First Monte Carlo beam model for ultra-high dose rate radiotherapy with a compact electron LINAC.

BACKGROUND: FLASH radiotherapy based on ultra-high dose rate (UHDR) is actively being studied by the radiotherapy community. Dedicated UHDR electron devices are currently a mainstay for FLASH studies. PURPOSE: To present the first Monte Carlo (MC) electron beam model for the UHDR capable Mobetron (FLASH-IQ) as a dose calculation and treatment planning platform for preclinical research and FLASH-radiotherapy (RT) clinical trials. METHODS: The initial beamline geometry of the Mobetron was provided by the manufacturer, with the first-principal implementation realized in the Geant4-based GAMOS MC toolkit. The geometry and electron source characteristics, such as energy spectrum and beamline parameters, were tuned to match the central-axis percentage depth dose (PDD) and lateral profiles for the pristine beam measured during machine commissioning. The thickness of the small foil in secondary scatter affected the beam model dominantly and was fine tuned to achieve the best agreement with commissioning data. Validation of the MC beam modeling was performed by comparing the calculated PDDs and profiles with EBT-XD radiochromic film measurements for various combinations of applicators and inserts. RESULTS: The nominal 9 MeV electron FLASH beams were best represented by a Gaussian energy spectrum with mean energy of 9.9 MeV and variance (σ) of 0.2 MeV. Good agreement between the MC beam model and commissioning data were demonstrated with maximal discrepancy < 3% for PDDs and profiles. Hundred percent gamma pass rate was achieved for all PDDs and profiles with the criteria of 2 mm/3%. With the criteria of 2 mm/2%, maximum, minimum and mean gamma pass rates were (100.0%, 93.8%, 98.7%) for PDDs and (100.0%, 96.7%, 99.4%) for profiles, respectively. CONCLUSIONS: A validated MC beam model for the UHDR capable Mobetron is presented for the first time. The MC model can be utilized for direct dose calculation or to generate beam modeling input required for treatment planning systems for FLASH-RT planning. The beam model presented in this work should facilitate translational and clinical FLASH-RT for trials conducted on the Mobetron FLASH-IQ platform.

Solvent Dependence of Cationic-Exciplex Emission: Limitation of Solvent Polarity Functions and the Role of Hydrogen Bonding.

Conventional exciplexes are products of excited-state charge generation reactions between neutral reactants (e.g., A* + D → A•-D•+), whereas cationic exciplexes are products of charge shift reactions of cations with neutral donors (e.g., A+* + D → A•D•+). Compared herein is the solvent-dependent fluorescence of a cationic exciplex with extant data for conventional exciplexes. Although linear correlations of conventional exciplex emission maxima with the Lippert-Mataga solvent polarity function are well documented in low to moderate polarity solvents, the correlations are often poor in more polar solvents. A number of such plots in moderate to high polarity solvents show a strong curvature. Intriguingly, for these same cases, plots of emission maxima versus the solvent dielectric constant (ε) are remarkably linear. Interestingly, emission maxima for the cationic exciplex of 1+ in nitrile and alcohol solvents also correlate linearly with ε. The solvent dependency for cationic exciplex emission maxima on ε is ca 1/3 of that for conventional exciplexes, which is ascribed to solvent stabilization of both the excited state and the ground state for cationic exciplexes. Differences in exciplex emissions between nitrile and alcohol solvents for 1+ are attributed to hydrogen bonding, with larger differences in higher hydrogen-bond acidity solvents.

Transition from charge-transfer to largely locally excited exciplexes, from structureless to vibrationally structured emissions.

Exciplexes of 9,10-dicyanoanthracene (DCA) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge-transfer (CT) character, when the donor has a relatively low oxidation potential (Eox ), e.g. hexamethylbenzene (HMB). With increasing Eox and stronger mixing with a locally excited (LE) state, vibrational structure begins to appear with 1,2,3,5-tetramethylbenzene and becomes prominent with p-xylene (p-Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g., p-Xy(•+) , are stabilized more in acetonitrile than the heavily substituted HMB(•+) . Remarkably, ion pairing with DCA(•-) in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low-Eox donors are almost identical except for displacements that approximately equal the differences in Eox , even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.

Charge sensed Pauli blockade in a metal-oxide-semiconductor lateral double quantum dot.

