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1. Chromophores inspired by the colors of fruit, flowers and wine

   https://doi.org/10.1515/pac-2019-0226  

   Silva, Cassio P. ; Silva, Gustavo T. ; Costa, Tássia de ; Carneiro, Vânia M. ; Siddique, Farhan ; Aquino, Adelia J. ; Freitas, Adilson A. ; Clark, John A. ; Espinoza, Eli M. ; Vullev, Valentine I. ; et al ( May 2019 , Pure and Applied Chemistry)

   Abstract Anthocyanins, which are responsible for most of the red, blue and purple colors of fruits and flowers, are very efficient at absorbing and dissipating light energy via excited state proton transfer or charge-transfer mediated internal conversion without appreciable excited triplet state formation. During the maturation of red wines, grape anthocyanins are slowly transformed into pyranoanthocyanins, which have a much more chemically stable pyranoflavylium cation chromophore. Development of straightforward synthetic routes to mono- and disubstituted derivatives of the pyranoflavylium cation chromophore has stimulated theoretical and experimental studies that highlight the interesting absorption and emission properties and redox properties of pyranoflavylium cations. Thus, p-methoxyphenyl substitution enhances the fluorescence quantum yield, while a p-dimethylaminophenyl substituent results in fast decay via a twisted intramolecular charge-transfer (TICT) state. Unlike anthocyanins and their synthetic analogs (flavylium cations), a variety of pyranoflavylium cations form readily detectable excited triplet states that sensitize singlet oxygen formation in solution and exhibit appreciable two-photon absorption cross sections for near-infrared light, suggesting a potential for applications in photodynamic therapy. These excited triplet states have microsecond lifetimes in solution and excited state reduction potentials of at least 1.3 V vs. SCE, features that are clearly desirable in a triplet photoredox catalyst.
2. Using ultra-fast spectroscopy to probe the excited state dynamics of a reported highly efficient thermally activated delayed fluorescence chromophore

   https://doi.org/10.1039/c8tc05957h  

   Vázquez, Ricardo Javier ; Kim, Hyungjun ; Zimmerman, Paul M. ; Goodson, Theodore ( April 2019 , Journal of Materials Chemistry C)

   Multiple ultrafast spectroscopic techniques and quantum chemical simulations (QCS) were used to investigate the excited state dynamics of BCC-TPTA. This organic chromophore is believed to possess excited state dynamics governed by a thermally activated delayed fluorescence (TADF) mechanism with a reported internal quantum efficiency ( η IQE ) of 84%. In addition, a significant enhancement in its quantum yield ( Φ ) in solution after purging oxygen has been reported. This Φ enhancement has been widely accepted as due to a delayed fluorescence process occurring on the μs time-scale. The spectroscopic measurements were carried out both in solution and blended films, and from fs to μs time-scales. The excited state dynamics of Rhodamine B and Ir(BT) 2 (acac) were also probed for comparison. Investigations in the absence of oxygen were also carried out. Our time-correlated single photon counting (TCSPC) measurements revealed a lack of a long-lived emissive lifetime for BCC-TPTA in any of the media tested. Our ns transient absorption spectroscopy (ns TAS) experiments revealed that BCC-TPTA does not possess triplet transient states that could be linked to a delayed fluorescence process. Instead, the evidence obtained from our spectroscopic tools suggests that BCC-TPTA has the excited state dynamics of amore »typical fluorescence chromophore and that just comparing the Φ difference before and after purging oxygen from the solution is not an accurate method to claim excited state dynamics governed by a delayed fluorescence mechanism. Consequently, we believe that previous studies, in which the photo-physics of organic chromophores with TADF characteristics are reported, may have overlooked the influence of the host materials on the obtained optical properties in blended films.« less
3. Toward rational design of TADF two-coordinate coinage metal complexes: understanding the relationship between natural transition orbital overlap and photophysical properties

   https://doi.org/10.1039/d2tc00163b  

   Li, Tian-yi ; Schaab, Jonas ; Djurovich, Peter I. ; Thompson, Mark E. ( March 2022 , Journal of Materials Chemistry C)

