCPhCC−]n bridge

DiiodoBodipy-Rhodamine Dyads: Preparation and Study of the Acid-Activatable Competing Intersystem Crossing and Energy Transfer Processes

Iodo-bodipy/rhodamine dyads with cyanuric chloride linker were prepared with the goal of achieving pH switching of the triplet excited state formation. The pH switching takes advantage of the acid-activated reversible cyclic lactam↔opened amide transformation of the rhodamine unit and the fluorescence resonance energy transfer (FRET). The photophysical properties of the dyads were studied with steady-state and femtosecond/nanosecond time-resolved transient absorption spectroscopies, electrochemical methods, as well as TD-DFT calculations. Our results show that the model dyad is an efficient triplet state generator under neutral condition, when the rhodamine unit adopts the closed form. The triplet generation occurs at the iodo-bodipy moiety and the triplet state is long-lived, with a lifetime of 51.7 μs. In the presence of the acid, the rhodamine unit adopts an opened amide form, and in this case, the efficient FRET occurs from iodo-bodipy to the rhodamine moiety. The FRET is much faster (τ_FRET = 81 ps) than the intersystem crossing of iodo-bodipy (τ_ISC = 178 ps), thus suppressing the triplet generation is assumed. However, we found that the additional energy transfer occurs at the longer timescale, which eventually converts the rhodamine-based S₁ state to the T₁ state localized on the iodo-bodipy unit

Diiodobodipy-styrylbodipy Dyads: Preparation and Study of the Intersystem Crossing and Fluorescence Resonance Energy Transfer

2,6-Diiodobodipy-styrylbodipy dyads were prepared to study the competing intersystem crossing (ISC) and the fluorescence-resonance-energy-transfer (FRET), and its effect on the photophysical property of the dyads. In the dyads, 2,6-diiodobodipy moiety was used as singlet energy donor and the spin converter for triplet state formation, whereas the styrylbodipy was used as singlet and triplet energy acceptors, thus the competition between the ISC and FRET processes is established. The photophysical properties were studied with steady-state UV–vis absorption and fluorescence spectroscopy, electrochemical characterization, and femto/nanosecond time-resolved transient absorption spectroscopies. FRET was confirmed with steady state fluorescence quenching and fluorescence excitation spectra and ultrafast transient absorption spectroscopy (<i>k</i>_FRET = 5.0 × 10¹⁰ s^–1). The singlet oxygen quantum yield (Φ_Δ = 0.19) of the dyad was reduced as compared with that of the reference spin converter (2,6-diiodobodipy, Φ_Δ = 0.85), thus the ISC was substantially inhibited by FRET. Photoinduced intramolecular electron transfer (ET) was studied by electrochemical data and fluorescence quenching. Intermolecular triplet energy transfer was studied with nanosecond transient absorption spectroscopy as an efficient (Φ_TTET = 92%) and fast process (<i>k</i>_TTET = 5.2 × 10⁴ s^–1). These results are useful for designing organic triplet photosensitizers and for the study of the photophysical properties

Thermodynamic Hydricities of Biomimetic Organic Hydride Donors

Thermodynamic hydricities (Δ<i>G</i>_H^–) in acetonitrile and dimethyl sulfoxide have been calculated and experimentally measured for several metal-free hydride donors: NADH analogs (BNAH, CN-BNAH, Me-MNAH, HEH), methylene tetrahydromethanopterin analogs (BIMH, CAFH), acridine derivatives (Ph-AcrH, Me₂N-AcrH, T-AcrH, 4OH, 2OH, 3NH), and a triarylmethane derivative (6OH). The calculated hydricity values, obtained using density functional theory, showed a reasonably good match (within 3 kcal/mol) with the experimental values, obtained using “potential p<i>K</i>_a” and “hydride-transfer” methods. The hydride donor abilities of model compounds were in the 48.7–85.8 kcal/mol (acetonitrile) and 46.9–84.1 kcal/mol (DMSO) range, making them comparable to previously studied first-row transition metal hydride complexes. To evaluate the relevance of entropic contribution to the overall hydricity, Gibbs free energy differences (Δ<i>G</i>_H^–) obtained in this work were compared with the enthalpy (Δ<i>H</i>_H^–) values obtained by others. The results indicate that, even though Δ<i>H</i>_H^– values exhibit the same trends as Δ<i>G</i>_H^–, the differences between room-temperature Δ<i>G</i>_H^– and Δ<i>H</i>_H^– values range from 3 to 9 kcal/mol. This study also reports a new metal-free hydride donor, namely, an acridine-based compound 3NH, whose hydricity exceeds that of NaBH₄. Collectively, this work gives a perspective of use metal-free hydride catalysts in fuel-forming and other reduction processes

Tuning Photophysical Properties and Improving Nonlinear Absorption of Pt(II) Diimine Complexes with Extended π‑Conjugation in the Acetylide Ligands