We report Pauli blockade in a multielectron silicon metal-oxide-semiconductor double quantum dot with an integrated charge sensor. The current is rectified up to a blockade energy of 0.18 ± 0.03 meV. The blockade energy is analogous to singlet-triplet splitting in a two electron double quantum dot. Built-in imbalances of tunnel rates in the MOS DQD obfuscate some edges of the bias triangles. A method to extract the bias triangles is described, and a numeric rate-equation simulation is used to understand the effect of tunneling imbalances and finite temperature on charge stability (honeycomb) diagram, in particular the identification of missing and shifting edges. A bound on relaxation time of the triplet-like state is also obtained from this measurement.

Adiabatic thermo-optic Mach-Zehnder switch.

In this Letter, we propose and demonstrate a high-speed and power-efficient thermo-optic switch using an adiabatic bend with a directly integrated silicon heater to minimize the heat capacity and therein maximize the performance of the thermo-optic switch. A rapid, τ=2.4 µs thermal time constant and a low electrical power consumption of P(π)=12.7 mW/π-phase shift were demonstrated representing a P(π)τ product of only 30.5 mW·µs in a compact device with a phase shifter of only ~10 µm long.

Bimolecular electron transfers that deviate from the Sandros-Boltzmann dependence on free energy: steric effect.

As we reported recently, endergonic to mildly exergonic electron transfer between neutral aromatics (benzenes and biphenyls) and their radical cations in acetonitrile follows a Sandros-Boltzmann (SB) dependency on the reaction free energy (ΔG); i.e., the rate constant is proportional to 1/[1 + exp(ΔG/RT)]. We now report deviations from this dependency when one reactant is sterically crowded: 1,4-di-tert-butylbenzene (C1), 1,3,5-tri-tert-butylbenzene (C2), or hexaethylbenzene (C3). Obvious deviation from SB behavior is observed with C1. Stronger deviation is observed with the more crowded C2 and C3, where steric hindrance increases the interplanar separation at contact by ~1 Å, significantly decreasing the π orbital overlap. Consequently, electron transfer (k(et)) within the contact pair becomes slower than diffusional separation (k(-d)), causing deviation from the SB dependency, especially near ΔG = 0. Fitting the data to a standard electron-transfer theory gives small matrix elements (~5-7 meV) and reasonable reorganization energies. A small systematic difference between reactions of C3 with benzenes vs biphenyls is rationalized in terms of small differences in the electron-transfer parameters that are consistent with previous data. The influence of solvent viscosity on the competition between k(et) and k(-d) was investigated by comparing reactions in acetonitrile and propylene carbonate.

Vertical junction silicon microdisk modulators and switches.

Vertical junction resonant microdisk modulators and switches have been demonstrated with exceptionally low power consumption, low-voltage operation, high-speed, and compact size. This paper reviews the progress of vertical junction microdisk modulators, provides detailed design data, and compares vertical junction performance to lateral junction performance. The use of a vertical junction maximizes the overlap of the depletion region with the optical mode thereby minimizing both the drive voltage and power consumption of a depletion-mode modulator. Further, the vertical junction enables contact to be made from the interior of the resonator and therein a hard outer wall to be formed that minimizes radiation in small diameter resonators, further reducing the capacitance and drive power of the modulator. Initial simple vertical junction modulators using depletion-mode operation demonstrated the first sub-100 fJ/bit silicon modulators. With more intricate doping schemes and through the use of AC-coupled drive signals, 3.5 µm diameter vertical junction microdisk modulators have recently achieved a communications efficiency of 3 fJ/bit, making these modulators the smallest and lowest power modulators demonstrated to date, in any material system. Additionally, the demonstration was performed at 12.5 Gb/s, required a peak-to-peak signal level of only 1 V, and achieved bit-error-rates below 10(-12) without requiring signal pre-emphasis. As an additional benefit to the use of interior contacts, higher-order active filters can be constructed from multiple vertical-junction modulators without interference of the electrodes. Doing so, we demonstrated second-order active high-speed bandpass switches with ~2.5 ns switching speeds, and power penalties of only 0.4 dB. Through the use of vertical junctions in resonant modulators, we have achieved the lowest power consumption, lowest voltage, and smallest silicon modulators demonstrated to date.

Reexamination of the Rehm-Weller data set reveals electron transfer quenching that follows a Sandros-Boltzmann dependence on free energy.

