Charge-Transfer Controlled Exchange Interaction in Radical-Triplet Encounter Pairs as Studied by FT-EPR Spectroscopy

Dynamic Electron Polarization Lasting More Than 10 μs by Hybridizing Porphyrin and TEMPO with Flexible Linkers

Dynamic electron polarization (DEP), induced by quenching of photoexcited species by stable radicals, can hyperpolarize electron spins in solution at room temperature. Recently, development of technologies based on electron spin polarization such as dynamic nuclear polarization (DNP) has been progressing, where it is important to design molecules that achieve long-lasting DEP in addition to high DEP. Hybridization by linking dyes and radicals is a promising approach for efficient DEP, but strong interactions between neighboring dyes and radicals often result in the rapid decay of DEP. In this study, we introduce a flexible linker into the hybrid system of porphyrin and TEMPO to achieve both efficient DEP and long-lasting DEP. The structural flexibility of the linker switches the interaction between the radical and the triplet, which promotes the DEP process by bringing the radical and the triplet into close proximity, while avoiding abrupt relaxation due to strong interactions. As a result, the new hybridized system exhibits a larger DEP than the unlinked system, while at the same time achieving a DEP lasting more than 10 μs.

Singlet and triplet to doublet energy transfer: improving organic light-emitting diodes with radicals

Organic light-emitting diodes (OLEDs) must be engineered to circumvent the efficiency limit imposed by the 3:1 ratio of triplet to singlet exciton formation following electron-hole capture. Here we show the spin nature of luminescent radicals such as TTM-3PCz allows direct energy harvesting from both singlet and triplet excitons through energy transfer, with subsequent rapid and efficient light emission from the doublet excitons. This is demonstrated with a model Thermally-Activated Delayed Fluorescence (TADF) organic semiconductor, 4CzIPN, where reverse intersystem crossing from triplets is characteristically slow (50% emission by 1 µs). The radical:TADF combination shows much faster emission via the doublet channel (80% emission by 100 ns) than the comparable TADF-only system, and sustains higher electroluminescent efficiency with increasing current density than a radical-only device. By unlocking energy transfer channels between singlet, triplet and doublet excitons, further technology opportunities are enabled for optoelectronics using organic radicals.

Organic light-emitting diodes must be engineered to circumvent efficiency limits imposed by the ratio of triplet to singlet exciton formation, following electron-hole capture. Here, authors unlock energy transfer channels between singlet, triplet and doublet excitons using thermally activated delayed fluorescence and radical emitters towards more efficient light-emitting devices.

Harnessing Electron Spin Hyperpolarization in Chromophore-Radical Spin Probes for Subcellular Resolution in Electron Paramagnetic Resonance Imaging: Concept and Feasibility

Obtaining a subcellular resolution for biological samples doped with stable radicals at room temperature (RT) is a long-sought goal in electron paramagnetic resonance imaging (EPRI). The spatial resolution in current EPRI methods is constrained either because of low electron spin polarization at RT or the experimental limitations associated with the field gradients and the radical linewidth. Inspired by the recent demonstration of a large electron spin hyperpolarization in chromophore-nitroxyl spin probe molecules, the present work proposes a novel optically hyperpolarized EPR imaging (OH-EPRI) method, which combines the optical method of two-photon confocal microscopy for hyperpolarization generation and the rapid scan (RS) EPR method for signal detection. An important aspect of OH-EPRI is that it is not limited by the abovementioned restrictions of conventional EPRI since the large hyperpolarization in the spin probes overcomes the poor thermal spin polarization at RT, and the use of two-photon optical excitation of the chromophore naturally generates the required spatial resolution, without the need for any magnetic field gradient. Simulations based on time-dependent Bloch equations, which took into account both the RS field modulation and the hyperpolarization generation by optical means, were performed to examine the feasibility of OH-EPRI. The simulation results revealed that a spatial resolution of up to 2 fL can be achieved in OH-EPRI at RT under in vitro conditions. Notably, the majority of the requirements for an OH-EPRI experiment can be fulfilled by the currently available technologies, thereby paving the way for its easy implementation. Thus, the proposed method could potentially bridge the sensitivity gap between the optical and magnetic imaging techniques.

