The blinking and photobleaching dynamics of alizarin (1,2dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) are investigated using single-molecule spectroscopy. The time-dependent emission of alizarin and purpurin on glass under N 2 is analyzed using the change point detection (CPD) method to compile on-and off-event distributions. The number of distinct emissive events per molecule is about four times higher for alizarin relative to purpurin, consistent with an excited-state intramolecular proton transfer (ESIPT) process to populate an emissive tautomer state. To elucidate the mechanism for blinking (i.e., switching between on and off events), maximum likelihood estimation (MLE), goodness-of-fit tests based on the Kolmogorov−Smirnov (KS) statistic, and the loglikelihood ratio (LLR) tests are used to establish the best fits to the on-and offinterval probability distributions. For both alizarin and purpurin the on intervals are log-normally distributed, and off intervals are Weibull distributed, consistent with a dispersive electron-transfer (ET) kinetics model for blinking (i.e., involving Gaussian-like distributions of activation barriers to ET). Further analysis of the blinking dynamics reveals that ET to a long-lived dark state most often precedes molecular photobleaching, where extended residency in the dark state increases the probability of photobleaching. Based on these findings, mechanisms for the blinking and photobleaching of alizarin and purpurin are proposed. The ability of alizarin to undergo ESIPT enables fast excited-state decay and decreases the probability of ET. In contrast, purpurin exhibits faster injection and slower back ET relative to alizarin, leading to increased photobleaching via a dark radical cation state.
Balancing dipolar and exchange coupling is essential for efficient Cross Effect DNP. This explains the complex performance of standard radicals (AMUPOL and HyTek) at high magnetic field and fast spinning.
Dynamic nuclear polarization (DNP) under magic-angle spinning (MAS) is transforming the scope of solid-state NMR by enormous signal amplification through transfer of polarization from electron spins to nuclear spins. Contemporary MAS-DNP exclusively relies on monochromatic continuous-wave (CW) irradiation of the electron spin resonance. This limits control on electron spin dynamics, which renders the DNP process inefficient, especially at higher magnetic fields and non cryogenic temperatures. Pulse-shaped microwave irradiation of the electron spins is predicted to overcome these challenges but hitherto has never been implemented under MAS. Here, we debut pulse-shaped microwave irradiation using arbitrary-waveform generation (AWG) which allows controlled recruitment of a greater number of electron spins per unit time, favorable for MAS-DNP. Experiments and quantum mechanical simulations demonstrate that pulse-shaped DNP is superior to CW-DNP for mixed radical system, especially when the electron spin resonance is heterogeneously broadened and/or when its spin−lattice relaxation is fast compared to the MAS rotor period, opening new prospects for MAS-DNP.
A new design principle for a mixed broad (TEMPO) and narrow (Trityl) line radical to boost the dynamic nuclear polarization efficiency is electron spin density matching, suggesting a polarizing agent of one Trityl tethered to at least two TEMPO moieties.
Polymers
that are elastic while supporting charge transport are
desirable for flexible and soft electronics. Many polymers with bulky
and conjugated redox-active pendant units have high glass transition
temperatures (T
g) in their neutral form
that will not lead to elasticity at room temperature. Their behavior
in charged form in the solid state without an electrolyte has not
been extensively studied. Here, the design strategy of polymeric ionic
liquid where two weakly interacting ionic groups are used to maintain
a low T
g is shown to lead to flexible
redox active polymers. The use of a flexible ethylene backbone and
redox-active phenothiazine (PTZ)-based pendant group resulted in polymers
with relatively low T
g that are electrically
conductive. PTZ that was N-substituted with 2-(2-ethoxyethoxy)ethoxy)ethyl
was found to promote solubility of the polymer and lower the T
g of the neutral polymer by ∼150 °C
relative to that of the T
g of a variant
without the N-substituent. Doping with trifluoromethanesulfonimide
leads to an electrically conductive polymer without significantly
increasing the T
g. Physical characterization
by UV–vis–NIR spectroscopy, electron spin resonance
spectroscopy, and impedance spectroscopy verified that the molecular
design leads to an efficient charge hopping between the PTZ groups.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.