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A(•-)/D(•+) for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, k(q), to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm-Weller formula rests on an incorrect mechanism. We have remeasured k(q) for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and k(q), including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros-Boltzmann (SB) equation k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A(•-)/D(•+)). The quantity k(lim) represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A(•-)/D(•+) vs A(•-) + D(•+)). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of -0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A(•-)/D(•+) relative to the free A(•-) + D(•+), which opposes the Coulombic stabilization of A(•-)/D(•+). The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A(•-) + D(•+)). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of k(q) on ΔG.

Bimolecular electron transfers that follow a Sandros-Boltzmann dependence on free energy.

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A(•+) + B ⇌ A + B(•+)) in acetonitrile could be fit well by a simple Sandros-Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant k(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P(+)*) by neutral donors (P(+)* + D → P(•) + D(•+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k(q)) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (E(redox) - E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P(•)/D(•+)), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A(•-) + D(•+)) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log k(q) on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A(•-)/D(•+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A(•-)/D(•+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A(•-)/D(•+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ⇌ exciplex ⇌ A(•-)/D(•+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern-Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ∼+0.1 eV. Beyond this point, additional exciplex deactivation pathways-fluorescence, intersystem crossing, and nonradiative decay-are likely to dominate, resulting in a change in mechanism.

Photophysical properties of the series fac- and mer-(1-phenylisoquinolinato-N∧C2')(x)(2-phenylpyridinato-N∧C2')(3-x)iridium(III) (x = 1-3).

The photophysical properties of tris-cyclometalated iridium(III) complexes have been probed by chemical and geometric variation through the series fac- and mer-Ir(piq)(x)(ppy)(3-x) (x = 1-3; piq = 1-phenylisoquinolinato-N(∧)C(2'), ppy = 2-phenylpyridinato-N(∧)C(2')). The phosphorescent decays were recorded in solution at 295 K and in polymer films from 2 to 295 K. In the heteroleptic complexes, emission occurs based solely on the piq ligand(s), at least by the nanosecond time scale, as its excited states are the lowest energy. Because fac-Ir(piq)(3) and fac-Ir(ppy)(3) possess practically the same oxidation potential, comparison of photophysical properties through the series fac-Ir(piq)(x)(ppy)(3-x) (x = 1-3) revealed the effects of having one, two, or three emissive piq ligands with no confounding effects from differences in electron withdrawing or donating properties between the spectator ppy ligands and the piq ligands. Effects of placement of piq ligands in different coordination geometries were elucidated by comparisons to the mer series.

E-type delayed fluorescence of a phosphine-supported Cu2(mu-NAr2)2 diamond core: harvesting singlet and triplet excitons in OLEDs.

A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was shown to exhibit E-type delayed fluorescence by variable temperature emission spectroscopy and photoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet excited states were assigned as charge transfer states on the basis of theoretical calculations and the small observed S(1)-T(1) energy gap. Vapor-deposited OLEDs doped with the complex in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the delayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several physical methods, including electrically detected EPR, cyclic voltammetry, and photoluminescence in the presence of applied current.

Narrow-line and broadband spectra of iridium(III) complexes in a Shpol'skii matrix and an amorphous host.

Narrow-line emission and excitation spectra of the lowest triplet state have been studied in a Shpol'skii host (n-octane) for three cyclometalated IrL(2)acac complexes of current practical interest. The ligands L and acac are the C-deprotonated anions of 2-phenylpyridine, 2-benzothienylpyridine, 1-phenylisoquinoline, and acetylacetone. The occurrence of narrow lines is surprising for such nonplanar structures. Individual triplet sublevels are identified, and a striking intensity effect is observed in a magnetic field. Broadband spectra and lifetime data of the complexes in PMMA yield additional properties of the individual sublevels-radiative and nonradiative decay rates and emission spectra. The data are consistent with the assumption that these individual rates and spectra are approximately independent of temperature. For each complex, the decay rates vary by a factor of > or =20 among the three sublevels, and that of the lowest sublevel is the smallest, but rates of radiative and nonradiative decay generally vary in parallel for all three sublevels. The individual sublevels have significantly different spectra. These regularities appear to be universal characteristics of Ir(III) and Pt(II) complexes. They cannot be explained solely on the basis of the symmetries of the electronic states because the symmetry of the present complexes is too low.