O2 solvation cavity in voids of ionic liquids studied by the solvatochromic red shift of O2(1Δg) phosphorescence

Phosphorescence spectroscopy of singlet oxygen [=O₂(¹Δ_g)] was applied to study the solvation properties of small solute molecule, O₂, in ionic liquids. Unlike conventional molecular solvents, the spectral red shift of the O₂(¹Δ_g) phosphorescence in ionic liquids from the gas phase was found to depend not only on the refractive index of solvents but also on the vdW volume of anions. This unusual spectral shift of the O₂(¹Δ_g) luminescence is interpreted by considering the size of solvation cavities in voids, which is estimated by analyzing the free volume in ionic liquids. These results suggest the potential of the O₂(¹Δ_g) phosphorescence spectral shift measurement in the study of molecular-scale voids in ionic liquids.

Distance-dependent formation of electronic charge-transfer states in the ground states of anthracene and pyrene covalently linked to a TEMPO free radical

Charge-transfer (CT) electronic states are generally seen in molecules involving interactions between species of low ionization potential and high electron affinity. In this context, the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) free radical is not considered to be a typical molecule to form charge transfer states with aromatic hydrocarbons. Nevertheless, involvement of such CT states has been invoked in rationalising the spin-dependent photophysical quenching of excited states of aromatic systems by TEMPO during bimolecular collisions. Direct observation of such CT states, however, has been elusive until recently, with our first report on the observation of CT states involving naphthalene and TEMPO moieties covalently linked through a spacer group (Rane et al., J. Fluoresc., 2015, 25, 1351-1361). With a view to demonstrating more systems of CT states involving a TEMPO donor, and establishing a possible dependence on its distance from an acceptor chromophore, we have now extended our investigation to anthracene (An) and pyrene (Py) moieties linked to TEMPO, using two different spacer groups of different lengths. The molecules are An-CH2-O-TEMPO, Py-CH2-O-TEMPO, Py-(CH2)2-O-TEMPO, Py-(CH2)4-O-TEMPO, Py-CH2-CO-O-TEMPO and Py-(CH2)3-CO-O-TEMPO, where a linear alkyl chain containing an ether or an ester moiety constitutes the spacer group. We established the formation of CT states in their ground states by comparing their electronic absorption spectra, steady-state fluorescence spectra and time-resolved fluorescence signals with those of the parent molecules An-CH2-OH, Py-CH2-OH and Py-CH2-COOH. CT bands of appreciable intensity were seen only with An-CH2-O-TEMPO, Py-CH2-O-TEMPO and Py-CH2-CO-O-TEMPO, the molecules with the shortest spacer group. Approximate shapes of the absorption and emission bands of the CT states have been determined. For the rest, very weak bands were seen. Similar trends were seen in their fluorescence lifetimes also. Absorption intensities of the CT bands were found to decrease exponentially with the length of the spacer group. The presence of the ether or the ester moiety in the spacer groups showed little influence on the intensities of the CT bands. Our results are probably the first experimental demonstration of the expected exponential dependence of the efficiency of the formation of CT states on the length of the spacer groups of chromophore-TEMPO linked molecules.

Radical-triplet pair mechanism of electron spin polarization. Detailed theoretical treatment

Specific features of net chemically induced dynamic electron spin polarization (CIDEP) P(n), generated in liquid-phase triplet-radical (TR) quenching, are analyzed in detail within the general model, which allows for fairly simple analysis of CIDEP both numerically and analytically. This model enables one to accurately treat nonadiabatic transitions between the terms of TR-pair spin Hamiltonian, resulting in CIDEP generation. The proposed theory predicts fairly simple analytical dependence of P(n) on parameters of the model. In particular, it is shown that within the wide region of parameters the P(n) dependence on the coefficient of relative TR diffusion D(r) is described by simple linear relation P(n)(-1)(D(r)) ≈ Q0 + q̅(n)D(r) (Q0 and q̅(n) are independent of D(r)). It is also demonstrated that obtained numerical and analytical results are very helpful for the analysis of experimental data, which is demonstrated by analyzing the experimental D(r)-dependence of P(n).