Advances in quantum information science (QIS) require the development of new molecular materials to serve as microwave addressable qubits that can be read out optically. Laser photoexcitation of organic π-conjugated molecules often results in spin-polarized phosphorescent triplet states that can be readily observed and manipulated using time-resolved electron paramagnetic resonance (EPR) techniques. Photoexcitation of N-mesityl-1,8-naphthalimide (M-NMI) and its phosphorus analogues, 2-mesitylbenzoisophosphinoline (M-BIPD) and 2-mesitylbenzoisophosphinoline oxide (M-BIPDO) results in ultrafast spin-orbit charge-transfer intersystem crossing to form the corresponding phosphorescent triplet states M-3*NMI, M-3*BIPD and M-3*BIPDO. The ultrafast triplet formation dynamics, phosphorescence, and spin-polarized EPR spectra of these triplet states were examined. The most promising qubit candidate, M-3*BIPD, was examined using pulse-EPR to measure its spin relaxation times, and pulse electron-nuclear double resonance spectroscopy to perform a two-qubit CNOT gate using the phosphorus nuclear spin.
There has been increasing interest in the excited states of stable diradicals as means of manipulating their spin states for potential applications in quantum information science (QIS). In this work, we examine a set of diradicals composed of two stable naphthalene-1,8:4,5-bis(dicarboximide) radical anions (NDIâ¢-) bound either directly at their imide nitrogen atoms or through a series of benzene spacers resulting in diradicals with either singlet or triplet ground states. We use time-resolved near-UV, visible, near-IR, and mid-IR spectroscopy to show that the population in the singlet ground state can undergo photoinduced electron transfer upon excitation of one of the NDIâ¢- radicals to produce the NDI0-NDI2- moiety, while the corresponding triplet population cannot. In particular, spectroscopy in the wavelength region 330-450 nm and in the energy range 1450-1750 cm-1 is critical to distinguishing the two populations. By varying the connectivity between the two radical anions, we vary both the sign and magnitude of the singlet-triplet energy splitting (2J) of the diradicals, thereby varying the proportion of singlet and triplet ground state populations that are detected optically. EPR spectroscopy provides corroborating evidence for the ground spin state of the diradicals. This result has implications for using photoexcitation to manipulate the spin states of diradicals for QIS applications.
Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4'-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S1) energy of 4,4'-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps)-1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4'-methoxy-BPEA and 4,4'-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps)-1 and (120 ± 10 ps)-1, and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4'-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids.
The 1-substituted-4-imino-1,2,3-triazole motif is an established component of coordination compounds and bioactive molecules, but depending on the substituent identity, it can be inherently unstable due to Dimroth rearrangements. This study examined parameters governing the ring-degenerate rearrangement reactions of 1-substituted-4-imino-1,2,3-triazoles, expanding on trends first observed by L'abbé et al. The efficiency of condensation between 4-formyltriazole and amine reactants as well as the propensity of imine products towards rearrangement was each strongly influenced by the substituent identity. It was observed that unsymmetrical condensation reactions conducted at 70 °C produced up to four imine products via a dynamic equilibrium of condensation, rearrangement and hydrolysis steps. Kinetic studies utilizing 1-(4-nitrophenyl)-1H-1,2,3-triazole-4-carbaldehyde with varying amines showed rearrangement rates sensitive to both steric and electronic factors. Such measurements were facilitated by a high throughput colorimetric assay to directly monitor the generation of a 4-nitroaniline byproduct.
We demonstrate that the 10-phenyl-10 H-phenothiazine radical cation (PTZ+â¢) has a manifold of excited doublet states accessible using visible and near-infrared light that can serve as super-photooxidants with excited-state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photoexcitation of PTZ+⢠in CH3CN with a 517 nm laser pulse populates a Dn electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D1' (τ < 0.3 ps), followed by relaxation to D1 (τ = 10.9 ± 0.4 ps), which finally decays to D0 (τ = 32.3 ± 0.8 ps). D1' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D1'âD1 relaxation time (τ = 19 ± 2 ps). To probe the oxidative power of PTZ+⢠photoexcited doublet states, PTZ+⢠was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenylanthracene (DPA), and 10-phenyl-9-anthracenecarbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs SCE, respectively. In all three cases, photoexcitation wavelength dependent ultrafast hole transfer occurs from Dn, D1', or D1 of PTZ+⢠to Per, DPA, and ACN. The ability to take advantage of the additional oxidative power provided by the upper excited doublet states of PTZ+⢠will enable applications using this chromophore as a super-oxidant for energy-demanding reactions.
A tandem method for preparing 4-formyl-1,2,3-triazoles via a two-step one-pot acetal cleavage/CuAAC reaction was developed. Using this method, 4-formyl-1,2,3-triazole analogs with both electron-withdrawing and electron-donating substituents were prepared in good yield and purity. Expansion of this method to a three-step tandem reaction that incorporates an additional step of azide substitution was also successful, circumventing the need for organic azide isolation. This one-pot method, noteworthy in its simplicity and mild conditions, utilizes practical, readily available reactants and relies on protic solvent to promote acid-catalyzed acetal cleavage.
A series of ethynylarene compounds containing 2-(1,2,3-triazol-4-yl)pyridine chelating units were studied as fluorescent chemosensors for metal cations in aqueous solution. Analogs possessing two chelating units bridged by either 1,4-diethynylphenyl or 2,7-diethynylnaphthyl subunits displayed large hypsochromic shifts coupled with signal intensification when exposed to increasing concentrations of Ni(II), a unique response among 22 metal cation analytes. This response was shown to be reversible, and is proposed to derive from disruption of aggregate formation upon Ni(II) binding at the peripheral chelating units.