Two new Pt­(II) 4,4′-di­(5,9-diethyltridecan-7-yl)-2,2′-bipyridine complexes (1 and 2) bearing 9,9-diethyl-2-ethynyl-7-(2-(4-nitrophenyl)­ethynyl)-9H-fluorene ligand and N-(4-(2-(9,9-diethyl-7-ethynyl-9H-fluoren-2-yl)­ethynyl)­phenyl)-N-phenylbenzeneamine ligand, respectively, were synthesized and characterized. Their photophysical properties were investigated systematically by UV–vis absorption, emission, and transient absorption (TA) spectroscopy, and the nonlinear absorption was studied by nonlinear transmission technique. Theoretical TD-DFT calculations using the CAM-B3LYP functional were carried out to determine the nature of the singlet excited electronic states and to assist in the assignment of significant transitions observed in experiments. Complex 1 exhibits an intense, structureless absorption band at ca. 397 nm in CH2Cl2 solution, which is attributed to mixed metal-to-ligand charge transfer (1MLCT)/ligand-to-ligand charge transfer (1LLCT)/intraligand charge transfer (1ILCT)/1π,π* transitions, and two 1MLCT/1LLCT transitions in the 300–350 nm spectral region. Complex 2 possesses an intense acetylide ligand localized 1π,π* absorption band at ca. 373 nm and a moderately intense 1MLCT/1LLCT tail above 425 nm in CH2Cl2. Both complexes are emissive in solution at room temperature, with the emitting state being tentatively assigned to the predominant 3π,π* state for 1, whereas the emitting state of 2 exhibits a switch from 3π,π* state in high-polarity solvents to 3MLCT/3LLCT state in low-polarity solvents. Both 1 and 2 exhibit strong singlet excited-state TA in the visible to NIR region, where reverse saturable absorption (RSA) is feasible. The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these Pt complexes can be tuned drastically by extending the π-conjugation of the acetylide ligands. In addition, strong RSA was observed at 532 nm for nanosecond (ns) laser pulses from 1 and 2, demonstrating that the RSA of the Pt­(II) diimine complexes can be improved by extending the π-conjugation of the acetylide ligands

Mechanistic Studies of Electrode-Assisted Catalytic Oxidation by Flavinium and Acridinium Cations

Electrochemical behavior of flavinium (Et-Fl⁺) and acridinium (Acr⁺) cations is presented, in order to investigate their activity toward catalytic water oxidation. Cyclic voltammograms of Acr⁺ and Et-Fl⁺ in acetonitrile are qualitatively similar, with oxidation peaks at highly positive potentials, and these oxidation peaks depend strongly on the type of the working electrode being used. However, the two model compounds exhibit different behaviors in the presence of water: while Et-Fl⁺ facilitates electrocatalytic water oxidation through an electrode-assisted mechanism, water oxidation is not accelerated in the presence of Acr⁺. A comparative study of variable scan-rate cyclic voltammetry, concentration dependence, and spectroelectrochemical behavior of two model compounds suggest that Et-Fl⁺ and Acr⁺ exhibit different reaction pathways with the electrode surface. On the basis of the experimental results, a mechanism is proposed to account for the observed differences in electrocatalysis

Photoregeneration of Biomimetic Nicotinamide Adenine Dinucleotide Analogues via a Dye-Sensitized Approach

Two-step photochemical reduction of an acridinium-based cation 2O+ to the corresponding anion 2O– was investigated using a dye-sensitized approach involving 2O+–COOH attached to the surface of a wide-bandgap semiconductor, p-NiO. The cation 2O+ and corresponding one-electron reduced radical form, 2O• were synthesized and characterized using steady-state UV/vis and electron paramagnectic resonance spectroscopy. The thermodynamics for the photoinduced hole injection from 2O+ and 2O• were evaluated and found to be favorable. Subsequent femtosecond transient absorption spectroscopy was utilized to evaluate the photoinduced hole injection into NiO, starting from 2O+–COOH/NiO and 2O•–COOH/NiO samples. The excitation of 2O+–COOH at 620 nm initiated fast (2.8 ps) hole injection into NiO. However, 90% of the charge-separated population recombined within ∼40 ps, while ∼10% of the charge-separated population exhibited lifetimes longer than the time scale of our instrument (1.6 ns). In the case of 2O•–COOH/NiO, the light absorption occurs predominantly by NiO (2O•–COOH absorbs at 310 nm) and is associated with the electron transfer from the conduction band of NiO to the radical. The charge-separated state in this case appears to be long-lived, based on the slow (ns) growth of the trapped carriers formed on the NiO surface. The results of this work indicate that the photochemical reduction of 2O+ to the corresponding hydride form (2OH) can be achieved, opening the possibility of using such a dye-sensitized approach for regeneration of nicotinamide adenine dinucleotide analogues in enzymatic and chemical catalysis

Pinpointing the Extent of Electronic Delocalization in the Re(I)-to-Tetrazine Charge-Separated Excited State Using Time-Resolved Infrared Spectroscopy

Pinpointing the Extent of Electronic Delocalization in the Re(I)-to-Tetrazine Charge-Separated Excited State Using Time-Resolved Infrared Spectroscop