   A series of twelve two-coordinate coinage metal, Cu, Ag and Au, complexes with carbene-metal-amide structures were prepared. The complexes all display thermal assisted delayed fluorescence (TADF) emission at room temperature from interligand charge transfer (ICT) excited state with short lifetimes (less than 2 μs) and photoluminescent quantum yields that reach near unity. Owing to the involvement of the substituents in the emissive transitions and different metal ion volume, the natural transition orbital (NTO) overlap of the emissive state can be adjusted in a wide range from 0.21 to 0.41. Investigations on the relationship between the NTO overlap of the emissive state and key TADF photophysical properties demonstrated that both singlet–triplet energy gap and radiative decay rate of S 1 state increase along with the NTO overlap exponentially. Consequently, the overall TADF radiative decay rate leads to a maximum when plotted against the NTO overlap, giving the ideal zone from 0.25 to 0.30 for high TADF radiative decay rate in this class of two-coordinate coinage metal complex luminophores.
4. Experimental study of the proton-transfer reaction C + H ₂ ⁺ → CH ⁺ + H and its isotopic variant (D ₂ ⁺ )

   https://doi.org/10.1039/D0CP04810K  

   Hillenbrand, Pierre-Michel ; Bowen, Kyle P. ; Dayou, Fabrice ; Miller, Kenneth A. ; de Ruette, Nathalie ; Urbain, Xavier ; Savin, Daniel W. ( December 2020 , Physical Chemistry Chemical Physics)

   We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of E r ∼ 0.01–10 eV. We used photodetachment of C − to produce a pure beam of atomic C in the ground electronic 3 P term, with statistically populated fine-structure levels. The H 2 + and D 2 + were formed in an electron impact ionization source, with well known vibrational and rotational distributions. The experimental work is complemented by a theoretical study of the CH 2 + electronic system in the reactant and product channels, which helps to clarify the possible reaction mechanisms underlying the ICS measurements. Our measurements provide evidence that the reactions are barrierless and exoergic. They also indicate the apparent absence of an intermolecular isotope effect, to within the total experimental uncertainties. Capture models, taking into account either the charge-induced dipole interaction potential or the combined charge-quadrupole and charge-induced dipole interaction potentials, produce reaction cross sections that lie a factor of ∼4 above the experimental results. Based on our theoretical study, we hypothesize that the reaction is most likely to proceed adiabatically through the 1 4 A′ and 1 4 A′′more »states of CH 2 + via the reaction C( 3 P) + H 2 + ( 2 Σ+g) → CH + ( 3 Π) + H( 2 S). We also hypothesize that at low collision energies only H 2 + ( v ≤ 2) and D 2 + ( v ≤ 3) contribute to the titular reactions, due to the onset of dissociative charge transfer for higher vibrational v levels. Incorporating these assumptions into the capture models brings them into better agreement with the experimental results. Still, for energies ≲0.1 eV where capture models are most relevant, the modified charge-induced dipole model yields reaction cross sections with an incorrect energy dependence and lying ∼10% below the experimental results. The capture cross section obtained from the combined charge-quadrupole and charge-induced dipole model better matches the measured energy dependence but lies ∼30–50% above the experimental results. These findings provide important guidance for future quasiclassical trajectory and quantum mechanical treatments of this reaction.« less
5. Single occupancy spectroelectrochemistry of freely diffusing flavin mononucleotide in zero-dimensional nanophotonic structures

   https://doi.org/10.1039/c5fd00072f  

   Zaino, Lawrence P. ; Grismer, Dane A. ; Han, Donghoon ; Crouch, Garrison M. ; Bohn, Paul W. ( January 2015 , Faraday Discussions)

   Zero-mode waveguides (ZMW) have the potential to be powerful confinement tools for studying electron transfer dynamics at single molecule occupancy conditions. Flavin mononucleotide contains an isoalloxazine chromophore, which is fluorescent in the oxidized state (FMN) while the reduced state (FMNH 2 ) exhibits dramatically lower light emission, i.e. a dark-state. This allows fluorescence emission to report the redox state of single FMN molecules, an observation that has been used previously to study single electron transfer events in surface-immobilized flavins and flavoenzymes, e.g. sarcosine oxidase, by direct wide-field imaging of ZMW arrays. Single molecule electron transfer dynamics have now been extended to the study of freely diffusing molecules using fluorescence measurements of Au ZMWs under single occupancy conditions. The Au in the ZMW serves both as an optical cladding layer and as the working electrode for potential control, thereby accessing single molecule electron transfer dynamics at μM concentrations. Consistent with expectations, the probability of observing single reduced molecules increases as the potential is scanned negative, E appl < E eq , and the probability of observing emitting oxidized molecules increases at E appl > E eq . Different single molecules exhibit different electron transfer properties as reflected in the position ofmore »E eq and the distribution of E eq among a population of FMN molecules. Two types of actively-controlled electroluminescence experiments were used: chronofluorometry experiments, in which the potential is alternately stepped between oxidizing and reducing potentials, and cyclic potential sweep fluorescence experiments, analogous to cyclic voltammetry, these latter experiments exhibiting a dramatic scan rate dependence with the slowest scan rates showing distinct intermediate states that are stable over a range of potentials. These states are assigned to flavosemiquinone species that are stabilized in the special environment of the ZMW nanopore.« less